Clarifying the roles of hydrogen and peroxyl radicals in the degradation of aromatic hydrocarbon polymer-based proton exchange membranes

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 initiators, 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 tert -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.