Electrode coating and method of use in a reverse polarity electrolytic cell

Abstract

An electrode having an electrocatalytic surface or coating composed of a mixture with iridium oxide is used in a reversible polarity electrolytic cell to selectively produce an alkali metal hypohalite, preferably sodium hypochlorite, from brine made from hard water. The mixture also may have a platinum group metal oxide and a valve metal oxide, preferably, ruthenium oxide and titanium oxide respectively.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to an electrode coating for use in reverse polarity electrolytic cells and, more particularly, to an electrode coating comprising iridium oxide for use in reverse polarity electrolytic cells for producing a hypohalite.

[0003] 2. Description of the Related Art

[0004] An electrolytic cell is an electrochemical device that may be used to overcome a positive free energy and force a chemical reaction in the desired direction. For example, Stillman, in U.S. Pat. No. 4,790,923, and Silveri, in U.S. Pat. No. 5,885,426, describe an electrolytic cell for producing a halogen.

[0005] The design of electrolytic cells depends on several factors including, for example, construction and operating costs, desired product, electrical, chemical and transport properties, electrode materials, shapes and surface properties, electrolyte pH and temperature, competing undesirable reactions and undesirable by-products. Some efforts have focused on developing electrode coatings. For example, Beer et al., in U.S. Pat. Nos. 3,751,296, 3,864,163 and 4,528,084 teach of an electrode coating and method of preparation thereof. Also, Chisholm, in U.S. Pat. No. 3,770,613, Franks et al., in U.S. Pat. No. 3,875,043, Ohe et al., in U.S. Pat. No. 4,626,334, Cairns et al., in U.S. Pat. No. 5,334,293, Hodgson, in U.S. Pat. No. 6,123,816, Tenhover et al., in U.S. Pat. No. 4,705,610, and de Nora et al., in U.S. Pat. No. 4,146,438, disclose other electrodes. Alford et al., in U.S. Pat. No. 5,017,276, teach a metal electrode with a coating consisting essentially of a mixed oxide compound ruthenium oxide with a compound of the general formula ABO4 and titanium oxide. In the ABO4 compound, A is a trivalent metal and B is antimony or tantalum.

[0006] As mentioned, an electrochemical device may produce a desired chemical product; in particular, an electrolytic cell may produce an alkali metal hypohalite, for example, potassium hypochlorite, lithium hypobromite, sodium hypochlorite and sodium hypobromite. Typically, a sodium hypochlorite electrolytic cell will find use where there is a need to treat or disinfect water sources such as in drinking and service water treatment, sewage treatment, in-land and offshore installations, swimming pools and spas. Sodium hypochlorite cells also may find use in pulp and textile bleaching operations. However, the brine electrolyte used in such cells typically has impurities that interfere with the electrolysis of the electrolyte. In particular, the brine may have hardness ions. These hardness ions typically precipitate or deposit on the electrically conductive surface of an electrode. This typically creates operational problems such as electrode short-circuiting, electrode passivation, reduced production capacity and efficiency, increased power consumption and reduced service life. As noted by Silveri, in U.S. Pat. No. 5,885,426, several techniques have been used to address electrode deposition or scaling including, for example, electrode acid cleaning, electrode configuration and design, electrolyte flow design and voltage polarity reversal. Each of these techniques typically complicates electrolytic cell operation and/or increases operating costs. In particular, reversing the polarity typically results in shortened electrode life. Accordingly, a need persists in developing electrodes or electrode surfaces or coatings for use in reverse polarity electrolytic cells having extended service life, long-term stability and high conversion efficiency and selectivity.

SUMMARY OF THE INVENTION

[0007] In accordance with one embodiment, the invention provides a reversible polarity electrolytic cell comprising an electrolyte in a cell compartment, electrodes immersed in the electrolyte, a power source for applying a current to the electrodes at a first polarity and means for reversing the polarity of the current. The electrodes are coated with a mixture comprising iridium oxide.

[0008] The invention also provides a method of producing a hypohalite comprising the steps of immersing electrodes in an electrolyte, supplying a current to the electrodes at a first polarity and reversing the polarity of the current. The electrodes are coated with a mixture comprising iridium oxide.

