Benchmarking the Activity, Stability, and Inherent Electrochemistry of Amorphous Molybdenum Sulfide for Hydrogen Production

Anodically electrodeposited amorphous molybdenum sulfide (AE-MoSx) has attracted significant attention as a non-noble metal electrocatalyst for its high activity toward the hydrogen evolution reaction (HER). The [Mo3S13](2-) polymer-based structure confers a high density of exposed sulfur moieties, widely regarded as the HER active sites. However, their intrinsic complexity conceals full understanding of their exact role in HER catalysis, hampering their full potential for water splitting applications. In this report, a unifying approach is adopted accounting for modifications in the inherent electrochemistry (EC), HER mechanism, and surface species to maximize the AE-MoSx electroactivity over a broad pH region (0-10). Dramatic enhancements in HER performance by selective electrochemical cycling within reductive (overpotential shift, eta(HER) approximate to -350 mV) and electro-oxidative windows (eta(HER) approximate to -290 mV) are accompanied by highly stable performance in mildly acidic electrolytes. Joint analysis of X-ray photoelectron spectroscopy, Raman, and EC experiments corroborate the key role of bridging and terminal S ligands as active site generators at low pH, and reveal molybdenum oxysulfides (Mo5+OxSy) to be the most active HER moiety in AE-MoSx in mildly acidic-to-neutral environments. These findings will be extremely beneficial for future tailoring of MoSx materials and their implementation in commercial electrolyzer technologies.