Morphology and Structure of SOFC Components Fabricated By Spray Pyrolysis

Abstract

Solid oxide fuel cells are expected to play a significant role in the future energy sector mainly due to their inherent high efficiency and low pollutants emissions. These devices typically operate above 800°C by means of a solid oxide electrolyte, typically yttria stabilized zirconia (YSZ) that conducts oxygen ions at sufficiently high conductivities at these temperatures. The cathode is exposed to air or pure oxygen which is reduced to O2 -. State of the art cathodes are based on the perovskite lanthanum strontium manganite La0.9Sr0.1MnO3 (LSM) in a mixture with YSZ which exhibits adequate electronic conductivity at 1000°C, is stable under oxidizing conditions and has a good thermal expansion match with YSZ. The anodic electrode is exposed to the fuel mixture that can even consist of CO which is normally, deleterious for other types of fuel cells. In fact, one major advantage of SOFCs is their suitability to oxidize conventional hydrocarbons and/or biofuels which renders them ideal candidates in connection with renewable sources (see Figure 1). Due to the difference in the oxygen chemical potential between the anode and the cathode, part of the free energy of the oxidation reaction is converted directly to electricity. State of the art anodes, have been based on Ni due to its low cost, again in a mixture with YSZ[i]. Oxidizing directly (i.e. without an intermediate reforming step) conventional hydrocarbons in SOFCs offers a number of advantages that relate to the existing infrastructure for these fuels that in this case, can be used more efficiently and environmentally consciously until their reserves are depleted. Nickel cermets, however, have to be replaced with other types of mixed conducting (i.e. ionic and electronic) electrodes that do not catalyze carbon formation such as composite anodes of the type Cu/CeO2 /YSZ[ii] or materials based on lanthanum chromate such as the perovskite (La0.75Sr0.25)Cr0.5Mn0.5O3 -δ (LSCM)[iii]. High materials and fabrication costs have prevented wide commercialization of these type of fuel cells and, thus, research efforts have been intensified in recent years by many laboratories that address the above issues. Regarding fabrication, there exist two major categories of techniques, namely conventional which are based on a particulate approach and deposition which entail the formation of cell components on a support via a physical and/or chemical process. The latter offer the advantage of in situ and directly controlling final particle size and morphology by appropriately tuning the characteristics of the physicochemical processes involved during deposition. The purpose of this research is to explore the potential of the spray pyrolysis (SP) technique which is a deposition technique for the fabrication of SOFC components of flat plate geometry. It involves spraying of an appropriate solution of ionic salts on a suitable substrate at certain temperature for the production of oxides in film form[iv]. The technique has the potential for the fabrication of electrolytic films with low thickness resulting in lower ohmic losses and for the fabrication of all the cell components in situ. It also offers great flexibility in terms of materials selection and processing parameters to fabricate suitable electrodes and electrolytes for SOFCs. In the present work, we focus on the effect of the substrate temperature and solution concentration on the final morphology of anodic, cathodic and electrolytic films fabricated by SP using environmentally friendly aqueous solutions. Results are presented of the application of this method for the production of anodic film cermets such as Cu-CeO2, Co-CeO2 and Cu-La0.75Sr0.25Cr0.5Mn0.5O3-δ (Cu-LSCM) on dense YSZ substrates. In addition, cathodic electrodes of La0.75Sr0.25MnO3(LSM) on dense YSZ and porous LSM substrates were fabricated. Finally, YSZ films as well as composite structures of the type Cu-LSCM/YSZ were deposited on porous LSM pellets. The effort is concentrated to identify optimized processing conditions that lead to good adhesion. The produced films were characterized by X-ray diffraction and scanning electron microscopy (SEM). [i] Kiratzis N. E., Connor P., Irvine J. T. S.(2010) Preparation and characterization of copper based cermet anodes for use in solid oxide fuel cells at intermediate temperatures. J. Electroceram. 24:270-287. [ii] Park S.D., Vohs J.M., Gorte R.J. (2000) Direct oxidation of hydrocarbons in a solid-oxide fuel cell. Nature 404: 265-267. doi:10.1038/35005040 [iii] Plint S.M., Connor P.A., Tao S., Irvine J.T.S. (2006) Electronic transport in the novel SOFC anode material La1−xSrxCr0.5Mn0.5O3±δ. Solid State Ion. 177: 2005-2008. doi:10.1016/j.ssi.2006.02.025 [iv] Papastergiades E., Argyropoulos S., Rigakis N., Kiratzis N. E. (2009) Fabrication of ceramic electrolytic films by the method of solution aerosol thermolysis (SAT) for solid oxide fuel cells (SOFC). Ionics 15: 545-554. Figure 1