Ionic conductivity and thermal expansion of anion-deficient Sr 11 Mo 4 O 23 perovskite
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ORIGINAL PAPER
Ionic conductivity and thermal expansion of anion-deficient Sr11Mo4O23 perovskite Vladislav V. Kharton 1
&
Ekaterina V. Tsipis 1 & Vladislav A. Kolotygin 1 & Maxim Avdeev 2,3 & Brendan J. Kennedy 3
Received: 16 April 2020 / Revised: 16 April 2020 / Accepted: 31 May 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Transport properties of perovskite-type Sr11Mo4O23 and composite Sr11Mo4O23 - 1 wt% Al2O3 were studied at 400–1300 K in the oxygen partial pressure range from 0.21 down to 10−19 atm. The electromotive force and faradaic efficiency measurements, in combination with the energy-dispersive spectroscopy of the fractured electrochemical cells, unambiguously showed prevailing role of the oxygen ionic conductivity under oxidizing conditions. At temperatures above 600 K, protonic and cationic transport can be neglected. The oxygen ion transference numbers vary in the range of 0.95–1.00 at 973–1223 K. At temperatures lower than 550 K, the total conductivity of Sr11Mo4O23 - 1 wt% Al2O3 composite measured by impedance spectroscopy tends to increase in wet atmospheres, thus indicating that hydration and protonic transport become significant. Reducing oxygen partial pressure below 10−10–10−9 atm leads to a significant increase in the n-type electronic conduction. The average thermal expansion coefficients in oxidizing atmospheres are (14.3–15.0) × 10−6 K−1 at 340–740 K and (18.3–19.2) × 10−6 K−1 at 870–1370 K. Keywords Solid oxide electrolyte . Strontium molybdate . Transference numbers . Oxygen ionic conductivity . Thermal expansion . Mixed conductor . Faradaic efficiency
Introduction Developments of high-efficiency electrode systems for solid oxide fuel cells (SOFCs), electrolysis cells (SOECs) and other electrochemical applications make it necessary to search for oxide materials with an improved tolerance to carbon deposition, high electrochemical activity, mixed ionic-electronic conductivity and thermodynamic stability in a wide range of oxygen chemical potentials [1–6]. The materials derived from alkaline earth molybdates with perovskite-related structures attract significant interest for these applications [3–8]. In particular, the double perovskite Sr2MgMoO6 with rock-salt like ordering of Mg and Mo cations was reported as a candidate for SOFC anodes [3–8]. The high electrochemical performance of
* Vladislav V. Kharton [email protected] 1
Institute of Solid State Physics RAS, 142432 Chernogolovka, Moscow District, Russia
2
Australian Nuclear Science and Technology Organisation, Lucas Heights, Australia
3
School of Chemistry, The University of Sydney, Sydney, Australia
Sr2MgMoO6 was attributed to the presence of anion vacancies providing oxygen diffusion and to the redox activity of 5d0 Mo6+-5d1 Mo5+ couple responsible for electronic transport. Reduction at temperatures above 1173 K results, however, in decomposition of Sr2MgMoO6−δ into a mixture of MgO, metallic Mo and n = 2 Ruddlesden-Popper phase with a substantially higher Mo/Mg cation ratio; the reduced m
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