WO2012159206A1

WO2012159206A1 - Electrolytic cells and methods for minimizing the formation of deposits on diamond electrodes

Info

Publication number: WO2012159206A1
Application number: PCT/CA2012/000503
Authority: WO
Inventors: Thomas Urbanek
Original assignee: Klaris US Corp
Current assignee: Pattern Bioscience Inc
Priority date: 2011-05-25
Filing date: 2012-05-25
Publication date: 2012-11-29
Anticipated expiration: 2013-11-25
Also published as: CA2740759A1

Abstract

Disclosed are electrolytic cells and methods for minimizing the formation of deposits on diamond electrodes, particularly nanocrystalline doped diamond electrodes.

Description

ELECTROLYTIC CELLS AND METHODS FOR MINIMIZING THE FORMATION OF DEPOSITS ON DIAMOND ELECTRODES

FIELD OF INVENTION

The present invention relates to methods to minimize the build-up of deposits, particularly scale, on doped diamond electrodes.

BACKGROUND OF THE INVENTION

During electrochemical operations undesirable deposits may form on electrodes with time. Scale, for instance, may quickly form on cathodes when they are operated in hard water. Deposits may be well adhered and destructive, particularly in tightly spaced electrode arrays. With the formation of deposits, higher electrical potentials are required to maintain a specified current density. As a result, electrochemical processes become less efficient and economical. A number of methods and devices have been developed to deal with these problems.

US patent 4,789,448 to Woodhouse discloses a device comprising a sacrificial anode that releases a salt into the electrolyte. The salt forces scale-forming solutes to precipitate from the electrolyte. Thus, the formation of scale on electrodes is reduced.

US patent 5,062,940 to Davies discloses polarity reversals as a method and device to remove deposits from electrodes.

US patent 5,439,566 to Zucker discloses vibrating electrodes and the use of baffles as a method and device to remove scale from electrodes.

US patent 7,566,387 to Nam et al. discloses the introduction of an auxiliary electrode as a device to keep scale from forming at the cathodes of electrolytic ozone generators.

Other methods commonly practiced include the use of scale inhibitors, deionized water, and acid cleaning. The present invention seeks to address the perceived limitations in the art by providing a novel method to minimize the build-up of deposits, such as scale, on doped diamond electrodes.

SUMMARY OF THE INVENTION

The present invention relates in a particular embodiment to operating doped diamond electrodes in contact with an electrolyte comprising at least one substance that can form deposits, such as scale, on said electrodes.

In an embodiment of the invention, at least one electrode of the electrolytic cell comprises nanocrystalline doped diamond; other electrodes of the cell may comprise any material suited to meet the requirements of the intended application.

In a preferred method of operation, deposits on diamond electrodes are removed by reversing the polarity of the electrical potential from time to time. By utilizing nanocrystalline diamond the process of removing deposits is significantly improved.

This summary of the invention does not necessarily describe all features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent from the following description in which reference is made to the following appended drawings:

Figure 1. Microcrystalline diamond electrodes, water supply: Ottawa drinking water, flow rate: 20cm/s, electrode area: 3cm², electrode spacing: 0.2cm, current density: 200mA/cm², frequency of polarity reversals: 4/hr, power supply/recorder: Solartron 1287.

Figure 2. Nanocrystalline diamond electrodes, water supply: Ottawa drinking water, flow rate: 20cm/s, electrode area: 3cm², electrode spacing: 0.2cm, current density: 200mA/cm², frequency of polarity reversals: 4/hr, power supply/recorder: Solartron Figure 3. Nanocrystalline diamond electrodes, water supply: Calgary drinking water, flow rate: 16cm/s, square plate electrode, electrode area: 9cm², electrode spacing: 0.1cm, current densities: 1 10mA/cm² and 170m A/cm², power supply: Lambda GEN 40-38.

Figure 4. Nanocrystalline diamond electrodes, water supply: Calgary drinking water, flow rates: 1 l cm/s and 19cm/s, square electrode, electrode area: 9cm², electrode spacing: 0.1cm, current density: 170mA/cm², power supply: Lambda GEN 40-38.

