Symposium EN01—Sustainable Catalysis—Novel Materials for Energy Conversion Beyond Photocatalysis

Electro-reduction of CO₂ in a highly selective and efficient manner is a crucial step toward the utilization of CO₂ but still requires the development of novel fabrication strategies for improving the activity of the selectivity of the catalytic materials. The catalytic properties are largely dictated by the electronic structure because the performance of catalysts follows the Sabatier principle, which predicts that interactions between reactants (and intermediates) and the catalyst surface must be ideally balanced¹. Owing to their reduced dimensionality, two-dimensional (2D) materials have emerged as interesting platforms for studying electrocatalysis².
In this context, nanostructured Ag catalysts have been found to be effective candidates for CO₂ to CO conversion³. However, the ambiguous determination of the CO₂ reduction active sites and the maximization of the density of exposed active sites has greatly limited the use of Ag towards the realization of practical electrocatalytic devices. Herein, we report a superstructure design strategy prepared by the self-assembly of two-dimensional Ag nanoprisms for maximizing the exposure of active edge ribs⁴. The self-assembled Ag nanoprisms allow exposing > 95% of the edge sites which translates into high selectivity and activity towards the production of CO from CO₂. Our first principle calculations combined with electrocatalytic measurements point out the reduced binding energy of COOH* intermediate on the low-coordinated Ag surface atoms at the corner and edges ribs of the nanoprisms. Electrochemical measurements on individuals nanoprisms and the corresponding superstructures allowed us to identify the edge ribs as the active sites with a onsetpotential for the CO2RR reaction of 190 mV and a turnover frequency of 5.2 × 10^-3 ± 2.8 × 10^-3 s^-1 at an overpotential of 0 mV. When tested in an H-cell, the Ag superstructure demonstrates a selectivity over 90% for 100 hours together with a current retention of ≈ 94% at -600 mV vs. RHE.


References:
Sabatier, P. La Catalyse en Chimie Organique. (Paris & Liége Ch. Béranger Editeur, 1920).
Voiry, D.; Shin, H. S.;Loh, K. P.; Chhowalla, M. Nature Reviews Chemistry 2018, 2, 0105.
Liu, S. et al. Chem. Soc. 2017, 139, 6
Qi, K. et al. In Preparation, 2020

Circular economy of CO₂ is nowadays a hot issue for our society. Nevertheless, its successful implementation still requires a lot of new knowledge of reliable systems. Electrochemical CO₂ reduction (ECO₂R) is one of several promising strategies to mitigate CO₂ emissions. Developing novel advanced functional nanostructured catalyst materials and systems for electro conversion of CO₂ to alternative fuels or value-added products in order to meet the global energy needs and so as a means to curb the increasing amounts of CO₂ in the atmosphere is of utmost importance. To date, Copper (Cu) and Copper-based materials are the only heterogeneous catalyst systems that have shown a propensity to produce valuable hydrocarbons and alcohols such as ethylene and ethanol.
Copper nanoparticles were synthesized by a simple wet chemical reduction method at mild temperatures using L-Ascorbic acid, a widely used green reducing agent, and different additives for the morphological control. X-Ray Diffraction studies revealed the presence of Cu₂O particles with varied crystallographic orientation, which can be selective towards ethylene and C₂₊ products, while SEM analysis showed the formation of specific morphologies such as cubes, cuboctahedrons, and truncated octahedron morphologies. The as-synthesized nanoparticles were directly drop cast on 3D porous substrates (e.g., carbon Toray and copper foam). The electrodes were evaluated in conventional H-type and flow filter-press cells with Gas Diffusion electrodes under neutral and alkaline electrolyte conditions. A clear correlation between working voltage, structural electrocatalyst properties, pH, and product distribution was observed, with higher selectivity towards ethylene generally obtained at intermediate potentials. Other aspects such as hindering the Hydrogen Evolution Reaction and evaluating the role of halide species were also considered. The results and both selectivity favoring and limitation factors were discussed and potential solutions were addressed here.

This work is supported by European Union's Horizon 2020 DOC-FAM program under the Marie Sklodowska-Curie Actions Grant Agreement No 754397.

The design and development of active, stable, and selective electrocatalysts for renewable energy conversion reactions is key for the transition towards a decarbonised future. This talk will present some recent strategies aiming to understand and engineer the interfacial structure and properties for oxygen and carbon monoxide/carbon dioxide electrocatalysis.
The first part will be focused on the development of self-supported high surface area nanostructured catalysts for the oxygen reduction and evolution reactions (ORR and OER, respectively). In the second part, I will present our recent work on Cu-based well-defined electrodes in contact with different electrolytes aiming to understand the structure sensitivity for CO₂ and CO reduction. We show how model studies are essential to understand the structure-property relationships and design efficient electrocatalysts for sustainable energy conversion.

The industrialization of the planet has created an increasing demand for energy which unfortunately has been linked to rising levels of carbon dioxide (CO₂) in the atmosphere. Thus, incentives to utilize alternative methods of energy generation is becoming more and more important. While renewable energy technologies are developing rapidly, they still suffer from intermittency. They will therefore be dependent on the development of new energy storage technologies if the transition to a renewable energy economy should be successful.

Electrochemical CO₂ reduction is a promising candidate for energy storage since it can generate valuable chemicals and fuels (such as hydrocarbons) while at the same time close the carbon cycle. The only metal catalyst capable of efficient hydrocarbon production is copper, despite being unselective and known to generate a wide range of products simultaneously. This greatly reduces the efficiency of the desired product, and much effort has been aimed at tuning the selectivity of copper-based catalysts. However, a highly selective Cu based catalyst that is functional over a prolonged period without suffering deactivation is yet to be found.

Copper hydroxide has shown great promise for CO₂ reduction reaction (CO₂RR), and have a greater affinity towards C₂ products such as ethylene at the cost of methane formation [1]. Similar to oxide-derived Cu (OD-Cu), the oxidation state of the surface is believed to play an important role in the enhanced selectivity towards ethylene. However, the existence of sub-surface oxide during reaction conditions is a controversial topic, and factors like increased surface roughness is also suggested as being responsible. Nevertheless, while favorable C-C coupling energetics are observed for Cu(OH)₂ based catalyst compared to polycrystalline Cu, the Faradaic efficiency (FE) for ethylene is still relatively low (38.1%) [2].

In this work we attempt to further enhance the C₂ selectivity of Cu(OH)₂ nanoneedles by depositing a thin Au coating. The Au sites will generate CO, which can be transferred to adjacent Cu sites by either readsorption or surface diffusion to undergo further reduction. The Cu(OH)₂ nanoneedles were synthesized by anodizing Cu foils in 3M KOH at a constant current density (~3.2 mA/cm²) for 10 minutes. Subsequently, the Cu(OH)₂ nanoneedles were decorated with Au using DC magnetron sputtering. The catalyst performance was evaluated in an H-cell and the gaseous products were quantified by gas chromatography in an on-line configuration .


References

1. Iijima, G., et al., Role of a Hydroxide Layer on Cu Electrodes in Electrochemical CO2 Reduction. ACS Catalysis, 2019. 9(7): p. 6305-6319.
2. Lee, S.Y., et al., Mixed Copper States in Anodized Cu Electrocatalyst for Stable and Selective Ethylene Production from CO2 Reduction. Journal of the American Chemical Society, 2018. 140(28): p. 8681-8689.

Organic photovoltaic cells are maturing from the academic research into the industrial development, entering the markets. Pure organic nanostructures and organic/inorganic hybrid nanostructures are comparatively studied for such devices. This talk gives an overview of materials’ aspect and devices.
In order to account for a sustainable future, the application of biodegradable and biocompatible systems for organic optoelectronics are needed. The use of cheap electronic devices in a large scale will introduce a “consumable electronics” into the market of “consumer electronics”. Therefore environmentally friendly materials are important to use. This is a next great challenge to material science in organic electronics. New developments of bio-inspired and/or bio-origin, bio-compatible materials from our institute will be reported. Such materials can also be used to interface the biological and biomedical research with the organic electronics field.
Last but not least the conversion of CO₂ to methane (or other synthetic fuels) using solar energy is an important step to make an efficient, large scale energy storage. At the same time this will make a cyclic and sustainable CO₂ economy. We report organic as well as bio-organic catalysts which can be used in photo-electro-catalytic conversion devices. Such bio-catalysts can be enzymes as well as living bacteria immobilized on electrodes. Selectivity of such bio-catalysts is very high and combined with the room temperature operation of such bio-electro-catalytic systems makes them industrially highly attractive.

Spin based properties, applications, and devices are commonly related to magnetic effects and to para or ferro magnetic materials. However, we found that chiral organic molecules act as spin filters for photoelectrons transmission, in electron transfer, in electron transport. The effect, termed Chiral Induced Spin Selectivity (CISS), [[1],[2]] lead to the discovery that when chiral molecules are charge polarized, there is a transient spin polarization [[3]]. At each electrical pole, the (partially) unpaired electron is spin polarized with opposite polarization at the positive and negative poles. Which spin is associate with which pole depends on the handedness of the molecule. This finding sheds new light on enantio-specific interactions and it allows to construct novel methods for enantio-separation.[[4],[5]] It also opens new ways to induce spin polarization in semiconductors and to obtain temperature activated ferromagnetism in chiral matallo-organic crystals [[6]].



[[1]] R. Naaman, Y.Paltiel, David Waldeck, Nature Reviews Chemistry 3, 250 (2019).

[[2]] R. Naaman, D. H. Waldeck Ann. Rev. Phys. Chem. 66, 263 (2015).

[[3]] A. Kumar, E. Capua, M. K. Kesharwani, J. M. L. Martin, E. Sitbon, D. H. Waldeck, R. Naaman, PNAS, 114, 2474–2478 (2017).

[[4]] K. Banerjee-Ghosh, O. Ben Dor, F. Tassinari, E. Capua, S. Yochelis, A. Capua, S.-H. Yang, S. S. P. Parkin, S. Sarkar, L. Kronik, L. T. Baczewski, R. Naaman, Y. Paltiel, Science 360, 1331 (2018).

[[5]] F. Tassinari, J. Steidel, S. Paltiel, C. Fontanesi, M. Lahav, Y. Paltiel, R. Naaman, Chemical Science, 10, 5246–5250 (2019).

[[6]] A. K. Mondal et al. ACS Nano (2020) in press. DOI: 10.1021/acsnano.0c07569.

