Influence of Surface Adsorption on the Oxygen Evolution Reaction on IrO2(110)

Performance of Nitrogen-Doped Carbon Nanoparticles Carrying FeNiCu as Bifunctional Electrocatalyst for Rechargeable Zinc-Air Battery

Catalysts for zinc-air batteries (ZABs) must be stable over long-term charging-discharging cycles and exhibit bifunctional catalytic activity. In this study, by doping nitrogen-doped carbon (NC) materials with three metal atoms (Fe, Ni, and Cu), a single-atom-distributed FeNiCu-NC bifunctional catalyst is prepared. The catalyst includes Fe(Ni-doped)-N4 for the oxygen evolution reaction (OER), Fe(Cu-doped)-N4 for the oxygen reduction reaction (ORR), and the NiCu-NC catalytic structure for the oxygen reduction reaction (ORR) in the nitrogen-doped carbon nanoparticles. This single-atom distribution catalyst structure enhances the bifunctional catalytic activity. If a trimetallic single-atom catalyst is designed, it will surpass the typical bimetallic single-atom catcalyst. FeNiCu-NC exhibits outstanding performance as an electrocatalyst, with a half-wave potential (E1/2) of 0.876 V versus RHE, overpotential (Ej = 10) of 253 mV versus RHE at 10 mA cm-2, and a small potential gap (ΔE = 0.61 V). As the anode in a ZAB, FeNiCu-NC can undergo continuous charge-discharged cycles for 575 h without significant attenuation. This study presents a new method for achieving high-performance, low-cost ZABs via trimetallic single-atom doping.

Role of Electrolyte pH on Water Oxidation for Iridium Oxides

Understanding the effect of noncovalent interactions of intermediates at the polarized catalyst-electrolyte interface on water oxidation kinetics is key for designing more active and stable electrocatalysts. Here, we combine operando optical spectroscopy, X-ray absorption spectroscopy (XAS), and surface-enhanced infrared absorption spectroscopy (SEIRAS) to probe the effect of noncovalent interactions on the oxygen evolution reaction (OER) activity of IrOx in acidic and alkaline electrolytes. Our results suggest that the active species for the OER (Ir4.x+-*O) binds much stronger in alkaline compared with acid at low coverage, while the repulsive interactions between these species are higher in alkaline electrolytes. These differences are attributed to the larger fraction of water within the cation hydration shell at the interface in alkaline electrolytes compared to acidic electrolytes, which can stabilize oxygenated intermediates and facilitate long-range interactions between them. Quantitative analysis of the state energetics shows that although the *O intermediates bind more strongly than optimal in alkaline electrolytes, the larger repulsive interaction between them results in a significant weakening of *O binding with increasing coverage, leading to similar energetics of active states in acid and alkaline at OER-relevant potentials. By directly probing the electrochemical interface with complementary spectroscopic techniques, our work goes beyond conventional computational descriptors of the OER activity to explain the experimentally observed OER kinetics of IrOx in acidic and alkaline electrolytes.

Theoretical Insights on the Charge State and Bifunctional OER/ORR Electrocatalyst Activity in 4d-Transition-Metal-Doped g-C3N4 Monolayers

Exploring efficient and stable electrocatalysts for the bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is vital to developing renewable energy technologies. However, due to the substantial and intricate design space associated with these bifunctional OER/ORR electrocatalysts, their development presents a formidable challenge, resulting in their cost-prohibitive nature in both experimental and computational studies. Herein, using the defect physics method, we systematically investigate the formation energies and bifunctional overpotential (ηBi) of 4d-transition-metal (4d-TM, 4d-TM = Zr, Nb, Mo, Ru, Rh, Pd, and Ag)-doped monolayer supercell g-C3N4 (4d-TM@C54N72) based on the density functional theory (DFT) calculations. Under N-rich and C-rich conditions, we find that the formation energies of RhN@C54N71 (Rh occupation N) and PdN@C54N71 (Pd occupation N) are smaller than that of other 4d-TMN@C54N71 (4d-TM occupation N site); for the 4d-TMint@C54N72 (4d-TM interstitial site occupation), the lowest-formation energy defects are Pdint@C54N72. These results indicate that they have better stabilities. Interestingly, for these formation energy lower systems, Pd0int@C54N72 (ηBi = 1.00 V) and Rh1+N@C54N71 (ηBi = 0.73 V) have ultralow overpotential and can be great candidates for bifunctional OER/ORR electrocatalysts. We find the reason is that adjusting the charge states of 4d-TM@C54N72 can tune the interaction strength between the oxygenated intermediates and the 4d-TM@C54N72, which plays a crucial role in the activity of reactions. Additionally, the data obtained through machine learning (ML) application suggest that the electronegativity (Nm) and bond length of 4d-TM and coordination atoms (dTM-OOH) are primary descriptors characterizing the OER and ORR activities, respectively. The charged defect tuning of the bifunctional OER/ORR activity for 4d-TM@C54N72 would enable electrocatalytic performance optimization and the development of potential electrocatalysts for renewable energy applications.

