Layered Tin Chalcogenide Electrochemistry: Fundamentals and Implications on Energy-Related Applications

Abstract

Despite the prominent applications of layered tin (Sn) chalcogenides in batteries and supercapacitors, there is limited discourse on the relevance of Sn valence or structural differences to their electrochemical and electrocatalytic properties. To address this gap in literature, we investigate the electrochemistry of layered Sn chalcogenides; orthorhombic SnS and hexagonal SnS2, and establish implications to catalytic applications in oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Between the two layered Sn chalcogenides, which show distinct signals in the cathodic region, the inherent electrochemistry is more accentuated in SnS2. SnS2 manifests three characteristic signals placed in juxtaposition to a single reduction peak in SnS. Therefore, SnS shows a wider inert operating window compared to SnS2. In the aspect of heterogeneous electron transfer (HET) rates, SnS2 is an order of magnitude faster than SnS. Due to the high onset potentials and low current, both SnS and SnS2 are rendered unsuitable catalysts for ORR and OER. Conversely, both Sn chalcogenides are moderate catalysts for HER that outperform the bare glassy carbon (GC) electrode. In particular, SnS2 emerged a stronger contender for HER than SnS given the lower HER overpotential required in SnS2. However, it is important to recognise that the overpotential of SnS2 is remote from the ideal Pt catalyst. We perform electrochemical impedance spectroscopy (EIS) and density function theory (DFT) calculations to elucidate the variations in the HER performance of the layered Sn chalcogenides. By EIS study, SnS2 unveiled strikingly lower charge transfer resistance and hence, faster HER kinetics that could have contributed to its higher HER catalytic efficiency. Likewise, DFT studies disclose favourable Gibbs free energy of adsorbed hydrogen (∆GH) at the S edges for SnS2 whereas ∆GH at all edges of SnS are incompatible for HER. Our findings offer fundamental insights to layered Sn chalcogenides and provide an initial assessment of their electrochemical properties that would be beneficial for their future development in energy-related applications.