CA1265471A

CA1265471A - Electrode with reversible polarity

Info

Publication number: CA1265471A
Application number: CA000466840A
Authority: CA
Inventors: Peter Fabian; Stephan Kotowski; Bernd Busse
Original assignee: Heraeus Elektroden GmbH
Current assignee: De Nora Deutschland GmbH
Priority date: 1983-11-02
Filing date: 1984-11-01
Publication date: 1990-02-06
Anticipated expiration: 2007-02-06
Also published as: ES291668Y; AU3614484A; WO1985001965A1; DE3476823D1; ES291668U; EP0140287B1; AU568172B2; EP0140287A3; EP0140287A2

Abstract

ABSTRACT OF THE DISCLOSURE
Electrode of reversible polarity for electro-chemical applications, especially those where the electrolyte contains also calcium and magnesium. A
multilayer structure is formed that comprises a carrier (1) of a valve metal, a conductive ceramic layer (2) applied onto the carrier and an electrocatalytically active top layer (3). This structure results, on the one hand, in protecting of the carrier (1) and, on the other hand, in the stability of the catalytic layer (3) also in cathodic operation. A continuous charge reversal does not damage the electrode. The equal surfaces of anodes and cathodes may be positioned parallel or substantially parallel facing each other.

Description

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The invention relates to an electrode of reversible polarity for electrochemical use, expecially to be used in an electro-flotation process, which electrode comprises a carrier, an intermediate layer and an electrocatalytic top layer.
The invention is directed to electrodes o reversible polarity i.e. those which can work successively as an anode and as a cathode within a given amount of time. Applications of such electrodes are also taught herein-There are numerous electrochemical methods inwhich a certain content of calcium and magnesium can be eliminated from the electrolyte.
An example of such a method is, firstly, the production of hypochlorite by the electrolysis of sea water, and secondly, the electrolysis of waste water whereby oxygen and hydrogen are produced which, in turn, are used for electro-flotation.
In such cases, compounds containing calcium and/or magnesium often deposit on the cathode, thus strongly restraining further flow of current through the cathode and causing an intolerable increase in the cell voltage.
Water softening is a third problem. However, in this case the calcification of the cathodes is a desirable efect since the depositing of calcium and or magnesium compounds on the cathode leads to a decrease in water hardness.
Various methods of the removal of calcium containing deposits have been proposed thus far:
a) scrubbing the electrolysers with an acid (preferably HCl) from time to time or periodical cleaning with hydrochloric acid.
b) applying an auxiliary cathode to function a~
an anode against the actual cathode, wherein the auxiliary ~s~
-- 2 ~
cathodeand the actual cathoceare periodically automatically interchanged in ihe electrical sense (West German Offenlegungsschrift 3043571, issued July 1,1982, inventor ~.J. Jansen) c) automatic mechanical cleaning of the electrodes.
d) an electrode of reversible polarity, made of a solid noble metal or provided with a noble-metal coating of such a thickness that neither the electrolyte nor daposits, if any, can penetrata the coating.
The method a) involves temporary interruption of the operation and also storage as well as disposal of hydrochloric acid. I~ is, therefore, practicable to use this method only in l~rge plants where permanent staff is available.
lS The method b) calls for complex mechanical and electrical equipment; moreover, an increased corrosion of the cathodes cannot be avoided.
The method c) is practicable only when the content of calcium or magnesium in the solution is very low.
As regards the method d), based on the prior art, the reversible-polarity electrode would have had to be made of solid noble metal such as Pt or Ir, since only these metals have sufficient electrical conductivity to match the required level of current and also sufficient electro-chemical resistivity to be able to function simultaneously alternately as anode or cathode. Such electrodes, however, are very expensive.
It is further Xnown in the prior art to install parallel wires made of, for instance, titanium coated with an electrocatalytic layer, to function as anodes or cathodes in the electrolysis of liquid sewage to produce oxygen and hydrogen to be used, in turn, for electro-flotation, or also in the electrolysis of other low-conducting aqueous solutions for other purposes, e.g.
prèparation of drinking water.

