BACKGROUND OF THE INVENTION

THIS invention relates to electrodes for use in electrochemical applications.

There are three main applications utilising electrochemical techniques. These are the synthesis of new materials, the destruction of waste materials, and the detection and measurement of materials, often present in low concentration.

In detection or sensor applications, electrochemical analysis utilises the relationship between current and voltage measured on immersed electrodes to characterise the solution in which the electrode is immersed. Dependent on the application, one of the current or voltage may be fixed and the other parameter allowed to vary, for example as the solution varies. Alternatively, the solution may be essentially fixed, and one of the current or voltage may be swept across a range of values and the response in the other parameter recorded in the form of a time/current plot or voltammogramme.

In material synthesis or destruction processes, electrochemical processing comprises modifying materials in liquid form, by the passage of an electric current through them or by application of an electric voltage across them, using electrodes in electrical contact with them. Typical examples are the modification of waste materials to reduce their environmental impact before disposal, or the production of desirable chemicals by electrochemical means.

In recent years, substantial interest has been generated in the use of diamond as an electrode material, as it possesses a range of advantages over other materials. In these applications diamond may be used as a free¬ standing layer or as a coating onto a substrate material such as silicon, molybdenum or tungsten. As an example, commercial exploitation of this technology has been established by Permalec Electrode Ltd, with the DiaCell for water treatment to control the development of Legionella Pneumophia without adding large quantities of chemicals. The B doped diamond is grown onto Si wafers. A further example is Condias GmbH with their DiaChem electrodes and Condiacell electrochemical cell for waste water treatment. One application is the disinfection of seawater held in ships' hulls which is a potential cause of contamination as ships move around the world. Although functional, these electrodes are reported to show significant loss of weight and thickness at a rate related to the current, with the diamond film eroding and perforating.

SUMMARY OF THE INVENTION

According to the invention there is provided an electrode comprising an electrode body having a region of heavily doped CVD diamond and a layer of lightly doped CVD diamond covering at least a portion of the region of heavily doped CVD diamond.

The electrode body preferably comprises an electrically conductive support layer coated with a heavily doped CVD diamond layer, which diamond layer provides the region of heavily doped CVD diamond. These two layers preferably form the majority of the thickness of the final electrode, with the thin lightly doped CVD diamond layer providing the final layer that may be contacted with an electrochemical environment. Typically, the doping of the diamond is provided by using S, P, N or B, although boron is preferred. Different dopants can be used in each or all of the layers.

In one embodiment of the invention, the support layer is formed of electrically conducting metal or diamond. In a particularly preferred embodiment the electrically conductive support layer is itself heavily boron doped CVD diamond, so that the final structure comprises entirely diamond with the majority of the thickness of the layer being heavily doped CVD diamond and the final layer in contact with the electrochemical environment being lightly doped CVD diamond.

The device may include only one working side, that is the lightly doped CVD diamond layer may be applied to one side of the electrode and only this side exposed to the electrochemical environment. More typically, however, the electrode will have two working sides, with the heavily boron doped electrode body covered on both major surfaces by the lightly boron doped diamond. In this configuration edges may also be coated with lightly doped diamond or with some other passivation or protective layer, such as undoped CVD diamond, for example.

Electrical contact to the electrode structure is to the conductive electrode body, either to the metal or heavily doped diamond portion of the electrode body, with this contact either not physically in contact with the electrochemical fluids or protected from them by a protective layer.

Whilst in the majority of applications requiring this invention the main objective is to provide a large surface area through which a heavy current can be passed, and of consequence a simple one sided or two sided electrode is typical, the use of non-conducting passivation or protective layers can extend to providing a more detailed geometric arrangement to the surface area of the whole electrode structure which will pass current and participate in the electrochemistry. Preferably, such a protective or passivation layer is undoped CVD diamond. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This invention provides for the use of lightly doped CVD diamond layers in heavily doped CVD diamond electrodes to control the erosion mechanisms that cause significant loss of mass and thickness of the electrodes in the application.

Characterisation of the mass loss mechanisms of CVD diamond electrodes has shown that chemical attack is enhanced in heavily doped regions. Typically, where the diamond is a coating, this perforation penetrates the diamond and attacks the diamond-substrate interface causing delamination. Even in the cases where the electrode is solid diamond the erosion can cause weakening of the electrode, or modify the surface characteristics and thus its utility as an electrode material.

However, to function as an electrode or electrode coating the diamond must be conducting, since otherwise the electrochemical process could not proceed.

