Ion exchange filters useful for electrochemical systems

Abstract

Provided herein are ion exchange filters comprising mixed beds of strong acid cation resins in H form, strong base anion resins in OH form and strong base anion resins in HCO3 form, in specific proportions. The mixed resin beds provide eluates having greater thermal stability and lower electrical conductivity compared to those processed through mixed beds comprising different resin components and proportions. Further provided herein is a method of purifying an aqueous fluid, for example a stream in a fuel cell, a battery, a battery charger or an electrolyzer, by contact with the mixed resin beds of the ion exchange filters described herein.

Description

[0001] Title of the Invention Ion exchange filters useful for electrochemical systems Cross-Reference to Related Application The present application claims priority under 35 U.S.C. § 365(c) to European Patent Application No.24305130, filed on January 23, 2024, which is incorporated herein by reference in its entirety. Field of the Invention The present invention relates to the field of ion exchange filters, in particular ion exchange filters having mixed resin beds, and to the use of these ion exchange filters to purify aqueous fluids, for example streams such as coolant streams in fuel cells, batteries or electrolyzers. Further provided are methods of using the ion exchange filters to purify the aqueous fluids. Background of the Invention Several patents, patent applications and publications are cited in this description in order to more fully describe the state of the art to which this invention pertains. The entire disclosure of each of these patents, patent applications, and publications is incorporated by reference herein. Electrochemical systems involve the conversion of chemical energy into electrical energy or vice versa through electrochemical reactions. Examples of electrochemical systems include batteries, battery chargers, fuel cells, and electrolyzers. These systems are widely used in various applications, such as in portable electronic devices, electric vehicles, and renewable energy technologies. Batteries convert chemical energy into electrical energy through a process of electrochemical reactions between two or more electrodes and an electrolyte. Inside the battery, chemical reactions occur to produce a flow of electrons from the negative electrode (anode) to the positive electrode (cathode). This flow of electrons creates a current that can be used to power an external device. The electrolyte acts as a medium for the transfer of ions between the anode and cathode. As the battery discharges, the chemical reactions that produce the current may begin to slow down, and the battery eventually runs out of charge. The battery can be recharged by a charger that applies an external electrical current that forces the electrochemical reactions to proceed in reverse. Coolants are needed in batteries and chargers to help manage their temperature during charging and discharging. Batteries and chargers can generate excessive heat during operation, particularly if they are being discharged or charged rapidly. This heat can lead to a degradation of the battery’s or charger’s performance and lifespan, and in extreme cases can even cause safety issues such as thermal runaway or explosions. Coolants are used to help dissipate heat from the battery or charger by absorbing and transferring it away from the internal elements where energy is dissipated as heat rather than electrical potential, for example, battery cells or plates. Thus, coolants help to maintain the battery’s or charger’s operating temperature within a safe range. In addition, coolants can also help to prevent freezing or boiling of the electrolyte in extreme temperature environments. More specifically, electric car chargers convert alternate current (AC) to direct current (DC), which is compatible with the electric car battery. This conversion generates heat. For example, a typical 22 kW AC charger provides enough charge in 120 minutes to provide an additional 200 km of vehicle range. To reduce the 200 km range charge time to only 16 minutes, a 150 kW DC charging station is required. At this level of power, and to avoid overheating, an effective and improved thermal management system is needed as the temperature of the battery pack may increase to more than 270°C during a 10-minute fast charge, according to a report from the U.S. Department of Energy. See, for example, Enabling Fast Charging: A Technology Gap Assessment, published by the Office of Energy Efficiency and Renewable Energy of the U.S. Department of Energy, found at https: / / www.energy.gov / sites / prod / files / 2017 / 10 / f38 / XFC%20Technology%20Gap%20Assessment%20Report_FINAL_10202017. pdf Traditionally, air cooling has been the preferred solution, but liquid cooling has been shown to be the most effective. The heat capacity of water, that is, the ratio of heat absorbed by the water to the resulting temperature change, is also 3,500 times greater than that of air. This is one reason why water is up to 10 times more effective at dissipating heat from the source. Furthermore, liquid cooling enables working with pre-filled systems, which will facilitate the maintenance, expedite any necessary replacements, enable quick initial installation, and expedite upgrades. Accordingly, liquid cooling can be found in many application areas, such