Electrolytic hydrogen production faces technological challenges to improve efficiency, economic value and rapid scale up. In conventional water electrolysis, the water oxidation and reduction reactions are coupled in both time and space, as they occur concurrently at an anode and a cathode in the same cell. This introduces challenges such as product separation, and sets strict constraints on material selection and process conditions. Another major challenge is to improve efficiency, which is limited by the large (> 400 mV) overpotential loss of the four-electron oxygen evolution reaction (OER).
The Electrochemical – Thermally-Activated Chemical (E-TAC) water splitting cycle decouples these reactions by dividing the process into two stages; an electrochemical (E) stage that reduces water at the cathode and charges (oxidizes) a nickel hydroxide (Ni(OH)₂) anode to nickel oxyhydroxide (NiOOH), followed by a chemical (TAC) stage that reduces the charged anode (NiOOH) spontaneously (without applied bias) back to its initial state (Ni(OH)₂) by oxidizing water. This chemical reaction is accelerated at elevated temperatures (60-100°C), providing a handle to control the evolution of oxygen in the cell so as to avoid mixing with hydrogen. The E-TAC cycle enables overall alkaline water splitting at an average cell voltage of ~1.5 V in a membraneless cell architecture that offers potential for cost reduction by eliminating membranes and sealing components, and supports high-pressure hydrogen production. High electrolytic efficiency of 98.7%_HHV is achieved by dividing the four-electron OER into four one-electron reactions wherein four Ni(II) sites are charged (oxidized) to Ni(III). The operational challenges that arise from swinging between the E and TAC stages and the material challenges that arise from the finite capacity of the nickel (oxy)hydroxide anode will be discussed in the talk.

A combination of the increased awareness about nitrogen-oxyanions contamination in waters and the value of certain nitrogen-based products as key commodities, potential fuel or chemical precursors, have opened a novel approach line within the circular economy, framed in the development of both: novel materials and optimal processes for nitrate electroreduction to ammonia as main reaction product. The vast literature in the field of water denitrification points to the most promising materials for nitrate electroreduction. Pure transition and noble metals, in combination with organic and inorganic substrates have been tested in this process with different outcomes, among which the most remarkable are the catalytic activities of copper, gold, silver and iron. However, most of the materials that showed excellent properties for one step of the complete reduction of nitrate, did not show good results in terms of faradaic efficiencies towards most of the nitrogen-based valuable products. In this work, ammonia is presented as main product given its essential role in modern agriculture and as a potential energy carrier. In this context, in a sustainable process, not only the catalytic activity defines the ideal material, but its combination with the faradaic efficiency and stability of the electrocatalyst. Having copper as the most active material for nitrate electroreduction, it is important to combine it with a substrate that provides stability, resistance to operative conditions, and selectivity to ammonia. The reasons stated above have conducted the research to define materials for nitrate electroreduction as copper nanoparticles supported by carbon-based and titanium structures. Copper nanoparticles obtained by electroless reduction and deposited on 3D carbon and metallic substrates, combined with the optimal electrochemical conditions of working potential, nitrate concentration and ammonia recovery system are presented in this work to open a potential alternative route to the traditional high energy consuming Haber-Bosch process for ammonia production.

Catalysts undergo poisoning and degradation during their utilization in many different fields. Common examples are the CO poisoning of Pt, or the degradation of Pt/C catalysts during start-stop procedures, both occurring in polymer electrolyte fuel cells (PEMFCs). Another energy technology that features prominent catalyst degradation is the hydrogen bromine redox flow battery (H₂-Br₂ RFB). H₂-Br₂ RFBs are a cheap and efficient solution to large scale energy storage. The main hinderance of the technology is the poisoning of the hydrogen evolution reaction/hydrogen oxidation reaction (HER/HOR) catalyst by bromine species which have crossed over the proton exchange membrane. Without a revolution in membrane development, this crossover appears unpreventable. Therefore, we sought to protect the catalyst locally. Two possible solutions were found to impart catalyst active site selectivity: Pt encapsulation in single walled carbon nanotubes (SWCNTs) and metal oxide deposition on catalysts via atomic layer deposition (ALD).
Platinum nanoparticles were synthesized within the internal cavities of small diameter SWCNTs, through a simple impregnation and drying procedure. High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and scanning tunneling electron microscopy (STEM) were used to characterize the particles and demonstrate that they were confined within the SWCNTs. The typical hydrogen under potential deposition peaks were observed on a cyclic voltammogram of the sample, whereas the oxide region was heavily suppressed compared to Pt/C. Some diffusion limitation was observed during the HOR, indicating that the electrolyte diffusion pathway is through the SWCNTs, but the same mass transport limited current was attained. The oxygen reduction reaction (ORR) mass transport limited current was much lower than expected for Pt (2 mA cm^-2), indicating a transport selectivity for hydrogen over oxygen. The stability of these platinum nanoparticles in the presence of bromide/tribromide solution was vastly increased compared to the standard 50% Pt/C catalyst, shown by x-ray photoelectron spectroscopy (XPS) and electrochemistry. It is proposed that this effect is caused by steric and electrostatic repulsion of the large tribromide ion by the SWCNT cavity (internal diameter of 2 nm). The encapsulated platinum also features a vastly higher mass activity when cycled in a cell, indicating better Pt utilization due to the small particle size. This opens a new route for imparting selective access to active sites of a catalyst, hence increasing the stability of the catalyst, a potential solution to many problems faced by technologies that rely on catalysts.
Another possible solution to prevent catalyst poisoning is through a protective oxide coating on its surface. ALD was chosen due to its highly controllable, conformal deposition, but also as it can be applied to a wide range of commercial catalysts, making it highly applicable for real world applications. Different thicknesses of vanadium oxide were deposited on a commercial 50% Pt/C catalyst. XPS and HRTEM confirmed the ALD process had deposited vanadium oxide species on the catalyst. The HOR was unaffected by this deposition, indicating diffusion of hydrogen could occur through the oxide layer. Stability of the material in the presence of bromide/tribromide solution was superior to the uncoated commercial catalyst, showing the oxide coating successfully protected the catalyst. Again, whilst it has been demonstrated for bromine poisoning of Pt, this approach should be applicable to many poisoning problems facing other catalysts/applications.

We present the development of electron-transporting polymeric organic semiconductors as a new class of metal-free electrocatalysts for the oxygen reduction reaction in aqueous electrolytes. The polymeric organic semiconductors are based on conjugated polymers with large electron affinities (equivalent to materials with low lying lowest unoccupied molecular orbital (LUMO)), where polar side chains are attached to the backbone to process materials from solution and to improve the ionic charge transport properties.^[1] This design concept results in fast charging polymer electrodes where volumetric charging of thick electrodes (> 1 μm) is achieved due to the balanced ionic and electronic charge transport properties of the polymer. The outstanding mixed ionic/electronic transport properties also enable the utilization of single-phase electrodes where no additives or binders are needed for the electrode to function in aqueous electrolytes.
We will further explain the working principle of the polymeric electrocatalyst, for which the polymeric organic semiconductor is first activated by an electrochemical doping reaction (reduction, n-type doping) that increases the reactivity of the material towards molecular oxygen. By employing in-situ spectroelectrochemical measurements and rotating ring disk electrode (RRDE) measurements, we find that the polymer achieves its highest performance when charged to the polaronic, singly charged, state. The polymers predominantly yield hydrogen peroxide through the 2-electron reduction of oxygen. The selectivity towards peroxide and water (4-electron product) is influenced by electrolyte pH. We hypothesize that chemically tuning the polymer's energy levels and side chains will pave the way for the successful development of low-cost, metal-free, and solution-processable electrocatalysts for energy conversion technologies.
[1] A. Giovannitti, C. B. Nielsen, D.-T. Sbircea, S. Inal, M. Donahue, M. R. Niazi, D. A. Hanifi, A. Amassian, G. G. Malliaras, J. Rivnay, I. McCulloch, Nat. Commun. 2016, 7, 13066.
[2] D. Moia, A. Giovannitti, A. A. Szumska, I. P. Maria, E. Rezasoltani, M. Sachs, M. Schnurr, P. R. F. Barnes, I. McCulloch, J. Nelson, Energy Environ. Sci. 2019, 12, 1349.

The most active and efficient catalysts for the electrochemical hydrogen evolution reaction rely on noble metals, a fact that increases the cost of producing hydrogen and thereby limits the widespread adoption of this fuel. Here we present metal-free polydopamine and polyguanine as selective organic hydrogen electrocatalysts^1–3. The conducting functional polymers incorporate selective hydrogen-affine hydrogen bonds that possess a similar hydrogen binding energies and work function as e.g. platinum or palladium. We report the synthesis of hydrogen-selective electrocatalytic polyguanine and polydopamine and demonstrate the enhancement of the rate-determining step in the proton reduction. We further present mechanistic spectral IR-operando studies on the catalytic hydrogen bonded motifs and showcase the surface tunability between hydrogen evolution and hydrogen electrosorption including steps towards scaling the material for continuous electrolysis for several 100 hours/cycles without notable degradation.
(1) Coskun, H.; Aljabour, A.; Schoefberger, W.; Hinterreiter, A.; Stifter, D.; Sariciftci, N. S.; Stadler, P. Cofunction of Protons as Dopant and Reactant Activate the Electrocatalytic Hydrogen Evolution in Emeraldine-Polyguanine. Adv. Mater. Interfaces 2020, 7 (2), 1901364 DOI: 10.1002/admi.201901364.
(2) Coskun, H.; Aljabour, A.; Luna, P.; Sun, H.; Nishiumi, N.; Yoshida, T.; Koller, G.; Ramsey, M. G.; Greunz, T.; Stifter, D.; Strobel, M.; Hild, S.; Hassel, A. W.; Sariciftci, N. S.; Sargent, E. H.; Stadler, P. Metal-Free Hydrogen-Bonded Polymers Mimic Noble Metal Electrocatalysts. Adv. Mater. 2020, 32 (25), 1902177 DOI: 10.1002/adma.201902177.
(3) Coskun, H.; Aljabour, A.; Greunz, T.; Kehrer, M.; Stifter, D.; Stadler, P. Electrochemical Hydrogen Storage in Amine-Activated Polydopamine. Adv. Sustain. Syst. 2020 DOI: 10.1002/adsu.202000176.

Carbon materials have been awakening scientific interest for research because it allows chemical functionalization for multiple applications in the sciences, especially in energy applications. Carbon Nano-onions (CNO) are spherical structures composed of multilayers of fullerenes, these layers are connected in a way that shows the shape of an onion. Its development begins with the use of nano-diamonds, a carbon material of strong structure which it forms in a very violent environment. The nano-diamonds are taken to a furnace at a temperature of 1650°C to finally obtain the CNO. Janus particles are receiving increasing attention because of their dual properties, where each side can be functionalized to have distinctive characteristics. The modifications on the surface of these nanoparticles can provide different chemical and physical properties. The interesting properties of Janus nanoparticles are that they have different sizes and shapes which have now been able to be studied in more detail. The purpose of this project is to use asymmetrically modified CNO as a support for metal nanoparticles to avoid agglomeration and, thus, increase their surface area and efficiency. Janus nanoparticles will be designed by a Wax-paraffin Pickering Emulsion process using CNO on its surface. The deposition of Platinum (Pt) was carried out by a chemical process using sodium borohydride. The removal process of the paraffin involves the dispersion of the wax-paraffin/CNO-Pt particles in Chloroform, repeating the process by six times and rinse with isopropanol to finally obtain the amphiphilic nanoparticle. The differences on the surface of the particles before and after removing the paraffin were observed by Scanning Electron Microscopy (SEM). The Energy-Dispersive Spectroscopy was used to validate the elemental information of the particles and assure the deposition of 20% of Pt on the surface of the particles. Transmission Electron Microscopy (TEM) provided us with information on the dispersion of Pt on the surface of CNO. Through X-ray diffraction (XRD) and Raman Spectroscopy, we were able to confirm the paraffin removal and presence of Pt in these particles. Cyclic Voltammetry was used to characterize the proposed catalyst and comparison with the commercial catalyst Pt/Vulcan. The Pt-CNO/CNO catalyst was tested for its performance for the Oxygen Reduction Reaction (ORR) using the Rotating Ring Disk Electrode (RRDE). Polarization curves of ORR were obtained with RRDE rotations at 0,100,400,900,1600 and 2500rpm in a O₂-saturated 0.1M KOH solution.

