Symposium EN11—Electrocatalytic Materials to Sustainably Convert Atmospheric C, H, O and N into Fuels and Chemicals

The novel use of CO₂ electroreduction to produce carbon-based components that do not contribute to the Greenhouse effect has encouraged the vision of carbon dioxide as a renewable feedstock with the goal to reach net zero emissions into the atmosphere.
The variety of useful products that may be obtained from such an electrochemical process strongly depends on the material of selection as electrocatalyst. In this research, we have focused on the `coinage’ metals, i.e. copper, silver and gold, due to their accessibility and the vast literature available. Whereas gold and silver are very selective to carbon monoxide, copper produces wide variety of components, covering liquids (such as methanol, formate and ethanol) and gases (such as carbon monoxide, methane and ethylene). However, one aspect that has not been yet addressed in detail and which heavily affects the chemical reaction efficiency is the CO₂ availability to the working electrode. In this aspect, the performance of any chemical reaction is significantly affected by the concentration of reactant gas in the proximity of the reacting site. Therefore, the overall chemical reaction efficiency strongly depends on the mass transfer capabilities of the system. A means of measuring the performance of the carbon dioxide electroreduction process is by comparing the total amount of generated carbon-based products and the amount of produced hydrogen. Hydrogen is originated at the parallel side-reaction occurring at the same time than CO₂ electroreduction. Its formation implies then that not all the current invested in the system is dedicated to consume carbon dioxide. Even though H₂ co-evolution is unavoidable with current techniques, other novel technologies (such as the gas diffusion electrode) are being developed to optimize the partial current exclusively dedicated to reduce CO₂. Here, mass transfer capabilities play a major role: the reaction consumes CO₂ in areas close to the electrode surface and originates a gas depletion environment that needs to be compensated. Therefore, in quiescent electrochemical cells, gas depletion drastically affects the overall chemical efficiency, especially in the long run.
In this research, we present a detailed description of the mass transport limitations originated by diffusion alone compensating for gas depletion in the vicinity of the electrode. This investigation is particularly important and relevant for the cases in which a reaction is maintained over long periods of time, as it would be the case for industrial scale CO₂ electroreduction. More specifically, this work focuses on the evolution of gas depletion over time as the reaction consumes the available CO₂ and provides an indirect accurate means of measuring that evolving gas concentration by pH. Indeed, the level of pH near the electrode’s surface is based on two major contributions: the decrease in the CO₂ concentration due to gas depletion and the generation of ions during chemical reaction. We develop a mathematical model that accounts for the evolution of the gas concentration in areas surrounding the electrode and refer it to the experimentally-measured pH.
Finally, we conclude that CO₂ depletion is the cause of the strong decay in the chemical efficiency of a continuous chemical reaction. Pulsed electrochemistry is then proposed as an alternative method to compensate for such an effect. In this scenario, a certain current is cycled on and off periodically, allowing the electrodes to react for a certain period of time, letting the system rest afterwards. This way, gas diffusion through the liquid electrolyte is able to partially compensate for the consumed CO₂ in the vicinity of the electrode. The overall chemical efficiency can be then maintained over longer periods of time. However, the total production is compromised. A discussion over the preferability of efficiency over production rate is then debated at the end and open to discussion.

The electrochemical reduction of CO₂ offers the dual benefits of recycling CO₂, thereby mitigating emissions, and producing intermediates for valuable fuels and chemicals such as CO, formic acid, ethylene, and ethanol in a carbon-neutral manner.^1,2 Previous research has focused on the application of gas diffusion electrodes (GDEs) in CO₂ reduction systems, which has enabled the conversion of CO₂ into products at a high rate.^3,4 Despite these GDEs requiring a lifetime of at least 3000 hours for economic feasibility, their durability has not been studied widely.^4,5 In fact, many GDEs studied to date show declining performance over time, the causes of which have not been determined. Progress in this area, in particular with respect to understanding GDE degradation mechanisms, will be crucial for further development of CO₂ reduction technology.
This presentation will discuss post-testing changes in GDE cathode surface morphology and composition as a function of electrolyte composition and applied current density/potential. We investigated these changes via a variety of surface and bulk characterization techniques (SEM, EDS, Micro-CT, XRD). This approach identified degradation mechanisms including carbonate deposit formation, surface passivation, and catalyst layer restructuring, leading to loss of catalyst availability. In particular, we determined that a highly alkaline electrolyte, although beneficial for achieving high current densities and efficiency for CO production, causes the rapid formation of carbonate deposits on the GDE surface due to high local pH at the reaction interface.⁷ The extent of carbonate deposit formation depends heavily on electrolyte cation identity and concentration. We also explored the impact of potential cycling on the catalyst later, noting significant restructuring and loss of catalyst layer when more positive potentials are repeatedly applied to the cathode. By using a combination of materials characterization techniques and electrochemical methods, we are able to image the evolution of the GDE surface, determine which conditions lead to loss of catalyst surface area, and better understand how to develop long-lasting electrodes for electrochemical reduction of CO₂ to value-added chemicals.

References:
Lee, M.-Y., Park, K. T. Lee, W., Lim H., Kwon, Y., Kang, S., Crit Rev Env Sci Tec 50, 789-815 (2019).
Endrodi, B., Bencsik, G., Darvas, F., Jones, R., Rajeshwar, K., Janaky, C., PECS 62, 133-154 (2017).
Weng, L.-C., Bell, A., Weber, A., Phys. Chem. Chem. Phys. 20, 16973-16984 (2018).
Cofell, E., Nwabara, O., Negro, E., Verma, S., Kenis, P. J. A., Chemsuschem 13, 855-875 (2020).
Verma, S., Kim, B., Jhong, H., Ma, S. C. & Kenis, P. J. A. Chemsuschem 9, 1972-1979 (2016).
Kim, B., Hillman, F., Ariyoshi, M., Fujikawa, S. & Kenis, P. J. A. J Power Sources 312, 192-198 (2016)
Cofell, E., Nwabara, O., Bhargava, S., Henckel, D., Kenis, P. J. A., ACS Appl. Matter. Interfaces 13, 15132-15142 (2021).

Carbon dioxide reduction in a gas diffusion electrode is a complex chemical process with potential for achieving electrocatalytic product synthesis with high throughput and selectivity. Spatially and time-resolved chemical measurements have the potential to elucidate the kinetic and transport processes in this complex environment. To this end, we report results of operando local pH measurements with confocal fluorescence microscopy around an operating CO₂ reduction gas diffusion electrode that enable understanding about CO₂ reduction electrocatalysis. Because the pH varies on the micrometer-scale close to the surface of an electrode, measuring the bulk pH does not provide a sufficiently detailed picture to understand the local reaction conditions. In our first experiments we have compiled three-dimensional pH maps with micron-scale resolution that are indicative of CO₂ flow through the inhomogeneous morphology of a porous gas diffusion electrode permeated by a periodic array of cracks in the microporous layer. These maps provide clear evidence for inhomogeneous CO₂ diffusion in the microporous layer, such that CO₂ diffuses faster through cracks and larger pores in the electrode. As CO₂ travels through a gas diffusion electrode and dissolves in the electrolyte, it lowers the local pH due to carbonate reactions. Therefore, the local pH can provide valuable information about the diffusion of CO₂ though different electrode morphologies. However, when an electrical current is applied, the pH around a gas diffusion electrode increases because CO₂ reduction reaction and the hydrogen evolution reaction both consume protons. The interplay between these two effects is crucial to understand and tailor the reaction microenvironment around the working electrode surface.
We use the two-color fluorescent pH sensitive dye 8-Aminopyrene-1,3,6-trisulfonic-acid-trisodium-salt (APTS) that is sensitive between pH 11.5 and 14 to spatially resolve the pH close to the surface of an operating gas diffusion electrode. The dye is dissolved with a concentration of 125 µM in 2 M KOH electrolyte and excited by a laser at 458 nm under a confocal microscope. The fluorescence of the dye is collected separately for two wavelength intervals from 480-550 nm and 551-754 nm and the ratio of the emission collected from the two excitations is a measure for the pH. The confocal microscope allows to excite the dye at different heights in z above and in cracks in porous gas diffusion electrodes. Through movement of the stage in x- and y-direction and taking measurements consecutively, we can map the pH time-resolved in all three spatial dimensions. With this setup, we performed time-dependent measurements without any applied current to investigate how the CO₂ flux through different gas diffusion electrode substrates made of porous PTFE and carbon paper influences the pH. We found that CO₂ dissolves faster through gas diffusion electrodes made of PTFE due to its larger pore size. We also investigated how the CO₂ permeates through different cracks 10s of microns wide in the carbon paper support and found that the pH first changes locally within the crack relative to the planar electrode surface. Additionally, cracks promote faster CO₂ transport through the electrode. To investigate if this is advantageous for CO₂ reduction, we will report results of experiments with current densities in the 0-30 mA/cm² range. Further, we will discuss the local pH at the electrode surfaces for different gas diffusion electrode structures with the goal to identify ideal gas diffusion electrode designs. For this, we will present three-dimensional pH maps with different applied current densities for PTFE working electrode supports with different pore sizes.
We expect that these results will provide valuable insights into the interplay between local pH and CO₂ reduction mechanisms for differently structured gas diffusion electrodes.

CO₂ electroreduction offers the opportunity to convert a waste and greenhouse gas into valuable products using renewable electricity. CO₂ electroreduction on copper results in a plethora of carbonaceous products unselectively and generates hydrogen. Methods for tuning the selectivity include controlling the reduction potential, modifying the catalyst morphology, adjusting the electrolyte pH, changing the reactor design, and altering the electrolyte composition. By including additives in the electrolyte, the CO₂ solubility, pH, conductivity and interactions at the electrode interface can be modified, resulting in changes to the product selectivity in CO₂ electroreduction. Salts and ionic liquids, added at low concentrations to aqueous bicarbonate electrolytes, can be promising modifications to the electrolyte while still maintaining the favorable buffering ability that bicarbonates bring. The hydrophilicity of the anion plays a significant role in the product selectivity on copper. For example, when the highly hydrophilic dicyanamide ([DCA]^-) – based additives are used, CO₂ electroreduction is effectively shutoff and hydrogen evolution is enhanced. In contrast, highly hydrophobic bis(trifluoromethylsulfonyl)imide ([NTF₂]^-)-based additives lower hydrogen evolution activity and increase formate production. XPS, EQCM and FTIR suggest that the surface coverage of the additives significantly influences CO₂ electroreduction.

Electrochemical reduction of CO₂ to CO, CH₄, and other value-added products provides compelling opportunities to realize the sustainable production of commodity chemicals. While increasingly demonstrated in lab-scale systems, a critical roadblock slowing the development of industrial processes is a poor understanding of how molecular assembly at electrode-electrolyte interfaces contributes to catalytic activity. Recently, ionic liquids have emerged as promising CO₂ reduction electrolytes, since low overpotentials and enhanced product selectivity has been observed for several classes of ionic liquids. However, the mechanisms of ionic liquid-mediated catalysis remain subject to ongoing discussion. Here, we present our research on tuning the collective assembly of ionic liquids to understand mechanisms of ionic liquid-mediated CO₂ reduction at silver, copper, and carbon surfaces. By changing ion structures, concentrations, and solvent environments, we reveal unexpected scaling relationships between structure, concentration, and reactivity. Notably, our findings point to collective assembly as a key factor underpinning structure-reactivity relationships for CO₂ electro-reduction. To highlight the impact of this concept, we will show how our findings help resolve mechanistic puzzles in the ionic liquids electrocatalysis community and suggest new strategies for controlling the performance of sustainable CO₂ electrochemical upgrading.

Local reaction environments where catalysts operate have been considered as critical as intrinsic active site nature of the catalysts in governing overall catalytic performance. For instance, enzymes are known to attain superior catalytic specificity and activity by creating optimal local reaction environments around their active sites using amino acid side chains. However, creating such ideal microenvironment for synthetic catalysts has been a grand challenge. Recently, we have found that nanoparticle surface ligands (e.g., alkylphosphonic acid), inevitably introduced for colloidal nanoparticle synthesis, in fact can actively create favorable microenvironment under electrochemical bias for selective CO₂-to-CO electroconversion. This microenvironment, so-called nanoparticle/ordered-ligand interlayer (NOLI), is composed of a metal (e.g., silver) nanoparticle and a structurally ordered ligand layer in a detached state. The interlayer created between the nanoparticle surface and the ordered-ligand layer encompasses dehydrated cations, associated with the ligand layer in the vicinity, exhibiting pseudocapacitive characteristics. This microenvironment offers unique active site centers through enhanced electrostatic interactions between the NOLI constituents and adsorbed CO₂ molecules, significantly enhancing catalytic turnover. In this talk, I will present how this catalytic interlayer can be created and its catalytic effect for selective CO₂ electroreduction. We also show the generality of the NOLI catalyst design, which is demonstrated across several transition metal nanoparticles, ultimately achieving up to 99% selectivity towards CO at low overpotentials. Furthermore, the NOLI catalyst is translated to a gas-diffusion environment, retaining nearly unit CO selectivity up to high current densities in neutral media (e.g., 98.1% at 400 mA/cm² in 1 M KHCO₃). In addition, I will also present our recent spectroscopic results that give insights into how initial nanoparticle assembly on a support, found necessary for the formation of the structurally ordered ligand layer and thus the NOLI, can enhance intermolecular ligand interactions and promote collective behavior of the ligands. I will conclude by giving a broad perspective on new opportunities in harnessing nanoparticle ligand interactions for developing advanced electrocatalysts.

With continuously decreased price of renewable electricity, it becomes increasingly attractive to utilize electrical energy for the production of fundamental chemical feedstocks or fuels. Electrochemical CO₂ reduction reaction (CO₂RR) provides a green and sustainable “bottom-up” pathway to generate basic chemical feedstocks ranging from C₁ to C₃ products. Liquid fuels from CO₂RR, such as formic acid, methanol, acetic acid, ethanol, and propanol, are of particular interest due to their high energy densities and ease of storage and distribution. However, this route is still retarded by the following two long-standing challenges in the field: there is still a lack of highly selective, active, and stable catalysts for CO₂RR to produce target liquid fuels, especially for high-value products beyond formic acid; another challenge is that the liquid fuels generated from CO₂RR electrolysis are always in a mixture with conventional liquid electrolyte, which subsequently necessitates energy- and cost-intensive downstream separation processes to recover pure liquid fuels.
Through integrating rational designs in both catalyst and reactor, here we demonstrate a direct and continuous generation of high-purity and high-concentrated liquid fuels via electrochemical CO₂/CO reduction. By designing a novel porous solid electrolyte (PSE) layer between cathode and anode, other than traditional liquid electrolyte, efficient ionic transport was successfully maintained while no impurity ions (e.g. K⁺, Na⁺, HCO₃^-, OH^-…) were mixed into generated liquid fuel. When employed in this novel PSE reactor, we successfully demonstrated directly producing pure formic acid solutions from CO₂RR on Bi-based catalysts with nearly 100 % purity. Through flexible tuning of the carrier gas stream, we also achieved an ultra-high concentration up to nearly 100 wt.%. Meanwhile, by tuning the edge-to-surface ratio of Cu nanocube catalyst, we demonstrated an unprecedented acetate performance in neutral pH with an ultrahigh relative purity of up to 98% and concentrations up to 2 wt.%, with excellent stability of over 150 h continuous operation.

Atomically dispersed and nitrogen coordinated single metal sties (MN_x: M=Ni, Co, and Fe) embedded in carbon support have exhibited exceptional catalytic activity and selectivity for the electrochemical CO ₂ reduction reaction (CO2RR) to CO. However, the understanding of the intrinsic and morphological factors contributing to the catalytic properties of MN_x sites is still lacking. Using a Fe-N-C model catalyst derived from the ZIF-8, we deconvoluted three key morphological and structural elements of FeN ₄ sites, including particle sizes of catalysts, Fe content, and Fe-N bond structures. Their monotonous effects on the CO2RR were comprehensively elucidated. Engineering the particle size and Fe doping is critical to control extrinsic morphological factors of FeN ₄ sites for optimal porosity, electrochemically active surface areas, and the graphitization of the carbon support. In contrast, the intrinsic activity of FeN ₄ sites was only tunable by varying thermal activation temperatures during the formation of FeN ₄ sites, which impacted the length of the Fe-N bonds and the local strains. The structural evolution of Fe-N bonds was characterized at the atomic level. First-principles calculations further elucidated the origin of intrinsic activity improvement associated with the optimal local strain on the Fe-N bond. The obtained understanding can further apply to the design of other singles (e.g., Ni) and dual metal sites (Ni-Fe) catalysts

Today, the chemical industry manufactures most chemicals and fuels using energy-intense thermal / catalytic processes that are responsible for a significant fraction of anthropogenic CO₂ emissions. Impending climate change necessitates a drastic reduction of greenhouse gas emissions over the next few decades. Electrifying manufacturing of some intermediates or products holds promise to significantly reduce the carbon footprint of the chemical industry. While techno-economic (TEA) and life-cycle (LCA) analyses of the electrochemical reduction of CO₂ to products such as CO, formic acid, ethylene, and ethanol indicate their promise, these also highlight the challenge to compete in terms of cost with present-day chemical conversion processes operated at scale [1].

This contribution will elaborate on three approaches to bring economically feasible, (close to) carbon-neutral electrified chemical manufacturing (electrolysis) closer. First, while active and selective catalysts now exist for electroreduction of CO₂ to products such as CO, formic acid, ethanol, and ethylene, the electrode technology enabling electrolysis over 1000s of hours is still lacking. This presentation will summarize the range of encountered electrode degradation mechanisms [2], elaborate on accelerated durability CO₂ electrolysis testing methods, and present a few approaches to mitigate electrode degradation.
Second, the electroreduction of CO₂ is highly dependent on the exact operating conditions of the process. This presentation will summarize system design rules that enhance CO₂ electrolysis performance (Faradaic efficiency, energy efficiency, conversion efficiency) through fine-tuning of a combination of parameters [3]: (i) Electrolyte composition and pH; (ii) Electrolyte flowrate; (iii) CO₂ feed flowrate; (iv) Catalyst loading; and (v) The use of magnetic fields to enhance mass transfer.
Third, this presentation will elaborate on how performing desired chemical conversions on both electrodes will drastically reduce the energy required to drive these electrolysis processes individually. We found, for example, that co-conversion of CO₂ (reduction at the cathode) with organic waste molecules (oxidation at the anode) reduced the overall energy consumption by 40-50%.[4] Identifying suitable anode catalysts and electrodes for selective organic oxidations will be a major materials research challenge for the next decade. Co-conversion electrolysis process technology could be the starting point for a new generation of chemical manufacturing technology that is significantly more energy efficient than the traditional thermochemical processes, while also able to use renewable feedstocks, such as CO₂, water, biomass adducts, or organic waste streams.

