Solid state proton conductors: hydrated perovskites and hydrated alkali thio-hydroxogermanates

Doctoral thesis, 2007

This thesis concerns experimental studies of hydrated perovskites and hydrated alkali thio-hydroxogermanates, two classes of solid state proton conductors which are promising to be used as electrolytes in future intermediate temperature (200-500°C) fuel cells. This is an attractive temperature range for many applications but for which today's available electrolytes do not show satisfactory performance. For the development of new materials with better performances a deeper understanding of the structure and proton dynamics of the material is essential. Incorporation of protons in perovskites, A2+B4+O2-3, requires the formation of oxygen deficient structures through acceptor doping, AMxB1-xO3-d (M = Y3+, Yb3+, In3+, Ga3+ etc.), and subsequent hydration. Key issues to understand are how the type and concentration of dopant atoms and the presence of oxygen vacancies influence the structure and proton dynamics of the material. These issues have been addressed in this thesis, in particular for the proton conducting BaInxZr1-xO3-x/2 (0.10 < x 0.75) system. Using a combination of vibrational spectroscopy and neutron scattering the local structure and the proton dynamics, from fast vibrational motions to the long-range diffusion, have been investigated and related to the macroscopic proton conductivity. The investigations of the novel class of hydrated alkali thio-hydroxogermanates, MxGeSx(OH)4-x*yH2O (1 < x < 4, 0 < y < 8, M = Na, K, Rb and Cs), have focused on the structure and the thermal stability using a combination of infrared spectroscopy and neutron diffraction. The results suggest that the structure is built up of dimers of thio-hydroxogermanate anions, which are connected to each other through hydrogen bonds via the molecular water, and alkali ions, which act as ''space fillers'' in voids formed by the thio-hydroxogermanate dimers. The results also show that the molecular water can be removed upon heating the material to 180°C with no significant effects such as phase transitions or structural degradation occurring.