Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells

Through the use of ambient pressure X-ray photoelectron spectroscopy (APXPS) and a single-sided solid oxide electrochemical cell (SOC), we have studied the mechanism of electrocatalytic splitting of water (H2O + 2e– → H2 + O2–) and electro-oxidation of hydrogen (H2 + O2– → H2O + 2e–) at ∼700 °C in 0.5 Torr of H2/H2O on ceria (CeO2–x) electrodes. The experiments reveal a transient build-up of surface intermediates (OH– and Ce3+) and show the separation of charge at the gas–solid interface exclusively in the electrochemically active region of the SOC. During water electrolysis on ceria, the increase in surface potentials of the adsorbed OH– and incorporated O2– differ by 0.25 eV in the active regions. For hydrogen electro-oxidation on ceria, the surface concentrations of OH– and O2– shift significantly from their equilibrium values. These data suggest that the same charge transfer step (H2O + Ce3+ ⇔ Ce4+ + OH– + H•) is rate limiting in both the forward (water electrolysis) and reverse (H2 electro-oxidation)...