Strong-Proton-Adsorption Co-Based Electrocatalysts Achieve Active and Stable Neutral Seawater Splitting

Fe doping intensifies the built-in electric field for tailoring the reconstruction of sulfides towards efficient oxygen evolution

The traditional view of sulfides as stable active centers has hindered the development of a clear structure-activity relationship and the rational design of high-performance oxygen evolution reaction (OER) catalysts. In this study, we focus on regulating sulfide reconstruction and have synthesized a Fe-Ni3S4/Cr2O3 pre-catalyst. Under the combined influence of the built-in electric field (BIEF) at the heterogeneous interface and Fe doping, both the sulfide reconstruction process and the electronic structure of the reconstructed product, namely Fe-NiOOH, were effectively tuned. The enhanced BIEF induced by Fe doping generated electron-rich regions on the sulfide surface, stabilizing the reconstruction process. Fe doping into the sulfide induced the incorporation of Fe into NiOOH, modulating the electronic states near the Fermi level of the metal-oxygen bond and subsequently activating the lattice oxygen mediated mechanism (LOM) of Fe-NiOOH, which serves as the true active center. Additionally, the BIEF optimized OH- diffusion dynamics and the energy consumption of hydroxyl deprotonation, reducing the energy barrier of the rate-limiting step of the LOM process, further enhancing OER activity. Remarkably, Fe-Ni3S4/Cr2O3 demonstrated excellent OER activity and commercial viability. This work offers a new perspective on the regulation of reconstruction products of pre-catalyst, providing fresh insights for the design of efficient OER catalysts.

Corrosion-resistant single-atom catalysts for direct seawater electrolysis

Direct seawater electrolysis (DSE) for hydrogen production is an appealing method for renewable energy storage. However, DSE faces challenges such as slow reaction kinetics, impurities, the competing chlorine evolution reaction at the anode, and membrane fouling, making it more complex than freshwater electrolysis. Therefore, developing catalysts with excellent stability under corrosion and fulfilling activity is vital to the advancement of DSE. Single-atom catalysts (SACs) with excellent tunability, high selectivity and high active sites demonstrate considerable potential for use in the electrolysis of seawater. In this review, we present the anodic and cathodic reaction mechanisms that occur during seawater cracking. Subsequently, to meet the challenges of DSE, rational strategies for modulating SACs are explored, including axial ligand engineering, carrier effects and protective layer coverage. Then, the application of in-situ characterization techniques and theoretical calculations to SACs is discussed with the aim of elucidating the intrinsic factors responsible for their efficient electrocatalysis. Finally, the process of scaling up monoatomic catalysts for the electrolysis of seawater is described, and some prospective insights are provided.

Enhancing Photoelectrochemical Seawater Splitting Efficiency by a Dual-Strategy Approach of W Doping and CoOOH Layer Deposition on BiVO4 Photoanodes

Photoelectrochemical (PEC) seawater splitting offers a sustainable pathway for hydrogen production, yet its practical application is hindered by sluggish reaction kinetics and severe photocorrosion in chloride-rich environments. This study presents a dual-strategy modification of BiVO4 photoanodes through tungsten (W) doping and cobalt oxyhydroxide (CoOOH) nanolayer deposition to synergistically enhance the PEC performance and stability in natural seawater. W doping optimizes the electronic structure of BiVO4 by reducing the bandgap from 2.4 to 2.35 eV and increasing carrier concentration from 1.41 × 1021 to 3.31 × 1021 cm-3, while CoOOH acts as a dual-functional layer that suppresses surface recombination via oxygen vacancy formation and protects against chloride-induced corrosion. The optimized CoOOH/W-BVO photoanode achieves a photocurrent density of 3.77 mA cm-2 at 1.23 V vs reversible hydrogen electrode (RHE) with 96 h stability in natural seawater, outperforming pristine BiVO4 by 150% and single-modified counterparts by 40-60%. Mechanistic analyses reveal that W6+ substitution elongates V-O bonds, thereby enhancing the bulk charge separation. Concurrently, CoOOH facilitates hole extraction through oxygen vacancies, with oxygen vacancy content increasing from 3.9% to 24.3%. The dual modification also reduces interfacial charge-transfer resistance to 94.44 Ω and shifts the flat-band potential negatively to 0.15 V vs RHE, improving light absorption and charge utilization efficiency (applied bias photocurrent efficiency (ABPE) of 0.95% at 0.77 V). This work provides a robust strategy for designing efficient and durable photoanodes, advancing marine-resource-utilized renewable energy technologies.

