NON-GAS-EMITTING ELECTRODES FOR USE IN ELECTRODIALYSIS AND ELECTRODIONIZATION DESALINATION SYSTEMS

Abstract

Non-gas emitting electrodes having a very high surface area, high electric capacitance, and low electric resistance are integrated with silver and/or silver chloride for use in electrodialysis/electrodeionization cells, or in any other system requiring the generation of electric fields through electrolyte solutions, and are capable of generating an electric field for extensive periods of time without generation of gases, and without the occurrence of water splitting electrode reactions. Each electrode is highly porous and highly conductive, such as a carbon aerogel electrode, and thus has a very large internal surface area, which is infused with silver and/or silver chloride. This combination supercapacitor and pseudocapacitor electrode can sustain electrode reactions for longer periods of time, and at much higher current densities, as compared to conventional (solid) silver/silver chloride electrodes.

Claims

1. A non-gas-emitting electrode for use in generation of electric fields within electrodialysis and electrodeionization systems, the electrode comprising an Electric Double Layer Capacitor (EDLC) electrode infused with silver or silver chloride, wherein the electrode has the properties of a supercapacitor electrode with the reversibility of a pseudocapacitor electrode for generating an electric field for extensive periods of time without generation of gases, and without the occurrence of water splitting electrode reactions.

2. The non-gas-emitting electrode of claim 1, wherein a pair of the non-gas-emitting electrodes are included in an apparatus for desalinating and deionizing a feed solution, wherein a first of the electrode pair is infused with silver and a second of the electrode pair is infused with silver chloride.

3. The non-gas-emitting electrode of claim 1, wherein the electrode comprises porous carbon materials including three-dimensional porous networks having a high specific surface area.

4. The non-gas-emitting electrode of claim 2, wherein the electrode material is selected from the group consisting of activated carbon, carbon aerogel and a carbon aerogel composite.

5. The non-gas-emitting electrode of claim 1, wherein the electrode is a carbon aerogel electrode having a substantial amount of its porous volume infused with silver or silver chloride.

6. A process for the manufacture of a non-gas-emitting electrode for electrodialysis and electrodeionization systems, the process comprising depositing silver or silver chloride within the structure of a carbon aerogel Electric Double Layer Capacitor (EDLC) electrode.

7. The process of claim 6, wherein a pair of the non-gas-emitting electrodes are manufactured for inclusion in an apparatus for desalinating and deionizing a feed solution, wherein a first of the electrode pair is infused with silver and a second of the electrode pair is infused with silver chloride.

8. The process of claim 6, wherein silver is deposited within the structure of the EDLC electrode by the electrolysis of silver nitrate with the carbon aerogel EDLC electrode acting as the cathode, and using a silver, titanium, carbon or graphite anode.

9. The process of claim 6, the process comprising the steps of: a) soaking the electrode in a silver nitrate solution to saturate and infuse the electrode with silver nitrate; b) rinsing the electrode with water; c) drying the electrode; and d) heating the electrode to above the decomposition temperature of silver nitrate to break down the infused silver nitrate into elemental silver and nitrogen dioxide and oxygen gases, leaving a finely dispersed deposit of silver within the structure of the electrode.

10. The process of claim 6, the process comprising the steps of: a) soaking the electrode in a silver nitrate solution to saturate and infuse the electrode with silver nitrate; b) rinsing the electrode with water; c) drying the electrode; and d) submerging the electrode in a high concentration solution of sodium chloride, resulting in the precipitation of silver chloride within the structure of the electrode.

11. An apparatus for desalinating and deionizing a feed solution, the apparatus comprising: a) a plurality of spaced apart ion exchange membranes arranged adjacent to one another with spacers in between forming a stack, each of the plurality of ion exchange membranes creating a concentrate compartment on one side and a dilute compartment on the other side when the apparatus is filled with a feed solution and acted upon by a direct current passing therethrough; b) a first non-gas-emitting electrode housed in a first endplate positioned on one side of the stack; c) a second non-gas-emitting electrode housed in a second endplate positioned on the other side of the stack; d) a first end spacer positioned between the first endplate and the first ion exchange membrane; e) a second end spacer positioned between the second endplate and the last ion exchange membrane, wherein the stack is arranged between the first and second endplates; f) a frame for compressing and sealing together the first endplate, the second endplate, and the stack of ion exchange membranes; g) a plurality of input and output passages leading into and out of the end plates and each compartment; and h) a direct current electric power supply capable of polarity reversal for charging and establishing a potential difference between the first non-gas-emitting electrode and the second non-gas-emitting electrode to cause the passage of electric current through the feed solution.