The key to this invention is the determination that a thin surface layer of diamond with relatively low electrical conductivity is tolerable, and in some circumstances beneficial, to the electrochemical process provided that the overall impedance of the system is not radically increased. Since the majority of the current within a large planar electrode is parallel to the plane of the electrode, to provide current to all areas of the surface, the central portion of the electrode needs to remain highly conducting. This is achieved by using a non-diamond conducting core (such as a metal) or by using a heavily doped diamond conducting core. Where a non-diamond conducting core is used, nucleation of a polycrystalline diamond layer on it means that the grain structure is not initially ideal to form an impermeable surface coating with the required electrochemical properties. A minimum thickness of diamond coating is therefore required, which is beneficially heavily doped to provide sufficient electrical conductivity. The surface layer of the diamond coating or solid diamond electrode can then be covered with a low or lightly doped diamond layer which is more resistant to electrochemical etching.

Typically, where a non-diamond core is used, it forms a substantial portion and more typically the majority of the thickness and provides the mechanical strength of the final electrode. This non-diamond core can be locally in the form of a planar structure, such as a sheet or object shaped from sheet, or it can be locally three dimensional, such as a wire grid or mesh, or perforated sheet, or aperture, or the like.

The heavily doped diamond layer, when used as an intermediate layer between a non-diamond core and the outer lightly doped layer, typically forms the majority of the total thickness of the diamond layers. Exact dimensions are very application specific, as electrodes can themselves be fabricated over a wide range of dimensions, but typically the thickness of the heavily doped diamond layer exceeds 10 μm, more typically exceeds 20 μm, even more typically exceeds 50 μm, and most typically exceeds 100 μm.

The use of a lightly doped layer, sufficiently thin not to have a significant impact on overall system impedance, that is deposited directly on to a suitable electrically conductive metallic substrate is not desirable. Thin (typically, but not exclusively, less that 20 μm) CVD diamond layers can contain pinholes, with the result that the metallic substrate material is preferentially attacked during the electrolysis process. Localised galvanic cells form such that parts of the substrate dissolve rapidly and the integrity of the substrate is lost. By having a highly doped, low impedance layer immediately beneath the thin lightly doped layer, any pinholes or other imperfections in the lightly doped layer are in contact with a much more chemically resistant material and localised rapid corrosion of the substrate material is thereby prevented.

The heavily doped diamond layer when used as the core, typically forms the majority of the total thickness of the diamond layers. Exact dimensions are very application specific, as electrodes can themselves be fabricated over a wide range of dimensions. The thickness of the heavily doped diamond layer typically exceeds 100 μm, more typically exceeds 200 μm, even more typically exceeds 300 μm, and most typically exceeds 500 μm.

In comparison the lightly doped layer is typically thinner than the heavily doped layer, with an upper limit on the thickness of the layer of typically 50 μm, more typically 20 μm, even more typically 10 μm, even more typically 5 μm, and most typically 2 μm, and with a lower limit on the thickness of the lightly doped layer of typically 100 nm, more typically 200 nm, even more typically 500 nm, and most typically 900 nm.

Doping levels for the doped diamond layers are dependent on the dopant in use, but in all cases the dopant concentration in the heavily doped layer exceeds that of the lightly doped layer by a factor of 5, preferably by a factor of 10, more preferably by a factor of 30, even more preferably by a factor of 100, even more preferably by a factor of 300, and most preferably by a factor of 1000.

Dopants useful in this application include all the dopants of diamond, including the dopants S, P, N, and B. The dopant most typically used is B. Where the dopant is boron then typical concentrations in the different layers of the electrode are as follows:

a) in the heavily doped diamond layer the upper limit of the average boron concentration in the diamond is preferably 3 x 1021 atoms/cm3, more preferably 1 x 1021 atoms/cm3, even more preferably 5 x 1020 atoms/cm3, even more preferably 2 x 1020 atoms/cm3, and most preferably 1 x 1020 atoms/cm3, and the lower limit of the average boron concentration in the diamond is preferably 1 x 1018 atoms/cm3, more preferably 3 x 1018 atoms/cm3, even more preferably 1 x 1019 atoms/cm3, and most preferably 3 x 1019 atoms/cm3; and b) in the lightly doped diamond layer the upper limit of the average boron concentration in the diamond is preferably 3 x 1019 atoms/cm3, more preferably 1 x 1019 atoms/cm3, even more preferably 3 x 1018 atoms/cm3, even more preferably 1 x 1018 atoms/cm3, and most preferably 3 x 1017 atoms/cm3, and the lower limit of the average boron concentration in the diamond is preferably 1 x 1015 atoms/cm3, more preferably 3 x 1015 atoms/cm3, even more preferably 1 x 1016 atoms/cm3, even more preferably 3 x 1016 atoms/cm3, and most preferably 1 x 1017 atoms/cm3.

