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

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.