EP2710169A2

EP2710169A2 - Electrochemical cell and method for operation of the same

Info

Publication number: EP2710169A2
Application number: EP12727153.4A
Authority: EP
Inventors: Patrick Bray
Original assignee: A Gas International Ltd
Current assignee: A Gas International Ltd
Priority date: 2011-05-17
Filing date: 2012-05-16
Publication date: 2014-03-26
Also published as: WO2012156671A2; GB201108254D0; GB2490913B; WO2012156671A3; GB2490913A; ZA201309362B

Abstract

There is disclosed a method of operating an electrochemical cell for use in the electrolysis of water to produce ozone, the cell comprising a first electrode and a second electrode separated by a Cation Exchange Membrane, wherein the first and second electrodes each has an active surface formed from electrically conductive diamond, in a first mode of operation the electrochemical cell having a flow of electrical current in a first polarity, whereby the first electrode functions as an anode and the second electrode is a cathode; the method comprising providing the electrochemical cell in the first mode of operation with a supply of current to the electrodes in the first polarity to produce ozone at the first electrode for a first period of time; and providing the electrochemical cell in a second mode of operation with a supply of current to the electrodes in a second polarity, opposite to the first polarity, for a second period of time. An electrochemical cell for use in the aforementioned method is also disclosed.

Description

ELECTROCHEMICAL CELL AND METHOD FOR OPERATION OF THE SAME

The present invention relates to an electrochemical cell and a method of operating the same. The present invention concerns in particular an electrochemical cell for the production of ozone and to a method of operating the cell.

Electrochemical cells find use in a range of applications for conducting a variety of electrochemical processes. In general, the cells comprise an anode and a cathode, separated by a semi-permeable membrane, in particular a Cation Exchange Membrane that may also be described as a Proton Exchange Membrane. One particular application for electrochemical cells is the production of ozone by the electrolysis of water. Ozone is one of the strongest and fastest acting oxidants and disinfectants available for water treatment. Although ozone is only partially soluble in water, it is sufficiently soluble and stable to disinfect water contaminated by pathogenic microorganisms and can be utilised for a wide range of disinfection applications including sterilisation. Microorganisms of all types are destroyed by ozone and ozonated water including bacteria, viruses, fungi and fungal spores, oocysts, protozoa and algae.

Ozone decomposes rapidly in water into oxygen and has a relatively short half life. The half life of ozone in water is dependant upon temperature, pH and other factors. However, the short half-life of ozone is a further advantage, as once treatment has been applied, the ozone will rapidly disappear, rendering the treated water safe. Once treatment has been applied, ozone that remains in solution will rapidly decay to oxygen. Unlike chorine based disinfectants, ozone does not form toxic halogenated intermediates and undesirable end products such as Tnhalomethanes (THMs).

The concentration of ozone dissolved in water determines the rate of oxidation and the degree of disinfection in any given volume of water, with the higher the concentration ozone, the faster the rate of disinfection of micro-organisms. Electrolysis of water at high electrode potential produces ozone at the anode in an electrochemical cell according to the following equations:

3H₂0 -> 0₃ + 3H⁺ + 6e- and

2H₂0 -» 0₂ + 4H⁺ + 4e^" (E₀ = 1.23 V_SHe)

H₂0 + 0₂ → 0₃ + 2H⁺ + 2e^" (E₀ = 2.07 V_SHE)

Ozone may be produced at higher current efficiencies and in higher concentrations from low conductivity water, deionised water, demineralised water, and softened water. The ozone may be produced in situ in the water stream by the above-mentioned anodic reactions as water flows through the electrochemical cell past the anode. Ozone dissolved in water is described as ozonated water. The production of ozone and ozonated water by electrolysis using an electrochemical cell is known in the art. DE 10025167 discloses an electrode assembly for use in a cell for the electrolytic production of ozone and/or oxygen. The cell comprises an anode and a cathode separated by a membrane in direct contact with each of the electrodes.

WO 2005/058761 discloses an electrolytic cell for the treatment of contaminated water. The cell comprises an anode and a cathode, with water being passed between the two electrodes. The cathode is preferably formed from nickel, titanium, graphite or a conductive metal oxide. The cathode is provided with a coating, preferably boron doped diamond (BDD), activated carbon or graphite. The anode is preferably formed from titanium, niobium, or a conductive non-metallic material, such as p-doped silicon. The anode is preferably provided with a coating, with preferred coatings being boron doped diamond (BDD), lead oxide (Pb0₂), tin oxide (Sn0₂), platinised titanium, platinum, activated carbon and graphite.

US 2008/156642 concerns a system for the disinfection of low-conductivity liquids, in particular water, the system comprising an electrochemical cell in which electrodes are arranged to allow the liquid to flow therearound. Oxidizing agents, such as ozone, are produced from the liquid by the application of an electrical current.

