Proton Hole Mixed Conduction

It is likely that there have been no accepted materials so far as the protonic-electronic mixed conductors that allow high hydrogen transfer. Therefore, in contrast to the conventional high-temperature proton conductors (HTPCs), the protonic-electronic mixed conductors are not yet established. We recently found that the protonic-electronic mixed conduction occurs in ruthenium-doped perovskites, BaCco.g Yo iRu Os (x = (M).l) [35] below is the summary of our findings. 

Hydrogen permeation can be evaluated for a specimen using the following cell. 

If one assumes proton (H ), oxide ion (0 ), conduction electron (e), and electron hole (h) as ionic and electronic charge carriers, hydrogen evolution at the permeating side will obey the ambipolar diffusion mechanism, expressed by Eq. (12.17) [36]  

As explained in previous sections, protonic and oxide-ionic change carriers are assumable for the proton-conducting perovskite oxides the SrCeo.95 Ybo.osOs a case has been shown in Fig. 12.2. By observing the change in water vapor pressure in the permeating compartment, 0.5 of the ionic 

It is not clear how the Ru doping results in the formation of an electron hole in reducing atmosphere. However, it is possible to assume a kind of energy level produced by the Ru-4d orbitals, which locate just above the valence band and accept electrons from the valence band to generate eleetron holes. Additional experiments, e.g., determination of the valence of Ru and the conductivity dependences on XO2) and / (H20), will provide a definite model for the protonic-electronic mixed conduction in Ru-doped perovskites. 

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Thus, measurement of the total conductivity together with the cell voltage allows the transport numbers of the ions to be determined (Fig. 8.17). The results show that at lower temperatures proton conductivity is of greatest importance, at middle temperatures oxygen ion conductivity becomes dominant, and at high temperatures the material is predominantly a hole conductor. Between these temperatures, at approximately 350°C the solid is a mixed H+ and O2- conductor while at approximately 650°C it is a mixed hole and O2- conductor. 

Proton conduction at high temperatures occurs in certain perovskites such as doped strontium cerate, Sr Ceo.95Ybo.o503 t. In air, this material is primarily an electronic conductor due to the mixed valence of Ce. In the presence of moisture, water is absorbed by the reaction with positive holes to generate protons ... 

In general the charge carriers in mixed proton-electron conducting materials could be protons, oxygen ions (or oxygen vacancies), hydroxyl ions, electronic-holes and electrons. To determine the nature of the charge carriers and their transport numbers, various defect structure models have been presented [36-38]. These defect models were fitted to the total electrical conductivity data measured... 

Figure 5.7 The variation in transport number for proton, oxygen ion and hole conductivity for the mixed proton conductor SrZr YO (Data from Huang and Petrie (1995))...

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