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

Abstract

As a replacement for renewable energy sources, an earth-abundant electrocatalyst for water splitting is effectively explored. In this work, Ni9S8 and cobalt-doped Ni9S8 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 Co0.05Ni8.95S8 is an optimized material for both hydrogen and oxygen evolution. The electrocatalyst Co0.05Ni8.95S8 only requires −0.151 V versus RHE (reversible hydrogen electrode) to obtain a 10 mA/cm2 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/cm2 current density. The corresponding Tafel slope values for the HER and OER are 125 and 49.8 mV/dec, respectively, obtained by using Co0.05Ni8.95S8 electrocatalysts in 1.0 M KOH solution. The stability of Co0.05Ni8.95S8 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/cm2 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 Ni9S8 facilitates efficient electrocatalytic activity. The optimum Gibbs free energy and hydrogen and oxygen coverage calculations also prove that the optimized Co0.05Ni8.95S8 electrocatalyst exhibits the best HER and OER activity. Therefore, this work provides a robust electrocatalyst for the electrocatalytic water splitting reaction.