Thermodynamically and Dynamically Boosted Electrocatalytic Iodine Conversion with Hydroxyl Groups for High-Efficiency Zinc–Iodine Batteries

Rechargeable zinc–iodine (Zn–I₂) batteries have shown immense potential for grid-scale energy storage applications, but there remain challenges of improving efficiency and cycling stability due to the sluggish iodine reduction reaction (IRR) kinetics and serious shuttle problem of polyiodides. We herein demonstrate an efficient metal-free hydroxyl (−OH)-functionalized carbon catalyst that effectively boosts the performance of Zn–I₂ batteries. It has been found that the obtained electrocatalytic performance is strongly correlated with the surface oxygen chemical environment in the carbon matrix. Both theoretical calculations and experimental measurements have uncovered that the −OH group, rather than carbonyl (−C═O) and carboxyl (−COOH), provides the active electrocatalytic site for IRR, improves the iodine redox kinetics and the electrochemical reversibility, and facilitates I₂ nucleation. As confirmed by a series of in situ and ex situ spectroscopy techniques, due to the favorable reaction thermodynamics and the lowered energy barrier for I₃^– dissociation, the O–H···I channels can effectively trigger the direct transformation of I₂/I^– and avoid the formation of stable polyiodides. As a result, the as-assembled battery of I₂/oxygen-functionalized carbon cloth (I₂/OCC-2)//Zn exhibits a high capacity of 2.27 mA h cm^–2 at 1 mA cm^–2, outstanding rate capability with 89.0% capacity retention at 20 mA cm^–2, and long-term stability of 10,000 cycles.