Room-Temperature Synthesis of Amorphous Fe-Doped Nickel Phosphate for High-Efficiency Alkaline Seawater Oxidation

The construction of an efficient and durable oxygen evolution reaction (OER) electrocatalyst through a simple synthesis strategy is crucial for the hydrogen produced from seawater splitting. However, achieving this goal remains a great challenge. Herein, the synthesis of amorphous Fe-doped nickel phosphate (Fe-NixPO4) as a high-performance OER electrocatalysts for alkaline freshwater/seawater splitting is presented using a straightforward co-precipitation method at room-temperature. Experimental results reveal the structural reconstruction of Fe-NixPO4 into Fe-doped NiOOH decorated with PO4 3-. The collaborative interplay between Ni2+ and Fe3+, along with the decoration of PO4 3-, can effectively modulate the electronic environment of the electrocatalyst. Consequently, the optimized Fe-NixPO4 exhibits exceptional OER activity, requiring overpotentials of 359 and 422 mV to generate 1000 mA cm-2 in alkaline freshwater and alkaline seawater, respectively. Moreover, Fe-NixPO4 also displays outstanding stability for 100 h at 100 mA cm-2 in alkaline seawater. This research presents a viable approach for fabricating OER electrocatalysts with exceptional efficiency for seawater electrolysis.

P-Doped NiFe Alloy-Based Oxygen Evolution Electrocatalyst for Efficient and Stable Seawater Splitting and Organic Electrosynthesis at Neutral pH

Development of high-performance and inexpensive electrocatalysts for oxygen evolution reaction (OER) at neutral pH is important for direct seawater splitting and organic electrosynthesis but remains challenging due to the sluggish OER kinetics and diverse side reactions inherent to the constituents of working electrolytes. Herein, we report on a P:NiFe electrode, containing P-doped NiFe alloy, as an excellent electrocatalyst for hydrogen evolution reaction (HER) and OER pre-catalyst for efficient OER in both seawater and organic electrolyte for adiponitrile (ADN) electrosynthesis at neutral pH. Fe and P species modulate the coordination environment of nickel sites, which enables the simultaneous formation of OER-active nickel species and FePOx passivation layer, thus transforming HER-active P:NiFe to OER-active a-P:NiFe. Besides, the redox-dependent interconversion between HER-active P:NiFe and OER-active a-P:NiFe is fast and reversible. Finally, efficient and stable overall seawater-splitting and ADN electrosynthesis at an industrially relevant current density (100 mA cm-2) in pH-neutral media are also demonstrated.

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

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

Long-term Durability of Seawater Electrolysis for Hydrogen: From Catalysts to Systems

Direct electrochemical seawater splitting is a renewable, scalable, and potentially economic approach for green hydrogen production in environments where ultra-pure water is not readily available. However, issues related to low durability caused by complex ions in seawater pose great challenges for its industrialization. In this review, a mechanistic analysis of durability issues of electrolytic seawater splitting is discussed. We critically analyze the development of seawater electrolysis and identify the durability challenges at both the anode and cathode. Particular emphasis is given to elucidating rational strategies for designing electrocatalysts/electrodes/interfaces with long lifetimes in realistic seawater including inducing passivating anion layers, preferential OH-adsorption, employing anti-corrosion materials, fabricating protective layers, immobilizing Cl- on the surface of electrocatalysts, tailoring Cl- adsorption sites, inhibition of OH- binding to Mg2+ and Ca2+, inhibition of Mg and Ca hydroxide precipitation adherence, and co-electrosynthesis of nano-sized Mg hydroxides. Synthesis methods of electrocatalysts/electrodes and innovations in electrolyzer are also discussed. Furthermore, the prospects for developing seawater splitting technologies for clean hydrogen generation are summarized. We found that researchers have rethought the role of Cl- ions, as well as more attention to cathodic reaction and electrolyzers, which is conducive to accelerate the commercialization of seawater electrolysis.

