Hierarchical Interconnected NiMoN with Large Specific Surface Area and High Mechanical Strength for Efficient and Stable Alkaline Water/Seawater Hydrogen Evolution

Enhancing hydrogen evolution by heterointerface engineering of Ni/MoN catalysts

Molybdenum nitrides have garnered significant attention for their potential in the hydrogen evolution reaction (HER) due to their metallic behavior, abundant reserves, and pH-universal stability. However, their unsatisfactory hydrogen adsorption limits industrial applications. Heterostructures can be designed to introduce defects and modulate the electronic structure of catalysts to optimize hydrogen adsorption to enhance HER. Nevertheless, the exact active sites at the heterointerface and the fundamental mechanisms underlying the HER process remain inadequately understood. Herein, a composite electrocatalyst in which metallic Ni and MoN phases (Ni/MoN) form the heterointerface between them is fabricated. The heterointerface produces strong electronic interactions between Ni and MoN to facilitate electron transfer from MoN to Ni, and the built-in electric field facilitates charge transfer during electrocatalysis. This optimized electronic configuration with abundant active sites delivers excellent performance in alkaline HER. Density-functional theory calculations demonstrate that H2O dissociates at the Ni site, whereas H2 desorption occurs at the Mo site. As a result, Ni/MoN/CC requires an overpotential of only 95 mV to achieve a current density of 10 mA cm-2 and a Tafel slope of 104 mV dec-1. Moreover, it maintains a high current density of 100 mA cm-2 for 100 h with negligible morphological or compositional changes. The strategy of modulating the electronic structure of low-cost, transition metal-based heterostructured electrocatalysts is an effective and commercially viable means to design and develop high-performance electrocatalysts for water splitting.

Dual Active Site Engineering in Porous NiW Bimetallic Alloys for Enhanced Alkaline Hydrogen Evolution Reaction

Utilizing dual active sites in electrocatalysts creates a synergistic effect, enabling the independent optimization of H2O dissociation and intermediate adsorption/desorption, which in turn enhances the efficiency of the hydrogen evolution reaction (HER). Herein, a porous NiW bimetallic alloy electrocatalyst using a dynamic H2 bubble template (DHBT) strategy is fabricated. This electrocatalyst capitalizes on the synergistic effect of dual active sites, achieving industrial-level current densities of 500 and 1000 mA cm-2 for HER in 1.0 M KOH, with low overpotentials of 198 and 264 mV, respectively. It also demonstrates excellent stability over a 200 h test. Theoretical studies reveal that alloying Ni with W shifts the d-band center (εd) of the W 5d orbital downward, which enhances *OH intermediate desorption and promotes H2O adsorption and dissociation at the W site, leading to increased active site availability. Meanwhile, this shift provides more accessible H* intermediates, further enhancing H2 production at the Ni2W1 hollow site. When the porous NiW bimetallic alloy electrocatalyst is implemented in a solar-driven water splitting system, it achieves a high solar-to-hydrogen (STH) conversion efficiency of 16.59%. This work underscores the effectiveness of dual active site electrocatalysts for sustainable H2 production.

Multiscale Biomimetic Evaporators Based on Liquid Metal/Polyacrylonitrile Composite Fibers for Highly Efficient Solar Steam Generation

Solar steam generation (SSG) offers a cost-effective solution for producing clean water by utilizing solar energy. However, integrating effective thermal management and water transportation to develop high-efficiency solar evaporators remains a significant challenge. Here, inspired by the hierarchical structure of the stem of bird of paradise, a three-dimensional multiscale liquid metal/polyacrylonitrile (LM/PAN) evaporator is fabricated by assembling LM/PAN fibers. The strong localized surface plasmon resonance of LM particles and porous structure of LM/PAN fibers with interconnected channels lead to efficient light absorption up to 90.9%. Consequently, the multiscale biomimetic LM/PAN evaporator achieves an outstanding water evaporation rate of 2.66 kg m-2 h-1 with a solar energy efficiency of 96.5% under one sun irradiation and an exceptional water rate of 2.58 kg m-2 h-1 in brine. Additionally, the LM/PAN evaporator demonstrates a superior purification performance for seawater, with the concentration of Na+, Mg2+, K+ and Ca2+ in real seawater dramatically decreased by three orders to less than 7 mg L-1 after desalination under light irradiation. The multiscale LM/PAN evaporator with hierarchical structure regulates the water transportation as well as thermal management for highly effective solar-driven evaporation, providing valuable insight into the structural design principles for advanced SSG systems.

