In-situ electrosynthetic fabrication of highly crystalline, aqua-compatible palladium halide perovskite films for photoelectrocatalysis

Semiconductors provide the material foundation for solar energy conversion, enabling the direct transformation of sunlight into electrical or chemical energy. Among emerging semiconductor classes, metal halide perovskites (HaPs) have attracted intense attention owing to their exceptional optoelectronic properties; however, their instability in aqueous media severely limits their application in photoelectrochemical energy conversion. We report electrosynthetic perovskite in-situ crystallization and film fabrication (EPIC-Fab), a scalable aqueous electrosynthesis and direct deposition strategy for palladium-based vacancy-ordered double perovskites (VODPs), specifically Cs2PdCl6, Cs2PdBr6, and Cs2PdI6. This method enables direct growth of highly crystalline films on a wide range of conductive substrates, including uneven tubular geometries, using water as the sole solvent and requiring no post-deposition annealing. Film thickness and morphology can be tuned through deposition parameters and duration. Unlike conventional lead halide perovskites, the resulting VODP films exhibit intrinsic high thermal and water stability. We demonstrate that these films function as efficient photoelectrocatalysts for hydrogen evolution (Faradic efficiency >94%) without additional cocatalysts or protective layers. Notably, a light-assisted self-repair process enables maintaining operational stability during continuous photoelectrochemical hydrogen evolution for > 100 hours. This work establishes an aqueous, surface-agnostic synthetic paradigm for vacancy-ordered double perovskites for optoelectronic and energy conversion applications.