Performance of (Ln 0.5 M 0.5 )FeO 3-δ Perovskites as Cathode for SOFCs: Effect of Mean Radius of the A Site Cations K. Vidal a , L. M. Rodríguez-Martínez b , L. Ortega-San-Martín c , A. Martínez-Amesti a M. Luisa Nó a , T. Rojo a , A. Laresgoiti b and M. I. Arriortua a a Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea, Apdo. 644, E-48080 Bilbao, Spain b Ikerlan, Centro Tecnológico, Parque Tecnológico de Alava Juan de la Cierva 1, Miñano 01510, Álava, Spain c Instituto de Ciencia de Materiales de Aragón (ICMA-CSIC) c/ María de Luna 3, Zaragoza 50018, Spain Crystal structure, microstructure, electrical conductivity and electrochemical performance of a new family of iron perovskites with the general formula Ln 0.5 M 0.5 FeO 3-δ (Ln = La and/or Pr; M = Sr, Ca or/and Ba) has been studied as function of mean A cation radius <r A >. All the samples have a rhombohedral structure (space group: R-3c). SEM images show that the average grain size decreases with increasing <r A >. The DC four-probe measurements indicate that all samples have high electrical conductivity, increasing with <r A >. The electrochemical behaviour of these cathode materials was evaluated by using I-V curve measurements and AC impedance spectroscopy over three electrodes electrolyte supported cells. The results show a great improvement in cathode performance and a significant decrease in polarization resistance with increasing mean A cation radius <r A >. Introduction The search for new materials to improve the existing ones is a constant in all fields of applied science. In the case of Solid Oxide Fuel Cells (SOFC), this search is directed towards three main areas: electrodes, interconnects and electrolytes. The present investigation is focused on the electrodes, particularly on the cathode side. Present day cathodes for SOFCs are mainly based on A-site substituted A 1-x A´ x BO 3 perovskite oxides where A and A’ are mostly La and Sr cations and B are transition metals such as Mn, Co and Fe (or combinations between them) resulting in the widespread notation of LSM, LSC and LSF, respectively (1,2). The most common cathodes are those based on LSM compounds although both ferrites and cobaltites show oxide ion diffusion that is some orders of magnitude higher than in manganites (3). This makes them promising candidates to substitute LSM systems. In fact we have selected LSF compounds because they have been reported to have a large number of oxygen vacancies, coupled with good electrocatalytic activity, high oxide ion transport and high electronic conductivity at relatively low temperatures (4,5). Given that there is a tendency towards lowering the operating temperature (and costs) of SOFCs from the traditional 1000ºC of yttria stabilised zirconia (YSZ) containing devices iron based compounds are in a good condition to be used in SOFCs (6,7). Moreover, it has been indicated that at present operation temperatures Sr-doped LaFeO 3 compounds react more slowly with YSZ ECS Transactions, 25 (2) 2427-2434 (2009) 10.1149/1.3205797 © The Electrochemical Society 2427 Downloaded 24 Nov 2009 to 158.227.47.102. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp