WO2009010720A1

WO2009010720A1 - Apparatus and method for measuring electrical potential of electrolyte

Info

Publication number: WO2009010720A1
Application number: PCT/GB2008/002370
Authority: WO
Inventors: Ian David Nickson; Howard Stuart Atkin
Original assignee: Add Power Technologies Limited
Priority date: 2007-07-17
Filing date: 2008-07-10
Publication date: 2009-01-22
Also published as: GB0713859D0

Abstract

According to a first aspect of the present invention there is provided an apparatus for measuring the electrical potential in an electrolyte (13) in a capacitor (9), the apparatus comprising: a reservoir (1) for retaining a first portion (13a) of the electrolyte outside of the capacitor that is in electrical connection with a second portion (13b) of the electrolyte contained in the capacitor; and a reference electrode (7), the reference electrode in. use being located in the reservoir and being in electrical connection with the first portion of the electrolyte, wherein the reference electrode is arranged to measure the electrical potential in the first portion of the electrolyte, so as to provide a measurement of the electrical potential in the second portion of the electrolyte contained in the capacitor.

Description

APPARATUS AND METHOD FOR MEASURING ELECTRICAL POTENTIAL OF ELECTROLYTE

The present invention relates to an apparatus and method for measuring the electrical potential of an electrolyte of a capacitor. In particular, the present invention relates to the measuring of the electrical potential of electrolyte contained in a capacitor.

In a simple parallel plate capacitor, two metal plates are arranged in face-to-face relationship with a small gap between them. The capacitance of the resultant device is directly proportional to the cross-sectional area of the plates and inversely proportional to the spacing between the plates.

The capacitance of a parallel plate capacitor can be increased by placing a thin layer of electrically insulating dielectric material between the plates. The dielectric constant of the dielectric material determines the electrostatic energy which may be stored in that material per unit volume for a given voltage. The use of such a dielectric layer has the additional benefit of providing a way of ensuring plate separation.

Although for the purposes of explanation reference is made above to parallel plate capacitors, it will be appreciated that the same principles apply to for example cylindrical capacitors in which flexible "plates" separated by flexible sheets of dielectric material are rolled up to form a cylindrical structure.

Electrolytic capacitors are also known in which one terminal of the capacitor is connected to an electrically conductive substrate on a surface of which a thin dielectric layer has been deposited. The other terminal of the capacitor is connected to an electrode which is in electrical contact with an electrolyte that also contacts the dielectric layer. Electrostatic energy is stored in the dielectric layer and in the electrolyte. The energy density of the device is a function of the effective area of the dielectric layer and the area of the electrolyte in contact with the dielectric layer. It is known to increase the effective area by forming the dielectric layer on a porous or sponge-like structure formed on a substrate, for example by forming the dielectric layer on a porous layer of porous metal, metal particles or metal fibres. Electrolyte can penetrate into voids defined between the particles or fibres. Thus the surface area presented by the porous layer to the electrolyte is substantially greater than the area of the plates on which those layers are formed.

Electrochemical capacitors are also known and are generally referred to as "supercapacitors". In a conventional double-layer supercapacitor, two spaced apart electrodes are provided each of which comprises an electrically conductive substrate upon which a high surface area porous conductive layer has been built up, the porous layer being made of for example carbon particles or fibres. The two electrodes are separated by an electrically insulating separator. The space between the two substrates is flooded with an electrolyte which penetrates into the spaces between the carbon particles or fibres. The separator is an ionic conductor so that, although a direct electrical connection cannot be made between the two porous layers, ions formed in the electrolyte can be conducted through the separator. Such supercapacitors have a very high capacitance per unit volume.

The electrolytes employed in many electrochemical capacitors often comprise a solvent into which is dissolved an ionisable salt. When the salt dissolves it ionises such that the application of a voltage between electrodes of the capacitor causes the formation of layers of ions within the electrolyte in the vicinity of each electrode of the capacitor.

