Symposium ET03 : Application of Nanoscale Phenomena and Materials to Practical Electrochemical Energy Storage and Conversion

Fuel produced from the electrochemical splitting of water can be used to power a wide variety of technologies including information communication technologies infrastructure. The slow kinetics of the oxygen evolution reaction (OER) is one of the performance-limiting factors for hydrogen production through electrolysis. OER catalysts are often unstable in alkaline environments exhibiting deactivation and structural transformation causing a significant challenge for use in photoeletrochemical cells and water electrolyzers. Moreover, in nanoscopically thin catalyst layers, OER activity also decreases due to inefficient charge transfer to the electrolyte-catalyst interface. Epitaxial heterostructures are promising to solve these issues, though recent attempts yielded improved stability only at the expense of greatly reduced OER activity. In this presentation, we elucidate the competing factors for deactivation of La_xSr_1-xCoO₃ (LSCO) in nanoscopically thin layers supported on conducting perovskite substrates, and demonstrate heterostructured anodes with simultaneously high activity and stability during electrochemical water splitting in alkaline environments (pH = 13).
Epitaxial thin films of La_1-xSr_xCoO₃, Ba_1-xLa_xSnO₃, and Sr_1-xLa_xTiO₃ were grown by pulsed laser epitaxy. Interface energetics were characterized using in situ X-ray and ultraviolet photoelectron spectroscopies. Scanning transmission electron microscopy and electron energy loss spectroscopy were used to resolve the atomic structures, and scanning nonlinear dielectric microscopy used to probe the nature of the charge carrier character on the heterostructured catalyst surface. Density functional theory calculations were used to assess the impact of the electronic structure of the heterostructured catalyst layers on the overpotential and OER catalytic activity.
While the LSCO undergoes dramatic structural and electronic changes during electrolysis, including leaching of La and Sr from the film to yield a layer of cobalt oxyhydroxide, the thickness dependence of the OER activity will be revealed to be due to inefficiency of charge carrier transport to active sites. We demonstrate engineering of depletion layers widths and chemical stability using heterostructures comprised of nanoscopically thin epitaxial layers of degenerately doped stannate and titanate perovskite structure oxides to yield low overpotentials ~ 300 mV at current densities (~10 mA/cm²) relevant for hydrogen production in electrolyzers and photo-electrochemical cells, at hundreds of hours operations in nanoscopically thin active layers. Implications of the results for applications of nanoscopically thin oxide heterostructures for designs of high activity and stable anodes for carbon neutral energy production via the electrochemical splitting of water will be discussed.

Acknowledgement
C.H. would like to thank the Japan Trust program of the National Institute of Information and Communication Technologies (NICT) for funding.

The development of OER catalysts for PEM water electrolysis requires both activity and stability testing methods. The OER catalyst activity can be estimated by using rotating disk electrode (RDE), flow-channel methods, or in an electrolyzer. The evaluation of catalyst stability is realized using accelerated tests as testing over the whole lifetime (5-10 years) under realistic conditions is not practical.^{1, 2} A protocol for the OER catalyst stability using RDE was proposed by the JCAP group and is now used by other researchers. In this protocol, a constant current is applied in a half-cell configuration and the change in potential is monitored until a sudden increase in potential indicates complete catalyst degradation.³ It was shown recently that the measured catalyst life-time depends on the nature of the electrode substrate onto which the catalyst powder is being supported. Based on this, it was recommended that Au and boron-doped diamond be used as they show better stability of the catalyst under investigation, while glassy carbon and fluorine doped tin oxide electrodes were deemed unsuitable in such stability test (St. T.).²
Here we present a careful examination of the use of RDE for investigating OER catalyst stability. Although the increase in potential in an RDE St. T. is thought to be due to catalyst degradation (dissolution), our findings provide a clear evidence that the change in potential is rather due to an experimental artifact. The source of this artifact are nano- and micro-bubbles formed within the pores of the catalyst layer during the OER that cannot be removed by electrode rotation. These bubbles accumulate and block the OER active sites, resulting in a potential increase, which is mistakenly interpreted as catalyst degradation. Our findings indicate that almost no catalyst degradation takes place at the first phase of the St. T., which can last for several hours. In this phase, the bubbles accumulate and block the active sites, resulting in an artificial increase in the potential. The second phase of the St. T. starts once a threshold potential is realized, where a rapid potential increase is observed, due to catalyst dissolution at high potentials. Gas bubbles accumulation is responsible for the increase in potential, and ultimately resulting in the full degradation of the catalyst layer. A proper St. T. using RDE technique should avoid the accumulation of oxygen bubbles, which is currently under investigation and preliminary results will be also presented.

1. H.-S. Oh, H. N. Nong, T. Reier, A. Bergmann, M. Gliech, J. Ferreira de Araújo, E. Willinger, R. Schlögl, D. Teschner and P. Strasser, J. Am. Chem. Soc., 138(38), 12552–12563 (2016).
2. S. Geiger, O. Kasian, A. M. Mingers, S. S. Nicley, K. Haenen, K. J. J. Mayrhofer and S. Cherevko, ChemSusChem, 41, 15 (2017).
3. C. C. L. McCrory, S. Jung, J. C. Peters and T. F. Jaramillo, Journal of the American Chemical Society, 135(45), 16977–16987 (2013).

Modern societies face the challenge of supplying a continuously increasing energy demand. Intensive research is being conducted all around the globe to ensure that these needs can be satisfied while minimizing carbon emissions and other environmental threats. Electrochemistry holds promise to satisfy this requirement, either by using renewable energy to obtain fuels or store electricity or by converting fuels into electricity. Here catalysis research is pivotal, providing key advances in electrolyzers, fuel cells, and metal-air batteries.

Nature Catalysis, a new journal from the Nature Research group launched in January 2018, provides coverage of top research from the area of electrocatalysis, as well as all other areas of catalysis. Our broad scope, drawing from the work of scientists, engineers and researchers in industry and academia, ensures that published work reaches the widest possible audience. Nature Catalysis brings together researchers from across all chemistry and related fields, publishing work on homogeneous, heterogeneous, and biocatalysis, incorporating both fundamental and applied studies that advance our knowledge and inform the development of sustainable industries and processes.

The need for a sustainable source of hydrogen has been widely recognized, not just as a potential transportation fueling vehicles, but to limit CO₂ production and fossil fuel consumption from existing industrial processes such as ammonia generation. Currently over 95% of hydrogen is made from fossil fuels through natural gas reforming or coal gasification. However, significant growth has occurred in recent years in water electrolysis research, especially in catalyst research for the hydrogen (HER) and oxygen (OER) evolution reactions. Proton exchange membrane (PEM)-based systems are relatively mature in that the technology has been commercialized, but further research and development can achieve significant impact; for example, order of magnitude reductions in catalyst loading. Anion exchange membranes based systems are still under development, with membrane and ionomer stability in the operating environment being a critical issue. In both the AEM and PEM case, there are complex interactions at the electrode level which need to be considered in catalyst and membrane development. First, the liquid electrolyte environment used for catalyst activity screening, where all of the catalyst surface is accessible to the reactant is often not comparable to a complex, 3-dimensional, ionomer-based electrode. Also, similar to automotive fuel cells, the operating environment is highly important and should be considered when claiming improvements over state of the art. For example, catalyst performance at very low current densities may indicate inherent activity but may not represent capability at typical device operating currents. Similarly, a membrane which cannot operate at differential pressure may be highly limited in utility even if more efficient than current commercial solutions. This talk will describe some of the complex interactions that need to be considered, typical operating requirements, and stages of development where relevant conditions should be introduced.

The catalytic oxygen evolution reaction (OER) to extract electrons from water molecules is important for the artificial photosynthesis to generate useful chemicals such as hydrogen and organic compounds [1, 2]. In terms of elements strategy, utilization of abundant element for OER catalysts is remarkably advantageous for future low-costly artificial photosynthetic system. Fe-based OER catalysts, based on the fourth most earth-abundant element, are attractive, but are known to suffer from low OER activity due to limited electrical conductivity and non-ideal electronic structures near the surfaces of these catalysts.
Here, we report a highly crystalline, 10 nm-sized red rust OER catalyst composed of pure β-phase FeOOH(Cl) nanorods (ca. 3 × 13 nm) doped with Ni ions (β-FeOOH(Cl):Ni) [3] and surface-modified with amorphous Ni(OH)₂ (a-Ni(OH)₂, at a Ni to Fe ratio of 22 at.%), which shows the highest level of performance among Fe-rich oxides and oxyhydroxides. This catalyst can be synthesized by a facile one-pot process at room temperature, and colloidal aqueous solutions of the β-FeOOH(Cl)Ni/a-Ni(OH)₂ nanorods are very stable, with no apparent precipitation over a time span of at least one month.
Electrochemical measurements for β-FeOOH:Ni/a-Ni(OH)₂ stacked nanorod anodes deposited on carbon paper (CP) were performed in a 3-electrode configuration using a Ag/AgCl reference electrode and a Pt wire counter electrode. The overpotential during the electrochemical OER over the anodes was 170 mV, and an OER current of 10 mA/cm² was obtained at an overpotential of 430 mV(+1.66 V vs. RHE) in 0.1 M KOH (without subtracting the iR drop). It is suggested that the surface modification with the a-Ni(OH)₂ lowered the OER overpotential of β-FeOOH(Cl):Ni, resulting in the very high current density at low potential compared with Fe-rich oxide and oxyhydroxide electrodes reported previously. Mössbauer spectroscopy also suggested electronic interaction between Fe and Ni species, which may be crucial evidence for the enhanced activity in the Fe-rich OER system [4].
The present cost-effective Fe-based OER catalysts can be widely applied to construct artificial photosynthetic systems for solar fuel generation by combination with CO₂ reduction catalysts.

References
[1] T. R. Cook, et al., Chem. Rev., 110 (2010) 6474. [2] C. C. L. McCrory, et al., J. Am. Chem. Soc., 136 (2013) 16977. [3] T. M. Suzuki, T. Morikawa, et al., Sustainable Energy Fuels, 1 (2017) 636. [4] T. M. Suzuki, T. Morikawa, et al., Bull. Chem. Soc. Jpn., 91 (2018) 778.

Two-dimensional (2D) materials have attracted great interest in catalyzing electrochemical reactions such as water splitting, oxygen reduction, and carbon dioxide reduction. Quantum mechanical simulations have been extensively employed to study the catalytic mechanisms. However, these calculations typically assume that the catalyst has a zero/constant charge for computational simplicity, while in reality, the catalyst usually has a varying charge as the reaction proceeds due to the match between its Fermi level and the applied electrode potential. These contradictions urge an evaluation of the charge effects.
Here using grand canonical density functional theory calculations, we show that the charge on 2D materials can have a much stronger impact on the electrochemical reaction than the charge on 3D metals, which arises from the unique electronic properties of 2D materials. Our work calls for reconsideration of some of the previously suggested electrocatalytic mechanisms of 2D materials by incorporating the charge effects. [1]
[1] Donghoon Kim, Jianjian Shi, Yuanyue Liu, submitted

One of the grand challenges facing humanity today is the development of an alternative energy system that is safe, clean, and sustainable and where combustion of fossil fuels no longer dominates. A distributed renewable electrochemical energy and mobility system (DREEMS) based on cheap renewable electricity could meet this challenge. At the foundation of this new energy system, we have chosen to study a number of electrochemical devices including fuel cells, electrolyzers, and flow batteries. We have been working on the development of hydroxide exchange membrane fuel cells (HEMFCs) and electrolyzers (HEMELs) which can work with nonprecious metal catalysts and inexpensive hydrocarbon polymer membranes. We have developed roadmaps for HEMFCs and HEMELs, the most chemically stable membranes, and the most active nonprecious metal catalysts. We have also studied why hydrogen oxidation and evolution reactions (HOR/HER) are slower in base than in acid for precious metal catalysts. For flow batteries we have developed novel designs, chemistries and cost models e.g., double-membrane aqueous flow batteries with high voltages (i.e., 3 V), single-element-mimic redox pairs, and user friendly physics-based analytical cost models. In this presentation, I will focus on our HEMEL work highlighting a new class of membranes, nonprecious metal catalysts and base/salt-free HEMEL cells for hydrogen production.

Replacing fossil fuels with renewable resources to meet the global need requires a technology that is scalable to the unprecedented level of several terawatts. Natural photosynthesis is the sole existing technology that produces energy dense chemicals on the terawatt scale (> 100 TW). Its key design feature is the closed cycle of H₂O oxidation and formation of the primary reduction products on the shortest possible length scale, the nanometer scale, while separating the incompatible redox environments by an ultrathin membrane. This offers the advantage of minimizing efficiency-degrading mass transport processes and unwanted side reactions.
To incorporate the key feature into artificial photosystems, we assembled ultrathin (2 nm), pinhole-free, molecular wires-embedded SiO₂ membrane on planar and nanotube constructs. This membrane system spatially separates the H₂O oxidation CO₂ reduction, but enables (photo-)electrochemical communication between the incompatible redox reactions by transmitting protons and electrons in a precisely controlled manner, while preventing O₂ transport causing unwanted reverse reactions. This unique mass-transport behavior on planar and nanotube configurations was systematically studied via cyclic voltammetry, electrochemical impedance spectroscopy, and visible light-sensitized short circuit current experiments. The embedded molecular wires’ integrity before and after the mass-transport process was confirmed by time-resolved optical spectroscopy and grazing angle ATR-FT-IR or IRRAS characterization.

