Fundamental Studies of Planar Single-Crystalline Oxide Model Electrodes (RuO2, IrO2) for Acidic Water Splitting

Iron Doping of 2D Nickel-Based Metal-Organic Frameworks Enhances the Lattice Heterogeneous Interface Coupling Effect for Improved Electrocatalytic Oxygen Evolution

The coupling of lattice and heterostructure interfaces represents an effective strategy for disrupting the so-called scalar relationship and accelerating reactions involving multiple intermediates. In view of this, a lattice-heterostructure interfacial catalyst consisting of a crystalline Fe/Ni bimetallic MOF and amorphous Fe-MOF was designed in this paper for high-performance alkaline oxygen evolution reaction electrocatalysis. The strongly coupled lattice-heterostructure interface induces a unique synergistic effect that promotes electron transfer of the catalyst. The resulting catalyst exhibits exceptionally high catalytic activity for the oxygen evolution reaction in alkaline media, the Ni9Fe1-BDC-1@Fe-MOF coated on a glassy carbon electrode has an overpotential of 257 mV at a current density of 10 mA cm-2. Furthermore, this catalyst demonstrates a high electrochemical stability. These research results highlight the superiority of lattice-heterostructure interfaces in the development of advanced catalysts.

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.

Structure-Activity Relationships in Oxygen Electrocatalysis

Oxygen electrocatalysis, as the pivotal circle of many green energy technologies, sets off a worldwide research boom in full swing, while its large kinetic obstacles require remarkable catalysts to break through. Here, based on summarizing reaction mechanisms and in situ characterizations, the structure-activity relationships of oxygen electrocatalysts are emphatically overviewed, including the influence of geometric morphology and chemical structures on the electrocatalytic performances. Subsequently, experimental/theoretical research is combined with device applications to comprehensively summarize the cutting-edge oxygen electrocatalysts according to various material categories. Finally, future challenges are forecasted from the perspective of catalyst development and device applications, favoring researchers to promote the industrialization of oxygen electrocatalysis at an early date.

Atomic Insights into the Competitive Edge of Nanosheets Splitting Water

The oxygen evolution reaction (OER) provides the protons for many electrocatalytic power-to-X processes, such as the production of green hydrogen from water or methanol from CO2. Iridium oxohydroxides (IOHs) are outstanding catalysts for this reaction because they strike a unique balance between activity and stability in acidic electrolytes. Within IOHs, this balance varies with the atomic structure. While amorphous IOHs perform best, they are least stable. The opposite is true for their crystalline counterparts. These rules-of-thumb are used to reduce the loading of scarce IOH catalysts and retain the performance. However, it is not fully understood how activity and stability are related at the atomic level, hampering rational design. Herein, we provide simple design rules (Figure 12) derived from the literature and various IOHs within this study. We chose crystalline IrOOH nanosheets as our lead material because they provide excellent catalyst utilization and a predictable structure. We found that IrOOH signals the chemical stability of crystalline IOHs while surpassing the activity of amorphous IOHs. Their dense bonding network of pyramidal trivalent oxygens (μ3Δ-O) provides structural integrity, while allowing reversible reduction to an electronically gapped state that diminishes the destructive effect of reductive potentials. The reactivity originates from coordinative unsaturated edge sites with radical character, i.e., μ1-O oxyls. By comparing to other IOHs and literature, we generalized our findings and synthesized a set of simple rules that allow prediction of stability and reactivity of IOHs from atomistic models. We hope that these rules will inspire atomic design strategies for future OER catalysts.

Recent Research on Iridium-Based Electrocatalysts for Acidic Oxygen Evolution Reaction from the Origin of Reaction Mechanism

As the anode reaction of proton exchange membrane water electrolysis (PEMWE), the acidic oxygen evolution reaction (OER) is one of the main obstacles to the practical application of PEMWE due to its sluggish four-electron transfer process. The development of high-performance acidic OER electrocatalysts has become the key to improving the reaction kinetics. To date, although various excellent acidic OER electrocatalysts have been widely researched, Ir-based nanomaterials are still state-of-the-art electrocatalysts. Hence, a comprehensive and in-depth understanding of the reaction mechanism of Ir-based electrocatalysts is crucial for the precise optimization of catalytic performance. In this review, the origin and nature of the conventional adsorbate evolution mechanism (AEM) and the derived volcanic relationship on Ir-based electrocatalysts for acidic OER processes are summarized and some optimization strategies for Ir-based electrocatalysts based on the AEM are introduced. To further investigate the development strategy of high-performance Ir-based electrocatalysts, several unconventional OER mechanisms including dual-site mechanism and lattice oxygen mediated mechanism, and their applications are introduced in detail. Thereafter, the active species on Ir-based electrocatalysts at acidic OER are summarized and classified into surface Ir species and O species. Finally, the future development direction and prospect of Ir-based electrocatalysts for acidic OER are put forward.