In situ ammonium formation mediates efficient hydrogen production from natural seawater splitting

Seawater electrolysis using renewable electricity offers an attractive route to sustainable hydrogen production, but the sluggish electrode kinetics and poor durability are two major challenges. We report a molybdenum nitride (Mo2N) catalyst for the hydrogen evolution reaction with activity comparable to commercial platinum on carbon (Pt/C) catalyst in natural seawater. The catalyst operates more than 1000 hours of continuous testing at 100 mA cm-2 without degradation, whereas massive precipitate (mainly magnesium hydroxide) forms on the Pt/C counterpart after 36 hours of operation at 10 mA cm-2. Our investigation reveals that ammonium groups generate in situ at the catalyst surface, which not only improve the connectivity of hydrogen-bond networks but also suppress the local pH increase, enabling the enhanced performances. Moreover, a zero-gap membrane flow electrolyser assembled by this catalyst exhibits a current density of 1 A cm-2 at 1.87 V and 60 oC in simulated seawater and runs steadily over 900 hours.

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.

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.

Accelerated Proton Transfer in Asymmetric Active Units for Sustainable Acidic Oxygen Evolution Reaction

The poor durability of Ru-based catalysts limits the practical application in proton exchange membrane water electrolysis (PEMWE). Here, we report that the asymmetric active units in Ru1-xMxO2 (M = Sb, In, and Sn) binary solid solution oxides are constructed by introducing acid-resistant p-block metal sites, breaking the activity and stability limitations of RuO2 in acidic oxygen evolution reaction (OER). Constructing highly asymmetric Ru-O-Sb units with a strong electron delocalization effect significantly shortens the spatial distance between Ru and Sb sites, improving the bonding strength of the overall structure. The unique two-electron redox couples at Sb sites in asymmetric active units trigger additional chemical steps at different OER stages, facilitating continuous proton transfer. The optimized Ru0.8Sb0.2O2 solid solution requires a superlow overpotential of 160 mV at 10 mA cm-2 and a record-breaking stability of 1100 h in an acidic electrolyte. Notably, the scale-prepared Ru0.8Sb0.2O2 achieves efficient PEMWE performance under industrial conditions. General mechanism analysis shows that the enhanced proton transport in the asymmetric Ru-O-M unit provides a new working pathway for acidic OER, breaking the scaling relationship without sacrificing stability.

Multicomponent nickel-molybdenum-tungsten-based nanorods for stable and efficient alkaline seawater splitting

The electrolysis of seawater for hydrogen production holds promise as a sustainable technology for energy generation. Developing water-splitting catalysts with low overpotential and stable operation in seawater is essential. In this study, we employed a hydrothermal method to synthesize NiMoWOX microrods (NiMoWOX@NF). Subsequently, an annealing process yielded a composite N-doped carbon-coated Ni3N/MoO2/WO2 nanorods (NC@Ni3N/MoO2/WO2@NF), preserving the ultrahigh-specific surface area of the original structure. A two-electrode electrolytic cell was assembled using NC@Ni3N/MoO2/WO2@NF as the cathode and NiMoWOX@NF as the anode, demonstrating exceptional performance in seawater splitting. The cell operated at a voltage of 1.51 V with a current density of 100 mA·cm-2 in an alkaline seawater solution. Furthermore, the NC@Ni3N/MoO2/WO2@NF || NiMoWOX@NF electrolytic cell exhibited remarkable stability, running continuously for over 120 h at a current of 1100 mA·cm-2 without any observable delay. These experimental results are corroborated by density functional theory calculations. The NC@Ni3N/MoO2/WO2@NF || NiMoWOX@NF electrolyzer emerges as a promising option for industrial-scale hydrogen production through seawater electrolysis.

Energy-saving hydrogen production by seawater electrolysis coupling tip-enhanced electric field promoted electrocatalytic sulfion oxidation

Hydrogen production by seawater electrolysis is significantly hindered by high energy costs and undesirable detrimental chlorine chemistry in seawater. In this work, energy-saving hydrogen production is reported by chlorine-free seawater splitting coupling tip-enhanced electric field promoted electrocatalytic sulfion oxidation reaction. We present a bifunctional needle-like Co3S4 catalyst grown on nickel foam with a unique tip structure that enhances the kinetic rate by improving the current density in the tip region. The assembled hybrid seawater electrolyzer combines thermodynamically favorable sulfion oxidation and cathodic seawater reduction can enable sustainable hydrogen production at a current density of 100 mA cm-2 for up to 504 h. The hybrid seawater electrolyzer has the potential for scale-up industrial implementation of hydrogen production by seawater electrolysis, which is promising to achieve high economic efficiency and environmental remediation.