[0009] In another embodiment, the invention provides a method of producing an electrolytic product comprising the steps of immersing a first electrode and a second electrode in an electrolyte, applying a current at a first polarity to the first electrode and the second electrode to populate the first electrode with electron donors and populate the second electrode with electron acceptors and changing the first polarity to populate the first electrode with electron acceptors and populate the second electrode with electron donors. The first and second electrodes are coated with a mixture comprising iridium oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Preferred, non-limiting embodiments of the present invention will be described by way of examples with reference to the accompanying drawings, in which:

[0011] FIG. 1 is a schematic diagram of one embodiment of a reverse polarity electrolytic apparatus of the present invention;

[0012] FIG. 2 is a graph of the measured current across the electrodes used in the apparatus of FIG. 1 for a period of over 28 days; and

[0013] FIG. 3 is a graph of the measured sodium hypochlorite concentration in the electrolyte of the apparatus used in FIG. 1 for a period of over 28 days.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The invention is directed to an electrolytic cell for producing alkali metal hypohalites and, more particularly, to using an electrode with an electrocatalytic surface in a reversible polarity electrolytic cell to electrolyze a brine electrolyte made with hard water to produce sodium hypochlorite or sodium hypobromite. The electrode has an electrocatalytic surface or coat composed of a mixture having iridium oxide. In addition to iridium oxide, the mixture may comprise another electrocatalyst, for example a platinum group metal or its oxide, and a binder to maintain the structural stability of the surface. Generally, the binder is a valve metal or its oxide. Preferably, the electrocatalyst is ruthenium oxide and the binder is titanium oxide. The mixture exhibits surprising stability and selectivity because those practicing the art seek to avoid the use of iridium oxide mixtures because of their known instability in reverse polarity systems.

[0015] The various aspects and embodiments of the invention can be better understood with the following definitions. As used herein, an “electrolytic cell” generally refers to an apparatus that converts electrical energy into chemical energy or produces a chemical products or an electrocatalytic product through a chemical reaction. The electrolytic cell may have “electrodes” or surfaces which are electrically conductive. “Current density” is the current passing through an electrode per unit area of the electrode. Typically, the current is a direct current which is a continuous unidirectional current flow rather an alternating current which is an oscillating current flow. Notably, reversing the polarity of the potential or voltage involves changing the direction of applied current flowing through the electrolytic cell.

[0016] The reactions in the electrolytic cell typically involve at least one oxidation reaction and at least one reduction reaction where the material or compound loosing an electron or electrons is being oxidized and the material gaining an electron or electrons is being reduced. An “anode” is a surface around which oxidation reactions occur and is typically the positive electrode in an electrolytic cell. Correspondingly, a “cathode” is a surface around which reduction reactions typically occur and is typically the negative electrode. “Electrocatalysis” is the phenomena of increasing the rate of an electrochemical reaction. Hence, an electrocatalytic material increases the rate of an electrochemical reaction. In contrast, passivation is the process whereby a material looses its active properties including, for example, its electrocatalytic properties.

[0017] Dissolved polyvalent metal ions, typically cations, cause water “hardness” and frequently interfere with the preferred electrochemical reaction. Thus, “hard water” is water with dissolved polyvalent metal ions. These ions typically precipitate on a surface as calcium and magnesium hydroxides or carbonates. Notably, applying an electrical current on a surface may promote the chemical reduction, hence precipitation, of hardness ions. Conversely, applying a current of reverse polarity promotes dissolution of the precipitated hardness ions. This technique of reversing the polarity of the applied voltage is well-known in the art, and incorporated herein, as one way to extend the operating life of electrodes. “Selectivity” is the degree to which a material prefers one property to others or the degree to which a material promotes one reaction over others. “Stability” refers to the ability of a material to resist degradation or to maintain its desired operative properties. “Platinum group metals” are those metals, typically in the Group VIII of the periodic table, including ruthenium (Ru), rhodium, palladium, osmium, iridium, and platinum. “Valve metals” are any of the transition metals of Group IV and V of the periodic table including titanium (Ti), vanadium, zirconium, niobium, hafnium and tantalum.

