Resolving ion-coupled electron transfer in bipolar organic electrodes in aqueous electrolytes by single-particle electrochemistry

Abstract

Bipolar organic electrode materials, integrating both n-type and p-type redox centers within a single molecular framework, offer compelling opportunities for high-capacity aqueous energy storage. Yet the ion-coupled electron transfer mechanisms that govern each center in aqueous environments are still not fully understood. Here, we employ scanning electrochemical cell microscopy (SECCM) to interrogate bipolar redox processes in a perylene diimide–triphenylamine (PDI–TPA) organic electrode at the single-particle level in aqueous media. Electrochemical mapping directly reveals sub-particle heterogeneity governed by particle thickness, establishing electron transport length as a key descriptor of redox utilization in π-stacked organic crystals. Systematic pH-dependent results reveal a mechanistic transition in PDI carbonyl reduction from proton-coupled electron transfer to cation-mediated charge compensation, accompanied by a five-order-of-magnitude suppression in apparent rate constants. The anomalous pH dependence arises from kinetical asymmetry and proton-transport-limited cathodic activation. In contrast, TPA oxidation is governed by anion uptake, with activity strongly dependent on anion valence and coordination strength. Comparative studies with a less conjugated analogue further demonstrate that extended π-conjugation enhances both electronic accessibility and interfacial kinetics. By acquiring statistically robust kinetic data across large ensembles of individual particles, this single-particle framework delivers mechanistic resolution that is inaccessible to bulk electrochemistry. This work provides a clear molecular design roadmap for next-generation bipolar organic electrodes.