Dual-sulfonated MOF/polysulfone composite membranes boosting performance for proton exchange membrane fuel cells

Nafion is the most widely used polymer electrolyte membrane (PEM) owing to its excellent physical stability and superior proton conductivity in the presence of long-side-chain perfluorinated sulfonic acid. However, commercially available Nafion is very expensive, limiting its practical application. Thus, a new proton exchange membrane must be developed to replace conventional Nafion for practical electrochemical system operations. In this study, we rationally designed dual-sulfonated polysulfone (PSF)/metal–organic framework (MOF) composite membranes for use as PEMs. Sulfonated PSF (sPSF) was systematically prepared with a controllable sulfonation degree. Additionally, sulfonated MOFs (sMOF; MIL-101(Cr)-SO3H and UiO-66(Zr)-SO3H) were incorporated into the sPSF matrix to form dual-sulfonated PSF/MOF composite membranes. The sMOF offers a high Brunauer-Emmett-Teller (BET) surface area with sulfonated sites that can effectively control water molecules and ion transport, increasing proton conductivity and dimensional stability. With an optimal MOF loading (∼3 wt%), the sPSF/sMOF composite membranes exhibited a high proton conductivity (∼0.18 S/cm) at room temperature under fully humid conditions. Additionally, the sPSF/sMOF composite membranes showed a moderate swelling ratio owing to the high dimensional stability due to the adsorbed molecules in the MOF framework. Particularly, the sPSF/sMOF composite membranes also exhibited a high proton conductivity at high temperatures (∼75 °C) and moderate relative humidity (∼60–80 % RH), demonstrating their high potential for practical operation. Thus, the sPSF/sMOF composite membranes show good potential as a novel PEM for electrochemical applications.