GB2098238A

GB2098238A - An electrochemical cell

Info

Publication number: GB2098238A
Application number: GB8113968A
Authority: GB
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1981-05-07
Filing date: 1981-05-07
Publication date: 1982-11-17
Also published as: DE3277878D1; JPS5831090A; GB2098238B; US4472255A; JPS627275B2; EP0064417A1; EP0064417B1

Description

1

SPECIFICATION An electrochernical cell

GB 2 098 238 A 1 This invention relates to electrochemical cells useful for a variety of purposes, for example electrochemical reduction, or electrochemical oxidation.

In an electrochemical cell, the current efficiency is determined by the relative rates at which the 5 various ions present are discharged at the electrodes. One method of increasing current density which has been proposed and is well documented in the scientific literature (for example J. Applied Electrochem 7, 473 (1977); Desalination 13, 171 (1973); Electro Chimica Acta 22, 1155 (1977)) is the use of a so called "turbulence promotor" usually in the form of a mesh of plastic or some other inert material.

We have now discovered that, by arranging for a cell having a flowpath for electrolyte over at least one of its electrodes to be provided with a turbulence promotor substantially filling the said flowpath, a great increase in current efficiency can be achieved.

The invention therefore provides an electrochemical cell having an anode and a cathode, at least one flowpath over the anode or the cathode or both for electrolyte through the cell, and a turbulence 15 promotor in the said flowpath positioned so as to generate turbulence in substantially all the electrolyte flowing through the said flowpath.

We have also found that a particularly advantageous arrangement for a circulatory electrochemical cell, particularly a cell arranged as a bipolar stack, can be provided by providing electrodes with a dished formation, which are accommodated in generally rectangular frame members, 20 having a complimentary shape.

Accordingly, a second aspect of the invention provides an electrochemical cell comprising an electrically insulating frame member having a generally rectangular opening therein, and a pair of electrodes, at least one of the electrodes of the pair being engaged with the edges of the frame member and having a dished formation protruding into the said opening to define in the said opening one side of 25 a flowpath for a liquid electrolyte through the cell, the interior surface of one pair of opposed edges of the frame member being so shaped as to define between the said surface and corresponding portions of the said electrode, plenum chambers for electrolyte flowing through the cell, the interior surface of the other pair of opposed edges of the frame member being so shaped as to fle relatively close to corresponding portions of the said electrode so as to define at most only a relatively narrow flowpath for 30 electrolyte between the said electrode and the interior surface of the said other pair of opposed edges of the frame member. This configuration enables the provision of a narrow flowpath (with consequent high linear flow rates for a given rate of bulk electrolyte circulation), and also provides advantages in enabling a bipolar cell assemly to be operated with a small inter- electrode gap, whilst retaining a conventional electrolyte manifold system.

Advantageously, the turbulence promotor arrangement may be used with a dished electrode cell of the kind described above.

Cells according to the invention may preferably be provided with a cell divider, for example of an ion exchange membrane, when species existing in the anode and cathode compartments are mutually incompatible. The turbulence promoter maybe provided either on the cathode or on the anode side of 40 the cell divider, depending on which of the cell reations taking place it is desired to affect.

The cell frame members are preferably constructed of an insulating material, for example polytetrafluoroethylene, high density polyethylene, polypropylene, or polyvinyl chloride.

The cell anodes are preferably coated titanium e.g. titanium coated with ruthenium dioxide, platinum irridium, platinised titanium, lead dioxide, or anodised lead.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which:

FIGURE 1 is a vertical section through a part of a cell according to the invention, FIGURE 2 is a perspective view of a frame member used in the cell of Figure 1, FIGURE 3 is a section on 3-3 of Figure 1, FIGURE 4 is an enlarged view of part of Figure 3, showing the frame at member and sealing arrangement, and FIGURE 5 is a view similar to the view of Figure 1 of an alternative embodiment of a cell according to the invention.

Figure 1 shows one sub-cell of a bipolar stack which consists of a large number of individual 55 compartments 20 and 21, separated from each other by electrodes (for example 1 and 2). In a practical cell, a large number of sub-cells as shown in Figure 1 are assembled end to end, with the electrode providing the cathode of one sub-cell also providing the anode of the adjacent sub-cell. An external voltage is then applied across the end electrodes, so that each individual electrode polarises as shown in Figure 1. Electrodes 1 and 2 are dished to provide anode surfaces and cathode surfaces 6 and 4 and 5 60 and 3 respectively. The edges are sealed by welding, a small hole being left for expansion. The space between the two surfaces 5 and 6 (and 3 and 4) is filled with a polyurethane foam.

