Tuning of lattice oxygen reactivity and scaling relation to construct better oxygen evolution electrocatalyst

Triggering the oxide path mechanism of oxygen evolution reaction: Introducing compressive strain on NiFe-LDH by partial replacement using Ba cations

NiFe layered double hydroxides (NiFe-LDHs) as an oxygen evolution reaction (OER) catalyst show great potential in alkaline water electrolysis. However, its electrocatalytic activity along with good stability is still an obstacle for practical utilization. In this work, we partially replace the metal ions of original NiFe-LDH with Ba cations to construct compression strain and trigger the oxide path mechanism (OPM) of OER. The experimental and theoretical calculation results show that the replacement with Ba cations in the original NiFe-LDH results in shortened distance of two adjacent Ni sites and a decreased d-band center, which promote the direct coupling of *O-*O radical and enhance O2 desorption ability. Attributable to the improved catalytic activity and kinetics of OER, the NiFe-LDH partially replaced with Ba cations presents a low overpotential of 241 mV at 500 mA cm-2 and remains stable for 100 h at 200 mA cm-2. Our study provides a chemical after-treatment method to introduce compressive strain in the NiFe-LDH and trigger the OPM in OER for alkaline water electrolysis.

Enhanced oxygen evolution reaction through improved lattice oxygen activity via carbon dots incorporation into MOFs

Emerging of the lattice oxygen mechanism (LOM) provides a new opportunity for enhancing oxygen evolution reaction (OER) activity. However, its stability suffers from metal cation dissolution and lattice oxygen anionic redox chemistry. In this paper, carbon dots (CDs)-modified nickel-iron MOF (Metal-Organic Framework) nanosheets (NiFe-BDC/CDs) were prepared for efficient OER electrocatalysis. The introduction of CDs promotes the hybridization of the O 2p band in the MOF with the metal 3d band near the Fermi level, leading to improved involvement of lattice oxygen in the oxygen evolution reaction. Additionally, C-M bonds formed between CDs and metal sites in MOF enhanced the stability of electrocatalyst. As results, the prepared NiFe-BDC/CDs electrodes demonstrated a current density of 100 mA cm-2 at overpotentials of 235 and 250 mV in alkaline freshwater and alkaline seawater, respectively, and a remarkable stability in alkaline seawater at 500 mA cm-2 for >100 h. This study provides a simple and versatile strategy for the design of OER electrocatalysts with highly active transition metal-based MOFs.

Fe-Doped Ni-Phytate/Carbon Nanotube Hybrids Integrating Activated Lattice Oxygen Participation and Enhanced Photothermal Effect for Highly Efficient Oxygen Evolution Reaction Electrocatalyst

Developing highly efficient oxygen evolution reaction (OER) electrocatalysts is critical for hydrogen production through electrocatalytic water splitting, yet it remains a significant challenge. In this study, a novel OER electrocatalyst, Fe-doped Ni-phytate supported on carbon nanotubes (NiFe-phy/CNT), which simultaneously follows lattice oxygen mechanism (LOM) and exhibits a photothermal effect, is synthesized through a facile and scalable co-precipitation method. Experimental results combined with theoretical calculations indicate that introducing Fe can facilitate the structural reconstruction of NiFe-phy/CNT to form highly active NiFe oxyhydroxides, switch OER pathway to LOM from the adsorbate evolution mechanism, and reinforce the photothermal effect to counterbalance the enthalpy change during OER process while reducing its activation energy. Therefore, under near-infrared light irradiation, NiFe-phy/CNT demonstrates exceptional OER activity, featuring low overpotentials of 237, 275, and 286 mV at 100, 500, and 1000 mA cm-2, respectively. Moreover, this electrocatalyst demonstrates the capability of large-scale synthesis and can be stored for over 120 days with a negligible decrease in activity. This work presents a novel conceptual approach to integrate lattice oxygen redox chemistry with photothermal effect for designing highly efficient OER electrocatalysts.

Unraveling Compressive Strain and Oxygen Vacancy Effect of Iridium Oxide for Proton-Exchange Membrane Water Electrolyzers

Iridium-based electrocatalysts are commonly regarded as the sole stable operating acidic oxygen evolution reaction (OER) catalysts in proton-exchange membrane water electrolysis (PEMWE), but the linear scaling relationship (LSR) of multiple reaction intermediates binding inhibits the enhancement of its activity. Herein, the compressive strain and oxygen vacancy effect exists in iridium dioxide (IrO2)-based catalyst by a doping engineering strategy for efficient acidic OER activity. In situ synchrotron characterizations elucidate that compressive strain can enhance Ir─O covalency and reduce the Ir─Ir bond distance, and oxygen vacancy (Ov) as an electronic regulator causes rapid adsorption of water molecules on the Ir and adjacent Ov (Ir─Ov) pair site to be coupled directly into *O─O* intermediates. Importantly, hence, volcano-shape curves are established between the compressive strain/oxygen vacancy and OER current using OER as the probe reaction. Theoretical calculation reveals Ni dopant can modulate Ir 5d- and O 2p-band centers for increasing overlap of Ir 5d and O 2p orbits to trigger a continuous metal site-oxygen vacancy synergistic mechanism (MS-OVSM) pathway, successfully breaking the LSR of intermediates binding during OER. Therefore, the resultant proton-exchange membrane water electrolysis (PEMWE) device fabricated using T-0.24Ni/IrO2 delivers a current density of 500 mA cm-2 and operates stably for 500 h.

Feasibility of Active and Durable Lattice Oxygen-Mediated Oxygen Evolution Electrocatalysts in Proton Exchange Membrane Water Electrolyzers Through d0 Metal Ion Incorporation

The primary hurdle faced in the practical application of proton exchange membrane water electrolyzer (PEMWE) involves improving the intrinsic kinetic activity of oxygen evolution reaction (OER) electrocatalysts while concurrently enhancing their durability. Although electrocatalysts based on lattice oxygen-mediated mechanism (LOM) have the potential to significantly enhance the activity in OER without being restricted by scaling relationships, they are neglected in acidic electrolytes due to limited durability. In this study, an innovative approach is presented to simultaneously promote the activation of lattice oxygen and improve the durability of LOM-based OER electrocatalysts by incorporating d0 metal ions into the RuO2 electrocatalyst. Leveraging the unique electronic properties of the d0 metal ion, the O 2p band center and Ru-O covalency of the electrocatalyst are successfully engineered, resulting in the change in OER mechanism. Furthermore, in a single cell of PEMWE, the LOM-based electrocatalyst demonstrates outstanding performance, achieving 3.0 A cm-2 at 1.81 V and maintaining durability for 100 h at 200 mA cm-2, surpassing commercial RuO2. This innovative strategy challenges the traditional viewpoint that suppressing lattice oxygen activation in OER is essential for enhancing PEMWE durability, offering new perspectives for the development of OER electrocatalysts in acidic electrolytes.

Heterostructured Electrocatalysts: from Fundamental Microkinetic Model to Electron Configuration and Interfacial Reactive Microenvironment

Electrocatalysts can efficiently convert earth-abundant simple molecules into high-value-added products. In this context, heterostructures, which are largely determined by the interface, have emerged as a pivotal architecture for enhancing the activity of electrocatalysts. In this review, the atomistic understanding of heterostructured electrocatalysts is considered, focusing on the reaction kinetic rate and electron configuration, gained from both empirical studies and theoretical models. We start from the fundamentals of the microkinetic model, adsorption energy theory, and electric double layer model. The importance of heterostructures to accelerate electrochemical processes via modulating electron configuration and interfacial reactive microenvironment is highlighted, by considering rectification, space charge region, built-in electric field, synergistic interactions, lattice strain, and geometric effect. We conclude this review by summarizing the challenges and perspectives in the field of heterostructured electrocatalysts, such as the determination of transition state energy, their dynamic evolution, refinement of the theoretical approaches, and the use of machine learning.

