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Artur Braun1 Qianli Chen1 2

1, Empa. Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, , Switzerland
2, University of Michigan - Shanghai Jiao Tong University Joint Institute, Shanghai, , China

We present a detailed and instrumentally and conceptually complex study on the structure and transport properties of metal oxides with x-ray and neutron and electroanalytical methods in situ and operando 1. Pressure and temperature were employed for a parameterization of the processes taking place in ceramic proton conductors<!--[endif]---->2. The proton is a structural component in many materials and an electric charge carrier in proton conducting ceramic membranes. We have investigated yttrium doped zirconia and ceria for such electrolyte membrane applications in situ and operando under realistic transport conditions. At low temperatures in humid ambient, water molecules enter the membrane and the oxygen ions of the H2O occupy oxygen vacancies while the proton forms OH bridges in the material with a crystallographic superstructure imposed by the doping, as determined with neutron diffraction and resonant/anomalous XRD 3. As such it is a structural component. Upon rising temperature, lattice vibrations form phonons which couple to the protons and propel it as polaron 4 through the lattice, which we have determined with quasi-elastic neutron scattering along with impedance spectroscopy. The transition of the proton from being a structural component to becoming a charge carrier is monitored with ambient pressure resonant XPS on polarized electrolytes. For the particular compounds we could determine the temperature range for this transition to around 700 K. The use of resonant valence band photoemission spectroscopy in situ allowed us to sketch a complete transition of the density of states during the protonation and deprotonation <!--[endif]---->5. <!--![endif]----><!--![endif]---->
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1. A. Braun: X-ray Studies on Electrochemical Systems - Synchrotron Methods for Energy Materials, (Walter De Gruyter GmbH, Berlin/Boston, 2017).
2. Q.L. Chen, S. Holdsworth, J. Embs, V. Pomjakushin, B. Frick and A. Braun: High-temperature high pressure cell for neutron-scattering studies. High Pressure Research 32, 471 (2012).
3. A. Braun, A. Ovalle, V. Pomjakushin, A. Cervellino, S. Erat, W.C. Stolte and T. Graule: Yttrium and hydrogen superstructure and correlation of lattice expansion and proton conductivity in the BaZr[sub 0.9]Y[sub 0.1]O[sub 2.95] proton conductor. Applied Physics Letters 95, 224103 (2009).
4. A. Braun and Q. Chen: Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors. Nature Communications 8, 15830 (2017).
5. Q.L. Chen, F. El Gabaly, F.A. Akgul, Z. Liu, B.S. Mun, S. Yamaguchi and A. Braun: Observation of Oxygen Vacancy Filling under Water Vapor in Ceramic Proton Conductors in Situ with Ambient Pressure XPS. Chemistry of Materials 25, 4690 (2013).

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