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.