On the Role of Hydrogen and Peroxyl Radicals in the Degradation of Aromatic Hydrocarbon-Based Proton Exchange Membranes

<title>Abstract</title> <p> Chemical degradation by radicals limits the durability of aromatic hydrocarbon-based proton exchange membranes used in fuel cells and water electrolyzers. While hydroxyl radicals (HO ^• ) are recognized as important degradation agents, the roles of hydrogen (H ^• ) and peroxyl radicals (HOO ^• /ROO ^• ) remain poorly understood. Here, γ-radiolysis of water was used to selectively generate H ^• and HOO ^• /ROO ^• along with HO ^• under acidic conditions relevant to device operation. Two aromatic sulfonate model compounds representing structural motifs of hydrocarbon ionomers were exposed to radicals in N ₂ saturated and O ₂ containing solutions at pH 0 and 2. In the presence of HO ^• , degradation increased with radical dose and was enhanced by oxygen and increasing pH, consistent with established HO ^• -induced degradation pathways. Selective suppression of HO ^• using <italic>tert</italic> -butanol showed that H ^• radicals cause negligible degradation of the aromatic substrates, while HOO ^• and organic peroxyl radicals (ROO ^• ) exhibit only minor reactivity. These results indicate that HO ^• is the principal radical responsible for degradation of the investigated aromatic hydrocarbon model compounds, whereas H ^• and HOO ^• /ROO ^• contribute little to substrate loss. The findings improve the mechanistic understanding of radical-induced degradation and provide guidance for the development of more durable hydrocarbon membranes. </p>