A Mixed Conducting Polymer with pH-Dependent Selectivity for the Oxygen Reduction Reaction

Achieving 4-electron oxygen reduction with metal-free electrodes has remained a longstanding challenge. Here, we show that poly(benzimidazobenzophenanthroline) (BBL), an n-type ladder polymer, breaks this barrier through an electrocatalytic inner-sphere 2 + 2 series mechanism─a mode of reactivity unprecedented among organic mixed ionic-electronic conductors (OMIECs). Using pH-dependent rotating ring-disk voltammetry, we find that this series mechanism occurs over separate potential windows on BBL. At more positive potentials, an initial reduction wave correlated with electron polaron formation in BBL produces exclusively hydro(gen) peroxide across all pH values, followed by a second, pH-dependent wave for hydro(gen) peroxide reduction at more negative potentials. At pH 14.0, this second wave enables complete four-electron reduction of O2 (n ≈ 4) to OH–, driven by formation of a bipolaronic state in BBL. Decreasing the pH instead stabilizes a hydrogenated state in BBL incapable of O–O bond cleavage, reverting selectivity to n ≈ 2. These insights reveal that the coupling between ion selectivity during electrochemical ion-insertion and product selectivity during ORR catalysis shapes active-site structure and poisoning, establishing a new design principle in the emerging field of OMIEC electrocatalysis.