Due to low conductivity of the solutions used, the transfer of current is concentrated on these parts of the wires that are disposed directly opposite each other, while those parts that are still opposite each other but are more spaced from each other contribute almost nothing to the transfer or current through the solution.
It is an object of the present invention to provide a long-life ~Qlectrode from which the aforesaid deposits can be removed in a simple manner and which is stable under operatiny conditions. Moreover, elimination of the potential drop between individual conductors of the electrode over the cross-section ("current shadows") is also desired.
According to the invention, an electrode of reversible polarity is provided for electrochemical use, particularly for the electro-flotation, comprising a carrier, an intermediate layer and an electrocatalytically active top layer, comprising in combination: a~ a carrier made of valve metals and having the form of plates,

2~ gratings, lattices, nets, meshes, perforated disks, sectional shaped elements, rods, bars, wires etc.; b) a conductive ceramic layer as an intermediate layer, applied by flame spraying or plasma spraying and selected from the group consisting of an o~ide, nitride, boride, carbide, and silicide of a valve metal; c) an electrocatalytic active layer as a cover layer, made of metal or metals of the platinum group, rhenium, mixtures or solid solutions of the compounds of said metals with the said valve metal compounds.
In one preferred embodiment, the electrode constitutes an arrangement of rods or bars of rectangular profile or cross-section, the rods being fastened to bus bars in a comb-like fashion, wherein two such electrodes face each other with their equal surfaces disposed generally parallel to each other.

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The application of rods of rectangular profile, disposed substantially parallel and always at an equal distance to each other results in the elimination of "current shadows" and thus in a substantial increase of 5 the total e~ficiency of the electrode.
According to the invention, cell voltage is prevented from rising since any cathodic build-up or incrustation is in turn dissolved in anodic operation.
It is to be mentioned tha~ in the above examples the electrolyte is acidic at the electrode that acts as an anode and alkaline at the electrode that acts as a cathode. The deposits under discussion build up in the alkaline solution and dissolve in the acidic solution.
~ow, while a quick reversal of polarity hinders the build-up of calcium and/or magnesium compounds on the electrodes whereby the first and second of the aforesaid problems are solved, a relatively slower reversal of polarity makes the deposited compounds fall off so that the compounds can be subsequently filtered off whereby the third problem is eliminated.
The invention is explained below in more detail in conjunction with figures 1-3 of the drawing.
FIGURE l shows the layered structure of an electrode of the invention; FIGURE 2A is a cross-sectional view of two parallel round wires as electrodes, FIGURE 2B
is a cross-sectional view of two parallel electrodes of rectangular cross-section; FIGURE 3 shows the layout of individual electrode bars and their power supply means, arranged horizontally or vertically in the electrolyzer.
Referring to FIGURE l, electrodes of reversible polarity may be made of a valve metal that acts as a mechanical carrier 1 and as an electric power supply means. Onto the valve metal is applied a conductive ceramic layer 2; further, the ceramic layer 2 is provided with an electrocatalytic layer 3 which is active in both anodic and cathodic mode.

Examples of valve metals to make the carrier of are Ti, Zr, Hf, Nb, Ta, W as well as their mixtures or alloys inasmuch as they are anodically stable (in anodic operation).
The materials suitable for the ceramic layer 2 are, for instance, partially reduced titanium dioxide (TiO2 x) or TiC or also other compounds of such elements as Ti, Nb, Ta, Zr, Hf, W and Mo on the one hand and 2' ~, B, Si or C on the other hand.
By way of example, the electrocatalytic active layer 3 is a coatin~ that consists of oxides of valve metals such as, eOg. Ti, ~b, Zr, Ta, Hf, W, M0 and oxides of platinum group metals such as Pd, Pt, Rh, Ru, Ir and Os and also of Re, wherein also matter not yet oxidic may be lS present. Mixed oxides of the above-mentioned metals are particularly active. Layers consisting only of metals such as Pt or Ir may also be used.
Likewise, layers consisting of platinum-metal oxides and platinum metals may be used, e.g. a layer ~0 comprising platinum oxide in addition to Pt and Ir.
The ceramic layer 2 can be applied, for instance, by means of plasmatic spraying.
The electro-catalytic active layer 3 can be ~pplied in a following manner~
~5 a solution containing compounds of one or more metals selected from the group of Ti, Nb, Zr, Hf, W, Mo and of one or more elements selected from the group of Pd, Pb, Rh, Ru, Ir, Os and Re is applied onto a suitable ceramlc coated carrier, dried out and subsequently txeated at the temperat~ure of ca. 500C for about 15 minutes under air.
The compounds suitable for this application are e.g.chlorides; mixtures of butanol and hydrochloric acid may be used as solvents. A conductive layer of partly reduced titanium dioxide applied by p]asma spraying is an example of a suitable substratum.

