Carbon dioxide reduction on gadolinia-doped ceria cathodes

This work describes an investigation of CO-CO2 exchange on 40 mol% gadolinia-doped ceria (GDC) electrodes for potential application as a CO2 reduction cathode for the solid oxide electrolysis of CO 2.;A computational analysis was performed on the thick electrolyte cylindrical pellet test cell geometry to investigate the effects of this cell geometry on Electrochemical Impedance Spectroscopy (EIS) due to non-linear current distribution. The analysis showed the particular cell geometry selected does induce an error of 15% on the impedance measurements, but in a predictable linear manner. Additional parametric analyses indicate that impedance errors for the cylindrical cell geometry can be reduced by covering the faces of the pellet with the working and counter-electrodes, centering the reference electrode hole equidistant from working and counter-electrodes, or utilizing a reference electrode mounted at the midpoint edge of the pellet.;A continuum-based model is described for equilibrium CO-CO2 exchange on a mixed-conducting electrode utilizing porous electrode theory. The resulting three-parameter model is expressed in terms of a characteristic resistance (Rchem), a characteristic time constant (tchem), and a utilization thickness ratio (&phis;), that can be related to physiochemical properties.;EIS measurements were performed on the 40 mol% GDC electrodes on yttria stabilized zirconia (YSZ) electrolytes at 700-950°C in reducing CO/CO 2 atmospheres. Area-specific-resistance (ASR) values for this electrode were in the range of 0.8-37 ohm-cm2, about two orders of magnitude lower than measurements on Pt electrodes and slightly lower than data on Ni-YSZ electrodes in the literature under similar temperature and partial pressure of oxygen ( PO2 ) conditions. An analysis utilizing the continuum-based porous electrode model was performed to extract the vacancy diffusion coefficient ( Dv) and surface exchange rate coefficient (reals 0) as a function of temperature and PO2 , from the impedance results. The Dv data agree well with published measurements of the tracer diffusion coefficient (D*) based on isotope profiling by secondary ion mass spectroscopy (SIMS), and conductivity measurements on 40 mol% GDC. The reals0 values are a factor of 3 lower than published measurements of the surface reaction rate (k) obtained from isothermal thermogravimetric relaxation and decrease with increasing PO2 .