Synergistic effect of composition gradient and morphology on the catalytic activity of amorphous FeCoNi-LDH

The rational design of electrocatalysts with well-designed compositions and structures for the oxygen evolution reaction (OER) is promising and challenging. Herein, we developed a novel strategy - a one-step double-cation etching sedimentation equilibrium strategy - to synthesize amorphous hollow Fe-Co-Ni layered double hydroxide nanocages with an outer surface of vertically interconnected ultrathin nanosheets (Fe-Co-Ni-LDH), which primarily depends on the in situ etching sedimentation equilibrium of the template interface. This unique vertical nanosheet-shell hierarchical nanostructure possesses enhanced charge transfer, increased active sites, and favorable kinetics during electrolysis, resulting in superb electrocatalytic performance for the oxygen evolution reaction (OER). Specifically, the Fe-Co-Ni-LDH nanocages exhibited remarkable OER activity in alkaline electrolytes and achieved a current density of 100 mA cm-2 at a low overpotential of 272 mV with excellent stability. This powerful strategy provides a profound molecular-level insight into the control of the morphology and composition of 2D layered materials.

Importance of the Walden Inversion for the Activity Volcano Plot of Oxygen Evolution

Since the birth of the computational hydrogen electrode approach, it is considered that activity trends of electrocatalysts in a homologous series can be quantified by the construction of volcano plots. This method aims to steer materials discovery by the identification of catalysts with an improved reaction kinetics, though evaluated by means of thermodynamic descriptors. The conventional approach for the volcano plot of the oxygen evolution reaction (OER) relies on the assumption of the mononuclear mechanism, comprising the * OH, * O, and * OOH intermediates. In the present manuscript, two new mechanistic pathways, comprising the idea of the Walden inversion in that bond-breaking and bond-making occurs simultaneously, are factored into a potential-dependent OER activity volcano plot. Surprisingly, it turns out that the Walden inversion plays an important role since the activity volcano is governed by mechanistic pathways comprising Walden steps rather than by the traditionally assumed reaction mechanisms under typical OER conditions.

Kinetics of the Oxygen Evolution Reaction (OER) on Amorphous and Crystalline Iridium Oxide Surfaces in Acidic Medium

Amorphous and crystalline IrO2 catalysts are synthesized by the Adams method and characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The oxygen evolution reaction (OER) is investigated on both the catalyst surfaces in 0.5 M H2SO4 electrolyte. The Tafel slope estimated in the temperature range of 293-333 K on the two surfaces indicates a change in the rate-limiting steps. The data are also analyzed in terms of the Eyring equation to estimate the activation enthalpy (ΔH#) and pre-exponential factor (Af) as a function of overpotential and therefore the charge-transfer coefficient (α). The estimated α values suggest strong electrocatalysis on both the surfaces. While the ΔH# plays a decisive role in the electrocatalysis on the amorphous sample, the trend of Af indicates that an increase in the entropy on the crystalline surface is pivotal in reducing the reaction barrier.

Acid-Base Chemistry of a Model IrO2 Catalytic Interface

Iridium oxide (IrO2) is one of the most efficient catalytic materials for the oxygen evolution reaction (OER), yet the atomic scale structure of its aqueous interface is largely unknown. Herein, the hydration structure, proton transfer mechanisms, and acid-base properties of the rutile IrO2(110)-water interface are investigated using ab initio based deep neural-network potentials and enhanced sampling simulations. The proton affinities of the different surface sites are characterized by calculating their acid dissociation constants, which yield a point of zero charge in agreement with experiments. A large fraction (≈80%) of adsorbed water dissociation is observed, together with a short lifetime (≈0.5 ns) of the resulting terminal hydroxy groups, due to rapid proton exchanges between adsorbed H2O and adjacent OH species. This rapid surface proton transfer supports the suggestion that the rate-determining step in the OER may not involve proton transfer across the double layer into solution, as indicated by recent experiments.

On the mechanistic complexity of oxygen evolution: potential-dependent switching of the mechanism at the volcano apex

The anodic four-electron oxygen evolution reaction (OER) corresponds to the limiting process in acidic or alkaline electrolyzers to produce gaseous hydrogen at the cathode of the device. In the last decade, tremendous efforts have been dedicated to the identification of active OER materials by electronic structure calculations in the density functional theory approximation. Most of these works rely on the assumption that the mononuclear mechanism, comprising the *OH, *O, and *OOH intermediates, is operative under OER conditions, and that a single elementary reaction step (most likely *OOH formation) governs the kinetics. In the present manuscript, six different OER mechanisms are analyzed, and potential-dependent volcano curves are constructed to comprehend the electrocatalytic activity of these pathways in the approximation of the descriptor G_max(U), a potential-dependent activity measure based on the notion of the free-energy span model. While the mononuclear description mainly describes the legs of the volcano plot, corresponding to electrocatalysts with low intrinsic activity, it is demonstrated that the preferred pathway at the volcano apex is a strong function of the applied electrode potential. The observed mechanistic complexity including a switch of the favored pathway with increasing overpotential sets previous investigations aiming at the identification of reaction mechanisms and limiting steps into question since the entire breadth of OER pathways was not accounted for. A prerequisite for future atomic-scale studies on highly active OER catalysts refers to the evaluation of several mechanistic pathways so that neither important mechanistic features are overlooked nor limiting steps are incorrectly determined.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Electrolyte Engineering Stabilizes Photoanodes Decorated with Molecular Catalysts

