Effects of PFSA Ionomer Side-Chain Structure on PEMFC Performance and Transport Properties Under Near-Boiling-Point Conditions

Operating proton-exchange membrane fuel cells (PEMFCs) under near-boiling-point conditions is considered promising for heavy-duty applications; however, elevated temperatures and insufficient hydration can intensify mass transport limitations in the catalyst layer (CL). Although regulating the side-chain structure of PFSA ionomers is an effective strategy to improve high-temperature performance, the relationship between ionomer architecture, CL microstructure, and transport behavior remains insufficiently understood. Herein, a systematic comparison between a long-side-chain (LSC) ionomer (Nafion-117) and a short-side-chain (SSC) ionomer (D79) was conducted using single-cell measurements at 75 and 105 °C, with polarization tests performed at 40% and 80% RH and cyclic voltammetry (CV) measurements conducted at 100% RH. The effects of ionomer structures on catalyst ink dispersion, CL morphology, pore characteristics, and electrochemical transport behavior were investigated. The results demonstrate that D79 exhibits superior performance under near-boiling-point conditions, accompanied by higher Pt utilization and lower ohmic and concentration losses, particularly at high current densities and low humidity. D79 also forms a more open and heterogeneous pore structure, which is consistent with its more favorable high-current-density transport behavior. Polarization loss decomposition further indicates that the performance advantage of D79 is mainly associated with reduced ohmic and concentration polarization losses under near-boiling-point and low-humidity conditions. These findings clarify the influence of PFSA side-chain structure on CL microstructure and electrochemical transport behavior and provide experimental insights for the design of PFSA ionomers for near-boiling-point PEMFC operation.