When the electrode is charged a balancing counter charge develops in the liquid electrolyte. The charges are not uniformly distributed throughout the electrolyte, but will be concentrated near the charged surface of the electrode and form what is known as a double layer. Thus, a small but finite volume of electrolyte near the charged surface of the electrode is different from the extended electrolyte. Many models have been formulated to explain this phenomenon, including the Helmholtz double layer model, the Gouy-Chapman double layer model, and a modification of the Gouy- Chapman double layer model by Stern.

The practical effect of the formation of such double layers is that the performance and properties of the capacitors is affected. If an 'ideal' simple parallel plate capacitor which contains electrolytes is examined, the absolute value of the electrical potential difference between electrolyte located half way between the parallel plates and each of the plates themselves should be equivalent. However, in a real capacitor where double layers form, this is not the case. The electrical potential between the electrolyte and the positive electrodes of the capacitor may be different from the electrical potential between the electrolyte and the negative electrode of the capacitor. The electrical potential difference between the electrolyte and the positive electrode is often referred to as the positive differential voltage (or positive voltage differential). The electrical potential difference between the electrolyte and the negative electrode is often referred to as the negative differential voltage (or negative voltage differential).

The positive and negative voltage differentials for any given capacitor may vary depending on, amongst other things, the electrolyte which the capacitor contains. This may be for a number of reasons, for example how the electrolyte used effects or forms the double layers mentioned above. Presently, in order to measure the positive and/or negative voltage differentials of a capacitor, the capacitor needs to be taken apart. Having to take the capacitor apart to measure the positive and/or negative voltage differentials is not ideal. Firstly, there is no guarantee that the measured positive and/or negative voltage differentials of a deconstructed capacitor are the same as that of an intact working capacitor. For example, the deconstruction of a capacitor could affect the surface areas of plates of the capacitor, making any results obtained invalid. Customers, manufacturers, etc. may therefore be unwilling to rely on results obtained using a deconstructed capacitor. Furthermore, it takes a significant amount of time to deconstruct a capacitor for testing, and, if necessary, reconstruct it for further use.

It is an aim of the present invention to obviate or mitigate at least one of the disadvantages of the prior art, whether identified herein or elsewhere.

According to a first aspect of the present invention there is provided an apparatus for measuring the electrical potential of an electrolyte of a capacitor, the apparatus comprising: a reservoir for retaining a first portion of the electrolyte outside of the capacitor that is in electrical connection with a second portion of the electrolyte contained in the capacitor; and a reference electrode, the reference electrode in use being located in the reservoir and being in electrical connection with the first portion of the electrolyte, wherein the reference electrode is arranged to measure the electrical potential of the first portion of the electrolyte, so as to provide a measurement of the electrical potential of the second portion of the electrolyte contained in the capacitor.

Advantageously, the apparatus allows the electrical potential of electrolyte contained in a working capacitor (i.e. the second portion of the electrolyte) to be measured without having to deconstruct the capacitor.

The reservoir may comprise, form, or be provided with a conduit. In use, the conduit maybe arranged to extend into the capacitor. The conduit maybe formed from an electrical insulator. The conduit maybe tube-shaped. The conduit may be a capillary tube.

The reservoir maybe formed from an electrical insulator. The reservoir maybe provided with an inlet port. The inlet port maybe provided with a tap controllable to allow or prevent the passage of electrolyte into the reservoir.

According to a second aspect of the present invention there is provided a method of measuring the electrical potential of an electrolyte of a capacitor, the method comprising: measuring the electrical potential of a first portion of the electrolyte located outside of the capacitor that is in electrical connection with a second portion of the electrolyte contained in the capacitor, so as to provide a measurement of the electrical potential of the second portion of the electrolyte contained in the capacitor.

Advantageously, the method allows the electrical potential of electrolyte contained in a working capacitor (i.e. the second portion of the electrolyte) to be measured without having to deconstruct the capacitor.

Prior to measuring the electrical potential of the first portion of the electrolyte, the capacitor maybe at least partially immersed in the first portion of the electrolyte to bring the first portion of the electrolyte into electrical connection with the second portion of the electrolyte. Alternatively, prior to measuring the electrical potential of the first portion of the electrolyte, the capacitor maybe filled with the second portion of electrolyte, and the second portion of the electrolyte is then brought into electrical connection with the first portion of the electrolyte using a reservoir.