In-Situ Construction of Fe-Doped NiOOH on the 3D Ni(OH)2 Hierarchical Nanosheet Array for Efficient Electrocatalytic Oxygen Evolution Reaction

Accessible and superior electrocatalysts to overcome the sluggish oxygen evolution reaction (OER) are pivotal for sustainable and low-cost hydrogen production through electrocatalytic water splitting. The iron and nickel oxohydroxide complexes are regarded as the most promising OER electrocatalyst attributed to their inexpensive costs, easy preparation, and robust stability. In particular, the Fe-doped NiOOH is widely deemed to be superior constituents for OER in an alkaline environment. However, the facile construction of robust Fe-doped NiOOH electrocatalysts is still a great challenge. Herein, we report the facile construction of Fe-doped NiOOH on Ni(OH)2 hierarchical nanosheet arrays grown on nickel foam (FeNi@NiA) as efficient OER electrocatalysts through a facile in-situ electrochemical activation of FeNi-based Prussian blue analogues (PBA) derived from Ni(OH)2. The resultant FeNi@NiA heterostructure shows high intrinsic activity for OER due to the modulation of the overall electronic energy state and the electrical conductivity. Importantly, the electrochemical measurement revealed that FeNi@NiA exhibits a low overpotential of 240 mV at 10 mA/cm2 with a small Tafel slope of 62 mV dec-1 in 1.0 M KOH, outperforming the commercial RuO2 electrocatalysts for OER.

Iron-Nickel synergistic catalysis growth of (Fe,Ni)9S8/Ni3S2@N,S codoped carbon bridged nanowires enhanced oxygen evolution reaction performance

Improving the conductivity of the electrocatalyst itself is essential for enhancing its performance. In this work, N, S-rich 6-thioguanine (TG) is selected as the ligand to synthesize a Fe, Ni bimetallic porous coordination polymer (PCP), which is then derived to fabricate N,S codoped carbon (NSC)-coated (Fe,Ni)9S8/Ni3S2 bridged nanowires. The (Fe,Ni)9S8/Ni3S2@NSC bridged nanowires obtained through bimetallic synergistic catalysis and self-sulfurization processes not only introduced additional electrocatalytic active sites but also significantly enhance the overall conductivity of the catalyst due to the interconnected nanowire structure. The resulting (Fe,Ni)9S8/Ni3S2@NSC demonstrates remarkable oxygen evolution reaction (OER) performance, exhibiting an overpotential as low as 252 mV at a current density of 10 mA cm-2. This work proposes a novel strategy for enhancing the overall conductivity of catalysts by growing bridged nanowires, providing valuable insights and inspiration for the design and preparation of advanced transition metal sulfide electrocatalysts.

Unilamellar MnO2 nanosheets confined Ru-clusters combined with pulse electrocatalysis for biomass electrooxidation in neutral electrolytes

The electrochemical oxidation of 5-hydroxymethylfurfural (HMFOR) in alkaline electrolyte is a promising strategy for producing high-value chemicals from biomass derivatives. However, the disproportionation of aldehyde groups under strong alkaline conditions and the polymerization of HMF to form humic substances can impact the purity of 2,5-furandicarboxylic acid (FDCA) products. The use of neutral electrolytes offers an alternative environment for electrolysis, but the lack of OH- ions in the electrolyte often leads to low current density and low yields of FDCA. In this study, a sandwich-structured catalyst, consisting of Ru clusters confined between unilamellar MnO2 nanosheets (S-Ru/MnO2), was used in conjunction with an electrochemical pulse method to realize the electrochemical conversion of 5-hydroxymethylfurfural into FDCA in neutral electrolytes. Pulse electrolysis and the strong electron transfer between Ru clusters and MnO2 nanosheets help maintain Ru in a low oxidation state, ensuring high activity. The increased *OH generation led to a groundbreaking current density of 47 mA/cm2 at 1.55 V vs. reversible hydrogen electrode (RHE) and an outstanding yield rate of 98.7 % for FDCA in a neutral electrolyte. This work provides a strategy that combines electrocatalyst design with an electrolysis technique to achieve remarkable performance in neutral HMFOR.

Best practices of modeling complex materials in electrocatalysis, exemplified by oxygen evolution reaction on pentlandites

Pentlandites are natural ores with structural properties comparable to that of [FeNi] hydrogenases. While this class of transition-metal sulfide materials - (Fe,Ni)9S8 - with a variable Fe : Ni ratio has been proven to be an active electrode material for the hydrogen evolution reaction, it is also discussed as electrocatalyst for the alkaline oxygen evolution reaction (OER), corresponding to the bottleneck of anion exchange membrane electrolyzers for green hydrogen production. Despite the experimental evidence for the use of (Fe,Ni)9S8 as an OER catalyst, a detailed investigation of the elementary reaction steps, including consideration of adsorbate coverages and limiting steps under anodic polarizing conditions, is still missing. We address this gap in the present manuscript by gaining atomistic insights into the OER on an Fe4.5Ni4.5S8(111) surface through density functional theory calculations combined with a descriptor-based analysis. We use this system to introduce best practices for modeling this rather complex material by pointing out hidden pitfalls that can arise when using the popular computational hydrogen electrode approach to describe electrocatalytic processes at the electrified solid/liquid interface for energy conversion and storage.

