Elucidating Oxygen Reduction Mechanisms on Organic Mixed Ionic-Electronic Conducting Polymers

Abstract

Atmospheric oxygen is an attractive oxidant for the generation of electricity from chemical fuels and for the electrochemical production of hydrogen peroxide. Although not universal, it is generally assumed that an electrocatalyst is needed to reduce oxygen. Recently, a variety of organic mixed ionic-electronic conducting polymers (OMIECs) have been reported to exhibit catalytic behavior for both the 4-electron/4-proton and the 2-electron/2-proton oxygen reduction reaction (ORR).This work sets out to identify the oxygen reduction mechanism of OMIECs through a multi-faceted experimental and theoretical approach. We primarily focus our efforts on p(NDI-T2 P75)—a random copolymer comprised of naphthalene diimide (NDI) and bithiophene (T2) units with 75% polar sidechains—and then expand the understanding built on this system to a range of other OMIECs. Using rotating ring disk electrochemistry, we find that the investigated OMIECs exhibit a high selectivity towards the 2-electron ORR product. Using operando UV-Vis and Raman spectroscopy measurements, we identify changes in OMIEC performance when in the presence of O2. To gain further insights about the catalytic reaction path, we construct and test a microkinetic model. Through this model, we show that the performance of the tested OMIECs can be rationalized through an EC’D reaction mechanism. Our results suggest that these materials do not operate as electrocatalysts for the initial reduction of oxygen to superoxide but do function as catalysts for the disproportionation of superoxide.