How many surface atoms in Co₃O₄ take part in oxygen evolution? Isotope labeling together with differential electrochemical mass spectrometry

Understanding the mechanism underlying the oxygen evolution reaction (OER) on oxides is crucial for the development of many energy storage systems. Here, the mechanism of OER on a Co₃O₄ spinel catalyst is investigated in alkaline media using ¹⁸O-labeling combined with differential electrochemical mass spectrometry (DEMS). This work unravels the role of surface oxygen of the oxide in the OER. It is shown that in H₂¹⁸O-containing electrolyte the amount of ¹⁸O¹⁶O evolved increases from cycle to cycle together with a concomitant decrease of the amount of ¹⁶O₂ with each cycle before reaching a steady-state value. ¹⁸O¹⁶O is also evolved from a H₂¹⁶O solution on a Co₃O₄ electrode pre-treated in H₂¹⁸O-containing solution, indicating the formation of the ¹⁸O-labeled oxide in the previous step. Therefore, the oxide layer takes part in OER via an oxygen exchange mechanism. The total number of oxygen atoms of the oxide participating in OER is 0.1 to 0.2% of the total oxide loading, corresponding to about 10-30% of the surface atoms; these represent the catalytically active sites. Moreover, the real surface area of the catalyst is estimated using different methods (namely the ball model, double layer capacitance method, redox peak method, isotope exchange), and compared to the BET data. The surface areas calculated from the BET data, ball model and redox peak method are similar for small particles, which indicates their smooth surface; however they are smaller than that estimated from double-layer capacitance. For larger particles, the much larger surface area estimated from the redox peak in comparison to that expected from the ball model seems to be due to their roughness. Thus, this work highlights the importance of probing the mechanism when investigating the OER activity of a catalyst.