Metal–Organic Frameworks as Advanced Photoelectrodes for Photoelectrochemical Water Splitting: Design, Mechanisms, and Challenges

Photoelectrochemical (PEC) water splitting represents a compelling pathway for sustainable hydrogen production, yet its large-scale deployment remains constrained by inefficient charge separation, sluggish interfacial kinetics, and limited material stability. Metal–organic frameworks (MOFs) have recently emerged as transformative photoelectrode platforms owing to their modular chemistry, ultrahigh surface areas (>1000 m² g^–1), and precisely tunable electronic structures. This review critically examines the evolution of MOFs from photoactive frameworks to high-performance PEC photoelectrodes, emphasizing mechanistic insights that govern light absorption, carrier dynamics, and catalytic reaction pathways. Quantitative analysis reveals that MOF–semiconductor heterojunctions and MOF-derived oxides can deliver photocurrent densities exceeding 6–7 mA cm^–2 at 1.23 V vs RHE, with onset potential shifts up to 400 mV and applied bias photon-to-current efficiencies approaching 1%. Advanced architectures─including S-scheme/Z-scheme junctions, defect-engineered MOFs, and conductive MOF-derived catalysts─demonstrate hydrogen evolution rates as high as 68 mmol g^–1 h^–1 and solar-to-hydrogen efficiencies nearing 10–11% in tandem configurations. By correlating framework chemistry, band energetics, and interfacial charge-transfer mechanisms, this review establishes design principles for next-generation MOF photoelectrodes and outlines scalable strategies to surpass the 10% STH benchmark, positioning MOFs as key enablers for practical solar fuel technologies.