[0018] FIG. 1 is a schematic diagram of an electrolytic apparatus, specifically a reverse polarity electrolytic cell 10. The cell has electrodes 12 immersed in an electrolyte 14 contained in a cell compartment 16. The embodiment shown in FIG. 1 also shows a power source 18 for supplying a current through electrodes 12. The electrodes have a surface 20, and optionally a coating 22, where electrochemical reactions may occur. Preferably, surface 20 and coating 22 are electrocatalytic. Effectively, surface 20 and coating 22 perform as the electrocatalytic site where electrochemical reduction and oxidation reactions may be catalyzed.

[0019] In one embodiment, the electrolytic cell, having electrodes coated with an electrocatalytic coating 22 comprising a mixture comprising iridium oxide, electrolyzes brine made from hard water to produce sodium hypohalite, for example, hypochlorite, hypoiodite and hypobromite. Preferably, the mixture also has a binder comprising a valve metal, a valve metal oxide or a combination of a valve metal and a valve metal oxide. More preferably, the mixture has another electrocatalyst comprising a precious metal, a precious metal oxide, a platinum group metal, a platinum group metal oxide or a combination thereof. More preferably still, the binder is titanium oxide and the electrocatalyst is ruthenium oxide. And more preferably still, the iridium oxide in the mixture is between about 0.5 to about 10 mole percent.

[0020] The electrolytic cell may have meters, voltmeter 24 and ammeter 26 for example, measuring the applied voltage potential and the amount and direction of flowing current respectively. In the embodiment of FIG. 1, the electrolytic cell has a timer 28 controlling the closing and opening of contact switches 30 thereby dictating the direction of current flow. Thus, in operation, one electrode performs as an anode while the other performs as a cathode, depending on the polarity of the applied current. In another embodiment, the polarity or direction of the applied current from power source 18 changes so that the electrode formerly performing as the anode now performs as the cathode and the electrode formerly performing as the cathode now performs as the anode.

[0021] Typically, when power source 18 applies a current at a first polarity on surface 20 or coating 22, a first region of surface 20 or coating 22 of one electrode may populate with electron donors, or charge donors, and a second region of surface 20 or coating 22 of another electrode may populate with electron acceptors, or charge acceptors. In another sense, surface 20 or coating 22 may be cation-rich in the first region and may be anion-rich in the second region. In addition, the first region of surface 20 or coating 22 may electrocatalyze an oxidation reaction while the second region of surface 20 or coating 22 may electrocatalyze the corresponding reduction reaction.

[0022] When the polarity of the current from power source 18 changes, preferably the current direction reverses, the first region formerly populated with electron donors may populate with electron acceptors while the second region formerly populated with electron acceptors may become populated with electron donors. Or, the first region formerly electrocatalyzing the oxidation reaction now electrocatalyzes the reduction reaction and the second region now electrocatalyzes the corresponding oxidation reaction. Thus, after the polarity change, the first region, which may have an electron donor, would have an electron acceptor. Correspondingly, the second region, which may have an electron acceptor, may have an electron donor.

[0023] In another embodiment, the electrolytic cells may be used in electrochlorination systems for treatment of sea, fresh and municipal water systems such as in cooling systems and fire protection systems. The design and operation of these systems are influenced by, among other things, the extent of hardness in the electrolyte. For example, where sea water chlorination is desired, the system may include a narrow gap between the electrodes and an electrolyte flowing through the electrode gap at a high rate. In this way, deposition or precipitation of scale is inhibited especially around the electrode surfaces. Alternatively, such electrochlorination systems may be operated using brine made from softer water and thus require only occasional cleaning. Typically in such systems, the scale deposited on electrodes are cleaned in an acid wash. Consequently, the entire system must be placed out of service. This leads to reduced capacity and higher maintenance cost.

