One-pot formation of ultra-thin Ni/Co hydroxides with a sheet-like structure for enhanced asymmetric supercapacitors

In our work, ultra-thin Ni/Co hydroxides (Ni1Co2) with a sheet-like structure are successfully grown on the surface of conductive nickel foam via a one-pot hydrothermal reaction, avoiding the need for binders and conducting agents. In the reaction, NO3- ions from reactants release OH- ions partially with Co2+ ions turning into Co3+ ions and then OH- ions can react with Ni2+, Co2+ or Co3+ in a mixed solvent of water and ethanol, without the necessary addition of an extra alkali source. When used for supercapacitors, the resulting free-standing Ni1Co2 composites exhibit an impressive specific capacitance of 2654.9 F g(-1) at a current density of 1 A g(-1) and a favorable cycling stability with 77% retention even at a high current density of 10 A g(-1) after 1500 cycles. In order to do research into the electrochemical properties of Ni1Co2 composites, a simple asymmetric supercapacitor (ASC) is constructed, using the Ni1Co2 nanosheets as the positive electrode and activated carbon (AC) as the negative electrode. This ASC with an extended voltage window of 0-1.6 V presents an outstanding energy density of 42.4 W h kg(-1) (at a power density of 823.2 W kg(-1)) and still retains 24.8 W h kg(-1) (at 10 170.8 W kg(-1)). Meanwhile, the excellent cycling stability of this ASC device has been revealed via a great specific capacitance retention of 94% after 3000 cycles (at a current density of 5 A g(-1)). So the Ni1Co2-based composites could be one of the potential materials for high-performance energy storage.