We present results elucidating the role of magnetic moment on catalytic activation energy barriers with a first principles approach. While many structure-activity relationships have been uncovered, further insight is still needed to design new catalysts from the large elemental design space on high surface area supports with performance comparable to platinum group metals. Specifically, single atom transition metal catalysts on graphene defect moieties with different magnetic moments, but similar charge transfer, are found to exhibit differing degrees of asymmetry of spin states near the Fermi energy. The effect of spin-dependent chemical pathways for transition metal SAC supported on graphene defects has been examined only recently. Previously, chemical reactions undergoing spin crossing have been considered to be spin forbidden or too slow to be practical. However, transition metal SAC have been demonstrated to facilitate spin crossing at points of minimal energy difference (<0.1 eV), allowing reactions such as CO oxidation to proceed along pathways with lower energy barriers. We utilize a first principles approach to examine how the spin state of the SAC and reactants can affect catalytic energy barriers of V, Fe, Mo, and Ta on two different graphene defects with differing magnetic moments. Single vacancy and pyridinic N-doped single vacancy defect moieties were investigated since they have similar charge transfer to metal adatoms but differing magnetic moments. Climbing image-nudged elastic band (CI-NEB) calculations of activation energy barriers of CO oxidation on all transition metal SAC in relaxed geometries on both defect moieties show all barrier energies on N-doped single vacancy defects were 0.8 eV or less. These low activation energy barriers suggest that the CO oxidation reaction would proceed at room temperature with pyridinic N-dopants present in the defect environment. Due to the similar charge transfer calculated for both defect moieties, changes in activation barrier energy values were correlated with differences in the spin state of the frontier orbitals. Spin polarized projected density of states calculations demonstrate relatively lower activation energy barriers for systems with higher spin state asymmetry near the Fermi energy; CO oxidation on Ta and V SAC have decreases in activation barrier energies of 27% and 44%, respectively. This motivated additional NEB calculations to compare the activation barrier energies of spin-constrained reaction pathways having differing spin states. By constraining the magnetic moment of the CO oxidation reaction to be either 0 (singlet), 1 (doublet), or 3 (quartet) states, the energy barriers of the doublet and quartet spin states were found to be quite similar with less than 0.05 eV of difference. However, both magnetic states had significantly lower energy barriers compared to the non-magnetic spin state, over 0.5 eV of difference. Since mainly the frontier orbital affects bonding, magnetic moment alone does not explain differences in chemical activity. We will present results suggesting that the magnetic moment of frontier orbitals could play a significant role in enhancing catalytic performance, as well as more traditionally understood mechanisms such as charge transfer and the proximity of frontier d-orbitals to the Fermi level. Computational design of materials is used to define processing conditions for three-dimensional porous graphene structures. Fabrication and characterization of macroscopic graphene structures with high surface area and architectures to facilitate mass transport will also be presented.

The rational design of high-performance electrochemical systems requires a fundamental understanding of the electrode-electrolyte interface. One of the key parameters essential for this understanding is the potential of zero charge (U_pzc). We present a methodology for characterizing this parameter. Our approach is based on electric-field induced second-harmonic generation (EFISH), a nonlinear optical method specific to the solid-liquid interface. We will discuss the physics behind this effect and how a potential-dependent EFISH experiment can reveal the electric field inside the electrochemical double layer and consequently U_pzc. We will further compare the results to the values obtained from other experimental approaches and discuss the implication from the work function analysis. Our work demonstrates EFISH as a facile method for measuring U_pzc and studying how the interfacial electric field influences electrocatalysis.

The optimization of the electrochemical reduction of CO₂ to value-added products using Cu catalysts requires tailoring past the surface. Although often overlooked in heterogeneous catalysis, the microenvironment confined near the surface has been recognized as a necessary component of catalytic performance. Despite its importance, this regime still remains understudied due to a lack of suitable characterization techniques. A better understanding of the microenvironment formed near the catalyst surface is especially relevant to multistep reactions like the electroconversion of CO₂ to multicarbon (C₂₊) products during which its formation is likely inherent to the reaction pathway. In this work, we report on-stream substitution of reactant isotope (OSRI), a new method that uses the subsequent introduction CO₂ isotopes and mathematical modeling to shed a new light on the unique microenvironment formed near Cu surfaces during CO₂ electrolysis. This method reveals the presence of an intermediary CO reservoir concentrated near the Cu surface during the reaction that influences C₂₊ formation. Applied to a Cu nanoparticle (NP) ensemble and Cu foil, the OSRI method indicates that more than a monolayer of CO intermediates is necessary to facilitate CO₂-to-C₂₊ conversion. Specifically, the intrinsic C₂₊ activity only increased after reaching a density of ~100 CO molecules per surface Cu atom. The Cu NP ensemble is found to satisfy this criterion at a lower overpotential than the Cu foil thus making it a better candidate for efficient C₂₊ production. Furthermore, given the same reservoir size, the ensemble’s intrinsically higher C-C coupling ability is highlighted by the 4-fold higher C₂₊ turnover it achieves at a more positive potential. Applicable to any Cu surfaces, this method offers a novel approach to distinguish the contribution of the microenvironment from the Cu surface’s intrinsic C-C coupling ability. OSRI provides new insights on how the presence of CO intermediates in the microenvironment impacts C₂₊ formation during the electroreduction of CO₂ on Cu surfaces and thus, highlights an additional dimension to consider when pushing future CO₂ reducing-catalysts designs beyond the surface.

We will discuss how to tune oxide electronic structure and design oxide surfaces to control the kinetics of key reactions central to making of chemicals and fuels using electrons from renewables (Kuznetsov et al. Joule 2018 and Hwang et al. Science 2017). Increasing metal-oxygen covalency (smaller energy gap between metal d and O 2p states) enhances activity for oxygen evolution upon water splitting but beyond an optimal value reduces oxide stability. Exploiting this concept to examine a series of oxides not only sets record catalytic activity but also establishes a new reaction mechanism for the most active oxides, where both metal and oxygen sites can catalyze oxygen evolution (Grimaud et al. Nature Chemistry 2017) and deprotonation from oxide surface can be rate-limiting (Hong et al. EES 2017) – contrary to long-standing belief. Tuning metal-covalency and activating surface oxygen sites points to a new direction to increase oxide activity and stability (Kuznetsov et al., JPCC 2018). Such concepts have been applied in the design of oxide surfaces to enhance the catalytic activity of NO oxidation to NO₂ (Nature Catalysis 2021) and suppress the dehydrogenation of electrolytes to enhance the lifetime of high-energy Li-ion batteries (Giordano et al. JPCL 2017, and Zhang et al., EES 2020).

Kuznetsov, D. A., B. Han, Y. Yu, R. R. Rao, J. Hwang, Y. Román-Leshkov, and Y. Shao-Horn, Tuning Redox Transitions via Inductive Effect in Metal Oxides and Complexes, and Implications in Oxygen Electrocatalysis, Joule, 2, 1–20 February 2018.
Hwang, J., R.R. Rao, L. Giordano, Y. Katayama, Y. Yu, and Y. Shao-Horn, Perovskites in Catalysis and Electrocatalysis, Science, 358, 751-756 November 2017.
Hong, W., K.A. Stoerzinger, Y-L. Lee, L. Giordano, A.J.L. Grimaud, A.M. Johnson, J. Hwang, E. Crumlin, W. Yang, Y. Shao-Horn, Charge-transfer-energy-dependent oxygen evolution reaction mechanisms for perovskite oxides, Energy & Environmental Science, 10, 2190-2200 October 2017.
Giordano, L., P. Karayaylali, Y. Yu, Y. Katayama, F. Maglia, S. Lux, and Y. Shao-Horn, Chemical Reactivity Descriptor for the Oxide-Electrolyte Interface in Li-Ion Batteries, Journal of Physical Chemistry Letters, 8, 3881-3887 August 2017.
Grimaud, A., O. Diaz-morales, B.H. Han, W. T. Hong, Y.L. Lee, L. Giordano, K. A. Stoerzinger, M.T.M. Koper, Y. Shao-Horn, Activating lattice oxygen redox reactions in metal oxides to catalyze oxygen evolution, Nature Chemistry, 9, 457-465 May 2017.
Kuznetsov, D.A., J. Peng, L. Giordano, Y. Román-Leshkov and Y. Shao-Horn, Bismuth Substituted Strontium Cobalt Perovskites for Catalyzing Oxygen Evolution, Journal of Physical Chemistry C, 124, 6652-6570, February 2020.
Y. Zhang, Y. Katayama, R. Tatara, L. Giordano, Y. Yu, D. Fraggedakis, J. Sun, F. Maglia, R. Jung, M.Z. Bazant and Y. Shao-Horn, Revealing Electrolyte Oxidation via Carbonate Dehydrogenation on Ni-based Oxides in Li-ion Batteries by in situ Fourier Transform Infrared Spectroscopy, Energy and Environmental Science, 13, 183-199, January 2020.
J. Hwang, R.R. Rao, L. Giordano, K. Akkiraju, X.R. Wang, E. Crumlin and Y. Shao-Horn, Regulating oxygen activity of perovskites to promote NO_x oxidation, Nature Catalysis, in press 2021.

During this century, climate change and energy demand have been becoming one of the major global concerns for the human beings. Taking inspiration from nature, the production of fuels and chemicals through CO₂ conversion or water splitting under the solar light can counteract these challenges.¹ So far, although TiO₂ is one of the most studied and applied photocatalysts due to low toxicity and cost and high biocompatibility and stability, some scientific questions are still unresolved. Several physico-chemical aspects can affect its photocatalytic performance, such as particles morphology, crystalline phases, band gap and specific surface area. Among these, by applying different sol-gel synthesis methods,² we study how the presence of polymorphs and Ti⁴⁺/Ti³⁺ defects can play a role in the photocatalytic efficiency for solar fuels production when they act alone or coupled with quantum dots (QDs).³ TiO₂ mainly exists in three different crystalline polymorphs: anatase and rutile (tetragonal structure) and brookite (orthorhombic structure), where the latter two phases have a direct band gap whereas the former phase shows an indirect band gap, with an average value from 3.0 eV up to 3.4 eV. The photogenerated electrons and holes show different behavior among the polymorphs due to different electron-trap depths and possible defects, which influence the final photocatalytic performance.⁴ Moreover, to absorb a wider part of the solar spectrum and to fast deliver photons to the photocatalytic centers, copper-based QDs at different wavelength emissions in the visible and NIR range are chosen as harvesting antennas.
To control the presence of the polymorphs and the coupling with QDs, we change the titanium precursors and the type of template, by also adopting a greener template-free procedure in mild calcination conditions. The photocatalysts are characterized by means of X-ray powder diffraction (XRPD), quantitative phase analysis as obtained by Rietveld refinement, diffuse reflectance (DR) UV−Vis spectroscopy, N₂ adsorption/desorption at −196 °C, electrophoretic mobility in water (ζ-potential), X-rays photoelectron spectroscopy (XPS) and electron microscopy. Charge transfer and recombination processes are characterized using transient absorption spectroscopy. The photocatalytic behavior of these nanostructures is assessed under simulated sunlight at 1 Sun (i.e. 100 mW/cm² of intensity).

(1) Freyria, F. S. . In Nanostructured Catalysts for Environmental Applications; Eds.; Springer International Publishing, 2021; pp 214–248.
(2) Freyria, F. S.; Blangetti, N.; Esposito, S.; Nasi, R.; Armandi, M.; Annelio, V.; Bonelli, B. ChemistryOpen 2020, 903–912.
(3) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. J. Am. Chem. Soc. 2011, 133 (5), 1176–1179.
(4) Vequizo, J. J. M.; Matsunaga, H.; Ishiku, T.; Kamimura, S.; Ohno, T.; Yamakata, A. ACS Catal. 2017, 7 (4), 2644–2651.

Inorganic electrides are solid-state sources of solvated electrons which serve as powerful reducing agents in organic transformations. Since electrons in electrides are strongly confined within the interstitial sites, their effective detrapping is the key to harnessing the strong reducing power of solvated electrons. Recent studies have revealed that two-dimensional (2D) electrides, such as [Ca₂N]⁺e^–, exhibit superior electron-donating ability beyond the conventional [Ca₂₄Al₂₈O₆₄]⁴⁺4e^– electride, because of extremely high densities of anionic electrons that are delocalized over the interlayer space between the cationic layers. However, the inherent instability of [Ca₂N]⁺e^– in organic solvents retards utilization of their superior electron-donating ability.
To overcome this limitation, we propose a strategy that uniquely combines a photoredox catalyst molecule and a chemically stable 2D electride. [Gd₂C]²⁺2e^- was chosen because it exhibits high robustness against air and moisture. The notable stability was ascribed to anionic electrons being weakly localized at the Gd sites between layers of [Gd₂C]²⁺. In addition, [Gd₂C]²⁺ has a rigid layer framework with strong covalent bonding between the Gd and C atoms. [Gd₂C]²⁺2e^- slowly decomposed in 2,2,2-trifluoroethanol (TFE) to release solvated electrons. The effective diffusion length of the solvated electrons should remain small due to the slow alcoholysis. To utilize the released solvated electrons, we added Pt(II) complexes of cyclometalating ligands, including 2-phenylpyridinate (Ptppy), 2-(4,6-difluorophenyl)pyridinate (Ptdfppy), and 2-(3-methoxyphenyl)pyridinate (PtOMe), into the [Gd₂C]²⁺2e^- suspension. The planar and electron-rich Pt(II) complex adsorbed onto the surface of the [Gd₂C]²⁺2e^- electride, and rapidly accepted one electron at a rate of 10⁷ s^-1 upon photoexcitation, which is greater than the diffusion rate in TFE (~10⁵ s^-1). The one-electron-reduced Pt complexes were electrochemically stable enough to deliver the electron to benzyl bromide substrates in the bulk with very negative reduction potentials (E_red, -2.20 ~ - 2.46 V vs standard calomel electrode (SCE)). This electron transfer recovered the neutral Pt(II) complexes. Therefore, the cycle enabled photogated electron transport from the stable [Gd₂C]²⁺2e^- electride to substrates. The key benefit of this reducing system was the suppression of undesirable charge recombination. The charge recombination by back electron transfer was prohibited due to the irreversible disruption of the [Gd₂C]²⁺2e^- electride after the electron ejection.
The desirable properties collectively served as the photoredox catalysis principle for the reductive generation of the benzyl radical from benzyl bromide, which is the key intermediate of dehalogenation or homocoupled products. We examined the photoinduced reducing ability of the combined [Gd₂C]²⁺2e^-/Pt complex system for a variety of benzyl bromides. Our optimized reaction conditions included deaerated CH₃CN:TFE (3:1, v/v) containing 0.25 M substrate, [Gd₂C]²⁺2e^- (4e^- mmol), and Ptdfppy (2 mol %), which was photoirradiated at a wavelength of 365 nm (4 W) at room temperature for 18 h. The conditions yielded dehalogenated and homocoupled products in yields as high as 98%. These results successfully demonstrated the effectiveness of our photoreducing system. We expect that our strategy will stimulate renewed research interest in the synthetic applications of stable yet unreactive 2D electrides.

For decades, the vast ignition of non-renewable energy sources represents an extreme risk to the world by discharging gases that cause air contamination and global warming. Using a facile pulsed laser deposition process, nickel oxide (NiO) thin films has been deposited on strontium titanate, lanthanum aluminate, and sapphire substrates. The substrate–film lattice mismatch and how it influences the electrochemical properties of NiO thin film electrocatalyst have been studied. X-ray diffraction measurements were carried out to confirm the phases and determine interfacial lattice strain. AFM measurements were used to analyze the surface topography of the catalyst and determine the surface roughness of the film. The electrochemical analysis was performed in one molar potassium hydroxide solution. It was observed that the electrocatalytic activities of the NiO thin films exhibit a strong sensitivity to strain; the NiO thin film samples with the smallest strain recorded the lowest overpotential for the half-cell hydrogen evolution reaction. Additionally, the charge storage capacity of the NiO electrocatalyst has been explored and might be a function of surface roughness. The electrochemical properties and simple synthesis make these NiO-based catalyst thin films helpful in incorporating as high-surface-area electrodes for water electrolysis.

The self-assembled monolayer (SAM) has been widely applied to modulate surface science for heterogeneous catalysis. In particular, 4-Mercaptobenzonitrile (4-MBN) SAM features a highly delocalized electron environment along with the benzene-nitrile head group, and its tail group – sulfur strongly bonds to the metal surface, i.e., Ag. Thus, 4-MBN SAM becomes a promising stable modifier to fine-tune the catalytic activity and selectivity of metal surfaces for the chemical process involving electron-transfer step, i.e., electrocatalysis. The electro-reduction reaction of CO₂ to CO (CO₂RR-to-CO) over Ag provides a sustainable approach to utilize renewable energy and generate value-added chemicals. However, there are major limitations of CO₂RR-to-CO over Ag: (1) low stability of Ag surface-active sites under high overpotentials due to the surface reconstruction; (2) high energy requirement of the potential rate-limiting step (RDS) – initial CO₂ activation.
To address the above challenges, we hypothesize that a highly ordered 4-MBN SAM will stabilize the Ag surface-active sites via strong thiol-metal chemisorption and promote initial CO₂ activation via the enhanced electron transfer between OCOH^* and 4-MBN SAM. To examine our hypothesis, we first applied DFT calculations to investigate the stabilities of 4-MBN SAMs modulated Ag catalysts. The DFT results show that 4-MBN SAM strongly bonds to Ag surfaces, with the adsorption energies of ~-2 eV at a low 4-MBN surface coverage (< ¼ monolayer). With an increase of the 4-MBN surface coverage over ¼ monolayer, 4-MBN SAMs stabilize active and less thermo-favored surfaces (i.e., Ag100 and Ag211), and will potentially inhibit the surface reconstruction and stabilize Ag surface active sites. In addition, we examined the potential RDS of CO₂RR-to-CO (i.e., CO₂+H^*àOCOH^*) over 4-MBN modulated Ag catalysts. Interestingly, at a high surface coverage of 4-MBN (> ¼ monolayer), the adsorption of OCOH^* is more favorable over the organic active sites – nitrogen of 4-MBN, and the stabilized OCOH^* intermediate results in lower reaction energies of CO₂+H^*àOCOH^*, as compared to the pristine Ag scenario. This improvement can be explained via a large ET region at the high surface coverages of 4-MBN, which greatly strengthens the electronic interaction between the nitrogen of 4-MBN and the carbon of OCOH^*. In summary, our work provides a systematic understanding of the physical and chemical properties of 4-MBN modulated Ag catalyst and its potential catalytic performance of CO₂RR-to-CO at the atomic scale. This work also guides a new strategy for the (electro)catalyst design with improved stability and activity via the SAMs modification as compared to the conventional metal catalysts.

