Differential Electrochemical Mass Spectrometry Coupled with Linear and Non-Linear Electrochemical Impedance Spectroscopy of Gadolinia-Doped Ceria: CO2 and H2o Co-Electrolysis

Abstract

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 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 co-electrolysis of CO2 and water on GDC have been compared with model NLEIS spectra developed to include CO­2 reduction, water reduction, and reverse water gas shift reaction mechanisms and rate determining steps. A rough vacuum and low volume reactor was developed to cross-check the proposed mechanisms 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.