Monolayer NiIr-Layered Double Hydroxide as a Long-Lived Efficient Oxygen Evolution Catalyst for Seawater Splitting

V-Doping induced surface electron modulation and nanostructure design for Ni(OH)2/GO towards efficient urea electro-oxidation

The V-α-Ni(OH)2/GO nanoarrays prepared by simple coprecipitation show excellent catalytic properties in urea electro-oxidation, ascribed to the dual modulation of d-orbital electron regulation and ultrathin hierarchical nanostructure construction, which is caused by the introduction of V.

Upcycling of Spent LiFePO4 Cathodes to Heterostructured Electrocatalysts for Stable Direct Seawater Splitting

The pursuit of carbon-neutral energy has intensified the interest in green hydrogen production from direct seawater electrolysis, given the scarcity of freshwater resources. While Ni-based catalysts are known for their robust activity in alkaline water oxidation, their catalytic sites are prone to rapid degradation in the chlorine-rich environments of seawater, leading to limited operation time. Herein, we report a Ni(OH)2 catalyst interfaced with laser-ablated LiFePO4 (Ni(OH)2/L-LFP), derived from spent Li-ion batteries (LIBs), as an effective and stable electrocatalyst for direct seawater oxidation. Our comprehensive analyses reveal that the PO4 3- species, formed around L-LFP, effectively repels Cl- ions during seawater oxidation, mitigating corrosion. Simultaneously, the interface between in situ generated NiOOH and Fe3(PO4)2 enhances OH- adsorption and electron transfer during the oxygen evolution reaction. This synergistic effect leads to a low overpotential of 237 mV to attain a current density of 10 mA cm-2 and remarkable durability, with only a 3.3 % activity loss after 600 h at 100 mA cm-2 in alkaline seawater. Our findings present a viable strategy for repurposing spent LIBs into high-performance catalysts for sustainable seawater electrolysis, contributing to the advancement of green hydrogen production technologies.

Optimizing the Intermediates Adsorption by Manipulating the Second Coordination Shell of Ir Single Atoms for Efficient Water Oxidation

The precise regulation of single-atom catalysts (SACs) with the desired local chemical environment is vital to elucidate the relationship between the SACs structure and the catalytic performance. The debate on the effect of the local coordination environment is quite complicated even for the SACs with the same composition and chemical nature, calling for increased attention on the regulation of the second coordination shell. For oxide-supported SACs, it remains a significant challenge to precisely manipulate the second coordination shell of single atoms supported on oxides due to the structural robustness of oxides. Here, Ir single atoms were anchored on NiO supports via different bonding strategies, resulting in the diverse Ir-O-Ni coordination numbers for Ir sites. Specifically, Ir1/NiO, Ir1-NiO, and Ir1@NiO SACs with increasing Ir-O-Ni coordination numbers of 3, 4, and 5 were synthesized, respectively. We found that the activity of the three samples towards oxygen evolution reaction (OER) exhibited a volcano-shaped relationship with the Ir-O-Ni coordination number, with Ir1-NiO showing the lowest overpotential of 225 mV at 10 mA cm-2. Mechanism investigations indicate that the moderate coordination number of Ir-O-Ni in Ir1-NiO creates the higher occupied Ir dz2 orbital, weakening the adsorption strength for *OOH intermediates and thereby enhancing the OER activity.

The Construction of Face-to-Face Combination between NiFe-layered Double Hydroxide Nanosheets and Monolayer rGO for Efficient Water Splitting and Oxygen Reduction

Developing cost-effective and efficient electrocatalysts is essential for advancing a green energy future. Herein, a NiFe-layered double hydroxide loaded on reduced graphene oxide (NiFe-LDHs@rGO) hybrid was synthesized using a straightforward three-step process involving exfoliation tearing, electrostatic self-assembly, and chemical reduction. The face-to-face packing and ultrathin exfoliation enable strong heterogeneous interactions, fully harnessing the potential of these complementary two-dimensional counterparts. Consequently, the resultant catalyst displays outstanding oxygen evolution reaction (OER) catalytic activity and stability, whose overpotential is as low as 241 mV at 30 mA cm-2 and 255 mV at 50 mA cm-2 with a low Tafel slope of 62.1 mV dec-1. Both the experimental results and density functional theory (DFT) calculations reveal that the face-to-face assembly strengthens the electronic interactions between NiFe-LDHs and rGO, which effectively modulates the d-band center of Ni and Fesites and improves the reaction kinetics for OER. Moreover, the resultant NiFe-LDHs@rGO hybrids exhibit excellent multifunctional catalytic performance. Its hydrogen evolution reaction (HER) activity is endowed by Fe-site of NiFe-LDHs and defect states rGO and achieves a low voltage of 1.68 V to drive a current density of 10 mA cm-2 for overall water splitting. The face-to-face heteroassembly also imparts NiFe-LDHs@rGO with superior oxygen reduction reaction (ORR) activity, with a half-wave potential of 0.70 V and a limiting current density of 4.2 mA cm-2. Its ORR primarily follows a four-electron transfer pathway with a minor contribution from a two-electron process. This study establishes the groundwork for optimizing two-dimensional heterogeneous interfaces in LDH@carbon-based materials for advanced energy conversion.

Citrate ions-modified NiFe layered double hydroxide for durable alkaline seawater oxidation

Seawater electrolysis taking advantage of coastal/offshore areas is acknowledged as a potential way of large-scale producing H2 to substitute traditional technology. However, anodic catalysts with high overpotentials and limited lifespans (caused by chloride-induced competitive chemical reactions) hinder the system of seawater electrolysis for H2 production. Herein, we present a citrate anion (CA) modified NiFe layered double hydroxide nanosheet array on nickel foam (NiFe LDH@NiFe-CA/NF), which serves as an efficient and stable electrocatalyst towards long-term alkaline seawater oxidation. It requires only a low overpotential of 387 mV to achieve a current density of 1000 mA cm-2, outperforming NiFe LDH/NF (414 mV). Moreover, NiFe LDH@NiFe-CA/NF exhibits continuous oxygen evolution testing for 300 h at 1000 mA cm-2 due to its anti-corrosion characterization. Additionally, the fabricated cell can stably operate at 300 mA cm-2 (60 °C, 6 M KOH + seawater) and only require 1.69 V, achieving low energy consumption of seawater splitting.

Vanadium-Doped Heterogeneous Bimetallic Phosphides Derived from Layered Double Hydroxides for Saline Water Splitting

The development of energy- and time-saving synthetic methods to prepare bifunctional and high stability catalysts are vital for overall water splitting. Here, V-doped nickel-iron hydroxide precursor by etching NiFe foam (NFF) at room temperature with dual chloride solution ("NaCl-VCl3"), is obtained then phosphating to obtain V-Ni2P-FeP/NFF as efficient bifunctional (oxygen/hydrogen exchange reaction, OER/HER) electrocatalysts, denoted as NFF(V, Na)-P. The NFF(V, Na)-P requires only 185 and 117 mV overpotentials to reach 10 mA cm-2 for OER and HER. When used as a catalyst for water splitting in a full cell, it can be stably sustained for more than 1000 h in alkaline brine electrolysis at both current densities of 100 and 500 mA cm-2. In situ Raman analyses and density functional theory (DFT) show that the V-doping-induced surface remodeling generates hydroxyl oxides as the true catalytic active centers, which not only enhances the reaction kinetics, but also reduces the free energy change in the rate-determining step. This work provides a cost-effective substrate self-derivation method to convert commercial NFF into a powerful catalyst for electrolytic brine, offering a unique route to the development of efficient electrocatalysts for saline water splitting.

