The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as “Pre-catalysts”

Oxygen evolution catalysts composed of a metal (Ni, Co, or Fe) and a pnictide or chalcogenide (P, S, or Se) counterion are a promising class of electrocatalysts for the oxygen evolution reaction (OER), an important reaction for the photoelectrochemical splitting of water. We synthesized a nickel-based oxygen evolution catalyst derived from pulse-electrodeposited nickel sulfide. This catalyst was found to produce current densities of 10 mA/cm2 at the relatively low overpotential of 320 mV in alkaline electrolyte (1 M KOH). Importantly, we found that the sulfur anion in the nickel sulfide is depleted in the active form of the electrocatalyst and that the NiS is converted into an amorphous nickel oxide in the potential range where water is oxidized to oxygen. The superior catalytic activity of this nickel sulfide is thus unrelated to the sulfur anions in the active catalyst but is instead related to the metal sulfide's ability to act as a precursor to a highly active nickel oxide OER electrocatalyst. The nickel oxide derived from nickel sulfide was found to be amorphous with a relatively high surface area, two factors that have been previously shown to be important in oxygen evolution electrocatalysis.