Crystal Structure-Controlled Electrocatalysis on Iron-Based Oxides Toward Oxygen Evolution in Alkaline Media: Trend and Mechanism

Abstract

Present energy issues have increasingly stimulated the demand for the development of clean and sustainable energy platforms which can achieve net zero carbon emissions. Renewable energy resources are quite promising candidates for the construction of sustainable decarbonized society. One of examples of harnessing renewable energy-derived electricity is the electrolysis of earth-abundant water into hydrogen, which is a prominent carbon-free chemical fuel with a high weight energy density. Compared with polymer electrolyte water electrolysis in acidic environment, electrochemical alkaline water splitting can employ inexpensive nonprecious metals as electrocatalysts, which can make this technology a large-scale and cost-effective process. However, the anodic oxygen evolution reaction (OER) in the water splitting has a large overpotential, and it is a great bottleneck for widespread use. Thus, highly active and inexpensive electrocatalysts for OER are desired to overcome this concern.Iron (Fe) is a very earth-abundant element that is a quite cheap and non-toxic metal, and thus it is a potential candidate as electrocatalyst for OER. Previous studies have reported several multimetal oxides with Fe and other metallic elements, such as perovskite and brownmillerite types, which have attracted great attentions due to their facile synthesis and easily tunable OER activities by element substitution and doping. However, most of these studies focused on the choice of elements. Contrarily, the effects of crystal structures on OER activity have not yet been sufficiently interpreted, and it can be a crucial aspect to understand the OER electrocatalysis on Fe-based oxides and for the rational design of outstanding OER catalysts.In this study, we aim to comprehensively understand the effects of crystal structures on electrocatalytic OER activities on Fe-based oxides. To obtain versatile insights into OER electrocatalysis, we collected OER performances of Fe-based simple and bimetal oxides from our experiments and literature, and their structural characteristics were obtained from databases for inorganic compounds. Thus, we found that OER efficiencies on the Fe-based oxides were correlated with Fe–O bond lengths in their crystals; i.e., a shorter Fe–O bond length led to a higher OER activity.[1] We then exploited databases to mine potential candidates based on the structure–activity relationship and selected unreported Fe-based bimetal and trimetal oxides with various structures, which can dramatically accelerate our catalyst research. We synthesized these Fe-based multimetal oxides and evaluated their OER activity in alkaline media; remarkably, the OER efficiency on the unreported Fe-based multimetal oxides also exhibited excellent correlation with Fe–O bond lengths, as plotted in Figure 1. It is notable that the trend in the Fe–O bond length and OER activity is applicable to a wide variety of Fe-based simple, bimetal, and trimetal oxides regardless of chemical compositions, crystal categories, and Fe valence states. Furthermore, machine learning analysis proved that Fe–O bond length is the most dominant structural descriptor for OER efficiency compared with other structural parameters.[2] However, it is required to overcome the abovementioned catalytic trend because theoretical Fe–O bond length in Fe-based oxides is limited, and superior OER catalysts cannot be achieved according to this design guideline. Hence, we attempted to switch the OER electrocatalytic mechanism on a Fe-based oxide by the arrangement of crystal structure because different mechanisms should be involved in different descriptors; i.e., the mechanism switch could surpass the abovementioned catalytic trend. Thus, we found post-spinel CaFe2O4 that comprises a large amount of edge-shared FeO6 connectivity in its crystal. DFT calculations unveiled that the structural characteristics induced an unprecedented OER mechanism, multi-iron-site mechanism, where OER process goes through a reaction intermediate with an O–O bridge on multiple Fe sites as depicted in upper side in Figure 1. Consequently, CaFe2O4 exhibited prominent OER activity despite long Fe–O bond length in its crystal.[3] These findings provide a novel insight into the OER electrocatalysis on Fe-based oxides and contribute to the innovation of electrochemical hydrogen production.The part of this paper is based on results obtained from a project, JPNP14021, commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Authors acknowledge the PRESTO program (No. JPMJPR15S3) of the JST. The part of DFT calculations was carried out on the supercomputers at NIMS, Hokkaido Univ., Kyushu Univ., and the HPCI systems (project codes: hp180115, hp200131). The part of this work was supported by JSPS KAKENHI Grant Number 20H00314 and 20K15087.