Polymer-catalyst interfaces control the energy efficiency of many energy conversion and storage device. The interfacial polymer layers are very thin (typically less than one micron thick). Many interesting structural, mechanical and transport properties in such thin ion containing polymer (ionomer) layers evolve as a result of complex multimodal interfacial interactions, unusual hydration behavior and confinement. Especially ion conductivity at the interface can be drastically different from that in the bulk membranes and the route to this poor ion conduction behavior is not well-understood. It is thus highly needed to systematically study how the ion conduction environment and water uptake change with the change in ionomer structure and film thickness. In this work, three potential fluorocarbon based hydrogen fuel cell ionomers (Nafion, 3M PFIA, 3M PFSA) having single/multiple acids at side chain were studied in sub-micron thick films. All three ionomers have fluorocarbon (PTFE) backbones. The difference between Nafion and 3M PFSA is in the side chain structure, but both has single acid group at the side chains. On the other hand, 3M PFIA has bis(sulfonyl)imide group in addition to perfluorosulfonic acid which makes the polymer more acidic. By tracking the fluorescence response of photoacid dye HPTS incorporated within hydrated ionomer thin films, very interesting trends were obtained regarding the extent of proton transfer. The results, when combined with the information on nanoscale structure and water sorption, clearly indicated that there are many factors controlling the proton conduction behavior in thin ionomer films, in addition to water uptake.

Nanoscale transport and materials are critical to electrochemical energy storage, such as power density, cycling life and safety. In this talk, I will present two examples on advanced tools and fabrication to understand nanoscale transport phenomena and designing of nanoscale materials. The first one is based on an emerging Stimulated Raman Scattering Microscopy, which is three orders of magnitude faster than traditional Raman microscopy. Therefore it can clearly track ion transport in electrolyte together with lithium dendrite, to illustrate their correlations. A positive feedback mechanism has been visualized, which guide methods to suppress lithium dendrite. The second example is through designing nanoscale modification of interfaces between battery electrodes and solid electrolyte. Therefore, the stability between electrodes and electrolyte and the cycling life of corresponding full cells are significantly improved.

The design/discovery of layered materials for applicability in next-generation battery technologies requires a fundamental understanding of the links between the atomic-scale structure, chemistry and the mechanisms and energetics of intercalation and de-intercalation reactions, and a consideration of other solid-state reactions that might compete. The goal of our research is to design/discover layered material microstructures as alternatives to graphite using an innovative combination of atomic-scale modeling, experimental in-situ characterization of the microstructural evolution during (de)intercalation reactions. Density functional theory (DFT) simulations are carried out to investigate the structural accommodation of the layered material during insertion and exertion of the intercalating species (energy barriers, volumetric expansion, and phase transformations). The structural stability of the 2H and 1T phases of MoS₂ during lithiation suggests that a phase transformation of the 2H phase of MoS₂ to the 1T phase may occur when MoS₂ is reacted with Li; the computational study allows different dosages of Lithium ion to be assessed with the aim of testing these the validity of these models using in-situ characterization of the solid-state reactions between Li and MoS₂ in the transmission electron microscope (TEM). The mechanisms of strain relaxation and the energetics of Li intercalation-induced phase transformations in MoS₂ at the atomic scales will be presented. This work is supported by NSF grant No. 1820565.

Understanding the structure and phase changes associated with two-dimensional (2D) layered transition metal dichalcogenides (TMDs) is critical in optimizing performance in lithium-ion batteries. The large interlayer spacing in MoS₂ (∼0.65nm) accommodates species such as alkali metal ions (Li⁺/Na⁺/K⁺) during intercalation. Intercalation is reported to change the electronic structure of the host molecule, resulting in variations in their electrical and optical properties. In this work, we examine the solid-state reactions between Li and MoS₂. Li⁺ ions can be inserted into vdW gap; the reaction is still unclear. Plan-view imaging has been extensively used, however, it is essential to visualize the process with the electron beam being parallel to the basal planes of the layer material to understand the reaction process. Lattice-fringe images have been discussed for several systems but relying on microtoming or simply using curved thin layers, the orientation of the specimen was less than ideally uncontrolled. Here, TEM specimens are made using FIB, and oriented for detailed study of the intercalation process. This study of TMDs uses a Tecnai F30 and a Cs/image-corrected Titan equipped with a direct electron detector camera, K2. This camera has two major advantages: the electron dose can be minimized and quick changes during reactions are recorded; both instruments have EELS and XEDS capabilities. DFT calculations are used to probe the structure and bonding changes during these reactions. Volumetric expansion, energy barriers, phase transformations and the role of doping, defects and interfaces can be modeled. The dynamics of the structural response are modeled using ab initio MD simulations. Electrochemical aspects can be monitored in situ in real-time and at atomic scale to provide understanding of lithium-ion storage mechanisms in these solid-state reactions and thus to test the modeling-based results.
In plan-view specimen, variations normal to the basal plane are not seen. Defects associated with the reactions were monitored real-time. As the reaction between MoS₂ and Li proceeds, white-line defects were observed under high-resolution imaging by TEM. Lower-magnification images show that the defects are not equally spaced and do not correspond to ‘stage’ development. These defects can cross several basal planes in the MoS₂ (either forwards or backwards) but maintain essentially the same width after the step; they are not completely constrained to the vdW gap.
This work is funded by NSF grant No. 1820565. MTJ is at LANL. TEM is at CINT, an Office of Science User Facility operated for the U.S. DOE. Sandia NL is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s NNSA under contract DE-NA-0003525. The views expressed in the abstract do not necessarily represent the views of the U.S. DOE or the U.S. Government.

Electrochemical processes at electrode/electrolyte interfaces (e.g. electric double layer charging, heterogeneous charge transfer and surface binding of reaction species on the electrode) are of vital importance to energy conversion and storage systems including batteries, supercapacitors and electrocatalytic production of fuels. It has been widely acknowledged that the kinetics of the interfacial electrochemical processes are largely determined by the electronic structure (e.g. density of states and electronic occupation) at the electrode/electrolyte interface. We have developed a back-gated electrode structure that utilizes electrostatic charging (induced by a gate bias) to control electrochemical kinetics on ultrathin or 2D materials (5-nm-thick ZnO, monolayer MoS₂ and graphene).^1,2 Such back-gated electrodes are fabricated with nanometer-thick semiconductors on SiO₂/degenerate Si substrates, analogous to the metal–oxide–semiconductor stack in the CMOS technology. Due to the extreme thinness of the electrode materials, the alignment of electronic bands as well as the electronic occupation, at the electrode/electrolyte interface, can be dramatically altered by the gate-induced charge carriers. Thus, real-time, continuous and efficient modulation of reaction kinetics can be achieved on 2D materials by varying the gate bias.
In this presentation, we will use back-gated monolayer MoS₂ as an example to demonstrate how the applied gate bias affects the kinetics of heterogeneous charge transfer and surface binding processes. Specifically, the standard charge transfer rate constant between MoS₂ and ferrocene/ferrocenium redox couple can be tuned by over two orders of magnitude and the catalytic overpotential of hydrogen evolution reaction on 2H-MoS₂ can be reduced by more than 150 mV. Overall, the approach introduced here is generally applicable to investigation and optimization of interfacial electrochemical phenomena in a wide range of electrochemical systems. With the ability to control the band alignment and electronic occupation independent of the electrode potential, the back-gated 2D electrodes will provide new insights to rational design of electrode materials.


(1) Kim, C.-H.; Frisbie, C. D. Field Effect Modulation of Outer-Sphere Electrochemistry at Back-Gated, Ultrathin ZnO Electrodes. J. Am. Chem. Soc. 2016, 138 (23), 7220–7223.
(2) Wang, Y.; Kim, C.-H.; Yoo, Y.; Johns, J. E.; Frisbie, C. D. Field Effect Modulation of Heterogeneous Charge Transfer Kinetics at Back-Gated Two-Dimensional MoS₂ Electrodes. Nano Lett. 2017, 17 (12), 7586–7592.

The performance of electrochemical energy materials depends crucially on the underlying nanoscale processes. The charge-discharge cycles of batteries result in gradual changes in nanoscale structure and chemistry of the different electrode layers with often detrimental consequences for the electrochemical properties [1]. To understand the nanoscale mechanisms causing the degradation of the battery materials and to develop strategies to counteract, high resolution imaging and analysis techniques are indispensable. While high-resolution Transmission Electron Microscopy (TEM) enables imaging of the nanostructures down to atomic resolution, analysis of light elements (Z < 6) and low concentrations (< 0.1 at. %) are difficult using typing analytical tools in a TEM such as Energy Dispersive X-ray Spectroscopy. In comparison, Secondary Ion Mass Spectrometry (SIMS) has an excellent sensitivity (can be as low as ppm range) and all the elements (including isotopes) of the periodic table can be analysed. However, the SIMS image resolution is limited to ~ 50 nm in most commercial SIMS instruments (except some new developments [2] where resolution < 20 nm has been demonstrated). Nevertheless, the resolution is still more than 2 orders of magnitude poorer than TEM imaging. To complement the strengths of TEM and SIMS in the same instrument, we developed an in-situ correlative microscopy technique combining TEM-SIMS [3, 4]. In this presentation, we will demonstrate the application of this new nanoscale characterization technique to elucidate the structural and chemical changes occurring in Li ion battery cathodes containing LiV₃O₈ thin film with different initial microstructures obtained by thermal annealing. Bright-Field TEM and corresponding SIMS images (e.g. Li⁺ and V⁺ maps) from uncycled and cycled samples were obtained to investigate the underlying materials phenomena (such as vanadium dissolution) in the cycled cathodes and to correlate the nanoscale processes with macroscopic electrochemical performance [5].
References:
[1] Q. Zhang et al, Journal of The Electrochemical Society, 164 (7) A1503-A1513 (2017)
[2] D. Dowsett, T. Wirtz, Anal. Chem. 89 (2017) 8957-8965
[3] T. Wirtz, P. Philipp, J.-N. Audinot, D. Dowsett, S. Eswara, Nanotechnology, Vol. 26, 434001, 2015.
[4] L. Yedra, S. Eswara, D. Dowsett, T. Wirtz, Sci. Rep. 6, 28705, 2016
[5] Acknowledgements: LVO cathode samples are synthesized as part of the Center for Mesoscale Transport Properties, an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award #DE-SC0012673.

Development of a stable energy-storage device is a fundamental approach to solve energy-related issues. Lithium-ion batteries (LIBs) are one of the most promising candidates because of their high energy density and long cycle life. From electrochemical impedance spectroscopic measurements, the cell resistance of conventional LIB is dominated by charge transfer resistance at electrode/electrolyte interfaces. [1,2] Therefore, we investigate the charge transfer process, i.e. Li insertion/desorption process, at the interface between a graphite anode and 1 M LiPF₆EC electrolyte. The density functional theory (DFT) with effective screening medium (ESM) method [3] combined with the reference interaction site model (RISM), called ESM-RISM, is employed to simulate the Li insertion/desorption process. [4] In this method, the graphite surface (Li_xC₆slab and additional Li⁺) and liquid solution (1 M LiPF₆EC) are represented as quantum mechanical and implicit classical solvation, respectively. The energy landscapes of reaction are revealed under constant electron chemical potential conditions at the interface. Across the transition state where the Li forms a half solvation shell, the reacting Li inside the electrode changes to a full solvation structure in the solution accompanied by electron transfer. The activation energies at the equilibrium potentials of the charge transfer reaction are approximately 0.6 eV, [5] which is consistent with the electrochemical impedance spectroscopy measurements. In the presentation, we explain the details of the ESM-RISM simulation and introduce the energy profiles of the Li insertion/desorption path at the LiC₆/EC LiPF₆interface.

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[4] S. Nishihara and M. Otani, Phys. Rev. B 96, 115429 (2017).
[5] J. Haruyama, T. Ikeshoji, and M. Otani, J. Phys. Chem. C 122, 9804 (2018).

