Enhancing the proton conduction performance of Nafion composite membranes through doping simple and low-cost hydrated calcium terephthalate

Abstract

Metal–organic frameworks (MOFs) have emerged as promising candidates for enhancing the performance of Nafion proton exchange membranes (PEMs). Increasing efforts have been devoted to investigating the synthesis of MOF fillers and their impact on improving the σ_H⁺ values of composite membranes. In this study, a low-cost alkaline earth metal ion, Ca(II), with a relatively large radius and multiple coordination sites was selected to react with terephthalic acid under solvothermal conditions. The resulting Ca–MOF exhibited abundant hydrophilic groups and continuous hydrogen bonds in its crystal structure. Various amounts of Ca–MOF particles were incorporated into Nafion to fabricate Ca–MOF/Nafion-x composite membranes, which were characterized using SEM, PXRD, IR, TG, etc. The proton conductivity of the Ca–MOF/Nafion-7 composite membrane was found to be highest at 353 K, reaching 1.95 times that of the pure Nafion membrane. Our analysis suggests that coordination water molecules and uncoordinated carboxylate groups play a significant role in determining the relationship between the structure and property in Ca–MOF/Nafion-x composite membranes. Despite its simple crystal structure and use of low-cost raw materials and straightforward synthesis methods, our findings demonstrate that Ca–MOF exhibits comparable or even superior proton conduction properties compared to more complex MOFs modified by functional groups or synthesized using expensive raw materials. This study provides valuable insights for designing cost-effective MOFs with enhanced proton conduction properties.