【Physical,Properties,of,the,Cathode,Materials,in,Fuel,Cells】in the case of

　　Abdelmadjid Temagoult1 and Kafia Oulmi2 　　1. Laboratory of Industrial Energy Systems (LESEI), Mechanical Department, Faculty of Engineering Sciences, University of Batna, Batna 05000, Algeria
　　2. Laboratory of Chemistry and Environmental Chemistry (LCCE), Faculty of Science, University of Batna, Batna 05000, Algeria.
　　Received: November 5, 2010 / Accepted: July 4, 2011 / Published: February 20, 2012.
　　Abstract: The materials used in fuel cells are currently the subject of much research, particularly those of the cathode which is a key element for the different functions that it provides. In our work the authors became interested in the different materials used for the cathode, which are usually ceramic, and some of their physical properties between different electrical conductivity (electronic, ionic), the coefficient of thermal expansion and chemical compatibility between different materials used in the stack. Not to mention, however, the various parameters that influence these properties, such as structure, the sintering temperature, dope, and the operating temperature of the battery. The main objective of research in this area is to improve battery performance by researching new materials and new manufacturing technologies that will increase the electrical conductivity while trying to lower the temperature operating the latter as much as possible while keeping it above 650 °C. In doing so, the longevity of the battery will be increased which will have a direct impact on manufacturing costs of the battery, and thus greater use thereof.
　　Key words: Ceramic, fuel cell, cathode, electrical conductivity, thermal expansion.
　　 1. Introduction??
　　The fuel cell is one of the promising sources of clean energy using renewable resources. They produce electricity through an electrochemical process. There are different types of fuel cells that are distinguished by their temperatures. As for the fuel cell Solid Oxide(SOFC), called cells at high temperature, it is traditionally around 1,000 degrees. This is not without causing some problems, including aging too fast and too early of materials in general, but especially the electrolyte, caused by excessive chemical reaction between certain components. Cycles in temperature can also cause damage due to poor mechanical strength of certain materials (significant differences between the coefficients of thermal expansion of components). Furthermore, operation at these temperatures requires the use of expensive interconnect materials (lanthanum chromite refractory or steel).
　　The problem of temperature has been studied in our laboratory. Thus, in our previous work [1-4], the objective was to visualize the temperature field and the calculation of the performance of SOFC. But the major problem for this type of battery, it is the materials used for the various components thereof namely the electrodes (anode and cathode), the electrolyte and interconnect plates. It is the intent of this study.
　　 2. The Cathode Materials
　　On the cathode, its performance must be improved especially those of electrical conduction (electronic and ionic) and reductive cleavage of oxygen. Thus the specification of compounds used as cathode materials indicates that the compounds must have high electronic conductivity (≥ 100 S·cm-1), and ionic conductivity (σO2- of about 0.1 S·cm-1) there by improving the response of the cathode material [5].
　　The cathode materials operate under the conditions very oxidizing (air or oxygen and high temperature), which prohibits the use of traditional materials and requires the use of noble materials or exotic (oxide semiconductors, metal oxides drivers), but which are very expensive. The most widely used material for the cathode so far is a lanthanum magnatite doped with strontium [6] and much research has developed, but no new material has really emerged to replace it [7, 8].
　　This work is above all work of literature on physical properties of cathode materials including electrical conductivity, coefficient of thermal expansion and chemical compatibility between materials of different components of the fuel cell. This has been done on the basis of a study and analysis of a large number of publications, spanning a broad period between 1996 and 2008. The Table 1 shows the cathode materials. These materiels have a perovskite structure of different types AA’BO4, AA’BO3 and ABO3. Its classified into categories corresponding to the type of structure and base material and chronological order, that is to say from the oldest to the most recent. Note however that the basic materials are the lanthanides for the most part (La, Pr, Gd, Nd, Sm), alkaline earth metals (Ba, Sr) and transition element (Y).
　　Regarding the crystal structure for the majority it is orthorhombic type, and can have very symmetrical cubic or hexagonal, but for others materials, the structure is more complex and changes depending on the item. As an example, for the material B-5-31 in Table 1, it has a hexagonal structure for the element La, an orthorhombic structure for the elements Pr and Sm, Gd and finally a structure of two orthorhombic and cubic phases. Transitions are observed as a function of temperature, or the value of x for some materials (Table 1).
