A cobalt–manganese modified theophrastite phase of nickel hydroxide nanoflower arrays on nickel foam as a self-standing bifunctional electrode for overall water electrolysis

Abstract

Designing and developing self-supported electrodes for water electrolysis is attractive as compared to conventional catalyst-coated electrodes. Herein, a (Co_0.3Mn_0.1Ni_0.6)(OH)₂/NF nanocomposite is prepared by anchoring (Co_0.3Mn_0.1Ni_0.6)(OH)₂via simple one-pot hydrothermal synthesis on nickel foam. The presence of Mn induces the external electronic structure of Co(OH)₂, and this also improves the electrochemically active surface area (ECSA). These favor better accessibility of active sites and increased intrinsic activity for the OER and HER. (Co_0.3Mn_0.1Ni_0.6)(OH)₂/NF has shown promising electrochemical activity toward the OER and HER in a 1 M KOH electrolyte, with an overpotential of 270 mV for the OER and 163 mV for the HER to deliver 10 mA cm⁻² current density. The overall water splitting was performed by employing (Co_0.3Mn_0.1Ni_0.6)(OH)₂/NF as both the cathode and anode by displaying a voltage of 1.62 V at 10 mA cm⁻², which is comparable to that extracted from a similar system based on the state-of-the-art Pt/C@NF cathode and RuO₂@NF anode (1.60 V at 10 mA cm⁻²) standard electrode pair. Interestingly, at high current densities, the (Co_0.3Mn_0.1Ni_0.6)(OH)₂/NF//(Co_0.3Mn_0.1Ni_0.6)(OH)₂/NF system displayed better overall water splitting performance (1.78 V at 100 mA cm⁻²) compared to its Pt/C@NF//RuO₂@NF (1.89 V at 100 mA cm⁻²) counterpart while displaying a stable output during the evolution period that lasted for 24 h. The amounts of H₂ and O₂ produced are estimated to be 701.2 and 358.6 mmol, respectively, at a time interval of 1 h; these amounts correspond to an ∼1 : 2 ratio of O₂ and H₂, respectively. This study confirms the capability for employing (Co_0.3Mn_0.1Ni_0.6)(OH)₂/NF as a bi-functional and self-standing electrode for the realistic demonstrations of overall water splitting applications.