Proton conducting oxides, and in particular acceptor doped BaZrO₃, are materials of great interest because of their huge potential as electrolyte in several energy related technologies, such as hydrogen sensors, membrane reactors, steam electrolyzers, and intermediate-temperature (200 - 500 °C) solid oxide fuel cells. A key requirement for the electrolyte is a high proton conductivity (>10^-2 Scm^-1), but even the best materials available today show conductivity values still too low for practical applications. The development of new materials, meeting the requirements for applications, depends on a detailed understanding of the fundamental processes underlying proton conductivity, but the local structure and chemistry complicate the description about proton conductivity on the atomic scale. Here, we have focused on investigating the relationship between local structure, vibrational dynamics and proton conductivity, which is key to future materials developments.

Due to the very large neutron cross section for hydrogen, inelastic neutron scattering (INS) is a powerful tool for the investigation of the local structure and vibrational dynamics in proton conducting oxides. Recently, we presented a combined INS and computer simulation study of the proton conducting oxide BaZr_0.5In_0.5O₃H_0.5 [1]. We showed that the material is characterized by a range of different local proton configurations. Analyses of the experimental and theoretical data provide a complete assignment of the vibrational spectra and establish key structural properties, such as the nature and occupancies of the different proton sites. In particular, we have developed a novel method that allows for the distinction of fundamental modes and higher-order transitions in the vibrational spectra, which is based on the analysis of the momentum (Q) dependence of the INS intensity (Figure 1a). Further, we found that, converse to more weakly doped proton conductors, such as BaZr_0.8In_0.2O₃H_0.2, the dopant-proton association effect does not hinder the diffusion of protons due to the presence of percolation paths of dopant atoms throughout the perovskite lattice (Figure 1b).

This project was carried out by researchers at the Department of Chemistry and Chemical Engineering at Chalmers (M. Karlsson group). The INS data were collected at the MERLIN and TOSCA spectrometers at the ISIS Pulsed Neutron and Muon Source (STFC Rutherford Appleton Laboratory, U.K.), whereas for the computer simulations the Swedish National Infrastructure for Computing (SNIC) at the PDC Center for High Performing Computing (PDC-HPC) was used. The project was supported by funds from the Swedish Research Council and the Swedish Foundation for Strategic Research.

Figure 1.  (a) INS map I(Q,w) at T = 10 K of BaZr_0.5In_0.5O₃H_0.5 as recorded on MERLIN, together with schematics showing the geometry of the O-H stretching, n(O-H), and O-H bending, d(O-H), modes. (b) Schematic 2-D illustration of a conduction pathway for protons in the presence of a network of dopant atoms (blue coloring).