Updating the sub-nanometric cognition of reconstructed oxyhydroxide active phase for water oxidation

Unveiling structure-activity correlations at the sub-nanoscale remains an essential challenge in catalysis science. During electrocatalysis, dynamic structural evolution drives the ambiguous entanglement of crystals and electrons degrees of freedom that obscure the activity origin. Here, we track the structural evolution of Ni-based model pre-catalysts (Ni(OH)2, NiS2, NiSe2, NiTe), detailing their catalytically active state during water oxidation via operando techniques and theoretical calculations. We reveal the sub-nanometric structural difference of NiO6 unit with a regular distortion in the reconstructed active phase NiOOH, codetermined by the geometric (bond lengths) and electronic (covalency) structure of the pre-catalysts on both spatial and temporal scales. The symmetry-broken active units induce the delicate balance of the p and d orbitals in NiOOH, further steering the modulation of catalytic intermediate configurations and mechanisms, with improved performance. This work recognizes the fine structural differences of the active phases from the sub-nanometer scale, and quantitatively explains their influence on activity. Our findings provide a more intuitive design framework for high-efficiency materials through targeted symmetry engineering of active units.

Maximising the Potential of Reactive Carbon Support with Cobalt Active Phase for the Oxygen Evolution Reaction

A growing interest in novel noble metal-free electrocatalysts is fuelled by the pressing need to overcome the drastic demand for sustainable energy sources. To this end, the oxygen evolution reaction (OER) utilising transition metal oxide-carbon composites in alkaline media is considered a robust technology. In many such systems, carbon is used as a conductive additive or support, and the interactions between carbon support materials and the active phase affect the efficiency of the electrocatalyst. Cobalt forms some of the most active and stable electrocatalysts for OER. In carbon-supported systems, the dispersion of the cobalt phase on the carbon surface is a key factor in influencing the catalyst activity in water-splitting reactions. In this study, a low-temperature plasma treatment is used to boost the efficiency of the cobalt active phase by functionalising the carbon support with various oxygen groups. We used a simple deposition-precipitation method to obtain cobalt hydroxide active phase over graphene nanoparticles. The activation of graphene nanoparticles with oxygen plasma allowed us to obtain a catalyst that showed only 317 mV@10 mA·cm-2. More importantly, in the series of plasma-activated samples, the OER activity was very high in a range of cobalt phase loadings, yielding a material with 2.4 wt.% of cobalt and an overpotential of only 327 mV@10 mA·cm-2. The results indicate that plasma activation of GNP support maximises the usage of the transition metal active phase, which allows for an improvement in area-normalised and a dramatic improvement in the mass-normalised OER electrocatalytic activity.

Unveiling the critical role of surface adsorbed oxygen species for efficiently photothermocatalytic oxidation of VOCs: Replenishing the active surface lattice oxygen sites

Surface oxygen species play a crucial role in the photothermocatalytic oxidation of volatile organic compounds (VOCs), but their exact functions and evolutionary processes remain unclear. Herein, a series of spinel CoxMn3-xO4 catalysts are synthesized and employed for photothermocatalytic oxidation of toluene. Co1.5Mn1.5O4 catalysts achieve 91.6 % toluene degradation and 81.2 % CO2 yield in a continuous flow reaction under 400 mW/cm2 light intensity, as well as remarked stability and water resistance. During the reaction, surface lattice oxygen on CoxMn3-xO4 serves as the active sites, directly participating in the oxidation of VOCs. The replenishment pathway of surface lattice oxygen is investigated through a series of designed in situ experiments, revealing O2 molecules adsorbed on the catalyst surface to be O2- species, which are then activated to O- species via increase in temperature. The active O- species effectively replenish the consumed surface lattice oxygen species, facilitating subsequent oxidation reactions. This study provides valuable insight into the replenishment mechanism of surface lattice oxygen during oxidation of VOCs.

Cation Migration-Induced Lattice Oxygen Oxidation in Spinel Oxide for Superior Oxygen Evolution Reaction

Activating the lattice oxygen can significantly improve the kinetics of oxygen evolution reaction (OER), however, it often results in reduced stability due to the bulk structure degradation. Here, we develop a spinel Fe0.3Co0.9Cr1.8O4 with active lattice oxygen by high-throughput methods, achieving high OER activity and stability, superior to the benchmark IrO2. The oxide exhibits an ultralow overpotential (190 mV at 10 mA cm-2) with outstanding stability for over 170 h at 100 mA cm-2. Soft X-ray absorption- and Raman-spectroscopies, combined with 18O isotope-labelling experiments, reveal that lattice oxygen activation is driven by Cr oxidation, which induces a cation migration from CrO6 octahedrons to CrO4 tetrahedrons. The geometry conversion creates accessible non-bonding oxygen states, crucial for lattice oxygen oxidation. Upon oxidation, peroxo O-O bond is formed and further stabilized by Cr6+ (CrO4 tetrahedra) via dimerization. This work establishes a new approach for designing efficient catalysts that feature active and stable lattice oxygen without compromising structural integrity.

Cerium Oxide-Induced Synchronous Lattice Oxygen Activation and Accelerated Deprotonation Kinetics in Cobalt (oxy)Hydroxide for Robust Water Oxidation

Theoretically, triggering the lattice oxygen mechanism (LOM) of the catalysts during the alkaline oxygen evolution reaction (OER) can effectively break through the thermodynamic limitations, while following this path, the rate of simultaneous deprotonation also determines the overall kinetics. A cerium oxide units-modified cobalt (oxy)hydroxide nanocomposite of CeO2-CoOOH/NF is proposed, where the Ce(4f)-O(2p)-Co (3d) coupling with sites interaction mediates the Co─O Mott-Hubbard splitting state to trigger efficient LOM. Meanwhile, the 4f orbital electron-rich state near the Fermi level is favorable for proceeding the electron-involved deprotonation behavior. All these empower CeO2-CoOOH/NF with considerable OER activity, which delivers an overpotential of 249 mV at 10 mA cm-2, and coupling with commercial Pt/C in anion exchange membrane water electrolyze (AEMWE) to realize energy-saving hydrogen production. This work is instructive for the design of high-performance OER catalysts through controlling the electron orbitals hybridization state of the catalysts to synchronously accelerate the kinetics of each link in OER.

Dual Doping in Precious Metal Oxides: Accelerating Acidic Oxygen Evolution Reaction

Developing a highly active and stable catalyst for acidic oxygen evolution reactions (OERs), the key half-reaction for proton exchange membrane water electrolysis, has been one of the most cutting-edge topics in electrocatalysis. A dual-doping strategy optimizes the catalyst electronic environment, modifies the coordination environment, generates vacancies, and introduces strain effects through the synergistic effect of two elements to achieve high catalytic performance. In this review, we summarize the progress of dual doping in RuO2 or IrO2 for acidic OERs. The three main mechanisms of OERs are dicussed firstly, followed by a detailed examination of the development history of dual-doping catalysts, from experimentally driven dual-doping systems to machine learning (ML) and theoretical screening of dual-doping systems. Lastly, we provide a summary of the remaining challenges and future prospects, offering valuable insights into dual doping for acidic OERs.

Reconstructing the Coordination Environment of Fe/Co Dual-atom Sites towards Efficient Oxygen Electrocatalysis for Zn-Air Batteries

Dual-atom catalysts with nitrogen-coordinated metal sites embedded in carbon can drive the oxygen reduction and evolution reactions (ORR/OER) in rechargeable zinc-air batteries (ZABs), and the further improvement is limited by the linear scaling relationship of intermediate binding energies in the absorbate evolution mechanism (AEM). Triggering the lattice oxygen mechanism (LOM) is promising to overcome this challenge, but has yet been verified since the lacking of bridge oxygen (O) in the rigid coordination environment of the metal centers. Here, we demonstrate that suitably tailored dual-atom catalysts of FeCo-N-C can undergo out-plane and in-plane reconstruction to form the both axial O and bridge O at the metal centers, and thus activate the LOM pathway. The tailored FeCo-N-C with shortened Fe-N bonds also favors the ORR process, therefore is a promising dual-atom oxygen catalyst. The assembled rechargeable ZABs demonstrate a peak power density of 332 mW cm-2, and exhibit no notable decline after ~720 h of continuous cycling.

