Auto-selective reaction mechanism on Al-substituted ZnFe2O4 spinel electrode and sustainable water oxidation by oxygen vacancy transition

Abstract

This study aims to optimize the oxygen evolution reaction (OER) by maximizing OH– adsorption on the mixed spinel ZFAO surface, in which Al3+ is substituted for Fe3+ in the ZnFe2O4 (ZFO) spinel structure composed of inexpensive transition metals. The substituted Al3+ ions simultaneously occupy the tetrahedral and octahedral sites of Fe in the ZFO spinel lattice. As the amount of substituted Al3+ increases, the amount of OH– adsorbed on the crystal surface increases significantly, and the OER activity is maximized at the ZFA0.75O/NF electrode, in which 0.75 M of Al is substituted. The overpotential is greatly reduced to 270 mV at 10 mA cm−2, and the Tafel slope is very low at 79 mA dec−1. These results were constant even after 10 d, which means that the charge transfer reaction proceeds rapidly between the ZFAO spinel catalyst and aqueous solution during the OER without side reactions. In particular, this study confirms that the main OER active sites of the ZFAO catalyst are the Al-tetrahedral and Fe-octahedral sites, and oxygen through the lattice vacancies rapidly transfers, maintaining a stable spinel structure. This study also reveals that the OER selectively pursues two mechanisms: an adsorbate evolution mechanism (AEM) on the Al-tetrahedral site and a lattice oxygen participation mechanism (LOPM) on the Fe-octahedral site. This study contributes to making the catalyst cost-effective, which has been an obstacle to the commercialization of water electrolysis technology, and securing its long-term durability, thereby opening the door to the mass production of clean hydrogen energy.