Molecular catalysts are promising oxygen evolution promoters in conjunction with photoanodes for solar water splitting. Maintaining the stability of both photoabsorber and cocatalyst is still a prime challenge, with many efforts tackling this issue through sophisticated material designs. Such approaches often mask the importance of the electrode-electrolyte interface and overlook easily tunable system parameters, such as the electrolyte environment, to improve efficiency. We provide a systematic study on the activity-stability relationship of a prominent Fe₂ O₃ photoanode modified with Ir molecular catalysts using in situ mass spectroscopy. After gaining detailed insights into the dissolution behavior of the Ir cocatalyst, a comprehensive pH study is conducted to probe the impact of the electrolyte on the performance. An inverse trend in Fe and Ir stability is found, with the best activity-stability synergy obtained at pH 9.7. The results bring awareness to the overall photostability and electrolyte engineering when advancing catalysts for solar water splitting.

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.

Volcano relationships and a new activity descriptor of 2D transition metal-Fe layered double hydroxides for efficient oxygen evolution reaction

Iron (Fe) sites play a critical role in boosting the catalytic activity of transition metal layered double hydroxide (LDH) electrocatalysts for the oxygen evolution reaction (OER), but the contribution of the Fe content to the catalysis of Fe-doped LDHs is still not well understood. Herein, a series of two-dimensional (2D) Fe-doped MFe-LDHs (M = Co, Ni, Cu, and Mn) was synthesized via a general molecular self-assembly method to track the role of Fe in their electrocatalytic OER activities. Besides the revelation of the intrinsic activity trend of NiFe > CoFe > MnFe > CuFe, volcano-shaped relationships among the catalytic activity descriptors, i.e., overpotential, Tafel slope, and turnover frequency (TOF), and the Fe-content in MFe-LDHs, were identified. Specifically, a ∼20% Fe content resulted in the highest OER performance for the LDH, while excess Fe compromised its activity. A similar volcano relationship was determined between the intermediate adsorption and Fe content via operando impedance spectroscopy (EIS) measurements, and it was shown that the intermediate adsorption capacitance (CPE_ad) can be a new activity descriptor for electrocatalysts. In this work, we not only performed a systematic study on the role of Fe in 2D Fe-doped LDHs but also offer some new insights into the activity descriptors for electrocatalysts.

Electrochemically Reconstructed Vanadic Oxide-Doped Cobalt Pyrophosphate as an Electrocatalyst for the Oxygen Evolution Reaction

More and more attention has been paid to the development of the efficient electrocatalysts for the oxygen evolution reaction (OER). Herein, a porous vanadic oxide-doped cobalt pyrophosphate electrocatalyst, namely V₂O₅-Co₂P₂O₇, was exploited by using the electrochemical reconstruction method in the alkaline electrolyte and selecting a cobalt vanadium phosphate Co(H₂O)₄(VOPO₄)₂ as a precursor. The reconstructed vanadic oxide-doped cobalt pyrophosphate catalyst V₂O₅-Co₂P₂O₇ exhibited efficient electrocatalytic activity for the OER in 1.0 M KOH, requiring a low overpotential of 199 mV at 10 mA cm^-2, compared to the reported pyrophosphate electrocatalysts. The porous morphology and doping of vanadic oxide after electrochemical reconstruction were beneficial to enhance the electrocatalytic performance for the OER, through improving the surface area to bring in more accessibly active sites and regulating the electronic structures. The results provided a promising strategy to prepare the pyrophosphate electrocatalysts and improve the performance of the OER catalyst.

Electrochemical Reconstruction of NiFe/NiFeOOH Superparamagnetic Core/Catalytic Shell Heterostructure for Magnetic Heating Enhancement of Oxygen Evolution Reaction

Although (oxy)hydroxides generated by electrochemical reconstruction (EC-reconstruction) of transition-metal catalysts exhibit highly catalytic activities, the amorphous nature fundamentally impedes the electrochemical kinetics due to its poor electrical conductivity. Here, EC-reconstructed NiFe/NiFeOOH core/shell nanoparticles in highly conductive carbon matrix based on the pulsed laser deposition prepared NiFe nanoparticles is successfully confined. Electrochemical characterizations and first-principles calculations demonstrate that the reconstructed NiFe/NiFeOOH core/shell nanoparticles exhibit high oxygen evolution reaction (OER) electrocatalytic activity (a low overpotential of 342.2 mV for 10 mA cm^-2 ) and remarkable durability due to the efficient charge transfer in the highly conductive confined heterostructure. More importantly, benefit from the superparamagnetic nature of the reconstructed NiFe/NiFeOOH core/shell nanoparticles, a large OER improvement is achieved (an ultralow overpotential of 209.2 mV for 10 mA cm^-2 ) with an alternating magnetic field stimulation. Such OER improvement can be attributed to the Néel relaxation related magnetic heating effect functionalized superparamagnetic NiFe cores, which are generally underutilized in reconstructed core/shell nanoparticles. This work demonstrates that the designed superparamagnetic core/shell nanoparticles, combined with the large improvement by magnetic heating effect, are expected to be highly efficient OER catalysts along with the confined structure guaranteed high conductivity and catalytic stability.

