(208c) Intermediate Temperature Solid Oxide Fuel Cell Based on Proton-Conducting Ceramic Electrolyte

Fuel cells cleanly and efficiently convert chemical energy directly to electrical energy. With highly efficient energy conversion and very low greenhouse gas emissions, fuel cells can play a pivotal role in reducing our nation's dependence on foreign oil while lessening the emission of greenhouse gases. To now, however, material issues have impeded the commercialization of fuel cells.  An oxygen-ion conducting electrolyte, such as yttria-stabilized zirconia, has been traditionally used for the fabrication of solid oxide fuel cells (SOFCs).  Due to the high activation energy for conduction, SOFCs based on stabilized zirconia are normally operated at temperatures approaching 1000°C.  This high operating temperature leads to the corrosion and incompatibility of materials, severely limits the selection of materials that might be suitable as the electrical interconnect, and increases the difficulty of forming seals and manifolding.  To overcome these problems, efforts are underway to develop electrolytes that can operate at intermediate temperatures.  Proton-conducting oxides have attracted considerable attention as an electrolyte for SOFCs that operate at intermediate-temperature (≈600°C).  Use of a proton-conducting electrolyte in SOFCs eliminates the dilution of fuel by water, which occurs at the anode in conventional SOFCs based on an oxygen-ion-conducting electrolyte.  We have fabricated a SOFC using a BaCe_0.8Y_0.2O_x (BCY) proton conductor as the electrolyte.  A dense BCY film (≈10-μm thick) was deposited on a porous Ni/BCY cermet (i.e., ceramic/metal composite) substrate by a dip-coating process.  The gas-permeable Ni/BCY cermet substrate backed with nickel mesh was used as the anode, and platinum paste backed with platinum mesh served as the cathode.  The current-voltage characteristics of the BCY-based SOFC were measured in the temperature range 450-800°C using wet air on the cathode side and hydrogen on the anode side.  The open-circuit voltage was close to the theoretical value at lower temperatures (<600°C) and was about 85% of the theoretical value at 800°C.  The lower measured voltages at high temperature may be due to the development of electronic conductivity in the BCY electrolyte.  Gas leakage due to an incomplete seal may also contribute to the deviation from the theoretical voltage at high temperature.  The peak power density of the fuel cell was ≈90 and ≈1500 mW/cm² at 450 and 800°C, respectively.  The power density increased as temperature increased because the cell resistance decreased.   Our efforts to develop SOFCs based on proton conductors will be summarized in this talk.

Work supported in part by the U.S. Department of Energy, Advanced Research Projects Agency – Energy (ARPA-E) under Contract DE-AC02-06CH11357.