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Proton-exchange membrane fuel cells (PEMFCs) are an electrochemical energy conversion device that has no carbon emissions provided it uses green hydrogen as a fuel. During fuel cell operation, the PEM undergoes mechanical, thermal, and chemical degradation. The chemical degradation is attributed to reactive oxygen species (ROS) that are generated in situ through both chemical and electrochemical pathways during fuel cell operation. Hydrogen peroxide (H₂O₂) is formed as an intermediate and reacts with trace transition metal ions like Fe²⁺ or Fe³⁺ to generate ROS. The ROS mainly comprise of hydroxyl and hydroperoxyl radicals and these radicals initiate oxidative degradation of both the PEM backbone and the side chains that contain ionic groups that are essential for proton conduction. In this study, Nafion-phosphonic acid ionomer blends were prepared into composite PEMs. The phosphonic acid ionomers were polyvinyl phosphonic acid (PVPA) and polytetrafluorostyrene phosphonic acid (PTFSPA). Using fluorescence spectroscopy, we observe scavenging of ROS species through the incorporation of phosphonic acid ionomers. The scavenging ability was observed to be on par, or to exceed, cerium oxide nanoparticles. The distribution and mobility of the phosphonic acid ionomer within Nafion were assessed using STEM EDX mapping. The incorporation of phosphonic acid ionomers, in some cases, were shown to improve membrane proton conductivity.