Proton conducting ceramics: Recent advances

A general introduction into the field of high temperature ceramic proton conductors (HTPCs) is given. Results of two typical studies involving these HTPCs are discussed. In the first part a study of the processes occurring during water vapor exposure of bulk ceramic proton conductors, BaCe_0.9Y_0.1O_2.95 (BCY10) and BaCe_0.8Y_0.2O_2.9 (BCY20), is presented. A disc of a HTPC was connected to a mass spectrometer in a vacuum system permitting the identification of the species crossing the ceramic-vacuum interface. Exposing the other side of the sample to D₂O led to a strong signal of D₂O⁺ after a certain lag time. From these lag times the tracer diffusivity of hydrogen could be determined as a function of temperature. The permeation of steam consisted of two components: a fast component, given by the diffusivities of deuterons (protons), and a slow component, assigned to chemical diffusion of deuterons (protons) coupled to oxygen vacancies. The data also suggested the possibility of participation of even more complex defects in this chemical diffusion. Dilatometry measurements of different specimens of BCY10 and BCY20 also revealed quite clearly this two-phase pattern during protonation. Diffusion measurements on protonic ceramic membranes using H ₂ ¹⁸ O permitted the determination of the tracer diffusivity of oxygen. All of the above measurements were interpreted in the light of the chemical diffusion model developed by Kreuer et al. The second part deals with composites of proton conductors and inorganic compounds such as carbonates, hydroxides, chlorides, and fluorides following the work of B. Zhu. Conductivities of such composites are presented. Nernst potentials of various electrochemical cells with these composites are discussed. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.