CHEMICAL ENGINEERING TRANSACTIONS VOL. 52, 2016 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Peng-Yen Liew, Jun-Yow Yong, Jiří Jaromír Klemeš, Hon Loong Lam Copyright © 2016, AIDIC Servizi S.r.l., ISBN 978-88-95608-42-6; ISSN 2283-9216 Determination of Optimum Production Conditions of Casting Slurry in the Manufacture of Molten Carbonate Fuel Cell Electrodes with the Tape Casting Technique Gülay Özkanb*, Selim Gemicib, Erdi Başarırb, Goksel Özkan a aDepartment of Chemical Engineering, Gazi University, 06570 Maltepe, Ankara, Turkey bDepartment of Chemical Engineering, Ankara University, 06100 Tandoğan, Ankara, Turkey gozkan@eng.ankara.edu.tr In this study, Ni anode and LiAlO2 electrolyte green sheets were manufactured for MCFC system. Experimental scale production of green sheets was carried out by using tape casting technique. The method of casting slurry, which is the first stage of preparation, has a direct impact on the final product. Therefore, cast slurries with different ratios of a solvent, a binder, a dispersant, plasticizer loading to the organic compounds were prepared to produce green sheets with the technique of tape casting in order to investigate the viscosity, shear stress, thixotropy and flow characteristics. After sheets were dried, physical properties (mechanical tests) were determined. Mixing method (balling mill and mechanical stirrer) and mixing duration of slurry, mixing weight ratio of binder, solvent, plasticizer, dispersant, Li/K carbonate, Ni, and LiAlO2 powder in the slurry was examined. 1. Introduction Molten Carbonate Fuel Cells (MCFC) are called according to its electrolyte as well as other fuel cells. MCFC has high energy conversion efficiency and operates between 650 - 700 °C. Usually, it comes at the beginning of the systems used in the local and regional level to high energy needs. In recent years, fuel cell electrode and electrolyte materials in the production of tape casting technique are known to find the area of intensive use. Nowadays tape casting technique is especially widely used in the production of plate and coating processes such as thin ceramic coating, polymeric coating, porous coating and the metal or alloy coating. Especially, this technique is used in the preparation of membrane, electrodes and electrolyte matrix of the polymeric membrane fuel cell (PEMFC) (Antolini, 2011), molten carbonate (MCFC) (Antolini, 1996) and solid oxide (SOFC) fuel cell (Özkan et al., 2015) and further discussion in (Özkan et al., 2016). In this study, Ni anode and LiAlO2 electrolyte green sheets were manufactured for MCFC system. Experimental scale production of green sheets was carried out by using tape casting technique. The method of casting slurry, which is the first stage of preparation, has a direct impact on the final product. Therefore, cast slurries with different ratios of a solvent, a binder, a dispersant, plasticizer loading to the organic compounds were prepared to produce green sheets with the technique of tape casting in order to investigate the viscosity, shear stress, thixotropy and flow characteristics. After sheets were dried, physical properties (mechanical tests) were determined. Mixing method (balling mill and mechanical stirrer) and mixing duration of slurry, mixing weight ratio of binder, solvent, plasticizer, dispersant, Li/K carbonate, Ni and LiAlO2 powder in the slurry and rheological characterization of slurries were examined. The optimum amount of cast slurries with different ratios of a solvent, a binder, a dispersant, plasticizer loading to the organic compounds was selected. The purpose of this study was to investigate the effect of green tensile strength because of good indicator of green tape homogeneity, which will also affect the sintering behavior. The ultimate tensile stress is important for handling. High strain to failure in the tape is necessary for successful removal of tapes from the carrier substrate and subsequent handling. Tape casting is DOI: 10.3303/CET1652146 Please cite this article as: Özkan G., Gemici S., Başarır E., Özkan G., 2016, Determination of optimum production conditions of casting slurry in the manufacture of molten carbonate fuel cell electrodes with the tape casting technique, Chemical Engineering Transactions, 52, 871-876 DOI:10.3303/CET1652146 871 a powerful method for manufacture of nickel green sheet which is sintered at high temperature for example 800 °C. 2. Materials and Methods Anode material was prepared by milling nickel, polyethylene glycol, polyvinylbutyral, glycerol in an organic medium (ethanol). Sintering process was carried out at 800 °C. The matrices for the molten carbonate fuel cell were manufactured by tape casting method of a slurry that consisted of an organic solvent (ethanol), binder (polyvinyl butyral), plasticizer (polyethylene glycol), dispersant (glycerol) and lithium aluminate, potassium carbonate, lithium carbonate. Ethanol was used as a solvent in the slurry when mixing period and method was changed. Slurry had been mixed in the ball-milling before it was mixed with mechanical stirrer. The ratio of the nickel powder that is 32 - 53 % was used in the anode slurry. The best weight ratio is 32 % for nickel powder. In addition, 13 - 52 % by weight of ethanol was used in the anode slurry. The best weight ratio is 41 % for the solvent (ethanol). 