The reservoir may comprise, form, or be provided with a conduit. The conduit may extend into the capacitor when the capacitor is being filled with second portion of the electrolyte.

A reference electrode maybe located in the first portion of the electrolyte.

The electrical potential of the first portion of the electrolyte maybe measured relative to a positive plate or electrode of the capacitor to determine a positive voltage differential.

The electrical potential of the first portion of the electrolyte maybe measured relative to a negative plate or electrode of the capacitor to determine a negative voltage differential.

The electrical potential of the first portion of the electrolyte maybe measured when a test voltage is applied between plates of the capacitor, and across the second portion of the electrolyte.

Prior to measuring the electrical potential of the first portion of the electrolyte, the test voltage maybe applied between the plates of the capacitor for a period of between about 1 hour and about 48 hours.

The electrical potential of the first portion of the electrolyte maybe measured every 5, 10, 15 or 20 minutes.

A stabilised electrical potential of the first portion of the electrolyte maybe determined. The stabilised electrical potential of the first portion of the electrolyte maybe determined when the measured electrical potential of the first portion of the electrolyte is the same for at least two or three successive measurements. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which like features have been given the same reference numerals, and in which:

Figure 1 depicts an apparatus for measuring the electrical potential of an electrolyte contained in a capacitor according to an embodiment of the present invention; and

Figures 2a-2b, 3a-3b and 4a-4b depict results obtained using the apparatus of Figure 1.

It should be noted that the Figures are not necessarily drawn to scale, and in some circumstances may deliberately be drawn to an inaccurate scale to highlight the features of one or more parts of the Figures.

Throughout the description of embodiments of the present invention, abbreviations will be used to describe electrolyte solvents and salts/additives as follows:

Electrolyte Solvents

AN = acetonitrile

BL = γ-butyrolactone

PC = propylene carbonate

ETFEC = ethyl-2,2,2-trifluoroethyl carbonate

ETFPC = ethyl-2,2,3,3-tetrafluoropropyl carbonate EPFPC = ethyl-2,2,3,3,3-pentafluoropropyl carbonate

Electrolyte Additives

M l = l-Butyl-l-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate M 4 = Trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate

Figure 1 depicts apparatus for measuring the electrical potential of an electrolyte contained in a capacitor. Figure 1 shows a reservoir 1. The reservoir 1 is provided with an inlet port 2 and an outlet port 3. The inlet port 2 can be opened and closed using a tap 4. The outlet port 3 opens into a conduit 5. The conduit 5 is tube-shaped (e.g. a capillary tube or the like) and is provided with an opening 5a remote from the reservoir 1. The reservoir 1 is also provided with a sealed opening 6. A reference electrode 7 is located in the reservoir 1, and electrical connection to the reference electrode 7 is made via the sealed opening 6.

The reservoir 1 and conduit 5 may be separate bodies connected to one another, or may be formed integrally. Preferably the conduit 5 and the reservoir 1 are formed from materials so as to not conduct electricity, the significance of which will be described in more detail below. For example, the conduit 5 and/or the reservoir 1 may be formed from glass, plastic, or any other electrical insulator.

The sealed opening 6 and inlet port 2, when the tap 4 is closed are substantially air tight, so that no fluid within the reservoir 1 can evaporate away.

It can be seen from the Figure that the conduit 5 extends into a capacitor 9. The capacitor 9 is a cylindrical capacitor 9 provided with cylindrically shaped plates 10. The capacitor is a Maxwell 350F capacitor. The capacitor 9 is also provided with a first electrode 11 and a second electrode 12, via which electrical connection to the capacitor 9 can be made.

If the capacitor 9 needs to be dried of electrolyte, it can be dried in any one of a number of ways. One example of how the capacitor 9 may be dried of electrolyte is now given. The capacitor 9 may be dried under vacuum at 100⁰C for 48 hours in a desiccator oven. The capacitor 9 may be removed from the desiccator oven by back filling the oven with argon, prior to opening the oven. Once the oven is opened, the capacitor 9 is sealed to ensure that the plates 10 of the capacitor 9 are kept under a dry argon atmosphere. The capacitor 9 is then allowed to cool.

