Additionally, although surface chemical and structural rearrangement of Co-based spinel oxides has been recently observed 4, 15, 16, 17, 18, 19, 20, the surface reconstruction or phase transformation responsible for the change in OER activity and stability has not yet been studied in-depth. However, the role of Fe of mixed Co-Fe oxides or (oxy)hydroxides in catalysing OER is poorly understood, being the subject of ongoing and intense debate 4, 5, 9, 10, 11, 12, 13, 14. The addition of small amounts of Fe in Co 3O 4 has been found to reduce the overpotential, while excess Fe increases the overpotential 7, 8. Depending on the composition, two spinel structures can be formed: (i) spinel, whereby a divalent cation, e.g., Co II, is located at the tetrahedral site, and trivalent Fe III at the octahedral site, and (ii) inverse spinel, whereby Co II is located at the octahedral site and Fe III at both the tetrahedral and octahedral sites 6.
Mixed 3d transition metal oxides, such as mixed Co-Fe spinel oxides, have attracted much attention in the context of OER electrocatalysts due to their high abundance, low cost and rich redox chemistry 3, 4, 5 these characteristics make them attractive alternatives to the high-cost benchmark noble metal-based oxides, i.e., IrO 2 and RuO 2. Therefore, to develop high-performance OER electrocatalysts, it is imperative to thoroughly evaluate the contribution made by individual atoms during reactions to the relationships between catalytic activity and stability. In addition, the electrocatalyst surfaces undergo drastic structural and compositional changes during OER.
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However, it is notoriously challenging to perform a full three-dimensional (3D) structural and chemical characterisation of the topmost atomic layers of electrocatalysts, especially for catalyst nanoparticles <100 nm in diameter. Optimisation of OER electrocatalysts requires a detailed understanding of the correlation between the surface composition of electrocatalysts and their activity and stability. One of the major hurdles is the limitation in the performance of anode electrocatalysts, where the oxygen evolution reaction (OER) takes place 1, 2. Although water electrolysis is a key technology in the production of hydrogen, it remains inefficient, and there are many complex challenges to improve its efficiency. Hydrogen has long been proposed as a clean energy carrier within sustainable energy infrastructure. Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER.
In contrast, there is negligible elemental redistribution for CoFe 2O 4 after OER, except for surface structural transformation towards (Fe III, Co III) 2O 3. However, the activity of Co 2FeO 4 drops considerably due to concurrent irreversible transformation towards Co IVO 2 and pronounced Fe dissolution. The interfaces of Co-rich and Fe-rich nanodomains of Co 2FeO 4 become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe 2O 4.
We reveal nanoscale spinodal decomposition in pristine Co 2FeO 4. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized Co 2FeO 4 and CoFe 2O 4 nanoparticles during oxygen evolution reaction (OER). Optimising the performance of electrocatalysts requires atomic-scale information, but it is difficult to obtain. The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability.
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