Selective Oxygen Reduction to Hydrogen Peroxide on Organic Mixed Ionic-Electronic Conducting Polymers

The oxygen reduction reaction (ORR) can involve the transfer of four electrons, resulting in the formation of H₂O, or two electrons, yielding hydrogen peroxide (H₂O₂). For either product, several reaction schemes with different intermediates are possible, complicating the identification of a detailed mechanism for a given material. Nevertheless, mechanistic studies of ORR-active materials are critical for optimizing of the use-cases for the ORR, whether it be fuel cells or electrochemical H₂O₂ synthesis. Developing a rigorous framework for the study of oxygen reduction is crucial to understanding whether the reaction proceeds via an electrocatalytic mechanism.<br/>Using a multi-faceted approach to study oxygen reduction, our work sets out to identify the operating mechanism of hydro(gen)peroxide production on organic mixed ionic-electronic conducting polymers (OMIECs). As a model system, we primarily study p(NDI-T2 P75)—a random copolymer comprised of naphthalene diimidie (NDI) and bithiophene (T2) units, where 75% of the NDI units have polar sidechains. Through the use of rotating ring disk electrode measurements, we find that this OMIEC is capable of reducing oxygen and that it is highly selective for the two-electron reaction. Further, using operando UV-Vis and Raman spectroscopy measurements, we find the mechanisms through which this OMIEC becomes active. To gain further insights about the reaction path, we construct and test a microkinetic model. Through this model, we show that this material, as well as a number of other OMIECs, reduces oxygen through a non-electrocatalytic, outer-sphere electron transfer mechanism.