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

Herein, a facile and efficient synthesis of microstructured Co3 O4 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 Co3 O4 . 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 Co3 O4 . The as-prepared Cl-doped Co3 O4 hierarchical nanospheres (Cl-Co3 O4 -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-Co3 O4 -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-Co3 O4 -h as both cathode and anode can be driven by Cl-Co3 O4 -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.