Synergistic niobium and manganese co-doping into RuO2 nanocrystal enables PEM water splitting under high current

Low-cost ruthenium-based catalysts with high activity have emerged as promising alternatives to iridium-based counterparts for acidic oxygen evolution reaction (OER) in proton exchange membrane water electrolyzers (PEMWE), but the poor stability under high current density remains as a key challenge. Here, we utilize the synergistic complementary strategy of introducing earth-abundant Mn and Nb dopants in ruthenium dioxide (RuO2) for Nb0.1Mn0.1Ru0.8O2 nanoparticle electrocatalyst that exhibits a low overpotential of 209 mV at 10 mA cm-2 and good stability of > 400 h at 0.2 A cm-2 in 0.5 M H2SO4. Significantly, a PEMWE device fabricated with Nb0.1Mn0.1Ru0.8O2 anode can operate continuously at least for 1000 h at 0.5 A cm-2 with 59 μV h-1 decay rate. Operando Raman spectroscopy analysis, differential electrochemical mass spectroscopy measurements, X-ray absorption spectroscopy analysis and theoretical calculations indicate that OER reaction on Nb0.1Mn0.1Ru0.8O2 primarily follows the adsorbate evolution mechanism with much favorable energy barrier accompanied by a locally passivated lattice oxygen mechanism (AEM-LPLOM) and the co-existed Nb and Mn in RuO2 crystal lattice could not only stabilize the lattice oxygen, but also relieve the valence state fluctuation of Ru site to stabilize the catalyst during the reaction.

Materials for Electrocatalysis: Future Prospects in Energy Conversion

Electrocatalysts play a pivotal role in various energy conversion processes, such as water splitting, batteries, carbon dioxide reduction, and fuel cell reactions, by significantly reducing the energy barrier and enhancing reaction kinetics. This review highlights the potential of earth-abundant electrocatalysts, with a particular focus on their capabilities in critical electrochemical reactions, including oxygen evolution reaction, carbon dioxide reduction reaction, oxygen reduction reaction and hydrogen evolution reaction. Emphasis is also placed on bifunctional, trifunctional, and tetrafunctional performance, showcasing their adaptability and effectiveness across diverse energy applications. Exploration is done on a range of promising materials, including transition metal chalcogenides, MXenes, metal-organic frameworks, covalent organic frameworks, and layered double hydroxides. By examining their intrinsic properties, structural versatility, and surface engineering strategies, this review sheds light on the factors that govern their catalytic efficiency and stability. The integration of experimental advancements with theoretical insights provides a deeper understanding of mechanisms driving their catalytic activity. Additionally, we address the scalability, cost-effectiveness, and environmental impact of these materials, underlining their potential for large-scale deployment. By synthesizing recent progress and identifying challenges, this work delivers a roadmap for the model and application of multifunctional electrocatalysts, fostering innovations that align with the goals of sustainable energy systems.

Direct Electrosynthesis of an Amino Acid from a Biomass Derivative

The electrochemical synthesis of nitrogen-containing molecules from biomass-derived compounds under ambient conditions is demonstrated, relying only on green sources of feedstock, renewable energy, and water. In this study, we report a two-step method of electrochemically synthesizing 5-(aminomethyl)furan-2-carboxylic acid (AFCA) from 5-hydroxymethylfurfural (HMF) using hydroxylamine (NH2OH) as the nitrogen source in an acidic electrolyte. In the first step, HMF was reductively aminated into (5-(aminomethyl)furan-2-yl)methanol (HMFA) using NH2OH as the source of nitrogen. This was followed by a second step, involving the oxidation of HMFA to AFCA on a manganese oxide (MnO x ) anode at the same pH. MnO x was able to selectively oxidize the alcohol group on HMFA to produce AFCA with 35% Faradaic efficiency without affecting the amine group. As both of these reactions are completed in a pH 1 electrolyte, it eliminates the need to separate HMFA before proceeding with the second reaction. Among different metal electrodes (Ag, Au, Cu, Pb, Pt and Sn) tested for the electrochemical reductive amination reaction, Ag electrodes displayed the best performance to selectively aminate HMF to the intermediate species, HMFA, with up to 69% Faradaic efficiency at mild potentials of -0.50 VRHE. Density functional theory calculations were carried out to explore a possible reaction pathway for the reductive amination on Ag(111), which suggests a thermodynamically feasible reaction even at 0 VRHE. The cathodic experimental reaction parameters were optimized to reveal that an electrolyte pH of 1 is optimal for the electrochemical reductive amination reaction. Our work shapes the future possibility of an electrochemical synthesis to produce AFCA without the need for any product separation between steps by combining the Ag cathode reaction to the MnO x anode reaction sharing the same electrolyte. Since both the cathode and anode reactions both involve four electrons transferred, combining both half reactions in a single electrochemical reactor can eliminate the need for energy-wasting auxiliary counter reactions such as hydrogen evolution or water oxidation.

Layered double hydroxide-derived bimetallic-MOF as a promising platform: Urea-coupled water oxidation and supercapattery-driven water electrolyzer

Developing a two-dimensional (2D) ultrathin metal-organic framework plays a significant role in energy conversion and storage systems. This work introduced a facile strategy for engineering ultrathin NiMn-MOF nanosheets on Ni foam (NF) via in situ conversion from NiMn-layered double hydroxide (LDH). The as-synthesized LDH-derived NiMn-MOF (LDH-D NiMn-MOF) nanosheet exhibited an overpotential of 350 mV to drive a current density of 100 mA cm-2 during oxygen evolution reaction (OER) owing to its better redox activity, hierarchical architecture, and intercalating ability. The similar effective catalytic trend was noticed during the urea-assisted water oxidation process. The developed catalyst required only a potential of 1.39 V vs. RHE at 100 mA cm-2 towards urea oxidation reaction (UOR). Moreover, the urea-assisted overall water-splitting voltage was found to be 1.5 V at the current density of 10 mA cm-2. Furthermore, the same catalyst was explored as an energy-storage material for supercapattery application with an aerial specific capacity value of 2613.9 mC cm-2 at 1 mA cm-2 which was found to be 1.5 times higher than NiMn-LDH (1724.3 mC cm-2). Additionally, an aqueous asymmetric supercapattery device was fabricated which demonstrated the best electrochemical performance and provided a maximum energy density of 64.1 Wh kg-1 at a power density of 493 W kg-1 with 77.8 percent capacity retention after a continuous run of 8000 cycles at 10 mA cm-2 current density. Hence, the multifaceted properties of energy conversion and storage of LDH-D NiMn-MOF outline its performance in real-world applications.

