Gelcasting as shaping technique for LSM-GDC supporting cathodes

In this work the attention was focused on the production of large area planar La0.8Sr0.2MnO3-Ce0.8Gd0.2O2 (LSM-GDC) cathodes for cathode supported solid oxide fuel cells (SOFCs). The obtained results were compared with the ones coming from the tape casting technique, the most common ceramic technique used to produce planar SOFC electrodes. The gelcasting process involved the optimization of the ceramic slurry in terms of organics nature and amount (in particular the monomer-cross-linker couple) and of the thermal treatments (drying, de-binding and sintering) needed to produce good strength, crack-free LSM-GDC green bodies. The type of reticulation adopted for this technique can be either radicalic or thermally induced. The first uses industrial monomers while the second one natural gelling agents in water. Only with the traditional gelcasting process was possible to produce a large area cathode. The "green" approach that involves agaroids as gelling agents and water as solvent and is therefore generally considered more environmentally friendly and economic leaded in fact to completely cracked samples. The water of the gelling solution induced the leaching of lanthanum ions of the LSM phase during the mixing step of the process causing the destruction of the perovskite crystal lattice and therefore the break-up of the ceramic sample after sintering. For the radicalic process, the critical step was identified to be the drying of the green body that had to be done in two subsequent steps to prevent defects such as warping and cracks. In the first the gelcasted sample was left at room temperature into a controlled humidity chamber whereas in the second, the partially dried sample was placed in a ventilated oven (25-40°C). A careful control of the suspension parameters and thermal treatments allowed the production of large area LSM-GDC supporting cathode suitable for SOFC applications. Solid oxide fuel cells (SOFCs) have attracted considerable research interest for their many advantages as green energy conversion devices. La1-xSrxMnO3-?-based materials are commonly used for cells that make use of zirconia as electrolyte. The two materials however can react forming an highly resistive La2Zr2O7 phase that is detrimental for the electrochemical performances. For this reason, a layer of gadolinia-doped ceria (GDC) is commonly interposed between the two layers. In this configuration, a mixture of LSM-GDC can be used to better match the thermo-mechanical properties of the additional GDC layer and to improve the electronic and catalytic properties of the electrode. A cathode-supported cell offers advantages over the anode-supported one in terms of structural stability as it does not suffer from volume contraction and expansion induced by the redox cycle of Ni-based supported cells. In addition, because no vapor is formed at the cathode, the size of its pores can be smaller than the ones of a supporting anode, and therefore it can be thinner than the anode retaining the same mechanical strength. Although tape casting is the most common technique used for producing supporting elements for SOFC, gelcasting can be considered an interesting alternative. Gelcasting is a near net shape technique that merges concepts derived from traditional ceramic processing to the ones derived by polymer chemistry. The increased interest for the gelcasting technique is motivated by: (i) possibility to use low-cost mould material, (ii) capability of producing simple and complex parts with large sections and good green strength, (iii) excellent green machinability, (iv) homogeneous material properties, (v) low organic content (easy binder removal), (vi) low investment in equipments and applicability for both ceramic and metal powders. The first gelcasting process (named "traditional") was developed at Oak Ridge National Laboratory (ORNL, USA) and it was based on the in-situ radical polymerization of a concentrated casting slurry, containing powder, aqueous (or nonaqueous) solvent, dispersant and a solution of organic monomers (e.g. acrylamide or methacrylamide with N,N'-methylenebisacrylamide) in a non-porous mold. The need of more eco-friendly processes leads to the exploration of possible innovative gelcasting methods. In this respect, lot of attention is nowadays focused on "green" gelcasting processes that use water as solvent and natural polysaccharides as gelling agents. Thermogelling polysaccharides (agarose and k-carrageenan) that are soluble in hot water and gelify at temperature below 40-50°C are the most considered ones. The use of a water-based environment however is not always feasible in particular when the process involves the use of hygroscopic powders such as lanthanum strontium manganite used in this process.