Rapidly increasing demand for low-cost, high energy density energy storage motivates researchers to develop advanced and reliable anode materials. Lithium alloying anodes such as Si, Sn, or Ge has three to ten times of charge capacity compared to the traditional graphite electrodes, and thus gathered enormous research interest. One of the major challenges associated with the lithium alloying anodes originates from de/lithiation-induced large volume change (~300%). Such volume change applies excessive cyclic strain on solid electrolyte interphase (SEI) to cause its mechanical failure and continued formation, resulting in poor cycle life. Several electrolyte additives such as fluoroethylene carbonate (FEC) or vinylene carbonate (VC) have been investigated and demonstrated to improve cyclic performance of Si electrodes. However, quantitative evaluation on influence of the additives on mechanical properties of SEI is still challenged.
With this background, we have developed an experimental approach to characterize elastic modulus, yield stress, inelastic deformation behavior, and crack density evolution of SEI formed with carbonate-based electrolytes. An SEI (~100nm) is prepared by lithium thin film - electrolyte (1.2M LiPF6 in ethylene carbonate) reactions on a rectangular free-standing polydimethylsyloxane (PDMS) membrane (~300 - 400nm in thickness). The prepared sample is subjected to bulge testing in an inert environment; various level of controlled pressure is applied to the SEI/PDMS membrane and the corresponding deflection is measured by the atomic force microscopy (AFM). The plane strain elastic modulus and the yield stress of SEI are evaluated from the pressure-deflection relation from the bulge testing. Moreover, a careful observation of SEI surface topography yields the evolution of crack density as a function of applied strain. The experiment is repeated using FEC added electrolytes to investigate the influence of the FEC additive on mechanical stability of SEI.

Multivalent ion batteries (MVIB), or those utilizing Mg, Ca, Zn, and Al, are garnering increasing attention as alternatives to Li-ion batteries in applications where portability is not an issue owing to their high energy density, cost efficiency, and abundance. However, the lack of suitable electrolytes allowing for the reversible plating of metallic anodes has limited the development of MVIBs, especially those involving Ca. This is primarily due to the fact that the solid-electrolyte interphase (SEI), a passivating layer which forms between the electrolyte and anode, often does not allow for the migration of ions in MVIBs. [1,2] In this work, we develop an understanding of the SEI in MVIB systems using a computational approach combining density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. [3] We first identify the principle components of the SEI by studying the decomposition of the solvents and salts comprising various electrolytes on Li, Ca, and Al surfaces using AIMD. Following this, we identify electrolytes which can be used with a Ca metal anode by investigating the diffusion of Ca ions through the likely inorganic compounds produced using DFT. Finally, we investigate the decomposition of these electrolytes in the presence of external electric fields to more fully understand these reactions in electrochemical systems. We anticipate the promising new electrolytes proposed in this work will help guide experimentalists in the development of rechargeable MVIBs.

J.Y. and M.S. were supported by funds from Binghamton University. DFT calculations were performed on the Spiedie cluster at Binghamton University, as well as the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF Grant No. ACI-1053575, under allocations TG-DMR170127 and TG-DMR180009.

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[3] J. Young and M. Smeu, J. Phys. Chem. Lett. 9 3295 (2018)

Room temperature ionic liquids (ILs) have recently emerged as highly promising electrolytes for a wide range of emerging energy technologies, including next-generation supercapacitors and ion-batteries, due to their high thermal stability, ionic conductivity and wide electrochemical windows. The chemical and structural diversity of ILs creates a vast design space that could be exploited to optimize the device performance and stability. However, many mechanistic details remain enigmatic, including the fundamental nature of the cation-anion interactions and their relevance in determining structural and electronic properties of the liquids. Having this detailed information for the bulk liquid is a prerequisite for eventually deciphering the complexity the arises at nanostructured electrode interfaces, which are ubiquitous among energy storage devices. In this presentation, we combine high-level first-principles simulations and synchrotron X-ray characterization experiments to unravel the key structural, chemical and electronic properties of several archetypal ILs comprised of imidazolium-based ILs. In particular, we utilize extensive ab initio molecular dynamics simulations to probe the local density distribution and medium-range order of the ILs, which can be directly compared and validated by X-ray scattering measurements. Soft and tender X-ray absorption spectroscopy at the K-edge of fluorine, phosphorus, and sulfur contained on the anion also complements the chemical and electronic picture from the simulations. Our integrated theoretical and experimental approach relates these structural and chemical signatures with the intrinsic cation-anion interactions, by considering ILs with anions having significant differences in the molecular geometry, chemical composition, and charge distribution.

This work was supported by the U.S. Department of Energy at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Electrocatalytic CO₂ reduction has the dual-promise of neutralizing carbon emissions in the near future, while providing a long-term pathway to create energy-dense chemicals and fuels from atmospheric CO₂. The field has advanced immensely in recent years, taking significant strides towards commercial realization. While catalyst innovations have played a pivotal role in increasing the product selectivity and activity of both C1 and C2 products, innovations at the systems level have resulted in the scaling up of CO₂ reduction processes to industrially relevant current densities. This has been achieved by using membrane-electrode assemblies, gas-diffusion electrodes and pressurized systems to provide ample CO₂ to the catalyst. The immediate gains in performance metrics offered by these system-integrated catalytic configurations, however, go beyond reduced system losses and high current densities, suggesting that different reaction pathways and improved kinetics are possible when catalysts are tested under these conditions.
In this work we discuss the underlying reasons for the observed changes in catalytic activity between low and high current density studies. In particular, nanoscale transport phenomena is combined with experimental observations to show that the local reaction environment around a CO₂ reduction catalyst is substantially different at low current densities (<20 mA/cm²) from those at practical applied current densities (>200 mA/cm²). Specifically, the combined requirements for both CO₂ and protons (H+) at a catalyst’s surface in CO₂ reduction leads to substantial cation, anion and pH gradients at the surface. At substantial current densities this results in alkaline reaction conditions at the catalyst’s surface regardless of the chosen material, configuration or electrolyte. These conditions alter the binding energy of reactants and intermediates as compared to low current operating conditions, resulting in different product distributions and reaction efficiencies than are observed in the classical H-cell testing configuration.
Further, we extend these findings to design a practical electrochemical cell configuration (e.g. large electrode areas of >100 cm² operating at current densities of 200 mA/cm²). We find that while the majority of catalytic materials are tested under ideal ‘inlet’ conditions (e.g. saturated CO₂, ideal electrolyte conditions, no reaction products), a catalytic material also needs to be designed to operate with high performance under the ‘outlet’ conditions of an electrochemical device (e.g. the segment of active catalyst area near the outlet of the cell). We discuss how the deviation between inlet and outlet conditions has large implications for the required functionality of the catalyst, and the maximum size of a CO₂ reduction unit cell.

Transition metal Ni-based catalysts are potential candidates to replace expensive and scarce noble metal-based oxygen evolution reaction (OER) catalysts such as RuO₂ and IrO₂. Herein, we report a facile and environment-friendly method to synthesize core-shell nanoparticles consisting of Ni₃P core and NiFeOOH thin shell homogeneously studded in N-doped carbon matrix (Ni₃P@NiFeOOH/C). The obtained Ni₃P@NiFeOOH/C catalyst exhibits excellent OER activity and stability in both alkaline and neutral electrolytes, and is among the most active OER electrocatalysts yet reported in literature. Ni₃P@NiFeOOH/C reaches a 10 mA cm^-2 current density at a potential of 1.49 V (vs. RHE) in alkaline electrolyte and achieves 1 mA cm^-2 at 1.56 V (vs. RHE) in neutral electrolyte, respectively, outperforming the noble metal oxide catalysts, RuO₂ and IrO₂. The superior performance can be ascribed to the synergetic effect of Ni₃P supporting catalyst, NiFeOOH shell, and N-doped carbon matrix. A two-electrode electrochemical water-splitting device combining Ni₃P@NiFeOOH/C OER catalyst with Ni₃P hydrogen evolution reaction (HER) catalyst delivers a stable current density of 10 mA cm^-2 at 1.59 V and 1.75 V for over 17 h in alkaline and neutral electrolyte, respectively. Our design and synthesis strategy offer insights for developing new highly active electrocatalysts for water splitting and CO₂ reduction.

This presentation presents an ionic-transport measurement in aqueous electrolytes that are confined within the cavities of nanoporous solids. Ionic transport plays an important role in electrochemical energy storage devices. Many studies have reported high ionic transport in nanoporous materials, for example, in porous carbon and in metal-organic framework materials. However, very little is understood how the confinements affect ionic transport. In this presentation, we present our measurement of the ionic transport through different nanoporous materials with controllable pore sizes and surface charges. We use these experimental measurements to establish the ionic transport ability and the role of electrical-double layer on ionic diffusions. These results provide insights into the elusive role of the electrical-double layer on confined ionic diffusions and a design strategy of future nanoporous materials for energy storage devices.

Proton exchange membrane fuel cells (PEMFCs) has attracted tremendous attentions as energy conversion device due to its high energy density, low operating temperature and environmentally friendly emission. Numerous efforts have been made to explore efficient catalysts for the reaction when valuable progress brings the large-scale commercialization of PEMFCs to the table. However, durability of the PEMFCs hindered the commercialization process. Here, we reported a simple, low-cost and readily scalable method to mitigate this effect by the incorporation of graphene oxide (GO). In our study, GO is deposited on the Nafion membrane or into the catalysts layer. The maximum power output of the cell under H₂/air atmosphere showed an enhancement over 20%. More importantly, the durability of PEMFCs was significantly improved by the GO introduction. 26.1% of maximum power degradation was observed for the cell without GO while only half amount decrease obtained for the cell with GO after 30K cycles of accelerated stress test based on DOE2020 protocol. Nearly 100% enhancement for the durability of PEMFCs can be attributed to the prohibition of H₂O₂ production. The promising durability promotion effect induced from low-cost GO involvement will help to accelerate the large-scale commercialization of PEMFCs.

Environment friendly, low-cost and high performance energy storage systems have been progressively required because of the global warming which has become an imperative and unavoidable factor. The increasing environmental setbacks along with the depletion of fossil fuels necessitate the tradition of solar photovoltaic and wind power as a source of electrical energy in practice[1, 2]. But, the unavailability of power from photovoltaic systems during night time and fluctuation in wind speed of the deliverable power have directed the researchers towards the usage of energy storage devices. Moreover, to maximize the productive use of electric vehicles, it is very essential to meet up with the intermittent energy needs and variable power demands. Correspondingly, high power delivery and long cycling stability is some of the most important criterion to be fulfilled by an energy storage device.[3]. To accomplish these demands, devices which could entail huge initial power to start up and show capability of charging with quicker rate are prerequisite. Therefore, supercapacitors have been explored as an upgraded energy storage devices to replace batteries and conventional capacitors with superior features[4].
In this work, different sized CeO₂ nanoparticles were synthesized using one step low-cost solvothermal method with various reaction time. Defect states were induced due to the reduction of Ce⁴⁺ into Ce³⁺ valence state. X-ray photoelectron spectroscopy results recommend that Ce³⁺ valence states and defects in the form of oxygen vacancies be present on the surface of CeO₂ nanoparticles. Such availability of oxygen vacancies provided high specific capacitance 142.5Fg^-1 at a current density of 0.25Ag^-1 in three electrode system using 1 M Na₂SO₄ electrolyte. There is an increase in faradaic reactions taken place on the surface which is attributed to the high surface area, more oxygen vacancies, and increased diffusion rate. The highest energy density is obtained to be ~12.68 Wh/kg, and the stability result confirmed that the capacitance retention is ~75 % after 1000 cycles of operation indicating that well optimized CeO₂ is a potential candidate as electrode materials for supercapacitor applications due to their fast mutation between Ce⁴⁺ to Ce³⁺ oxidation state.

Reference
[1] Xu Y et al., Journal of Materials Chemistry A 2014;2:16480-8.
[2] Liu M et al., Journal of Materials Chemistry A 2014;2:2555-62.
[3] Chiam S et al., Scientific reports 2018;8:3093.
[4] Wang F et al., Chemical Society Reviews 2017;46:6816-54.

One of the best catalyst for alkaline OER anode, nickel exist in various form of nickel oxide, hydroxide and oxyhydroxide in alkaline aqueous solution. Particularly facet-controlled surface of β-NiOOH is expected to have the best heterocatalytic performance. However, due to the great stability of layered structure of hcp crystal, Ni(OH)₂ or NiOOH naturally prefer sheet-shape which is fully coordinated but difficult to form uncoordinated facet enclosure. Moreover, β-NiOOH inevitably oxidize to γ-NiOOH in alkaline OER condition. Herein, we demonstrate {111}fcc facet controlled Ni nanoparticles and characteristic heteroepitaxy between {111}_fcc facet of rock salt NiO and layered Ni(OH)₂/NiOOH during the sequential oxidation in alkaline OER. As the result, {100}_hcp faceted β-NiOOH surface formed on Ni octahedral nanoparticles that have resistance further oxidation. β-NiOOH/Ni octahedral nanoparticles showed excellent electrocatalytic activity and stability for OER in an alkaline electrolyte, requiring an overpotential of 0.32 V at 10 mA cm^-2 after 2 h of chronopontentiometric stability test. The facet-dependent heteroepitaxy of ionic crystals is responsible for the excellent electrocatalytic activity and stability.

