Per- and polyfluoroalkyl substances (PFAS) are generally employed as proton exchange membranes in fuel cell gas sensors, but they suffer from environmental persistence, safety concerns, and high cost. Here, sulfonated poly(ether ether ketone) (SPEEK) is employed to replace the PFAS and a chemically compatible hydroxylated SPEEK ionomer binder is designed to reduce the interfacial resistance at the catalyst–membrane boundary. Due to the high proton conductivity, improved interfacial proton transport and enhanced intrinsic hydrophilicity of SPEEK, the resulting PFAS-free gas sensor exhibits a twofold higher response at 100 ppm H₂S compared to a commercial Nafion benchmark, along with high sensitivity (0.635 µA·ppm^–1) and faster response time (31 s). The device operates stably across 15–95% RH and retains over 90% of its initial response after 60 days. This work establishes SPEEK as a cost-effective, eco-friendly alternative to fluorinated membranes and provides a general interface-engineering strategy for high-performance electrochemical gas sensing.