Atomic-scale insights into surface reconstruction and transformation in Co-Cr spinel oxides during the oxygen...

Optimizing the activity and longevity of oxygen evolution reaction (OER) electrocatalysts requires an atomic-scale understanding of multiple reconstruction and transformation processes occurring in the surface and sub-surface regions of the electrocatalyst. Herein, a multimodal method combining X-ray absorption fine structure and photoemission spectroscopy, in situ Raman spectroscopy, transmission electron microscopy and atom probe tomography with electrochemical measurements is employed to unveil how the changes in oxidation states, atomic coordination, structure and composition on ~20 nm CoCr₂O₄ and Co₂CrO₄ spinel nanoparticle surfaces affect OER activity and stability in alkaline media. CoCr₂O₄ undergoes an activation process and subsequently retains high OER activity for extended durations. The activation of CoCr₂O₄ is induced by a steady and substantial Cr dissolution that facilitates bulk incorporation and intercalation of hydroxide ions, coupled with the highly reversible ( ${\mathrm{Co}}_{\mathrm{Td}}^{\mathrm{II}}$ ,Cr)(OH)₂ ↔ ( ${\mathrm{Co}}_{\mathrm{Oct}}^{\mathrm{III}}$ ,Cr)OOH transformation, which enhances OER activity and stability_. In comparison, a ~ 2 nm thick amorphous self-limiting Cr-based (oxy)hydroxide forms on Co₂CrO₄ upon cycling, contributing to OER activity. As OER proceeds, such Cr-based (oxy)hydroxide layers on Co₂CrO₄ are depleted from the surfaces, leading to deteriorating activity. Overall, this study demonstrates that continuous Cr dissolution triggers an intercalation-assisted ( ${\mathrm{Co}}_{\mathrm{Td}}^{\mathrm{II}}$ ,Cr)(OH)₂ ↔ ( ${\mathrm{Co}}_{\mathrm{Oct}}^{\mathrm{III}}$ ,Cr)OOH transformation that can promote the OER activity and stability of Co-based spinels.