US 2007/0023273 concerns a method of sterilization and an electrolytic water ejecting apparatus. Raw water is sterilized by electrolysis in a unit comprising a cell having a cathode and an anode at least having a part containing a conductive diamond material.

WO 2010/037391 discloses a device and process for removing microbial impurities in water. The device comprises a disinfection chamber having a stack of electrodes of a least two perforated electrode plates of an electrically conductive material.

An electrolytic apparatus and an electrolytic method are disclosed in

JP 2011038145.

US 2010/0006450 discloses a diamond electrode arrangement for use in an electrochemical cell for the treatment of water and/or the production of ozone. The cell comprises an anode and a cathode separated by a proton exchange membrane (PEM). The electrode is formed with a diamond plate and is configured to have one or more slots (described as elongated apertures) therein, to provide a minimum specified apertures length per unit of working area of the electrode.

During the production of ozone at the anode in the electrochemical cell, the metal anions in solution, such as calcium, magnesium, iron and manganese, migrate to the cathode, causing a build up of these metals and their compounds on the active surface of the cathode. The deposition of these metals and their compounds individually and collectively causes passivation of the cathode and a consequential reduction in the flow of current through the electrochemical cell. This process of electro-deposition of materials on the cathode passifies the electrodes in the electrochemical cell causing the current flowing through the cell to reduce over a period of time, thereby reducing the productivity of the cell over time, to the point when ozone may no longer be produced at the anode. Further, unrestricted cathodic electro-deposition of metals and their compounds in the electrochemical cell causes W

4

the liquid flow channels to become restricted by the build-up of deposited substances and even, in extreme cases, to become completely blocked.

Compounds of calcium and magnesium are found in significant concentration in hard water and it is known that these compounds are the principal cause of electrode passivation within electrochemical cells used in the production of ozone or ozonated water. In particular, it is known that calcium cations readily pass through the Cation Exchange Membrane present between the electrodes in the cell and that calcium is rapidly deposited on the cathode within the electrochemical cell.

In the absence of a cathode cleaning system, the cathodes in an electrochemical cell become passified by the metal cations in solution in the feed water. The build up of substances on the cathode will inevitably cause the cell to fail. Accordingly, there is a need to address the problem of the electro-deposition of substances at the electrodes of an electrochemical cell, in particular the cathode, and the resultant passivation of the cell and reduction or loss of overall performance.

Chemical cleaning systems for cathodes are known in the art and have been used. Chemical cleaning systems rely on the use of agents in the catholyte to remove material deposited on the cathodes. Chemical agents that may be used in such chemical cleaning systems include acids of various types, in particular mild organic acids such as citric acid, ethylenediaminetetraactic acid (EDTA) and other mild acids. Citric acid and EDTA have a further benefit as they also act as a chelating agent, removing metal cations from solution.

Chemical cleaning systems for cathodes require a separate catholyte circuit. The catholyte is the aqueous solution that flows past the cathodes in the electrochemical cell. The concentration of the chemi^'cai cleaning agent in the catholyte must be maintained at the required level in order that the performance of the electrochemical cell is maintained. The use of a catholyte circuit with chemical cleaning system increases the complexity of the electrochemical cell and its operation. Surprisingly, it has been found that the passivation of an electrochemical cell for use in the production of ozone or ozonated water by the electrolysis of water may be reduced or prevented entirely by the periodic reversal of the electric current flowing through the cell, provided the electrodes of the cell are provided with an active surface formed from diamond, preferably polycrystalline boron doped diamond (BDD).

Accordingly, in a first aspect, the present invention provides a method of operating an electrochemical cell for use in the electrolysis of water to produce ozone, the cell comprising a first electrode and a second electrode separated by a Cation Exchange Membrane, wherein the first and second electrodes each has an active surface formed from electrically conductive diamond, in a first mode of operation the electrochemical cell having a flow of electrical current in a first polarity, whereby the first electrode functions as an anode and the second electrode is a cathode; the method comprising:

providing the electrochemical cell in the first mode of operation with a supply of current to the electrodes in the first polarity to produce ozone at the first electrode for a first period of time; and

providing the electrochemical cell in a second mode of operation with a supply of current to the electrodes in a second polarity, opposite to the first polarity, for a second period of time.

In a further aspect, the present invention provides an electrochemical cell for use in the production of ozone in normal operation, the electrochemical cell comprising:

a first electrode and a first fluid conduit for bringing fluid into contact with the first electrode, the first electrode having an active surface formed from electrically conductive diamond;

a second electrode and a second fluid conduit for bringing fluid into contact with the second electrode, the second electrode having an active surface formed from electrically conductive diamond;

a Cation Exchange Membrane extending between the first electrode and the second electrode and separating the first fluid conduit from the second fluid conduit; an electrical supply system for providing an electric current to the first and second electrodes, the electrical supply system operable to provide the electric current to the electrodes with a first polarity during a first mode of operation and to provide an electric current to the electrodes with a second polarity opposite to the first polarity during a second mode of operation.

