Zinc-induced polycrystalline transformation of high-entropy fluorides and derived regulatory mechanisms for bifunctional oxygen electrocatalysis

Abstract

Designing efficient, stable and low-cost bifunctional oxygen electrocatalysts for metal air batteries is crucial and challenging. The study reports a simple and controllable synthesis strategy of polycrystalline high entropy fluoride modified porous carbon nanofibers ((MnNiCuCoZn)F₂-PCNFs) for bifunctional oxygen electrocatalysis. The (MnNiCuCoZn)F₂-PCNFs feature continuous and facilitated pathways and a porous structure for electron transportation and mass diffusion, providing an ideal environment for electrocatalytic reactions. The multi-principal element characteristic of high-entropy fluoride also offers a wealth of active sites to enhance the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The zinc component leads to the transformation of high-entropy fluoride nanoparticles from single-crystal to polycrystalline, thereby effectively increasing the likelihood of exposing electrocatalytically active sites. Therefore, the prepared (MnNiCuCoZn)F₂-PCNFs exhibit superior electrocatalytic performance for the ORR (E_1/2 = 0.852 V) and OER (η₁₀ = 310 mV). The assembled (MnNiCuCoZn)F₂-PCNF-based ZABs achieve a high peak power density of 206 mW cm⁻² and stable operation for more than 400 h. The corresponding DFT calculations further reveal that the zinc component in (MnNiCuCoZn)F₂-PCNFs can lead to a continuous distribution of the density of electronic states at the Fermi level, thereby facilitating electron transfer. It can also significantly optimize the energy layout of reaction intermediates, substantially reducing the energy barriers for the ORR and OER (from 1.80 eV to 1.03 eV for the ORR and from 1.35 eV to 0.73 eV for the OER). The study proposes a “polycrystalline transition” that paves a novel pathway for the structural design of high-entropy nanoparticles and holds significant importance for the development of advanced multifunctional electrocatalysts.