WO2001089024A1

WO2001089024A1 - Electrochemical methods and cells

Info

Publication number: WO2001089024A1
Application number: PCT/GB2001/002091
Authority: WO
Inventors: Sean Patrick Mccormack; Daren Joseph Caruana
Original assignee: University College London
Priority date: 2000-05-16
Filing date: 2001-05-14
Publication date: 2001-11-22
Also published as: AU2001254976A1

Abstract

A method for carrying out electrochemical reactions in an electrochemical cell is disclosed. The method includes the step of enveloping in a plasma a respective part of each of two electrodes. The plasma is moved to the electrodes and acts as an electrolyte between the electrodes in the electrochemical cell. In particular, the plasma may be a flame. The invention may be used, for example, for power generation, electroplating, electrochemical conversion of noxious gases, or in analytical techniques for concentration measurement and/or identification of chemical species.

Description

ELECTROCHEMICAL METHODS AND CELLS

FIELD OF THE INVENTION

The present invention relates to electrochemical methods and electrochemical cells. More specifically, it relates to an electrochemical method and an electrochemical cell with a novel electrolyte in comparison with a conventional electrochemical cell.

BACKGROUND OF THE INVENTION

An electrochemical cell consists of electrodes and an electrolyte. An electromotive force (e.m.f.) developed between the anode and the cathode may be used to drive a current through an external circuit. An example of an electrochemical cell is the

Daniell cell. A copper/zinc Daniell cell is shown in Fig. 1. A zinc electrode 2 is partially immersed in an aqueous solution 4 of Zn²⁺ ions and a copper electrode is partially immersed in an aqueous solution 8 of Cu²⁺ ions. When a circuit incorporating the electrodes 2,6 is completed (Fig. 1(b)), a current flows around the circuit and may be used to power, for example, a resistive electrical component 10. At the copper electrode 6, the following electrochemical reaction occurs: Cu²⁺(aq) + 2e^" → Cu(s)

At the zinc electrode 2, the following electrochemical reaction occurs:

Zn(s ) → Zn²⁺(aq) + 2e^~ The zinc solution 4 and the copper solution 8 are electrically connected via a salt bridge 12.

A conventional electrochemical cell may also be used for electroplating. Here a conducting substrate forms one of the electrodes and an external power source is used to drive a current between the electrodes. This brings about a desired chemical reaction wherein, for example, metallic cations are reduced to the metal, plating the cathode with a layer of that metal.

Electrochemical reactions may be studied in an electrochemical cell using a three-electrode system. A schematic circuit diagram for a three-electrode electrochemical cell is shown in Fig. 2. The cell 20 has three electrodes: the working electrode 22, the reference electrode 24 and the counter electrode 26. Using a power supply 28, the working electrode is polarised against the reference electrode . The current that flows through the working electrode 22 is measured using a current meter 30. The potential of the working electrode 22 is monitored with reference to the reference electrode 24 by a voltmeter 32.

A conventional electrochemical cell may further be used to detect concentrations and/or identities of species. A typical example of such a use is the technique of anodic stripping voltammetry. In a three- electrode electrochemical cell, the species of interest is introduced into the electrolyte, typically in solution in a liquid electrolyte. Subsequently, the potential of the working electrode is altered so that the species undergoes an electrochemical reaction (e.g. reduction) and is plated on an electrode. A sufficient change in the potential of the plated electrode allows the plated material to undergo the reverse electrochemical reduction (i.e. oxidation in this case). This is called

"stripping" the electrode. The current through the circuit is monitored. A current peak or trough is measured when stripping of the electrode occurs. The potential at which this stripping occurs is characteristic of the material stripped from the electrode . The magnitude of the current measured during stripping is indicative of the quantity of material stripped. Therefore, the identity and concentration of the species in solution are determinable by this technique.

Conventional electrochemical cells use liquid or even solid electrolytes. Commonly, a liquid electrolyte is an aqueous ionic solution. Passage of an electric current through the electrolyte is made possible by the migration of ions in the solution. The current density in the electrolyte of the above electrochemical cells can be limited by inefficient mass transport in the electrolyte. Consequently the rate of the electrochemical reactions at the electrode surfaces can be limited by this inefficient mass transport in the electrolyte.

