Abstract
Two-dimensional metallic transition metal dichalcogenides (2D TMDs) such as 1T phasesof MoS2 and WS2, NbS2, TaS2 and VS2 have been studied as potentiallyinexpensive and earth abundant electrocatalysts for the hydrogen revolution reaction(HER). The performance of HER catalysts is typically evaluated in terms of overpotentialat which the reaction starts and the Tafel slope – a measure of the over-potential requiredto increase a reaction by a factor of 10. The overpotential and Tafel slope values of metallicphases and edges9 of 2D TMDs approach those of Pt nanoparticles – the best HER catalyst.However, despite substantial progress the overall current density of 2D TMD catalystsremains orders of magnitude lower (~ 10 – 100 mA-cm-2) than industrial Pt and Irelectrolyzers (> 1,000 mA-cm-2) 10,11. Here, we report the synthesis of three dimensional(3D) niobium disulfide (Nb1+xS2 where x is ~ 0.35)12 as a HER catalyst that is capable ofevolving hydrogen at current densities of > 5,000 mA-cm-2 at an overpotential of ~420 mVversus reversible hydrogen electrode (RHE). We find the exchange current density at 0 Vfor 2H phase Nb1.35S2 catalysts to be ~ 0.8 mA-cm-2 (comparable to that of noble metals),corresponding to a turnover frequency of ~ 0.2 s-1. We demonstrate a proof of conceptelectrolyzer based on 2H Nb1.35S2 cathode that is capable of generating current densities of1000 mA-cm-2. Our theoretical results reveal that the Nb1.35S2 with Nb terminated surfacehas free energy for hydrogen adsorption that is close to thermoneutral, facilitating HER.The Nb1+xS2 could therefore be a viable non-precious metal catalyst for practicalelectrolyzers used to generate hydrogen.