Unlocking Potential of Pyrochlore in Energy Systems via Soft Voting Ensemble Learning

In traditional machine learning (ML)-based material design, the defects of low prediction accuracy, overfitting and low generalization ability are mainly caused by the training of a single ML model. Here, a Soft Voting Ensemble Learning (SVEL) approach is proposed to solve the above issues by integrating multiple ML models in the same scene, thus pursuing more stable and reliable prediction. As a case study, SVEL is applied to develop the broad chemical space of novel pyrochlore electrocatalysts with the molecular formula of A2B2O7, to explore promising pyrochlore oxides and accelerate predictions of unknown pyrochlore in the periodic table. The model successfully established the structure-property relationship of pyrochlore, and selected six cost-effective pyrochlore from the periodic table with a high prediction accuracy of 91.7%, all of which showed good electrocatalytic performance. SVEL not only effectively avoids the high costs of experimentation and lengthy computations, but also addresses biases arising from data scarcity in single models. Furthermore, it has significantly reduced the research cycle of pyrochlore by ≈ 22 years, offering broad prospects for accelerating the development of materials genomics. SVEL method is intended to integrate multiple AI models to provide broader model training clues for the AI material design community.

Honeycomb-Structured IrOx Foam Platelets as the Building Block of Anode Catalyst Layer in PEM Water Electrolyzer

Achieving robust long-term durability with high catalytic activity at low iridium loading remains one of great challenges for proton exchange membrane water electrolyzer (PEMWE). Herein, we report the low-temperature synthesis of iridium oxide foam platelets comprising edge-sharing IrO6 octahedral honeycomb framework, and demonstrate the structural advantages of this material for multilevel tuning of anodic catalyst layer across atomic-to-microscopic scales for PEMWE. The integration of IrO6 octahedral honeycomb framework, foam-like texture and platelet morphology into a single material system assures the generation and exposure of highly active and stable iridium catalytic sites for the oxygen evolution reaction (OER), while facilitating the reduction of both mass transport loss and electronic resistance of catalyst layer. As a proof of concept, the membrane electrode assembly in single-cell PEMWE based on honeycomb-structured IrOx foam platelets, with a low iridium loading (~0.3 mgIr/cm2), is demonstrated to exhibit high catalytic activity at ampere-level current densities and to remain stable for more than 2000 hours.

Construction of "Metal Defect/Oxygen Defect Junction" in ZnFe2O4-NiCo2O4 Heterostructures for Enhancing Electrocatalytic Oxygen Evolution

Defect engineering is a promising approach to improve the conductivity and increase the active sites of transition metal oxides used as catalysts for the oxygen evolution reaction (OER). However, when metal defects and oxygen defects coexist closely within the same crystal, their compensating charges can diminish the benefits of both defect structures on the catalyst's local electronic structure. To address this limitation, a novel strategy that employs the heterostructure interface of ZnFe2O4-NiCo2O4 to spatially separate the metal defects from the oxygen defects is proposed. This configuration positions the two types of defects on opposite sides of the heterojunction interface, creating a unique structure termed the "metal-defect/oxygen-defect junction". Physical characterization and simulations reveal that this configuration enhances electron transfer at the heterostructure interface, increases the oxidation state of Fe on the catalyst surface, and boosts bulk charge carrier concentration. These improvements enhance active site performance, facilitating hydroxyl adsorption and deprotonation, thereby reducing the overpotential required for the OER.

Zn-facilitated surface reconstruction of Ni-MOF for an enhanced oxygen evolution reaction

Facilitating the surface reconstruction of pre-catalysts has been considered an effective strategy for constructing low-cost and highly efficient OER electrocatalysts. Metal doping is a feasible way to activate the surface reconstruction, thus enhancing the OER performance. Herein, we report a facile hydrothermal method to synthesize a series of Zn-doped Ni-MOF on nickel foam (NiZn-MOF/NF) as promising pre-catalysts toward the oxygen evolution reaction (OER). The Zn leaching of NiZn-MOF/NF can promote the surface self-reconstruction of NiZn-MOF/NF into oxygen-vacancy-rich NiOOH after electrochemical activation. Benefiting from the optimized electronic structure, abundant defects, more accessible active sites, and enhanced electrical conductivity, the reconstructed metal oxyhydroxide hybrids exhibit better electrocatalytic activity than the catalysts transformed from Ni-MOF/NF without Zn doping. The optimized NiZn-MOF/NF-OH as an OER catalyst has an overpotential of 336 mV at 100 mA cm-2, and a Tafel slope of 65.9 mV dec-1, as well as stability over 12 h. This work reveals that Zn cation-doping/leaching induces the surface reconstruction of pre-catalysts for enhanced oxygen catalytic activity, which provides a new approach for the development of advanced electrocatalysts.

Progress in Anode Stability Improvement for Seawater Electrolysis to Produce Hydrogen

Seawater electrolysis for hydrogen production is a sustainable and economical approach that can mitigate the energy crisis and global warming issues. Although various catalysts/electrodes with excellent activities have been developed for high-efficiency seawater electrolysis, their unsatisfactory durability, especially for anodes, severely impedes their industrial applications. In this review, attention is paid to the factors that affect the stability of anodes and the corresponding strategies for designing catalytic materials to prolong the anode's lifetime. In addition, two important aspects-electrolyte optimization and electrolyzer design-with respect to anode stability improvement are summarized. Furthermore, several methods for rapid stability assessment are proposed for the fast screening of both highly active and stable catalysts/electrodes. Finally, perspectives on future investigations aimed at improving the stability of seawater electrolysis systems are outlined.

Complementary Multisite Turnover Catalysis toward Superefficient Bifunctional Seawater Splitting at Ampere-Level Current Density

The utilization of seawater for hydrogen production via water splitting is increasingly recognized as a promising avenue for the future. The key dilemma for seawater electrolysis is the incompatibility of superior hydrogen- and oxygen-evolving activities at ampere-scale current densities for both cathodic and anodic catalysts, thus leading to large electric power consumption of overall seawater splitting. Here, in situ construction of Fe4N/Co3N/MoO2 heterostructure arrays anchoring on metallic nickel nitride surface with multilevel collaborative catalytic interfaces and abundant multifunctional metal sites is reported, which serves as a robust bifunctional catalyst for alkaline freshwater/seawater splitting at ampere-level current density. Operando Raman and X-ray photoelectron spectroscopic studies combined with density functional theory calculations corroborate that Mo and Co/Fe sites situated on the Fe4N/Co3N/MoO2 multilevel interfaces optimize the reaction pathway and coordination environment to enhance water adsorption/dissociation, hydrogen adsorption, and oxygen-containing intermediate adsorption, thus cooperatively expediting hydrogen/oxygen evolution reactions in base. Inspiringly, this electrocatalyst can substantially ameliorate overall freshwater/seawater splitting at 1000 mA cm-2 with low cell voltages of 1.65/1.69 V, along with superb long-term stability at 500-1500 mA cm-2 for over 200 h, outperforming nearly all the ever-reported non-noble electrocatalysts for freshwater/seawater electrolysis. This work offers a viable approach to design high-performance bifunctional catalysts for seawater splitting.

Confined Growth of Highly Ordered Metal Atomic Arrays for Seawater Oxidation

Metal atom catalysts have been among the most important research objects due to their specific physical and chemical properties. However, precise control of the anchoring of metal atoms is still challenging to achieve. Cobalt and iridium atomic arrays formed sequentially ordered stable arrays in graphdiyne (GDY) triangular cavities depending on their intrinsic chemical properties and interactions. The success of this method was attributed to multifunctional integration of GDY, enabling selective growth from one to several atoms and various atomic densities. The bimetallic atom arrays show several advantages resulting from reducibility of acetylene bonds, space limiting effect, incomplete charge transfer between GDY and metal atoms, and sp-C hybridized triple bond skeleton. This well-designed system exhibits unprecedented oxygen evolution reaction (OER) performance with a mass activity of 2.6 A mgcat. -1 at a low overpotential of 300 mV, which is 216.6 times higher than the state-of-the-art IrO2 catalyst, and long-term stability.

