Molybdenum Site Incorporation into a Mixed-Valence Copper Coordination Polymer Enhances Asymmetric Charge Distribution for Stable Photoelectrocatalytic Proton-Coupled Electron Transfer

Abstract

Visible-light-active photoelectrodes, which display simultaneous photoelectroactivity and stability over a wide potential window and pH range, are essential for solar-to-chemical energy conversion in photoelectrosynthesis devices. Herein, we report a p-type nanoporous photocathode material, showing repeatable photocurrent responses in pH 0–14 electrolytes. The photoelectroactive Cu-MoDMcT catalyst is synthesized by coordination of copper and molybdenum through sulfur- and nitrogen-rich 2,5-dimercapto-1,3,4-thiadiazole (DMcT). The UV–visible spectral analysis evidences the visible light activity with a clear bathochromic shift in the absorption band upon coordination with heterometal ions. XPS and XAS analyses reveal that asymmetric charge distribution is amplified upon incorporation of trace amounts of Mo sites into the copper-containing coordination system, which further enhances the built-in electric field; this plays a critical role in improving photoelectroactivity and proton-coupled electron transfer (PCET) reactions. In contrast to a well-known p-type copper oxide photocathode, Cu-MoDMcT shows a repeatable photoresponse of ∼140 µA cm–2 in the acidic electrolyte for a prolonged duration. The excellent stability in acidic media is attributed to the robust coordination of isolated Cu and Mo coordination sites within the thiadiazole framework, while the tautomeric proton shuttle between N and S along with charge redistribution enhances PCET. Additionally, the nanoporous structure formed by the functional ligand network enables facile electrolyte access to active sites. The presence of molybdenum redox sites in coordination systems exhibits a further anodic shift in the onset oxygen reduction potential to +0.65 V vs RHE with a relatively high reduction current density. Furthermore, the Cu-MoDMcT photocathode exhibits photoinduced oxygen reduction activity.