Micro-Structural Optimization of MoO2-Based Anode for SOFC Applications

Micro-structural optimization of MoO₂-based anode for SOFC applications

Byeong Wan Kwon^a, Oscar Marin-Flores^a,b, M. Grant Norton^b, Su Ha^a

^aThe Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164

^bSchool of Mechanical and Materials Engineering, Washington State University Pullman, WA, 99164-2920

Solid oxide fuel cells (SOFCs), which typically run at temperatures in the range of 600-1000°C, can be operated with a variety of fuels, such as liquid hydrocarbons and synthetic liquid bio-fuels. The SOFCs are composed of a solid dense electrolyte sandwiched by two electrodes, an anode and a cathode. These electrodes should have a proper porosity to allow an efficient transport of both the reactants and byproducts, while possessing a proper sintering to allow high ionic and electronic conductivities. Therefore, their optimized micro-structures can lead to improve overall performances of SOFCs.

Recently, our group has successfully fabricated a novel MoO₂-based anode for the SOFC application and operated it with both the model aviation fuel and bio-diesel. In this study, we have improved the overall performance of our MoO₂-based SOFC by controlling the micro-structure of the MoO₂-based anode using a pore former such as a polyvinyl alcohol (PVA). Typically, MoO₂-based anodes were manufactured using MoO₂ nanoparticles to forma seed layer structure with an average thickness of 8 µm via electrostatic spray deposition (ESD) method. Over this seed layer, additionally catalyst inks containing both MoO₂ and different concentrations of PVA were painted to determine the effect of PVA concentration on anode morphology and power density output. We have utilized n-dodecane as a model fuel to assess the performance of SOFCs. The power density value at 0.6V improves from 540mW cm^-2 to 2,168mW cm^-2 as the PVA concentration increases from 6wt% to 12 wt%. However, further increasing the PVA concentration up to 21wt% decreases the power density output. Hence, the PVA concentration greatly influences the cell performance and a 12wt% PVA seems to be the optimum concentration that leads to the best micro-structure of the MoO₂-based anode. As observed in SEM images of 24h tested cells, the anode structure with a low PVA content of 6wt% is much denser compared to the ones with a higher PVA content, which can be attributed to a deficit amount of PVA required to produce a proper micro-porous structure in the anode. Therefore, a low porosity in the anode significantly limits mass transfer process, which leads to a larger anode overpotential (i.e., a larger anode charge transfer resistance from the impedance measurements) and lower power density output. Hence, the PVA concentration plays an important role in the performance and morphology of MoO₂-based anode. The MoO₂-based SOFC with the optimized micro-pore structure also showed a very stable performance over 24h test without showing any coking. Under the similar operating conditions used for the MoO₂-based SOFC, the commercial Ni-based SOFC showed a much lower initial performance and deactivated immediately due to the anode coking.