Figure 5. Nanocrystalline diamond electrodes, water supply: Calgary drinking water, added salt: sodium chloride, flow rate: 1 lcm/s, square electrode, electrode area: 9cm², electrode spacing: 0.1cm, current density: 170m A/cm², power supply: Lambda GEN 40-38.

Figure 6. Typical performance of the reverse polarity power supply. DETAILED DESCRIPTION

The following is a description of a preferred embodiment.

The terms 'hardness of water' or 'hard water' are commonly defined as water containing a concentration of multivalent cations, and water containing a higher concentration of these cations is considered harder. Multivalent cations are ions with a charge greater than +1 , such as calcium and magnesium ions. Minerals comprising multivalent cations can deposit on electrodes, and the deposits are commonly referred to as scale. Other deposits, for example, may comprise biofilms or polymers that form via electropolymerization processes.

Doped diamond recently attracted much attention as a new electrode material.

Electrodes comprising doped diamond are commonly referred to as diamond electrodes. Most diamond electrodes comprise microcrystalline doped diamond with a nominal grain size above Ι ιη. More recently nanocrystalline doped diamond electrodes were developed. Nanocrystalline doped diamond is commonly defined by having a nominal grain size of less than 100 nm, for example 100, 90, 80, 70, 60 nm, etc., and more preferably less than 50 nm, for example 50, 40, 30, 20, 10, 5 nm, etc. Diamond electrodes have unique properties, such as a wide potential window, and high chemical and physical stabilities. These properties make diamond electrodes eminently suited for synthetic and analytical processes. The high oxygen

overpotential of the electrodes permits the efficient generation of strong oxidants, such as hydroxyl radicals, ozone, persulfates, and hypochlorites in aqueous media.

US patent 7,309,441 to Rychen et al. discloses the use of diamond electrodes to treat water supplies that form aerosols and, thereby, are liable to spread Legionella bacteria. Specifically cited are the treatment of water in air conditioners, hot water supplies, circulation baths, ornamental waterscapes, cooling towers, and other processes where water is recirculated.

Some of the applications cited by Rychen et al. present a specific problem: the hardness of water. Cooling towers, ornamental waterscapes, and swimming pools commonly comprise cementitious materials, such as concrete, mortar, or grout. If the contained water were low in hardness, detrimental erosion of these materials would set in. To avoid erosion, a certain degree of water hardness, calculable with the 'Langelier Saturation Index Calculator', must be maintained. The hardness, however, causes scale to form on diamond and other electrodes. Since scale affects the electrodes' performance, they must be cleaned from time to time.

It would be advantageous if a convenient method, such as polarity reversals, could be used to remove the deposits, in particular from diamond electrodes. Over extended periods, however, polarity reversals are generally not sufficient to remove all scale from microcrystalline diamond electrodes. Since the performance of the electrodes deteriorates with the formation of scale, the use of more elaborate cleaning methods eventually becomes necessary. Such methods may involve taking the electrolytic cell and electrode array apart, treating the electrodes with acid, and reassembling the entire device.

We now found that the build-up of scale can be efficiently removed via polarity reversals when nanocrystalline diamond electrodes are used.

Figures 1 and 2 depict the performance of microcrystalline and nanocrystalline doped diamond electrodes under galvanostatic conditions in hard water. Expectedly, the floating electrical potential rises as deposits build up on both types of diamond electrodes. After a period of time, the polarity of the electrical potential is reversed to remove formed deposits. With the removal of deposits, the electrical potential required to maintain a specific current drops and establishes new minimum.

Remarkably, the floating electrical potential applied to the nanocrystalline diamond electrodes always reverts quickly back to the original minima, indicating a

substantially complete removal of deposits. The minima of the electrical potential applied to the microcrystalline diamond electrodes, on the other hand, numerically increase with time since the deposits are not completely removed. Figure 1 shows that the potential required to maintain a current density of 500mA/cm² increases by about 23% over the period of 20 hours. Thus, nanocrystalline doped diamond electrodes offer a significant advantage over microcrystalline doped diamond electrodes.