Adsorption Energy in Oxygen Electrocatalysis

Adsorption energy (AE) of reactive intermediate is currently the most important descriptor for electrochemical reactions (e.g., water electrolysis, hydrogen fuel cell, electrochemical nitrogen fixation, electrochemical carbon dioxide reduction, etc.), which can bridge the gap between catalyst's structure and activity. Tracing the history and evolution of AE can help to understand electrocatalysis and design optimal electrocatalysts. Focusing on oxygen electrocatalysis, this review aims to provide a comprehensive introduction on how AE is selected as the activity descriptor, the intrinsic and empirical relationships related to AE, how AE links the structure and electrocatalytic performance, the approaches to obtain AE, the strategies to improve catalytic activity by modulating AE, the extrinsic influences on AE from the environment, and the methods in circumventing linear scaling relations of AE. An outlook is provided at the end with emphasis on possible future investigation related to the obstacles existing between adsorption energy and electrocatalytic performance.

Synergistic effect of p-type and n-type dopants in semiconductors for efficient electrocatalytic water splitting

The main challenge for acidic water electrolysis is the lack of active and stable oxygen evolution catalysts based on abundant materials, which are globally scalable. Iridium oxide is the only material which is active and stable. However, Ir is extremely rare. While both active materials and stable materials exist, those that are active are usually not stable and vice versa. In this work, we present a new design strategy for activating stable materials originally deemed unsuitable due to a semiconducting nature and wide band gap energy. These stable semiconductors cannot change oxidation state under the relevant reaction conditions. Based on DFT calculations, we find that adding an n-type dopant facilitates oxygen binding on semiconductor surfaces. The binding is, however, strong and prevents further binding or desorption of oxygen. By combining both n-type and p-type dopants, the reactivity can be tuned so that oxygen can be adsorbed and desorbed under reaction conditions. The tuning results from the electrostatic interactions between the dopants as well as between the dopants and the binding site. This concept is experimentally verified on TiO₂ by co-substituting with different pairs of n- and p-type dopants. Our findings suggest that the co-substitution approach can be used to activate stable materials, with no intrinsic oxygen evolution activity, to design new catalysts for acid water electrolysis.

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.

Multistage Electron Distribution Engineering of Iridium Oxide by Codoping W and Sn for Enhanced Acidic Water Oxidation Electrocatalysis

Developing efficient and robust anodic electrocatalysts to implement the proton-exchange membrane (PEM) electrolyzer is critical for hydrogen generation. Nevertheless, the only known applicable anode catalyst IrO_x in PEM electrolyzers still requires high overpotential due to the weak binding energy between oxygen intermediates and active sites, limiting its wide applications. Herein, a ternary Ir_0.7 W_0.2 Sn_0.1 O_x nanocatalyst synthesized through a sol-gel strategy, exhibits a low overpotential of 236 mV (10 mA cm^-2 _geo ) for thoxygen evolution reaction (OER), accompanied with robust durability over 220 h at 1 A cm^-2 _geo in 0.5 m H₂ SO₄ . Moreover, the optimized Ir_0.7 W_0.2 Sn_0.1 O_x delivers a prominent mass activity of 722.7 A g^-1 _Ir at 1.53 V (vs RHE), which is around 34 times higher compared with that of IrO_x . The mircrostructural analyses reveal that codoping of W and Sn stabilizes Ir with a valence state lower than 4+ through multistage charge redistribution, avoiding the overoxidation of Ir above 1.6 V versus RHE and enhancing the acidic OER performance. Additionally, density functional theory calculations reveal that codoping of W and Sn moves the d band center of Ir to the Fermi level, thus enhancing the binding energies of oxygen intermediates with Ir sites and decreasing the energy barrier toward acidic OER.

Mass Transport Limitations in Electrochemical Conversion of CO2 to Formic Acid at High Pressure

