Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting

A Nd-doped NiCo spinel dual functional catalyst for both oxygen reduction reactions and oxygen evolution reactions: Enhanced activity through surface reconstruction

The design of efficient, low-cost, highly active and thermally stable electrocatalysts is critical for both oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). While some spinel metal oxides exhibit good activities for either ORR or OER, a bifunctional spinel metal oxide that can provide decent activities for both ORR and OER would be most desirable. To date, rare earth metal-modified spinel oxides have not been well-studied, but they are thought to be able to boost both ORR and OER simultaneously. Hence, a Nd-doped NiCo2O4 catalyst was synthesized in this work to evaluate its potential for improving both ORR and OER reactions. We hypothesized that this catalyst would be a viable option, as the highly oxidized Co4+ (hydroxycobalt oxide) generated from surface reconstruction could be an active site for OER while Ni2+ is intrinsically an active site for ORR. Amazingly, our study revealed that the addition of Nd in spinel metal oxides was able to inhibit the formation of Co4+ at low potentials while the Ni species promoted the formation of Co4+ from Co2+, thus achieving a balance between Co2+ and Co4+ which resulted in a multi-step oxidation process of Co2+ → Co3+ → Co4+. In addition, by tuning the amount of Nd doped, an optimum electrocatalyst Nd0.1Ni0.9Co2O4 with excellent activities for both ORR (i.e. the half-wave potential E1/2 = 0.735 V) and OER (i.e. the overpotential at 10 mA cm-2 E10 mA·cm-2 = 302 mV) in alkaline conditions was developed. In summary, this work may have opened a new pathway for applying spinel metal oxides as bifunctional catalysts in future commercial ORR and OER processes.

Femtosecond laser synthesis of metastable PtRu/graphene electrocatalysts for efficient hydrogen evolution reaction in acidic and alkaline solutions

Facile synthesis of nanoalloys with atomic dispersity is an effective means to engineer highly efficient electrocatalysts by maximizing the exposure of catalytically active sites. However, such effort faces challenges in practice. One key issue is the thermodynamic-driven phase separation because of differences in redox potentials across different elements. Conventional wet chemistry methods often lack the rapid high-energy input required to non-selectively reduce precursors and mix elements with differing crystallographic parameters or phases, processes hindered by kinetic barriers. Here, we employ a femtosecond (fs) laser liquid ablation technique to synthesize defect-rich PtRu alloys on carbon supports without the application of external reducing agents or capping ligands. The extreme light field generated by fs lasers facilitates the rapid synthesis and stabilization of nanoalloy particles. The synthesized PtRu nanoparticles exhibited remarkable hydrogen evolution reaction (HER) activity in 1 M KOH and 0.5 M H2SO4, with overpotentials of 15.5 mV and 13.6 mV at 10 mA cm-2, respectively. In alkaline and acidic solutions, the mass activity of PtRu/graphene catalyst was 4.7 and 4.3 times that of commercial Pt/C catalysts, respectively. The electrolyte/electrode interfacial properties were investigated using in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). It was found that alloying Pt with Ru activates water molecules and optimizes the interfacial water structure and hydrogen adsorption energy. Populated free water molecules at the electrolyte/electrode (PtRu/graphene) interface facilitate the transport of reaction intermediates. In-situ Raman spectroscopy reveals that Ru directly participates in the Volmer step, serving as the active site for water dissociation. Our study demonstrates the potential of using fs lasers for materials engineering and designing ligand-free metastable nanoalloys for various applications.

Unraveling Hydrogen Evolution in Ni-Doped SnSe: Mechanistic Insights into the Synergy of Crystal Facets, Doping, and External Stimuli Using On-Chip Microelectrochemical Cell

Understanding electrocatalytic processes at the microscale in 2D-layered architectures is crucial for catalyst design and investigating underlying mechanisms. In this study, pristine and nickel (Ni)-doped tin selenide (SnSe) flakes are analyzed using on-chip microelectrochemical measurements to explore the effects of defect and facet engineering on their hydrogen evolution reaction (HER) activity. The catalytic activity is found to be influenced by the exposed crystal facets, with the edges exhibiting higher activity than the basal planes. Deliberately exposing the (010) planes of SnSe flakes, having the highest density of dangling bonds, results in a noticeable improvement in HER performance. Additionally, Ni doping in SnSe enhances the HER performance by reducing the overpotential value required to achieve a current density of 100 mA cm-2 from 231 ± 24 to just 89 ± 35 mV versus the reversible hydrogen electrode, which is attributed to an increased number of active sites and lower semiconductor/electrolyte barrier height. Ni doping also induces a transition in p-type SnSe to n-type by substituting Sn sites and occupying Sn vacancies, which facilitates enhanced HER kinetics with ~ three times enhancement in current density. External electric fields and photoirradiation further modulate HER kinetics, highlighting the potential for tuning SnSe and similar 2D materials for electrocatalytic applications.

The role of carbon catalyst coatings in the electrochemical water splitting reaction

Designing inexpensive, sustainable, and high-performance oxygen-evolution reaction (OER) electrocatalysts is one of the largest obstacles hindering the development of new electrolyzers. Carbon-coated metal/metal oxide (nano)particles have been used in such applications, but the role played by the carbon coatings is poorly understood. Here, we use a carbon-coated catalyst comprising metal-oxide nanoparticles encapsulated within single-walled carbon nanotubes (SWNTs), to study the effects of carbon coatings on catalytic performance. Electrolyte access to the encapsulated metal oxides is shut off by plugging the SWNT ends with size-matched fullerenes. Our results reveal that the catalytic activity of the composite rivals that of the metal oxide, despite the fact that the metal oxides cannot access the bulk electrolyte. Moreover, the rate-determining step (RDS) of the OER matches that measured at empty SWNTs, indicating that electrocatalysis occurs on the carbon surface. Synergism between the encapsulated metal oxide and carbon coating was explored using electrochemical Raman spectroscopy and computational analysis, revealing that charge transfer from the carbon host to the metal oxide is key to the high electrocatalytic activity of carbon in this system; decreasing electron density on the carbon surface facilitates binding of -OH, accelerating the rate of the OER on the carbon surface.

The Role of Cu3+ in the Oxygen Evolution Activity of Copper Oxides

Cu-based oxides and hydroxides represent an important class of materials from a catalytic and corrosion perspective. In this study, we investigate the formation of bulk and surface Cu3+ species that are stable under water oxidation catalysis in alkaline media. So far, no direct evidence existed for the presence of hydroxides (CuOOH) or oxides, which were primarily proposed by theory. This work directly places CuOOH in the oxygen evolution reaction (OER) Pourbaix stability region with a calculated free energy of -208.68 kJ/mol, necessitating a revision of known Cu-H2O phase diagrams. We also predict that the active sites of CuOOH for the OER are consistent with a bridge O* site between the two Cu3+ atoms with onset at ≥1.6 V vs the reversible hydrogen electrode (RHE), aligning with experimentally observed Cu2+/3+ oxidation waves in cyclic voltammetry of Fe-free and Fe-spiked copper in alkaline media. Trace amounts of Fe (2 μg/mL (ppm) to 5 μg/mL) in the solution measurably enhance the catalytic activity of the OER, likely due to the adsorption of Fe species that serve as the active sites . Importantly, modulation excitation X-ray absorption spectroscopy (ME-XAS) of a Cu thin-film electrode shows a distinct Cu3+ fingerprint under OER conditions at 1.8 V vs RHE. Additionally, in situ Raman spectroscopy of polycrystalline Cu in 0.1 mol/L (M) KOH revealed features consistent with those calculated for CuOOH in addition to CuO. Overall, this work provides direct evidence of bulk electrochemical Cu3+ species under OER conditions and expands our longstanding understanding of the oxidation mechanism and catalytic activity of copper.