The formation of deposits on diamond electrodes may be encountered under many circumstances, some of which are outlined in US patent 7,309,441. Further examples include the treatment of ballast water on ships (Ph. Rychen et al., Water Treatment Applications with BDD Electrodes and the DiaCell Concept, New Diamond and Frontier Carbon Technology, Vol. 13, No. 2 2003 MYU Tokyo) and the destruction of cyanide-containing effluents (C. Comninellis et al., Electrochemical Behavior of Synthetic Diamond Thin Film Electrodes, Diamond and Related Materials, Volume 8. Issues 2-5. March 1999, 820-823). In general, deposits may form during the treatment of chemical and biological contaminants, specifically disinfection and sterilization processes, deodortzation and decolourization processes, the removal of ions from electrolytes, the electrochemical generation of oxidants, such as hydroxyl radicals, ozone, hypochlorites, persulfates, percarbonates, perborates, chlorine dioxide, and others, the syntheses of short-lived or stable chemicals, or combinations of these processes.

In a first embodiment of the invention, the anodes and cathodes of an electrolytic cell comprise nanocrystalline doped diamond. This embodiment is particularly useful as it may (a) permit the removal of formed deposits with the least interruption of ongoing electrochemical processes, (b) eliminate the cross-contamination of reaction products with other electrode materials, (c) extend the longevity of an electrolytic cell, (d) extend the interval between maintenance cycles of the electrolytic cell, and (e) permit the use of the electrolytic cell under more aggressive operating conditions.

In a further embodiment of the present invention, at least one electrode of an electrolytic cell comprises nanocrystalline doped diamond. Other electrodes of the electrolytic cell may comprise any suitable material, such as steel, stainless steel, tungsten, carbon, platinum, gold, a conductive ceramic, and others. This embodiment may be useful, for instance, to reduce the cost of the electrolytic cell or to perform electrochemical reactions that other electrodes are better suited for. Assuming that the diamond electrodes operate as anodes, scale may form on cathodes comprising other materials. The scale may be removed through polarity reversals. For the period that the diamond electrodes operate as cathodes, deposits, such as scale, may form on them. If microcrystalline diamond electrodes were used, the removal of formed deposits may require more elaborate cleaning processes than polarity reversals alone.

Electrodes of the disclosed electrolytic cell may have any suitable shape, dimension, numerical ratio, polarity, and spatial arrangement relative to each other. The electrolytic cells may also be divided or undivided, and may comprise ion-conductive membranes that are placed between the electrodes of the device.

Electrical potentials applied to the disclosed electrolytic cell may range from 1 to 1 15 Volt, for example, but not limited to 1 , 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 1 10, 1 15 Volt, or any value therein between, but more preferably from 1 to 40 Volt, for example, but not limited to 1, 2, 3, 5, 7, 9, 10, 12, 14, 15, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40 Volt, or any value therein between. Except for using polarity reversals, the electrolytic cell may arbitrarily be operated under potentiostatic or galvanostatic conditions, and others. Comninellis et al. (Electrocatalysis in the Electrochemical Conversion / Combustion of Organic Pollutants for Waste Water Treatment, Electrochim. Acta 39, 1857-1862 (1994)) showed that optimal current efficiencies are achieved by balancing the current densities with given contaminant concentrations. The floating electrical potential then depends on operating conditions, such as the electrical resistance of the cell, electrolyte, and ongoing physical and chemical processes and may vary over a wide range before and after the occurrence of polarity reversals. When operated under potentiostatic conditions, the electrical potentials may be adjusted to any arbitrary value, and their numerical values may be kept identical or dissimilar before and after polarity reversals. The floating electrical current then depends on the operating conditions and may vary over a wide range before and after the occurrence of polarity reversals.

