Anodes based on molybdenum-doper ceria for carbon-air fuel cell applications
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Molybdenum(Mo)-doped ceria (CMO) nanoparticles were synthesized by the combustion method with three different Mo concentrations: 5 wt.%, 7 wt.% and 10 wt.%. The catalytic activity of CMO for wet gasification of carbon was studied in a fluidized bed reactor, while the mechanical and electrical properties of this material were characterized using dense sintered CMO pellets. The Young s modulus increases with the Mo content; the highest measured value is 340.3 GPa for CMO with 10 wt.% Mo. Measurements of Vickers microhardness demonstrate that an increase in the Mo content produces a decrease in the microhardness of the material, suggesting that Mo confers semimetallic characteristics to CMO. The highest fracture toughness value, determined by the Niihara equation, is 4.65 MPa m^0.5 for CMO with 10 wt.% Mo. In addition, an increase in the molybdenum content produces an increase in the electrical conductivity under air and H2 atmospheres. The maximum electrical conductivities under air and H2 are found for CMO with 10 wt.% Mo at 800 °C: 1.87x10^-3 S cm^-1 and 9.37x10^-1 S cm^-1, correspondingly. Therefore, CMO with 10 wt.% Mo shows good catalytic activity during carbon gasification. Based on its mechanical, electrical and catalytic properties, CMO 10 wt.% Mo was selected to perform electrochemical impedance spectroscopy (EIS) measurements in symmetric-cells with CMO-based electrodes, and polarization curve measurements in SOFCs with CMO anode, YSZ electrolyte and LSM cathode. The area specific polarization resistance (ASPR) values measured in air and hydrogen-water atmospheres are 6.76 Ohm cm^2 and 5.68 Ohm cm^2 at 800 °C, respectively. These ASPR values are lower than those reported in the literature for the same system under study, which was related to the improve synthesis method proposed in this work (combustion method). The diffusion of gaseous oxygen was related to a Knudsen-type mechanism, while the diffusion of hydrogen-water was related to a mixed mechanism between molecular and Knudsen-type diffusion. The maximum electrical power density obtained in the SOFC (using CMO-YSZ as cell anode) was 48.9 mW cm^-2. This low value was related to cathode problems and inadequate SOFC fabrication procedures. The addition of molybdenum to cerium oxide improves its mechanical, electrical and catalytic properties, while the CMO anode in this work shows a lower polarization resistance compared to the results reported in the literature. These results show that the CMO is a good candidate for use as a carbon-air solid oxide fuel cell (CA-SOFC) anode. However, it is necessary to improve the SOFC fabrication method to determine if it is the reason for the low electrical power density obtained.
Tesis para optar al grado de Doctor en Ciencias de la Ingeniería, Mención Ingeniería Química y Biotecnología
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