Self-Sulfuration and Carbonization of a Mixed-Metal Aryl Sulfonium Polyoxometalate Hybrid: A Path to Electrocatalytically Active Ternary Composite

Self-assembly of an aryl sulfonium counterion (ASCI), (4-(decyloxy)phenyl)dimethylsulfonium (DPDS) with a tricobalt substituted polyoxometalate (POM) cluster [SiW₉Co₃(H₂O)₃O₃₇]^10- resulted in the formation of an aryl sulfonium polyoxometalate (ASPOM) hybrid (AS-CoPOM) with layered structure. Calcination of AS-CoPOM at different temperatures (500, 700, and 900 °C) gave three derivatives, AS-CoPOM500/700/900, respectively. The gradual formation of a ternary composite (CoWO₄/WS₂/WO₃@C) from the starting AS-CoPOM at higher temperatures was revealed from detailed spectroscopic and analytical studies. A well-defined ternary structure obtained for AS-CoPOM900 showed nanosheets of WS₂ decorated with spherical WO₃ particles and rod-like structures of CoWO₄ with an inhomogeneous presence of graphitic carbon. Further, AS-CoPOM900 also showed the best crystallinity, highest % of oxygen vacancy, and I_D/I_G ratio among the other synthesized samples. It exhibited the best electrocatalytic oxygen evolution reaction (OER) activity with the lowest Tafel slope (65 mV dec^-1), highest electrochemical surface area (0.7 mF cm^-2), and best OER stability among the other variants. The present approach reveals an alternate method for generating electrocatalytically active mixed-metal oxide/chalcogenide composites by self-carbonizing and sulfurating ASPOM precursors at elevated temperatures and their detailed structural transformation.