Multilayer Casting Strategy for Enhanced Cation Contamination Resistance in Proton Exchange Membranes

The proton exchange membrane (PEM) is critical for the operation of proton exchange membrane fuel cells (PEMFCs). However, cationic impurities (e.g., Ca²⁺ and Mg²⁺) in water or the environment readily bind to the PEM's sulfonic acid groups (−SO₃H), deactivating sites and reducing proton conductivity. This necessitates costly and high-precision water treatment. Conventional solutions, such as adding perfluorosulfonic acid (PFSA) chains or anti-hardness additives often raise costs and lower intrinsic conductivity. To address this issue, we developed a novel multilayer casting technique. This method used airborne moisture to drive −SO₃H group migration toward surfaces while simultaneously densifying fluorocarbon chains into a robust network. Repeated casting cycles created an internal multilayered barrier network within the PEM. This structure generated effective steric hindrance, physically blocking cation diffusion and significantly boosting hard water resistance. Performance tests demonstrated that after 4 h of heated immersion in 220 ppm hard water, the proton conductivity (8.0 S/m) maintained a value of 294%, which was higher than that of N117. The technique also established a gradient distribution of hydrophilic domains across the membrane. This optimized proton transport pathways enabling the MLM to achieve proton conductivity comparable to commercial Nafion membranes under standard conditions. This provided a path to more durable and cost-effective PEMFCs.