Nickel-Molybdenum-Based Three-Dimensional Nanoarrays for Oxygen Evolution Reaction in Water Splitting

Water splitting is an important approach to hydrogen production. But the efficiency of the process is always controlled by the oxygen evolution reaction process. In this study, a three-dimensional nickel-molybdenum binary nanoarray microstructure electrocatalyst is successfully synthesized. It is grown uniformly on Ni foam using a hydrothermal method. Attributed to their unique nanostructure and controllable nature, the Ni-Mo-based nanoarray samples show superior reactivity and durability in oxygen evolution reactions. The series of Ni-Mo-based electrocatalysts presents a competitive overpotential of 296 mV at 10 mA·cm-2 for an OER in 1.0 M KOH, corresponding with a low Tafel slope of 121 mV dec-1. The three-dimensional nanostructure has a large double-layer capacitance and plenty of channels for ion transfer, which demonstrates more active sites and improved charge transmission. This study provides a valuable reference for the development of non-precious catalysts for water splitting.

Ru-regulated electronic structure CoNi-MOF nanosheets advance water electrolysis kinetics in alkaline and seawater media

Herein, an ion-exchange strategy is utilized to greatly improve the kinetics of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by Ru-modified CoNi- 1,3,5-Benzenetricarboxylic acid (BTC)-metal organic framework nanosheets (Ru@CoNi-MOF). Due to the higher Ni active sites and lower electron transfer impedance, Ru@CoNi-MOF catalyst requires the overpotential as low as 47 and 279 mV, at a current density of 10 mA/cm2 toward HER and OER, respectively. Significantly, the mass activity of Ru@CoNi-MOF for HER and OER are 25.9 and 10.6 mA mg-1, nearly 15.2 and 8.8 times higher than that of Ni-MOF. In addition, the electrolyzer of Ru@CoNi-MOF demonstrates exceptional electrolytic performance in both KOH and seawater environment, surpasses the commercial Pt/C||IrO2 couple. Theoretical calculations prove that introducing Ru atoms in - CoNi-MOF modulates the electronic structure of Ni, optimizes adsorption energy for H* and reduces energy barrier of metal organic frameworks (MOFs). This modification significantly improves the kinetic rate of the Ru@CoNi-MOF during water splitting. Certainly, this study highlights the utilization of MOF nanosheets as advanced HER/OER electrocatalysts with immense potential, and will paves a way to develop more efficient MOFs for catalytic applications.

Boron modification promoting electrochemical surface reconstruction of NiFe-LDH for efficient and stable freshwater/seawater oxidation catalysis

Transition metal-based electrocatalysts generally take place surface reconstruction in alkaline conditions, but little is known about how to improve the reconstruction to a highly active oxyhydroxide surface for an efficient and stable oxygen evolution reaction (OER). Herein, we develop a strategy to accelerate surface reconstruction by combining boron modification and cyclic voltammetry (CV) activation. Density functional theory calculations and in-situ/ex-situ characterizations indicate that both B-doping and electrochemical activation can reduce the energy barrier and contribute to the surface evolution into highly active oxyhydroxides. The formed oxyhydroxide active phase can tune the electronic configuration and boost the OER process. The reconstructed catalyst of CV-B-NiFe-LDH displays excellent alkaline OER performance in freshwater, simulated seawater, and natural seawater with low overpotentials at 100 mA cm-2 (η100: 219, 236, and 255 mV, respectively) and good durability. This catalyst also presents outstanding Cl- corrosion resistance in alkalized seawater electrolytes. The CV-B-NiFe-LDH||Pt/C electrolyzer reveals prominent performance for alkalized freshwater/seawater splitting. This study provides a guideline for developing advanced OER electrocatalysts by promoting surface reconstruction.

Optimized Transition Metal Phosphides for Direct Seawater Electrolysis: Current Trends

Seawater electrolysis presents a viable route for sustainable large-scale hydrogen production, yet its practical application is hindered by several technical challenges. These include the sluggish kinetics of hydrogen evolution, poor stability, cation deposition at the cathode, electrode corrosion, and competing chloride oxidation at the anode. To overcome these obstacles, the development of innovative electrocatalysts is crucial. Transition metal phosphides (TMPs) have emerged as promising candidates owing to their superior catalytic performance and tunable structural properties. This review provides a comprehensive analysis of recent progress in the structural engineering of TMPs tailored for efficient seawater electrolysis. We delve into the catalytic mechanisms underpinning hydrogen and oxygen evolution reactions in different pH conditions, along with the detrimental side reactions that impede hydrogen production efficiency. Several methods to prepare TMPs are then introduced. Additionally, detailed discussions on structural modifications and interface engineering tactics are presented, showcasing strategies to enhance the activity and durability of TMP electrocatalysts. By analyzing current research findings, our review aims to inform ongoing research endeavors and foster advancements in seawater electrolysis for practical and ecologically sound hydrogen generation.

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.

Simultaneous Interface Engineering and Phase Tuning of CeO2-Decorated Catalysts for Boosted Oxygen Evolution Reaction

Heterogeneous catalysts have attracted extensive attention among various emerging catalysts for their exceptional oxygen evolution reaction (OER) capabilities, outperforming their single-component counterparts. Nonetheless, the synthesis of heterogeneous materials with predictable, precise, and facile control remains a formidable challenge. Herein, a novel strategy involving the decoration of catalysts with CeO2 is introduced to concurrently engineer heterogeneous interfaces and adjust phase composition, thereby enhancing OER performance. Theoretical calculations suggest that the presence of ceria reduces the free energy barrier for the conversion of nitrides into metals. Supporting this, the experimental findings reveal that the incorporation of rare earth oxides enables the controlled phase transition from nitride into metal, with the proportion adjustable by varying the amount of added rare earth. Thanks to the role of CeO2 decoration in promoting the reaction kinetics and fostering the formation of the genuine active phase, the optimized Ni3FeN/Ni3Fe/CeO2-5% nanoparticles heterostructure catalyst exhibits outstanding OER activity, achieving an overpotential of just 249 mV at 10 mA cm-2. This approach offers fresh perspectives for the conception of highly efficient heterogeneous OER catalysts, contributing a strategic avenue for advanced catalytic design in the field of energy conversion.

Regioselective super-assembly of Prussian blue analogue

The construction of high-asymmetrical structures demonstrates significant potential in improving the functionality and distinctness of nanomaterials, but remains a considerable challenge. Herein, we develop a one-pot method to fabricate regioselective super-assembly of Prussian blue analogue (PBA) -- a PBA anisotropic structure (PBA-AS) decorated with epitaxial modules--using a step-by-step epitaxial growth on a rapidly self-assembled cubic substrate guided by thiocyanuric acid (TCA) molecules. The epitaxial growth units manifest as diverse geometric shapes, which are predominantly concentrated on the {100}, {111}, or {100}+{111} crystal plane of the cubic substrate. The crystal plane and morphology of epitaxial module can be regulated by changing the TCA concentration and reaction temperature, enabling a high level of controllability over specific assembly sites and structures. To illustrate the advantage of the asymmetrical structure, phosphated PBA-AS demonstrates improved performance in the oxygen evolution reaction compared to simple phosphated PBA nanocube. This method offers valuable insights for designing asymmetrical nanomaterials with intricate architectures and versatile functionalities.

