OXIDATION

CROSS-REFERENCE TO RELATED APPLICATIO (S)

[0001] This application claims priority from United States provisional patent application serial no .61 /563 , 025 , entitled "In-situ Regenerating Diamond Electrodes After Anodic

Oxidation", filed November 22, 2011, which application is incorporated herein by reference, in its entirety.

TECHNICAL FIELD

[0002] This invention relates to regeneration of diamond electrodes after anodic oxidation during electrochemical processes .

BACKGROUND

[0003] The use of diamond electrodes, including microcrystalline diamond (MCD) and Ultra Nano Crystalline Diamond (UNCD) electrodes, offers significant advantages for many

electrochemical processes. Diamond electrodes often comprise a thin layer of conductive diamond deposited on a suitable conductive metal substrate. Thin film diamond can be deposited having high conductivity, and conductivity can be increased by suitable doping, e.g. boron-doped diamond (BDD) or nitrogen doped diamond. Diamond electrodes have been demonstrated to withstand high current densities, over significantly extended lifetimes relative to conventional electrode materials, as disclosed, for example, in PCT International patent

application no. PCT/US2012/033557 to Wylie et al. entitled "Electrochemical system and method for on-site generation of oxidants at high current density." Diamond is more resistant to oxidation that most other electrode materials. In

particular, diamond has significant resistance to the harsh conditions imposed on the anode and is typically more

resistant to anodic oxidation than other commonly used metal electrode materials. [0004] Nevertheless, it is well known that diamond does oxidize (Howe, J.Y., Jones, L.E, and Cormack, A.N., "The Oxidation of Diamond", School of Ceramic Engineering and Materials Science, Alfred University, Alfred, NY; 2001) . When thin film diamond is produced, e.g. by Chemical Vapor Deposition (CVD) processes from hydrocarbon reactants mixed with hydrogen (H ₂ ) gas, typically, the as-deposited diamond films are produced with hydrogen-terminated surfaces. Hydrogen termination of the surface results from atomic hydrogen etching that takes place during the deposition process principally caused by atomic hydrogen generated on the hot filaments from H ₂ , which removes non-diamond carbon species. The hydrogen terminated surface is stable in air for months (A. Kraft, "Doped Diamond : A

Compact Review on a New, Versatile Electrode Material" , Int. J. Electrochem. Sci., 2 (2007); p 364). Over time, in ambient air, the surface hydrogen is slowly replaced by oxygen from the air forming a hydroxyl (-OH) terminated surface.

[0005] During an electrochemical process, when a diamond electrode is operated as an anode, oxidation is accelerated by oxygen that is electrochemically generated at the electrode surface by anodic oxidation. Anodic oxidation of diamond is similar to the electrochemical oxidation of other electrode materials, such as metals. However, due to the electrochemical stability of sp3 carbon, oxidation of diamond tends to be much slower than anodic oxidation of other commonly used electrode materials. Nevertheless, it is known that anodic oxidation of diamond is catalyzed, i.e. accelerated, by small carboxylic acids such as acetic acid (Kraft, and "Anodic oxidation of phenol in the presence of NaCl for wastewater treatment" , Ch . Comninellis et al . , J. Applied Electrochemistry, 25 (1995) 23- 28) .

[0006] A hydrogen terminated diamond surface has high

conductivity, but as the diamond anode oxidizes, the diamond surface loses its conductivity (G. Salazar-Banda, et . al . ; "On the changing electrochemical behavior of boron-doped diamond surfaces with time after cathodic pre-treatments" ,

Electrochimica Acta; v. 51; p. 4612-4619, 2006) . The underlying layer of diamond is usually not significantly affected by the oxidation process but there is a change in surface termination and surface conductivity. For example, during electrochemical processes in the presence of oxygen, in aqueous solutions or other oxygen containing electrolytes, such as alcohols, a diamond anode oxidizes through formation of surface species such as ketones ( sesquinones ) and

carboxylic acid groups. These oxygen containing surface species have lower conductivity than diamond. Therefore, anodic oxidation of the diamond surface reduces the

conductivity of the diamond electrode, causing an increase in cell voltage and limiting its effectiveness in the

electrochemical process.

[0007] It is also known that oxidation of chemically oxidized diamond surfaces resulting in oxygen termination can be reversed, by a chemical treatment in strong acid, or by a hydrogen termination process (H-termination) such as hydrogen plasma or hot filament treatments with H ₂ gas. H-termination can also be achieved electrochemically by a cathodic treatment (CT) in an acidic electrolyte solution, e.g. 2M hydrochloric acid for 1 to 5 minutes at ~lAmp/cm ² (Rene Hoffmann et

al ., "Electrochemical hydrogen termination of boron-doped diamond", Applied Physics Letters 97, 052103 (2010)).

[0008] In electrochemical processes, it is possible to

regenerate diamond electrodes after anodic oxidation by removing the electrodes from the electrochemical cell and using Hot-Filament Chemical Vapor Deposition (HFCVD) for hydrogen treatment, or other electrode cleaning treatments. Alternatively, diamond anodes can be regenerated, either in situ or in another cell, by providing a chemical treatment or an electrochemical cathodic treatment (CT) in a strongly acidic solution, such as concentrated hydrochloric acid. Cathodic treatment provides H-termination of the surface, and extends the operational lifetime of the electrode. However, in

commercial applications, it is desirable to avoid handling of strong acids and/or the requirement to remove the electrodes from the electrochemical cell for regeneration. Significant capital and/or operational costs are associated with either in situ electrode regeneration processes (cathodic

treatments ) using strong acids, or cell disassembly and

electrode cleaning treatments in other equipment, particularly those requiring handling of concentrated acids or other hazardous chemicals.

[0009] Thus, there is a need for improved or alternative solutions which address the shortcomings of known methods to provide in situ regeneration of diamond electrodes after anodic oxidation.

SUMMARY OF INVENTION

[0010] The present invention seeks to mitigate the above mentioned problems, or at least provide an alternative method for regeneration of diamond electrodes.

[0011] A method is provided for operation of an electrochemical cell, comprising: a process cycle and a regeneration cycle, which enables in situ regeneration, or reactivation, of a diamond electrode after anodic oxidation, e.g. during

electrolysis of an aqueous electrolyte. The regeneration cycle is performed upon detecting a condition indicative of a threshold level or excessive level of anodic oxidation. The regeneration cycle comprises supplying a current to the electrodes under reverse polarity, to cause the diamond electrode to act as a cathode and electrolyzing an electrolyte to generate hydrogen on the diamond surface of a first

electrode to reduce said anodic oxidation and reactivate the electrode surface.

