GB1585070A - Electrochemical cell - Google Patents

Description

(54) ELECTROCHEMICAL CELL (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66-74 Victoria Street, London, S.W.1, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to electrochemical cells, particularly electrochemical cells capable of sensing the presence of an active component in a fluid, that is, an oxidisable or reducible component, such as hydrogen or oxygen in solution or in a gas mixture.

It is a disadvantage of many electrochemical cells that a background current flows which may vary during the life of the cell, and that the life may be short. A variation in background current may cause a variation in the operating potential of a reference electrode in the cell, and if a gas such as hydrogen or carbon monoxide is being sensed, then this variation is particularly deleterious because these gases are efficiently oxidisable only in a narrow range of potential, and drift out of the required range may occur.

One method of preventing a variation in background currents is by the use of an amperostatic circuit as described in the specification of U.K. Patent No. 1,385,201. However, such a circuit does not increase the life of the cell.

It is an object of the invention to provide an electrochemical cell in which background current is minimised by selection of the materials for its component parts.

According to the invention, an electrochemical cell sensitive to an active component of a fluid comprises a container for an electrolyte; a working electrode arranged for exposure to the fluid and to the electrolyte; and a platinum/platinum oxide reference electrode of area comparable to the working electrode positioned close to and parallel to the working electrode, and arranged for exposure to the electrolyte.

In a preferred form the working electrode is in the form of a metallised membrane, for example a metallised film of high polymeric material such as p.t.f.e. (polytetrafluoroethylene), and the electrolyte is a concentrated solution of an alkali. However, a dilute alkaline electrolyte or an acid electrolyte may also be used, and if fast response is not required, the working and reference electrodes may be conventional metal electrodes separated from the electrolyte by permeable membranes.

It is an advantage of the present invention in which the two electrodes are close and of similar size that distortion of the currentvoltage curves of the working electrode is reduced.

In a first form the invention further comprises a counter electrode arranged for exposure to the electrolyte, means for maintaining the working electrode at a potential which is fixed with respect to the reference electrode, and means for determining the current which flows through the working electrode. Said current is proportional to the concentration of active component in the fluid in contact with the working electrode. The means for maintaining the working electrode at the fixed potential may be a potentiostatic device electrically connected to the working electrode, the counter electrode, and the reference electrode which, by varying the potential applied to the counter electrode, maintains the working electrode at the required potential.

Such an arrangement is suitable for measuring the concentration of an oxidisable component of a fluid such as hydrogen or carbon monoxide in solution or in a gas mixture. The potential of the working electrode can be controlled to be in the potential range at which the oxidation of hydrogen or carbon monoxide is efficient.

Preferably the counter electrode is a palladium electrode. Alternatively the arrangement can be used to measure the concentration of a reducible component of a fluid, such as oxygen.

In a second form of the invention the electrochemical cell is capable of determining the concentration of hydrogen in a fluid. The working electrode comprises a metal capable of absorbing a substantial proportion of hydrogen, and there is provided means for determining the electrical potential of the working electrode with respect to the platinum/platinum oxide reference electrode. The working electrode is preferably palladium which may either be massive or deposited on a p.t.f.e. film. Alternatively the working electrode may be another metal having a lattice structure which is capable of expanding to accommodate absorbed hydrogen, such as, for example, titanium, zirconium, yttrium or nickel.The potential of the working electrode with respect to the reference electrode may be displayed on, for example, a chart recorder, and the difference between theta determined by eye. In such an arrangement, hydrogen from the fluid under test is absorbed by the working electrode and the potential of that electrode depends on the amount of absorbed hydrogen, provided that the hydrogen concentration does not exceed 0.03 when expressed in the ratio (hydrogen concentration in the alloy)/palladium concentration. When the source of hydrogen is removed, the absorbed hydrogen desorbs rapidly from the working electrode which returns to its original potential provided the concentration of hydrogen in the fluid was less than about 1%.

In a third form of the invention, the electrochemical cell is capable of determining the concentration of hydrogen in a fluid; the working electrode comprises a metal which does not appreciably absorb hydrogen, such as platinum, and there is provided means such as a chart recorder for determining the potential of the working electrode with respect to the platinum platinum oxide reference electrode. In such an arrangement, when the working electrode, again as a metallised membrane, is exposed to a hydrogen-containing fluid, the potential of the platinum working electrode is dependent on the hydrogen concentration.

It is an advantage of any form of electrochemical cell according to the invention that the use of a platinum/platinum oxide reference electrode provides a cell having improved stability, in that the background current does not vary, and that cell life can be long, e.g. 6 months. When a concentrated alkaline electrolyte is used, even if the electrode reactions are such that a small quantity of water is produced as a result of the electrode reactions involved, the pH is not affected. In any case, a platinum platinum oxide reference electrode is not very pH sensitive.