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The procedure is repeated until the electrocatalytic layer 3 reaches a desired thickness.
This method is well known in the technology of anode manufacturing for the chlorine-and-alkali industry.
Another method of forming the layer 3 involves the step of applying a coating in the above described manner, the coating subsequently being partly reduced under the current of hydrogen at a temperature of ca.
400C, where~y some platinum metal oxides liberate the metals. When this method is employed, the admixture of the value-metal oxides is often renounced.
A third method of forming the layer 3 consists of, e.g. the galvanic application of a platinum layer or a coating of an alloy of platinum metals.
As shown in FIGURE 2A, anodes 4 and cathodes 5 are made o titanium wires of circular cross-section and spaced a few millimeters from each other.
Due to the high electrical resistance, the potential between the wire and the adjacent solution at the contact places (current shadows) is lower by about 50-200 mV than at the reinforced peripheral areas 6,7 of anode 4 and cathode 5. A resistivity of 50 Q cm was assumed therefor as this is typical for such applications as electroflo~tation and preparation of drinking water. It is clear that in each case, only the reinforced peripheral area 6,7 of the wires contributes actively to the transfer of current.
As shown in FIGURE 2B, the electrodes 4', 5' are made of rectangular titanium bars. Owing to the parallel, planar arrangement of the ele trodes and the resulting uniform distance of their facing surfaces, the entire area of the facing surfaces of the electrodes contributes to the transfer of current.
The total radius of action is thus correspondingly enlarged.

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The wire-shaped electrodes 4,5, shown in FIG~RE
2A were spent after about eight weeks and had to be provided with a new electrocatalytic layer, while the electrocatalytic top layer on the rectangular titanium bar electrodes 4', 5' of FIGURE 2B still did not require to be renewed even after about thirty weeks. Other valve metals may also be used instead of titanium.
It is expedient to provide isolating spacers between the separate electrode bars since they are disposed paxallel at a distance of mere few millimeters rom each other.
It is advantageous to apply the layer that is active both in anodic and cathodic operation onto the entire surface of the bars, preferably by thermal spraying lS of the o~ides of platinum group metals in a mixture with the oxides of valve metals.
As shown in FIGURE 3, the rectangular electrodes 4', 5' are arranged ~s combs with meshing teeth wherein the comb backs serve as bus bars 8,9 to supply power to the electrodes 4', 5'.
In the course of experiments on electrodes of reversible polarity it appeared unexpectedly that some ceramic materials suitable for the conductive layer 2 are capable to protect the valve metals that constitute the carrier 1 from the formation of hydrides that tend to develop in these valve metals in the cathodic operation.
Although such cexamic materials have been known for a long time to be used in the production o~
electrodes, they have been deemed unsuitable for the cathodic process; due to the porosity and presence of crevices in the layers formed from these ceramic materials, they were assumed as not being capable of sufficient protection o~ the valve metal (carrier 1) against the penetration by hydrogen.

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The above holds true especially for plasma sprayed ceramic materials because they are much more porous than sintered materials or those applied by baking. On the other hand, however, even metal mesh or 5 wires of titanium can be coated using plasma spraying which is not possible by sintering.
The above-mentioned ceramic materials are completely insufficient as electrocatalysts both in anodic and in cathodic process. Therefore, it was necessary to provide the materials with the active electrocatalytic l~yer 3 as shown in FIGURE 1.
It appeared unexpectedly that anodic coatings, known to be stable at high current densitites in the chlorine-and-alkali electrolytic process, are also stable in cathodic operation provided that they are prepared in the above-described manner and that the density of current is sufficiently low.
Moreover, it turned out unexpectedly that the continuous alternation o~ charge does not damage the electrocatalytic layer 3.
This is at variance with the prior art teaching, se~ è.g. Juchnierwicz, Plat~ Met. Rev.6, 1962, 100-105, or R. Doblhofer et al., Ber. Buns. Ges. 82, (1978~, 10~6.
Examples.
The following examples are meant to explain the technical procedure in more detail; they do not constitute any limitation of the above-described embodiments of the electrodes.
A) A solution containing 8g NaCl per 1 liter of tap water (~ardness 15) was pumped through a cartridge cell such as described in West German Offenlegungsschrift 3138438 (issued April 14, 1983, inventors Fabian et al).
The volumetric flow as 6 l/h, the inner diameter of the cartridge was 40 mm and the length was 25 cm. A 10 A current (corresponding to about 600 A/M2) was passed through the cell.

