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

Abstract

Abstract 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 CoCr 2 O 4 and Co 2 CrO 4 spinel nanoparticle surfaces affect OER activity and stability in alkaline media. CoCr 2 O 4 undergoes an activation process and subsequently retains high OER activity for extended durations. The activation of CoCr 2 O 4 is induced by a steady and substantial Cr dissolution that facilitates bulk incorporation and intercalation of hydroxide ions, coupled with the highly reversible ( $${{{{\rm{Co}}}}}_{{{{\rm{Td}}}}}^{{{{\rm{II}}}}}$$ Co Td II ,Cr)(OH) 2  ↔ ( $${{{{\rm{Co}}}}}_{{{{\rm{Oct}}}}}^{{{{\rm{III}}}}}$$ Co Oct 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 Co 2 CrO 4 upon cycling, contributing to OER activity. As OER proceeds, such Cr-based (oxy)hydroxide layers on Co 2 CrO 4 are depleted from the surfaces, leading to deteriorating activity. Overall, this study demonstrates that continuous Cr dissolution triggers an intercalation-assisted ( $${{{{\rm{Co}}}}}_{{{{\rm{Td}}}}}^{{{{\rm{II}}}}}$$ Co Td II ,Cr)(OH) 2  ↔ ( $${{{{\rm{Co}}}}}_{{{{\rm{Oct}}}}}^{{{{\rm{III}}}}}$$ Co Oct III ,Cr)OOH transformation that can promote the OER activity and stability of Co-based spinels.