Using Metal Cation to Control the Microstructure of Cobalt Oxide in Energy Conversion and Storage Applications

Abstract 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.