Probing Interfacial Electrochemistry on a Co3O4 Water Oxidation Catalyst Using Lab-Based Ambient Pressure X-ray Photoelectron Spectroscopy

The design and mechanistic understanding of efficient and low-cost catalysts for the oxygen evolution reaction (OER) are currently the focus of electrochemical water splitting technology. Herein, we report the chemical transformations on the water-vapor/solid interface and catalytic performance of an OER catalyst consisting of Co3O4 nanoparticles on multiwalled carbon nanotubes (Co3O4-MINCNT). Using a specially constructed electrochemical cell incorporated to the lab-based ambient-pressure X-ray photoelectron spectroscopy (APXPS) to mimic operando conditions, we obtained experimental evidence for the formation of CoO(OH) as the catalytically active phase on a Co3O4-MVVCNT OER catalyst. Under water and applied potential conditions, CoO(OH) is formed, enriching the surface of Co3O4 nanoparticles with subnanometer thickness, and oxidizing H2O into O-2. However, immediately after the removal of the applied potential, the CoO(OH) phase is converted back to Co3O4. This back-conversion from CoO(OH) to Co3O4 is likely driven by locally concentrated protons (H+) in water vapor, which shows the necessity of an electrochemical bias to preserve the catalytically active phase. These results reveal the surface chemical identities of the Co3O4-MWCNT OER catalyst, which are in agreement with those obtained from in-situ APXPS studies of liquid/solid interfaces consisting of Co3O4 catalyst and disagree with those obtained from ex-situ ultrahigh vacuum (UHV) XPS. Thus, our results demonstrate the possibility of performing surface chemical analysis in simplified electrochemical systems and further reinforce the importance of performing mechanistic studies of electrochemical devices under in-situ conditions.