Investigation of Electrochemical Carbon Conversion in Hybrid Direct Carbon Fuel Cells

In recent years, direct carbon fuel cells (DCFCs) have received significant attention owing to an increasing need for efficient power generation from coals and other carbon-based solid fuels. In a DCFC with a solid oxide electrolyte (e.g., yttria-stabilized zirconia, YSZ), carbon conversion would take place over three-phase boundaries. There is, however, very little interaction between the solid carbon and the Ni-YSZ anode. A hybrid DCFC design combines the advantages of the solid oxide and molten carbonate fuel cell technologies, where the molten carbonate and solid oxide electrolytes are in direct physical contact. Oxide ions are transported through the solid oxide electrolyte toward the molten carbonate holding the dispersed carbon particles inside the anode compartment. Although promising performances of hybrid DCFCs have been reported in the literature, the mechanistic understanding of the carbon conversion is incomplete. In fact, the reactions at the anode are quite complex and involve multistep transfer of electrons in a sequence of elementary reactions. In this paper, we study the electrochemistry of carbon conversion in a hybrid DCFC using Ni-YSZ anodes with various porous structures. Ni-YSZ anodes are fabricated using PMMA spheres with different diameters as a pore-former to tailor their porous structures (pore volume, size, distribution, and surface area). The polarization behaviors of the DCFC are measured as a function of the porous structure. The results show that among the parameters studied, the surface area plays the most critical role in determining the anodic polarization. The electrochemical carbon conversion reaction is discussed in detail, on the basis of the experimental data obtained from mercury porosimetry, polarization and impedance measurements, and gas chromatography.