[0012] Thus, a first aspect of the invention provides a method of operating an electrochemical cell for electrolyzing an aqueous electrolyte, the electrochemical cell comprising a current source and first and second electrodes, at least the first electrode comprising a diamond electrode, the method comprising the steps of:

in a process cycle, supplying a current to the electrodes under forward bias, comprising applying a potential to cause the first electrode to act as an anode and the second electrode to act a cathode thereby electrolyzing the electrolyte to produce a desired product; and

in a regeneration cycle, supplying a current to the electrodes under reverse polarity, comprising applying a potential to cause the first electrode to act a cathode and the second electrode to act as an anode, and electrolyzing the

electrolyte, i.e. with a sufficiently negative cathode

potential, to generate hydrogen on the diamond surface of the first electrode to diminish said anodic oxidation.

[0013] A second aspect of the invention provides a method of regeneration of a diamond electrode after anodic surface oxidation of the diamond electrode operated as an anode in an electrochemical cell, comprising the steps of:

performing a regeneration cycle comprising supplying a current to the diamond electrode, under reverse polarity, comprising applying a potential to cause the electrode to act as a cathode and electrolyzing an aqueous electrolyte, wherein the electrode is held at a sufficiently negative potential to generate hydrogen on the surface of the diamond electrode and diminish said surface oxidation.

[0014] A third aspect of the invention provides a control system or controller for operating an electrochemical cell for electrolyzing an aqueous electrolyte wherein the

electrochemical cell comprising a current source and first and second electrodes, at least the first electrode comprising a diamond electrode, and the controller comprising:

input means for receiving process parameters for operation in a process cycle or in a regeneration cycle, wherein

the process cycle comprises supplying a current to the

electrodes under forward polarity, comprising applying a potential to cause the first electrode to act as an anode and the second electrode to act a cathode thereby electrolyzing the electrolyte to produce a desired product; and there generation cycle comprises supplying a current to the

electrodes under reverse polarity, comprising applying a potential to cause the first electrode to act as a cathode and the second electrode to act as an anode, and electrolyzing the electrolyte to generate hydrogen on the diamond surface of first electrode to diminish said anodic oxidation;

means for switching the polarity of operation of the

electrochemical cell between a forward polarity for a process cycle and a reverse polarity for a regeneration cycle,

means for monitoring a condition indicative of a

threshold/excessive level of anodic oxidation; and

wherein the switching means is configured, on detection of said condition indicative of a threshold/excessive level of anodic oxidation, to switch polarity from a process cycle to a regeneration cycle, and on detection of a condition indicative of regeneration, to switch polarity back to a process cycle.

[0015] Operation of the diamond electrode, under reverse

polarity, i.e. comprising applying a potential to cause the electrode to act as a cathode, and electrolyzing an aqueous electrolyte, with the electrode is held at a sufficiently negative potential, causes generation of hydrogen on the surface of the diamond electrode, thereby reducing the surface oxidation and reactivating the electrode.

[0016] The regeneration cycle may be implemented after operating in a process cycle for a predetermined time or after detecting another condition indicative of a threshold/excessive level of anodic oxidation. The step of detecting a condition indicative of a threshold/excessive level of anodic oxidation comprises, for example: determining one of a prescribed operating time and/or a change in electrochemical potential between the anode and cathode indicative of a threshold level of anodic

oxidation, and/or an indicator of are duction in active area to less than a predetermined percentage, e.g. <~20% of active surface area.

[0017] Preferably, the regeneration cycle comprises

electrolyzing the same electrolyte as the process cycle. For example, in the regeneration cycle, in an aqueous electrolyte, e.g. containing chloride ions, such as sodium chloride

solution at pH 7 to 9, applying a sufficiently negative potential to the diamond electrode so that it acts as a cathode will result in generation of hydrogen at the electrode, as evidenced by visible hydrogen bubbles in the vicinity of the first electrode, and reactivation of the diamond surface.

[0018] Thus regeneration can be carried out in situ, e.g.

without need to flush the cell with strong acids, and avoiding the need to remove the electrodes from the cell for

regeneration .

[0019] Alternatively, the regeneration cycle may comprise flushing the cell and regenerating the electrode in situ by electrolyzing a regeneration electrolyte.

[0020] The regeneration cycle comprises electrolyzing an electrolyte having a pH in the range pH >4, e.g. in a range from 4 to 12, and preferably in the range 4 to 9, and more preferably in the pH range from 7 to 9.

[0021] A process cycle may comprise supplying current of a current density in the range of 10-1000 mA/cm ² , and more preferably in the range from 250-500 mA/cm ² . The regeneration cycle is preferably carried out under reverse polarity, but otherwise using the same current density and other process parameters as the process cycle. The regeneration cycle may be carried out at reduced current density, e.g. about 25% of the current density of the process cycle, but preferably the regeneration cycle is run at least at 50% or >75% of the current density of the process cycle.

[0022] A regeneration cycle may be applied after a prescribed operating time, or after detecting a condition indicative of a threshold level or excessive level of anodic oxidation. For example, a regeneration cycle may be applied after an extended operating time when it is expected under typical operation that the active area of the electrode has diminished to a certain level, e.g. less than -20% of active surface area, or periodically at shorter intervals as preventative maintenance, e.g. to maintain a higher percentage of active surface area.

[0023] Alternatively, a condition indicative of a threshold level of anodic oxidation, or an excessive level of anodic oxidation may, for example, comprise detecting a change in electrochemical potential of the cell ≥ 3 volts relative to a baseline or reference electrochemical potential of the cell. Then, the regeneration cycle may be applied until the

electrochemical potential of the cell returns to within 0.5 volts of the baseline electrochemical potential.

[0024] For example, the method may comprise operating in regeneration cycle for a time of <1% of the preceding process cycle time, and more preferably for a time of <0.1% of the preceding process cycle time; or cycling between the process cycle and regeneration cycle at predetermined periodic intervals; or cycling between the process cycle and

regeneration cycle at predetermined periodic intervals, comprising reversing polarity while otherwise maintaining the same current density and other operational parameters. The regeneration cycle time may be in the range from 1 to 60 minutes, or more preferably from 2 to 5 minutes, depending on the capability of the other electrode to withstand reverse polarity operation.

[0025] Such a method of regeneration or reactivation of a diamond electrode after anodic oxidation is applicable to diamond electrodes such as electrodes comprising UNCD or conductive diamond having an average grain side of ≤10nm;

electrodes comprising MCDor conductive diamond having an average grain size of in the range from 20nm to 500nm; and/or boron-doped diamond in general.

[0026] Advantageously, the electrochemical cell comprises a control system comprising means for monitoring a condition indicative of a threshold level of anodic oxidation of the diamond anode, and switching means for automatically switching polarity of operation as needed, between the process cycle and regeneration cycle.

[0027] The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings, of embodiments of the

invention, which description is by way of example only. BRIEF DESCRIPTION OF DRAWINGS

[0028] In the drawings, identical or corresponding elements in the different Figures have the same reference numeral.