Metal-free carbon-based porous materials, promising electrocatalysts for hydrogen fuel production

The development of effective and sustainable electrocatalysts for hydrogen fuel production is crucial for advancing clean energy technologies. Metal-free porous carbon materials have emerged as promising alternatives to traditional metal-based catalysts due to their low cost, high surface area, tunable porosity, and excellent electrochemical stability. This review provides a comprehensive overview of the latest advancements in metal-free porous carbon electrocatalysts for the hydrogen evolution reaction (HER). A comprehensive discussion is provided regarding how heteroatom doping, defect engineering, and surface functionalization improve catalytic activity. Additionally, we compare the performance of these materials and highlight recent strategies for improving their efficiency and durability. Future perspectives on optimizing metal-free porous carbons for large-scale hydrogen production are also explored. This review intends to give a clear view on the rational design of next-generation electrocatalysts for sustainable hydrogen energy applications.

Photocatalytic Hydrogen Production Using Semiconductor (CdSe)13 Clusters

Atomically precise (CdSe)13 clusters, the smallest CdSe semiconductors, represent a unique class of materials at the boundary between nanocrystals and molecules. Despite their promising potential, low structural stability limits their applications as photocatalysts. Herein, we report photocatalytic hydrogen production using atomically precise (CdSe)13 clusters. To improve stability in aqueous environments, we induce self-assembly into suprastructures, making them suitable for water splitting. Our findings demonstrate that Co2+ doping enhances the electrical properties of these clusters, while bipyridine serves as cocatalyst by interacting with Co2+ dopants and providing catalytic active sites. Through the synergistic effects of Co2+ doping and bipyridine, Co2+-doped (CdSe)13 suprastructures achieve promising hydrogen evolution activity, surpassing those of undoped suprastructures or nanoclusters. Theoretical calculations confirm that Co2+ doping and bipyridine incorporation lower the hydrogen adsorption energy, consistent with the experimental results. These results highlight the potential of semiconductor (CdSe)13 clusters as photocatalysts for sustainable hydrogen production.

A Review on Electrochemical Water Splitting Electrocatalysts for Green H2 Production: Unveiling the Fundamentals and Recent Advances

Green H2 production via electrochemical water splitting has emerged as a pivotal solution for achieving a sustainable energy future. This Review delves into the fundamentals of water splitting, focusing on the O2 evolution reaction (OER) and H2 evolution reaction (HER), and focuses on the critical role of electrocatalysts in these processes. Precious metals such as paltinum and iridium remain the benchmarks for catalytic performance; however, their scarcity and high cost necessitate the development of alternative materials. Recent advances in Earth-abundant catalysts, including transition-metal oxides, carbides, nitrides, and sulfides, have shown promise in balancing activity, durability, and affordability. The integration of nanostructuring techniques and computational modeling has enabled the design of catalysts with enhanced active site exposure and electronic properties. Furthermore, the Review highlights challenges such as material degradation, high overpotentials, and gas crossover, along with potential solutions like protective coatings, bifunctional catalysts, and advanced electrolyzer designs. Future prospects emphasize the role of artificial intelligence, hybrid systems, and sustainable manufacturing in accelerating progress. This comprehensive review underscores the significance of bridging fundamental research with technological innovations to scale up green hydrogen production, addressing energy demands while mitigating environmental impacts.

Applications, performance enhancement strategies and prospects of NixPy in electrocatalysis

Developing low-cost and high-efficiency electrocatalysts is the key to making energy-related electrocatalytic technologies commercially feasible. In recent years, nickel phosphide (NixPy) electrocatalysts have received extensive attention due to their multiple active sites, adjustable structure and composition, and unique physicochemical properties. In this review, the latest progress of NixPy in the field of electrocatalysis is reviewed from the aspects of the properties of NixPy, different synthesis methods, and ingenious modulation strategies. The significant enhancement effects of elemental doping, vacancy defect, interfacial engineering, synergistic effect, and the external magnetic field excitation-enhanced strategy on the electrocatalytic performance of NixPy are emphasized, Moreover, a forward-looking outlook for its future development direction is provided. Finally, some basic problems and research directions of NixPy in high-efficiency energy electrocatalysis are presented.

Simultaneous Electrochemical Upgrading of Biomass and CO2 Utilization Using Fe/Ni-Derived Carbon Nanotubes Derived from CO2

Fossil fuel consumption has caused petroleum shortages and increased carbon emissions; thus, utilizing renewable resources in biorefineries for biomass-derived chemical synthesis is promising. Among them, 2,5-furandicarboxylic acid (FDCA) is a key alternative to terephthalic acid (PTA) for sustainable polyester production. In this work, we demonstrate an efficient approach for the simultaneous production of FDCA while utilizing carbon dioxide (CO₂) via an electrochemical approach. Complete electrooxidation of hydroxymethylfurfural (HMF) at the anode yields FDCA, while CO₂ reduction at the cathode produces valuable compounds such as carbon monoxide (CO). This concurrent HMF electrooxidation and CO₂ electroreduction strategy enables high-value chemical production at mild conditions. In addition, we developed efficient single catalysts, FeNi metals supported on CO₂-derived multi-walled carbon nanotubes deposited on nickel foam (FeNiCNTs/NF) as both the anode and the cathode for HMF oxidation and CO2 reduction, respectively. Remarkably, faradaic efficiencies reached 99.60% for FDCA (FEFDCA) at the anode and 96.25% for CO (FECO) at the cathode. This study highlights the effective use of synthesized non-noble metals supported on CO₂-derived CNTs for integrated biorefinery and CO₂ utilization.

Enhancing Photocatalytic Hydrogen Evolution by Improving the Morphology of Organic Semiconductor Nanoparticles with TCB Additive

Organic semiconductor nanoparticles (NPs) are promising organic photocatalysts for water splitting. However, effective organic semiconductor NPs require efficient charge dissociation and transport at the heterojunction interface. Common core-shell structured NPs exhibit low hydrogen evolution rates (HERs) due to limited charge dissociation efficiency. Here, this challenge is addressed by introducing 1,3,5-trichlorobenzene (TCB) additive into organic semiconductor NPs to improve their heterojunction morphology. As a result, PM6:Y6 NPs with TCB have a more intimately blended morphology, which enhances charge dissociation and transport. These NPs achieve a HER of 16,490 µmol g-1 h-1, which is more than twice that of NPs without TCB. Further optimization of the NPs concentration led to a remarkable HER of 58,400 µmol g-1 h-1. Moreover, the PM6:Y6 NPs with TCB exhibit better operational stability due to their enhanced morphological stability. This study demonstrates the effectiveness of the additive strategy in improving the heterojunction morphology of organic semiconductor NPs to overcome key limitations of their photocatalytic hydrogen evolution efficiency and provides valuable insights for the development of high-performance organic photocatalysts.

Hydrazine-assisted water electrolysis system: performance enhancement and application expansion

Powered by renewable energy sources, water electrolysis has emerged as a highly promising technology for energy conversion, attracting significant attention in recent years, but it faces severe challenges, especially at the anode. Accordingly, hydrazine-assisted water electrolysis, incorporating the electro-oxidation of hydrazine at the anode, holds great promise for greatly reducing the input voltage and optimizing the system by application expansion. In this review, we present an in-depth overview of hydrazine-assisted water electrolysis, introducing its reaction mechanisms, basic parameters, specific advantages compared with conventional water electrolysis and other hybrid water electrolysis systems, strategies for developing efficient electrocatalysts with enhanced electrocatalytic performances, and especially its potential application expansion. An analysis of its technical and economic aspects, feasibility studies, mechanistic investigations, and relevant comparisons are also presented for providing a deeper insight into hydrazine-assisted water electrolysis. Finally, the potential avenues and opportunities for future research on hydrazine-assisted water electrolysis are discussed.

