In Situ FeO(OH) Formation Drives Unconventional Capacity Growth in Aqueous Iron-Based Ion Hybrid Supercapacitors

Aqueous iron-based energy storage devices have garnered significant attention in the energy storage sector due to their low cost and high safety profile. However, the susceptibility of Fe2+ to oxidation and the sluggish intercalation kinetics have hindered comprehensive investigations into iron-based energy storage devices. Moreover, several proposed energy storage mechanisms remain undisclosed or are mutually inconsistent. In this work, biomass-derived nitrogen-doped porous carbon (NPC) was employed as the cathode material, while pure iron sheets served as the anode, to construct aqueous iron-based ion hybrid supercapacitors (IHS). Herein, we elucidate the energy storage mechanism involving Fe2+/H+ coadsorption, and density functional theory (DFT) calculations confirmed that H+ exhibits a higher adsorption affinity toward the NPC surface. IHS demonstrates superior electrochemical performance, including a high discharge capacity of 58.9 mAh g–1 and a high power density of 1548 W kg–1. However, during the cycling test, an unexpected observation was made: the capacity of IHS continued to increase even after nearly 30 days of continuous operation. This phenomenon is both anomalous and scientifically intriguing. Based on this finding, we verified that the enhanced capacity originates from the in situ generation of the active material FeO(OH) on the NPC surface during the charging process. Overall, our IHS energy storage system challenges the conventional understanding that the energy storage capacity of traditional batteries or supercapacitors inevitably deteriorates over time during operation. This work elucidates a novel energy storage mechanism for aqueous iron-based energy storage systems and opens up new avenues for future research.