　　After that it was interested in the electrical conductivity, and coefficient of thermal expansion, which is the main objective of our study, or there were values for these different materials listed above. The authors note that in most publications on the measures are followed electrical conductivity measurements on the thermal and thermo mechanical behaviour of materials, this has also revealed the values for the coefficient of thermal expansion (TEC) of material cathode and the electrolyte. This parameter helps answer to the question of thermal compatibility between different elements of the stack. 2.1 The Electrical Conductivity
　　As a whole the cathode is made of different types of ceramic materials, must be a very good electrical conductor, and must be very porous to allow oxygen gas to move and spread to the point of reaction. The cathode is the negative side of the cell through which electrons flow. This is the side exposed to the air whose role is to use electrons to reduce molecular oxygen (O2) oxygen ion (O2-). The cathode is the seat of the chemical reaction. The oxygen is adsorbed and dissociated ions and reduced O2- thanks to the presence of oxygen vacancies. The place where this reaction occurs and which are present simultaneously electrons from the cathode, the oxygen vacancies in the electrolyte and oxygen gas is called the triple point or triple touch (TPB for Triple Phase Boundary).
　　Te cathode is thus simultaneously a collector load and the headquarters of the reduction of oxygen which then diffuses to the state of O2- ions through the electrolyte. This dual property (electronic conductor and catalyst for oxygen reduction) is achieved most commonly by oxides of perovskite structure type ABO3 is the case for virtually all materials listed above.
　　For example, the authors cite the magnates of lanthanum-doped La1-xSrxMnO3 strontium (LSM) which are the materials [5] privileged Cathode for SOFC and the currently most used for their high electrical conductivities (the order of σ = 200 S·cm-1 at 900 °C) and especially in an oxidizing atmosphere. This comes from the mixed-valence manganese Mn3+/Mn4+. They have a perovskite structure type ABO3, which may be biased atomic. Thus, the non LaMnO3 dope is orthorhombic at room temperature, and shows a crystallographic transition orthorhombicrhombohedral at about 387 °C. However, it should be noted that the rate of strontium which is observed the maximum conductivity is not yet established.
　　Table 1 List of cathode materials.
　　
　　Table 2 Values of electrical conductivity and coefficient of thermal expansion for different cathode materials.
　　
　　In general, the electrical conductivity of a material can be increased by substituting an ion with lower valence electrons on either site A or site B. For example, the lanthanum manganite LaMnO3 is doped with different cations of lower valence such as strontium, calcium, barium, nickel or magnesium and this is the case of certain materials identified in this work. The electrical conductivity is also a function of very important parameters that are operating temperature, the sintering temperature.
　　The authors wanted to make an inventory in this area trying to collect in Table 2, materials used, their maximum electrical conductivity and the coefficient of thermal expansion (TEC) at different temperatures. As regards the electrolyte most often encountered electrolytes such TCS, CGO, and YSZ especially with values for the TEC found in the bibliography are to YSZ, 10.7 × 10-6/°K [13], 12.5 × 10-6/°K [30], 11.5 ×10-6/°K [31], for TCS, 12.3 × 10-6/°K between 30 and 800 °C [27], and finally to OGS entre12. 0 and 12.5 ×10-6/°K [20, 39]. In terms of the LSGM electrolyte, the values are between 9.50 and 13.0 × 10-6/°K and this at 800 °C [15, 20].
　　2.2 Coefficient of Thermal Expansion
　　Regarding thermal compatibility between different element of the stack and given the specification very rigorous in this sense, many problems sometimes appear, and are observed due to too large differences between the coefficient of thermal expansion of the cathode and the electrolyte. Indeed the very important differences can distort too large, which may cause disruption of the cell in the physical sense of the term, and often for very important differences not the material is machined which allows to minimize this defect [43].
　　Thus, in some cases, adding the dope increases the coefficient of thermal expansion of the material (as in LaMnO3 dope with strontium) the coefficient is even larger than the amount of strontium introduced is important. It was noted that the values of the coefficient of thermal expansion for the cathode materials become too important in some cases and for certain material differences exceeding 100%, far short of accepted values.
　　2.3 Chemical Compatibility
　　The chemical compatibility between different elements of the stack, and in our case between the cathode and the electrolyte used also requires special attention.
　　Thus, among the materials lists in Table 1 almost half presents the chemical compatibility problem. The authors note that for many materials there is no reaction is to say, a good chemical compatibility between the material of cathode and electrolyte.
　　However, for other reactivity exists and appears in different forms. For the following materials, and to mention that the reaction appears as an impurity in perovskite structure following the passage of ions from the electrolyte (SDC) and which are incorporated in the A site of the cathode material, the case of(Ba0.5Sr0.5) ×Co0.8Fe0.2O3-1-δ [25], number D-5-5O, but this does not affect so much and in this case the performance of the cathode long term. The material Gd0.8Sr0.2Co1-yMnyO3, No. F-1-8, which reveals the appearance of reactivity with the electrolyte (YSZ) after annealing at 1,000 °C form a transition compound [32]. This compound becomes unstable and dissolves into the matrix of the electrolyte at higher temperatures. The material Gd1-xSrxMnO3, No. F-2-10 reacts with 8YSZ, but is compatible with OGS [33].