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. g In the first embodiment the current was alternated every 15 minutes (the polarity was reversed);
the current was not alternated in the comparative e~periment.
In the first experiment no deposit was visible on either electrode even after 8 hours while in the comparative experiment the cathode side became light-white of calcium and/or magnesium compounds deposited thereon.
The cell voltage began to rise.
A continuous test was run in which the polarity was reversed, the same cell worked several months faultlessly under these conditions. The experiment was terminated when it became clear that no calcification or other damage to the electrodes would occur.
B) A similar system as in Example A was used, except that the current was reversed only every 2-3 hours. The electrolyte was tap water and the density of current was only about 100 A/m . Elere, the calcium and/ or magnesium compounds that deposited on the cathode blew off ~0 in a filterable form whereby the hardness of the effluent water decreased correspondingly.
Should the cathodic deposits partly dissolve when the process is reversed into anodic, then that part of water which contains the dissolved material could be discarded and ~5 thus the amount of softened water reduced correspondingly.
C) Titanium rods were provided with a plasma sprayed layer (intermediate layer) of partly reduced ~itanium dioxide. The rods were subse~uently coated with an electrolytically active coating as described in West German ~a Ofenlegungsschrift 2300422 (issued August 1, 1974, inventor Hund et al). For comparison purposes, other titanium rods were provided with the same electrocatalytic layer but the intermediate TiO2 layer was not applied.
The rods with the intermediate layer were welded together to form an electrode so that they were comb-shaped. The rods were interpositioned with the other ~ 7~

electrode rods while the backs of the rods served as electric terminals ~power supply means~.
A direct voltage was applied to the terminals and its polarity was reversed every 15 minutes. The density of current was 100 A/m2. The electrodes worked in tap water of hardness 15 and also in an industrial wastewater. The cell voltage in the tap water was about 6-7 V; in the wastewater, about 3-4 V.
The comparative electrodes worked in an identical system. The electrodes were considered out of duty when the cell voltage rose to ca. 15 V. Following results were obtained under these conditions.
Table Type of electrodeElectrolyte Longevity (day~
15 with intermediate layertap water 102 with intermediate layerwaste water 30 w/o intermediate layertap water 38 w/o intermediate layerwaste water 14 It may be appreciated that the use of the intermediate layer more than doubles the working life of the electrodes.

Claims

The embodiments of the invention in which an exclusive right or privilege is claimed are defined as follows:

1. In a method of electrolysis comprising passing electrical current from electrodes acting as anodes to electrodes acting as cathodes through an electrolyte containing impurities capable of being cathodically deposited and thereby increasing the electrical potential required to carry out said electrolysis, and wherein the polarities of said electrodes are intermittently reversed to inhibit excessive growth of said cathodically deposited impurities by redissolution or removal thereof during anodic polarization, the provision of at least one pair of substantially identical reversible polarity electrodes comprising:
- a valve metal substrate in form of at least one of: a plate, a grid, a rod, a wire;
- an intermediate layer in form of a conductive ceramic layer formed of an oxide, nitride, boride, carbide or silicide of a valve metal;
said intermediate layer having been applied to the support by flame spraying or plasma spraying; and - an electrolytically active covering layer comprising at least one of the group consisting of platinum group metals; rhenium; mixtures and mixed crystals of compounds of platinum group metals or rhenium with an oxide of a valve metal.

2. The method of claim 1, wherein each electrode comprises a holder element forming an electrical terminal for said electrode, a plurality of rods being arranged on said holder elements in essentially perpendicular relation thereto and at least essentially parallel to each other, each rod having essentially rectangular cross-section, a pair of electrodes comprising two holder elements opposing each other in parallel relationship with said rods of each holder element directed towards the other holder element thereby interengaging said rods of both holder elements in double-comb-like fashion.

3. The method of claim 2, wherein said rectangular rods of either electrode comprise surfaces opposing like surfaces of the other electrode in parallel relationship, the size of said opposing surfaces being essentially the same at both electrodes.