Manipulating Dual-Metal Catalytic Activities toward Organic Upgrading in Upcycling Plastic Wastes with Inhibited Oxygen Evolution

Electrorefinery of polybutylene terephthalate (PBT) waste plastic, specifically conversion of a PBT-derived 1,4-butanediol (BDO) monomer into value-added succinate coupled with H2 production, emerges as an auspicious strategy to mitigate severe plastic pollution. Herein, we report the synthesis of Mn-doped NiNDA nanosheets (NDA: 2,6-naphthalenedicarboxylic acid), a metal-organic framework (MOF) through a ligand exchange method, and its utilization for electrocatalytic BDO oxidation to succinate. Interestingly, the transformation of doped layered-hydroxide (d-LH) precursors to MOF promotes BDO oxidation while hindering the competitive oxygen evolution reaction. Experimental and theoretical results indicate that the MOF has a higher affinity (i.e., alcoholophilic) for BDO than the d-LH, while Mn doping into NiNDA results in electron accumulation at Ni sites with an upward shift in the d-band center and convenient spin-dependent charge transfer, which are all beneficial for BDO oxidation. The as-constructed two-electrode membrane-electrode assembly (MEA) flow cell, by coupling BDO oxidation and hydrogen evolution reaction, attains an industrial current density of 1.5 A cm-2@1.82 V at 50 °C, corresponding to a specific energy consumption of 3.68 kWh/Nm3 H2. This represents an energy saving of >25% for hydrogen production on an industrial scale compared to conventional water electrolysis (∼5 kWh/Nm3 H2) in addition to the production of valuable chemicals.

Engineering interfacial sulfur migration in transition-metal sulfide enables low overpotential for durable hydrogen evolution in seawater

Hydrogen production from seawater remains challenging due to the deactivation of the hydrogen evolution reaction (HER) electrode under high current density. To overcome the activity-stability trade-offs in transition-metal sulfides, we propose a strategy to engineer sulfur migration by constructing a nickel-cobalt sulfides heterostructure with nitrogen-doped carbon shell encapsulation (CN@NiCoS) electrocatalyst. State-of-the-art ex situ/in situ characterizations and density functional theory calculations reveal the restructuring of the CN@NiCoS interface, clearly identifying dynamic sulfur migration. The NiCoS heterostructure stimulates sulfur migration by creating sulfur vacancies at the Ni3S2-Co9S8 heterointerface, while the migrated sulfur atoms are subsequently captured by the CN shell via strong C-S bond, preventing sulfide dissolution into alkaline electrolyte. Remarkably, the dynamically formed sulfur-doped CN shell and sulfur vacancies pairing sites significantly enhances HER activity by altering the d-band center near Fermi level, resulting in a low overpotential of 4.6 and 8 mV at 10 mA cm-2 in alkaline freshwater and seawater media, and long-term stability up to 1000 h. This work thus provides a guidance for the design of high-performance HER electrocatalyst by engineering interfacial atomic migration.

Next-Generation Green Hydrogen: Progress and Perspective from Electricity, Catalyst to Electrolyte in Electrocatalytic Water Splitting

Green hydrogen from electrolysis of water has attracted widespread attention as a renewable power source. Among several hydrogen production methods, it has become the most promising technology. However, there is no large-scale renewable hydrogen production system currently that can compete with conventional fossil fuel hydrogen production. Renewable energy electrocatalytic water splitting is an ideal production technology with environmental cleanliness protection and good hydrogen purity, which meet the requirements of future development. This review summarizes and introduces the current status of hydrogen production by water splitting from three aspects: electricity, catalyst and electrolyte. In particular, the present situation and the latest progress of the key sources of power, catalytic materials and electrolyzers for electrocatalytic water splitting are introduced. Finally, the problems of hydrogen generation from electrolytic water splitting and directions of next-generation green hydrogen in the future are discussed and outlooked. It is expected that this review will have an important impact on the field of hydrogen production from water.

Recent Developments in Nickel-Based Layered Double Hydroxides for Photo(-/)electrocatalytic Water Oxidation

Layered double hydroxides (LDHs), especially those containing nickel (Ni), are increasingly recognized for their potential in photo(-/)electrocatalytic water oxidation due to the abundant availability of Ni, their corrosion resistance, and their minimal toxicity. This review provides a comprehensive examination of Ni-based LDHs in electrocatalytic (EC), photocatalytic (PC), and photoelectrocatalytic (PEC) water oxidation processes. The review delves into the operational principles, highlighting similarities and distinctions as well as the benefits and limitations associated with each method of water oxidation. It includes a detailed discussion on the synthesis of monolayer, ultrathin, and bulk Ni-based LDHs, focusing on the merits and drawbacks inherent to each synthesis approach. Regarding the EC oxygen evolution reaction (OER), strategies to improve catalytic performance and insights into the structural evolution of Ni-based LDHs during the electrocatalytic process are summarized. Furthermore, the review extensively covers the advancements in Ni-based LDHs for PEC OER, including an analysis of semiconductors paired with Ni-based LDHs to form photoanodes, with a focus on their enhanced activity, stability, and underlying mechanisms facilitated by LDHs. The review concludes by addressing the challenges and prospects in the development of innovative Ni-based LDH catalysts for practical applications. The comprehensive insights provided in this paper will not only stimulate further research but also engage the scientific community, thus driving the field of photo(-/)electrocatalytic water oxidation forward.

Electronic Modulation in Cu Doped NiCo LDH/NiCo Heterostructure for Highly Efficient Overall Water Splitting

Layered double hydroxides (LDHs), promising bifunctional electrocatalysts for overall water splitting, are hindered by their poor conductivity and sluggish electrochemical reaction kinetics. Herein, a hierarchical Cu-doped NiCo LDH/NiCo alloy heterostructure with rich oxygen vacancies by electronic modulation is tactfully designed. It extraordinarily effectively drives both the oxygen evolution reaction (151 mV@10 mA cm-2) and the hydrogen evolution reaction (73 mV@10 mA cm-2) in an alkaline medium. As bifunctional electrodes for overall water splitting, a low cell voltage of 1.51 V at 10 mA cm-2 and remarkable long-term stability for 100 h are achieved. The experimental and theoretical results reveal that Cu doping and NiCo alloy recombination can improve the conductivity and reaction kinetics of NiCo LDH with surface charge redistribution and reduced Gibbs free energy barriers. This work provides a new inspiration for further design and construction of nonprecious metal-based bifunctional electrocatalysts based on electronic structure modulation strategies.

Unraveling the crucial contribution of additive chromate to efficient and stable alkaline seawater oxidation on Ni-based layered double hydroxides

Seawater electrolysis to generate hydrogen offers a clean, green, and sustainable solution for new energy. However, the catalytic activity and durability of anodic catalysts are plagued by the corrosion and competitive oxidation reactions of chloride in high concentrations. In this study, we find that the additive CrO42- anions in the electrolyte can not only promote the formation and stabilization of the metal oxyhydroxide active phase but also greatly mitigate the adverse effect of Cl- on the anode. Linear sweep voltammetry, accelerated corrosion experiments, corrosion polarization curves, and charge transfer resistance results indicate that the addition of CrO42- distinctly improves oxygen evolution reaction (OER) kinetics and corrosion resistance in alkaline seawater electrolytes. Especially, the introduction of CrO42- even in the highly concentrated NaCl (2.5 M) electrolyte prolongs the durability of NiFe-LDH to almost five times the case without CrO42-. Density functional theory calculations also reveal that the adsorption of CrO42- can tune the electronic configuration of active sites of metal oxyhydroxides, enhance conductivity, and optimize the intermediate adsorption energies. This anionic additive strategy can give a better enlightenment for the development of efficient and stable oxygen evolution reactions for seawater electrolysis.