SUMMARY OF THE INVENTION

Accordingly, in its broadest aspect, the present invention consists in a method for carrying out an electrochemical reactions in an electrochemical cell, the method including the step of enveloping in a plasma a respective part of each of two electrodes, the plasma being an electrolyte between the electrodes in the electrochemical cell.

Particularly, but not exclusively, the plasma may be a flame. Since a flame contains charged species, these charged species can carry current between the first and the second electrodes. The diffusion rate or speed in the flame can be significantly greater than that in a liquid electrolyte, allowing faster electrode reactions in the electrolyte.

Alternatively other methods of generating a plasma may be employed, e.g. by inductive coupling such as microwave/magnetic field coupling.

Particularly the method involves generating the plasma at a location away from the electrodes and moving it to the electrodes . It is preferred that the electrodes of the electrochemical cell are electrically independent of plasma generating means, e.g. of electrodes generating the plasma.

The work here described shows that electrochemical processes can be carried out on a useful scale and in a controlled manner using a plasma, in particular a flame, as an electrolyte.

In one embodiment of the invention, an electrochemical reaction in the electrochemical cell is driven by a difference in electrochemical potential between the electrodes. For example, an emf generated between the electrodes is used to drive a current through an external circuit.

In another embodiment of the invention an electrochemical reaction in the electrochemical cell is driven by a potential difference applied between the electrodes. Typically, a species is introduced into the plasma and undergoes an electrochemical reaction. An electrochemical reaction product of the species may be electrochemically plated onto one of the electrodes.

Alternatively, the species may be an oxide of nitrogen or sulphur dioxide. In a further embodiment of the invention, an electrochemical reaction in the electrochemical cell is driven by a difference in electrochemical potential between the electrodes wherein the method includes the steps of measuring a first emf between the two electrodes, feeding a species into the plasma, and measuring a second emf between the two electrodes with the species in the plasma, wherein the difference between the first emf and the second emf provides an indication of the concentration of species in the plasma. In another embodiment of the invention an electrochemical reaction in the electrochemical cell is driven by a potential difference applied between the electrodes wherein the method includes the steps of introducing a species into the plasma, controlling the potential difference applied between the electrodes, and measuring the current flowing between the electrodes at different applied potential differences, wherein the current measurement providing an indication of the concentration of the species. Preferably, the potential difference is controlled as follows: (i) a first potential difference is applied whereby an electrochemical reaction product of the species is coated onto an electrode, and (ii) a second potential difference is applied whereby the electrochemical reaction product is electrochemically stripped from the electrode at a potential difference characteristic of the electrochemical reaction product.

The method may involve determining a measurable quantity by electrochemical measurement, e.g. by measuring emf or voltage directly or by a polarimetric method. For all the above embodiments of the present invention, the plasma preferably is generated at a pressure (absolute) of at least 0.1 atmosphere and more preferably at a pressure (absolute) of at least 0.5 atmosphere. Furthermore, the plasma preferably has an upward convective flow. The plasma may, for example, be a flame.

In a second aspect, the invention provides an electrochemical cell including at least two electrodes and a plasma-producing means, wherein in use the electrodes contact a plasma produced by the plasma- producing means and the plasma acts as an electrolyte for the electrochemical cell. Preferably, the plasma- producing means is a flame-producing means, but other plasma generating means may be employed, as described above .

Each respective electrode may be made, for example, from a respective metal or alloy having a melting point in excess of 500°C and preferably in excess of 1000°C. The respective metal or alloy may comprise at least one or more metal selected from the group consisting of Ti, Nb, Mo, Pt, W, Rh, Ta, Hf . A high melting point is useful since the electrode should withstand the heat of the flame. A low ionisation potential is preferable since this means that the material of the electrode is ionised around the electrode due to the heat of the flame.