Pd and Ru, which are two neighbouring elements of Rh, are highly attractive for their wide applications in catalysis and energy area. However, the two metals are immiscible at the atomic level in bulk state even at the melting point of Pd. Very recently, our group synthesized PdRu non-equilibrium solid-solution nanoalloys over the whole composition range and demonstrated their applications in CO oxidation, formic acid electrooxidation and Suzuki–Miyaura cross-coupling reaction.[1-3] In view of both fundamental research and practical application, the influence of particle size on catalytic performance should be an interesting and significant subject. The surface area, electronic state and adsorption/desorption energy, which are important factors in catalysis, will change with size, especially in the sub-10 nm size range. Since Pd and Ru are immiscible metals in their bulk form, it is difficult to simultaneously control the metal composition and size of the PdRu solid-solution nanoparticles. To date, there is no report on the size effect in PdRu solid-solution system. Herein, fixing the Pd/Ru molar ratio at 1:1, we successfully synthesized Pd_0.5Ru_0.5 solid-solution nanoparticles from 2 to 15 nm with narrow size distribution via a simple one-pot reaction. The relationship between size and catalytic properties is discussed.
References:
K. Kusada, H. Kobayashi, R. Ikeda et al. J. Am. Chem. Soc. 136 (2014) 1864-1871.
D. Wu, M. Cao, M. Shen et al. ChemCatChem, 6 (2014) 1731-1736
M. Kutubi, K.Sato, K. Wada et al. ChemCatChem, 7(2015), 3887-3894

Efficient electro-catalysts are highly demanded in many energy conversion devices, such as proton exchange membrane fuel cell, rechargeable batteries, water splitting devices, etc. Electrocatalytic process can be seen in this way¹: electrons are transported to (or removed from) the catalysts under external voltage, then catalysts deliver electrons to (or attract from) the surface active sites and then to the reactants, the reactants receive (or donate) electrons and proceed the catalytic reactions. Thus the methods to enhance electrochemical activities mainly rely on four aspects: (1) enhancing the conductivity of catalysts; (2) increasing the intrinsic activities of active centers; (3) augmenting the amount or concentration of active centers; (4) optimizing the mass transfer during the reaction. Tuning one or multiple factors in these four could help to enhance the electrocatalytic activities.
On the other hand, reducing costs while maintaining high activity and stability of the catalysts is also vital to applications. Guiding by the aforementioned aspects, we combined the virtue of zeolitic imidazolate frameworks (ZIFs, potential active centers) and commercial carbon black (CB, good conductor) to realize efficient and cheap oxygen reduction reaction (ORR) catalysts in alkali solution^2,3. The modified CB (MCB) was prepared by sequentially soaking the CB in cations (Co²⁺ and/or Zn²⁺) and ligand (2-methylimidazole) solutions, which was followed by a pyrolysis process. The sequential soaking process enabled a thin-layer coating of ZIF on the CB, and the amounts of ZIF were trace which could contribute to the great reduction of the costs (the total costs of the catalysts were less than 70RMB/kg). After the pyrolysis process, the obtained MCB showed large diffusion-limited current density (6.18mA/cm²), half-wave potential (0.858V vs RHE), and no obvious decay after 20000 cyclic voltammetry cycles. The MCB also displayed high resistance to methanol poison.
The combined experimental and theoretical studies illustrated that the C-O bond formed on the CB surface by the modification process was the main reason for the high activity, but not single-atom implanted carbon structures or metal oxides. The pristine CB surface had a large content of C=C double bonds or sp² carbon, while the MCB had a larger content of C-O bonds which are more active in ORR, and the activity increased with the increasing content of C-O bonds. On the other hand, when this kind of materials is in acidic conditions, the C-O active sites would be protonated and lose activity.


1. B. Ni, K. Wang, T. He, Y. Gong, L. Gu, J. Zhuang, X. Wang, Adv. Energy Mater., 2018, 8, 1702313
2. B. Ni, C. Ouyang, X. Xu, J. Zhuang, X. Wang, Adv. Mater. 2017, 29, 1701354
3. C. Ouyang, B. Ni, J. Zhuang, H. Xiao, X. Wang, submitted.

The production of ammonia (NH₃) by the well-known Haber-Bosch process from N₂ and H₂ has marked over a century success for providing > 80% nitrogen source for fertilizer and an alternative energy carrier with large energy density. In spite of the natural abundance of N₂, the high bond energy of NΞN (940.95 kJ mol⁻¹) prevents it as a reactive form and thus requires a significant amount of the global energy cost annually. In addition, the use of fossil fuels to produce H₂ reactant also leads to a significant level of CO₂ release. The electrochemical N₂ reduction reaction (NRR) can be processed in ambient conditions and use inexpensive aqueous electrolytes as proton source, and thus is regarded as a promising approach. However, the development of high selective electrocatalysts for simultaneously producing NH₃ rather than the competing hydrogen evolution reaction (HER), is still challenging. Herein, combined with activity of Ti³⁺ species of titanium oxides, and more strong N-adatoms binding strength than H-adatoms on the surface of Ti and Zr, we developed a type of hybrid oxide Zr-doped TiO₂ nanotubes via a facile wet-chemical process towards a NRR catalyst. N₂ and water were used as nitrogen and protons sources, respectively. The replacement of Ti⁴⁺ with larger diameter of Zr⁴⁺ leads to the formation of adjacent oxygen vacancies and the increase of Ti³⁺ concentration. In contrast, further increase of the dopant size (such as Ce⁴⁺) is not capable of incorporating dopants into the original TiO₂ lattice structure. As a result, the Zr-doped TiO₂ NTs exhibited excellent catalytic ammonia synthesis performances (NH₃ yield: 9.07 µg h⁻¹ cm^-2cat., Faradaic efficiency: 4.83%) at -0.45 V vs. reversible hydrogen electrode (RHE) in 0.1 M KOH electrolyte at under ambient conditions. This size-dependent doping strategy suggests an attractive means in tuning the active catalytic centers of electrocatalysts.

Water splitting provides a clean and renewable way to produce high-purity hydrogen, but the slow kinetics and poor stability of electrocatalysts limit its practical application. Here we report a class of open hollow Co-Pt nanoclusters embedded in N-doped carbon nanoflake arrays aligned on carbon cloth (Co-Pt/C NAs, 2.5 wt% Pt), which display low overpotentials (50mV for hydrogen evolution, 320 mV for oxygen evolution, vs RHE) in alkaline media. The high performance arises from the unique nanostructure and the synergy of Co-Pt promoting water dissociation demonstrated by density functional theory (DFT) calculation results. It can be directly utilized as a highly efficient bifunctional electrocatalyst for overall alkaline water splitting and outperform noble-metal-based materials in terms of much lower operation voltage (1.54 V at 10 mA cm^-2) and higher stability (no degradation at constant current or voltage up to 120 hrs.), representing a highly promising electrode for electrochemical energy conversion.

Nowadays the need for new energy sources required to let us overcome the intensive use of fossil fuels is more actual than ever. Some efforts have been focused on the development of a system to produce and store hydrogen in an affordable way; right now, though, this gaseous fuel is still produced by the decomposition of biomass.
A strategy then being adopted by scientists is to develop new materials capable of producing hydrogen directly from the catalytic reduction of water, a process commonly known as water splitting. Being hydrogen a very powerful combustible that only produces water when burned, giving rise to a so-called hydrogen cycle, it could be the green fuel of the future.
In the field of nanotechnologies, among some of the most interesting materials studied for this very purpose are Polyoxometalates (POM), a class of polyanionic clusters with rich redox and photochemical properties that can be opportunely tuned modifying their structure and composition [1]; despite several reports confirming their ability to catalyze water decomposition, their main drawback lies in their sensibility to light, limited only to the UV part of the spectrum. In order to make use of their appealing properties via the employment of the full solar radiation, several strategies have thus been approached. In this regard, many efforts have been made to obtain an effective coupling with different photosensitizers [2]; with this in mind, the recent discovery of Carbon Dots (CDs) opens completely new perspectives.
CDs are a novel class of carbon-based nanostructures, very cheap to synthesize, often non-toxic and that usually show an intense and tunable fluorescence when excited by both UV and visible light. Also, it has been reported they possess a remarkable tendency to transfer their excitation state when coupled to other species [3]. Making use of the broad distribution of molecular moieties found on CDs outer shell which allows an electrostatic interaction, we have been able to obtain a static coupling between these two species, as shown both by steady state and time resolved spectroscopy.
This nanohybrid has thus been used to prepare photoelectrodes that, studied in a common photoelectrocatalytic cell, show activity when irradiated by UV-Vis light; such a device could then be employed for the solar driven photocatalytic splitting of water, providing molecular hydrogen which could be used as a green energy source alternative to fossil fuels.

[1] T. Yamase, X. Cao, S. Yazaki, J. Mol. Catal. A: Chem. 262, 119 (2007)
[2] D. Schaming, C. Allain, R. Farha, M. Goldmann, S. Lobstein, A. Giraudeau, B. Hasenknopf and L. Ruhlmann, Langmuir 26(7), 5101 (2010)
[3] A. Sciortino, A. Madonia, M. Gazzetto, L. Sciortino, E. J. Rohwer, T. Feurer, F. M. Gelardi, M. Cannas, A. Cannizzo and F. Messina, Nanoscale 9, 11902 (2017)

Carbon dioxide (CO₂) is one of the main greenhouse gases accumulating from fossil fuel consumption has been caused urgent energy crisis and serious global warming problem,which represent two major challenges of the world. The electrochemical reduction of CO₂ into value-added chemicals and fuels provides both an attractive strategy for industrial-scale means and a candidate for energy storage. Copper (Cu) has been known as an unique metal catalyst in its ability of directly conveting CO₂ into a high quantity of fuels from the electrochemical reduction of CO₂. However, poor selectivity and activity degradation are two remaining restrains for practical application. Upon most occasions, the CO₂ molecules obtain an electron to form CO_2^- intermediate has been regarded as a rate-determining step. However, the *CO intermediate is the key role to obtain deep reduction products. For instance, to formation of C₂H₄ , the selectivity-determining step is the generation of the *COH intermediate by protonation of *CO and the subsequent converting into *CH₂ intermediate, which can gain ethylene through non electrochemical *CH₂ dimerization. Conversely, when the ^*CO intermediate is more and strongly enough bound on the befitting facets of Cu catalyst and the adsorbed ^*CO species couple with each other to yield ^*CO dimer, which will hydrogenate with the ^*CO dimer that can tailored by adjusting the applied potential and the subsequent to generate ethanol. To investigate the absorbed CO on the copper surface and discuss different reaction pathway through the ^*CO intermediate. We designed facile electrochemical deposition strategy to synthesize Cu nanodendrites by carbonyl coordination to expose the adsorbed CO site. Meanwhile, the adsorbed CO severely restricts hydrogen formation and the ^*CO intermediate adsorption strength can tailored by adjusting the applied potential.

The severe environmental pollution and the rapid development of renewable energy have stimulated the demand and research of energy storage technology in recent years. Lithium ion battery has huge demand for all kinds of electronics and energy storage devices. However, the lithium resource is rare, which will result in high cost and cannot meet the development in future. Sodium-based energy storage, as a potential alternative, has attracted enormously attention because of the abundance nature and similar storage mechanism.

Two-dimensional (2D) layered transition-metal dichalcogenides has been regarded as highly promising electrode materials for fast-rate Li-ion and Na-ion batteries. Monolayer or multilayer MoS₂ nanoflakes have been employed for metal ion batteries, but the material suffers from poor cyclic stability due to damage of the layered structure in a decomposition reaction. Herein, we judiciously synthesize ultrathin MoS_2-xSe_x nanoflakes quasi-vertically aligned on the 3D graphene-like carbon foam backbone (the obtained material is referred to as MoS_2-xSe_x/GF). The MoS_2-xSe_x/GF electrode with a Se concentration (x=0.9) exhibits enhanced rate performance with a higher reversible capacity and capacity retention compared to pure nanoflake MoS₂/GF electrodes. Quantitative analysis reveals that the improved pseudocapacitive contribution, derived from enlarged interlayer spacing by selenium substitution, is the origin of good rate performance. We also investigate the decomposition reaction of MoS_2-xSe_x with in-situ Raman and ex-situ XRD measurements in different potential ranges (0.01-3.0 V and 0.5-3.0 V vs. Na⁺/Na), which reveals that the 2D structure in MoS_2-xSe_x can be preserved due to Na-ion intercalation process in the potential range above 0.5 V. Discharge to 0.01 V leads to damage of the 2D structure and aggregation. So we can maintain the 2D layered structure and thus significantly improve the capacity retention by choosing appropriate potential ranges. This study sheds new light on better understanding of the metal ion storage mechanism of 2D transition metal chalcogenides that are being widely investigated.

