Differential Electrochemical Mass Spectrometry Coupled with Linear and Non-Linear Electrochemical Impedance Spectroscopy of Gadolinia-Doped Ceria: Deconvolution of CO2 and H2o Co-Electrolysis
Authors: Witt Jonathan Michael , Stuve Eric M. , Adler Stuart B. 
Published: February 2020

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 development2,3. 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 work3,4 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

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

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

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

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).