Electrocatalysis of layered Group 5 metallic transition metal dichalcogenides (MX2, M = V, Nb, and Ta; X = S, Se, and Te)

Abstract

The revelation of MoS₂ as an efficient electrocatalyst for the hydrogen evolution reaction (HER) has ratcheted up interest in other transition metal dichalcogenides (TMDs). To date, extensive studies have been focused towards semiconducting Group 6 TMDs while research into metallic Group 5 TMDs has been comparatively limited. Past computational screening of Group 5 TMDs showed propitious Gibbs free energy of the adsorbed hydrogen (ΔG_H) for HER, especially for VS₂, which prompted us to experimentally explore their HER efficiency. In addition to the HER electrocatalytic performance, we examine the inherent electrochemistry and the charge-transfer property of the entire set of Group 5 TMDs in the bulk form: VS₂, VSe₂, VTe₂, NbS₂, NbSe₂, NbTe₂, TaS₂, TaSe₂ and TaTe₂. We demonstrate that the nine Group 5 TMDs show distinctive inherent electroactivities arising from their intrinsic electrochemical processes or surface oxides. TaS₂ possesses the fastest heterogeneous electron transfer (HET) rate at 3.4 × 10⁻³ cm s⁻¹ amongst the Group 5 TMDs and may be ideal for electrochemical sensing. Chalcogen dependence is evident in the electrochemical charge-transfer ability of the Group 5 TMDs whereby tellurides show slower HET rates than sulfides and selenides. We identify VTe₂ as the best-performing material for HER contrary to the widely predicted VS₂. VTe₂ manifests the lowest HER overpotential at 0.5 V vs. RHE and Tafel slope of 55 mV dec⁻¹. Interestingly, the HER performance of vanadium dichalcogenides and Group 5 tellurides shows chalcogen- and transition metal- dependence, respectively. Reasons behind their HER performance have also been proposed from our theoretical studies found on thermodynamics and kinetics. Broadly, the HER performances of bulk Group 5 TMDs are less outstanding than those expected despite being true metals. This fundamental study provides fresh insights into the electrochemical and electrocatalytic characteristics of metallic Group 5 TMDs that will be indispensable for the development of TMDs in future applications.