Chemical compatibility study of melilite-type gallate solid electrolyte with different cathode materials Alessandro Mancini a , Valeria Felice b , Isabella Natali Sora b , Lorenzo Malavasi a , Cristina Tealdi a,n a INSTM R.U. and Department of Chemistry–Physical Chemistry Division, University of Pavia, Pavia I-27100, Italy b INSTM R.U. and Department of Engineering, University of Bergamo, Dalmine, Bergamo I-24044, Italy article info Article history: Received 20 January 2014 Received in revised form 27 February 2014 Accepted 3 March 2014 Available online 12 March 2014 Keywords: Melilite-type gallate Solid oxide fuel cells Electrode–electrolyte chemical compatibility X-ray diffraction abstract Chemical reactivity between cathodes and electrolytes is a crucial issue for long term SOFCs stability and performances. In this study, chemical reactivity between selected cathodic materials and the ionic conducting melilite La 1.50 Sr 0.50 Ga 3 O 7.25 has been extensively investigated by X-ray powder diffraction in a wide temperature range (up to 1573 K). Perovskite-type La 0.8 Sr 0.2 MnO 3 Àd and La 0.8 Sr 0.2 Fe 0.8 Cu 0.2 O 3 Àd and K 2 NiF 4 -type La 2 NiO 4 þd were selected as cathode materials. The results of this study allow identifying the most suitable electrode material to be used in combination with the melilite-type gallate electrolyte and set the basis for future work on this novel system. & 2014 Elsevier Inc. All rights reserved. 1. Introduction The melilite series La 1 þ x Sr 1 Àx Ga 3 O 7 þ x/2 was proposed as an electrolyte in solid oxide fuel cells (SOFCs) in 2008 [1], although the first detailed study on the electrical conduction behavior of La 1 þx Sr 1 Àx Ga 3 O 7 þ x/2 was reported by Rozumek et al. [2], who observed an increase of electrical and predominantly ionic con- ductivity when the degree of strontium substitution for lanthanum is increased. Kuang et al. [1] reported an oxide ion conductivity of 0.02–0.1 S cm À1 between 873 K and 1173 K for x ¼ 0.54 suitable for intermediate temperature (IT) SOFCs. The solid solution extends to x ¼ 0.64, and for x r0.6 exhibits tetragonal symmetry [3]. The parent compound LaSrGa 3 O 7 has been known for considerable time since it is observed as secondary phase in the preparation of another gallate series, the perovskite-type electrolyte La 1Àx Sr x Ga 1Ày Mg y O 3Àd (LSGM) [4]. The structure of LaSrGa 3 O 7 is isostructural with melilite, and consists of Ga 3 O 7 layers composed entirely of distorted rings of five GaO 4 tetrahedra. Along the c axis the Ga 3 O 7 layers are separated by the larger La and Sr cations in position of 8-coordination. By increasing the La/Sr ratio 41 interstitial oxide ions are introduced in the structure of La 1þ x Sr 1Àx Ga 3 O 7þ x/2 within the pentagonal rings. The ionic transport mechanism is related to the migration of oxide ions in the Ga 3 O 7 layers (ab plane) [1,3,5]. Combined atomistic and molecular dynamics simulations confirmed this anisotropy [6]. Moreover, it was found that the oxide-ion conduction occurs through an interstitialcy or cooperative mechanism involving the knock-on motion of interstitial and lattice oxide ions. It is worth to note that the structural and transport properties of the La 1þ x Sr 1Àx Ga 3 O 7 þx/2 series are very different with respect to those of conventional electrolyte materials for SOFCs such as yttria stabilized zirconia (YSZ) and strontium and magnesium doped lanthanum gallate (LSGM). The structure-types of YSZ and LSGM are fluorite and perovskite, respec- tively, and their oxide-ion conductivity is based on the presence of oxygen vacancies. A small change of the La/Sr ratio in the solid solution La 1 þx Sr 1 Àx Ga 3 O 7 þ x/2 influences its oxide-ion conductivity [1–3]. More- over, the oxide-ion conductivity of La 1.54 Sr 0.46 Ga 3 O 7.27 is decreased drastically by replacing La with other trivalent elements (Pr, Nd, Eu, Gd, Dy, Yb, and Y) with smaller ionic radius [7], while Ga deficiency and/or Ga partial substitution have/has a beneficial effect on high temperature conductivity [8]. The formation of low conductive phases at the electrolyte– electrode interface due to high temperature reaction of the compo- nents can be detrimental to the fuel cell performance. To the best of our knowledge, studies on the chemical reactivity of La 1 þ x Sr 1Àx Ga 3 O 7 þ x/2 with cathode materials are unavailable in the current literature. At present, the most used cathodes for SOFCs are perovskite oxides such as La 1Àx Sr x MnO 3Àd (LSM). LSM is mainly an electronic conductor successfully applied as cathode material in YSZ-based high temperature (HT) SOFCs, while it shows higher Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry http://dx.doi.org/10.1016/j.jssc.2014.03.006 0022-4596/& 2014 Elsevier Inc. All rights reserved. n Corresponding author. Tel.: þ39 0382 987569. E-mail address: cristina.tealdi@unipv.it (C. Tealdi). Journal of Solid State Chemistry 213 (2014) 287–292