Once the capacitor 9 has cooled, it is filled with electrolyte. The capacitor 9 is filled with electrolyte using the reservoir 1 and conduit 5 shown in Figure 1. Firstly, the conduit 5 is passed through the opening 8 of the capacitor 9 such that the conduit opening 5 a is located within the capacitor 9. The tap 4 is opened to allow electrolyte 13 to be passed through the inlet port 2 and into the reservoir 1. The electrolyte 13 .

flows down and through the conduit 5 and into the capacitor 9. The filling of the capacitor 9 with electrolyte 13 is performed in a number of steps to ensure that the material forming the plates 10 is fully wetted. Initially, the capacitor is filled up to the level of the opening 8. Due to the nature of the material forming the plate 10, the electrolyte 13 slowly wicks into the plates (due to the porous structure of, for example, carbon which forms the plates 10), and hence the level of the electrolyte 13 needs to be periodically topped up until no more electrolyte is required. This process may take around eight hours. Once the level of the electrolyte 13 has stabilised, more electrolyte 13 is passed through the inlet port 2 and into the reservoir 1 such that the referenced electrode 7 is immersed in the electrolyte 13. The tap 4 is then closed such that no more electrolyte 13 may pass into the reservoir 1 and capacitor 9. Electrolyte 13 cannot evaporate from the reservoir 1 and through the inlet port 2, because the tap 4 is closed. Any gaps around the conduit 5 in the vicinity of the opening 8 of the capacitor 9 maybe sealed with, for example, pliable putty or the like, such that no electrolyte 13 can evaporate from the capacitor 9.

The plates 10 of the capacitor 9 have a very large surface area. As a consequence of the large surface area of the plates 10, a conditioning period is required prior to undertaking any tasks on the capacitor 9. The conditioning period involves the application of a voltage between the plates 10 of the capacitor 9 (via the first electrode 11 and second electrode 12) for a prolonged period of time, say at least about one hour to forty eight hours depending on the size of the capacitor, prior to the taking of any measurements associated with the operation of the capacitor 9. For example, a voltage of 2.5V may be applied between the first electrode 11 and second electrode 12 prior to the measurement of any electrical properties of the capacitor 9.

The electrolyte 13 is electrically conductive. Consequently, even though the reference electrode 7 is located in a first portion of the electrolyte 13a located outside of the capacitor 9, the reference electrode 7 is still in electrical connection with a second portion of the electrolyte 13b located inside of the capacitor 9. This means that the electrical potential of the second portion of the electrolyte 13b located inside of the capacitor can be measure in-situ, e.g. in a working capacitor. The capacitor 9 may be tested in any appropriate way once the conditioning period is completed. When the conditioning voltage of 2.5V is applied between the plates 10 of the capacitor 9, the electrical potential of the electrolyte 13 relative to a positive electrode (for example, the second electrode 12) is then measured to determine the positive voltage differential. This may be achieved by measuring the electrical potential difference between the reference electrode 7 and the second electrode 12 using, for example, a digital voltmeter (not shown). Alternatively, the electrical potential of the electrolyte 13 relative to a negative electrode (for example, the first electrode 11) maybe measured to determine the negative voltage differential. This may be achieved by measuring the electrical potential difference between the reference electrode 7 and the first electrode 11 using, for example, a digital voltmeter (not shown).

A measurement of the electrical potential may be undertaken every five, ten, fifteen, or twenty minutes. A stable electrical potential is determined when the measured electrical potential is the same over, for example, two or three consecutive measurements. The overall test time at a particular test voltage maybe around ninety minutes.

The positive voltage differential may be determined for any electrical potential difference applied between the first electrode 11 and second electrode 12. If a different voltage is to be applied between the first electrode 11 and second electrode 12, the conditioning period mentioned above should be undertaken at the different test voltage prior to the determination of the positive (or indeed negative) voltage differentials. For example, the positive voltage differential may be measured when a voltage of 2.5V is applied across the first electrode 11 and second electrode 12, and then 2.6V, 2.7V, 2.8V, 2.9V, and 3V. This process, or a similar process, can be undertaken for different electrolytes, for example, electrolytes having different levels of additives. Examples of electrolytes, and results obtained using those electrolytes are given below.