In some embodiments, a first electrode has a first electrochemical potential and a second electrode has a second electrochemical potential not equal to the first electrochemical potential. Additionally or alternatively, the electrochemical cell further includes a third electrode, wherein in use the third electrode contacts the plasma and acts as counter electrode.

The flame producing means typically includes feed means for feeding gaseous or liquid or solid oxidisable fuel and gaseous or liquid or solid oxidant to a flame region. Preferably, the feed means has two portions, a first portion including means whereby a reactable species is feedable into the flame region.

INTRODUCTION OF THE DRAWINGS

Several specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 shows a conventional Cu/Zn Daniell cell (a) with the circuit incomplete, and (b) with the circuit completed with a resistive electrical component.

Fig. 2 shows a typical schematic circuit diagram for a three-electrode electrochemical cell.

Fig. 3 shows electrochemical series for several metals compiled using the potential difference measured between two electrodes in an electrochemical cell using a flame as an electrolyte. Each metal was measured against platinum and niobium.

Fig. 4 shows a schematic diagram of a gaseous electrochemical cell with dissimilar electrodes.

Fig. 5 shows the dependence of electrochemical potential on the mole fraction of rhodium in a platinum/rhodium alloy electrode at two different temperatures, the potential difference being measured against a niobium electrode in an electrochemical cell using a flame as an electrolyte.

Fig. 6 shows a schematic diagram of a gaseous electrochemical cell for electroplating (a) with no potential difference applied between the electrodes, and (b) with a potential difference applied between the electrodes . Fig. 7 shows the schematic arrangement of electrodes in a flame above a burner.

Fig. 8 shows cyclic voltammograms between -5 V to -1 V, with and without 0.1 M cupric salt added to a flame in a gaseous electrochemical cell. Fig. 9 shows a gaseous electrochemical cell as a potentiometric sensor.

Fig. 10 shows a gaseous electrochemical cell as an amperometric sensor.

Fig. 11 shows a gaseous electrochemical cell used for anodic stripping voltammetry.

Fig. 12 shows a gaseous electrochemical cell as an environmental cell.

DESCRIPTION OF EMBODIMENTS The present invention makes use of the electrochemical properties of plasmas. An example of a plasma is a flame. A flame is usually a luminous region in which gases undergo combustion. It is known that charged species (ions) exist in flames. Generally, a flame can be considered as a weak plasma which is overall electrically neutral with the negative species being predominantly electrons and the positive species being complex ions. In some areas within a flame, ionic concentrations may be as high as 0.1 mol dm^""3. The lower limit of ionic concentration for a flame is about 0.05 nmol dm^"3. The extent of ionisation in hydrocarbon flames is not large (typically about 10^"7 mole fraction) compared to other flame products such as water (typically about 0.16 mole fraction) or carbon monoxide (typically about 0.06 mole fraction). The extent of ionisation tends to vary greatly between different flame systems. The present invention has been found to work best in flames with ionic concentrations in the range 0.01 to 1 μmol dm^"3, e.g. approximately 0.1 μmol dm^"3.

The predominant cations in hydrocarbon flames are CH0⁺ and H₃0⁺, produced by chemi-ionisation. The chemical rearrangement releases energy leading to the ionisation of the product species. For example:

CH + O → CH0⁺ + e^" Ions are produced in the primary reaction zone of the flame in concentrations in excess of the concentrations expected from the chemical equilibrium. This shows that some non-equilibrium ionisation processes take place. Such non-equilibrium ionisation processes include thermal or collision ionisation. Ionisation generally requires energies in the range 4-20 eV. Collision ionisation of species A may take place, for example, by collision with an electron or with another uncharged species B, e.g.

A + e^" → A⁺ + 2e^"

A + B → A⁺ + B + e^"

Electron transfer can be an endothermic process where the ionisation potentials are larger than the electron attachment energies, e.g.

K + Cl → K⁺ + Cl^" Ionisation can also occur via the transfer of excitation energy, e.g. Ne* + A → A⁺ + Ne + e^"

Metals in flames generally are ionised either by thermal ionisation or by charge or proton transfer. Usually, metals with a lower ionisation potential are ionised thermally.