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.

Construction of Heterojunction-Rich Metal Nitrides Porous Nanosheets Electrocatalyst for Alkaline Water/Seawater Splitting at Large Current Density

The exploiting electrocatalysts for water/seawater electrolysis with remarkable activity and outstanding durability at industrial grade current density remains a huge challenge. Herein, CoMoNx and Fe-doped CoMoNx nanosheet arrays are in-situ grown on Ni foam, which possess plentiful holes, multilevel heterostructure, and lavish Co5.47N/MoN@NF and Fe-Co5.47N/MoN@NF interfaces. They require low overpotentials of 213 and 296 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline media to achieve current density of 800 mA cm-2, respectively, and both possess low Tafel slopes (51.1 and 49.1 mV dec-1) and undiminished stability over 80 h. Moreover, the coupled Co5.47N/MoN@NF and Fe-Co5.47N/MoN@NF electrolyzer requires low voltages of 1.735 V to yield 500 mA cm-2 in alkaline water. Notably, they also exhibit exceptional electrocatalytic properties in alkaline seawater (1.833 V@500 mA cm-2). The experimental studies and theoretical calculations verify that Fe doping does reduce the energy barrier from OH* to O* intermediates during OER process after catalyst reconstruction, and the non-metallic N site from MoN exhibits the lowest theoretical overpotential. The splendid catalytic performance is attributed to the optimized local electron configuration and porous structure. This discovery provides a new design method toward low-cost and excellent catalysts for water/seawater splitting to produce hydrogen.

A Dual-Cation Exchange Membrane Electrolyzer for Continuous H2 Production from Seawater

Direct seawater splitting (DSS) offers an aspirational route toward green hydrogen (H2) production but remains challenging when operating in a practically continuous manner, mainly due to the difficulty in establishing the water supply-consumption balance under the interference from impurity ions. A DSS system is reported for continuous ampere-level H2 production by coupling a dual-cation exchange membrane (CEM) three-compartment architecture with a circulatory electrolyte design. Monovalent-selective CEMs decouple the transmembrane water migration from interferences of Mg2+, Ca2+, and Cl- ions while maintaining ionic neutrality during electrolysis; the self-loop concentrated alkaline electrolyte ensures the constant gradient of water chemical potential, allowing a specific water supply-consumption balance relationship in a seawater-electrolyte-H2 sequence to be built among an expanded current range. Even paired with commercialized Ni foams, this electrolyzer (model size: 2 × 2 cm2) continuously produces H2 from flowing seawater with a rate of 7.5 mL min-1 at an industrially relevant current of 1.0 A over 100 h. More importantly, the energy consumption can be further reduced by coupling more efficient NiMo/NiFe foams (≈6.2 kWh Nm-3 H2 at 1.0 A), demonstrating the potential to further optimize the continuous DSS electrolyzer for practical applications.

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.

Efficient and durable seawater electrolysis with a V2O3-protected catalyst

The ocean, a vast hydrogen reservoir, holds potential for sustainable energy and water development. Developing high-performance electrocatalysts for hydrogen production under harsh seawater conditions is challenging. Here, we propose incorporating a protective V2O3 layer to modulate the microcatalytic environment and create in situ dual-active sites consisting of low-loaded Pt and Ni3N. This catalyst demonstrates an ultralow overpotential of 80 mV at 500 mA cm-2, a mass activity 30.86 times higher than Pt-C and maintains at least 500 hours in seawater. Moreover, the assembled anion exchange membrane water electrolyzers (AEMWE) demonstrate superior activity and durability even under demanding industrial conditions. In situ localized pH analysis elucidates the microcatalytic environmental regulation mechanism of the V2O3 layer. Its role as a Lewis acid layer enables the sequestration of excess OH- ions, mitigate Cl- corrosion, and alkaline earth salt precipitation. Our catalyst protection strategy by using V2O3 presents a promising and cost-effective approach for large-scale sustainable green hydrogen production.