[0029] Figure 1 illustrates schematically an electrochemical system for performing electrochemical processes using diamond electrodes and for implementing a method, according to an embodiment of the present invention, comprising in situ regeneration of the diamond electrodes after anodic oxidation, using a batch or recycle mode;

[0030] Figure 2 illustrates schematically another

electrochemical system for performing electrochemical

processes using diamond electrodes and for implementing a method, according to an embodiment of the present invention, comprising in situ regeneration of the diamond electrodes after anodic oxidation, in single-pass or continuous-flow mode .

[0031] Figure 3 shows a plot representing the change in

electrode potential during operation of an electrochemical cell comprising a diamond electrode, during successive cycles of a method according to an embodiment;

[0032] Figure 4 shows a graph of data for the peak current response Ip (μΑ) of a UNCD electrode by cyclic voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ;

[0033] Figure 5shows a graph of data for the peak current response Ιρ (μΑ)οΐ the UNCD electrode by differential pulse voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ;

[0034] Figure 6 shows a graph of data for the peak voltage difference ΔΕ _Ρ (mV) of the UNCD electrode by cyclic voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ;

[0035] Figure 7shows a graph of data for the peak voltage change ΔΕ _Ρ (mV) of the UNCD electrode by differential pulse voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ; [0036] Figure 8shows a graph of data for the peak current response Ιρ (μΑ)οΐ aMCD electrode by cyclic voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ;

[0037] Figure 9shows a graph of data for the peak current response Ιρ (μΑ)οΐ the MCD electrode by differential pulse voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ;

[0038] Figure 10 shows a graph of data for the peak voltage difference AE _p Of the MCD electrode by cyclic voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ;

[0039] Figure llshows a graph of data for the peak voltage difference ΔΕ _Ρ (mV) of the MCD electrode by differential pulse

voltammetry through a sequence of process cycles (white points) and regeneration cycles (black points) ;

[0040] Figure 12 shows a cyclic voltammogram for A. fresh and B. anodically oxidized surfaces of the UNCD electrode; and

[0041] Figure 13 shows differential pulse voltammetry scans for A. fresh and B. anodically oxidized surfaces of the UNCD electrode .

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] An electrochemical system 100 suitable for performing a method according to a first embodiment of the present

invention, comprising electrolyzing an aqueous electrolyte to produce a product, is shown in Figure 1. For example, such a system may be used for on-site generation of oxidants

comprising free active chlorine (FAC) by electrolysis of electrolytes comprising aqueous salt solutions, e.g. sodium chloride (NaCl) or other chloride containing electrolytes.

Other oxidants may also be generated in such cells which may also cause oxidation of the diamond surface, including:

peroxodisulphate ( "persulfate" , S ₂ 0 ₈ ^2~ ) , periodate (Ι0 ₄ ^1_ ), hypobromite (BrO-) , chlorine dioxide (CIO ₂ ) and similar oxidants .

[0043] The electrochemical system 100 comprises a reaction tank or electrolysis cell 110 containing electrodes 120 and 130 together with a feedstock reservoir 140 containing electrolyte solution 150. The electrode pair 120/130 comprises at least one diamond electrode 120 which acts as the anode, and may comprise a matched diamond anode 120 and diamond cathode

130. Preferably, the anode comprises a conductive diamond electrode, which may comprise a microcrystalline diamond (MCD) or ultrananocrystalline diamond (UNCD) layer on a suitable substrate. The anode may, for example, comprise conductive boron-doped diamond, having an average grain size of less than 500 nm, and in some embodiments it may be UNCD having an average grain size of lOnm or less.

[0044] Conductive diamond is chemically and electrochemically inert and, in particular, has enhanced adhesion to metal substrates such as niobium and tantalum. Since diamond electrodes typically fail by delamination of the diamond layer, an electrode comprising UNCD on a metal substrate such as niobium or tantalum can provide reliable operation over an extended lifetime. The cathode 130 may be a lower cost electrode, e.g. an inexpensive stainless steel electrode or one comprising other more traditional known electrode

materials, such as tungsten (W) , high quality graphite, zirconium (Zr) , titanium (Ti) , or other suitable metals. In some processes it is beneficial to use aMCD or UNCD diamond on metal anode with a matched MCD or UNCD cathode, to provide a similar operational life for both electrodes and enable reliable operation of the cell with extended periods between maintenance .

[0045] The system shown in Figure 1 is configured for operation in a recycle mode, or batch mode. The electrolyte feedstock 150 held in the reservoir 140 is injected into the reaction tank 110 between electrodes 120 and 130, via pump 144 and conduit 146 at the bottom of the cell 110. The feedstock electrolyte 150 is injected, by the pump 144, for example, at a rate from about 0.2 liters/minute (very low flow) to about 9 liters/minute (high flow) , and typically about 1 to 5

liters/minute, for each 200 cm ² of active area of the cell. Feedstock electrolyte 150 exits the reaction tank 110 via conduit 112 at the top of the cell and is returned to the reservoir 140 after processing. The reservoir 140 also includes ports 114 and 148 at the bottom and top of the chamber respectively, e.g. for addition of feedstock, or output of the processed electrolyte solution, e.g. after a particular processing time, when the product concentration has reached a desired level.

[0046] The controller 160 comprises a low-voltage, high-current power supply for supplying a required DC current density, which may be around 300-500 mA/cm ² , for example, depending on the requirements of the electrochemical process, such as the requirement to generate a product at a given rate and/or other process parameters. The current density may be in the range of 150 mA/cm ² and up to 1000mA/cm ² ofactive area, with an operating voltage, typically, of <8 V with a cell gap between anode and cathode of 6mm. A smaller gap can result in a lower operating voltage, however, this can result in gas bubble hindrance of flow through the active volume of the cell and therefore there is a lower limit to the gap which is a function of the

geometry of the cell, the overall flow rate of the system and the current density. For a cell having an active area of 100 cm ² to 200 cm ² the required current would be in the range from about 15 A to 200 A.

[0047] Under normal operating conditions (DC, forward voltage) , one electrode, e.g. the electrode 120, operates as the anode in the electrochemical process and the other electrode 130 as a cathode. In an electrochemical cell, current flows from the anode to the cathode. According to convention, the flow of electrons in an electrical circuit is in the opposite

direction to the flow of current. Thus, in the

electrochemical cell, referring to Figure 1, when the

electrode 120 is operated as an anode, electron flow is in the direction from the anode to the power supply and from the power supply to the cathode 130.

[0048] An advantage of using diamond electrodes is that they may be operated under reverse polarity for reasonable periods of time (i.e. up to several hours) without deleterious effects. The system also provides for periodic reverse polarity

operation, to provide a regeneration cycle, as will be

described in detail below. A reverse polarity regeneration cycle contributes to improved long-term reliability and reduced maintenance.

