Ternary Pentlandites as Hydrogen Evolution Catalysts in Alkaline Media

Abstract

Metal sulfides are promising alternatives to noble metal electrocatalysts for water-based hydrogen evolution. Pentlandites, notably, exhibit high activity in acidic environments. To explore their potential in alkaline conditions, pentlandite electrodes are tested in both conventional three-electrode setups and scaled up to a 12.6 cm${}^2$ membrane electrode assembly (MEA). Optimized pentlandites with a stochiometry of $M_9{S}_8$, containing Fe, Ni, and Co, show reduced overpotentials for hydrogen evolution with higher Fe and Ni contents. However, a minimum Co content of three equivalents is necessary for peak hydrogen evolution reaction activity with −0.40 V versus reversible hydrogen electrode at −300 mA cm${}^{-2}$. Stability assessments via X-ray photoelectron and Raman spectroscopy reveal minor surface changes for Fe and Ni species but significant leaching of cobalt from Co${}_{4.5}$Ni${}_{4.5}$S${}_8$ surfaces postelectrolysis. Selected pentlandite catalysts are integrated into MEAs, with ${\text{Fe}}_4{\text{Co}}_3{\text{Ni}}_2{S}_8$ achieving 1 A cm${}^{-2}$ at 2.2 V with minimal potential decay of 50 μV h${}^{-1}$ alongside a La${}_{0.8}$Sr${}_{0.2}$CoO${}_3$ anode. These findings underscore the suitability of pentlandite catalysts for water splitting at industrial scales under alkaline conditions.