High temperature fuel cells (SOFC) operate at temperatures from 500°C to 1000°C. The electrolytes of these electrochemcial energy converters are thin ceramic metal oxide membranes (zirconia or cerium oxides) membranes in which oxygen ions and vacancies are the electric charge carriers. Their mobility is thermally activated which mandates the high temperature. Under humid atmosphere, numerous metal oxides show proton conductivity at a temperature considerably lower than for oxygen ion conductivity. Yet, fuel cell devleopers hope for substantial improvement of the conductivity to make such electrolyte membranes before they are fit for deployment in SOFC.
We have recently shown that the protons in such ceramic membranes couple to the phonons generated by thermal excitation in a way that they form new quasi-particles which follow characteristically the Holstein polaron concept <!--[endif]---->1, 2. With a suite of analytical techniques we could identify which particular phonon modes propell the protons through the electrolyte membrane during SOFC operation. As these phonon modes scale characteristically with the lattice constant of the proton conductor, we could show that external lattice strain considerable affects the proton conductivity. We have demonstrated this expermentally for compressive strain. Extrapolation of the activation energy to tensile strain in ultrathin membranes suggests that epitxial films with the desirable orientation of lattice planes could further lower the activation energy for proton transport not too distant from the ambient temperature <!--[endif]---->3, 4.<!--![endif]----><!--![endif]---->
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