Mn0.75Ru0.25O2 with Low Ru Concentration for Active and Durable Acidic Oxygen Evolution

Ruthenium has emerged as a promising alternative to iridium in water-splitting anodes. However, it becomes overoxidized and dissolves at industry-relevant working conditions. To enhance the activity and stability of electrocatalysts for oxygen evolution reaction, an isostructural rutile MnRu oxide with low Ru concentration (Mn0.75Ru0.25O2) is synthesized and an asymmetric Mn-O-Ru dual-site active center is developed. It exhibits 154 mV overpotential at 10 mA cm-2 and can operate stably at 200 mA cm-2 for 670 h with a degradation rate of 29 uV/h-1. A proton exchange membrane water electrolyzer achieves stable operation at 1 A cm-2 for 700 h with a degradation rate of 53 uV h-1. Structural analysis and isotopic labeling correlate the asymmetric nature of the Mn-O-Ru dual-site active center, which facilitates the oxygen evolution reaction along the radical coupling pathway, with the stabilization of the cations and the lattice oxygen in isostructural rutile Mn0.75Ru0.25O2.

Electrocatalysis: From Planar Surfaces to Nanostructured Interfaces

The reactions critical for the energy transition center on the chemistry of hydrogen, oxygen, carbon, and the heterogeneous catalyst surfaces that make up electrochemical energy conversion systems. Together, the surface-adsorbate interactions constitute the electrochemical interphase and define reaction kinetics of many clean energy technologies. Practical devices introduce high levels of complexity where surface roughness, structure, composition, and morphology combine with electrolyte, pH, diffusion, and system level limitations to challenge our ability to deconvolute underlying phenomena. To make significant strides in materials design, a structured approach based on well-defined surfaces is necessary to selectively control distinct parameters, while complexity is added sequentially through careful application of nanostructured surfaces. In this review, we cover advances made through this approach for key elements in the field, beginning with the simplest hydrogen oxidation and evolution reactions and concluding with more complex organic molecules. In each case, we offer a unique perspective on the contribution of well-defined systems to our understanding of electrochemical energy conversion technologies and how wider deployment can aid intelligent materials design.

Synthesis, structure, oxygen evolution reaction (OER) and visible-light assisted organic reaction studies on A2M2TeB2O10 (A = Ba and Pb; M = Mg, Zn, Co, Ni, Cu, and Fe)

Compounds with the general formula A2M2TeB2O10 (A = Ba and Pb; M = Mg, Zn, Co, Ni, Cu, and Fe) have been synthesised via solid-state techniques and characterised. The structure exhibits M2B2O10 layers connected by TeO6 octahedra giving rise to a three-dimensional structure with voids, where Ba2+ ions reside. Substitution of Mg by transition elements (M = Co, Ni, and Cu) in Ba2Mg2TeB2O10 and (Ba0.5Pb1.5)Mg2TeB2O10 gives rise to interesting colored compounds. NIR reflectivity studies indicated that white-colored compounds exhibited good NIR reflectivity, which was is comparable to that of TiO2. Dielectric studies indicated reasonable values with low dielectric loss at low frequencies. The cobalt-substituted compounds Ba2(MgCo)TeB2O10 and (Ba0.5Pb1.5)(MgCo)TeB2O10 were explored towards the oxygen evolution reaction (OER) in an alkaline medium. The compound (Ba0.5Pb1.5)(MgCo)TeB2O10 was found to be a good electrocatalyst for the OER with a faradaic efficiency of ∼96%. The Cu-substituted compound Ba2(Mg1.5Cu0.5)TeB2O10 was found to be a good photocatalyst for the formation of α-chloroketones under visible light in the presence of molecular oxygen.

Disentangling the Pitfalls of Rotating Disk Electrode-Based OER Stability Assessment: Bubble Blockage or Substrate Passivation?

Oxygen evolution reaction (OER) catalyst stability metrics derived from aqueous model systems (AMSs) prove valuable only if they are transferable to technical membrane electrode assembly (MEA) settings. Currently, there is consensus that stability data derived from ubiquitous rotating disk electrode (RDE)-based investigations substantially overestimate material degradation mainly due to the nonideal inertness of catalyst-backing electrode materials as well as bubble shielding of the catalyst by evolved oxygen. Despite the independently developed understanding of these two processes, their interplay and relative impact on intrinsic and operational material stability have not yet been established. Herein, we employ an inverted RDE-based approach coupled with online gas chromatographic quantification that exploits buoyancy and anode hydrophilicity existing under operating acidic OER conditions and excludes the influence of bubble retention on the surface of the catalyst. This approach thus allows us to dissect the degradation process occurring during the RDE-based OER stability studies. We demonstrate that the stability discrepancy between galvanostatic nanoparticle (NP)-based RDE and MEA data does not originate from the accumulation of bubbles in the catalyst layer during water oxidation but from the utilization of corrosion-prone substrate materials in the AMS. Moreover, we provide mechanistic insights into the degradation process and devise experimental measures to mitigate or circumvent RDE-related limitations when the technique is to be applied to an OER catalyst stability assessment. These findings should facilitate the transferability between AMS and MEA approaches and promote further development of the latter.