Electrodes 1 and 2 are spaced from each other, and from cell divider 7 by frame members 8.

Frame members 8 have a square recess 9 on each of their faces, to accommodate a sealing ring 10, to 2 GB 2 098 238 A 2 prevent leakage of electrolyte from the cell. it is preferred that the sealing ring 10 has a square section, rather than the more conventional "0" ring section, as this provides a larger area of contact with electrodes 1 and 2. and shows less tendency to cut through the cell divider 7.

Frame member 8 is generally rectangular in shape, and has horizontal arms 11 and 12, and vertical arms 13 and 14. Horizontal arms 11 and 12 are generally square in cross section, as shown in Figure 1. Vertical arms 13 and 41 are generally trapezoidal in cross- section, as shown in Figure 3. In Figure 4, it can be seen that the trapezoidally shaped limbs 13 and 14 are formed by securing a portion of triangular section, which is secured to a rectangular frame portion 16 by means of countersunk screws 17. The triangular section 15 may thus be removed and replaced by a different section depending on the shape of the electrode being used. Alternatively, section 15 may be secured to limb 10 16 to form the trapezoidal members 13 and 14 by an adhesive, or by welding. The frame member 8 may be formed of any suitable electrically insulating material, for example a plastic such as polypropylene or polyethylene. Each frame member 8 has provided therein inlets 18 and outlets 19 for electrolyte as can be seen in Figure 1, both inlets 18 and outlets 19 open into a plenum chamber defined by frame member 8, a 15 part of the electrodes 1 and 2, and the cell divider 7. Because of the trapezoidal shape of vertical limbs 13 and 14 of the frame 8, there is no corresponding chamber adjacent the vertical edges of the electrode. This arrangement ensures that electrolyte entering plenum chambers 20 and 21 via inlets 18 flow evenly over the surfaces 4 and 5 of electrodes 1 and 2.

As can be seen in Figure 4, the gap between the trapezoidal limbs 13 and 14 of the frame member 20 and the corresponding section 20 of the adjacent electrode is somewhat smaller in width than the distance between the cell membrane and the surface of the electrode in the region where the electrode is flat. If the gap is too wide, flow is lost from the active part of the face of the electrode, and if the gap is too small, or the triangular section 15 is of such a shape that no gap at all is formed, corrosion has been found to take place on the sides of the electrode.

Between the anode surface 4 and the cell divider 7 (i.e. in the cell anode compartment) there is provided a turbulence promotor 2 1. The turbulence promotor is preferably of a plastic mesh, such as PVC, polypropylene, polyethylene, polypropylene polyethylene copolymer, polytetrafluoroethylene, or, for non-acidic environments, nylon. The turbulence promotor substantially fills the whole of the electrolyte flowpath, i.e. the whole of the gap between anode surface 4, and the cell divider 7. Thus, substantially all of the electrolyte pumped through inlets 18, and out of outlets 19 during operation of the cell is caused to interact with the turbulence promotor.

Turbulence promotor 21 is on the anode side of cell divider 7 in the embodiment shown, because the reaction of interest (i.e. the reaction for which it is desired to achieve high current efficiency) is that taking place at the anode (e.g. the oxidation of metallic cations). If the cathodic reaction is of interest, a 35 turbulence promotor may be provided between cathode surface 5, and cell divider 7. Furthermore, if the cell reactions are such that a cell divider is not required, the turbulence promotor may fill the whole of the space between anode surface 5 and cathode surface 4.

The inlets 18 feeding cathode compartments are preferably connected together, as are the inlets to anode compartments. Similarly, cathode outlets 19 are generally interconnected, as are anode 40 outlets 19. A single circulatory pump may then be used to pump electrolyte through the cell compartments.

The cell illustrated in Figure 5 is in all respects similar to that illustrated in Figures 1 to 4, except that only the cathode surface 35 of each sub-cell has the dished shape, the anode surface 34 being flat, and no cell divider is used. The vertical arms (not shown) of the frame members 30, are again trapezoidal in shape so that the turbulence promotor 35 substantially fills the electrolyte flowpath from inlet 33 to outlet 32. Again, square section sealing rings 31 are used.