[0049] By way of example only, the electrolyte volume of the system is, for example, from -500 mL to -30 L, and the size of cell 110 is approximately 12" high by 6" wide (30 cm x 15 cm) , and the electrodes 120 and 130 have an active area of -100-200 cm ² and an electrode separation of ~4-8mm.

[0050] In exemplary systems and methods used to obtain data shown in the accompanying figures, the diamond electrodes have an active area of either 100 cm ² (e.g. "Julius") or 10 cm ² (e.g. "Nero"R+D test systems) or 210 cm ² (e.g. "Diamonox 200", 6L or 12L batch/recycle mode system) and the electrode

separation is set at 4mm, 8mm, or 15mm, for example.

[0051] The system may optionally comprise a cooling system, e.g. using water cooling (not shown) to maintain the electrolyte at a desired operating temperature.

[0052] The pump 144 preferably allows for adjustment of the flow rate for control of the reaction rates and/or rate of

production of the product. In some processes, with operation at elevated current densities, the reaction process may not require additional mixing. For example, hydrogen (H ₂ )

production on the cathode is sufficiently energetic to produce adequate mixing for continuing reaction. Beneficially, the electrode spacing and the flow rate is set to maintain flow between the electrodes, in the direction in which the hydrogen bubbles tend to move against gravity. Thus, as illustrated in Figure 1, the feedstock is injected at the bottom, flows upward between the electrodes and exits at the top of the cell 110. The closed loop configuration shown in Figure 1 allows for recirculation of a batch of electrolyte solution 150 to enable operation in a recycle mode or batch mode.

[0053] Another system 200 is shown in Figure 2, configured for single-pass or continuous-flow operation. Elements of Figure 2 are labeled similarly to corresponding elements of Figure 1, with reference numerals incremented by 100. In this

embodiment, the cell 210 comprises an anode 220 and cathode 230 with connections to a controller 260 comprising a power supply for supplying operating current and voltage, as in the system of the first embodiment. In this configuration, feedstock electrolyte solution 250 is supplied to the cell 210 at a controlled flow rate via the pump 244 to an input port of the cell 246. After electrolysis of electrolyte 250, the processed electrolyte solution containing product 252 exits cell 210 via conduit 212 at the top of the cell and is

delivered to the reservoir 240, with ports 214 and 248 for output or connection as required.

[0054] While a pump is illustrated (i . e .144 or 244 in Figures 1 and 2 respectively) , it will be appreciated that alternative flow control means may be used, such as a gravity feed system or other fluid delivery system that can supply the feedstock electrolyte solution to the cell at a an appropriate flow rate .

[0055] In the following description, methods according to embodiments of the invention will be described with reference to an electrochemical system 100 configured as shown in Figure 1, and in which the controller 160 is configured to provide for switching between operation under forward polarity and reverse polarity. As described below forward polarity

operation is used under normal operating conditions, i.e. for a process cycle for generating a product, and reverse polarity operation is used for a regeneration cycle. Such methods may alternatively be carried out in a system configured as shown in Figure 2, with a controller that allows for operation under forward or reverse polarity. [0056] Method for operating an electrochemical cell comprising cycling between process cycles and in situ regeneration cycles .

[0057] Example A: On-site generation (OSG) of oxidants

comprising chlorine, at high current density.

[0058] A method according to an embodiment of the invention, provides for operation of a system 100, comprising an

electrochemical cell llOsuch as illustrated in Figure 1, comprising cycling between process cycles and in situ

regeneration cycles.

[0059] During a process cycle, an aqueous electrolyte 150, e.g. comprising 1M sodium chloride is electrolyzed to produce an oxidant comprising a solution of free and active chlorine (FAC) . Electrolysis is preferably carried out at high current density, as described in detail in the above referenced PCT applicationPCT/US2012/ 033557.

[0060] Typically, a cell is operated in constant current mode with a floating voltage. During the process cycle, the anode 120 is at a positive potential and the cathode 130 is at a negative potential. Thus negatively charged ions, such as chloride ions (Cl ^~ ) in the electrolyte solution, will move towards the anode, become oxidized, i.e. give up electrons at the anode, and generate chlorine. At the cathode, positively charged ions, such as hydronium ions, (H ₃ 0 ⁺⁾ will be reduced, i.e. receive electrons, to produce hydrogen. Accordingly, in the external circuit, electrons flow from the anode to the power supply to the cathode. During the process cycle, negatively polarized oxygen containing species in the

electrolyte will also tend to move towards the anode or be generated on the anode surface and react with anode material, i.e. resulting in anodic oxidation of the diamond surface as mentioned above.

[0061] Since diamond is relatively resistant to anodic

oxidation, the cell 110 may typically be operated in a process cycle for extended periods, e.g. for several hundred hours at a time. However, after operation of the cell 110 for an extended time, evidence of anodic oxidation of the diamond electrode 120, which acts as the anode during a process cycle, will be observed. For example, the operator may observe a rise in cell voltage indicative of a condition of excessive anodic oxidation, e.g. an increase of ≥3V relative to a baseline electrochemical potential. Such electrode significant rise in cell voltage may be indicative of irreversible oxidation of the diamond or degradation of the electrode, e.g. by

delamination of the diamond layer. A rise in cell voltage may also be indicative of scaling of the cathode. A condition of unacceptable or excessive anodic oxidation, or a build-up of anodic oxidation above a threshold level, may also be expected after a prescribed or predetermined operational time or at scheduled maintenance intervals, e.g. as determined by prior monitoring and test results over an extended period. Thus, in practice, e.g. for preventative maintenance, it is desirable to operate the cell in a regeneration cycle well before significant irreversible anodic oxidation or degradation of the diamond electrode occurs. For example, based on such testing, the regeneration cycle may be carried out after detecting condition such as an operational time or other parameter indicative of a threshold level of anodic oxidation, e.g. a level of anodic oxidation that is substantially

reversible by a short regeneration cycle.

[0062] In the process cycle, current is supplied to the

electrodes under forward bias, i.e. applying a potential to cause the first electrode 120 to act as an anode and the second electrode 130 to act as a cathode thereby electrolyzing the electrolyte 150 to produce a desired product. Then, at a prescribed time, or on detection of another indication of a threshold level of anodic oxidation, the cell 110 is operated in a regeneration cycle. A condition indicative of a threshold level, or excessive level, of anodic oxidation may be

detected, for example, when there is a change in

electrochemical potential of the cell between the anode and cathode. Such a condition may be apparent after a prescribed operating time, e.g. based on prior process monitoring and voltammetry test results. [0063] The regeneration cycle comprises supplying a current to the electrodes under reverse polarity, i.e. applying a

potential to cause the first electrode 120 to temporarily act as a cathode and the second electrode 130 to act as an anode. The electrolyte 150 is then electrolyzed by applying a cathode potential that is sufficiently negative to generate hydrogen on the diamond surface of first electrode 120, to reduce or diminish the anodic oxidation of the diamond

surface. By applying the regeneration cycle, for a short time, e.g. <1% of the process cycle time, the diamond

electrode surface is reactivated, i.e. the conductivity of the diamond surface appears to be substantially restored. By way of explanation, it appears that, for example, oxygen

termination of the surface caused by anodic oxidation of the diamond electrode is replaced with hydrogen termination, or lower conductivity surface species, containing e.g. carboxyl or ether groups (sesquinones) , are replaced with more

conductive surface species.