Achieving a Thermodynamic Self-Regulation Dynamic Adsorption Mechanism for Ammonia Synthesis through Selective Orbital Coupling

With the continuous pursuing on the improvement of catalytic activity, a catalyst performed exceeding catalytic volcano plots is desired, while it is impeded by the adsorption-energy scaling relations of reaction intermediates. Numerous efforts have been focused on optimizing the initial and final intermediates to circumvent the scaling relations for an improved performance. For a step forward, simultaneously optimizing all intermediates is essential to explore the theoretical maximum of catalytic activity. Herein, we proposed a dynamic adsorption mechanism (DAM) to independently regulate the adsorption configurations of all intermediates of electrochemical nitrogen reduction reaction (NRR). To demonstrate the DAM, a multi-site NbNi3 intermetallic is developed, which enables suitable adsorption energies of different intermediates via modulating orbital coupling mechanisms. As a result, NbNi3 achieves an ultra-low limiting potential of NRR of -0.11 V vs. reversible hydrogen electrode (RHE). Strikingly, the theoretical result is confirmed by a proof-of-concept experiment, wherein the nanoporous NbNi3 electrode exhibits a remarkable NH3 yield rate of 25.89 μg h-1 cm-2 with the Faradaic efficiency of 33.15 % at -0.25 V vs. RHE. Overall, this work brings out a new strategy to avoid the scaling relations, and opens up a promising avenue toward high-efficiency NRR catalysts.

MXene-Assisted NiFe sulfides for high-performance anion exchange membrane seawater electrolysis

Anion exchange membrane seawater electrolysis is vital for future large-scale green hydrogen production, however enduring a huge challenge that lacks high-stable oxygen evolution reaction electrocatalysts. Herein, we report a robust OER electrocatalyst for AEMSE by integrating MXene (Ti3C2) with NiFe sulfides ((Ni,Fe)S2@Ti3C2). The strong interaction between (Ni,Fe)S2 and Ti3C2 induces electron distribution to trigger lattice oxygen mechanism, improving the intrinsic activity, and particularly prohibits the dissolution of Fe species during OER process via the Ti-O-Fe bonding effectively, achieving notable stability. Furthermore, the good retention of sulfates and the abundant groups of Ti3C2 provide effective Cl- resistance. Accordingly, (Ni,Fe)S2@Ti3C2 achieves high OER activity (1.598 V@2 A cm-2) and long-term durability (1000 h) in seawater system. Furthermore, AEMSE with industrial current density (0.5 A cm-2) and durability (500 h) is achieved by (Ni,Fe)S2@Ti3C2 anode and Raney Ni cathode with electrolysis efficiency of 70% and energy consumption of 48.4 kWh kg-1 H2.

Breaking linear scaling relationships in oxygen evolution via dynamic structural regulation of active sites

The universal linear scaling relationships between the adsorption energies of reactive intermediates limit the performance of catalysts in multi-step catalytic reactions. Here, we show how these scaling relationships can be circumvented in electrochemical oxygen evolution reaction by dynamic structural regulation of active sites. We construct a model Ni-Fe2 molecular catalyst via in situ electrochemical activation, which is able to deliver a notable intrinsic oxygen evolution reaction activity. Theoretical calculations and electrokinetic studies reveal that the dynamic evolution of Ni-adsorbate coordination driven by intramolecular proton transfer can effectively alter the electronic structure of the adjacent Fe active centre during the catalytic cycle. This dynamic dual-site cooperation simultaneously lowers the free energy change associated with O-H bond cleavage and O-O bond formation, thereby disrupting the inherent scaling relationship in oxygen evolution reaction. The present study not only advances the development of molecular water oxidation catalysts, but also provides an unconventional paradigm for breaking the linear scaling relationships in multi-intermediates involved catalysis.

Highly dispersed Ir nanoparticles on Ti3C2Tx MXene nanosheets for efficient oxygen evolution in acidic media

The industrialization of hydrogen production technology through polymer electrolyte membrane water splitting faces challenges due to high iridium (Ir) loading on the anode catalyst layer. While rational design of oxygen evolution reaction (OER) electrocatalysts aimed at effective iridium utilization is promising, it remains a challenging task. Herein, we present exfoliated Ti3C2Tx MXene as a highly conductive and corrosion-resistant support for acidic OER. We develop an alcohol reduction method to achieve uniform and dense loading of ultrafine Ir nanoparticles on the MXene surface. The IrO2/TiOx heterointerface is formed in situ on the Ir@Ti3C2Tx MXene surface, acting as a catalytically active phase for OER during electrocatalysis. The electron interactions at the IrO2/TiOx heterointerface create electron-rich Ir sites, which reduce the adsorption properties of oxygen intermediates and enhance intrinsic OER activity. Consequently, the prepared Ir@Ti3C2Tx exhibits a mass activity that is 7 times greater than that of the benchmark IrO2 catalyst for OER in acidic media. In addition, the /Ti3C2Tx MXene support can stabilize the Ir nanoparticles, so that the stability number of Ir@Ti3C2Tx MXene is about 2.4 times higher than that of the IrO2 catalyst.

1D Co6Mo6C-Based Heterojunctional Nanowires from Pyrolytically "Squeezing" PMo12/ZIF-67 Cubes for Efficient Overall Water Electrolysis

The bi-transition-metal interstitial compounds (BTMICs) are promising for water electrolysis. The previous BTMICs are usually composed of irregular particles. Here, this work shows the synthesis of novel 1D Co6Mo6C-based heterojunction nanowires (1D Co/Co6Mo6C) with diameters about 50 nm and a length-to-diameter ratio about 20 for efficient water electrolysis. An interesting growth process based on pyrolytically "squeezing" PMo12 (Phosphomolybdic acid)/ZIF-67 (Zeolitic Imidazolate Framework-67) cube precursor is demonstrated. The "squeezing" growth is related to the role of Mo species for isolating Co species. A series of tests and theoretical calculation show the mutual regulation of Co and Mo to optimize the electronic structure, accelerating H2O dissociation and the reduction kinetics of H+. Additionally, the nanowires provide pathways for electron transfer and the transmission of reactants. Consequently, the 1D Co/Co6Mo6C exhibits high activity for hydrogen evolution reaction (η10 of 31 mV) and oxygen evolution reaction (η10 of 210 mV) in 1 m KOH. The electrolytic cell based on 1D Co/Co6Mo6C requires a low voltage of 1.43 V to drive 10 mA cm-2. The catalyst also exhibits good HER performance in 1 m phosphate-buffered saline solution, exceeding Pt/C at a current density >42 mA cm-2.

Triggering Oxygen Redox Cycles in Nickel Ferrite by Octahedral Geometry Engineering for Enhancing Oxygen Evolution

Spinel-type nickel ferrite (NixFe3-xO4, x≤1) is a widely used electrocatalyst for the oxygen evolution reaction (OER). Due to the lower hybridization of metal-d and oxygen-p orbitals, the OER process on NixFe3-xO4 follows the sluggish adsorbate evolution mechanism (AEM). Generally, activating the lattice oxygen to trigger the lattice-oxygen-mediated mechanism (LOM) can enhance the OER activity. Herein, to trigger the LOM pathway while maintaining high stability, iron foam (IF)-supported Ni0.75Fe2.25O4 (NiFeO) with geometrical defects of [NiO6] (nickel cation coordinated with six oxygen anions) units and higher ratio of Fe to Ni cations in octahedral sites (d-NiFeHRO/IF) is prepared by ion-exchanging with polar aprotic solvent followed by annealing. As a result, as-synthesized d-NiFeHRO/IF exhibits excellent activity (at 295 mV overpotential to achieve 100 mA cm-2), fast kinetics (Tafel slope of only 34.6 mV dec-1), and high stability (maintaining a current density of 100 mA cm-2 over 130 h) for the OER. The theoretical calculations reveal that the construction of octahedral defect in NixFe3-xO4 enhances the overlap of Fe-d and O-p orbitals, which can activate the lattice oxygen. Therefore, increasing the ratio of Fe to Ni will further improve the lattice oxygen redox activity, and thus trigger the fast LOM pathway, ultimately facilitating the OER process.

