Refining Mn(III) Active Sites in MnO2-Based Self-Assembled Electrodes Via Oxygen Vacancy Induction to Enhance Oxygen Reaction Performance in Zinc-Air Batteries

Abstract

Rechargeable zinc-air batteries are considered as a promising candidate for next-generation electrochemical energy conversion devices due to their safety, low cost, and high theoretical capacity/ energy density. Over the past few decades, α-MnO2-based electrodes have demonstrated their excellent performance and competitiveness in oxygen catalytic reactions. (1) According to eg filling theory, Mn(III) (t2g3eg1) sites in [MnO6] octahedral unit deliver superior bifunctional activity compared to Mn(II) (t2g3eg2) and Mn(IV) (t2g3eg0) sites.(2) However, the presence of unpaired single electrons of the Mn(III) site leads to low thermodynamic stability. Introducing oxygen vacancies to promote the electron rearrangement of the [MnO6] unit is a promising approach for enhancing the durability of Mn(III) active sites. Herein, we construct an oxygen-vacancy-enhanced MnO2-based electrode via a self-assembly process (NiCo2O4-MnO2@Ni foam). The intrinsic oxygen catalytic activity of NiCo2O4-MnO2@Ni foam is significantly improved by forming abundant highly stable Mn(III) active sites, which accelerate the mass transfer ability. As expected, the NiCo2O4-MnO2@Ni foam shows good bifunctional activity with a low overpotential of 0.69 V. The zinc-air battery assembled by NiCo2O4-MnO2@Ni foam exhibits a high peak power density of 576 mW cm-2, surpassing that of the Pt/C-Ir/C electrode. Additionally, benefiting from its stable flower-shaped hierarchical structure, the self-assembled electrode demonstrates long-term charge-discharge cycling stability, which lasts over 800 hours. This work provides innovative insights into enhancing MnO2-based electrodes by oxygen vacancy introduction.References N. Xu, Q. Nie, L. Luo, C. Z. Yao, Q. Gong, Y. Liu, X. Zhou, and J. Qiao, ACS applied materials & interfaces (2018).Z. Yin, R. He, H. Xue, J.-M. Chen, Y. Wang, X. Ye, N. Xu, J. Qiao and H. Huang, Energy Materials (2022).