Typical resistivity values for low resistivity heavily doped diamond are in the range 10"2- lO^ohm.m, with more typical values being 3 x10~3- 3 x 10' 4 ohm.m. Suitable resistivity values for the lightly doped layer are generally in the range 103- 10"2 ohm.m, depending on the application and the thickness of the layer. For example, assuming that an application requires a current density J = 1000 A/m2 over a square electrode of side 100 mm and area A = 0.01 m2, where the lightly doped layer has a thickness of t = 50 μm and a resistivity p.

Power dissipated is W = I2R I = current, R = resistance W = AJ2pt

Thus, power dissipated in the thin overlayer = 0.5 x p Watts. For p = 103 this gives 500 W, and for p = 10"2 this gives 5 mW. Lower or higher values would be obtained for different thicknesses of coating. Typically, in an application the power loss due to this mechanism would preferably be below 5% of the total power used, more preferably less than 1 %, and even more preferably less than 0.2%. Thus, the preferred upper limit on the resistivity of the lightly doped diamond layer is 103 ohm.m, more preferably 102 ohm.m, even more preferably 10 ohm.m, even more preferably 1 ohm.m, and most preferably 0.3 ohm.m, and the preferred lower limit on the resistivity of the lightly doped diamond layer is 5 x 10"3 ohm.m, more preferably 1 x 10"2 ohm.m, even more preferably 3 x 10"2 ohm.m, even more preferably 1 x 10"1 ohm.m, and most preferably 2 x 10~1 ohm.m.

The specification of the lightly boron doped layer refers to the average values of the layer taken through its thickness. In some instances it is advantageous to grade the lightly doped layer, typically with the level of doping reducing towards the outer surface. In some instances there may be no sharp boundary between the heavily doped region and the lightly doped surface layer since the whole variation of doping concentration and electrical conductivity could be graded or continuously varying. In these circumstances, placing a notional divide between the heavily doped region and the lightly doped region, and then taking average values for the doping and conductivity, such a structure can again be seen to conform to the general outline of the characteristics of the heavily and lightly doped layers. Alternatively there may be other variation in detail, such as using further layers in the structure to provide three or more layers of decreasing conductivity as the surface in contact with the electrochemical process is approached. Again, by notionally grouping such layers into two groups, and taking a thickness weighted average of the dopant concentrations and electrical conductivities, such a structure can again be seen to conform to the general outline of the characteristics of the heavily and lightly doped layers.

Since the dopant concentration of the bulk of the diamond and the surface of the diamond are now decoupled, it is possible to reduce the overall impedance of the system by increasing the dopant concentration in the bulk to more than compensate for the increase in impedance adjacent to the surface due to the lightly doped layer.

The invention is directed mainly at polycrystalline diamond layers. By definition, in the case of coatings onto a non-diamond core, heterogeneous nucleation of the diamond results in a polycrystalline diamond layer. However, the design also has utility in solid single crystal diamond electrodes, particularly for small area electrodes, and in addition to reducing erosion can modify the behaviour of the electrode and improve uniformity of response.

An additional benefit arising from this invention is the improved uniformity in electrochemical processing across the major surfaces of a large electrode. By providing a highly conducting core and then uniform resistance across the surface of that core, the uniformity of the current density across the electrode is improved. By protecting the core from etching using the lightly doped CVD diamond layer, the concentration of the dopant in the core diamond layer can be increased without exacerbating the etching problems.

Fabrication of the electrode of the invention is relatively easily achieved by modifying the rate of dopant addition to the CVD process used to produce the electrodes. Many such CVD processes for the production of typically boron doped diamond are now known in the art, with the source of boron generally being diborane although other gaseous, liquid/vapour or solid sources can be used. One advantageous method of producing the lightly doped layer is to largely or wholly remove the source of new boron dopant in the process and rely on the long residence time boron generally shows in such processes to produce a lightly doped layer with decreasing boron concentration throughout the layer.

Thus the invention can be practiced using microwave synthesis, hot filament synthesis, jet synthesis and any other means by which doped CVD diamond can be produced. The preferred method varies with the application and size of electrode required, for example hot filament synthesis being preferred in some large area applications, whereas microwave or jet techniques may be preferred for smaller polycrystalline and single crystal diamond electrodes.

