Electrolysis of water on oxide surfaces

Electron donation from carbon support enhances the activity and stability of ultrasmall ruthenium dioxide nanoparticles in acidic oxygen evolution reaction

Developing non-iridium (Ir)-based electrocatalysts with good stability and activity for acid oxygen evolution reaction (OER) is of great importance for electrocatalytic water splitting. Ruthenium dioxide (RuO2), which has lower price and higher OER activity, has been recognized as an attractive alternative to Ir-based electrocatalyst for acidic OER. However, the stability of most Ru-based electrocatalysts faces a great challenge in acidic condition. Here, a highly stable and active RuO2-based catalyst, tiny RuO2 nanoparticles inlaid onto carbon support (RuO2/C), is successfully prepared for acidic OER. Such a structure can efficiently inhibit the over-growth of RuO2 nanoparticles and prevent the agglomeration of RuO2 nanoparticles. Moreover, it is found that carbon support donate electron to RuO2 nanoparticles, which enhances the OER activity and stability of RuO2 during acidic OER. The RuO2/C exhibits an impressive OER performance with a low overpotential (197 mV at 10 mA cm-2) and low degradation rate (0.035 mV h-1) over a 450-h stability test in 0.5 M H2SO4, which are much better than the commercial Ir/C, RuO2 and the reported Ru-based electrocatalysts. This work provides an efficient strategy to simultaneously improve both stability and activity of Ru-based catalysts for acidic water oxidation.

Boosting selective chlorine evolution reaction: Impact of Ag doping in RuO2 electrocatalysts

The chlor-alkali process is critical to the modern chemical industry because of the wide utilization of chlorine gas (Cl2). More than 95 % of global Cl2 production relies on electrocatalytic chlorine evolution reaction (CER) through chlor-alkali electrolysis. The RuO2 electrocatalyst serves as the main active component widely used in commercial applications. However, oxygen evolution reaction (OER) generally competes with CER electrocatalysts at RuO2 electrocatalyst owing to the intrinsically scaling reaction energy barrier of *OCl and *OOH intermediates, leading to decreased CER selectivity, high energy consumption, and increased cost. Here, the effect of Ag doping on selective CER over RuO2 electrocatalysts prepared by a sol-gel method has been systematically studied. We found that Ag-doping can effectively improve the Faradaic efficiency of RuO2 electrocatalyst for CER. Furthermore, the improved CER selectivity of Ag-doped RuO2 electrocatalysts is highly dependent on the Ag-doping concentration. The optimized Ag0.15Ru0.85O2 electrocatalyst displays an overpotential of 105 mV along with a selectivity of 84.64 ± 1.84 % in 5.0 M NaCl electrolyte (pH = 2.0 ± 0.05), significantly outperforming undoped one (142 mV, 72.75 ± 1.52 %). Our experiments and density functional theory (DFT) calculations show electron transfer from Ag+ to Ru4+ suppresses *OOH intermediates desorption on Ag-doped RuO2, enabling improved CER selectivity. Such designs of Ag-doped RuO2 electrocatalysts are expected to be favorable for practical chlor-alkali applications.

MnO2-Catalyzed electrocatalytic mineralization of triclosan in chlorinated wastewater

The rising concentrations of xenobiotic aromatic compounds in the environment pose significant risks to human and ecosystem health. Developing a universal, environmentally benign, and scalable platform for mineralizing organic pollutants before their release into the environment is therefore crucial. Electrocatalysis can be highly advantageous for wastewater treatment because it is immediately responsive upon applying potential, requires no additional chemicals, and typically uses heterogeneous catalysts. However, achieving efficient electrochemical mineralization of wastewater pollutants at parts-per-million (ppm) levels remains a challenge. Here, we report the use of manganese dioxide (MnO2), an Earth-abundant, chemically benign, and cost-effective electrocatalyst, to achieve over 99 % mineralization of triclosan (TCS) and other halogenated phenols at ppm levels. Two highly active MnO2 phases-α-MnO2-CC and δ-MnO2-CC-were fabricated on inexpensive carbon cloth (CC) support and evaluated for their ability to oxidatively degrade TCS in pH-neutral conditions, including simulated chlorinated wastewater, real wastewater, and both synthetic and real landfill leachates. Total organic carbon analysis confirmed the effective degradation of TCS. Electron paramagnetic resonance and ultraviolet-visible spectroscopy identified reactive oxygen species, enabling the construction of a detailed TCS degradation pathway. Upon optimization, the TCS removal rate reached 38.38 nmol min-1, surpassing previously reported rates achieved with precious and toxic metal co-catalysts. These findings highlight MnO2-CC as a promising, eco-friendly electrocatalyst with strong potential for upscaled remediation of organic pollutants in wastewater treatment.

Polyoxometalated metal-organic framework superstructure for stable water oxidation

Stable, nonprecious catalysts are vital for large-scale alkaline water electrolysis. Here, we report a grafted superstructure, MOF@POM, formed by self-assembling a metal-organic framework (MOF) with polyoxometalate (POM). In situ electrochemical transformation converts MOF into active metal (oxy)hydroxides to produce a catalyst with a low overpotential of 178 millivolts at 10 milliamperes per square centimeter in alkaline electrolyte. An anion exchange membrane water electrolyzer incorporating this catalyst achieves 3 amperes per square centimeter at 1.78 volts at 80°C and stable operation at 2 amperes per square centimeter for 5140 hours at room temperature. In situ electrochemical spectroscopy and theoretical studies reveal that the synergistic interactions between metal atoms create a fast electron-transfer channel from catalytic iron and cobalt sites, nickel, and tungsten in the polyoxometalate to the electrode, stabilizing the metal sites and preventing dissolution.

Sub-4 nm Ru-RuO2 Schottky Nanojunction as a Catalyst for Durable Acidic Water Oxidation

RuO2 with high intrinsic activity for water oxidation is a promising alternative to IrO2 in proton exchange membrane (PEM) electrolyzer, but it suffers from long-term stability issues due to overoxidation. Here, we report a sub-4 nm Ru-RuO2 Schottky nanojunction (Ru-RuO2-SN) prepared by a microwave reaction that exhibits high activity and long-term stability in both three-electrode systems and PEM devices. The lattice strain and charge transfer induced by the metal-oxide SN increase the work function of the Ru-RuO2-SN, optimize the local electronic structure, and reduce the desorption energy of the metal site to the oxygen-containing intermediates; as a result, it leads to the oxide path mechanism (OPM) and inhibits the excessive oxidation of surface ruthenium. The Ru-RuO2-SN requires only 165 mV overpotential to obtain 10 mA·cm-2 with 1400 h stability without obvious activity degradation, achieving a stability number (6.7 × 106) matching iridium-based catalysts. In a PEM electrolyzer with Ru-RuO2-SN as an anode catalyst, only 1.6 V is needed to reach 1.0 A·cm-2 and it shows long-term stability at 100 mA·cm-2 for 1100 h and at 500 mA·cm-2 for 100 h. The reaction mechanism for the high stability of Ru-RuO2-SN was analyzed by density functional theory calculations. This work reports a durable, pure Ru-based water-oxidation catalyst and provides a new perspective for the development of efficient Ru-based catalysts.

Reaction-driven formation of anisotropic strains in FeTeSe nanosheets boosts low-concentration nitrate reduction to ammonia

FeM (M = Se, Te) chalcogenides have been well studied as promising magnets and superconductors, yet their potential as electrocatalysts is often considered limited due to anion dissolution and oxidation during electrochemical reactions. Here, we show that by using two-dimensional (2D) FeTeSe nanosheets, these conventionally perceived limitations can be leveraged to enable the reaction-driven in-situ generation of anisotropic in-plane tensile and out-of-plane compressive strains during the alkaline low-concentration nitrate reduction reaction (NO3-RR). The reconstructed catalyst demonstrates enhanced performance, yielding ammonia with a near-unity Faradaic efficiency and a high yield rate of 42.14 ± 2.06 mg h-1 mgcat-1. A series of operando synchrotron-based X-ray measurements and ex-situ characterizations, alongside theoretical calculations, reveal that strain formation is ascribed to chalcogen vacancies created by partial Se/Te leaching, which facilitate the adsorption and dissociation of OH-/NO3- from the electrolyte, resulting in an O(H)-doped strained lattice. Combined electrochemical and computational investigations suggest that the superior catalytic performance arises from the synergistic contributions from the exposed strained Fe sites and surface hydroxyl groups. These findings highlight the potential of 2D transition metal chalcogenides for in-situ structural engineering during electrochemical reactions to enhance catalytic activity for NO3-RR and beyond.