[0064] Operation of the system 100 comprising electrochemical cell 110 according to this embodiment comprises sequentially cycling between process cycles, e.g. for generating product and in situ regeneration cycles, i.e. to maintain the

electrodes 120 and 130 within a desirable operational range of surface conductivity, electrochemical potential or other operational parameters. For example, the cathodic regeneration cycle may be carried out to restore the electrochemical potential to within 0.5 volts of the baseline electrochemical potential of the cell 110.

[0065] Figure 3 shows a plot representing schematically the change in electrode potential during operation of an

electrochemical cell comprising a diamond electrode, during successive cycles of a method according to an embodiment, comprising process cycles wherein the diamond electrode acts as an anode and regeneration cycles for reactivation of the diamond electrode. Each regeneration cycle is typically very short compared to the preceding process cycle. While the electrochemical potential may rise during extended operation in the process cycle, in part due to anodic oxidation and in part due to e.g. scaling of the cathode, desirably, the regeneration cycle is applied well before significant

irreversible anodic oxidation occurs.

[0066] The regeneration cycle is preferably carried out using the same electrolyte 150 as the process cycle. For example, for regeneration, electrolysis of a 1. OM NaCl solution at current densities between 250 mA/cm ² and 1000 mA/cm ² , for times in the range between 1 minute and 60 minutes, more preferably a shorter time, e.g. for 2 to 5 minutes, was demonstrated to regenerate anodically oxidized MCD and UNCD electrodes, when the pH of the electrolyte solution was between 7 and 9.

Effective regeneration of the electrode occurs if a

sufficiently negative potential is applied during electrolysis in an aqueous electrolyte containing sufficient hydrogen ion to cause generation of hydrogen, e.g. as observed by

generation of visible hydrogen bubbles/effervescence, at the oxidized electrode being regenerated. Ideally the

regeneration cycle simply involves reversing polarity of the electrodes 120 and 130 while maintaining similar operational parameters such as current density, that are used for the process cycle that is used for electrochemical generation of product. This simplifies operation by minimizing required processing adjustments for the regeneration cycle.

[0067] Advantageously the controller 160 of the electrochemical cell comprises means for monitoring a condition indicative of a threshold level of anodic oxidation of the diamond anode, and switching means for automatically switching polarity of operation as needed, between the process cycle and

regeneration cycle.

[0068] To facilitate regeneration, the cell 110 may be flushed with a regeneration electrolyte during the regeneration cycle, for example by adding a weak acid to the electrolyte.

However, effective regeneration was demonstrated using weakly acidic or non acidic solutions having a pH ≥ 4. Conventional strong acid electrolytes or flushing the cell with cleaning solutions, such as concentrated hydrochloric acid, are not required .

[0069] As an example, if the surface of the electrode 120, which acts as the anode during a process cycle, becomes oxidized to a predetermined "oxidation density" or after a predetermined change in the cell potential is observed, the current is reversed from operating conditions to operate the electrode 120 as the cathode. Thus the regeneration process may be alternatively be referred to as a cathodic treatment.

[0070] The extent of anodic oxidation, which may be referred to as "oxidation density", of the diamond electrode may be assessed experimentally by DPV (Differential Pulse

Voltammetry) in a reference system, e.g. for conversion of Fe (CN) ₆ ^~ to Fe (CN) ₆ ^3~ , to assess the percentage of the

electrode surface area that is inactive. After operation of the diamond electrode, i.e. anode 120, for several hundred hours such measurements can indicate 90% of the diamond surface is inactive. A regeneration cycle restores the active surface area to 90%, i.e. that only 10% of the original (non- oxidized) surface area of the electrode is inactive, as indicated by this sensitive DPV test. Thus, the electrodes may be calibrated for a particular On-Site Generation

(OSG) process . For example, if the oxidation density reaches 90% after several hundred hours, a regeneration cycle may desirably be applied at about half that time, e.g. atlOO hours, well before the active area of the electrode is significantly diminished. Alternatively, timing of the

regeneration cycle may be scheduled, for example, after detecting a parameter such as a predetermined change in the cell potential, which has been determined to be indicative of a threshold level of inactivation of the diamond surface.

[0071] In the process cycle, a current density may be selected between 10 and 1000 mA/cm ² , and would typically be in a range between 300 and 500 mA/cm ² . Depending on current density and the extent of anodic oxidation, the cathodic regeneration cycle may be of a duration from as short as a minute to one hour or longer. It may be preferable to limit treatments to periodic treatments of a few minutes to 30 minutes, for example. Since the other electrode 130, which normally acts as the cathode in the process cycle, is usually a conventional metal electrode such as a low-cost stainless steel electrode or tungsten electrode, the capacity of the material of the other electrode 130 to endure anodic polarization may be a significant factor. Typical durations of reverse polarity operation before damage (electro-etching) for a tungsten cathode operated at between 300 and 450 mA/cm ² have been measured to be in the range of approximately 2 to 5 minutes and less for stainless steel. Thus, more frequent and shorter regeneration cycles may be preferred if the electrode 130, which is normally operated as a cathode, cannot withstand prolonged periods of reverse polarity during the regeneration cycle.

[0072] The reverse polarity regeneration cycle is preferably carried out at the same current density as the process cycle. However, if required, the regeneration cycle may be carried out at a lower current density, e.g. if the electrode that acts as a cathode in the process cycle cannot withstand full current density under reverse polarity for the duration of the regeneration cycle, it may be desirable to operate the

regeneration cycle at a lower current density. Desirably the current density for the regeneration cycle is at least 25%, or more preferably, at least 50% or >75% of that used for the process cycle.

[0073] After the regeneration cycle is complete, e.g. after a predetermined time, or when the electrochemical potential of the cell 110 has been restored to an acceptable value, the polarity of the cell 110 is returned to forward bias, for a process cycle, i.e. with the electrode 120 acting as the anode, and 130 as the cathode.

