Bifurcated electrode of use in electrolytic cells

Details Bifurcated electrode of use in electrolytic cells Bifurcated electrode of use in electrolytic cells

2001-03-22

2003-03-04

Bell, Bruce F. (Department: 1741)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S288000, C204S288200, C204S289000, C204S280000, C204S275100, C204S278000, C204S278500

Reexamination Certificate

active

06527923

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to bifurcated metal electrodes, particularly formed of nickel of use in electrolytic cells, particularly water electrolyser cells for the production of hydrogen.
BACKGROUND OF THE INVENTION
In the field of electrolysis, there are two types of electrochemical cell stacks, namely, monopolar cell stacks and bipolar cell stacks. Bipolar cells have electrodes that operate with one side of the electrode as an anode while the other side operates as a cathode. Monopolar electrodes operate with one side or both sides as either an anode or as a cathode, but not both. The difference in these electrode types means that bipolar cells are connected in series with only 2 electrical connections one at each end of the cell stack while monopolar cells have multiple parallel type electrical connections. Stack construction is simpler and usually less expensive for bipolar cells than for monopolar cells.
With either type of aforesaid cell, electrolyte channel(s) must be present for each electrode. For cells which produce products at the anode and cathode surfaces that should not be mixed, a membrane, diaphragm or separator is placed in the electrolyte channel which separates the cell into anolyte and catholyte compartments. The membrane allows only selective ions to pass between compartments, the diaphragm allows electrolyte to pass but prevents gaseous products from passing between compartments, while a separator can be used to isolate cell compartments in some monopolar designs. The electrolyte channel is necessary for the operation of the cell since the electrolyte has the ions required to complete the electrical circuit. Both types of cells, therefore, have voltage losses due to the resistance of the electrolyte. Decreasing the electrolyte channel width by placing the electrodes closer together reduces this electrolyte resistance but increases the difficulty in supplying sufficient electrolyte to allow the products to escape from the cell.
Electrodes can be made from rods, tubes, meshes and conductive particulates. However, for commercial high production rates the filterpress design with plate electrodes is most common. Porous three-dimensional electrodes having high surface area have been described. U.S. Pat. No. 6,086,733 describes an electrochemical cell with a porous material covering the cathode in which fluid is forced to travel and improve mass transfer. These electrodes are particularly useful for plating of very dilute species from aqueous waste waters. Unfortunately, with gas evolving reactions, porous electrodes are not desirable because they trap the gases produced and, hence, increase the resistance of the gas-filled electrolyte.
Other electrode shapes have been described to direct the flow of gas evolved from the electrode surface. U.S. Pat. No. 4,464,242 teaches the use of an electrode, which has essentially fins to direct the flow of gas and electrolyte. However, this electrode does not decrease electrolyte resistance in the cell.
U.S. Pat. No. 5,480,515 teaches the use of a shaped electrode to move electrolyte to the electrode surface under the influence of gas release by means of channels. The electrode design does not reduce the electrolyte resistance.
U.S. Pat. No. 4,424,106 teaches a shaped electrode for anodes and cathodes in a filter press cell stack for improvement of electrolyte and gas flow within the cell. Mixing and fluid flow are affected by the shaped electrodes but there is no reduction in cell resistance with this design.
U.S. Pat. No. 4,469,580 teaches an electrode design which increases surface area and is self-supporting. The increase in surface area is achieved by having a ribbon-like screen project into the flow channel. The screen must be used in a monopolar cell and must have flow on both sides of the electrode. The electrode shape is not suitable for narrow gap cell designs because of the physical width of the electrode and the need for electrolyte flow on both sides of the electrode. Any advantage in electrolyte resistance reduction due to projection of the screen ribbon structure will be offset by gas hold-up due to the same.
Thus, the prior art does not teach how to, both, reduce electrolyte resistance and to keep an open electrolyte channel for escape of gaseous products.
There is, therefore, a need for an improved electrode which addresses both of the aforesaid and other issues.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a bifurcated electrode that offers substantial improvements in voltage and efficiency in a water electrolyser cell. It also provides longer-term electrode stability, opportunity for periodic depolarization and increases cell efficiency. It also may utilize relatively cheap nickel and its fabrication is compatible with present cell designs.
Accordingly, in one aspect the invention provides an electrode of use in an electrolytic cell comprising:
an electrically conductive first metal sheet having an electrochemically active gas-evolving planar surface;
an electrically conductive second metal electrochemically active gas-evolving screen intimately adjacent and parallel to said planar surface to define an electrolyte and gas-evolving chamber between said sheet and said screen having a narrow width.
The term “screen” includes, for example, mesh, grid and the like. The term “sheet” includes, for example, plate, foil, or other members having a planar surface.
Preferably, the sheet has peripheral portions to which, upstanding, screen portions, are welded to form an electrode of unitary construction having an aqueous electrolyte and gas-evolving space, channel or chamber of a sufficiently narrow width as to be practicable in providing minimal inter sheet-screen separation. Useful efficacious widths are selected from 0.3 mm−2 cm.
The bifurcated electrode, according to the invention, reduces the electrolyte resistance, but still maintains an electrolyte flow channel for electrolyte movement and escape of reactant products. The bifurcated electrode is particularly suited to electrolysis which produces gases, since the presence of non-conductive gas bubbles in a narrow electrolyte flow channel increases cell resistance substantially for a conventional electrode. Thus, the bifurcated electrode has two, in effect, distinct structures wherein one part is a conventional or primary plate electrode and the second part is a metallic screen or perforated plate responsible for the reduction of cell resistance. The metallic screen is electrically connected to the primary plate, preferably, near the edge of the electrolyte-exposed active plate area and parallel to the flow channel and extends across the flow channel from the plate to a membrane/separator, then along the surface of the membrane/separator and returns to the electrode plate essentially covering the boundary of the flow channel wall, but not interfering with the flow channel. By virtue of the screen's contact with the electrode plate, the current is conducted to the electrolyte adjacent the membrane/separator and, thus, reduces the resistance path of the electrolyte. A similar arrangement can be used in the other electrode compartment to reduce the total cell resistance.
The bifurcated electrode, thus, increases the electrode surface area and provides an electrode which is placed very closely to the membrane/separator without blocking the electrolyte flow or creating a flow channel which is most often too small to permit product from being efficiently removed. The extension to the primary electrode must, of course, be of a material which is conductive and screen-like. While a conductive sponge placed in the electrolyte flow channel would provide reduced cell resistance, it would trap gaseous products leading to displacement of the electrolyte and reduction or termination of the electrochemical reaction. Thus, a primary benefit of the bifurcated electrode of the invention is that the flow channel is left open while the electrolyte resistance path is minimized. Thus, the term “screen” as used herein does

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