Alternative anode materials for methane oxidation in solid oxide fuel cells
Fuel Cells are electrochemical devices that are able to directly convert chemical energy to electrical energy, without any Carnot limitation. Hence, their energy efficiencies are relatively high. Among the various types of fuel cells, solid oxide fuel cells (SOFC) are operated at high temperatures and in principle can run on various fuels such as natural gas and hydrogen. As natural gas is sought to become one of the main fuels of the next decades, its direct feed to a SOFC is desirable as the reaction free energy is high CH4 + 2O2 = CO2 + 2H2O, ΔrG°800°C = -800.8kJ/mol At present, conventional SOFC are operated with pure hydrogen or partially to fully reformed natural gas implying an efficiency penalty. SOFC anodes are generally made of electrocatalytically active Ni-YSZ cermet. Pure CH4 is not fed directly to the anode, because of problems associated with the anode deactivation and coking as CH4 leads to the detachment of Ni particles from the YSZ support and their encapsulation by carbon. To overcome this problem, partially oxidized or reformed CH4 is used. For SOFCs running on direct CH4 feed, alternative anode materials to Ni-YSZ, or new anode formulations are necessary. However, in this case, several parameters influence the anode performance and stability. The anodes should withstand reduction at a low PO2 of around 10-24 atm, be compatible with the SOFC electrolyte, possess an acceptable conductivity and thermal expansion coefficient, and appropriate catalytic and electrocatalytic properties along with low coking activity. LaCrO3 and CeO2-based materials as well as some metal catalysts were studied here for their potential application as anodes in direct oxidation of CH4. This choice of materials was made as lanthanum chromite and ceria-based materials are mixed conductors known to resist quite well to the very reducing conditions in SOFC as they are commonly used for interconnect and electrolyte materials respectively. Moreover, these compounds possess some catalytic activity for CH4 activation and combustion. LaCrO3 and CeO2-based materials show rather low electrical conductivity (on the order of 1 S/cm in reducing conditions), necessitating the application of an extra material for proper current collection. Nevertheless, these oxides possess many of the stringent characteristics cited above. For SOFC anode purposes, LaCrO3-based compounds, substituted with Ca, Sr, Mg, Mn, Fe, Co and Ni, were synthesized by a modified citrate route from nitrate precursors. An optimal calcination temperature of 1100°C was found to be necessary to obtain XRD-pure yet sinteractive compounds. Conductivity measurements in air and in reducing atmospheres of 10-21 atm showed that the La A-site substituents (Ca and Sr) influenced the conductivity more than the Cr B-site substituents (Mg, Mn, Fe, Co and Ni). In humidifed H2, the total conductivity was maximally of 6.5 S/cm. Surface reaction with YSZ electrolyte was evidenced by SEM, XPS and SIMS, in reducing conditions, especially under current load, when these powders were applied as anodes. A current induced demixing effect was noted for heavily substituted LaCrO3. Ceria-based compounds, doped with Y, Nb, Pr and Gd, were synthesized through 3 different techniques: the solid-state method, coprecipitation and the NbCl5 route. The coprecipitation route was found to be most appropriate for Y, Pr and Gd, whereas for Nb a modified precipitation and the NbCl5 technique were the most efficient. Co-doping of Nb and Gd or Pr was studied in order to increase the electronic as well as the ionic conductivity in CeO2. Among all dopants, Nb was found to promote most the adhesion to YSZ electrolyte sheets. Adding Nb to Gd-doped ceria improved the electrode adhesion on YSZ sheets, but deteriorated its conductivity in oxidizing conditions. No interfacial reaction with YSZ was observed by HRTEM when Nb-doped ceria anodes were sintered on YSZ at 1200°C/4h. Catalytic measurements were undertaken with these materials in CH4 rich atmospheres. Different reaction mixtures were chosen to simulate the various SOFC operating conditions: partial oxidation, CO2 reforming by recycling and H2O reforming. Among the different elements, Sr and Ni were found to be the most active substituents for LaCrO3, as they