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-MWCNT). 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-MWCNT 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 O₂. 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.