Ru II ‐Based Soft Metallo‐Supramolecular Polymer Electrode for Photoelectrochemical Water Oxidation: Mechanistic Insights Through in Situ Spectroscopic Studies

ABSTRACT Photoelectrochemical (PEC) water splitting offers a promising solar‐driven route for hydrogen production, but its efficiency is limited by poor semiconducting properties, sluggish kinetics, and high overpotential of the oxygen evolution reaction (OER). Herein, we explored a Ru‐based coordination polymer gel (Ru‐TTN‐CPG), prepared by self‐assembly of TTN low molecular weight gelator (TTN: naphthalenediimide (NDI) core connected with four terpyridine moieties through alkyl amide chains) with Ru 2+ as a soft, processible hybrid photoanode material for PEC water oxidation. The crosslinked nanofibrous CPG promotes efficient photogenerated charge separation and transport while providing an abundant catalytic site, and the Ru‐terpyridine units enable visible‐light absorption through metal‐to‐ligand charge transfer. A type‐II heterojunction photoanode is fabricated by introducing TiO 2 as an electron‐transporting layer, which facilitates charge separation, suppresses interface recombination, and significantly improves photocurrent density. Moreover, in situ FTIR spectroscopy reveals water oxidation on the Ru‐TTN‐CPG surface under illumination, enabling identification of key intermediates. Complementary in situ XAS measurements support oxidation state changes of the Ru 2+ ‐center under operating conditions, conclusively establishing Ru 2+ as a catalytically active site. Guided by experimental observations, theoretical calculations elucidate the mechanistic pathway of water oxidation. Overall, this work highlights the potential of soft hybrid metallo‐supramolecular polymers for advancing next‐generation PEC energy‐conversion systems.