Understanding the Structural Influence of Fe Incorporation in Co-Based Spinel Oxides as Oxygen Evolution Reaction Catalysts

The growing global energy demand emphasized the need for sustainable energy technologies. Water electrolysis is a promising method for clean hydrogen production. However, the slow kinetics of the oxygen evolution reaction (OER) makes it the rate-limiting step of this process, necessitating the development of efficient OER electrocatalysts. Cobalt oxides (Co3O4) are promising OER catalysts in alkaline media due to their high catalytic activity, chemical stability, and cost-effectiveness1. Incorporating Fe into the Co3O4 spinel structure enhances their performance, leading to the development of iron cobaltites (FexCo3−xO4) with huge potential as OER electrocatalysts. There are extensive studies exploring FexCo3−xO4 system, highlighting the structural diversity between normal and inverse spinel configurations2. The conventional description of normal and inverse spinels does not fully capture the complexities introduced by Fe incorporation into the Co3O4 framework. The FexCo3−xO4 system is better described by a combination of diverse spinel structures, with Fe incorporation altering cation distribution in the octahedral and tetrahedral sites, oxidation states, and electronic configurations. These changes influence spin states and magnetic properties, which have been linked to these materials’ OER activity and stability3. By varying Fe content and annealing conditions, this study seeks to understand the structural-catalytic behavior of FexCo3−xO4 and reveal the correlation between spin states and OER catalytic performance. To address these objectives, we delve into the synthesis and characterization of FexCo3-xO4 binary oxide series. X-ray diffraction (XRD) analyses have confirmed the occurrence of spinodal decomposition, a phase separation (into Co-rich spinel and Fe-rich spinel) phenomenon that occurs within a specific temperature region of the Fe–Co–O system2. Changing annealing temperature has allowed successful incorporation of Fe to the Co3O4 structure which enables further exploration of the Fe-Co-O phase diagram. Low annealing temperatures (around 400 °C) do not provide enough thermodynamic energy to form the desired pure spinel structure and fail to incorporate all the Fe into the spinel, resulting in formation of other iron oxides, such as hematite. Our systematic study shows spinodal decomposition for compositions between 0.25 ≤ x ≤ 1.5 (FexCo3−xO4) in the temperature region of 800 to 950 °C. Structural data obtained using XRD of samples annealed at high temperatures (800 °C and above) provided critical insights of their structure. The lattice parameters obtained are 8.0839 Å for Co3O4, 8.1249 Å for Fe0.25Co2.75O4 and 8.1708 Å for Fe0.5Co2.5O4. The increase in lattice parameters confirms successful Fe incorporation, as it aligns with the expected lattice expansion caused by the larger ionic radius of Fe compared to Co. To further confirm, for low Fe containing compositions (FexCo3-xO4 for x < 1), lattice parameters fall between those of the end members: Co3O4 (~8.08 Å) and FeCo2O4 (~8.24 Å), supporting the successful Fe incorporation into the spinel structure4. Rietveld refinements confirm the phase purity of these compositions. To form pure phases of low Fe containing compositions (Fe0.25Co2.75O4 and Fe0.5Co2.5O4) a higher annealing temperature of 875 °C is required. However, the synthesis of pure FeCo2O4 has yet to be achieved suggesting that an optimal annealing temperature ranges between 875 °C and 950 °C. 57Fe Mössbauer spectroscopy was performed on the FexCo3-xO4 binary oxide series to elucidate the magnetic properties, local environment and oxidation states of Fe. 57Fe Mössbauer studies performed at the room temperature agree with XRD analysis. Most compositions show both magnetic and paramagnetic components, shedding light on the interplay between Fe content and different phase formation. Co-rich samples are purely paramagnetic, consistent with the literature4. FeCo2O4 shows both paramagnetic and magnetic properties due to the existence of both Co rich and Fe rich domains while Fe2CoO4 is only magnetic. During this presentation, the electrocatalytic activities of these materials towards the OER will be also discussed in term of the physical properties discussed above and compared with those of other cobalt-rich spinel oxide electrocatalysis. References Badruzzaman, A. et al. Recent advances in cobalt based heterogeneous catalysts for oxygen evolution reaction. Inorganica Chim Acta 511, 119854 (2020). Dinh, T. M. C. et al. FIB plan view lift-out sample preparation for TEM characterization of periodic nanostructures obtained by spinodal decomposition in Co1.7Fe1.3O4 thin films. CrystEngComm 20, 6146–6155 (2018). Gao, X. et al. Optimized Co2+(Td)-O-Fe3+(Oh) electronic states in a spinel electrocatalyst for highly efficient oxygen evolution reaction performance. Inorg Chem Front 6, 3295–3301 (2019). Le Trong, H. et al. Mössbauer characterisations and magnetic properties of iron cobaltites CoxFe3-xO4 (1≤x≤2.46) before and after spinodal decomposition. J Magn Magn Mater 334, 66–73 (2013).