Bulk Electrode Research Articles

Elucidating Factors Governing the Discrepancy of Oxygen Vacancy Concentration Between La0.6Sr0.4Co1-yFeyO3-δ (y = 0, 0.2, 0.4, 0.6, 0.8, 1) Dense Film and Bulk Electrodes

Abstract Dense film electrodes are used to understand the reaction mechanisms of solid oxide cells. However, the oxygen vacancy concentration, δ, in films differ from that in bulk materials, which can mislead the understanding of the reaction mechanism. Therefore, a comprehensive study was conducted to elucidate the differences in δ for the air electrode, La0.6Sr0.4Co1−yFeyO3−δ (LSCF, y=0, 0.2, 0.4, 0.6, 0.8, and 1), between film and bulk electrodes. Two key findings emerged: (1) For Co-rich compositions (y ≤ 0.5), significant differences in δ were observed between films and bulk electrodes, while (2) for Fe-rich compositions (y ≥ 0.6), there was almost no difference in δ. Plausible reasons for these discrepancies were investigated: (1) lattice stress in the film, (2) compositional variation within the film electrode, and (3) changes in the electronic conduction. High-temperature X-ray diffraction revealed no significant differences in lattice constant between films and bulk electrodes, and X-ray photoelectron spectroscopy and X-ray absorption spectroscopy indicated no changes in electronic structure. However, scanning transmission electron microscopy/energy dispersive X-ray spectroscopy revealed the presence of Co3O4 formation within the film, suggesting local compositional variations. Therefore, the results indicate that local compositional differences are the most likely factor affecting δ in LSCF films.

Freeze-thaw synergistic salt template prepared self-supporting MXene/wood-derived porous carbon electrode for supercapacitors

Supercapacitors are anticipated to be utilized in the upcoming generation of emerging energy storage devices. The electrode stands out as the most crucial component of the supercapacitor, and a bulk electrode containing sufficient electrochemically active substances is beneficial for enhancing the energy density of the energy storage device. Furthermore, the pore structure and wettability regulation of the electrode are pivotal factors that determine its electrochemical performance. In this study, we propose a monolithic electrode which was created by impregnating different concentrations of MXene dispersions into wood-based carbon materials co-treated with freeze-thaw/LiCl salt template, assisted by freezing treatment. The integration of MXene and freeze-thaw/LiCl salt template wood results in a layered porous structure that enhances both wettability and conductivity of the electrode. Freeze-thaw/LiCl salt template carbonized wood@MXene-4 (FCW@MXene-4) demonstrates an impressive electrochemical performance reaching 7.7 F cm−2 (184.5 F g−1) at a current density of 1 mA cm−2. Additionally, when assembled into a symmetric supercapacitor (SSC), it exhibits high specific capacitance at 3.4 F cm−2 (34.1 F g−1/17 F cm−3) and high energy density at 0.478 mWh cm−2 (6 Wh kg−1/2.39 mWh cm−3) at 1 mA cm−2, with a capacity retention rate of 88 % after 10,000 cycles. This research introduces an innovative self-supporting thick electrode design strategy for high-performance energy storage devices.

Equivalent Circuit Modelling for Electrode-Supported Solid Oxide Cell Based on Analysis of Gas and Temperature Dependence