as, without limitation, thermal battery packs, vehicle inverters, and charging station power electronics. The coolant used for liquid cooling of the fast charger is typically a water / glycol mixture. The use of water / glycol mixtures prevents the coolant from freezing and increases its boiling point, compared to water alone. It is apparent that technical malfunctions in the battery charger can create additional heat. For example, if the voltage is not stable then a charger that is programmed to maintain a constant charging rate will generate excess heat. Moreover, impurities will elute from plastic surfaces used in the cooling system as well as metals originating from metal parts such as coolers. The non-conductive coolant itself is subject to thermal degradation, releasing conductive components into the cooling loop. Glycolic and formic acids are typical examples formed in the degradation mechanism of ethylene glycol (“monoethylene glycol” or “MEG”), and lactic and acetic acids are formed in the degradation mechanisms of propylene glycol (“PG”). See, for example, AN INVESTIGATION OF THE DEGRADATION OF AQUEOUS ETHYLENE GLYCOL AND PROPYLENE GLYCOL SOLUTIONS USING ION CHROMATOGRAPHY Walter J. ROSSITER, Jr., McClure GODETTE, Paul W. BROWN and Kevin G. GALUK Building Materials Division, Center for Building Technology, National Bureau of Standards, Gaithersburg, MD 10899, USA; Received 17 May 1984; revised 30 August 1984, available online 5 March 2003; Solar Energy Materials, Volume 11, Issues 5–6, January–February 1985, Pages 455-467 accessible at https: / / doi.org / 10.1016 / 0165-1633(85)90016-4. Fuel cell systems perform electrochemical oxidation of a fuel e.g. hydrogen with oxygen resulting in electrical energy and water. The system can include several subsystems, for example: • Subsystem for managing the flow of hydrogen and oxygen: This may include gas compression and control systems to deliver the gases to the subsystem for electrical energy generation. In some cases, hydrogen is generated on site using gas reformers that extract hydrogen from hydrocarbons such as natural gas. Oxygen can be supplied directly from the air. • Subsystem for electrical energy generation: this is commonly in the form of a fuel cell stack which comprises multiple individual fuel cells. Each cell contains an anode and cathode separated by an electrolyte and a proton-selective membrane. When hydrogen gas contacts the anode, it splits into protons (H+ions) and electrons. The protons pass through the electrolyte and the proton selective membrane to the cathode, where they react with oxygen and electrons from the anode to generate electricity, heat and water. • Subsystem for managing the flow of water: water is generated as a product of electrochemical oxidation of hydrogen in a fuel cell. This water may be released as vapor from the fuel cell stack, or condensed and reused in the system for example as a coolant or humidifier to the incoming air. • Liquid coolant subsystem: This usually includes: o a means of exchanging heat via a liquid coolant: Fuel cells produce heat as they perform electrochemical oxidation. Heat typically improves the efficiency of the fuel cells, and this may be regarded as a beneficial way to operate. However, heat can degrade components such as the ion exchange resin, electrolysis membrane and other materials of construction. Heat management is crucial to ensure the fuel cell performance and lifetime. Thus, a cooling system is necessary to preserve and extend the operation of the electrochemical system. Cooling systems that are suitable for use with the ion exchange filters described herein are not particularly limited and typically comprise a coolant re-circulating within pipes or tubing between a remote heat exchanger and the fuel cell. The heat exchanger may be in fluid communication with one or more of a radiator, heat regulators, or by-pass means for selectively filtering circulating coolant. The coolants that are suitable for use with the ion exchange filters described herein are not particularly limited but typically comprises low conductivity fluids such as purified water or an aqueous alkylene glycol mixture. Various additives may be included in the coolant. Other designs and coolants are well known and may also be suitable, including those described in EP1791206, for example. o a means of treating said liquid coolant: during the circulation of the cooling liquid, ions from metal and organics from plastic surfaces may diffuse into the cooling liquid, or the glycol may thermally degrade into organic acids, such as glycolic acid or formic acid, for example, or both of these phenomena may occur. Each of these mechanisms result in a progressive increase of electrical conductivity in the coolant liquid. This electrical conductance imposes safety and reliability risks, and therefore the coolant must be treated to decrease or remove the impurities. This treatment is often performed using ion exchange resins. Electrolyzers perform electrochemical reduction of water to produce hydrogen and oxygen. The electrolysis system may include several subsystems, for example: • Subsystem for managing the flow of water: a system of pumps and water quality control may be necessary to provide a reliable supply of water to the electrolyzer. • Subsystem for electrical energy consumption: this is commonly in the form of an electrolysis cell which contains an anode and cathode separated by an electrolyte