The process of reversing the polarity of the electrical potential may be performed manually, automatically after certain intervals, or automatically triggered when the floating electrical potential or current reaches a preset value. The frequency of the polarity reversals may be adjusted according to the rate at which deposits on the electrodes are formed and may vary from seconds and minutes, to hours and many days, for example, but not limited to 1 , 5, 10, 20, 30, 45, 60 seconds, 1.5, 2, 2.5, 5, 10, 15, 20, 30, 40, 45, 50, 60 minutes, 1.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 1 1 , 12 hours, 1 , 2, 3, 4, 5, 6, 7 days, and the like. The duration of the polarity reversal may also occur for a similar time period as described above.

Other methods and devices useful to minimize and remove deposits from

nanocrystalline diamond electrodes include, but are not limited to, laminar or turbulent flows of the electrolyte over the electrodes, continuous or intermittent streams of gas bubbles contacting the electrodes, baffles, Venturis, or static mixers, and various combinations thereof. Figures 3 to 5 show that the electrical potential increases at lower rates, and less deposits are formed, when salt concentrations or flow rates are higher, and when current densities are lower. These methods and devices, and others, may be applied in conjunction with the methods described herein.

EXAMPLE 1

Electrolytic cells comprising diamond electrodes were powered by a device that converts 120 VAC to a DC output in the range of 12 to 40VDC and provides constant currents, which can be preset to an arbitrary value between 5 and 20A. The power supply was also capable of reversing the polarity of the voltage applied to the electrodes after preset time periods ranging from 0.5 to 60 minutes. Before each polarity reversal and for a preset period, the potential was kept at zero. During this period the electrolytic cell, which acts as a capacitor, was allowed to discharge. If the external load were to exceed 20A, the power supply voltage would fold back for protection. The power supply would resume its normal operating condition when the overload or short condition was removed. All of the cited functions were provided automatically.

The power supply started up by applying a normal polarity and a constant electrical current to the electrodes for a preset period. The electrical potential was allowed to float within preset limits, and it increased as deposits on the electrodes were formed. Before reversing the polarity, the electrical potential was kept at zero for a preset period. Thereafter, the polarity was reversed and a constant electrical current flowed in the opposite direction for a preset period. Then, another period followed where the electrical potential was set to zero for a preset period. Afterward, the cycle began again. Any of the time periods may be chosen as equal in length to previous periods, but they may also be different, and the electrical currents may also be equal or different before and after polarity reversals. Electrolytic cells comprising

nanocrystalline diamond electrodes and employing the polarity reversal technology work well. Figure 6 shows the performance of the power supply in dependence of time. Shown are two 30s intervals before polarity reversals during which no voltage is applied to the electrodes.

It was also contemplated that a polarity reverser may change the polarity when it senses that a preset limit of the floating electrical potential or current is reached. This embodiment works on the principal that the potential increases or that the electrical current decreases as deposits are formed. Thus, it is possible to monitor these changes and to reverse the polarity when a preset limit is reached, or the like.

The present invention has been described with regard to one or more methods and embodiments. However, it will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.

All citations are herein incorporated by reference.

Claims

WHAT IS CLAIMED IS:

1. A method of removing deposits from one or more electrodes of an electrochemical cell, whereby one or more electrodes of the cell comprise nanocrystalline doped diamond, and the method comprises reversing the polarity of the electrical potential applied to the electrolytic cell, manually or automatically performed from time to time.

2. The method of claim 1 , wherein the electrodes of the electrolytic cell are in

contact with an electrolyte comprising at least one substance that can form deposits on the electrodes of the cell.

3. The method of claim 2, wherein the deposits comprise scale.

4. The method of claim 1 , wherein the cell is used to treat recreational waters.

5. The method of claim 4, wherein the recreational waters comprise water contained in pools, spas, jetted tubs, hot tubs, fountains, reflecting pools, ornamental ponds, or artificial waterfalls.

6. The method of claim 1 , wherein the electrodes of the electrolytic cell have a

suitable shape, dimension, numerical ratio, polarity, and spatial arrangement relative to each other.

7. The method of claim 1 , wherein the polarity of the applied electrical potential to the electrolytic cell is manually or automatically reversed from time to time by the power supply, and whereby the polarity reversals facilitate the removal of deposits from the electrodes.