Electrochemical impedance spectroscopy (EIS) measurement is used to determine the performance of solid oxide cells (SOCs). In the commercial-type of SOC such as an electrode-supported cell (ESC), all reaction resistance is sometimes combined into one spectrum, making it difficult to deconvolute the reaction resistance. The distributed-relaxation times (DRT) is often used to deconvolute the impedance spectra into a several reaction resistances. However, the DRT analysis cannot give any physical meaning to each reaction resistance of SOC’s components. Thus, it is necessary to build an equivalent circuit model to describe the physical meaning of each resistance.In this study, the ESC (Nexceris, USA) with a composition of Ni-YSZ (yttria stabilized zirconia) as a fuel electrode, 8YSZ as an electrolyte, Ce0.9Gd0.1O1.95 as an interlayer, and La0.6Sr0.4CoO3- δ (LSC) as an electrode was measured by EIS as a function of oxygen partial pressure, hydrogen partial pressure, water vapor partial pressure, temperature, and carbon dioxide partial pressure under open circuit voltage (OCV). At first, the impedance spectra were analyzed by the DRT method using software “FTIKREG” [1] based on Tikhonov regularization. After that, the complex nonlinear least square (CNLS) equivalent circuit was developed and fitted to the obtained impedance spectra.The gas conversion and diffusion at a very low-frequency region were analyzed by the Warburg impedance, meanwhile, the electrode resistance was perfectly fitted by the Gerischer impedance. The analysis result on the air electrode resistance of LSC showed the transport properties such as oxygen exchange coefficient and oxide ion diffusivity of the LSC air electrode close to the reported data on the bulk electrode [2]. Meanwhile, the analysis result on the Ni/YSZ fuel electrode resistance showed a good agreement on the reaction resistance and chemical capacitance from the reported data [3].Despite the developed model is “well-fitted” with the obtained impedance spectra, the analysis only done at OCV. In the real operating condition such as fuel cell or electrolysis mode the applied bias is used. Thus, the impedance spectra will slightly be different to those obtained under OCV. Therefore, the developed model will be also fitted to the impedance spectra under applied bias. The analysis result will be used to discuss the application of the developed equivalent circuit model. Acknowledgement This study was supported by the New Energy and Industrial Technology Development Organization (NEDO), grant number JPNP16002. Reference [1] J. Weese, Computer Physics Communications, 69 (1992) 99-111.[2] D. K. Hohnke et al., Solid State Ionics,5 (1981) 531-534.[3] M. Takeda, K. Yashiro, R. A. Budiman, S. Hashimoto, T. Kawada, J. Electrochem. Soc., 170 (2023) 034505.

Fast Li Ion Transportation via Dielectric Nanoislands

Drastic enhancement in the high rate capability of lithium ion battery up to the level of supercapacitors has been required while maintaining its high energy density, towards the next generation power sources. The strategy to incorporate the dielectric nanoisland including benchmarking dielectric compound, BaTiO3 (BTO), into the active materials–electrolyte interface delivers the ultrafast charge transfer pathway via the dialectic layer [1-3]. A series of experimental results and density functional theory and molecular dynamics (DFT-MD) based calculations demonstrated the activated interfacial charge transfer pathway. For instance, in charging, The solvated Li adsorbed onto the dielectric surface and then desolvated at the same surface. The naked Li preferentially intercalates into the electrode bulk via the triple phase interface (TPI): dielectrics-active materials-electrolyte interface.The idea of fast charge transfer architecture via the mentioned TPI, involving activated electrochemical reaction on dielectric surface, has been utilized to Li ion batteries (LIB) and capacitors, to further enhance their cell performances. For instance, the BTO nanocube (NC) decoration onto cathodes for LIB yields to many unequalled advantages that the conventional nanoparticles hardly achieve; most importantly, the NC displays a high dispersibility owing to the steric hindrance effect originating from the bulky oleic acids on the NC surface. In fact, the NC with the cube length, ca. 25 nm decorated LiCoO2 (LCO) displays significantly enhanced high rate capability as the LIB cathode. The pulsed-laser-deposition (PLD) based nanodecoration technique also effectively increased TPI density. 3D nanodecorated LCO (BTO nanodots were decorated onto LCO raw powder prior to its processing to form a working cathode) were found to exhibit notably higher capacity retention value at 10C rate, namely ~47% higher than that of their sol-gel processed cathode. The activated dielectric interface was also utilized to the lithium ion capacitor (LIC) cathode, activated carbon (AC). Optimized capacity of the BTO-AC composite was 35% higher than that of the bare AC. The strengthened LIC capacity is responsible for the enhancement of the Li desolvation activities on the dielectric surface, rather than that in the AC micropores.[1] T. Teranishi et al., Appl. Phys. Lett.,105,143904 (2014). [2] T. Teranishi et al., Adv. Electron. Mater. 4, 1700413 (2018). [3] T. Teranishi et al., J. Power Sources 494, 229710 (2021). [4] T. Teranishi et al., J. Appl. Phys. 131, 124105 (2022). [5] Y. Toyota et al., ACS Appl. Energy Mater. 7, 1440 (2024).