In the present invention, the electrochemical cell is operated in a first mode, that is with a current density applied to the electrodes in a first polarity, whereby the first electrode acts as the anode and the second electrode acts as the cathode. When water is fed through the first conduit, ozone is produced at the anode, according to the electrochemical reactions described above. The operation of the electrochemical cell in the first mode is continued for a first period of time. During this mode of operation, material is deposited on the second electrode acting as the cathode, as described above. When it is required to remove material deposited on the second electrode, acting as the cathode in normal operation, the polarity of the current applied to the electrodes is reversed and the electrochemical cell operated in the second mode for a second period of time. This polarity reversal removes the materials deposited on the cathode during the first mode of operation of the cell. The use of reverse polarity to clean the electrodes of electrochemical cells is known in the art. A system for applying a reverse polarity to an electrochemical cell is described in US 2009/0229992. However, in general, it is known that reversing the polarity of electrodes in an electrochemical cell may cause damage to the structure and fabric of the electrodes within the cell, particularly the active surfaces of the electrodes, typically causing erosion and attrition of the surface of the electrodes, thereby reducing the operating lifetime of the electrodes and also reducing the performance of the electrochemical cell.

In particular, it is known that electrode materials, notably materials with metal oxide and mixed metal oxide coatings, are unsuitable for electrochemical processes where polarity reversal is required. For example, titanium electrodes that have a single or mixed metal oxide coating of ruthenium, iridium and tantalum, such as DSA™ electrodes, the active surface oxide layer breaks down as a result of frequent polarity reversals. Lead dioxide (Pb0₂) has been used as an anode material in electrochemical cells for the production of ozone and ozonated water. However, this material is unstable and erodes rapidly over time under the application of high current densities. It is has also been found that polarity reversal increases the rate of erosion of the lead dioxide electrode. Further, it is accepted that lead, as an electrode material, is unsuitable for application in drinking water processes as a result of its known toxicity.

However, surprisingly, it has been found that when employing electrodes having an active surface formed from diamond, in particular polycrystalline boron doped diamond (BDD) the aforementioned erosion of the electrodes is not observed.

As noted above, during the first mode of operation of the electrochemical cell, ozone is produced at the first electrode acting as the anode. When the polarity of the electrical current is reversed through the cell, the first electrode becomes the cathode, and the second electrode becomes the anode. The conditions prevailing in the vicinity of the anode in the electrochemical cell are acidic as a result of the generation of hydrogen ions on the surface of the anode. When the polarity of the applied current is reversed and the second electrode becomes the anode, acidic conditions are quickly established in the vicinity of the second electrode. Any substance that has been deposited on the electrode by the process of electro- deposition is dissolved and removed by the acidic conditions prevailing at the electrode. In particular, calcium and magnesium compounds formed on the surfaces of the electrodes are simply removed each time the polarity of the current applied to the electrochemical cell is reversed.

The process of polarity reversal is an effective means of cleaning the working electrodes in the electrochemical cell. The performance and current efficiency of the cell and, hence, the production of ozone is maintained in this way. One significant advantage of polarity reversal of the applied electrical current is that no chemical cleaning agents are required to maintain the performance of the electrochemical cell and the production of ozone at the anodes. In general, in accordance with the present invention, the electrochemical cell is operated for repeated periods of time in the first mode with a first operating current polarity, separated by operation for a second period of time in the second mode with the polarity of the current reversed. The length of operation of the cell in the first and second modes and the frequency at which the polarity of the applied current is reversed in the electrochemical cell can be varied to optimise the performance and efficiency of the cell, according to the prevailing operating conditions. In particular, the length of operation in each of the first and second modes may be increased or decreased to optimise performance and current efficiency.

The time intervals between successive polarity reversals can be varied within wide limits, in particular to optimise cell performance and take account of such operating parameters as the concentration of metal cations, such as calcium, magnesium, iron and manganese, and other substances present in the feed water serving as the anolyte to the cell.

The length of time that the cell is operated in the first mode of operation, so as to produce ozone at the first electrode, may be determined by monitoring the condition of the second electrode and the amount of substances deposited thereon. This may be achieved, for example,, by monitoring one or more operating parameters of the cell, such as the electrical current, measured in Amps, and the potential of the cell, measured in Volts. The condition of the second electrode may be similarly monitored during the operation in the second mode of operation.

The length of time of the first period of operation in the first mode, that is the time between successive polarity reversals may be varied as required to maintain efficient operation of the cell. This time may be from several seconds to one or more hours, depending upon the operation conditions of the cell. The /ength of time between successive polarity reversals is preferably in the range of from 5 seconds to 60 minutes, depending upon the purity of the water and, in particular, the concentration of metal cations, such as calcium, magnesium, iron and manganese, and other substances present in the water being fed to the cell, more preferably from 5 seconds to 30 minutes, still more preferably from 5 seconds to 10 minutes, still more preferably from 5 seconds to 5 minutes, still more preferably from 5 seconds to 60 seconds. In the case of 'soft' water, the length of time of the first period of operation in the first mode is preferably from 10 seconds to 60 seconds. In the case of 'hard' water, the length of time of operation in the first mode is more preferably from 5 seconds to 30 seconds.