8. The method of claim 7, wherein the frequency of polarity reversals is adjustable according to the rate at which deposits on the electrodes of the electrolytic cells are formed.

9. The method of claim 8, wherein the frequency of polarity reversals is in the range of from seconds to days.

10. The method of claim 8, wherein the frequency of polarity reversals is in the range of from minutes to hours.

1 1. The method of claim 1, wherein the electrical potential applied to the electrodes of the electrolytic cell ranges from 1 to 1 15 Volts.

12. The method of claim 1 1 , wherein the electrical potential applied to the electrodes of the electrolytic cell ranges from 1 to 40 Volts.

13. The method of claim 1 , wherein at least one ion-conductive membrane is placed between electrodes of the cell.

14. The method of claim 1 , whereby the cell is used to treat chemical and biological contaminants.

15. The method of claim 14, whereby the cell is used to disinfect, sterilize, deodorize, decolorize, remove ions from electrolytes, synthesize short-lived or stable chemicals, or combinations thereof.

16. The method of claim 1 , wherein the cell is used to produce oxidants.

17. The method of claim 16, wherein the cell is used to produce hydroxyl radicals, ozone, hypochlorites, persulfates, percarbonates, perborates, or chlorine dioxide.

18. The method of claim 1 , wherein the cell is used to treat the water in cooling

towers.

19. The method of claim 1 , wherein the cell is used to treat the ballast water on ships.

20. The method of claim 1 , wherein the cell is used to produce oxidants from seawater or to treat seawater that comprises contaminants.

21 . An electrochemical cell comprising at least one electrode which comprises

nanocrystalline doped diamond, the electrochemical cell being adapted to reduce or remove deposits from the at least one electrode by reversing polarity of the electrical potential applied to the electrochemical cell, manually or automatically performed from time to time.

22. The electrochemical cell of claim 21 , wherein electrodes of the electrochemical cell are adapted for contact with an electrolyte comprising at least one substance that can form deposits on the electrodes of the cell.

23. The electrochemical cell of claim 22, wherein the deposits comprise scale.

24. The electrochemical cell of claim 1 , wherein the electrochemical cell is for

treating recreational waters.

25. The electrochemical cell of claim 24, wherein the recreational waters comprise water contained in pools, spas, jetted tubs, hot tubs, fountains, reflecting pools, ornamental ponds, or artificial waterfalls.

26. The electrochemical cell of claim 1 , wherein the electrodes of the electrolytic cell have a suitable shape, dimension, numerical ratio, polarity, and spatial arrangement relative to each other.

27. The electrochemical cell of claim 21 , adapted for manual or automatic reversal of the polarity of the electrical potential applied to the electrolytic cell from time to time by the power supply, wherein reversal of the polarity facilitates removal of deposits from the electrodes.

28. The electrochemical cell of claim 27, adapted for adjustment of the frequency of polarity reversals according to the rate at which deposits on the electrodes of the electrolytic cells are formed.

29. The electrochemical cell of claim 21 , wherein at least one ion-conductive

membrane is placed between electrodes of the cell.

30. The electrochemical cell of claim 21, wherein the electrochemical cell is for

treating chemical and biological contaminants.

31. The electrochemical cell of claim 30, wherein the electrochemical cell is for

disinfecting, sterilizing, deodorizing, decolorizing, removing ions from electrolytes, synthesizing short-lived or stable chemicals, or combinations thereof.

32. The electrochemical cell of claim 21 , wherein the electrochemical cell is for producing oxidants.

33. The electrochemical cell of claim 32, wherein the electrochemical cell is for producing hydroxyl radicals, ozone, hypochlorites, persulfates, percarbonates, perborates, or chlorine dioxide.

34. The electrochemical cell of claim 21 , wherein the electrochemical cell is for treating the water in cooling towers.

35. The electrochemical cell of claim 21 , wherein the electrochemical cell is for treating the ballast water on ships.

36. The electrochemical cell of claim 21 , wherein the electrochemical cell is for producing oxidants from seawater or for treating seawater that comprises contaminants