Mass transport of different species plays a crucial role in electrochemical conversion of CO2 due to the solubility limit of CO2 in aqueous electrolytes. In this study, we investigate the transport of CO2 and other ionic species through the electrolyte and the membrane, and its impact on the scale-up process of HCOO−/HCOOH formation. The mass transport of ions to the electrode and the membrane is modelled at constant current density. The mass transport limitations of CO2 on the formation of HCOO−/HCOOH is investigated at different pressures ranges from 5–40 bar. The maximum achievable partial current density of formate/formic acid is increased with increasing CO2 pressure. We use an ion exchange membrane model to understand the ion transport behaviour for both the monopolar and bipolar membranes. The cation exchange (CEM) and anion exchange membrane (AEM) model show that ion transport is limited by the electrolyte salt concentrations. For 0.1 M KHCO3, the AEM reaches the limiting current density more quickly than the CEM. For the BPM model, ion transport across the diffusion layer on either side of the BPM is also included to understand the concentration polarization across the BPM. The model revealed that the polarization losses across the bipolar membrane depend on the pH of the electrolyte used for the CO2 reduction reaction (CO2RR). The polarization loss on the anolyte side decreases with an increasing pH, while, on the cathode side, it increases with increasing catholyte pH. With this combined model for the electrode reactions and the membrane transport, we are able to account for the various factors influencing the polarization losses in the CO2 electrolyzer. To complete the analysis, we simulated the full cell polarization curve and fitted with the experimental data.

Core-Shell Carbon-Based Bifunctional Electrocatalysts Derived from COF@MOF Hybrid for Advanced Rechargeable Zn-Air Batteries

The development of highly active carbon-based bifunctional electrocatalysts for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is highly desired, but still full of challenges in rechargeable Zn-air batteries. Metal organic frameworks (MOFs) and covalent organic frameworks (COFs) have gained great attention for various applications due to their attractive features of structural tunability, high surface area and high porosity. Herein, a core-shell structured carbon-based hybrid electrocatalyst (H-NSC@Co/NSC), which contains high density active sites of MOF-derived shell (Co/NSC) and COF-derived hollow core (H-NSC), is successfully fabricated by direct pyrolysis of covalently-connected COF@ZIF-67 hybrid. The core-shell H-NSC@Co/NSC hybrid manifests excellent catalytic properties toward both OER and ORR with a small potential gap (∆E = 0.75 V). The H-NSC@Co/NSC assembled Zn-air battery exhibits a high power-density of 204.3 mW cm^-2 and stable rechargeability, outperforming that of Pt/C+RuO₂ assembled Zn-air battery. Density functional theory calculations reveal that the electronic structure of the carbon frameworks on the Co/NSC shell can be effectively modulated by the embedded Co nanoparticles (NPs), facilitating the adsorption of oxygen intermediates and leading to enhanced catalytic activity. This work will provide a strategy to design highly-efficient electrocatalysts for application in energy conversion and storage.

Sustainable oxygen evolution electrocatalysis in aqueous 1 M H2SO4 with earth abundant nanostructured Co3O4

Earth-abundant electrocatalysts for the oxygen evolution reaction (OER) able to work in acidic working conditions are elusive. While many first-row transition metal oxides are competitive in alkaline media, most of them just dissolve or become inactive at high proton concentrations where hydrogen evolution is preferred. Only noble-metal catalysts, such as IrO₂, are fast and stable enough in acidic media. Herein, we report the excellent activity and long-term stability of Co₃O₄-based anodes in 1 M H₂SO₄ (pH 0.1) when processed in a partially hydrophobic carbon-based protecting matrix. These Co₃O₄@C composites reliably drive O₂ evolution a 10 mA cm^-2 current density for >40 h without appearance of performance fatigue, successfully passing benchmarking protocols without incorporating noble metals. Our strategy opens an alternative venue towards fast, energy efficient acid-media water oxidation electrodes.

Rate and Mechanism of Electrochemical Formation of Surface-Bound Hydrogen on Pt(111) Single Crystals

The formation of surface-bound hydrogen from one proton and one electron plays an enabling role in renewable hydrogen production. Quantifying the surface-bound hydrogen formation, however, requires decoupling the delicate interplay of numerous processes. We study cyclic voltammetry (CV) at fast scan rates to characterize the rate constant for the surface-bound hydrogen formation (also known as underpotential deposition hydrogen, UPD H_ad). We find that the formation of H_ad on Pt(111) single crystals is ∼100× faster in acid than in base. Reaction-order analysis indicates that the formation of H_ad occurs as a standard proton-coupled electron transfer (PCET) reaction in acid, whereas in base, it displays a pH-independent rate constant, indicating the presence of a chemical step such as the reorganization of interfacial water. Our results provide a methodology for quantifying the interfacial PCET kinetics and reveal the mechanistic nature of the UPD H_ad formation as the reason the hydrogen evolution electrocatalysis on Pt is faster in acid than in base.

The Ir-OOOO-Ir transition state and the mechanism of the oxygen evolution reaction on IrO2(110)