Influence of Transition Metal Doping on the Oxygen Evolution Reaction Activity of Nickel Phosphate Surface

This study presents a comprehensive investigation of the oxygen evolution reaction (OER) activity on nickel phosphate (NiPO) surfaces doped with transition metals (Mn, Fe, Co, and Cu). By combining density functional theory calculations, the computational hydrogen electrode approximation, and microkinetic simulations, we demonstrate that transition metal doping significantly enhances OER performance compared to the pristine NiPO surface. The observed trends in overpotential values align with the oxygen adsorption energies on the doped surfaces, indicating a consistent improvement in catalytic activity. Despite the incorporation of different transition metals, the electronic profiles of surface nickel atoms remain largely unchanged, resulting in similar overpotential values at these sites. This suggests that the enhanced OER activity is primarily driven by the localized electronic states of the embedded transition metal dopants rather than changes in the nickel sites. Among the dopants studied, Fe and Mn exhibit the best OER performance, followed by Co and Cu.

Recent Development of Ir- and Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction

Proton exchange membrane (PEM) water electrolyzers are one type of the most promising technologies for efficient, nonpolluting and sustainable production of high-purity hydrogen. The anode catalysts account for a very large fraction of cost in PEM water electrolyzer and also determine the lifetime of the electrolyzer. To date, Ir- and Ru-based materials are types of promising catalysts for the acidic oxygen evolution reaction (OER), but they still face challenges of high cost or low stability. Hence, exploring low Ir and stable Ru-based electrocatalysts for acidic OER attracts extensive research interest in recent years. Owing to these great research efforts, significant developments have been achieved in this field. In this review, the developments in the field of Ir- and Ru-based electrocatalysts for acidic OER are comprehensively described. The possible OER mechanisms are first presented, followed by the introduction of the criteria for evaluation of the OER electrocatalysts. The development of Ir- and Ru-based OER electrocatalysts are then elucidated according to the strategies utilized to tune the catalytic performances. Lastly, possible future research in this burgeoning field is discussed.

The Improved Covalent Coupling Induced by Bi Cations in CoBiSex Nanosheet Arrays for Stable Water Electrolysis

Transition metal selenides (TMSes) with high electrical conductivity, easily regulated morphological structure, and unique electronic structure have been explored as promising electrocatalysts for electrolytic water, while the lower intrinsic electrocatalytic capability has limited their wide application. Herein, we obtained CoSe nanosheet arrays with Bi cations incorporation (CoBiSex nanosheet arrays) driven by spinel oxides. Compared with spinel oxides, the low electronegativity of Se in CoBiSex nanosheet arrays leads to a portion of Co2+ in the low spin states (t2g6eg1) due to the high covalent interaction between Co and Se, which can facilitate charge transfer. Density functional theory (DFT) calculation results further identify the redistribution of electron density from Se to Co, intensifying the adsorption of OOH* during the electrolytic water process with a 140 h robust catalytic performance. This work offers a unique strategy for exploring TMSe materials and their potential applications in the electrolytic water process.

Improving the OER Activity of Titania Via Doping and Adlayers

The oxygen evolution reaction (OER) was investigated theoretically on modified rutile(110) surfaces at density functional theory level in search for inexpensive but active catalyst materials required for water electrolysis. Ti substitution by Nb in rutile and furthermore adding adlayers of transition metal (TM) oxides, with TM = ${ = }$ Ir, Ru and Rh, substantially improves titania OER activity. The catalytic activity was assessed by the overpotential of the OER which was calculated from adsorption energies of the intermediates M-O, M-OH and M-OOH. Different reaction mechanisms were suggested depending on the presence or absence of M-OOH. Materials with iridium dioxide in the top layer have similar overpotentials, both as adlayer on (doped)TiO 2 ${{\rm{TiO}}_{\rm{2}} }$ and as pureIrO 2 ${{\rm{IrO}}_{\rm{2}} }$ . Thus, the percentage of this expensive and scarce element can be drastically reduced without deteriorating the activity. A monolayer ofRuO 2 ${{\rm{RuO}}_{\rm{2}} }$ on rutileTiO 2 ${{\rm{TiO}}_{\rm{2}} }$ has an even lower overpotential compared to pureRuO 2 ${{\rm{RuO}}_{\rm{2}} }$ . In addition,RhO 2 ${{\rm{RhO}}_{\rm{2}} }$ andRhO 2 ${{\rm{RhO}}_{\rm{2}} }$ :Nb 1 / 3 Ti 2 / 3 O 2 ${{\rm{Nb}}_{{\rm{1/3}}} {\rm{Ti}}_{{\rm{2/3}}} {\rm{O}}_{\rm{2}} }$ were identified as catalysts with higher OER activity thanIrO 2 ${{\rm{IrO}}_{\rm{2}} }$ .

Microkinetic Modelling of Electrochemical Oxygen Evolution Reaction on Ir(111)@N-Graphene Surface

We have explored the thermodynamics and microkinetic aspects of oxygen evolution catalysis on low loading of Ir(111) on nitrogen-doped graphene at constant potential. The electronic modification induced by N-doping is the reason for the reduced overpotential of OER. The N-induced defect in the charge density is observed with increasing charge-depleted region around the Ir atoms. The lattice contraction shifts the d-band center away from the Fermi level, which increases the barrier for OH* and O* formation on Ir(111) supported on NGr (Ir(111)@NGr). Thus, highly endothermic O* formation reduces the OOH* formation, which is the potential determining step. For comparison, all electronic and binding energy calculations were also performed against Ir NP supported on Gr (Ir(111)@Gr). The stepwise potential-dependent activation barrier (G a ${{G}_{a}}$ ) was obtained using the charge extrapolation method. The third step remains the RDS in all ranges of water oxidation potentials. The potential dependentG a ${{G}_{a}}$ is further applied to the Eyring rate equation to obtain the current density (j O E R ${{j}_{OER}}$ ) and correlation betweenj O E R ${{j}_{OER}}$ and pH dependence, i. e., OH- concentration. The microkineticj O E R ${{j}_{OER}}$ progression leads to a Tafel slope value of 30 mV dec-1 at pH=14.0, requiringη k i n e t i c = 0 . 33 V ${{\eta }_{kinetic}=0.33\ V}$ .

Oxidation of interfacial cobalt controls the pH dependence of the oxygen evolution reaction

Transition metal oxides often undergo dynamic surface reconstruction under oxygen evolution reaction conditions to form the active state, which differs in response to the electrolyte pH. The resulting pH dependency of catalytic activity is commonly observed but poorly understood. Herein we track Co oxidation state changes at different pH-directed (hydr)oxide/electrolyte interfaces using operando X-ray absorption spectroscopy characterizations. Combined with in situ electrochemical analyses, we establish correlations between Co redox dynamics, the flat band potential and Co oxidation state changes to explain the pH dependency of the oxygen evolution activity. Alkaline environments provide a low flat band potential that yields a low-potential Co redox transformation, which favours surface reconstruction. Neutral and acidic environments afford an anodic shift of the Co redox transformation that increases the catalytic overpotential. The larger overpotential in neutral environments is attributable to poor Co atom polarizability and slow Co oxidation state changes. These findings reveal that interfacial Co oxidation state changes directly determine the pH dependency of the oxygen evolution reaction activity.

Inhibiting Overoxidation of Dynamically Evolved RuO2 to Achieve a Win-Win in Activity-Stability for Acidic Water Electrolysis

Proton exchange membrane (PEM) water electrolysis offers an efficient route to large-scale green hydrogen production, in which the RuO2 catalyst exhibits superior activity but limited stability. Unveiling the atomic-scale structural evolution during operando reaction conditions is critical but remains a grand challenge for enhancing the durability of the RuO2 catalyst in the acidic oxygen evolution reaction (a-OER). This study proposes an adaptive machine learning workflow to elucidate the potential-dependent state-to-state global evolution of the RuO2(110) surface within a complex composition and configuration space, revealing the correlation between structural patterns and stability. We identify the active state with distorted RuO5 units that self-evolve at low potential, which exhibits minor Ru dissolution and an activity self-promotion phenomenon. However, this state exhibits a low potential resistance capacity (PRC) and evolves into inert RuO4 units at elevated potential. To enhance PRC and mitigate the overevolution of the active state, we explore the metal doping engineering and uncover an inverse volcano-type doping rule: the doped metal-oxygen bond strength should significantly differ from the Ru-O bond. This rule provides a theoretical framework for designing stable RuO2-based catalysts and clarifies current discrepancies regarding the roles of different metals in stabilizing RuO2. Applying this rule, we predict and confirm experimentally that Na can effectively stabilize RuO2 in its active state. The synthesized Na-RuO2 operates in a-OER for over 1800 h without any degradation and enables long-term durability in PEM electrolysis. This work enhances our understanding of the operando structural evolution of RuO2 and aids in designing durable catalysts for a-OER.

