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One of the most important areas under investigation is the role that porosity plays in the performance of the individual solid oxide fuel cell (SOFC) components and finished stack.
The Role of PorosityThe SOFC is a layered ceramic consisting of at least three discrete materials and functionalities. Like all other fuel cell designs, the three essential components are the cathode, the anode and an electrolyte membrane between the two electrodes. Unlike other fuel cell designs, the operating temperature is extremely high-around 800ºC-so all the materials of construction, including the interconnects (which "wire" together individual cells into a stack) must be dimensionally stable and resistant to sintering.
The ElectrolyteThis component is not an electron conductor but an ion conductor and passes O2- ions, created at its interface with the cathode, to its interface with the fuel-rich anode. The material most commonly employed is fully dense (i.e., non-porous) yttria-stabilized zirconia (YSZ). The lack of porosity is essential in preventing the reactive gases from passing directly through from electrode to electrode-in other words, creating a short circuit.
The CathodeLanthanum mixed-metal oxides with a perovskite structure are the materials of choice for the cathode, where oxygen molecules are reduced to anions by electrons from the external circuit. In stark contrast to the electrolyte, therefore, the cathode must be both electrically conductive and highly permeable to gases (air). The porosity ideally should be high, interconnected yet non-tortuous in nature, and must extend all the way to the interface with the electrolyte.
The AnodeConductive materials that are stable under reducing conditions and able to adsorb fuel gases such as hydrogen are required for the anode, which would tend to indicate that porous metals should be suitable. However, the significant differences in thermal expansion between metals and ceramic components, plus the high operating temperature (which leads to rapid sintering and loss of porosity) make metals unsuitable. A modified electrolyte material in the form of metal-doped (typically with nickel) YSZ is popular. During synthesis, organic or carbon pore-formers are added and are subsequently burned out to ensure that the porosity of the anode is retained.
Materials CharacterizationMost commonly used characterization and testing procedures are related to the electrical and energy performance of SOFC assemblies. However, in developing and characterizing the materials and components, two main areas are especially important-porosity characterization and reactive anode characterization.
The electrode material must allow for the transport of gaseous species. In this role, the critical factors that affect diffusion and flow of fluids are overall porosity and pore size distribution. These essential characteristics can be readily obtained using mercury intrusion porosimetry and gas sorption analyzers.
Reactive Anode Characterization
In many respects, the SOFC anode is no different than most reducing catalysts. It must be capable of adsorbing hydrogen (or hydrocarbon fuel) and promoting its reaction with a reducible species-in this case, the oxygen anion. The gas-accessible active area can be determined quite easily by today's modern, fully automated gas chemisorption instruments. These instruments will also reveal the effective (metal) nanoparticle size.
Looking ForwardFuel cell development is no different than other applied technologies based on surface phenomena. Other advanced ceramic industry sectors have long recognized the impact that pore structure has on material performance, and routinely use the analysis techniques outlined in this article. Those in the fuel cell arena must adopt similar capabilities to quickly advance the understanding of material properties to meet the commercial need for rapid development into the marketplace.
For more information about pore characterization and analysis, contact Quantachrome Instruments at 1900 Corporate Dr., Boynton Beach, FL 33426; (561) 731-4999; fax (561) 732-9888; e-mail QC.firstname.lastname@example.org.