High entropy spinel oxide (Ni_0.2Co_0.2Zn_0.2Cu_0.2Mg_0.2)Fe₂O₄ nanofibers for efficient oxygen evolution reaction

Developing efficient oxygen evolution reaction (OER) catalysts is urgent for the production of clean hydrogen energy. High-entropy oxides (HEOs) have become a focus of interest, and have been widely used for OER. HEOs would provide multiple degrees of freedom, allowing modification of the composition and atomic arrangement to fine-tune the electronic structure or active sites to optimize catalytic activity in OER. However, achieving multi-ion crystallization in HEOs while maintaining porous or nanostructured morphology still remains a challenge. In this work, (Ni_0.2Co_0.2Zn_0.2Cu_0.2Mg_0.2)Fe₂O₄ nanofibers were prepared by the electrospinning method. (Ni_0.2Co_0.2Zn_0.2Cu_0.2Mg_0.2)Fe₂O₄ exhibited enhanced OER activity (η₁₀ = 286 mV, Tafel slope = 136 mV dec⁻¹) and strong catalytic stability compared with single, binary, ternary, and quaternary oxides. The oxygen vacancies generated during the OER were confirmed by EPR experiments. XPS, TEM and in situ Raman spectroscopy confirmed the self-reconstruction of (Ni_0.2Co_0.2Zn_0.2Cu_0.2Mg_0.2)Fe₂O₄ during the OER. DFT calculations revealed that the high entropy structure would promote the shift of the D-band center towards the Fermi level and reduce the ΔG_max, which is consistent with the catalytic performance. This research demonstrates the importance of the concept of high entropy to increase the performance of high entropy materials for electrochemical application.