Modulation of the Electronic Properties of Co3O4 through Bi Octahedral Doping for Enhanced Activity in the Oxygen Evolution Reaction

Developing a highly active and stable electrocatalyst for the oxygen evolution reaction (OER) is essential for efficient hydrogen production through anion exchange membrane water electrolysis powered by renewable electricity. Recently, there has been a renewed interest in designing electrocatalysts based on their work function optimization. The insights into the materials' electronic properties gained from developing other heterogeneous catalysts, such as those used for N2O decomposition, can be thus leveraged to enhance the performance of the OER electrocatalysts. Knowing that Bi enhances the catalytic activity of Co3O4 in N2O decomposition, where the surface electronic properties play a crucial role, we hypothesized that it might also improve the electroactivity of the OER electroactivity. Therefore, we synthesized Bi-doped Co3O4 with different bismuth contents and studied the sample with a complementary set of physicochemical, electrochemical, and computational techniques. We found that promoting Co3O4 with atomically dispersed bismuth enhances its OER electrocatalytic properties by reducing the energy of the potential-determining step and improving electron charge transfer properties. Bismuth atoms enter octahedral sites in Co3O4, creating Bi active centers and enhancing the activity of vicinal Co sites in the OER. The Bi and modified Co centers are characterized by increased binding energy of the intermediate state of the metal-oxygen intermediate and increased density of states at the Fermi level. The former reduces the overpotential required for the OER, whereas the latter improves the reaction kinetics by decreasing the charge transfer resistance.

DFT-metadynamics insights on the origin of the oxygen evolution kinetics at the (100)-WSe2 surface

Water oxidation or oxygen evolution reaction (OER) in electrochemical cells is considered to be a major bottleneck in the way of hydrogen production by electro-synthesis, mainly due to a sluggish kinetics that characterizes the OER steps. Layered transition metal dichalcogenides, such as WSe2, are emerging as promising non-precious electrocatalysts for water splitting due to their excellent activity and stability. This paper aims to shed light on the (100) WSe2-aqueous interface in catalyzing the slow kinetics of the OER in the context of water splitting electro-catalysis. We employ state-of-the-art DFT-metadynamics to explore reaction mechanisms, activation free energies, and catalytic sites. This study reveals an energetically preferred water-assisted OER mechanism, where proton transfer is facilitated by the surrounding aqueous environment. Our findings not only provide insights into the OER process but also offer a design strategy for optimizing WSe2-based catalysts and a modeling protocol for future DFT-based OER investigations.

Constructing Janus Structures and Rich Electron Pool in 2D TMTe Nanostructures To Achieve OER/ORR Electrocatalysts

Through first-principles structure search calculations, we have identified ten hitherto unknown two-dimensional (2D) Janus-wrinkled TMTe monolayers (TM = Ni, Pd, Pt, Co, Rh, Ir, Fe, Ru, Os, and Hf) by screening 3d, 4d, and 5d transition metal atoms. These monolayers exhibit high stability and metallic conductivity. Among the discovered materials, the 2D PdTe (ηOER/ORR = 0.46/0.22 V) and PtTe (ηOER/ORR = 0.46/0.32 V) monolayers can demonstrate superior bifunctional catalytic performance for oxygen evolution and oxygen reduction reactions (OER/ORR), with lower overpotential than the state-of-the-art IrO2 for OER and Pt (111) for ORR, respectively. The TM- and Te-sides originating from the unique Janus configurations play a crucial role in the high OER and ORR catalytic activities, respectively. Furthermore, by stacking the monolayer structures, eight new (TMTe)2 bilayers with high stability and metallic conductivity can be achieved, which possess an internal metal layer, forming a rich electron pool. This effectively improves oxygen adsorption and activity on some bilayers, including (PdTe)2, (PtTe)2, (RhTe)2, and (IrTe)2, by transferring more electrons to the adsorbed O2 molecule, leading to considerably high ORR catalytic performance (ηORR = 0.16-0.44 V). Moreover, detailed analyses of the catalytic mechanisms have been conducted. These intriguing findings can offer new insights for designing low-cost and high-performance electrocatalysts for OER and ORR reactions, with the potential to replace related noble metal catalysts used in water splitting, fuel cells, metal-air batteries, etc.

Phase-Engineered Bi-RuO2 Single-Atom Alloy Oxide Boosting Oxygen Evolution Electrocatalysis in Proton Exchange Membrane Water Electrolyzer

Engineering nanomaterials at single-atomic sites can enable unprecedented catalytic properties for broad applications, yet it remains challenging to do so on RuO2-based electrocatalysts for proton exchange membrane water electrolyzer (PEMWE). Herein, the rational design and construction of Bi-RuO2 single-atom alloy oxide (SAAO) are presented to boost acidic oxygen evolution reaction (OER), via phase engineering a novel hexagonal close packed (hcp) RuBi single-atom alloy. This Bi-RuO2 SAAO electrocatalyst exhibits a low overpotential of 192 mV and superb stability over 650 h at 10 mA cm-2, enabling a practical PEMWE that needs only 1.59 V to reach 1.0 A cm-2 under industrial conditions. Operando differential electrochemical mass spectroscopy analysis, coupled with density functional theory studies, confirmed the adsorbate-evolving mechanism on Bi-RuO2 SAAO and that the incorporation of Bi1 improves the activity by electronic density optimization and the stability by hindering surface Ru demetallation. This work not only introduces a new strategy to fabricate high-performance electrocatalysts at atomic-level, but also demonstrates their potential use in industrial electrolyzers.

Oxygen evolution reaction on NiFe-LDH/(Ni,Fe)OOH: theoretical insights into the effects of electronic structure and spin-state evolution

NiFe-layered double hydroxides (NiFe-LDH) have emerged as promising oxygen evolution reaction (OER) catalysts in alkaline medium, but their commercial applications are limited due to the decrease in their activity as the electrolyte becomes less alkaline. Thus, a comprehensive understanding of the OER mechanism of NiFe-LDH in alkaline medium is desirable for the rational design of new catalysts with improved performances. Especially, their spin-related factors have rarely been systematically investigated during the OER (diamagnetic H2O → paramagnetic O2). Herein, we simulated the OER performance of NiFe-LDH and (Ni,Fe)OOH as NiFe-LDH underwent surface-reconstruction and formed (Ni,Fe)OOH under alkaline conditions. Results demonstrated an enhanced OER performance on (Ni,Fe)OOH, and the Fe active site of NiFe-LDH on losing 3H (namely, NiFe(OH)2 - 3H) showed the lowest overpotential for OER because the d-orbital electron of the Fe atom shifted up to the Fermi level. Notably, the electronic interaction between Fe and OOH induced a change in the spin state of Fe, which further decreased the overpotential for the OER. Thus, the overpotential of the Fe site on NiFe(OH)2 - 3H decreased from 0.55 eV to 0.46 eV. The density of states (DOS) analysis revealed that the spin flip of Fe promoted the formation of bonding states between Fe and OOH, endowing the catalyst with a better OER performance. Our findings can help pave the way for the development of high-performance OER catalysts at the spintronic level.

Achieving Higher Activity of Acidic Oxygen Evolution Reaction Using an Atomically Thin Layer of IrOx over Co3O4

The development of electrocatalysts with reduced iridium (Ir) loading for the oxygen evolution reaction (OER) is essential to produce low-cost green hydrogen from water electrolysis under acidic conditions. Herein, an atomically thin layer of iridium oxide (IrOx) has been uniformly dispersed onto cobalt oxide (Co3O4) nanocrystals to improve the efficient use of Ir for acidic OER. In situ characterization and theoretical calculations reveal that compared to the conventional IrOx cluster, the atomically thin layer of IrOx shows stronger interaction with the Co3O4 and consequently higher OER activity due to the Ir-O-Co bond formation at the interface. Equally important, the facile synthetic method and the promising activity in the proton exchange membrane water electrolyzer, reaching 1 A cm-2 at 1.7 V with remarkable durability, enable potential scale-up applications. These findings provide a mechanistic understanding for designing active, stable and lower-cost electrocatalysts with well-defined structures for acidic OER.