Exploring P-(Fe,V)-Codoped Metastable-Phase β-NiMoO4 for Improving the Performance of Overall Water Splitting

It is especially essential to develop high-performance and low-cost nonprecious metal catalysts for large-scale hydrogen production. A large number of electrochemical catalysts composited by transition metal centers has been reported; however, it is still a great challenge to design and manipulate target electrocatalysts to realize high overall water-splitting activity at the atomic level. Herein, we develop totally new P-(Fe,V)-codoped metastable-phase β-NiMoO4. As an electrocatalyst, it can realize oxygen evolution at only 163 mV and hydrogen evolution at only 44 mV at 10 mA cm-2. It, as both an anode and a cathode, is fabricated into a cell for overall water splitting, which has an ultralow voltage value of 1.48 V to drive a current density of 10 mA cm-2 and can remain stable for at least 100 h. In the target electrode, the P element plays three important roles: (1) it can stabilize the metastable-phase structure of β-NiMoO4; (2) it can further optimize the electronic structure; and (3) it can provide more active sites. The synergistic effect for multimetal centers with different redox couples is key for the great improvement of catalytic activity. The related mechanism is discussed in detail.

Comprehensive Chlorine Suppression: Advances in Materials and System Technologies for Direct Seawater Electrolysis

Seawater electrolysis offers a promising pathway to generate green hydrogen, which is crucial for the net-zero emission targets. Indirect seawater electrolysis is severely limited by high energy demands and system complexity, while the direct seawater electrolysis bypasses pre-treatment, offering a simpler and more cost-effective solution. However, the chlorine evolution reaction and impurities in the seawater lead to severe corrosion and hinder electrolysis's efficiency. Herein, we review recent advances in the rational design of chlorine-suppressive catalysts and integrated electrolysis systems architectures for chloride-induced corrosion, with simultaneous enhancement of Faradaic efficiency and reduction of electrolysis's cost. Furthermore, promising directions are proposed for durable and efficient seawater electrolysis systems. This review provides perspectives for seawater electrolysis toward sustainable energy conversion and environmental protection.

Phosphorus and sulfur co-doped nickel molybdate with rich-oxygen vacancies for efficient water splitting

The rational design of high-performance electrocatalysts is essential for promoting the industrialization of electrocatalytic water-splitting technology. Herein, phosphorus and sulfur co-doped nickel molybdate with rich-oxygen vacancies (P, S-NiMoO4) was prepared as an efficient bifunctional self-supporting water-splitting catalyst from the perspective of enhancing the conductivity and optimizing the electronic configurations. The incorporation of P, S and oxygen vacancies greatly enhances the conductivity and charge-transfer efficiency of NiMoO4. Additionally, P and S can serve as proton carriers and electron acceptors to enhance the catalytic activity by accelerating proton activation and high-valent metal generation in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As expected, P, S-NiMoO4 demonstrates efficient bifunctional catalytic activity with an overpotential of only 31/206 mV at 10 mA cm-2 for HER/OER in 1 M KOH. Meantime, the electrolyzer assembled with P, S-NiMoO4 as electrodes requires a voltage of only 1.55 V to achieve a water-splitting current density of 50 mA cm-2 along with good stability over 110 h. This work puts forward a novel approach based on elemental doping and vacancy engineering for the design of effective and enduring catalysts for water splitting.