Metallic Ru─Ru Interaction in Ruthenium Oxide Enabling Durable Proton Exchange Membrane Water Electrolysis

Developing efficient and robust electrocatalysts toward the oxygen evolution reaction (OER) is critical for proton exchange membrane water electrolysis (PEMWE). RuO2 possesses intrinsically high OER activity, but the concurrent electrochemical dissolution leads to rapid deactivation. Here a unique RuO2 catalyst containing metallic Ru─Ru interactions (m-RuO2) is reported, which maintains stability in practical PEMWE for 100 h at 60 °C and 1 A cm-2. Experimental and theoretical investigations suggest that the presence of Ru─Ru interactions significantly increases the energy barrier for the formation of RuO2(OH)2, which is a key intermediate for Ru dissolution, and hence substantially mitigates the electrochemical corrosion of m-RuO2. Meanwhile, the Ru4d band center downshifts, accordingly, ensuring the high OER activity, and the participation of lattice oxygen in the OER is also suppressed at the Ru─Ru sites, further contributing to the enhanced durability. Interestingly, such enhanced stability is also dependent on the size of metallic Ru─Ru cluster, where the energy barrier is further increased for Ru3, but is decreased for Ru5. These results highlight the significance of local coordination structure modulation on the electrochemical stability of RuO2 and open a feasible avenue toward the development of robust OER electrocatalysts for high-performance PEMWE.

Robust High-performance Bifunctional Porous Cobalt MOF-Based Catalysts for Overall Water Splitting

MOF-based materials, as bifunctional catalysts for electrocatalytic water splitting, play an important role in the application and development of clean fuel hydrogen energy. This study presents a series of novel 3D Co-based MOFs with layered networks, including [Co(4,4'-bipy)0.5(aip)(CH3OH)·H2O]n (Co-MOF 1), [Co2(1,3'-bit)(aip)2(CH3OH)·H2O]n (Co-MOF 2), [Co(4,4'-bipb)(aip)]n (Co-MOF 3), and [Co2(4,4'-bipe)(aip)2·1.5H2O]n (Co-MOF 4). Their single-crystal structures of Co-MOFs 1-4 are characterized and analyzed before being applied in alkaline solutions for water decomposition (OER and HER). The electrocatalytic tests indicate that Co-MOFs 1-4 exhibit a good performance. Notably, Co-MOF 4 exhibits great behavior which has low overpotentials of 94 and 188 mV (OER) as well as 185 and 352 mV (HER) at the currents of 10 and 100 mA cm-2, respectively. In comparison with Co-MOFs 1-3, Co-MOF 4 has the lowest Tafel slopes, highest ECSA, and smallest resistance. The immanent qualities, such as distinct interwoven long chain layered structure, unsaturated coordination modes, and synergistic catalytic qualities among Co ions, contribute to explaining the results. The fundamentals provide valuable information for the investigation of innovative MOF-based bifunctional electrocatalysts for overall water splitting.

Enhancing the acidic oxygen evolution reaction performance of RuO2-TiO2by a reduction-oxidation process

As a promising alternative to Ir based acidic oxygen evolution reaction (OER) catalysts, Ru suffers from severe fading issues. Supporting it on robust oxides such as TiO2is a simple and effective way to enhance its lifetime. Here, we find that a simple reduction-oxidation process can further improve both activity and stability of RuO2-TiO2composites at high potentials. In this process, the degree of oxidation was carefully controlled to form Ru/RuO2heterostructure to improve OER activity. Moreover, due to the oxophilicity difference of Ru and Ti, the structure of catalysts was changed from supported to embedded, which enhanced the protective effect of TiO2and mitigated the dissolution of Ru element in acidic electrolyte, making as-prepared Ru/RuO2-TiO2with better durability at all tested potentials.

Synergistic strategy of solute environment and phase control of Pb-based anodes to solve the activity-stability trade-off

The contradiction between the activity and stability of metal anodes exists extensively, especially in acid electrooxidation under industrial-level current density. Although the anode modification enhanced the initial activity of anodes, its long-term activity is limited by anode slime accumulation. Herein, a synergistic strategy, coupling the solute environment with the phase control of anodes, is proposed to achieve the trade-off between activity and stability of Pb-based anodes in concentrated sulfuric acid electrolysis. Non-exogenous Mn2+ motivated a series of positive behaviours of reactive-oxygen-species capture, anode reconstruction and corrosion-dependent activity alleviation. The synergistic effects, which are crystal phase-dependent, mainly benefit from the continuous self-healing ability of the specific crystal phase of MnO2 on the anodes by the coexisted Mn2+. Compared with Mn2+/α-MnO2, Mn2+/γ-MnO2 exhibited outperformed activity and stability in boosting oxygen evolution reaction (OER) and reducing hazardous pollutants, which resulted from the energy difference in the rate-determining step of OER and in the selectivity priority of Mn2+/MnO2 oxidation pathway. Interestingly, the pre-coated γ-MnO2 on the anode also presents excellent inheritance, guaranteeing the unchanged crystal phase of MnO2 and the high performance in ultra-low hazardous slime generation in subsequent Mn2+ oxidation. The sustainability of Mn2+/γ-MnO2 was proved in the operating hydrometallurgy conditions on Pb-based anodes. This strategy offers a promising approach for this common issue in electrooxidation-related areas.

Four Generations of Volcano Plots for the Oxygen Evolution Reaction: Beyond Proton-Coupled Electron Transfer Steps?