　　 3. Conclusion
　　During the preparation of this work the authors note first time the complexity of the issue of choice of materials for both the cathode and for other elements of the stack was towards the specification and the needs of its observation of a very rigorous. Many parameters have been taken into consideration among others the operating temperature of the battery, the sintering temperature, the material structure and doping. Note however that these cells operate at high temperature which is necessary for the proper functioning of the electrolyte, but creates much inconvenience to both the electrolyte itself and for other elements. To overcome these drawbacks, a decrease in operating temperatures up to 650-700 °C is highly desired. This is a milestone in the development of SOFCs operating at medium temperatures (600 to 700 °C), and right through the development and characterization of new materials performance at 700 °C, including the electrolyte and cathode, and a Master of formatting elements in thin or thick, dense or porous. The target is an ionic conductivity of about 0.1 S·cm-1 and E-100 S·cm-1.
　　Despite extensive research no new material has really emerged to replace the conventional compound, the lanthanum magnetite (LSM). A more innovative is to seek new structures of materials, such as family A2MO4 + δ with A = La, Pr or Nd and M = Ni or Cu. The originality of these compounds is that they are not sub-stoichiometric oxygen, but to present an overanion stoichiometry. They are the subject of recent technological developments. The two selected ways to lower the temperature are: the search for an electrolyte remaining ionic conductor at lower temperatures, this research is hampered by the fact that the electrolytes used (based on rare earth) have the annoying property of being sufficiently electronic conductors which reduces significantly the performance conductor at lower temperatures.
　　The implementation of very thin layers with the conventional electrolyte (yttria-stabilized zirconia) reduces the internal resistance. This last technology shows the most promising results.
　　 References
　　[1] H. Benmoussa, B. Zitouni, K. Oulmi, B. Mahmah, M. Belhamel, P. Mandin, Hydrogen consumption and power density in a co-flow planar SOFC, Int. J. of Hydrogen Energy 34 (2009) 5022-5031
　　[2] B. Zitouni, H. Benmoussa, K. Oulmi, S. Saighi, K. Chetehouna, Temperature field, H2 and H2O mass transfer in SOFC single cell: Electrode and electrolyte thickness effects, Int. Journal of Hydrogen Energy 34(2009) 5032-5039
　　[3] B. Zitouni, H. Benmoussa, K. Oulmi, Studing on the increasing temperature in IT-SOFC: Effect of heat sources, Int. Applied Physics & Engineering J. 8 (2007) 1500-1505.
　　[4] B. Zitouni, H. Benmoussa, K. Oulmi, S. Saighi, The performance of SOFC based on the type of electrode materials and geometric configuration, in: AlgerianFrench Symposium on Materials and Corrosion: Multifunctional Application, Bejaia, 2006, pp. 522-525.
　　[5] N. Preux, Unit of Catalysis and Solid State Chemistry UMR 8181 [Online], http://uccs.univlille1.fr/spip.php?article172&lang=fr.
　　[6] T. Alleau, Memento Hydrogen, [Online], www.省略/.../Fiche%205.2.3%20AFC%20rev%20avr il%202008.pdf.
　　[7] C. Lamy, Fuel Cells and Their Management (FCM), University of Poitiers, Final report, July 2002-June 2004.
　　[8] K.C. Wincewicz, J.S. Cooper, Taxonomies of SOFC material and manufacturing alternatives, J. of Power Sources 140 (2005) 280-296.
　　[9] H. Ullmanna, N. Trofimenkoa, F. Tietzb, D. Stoerb, A. Ahmad-Khanloub, Correlation between thermal expansion and oxide ion transport in mixed conducting perovskite-type oxides for SOFC cathodes, Solid State Ionics 138 (2000) 79-90.
　　[10] Y. Wang, H. Nie, S. Wang, T.L. Wen, U. Guth, V. Valshook, A2?αAα′BO4-type oxides as cathode materials for IT-SOFCs (A = Pr, Sm; A′ = Sr; B = Fe, Co), Materials Letters 60 (2006) 1174-1178.
　　[11] H.W. Nie, T.L. Wen, S.R. Wang, Y.S. Wang , U. Guth, V. Vashook, Preparation, thermal expansion, chemical compatibility, electrical conductivity and polarization of A2?αA′αMO4 (A = Pr, Sm; A′ = Sr; M = Mn, Ni; α = 0.3, 0.6) as a new cathode for SOFC, Solid State Ionics 177(2006) 1929-1932.
