The growing interest in proton conducting oxides is driven by the desire to reduce the solid oxide fuel cell (SOFC) operating temperature from >973 K to the intermediate temperature range (673 – 873 K). Fuel cells operating in this range would maintain the benefits of high temperature operation, such as increased cell and system efficiency, fuel flexibility, use of transition metal catalysts, CO and S tolerance and dry solid-state ion transport with no water management problems, while gaining the benefits of lower temperature operation - increased lifetime, reduced material costs and reduced startup/shutdown time.
Perovskite structured oxides in the series BaCe1-x-zZrxYzO3-d (BCZY) have shown to provide technologically relevant proton conductivity in the target temperature range. The primary barriers to their application are stability in CO2 containing atmospheres, low grain boundary conductivity, and the high sintering temperature required to produce dense electrolytes, typically >1973 K. In this study, we have utilized cobalt doping to lower this sintering temperature to <1698 K in BaCe0.5Zr0.4(Y,Yb)0.1-xCoxO3-d.
The materials were synthesized using a modified Pechini procedure and were confirmed as phase pure cubic perovskites (space group Pm-3m) by X-ray diffraction. Density of >95% of theoretical was achieved by sintering at 1698 K or below. AC and DC conductivity measurements, performed in dry and humidified air, H2 and Ar/N2, demonstrate proton conductivities comparable with the best undoped proton conductors sintered at high temperatures. DC conductivity values are higher in humidified atmospheres; consistent with a proton-conducting mechanism. Proton conduction was confirmed by Nernst potential measurements, conducted in a dual chamber system as a function of pH2 and pO2 driving force and the materials were shown to be phase stable in the presence of hydrogen and steam. The upper limit for electrolyte applications was found to be between 5 and 10% cobalt doping on the B-site due to decreased stability and increased electronic contribution to the total conductivity above 5%.
Finally proton conducting SOFC were fabricated using a dual-layer tape-casting technique that allows flexible composition in the anode. The cells consisted of a 0.04 mm thick electrolyte supported on a 0.3 mm thick porous Ni/BCZY anode with an La0.8Sr0.2CoO3-d/BCZY cathode. The cell performance was measured with humidified H2 fuel and methanol/water fuel at 873 K. The maximum open circuit potential (OCP) of 1.04 V was achieved at 873 K in 3% humidified H2 fuel with an accompanying peak power density of 89 mW/cm2. The total cell polarization resistance was 0.83 Ω·cm2. Impedance measurements showed decreasing polarization and ohmic resistance with increasing current density. Cell operation via internal steam reforming of methanol was demonstrated using a methanol/water feed with 3:1 and 2:1 steam-to-carbon (S:C) ratios; however, the OCP was reduced to 0.97 and 0.95 V and peak power density decreased to 65 and 58 mW/cm2 respectively. This was primarily attributed to a lower H2 concentration in the feed stream. Methanol reforming and carbon deposition rates were also assessed over Ni/BCZY catalysts for varying methanol/water S:C ratios.