Bottom-up Electrosynthesis of Highly Active Tungsten Sulfide (WS_3-x) Films for Hydrogen Evolution

Transition metal dichalcogenides have been extensively studied as promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER). However, despite the intention to achieve sustainable energy generation, conventional syntheses typically use environmentally damaging reagents and energy-demanding preparation conditions. Hence, we present electrochemical synthesis as a green and versatile alternative to traditional methods. In this fundamental study, we demonstrated the bottom-up synthesis of a mixed WS₂/WS₃ film-like material via cyclic voltammetry (CV). The film-like material can be directly electrosynthesized on any conductive substrates and renders the catalyst immobilization step redundant. Through stepwise analysis of deposition voltammograms facilitated by straightforward modification of CV conditions, and characterization using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), a two-step mechanism involving the initial WS₃ deposition and subsequent partial reduction to WS₂ was proposed. The WS₂/WS₃ material was determined to possess composition of WS_2.64. Compared to non-electrosynthesized WS_x materials, its predominantly basal orientation limited the heterogeneous electron transfer rate toward surface-sensitive redox couples. However, WS_2.64 demonstrated excellent HER activity, with the lowest Tafel slope of 43.7 mV dec^-1 to date; this was attributed to different metal-chalcogen binding strengths within WS_2.64. Fundamental understanding of the electrosynthesis process is crucial for green syntheses of inexpensive and highly electrocatalytically active materials for sustainable energy production. Albeit, the process may be different for a myriad of nanomaterials, this study can be exploited for its analyses from which the conclusions were made, to empower electrochemical synthesis as the prime fabrication approach for HER electrocatalyst development.