The material of the counter electrode, when provided, must be chosen depending on the electrolyte. When oxidation occurs at the working electrode, there is reduction at the counter electrode, therefore it must be composed of reducible ions otherwise unwanted gaseous evolution may occur. When a palladium counter electrode is used, protons are consumed, at the parts per million level and hydrogen is produced which is absorbed by the palladium. If a lead/lead oxide counter electrode is used, the lead oxide would be reduced to lead.

An electrochemical cell according to the invention is expected to find its most important use in measurement of oxidisable or reducible substances at parts per million concentrations, but it could be also capable of operating at higher concentrations; then some drift would be encountered for the invention of the second type; electronic means would be required to return the device to zero.

The invention will now be described by way of example with reference to the accompanying drawings in which: - Figures 1 (a) and 1 (b) illustrate the physical arrangement of one type of electrochemical cell according to the invention; and Figure 2 illustrates the electrical arrangement of the electrochemical cell and its control and measuring circuit.

In Figure 1, an electrochemical cell is shown in its three basic constituent parts, a central cylindrical body 10 shown in vertical section in Figure 1(a) and two outer half cylinders 12,14 shown trimetrically in Figure 1(b).

The central body 10 is hollow and in use contains an electrolyte which can pass to the outside of the cylinder through the plurality of apertures 16 spaced around the circumference of the cylinder. Two circumferential grooves 30, 32 encircle the outside of the cylinder above and below the apertures 16. The cell can be filled with electrolyte through filler holes 18, shown plugged. At the top of the cell is an insulating insert 20 which carries electrode mounts 22, 24, 26 for the counter, reference and working electrodes respectively. The mount 22 is connected to a palladium counter electrode 23 within the central body. The mounts 24, 26 are respectively connected through connecting wires 28, 29 to thin layers of cured conducting resin 31, 33 such as 'Araldite' (Registered Trade Mark) on the surfaces of the grooves 30, 32.The resin forms the contacts for working and reference electrodes (see below). The electrode mounts are covered by a protective cap 34 through which connecting wires 36 connect the electrochemical cell to the control and measuring circuits. At one point on the outer surface of the central body is a longitudinal groove 38 between the circumferential grooves 30, 32.

The inner sides of the outer half cylinders 12, 14 both have upper and lower circumferential grooves 40, 42, which match the grooves 30, 32 in the body 10, and have longitudinal flanges 44 having bolt holes 46 by which the half cylinders can be bolted together.

One half 12 also has on its inner surface a longitudinal raised strip 48 of length equal to the groove 38 in body 10. On either side of the raised strip 46 is a fishtail inlet 50 and outlet 51.

The reference electrode comprises a layer 25 of platinum/platinum oxide deposited on the outside of the central body 10, and in electrical contact with the resin 33 in the lower groove 32. In use a strip of filter paper 52 is wetted with electrolyte, such as 1.3 molar potassium hydroxide solution, and wrapped round the central body 10 to cover the apertures and the reference electrode 25. A strip of p.t.f.e. 54 is wrapped around the filter paper, the strip overlapping itself in the groove 38, and being wide enough to cover both the circumferential grooves. The strip 54 is held in place by 'O' rings 56, 58 which co-operate with the upper and lower grooves 30, 32 respectively.

The inner surface of the strip 54 is metallised as indicated by reference 55, the metallised area being such as to surround the body 10 covering the filter paper 52 and being in contact with the upper groove 30 so as to make electrical contact with the resin layer 31, through which the metallised area is connected to the working electrode mount 26 via connecting wire 28. The area of p.t.f.e. adjacent the lower groove 32 is not metallised.

The metallised area 55 constitutes the working electrode, and is parallel to, close to and of similar area to the reference electrode 25; the electrodes are electrically connected only through the electrolyte.

The two half cylinders 12, 14 are placed round the central body 10 and the halves are bolted together. The raised strip 48 co-operates with the groove 38 to assist with the location of the p.t.f.e strip 54. The cell is filled with the electrolyte through the filler holes 18, and a liquid under test is connected to the fishtail inlet 50 to flow around the cell in a narrow cylindrical space between the p.t.f.e. film 54 and the outer half cylinders and out of the fishtail outlet 51, the internal diameters of the half cylinders and the size of 'O' rings 56, 58 being chosen to provide this space and give a liquid-tight seal.

The liquid under test can contact the platinum working electrode 55 through the thin p.t.f.e. film, and this electrode contacts the electrolyte through the electrolye-soaked filter paper, which also holds the electrolyte in contact with the reference electrode 25.

