pH-Dependent Electrochemistry of Organic Electrode Materials in Aqueous Electrolytes

Abstract

Commercial rechargeable batteries predominantly use organic and non-aqueous electrolytes, which have significant potential in terms of redox voltage and energy density. However, aqueous electrolytes offer distinct advantages, such as reduced costs, decreased flammability, and enhanced environmental safety. In this study, we delve into the thermodynamic and electrochemical kinetics of an organic electrode material in aqueous electrolytes. Specifically, DAPT synthesized from perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and N1, N1-diphenylbenzene-1,4-diamine (DPA) was employed as the model system. The redox chemistry of DAPT involves ion-coupled electron transfer at the active redox carbonyl groups. A tunable parameter, pH, was employed to regulate the availability of protons within the reaction mechanism. Accordingly, glassy carbon electrodes coated with drop-casted DAPT were tested using cyclic voltammetry in sodium-phosphate buffers with pH values ranging from 2.0 to 12.0. The equilibrium potential (E_eq) dependence on pH reveals the proton-coupled electron transfer reaction mechanism, which operates with a 1:1 proton/electron ratio within the pH range of 2–7. At pH > 7, E_eq becomes independent of pH, indicating a transition to sodium ion-coupled electron transfer. The reaction kinetics determined from the peak separation (ΔE_p) exhibit a strong dependence on pH, with a decreasing kinetics within pH 2–9 and a reversed trend at pH 9–12. These findings underscore the crucial roles of cation type and concentration in influencing the thermodynamic and electrochemical kinetics of organic electrode materials. This research is expected to advance the characterization of this material as a sustainable and safe candidate for aqueous electrolyte batteries.