[0074] Figure 3 shows a plot representing schematically the switching of the electrode potential of the electrode 120 during operation of an electrochemical cell comprising a diamond anodel20 during successive cycles comprising: a process cycle wherein the diamond electrode 120 acts as an anode; a regeneration cycle during cathodic treatment of the diamond electrode; and another process cycle with operation of the diamond electrode as the anode; and an inactive period followed by another regeneration cycle and another process cycle. The potential on the electrode 120 during a process cycle is generally positive (e.g. ~7V) and is negative (e.g. ~-5V) during the regeneration cycle (cathodic treatment) . In practice, since the cell is usually operated at constant current, the voltage may be observed to increase (not shown in Figure 3) after extended operation, e.g. due to oxidation and inactivation of the diamond surface of the anode and/or scaling of the cathode. After operating in a process cycle, a scheduled time, or when the cell voltage increases more than a predetermined value, the cell polarity may be reversed to provide a short regeneration cycle.

[0075] In a process cycle for electrolysis of an aqueous

electrolyte, e.g. for generation of a product such as FAC or other process, the diamond electrode 120is operated with a positive potential. Under typical process conditions, a diamond electrode may be operated for weeks or months before anodic oxidation reaches a level that requires regeneration. Anodic oxidation may be indicated by a change in cell voltage AV, which may be significantly below a change of ≥3V above a baseline or initial value, which could indicate irreversible oxidation or degradation of the electrode. The regeneration cycle is typically much shorter (minutes) , and comprises less than 1% and preferably less than 0.1% of the process cycle time.

[0076] It should be noted that reduced effectiveness of the cell may also result from build-up of scale on the cathode, e.g. calcium carbonate deposits, from impurities in the water supply or from use of impure salt feedstocks. Such scaling of the cathode may cause a more significant increase in cell voltage relative to deactivation of the diamond anode.

However, a change in cell voltage due to scaling may also conveniently be used as an indicator for performing a

regeneration cycle to reactivate the diamond electrode. As disclosed in the above referenced PCT application, a short period of reverse polarity operation during a regeneration cycle may additionally be beneficial in removing scale build up from electrode 130 that acts as the cathode during the process cycle. Such a cycle may be as short as a few seconds to a few minutes. The regeneration cycle for reactivation and hydrogen termination of the anode is typically required to be somewhat longer to produce reactivation of the diamond

electrode. Thus, a regeneration cycle under conditions that provide for reactivation of the diamond anode may also be beneficial in reducing scaling of the cathode.

[0077] OSG of salt solutions for generation of chlorine or mixed oxidants typically takes place in a pH range from pH 7 to pH 9. That is, the solution is initially neutral, but as hydrogen is evolved at the cathode, the pH of the solution gradually increases during processing. During the

regeneration cycle, provided a sufficiently negative potential is applied to the electrode to be regenerated, and there are sufficient hydrogen ions in solution, hydrogen will be

generated in sufficient quantity by electrolysis at high current density to regenerate the electrode, e.g. by

hydrogenation or H-termination of the diamond surface.

[0078] Clearly regeneration will be accelerated at lower pH, e.g. by acidifying the solution during the regeneration cycle, but it is demonstrated herein that electrolysis in non-acidic solutions, e.g. pH 7 to 9 is effective. Electrolysis in low pH solutions, e g. weakly acidic solutions of pH≥4 may be beneficial. However the process avoids the need for use of strong acids, or the handling of concentrated acids for chemical regeneration of the diamond surface. When the

regeneration cycle is carried out in situ using a process electrolyte, minimal interruption of the process cycle is required, and down time for changing the electrolyte or removal and regeneration of the electrodes is avoided.

[0079] Example B: Oxidation of organic contaminants in waste water

[0080] Electrochemical oxidation may be used for electrochemical breakdown of organic contaminants such as phenols in waste water or industrial effluent, using a diamond anode (M. Fryda et al . , "Wastewater treatment with diamond electrodes" ,

Electrochemical Society Proceedings Volume 99-33) . However, such a process tends to generate organic acids, which can catalyze anodic oxidation of diamond, resulting in more rapid anodic oxidation and deactivation of the diamond anode. In such a process, periodic reverse polarity regeneration cycles after each process cycle, similar to Example A, would be beneficial for reactivation of the diamond electrode.

[0081] The following test results illustrate the effectiveness of extended operation using regeneration cycles for

regeneration and reactivation of diamond anodes.

[0082] Example C: Accelerated test results

[0083] To assess the long term effectiveness of the regeneration treatment, tests were conducted with repeated cycling between a process cycle causing anodic oxidation and a regeneration cycle. To accelerate testing, the process cycle was

simulated by a process using extreme conditions that causes rapid anodic oxidation of the diamond anode. Thus, for the tests the "process cycle" comprised electrolysis of 0.1M sulfuric acid containingO .3M acetic acid as a catalyst for anodic oxidation, for process cycle times in the range from 10 min to 20 minutes. The "regeneration cycle" comprised

electrolysis of 1M aqueous sodium chloride solution for cycle times of 20 minutes.

[0084] Experiments were performed in an electrochemical system 100 as illustrated in Figure 1, comprising two

electrodesl20, 130 comprising a first electrode 120, which acts as the anode during the process cycle, being a diamond anode and the second electrode 130 being a stainless steel cathode or tungsten cathode. To demonstrate the effectiveness of operation comprising repeated cycling of process cycles, causing anodic oxidation, and regeneration cycles, for

regeneration and reversal of anodic oxidation on the diamond electrode, two series of tests were conducted using different diamond electrodes: the UNCD electrode had an average grain size of <10nm and the MCD electrode had an average grain size of 100 nm-500 nm (grain size increased with the thickness of the film) .

[0085] After each process cycle and regeneration cycle, data was obtained using both Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) to assess how well the diamond

electrode withstood repeated surface oxidation and

regeneration cycles. To accelerate testing, the "process" cycle, or anodic oxidation cycle using the diamond electrode as the anode, comprised 0.3Macetic acid catalyzed electrolysis of a 0.1M sulfuric acid solution, to speed up anodic

oxidation, thus simulating extended operation during a typical electrolysis process to destroy/oxidize an organic material producing high concentrations of acetic as a breakdown

product. After the anodic oxidation cycle, the electrode was tested by CV and DPV. Then after a regeneration cycle in 1.0 M aqueous NaCl (initial pH= 7) the extent of regeneration or recovery from the acetic acid catalyzed anodic oxidation was assessed again by both CV and DPV. [0086] Summary of Test Results

[0087] Figures 4 to 7show CV and DPV results for the UNCD electrode, and Figures 8 to 11 show CV and DPV results for the MCD electrode. Tables 1 and 3 provide a summary of test parameters for the test cycles of process and regeneration cycles for each experiment, for the UNCD and MCD electrodes respectively. Tables 2 and 4 summarize the CV and DPV test results for the UNCD and MCD electrodes respectively. Figure 12 shows cyclic voltammograms for the UNCD electrode,

comparing that of fresh surfaces (A) and anodically oxidized surfaces (B) . Figure 13 shows differential voltammetry scans for the same UNCD electrode surfaces comparing the fresh surface (A) and oxidized surfaces (B) .

