CA2740759A1 - Electrolytic cells and methods for minimizing the formation of deposits on diamond electrodes - Google Patents

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

s FIELD OF INVENTION

[0001] The present invention relates to devices and methods to minimize the build-up of deposits, particularly scale, on doped diamond electrodes during their use.
BACKGROUND OF THE INVENTION

[0002] 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 targeted current densities. As a result, the ongoing electrochemical processes become less efficient and economical. A number of methods and devices have been developed to deal with these problems.

[0003] 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.

[0004] US patent 5,062,940 to Davies discloses polarity reversals and the introduction of baffles into the flow of the electrolyte as a method and device to remove deposits from electrodes.

[0005] 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.

[0006] 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.

OTT_LAW\ 2812813\1 [0007] Other methods commonly practiced include the use of scale inhibitors, deionized water and acid cleaning.

[0008] The present invention seeks to address the perceived limitations in the art by providing a novel method and device to minimize the build-up of deposits, such as scale, on doped diamond electrodes.

SUMMARY OF THE INVENTION

[0009] 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.

[0010] 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.
[0011 ] 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.

[0012] This summary of the invention does not necessarily describe all features of the invention.

DETAILED DESCRIPTION

[0013] The following is a description of a preferred embodiment.

[0014] 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 cations with a charge greater than +1, including calcium or magnesium ions.
Multivalent cations can deposit on electrodes and are commonly referred to as scale. Other deposits may comprise organic materials that, for instance, may form subsequent to electropolymerization processes.

OTT_LAW\ 2812813\1 [0015] 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 I m. 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.
[0016] 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.

[0017] 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.

[0018] Some of the applications cited by Rychen et al. present a specific problem: the hardness of water. Cooling towers and ornamental waterscapes, but also 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.

[0019] 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 polarity reversals, however, are not sufficient to remove all scale from microcrystalline diamond electrodes. Since the performance of the electrodes deteriorates, the use of more elaborate cleaning methods eventually becomes OTT_LAW\ 2812813\1 necessary. Such methods may involve taking the electrolytic cell and electrode array apart, treating the electrodes with acid, and reassembling the entire device.

[0020] We have now found that the build-up of scale can be efficiently removed when nanocrystalline diamond electrodes are used.

[0021] Figures 1 and 2 depict the performance of microcrystalline and nanocrystalline doped diamond electrodes under galvanostatic conditions in hard water, respectively.
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.
Notably, the floating electrical potential applied to the nanocrystalline diamond electrodes always reverts back to the original minimum, 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. Thus, as can be seen from Figures 1 and 2, nanocrystalline doped diamond electrodes offer advantages over the use of microcrystalline doped diamond electrodes.

[0022] The formation of deposits on diamond electrodes may be encountered under many circumstances, some of which are outlined in US patent 7,309,441. Others may include the treatment of chemical and biological contaminants, specifically disinfection and sterilization processes, deodorization and decolourization processes, the removal of ions from electrolytes, the electrochemical syntheses of short-lived or stable chemicals or combinations of these processes.

[0023] In a first embodiment of the invention, the anodes and/or cathodes of the devices comprise nanocrystalline doped diamond. This embodiment is particularly useful as it (a) permits the removal of formed deposits with the least interruption of ongoing electrochemical processes, (b) eliminates the cross-contamination of reaction products with other electrode materials, (c) may possibly extend the longevity of the electrolytic cell, (d) may possibly extend the interval between maintenance cycles of OTT_LAW\ 2812813\1 the electrolytic cell, and (d) may permit the use of the electrolytic cell under more aggressive operating conditions.

[0024] In a further embodiment of the present invention, at least one electrode of the electrolytic cell comprises nanocrystalline doped diamond. Other electrodes of the electrolytic cell may comprise any suitable material, such as steel, stainless steel, modifications of 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 anodic or cathodic electrochemical processes that other electrodes are better suited for. Also, assuming that the diamond electrodes operate as anodes, scale may form on cathodes comprising other materials. The scale may be removed by reversing the polarity of the potential. For the period that the diamond electrodes then operate as cathodes, deposits, such as scale, could form on them. If microcrystalline diamond electrodes were used, build up of deposits may potentially occur that may not be minimized or substantially removed upon polarity reversal.

