Fluorinated graphene electrocatalyst boosts zinc-air battery performance

A research team led by Professors WEN Zhenhai and HUANG Junheng from the Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences has developed a fluorine-engineered graphene electrocatalyst. This catalyst, featuring precisely tailored sp2-C-F bonding, enhances the performance of zinc-air batteries.

Rechargeable zinc-air batteries (ZABs) are considered promising for next-generation energy storage due to their high theoretical energy density, operational safety, and the abundance of zinc resources. However, their practical application is limited by slow four-electron oxygen electrocatalysis, which includes the oxygen reduction reaction (ORR) during discharge and the oxygen evolution reaction (OER) during charge. These reactions face high energy barriers, causing large voltage polarization and reduced efficiency.

The two-electron O2-H2O2 interconversion pathway offers faster kinetics and better reversibility. Despite this, developing high-performance bifunctional catalysts is challenging. Metal-based catalysts can suffer from performance-degrading side reactions, while metal-free carbon catalysts often face a trade-off between conductivity and selectivity.

The researchers employed a thermal defluorination strategy to synthesize fluorinated graphene (FG-X) with varying fluorine contents. Using highly fluorinated graphene as a precursor, stepwise pyrolysis under an inert atmosphere adjusted C-F bonding from insulating sp3-C-F to conductive sp2-C-F, while restoring the conjugated carbon network.

The optimized FG-3 (~3.1 at.% F) achieved a balance between electrical conductivity and catalytic activity, featuring a well-recovered graphitic structure, uniform fluorine distribution, high surface area, and improved wettability. Electrochemical evaluations showed FG-3's exceptional bifunctional activity for 2e-ORR and POR, with 97.4% H2O2 selectivity and 98.0% Faradaic efficiency.

The assembled primary zinc-oxygen battery delivered a peak power density of 569 mW cm-2. The rechargeable version achieved a 98.0% round-trip efficiency at 2 mA cm-2, retained 97.6% efficiency after 1500 cycles, and operated stably for over 820 hours in ambient air.

Mechanistic studies using operando spectroscopy and first-principles simulations revealed that sp2-C-F bonds induce localized electrostatic polarization on adjacent carbon sites, stabilizing the key *OOH intermediate and lowering the kinetic barrier. This research establishes a structure-property relationship for fluorinated carbon electrocatalysis, providing a strategy for carbon material engineering and advancing metal-air batteries and on-site H2O2 electrosynthesis.