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carbon-intensive fuels. Hydrogen represents a versatile fuel with high energy density and low carbon emission. In 2020, the global hydrogen generation market size was valued at ≈120 billion USD and is expected to rapidly expand. Presently, industrial hydrogen is primarily acquired from natural gas reforming that involves energy-consuming processes with extensive greenhouse gas release. Alternatively, electrochemical water splitting is a sustainable technique to store the intermittent electricity and produce high-purity hydrogen under ambient condition. [1] The water-splitting process consists of anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), in which electrocatalysts are particularly vital to lower their energy barriers and reduce the required overpotentials. [2] Currently, precious metal (e.g., Pt) and metal oxides (e.g., RuO 2 , and IrO 2 ) hold the benchmark performance toward HER and OER, respectively. [3] However, their nature scarcity and high cost considerably impede their widespread utilization. Therefore, it is desirable to develop costeffective OER/HER catalysts with active, abundant, and robust surface reactive sites. Non-precious transition metal-based materials, such as oxides, [4] sulfides, [5] LDHs, [6] MOFs, [7] and their composites, [8] have received intensive research in water splitting due to their tunable structure, stability, as well as high reserve. Strategies including morphology and components tuning are applied for advanced electrocatalysts. High material surface area and porosity benefit the intermediate accessibility and fast mass transport during the electrochemical reactions. Researchers pursuit improved electrocatalysis by fabricating porous or hierarchical catalysts with highly exposed reactive sites. [9] However, the synthesis of the highly porous transition metal catalysts with the controlled crystalline phase using the traditional template strategy often needs tedious and timeconsuming manufacturing. In addition, regarding the required bifunctionality (i.e., OER and HER) for water splitting, the integration of different nanostructures by heterostructure fabrication is adopted for further optimizing the electrocatalytic performance. The interface engineering and electronic regulation facilitate the optimized chemisorption of the reaction Developing efficient bifunctional electrocatalysts toward oxygen/hydrogen evolution reactions is crucial for electrochemical water splitting toward hydrogen production. The high-performance electrocatalysts depend on the catalytically active and highly accessible reaction sites and their structural robustness, while the rational design of such electrocatalysts with desired features avoiding tedious manufacture is still challenging. Here, a facile method is reported to synthesize mesoporous and heterostructured transition metal oxides strongly anchored on a nickel skeleton (MH-TMO) containing identified Fe-Cu oxide interfaces with high intrinsic activity, easy accessibility for reaction intermediates, ...

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Rational heterointerface engineering is crucial for superior and robust hydrogen evolution reaction (HER). Herein, a delicate organic-inorganic hybrid heterojunction based on the assembly of oxalate with polyaniline (PANI) for HER at high-current-densities is envisioned. Strong π-d electron coupling is achieved between the delocalized π electrons of PANI and the localized d electrons of oxalate metal sites. The CoC 2 O 4 nanosheets are grown on nickel foam (NF) with Ni 2+ ions substitution by the precursor etching. By virtue of the synergy of hetero ions and π-d electron coupling, metal sites obtain sufficient exposure and electronic structure optimization. Surprisingly, the phase transition of oxalate during HER in the alkaline environment does not weaken the π-d electronic coupling of the organicinorganic hybrid interfaces. Inheritable interfacial electron interaction provides a reliable guarantee for robust stability at high-current-densities while endowing the hybrid materials with extremely low overpotentials. As expected, post-phase reconstructed Co 0.59 Ni 0.41 (OH) 2 @PANI/NF displays impressive HER activity, with a low overpotential of 43 mV@−10 mA cm −2 and robust stability at −1000 mA cm −2 for 30 h in the alkaline environment. This study sheds light on the rational heterostructure interface design and promotes the architecture of an impressive electrocatalysts system.