[0088] Process cycle causing anodic oxidation

[0089] To encourage fast oxidation of the diamond electrode 120, during the process cycle (anodic oxidation cycle) , the electrolyte comprised a solution of 0.3M acetic acid with 0.1M sulfuric acid (H ₂ SO ₄ ) (data shown as white points) . Acetic acid is known from the literature to accelerate (catalyze) the oxidation of diamond. The diamond electrode was held at a positive potential while current was supplied to the

electrodes at a current density of 500mA/cm ² for a period of 10 minutes for electrochemical anodic oxidation of the diamond surface. The spacing or gap between the electrodes in the electrochemical cell was such that a potential of 28V was produced between the cathode 130 and the diamond anode 120 using this relatively dilute electrolyte. CV and DPV

measurements were conducted at the end of the cycle, after oxidation of the diamond electrode. [0090] Regeneration cycle

[0091] The diamond electrode was treated by electrolysis in an electrolyte comprising a 1M aqueous solution of NaCl, to provide a regeneration cycle (i.e. a cathodic treatment) during which current was supplied to the cell at a current density of 300mA/cm ² for 20 minutes while the diamond electrode 120 was held at a sufficiently negative potential to generate hydrogen. The spacing between the electrodes was such that a potential of -5V was produced between the electrode 130which acted as the anode during the regeneration cycle (i.e. the cathode during the process cycle) and the other diamond electrode 120 which acted as the cathode during the

regeneration cycle.

[0092] Cyclic Voltammetry (CV) and Differential Pulse

Voltammetry (DPV) Measurements

[0093] UNCD electrode

[0094] Figures 4 and 5 show the peak current response Ip (μΑ) using CV and DPV respectively. Figures 6 and 7 show the peak voltage change ΔΕ _Ρ (mV) for CV and DPV respectively. Each data point was taken at the end of the respective cycle with the first data point in each plot being the value of the unused (fresh) or uncycled UNCD diamond electrode 120. The electrode was cycled for 10 minutes in the acetic acid solution to oxidize it (process cycle/anodic treatment) then tested (second data point) and then cycled for 20 minutes in 1M NaCl solution with reversed potential (regeneration cycle/cathodic treatment) to regenerate the electrode and then tested again (third point) .

Each cycle was performed seven times. White data points are from tests at the end of the process cycles/anodic treatments and the black data points are from the tests after the

subsequent regeneration cycles/cathodic treatments.

Table 1 : Test sequence for UNCD electrode

for CV and DPC measurements shown in Figures 4 to 7

[0095] Figures 8 to 11 show the results of similar tests on an MCD electrode with an average diamond grain size in the range from 100 nm to 500 nm. For the process cycle, acetic acid catalyzed anodic oxidation of the MCD electrode 120 was carried out with electrolyte solutions comprising either 0.3M or 0.03M acetic acid mixed with 0.1M H ₂ S0 ₄ at a current density of 500mA/cm ² .For the regeneration cycle, or cathodic treatment, electrolysis of an electrolyte comprising a 1M aqueous NaCl solution at a current density of 300mA/cm ² for 20 minutes was carried out. The spacing between the electrodes was such that a potential of -5V is produced between the electrode 130and the diamond electrode 120.

[0096] Cycling between the process cycle and regeneration cycle was repeated multiple times to assess regeneration of the diamond electrodeby both CV and DPV.

[0097] Referring to Figures 4 and 5, the plots of Ip values measured by CV and DPV through successive cycles show that the UNCD electrode shows excellent recovery over multiple process and regeneration cycles. However, it is apparent there is an overall downward trend in measured values of Ip relative to the initial value in successive cycles. Referring to

Figures 6 and 7, showing CV and DPV test results, the peak voltage difference ΔΕ _Ρ (mV) shows excellent recovery after regeneration to close to initial value (~200mV) after the first 5 cycles. However values begin to decline in the last two cycles.

[0098] Table 2 show a comparison of the voltammetry tests of the diamond electrode after repeated cycling, i.e. after the final process cycle (anodic treatment) atl60 minutes and after the final regeneration cycle (cathodic treatment) atl80 minutes.

Table 2: Voltammetry data from testing of the UNCD

electrode

ΔΕ _Ρ (mV) Ip (μΑ) ΔΕ _Ρ (mV) I P (μΑ) by CV by CV by DPV by DPV

New surface 193 666 169 211

After final

process

cycle/anodic 317 424 185 38

oxidation

treatment

After final

regeneration

164 471 142 144 cycle/cathodic

treatment

[0099] Figure 12 compares cyclic voltammograms for (A) the fresh/uncycled surface of the UNCD electrode and (B) the surface of the same UNCD electrode, after repeated

process/regeneration cycle, as illustrated in Figures 4 to 6.

The spread between the peak of the voltammogram for the surface that was repeatedly anodically oxidized (B) and for the fresh surface (A) indicates a certain degree of

inactivation on the surface, e.g. resulting from surface changes because anodic oxidation isnot entirely reversible after the repeated anodic/cathodic cycles of the

oxidation/regeneration treatment protocol.

[00100] The degree of oxidation of the surface of the diamond electrode relative to a fresh diamond electrode can be measured by ΔΕ _Ρ (mV) of DPV and CV and is apparent from these scans, which show the ΔΕ _Ρ increases with degree of surface oxidation (measured relative to a Standard Calomel Electrode (SCE) as reference and platinum wire as counter electrode) .

[00101] Figure 13compares DPV scans, Ip vs . V (vs. SCE), for (A) the fresh uncycled surfaces of the UNCD electrode and (B) the same UNCD electrode after a process cycle causing anodic oxidation of the diamond surface. The peak current value is indicative of the amount of active area on the surface: higher peak current values correspond to higher percentages of active area. For curve (B) , oxidation of the anodically oxidized surface was performed for one cycle in 0.3M acetic acid plus 0.1M sulfuric acid, at 500mA/cm ² for 10 minutes. The decrease in Ip after oxidation is clearly apparent. The ratio of the peak current value before and after oxidation, can be used to estimate the extent of activation or deactivation of the diamond electrode i.e. as shown in Figure 13, about Ip _B /IP _A = 0.8/2.8 or -28% activated/~72% deactivated.

[00102] The CV and DPV results illustrated in Figures 4 to 6, 12 and 13 and shown in Table 2indicate the UNCD electrode shows excellent recovery after the first several cycles, and good recovery even after multiple extended regeneration cycles.

[00103] MCD electrode

[00104] Figures 8 and 9 show the peak current response Ip (μΑ) by CV and DPV measurements respectively. Figures 9 and lOshow the peak voltage change ΔΕ _Ρ (mV) by CV and DPV

measurements respectively. Each data point is the measurement taken at the end of the cycle. The first data point on each plot is the value for the unused or uncycled diamond electrode 130.

