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

Herein, a facile and efficient synthesis of microstructured Co 3 O 4 for both supercapacitor and water-splitting applications is reported. Metal cations (Fe 3+ , Cu 2+ ) serve as structure-directing agents regulating the structure of Co compounds, which are subsequently annealed to yield Co 3 O 4 . 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 Co 3 O 4 . The as-prepared Cl-doped Co 3 O 4 hierarchical nanospheres (Cl-Co 3 O 4 -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-Co 3 O 4 -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-Co 3 O 4 -h as both cathode and anode can be driven by Cl-Co 3 O 4 -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.