(33d) Ceramic Proton Conductors for Intermediate Temperature Solid Oxide Fuel Cells

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.