12. The apparatus of claim 11, wherein each of the first non-gas-emitting electrode and the second non-gas-emitting electrode comprises an Electric Double Layer Capacitor (EDLC) electrode having a substantial amount of its inner porous volume infused with silver or silver chloride for generating an electric field for extensive periods of time without generation of gases, and without the occurrence of electrode reactions.

13. The apparatus of claim 11, wherein the feed solution is selected from the group consisting of seawater, wastewater, brackish water, and hard water.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawing illustrates a preferred embodiment of the invention and, together with a general description of the invention given above, and the detailed description given below, serves to explain the principles of the invention.

[0025] FIG. 1 illustrates how current flow decreases over time at constant voltage when chlorine is electrochemically deposited on a silver electrode.

[0026] FIG. 2 illustrates the variation of the developed voltage over time between conventional EDLC electrodes and the electrodes of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Definitions—As defined herein, the terms “ion” or “ions” refer to atoms or molecules with a net electric charge due to the loss or gain of one or more electrons. In electrolytes, ions are hydrated ions which means that they are covered by a shell of water molecules. The amount of charge of an ion depends on the number of electrons lost or gained. For any ion missing or gaining one electron, the net charge is equal to that of an electron, which is equal to 1.60217662×10-19 Coulombs. This results in the fact that one mole of electrons is equivalent to Avogadro's number (6.02214×1023) of electrons or 96,485.3 Coulombs.

[0028] As used herein, the terms “electrolyte” and “electrolyte solution” are interchangeable when used in this document and are therefore applicable to any solute or chemically defined salt dissolved in any polar liquid, wherein the result is the formation of an electrolyte solution. Therefore, when referring to “ion-containing” or “salty” waters, irrespective of the number of salts present in unit volume of the liquid, it is to be interpreted as to mean and include an electrolyte solution. As such, the term “water” can mean any polar solvent and the term “salt” can mean any solute that together with a polar solvent forms an electrolyte solution.

[0029] As used herein the terms “ion exchange membranes” and “ion selective membranes” are interchangeable, and include cation exchange membranes, anion exchange membranes, cation selective membranes, and anion selective membranes.

[0030] As used herein the terms “electroactive media” and “ion exchange resin beads” are interchangeable and can include any shape or form which can perform the intended function of conducting ions in a sparingly conductive solution under the influence of an electric field, while maintaining sufficient mechanical integrity. Many types of electroactive media can be used to define a lower resistance path for ion flow. The most common type is in the form of ion exchange resin beads, but electroactive media can also be in the form of beads bonded to one another by a bonding agent, or in the form of fabrics, and depending on the specifics of a design could be mixed anion and cation exchange beads or singular polarity bead layers filling one compartment or distinct sections of both types of resin beads in a single compartment.

[0031] The terms “electrodeionization” and “electrodialysis” as used in this document are technically different, but similar in design and so can be referred to interchangeably herein as electrodialysis/electrodeionization cells or systems.

[0032] The present invention provides non-gas emitting electrodes for generation of electric fields within electrodialysis/electrodeionization cells. The electrodes can be used for desalinating and deionizing feed electrolytes, including but not limited to seawater, wastewater, brackish water, hard water and feed water used for the production of ultrapure water, and for processing of industrial electrolytes needing to be concentrated or diluted. The non-gas emitting electrodes combine supercapacitance with pseudocapacitance, as explained in detail herein.

[0033] Supercapacitors, or ultracapacitors, are generally divided into two types: (1) Electric Double Layer Capacitor electrodes, or EDLC's, which are supercapacitors which store charges electrostatically in a double layer (Helmholtz Layer); and (2) Pseudocapacitors, such as silver chloride electrodes, which store charges through reversible Faradic reactions. As noted above, EDLC's and their use have been described in the U.S. Pat. No. 10,329,174 to Yazdanbod, and pseudocapacitive electrodes benefitting from reversible silver-silver chloride reactions have been described in U.S. Pat. No. 10,604,426 to Connor, Jr., et al.

[0034] It has been discovered that the very large, porous, internal surface area of EDLC electrodes such as carbon aerogel electrodes can be covered or infused with silver and/or silver chloride, such that a substantial amount of silver and/or silver chloride can be made available to sustain electrode reactions for longer periods of time, and at much higher current densities, as compared to conventional (solid) silver/silver chloride electrodes. The inventive electrodes can combine the capacitances of EDLC's with the reversibility of pseudocapacitors, increasing their functionality as electrodes for use in electrodialysis and electrodeionization cells without the generation of gases or the occurrence of water splitting Redox Reactions.