The polarity of the applied current is reversed and the cell is operated in the second mode with the reversed polarity for a sufficient period of time to reduce or remove the material deposited on the second electrode. Again, this may be achieved, for example, by monitoring one or more operating parameters of the cell, such as the electrical current or the potential of the cell. Once the second electrode has been cleaned by operation in the second mode, operation of the cell may be switched to the first mode, which in this case may be considered to be normal operation. Typically, to remove deposits and clean the second electrode, the cell will require operation in the second mode with the reversed polarity of current for the same periods of time required for the periods of operation in the first mode. Preferably, to remove substances deposited on the second electrode, the cell is operated in the second mode with the polarity of the current reversed for a period of time in the range of from 5 seconds to 60 minutes, more preferably from 5 seconds to 30 minutes, still more preferably from 5 seconds to 10 minutes, still more preferably from 5 seconds to 5 minutes, still more preferably from 5 seconds to 60 seconds.

The operating current density, measured in Amps/cm², at the electrodes is a function of the electrical current applied to the cell, measured in Amps, from the power supply, divided by the active surface area of the diamond anodes. The current applied to the electrochemical cell, and therefore the current density at the anodes, during each of the first and second modes of operation may be selected to optimise the performance of the cell and to optimise the production of ozone and ozonated water.

The electrochemical cell may be operated at current densities up to 5.0 Amps/cm², depending upon the size and duty of the cell. Preferably, the current density is in the range of from 0.1 to 5.0 Amps/cm², more preferably from 0.4 to 3.2 Amps/cm², and still more preferably in the range 0.5 to 1.2 Amps/cm² for the production of ozonated water for most disinfection applications.

The current density applied during the second mode of operation is preferably the same or substantially the same as the current density applied in the first mode of operation, in particular, when the production of ozone and/or ozonated water is required in both the first and second modes of operation.

The electrochemical cell may be operated at applied voltages up to 50 Volts, depending upon the conductivity of the water stream being treated. According to the operating conditions the voltage is preferably at least 10 Volts, more preferably at least 12 Volts, still more preferably at least 15 Volts, still more preferably at least 20 Volts. Voltages in excess of 25 Volts may also be applied, for example a voltage up to 30 Volts or up to 40 Volts, as required. A voltage of between 12 and 24 Volts is particularly preferred.

As noted above, the present invention requires that the electrochemical cell has both the first and the second electrodes are provided with active surfaces formed from diamond.

Suitable diamond materials for forming the active surface of each electrode are known in the art and are commercially available. The electrically conductive diamond material may be a layer of single crystal synthetic diamond, natural diamond, in particular Type I IB diamond, or polycrystalline diamond. Polycrystalline diamond is particularly preferred. The diamond may be naturally occurring diamond, high pressure high temperature (HPHT) synthesised diamond or chemical vapour deposition (CVD) diamond, with CVD diamond again being preferred. The diamond material may consist essentially of carbon, but is doped with one or more elements that provide electrical conductivity. Suitable dopants to provide the diamond with electrical conductivity are known in the art. Diamond is preferably doped with boron to confer electrical conductivity and is described as boron doped diamond (BDD).

A particularly suitable and preferred diamond material is polycrystalline boron doped diamond (BDD). Again, BDD is a known and commercially available material. The active electrodes of the cell may be of a solid diamond material or a substrate material coated with diamond. The preferred electrode material is electrically conductive, solid, free standing polycrystalline Boron-doped diamond. This diamond material is manufactured by way of a process of chemical vapour deposition in a microwave plasma system. This diamond material is preferably from 200 to 1000 microns in thickness, more preferably from 300 to 800 microns thick. It is particularly preferred that the solid diamond material is a layer of thickness from 400 to 700 microns, more particularly from 500 to 600 microns.

Alternatively, the active electrode material may be a substrate material coated with conductive diamond. The substrate material may be any suitable material, examples of which include silicon (Si), tungsten (W), niobium (Nb), molybdenum (Mo) or tantalum (Ta). This diamond material is manufactured by known techniques, for example by way of a process of chemical vapour deposition in a hot filament system. The active diamond surface of the electrode material, in this case, is typically from 1 to 10 microns in thickness, more preferably from 3 to 5 microns thick.

Suitable techniques for manufacturing both solid free-standing electrically conductive Boron-doped diamond material and diamond coated material are known in the art. It has been found that diamond material provided as a layer formed on the substrate material is eroded under the conditions prevailing in the electrochemical cell during operation. This in turn reduces the longevity and operating life of the cell. Accordingly, it is preferred that the diamond material is provided as a layer of pre- formed solid diamond, such as the Boron-doped diamond material referred to hereinbefore.

