Proton H Ion Conductors

This technique, known as coulometric titration, allows the composition of a non-stoichiometric oxide to be varied in a precise way. The oxide in question is set as the anode of a cell with oxygen as the cathode  

Proton, that is, H+ ion, conductors are of importance as potential electrolytes in fuel cells. There are a number of hydroxides, zeolites, and other hydrated materials that conduct hydrogen ions, but these are not usually stable at moderate temperatures, when water or hydroxyl tends to be lost, and so have only limited applicability. 

This section will be limited to oxide materials that are inherently not H+ ion conductors but can be made so by the introduction of suitable defects. These materials generally maintain H+ conductivity to high temperatures. 

The most important of these are perovskite structure solids with a formula A2+b4+o3 that can be typified by BaCeC 3 and BaZrCV The way in which defects play a part in H+ conductivity can be illustrated by reference to BaCeCV BaCeC 3 is an insulating oxide when prepared in air. This is converted to an oxygen-deficient phase by doping the Ce4+ sites with trivalent M3+ ions (Sections 8.2 and 8.6). The addition of the lower valence ions is balanced by a population of vacancies. A simple substitution reaction might be formulated  

Note that this simple formalism disguises the fact that a considerable amount of chemical skill is involved in ensuring that the dopant M only occupies the Ce4+ sites. For example, when the apparently suitable dopant ion Nd3+ is used, it occupies both the Ce and Ba sites, thus suppressing vacancy formation  

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