Superhydrophilic/Superaerophobic Hierarchical NiP2@MoO2/Co(Ni)MoO4 Core-Shell Array Electrocatalysts for Efficient Hydrogen Production at Large Current Densities

Synergistic design of hierarchical and heterostructural P-NiMoO4@Net-like Ni2P for enhanced hydrogen evolution electrocatalysis

Hierarchical structure design and heterostructure engineering are effective strategies for enhancing hydrogen evolution reaction (HER) performance, yet their synergistic integration remains underexplored. In this work, a novel hierarchical P-NiMoO4@Net-like Ni2P heterostructure HER electrocatalyst was prepared via a two-step hydrothermal method followed by low-temperature phosphorization. The three-dimensional net like Ni2P was closely integrated with one-dimensional phosphorus-doped NiMoO4 micro/nanorod arrays, enabling hierarchical structural assembly and the formation of synergistic heterointerfaces. The hierarchical structure significantly increased active site exposure, with a double-layer capacitance of 254.4 mF cm-2, more than five times that of single-component Ni2P (48.4 mF cm-2). Density functional theory calculations revealed that the heterostructure lowered the d-band center of active Ni3 sites and optimized the hydrogen adsorption energy, thereby enhancing HER activity. The P-NiMoO4@Net-like Ni2P catalyst exhibited an alkaline HER overpotential of 49 mV at a current density of 10 mA cm-2. It also maintained stable operation for 100 h at 10mA cm-2 and 120 h at 100mA cm-2. This study demonstrates the potential of integrating hierarchical and heterostructural strategies, providing a reference for advanced nanostructured catalyst development.

Hierarchically Structured Catalysts Toward Sustainable Hydrogen Economy: Electro- and Thermo-Chemical Pathways

Hydrogen, as an important clean energy source, plays a more and more crucial role in decarbonizing the planet and meeting the global climate challenge due to its high energy density and zero-emission. The demand for sustainable hydrogen is increasing drastically worldwide as driven by the global shift towards low-carbon energy solutions. Thermochemical catalysis process dominates hydrogen production at scale given its relatively mature technology and commercialization status, as well as the established manufacturing infrastructure. While due to its environmentally friendly nature and growing abundant sources of renewable electricity, the electrochemical path for hydrogen production is rising as a major alternative to the thermochemical means. Nevertheless, hierarchically structured catalysts and devices have gradually taken the center stage toward replacing the traditional counterparts, especially with the rapid advancement of the design and manufacture of such ordered nanostructure assemblies toward high activity, efficient mass transport, and superb stability. In this review, the latest progress of the hierarchically structured catalysts for hydrogen production have been surveyed on electro- and thermo- chemical pathways comparatively. It covers the structure designs of atomic dispersion, nanoscale surfaces and interfaces for achieving highly active and durable catalysts, components, and devices. Both electrochemical and thermochemical approaches are reviewed in terms of the vast design details, engineered benefits, and understandings of various Pt-group metal (PGM) and non-PGM based transition metal catalysts for hydrogen production. As the growing trend, brief discussions are also presented toward the high-level assembly and manufacture of complexly structured components and devices at scale in the electrochemical and thermochemical energy systems.

A Ni2P/NiMoOx nanocone electrocatalyst for efficient hydrogen evolution: tip-enhanced local electric field effect

The sluggish kinetics of the hydrogen evolution reaction (HER) result in a high overpotential in alkaline solutions. A high-curvature metal oxide heterostructure can effectively boost the electrocatalytic HER by leveraging the tip-enhanced local electric field effect. Herein, Ni2P/NiMoOx nanocones were synthesised on a nickel foam (NF) substrate by etching a metal-organic framework template. The Ni2P/NiMoOx nanocones on the NF substrate served as an advanced electrocatalyst for the HER. Analysis using the finite element method indicated that the high-curvature tips of the Ni2P/NiMoOx nanocones enhanced the local electric field, resulting in a higher concentration of hydrated K+ ions (K(H2O)6+), which facilitated water dissociation and accelerated the reaction kinetics. The tip-enhanced local electric field effect accelerates the mass transfer rate, and the heterostructure promotes charge transfer to activate the active center, thereby synergically enhancing the electrocatalytic reaction. The Ni2P/NiMoOx nanocone electrocatalyst exhibited low overpotentials of 49, 137 and 274 mV at 10, 100 and 500 mA cm-2, respectively, under alkaline conditions for the HER. In addition, the electrocatalyst demonstrated excellent stability over 200 h at 300 mA cm-2. This study provides a promising approach for developing efficient electrocatalysts that facilitate the HER in alkaline solutions.

Synergizing superwetting and architected electrodes for high-rate water splitting

Water splitting is one of the most promising technologies for generating green hydrogen. To meet industrial demand, it is essential to boost the operation current density to industrial levels, typically in the hundreds of mA cm-2. However, operating at these high current densities presents significant challenges, with bubble formation being one of the most critical issues. Efficient bubble management is crucial as it directly impacts the performance and stability of the water splitting process. Superwetting electrodes, which can enhance aerophobicity, are particularly favorable for facilitating bubble detachment and transport. By reducing bubble contact time and minimizing the size of detached bubbles, these electrodes help prevent blockage and maintain high catalytic efficiency. In this review, we aim to provide an overview of recent advancements in tackling bubble-related issues through the design and implementation of superwetting electrodes, including surface modification techniques and structural optimizations. We will also share our insights into the principles and mechanisms behind the design of superwetting electrodes, highlighting the key factors that influence their performance. Our review aims to guide future research directions and provides a solid foundation for developing more efficient and durable superwetting electrodes for high-rate water splitting.

