Recent advances in carbon capture create new opportunities for recycling CO₂ into liquid fuels to store intermittent electrical energy¹. One option is to co-electrolyze CO₂ and H₂O at high temperature using solid oxide electrolysis cells (SOECs). Although promising, the factors controlling rates of CO₂ and H₂O reduction in SOECs are not well understood, hampering development^2,3. Traditional electrochemical techniques have difficulty resolving the mechanisms and kinetic parameters of the individual steps governing CO₂ and H₂O reduction. This limitation can potentially be overcome using linear and non-linear electrochemical impedance spectroscopy (EIS and NLEIS) in conjunction with dynamic measurement of gas-phase species using differential electrochemical mass spectrometry (DEMS).

Regarding co-electrolysis, mixed ionic electronic conductors (MIEC) have gained interest as alternatives to nickel-yttria stabilized zirconia (Ni-YSZ) because the active region is not limited to the triple phase boundary and they are more stable in reducing environments³. The MIEC studied here, gadolinia-doped ceria (GDC), has been well characterized as an electrolyte³ and is a promising cathode in SOECs. Unlike Ni-YSZ, it does not coke in carbon environments or oxidize completely in a feed of water and CO₂.

We have performed NLEIS and EIS measurements of CO₂ reduction, water reduction, and co-electrolysis on button cells composed of GDC as the working electrode with electrolyte YSZ under various temperatures and gas compositions. Gas atmospheres surrounding the cells contain mixtures of water, CO₂, CO, H₂, and carrier gas such as Ar or N₂, used to manipulate the oxygen partial pressure of the system and thus the oxygen vacancy concentration of the surface and bulk electrode. The data we have collected on CO₂ and water reduction on GDC have been compared with model NLEIS spectra developed in our previous work^3,4 in the context of a reaction mechanism and a rate determining step. Additionally, these mechanisms were cross-checked through measuring a gas phase response in DEMS and a micro-kinetic model was developed to predict product distributions corresponding to the mechanism that most closely matched the results.

References

1. Graves, C., Ebbesen, S. D., Mogensen, M., & Lackner, K. S. (2011). Sustainable hydrocarbon fuels by recycling CO2 and H2O with renewable or nuclear energy. Renewable and Sustainable Energy Reviews, 15(1), 1–23. https://doi.org/10.1016/j.rser.2010.07.014

2. Graves, C., Chatzichristodoulou, C., & Mogensen, M. B. (2015). Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes. Faraday Discussions, 182(0), 75–95. https://doi.org/10.1039/C5FD00048C

3. Valdés-Espinosa, H., Stuve, E. M., & Adler, S. B. (2015). Modeling Water Reduction on 10 Mole% Gadolinia-Doped Ceria (GDC10) Porous Electrodes. ECS Transactions, 66(2), 229–251. https://doi.org/10.1149/06602.0229ecst

4. Witt, J., Stuve, E.M., & Adler, S.B. (2017). Modeling CO₂ Electrolysis on Gadolinia Doped Ceria Porous Electrode using a 1-D Macro-Homogeneous Model and Linear and Non-Linear Electrochemical Impedance Spectroscopy. ECS Transactions (accepted and awaiting publication).