Evaluating the Redox Behavior of Doped Ceria for Thermochemical CO2 Splitting Using Time-Resolved Raman Spectroscopy

Abstract

Solar-to-fuel technology promises to play a key role in realizing a carbon-neutral future by enabling renewable fuel processing for capacity-independent storage beyond current battery technologies. Redox metal oxides enable the two-step thermochemical cycle at the heart of this technology by facilitating the reduction of H2O and CO2 to produce H2 and CO. Currently, ceria is the material of choice for this technology; however, the low oxygen exchange capacity of CeO2 has motivated its doping with tetravalent cations as a strategy to increase the maximum fuel yield per cycle. It is challenging to probe the defect chemical and short-range structural changes in CeO2 during thermochemical cycling. However, these are crucial for understanding the reasons behind a material's oxygen exchange capacity and redox kinetics. To gain more insights about of the effects of isovalent doping, in this work, we demonstrate how to access the defect chemical changes during fuel production by Raman spectroscopy. The results illuminate the reaction mechanisms of oxygen vacancy formation and CO2 splitting in doped ceria solid solutions of Ce0.9Hf0.1O2, undoped ceria, and trivalently doped Ce0.9La0.1O2. By tracking the frequency shift of the F2g mode during thermochemical cycling under oxidizing CO2 and reducing H2, we estimate the oxygen exchange capacity due to cation expansion and vacancy formation. We connect these results with the fuel production performance of these materials in a fixed-bed reactor and corroborate the kinetic effects of +3 and +4 doping. The methodology presented here could be extrapolated to other redox systems used for thermochemical syngas production and energy conversion, such as chemical looping combustion or electrochemical fuel production, proving fundamental insights into vacancy formation that could be useful in the design of new materials and understanding reaction pathways.