Comparative Study of the Performance of Syngas-Fueled Carbon-Air Solid Oxide Fuel Cells Using Anodes Based on M-Doped CeO2 (M: Mo or Mo-Cu)

Abstract

The use of syngas to feed solid oxide fuel cells (SOFCs) appears as an attractive alternative due to the possibility of being obtained from renewable sources such as biomass or from other solids of carbonaceous composition such as solid wastes through reforming reactions. The use of syngas in SOFCs has been studied in several previous works, directing the efforts to obtain an anode material with the appropriate electrocatalytic activity for the electrooxidation of syngas, able to tolerate carbon deposition and avoid sulfur poisoning [1]. The Ni-YSZ anode has been studied in the electrooxidation of syngas, being one of its advantages the catalysis of the water gas shift (WGS) reaction, generating more H2 with respect to CO and therefore improving the reaction kinetics [2]. One of the disadvantages presented by Ni-YSZ is the decrease of its performance in long term tests, due to the catalytic activity of Ni towards the reverse Boudouard reaction, so that carbon deposits block the triple phase boundaries (TPBs) of the anode [3]. CeO2-based materials are suitable for use as anodes in fuel cells fed with carbonaceous fuels due to their resistance to carbon deposition and sulfur poisoning [4, 5]. Recently it has been reported that the addition of molybdenum to CeO2 (Mo-doped CeO2 or CMO) confers catalytic activity to the carbon gasification reaction and an increase in the electrical conductivity of CeO2 [6]. Furthermore, it has been reported that the addition of copper nanoparticles to Mo-doped CeO2 increases about 10 times its electrical conductivity [7].Two nanomaterials for SOFC anodes were synthesized: Mo-doped CeO2 (CMO) and Cu,Mo-doped CeO2 (CMCuO). Both nanomaterials were synthesized by the combustion method with 10 wt.% total metal loading following the experimental procedures detailed in [6] and [7] correspondingly. The SOFCs were fabricated using a CMO-YSZ or CMCuO-YSZ composite as anode, a dense YSZ electrolyte and a LSM-YSZ cathode. Both SOFCs were fed with air at the cathode and humidified syngas at the anode (syngas + 3 vol.% H2O). The morphology of the SOFCs was studied by SEM, while the crystal structure and chemical stability of the materials were studied by XRD and EDS. The cells were characterized by EIS at OCV condition at different temperatures and syngas concentrations, while the cell performance was studied by linear sweep voltammetry.An ADIS analysis obtained from the EIS data for both fuel cells in Figure 1 clearly shows the presence of two phenomena at high frequencies related to the electrooxidation reactions of H2 and CO, these phenomena are sensitive to temperature changes, the differences in their charge transfer resistance values suggest that both reactions are carried out with different kinetics. The processes shown at low frequencies are related to diffusional limitations of each gas towards triple phase boundaries (TPBs), processes sensitive to changes in syngas concentration. Cell tests, operating at 800°C, showed a maximum electrical power density of 114 mW cm-2 at a current density of 208 mA cm-2 and a cell voltage of 549 mV for SOFC with CMO-YSZ anode and a maximum electrical power density of 120 mW cm-2 at a current density of 200 mA cm-2 and a cell voltage of 600 mV for SOFC with CMCuO-YSZ. The addition of copper to the CMO produces a slight improvement in the electrocatalytic activity of the material and reduced ohmic resistance by almost 50% (see Figure 2), however the ASR of the cell with CMCuO anodes does not show a substantial improvement with respect to the cell with CMO anodes. In addition, the ohmic resistance is the most important component in the ASR of both cells. SEM images of the cells after tests show that the CMCuO anode surface exhibits fractures and delamination of the material, which is not observed on the CMO anode surface. The X-ray diffraction spectra do not show the presence of new phases in any of the two anode materials so the delamination of the CMCuO anode is related to the differences in thermal expansion coefficients (TECs) of the materials whose value must be measured to prove this hypothesis.[1] T.M. Gür (2013) Chem. Rev., 113(8), 6179-6206.[2] O. Costa Nunez et al. (2005) J. Power Sources, 141(2), 241-249.[3] T.M. Gür et al. (2010) J. Power Sources, 195(4), 1085-1090.[4] I. Sreedhar et al. (2019), J. Electroanal. Chem., 848, 113315.[5] N. Mahato et al. (2015) Prog. Mater. Sci., 72, 141-337.[6] I. Díaz‐Aburto et al. (2019) Fuel Cells, 19(2), 147-159.[7] I. Díaz-Aburto et al. (2021), J. Electrochem. Sci. Technol., [epub ahead of print: https://doi.org/10.33961/jecst.2020.01571]. Figure 1