Electrodeposition synthesis of cobalt-molybdenum bimetallic phosphide on nickel foam for efficient water splitting

A reasonable design of excellent bifunctional catalyst is an effective strategy for large-scale hydrogen production. In this study, a two-stage electrodeposition method was used to prepare a crystalline-amorphous structure cobalt molybdenum phosphide layered particles with different sizes on a nickel foam (NF) substrate. Electron rearrangement at the Co/CoMoP2@CoMoO4 heterogeneous interface can reduce the reaction energy barrier for HER and OER, and accelerate the catalytic reaction kinetics. The doping of Mo can promote the synergistic effect between Co and Mo, thereby optimizing the Gibbs free energy of hydrogen adsorption/desorption. This layered arrangement of different size particles greatly improves the active area of the catalyst. In alkaline solution, achieving a current density of 10 mA cm-2 only required overpotentials of 40 mV for HER and 278 mV for OER, respectively. The cell voltage required for the CoMo-P/NF||CoMo-P/NF electrolytic cell is only 1.53 V at 10 mA cm-2. This study provides a reference for the rapid, efficient, and environmentally friendly preparation of high-activity water splitting catalysts with large surface areas.

Multifunctional molybdenum-tuning porous nickel-cobalt bimetallic phosphide nanoarrays for efficient water splitting and energy-saving hydrogen production

The sluggish kinetics of the hydrogen evolution reaction (HER) and substantial barriers in the oxygen evolution reaction (OER) significantly impede its application in hydrogen production. To address this issue and enhance energy efficiency in hydrogen generation, we explored a high-activity alkaline HER catalyst while concurrently coupling it with the urea oxidation reaction (UOR). In this work, we designed and synthesized porous molybdenum (Mo)-modulated nickel-cobalt bimetallic phosphide nanoarrays (M0.3NCP@NF). This multifunctional self-supported electrocatalyst demonstrates superior performance in HER, OER, and UOR. The introduction of Mo, in the form of CoMoO4 nanoparticles, promotes interfacial electron transfer to reduce the electron density around the cations in phosphides, enhancing the kinetics and intrinsic activity. Furthermore, the morphological changes induced by Mo accelerate both electron and mass transfer processes. Density functional theory and operando electrochemical impedance spectroscopy indicate that Mo introduction optimizes the interaction with HER intermediate H*, facilitating the conversion to a high-valent active intermediate for OER and accelerating UOR kinetics. Benefiting from dual optimization of morphology and structure, the as-prepared M0.3NCP@NF electrocatalyst demonstrates outstanding HER, OER, and UOR performances. Notably, a full urea electrolysis device powered by M0.3NCP@NF operates with a cell voltage of only 1.53 V to achieve a current density of 100 mA cm-2. which is 240 mV lower than that of conventional water electrolysis, demonstrating the competitive potential of our approach for efficient and energy-saving hydrogen production, along with simultaneous urea wastewater remediation.

Efficient Self-Powered Overall Water Splitting by Ni4 Mo/MoO2 Heterogeneous Nanorods Trifunctional Electrocatalysts

The exploration of cost-effective multifunctional electrodes with high activity toward energy storage and conversion systems, such as self-powered alkaline water electrolysis, is very meaningful, although studies remain quite limited. Herein, a heterogeneous nickel-molybdenum (NiMo)-based electrode is fabricated for the first time as a trifunctional electrode for asymmetric supercapacitor (ASC), hydrogen evolution reaction, and oxygen evolution reaction. The trifunctional electrode consists of Ni₄ Mo and MoO₂ (denoted Ni₄ Mo/MoO₂ ) with hierarchical nanorod heterostructure and abundant heterogeneous nanointerfaces creating sufficient active sites and efficient charge transfer for achieving high performance self-power electrochemical devices. The ASC consists of the as-prepared Ni₄ Mo/MoO₂ positive electrode, showing a broad potential window of 1.6 V, and a maximum energy density of 115.6 Wh kg^-1 , while the alkaline overall water splitting (OWS) assembled using the as-prepared Ni₄ Mo/MoO₂ as bifunctional catalysts only requires a low cell voltage of 1.48 V to achieve a current density of 10 mA cm^-2 in aqueous alkaline electrolyte. Finally, by integrating the Ni₄ Mo/MoO₂ -based ASC and OWS devices, an aqueous self-powered OWS is assembled, which self-power the OWS to generate hydrogen gas and oxygen gas, verifying great potential of the as-prepared Ni₄ Mo/MoO₂ for sustainable and renewable energy storage and conversion system.

Engineered Superhydrophilic/Superaerophobic Catalyst: Two-Dimensional Co(OH)2-CeO2 Nanosheets Supported on Three-Dimensional Co Dendrites for Overall Water Splitting

Efficient electrocatalysts require not only a tunable electronic structure but also great active site accessibility and favorable mass transfer. Here, a two-dimensional/three-dimensional (2D/3D) hierarchical electrocatalyst consisting of Co(OH)₂-CeO₂ nanosheet-decorated Co dendrites is proposed, named as Co(OH)₂-CeO₂/Co. Based on the strong electronic interaction of the Co(OH)₂-CeO₂ heterojunction, the electronic structure of the Co site is optimized, which facilitates the adsorption of intermediates and the dissociation of H₂O. Moreover, the open 2D/3D structure formed by introducing the Co substrate further reduces the accumulation of heterogeneous nanosheets and promotes the radial diffusion of the electrolyte, significantly improving the utilization of active sites and shortening the electron transfer pathway. In addition, the superhydrophilic/superaerophobic interface achieved by constructing the hierarchical micro-nanostructure is beneficial to electrolyte infiltration and bubble desorption, thus ensuring favorable mass transfer. Therefore, Co(OH)₂-CeO₂/Co exhibits an excellent overall water-splitting activity in alkaline solution.