ConspectusDue to its importance for electrolyzers or metal-air batteries for energy conversion or storage, there is huge interest in the development of high-performance materials for the oxygen evolution reaction (OER). Theoretical investigations have aided the search for active material motifs through the construction of volcano plots for the kinetically sluggish OER, which involves the transfer of four proton-electron pairs to form a single oxygen molecule. The theory-driven volcano approach has gained unprecedented popularity in the catalysis and energy communities, largely due to its simplicity, as adsorption free energies can be used to approximate the electrocatalytic activity by heuristic descriptors.In the last two decades, the binding-energy-based volcano method has witnessed a renaissance with special concepts being developed to incorporate missing factors into the analysis. To this end, this Account summarizes and discusses the different generations of volcano plots for the example of the OER. While first-generation methods relied on the assessment of the thermodynamic information for the OER reaction intermediates by means of scaling relations, the second and third generations developed strategies to include overpotential and kinetic effects into the analysis of activity trends. Finally, the fourth generation of volcano approaches allowed the incorporation of various mechanistic pathways into the volcano methodology, thus paving the path toward data- and mechanistic-driven volcano plots in electrocatalysis.Although the concept of volcano plots has been significantly expanded in recent years, further research activities are discussed by challenging one of the main paradigms of the volcano concept. To date, the evaluation of activity trends relies on the assumption of proton-coupled electron transfer steps (CPET), even though there is experimental evidence of sequential proton-electron transfer (SPET) steps. While the computational assessment of SPET for solid-state electrodes is ambitious, it is strongly suggested to comprehend their importance in energy conversion and storage processes, including the OER. This can be achieved by knowledge transfer from homogeneous to heterogeneous electrocatalysis and by focusing on the material class of single-atom catalysts in which the active center is well defined. The derived concept of how to analyze the importance of SPET for mechanistic pathways in the OER over solid-state electrodes could further shape our understanding of the proton-electron transfer steps at electrified solid/liquid interfaces, which is crucial for further progress toward sustainable energy and climate neutrality.

Ultrasound-Assisted Preparation and Performance Regulation of Electrocatalytic Materials

With the advancement of scientific research, the introduction of external physical methods not only adds extra freedom to the design of electro-catalytical processes for green technologies but also effectively improves the reactivity of materials. Physical methods can adjust the intrinsic activity of materials and modulate the local environment at the solid-liquid interface. In particular, this approach holds great promise in the field of electrocatalysis. Among them, the ultrasonic waves have shown reasonable control over the preparation of materials and the electrocatalytic process. However, the research on coupling ultrasonic waves and electrocatalysis is still early. The understanding of their mechanisms needs to be more comprehensive and deep enough. Firstly, this article extensively discusses the adhibition of the ultrasonic-assisted preparation of metal-based catalysts and their catalytic performance as electrocatalysts. The obtained metal-based catalysts exhibit improved electrocatalytic performances due to their high surface area and more exposed active sites. Additionally, this article also points out some urgent unresolved issues in the synthesis of materials using ultrasonic waves and the regulation of electrocatalytic performance. Lastly, the challenges and opportunities in this field are discussed, providing new insights for improving the catalytic performance of transition metal-based electrocatalysts.

Computational chemistry for water-splitting electrocatalysis

Electrocatalytic water splitting driven by renewable electricity has attracted great interest in recent years for producing hydrogen with high-purity. However, the practical applications of this technology are limited by the development of electrocatalysts with high activity, low cost, and long durability. In the search for new electrocatalysts, computational chemistry has made outstanding contributions by providing fundamental laws that govern the electron behavior and enabling predictions of electrocatalyst performance. This review delves into theoretical studies on electrochemical water-splitting processes. Firstly, we introduce the fundamentals of electrochemical water electrolysis and subsequently discuss the current advancements in computational methods and models for electrocatalytic water splitting. Additionally, a comprehensive overview of benchmark descriptors is provided to aid in understanding intrinsic catalytic performance for water-splitting electrocatalysts. Finally, we critically evaluate the remaining challenges within this field.

Mo-Doped Mesoporous RuO2 Spheres as High-Performance Acidic Oxygen Evolution Reaction Electrocatalyst

The development of highly active and acid-stable electrocatalysts for oxygen evolution reaction (OER) is of great significance for water electrolysis technology. Herein, a highly efficient molybdenum-doped mesoporous ruthenium dioxide sphere (Mo-RuO2 ) catalyst is fabricated by a facile impregnation and post-calcination method using mesoporous carbon spheres to template the mesostructure. The optimal Mo0.15 -RuO2 catalyst with Mo doping amount of 15 mol.% exhibits a significantly low overpotential of 147 mV at 10 mA cm-2 , a small Tafel slope of 38 mV decade-1 , and enhanced electrochemical stability in acidic electrolyte, far superior to the commercial RuO2 catalyst. The experimental results and theoretical analysis reveal that the remarkable electrocatalytic performance can be attributed to the large surface area of the mesoporous spherical structure, the structural robustness of the interconnected mesoporous framework, and the change in the electronic structure of Ru active sites induced by Mo doping. These excellent advantages make Mo-doped mesoporous RuO2 spheres a promising catalyst for highly efficient electrocatalytic OER in acidic media.

