Composite Proton Exchange Membrane Featuring a Three-Layer Structure: Enhanced Thermal Stability, Proton Conductivity, and Fuel Cell Performance

The optimal operating temperature for contemporary perfluoro-sulfonic acid (PFSA)-based proton exchange membranes (PEMs) is identified to range from 60 to 80 °C. However, operating at temperatures exceeding this threshold could offer substantial advantages. Therefore, the development of PEMs that can maintain performance at elevated temperatures is imperative. This study introduces novel SUS composite proton exchange membranes with a three-layer architecture. These membranes feature a central UIO-66-NH2/Nafion composite layer (U), bordered by sulfonated carbon-nanotubes/Nafion composite layers (S) on both sides. The SUS PEMs demonstrate improved proton conductivity, long-term stability, fuel cell efficiency, and gas barrier properties. Notably, at the elevated temperature of 145 °C, attributable to enhanced water retention capabilities, these membranes exhibit significant proton conductivity, reaching 0.428 S cm−1. For fuel cell evaluations, the SUS PEMs exhibited optimal performance (0.940 W cm−2) at the elevated temperature of 115 °C. These improvements are attributed to the dense S layer, which regulates diffusion rates of both water and gas molecules, and the U layer, which serves as a water reservoir due to its high retention capacity. These conclusions have been validated through computational simulations and further supported by positron annihilation spectroscopy.