[0035] Silver can be deposited within the structure of various types of carbon aerogel electrodes via the electrolysis of silver nitrate with a carbon aerogel EDLC electrode acting as the cathode, and using a silver, titanium, carbon or graphite anode. Another means for infusing a carbon aerogel EDLC electrode with silver is by saturating the electrode with a silver nitrate solution and then heating it to above the boiling/decomposition temperature of silver nitrate (440 to 550 degrees C.). At this temperature, silver nitrate decomposes and gives off nitrogen dioxide and oxygen gases, and leaves a finely dispersed deposit of silver within the structure of the electrode.

[0036] Silver chloride may also be placed within the structure of various types of carbon aerogel electrodes by saturating each with a silver nitrate solution. By placing these electrodes in such a solution, silver nitrate is dispersed within the aerogel's complex structure and pores. Placing this electrode in a solution of sodium chloride will then result in the precipitation of silver chloride within the structure of the electrode, according to the following reaction.

AgNO.sub.3(aq)+NaCl.sub.(aq).fwdarw.AgCl.sub.(s)+NaNO.sub.3(aq)  (Equation 3)

[0037] This can be done when the electrode is still saturated with silver nitrate solution, or preferably after partial or full drying of the electrode. Yet another method for deposition of the silver chloride within carbon aerogel electrodes is through the use of a carbon aerogel electrode having silver already deposited within it as anode, with hydrochloric acid solution as the solution, and any other electrode as the cathode in an electrochemical cell.

[0038] Although the abovementioned processes have been proven to be very effective in forming a dispersed collection of silver and silver chloride within carbon aerogel electrodes, it has been found by the inventors that during the manufacture of these electrodes the higher concentration of silver and silver chloride near the surface can lead to the formation of superficial layers of silver chloride on these electrodes, which may block the accessibility of silver and silver chloride to the electrolyte solution bathing the electrode body. This problem can be solved by rinsing the electrodes with water after their saturation with silver nitrate solution. This rinsing action, when combined with simple rubbing of the external electrode surfaces, has been found to eliminate the formation of a silver chloride layer in the subsequent electrode preparation steps for silver chloride electrodes, and the formation of the same when electrodes with infused silver deposits within them act as anodes.

[0039] The electrodes described herein are for use in electrodialysis/electrodeionization cells, or in any other system requiring the generation of electric fields through electrolyte solutions. The incorporation of silver and/or silver chloride in highly porous and conductive electrodes, such as carbon aerogel electrodes, the required electric field can be generated for extensive periods of time without generation of gases, and without the occurrence of water splitting Redox Reactions at these electrodes.

[0040] EauiDment and Materials—in the tests described herein, the following equipment and materials were used: The power supply used was a Reference 3000 Potentiostat manufactured by GAMRY Instruments Inc. of Pennsylvania, USA. This device can supply up to 3.0 Amperes of current to each electrode and had a maximum active electrode voltage of +/−6.5 Volts. This device is classified as a high precision laboratory measurement tool.

[0041] The EDLC electrode used for infusion of silver and silver chloride as electrodes for generation of electric fields in electrodialysis cells under this invention were a 50 mm by 100 mm and 10 mm thick carbon aerogel-graphite composite electrode. These composites were prepared with the specific intention of allowing for the use of Resorcinol-Formaldehyde aerogels as the dominant phase in order to benefit from their high surface area, high electric capacitance and low electric resistance. Further and in order to avoid the complications and high cost associated with supercritical drying usually used to reduce the volume shrinkage associated with drying of such aerogels and aerogel composites, and in order to further enhance their electric conductivity, graphite powder filler material was incorporated into the mix. The graphite powder fill used was laboratory grade #38 commercially available from Fisher Scientific, Canada.

[0042] The typical composition of Resorcinol Formaldehyde (RF) aerogel base material used was 12.35 grams of resorcinol for 17.91 grams of 37% methane stabilized Formaldehyde in water solution and 8.7 grams of 25% solution of Cetyltrimethylammonium Chloride (CTAC) solution in water (all three lab grade purchased from Sigma Aldrich) which were mixed and stirred until the Resorcinol was totally dissolved. Then 1.5 grams of 0.4 Mole solution of sodium carbonate in water was added as catalyst.

