Investigation of the stability of Co-doped apatite ionic conductors in NH 3 D.A. Headspith a , A. Orera b , P.R. Slater c , N.A. Young a , M.G. Francesconi a,n a Department of Chemistry, University of Hull, Cottingham Road, Hull HU6 7RX, UK b Instituto de Ciencia de Materiales de Arago ´n, Dpto de Ciencia y Tecnologı ´a de Materiales y Fluidos Ed. Torres, Quevedo c/Marı ´a de Luna 3, 50018 Zaragoza, Spain c School of Chemistry, University of Birmingham, Birmingham B15 2TT, UK article info Article history: Received 27 May 2010 Received in revised form 26 August 2010 Accepted 27 August 2010 Available online 8 October 2010 Keywords: Apatite Ammonia Solid oxide fuel cell abstract Hydrogen powered solid oxide fuel cells (SOFCs) are of enormous interest as devices for the efficient and clean production of electrical energy. However, a number of problems linked to hydrogen production, storage and transportation are slowing down the larger scale use of SOFCs. Identifying alternative fuel sources to act as intermediate during the transition to the full use of hydrogen is, therefore, of importance. One excellent alternative is ammonia, which is produced on a large scale, is relatively cheap and has the infrastructure for storage and transportation already in place. However, considering that SOFCs operate at temperatures higher than 500 1C, a potential problem is the interaction of gaseous ammonia with the materials in the cathode, anode and solid electrolyte. In this paper, we extend earlier work on high temperature reactions of apatite electrolytes with NH 3 to the transition metal (Co) doped systems, La 9.67 Si 5 CoO 26 and La 10 (Si/Ge) 5 CoO 26.5 . A combination of PXRD, TGA and XAFS spectroscopy data showed a better structural stability for the silicate systems. Apatite silicates and germanates not containing transition metals tend to substitute nitride anions for their interstitial oxide anions, when reacted with NH 3 at high temperature and, consequentially, lower the interstitial oxide content. In La 9.67 Si 5 CoO 26 and La 10 (Si/Ge) 5 CoO 26.5 reduction of Co occurs as a competing process, favouring lower levels of nitride–oxide substitution. & 2010 Elsevier Inc. All rights reserved. 1. Introduction Apatite-type materials have recently been investigated for their oxide ion conductivity and, hence, applicability as an electrolyte in areas such as solid oxide fuel cells (SOFCs) [1]. Hydrogen powered SOFCs are of enormous interest as devices for the production of clean energy. However, a number of problems linked to hydrogen production, storage and transportation are slowing down the large scale use of SOFCs. Hydrogen is a potentially explosive gas, making handling and transport difficult for large scale technological applications. Therefore, there is growing interest in alternative fuels, one example being ammonia, as a carbon free fuel source with a similar performance to hydrogen [2]. The use of ammonia in fuel cells can potentially yield only N 2 and H 2 O emissions and hence reduce greenhouse gas emissions, relative to burning fossil fuels [3]. Ammonia is currently produced in large quantities, with well established storage and distribution infrastructure because of its importance in the fertiliser industry. It can be liquefied under relatively mild conditions (10 atmospheres of pressure, or at  33 1C at atmospheric pressure) for transport or alternatively dissolved in water for safe delivery [2–4]. While ammonia cannot be used in low temperature polymer fuel cells, due to poisoning of the system, this is not a problem with high temperature cells, and in this respect ammonia has been demonstrated by a number of groups as a suitable fuel for use in solid oxide fuel cells [5]. There is still, however, a need for fundamental studies of the interaction of NH 3 fuels with materials in the cell. In particular, high temperature heat treatment in NH 3 is a known route for the synthesis of oxide nitrides, [6,7] and consequently it is possible that use of NH 3 may lead to nitridation occuring on the anode side of the cell under operating conditions. In this respect, it has been recently shown that NH 3 heat treatment will lead to significant nitridation of CeO 2 and the apatite-type oxide ion conductors (La/Sr) 10x Si 6 O 26+ z [8–10]. The latter apatite systems are new electrolytes for potential use in solid oxide fuel cells [11–15]. They are non-conventional electrolytes in the sense that the oxide ion conduction proceeds via interstitial oxide ions, in contrast to the traditional fluorite and perovskite systems, which are vacancy mediated oxide ion conductors. Apatite materials take the general formula A 10 (MO 4 ) 6 X 2 , where A is an alkali, alkaline earth, rare-earth metal, lead or bismuth, M is silicon, germanium, phosphorus or vanadium and X is oxygen, fluorine, chlorine, bromine, iodine or hydroxide. The apatite structure (Fig. 1) consists of isolated MO 4 units with A cations in Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry 0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jssc.2010.08.033 n Corresponding author. E-mail address: m.g.francesconi@hull.ac.uk (M.G. Francesconi). Journal of Solid State Chemistry 183 (2010) 2746–2758