Thick films cathodes for Solid Oxide Fuel Cells (SOFC) are prepared by dip-coating slurries made of several lanthanum nickelate oxide powders onto yttria stabilized zirconia (YSZ) substrates. The processing parameters for the slurries preparation and the multilayers coating have been optimized to obtain homogeneous, crack-free, thick and adherent films after heat treatment

Ansart, Florence

Bacchin, Patrice

Castillo, Simone

Cienfuegos, René Fabian

Fontaine, Marie-Laure

Lenormand, Pascal

International audienceThick films cathodes for Solid Oxide Fuel Cells (SOFC) are prepared by dip-coating slurries made of several lanthanum nickelate oxide powders onto yttria stabilized zirconia (YSZ) substrates. The processing parameters for the slurries preparation and the multilayers coating have been optimized to obtain homogeneous, crack-free, thick and adherent films after heat treatment

Castillo, S.

Cienfuegos, R.F.

Fontaine, M.L.

Lenormand, P.

HAL Université de Toulouse, et Toulouse INP

Influence of the processing parameters of slurries for the deposit of nickelate thick films

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   Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.  This is an author-deposited version published in: http://oatao.univ-toulouse.fr/  Eprints ID :  2454  To link to this article :  URL : http://dx.doi.org/10.1016/j.materresbull.2007.01.011 To cite this version : Castillo, Simone and Cienfuegos, R. F. and Fontaine, Marie-Laure and Lenormand, Pascal and Bacchin, Patrice and Ansart, Florence ( 2007) Influence of the processing parameters of slurries for the deposit of nickelate thick films. Materials Research Bulletin, vol. 42 (n° 12). pp. 2125-2131. ISSN 0025-5408 Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@inp-toulouse.fr   Influence of the processing parameters of slurries for thedeposit of nickelate thick filmsS. Castillo a,*, R.F. Cienfuegos a, M.L. Fontaine b, P. Lenormand a,P. Bacchin c, F. Ansart aaCIRIMAT Laboratory, UMR CNRS 5085, Paul Sabatier University, 118 route de Narbonne, 31062 Toulouse cedex 4, Franceb SINTEF, Pb. 124 Blindern, NO-0314 Oslo, NorwaycLGC UMR CNRS 5503, Paul Sabatier University, 118 route de Narbonne, 31062 Toulouse cedex 4, FranceAbstractThick films cathodes for Solid Oxide Fuel Cells (SOFC) are prepared by dip-coating slurries made of several lanthanumnickelate oxide powders onto yttria stabilized zirconia (YSZ) substrates. The processing parameters for the slurries preparation andthe multilayers coating have been optimized to obtain homogeneous, crack-free, thick and adherent films after heat treatment.Keywords: A. Multilayers; A. Oxides; B. Sol–gel chemistry; C. Electron microscopy; D. Microstructure1. IntroductionThe development of new conversion energy systems like Solid Oxide Fuel Cells (SOFC) requires to drasticallydecrease their working temperature from 1000 8C to 700 8C in order to reduce their manufacturing costs and tooptimize materials lifetime. In such systems, the cathode preferentially consists of a MIEC active material (mixedionic and electronic conductor) coated onto an oxygen ionic conducting electrolyte, which is commonly yttriastabilized zirconia (YSZ) [1–5]. Use of MIEC allows to not limit the oxygen reduction to occur at the triple phaseboundaries region. An other way to improve the electrochemical performances of the cathode is to increase the amountof active material by the deposition of porous films with high effective surface area. In this work, these two approacheshave been combined. Cathode materials were two Ruddlesden-Popper phases, such as La2NiO4+d and La4Ni3O10.Previous work showed that the ratio La/Ni influences the cathode conductivity [4,6]. Therefore, cathodes withdifferent La/Ni ratio have been prepared as monophasic multilayers and graded multilayers. To control the effectivesurface area of the cathode, a processing method based on the suspension deposition