High buffering capacity cobalt-doped nickel hydroxide electrode as redox mediator for flexible hydrogen evolution by two-step water electrolysis.

Two-step water electrolysis has been proposed to tackle the ticklish H/O mixture problems in conventional alkaline water electrolysis recently. However, low buffering capacity of pure nickel hydroxide electrode as redox mediator limited practical application of two-step water electrolysis system. A high-capacity redox mediator (RM) is urgently needed to permit consecutive operation of two-step cycles and high-efficiency hydrogen evolution. Consequently, a high mass-loading cobalt-doped nickel hydroxide/active carbon cloth (NiCo-LDH/ACC) RM is synthesized via a facile electrochemical method. The proper Co doping can apparently enhance the conductivity and simultaneously remain the high-capacity of the electrode. Density functional theory results further confirms more negative values in redox potential of NiCo-LDH/ACC than Ni(OH)/ACC on account of the charge redistribution induced by Co doping, which can prevent the parasitic O evolution on RM electrode during decoupled H evolution step. As a result, the NiCo-LDH/ACC combined the superiorities of high-capacity Ni(OH)/ACC and high-conductivity Co(OH)/ACC, and the NiCo-LDH/ACC with 4:1 ratio of Ni to Co presented a large specific capacitance of 33.52F/cm for reversible charge-discharge and high buffering capacity with two-step H/O evolution duration of 1740 s at 10 mA/cm. The necessary input voltage (2.00 V) of the whole water electrolysis was broken into two smaller ones, 1.41 and 0.38 V, for H and O production, respectively. NiCo-LDH/ACC provided a favorable electrode material for the practical application of two-step water electrolysis system.