The promise of chiral electrocatalysis for efficient and sustainable energy conversion and storage: a comprehensive review of the CISS effect and future directions

The integration of chirality, specifically through the chirality-induced spin selectivity (CISS) effect, into electrocatalytic processes represents a pioneering approach for enhancing the efficiency of energy conversion and storage systems. This review delves into the burgeoning field of chiral electrocatalysis, elucidating the fundamental principles, historical development, theoretical underpinnings, and practical applications of the CISS effect across a spectrum of electrocatalytic reactions, including the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). We explore the methodological advancements in inducing the CISS effect through structural and surface engineering and discuss various techniques for its measurement, from magnetic conductive atomic force microscopy (mc-AFM) to hydrogen peroxide titration. Furthermore, this review highlights the transformative potential of the CISS effect in addressing the key challenges of the NRR and CO2RR processes and in mitigating singlet oxygen formation in metal-air batteries, thereby improving their performance and durability. Through this comprehensive overview, we aim to underscore the significant role of incorporating chirality and spin polarization in advancing electrocatalytic technologies for sustainable energy applications.

Optimal Coatings of Co3O4 Anodes for Acidic Water Electrooxidation

Implementation of proton-exchange membrane water electrolyzers for large-scale sustainable hydrogen production requires the replacement of scarce noble-metal anode electrocatalysts with low-cost alternatives. However, such earth-abundant materials often exhibit inadequate stability and/or catalytic activity at low pH, especially at high rates of the anodic oxygen evolution reaction (OER). Here, the authors explore the influence of a dielectric nanoscale-thin oxide layer, namely Al2O3, SiO2, TiO2, SnO2, and HfO2, prepared by atomic layer deposition, on the stability and catalytic activity of low-cost and active but insufficiently stable Co3O4 anodes. It is demonstrated that the ALD layers improve both the stability and activity of Co3O4 following the order of HfO2 > SnO2 > TiO2 > Al2O3, SiO2. An optimal HfO2 layer thickness of 12 nm enhances the Co3O4 anode durability by more than threefold, achieving over 42 h of continuous electrolysis at 10 mA cm-2 in 1 m H2SO4 electrolyte. Density functional theory is used to investigate the superior performance of HfO2, revealing a major role of the HfO2|Co3O4 interlayer forces in the stabilization mechanism. These insights offer a potential strategy to engineer earth-abundant materials for low-pH OER catalysts with improved performance from earth-abundant materials for efficient hydrogen production.

Automated monitoring of electrocatalyst corrosion as a function of electrochemical history and electrolyte formulation

Automated platforms assessing the stability of electrocatalysts are key to accelerate the deployment of clean energy technologies. Here, we present a robust system that allows the study of corrosion behavior in conjunction with the electrochemical protocol and electrolyte composition over many individual electrodes. Oxygen reduction reaction on Pt is used as a proof-of-concept platform, where the influence of the potential window and phosphoric acid (PA) addition on Pt dissolution is probed. A total of 72 hours of automated operation was realized with actions including liquid management, cell cleaning, aliquoting, PA injection, and bubble detection and removal, demonstrating further advancements in automated stability testing for electrocatalysts.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Nanoengineered Cobalt Electrocatalyst for Alkaline Oxygen Evolution Reaction

The alkaline oxygen evolution reaction (OER) remains a bottleneck in green hydrogen production owing to its slow reaction kinetics and low catalytic efficiencies of earth abundant electrocatalysts in the alkaline OER reaction. This study investigates the OER performance of hierarchically porous cobalt electrocatalysts synthesized using the dynamic hydrogen bubble templating (DHBT) method. Characterization studies revealed that electrocatalysts synthesized under optimized conditions using the DHBT method consisted of cobalt nanosheets, and hierarchical porosity with macropores distributed in a honeycomb network and mesopores distributed between cobalt nanosheets. Moreover, X-ray photoelectron spectroscopy studies revealed the presence of Co(OH)2 as the predominant surface cobalt species while Raman studies revealed the presence of the cubic Co3O4 phase in the synthesized electrocatalysts. The best performing electrocatalyst required only 360 mV of overpotential to initiate a current density of 10 mA cm-2, exhibited a Tafel slope of 37 mV dec-1, and stable OER activity over 24 h. The DHBT method offers a facile, low cost and rapid synthesis approach for preparation for highly efficient cobalt electrocatalysts.

Nanostructured Niobium and Titanium Carbonitrides as Electrocatalyst Supports

Nanostructured niobium-titanium carbonitrides, (Nb,Ti)C1-xNx, with the cubic-rock salt structure are prepared without the use of reactive gases via thermal treatment (700-1200 °C) under nitrogen of mixtures of guanidine carbonate and ammonium niobate (V) oxalate hydrate, with addition of ammonium titanyl oxalate monohydrate as a titanium source. The bulk structure and chemical composition of the materials are characterized using powder X-ray diffraction (XRD) and powder neutron diffraction, elemental homogeneity is studied using energy dispersive spectroscopy (EDS) mapping using transmission electron microscopy (TEM), and surface chemical analysis is examined using X-ray photoelectron spectroscopy (XPS). Nanoscale crystallites of between 10 and 50 nm are observed by TEM, where EDS reveals the homogeneity of metal distribution for the mixed-metal materials. Titanium carbonitrides are found to be air sensitive, reacting with air under ambient conditions, while titanium-niobium carbonitrides are found to degrade in aqueous sulfuric acid. The niobium carbonitrides, however, show some stability toward acidic solutions. Materials are produced with composition NbC1-xNx with x between 0.35 and 0.45, and more carbon-rich materials (x ≈ 0.35) are found as the synthesis temperature is increased, as proven by Rietveld refinement of crystal structure against powder neutron diffraction data. Despite phase purity seen by diffraction and negligible bulk carbon content, XPS shows a complex surface chemistry for the NbC1-xNx materials, with evidence for Nb2O5-like oxide species in a carbon-rich environment. The NbC1-xNx prepared at 900 °C has a surface area around 50 m2 g-1, making it suitable as a catalyst support. Loading with iridium provides a material active for the oxygen evolution reaction in 0.1 M sulfuric acid, with minimal leaching of either Nb or Ir after 1000 cycles.