In this talk, I will highlight some recent advances in our understanding of the catalytic reaction mechanism of the direct electrochemical reduction of CO₂ and of related mixed feeds into value-added fuels and chemicals on smooth polycrystalline metallic surfaces, on non-metallic, single metal-site electrocatalysts, and on metallic/non-metallic tandem catalyst schemes. Methods used include in situ X-ray analytical techniques and time-resolved Differential Electrochemical Mass Spectrometry (DEMS) conducted in novel capillary flow cells. The DEMS flow cells enable milli-second resolved analysis of reaction products under stationary and transient conditions, providing access to accurate onset potentials of a wide variety of reaction products.

Combinatorial materials science (CMS) is a highly promising method for fast discovery of new functional materials, such as low T_c superconductors, shape-memory alloys, and photoabsorbers. CMS has been used to form thin films with composition and thickness gradients, consequently, synthesizing, on a single substrate, a range of samples with systematically varying properties, which is the first step in finding new materials and device structures. Apart from the composition or thickness, film morphology and nanostructuring can be especially important for an assortment of applications ranging from catalysis and photovoltaics to magnetic materials, as morphology governs the chemical reactivity, determines the surface area, and is important for charge mobility and recombination processes. However, heretofore CMS research did not encompass film morphology as a study parameter.
Here we describe how we vary nano-scale morphology and material composition at the same time using an adapted shadow growth method based on glancing angle deposition (GLAD), which eliminates the commonly used wet chemical steps for nanostructure synthesis. In a one-step well-controlled growth we quickly obtain a large number of nano-columnar structures, including nanorods, nanohelices, and nano-zigzags, with varying material compositions. Adapting GLAD and introducing it into CMS, with accompanying high-throughput characterization, constitutes an integrated approach for discovering new materials and structures for a multitude of applications in many scientific fields.
We use this method to fabricate a multi-component nanocomposite electrolyzer and study its compositional and structural variations. The system is a multinary elemental metal-based library, where each material has an impact on the resulting nanostructure as well and the chemical composition and state. After investigating the physical and chemical properties of the library, it is then examined as an electrocatalyst for oxygen evolution reaction (OER). The OER activity shows a dependence on the nanostructuring of the library as well as on the chemical and compositional variation. By using CMS and high-throughput analysis, we are able to gain insights that the standard experimental techniques would not be able to achieve, thus indicating the importance and impact CMS has in the field of electrolyzers for the future.

Electrocatalysis of hydrazine oxidation, a promising non-carbon fuel, is both a practical goal, and a scientific paradox. Hydrazine is a famous reductant in chemical synthesis, yet the onset potential for its electro-oxidation reaction is highly dependent on the catalytic surface. Some excellent catalysts have been developed in recent years towards direct hydrazine fuel cells. Most of these catalysts, however, are either prohibitively expensive (e.g. Pt-based ones), or easily deactivated (e.g Ni-based ones).

We have recently discovered a family of multi-doped carbons with record-breaking electrocatalytic activity towards hydrazine oxidation in alkaline pH.^1,2 They contain many components, all of which postulated to be possible candidates for hydrazine oxidation active sites: from Mo-doped Fe₃C nanoparticles, to the N-doped, graphitic, hierarchically porous carbon. They provide the first example of hydrazine oxidation on carbides. Moreover, they are stable, efficient, and easy to make on a large scale.

In our quest to understand the source of activity in these fascinating materials, we launched a systematic study into each of the component, and into their combination. On the way, we discovered how Mo-doping is actually unnecessary, and how Zn and Cu direct the nanostructure by different mechanisms.² Furthermore, we demonstrated that the carbon matrix itself is highly active toward the reaction.³ At this point, we raised even larger questions: are the carbide nanoparticles even participate in the reaction? What does it really take for a carbon material to electro-oxidize hydrazine? To answer these questions, we combined electrochemical measurements, broad scope material characterization, and mechanistic quantum-mechanical calculations.⁴

In this talk I will present unpublished and recently published results, clearing the field of hydrazine oxidation on doped carbons, and explaining the individual catalytic, cooperative and structural roles of each component. While much remains to be understood about the activity and selectivity of these elusively simple material, and their excellent performance makes this challenge both technologically and scientifically appealing.

(1) Ojha, K.; Farber, E. M.; Burshtein, T. Y.; Eisenberg, D. Angew. Chem. Int. Ed. 2018, 57, 17168.(2) Burshtein, T. Y.; Farber, E. M.; Ojha, K.; Eisenberg, D. J. Mater. Chem. A 2019, 7, 23854.(3) Farber, E. M.; Ojha, K.; Burshtein, T.; Eisenberg, D., J. Electrochem. Soc. 2020, 157, 064517.(4) Burshtein, T.; Tamakuwala, K.; Sananis, M.; Ioffe, K.; Hirsch, S.; Farber, E. M.; Grinberg, I.; Eisenberg, D., under preparation.

To pull through the emerging energy crisis, cost-effective, stable, and highly active electrocatalysts are required. Hydrogen evolution reaction (HER) is one of the electrochemical processes which allows direct conversion of electrical energy to chemical energy. However, the efficiency of this conversion process is limited by the overpotential required to reach a high enough current density. Up to date, transition metals such as platinum having the narrow d-orbital have been treated as a good electrocatalyst. They enable the activation energy to be reduced by lowering the energy of the transition state via strong interaction between the adsorbed hydrogen atoms and the energy states of the catalyst. However, the high cost as well as low abundance in the Earth has been limiting their widespread use in commercial electrocatalysis. Herein, we show that the turn on voltage of the electrochemical reaction can be adjusted in a semiconductor-insulator-plasma etched graphene (SIEG) device. O₂ plasma etched graphene was introduced as a top electrode to increase the number of electrochemical active sites. This breaks the limitation of the catalytic property of pristine graphene limited by its large hydrogen adsorption energy and lack of electrochemically active sites. The Oxide-semiconductor layer plays a role to tune the hot electron population by biasing the graphene-semiconductor junction. The shift of the onset potential of HER can be achieved up to ~0.8V while reaching a current density of 90 mA/cm² at an overpotential of -0.5V vs RHE. Importantly, this occurs without the assist of any noble metal catalyst and uses only abundant elements such as silicon, aluminum, and carbon. This potentially introduces a new pathway in which electrocatalysts could be engineered through control over the number of active sites and electron distribution.

Electrocatalytic water splitting has received a great deal of attention as an attractive way of storing energy in the form of pure hydrogen. RuO₂ and IrO₂ catalysts are archetypical materials used for the anodic oxygen evolution reaction (OER) in acidic media, but even for the most stable oxides the long-term stability represents a critical issue. Theoretical analysis based on the use of Pourbaix diagrams have emerged as a powerful tool to identify acid-stable materials for electrocatalysis. Despite the great utility of these diagrams, they do not capture the whole complexity of reaction conditions that may affect materials stability such as highly oxidizing non-equilibrium conditions of the OER. For example, it has been recently demonstrated by both simulations and experiments for a number of complex oxides including rutile IrO₂ that stability of a catalyst can suffer from the active participation of lattice oxygen in the OER. In this work we employ density functional theory (DFT) calculations to investigate the thermodynamics of the OER through both the conventional and the lattice oxygen evolution reaction mechanisms across a series of iridium-oxide based catalysts. We show that lattice oxygen participation should be attainable for a number of iridium-oxide phases that are predicted to be thermodynamically stable based on the Pourbaix diagrams. These results suggest that lattice oxygen evolution reaction should be taken into account when analyzing electrode stability under electrochemical conditions. The obtained theoretical results will be discussed in the context of available experimental data.

Single-atom catalyst (SAC) is a key player in catalysis these days owning a tiny amount of metal atoms could increase its intrinsic activity towards efficient catalyst in numerous reactions. However, it is the remaining challenge and questionable to control its stability, benchmark performance, superior energy consumption during the complex synthesis process, and above the average temperature that encourages single metal atoms to become agglomerate. Herein we report a new approach based on laser ablation techniques to make laser irradiated doped holey graphene support (LGO), and subsequently make it hetero-doped laser irradiated macroporous graphene (NLG). Used the fundamental dangling bonds concept of nanocarbons to trap the metal atoms from Iron and Cobalt foams. We applied the optimized process conditions so that M⁰ could transfer electrons to NLG via dangling oxygen groups to become M^σ+. These dangling surface bonds of oxygen synchronizes with M^σ+ to make metal-oxide (M-O) bonds. Dry the material at ambient conditions to make stronger dangling bonds between M-O, and then sonicate it afterwards to take away metal atoms from bulk counterparts to metal single atoms (MSA) anchored on the NLG via dangling bonds associated with oxygen groups. This new synthesis approach for making SAC is easy, economical, and sustainable as the metal foam is working here like photocopying machine, and we can use the same foam for a long time as a metal precursor. Furthermore, we demonstrated this idea on the synthesis of two catalysts, namely Co-SAC@NLG towards hydrogen evolution reaction (HER) and Fe-SAC@NLG for oxygen reduction reaction (ORR) electrocatalysts. The morphology was experimentally verified by different tools, including aberration-corrected scanning transmission electron microscopy (AC-STEM). At the same time, bonding states, and surrounding environment of MSA are dually confirmed by X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). For mechanism understanding, first-principle calculation density functional theory (DFT) was applied to justify its working mechanism in detail based on thermodynamics using grand canonical potential kinetics (GCP-K) quantum mechanics to obtain for various overpotentials to get minimum free energy using Legendre transformation (LT) to relate applied voltage and net charge of the structure. Apart from the demonstration of a new synthesis approach, the detailed experimental insights, and understanding the theoretical mechanism from this study will expand the existing knowledge in the field of heterogeneous single-atom catalyst towards different reactions.