Figures 2a and 2b are graphs showing the results obtained for the capacitor 9 of Figure 1 which is filled with an electrolyte comprising an AN solvent with an Ml additive. Figure 2a shows a graph of the results obtained for a capacitor having a voltage of 2.5V applied across the plates 10 of the capacitor 9. It can be seen that an increase in the concentration of Ml in the electrolyte 13 has the effect of reducing the positive voltage differential which may be desirable where a reduction in the positive voltage differential is required in order to access wider operational voltage windows for an electrolyte. Figure 2b illustrates the results obtained from the capacitor 9 when the voltage applied across the plates 10 is 2.8V. Again, it can be seen that the positive voltage differential decreases as the concentration of Ml in the electrolyte 13 increases.

Figures 3 a and 3b depict results obtained from the capacitor 9 of Figure 1 which is filled with an electrolyte comprising an AN solvent with an M4 additive. Figure 3 a depicts the results obtained for varying concentrations of M4 when the voltage applied between the plates 10 of the capacitor 9 is 2.5V. Figure 3b depicts the results obtained for varying concentrations of M4 when the voltage applied between the plates 10 of the capacitor 9 is 2.8 V. It can be seen from both graphs that, initially, the addition of the additive M4 increases the positive voltage differential, but that the positive voltage differential decreases when the concentration of M4 exceeds about 6%. Varying the percentage of the additive M4 may be desirable where a change in the positive voltage differential is required in order to access wider operational voltage windows for an electrolyte.

The differences in the results obtained for different voltages applied between the plates 10 of the capacitor 9 can be used to infer information about the behaviour of the electrolyte 13 and/or the capacitor 9 as a whole. For example, the results obtained for different applied voltages and additive concentrations can be used to choose a suitable additive for use in an electrolyte to optimise the performance (e.g. operating voltage) of a capacitor. To this end, it will be appreciated that the ability to modify (i.e. increase or decrease) the positive and/or negative voltage differentials of the electrolyte enables the working voltage of the capacitor to be increased. Since the working voltage of the capacitor may be increased, so may the charge which the capacitor can store.

Figures 4a and 4b depict further results obtained using the apparatus and method described above. Figure 4a is a graph depicting the discharge of capacitors that contain different electrolytes. Each capacitor was tested according to European Standard IEC 62391-1, and specifically in the method outlined in paragraph 4.5.1. The graph shows three data sets. One data set shows the results obtained for a number of Maxwell 350F capacitors having an AN solvent with 14% Ml additive. These capacitors were provided with this solvent-additive mixture using the filling steps described above. The capacitors were tested at an operating voltage of 2.8V, and were found to have a positive voltage differential of 1.55V and a negative voltage differential of -1.25V. Another data set shows the results obtained for a number of Maxwell 350F capacitors having an AN solvent with 2% M4 additive. These capacitors were provided with this solvent- additive mixture using the filling steps described above. The capacitors were tested at an operating voltage of 2.8V, and were found to have a positive voltage differential of 1.8V and a negative voltage differential of -1.0V. The last data set shows the results obtained for a number of Maxwell 350F capacitors having the electrolyte with which they were initially provided (in other words, standard capacitors). These capacitors were tested at an operating voltage of 2.8V, and were found to have a positive voltage differential of 1.71V and a negative voltage differential of -1.09V.

All of the tests were undertaken at an elevated temperature of 65⁰C. The temperature at which the testing was undertaken was elevated to provide an indication of the long term performance of the capacitors. For instance, it is known that the testing of the capacitors at 65⁰C over ninety days is equivalent to the capacitors being tested at around room temperature over a period of around ten years. Over the testing period of ninety days, manufacturers of capacitors would like the storage capacity of the capacitor to drop by no more than 25%. In other words, if the storage capacity decreases by 25% or less over the testing period of ninety days, the capacitor is taken to be durable enough for use, and is therefore taken to be satisfactory.