Real-Time Monitoring of Fe-Induced Stable γ-NiOOH in Binder-Free FeNi MOF Electrocatalysts for Enhanced Oxygen Evolution

Hydrogen energy is a promising renewable source, and metal-organic frameworks (MOFs) are considered potential electrocatalysts for water electrolysis due to their abundant active sites, high porosity, and large surface area. The synthesis of bimetallic iron-nickel-benzene-1,3,5-tricarboxylate/nickel foam (FeNi-BTC/NF) MOF is reported using a binder-free one-pot method by immersing nickel foam (NF) into a solution of benzene-1,3,5-tricarboxylic acid (BTC), N,N-dimethylformamide (DMF), and iron (Fe) salts. FeNi-BTC/NF exhibits a low overpotential of 276 mV at 100 mA cm- 2, a Tafel slope of 94 mV dec-1, and stability exceeding 120 h. The Fe-Ni interaction facilitates the formation of a stable gamma-nickel oxyhydroxide (γ-NiOOH) phase, preventing its reversion to nickel hydroxyide (Ni(OH)₂), which is crucial for improving oxygen evolution reaction (OER) performance. This phase transition, revealed via in situ Raman spectroelectrochemical analysis, enhances electrocatalytic activity. Additionally, high-valent Fe modulates the electronic structure of Ni, enabling FeNi-BTC/NF to transform into γ-NiOOH at higher potentials, with Fe and γ-NiOOH synergistically boosting OER efficiency. The findings offer insights into Fe/Ni atom interactions and phase transformations in FeNi-BTC/NF MOFs for enhanced water splitting.

A mechanism-guided descriptor for the hydrogen evolution reaction in 2D ordered double transition-metal carbide MXenes

Selecting effective catalysts for the hydrogen evolution reaction (HER) among MXenes remains a complex challenge. While machine learning (ML) paired with density functional theory (DFT) can streamline this search, issues with training data quality, model accuracy, and descriptor selection limit its effectiveness. These hurdles often arise from an incomplete understanding of the catalytic mechanisms. Here, we introduce a mechanism-guided descriptor (δ) for the HER, designed to enhance catalyst screening among ordered transition metal carbide MXenes. This descriptor integrates structural and energetic characteristics, derived from an in-depth analysis of orbital interactions and the relationship between Gibbs free energy of hydrogen adsorption (ΔG H) and structural features. The proposed model (ΔG H = -0.49δ - 2.18) not only clarifies structure-activity links but also supports efficient, resource-effective identification of promising catalysts. Our approach offers a new framework for developing descriptors and advancing catalyst screening.

Role of Water in Green Carbon Science

Within the context of green chemistry, the concept of green carbon science emphasizes carbon balance and recycling to address the challenge of achieving carbon neutrality. The fundamental processes in this field are oxidation and reduction, which often involve simple molecules such as CO2, CO, CH4, CHx, and H2O. Water plays a critical role in nearly all oxidation-reduction processes, and thus, it is a central focus of research in green carbon science. Water can act as a direct source of dihydrogen in reduction reactions or participate in oxidation reactions, frequently involving O-O coupling to produce hydrogen peroxide or dioxygen. At the atomic level, this coupling involves the statistically unfavorable proximity of two atoms, requiring optimization through a catalytic process influenced by two types of factors, as described by the authors. Extrinsic factors are related to geometrical and electronic criteria associated with the catalytic metal, involving its d-orbitals (or bands in the case of zerovalent metals and electrodes). Intrinsic factors are related to the coupling of oxygen atoms via their p-orbitals. At the mesoscopic or microscopic scale, the reaction medium typically consists of mixtures of lipophilic and hydrophilic phases with water, which may exist under supercritical conditions or as suspensions of microdroplets. These reactions predominantly occur at phase interfaces. A comprehensive understanding of the phenomena across these scales could facilitate improvements and even lead to the development of novel conversion processes.

Vanadium-Stabilized MoB Nanoparticles Enable Hydrogen Evolution at Industry-Relevant High Current Densities

Bulk molybdenum boride electrocatalysts have emerged as as cost-effective alternatives to platinum-based catalysts toward the hydrogen evolution reaction (HER), particularly under harsh industrial conditions requiring high current densities. However, differences in electrode preparation methods between molybdenum borides and Pt/C thus complicate direct activity comparison. In this study, vanadium-stabilized molybdenum monoboride (V0.3Mo0.7B) nanoparticles are synthesized and shown to outperform Pt/C at industrially relevant current densities under the same experimental conditions, achieving 1000 mA cm-2 with an overpotential of just 0.452 V compared to 0.837 V for Pt/C. Our density functional theory (DFT) calculations demonstrate that V0.31Mo0.69B exhibits improved Gibbs free energy for HER (ΔGH = -0.12 eV) at high hydrogen coverages (80 to 100%), showcasing its superior catalytic activity at high current densities. Stability tests demonstrate that the V0.3Mo0.7B electrode retains 97% of its performance after ≈28 h of operation at 1000 mA cm-2, positioning it as a compelling candidate for sustainable hydrogen production.

Titanium‒Nickel Dual Active Sites Enabled Reversible Hydrogen Storage of Magnesium at 180 °C with Exceptional Cycle Stability

Enhancing hydrogenation and dehydrogenation (de/hydrogenation) kinetics without compromising cycle stability is a major challenge for Mg-based hydrogen storage materials (Mg/MgH2). The de/hydrogenation reactions of Mg/MgH2 are one of the gas-solid reactions involving hydrogen adsorption, dissociation, diffusion, and nucleation, which often results in the catalysts being unable to simultaneously accelerate these distinct kinetic processes. Here, the Mg2Ni@Ti─MgO catalyst with dual active sites is reported to be designed to address this issue. The stabilization of Ti2+ and Ti3+ valence states in the MgO lattice simultaneously accelerates hydrogen adsorption and dissociation. Additionally, Mg2Ni serves as a hydrogen diffusion and nucleation center, synergistically enhancing de/hydrogenation reactions. Consequently, it enables MgH2 to release 5.28 wt.% H2 in 2 min at 280 °C, and achieves 1.96 wt.% H2 of hydrogen release in 60 min at 180 °C. The Mg2Ni@Ti─MgO catalyst exhibits remarkable chemical stability at the interfacial structure, minimizing structural and chemical degradation impact, and realizing excellent de/hydrogenation performance over 1000 cycles. These results provide a new methodology for optimizing multiple kinetic steps, attaining highly efficient and stable de/hydrogenation reactions.

Lamellar Nanoporous Intermetallic Cobalt-Titanium Multisite Electrocatalyst with Extraordinary Activity and Durability for the Hydrogen Evolution Reaction

Constructing well-defined multisites with high activity and durability is crucial for the development of highly efficient electrocatalysts toward multiple-intermediate reactions. Here we report negative mixing enthalpy caused intermetallic cobalt-titanium (Co3Ti) nanoprecipitates on a lamellar hierarchical nanoporous cobalt skeleton as a high-performance nonprecious multisite electrocatalyst for an alkaline hydrogen evolution reaction. The intermetallic Co3Ti as a robust multisite substantially boosts the reaction kinetics of water dissociation and hydrogen adsorption/combination by unisonous adsorptions of hydrogen and hydroxyl intermediates with proper binding energies. By virtue of a bicontinuous and hierarchical nanoporous cobalt skeleton that enables sufficiently accessible Co3Ti multisites and facilitates electron transfer and ion/molecule transportation, a self-supported nanoporous cobalt-titanium heterogeneous electrode exhibits extraordinary electrocatalytic activity and durability toward the hydrogen evolution reaction in 1 M KOH. It reaches a current density of as high as ∼3.31 A cm-2 at a low overpotential of 200 mV and maintains exceptional stability at ∼1.33 A cm-2 for >1000 h.

