Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1995-07-31
1997-10-14
Valentine, Donald R.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
4271263, 4271265, 4271266, C25B 900, C25B 1104, B05D 512
Patent
active
056768060
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method according to the precharacterizing clause of claim 1.
A method of this type, followed by the reduction of the metal oxides to the metal is employed for fabricating electrodes in electrochemical reactors. An example of this is given in the article "Characteristics of Slurry-Coated Nickel Zirconia Cermet Anodes for Solid Oxide Fuel Cells" by Tasuya Kawada et. al. in J. Electrochem. Soc., Vol. 137, No. 10, October 1990, pp. 3042-3047. In this case, the metal oxide employed was nickel oxide, and the oxygen ion-conducting oxide employed was yttrium-stabilized zirconium oxide (YSZ). A mixture of nickel oxide and YSZ is calcined after mixing, is applied to the electrolyte in the form of a slurry and is then sintered, a reduction treatment finally taking place to convert the metal oxide into metal.
For electrodes of this type, which are used, in particular, as plate anodes in solid-oxide fuel cells, the lateral electron conduction is important, in order to be able to take off current. Moreover, in connection with promoting the electrochemical reaction, a high catalytical activity is important. Because fuel cells of this type are generally operated at an elevated temperature (from 800.degree. C.), it is important that the coefficient of expansion of the layer and of the electrolyte which serves as the support of the anode are approximately equal in order to avoid, as far as possible, thermal stresses during the heating and cooling cycles. Finally it is important that no shrinkage arises during sintering.
The method described in the abovementioned publication by Kawada is particularly suitable for fabricating small electrode. However, when aiming for putting larger-scale fuel cells into practice, it is important likewise to be able to coat in this manner electrolytes having a larger surface area. It was found that, when an electrolyte having a larger surface area was coated in this manner, problems arose as a result of the considerable sintering shrinkage during sintering and of the lack of lateral conductance after reduction, so that the electrode thus fabricated will fail.
The German Offenlegungsschrift 2,852,638 discloses a method according to the precharacterizing clause of claim 1. The starting point for the sensors fabricated according to that patent specification is formed by metals or semiprecious metals. According to this German Offenlegungsschrift, a relatively high sintering temperature is employed. The properties required of gas sensors are completely different from the demands made of electrochemical cells. In the case of a gas sensor, the determination of an electromotoric force between the gas to be analysed and a reference gas is the only thing that matters. In an electrochemical cell, the electrodes must be suitable for high current densities in order to produce a sufficient output. In order to achieve such a high current density, relatively coarse ion-conducting oxides should be present and the metal or precious-metal particles should be as small as possible.
The object of the present invention is to provide a method by which it is possible to eliminate the drawbacks existing for gas sensors for use in electrochemical cells. This object is achieved in a method described hereinabove having the characterizing features of claim 1.
It has been found that it is possible, by precalcining the ion-conducting oxide, to adjust, independently of the metal oxide, the particle size thereof. The higher the precalcination temperature, the larger the particle size. By starting from oxides of precious metals or semiprecious metals, simpler grinding is possible. The lower sintering temperature, compared to the prior art, prevents the oxide particles, after sintering, from adhering to one another in such a way that after reduction lateral conductance is no longer possible.
The ion-conducting oxide may be a member of the crystal structure class of the perovskites and fluorites which can be formed from the transition metals, the rare earth metals and the alkaline earth metals.
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T. Kawada et al., "Characteristics of Slurry-Coastel Nickel Zirconia Cermet Anodes for Solid Oxide Fuel Cells", Journal of the Electrochemical Society, vol. 137, No. 10, Oct. 1990, pp. 3042-3047.
T. Setoguchi et al., "Effects of anode Material and Fuel on anodic Reaction of Solid Oxide Fuel Cells", Journal of the Electrochemical Society, vol. 139, No. 10, Oct. 1992, pp. 2875-2880.
T. Kawada et al., "Structure and Polarization Characteristics of Solid Oxide Fuel Cells Anodes", Solid State Ionics, vol. 40/41, 1990, pp. 402-406.
D.W. Dees et al., "Conductivity of Porous Ni/ZrO.sub.2 -Y.sub.2 O.sub.3 Cermets", Journal of the Electrochemical Society, vol. 134, No. 9, Sep. 1987, pp. 2141-2146.
S. Murakami et al., "Development of a Solid Oxide Fuel Cell With Composite Anodes", pp. 561-568 No Date Available.
De Jong Jan Peter
Huijsmans Jozef Peter Paul
Van Berkel Franciscus Petrus Felix
Stichting Energieonderzoek Centrum Nederland
Valentine Donald R.
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