1,5 PVB (Polyvinyl butyral) / PEG (Polyethylene glycol) (w/w) was optimized for anode. 22 - 32.3 % by weight of lithium aluminate (LiAlO2) powder was used in the electrolyte slurry. The best weight ratio is 32 % for lithium aluminate powder. An amount varying between 23 - 52.3 % by weight of ethanol was used in the electrolyte slurry. The best weight ratio is 45.7 % for the solvent and 4.4 : 3.8 wt % for K/Li carbonate, 6.2 wt % for PVG, 7.1 wt % for PEG and 0.5 wt % for glycerol was used in the electrolyte mixture. Table 1: Material list Binder Solvent Powde Dispersant Plasticize Anode PVB Ethanol Ni Glycerol PEG Electrolyte PVB Ethanol LiAlO2 Glycerol PEG Anode materials are given in Table 1.The process of tape casting was performed on nickel powder that shows in Figure 1.  20 g Ni powder added to the milling.  15.8 g Ethanol was used for nickel powder in the milling  This slurry was ball milled for 4 h in order to separate nickel particles and to break up weak agglomerates.  Then 1 g binder PVB ,0.5 g PEG and 0.5 g glycerol added to the mill.  15 pieces ball used in the mill and slurry was milled for 48 h at 47 rpm.  The slurry was then cast on the glass surface and dr blade was used to prepare green sheet.  Green sheet was sintered at 800 °C with the CO2 and O2 in the furnace Also, electrolyte materials are given in Table 1.The process of tape casting was performed on lithium aluminate powder that shows in Figure 2  22 g LiAlO2 powder added to the milling.  46 g Ethanol was used for LiAlO2 powder in the milling  5 g LiCO3 and 44 g K2CO3 added to the mill.  This slurry was ball milled for 4 h in order to separate LiAlO2 particles and to break up weak agglomerates.  Then 7 g binder PVB, 8 g PEG and 1 g glycerol added to the mill.  15 pieces ball used in the mill and slurry was milled for 48 h at 47 rpm.  The slurry was then cast on the glass surface and dr blade was used to prepare green sheet.  Green sheet was sintered at 800 °C with the CO2 and O2 in the furnace 872 Figure 1: Experimental procedure for anode Figure 2: Experimental procedure for electrolyte 3. Result and Discussion 3.1 Tensile test Tensile strength, displacement, stress, strain and thickness of the green sheet were analyzed by performing tensile test. Green sheet samples were pulled off by test device with 1 mm/min. Thickness was 0.4 mm in experiment 1 on the other hand thickness of the green sheet was increased in experiment 2. Maximum displacement of the sample was decreased from 4,421 mm to 3,877 mm and tensile strength was decreased. Maximum stress was decreased from 4.4244 N/mm2 to 0.819 N/mm2 on the other hand maximum strains was increased from 6, 3 % to 12, and 9 %. The tensile strength and strain to failure off green sheets were measured. As can be seen in Table 2, the strength of the green sheet decreased and amount of PVB was decreased in the mixture totally. Table 2: Physical Properties of Anode Experiment No 1 Experiment No 2 PVB (g) 4.8 (15 %) 1.5 (3.9 %) PEG (g) 3.2 (10 %) 0.5 (1.3 %) Ethanol (g) 13 (41 %) 15.8 (41 %) Glycerol (g) 0.25 (0.8 %) 0.5 (1.3%) Ni (g) 10 (32 %) 20 (52 %) Maximum Strength (N) 53.0938 49.3156 Maximum Displacement (mm) 4.421 3.877 Maximum Stress (N/mm2) 4.424 0.819 Maximum Strain (%) 6.315 12.923 Thickness (mm) 0.4 1.4 Width × Length (mm) 30 × 70 43 × 30 Tensile Strength/ (Width × Length) (N/mm2) 0.025 0.038 873 Obviously, the binder to plasticizer ratio plays an important role in mechanical properties of green sheet. In the range less than 50 wt % PVB (50 : 50 PVB:PEG (wt/wt)), the green sheet had low strength and high flexibility because of insufficient binder among the nickel particles. On the other hand, the green sheet with PVB content than 50 wt % had high strength and stiffness due to insufficient plasticizer. Plasticizer can reduce the molecular chain length of PVB by breaking the chain and decrease the Van der Waals forces between large binder molecules. According to Table 3, LiAlO2 percentage was reduced (Experiment 4), it was observed that the tensile strength was dropped. While the dispersant ratio was rising, tensile force was also increasing. Pressure on unit surface was also increased. In case of optimum proportions of material for the dispersant or other mixture ratios, it is found to provide a homogeneous mixture. Thickness of the green sheet was 0.5 mm in experiment 3 and thickness was increased to 1 mm in Experiment 4. Maximum displacement