Considering Figure and Figure , Zelenay and coworkers also observed from ex situ ICP-MS that HClO 4 dissolved more Fe from their polyaniline-derived Fe-N-C than H 2 SO 4 , which they atributed to differences in solubility of Fe perchlorates and sulfates. We suggest this observa�on could also be atributed to the stronger SO 4 2-binding on the Fe site, whereas ClO 4 -has been proposed to mimic non-specifically adsorbing proper�es of perfluoro sulfonic acid ionomers. If true, this would imply AST measurements in H 2 SO 4 in RDE and GDE would lead to slower FeNC degrada�on than in HClO 4 (at the same pH), when Fe dissolu�on is the main degrada�on mechanism.