Identifying the in situ protection role of MnO2 nanosheets on Co oxide for superior water oxidation

Abstract

As a limiting step of electrochemical water splitting, the oxygen evolution reaction calls for efficient catalysts to surmount the high energy barrier. In this work, the synthesis of O-deficient Co3O4 nanoparticles with abundant surface area was realized by applying two-dimensional MnO2 nanosheets as a self-protection agent. The strong interaction between Mn and Co was confirmed by our experiments and DFT calculations that the valence state is increased for Co while it is reduced for Mn during the annealing in reducing atmosphere. Mn3+δ with stronger Lewis acidity can protect Co2+ from being reduced when subjected to the H2 treatment. The H2– reduced CoMn oxide under 300 °C (CMO-H300) shows the best OER activity with an overpotential of 280 mV in 1 M KOH and 460 mV in 1 M phosphate buffer (PBS) at 10 mA cm-2 geo, which can be attributed to the optimal chemical state, appropriate oxygen vacancies and highly exposed surface area. Operando Raman spectroscopy combined with ex-situ XPS revealed the manner of active species evolution in CMO-FD (Freezing-drying CoMn oxide) and CMO-H300 under the OER process, which accounted for the dynamic change of chronoamperometry. Our findings provide a different viewpoint for novel electrocatalyst fabrication by introducing a self-sacrifice agent that in situ promotes electronic structure modulation and high surface area preservation.