Carefully assessing the energetics along the pathway of the oxygen evolution reaction (OER), our computational study reveals that the "classical" OER mechanism on the (110) surface of iridium dioxide (IrO₂) must be reconsidered. We find that the OER follows a bi-nuclear mechanism with adjacent top surface oxygen atoms as fixed adsorption sites, whereas the iridium atoms underneath play an indirect role and maintain their saturated 6-fold oxygen coordination at all stages of the reaction. The oxygen molecule is formed, via an Ir-OOOO-Ir transition state, by association of the outer oxygen atoms of two adjacent Ir-OO surface entities, leaving two intact Ir-O entities at the surface behind. This is drastically different from the commonly considered mono-nuclear mechanism where the O₂ molecule evolves by splitting of the Ir-O bond in an Ir-OO entity. We regard the rather weak reducibility of crystalline IrO₂ as the reason for favoring the novel pathway, which allows the Ir-O bonds to remain stable and explains the outstanding stability of IrO₂ under OER conditions. The establishment of surface oxygen atoms as fixed electrocatalytically active sites on a transition-metal oxide represents a paradigm shift for the understanding of water oxidation electrocatalysis, and it reconciles the theoretical understanding of the OER mechanism on iridium oxide with recently reported experimental results from operando X-ray spectroscopy. The novel mechanism provides an efficient OER pathway on a weakly reducible oxide, defining a new strategy towards the design of advanced OER catalysts with combined activity and stability.

Controlled Atmosphere Corrosion Engineering toward Inhomogeneous NiFe-LDH for Energetic Oxygen Evolution

The "Fe effect" can maximize the activity of nickel-iron layered double hydroxides (NiFe-LDH) toward oxygen evolution reaction (OER) when the iron content, the lattice distortion, the conductivity, and other related factors are well balanced. It is difficult for the homogeneous NiFe-LDH to take good care of the above requirements at the same time. Herein, we proposed an elaborate atmosphere corrosion strategy to construct porous NiFe-LDH with rich edge/surface-Fe defects on Ni foam (NF). Such edge/surface-Fe defects, mainly caused by the local unequal-stoichiometric ratio of Fe/Ni in the nanometer or subnanometer region, are determined by the unbalanced permeating of the acid vapor and the confined reaction of local Fe and Ni species ionized by the acid vapor. Benefiting from the abundant and fantastic edge/surface-Fe defects, the optimal NiFe-LDH prepared by atmosphere corrosion is more energetic for OER than that synthesized in conventional liquid phase, only a potential of 1.481 and 1.552 V_RHE to respectively achieve the current density of 100 and 1000 mA cm^-2 as well as a satisfactory stability and reproducibility. An overall water-splitting system assembled by inhomogeneous NiFe-LDH and commercial Pt-C can reach a current density of 100 mA cm^-2 at a solar cell of 1.72 V. Additionally, the atmosphere corrosion is very suitable for the large-scale, green, and economic synthesis of metal-based catalysts with high enrichment of defects, highlighting its potential for device and industrial applications.

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.

Theoretical Optimization of Compositions of High-Entropy Oxides for the Oxygen Evolution Reaction

High‐entropy oxides are oxides consisting of five or more metals incorporated in a single lattice, and the large composition space suggests that properties of interest can be readily optimised. For applications within catalysis, the different local atomic environments result in a distribution of binding energies for the catalytic intermediates. Using the oxygen evolution reaction on the rutile (110) surface as example, here we outline a strategy for the theoretical optimization of the composition. Density functional theory calculations performed for a limited number of sites are used to fit a model that predicts the reaction energies for all possible local atomic environments. Two reaction pathways are considered; the conventional pathway on the coordinatively unsaturated sites and an alternative pathway involving transfer of protons to a bridging oxygen. An explicit model of the surface is constructed to describe the interdependency of the two pathways and identify the composition that maximizes catalytic activity.

Selection of oxygen reduction catalysts for secondary tri-electrode zinc-air batteries

Oxygen reduction reaction (ORR) electrocatalysts, which are highly efficient, low-cost, yet durable, are important for secondary Zn–air cell applications. ORR activities of single and mixed metal oxide and carbon electrocatalysts were studied using rotating disc electrode (RDE) measurements, Tafel slope and Koutecky–Levich plots. It was found that MnO_x combined with XC-72R demonstrated high ORR activity and good stability—up to 100 mA cm⁻². The performance of the selected ORR electrode and a previously optimised oxygen evolution reaction (OER) electrode was thereafter tested in a custom-built secondary Zn–air cell in a tri-electrode configuration, and the effects of current density, electrolyte molarity, temperature, and oxygen purity on the performance of the ORR and OER electrode were investigated. Finally, the durability of the secondary Zn–air system was assessed, demonstrating energy efficiencies of 58–61% at 20 mA cm⁻² over 40 h in 4 M NaOH + 0.3 M ZnO at 333 K.

Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV–vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni²⁺/Ni³⁺ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s^–1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O–O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Cu2S Nanoflakes Decorated with NiS Nanoneedles for Enhanced Oxygen Evolution Activity

Metal sulfides are considered excellent materials for oxygen evolution reaction because of their excellent conductivity and high electrocatalytic activity. In this report, the NiS-Cu₂S composites were prepared on copper foam (NiS-Cu₂S-CF) using a facile synthetic strategy. The scanning electron microscopy results confirmed that the NiS nanoneedles were successfully grown on Cu₂S nanoflakes, greatly increasing the active sites. Particularly, the optimized 15% NiS-Cu₂S-CF composite demonstrated excellent oxygen evolution activity with a small overpotential of 308 mV@20 mA cm⁻², which is significantly smaller than that of noble metal-based electrocatalysts and other NiS-Cu₂S-CF composites. The enhanced oxygen evolution activity is attributed to the unique morphology that can provide ample active sites, rich ion-transfer pathways, and the synergistic effect between NiS and Cu₂S, which can boost the electron transfer rate.