The ionisation, which results from the cascade of chemical reactions in a flame, is enough to carry an electrical current between two electrodes and to support reduction/oxidation (redox) reactions at an electrode surface.

The present work shows that it is possible to measure electrochemical potentials in a gaseous flame. These electrochemical potentials are measured as a manifestation of the ionisation of metal from the surface of a metal electrode. The potential difference measured is analogous to a potential difference measured in conventional electrochemical cells using liquid electrolyte.

In an embodiment of the invention, two metals which have low ionisation potentials (typically less than about 8 eV) are inserted into the flame . Through thermal ionisation of each metal, each is shrouded in an atmosphere of metal ions. For example, if molybdenum and niobium electrodes are used, the electrochemical cell is:

Mo_(metal) |Mo⁺ _(gas) I |Nb⁺ _(gas) |Nb_(metaZ) Metals which are suitable for use as electrodes in the gaseous electrochemical cell should have high melting points. In addition, they should also have relatively low ionisation potentials (typically less than about 8 eV) . Alloys may be employed. An electrochemical series has been formulated on the basis of the potential difference measured across a gaseous electrochemical cell. The results are shown in Fig. 3. The experiments were carried out at an absolute pressure of 1 atmosphere. Note that the right hand list (Vs Mo) is not on the same scale as the left hand list. The potential differences for Ti, Nb, Mo, W, Ta and Hf are shown as measured with reference to Hf and Mo. Fig. 3 shows that the general trends are the same although the potential differences are not precise. The magnitude of the potential difference is determined by the electrochemical potentials of the electrodes used.

A gaseous electrochemical cell may be used to generate electrical power. A voltaic cell may be constructed using two electrodes made from dissimilar metals. These are inserted into a flame. The resulting e.m.f. may be used to drive a current around an external circuit. Due to the efficient mass transport in the flame, the current density may be significantly greater than the current density when a liquid electrolyte is used.

Fig. 4 shows a schematic diagram of a voltaic cell 40, one embodiment of the present invention. Electrode 42 is made from metal N and electrode 44 is made from metal M. The plasma 46 has an upward convective flow, and the plasma is preferably a flame. The direction of the flame is shown by the bold arrow. The heat of the flame (for example, the flame is due to combusting methane gas) is sufficient to evaporate and ionise some atoms of the M and N metals to M⁺ and N⁺ ions, respectively. Due to the flow of the flame, these M⁺ and N⁺ ions are confined to regions around the M and N electrodes, respectively. These regions are schematically shown in Fig. 4 as regions 48 and 50, respectively. The plasma 46 acts as an electrolyte between electrodes 42 and 44 and completes the circuit. Current flows around the circuit due to the emf generated by the difference in electrochemical potential between electrodes 42 and 44.

With precise control of the metal ion concentration around the electrodes, it has been possible to measure Nernstian electrochemical potentials in the gas phase. The electrodes used to measure these electrochemical potentials were alloys of platinum containing a potential-determining ion. Generally, platinum will not ionise via thermal processes. In this case, the potential-determining ion was rhodium. The percentages of rhodium in the platinum/rhodium alloys used were 6 %, 10 %, 20 % and 30 %. Fig. 5 shows the linear relationship obtained for the potential difference between a platinum/rhodium electrode and a niobium electrode versus the concentration in mole fraction rhodium. Line A is for measurements taken at 1223 K and line B is for measurements taken at 1298 K. The plot clearly shows a linear trend with a gradient of the correct magnitude to that expected from the Nernst equation. At 1223 K there is a 0.137 V change per decade change in rhodium concentration. The experiment was carried out at 1 atmosphere absolute pressure.

A gaseous electrochemical cell may be used to conduct redox reactions at a conducting electrode. There are significant departures in electrode reactions in a gaseous electrochemical cell compared to electrode reactions in liquid electrochemical cells. The factors which influence the dynamics of electrochemical reactions at electrochemical surfaces in a flame include : the equivalent of an electrode double layer will be much thicker than that found in liquids; mass transport may reach 20-200 cm s^"1, depending on the fuel; fast diffusion (typically about 0.1 cm² s^"1 for gases) and migration effects .