Modulating Electronic Correlations in Ruthenium Oxides for Highly Efficient Oxygen Evolution Reaction

Elucidating the electronic factors dominating the adsorption properties of transition-metal oxides is essential to construct highly efficient oxygen-evolving catalysts for hydrogen production by water splitting but remains a great challenge. Electron correlation from on-site Coulomb repulsion (U) among d-electrons is generally believed to significantly affect the electronic structure of these materials; however, it has long been neglected in studying their adsorption properties. Here, by choosing ruthenium oxide as a model system, we demonstrate the role of electron correlation on the electrocatalytic activity toward oxygen evolution reaction (OER). Our density functional theory plus U calculations on rutile RuO2 reveal that the electron correlation can tune the adsorption energies for oxygenated intermediate and optimize them after the metallic oxide being a Mott insulator upon increasing U. By regulating the RuO6 octahedral network, we constructed and synthesized a series of strongly correlated ruthenium oxides, where the Mott insulating ones indeed exhibit a superior OER performance to the metallic RuO2. Our work builds a bridge between the electrochemistry and Mott physics for transition-metal oxides, opening a new avenue for designing advanced catalysts.

Impacts of Surface Reconstruction and Metal Dissolution on Ru1-x Ti x O2 Acidic Oxygen Evolution Electrocatalysts

Improved oxygen evolution reaction (OER) electrocatalysts based on an additional understanding of surface changes that occur upon metal dissolution are needed to enable the efficient use of electrochemical water splitting. This work integrates theoretical and experimental studies of the effects of metal dissolution from the RuO2 and Ru1-x Ti x O2 surfaces on the OER activity and electrochemical stability. Our computational analysis shows that the energetic barriers for metal dissolution depend highly on the surface site and Ti-substituent location. Metal dissolution induces the formation of new active surface sites with different electronic density distributions. In addition to dissolution-induced changes to the surface composition, electron density changes occur in the interfacial electrolyte components. Surface reconstruction changes the activation barriers for the OER steps. Our experimental analysis of RuO2 and Ru0.8Ti0.2O2 using a two-step durability test in acidic electrolytes shows that the OER activity, surface, and metal dissolution change over the durability tests. Ti-substitution exhibits improved electrochemical stability with cycling. For RuO2, changes in the mass activity of RuO2 with cycling are directly correlated with Ru dissolution and lowering of the electrochemical surface area (ECSA). In contrast, Ru0.8Ti0.2O2 showed a 19 times lower Ru dissolution rate, and metal dissolution results in increasing the ECSA and new active sites. Our STEM and EELS analysis supports that repeated cycling under OER conditions results in surface reconstruction for both RuO2 and Ru0.8Ti0.2O2, with the formation of a disordered RuO2 surface and changes to the distribution of Ru and Ti at the Ru0.8Ti0.2O2 surface. The experimentally observed changes in activity and surface structure after cycling are consistent with computational analysis, which shows how metal dissolution may alter the OER activation barriers. Combining experimental and computational insights, this work reveals the effects of metal dissolution on the surface atomic and electronic structure and OER activity and advances our comprehension of metal dissolution dynamics and surface reconstruction, which may have implications for other catalytic processes.

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.

Unravelling the pH-dependent mechanism of ferroelectric polarization on different dynamic pathways of photoelectrochemical water oxidation

Ferroelectric polarization is considered to be an effective strategy to improve the oxygen evolution reaction (OER) of photoelectrocatalysis. The primary challenge is to clarify how the polarization field controls the OER dynamic pathway at a molecular level. Here, electrochemical fingerprint tests were used, together with theoretical calculations, to systematically investigate the free energy change in oxo and hydroxyl intermediates on TiO2-BaTiO3 core-shell nanowires (BTO@TiO2) upon polarization in different pH environments. We demonstrate that the adsorbate evolution mechanism (AEM) dominated in acidic environments, and both positive and negative polarization resulted in a reduction in the oxo-free energy, which inhibited the reaction kinetics. In the oxide path mechanism (OPM) that occurs in alkaline conditions, the ferroelectric polarization exhibits repulsive adsorbate-adsorbate interaction for OH- coverage and free energy shift of the OH- groups. We elucidate that a weakly alkaline electrolyte is the optimal environment for ferroelectric polarization because the positive polarization promotes OH- coverage and facilitates reaction pathway transfer from AEM to OPM; therefore, BTO@TiO2 exhibited a record polarization enhancement to 0.52 mA cm-2 at 1.23 VRHE in pH = 11. This work provides a more accurate insight into the pH-dependent effect of ferroelectric polarization on the OER dynamic pathway than conventional models that are based solely on the regulation of band bending.

Atomic Imaging of the Surface Termination and Reconstruction of Low and High Index Iridium Oxide Surfaces and Insights into Their Facet-Dependent Oxygen Evolution Activities

Resolving the atomic surface structure, particularly surface termination or reconstruction, is essential for understanding the catalytic properties of metal oxides. Although rutile phase iridium dioxide (IrO2) is the state-of-the-art electrocatalyst for the oxygen evolution reaction (OER) in water splitting, the atomic-level surface structures of IrO2 remain largely unexplored, limiting our understanding of its facet-dependent OER activities. Herein, we perform aberration-corrected integrated differential phase contrast scanning transmission electron microscopy of the low- and high-index surface structures of spindle-shaped IrO2 nanorods and reveal distinct surface terminations and/or reconstructions on different surfaces. Notably, the (110) surface shows a predominantly top-/bridge-oxygen termination and high structural stability without obvious surface reconstruction. In contrast, the (001) and (101) surfaces, where all surface Ir atoms are coordinatively unsaturated, undergo significant reconstruction. Additionally, a high-index (321) surface composed of (110) terraces is identified and exhibits a distinct [IrO] surface termination, indicating a weaker binding energy between Ir with O. Density functional theory calculations reveals weakened oxygen-binding energies on both the reconstructed (101) surface and the high-index (321) surface, predicting substantially lower limiting OER overpotentials compared to the (110) surface. These findings provide an important structural basis for understanding the OER activities of IrO2 surfaces.

Computational Engineering of Non-van der Waals 2D Magnetene for Enhanced Oxygen Evolution and Reduction Reactions

Non-van der Waals two-dimensional materials containing exposed transition metal atoms are promising catalysts for green energy storage and conversion. For instance, hematene and ilmenene have been successfully applied as catalysts. Building on these reports, this work is the first investigation of recently synthesized magnetene towards the Oxygen Evolution Reaction (OER) and Oxygen Reduction Reaction (ORR). Using Density Functional Theory (DFT) calculations, we unveil the mechanism, performance and ideal conditions for OER and ORR on magnetene. With overpotentials of ηOER=0.50 V and ηORR=0.41 V, the material is not only a bifunctional catalyst, but also superior to state-of-the-art systems such as Pt and IrO2. Additionally, its catalytic properties can be further enhanced through engineering strategies such as point defects and in-plane compression. It reaches ηORR=0.28 V at a compressive strain of only 2 %, while the presence of Ni boosts it to ηOER=0.39 V and ηORR=0.31 V, comparable to many reported single-atom catalysts. Overall, this work demonstrates that magnetene is a promising bifunctional catalyst for applications such as regenerative fuel cells and metal-air batteries.

Expanding the frontiers of electrocatalysis: advanced theoretical methods for water splitting

Electrochemical water splitting, which encompasses the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), offers a promising route for sustainable hydrogen production. The development of efficient and cost-effective electrocatalysts is crucial for advancing this technology, especially given the reliance on expensive transition metals, such as Pt and Ir, in traditional catalysts. This review highlights recent advances in the design and optimization of electrocatalysts, focusing on density functional theory (DFT) as a key tool for understanding and improving catalytic performance in the HER and OER. We begin by exploring DFT-based approaches for evaluating catalytic activity under both acidic and alkaline conditions. The review then shifts to a material-oriented perspective, showcasing key catalyst materials and the theoretical strategies employed to enhance their performance. In addition, we discuss scaling relationships that exist between binding energies and electronic structures through the use of charge-density analysis and d-band theory. Advanced concepts, such as the effects of adsorbate coverage, solvation, and applied potential on catalytic behavior, are also discussed. We finally focus on integrating machine learning (ML) with DFT to enable high-throughput screening and accelerate the discovery of novel water-splitting catalysts. This comprehensive review underscores the pivotal role that DFT plays in advancing electrocatalyst design and highlights its potential for shaping the future of sustainable hydrogen production.

