Zirconia electrolytes film fabrication

Basu, R.N., Randall, C. A, and Mayo, M.J. (2001), Fabrication of dense zirconia electrolyte films for tubular solid oxide fuel cells by electrophoretic deposition, J. Am. Ceram. Soc. 84, 33-40. 

The electrolyte film can be fabricated by a number of processes depending upon the configuration of the cells. For tubular SOFCs, an electrochemical vapour deposition (EVD) technique was developed by Westinghouse Electric Corporation (now Siemens Westinghouse Power Corporation) in 1977 [40] to fabricate gas-tight thin layers of doped zirconia. This EVD process involved growing a dense oxide layer on a porous substrate at elevated temperatures and reduced pressures, as described in Figure 4.12 [41]. 

Empirical development of the nickel-zirconia anode over several decades has led to solid oxide fuel cells with adequate service life and performance, but fuel reforming is still required to operate with commercially available hydrocarbon fuels. It has become evident that the anode reactions are dominated by the three-phase boundary and that the microstructure of the composite cermet anodes is pivotal. Consequently, the processing methods used for making the anode powders, and the fabrication techniques used for deposition on the electrolyte are critical in making high performance anodes. Anode-supported cells with very thin electrolyte films are becoming interesting for operation at lower temperatures. 

The tape-casting method makes possible the fabrication of films in the region of several hundred micrometers thick. The mechanical strength allows the use of such a solid electrolyte as the structural element for devices such as the high-temperature solid oxide fuel cell in which zirconia-based solid electrolytes are employed both as electrolyte and as mechanical separator of the electrodes. 

Instead of the system silica/silicate also other systems such as titania/titanate, zirconia/zirconate can be used as a reference system [xiv]. The response time of freshly fabricated thick-film sensors based on thin-film /3-alumina is very short (about 15 ms at 650 °C). After several weeks of operating this time increases 10 times (150 ms) [xv]. Solid electrolyte C02 sensors using Ni/carbonate composite as measuring electrode are suited for measuring of C02 in equilibrated water gases [xiv]. Using semiconducting oxides and carbonates like ITO (indium tin oxide) Nasicon-based C02 sensors are able to measure at room temperature [xvi]. 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

These devices feature the use of micro-fabrication techniques adapted from the micro-electronics industry. These encompass substrate etching, thin-fihn deposition, lithography, and film-etching steps. This field has recently been reviewed by Evans et al. [11], and all devices exhibit the beautiful structural quality resulting from the micro-fabrication techniques. An example of a micro-planar SOFC fabricated on a silicon substrate is illustrated in Fig. 19.4 [12]. Figure 19.4a shows the sequence of fabrication steps used to make the edge-supported SOFC membrane which spans an aperture with dimensions 600 x 600 pm. The yttria stabilized zirconia (YSZ) electrolyte, which is only 70-nm thick, was deposited by... 

In order to improve the performance of SOFC, a thirmer yttria-stabilized zirconia (YSZ) electrolyte is considered for lower ohmic resistance and for operation in the intermediate temperature range of 500°C-800°C. Ionic conductivity decreases with decrease in temperature and hence the area-specific resistance (ASR) of an electrolyte increases with lower operating temperature. Fabricating the electrolyte in a dense and thinner film reduces the ASR or the resistance to ionic transport, allowing a lower operating temperature. For this purpose, efforts are being made in fabricating SOFC cell on the basis of either a thicker anode-supported or a thicker cathode-supported SOFC... 

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