Auto-selective reaction mechanism on Al-substituted ZnFe2O4 spinel electrode and sustainable water oxidation by oxygen vacancy transition (2023)

Owing to the large-scale burning of fossil fuels to produce energy, greenhouse gas emissions have increased alarmingly. Reducing these emissions has become a common concern. Hydrogen energy, a new and renewable energy source, is being actively used nowadays in hydrogen cars and various other means of transportation, such as ships, trains, and airplanes [1]. It has the potential to be applied as a key energy source in the carbon-neutral era. However, owing to the current hydrogen production methods, hydrogen energy has been criticized for not being a clean energy source. According to the report ‘The Future of Hydrogen’ published by the International Energy Agency (IEA) [2], 76% of the world’s 70 million tons of hydrogen production is currently extracted from natural gas and the remaining 23% from coal. Approximately 830 million tons of carbon dioxide are generated annually, and less than 0.1% of the world's hydrogen production is green hydrogen. Therefore, to use hydrogen as a clean energy source, the proportion of green hydrogen production must increase. Water electrolysis technology, which produces hydrogen by electrolyzing water and is a method of producing green hydrogen, is actively being researched for various applications, such as electrodes, catalysts, separators, electrolytes, and electrolytic cells (reactors). The water electrolysis oxygen evolution reaction (OER) under alkaline conditions starts with the adsorption of abundant OH^- ions on the catalyst surface (or O vacancy site) (1) as follows [3]: * + OH⁻ →*OH + e^- (1), *OH →*O + H⁺ + e⁻ (2), *O + OH⁻ →*OOH + e⁻ (3), *OOH →* + O₂ + H⁺ + e⁻ (4) (where * represents a catalyst adsorption (or active) site). The adsorbed *OH undergoes oxidative deprotonation to form *O (2). Then, *O reacts with other OH^- to form *OOH intermediate (3), and in the last step, *OOH is deprotonated to regenerate the active site, and O₂ is generated (4). The OER is the most important process in water electrolysis, involving the generation of four electrons, and occurs more slowly than the hydrogen generation reaction (HER). The main intermediates observed in the OER are HO^⁎, O^⁎, and HOO^⁎, and in the OER reaction, the binding interaction with MO on the catalyst surface plays an important role in stabilizing these reaction intermediates, which eventually has a significant effect on the overall water splitting efficiency [4]. Because the fundamental overpotential of the reaction can be overcome by stabilizing HOO^⁎ on the catalyst surface [5], developing an OER catalyst that lowers the overpotential in water electrolysis is crucial.

Metal oxide catalysts, such as RuO₂, IrO₂, metal oxides, and oxyhydroxides, are the commonly used electrocatalysts in an OER [6], [7], [8], [9], [10]. However, noble metals, such as Ir, Ru, IrO₂, and RuO₂, which are widely used as state-of-the-art catalysts in OER processes to increase energy conversion efficiency, cannot be mass-produced owing to their scarcity, high cost, and poor durability. Therefore, it is important to explore noble metal-free catalysts that promote kinetically slow OER. Contrarily, oxides or layered hydroxides (CoOOH, FeOOH, or NiOOH) containing transition metal elements, such as Co, Ni, and Fe, have relatively stable activities, and they have been widely studied as OER catalysts [11], [12], [13]. Furthermore, they are abundant, inexpensive, and environmentally friendly; and because they exhibit semiconductor characteristics, they have the advantage of enhancing OER activity by promoting charge transfer when grafted with other conductive materials [14]. In general, two main strategies are used to continuously improve the electrocatalytic activity: “active site maximization and conductivity optimization” [15]. First, to maximize the density of active sites, the catalyst crystal size, shape, intrinsic lattice defects, and crystal phase must be well controlled [16]. Recently, studies on spinel-type metal oxide nanostructure designs with a large surface area and many active sites have been reported. In particular, mixed transition metal oxides, which have typical spinel structures, are more valuable as OER electrode catalysts than single transition metal oxides [17]. For example, the synthesis of multi-layered oxygen vacancy-rich NiCo₂O₄ spinel nanosheets has been reported [18], [19], where it was revealed that oxygen vacancies trapped in ultrathin nanosheets can lower the H₂O adsorption energy and increase the OER efficiency.

This study attempts to optimize the OER by inducing lattice defects and maximizing OH⁻ ion adsorption on the crystal surface by adding Al to the ZnFe₂O₄ spinel structure, which is composed of transition metals that are cheaper than Ni or Co. In general, because the OER catalyst needs to oxidize the adsorbed OH⁻ to O₂, the catalyst itself needs to undergo significant reduction. Additionally, because the electrons generated at this time must be well passed to the external circuit, it should also be able to self-oxidize. That is, the catalyst must have a reducing component that attracts electrons well and an oxidizing component that provides electrons well, simultaneously. Therefore, the catalyst in this study was composed of bi-metals, using Fe³⁺ as the reducing metal and Zn²⁺ as the oxidizing metal. To induce an advantage owing to the oxygen defects, the crystal was formed into a spinel structure. Furthermore, to maximize the adsorption of OH⁻, the catalyst surface was acidic, and Al³⁺ ions were partially substituted into the framework. Finally, we attempted to determine the effect of lattice defects and OH⁻ adsorption capacity of the designed catalyst surface on the OER performance through various analyses.