Developing substituent of monometallic platinum (Pt) precious metal as promising cathode catalysts in direct methanol fuel cells (DMFCs) have attracted great interests underlying sustainable energy applications. Numerous strategies have been adopted to produce active Pt-based binary and ternary alloys to overcome the faced challenges in oxygen reduction reactions (ORR). To this end, we have introduced a facile strategy to develop an efficient electrocatalyst with enhanced ORR performance and negligible methanol crossover effect to maintain the overall cell voltage as a stable in DMFCs. Surface tailoring of mixed-phase manganese-cerium oxides were induced by fabrication of ultra-low and smaller sized Pt nanoparticles by capping agent free dry-chemistry reduction process. The geometry and structural analysis of as-obtained electrocatalyst were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS). In addition, the precise surface tailoring was screened out by HAADF-STEM element mapping and atomic force microscopy (AFM). The cathode catalyst performance of Pt/Manganese-cerium oxides nanocomposite was analyzed by electro-reduction of oxygen with and without presence of methanol in alkaline medium to evaluate the negligence of methanol crossover from anode to cathode. The higher ORR performance with excellent methanol tolerance behavior compared with commercial Pt/C catalyst proved the promising nature and applications of present material in DMFCs.

Proton exchange membrane (PEM) electrolyzers are a promising route to large-scale energy storage. Platinum is the state-of-the-art hydrogen evolution reaction (HER) catalyst used in commercial PEM electrolyzers today owing to its excellent activity and stability. However, the cost and scarcity of Pt motivate research into non-platinum group (NPG) electrocatalysts. For decades, electrocatalysis research has focused on developing cheaper, efficient HER catalysts to reduce the capital cost of PEM electrolyzers¹. However, a gap in this research has been demonstrating the stability of a non-platinum group (NPG) HER catalyst in a commercial-grade electrolyzer operating under conditions much different than lab-based tests².

Cobalt phosphide is among the most active NPG catalysts for HER and has been well-studied and characterized in the literature³. In this work, we develop a synthesis technique to prepare large batches of nanoparticulate CoP on a high surface area carbon support. In this form, we were able to directly integrate the nanomaterial into an 86 cm² commercial PEM membrane electrode assembly (MEA) electrolyzer without making significant changes to the fabrication process. This CoP PEM MEA demonstrated excellent stability by maintaining a nearly constant voltage for > 1500 hours of continuous operation at a current density of 1.86 A.cm^-2 and elevated temperature and pressure. To the best of our knowledge, this is the first demonstration of a stable NPG HER catalyst in a commercial PEM MEA electrolyzer.

For the purpose of energy storage, PEM electrolyzers will likely operate intermittently, following the flux in renewable power generation. We therefore also investigate lab-based accelerated stress tests (ASTs) to probe degradation of CoP to better understand failure mechanisms and predict catalyst performance when translated from the lab to a commercial device. Pre- and post-test characterization techniques such as x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and inductively coupled plasma mass spectrometry (ICP-MS) were used to observe and quantify degradation. Establishing standard lab-scale ASTs for HER catalysts is essential for benchmarking stability across a wide range of materials.

References
1. Anantharaj, S. et al. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 6, 8069–8097 (2016).
2. Vesborg, P. C. K., Seger, B. & Chorkendorff, I. Recent development in hydrogen evolution reaction catalysts and their practical implementation. J. Phys. Chem. Lett. 6, 951–957 (2015).
3. Callejas, J. F., Read, C. G., Roske, C. W., Lewis, N. S. & Schaak, R. E. Synthesis , Characterization , and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction. Chem. Mater. (2016). doi:10.1021/acs.chemmater.6b02148.

Currently considerable efforts are made to develop renewable ways to replace the fossil-based route for producing hydrogen. As a well-studied option, water photoelectrolysis is restricted by its low energy efficiency and low output. Water electrolysers are promising for producing renewable hydrogen based on acidic proton-exchange membranes (PEMs) or alkaline water electrolyser. However, the electrode materials of the PEM electrolysers contain platinum group metals (PGMs), which are relatively scarce and expensive thus hindering mass commercialization. By contrast, both cathode and anode materials for alkaline water electrolysers can be produced from non-noble transition metal compounds, suitable for large scale production. However, the oxygen evolution reaction (OER) in alkaline solution is still sluggish and needs a high overpotential (>350 mV) to reach a current density of 10 mA cm^-2. Developing the right materials with favorable structures and properties can help improve the OER performance of catalysts in alkaline solutions.

As reported by Chatenet group¹, magnetic nanoparticles generate magnetic heating under high-frequency alternating magnetic fields. The localized heating to the catalysts in an electrolyser can considerably reduce the overpotential for OER. Besides, introducing Fe into Ni-based materials may improve its conductivity and lead to an activation effect on Ni, resulting in a dramatically enhancement of the OER performance.² In addition, hierarchical structures could facilitate the electrochemical catalysis process, with active sites locating at the micro- and mesopores (nanopores) and the macropores promoting facile diffusion of species towards these active sites.³

Here we report a synthesis method that Fe was introduced into the oxide precursor (NiMoO₄) by an ion exchange method to form Prussian blue analogues (PBA, KNiFe(CN)₆) which was further transformed into hierarchical hollow sub-microwires composed of NiFe metal nanoparticles without the need for hydrogen post-treatment. The obtained hierarchical NiFe hollow sub-microwires, with a diameter of ~200 nm, had a high specific surface area (53 m² g^-1) which provided lots of active sites. These sub-units--NiFe nanoparticles, with a size of 3-5 nm, acted as precursors and experienced activation to form NiFe oxide in situ on the surface, favorable to OER.

This work introduces a facile and versatile way to introduce Fe into the precursor by an ion exchange method from different transition metal oxides, and also opened up new routes for synthesizing magnetic hierarchical hollow sub-microwires based on metal nanoparticles by directly annealing from their corresponding PBA precursors without introducing explosive hydrogen for heat-treatment.

1. C. Niether, et al., Nature Energy, 2018, DOI: 10.1038/s41560-018-0132-1.
2. L. Trotochaud, et al., Journal of the American Chemical Society, 2014, 136, 6744-6753.
3. T. Panagiotis, et al., Angewandte Chemie International Edition, 2016, 55, 122-148.

Effective surface protection of lithium metal anode is the enabling factor for next-generation high-energy batteries. However, the exacting requirements on the stability, mechanical properties and homogeneities of the protection layer hinder the realization of an ideal artificial interface. Among all the material choices, diamond with its renowned mechanical strength and exceptional electrochemical inertness is a prime candidate for lithium metal stabilization. Herein, by special synthetic techniques and rational design, we successfully rendered this desirable material compatible as lithium metal interface, which could strictly satisfy all the critical requirements. By fabricating high-quality nanodiamond film with long-range homogeneity but weak adhesion to the current collector, lithium can be electroplated solely underneath the film and effectively protected from parasitic reactions with liquid electrolyte. Notably, the nanodiamond interface possessed the highest modulus of all the lithium metal interfaces reported so far (>200 GPa), which can effectively arrest dendrite propagation to afford a dense deposition morphology. And the good flexibility of the thin film can accommodate the volume change of electrode during cycling. Importantly, since pinholes and mechanical damages during cycling are the main failure mechanisms of the artificial coatings developed so far, a unique double-layer nanodiamond design was proposed for the first time to enhance the defect tolerance of the nanodiamond interface, which enabled more uniform ion flux and mechanical properties as confirmed by both simulation and experimental results. Thanks to the multifold advantages of the double-layer nanodiamond interface, high Coulombic efficiency of over 99.4% can be obtained at a current density of 1 mA cm^-2 and an areal capacity of 2 mAh cm^-2. Moreover, with ~250% excess Li, more than 400 stable cycles can be realized in prototypical lithium-sulfur cells at a current density of 1.25 mA cm^-2, corresponding to an average anode Coulombic efficiency of above 99%.

Sodium (Na) is most appealing as a replacement for Li batteries owing to its high similarity in chemical and physical properties as Li, low cost, high natural abundance and accessibility. However, the extremely high reactivity of Na metal with organic electrolyte leads to the formation of unstable solid electrolyte interphase (SEI) and growth of Na dendrites upon repeated electrochemical stripping/plating, resulting in poor cycling performance and serious safety issues. Here in this paper, highly stable and dendrite-free Na metal anodes over a wide current range and long-term cycling were presented by employing free-standing graphene films with tunable thickness (through controlling the number of graphene layers) on Na metal surface. To reveal the critical role of graphene thickness in detail, we systematically investigated the dependence of Na anode stability on the graphene thickness at different current densities and capacities. We discovered that the optimal cycling performance of Na anode highly depends on the thickness of the graphene film on Na surface: a thickness difference in only a few nanometers (~2-3 nm) can have decisive influence on the stability and rate capability of Na anode. We demonstrate that with a multi-layer graphene film (~5 nm in thickness) as a protective layer on Na metal surface, a stable Na cycling behavior was first achieved in carbonate electrolyte without any additives over 300 hours at a relatively high current density of 2 mA cm^-2 with a high cycling capacity of 3 mAh cm^-2. We believe that our facile and cost-effective method could be a viable route towards high-energy Na battery systems, and can provide valuable insights into the Li batteries as well.

Nanostructured materials have been found to be critical in promoting the performance of energy storage and conversion devices, such as batteries. Here, Si anodes for Li-ion batteries (LIBs) and high capacity alloy anodes for Na-ion batteries (SIBs) are discussed as examples for the importance of design electrode materials at nanoscale.
For LIBs, we designed porous structured Si-C composite materials as high performance anodes. In one effort, the large mesoporous silicon sponge of tens of microns prepared have controlled porosity and pore size, which can limit the particle volume expansion at full lithiation to ~30% and prevent pulverization of bulk particles. The electrodes with the loading of 1.5 mAh per cm2 demonstrated ~92% capacity retention over 300 cycles. The composite electrodes of porous Si and graphite (~3 mAh per cm² loading) with a specific capacity of ~650 mAh per gram demonstrate ~82% capacity retention over 450 cycles. In another effort, hierarchical structured Si-MWNT microspheres developed not only have good porous structure to accommodate the volume expansion and achieve ~30% apparent particle swell at full lithiation, but also demonstrate good mechanical integrity with the structure sustained up to ~220 MPa pressure. The anodes deliver a high specific capacity of ~1500 mAh per gram and 85% capacity retention over 200 cycles at the areal loading of ~3 mAh per cm².
For SIBs, yolk-shell structured Sb@C particles and pomegranate microspheres have been prepared as high capacity anodes. With well-controlled nanostructure, these materials render stable cycling performance. The Sb@C yolk-shell structure prepared by controlled reduction and selective removal of Sb₂O₃ from carbon coated Sb₂O₃ nanoparticles can accommodate the Sb swelling upon sodiation and improve the structural/electrical integrity against pulverization. It delivers a high specific capacity of ~554 mAh per gram and long cyclability with 92% capacity retention over 200 cycles. Sb@C yolk-shell microspheres by the emulsion method further improved the packing density and the cycling stability with ~99% capacity retention over 200 cycles.

Electrochemical devices, such as batteries and fuel cells, are commonly used for energy storage and conversion. Generally, such devices are operated through the separation, translocation, and recombination of electrons and ions (e.g., protons and lithium ions) between the electrodes. Building devices with roust and effective transport pathways is the key towards high power performance and long cycling life. In this context, this presentation will discuss the design and synthesis of materials, as well as the engineering of electrode structure, towards electrochemical devices with improved performance. Three main topics will be covered, including 1) the design and synthesis of anode and cathode materials, 2) novel solid electrolytes and functional separators, and 3) novel hydrogen fuel cells with multifunctional anodes for enhanced transient power and prolonged lifetime.

Battery materials exhibit high energy density by utilizing reversible redox reactions, but their slow ionic diffusion leads to long charging times. Electrochemical double layer capacitors (ELDCs) offer some advantages over batteries, including fast charging times (<1 minute) and long lifetimes (<500,000 cycles). However, ELDCs do not involve redox reactions and as a result they have lower energy densities compared to batteries. For this reason, there is widespread interest in pseudocapacitance, a faradaic process involving surface or near-surface redox reactions, that can lead to high energy density at high charge-discharge rates. In recent work, we have suggested that pseudocapacitive materials can be classified as extrinsic or intrinsic; in the latter case, pseudocapacitive behavior is dependent on particle size, as fundamental changes in redox reactions may occur in finite sized systems. This paper will review our work on identifying Li⁺ conducting nanoscale materials which exhibit pseudocapacitive behavior. One key feature associated with pseudocapacitance is that the rate of charge storage is determined by surface-like kinetics rather than semi-infinite diffusion as occurs with battery materials. In this regard, the presence of two-dimensional pathways in the structure of the oxide or sulfide material seems to be favorable for obtaining a pseudocapacitive response. In addition, when materials are reduced to nanoscale dimensions, they may begin to exhibit pseudocapacitive characteristics because of the large number of surface sites or because phase transitions which occur in the corresponding bulk materials are suppressed. MoO₂ is a good example of this behavior as micron-sized particles exhibit battery-like properties while nanosized materials exhibit pseudocapacitive responses and operate at high charge/discharge rates without decreasing the level of charge storage. Morphology is another parameter that has been used to develop pseudocapacitive responses in a variety of systems. Mesoporous materials, which possess an interconnected pore network that provides electrolyte access to thin (<15 nm) redox-active walls, lead to a pseudocapacitive response while two-dimensional nanosheets of transition metal oxides exhibit surface-controlled kinetics indicative of pseudocapacitive behavior. The ensemble of these results suggests that we can expect an increasing number of nanoscale materials to be developed that retain high energy density at charge/discharge rates which are well above those of battery materials.