Rationally Designed Ag@polymer@2-D LDH Nanoflakes for Bifunctional Efficient Electrochemical Sensing of 4-Nitrophenol and Water Oxidation Reaction

The rational design and demonstration of a facile sequential template-mediated strategy to construct noble-metal-free efficient bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and electrocatalytic detection of hazardous environmental 4-nitrophenol (4-NP) have continued as a major challenging task. Herein, we construct a novel Ag@polymer/NiAl LDH (designated as APL) nanohybrid as an efficient bifunctional electrocatalyst by a simple hydrolysis method. The well-fabricated APL/GCE exhibited an extensive linear range from 0.1 to 100 μM in optimized conditions. It showed a detection limit (LOD) of 0.0096 μM (9.6 nM) (S/N = 3) for 4-NP in pH 6 by differential pulse voltammetry (DPV). Meanwhile, the newly fabricated APL exhibited outstanding OER activity with a very low overpotential of 259 mV to deliver 10 mA cm^-2 current density (J) at a scan rate of 5 mV/s. The Tafel plot value of APL is low (97 mV/dec) compared to that of the benchmark RuO₂ due to a fast kinetic reaction. Besides, the durability of the electrocatalyst was assessed by a chronoamperometry test (CA) for 36 h at 1.55 mV vs RHE, and the long-term cycling stability was analyzed by using cyclic voltammetry (CV); after 5000 cycles, the electrocatalyst was highly stable. These demonstrated results could lead to an alternative electrocatalyst construction for the bifunctionally efficient electrochemical sensing of 4-nitrophenol and oxygen evolution reaction.

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.

Morphology-oxygen evolution activity relationship of iridium(iv) oxide nanomaterials

This work demonstrated shape tuning of IrO2 nanoparticles to nanocube and nanorods in molten salt and demonstrated the exemplary performance of IrO2 nanorods as an electrocatalyst for oxygen evolution reaction even surpassing commercial IrO2.

Multiscale structural regulation of Metal-organic framework nanofilm arrays for efficient oxygen evolution reaction

A novel MOF nanofilm arrays (NiCoBDC-Fc) grown on Ni foam via a multiscale structural regulation strategy. The introducing of metal doping and defects regulated the morphology structure of NiBDC for...

Lattice site-dependent metal leaching in perovskites toward a honeycomb-like water oxidation catalyst

Metal leaching during water oxidation has been typically observed in conjunction with surface reconstruction on perovskite oxide catalysts, but the role of metal leaching at each geometric site has not been distinguished. Here, we manipulate the occurrence and process of surface reconstruction in two model ABO₃ perovskites, i.e., SrSc_0.5Ir_0.5O₃ and SrCo_0.5Ir_0.5O₃, which allow us to evaluate the structure and activity evolution step by step. The occurrence and order of leaching of Sr (A-site) and Sc/Co (B-site) were controlled by tailoring the thermodynamic stability of B-site. Sr leaching from A-site mainly generates more electrochemical surface area for the reaction, and additional leaching of Sc/Co from B-site triggers the formation of a honeycomb-like IrO_xH_y phase with a notable increase in intrinsic activity. A thorough surface reconstruction with dual-site metal leaching induces an activity improvement by approximately two orders of magnitude, which makes the reconstructed SrCo_0.5Ir_0.5O₃ among the best for water oxidation in acid.

Bimetallic-MOF-Derived Amorphous Zinc/Cobalt-Iron-Based Hollow Nanowall Arrays via Ion Exchange for Highly Efficient Oxygen Evolution

Oxygen evolution reaction (OER) is critical for optimizing renewable energy systems, including metal-air batteries and water electrolysis. One major challenge for OER is to develop durable and cost-effective electrocatalysts with high catalytic performance. Herein, a controllable ion-exchange method to synthesize amorphous zinc/cobalt-iron hydroxide-based hollow nanowall arrays (A-Zn/Co-Fe HNAs) derived from bimetallic metal-organic frameworks (MOFs) on carbon cloth is reported. The amorphous characteristic enables the presented materials with more electrocatalytic sites and short diffusion paths for rapid access to the electrolyte, achieving efficient charge transfer for OER. The optimized nanostructure of A-Zn/Co-Fe HNAs via tuning the amount of iron sulfate in the reaction solution delivers a low overpotential of 226 mV to reach a current density of 10 mA cm^-2 with a small Tafel slope of 37.81 mV dec^-1 while exhibiting high durability at varied current densities over 80 h. The remarkable electrochemical performance can be attributed to the synergistic effect from chemical elements of Zn, Co-Fe, and a robust hollow structure. This simple method of fabricating bimetallic-MOF-derived amorphous Zn/Co-Fe HNAs on carbon cloth can be applied as a practical platform for other OER electrocatalysts.