Human-induced climate change, driven by rising levels of carbon dioxide in the atmosphere, is one of the greatest challenges humanity has ever faced. Electrolysis holds the potential to be a valuable option for carbon-free energy storage especially when coupled with renewable electricity sources. Hydrogen produced by electrolysis can be used to generate electricity in a fuel cell or as a feedstock for vital chemicals. Current electrolysis technology lacks the necessary efficiency to be economically viable at an industrial scale due to the lack of adequate catalysts for the Oxygen Evolution Reaction (OER), the more complex of the necessary half-reactions, in acidic environments.
The best catalysts for the OER in acid are iridium- and ruthenium-based. By pushing these compounds to higher performance, we can identify which material properties are responsible for high OER activity. From recent literature on cobalt-based OER catalysts in alkaline conditions, increased covalency of the metal-oxygen bond results in involvement of lattice oxygen in the reaction and decreased formation energies for intermediate oxygen species, both resulting in higher activity. We examine whether similar effects of metal-oxygen bond covalency occur in the analogous system of iridium-based catalysts in acidic environments, recognizing that Ir and Co are both group 9 transition metals. We build upon this test by tuning the O 2p band through manipulation of the anion sub-lattice of the exceptionally active perovskite catalyst, orthorhombic SrIrO3 (SIO). Precise control and variation of the concentration of oxygen vacancies allows us to perturb the electronic structure, compare the results with a concurrent DFT study, and measure the resulting changes in catalytic activity of SIO. This study first requires a reliable deposition method, which has been achieved by investigating factors that control the growth, crystallinity, and composition of the films.
We developed a consistent method for depositing atomically smooth thin films of SIO via pulsed laser deposition. The films have been extensively characterized using x-ray diffraction, spectroscopy methods, and multiple forms of microscopy, revealing several trends that highlight key deposition parameters. For example, the x-ray photoelectron spectroscopy of the Ir4f peaks signifies an unexpected but critical dependence of the film’s iridium content on the position of the substrate within the vapor plume during deposition. This phenomenon is likely due to the relative masses of the vaporized species. This revelation, along with other trends, led to the consistent deposition protocol that provided a platform to vary oxygen content in SIO films.
Deliberate formation of oxygen vacancies typically includes doping with foreign cations, which may result in unintended changes to electronic structure and catalytic activity, overshadowing the effect of the oxygen vacancies. Thus, we investigate alternative routes to control oxygen content without introduction of foreign species: 1) varying the oxygen partial pressure during deposition, and 2) annealing films in mixtures of O2 and N2 post-deposition. Characterizing the oxygen-deficient films, such as with x-ray absorption spectroscopy, in conjunction with electrochemical OER testing allows us to investigate the electronic structure changes and their effects on catalytic activity. For example, a clear relationship has been identified correlating decreased oxygen content with increased Ir4f binding energy, making plain the influence of anion stoichiometry on electronic structure. Exploring these relationships enhances our understanding of the drivers of OER catalytic activity and helps in the development of better catalysts. Improvement of OER technology will enable electrolysis to become a financially viable option for energy storage and sustainable chemical production, aiding in our fight against climate change.

We report on electrochemical analysis and stress measurements of hydrogen and CO₂ reactions on Au and Cu surfaces in aqueous solutions with different pHs. Hydrogen and CO₂ reactions are important for the production of chemical fuels using water electrolysis and reduction of CO₂ to CO or small hydrocarbons. However, room temperature electrocatalysis of H₂ production and CO₂ reduction on earth-abundant surfaces remain challenging. Hydrogen reactions (2H⁺ + 2e^– ↔ H₂) are efficient on Pt and Pd surfaces but are not as feasible on many earth-abundant transitions metal surfaces particularly at higher pHs. CO₂ reduction is slow on even most active surfaces and requires transfer of multiple electrons and protons (CO₂ + 2H⁺ + 2e⁻ → CO + H₂O). Interestingly, surfaces with high activity toward CO₂ (e.g. Au and Cu) are not active toward hydrogen reactions. Here, we explore the reaction mechanism and early stages of catalysts activation during hydrogen and CO₂ reactions on these surfaces using electrochemical analysis and in-situ stress measurements.
Well-defined Au and Cu surfaces are prepared using evaporation and atomic layer electrochemical deposition. Electrochemical analysis shows changes in surface reactivity toward HER and CO₂ reduction as the surface composition and structure is altered. Our stress analysis shows that hydrogen activation on surfaces causes a compressive stress, that can be used as a signature for activity toward hydrogen reactions. Interestingly, in-situ stress measurements of Au surfaces do not show proton activation even at low pHs. However, our analysis reveals stress responses corresponding to CO₂ interaction with Au surface at low pHs. The CO₂ stress responses are evident at ca. 0.2-0 V vs. RHE. The stress response is different from what is observed for hydrogen activation on surfaces. We will particularly discuss how these signature features of hydrogen and CO₂ reactions changes as a function of solution pH. We also use rate dependence of stress response to identify surface charge effects of primary activation steps. Our studies show that at higher pHs water is involved in the hydrogen reactions even on Pt surfaces. We will discuss how CO₂ activation changes surface charge dynamics during activation steps. These studies will provide insight into activity and selectivity descriptors for hydrogen production and CO₂ reduction reactions in aqueous media.

The increasing global population and advancement in energy-dependent devices have caused increased energy use in consumer and industrial appliances, electronic devices, and automobiles, creating an urgent need for clean and renewable energy sources. Electrochemical water-splitting is one of the greenest ways to generate clean and high-performance fuel. Water-splitting produces hydrogen and oxygen gases. The generated hydrogen gas can be used as fuel, whereas evolved oxygen gas can be used in metal-air batteries or released in the atmosphere as a clean gas. The electrocatalytic properties of most of the materials for water splitting depend upon several factors such as morphology, phase purity, defects, etc. Additionally, electrochemical energy storage devices garner considerable research interest because of their high storage energy and long lifecycle; supercapacitor's global market reached $2 billion in 2015. This project has synthesized metal-organic framework (MOF) derived cobalt oxide and cobalt sulfide using a facile method. 2-methyl imidazole and cobalt nitrate hexahydrate were used for the synthesis of MOF-derived cobalt oxide and MOF-derived cobalt sulfide electrodes. The electrode with MOF-derived cobalt oxide went through a solvothermal process while the electrode with MOF-derived cobalt sulfide was sulfurized hydrothermally. The structural and electrochemical properties of these electrodes were studied in detail. The MOF-derived cobalt oxide and sulfide's electrocatalytic activities were studied in 1M KOH solution for oxygen evolution reaction and 3M KOH solution for capacitive behaviors and storage capabilities. MOF-derived cobalt oxide showed an overpotential of ~375 mV to achieve a current density of 10 mA/cm². A significant improvement in electrocatalytic properties was observed with the electrode that went through sulfurization. MOF-derived cobalt sulfide displayed an overpotential of ~278 mV at 10 mA/cm². The specific capacitance obtained by the cobalt sulfide-based electrode (MOF-derived) was ~2537 F/g at 1A/g while the cobalt oxide-based electrode (MOF-derived) was ~484 F/g at 1 A/g. Our results suggest that a facile method of sulfurization of the MOF-derived compound is a way to achieve high electrocatalytic activities for oxygen evolution reaction in the water-splitting process and increased capacitive capabilities.

Copper is an attractive electrocatalyst for CO₂ conversion to valuable carbonaceous products. However, the poor selectivity remains as a challenge to achieve high energetic efficiency. In this work, using model Cu (001) surfaces, we clarify how tensile strain influences CO₂ reduction reaction pathway, shifting products selectivity away from single-carbon to value-added, multi-carbon products.
We establish varying built-in tensile strain on epitaxially grown Cu (001) surfaces on single-crystal Si substrate by changing film thickness. With decreasing film thickness, we observe increasing in-plane tensile strain at the surface that shifts the Cu d-band center toward the Fermi level, in good agreement with d-band theory. In CO₂ electrolysis at moderate overpotential, we find a suppression of single-carbon products with this upshifting d-band center. The change in selectivity indicates a change in adsorption energy for reaction intermediates and perhaps the promotion of hydrogenation of *CO-to-*CHO, one of the key descriptors for CO₂ conversion to multi-carbon products. These findings provide direct experimental evidence that strain can tune the CO₂ conversion reaction pathway to produce energy dense products, providing new opportunities to design efficient and selective catalysts even without changing catalyst composition.

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 hydrogen (H₂) 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. 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 (3660 moles per kilogram of catalyst per hour). A wide range of pure H₂O₂ concentrations up to 20 wt.% could be obtained by tuning the water flow through the solid electrolyte, and the catalyst retained activity and selectivity for 100 hours. The as-generated H₂O₂ solutions were also demonstrated to reduce the Total Organic Carbon (TOC) level of local rainwater to that of drinking water.

We present the use of ab-initio calculations to calculate the coverage-dependent catalytic activity of alloy nanoparticles with realistic sizes (5 nm-10 nm) by explicitly predicting atomic-scale structures and adsorbate binding energies. We demonstrate our approach using Pt–Ni nanoparticles as catalysts for the oxygen reduction reaction (ORR). We achieve our results by constructing a quaternary Pt-Ni-OH-Vacancy cluster expansion model to explicitly predict OH adsorption energies on nanoparticles of varying shape, size, and atomic structure. The OH coverage-dependent ORR activity is calculated through a kinetic Monte Carlo (KMC) simulation. This model enables us to accurately investigate the catalytic activity of various surface sites with different coordination numbers and local atomic environments. Using this model, we evaluate how different parameters affect the ORR activity of Pt-Ni nanoparticles, including size (2 nm-10 nm), Pt composition (60%-100%), and shape. Through the use of KMC-enabled kinetic simulations of structural evolution we evaluate how the activities of the particles change due to Ni dissolution. Our approach identifies OH coverage at the atomic scale and provides theoretical insights into how to tune the structures of alloy nanoparticles to optimize catalytic activity.

Many catalysts for energy related applications, in particular metallic nanoalloys, readily undergo atomic-level changes during electrochemical reactions. The origin, dynamics and implications of the changes for the performance of the catalysts inside operating devices though are not well understood. This is largely because they are studied on model nanocatalysts under controlled laboratory conditions. We will present results from combined x-ray specroscopy and total scattering studies on the dynamic behavior of Pt/Pd-3d transition metal nanoalloys inside an operating proton exchange membrane fuel cell [1]. The results indicate that the catalysts change profoundly under the erosive conditions inside the cell, including leaching of transition metal species and continuous re-alloying leading to the emergence of structure states with an improved activity and stability.