A novel propitious nanoporous anodized stainless steel 316L (NASS316L) photoanode was developed for water splitting. The anodization could successfully produce a uniform nanoporous (∼ 90 nm in pore diameter) array (∼ 2.0 μm thick) of NASS316L with a high pore density. Several techniques, including FESEM, EDX, XRD, XPS, ICP-OES, and UV–vis-NIR spectrophotometry, were employed to characterize the catalyst and to assess and interpret its activity toward water splitting. Surprisingly, the NASS316L retained almost the same composition of the bare stainless steel 316L, which recommended a symmetric dealloying mechanism during anodization. It also possessed a narrow band gap energy (1.77 eV) and a unique photoelectrocatalytic activity (∼ 4.1 mA cm^–2 at 0.65 V versus Ag/AgCl, 4-fold to that of α-Fe₂O₃) toward water splitting. The onset potential (−0.85 V) in the photocurrent–voltage curve of the NASS316L catalyst demonstrated a negative shift in its Fermi level when compared to α-Fe₂O₃. The high (23% at 0.2 V vs Ag/AgCl) incident-photon-to-current conversion efficiency and the robust durability revealed from the in situ analysis of the produced H₂ gas continued recommending the peerless inexpensive and abundant NASS316L catalyst for potential visible-induced solar applications.

Liquid phase CH4 oxidation under mild conditions is a promising field for energy-efficient conversion. Although highly active electrochemical catalysts have achieved methane-alcohol conversion at room temperature and pressure, high alcohol production remains a challenge. We have confirmed the sustainable electrochemical conversion of methane to alcohol using IrO2 catalyst. IrO2 has an activation energy for C-H bond of 9.5 kJ/mol, which is very low compared to conventional metal oxide catalysts. It is a catalyst known to facilitate methane activation reaction due to strong interaction between Ir_cus sites and methane molecules. Nevertheless, IrO2 has not yet been reported as an electrochemical catalyst for methane oxidation. The surface phase of IrO2 is determined by the sintering temperature, and we selectively fabricated the IrO2 (110) surface, which is the most effective surface for CH4 activation. Higher methane oxidation activity was confirmed compared to previously reported studies, and 216.9% higher alcohol production was achieved at room temperature and pressure, and the mechanism of alcohol formation by oxidation of methane by active oxygen in the catalyst was confirmed.

SnO₂ has been identified as one of the promising and suitable oxide-based photocatalytic semiconductors. However, the large bandgap limits its application in the visible region. Doping is an efficient way of optimizing the materials properties and improving the photocatalytic performance. Here, using the first-principles density functional theory calculations, we considered the substitutional and interstitial doping of SnO₂ with Fe and Cr atoms. We find that Fe atoms tend to occupy interstitial sites while Cr atoms prefer to occupy substitutional sites with the different amount of oxygen vacancies. Bandgap narrowing, O-K edge electron energy loss spectra due to the oxygen vacancy, and resulting enhanced photocatalytic behavior of visible light will be discussed and associated underlying mechanism will be introduced along with scanning transmitted electron microscopy measurements.

The development of selective and stable catalysts for electrochemical CO₂ reduction reaction (CO₂RR) in CO₂-purged electrolyte that contains sulfur is essential for practical application because CO₂ gas emitted from industry inevitably contains hundreds ppm-level of H₂S gas. Despite many researchers have been demonstrated selective and stable catalysts for electrochemical reduction of pure CO₂ gas, the conversion of industrial CO₂ gas has been limited. Here, we demonstrated the copper sulfide (CuS_x) catalysts which were spontaneously formed by dipping the Cu foil into laboratory-prepared industrial CO₂-purged in 0.1 M KHCO₃ electrolyte. Since the industrial CO₂ contains H₂S gas, sulfur species dissolved in the electrolyte can easily react with Cu foil. As the concentration of dissolved sulfur species increased, the reaction between Cu foil and sulfur enhanced. As a result, the average size and surface density of CuS_x increased to 133.2<!--[if gte msEquation 12]><m:oMath><i style='mso-bidi-font-style:normal'><span lang=EN-US style='font-family:"Cambria Math",serif; mso-fareast-language:KO;layout-grid-mode:line'><m:r>±</m:r></span></m:oMath><![endif]--><!--[if gte vml 1]><v:shapetype id="_x0000_t75" coordsize="21600,21600" o:spt="75" o:preferrelative="t" path="m@4@5l@4@11@9@11@9@5xe" filled="f" stroked="f"> <v:stroke joinstyle="miter"/> <v:formulas> <v:f eqn="if lineDrawn pixelLineWidth 0"/> <v:f eqn="sum @0 1 0"/> <v:f eqn="sum 0 0 @1"/> <v:f eqn="prod @2 1 2"/> <v:f eqn="prod @3 21600 pixelWidth"/> <v:f eqn="prod @3 21600 pixelHeight"/> <v:f eqn="sum @0 0 1"/> <v:f eqn="prod @6 1 2"/> <v:f eqn="prod @7 21600 pixelWidth"/> <v:f eqn="sum @8 21600 0"/> <v:f eqn="prod @7 21600 pixelHeight"/> <v:f eqn="sum @10 21600 0"/> </v:formulas> <v:path o:extrusionok="f" gradientshapeok="t" o:connecttype="rect"/> <o:lock v:ext="edit" aspectratio="t"/> </v:shapetype><v:shape id="_x0000_i1025" type="#_x0000_t75" style='width:9pt; height:14.25pt'> <v:imagedata src="file:///C:/Users/JW/AppData/Local/Temp/msohtmlclip1/01/clip_image001.png" o:title="" chromakey="white"/> </v:shape><![endif]-->33.1 nm and 86.2<!--[if gte msEquation 12]><m:oMath><i style='mso-bidi-font-style: normal'><span lang=EN-US style='font-family:"Cambria Math",serif;mso-fareast-language: KO;layout-grid-mode:line'><m:r>±</m:r></span></m:oMath><![endif]--><!--[if gte vml 1]><v:shape id="_x0000_i1025" type="#_x0000_t75" style='width:9pt;height:14.25pt'> <v:imagedata src="file:///C:/Users/JW/AppData/Local/Temp/msohtmlclip1/01/clip_image001.png" o:title="" chromakey="white"/> </v:shape><![endif]-->3.3%, respectively. Due to the larger amount of sulfur content and the enlarged electrochemical surface area of CuS_x, the faradaic efficiency of formate was improved from 22.7 to 72.0% at -0.6 V_RHE. Additionally, CuS_x catalysts showed excellent stability for reducing the industrial CO₂ to formate. Change in faradaic efficiency was hardly observed even after long-term (72 h) operation. This study experimentally demonstrated that spontaneously formed CuS_x catalysts are efficient and stable for reducing the industrial CO₂ gas to formate.

Structured copper substrates have gained scientific attraction as electrodes for catalysis, especially for CO₂ reduction (CO₂RR). Dendritic copper foams offer many of the functional aspects considered to be beneficial for activity and selectivity for CO₂RR, including three-dimensional structure, high surface areas, and a large density of grain boundaries. The typical fabrication method for dendritic copper foams is via high-rate cathodic electrodeposition in acidic solutions of Cu²⁺ ions. The competing hydrogen evolution introduces large voids to the foam structure because of bubbles evolving from the surface, which can add to the hierarchical structure of the electrode, but also provides ready-made bubble nucleation sites for hydrogen evolution during CO₂RR. Here, we consider how to fabricate copper foams with intentional control over these bubble nucleation sites. Namely, we explore high-rate Cu electrodeposition in lactate-stabilized, alkaline copper solutions and its effects on the hierarchical structure on multiple length scales. We will show the control that the electrochemical growth conditions exert on the nanoscale, microscale, and macroscale structure of the Cu foams. The electrochemical nature of the Cu-lactate complex and the significant reduction of hydrogen evolution during deposition result in copper dendrites that are significantly denser and more fine-grained than Cu foams derived from acidic solutions, as indicated by tomographic studies of synchrotron-based, transmission x-ray microscopy computed tomography reconstructions. We will also demonstrate that this morphological control has consequences for both the electrocatalytic and mass transport behavior for the Cu electrocatalysts in both hydrogen evolution and in selectivity in CO₂RR reactions.

Recently, ternary oxide semiconductors have received increasing attention as a promising photocathode material for photoelectrochemical (PEC) water splitting. There exists large room to tune their electronic structures which can directly affect PEC properties. However, developing a ternary oxide photoelectrode showing improved PEC performance still remains at a challenging stage. Here, we report the success in the synthesis of high-density Ca₂Fe₂O₅ (CFO) nanorods on a NiO/FTO layer that is an effective hole transport layer (HTL) using a wet chemical approach. Also, we propose a two-step annealing strategy to further increase the PEC efficiency of Ca₂Fe₂O₅ (CFO) photocathode. It was elucidated that two-step annealing was beneficial to their PEC properties in various aspects such as morphologies, chemical states, optical properties, and impedances. In addition, the NiO buffer layer further enhanced their charge transport properties. Enhanced PEC performance was observed by LSV and IPCE measurements. CFO on NiO HTL fabricated via two-step annealing demonstrates a photocurrent density of up to -313.7 µA/cm², which is 76 % higher than that of CFO without HTL fabricated via single-step annealing. The mechanism that the annealing process modifies material characteristics and affects the PEC performance was discussed in detail.

Developing an efficient chemical transformation pathway of ambient CH₄ functionalization is an ongoing challenge and the key resides on the discovery of new catalytic systems. The hard-soft acid-base (HSAB) theory suggests that high-valent transition metals, as soft class (b) Lewis acids based on Pearson's classification, are suitable candidates for CH₄ activation due to methyl moiety's relatively low value of chemical hardness. While most of class (b) transition metals have been studied, divalent silver (Ag^II), possibly due to its reactive nature, is the only class (b) high-valent transition metal center that is yet reported to exhibit reactivities towards CH₄ activation. Inspired by such notion, here we report that electrochemically generated Ag^II metalloradical readily functionalizes CH₄ into methyl bisulfate (CH₃OSO₃H) at ambient conditions in 98% H₂SO₄. Mechanistic investigation experimentally unveils a low activation energy of 13.1 kcal mol⁻¹, a high pseudo-first-order rate constant of CH₄ activation up to 2.8×10³ h⁻¹ at room temperature and a CH₄ pressure of 85 psi, and two competing reaction pathways preferable towards CH₄ activation over solvent oxidation. Reaction kinetic data suggest a Faradaic efficiency exceeding 99% beyond 180 psi CH₄ at room temperature for potential chemical production from widely distributed natural gas resources with minimal infrastructure reliance.

The successful use of electrolysis processes at an industrial scale, such as aluminum electrorefining, brine electrolysis, and water splitting, has increased the interest of using electrochemistry to promote other promising reactions, including hydrogen evolution, CO₂ reduction, and ammonia synthesis. To design electrocatalysts which selectively produce fuel from aqueous media, the tuning of local surface coordination and electronic structure becomes imperative. In this work, we tune the surface electronic structure by changing chalcogen composition in different molybdenum chalcogenides systems to understand proton adsorption interactions. The molybdenum chalcogenides evaluated were synthesized successfully via high-temperature microwave-assisted and template-controlled approaches. Their ability to stabilize proton adsorption was studied under relevant conditions for hydrogen production in acidic media. Electrochemical characterization showed that increasing the electronegativity of the chalcogen increases the hydrogen adsorption strength due to the higher relative basicity of the chalcogen. The results presented here provide guidelines to improve proton adsorption in metal chalcogenide systems and increase selectivity in various electrochemical reactions.

Nitrogen in biological molecules ultimately comes from the reduction of dinitrogen. Interfacing biological catalysts with inorganic materials is constructive towards efficient and customizable chemical production at ambient temperature and pressure. Here, we demonstrate a microbial-material hybrid system with an appreciable internal quantum efficiency of (19.8 ± 2.6)% for photocatalytic nitrogen fixation. This hybrid system consists of nitrogen-fixing microbes and semiconductor quantum dots with a unique interfacial charge transfer behavior at the microbial-material interface. Metabolomics and proteomics studies reveal a metabolism favoring nitrogen accumulation, pertinent to our observed efficiency of nitrogen fixation.

The formation of a surface-bound hydrogen atom from one proton and one electron plays an enabling role in renewable hydrogen production. Quantifying the surface-bound hydrogen kinetics, however, requires decoupling the delicate interplay of numerous processes. In this work, we describe a cyclic-voltammetry method for measuring the rate constant for the formation of surface-bound hydrogen atoms (“hydrogen electro-adsorption”). We find that the hydrogen electro-adsorption on platinum single crystals is ~100x faster in acid than in base. Reaction-order analysis suggests that the electro-adsorption occurs as a proton-coupled electron transfer (PCET) reaction in acid, whereas in base, the electro-adsorption requires the interfacial water reorganization. This work demonstrates a methodology for quantifying the interfacial PCET kinetics and reveals the hydrogen electro-adsorption kinetics as the reason why the hydrogen evolution electrocatalysis on platinum is faster in acid than in base.

The ever-increasing human demand for energy and constant depletion of fossil fuels have sparked a global crisis with urgent attention. Global warming has also become an increasingly serious environmental concern due to its excessive emission of carbon dioxide and similar greenhouse gases. Currently, the conversion of CO₂ into renewable energy in the presence of sunlight is considered to be a viable technique to reduce global warming. Photocatalytic reduction of CO₂ has recently attracted substantial interest, but the activity, stability, and stability of products were severely determined by the efficiencies of light-harvesting, charge migration, and surface reactions. The rational design and preparation of dissimilar dimensional materials (1D/2D) have been therefore extensively investigated as heterogeneous photocatalysts. 1D materials possess distinct advantages of efficient electron transport and optical excitation while suffering from the drawback of low surface area. Meanwhile, 2D materials show a large surface area but tend to agglomerate. Interfacial engineering is a promising technique to take full advantage of the unique dimensionality-dependent advantage while mitigating the disadvantage during sustainable energy applications.

Herein, a highly synergized NiTiO₃/MoS₂ (1D/2D) heterostructure was synthesized using a two-step process. In the first step, NiTiO₃ nanofibers were synthesized through electrospinning which was later combined with 2D flower-like MoS₂ through a hydrothermal process. The morphologies and optical properties of as-synthesized photocatalysts were analyzed by a series of characterizations. The selective growth of highly reactive 1T along honeycomb-like 2H phases were also observed through XPS and TEM analysis. The hybrid NiTiO₃/MoS₂ exhibited a red-shift to the visible light region with an enhanced absorption capacity. At the optimum loading of 2wt.% MoS₂, NiTiO₃/MoS₂ exhibited the highest CO₂ reduction producing CO and CH₄ gases of 130 μmolg^-1h^-1 and 55 μmolg^-1h^-1 which are 3.8 (34 μmolg^-1h^-1) and 3.6 (15 μmolg^-1h^-1)-fold higher than those of pristine NiTiO₃ nanofibers respectively, with overall CO₂ selectivity of 84%. The enhanced photoreduction performance could be attributed to the enhanced light absorption, higher surface area and coexistence of 1T and 2H phases of MoS₂ in the hybrid structures. Moreover, the significantly accelerated charge separation and reduced recombination of charge carriers on account of the strong electronic coupling is verified through photoluminescence and EIS analysis. This work contributes to the development of efficient and stable photocatalytic materials which could also be used further in water splitting, H₂ evolution, and other photocatalytic activities as an efficient photocatalyst.

Ammonia is one of the most widely produced chemical in the world. Most ammonia (over 66%) is now produced through the Haber−Bosch process, consuming approximately 5% of the global natural gas production and contributes to over 3% of global carbon dioxide emissions. In an effort to develop an electrochemical alternative, we have engineered cyanobacterium to perform nitrogen fixation under ambient conditions without consuming natural gas, in stark contrast to the Haber-Bosch process that requires extreme temperatures and pressures to the same end. To realize the heterologous expression of nitrogenase, we transformed the cyanobacterium S. elongatus PCC 7942 with the minimal nitrogen fixation (nif) gene cluster. Biosynthesis of Mo-nitrogenase generally requires a large number of nif genes. However, the transformation and transcription of such a larger set of gene cluster into the cyanobacterium is challenging. In this work, a minimal nif cluster with reduced genetic complexity was chosen to accomplish the transformation of the cyanobacterium S. elongatus PCC 7942. The cyanobacterium S. elongatus PCC 7942 was transformed with nif genes and it was shown to be able to express active nitrogenase. The engineered S. elongatus PCC 7942 was immobilized on an electrode to accelerate the microbial conversion of nitrogen into ammonium
using electrichemical driving forces. Methyl viologen (MV) was added as an electron mediator to shuttle electrons from the electrode to the enzyme active center, since MV can go through the cell membrane and deliver continuous electrons to nitrogenase. In this talk, we will compare and contrast this strategy to other bioelectrocatalytic strategies for nitrogen reduction.

Microorganisms precisely control biological processes, including biosynthesis and cell growth, by regulating the redox state of different biomolecules. These processes can be modulated by electrochemically coupling intracellular biomolecules to an external electrode, but current approaches afford only limited control and specificity. Here we describe using electrical current to reduce specific biomolecules, allowing control of biosynthesis and cell growth in two industrially relevant microorganisms, Escherichia coli and Lactobacillus plantarum.
To enable electrochemical control of E. coli, we used synthetic biology to introduce a heterologous electron transfer pathway. E. coli expressing mtrCAB from Shewanella oneidensis MR-1 consumed electrons directly from a cathode when fumarate or nitrate, both intracellular electron acceptors, were present. The fumarate-triggered current consumption occurred only when fumarate reductase was present, indicating all the electrons passed through this enzyme. Moreover, MtrCAB-expressing E. coli used current to stoichiometrically produce ammonia. Thus, our work introduces a modular genetic tool to reduce a specific intracellular redox molecule with an electrode, opening the possibility of electronically controlling biological processes such as biosynthesis and growth in any microorganism.
Complementing this approach, we discovered that L. plantarum, a lactic acid bacteria used industrially to produce fermented foods, can uptake electrons. This electron uptake occurs under anaerobic conditions when a terminal electron acceptor and its corresponding oxidioreductase are present. Unlike other anaerobic respiratory modes in L. plantarum, this current consumption does not require addition of co-factors such as heme or riboflavin. Electron uptake promotes cell growth and acidification of the media, both processes essential for food fermentation. We find that the L. plantarum metabolism is shifted towards ATP producing pathways in the presence of both a cathode and a suitable electron acceptor. This surprising discovery opens the possibility of using electrical current to drive industrial production of fermented foods using lactic acid bacteria.