Interfacial processes in electrochemical energy systems

Electrochemical energy systems such as batteries, water electrolyzers, and fuel cells are considered as promising and sustainable energy storage and conversion devices due to their high energy densities and zero or negative carbon dioxide emission. However, their widespread applications are hindered by many technical challenges, such as the low efficiency and poor long-term cyclability, which are mostly affected by the changes at the reactant/electrode/electrolyte interfaces. These interfacial processes involve ion/electron transfer, molecular/ion adsorption/desorption, and complex interface restructuring, which lead to irreversible modifications to the electrodes and the electrolyte. The understanding of these interfacial processes is thus crucial to provide strategies for solving those problems. In this review, we will discuss different interfacial processes at three representative interfaces, namely, solid-gas, solid-liquid, and solid-solid, in various electrochemical energy systems, and how they could influence the performance of electrochemical systems.

Coherent Acoustic Interferometry during the Photodriven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate (Ti-OH*)

The oxygen evolution reaction (OER) from water requires the formation of metastable, reactive oxygen intermediates to enable oxygen-oxygen bond formation. Conversely, such reactive intermediates could also structurally modify the catalyst. A descriptor for the overall catalytic activity, the first electron and proton transfer OER intermediate from water, (M-OH*), has been associated with significant distortions of the metal-oxygen bonds upon charge-trapping. Time-resolved spectroscopy of in situ, photodriven OER on transition metal oxide surfaces has characterized M-OH* for the charge trapping and the symmetry of the lattice distortions by optical and vibrational transitions, respectively, but had yet to detect an interfacial strain field arising from a surface coverage M-OH*. Here, we utilize picosecond, coherent acoustic interferometry to detect the uniaxial strain normal to the SrTiO₃/aqueous interface directly caused by Ti-OH*. The spectral analysis applies a fairly general methodology for detecting a combination of the spatial extent, magnitude, and generation time of the interfacial strain through the coherent oscillations' phase. For lightly n-doped SrTiO₃, we identify the strain generation time (1.31 ps), which occurs simultaneously with Ti-OH* formation, and a tensile strain of 0.06% (upper limit 0.6%). In addition to fully characterizing this intermediate across visible, mid-infrared, and now GHz-THz probes on SrTiO₃, we show that strain fields occur with the creation of some M-OH*, which modifies design strategies for tuning catalytic activity and provides insight into photo-induced degradation so prevalent for OER. To that end, the work put forth here provides a unique methodology to characterize intermediate-induced interfacial strain across OER catalysts.

Increasing Iridium Oxide Activity for the Oxygen Evolution Reaction with Hafnium Modification

Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrO_x) due to its superior performance and excellent stability. However, large scale applications using IrO_x are impractical due to its low abundance and high cost. Herein, we report a highly active hafnium-modified iridium oxide (IrHf_xO_y) electrocatalyst for OER. The IrHf_xO_y electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrO_x electrocatalyst. The highest intrinsic mass activity of the IrHf_xO_y catalyst in acid conditions was calculated as 6950 A g_IrOx^-1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir-O species for both IrO_x and IrHf_xO_y catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, shortening of the bond lengths of Ir-O, and lowers free energies for OER intermediates that accelerate the OER process.

Lifting the discrepancy between experimental results and the theoretical predictions for the catalytic activity of RuO2(110) towards oxygen evolution reaction

Developing new efficient catalyst materials for the oxygen evolution reaction (OER) is essential for widespread proton exchange membrane water electrolyzer use. Both RuO₂(110) and IrO₂(110) have been shown to be highly active OER catalysts, however DFT predictions have been unable to explain the high activity of RuO₂. We propose that this discrepancy is due to RuO₂ utilizing a different reaction pathway, as compared to the conventional IrO₂ pathway. This hypothesis is supported by comparisons between experimental data, DFT data and the proposed reaction model.

Electronic modulation and proton transfer by iron and borate co-doping for synergistically triggering the oxygen evolution reaction on a hollow NiO bipyramidal prism

Designing an Earth-abundant and inexpensive electrocatalyst to drive the oxygen evolution reaction (OER) for high-purity hydrogen production is of great importance. Herein, the cation (iron) and anion (borate) co-doping strategy was proposed to effectively trigger the OER performance on a low-cost NiO material. The optimal hollow Fe/B_i-NiO bipyramidal prism shows superior OER performance, and displays a low overpotential (261 mV) at 10 mA cm^-2, accompanied by a low Tafel slope (46 mV dec^-1), excellent intrinsic activity and robust stability. The overall alkaline water splitting using Fe/B_i-NiO/NF as an anode affords low cell voltages of 1.50 and 1.63 V at 10 and 100 mA cm^-2, and operates steadily at a high current density of 100 mA cm^-2 for 55 h without decay. The excellent electrocatalytic activity could be ascribed to the hollow structure to shorten the mass transfer pathway, the electronic modulation by Fe doping, the increased accessible electroactive sites created by oxygen vacancies through borate doping, and the formation of BO₃^3--OH^- to accelerate the deprotonation of OH_ads.