Oxygen Nonstoichiometry of Bulk and Thin Film of Mixed Ionic and Electronic Conductors

One of the most important properties of mixed ionic and electronic conductor is oxygen non-stoichiometry which defines the oxygen reduction reaction (ORR) process in solid oxide fuel cell (SOFC) cathode. Typically, to understand the ORR of SOFC cathode, a thin film electrode is preferred to be used than a porous electrode due to the simpler microstructure and geometry. However, one of the most used SOFC cathodes, La0.6Sr0.4CoO3-δ (LSC64), has been reported to have different oxygen nonstoichiometry in thin films and bulk [1][2]. This has been hypothesized that the discrepancy might be due to the segregation of Sr in the thin film resulting in an inhomogeneous distribution of the composition [2]. The cause of the Sr segregation could be due to the Sr having a larger cation radius than La. Instead of Sr, Ca that has a cation radius closer to the La is expected to have higher stability than Sr that have attracts attention recently [3]. Thus, the oxygen nonstoichiometry of LSC64 and La0.6Ca0.4CoO3 - δ (LCC64) was studied to understand the discrepancy factor in bulk and thin film electrodes. In this study, the oxygen nonstoichiometry of the LCC64 bulk material was measured by a high-temperature gravimetry (TG) (NETZSCH, STA449F1-K21Jupiter, Germany) and Coulometric titration (CT) as a function of temperature and oxygen partial pressure (p(O2). Meanwhile the oxygen nonstoichiometry for LSC64 bulk from reference [4] was used in this study. The thin film LSC64 and LCC64 electrodes were prepared by pulsed-laser deposition on the top of Ce0.9Gd0.1O1.95(GDC) substrate. The thin film electrodes were measured their electrochemical performance by electrochemical impedance spectroscopy as a function of temperature and p(O2). For the AC impedance method, an experimental system combining an impedance analyzer (Solartron, SI1255B) and a potentiogalvanostat (Solartron, SI1287) was used. The chemical capacitance from impedance measurement was used to determine the oxygen nonstoichiometry as reported by Kawada et al [1]. In addition, the compositions were analyzed by SEM-EDX (JEOL, JSM-7001F) and STEM-EDX (Jeol, JEM2100F). As a result, the oxygen nonstoichiometry in the LSC64 thin film were not significantly different from those in the bulk at low p(O2) (10-3 – 10-4 bar) range for all measured temperature, but at high p(O2) range, a large discrepancy was observed as reported in our earlier paper [1]. Oxygen vacancy formation enthalpy determined for the thin film was higher than the reported value for bulk of LSC64[4]. SEM-EDX and STEM-EDX observations on the LSC64 thin film confirmed that the as-prepared thin film was homogeneously formed on both surface and inside the thin film while the film after EIS showed segregation of Sr on the surface and Co3O4 near the interface with the substrate. The compositional deviation in thin film could be one of the factors of the discrepancy in oxygen nonstoichiometry. In the other side, careful measurement of the bulk nonstoichiometry of LCC64 was performed in its stability range which was narrower than LSC64. It showed that the oxygen vacancy concentration (δ) in the bulk was slightly smaller than that of LSC64. What is noteworthy is that the chemical capacitance of LCC64 thin film at 773 K matched with that estimated from oxygen nonstoichiometry of LCC64 bulk. To confirm the effect of compositional change on the discrepancy of oxygen nonstoishiometry, further study is necessary to determine by EIS at different temperatures with careful observation of compositional change before and after the measurement. The factor that governs the defect equilibrium of bulk and thin film electrode will be discussed.

Advanced Analysis of Binder Distribution within Multilayer Composite Electrodes of Lithium Ion Batteries

The loading levels of composite electrodes are increasing continuously to satisfy the energy density requirements of lithium-ion batteries (LIBs) in electric vehicles (EVs). Furthermore, a faster coating and drying process in the mass-production line yields a nonuniform binder distribution. Thus, it is necessary to understand its distribution within the composite electrode and control it for a better and more reliable electrochemical performance. Therefore, we propose the utilization of an advanced multilayer electrode model consisting of several electrode layers with different binder contents. Using these controlled electrode models, the adhesive strength within each layer was examined using a surface and interfacial cutting analysis system (SAICAS). This was followed by a composition analysis using EDX on each surface. Subsequently, the electronic conductivities of the model electrodes were measured using an electrode resistance meter to determine the bulk and interfacial electrode resistances. Furthermore, the electrochemical properties of each model electrode were evaluated to correlate their relationships and design the optimum binder distribution. Thus, this multilayer model provides a highly effective platform for determining the optimum binder distribution in highly loaded composite electrodes for high-energy–density and long-lasting LIBs.

