Radical initiated polymerization of p-styrenesulfonate on graphitic carbon nitride for interconnected water networks in short-side-chain PFSA membranes for low-humidity hydrogen fuel cells†

Interfacial ionic transport resistance, caused by sparsely connected water networks in polymer electrolyte membranes (PEMs) at low relative humidity (RH), limits the performance of hydrogen fuel cells. This challenge is addressed by employing a radical-initiated polymerization of p-styrenesulfonate (SS) on graphitic carbon nitride (CN) to enrich sulfonic acid groups via covalent grafting which are then incorporated into a short-side chain perfluoro sulfonic acid (SSC PFSA) ionomer matrix. This promotes the formation of interconnected water networks, even at low RH, and reduces the activation energy without negatively impacting the transport-stability trade-off. With the synergistic improvement in proton conductivity, water retention and mechanical stability, at 0.6 V, composite membranes demonstrated a 30% improvement in current density (1.12 A cm⁻²) at 30% RH and a 42% improvement (0.93 A cm⁻²) under dry gas conditions. The peak power density achieved for the composite membrane was 1.3 W cm⁻² at 100% RH. Furthermore, the composite membrane reinforces critical mechanical properties such as Young's modulus, tensile strength and dimensional stability, ensuring durability under operational stresses, evidenced by only a 10% reduction in the initial Open Circuit Voltage (OCV) during the accelerated stress test. Current density comparisons before and after the stability test also showed minimal losses, attributed to the ability of the additive to maintain interconnected water networks and reduce ionic transport resistance, thus enhancing proton conduction and fuel cell performance, particularly in low RH environments.