Mo2O2S8]2− small molecule dimer as a basis for hydrogen evolution reaction (HER) catalyst materials

Abstract

Molybdenum sulfides have emerged as promising, low cost alternatives to platinum for catalysis of the electrochemical hydrogen evolution reaction (HER). A wide variety of sulfides already achieve impressive catalysis, but further development could be enabled by an understanding of the relative activities of different sulfur sites. To contribute towards a better understanding, the small molecule dimer [Mo2O2S8]2−, which is rich in terminal and unsaturated sulfur, has been investigated for HER. Homogeneous catalysis has been attempted in acidified DMF but is precluded by electrodeposition. The resulting deposit ultimately proves to be an active heterogeneous HER catalyst in aqueous acid and is compared to dropcasts of the molecular dimer which are also HER catalysts. XPS elucidates the end structures of the electrodeposits and dropcasts showing a shift away from molecular features in both cases. Investigation of HER performance reveals that the activity of both dimer-based heterogeneous catalysts lags behind that of other reported molybdenum sulfides. However, detailed performance comparisons allow for consideration of key design parameters. Density functional theory calculations are presented for the reduced molecular dimer as well as possible molecular catalytic pathways. These give insight into the cathodic structural transformations of the dimer as well as the observation of heterogeneous but not homogeneous catalysis.