[0024] Advantageously, reverse polarity may be used to clean or remove any precipitated scale at significantly lower maintenance costs. For example, the electrolytic cell may be used in industrial systems where the current density is at least 1,000 amperes/m2 (A/m2). In such systems, reverse polarity operation may be performed at a lower current density, at about less than 500 A/m2 for example, in order to remove or dissolve precipitated scale. After the low-current density operation, the cells may be switched back to a normal operation mode at higher current density. Notably, the current applied at the low density and reverse direction is sufficient to dissolve at least a portion of any precipitated scale without damaging the iridium comprising coating.

[0025] The electrochemical device may further include other process sensing elements, as is well known in the art, measuring any of the electrolytic cell operating parameters including, for example, the concentration of a species in the electrolyte, the voltage, the cell resistance, the pH and the current flow. Notably, the sensing element may be a combination of sensors measuring the cell operating parameters in addition to those noted. In one embodiment, the sensing elements and may be controlled or triggered to change the polarity of the current when a predetermined condition has been satisfied. For example, the electrolytic cell may have a control system that changes the polarity of the current according to a predetermined sequence or when the concentration of a particular species, the desired product for example, has reached a predetermined level. In the former, the predetermined sequence may be set by an operator according to empirical or other information. In the latter, the control system may include a control loop incorporating, for example, a computer with a control loop around a set-point. As with the predetermined period, the set-point may be set by the operator or may be set according to other requirements. The control system typically includes such systems well-known control in the art such as a control loop incorporating any of proportional, integral and derivative control, or a combination thereof, or may be based on, for example, fuzzy logic or artificial intelligence control.

[0026] In an embodiment related to coating the substrate, the substrate, preferably an electrically conductive substrate and more preferably a titanium substrate, may be cleaned in a cleaning bath apparatus to remove or minimize contaminants that may hinder proper adhesion of the coating to the substrate surface. For example, the substrate may be placed in an alkaline bath for at least 20 minutes at a temperature of at least 50° C. The substrate surface may then be rinsed with deionized (DI) water and air dried. Preferably, the substrate surface is further treated by grit blasting with aluminum oxide grit or by chemical etching. The chemical etching may comprise washing the substrate surface with an acid, such as oxalic, sulfuric, hydrochloric or a combination thereof, at a temperature of at least about 40° C. for several minutes, preferably several hours, depending on the desired substrate surface characteristics. Further, the chemical etch may be followed by one or several DI water rinses.

[0027] An iridium salt may be dissolved in an alcohol to produce a homogeneous alcohol salt mixture which may be applied to the substrate surface. Thus, in one embodiment, the alcoholic salt mixture is prepared by dissolving iridium chloride salt in n-butanol or other suitable solvent known in the art such as ethanol, n-propanol and isopropanol. In another embodiment, the alcoholic salt mixture may further comprise salts of a valve metal, preferably, titanium and a platinum group metal, preferably ruthenium. This mixture may be applied to the cleaned substrate surface. Typically, each application produces a coat of about 1 to 6 g/m2 (dry basis). The wet coated substrates are typically allowed to air dry before being heat-treated. The heat treatment may involve placing the air-dried substrate in a furnace for at least about 20 minutes at a temperature of at least about 400° C. The alcoholic salt mixture may be reapplied several times to obtain a total coating loading of at least 10 and preferably, at least 20 g/m2. After the last application and heat treatment, the coated substrate is typically exposed to a final thermal treatment at a temperature sufficient to convert the salts to their corresponding oxides. Preferably, the final thermal treatment is performed at a temperature of at least 400° C.

[0028] The invention may be further understood with reference to the following examples. The examples are intended to serve as illustrations and not as limitations of the present invention as defined in the claims herein.

EXAMPLE 1

[0029] An electrode with an electrocatalytic surface embodying features of the invention was prepared by coating a substrate of commercial Grade 2 titanium. The titanium substrate was cleaned in a commercially available alkaline cleaning bath for 20 minutes at a temperature of 50° C. and then rinsed with DI water. After air drying, the substrate was grit blasted with aluminum oxide grit.