Ultralow-Overpotential Acidic Oxygen Evolution Reaction Over Bismuth Telluride-Carbon Nanotube Heterostructure with Organic Framework

The state-of-the-art iridium and ruthenium oxides-based materials are best known for high efficiency and stability in acidic oxygen evolution reaction (OER). However, the development of economically feasible catalysts for water-splitting technologies is challenging by the requirements of low overpotential, high stability, and resistance of catalysts to dissolution during the acidic oxygen evolution reaction . Herein, an organometallic core-shell heterostructure composed of a carbon nanotube core (CNT) and bismuth telluride (Bi2Te3) shell (denoted as nC-Bi2Te3) is designed and use it as a catalyst for the acidic OER. The proposed catalyst achieves an ultralow overpotential of 160 mV at 10 mA cm-2 (geometrical), thereby outperforming most of the state-of-the-art precious-metal-based catalysts. The low Tafel slope of 30 mV dec-1 and charge transfer resistance (RCT) of 1.5 Ω demonstrate its excellent electrocatalytic activity. The morphological and chemical compositions of nC-Bi2Te3 enable the generation of ─OH functional group in the Bi─Te sections formed via a ligand support, which enhances the absorption capacity of H+ ions and increases the intrinsic catalytic activity. The presented insights regarding the material composition-structure relationship can help expand the application scope of high-performance catalysts.

Electron-Sponge Nature of Polyoxometalates for Next-Generation Electrocatalytic Water Splitting and Nonvolatile Neuromorphic Devices

Designing next-generation molecular devices typically necessitates plentiful oxygen-bearing sites to facilitate multiple-electron transfers. However, the theoretical limits of existing materials for energy conversion and information storage devices make it inevitable to hunt for new competitors. Polyoxometalates (POMs), a unique class of metal-oxide clusters, have been investigated exponentially due to their structural diversity and tunable redox properties. POMs behave as electron-sponges owing to their intrinsic ability of reversible uptake-release of multiple electrons. In this review, numerous POM-frameworks together with desired features of a contender material and inherited properties of POMs are systematically discussed to demonstrate how and why the electron-sponge-like nature of POMs is beneficial to design next-generation water oxidation/reduction electrocatalysts, and neuromorphic nonvolatile resistance-switching random-access memory devices. The aim is to converge the attention of scientists who are working separately on electrocatalysts and memory devices, on a point that, although the application types are different, they all hunt for a material that could exhibit electron-sponge-like feature to realize boosted performances and thus, encouraging the scientists of two completely different fields to explore POMs as imperious contenders to design next-generation nanodevices. Finally, challenges and promising prospects in this research field are also highlighted.

Co-doped RuO2 nanoparticles with enhanced catalytic activity and stability for the oxygen evolution reaction

Efficient and durable electrocatalysts for the oxygen evolution reaction (OER) play an important role in the use of hydrogen energy. Rutile RuO2, despite being considered as an advanced electrocatalyst for the OER, performs poorly in stability due to its easy oxidative dissolution at very positive (oxidizing) potentials. Herein, we report a type of Co-doped RuO2 nanoparticle for boosting OER catalytic activity and stability in alkaline solutions. The replacement of Ru by Co atoms with a lower ionic valence and smaller electronegativity can promote the generation of O vacancies and increase the electron density around Ru, thus enhancing the adsorption of oxygen species and inhibiting the peroxidative dissolution of RuO2 during the OER process. It was found that Ru0.95Co0.05Oy exhibited excellent OER performance with overpotentials as low as 217 mV at 10 mA cm-2 and 290 mV at 100 mA cm-2 in 1 M KOH, as well as outstanding stability in continuous testing for 50 h at a current density of 100 mA cm-2, and nearly no significant degradation after the accelerated durability test of 2000 cycles.

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.

Unveiling the Hidden Energy Profiles of the Oxygen Evolution Reaction via Machine Learning Analyses

The oxygen evolution reaction (OER) is a crucial electrochemical process for hydrogen production in water electrolysis. However, due to the involvement of multiple proton-coupled electron transfer steps, it is challenging to identify the specific elementary reaction that limits the rate of the OER. Here we employed a machine-learning-based approach to extract the reaction pathway exhaustively from experimental data. Genetic algorithms were applied to search for thermodynamic and kinetic parameters using the current-electrochemical potential relationship of the OER. Interestingly, analysis of the datasets revealed the energy state distributions of reaction intermediates, which likely originated in the interactions among intermediates or the distribution of multiple sites. Through our exhaustive analyses, we successfully uncovered the hidden energy profiles of the OER. This approach can reveal the reaction pathway to activate for efficient hydrogen production, which facilitates the design of catalysts.

A Quaternary mixed oxide protective scaffold for ruthenium during oxygen evolution reaction in acidic media

Proton exchange membrane water electrolysis is widely used in hydrogen production, but its application is limited by significant electrocatalyst dissolution at the anode during the oxygen evolution reaction (OER). The best performing electrocatalysts to date are based on ruthenium and iridium oxides, but these experience degradation even at moderate cell potentials. Here we investigate a quaternary Sn-Sb-Mo-W mixed oxide as a protective scaffold for ruthenium oxide. The acid-stable mixed oxide consists of an interconnected network of nanostructured oxides capable of stabilizing ruthenium into the matrix (Ru-MO). In combination with titanium fibre felt, we observed a lower degradation in the oxygen evolution reaction activity compared to unprotected ruthenium oxide after the electrochemical stress test. The superior stability of Ru-MO@Ti is attributed to the presence of MO which hinders the formation of reactive higher valence ruthenium (Ru+8). Our work demonstrates the potential of multi-metal oxides to extend the lifetime of the OER active metal and the titanium support.

Oxygen evolution electrocatalysts for proton exchange membrane water electrolysis encounter degradation even at moderate cell potentials. Piñeiro-García and colleagues develop a quaternary Sn-Sb-Mo-W mixed oxide scaffold to protect RuO2 against early dissolution under harsh acid conditions, extending the lifetime of catalysts as well as the titanium supports used in water electrolysis cells.