1. V. Petkov et al. Nanoscale 11 (2019) 5512.

The sluggish kinetics of hydrogen oxidation reaction (HOR) at the anode in alkaline electrolytes are one of the biggest hurdles in the development of next generation non-Pt catalyst for anion Exchange Membrane Fuel Cells (AEMFCs) having an efficient HOR catalyst^1–3. Lately, Pd has been extensively used because of the need of developing HOR electrocatalysts based on more abundant and cheaper elements than Pt. Additionally, in order to increase the HOR kinetics of Pd, previous studies focused their efforts on the development of Pd-CeO₂ composites because CeO₂ is an oxygen-deficient compound that allows for a fast OH− saturation. Lately, Pd has been extensively used because of the need of developing HOR electrocatalysts based on more abundant and cheaper elements than Pt. Additionally, to increase the HOR kinetics of Pd, previous studies focused their efforts on the development of Pd-CeO₂ composites because CeO₂ is an oxygen-deficient compound that allows for a fast OH− saturation. Controlled Surface Reactions (CSR) process has been used to selectively deposit different atomic ratios (0, 0.24, 0.38 and 0.59 at. ratio ICP-AES Measured bulk Ce/Pd) between CeO_x onto carbon supported Pd catalysts nanoparticles with the main goal of improving the efficiency of HOR catalysts expecting a homogenous distribution of CeO_x nano-islands preferentially attached to Pd nanoparticles (NPs) in order to achieve highly active CeO_x-Pd/C catalysts for HOR⁴. In recent years, when it comes to study the relationship between morphology and structure of nanoparticles that constitute the fuel cells catalyst and the correlation to their electrocatalytic activity, Transmission Electron Microscopy (TEM) has become the state-of the art characterization technique. Here I present a comprehensive characterization approach for the synthesized highly active catalyst, and correlate obtained structural/compositional parameters to the performance. The characterization of the catalysts was carried out via Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), High-Resolution Transmission Electron Microscopy (HR-TEM), Scanning Transmission Electron Microscopy (STEM) - Energy Dispersive Spectroscopy (EDS), Electron Energy Loss Spectroscopy (EELS), and X-ray Photoelectron Spectroscopy (XPS) to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation state and surface chemical states, respectively. The HRTEM images indicated that Pd NPs were uniformly distributed on the carbon support with only some minor agglomeration. Additionally, the achieved high interfacial contact between CeO_x and Pd acquired on single NPs was, for the first time, segmented and calculated using High-resolution STEM-EDS maps and Image J processing program by measuring the overlap intensities between Pd and Ce NPs; the results clearly showed that CeOx NPs were in intimate contact with Pd and their interfacial contact area increased with the addition of CeOx, reached a maximum at a ratio 0.38 CeOx -Pd/C, then decreased due to the formation of large CeOx islands upon the further addition of CeO_x. The attained interfacial contact area also seems to be much higher than other previously reported Pd-CeO₂ catalysts synthesized by other methods^4–7.
REFERENCES: 1.Davydova, et al. ACS Catal 2018. 8, 6665–6690; 2.Dekel, Curr. Opin. Electrochem 2018. 12, 182–188; 3.Dekel, J. Power Sources 2018. 375, 158–169; 4.Singh, et al. Adv. Funct. Mater 2020. 30, 1–11; 5.Miller, et al. Angew. Chemie 2016. Int. 55, 6004–6007; 6.Hamish et al. Nano Energy 2017, 293–305; 7.Yu, et al. Nano Energy 2019. 57, 820–826.

Proton-exchange membrane fuel cell (PEMFC) is one of the most important sources of clean energy especially for automotive applications, which currently utilizes platinum nanoparticles dispersed on carbon (Pt/C) as a catalyst. However, the catalyst activity and durability need to be improved and the cost of the fuel cell need to be reduced for successful commercialization. Extensive research has been done to improve the catalyst activity and durability, reduce the amount of platinum used and reduce its manufacturing cost. Continuous thin film layer approach is a promising candidate for non-conventional catalysts to address these challenges. For this purpose, nanocolumnar Pt thin film (Pt-TF) layers supported on carbon was fabricated by high pressure sputtering (HIPS) and investigated as oxygen reduction reaction electrocatalysts for PEMFCs. HIPS is a simple physical vapor deposition method that is scalable and easily applicable to industrial sputter deposition systems, in which atoms come to the substrate surface with oblique angles and form columnar structures. Different Pt-TF/C weight ratios ranging from 5% to 20% and Pt: Ni (1:3) TF/C were studied. Weight loading was controlled by changing the sputter deposition time. X-ray diffraction analysis revealed the existence of Pt and formation of the Pt: Ni alloy on carbon support. Electrochemical characterization of the carbon-supported Pt-TF samples was conducted by cyclic voltammetry and rotating disk electrode measurements in an aqueous perchloric acid electrolyte. The electrochemically active surface area, mass activity and specific activity of the Pt-TF/C samples were found to be increasing as the Pt-TF/C ratio was increased.

Ytterbium Oxide (Yb₂O₃) is a rare earth oxide that has been used in various applications, including sensor and laser technology, sintering aid and doping of different ceramics, and catalytic applications. Although Yb₂O₃ has been considered a potential catalyst for reactions such as vapor phase catalytic dehydration, to the best of our knowledge, there is no reported research for the use of Yb₂O₃ in the hydrogenation of syngas in Fisher-Tropsch chemical reactions. In order to add to the current state of knowledge, the possibility of using porous Yb₂O₃ as a catalyst for converting syngas (CO+H₂) into liquid hydrocarbons using the Fischer-Tropsch process was explored. 99.99% pure Yb₂O₃ ceramic powder was pressureless sintered at 900 °C for four hours in the air to create cylinders with an average porosity of 45%. The crystal structure and lattice parameters of Yb₂O₃ were then measured using neutron diffraction and Yb₂O₃ was confirmed to be cubic (Ia-3) with c=10.43731 Å lattice parameter. The spectral vibrational signature of Yb₂O₃ attained by micro-Raman spectroscopy corresponded fully to the one published in the literature. Two plug flow catalytic experiments, at temperatures of 250 °C and 500 °C, were performed for the catalytic studies. It was found that Yb₂O₃ is catalytically active and can be used to convert syngas into useful hydrocarbons. Production of methane, ethene, and ethane was detected in the catalytic experiment at 500 °C, but propane, propene, butane, and methanol were also detected in the experiment at 250 °C. It was concluded that more data points are required to be collected to determine if the products' mass distribution followed Flory-Schulz's distribution. Raman spectroscopy and neutron diffraction were also performed on samples used in the catalytic experiment. The crystal structure did not change during the catalytic experiment and the lattice parameter was c=10.43582 Å. A slight shift in the Raman active peaks was detected in the Raman spectra and an extra peak had to be added to obtain the curve fitting.

Conventionally, the operation of electrochemical energy conversion and storage devices is inherently dictated and constrained by the redox reactions at electrode-electrolyte interface. The redox-mediated process, a chemical reaction between an electrolyte-borne redox species electrochemically generated on electrode and a material (either soluble or insoluble in electrolyte) off the electrode, provides additional flexibility in circumventing the constraints intrinsically confronted by the conventional electrochemical devices. One example is the redox-mediated hydrogen and oxygen evolution reactions (HER & OER) for spatially decoupled water electrolysis. The concurrent electrochemical-chemical cycle enables continuous reactions between an electrolyte-borne redox mediator and a HER/OER catalyst loaded in a fixed-bed reactor spatially separated from the cell, which is believed to be advantageous to enhanced safety, operation flexibility and H₂ purity. Another example is redox-mediated nitrogen reduction reaction (NRR) for ammonia synthesis. Judiciously selected redox species serve both as electron and proton carriers circulating between the cell and N₂-filled catalyst bed and present considerably promoted NRR reaction yield. In this talk, I will report our latest progress in the above areas. In addition, I will briefly introduce some other studies on redox-mediated reactions, such as redox targeting-based battery for low-grade waste heat harnessing based on a thermal-electrochemical cycle.

References:
1. F. Zhang, S. Huang, X. Wang, C. Jia, Y. Du, and Q. Wang, Redox-Targeted Catalysis for Vanadium Redox-Flow Batteries. Nano Energy, 52, 292-299 (2018).
Y. G. Zhu, F. W. Goh, R. Yan, S. Wu, S. Adams, and Q. Wang, Synergistic Oxygen Reduction of Dual Redox Catalysts Boosting the Power of Lithium-air Battery. Phys. Chem. Chem. Phys., 20 (44), 27930-27936 (2018).
R. Yan and Q. Wang, Redox-Targeting-Based Flow Batteries for Large-Scale Energy Storage. Adv. Mater., 30, 1802406 (2018).
Y. Chen, M. Zhou, Y. Xia, X. Wang, Y. Liu, Y. Yao, H. Zhang, Y. Li, S. Lu, W. Qin, X. Wu, and Q. Wang, A Stable and High Capacity Redox Targeting-based Electrolyte for Aqueous Flow Batteries. Joule, 3 (9), 2255-2267 (2019).
M. Zhou, Y. Chen, M. Salla, H. Zhang, X. Wang, S. R. Mothe, Q. Wang, Single-Molecule Redox-Targeting Reactions for a pH-Neutral Aqueous Organic Redox Flow Battery. Angew. Chem. Int. Ed., 59 (34), 14286-14291(2020).

Electrochemical reduction of N₂ to NH₃ has recently received considerable attention, because it may enable sustainable, distributed production of NH₃ when powered by solar- or wind-generated electricity. However, typical catalysts show a low activity and selectivity for N₂ reduction reaction (NRR) due to the barrier for N₂ activation and the competing hydrogen evolution reaction (HER). A rational design of NRR catalysts relies on our understandings of structure-activity relationships and active sites for the NRR, and such study requires model catalysts with well-defined structures. Here we present a study of size-dependent activity for the NRR on Ru nanoparticle catalysts. We first tried colloidal synthesis method with polyvinylpyrrolidine (PVP) as a surfactant to control the size of Ru nanoparticles, while the derived Ru catalysts showed negligible activity for NRR, which was attributed to residual surfactant molecules that blocked catalyst surfaces. Therefore, we used atomic layer deposition (ALD) method to prepare Ru nanoparticles with controlled sizes and clean surfaces. We also quantified the electrochemical active surface areas of Ru samples and measured surface-area-normalized activity for the NRR. Consequently, the effect of Ru nanoparticle size on the NRR activity and Faradaic efficiency was revealed, which can provide insights into the active sites for the NRR and guidance on the design of NRR catalysts via surface site engineering.

To substitute the energy-intensive Haber-Bosch process for the synthesis of ammonia, some labile techniques, such as photocatalysis, electrocatalysis, photoelectrocatalysis, and photothermocatalysis, have emerged and attracted intense research interest. However, the contamination of the reaction system is one of the major concerns on how to reliably and accurately evaluate the performance of these catalysts, which is why various control studies are involved. Isotopic labelling studies are one of the most reliable control strategies in nitrogen fixation experiments, to ensure the fact that N₂ is exclusively the source of the generated ammonia. As a convenient, sensitive and accurate technique distinguished with a quantitative atomic mass resolution, liquid chromatography-mass spectrometry (LC-MS) has been extensively employed for the detection of ammonia in aqueous electrolyte systems. However, the previously reported protocols for ¹⁵N₂ isotopic analysis using LC-MS either involved hazardous procedures which could potentially damage the instrument, or lacked in their experimental verification using real samples. In this work, we present a safe, reproducible and economical protocol for the detection of ammonia using LC-MS, exhibiting an exponentially steep progressive detectivity of ¹⁵N abundance, which was well verified with a series of experimental results for nitrogen reduction reactions. This is expected to provide a prudent, cost-effective and sustainable gateway into isotopic analysis.

