Plant-Derived Functional Molecules for Photoelectrochemical Water Splitting: From Molecular Design to Hybrid...

Plant-derived molecules offer tunable coordination and surface chemistry for photoelectrochemical (PEC) water splitting. A Scopus survey retrieved 45 documents linking both topics in 2005-2025, equivalent to 0.34% of the PEC literature. A supplementary search added 15 records, and screening retained 26 research articles. The review organizes molecular functions into light harvesting, charge-transfer mediation, coordination of catalytic overlayers, and passivation. A molecular readiness scale and a validation protocol connect molecular identity to verified device performance. Hybrid layers combining plant ligands with Co, Ni, or Fe raised BiVO₄ photocurrents to approximately 3-6 mA cm^-2 at 1.23 V vs. the reversible hydrogen electrode (RHE). Comparable inorganic overlayers preclude a general claim of superior activity. Ambient solution processing and control of interfacial coordination provide the principal design opportunities. Two-electron phenol/quinone potentials do not establish the feasibility of one-electron hole relay, which requires independent kinetic and redox evidence. At a coverage of 0.1 nmol cm^-2, a chlorophyll layer has a calculated photocurrent upper bound of approximately 0.05 mA cm^-2 within 640-680 nm alone. Broadband enhancement requires wavelength-resolved attribution. Operation for 1000 h at 1 mA cm^-2 and a loading of 1 nmol cm^-2 corresponds to approximately 3.7 × 10⁷ electrons per initially loaded molecule. The proposed framework links molecular structure, interfacial function, and product verification to guide durable molecular photoelectrodes.