Climate change is a pressing global dilemma driven by the combustion of hydrocarbon fuels to CO₂, a potent greenhouse gas. CO₂ reduction has been the focus of intensive research with the goal of closing the carbon loop. A promising avenue to CO₂ conversion encompasses the materials-biology nexus. Microorganisms evolved over millions of years to fix CO₂, H₂O and N₂ with high selectivity and low substrate activation while self-regenerating. We have designed light-active nanomaterials that pair biocompatibly with these microorganisms in order to harness their biocatalytic activity. We demonstrate that by controlling the microenvironment in a silicon nanowire – bacteria ensemble, we can boost the solar-driven turnover rate of CO₂ reduction. Furthermore, adaptation of whole-cell autotrophic biocatalysts to a cytotoxic environment increases their ability to process CO₂. In addition to optimizing the CO₂ conversion rate by improving the material-biology interface and biocatalyst robustness, we establish synergistic co-cultures with guidable dynamics and outputs. We firstly present an integrated process for the production of 3D-printable biopolymer from CO₂. An autotrophic bacteria-mediated process converts CO₂ to acetate which is then directly used as the primary carbon source for biopolymer production by secondary bacteria without any processing of the culture medium between microbial production steps. We also showcase a consortium involving acetogenic Sporomusa ovata and diazotrophic Rhodopseudomonas palustris for tandem CO₂ and N₂ fixation to tunable value-added products. Co-culture dynamics and products can be directed by substrate availability and electrochemical inputs. Finally, we illustrate an electrochemically catalyzed abiotic pathway for sugar generation from CO_2. The sugars are employed as an Escherichia coli feedstock for the production of value-added materials.

The rising demands in providing energy, food, and water for the rapid-growing world population while addressing climate change and environmental pollution requires dramatic acceleration in technical innovations. Green and sustainable production of renewable energy is critical in solving the imminent challenges. Electrocatalysis plays a central role in diverse energy technologies that are of increasing importance for applications in mobile electronics, electric vehicles, as well as renewable energy industry. Despite considerable progresses, many electrocatalytic systems are still plagued by insufficient catalytic efficiency, poor stability or high cost of the precious metal catalysts used. In this talk I will share our research centred around pushing the limits of novel catalysts based on the understanding of the fundamental physical and chemical processes at the catalytic surfaces which can vastly improve the efficiency of diverse electrocatalytic systems, leading to a sustainable ad energy efficient society. In particular, I will discuss our efforts in promoting the mass activity and enhancing the stability of Pt-based cathode catalysts for hydrogen fuel cell applications.

Highly dispersed iron-based quantum dots (QDs) onto nanocarbons were successfully electrodeposited by the Rotating Disk Slurry Electrodeposition (RoDSE) technique. Our findings through chemical physics characterization revealed that the continuous electron pathway interaction between the interface metal-carbon was controlled.

The rotating ring-disk electrode (RRDE) and the prototype generation unit (PGU) of in-situ H₂O₂ generation in fuel cell experiments revealed a high activity for the oxygen reduction reaction (ORR) via two-electron pathway. These results establish the Fe/C catalyst at a competitive level for space and terrestrial new materials carriers, specifically for the in-situ H₂O₂ production. Transmission electron microscopy (TEM) analysis reveals the well-dispersed Fe-based quantum dots with a particle size of 4 nm. The structural and chemical-physical characterization through induced coupled plasma-optical emission spectroscopy (ICP-OES), transmission scanning electron microscopy (STEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS); reveals that, under atmospheric conditions, our quantum dots system is a Fe^2+/3+/Fe³⁺ combination. The QDs oxidation state tunability was showed by the applied potential.

The obtention of H₂O₂ under the compatibility conditions of the drinking water resources available in the International Space Station (ISS) enhances the applicability of this iron- and carbon-based materials for in-situ H₂O₂ production in future space scenarios. Terrestrial and space abundance of iron and carbon, combined with its low toxicity and high stability, consolidates this present work to be further extended for the large-scale production of Fe-based nanoparticles for several applications.

Our approach is to switch the nanocarbon source and metal-carbon configurations, to control the two-electrons route in the ORR kinetics for fuel cells in space technologies. This consolidates our present work as a large-scale production process of highly active Fe nanoparticles for in situ H₂O₂ generation.

Likewise, the origin of the current transients, generated during the RoDSE technique, are associated with the Fe/C and H₂O/H⁺ couple interactions through electrocatalytic amplification. The enhanced current transients open the possibility to design a fluidized bed reactor-like, to study the electrochemical amplification reactions where the electrocatalyst is used as a label to obtain higher yields. Accurately, large-scale production of treasured gases for human space missions via electrochemical amplification of the HER and OER, to produce large quantities of gases such as H₂ and O₂, represents a fascinating research field.


References:
1. Peña-Duarte, A.; Vijapur, S. H.; Hall, T. D.; Hayes, K. L.; Larios-Rodríguez, E.; Pilar-Albaladejo, J. D.; Santiago, M. E. B.; Snyder, S.; Taylor, J.; Cabrera, C. R., Iron Quantum Dots Electro-Assembling on Vulcan XC-72R: Hydrogen Peroxide Generation for Space Applications. ACS Applied Materials & Interfaces 2021.
2. Nelson, G. J.; Vijapur, S. H.; Hall, T. D.; Brown, B. R.; Peña-Duarte, A.; Cabrera, C. R., Electrochemistry for Space Life Support. Electrochemical Society Interface 2020, 29 (1), 47-52.

The broad dissemination of proton-exchange membrane fuel cells (PEMFCs) is largely limited by the high cost of platinum (Pt)-based catalysts that are used to accelerate the sluggish oxygen reduction reaction (ORR) at the cathode. While the theoretical descriptor, △E_O (oxygen binding energy to the catalyst surface), has been widely adopted for predicting the activity, its experimental analogue has met very limited success and the durability of the resulting catalysts is yet difficult to predict. To date, the developments of highly active and stable Pt-alloy nanocatalysts have largely relied on anecdotal discoveries. Therefore, it is essential to develop experimentally tangible descriptors that capture key parameters for efficient and rapid catalyst design. Herein, by combining theoretical modeling and experimental observations, we developed a binary experimental descriptor that directly reflects the calculated △E_O on Pt-alloy catalyst surface, leading to an experimental Sabatier plot that can be used to comprehensively predict both the catalytic activity and stability of a wide range of Pt-alloy ORR catalysts. Furthermore, based on the established Sabatier plot, we demonstrated the design of a PtNiCo catalyst that simultaneously delivers a superior activity and robust stability, approaching the peak of the Sabatier plot.

Hydrogen peroxide (H₂O₂) is a crucial chemical with a wide range of applications in civil and industrial fields. It is currently produced from the industrial energy- and waste-intensive anthraquinone process. Its centralized feature also makes it rely heavily on the storage and transportation of H₂O₂, which is unstable and hazardous. Electrocatalytic oxygen reduction reaction (ORR) to H₂O₂ provides an alternative to realize green and delocalized production, with the only inputs from renewable electricity, water and air. However, this route still faces two challenges: 1) lack of catalysts which selectively drive the 2e^- ORR towards H₂O₂ (instead of H₂O); 2) generated H₂O₂ are typically in mix with solutes in traditional electrolyzers, which necessitates complicated separation processes to recover pure H₂O₂ solutions for applications.

To make the electrochemical route more reliable in the future scaling-up, we reported a direct and continuous production of pure H₂O₂ solutions for the first time, through rational design of both catalyst and reactor. Here, we report a direct electrosynthesis strategy that delivers separate water (H₂O) and oxygen (O₂) streams to an anode and cathode separated by a porous solid electrolyte, wherein the electrochemically generated H⁺ and HO₂^– recombine to form pure aqueous H₂O₂ solutions with different concentrations. By optimizing a functionalized carbon black catalyst, we achieved over 90% selectivity for pure H₂O₂ at current densities up to 200 mA cm^-2, which represents a H₂O₂ productivity of 3.4 millimoles per square centimeter per hour.

To further improve the activity and lower the overpotential at high current densities for industrial-level applications, we introduced boron-doped carbon catalyst which can achieve better activity under industrial-relevant current densities (saving over 200 mV overpotential at up to 300 mA cm^-2) compared to the state-of-the-art oxidize carbon catalyst while maintaining a high selectivity (up to 90%). By incorporating the boron-doped carbon catalyst into our solid electrolyte cell setup, we achieved high partial H₂O₂ current densities (400 mA cm^-2) with high selectivity (up to 95%), which represents a H₂O₂ productivity of 7.36 millimoles per square centimeter per hour. The setup can also undergo 200-hour stability test without degradation, making it a good candidate to be further improved in the aspect of potential scaling-up for electrochemical H₂O₂ production in industrial level with long-lasting performance.

Development of efficient, selective and cost-effective electrocatalysts is central to the efficient production of sustainable fuels and industrial feedstocks. While significant progress has been made in the hydrogen evolution reaction (HER) for water splitting, the direct synthesis of industrial fuels and feedstocks by the CO2 reduction reaction (CO₂RR) still presents significant challenges related to the limited number of metals that maintain selectivity over many reaction steps while sufficiently inhibiting competing HER. In this presentation we will show our success in promoting CO₂RR over HER as well as H₂O₂ over H₂O in the oxygen reduction reaction (ORR) using metal-phosphorus nanocrystalline catalysts based. Taken together, these three examples show the promise of controlled stoichiometric control in promoting desirable catalytic properties.

Recently (Nature Communications 2019) we demonstrated that CO₂ reduction on AgP₂ NCs shows a 3-fold reduction in Faradaic CO overpotential compared to Ag NCs. The reduced overpotential is consistent with the free-energies of the intermediates calculated with density functional theory in which the calculated free energy for COOH* is roughly three times lower for both the AgP₂ (111) and AgP₂ (211) surfaces compared to Ag (111). Operando X-ray absorption spectroscopy (XAS) gave insight into the role of P in stabilizing positive Ag partial oxidation states under reductive potentials, where at -0.8 V vs. RHE (corresponding to the peak in selectivity for CO2-to-CO), the Ag oxidation state in AgP2 NCs is +1.08 and stable for 2 h. In comparison, AgO and Ag₂O are reduced to Ag0 in less than 50 s at -0.8 V vs. RHE. Consistent with experiment, Bader charge analysis of COOH* on Ag and AgP₂ also shows a higher Ag oxidation state of AgP₂ versus metallic Ag. Notably, the decrease in overpotential was observed for a system that still maintained selectivity for CO₂RR over HER, despite the presence of P atoms on the surface. Free energy calculations for H* on Ag and P suggest Ag remains inactive for HER and that for the Ag(211) surface P over-binds H, consequently limiting HER.

Pushing towards selective C₂ product formation requires efficient C-C bond formation at sufficiently low overpotentials to avoid methane formation. Current work demonstrates a 280 mV reduction in ethylene overpotential on Cu3P compared to Cu nanocrystals, with C₂H₄ selectivity reaching 68%. Encouragingly, HER is again limited. The mechanism behind the observed increase in C₂H₄ selectivity is partially revealed by density functional theory and in situ FTIR, which suggests the reduction in the barrier for formation of CHO* creates a low overpotential regime in which a CO* and CHO* rich surface facilitates the bimolecular C-C bond formation reaction.

The role of P in templating metal atoms to control selectivity is demonstrated in the selective H₂O₂ ORR by PtP₂ NCs (Nature Communications 2020). For Pt-based oxygen reduction, the selectivity for the 2 e- H₂O₂ pathway versus the 4 e- H₂O pathway depends critically on the comparative strength of the Pt-OOH and the O-OH bonds. On pure Pt, strong bridge binding of OOH* decreases the chance of desorption and weakens the OOH bond which increases reactivity in the subsequent 4 e- pathway steps. PtP₂ increases the Pt separation and destabilizes the OOH* bridge bond based on DFT calculated bond distances. The result is a dramatic change in selectivity toward H₂O₂ which reaches 98%. Atomic layer deposition of Al₂O₃ prevents NC aggregation and enables application in a polymer electrolyte membrane fuel cell (PEMFC) with a maximum r(H₂O₂) of 2.26 mmol h^-1 cm^-2 and a current efficiency of 78.8% even at a high current density of 150 mA cm^-2. Catalyst stability enables an accumulated neutral H₂O₂ concentration in 600 mL of 3.0 wt% (pH = 6.6).

Decarbonization of the chemical industry requires transitioning from thermochemical transformations towards electrochemical pathways that use “green” electricity generated from renewables. These electrochemical processes focus on converting abundant natural chemical resources, such as the atmospheric gases O₂, N₂, CO₂, and H₂O, into fuels and valuable commodity chemicals, such as H₂, H₂O₂, NH₃, and multi-carbon products. But these processes are kinetically unfavorable and require the use of electrocatalysts for reasonable efficiencies. An ideal electrocatalyst should be (1) highly active and selective for generation of the desired end product with near 100% Faradaic efficiency at low overpotentials, (2) able to transport electrons (or holes), ions, and molecules to/from the catalytic site, and (3) affordable—meaning that they should only contain elements with reasonably high abundance and use synthetic methods that can scale to meet global demand. Unfortunately, simultaneously achieving all three desired qualities is difficult and often multi-component electrodes are necessary—for example Pt is highly active and selective for the full 4 e^- reduction of O₂ to H₂O in fuel cell electrodes but requires the incorporation of carbon black additives to minimize Pt content and reduce costs. Additionally, Pt is not ionically conductive and requires Nafion polymeric binders to efficiently transport ions to the reaction sites.¹ Inspired by nature’s design of enzymatic catalytic sites, we set out to synthesize organic electrocatalytic structures that, in principle, may achieve all three desired qualities.²

In this talk, we will describe the development of electrocatalysts based on n-type (electron conducting) organic semiconducting (OSC) polymers and their applications towards the production of H₂O₂ from O₂. These polymeric OSC’s are mixed ionic-electronic conductors with redox active polaronic states formed through electrochemical ion insertion reactions. Rational design of the chemical structure of the OSC polymer backbone can be used to tune the energy (reduction potential) of these polaronic states and the adsorption strength of reactive intermediates to optimize selectivity and catalytic activity. Simultaneously, the attachment of a mixture of polar and non-polar side-chains to the backbone modifies the hydrophilicity of the polymer to tune the ionic and molecular transport properties through the bulk of the electrode. This approach enables full utilization of electrochemically active redox sites while simultaneously ensuring robust mechanical and chemical stability.³ Operando UV-Vis spectroelectrochemistry and transport measurements performed during active electrochemical oxygen reduction in electrolytes at various pH’s yield insights into the nature of the reaction site and mechanism of the reaction, providing chemical design principles for future development of this emerging class of electrocatalysts.

1. F. Barbir, PEM Fuel Cells: Theory and Practice, 2^nd Edition (Elsevier, 2012).
2. R. Morris Bullock et al. Science 369, eabc3183 (2020)
3. A. Giovannitti et al. Nat. Commun. 7:13066 (2016)

Hydrogen peroxide (H₂O₂) is a widely produced green oxidant with applications in paper bleaching, chemical synthesis, and disinfection. H₂O₂ could be electrochemically produced safely and on-demand by using a catalyst selective for the two-electron oxygen reduction reaction (2e^- ORR) as opposed to the four-electron (4e^-) pathway, though development of such catalysts has been hindered by lack of systematic design principles. Recently, well-defined crystalline two-dimensional conductive metal organic frameworks (2D-MOFs) with a square planar metal-nitrogen (M-N₄) motif have shown selective 2e^- ORR but the origins of the catalytic activity and selectivity remain unclear. Here, we demonstrate that one such 2D-MOF, Ni-HAB (HAB = hexaaminobenzene), has high selectivity compared to other similar 2D-MOFs with M-N₄ motifs. Bulk electrolysis experiments prove the practical efficacy of Ni-HAB as a 2e^- ORR catalyst in buffered near-neutral electrolyte and reveal a strong dependence on operating potential. Operando X-ray absorption spectroscopy reveals highly stable metal coordination sites and charging of the organic linkers that occurs during ORR that directly competes with the production of H₂O₂. These results confirm the dominant active site during ORR in these MOF catalysts and reveal a unique interplay between the material’s catalytic and redox properties that can guide the future design of similar MOFs for enhanced catalytic activity.

Oxygen evolution reaction (OER) is an essential reaction and often a limiting step in many electrochemical devices that hold great potential for clean energy conversion and fuel transformation. The two paramount questions remain unclear in OER heterogeneous catalyst design: (1) how to identify and construct well-defined catalytic centers for efficient OER catalysis, and (2) how to delineate unambiguously the atomistic mechanism and kinetics for OER and competitive reactions at this center. In this work, we developed a new synthesis for the single-site cobalt (Co) doped TiO₂ nanorods with well-defined catalytic surface (210). This material was systematically studied with a suite of operando probes under electrochemical conditions and exhibited a record-high intrinsic activity for the OER reported to date. Moreover, based on grand canonical quantum mechanics (GCQM) calculations on well-defined single-site Co atomic structure, we establish a full description of reaction kinetics for the Co-TiO2 as a function of applied potential, revealing an adsorbate evolution mechanism for the OER. The computationally predicted Tafel slope and TOFs also exhibit unprecedently good agreement with experiment. Moreover, we demonstrated a methodological advance for the study of OER by combining the atomic-precision synthesis of nanocrystals and GCQM calculations. With the accuracy of GCQM calculations validated here, it can potentially allow us to perform high-accuracy in-silico design of rational catalyst and subsequently maximize its benefit through controlled synthesis of nanocatalysts.