Engineering highly selective CO2 electroreduction in Cu-based perovskites through A-site cation manipulation

Perovskites exhibit considerable potential as catalysts for various applications, yet their performance modulation in the carbon dioxide reduction reaction (CO2RR) remains underexplored. In this study, we report a strategy to enhance the electrocatalytic carbon dioxide (CO2) reduction activity via Ce-doped La2CuO4 (LCCO) and Sr-doped La2CuO4 (LSCO) perovskite oxides. Specifically, compared to pure phase La2CuO4 (LCO), the Faraday efficiency (FE) for CH4 of LCCO at -1.4 V vs. RHE (reversible hydrogen electrode) is improved from 38.9% to 59.4%, and the FECO2RR of LSCO increased from 68.8% to 85.4%. In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy spectra results indicate that the doping of A-site ions promotes the formation of *CHO and *HCOO, which are key intermediates in the production of CH4, compared to the pristine La2CuO4. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and double-layer capacitance (Cdl) outcomes reveal that heteroatom-doped perovskites exhibit more oxygen vacancies and higher electrochemical active surface areas, leading to a significant improvement in the CO2RR performance of the catalysts. This study systematically investigates the effect of A-site ion doping on the catalytic activity center Cu and proposes a strategy to improve the catalytic performance of perovskite oxides.

In-situ direct seawater electrolysis using floating platform in ocean with uncontrollable wave motion

Direct hydrogen production from inexhaustible seawater using abundant offshore wind power offers a promising pathway for achieving a sustainable energy industry and fuel economy. Various direct seawater electrolysis methods have been demonstrated to be effective at the laboratory scale. However, larger-scale in situ demonstrations that are completely free of corrosion and side reactions in fluctuating oceans are lacking. Here, fluctuating conditions of the ocean were considered for the first time, and seawater electrolysis in wave motion environment was achieved. We present the successful scaling of a floating seawater electrolysis system that employed wind power in Xinghua Bay and the integration of a 1.2 Nm3 h-1-scale pilot system. Stable electrolysis operation was achieved for over 240 h with an electrolytic energy consumption of 5 kWh Nm-3 H2 and a high purity (>99.9%) of hydrogen under fluctuating ocean conditions (0~0.9 m wave height, 0~15 m s-1 wind speed), which is comparable to that during onshore water electrolysis. The concentration of impurity ions in the electrolyte was low and stable over a long period of time under complex and changing scenarios. We identified the technological challenges and performances of the key system components and examined the future outlook for this emerging technology.

Self-protecting CoFeAl-layered double hydroxides enable stable and efficient brine oxidation at 2 A cm-2

Low-energy consumption seawater electrolysis at high current density is an effective way for hydrogen production, however the continuous feeding of seawater may result in the accumulation of Cl-, leading to severe anode poisoning and corrosion, thereby compromising the activity and stability. Herein, CoFeAl layered double hydroxide anodes with excellent oxygen evolution reaction activity are synthesized and delivered stable catalytic performance for 350 hours at 2 A cm-2 in the presence of 6-fold concentrated seawater. Comprehensive analysis reveals that the Al3+ ions in electrode are etched off by OH- during oxygen evolution reaction process, resulting in M3+ vacancies that boost oxygen evolution reaction activity. Additionally, the self-originated Al(OH)n- is found to adsorb on the anode surface to improve stability. An electrode assembly based on a micropore membrane and CoFeAl layered double hydroxide electrodes operates continuously for 500 hours at 1 A cm-2, demonstrating their feasibility in brine electrolysis.

A self-supported porous NiMo electrocatalyst to boost the catalytic activity in the hydrogen evolution reaction

To develop hydrogen energy production and address the issues of global warming, inexpensive, effective, and long-lasting transition metal-based electrocatalysts for the synthesis of hydrogen are crucial. Herein, a porous electrocatalyst NiMo/Ni/NF was successfully constructed by a two-step electrodeposition process, and was used in the hydrogen evolution reaction (HER) of electrocatalytic water decomposition. NiMo nanoparticles were coated on porous Ni/NF grown on nickel foam (NF), leading to a resilient porous structure with enhanced conductivity for efficient charge transfer, as well as distinctive three-dimensional channels for quick electrolyte diffusion and gas release. Notably, the low overpotential (42 mV) and fast kinetics (Tafel slope of 44 mV dec-1) at a current density of 10 mA cm-2 in 1.0 M KOH solution demonstrate the excellent HER activity of the electrode, which was superior to that of recently reported non-noble metal-based catalysts. Additionally, NiMo/Ni/NF showed extraordinary catalytic durability in stability tests at a current density of 10 mA cm-2 for 70 h. The porous structure catalyst and the electrodeposition-electrocatalysis technique examined in this study offer new approaches for the advancement of the electrocatalysis field because of these benefits.

Regulation of the d-band center of metal-organic frameworks for energy-saving hydrogen generation coupled with selective glycerol oxidation

The hybrid electrolyzer coupled glycerol oxidation (GOR) with hydrogen evolution reaction (HER) is fascinating to simultaneously generate H2 and high value-added chemicals with low energy input, yet facing a challenge. Herein, Cu-based metal-organic frameworks (Cu-MOFs) are reported as model catalysts for both HER and GOR through doping of atomically dispersed precious and nonprecious metals. Remarkably, the HER activity of Ru-doped Cu-MOF outperformed a Pt/C catalyst, with its Faradaic efficiency for formate formation at 90% at a low potential of 1.40 V. Furthermore, the hybrid electrolyzer only needed 1.36 V to achieve 10 mA cm-2, 340 mV lower than that for splitting pure water. Theoretical calculations demonstrated that electronic interactions between the host and guest (doped) metals shifted downward the d-band centers (εd) of MOFs. This consequently lowered water adsorption and dissociation energy barriers and optimized hydrogen adsorption energy, leading to significantly enhanced HER activities. Meanwhile, the downshift of εd centers reduced energy barriers for rate-limiting step and the formation energy of OH*, synergistically enhancing the activity of MOFs for GOR. These findings offered an effective means for simultaneous productions of hydrogen fuel and high value-added chemicals using one hybrid electrolyzer with low energy input.

Efficient bubble/precipitate traffic enables stable seawater reduction electrocatalysis at industrial-level current densities

Seawater electroreduction is attractive for future H2 production and intermittent energy storage, which has been hindered by aggressive Mg2+/Ca2+ precipitation at cathodes and consequent poor stability. Here we present a vital microscopic bubble/precipitate traffic system (MBPTS) by constructing honeycomb-type 3D cathodes for robust anti-precipitation seawater reduction (SR), which massively/uniformly release small-sized H2 bubbles to almost every corner of the cathode to repel Mg2+/Ca2+ precipitates without a break. Noticeably, the optimal cathode with built-in MBPTS not only enables state-of-the-art alkaline SR performance (1000-h stable operation at -1 A cm-2) but also is highly specialized in catalytically splitting natural seawater into H2 with the greatest anti-precipitation ability. Low precipitation amounts after prolonged tests under large current densities reflect genuine efficacy by our MBPTS. Additionally, a flow-type electrolyzer based on our optimal cathode stably functions at industrially-relevant 500 mA cm-2 for 150 h in natural seawater while unwaveringly sustaining near-100% H2 Faradic efficiency. Note that the estimated price (~1.8 US$/kgH2) is even cheaper than the US Department of Energy's goal price (2 US$/kgH2).

Controlled Growth of Metal Atom Arrays on Graphdiyne for Seawater Oxidation

Advanced atomic-level heterointerface engineering provides a promising method for the preparation of next-generation catalysts. Traditional carbon-based heterointerface catalytic performance rely heavily on the undetermined defects in complex and demanding preparation processes, rendering it impossible to control the catalytic performance. Here, we present a general method for the controlled growth of metal atom arrays on graphdiyne (GDY/IrCuOx), and we are surprised to find strong heterointerface strains during the growth. We successfully controlled the thickness of GDY to regulate the heterointerface metal atoms and achieved compressive strain at the interface. Experimental and density functional theory calculation results show that the unique incomplete charge transfer between GDY and metal atoms leads to the formation of strong interactions and significant heterointerface compressive strain between GDY and IrCuOx, which results in high oxidation performances with 1000 mA cm-2 at a low overpotential of 283 mV and long-term stability at large current densities in alkaline simulated seawater. We anticipate that this finding will contribute to construction of high-performance heterogeneous interface structures, leading to the development of new generation of GDY-based heterojunction catalysts in the field of catalysis for future promising performance.