Figure 4a shows the results for the capacitors over a period of fifteen days. It can be seen that even over fifteen days, the capacitors having electrolytes with the Ml and M4 additive are already outperforming the standard capacitors. That is, the storage capacity of the capacitors having electrolytes with the Ml and M4 additive decreases more slowly over time than the storage capacity of the standard capacitors. It can be seen that this benefit is enjoyed from the very first day of the test onward. It can also be seen that the capacitors having electrolyte comprising an AN solvent with 14% Ml additive outperforms those having electrolyte comprising an AN solvent with 2% M4 additive.

Figure 4b shows the extrapolation over ninety days of the results obtained over fifteen days. It can readily be seen that the standard capacitors are not satisfactory, in that there storage capacity decreases by more than 25% in ninety days. Indeed, it can be seen that the standard capacitors lose 25% of their storage capacity in just over fifty days. In stark contrast, it can be seen that the capacitors having electrolyte comprising an AN solvent with 14% Ml additive lose 25% of their storage capacity at the ninety day mark, and are therefore satisfactory.

After the capacitors had been tested, it was found that the capacitors containing an electrolyte having either an Ml or M4 additive contained 70ppm (parts per million) water. In comparison, the electrolytes of the standard capacitors were found to contain 25ppm (parts per million) water. It is commonly believed that the more water an electrolyte of a capacitor contains, the quicker the storage capacity of the capacitor will degrade. However, it can be seen from Figures 4a and 4b that this is not the case for the capacitors containing an electrolyte having either an Ml or M4 additive. This result can be interpreted as meaning that the use of the additive circumvents the problem of additional water, or that the effect of the additive on the long term storage capacity of the capacitor is so great as to mask any degradation in performance caused by the increased amount of water.

In the above embodiment, the apparatus has been described as having a reservoir 1 and a conduit 5 extending from the reservoir 1. This is not essential, and the reservoir 1 could be shaped to come to a point or to form a thin channel (e.g. to form a conduit) which could be inserted into any appropriate opening in the capacitor 9. The overriding function of the apparatus shown in Figure 1 is to measure the electrical potential of the electrolyte in the (working) capacitor 9 without having to deconstruct or modify the capacitor 9. That is, a portion of the electrolyte outside of the capacitor 9 in which is located the reference electrode 7 is used to conduct the electrical potential of the portion of the electrolyte within the capacitor to the reference electrode 7.

The capacitor of Figure 1 was described as being filled from a reservoir in which a reference electrode was located. This is not essential. The capacitor of Figure 1 could be filled from any suitable reservoir and in any suitable manner, for example following the filling steps mentioned above. The conduit of the apparatus of Figure 1 could then be inserted into the capacitor, and the reservoir filled with electrolyte to immerse the reference electrode and bring the reference electrode into electrical connection with electrolyte located in the capacitor.

An alternative arrangement to that shown in Figure 1 would be where a capacitor is fully or partially immersed in a reservoir of electrolyte, with a reference electrode being located in the electrolyte in the reservoir, but external to the capacitor. The reference electrode is still then in electrical connection with electrolyte located in the capacitor but the reference electrode does not need to be placed inside the capacitor to measure the electrical potential of the electrolyte. Although such a measurement technique and apparatus is possible, it may be more practical to undertake measurements using the apparatus described in relation to Figure 1. This is because it may be undesirable or impractical to fully or partially immerse the capacitor in the electrolyte.

Although the method and apparatus described above have made reference to certain electrolyte solvents and electrolytes additives, it will be appreciated that the invention as described is equally applicable to other electrolytes. For example, instead of using AN as the solvent, the solvent could be one of BL, PC, ETFEC, ETFPC or EPFPC, or any mixture thereof. Additives or combinations of additives other than Ml and M4 may be added to the solvent or solvents used. The capacitors tested in the embodiments described above had, as well as a solvent and an additive, a salt, namely tetraethyl ammonium tetrafluoroborate. It will be appreciated that the testing methods and apparatus described above are equally applicable to electrolytes having different salts. In the embodiments mentioned above, the measurements of the potential of an electrolyte have been described in relation to a specific capacitor. However, the invention is not limited to the use of this specific capacitor, and the invention may be used to measure the potential of electrolyte contained in any capacitor. The capacitor to be tested may already be provided with an opening for insertion of, for example, a conduit. Alternatively, an opening may be made in the capacitor.