The integration of chemo and biocatalysis for the sustainable production of high-value chemicals and fuels from bio-renewable resources like lignocellulosic biomass has attained notable attention in the past decade. The catalytic reactions for transforming platform chemicals obtained from fermentation are often carried out in the liquid phase. Catalyst deactivation caused by the biogenic impurities such as amino acids and proteins arriving from the fermentation media is a key challenge in this line. To understand the role of solvents and biogenic impurities, the hydrogenation reaction of fermentation-derived 6-amyl-α-pyrone (6PP) was explored. Reactions using pure 6PP was carried out in solvents with different dielectric constants. Cyclohexane was found to be the best solvent providing >99% 6PP conversion and 79% DDL yield in 10 min using (10%) Pd/C. Further, the same reaction was carried out at different temperatures and almost complete conversion was obtained at 433 K. With an increase in temperature 6PP conversions increased along with DDL yield. However, reactions using fermentation-derived 6PP showed reduced conversion and product yield. Experiments revealed that even trace quantities of biogenic impurities such as amino acids and proteins from the fermentation media interact with the catalyst and cause significant deactivation of the catalyst. DFT simulations using the Vienna ab initio simulation package (VASP) was employed to unravel the interaction of amino acids on the catalyst surface. Amino acid Methionine (Met), bound through sulfur atom in its most stable adsorption mode with a binding energy=-186 kJ/mol, and it underwent C-S bond cleavage to form SCH₃ species with an activation barrier E_a =136 kJ/mol. This species was found to be stable on the surface. A plausible mechanism for the dissociation of Met on Pd (111) surface is proposed capturing the energetics of the dissociation steps. Much stronger adsorption and relatively lower dissociation barriers were found in the case of Cys on the same surface. Interestingly the observed experimental trend in deactivation also aligns well with simulation results. A combined experimental and computational approach is used to develop deeper insights into the interaction of solvents and biogenic impurities in heterogeneous catalysis.

Reducing atmospheric carbon dioxide (CO₂) is one of the most important challenges for sustainable society. Electrocatalysis for CO₂ reduction reaction (CO₂RR) to energy-rich chemicals has been intensively studied. The catalyst needs to be developed out of earth-abundant elements and should be efficient, fast and selective for useful product.
Recently, Coskun et al. have reported high catalytic activity of metal-free polydopamine towards CO₂RR [1]. Aside from its conductivity, its hydrogen-bonding nature is supposed to be important for stabilization of reaction intermediates. Among traditional electrocatalytic metals, copper (Cu) is outstanding to yield useful hydrocarbons, although its product selectivity is limited. If we can now combine both of them, namely, by introducing hydrogen-bonding organic molecules into Cu, selective production of hydrocarbon such as methane may be achieved by forming concerted catalytic sites.
In this study, the above-mentioned hybrid catalyst has been aimed by employing electrochemical self-assembly (ESA) of CuSCN / neutral red (NR) hybrid thin films. Minor addition of NR, one of phenazine dyes bearing many amino groups in its structure, to the bath for cathodic electrodeposition of CuSCN resulted in ESA of red-colored and nanostructured CuSCN/NR hybrid thin films, which is then electrochemically transformed into Cu/NR hybrid by electrolysis at -1.0 V vs. Ag/AgCl in an aqueous 0.1 mol dm^-3 KCl under N₂. Formation of metallic Cu and remainder of NR have been confirmed by XRD and FT-IR, respectively.
Thus prepared thin film electrodes were tested for CO₂RR in an aqueous 0.1 mol dm^-3 KCl (pH 6.8) While nanostructured Cu made by reductive conversion of NR-free CuSCN showed a reasonable activity to achieve -1.3 mA cm^-2 at -1.2 V (vs. Ag/AgCl) under CO₂, about 3 times enhanced from that under N₂, the Cu/NR hybrid showed a unique feature to reduce potential (for -0.75 mA cm^-2) from the initial -1.01 to -0.73 V upon 10 times repetition of voltammetric potential scanning, indicating its activation for CO₂RR. It is possible that NR molecules re-organize to form catalytic sites during the electrolysis.
At this stage, we haven’t identified the products and their selectivity. These preliminary results, however, already indicate successful strategy to obtain hybrid electrocatalyst to combine Cu and hydrogen-bonding organic molecules to enhance the activity for CO₂RR.

[1] Halime Coskun et al., Science Advances, Vol. 3, no. 8, e1700686 (2017).

The mechanistic understanding of defects engineering in nanocarbons, and its hybrid with single metal atoms towards electrocatalysis are vital to study in-depth, and still its remains very challenging. Herein, we advocate the fundamental understanding of the nature of active sites in ample edge defects nitrogen-doped graphene (DG) fascinated with specific coordination of carbon atoms rings. The concept of defect mechanism revealed that the topological defect (e.g. multiple edge pentagon, pentagon—octagon—pentagon or pentagon—heptagon—pentagon rings) appeared correspondingly in the combination of carbon atoms that avoided random dislocations and disclinations that brought active sites during reactions. We anchored Ni and Fe isolated single metals on DG support towards enhanced O₂ response. X-ray absorption spectroscopy evidences the embedding nature of NiFe metal atoms within the DG surface. The precise structural characterizations using high-resolution transmission electron microscopy/high-angle annular dark-field (HAADF) images in scanning transmission electron microscopy (STEM) techniques to probe the atomic level understanding of such defects, its different coordination of atoms. Density functional theory (DFT) further trust these specific grouping of carbon atoms and frilling of NiFe single atoms on the tip-enhanced local electric field of the carbon support that active upsurge sites and promotes the kinetics for ORR and OER. This study offers new prospects and underscores the importance of identifying topological defects in nanocarbons with the addition of single metal atoms towards the active species for multiple reaction catalysts.

Reduction of carbon dioxide has as main objective the production of useful organic compounds and fuels - renewable fuels - in which solar energy would be stored. Molecular catalysts can be employed to reach this goal. They may in particular provide excellent selectivity thanks to easy tuning of the electronic properties at the metal and of the ligand second and third coordination sphere. Hybridization of these catalysts with conductive or semi-conductive materials may lead to enhance stability and new catalytic properties. This approach bridges between homogeneous and heterogeneous, and it raises new fundamental questions that may further lead to breakthrough in CO₂ reduction chemistry.
Our recent results in this area will be discussed. [1-5]

1. M. Wang, L. Chen, T-C. Lau, M. Robert, Angew. Chem. Int. Ed. 2018, 57, 7769-7773.
2. S. Ren, D. Joulie, D. Salvatore, K. Torbensen, M. Wang, M. Robert, C. Berlinguette, Science 2019, 365, 367-369.
3. E. Boutin, M. Wang, J. C. Lin, M. Mesnage, D. Mendoza, B. Lassalle-Kaiser, C. Hahn, T. F. Jaramillo, M. Robert, Angew. Chem. Int. Ed. 2019, 58, 16172-16176.
4. B. Ma, G. Chen, C. Fave, L. Chen, R. Kuriki, K. Maeda, O. Ishitani, T-C. Lau, J. Bonin, M. Robert, J. Am. Chem. Soc. 2020, 142, 6188-6195
5. P. B. Pati, E. Boutin, R. Wang, S. Diring, S. Jobic, N. Barreau, F. Odobel, M. Robert, Nat. Commun. 2020, 11:3499.

The efficient electrochemical conversion of CO₂ (CO₂RR) to valuable fuels and feedstocks is a highly sought process towards the minimization of the carbon footprint. However, higher selectivity towards C₂₊ products must be achieved before a broad industrial use can be envisioned. Better efficiency can be attained by tuning the morphology (size, shape, dispersion), oxidation state, composition of the catalysts, NP/support interactions, and by a rational selection of the electrolyte. In addition, understanding the changes that a catalyst may experience on its surface during a reaction is crucial in order to stablish structure/composition-reactivity correlations. Here, mechanistic insight into CO₂RR will be provided by using as target materials size- and shape-controlled mono and bimetallic NPs including Cu, Cu₂O, Zn, Cu-Zn, Cu-Ag NPs with spherical, cubic and triangular-base prism shapes, and Cu₂O cubes decorated with monodispersed Ag nanoparticles on the surface.
A synergistic combination of spectro-electrochemical methods, in situ microscopy (EC-AFM, L-TEM), operando X-ray absorption spectroscopy (XAS), operando high-energy X-ray diffraction (HE-XRD), operando Raman spectroscopy and quasi in situ X-ray photoelectron spectroscopy (XPS) were used to gain insight into the morphological, structural, and chemical transformations undergone by the NPs during CO₂RR. I will illustrate that the as-prepared state of the mono and bimetallic NPs is drastically different from the structure and surface composition of the working catalyst. Thus, our study gives a comprehensive insight into C₂₊-forming state of Cu-based catalysts and sheds light into the selectivity-determining catalyst properties for CO₂RR.

Hydrogen peroxide (H₂O₂) is a versatile and green oxidant with a variety of distributed applications such as environmental remediation, disinfection, and household sanitation, but its centralized chemical production via the anthraquinone process poses significant cost, energy, and safety concerns. Decentralized electrosynthesis using renewable electricity to selectively reduce O₂ to H₂O₂ via the two-electron oxygen reduction reaction (2e^- ORR) could better satisfy end-user demands on-site, yet robust, earth-abundant catalysts that are active and selective in acidic (or neutral) solutions are lacking. Here we present our recent joint efforts combining theory and experiments to establish rational design rules for selective and stable acidic 2e^- ORR electrocatalysts based on earth-abundant metal chalcogenide compounds. We first showed that pyrite-type cobalt disulfide (CoS₂) selectively catalyzes the acidic 2e^- ORR at low overpotentials due to the spatial separation of active metal sites by anions, which kinetically suppresses the scission of O-O bond in OOH* adsorbate and the undesired 4e^- ORR. We further established both pyrite- and marcasite-type cobalt diselenide (CoSe₂) polymorphs as more stable and leaching-resistant acidic 2e^- ORR catalysts because of the much weaker binding of O* adsorbate to Se sites. This stability allows for the bulk accumulation of practically useful 547 ppm H₂O₂ and the effective electro-Fenton degradation of organic pollutant for on-site environmental remediation. Building on the new mechanistic understanding, our ongoing developments and future perspectives of earth-abundant metal compound-based 2e^- ORR electrocatalysts will also be discussed.

