Operando XAS Investigation of Bimetallic Iron−Molybdenum Sulfide Electrocatalysts for the Hydrogen Evolution Reaction

Abstract

Nowadays green hydrogen production has been seen as a key element for the future development and large-scale implementation of fuel cell technology, while electrolyzer technologies represent the most advanced methods for producing pure hydrogen from water. The preparation of efficient, cost-effective, and stable noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) at the cathode of proton exchange membrane electrolyzers (PEMEL) remains one of the main issues that need to be solved to allow the widespread use of hydrogen as an energy carrier.Molybdenum sulfide (MoSx) is a well-known material showing promising HER catalytic activity, especially in its amorphous state and when doped with other transition metals1. The successful implementation of a bimetallic iron-molybdenum sulfide electrocatalyst for HER into a PEM electrolyzer has been recently reported2. However, the structure of their active sites requires a more elaborate investigation. In this study, we employed operando X-ray absorption spectroscopy to identify potential-induced structural and electronic changes in FeMoSmw bimetallic iron−molybdenum sulfides and MoSmw obtained by a microwave irradiation synthetic approach 2. Extended X-ray absorption fine structure (EXAFS) spectra at Mo and Fe K-edges under operando conditions provided us with accurate structural characterization of the catalytic active sites: while an amorphous MoS3 phase is detected and retained in both MoSmw and FeMoSmw at low potential, a FeS phase is observed at the Fe K-edge, likely explaining the better performance of the bimetallic iron-molybdenum sulfides electrocatalysts compared to iron-free molybdenum sulfides.