Chemistry: electrical and wave energy – Processes and products
Patent
1986-11-24
1989-01-31
Andrews, R. L.
Chemistry: electrical and wave energy
Processes and products
204 59R, 204129, 204130, 204252, 204282, 204283, 204295, 204296, 521 27, 564291, 564296, C25G 102
Patent
active
048013595
DESCRIPTION:
BRIEF SUMMARY
The invention has as an object solid compositions based on superoxides having high ionic conductivity, their method of manufacture, and their electrochemical applications.
It addresses, in particular their applications as electrolytes in devices for the separation or analysis of oxygen.
Molecular oxygen O.sub.2 is easily reducible to the anionic state by reason of its very electro-active character which distinguishes it from other constituents of air (nitrogen, rare gases) and numerous other gases.
The successivee anionic states of the reduction of O.sub.2 lead to O.sub.2.sup.- (superoxide ion), O.sub.2.sup.-- (peroxide ion) or O.sub.2 H.sup.- (hydroperoxide ion) and, with the breaking of the O--O bond, to 2 O.sup.-- or 2 OH.sup.- (oxide and hydroxide ions respectively).
Electro-diffusion of oxygen in anionic form has already been envisioned for the separation of oxygen from a given medium.
In a general manner, a first electrode or cathode permits the reduction of the molecular oxygen to an oxygen anion which migrates through an electrolyte to a second electrode or anode where the oxide is reformed to molecular oxygen.
Several types of electrolytes have been proposed for separating out the oxygen by electro-diffusion.
The most widely used electrolytes in liquid phase are constituted by aqueous solutions of potassium hydroxide and in solid phase by zirconia doped with oxides such as those of yttrium Y.sub.2 O.sub.3 or calcium CaO. The oxygen anions involved correspond respectively to the hydroxide ion OH.sup.- and the oxide ion O.sup.--.
In these two cases, the molecular oxygen undergoes a reduction by four electrons per molecule and the oxygen molecule is severed.
The very great overvoltages arising from the need to activate the aggregate of chemical and electrochemical reactions at the electrodes entails an energy consumption much higher than the theoretical energy for separation of the oxygen.
For example, in the case of an extractor of oxygen by electro-diffusion in a potassium hydroxide solution, the voltage to be applied for producing oxygen at one bar from atmospheric air and the corresponding consumption of energy are about 100 times the minimal value given by the thermodynamics.
In the case of the zirconia that has very low conductivity at ambient temperature, it is necessary to heat above 600.degree. C. to obtain a sufficient ionic conductivity and to activate the electrode reactions. The use of high temperatures entails moreover, problems of chemical reactivity in gaseous phase and problems of corrosion of the electrode materials.
Other media have been envisioned but have not been able to be developed such as the molten salts based on alkali nitrates functioning at temperatures of about 250.degree. C. In this case, the corrosion of the electrodes and the precipitation of peroxides have prevented the application of these media to the production of oxygen. Moreover, in an application as a gauge of the partial pressure of oxygen, these media have response times on the order of several hours, thus rendering them unusable in practice.
The separation of gases, and in particular, the extraction of oxygen from the air are processes generally costly in energy. The processes of electro-diffusion mentioned above are particularly so because of the great overvoltages of the electrodes and also because of the significant quantities of current required, being 4 faradays per mole of oxygen transported.
Although significant efforts to reduce the over-voltages have been undertaken in the field of oxygen electrodes for fuel cells and in the field of oxygen anodes for the electrolysis of water, the solutions most frequently proposed resort to an electrocatalysis using costly compounds based on precious metals and/or organic molecules of delicate synthesis.
Furthermore, the high intensities of current necessary when the transport of one mole of oxygen involves 4 faradays entail losses of energy by the Joule effect both in the electrolytic medium and in the conductors.
U.S. Pat. No. 4,475,994 proposes an electrochemic
REFERENCES:
patent: Re28792 (1976-04-01), Ruka et al.
patent: 4167457 (1979-09-01), Giner
patent: 4475994 (1984-10-01), Gagne et al.
patent: 4680101 (1987-07-01), Darlington et al.
Jeanne Francis
Lombard Serge
Schmidt Emmanuel
Andrews R. L.
L'Air Liquide, Societe Anonyme Pour L'Etude et Exploitation Des
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