Ultrafast Synthesis of Single-Atom Catalysts for Electrocatalytic Applications

A recent development in catalytic research, single-atom catalysts (SACs) are one of the most significant categories of catalytic materials. During preparation, individual atoms migrate and agglomerate due to the high surface free energy. The rapid thermal shock strategy addresses this challenge by employing instantaneous high-temperature pulses to synthesize SACs, while minimizing heating duration to prevent metal aggregation and substrate degradation, thereby preserving atomic-level dispersion. The resultant SACs exhibit exceptional catalytic activity, remarkable selectivity, and long-term stability, which have attracted extensive attention in electrocatalysis. In this paper, cutting-edge ultrafast synthesis techniques such as Joule heating, microwave radiation, pulsed discharge, and arc discharge are comprehensively analyzed. Their ability is emphasized to achieve uniform dispersion of separated metal atoms and optimize the catalytic activity for electrocatalytic applications. A systematic summary of SACs synthesized by these rapid methods is provided, with particular emphasis on their implementation in carbon dioxide reduction reaction (CO2RR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR) systems. The review provides an in-depth discussion on the rapid synthesis strategy for development trend, remaining challenges, and the application prospects in electrocatalysis.

Coarse-Grained Molecular Dynamics Simulation of Nucleation and Stability of Electrochemically Generated Nanobubbles

With growing concerns over environmental pollution associated with fossil fuels, hydrogen (H2) energy has emerged as a promising alternative. Water electrolysis, a key hydrogen production method, is fundamentally governed by the nucleation and stability of electrochemically generated nanobubbles. This study employs coarse-grained molecular dynamics (MD) simulations incorporating a self-programming gas generation algorithm to investigate the nucleation and growth dynamics of nanobubbles on hydrophilic and hydrophobic electrodes. Key parameters, such as contact angle, electric current, and nanobubble number density, were computed to validate the MD model. The findings reveal a three-stage nucleation process: (i) induction─gas molecules accumulate to form a nucleus, (ii) nucleation and growth─gas nuclei expand into nanobubbles, and (iii) stationary state─nanobubble growth ceases. Increased electrode hydrophilicity resulted in larger nanobubble contact angles, aligning well with classical nucleation theory (CNT) at the nanoscale. Three distinct nanobubble types─surface, solution, and pancake nanobubbles─were identified, each exhibiting unique interfacial behaviors based on electrode properties. Solution nanobubbles primarily formed on hydrophilic electrodes, pancake nanobubbles adhered to hydrophobic electrodes, and surface nanobubbles appeared as spherical caps. Energy analysis and phase mapping further delineated the critical parameter ranges for these nanobubble modes, providing valuable insights for optimizing electrode materials to enhance hydrogen production efficiency.

High-entropy alloy enables multi-path electron synergism and lattice oxygen activation for enhanced oxygen evolution activity

Electrocatalytic oxygen evolution reaction (OER) is key to several energy technologies but suffers from low activity. Leveraging the lattice oxygen activation mechanism (LOM) is a strategy for boosting its activity. However, this approach faces significant thermodynamic challenges, requiring high-valent oxidation of metal ions without compromising their stability. We reveal that high-entropy alloys (HEAs) can efficiently activate the LOM through synergistic multi-path electron transfer. Specifically, the oxidation of nickel is enhanced by this electron transfer, aided by the integration of weaker Co-O bonds, enabling effective LOM at the Ni-Co dual-site. These insights allow the design of a NiFeCoCrW0.2 HEA that exhibits improved activity, achieving an overpotential of 220 mV at a current density of 10 mA cm-2. It also demonstrates good stability, maintaining the potential with less than 5% variation over 90 days at 100 mA cm-2 current density. This study sheds light on the synergistic effects that confer high activity in HEAs and contribute to the advancement of high-performance OER electrocatalysts.

Stabilizing the Unstable: Enhancing OER Durability with 3d-Orbital Transition Metal Multielemental Alloy Nanoparticles by Atomically Dispersed 4d-Orbital Pd for a 100-Fold Extended Lifetime

Earth-abundant 3d-orbital late transition metals are the most used and highly desired catalysts for the oxygen evolution reaction (OER) but are prone to quick oxidative dissolution, leading to poor durability. We first report that FeCoNiCu multielemental alloy nanoparticles (MEA NPs) can be stabilized with only 0.3 at. % Pd, a 4d-orbital element. Although pure Pd is known for extremely poor OER activity and durability, Pd-FeCoNiCu sustains 1000 h at 10 mA cm-2. In an accelerating durability test (ADT) at 100 mA cm-2, it exhibits a mere 8.9 mV increase over 25 h with a degradation rate of 0.356 mV h-1, which is 1/350th that of FeCoNiCu (125 mV h-1) and among the most stable OER catalysts reported so far. Aberration-corrected HAADF-STEM and X-ray absorption fine structure (XAFS) reveal that atomically dispersed Pd atoms, surrounded by Fe, Co, Ni, and Cu atoms, contributed to a more delocalized electronic structure and stronger bonding via strong d-d/sp hybridization and the vibronic coupling induced by atomic displacement. The altered local density of states (LDOS) of Fe, Co, Ni, and Cu mitigates the oxidation of FeCoNiCu in OER by over 50%, quantified by hard X-ray photoelectron spectroscopy (HAXPES), making the combination of these five dissoluble elements a durable catalyst.

Charge transport channel and nonmetallic plasmon synergistically augment surface reaction kinetics and charge separation for efficient photoelectrochemical hydrogen evolution of CdS/TiN-sensitized Fe2V4O13 photoanode

Achieving simultaneous enhancement in the light energy utilization efficiency, bulk charge carrier separation and surface charge carrier injection efficiency as well as the surface reaction kinetics of water oxidation is a formidable challenge for photoanodes in photoelectrochemical (PEC) water splitting hydrogen generation. Herein, nanoparticle-assembled flower-like CdS spheres and nonmetallic plasmonic TiN nanoparticles are exploited to successively sensitize Fe2V4O13 nanoporous film (NPF) photoanode for achieving efficient PEC hydrogen evolution. The sensitization of TiN and CdS simultaneously integrates type-II band structure, surface plasmon resonance and Schottky junction into Fe2V4O13 NPF photoanode, synergistically achieving simultaneous enhancement in the light energy utilization efficiency, bulk charge carrier separation efficiency, surface reaction kinetics of water oxidation and surface charge carrier injection efficiency. As a result, the highest charge separation and injection efficiencies of CdS/TiN-sensitized Fe2V4O13 NPF photoanode are respectively increased by 25.5 and 1.96 times to those of bare Fe2V4O13 NPF photoanode. Furthermore, the designed and constructed CdS/TiN-sensitized Fe2V4O13 NPF photoanode exhibits substantially boosted unbiased solar-light-driven PEC hydrogen evolution ability with a photocurrent density of 2.12 mA/cm2, which is two orders of magnitude (662 times) higher than that of the unsensitized Fe2V4O13 NPF photoanode. The findings in this work provide a novel and promising strategy to design and construct high-performance Fe2V4O13-based nonmetallic plasmonic photoanodes for potential application in PEC hydrogen evolution.

Uncovering Interfacial Oxygen-Bridged Binuclear Metal Centers of Heterogenized Molecular Catalyst for Water Electrolysis

The success of different heterogeneous strategies of organometallic catalysts has been demonstrated to achieve high selectivity and activity in photo/electrocatalysis. However, yielding their catalytic mechanisms at complex molecule-electrode and electrochemical interfaces remains a great challenge. Herein, shell-isolated nanoparticle-enhanced Raman spectroscopy is employed to elucidate the dynamic process, interfacial structure, and intermediates of copper hydroxide-2-2' bipyridine on Au electrode ((bpy)Cu(OH)2/Au) during the oxygen evolution reaction (OER). Direct Raman molecular evidences reveal that the interfacial (bpy)Cu(OH)2 oxidizes into Cu(III) and bridges to Au atoms via oxygenated species, forming (bpy)Cu(III)O2-Au with oxygen-bridged binuclear metal centers of Cu(III)-O-Au for the OER. As the potential further increases, Cu(III)-O-Au combines with surface hydroxyl groups (*OH) to form the important intermediate of Cu(III)-OOH-Au, which then turns into Cu(III)-OO-Au to release O2. Furthermore, in situ electrochemical impedance spectroscopy proves that the Cu(III)-O-Au has lower resistance and faster mass transport of hydroxy to enhance OER. Theoretical calculations reveal that the formation of Cu(III)-O-Au significantly modify the elementary reaction steps of the OER, resulting in a lower potential-determining step of ≈0.58 V than that of bare Au. This work provides new insights into the OER mechanism of immobilized-molecule catalysts for the development and application of renewable energy conversion devices.