In Figure 2, the electrochemical cell is indicated by reference 8, and the counter, reference and working electrodes 23, 25 and 55 are shown.

The counter electrode 23 is supplied through a first operational amplifier 60, such as a type 741, connected differentially to the reference electrode 25 and to a potentiometer 62 which is supplied with a constant voltage of 5.6 volts by a transistor 64 connected to a 12 volt supply. Use of the potentiometer 62 alters one input to the amplifier 60 and causes the amplifier to supply to the counter electrode 23 a current to equalise the inputs, thus maintaining the reference electrode, and the working electrode, at required potentials.

The working electrode 55 is connected differentially to the inverting input of a second operational amplifier 66, such as a type 741.

The other input is earthed, and a feedback circuit is supplied in the form of a selection switch 68 arranged to select one of three feedback resistors 70, 72, 74. The feedback resistors provide a scale-changing facility for measuring the current through the working electrode.

The output amplifier 66 is connected to the non-inverting input of a third amplifier 76, such as a type 741, and through a 10k resistor 78 to ground. The inverting input of amplifier 76 is connected to a 'set zero' arrangement comprising a potentiometer 80 supplied, via resistors 82, and 84, with constant voltage by a transistor 86 supplied from a 12 volt source.

The output current of the measuring circuit is supplied to terminals 88, so that a meter display can be provided which indicates the current through the working electrode; the meter can, if necessary, be calibrated to indicate directly the amount of oxidisable ion present at the working electrode.

In order to measure very low concentrations of an oxidisable ion, a substantially ripple free power supply must be provided.

If the cell is to be used in its second or third forms, then in Figure 1 the counter electrode 23 is removed, the entire circuit shown in Figure 2 is omitted, and the potential difference generated in the cell between the working electrode 55 and the reference electrode 25 can be connected directly to a differential voltmeter (or an electronic buffer amplifier to prevent polarisation of the electrodes) and the voltage displayed on a suitable chart recorder; this potential difference is related to the concentration of hydrogen at the working electrode. At low concentrations of hydrogen, at the parts per million level, the potential of the working electrode decreases when the source of hydrogen is removed.Even at higher concentrations, but still less than 1%, the decrease occurs when a platinum working electrode is used, but with a palladium working electrode there may be some zero drift at higher concentrations of hydrogen. At per cent concentrations, hydrogen may not desorb fast enough from either metal, and compensating circuitry may be necessary.

Typical results are, for an increase in hydrogen concentration in aqueous solution from zero to 1 part per million, 90% response in about 1 minutes, a change of 2.3 microamps per square centimetre of the working electrode and 6 millivolts; for an increase of hydrogen concentration from 103 to 104 parts per million in air, a change of 0.1 microamps per square centimetre and 9 millivolts.

WHAT WE CLAIM IS: - 1. An electrochemical cell comprising a container for an electrolyte; a working electrode arranged for exposure to a fluid under test and to the electrolyte; and a platinum/ platinum oxide reference electrode of area comparable to the working electrode positioned close to and parallel to the working electrode, and arranged for exposure to the electrolyte.