[0025] 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 at least one ion-conductive membrane between electrodes of the device.

[0026] Electrical potentials applied to the disclosed electrolytic cell may range from 1 to 115 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, 110, 115 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. The disclosed electrolytic cell may be operated under potentiostatic or galvanostatic conditions, or both. 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 adjusting and maintaining the current densities in proportion to contaminant concentrations, i.e. under galvanostatic conditions.
The floating electrical potential then depends on the electrical resistance of the device and electrochemical process and can vary over a wide range before and after the OTT_LAW\ 2812813\1 occurrence of polarity reversals. When operated under potentiostatic conditions, the electrical potentials may be adjusted to any arbitrary value, and their numerical values can be identical or dissimilar before and after polarity reversals.

[0027] The process of reversing the polarity of the electrical potential can be performed manually, automatically after certain intervals, or automatically triggered when the floating electrical potential reaches a preset value. The frequency of the polarity reversals may be adjusted according to the rate at which deposits on the electrodes of the electrolytic cells are formed and can vary from seconds and minutes, to hours and many days, for example, but not limited to 1, 5, 10, 20, 30, 45, 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, 11, 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.

[0028] Other methods and devices useful to minimize and remove deposits from the 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 and baffles, venturis, or static mixers, but also various combinations thereof. Those methods and devices may be applied concurrently with the methods and devices as described herein.

[0029] EXAMPLE I

[0030] An electrolytic device comprising a nanocrystalline diamond electrode was employed using a power supply that converts 120Vac to a DC output voltage in the range of 12 to 40Vdc and provides constant currents, which can be preset to an arbitrary value between 5 and 20 Amps. 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 the period of 30 seconds, no voltage was applied to the electrodes. During this period the electrolytic cell, which acts as a capacitor, was allowed to discharge. If the external load were to exceed 20 Amps, 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.
OTT_LAW\ 2812813\1 [0031 ] 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 polarity reversals, the electrical potential was set to zero for a preset period. Thereafter, the polarity reversed and the 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 devices employing the polarity reversal technology worked well. Figure 3 shows the performance of the power supply in dependence of time. Shown are two 30s intervals before voltage reversals where no voltage is applied to the electrodes.

[0032] It was also contemplated that a polarity reverser may change the polarity when it senses that a preset upper limit of the floating electrical potential is reached. This embodiment works on the principal that the potential increases as deposits are formed on the electrodes. Thus, it is possible to monitor increases of the potential due to the build-up of scale and reverse the polarity when the floating electrical potential reaches a preset limit or the like.

[0033] 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.

[0034] All citations are herein incorporated by reference.
OTT_LAW\ 2812813\1

Claims (8)

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

2. The electrochemical device of claim 1, wherein the electrolytic cell comprises an electrolyte comprising at least one substance that can form deposits, particularly scale, on the electrodes of the cell.

3. The electrochemical device of claim 1, whereby the device is used to treat recreational waters, such as they are contained in pools, spas, jetted tubs, hot tubs, fountains, reflecting pools, ornamental ponds, and artificial waterfalls.

4. The electrochemical device of claim 1, whereby the electrodes of the electrolytic cell may have any suitable shape, dimension, numerical ratio, polarity, and spatial arrangement relative to each other.

5. The electrochemical device 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, and whereby the frequency of polarity reversals may be adjusted according to the rate at which deposits on the electrodes of the electrolytic cells are formed and can vary from seconds and minutes, to hours and many days.

6. The electrochemical device of claim 1, wherein the electrical potential applied to the electrodes of the electrolytic cell ranges from 1 to 115 Volts, and more preferably from 1 to 40 Volts.

7. The device of claim 1, wherein at least one ion-conductive membrane is placed between electrodes of the device.

8. The device of claim 1, whereby the device is used to treat chemical and biological contaminants, specifically to disinfect and sterilize, deodorize and decolorize, and also to remove ions from electrolytes, synthesize short-lived or stable chemicals, or perform combinations of these processes.