Heterometallic Electrocatalysts Derived from High-Nuclearity Metal Clusters for Efficient Overall Water Splitting

The development of cost-effective electrocatalysts with an optimal surface affinity for intermediates is essential for sustainable hydrogen fuel production, but this remains insufficient. Here we synthesize Ni2P/MoS2-CoMo2S4@C heterometallic electrocatalysts based on the high-nuclearity cluster {Co24(TC4A)6(MoO4)8Cl6}, in which Ni2P nanoparticles were anchored to the surface of the MoS2-CoMo2S4@C nanosheets via strong interfacial interactions. Theoretical calculations revealed that the introduction of Ni2P phases induces significant disturbances in the surface electronic configuration of Ni2P/MoS2-CoMo2S4@C, resulting in more relaxed d-d orbital electron transfers between the metal atoms. Moreover, continuous electron transport was established by the formation of multiple heterojunction interfaces. The optimized Ni2P/MoS2-CoMo2S4@C electrocatalyst exhibited ultralow overpotentials of 198 and 73 mV for oxygen and hydrogen evolution reactions, respectively, in alkaline media, at 10 mA cm-2. The alkali electrolyzer constructed using Ni2P/MoS2-CoMo2S4@C required a cell voltage of only 1.45 V (10 mA cm-2) to drive overall water splitting with excellent long-term stability.

Challenges and progress in oxygen evolution reaction catalyst development for seawater electrolysis for hydrogen production

Production of green hydrogen on a large scale can negatively impact freshwater resources. Therefore, using seawater as an electrolyte in electrolysis is a desirable alternative to reduce costs and freshwater reliance. However, there are limitations to this approach, primarily due to the catalyst involved in the oxygen evolution reaction (OER). In seawater, the OER features sluggish kinetics and complicated chemical reactions that compete. This review first introduces the benefits and challenges of direct seawater electrolysis and then summarises recent research into cost-effective and durable OER electrocatalysts. Different modification methods for nickel-based electrocatalysts are thoroughly reviewed, and promising electrocatalysts that the authors believe deserve further exploration have been highlighted.

1D/3D Heterogeneous Assembling Body of Cobalt Nitrides for Highly Efficient Overall Hydrazine Splitting and Supercapacitors

Herein, the construction of a heterostructured 1D/3D CoN-Co2 N@NF (nickel foam) electrode used for thermodynamically favorable hydrazine oxidation reaction (HzOR), as an alternative to sluggish anodic oxygen evolution reaction (OER) in water splitting for hydrogen production, is reported. The electrode exhibits remarkable catalytic activities, with an onset potential of -0.11 V in HzOR and -71 mV for a current density of 10 mA cm-2 in hydrogen evolution reaction (HER). Consequently, an extraordinary low cell voltage of 53 mV is required to achieve 10 mA cm-2 for overall hydrazine splitting in a two-electrode system, demonstrating significant energy-saving advantages over conventional water splitting. The HzOR proceeds through the 4e- reaction pathway to release N2 while the 1e- pathway to emit NH3 is uncompetitive, as evidenced by differential electrochemical mass spectrometric measurements. The X-ray absorption spectroscopy, in situ Raman spectroscopy, and theoretical calculations identify cobalt nitrides rather than corresponding oxides/(oxy)hydroxides as catalytic species for HzOR and illustrate advantages of heterostructured CoN-Co2 N in optimizing adsorption energies of intermediates/reagents and promoting catalytic activities toward both HzOR and HER. The CoN-Co2 N@NF is also an excellent supercapacitive material, exhibiting an increased specific capacity (938 F g-1 at 1 A g-1 ) with excellent cycling stability (95.8%, 5000 cycles).

Carbon Oxyanion Self-Transformation on NiFe Oxalates Enables Long-Term Ampere-Level Current Density Seawater Oxidation

Seawater electrolysis is an attractive way of making H2 in coastal areas, and NiFe-based materials are among the top options for alkaline seawater oxidation (ASO). However, ample Cl- in seawater can severely corrode catalytic sites and lead to limited lifespans. Herein, we report that in situ carbon oxyanion self-transformation (COST) from oxalate to carbonate on a monolithic NiFe oxalate micropillar electrode allows safeguard of high-valence metal reaction sites in ASO. In situ/ex situ studies show that spontaneous, timely, and appropriate COST safeguards active sites against Cl- attack during ASO even at an ampere-level current density (j). Our NiFe catalyst shows efficient and stable ASO performance, which requires an overpotential as low as 349 mV to attain a j of 1 A cm-2 . Moreover, the NiFe catalyst with protective surface CO3 2- exhibits a slight activity degradation after 600 h of electrolysis under 1 A cm-2 in alkaline seawater. This work reports effective catalyst surface design concepts at the level of oxyanion self-transformation, acting as a momentous step toward defending active sites in seawater-to-H2 conversion systems.

In-situ fabrication of bimetallic FeCo2O4-FeCo2S4 heterostructure for high-efficient alkaline freshwater/seawater electrolysis

Rational construction of bifunctional electrocatalysts with long-term stability and high electrocatalytic activity is of great importance, but it is challenging to obtain highly efficient non-precious metal-based catalysts for overall seawater electrolysis. Herein, a nickel foam (NF) self-supporting CoFe-layered double hydroxide (CoFe-LDH/NF) was directly converted into FeCo2O4-FeCo2S4 heterostructure via hydrothermal method in 50 mM Na2S solution, instead of FeCo2O4@FeCo2S4 core-shell structure. The FeCo2O4-FeCo2S4 heterojunction shows nanosheets structure with rough surface (the thickness of ∼ 198.9 nm), which provides rich oxide/sulfide interfaces, high electrochemical active area, a large number of active sites, as well as fast charge and mass transfer. In 1.0 M KOH solution, 1.0 M KOH + 0.5 M NaCl, and alkaline natural seawater, the FeCo2O4-FeCo2S4 heterojunction exhibits eminently electrocatalytic performance, with overpotentials of η-100 = 225 mV, η-100 = 233 mV, and η-100 = 238 mV for OER, as well as η-100 = 271 mV, η-100 = 273 mV, and η-100 = 277 mV for HER, respectively. Furthermore, self-supporting FeCo2O4-FeCo2S4 electrode (FeCo2O4-FeCo2S4/NF) as the cathode and anode of an electrolyzer exhibits a lower cell voltage of E-100 = 1.75 V in alkaline seawater than those of FeCo2S4/NF (1.77 V), CoFe-LDH/NF (1.87 V), and FeCo2O4/NF (1.91 V). Specifically, FeCo2O4-FeCo2S4 electrolyzer can stably produce hydrogen for over 48 h in alkaline freshwater/seawater electrolyte. These outstanding electrocatalytic performances and corrosion resistance to salty-water can be attributed to the surface reconstruction behavior of the FeCo2O4-FeCo2S4/NF catalyst during OER, which leads to the in-situ formation of metal oxyhydroxides. In particular, the FeCo2O4-FeCo2S4 heterojunction is also very competitive among most state-of-the-art non-noble metal-based catalysts, whether in KOH or alkaline salty-water electrolytes.

