Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-04-25
2002-04-09
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S265000, C204S266000, C204S261000, C204S283000
Reexamination Certificate
active
06368473
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a sodium chloride electrolytic cell provided with a gas diffusion electrode. More particularly, the present invention relates to a sodium chloride electrolytic cell provided with a gas diffusion electrode which allows smooth supply and discharge of catholyte as well as allows oxygen gas to come in good contact therewith.
BACKGROUND ART
A gas diffusion electrode is normally used as an oxygen electrode for fuel cell or electrolysis of sodium chloride and is internally composed of a gas supply layer and a reaction layer.
The outline of the function and structure of a gas diffusion electrode is described below taking as an example an oxygen cathode to be used as a cathode in the ion exchange membrane process electrolysis of sodium chloride. In general, the ion exchange membrane process electrolysis of sodium chloride involves electrolysis in an electrolytic cell comprising an anode chamber and a cathode chamber divided by a cation exchange member, the anode chamber being provided with an anode and filled with an aqueous solution of sodium chloride and the cathode chamber being provided with a cathode and filled with an aqueous solution of caustic soda. One of these ion exchange membrane process sodium chloride electrolytic cells is an electrolytic cell comprising as a cathode a gas diffusion electrode which supplies a gas containing oxygen, i.e., oxygen cathode. This type of an electrolytic cell comprises a cathode chamber provided with a gas supply chamber and composed of a gas diffusion electrode arranged to supply an oxygen-containing gas onto the cathode therefrom and an electrolytic solution chamber filled with an aqueous solution of caustic soda.
In this arrangement, the use of a gas diffusion electrode arranged to supply an oxygen-containing gas onto the cathode (gas diffusion electrode made of a porous material which supplies an oxygen-containing gas from the gas supply chamber, hereinafter simply referred to as “oxygen cathode”) in electrolysis in the electrolytic cell involving energization across the gap between the anode and the cathode gives an advantage that the reduction reaction of oxygen by hydrogen takes place on the oxygen electrode to lower the cathode potential, remarkably lowering the required electrolysis voltage.
The oxygen cathode comprises a thin layer mainly composed of a porous conductor. In the oxygen cathode, the conductor layer is hydrophobic on the gas supply chamber side while the conductor layer is hydrophilic on the electrolytic solution side. Further, the cathode is air-permeable as a whole. Moreover, the cathode is permeable to electrolytic solution on the electrolytic solution side conductor layer. The electrolytic solution side conductor layer in contact with the electrode electrolytic solution, e.g., aqueous solution of caustic soda in the case of electrolysis of sodium chloride, is internally provided with a collector made of a metal gauze.
In general, the foregoing porous conductor is mainly made of carbon black. The porous conductor comprises a catalyst made of a noble metal such as platinum supported thereon in the pores. The oxygen cathode is made of a water-repellent porous thin layer which causes no leakage of electrolytic solution on the oxygen-containing gas supply side thereof. The foregoing water-repellent porous thin layer is normally prepared by forming a mixture of a particulate fluororesin-based polymer resistant to redox reaction and water-repellent carbon black.
The foregoing porous thin layer having such a catalytic activity has an integrated structure obtained by forming a mixture of hydrophobic carbon, water-repellent carbon and particulate fluororesin such that the composition of the layer shows a stepwise change from the hydrophilic surface in contact with the electrolytic solution to the water-repellent porous thin layer on the gas supply chamber side. Accordingly, the porous oxygen cathode can efficiently supply an oxygen-containing gas from the oxygen-containing gas supply side to the side in contact with the electrolytic solution. Further, the electrolytic solution can easily penetrate and diffuse into the electrode from the side in contact with the electrolytic solution but doesn't leak into the gas supply chamber.
Thus, in the presence of sodium ion supplied from the side in contact with the electrolytic solution and the foregoing catalyst, water is oxidized in the oxygen cathode to hydroxyl group, producing caustic soda.
Further, unlike the earlier process of electrolysis of an aqueous solution of sodium chloride free from oxygen cathode involving the production of hydrogen at the cathode, the foregoing electrolysis process using an oxygen cathode is not liable to production of hydrogen, making it possible to lower the electrolysis voltage.
Thus, the outline of the function and structure of the oxygen cathode (gas diffusion electrode arranged to supply an oxygen-containing gas) used in an ion exchange membrane process sodium chloride electrolytic cell has been described. The outline of the function and structure of an ordinary gas diffusion electrode is similar to that described above.
In the case where a gas diffusion electrode is used as an oxygen cathode in the conventional ion exchange membrane type sodium chloride electrolytic cell, a liquid-impermeable gas diffusion electrode is normally used to form a three-chamber structure. In practical sodium chloride electrolytic cells, e.g., vertical electrolytic cell having a height as great as 1.2 m or more, electrolysis is conducted with the electrolytic solution chamber filled with the electrolytic solution. Thus, the gas diffusion electrode is subject to liquid pressure developed by the electrolytic solution at the lower portion thereof. In other words, the liquid pressure on the upper portion of the gas diffusion electrode in the vicinity of the surface of the electrolytic solution in the cathode chamber is closed to atmospheric pressure, but the liquid pressure on the lower portion of the gas diffusion electrode in the vicinity of the bottom of the cathode chamber is the sum of atmospheric pressure and liquid pressure based on the height of the electrolytic solution (liquid head)
When the vertical electrolytic cell is provided with a gas diffusion electrode as oxygen cathode and is then supplied with the electrolytic solution, the gas diffusion electrode is subject to a great liquid pressure at the lower portion thereof but is subject to little liquid pressure at the upper portion thereof, making a pressure differential between the two portions. This pressure differential causes liquid leakage from the catholyte chamber to the gas chamber at the lower portion of the gas diffusion electrode. When the liquid pressure and the gas pressure are adjusted equal to each other at the lower portion of the catholyte chamber to prevent liquid leakage, the gas pressure in the gas diffusion electrode is higher than the liquid pressure at the upper portion of the catholyte chamber, causing the leakage of gas into the electrolytic solution at the upper portion of the gas diffusion electrode.
Further, when operation is conducted with the liquid pressure being higher than the gas pressure, if the gas diffusion electrode is not highly water-resistant and sufficiently sealed, the electrolytic solution leaks into the gas chamber in a large amount, inhibiting the supply of gas and hence deteriorating the electrode performance and life. In particular, the use of a gas diffusion electrode having a low resistance to water pressure is restricted.
As shown in
FIG. 11
, the cathode chamber in the foregoing conventional electrolysis chamber comprises a sheet-shaped gas diffusion electrode
31
placed on a cathode metal gauze
32
mounted on a cathode chamber frame (not shown). In this arrangement, when a pressure is applied to the gas diffusion electrode
31
at the caustic chamber
33
side thereof, the gas diffusion electrode
31
is pressed against the cathode metal gauze
32
to come in contact with the cathode metal gauze
32
so
Aikawa Hiroaki
Furuya Nagakazu
Katayama Shinji
Saiki Koji
Sakata Akihiro
Bell Bruce F.
Furuya Nagakazu
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