Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1989-10-12
1992-06-30
Niebling, John
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204284, 429 42, 429 43, 429 41, C25B 1100
Patent
active
051260310
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to gas diffusion electrodes and, more particularly, this invention relates to gas diffusion electrodes adapted for use in electrochemical cells utilizing an aqueous alkaline electrolyte and consuming or generating a gas via the electrochemical process occurring within the gas diffusion electrode.
2. Description of Related Art
The use of gas diffusion electrodes in fuel cells and metal-air batteries is well known. Gas diffusion electrodes have also been used in the electrolysis, either oxidation or reduction, of gaseous reactants. It is also possible to generate gases in such electrodes. In general, gas diffusion electrodes take the form of solid porous (gas and liquid permeable) bodies formed at least in part of an electronically conductive, electrochemically active material, and may include a catalyst. Such electrodes generally define an electrolyte contacting surface and a gas contacting surface. Electrochemical oxidation and reduction occur at the points in the electrode where the gas to be oxidized or reduced contacts both the electrolyte and the active material of the electrode. In the case of gas generation, electrolyte contacts the active material, and gas is generated at this interface.
Electrochemical cells utilizing such electrodes generally comprise the gas diffusion electrode, a spaced counter electrode, a liquid electrolyte (which is generally aqueous) which contacts both the counter electrode and the gas diffusion electrode, and a gas which contacts the gas diffusion electrode either (1) for reduction or oxidation of the gas or (2) produced via electrolytic generation. Circuit connections are disposed between the counter and gas diffusion electrodes. Additionally, the counter electrode may also be a gas diffusion electrode. A well-known example of such a design is the H.sub.2 /O.sub.2 fuel cell.
Electrochemical batteries, for example, the metal-air type, commonly utilize either an aqueous alkaline or neutral (e.g., saline) electrolyte, while fuel cells may commonly utilize either acidic electrolytes or alkaline electrolytes. Other types of electrolytes are also used, depending upon the specific gas which is consumed or generated.
The use in electrochemical batteries of an oxygen-containing gas, such as air, which is reduced at the gas diffusion electrode is well known. However, the gas need not be oxygen-containing nor need it be reduced at the gas diffusion electrode. For example, hydrogen gas is oxidized in some fuel cells. The present invention is generally applicable to all such types of gas diffusion electrodes and cells.
The electronically conductive material in a gas diffusion electrode typically may be carbon. Additionally, a wide variety of catalysts, such as platinum or transition metal organometallic catalysts (such as porphyrins), are available.
In various applications, it is desirable that either or both the liquid electrolyte and the gaseous electrode reactant be flowed through the body of the cell over the electrode surfaces. Flowing electrolyte and/or flowed gaseous reactant are, of course, accompanied by a pressure drop across the cell, especially on the electrolyte side. This can be lead to excess pressures either on the gas-side or the electrolyte-side of the electrode. Furthermore, it may be desirable in certain circumstances to operate at an elevated gas pressure with respect to the electrolyte pressure. One example of such a situation would be one in which the performance is increased by pressurizing the gaseous reactant. In battery and fuel cell applications, it is desirable to obtain as high a cell voltage as possible at any given current density. One means of accomplishing this is to utilize a relatively high gas pressure or flow rate.
The use of a porous (e.g., typically 30%-60% porosity) gas diffusion electrode, however, poses difficult flow management problems. When gas pressure exceeds liquid electrolyte pressure by a sufficient amount, "blow-through" of gas through the electro
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Gordon Arnold Z.
Hossain M. Sohrab
Tryk Donald A.
Yeager Ernest B.
Gorgos Kathryn
Niebling John
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