Energy-Efficient Electrosynthesis of High Value-Added Active Chlorine Coupled with H2 Generation from Direct Seawater Electrolysis through Decoupling Electrolytes

Direct saline (seawater) electrolysis is a well-recognized system to generate active chlorine species for the chloride-mediated electrosynthesis, environmental remediation and sterilization over the past few decades. However, the large energy consumption originated from the high cell voltage of traditional direct saline electrolysis system, greatly restricts its practical application. Here, we report an acid-saline hybrid electrolysis system for energy-saving co-electrosynthesis of active chlorine and H2. We demonstrate that this system just requires a low cell voltage of 1.59 V to attain 10 mA cm-2 with a large energy consumption decrease of 27.7 % compared to direct saline electrolysis system (2.20 V). We further demonstrate that such acid-saline hybrid electrolysis system could be extended to realize energy-saving and sustainable seawater electrolysis. The acidified seawater in this system can absolutely avoid the formation of Ca/Mg-based sediments that always form in the seawater electrolysis system. We also prove that this system in the half-flow mode can realize real-time preparation of active chlorine used for sterilization and pea sprout production.

A silicon photoanode protected with TiO2/stainless steel bilayer stack for solar seawater splitting

Photoelectrochemical seawater splitting is a promising route for direct utilization of solar energy and abundant seawater resources for H2 production. However, the complex salinity composition in seawater results in intractable challenges for photoelectrodes. This paper describes the fabrication of a bilayer stack consisting of stainless steel and TiO2 as a cocatalyst and protective layer for Si photoanode. The chromium-incorporated NiFe (oxy)hydroxide converted from stainless steel film serves as a protective cocatalyst for efficient oxygen evolution and retarding the adsorption of corrosive ions from seawater, while the TiO2 is capable of avoiding the plasma damage of the surface layer of Si photoanode during the sputtering of stainless steel catalysts. By implementing this approach, the TiO2 layer effectively shields the vulnerable semiconductor photoelectrode from the harsh plasma sputtering conditions in stainless steel coating, preventing surface damages. Finally, the Si photoanode with the bilayer stack inhibits the adsorption of chloride and realizes 167 h stability in chloride-containing alkaline electrolytes. Furthermore, this photoanode also demonstrates stable performance under alkaline natural seawater for over 50 h with an applied bias photon-to-current efficiency of 2.62%.

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).

Direct Seawater Electrolysis: From Catalyst Design to Device Applications

Direct seawater electrolysis (DSE) for hydrogen production, using earth-abundant seawater as the feedstock and renewable electricity as the driving source, paves a new opportunity for flexible energy conversion/storage and smooths the volatility of renewable energy. Unfortunately, the complex environments of seawater impose significant challenges on the design of DSE catalysts, and the practical performance of many current DSE catalysts remains unsatisfactory on the device level. However, many studies predominantly concentrate on the development of electrocatalysts for DSE without giving due consideration to the specific devices. To mitigate this gap, the most recent progress (mainly published within the year 2020-2023) of DSE electrocatalysts and devices are systematically evaluated. By discussing key bottlenecks, corresponding mitigation strategies, and various device designs and applications, the tremendous challenges in addressing the trade-off among activity, stability, and selectivity for DSE electrocatalysts by a single shot are emphasized. In addition, the rational design of the DSE electrocatalysts needs to align with the specific device configuration, which is more effective than attempting to comprehensively enhance all catalytic parameters. This work, featuring the first review of this kind to consider rational catalyst design in the framework of DSE devices, will facilitate practical DSE development.

