Developments of Proton-Conducting SOFCs

In 1995, Bonanos et al. [68] reviewed the literature and their own results and tabulated performances reported for a number of H2-O2 or H2-air fuel cells with 0.4- to 0.5-mm-thick, 10% 20% Gd- or Nd-doped BaCeOa electrolytes, Pt anodes, and Pt or Ag cathodes, operated at 800°C. At 700 mV cell voltage, they delivered from 70 to 285 mA/cm of current density, corresponding to 50-200 mW/cm of power density and effective electrolyte conductivities of around 10-40 mS/cm. These conductivities are, to a first approximation, in agreement with bulk (grain interior) conductivity data for doped BaCeOaS, indicating that electrode and grain boundary impedances were small. 

Some years later, the company Protonetics Inc. pursued commercialization of fuel cells based on BaCeOs and reported their own data on running the cells on hydrogen or methane [69]. Power densities in the lower range of those mentioned above were obtained. 

Kreuer [70] reported a test of a Ba(Ce,Zr)03 fuel cell and obtained a modest power density. It was shown that this was much lower than expected from bulk (grain interior) conductivity and in agreement with expectations from electrical properties, including the grain boundary resistance. 

In the aforementioned studies, Coors and Kreuer bring up the issue of mixed proton-oxide ion conduction of BaCeOs-based materials while proton conduction is beneficial for efficiency in hydrogen-burning fuel cells, some oxide ion transport is beneficial for providing water vapor on the anode side for reforming and shifting carbon-based fuels such as methane. 

Ito et al. [9] made a laboratory fuel cell with a 0.7-pm-thick, BaCeOs-based electrolyte deposited on a Pd anode substrate and with a noble metal cathode. The cell showed high power densities, more than 1 W/cm at 600°C and about 0.7 W/cm at as low as 400°C, but substantial lifetimes were not reported. 

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