The electrochemical cell comprises first and second electrodes, as noted above. Each of the first and second electrodes may comprise a single electrode or, more preferably a plurality of electrodes electrically connected to act together.

The electrochemical cell comprises a Cation Exchange Membrane disposed between the electrodes. The membrane permits the movement of cations, including hydrogen ions (protons) and positively charged metal cations, such as calcium, magnesium, iron and manganese, in the anolyte to pass through the membrane to the cathode, in either direction, depending upon the polarity of the current applied to the cell at any given time. The membrane is most preferably in contact with each electrode. Each electrode has a conduit for providing a fluid to contact the respective electrode, the two conduits being separated by the membrane. As described in US 2010/0006450, each electrode is preferably formed to have edges to the active surface of the diamond, with the semi-permeable membrane being in contact with the edges of the diamond material. In this way, at the interface between the anode, the membrane and the water in the conduit adjacent the anode, ozone is produced in the water stream (ozonated water). Hydrogen ions (protons) pass through the membrane to the cathode side of the cell where hydrogen gas is produced. Other positively charged metal cations such as calcium, magnesium, iron and manganese also pass through the membrane and are deposited on the cathode, as noted above.

Suitable materials for the membrane are known in the art and are commercially available. One particularly preferred class of materials for use in the membrane are sulfonated tetrafluoroethylene-based fluoropolymers. Such materials are known in the art and are commercially available, for example the Nafion™ range of products available from Dow Chemical.

During operation of the cell in the first mode, water is supplied to the conduits on the side of the membrane adjacent the first electrode acting as the anode and forms the anolyte. Water is also supplied to the conduits on the side of the membrane adjacent the second electrode acting as the cathode and forms the catholyte. In a preferred embodiment, the electrochemical cell is operated under a regime of forced flow circulation. Water is supplied to the cell under pressure and is evenly distributed either side of the membrane by means of . inlet manifolds connected to the conduits adjacent to both the first and second electrodes. The electrochemical reactions that take place when the cell is operating are therefore uniformly distributed in the conduits adjacent the edges of the electrodes. Preferably, the water supplied to the electrochemical cell is divided equally between the anolyte and cathol te sides of the cell. In this way, the water pressure either side of the membrane in the cell is equal, regardless of the rate of flow of water through the cell. Therefore, there is no hydrostatic force driving anolyte through the membrane into the catholyte side of the cell. This is in contrast to electrochemical cells that have separate catholyte circuits, such as required when using chemical cleaning systems for the cathode. Further, as the water supply is common to both sides of the membrane, the concentration of the anolyte and catholyte may also be maintained substantially equal. Therefore, there is no osmotic force present to drive water from the anolyte side of the cell into the catholyte side of the membrane.

The cell may be provided with water at any suitable flowrate. The water flow through the electrochemical cell is determined several factors including the size of the cell, the number of flow channels through the cell and the dimensions of the flow channels. The generation of ozone in the cell and therefore cell performance increases as the rate of mass transport increases at the interface of the anode, the membrane and the water flowing through the cell. Increasing the rate of flow of water to the optimum increases mass transport within the cell, and therefore cell efficiency, and the production of ozone. The range of flow is between 0.5 and 50 litres/minute depending upon the size of the electrochemical cell and the factors described above. The water flow through the cell is preferably at least 1 litre/minute, more preferably at least 5 litres/minute. Higher water flow rates may also be employed, subject to the size and capacity of the cell and particularly its flow channels, such as flow rates of at least 10 litres/minute, more preferably at least 20 litres/minute, up to 40 litres/minute.

The electrochemical cell can be operated at a wide range of water pressures, most conveniently at mains water pressure. Water pressures of at least 1 Bar may be employed, preferably 2 Bar, more preferably 4 Bar, still more preferably 6 Bar, and up to 8 Bar. To obtain the optimum efficiency of the electrochemical cell the water pressure should be regulated to maintain the optimum flow as described above.

In one preferred embodiment, the cell is operated with a supply of water to the conduits on both sides of the membrane within the cell, as described above. Further, the liquid outlets from each side are provided with means to divert the liquid stream leaving each side of the cell to either an anolyte system or a catholyte system. In operation, anolyte produced by the cell is the required product, in particular ozonated water. The catholyte is removed from the cell and is preferably passed to a degasser to remove hydrogen generated at the cathode. Thus, in this embodiment, the cell is operated for a first period of time in the first mode, with the first electrode as the anode and producing anolyte from its respective conduit to be fed to the anolyte system, while the second electrode acts as the cathode and produces catholyte from its respective conduit, to be fed to the catholyte system, such as the hydrogen degasser. The polarity of the current applied to the electrodes is reversed and the cell operated in the second mode, for a second period of time. In this second mode, the first electrode acts as the cathode and has the catholyte removed from its respective conduit directed to be processed in the catholyte system. The second electrode acts as the anode in the second mode and produces anolyte within its respective conduit, which is directed to the anolyte system as product of the cell. In this way, the cell may be operated in a particularly efficient manner, with each electrode of the cell operating as either the anode or the cathode, depending upon the mode of operation, and producing the required product throughout the entire operation.

