Protonic defects stability

The key to the development of C02-resistant protonconducting oxides was the maximization of the en-tropic stabilization of protonic defects. If this approach also led to stable hydroxides with sufficiently high conductivity, AFCs using such electrolytes may operate even with air as the cathode gas. This would be tremendously advantageous, because fuel cells with nonacidic electrolytes may operate with non-noble-metal catalysts such as nickel for the anode and silver for the cathode. 

Another key feature is the availability of a nearly perfect acceptor dopant (i.e. a dopant which does not change the electronic structure of the oxygen). While in all other reported cases, the increase of the acceptor dopant concentration leads to a reduction of the proton mobility and an entropic destabilization of protonic defects [222], both the proton mobility and the thermodynamics of hydration are practically unchanged for dopant levels up to 20% Y in BaZr03 (Fig. 3.2.13). High proton mobility and entropically stabilized protonic defects even at high dopant concentrations and the high-solubility limit lead to the enormous proton... 

In pure water, excess protons (H3O+, H502 ) and defect protons (OH ) are present at identical concentrations however, because of their low concentration (10 M under ambient conditions), the diffusion of these defects may be considered to be quasi-independent. Only in small water clusters do zwitterions show a remarkable stability. ... 

Although it was not suggested that defects are required for selective oxidation over other catalysts, the results indicated that defects and bismuth must be present for high activity and selectivity over scheelite-type catalysts. The authors concluded that the defects which were introduced into the bulk of these phases must manifest themselves in some manner at the surface. The question of how the introduction of defects into these phases affected their catalytic properties was not resolved. However, the active site for catalysis was suggested as a cation vacancy which could abstract a proton from an olefin to form the well-established allyl intermediate and should offer considerable stabilization to a surface hydroxyl group. 

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