The oxygen evolution reaction (OER) is a crucial process in water electrolysis for hydrogen production, yet its efficiency is hampered by the limitations of anode electrocatalysts. Optimizing OER electrocatalysts requires understanding the relationship between their surface composition and activity/stability. However, achieving a complete three-dimensional (3D) structural and chemical characterization of electrocatalyst surfaces, especially for nanoparticles under 100 nm, is challenging, particularly given the drastic structural and compositional changes these surfaces undergo during OER. Mixed 3d transition metal oxides, such as mixed Co-Fe spinel oxides, are promising alternatives to expensive noble metal-based oxides due to their abundance, low cost, and redox chemistry. Two spinel structures are possible depending on composition: spinel (CoII in tetrahedral, FeIII in octahedral sites) and inverse spinel (CoII in octahedral, FeIII in tetrahedral and octahedral sites). While adding small amounts of Fe to Co₃O₄ reduces overpotential, excess Fe increases it. The role of Fe in OER catalysis remains debated. Although surface rearrangement of Co-based spinel oxides has been observed, the underlying reconstruction or phase transformation impacting activity and stability lacks in-depth study. This research aims to correlate OER performance changes with structural and compositional evolution of Co₂FeO₄ and CoFe₂O₄, elucidating deactivation processes and the role of Fe in OER activity.

Extensive research has focused on mixed Co-Fe spinel oxides for OER catalysis due to their cost-effectiveness and rich redox properties. Studies have shown that the addition of small amounts of iron can enhance the catalytic activity of cobalt oxides, while excessive amounts can lead to a decrease in performance. However, a comprehensive understanding of the correlation between the atomic-scale structure and the catalytic activity and stability of these materials is still lacking. This research builds upon previous work that suggests surface restructuring and compositional changes play a significant role in OER activity. While some studies have employed various advanced techniques, the current understanding of the 3D atomic-scale structure and dynamics of these nanoparticles is insufficient for effective catalyst design.

This study employed a multi-faceted approach combining various characterization techniques to investigate the 3D atomic structure and compositional changes in Co₂FeO₄ and CoFe₂O₄ nanoparticles during OER. The nanoparticles were synthesized using a hydrothermal method. Their structure, size, and morphology were initially characterized using X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Electrochemical performance was evaluated via linear sweep voltammetry (LSV) and cyclic voltammetry (CV) in 1.0 M KOH, determining overpotential and Tafel slopes. X-ray photoelectron spectroscopy (XPS) probed the surface oxidation states of Co and Fe before and after CV cycles under OER conditions. X-ray absorption spectroscopy (XAS) provided bulk oxidation state information. Crucially, atom probe tomography (APT) offered 3D atomic-scale compositional mapping of individual nanoparticles, revealing the distribution of Co, Fe, O, and (in deuterated electrolyte) hydroxyl groups. Electrochemical impedance spectroscopy (EIS) provided insights into electrochemical sub-processes and charge transfer kinetics. HRTEM was used to analyze changes in surface structure. The combination of these techniques allowed the researchers to comprehensively characterize the materials at multiple length scales and reaction stages.

The study revealed several key findings: 1. **Nanoscale spinodal decomposition**: Pristine Co₂FeO₄ nanoparticles exhibited nanoscale compositional modulation with Co-rich and Fe-rich nanodomains. The interfaces between these domains acted as trapping sites for hydroxyl groups, contributing to higher initial OER activity compared to CoFe₂O₄. 2. **Compositional and Structural Changes during OER**: Co₂FeO₄ underwent significant changes during OER. There was pronounced Fe dissolution from the Co-rich nanodomains, coupled with irreversible structural transformation towards Co₃O₄. This led to a substantial decrease in OER activity with increasing CV cycles. In contrast, CoFe₂O₄ showed negligible Fe loss, instead undergoing surface transformation to (FeIII, CoIII)₂O₃, resulting in a less severe activity decline. 3. **Active Site Identification**: The 3D APT data indicated that hydroxyl groups preferentially accumulated at the interfaces between the Co-rich and Fe-rich nanodomains in Co₂FeO₄, suggesting these interfaces are active sites for OER. 4. **Electrochemical Sub-processes**: EIS analysis revealed different electrochemical sub-processes for Co₂FeO₄ and CoFe₂O₄. Co₂FeO₄ exhibited pseudocapacitive behavior associated with Co oxidation state changes, which was linked to its higher initial activity. The increase in resistance with CV cycles for Co₂FeO₄ supported the observed structural transformation and deactivation. 5. **Role of Iron**: The different levels of Fe dissolution in various regions (O-rich regions of CoFe₂O₄ < O-rich regions of non-segregated Co₂FeO₄ < Fe-rich domains in segregated Co₂FeO₄ < Co-rich domains in segregated Co₂FeO₄) suggest a correlation between Fe loss and OER activity. The authors proposed that Fe plays a vital role in activating Co sites for OER catalysis, possibly by promoting the formation of more active CoIII FeOOH species. The optimal ratio of Fe to Co in Co₂FeO₄, along with the additional active sites at nanodomain interfaces, contributed to its superior initial OER activity.

The findings address the research question by directly linking the atomic-scale structure and compositional changes of Co-Fe spinel oxide nanoparticles to their OER activity and stability. The observed spinodal decomposition in Co₂FeO₄ provides a unique insight into the origin of its high initial activity. The trapping of hydroxyl groups at nanodomain interfaces highlights the importance of nanoscale structural features in electrocatalysis. The contrasting behavior of Co₂FeO₄ and CoFe₂O₄ under OER conditions emphasizes the crucial role of Fe content and its interaction with Co in determining catalyst stability. The identified structural transformations and Fe dissolution in Co₂FeO₄ explain the activity decline, while the formation of (FeIII, CoIII)₂O₃ in CoFe₂O₄ accounts for its moderate performance drop. The study's significance lies in its atomic-scale resolution, providing a level of detail previously unavailable for understanding OER catalyst behavior. This detailed understanding is crucial for designing more durable and efficient OER catalysts.

This study provides unprecedented 3D atomic-scale insights into the structure-activity relationships of Co-Fe spinel oxide nanoparticles during OER. The research highlights the importance of spinodal decomposition, the role of Fe in enhancing Co-based OER activity, and the deactivation mechanisms caused by structural transformations and Fe dissolution. The use of APT, combined with other techniques, offers a powerful approach for studying electrocatalyst behavior at the atomic level. Future research could focus on investigating other mixed-metal oxides with similar spinodal decomposition behavior, tuning the nanoscale composition to further optimize OER activity and stability, and exploring strategies to mitigate Fe dissolution.

The study primarily focuses on two specific Co-Fe spinel compositions (Co₂FeO₄ and CoFe₂O₄). While the findings are valuable, they may not be directly generalizable to all Co-Fe spinel oxides with varying compositions. The hydrothermal synthesis method may also influence the resulting nanoparticle morphology and properties, impacting the overall results. The experimental conditions (e.g., electrolyte concentration, temperature) were held constant; investigation under different conditions could reveal further details of the catalyst behavior. Although the authors attempt to control the effect of carbon contamination during the HRTEM measurements, this factor might slightly affect some of the structural analysis.