and an ion-selective membrane, such as a proton exchange membrane (PEM) for protons. In the case of a proton selective membrane, when water contacts the anode, it splits into hydrogen ions (H+) and oxygen gas. The hydrogen ions travel through the electrolyte and proton-selective membrane to the cathode, where they are converted to hydrogen gas. The oxygen gas is collected at the anode. • Subsystem for managing the flow of hydrogen and oxygen: This may include gas separation, purification, and compression as well as control systems to deliver the gases to their destinations. • Liquid coolant subsystem: This usually includes: o a means of exchanging heat via a liquid coolant: Heat is generated in an electrolyzer due to the resistance of the electrolyte solution to the passage of electricity. Heat typically improves the efficiency of the electrolysis process and this may be regarded as a beneficial way to operate. However, heat can degrade components such as the ion exchange resin, electrolysis membrane and other materials of construction. Thus, a cooling system is necessary to preserve and extend the operation of the electrochemical system. o Some heat is absorbed by the water circulating through the electrolyzer. Typically, not all the water fed to the electrolyzer is converted to hydrogen and oxygen. A portion of the unconverted water may be treated with ion exchange resins for the removal of contaminants. The converted water may be replenished by a makeup water stream. o Liquid or air cooling may be additionally employed. Cooling systems that are suitable for use with the ion exchange filters described herein are not particularly limited and typically comprise a coolant re-circulating within pipes or tubing between a remote heat exchanger and the electrolyzer. The heat exchanger may be in fluid communication with one or more of a plate heat exchanger, a heat regulator, or by-pass means for selectively filtering circulating coolant. The coolant system or loop, which is distinct from the loops that contain the electrolysis feed and product streams, is not particularly limited but typically comprises low conductivity fluids such as purified water. Various additives may be included in the coolant. Other designs and coolants are well known and may be used, including several that are similar to those described in EP1791206 with respect to fuel cells. Water is commonly used to transfer the heat from electrolyzers to one or more coolers. o Often further included is a means of treating the feed water recirculating through the electrolyzer or the coolant stream, which may become contaminated by ions from metal surfaces, organics from plastic surfaces, other chemicals, or microbes. This contamination can lower heat transfer efficiency, or cause fouling, scaling, corrosion, or safety hazards. This treatment is often performed using ion exchange resins. In all of these electrochemical systems, however, coolant that includes an impurity can cause a short circuit by conductance. The short circuit may merely cause an inefficiency, or it may cause a complete failure of the system. Dangerous electrical fires may also result from short circuits. Alternatively, the impurities can foul the anode, cathode, or membrane, leading to inefficient operation and shortening the lifetime of the system. It is therefore apparent that coolant purification is an important aspect of many electrochemical systems. One of the common methods to remove contaminants from coolant streams is an ion exchange treatment. Several publications describe the use of ion exchange resins to remove debris and contaminants from coolant streams. See, for example, US8808931; US7261816; US6673482; US6663993 and EP1791206. In particular, US8808931 describes the use of strong base anion (SBA) exchange resins in HCO3 form to treat a fuel cell coolant. Using the HCO3form anion resin increased the thermal stability of the coolant, compared to coolant treated with the typically used OH form anion resin. However, using the HCO3 form SBA in the fuel cell coolant also increased the conductivity of the coolant. As discussed above, increased conductivity is undesired as it can lead to short circuits, other unsafe circumstances, and fuel cell shut down. US11165074 describes the use of a combination of strong acid cation, strong basic anion and weak basic anion exchange resins to treat a coolant stream. It is postulated that weak base anion resins may have higher thermal stability than strongly basic anion resins. Nevertheless, there are limitations associated with weak base anion resins. In the coolant loops for fuel cells and batteries for electric vehicles (EV), for example, the main contaminant (a weakly associated organic acid, mainly glycolic acid) is related to the thermal degradation of glycol. A weak base anion resin containing secondary or tertiary amines would not be able to remove weakly acidic ions such as glycolic acid having a pKa of 3.83 from the water effectively using ion exchange at near neutral or alkaline pH. Specifically, the glycolic acid and its derivative impurities are not significantly present in the free acidic form as the pH exceeds their pKa value. At conditions of near-neutral pH, therefore, the conjugate bases of the glycolic acid and the derivative acids cannot protonate the secondary or tertiary amine to allow the adsorption of the glycolic acid and other acid impurities to the weak base resin. Thus, the glycolic acid