It will be appreciated that the above embodiments have been described by way of example only. It will be appreciated that various modifications may be made to these and indeed other embodiments, without departing from the invention which is defined by the claims that follow.

Claims

1. An apparatus for measuring the electrical potential of an electrolyte of a capacitor, the apparatus comprising: a reservoir for retaining a first portion of the electrolyte outside of the capacitor that is in electrical connection with a second portion of the electrolyte contained in the capacitor; and a reference electrode, the reference electrode in use being located in the reservoir and being in electrical connection with the first portion of the electrolyte, wherein the reference electrode is arranged to measure the electrical potential of the first portion of the electrolyte, so as to provide a measurement of the electrical potential of the second portion of the electrolyte contained in the capacitor.

2. The apparatus of claim 1, wherein the reservoir comprises, forms, or is provided with a conduit.

3. The apparatus of claim 2, wherein, in use, the conduit is arranged to extend into the capacitor.

4. The apparatus of claim 2 or clam 3, wherein the conduit is formed from an electrical insulator.

5. The apparatus of any of claims 2 to 4, wherein the conduit is tube-shaped.

6. The apparatus of claim 5, wherein the conduit is a capillary tube.

7. The apparatus of any preceding claim, wherein the reservoir is formed from an electrical insulator.

8. The apparatus of any preceding claim, wherein the reservoir is provided with an inlet port.

9. The apparatus of claim 8, wherein the inlet port is provided with a tap controllable to allow or prevent the passage of electrolyte into the reservoir.

10. A method of measuring the electrical potential of an electrolyte of a capacitor, the method comprising: measuring the electrical potential of a first portion of the electrolyte located outside of the capacitor that is in electrical connection with a second portion of the electrolyte contained in the capacitor, so as to provide a measurement of the electrical potential of the second portion of the electrolyte contained in the capacitor.

11. The method of claim 10, wherein, prior to measuring the electrical potential of the first portion of the electrolyte, the capacitor is at least partially immersed in the first portion of the electrolyte to bring the first portion of the electrolyte into electrical connection with the second portion of the electrolyte.

12. The method of claim 10, wherein, prior to measuring the electrical potential of the first portion of the electrolyte, the capacitor is filled with the second portion of electrolyte, and the second portion of the electrolyte is then brought into electrical connection with the first portion of the electrolyte using a reservoir.

13. The method of claim 12, wherein the reservoir comprises, forms, or is provided with a conduit.

14. The method of claim 13, wherein the conduit extends into the capacitor when the capacitor is being filled with second portion of the electrolyte.

15. The method of any one of claims 10 to 14, wherein a reference electrode is located in the first portion of the electrolyte.

16. The method of any of claims 10 to 15, wherein the electrical potential of the first portion of the electrolyte is measured relative to a positive plate or electrode of the capacitor to determine a positive voltage differential.

17. The method of any of claims 10 to 15, wherein the electrical potential of the first portion of the electrolyte is measured relative to a negative plate or electrode of the capacitor to determine a negative voltage differential.

18. The method of any of claims 10 to 17, wherein the electrical potential of the first portion of the electrolyte is measured when a test voltage is applied between plates of the capacitor, and across the second portion of the electrolyte.

19. The method of claim 18, wherein, prior to measuring the electrical potential of the first portion of the electrolyte, the test voltage is applied between the plates of the capacitor for a period of between about 1 hour and about 48 hours

20. The method of any preceding claim, wherein the electrical potential of the first portion of the electrolyte is measured every 5, 10, 15 or 20 minutes.

21. The method of any of claims 10 to 20, comprising determining a stabilised electrical potential of the first portion of the electrolyte.

22. The method of claim 21, wherein the stabilised electrical potential of the first portion of the electrolyte is determined when the measured electrical potential of the first portion of the electrolyte is the same for at least two or three successive measurements.

23. An apparatus for measuring the electrical potential of an electrolyte substantially as hereinbefore described with reference to the accompanying Figures.

24. A method of measuring the electrical potential of an electrolyte substantially as hereinbefore described with reference to the accompanying Figures.