[0030] A mixture was prepared by dissolving 0.7 g of chloroiridic acid (H2IrCl6.4H2O) with ruthenium chloride (RuCl3.3H2O) and titanium tetraorthobutanate (Ti(C4H9O)4) in 1.0 ml of DI water and 73 ml of n-butanol. This mixture was applied to the cleaned substrate to achieve a loading of about 1 to 6 g/m2 per coat on a dry basis. The wet coated substrate was allowed to air dry before being placed in a furnace where it was heat treated for 20 minutes at a temperature of about 450° C.

[0031] The mixture was reapplied several times to obtain a total coating loading of at least 10 g/cm2. After the last application, the coated substrate was thermally treated for at least 10 minutes at a temperature of about 450° C. to oxidize the salts.

[0032] The resultant coating had about 1.5 mole percent iridium oxide.

EXAMPLE 2

[0033] The electrodes as prepared Example 1 were evaluated in a reverse polarity electrolytic cell similar to one shown in the schematic of FIG. 1. As described herein, Example 2 summarizes the results of an accelerated test designed to test the electrodes at the conditions more sever than a real application. In particular, the tests were performed at low sodium chloride concentrations, which promotes an undesirable oxygen producing side reaction.

[0034] A plastic tank was filled with 40 liters of tap water. To the water, 80 grams of sodium chloride was added to produce a 2,000 ppm NaCl solution. Two electrochemical cells were submersed into the tank. The cell with electrodes with the coating prepared in Example 1 was designated as “A.” The other cell had electrodes with a similar coating, designated as “B.” Notably, the “B” coating was prepared similarly as in the coating of Example 1 except that no iridium was added.

[0035] Each cell had two electrodes each with a 1-inch×3-inch electrocatalytic coating on either one or both sides. The gap between electrodes, the interelectrode gap, was ¼ inch. Each cell had a separate current power supply and a control panel. In addition, each cell was operated at constant cell voltage and at a current sufficient to provide a required current density of approximately 2,500 A/m2.

[0036] Every 90 minutes the direction of the current was automatically switched by reversing the polarity of the electrodes. During the test, the solution level in the tank was maintained by adding fresh unconditioned tap water. No salt was added after the start-up. Also, the cell voltage, current, temperature and sodium hypochlorite concentrations were monitored throughout the test. The results are graphically presented in FIGS. 2 and 3.

[0037] With time, the measured current began to decline as illustrated in FIG. 2. When the absolute value of the current falls from approximately 2.3-2.5 A to about 1 A, the anode is considered to have failed. The time from the start of the test to the anode failure is called the anode's lifetime. Anode lifetime is one measure of the stability of the anode. FIG. 2 shows two test runs for each of A and B. In both sets of test runs, the A coating significantly outlasted the B coating. Thus, the coating of Example 1 out performed the coating typically used in sodium hypochlorite production.

[0038] FIG. 3 shows the sodium hypochlorite concentration in the electrolyte during the test runs. The data shows that the hypochlorite production of the A coating compared favorably with the B coating.

[0039] In summary, the coating of the invention, as prepared in the embodiment of Example 1, showed improved stability with little or no reduction in electrolytic efficiency.

EXAMPLE 3

[0040] An electrochlorination system was designed to use the electrodes as coated in Example 1. The electrochlorination system was designed to provide hypochlorite continuously at a level sufficient to disinfect an industrial seawater system. The design considerations for such a system included: 1 Salt concentration: about 30,000 ppm Operating temperture: 10° to 35° C. Normal current density: at least 1,300 A/m2 (high density)

[0041] The design of this electrochlorination system further included a control system for reversing the polarity of the applied current. The electrochlorination system was designed such that a normal applied current would be applied for a predetermined period, typically several weeks, at a high current density. At the end of the normal current period, the current would be reversed and would be applied at a low current density, typically at less than about 500 A/m2, for several hours. The system would then be switched back to normal operation at high current density. The expected operating life of the coated electrodes is at least five years with none or minimal cleaning. This reverse current is expected to be sufficient to dissolve any deposited scale without damaging or shortening the operating life of the electrode coatings.

[0042] Further modifications and equivalents of the invention herein disclosed will occur to persons skilled in the art using no more than routine experimentation and all such modifications and equivalents are believed to be within the spirit and scope of the invention as defined by the following claims.