Enhanced the Overall Water Splitting Performance of Quaternary NiFeCrCo LDH: Via Increasing Entropy

The construction of high-performance catalysts for overall water splitting (OWS) is crucial. Nickel-iron-layered double hydroxide (NiFe LDH) is a promising catalyst for OWS. However, the slow kinetics of the HER under alkaline conditions seriously hinder the application of NiFe LDH in OWS. This work presents a strategy to optimize OWS performance by adjusting the entropy of multi-metallic LDH. Quaternary NiFeCrCo LDH was constructed, which exhibited remarkable OWS activity. The OER and HER of NiFeCrCo LDH were stable for 100 h and 80 h, respectively. The OWS activity of NiFeCrCo LDH//NiFeCrCo LDH only required 1.42 V to reach 10 mA cm-2, and 100 mA cm-2 required 1.54 V. Under simulated seawater conditions, NiFeCrCo LDH//NiFeCrCo LDH required 1.57 V to reach 10 mA cm-2 and 1.71 V to reach 100 mA cm-2. The introduction of Co into the structure induced Cr to provide more electrons to Fe, which regulated the electronic state of NiFeCrCo LDH. The appropriate electronic state of the structure is essential for the remarkable performance of OWS. This work proposes a new strategy to achieve excellent OWS performance through entropy-increase engineering.

The investigation of Co6-xFexW6N (x = 0, 3, 6) as electrocatalysts for the hydrogen evolution reaction

Proton exchange membrane (PEM) water electrolysers are considered the most promising devices for hydrogen production when operated in tandem with renewable energy sources. However, their efficiency depends on catalysts used on the anode and cathode, but the acidic conditions at the membrane restrict the catalysts to noble metals. Hence, the search for non-noble metal catalysts that are active and stable under acidic conditions is important. In this work we demonstrate that phase pure Co6W6N (prepared by the nitridation of an oxide precursor) remains stable in 0.5 M H2SO4, suggesting that it is a suitable electrocatalyst for the hydrogen evolution reaction (HER) in acidic media. Moreover, it shows a comparatively low overpotential of -150 ± 8 mV at a benchmark current density of 10 mA cm-2. Furthermore, two isostructural catalysts with the compositions Co3Fe3W6N and Fe6W6N showed overpotentials of -225 ± 8 mV and -414 ± 18 mV at 10 mA cm-2, respectively, suggesting that Co-sites are responsible for the catalytic performance. This was further confirmed by X-ray photoelectron spectroscopy (XPS) which showed that W-oxidation states in Co6W6N and Fe6W6N are practically identical and hence, cannot be the cause for the overpotential's increase upon the substitution. In this context, Co6W6N appears an optimal target for future tests in full-scale electrolysis systems.

Diiron(IV)-Oxo Species and Water Oxidation: How Crucial is Electronic Cooperativity?

Water splitting, crucial for generating oxygen and hydrogen, remains a central challenge in chemistry due to its importance in developing sustainable energy sources and addressing environmental concerns. Consequently, numerous complexes have been developed to split water and release oxygen and hydrogen, albeit typically requiring external sources such as thermal, photo, or electrochemical methods. In this context, the discovery of a (μ-oxo)bis(μ-carboxamido) diiron(IV) complex, [FeIV₂O(L)₂]2+ (L=N,N-bis-(3',5'-dimethyl-4'-methoxypyridyl-2'-methyl)-N'-acetyl-1,2-diaminoethane), which activates both C-H and O-H bonds without external stimuli, has attracted significant attention. Notably, this complex generates hydroxyl radicals (⋅OH) without O₂ evolution and displays termolecular kinetics, presenting a rare and intriguing mechanistic puzzle. In this work, we explore the catalytic mechanism of water oxidation by this diiron(IV) complex using DFT methods. Our computational findings validate experimental observations regarding the necessity of a second water molecule in the reaction, revealing a bifurcated electron-proton transfer (BEPT) pathway driven by termolecular reactivity. Moreover, we highlight the crucial role of excess water molecules in stabilising the reaction intermediates, particularly via interaction with the -OMe groups to form a water cluster model. The inclusion of explicit water molecules was found to reduce the activation barrier to 23.5 kJ/mol from the reactant and 62.7 kJ/mol from the reactant complex, whereas, with only one water molecule present, the barrier was 344.3 kJ/mol, highlighting the critical role of the adventitious water molecule at the active site. Our study underscores the importance of metal-metal cooperativity, ligand design, spin-state modulation, and second-sphere effects in shaping the catalytic behaviour. These insights provide a detailed understanding of the electronic structure and reactivity, offering valuable guidelines for future catalyst design in water oxidation and beyond.

Rerouting charge transfer for pharmaceutical wastewater electrochemical treatment via interfacial cocatalyst modification

Electrochemical oxidation stands as a pivotal technology for refractory wastewater treatment. However, the high cost and low elemental abundance of commercial electrodes limit its widespread application. This work tries to address this by introducing a charge-transfer rerouting strategy via cocatalyst modification using earth-abundant elements. Here, we uncover the role of the cocatalyst in enhancing electrode performance. The in-situ reconstructed cocatalyst induces a substantial rerouting of the charge transfer pathway, facilitating the mass/charge transfer of organics while concurrently suppressing the oxygen evolution side reaction. The Ti-Fe2O3 electrode, loaded with the cocatalyst PbO2, exhibits both high current efficiency (∼45.4 %) and low energy requirement (∼31.8 kW h kg-1 COD), surpassing other reported electrodes and displaying great versatility in various scenarios with good stability and reusability. Moreover, this charge-transfer rerouting strategy holds promise for synergy with other methodologies, such as nanostructure engineering and molecular imprinting, to further enhance the reactivity and selectivity of electrocatalysts in environment and energy-related domains.

Challenges and Opportunities for Rechargeable Aqueous Sn Metal Batteries

Rechargeable aqueous batteries based on metallic anodes hold tremendous potential of high energy density enabled by the combination of relatively low working potential and large capacity while retaining the intrinsic safety nature and economical value of aqueous systems; However, the realization of these promised advantages relies on the identification of an ideal metal anode chemistry with all these merits. In this review, the emerging Sn metal anode chemistry is examined as such an anode candidate in both acidic and alkaline media, where the inertness of Sn toward hydrogen evolution, flat low voltage profile, and low polarization make it a unique metal anode for aqueous batteries. From a panoramic viewpoint, the key challenges and detrimental issues of Sn metal batteries are discussed, including dead Sn formation, self-discharge, and electrolyte degradation, as well as strategies for mitigating these issues by constructing robust Sn anodes. New design approaches for more durable and reliable Sn metal batteries are also discussed, with the aim of fully realizing the potential of Sn anode chemistry.

High-Entropy Selenides with Tunable Lattice Distortion as Efficient Electrocatalysts for Oxygen Evolution Reaction

Developing stable and active electrocatalysts is crucial for enhancing the oxygen evolution reaction (OER) efficiency, which sluggish kinetics hinder sustainable hydrogen production. High entropy selenides (HESes) feature with random distribution of multiple metals cations and unique electronic and size effect of Se anion, allowing for precious regulation of their catalytic properties towards high OER activity. In this work, we report a series of high-entropy selenides catalysts with tunable lattice strain for electrocatalytic oxygen evolution. Electrochemical measurements show that the quinary (NiCoMnMoFe)Sex requires only 291 mV to reach 10 mA cm-2 and exhibits a superior stability with negligible current decay during 100 h's continuous operation. By combining experimental measurements and theoretical calculation, the study reveals that the lattice distortion, reflected by the local microstrain near the active site, plays a vital role in boosting the OER activity of HESes.