An Ultrafast and Ultralow‐Temperature 3D‐Printed All‐Organic Proton Pseudocapacitor

AbstractA critical challenge for pseudocapacitors applications is the rapid capacitance fading under extreme environments, which originates from sluggish diffusion kinetics of inorganic materials and tortuous ionic channels in conventional bulk electrodes. Herein, a novel 3D‐printed all‐organic proton pseudocapacitor (composed of 2,6‐diaminoanthraquinone (DQ)‐based anode and polyaniline‐based cathode) with chemical and structural stability is developed, which exhibits an extraordinary rate performance and cycle stability under ultralow temperature. The DQ molecules are anchored on reduced graphene oxide, which enhances the electronic conductivity and structural stability. Theoretical calculation and spectroscopic characterization reveal that the two‐electron transfer process involves quinone/hydroquinone transition. Exploiting the synergy of fast reaction kinetics of organic and the efficient ion diffusion paths of the 3D architecture, the 3D‐printed anode achieves an impressive areal capacitance of 10.14 F cm−2 at high mass loading (28.73 mg cm−2). The 3D‐printed all‐organic proton pseudocapacitor shows stable cycling performance at −80 °C and releases a high energy density of 0.76 mWh cm−2 at −60 °C. This work is instructive for the development of competitive ultra‐low temperature energy storage devices via integrating organic materials and 3D architectural electrode designs.

Binder-free tin (IV) oxide coated vertically aligned carbon nanotubes as anode for lithium-ion batteries

Despite the tremendous potential of tin oxide (SnO2) as an anode material, irreversible capacity loss due to the sluggish kinetics and structural pulverization as a result of the substantial volume alteration during redox reactions limits its use in lithium-ion batteries. The typical layered design of an electrode consisting of binder and conductive additive can lower the practical capacity of high-capacity electrode materials. We synthesized a binder and conductive additive-free, self-standing core-shell vertically-aligned carbon nanotubes (VACNTs)-SnO2 anode (SnO2-VACNTs) on 3D nickel foam using plasma-enhanced chemical vapor deposition and wet chemical method. The SnO2-VACNTs exhibited excellent cyclability with a specific capacity of 1512 mAh g−1 at 0.1 A g−1 after 100 cycles and 800 mAh g−1 at 1 A g−1 after 200 cycles. The ultra-fine SnO2 particles (<5 nm) shortened the Li+ diffusion paths into the bulk electrode and alleviated the volume alteration by lowering the strains during the redox reactions. Also, proper inter-tube distance between individual SnO2-VACNTs buffered the volume instability and offered better electrolyte accessibility. Direct connection of VACNTs with the current collector ensured an uninterrupted electron conducting path between the current collector and active material, thus offering more efficient charge transportation kinetics at the electrode/electrolyte interfaces.

Curved surface coupled structural battery composites manufactured by resin transfer molding process: Microstructure and multifunctional performance

To bridge industrial production and lab-scale research, this work demonstrates a technology to manufacture curved surface structural battery composites (CSBCs) that can simultaneously achieve electrochemical energy storage and load-bearing. The curved-surface carbon fiber structural anode and cathode are fabricated by coating the active materials on carbon fiber fabric with a vacuum-bag-assisted technique. The resin transfer molding (RTM) process is conducted to manufacture the coupled CSBCs by infusing bi-continuous phase epoxy resin electrolyte and curing at high temperatures. The microstructure of structural electrodes and CSBCs is characterized by scanning electron microscopy (SEM). Due to good interfacial compatibility between high mechanical strength carbon fiber structural electrode and high ionic conductivity solid polymer electrolyte bulk for load support, the fabricated CSBCs demonstrate a high density of 294 mWh kg−1 based on the whole mass of devices, a tensile strength of 257.4 MPa with Young's modulus of 12.9 GPa and a flexural strength of 194.1 MPa with flexural modulus of 11.1 GPa. In situ electrochemical-mechanical tests further confirm the durability of CSBCs under mechanical loads with a multifunctional efficiency of 1.07, suggesting the effectiveness of the introduced manufacturing techniques for coupled structural battery composites.

Application of metal oxide/porous carbon nanocomposites in electrochemical capacitors: A review

In the past decade, electrochemical capacitors (ECs) have emerged as innovative energy storage devices, attracting considerable interest from both industrial and academic sectors due to their prolonged cyclic stability and impressive power density. Recently, there has been a focus on advancing high-performance ECs by developing electrode material composed of metal oxide incorporated with porous carbon. Integrating metal oxides into a porous carbon structure enhances capacitance, charge storage, and cyclic stability. Furthermore, the porous structure inherent in carbon materials offers efficient ion diffusion and large surface area, thereby augmenting the overall effectiveness of the nanocomposites. Efficient electron and ion transport within the electrode's bulk is achieved through the interaction between electrodes and electrolytes occurring at the electrode-electrolyte interface, subsequently increasing the capacitance. This paper summarizes the latest developments in ECs, focusing on key performance metrics such as cyclic stability and capacitance. It also summarizes the recent advancements made in electrode materials, emphasizing the utilization of sustainable feedstocks. As the research landscape shifts towards more environmentally friendly solutions, the incorporation bio-materials into electrode design has gained significant attention. It particularly highlights underlying mechanisms governing the performance of metal oxides/porous carbon nanocomposites in ECs and outlines future research directions aimed at maximizing their potential for next-generation energy storage devices. Additionally, the paper delves into the possibilities and challenges associated with the preparation, and optimization of the structure of metal oxide electrodes incorporating porous carbon to improve energy storage performance in ECs.