Carbothermal Shock Synthesis of Lattice Oxygen-Mediated High-Entropy FeCoNiCuMo-O Electrocatalyst with a Fast Kinetic, High Efficiency, and Stable Oxygen Evolution Reaction

Efficient oxygen evolution reaction (OER) catalysts with fast kinetics, high efficiency, and stability are essential for scalable green production of hydrogen. The rational design and fabrication of catalysts play a decisive role in their catalytic behavior. This work presents a high-entropy catalyst, FeCoNiCuMo-O, synthesized via carbothermal shock. Synergistic optimization of the adsorption evolution mechanism (AEM) and lattice oxygen mechanism (LOM) was realized and demonstrated through the combination of in situ spectra/mass spectrometry and chemical probe analysis in FeCoNiCuMo-O. Furthermore, the robust stability is reinforced by the inherent properties conferred by the high-entropy design. The catalyst exhibits outstanding performance metrics, featuring an exceptionally low Tafel slope of 41 mV dec-1, a low overpotential of 272 mV at 10 mA cm-2, and a commendable endurance (a mere 2.2% voltage decline after a 240-h continuous chronopotentiometry test at 10 mA cm-2). This study advances the development of efficient, durable OER electrocatalysts for sustainable hydrogen production.

Transforming Adsorbate Surface Dynamics in Aqueous Electrocatalysis: Pathways to Unconstrained Performance

Developing highly efficient catalysts to accelerate sluggish electrode reactions is critical for the deployment of sustainable aqueous electrochemical technologies, yet remains a great challenge. Rationally integrating functional components to tailor surface adsorption behaviors and adsorbate dynamics would divert reaction pathways and alleviate energy barriers, eliminating conventional thermodynamic constraints and ultimately optimizing energy flow within electrochemical systems. This approach has, therefore, garnered significant interest, presenting substantial potential for developing highly efficient catalysts that simultaneously enhance activity, selectivity, and stability. The immense promise and rapid evolution of this design strategy, however, do not overshadow the substantial challenges and ambiguities that persist, impeding the realization of significant breakthroughs in electrocatalyst development. This review explores the latest insights into the principles guiding the design of catalytic surfaces that enable favorable adsorbate dynamics within the contexts of hydrogen and oxygen electrochemistry. Innovative approaches for tailoring adsorbate-surface interactions are discussed, delving into underlying principles that govern these dynamics. Additionally, perspectives on the prevailing challenges are presented and future research directions are proposed. By evaluating the core principles and identifying critical research gaps, this review seeks to inspire rational electrocatalyst design, the discovery of novel reaction mechanisms and concepts, and ultimately, advance the large-scale implementation of electroconversion technologies.

Iron-Induced Localized Oxide Path Mechanism Enables Efficient and Stable Water Oxidation

The sluggish reaction kinetics of the anodic oxygen evolution reaction (OER) and the inadequate catalytic performance of non-noble metal-based electrocatalysts represent substantial barriers to the development of anion exchange membrane water electrolyzer (AEMWE). This study performed the synthesis of a three-dimensional (3D) nanoflower-like electrocatalyst (CFMO) via a simple one-step method. The substitution of Co with Fe in the structure induces a localized oxide path mechanism (LOPM), facilitating direct O-O radical coupling for enhanced O2 evolution. The optimized CFMO-2 electrocatalyst demonstrates superior OER performance, achieving an overpotential of 217 mV at 10 mA cm-2, alongside exceptional long-term stability with minimal degradation after 1000 h of operation in 1.0 M KOH. These properties surpass most of conventional noble metal-based electrocatalysts. Furthermore, the assembled AEMWE system, utilizing CFMO-2, operates with a cell voltage of 1.65 V to deliver 1.0 A cm-2. In situ characterizations reveal that, in addition to the traditional adsorbate evolution mechanism (AEM) at isolated Co sites, a new LOPM occurred around the Fe and Co bimetallic sites. First-principles calculations confirm the LOPM greatly reduced the energy barriers. This work highlights the potential of LOPM for improving the design of non-noble metal-based electrocatalysts and the development of AEMWE.

Beyond conventional structures: emerging complex metal oxides for efficient oxygen and hydrogen electrocatalysis

The core of clean energy technologies such as fuel cells, water electrolyzers, and metal-air batteries depends on a series of oxygen and hydrogen-based electrocatalysis reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which necessitate cost-effective electrocatalysts to improve their energy efficiency. In the recent decade, complex metal oxides (beyond simple transition metal oxides, spinel oxides and ABO3 perovskite oxides) have emerged as promising candidate materials with unexpected electrocatalytic activities for oxygen and hydrogen electrocatalysis owing to their special crystal structures and unique physicochemical properties. In this review, the current progress in complex metal oxides for ORR, OER, and HER electrocatalysis is comprehensively presented. Initially, we present a brief description of some fundamental concepts of the ORR, OER, and HER and a detailed description of complex metal oxides, including their physicochemical characteristics, synthesis methods, and structural characterization. Subsequently, we present a thorough overview of various complex metal oxides reported for ORR, OER, and HER electrocatalysis thus far, such as double/triple/quadruple perovskites, perovskite hydroxides, brownmillerites, Ruddlesden-Popper oxides, Aurivillius oxides, lithium/sodium transition metal oxides, pyrochlores, metal phosphates, polyoxometalates and other specially structured oxides, with emphasis on the designed strategies for promoting their performance and structure-property-performance relationships. Moreover, the practical device applications of complex metal oxides in fuel cells, water electrolyzers, and metal-air batteries are discussed. Finally, some concluding remarks summarizing the challenges, perspectives, and research trends of this topic are presented. We hope that this review provides a clear overview of the current status of this emerging field and stimulate future efforts to design more advanced electrocatalysts.

Importing Atomic Rare-Earth Sites to Activate Lattice Oxygen of Spinel Oxides for Electrocatalytic Oxygen Evolution

Spinel oxides have emerged as highly active catalysts for the oxygen evolution reaction (OER). Owing to covalency competition, the OER process on spinel oxides often follows an arduous adsorbate evolution mechanism (AEM) pathway. Herein, we propose a novel rare-earth sites substitution strategy to tune the lattice oxygen redox of spinel oxides and bypass the AEM scaling relationship limitation. Taking NiCo2O4 as a model, the incorporation of Ce into the octahedral site induces the formation of Ce-O-M (M=Ni, Co) bridge, which triggers charge redistribution within NiCo2O4. The developed Ce-NiCo2O4 exhibits remarkable OER activity with a low overpotential, satisfactory electrochemical stability, and good practicability in anion-exchange membrane water electrolyzer. Theoretical analyses reveal that OER on Ce-NiCo2O4 surface follows a more favorable lattice oxygen mechanism (LOM) pathway and non-concerted proton-electron transfers compared to pure NiCo2O4, as also verified by pH-dependent behavior and in situ Raman analysis. The 18O-labeled electrochemical mass spectrometry provides direct evidence that the oxygen released during the OER originates from the lattice oxygen of Ce-NiCo2O4. We discover that electron delocalization of Ce 4f states triggers charge redistribution in NiCo2O4 through the Ce-O-M bridge, favoring antibonding state occupation of Ni-O bonding in [Ce-O-Ni] unit site, thereby activating lattice oxygen redox of NiCo2O4 in OER. This work provides a new perspective for designing highly active spinel oxides for OER and offers significant insights into the rare-earth-enhanced LOM mechanism.