Metallic lithium (Li) has attracted extensive attentions for Li-S, Li-air and solid-state Li batteries, due to its high theoretical capacity and low redox potential [1-3]. However, several challenges substantially hinder the real application of Li anodes, such as dendrite formation and unfavorable reaction between Li and electrolyte. In fact, the real problem is originated from the unavoidably reacts of Li with electrolyte and the non-uniform distribution of electrochemical active sites for Li plating/stripping during cycling.
Formation of a thin and stable protection layer with uniform and high Li ion flux on the surface of Li metal may address all of the main problems of Li anode. Sulfide-based solid-state electrolyte possesses reasonably high ionic conductivity (especially for nanostructured layer, which can achieve ionic conductivities higher than 10 mS cm^-1 at room temperature), which is a good choice for the materials of Li metal protection layer. While, the formation of a thin sulfide-based solid-state electrolyte layer, especially for in-situ or adjustable layer with nanosize and different thickness, on the surface of Li metal is still a challenge due to the high chemical active of metallic Li and the difficult synthesized conditions of sulfide-based solid-state electrolyte.
Here, we show an in-situ depositional strategy to form a sulfide solid-state electrolyte protection layer on the surface of metallic Li to address the dynamic Li plating/stripping process. Taking adjustable Li₃PS₄ solid-state electrolyte layer as an example, due to the high ionic conductivity and low electrochemical activity of Li₃PS₄, the intimate protection layer of Li₃PS₄ between Li metal and liquid organic electrolyte can not only restrain the formation of Li dendrite, but also reduce the SEI formation and prevent further corrosion of Li metal during battery cycling. Thus, excellent electrochemical performance has been achieved: (1) Symmetric cells with the Li₃PS₄ protection layer can deliver stable Li plating/stripping for 2000 h with voltage hysteresis as low as ~10 mV; (2) Full cells assembled with the Li₃PS₄-protected Li exhibit two times higher capacity retention in Li-S batteries (~ 800 mAh g^-1) at 5 A g^-1 for over 400 cycles compared to their bare Li counterparts; (3) High rate performances can be achieved with Li-Li₃PS₄/LiCoO₂ cells, which are capable of cycling at rates as high as 20 C.[4]

Reference
1. Lin D. et al. Nat. Nanotechno. 2017, 12, 194-206.
2. Wang Z. et al Chem. Soc. Rev. 2014, 43, 7746-7786.
3. Liu W. J. Am. Chem. Soc. 2016, 138, 15443-15450.
4. Liang J, Li X, Sun X, et al. submitted, 2018.

In essence, nanocrystalline metals are promising building units to realize high-capacity anodes are enabling ever-increasing gravimetric and volumetric energy densities in lithium-ion batteries (LIBs), but their fading capacities upon both pulverization and electrical disconnection caused by large volume changes during repetitive lithiation/delithiation reactions must be remedied. Another challenge is the lack of a fast and scalable process to fabricate nanocrystalline metals into real electrodes. Herein, we report graphene pliable pockets (GPPs) remedying the limitations of nanocrystalline metals for high-performance LIBs, and Metal_encapsulated GPPs (M_GPPs) can be fabricated via the ultrafast dynamic polymerization and evaporation of specific polymers on nanocrystalline metals. This process is also shown to enable scalable mass production upon increasing the batch size.

We applied the GPP structure to silicon, a most promising but also difficult to handle electrode material. Utilizing Si_GPPs with high tap densities exhibits excellent rate capability and robust cycle life. We discover that the inner graphene pliable layers allow electrical conductance to Si and the outer GPP controls formation of solid-electrolyte interface (SEI) layers, while both of them provide pliable compartments to prevent volume expansion and pulverization of Si nanocrystals during repeated lithiation/delithiation cycles. Full-cell LIBs of the Si_GPP electrodes assembled with representative cathodes of LiCoO2 (LCO), LiMnO2 (LMO), and LiFePO4 (LFP) demonstrate remarkably high gravimetric and volumetric energy densities. Moreover, Si_GPPs can be used as the battery-type electrodes for lithium-ion capacitors (LIC), such an attempt lead to a result in much faster charging/discharging and strikingly longer life performance while maintaining high energy densities. GPP structures are superior and feasible than other promising anode structures, thereby applicable to industries and attainable shortly.

In situ sodiation experiments were performed on the Na-Sn system to evaluate the rate capability and cycle stability of the anode material. Experiments showed that the sodiation rate of crystalline Sn (c-Sn) is 2-3 orders of magnitude faster than the lithiation rate of c-Si with the same diameters. Furthermore, the observed rates were nearly the same regardless of the orientation of c-Sn, causing the Sn anode to swell in an isotropic manner and thus mitigating pulverization. Here, using atomic simulations and advanced analysis techniques, we elucidated the mechanistic origins responsible for the ultrafast sodiation and isotropic swelling observed from for the Sn anode by clarifying the diffusion kinetics at the Na-Sn diffusion couple. It was found that both the crystalline-to-amorphous phase transformation at thin layers of c-Sn near the propagating interface and pipe diffusion through sodiation-induced dislocations are the two dominant structural features occurring during sodiation. These sodiation behaviors observed from the Na/Sn interfacealleviate the rate-limiting behavior of the propagating interface, while nullifying the orientation effect of diffusion in c-Sn. This promotes the Na diffusion to c-Sn at unprecedented rates and enables isotropic swelling of c-Sn. The observed phenomena provide insight into the design of anode materials for realizing batteries with high rate performance and cycle stability.

Platinum (Pt) is by far the best catalyst for the oxygen reduction reaction (ORR) occurring on the cathode of a proton exchange membrane fuel cell (PEMFC). Its low abundance, limited supply, and ever-increasing price have kept motivating researchers to minimize the loading of this precious metal in the catalyst. In this talk, I will discuss a number of strategies for greatly increasing the mass activity and durability of Pt-based ORR catalysts, including facet-controlled synthesis, increase of dispersion by forming a core-shell or hollow structure, manipulation of surface strain, electronic coupling through the incorporation of a second metal, and use of particles with an uniform, optimal size. These strategies have resulted in the development of advanced ORR catalysts, enabling the society to achieve a sustainable use of precious metals such as Pt in energy conversion, industrial catalysis, and related applications

Cost and life-time are key factors in limiting the broad adoption of polymer electrolyte fuel cells for cars. In response, we have been investigating the morphology of noble metal nanoparticles during surface deposition. In particular, we explored the influence of precursor-substrate and precursor-deposit interactions. Depositions can be improved through a variety of means, including tailoring the surface energy of a substrate to improve precursor wettability, or by modifying the surface energy of the deposits themselves. Here, we show that carbon monoxide can be used as a passivation gas during atomic layer deposition to modify the surface energy of already deposited Pt nanoparticles to assist direct deposition onto a carbon catalyst support. The passivation process promotes two-dimensional growth leading to Pt nanoparticles with suppressed thicknesses and a more than 40% improvement in Pt surface-to-volume ratio. This approach to synthesizing nanoparticulate Pt/C catalysts achieved high Pt mass activities for the oxygen reduction reaction (ORR), along with excellent stability likely facilitated by strong catalyst-support interactions afforded by this synthetic technique.

The high surface area per unit volume and large mass-transfer rates offered by porous, flow-through electrodes have resulted in their use in a wide variety of electrochemical processes, including organic electrosynthesis, water electrolysis, water treatment, fuel cells, and redox flow batteries. Many types of porous electrodes are commercially available, including carbon paper, graphite felt, reticulated vitreous carbon (RVC), metal mesh, and metal foam. Metal foam offers relatively high conductivity but a low surface area, whereas carbon paper has one of the highest surface areas but lower conductivity.

This presentation will describe the characteristics of a Cu nanowire flow-through electrode that has 15 times more surface area and is 32 times more conductive than carbon paper. The improvement in surface area is due to the small diameter of the nanowires relative to carbon fibers. The higher conductivity is due to the intrinsically higher conductivity of Cu, and the fact that the metal nanowires can be sintered together. The nanowire electrode has a porosity of 94%, but its hydraulic permeability was 89 times lower than carbon paper. For Cu ion reduction, the Cu nanowire electrode can achieve the same single-pass conversion as carbon paper at flow rates 300 times greater under mass transport-limited conditions, and 10 times greater under kinetically limited conditions. We will also report the performance of the flow-through nanowire electrodes for organic electrosynthesis and for water splitting. The high-conductivity, high surface area, and high porosity that can be achieved with metal nanowire electrodes create new opportunities for improving the performance of electrochemical systems for energy storage, hydrogen production, water treatment and the production of fine chemicals.

Improving the performance of cathodes for the electrochemical CO₂ reduction reaction (CO₂RR) will benefit from the discovery of new materials as well as the introduction of new paradigms. This presentation focuses on the synthesis and systematic study of the CO₂ reduction activity of a novel quasi-2D PdPt bimetallic ‘nanoclam’ catalyst synthesized on carbon cloth electrodes via a pulsed electrodeposition technique. These results highlight the importance of nanostructuring to improve selectivity and geometric activity for CO₂ reduction in this system through the comparison of the bulk and nanostructured PdPt. In addition, we also propose that the high activity of this catalyst at low overpotential is ideal for paring with biological systems to realize a hybrid system for CO₂ reduction.

The PdPt nanoclams have a unique tapered morphology that combines high surface area with exposure of numerous undercoordinated sites for CO₂ reduction with activity exceeding that of either Pd or Pt for CO₂ reduction to formate at 0.2 V vs RHE, which is a provocative result. In comparison with bulk Pd, PdPt, and Pt systems, we find that the interplay of multiple trends affects selectivity and activity:
1. Increasing Pt content shifts selectivity from formation of formate to hydrogen evolution in the bulk
2. Increasing Pt content increases overall activity and prevents catalyst deactivation by changing the energetics of hydride intercalation into PdPt
3. Nanostructured morphology of PdPt nanoclams increases selectivity to formate in PdPt nanoclams vs in bulk, planar PdPt.

In addition to this understanding, we report our initial results on the creation of a hybrid electrochemical-biological CO₂ reduction system in which formate and hydrogen are produced through electrochemical CO₂ reduction by PdPt nanoclams. These products are metabolized by methanogens in combination with CO₂ to yield ~100% faradaic efficiency to methane. Going forward, the integration of microbial communities with these nanostructured PdPt catalysts has the potential to combine the best-of-both-worlds from electrochemical and biological systems to achieve a regenerative catalytic system with high-selectivity, high activity, and low overpotential.

Ammonia is one of the most important chemicals used in the modern society. As an essential precursor used in the fertilizer production, the supply of ammonia is strongly associated to food furnishing.¹ Without ammonia, it is predicted that more than half of the world population would starve. However, the synthesis of ammonia is extremely challenging due to the great thermodynamic stability of the nitrogen triple bond. Currently, more than 90% of the global ammonia commodity is produced by the Haber-Bosch process which is responsible for the release of ~12 Gt (1.5% of global total greenhouse gas emisssions) of CO₂ into the atmosphere, annually.² Therefore, the synthesis of ammonia from atmospheric N₂ utilizing renewable energy sources with zero carbon footprint will become an important process in moving forward.³
Electrochemistry provides a direct pathway for N₂ conversion into NH₃ utilizing renewable electricity. Hitherto, the NH₃ electrosynthesis suffers from the drawbacks of low yield rate and selectivity (<10%), mainly due to the predominating hydrogen evolution reaction (HER).⁴ In this work, an electrochemical nitrogen reduction reaction (NRR) with a significantly enhanced selectivity and yield rate of NH₃ have been obtained via both electrode and electrolyte engineering. Surface area enhanced α-Fe@Fe₃O₄ nanorods were employed as the NRR cathode in an aprotic solvent – ionic liquid mixed electrolyte. Remarkably, a NH₃ yield rate of ~2.35 × 10^-11 mol s^-1 cm^-2 with a high selectivity of ~32% was achieved. This work reveals that the abilities to both regulate the proton availability and enhance the N2 solubility in the engineered electrolyte are imperative in achieving highly efficient NRR at room temperature and pressure.
References:
1. V. Smil, Scientific American, 1997, 277, 76-81.
2. L. Wang, M. Xia, H. Wang, K. Huang, C. Qian, C. T. Maravelias and G. A. Ozin, Joule, DOI: 10.1016/j.joule.2018.04.017.
3. C. Guo, J. Ran, A. Vasileff and S.-Z. Qiao, Energy & Environmental Science, 2017, DOI: 10.1039/C7EE02220D.
4. A. R. Singh, B. A. Rohr, J. A. Schwalbe, M. Cargnello, K. Chan, T. F. Jaramillo, I. Chorkendorff and J. K. Nørskov, ACS Catalysis, 2017, 7, 706-709

The hybridization of electrochemical interface and microbiological synthesis combines the high efficiency of electrochemical reactions and the catalytic capability of microbes to produce valuable chemicals. Such hybrid can be used for nitrogen fixation, which consumes renewable energy such as solar electricity, and enables distributed production of environmental-friendly fixed nitrogen species. However, a fundamental contradiction occurs as the crucial nitrogen fixing enzyme, nitrogenase, is incompatible with oxygen. This prevents nitrogen fixation via nitrogenase in air, the most abundant nitrogen source. Here we report a micro-structured electrochemical interface that solves this problem. Our design enables successful nitrogen fixation under ambient atmospheric conditions. We also prove that such interface is scalable, which enables potential applications.