[0043] The composite was then made using 50% by weight graphite powder detailed above and 50% aerogel base solution, also described above. The well mixed mixture of the two materials were then placed in a sealed steel mold 100 mm by 100 mm by 10 mm. The sealed mold was allowed to cure for about 24 hours at room temperature followed by 24 hours at 50 degrees Celsius (° C.) and 24 hours at 80° C., as is customary for RF aerogels. Once the mold was cool enough the components were taken out and soaked in acetone for 24 hours and then gradually dried in room temperature for 24 hrs and then heated up to 80° C. to remove the acetone and most of the remaining water. The components were placed in a refractory mould, covered with crushed activated carbon particles and were heated to 1100° C. This temperature was maintained for about 4 hours. After cooling to room temperature, the resulting parts were cut in half with a hand saw and were then used.

[0044] To infuse the silver and silver chloride into these electrodes, they were submerged in a 0.2 molar silver nitrate (more than 99% purity lab grade by ALPHACHEM) for 24 hours. The resulting electrodes were then rinsed with water and then dried at 110° C. Then one electrode was placed in 15000 ppm solution of sodium chloride to form silver chloride within the electrode, as described earlier. The other electrode was heated under a cover of activated carbon to over 500° C. to decompose the infused silver nitrate to nitrogen dioxide, oxygen and silver. This sample was also then placed in a 15000 ppm sodium chloride solution. For comparative tests, another set of similar electrodes were just placed in 15000 ppm sodium chloride solution after kilning at 1100° C.

[0045] Test 1—FIG. 1 presents the results of an electrochemical deposition test in which silver chloride was deposited onto a silver rod. In this test a 10 mm length of 3.14 mm in diameter pure silver rod (ROSS METALS, NY, NY) was exposed as the anode to a 10% hydrochloric acid solution (pH=0.84). The other electrode (cathode) in this cell was a graphite rod. The applied voltage using a GAMRY 3000 potentiostat was 0.5 volts operated in chrono amperometry mode. This figure shows that as chlorine was deposited on the silver electrode, the resistance of the cell increased as demonstrated by the drop in current. After the test the diameter of the 10 mm long rod was measured to be 3.17 mm. This test confirms that as silver chloride is deposited on the silver anode (according to Equation 1), its low conductivity increases cell resistance, as discussed earlier.

[0046] Test 2—included two experiments which evaluated the electric capacitance of a pair of plain carbon aerogel-graphite composite EDLC electrodes compared to a substantially identical carbon aerogel-graphite composite EDLC electrode pair that had been modified with silver/silver chloride according to the teachings of this invention. The test results are presented in FIG. 2, in which curve 1 and curve 2 both represent the variation of the developed voltage over time, when a constant current of 0.5 amps was passed between one electrode pair placed in a 15000 ppm sodium chloride solution measured using the potentiostat noted earlier.

[0047] The first pair of electrodes used were EDLC carbon aerogel-graphite composite electrodes prepared as described earlier with dimensions of 100 mm by 50 mm by 10 mm. The second curve represents the same for another pair of very similar electrodes that were prepared according to the teachings of this invention and after full kilning at a maximum temperature of about 1100 degrees C. Accordingly, the second set of electrodes was saturated with a 0.2 molar solution of silver nitrate by soaking in this solution for over 24 hours. Both these silver nitrate infused electrodes were then dried at 110 degrees C. One of these electrodes was then further heated to about 550 degrees C. for more than two hours to break down its infused silver nitrate into elemental silver and nitrogen dioxide and oxygen gases (which is assumed to have left the electrode). Both electrodes were then placed in 15000 ppm sodium chloride solution in a test cell. Placement of the silver infused electrode in 15000 ppm solution just infused it with sodium chloride while the same for the silver nitrate infused electrode resulted in the formation of silver chloride precipitate within the electrode according to the reaction noted in Equation 3. This second pair of electrodes were then subjected to a current of 0.5 Amps while measuring the developed voltage between them using the potentiostat identified earlier. The maximum voltage allowed to develop between the electrodes was limited to 0.8 volts above the base system voltage (the voltage developed at the instant the current was applied to the cell that indicates the required voltage to overcome the resistive component of the electric circuit used). This way the maximum voltage allowed to develop between the electrodes was below the minimum voltage required for water splitting. The test results clearly indicate a marked increase in electric capacitance of EDLC electrodes when manufactured and used in accordance with the teachings of this invention.

[0048] While the present invention has been illustrated by the description of the various embodiments and examples thereof, it is not intended to restrict or in any way limit the scope of the accompanying claims to such detail. Additional advantages and modifications will be readily apparent to those skilled in the art. Accordingly, departures may be made from such details without departing from the scope of the invention.