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

Abstract

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 SnS 2 , 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 SnS 2 as indicated by three distinct cathodic signals juxtaposed with a broad reduction peak in the largely electro-inactive SnS. In addition, SnS 2 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 SnS 2 as electrocatalysts for ORR and OER. Contrarily, both Sn chalcogenides fare better as HER electrocatalysts, surpassing the GC electrode. SnS 2 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), SnS 2 demonstrates significantly faster HER kinetics than SnS. The DFT study unveiled that the high electrocatalytic showing of SnS 2 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.

Role of the Edge Properties in the Hydrogen Evolution Reaction on MoS2

“…Recent experimental studies of the HER reaction catalyzed by platinum dichalcogenides, group V dichalcogenides, and also less frequently used tin chalcogenides have not, however, found any clear relation between Δ G H and HER performance, which might question the usefulness of Δ G H . Nevertheless, a computational model used to mimic an edge can influence calculated hydrogen binding properties and this factor has been largely overlooked.…”

“…[12] Gao et al [13] used the calculated values of DG H to predict that the catalytic activity of 1T'-MoS 2 would be superior to that of 2H-MoS 2 ,w hich has been confirmed in recent experiments. [14] Recent experimental studies of the HER reactionc atalyzed by platinum dichalcogenides, [15] group Vd ichalcogenides, [16] and also less frequently used tin chalcogenides [17] have not, however,f ound any clear relation between DG H and HER performance, which might question the usefulness of DG H .N ever-theless, ac omputational model used to mimic an edge can influence calculated hydrogen bindingp roperties and this factor has been largely overlooked. The edge of ac rystal represents aq uasi-one-dimensionalo bject, which complicates its DFT computations, because these calculations are feasible only for systemsc omprising of severalt ens or hundreds of atoms.…”

Structural Transformation of SnS2 to SnS by Mo Doping Produces Electro/Photocatalyst for Hydrogen Production

“…In 5 % Mo‐SnS, a pair of doublet signals appears, at binding energies of 485.8 and 494.3 eV corresponding to 3d 5/2 and 3d 3/2 of Sn 0 and at 486.9 and 495.3 eV for the 3d 5/2 and 3d 3/2 of Sn II . Similarly, in 10 % Mo‐SnS, this doublet pair is located at 485.9 and 494.3 eV, and at 486.8 and 495.2 eV, corresponding to the 3d 5/2 and 3d 3/2 of Sn 0 and Sn II , respectively . The higher intensity of the Sn II signal in 10 % Mo‐SnS relative to the 5 % Mo‐SnS, suggests higher fraction of Sn II in the 10 % Mo‐SnS sample.…”

“…The 10 % Mo‐doped SnS catalyst showed the best electrocatalytic activity among the set of samples, with lower over potential and Tafel slope. Previous work on the electrochemical HER activity of pristine SnS and SnS 2 flat electrodes concluded that SnS 2 demonstrate higher HER efficiency than SnS . DFT calculation demonstrates that the active sites for H adsorption, according to the Δ G H value, are the S edges in SnS 2 , while Δ G H values for SnS render it incompatible for HER electrocatalysis In addition, it was reported that SnS 2 exhibits lower charge transfer resistance compared to SnS .…”

“…The long‐term cyclic stability of the 10 % Mo‐SnS catalyst was verified by performing continuous 3000 CV cycling between −0.4 and 0.0 V vs. RHE at a 100 mVs −1 scan rate in 0.5 m H 2 SO 4 solution (Figure c). The obtained electrochemical HER activity of the pristine SnS 2 in our work is better than the reported values for bulk electrodes and comparable with nanostructures of SnS 2, and the 10 % Mo‐doped SnS is better than pristine values reported, see Table S3 . Electrochemical impedance spectroscopy (EIS) was performed in order to gain more insights into the electron‐transfer kinetics of HER (Figure d).…”