Enhancing the oxygen evolution reaction activity and stability of high-valent CoOOH by switching the catalytic pathway through doping low-valent Cu

The oxygen evolution reaction (OER) is a critical process in electrochemical energy storage and conversion systems. The adsorbate evolution mechanism (AEM) pathway possesses the characteristics of high stability but slow catalytic kinetics. We propose that combining AEM with the lattice oxidation mechanism (LOM) pathway can potentially enhance the OER catalytic activity and stability. However, the triggering of LOM is an important challenge due to the high thermodynamic activation barrier of lattice oxygen. To solve this problem, we performed theoretical calculations and experiments which suggest that the introduction of low-valent Cu in CoOOH (CuxCo1-xOOH) could directionally modulate the local coordination environment of CoO bonds. This approach can activate lattice oxygen and generate oxygen vacancies to enhance the nucleophilic attack of *OH and directly establish OO coupling, thereby facilitating the smoothly switching from AEM to LOM pathway by increasing voltage and thus activating lattice oxygen in CuxCo1-xOOH. The switching of AEM and LOM enables CuxCo1-xOOH showing an outstanding overpotential of only 252 mV (10 mA cm-2) and durability of only 2.80 % degradation after 280h. This work provides a new way for designing efficient and stable electrocatalysts with AEM and LOM pathway switching.

Expanded Negative Electrostatic Network-Assisted Seawater Oxidation and High-Salinity Seawater Reutilization

Coastal/offshore renewable energy sources combined with seawater splitting offer an attractive means for large-scale H2 electrosynthesis in the future. However, designing anodes proves rather challenging, as surface chlorine chemistry must be blocked, particularly at high current densities (J). Additionally, waste seawater with increased salinity produced after long-term electrolysis would impair the whole process sustainability. Here, we convert seawater to O2 selectively, on hydroxides, by building phytate-based expanded negative electrostatic networks (ENENs) with electrostatically repulsive capacities and higher negative charge coverage ranges than those of common inorganic polyatomic anions. With surface ENENs, even typically unstable CoFe hydroxides perform nicely toward alkaline seawater oxidation at activities of >1 A cm-2. CoFe hydroxides with phytate-based ENENs exhibit prolonged lifespans of 1000 h at J of 1 A cm-2 and 900 h at J of 2 A cm-2 and thus rival the best seawater oxidation anodes. Direct introduction of trace phytates to seawater weakens corrosion tendency on conventional CoFe hydroxides as well, extending the life of hydroxides by ∼28 times at J of 2 A cm-2. A wide range of materials all obtain prolonged lifetimes in the presence of ENENs, validating universal applicability. Mechanisms are studied using theoretical computations under working conditions and ex situ/in situ characterizations. We demonstrate a potentially viable way to sustainably reutilize high-salinity wastewater, which is a long-standing but neglected issue. Series-connected devices exhibit good resistance to low temperature operation and are more eco-friendly than current organic electrolyte-based energy storage devices.

Operando monitoring of the functional role of tetrahedral cobalt centers for the oxygen evolution reaction

The complexity of the intrinsic oxygen evolution reaction (OER) mechanism, particularly the precise relationships between the local coordination geometry of active metal centers and the resulting OER kinetics, remains to be fully understood. Herein, we construct a series of 3 d transition metal-incorporated cobalt hydroxide-based nanobox architectures for the OER which contain tetrahedrally coordinated Co(II) centers. Combination of bulk- and surface-sensitive operando spectroelectrochemical approaches reveals that tetrahedral Co(II) centers undergo a dynamic transformation into highly active Co(IV) intermediates acting as the true OER active species which activate lattice oxygen during the OER. Such a dynamic change in the local coordination geometry of Co centers can be further facilitated by partial Fe incorporation. In comparison, the formation of such active Co(IV) species is found to be hindered in CoOOH and Co-FeOOH, which are predominantly containing [CoIIIO6] and [CoII/FeIIIO6] octahedra, respectively, but no mono-μ-oxo-bridged [CoIIO4] moieties. This study offers a comprehensive view of the dynamic role of local coordination geometry of active metal centers in the OER kinetics.

Ultrastable Seawater Oxidation at Ampere-level Current Densities with Corrosion-resistant CoCO3/CoFe Layered Double Hydroxide Electrocatalyst

Hydrogen is an essential energy resource, playing a pivotal role in advancing a sustainable future. Electrolysis of seawater shows great potential for large-scale hydrogen production but encounters challenges such as electrode corrosion caused by chlorine evolution. Herein, a durable CoCO3/CoFe layered double hydroxide (LDH) electrocatalyst is presented for alkaline seawater oxidation, showcasing resistance to corrosion and stable operation exceeding 1,000 h at a high current density of 1 A cm-2. The results indicate that CoCO3 within the electrocatalyst undergoes conversion into CoOOH and releases CO3 2- during electrolysis. The incorporation of CO3 2- within its layers and the anchoring of the electrocatalyst's surface prevent the adverse adsorption of chloride ions, enhancing resistance to chloride ion corrosion, thereby protecting the active sites of the electrocatalyst effectively.

Edge-Induced Synergy of Ni-Ni Defects in NiFe Layered-Double-Hydroxide for Electrocatalytic Water Oxidation Reaction

Defect engineering is widely regarded as a promising strategy to enhance the performance of electrocatalysts for water splitting. In this work, defective NiFe layered double hydroxide (NiFe LDH) with a high density of edge sites (edge-rich NiFe LDH) is synthesized via a simple reduction process during the early stages of nucleation. The introduction of edges into oxygen evolution reaction (OER) catalysts modulates the electronic structure of the active sites. X-ray absorption spectroscopy (XAS) analyses revealed that the edges facilitated the formation of unsaturated Ni-Ni coordination, which is crucial for promoting the deprotonation of the OH* intermediate. Consequently, the edge-rich NiFe LDH exhibited a significantly lower overpotential of 228 mV to achieve a current density of 10 mA cm⁻2, compared to 275 mV for pristine NiFe LDH. The assembled membrane electrode can reach a current density of 1000 mA cm⁻2 at a cell voltage of 2.5 V. This study highlights the role of edge effects in defect engineering to enhance OER activity and provides valuable theoretical insights for the design of efficient electrocatalysts.

Oxygen Vacancy: How Will Poling History Affect Its Role in Photoelectrocatalysis

Oxygen vacancy (VO) has been recognized to possess an effect to promote the charge separation and transfer (CST) in various n-type semiconductor based photoelectrodes. But how external stimulus will change this VO effect has not been investigated. In this work, external polarization is applied to investigate the effect of VO on the CST process of a typical ferroelectric BiFeO3 photoelectrode. It is found that negative poling treatment can significantly boost VO effect, while positive poling treatment will deteriorate the CST capability in BiFeO3 photoelectrodes. This poling history determined VO effect is rooted in the VO induced defect dipoles, wherein their alignment produces a depolarization electric field to modulate the CST driving force. This finding highlights the significance of poling history in functionalizing the VO in a photoelectrode.