Unraveling the catalytic performance of RuO2(1 1 0) for highly-selective ethylene production from methane at low temperature: Insights from first-principles and microkinetic simulations

Despite significant progress in low-temperature methane (CH4) activation, commercial viability, specifically obtaining high yields of C1/C2 products, remains a challenge. High desorption energy (>2 eV) and overoxidation of the target products are key limitations in CH4 utilization. Herein, we employ first-principles density functional theory (DFT) and microkinetics simulations to investigate the CH4 activation and the feasibility of its conversion to ethylene (C2H4) on the RuO2 (1 1 0) surface. The CH activation and CH4 dehydrogenation processes are thoroughly investigated, with a particular focus on the diffusion of surface intermediates. The results show that the RuO2 (1 1 0) surface exhibits high reactivity in CH4 activation (Ea = 0.60 eV), with CH3 and CH2 are the predominant species, and CH2 being the most mobile intermediate on the surface. Consequently, self-coupling of CH2* species via CC coupling occurs more readily, yielding C2H4, a potential raw material for the chemical industry. More importantly, we demonstrate that the produced C2H4 can easily desorb under mild conditions due to its low desorption energy of 0.97 eV. Microkinetic simulations based on the DFT energetics indicate that CH4 activation can occur at temperatures below 200 K, and C2H4 can be desorbed at room temperature. Further, the selectivity analysis predicts that C2H4 is the major product at low temperatures (300-450 K) with 100 % selectivity, then competes with formaldehyde at intermediate temperatures in the CH4 conversion over RuO2 (1 1 0) surface. The present findings suggest that the RuO2 (1 1 0) surface is a potential catalyst for facilitating ethylene production under mild conditions.

Smallest [5,6]Fullerene as Building Blocks for 2D Networks with Superior Stability and Enhanced Photocatalytic Performance

The assembly of molecules to form covalent networks can create varied lattice structures with physical and chemical properties distinct from those of conventional atomic lattices. Using the smallest stable [5,6]fullerene units as building blocks, various 2D C24 networks can be formed with superior stability and strength compared to the recently synthesized monolayer polymeric C60. Monolayer C24 harnesses the properties of both carbon crystals and fullerene molecules, such as stable chemical bonds, suitable band gaps, and large surface area, facilitating photocatalytic water splitting. The electronic band gaps of C24 are comparable to those of TiO2, providing appropriate band edges with sufficient external potential for overall water splitting over the acidic and neutral pH range. Upon photoexcitation, strong solar absorption enabled by strongly bound bright excitons can generate carriers effectively, while the type-II band alignment between C24 and other 2D monolayers can separate electrons and holes in individual layers simultaneously. Additionally, the number of surface-active sites of C24 monolayers are three times more than that of their C60 counterparts in a much wider pH range, providing spontaneous reaction pathways for the hydrogen evolution reaction. Our work provides insights into materials design using tunable building blocks of fullerene units with tailored functions for energy generation, conversion, and storage.

Nickel Phosphide: The Effect of Phosphorus Content on the Activity and Stability toward Oxygen Evolution Reaction in Alkaline Medium

In this work a systematic study of the effect of the metal to phosphorus ratio in Ni-P nanoparticles on their catalytic activity with respect to the OER is reported. To this end, nickel phosphide nanoparticles are synthesized through two different synthesis routes, one involving in-situ phosphidation and one involving ex-situ phosphidation. In-situ phosphidation is performed via two steps route in a one-pot synthesis, in which Ni nanoparticles are formed at 220 °C, but not isolated, and then transformed to phase-pure either Ni12P5 or Ni2P nanocrystallites. In the second synthesis method (ex-situ phosphidation), nickel nanoparticles with an excess amount of trioctylphosphine (TOP) as a capping agent are synthesized and separated from the solution, then subsequently annealed in three different atmospheres, leading to the formation of three types of NixPy viz. [NixPy-H2/Ar], [NixPy-Ar], and [NixPy-air]. [NixPy-air] nanoparticles shows the best electrocatalytic activity among the annealed nanoparticles in Ar and H2/Ar but lower than Ni12P5 nanoparticles. However, [NixPy-air] shows very high stability in comparison with other synthesized nanoparticles. Moreover, the effect of the adventitious and spiked Fe in the electrolyte is studied on the electrocatalytic activity of all synthesized nanoparticles.

Optimizing Acidic Oxygen Evolution with Manganese-Doped Ruthenium Dioxide Assembly

Ruthenium dioxide (RuO2) is one of the promising catalysts for the acidic oxygen evolution reaction (OER). However, designing RuO2 catalysts with good activity and stability remains a significant challenge. In this work, we propose the manganese (Mn)-doped RuO2 assembly as a catalyst for the OER with improved activity and stability. Consequently, the optimized 7% Mn-RuO2 exhibits exceptional OER activity in 0.5 M H2SO4, delivering a low overpotential of 195 mV to achieve a current density of 10 mA cm-2. Furthermore, it displays the highest mass activity among all the tested catalysts, reaching 587.9 A gRu-1 at 1.5 V versus the reversible hydrogen electrode (vs RHE), which is 7.8 and 139.8 times higher than those of undoped RuO2 and commercial RuO2, respectively. Moreover, 7% Mn-RuO2 demonstrates remarkable stability over a continuous operation to 100 h (at 10 mA cm-2) without significant performance attenuation. Additionally, theoretical calculations indicate that Mn doping weakens the adsorption of the OER intermediates and modifies the potential-determining step (PDS) of the OER, thereby reducing the OER overpotential. Consequently, strategies involving Mn doping can effectively enhance the overall kinetics of the OER. This work offers a promising approach for the design of efficient water electrolysis catalysts.

Dynamically Reconstructed Fe-CoOOH Semi-Crystalline Electrocatalyst for Efficient Oxygen Evolution Reaction

The development of robust and efficient electrocatalysts for the oxygen evolution reaction (OER) has been the main focus of water electrolysis but remains a great challenge. Here, the synthesis of a highly active and ultra-stable Fe-CoOOH electrocatalyst is reported by steering raw cobalt foam via an in situ solution combustion method assisted by a galvanic replacement reaction and subsequent electrochemical reconstruction of the CoFeOx pre-catalyst. In/ex situ electrochemical analysis and physicochemical characterizations show that the CoFeOx undergoes quick chemical and slow morphological reconstruction to Fe-CoOOH nanosheets. The Fe-CoOOH possesses a semi-crystalline nature with distinct short-range ordering and outstanding OER activity with overpotentials as low as 271 and 291 mV at current densities of 500 and 1,000 mA cm-2, respectively. The remarkable stability under 1,000 mA cm-2 for at least 700 h is achieved. Theoretical calculations confirm the crucial role of Fe doping in facilitating surface reconstruction, enhancing OER activity, and improving the stability of the Fe-CoOOH. Comparative analysis with other transition metals doping reveals the unique ability of Fe to adsorb onto the CoOOH surface, thereby modulating the electronic density and facilitating faster adsorption of reaction intermediates. This work represents valuable insights into the surface reconstruction and doping processes.

Breaking the Mutual-Constraint of Bifunctional Oxygen Electrocatalysis via Direct O─O Coupling on High-Valence Ir Single-Atom on MnOx

Insufficient bifunctional activity of electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is the major obstruction to the application of rechargeable metal-air batteries. The primary reason is the mutual constraint of ORR and OER mechanism, involving the same oxygen-containing intermediates and demonstrating the scaling limitations of the adsorption energies. Herein, it is reported a high-valence Ir single atom anchored on manganese oxide (IrSA-MnOx) bifunctional catalyst showing independent pathways for ORR and OER, i.e., associated 4e- pathway on high-valence Ir site for ORR and a novel chemical-activated concerted mechanism for OER, where a distinct spontaneous chemical activation process triggers direct O─O coupling. The IrSA-MnOx therefore delivers outstanding bifunctional activities with remarkably low potential difference (0.635 V) between OER potential at 10 mA cm-2 and ORR half-wave potential in alkaline solution. This work breaks the scaling limitations and provides a new avenue to design efficient and multifunctional electrocatalysts.