The flame systems which may be used include the Meker burner which produces a uniform laminar flame. This burner allows chemical additives to be introduced in specific areas of the flame. Other flame systems which may be used are the Opposed Jet Flame or the Wolfhard- Parker burner. A burner called the Putnam burner produces a spherical flame with a defined volume and no flame front. It is considered to be a perfectly stirred reactor which ensures that the electrodes are in similar chemical environments.

A system for the electro-reduction of metallic ions is demonstrated by the electro-reduction of copper ions introduced to a premixed methane/air Bunsen burner flame. Figure 8 shows cyclic voltammograms obtained using a three-electrode system in the presence of copper ions. Figure 8 clearly shows a reduction process at -3.5 V when a copper salt is added to the flame. The polarographic wave is similar in character to that seen when using liquid electrolytes with efficient mass transport.

The electrode assembly should be constructed to minimise flame disturbances and to withstand the temperature (typically 1600°C) . The influence of the temperature on the galvani potential of the bulk metal itself is essential to enable the potentiometric control of the redox reaction. If the galvani potential is higher in energy than the potential of the redox reaction of interest then the redox reaction will occur at any applied potential. The galvani potential of an electrode can be controlled either by maintaining the temperature of the electrode as low as possible whilst it is in the flame, or by choice of electrode material with the appropriate work function.

Fig. 6 shows a schematic diagram of a typical electrochemical cell for electroplating, another embodiment of the present invention. The gaseous electrochemical cell to be used for electroplating typically utilises three electrodes. These electrodes are called the working electrode 60 , reference electrode 62 and the counter electrode 64. Fig. 6(a) shows a gaseous electrochemical cell to be used for electroplating with no potential difference applied between the electrodes 60 and 62. Species S⁺ _gas is present in the plasma e.g. flame plasma 66 (the direction of the flowing plasma or flame is shown by the bold arrow). In Fig. 6(b), a potential difference is applied between electrodes 60 and 62. Species S⁺ _gas undergoes an electrochemical reaction (reduction) at electrode 60 to form a plating layer of S on electrode 60.

Fig. 7 shows the arrangement of electrodes in a typical 3 -electrode gaseous electrochemical cell embodying the present invention. The working electrode 70 is placed close to the reference electrode 72 to try to minimise temperature differences between them. The counter electrode 74 is typically placed as shown. 76 is the burner and the outline of the flame is shown by the dotted line 78.

It is important to design the electrode geometry to reduce flame disturbances since such disturbances can introduce electrochemical noise in the current response. Here the working electrode is in the form of a metallic disc of about 100 μm diameter. This is sealed in ceramic or other heat resistant insulating material. The small size ensures low current density in order to reduce potential drop effects.

The reference electrode 72 is preferentially a sacrificial type electrode made from a metal with a low ionisation potential to ensure that the electrode surface is shrouded in metal ions to maintain a stable electrochemical potential in use. The reference and working electrodes are placed as close to each other as possible to ensure that the temperatures observed by both electrodes is the same. The counter electrode may be the burner plate itself or a large surface area platinum flag 74 placed downstream from the working and reference electrodes .

Fig. 8 shows a plot of current i against potential E applied to a platinum electrode (E measured relative to a Mo electrode) . Plot A shows the behaviour with no species added to the flame. Plot B shows the behaviour with Cu⁺ species (from CuS0₄ salt added to the flame) . Plot B clearly shows a step at about -3.5 V, corresponding to the reduction of Cu⁺ at the working electrode .