Regulating Local Coordination Sphere of Ir Single Atoms at the Atomic Interface for Efficient Oxygen Evolution Reaction

Single-atom catalysts dispersed on an oxide support are essential for overcoming the sluggishness of the oxygen evolution reaction (OER). However, the durability of most metal single-atoms is compromised under harsh OER conditions due to their low coordination (weak metal-support interactions) and excessive disruption of metal-Olattice bonds to enable lattice oxygen participation, leading to metal dissolution and hindering their practical applicability. Herein, we systematically regulate the local coordination of Irsingle-atoms at the atomic level to enhance the performance of the OER by precisely modulating their steric localization on the NiO surface. Compared to conventional Irsingle-atoms adsorbed on NiO surface, the atomic Ir atoms partially embedded within the NiO surface (Iremb-NiO) exhibit a 2-fold increase in Ir-Ni second-shell interaction revealed by X-ray absorption spectroscopy (XAS), suggesting stronger metal-support interactions. Remarkably, Iremb-NiO with tailored coordination sphere exhibits excellent alkaline OER mass activity and long-term durability (degradation rate: ∼1 mV/h), outperforming commercial IrO2 (∼26 mV/h) and conventional Irsingle-atoms on NiO (∼7 mV/h). Comprehensive operando X-ray absorption and Raman spectroscopies, along with pH-dependence activity tests, identified high-valence atomic Ir sites embedded on the NiOOH surface during the OER followed the lattice oxygen mechanism, thereby circumventing the traditional linear scaling relationships. Moreover, the enhanced Ir-Ni second-shell interaction in Iremb-NiO plays a crucial role in imparting structural rigidity to Ir single-atoms, thereby mitigating Ir-dissolution and ensuring superior OER kinetics alongside sustained durability.

Manipulating the d- and p-Band centers of amorphous alloys by variable composition for robust oxygen evolution reaction

Amorphous electrocatalysts display several unique advantages in electricity-driven water splitting compared to their crystalline analogs, but understanding their structure-activity relationships remains a major challenge. Herein, we show that the d- and p-electronic states of amorphous Ni-Fe-B can be subtly manipulated by varying the Ni and Fe contents. The optimal Ni-Fe-B alloy exhibits a high performance in the oxygen evolution reaction (OER), as supported by its impressive stability (no clear degradation after 100 h) and considerably lower overpotential compared to those of its crystalline analogs. Based on theoretical calculations, different Ni and Fe contents can cause significant shifts in the d-band levels of Ni and Fe and the p-band level of B, thus altering the OER activity. Additionally, the energy difference between the d- and p-band centers (ΔEad-p) may be an effective index for use in reflecting the structure-activity relationship of an amorphous Ni-Fe-B alloy in the OER. An amorphous Ni-Fe-B alloy with a smaller ΔEad-p displays a higher intrinsic activity. This study supplies a unique direction for use in constructing the structure-activity relationships of amorphous electrocatalysts by revealing the role of ΔEad-p, which promotes fundamental research and the practical application of amorphous electrocatalysts.

Ternary g-C3N4/Co3O4/CeO2 nanostructured composites for electrochemical energy storage supercapacitors

Extensive use of fossil fuels causes heavy discharge of carbon dioxide, depleting energy resources and this requires environmentally friendly and effective energy storage materials. Hybrid supercapacitors (HSCs) are recently developed as effective energy storage materials enabling high capacitance retention rate and quick charging. Herein, synthesis of two-dimensional g-C3N4 nanosheets supported onto three-dimensional flower-like Co3O4/CeO2 (CoCe) ternary synergistic heterostructures are developed as effective electrodes for hybrid supercapacitor applications. Addition of g-C3N4 produces substantial surface active sites, enabling its synergistic effect with CoCe to enhance electrochemical performance having exceptional conductivity. The CoCe/g-C3N4 ternary composite electrode exhibits a higher specific capacitance of 1088.3 F g-1 at 1 A g-1 with 96 % of recycling stability over 5000 cycles, which is ∼5.5 and ∼5 folds higher specific capacitance than the pristine g-C3N4 and CoCe electrodes. EIS analysis revealed that CoCe/g-C3N4 electrode offered reduced charge transfer resistance compared to pristine electrodes. The fabricated two-electrode HSC device displays outstanding retention after 10,000 cycles with an ultra-high specific capacitance of 119.8 F g-1, excellent energy density 37.4 Wh kg-1 and power density of 749.9 W kg-1. This research showcases the perspectives of CoCe/g-C3N4 ternary electrodes in hybrid supercapacitors and other renewable energy storage devices.

Iridium Single-Atom-Ensembles Stabilized on Mn-Substituted Spinel Oxide for Durable Acidic Water Electrolysis

Exploring single-atom-catalysts for the acidic oxygen evolution reaction (OER) is of paramount importance for cost-effective hydrogen production via acidic water electrolyzers. However, the limited durability of most single-atom-catalysts and Ir/Ru-based oxides under harsh acidic OER conditions, primarily attributed to excessive lattice oxygen participation resulting in metal-leaching and structural collapse, hinders their practical application. Herein, an innovative strategy is developed to fabricate short-range Ir single-atom-ensembles (IrSAE) stabilized on the surface of Mn-substituted spinel Co3O4 (IrSAE-CMO), which exhibits excellent mass activity and significantly improved durability (degradation-rate: ≈2 mV h-1), outperforming benchmark IrO2 (≈44 mV h-1) and conventional Irsingle-atoms on pristine-Co3O4 for acidic OER. First-principle calculations reveal that Mn-substitution in the octahedral sites of Co3O4 substantially reduces the migration energy barrier for Irsingle-atoms on the CMO surface compared to pristine-Co3O4, facilitating the migration of Irsingle-atoms to form strongly correlated IrSAE during pyrolysis. Extensive ex situ characterization, operando X-ray absorption and Raman spectroscopies, pH-dependence activity tests, and theoretical calculations indicate that the rigid IrSAE with appropriate Ir-Ir distance stabilized on the CMO surface effectively suppresses lattice oxygen participation while promoting direct O─O radical coupling, thereby mitigating Ir-dissolution and structural collapse, boosting the stability in an acidic environment.

Topological Synthesis of 2D High-Entropy Multimetallic (Oxy)hydroxide for Enhanced Lattice Oxygen Oxidation Mechanism

Owing to sluggish reaction kinetics and high potential, oxygen evolution reaction (OER) electrocatalysts face a trade-off between activity and stability. Herein, an innovative topological strategy is presented for preparing 2D multimetallic (oxy)hydroxide, including ternary CoFeZn, quaternary CoFeMnZn, and high-entropy CoFeMnCuZn. The key to the synthesis lies in using Ca-rich brownmillerite oxide as a precursor, which possesses inherent structural flexibility enabling tailored elemental adjustments and topologically transforms from a point-shared structure of metal-oxygen octahedrons into an edge-shared structure under alkaline conditions. The presence of Zn in the catalysts causes a shift in the center of the O2p band toward the Fermi level, resulting in more Co4+ species, which drive holes into oxygen ligands to promote intramolecular oxygen coupling. The triggered lattice oxidation mechanism is identified by detecting peroxo-like (O2 2-) negative species using tetramethylammonium chemical probe, along with 18O isotope labeling experiments. As a result, the catalyst demonstrates an overpotential of 267 mV at 10 mA cm-2, ranking it among the top-performing non-Ni-based catalysts. Importantly, the catalysts also show high Fe-leaching resistance during OER compared to conventional NiFe and CoFe hydroxides/(oxy)hydroxides. The assembled zinc-air battery enables stable operation for over 225 h at a low charging voltage of 1.93 V.

Design and regulation of defective electrocatalysts

Electrocatalysts are the key components of electrochemical energy storage and conversion devices. High performance electrocatalysts can effectively reduce the energy barrier of the chemical reactions, thereby improving the conversion efficiency of energy devices. The electrocatalytic reaction mainly experiences adsorption and desorption of molecules (reactants, intermediates and products) on a catalyst surface, accompanied by charge transfer processes. Therefore, surface control of electrocatalysts plays a pivotal role in catalyst design and optimization. In recent years, many studies have revealed that the rational design and regulation of a defect structure can result in rearrangement of the atomic structure on the catalyst surface, thereby efficaciously promoting the electrocatalytic performance. However, the relationship between defects and catalytic properties still remains to be understood. In this review, the types of defects, synthesis methods and characterization techniques are comprehensively summarized, and then the intrinsic relationship between defects and electrocatalytic performance is discussed. Moreover, the application and development of defects are reviewed in detail. Finally, the challenges existing in defective electrocatalysts are summarized and prospected, and the future research direction is also suggested. We hope that this review will provide some principal guidance and reference for researchers engaged in defect and catalysis research, better help researchers understand the research status and development trends in the field of defects and catalysis, and expand the application of high-performance defective electrocatalysts to the field of electrocatalytic engineering.