promote all three types of reactions. The combination of both elements is favorable not only for catalytic use, but also for electrode application as they tend to increase the electronic conductivity of the material. Temperature programmed oxidation (TPO) and TEM made on the catalysts after runs in CH4, showed a very low carbon coverage over these oxides, except in the case of Fe substitution, over which carbon deposition was promoted. XPS analysis showed also that all B-site substituted compounds did not segregate after the catalytic runs whereas Ca and Sr (A-site substituents) tended to segregate in H2 and H2O rich atmospheres. Thermodynamic calculations, made using correlations developed in literature, show that Ca and Sr are expected to segregate in H2 and H2O rich atmospheres as they tend to form volatile hydroxyl species. Also, the thermodynamic stability of the LaCrO3 was observed to depend much on the substitution. For CeO2-based oxides, the activity for steam reforming increased from Nb<Gd<Pr, but in all cases the activity towards CH4 was poor. All three solid solutions did not deposit carbon even under the most reducing conditions as observed by TPO and TEM. From temperature programmed reduction (TPR), Pr-doped ceria showed a higher degree of reduction leading to a higher vacancy concentration. Metallic loading of these catalysts was very sensitive to the sintering and reducing temperature, but in all cases lead ultimately to carbon deposition. Sintering at higher temperatures gave rise to a higher interaction with the CeO2-based support, as the induction time for carbon deposition was increased. Cu-based alloys loading showed a high resistance to carbon deposition when compared to Ni. By adding Cu to Ni, the overall activity towards CH4 was reduced and concomitantly the ability to form carbon was reduced. Electrochemical measurements (I/V curves, impedance spectroscopy) were made on Ca, Sr, Mg, Fe, Co, and Ni substituted LaCrO3 anodes deposited on 8YSZ sheets. The effects of the sintering temperature, the polarization, the gas and the anode composition, as well as the presence of an adhesion layer made of LaCrO3-based powder and 8YSZ were taken into account. Ni and Sr substituents were observed to improve the electrocatalytic activity in H2 as well as in CH4. The electrocatalytic trend followed almost the catalytic trend in CH4, LaCrO3 ≈ LaCr0.9Co0.1O3 < LaCr0.9Ni0.1O3 < La0.85Sr0.15CrO3 < La0.85Sr0.15Cr0.9Ni0.1O3, and was influenced by the anodes conductivity. The application of an adhesion layer made of the anode material and YSZ fine powders improved the stability of the system. The best cell gave at 877°C, a power output of 450 mW/cm2 and 300 mW/cm2, with a short circuit current of 1950 mA/cm2 and 900 mA/cm2, in humidified (3%H2O) H2 and CH4 respectively. CO oxidation rate was observed to be slower than CH4, and CH4 slower than H2. As for the catalytic study, CO2 had an inbiting effect on the performance of LaCrO3, possibly due to a blocking effect of surface carbonates. H2O had no measurable effect on the H2 reaction, whereas its effect was observable at high current densities for CH4. For ceria, the electrocatalytic properties of Nb and Gd-doped ceria anodes deposited on 8YSZ and 20CGdO electrolytes were analyzed. The effects of the electrode thickness and morphology, the addition of a pore former and Ni or Cu metal catalysts, the polarization as well as the gas composition (H2, H2O, CH4 and CO2) were taken into account. The low activity of Nb and Gd-doped ceria for the oxidation of CH4 confirmed the catalytic measurements. No carbon deposition was observed. Similarly to LaCrO3-based anodes, ceria anodes performance in CH4 depended much on H2 oxidation at least at open circuit potential (OCV). Oxygen ion diffusion in the ceria anode is thought to participate in the overall polarization losses, indicating that the anode reaction spreads far away from the triple phase boundary. The best performance reached was of 430 mW/cm2 between 850-900°C in H2. The performance in CH4 depended much on the presence of a catalyst. At 900°C, with a Pt mesh current collector, the performance was of 350 mW/cm2 with a short circuit current of 1050 mA/cm2, whereas with an Au mesh it was only of 100 mW/cm2 and 420 mA/cm2. Cu addition increased the activity of the anode when using an Au mesh.