In Situ and Operando X-ray Scattering Methods in Electrochemistry and Electrocatalysis

Electrochemical and electrocatalytic processes are of key importance for the transition to a sustainable energy supply as well as for a wide variety of other technologically relevant fields. Further development of these processes requires in-depth understanding of the atomic, nano, and micro scale structure of the materials and interfaces in electrochemical devices under reaction conditions. We here provide a comprehensive review of in situ and operando studies by X-ray scattering methods, which are powerful and highly versatile tools to provide such understanding. We discuss the application of X-ray scattering to a wide variety of electrochemical systems, ranging from metal and oxide single crystals to nanoparticles and even full devices. We show how structural data on bulk phases, electrode-electrolyte interfaces, and nanoscale morphology can be obtained and describe recent developments that provide highly local information and insight into the composition and electronic structure. These X-ray scattering studies yield insights into the structure in the double layer potential range as well as into the structural evolution during electrocatalytic processes and phase formation reactions, such as nucleation and growth during electrodeposition and dissolution, the formation of passive films, corrosion processes, and the electrochemical intercalation into battery materials.

The Promising Seesaw Relationship Between Activity and Stability of Ru-Based Electrocatalysts for Acid Oxygen Evolution and Proton Exchange Membrane Water Electrolysis

The development of electrocatalysts that are not reliant on iridium for efficient acid-oxygen evolution is a critical step towards the proton exchange membrane water electrolysis (PEMWE) and green hydrogen industry. Ruthenium-based electrocatalysts have garnered widespread attention due to their remarkable catalytic activity and lower commercial price. However, the challenge lies in balancing the seesaw relationship between activity and stability of these electrocatalysts during the acid-oxygen evolution reaction (OER). This review delves into the progress made in Ru-based electrocatalysts with regards to acid OER and PEMWE applications. It highlights the significance of customizing the acidic OER mechanism of Ru-based electrocatalysts through the coordination of adsorption evolution mechanism (AEM) and lattice oxygen oxidation mechanism (LOM) to attain the ideal activity and stability relationship. The promising tradeoffs between the activity and stability of different Ru-based electrocatalysts, including Ru metals and alloys, Ru single-atomic materials, Ru oxides, and derived complexes, and Ru-based heterojunctions, as well as their applicability to PEMWE systems, are discussed in detail. Furthermore, this paper offers insights on in situ control of Ru active sites, dynamic catalytic mechanism, and commercial application of PEMWE. Based on three-way relationship between cost, activity, and stability, the perspectives and development are provided.

Surface Reconstruction Facilitated by Fluorine Migration and Bimetallic Center in NiCo Bimetallic Fluoride Toward Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is a critical anodic reaction of electrochemical water splitting, developing a high-efficiency electrocatalyst is essential. Transition metal-based catalysts are much more cost-effective if comparable activities can be achieved. Among them, fluorides are rarely reported due to their low aqueous stability of coordination and low electric conductivity. Herein, a NiCo bimetallic fluoride with good crystallinity is designed and constructed, and significantly enhanced catalytic activity and conductivity are observed. The inevitable oxidation of transition metal ions at high potential and the dissociation of F- are attributed to the low aqueous stability of coordination. The theoretical researches predicte that transition metal fluorides should have a strong tendency to electrochemical reconstruction. Therefore, based on the observations on their electrochemical behavior, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and bode plots, it is further demonstrated that surface reconstruction occurred during the electrochemical process, meanwhile a significant increase of electrochemically active area, which is created by F migration, are also directly observed. Additionally, DFT calculation results show that the electronic structure of the catalysts is modulated by the bimetallic centers, and this reconstruction helps optimizing the adsorption energy of oxygen-containing species and improves OER activity.

Frank Partial Dislocations in Coplanar Ir/C Ultrathin Nanosheets Boost Hydrogen Evolution Reaction

Developing highly active and stable acidic hydrogen evolution catalysts is of great significance and challenge for the long-term operation of commercial proton exchange membrane (PEM) electrolyzers. In this work, coplanar ultrathin nanosheets composed of rich-Frank partial dislocations (FPDs) are first synthesized. Ir nanoparticles and carbon (Dr-Ir/C NSs) use a nonequilibrium high-temperature thermal shock method (>1200 °C) and KBr template-assisted techniques. Dr-Ir/C NSs exhibit excellent hydrogen evolution reaction (HER) performance with a remarkably high mass activity of 6.64 A mg-1 at 50 mV, which is among the best Ir-based catalysts.In addition, Dr-Ir/C NSs are able to operate stably at 1.0 A cm-2 for 200 h as a cathode in a PEM electrolyser, and the original coplanar ultrathin nanosheets structure are maintained after the test, demonstrating excellent stability against stacking and agglomeration. Geometrical phase analysis and theoretical calculations show that the FPDs produce a 4% compressive strain in the Dr-Ir/C NSs, and the compressive strain weaken the adsorption of H* by Ir, thus increasing the intrinsic activity of the catalyst.

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.