Elucidation of the Activity and pH Stability Limits of Polyoxometalate-Intercalated Layered Double Hydroxide Nanocomposites toward Water Oxidation Catalysis

The inclusion of water oxidation active polyoxometalates (POMs) inside layered materials is a promising strategy to increase their catalytic efficiency while overcoming their fragility under homogeneous conditions. In this sense, intercalation of POMs in the interlaminar space of layered double hydroxides (LDHs), formed by positively charged brucite-type inorganic layers, is a very interesting strategy that is gaining attention in the field. Despite their huge potential, there is a lack of accurate characterization of the materials, especially after their use as water oxidation catalysts under pH conditions in which the POM counterpart has been demonstrated to be unstable (strong alkali media). For this reason and as a proof of concept, we have intercalated the well-known [Co4(H2O)2(PW9O34)2]10- POM (Co4-POM) in the lamellar space of the Mg2Al-LDH, to study its catalytic capabilities and stability. Remarkably, the nanocomposites show improved water oxidation efficiencies with excellent stability in close-to-neutral media compared with the water-insoluble cesium salt of Co4-POM or commercial Co3O4. However, thorough postcatalytic characterization of the nanocomposites demonstrates that the polyoxotungstate framework of the POM suffers from hydrolytic instability in strong alkali conditions, leading to the formation of a mixed-valence cobalt(II/III) oxide in the interlayer space of Mg2Al-LDH. This work highlights the importance of accurately assessing the fate of the catalytic POM after the catalytic reaction, especially when conditions are employed outside the pH stability window of the POM, which can lead to erroneous conclusions and mistaken catalytic activities.

Probing the catalytic heterogeneity of single FeCo and FeCoNi hydroxide nanoneedles by scanning electrochemical microscopy

Bimetallic and trimetallic alloys are widely used as catalysts for the oxygen evolution reaction (OER) and other electrocatalytic processes. We employed a scanning electrochemical microscope (SECM) and finite-element simulations to investigate the OER at single FeCo and FeCoNi hydroxide nanoneedles and observed different distributions of catalytic activity on bimetallic and trimetallic particles.

Photo-Driven Ammonia Synthesis via a Proton-Mediated Photoelectrochemical Device

N2 reduction reaction (NRR) by light is an energy-saving and sustainable ammonia (NH3) synthesis technology. However, it faces significant challenges, including high energy barriers of N2 activation and unclear catalytic active sites. Herein, we propose a strategy of photo-driven ammonia synthesis via a proton-mediated photoelectrochemical device. We used redox-catalysis covalent organic framework (COF), with a redox site (-C=O) for H+ reversible storage and a catalytic site (porphyrin Au) for NRR. In the proton-mediated photoelectrochemical device, the COF can successfully store e- and H+ generated by hydrogen oxidation reaction, forming COF-H. Then, these stored e- and H+ can be used for photo-driven NRR (108.97 umol g-1) under low proton concentration promoted by the H-bond network formed between -OH in COF-H and N2 on Au, which enabled N2 hydrogenation and NH3 production, establishing basis for advancing artificial photosynthesis and enhancing ammonia synthesis technology.

Leveraging Iron in the Electrolyte to Improve Oxygen Evolution Reaction Performance: Fundamentals, Strategies, and Perspectives

Electrochemical water splitting is a pivotal technology for storing intermittent electricity from renewable sources into hydrogen fuel. However, its overall energy efficiency is impeded by the sluggish oxygen evolution reaction (OER) at the anode. In the quest to design high-performance anode catalysts for driving the OER under non-acidic conditions, iron (Fe) has emerged as a crucial element. Although the profound impact of adventitious electrolyte Fen+ species on OER catalysis had been reported forty years ago, recent interest in tailoring the electrode-electrolyte interface has spurred studies on the controlled introduction of Fe ions into the electrolyte to improve OER performance. During the catalytic process, scenarios where the rate of Fen+ deposition on a specific host material outruns that of dissolution pave the way for establishing highly efficient and dynamically stable electrochemical interfaces for long-term steady operation. This review systematically summarizes recent endeavors devoted to elucidating the behaviors of in situ Fe(aq.) incorporation, the role of incorporated Fe sites in the OER, and critical factors influencing the interplay between the electrode surface and Fe ions in the electrolyte environment. Finally, unexplored issues related to comprehensively understanding and leveraging the dynamic exchange of Fen+ at the interface for improved OER catalysis are summarized.

Overcoming the Tradeoff Between Reaction Rate and Overpotential in Dinuclear Cobalt Complex Catalyzed Electrochemical Water Oxidation

This study focuses on enhancing the water oxidation reaction (WOR) efficacy of dinuclear cobalt complex catalysts from both kinetic (turnover frequency, TOF) and thermodynamic (overpotential, η) perspectives. For this purpose, we synthesized six dinuclear cobalt complexes 1-6 comprising non-innocent ligands with different electronically active substituents (-OMe (1), -Me (2), -H (3), -F (4), -Cl (5), and -CN (6)). The electronic effects on the electrochemical WOR under neutral, acidic, and alkaline conditions were investigated experimentally and computationally. X-ray crystallography revealed that the valence state of the cobalt complexes was affected by electronic effects, affording different catalytic potentials, as evidenced by electrochemical measurements. Kinetic and spectroscopic studies confirmed that the synergistic effect of catalyst identity and reaction media changes the reaction mechanism of each catalyst. The WOR performance of 1-6 was tunable through ligand electronics and solvent effects, with the log(TOF)-η analysis highlighting an operational η as low as 40 mV. This study provides valuable insights into optimizing WOR catalysis through molecular design and environmental tuning.

Oxygen Evolution Reaction of Amorphous/Crystalline Composites of NiFe(OH)x/NiFe2O4

Orbital structures are strongly correlated with catalytic performance, whereas their regulation strategy is still in pursuit. Herein, the Fe 3d and O 2p orbital hybridization was optimized by controlling the content of amorphous NiFe(OH)x (a-NiFe(OH)x), which was grown in situ on crystalline NiFe2O4 (c-NiFe2O4) using an ultrasonic reduction method. The results of electron energy loss spectroscopy (EELS) and X-ray absorption spectra (XAS) revealed that the Fe-Oa orbital hybridization in a-NiFe(OH)x is effectively strengthened by jointing with the adjacent oxygen (Oc) in c-NiFe2O4, which is further confirmed by the higher antibonding orbital energies based on density functional theory (DFT) calculations. The resultant Oa-Fe-Oc at the composite interface leads to balanced adsorption and desorption energies. Accordingly, the optimal composite with strong Fe 3d-O 2p hybridization results in enhanced OER performance, and the overpotential is 150 mV, lower than that of the pristine sample. This work represents a promising approach to orbital hybridization via the introduction of an amorphous phase to construct highly efficient catalysts.

An MIL-53(FeNiCo) decorated BiVO4 photoanode for efficient photoelectrochemical water oxidation

BiVO4 is considered as one of the important candidate materials for photoelectrochemical water splitting technology. However, the low efficiency of charge separation and poor kinetics of water oxidation limit its performance in PEC water splitting. In this work, a BiVO4/MIL-53(FeNiCo) photoanode was constructed by a facile hydrothermal deposition method, exhibiting excellent water oxidation ability under AM 1.5 G light irradiation, and achieving a photocurrent density of 3.53 mA cm-2 at 1.23 VRHE, which is about 3.2 times that of pure BiVO4. The electrochemical test results demonstrate that the decoration of MIL-53(FeNiCo) could enhance the charge transfer and accelerate the kinetics of water oxidation reaction, thus leading to excellent PEC water splitting performance. The PEC stability is also significantly improved, indicating that MIL-53(FeNiCo) allows BiVO4 to be used for water oxidation for stable PEC water oxidation.

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.

