Designing Hydrostable MOFs for Photoelectrochemical Water Splitting: A Review on Progress, Stability Challenges, and Future Directions

The growing global energy demand and climate concerns have intensified interest in sustainable hydrogen production. Photoelectrochemical (PEC) water splitting offers a direct solar-driven pathway but remains limited by low efficiency, poor long-term stability, and material cost constraints. Metal–organic frameworks (MOFs) have emerged as promising photoelectrode materials owing to their high porosity, tunable structures, and versatile chemistry. In particular, hydrostable MOFs are essential for maintaining structural integrity, catalytic activity, and efficient charge transport under illuminated aqueous conditions. This review summarises the fundamentals and key challenges of PEC water splitting and critically examines recent advances in MOF functionalisation strategies, including ligand engineering, metal node exchange, doping, heterostructure formation, and pre- and post-synthetic modification. These approaches are discussed in relation to their roles in enhancing light absorption, charge separation, interfacial charge transfer, and hydrostability. Advances in MOF-derived materials are also highlighted, demonstrating how framework transformation can produce conductive and catalytically active architectures. Despite notable progress, challenges such as limited long-term aqueous hydrostability, insufficient mechanistic understanding of structural degradation, modest improvements in charge-transfer kinetics, and lack of cost and sustainability assessments remain. This review provides a comprehensive perspective on how rational MOF engineering improves PEC performance and outlines future directions for developing durable and efficient solar fuel materials.