Multiple-Strategy Design of MOF-Derived N, P Co-Doped MoS2 Electrocatalysts Toward Efficient Alkaline Hydrogen Evolution and Overall Water Splitting

The multiple strategy design is crucial for enhancing the efficiency of nonprecious electrocatalysts in hydrogen evolution reaction (HER). In this work, we successfully synthesized N, P-codoped MoS2 nanosheets as highly efficient catalysts by integrating doping effects and phase engineering using a porous metal-organic framework (MOF) template. The electrocatalysts exhibit excellent bifunctional activity and stability in alkaline media. The N, P codoping induces electron redistribution to enhance conductivity and promote the intrinsic activity of the electrocatalysts. It optimizes the H* adsorption free energy and the dissociative adsorption energy, resulting in significant enhancement of HER activity. Moreover, the porous MOF structure exposes a large number of electrochemically active sites and facilitates the diffusion of ions and gases, which improve charge transfer efficiency and structural stability. Specifically, at a current density of 10 mA cm-2, the overpotential of the HER is only 32 mV, with a Tafel slope of 47 mV dec-1 and Faradaic efficiency as high as 98.51% (at 100 mA cm-2). Only a 338 mV overpotential is required to achieve a current density of 50 mA cm-2 for oxygen evolution reaction (OER), and a potential of 1.49 V (at 10 mA cm-2) is sufficient to drive overall water splitting. Further experimental measurements and first-principles calculations evidence that the exceptional performance is primarily attributed to the dual functionality of N and P dopants, which not only activate additional S sites but also initialize the phase transition of 2H to 1T-MoS2 to facilitate the rapid charge transfer. Through in-depth exploration of the combined design of multiple strategies for efficient catalysts, our work paves a new way for the development of future efficient nonprecious metal catalysts.

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.

Defect- and oxygen-rich nanocarbon derived from solution plasma for bifunctional catalytic activity of oxygen reduction and evolution reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are key for renewable energy systems, including metal-air batteries, fuel cells, and water electrolysis. In particular, metal-air batteries require multiple catalysts for the ORR and OER. Thus, bifunctional catalysts are required to improve efficiency and simplify catalytic systems. Hence, we developed defect- and oxygen-rich nanocarbons as bifunctional catalysts through a one-pot formation by applying plasma discharge in mixed solvents of benzene with crown ether. Raman and X-ray photoelectron spectroscopy results confirmed that oxygen was embedded and functionalized into the carbon matrix and abundant defects were formed, which highly affected the catalytic activity of the ORR and OER. The obtained CNP-CEs revealed a tuned electron transfer trend to a rapid four-electron pathway (n = 3.5) for the ORR, as well as a decreased onset potential and Tafel slope for the OER. Consequently, CNP-CE-50 exhibited an improved bifunctional catalytic characteristic with the narrowest potential gap between the ORR and OER. We believe that our findings suggest new models for carbon-based bifunctional catalysts and provide a prospective approach for a synthetic procedure of carbon nanomaterials.

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.

Regulable in-situ autoredox for anchoring synergistic Ni/NiO nanoparticles on reduced graphene oxide with boosted alkaline electrocatalytic oxygen evolution

To develop ideal alternatives to noble metal catalysts, transition metal catalysts supported on graphene have been receiving extensive attention in the field of electrochemical energy. In this work, using graphene oxide (GO) and nickel formate as precursors, Ni/NiO synergistic nanoparticles with regulable composition are anchored on reduced graphene oxide (RGO) to prepare Ni/NiO/RGO composite electrocatalysts through in-situ autoredox. Thanks to the synergistic effect of Ni³⁺ active sites and Ni electron donors, the as-prepared Ni/NiO/RGO catalysts exhibit efficient electrocatalytic oxygen evolution performance in 1.0 M KOH electrolyte. The optimal sample has an overpotential of only 275 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 90 mV dec^-1, which are very comparable to those of commercial RuO₂ catalyst. Additionally, the catalytic capacity and structure remain stable after 2000 cyclic voltammetry cycles. For the electrolytic cell assembled with the best-performing sample as anode and commercial Pt/C as cathode, the current density can reach 10 mA cm^-2 at a low potential of 1.57 V and remains stable after 30 h of continuous work. It would be expected that the as-developed Ni/NiO/RGO catalyst with high activity should have broad application prospects.

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

Activity-Stability Trends of the Sb-SnO2@RuOx Heterostructure toward Acidic Water Oxidation

Accomplishments of enhanced activity and durability are a major concern in the design of catalysts for acidic water oxidation. To date, most studied supported metal catalysts undergo fast degradation in strongly acidic and oxidative environments due to improper controlling of the interface stability caused by their lattice mismatches. Here, we evaluate the activity-stability trends of in situ crystallized antimony-doped tin oxide (Sb-SnO₂)@RuO_x (Sb-SnO₂@RuO_x) heterostructure nanosheets (NSs) for acidic water oxidation. The catalyst prepared by atomic layer deposition of a conformal Ru film on antimony-doped tin sulfide (Sb-SnS₂) NSs followed by heat treatment highlights comparable activity but longer stability than that of the ex situ catalyst (where Ru was deposited on Sb-SnO₂ followed by heating). Air calcination for in situ crystallization allows the formation of hierarchical mesoporous Sb-SnO₂ NSs from as-prepared Sb-SnS₂ NSs and parallel in situ transformation from Ru to RuO_x, resulting in a compact heterostructure. The significance of this approach significantly resists corrosive dissolution, which is justified by the enhanced oxygen evolution reaction (OER) stability of the catalyst compared to most of the state-of-the-art ruthenium-based catalysts including Carbon@RuO_x (which shows ∼10 times higher dissolution) as well as Sb-SnO₂@Com. RuO_x and Com. RuO₂. This study demonstrates the controlled interface stability of heterostructure catalysts toward enhancing OER activity and stability.

