Here, a sol-gel method is used to prepare a Prussian blue analogue (NiFe-PBA) precursor with a 2D network, which is further annealed to an Fe 3 O 4 /NiC x composite (NiFe-PBA-gel-cal), inheriting the ultrahigh specific surface area of the parent structure. When the composite is used as both anode and cathode catalyst for overall water splitting, it requires low voltages of 1.57 and 1.66 V to provide a current density of 100 mA cm −2 in alkaline freshwater and simulated seawater, respectively, exhibiting no obvious attenuation over a 50 h test. Operando Raman spectroscopy and X-ray photoelectron spectroscopy indicate that NiOOH 2-x active species containing high-valence Ni 3+ /Ni 4+ are in situ generated from NiC x during the water oxidation. Density functional theory calculations combined with ligand field theory reveal that the role of high valence states of Ni is to trigger the production of localized O 2p electron holes, acting as electrophilic centers for the activation of redox reactions for oxygen evolution reaction. After hydrogen evolution reaction, a series of ex situ and in situ investigations indicate the reduction from Fe 3+ to Fe 2+ and the evolution of Ni(OH) 2 are the origin of the high activity.

Herein, a facile and efficient synthesis of microstructured Co3O4 for both supercapacitor and water‐splitting applications is reported. Metal cations (Fe3+, Cu2+) serve as structure‐directing agents regulating the structure of Co compounds, which are subsequently annealed to yield Co3O4. Detailed characterizations and density functional theory (DFT) calculations reveal that the in situ Cl‐doping introduces oxygen defects and provides abundant electroactive sites, and narrows the bandgap, which enhances the electron excitation of the as‐formed Co3O4. The as‐prepared Cl‐doped Co3O4 hierarchical nanospheres (Cl‐Co3O4‐h) display a high specific capacitance of 1629 F g−1 at 1 A g−1 as an electrode for supercapacitors, with excellent rate capability and cyclability. The Cl‐Co3O4‐h//activated carbon (AC) asymmetric supercapacitor (ASC) electrode achieves a specific capacitance of 237 F g−1 at 1 A g−1, with an energy density of 74 Wh kg−1 at a power density of 807 W kg−1 and even maintains 47 Wh kg−1 at the higher‐power density of 24.2 kW kg−1. An integrated electrolyzer for water‐splitting with Cl‐Co3O4‐h as both cathode and anode can be driven by Cl‐Co3O4‐h//AC ASC. The electrolyzer provides a high current density of 35 mA cm–2 at a cell voltage of 1.6 V, with good current density retention over 50 h.