Mechanisms of self-discharge and capacity loss in organic electrodes for aqueous batteries

The stability of electrode materials in aqueous environments presents a significant challenge for the long-term performance of energy storage systems, particularly when operating at potentials that promote water electrolysis. Many electrode materials undergo spontaneous self-discharge, resulting in a gradual loss of stored charge. While previous studies have shown that metallic and inorganic electrodes in aqueous solutions can experience significant self-discharge, much less is known about this phenomenon in organic electrodes. To bridge this gap, this study investigates the self-discharge behavior of polyimide (PI)-based electrodes, focusing on 1,4,5,8-naphthalenetetracarboxylic dianhydride-derived polyimide (PNTCDA) in aqueous electrolyte solutions. Through a systematic evaluation of charge loss, we demonstrate that while water reduction primarily drives reversible self-discharge, it also indirectly contributes to irreversible capacity loss by generating reactive species and conditions that accelerate the hydrolytic degradation of the polymeric structure. These processes are particularly pronounced when the anode material is in its electrochemically reduced state at low potentials. Comparisons with nonaqueous systems reveal that even small amounts of water can significantly accelerate capacity loss, underscoring the susceptibility of organic-based electrodes to instability when operating within potential windows where water is reduced. These findings highlight the critical need for strategies to mitigate both reversible self-discharge and irreversible degradation processes in aqueous battery systems.