Comparing the trends in bulk redox and oxygen reduction, we observe an ~59 mV pH -1 positive shift in voltage versus RHE for both the electron polaron/bipolaron reactions and oxygen reduction (Figure ). Although the overall ORR and HPPR are both proton-coupled, the observed pH-dependence reflects that both the bulk ion-insertion reactions and the rate-limiting step of oxygen reduction do not involve proton transfer. Instead, charge compensation for the electron polaron/bipolaron reaction is likely provided by K + , the cation with majority concentration in these experiments.