Anchoring Ni(OH)2-CeOx Heterostructure on FeOOH-Modified Nickel-Mesh for Efficient Alkaline Water-Splitting Performance with Improved Stability under Quasi-Industrial Conditions

Developing low-cost and industrially viable electrode materials for efficient water-splitting performance and constructing intrinsically active materials with abundant active sites is still challenging. In this study, a self-supported porous network Ni(OH)2-CeOx heterostructure layer on a FeOOH-modified Ni-mesh (NiCe/Fe@NM) electrode is successfully prepared by a facile, scalable two-electrode electrodeposition strategy for overall alkaline water splitting. The optimized NiCe0.05/Fe@NM catalyst reaches a current density of 100 mA cm-2 at an overpotential of 163 and 262 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively, in 1.0 m KOH with excellent stability. Additionally, NiCe0.05/Fe@NM demonstrates exceptional HER performance in alkaline seawater, requiring only 148 mV overpotential at 100 mA cm-2. Under real water splitting conditions, NiCe0.05/Fe@NM requires only 1.701 V to achieve 100 mA cm-2 with robust stability over 1000 h in an alkaline medium. The remarkable water-splitting performance and stability of the NiCe0.05/Fe@NM catalyst result from a synergistic combination of factors, including well-optimized surface and electronic structures facilitated by an optimal Ce ratio, rapid reaction kinetics, a superhydrophilic/superaerophobic interface, and enhanced intrinsic catalytic activity. This study presents a simple two-electrode electrodeposition method for the scalable production of self-supported electrocatalysts, paving the way for their practical application in industrial water-splitting processes.

Scalable Ir-Doped NiFe2O4/TiO2 Heterojunction Anode for Decentralized Saline Wastewater Treatment and H2 Production

Wastewater electrolysis cells (WECs) for decentralized wastewater treatment/reuse coupled with H2 production can reduce the carbon footprint associated with transportation of water, waste, and energy carrier. This study reports Ir-doped NiFe2O4 (NFI, ~ 5 at% Ir) spinel layer with TiO2 overlayer (NFI/TiO2), as a scalable heterojunction anode for direct electrolysis of wastewater with circumneutral pH in a single-compartment cell. In dilute (0.1 M) NaCl solutions, the NFI/TiO2 marks superior activity and selectivity for chlorine evolution reaction, outperforming the benchmark IrO2. Robust operation in near-neutral pH was confirmed. Electroanalyses including operando X-ray absorption spectroscopy unveiled crucial roles of TiO2 which serves both as the primary site for Cl- chemisorption and a protective layer for NFI as an ohmic contact. Galvanostatic electrolysis of NH4+-laden synthetic wastewater demonstrated that NFI/TiO2 not only achieves quasi-stoichiometric NH4+-to-N2 conversion, but also enhances H2 generation efficiency with minimal competing reactions such as reduction of dissolved oxygen and reactive chlorine. The scaled-up WEC with NFI/TiO2 was demonstrated for electrolysis of toilet wastewater.

Regulating Mo-based alloy-oxide active interfaces for efficient alkaline hydrogen evolution assisted by hydrazine oxidation

Improving the efficiency of overall water splitting (OWS) is crucial due to the slow four-electron transfer process in the oxygen evolution reaction (OER). The coupling of the thermodynamically favorable hydrazine oxidation reaction (HzOR) with the hydrogen evolution reaction (HER) significantly boosts hydrogen production. A Ru-decorated MoNi/MoO2 micropillar (Ru-MoNi/MoO2) has been synthesized using a hydrothermal followed by reduction annealing. Benefiting from Ru moderating the active interface of Mo-based alloys/oxides and the unique one-dimensional micropillar morphology. The synthesized Ru-MoNi/MoO2 exhibits outstanding bifunctional activity for HER and HzOR, achieving 10 mA cm-2 at merely -13 mV and -34 mV in 1 M KOH and 1 M KOH + 0.5 M N2H4, respectively. Notably, with Ru-MoNi/MoO2 in a dual-electrode setup, only 0.57 V is needed to achieve 50 mA cm-2, demonstrating good stability and facilitating hydrazine-assisted water splitting (OHzS). This work offers insights into the modulation of alloy/metal oxide active interfaces, contributing to the development of efficient bifunctional catalysts for HER and HzOR.