technique [7,8] was developed toprepare thick cathodes with graded compositions.Within this process, the first objective was the obtention of stable suspensions. These suspensions were preparedfrom the dispersion of lanthanum nickelates crystallites (La2NiO4+d and La4Ni3O10) [9,10] synthesized by a sol–gelroute, in a solution made of a solvent, a dispersant and some organic additives [11,12]. The stability of the suspensions* Corresponding author. Tel.: +33 5 61 55 61 09; fax: +33 5 61 55 61 63.E-mail address: castillo@chimie.ups-tlse.fr (S. Castillo).doi:10.1016/j.materresbull.2007.01.011was achieved by adjusting the dispersant concentration [12]. The stable slurries were then used to coat thick cathodiclayers onto YSZ electrolyte, using the dip-coating technique and a controlled withdrawal speed [13].The second objective was the preparation of different cathodic architectures by depositing several layers ofLa2NiO4+d and La4Ni3O10 materials onto YSZ electrolyte. In some cases, the suspensions were also deposited onto athin cathodic interlayer obtained by dip-coating in a polymeric sol of La2NiO4+d or La4Ni3O10 precursors [5]. Thisduplex microstructure design is expected to enhance the cathode performances due to a more intimate cathode/electrolyte interface [6,14]. The microstructure of different architectures was investigated by field emission gunscanning electron microscopy (FEG-SEM) and X-ray diffraction (XRD).2. Experimental procedureThe raw material, in the form of fine powder, was prepared by a derived sol–gel method [2]. The flow chart of thepowders preparation is reported in Fig. 1. The powders were characterized by X-ray diffraction (BRUKER axs D4ENDEAVOR diffractometer), thermal analysis (SETARAM TGA 92), Zeta potential and granulometry (Zetasizer300HS, Malvern Instruments Ltd.), Scanning Electron Microscopy (SEM) (JEOL-6700F) to determine themorphology of the particles after calcination.The crystallized powders were dispersed into an azeotropic mixture of methylethylketone (MEK) and ethanol(ETOH) also containing a commercial dispersant C213 and two additives such as PVB binder and phthalate plasticizerto achieve adequate viscosity and cohesion in the slurries [10,12]. A 3 min ultrasonic step (SinapTec Power UnitNEXUS 198) was required to obtain an homogeneous slurry. The composition of the slurries prepared with differentamounts of dispersant is reported in Table 1.The slurries were characterized by thermogravimetric and thermodifferential analyses in air, during heating with arate of 5 K/min from 150 8C to 1000 8C (preliminary evaporation of solvent at 150 8Cwas achieved in a drying oven inorder to obtain dried mixture). The size and the charge of aggregates were obtained by Zeta potential andgranulommetry measurements, respectively, in dilutedMEK/ETOH solutions (0.1 mg/mL). The best compositions forpreparing stable slurries were chosen from these experiments.The films were dip-coated according to the following procedure: YSZ substrates were immersed into the slurry andwithdrawn at a controlled speed (3 cm/min). In order to avoid cracks during annealing, the heating treatment of themonolayer was defined according to the thermal decomposition profile of the slurry. Thick layers were obtained byFig. 1. Flow chart of powder preparation.repeating the dip-coating and thermal treatment steps. After three deposits, various cathode architectures wereprepared as illustrated in Fig. 2.One architecture was also built on a cathodic interlayer obtained by dip-coating YSZ electrolyte in a precursorlanthanum nickelate oxide sol, heated slowly up to 700 8C (50 8C/h), then annealed at 1000 8C for 2 h with an heatingrate of 100 8C/h [5].The cathodes thickness was measured by optical interferometry (Dektak 3030 ST).3. Results and discussionThe powders were obtained as single phases of La2NiO4+d and La4Ni3O10 oxides. The corresponding patternsare reported in Fig. 3 and are