Late transition metal oxides are reported to be the most active non-precious catalysts for oxygen evolution reaction (OER),^[1] but the most active oxides based on Ni or Co^[2] still have turnover frequencies per metal site at least one order of magnitude lower than that of oxygen-evolving complex (OEC) in biological systems.^[3] The OEC contains an intricate manganese-calcium-oxo cluster, and its unparalleled OER activity can be attributed to the unique characteristics of Mn centers regulated by Ca²⁺ cations.^[4,5] Such electronic structure tuning can be generalized as the inductive effect.^[6] Specifically, substituting metal oxides and complexes using foreign metals with higher electronegativity can effectively increase their redox potentials and promote their activity. We have recently employed this concept to design bismuth-substituted strontium cobaltite, where the strongly electronegative Bi³⁺ ions give rise to Co centers with record intrinsic OER activity in alkaline solutions.^[7]

Unfortunately, metal substitution in oxides exhibits a much smaller tunability of electronic structures than that found in metal-oxo clusters. For instance, Ca²⁺ substitution of Mn⁴⁺ in synthetic [CaMn₃O₄] clusters can tune the Mn^3+/4+ redox potential by ~1 V,^[4,5] whereas negligible changes of ~0.02 V are typically observed for oxides.^[6] To tackle this limitation, we have designed metal-hydroxide-organic frameworks (MHOFs) that combine the great tunability of enzymatic systems with known oxide-based chemistries. A series of MHOFs were constructed by transforming layered hydroxides into 2D sheets composed of metal-octahedra chains cross-linked with neighboring chains using organic linkers. MHOFs can act as a tunable platform for OER, where the nature of π-π-interactions between adjacent stacked linkers and the transition metals in the layered metal hydroxides dictate stability and activity, respectively. Substituting MHOF nanosheets with more electron-withdrawing cations increased their OER activity, where Fe-substituted Ni-based MHOFs exhibited three orders of magnitude enhancement in activity per metal site, rivaling those of state-of-the-art OER catalysts. This enhancement was correlated with the MHOF-based modulation of Ni redox potentials and the optimized binding of reaction intermediates. These results represent a step forward towards designing active metal centers with ligand fields akin to those in homogenous/enzymatic systems, where MHOFs can act as a versatile platform to design catalysts with unparalleled tunability.

References:
[1] J. Hwang, R. R. Rao, L. Giordano, Y. Katayama, Y. Yu, Y. Shao-Horn, Science 2017, 358, 751.
[2] C. Wei, R. R. Rao, J. Peng, B. Huang, I. E. L. Stephens, M. Risch, Z. J. Xu, Y. Shao-Horn, Adv. Mater. 2019, 31, 1806296.
[3] W. T. Hong, M. Risch, K. A. Stoerzinger, A. Grimaud, J. Suntivich, Y. Shao-Horn, Energy Environ. Sci. 2015, 8, 1404.
[4] E. Y. Tsui, R. Tran, J. Yano, T. Agapie, Nat. Chem. 2013, 5, 293.
[5] E. Y. Tsui, T. Agapie, Proc. Natl. Acad. Sci. 2013, 110, 10084.
[6] D. A. Kuznetsov, B. Han, Y. Yu, R. R. Rao, J. Hwang, Y. Román-Leshkov, Y. Shao-Horn, Joule 2018, 2, 225.
[7] D. A. Kuznetsov, J. Peng, L. Giordano, Y. Román-Leshkov, Y. Shao-Horn, J. Phys. Chem. C 2020, 124, 6562.

The production of molecular hydrogen by catalyzing water splitting is central to achieving the decarbonization of sustainable fuels and chemical transformations. While the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) have been studied on a variety of electrode materials and pH, the mechanisms at play in the electrochemical double-layer and the roles of water structure are poorly understood. Especially changes in water structure have gained interest because of its direct implications on many of its properties, such as diffusion, dielectric function and H-bond dynamics. In this work, we achieved to confine water in organic solvents, and tune the water structure and hydrogen-bonding networks by changing the water concentration and organic solvents. Fourier-transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) reveals the change of water structure confined in organics, and the HER/HOR kinetics are altered accordingly. We further perform surface-enhanced infrared absorption spectroscopy (SEIRAS) measurements to probe the water structure change during the reactions, and understand the role of water structure, and more specifically confined water, in reaction kinetics. This work highlights immense opportunities to control the water activity and HER/HOR kinetics by tuning interfacial structures of water and solvents. The understanding in controlling water activity would also be impactful across other reactions crucial to improving decarbonizing efforts in energy storage such as CO₂ reduction and aqueous batteries.

Understanding the underlying mechanisms for the electrocatalytic transformation of CO₂ to fuels and chemicals, the electrochemical oxidation of methane to methanol or the electrocatalytic transformation of nitrogen to ammonia is a significant challenge for the scientific community. In our group, we have recently discovered that the H-type and gas diffusion electrode cells broadly used for the study of electrocatalytic transformations suffer from ill-defined mass and heat transfer characteristics obscuring the results obtained in kinetic studies and thus complicating further development of the field.
The complexity of electrocatalytic reactions is due to the coupling between reaction and diffusion and requires a profound understanding of processes occurring at multiple scales. In this work, a novel electrochemical cell is presented with well-defined mass transport properties where the hydrodynamics of the cell can be controlled from Reynolds numbers from 200 to 20,000 while the dimensionless mass transfer coefficient, the Sherwood number, is varied over an order of magnitude. Using this advanced electrochemical cell, we show that the product distribution obtained on a flat polycrystalline electrode for the electrocatalytic reduction of CO₂ varies widely under different mass transport regimes. Under the same applied potential, low Sherwood numbers favor the generation of >2 electrons reduction products such as ethylene and alcohols while high Sherwood numbers favor the production of hydrogen, carbon monoxide and formate. We also show that changes in the hydrodynamics of the cell can be used to change the selectivity of transition metal oxides for the electrocatalytic oxidation of methane to methanol. We show for the first time that various transition metal oxides are indeed selective for the transformation of methane to methanol under optimized conditions of applied potential and mass transfer. Through the study of these systems at different temperatures it is possible to decouple thermal from electron transfer steps in the electrochemical activation and oxidation of methane and extract intrinsic kinetics for this process. Understanding the coupling between reaction and diffusion should facilitate the study of electrocatalytic materials and the scale-up of electrocatalytic reactors in the near future.

In recent years, a growing emphasis has been put on developing clean energy technologies and processes based on renewable energy sources. Solar power is the key to a clean energy future.¹ Artificial systems that mimic natural photosynthesis can be used to convert the Sun’s energy to chemical or electrical energy. However, the effective conversion of light into a usable form of energy is mainly limited due to non-efficient charge separation and poor light-harvesting architecture.^2,3 This issue might be overcome by the proper selection of photoactive materials and the suitable engineering of photoreactors. Consequently, the challenge of developing successful solar-driven technology starts from understanding the nature of the performed chemical processes and the design of devices which can be implemented with photo-catalytic technologies. Following this trend, we have decided to examine how one of the most promising materials for alcohol photo/electrooxidation, metal-doped carbon nitrides (g-C₃N₄), can be applied to microfluidic fuel cells.⁴ Our research indicates that among various materials, Ni-doped g-C₃N₄ exhibits outstanding catalytic activity towards methanol photo/electrooxidation upon batch reaction conditions.⁵ Therefore, this stable and efficient catalytic reaction system has been transferred towards continuous flow technology via adopting a microfluidic system.

[1] G. Ciamician, Science, 1912, 36 (926), 385.
[2] V. Balzani, G.Bergamini, P. Ceroni, Angew. Chem., 2015, 54, 11320.
[3] C.C. Chua, D.M. Bassani, Photochem. Photobiol. Sci., 2008, 7, 521.
[4] E. Kuna, D. Mrdenovic, M. Jonsson-Niedziolka, P. Pieta, I.S Pieta, Nanoscale Adv., 2021,3, 1342.
[5] I.S.Pieta, P. Pieta, R. Nowakowski ,M. Holdynski, M. Pisarek, A Kaminska, M.B.Gawande, R. Zboril, Appl. Catal. B., 2019, 244, 272.

In recent years, considerable efforts have been made to improve the performance of photoactive nanostructured materials for water splitting applications. Herein, we report on the assembly and use of a bacteriorhodopsin (bR)/TiO₂ nanotube array hybrid electrode system. Photoanode materials composed of ∼ 7μm long self-ordered and vertically oriented nanotube array of titanium dioxide films were fabricated via the anodization of Ti foil in formamide electrolytes containing NH₄F at room temperature followed by sensitization of the electrodes with bR. The stability of bR on the TiO₂ surface was found to depend on the pretreatment process of the TiO₂ films. Our results demonstrate the opportunity to fabricate fairly stable bR/TiO₂ hybrid electrodes that can be used as photoanodes for photoelectrochemical water splitting. Under AM 1.5 illumination (100 mW/cm²), the hybrid electrodes achieved a photocurrent density of 0.65 mA/cm² which is a ∼ 50% increase over that measured for pure TiO₂ nanotubes (0.43 mA/cm²) fabricated and tested under the same conditions. In the presence of a redox electrolyte, the photocurrent increased to 0.87 mA/cm². To the best of our knowledge, this is the first report on the use of bR/TiO₂ hybrid electrodes in photoelectrochemical water oxidation cells. We believe the proton pumping property of bR can be used in a variety of applications, especially those related to third generation photovoltaic cells.

There has been a vast improvement in photovoltaic (PV) technology in the last couple of decades; however, solar PV technology has a lot of drawbacks, including that it is intermittent, broadly dispersed, and non-transportable. Hence, cost-effective solar energy storage is critical for widespread implementation of solar energy as the primary energy source.
Slurries of semiconductor particles individually capable of unassisted light-driven water-splitting are modeled to have a promising path to low-cost solar hydrogen generation, but they have had poor efficiencies. Tandem microparticle systems are a clear direction to pursue to increase the efficiency. However, light absorption must be carefully managed in a tandem to prevent current mismatch in the subcells, which presents a possible challenge for tandem microwire particles suspended in a liquid. In this work, a Ni-catalyzed Si/TiO₂ tandem microwire slurry was used as a stand-in for an ideal bandgap combination to demonstrate proof-of-concept alignment of unassisted water-splitting microwires with an external magnetic field. The Ni hydrogen evolution catalyst was selectively photodeposited at the exposed Si microwire core to serve as the cathode site as well as a handle for magnetic orientation. The frequency distribution of the suspended microwire orientation angles was determined as a function of magnetic field strength under dispersion with and without uplifting microbubbles. After magnetizing the Ni bulb, tandem microwires could be highly aligned in water under a magnetic field despite active dispersion from bubbling or convection.

Water oxidaiton is an important first step toward artificial photosynthesis. A key challenge in this field is the lack of clarity on how water is oxidized. As a result, a durable but highly active heterogeneous catalyst that is also low-cost has not been found. Important questions on why certain catalysts are more active than others also remain unanswered. This problem has been difficult to solve because, at least in part, there is a lack of clarity on the structure of the active sites. The fact that the catalyst structure also evolves during the catalytic reaction further excerbates the situation. Here we present a new catalyst platform that has the potential to address this issue. Starting from molecular catalysts whose structure is well resolved, we synthesize heterogeneous catalysts that preserve the active site structure. The resulting material is a dinuclear catalyst that is highly active but also durable. More importantly, it provides an opportunity to test the various hypotheses concerning water oxidation mechanisms.

Nitrate (NO₃^-) pollution of groundwater is an arising issue due to the high input of nitrogen-rich agriculture fertilizers. Furthermore, nitrite (NO₂^-), which is the product of NO₃^- reduction, is also a major threat to human health such as methemoglobinemia (“blue baby syndrome”) and cancer. Electrochemical conversion of harmful nitrate has attracted attention to produce benign dinitrogen (N₂) and ammonia (NH₃) as a means of transforming waste into fertilizers. Designing structure of PdCu bimetallic nanoparticles is important for developing highly efficient and selective electrocatalysts. Here, we engineered the atomic structure of PdCu and controlled Cu surface coverage on Pd nanocube nanoparticles for enhancing the NO₃^- conversion rate and steer the selectivity of N₂/NH₃. Highly ordered structure of PdCu nanoparticles showed high nitrate reduction activity, excellent dinitrogen selectivity and robust stability in a neutral electrolyte compared to disordered PdCu nanoparticles. We also develop Pd nanocube nanoparticles and introduce Cu metals on the surface by underpotential deposition (UPD) method. The decorated Cu metals catalyze the reduction of NO₃^- to NO₂^- and the Pd (100) enhances the activity and selectivity of NO₂^- to N₂. Rotating disk electrode (RDE) and H-cell tests were conducted for NO₃^-/NO₂^- reduction activity and the selectivity of PdCu bimetal electrocatalysts were examined by UV-vis and in-situ mass spectrometry. This study demonstrates that designing nanostructure of PdCu is an efficient approach to facilitate the nitrate reduction process and control the selectivity of N₂/NH₃ as reducing environmental issues associated with removing NO₃^- and NO₂^- from water.

Nitrate contamination in groundwater due to agricultural runoff is one of the most significant modern environmental challenges, with substantial implications on wildlife and human health. The electrochemical reduction of nitrate has demonstrated promise for its elimination; however, developing methods for improving selectivity to desired products is paramount toward its effectiveness. Nitrogen gas (N₂) is benign and a desirable target for wastewater remediation, though maintaining its production at sufficient activity and current efficiency has been elusive.

This presentation describes our recent computational and experimental work¹ in developing transition metal nitrate reduction catalysts. Our experimental results, combining rigorous electrochemistry with online gas chromatography, have found that Ag electrocatalysts demonstrate a unique activity and selectivity for nitrate to nitrite not found on other transition metal catalysts. In particular, an oxide-derived (OD-)Ag catalyst demonstrated up to 98% nitrate-to-nitrite selectivity with 95% faradaic efficiency. The product selectivity can be tuned simply by changing the electrode potential to more reducing conditions, yielding ammonia with faradaic efficiency of 89%. Alternatively, the nitrite generated by the electrocatalytic process can be efficiently and selectively reduced to N₂ in a separate (nonelectrochemical) catalytic process on Pd/C to yield a clean two-step process for nitrate reduction.

Density functional theory (DFT) calculations on model surfaces show that the undercoordinated nature of the OD-Ag catalysts is particularly advantageous for nitrate reduction relative to traditional Ag nanoparticles. The Ag-based catalysts have difficulty in directly activating N-O bonds, instead relying on proton-mediated pathways to perform N-O bond dissociation. The unique selectivity arises from the relative ease of nitrate vs. nitrite reduction along these proton-mediated pathways, with the undercoordinated nature of OD-Ag aiding the electrocatalytic performance due to the preferential adsorption of nitrate. In contrast, Cu-based catalysts demonstrate poorer selectivity to nitrate due to their ability to directly activate N-O through non-electrochemical routes. Together, these results open the possibility of electrochemical nitrate remediation to close the nitrogen cycle (by forming N₂) and/or the potential use of nitrite as a platform for the generation of other N-containing compounds (e.g., ammonia, hydroxylamine) when treating concentrated nitrate sources.

¹H Liu, J Park, Y Chen, Y Qiu, Y Cheng, K Srivastava, S Gu, B H Shanks, L T Roling, W Li, ACS Catalysis (accepted), https://doi.org/10.1021/acscatal.1c01525

High current density electrochemical CO₂ reduction (CO₂R) (~1 A cm^-2) can create concentrated hydroxide at the cathode, causing the activity coefficient of water and hydroxide to deviate from the “dilute solution approximation.” Also, for CO₂ reduction to CO, as the ratio of CO₂ to CO changes along the gas flow channel at a gas-diffusion electrode the reaction equilibrium potential will shift. Here I discuss the implications of these effects for CO₂R energy efficiency and product selectivity based on Nernstian analysis. Promising reactor designs are suggested, accordingly.

The influences of various transition metal ions (Ni, Fe, Co, Cu, Mn, Zn, and Sn) in metal- and nitrogen-doped carbon (M-N-C) are systematically investigated for the electrochemical reduction of water and CO₂ (CO₂RR). By careful pyrolysis of carbon-supported porphyrins at low loading predominately M-N₄-centers embedded in a carbon matrix are achieved, while avoiding significant amounts of undesired metal- and nitrogen byproducts. The catalysts were electrochemically investigated in a Gas Diffusion Electrode (GDE) arrangement coupled in-line to a mass spectrometer, allowing the almost simultaneous detection of product selectivity, Faraday efficiencies and current densities in linear sweep voltage (LSV) experiments. Among these catalysts, Fe– and Ni-based samples have shown remarkable high Faraday efficiencies of > 90% for the formation of CO even at current densities exceeding 175 mA cm^-2 in pH 7.5 carbonate buffer. The comparison of measurements with different gas supply (Ar vs CO₂) underlines that this is achieved by an almost completely suppressed hydrogen evolution in the presence of CO₂, as theoretically expected for single site centers like MeN₄. The Fe-N-C and the Cu-N-C catalyst show a change in reaction mechanism for potentials more negative than -1.5V/NHE accompanied by a steep increase of the formation of CH₄, H₂ (and C₂H₄ on Cu-N-C) whereas CO formation is partially suppressed. Post mortem XPS analysis of Cu-N-C reveals that Cu²⁺ of the Cu-N₄ centers is partially reduced to Cu° in that potential range, which led us assume that also in M-N-C catalysts Cu° structures are involved in the formation of hydrocarbons. While Zn-N-C and Mn-N-C in our study generally show only little CO2RR activity, Sn-N-C produces as main product formate with an FE of about 40% and production rates of about 20 µmol h^-1 cm^-2 at potentials smaller than -1.5V/NHE. Our work allows new insights into the potential dependent behavior of M-N-C catalysts and reveals the high selectivity of MN₄ centers for the CO2RR.

The electrochemical oxidation of biomass platform 5-hydroxymethyl furfural (HMF) to 2,5-furandicarboxylic acid (FDCA), is an important reaction in the emerging area of renewably powered biomass valorization. To this end, the efficiency of catalytic systems still needs to be improved to render these systems economically competitive. In the electrochemical HMF oxidation reaction (HMFOR), the utilization of well-defined active sites is critical for fundamental insights into their function and subsequently rationally turning them to maximize catalytic activity. However, heterogeneous catalysis of HMF conversation often feature poorly defined catalytic surfaces, hampering the community's complete understanding of the reaction mechanism. To the end, we report the use of reticular chemistry to design a conductive metal organic framework-based electrocatalytic model system with well-defined M-O₄ active sties for HMFOR. The activity of MOFs bearing Ni-O₄ (Ni-CAT) and Co-O₄ (Co-CAT) active sites were analyzed with electrochemical and operando spectroscopic techniques to elucidate the reaction mechanism occurring on the surface. Electrochemical experiments reveal that Co-CAT has an earlier onset potential for enabling HMFOR, relative to other known catalysts thus far, while the Ni-CAT shows faster kinetics for the conversion of HMF to FDCA.
Also, Ni-CAT exhibits efficient charge transfer properties and intrinsically high kinetic towards HMFOR. It achieved FDCA yields of 98.7% yield with 86.8% faradic efficiency at 1.42 V vs RHE.
Infrared spectroscopy captures the surface-bound aldehyde group as the key intermediate in the catalytic cycle, which forms once M(II\III) oxidation occurs. This work illustrates the advantage of utilizing molecularly defined active sites coupled with operando spectroscopy to provide fundamental insights into a variety of electrosynthetic reactions and pave the way for future catalyst design.