Operando CO Infrared Spectroscopy and On-Line Mass Spectrometry for Studying the Active Phase of IrO2 in the Catalytic CO Oxidation Reaction

We combine operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with on-line mass spectrometry (MS) to study the correlation between the oxidation state of titania-supported IrO2 catalysts (IrO2@TiO2) and their catalytic activity in the prototypical CO oxidation reaction. Here, the stretching vibration of adsorbed COad serves as the probe. DRIFTS provides information on both surface and gas phase species. Partially reduced IrO2 is shown to be significantly more active than its fully oxidized counterpart, with onset and full conversion temperatures being about 50 °C lower for reduced IrO2. By operando DRIFTS, this increase in activity is traced to a partially reduced state of the catalysts, as evidenced by a broad IR band of adsorbed CO reaching from 2080 to 1800 cm−1.

Skeleton-coated CoCu-Based bimetal hollow nanoprisms as High-Performance electrocatalysts for oxygen evolution reaction

CoSx materials with high catalytic activity are considered as promising HER electrocatalysts, but their inherent low electrical conductivity and easy loss of active sites have greatly limited their applications in OER electrocatalysis. Herein, we present a convenient method to synthesize Co-Cu hollow nanoprisms after wrapping and calcining with trithiocyanuric acid (C3H3N3S3) (denoted N-Co-Cu-S-x HNs). The results showed that Cu doping modified the charge density of Co center, leading to the enhancement of the intrinsic activity of the Co3S4 active center, meanwhile wrapping trithiocyanuric acid on the surfaces and calcinating to form N-containing C skeleton as a flexible substrate to encapsulate the catalysts, which effectively protected the active sites inside the catalysts. Notably, the OER catalyst that was optimized by adjusting the metal ratio and controlling the trithiocyanuric acid incorporation exhibited a low overpotential of 306 mV under a current density of 10 mA cm-2 and showed a superior durability of more than 27 h. This work may provide some insights into the preparation of oxygen evolution reaction catalysts with excellent performance through doping transition metals and protecting the internal active sites strategies.

Materials Research Directions Toward a Green Hydrogen Economy: A Review

A constellation of technologies has been researched with an eye toward enabling a hydrogen economy. Within the research fields of hydrogen production, storage, and utilization in fuel cells, various classes of materials have been developed that target higher efficiencies and utility. This Review examines recent progress in these research fields from the years 2011-2021, exploring the most commonly occurring concepts and the materials directions important to each field. Particular attention has been given to catalyst materials that enable the green production of hydrogen from water, chemical and physical storage systems, and materials used in technical capacities within fuel cells. The quantification of publication and materials trends provides a picture of the current state of development within each node of the hydrogen economy.

Alpha-Nickel Hydroxide Coating of Metallic Nickel for Enhanced Alkaline Hydrogen Evolution

In this work, alkaline hydrogen evolution reaction (HER) processes of three typical nickel-based electrocatalysts [i. e., Ni, α-Ni(OH)₂ , and β-Ni(OH)₂ ] were investigated to probe critical factors that determine the activity and durability. The HER activity trend was observed as Ni≫α-Ni(OH)₂ >β-Ni(OH)₂ , likely attributed to a synergy between metallic Ni and Ni(OH)₂ components on the Ni surface and fast water dissociation kinetics on the α-Ni(OH)₂ surface. With the HER proceeding, the metallic Ni surface, however, gradually became α-Ni(OH)₂ , and α-Ni(OH)₂ surface ultimately transformed into β-phase, leading to a dramatic activity decrease of Ni electrodes. Therefore, Ni electrodes were coated with α-Ni(OH)₂ nanosheets to slow down the nickel hydroxylation and optimize the surface ratio of Ni(OH)₂ to metallic Ni. This simple coating procedure enhanced both activity and durability of Ni electrocatalysts.

Interface engineering breaks both stability and activity limits of RuO2 for sustainable water oxidation

Designing catalytic materials with enhanced stability and activity is crucial for sustainable electrochemical energy technologies. RuO2 is the most active material for oxygen evolution reaction (OER) in electrolysers aiming at producing 'green' hydrogen, however it encounters critical electrochemical oxidation and dissolution issues during reaction. It remains a grand challenge to achieve stable and active RuO2 electrocatalyst as the current strategies usually enhance one of the two properties at the expense of the other. Here, we report breaking the stability and activity limits of RuO2 in neutral and alkaline environments by constructing a RuO2/CoOx interface. We demonstrate that RuO2 can be greatly stabilized on the CoOx substrate to exceed the Pourbaix stability limit of bulk RuO2. This is realized by the preferential oxidation of CoOx during OER and the electron gain of RuO2 through the interface. Besides, a highly active Ru/Co dual-atom site can be generated around the RuO2/CoOx interface to synergistically adsorb the oxygen intermediates, leading to a favourable reaction path. The as-designed RuO2/CoOx catalyst provides an avenue to achieve stable and active materials for sustainable electrochemical energy technologies.

