Novel K 2 NiF 4 -Type Materials for Solid Oxide Fuel Cells: Compatibility with Electrolytes in the Intermediate Temperature Range R. Sayers 1 , J. Liu 1 , B. Rustumji 1 , and S. J. Skinner 1 * 1 Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK Received March 04, 2008; accepted May 29, 2008 1 Introduction Development of new materials for solid oxide fuel cell ap- plications is of critical importance to the deployment of next- generation devices with enhanced performance and lifetime. Current trends in device development include the require- ment to increase durability and potentially lower operating temperature. There are many areas that are currently receiv- ing attention in terms of materials development with signifi- cant work in the electrolyte, cathode and anode areas. Cur- rent collection is also viewed to be of considerable importance and both anode and cathode current collectors are the subject of intensive research efforts. In recent years, a number of potential new materials have been developed for both electrolyte and cathode applications with the focus being on the reduction of device operating temperature. Lowering the operating temperature not only brings certain benefits in terms of durability and sealing of cells, but also presents challenges such as with the kinetics of cathode reactions being reduced. One of the most significant developments in low temperature SOFCs has been the exploi- tation of ceria-based electrolytes [1–5] and the potential to use these as composites in cathode applications. Indeed, success has been achieved through the use of ceria gadolinium oxide (CGO) with a La 1– x Sr x Co 1– y Fe y O 3– d (LSCF) mixed conduc- tor as a composite cathode for use with CGO electrolytes [6]. Potential new SOFC materials that have received consider- able attention recently are the La 2 Mo 2 O 9 -based electrolytes first identified by Lacorre and coworkers [7–12] and the Ln 2 NiO 4+ d -based interstitial oxide ion mixed conductors [13–20]. Building on this there has also been growing interest in related layered structures such as the double perovskites [21–23]. Tarancon et al. [22, 23] demonstrated excellent cath- ode performance with a Gd 2 BaCoO 5+ d composition and Kim et al. [24–26] have also found attractive performance with the Pr analogue. Much of the work on these new materials has – [ * ] Corresponding author, s.skinner@imperial.ac.uk Abstract Many new materials have been identified as being of poten- tial interest to solid oxide fuel cell developers. Recent work has suggested that materials of the La 2 NiO 4 composition possess sufficient activity to act as an effective cathode. Whilst there have been many studies of the transport prop- erties of materials of this family the reactivity with electro- lyte compositions has received relatively little attention. In this work we have investigated the compatibility of La 2 NiO 4+ d (LNO) with LSGM and ceria gadolinium oxide (CGO) electrolytes. It is clear that under an air atmosphere LNO reacts readily with CGO electrodes with the formation of a higher order Ruddlesden–Popper (La n +1 Ni n O 3n +1 ) phase as one of the reaction products. In contrast, there is no evidence of secondary phase formation with LSGM electro- lytes from the obtained diffraction data over a period of 72 h at temperatures up to 1,273 K. Keywords: CGO, Compatibility, Composites, Impedance Spectroscopy, La 2 NiO 4+ d , LSGM, XRD 338 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim FUEL CELLS 08, 2008, No. 5, 338–343 ORIGINAL RESEARCH PAPER DOI: 10.1002/fuce.200800023