and related impurities will not be removed from the coolant stream by a weak base anion resin when the coolant’s pH is near neutral or slightly alkaline. Consequently, there remains a need for ion exchange filters comprising resins that have improved thermal stability, that produce eluates having lower electrical conductivity, and that produce aqueous streams of superior purity for use in a variety of electrochemical systems. Summary of the Invention Accordingly, provided herein are electrochemical systems, such as fuel cells, batteries, battery chargers, and electrolyzers, comprising ion exchange filters having mixed beds of strong acid cation resins in H form together with strong base anion resins in OH form, strong base anion resins in HCO3 form, or a combination of both of these types of strong base anion resins, in specific proportions. Further provided are methods for treating coolant water streams, for example coolant water streams from electrochemical systems, using the ion exchange filters described herein. Brief Description of the Drawings Fig.1 is a graph of conductivity vs. total amount of hydroxy glycolic acid (HGA) removed from 0.1M aqueous solution, showing conductivity trends in the product stream from various mixed beds used to treat a contaminated coolant stream. Fig.2 is a graph of tolytriazole retention vs. time for various mixed beds. Detailed Description of the Invention Mixed bed resin cartridges are commonly used for the treatment of coolant streams. The mixed beds known in the art generally include a cation resin in H form and an anion resin in OH form. The resin bead size is not particularly limited and can be selected based upon the operating conditions. Bead diameters of preferred anion exchange resins are from about 300 to 1000 microns. Uniform particle size (UPS) resins may be used, or resins having mixtures of beads sizes may be used, for example those having gaussian particle size distributions. The high temperature in the cooling circuits of electrochemical systems (approximately 60 to 1050C) will cause thermal degradation of the anion functionality in the OH-form resin, thus shortening the resin lifetime. To minimize the effects of the thermal degradation process, an HCO3 form anion resin may be used. When the mixed bed with the strong acid cation in H form and strong base anion in HCO3form is used in the coolant loop, however, the HCO3 equilibrium causes the conductivity baseline of the coolant to shift from < 0.1 uS / cm to the range 1.0 to 5 uS / cm or 0.3 – 5 uS / cm, which is undesirable. Conductivity is measured with an inoLab™ Cond 7310P conductivity meter equipped with conductivity probe WTWLR325 / 01 and having a cell constant of 0.1 cm-1. It has now surprisingly been found that an ion exchange filter comprising a mixed bed of ion exchange resins addresses the thermal stability and conductivity concerns, while maintaining the high standard of water resistivity required for use in electrochemical systems. In general, water having a resistivity greater than or equal to 12, 15, 17, or 19 MOhm-cm is considered to meet this standard. Resistivity is the numerical inverse of conductivity; thus, it is determined by the same experimental method as conductivity. The mixed bed described herein comprises a strong acid cation resin in H form together with a strong base anion resin in OH form, a strong base anion resin in HCO3 form, or a combination of both of these types of strong base anion resins, in specific proportions. Suitable strong acid and strong base ion exchange resins for use in the present invention and methods of synthesizing the resins are described in detail in U.S. Pat. Nos.8,808,931, issued to Golz et al.; and 6,784,213, issued to Rohrbach et al.; and references cited in these patents. In addition, suitable acid and base resins are commercially available, for example from DuPont de Nemours, Inc., of Wilmington, DE (hereinafter “DuPont”). Moreover, those of skill in the art are familiar with the definitions of various terms used herein to describe ion exchange resins and uses for the resins, for example “make-up water”, “Type I resins,” “Type II resins,” “gel resins,” and the like. These terms are also defined and used in Product Data Sheets and other literature that is made publicly available by manufacturers of ion exchange resins. See, for example, https: / / www.dupont.com / water / technologies / ion-exchange-ix.html. Preferably, the strong acid ion exchange resins used herein are suitable for use in one or more of industrial utility water make-up applications, condensate polishing applications, and in 18 MOhm-cm water production for the semiconductor industry. Also preferably, the strong acid cation resin is characterized by one or more of the following properties: it is a gel-type resin; it is a cross-linked copolymer of styrene and divinyl benzene; it is a sulfonated polymer; it is in protonated (H) form; it may have any particle size distribution, for example a uniform particle size or a gaussian particle size distribution, or it may be sieved to achieve a different type of particle size distribution; and it has a total ion-exchange capacity of about 1.0 to about 2.65, more preferably about 1.5 to about 2.65, still more preferably about 1.8 to about 2.3 equivalents per liter of resin (eq / L) in H form. The term “total ion exchange capacity” is synonymous and used interchangeably herein with the terms “total capacity of cation