As indicated above, the use of turbulence promotors has been previously proposed, to increase the current efficiency of electrolytic reactions, which are mass transport limited. However, we have discovered that using the apparatus described above, an increase in current efficiency can be obtained 50 with electrolytic reactions which are not normally considered to be limited by mass transport. A good illustration of this is the oxidation of chromous (Cr3+) to chromic CrIl in aqueous sulphuric acid. This reaction is not mass transport dependant, but as can be seen by the results presented in Table 1 below, a significant increase in current efficiency of the process was obtained over conventional tank type and plate and frame type electrolytic cells, using the cell shown in Figures 1 to 4 above.

EXAMPLE 1

Using a cell as shown in Figures 1 to 4, and consisting of 4 bipolar electrodes, separated by cell dividers (nafion ion exchange resin) a 0.5 M solution of Cr31 in HIS04 (1 509/L) was pumped through the anode compartment of the cell, at a rate such as to give a linear flow rate of approximately 30 centimetres per second. The total applied voltage across the bipolar stack was 12 volts (i.e. 3 volts per 60 sub-cell).

The electrodes used were lead (99.9% purity), and the operating temperature was 4WC. Aqueous suifuric acid (5g/L) was pumped through the cathode compartments.

The current efficiency for two current densities is shown in Table 1, as compared with 3 GB 2 098 238 A 3 conventional tank type and plate and frame type electrolytic cells. The plate and frame type may be likened directly to a cell as shown in the accompanying drawings, but without the turbulence promotor.

TABLE 1

Current Density Current Efficiency Cell (a/M') (%) Tank type 1000 46 2000 30 Plate and Frame 1000 45 2000 50 Cell of Figure 1 1000 95+ turbulence promotor 2000 95+ As shown in Table 1, even at at a current density as high as 2000 A/M2, almost theoretical current efficiencies may be achieved.

EXAMPLE 2

A reaction which is normally mass transport dependant is the oxidation of cerous (Ce 3+) to ceric (Ce 4+) in aqueous sulphuric acid. A solution of 0.125 M Ce 31 in H2Sol (100 g/L) was oxidised to Ce 4+ in a cell of the kind described, using a current density of 1500 AM, at a cell temperature of 501C. The 10 current efficiency for various flow rates was as shown in Table 2.

TABLE 2

Cell Flow Rate Current Efficiency (cm/sec.) (%) Plate & Frame 10.5 30 Plate & Frame 19.3 30 Plate & Frame 21.5 30 Plate & Frame 30.5 42 Cell of Figure 1 with turbulence promotor 10.5 47.5 Cell of Figure 1 with turbulence promotor 21.5 62 Cell of Figure 1 with turbulence promotor 30.5 65 As the Table demonstrates, high current efficiencies can be obtained using the cell according to the invention, even at low flow rates.

EXAMPLE 3

Using a cell generally as shown in Figures 1 to 4 but consisting of only one pair of electrodes 15 separated by a cell divider consisting of a polyamide coated cation selective membrane metallic tin and bromine were recovered from a solution of tin bromide in dimethyiformamide.

The cathode was an acid resistant grade of stainless steel (grade 316) although any acid-resistant grade would be suitable, and the anode was titanium coated with ruthenium dioxide, alternative anode materials are other coated-titanium substrates such as platinised titanium or platinium irridium coated 20 titanium. The solution of stannous bromide in dimethylformamide (200 g/1) was pumped through the cathode compartment of the cell at a linear flow rate of 30 cm sec. An aqueous solution of sulphuric acid (5g/1) was pumped at a similar rate through the anode compartment of the cell. When the current was switched on the cell voltage was 3.5 V at a current density of 200 A/W. Metallic tin was deposited 4 GB 2 098 238 A 4 on the cathode at a current efficiency of 95% and bromine was evolved from the anode at a similar current efficiency. The metallic tin was recovered by dismantling the cell.

EXAM P LE 4 A cell as shown in Figure 5 was constructed from the following materials. The cell frame members were constructed from high grade chemically resistant High Density Polyethyiene. The anode was platinum-coated titanium and the cathode was a suitable acid-resistant stainless steel (316). The mesh type turbulence promotor 35 had a mesh size of 25 X 25 mm and was made from a high grade plastic material.

