Highlights Three-dimensional (3D) core‐shell heterostructured NixSy@MnOxHy nanorods grown on nickel foam (NixSy@MnOxHy/NF) were successfully fabricated via a simple hydrothermal reaction and a subsequent electrodeposition process. The fabricated NixSy@MnOxHy/NF shows outstanding bifunctional activity and stability for hydrogen evolution reaction and oxygen evolution reaction, as well as overall‐water‐splitting performance. The main origins are the interface engineering of NixSy@MnOxHy, the shell‐protection characteristic of MnOxHy, and the 3D open nanorod structure, which remarkably endow the electrocatalyst with high activity and stability. Abstract Exploring highly active and stable transition metal-based bifunctional electrocatalysts has recently attracted extensive research interests for achieving high inherent activity, abundant exposed active sites, rapid mass transfer, and strong structure stability for overall water splitting. Herein, an interface engineering coupled with shell-protection strategy was applied to construct three-dimensional (3D) core‐shell NixSy@MnOxHy heterostructure nanorods grown on nickel foam (NixSy@MnOxHy/NF) as a bifunctional electrocatalyst. NixSy@MnOxHy/NF was synthesized via a facile hydrothermal reaction followed by an electrodeposition process. The X-ray absorption fine structure spectra reveal that abundant Mn‐S bonds connect the heterostructure interfaces of NixSy@MnOxHy, leading to a strong electronic interaction, which improves the intrinsic activities of hydrogen evolution reaction and oxygen evolution reaction (OER). Besides, as an efficient protective shell, the MnOxHy dramatically inhibits the electrochemical corrosion of the electrocatalyst at high current densities, which remarkably enhances the stability at high potentials. Furthermore, the 3D nanorod structure not only exposes enriched active sites, but also accelerates the electrolyte diffusion and bubble desorption. Therefore, NixSy@MnOxHy/NF exhibits exceptional bifunctional activity and stability for overall water splitting, with low overpotentials of 326 and 356 mV for OER at 100 and 500 mA cm–2, respectively, along with high stability of 150 h at 100 mA cm–2. Furthermore, for overall water splitting, it presents a low cell voltage of 1.529 V at 10 mA cm–2, accompanied by excellent stability at 100 mA cm–2 for 100 h. This work sheds a light on exploring highly active and stable bifunctional electrocatalysts by the interface engineering coupled with shell-protection strategy.

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Hydrogen (H 2 ) has been proposed as a future energy carrier in the transition from the current hydrocarbon economy. [2] In particular, the production of molecular H 2 from electrocatalytic water splitting is an attractive solution. [3] However, because of the sluggish reaction kinetics caused by high energy barriers, H 2 production from water splitting is not economical, which hinders its large-scale application. Water splitting consists of two half-reactions, including the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). Their sluggish kinetics, especially for OER, lead to large overpotentials that require higher energy consumption to drive an electrolytic cell. High-performance electrocatalysts for water splitting can reduce overpotentials efficiently, which can increase the efficiency of the electrochemical process. [4] As yet, precious-metal-based materials, such as Pt, Ru, and Ir, are still the most potential electrocatalysts for water splitting, but the scarcity and high cost of these materials prohibit their wide-scale industrial application for electrolysis. [4,5] Meanwhile, producing different catalysts for OER and HER requires different equipment and processes, which could increase the manufacturing cost. [6] As a result, these above disadvantages have motivated extensive research interest to synthesize various earth-abundant bifunctional electrocatalysts with high activities and stabilities for water splitting in the past few years.Until now, tremendous efforts have been made to develop various transition metals and their derivatives to prepare lowcost and high-performance water-splitting electrocatalysts, such as transition-metal oxides, [7] chalcogenides, [8] phosphides, [9] and nitrides, [10] in which transition-metal phosphides, especially Ni 2 P, [11] have recently attracted increasing research interest owing to their moderate interaction with hydrogen, relatively high mechanical strength, electrical conductivity, and chemical stability. In addition, it has been reported by a large number of articles that Fe can efficiently enhance the OER activities of electrocatalysts. [12] Accordingly, through integrating the above advantages, it may be a promising route to develop nickel-ironbased phosphides as bifunctional electrocatalysts for overall water splitting. In addition, for commercial applications, Exploring highly active and stable bifunctional water-splitting electrocatalysts at ultra-high current densities is remarkably desirable. Herein, 3D nickel-iron phosphides nanosheets modified by MnO x nanoparticles are grown on nickel foam (MnO x /NiFeP/NF). Resulting from the electronic coupling effect enabled by interface modifications, the intrinsic activities of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are improved. Meanwhile, 3D nanosheets provide abundant active sites for HER and OER, leading to accelerating the reaction kinetics. Besides, the shell-protection characteristic of MnO x improves the durability of MnO x /NiFeP/...

Efficient, low-cost, and stable electrocatalysts for water splitting are highly desirable. Herein, three-dimensional (3D) Ni 2 P nanosheet arrays were fabricated and simultaneously modulated by heterostructure engineering and Mn doping (Mn-doped Ni 2 O 3 /Ni 2 P and Mn-doped Ni x S y /Ni 2 P) via a facile hydrothermal reaction and subsequent phosphorization and sulfurization. Due to the Mn doping, synergistic effect in the heterostructures, and abundantly exposed active sites from the 3D-nanosheet arrays, Mn-doped Ni 2 O 3 /Ni 2 P and Mn-doped Ni x S y /Ni 2 P exhibit excellent properties for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. The former achieves an excellent current density of −10 mA cm −2 at a low overpotential of 104 mV for HER, while the latter attains 100 mA cm −2 for OER at an ultralow overpotential of 290 mV and exhibits superior stability at 50 mA cm −2 for 160 h. Impressively, the Mndoped Ni 2 O 3 /Ni 2 P//Mn-doped Ni x S y /Ni 2 P couple show high overall-water-splitting activity with a cell voltage of 1.65 V at 10 mA cm −2 and outstanding durability at 50 mA cm −2 for 120 h in an alkaline electrolyzer. This work presents an effective strategy to design and synthesize low-cost and highly active non-noble metal electrocatalysts for overall water splitting through the simultaneous application of heterostructure engineering, foreign-metal-atom doping, and a 3Dnanoarray structure. The strategy brings a paradigm shift toward the mass production of low-cost non-noble metal electrocatalysts for renewable energy devices.

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