Nanotubular TiO x N y -Supported Ir Single Atoms and Clusters as Thin-Film Electrocatalysts for Oxygen Evolution in Acid Media

A versatile approach to the production of cluster- and single atom-based thin-film electrode composites is presented. The developed TiO _x N _y -Ir catalyst was prepared from sputtered Ti-Ir alloy constituted of 0.8 ± 0.2 at % Ir in α-Ti solid solution. The Ti-Ir solid solution on the Ti metal foil substrate was anodically oxidized to form amorphous TiO₂-Ir and later subjected to heat treatment in air and in ammonia to prepare the final catalyst. Detailed morphological, structural, compositional, and electrochemical characterization revealed a nanoporous film with Ir single atoms and clusters that are present throughout the entire film thickness and concentrated at the Ti/TiO _x N _y -Ir interface as a result of the anodic oxidation mechanism. The developed TiO _x N _y -Ir catalyst exhibits very high oxygen evolution reaction activity in 0.1 M HClO₄, reaching 1460 A g^-1 _Ir at 1.6 V vs reference hydrogen electrode. The new preparation concept of single atom- and cluster-based thin-film catalysts has wide potential applications in electrocatalysis and beyond. In the present paper, a detailed description of the new and unique method and a high-performance thin film catalyst are provided along with directions for the future development of high-performance cluster and single-atom catalysts prepared from solid solutions.

Recent Advances of Transition Metal Basic Salts for Electrocatalytic Oxygen Evolution Reaction and Overall Water Electrolysis

Electrocatalytic oxygen evolution reaction (OER) has been recognized as the bottleneck of overall water splitting, which is a promising approach for sustainable production of H2. Transition metal (TM) hydroxides are the most conventional and classical non-noble metal-based electrocatalysts for OER, while TM basic salts [M2+(OH)2-x(Am-)x/m, A = CO32-, NO3-, F-, Cl-] consisting of OH- and another anion have drawn extensive research interest due to its higher catalytic activity in the past decade. In this review, we summarize the recent advances of TM basic salts and their application in OER and further overall water splitting. We categorize TM basic salt-based OER pre-catalysts into four types (CO32-, NO3-, F-, Cl-) according to the anion, which is a key factor for their outstanding performance towards OER. We highlight experimental and theoretical methods for understanding the structure evolution during OER and the effect of anion on catalytic performance. To develop bifunctional TM basic salts as catalyst for the practical electrolysis application, we also review the present strategies for enhancing its hydrogen evolution reaction activity and thereby improving its overall water splitting performance. Finally, we conclude this review with a summary and perspective about the remaining challenges and future opportunities of TM basic salts as catalysts for water electrolysis.

Identification of the Origin for Reconstructed Active Sites on Oxyhydroxide for Oxygen Evolution Reaction

The regulation of atomic and electronic structures of active sites plays an important role in the rational design of oxygen evolution reaction (OER) catalysts toward electrocatalytic hydrogen generation. However, the precise identification of the active sites for surface reconstruction behavior during OER remains elusive for water-alkali electrolysis. Herein, irreversible reconstruction behavior accompanied by copper dynamic evolution for cobalt iron layered double hydroxide (CoFe LDH) precatalyst to form CoFeCuOOH active species with high-valent Co species is reported, identifying the origin of reconstructed active sites through operando UV-Visible (UV-vis), in situ Raman, and X-ray absorption fine-structure (XAFS) spectroscopies. Density functional theory analysis rationalizes this typical electronic structure evolution causing the transfer of intramolecular electrons to form ligand holes, promoting the reconstruction of active sites. Specifically, unambiguous identification of active sites for CoFeCuOOH is explored by in situ ¹⁸ O isotope-labeling differential electrochemical mass spectrometry (DEMS) and supported by theoretical calculation, confirming mechanism switch to oxygen-vacancy-site mechanism (OVSM) pathway on lattice oxygen. This work enables to elucidate the vital role of dynamic active-site generation and the representative contribution of OVSM pathway for efficient OER performance.

Challenges and Opportunities of Transition Metal Oxides as Electrocatalysts

As a sustainable energy technology, electrocatalytic energy conversion and storage has become increasingly prominent. The oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), nitrogen reduction reaction (NRR), and carbon dioxide reduction reaction (CO2 RR) are the key steps in the industrial applications of energy conversion and storage. Compared to the widely used precious metal catalysts, less-noble transition metal oxides (TMOs) and TMO-like materials have attracted broad attention as electrocatalysts in the above reactions. In this concept, we summarize the challenges and opportunities of some typical TMOs in electrocatalysis, and modification strategies of TMOs as electrocatalysts are discussed.

Facile Fabrication of Ni9 S8 /Ag2 S Intertwined Structures for Oxygen and Hydrogen Evolution Reactions

Here, we report the fabrication of the unique intertwined Ni₉ S₈ /Ag₂ S composite structure with hexagonal shape from their molecular precursors by one-pot thermal decomposition. Various spectroscopic and microscopic techniques were utilized to confirm the Ni₉ S₈ /Ag₂ S intertwined structure. Powder X-ray Powder Diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis suggest that there is an enrichment of Ni₉ S₈ phase in Ni₉ S₈ /Ag₂ S. The presence of Ag₂ S in Ni₉ S₈ /Ag₂ S improves the conductivity by reducing the interfacial energy and charge transfer resistance. When Ni₉ S₈ /Ag₂ S is employed as an electrocatalyst for electrochemical oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activity, it requires a low overpotential of 152 mV for HER and 277 mV for OER to obtain the geometrical current density of 10 mA cm^-2 , which is definitely superior to that of its components Ni₉ S₈ and Ag₂ S. This work provides a simple design route to develop an efficient and durable electrocatalyst with outstanding OER and HER performance and the present catalyst (Ni₉ S₈ /Ag₂ S) deserves as a potential candidate in the field of energy conversion systems.

