Compatibility of proton conducting La 6 WO 12 electrolyte with standard cathode materials Eric Quarez ⁎, Kostiantyn V. Kravchyk, Olivier Joubert Institut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, 2, rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France abstract article info Article history: Received 30 June 2011 Received in revised form 27 October 2011 Accepted 2 November 2011 Available online 9 December 2011 Keywords: La6WO12 Solid oxide fuel cells Proton conductivity Compatibility Cathode Mixed ionic and electronic conductors (MIEC) such as LSM (La 0.7 Sr 0.3 MnO 3 −δ ), LSCM (La 0.75 Sr 0.25 Cr 0.5 Mn 0.5- O 3 −δ ) and BSCF (Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 −δ ) have been investigated as potential cathode materials with La 6 WO 12 (LWO) proton conducting electrolyte for use in protonic ceramic fuel cells (PCFC). Different cathode — LWO powder mixtures have been subjected to high temperature treatment (1150 °C during 144 h in air) and studied by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). The analysis of the results reveals that LWO is chemically and mechanically stable with LSM and LSCM but reacts with BSCF. Symmetrical cells cathode/LWO/cathode have been studied by electrochemical impedance spectrosco- py (EIS). The minimum of area specific resistance (ASR) values have been found for LSM cathode (in humid- ified air at 750 °C: ASR LSM = 4.3 Ω·cm 2 ; ASR LSCM = 15.5 Ω·cm 2 ; ASR BSCF = 9.7 Ω·cm 2 ). Composite cathodes using a two-phase ceramic/ceramic (cer–cer) approach were also studied. In the case of LSM/LWO cathode cer–cer, it allows a significant decrease of the ASR value. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Protonic ceramic fuel cells (PCFC) are some of the most important electrochemical devices for electrochemical conversion of chemical into electrical energy. The main advantages of proton conducting fuel cells versus oxygen conduction-based solid oxide fuel cells are the absence of fuel dilution as water formation takes place at the cathode side and lower working temperatures. The latter should be high enough to assure high level of proton diffusion but relatively low to avoid the degradation of the fuel cell components and inter- connectors. The increase of the ionic conductivity and stability of the electrolyte material is one efficient way to improve the perfor- mance of the fuel cells. However, it is well known that polarization re- sistance of electrodes plays an important role as limiting factor and is another critical parameter in order to get a high efficient fuel cell. The cathode material used is a mixed ionic–electronic conductor for which chemical and mechanical compatibility with electrolyte is es- sential to assure good performance and a significant lifetime of the fuel cell. The presence of chemical reaction between electrolyte and cathode or cation diffusion with formation of high resistive phases as well as interface delamination can decrease the fuel cell efficiency. If interfacial phases are generated, it is important to know their elec- trical nature since their presence traditionally increases the polariza- tion resistance of the system. For instance, a widely studied case of dramatic increase of polarization resistance due to the interdiffusion of elements is the formation of the insulating phase La 2 Zr 2 O 7 at the (ZrO 2 ) 0.92 (Y 2 O 3 ) 0.08 /LaMnO 3 interface [1]. The PCFC technology is a relatively recent technology and only a few cathode materials exhibiting mixed electron/proton conductivity have been studied as potential cathode materials for PCFC: La 0.6 Ba 0.4 CoO 3 [2], BaPr 0.8 Gd 0.2 O 2.9 [3], Ba 0.5 Pr 0.5 CoO 3 [4], BaCe 0.4 Pr 0.4 Y 0.2 O 3 − δ [5], BaCe 0.5 Bi 0.5 O 3 − δ [6], BaCe 0.5 Fe 0.5 O 3 − δ [7] and BaZr 0.1 Ce 0.7 Co 0.2 O 3 − δ [8]. Many research groups have also performed the study of PCFC with the use, as a cathode, of a mixed electron/oxygen conductor. Among them, it is interesting to note that the electrochemical tests of complete PCFC lead to relatively high values of peak power density: cathode (Pmax (mW cm −2 ), T°C): La 0.8 Sr 0.2 MnO 3 − δ (590, 700 °C) [9], Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 − δ (377, 600 °C) [10] and Sm 0.5 Sr 0.5 CoO 3 − δ (750, 600 °C) [11]. La 6 WO 12 (LWO) could be described as a face-centered cubic struc- ture with an inherently deficient oxygen sublattice which can be filled by OH• defects in presence of water vapor. Proton conductivity of LWO dominates below roughly 800 °C with a maximum of approxi- mately 1∙10 −3 S·cm −1 at T = 600 °C [12]. Among mixed ionic and electronic conductors (MIEC) proposed as cathodes for PCFC based on LWO electrolyte, LSM and LSCM seem to be good candidates due to their close match in thermal expansion coeffi- cients (TEC) with that of LWO (TEC LWO =11×10 −6 (300–1073 K); TEC LSM = 11.7·10 −6 K −1 (300–1273 K) [13]; TEC LSCM = 11.8·10 −6 K −1 (300–1173 K) [14]). These perovskite cathode materials are known (i) to have adequate catalytic activity for oxygen reduction as SOFC cathodes at temperatures above 700 °C; (ii) to retain their oxygen deficiency and high oxygen-self-diffusion coefficients even in an oxidizing atmosphere; Solid State Ionics 216 (2012) 19–24 ⁎ Corresponding author. Tel.: + 33 2 40 37 39 13. E-mail address: Eric.Quarez@cnrs-imn.fr (E. Quarez). 0167-2738/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2011.11.003 Contents lists available at SciVerse ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi