Evaluating the Behavior of CO/CO2 in Ni/GDC Solid Oxide Fuel Cell Anodes

Ni/GDC anodes for solid oxide fuel cells have been reported to be more resistant to carbon deposition than conventional Ni/YSZ anodes. To facilitate understanding the nature of carbon chemistry in Ni/GDC anode functional layers, we have studied the behavior of CO oxidation and CO2 dissociation on thin-film Ni/GDC electrodes.  Sputtered thin-film GDC electrodes with open Ni sputtered overlayers provide an optically accessible and well-characterized electrode in which to study fundamental reactions of CO and CO2 in Ni/GDC systems for both dry and wet environments. To further separate out the behavior of the GDC surface chemistry from the Ni chemistry, Au overlayers were deposited on the thin-film GDC for an alternative electrode with the Au serving as a low-activity current collector. Results indicate that dry CO oxidation and CO2 dissociation are substantially slower than their H2 and H2O analogs on Ni/GDC with fitted exchange current densities for CO/CO2 approximately one order of magnitude lower than H2/H2O under similar partial pressures and temperatures ranging from 600 to 700 deg. C. By replacing the CO2 with H2O, exchange current densities associated with CO oxidation rise dramatically although still lower than H2 oxidation at similar conditions. By using our studies with ambient pressure XPS, we are able to obtain local overpotentials across the Ni/GDC for some test conditions and thereby extract fundamental information about the CO and CO2 chemistry in the Ni/GDC cell.

Although the CO/CO2 reactions are slow, they may play a vital role in the suppression of carbon formation in Ni/GDC cells.  Thus, to better understand this, we fit our experimental data for the thin-film electrodes to a microkinetic model that includes CO/CO2 chemistry on both the Ni and GDC.  By fitting these rates to match the model results to the performance of Ni/GDC and Au/GDC electrodes (and assuming the Au functions as an inert current collector), we have developed kinetic expressions for surface chemistry of CO and CO2 on the GDC surface. Coupling this surface chemistry to previous Ni surface chemistry and to our own developed mechanism for H2 oxidation on Ni/GDC provides a basis for exploring how the Ni and GDC surface chemistry interact with exposure to carbon-rich syngas.  The fitted model for the thin-film Ni/GDC performance suggests that the presence of surface hydrogen and hydroxyls on the Ni can have significant boosts to CO conversion to CO2 on both the GDC and Ni surfaces. These synergies may be critical for providing rapid CO conversion in lower temperature SOFCs which rely on GDC as part of the anode and electrolyte materials.