Both the impelling need for a constant reduction of pollution in large cities and carbon dioxide in the atmosphere, as well as the continuous increase in petrol cost, have reinforced the interest in more efficient, clean, and sustainable systems such as fuel cells (FCs) for the conversion of electrical energy. However, the present FC technology does not give till now the wished performances and the complete assurance for a long durability in all climatic conditions. The proton conducting separators preferred by cars producers are perfluorosulfonic acid (PFSA) membranes at temperatures of about 80 °C. The main target of the use of the INCA Method (Ionomer Nc Analysis) is the study and the understanding of ionomers in the conditions of use and the improvement of ionomeric membranes under operating conditions. In this presentation we will illustrate the results obtained comparing pristine Nafion 1100, oriented Nafion 1100 prepared in our laboratory, and stabilized (crystalline) Nafion 1100 membranes treated with special annealing agents. We will propose the best taylor made annealing in order to avoid critical degradations of mechanical properties and ionic conductivity due to the formation of layered morphologies, prevalently oriented in the direction parallel to the membrane surface. Hence commercial Nafion 1100 membranes were treated with dimethylsulfoxide (DMSO) for 7 days at 140 °C, to increase the thermal stability because of the formation of semi-crystalline phase (physical crosslinking), followed by hydrothermal annealing in liquid water to obtain a suitable water Uptake (WU). The results obtained with the INCA Method will be compared with the Dynamic Mechanical Analysis (DMA) and tensile stress measurements. The INCA analysis was also used to study the effect of the presence of H-bond and/or crystallinity in un-crystallized Nafion 1100 prepared from the solution. This material, prevalently amorphous, has a melting temperature (Tm) lower of about 50 °C respect to commercial Nafion 1100, but presents a high WU in the same condition of temperature and relative humidity. At the same time we will present the effect of the variation of the equivalent weight (EW) on the Tm: the lowering of EW causes a decrease in Tm of 10 °C. The knowledge of this effect on Tm is fundamental for Aquivion membranes, which have a wide range of EW. The INCA method allowed us to evaluate the behavior of Aquivion membranes in fuel cell conditions and compare them with Nafion membranes. The results permit to suggest the most appropriate thermal/hydrothermal treatments to stabilized these ionomers, where the mechanical stability play a fundamental role.

Rational design of active, durable and low-cost catalysts for oxygen reduction reaction (ORR) is highly desirable in fuel cell research. Nanoparticles with a core (Pt-based alloy)/shell (few layers of Pt skin) structure have attracted much attention due to their lower cost than pure Pt and potential benefits of activity tuning afforded by careful configuration of the core alloys. Previous theoretical work studying the enhancement effects has primarily focused on core alloys with cubic structures, i.e. disordered alloy or L1₂ ordered structure. In this work, using ab initio calculations, we have systemically investigated the structure-activity relationship of a new class of Pt_0.5M_0.5 (M=V, Cr, Fe, Co, Ni, and Cu) core alloy that has a low-temperature tetragonal L1₀ intermetallic structure. We have calculated the adsorption energies of O, OH, and OOH on various Pt skins and the underlying tetragonal structured alloys. We comprehensively explore the interaction between Pt layers and host materials to acquire the best Pt thickness associated with certain host material, tuning the ORR activity toward the peak of the ORR volcano plot. We further decompose the enhancement factor into ligand, normal and shear strain effects in these systems and clarify the origin of the improved activity of this class of catalysts. Our results could facilitate future design of ordered intermetallic FCT structure in ORR applications.

In lithium-ion batteries, the solid electrolyte interphase (SEI) is an important passivating layer formed on the anode from electrolyte decomposition products. On silicon anodes, the SEI is believed to be critical to battery reliability and performance. SEI must be both electronically insulating to prevent further electrolyte decomposition and ionically conductive to permit the flow of lithium ions. While electronic resistivity is a critical intrinsic property of SEI, characterization tools to investigate the electronic properties of this thin, reactive layer have been limited.
To advance understanding of the electronic properties of SEI, our group has developed an instrumental approach utilizing scanning spreading resistance microscopy (SSRM) to characterize electronic properties in three dimensions in the nanoscale. Resistivity vs. depth profiling results originating from this technique have shown that electronic resistivity decreases from the surface of SEI moving towards the SEI/Si interface. Moreover, total SEI thickness is readily calculated by the identification of electronically conductive silicon beneath the insulating SEI. Thickness of SEI is highly variable based on the electrolyte and the cycling conditions utilized. Additionally, two-dimensional mapping of electronic resistivity at varied depths within the SEI gives quantitative and qualitative data regarding the heterogeneity of SEI structures in the nanometer regime.
Investigation of reference materials with known electronic properties has allowed for determination of instrumental resolution. Characterization of model SEI systems with SSRM alongside transmission electron microscopy (TEM), X-ray photoelectron microscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) permits the association of electronic properties with specific SEI chemical components.

Lithium ion batteries are ubiquitous in today’s technology: they power our phones, laptops and are finding greater use for larger scale applications such as electric vehicles. Developments in sodium ion batteries are also of great current interest for use in stationary energy storage technologies such as grid storage for renewable energy sources.¹ The movement of Li⁺ or Na⁺ ions through the active electrode material in a battery is crucial for its electrochemical performance, yet quantifying this behaviour can be very technique dependant. For example, macroscopic ionic diffusion measurements can indicate greater activation energies for ion diffusion due to the extra impedance imparted by macroscopic grain boundaries. Here, we demonstrate the use of muon spin relaxation spectroscopy (µ⁺-SR) as a valuable local tool to probe the Li⁺ and Na⁺ diffusion dynamics in battery electrode materials. The µ⁺-SR method allows one to probe the nanoscale diffusion of ions via the muon spin perturbation due to diffusing ions nearby.² Developing a clearer understanding of these diffusion processes at a local scale can provide us insights into electrochemical behaviour and the opportunity to optimise and tailor performance.

We present the investigation of local ion diffusion in the polyanionic positive insertion electrode materials LiFeSO₄F and Na₂FePO₄F using µ⁺-SR for the first time. It is predicted that these structures possess multi-dimensional ionic diffusion pathways, suggesting facile ion diffusion.^3-4 In addition to enhanced ionic diffusion, these compounds display high energy densities driven by the inductive effect where the presence of electronegative species such as fluoride changes the iono-covalent nature of bonds. The results obtained from these µ⁺-SR studies indicate lower activation energies and larger diffusion coefficients at room temperature compared to values obtained by computational and bulk characterisation means. This indicates intrinsically favourable diffusion dynamics which could be accessed by improvements in, for example, engineering of materials to reduce impedance effects from grain boundaries.

References
[1] Palomares, V. 2012. Energy Environ. Sci, 5(3), pp.5884-5901.
[2] Baker, P. J. 2011. Phys. Rev. B, 84(17), p.174403.
[3] Tripathi, R., Gardiner, G.R., Islam, M.S. and Nazar, L.F. 2011. Chem. Mater, 23(8), pp.2278-2284.
[4] Tripathi, R., Wood, S.M., Islam, M.S. and Nazar, L.F. 2013. Energy Environ. Sci, 6(8), pp.2257-2264.

Although the rechargeable battery industry is currently dominated by lithium-ion battery technology, sodium-ion batteries are expected to play a key role in the near future, owing to the high abundance of raw sodium resources. Achieving high energy densities in sodium-ion batteries equal to, or exceeding lithium-ion batteries requires alloy-type high-capacity anode materials such as Sb, Sn, P, Ge [1]. However, alloying reaction of sodium with these materials result in significant phase transformation-induced stresses and volume changes, which ultimately cause a rapid capacity fading. In this work, nanoporous germanium was made by selective alloy corrosion and used to prepare sodium-ion battery composite slurry anodes. The performance of this composite electrode for reversible sodium storage was investigated by in situ electrochemical dilatometry, during which the (de)sodiation-induced macroscopic dimensional changes were recorded simultaneously during (dis)charging cycles, and for the first 200 cycles. (De)sodiation-induced dimensional changes and capacity fading were found to be proportional [2].
References
[1] Eric Detsi, Xavier Petrissans, Yan Yan, John B. Cook, Ziling Deng, Yu-Lun Liang, Bruce Dunn and Sarah H. Tolbert: “Tuning Ligament Shape in Dealloyed Nanoporous Tin and the Impact of Nanoscale Morphology on Its Applications in Na-Ion Alloy Battery Anodes”, Phys. Rev. Materials, 2 (2018) 055404.
[2] Manni Li, Eric Detsi: “In Situ Electrochemical Dilatometry Study of Capacity Fading in Nanoporous Ge-based Na-ion Battery Anode during Sodiation-Desodiation Cycling” (Under review).

Lithium-Ion batteries (LIBs) are one of the most predominant energy storage systems for portable to stationary electronic devices. The LIBs are indispensable to laptops, mobile phones, and electric vehicles due to their high energy/power density and long cycle life. Accordingly, there is a continuing increase in the technical demand for developing higher capacity/power LIBs. Especially, silicon (Si) has been intensively pursued as the most promising anode material for its high specific capacity (> 3500 mAh/g) and abundance. Despite its high capacity, Si suffers from fast capacity loss caused by its large volume change (> 300%), unstable solid electrolyte interphase (SEI) and physical disintegration (cracking and crumbling) of the electrode structure during lithiation/delithiation processes. Therefore, there are various research activities to control the physico-chemical stabilities of Si anode material. Currently, Si nanostructures such as nanowires and nanoparticle carbon composites are proved to be effective methods for improving capacity and cycling stability, since nano-sized Si can alleviate mechanical fractures during volume changes. In this work, we conducted a series of computational simulations to understand how the physico-chemical stabilities of nanostuctured Si anodes are associated with the high surface ratio of Si nanostructures. Using Monte Carlo simulations within the first-principles based reactive force-field (ReaxFF) framework, we determined atomic structures of Si nanoparticles in terms of particle size and lithiation ratio. Then, we elucidated a unique relationship between the structure of Si nanoparticle anodes and their physico-chemical properties such as theoretical charge/discharge potentials, by using the density-functional theory (DFT) calculations. Our theoretical findings will provide future guidance for developing next-generation Si nanomaterials, which can be successfully applied to commercialization of Si anode materials.

The development of flexible and conductive electrodes is crucial for rapidly evolving flexible/wearable electronic applications. For this purpose, various Ag nanomaterials with high electrical conductivity, such as nanoparticles and nanowires, have been physically deposited onto flexible substrates, but the electrical conductivity and mechanical stability of these electrodes are still restricted. Including Ag nanomaterials, commercially available cellulose papers have received great attention as promising substrates for next-generation flexible electrodes due to their high flexibility, large specific surface area, lightweight and low cost. Herein, we introduce a highly porous paper electrode with bulk metal-like electrical conductivity using room-temperature metallic fusion-assisted layer-by-layer (LbL) assembly. The newly synthesized tetraoctylammonium thiosulfate (TOAS) stabilized-Ag NPs were LbL-assembled with tris(2-aminoethyl) amine (TAA), allowing the almost perfect ligand exchange reaction between bulky/insulating TOAS ligands and small TAA molecules. The introduced small TAA molecules connect between neighboring Ag NPs, significantly decreasing the interparticle distance of Ag-Ag NPs. In addition to the minimized interparticle distance of Ag-Ag NPs, the low cohesive energy of Ag NPs strongly induces metallic fusion between closely packed Ag NPs at room temperature without any additional treatments. The resultant multilayers of (TOAS-Ag NP/TAA)_n exhibits the bulk metal-like electrical conductivity of ~ 1.60 x 10⁵ S cm^-1. When depositing the Ag NPs onto cellulose papers through our approach, the insulating papers can be converted to flexible and bulk metal-like conductive papers that can be used as 3D current collectors for high-performance energy storage devices.