An efficient cerium dioxide incorporated nickel cobalt phosphide complex as electrocatalyst for All-pH hydrogen evolution reaction and overall water splitting

Transition metal phosphides (TMPs) have been considered as potential electrocatalysts with adjustable valence states, metal characteristics, and phase diversity. However, it is necessary but remains a major challenge to obtain efficient and durable TMPs catalysts, which can realize efficiently for not only all-pH hydrogen evolution reaction (HER), but also oxygen evolution reaction (OER). Hence, cerium dioxide incorporated nickel cobalt phosphide growth on nickel foam (CeO2/NiCoP) is fabricated by hydrothermal and phosphating reaction. CeO2/NiCoP shows excellent activity for all-pH HER (overpotentials of 48, 58 and 72 mV in alkaline, neutral and acidic solution at the current density of 10 mA cm-2), and has a small OER overpotential (231 mV @ 10 mA cm-2). Moreover, the voltage of overall water splitting in alkaline solution and simulated seawater electrolyte is only 1.46 and 1.41 V (10 mA cm-2), respectively, coupled with outstanding operational stability and corrosion resistance. Further mechanism research shows that CeO2/NiCoP possesses rich heterointerfaces, which serves more exposed active sites and possesses a promising superhydrophilic and superaerophobic surface. Density functional theory calculations manifest that CeO2/NiCoP has appropriate energy for intermediates of reactions. This work provides a deep insight into the CeO2/NiCoP catalyst for high-performance water/seawater electrolysis.

Ag Nanoparticle-Induced Surface Chloride Immobilization Strategy Enables Stable Seawater Electrolysis

Although hydrogen gas (H2 ) storage might enable offshore renewable energy to be stored at scale, the commercialization of technology for H2 generation by seawater electrolysis depends upon the development of methods that avoid the severe corrosion of anodes by chloride (Cl- ) ions. Here, it is revealed that the stability of an anode used for seawater splitting can be increased by more than an order of magnitude by loading Ag nanoparticles on the catalyst surface. In experiments, an optimized NiFe-layered double hydroxide (LDH)@Ag electrode displays stable operation at 400 mA cm-2 in alkaline saline electrolyte and seawater for over 5000 and 2500 h, respectively. The impressive long-term durability is more than 20 times that of an unmodified NiFe-LDH anode. Meticulous characterization and simulation reveals that in the presence of an applied electric field, free Cl- ions react with oxidized Ag nanoparticles to form stable AgCl species, giving rise to the formation of a Cl- -free layer near the anode surface. Because of its simplicity and effectiveness, it is anticipated that the proposed strategy to immobilize chloride ions on the surface of an anode has the potential to become a crucial technology to control corrosion during large-scale electrolysis of seawater to produce hydrogen.

Amorphous Co-Mo-P film on nickel foam: a superior bifunctional electrocatalyst for alkaline seawater splitting

Seawater splitting is a compelling avenue to produce abundant hydrogen, which requires high-performance and cost-effective catalysts. Constructing bimetallic transition metal phosphides is a feasible strategy to meet the challenge. Here, an amorphous Co-Mo-P film supported on nickel foam (Co-Mo-P/NF) electrode is developed with bifunctional properties for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline seawater. Corresponding results indicate that the introduction of Mo can improve the active sites and regulate the intrinsic activity. Such a Co-Mo-P/NF behaves with prominent electrocatalytic activity towards both HER and OER, demanding low overpotentials of 193 and 352 mV at 100 mA cm‒2in alkaline seawater, respectively. Furthermore, the assembled electrolyzer demands a pronounced overall seawater splitting activity with a low cell voltage of 1.76 V to deliver 100 mA cm-2presenting excellent durability without obvious attenuation after 24 h continuous stability test. This work expands the horizon to develop transition metal-phosphorus electrocatalysts with robust and efficient activity for overall seawater splitting.

Interfacial Electron Regulation and Composition Evolution of NiFe/MoC Heteronanowire Arrays for Highly Stable Alkaline Seawater Oxidation

In alkaline seawater electrolysis, the oxygen evolution reaction (OER) is greatly suppressed by the occurrence of electrode corrosion due to the formation of hypochlorite. Herein, a catalyst consisting of MoC nanowires modified with NiFe alloy nanoparticles (NiFe/MoC) on nickel foam (NF) is prepared. The optimized catalyst can deliver a large current density of 500 mA cm-2 at a very low overpotential of 366 mV in alkaline seawater, respectively, outperforming commercial IrO2 . Remarkably, an electrolyzer assembled with NiFe/MoC/NF as the anode and NiMoN/NF as the cathode only requires 1.77 V to drive a current density of 500 mA cm-2 for alkaline seawater electrolysis, as well as excellent stability. Theory calculation indicates that the initial activity of NiFe/MoC is attributed to increased electrical conductivity and decreased energy barrier for OER due to the introduction of Fe. We find that the change of the catalyst in the composition occurred after the stability test; however, the reconstructed catalyst has an energy barrier close to that of the pristine one, which is responsible for its excellent long-term stability. Our findings provide an efficient way to construct high-performance OER catalysts for alkaline seawater splitting.

Design of polymetallic sulfide NiS2@Co4S3@FeS as bifunctional catalyst for high efficiency seawater splitting

The shortage of freshwater resources in the world today has limited the development of water splitting, and our eyes have turned to the abundant seawater. The development of relatively low-toxicity and high-efficiency catalysts is the most important area in seawater electrolysis. In this paper, the preparation of NiS2@Co4S3@FeS via a hydrothermal method on nickel foam has been studied for the first time. In the process of vulcanization, Fe will first generate FeS by virtue of its high affinity for vulcanization. Once Fe is vulcanized, the residual sulfur will be used to generate NiS2, while the vulcanization of Co requires a higher sulfur concentration and reaction temperature; thus, Co4S3 will be generated last. NiS2@Co4S3@FeS is confirmed to have excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic properties in alkaline seawater. Its unique structure allows it to expose more reaction centres, and the synergies between the multiple metals optimize the charge distribution of the material and accelerate the OER and HER kinetics. NiS2@Co4S3@FeS requires overpotentials of only 122 mV and 68 mV for the OER and HER when reaching 10 mA cm-2, which is superior to most catalysts reported to date for seawater electrolysis, and the material displays acceptable stability. In an electrolytic cell composed of both positive and negative electrodes, when the current density is 10 mA cm-2, the NiS2@Co4S3@FeS material displays a low overpotential of only 357 mV for seawater splitting. Density functional theory shows that the FeS electrode has the optimum Gibbs free energy of H to accelerate reaction kinetics, and the synergistic catalysis of the NiS2, Co4S3 and FeS materials promotes the hydrogen production activity of the NiS2@Co4S3@FeS electrode. This work proposes a novel idea for designing environmentally friendly seawater splitting catalysts.

Enhancing Oxygen Evolution Reaction Performance in Prussian Blue Analogues: Triple-Play of Metal Exsolution, Hollow Interiors, and Anionic Regulation

Prussian blue analogs (PBAs) are promising catalysts for green hydrogen production. However, the rational design of high-performing PBAs is challenging, which requires an in-depth understanding of the catalytic mechanism. Here FeMn@CoNi core-shell PBAs are employed as precursors, together with Se powders, in low-temperature pyrolysis in an argon atmosphere. This synthesis method enables the partial dissociation of inner FeMn PBAs that results in hollow interiors, Ni nanoparticles (NPs) exsolution to the surface, and Se incorporation onto the PBA shell. The resulting material presents ultralow oxygen evolution reaction (OER) overpotential (184 mV at 10 mA cm-2 ) and low Tafel slope (43.4 mV dec-1 ), outperforming leading-edge PBA-based electrocatalysts. The mechanism responsible for such a high OER activity is revealed, assisted by density functional theory (DFT) calculations and the surface examination before and after the OER process. The exsolved Ni NPs are found to help turn the PBAs into Se-doped core-shell metal oxyhydroxides during the OER, in which the heterojunction with Ni and the Se incorporation are combined to improve the OER kinetics. This work shows that efficient OER catalysts could be developed by using a novel synthesis method backed up by a sound understanding and control of the catalytic pathway.

