Mixed electronic and protonic conductivity

Our approach is to separate the conduction paths for H+ ions and electrons through the incorporation of a ceramic second phase. This approach essentially eliminates the combined dependence of hydrogen flux on electronic and proton conductivities. The approach is to short-circuit the electron flow-paths so that the overall flux is limited only by the proton conductivity. A similar mixed conducting requirement exists for electrodes in high-temperature proton conducting fuel cells, and some work has been carried out to develop mixed conductors as electrodes [24]. 

Both electrodes must present a mixed conductivity - electronic and protonic - with an internal RedOx couple giving a potential high enough (versus a hydrogen electrode) for the positive electrode and sufficiently low for the negative one. 

Sutija, D. P., Norby, T., Bjombom, P. (1995). Transport number determination by the concentration-ceU/open-circuit voltage method for oxides with mixed electronic, ionic and protonic conductivity. Solid State Ionics, 77, 167—174. 

A series of perovskite compositions were synthesized using oxides and carbonates of the cations by conventional ceramic process. The synthesized powders were characterized using powder x-ray diffraction technique to ensure phase purity. Conductivity measurements were made in H2-H2O atmosphere to determine proton conductity. As the perovskite compositions are inherently mixed conducting, the transference numbers for proton and electron conduction were also determined by varying the partial pressures of hydrogen and steam across the membrane. 

With the requirement of electronic conductivity, oxides containing cations with mixed valence and, in particular, reducible cations are preferable. Oxides containing transition metals are therefore appropriate alternatives. There are indications based on conductivity measurements that Ti02 could be a possible candidate [83], but no direct measurements of hydrogen permeability have been reported. Tita-nates, in general, however, are interesting because there are a number of materials classes that accommodate oxygen vacancies and may dissolve protons. 

---