GB2127856A - Electrochemical cell - Google Patents

Abstract

An electrochemical cell comprises an insulating frame member 8 having a rectangular opening therein, and a pair of electrodes 4, 5, 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 opening to define in the opening one side of a flowpath (e.g. from inlet 18 to outlet 19 for an electrolyte through the cell, the interior surface of one pair 13, 14 of opposed edges of the frame member being so shaped as to define between the surface and corresponding portions of the said electrode, plenum chambers for the 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 electrode, so as to define at most only a narrow flowpath for electrolyte between the electrode and the interior surface of the other pair of opposed edges of the frame member. The narrow flowpath results in high electrolyte flow rates and a small inter-electrode gap. High current efficiencies can be obtained. The cell may include an ion exchange membrane 7 and a turbulence promoting mesh. <IMAGE>

Description

SPECIFICATION An electrochemical cell 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 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, (1973); Electro Chimica Acta 22, 1155 (1977)) is the use of a so-called "turbulence promotor" usually in the form of a mesh of plastics 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 such a turbulence promotor extending across substantially all of the said flowpath, a great increase in current efficiency can be achieved.

In accordance with the invention claimed in copending application No. 8113968 there is provided 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 mesh extending across substantially all of the said flowpath and arranged to generate turbulence in substantially all the electrolyte flowing through the said flowpath.

We have also found that a particular 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, having a complementary shape.

In accordance with the present invention there is provided 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 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 elecrode 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 assembly to be operated with a small inter-electrode gap, whilst retaining a conventional electrolyte manifold system.

Advantageously, the mesh 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 mesh may be provided either on the cathode or on the anode side of the cell divider, depending on which of the cell reactions 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, iridium, 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 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 subcell also providing the anode of the adjacent subcell. An external voltage is then applied across the end electrodes, so that each individual electrode polarises as shown in Figure 1. Electrodes 2 and 1 are dished to provide anode surfaces and cathode surfaces 6 and 4 and 5 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 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 1 3 and 1 4. 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 1 5 of triangular section, which is secured to a rectangular frame portion 16 by means of countersunk screws 1 7. The triangular section 1 5 may thus be removed and replaced by a different section depending on the shape of the electrode being used. Alternatively, section 1 5 may be secured to limb 1 6 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 plastics material such as polypropylene or polyethylene.

Each frame member 8 has provided therein inlets 1 8 and outlets 1 9 for electrolyte as can be seen in Figure 1, both inlets 1 8 and outlets 19 open into a plenum chamber defined by frame member 8, a 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 1 8 flows 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 1 3 and 14 of the frame member and the corresponding section 4a 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 1 5 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 promoting mesh 21A.

The mesh is preferably of a plastics material, such as PVC, polypropylene, polyethylene, polypropylene/polyethylene copolymers, polytetrafluoroethylene, or, for non-acidic environments, nylon. The mesh size is preferably from 1 to 2 centimeters. The mesh 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 mesh.

The mesh 21 A 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 metalic cations). If the cathodic reaction is of interest, a mesh 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 mesh may fili the whole of the space between anode surface 5 and cathode surface 4.

The inlets 1 8 feeding cathode compartments are preferably connected together, as are the inlets to anode compartments. Similarly, cathode outlets 1 9 are generally interconnected, as are anode outlets 1 9. 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 mesh 36 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 promoting meshes 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 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 (Cre+) in aqueous sulphuric acid. This reaction is not mass transport dependent, 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 Cr3+ in H2SO4 (150 g/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 sub-cell).

The electrodes used were lead (99.9% purity), and the operating temperature was 400 C.

Aqueous sulfuric acid (5 g/L) was pumped through the cathode compartments.

The current efficiency for two current densities is shown in Table 1, as compared with 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 mesh.

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

EXAMPLE2 A reaction which is normally mass transport dependent is the oxidation of cerous (Ce3+) to ceric (Ce4+) in aqueous sulphuric acid. A solution of 0.125 M Ce3+ in H2SO4 (100 g/L) was oxidised to Ce4+ in a cell of the kind described, using a current density of 1 500 AIM2, at a cell temperature of 500 C. The current efficiency for various fjow rates was as shown in Table 2.

TABLE 2 Current Flow Rate Efficiency Cell (cm/sec.) (%) Plate 8 Frame 10.5 30 Plate 8 Frame 19.3 30 Plate 8 Frame 21.5 30 Plate s Frame 30.5 42 Cell of Figure 1 10.5 47.5 with mesh Cell of Figure 1 21.5 62 with mesh Cell of Figure 1 30.5 65 with mesh 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 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 dimethylformamide.

The cathode was an acid resistant grade of stainless steel (grade 316) although any acidresistant grade would be suitable, and the anode was titanium coated with ruthenium dioxide, alternative anode materials are other coatedtitanium substrates such as platinised titanium or platinium irridium coated titanium. The solution of stannous bromide in dimethylformamide (200 g/l) was pumped through the cathode compartment of the cell at a linear flow rate of 30 cm sec. An aqueous solution of sulphuric acid (5 g/l) 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/M2. Metallic tin was deposited 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.

EXAMPLE 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 Polyethylene.

The anode was platinum-coated titanium and the cathode was a suitable acid-resistant stainless steel (316). The mesh 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/l) and sodium bromate (200 g/l) was pumped through the cell at a flow rate of 30 cm/sec and current was passed to oxidise the bromide to bromate. Fresh sodium bromide was added periodically and electrolyte bled off to maintain the concentration at the same level. At a temperature of 600C 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 g/l) was pumped through the cell at a flow rate of 30 cm/sec at a temperature of 800C. At a current density of 3000 A/M2 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 M2 in area. The narrow inter-electrode gap lowers the cell potential, and thus leads to high power efficiencies. This is often 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.

Cells as described above have, in particular, been found useful for the processes described in British Patent Serial No. 2065702, the disclosure of which is incorporated herein by reference.

Claims (10)

1. 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 between the said surface and corresponding portions of the said electrode, plenum chambers for the 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.

2. A cell as claimed in claim 1, including a cell divider between the electrodes of the pair.

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

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

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

6. A cell as claimed in claim 5 being a cell as claimed in claim 2 or claim 3, wherein the cell dividers are disposed between alternate frame members.

7. A cell as claimed in any one of the preceding claims including a mesh, filiing at least a portion, but not substantially all, of the electrolyte flowpath.

8. A cell as claimed in claim 7 wherein the mesh is composed of PVC, polypropylene, polyethylene, a polyethylenelpolypropylene copolymer, polytetrafl uoroethylene, or nylon.

9. A cell as claimed in any one of the preceding claims 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 shapes.

10. An electrochemical cell as claimed in claim 1 and substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.