Electrocatalytic decomposition of water is a promising route for hydrogen production, which nowadays is one of the best candidates as energy vector for a future carbon-free economy. The involved catalysts must have high electrocatalytic activity (and high selectivity) for lowering the applied voltage to the cell and long-term stability for the durability of the device. Apart from that, the materials should be composed by Earth-abundant elements and be environmentally friendly. Besides, the synthetic route should be cheap and scalable for industrial production. For the oxygen evolution reaction (OER), the Ir/Ru oxides show excellent electrocatalytic performance for water oxidation in alkaline media, however, the scarcity of these metals and their high cost are detrimental for practical applications. Fueled by a growing interest in new materials that could be advantageous against precious metal-based catalysts, metal phosphides have attracted particular attention. Most of them exhibit good catalytic properties toward hydrogen evolution reaction (HER) and OER and a high conductivity compared with the corresponding metal oxides/hydroxides. In this work, we investigate CoFeP nanoparticles supported on two different 3D electrodes, nickel foam and carbon felt substrates. Electrodes using nickel foam display higher current densities due to a lowered Tafel slope and higher conductivity of the substrate. For CoFeP deposited on nickel foam, overpotentials lower than 300 mV are achieved for 10 mA/cm². The influence of the Co/Fe ratio, particle size, morphology and structure of CoFeP nanoparticles deposited on the electrode supporting materials have been investigated and compared with the monometallic phosphides emphasizing the role of the use of bimetallic based systems combined with the advantageous expected role of the phosphides.

Artificial photosynthesis of ammonia using atmospheric nitrogen and water is a sustainable but challenging alternative to the Haber-Bosch process. Bismuth oxyiodide (BiOI) is a promising candidate due to its superior light absorption capability (bandgap of around 1.8 eV) and abundant surface oxygen vacancies. However, its improper band edge positions made it inactive for overall water-splitting and N₂ fixation. In this work, ultrathin BiOI nanosheets were synthesized through a simple surfactant (PVP) assisted hydrothermal route. The nanosheets split pure water into H₂ and O₂ and converted nitrogen and water into ammonia under visible or near-infrared light. PVP capping not only reduced the thickness of BiOI sheets but also upshifted its band edge positions due to the surface electric dipole induced by polyvinyl pyrrolidone molecules on the BiOI surface, thus enabling the water oxidation and nitrogen reduction half-reactions simultaneously.

The efficient reduction of protons and CO₂ under mild conditions is a current challenge for modern society. Nature utilizes enzymatic machineries that comprise iron- and nickel- containing active sites to perform these transformations.^[1] Recently, we reported on the formidable HER activity of bulk Fe_4.5Ni_4.5S₈ electrodes revealing similar structural and functional properties of the enzymes.^[2,3] We herein set out to explore the influence of the Fe : Ni ratio on the performance of the electrocatalyst.^[4] Using linear sweep voltammetry, we show that the increase in the Fe or Ni content, respectively, lowers the activity of the bulk electrocatalyst towards HER. Additionally, with increasing Se content in Fe_4.5Ni_4.5S_8-xSe_x, the HER performance is significantly lowered.^[5,6] Thus, specific Fe-Ni interactions seem to be the key for materials reactivity.
In addition, we show that a temperature increase leads to a significant decrease of the overpotential. Furthermore, due to the resemblance of such sulfides with CO₂ converting enzymes, we likewise investigated Fe_4.5Ni_4.5S₈ electrodes to perform CO₂ reduction.^[7] In non-aqueous conditions as well as in supercritical CO₂, this material is indeed a potential catalyst affording CO or formic acid, respectively, as main product with high Faradaic yields.
Notably, the reactivity of the pentlandite materials can be further tuned by the reactor environment as well as the electrodes shape which was found to be equally important as the catalyst.

References.
[1] Möller, F. ; Piontek, S. ; Miller, R. G. ; Apfel U.-P. ; Chem. Eur. J. 2018, 24, 1471-1493.
[2] Konkena, B.; junge Puring, K.; Khavryuchenko, O.; Sinev, I. ; Piontek, S.; Muhler, M.; Schuhmann, W.; Apfel, U.-P. ; Nature Commun.2016, 7:12269, DOI: 10.1038/ncomms12269.
[3] Zegkinoglou, I.; Zendegani, A.; Sinev, I.; Kunze, S.; Mistry, H.; Zhao, J.; Hu, M. Y.; Alp, E. E.; Piontek, S.; Smialkowski, M.; Apfel, U.-P.; Hickel, T.; Neugebauer, J.; Roldan Cuenya, B.; J. Am. Chem. Soc. 2017, 139, 14360-14363.
[4] Piontek, S.; Andronescu, C.; Zaichenko, A.; Konkena, B.; junge Puring, K.; Marler, B.; Antoni, H.; Sinev, I.; Muhler, M.; Mollenhauer, D.; Roldan Cuenya, B.; Schuhmann, W.; Apfel, U.-P.; ACS Catalysis 2018, 8, 987-966.
[5] Smialkowski, M.; Siegmund, D.; Pellumbi, K.; Hensgen, L.; Antoni, H.; Muhler, M.; Apfel, U.-P.; Chem. Comm. 2019, 55, 8792-8795.
[6] Pellumbi, K.; Smialkowski, M.; Siegmund, D.; Apfel, U.-P. ; Chem. Eur. J. 2020, 26, 9938 –9944.
[7] Piontek, S.; junge Puring, K.; Siegmund, D.; Smialkowski, M.; Sinev, I.; Roldan Cuenya, B.; Apfel, U.-P.; Chem. Sci. 2019, 10, 1075–1081.

The deployment of carbon-free energy technologies that could compete with the price of fossil fuels is expected to be a key factor for sustaining the future energy requirements of the world population. In this regard, H₂ is a promising energy vector that could be produced using solar energy, and one of the approaches consists on coupling an electrolyzer to a photovoltaic cell. Among electrode materials used as cathodes, iron phosphide has engaged interest in the last years. Apart from the fact that it is composed of two elements highly abundant in the Earth crust, the new synthetic routes are making its synthesis simpler and cheaper. In this work, a 3D electrode composed by FeP directly deposited on carbon fibers is fabricated by a simple impregnation method and subsequent heat treatment. The intermediate iron species are converted to FeP by in situ PH₃ generation at 300-400 °C, leading to a highly porous electrode with a high surface area. Electrodes are optimized by exploring different synthetic conditions, such as heat treatment temperatures or catalyst loading. The intermediate Fe species are found to influence the final FeP/C configuration. Optimized FeP/C electrodes exhibit high activity towards hydrogen evolution reaction (HER) in both acidic and alkaline electrolytes displaying low overpotentials and excellent stability. Additionally, the doping with some particular metals is also studied, unveiling the role of the dopant in the HER activity. A further demonstration of performance in a flow cell is presented, confirming the feasibility of our electrodes as cathodes for HER in continuous operation.

It has been demonstrated that topological nontrivial surface states can favor heterogeneous catalysis processes such as the hydrogen evolution reaction (HER), but a further decrease in mass loading and an increase in activity are still highly challenging. The observation of massless chiral fermions associated with large topological charge and long Fermi arc (FA) surface states inspires the investigation of their relationship with the charge transfer and adsorption process in the HER. In this study, it is found that the HER efficiency of Pt-group metals can be boosted significantly by introducing topological order. A giant nontrivial topological energy window and a long topological surface FA are expected at the surface when forming chiral crystals in the space group of P2₁3 (#198). This makes the nontrivial topological features resistant to a large change in the applied overpotential. As HER catalysts, PtAl and PtGa chiral crystals show turnover frequencies as high as 5.6 and 17.1 s^-1 and an overpotential as low as 14 and 13.3 mV at a current density of 10 mA cm^-2. These crystals outperform those of commercial Pt and nanostructured catalysts. This work opens a new avenue for the development of high-efficiency catalysts with the strategy of topological engineering of excellent transitional catalytic materials.

Electrocatalytic water splitting using renewable energies to form green hydrogen and oxygen has attracted a widespread interest to replace fossil fuels as major energy carrier. Currently, the oxygen evolution half reaction (OER) is regarded to be the major bottle-neck of water splitting due to its hampered kinetics involving a multi-step proton and electron transfer. Noble metal electrocatalysts, such as RuO₂ and IrO₂ are widely used for OER, however, their prohibitive scarcity and cost limit their potential widespread use in electrolyzers.
Among multiple transition metal oxides, ferrites with the general formula M(II)Fe₂O₄ (M = Ca, Zn, Mg, Ni, Co, Mn etc.) have gained considerable attention due to their compositions made up from earth-abundant elements and widespread application fields including electrocatalysis.[1-4] Especially nickel ferrite has been considered as efficient OER electrocatalyst, with outstanding high stability in alkaline media, excellent redox properties, and ferromagnetism facilitating the catalyst separation from solution. [5]
We will report on the successful synthesis of mesoporous NiFe₂O₄ materials with narrow pore size distribution for the oxygen evolution reaction. The materials are prepared by a soft-templating strategy using citric acid and the optional addition of the commercially available block copolymer Pluronic® P-123, followed by calcination. The mesopore evolution during thermal treatment is examined systematically giving insights into the formation process of mesoporous NiFe₂O₄. The formation of intermediate carbonate species induced by the use of citric acid in the synthesis plays a key role in the formation mechanism of the templated mesoporous structure. Furthermore, citric acid is also crucial to obtain phase-pure NiFe₂O₄.
Detailed nitrogen physisorption analysis including desorption scanning experiments will be presented to reveal the exceptional accessibility of the mesopores generating surface areas of up to 200 m2/g. The ability of the NiFe₂O₄ powders to perform electrocatalytic oxygen evolution reaction under alkaline conditions was investigated, highlighting the advantages of mesopore insertion. The performance was ascribed to be dependent of the electrochemical surface area, which increased with the relative surface area of the prepared materials. In general, the highly accessible mesoporous and amorphous structure with large surface areas seem to be key parameters for OER, whereas high crystallinity turns out to be not beneficial.

References
[1] X. F. Lu, L. F. Gu, J. W. Wang, J. X. Wu, P. Q. Liao, G. R. Li, Adv. Mater. 2017, 29, 1604437.
[2] G. Liu, K. Wang, X. Gao, D. He, J. Li, Electrochim. Acta 2016, 211, 871–878.
[3] Q. Qin, L. Chen, T. Wei, Y. Wang, X. Liu, Catal. Sci. Technol. 2019, 9, 1595–1601.
[4] M. Li, Y. Xiong, X. Liu, X. Bo, Y. Zhang, C. Han, L. Guo, Nanoscale 2015, 7, 8920–8930.
[5] Z. Wu, Z. Zou, J. Huang, F. Gao, ACS Appl. Mater. Interfaces 2018, 10, 26283

Transition-metal phosphides (MP_x) have received significant scrutiny as water splitting electrocatalysts, particularly those with nanostructured morphologies that facilitate the electrocatalytic hydrogen evolution reaction (HER). The most synthetically accessible structures are those from the metal-rich side of metal-phosphorus phase diagrams (e.g., FeP, CoP, Ni₂P, or Cu₃P). Our group has developed a solvent-free chemical exchange synthesis for metal phosphides that can directly produce both crystalline metal-rich and less-studied phosphorus-rich metal phosphides at moderate temperatures near 500 °C. This reaction relies on the direct insertion/exchange reaction between metal halides and elemental phosphorus to form PCl₃ and the MP_x products. In some cases, addition of a tin flux facilitates redox processes and MP_x growth along with SnCl₂ byproduct formation. Single metal phosphides from the 3d metal group can be targeted through changes in synthetic conditions to form metal-rich and phosphorus-rich crystalline materials such as FeP versus FeP₂, CoP versus CoP₃, and Ni₂P versus NiP₂ or NiP₃. The synthesis and characterization of different compositions in the Fe/Co/Ni phosphide families will be described.

