Ferrocyanide "Skin"-Mediated Anticatalysis: Mitigating Self-Discharge in Aqueous Electrochemical Devices

The interest in aqueous energy storage devices is surging due to their exceptional safety profile. However, in aqueous energy storage systems, interfacial side reactions, predominantly attributed to the oxygen evolution reaction (OER), result in significant self-discharge, which is concomitant with the deterioration of both voltage and capacity. Herein, we propose the construction of a ferrocyanide "skin" on transition metal compounds (TMCs) to mitigate this issue. This engineered "skin" creates Fe-C≡N terminations, initiating a new reaction pathway featured by the bonding process of N-O and N-H bonds. This reaction pathway presents a significant energy barrier, effectively shielding the active sites for the OER from H₂O molecules and hydroxyl ions. Taking NiO as an example, the ferrocyanide "skin" effectively suppresses the undesired phase transition from NiOOH to Ni(OH)₂ during the idling process of a fully charged electrode, enabling the as-modified electrode to achieve a remarkable voltage retention of 80.0% after 1 week of idling within a device. Furthermore, this concept demonstrates extensive applicability, extending to a range of TMC materials, including but not limited to manganese oxide, vanadium oxide, and nickel cobalt oxide. These findings highlight the efficacy of the ferrocyanide "skin" design strategy as a broadly applicable paradigm for suppressing H₂O-induced undesirable phase transitions in aqueous energy storage devices.