Stability of electrocatalytic OER: from principle to application

Hydrogen energy, derived from the electrolysis of water using renewable energy sources such as solar, wind, and hydroelectric power, is considered a promising form of energy to address the energy crisis. However, the anodic oxygen evolution reaction (OER) poses limitations due to sluggish kinetics. Apart from high catalytic activity, the long-term stability of electrocatalytic OER has garnered significant attention. To date, several research studies have been conducted to explore stable electrocatalysts for the OER. A comprehensive review is urgently warranted to provide a concise overview of the recent advancements in the electrocatalytic OER stability, encompassing both electrocatalyst and device developments. This review aims to succinctly summarize the primary factors influencing OER stability, including morphological/phase change and electrocatalyst dissolution, as well as mechanical detachment, alongside chemical, mechanical, and operational degradation observed in devices. Furthermore, an overview of contemporary approaches to enhance stability is provided, encompassing electrocatalyst design (structural regulation, protective layer coating, and stable substrate anchoring) and device optimization (bipolar plates, gas diffusion layers, and membranes). Hopefully, more attention will be paid to ensuring the stable operation of electrocatalytic OER and the future large-scale water electrolysis applications. This review presents design principles aimed at addressing challenges related to the stability of electrocatalytic OER.

The dual-functional role of carboxylate in a nickel-iron catalyst towards efficient oxygen evolution

The efficiency of the oxygen evolution reaction (OER) is severely limited by the sluggish proton-coupled electron transfer processes and inadequate long-term stability. Herein, we introduce a carboxylate group (TPA) to modify NiFe layered double hydroxide (NiFe LDH@TPA), resulting in notable improvements in both activity and stability. A combination of spectroscopic and theoretical investigations reveals the dual-functional role of incorporated TPA. It facilitates the deprotonation of OER intermediates while strengthening the Fe-O bond and acting as a molecular fence, ensuring superior OER kinetics and anti-dissolution properties. NiFe LDH@TPA delivers a low overpotential of 200 mV at 10 mA cm-2 and an impressive long-term stability of 500 h at 150 mA cm-2, significantly outperforming its unmodified counterpart. Furthermore, operating in an anion exchange membrane water electrolyzer, it affords prolonged stability at an industrial-scale current density of 1 A cm-2, sustaining performance for over 120 hours. This strategy offers a promising avenue for the development of durable and efficient OER catalysts.

Sequential oxygen evolution and decoupled water splitting via electrochemical redox reaction of nickel hydroxides

Alkaline water electrolysis is a promising low-cost strategy for clean and sustainable hydrogen production but is largely limited by the sluggish anodic oxygen evolution reaction and the challenges in maintaining adequate separation between H2 and O2. Here, we reveal an anodic-cathodic sequential oxygen evolution process via electrochemical oxidation and subsequent reduction of Ni hydroxides, enabling much lower overpotentials than conventional anodic oxygen evolution. By using (isotope-labeled) differential electrochemical mass spectrometry and in situ Raman spectroscopy combined with density functional theory calculations, we evidence that the sequential oxygen evolution originates from the electrochemical oxidation of Ni hydroxides to NiOO- active species while undergoing a different, reductive step of NiOO- for the final release of O2 due to weakened Ni-O covalency. Based on this sequential process, we propose and demonstrate a hybrid water electrolysis and energy storage device, which enables time-decoupled hydrogen and oxygen evolution and electrochemical energy storage in the Ni hydroxides.

Fe-S dually modulated adsorbate evolution and lattice oxygen compatible mechanism for water oxidation

Simultaneously activating metal and lattice oxygen sites to construct a compatible multi-mechanism catalysis is expected for the oxygen evolution reaction (OER) by providing highly available active sites and mediate catalytic activity/stability, but significant challenges remain. Herein, Fe and S dually modulated NiFe oxyhydroxide (R-NiFeOOH@SO4) is conceived by complete reconstruction of NiMoO4·xH2O@Fe,S during OER, and achieves compatible adsorbate evolution mechanism and lattice oxygen oxidation mechanism with simultaneously optimized metal/oxygen sites, as substantiated by in situ spectroscopy/mass spectrometry and chemical probe. Further theoretical analyses reveal that Fe promotes the OER kinetics under adsorbate evolution mechanism, while S excites the lattice oxygen activity under lattice oxygen oxidation mechanism, featuring upshifted O 2p band centers, enlarged d-d Coulomb interaction, weakened metal-oxygen bond and optimized intermediate adsorption free energy. Benefiting from the compatible multi-mechanism, R-NiFeOOH@SO4 only requires overpotentials of 251 ± 5/291 ± 1 mV to drive current densities of 100/500 mA cm-2 in alkaline media, with robust stability for over 300 h. This work provides insights in understanding the OER mechanism to better design high-performance OER catalysts.

Operando identification of the oxide path mechanism with different dual-active sites for acidic water oxidation

The microscopic reaction pathway plays a crucial role in determining the electrochemical performance. However, artificially manipulating the reaction pathway still faces considerable challenges. In this study, we focus on the classical acidic water oxidation based on RuO2 catalysts, which currently face the issues of low activity and poor stability. As a proof-of-concept, we propose a strategy to create local structural symmetry but oxidation-state asymmetric Mn4-δ-O-Ru4+δ active sites by introducing Mn atoms into RuO2 host, thereby switching the reaction pathway from traditional adsorbate evolution mechanism to oxide path mechanism. Through advanced operando synchrotron spectroscopies and density functional theory calculations, we demonstrate the synergistic effect of dual-active metal sites in asymmetric Mn4-δ-O-Ru4+δ microstructure in optimizing the adsorption energy and rate-determining step barrier via an oxide path mechanism. This study highlights the importance of engineering reaction pathways and provides an alternative strategy for promoting acidic water oxidation.

Anion Structure Regulation of Cobalt Silicate Hydroxide Endowing Boosted Oxygen Evolution Reaction

Transition metal silicates (TMSs) are attempted for the electrocatalyst of oxygen evolution reaction (OER) due to their special layered structure in recent years. However, defects such as low theoretical activity and conductivity limit their application. Researchers always prefer to composite TMSs with other functional materials to make up for their deficiency, but rarely focus on the effect of intrinsic structure adjustment on their catalytic activity, especially anion structure regulation. Herein, applying the method of interference hydrolysis and vacancy reserve, new silicate vacancies (anionic regulation) are introduced in cobalt silicate hydroxide (CoSi), named SV-CoSi, to enlarge the number and enhance the activity of catalytic sites. The overpotential of SV-CoSi declines to 301 mV at 10 mA cm-2 compared to 438 mV of CoSi. Source of such improvement is verified to be not only the increase of active sites, but also the positive effect on the intrinsic activity due to the enhancement of cobalt-oxygen covalence with the variation of anion structure by density functional theory (DFT) method. This work demonstrates that the feasible intrinsic anion structure regulation can improve OER performance of TMSs and provides an effective idea for the development of non-noble metal catalyst for OER.

