Tuning Porphyrin-Based COFs for High Iodine Loading and Efficient Redox Kinetics in Aqueous Zinc-Iodine Batteries.

The performance of aqueous zinc-iodine batteries (AZIBs) has historically been limited by poor iodine infiltration and slow reaction kinetics at the I2 cathode. To address these issues, developing an effective cathode host that can achieve both high iodine loading and rapid redox kinetics has become a pivotal challenge in the field. In this study, a porphyrin-based covalent organic framework (COF366) was synthesized with tunable in-wall functional groups, which served as an iodine host. The hydroxyl-functionalized version of COF366, designated as COF366-OH, demonstrated a remarkable iodine loading capacity of 52 wt %. This high ratio can be credited to a combination of mechanisms, including physical confinement of iodine and favorable Lewis acid-base interactions with the framework. The resultant synergy from these interactions not only improved iodine immobilization but also reduced voltage polarization and sped up the redox conversion process. As a result, the COF366-OH/I2 cathode exhibited an impressive specific capacity of 157 mAh g-1 after 500 cycles. This performance surpassed that of the pristine COF366, highlighting significant improvements in both rate capability and cycling stability. In situ Raman spectroscopy analyses indicated that I5- was the primary oxidation product formed during the discharge process, which aligns with the strong binding energy observed between COF366-OH and I5-. This robust interaction not only stabilized the I5- species but also effectively mitigated the shuttle effect, thus contributing to a longer cycling life and improved charge-transfer kinetics. This research illustrates a synergistic approach through structural modifications in order to boost the performance of iodine species in AZIBs, showcasing the potential benefits of tailored COFs in energy storage applications.