CO₂ electrolysis is a promising technique for CO₂ recycling and utilization offering carbon-neutral chemical feedstocks for downstream applications. Electrocatalyst development for CO₂ reduction reaction (CO₂RR) toward single carbon products has achieved enormous progress. However, copper has been found the only metal that can efficiently catalyze CO₂ to higher-value and energy concentrated multicarbon products (e.g., ethylene, ethanol, acetate). Therefore, how to tune the selectivity and boost productivity towards multicarbons based on Cu catalysts becomes a central question in CO₂RR. Here, we highlight that tandem electrocatalysis can be a promising approach towards this target. With tandem strategy, a chemically complicated pathway can be decoupled into individual steps within via multicomponent catalyst design. As a demonstration, we show that a Cu-Ag tandem catalyst on a GDE can enhance the multicarbon production rate from CO₂ through CO₂ reduction to CO on Ag and subsequent carbon coupling on Cu. With added Ag, the multicarbon partial current over a Cu surface increases by 4-fold at -0.70 V vs RHE in 1M KOH. Corresponding CO reduction experiments have been conducted to show that no mutual interference between these two metals can be observed at the catalytic level. More interestingly, the intrinsic C₂H₄ and C₂H₅OH turnover frequency in the tandem platform are significantly higher than Cu alone under either pure CO₂ or CO atmosphere. Such results highlight the unexpected superiority of the tandem strategy for CO₂RR in a flow cell, which offers the opportunity to enhance the efficiency of C-C bonding on a Cu surface beyond conventional CORR during high-rate electrolysis. Considering that the tandem strategy is an independent approach and can be further conjugated with catalyst surface engineering, we aim to optimize the Cu-Ag nanostructure and further boost the catalyst performance. Meanwhile, operando spectroscopy can be a powerful method to explore the microenvironment tunability and structure-catalysis relationship.

In the electrochemical CO2 reduction reaction (CO2RR) selectivity still remains an important issue.
In this talk, I will showcase a few examples which highlight how shape-controlled nanocrystals can contribute to address this challenge. First of all, I will discuss how size control of Cu nanocubes (Cu_cub) and Cu octahedra (Cu_oh) has revealed the importance of facet-ratio to maximize the selectivity towards ethylene and methane, respectively. Second, I will present our recent computational-experimental efforts towards using well-defined NCs to elucidate selectivity rules at the hydrocarbons/alcohols branching nodes in the CO₂RR pathway. The formation of ethanol via *CH_x-*CO coupling or *CO-*CO coupling is an open debate in the literature. As a platform to address this question, we have used CH₄ favoring (i.e. *CH_x populated) Cu_oh and C₂H₄ favoring (i.e. *CO-*CO populated) Cu_cub under enhanced *CO coverage induced by the presence of Ag NCs. The selective promotion of ethanol in the Cu_oh provided evidence for *CH_x-*CO coupling being the preferred pathway. Having learned this, we have also explored the size-dependent edge/face ratio of the Cu_cub to interrogate the reactivity of the high index (711) copper surface. The results of this last study indicate the propensity of (711) step sites towards ethanol production.

The electrocatalytic (EC) CO₂ reduction (CO₂R) driven by renewable energy can be exploited for the future energy transition, for the carbon storage into valuable products like syngas (H₂/CO mixtures), organic acids (formic acid) and chemicals/fuels (C₁₊ alcohols).^1,2 A big challenge for the industrialization of this technology is to find low-cost electrocatalyst, efficient reactors and process conditions. In our lab, we are exploiting the current knowledge of the thermocatalytic CO₂ hydrogenation to develop noble-metal-free CO₂R electrocatalysts.³ Our results open a promising path for the prospective implementation of metal-oxides nanostructures for the CO₂ conversion to the chemicals and fuels of the future.
We have developed novel structures of Cu/Zn/Al-based catalysts, which produce methanol and CO from the CO₂ thermocatalytic (TC) hydrogenation (at H₂ pressure (P) of 30 bar and temperature (T) > 200 ^oC) and are able to generate H₂, CO and other C₁ to C₃ liquid products like alcohols, during the electrochemical CO₂R in a gas-diffusion-electrode (GDE) system at ambient T,P. We have also developed B,N-doped Cu oxide catalysts, inducing the formation and stabilization of Cu⁺¹/Cu⁰ interfaces and promoting the dimerization of the *CO intermediate. Incorporating both B and N heteroatoms led to greater selectivity for reducing CO₂ to products of interest: that is, a total FE of 64% to CO₂ reduction products with 49% of liquid products (i.e. 77% selectivity towards formate and alcohols) at a relevant current density of 23 mA cm^-2. The catalysts and electrodes have been prepared by scalable techniques (i.e. co-precipitation and spray-coating) and characterized ex-situ by different physico-chemical methods like X-ray diffraction, BET, porosimetry, filed-emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS) among others. A correlation has been found between their activity and selectivity with respect to their porosity and superficial Cu states. The physical and chemical properties of these Cu-based materials can be manipulated to tune the performance of the electrochemical reaction. In addition, the electrochemical reactor configuration (GDE vs. CO₂ dissolved in liquid electrolyte) also play an important role in the final selectivity and productivity.⁴ These interesting results could help find a suitable electrocatalytic systems for the future implementation of this technology at industrial level.
References:
1 H. Guzmán, M. A. Farkhondehfal, K. Rodulfo Tolod, N. Russo and S. Hernández, in Solar Hydrogen Production Processes, Systems and Technologies, Elsevier Inc., 2019, p. 560.
2 H. Guzmán, N. Russo and S. Hernández, Green Chem., 2021, 23, 1896–1920.
3 H. Guzmán, F. Salomone, E. Batuecas, T. Tommasi, N. Russo, S. Bensaid and S. Hernández, Chem. Eng. J., 2020, 127973.
4 H. Guzmán, F. Zammillo, D. Roldán, C. Galletti, N. Russo and S. Hernández, Catalysts, 2021, 11, 482.

Acknowledgements: This work has received financial support by the EU H2020 Project SunCOChem grant agreement: 862192 and Eni SpA: R&D Program Energy Transition (Cattura e Utilizzo CO₂).

CO₂ conversion has been proposed as a potential way to close the carbon cycle and generate chemical fuels. The electrocatalytic reduction of carbon dioxide, powered by renewable electricity, to produce valuable fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of energy produced by intermittent renewable sources.
The development of such a technology is currently hampered by the lack of catalysts, which can drive the reaction at industrially relevant current densities with high efficiency and selectivity. Examples of strategies for optimizing the CO₂RR performance include alloying, surface doping, ligand modification, and interface engineering. Our investigations focus on exploring the role of functionalization on the catalytic activity of Cu towards the conversion of CO₂ to hydrocarbon products.
We developed a method to modify the surface of bimetallic silver-copper (Ag-Cu) catalyst with aromatic heterocycles such as thiadiazole and triazole derivatives. By combining operando Raman and X-ray absorption spectroscopy with electrocatalytic measurements and analysis of the reaction products, we identified that the electron withdrawing nature of functional groups orients the reaction pathway towards the production of C₂₊ species such as ethanol and ethylene and enhances the reaction rates on the surface of the catalyst. As a result, we achieve a maximum Faradaic efficiency for the formation of C₂₊ of ≈ 80% and full-cell energy efficiency of 20.3% with a specific current density of 261.4 mA cm^-2 for C₂₊ using functionalized Ag-Cu electrodes, compared to only 33.8% and 70.6 mA cm^-2 for the pristine Ag-Cu electrodes. We anticipate that our strategy can further be extended in order to improve the selectivity of the reaction towards the production of specific multicarbon molecules.

Carbon monoxide electroreduction is a rapidly developing field for the production of valuable commodity chemicals. When coupled with CO₂ capture and electrochemical conversion, it can be used as a clean CO₂ negative process for the production of a variety of products. Carbon monoxide reduction has the distinct advantage over direct electrochemical CO₂ reduction to multi-carbon products (C₂₊) in that it does not suffer from carbonate formation, allowing for higher single pass conversion and electrolyte stability. This work focuses on the design and production of a zero gap membrane electrode assembly carbon monoxide electrolyzer, which operates at low cell potentials (< 2.5 V), and industrially relevant current densities (> 200 mA/cm²) for the production of acetate and ethylene. Acetate and ethylene are valuable target products due to their high value and market potential. Zero gap electrolyzers place the cathode and anode in direct contact with a conductive polymer membrane, whereas conventionally carbon monoxide electrolyzers have relied on a three-compartment design where the cathode is in contact with a liquid electrolyte layer.
By utilizing the zero-gap design, liquid products can be collected in either cathode or anode chamber. To enhance product selectivity, we choose suitable electrocatalysts for both cathode and anode to maximize the selectivity towards carboxylates. By utilizing a suitable combination of electrocatalysts, we have successfully improved the product selectivity towards acetate substantially. The system was also found to have improved current densities and cell voltages when compared to conventional three-compartment carbon monoxide electroreduction systems. Increased residence time of the anolyte by recirculation also allowed for the production of highly concentrated acetate streams with >90% purity relative to other liquid CO reduction products.

I will review progress in CO2 to ethanol, and to n-propanol, in electrochemical CO2RR and CORR systems.

There is significant interest in developing efficient electrochemical processes for commodity chemical manufacturing, all directly powered by renewable electricity. Here we report a one-step electrochemical route at ambient temperature and pressure in aqueous media for the selective anodic partial oxidation of ethylene to ethylene glycol with co-production of hydrogen at the cathode^[1]. Tuning of the catalyst OH binding energy was hypothesized to be crucial for facilitating the transfer of OH to *C₂H₄OH to form ethylene glycol. Computational studies suggested that a gold-doped palladium catalyst could perform this step efficiently, and experimentally we found it to exhibit an approximate 80% Faradaic efficiency to ethylene glycol, retaining its performance for 100 hours of continous operation. I will also describe our efforts in developing an electrochemical system for the electrosynthesis of ethylene oxide from ethylene using a chloride mediator^[2]. A 2 step integrated system involving CO2-to-ethylene conversion and subsequent conversion to ethylene oxide was also demonstrated. In summary, these findings represent a significant advance in the development of selective anodic partial oxidation reactions in aqueous media under mild conditions.

References
[1] Lum, Y.* ; Huang, J. E.* and Sargent, E. H et al. Tuning OH binding energy enables selective electrochemical oxidation of ethylene to ethylene glycol. Nature Catalysis 2020, 3 (1), 14–22.

[1] Leow, W.* ; Lum, Y.* and Sargent, E.H et al. Chloride-mediated selective electrosynthesis of ethylene and propylene oxides at high current density. Science 2020, 368 (6496), 1228-1233.

In this study, a bifunctional electrocatalyst for CO₂ reduction and the O₂ evolution reaction (OER) was constructed from the electrodeposition of cuprous oxide (Cu₂O) and Ni on a carbon substrate. Different Ni thicknesses on Cu₂O were achieved by varying the time of chronopotentiometric deposition of Ni. Electrochemical CO₂ reduction was carried out at -0.89 V and -1.89 V vs. RHE, and it was found that formate and CO were the two major products. Cu₂O modified with a Ni overlayer with a thickness of ~700 nm resulted in the highest formate Faradaic efficiency of 18%, and Cu₂O resulted in highest CO Faradaic efficiency of 7.9%. The enhanced Faradaic efficiency for formate is attributed to the synergistic effect between Ni and Cu₂O due to maximized amounts of exposed bimetallic sites that facilitate CO₂ reduction. The electrocatalyst also produces ~9 times more current density than previous studies using Ni-Cu₂O electrocatalysts for the OER. The ability of the Ni-Cu₂O thin films to catalyze both the OER and CO₂ reduction allows them to be incorporated in the first demonstration of a two-electrode CO₂ conversion device with a bifunctional catalyst.

Electrocatalytic water splitting can be regarded as promising method to store electrical energy in form of hydrogen as a sustainable green energy carrier. However, water splitting typically suffers from the sluggish reaction kinetics of the oxygen evolution half reaction (OER). By employing electrocatalysts, the required overpotential of the OER can be decreased significantly.
In recent years, earth-abundant spinel ferrites have emerged as auspicious materials for applications in photoelectrochemistry and electrocatalysis. Spinel-type first-row transition metal oxides (TMOs) or their composite compounds containing nickel, manganese, iron, or cobalt are attractive materials for the electrocatalytic oxygen evolution reaction in alkaline media. However, Mn-based electrocatalysts often lack due to stability issues, and the use of cobalt is further discussed critically due to its toxicity hazards and limited availability. Therefore, low cost and high abundant bimetallic NiFe spinel oxide with the elemental composition NiFe₂O₄ has gained attention in the field of alkaline water electrolysis, combining remarkable stability in alkaline media with excellent redox properties.[1,2]
We developed a fast microwave-assisted synthesis yielding phase-pure spinel ferrite nanoparticles of e.g. MgFe₂O₄, CoFe₂O₄, NiFe₂O₄ and ZnFe₂O₄ at temperatures as low as 170-200 °C.[3-6] The crystallite size can be tailored by post-synthetic heat treatment, however the materials are already (partly) crystalline as-prepared, with specific surface areas of around 200 m²/g and good colloidal stability. Photocatalytic and electrocatalytic experiments will be presented, as well as the conversion of some spinel oxides into sulfides, e.g. pendladites.[7] A new direct microwave synthesis for nickel-iron sulfides for electrocatalytic CO₂ reduction will also be presented.[8]

[1] M. Gong, H. Dai, Nano Res. 2015, 8, 23
[2] L. Gong et al., Adv. Sustain. Syst. 2021, 5, 2000136.
[3] K. Kirchberg et al., J. Phys. Chem. C 121 (2017) 27126
[4] C. Simon et al., in revision
[5] P. Dolcet et al., Inorg. Chem. Front. 6 (2019) 1527
[6] A. Bloesser et al., ACS Appl. Nano Mater. 3 (2020) 11587
[7] D. Tetzlaff et al., Faraday Discussions 215 (2019) 216
[8] C. Simon et al, in revision

We are studying new applications of layered zirconium phosphate (ZrP) inorganic
nanomaterials. The θ phase of ZrP can be directly ion-exchanged with large metal complexes, producing intercalated phases useful for artificial photosynthesis schemes for water splitting, amperometric biosensors, and drug delivery applications. Recently, we have demonstrated improved electrocatalytic activity of ZrP nanomaterials loaded with metal ions suitable for the oxygen evolution reaction (OER) of water splitting.
Adsorbing Co and Ni catalysts on the ZrP nanoparticles surface proved to improved OER activity compared to intercalated catalysts. A comparison between adsorbed Co or Ni catalysts and those catalysts on exfoliated ZrP nanoplatelets proved that those on exfoliated nanoplatelets were more active, with diminished overpotentials and reduced Tafel plot slopes as well as higher mass activities. More recently, comparison
between Co and Ni catalyst on ZrP particles with different morphologies (hexagonal platelets, rods, cubes, and spheres) revealed that the more active Co catalysts are those on hexagonal ZrP platelets, whereas the best Ni catalysts are those on ZrP spheres. We are exploring the OER activities of mixed-metal catalysts and porphyrins, as well as operando synchrotron X-ray absorption spectroscopy studies to elucidate the nature of the active catalysts.

Electrochemical conversion of abundant feedstocks to fuels and value-added chemicals is rapidly gaining significance as a promising method to harness renewable electricity. Specific reactions within this context that my research group is focused on are the reduction of CO₂ and oxidation of biomass platforms to fuels and value-added chemicals. Because the design of new catalytic systems is inherently linked to a precise understanding of how these reactions proceed on heterogeneous surfaces, we put considerable efforts in developing methodology for operando probing with vibrational spectroscopy. In all, I show how using these experiments provide the key mechanistic information on surface reaction mechanisms that enhance our understanding of functional hybrid interfaces and how the research provide avenues for future materials design within the context of electrosynthesis of fuels and chemicals.

CO₂ electrolyzers operating at high current densities and product selectivities are essential for the commercialization of electrochemical CO₂ reduction (CO₂R) systems to chemicals and fuels. An electrolyzer based on a membrane-electrode assembly (MEA) enables the achievement of these desired criteria. A critical component of an MEA is its catalyst layers, which are composed of catalyst nanoparticles and an ionomer. The latter component provides a pathway for ion transport from the catalyst to the ion-conducting membrane.

The materials design parameters that characterize these ionomer-based catalyst-layers are the ionomer-to-catalyst ratio (I:Cat), catalyst loading, and catalyst-layer thickness. In this study, we systematically analyze the effects of these parameters on the activity and product selectivity of a Ag cathode CO₂R MEA. An optimum I:Cat ratio of 3 is rationalized based on electrochemically-active surface area (ECSA) and ionomer coverage/distribution arguments, where the extent of this coverage and the resulting ionomer-catalyst interaction significantly impacts the total current density and CO selectivity. Coverage regimes range from a patchy/sparse ionomer distribution at low I:Cat ratios (i.e., I:Cat < 3) suffering from poor ECSA and ionic pathway to excessive coverages at high I:Cat ratios (i.e., I:Cat > 3) suffering from poor electronic contact and CO₂ utilization. The effect of decreasing the catalyst loading and/or catalyst-layer thickness changes mainly the CO₂R product selectivity (in agreement with previous modeling work) and the CO FE is shown to be highly sensitive to this metric. By changing these catalyst-layer design parameters, our results demonstrate that the MEA architecture can behave similarly to traditional cells in their ability to highly tune CO₂R selectivity with their electrode/electrolyte interfaces.

The crucial insights gained from the reported trends are compounded to yield an efficient Ag CO₂R MEA cell that operates at current densities of 200 mA cm^-2 to 1000 mA cm^-2, CO faradaic efficiencies of 78 to 91%, and an area-specific resistance under 1 Ω cm². These findings can be applied to a broad range of CO₂R MEA-based devices, including CO₂-to-CO tandem devices and flexible manufacturing systems where changing the aforementioned catalyst-layer properties can effectively tune the outlet CO:H₂ ratio for various syngas applications. The scientific and experimental design learnings from our Ag-cathode MEA studies are subsequently applied to a Cu-cathode MEA, which has farther reaching and impactful applications due to its ability to produce value-added chemical precursors such as ethylene and ethanol.

