Ion-Transport Engineering of Alkaline-Earth Hydrides for Hydride Electrolyte Applications

The heavier alkaline-earth hydrides (AeH2; Ae = Ca, Sr, Ba) are promising materials for hydrogen energy applications, due to their excellent ionic conductivity and thermal stability. We use first-principles calculations to elucidate their defect chemistry and ion-transport mechanisms. On the basis of calculated formation energies, we find that hydrogen vacancies (VH) are present in large concentrations, and hence, transport is vacancy-mediated. The vacancy migration barrier depends strongly on the charge state of the vacancy, with VH+ yielding the lowest barriers. The activation energy for ionic transport, which is the sum of the migration barrier and formation energy, can be further reduced by lowering the formation energy of the vacancies. This goal can be achieved by doping with acceptors, such as alkali-metal impurities, which induces larger concentrations of VH+. We show that, with optimized conditions for doping, ionic conductivities can be improved by several orders of magnitude; for BaH2, the conductivity is on par with proton conductivity in the best proton-conducting oxides. For BaH2, we find K to be the optimal dopant for enhancing conductivity, and for CaH2 and SrH2, we find Na to be optimal. The resulting large ionic conductivity of hydrogen makes the heavier alkaline-earth hydrides strong candidates for electrolytes in hydrogen fuel cells.