The high operating temperature (>700^oC) of oxygen ion conducting SOFC enables the use of CO and S-tolerant transition metal catalysts, increased efficiency and utilization of waste heat; however, such high temperatures require expensive materials of construction, complicate sealing and shorten cell lifetime. Therefore, there is great impetus to reduce the cell operating temperature to 500-600^oC. Perovskite structured oxides in the series BaCe_1-x-zZr_xY_zO_3-d (BCZY), are among the most promising materials for application as the electrolyte in intermediate temperature proton conducting solid oxide fuel cells (H⁺-SOFC). These materials have shown technologically relevant proton conductivity in the target temperature range. The primary barriers to application of these materials are stability in CO₂ containing atmospheres, low grain boundary conductivity and, perhaps most significantly, the high sintering temperature required to produce dense electrolytes, typically >1700ºC. In this study, we have utilized transition metal doping to lower this sintering temperature to <1425ºC in BaCe_0.5Zr_0.4Y_0.1-xM_xO_3-d (BCZY), where M is the transition metal.

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 is achieved by sintering at 1425ºC or below. Energy dispersive X-ray spectroscopy analysis shows homogeneous distribution of the elements with no evidence for dopant leaching or segregation to the grain boundaries. 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. These experiments were complemented by TGA analysis, indicating that changes in the conductivity mechanism occur upon reduction of the dopant and constituent cations. Finally, the materials were shown to be phase stable with X-ray diffraction in the presence of hydrogen and steam.

H⁺-SOFC were fabricated using a dual-layer tape-casting technique. The cells consisted of a 0.05mm thick electrolyte supported on a 0.3mm thick porous Cu/BCZY anode with an La_0.8Sr_0.2CoO_3-d/BCZY cathode. The maximum power density of these un-optimized fuel cells with 3% humidified hydrogen fuel was 55 mW/cm² with an open circuit voltage (OCV) of 0.95V. Impedance spectroscopy indicated that the resistance of the electrolyte (2.7 Ω.cm²) was the primary limiting factor.