In a further aspect, the present invention provides a method for removing deposits from the cathode of an electrochemical cell, the cathode having an active surface formed by diamond, the method comprising reversing the polarity of the current applied to the electrodes of the cell during normal operation for a sufficient time to remove material deposited on the cathode during normal operation.

An embodiment of the method and the electrochemical cell of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which.

Figure 1 is a diagrammatical cross-sectional view of an electrochemical cell of one embodiment of the present invention; and Figure 2 is a schematic representation of a system for producing ozonated water according to one embodiment of the present invention.

Turning to Figure 1 , there is shown an electrochemical cell, generally indicated as 2, for the production of ozone and/or ozonated water by the electrolysis of water. The cell 2 comprises a pair of opposing cell bodies 4, 6 held together by means of a number of through bolts 12. The first and second cell bodies 4, 6 may be prepared from any suitable material that is chemically resistant to strong oxidants, in particular ozone, generated within the cell. Thermoplastic fluoropolymers are a preferred class of materials for forming the cell bodies, in particular polyvinylidene fluoride (PVDF). Another suitable material is polymethylmethacrylate (PMMA) known as acrylic.

Each cell body 4, 6 is provided with a plurality of cavities in a major face, with each adjacent pair of cavities as shown in Figure 1 being separated by a land. The cavities and lands are arranged in the face of the cell bodies 4, 6 such that the cavities in the cell bodies are arranged in opposing pairs. Each cavity forms a conduit 14 for the passage of water therethrough. As shown in Figure 1 , the conduits 14 have a generally rectangular cross-section. Similarly, the lands in the face of each cell body 4, 6 are arranged to form opposing pairs. The surface of each land is provided with a layer of boron doped diamond (BDD) 16. The layers of BDD on the first cell body 4 form a plurality of first electrodes, while the layers of BDD on the second cell body 6 form a plurality of second electrodes. A semi-permeable membrane 18 extends between the first and second cell bodies 4, 6. The membrane 18 is formed from a sulfonated tetrafluoroethylene- based fluoropolymer (Nafion® N-117 membrane). The membrane 18 divides the opposing conduits 14 in the faces of the cell bodies. Further, the membrane 18 extends between the opposing pairs of BDD electrodes. The cell bodies 4, 6, the BDD electrodes 16 and the membrane 18 are arranged such that the surface of each of the BDD electrodes is in contact with the adjacent portion of the membrane.

Each cell body 4, 6 of the cell 2 is provided with a busbar 20, from which extend current feeders 22 connecting the busbar 20 with respective BDD electrodes 16. The busbar 20 and current feeders 22 may be formed from any suitable conductor, for example stainless steel, aluminium, copper or brass. Each busbar 20 is further provided with a current connector 24, allowing the busbar to be connected to an electrical supply system. The current feeders 22 and busbars 20 are sealed within the cell bodies 4, 6 to prevent leakage from the conduits 14.

In use, the electrochemical cell 2 shown in Figure 1 is connected to a system as shown in Figure 2. Referring to Figure 2, the electrochemical cell 2 is provided with a supply of fresh water, for example mains water, via a tank 101. The water is supplied to the cell through a line 102 and divided equally between lines 104 and 106, each feeding water from the supply to the conduits 14 in respective cell bodies 4, 6. In this way, the conduits and electrodes on each side of the membrane are provided with water having the same composition and under the same conditions of pressure and flowrate.

Liquid leaves the conduits 14 in the cell bodies 4, 6 through respective lines 114, 1 6, each provided with a solenoid valve 18, 120. Each solenoid valve 118, 120 is operable to direct the liquid leaving the cell 2 to either an ozonated water product line 122 or a hydrogen degassing column 124. Thus, liquid may be directed from the solenoid valve 118 along either a line 134a to the hydrogen degassing column 124 or a line 34b to the product line 122. Similarly, liquid may be directed from the solenoid valve 120 along a line 136a to the hydrogen degassing column 24 or a line 136b to the product line 122. Hydrogen gas may be recovered from the hydrogen degassing column 124, for further processing and/or use. Alternatively, the hydrogen may be safely vented to the atmosphere. Water recovered from the hydrogen degassing column may be used elsewhere, disposed of or recycled to the inlet of the system, in particular returned to the tank 101 for further use in the electrochemical cell.

The product line 122 returns ozonated water from the electrochemical cell to the tank 101 , where it is diluted with fresh mains water, to achieve the desired ozone concentration. An ozonated and disinfected water product is removed from the tank 101. A supply of direct current is provided to the electrochemical cell 2 by a DC electric supply system 150, connected by cables 152 to the current connectors 24 of each cell body 4, 6. The electric supply system 150 is operable to provide current to the electrodes as required and with either of two polarities.

