Shape Forming of Ceramic Tubes for Electrochemical Reactors by Gel-Casting Method
Authors: Chalkia Vasiliki , Pandis Pavlos , Stathopoulos Vassilis 
Published: February 2020

Three-dimensional structures are required as electrodes to increase the space time yield of electrochemical reactors when the reactions take place at low current densities. However, a tubular arrangement is necessary in some cases even in the industrial practice. Tubular Electrochemical Reactors (TERs) consist of a dense co-ionic ceramic membrane and two porous electrodes [1,2]. Gel-casting is a promising method of producing tailor-made dimensions of electrolyte and/or electrode tubes for the needs of the above reactors. The basic principle of gel-casting concerns a combination of a solvent, a dispersant, organic monomers and low particle sized ceramic powders to form a high-solids-content, fluid slurry. The slurry is poured under conditions into a casting mold where by an initiator and a catalyst the organic monomers polymerize to form a 3-D polymer network of a solid gel in the shape of the mold.
In particular, an aqueous solution of water soluble monomer acrylamide (AM) and cross-linker methylene–bis-acrylamide (MBAM) in appropriate ratio was prepared. Darvan C was used as dispersant and added to the AM and MBAM solution. Subsequently, the ceramic powder was added (i.e 50-80 wt%) and the system was mixed thoroughly to a stable slurry. Finally initiator and catalyst were added to the slurry, respectively ammonium persulfate (APS) and tetra methyl ethylene diamine (TEMED)/g of slurry. The casting slurry is poured into molds of various shapes to form within minutes a gelled green tubular body. Prior any further heating treatment the green bodies may subject to machining i.e cutting and drilling. All volatiles are removed by a heating treatment up to 6000C. Calcination at higher temperatures was applied in order to introduce mechanical strength to the shaped structures through controlled sintering [3]. The sintering temperature is totally depended on the composition of the ceramic powder. Major crucial factors in the overall success of the process are particle size distribution of the ceramic powder, drying conditions, slurry rheology, mold features and sintering parameters.
In this study, ceramic powders of an oxygen ion conductor such as apatite-type lanthanum silicates (ATLS) and a proton conductor such as yttrium-doped barium zirconate (BZY) where prepared, optimized and used in gel-gasting methods. They are considered promising electrolytes for intermediate temperature (IT-SOFCs) solid oxide fuel cells. ATLS have attracted interest as promising electrolytes due to their thermodynamic stability, robustness (as ceramics) and their ability to maintain distorted crystal structure by doping but more important due to their high oxide ion conductivity at intermediate temperatures. Additionally they exhibit satisfactory refractoriness with high ionic but low electronic conductivity, low cost and good availability of raw materials [4,5]. On the other hand BZY proton conducting ceramics are regarded as promising electrolyte materials for low temperature proton conducting solid oxide fuel cells due to their higher ionic conductivities and lower activation energy as compared to conventional oxygen-ion conducting electrolytes [6-8]. The above materials were synthesized from various routes, providing thus singled-phased structures and a suitable particle sized distribution for their use in the shape-forming with gel-casting.
Powders are verified by XRD, SEM and particle size laser distribution analysis. In addition, rheological measurements were performed at the slurries in order to evaluate the suitable viscosity for a successful shape-forming casting. Different sintering protocols were applied. The porosity of the produced tubes was evaluated by Archimedes method while the microstructure was studied by SEM.

References:

[1] C. Peng, et al., Journal of Power Sources, 2009. 190(2): p. 447-452
[2] D. Gao, and R. Guo, Journal of Alloys and Compounds, 2010. 493(1–2): p. 288-293
[3] A.C. Young, O.O. Omatete, M.A. Janney and P.A. Menchhofer, Journal of American Ceramic Society, 1991. 74 (3):p. 612-618
[4] X. Li et al., Electrochemistry Communications, 2011. 13 :p. 694–697
[5] Y. Yoo, N. Lim, Journal of Power Sources, 2013. 229 :p. 48-57
[6] S.-S. Baek et al., Acta Materialia 2014. 66 :p.273–283
[7] H. Gasparyan, et al, Solid State Ionics, 2011. 192: p. 158.
[8] S. Bebelis et al, ECS Transactions, 2009. 25: p.2681.

Acknowledgements

Financial support by the programs Archimedes III implemented within the framework of Education and Lifelong Learning Operational Programme, co-financed by the Hellenic Ministry of Education, Lifelong Learning and Religious Affairs and the European Social Fund, Project: ‘Synthesis, Characterization and study of properties of solid electrolytes of the apatite structure for fuel cell applications - APACELL’, is gratefully acknowledged.