(641d) Proton Conducting It-SOFC Utilizing Internally Steam Reformed Alcohol Fuels

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 BaCe_1-x-zZr_xY_zO_3-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 BaCe_0.5Zr_0.4(Y,Yb)_0.1-xCo_xO_3-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 La_0.8Sr_0.2CoO_3-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.