exchange resins”, “total capacity of anion exchange resins”, and “total and salt-splitting capacities of anion exchange resins”, depending on the cationic or anionic nature of the resin under discussion. The total ion exchange capacity may be measured according to one or more of the methods set forth in ASTM Standard No. D2187-17 (Standard Test Methods and Practices for Evaluating Physical and Chemical Properties of Particulate Ion-Exchange Resins), or by another suitable method. Preferably, the strong base ion exchange resins used herein are also suitable for use in one or more of industrial utility water make-up applications, condensate polishing applications, or 18 MOhm-cm water production for the semiconductor industry. Also preferably, the strong base anion resin is characterized by one or more of the following properties: it is a Type I resin; it is a gel-type resin; it is a cross-linked copolymer of styrene and divinyl benzene; it is in OH or HCO3 form; it has a uniform particle size or a gaussian particle size distribution; and it has a total ion-exchange capacity of about 0.9 to about 1.8, more preferably about 0.9 to about 1.6, still more preferably about 1.0 to about 1.4 eq / L in OH form. The mixed beds described herein include a strong acid cation resin (SAC) in H form and a strong base anion (SBA) in OH form or in OH and HCO3forms. Specifically, when the electrochemical system is a fuel cell, a battery, or a battery charger, the mixed bed comprises: a) a strong acid cation resin in H form; b) a strong base anion resin in OH form; and c) a strong base anion resin in HCO3 form. When the electrochemical system is an electrolyzer, however, the mixed bed comprises: a) a strong acid cation resin in H form; b) a strong base anion resin in OH form; and optionally c) a strong base anion resin in HCO3form. The mixed bed is preferably anion dominated. Stated alternatively, in a given volume of the mixed bed, the sum of the volumes of the strong base anion resins in OH and HCO3form is preferably greater than or equal to the volume of the strong acid cation resin in H form. Preferably, the volume of the cation resin is greater than 7.5 vol% and less than 25.0 vol%, greater than 10.3 vol% and less than 25.0 vol%, greater than 15 vol% and less than 20 vol%, based on the total volume of the mixed bed. Complementarily, the volume of the anion resin(s) is more preferably greater than 75.0 vol% and less than 92.5 vol%, greater than 75.0 vol% and less than 89.7 vol%, or greater than 80 vol% and less than 85 vol%, based on the total volume of the mixed bed. The sum of the volume percentages of the resins in the mixed bed is 100 vol%. The term “complementary”, as used herein alone or in derived form, such as “complementarily”, refers to percentages having a sum of 100%, such as, for example, 10.3% and 89.7%. Also preferably, in a given volume of the anion dominated mixed bed, for each equivalent of total ion exchange capacity of the strong acid cation resin in that mixture, strong base anion resins have a total ion exchange capacity that is greater than one equivalent. For example: in a 100 ml mixed bed comprising 75 ml of an anion resin having an exchange capacity of 1.0 equiv / L and 25 ml of a cation resin having an exchange capacity of 2.0 equiv / L, the volume of the mixture comprises 75x1 / 25 x 2 = 1.5 equivalent of anion capacity per equivalent of cation capacity. More preferably, in a given volume of the anion-dominated mixed bed, both of the following conditions are met: the sum of the volume percentage(s) of the strong base anion exchange resin(s) is greater than the volume percentage of the strong acid cation exchange resin, based on the total volume of the mixed bed; and, the total ion exchange capacity(ies) of the strong base anion exchange resin(s) is greater than the total ion exchange capacity of the strong acid cation exchange resin. Still more preferably, in a given volume of the anion dominated mixed bed, the number of moles of anion exchange sites exceeds the number of moles of cation exchange sites. Suitable designs for ion exchange cartridges used to purify a fuel cell coolant stream are described in U.S. Pat. No.8,808,931, for example. Similar design principles can be used to adapt this system for use to purify the coolant stream of other electrochemical systems, for example batteries, battery chargers, and electrolysis devices. Suitable coolants for use in the electrochemical systems described herein are well known in the art. Briefly, however, coolants generally comprise water, one or more glycols, or a combination of water and one or more glycols. Suitable glycols include alkylene glycols. The alkylene groups may be linear or branched. Preferred alkylene groups include 2 to 10 carbon atoms, more preferably 2 to 4, still more preferably 2 to 3 carbon atoms. In addition, suitable coolants may include a number of additives or stabilizers that reduce or prevent thermal or oxidative degradation of the glycols. In general, additive packages are held as the trade secrets of the coolant manufacturers. Nevertheless, the coolant may comprise one or more of a thermal stabilizer and a corrosion inhibitor, for example. The additive packages are typically divided into three groups, IAT (inorganic additive technology), OAT (organic additive technology), and HOAT (hybrid OAT). The OAT and HOAT types have the lowest conductivity and are mostly used for fuel cells and electric vehicles. A