Observation on Microenvironment Changes of Dynamic Catalysts in Acidic CO2 Reduction

Electrochemical CO2 reduction reaction (CO2RR) in acid can solve alkalinity issues while highly corrosive and reductive acidic electrolytes usually cause catalyst degradation. Inhibiting catalyst degradation is crucial for the stability of acidic CO2RR. Here, we reveal the microenvironment changes of dynamic Bi-based catalysts and develop a pulse chronoamperometry (CA) strategy to improve the stability of acidic CO2RR. In situ fluorescence mappings show that the local pH changes from neutral to acid, and the in situ Raman spectra reveal the dynamic evolution of interfacial water structures in the microenvironment. We propose that the surface charge properties of dynamic catalysts affect the competitive adsorption of K+ and protons, thereby causing the differences in local pH and CO2RR intermediate adsorption. We also develop a pulse CA strategy to reactivate catalysts, and the stability of acidic CO2RR is improved by 2 orders of magnitude for 100 h operation, which is higher than most reports on the stability of acidic CO2RR. This work gives insights on how microenvironment changes affecting the stability of acidic CO2RR, and provides guidance for designing stable catalysts in acidic electrolytes.

Lewis Acid Driving Asymmetric Interfacial Electron Distribution to Stabilize Active Species for Efficient Neutral Water Oxidation

Neutral oxygen evolution reaction (OER) with unique reactive environments exhibits extremely slow reaction kinetics, posing significant challenges in the design of catalysts. Herein, a built-in electric field between the tungstate (Ni-FeWO4 ) with adjustable work function and Lewis acid WO3 is elaborately constructed to regulate asymmetric interfacial electron distribution, which promotes electron accumulation of Fe sites in the tungstate. This decelerates the rapid dissolution of Fe under the OER potentials, thereby retaining the active hydroxyl oxide with the optimized OER reaction pathway. Meanwhile, Lewis acid WO3 enhances hydroxyl adsorption near the electrode surface to improve mass transfer. As expected, the optimized Ni-FeWO4 @WO3 /NF self-supporting electrode achieves a low overpotential of 235 mV at 10 mA cm-2 in neutral media and maintains stable operation for 200 h. Furthermore, the membrane electrode assembly constructed by such self-supporting electrode exhibits robust stability for 250 h during neutral seawater electrolysis. This work deepens the understanding of the reconstruction of OER catalysts in neutral environments and paves the way for development of the energy conversion technologies.

Metal nitrides for seawater electrolysis

Electrocatalytic high-throughput seawater electrolysis for hydrogen production is a promising green energy technology that offers possibilities for environmental and energy sustainability. However, large-scale application is limited by the complex composition of seawater, high concentration of Cl- leading to competing reaction, and severe corrosion of electrode materials. In recent years, extensive research has been conducted to address these challenges. Metal nitrides (MNs) with excellent chemical stability and catalytic properties have emerged as ideal electrocatalyst candidates. This review presents the electrode reactions and basic parameters of the seawater splitting process, and summarizes the types and selection principles of conductive substrates with critical analysis of the design principles for seawater electrocatalysts. The focus is on discussing the properties, synthesis, and design strategies of MN-based electrocatalysts. Finally, we provide an outlook for the future development of MNs in the high-throughput seawater electrolysis field and highlight key issues that require further research and optimization.

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.

NiCo layered double hydroxides/NiFe layered double hydroxides composite (NiCo-LDH/NiFe-LDH) towards efficient oxygen evolution in different water matrices

Engineering robust non-noble metal electrocatalysts towards efficient impure water (e.g., seawater, wastewater) oxidation is a prospective approach to achieve carbon neutrality via accelerating green hydrogen energy development. Herein, a NiCo layered double hydroxides (LDH)/NiFe LDH composite (NiCo-LDH/NiFe-LDH) was developed for oxygen evolution reaction (OER) via a hydrothermal process-electrodeposition method. The optimal NiCo-LDH/NiFe-LDH-30 composite only needed an overpotential (η) of 240 mV to drive 100 mA/cm2 in alkalized freshwater, with a low Tafel slope of 16.6 mV/dec and good stability for over 90 h. Further analyses suggested that the strong interface interaction between NiCo-LDH and NiFe-LDH accelerated the oxygen gas bubble evolution and boosted interfacial charge transfer, and the formed built-in electric field and higher oxidation state species (metal oxyhydroxides) contributed to the high intrinsic catalytic activity. The NiCo-LDH/NiFe-LDH-30 composite also held excellent OER activities in different impure water environments, including alkaline 0.5 M NaCl solution (η100 = 333 mV), alkaline lake water (η100 = 345 mV), and alkaline wastewater treatment plant (WWTP) effluent (η100 = 320 mV). More importantly, the potential effects of Cl- and CO32- in impure water were revealed during the OER process. This work elaborates on the role of built-in electric field and the strong coupling interaction in composite catalysts, which pave the way for the design of cost-effective catalysts with excellent adaptability in different water environments.