The electrode of the invention can be used in a number of different configurations. In one embodiment, two electrodes of the invention are immersed in the liquid to be treated such that their large planar, diamond- coated surfaces are parallel and adjacent, and separated by a distance of between 1 mm and 200 mm depending on the nature of the process and liquid. A voltage (or potential difference) of typically between 0.5 V and 20 V is applied between the electrodes by means of an external circuit. In this configuration, one electrode is the cathode and the other is the anode, and oxidative and reductive chemical reactions occur at the surfaces of the electrodes depending upon the nature of the electrolyte and the applied potential difference.

In a second embodiment, a plurality of planar electrodes of the invention are coated with diamond on both large planar surfaces, and are arranged parallel and adjacent to each other to form an electrode stack. The number of electrodes in the stack can be between three and typically less than 500. Adjacent electrodes in the stack are separated by between 1 mm and 200 mm along the axis of the stack. It may be arranged so that the liquid being electrolysed flows in a serpentine manner between the first and second electrodes, then between the second and third electrodes and so on. until it has passed through the whole stack, thus putting the spaces between the electrodes effectively in series to increase the treatment time of a particular volume. Alternatively the spaces between the electrodes may be effectively used in parallel, simply to increase the volume of material treated.

In either case it is arranged that the flow between the first and second electrodes and second and third electrodes, and so on, does not provide a 'short circuit' for electrical current. A potential difference is then applied between the first and last electrodes of the electrode stack. The applied potential difference results in an electric field between the first and last electrodes. The resultant electric field causes the intermediate electrodes to adopt potentials intermediate between those of the first and last electrodes, the magnitude of which depends on the intermediate electrodes' positions. Intermediate electrodes therefore become simultaneously anodes with respect to adjacent cathodes and cathodes with respect to adjacent anodes. For example in a three electrode cell, the first electrode is a cathode, the third electrode is an anode and for the second electrode, the surface closest to the first electrode (a cathode) is an anode and the surface closest the third electrode (an anode) is a cathode. The electrode stack therefore consists of two anode-cathode pairs between which the electrolyte flows sequentially. The potential difference between the first electrode (cathode) and last electrode (anode) is selected such that the potential difference between any two adjacent electrodes is in the range 0.5 V to 20 V.

In a further embodiment, the intermediate electrodes described above could be deliberately biased using a separate electrical connection to each so that the potential difference between different pairs of adjacent electrodes varies so that different reactions can be targeted in different parts of the electrode stack, for example a series of oxidation processes.

Example 1

This example relates to the manufacture of an electrode of the invention and its performance as an electrode in an electrochemical reaction.

A number of electrodes were prepared using the method described below. A high purity tungsten substrate 100 mm in diameter and 1.0 mm thick was prepared by electro-discharge machining from a larger blank. The surfaces of the tungsten disc were lapped to a surface roughness (as defined by the Ra of the surface) of between 0.5 μm and 2 μm. The tungsten disc was then thoroughly cleaned using iso-propanol. Methods known in the art were used to seed the tungsten substrate prior to diamond deposition.

A microwave plasma CVD technique was used to deposit the highly and lightly doped diamond layers. Diamond deposition was performed in a microwave CVD deposition system such as is well known in the art. The total gas flow was in the region of 3000 seem, comprising 1% methane, 1% argon, balance hydrogen with a diborane (B2H6) added such that the diborane to methane ratio is 0.06%. The exact diborane to methane ratio required to achieve a given resistivity is a sensitive function of the exact deposition conditions and, as those skilled in the art will be aware, can vary substantially between synthesis systems. The diborane was added with hydrogen as a dilutant, in this case as 500 ppm diborane in hydrogen. The total hydrogen in the gas mixture includes the hydrogen used to dilute the diborane. The pressure in the chamber during deposition was 18 x 103 Pa (140 torr). Deposition continued until the layer thickness was between 40 and 50 μm, the exact thickness being unimportant. Once the desired thickness was reached, the ratio of diborane to methane was reduced to 0.001 % and growth continued for a duration sufficient to deposit a further layer of boron doped diamond with a thickness of approximately 2 μm over the whole surface.

The resultant electrode consisted of a 1.0 mm thick tungsten substrate covered on one surface with a 40-50 μm thick, highly doped, layer of CVD diamond with a boron concentration measured by secondary ion mass spectroscopy (SIMS) of 1020 atoms/cm3 and a 2 μm thick lightly doped capping layer having a boron concentration measured by SIMS of approximately 1018 atoms/cm3.