pure glycol itself is colorless, but for the purpose of recognizing the liquid and for leak detection, it may be colored with a dye such as a rhodamine. Thus, the additive package may further include a dye. Other suitable additives include thermal stabilizers, which are often described as silica based, and antifoam additives, which are often described as silicones or poly glycols. Suitable corrosion inhibitors, which prevent oxidative degradation, include azoles, carboxylates and triazoles. Suitable additives and stabilizers and suitable amounts of these additives and stabilizers are known in the art. See, for example, the Kirk-Othmer Encyclopedia of Chemical Technology and PCT Intl. Appln. Publn. No. WO2017080542. Briefly, however, the total amount of additives and stabilizers in the coolant is preferably less than 5 wt%, less than 3 wt%, less than 2 wt%, or less than 1 wt%, based on the total weight of the coolant. When the additives such as corrosion inhibitors and thermal stabilizers remain in the coolant stream, the glycol-containing coolant will last longer and will perform better. When a glycol is exposed to high temperatures, however, it will degrade. The degradation products include low molecular weight organic acids, which can cause corrosion of the metal surface present in the coolant loop, for example aluminium-copper coolers. The dissolved or suspended metals that are the products of corrosion catalyze the degradation of the glycol(s) in the coolant, thus increasing the thermal degradation rate of the glycol(s) and the formation of conductive organic acids that enhance the corrosion. This negative feedback loop may be prevented or decelerated by preserving an optimal level of stabilizers in the coolant. For this reason, stabilized coolants are commercially available, such as for example Glysantin™ FC G 20-00 / 50 inhibited, ready-to-use, low-conductivity glycol coolant by BASF SE of Ludwigshafen, Germany (hereinafter “BASF”). For a demonstration of the effect of stabilizers, see, for example, Technische Information TI / EVO e for Glysantin™ FC G 20-00 / 50, published by BASF in January, 2016. Current state of the art ion exchange resins in H / OH form effectively remove these additives from the glycol, however. This phenomenon has been described in Rossiter et al., cited above, for example. In contrast, the mixed bed ion exchange resins described herein surprisingly and advantageously do not remove or remove lower amounts of additives such as corrosion inhibitors and stabilizers from the coolant stream, thus extending the useful life of the coolant and the coolant loop. Three preferred embodiments of the mixed bed ion exchange filter provided herein are described below. Embodiment 1: the mixed bed comprises the entirety of the bed of ion exchange resins. In this embodiment: • The mixed bed is preferably anion dominated. This can be described thus: the sum of the volume of the strong base anion resins in OH form and HCO3 form is equal to or greater than the volume of the strong acid cation resin in H form. • For the anion exchange resins, the proportion of the OH form should be in a range of 33 to 75 vol% of the total volume of anion resins in the mixed bed. This can be described further as: the volume of the strong base anion resin in OH form is greater than 33 % and less than 75% of the sum of the volumes of the strong base anion resin in OH form and the strong base anion resin in HCO3 form. Embodiment 2: the bed of ion exchange resins contains a mixed bed section and an unmixed bed section, where the unmixed bed section is above the mixed bed section. As used herein, the term “above” refers to an upstream position, and the term “below” refers to a downstream position. The ion exchange filter need not be placed vertically for these terms to apply. For example, in this Embodiment 2, the coolant water that is flowed through the ion exchange filter passes first through the unmixed bed section, which is above the mixed bed section. Complementarily, the mixed bed section is below the unmixed bed section, that is, water that passes through the mixed bed section has already passed through the unmixed bed section. In this Embodiment 2: • The mixed bed section is preferably anion dominated. This can be described thus: the sum of the volumes of the strong base anion resins in OH form and HCO3form is equal to or greater than the volume of the strong acid cation resin in H form. • The unmixed bed section is preferably comprised of one or more strong base anion resins in OH form. This resin may be the same as or different from the strong base anion resin in OH form in the mixed bed. • For the anion exchange resins in the mixed bed section, the proportion of the OH form should be in a range of 33 to 75 vol%. This can be described further as: the volume of the strong base anion resin in OH form is greater than 33 % and less than 75% of the sum of the volumes of the strong base anion resin in OH form and the strong base anion resin in HCO3 form. • The volume of the strong base anion resin in OH form in the mixed bed section is greater than 40% of the sum of the volumes of the strong base anion resins in OH form in the unmixed bed section and in the mixed bed section. Embodiment 3: the bed of ion exchange resins contains a mixed bed section and an unmixed bed section, where the unmixed bed section is below the mixed bed section. In this embodiment: • The mixed bed section is preferably