in good agreement with literature data [3,9]. The crystallite sizes were calculated fromTable 1Slurry compositionComposition (1) (2) (3)g g gPowder (La2NiO4+dor La4Ni3O10) 1.00 1.00 1.00Solvent (MEK/ETOH) 1.00 1.00 1.00Dispersant (C213) 0.02 0.04 0.06Binder (poly vinyl butyral, PVB) 0.12 0.12 0.12Plasticizer (dioctyl phthalate) 0.02 0.02 0.02Fig. 2. Layer array.Fig. 3. X-ray diffraction patterns of powders.XRD pattern using Williamson and Hall diagram obtained by peak diffraction modelisation. Both powderscalcined at 1000 8C in air for 2 h (Fig. 3) have a crystallite size of about 280 nm for La2NiO4+d, while discrepanciesin the 3D dimensions were found for La4Ni3O10 leading to a crystallite size of about 150 nm ? to caxis and550 nmjjcaxis.La2NiO4+d powder has spherical grains shape (Fig. 4). The mean grain size determined from SEMmicrograph wasapproximately 250–280 nm. These values are in good agreement with the crystallite size determined from XRDanalysis, indicating that the grains are monocrystalline. La4Ni3O10 particles are platelet shaped grains with dimensionsof about 200 nm long and 600 nm large; these values are also in good agreement with those determined from XRDanalysis, allowing similar conclusion than for La2NiO4+d to be drawn.Preliminary granulometry measurements performed on diluted suspensions made of powder dispersed in solvent,indicated a mean particle size of 1000 nm and 385 nm for La2NiO4+d and La4Ni3O10, respectively. Considering ourprevious results, one may thus conclude that La2NiO4+d particles were more aggregated in suspension thanLa4Ni3O10.In order to improve the dispersion state of powders and to optimize the stabilization of the suspensions, anincreasing amount of dispersant ratio (2, 4, 6 wt.% of powder) was added in the suspensions, as detailed in Table 1.Suspensions granulometry and Zeta potential measurements are reported in Fig. 5. Accordingly, the best dispersantconcentration which corresponds to smaller aggregates and negative surface charge of particles, is 4% for La2NiO4+d,Fig. 4. SEM micrographs of La2NiO4+d and La4Ni3O10.Fig. 5. Granulometry and Zeta potential measurements performed on powders dispersed in suspensions prepared with different amounts ofdispersant.and 2% for La4Ni3O10. It is also worthwhile to notice that, for these ratio, the mean aggregate size of both oxides(about 250 nm for La2NiO4 and 350 nm for La4Ni3O10) is in good agreement with the values determined from SEMand XRD analyses on powders. Thus, this shows that both powders were successfully dispersed in the suspensionswhile using an optimal amount of dispersant.Suspensions decomposition in air was followed up by thermogravimetric (TG) and thermodifferential (TD)analyses as a function of increasing temperature in order to determine a suitable treatment for the annealing ofLa2NiO4+d and La4Ni3O10 films (Fig. 6). Accordingly, the decomposition of both suspensions is exothermic withcritical step occuring in between 200 8C and 400 8C. Above this temperature, nomore signal is observed. On this basis,an appropriate thermal profile was defined for the films annealing, as described in Fig. 6. A low heating rate was usedbelow 500 8C during the burnout of organics in order to avoid cracks formation in films.Surface and cross section micrographs of the different architectures I to IV are shown in Fig. 7. Their thicknessis reported in Table 2. Accordingly, the multilayers are homogeneous, crack-free and adherent to the YSZsubstrate.The average thickness after three layer depositions is about 3.5 mm. As expected from the peculiar morphology ofLa2NiO4+d and La4Ni3O10 grains, thick films exhibit different microstructures depending on the selectedarchitecture. The deposit of ceramic suspensions and the subsequent annealing of the films did not alter the shape ofthe grains. Indeed, Fig. 7 evidences the spherical and platelet shapes of La2NiO4+d and La4Ni3O10 grains in theelectrodes (architectures I and II, respectively). Architectures III and IV made by combining La2NiO4+d andFig. 