4. The method of claim 1, 2 or 3, for the production of hypochlorite or oxygen or hydrogen from seawater, for decalcification of an aqueous solution or for electroflotation.

5. In an electrolysis cell comprising anodes and cathodes arranged to pass electrical current through an electrolyte containing impurities capable of being cathodically deposited and thereby increasing the electrical potential required to carry out said electrolysis, and wherein the polarities of said anodes and cathodes are intermittently reversed to inhibit excessive growth of said cathodically deposited impurities by redissolution or removal thereof during anodic polarization, the provision of at least one pair of substantially identical reversible electrodes, said pair comprising an anode and a cathode; each of said anode and said cathode comprising:
- a valve metal substrate in form of at least one of: a plate, a grid, a net, an expanded net, a perforated disc, a profiled rail, and a rod or a wire, - an intermediate layer in the form of a conductive ceramic layer formed of an oxide, nitride, boride, carbide or silicide of a valve metal;
said intermediate layer having been applied to the support by plasma spraying, and - an electrolytically active covering layer comprising at least one of the group consisting of platinum group metals;
rhenium; mixtures and mixed crystals of compounds of platinum group metals or rhenium with an oxide of a valve metal.

6. Apparatus for the production of hypochloride, or oxygen and/or hydrogen from sea water, for decalcification of an aqueous solution or for electro-flotation, said apparatus comprising an electrolysis cell which includes anodes and cathodes arranged to pass electrical current through an electrolyte containing impurities capable of being cathodically deposited and thereby increasing the electrical potential required to carry out said electrolysis, and wherein the polarities of said anodes and cathodes are intermittently reversed to inhibit excessive growth of said cathodically deposited impurities by redissolution or removal thereof during anodic polarization, the provision of at least one pair of substantially identical reversible electrodes, said pair comprising an anode and a cathode; each of said anode and said cathode comprising:
- a valve metal substrate in form of at least one of: a plate, a grid, a net, an expanded net, a perforated disc, a profiled rail, and a rod or wire, - an intermediate layer in the form of a conductive ceramic layer formed of an oxide, nitride, boride, carbide or silicide of a valve metal;
said intermediate layer having been applied to the support by plasma spraying, and - an electrolytically active covering layer comprising at least one of the group consisting of platinum group metals;
rhenium; mixtures and mixed crystals of compounds of platinum group metals or rhenium with an oxide of a valve metal.

7. Tile apparatus of claim 6, wherein each electrode comprises a holder element forming an electrical terminal for the respective electrode, a plurality of rods being arranged on each said holder element in essentially perpendicular relation thereto and essentially parallel to each other, each rod having essentially rectangular cross-section, a pair of said electrodes comprising two holder elements opposing each other in parallel relationship with said rods of each holder element directed towards the other holder element of the pair thereby interengaging said rods of both holder elements in double-comb-like fashion.

8. The apparatus as claimed in claim 7, wherein said rods of one holder element comprise surfaces opposing similar surfaces of the rods of the other holder element, in parallel relationship, the size of said opposing surfaces being essentially the same at each rod.

9. Method for operating an electrochemical cell comprising electrodes, the electrolyte of which contains calcium and/or magnesium, wherein electrodes are used which on a carrier of valve metal comprise a conductive ceramic intermediate layer from the group comprising an oxide, a nitride, a boride, a carbide or a silicide of a valve metal, applied by plasma spraying, and an electrocatalytically active layer of metal(s) of the platinum group, rhenium, mixtures or mixed crystals of compounds of the aforesaid metals with the said valve metal compounds applied thereon as a coating layer, and wherein the electrodes are operated as anodes and cathodes with changing polarity.

10. Method according to claim 9, wherein the cell is equipped with electrodes comprising bars of rectangular profile or cross-section which are secured in a comb-like manner on contact bars two of these electrodes having electrode surfaces of the same size opposing one another in parallel or almost parallel positions.

11. Method according to claim 1 or 2 which is used for the production of hypochlorite from sea water or other waters containing calcium and/or magnesium.

12. Method according to claim 1 or 2 which is used for the production of oxygen and/or hydrogen from aqueous solutions containing calcium and/or magnesium.

13. Method according to claim 1 or 2 which is used for the decalcification of aqueous solutions.