Entropy-increased LiMn2O4-based positive electrodes for fast-charging lithium metal batteries

Fast-charging, non-aqueous lithium-based batteries are desired for practical applications. In this regard, LiMn2O4 is considered an appealing positive electrode active material because of its favourable ionic diffusivity due to the presence of three-dimensional Li-ion diffusion channels. However, LiMn2O4 exhibits inadequate rate capabilities and rapid structural degradation at high currents. To circumvent these issues, here we introduce quintuple low-valence cations to increase the entropy of LiMn2O4. As a result, the entropy-increased LiMn2O4-based material, i.e., LiMn1.9Cu0.02Mg0.02Fe0.02Zn0.02Ni0.02O4, when tested in non-aqueous lithium metal coin cell configuration, enable 1000 cell cycles at 1.48 A g-1 (corresponding to a cell charging time of 4 minutes) and 25°C with a discharge capacity retention of about 80%. We demonstrate that the increased entropy in LiMn2O4 leads to an increase in the disordering of dopant cations and a contracted local structure, where the enlarged LiO4 space and enhanced Mn-O covalency improve the Li-ion transport and stabilize the diffusion channels. We also prove that stress caused by cycling at a high cell state of charge is relieved through elastic deformation via a solid-solution transition, thus avoiding structural degradation upon prolonged cycling.

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

Boosting the Electrochemical Oxygen Evolution with Nickel Oxide Nanoparticle-Modified Glassy Carbon Electrodes in Alkaline Solutions

The present work investigates the electrocatalysis of oxygen evolution (OE) on a glassy carbon electrode modified with nickel oxide nanoparticles (NPs) (nano-Ni) in an alkaline solution. The nano-Ni is electrodeposited from an acidic sulfate electrolyte containing various additives, such as glucose, glycerol, and dimethyl glyoxime. The NPs are characterized morphologically and electrochemically using scanning electron microscopy and cyclic voltammetry. The elemental composition and electronic state of the modified electrodes were analyzed using X-ray photoelectron spectroscopy. A considerable enhancement in electrocatalytic activity, depending on the additives used, is observed. The study also explores the effect of nickel oxide loading to optimize the process. The highest cathodic shift in the onset potential of the oxygen evolution reaction is achieved with nickel oxide deposited in the presence of ethylene glycol.

Electrosynthesis of Amides through Cu- and Co-Incorporated Nickel Hydroxide-Catalyzed Oxidation of Primary Amines Coupled with Hydrogen Evolution

The electrocatalytic oxidation of organic molecules coupled with hydrogen evolution reaction can reduce overpotential and can be connected in series with nonelectrochemical processes to achieve the preparation of more high-value compounds. Herein, Cu- and Co-incorporated nickel hydroxide (CuCo-Ni(OH)2) was synthesized and applied to the anodic benzylamine oxidation reaction, which is 280 mV lower than the corresponding oxygen evolution reaction to reach the current density of 50 mA cm-2. When the electrocatalytic oxidation of benzylamine and hydrogen evolution reaction are coupled to form an electrolytic cell, the potential to reach 10 mA cm-2 is reduced by 197 mV compared to the overall water splitting. The benzylamine is converted to benzamide with 99.3% conversion and 90.2% faraday efficiency under 1.45 V constant voltage electrolysis, and the catalytic performance remains at a high level after 4 cycles. The characterization and density functional theory calculations show that Cu and Co share the transfer charge from Ni, making it easy for CuCo-Ni(OH)2 to deprotonate Ni-O* sites. The formed Ni-O* sites exhibit lower energy barriers in the proton transfer of benzylamine to benzonitrile and hydration intermediates, resulting in a better catalytic performance of CuCo-Ni(OH)2 than Ni(OH)2 in the electrocatalytic oxidation of benzylamine to benzamide.

Stabilizing NiFe sites by high-dispersity of nanosized and anionic Cr species toward durable seawater oxidation

Electrocatalytic H2 production from seawater, recognized as a promising technology utilizing offshore renewables, faces challenges from chloride-induced reactions and corrosion. Here, We introduce a catalytic surface where OH- dominates over Cl- in adsorption and activation, which is crucial for O2 production. Our NiFe-based anode, enhanced by nearby Cr sites, achieves low overpotentials and selective alkaline seawater oxidation. It outperforms the RuO2 counterpart in terms of lifespan in scaled-up stacks, maintaining stability for over 2500 h in three-electrode tests. Ex situ/in situ analyses reveal that Cr(III) sites enrich OH-, while Cl- is repelled by Cr(VI) sites, both of which are well-dispersed and close to NiFe, enhancing charge transfer and overall electrode performance. Such multiple effects fundamentally boost the activity, selectively, and chemical stability of the NiFe-based electrode. This development marks a significant advance in creating durable, noble-metal-free electrodes for alkaline seawater electrolysis, highlighting the importance of well-distributed catalytic sites.

Advances in Noble Metal Electrocatalysts for Acidic Oxygen Evolution Reaction: Construction of Under-Coordinated Active Sites

Renewable energy-driven proton exchange membrane water electrolyzer (PEMWE) attracts widespread attention as a zero-emission and sustainable technology. Oxygen evolution reaction (OER) catalysts with sluggish OER kinetics and rapid deactivation are major obstacles to the widespread commercialization of PEMWE. To date, although various advanced electrocatalysts have been reported to enhance acidic OER performance, Ru/Ir-based nanomaterials remain the most promising catalysts for PEMWE applications. Therefore, there is an urgent need to develop efficient, stable, and cost-effective Ru/Ir catalysts. Since the structure-performance relationship is one of the most important tools for studying the reaction mechanism and constructing the optimal catalytic system. In this review, the recent research progress from the construction of unsaturated sites to gain a deeper understanding of the reaction and deactivation mechanism of catalysts is summarized. First, a general understanding of OER reaction mechanism, catalyst dissolution mechanism, and active site structure is provided. Then, advances in the design and synthesis of advanced acidic OER catalysts are reviewed in terms of the classification of unsaturated active site design, i.e., alloy, core-shell, single-atom, and framework structures. Finally, challenges and perspectives are presented for the future development of OER catalysts and renewable energy technologies for hydrogen production.

Tungstate Intercalated NiFe Layered Double Hydroxide Enables Long-Term Alkaline Seawater Oxidation

Renewable electricity-driven seawater splitting presents a green, effective, and promising strategy for building hydrogen (H2)-based energy systems (e.g., storing wind power as H2), especially in many coastal cities. The abundance of Cl- in seawater, however, will cause severe corrosion of anode catalyst during the seawater electrolysis, and thus affect the long-term stability of the catalyst. Herein, seawater oxidation performances of NiFe layered double hydroxides (LDH), a classic oxygen (O2) evolution material, can be boosted by employing tungstate (WO4 2-) as the intercalated guest. Notably, insertion of WO4 2- to LDH layers upgrades the reaction kinetics and selectivity, attaining higher current densities with ≈100% O2 generation efficiency in alkaline seawater. Moreover, after a 350 h test at 1000 mA cm-2, only trace active chlorine can be detected in the electrolyte. Additionally, O2 evolution follows lattice oxygen mechanism on NiFe LDH with intercalated WO4 2-.

In Situ Tracking of Water Oxidation Generated Nanoscale Dynamics in Layered Double Hydroxides Nanosheets

Layered double hydroxides (LDHs) are potential catalysts for water oxidation, and it is recognized that they undergo dynamic evolution during the operation. However, little is known about the interfacial behaviors at the nanoscale under working conditions nor the underlying effects on electrocatalytic performance. Herein, using electrochemical atomic force microscopy, we in situ visualize the heterogeneous evolution of LDH nanosheets during oxygen evolution reaction (OER). By further combining density functional theory calculations, we elucidate the origin of the heterogeneous dynamics and their impact on the OER efficiency. Our findings demonstrate that NiCo LDHs transform to the catalytically active NiCoOx(OH)2-x phase during OER, and the redox transition between is accompanied by compressive and tensile strain, leading to in-plane contraction and reversible expansion of the nanosheets. Nonisotropic strain and out-of-plane strain relaxation due to defects and interparticle interactions result in cracking and wrinkling in the nanostructure, which is responsible for the partial activation and long-term deterioration of LDH electrocatalysts toward the OER. With this knowledge, we suggest and validate that engineering defects can precisely tune these dynamic behaviors, improving the OER activity and stability among LDH-based electrocatalysts.