Materials Design and System Innovation for Direct and Indirect Seawater Electrolysis

Green hydrogen production from renewably powered water electrolysis is considered as an ideal approach to decarbonizing the energy and industry sectors. Given the high-cost supply of ultra-high-purity water, as well as the mismatched distribution of water sources and renewable energies, combining seawater electrolysis with coastal solar/offshore wind power is attracting increasing interest for large-scale green hydrogen production. However, various impurities in seawater lead to corrosive and toxic halides, hydroxide precipitation, and physical blocking, which will significantly degrade catalysts, electrodes, and membranes, thus shortening the stable service life of electrolyzers. To accelerate the development of seawater electrolysis, it is crucial to widen the working potential gap between oxygen evolution and chlorine evolution reactions and develop flexible and highly efficient seawater purification technologies. In this review, we comprehensively discuss present challenges, research efforts, and design principles for direct/indirect seawater electrolysis from the aspects of materials engineering and system innovation. Further opportunities in developing efficient and stable catalysts, advanced membranes, and integrated electrolyzers are highlighted for green hydrogen production from both seawater and low-grade water sources.

Emerging materials and technologies for electrocatalytic seawater splitting

The limited availability of freshwater in renewable energy-rich areas has led to the exploration of seawater electrolysis for green hydrogen production. However, the complex composition of seawater presents substantial challenges such as electrode corrosion and electrolyzer failure, calling into question the technological and economic feasibility of direct seawater splitting. Despite many efforts, a comprehensive overview and analysis of seawater electrolysis, including electrochemical fundamentals, materials, and technologies of recent breakthroughs, is still lacking. In this review, we systematically examine recent advances in electrocatalytic seawater splitting and critically evaluate the obstacles to optimizing water supply, materials, and devices for stable hydrogen production from seawater. We demonstrate that robust materials and innovative technologies, especially selective catalysts and high-performance devices, are critical for efficient seawater electrolysis. We then outline and discuss future directions that could advance the techno-economic feasibility of this emerging field, providing a roadmap toward the design and commercialization of materials that can enable efficient, cost-effective, and sustainable seawater electrolysis.

Ferricyanide Armed Anodes Enable Stable Water Oxidation in Saturated Saline Water at 2 A/cm2

The direct seawater electrolysis at high current density and low overpotential affords an effective strategy toward clean and renewable hydrogen fuel production. However, the severe corrosion of anode as a result of the saturation of Cl- upon continuous seawater feeding seriously hamper the electrolytic process. Herein, cobalt ferricyanide / cobalt phosphide (CoFePBA/Co2 P) anodes with Cap/Pin structure are synthesized, which stably catalyze alkaline saturated saline water oxidation at 200-2000 mA cm-2 over hundreds of hours without corrosion. Together with the experimental findings, the molecular dynamics simulations reveal that PO4 3- and Fe(CN)6 3- generated by the electrode play synergistic role in repelling Cl- via electrostatic repulsion and dense coverage, which reduced Cl- adsorption by nearly 5-fold. The novel anionic synergy endow superior corrosion protection for the electrode, and is expected to promote the practical application of saline water electrolysis.

Lattice Strain Engineering of Ni2 P Enables Efficient Catalytic Hydrazine Oxidation-Assisted Hydrogen Production

Hydrazine-assisted water electrolysis provides new opportunities to enable energy-saving hydrogen production while solving the issue of hydrazine pollution. Here, the synthesis of compressively strained Ni2 P as a bifunctional electrocatalyst for boosting both the anodic hydrazine oxidation reaction (HzOR) and cathodic hydrogen evolution reaction (HER) is reported. Different from a multistep synthetic method that induces lattice strain by creating core-shell structures, a facile strategy is developed to tune the strain of Ni2 P via dual-cation co-doping. The obtained Ni2 P with a compressive strain of -3.62% exhibits significantly enhanced activity for both the HzOR and HER than counterparts with tensile strain and without strain. Consequently, the optimized Ni2 P delivers current densities of 10 and 100 mA cm-2 at small cell voltages of 0.16 and 0.39 V for hydrazine-assisted water electrolysis, respectively. Density functional theory (DFT) calculations reveal that the compressive strain promotes water dissociation and concurrently tunes the adsorption strength of hydrogen intermediates, thereby facilitating the HER process on Ni2 P. As for the HzOR, the compressive strain reduces the energy barrier of the potential-determining step for the dehydrogenation of *N2 H4 to *N2 H3 . Clearly, this work paves a facile pathway to the synthesis of lattice-strained electrocatalysts via the dual-cation co-doping.

