Proton Conduction with Metal-Organic Frameworks

Further development of microporous crystalline materials as proton conductors may lead to better electrolyte membranes for fuel cells. Proton-exchange membrane fuel cells (PEMFCs) generate electricity because the electrons generated by the reaction of hydrogen and oxygen must travel through an external circuit; the membrane electrolyte only transfers protons. The membrane materials of choice have been ionomeric polymers, such as sulfonated fluoropolymers (Nafion), that achieve proton conductivities of up to 1 S cm −1 , but the requirement to keep these materials hydrated limits their operating temperature and efficiency. Metal-organic frameworks (MOFs), in which inorganic assemblies are joined by organic linkers, have inherent porosity that could be exploited for the development of proton-conducting membranes. Among recent studies of experimental proton-conducting MOFs [e.g., ( 1 )], two general targets for PEMFC operation have emerged: developing better materials for operations under humid conditions (below 100°C), and developing efficient anhydrous proton conductors that could unlock the cost efficiencies enabled by humidity-independent operation above 100°C.