Green H2 Generation from Seawater Deploying a Bifunctional Hetero-Interfaced CoS2-CoFe-Layered Double Hydroxide in an Electrolyzer

This work illustrates the practicality and economic benefits of employing a hetero-interfaced electrocatalyst (CoS2@CoFe-LDH), containing cobalt sulphide and iron-cobalt double-layer hydroxide for large-scale hydrogen generation. Here, the rational synthesis and detailed characterization of the CoS2@CoFe-LDH material to unravel its unique heterostructure are essayed. The CoS2@CoFe-LDH operates as a bifunctional electrocatalyst to trigger both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline seawater (pH 14.0) while showcasing low overpotential requirement for HER (311 mV) and OER (450 mV) at 100 mA cm- 2 current density. The identical CoS2@CoFe-LDH on either electrode in an H-cell setup results in simultaneous H2 and O2 production from seawater with a ≈98% Faradaic efficiency with an applied potential of 1.96V@100 mA cm- 2. Next, this CoS2@CoFe-LDH catalyst is deployed on both sides of a membrane electrode assembly in a one-stack electrolyzer, which retains the intrinsic bifunctional reactivity of the catalyst to generate H2 and O2 in tandem from alkaline seawater with an impeccable energy efficiency (50 kWh kg-1-of-H2). This electrolyzer assembly can be directly linked with a Si-solar cell to produce truly green hydrogen with a solar-to-hydrogen generation efficiency of 15.88%, highlighting the potential of this converting seawater to hydrogen under solar irradiation.

High-Density CuBi2O4 Photocathodes Using Well-Textured Buffer Layers and Their Unassisted Solar Hydrogen Production Performances

Solar hydrogen production using photoelectrochemical (PEC) cells requires the selection of cost-effective materials with high photoactivity and durability. CuBi2O4 photocathodes possess an appropriate bandgap for efficient hydrogen production. However, their performance is limited by poor charge transport and interface voids formed due to the porous structure during annealing, which complicates the deposition of passivation overlayers. To address this, effective suppression of the porous structure in CuBi2O4 is essential. Here, the study proposes the strategic use of an Sb-Cu2O buffer layer with a uniform (111) crystal orientation prior to the electrodeposition of Cu-Bi-O. This buffer layer facilitates 2D film growth during electrodeposition, enhancing Cu supply via out-diffusion from the buffer during annealing. Moreover, the uniform orientation of the buffer layer promotes the crystallization of CuBi2O4, significantly improving charge transport efficiency. By incorporating an Al-ZnO/TiO2 overlayer, the study achieves a photocurrent of 2.56 mA cm-2 at 0 VRHE and an onset potential of 1.04 VRHE, with excellent stability exceeding 60 hours. In a glycerol oxidation reaction coupled with hydrogen production, an unassisted PEC cell with a BiVO4 photoanode demonstrates the highest H2 production (750.5 µmol cm-2) among Cu-based ternary oxides, with 97% Faradaic efficiency over 20 hours while producing DHA, and formic acid.

Edge-doped substituents as an emerging atomic-level strategy for enhancing M-N4-C single-atom catalysts in electrocatalysis of the ORR, OER, and HER

M-N4-C single-atom catalysts (MN4) have gained attention for their efficient use at the atomic level and adjustable properties in electrocatalytic reactions like the ORR, OER, and HER. Yet, understanding MN4's activity origin and enhancing its performance remains challenging. Edge-doped substituents profoundly affect MN4's activity, explored in this study by investigating their interaction with MN4 metal centers in ORR/OER/HER catalysis (Sub@MN4, Sub = B, N, O, S, CH3, NO2, NH2, OCH3, SO4; M = Fe, Co, Ni, Cu). The results show overpotential variations (0 V to 1.82 V) based on Sub and metal centers. S and SO4 groups optimize FeN4 for peak ORR activity (overpotential at 0.48 V) and reduce OER overpotentials for NiN4 (0.48 V and 0.44 V). N significantly reduces FeN4's HER overpotential (0.09 V). Correlation analysis highlights the metal center's key role, with ΔG*H and ΔG*OOH showing mutual predictability (R2 = 0.92). Eg proves a reliable predictor for Sub@CoN4 (ΔG*OOH/ΔG*H, R2 = 0.96 and 0.72). Machine learning with the KNN model aids catalyst performance prediction (R2 = 0.955 and 0.943 for ΔG*OOH/ΔG*H), emphasizing M-O/M-H and the d band center as crucial factors. This study elucidates edge-doped substituents' pivotal role in MN4 activity modulation, offering insights for electrocatalyst design and optimization.

Photoelectrochemical Ethylene Glycol Oxidization Coupled with Hydrogen Generation Using Metal Oxide Photoelectrodes

Photoelectrochemical (PEC) water splitting represents a promising approach for harnessing solar energy and transforming it into storable hydrogen. However, the complicated 4-electron transfer process of water oxidation reaction imposes kinetic limitations on the overall efficiency. Herein, we proposed a strategy by substituting water oxidation with the oxidation of ethylene glycol (EG), which is a hydrolysis byproduct of polyethylene terephthalate (PET) plastic waste. To achieve this, we developed and synthesized BiVO4/NiCo-LDH photoanodes capable of achieving a high Faradaic efficiency (FE) exceeding 85 % for the oxidation of EG to formate in a strongly alkaline environment. The reaction mechanism was further elucidated using in situ FTIR spectroscopy. Additionally, we successfully constructed an unassisted PEC device for EG oxidation and hydrogen generation by pairing the translucent Mo : BiVO4/NiCo-LDH photoanode with a state-of-the-art Cu2O photocathode, resulting in an approximate photocurrent density of 2.3 mA/cm2. Our research not only offers a PEC pathway for converting PET plastics into valuable chemicals but also enables simultaneous hydrogen production.

Synthesis of an imine-type nickel complex and investigation of its electrocatalytic activity for H2 evolution

We report on the synthesis and characterization of an imine-type nickel complex produced via the complexation of an in situ generated 2-(iminomethyl)phenol ligand with NiII ion. The use of this complex as an electrocatalyst for H2 evolution in a DMF solution, with acetic acid as the proton source, was investigated in detail, employing both experimental analyses (electrochemical analysis, spectroscopy analysis) and theoretical analysis (plateau current analysis). The overpotential required for H2 evolution is about 590 mV with a faradaic efficiency of 49% after 3 hours bulk electrolysis, competing with the two-electron reduction of free-imine groups in the ligand. The maximal TOF was estimated to be about 12 860 s-1 on the basis of a CECE mechanism.

Earth: An Oxidative Planet with Limited Atom Resources and Rich Chemistry

Humanity faces an unprecedented survival challenge: climate change, driven by the depletion of natural resources, excessive waste generation, and deforestation. Six out of nine planetary boundaries have been exceeded, signaling that Earth is far from a safe operating space for humanity. In this Viewpoint Article we explore three critical "atomic-molecular" challenges: Earth's limited atomic resources, its oxidative nature, and very rich chemistry. Addressing these requires a transformation in how we produce and consume, emphasizing sustainable practices aligned with the United Nations' 17 goals. The advancement of science and technology has extended human life expectancy and improved quality of life. However, to ensure a sustainable future, we must move towards less oxidative chemical processes, incorporate CH4-CO2 redox chemistry into the circular economy, and transition from a linear, fossil fuel-dependent economy to a circular bioeconomy. Reforestation and the recovery of degraded lands are essential, alongside the shift towards green and sustainable chemistry. Earth's dynamic chemistry is governed by the principles of thermodynamics and kinetics, but science alone is insufficient. Achieving global sustainability requires coordinated economic, political, and social decisions that recognize Earth's limited resources and oxidative nature. Together, these efforts will position humanity to meet the challenges of climate change and secure a sustainable future.