Progress of Heterogeneous Iridium-Based Water Oxidation Catalysts

The water oxidation reaction (or oxygen evolution reaction, OER) plays a critical role in green hydrogen production via water splitting, electrochemical CO₂ reduction, and nitrogen fixation. The four-electron and four-proton transfer OER process involves multiple reaction intermediates and elementary steps that lead to sluggish kinetics; therefore, a high overpotential is necessary to drive the reaction. Among the different water-splitting electrolyzers, the proton exchange membrane type electrolyzer has greater advantages, but its anode catalysts are limited to iridium-based materials. The iridium catalyst has been extensively studied in recent years due to its balanced activity and stability for acidic OER, and many exciting signs of progress have been made. In this review, the surface and bulk Pourbaix diagrams of iridium species in an aqueous solution are introduced. The iridium-based catalysts, including metallic or oxides, amorphous or crystalline, single crystals, atomically dispersed or nanostructured, and iridium compounds for OER, are then elaborated. The latest progress of active sites, reaction intermediates, reaction kinetics, and elementary steps is summarized. Finally, future research directions regarding iridium catalysts for acidic OER are discussed.

Enhancing the catalytic OER performance of MoS2via Fe and Co doping

Enhancing the sluggish kinetics of the electrochemical oxygen evolution reaction (OER) is crucial for many clean-energy production technologies. Although much progress has been made in recent years, developing active, stable, and cost-effective OER electrocatalysts is still challenging. The layered MoS₂, based on Earth-abundant elements, is widely explored as a promising hydrogen evolution electrocatalyst but exhibits poor OER activity. Here, we report a facile strategy to improve the sluggish OER of MoS₂ through co-doping MoS₂ nanosheets with Fe and Co atoms. The synergistic effect obtained by adjusting the Co/Fe ratio in the Fe-Co doped MoS₂ induces electronic and structural modifications and a richer active surface area morphology resulting in a relatively low OER overpotential of 380 mV (at 10 mA cm^-2). The electronic modulation upon doping was further supported by DFT calculations that show favorable interaction with the OER intermediate species, thus reducing the energy barrier for the OER. This work paves the way for future strategies for tailoring the electronic properties of transition-metal dichalcogenides (TMDCs) to activate the structure for the sluggish OER with the assistance of non-noble-metal materials.

Tailoring the active site for the oxygen evolution reaction on a Pt electrode

Highly active electrocatalysts for the oxygen evolution reaction (OER) are essential to improve the efficiency of water electrolysis. The properties of OER active sites on single-crystal Pt electrodes were examined herein. The OER is markedly enhanced by repeated oxidative and reductive potential cycles on the Pt(111) surface. The OER activity on Pt(111) is nine times higher in the third cycle than that before the potential cycles. OER activation by potential cycling depends on the (111) terrace width, with wider (111) terraces significantly enhancing the OER. The oxidation/reduction of the Pt(111) surface produces atomic-sized vacancies on the terraces that activate the OER. Structural analysis using X-ray diffraction reveals that the active sites formed by potential cycling are defects in the second subsurface Pt layer. Potential cycling induces the bowl-shaped roughening of the electrode surface, wherein high-coordination number Pt atoms at the bottom of the cavities activate the OER.

Tuning Electronic Structure and Composition of FeNi Nanoalloys for Enhanced Oxygen Evolution Electrocatalysis via a General Synthesis Strategy

Developing low-cost and efficient oxygen evolution electrocatalysts is key to decarbonization. A facile, surfactant-free, and gram-level biomass-assisted fast heating and cooling synthesis method is reported for synthesizing a series of carbon-encapsulated dense and uniform FeNi nanoalloys with a single-phase face-centered-cubic solid-solution crystalline structure and an average particle size of sub-5 nm. This method also enables precise control of both size and composition. Electrochemical measurements show that among Fe_x Ni_{(1- x )} nanoalloys, Fe_0.5 Ni_0.5 has the best performance. Density functional theory calculations support the experimental findings and reveal that the optimally positioned d-band center of O-covered Fe_0.5 Ni_0.5 renders a half-filled antibonding state, resulting in moderate binding energies of key reaction intermediates. By increasing the total metal content from 25 to 60 wt%, the 60% Fe_0.5 Ni_0.5 /40% C shows an extraordinarily low overpotential of 219 mV at 10 mA cm^-2 with a small Tafel slope of 23.2 mV dec^-1 for the oxygen evolution reaction, which are much lower than most other FeNi-based electrocatalysts and even the state-of-the-art RuO₂ . It also shows robust durability in an alkaline environment for at least 50 h. The gram-level fast heating and cooling synthesis method is extendable to a wide range of binary, ternary, quaternary nanoalloys, as well as quinary and denary high-entropy-alloy nanoparticles.

Isovalent anion-induced electrochemical activity of doped Co3V2O8 for oxygen evolution reaction application

The activity of an OER electrocatalyst is a strong function of the reaction kinetics at the active sites, which can be influenced by catalytic engineering (e.g., heterostructure, doping, and the addition of cocatalysts). Herein, we report the improved reaction kinetics of cobalt oxide for the OER via the addition of high valence vanadium and thereafter doping with sulphur (S-Co₃V₂O₈). The addition of vanadium increases the oxygen vacancy while the doping of sulphur increases the electronic conductivity of the electrocatalyst. The synergic effect of the oxygen vacancy and electronic conductivity increases the activity of S-Co₃V₂O₈. Furthermore, S-Co₃V₂O₈ showed the least Tafel slope, which showed the activity enhancement towards the oxygen evolution reaction. Moreover, the underlying reaction mechanism is explored by electrochemical impedance spectroscopy, which reveals that the ratio of polarisation resistance to double-layer capacitance is minimum for S-Co₃V₂O₈, indicating the highest activity.