An electrolyte containing sodium bromide (140 g/1) and sodium bromate (200 g/1) was pumped through the cell at a flow rate of 30 cm/sec and current was passed to oxidise the bromide to bromite.10 Fresh sodium bromide was added periodically and electrolyte bled off to maintain the concentration at the same level. At a temperature of 601C and a current density of 2500 A/M2 the cell potential was less than three volts and the current efficiency was higher than 90%.

EXAMPLE 5

In a similar experiment using the cell as shown in Figure 5, a solution of sodium chloride (110 9/1) 15 was pumped through the cell at a flow rate of 30 cm/sec at a temperature of 8WC. At a current density of 3000 A/W the cell potential was 2.5 V and the current efficiency for sodium chlorate production was better than 95%.

High current efficiencies have been obtained using electrodes as large as 1 M' in area. The narrow inter-electrode gap lowers the cell potential, and thus leads to high power efficiencies. This is often 20 essential in situations where the species of interest in the electrolyte are present only in low concentrations, for example in the recovery of metals from dilute or poorly conducting non aqueous solutions, or in the oxidation or reduction of organic compound, where a non aqueous or mixed electrolyte of low conductivity is used.

Cellas as described above have in particular been found useful for the processes described in 25 British Patent Application No. 7942661, the disclosure of which is incorporated herein by reference.

Claims

1. An electrochemical cell having an anode and a cathode, at least one flowpath over the anode or the cathode or both for electrolyte through the cell, and a turbulence promotor in the said flowpath positioned so as to generate turbulence in substantially all the electrolyte flowing through the said 30 flowpath.

2. A cell as claimed in claim 1, wherein the flowpath is defined between the anode or the cathode and a cell separator.

3. A cell as claimed in claim 2, wherein the separator is an ion exchange membrane.

4. A cell as claimed in anyone of the preceding claims, wherein the turbulence promotoris a mesh 35 of PVC, polypropylene, polyethylene, a poiyethylene/polypropylene copolymer, polytetrafluoroethylene, or nylon. A cell as claimed in any one of the preceding claims, wherein the turbulence promotor has a mesh size of from 1 to 2 centimetres.

5. A cell as claimed in any one of the preceding claims, wherein at least one of the anode and cathode is made of lead.

6. An electrochemical cell comprising an electrically insulating frame member having a generally rectangular opening therein, and a pair of electrodes, at least one of the electrodes of the pair being engaged with the edges of the frame member and having a dished formation protruding into the said opening to define in the said opening one side of a flowpath for a liquid electrolyte through the cell, the interior surface of one pair of opposed edges of the frame member being so shaped as to define 45 between the said surface and corresponding portions of the said electrode, plenum chambers for electrolyte flowing through the cell, the interior surface of the other pair of opposed edges of the frame member being so shaped as to lie relatively close to corresponding portions of the said electrode, so as to define at most only a relatively narrow flowpath for electrolyte between the said electrode and the interior surface of the said other pair of opposed edges of the frame member.

7. A cell as claimed in claim 6, including a cell divider between the electrodes of the pair.

8. A cell as claimed in claim 7, wherein the cell divider is an ion exchange membrane.

9. A cell as claimed in any one of the claims 6 to 8 wherein both of the electrodes of the pair are dished and protrude into an opening in a frame member.

10. A cell as claimed in any one of the claims 6 to 9 including a plurality of electrodes mounted 55 between a plurality of the said frame members to form a bipolar stack.

11. A cell as claimed in claim 10 being a cell as claimed in claim 7 or claim 8, wherein the cell dividers are disposed between alternate frame members.

12. A cell as claimed in any one of the claims 6 to 11 including a turbulence promotor, filling at least a portion of the electrolyte flowpath.

13. A cell as claimed in claim 11 being a cell as claimed in claim 7 or claim 8, wherein the turbulence promotor substantially fills the gap between a dished electrode and the cell divider.

14 0 GB 2 098 238 A 5 14. A cell as claimed in any one of claims 6 to 13 wherein the said one pair of opposed edges of the frame member include an interchangeable portion to enable the frame member to be used with electrodes of differing shape.

15. An electrochemical cell substantially as hereinbefore described with reference to and as 5 illustrated by the accompanying drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1982. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained