Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-11-29
2002-05-07
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S265000, C204S266000
Reexamination Certificate
active
06383349
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electrolytic cells employing an oxygen cathode which are used for, e.g., sodium chloride electrolysis by the ion-exchange membrane method. More particularly, the invention relates to electrolytic cells employing a gas diffusion electrode as an oxygen cathode which can be improved in any of the following: a caustic solution can be effectively fed and discharged; caustic solution leakage through the gas diffusion electrode into the gas chamber can be effectively and appropriately coped with; a caustic chamber serving as an electrolytic solution passageway can be constituted so as to have an exceedingly small thickness; oxygen gas can be evenly fed to and discharged from the gas chamber having the gas diffusion electrode; a gas- and liquid-permeable gas diffusion electrode is used as the gas diffusion electrode to thereby enable a stable electrolytic operation to be continued at a high current efficiency; and power distribution in the electrolytic cell employing a gas diffusion electrode can be conducted so as to apply a voltage to a large area without considerably modifying the structure of a conventional electrolytic cell.
BACKGROUND ART
An electrolytic cell employing an anode, an ion-exchange membrane, and an oxygen cathode comprising a gas diffusion electrode has hitherto been proposed for use in sodium chloride electrolysis or Glauber's salt electrolysis.
In such a conventional electrolytic cell employing a gas diffusion electrode, e.g., an electrolytic cell for sodium chloride electrolysis, the electrolytic cell is constituted of elements including a cathode element, cathode collector frame, and caustic chamber frame and these elements have been assembled together with gaskets interposed therebetween. A caustic solution is fed and discharged through liquid inlets and outlets of a caustic chamber disposed in the cathode element. Since this electrolytic cell has the constitution described above, it necessitates gaskets for assembly.
Because of this, this electrolytic cell has a complicated structure and has had a problem that there is a high possibility that the caustic solution might leak out due to a decrease in sealing properties in the joints between members, e.g., in the gaskets.
This electrolytic cell has further had a problem that although there is a possibility that the caustic chamber of the cathode element might suffer electrolytic corrosion, it is difficult to plate the caustic chamber with a metal having resistance to corrosion by NaOH, e.g., silver, for corrosion prevention because the chamber has a complicated structure.
Furthermore, in the conventional ion-exchange membrane type electrolytic cell for sodium chloride electrolysis, in the case where a gas diffusion electrode is used as an oxygen cathode in place of the gas generation type cathode, a gas diffusion electrode which is liquid-impermeable is usually employed to constitute the electrolytic cell so as to have three chambers. In such a case, since the electrolytic cell for practical use has a height of 1.2 m or higher and the solution chamber thereof is filled with an electrolytic solution, a high fluid pressure attributable to the electrolytic solution is applied to a lower part of the gas diffusion electrode and this is causative of liquid leakage from the catholyte chamber to the gas chamber.
When a gas diffusion electrode is attached to such a vertical electrolytic cell and an electrolytic solution is fed thereto, then a difference in fluid pressure results. Namely, a high fluid pressure is applied to a lower part of the gas diffusion electrode as stated above, whereas almost no fluid pressure is applied to an upper part. This difference in fluid pressure is causative, in the lower part, of liquid leakage from the catholyte chamber to the gas chamber, and is causative, in the upper part, of gas leakage through the gas diffusion electrode to the electrolytic solution side.
Furthermore, when an actual electrolytic operation is conducted under such conditions that the fluid pressure is higher than the gas pressure for the gas diffusion electrode, then a large amount of the electrolytic solution (caustic solution) leaks out into the gas chamber in the case where the gas diffusion electrode has low water resistance and the sealing is insufficient. There has hence been a problem that this leakage inhibits gas feeding and reduces the electrode performance and electrode life.
In particular, gas diffusion electrodes having low water pressure resistance have limited uses.
In addition, if the gas chamber is filled with a caustic solution, this caustic solution further flows into a lower gas chamber for gas discharge or feeding (which has conventionally been formed in the frame of the electrolytic cell). In this case, since the lower gas chamber is corroded by the caustic solution, the inner surface of the lower gas chamber should be plated beforehand with a metal having resistance to corrosion by NaOH, e.g., silver. In the conventional electrolytic cell, however, it has been difficult to subject the inner surface of the lower gas chamber to corrosion-preventive plating because of the structure thereof. There has been a further problem that although the cathode collector frame has been sealed to the lower gas chamber with a gasket, insufficient sealing permits the caustic solution to flow into the cathode element and corrode the inside of the element. Furthermore, in some electrolytic cells, it has been difficult to attach a gas chamber to the existing cathode element due to the structure of the element.
Many of the gas diffusion electrodes for use in such electrolytic cells are usually composed of two layers, i.e., a reaction layer for subjecting a liquid reactant to an electrolytic reaction and a gas feed layer which is permeable to gases but impermeable to the electrolytic solution.
The reaction layer is constituted of a hydrophilic carbon black having a catalyst supported thereon, a hydrophobic carbon black, and polytetrafluoroethylene (PTFE). The reaction layer is produced by dispersing and self-organizing those materials in various proportions so as to form hydrophilic areas into which an electrolytic solution penetrates and hydrophobic areas to which a gas is fed. The reaction layer thus produced has been attached to a cell and used either as it is or after only the surface thereof is hydrophilized by adhering fine hydrophilic particles to the surface.
Moreover, a technique has been used in which a structure having through-holes and a high porosity is interposed between an ion-exchange membrane and the reaction layer of a gas diffusion electrode in order to secure electrolytic solution passageways between the ion-exchange membrane and the reaction layer of the gas diffusion electrode.
As a result, flows of an electrolytic solution have been secured. However, there has been a problem that the caustic chamber serving as a cathode chamber into which an electrolytic solution is to be introduced has an increased thickness and inevitably has increased electrical resistance and this necessitates use of a higher voltage.
With respect to a gas chamber having a gas diffusion electrode, it has conventionally been known that there is a relationship in which the higher the linear velocity of the oxygen which is in contact with the gas diffusion electrode serving as an oxygen cathode, the higher the rate of diffusion of the oxygen into the electrode.
Because of this, investigations have been made on: a technique for providing a gas chamber formed by press-molding a nickel sheet to form in a central part thereof a depression having the same size as a gas diffusion electrode, using the depression and the gas diffusion electrode to form a gas chamber, inserting into the chamber a nickel mesh serving as a spacer for securing oxygen passageways to constitute a gas chamber for the gas diffusion electrode and thereby form an exclusive gas chamber, forming in this gas chamber a space which enables oxygen to have a linear velocity necessary for sufficient diffusion into the
Aikawa Hiroaki
Katayama Shinji
Saiki Koji
Sakata Akihiro
Yamaguchi Kenzo
Bell Bruce F.
Toagosei Co. Ltd.
Young & Thompson
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