In operation, the electrochemical cell is operated in the first mode, with the water supplied to the conduits 14 in both cell bodies 4, 6. Current is supplied to the cell by the electric supply system 150 with a first polarity, such that the BDD electrodes in the first cell body 4 act as the anode and the BDD electrodes in the second cell body 6 act as the cathode. Anolyte produced in the conduits of the first cell body 4 leaves through the line 1 14 and is directed by the solenoid valve 1 18 to the line 134b to the product outlet line 122. Catholyte produced in the conduits of the second cell body 6 leaves through the line 1 16 and is directed by the solenoid valve 20 to the line 136a, to be fed to the hydrogen degassing column 124.

After a period of operation in the first mode, material is deposited on the electrodes in the second cell body 6, acting as the cathode. This in turn passivates the BDD electrodes, reducing the efficiency of operation of the cell. Once the operation of the cell is sufficiently impaired, the cell is switched into the second mode. In particular, the position of each of the solenoid valves 1 18 and 120 is changed and the polarity of the current supplied by the electric supply system 150. In this mode, the BDD electrodes in the second cell body 6 act as the anode and the BDD electrodes in the first cell body 4 act as the cathode. Anolyte produced in the conduits of the second cell body 6 leaves through the line 1 6 and is directed by the solenoid valve 120 to the line 136b to the product outlet line 122. Catholyte produced in the conduits of the first cell body 4 leaves through the line 1 14 and is directed by the soienoid valve 1 18 to the line 134a, to be fed to the hydrogen degassing column 124.

Operation of the system is controlled by means of a controller 180, in particular allowing the position of the solenoid valves 18, 120 to be changed and the DC power supply 150 to be controlled, in particular to change the current polarity. The controller 80 further controls the concentration of ozone in the water in the tank 101, by means of an Oxidation Reduction Potential (ORP) or Redox Sensor 200, or Ozone Sensor in the base of the tank 101. In operation, the controller 180 determines the concentration of ozone in the water in the tank 101 by means of the signal received from the sensor 200. The operation of the electrochemical cell is controlled to maintain the ozone concentration in the tank within the desired range, determined by the end use to be made of the ozonated water.

The cell may be operated in the second mode until the performance of the cell has been restored. Thereafter, operation may be switched to the first mode. Alternatively, the cell may be continued to be operated in the second mode, until the performance of the cell again deteriorates, due to the electro-deposition of material on the BDD electrodes in the first cell body 4. At this time, operation is switched to the first mode. The cell may be cycled between the first and second modes of operation in this manner. It has been found that the electrochemical cell employing the BDD electrodes may be operated in accordance with the present invention with a high overall efficiency and with substantially no attrition of the electrodes observed.

Claims

1. A method of operating an electrochemical cell for use in the electrolysis of water to produce ozone, the cell comprising a first electrode and a second electrode separated by a Cation Exchange Membrane, wherein the first and second electrodes each has an active surface formed from electrically conductive diamond, in a first mode of operation the electrochemical cell having a flow of electrical current in a first polarity, whereby the first electrode functions as an anode and the second electrode is a cathode; the method comprising:

2. The method according to claim 1, wherein the active surface is formed from polycrystalline boron doped diamond (BDD). 3. The method according to either of claims 1 or 2, wherein the first and/or the second period of time is kept constant.

4. The method according to either of claims 1 or 2, wherein the first and/or the second period of time are varied in length to maintain operating efficiency of the cell.

5. The method according to any preceding claim, wherein the condition of the second electrode is monitored during the operation in the first mode to determine the first period of time. 6. The method according to any preceding claim, wherein the condition of the first electrode is monitored during the operation in the second mode to determine the second period of time.

7. The method according to either of claims 5 or 6, wherein the condition of the first and/pr the second electrode is monitored by monitoring one or more of the electrical current supplied to the cell and the potential applied across the cell. 8. The method according to any preceding claim, wherein the first and second periods of time are from 5 seconds to 60 minutes.

9. The method according to claim 8, wherein the first and second periods of time are from 5 seconds to 30 minutes.

10. The method according to claim 9, wherein the first and second periods of time are from 5 seconds to 10 minutes.

11. The method according to claim 10, wherein the first and second periods of time are from 5 seconds to 60 seconds.

12. The method according to any preceding claim, wherein the first period of time is equal to the second period of time. 13. The method according to any preceding claim, wherein the current density during the first and/or second period of time is from 0.1 to 5.0 Amps/cm².

14. The method according to claim 13, wherein the current density during the first and/or second period of time is from 0.4 to 3.2 Amps/cm². 5. The method according to claim 14, wherein the current density during the first and/or second period of time is from 0.5 to 1.2 Amps/cm².