For Fig. 8, the scan rate used to produce the graphs was 0.1 Vs^"1. The three electrode assembly used consisted of: a 2 mm long, 0.1 mm diameter platinum wire (working electrode), 2 mm upstream was a 2 mm long, 0.1 mm diameter titanium wire serving as a pseudoreference electrode, and 5 mm upstream was a platinum flag (l x l cm) acting as a counter electrode. The working and pseudoreference electrodes were held in a ceramic tube positioned using a micromanipulator in the visible edge of the flame. The resistance was found to be between 800 and 1000 Ω. Another embodiment of the present invention is as an analytical sensor to measure the concentration of and/or identify ionic and even uncharged species introduced into a flame. Fig. 9 shows a potentiometric sensor embodying the invention. Electrodes 90 and 92 have different electrochemical potentials. Burner 94 has two portions 96 and 98. Fuel is admitted as a stream through both portions 96 and 98 for the single flame 100. An analyte A is introduced in the stream in portion 96 but not in portion 98. The analyte A is thermally ionised to A⁺ _gas in the flame 100. The A⁺ _gas is substantially confined in the portion of the flame around electrode 90. Measurement and comparison of the emf between electrodes 90 and 92 by voltmeter 102 with and without analyte A introduced gives an indication of the concentration of species A⁺ in the flame.

Fig. 10 shows an amperometric sensor embodying the invention. The cell is constructed similarly to Fig. 9 but the electrodes 110 (working) , 112 (reference) and 114 (counter) may be made from the same metal. Again, the burner 116 has two portions 118,120 to allow an analyte A to be introduced in part of the flame 122. A potential is applied between electrodes 110 and 112. The current flowing between electrodes 110 and 114 is measured by current meter 124. Comparison of the current measured with no analyte A introduced and with analyte A introduced gives an indication of the concentration of A in the flame. Fig. 11 shows an embodiment of the present invention which is a variation of the amperometric sensor. Here, the technique of anodic stripping voltammetry known from conventional electrochemical cells is applied to gaseous electrochemical cells. In this case, the species to be analysed is S⁺. This is added to the flame and the potential at electrode 130 is controlled by the variable power source 132 so that the species S⁺ is reduced and plated as S onto the surface of the electrode 130. Subsequently, the potential at electrode 130 is altered so that S is oxidised to S⁺ and is stripped from electrode 130. Measurement of the current during this stripping shows a peak at a potential characteristic of the electrochemical reaction:

S → S⁺ + e^" This potential indicates the identity of S and the magnitude of the current peak indicates the concentration of S⁺ in the flame.

A further embodiment of the present invention is as an environmental electrochemical converter. An electrochemical cell with two electrodes with a flame as an electrolyte may be used to convert noxious gases such as nitrogen oxides and sulphur dioxide, for example. The noxious gas is introduced into the flame between the electrodes. This gas is converted to less harmful substances via redox reactions in the electrochemical cell. For example, nitrogen oxide may be converted to molecular nitrogen and oxygen.

A schematic diagram of an environmental converter is shown in Fig. 12. Two large area electrodes 150 and 152 are contacted by a flame 154 with, for example, a substance A,C introduced into the flame. In the flame, substance A,C dissociates into A + C. A power supply 156 applies a potential difference between electrodes 150 and 152. This potential difference is sufficient to electrochemically convert A to B and C to D at electrodes 150 and 152, respectively. The products B and D are environmentally less harmful than A,C.

Whilst various embodiments of the present invention have been described in detail, modifications and adaptations of these and further embodiments will be apparent to those skilled in the art. In particular, it will be apparent to those skilled in the art that a discharge plasma could be used in place of a flame in the above described embodiments of the invention. However, it is to be expressly understood that such further embodiments are within the spirit and scope of the present invention.

Claims

1. A method of operating an electrochemical cell to carry out an electrochemical reaction and/or perform an electrochemical measurement, the method including the step of enveloping in a plasma a respective part of each of two electrodes, the plasma acting as an electrolyte between the electrodes in the electrochemical cell, wherein the plasma is generated at a location away from said electrodes of the cell and moves to the electrodes.

2. A method of operating an electrochemical cell to carry out an electrochemical reaction and/or perform an electrochemical measurement, the method including the step of enveloping in a plasma a respective part of each of two electrodes, the plasma acting as an electrolyte between the electrodes in the electrochemical cell, wherein the plasma is produced by plasma generating means, said electrodes of the electrochemical cell being electrically independent of the plasma generating means.

