Structure and Electrocatalytic Performance of Cocrystallized Ternary Molybdenum Oxosulfide Clusters for Efficient Water Splitting

Abstract

Polyoxometalates (POMs) belong to a class of metal oxyanion clusters that hold enormous promise for a wide range of catalytic reactions, due to their structural diversity and the presence of redox-active metal centers and heteroatomic sites within the framework. In this study, we successfully determined the structures of the first cocrystallized ternary molybdenum oxo-sulfide clusters: Mo12NaO54P8C48H40, Mo12NaS2O52P8C48H40, and Mo12NaS6O48P8C48H40, which are abbreviated as Mo12, Mo12@S2, and Mo12@S6, respectively. Together, they are referred to as Mo12-TC. These clusters exhibit nearly identical exterior structures, making them indistinguishable, leading to their cocrystallization in a single unit cell with 50%, 25%, and 25% occupancy for Mo12, Mo12@S2, and Mo12@S6, respectively, and could not be separated easily. To confirm their molecular formulae and occupancy within a crystal, we conducted single-crystal X-ray diffraction (SCXRD) and high-resolution electrospray ionization-mass spectrometry (ESI-MS) studies. The clusters exhibit a dumbbell-like shape, with each terminal of the dumbbell comprising a hexagonal Mo6 basal plane shielded by multiple oxo, and oxosulfide moieties for Mo12 and Mo12@S2/Mo12@S6 clusters, respectively. Additionally, the clusters are protected by a ligand shell consisting of vertically aligned phenylphosphonic acid (PPA). Mo12-TC demonstrates promising activity for electrochemical hydrogen and oxygen evolution reactions (HER and OER). Mo12-TC exhibits overpotentials of 0.262 and 0.413 V vs RHE to reach HER current densities (in H2SO4) of 10 and 100 mA cm-2, respectively, and overpotentials of 0.45 and 0.787 V vs RHE to reach OER current densities (in KOH) of 10 and 100 mA cm-2, respectively, stable up to 5000 cycles. Density functional theory (DFT) calculations further elucidate their electrocatalytic potential, revealing the presence of active sites within these molecular frameworks. © 2023 American Chemical Society.