Claims

1. A reversible polarity electrolytic cell comprising:

an electrolyte in a cell compartment;

electrodes immersed in the electrolyte;

a power source for applying a current to the electrodes at a first polarity; and

means for reversing the polarity of the current,

wherein the electrodes are coated with a mixture comprising iridium oxide.

2. The electrolytic cell as in claim 1, wherein the mixture further comprises a binder.

3. The electrolytic cell as in claim 2, wherein the binder is titanium oxide.

4. The electrolytic cell as in claim 3, wherein the mixture further comprises an electrocatalyst.

5. The electrolytic cell as in claim 4, wherein the electrocatalyst is ruthenium oxide.

6. The electrolytic cell as in claim 5, wherein the mixture is about 0.5 to about 10 mole percent iridium oxide.

7. The electrolytic cell as in claim 6, wherein the mixture is about 10 to about 30 mole percent ruthenium oxide.

8. The electrolytic cell as in claim 7, wherein the mixture is about 2 mole percent iridium oxide.

9. The electrolytic cell as in claim 8, wherein the electrodes are coated with a total coating load of at least 10 g/m2.

10. The electrolytic cell as in claim 9, wherein the total coating load is at least 20 g/m2.

11. The electrolytic apparatus as in claim 10, wherein the electrolyte comprises a sodium chloride solution.

12. The electrolytic apparatus as in claim 11, wherein the hypohalite is hypochlorite.

13. The electrolytic apparatus as in claim 11, wherein the hypohalite is hypobromite.

14. A method of producing a hypohalite comprising:

immersing electrodes in an electrolyte;

supplying a current to the electrodes at a first polarity; and

reversing the polarity of the current,

wherein the electrodes are coated with a mixture comprising iridium oxide.

15. The method as in claim 14, wherein the mixture further comprises ruthenium oxide.

16. The method as in claim 15, wherein the mixture further comprises titanium oxide.

17. The method as in claim 16, wherein the mixture is about 0.5 to about 10 mole percent iridium oxide.

18. The method as in claim 17, wherein the mixture is about 10 to about 30 mole percent ruthenium oxide.

19. The method as in claim 18, wherein the mixture is about 2 mole percent iridium oxide.

20. The method as in claim 19, wherein the electrodes are coated with a total coating load of at least 10 g/m2.

21. The method as in claim 20, wherein the total coating load is at least 20 g/m2.

22. The method as in claim 20, further comprising measuring at least one operating parameter in the electrolytic cell.

23. The method as in claim 22, wherein reversing the polarity of the current depends upon at least one measured operating parameter.

24. The method as in claim 20, wherein the hypohalite is hypochlorite.

25. The method as in claim 20, wherein the hypohalite is hypobromite.

26. A method of producing an electrolytic product comprising:

immersing a first electrode and a second electrode in an electrolyte;

applying a current at a first polarity to the first electrode and the second electrode to populate the first electrode with electron donors and populate the second electrode with electron acceptors;

changing the first polarity to populate the first electrode with electron acceptors and populate the second electrode with electron donors,

wherein the first and second electrodes are coated with a mixture comprising iridium oxide.

27. The method as in claim 26, wherein the mixture further comprises a binder.

28. The method as in claim 27, wherein the mixture further comprises ruthenium oxide.

29. The method as in claim 28, wherein the binder is titanium oxide.

30. The method as in claim 29, wherein the mixture is about 0.5 to about 10 mole percent iridium oxide.

31. The method as in claim 30, wherein the mixture is about 10 to about 30 mole percent ruthenium oxide.

32. The method as in claim 31, wherein the mixture is about 2 mole percent iridium oxide.

33. The method as in claim 32, wherein the electrodes are coated with a total coating load of at least 10 g/m2.

34. The method as in claim 33, wherein the total coating load is at least 20 g/m2.

35. The method as in claim 34, wherein the electrolytic product is an alkali metal hypohalite.

36. The method as in claim 35, wherein the electrolyte is a brine solution.

37. The method as in claim 36, wherein the alkali metal hypohalite is sodium hypochlorite.

38. The method as in claim 36, wherein the alkali metal hypohalite is sodium hypobromite.