3. A method according to claim 1 or claim 2 wherein the plasma is a combustion flame.

4. A method of operating an electrochemical cell to carry out an electrochemical reaction and/or perform an electrochemical measurement, the method including the step of enveloping in a plasma a respective part of each of two electrodes, the plasma acting as an electrolyte between the electrodes in the electrochemical cell, wherein the plasma is a combustion flame.

5. A method according to any one of claims 1 to 4 , wherein an electrochemical reaction in the electrochemical cell is driven by a difference in electrochemical potential between the electrodes.

6. A method according to claim 5 wherein an emf generated between the electrodes is used to drive a current through an external circuit .

7. A method according to any one of claims 1 to 4 , wherein an electrochemical reaction in the electrochemical cell is driven by a potential difference applied between the electrodes.

8. A method according to claim 7 wherein a species is introduced into the plasma and undergoes an electrochemical reaction.

9. A method according to claim 8 wherein an electrochemical reaction product of the species is electrochemically plated onto one of the electrodes.

10. A method according to claim 8 or claim 9 wherein the species is an oxide of nitrogen, sulphur dioxide or an organic compound, such as a hydrocarbon or an organohalogen compound.

11. A method according to any one of claims 1 to 4 wherein an electrochemical measurement is performed by the steps of :

(i) measuring a first emf between the two electrodes; (ii) feeding a species into the plasma;

(iii) measuring a second emf between the two electrodes with the species in the plasma, wherein the difference between the first emf and the second emf provides an indication of the concentration of species in the plasma.

12. A method according to claim 7 including the steps of: (i) introducing a species into the plasma;

(ii) controlling the potential difference applied between the electrodes; and

(iii) measuring the current flowing between the electrodes at different applied potential differences, wherein the current measurement providing an indication of the concentration of the species.

13. A method according to claim 12 wherein the potential difference is controlled as follows: (i) a first potential difference is applied whereby an electrochemical reaction product of the species is coated onto an electrode, and

(ii) a second potential difference is applied whereby the electrochemical reaction product is electrochemically stripped from the electrode at a potential difference characteristic of the electrochemical reaction product .

14. A method according to any one of the above claims wherein the plasma is generated at a pressure of at least 0.5 atmosphere .

15. A method according to any one of claims 1, 3 and 4, wherein the plasma is produced by plasma generating means, said electrodes of the electrochemical cell being electrically independent of the plasma generating means.

16. A method according to claim 2, wherein the plasma is generated at a location away from said electrodes of the cell and moves to the electrodes.

17. A method according to any of the above claims wherein the plasma has an upward convective flow.

18. An electrochemical cell arranged for carrying out the method of any one of claims 1 to 17 including at least two electrodes and a plasma-producing means, wherein in use the electrodes contact a plasma produced by the plasma-producing means and the plasma acts as an electrolyte for the electrochemical cell .

19. An electrochemical cell according to claim 18 wherein each respective electrode is made from a metal or alloy comprising or consisting of at least one or more metals selected from the group consisting of Ti, Nb, Mo, Pt, W, Rh, Ta and Hf .

20. An electrochemical cell according to claim 18 or claim 19 wherein a first electrode has a first electrochemical potential and a second electrode has a second electrochemical potential which may be equal to or not equal to the first electrochemical potential.

21. An electrochemical cell according to any one of claims 18 to 20, the electrochemical cell further including a third electrode, wherein in use the third electrode contacts the plasma and acts as a counter electrode .

22. An electrochemical cell according to any one of claims 18 to 21, wherein the plasma producing means is a flame producing means .

23. An electrochemical cell according to claim 22, the flame producing means including feed means for feeding gaseous oxidisable fuel and gaseous oxidant to a flame region.

24. An electrochemical cell according to claim 22 wherein the feed means has two portions, a first portion including means whereby a species is feedable into the flame region.