A Single Source, Scalable Route for Direct Isolation of Earth-Abundant Nanometal Carbide Water-Splitting Electrocatalysts

Single-phase M_xCs (M = Fe, Co, and Ni) were prepared by solvothermal conversion of Prussian blue single source precursors. The single source precursor is prepared in water, and the conversion process is carried out in alkylamines at reaction temperatures above 200 °C. The reaction is scalable using a commercial source of Fe-PB. High-resolution transmission electron microscopy, X-ray photoelectron microscopy, and powder X-ray diffraction confirm that carbides have thin oxide termination but lack graphitic surfaces. Electrocatalytic activity reveals that Fe₃C and Co₂C are oxygen evolution reaction electrocatalysts, while Ni₃C is a bifunctional [OER and hydrogen evolution reaction (HER)] electrocatalyst.

Stabilization of a Mn-Co Oxide During Oxygen Evolution in Alkaline Media

Improving the stability of electrocatalysts for the oxygen evolution reaction (OER) through materials design has received less attention than improving their catalytic activity. We explored the effects of Mn addition to a cobalt oxide for stabilizing the catalyst by comparing single phase CoOx and (Co0.7Mn0.3)Ox films electrodeposited in alkaline solution. The obtained disordered films were classified as layered oxides using X-ray absorption spectroscopy (XAS). The CoOx films showed a constant decrease in the catalytic activity during cycling, confirmed by oxygen detection, while that of (Co0.7Mn0.3)Ox remained constant within error as measured by electrochemical metrics. These trends were rationalized based on XAS analysis of the metal oxidation states, which were Co2.7+ and Mn3.7+ in the bulk and similar near the surface of (Co0.7Mn0.3)Ox, before and after cycling. Thus, Mn in (Co0.7Mn0.3)Ox successfully stabilized the bulk catalyst material and its surface activity during OER cycling. The development of stabilization approaches is essential to extend the durability of OER catalysts.

Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60-80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)2 on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm2, respectively.

RuO2 electronic structure and lattice strain dual engineering for enhanced acidic oxygen evolution reaction performance

Developing highly active and durable electrocatalysts for acidic oxygen evolution reaction remains a great challenge due to the sluggish kinetics of the four-electron transfer reaction and severe catalyst dissolution. Here we report an electrochemical lithium intercalation method to improve both the activity and stability of RuO₂ for acidic oxygen evolution reaction. The lithium intercalates into the lattice interstices of RuO₂, donates electrons and distorts the local structure. Therefore, the Ru valence state is lowered with formation of stable Li-O-Ru local structure, and the Ru–O covalency is weakened, which suppresses the dissolution of Ru, resulting in greatly enhanced durability. Meanwhile, the inherent lattice strain results in the surface structural distortion of Li_xRuO₂ and activates the dangling O atom near the Ru active site as a proton acceptor, which stabilizes the OOH* and dramatically enhances the activity. This work provides an effective strategy to develop highly efficient catalyst towards water splitting.

While water splitting in acid offers higher operational performances than in alkaline conditions, there are few high-activity, acid-stable oxygen evolution electrocatalysts. Here, authors examine electrochemical Li intercalation to improve the activity and stability of RuO₂ for acidic water oxidation.

The low overpotential regime of acidic water oxidation part I: the importance of O2 detection

The high overpotential required for the oxygen evolution reaction (OER) represents a significant barrier for the production of closed-cycle renewable fuels and chemicals. Ruthenium dioxide is among the most active catalysts for OER in acid, but the activity at low overpotentials can be difficult to measure due to high capacitance. In this work, we use electrochemistry - mass spectrometry to obtain accurate OER activity measurements spanning six orders of magnitude on a model series of ruthenium-based catalysts in acidic electrolyte, quantifying electrocatalytic O₂ production at potential as low as 1.30 V_RHE. We show that the potential-dependent O₂ production rate, i.e., the Tafel slope, exhibits three regimes, revealing a previously unobserved Tafel slope of 25 mV decade^-1 below 1.4 V_RHE. We fit the expanded activity data to a microkinetic model based on potential-dependent coverage of the surface intermediates from which the rate-determining step takes place. Our results demonstrate how the familiar quantities "onset potential" and "exchange current density" are influenced by the sensitivity of the detection method.

Spectroelectrochemistry of Water Oxidation Kinetics in Molecular versus Heterogeneous Oxide Iridium Electrocatalysts

Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir₂(pyalc)₂(H₂O)₄-(μ-O)]²⁺ (Ir_Molecular, pyalc = 2-(2′pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrO_x) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)–Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for Ir_Molecular contrasting with potential-dependent kinetics for IrO_x. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of Ir_Molecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.

Synergetic Effects of Mixed-Metal Polyoxometalates@Carbon-Based Composites as Electrocatalysts for the Oxygen Reduction and the Oxygen Evolution Reactions

The smart choice of polyoxometalates (POMs) and the design of POM@carbon-based composites are promising tools for producing active electrocatalysts for both the oxygen reduction (ORR) and the oxygen evolution reactions (OER). Hence, herein, we report the preparation, characterization and application of three composites based on doped, multi-walled carbon nanotubes (MWCNT_N6) and three different POMs (Na12[(FeOH2)2Fe2(As2W15O56)2]·54H2O, Na12[(NiOH2)2Ni2(As2W15O56)2]·54H2O and Na14[(FeOH2)2Ni2(As2W15O56)2]·55H2O) as ORR and OER electrocatalysts in alkaline medium (pH = 13). Overall, the three POM@MWCNT_N6 composites showed good ORR performance with onset potentials between 0.80 and 0.81 V vs. RHE and diffusion-limiting current densities ranging from −3.19 to −3.66 mA cm−2. Fe4@MWCNT_N6 and Fe2Ni2@MWCNT_N6 also showed good stability after 12 h (84% and 80% of initial current). The number of electrons transferred per O2 molecule was close to three, suggesting a mixed regime. Moreover, the Fe2Ni2@MWCNT_N6 presented remarkable OER performance with an overpotential of 0.36 V vs. RHE (for j = 10 mA cm−2), a jmax close to 135 mA cm−2 and fast kinetics with a Tafel slope of 45 mV dec−1. More importantly, this electrocatalyst outperformed not only most POM@carbon-based composites reported so far but also the state-of-the-art RuO2 electrocatalyst. Thus, this work represents a step forward towards bifunctional electrocatalysts using less expensive materials.