Deep reconstruction of crystalline-amorphous heterojunction electrocatalysts for efficient and stable water and methanol electrolysis

During electrocatalytic water splitting, surface reconstruction often occurs to generate truly active species for catalytic reactions, but the stability and mass activity of the catalysts is a huge challenge. A method that combines cation doping with morphology control strategies and constructs an amorphous-crystalline heterostructure is proposed to achieve deep reconstruction of the catalyst during the electrochemical activation process, thereby significantly improving catalytic activity and stability. Amorphous iron borate (FeBO) is deposited on cobalt-doped nickel sulfide (Co-Ni3S2) crystals to form ultrathin nanosheet heterostructures (FeBO/Co-Ni3S2) as bifunctional electrocatalysts for the OER and methanol oxidation reaction (MOR). During the OER process, FeBO/Co-Ni3S2 is deeply reconstructed to form a NiFeOOH/Co-Ni3S2 composite structure with ultrathin nanosheets with abundant amorphous-crystalline interfaces to ensure structural stability. Furthermore, Co-Ni3S2 electrocatalysts were synthesized via nickel foam (NF) self-derivation, which resulted in strong adhesion between the catalyst and substrate and formed a hierarchical structure consisting of interconnected nanosheets with excellent mass transfer and abundant active sites to increase the activity and stability of the electrocatalyst. The dual-electrode electrolyzer requires cell voltages of 1.58 and 1.44 V to achieve water and methanol overall electrolysis at a current density of 10 mA cm-2 and keep working over 100 and 25 h, respectively. This strategy provides a new way to promote reconstruction to construct excellent bifunctional electrocatalysts.

Multitasking-Effect Ca Ions Triggered Symmetry-Breaking of RuO2 Coordination for Acidic Oxygen Evolution Reaction

The development of highly active and stable electrocatalysts for the acid oxygen evolution reaction (OER) is both appealing and challenging. The generation of defects is an emerging strategy for improving the water oxidation efficiency. Herein, we introduced multitasking Ca ions to trigger oxygen vacancies in RuO2, resulting in vacancy-rich RuO2 (RuO2-Ov) nanoparticles with enhanced and sustainable OER activity. The oxygen vacancy in RuO2-Ov breaks the symmetry of the RuO6 octahedron, enhancing the d-band center of Ru and reducing the level of 4d-2p hybridization in Ru-O bonds. This effectively optimizes intermediate adsorption and inhibits Ru dissolution. The RuO2-OV catalyst achieves a current density of 10 mA/cm2 with an overpotential of only 198 mV, stabilizing for over 100 h (degradation rate: 0.2 mV/h). Its mass activity is 17.9 times higher than that of commercial RuO2. Our work highlights that multitasking atomic construction defect engineering effectively balances the seesaw relationship between catalytic activity and stability.

Graphene-supported MN4 single-atom catalysts for multifunctional electrocatalysis enabled by axial Fe tetramer coordination

Multifunctional electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) are crucial for development of the key electrochemical energy storage and conversion devices, for which single-atom catalyst (SAC) has present great promises. Very recently, some experimental works showed that structurally well-defined ultra-small transition-metal clusters (such as Fe and Co tetramers, denoted as Fe4 and Co4, respectively), can efficiently modulate the catalytic behavior of SACs by axial coordination. Herein, taking the graphene-supported MN4 SACs as representatives, we theoretically explored the feasibility of realizing multifunctional SACs for ORR, OER and HER by this novel axial coordination engineering. Through extensive first-principles calculations, from 23 candidates, IrN4 decorated with Fe4 (IrN4/Fe4) is identified as the promising trifunctional catalyst with the theoretical overpotential of 0.43, 0.51 and 0.30 V for OER, ORR and HER, respectively. RhN4/Fe4 and CoN4/Fe4 are recognized as potential OER and ORR bifunctional catalysts. In addition, NiN4/Fe4 exhibits the best ORR activity with an overpotential of 0.30 V, far superior to the pristine NiN4 SAC (0.88 V). Electronic structure analyses reveal that the significantly enhanced ORR/OER activity can be ascribed to the orbital and charge redistribution of Ni/Ir active center, resulting from its electronic interaction with Fe4 cluster. This work could stimulate and guide the rational design of graphene-based multifunctional SACs realized by axial coordination of small TM clusters, and provide insights into the modulation mechanism.

A Complex Oxide Containing Inherent Peroxide Ions for Catalyzing Oxygen Evolution Reactions in Acid

Proton exchange membrane water electrolyzers powered by sustainable energy represent a cutting-edge technology for renewable hydrogen generation, while slow anodic oxygen evolution reaction (OER) kinetics still remains a formidable obstacle that necessitates basic comprehension for facilitating electrocatalysts' design. Here, we report a low-iridium complex oxide La1.2Sr2.7IrO7.33 with a unique hexagonal structure consisting of isolated Ir(V)O6 octahedra and true peroxide O22- groups as a highly active and stable OER electrocatalyst under acidic conditions. Remarkably, La1.2Sr2.7IrO7.33, containing 59 wt % less iridium relative to the benchmark IrO2, shows about an order of magnitude higher mass activity, 6-folds higher intrinsic activity than the latter, and also surpasses the state-of-the-art Ir-based oxides ever reported. Combined electrochemical, spectroscopic, and density functional theory investigations reveal that La1.2Sr2.7IrO7.33 follows the peroxide-ion participation mechanism under the OER condition, where the inherent peroxide ions with accessible nonbonded oxygen states are responsible for the high OER activity. This discovery offers an innovative strategy for designing advanced catalysts for various catalytic applications.

Fe-Doped Ni-Based Catalysts Surpass Ir-Baselines for Oxygen Evolution Due to Optimal Charge-Transfer Characteristics

Ni-based catalysts with Co or Fe can potentially replace precious Ir-based catalysts for the rate-limiting oxygen evolution reaction (OER) in anion-exchange membrane (AEM) electrolyzers. In this study, density functional theory (DFT) calculations provide atomic- and electronic-level resolution on how the inclusion of Co or Fe can overcome the inactivity of NiO catalysts and even enable them to surpass IrO2 in activating key steps to the OER. Namely, NiO resists binding the key OH* intermediate and presents a high energetic barrier to forming the O*. Co- and Fe-substitution of Ni active sites allows for the stronger binding of OH* and preferentially activates O*/O2* formation, with Fe-substitution increasing the OER activity substantially as compared to Co-substitution. Whereas IrO2 requires an activation energy of 0.34-0.49 eV to form O2, this step is spontaneous on Fesub-NiO. Electrodeposition of polycrystalline electrodes and synthesized nanoparticles exploit the Co or Fe presence, with Fe particularly exhibiting greater activity: Tafel slopes indicate a significant change in the mechanism as compared to pure NiO, validating the theoretical predictions of OER activation at different steps. High-performing synthesized nanoparticles of 25% Fe-Ni exhibited a 4.6 times improvement over IrO2 and a 34% improvement over RuO2, showcasing that non-platinum group metal catalysts can outperform platinum group metals. High-resolution transmission electron microscopy further highlights the advantages of Fe-Ni oxide synthesized nanoparticles over commercial catalysts: small, randomly oriented nanoparticles expose greater edge sites than large nanoparticles typical of commercially available materials.

Encapsulating Nickel-Iron Alloy Nanoparticles in a Polysilazane-Derived Microporous Si-C-O-N-Based Support to Stimulate Superior OER Activity

The in situ confinement of nickel (Ni)-iron (Fe) nanoparticles (NPs) in a polymer-derived microporous silicon carboxynitride (Si-C-O-N)-based support is investigated to stimulate superior oxygen evolution reaction (OER) activity in an alkaline media. Firstly, we consider a commercial polysilazane (PSZ) and Ni and Fe chlorides to be mixed in N,N-dimethylformamide (DMF) and deliver after overnight solvent reflux a series of Ni-Fe : organosilicon coordination polymers. The latter are then heat-treated at 500 °C in flowing argon to form the title compounds. By considering a Ni : Fe ratio of 1.5, face centred cubic (fcc) NixFey alloy NPs with a size of 15-30 nm are in situ generated in a porous Si-C-O-N-based matrix displaying a specific surface area (SSA) as high as 237 m2 ⋅ g-1. Hence, encapsulated NPs are rendered accessible to promote electrocatalytic water oxidation. An OER overpotential as low as 315 mV at 10 mA ⋅ cm-2 is measured. This high catalytic performance (considering that the metal mass loading is as low as 0.24 mg cm-2) is rather stable as observed after an activation step; thus, validating our synthesis approach. This is clearly attributed to both the strong NP-matrix interaction and the confinement effect of the matrix as highlighted through post mortem microscopy observations.

