(658c) Combustion Synthesis of Solid Oxide Fuel Cell (SOFC) Cathode Materials

INTRODUCTION

Fuel cells generate electrical power via chemical reactions between fuels and oxygen (air).  A Solid Oxide Fuel Cell (SOFC) consists of ceramic cathode (e.g. LaMnO₃), anode (e.g. Ni/YSZ), and ceramic electrolyte (e.g. YSZ) and operates at a temperature above 700 °C.  Because there is no mechanical moving device in a fuel cell, a higher efficiency can be achieved compared to an internal combustion power generator.

The high cost of cathode materials has been identified as one of the bottle necks in Solid Oxide Fuel Cell commercialization by U.S. DOE's Solid-State Energy Conversion Alliance (SECA) consortium.  It costs more than 40% of the total cost in SOFC production.  The current market price of cathode materials is US$700-1,000/kg.  Self-propagating High-temperature Synthesis (SHS) process can produce high quality cathode powders ^{[1, 2]} at a cost less than US$100/kg.  A stable combustion front movement and uniform temperature distribution during the synthesis is needed to produce high quality cathode materials.  In order to improve and control the SHS of cathode materials, a mathematical model was developed to study the 3-dimensional temperature history during the SHS process.

Temperature distribution during SHS of LaMnO₃ from lanthanum oxide and manganese metal was modeled based on the following chemical equations:

                        La₂O₃ + 2 Mn + 0.75 NaClO₄   à  2 LaMnO₃ + 0.75 NaCl                                           (1)

                        La₂O₃ + 2 Mn + 1.5 O₂   à  2 LaMnO₃                                                                          (2)

In reaction (1), a solid oxidizer (NaClO₄) is used as the oxygen source and atmospheric pressure oxygen is used in reaction (2).  Reaction heat and other thermodynamic data, such as thermal conductivity were calculated based on experimental data ^{[3, 4]} and properties from handbooks ^{[5, 6]}.

Impacts of standard SHS variables, such as oxygen flow rate, pre-heating temperature, pellet diameter, and pellet density upon the temperature distribution were studied.

RESULTS

Figure 1 is an example of the calculated temperature profile inside a pellet of a 2 cm diameter at a time 2 seconds after one end of the pellet is ignited.  The initial pellet temperature and surrounding temperature are both 300 K and the ignition temperature is 773 K.  A maximum combustion temperature of 2,100 K can be reached when solid oxidizer (NaClO₄) is used as shown in Figure 1(a).  This temperature correlates well with the published experimental results ^[3], in which a highest measured temperature of 1,850 °C (2,123 K) was reported.  Figure 1(b) represents calculated temperature profile when gaseous oxidizer (oxygen at the atmosphere pressure) is used.  A higher maximum combustion temperature of 2,200 K can be reached in this reaction.

Figure 1: Temperature profile inside a pellet when (a) solid oxidizer (NaClO₄) and (b) gaseous oxidizer (O₂) is used as an oxidizer.

Besides the source of oxidizers, the 3-dimensional temperature profiles are modeled for various pellet diameter, oxygen flow rate, preheating temperature, and green density.  Our model suggests that pellet diameter has significant impact on the propagation of reaction front movement.    A minimum pellet diameter for a sustaining reaction front movement can be determined at different preheating condition.  In addition, by introducing a pellet holder was introduced underneath the reactant pellet in the model (quartz holder, for example) to simulate the real reaction system.  The presence of the pellet holder results in less symmetric temperature distribution inside the pellet.

CONCLUSION

Our model has been successfully used to model SHS of LaMnO₃ and other SHS reactions.  It is also used for studying different reaction conditions on the propagation and temperature distribution.

REFERENCES

1.   Ming, Q., M. Nersesyan, K. Ross, J. T. Richardson and D. Luss," Reaction Steps and Microstruture Formation during SHS of La_0.8Sr_0.2CrO₃", Comb. Sci. & Tech., 128, 279 (1997).

2.   Ming, Q., J., M.D. Nersesyan, S. Lin, J.T. Richardson, D. Luss, Chemical Rate processes Involved in SHS of La_0.9Sr_0.1CrO₃, Int. J. Of SHS, 7(4), 447 (1998).

3.   Kuznetsov, Maxim V., Ivan Parkin, Daren J. Caruana, Yuri G. Morozov, "Combustion Synthesis  of Alkaline-Earth Substituted Lanthanum Manganites ", J. of Mater. Chem., 14, 1377 (2004).

4.   Jacob, K. T., M. Attaluri, "Refinement of Thermodynamic Data for LaMnO₃"; J. of Meter. Chem., 13, 934 (2003).

5.   Yaws, Carl L., "Chemical Properties Handbook", McGraw-Hill Inc, New York, NY (1999), ISBN: 0070734011.

6.   Barin, Ihsan, "Thermochemical Data of Pure Substances", VCH, New York, NY (1993), ISBN: 1560817178.