Electrodeposition of porous metal-organic frameworks for efficient charge storage

Efficient charge storage is a key requirement for a range of applications, including energy storage devices and catalysis. Metal-organic frameworks are potential materials for efficient charge storage due to their self-supported three-dimensional design. MOFs are high surface area materials made up of coordination of appropriate amounts of metal ions and organic linkers, hence used in various applications. Yet, creating an effective MOF nanostructure with reduced random crystal formation continues to be a difficult task. The energy efficiency and electrochemical yield of bulk electrodes are improved in this study by demonstrating an effective technique for growing MOFs over a conducting substrate utilizing electrodeposition. An exceptionally stable asymmetric supercapacitor is created when activated carbon cloth is combined with the resulting MOF structure that was directly synthesized via an electrochemical method resulting in 97% stability over 5k cycles which is higher than conventional processes. High performance in supercapacitors is ensured by this practical approach for producing MOF electrodes, making it a suitable structure for effective charge storage.

Understanding Surface Degradation Mechanisms in Ni-Rich Cathodes for Li-Ion Batteries

High Ni-rich layered cathodes, such as; LiNi0.8Co0.15Al0.05O2 (NCA) and LiNi0.8Mn0.1Co0.1O2 (NMC811) materials, offer very high capacity and excellent rate capability fulfilling the pressing priorities. However, the cyclic performance is poor compared to other cathodes with lower Ni content. Due to high surface reactivity, first-cycle irreversibility is quite high in all these Ni-rich cathodes. Such surface reactions lead to different electrochemically inactive phases and reduce cyclic performance. Herein, we report a comprehensive analysis of plane selective cathode and electrolyte interface reactions and link the relationship between the formation of different cathode-electrolyte interphase (CEI) by-products with surface reconstruction mechanisms. Moreover, we established a guiding principle of coating strategy to enhance plane selective surface properties. Both bulk and thin-film Ni-rich NMC811 electrodes were used to investigate the surface properties. We fabricated epitaxial thin films of Ni-rich NMC811 cathodes of different orientations, such as; (104), (018), and (003). Al2O3 was chosen as a model coating material to understand how it modulates surface properties. The surface by-products depend highly on the surface charge and subsequently change with Al2O3 coating. Likewise, the surface reconstruction depends on these crystallographic planes and the coating layer exposed to the electrolyte. Keywords: NMC811; Thin film; Epitaxial; CEI; Surface reconstruction

Simulation of Electrical Conductivity of Porous Composite Electrodes for Solid Oxide Cells Using a Monte Carlo 3D Equivalent Electronic Circuit Networks