Oxygen electrode catalysis in N-doped graphene: the role of nitrogen coordination and solvation effects

The efficiency of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in energy conversion devices is often hindered by sluggish kinetics and high overpotentials. The role of different nitrogen coordination groups including graphitic N (GN), pyridinic N (PdN), and pyrrolic N (PrN) groups in N-doped carbon materials for these processes is still under debate. Using density functional theory (DFT) calculations, we explored graphene structures doped with in-plane GN, PdN, and PrN as cost-effective electrocatalysts for oxygen electrode reactions, respectively. Our results highlight the importance of explicit solvents in accurately describing the binding behaviours of ORR/OER intermediates, contrasting with vacuum modelling which ignores the hydrogen bond formed between the adsorbates and water layer. Utilizing an explicit water layer, PdN-doped graphene is theoretically recognized as a bifunctional electrocatalyst for oxygen electrode catalysis. Moreover, based on the frontier molecular orbital theory, the Highest Occupied Molecular Orbital (HOMO) energy level of active sites is the underlying factor for the strong binding affinity to oxygen-containing intermediates, leading to the improved ORR/OER activity. Our work provides insight into the identification of active sites in N-doped graphene and provides a theoretical guidance for the rational design of effective carbon-based ORR/OER catalysts.

Research Progress in Structure Evolution and Durability Modulation of Ir- and Ru-Based OER Catalysts under Acidic Conditions

Green hydrogen energy, as one of the most promising energy carriers, plays a crucial role in addressing energy and environmental issues. Oxygen evolution reaction catalysts, as the key to water electrolysis hydrogen production technology, have been subject to durability constraints, preventing large-scale commercial development. Under the high current density and harsh acid-base electrolyte conditions of the water electrolysis reaction, the active metals in the catalysts are easily converted into high-valent soluble species to dissolve, leading to poor structural durability of the catalysts. There is an urgent need to overcome the durability challenges under acidic conditions and develop electrocatalysts with both high catalytic activity and high durability. In this review, the latest research results are analyzed in depth from both thermodynamic and kinetic perspectives. First, a comprehensive summary of the structural deactivation state process of noble metal oxide catalysts is presented. Second, the evolution of the structure of catalysts possessing high durability is discussed. Finally, four new strategies for the preparation of stable catalysts, "electron buffer (ECB) strategy", combination strength control, strain control, and surface coating, are summarized. The challenges and prospects are also elaborated for the future synthesis of more effective Ru/Ir-based catalysts and boost their future application.

Identifying key intermediates for the oxygen evolution reaction on hematite using ab-initio molecular dynamics

Hematite is a well-known catalyst for the oxygen evolution reaction on photoanodes in photoelectrochemical water-splitting cells. However, the knowledge of hematite-water interfaces and water oxidation mechanisms is still lacking, which limits improvements in photoelectrochemical water-splitting performance. Herein, we use the Fe-terminated hematite (0001) surface as a model and propose a comprehensive mechanism for the oxygen evolution reaction on both non-solvated and solvated surfaces. Key reaction intermediates are identified through ab initio molecular dynamics simulations at the density functional theory level with a Hubbard U correction. Several notable intermediates are proposed, and the effects of water solvent on these intermediates and the overall reaction mechanisms are suggested. The proposed mechanisms align well with experimental observations under photoelectrochemical water oxidation conditions. Additionally, we highlight the potential role of O2 desorption in the oxygen evolution reaction on hematite, as O2 adsorption may block reaction sites and increases surface hydrophobicity, leading to an unfavorable pathway for oxygen evolution.

Recognizing the reactive sites of SnFe2O4 for the oxygen evolution reaction: the synergistic effect of SnII and FeIII in stabilizing reaction intermediates

Among the reported spinel ferrites, the p-block metal containing SnFe2O4 is scarcely explored, but it is a promising water-splitting electrocatalyst. This study focuses on the reaction kinetics and atomic scale insight of the reaction mechanism of the oxygen evolution reaction (OER) catalyzed by SnFe2O4 and analogous Fe3O4. The replacement of FeIIOh sites with SnIIOh in SnFe2O4 improves the catalytic efficiency and various intrinsic parameters affecting the reaction kinetics. The variable temperature OER depicts a low activation energy (Ea) of 28.71 kJ mol-1 on SnFe2O4. Experimentally determined second-order dependence on [OH-] and the prominent kinetic isotope effect observed during the deuterium labelling study implies the role of hydroxide ions in the rate-determining step (RDS). Using density functional theory, the reaction mechanism on the (001) surface of SnFe2O4 and Fe3O4 is modelled. The DFT simulated free energy diagram for the reaction intermediates shows an adsorbate evolution mechanism (AEM) on both the ferrites' surfaces where the formation of *OOH is the RDS on SnFe2O4 while *O formation is the RDS on Fe3O4. In contrast to other spinel ferrites, where individual metal sites act independently, in case of SnFe2O4, a synergy between FeIIIOh and the neighbouring SnIIOh atoms is responsible for stabilizing the OER intermediates, enhancing the catalytic OER activity of SnFe2O4 as compared to isostructural Fe3O4.

Iron-Based Layered Perovskite Oxyfluoride Electrocatalyst for Oxygen Evolution: Insights from Crystal Facets with Heteroanionic Coordination

Mixed-anion compounds have recently attracted attention as solid-state materials that exhibit properties unattainable with those of their single-anion counterparts. However, the use of mixed-anion compounds to control the morphology and engineer the crystal facets of electrocatalysts has been limited because their synthesis method is still immature. This study explored the electrocatalytic properties of a Pb-Fe oxyfluoride, Pb3Fe2O5F2, with a layered perovskite structure for oxygen evolution reaction (OER) and compared its properties in detail with those of a bulk-type cubic three-dimensional (3D) perovskite, PbFeO2F. A Pb3Fe2O5F2 electrode prepared with carbon nanotubes and a graphite sheet as a conductive support and a substrate, respectively, demonstrated better OER performance than a PbFeO2F electrode. The role of specific crystal facets of Pb3Fe2O5F2 in enhancing the OER activity was elucidated through electrochemical analysis. Density functional theory calculations indicated that the Pb3Fe2O5F2 (060) facet with Fe sites exhibited a lower theoretical overpotential for the OER, which was attributed to a moderately strong interaction between the active sites and the reaction intermediates; this interaction was reinforced by the strong electron-withdrawing behavior of fluoride ions. This finding offers new insights for developing efficient electrocatalysts based on oxyfluorides, leveraging the high electronegativity of fluorine to optimize the electronic states at active sites for the OER, without relying on precious metals.

Rationalizing Acidic Oxygen Evolution Reaction over IrO2: Essential Role of Hydronium Cation

The development of active, stable, and more affordable electrocatalysts for acidic oxygen evolution reaction (OER) is of great importance for the practical application of electrolyzers and the advancement of renewable energy conversion technologies. Currently, IrO2 is the only catalyst with high stability and activity, but a high cost. Further optimization of the catalyst is limited by the lack of understanding of catalytic behaviors at the acid-IrO2 interface. Here, in strong interaction with the experiment, we develop an explicit model based on grand-canonical density function theory (GC-DFT) calculations to describe acidic OER over IrO2. Compared to the explicit models reported previously, hydronium cations (H3O+) are introduced at the electrochemical interface in the current model. As a result, a variation in stable IrO2 surface configuration under the OER operating condition from previously proposed complete *O-coverage to a mixture coverage of *OH and *O is revealed, which is well supported by in situ Raman measurements. In addition, the accuracy of predicted overpotential is increased in comparison with the experimentally measured. More importantly, an alteration of the potential limiting step from previously identified *O→*OOH to *OH→*O is observed, which opens new opportunities to advance the IrO2-based catalysts for acidic OER.

