Single-atom catalysts (SACs) offer maximized atomic utilization and uniform active sites, beneficial for selectivity and mechanistic studies. However, true homogeneity in SACs is challenging because the support's diverse coordination environments influence catalytic performance. The support material's surface topology and defects create various anchoring sites, leading to differing coordination environments for the single atoms. For example, single-atom Fe on nitrogen-doped carbon exhibits different oxidation states (+3 and +2) depending on its coordination, affecting CO2 adsorption. Similarly, Pt single atoms coordinated with N and Mo show different oxygen reduction activity due to variations in Pt-N and Pt-Mo bond strengths. This necessitates precise control over the coordination environment to achieve true homogeneity in SACs. This study focuses on selectively anchoring Ir single atoms onto specific sites of defective CoOOH to examine the influence of anchoring site on the OER activity. CoOOH, with its (001) surface terminated with oxygen atoms and potential oxygen vacancies introduced during synthesis, provides diverse anchoring sites. The electrochemical deposition method offers control over the anchoring sites by using either cathodic or anodic deposition to drive the deposition of Ir cations or anions, respectively, onto the three-fold hollow sites or oxygen vacancies. This approach allows for the investigation of how different anchoring sites affect the OER performance.

The literature extensively documents the potential of SACs in various energy conversion reactions. Studies have demonstrated the advantages of maximized atomic utilization and uniform active sites in SACs, leading to improved selectivity and providing an ideal platform for mechanistic investigations. However, research has also highlighted the limitations of the first-order approximation of SAC homogeneity, where not all isolated metal centers interact identically with the support. Numerous studies have illustrated how the surrounding coordinated atoms significantly impact the catalytic performance of SACs, demonstrating the need for precise control over the coordination environment to achieve real homogeneity. Previous studies have shown how different coordination environments can lead to distinct oxidation states, influencing adsorption properties. For instance, the varying coordination of single-atom Fe on nitrogen-doped carbon results in different oxidation states and consequent differences in CO2 adsorption. Similarly, the coordination of Pt with N and Mo atoms affects the adsorption of oxygen reduction intermediates. These findings emphasize the crucial role of precise control over the coordination environment in achieving the desired catalytic properties. The use of CoOOH as a support material, with its potential for varying surface topologies and defects, has been explored, highlighting the inherent challenges in creating identical active sites in SACs. Electrochemical deposition methods have shown promise in selectively anchoring SACs, providing a pathway to precisely control the coordination environment of single atoms.

The CoOOH support was synthesized via a co-precipitation method, characterized by TEM, EDX mapping, XRD, ESR, and XPS to confirm its morphology, elemental composition, and presence of oxygen vacancies. Ir single atoms were then selectively anchored onto the CoOOH support using an electrochemical deposition method. Cathodic deposition produced Ir1/To-CoOOH, anchoring Ir atoms onto the three-fold hollow sites, while anodic deposition generated Ir1/Vo-CoOOH, placing Ir atoms at oxygen vacancies. The atomic dispersion of Ir was verified using aberration-corrected HAADF-STEM and EDX mapping. The electronic structure and coordination environments were investigated using XANES and EXAFS, revealing the different Ir oxidation states and coordination numbers in the two samples. DFT calculations were used to explain the selective anchoring of Ir at specific sites, considering the formation energies of IrCl3+ and Ir(OH)2- anions on different sites. XAS analyses and XPS were used to elucidate the metal-support interactions, revealing charge transfer in Ir1/To-CoOOH but not in Ir1/Vo-CoOOH. OER activity was evaluated using a standard three-electrode system in 1 M KOH electrolyte, measuring polarization curves and calculating overpotentials, specific activities, mass activities, and TOFs. Tafel slopes were determined to evaluate reaction kinetics. Durability tests were conducted to assess long-term stability. In-situ XANES, EXAFS, and Raman spectroscopy were employed to probe structural changes under OER conditions. Finally, DFT calculations were utilized to determine free energy diagrams and reaction pathways for the OER, examining the adsorption of intermediates on Co active sites and comparing the rate-determining step energy barriers for the two samples. The influence of hydrogen bonding in Ir1/Vo-CoOOH was explored to explain the improved OER performance.

HAADF-STEM and EDX mapping confirmed the successful single-atom dispersion of Ir on CoOOH in both Ir1/To-CoOOH and Ir1/Vo-CoOOH. XANES and EXAFS analysis revealed distinct Ir coordination environments: IrCl3O3 for Ir1/To-CoOOH and IrO6 for Ir1/Vo-CoOOH. DFT calculations supported the selective anchoring mechanisms, based on formation energies. Ir1/Vo-CoOOH exhibited superior OER activity compared to Ir1/To-CoOOH and CoOOH, with a 70 mV lower overpotential at 10 mA cm⁻². The specific activity of Ir1/Vo-CoOOH was 5.75 times higher than Ir1/To-CoOOH at an overpotential of 300 mV. Tafel slopes indicated faster reaction kinetics for Ir1/Vo-CoOOH. In-situ XAS revealed that both Co and Ir in both samples increased their oxidation states under OER conditions. DFT calculations showed that Ir1/To-CoOOH's improved activity resulted from a reduced band gap, enhancing electron transfer and intermediate adsorption, while Ir1/Vo-CoOOH's activity benefited from hydrogen bonding between the Ir center and intermediates, optimizing the energy barrier of the rate-determining step. The hydrogen bonding in Ir1/Vo-CoOOH was identified as a significant factor in its superior OER activity. The durability tests showed that Ir1/Vo-CoOOH maintained its activity and structure after 20 hours of operation.

The findings demonstrate that the anchoring site significantly influences the OER activity of Ir single-atom catalysts on CoOOH. The enhanced performance of Ir1/Vo-CoOOH compared to Ir1/To-CoOOH highlights the importance of considering the support's defect sites and their interaction with the anchored single atoms. The distinct mechanisms underlying the improved activity in both cases showcase the complexity of SAC catalysis. The strong electronic interaction in Ir1/To-CoOOH modifies the electronic structure, promoting electron transfer and intermediate adsorption. In contrast, the configural interaction via hydrogen bonding in Ir1/Vo-CoOOH stabilizes intermediates and lowers the energy barrier. These results demonstrate the need for precise control of the anchoring site to achieve optimal OER activity in SACs. The superior performance of Ir1/Vo-CoOOH demonstrates the potential of strategically utilizing support defects to enhance catalysis. This work advances the understanding of SAC design and paves the way for developing highly active and stable electrocatalysts for water splitting applications.

This study successfully demonstrated the site-selective anchoring of Ir single atoms on defective CoOOH for improved OER activity. The distinct OER mechanisms for Ir1/To-CoOOH (electronic effect) and Ir1/Vo-CoOOH (configural effect) highlight the critical role of anchoring site selection. The superior performance of Ir1/Vo-CoOOH suggests that engineering support defects is a promising strategy for designing highly efficient SACs. Future work could explore other support materials with controllable defect sites and investigate different single-atom species to further optimize OER activity and stability.

The study focused solely on Ir single atoms on CoOOH. Further investigation is needed to explore the generality of these findings to other metal-support combinations. The DFT calculations utilized simplified models and did not fully capture all aspects of the complex interactions involved in OER catalysis, such as the solvent effects. The experimental measurements were performed under specific conditions and may not directly translate to other environments.