Cobalt Phosphotungstate-Based Composites as Bifunctional Electrocatalysts for Oxygen Reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are key reactions in energy-converting systems, such as fuel cells (FCs) and water-splitting (WS) devices. However, the current use of expensive Pt-based electrocatalysts for ORR and IrO2 and RuO2 for OER is still a major drawback for the economic viability of these clean energy technologies. Thus, there is an incessant search for low-cost and efficient electrocatalysts (ECs). Hence, herein, we report the preparation, characterization (Raman, XPS, and SEM), and application of four composites based on doped-carbon materials (CM) and cobalt phosphotungstate (MWCNT_N8_Co4, GF_N8_Co4, GF_ND8_Co4, and GF_NS8_Co4) as ORR and OER electrocatalysts in alkaline medium (pH = 13). Structural characterization confirmed the successful carbon materials doping with N and/or N, S, and the incorporation of the cobalt phosphotungstate. Overall, all composites showed good ORR performance with onset potentials ranging from 0.83 to 0.85 V vs. RHE, excellent tolerance to methanol crossover with current retentions between 88 and 90%, and good stability after 20,000 s at E = 0.55 V vs. RHE (73% to 82% of initial current). In addition, the number of electrons transferred per O2 molecule was close to four, suggesting selectivity to the direct process. Moreover, these composites also presented excellent OER performance with GF_N8_Co4 showing an overpotential of 0.34 V vs. RHE (for j = 10 mA cm−2) and jmax close to 70 mA cm−2. More importantly, this electrocatalyst outperformed state-of-the-art IrO2 electrocatalyst. Thus, this work represents a step forward toward bifunctional electrocatalysts using less expensive materials.

Nickel-Based Metal-Organic Frameworks as Electrocatalysts for the Oxygen Evolution Reaction (OER)

The exploration of earth-abundant electrocatalysts with high performance for the oxygen evolution reaction (OER) is eminently desirable and remains a significant challenge. The composite of the metal-organic framework (MOF) Ni₁₀Co-BTC (BTC = 1,3,5-benzenetricarboxylate) and the highly conductive carbon material ketjenblack (KB) could be easily obtained from the MOF synthesis in the presence of KB in a one-step solvothermal reaction. The composite and the pristine MOF perform better than commercially available Ni/NiO nanoparticles under the same conditions for the OER. Activation of the nickel-cobalt clusters from the MOF can be seen under the applied anodic potential, which steadily boosts the OER performance. Ni₁₀Co-BTC and Ni₁₀Co-BTC/KB are used as sacrificial agents and undergo structural changes during electrochemical measurements, the stabilized materials show good OER performances.

Towards the Rational Design of Stable Electrocatalysts for Green Hydrogen Production

Now, it is time to set up reliable water electrolysis stacks with active and robust electrocatalysts to produce green hydrogen. Compared with catalytic kinetics, much less attention has been paid to catalyst stability, and the weak understanding of the catalyst deactivation mechanism restricts the design of robust electrocatalysts. Herein, we discuss the issues of catalysts’ stability evaluation and characterization, and the degradation mechanism. The systematic understanding of the degradation mechanism would help us to formulate principles for the design of stable catalysts. Particularly, we found that the dissolution rate for different 3d transition metals differed greatly: Fe dissolves 114 and 84 times faster than Co and Ni. Based on this trend, we designed Fe@Ni and FeNi@Ni core-shell structures to achieve excellent stability in a 1 A cm−2 current density, as well as good catalytic activity at the same time.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

A Protocol for Electrocatalyst Stability Evaluation: H2O2 Electrosynthesis for Industrial Wastewater Treatment

Electrocatalysis has been proposed as a versatile technology for wastewater treatment and reuse. While enormous attention has been centered on material synthesis and design, the practicality of such catalyst materials remains clouded by a lack of both stability assessment protocols and understanding of deactivation mechanisms. In this study, we develop a protocol to identify the wastewater constituents most detrimental to electrocatalyst performance in a timely manner and elucidate the underlying phenomena behind these losses. Synthesized catalysts are electrochemically investigated in various electrolytes based on real industrial effluent characteristics and methodically subjected to a sequence of chronopotentiometric stability tests, in which each stage presents harsher operating conditions. To showcase, oxidized carbon black is chosen as a model catalyst for the electrosynthesis of H₂O₂, a precursor for advanced oxidation processes. Results illustrate severe losses in catalyst activity and/or selectivity upon the introduction of metal pollutants, namely magnesium and zinc. The insights garnered from this protocol serve to translate lab-scale electrocatalyst developments into practical technologies for industrial water treatment purposes.

Principles of Water Electrolysis and Recent Progress in Cobalt-, Nickel-, and Iron-Based Oxides for the Oxygen Evolution Reaction

Water electrolysis that results in green hydrogen is the key process towards a circular economy. The supply of sustainable electricity and availability of oxygen evolution reaction (OER) electrocatalysts are the main bottlenecks of the process for large-scale production of green hydrogen. A broad range of OER electrocatalysts have been explored to decrease the overpotential and boost the kinetics of this sluggish half-reaction. Co-, Ni-, and Fe-based catalysts have been considered to be potential candidates to replace noble metals due to their tunable 3d electron configuration and spin state, versatility in terms of crystal and electronic structures, as well as abundance in nature. This Review provides some basic principles of water electrolysis, key aspects of OER, and significant criteria for the development of the catalysts. It provides also some insights on recent advances of Co-, Ni-, and Fe-based oxides and a brief perspective on green hydrogen production and the challenges of water electrolysis.

Electrochemical evaluation of the de-/re-activation of oxygen evolving Ir oxide

Understanding the influence of dynamic and stationary polarization on the deactivation of state-of-the-art IrOx catalysts is imperative for the design and operation of robust and efficient proton exchange membrane water...