Quantum dot solar cells offer a low-cost route to flexible photovoltaics, with additional effects such as the relative ease of multiple exciton generation (MEG) offering tantalising possibilities of breaking the shockley-queisser limit in the future. However, bottlenecks in device design and fabrication currently limit this technology's record efficiencies. Chief among those problems are the mobility of charges within the quantum dot layers of the device – causing a reduction in fill factor - and the need for stronger n-doping to improve open circuit voltages. One method of suppressing these problems is through ligand passivation, or combinations thereof. Current research has focused on ever-smaller ligands, down to individual halide ion passivation with dissociating organic salts, while research into the effects of substituent groups residing beyond the coordinating groups of small molecules has waned. This research begins with the introduction of novel ligand systems using electron-dense substituents outside the coordinating functional group, and follows with an exploration of the optoelectronic properties this class of ligands can provide to PbS quantum dot films, finishing with an overview of the photovoltaic performance of these systems.

In recent times, demand for portable electronic devices like mobile phones, cameras, and laptops, etc. is increasing day by day. Energy storage devices such as batteries and supercapacitors have significant importance because of their high energy density and high power density, respectively.¹ The supercapacitor is gaining a considerable amount of attention because it uses the less toxic material, offers high power density, excellent electrochemical stability, a wide range of operating temperatures and durability. Facile fabrication of low cost, efficient, stable, eco-friendly and earth-abundant electrode materials for supercapacitors is critical.² Layered double hydroxide (LDH) is a new class of material having positively charged hydrotalcite-like layers, weakly bound, intercalating charge compensating anions and water molecules, has shown tremendous supercapacitive performance.³ In this work, an ionic lamellar, two-dimensional (2D) nickel-vanadium layered double hydroxide (NV LDH) nanosheets have been synthesized via facile, cost-effective and potentially scalable hydrothermal method. The as-prepared 2D NV LDH nanosheets over carbon cloth (NVL@CC) was used as the supercapacitor electrode. The electrochemical characterization techniques such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were used to characterize the material for its electrochemical properties, while SEM, TEM, XRD, BET, and XPS, etc. techniques have been used for their morphological, structural and physical characterization. The high specific capacitance of ~2600 F g^-1 at the current density of 1 A g^-1 was observed in a three-electrode system using KOH as an electrolyte, which remained quite high at an increased current density of 10 A g^-1. This work demonstrates excellent potential for NV LDH nanosheets as an electrode material for supercapacitor applications.

References
[1] A. Tyagi, K. M. Tripathi, R. K. Gupta, J. Mater. Chem. A 3, (2015) 22507-22541.
[2] A. Tyagi, R. K. Gupta, Nanomaterials: A guide to fabrication and applications, (CRC Press), 261 (2015).
[3] K. Fan, H. Chen, Y. Ji, H. Huang, P. M. Claesson, Q. Daniel, B. Philippe, H. Rensmo, F. Li, Y. Luo, and L. Sun, Nat. commun. 7 (2016) 11981.

The interface between an electrode and electrolyte is crucial to the overall performance of lithium ion batteries (LIBs). Here we demonstrate chemical vapor deposition polymerization as a novel method to improve the performance of LiCoO₂ cathode electrode for LIBs, including both rate capability and high voltage (4.5 V) cycling stability. Three polymers are utilized to engineer the surface and CR2016 coin cells are fabricated to study the effect of these polymers on LIBs electrochemical performance. The three polymers are poly(3,4-ethylenedioxythiophene) (PEDOT), poly(divinylbenzene) (PDVB) and poly(divinylbenzene-co-1H,1H,2H,2H-perfluorodecyl acrylate (P(DVB-co-PFDA)). Our results show that the PEDOT improves both the rate and the high voltage cycling performance of LiCoO₂ electrodes, the PDVB has little effect on the both performances, while the P(DVB-co-PFDA) inhibits the performances. The PEDOT coating increases 10C rate capacity by 62% (from 34 to 55 m Ah/g). The high voltage cycling number is increased by 250% (from 40 to 100 times) when the capacity decreases to 50% of its initial capacity. X-ray photoelectron spectroscopy is applied to understand the improvement of cycling stability. The results suggest that chemical or coordination bonds form between Co in LiCoO₂ and O and S in the PEDOT film, while there is no bonds formation from the PDVB or the p(DVB-co-PFDA) coatings. The bonds stabilize the surface of LiCoO₂ and thus improve the high voltage cycling performance. This work introduces chemical vapor deposition polymerization as a new research tool for surface modification and interface engineering of lithium ion battery electrodes.

Silicon has received much attention as a promising negative electrode material, owing to its greater theoretical capacity than state-of-the-art graphite negative electrodes. Silicon, however, undergoes internal microstructural changes and large volume changes that induce substantial compressive stresses inside of particle-based electrodes. These are believed to contribute to poor cycling performance.
In the present work, in-situ stress and electrochemical measurements are monitored directly to capture stress evolution in silicon and silicon oxide composite electrodes with various particle sizes. Silicon nanoparticle electrodes reveal a stress response with notable hysteresis characterized as plastic flow. When silicon oxide(SiOx) is incorporated, the cycling performance improves and the amount of stress decreases. The stress response of electrodes based on SiOx composite particles also shows more reversible and elastic behavior compared to electrodes with silicon nano particles.

As a class of outstanding cathode material for lithium ionic (Li-Ion) batteries, LiFePO₄ has an obstacle of low rate capability due to its slow Li ionic diffusion and poor electronic conductivity as well as power density issue. To improve this, one should shorten the channels of Li-ion travelling (i.e. reduce the dimension of LiFePO₄ in <010>), and enhance the particle impact on the same crystallographic orintation. According to this hypothesis, one of the proposals is to produce 2D LiFePO₄ as the building block materials by exposing their {010} facets as the termination planes, and to further pack these 2D LiFePO₄ along the <010> direction efficiently. Herein, we report a novel synthesis of ultrathin 2D LiFePO₄ nanosheets using a modified hot organic solvent approach. The structure of the resultant LiFePO₄ is precisely controlled with some unique features: the nanosheets exhibit in shape of rectangle with a thickness of as small as ~10 nm. Importantly, the exposed planes are {010}, which can maximize the channels for the travel of Li-ions. Calcination of the oleylamine/oleic acid capped LiFePO₄ nanosheets resulted in densely hierarchical structures containing Layer-by-Layer stacked assemblies with a tap density as high as 1.3 g cm⁻³. In contrast, the tap density of freestanding LiFePO₄ nanomaterials is usually less than 1.0 g cm⁻³. The in-situ generated carbon blacks from the pyrolysis of the capping ligands act as not only a medium conductor for the electrons but also a baffle to possibly prevent the nanosheets from fusing into larger particles during the calcination. With such an in-situ carbon coating evenly on every single nanosheet, the 2D LiFePO₄/C composite nanosheets self-assembled into superstructures. The electrochemical evaluations on these LiFePO₄/C assemblies show a reversible specific capacity of as high as 105 mAh g⁻¹ at 10 C, as well as an excellent rate capability and cycling performance. Both the high tap density and high-rate capability are promising for the increase of volumetric power and energy density as Li-ion cathodes. Such layer-by-layer compacted patterns, rather than an accumulation of disordered building blocks, could also effectively increase both gravimetric and volumetric power densities of LiFePO₄ electrodes, paving the way for promoting high-rate capacity and minimizing the ionic diffusion issue by shortening the length of Li-ion travelling channels.

Controlling the electrical response at the electrode-electrolyte interface is key to the development of next-generation supercapacitors and other electrochemical storage systems. In this work, we utilize first-principles calculations to elucidate the key factors that determine the performance of C60-functionalized graphene as a promising carbon-based supercapacitor material. We show that, for the hybrid electrode, the surface morphology influences the electric double-layer (EDL) formation by affecting the charge localization character, which in turn significantly enhances the EDL contribution to the capacitance, in sharp contrast to pristine graphene. In addition, the electronic structure was found to govern not only the quantum capacitance but also the double-layer response. Our study highlights a complex interplay among surface morphology, electronic structure and functionality of the hybrid electrode, suggesting general improvement strategies for optimizing EDL and total capacitance in carbon-based supercapacitor materials.

Lithium-ion battery (LIB) is widely utilized in many modern applications as energy sources. Numerous efforts have been dedicated to increasing electrochemical performances, but improvement on battery safety remains a visible challenge. While new electrode materials have been developed, advancement in new separators for LIBs has remained relatively slow. A separator is the polymeric porous material that physically separates electrodes and allows free flow of ions through its structure. It is electrochemically inactive but essential for avoiding thermal runaway conditions. Besides its crucial functions, the separator has been known as the mechanically weakest component. Structural battery is a new approach that employs a multifunctional material concept to use LIB as a load-bearing material to minimize the weight of the complete system and maximize the efficiency. Separator materials are required to have good thermal stability, battery chemistry, and mechanical performance. This work aims at creating electrospun membranes with improved thermal resistance and structural integrity as the next generation LIB separators. The electrospinning (ES) is known as a versatile and straightforward technique to fabricate continuous fibers at nano- and micro- scales. Solution and process parameters including, type of polymer and solvent system, concentration of polymer solution, acceleration voltage, and solution feed rate have been studied to achieve the desirable membrane properties. Adding various ceramic materials to the polymers have been studied for improving the thermal shrinkage, increasing ionic conductivity and, decreasing interfacial resistance of the composite separators. In this study, nanofibrous membranes are created by the electrospinning process. Graphene oxide is used due to its high storage modulus. Synthesized, non-conductive graphene oxide and titanium dioxide nanoparticles (for comparisons) are incorporated into the polymer solutions for the electrospinning. In this presentation, we will discuss the control of electrospinning process, graphene oxide synthesis and properties of composite membranes for structural battery applications.

Mesoporous carbon materials have received intensive attention due to their wide applications including energy storage/conversion, catalysis and absorption. Herein, ultrathin and porous nitrogen enriched carbon hollow shells have been synthesized by a facile and cost effective sol-gel coating method using different ratio of carbon precursor and nitrogen precursor. As-synthesized hollow shells possessed uniform size of ~120nm in diameter and ∼4.5nm significantly thin, porous shells obtained from silica template that size controlled by solvent ratio. As the ratio of nitrogen precursor increased, thinner and smaller shells were synthesized. In lithium ion hybrid capacitor system, cathode electrode material is storing electrical energy through electrical double-layer capacitance mechanism of electrode surface. Based on these mechanisms, as-synthesized ultrathin and porous nitrogen enriched carbon hollow shells used as a cathode material of lithium ion hybrid capacitor. It is shown remarkable electrochemical performance, delivering a high reversible capacity (~48mAh/g in 3V~4.5V (Li/Li+) potential range) as well as superior rate performance for long cycles in an organic solvent system. These excellent results can be attributed to two major reasons. First, hollow and porous structures could improve the properties of surface adsorption for lithium ion capacitor by facilitating the accessibility of ions. Sites from both inside and outside of carbon shell can be accessed by PF6- ions, whereas the closed window of the pore for solid sphere can offer the sites for electrostatic adsorption only on the outside surface. Second, nitrogen contents could significantly change the interaction sites and enhance the adsorption of ion towards a carbon shell framework. These effects are also shown in capacity analysis of various synthesized samples with different amounts of nitrogen precursor. As the nitrogen content increased, the capacity tended to increase. Even in the case of carbon precursor only, the capacity is less than half of that of nitrogen contained hollow shells. This simple and cost effective strategy could be extended to synthesize tunable interior architecture mesoporous carbon composites like yolk-shell structure and core-shell structure with metal, metal oxide. Also, due to the characteristic of the nitrogen-containing material, it can be applied to various ions adsorption. By applying this phenomenon, it can be utilized not only for commercial carbon material like an electric double layer capacitor (EDLC) but also for various electrochemical energy storage and conversion application.

Two-dimensional (2D) MXenes (i.e. Ti₃C₂T_x) have shown fascinating performances as electrode materials for electrochemical capacitors (ECs). However, conventional methods for fabrication of freestanding MXene films results in their self-restacking, decreasing ions accessibility to their redox-active sites and limiting their rate-capability. These problems are more signified for thicker electrodes and, as a result, fabrication of MXene electrodes with thicknesses and mass loadings close to commercial EC electrodes is not feasible. Here, we demonstrate a strategy to fabricate hybrid electrodes of MXene and conductive polyaniline (PANI) with highly accessible surfaces and excellent electrochemical performance event at high mass-loadings and thicknesses. The freestanding and flexible films of Ti₃C₂T_x/PANI were fabricated through an oxidant-free in situ polymerization of aniline monomer on the surface of MXenes sheets. The fabricated hybrid electrodes delivered outstanding gravimetric and volumetric capacitances of 503 F g^-1 and 1682 F cm^-3, respectively, at a scan rate of 2 mV s^-1. Even at electrode thicknesses close to commercial ECs, these hybrid electrodes could maintain their high specific capacitance. For instance, a 90-µm-thick electrode with a mass loading of 23.82 mg cm^-2 could deliver a specific capacitance of about 336 F g^-1 and 888 F cm^-3 in a sulfuric acid electrolyte. The MXene /PANI hybrid electrodes also showed a long cycle life with a capacitance retention of 98.3% after 10,000 cycles.