Layered SnS versus SnS₂: Valence and Structural Implications on Electrochemistry and Clean Energy Electrocatalysis

Despite the far-reaching applications of layered Sn chalcogenides to date, their electrochemistry and electrochemical and electrocatalytic properties remain a mystery. The bulk of current research highlights promising uses of layered Sn chalcogenides with limited discourse on the relevance of Sn valency or crystal structures to their properties. We therefore examine the electrochemistry of orthorhombic SnS and hexagonal SnS₂, and determine the implications to their electrocatalytic applications, namely, oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Higher inherent electroactivity has been demonstrated in SnS₂ as indicated by three distinct cathodic signals juxtaposed with a broad reduction peak in the largely electro-inactive SnS. In addition, SnS₂ exhibits a faster heterogeneous electron transfer (HET) rate than SnS, though both are of less-than-sterling showing when compared to the glassy carbon (GC) electrode in terms of current intensity. The low onset potentials and current do not auger well for SnS and SnS₂ as electrocatalysts for ORR and OER. Contrarily, both Sn chalcogenides fare better as HER electrocatalysts, surpassing the GC electrode. SnS₂ exudes stronger HER electrocatalytic behavior than SnS. The differing HER performance is explained by means of HER electrode kinetics and density functional theory (DFT) calculation. Using electrochemical impedance spectroscopy (EIS), SnS₂ demonstrates significantly faster HER kinetics than SnS. The DFT study unveiled that the high electrocatalytic showing of SnS₂ originated from the propitious δG_H at the S edges. Conversely, δG_H of SnS at all edges are disadvantageous for HER. The results provide crucial knowledge on the electrochemistry and electrocatalysis of Sn chalcogenides and create opportunities for future developments.