[00105] Each graph in Figures 5 to 10 show four different sets of test data indicated by I, II, III, IV, with continual use of the same MCD electrode through each set of a sequence of process and regeneration cycles, as summarized in Table 3.

Table 3 : Test sequences for MCD electrode

for generating measurements shown in Figures 9 to 12

[00106] For the first data set, indicated by I, each process cycle, or anodic oxidation treatment, of the electrode was performed in a solution of 0.3M acetic acid with 0.1M H ₂ S0 for 10 minutes per cycle, and then the electrode was then tested with CV and DPV (white data points) . The regeneration cycle, or cathodic treatment of the electrode, was performed inlM NaCl solution for 20 minutes, and then the electrode was tested by CV and DPV. This cyclic process is repeated five times for the first set of data I. White data points

represent results after an anodic oxidation cycle and black data points after a regeneration cycle.

[00107] For the second data set, indicated by II, each process cycle used a solution of 0.03M acetic acid with 0.1M H ₂ S0 ₄ for the anodic treatment of 10 minutes per cycle. After a first process cycle, using the anodic treatment for 10 minutes, the electrode was then tested with CV and DPV, i.e. to provide the first white data point in this set. Subsequently the

electrode was given three more 10 minute anodic treatment cycles, and CV and DPV measurements were taken at the end of each process cycle, indicating increasing levels of oxidation. At the end of the fourth process cycle/anodic treatment cycle, and after the fourth CV and DPV tests, the electrode 120 was placed in the 1M NaCl for a regeneration

cycle/cathodic treatment for 20 minutes. The final data point (black point) in the second dataset II shows the recovery of the diamond electrode 120 after regeneration.

[00108] The third data set, indicated by III, used a solution of 0.3M acetic acid with 0.1M H ₂ SO ₄ for a process cycle/anodic treatment of 20 minutes per cycle. The electrode 120was tested with CV and DPV after one cycle to provide data for the first white square data point in the set. In the same manner as the second data set II, the anode was then subjected to two more 20 minute process cycles/anodic treatments, without regeneration between cycles, and CV and DPV measurements are taken at the end of each process cycle, showing an increasing degree of oxidation after each cycle. After the third

process/anodic treatment cycle, and after the third CV and DPV test, the electrode 120 is regenerated by a regeneration cycle/cathodic treatment in 1M NaCl solution cathodic

treatment for 20 minutes. The final data point (black point) of set III shows the recovery of the diamond electrode 120 after the regeneration cycle. [00109] The fourth data set IV used conditions similar to those of the first set I, with alternating process cycles and regeneration cycles. The process cycles used a solution of 0.3M acetic acid with 0.1M H ₂ S0 ₄ for the anodic treatment for 10 minutes per cycle. The regeneration cycles use a 1M NaCl solution, for cathodic treatment of20 minutes per cycle. At the end of each cycle, the electrode 120 is tested with CV and DPV. The last data point in data set IV after the last

regeneration cycle (black data point) shows the recovery of the diamond electrode after prolonged cycling after obtaining data sets I to IV.

[00110] Referring to Figures 8 and 9, and 10 and 11, the Ip and ΔΕ _Ρ values measured after the regeneration cycles, by both CV and DPV, are substantially unchanged even after repeated cycling,

indicating that the MCD electrode is effectively regenerated.

[00111] Table 4 shows a comparison of the voltammetry tests on the MCD electrode before cycling (at 0 minutes) , after the final process cycle/anodic treatment (at 330 minutes) and the recovery after the final regeneration cycle/cathodic treatment (at 350 minutes) .

Table 4: Voltammetry data from testing of the MCD electrode

ΔΕ _Ρ (mV) (CV) I _P (μΑ) (CV ΔΕ _Ρ (mV) (DPV I _P (μΑ) (DP

) ) V)

New surface 183 530 153 199

After final 237 467 165 136 process

cycle/anodic

oxidation

treatment

After final 183 516 155 197 regeneration

cycle/cathodic

treatment [00112] Thus, it is demonstrated that the MCD electrode shows excellent recovery, even after extended, rigorous and repeated cycling of process cycles/anodic treatments and regeneration cycles/cathodic treatments. In fact, results indicate

substantially full recovery to DPV and CV values indicative of a surface conductivity equivalent to that observed upon initial testing of the new electrode with a hydrogen- terminated surface. [00113] System controller

[00114] To carry out a method according to embodiments of the present invention, the electrochemical system, e.g. system 100 or 200 as shown in Figures 1 and 2, desirably comprises a control system, e.g. system controller 160, which provides for automatic switching of operation between a process cycle and a regeneration cycle, as required. For example, the controller 160 may comprise a user interface or other input means for receiving process parameters for operation in a process cycle or in a regeneration cycle as described above, and switching means for switching the polarity of operation of the

electrochemical cell between a forward polarity for a process cycle and a reverse polarity for a regeneration cycle, and means for monitoring a condition indicative of a

threshold/excessive level of anodic oxidation, which may for example detect an elapsed operating time during a process cycle or a predetermined change in cell potential. Upon detection of the condition indicative of a threshold/excessive level of anodic oxidation, the controller automatically switches to reverse polarity operation for a regeneration cycle, and after operation in a regeneration cycle for a prescribed time, or to return the cell voltage to a desired value, for example, the controller switches back to forward polarity for another process cycle.

[00115] For example, characterization of the electrochemical system for a particular process, and particular process conditions, may be made by CV and DPV measurements similar to those described above, to determine an expected operational time that causes a threshold or excessive level of

deactivation of the diamond surface, and a minimum time for a regeneration cycle to provide effective regeneration of the electrode. Thus, after such characterization, switching of the system between forward polarity and reverse polarity cycles may be carried out according to a timed maintenance schedule of process cycles and regeneration cycles.

INDUSTRIAL APPLICABILITY

[00116] A method of operation of an electrochemical cell is disclosed which provides for a regeneration cycle to reduce the effects of anodic oxidation of a diamond anode after a process cycle. An effective method is provided forin situ regeneration of the surface conductivity of a diamond

electrode after anodic oxidation which avoids the need for handling of concentrated acids and/or removing the electrodes from the electrochemical cell for cleaning and reactivation. Periodic cycling of the cell between process cycles and short regeneration cycles allow for prolonged operation of the cell while maintaining preferred operational parameters such as surface conductivity, electrochemical potential, and degree of anodic oxidation. Regeneration cycles may be carried out at predetermined intervals or after detecting a condition of excessive anodic oxidation. Such a method of operation

prolongs the useful lifetime of diamond electrodes, reduces operational costs and reduces down time for maintenance.

[00117] Although embodiments of the invention have been

described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and not to be taken by way of limitation, the scope of the present invention being limited only by the appended claims .