Controlled synthesis of highly active bifunctional electrocatalysts for overall water splitting using coal-based activated carbons

Abstract

We report a facile approach for the synthesis of highly active bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), by simply annealing mixtures of cheap coal-based activated carbons (CACs), ruthenium chloride and nickel chlorides in ammonia. The electrocatalysts consist of nitrogen doped CACs (NCACs), which are uniformly decorated with ruthenium (Ru) (with a low content of 0.3 wt%) and nickel nitride (Ni₃N) nanoparticles (Ni₃N/Ru/NCAC composites). The Ni₃N/Ru/NCAC composites have a large surface area (853 m² g⁻¹), which is proven to be attributable to the inherent large surface area of the CACs and the easy etching of CACs during an annealing process in ammonia. Electrochemical measurements reveal that OER electrocatalytic activities of the Ni₃N/Ru/NCAC composites remarkably outperform those of the state-of-the-art IrO₂ catalysts, and their HER activities were comparable to those of the benchmark Pt/C catalysts. Moreover, when the Ni₃N/Ru/NCAC composites are used as both anodes and cathodes of electrolyzers for overall water splitting (OWS), they delivered a lower voltage of 1.55 V at a current density of 10 mA cm⁻² and better durability than Pt/C(−)//IrO₂(+) electrodes. These outstanding OER/HER bifunctional activities and OWS performances of the Ni₃N/Ru/NCAC composites are ascribed to the collaborative contributions of N, Ru, Ni₃N and their large surface areas.