anion dominated. This can be described thus: For a given volume of the mixed bed resin, the sum of the volumes of the strong base anion resins in OH and HCO3form is greater than or equal to the volume the strong acid cation resin in H form. • The unmixed bed section is preferably comprised of one or more strong base anion resins in OH form. This resin may be the same as or different from the strong base anion resin in OH form in the mixed bed. • For the anion exchange resins in the mixed bed section, the proportion of the OH form should be in a range of 33 to 75 vol%. This can be described further as: the volume of the strong base anion resin in OH form is greater than 33 % and less than 75% of the sum of the volumes of the strong base anion resin in OH form and the strong base anion resin in HCO3form. • The volume of the strong base anion resin in OH form in the mixed bed section is greater than 60% of the sum of the volumes of the strong base anion resins in OH form in the unmixed bed section and in the mixed bed section. The following examples are provided to describe the invention in further detail. These examples, which set forth specific embodiments and a preferred mode presently contemplated for carrying out the invention, are intended to illustrate and not to limit the invention. EXAMPLES In the following Experiment Sets, the strong acid cation exchange resin was a sulfonated, divinylbenzene-crosslinked polystyrene gel polymer having a total ion exchange capacity of equal to or greater than 2.05 equivalents per liter in H form. The strong base anion exchange resins were Type 1, divinylbenzene-crosslinked polystyrene gel polymers having a total ion exchange capacity equal to or greater than 1.10 equivalents per liter in OH form. These ion exchange resins are commercially available, for example from DuPont, and represent a wide range of commercially available anion and cation resins characterized by different capacities but similar chemistry composition. The ultrapure water used in these Experiment Sets has a resistivity of 17 MOhm-cm or greater. Experiment Set 1: Contaminant removal using a fully mixed bed: strong acid cation (SAC) (H form), strong base anion (SBA) (OH and HCO3forms) • Experiments were performed in a glass column with 2 cm inner diameter, containing a total resin volume of 50 ml. A 380 ppm solution of mono glycolic acid (MGA; a known contaminant in fuel cell coolant cycle loops) in ultrapure water was prepared. The proportions of the resins in the mixed bed were varied as described in Table 1. The mixed bed was rinsed with 5 bed volumes (BV; here, 5•50ml = 250 ml) of ultrapure water before use. The MGA solution was injected at a flowrate of 1000 mL / hr (corresponding to 20 BV per hour) at the top of the mixed bed. The injection continued until the mixed bed resin was exhausted. Exhaustion, which was defined by the eluate having a conductivity at least 5 µS / cm, was reached after approximately 150 Bed Volumes were passed through the resin. The first 3 bed volumes of eluate were discarded and are not included in this measurement. The results of the experiments are shown in Table 1. The Average WeightedConductivityof the eluateis calculatedasthe sum of conductivity measuredfor each BV divided by the total number of BV passed through the mixed bed. The final value of the Average Weighted Conductivity is reported whenexhaustion is reached (that is, when the conductivity of the eluate is5 µS / cm).For example, when 0 - 10 BV at 1 uS = 10uS and 10 - 20 BV at 2 uS = 20uS, the weighted average at 20BV equals (10+20uS) / 20 BV = 1.5 uS. The conductivity of the 10th BV of eluate was considered to be the starting conductivity. It is shown that, when the amount of OH form resin in the Mixed Bed wasbetween 33 and 75 vol%, theAverage Weighted Conductivity remained low.Increasing the vol% OH form in the Mixed Bed above 75 vol% did not further decrease the Average Weighted Conductivity; however, these compositions are expected to decrease the thermal stability of the mixed bed.

[0002] Table 1: Resin combinations used to treat MGA stream (fully mixed bed) Test # 1 2 3 4 5 6 7 8 9 10 11 SAC H form volume (ml) 7.6 7.6 7.6 7.6 7.6 7.6 7.6 7.6 7.6 7.6 7.6 1.2 28.8 31.8 36.04 42.4 1.2 13.6 10.6 6.36 0 50 50 50 50 50 100 50 68 75 85 (= 0% HCO3) l Testing) .33 0.33 0.49 0.27 0.25 .87 1.58 1.39 1.39 1.73 Experiment Set 2: Thermal stability of resins in the mixed bed: SAC (H form), SBA (OH and HCO3forms) The resin capacity loss due to thermal aging is evaluated by calculating the difference between the total ion exchange capacity of a thermally aged ion exchange resin and the total ion exchange capacity of the same ion exchange resin prior to thermally aging and reporting that difference as a percentage of the original resin total exchange capacity. To determine the resin capacity loss due to thermal aging, 200 mL of mixed resin beds, with different amounts of OH and HCO3resins, were put in a closed glass bottle and placed in a furnace at 900C for 500 hours. The thermally-aged resins were regenerated, then tested to evaluate their ion exchange capacity. To regenerate the OH and HCO3resins to OH form, 10 ml of the OH form resin was stirred with 20 ml of HCl 1N for 10 minutes to react the present OH sites with HCl. After the 10 minutes of contact time, the remaining supernatant HCl solution was dosed with NaOH 1N solution (back titration) to a pH of 7.0. Strong acid cation resins