　　[12] Y. Sakaki, Y. Takeda, A. Kato, N. Imanishi, O. Yamamoto, M. Hattori, et al., Ln1-xSrxMnO3 (Ln5Pr, Nd, Sm and Gd) as the cathode material for solid oxide fuel cells, Solid State Ionics 118 (1999) 187-194.
　　[13] S.T. Aruna1, M. Muthuraman, K.C. Patil, Studies on strontium substituted rare earth manganit, Solid State Ionics 120 (1999) 275-280.
　　[14] T.L. Wen, H. Tu, Z. Xu, O. Yamamoto, A study of (Pr, Nd, Sm)1-xSrxMnO3 cathode materials for solid oxide fuel cell, Solid State Ionics 121 (1999) 25-30.
　　[15] W. Chen, T. Wen, H. Nie, R. Zheng, Materials research bulletin, JULLY 38 (8) (2003) 1319-1328.
　　[16] K. Kammer, Studies of Fe-Co based perovskite cathodes with different A-site cations, Solid State Ionics 177 (2006) 1047-1051.
　　[17] H.K. Lee, Electrochemical characteristics of La1?xSrxMnO3 for solid oxide fuel cell, Materials Chemistry and Physics 77 (2002) 639-646.
　　[18] H.C. Yu, K.Z. Fung, La1-xSrxCuO2.5-δ, as a new cathode materials for intermediate temperature solid oxide fuel cells, Materials Research Bulletin 38 (2003) 231-239.
　　[19] M. Mori, N.M. Sammes, E. Suda, Y. Takeda, Application of La0.6AE0.4MnO3 (AE = Ca and Sr) to electric current collectors in high-temperature solid oxide fuel cells, Solid State Ionics 164 (2003) 1-15.
　　[20] S. Wang, R. Zheng, A. Suzuki, T. Hashimoto, Preparation, thermal expansion and electrical conductivity of La0.6Sr0.4Co1-xGaxO3-d (x = 0.0-0.4) as a new cathode material of SOFC, Solid State Ionics 174(2004) 157-162.
　　[21] M. Zheng, X. Liu, W. Su, Preparation and performance of La1?xSrxCuO3?δ as cathode material in IT-SOFCs, Journal of Alloys and Compounds 395 (2005) 300-303.
　　[22] M.H. Hung, M.V. Madhava Rao, D.-S. Tsai, Microstructures and electrical properties of calcium substituted LaFeO3 as SOFC cathode, Materials Chemistry and Physics 101 (2007) 297-302.
　　[23] M.Z. Zheng, H. Xin, X.M. Liu, Q. Liu, D. Xu, W.H. Su, Electrochemical performance of La1-xSrxCuO3-δSm0.15Ce0.85O1.925 composite cathodes, IT-SOFCs Rare Metals 25 (2006) 256.
　　[24] P. Plonczak, M. Gazda, B. Kusz, P. Jasinski, Fabrication of solid oxide fuel cell supported on specially performed ferrite-based perovskite cathode, Journal of Power Sources 181 (2008) 1-7.
　　[25] B. Wei, Z. Lu, X. Huang, M. Liu, N. Li, W. Su, Synthesis, electrical and electrochemical properties of Ba0.5Sr0.5Zn0.2Fe0.8O3?δ perovskite oxide for IT-SOFC cathode, Journal of Power Sources 176 (2008) 1-8.
　　[26] A. Subramania, T. Saradha, S. Muzhumathi, Synthesis of nano-crystalline (Ba0.5Sr0.5)Co0.8Fe0.2O3?δ cathode material by a novel sol-gel thermolysis process for IT—SOFCs, Journal of Power Sources 165 (2007) 728-732.
　　[27] S. Li, Z. Lu, B. Wei, X. Huang, J. Miao, Z. Liu, et al., Performances of Ba0.5Sr0.5Co0.6Fe0.4O3?δ-Ce0.8Sm0.2O1.9 composite cathode materials for IT-SOFC, Journal of Alloys and Compounds 448 (2008) 116-121.
　　[28] S. Li, Z. Lü, X. Huang, W. Su, Thermal, electrical, and electrochemical properties of Nd-doped Ba0.5Sr0.5 Co0.8Fe0.2O3?δ as a cathode material for SOFC, Solid State Ionics 178 (2008) 1853-1858.
　　[29] W. Zhou, R. Ran, Z. Shao, W. Jin, N. Xu, Evaluation of A-site cation-deficient (Ba0.5Sr0.5)1?xCo0.8Fe0.2O3?δ (x>0), perovskite as a solid-oxide fuel cell cathode, Journal of Power Sources 182 (2008) 24-31.
