Ambipolar protonic-electronic

Figure 1.1a shows schematically the operation of a membrane that is permeable to hydrogen molecules (corresponding to a porous membrane or a dense material in which molecules dissolve and diffuse) or to neutral hydrogen atoms (corresponding to a material in which hydrogen dissolves dissociatively, as in a metal). Figure 1.1b shows schematically how a mixed proton-electron conductor performs the same process by so-caUed ambipolar diffusion of both protons and electrons in the same direction to maintain electroneutrality and zero net current. 

This is a good point at which to look a little closer at the ambipolar transport term that we have mentioned eartier in passing. The materials property of interest here is the protonic-electronic ambipolar conductivity term, which can be written in... 

By assuming that the transport number for oxygen ions is zero (cf. Eq. (1.32)), the only flux of hydrogen remaining is that of ambipolar transport of protons and electrons ... 

In ambipolar diffusion of protons and oxygen ions, the two types of ions move in the same direction for charge compensation, and the net result is permeation of water. If we also have electronic transport, then the transport of hydrogen and oxygen ions may be de-coupled. They may move in the same direction or in opposite directions and at different ratios. 

We note that the molecular flux is proportional to the difference in hydrogen pressure (as in gas phase transport through a piorous membrane), whereas atomic transport has a square root dependence, and as such would behave similarly to ambipolar transport of protons and electrons limited by the conductivity of a minority concentration of protons. 

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] ... 

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