Highly Durable and Efficient Seawater Electrolysis Enabled by Defective Graphene-Confined Nanoreactor

Direct seawater electrolysis is a promising technology for massive green hydrogen production but is limited by the lack of durable and efficient electrocatalysts toward the oxygen evolution reaction (OER). Herein, we develop a core-shell nanoreactor as a high-performance OER catalyst consisting of NiFe alloys encapsulated within defective graphene layers (NiFe@DG) by a facile microwave shocking strategy. This catalyst needs overpotentials of merely 218 and 276 mV in alkalized seawater to deliver current densities of 10 and 100 mA cm-2, respectively, and operates continuously for 2000 h with negligible activity decay (1.0%), making it one of the best OER catalysts reported to date. Detailed experimental and theoretical analyses reveal that the excellent durability of NiFe@DG originates from the formation of the built-in electric field triggered by the defective graphene coating against chloride ions at the electrode/electrolyte interface, thus protecting the active NiFe alloys at the core from dissolution and aggregation under harsh operation conditions. Further, a highly stable and efficient seawater electrolyzer is assembled with the NiFe@DG anode and the Pt/C cathode to demonstrate the practicability of the catalysts.

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.

Coordination Tuning of Metal Porphyrins for Improved Oxygen Evolution Reaction

The nucleophilic attack of water or hydroxide on metal-oxo units forms an O-O bond in the oxygen evolution reaction (OER). Coordination tuning to improve this attack is intriguing but has been rarely realized. We herein report on improved OER catalysis by metal porphyrin 1-M (M=Co, Fe) with a coordinatively unsaturated metal ion. We designed and synthesized 1-M by sterically blocking one porphyrin side with a tethered tetraazacyclododecane unit. With this protection, the metal-oxo species generated in OER can maintain an unoccupied trans axial site. Importantly, 1-M displays a higher OER activity in alkaline solutions than analogues lacking such an axial protection by decreasing up to 150-mV overpotential to achieve 10 mA/cm2 current density. Theoretical studies suggest that with an unoccupied trans axial site, the metal-oxo unit becomes more positively charged and thus is more favoured for the hydroxide nucleophilic attack as compared to metal-oxo units bearing trans axial ligands.

Single-Atom Pt Embedded in Defective Layered Double Hydroxide for Efficient and Durable Hydrogen Evolution

Electrocatalysis in neutral conditions is appealing for hydrogen production by utilizing abundant wastewater or seawater resources. Single-atom catalysts (SACs) immobilized on supports are considered one of the most promising strategies for electrocatalysis research. While they have principally exhibited breakthrough activity and selectivity for the hydrogen evolution reaction (HER) electrocatalysis in alkaline or acidic conditions, few SACs were reported for HER in neutral media. Herein, we report a facile strategy to tailor the water dissociation active sites on the NiFe LDH by inducing Mo species and an ultralow single atomic Pt loading. The defected NiFeMo LDH (V-NiFeMo LDH) shows HER activity with an overpotential of 89 mV at 10 mA cm-2 in 1 M phosphate buffer solutions. The induced Mo species and the transformed NiO/Ni phases after etching significantly increase the electron conductivity and the catalytic active sites. A further enhancement can be achieved by modulating the ultralow single atom Pt anchored on the V-NiFeMo LDH by potentiostatic polarization. A potential as low as 37 mV is obtained at 10 mA cm-2 with a pronounced long-term durability over 110 h, surpassing its crystalline LDH materials and most of the HER catalysts in neutral medium. Experimental and density functional theory calculation results have demonstrated that the synergistic effects of Mo/SAs Pt and phase transformation into NiFe LDH reduce the kinetic energy barrier of the water dissociation process and promote the H* conversion for accelerating the neutral HER.

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.