　　[30] G. Ch. Kostogloudis, N. Vasilakos, Ch. Ftikos, Preparation and characterization of Pr1-x, Srx, MnO1±δ (x= 0.0, 0.15, 0.3, 0.4, 0.5) as a potential SOFC cathode material perating at intermediate temperatures (500-700 °C), Journal of the European Ceramic Society 17(1997) 1513-1521.
　　[31] H.R. Rim, S.G. Jeung, E. Jung, J.S. Lee, Characteristics of Pr1-xMxMnO3 (M = Ca, Sr) as cathode material in solid oxide fuel cells, Materials Chemistry and Physics 52(1998) 54-59.
　　[32] X. Huang, L. Pei, Z. Liu, Z. Lu, Y Sui, Z. Qian, et al., A study on PrMnO-based perovskite oxides used in SOFC cathodes, Journal of Alloys and Compounds 345 (2002) 265-270.
　　[33] V. Vashook, J. Zosel, T.-L. Wen, U. Guth, Transport properties of the Pr2?xSrxNiO4±δ ceramics with x = 0.3 and 0.6, Solid State Ionics 177 (2006) 1827-1830.
　　[34] L Xiong, S. Wang, Y. Wang, T.L. Wen,(Pr0.7Ca0.3)0.9MnO3-δ–SDC cathode for IT-SOFC, Journal of Alloys and Compounds 453 (2008) 356-360.
　　[35] C. Zhu, X. Liu, D. Xu, D. Yan, D. Wang, W. Su, Preparation and performance of Pr0.7Sr0.3Co1?yCuyO3?δ as cathode material of IT-SOFCs, Solid State Ionics 179(2008) 1470-1473.
　　[36] M.B. Phillipps, N.M. Sammes, O. Yamamoto, Gd1?xAxCo1-yMnyO3 (A = Sr, Ca) as a cathode for the SOFC, Solid State Ionics 123 (1999) 131-138.
　　[37] H.S. Yoon, S.W. Choi, D. Lee, B.-H. Kim, Synthesis and characterization of Gd1-xSrxMnO3 cathode for solid oxide fuel cells, Journal of Power Sources 93 (2001) 1-7.
　　[38] C.R. Dyck, Z.B. Yu, V.D. Krstic, Thermal expansion matching of Gd1-xSrxCoO3-δ composite cathodes to Ce0.8Gd0.2O1.95 IT-SOFC electrolytes, Solid State Ionics 171 (2004) 17-23.
　　[39] G. Ch. Kostogloudis, Ch. Ftikos, Characterization of Nd1-xSrxMnO3±δ SOFC cathode materials, Journal of the European Ceramic Society 19 (1999) 497-505.
　　[40] K.T. Lee, A. Manthiram , Synthesis and characterization of Nd0.6Sr0.4Co1?yMnyO3?δ (0 ≤ y ≤ 1.0) cathodes for intermediate temperature Y1?xSrxMnO3 as SOFC cathode material, Materials Science and Engineering B103 (2003) 207-212.
　　[41] K.T. Lee, A. Manthiram, Effect of cation doping on the physical properties and electrochemical performance of Nd0.6Sr0.4Co0.8M0.2O3?δ (M = Ti, Cr, Mn, Fe, Co, and Cu) cathodes, Solid State Ionics 178 (2007) 995-1000.
　　[42] T.J. Huang, Y.S. Huang, Electrical conductivity and YSZ reactivity of solid oxide fuel cells, Journal of Power Sources 158 (2006) 1202-1208.
　　[43] J.E.H. Sansom, H.A. Rudge-Pickard, G. Smith, P.R. Slater, M.S. Islam, Perovskite related cuprate phases as potential cathode materials for solid oxide fuel cells, Solid State Ionics 175 (2004) 99-102.
　　[44] E.V. Tsipis, V.V. Kharton, J.R. Frade, Transport properties and electrochemical activity of YBa(Co,Fe)4O7 cathodes, Solid State Ionics 177 (2006) 1823-1826.
　　[45] H. Lv, Y.J. Wu, B. Huang, B.Y. Zhao, K.A. Hu, Structure and electrochemical properties of Sm0.5Sr0.5Co1? xFexO3? δ, cathodes for solid oxide fuel cells, Solid State Ionics 177(2006) 901-906.
　　[46] S. Liping, H. Lihua, Z. Hui, L. Qiang, Christophe Pijolat, La substituted Sr2MnO4 as a possible cathode material, SOFC Journal of Power Sources 179 (2008) 96-100.