Design Strategies of Hydrogen Evolution Reaction Nano Electrocatalysts for High Current Density Water Splitting

Hydrogen is now recognized as the primary alternative to fossil fuels due to its renewable, safe, high-energy density and environmentally friendly properties. Efficient hydrogen production through water splitting has laid the foundation for sustainable energy technologies. However, when hydrogen production is scaled up to industrial levels, operating at high current densities introduces unique challenges. It is necessary to design advanced electrocatalysts for hydrogen evolution reactions (HERs) under high current densities. This review will briefly introduce the challenges posed by high current densities on electrocatalysts, including catalytic activity, mass diffusion, and catalyst stability. In an attempt to address these issues, various electrocatalyst design strategies are summarized in detail. In the end, our insights into future challenges for efficient large-scale industrial hydrogen production from water splitting are presented. This review is expected to guide the rational design of efficient high-current density water electrolysis electrocatalysts and promote the research progress of sustainable energy.

Dual-strategy engineered nickel phosphide for achieving efficient hydrazine-assisted hydrogen production in seawater

Electrocatalytic hydrogen production in seawater to alleviate freshwater shortage pressures is promising, but is hindered by the sluggish oxygen evolution reaction and detrimental chloride electrochemistry. Herein, a dual strategy approach of Fe-doping and CeO2-decoration in nickel phosphide (Fe-Ni2P/CeO2) is rationally designed to achieve superior bifunctional catalytic performance for the hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR) in seawater. Notably, the two-electrode Fe-Ni2P/CeO2-based hybrid seawater electrolyzer realizes energy-efficient and chlorine-free hydrogen production with ultralow cell voltages of 0.051 and 0.597 V at 10 and 400 mA cm-2, which are significantly lower than those needed in the hydrazine-free seawater electrolyzer. Density functional theory calculations manifest that the combination of Fe doping and heterointerface construction between Fe-Ni2P and CeO2 can adjust the electronic structure of the Ni2P and optimize the water dissociation barrier and hydrogen adsorption free energy, leading to improvement of the intrinsic catalytic performance. This route affords a feasible solution for future large-scale hydrogen generation using abundant ocean water.

Interface engineering of three-phase nickel-cobalt sulfide/nickel phosphide/iron phosphide heterostructure for enhanced water splitting and urea electrolysis

Rational designing efficient transition metal-based multifunctional electrocatalysts is highly desirable for improving the efficiency of hydrogen production from water cracking. Herein, a self-supported three-phase heterostructure electrocatalyst of nickel-cobalt sulfide/nickel phosphide/iron phosphide (CoNi5S8-Ni2P-FeP2) was prepared by a two-step gas-phase sulfurization/phosphorization strategy. The heterostructure in CoNi5S8-Ni2P-FeP2 provides a favorable interfacial environment for electron transfer and synergistic interaction of multiphase active components, while the introduced electronegative P/S not only serves as a carrier for proton capture in the hydrogen evolution reaction (HER) process but also promotes the metal-electron outflow, which in turn accelerates the generation of high-valent Ni3+ species to enhance the catalytic activity of oxygen evolution reaction (OER) and urea oxidation reaction (UOR). As expected, CoNi5S8-Ni2P-FeP2 reveals excellent multifunctional electrocatalytic properties. An overpotential of 35/215 mV is required to reach 10 mA cm-2 for HER/OER. More encouragingly, a current of 100 mA cm-2 requires only 1.36 V for UOR with CoNi5S8-Ni2P-FeP2 as anode, which is much lower as compared to the OER (1.50 V). Besides, a two-electrode water/urea electrolyzer assembled based on CoNi5S8-Ni2P-FeP2 has a voltage of only 1.59/1.48 V when the system reaches 50 mA cm-2. This work provides a new idea for the design of energy-efficient water/urea-assisted water-splitting multifunctional catalysts with multi-component heterostructure synergistic interface engineering.

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).

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.