16. The method according to any preceding claim, wherein the current density applied during the first mode of operation is the same as the current density applied during the second mode of operation.

17. The method according to any preceding claim, wherein the voltage applied during the first and/or second period of time is at least 10 Volts.

18. The method according to claim 17, wherein the voltage applied during the first and/or second period of time is at least 20 Volts. 19. The method according to any preceding claim, wherein the first and the second electrodes are formed from a substrate material coated with electrically conductive diamond.

20. The method according to claim 19, wherein the substrate material comprises silicon (Si), tungsten (W), niobium (Nb), molybdenum (Mo) or tantalum (Ta).

2 . The method according to either of claims 19 or 20, wherein the electrically conductive diamond coating is from 1 to 10 microns in thickness. 22. The method according to claim 21 , wherein the electrically conductive diamond coating is from 3 to 5 microns in thickness.

23. The method according to any of claims 1 to 18, wherein the first and the second electrodes are formed from solid electrically conductive diamond material.

24. The method according to claim 23, wherein the diamond material is from 200 to 1000 microns in thickness.

25. The method according to claim 24, wherein the diamond material is from 300 to 600 microns in thickness.

26. The method according to any preceding claim, wherein one or both of the first and second electrodes comprises an electrode assembly having a plurality of electrodes electrically connected to each other.

27. The method according to any preceding claim, wherein each electrode comprises edges to the active surface of the diamond in contact with the Cation Exchange Membrane.

28. The method according to any preceding claim, wherein the Cation Exchange Membrane comprises a sulfonated tetrafluoroethylene-based fluoropolymer.

29. The method according to any preceding claim, wherein the cell is operated under a forced flow circulation regime.

30. The method according to any preceding claim, wherein the water pressure on both the anode and cathode side of the Cation Exchange Membrane is equal. 31. The method according to claim 30, wherein water from a common supply is fed to both the anode and cathode side of the Cation Exchange Membrane.

32. The method according to any preceding claim, wherein water is provided to the cell at a flowrate of from 0.5 to 50 litres/min.

33. The method according to any preceding claim, wherein water is supplied to the cell at a pressure in excess of 1 Bar.

34. A method for removing deposits from the cathode of an electrochemical cell, the cathode having an active surface formed by diamond, the method comprising reversing the polarity of the current applied to the electrodes of the cell during normal operation for a sufficient time to remove material deposited on the cathode during normal operation. 35. An electrochemical cell for use in the production of ozone in normal operation, the electrochemical cell comprising:

36. The electrochemical cell according to claim 35, wherein the active surface is formed from polycrystalline boron doped diamond (BDD). 37. The electrochemical cell according to either of claims 35 or 36, further comprising means for monitoring the condition of the second electrode during the operation in the first mode.

38. The electrochemical cell according to any of claims 35 to 37, further comprising means to monitor the condition of the first electrode during the operation in the second mode.

39. The electrochemical cell according to either of claims 37 or 38, wherein the means to monitor the condition of the first and/or the second electrode are operable to monitor one or more of the electrical current supplied to the cell and the potential applied across the cell.

40. The electrochemical cell according to any of claims 35 to 39, wherein the first and the second electrodes are formed from a substrate material coated with diamond.

41. The electrochemical cell according to claim 40, wherein the substrate material comprises silicon (Si), tungsten (W), niobium (Nb), molybdenum (Mo) or tantalum (Ta).

42. The electrochemical cell according to either of claims 40 or 41 , wherein the diamond coating is from 1 to 10 microns in thickness.

43. The electrochemical cell according to claim 42, wherein the diamond coating is from 3 to 5 microns in thickness.

44. The electrochemical cell according to any of claims 35 to 39, wherein the first and the second electrodes are formed from solid diamond materia).

45. The electrochemical cell according to claim 44, wherein the diamond material is from 200 to 1000 microns in thickness.

46. The electrochemical cell according to claim 45, wherein the diamond material is from 300 to 600 microns in thickness.

47. The electrochemical cell according to any of claims 35 to 46, wherein one or both of the first and second electrodes comprises an electrode assembly having a plurality of electrodes electrically connected to each other.

48. The electrochemical cell according to any of claims 35 to 47, wherein each electrode comprises edges to the active surface of the diamond in contact with the Cation Exchange Membrane.

49. The electrochemical cell according to any of claims 35 to 48, wherein the Cation Exchange Membrane comprises a sulfonated tetrafluoroethylene-based fluoropolymer. 50. The electrochemical cell according to any of claims 35 to 49, wherein the cell is operable under a forced flow circulation regime.

51. The electrochemical cell according to any of claims 35 to 50, wherein water from a common supply is fed to both the anode and cathode side of the Cation Exchange Membrane.

52. A method of producing ozonated water comprising employing a method according to any of claims 1 to 34.

53. The use of an electrochemical cell according to any of claims 35 to 51 for the production of ozonated water.