Acknowledgements
The authors gratefully acknowledge Lawrence Berkeley National Laboratory’s Laboratory Directed Research and Development (LDRD) Grant for funding. This material is also based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

The electrochemical CO₂ reduction reaction (CO₂RR), which converts CO₂ to value-added chemical products with high energy density, is a promising solution for fossil fuel-induced energy and environmental issues because it can help realize a sustainable energy cycle. A commercially feasible CO₂RR requires the development of an electrocatalyst that can steer the reaction pathway toward specific products on a multidimensional scale by controlling the binding energy of the reaction intermediates to design a reaction configuration. This enables economic earnings to be achieved by renewable energy conversion-induced electrochemical reactions in terms of energy efficiency, current density and stability.
For steady electroconversion to value-added chemical products with high efficiency, electrocatalyst reconstruction during electrochemical reactions is a critical issue in catalyst design strategies. Here, we report a reconstruction-immunized catalyst system in which Cu nanoparticles are protected by a quasi-graphitic C shell. This C shell epitaxially grew on Cu with quasi-graphitic bonding via a gas-solid reaction governed by the CO (g) - CO₂ (g) - C (s) equilibrium. The quasi-graphitic C shell-coated Cu was stable during the CO₂ reduction reaction and provided a platform for rational material design. C₂₊ product selectivity could be additionally improved by doping p-block elements. These elements modulated the electronic structure of the Cu surface and its binding properties, which can affect the intermediate binding and CO dimerization barrier. B-modified Cu attained a 68.1% Faradaic efficiency for C₂H₄ at -0.55 V (vs RHE) and a C₂H₄ cathodic power conversion efficiency of 44.0%. In the case of N-modified Cu, an improved C₂₊ selectivity of 82.3% at a partial current density of 329.2 mA/cm² was acquired. Quasi-graphitic C shells, which enable surface stabilization and inner element doping, can realize stable CO₂-to-C₂H₄ conversion over 180 hours and allow practical application of electrocatalysts for renewable energy conversion.

In the present talk I will illustrate the main challenges in the modeling of advanced electrocatalytic processes. To start with the changes induced by the rearrangement of the initial catalytic material under reaction conditions will be analyzed for oxygen-derived copper systems. In a second step the role of cations at the interface and how they affect adsorption and reactivity will be investigated. In the final part, the need for advanced fast screening reaction profiles for C3 and longer chain molecules will be presented.

Polymer electrolyte fuel cell (PEFC) is expected to the next generation power sources, but its high cost and low service life are inhibited the wide-spread commercialization. In particular, the platinum utilization and the durability of the carbon supported platinum (Pt/C) used as cathode catalyst are big problems for PEFC. To improve the catalyst activity and durability, many researchers have developed the new catalyst nanoparticle. However, the development of catalyst support is not enough in comparison with the catalyst nanoparticles. In general, the activated carbon, which has a high surface area, is employed as the catalyst support to realize the high dispersion supporting of catalyst nanoparticles. However, the activated carbon leads to the low platinum utilization by its high substance diffusion resistance attributed to the complex pore structure. And the functional groups on the surface conduces to the carbon corrosion. Graphitized carbon is well known as one of the high corrosion tolerance carbon materials. But it is hard to disperse catalyst nanoparticles highly due to the low surface area of the graphitized carbon. When developing the new catalyst support, it is necessary to consider the balance between the catalytic activity and the carbon-corrosion tolerance.
We developed the carbon-sphere composing with the reduced graphene oxide (rGO) wall and SiO₂ core as a new familiar of carbon materials [1]. The carbon-sphere is expected to have low substance diffusion resistance due to its simple pore structure compared to conventional carbon. Previous reports indicate that the rGO trends to disperse the platinum nanoparticles highly [2-8]. Thence, the carbon-sphere developed by our group can be expected to balance the catalytic activity and the carbon corrosion tolerance. In this study, the carbon-sphere supported platinum (Pt/CS) was synthesized and evaluated the oxygen reduction reaction (ORR) activity and durability.
Carbon sphere was prepared as previously reported. [1] Pt/CS was synthesized by an impregnation method. Morphology, crystalline structure and electronic state of Pt/Carbon-sphere were observed SEM, XRD, and XPS. The electrochemical measurement was conducted by the rotating disk electrode (RDE) method.
The morphology of Pt/CS was maintained to the uniform sphere shape. The presence of Pt was confirmed by XRD, SEM and XPS. The platinum particle size on CS was around 5 nm. The ORR activity of Pt/CS was higher than that of a commercial catalyst (TEC10E50E, TANAKA KIKINZOKU KOUGYOU). After the durability test, the activity of Pt/CS remained about 50% of the initial state, although a commercial catalyst showed almost no activity. We concluded that Pt/CS could balance the catalytic activity and the carbon corrosion tolerance.

[1] T. Saida, T. Kogiso, T. Maruyama, Chem. Lett., 45, 330-332, (2016).
[2] P. Kundu, C. Nethravathi, P. A. Deshpande, M. Rajamathi, G. Madras, and N. Ravishankar, Chem. Mater., 23, 2772-2780 (2011).
[3] J. Sato, K. Higurashi, K. Fukuda, W. Sugimoto, Electrochemistry, 79, 337-339 (2011).
[4] Y. Li, Y. Li, E. Zhu, T. McLouth, C.-Y. Chiu, X. Huang, Y. Huang, J. Am. Chem. Soc., 134, 12326-12329 (2012).
[5] K. Cheng, D. He, T. Peng, H. Lv, M. Pan, S. Mu, Electrochim. Acta, 132, 356-363 (2014).
[6] D. Higgins, M. A. Hoque, M. H. Seo, R. Wang, F. Hassan, J.-Y. Choi, M. Pritzker, A. Yu, J. Zhang, Z. Chen, Adv. Funct. Mater., 4, 4325-4336 (2014).
[7] P. Zhang, W. Tu, R. Wang, S. Cai, J. Wu, Q. Yan, H. Pan, H. Zhang, H. Tang, Int. J. Electrochem. Sci., 11, 10763-10778 (2016).
[8] P. Mardle, O. Fernihough, S. Du, Coatings, 8, 48-9 (2018).

MXenes, as the newest family of 2-dimensional materials, can pave the way toward passing the obstacles against developing novel energy materials. However, among different MXene compositions, negligible attention has been paid to Nb-based MXenes in electrocatalysis, where, Niobium possesses a high number of empty orbitals, providing more opportunities for properties engineering. In addition, various synthesis methods in exfoliating the Nb-based MAX phase can lead to the formation of different terminating groups, each of which can result in different catalytic properties. Up to now, no works have been devoted to perusing the role of terminating groups in varying the MXenes' electrocatalytic properties. Moreover, exploiting computational methods can facilitate the properties engineering of MXenes significantly, by which, the synthesis process can be optimized as a function of each catalytic reaction. In the presented work, by employing DFT calculations, the acidic and basic stability of Nb-based MXenes, terminated by F, S, Cl, FS, FCl, ClS groups, will be deciphered. Furthermore, the effect of each terminating groups on electronic properties and activity of the Nb2C Mxene will be revealed. Therefore, considering the descriptors of the electrocatalytic reactions, the suitablity of each Nb2C MXene in electrocatalysis will be studied.

The development of precisely tailored catalyst materials is predicated on a fundamental understanding of their composition-structure-function relationships. In this work, rapid micowave-assisted synthetic methods were employed to generate dimensionally controlled binary and ternary chalcogenide materials wherein compositional and structural variations modulated charge transport dynamics, interfacial reaction kinetics, as well as bulk and surface energetics. Through an interplay of synthetic materials chemistry, synchrotron X-ray spectroscopy analysis of electronic and atomistic structure, CO₂ reduction and hydrogen evolution electrocatalysis, as well as computational modeling, material design principles are established and the evolution of electrochemical reactivity is evaluated in order to gain fundamental insights into energy conversion and storage. Of note, catalytic performance metrics for the Chevrel-Phase M_yMo₆X₈ (M = alkali, alkaline, transition or post transition metal; y = 0-4; X = S, Se, Te) and Psuedo-Chevrel-Phase M₂Mo₆X₆ (M = alkali metal; X = S, Se, Te) composition spaces indicate that proton reduction overpotentials decrease significantly over M-X bridging sites with increased ionic character. Selective conversion of aqueous CO₂ to methanol and formate is also observed at mild applied potentials over metal-promoted Mo₆S₈ cluster frameworks, which is explored via electrochemistry as well as X-ray absorption spectroscopy in order to correlate frontier electronic structure with interfacial reactivity. Lastly, to further advance the discovery of new materials, we integrate a new feature regression machine learning methodology for extracting stability and synthesizability metrics for the expansive M_yMo₆X₈ composition space in order to expand the library of functional materials with applicaitons in energy conversion, energy storage, and electronic devices.

Molecular hydrogen is a sustainable energy carrier of primary interest to draw down carbon dioxide emissions and accelerate the transition to renewable modes of energy production for transportation, commercial, and residential applications. Photocatalytic materials use solar energy to split water and generate hydrogen, emitting oxygen as the only byproduct, but many of the known water-splitting photoactive semiconductors have limited solar-to-hydrogen conversion efficiency. We present a high-throughput screening protocol that was collaboratively developed to discover water-splitting photocatalysts [1] and yielded 7 newly predicted photoactive materials from an initial set of 70,150 candidate materials from the Materials Project database [2]. We also provide a comparison of the computational and experimental data to test the validity of computational predictions using a Hubbard-corrected density-functional theory (DFT+U) approach, in which the U parameters are obtained self-consistently from density-functional perturbation theory to impose the piecewise linearity of the total energy with respect to the occupations of a subset of electronic states in the system (typically, of d or f character) [3]. By systematically comparing DFT+U results obtained using nonorthogonalized and orthogonalized atomic orbitals as Hubbard projectors for a representative set of 20 compounds containing transition-metal or p-block (group III-IV) elements, including oxides, nitrides, sulfides, oxynitrides, and oxysulfides, we find that the predicted band gaps are extremely sensitive to the type of projector functions and that the orthogonalized projectors give the most accurate band gaps. These results demonstrate that DFT+U may serve as a useful method for high-throughput workflows that require reliable band gap predictions at moderate computational cost.

[1] Xiong, Y.; Campbell, et al., Optimizing accuracy and efficacy in data-driven materials discovery for the solar production of hydrogen, Energy Environ. Sci., 2021, 14, 2335-2348.
[2] Jain, A.; Ong, S.; Hautier, G.; Chen, W.; Richards, W.; Dacek, S.; Cholia, S.; Gunter, D.; Skinner, D.; Ceder, G.; et al., The Materials Project: A materials genome approach to accelerating materials innovation, APL Mater., 2013, 1, 011002.
[3] Kirchner-Hall, N.; Zhao, W.; Xiong, Y.; Timrov, I.; Dabo, I, Extensive benchmarking of DFT+U calculations for predicting band gaps, Appl. Sci., 2021, 11, 2395.

Electrochemical methods are currently seen as a promising approach to convert CO₂ into a variety of value-added chemical products such as formic acid, formaldehyde and carbon monoxide [1]. In addition, the electrochemical approach is commonly utilized for the removal of various contaminants/pollutants from wastewater [2]. However, for these processes to be efficient and economically viable, inexpensive and chemically robust electrodes with long operation life-time in harsh electrochemical conditions are required.

Recently, electrochemical reduction of CO₂ from the seawater and water-based electrolytes into various chemical products using boron-doped diamond (BDD) polycrystalline electrode has been reported [3]. In general, BDD electrodes are known for their superior chemical and mechanical characteristics, and such electrodes can be fabricated using chemical vapor deposition (CVD) techniques in a variety of forms (e.g. planar, porous) on different substrates and at different temperatures [4,5].

In this work, we fabricate nanocrystalline boron-doped diamond BDD electrodes of planar and porous (3D) geometries using the CVD method. Additionally, photoelectrodes based on Si solar cell absorber and BDD electrodes were constructed. We examine fabricated BDD electrodes for the (photo)electrochemical of CO₂ from the water-based electrolytes. Furthermore, fabricated BDD electrodes were utilized for the (photo)electrochemical treatment/degradation of organic pollutants (i.e. phenol) in water-based electrolytes.

This work has been supported by the Grant Agency of the Czech Republic (GACR) contract 19-09784Y.
[1] A. Goeppert et al., Chem. Soc. Rev. (2014) 43, 7995-8048.
[2] I. Sirés et al., Environ Sci Pollut Res (2014) 21: 8336.
[3] K. Nakata et al., Angew. Chem. Int. Ed. (2014) 53, 871 –874.
[4] P. Ashcheulov et al., Appl. Mater. Today (2020) 19, 100633
[5] Z. Vlckova Zivcova et al., Diam. Relat. Mater. (2018) 87, 61-69

The renewable energy-driven valorization of excess feedstocks into commodity chemicals and societally useful products constitutes a longstanding push in energy and sustainability research. To this end, this work pushes to expand the scope of green electrosynthesis by innovating a new approach to convert acetonitrile, industrially generated in excess and burned off, to in-demand ammonia and acetaldehyde products. Success here was enabled through the use of a Pd-membrane based reactor which abstracted hydrogen atoms from water, which subsequently diffused through to a separate organic compartment in which they carried out the hydrogenation reaction. In this geometry, the reaction proceeded at 5.2 mA/cm² partial current density and 60% Faradaic efficiency towards ammonia generation. Further, the transmembrane hydrogenation approach gave rise to an onset potential of 0.2V_Ag/AgCl, surpassing previous state-of-the-art systems by 0.7V. A customized infrared spectroelectrochemcal setup was built up to probe the mechanism of the reaction, which was shown to proceed through an imine hydrolysis-like pathway, with the hydrogenation of the NH_x species that remained to be the rate-limiting steps in the process. This work establishes a new route in electrochemical nitrile hydrogenation and general opens up promising avenues in green electrosynthesis.

CO₂ electroreduction reaction (CO₂RR) is a promising method to produce fuels or useful chemicals in a clean and sustainable way. Electricity can be derived from the conversion of other clean energy sources, such as solar or wind, and the extra electricity can be converted into chemical fuels and stored in the form of chemical bonds. The consumption of CO2 can also release greenhouse effect, thus this reaction has the potential to solve energy crisis and environmental problems. Varies of products (carbon monoxide, methanol, methane, ethanol, ethylene, n-propanol) can be produced by CO₂RR, but products with heteroatoms like acetamide or urea are more valuable than these products. Recently, nitrogen source (such as N2 or ammonia) was added into CO2RR system and products with C-N bonds was obtained, but the mechanism of the formation of C-N bonds is still unclear.
Herein, we combine electrochemical method with in-situ spectroscopy to reveal the intermediate of the reaction, and propose the possible reaction pathways. Cu or CuO nanoparticles (NPs) commercial powder was used as electrocatalyst, CO₂ and NH₄OH was used as the precursor, acetamide and formamide was observed as the final products with heteroatom. Electrolysis experiment shows that formamide is preferential to be formed on Cu catalyst, while CuO is more likely to produce acetamide. The faradaic efficiency (FE) is maximum for the formation of acetamide at -1.8V, which is 0.63%. While for Cu, the formation of formamide reach maximum (3.78% FE) at -2.0V. As the formation of acetamide and acetate share the same intermediate, the production of formamide and formate may also undergo the same reaction pathway at the initial stage of the reaction. In-situ infrared (IR) spectroscopy was conducted to prove this speculation.

Photoelectrochemical cells (PEC) are one of the most promising approaches for the production of alternative energy carrier in the global effort to diminish the usage of fossil fuels¹. In this context, III-V nanowires (NW) based photoelectrodes² are particularly attractive thanks to their high surface/volume ratio and their efficient charge separation and collection. However III-V NWs suffer from corrosion in aqueous electrolyte that prevents their utilization for long period. In order to avoid the surface degradation of the III-V NW under working conditions, a particular attention has to be given to their surfaces. We proposed to grow an oxide shell transparent to visible light and compatible with the carrier transfer from the III-V semiconductor to the electrolyte to increase the viability of these photoelectrodes.
GaP and GaAs NWs were grown by molecular beam epitaxy (MBE) using the vapor-liquid-solid (VLS) mechanism on silicon substrate and a TiO₂ shell was deposited by atomic layer deposition (ALD). The morphology, interface, and structure of the NWs were studied by scanning transmission electron microscopy and electron energy loss spectroscopy before and after the measurements of their photoelectrochemical activity after combination with suitable hydrogen or oxygen evolution catalysts.⁷

1 N. Armaroli et al, Angew. Chem. 46, (2007), 52
2 M. G. Kibria et al, Nat. Commun. 5 (2014), 3825
3 J. Kamimura et al, Semicond. Sci. Technol. 31 (2016), 074001
4 K. T. Fountaine et al, ACS Photonics 3 (2016), 1826
5 L. Gao et al, Adv. Funct. Mater. 26 (2016), 679
6 N. Kornienko et al, ACS Nano 10 (2016), 5525
7 This work was done as part of the ANR BEEP project (ANR-18-CE05-0017)

There are needs for finding anodic and cathodic electrocatalysts having high efficiencies and durability based on abundant, environmentally friendly materials and low-cost materials. Among the electrode materials fulfilling these requirements, iron phosphide has engaged interest in the last years as electrocatalyst in the cathode. It is composed of two elements highly abundant in the Earth’s crust and the synthesis routes are making this catalyst simpler and cheaper. Nevertheless, aside of these performances, support’s microstructure is also critical in determining the catalytic activity, selectivity and overall energy efficiency of the supported catalytic nanoassemblies. In this work, ultrathin interconnected carbon nanosheets (CN) are prepared and used as robust support for dispersion of iron phosphide (FeP) nanoparticles, and the resulting catalytic system is evaluated as a low-cost electrocatalyst for hydrogen evolution reaction (HER). A different approach consists of the direct deposition of FeP on the carbonaceous substrate composed by carbon microfibers. Firstly, intermediate iron species are deposited and then, they are converted to FeP by in situ PH₃ generation at 300-400 ^οC. In both cases, high coverage of the substrate is achieved as observed by SEM. The electrochemical current density-overvoltage curves for HER will be evaluated in both acidic and alkaline electrolytes. The differences between both approaches in terms of reference overpotentials at different low, medium and high current density values and stability tests under cathodic conditions will be addressed. Electrodes under the most outperformed conditions have been tested in a flow cell using a cation exchange membranes (CEM) or anion exchange membranes (AEM) to evaluate the feasibility of the cathodes for HER in continuous operation and the obtained data will be discussed according to the employed support material characteristics.

