On the use of Pd filters on proton conducting SOFC ceramics

Ceramic proton conductors are materials which conduct protons under an applied electric field and permit the construction of a SOFC in analogy to the oxygen ion conductor 8YSZ. Such protonic ceramic fuel cells have been labelled PCFCs to distinguish them from the standard SOFCs. PCFCs have several advantages when compared with standard SOFCs. First, there is no water contamination of the fuel gas. Second, the protonic conductivity may be higher in the intermediate temperature range (550 to 700 °C). Also, the activation energy for proton migration is much lower than that of oxygen ion conduction. In addition, internal reforming is also possible permitting the use of natural gas. An intriguing possibility is the use of palladium filters on the fuel side which only permit the diffusion of protons to the filter-electrolyte interface. This may reduce undesirable mixing of the fuel and the oxygen on either side of the electrolyte. In this work, it is shown that the entry of protons from the Pd into the ceramic conductor is straightforward. Prototype test PCFCs are shown to operate as required. To test the Pd - ceramic proton conductor system a study was undertaken using Pd tubes which were coated with thin layers of the proton conducting ceramics. When flushing the tube insides with hydrogen and exposing the outsides to air, Nernst voltages of such electrochemical cells could be recorded as a function of temperature. The present rather preliminary work, however, was hampered by leakage of the thin film HTPC's. Thus, the theoretical Nernst voltages were not attained. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.