Several of the MP₂/MP₃ phases show hydrogen evolution reaction (HER) activity analogous to that observed for their metal-rich counterparts despite lower metal content. Even for 3d metal catalysts, a decrease in metal content without large corresponding loss in surface catalytic activity is desirable for sustainable catalytic solutions. The P-rich structures appear to resist (electro)chemical degradation in acidic environments during electrocatalytic HER experiments over extended 18-hour periods. In contrast to the metal-rich phosphides with extensive metal-metal bonding, MP₂/MP₃ structures contain polyphosphide anions that may better encapsulate metal centers and can enable electrocatalytically useful polyphosphide anion redox activity. Comparisons of local structure and properties and their possible impact on surface reactions in electrocatalytic HER will be described.

Exotic electronic states are realized in various topological phases, from topological insulators to recently reported Weyl/Dirac semimetals, nodal line semimetals, and magnetic semimetals. They strongly influence the surface electronic structures of the investigated materials and could serve as a good platform to gain insight into the catalytic mechanism of surface reactions. Topological Semimetals such as PtSn₄ and Co₃Sn₂S₂ adopt quasi-two-dimensional structures and could expose the crystal surfaces constructing by transition metal atoms. Topological non-trivial surface states are observed at the crystal surfaces, which are located near the Fermi level. These topological surface states can act as both electron acceptors or donators for small adsorbed molecules, consequently tailoring the adsorption energy and Gibbs free energy in the electrochemical catalytic reactions.

Two dimensional materials, such as the transition metal dichalcogenides (TMDCs) are a good candidate for water splitting catalysts [1,2], as they often have larger band gaps than their bulk counterparts. However, this had to be balanced by the thin layers having a small absorption cross section and difficulties in mounting on a suitable substrate. PdSe₂ is being suggested as a potential water splitting candidate [3]. However its bulk band gap is too small for water splitting [4]. We propose to use this structure as a surface coating to a second TMDC with a larger band gap such as MoS₂ and use this as an example of how such heterostructures could function.

Using density functional theory, implemented in the Vienna Ab-initio Simulation Package, we have investigated the surfaces of TMDC monolayers MoS₂ and PdSe₂, and a Hetero-bilayer of the two, for their potential application as photocatalytic water splitters. The different functional groups involved in the Hydrogen and Oxygen evolution reactions have been added to the monolayers and the hetero-bilayer to determine their energetics. In addition to this, we have looked at how stable these materials are, to both adsorptions and substitutions, in both air and water environments.

References:
[1] Qing Tang and De En Jiang. ACS Catalysis, 6(8):4953–4961, Aug 2016.
[2] B. Amin, et al . Phys. Rev. B, 92:075439, Aug 2015.
[3] C. Long, et al . ACS Appl. Energy Mater., 2, 1, 513-520, 2019.
[4] G. Zhang, et al . Appl. Phys. Lett., 114, 253102, June 2019.

Molecular electrocatalysts are attractive due to their well-defined structures and easy regulation of chemical properties. However, in heterogeneous systems, molecular electrocatalysts are usually inferior to noble metal based materials in terms of activity and stability, and the molecular engineering of electrocatalytic performance remains a grand challenge. Herein, we present our recent work on the development of molecularly dispersive electrocatalysts (MDEs) and their molecular engineering regulation strategies for the carbon dioxide reduction reaction. These MDEs were constructed by anchoring metal phthalocyanine molecules on side walls of carbon nanotubes, which could effectively overcome the issues of poor electrical conductivity and molecular aggregations. The electrocatalytic performance could be further improved by tuning metal centers and substitution groups on phthalocyanine. The designed nickel phthalocyanine MDE system exhibited high selectivity and good stability for the conversion to carbon monoxide at high current densities in gas-diffusion electrodes, setting new records of noble-metal-free and molecule-based electrocatalysts. The cobalt phthalocyanine MDE system could catalyze the six-electron reduction of carbon dioxide to methanol with high selectivity and good stability, which had not been achieved by molecular electrocatalysts. The MDEs with well-defined active centers also facilitated understanding the underlying mechanism on structural factors affecting electrocatalytic performance with the help of in-situ/operando characterizations and theoretical calculations. These studies pave a new path for the development of high-performance electrocatalysts.

Discovery of 2D atomic structures, including graphene, transition metallic dichalcogenides (TMDs), h-boron nitride, phospherene, and mxene, has unveiled new possibilities in materials science. Unzipping of the basal plane is a general issue to uniquely control the material signatures of the 2D materials, as evidenced by the effective transformation of intrinsically metallic graphene into semiconductors, while unzipped into few-nanometer-wide graphene nanoribbons. Nonetheless, reliable unzipping has been reported for graphene and phosphorene thus far. Single elemental nature of those materials allows straightforward understanding of chemical reaction and property modulation involved with such geometric transformations. Here we present spontaneous linear ordered unzipping of bi-elemental 2D MX₂ transition metal chalcogenides as a general route to synthesize 1D nanoribbon structures. Strained metallic phases (1T′) of MX₂ are found to undergo highly specific longitudinal unzipping owing to the self-linearized oxygenation at chalcogenides. Stable dispersions of 1T′ MoS₂ nanoribbons with widths of 10-120 nm and lengths up to ~4 µm are produced in water. Edge abundant 1T′ MoS₂ nanoribbons reveal the hidden potential of idealized elecrocatalysis for hydrogen evolution in a competitive level with precious Pt catalyst.

Oxygen evolution reaction (OER) is the key anodic catalytic reaction for many important clean energy processes. To date, the search for an active, selective, and stable electrocatalysts has not ceased and a detailed atomic-level design of the OER catalyst remains an outstanding (if not, compelling) problem. Only recently, a computational high-throughput study of iridium oxides (for both IrO₂ and IrO₃) has highlighted the role of polymorphism and stoichiometry to precisely engineer iridium oxides for efficient and stable OER catalysis. However, it seems surprising that nanoporous (i.e. crystal structures containing nanopores and nanochannels) iridium oxides – which have been proposed in various experiments – were not examined in that study. In this work, we have further extended the previous computational report to include many metastable nanoporous iridium oxide polymorphs – inspired both from experiments and also analogous crystal structures from manganese oxides. Using van der Waals corrected density-functional theory calculations, we investigate the thermodynamic stability, intercalation properties, and electronic structure of these nanoporous iridium oxides. We focus on understanding how the charge, size, and concentration of the intercalation ions (e.g. K⁺, Ca²⁺, etc.) may be taken advantage of for engineering the desired OER descriptor – the ratio of Ir³⁺/Ir⁴⁺ in these intercalated polymorphs, for the neoteric OER electrocatalysts.

Transition metal carbides and nitrides are suggested as promising candidates for conductive low-cost heterogeneous catalyst supports given their superior structural durability and electronic structure to effectively promote charge transfer across the catalyst interface. To architecture the most optimal carbide and/or nitride for conductive supports, we consider a range of group VI and V metal carbides and nitrides (in their rock-salt bulk phase) and perform high-throughput first-principles density-functional theory (DFT) calculations using the Vienna Ab initio Simulation Package (VASP) within our automated high-throughput in-house python code via the Atomic Simulation Environment (ASE). Namely, from the low Miller-index surfaces of these carbides and nitrides, we calculate and analyze their surface thermodynamic stability, surface work function modulations, and electronic structure analysis to examine the magnitude/origin of charge transfer across the catalyst interface. Furthermore, we introduce point defects at the pristine surfaces of these carbides and nitrides to inspect the variations in surface work functions and electronic structure to anticipate a wider range of tunability of their support properties in heterogeneous catalysis. Through these high-throughput DFT calculations, we have successfully screened out the most promising candidates and propose an ab initio design rule to engineer the next-generation hybrid heterogeneous catalysts.

In the present era of depleting conventional energy resources, it is urgent to inspect and develop substitutional fuel resources and suitable energy storage and conversion systems for their exploration. Further, in light of the increasing pollution concerns, it is desirable that the alternative systems are suitable for environment-friendly energy conversion. Polymer electrolyte membrane fuel cell (PEMFC) and the metal-air batteries are the prospective devices, which need dedicated research efforts to overcome the shortcomings and reach commercialization. The major performance limiting factor is the sluggish reaction kinetics of the oxygen reduction reaction (ORR) at the cathode. Noble metal nanoparticles supported on carbon are employed as cathode catalyst to enhance the reaction rate. But, these catalysts are not economical and suffer from poisoning and fuel-crossover issues. Suitable cost-effective, highly active and durable catalyst system are very much needed to enhance the rate of the ORR.

Here, we are presenting an atomic Ni decorated N, S doped hierarchical porous networked carbon synthesized by employing an optimized inert pyrolysis followed acid-treatment strategy. Nickel nitrate and guanidine thiocyanate are chosen as precursors for metallic entity and doped porous support, respectively. The catalyst possesses a novel morphology with atomically dispersed Ni atoms and nickel sulfide entities on networked N, S-doped carbon backbone.

The sample synthesized at 750 ^oC, which is further treated with 1 M HCl (Ni-GT-750-A) exhibit superior pH-universal ORR performance with an onset potential (V_Onset) = 0.91 V (0.1 M KOH) and 0.89 V (0.1 M HClO₄) versus reversible hydrogen electrode (RHE). The excellent current stability (95.0 and 60 %) and resistance to methanol poisoning (90.6 and 80.3 %) are observed through chronoamperometric studies as comparison to the state-of-the-art catalyst (Pt/C) (65.0, 27, -33.0 and 16.5 %) in alkaline and acidic media, respectively. This electrochemical performance marks the suitability of the proposed catalyst in wide range of electrolytic media.

The observed ORR performance of the prepared electrocatalyst is attributed to the localized single-metal atom type ORR active sites over carbon matrix, which decreases the potential barriers for intermittent reactions due to enhanced atom utilization efficiency, which in turn increases the ORR kinetics. These atomic species are visualized using high-angle annular dark field- scanning transmission electron microscopy (HAADF-STEM). The hierarchical porous morphology as confirmed through N₂ adsorption-desorption isotherms and pore size distribution in addition to the defects in the carbon support owing to the present heteroatoms as can be visualized by Raman studies also contributed in the overall ORR performance in a synergistic manner. The explored physicochemical traits and ORR activity of Ni-GT-750-A paves the way for exploration of its catalytic activity for other electrochemical reactions and as electrode material for energy storage devices.

This work is published as:
A. Tyagi, K K. Kar, H. Yokoi, J. Colloid and Interface Sci., 571 (2020) 285-296
https://doi.org/10.1016/j.jcis.2020.03.043