Theoretical calculations-driven rational screening of d-block single-atom electrocatalysts based on d-p orbital hybridization for durable aqueous zinc-iodine batteries

Abstract

Aqueous Zn−iodine (Zn-I2) batteries, featuring intrinsically high-safety aqueous electrolytes and eco-friendly cathode/anode materials, however are restricted by the shuttling of polyiodide and sluggish redox kinetics of iodine redox. Although various single atom catalysts (SACs) have been proposed to improve the electrochemical performance, the underlying mechanisms of different SACs involved in iodine redox are not completely elucidated. Herein, the interaction between d-block SACs and polyiodide is demonstrated to follow d-p orbital hybridization theory, thus a series of SACs with different d-block transitional metal sites are pre-screened using DFT calculations to assess the hybridization effectiveness. Among these, Nb-NC is selected due to its numerous unfilled antibonding orbitals, which facilitates effective d-p hybridization between Nb-d and I-p orbitals. Accordingly, Nb-NC with a low d-band center of 0.271 eV exhibits the highest binding energy for polyiodide and the lowest reaction barrier for the rate-determining step (I3-→I-). These theoretical predictions are well corroborated by various in/ex situ characterizations, which confirms the suppressed shuttle effect and enhanced redox conversion of iodine species by using a free-standing Nb-NC/I2 cathode. Consequently, the Zn||Nb-NC/I2 battery can maintain an exceptional capacity of 140 mA h g-1 over 50,000 cycles at 10 A g-1, with only 0.00008% capacity decay per cycle.