Solid oxide cells (SOCs) show high potential in energy conversion applications necessary for the decarbonization of the economy. Their advantage consists in ability to operate at high temperatures, reaching up to 900 °C. Firstly, it results in accelerated electrode reactions kinetics, secondly, in favorable thermodynamic conditions decreasing the equilibrium voltage of the water splitting. As a result, SOCs are attractive candidates for both efficient electrolysis and fuel cell applications, offering a robust solution in the pursuit of clean energy. However, the elevated operational temperature also introduces material challenges that hinder the competitiveness of SOCs.Porous gas diffusion electrodes are used to accomplish the desired electrode reactions. Considering conventional exclusively electron-conductive electrode materials, triple phase boundary (TPB) is required for the reactions to take place – a simultaneous contact of electron-conductive, ion-conductive and gaseous phase. While the electrode and electrolyte phases are solid, the gaseous phase is represented by open porosity in the electrode body. TPB is located exclusively at the contact area of electrode and electrolyte components in the case of single-phase electrodes, limiting the electrode electrocatalytic performance. To avoid the limitation by TPB length, composite electrodes are frequently used. Due to the involvement of the ionically conductive phase, they exhibit improved electrocatalytic capabilities.Electrolyte phase, however, usually exhibits 4 orders of magnitude lower electrical conductivity than the electron-conductive phase, while gaseous phase is electrically non-conductive. Thus, an improper phase composition and morphology of the composites can lead to a significant drop in their electrical conductivity and thus in the cell performance. Simple and reliable method for the conductivity value prediction is up to now missing. At the same time, experimental determination is both expensive and time-consuming. The goal of this study was to develop simple, fast and reliable method for prediction of electrical conductivity of the porous electron-ion conductor composites.The key feature of the presented approach lies in the minimal requirements for input parameters. Only electrode phase composition and bulk electrical conductivity of each phase are required. Based on the input composition, the model generates a 3D artificial specimen, a domain discretized into cubic voxels each assigned randomly to a specific phase. The resulting structure is substituted with an equivalent circuit network with electric elements corresponding to the electrochemical properties of the material components: specific resistivity of electrical conductors and polarization resistance / capacitance of electrochemical reaction at material interface. In-silica electrochemical impedance spectroscopy measurement is performed by solving charge balance following Kirchhoff’s current law for multiple frequency values. Resulting impedance spectrum is fitted in order to obtain Ohmic and polarization resistance and the capacitance of the artificial specimen. This procedure is averaged over a large number of artificial specimens in a Monte Carlo manner.Conventionally used oxygen electrode composite material lanthanum strontium manganite (LSM) mixed with yttria-stabilized zirconia (YSZ) was used to produce validation dataset. LSM:YSZ powder mixtures of compositions between 1:0 and 0:1, chosen to produce samples of various degree of LSM percolation, were homogenized. The mixtures were fired at 1150 °C. Electrical conductivity of the pellets was determined at temperatures between 600 and 800 °C using electrochemical impedance spectroscopy.Experimental data obtained were confronted to the model results. The model demonstrated very good accuracy for a porosity value of up to 55%. Significant error was observed in the porosity range between 55% and 68%. Finally, the model failed to generate an artificial specimen with a porosity of 75%. As it was found, the limited applicability of the model for the materials characteristic for high porosity was caused by the coalescence of the void phase. This shortcoming of the model was solved by implementing morphological parameter describing degree of void phase coalescence in the electrode structure. Due to this modification, model allows to gain valuable information on the microstructure of the studied composite material on the base of the experimentally determined conductivity data.This work introduces a novel modelling approach with minimal amount of input parameters, streamlining the prediction of electrical conductivity of porous electron-ion conductor composites. This simplified yet effective methodology holds great promise for efficiently characterizing and optimizing materials for energy conversion applications, offering a valuable tool for advancing research in the field.This publication was supported by the project "The Energy Conversion and Storage", funded as project No. CZ.02.01.01/00/22_008/0004617 by Programme Johannes Amos Commenius, call Excellent Research.

Intermetallic Compounds Mo2 TMB2 (TM: Fe, Co, Ni) for Hydrogen Evolution Reaction

Due to rising energy demand, renewable energy sources integrated processes have gained great interest in the recent years. Water electrolysis powered with renewable energy have potential to contribute to the development of sustainable hydrogen economy enormously. Despite the fact that “green” hydrogen is produced in the water electrolyzers with eco-friendly product O2, the excess energy input still hinders this technology to replace the fossil fuel-based technologies. Even with tremendous attempts and considerable contribution to the electrocatalyst development/optimization/improvement over last decades, there is a continued massive endeavor on the way of noble-metal based catalyst replacement for oxygen evolution reaction (OER) as well as for hydrogen evolution reaction (HER). From this point of view, transition metal-containing intermetallic compounds with well-defined electronic and crystal structures, are considered as promising electrocatalysts or precursors for them [1, 3]. Up to now, Mo-Ni and Mo-B systems have been explored for the hydrogen evolution reaction, and noticeable HER activities have been demonstrated for the binary compounds in these systems [4-6].The chemical behavior of ternary borides Mo2FeB2, Mo2CoB2 and Mo2NiB2 under hydrogen evolution (HER) reactions is investigated. Material synthesis and electrode manufacturing include the arc melting of elements in 2:1:2 atomic ratio, homogenization annealing at 1300-1400 °C and densification of grinded powder into cylindrically shaped electrodes via spark plasma sintering (SPS). The electrochemical measurements are carried out in 1M KOH and ambient conditions. Electrocatalytic activity and stability are monitored with cyclic voltammetry (CV) and 2-hour chronopotentiometry (CP) measurements performed at reducing current density of 10 mA cm-2, which are considered as conditions of standard benchmarking HER experiment. Furthermore, long-term stability of HER activity at elevated reducing current densities such as 50, 100, 200 mA cm-2 are essential to prove the preserved catalytic performance under harsh reaction conditions. For the comprehensive pre- and post-characterization of the electrode materials, bulk and surface-sensitive techniques are utilized.The pristine Mo2FeB2, Mo2CoB2 and Mo2NiB2 in HER region outperform their constituent TM components and indeed, their HER activity is close to that of the state-of-art Pt catalyst (Figure). During short-term CP experiment, Mo2NiB2 and Mo2FeB2 show continuing activation during the measurement, whereas Mo2CoB2 maintained its stability. Energy dispersive X-ray analysis (EDXS) of electrochemically treated areas indicate an enrichment in Fe, Co and Ni due to the noticeable leaching of Mo into electrolyte. Also, chemical analysis via ICP-OES reveals only Mo among all three constituent elements in the exploited electrolyte solution.As a conclusion, Mo2 TMB2 intermetallic compounds show outstanding HER activity and its stability over 2h of CP with respect to corresponding reference materials. Long-term chronopotentiometry in alkaline media were carried out and will be extensively presented. Figure. HER activity of Mo2 TMB2 (TM: Fe, Co, Ni) with respect to elemental references Fe, Co, Ni and state-of-art Pt. Inlets: Backscattered secondary electron images after 2h CP, including energy dispersive X-ray (EDXS) analysis results.