Stabilization of High-Valent Molecular Cobalt Sites through Oxidized Phosphorus in Reduced Graphene Oxide for Enhanced Oxygen Evolution Catalysis

Heterogeneous molecular cobalt (Co) sites represent one type of classical catalytic sites for electrochemical oxygen evolution reaction (OER) in alkaline solutions. There are dynamic equilibriums between Co2+, Co3+ and Co4+ states coupling with OH-/H+ interaction before and during the OER event. Since the emergence of Co2+ sites is detrimental to the OER cycle, the stabilization of high-valent Co sites to shift away from the equilibrium becomes critical and is proposed as a new strategy to enhance OER. Herein, phosphorus (P) atoms were doped into reduced graphene oxide to link molecular Co2+ acetylacetonate toward synthesizing a novel heterogeneous molecular catalyst. By increasing the oxidation states of P heteroatoms, the linked Co sites were spontaneously oxidized from 2+ to 3+ states in a KOH solution through OH- ions coupling at an open circuit condition. With excluding the Co2+ sites, the as-derived Co sites with 3+ initial states exhibited intrinsically high OER activity, validating the effectiveness of the strategy of stabilizing high valence Co sites.

"Double-Use" Strategy for Improving the Photoelectrochemical Performance of BiVO4 Photoanodes Using a Cobalt-Functionalized Polyoxotungstate

Doping and surface-modification are well-established strategies for the performance enhancement of bismuth vanadate (BiVO4) photoanodes in photoelectrochemical (PEC) water splitting devices. Herein, a "double-use" strategy for the development of high-performance BiVO4 photoanodes for solar water splitting is reported, where a molecular cobalt-phosphotungstate (CoPOM = Na10[Co4(H2O)2(PW9O34)2]) is used both as a bulk doping agent as well as a surface-deposited water oxidation cocatalyst. The use of CoPOM for bulk doping of BiVO4 is shown to enhance the electrical conductivity and improve the charge separation efficiency, resulting in the enhancement of the maximum applied-bias photoconversion efficiency (ABPE) by a factor of ∼18 to 0.54% at 0.87 V vs. RHE, as compared to pristine BiVO4 (0.03% at 1.04 V vs. RHE). The ratio of W/Co on the surface of the photoanode is related to the activity and stability. In addition, modification of CoPOM-doped BiVO4 with CoPOM as a surface cocatalyst enhances the hole extraction and improves the water oxidation kinetics, resulting in the overall enhancement of the ABPE to 0.79% (at 0.82 V vs. RHE), i.e., by a factor of ∼26 with respect to pristine BiVO4. This study establishes the "double-use" strategy involving CoPOMs as an effective, straightforward, and easily scalable approach for the development of high-quality photoanodes for solar water splitting and highlights the future potential of utilizing well-designed polyoxometalates as precursors for the synthesis of energy materials.

Structural Regulation of NiFe LDH Under Spontaneous Corrosion to Enhance the Oxygen Evolution Properties

Electrochemical water splitting holds promise for sustainable hydrogen production but restricted by the sluggish reaction kinetics at the anodic oxygen evolution. Herein, we present a room-temperature spontaneous corrosion strategy to convert inexpensive iron (Fe) on iron foam substrates into highly active and stable self-supporting nickel iron layered hydroxide (NiFe LDH) catalysts. The corrosion evolution mechanisms are elucidated combining ex-situ scanning electron microscopy (SEM) and X-ray photo electron spectroscopy (XPS) techniques, demonstrating precise control over the concentration of Ni2+ and reaction time to achieve controllable micro-structures of NiFe LDH. Taking advantage of the self-supporting morphology and hierarchical micro-/nano- structure, the NiFe LDH with optimized Ni2+ concentration and reaction time exhibits significant small overpotentials of 160 mV and 200 mV for the OER at current densities of 10 mA cm-2 and 100 mA cm-2 respectively, showcasing excellent OER activities. Furthermore, this catalyst demonstrates superior reaction kinetics, high electrochemical stability, and excellent integral water splitting performance when coupled with a commercial Pt/C cathode. The energy-efficient, cost-effective, and scalable spontaneous corrosion strategy opens new avenues for the development of high-electrochemical-interface catalysts.

Lanthanum-Promoted Electrocatalyst for the Oxygen Evolution Reaction: Unique Catalyst or Oxide Deconstruction?

A conventional performance metric for electrocatalysts that promote the oxygen evolution reaction (OER) is the current density at a given overpotential. However, the assumption that increased current density at lower overpotentials indicates superior catalyst design is precarious for OER catalysts in the working environment, as the crystalline lattice is prone to deconstruction and amorphization, thus greatly increasing the concentration of catalytic active sites. We show this to be the case for La3+ incorporation into Co3O4. Powder X-ray diffraction (PXRD), Raman spectroscopy and extended X-ray absorption fine structure (EXAFS) reveal smaller domain sizes with decreased long-range order and increased amorphization for La-modified Co3O4. This lattice deconstruction is exacerbated under the conditions of OER as indicated by operando spectroscopies. The overpotential for OER decreases with increasing La3+ concentration, with maximum activity achieved at 17% La incorporation. HRTEM images and electron diffraction patterns clearly show the formation of an amorphous overlayer during OER catalysis that is accelerated with La3+ addition. O 1s XPS spectra after OER show the loss of lattice-oxide and an increase in peak intensities associated with hydroxylated or defective O-atom environments, consistent with Co(O)x(OH)y species in an amorphous overlayer. Our results suggest that improved catalytic activity of oxides incorporated with La3+ ions (and likely other metal ions) is due to an increase in the number of terminal octahedral Co(O)x(OH)y edge sites upon Co3O4 lattice deconstruction, rather than enhanced intrinsic catalysis.

SnS2 Nanoparticles Embedded in BiVO4 Surfaces via Eutectic Decomposition for Enhanced Performance in Photoelectrochemical Water Splitting

BiVO4 has garnered substantial interest as a promising photoanode material for photoelectrochemical water-splitting due to its narrow band gap and appropriate band edge positions for water oxidation. Nevertheless, its practical use has been impeded by poor charge transport and sluggish water oxidation kinetics. Here, a hybrid composite photoanode is fabricated by uniformly embedding SnS2 nanoparticles near the surface of a BiVO4 thin film, creating a type II heterostructure with strong interactions between the nanoparticles and the film for efficient charge separation. This structure forms via eutectic melting during atomic layer deposition of SnS2 with subsequent phase separation between SnS2 and BiVO4 at room temperature, offering greater advantages and flexibilities over conventional exsolution techniques. Furthermore, the SnS2/BiVO4 hybrid composite is coated with a thin amorphous ZnS passivation layer to accelerate charge transfer process and enhance long-term stability. The optimized BiVO4/SnS2/ZnS photoanode exhibits a photocurrent density of 5.44 mA cm-2 at 1.23 V versus RHE, which is 2.73 times higher than that of the BiVO4 photoanode, and a dramatic improvement in photostability retention at 1.23 V versus RHE, increasing from 55% to 91% over 24 hours. This method of anchoring nanoparticles onto host materials proves highly valuable for energy and environmental applications.

Rechargeable Hydrogen Gas Batteries: Fundamentals, Principles, Materials, and Applications

The growing demand for renewable energy sources has accelerated a boom in research on new battery chemistries. Despite decades of development for various battery types, including lithium-ion batteries, their suitability for grid-scale energy storage applications remains imperfect. In recent years, rechargeable hydrogen gas batteries (HGBs), utilizing hydrogen catalytic electrode as anode, have attracted extensive academic and industrial attention. HGBs, facilitated by appropriate catalysts, demonstrate notable attributes such as high power density, high capacity, excellent low-temperature performance, and ultralong cycle life. This review presents a comprehensive overview of four key aspects pertaining to HGBs: fundamentals, principles, materials, and applications. First, detailed insights are provided into hydrogen electrodes, encompassing electrochemical principles, hydrogen catalytic mechanisms, advancements in hydrogen catalytic materials, and structural considerations in hydrogen electrode design. Second, an examination and future prospects of cathode material compatibility, encompassing both current and potential materials, are summarized. Third, other components and engineering considerations of HGBs are elaborated, including cell stack design and pressure vessel design. Finally, a techno-economic analysis and outlook offers an overview of the current status and future prospects of HGBs, indicating their orientation for further research and application advancements.