High area-specific capacitance of Co(OH)₂/hierarchical nickel/nickel foam supercapacitors and its increase with cycling

Supercapacitors are an important energy storage systems due to their high power compared to batteries, giving them unique applications. The search for low-cost and high-surface-area pseudocapacitive metal oxides/hydroxides with good conductivity as supercapacitor materials is a developing trend. In this work, the nanometer-scale active material Co(OH)2 was deposited on well-designed hierarchical nickel/nickel foam (current collector) with significantly enhanced conductivity, and the electrodes exhibited excellent electrochemical performance, displaying a high area-specific capacitance of 3.17 F cm-2 under the current density of 5 mA cm-2. Importantly, the capacitance increased to 9.62 F cm-2 after 2000 cycles of galvanostatic charging–discharging, which is 3.03 times as great as the initial value. Such a capacitance increase could be attributed to the formation of porous Ni–Co mixed hydroxide with a micro/nanostructure during cycling, which was confirmed by scanning and transmission electron microscopy investigations. The formation mechanism of Ni–Co mixed hydroxide is also proposed. This result demonstrates that the electrochemical reactions during galvanostatic charging–discharging could represent a novel fabrication method to synthesize mixed hydroxides for energy storage and conversion materials in the future.