Metastable-state structures promote the surface reconstruction of spinel NiFe2O4 for efficient oxygen evolution reaction†

Abstract

Regulation of catalyst surface structure in a catalytic process is the basis for regulating catalytic performance. For spinel materials, the common active phases on their surfaces for the oxygen evolution reaction (OER) are metal hydroxyl oxides; hence, promoting their surface reconstruction to form the metal hydroxyl oxide for improving their catalytic activity is one of the commonly used modification methods. In this work, amorphous P-doped NiFe₂O₄ (A-P-NFO) was successfully prepared via a hydrothermal method followed by a phosphating treatment. The amorphous structure led to a strong polarization effect in A-P-NFO, where electrons were attracted from the antibonding orbitals to the empty orbitals, thus lowering the average valence state of nickel and activating the Ni–O bond. Unlike other reported work on NFO, this work demonstrated that the lower Ni and Fe valence states were more favorable for catalytic activity. In addition, the amorphous structure kept A-P-NFO in a metastable state, which favored the leaching of surface cations and acceleration of the surface reconstruction of A-P-NFO during the OER process, thereby playing a key role in the improvement of the catalytic performance of OER. Synchrotron radiation, in situ Raman spectra and DFT calculations demonstrated that the A-P-NFO promoted electron transfer and adsorption of electroactive species (OH*, O*, etc.) for OER. A-P-NFO exhibited excellent OER catalytic performance under alkaline electrolyte conditions, achieving a low overpotential of 240 mV at 10 mA cm⁻² and maintaining high catalytic stability over 100 h. Furthermore, its OER catalytic performance was superior to the recently reported catalysts of similar types. The method of turning the catalyst into a metastable-state structure via P doping and amorphization strategies is expected to provide new research ideas and application prospects for improving the catalytic performance of similar catalysts.