Controlled Electrode-Electrolyte Interphases (EEI) Formation Via Ex-Situ Interfacial Synthesis

The stability and electrochemical performance of energy storage systems are heavily influenced by their interfaces. The electrode-electrolyte interphases (EEIs) present at these interfaces are typically formed in-situ through the electrochemically induced complex interactions involving the electrode surface and bulk electronic structure, functional groups and chemical species present at the interface, and electrolyte. Given the importance of interface stability in energy storage, herein, multiple ex-situ approaches (electrochemical fluorination, ion-exchange metathesis, layer-by-layer coating) have been explored to form stable and controlled EEI that allow ion transport and limit the electron leakage. Such controlled EEI via ex-situ interfacial synthesis demonstrated significantly improved cycling stability compared to in-situ EEI formation.

A Self-Assembled Multiphasic Thin Film as an Oxygen Electrode for Enhanced Durability in Reversible Solid Oxide Cells.

The implementation of nanocomposite materials as electrode layers represents a potential turning point for next-generation of solid oxide cells in order to reduce the use of critical raw materials. However, the substitution of bulk electrode materials by thin films is still under debate especially due to the uncertainty about their performance and stability under operando conditions, which restricts their use in real applications. In this work, we propose a multiphase nanocomposite characterized by a highly disordered microstructure and high cationic intermixing as a result from thin-film self-assembly of a perovskite-based mixed ionic-electronic conductor (lanthanum strontium cobaltite) and a fluorite-based pure ionic conductor (samarium-doped ceria) as an oxygen electrode for reversible solid oxide cells. Electrochemical characterization shows remarkable oxygen reduction reaction (fuel cell mode) and oxygen evolution activity (electrolysis mode) in comparison with state-of-the-art bulk electrodes, combined with outstanding long-term stability at operational temperatures of 700 °C. The disordered nanostructure was implemented as a standalone oxygen electrode on commercial anode-supported cells, resulting in high electrical output in fuel cell and electrolysis mode for active layer thicknesses of only 200 nm (>95% decrease in critical raw materials with respect to conventional cathodes). The cell was operated for over 300 h in fuel cell mode displaying excellent stability. Our findings unlock the hidden potential of advanced thin-film technologies for obtaining high-performance disordered electrodes based on nanocomposite self-assembly combining long durability and minimized use of critical raw materials.

Phase engineering and surface reconstruction of FeNiMo alloys as high efficient electrode for oxygen evolution reaction

In the field of electrocatalysis, most of the electrodes have the problem of poor mechanical properties, which leads to the electrode can not be used on a large scale. At present, most bulk electrodes are used to study their mechanical properties due to their unique phase composition distribution and facile fabricate methods. But their novel properties in the field of electrocatalysts have not been fully developed. Multi-element bulk electrodes not only diversified the electronic states, but also build complex surface morphology by different dissolution rate between multi-phase distribution. In this paper, we synthesized the FeNiMo bulk electrode for oxygen evolution reaction (OER) with two-phase coexistence of Mo-doped face center cubic phase (FCC) and Mo-rich intermetallic compound (IMC) phase. During cyclic voltammetry activation, the surface reconstruction of Mo-rich IMC phases generated by ion leaching leaves Fe–Ni pore structures with more active area, which enhances OER performance. Meanwhile, the electron states get more diverse on the surface of remaining FCC phases due to Mo doping, which provides more suitable sites for four-step OER process. The FeNiMo electrode shows an ultra-low overpotential of 212 mV and 293.4 mV at a current density of 10 mA cm−2 and 100 mA cm−2, respectively. Moreover, it can maintain stability at 100 mA cm−2 for up to 72 h. This work provides a strategy for studying the relationship between phase composition and electrochemical performance of alloy bulk electrodes.