2. An electrochemical cell according to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. hydroxide solution, and wrapped round the central body 10 to cover the apertures and the reference electrode 25. A strip of p.t.f.e. 54 is wrapped around the filter paper, the strip overlapping itself in the groove 38, and being wide enough to cover both the circumferential grooves. The strip 54 is held in place by 'O' rings 56, 58 which co-operate with the upper and lower grooves 30, 32 respectively. The inner surface of the strip 54 is metallised as indicated by reference 55, the metallised area being such as to surround the body 10 covering the filter paper 52 and being in contact with the upper groove 30 so as to make electrical contact with the resin layer 31, through which the metallised area is connected to the working electrode mount 26 via connecting wire 28. The area of p.t.f.e. adjacent the lower groove 32 is not metallised. The metallised area 55 constitutes the working electrode, and is parallel to, close to and of similar area to the reference electrode 25; the electrodes are electrically connected only through the electrolyte. The two half cylinders 12, 14 are placed round the central body 10 and the halves are bolted together. The raised strip 48 co-operates with the groove 38 to assist with the location of the p.t.f.e strip 54. The cell is filled with the electrolyte through the filler holes 18, and a liquid under test is connected to the fishtail inlet 50 to flow around the cell in a narrow cylindrical space between the p.t.f.e. film 54 and the outer half cylinders and out of the fishtail outlet 51, the internal diameters of the half cylinders and the size of 'O' rings 56, 58 being chosen to provide this space and give a liquid-tight seal. The liquid under test can contact the platinum working electrode 55 through the thin p.t.f.e. film, and this electrode contacts the electrolyte through the electrolye-soaked filter paper, which also holds the electrolyte in contact with the reference electrode 25. In Figure 2, the electrochemical cell is indicated by reference 8, and the counter, reference and working electrodes 23, 25 and 55 are shown. The counter electrode 23 is supplied through a first operational amplifier 60, such as a type 741, connected differentially to the reference electrode 25 and to a potentiometer 62 which is supplied with a constant voltage of 5.6 volts by a transistor 64 connected to a 12 volt supply. Use of the potentiometer 62 alters one input to the amplifier 60 and causes the amplifier to supply to the counter electrode 23 a current to equalise the inputs, thus maintaining the reference electrode, and the working electrode, at required potentials. The working electrode 55 is connected differentially to the inverting input of a second operational amplifier 66, such as a type 741. The other input is earthed, and a feedback circuit is supplied in the form of a selection switch 68 arranged to select one of three feedback resistors 70, 72, 74. The feedback resistors provide a scale-changing facility for measuring the current through the working electrode. The output amplifier 66 is connected to the non-inverting input of a third amplifier 76, such as a type 741, and through a 10k resistor 78 to ground. The inverting input of amplifier 76 is connected to a 'set zero' arrangement comprising a potentiometer 80 supplied, via resistors 82, and 84, with constant voltage by a transistor 86 supplied from a 12 volt source. The output current of the measuring circuit is supplied to terminals 88, so that a meter display can be provided which indicates the current through the working electrode; the meter can, if necessary, be calibrated to indicate directly the amount of oxidisable ion present at the working electrode. In order to measure very low concentrations of an oxidisable ion, a substantially ripple free power supply must be provided. If the cell is to be used in its second or third forms, then in Figure 1 the counter electrode 23 is removed, the entire circuit shown in Figure 2 is omitted, and the potential difference generated in the cell between the working electrode 55 and the reference electrode 25 can be connected directly to a differential voltmeter (or an electronic buffer amplifier to prevent polarisation of the electrodes) and the voltage displayed on a suitable chart recorder; this potential difference is related to the concentration of hydrogen at the working electrode. At low concentrations of hydrogen, at the parts per million level, the potential of the working electrode decreases when the source of hydrogen is removed.Even at higher concentrations, but still less than 1%, the decrease occurs when a platinum working electrode is used, but with a palladium working electrode there may be some zero drift at higher concentrations of hydrogen. At per cent concentrations, hydrogen may not desorb fast enough from either metal, and compensating circuitry may be necessary. Typical results are, for an increase in hydrogen concentration in aqueous solution from zero to 1 part per million, 90% response in about 1 minutes, a change of 2.3 microamps per square centimetre of the working electrode and 6 millivolts; for an increase of hydrogen concentration from 103 to 104 parts per million in air, a change of 0.1 microamps per square centimetre and 9 millivolts. WHAT WE CLAIM IS: -

1. An electrochemical cell comprising a container for an electrolyte; a working electrode arranged for exposure to a fluid under test and to the electrolyte; and a platinum/ platinum oxide reference electrode of area comparable to the working electrode positioned close to and parallel to the working electrode, and arranged for exposure to the electrolyte.

2. An electrochemical cell according to

Claim 1 in which the working electrode comprises a metallised film of a plastics material.

3. An electrochemical cell according to either Claim 1 or Claim 2 further comprising a counter electrode arranged for exposure to the electrolyte; means for maintaining the working electrode at a potential which is fixed with respect to the reference electrode; and means for determining any current flow through the working electrode.

4. An electrochemical cell according to Claim 3 comprising a potentiostatic device electrically connected to the working electrode, the counter electrode and the reference electrode and arranged to vary the electrical potential applied to the counter electrode, whereby the working electrode is maintained at a required potential.

5. An electrochemical cell according the Claim 3 or Claim 4 in which the counter electrode is a palladium electrode.

6. An electrochemical cell according to Claim 1 or Claim 2 further comprising means for determining the electrical potential of the working electrode with respect to the reference electrode.

7. An electrochemical cell according to Claim 6 in which the working electrode comprises a metal capable of absorbing a substantial proportion of hydrogen.

8. An electrochemical cell according to Claim 7 in which the working electrode comprises a metal selected from the group consisting of palladium, titanium, zirconium, yttrium and nickel.

9. An electrochemical cell according to Claim 6 in which the working electrode comprises a metal which does not appreciably absorb hydrogen.

10. An electrochemical cell according to Claim 9 in which the working electrode is a platinum electrode.

11. An electrochemical cell substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.