Introducing Brønsted acid sites to accelerate the bridging-oxygen-assisted deprotonation in acidic water oxidation

Oxygen evolution reaction (OER) consists of four sequential proton-coupled electron transfer steps, which suffer from sluggish kinetics even on state-of-the-art ruthenium dioxide (RuO₂) catalysts. Understanding and controlling the proton transfer process could be an effective strategy to improve OER performances. Herein, we present a strategy to accelerate the deprotonation of OER intermediates by introducing strong Brønsted acid sites (e.g. tungsten oxides, WO_x) into the RuO₂. The Ru-W binary oxide is reported as a stable and active iridium-free acidic OER catalyst that exhibits a low overpotential (235 mV at 10 mA cm⁻²) and low degradation rate (0.014 mV h⁻¹) over a 550-hour stability test. Electrochemical studies, in-situ near-ambient pressure X-ray photoelectron spectroscopy and density functional theory show that the W-O-Ru Brønsted acid sites are instrumental to facilitate proton transfer from the oxo-intermediate to the neighboring bridging oxygen sites, thus accelerating bridging-oxygen-assisted deprotonation OER steps in acidic electrolytes. The universality of the strategy is demonstrated for other Ru-M binary metal oxides (M = Cr, Mo, Nb, Ta, and Ti).

While water electrolysis devices represent a technology for renewable energy, there are few stable catalysts that survive the acidic conditions. Here, authors enhance acidic oxygen evolution by introducing strong Brønsted acid sites into RuO₂ to accelerate bridging-oxygen-assisted deprotonation.

Recent Advances Regarding Precious Metal-Based Electrocatalysts for Acidic Water Splitting

Electrochemical water splitting has wide applicability in preparing high-density green energy. The Proton exchange membrane (PEM) water electrolysis system is a promising technique for the generation of hydrogen due to its high electrolytic efficiency, safety and reliability, compactness, and quick response to renewable energy sources. However, the instability of catalysts for electrochemical water splitting under operating conditions limits their practical applications. Until now, only precious metal-based materials have met the requirements for rigorous long-term stability and high catalytic activity under acid conditions. In this review, the recent progress made in this regard is presented and analyzed to clarify the role of precious metals in the promotion of the electrolytic decomposition of water. Reducing precious metal loading, enhancing catalytic activity, and improving catalytic lifetime are crucial directions for developing a new generation of PEM water electrolysis catalysts. A summary of the synthesis of high-performance catalysts based on precious metals and an analysis of the factors affecting catalytic performance were derived from a recent investigation. Finally, we present the remaining challenges and future perspectives as guidelines for practical use.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Morphologically Controlled Metal-Organic Framework-Derived FeNi Oxides for Efficient Water Oxidation

The complex oxygen evolution reaction (OER) is recognized as the most studied and explored electrochemical conversion, which plays a crucial role in energy-related applications. In this work, a series of metal-organic framework (MOF)-derived FeNi oxides from a barrel-shaped Ni-based BMM-10 precursor are conveniently obtained to show an excellent OER performance. Under mild Fe(III) etching, a type of core-shell Fe_0.5-BMM-10 can be well preserved and the coordination bond of the middle frame structure is decomposed. Furthermore, the Fe_x-BMM-10-T series is successfully synthesized with a well-preserved morphology compared to precursors after direct oxidation. Finally, followed by initial electrochemical activation, the decomposition of FeNi oxides generates active Fe-doped nickel oxyhydroxides for efficient water oxidation. The improved OER performance stems from the high specific surface area and abundant exposed active centers, as well as the significant synergistic effect between iron and nickel, which is further verified by the theoretical calculation. This approach can be extended to precisely adjust the morphology of MOFs and their derivatives that can result in superior electrocatalytic properties in terms of energy conversion and storage applications.

Epitaxial Core-Shell Oxide Nanoparticles: First-Principles Evidence for Increased Activity and Stability of Rutile Catalysts for Acidic Oxygen Evolution

Due to their high activity and favorable stability in acidic electrolytes, Ir and Ru oxides are primary catalysts for the oxygen evolution reaction (OER) in proton-exchange membrane (PEM) electrolyzers. For a future large-scale application, core-shell nanoparticles are an appealing route to minimize the demand for these precious oxides. Here, we employ first-principles density-functional theory (DFT) and ab initio thermodynamics to assess the feasibility of encapsulating a cheap rutile-structured TiO₂ core with coherent, monolayer-thin IrO₂ or RuO₂ films. Resulting from a strong directional dependence of adhesion and strain, a wetting tendency is only obtained for some low-index facets under typical gas-phase synthesis conditions. Thermodynamic stability in particular of lattice-matched RuO₂ films is instead indicated for more oxidizing conditions. Intriguingly, the calculations also predict an enhanced activity and stability of such epitaxial RuO₂ /TiO₂ core-shell particles under OER operation.