6. Layer thermal profile and TG-TD curves.Table 2Architectures CharacteristicsArchitecture Thickness for 3 layers ( 0.2 mm)I 4.1 mmII 3.6 mmIII 3.8 mmIV 3.3 mmLa4Ni3O10 layers exhibit both peculiar shapes with grains disposed in accordance with the stacking of the differentlayers (see Fig. 2).The porosity of these architectures consists of small pores of few hundred nanometers homogeneously distributedwithin the electrodes. Work is currently in progress for quantitative analysis of the porosity.Cross section micrograph of architecture V is shown in Fig. 8. It can be seen that the thick cathodic layers are welladherent to the thin interlayer and exhibit a distinct microstructure, allowing us to define a ‘‘duplex microstructure’’architecture for this cathode. The thin interlayer of nanoscale dimension exhibits an intimate contact interface withYSZ substrate, which is expected to reduce polarisation resistance by increasing the number of contact points at thecathode/electrolyte interface.Fig. 7. SEM micrographs of architectures I to IV.4. ConclusionWe synthesized La2NiO4+d and La4Ni3O10 oxides via a sol–gel route and the X-ray powder diffraction analysisrevealed single phases without impurities for both oxides. Both powders were shown to exhibit different morphologies(shape and size of grains). These lanthanum nickelates powders were mixed in an organic solvent using differentdispersant ratio and additives to prepare stable slurries. An optimum concentration of dispersant was determined bymeans of several relevant characterization techniques. The stable slurries were used to dip-coat graded or monophasemultilayered cathodes. After annealing, homogeneous, adherent and crack-free cathodes were obtained, exhibitingdifferent microstructures in accordance with the peculiar morphology of the raw powders and the stacking of thelayers. Duplexmicrostructure electrodes were designed by combining sol–gel route and slurry deposition technique. Athin interlayer of nanoscale dimension was coated onto the YSZ electrolyte in order to achieve intimate contactbetween the electrolyte and the cathode, and thick micro-scaled layers of tailored microstructure were added toincrease the amount of active surface materials. The average thickness of the cathodes in this work was in the range of3–4 mm.References[1] E. Boehm, J.M. Bassat, P. Dordor, F. Mauvy, J.C. Grenier, Ph Stevens, Solid State Sci. 5 (2003) 973–981.[2] J.M. Bassat, P. Odier, A. Villesuzanne, C. Marin, M. Pouchard, Solid State Ionics 167 (2004) 341–347.[3] M.L. Fontaine, C. Laberty-Robert, A. Barnabe´, F. Ansart, P. Tailhades, Ceram. Int. 30 (2004) 2087–2098.[4] ML Fontaine, PhD thesis (2004) Paul Sabatier University Toulouse III, France.[5] M.L. Fontaine, C. Laberty-Robert, F. Ansart, P. Tailhades, J. Power Sources 156 (1) (2006) 33–38.[6] M. Kleitz, F. Petitbon, Solid State Ionics 92 (1996) 65–74.[7] R. Moreno, Am. Ceram. Soc. Bull. 71 (10) (1992) 1521–1531.[8] R. Moreno, Am. Ceram. Soc. Bull. 71 (11) (1992) 1647–1657.[9] Z. Zhang, M. Greenblatt, J. solid state chem. 117 (1995) 236–246.[10] M. Greenblatt, Z. Zhang, M.H. Whangbo, Synth. Met. 85 (1997) 1451–1452.[11] T. Chartier, D. Merle, J.L. Besson, J. Eur. Ceram. Soc. 15 (1995) 101–107.[12] A. Navarro, J.R. Alcock, R.W. Whatmore, J. Eur. Ceram. Soc. 24 (2004) 1073–1076.[13] M.W. Murphy, T.R. Armstrong, P.A. Smith, J. Am. Ceram. Soc. 80 (1) (1997) 165–170.[14] E. Ivers-Tiffe´e, A. Weber, D. Herbstritt, J. Eur. Ceram. Soc. 21 (2001) 1805–1811.Fig. 8. SEM micrograph of architecture V.

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Influence of the processing parameters of slurries for the

deposit of nickelate thick films

Influence of the processing parameters of slurries for the deposit of nickelate thick films

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