Catalytic materials employed in both half-reactions of the water splitting must have high electrocatalytic activity and low Tafel slope for lowering the applied voltage to the electrochemical cell and long-term stability for the durability of the device vanishing the degradation mechanisms. Additionally, these catalyst materials must be composed by Earth-abundant elements and the synthetic route should be cheap and scalable for industrial production. In general, OER required higher overpotentials than HER, then mostly contributing to the required applied voltage in practical devices. Although Ir/Ru oxides have demonstrated good performance for OER, there is a need for research in cost-effective materials that could compete with the precious metal-based catalysts for OER in order to reduce electrode fabrication costs. Cobalt and iron elements have been reported as electrocatalysts active centers enhancing OER. In this work, we investigate the use of CoFeP nanoparticles to obtain ultra-disperse CoFeP particles supported on metallic foam. It allows achieving high current densities with lower overpotentials as the prepared electrode are showing a low Tafel slope. Under operando conditions for OER in alkaline media, chemical species are found to be different from the ones present in the freshly prepared electrode due to the reactivity of the electrodic material under anodic conditions. Nonetheless, after few initial minutes, the electrodes were stable for long-time tests under anodic currents. A deep analysis is performed for identifying the electrochemically active species for OER in alkaline media by XPS, TPR and IR spectroscopy and their functional role discussed taking into account the initial composition of the freshly prepared catalyst.

Some of the most important electrocatalytic reactions related to clean energy often involves gas reactants or products, including CO₂, H₂, O₂, N₂ mixed with liquid electrolyte and solid catalysts. The efficient electrocatalytic systems need to consider multiple length design from the atomic scale to 3D electrode integration. Here I will present key results from my lab, including: 1) Atomic design and understanding of catalysts for improving catalytic reaction selectivity and throughput; 2) Tuning the activity of catalysts by lithium ion interaction; 3) A novel concept of self-selective catalyst (SELF-CAT) to rapidly identify the proper catalyst for a certain chemical reaction by having the target reaction itself select its own catalyst. 4) 3D electrochemical concept to optimize the electron and mass transport involving three-phase reactions.

Efficient, stable, and low-cost oxygen evolution reaction (OER) catalysts are in great demand for the practical application of electrochemical water splitting for energy conversion and storage. This work reports a rationally designed and innovatively fabricated three-dimensional (3D) core-shell dendritically porous OER catalyst that exhibits outstanding and highly reproducible performance. The porous superstructure provides not only substantially enhanced loading capacity of electrochemical catalysts but also easy access of electrolyte to active catalysts. With a facile and designed sulfurization process followed by anodization, core-shell (FeCoNi)OOH@dendritic foam can be obtained and exhibits a remarkably low overpotential of 204.4 mV at a current density of 10 mA cm^-2, which are among the best of the reported FeCoNi OER catalysts. Such value is also highly reproducible and stable with over 18-hour operation time. This excellent performance could be attributed to the synergism of trimetal chemistry, the unique structure with metal/metal sulfide as the conductive core and (oxy)hydroxide catalysts as the active shell, and the enhanced superstructural surface area. This work points towards a synergistic way based on rational design and innovative fabrication for creating scalable, stable, low-cost, and high-performance OER catalysts.

CO₂ emissions have contributed largely to climate change (over 60%), negatively impacting the environment through its increase due to anthropogenic activities; its close relation to the energetic crisis, as a decrease in the supply of energy resources (oil shortage) and its economic impact, forces the search of sustainable and integral solutions. Transforming CO₂ (a very inert molecule with no direct use in the market) to value-added products (renewable fuels and other material precursors) is no easy task, as its reduction is thermodynamically and kinetically unfavored. Taking into consideration costs, mild reaction conditions, energetic requirements, and sustainability of the reaction system to be designed, photoelectrocatalytic (PEC) systems seem to be the pathway to follow towards environmentally-friendly reduction of CO₂; and microfluidic designs offer quite the advantages (lower energetic consumption, higher active surface equaling higher yields, and easier flow dynamics). Therefore a microfluidic PEC device for CO2-based reduction using SrTiO₃ based nanocomposites is proposed. As photocathodes, nanomaterials are a good choice as larger active surfaces mean greater catalytic activities, and perovskite oxides (SrTiO₃ NPs) are desired in novel PEC systems for their great stability, energy conversion, and low costs. Coupling with g-c3N4 NSs in order to generate a heterostructure enhances CO2 adsorption, and light absorption resulting in greater energetic conversion towards value-added products. The resulting hydrothermally synthesized composite (20% g-C₃N₄/SrTiO₃) has already shown enhanced photoelectrochemical activity, as well as adequate band gap (2.85 eV) and enhanced band alignment (E_CB -1.63 V vs. NHE) minimizing possible hydrogen evolution, and maximizing possible CO₂ reduction yield.

Electrochemical oxygen reduction to peroxide has garnered significant interest as a green alternative to the conventional anthraquinone production process. Electrochemical synthesis offers potentially economical and environmentally friendly route to peroxide (H₂O₂) synthesis. The significant challenge in electrochemical oxygen reduction reaction is to have selectivity for peroxide production vs oxygen reduction to water by modulating the binding strength of intermediates. Herein linker modulated Ni-based metal-organic nanosheets (Ni-MONs) catalysts were investigated as catalysts for peroxide synthesis. The electronic structure of the Ni active sites was tuned by constructing analogous Ni-MONs with electron-withdrawing groups - fluorine (Ni-F-MON) and electron-donating groups such as (hydroxyl, amine) (Ni-OH-MON, Ni-Amine-MON) grafted onto their linkers. The electronic structure of the Ni active sites of the MONs was investigated through a series of electrochemical experiments and X-ray photoelectron spectroscopy measurements. Amongst the Ni-MONs studied, the Ni-MONs with an electron-donating group –hydroxyl and amine have demonstrated higher activity and selectivity. In a gas diffusion electrode set up with Ni-Amine-MON partial current density of 21.7 mAcm^-2 were obtained. Subsequently the system is scaled up to obtain the higher partial current density. Our interpretation is that modifying the Ni electronic structure modulates the binding energy to the *OOH intermediate and is thus the determining factor for H₂O₂ vs. H₂O selectivity in this system and electron-donating groups (hydroxyl and amine) linkers regulated the Ni catalytic activity. This study puts forward design principles towards attaining higher selectivity of electrosynthesis of H₂O₂ through rational modulation of catalytic sites.

Reshaping our energy portfolio considering the sustainability of global energy resources is central to the European Energy Roadmap 2050. Hence, researchers need to identify efficient routes towards solar fuels production. Unlike H₂ evolution, CO₂ photoreduction has been poorly studied. Given the scope for CO₂ utilisation in a carbon-constrained future, there is an exciting opportunity to devote targeted research towards CO₂ photoreduction. Photocatalysis is one route towards CO₂ reduction. Yet, the design of a cost-effective, sustainable, efficient and robust photocatalyst remains a highly challenging task.

To date, the majority of studies on CO₂ reduction photocatalyst focuses on crystalline and non-porous materials, with TiO₂ remaining the 40-year old benchmark in the field. Yet, porous amorphous materials can present interesting photocatalytic features. The high surface area allows one to favour access to catalytic sites, tune the strength of adsorption/desorption of reactants/products and possibly control electronic transfer mechanisms. Scaling up amorphous materials synthesis can be easier than for crystalline materials.

Herein, we will discuss our recent work towards the development of CO₂ photoreduction catalysts that present both a porous and an amorphous character. We will highlight in particular our studies on boron nitride and hypercrosslinked polymers, two types of materials that do not involve any rare-earth metals. For both types, we show that the selective conversion of CO₂ into CO is possible under both UV and visible light. Using analytical and spectroscopic tools, we are starting to understand the mechanisms of reaction of these materials.

Biohybrid systems for solar energy conversion have the potential to combine the strengths of artificial and natural regimes to overcome their respective limitations and deliver efficient electricity and chemical generation. Already, efficient biohybrid approaches exist that couple synthetic ‘light’ reactions to microbial ‘dark’ reactions for chemical synthesis. Could photosynthetic microbial ‘light’ reactions also be coupled to other dark reactions for alternative biohybrid regimes? In this talk, I will show our recent efforts to electrochemically wire into the photosynthetic electron transfer chain in vivo, thereby applying photosynthetic microorganisms as living photoelectrocatalysts. First, we investigate how electron flux from the photosynthetic pathway can be streamlined via exogenous mediators, then we use 3D-printing to design bespoke electrode interfaces that can address the complex needs of the biofilm-electrode interface to optimise photocurrent output. Overall, we will show how this is a more efficient and sustainable means of utilising photosynthetic materials compared to classical bioenergy conversion.

Ammonia is highly used in the fertilizer industry, causing 1% of total GHG emissions. The design of proficient and low-cost catalysts is highly challenging for CO_x-free ammonia production. Alloying non-noble with noble metals is a good approach to obtain elevated catalytic activity. Ni-based materials are known to have a great potential to improve the catalytic activity by tuning the d-band center of Cu and enhance the adsorption energies of intermediate compounds. However, today’s development in understanding this reaction remains inexplicable by the lack of a clear image of how catalyst structure can be altered to enhance its properties. This research provides further information on catalytic properties of Cu/Ni alloys for Nitrate reduction reaction (NO₃^-RR). Using Density Functional Theory (DFT), different compositions of Cu/Ni alloys are modeled. In order to further understand the NO₃^-RR mechanism, the energy path of the reaction along with the active sites, adsorption energies of reactants and intermediate compounds are investigated and compared in pure Cu and Cu/Ni alloys. This research paves the way for further development and design of economical bimetallic catalysts for NO₃^-RR.

Photocatalytic ammonia production is an ideal solution to mitigate carbon dioxide emission in two different ways; firstly, by obsoleting the old Haber-Bosch-based factories and secondly, by storing clean hydrogen fuel in the form of ammonia. However, there is a long way to achieve these goals due to the inertness of nitrogen molecules and rapid recombination of the charge carriers in the photocatalyst nanoparticles. In this work, we have proposed a novel method to modify BiOCl nano-coins by cobalt doping for the photocatalytic nitrogen reduction reaction. The modified BiOCl nano-coins showed a notable photocatalytic NH₃ generation rate in pure water. The strong OER catalytic performance of nano-coins and boosted electron/hole separation efficiency, due to separation of NRR and OER active sites, allowed 4.6 times improvement of ammonia evolution rate. Thus, this study provides a blueprint for the development of new and practical photocatalysts for solar fuel production.

Highly selective electrocatalysis is one of the key challenges impeding sustainable chemical and fuel production. However, the product distribution of CO₂ and N₂ reduction systems is extremely sensitive to the physical and chemical environment at which the reaction takes place. Thus, control of the electrolyte ionic content at the vicinity of the electrodes can enhance their selectivity towards the desired products. In this contribution we show that ion pumps based on a ratcheting mechanism can drive ions selectively. The integration of ratchet based ion pumps (RBIPs) with electrochemical devices may provide a necessary step for product specific electrocatalysis.
Flashing ratchets are devices that utilize temporal modulation of a spatially asymmetric electric field to drive a non-zero time averaged current. Electronic flashing ratchets have been demonstrated experimentally by pumping electrons through organic semiconductors. We have recently demonstrated experimentally first-of-their-kind ratchet-based ion pumps. RBIPs were fabricated by coating the two surfaces of nano-porous alumina wafers with gold forming nano-porous capacitors. The electric field within the nano-pores is modulated by oscillating the capacitors voltage. Thus, when immersed in solution, ions within the pores experience a modulating electric field resulting in ratchet based ion pumping.
An important hallmark of ratchets is the ability to invert the direction of particle flow with a change in the input signal frequency. The current inversion frequency, which is the input signal frequency at which the particle flux changes its direction, is determined by the potential distribution and particles transport properties. As a result, for a given ratchet, there can be a frequency at which particles with the same charge but different diffusion coefficients are transported in opposite directions resulting in particle sorting. This effect was used to sort gold nanoparticles and micron scale colloidal particles according to size and shape. In this talk we will show how this effect can be used to selectively pump ions in different directions according to their diffusion coefficients. Selective ion pumping may allow tuning the electrolyte content at the vicinity of the electrodes performing the electrochemical reaction independent of their potential thus providing another degree of freedom for the electrochemical process.

Reports of photocatalytic nitrogen fixation on various environmental and synthetic photocatalyst have surfaced for decades. Demonstrations have occurred in both the gas and aqueous phase, and with various reactants (oxygen, nitrogen, water, hydrogen). Nearly all experimental observations suggest that photocatalytic nitrogen fixation is possible at ambient temperature and pressure, and that nitrogen is converted to ammonia through a direct reduction process. This is impactful as it promotes the possibility for ammonia based fertilizer production from environmentally abundant materials using only the sun as a source of energy. This is in contrast with state of the art methods for ammonia production, which are energy and carbon intensive. Despite this promise, there is lack of molecular-scale understanding on how this reaction proceeds, with the reaction pathway and active sites still largely misunderstood.

Given the growing interest in this field, it is critical to begin to demystify how this reaction proceeds through conducting fundamental studies aimed at observing molecular level processes. Thus the main goal of this presentation will be to outline what molecular-scale processes which may allow for nitrogen fixation to proceed on titania. We will provide an overview of a new reaction pathway which was observed through the use of advanced spectroscopy (XPS, IR Spectroscopy, EPR), and electroanalytical testing. Furthermore, we will highlight the role catalyst design may play in enabling aerobic nitrogen fixation. Finally, we will end on discussing the economic and systems related challenges which must be overcome to transition toward a real technology.

Electrochemical processes play an essential role in energy conversion reactions inevitable for the mitigation of climate change. To make these processes at the same time efficient, cost-effective and as climate-neutral as possible new catalysts are needed. One of the approaches in catalyst discovery is combining multiple chemical elements. Adjusting the configuration (i.e., which elements) and composition (i.e., the ratio of elements) allows tailoring the final material's properties. There are about 50 chemical elements that are available for the development of novel and sustainable catalysts (i.e. elements, which are abundant, non-radioactive, and non-toxic). If, for example, five of these potentially plausible elements are combined, there are more than 2 million possible combinations, and this is without considering different ratios of the elements. The various combinations lead to diverse electrochemical properties, making the material disparately suitable for a given reaction. Therefore, the challenge is to find a material from the large number of possible combinations that meets the given electrochemical requirements particularly well. This is like looking for a needle in a haystack, and testing many catalysts usually requires serious investment in time and money. Considering the number of possible compositions of new catalysts, which activity cannot be predicted, finding and synthesizing new materials needs to be coupled with high-throughput characterization to accelerate the discovery process. Speeding up the characterization process should not be accomplished by lowering the quality of measurements but by automatization and unsupervised operation.
Electrochemical characterization needs a three-electrode cell under potentiostatic control. In high-throughput processes, this is successfully realized with the use of a scanning droplet cell (SDC). The set-up consists of a positioning system, an electrochemical probe head and an electrolyte-delivery system controlled by the software. The electrolyte is replaced using a pump to the probe head after each measurement providing a fresh solution for reliable measurements. Reference electrode and counter electrode are built into the probe head. The working electrode is created on the sample by the tip, and its surface area is defined by the tip opening size and can be as small as 0.8 mm².
High-throughput screening of materials libraries by means of SDC was successfully used to discover novel complex solid solution electrocatalysts, e.g. (TiNi)-Cu-Hf-Pd-Zr, in an unconventional search space.[1] Introducing unusual elements allows the discovery of catalytic activity for hitherto unknown compositions. Moreover, it was shown that material libraries with very similar composition spreads could show different activity vs composition trends for different reactions. A combination of high-throughput experimentation with theoretical prediction led to identifying the optimal composition of Ag-Ir-Pd-Pt-Ru complex solid solution electrocatalyst.[2] Unsupervised machine learning together with high-throughput experimentation applied to the Rh-Ir-Pd-Pt-Ru system, revealed that electrochemical activity is governed by the complex interplay of chemical and structural factors.[3] Results obtained from high-throughput experimentation are crucial input data for future data-driven materials discoveries and will be shown in this contribution.
1. O.A. Krysiak,^# S. Schumacher,^# A. Savan, W. Schuhmann, A. Ludwig, C. Andronescu, accepted in Nano Research
2. T.A.A. Batchelor,^# T. Löffler,^# B. Xiao,^# O. A. Krysiak, V. Strotkötter, J.K. Pedersen, C.M. Clausen, A. Savan, Y. Li, W. Schuhmann, J. Rossmeisl, A. Ludwig, Angew. Chem. Int. Ed. 2021, 60, 1 – 7
3. L. Banko,^# O.A. Krysiak,^# B. Xiao, T. Löffler, A. Savan, J.K. Pedersen, J. Rossmeisl, W. Schuhmann, A. Ludwig, arXiv:2106.08776

The fast-increasing atmospheric carbon dioxide (CO₂) concentration is a critical challenge for human society. In the quest for developing techniques for carbon neutrality and CO₂ utilization, electrochemical reduction offers a potential route for the conversion of CO₂ into value-added chemicals and fuels is of interest to close the carbon cycle. While active and selective catalysts for CO₂ reduction to the C₁ product have been developed and shown economic output potentials over the past few years, the formation of multi-carbon (C₂₊) products at industry-relevant rates remain challenging.
Among the transition metals, copper-based catalysts are the only candidates able to generate C₂₊ products at significant rates. However, the use of copper is still hampered by poor selectivity towards high-value and high-energy-density multicarbon chemicals with 3 or more carbons (C₃₊). Besides, the most restrictive parameter for CO₂ mass transport resides in its aqueous solubility, which plagues the electrocatalytic performance of the catalysts. From a mechanical point of view, during the CO₂RR, the *CO intermediate has been identified as a key intermediate involved in the C-C coupling to produce multicarbon hydrocarbons and oxygenates. The moderate coverage of *CO on the surface of the catalyst was also found to be beneficial for the CO₂RR into C₃₊ products.
Herein, we report a co-deposition method using CO₂ as a surfactant for the growth of Cu-Ag alloy catalyst on the electrode. The use of CO₂ during the deposition protects the alloy and avoids the aggregation of Ag on the catalyst surface. Our operando Raman measurements revealed that the peculiar alloy structure of the catalyst gets rapidly covered with *CO on the bridge and atop sites, which translates in a record-high selectivity for the 2-propanol (C₃H₈O) of 57.61 % at a partial current density of 59.33 mA cm^-2. We finally report retentions of both the Faradaic efficiency and the current density for C₃₊ of 98.3 % and 97.5 %, respectively for 200 hours.