Phosphonate Metal-Organic Frameworks for Proton Conduction: Design Strategies from Crystalline Intermediates to Processable Membranes

Abstract

Phosphonate metal-organic frameworks (MOFs) have emerged as promising alternatives to conventional perfluorosulfonic acid-based proton exchange membranes because of their excellent chemical stability, tunable structures, and efficient proton transport properties. This review examines recent advances in the design of phosphonate MOFs for proton conduction, focusing on the progression from crystalline intermediate-directed synthesis to processable membrane architectures. These frameworks are evaluated against the temperature and humidity windows in which they must operate: 60–90 °C at 80–98% relative humidity for hydrated fuel cell duty and 100–160 °C at low or zero humidity for anhydrous operation. The role of linker chemistry, metal centres, framework topology and hydrogen-bonding networks on conductivity and stability is critically discussed, with special attention to the intermediate-directed crystallisation as a means to control framework order and proton-conduction pathways. Integration into mixed-matrix membranes, pure MOF films and hybrid composites is also evaluated alongside the characterisation methods used to probe proton transport and durability. This evidence is presented in an order-defect-interface paradigm suggesting a small but justifiable gap: No study has combined an intermediate-derived phosphonate framework with chitosan or related precursor-controlled order, defect population, interfacial chemistry and membrane mechanics to one another.  Despite significant progress, challenges remain in achieving efficient anhydrous proton conduction, standardised performance evaluation, scalable membrane fabrication, and long-term operational stability. Rational defect engineering, intermediate-directed synthesis and sustainable biopolymer-based fabrication should be combined in future research to bridge the gap between laboratory-scale materials and practical membranes for fuel cells and related electrochemical technologies.