was increased from 1.3 mm to 3.4 mm and tensile strength is decreased. Maximum stress was decreased from 1.53 N/mm2 to 0.18 N/mm2 and maximum strain was increased from 1.86 % to 4.25 %. Size of the green sheet was similar. Thickness of the green sheet was 1 mm in experiment 4 and thickness was increased to 1.3 mm in experiment 5. Maximum displacement was decreased from 3.4 mm to 0.305 mm and maximum strain was decreased from 4.25 % to 1.01 %. Tensile strength is increased. Maximum stress was increased from 0.187 N/mm2 to 0.339 N/mm2.The amounts of electrolyte material in the matrix slurries were 8.2 %, 9.4 % and 10.1. Each green matrix sheet had a thickness of 0.5 - 1.3 mm. Table 3: Physical Properties of Electrolyte Experiment No 3 Experiment No 4 Experiment No 5 PVB (g) 7.1 (6.2 %) 7.1 (7.1 %) 3.5 (7.5 %) PEG (g) 8.1 (7.1 %) 8.1 (8.1 %) 4 (8.6 %) Ethanol (g) 52.3 (45.7 %) 52.3 (52.6 %) 23 (49.3 %) Glycerol (g) 0.6 (0.5 %) 0.6 (0.6 %) 0.5 (1.1 %) Li2CO3 (g) 5 (4.4 %) 5 (5 %) 2.5 (5.4 %) K2CO3 (g) 4.4 (3.8 %) 4.4 (4.4 %) 2.2 (4.7 %) LiAlO2 (g) 36.9(32.3 %) 21.9 (22 %) 11 (23.6 %) Maximum Tensile Strength (N) 20.723 5.073 8.836 Maximum Displacement (mm) 1.304 3.405 0.305 Maximum Stress (N/mm2) 1.535 0.187 0.339 Maximum Strain (%) 1.862 4.256 1.016 Thickness (mm) 0.5 1 1.3 Width × Length (mm) 27 × 70 27 × 80 20 × 30 Tensile Strength/(Width × Length) (N/mm2) 0.011 0.0023 0.0147 3.2 Rheological characterization of slurries: The flow curves of the anodes and electrolyte slurries obtained by measurements performed by increasing the shear rate from 0 to 100 s-1 for anode and 0 to 1,000 s-1 for electrolyte and temperature maintained constant at 25 °C were shown in Figure 3 and 4. It is expected that the anode slurry high Ni shows the highest viscosity as well as some thixotropy which shows time dependent (Table 4). Both viscosity and thixotropy behavior decrease when solvent is added. Minimum average viscosity is reached for 32 % Ni slurry. Average viscosity of the best electrolyte (Experiment No 1) is 1.2 Pa.s. Experimental data were analyzed using different regression models. The best fitting was obtained for the Herschel–Bulkley model. As a result of researching the relation between sheer rate and sheer stress, flow behavior indexes were shown in Table 4. All the suspensions reveal a shear thinning (pseudo plastic) behavior. Shear - thinning flow behavior is described as;  Decreasing viscosity  Deformation in shear direction occurs disentanglement  Agglomerates are disintegrated,  Droplets are deformed and show the shape of ellipsoids.  Particles are orientated in flow direction. All of them, Shear - thinning flow behavior is appropriate for tape casting 874 Figure 3: Flow curves for anode Figure 4: Flow curves for electrolyte slurries Table 4: Thixotropy of samples Thixotropy (%) 52 % Anode 103,7 32 % Anode 60,78 Electrolyte 97.74 Table 5: Flow behavior index Flow behavior index n Flow behavior 52 % Anode 0.49 Shear - thinning 32 % Anode 0.76 Shear - thinning Electrolyte 0.94 Shear - thinning 4. Conclusions This study successfully prepared Ni powder green sheet and electrolyte for sintering process. Slurry compositions of the pure LiAlO2 matrix were optimized using tape casting method. The performance of unit cell with LiAlO2 matrices were much improved for the mechanical tests. Electrolyte matrices were manufactured with electrolyte materials in 62 : 38 ratios for the Li2CO3 and K2CO3. Electrolyte materials were homogeneously dispersed in the matrices. LiAlO2 matrices were optimized by quantification of the electrolyte 875 materials in matrices with 32.3 % (wt) solids (Experiment No 3). Binder and plasticizer were added to the slurry after the addition of the solvent and LiAlO2. It is important factor for the homogeneity of the matrix sheet. Anode electrodes were prepared with powder sintering and by tape casting methods. This had resulted in better performance for the mechanical strength test’s the nickel powder increased, tensile strength was decreased. PVB ratio and thickness of the green sheet affected the mechanical properties. The best mixing got from Experimental 1. Reference Antolini E., 2011, The stability of molten carbonate fuel cell electrodes: A review of recent improvements, Applied Energy, 88, 4274-4280 Antolini E.,1996, Preparation of porous nickel electrodes for MCFC cells by non-aqueous tape casting, J. Materials Science,31,2187-2190 Özkan G., Özkan G., İyidir U.C., 2015, Synthesıs and Characterızatıon of Molten Carbonate Fuel Cell Anode Materıals, Energy Sources, and Part A: Recovery, Utilization, and Environmental Effects, 37, 2487 – 2493. Özkan G., Özkan G., İncirkuş V., 2016, Synthesis and characterization of solid electrolyte structure material (LiAlO2) using different kinds of lithium and aluminum compounds for molten carbonate fuel cells, Indian Journal of Chemical Technology, 23, 227-231 876