Reactivity and Stability of Reduced Ir-Weight TiO2-Supported Oxygen Evolution Catalysts for Proton Exchange Membrane (PEM) Water Electrolyzer Anodes

Reducing the iridium demand in Proton Exchange Membrane Water Electrolyzers (PEM WE) is a critical priority for the green hydrogen industry. This study reports the discovery of a TiO2-supported Ir@IrO(OH)x core-shell nanoparticle catalyst with reduced Ir content, which exhibits superior catalytic performance for the electrochemical oxygen evolution reaction (OER) compared to a commercial reference. The TiO2-supported Ir@IrO(OH)x core-shell nanoparticle configuration significantly enhances the OER Ir mass activity from 8 to approximately 150 A gIr-1 at 1.53 VRHE while reducing the iridium packing density from 1.6 to below 0.77 gIr cm-3. These advancements allow for viable anode layer thicknesses with lower Ir loading, reducing iridium utilization at 70% LHV from 0.42 to 0.075 gIr kW-1 compared to commercial IrO2/TiO2. The identification of the Ir@IrO(OH)x/TiO2 OER catalyst resulted from extensive HAADF-EDX microscopic analysis, operando XAS, and online ICP-MS analysis of 30-80 wt % Ir/TiO2 materials. These analyses established correlations among Ir weight loading, electrode electrical conductivity, electrochemical stability, and Ir mass-based OER activity. The activated Ir@IrO(OH)x/TiO2 catalyst-support system demonstrated an exceptionally stable morphology of supported core-shell particles, suggesting strong catalyst-support interactions (CSIs) between nanoparticles and crystalline oxide facets. Operando XAS analysis revealed the reversible evolution of significantly contracted Ir-O bond motifs with enhanced covalent character, conducive to the formation of catalytically active electrophilic OI- ligand species. These findings indicate that atomic Ir surface dissolution generates Ir lattice vacancies, facilitating the emergence of electrophilic OI- species under OER conditions, while CSIs promote the reversible contraction of Ir-O distances, reforming electrophilic OI- and enhancing both catalytic activity and stability.

Greenly Synthesized Conducting Polymer Nanotunnels with Metal-Hydroxide Nanobundles in Single Dais for Unmitigated Water Oxidation

Electrochemical water splitting required efficient electrocatalysts to produce clean hydrogen fuel. Here, we adopted greenway coprecipitation (GC) method to synthesize conducting polymer (CP) nanotunnel network affixed with luminal-abluminal CoNi hydroxides (GC-CoNiCP), namely, GC-Co1Ni2CP, GC-Co1.5Ni1.5CP, and GC-Co2Ni1CP. The active catalyst, GC-Co2Ni1CP/GC, has low oxygen evolution reaction (OER) overpotential (307 mV) and a smaller Tafel slope (47 mV dec-1) than IrO2 (125 mV dec-1). The electrochemical active surface area (EASA) normalized linear sweep voltammetry (LSV) curve exhibited outstanding intrinsic activity of GC-Co2Ni1CP, which required 285 mV to attain 10 mA cm-2. At 1.54 V, the estimated turnover frequency (TOF) of GC-Co2Ni1CP/GC (0.017337 s-1) was found to be 3-fold higher than that of IrO2 (0.0014 s-1). Furthermore, the GC-Co2Ni1CP/NF consumed a very low overpotential (281 mV) with a small Tafel slope of 121 mV dec-1. The ultrastability of GC-Co2Ni1CP for industrial application was confirmed by durability at 10 and 100 mA cm-2 for the OER (GC/NF-8 h, 2.0%/100 h, 2.2%) and overall water splitting (100 h, 3.8%), which implies that GC-Co2Ni1CP had adequate kinetics to address the elevated rates of water oxidation. The effect of pH and addition of tetramethylammonium cation (TMA+) reveal that GC-Co2Ni1CP follows the lattice oxygen mechanism (LOM). The solar-powered water electrolysis at 1.55 V supports the efficacy of GC-Co2Ni1CP in the solar-to-hydrogen conversion. The environmental impact studies and solar-driven water electrolysis proved that GC-CoNiCP has excellent greenness and efficiency, respectively.

Simplification of solvation shell with water clusters in the simulation of electrochemical nitrogen reduction reaction

Electrochemical methods for nitrogen reduction have received extensive attention due to the mild reaction conditions. In order to gain an insight into the mechanism of the electrochemical nitrogen reduction process, theoretical simulations are necessary. However, current simulation studies contain many imprecise approximations that may hinder the real recognition of the reaction process. Although solvation methods have recently been developed to provide efficient descriptions, their further applications are hindered by controversy over modeling of solvents and the enormous computational effort required. In this work, we simplify the solvation conditions by using water clusters and compare them with an accurate water layer model. The results demonstrate that the simplified water clusters can effectively capture protons, simulate the surface electric environment, and enable the calculation of the activation energy of the reaction. This method offers an affordable approach for simulating the surface potentials and solvents and provides a new reference for the theoretical study of the electrochemical nitrogen reduction reaction.

Fe/Co/Ni modified Ti3C2Tx nanosheets accelerate alkaline hydrogen evolution reaction

The novel two-dimensional MXene material Ti3C2Tx boasts advantages such as large specific surface area, good electrical conductivity, and high stability, making it suitable for the field of electrocatalysis. However, Ti3C2Tx exhibits unacceptably slow kinetics during the electrocatalytic hydrogen evolution reaction (HER). Electron-metal-support interaction is an effective method for regulating the electronic state of active sites and enhancing HER performance. Therefore, in this study, Fe, Co, and Ni were respectively loaded onto Ti3C2Txvia electron beam deposition to form electron microscopy-supported interface (EMSI) effects, thereby improving the HER activity of Ti3C2Tx. The tests conclude that loading different transition metals (Fe, Co, Ni) onto Ti3C2Tx effectively enhances its HER performance. Experimental and theoretical studies further indicate that the electrocatalytic performance of Ni-loaded Ti3C2Tx is superior to that of Co-loaded and Fe-loaded Ti3C2Tx. This work presents a promising strategy for synthesizing metal-loaded MXene.

Origin of the Activity of Electrochemical Ozone Production Over Rutile PbO2 Surfaces

Ozonation water treatment technology has attracted increasing attention due to its environmental benign and high efficiency. Rutile PbO2 is a promising anode material for electrochemical ozone production (EOP). However, the reaction mechanism underlying ozone production catalyzed by PbO2 was rarely studied and not well-understood, which was in part due to the overlook of the electrochemistry-driven formation of oxygen vacancy (OV) of PbO2. Herein, we unrevealed the origin of the EOP activity of PbO2 starting from the electrochemical surface state analysis using density functional theory (DFT) calculations, activity analysis, and catalytic volcano modeling. Interestingly, we found that under experimental EOP potential (i. e., a potential around 2.2 V vs. reversible hydrogen electrode), OV can still be generated easily on PbO2 surfaces. Our subsequent kinetic and thermodynamic analyses show that these OV sites on PbO2 surfaces are highly active for the EOP reaction through an interesting atomic oxygen (O*)-O2 coupled mechanism. In particular, rutile PbO2(101) with the "in-situ" generated OV exhibited superior EOP activities, outperforming the (111) and (110) surfaces. Finally, by catalytic volcano modeling, we found that PbO2 is close to the theoretical optimum of the reaction, suggesting a superior EOP performance of rutile PbO2. All these analyses are in good agreement with previous experimental observations in terms of EOP overpotentials. This study provides the first volcano model to explain why rutile PbO2 is among the best metal oxide materials for EOP and provides new design guidelines for this rarely studied but industrially promising reaction.

Ir Doping Modulates the Electronic Structure of Flower-Shaped Phosphides for Water Oxidation

Electrolysis of water to produce hydrogen is an efficient, clean, and environmentally friendly hydrogen production method with unlimited development prospects. However, its overall efficiency is hampered by the slow oxygen evolution reaction (OER) with complex electron transfer processes. Therefore, designing efficient and low-cost OER catalysts is the key to solving this problem. In this paper, Ir-doped Co2P/Fe2P (abbreviated as Ir-CoFeP/NF) was grown on nickel foam through the strategies of low amount noble-metal doping and mild phosphating. Phosphide derived from a floral metal-organic framework (MOF) exhibits regular three-dimensional (3D) morphology and large active area, avoiding the stacking of active sites. The addition of Ir can effectively adjust the electronic structure, change the position of the d-band center, and increase active sites, thus enhancing the catalytic activity. Hence, the optimized catalyst exhibits unexpected electrocatalytic OER activity with an ideal overpotential of 213 mV at 10 mA cm-2, as well as a low Tafel slope of 40.63 mV dec-1. Coupling with Pt/C for overall water splitting (OWS), the entire device only needs an ultralow cell voltage of 1.50 V to achieve a current density of 10 mA cm-2. Besides, the OWS can be maintained for more than 70 h. This study demonstrates the superiority of Ir-doped phosphide in accelerating water oxidation.