Electrode/Electrolyte Synergy for Concerted Promotion of Electron and Proton Transfers toward Efficient Neutral Water Oxidation

Neutral water oxidation is a crucial half-reaction for various electrochemical applications requiring pH-benign conditions. However, its sluggish kinetics with limited proton and electron transfer rates greatly impacts the overall energy efficiency. In this work, we created an electrode/electrolyte synergy strategy for simultaneously enhancing the proton and electron transfers at the interface toward highly efficient neutral water oxidation. The charge transfer was accelerated between the iridium oxide and in situ formed nickel oxyhydroxide on the electrode end. The proton transfer was expedited by the compact borate environment that originated from hierarchical fluoride/borate anions on the electrolyte end. These concerted promotions facilitated the proton-coupled electron transfer (PCET) events. Due to the electrode/electrolyte synergy, Ir-O and Ir-OO- intermediates could be directly detected by in situ Raman spectroscopy, and the rate-limiting step of Ir-O oxidation was determined. This synergy strategy can extend the scope of optimizing electrocatalytic activities toward more electrode/electrolyte combinations.

Toward Understanding the Formation Mechanism and OER Catalytic Mechanism of Hydroxides by In Situ and Operando Techniques

Developing efficient and affordable electrocatalysts for the sluggish oxygen evolution reaction (OER) remains a significant barrier that needs to be overcome for the practical applications of hydrogen production via water electrolysis, transforming CO2 to value-added chemicals, and metal-air batteries. Recently, hydroxides have shown promise as electrocatalysts for OER. In situ or operando techniques are particularly indispensable for monitoring the key intermediates together with understanding the reaction process, which is extremely important for revealing the formation/OER catalytic mechanism of hydroxides and preparing cost-effective electrocatalysts for OER. However, there is a lack of comprehensive discussion on the current status and challenges of studying these mechanisms using in situ or operando techniques, which hinders our ability to identify and address the obstacles present in this field. This review offers an overview of in situ or operando techniques, outlining their capabilities, advantages, and disadvantages. Recent findings related to the formation mechanism and OER catalytic mechanism of hydroxides revealed by in situ or operando techniques are also discussed in detail. Additionally, some current challenges in this field are concluded and appropriate solution strategies are provided.

Concerning the stability of seawater electrolysis: a corrosion mechanism study of halide on Ni-based anode

The corrosive anions (e.g., Cl-) have been recognized as the origins to cause severe corrosion of anode during seawater electrolysis, while in experiments it is found that natural seawater (~0.41 M Cl-) is usually more corrosive than simulated seawater (~0.5 M Cl-). Here we elucidate that besides Cl-, Br- in seawater is even more harmful to Ni-based anodes because of the inferior corrosion resistance and faster corrosion kinetics in bromide than in chloride. Experimental and simulated results reveal that Cl- corrodes locally to form narrow-deep pits while Br- etches extensively to generate shallow-wide pits, which can be attributed to the fast diffusion kinetics of Cl- and the lower reaction energy of Br- in the passivation layer. Additionally, for the Ni-based electrodes with catalysts (e.g., NiFe-LDH) loading on the surface, Br- causes extensive spalling of the catalyst layer, resulting in rapid performance degradation. This work clearly points out that, in addition to anti-Cl- corrosion, designing anti-Br- corrosion anodes is even more crucial for future application of seawater electrolysis.

Dynamically Stabilized Electronic Regulation and Electrochemical Reconstruction in Co and S Atomic Pair Doped Fe3 O4 for Water Oxidation

The electronic regulation and surface reconstruction of earth-abundant electrocatalysts are essential to efficient oxygen evolution reaction (OER). Here, an inverse-spinel Co,S atomic pair codoped Fe3 O4 grown on iron foam (Co,S-Fe3 O4 /IF) is fabricated as a cost-effective electrocatalyst for OER. This strategy of Co and S atomic pair directional codoping features accelerates surface reconstruction and dynamically stabilizes electronic regulation. CoS atomic pairs doped in the Fe3 O4 crystal favor controllable surface reconstruction via sulfur leaching, forming oxygen vacancies and Co doping on the surface of reconstructed FeOOH (Co-FeOOH-Ov /IF). Before and after surface reconstruction via in situ electrochemical process, the Fe sites with octahedral field dynamically maintains an appropriate electronic structure for OER intermediates, thus exhibiting consistently excellent OER performance. The electrochemically tuned Fe-based electrodes exhibit a low overpotential of 349 mV at a current density of 1000 mA cm-2 , a slight Tafel slope of 43.3 mV dec-1 , and exceptional long-term electrolysis stability of 200 h in an alkaline medium. Density functional theory calculations illustrate the electronic regulation of Fe sites, changes in Gibbs free energies, and the breaking of the restrictive scaling relation between OER intermediates. This work provides a promising directional codoping strategy for developing precatalysts for large-scale water-splitting systems.

Ultralow-iridium content NiIr alloy derivative nanochain arrays as bifunctional electrocatalysts for overall water splitting

The development of low-cost and high-durability bifunctional electrocatalysts is of considerable importance for overall water splitting (OWS). This work reports the controlled synthesis of nickel-iridium alloy derivative nanochain array electrodes (NiIrx NCs) with fully exposed active sites that facilitated mass transfer for efficient OWS. The nanochains have a self-supported three-dimensional core-shell structure, composed of a metallic NiIrx core and a thin (5-10 nm) amorphous (hydr)oxide film as the shell (e.g., IrO2/NiIrx and Ni(OH)2/NiIrx). Interestingly, NiIrx NCs have bifunctional properties. Particularly, the oxygen evolution reaction (OER) current density (electrode geometrical area) of NiIr1 NCs is four times higher than that of IrO2 at 1.6 V vs. RHE. Meanwhile, its hydrogen evolution reaction (HER) overpotential at 10 mA cm-2 (η10 = 63 mV) is comparable to that of 10 wt% Pt/C. These performances may originate from the interfacial effect between the surface (hydr)oxide shell and metallic NiIrx core, which facilitates the charge transfer, along with the synergistic effect between Ni2+ and Ir4+ in the (hydr)oxide shell. Furthermore, NiIr1 NCs exhibits excellent OER durability (100 h @ 200 mA cm-2) and OWS durability (100 h @ 500 mA cm-2) with the nanochain array structure well preserved. This work provides a promising route for developing effective bifunctional electrocatalysts for OWS applications.

Flower-Like Amorphous MoO3- x Stabilized Ru Single Atoms for Efficient Overall Water/Seawater Splitting

Benefitting from the maximum atom utilization efficiency, special size quantum effects and tailored active sites, single-atom catalysts (SACs) have been promising candidates for bifunctional catalysts toward water splitting. Besides, due to the unique structure and properties, some amorphous materials have been found to possess better performance than their crystalline counterparts in electrocatalytic water splitting. Herein, by combining the advantages of ruthenium (Ru) single atoms and amorphous substrates, amorphous molybdenum-based oxide stabilized single-atomic-site Ru (Ru SAs-MoO3- x /NF) catalysts are conceived as a self-supported electrode. By virtue of the large surface area, enhanced intrinsic activity and fast reaction kinetics, the as-prepared Ru SAs-MoO3- x /NF electrode effectively drives both oxygen evolution reaction (209 mV @ 10 mA cm-2 ) and hydrogen evolution reaction (36 mV @ 10 mA cm-2 ) in alkaline media. Impressively, the assembled electrolyzer merely requires an ultralow cell voltage of 1.487 V to deliver the current density of 10 mA cm-2 . Furthermore, such an electrode also exhibits a great application potential in alkaline seawater electrolysis, achieving a current density of 100 mA cm-2 at a low cell voltage of 1.759 V. In addition, Ru SAs-MoO3- x /NF only has very small current density decay in the long-term constant current water splitting test.