Operando insights on CO2 electrolysis on Pr/Gd-doped ceria electrodes : the role of H2

Ceria-based mixed ionic electronic conductors (MIEC) provide a lucrative electrode material for high temperature (800 oC) CO2 electrolysis. Doping the CeOx electrodes with Gd or Pr improves both the electrochemical and thermochemical processes on the catalyst surface. Our study provides an in-depth analysis of the effects of reactant gas concentration and applied reducing bias on the nature of the products formed. The electrodes were stable under strong reducing biases (-2 A/cm2), and could operate without incurring any major overpotential penalties. Operando Raman spectroscopy and online mass spectrometry (MS) were utilised in conjunction with electrochemical measurements to help establish a relationship between the catalyst structure and its chemical behaviour. The reduction of Ce4+ sites to Ce3+ (creation of oxygen defects) was found to be correlated with the production of CO on applying reducing bias. The doped materials (Ce{Pr}Ox and Ce{Gd}Ox), in general, demonstrated enhanched defect formation. In the presence of H2, Ce{Pr}Ox was found to oxidise, whereas, both CeOx and Ce{Gd}Ox exhibited reduction. In Ce{Pr}Ox, the oxidation by H2 and reduction by bias were found to compete. Coke formation was observed on the catalyst surface at high CO2 concentrations for the doped ceria electrodes. This coke formation co-incided with the removal of carbonyl peaks and Ce3+ enrichment in the Raman spectra. We have observed that increasing H2 concentration affects coke formation differently for Ce{Pr}Ox and Ce{Gd}Ox: it delays the onset bias for coke formation for the former, while expediting it for the latter with increasing H2 concentration. Through first principles-based calculations, we have correlated the counter-intuitive behaviour of Ce{Pr}Ox in H2 to the nature of the incorporated hydrogen in the Ce{Pr}Ox-H adduct in a reducing environment. Incorporation of H is found to stabilise the nearby oxygen sites for Ce{Pr}Ox, whereas it makes oxygen vacancy formation (and thereby, Ce3+ formation) easier for CeOx, Ce{Gd}Ox remains largely unaffected. We have demonstrated the role of hydrogen in improving the nearby metal-oxygen bonding. Our study provides a first account of the nature of interaction of hydrogen with doped ceria-based materials through both experimental techniques and theoretical calculations, which in turn affect their activity towards CO2 electrolysis.