How the surface state of nickel/gadolinium-doped ceria cathodes influences the electrochemical performance in direct CO2 electrolysis

The effect of the surface state on the electrochemical performance of nickel gadolinium-doped ceria (NiGDC) cermet electrodes in direct CO2 electrolysis was studied by operando near ambient pressure X-ray photoelectron spectroscopy combined with on-line gas phase and electrical measurements. The CO2 electrolysis was limited at overpotentials below the carbon deposition threshold to avoid irreversible cathode deactivation. The results revealed the dynamic evolution of the NiGDC electrode surface and dis-close the side reactions associated to electrode activation in CO2 electrolysis. Comparison of reduced and oxidized electrodes shows that metallic Ni is a prerequisite for CO2 electrolysis, at least at low potentials, suggesting that CO2 electoreduction occurs primarily at the three phase boundaries between gas, metallic nickel and partially reduced ceria. We also provide evidences of in situ reduction of NiO upon polarization in CO2, implying that addition of reductive gases to CO2 is not indispensable to maintain the cermet elec-trode in the reduced state. Inspired by this observation, we use a conventional button cell setup to demonstrate improved i-V characteristics of NiGDC electrodes in direct CO2 electrolysis as compared to CO2/H2 fuel conditions and we rationalize this behavior based on NAP-XPS results. (c) 2021 Elsevier Inc. All rights reserved.