A pair of electrodes, made using the above technique, was used in an electrochemical cell containing a mixture of chemical species including water, sulphuric acid and water soluble organic compounds such as acetone and other ketones, esters and phenol-based compounds. The total concentration of organic compounds in the solution was approximately 25 g/dm3. Only the diamond coated surfaces of the electrodes were exposed to the electrolyte. The separation of the electrodes was 20 mm and the total cell volume was 0.5 dm3.

A controlled voltage, high current power supply was used to apply a voltage of approximately 8 V between the electrodes. This yielded a current density of 0.8 A/cm2. The organic compounds in the electrolyte were directly oxidised in the cell, such that after 60 minutes of operation, the phenol concentration had dropped to approximately 60% of the initial concentration. A further 60 minutes of operation further reduced the phenol concentration to approximately 30% of the initial concentration.

Example 2

A series of four electrodes were prepared as described in Example 1. Using the method described in Example 1 , two of the electrodes were coated with CVD diamond on both sides to yield a pair of electrodes coated on both major surfaces.

The four electrodes were used in an electrode stack as previously described, the electrodes coated on a single side forming the primary anode and cathode and the electrodes coated on both sides forming the self-biasing intermediate electrodes. The separation of the electrode plates was 20 mm.

An electrolyte, consisting of residue from dyestuff manufacturing with a chemical oxygen demand of approximately 15 g/dm3, was pumped between the electrodes at a flow rate of approximately 5 dm3/min and a voltage of approximately 20 V was applied between the primary anode and cathode.

Assay of the electrolyte after passage through the electrode stack showed the chemical oxygen demand of the residue was reduced to approximately 5 g/dm3 after 30 minutes of circulation and less than 1 g/dm3 after 60 minutes of circulation.

Examination of the electrodes after testing revealed no mass loss and scanning electron microscopy revealed no signs of erosion of the diamond.

Example 3

A pair of electrodes was prepared as in Example 1. Their mass wascarefully measured and their surfaces examined by scanning electron microscopy. The electrodes were used in a cell containing dilute sulphuric acid (0.1 molar) and phenol (C6H5OH) at a concentration of 0.05 M (4.7 g/dm3). A voltage of 7 V was applied between the electrodes with a resultant current density of approximately 0.75 A/cm2. The cell was operated for a period of 500 hours, the electrolyte being replaced every 8 hours. After use for 500 hours, the masses of the electrodes were again carefully measured and the surfaces examined by scanning electron microscopy. No signs of erosion of the surface or mass loss larger than the experimental error were observed.

Example 4

A number of electrodes were prepared using the method described below. A high purity tungsten substrate 100 mm in diameter and 6.2 mm thick was prepared by electro-discharge machining from a larger blank. The surfaces of the tungsten disc were lapped to a surface roughness (as defined by the Ra of the surface) of between 0.5 μm and 2 μm. The tungsten disc was then thoroughly cleaned using iso-propanol. Methods known in the art were used to seed the tungsten substrate prior to diamond deposition.

A microwave plasma CVD technique was used to deposit the highly and lightly doped diamond layers. Diamond deposition was performed in a typical microwave CVD deposition system as is well known in the art . The total gas flow was in the region of 3000 seem, comprising 1 % methane, 1% argon, balance hydrogen with a diborane (B2H6) added such that the diborane to methane ratio is 0.06%. The exact diborane to methane ratio required to achieve a given resistivity is a sensitive function of the exact deposition conditions and, as those skilled in the art will be aware, can vary substantially between synthesis systems. The diborane was added with hydrogen as a dilutant, in this case as 500 ppm diborane in hydrogen. The total hydrogen in the gas mixture includes the hydrogen used to dilute the diborane. The pressure in the chamber during deposition was 18 x 103 Pa (140 torr). Deposition continued until the layer thickness was between 600 and 700 μm, the exact thickness being unimportant. Once the desired thickness was reached, the ratio of diborane to methane was reduced to 0.001% and growth continued for a duration sufficient to deposit a further layer of boron doped diamond with a thickness of approximately 3 μm over the whole surface.

Synthesis was then terminated and the boron doped layer was separated from the tungsten substrate.

The surface of the boron doped disc originally adjacent to the tungsten substrate is then coated with a 3 μm thick layer of lightly boron doped diamond.

The resulting layer consisted of a highly boron doped layer between 600 and 700 μm covered with a lightly boron doped layer approximated 3 μm.

Electrodes made by this technique were used as electrochemical cells of the type described in Example 3 with similar results.