From Dark Photocatalysis to Photobattery-Type Operation: A Re-Functionalized Ti-MOF for Solar Energy Storage

Efficient harvesting, storage, and on-demand release of solar energy are essential for overcoming the intermittency of sunlight and enabling continuous solar energy-driven processes, yet achieving storage of photogenerated charge remains challenging. Here, we report a Ti-based bipyridine MOF, COK47-bpy, and its ReCl(CO)₃-functionalized analogue, ReCl47-bpy, as photochargeable materials for time-decoupled solar energy utilization. Re incorporation introduces localized photosensitizing and "electron sink" sites while preserving the parent framework structure, enabling visible-light response, faster charge accumulation, and improved charge retention. In colloidal suspension, ReCl47-bpy reaches an electron storage capacity of 19.5 C g^-1 _MOF under simulated solar irradiation and retains about 70% of the stored charge after 2.5 months. The stored electrons can be released on demand for dark hydrogen (H₂) evolution with electron-to-H₂ efficiencies exceeding 90%. Beyond colloids, when fabricated as robust FTO-supported layers, ReCl47-bpy|FTO photoelectrodes deliver capacities of up to ∼7.0 mAh g^-1 _MOF upon photocharging under 1 Sun irradiation. Overall, this work establishes Re-functionalized Ti-MOFs as versatile platforms for solar-driven charge storage, dark photocatalysis, and MOF-based photoelectrodes, toward the development of next-generation MOF-based energy devices.