Origins of hydrogen peroxide selectivity during oxygen reduction on organic mixed ionic-electronic conducting polymers

Electrochemical reduction of atmospheric oxygen provides carbon emission-free pathways for the generation of electricity from chemical fuels and for the distributed production of green chemical oxidants like hydrogen peroxide. Recently, organic mixed ionic-electronic conducting polymers (OMIECs) have been reported as a new class of active electrode materials for the oxygen reduction reaction. This work sets out to identify the operative oxygen reduction mechanism of OMIECs through a multi-faceted experimental and theoretical approach. Using a combination of pH-dependent electrochemical characterization, operando UV-Vis and Raman spectroscopy, and ab initio calculations, we find that the n-type OMIEC, p(NDI-T2 P75), displays pH-dependent activity for the selective reduction of oxygen to the 2-electron hydrogen peroxide product. We use microkinetic simulations of the electrochemical behavior to rationalize our experimental observations through a polaron-mediated, non-adsorptive pathway involving chemical reduction of oxygen to the 1-electron superoxide intermediate followed by pH-dependent catalytic disproportionation to hydrogen peroxide. Finally, this pathway is applied to understand the experimental oxygen reduction reactivity across several n- and p-type OMIECs. Through experiment and theory, this work explains how a set of OMIEC polymer electrodes selectively reduce oxygen to hydrogen peroxide.