We verified with pressure dependent experiments that application of compressive strain increases the activation energy for proton conductivity in perovskite-type metal oxides such as Y-substituted barium zirconate and barium cerate. The results suggest that tensile strain engineering in epitxial ultrathin films could significantly enhance the proton conductivity, thus opening novel opportunities for protonics devices.

Lay summary

Proton conduction occurs by the hopping of a proton from one oxygen position to the next. We propose to study protonated and deuterated samples with neutron scattering. By locating protons/deuterons, a conduction mechanism may be hypothesized.Recent impedance studies show a clear influence of the synthesis method (chemical pressure) on the conductivity. The total conductivity was dominated by the grain boundary contribution. The apparent grain boundary conductivity may be related to the microstructure, in particular to the density, the particle size and to the grain-to-grain connectivity. The latter effect would be pressure dependent, too, but have no direct link to the lattice constant.Open questions and aimsThe project aims at understanding the influence of the near neighborhood structure in proton conductors on the bulk proton conductivity by combining electrical measurements, and neutron scattering as a function of temperature and pressure.Is the enhanced proton conductivity effectively a consequence of the increased lattice constant (atomic scale), or is it a result of more intimate grain-to-grain contact, i.e. a so-called grain boundary effect?There exists unambiguity on the term "grain boundary" in ceramic proton conductor literature. Grains are usually identified by scanning electron microscopy, whereas the typically much smaller crystallites can only be identified with transmission electron microscopy. Both present imperfections and may act as scatter centers for proton transport.How do crystallites and grains impede the proton transport?Is it possible to simulate the "chemical" pressure - which is done by A-site substitution - by application of a external pressure?Significance and broader benefitOur results have the potential to fine-tune the synthesis and processing of BZY and tailor them to enable new applications in solid state electrochemical devices.A broader benefit to the society will be particularly for energy conversion devices such as solid oxide fuel cells and hydrogen generation electrolysers, since global demand for energy is increasing while resources are limited and the environment is impaired.Our activities on proton conductors are compatible with a hydrogen economy. The extra know-how that we gain by applying neutron scattering on proton conductors is an investment in collateral activities on hydrogen related research.


[1] Q. Chen, A. Braun, S. Yoon, N. Bagdassarov, T. Graule, Effect of lattice volume and compressive strain on the conductivity of BaCeY-oxide ceramic proton conductors, J. Eur. Ceram. Soc. (2011) 31 (14) 2657-2661. http://dx.doi.org/10.1016/j.jeurceramsoc.2011.02.014

[2] Q. Chen, A. Braun, A. Ovalle, C.-D. Savaniu, T. Graule, N. Bagdassarov, Hydrostatic pressure decreases the proton mobility in the hydrated BaZr0.9Y0.1O3 proton conductor, Appl. Phys. Lett. 97, 041902 (2010) http://link.aip.org/link/?APL/97/041902   http://arxiv.org/abs/1106.1091

[3] A. Braun, A. Ovalle, S. Erat, V. Pomjakushin, A. Cervellino, W. Stolte, and T. Graule, Yttrium and hydrogen superstructure and correlation of lattice expansion and proton conductivity in the BaZr0.9Y0.1O2.95 proton conductor, Appl. Phys. Lett., 95, 224103, 2009.  http://apl.aip.org/resource/1/applab/v95/i22/p224103_s1

[4] A. Braun, S. Duval, J.P. Embs, F. Juranyi, P. Ried,  P. Holtappels, R. Hempelmann, U. Stimming, Th. Graule. Proton diffusivity in the BaZr0.9Y0.1O3-delta proton conductor.  J. Appl.  Electrochem. 2009, 39(4), 471-475. http://arxiv.org/abs/1106.1924  http://www.springerlink.com/content/c884r27365325361/fulltext.pdf