Increasing the Porosity of Anode Material Could Lead to Better Performing Low-Temperature Solid Oxide Fuel Cells
|Schematic of the tubular SOFC and cross-sectional SEM image of the cell. Credit: Suzuki et al., Science. Click to enlarge.|
Researchers at Japan’s National Institute of Advanced Industrial Science and Technology have shown that increasing the porosity of the anode material of a solid oxide fuel cell can significantly improve its electrochemical performance at lower temperatures.
Toshio Suzuki and his colleagues found that with an anode with a highly porous microstructure, the SOFC had a power density of greater than 1 watt per square centimeter at an operating temperature as low as 600 °C with a conventional zirconia-based electrolyte, a nickel cermet anode, and a lanthanum ferrite perovskite cathode material. A report on their work was published in the 14 August issue of the journal Science.
While solid oxide fuel cells are attractive for use in certain applications because of their high efficiency and ability to use hydrocarbon fuels directly, their high operating temperatures, often in excess of 700 °C, can be a drawback.
...transition metals used in the electrode materials can diffuse into the electrolyte and lower performance and, ultimately, lifetime. Thus, lowering the operation temperature can be beneficial for the commercialization of SOFC systems, since it can offer quick start-up ability, which in turn can allow for their use in applications such as transportable power sources and auxiliary power units for automobiles.—Suzuki et al.
There has been a great deal of effort in recent years in researching new electrode and electrolyte materials to enable low- or intermediate-temperature operation of SOFCs, and one study showed a power density of more than 2W/cm2 with a novel electrolyte.
Although these approaches are promising, there are also advantages to using the materials in commercial SOFC systems—that is, zirconia-based electrolytes and NiO and (La, Sr)MnO3 or (La,Sr)(Co,Fe)O3 as the anode and cathode materials, respectively. These materials remain attractive because of their long-term reliability as well as cost factors; thus, specifically, the use of zirconia and Ni cermet remains interesting. In fact, SOFCs using such materials have been shown to be operable down to 650° to 750°C. Further lowering the cell operating temperature below 650°C and using zirconia-based electrolytes could be expected to accelerate the commercialization of the SOFC system for a variety of applications.—Suzuki et al.
The researchers improved SOFC performance by controlling the microstructure of an anode electrode with the conventional materials and fabrication techniques. The SOFCs have a microtubular design and consist of NiO-Sc-stabilized zirconia (ScSZ) and Ce-doped zirconia (10Sc1CeSZ) for the anode, 10Sc1CeSZ for the electrolyte, and (La, Sr)(Fe, Co)O3 (LSCF)-Gd–doped ceria (GDC) for the cathode, with an interlayer of GDC between the cathode and the electrolyte. All materials used for the SOFC fabrication are commercially available.
They prepared three kinds of cells (A, B and C) with different anode microstructures. The porosities of the anodes were 54, 47, and 37% for cells A, B, and C, respectively, before reduction. Cell A showed the greatest power density.
The outstanding performance of cell A seems to arise from the improvement of the anode microstructure (Ni particle size below 100 nm). The open-circuit voltages slightly decreased from 1.1 V for cell C to 1.0 V for cell A as sintering temperature decreased from 1400° to 1250°C. This change possibly relates to the density of the electrolyte. Bundling of the cells, stack design, and modularization are other challenges that need to be overcome for the realization of high-performance SOFC systems.—Suzuki et al.
Toshio Suzuki, Zahir Hasan, Yoshihiro Funahashi, Toshiaki Yamaguchi, Yoshinobu Fujishiro, Masanobu Awano (2009) Impact of Anode Microstructure on Solid Oxide Fuel Cells. Science Vol. 325. no. 5942, pp. 852 - 855 doi: 10.1126/science.1176404