How to Chemically Protect PFAS-Free Membranes in Fuel Cells: Radical Quenching Poly(vinylphosphonic acid) Layer

Hydrocarbon (HC) membranes are discussed for use in proton exchange membrane fuel cells as environmental benign alternative to perfluorosulfonic acid (PFSA) membranes. However, long-term chemical stability is still hindered by the formation of radical species during fuel cell operation. The fast reaction kinetics of aryl-based HC polymers towards radical attack render conventional mitigation strategies (e.g., blending of a radical scavenger into the membrane) ineffective. In this study, we present an approach to extend the operating lifetime of proton exchange fuel cell membranes by adding a radical quenching layer that reacts and deactivates radical species before they diffuse into the membrane. Poly(vinylphosphonic acid) (PVPA) is both an antioxidant and a proton conductor, which makes it a suitable material for a radical blocking barrier that still conducts protons. PVPA was applied in two different ways. First, directly as an ionomer in the catalyst layer (CL), where the bulk of radicals are formed, and secondly - to avoid the detrimental catalyst poisoning by phosphonic acid - as an interfacial layer between the CL and the membrane. Compared to non-stabilized membranes, especially the latter approach resulted in a substantial increase in membrane lifetime during accelerated chemical stress tests customized for HC membranes.