Electrode wettability and capacitance of electrical double layer capacitor: a classical density functional theory study

We suggest a coarse-grained water model for use in classical density functional theory (cDFT) to describe aqueous inorganic salt solutions that act as working electrolytes in electrical double-layer capacitors (EDLCs) with electrodes comprising two face-to-face doped carbon monolayers. Focus of the cDFT calculations lies on the influence of solvent electrode wettability (SEW) on capacitance and energy storage behaviors, while also considering its interaction with factors like electrolyte bulk concentration, pore size, electrode voltage, and temperature. New phenomena are disclosed theoretically. Remarkably, this study challenges the traditional notion that energy storage is consistently boosted by enhancing the electrode’s ionophobicity. Contrarily, the SEW effect reduces energy storage below the standard aqueous electrochemical window voltage (around 1.2 V) and only enhances the energy storage as the voltage surpasses a certain threshold up to the optimal window voltage (2 V–2.5 V). Furthermore, a non-monotonic SEW effect on energy storage is demonstrated under appropriate conditions, shedding new light on the complex relationship between ionophobicity and energy storage. Moreover, the present coarse-grained water model enables the prediction of the experimentally observed inverse relationship between temperature and capacitance. In contrast, the widely used electrolyte primitive model predicts the existence of a maximum value. The decisive factor for the impact of SEW on capacitance and energy storage is identified as congestion within the electrode pore, while other factors contribute by affecting this congestion. The present research offers valuable insights, highlighting the significance of SEW in the innovative and strategic design of aqueous inorganic EDLC devices.

Understanding the charge transfer effects of single atoms for boosting the performance of Na-S batteries.

The effective flow of electrons through bulk electrodes is crucial for achieving high-performance batteries, although the poor conductivity of homocyclic sulfur molecules results in high barriers against the passage of electrons through electrode structures. This phenomenon causes incomplete reactions and the formation of metastable products. To enhance the performance of the electrode, it is important to place substitutable electrification units to accelerate the cleavage of sulfur molecules and increase the selectivity of stable products during charging and discharging. Herein, we develop a single-atom-charging strategy to address the electron transport issues in bulk sulfur electrodes. The establishment of the synergistic interaction between the adsorption model and electronic transfer helps us achieve a high level of selectivity towards the desirable short-chain sodium polysulfides during the practical battery test. These finding indicates that the atomic manganese sites have an enhanced ability to capture and donate electrons. Additionally, the charge transfer process facilitates the rearrangement of sodium ions, thereby accelerating the kinetics of the sodium ions through the electrostatic force. These combined effects improve pathway selectivity and conversion to stable products during the redox process, leading to superior electrochemical performance for room temperature sodium-sulfur batteries.

Microwave sintering construct 3D NiFeAl micro/nano porous bulk electrode for effective oxygen evolution reaction

The development of oxygen evolution reaction (OER) electrocatalysts is an important strategy to produce hydrogen energy by water splitting. Based on the Kirkendall effect and the capillary action of Al melting, this paper introduced a proper amount of Al into NiFe, and the 3D porous bulk high-efficiency OER electrodes were successfully prepared by microwave sintering. The 3D porous structure can provide abundant active sites for OER. The porous channel accelerates the penetration of O2 and electrolyte, shortens the diffusion path of reagents and products, and reduces the charge transfer resistance. It is worth noting that the NiFeAl porous bulk electrode shows excellent OER performance. The overpotentials are 205 mV and 251 mV at the current density of 10 mA cm−2 and 100 mA cm−2, respectively, and the Tafel slope is as low as 39.3 mV dec−1. Due to the existence of Al in the electrode and continuous etching and reconstruction in 1 M KOH solution, the stability test of the electrode lasts for more than 80 h, and the stability rate is even as high as 99.6%. In this work, a new view is put forward in developing an efficient and stable OER electrocatalysts for the preparation of transition metals.