Insight into the Nature of Iron Sulfide Surfaces During the Electrochemical Hydrogen Evolution and CO₂ Reduction Reactions

Greigite and other iron sulfides are potential, cheap, earth-abundant electrocatalysts for the hydrogen evolution reaction (HER), yet little is known about the underlying surface chemistry. Structural and chemical changes to a greigite (Fe₃S₄)-modified electrode were determined at −0.6 V versus standard hydrogen electrode (SHE) at pH 7, under conditions of the HER. In situ X-ray absorption spectroscopy was employed at the Fe K-edge to show that iron–sulfur linkages were replaced by iron–oxygen units under these conditions. The resulting material was determined as 60% greigite and 40% iron hydroxide (goethite) with a proposed core–shell structure. A large increase in pH at the electrode surface (to pH 12) is caused by the generation of OH^– as a product of the HER. Under these conditions, iron sulfide materials are thermodynamically unstable with respect to the hydroxide. In situ infrared spectroscopy of the solution near the electrode interface confirmed changes in the phosphate ion speciation consistent with a change in pH from 7 to 12 when −0.6 V versus SHE is applied. Saturation of the solution with CO₂ resulted in the inhibition of the hydroxide formation, potentially due to surface adsorption of HCO₃^–. This study shows that the true nature of the greigite electrode under conditions of the HER is a core–shell greigite-hydroxide material and emphasizes the importance of in situ investigation of the catalyst under operation to develop true and accurate mechanistic models.