Exploring the integration of sulfonated poly(phenylene sulfone) ionomers into the cathode catalyst layers of proton exchange membrane fuel cells

Abstract

Increasing regulatory pressure on perfluorinated sulfonic acid (PFSA) ionomers demands alternative materials for catalyst-coated membranes (CCMs) in proton exchange membrane fuel cells. Sulfonated poly(phenylene sulfone) (sPPS) has emerged as a promising candidate, and this study compares physical-chemical and electrochemical properties of CCMs using platinum (Pt)-based catalysts in either PFSA- or sPPS-bonded cathodes. During break-in, PFSA-bonded cathode performance stabilizes after eight voltage cycles with low charge transfer resistance, while sPPS requires 30–40 cycles. Atomic force microscopy indicates electrodes undergo partial ionomer redistribution over cycling, affecting proton conduction and oxygen diffusion. Polarization curves indicate PFSA attains higher cell voltages at the low current densities, owing to a fourfold greater Pt mass activity compared to sPPS, despite comparable Tafel slopes. X-ray photoelectron spectroscopy suggests strong Pt-sPPS interactions, potentially reducing catalytic activity by covering active Pt-surface with sPPS. At high current densities, under fully humidified conditions, sPPS benefits from enhanced oxygen transport, mitigating mass transport limitations. Mercury intrusion porosimetry shows abundant macropores in sPPS-based cathodes, promoting oxygen transport, while PFSA's balanced meso-/macropore distribution supports hydration and ionic conductivity. Future efforts—e.g., deploying Pt-alloy catalysts, refining break-in protocols, and optimizing cathode architecture—could alleviate sPPS's kinetic constraints, supporting its viability as a PFSA alternative.