Co-Doped Ni9S8 Nanostructures for Electrocatalytic Water Splitting over a Wide pH Range

As a replacement for renewable energy sources, an earth-abundant electrocatalyst for water splitting is effectively explored. In this work, Ni 9 S 8 and cobalt-doped Ni 9 S 8 nanostructures are fabricated on carbon cloth using the hydrothermal technique. The developed electrocatalysts are characterized through various techniques, for example, powder X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, the Brunauer–Emmett–Teller method, and inductively coupled plasma atomic emission spectroscopy. Tuning of cobalt doping is performed to obtain the best optimized ratio of Co/Ni for electrocatalytic activity. All the developed materials are used for a water splitting reaction in an alkaline electrolyzer, and Co 0.05 Ni 8.95 S 8 is an optimized material for both hydrogen and oxygen evolution. The electrocatalyst Co 0.05 Ni 8.95 S 8 only requires −0.151 V versus RHE (reversible hydrogen electrode) to obtain a 10 mA/cm 2 current density in the hydrogen evolution reaction (HER), and in the oxygen evolution reaction (OER), it requires 1.557 V versus RHE to generate a 30 mA/cm 2 current density. The corresponding Tafel slope values for the HER and OER are 125 and 49.8 mV/dec, respectively, obtained by using Co 0.05 Ni 8.95 S 8 electrocatalysts in 1.0 M KOH solution. The stability of Co 0.05 Ni 8.95 S 8 is also checked, and it is stable for up to 60 and 80 h for the HER and OER, respectively. The cell voltage of 1.89 V is required to generate a 10 mA/cm 2 current density for the overall water splitting reaction. The electrocatalyst is also used for the HER and OER in a wide pH range for practical applicability. The overall experimental findings were verified by theoretical calculations, which state that the higher metallic nature of Co-doped Ni 9 S 8 facilitates efficient electrocatalytic activity. The optimum Gibbs free energy and hydrogen and oxygen coverage calculations also prove that the optimized Co 0.05 Ni 8.95 S 8 electrocatalyst exhibits the best HER and OER activity. Therefore, this work provides a robust electrocatalyst for the electrocatalytic water splitting reaction.