are regenerated with HCl 1N to be fully restored to H form, then rinsed with deionized water. H sites are then released with addition of 5g of NaCl before back titration with NaOH 1N solution to a pH of 7.0. The capacity of the mixed resins prior to thermal aging was calculated as the average of the initial capacity of each individual resin in the mixture, weighted by its volume percentage. For example, the initial capacity of the mixed resin of Test 1 is (55 • (initial capacity OH form) + 45 • (initial capacity HCO3form)) / 100, wherein the sum of the volume percentages of the OH and HCO3 resins is 100 vol%. The initial thermal capacities were also determined by the methods described above. Specifically, the resins were not heated; however, they were regenerated, and then their ion exchange capacity as equivalents per liter of OH functional groups were measured. The results are shown in Table 2. The resin capacity loss due to thermal aging was greater at 90°C when the Volume % of OH form exceeded 75 vol%. Table 2: Thermal stability testing results Experiment Set 3: Contaminant removal using a partially mixed bed with unmixed OH resins on top: SAC (H form), SBA (OH and HCO3 forms) • Experiments were performed in a glass column with 2 cm inner diameter, containing a total resin volume of 50 ml. A 380 ppm solution of hydroxy glycolic acid (HGA, a known contaminant in fuel cell coolant cycle loops) in Glysantin™ FC G20 coolant, available from BASF and believed to be a mixture comprising 50% ethylene glycol and 50% DI water, was prepared. The proportions of the resins in the mixed bed were varied as described in Table 3. The partially mixed bed was rinsed with 5 BV ultrapure water before use. The HGA solution was injected at a flowrate of 1000 mL / hr (corresponding to 20 Bed Volumes per hour) at the top of the partially mixed bed. The injection continued until the mixed bed resin was exhausted, as determined by the methods described above in Experiment Set 1. The results are shown in Table 3. Performance was measured as the number of bed volumes eluted before the5µS / cm endpoint indicating that exhaustion was reached. The results indicate that at most 60 vol% of the OH form can remain in the unmixed bed (or at least 40 vol% of the OH form must be present in the mixed bed) before a drop in performance was observed as a decrease in the eluant volume that is processed through the column before exhaustion. Table 3: Resin combinations used to treat HGA stream (partially mixed bed below unmixed OH resins) Test # 1 2 3 4 5 6 7 8 Total resin volume(mL) 507.5 15 42.585 29.25 69 13.2531 70 80 90 10020.48 23.40 26.33 29.2513.25 13.25 13.25 13.258.78 5.85 2.93 0 137 126 117 109 Experiment Set 4: Contaminant removal using a partially mixed bed with unmixed OH resins at the bottom : SAC (H form), SBA (OH and HCO3 forms) • Experiments were performed in a glass column with 2 cm inner diameter, containing a total resin volume of 50 ml. The HGA solution described in Experiment Set 3 was prepared. The proportions of the resins in the mixed bed were varied as described in Table 4. The partially mixed bed was rinsed with 5BV of ultrapure water before use. The HGA solution was injected at a flowrate of 1000 mL / hr (corresponding to 20 Bed Volumes per hour) at the top of the partially mixed bed. The injection continued until the mixed bed resin was exhausted, as determined by the methods set forth in Experiment Set 1, above. The results are shown in Table 4. Performance is measured as the number of bed volumes treated before the5µS / cm, indicating exhaustion was reached. The results indicate that at most 40 vol% of the OH form can remain in the unmixed bed in this configuration (or at least 60 vol% of the OH form must be present in the mixed bed) before a drop in performance is observed as a decrease in the eluant volume that is processed through the column before exhaustion.In summary, these data indicate that thereis an optimum range of the relativevolumes of the HCO3 and OH form anion resins that combines high thermal stability and good performance (as measured by the Resin Capacity Loss due to Thermal Aging and the Average Weighted Conductivity). There is also an optimal volume percentage range of mixing for OH resins to maintain good performance (as measured by removing a greater amount of impuritybefore exhaustion), which surprisingly depends onwhether the unmixed bedof OH form resin is above or below the mixed bed. Table 4: Resin combinations used to treat HGA stream (partially mixed bed above unmixed OH resins) Test # 1 2 3 4 5 Total resin volume (mL) 50 IRN99H C ti R i V l L 7.5 15 42.5 85 29.2569 13.25 31 040 60 100.8 11.7 17.5 29.25 .25 13.25 13.25 13.25 .45 17.55 11.75 040 138 122 123 Experiment Set 5: Contaminant removal using a mixed bed: strong acid cation (SAC) in H form, weak base anion (WBA) in free base form • This experiment set was performed to provide a comparative example. Experiments were performed in a glass column with 2 cm inner diameter, containing a total resin volume of 400 ml. The HGA solution described in Experiment Sets 3 and 4 was prepared. The proportion of the resins in the mixed bed was 50% SAC, 50% WBA by volume. The mixed bed was rinsed with 5 BV of ultrapure water before use. The HGA solution was injected at a flowrate of 600 Bed Volumes per hour at the top of the mixed bed. The injection continued until the mixed bed resin was exhausted, as described in Experiment Set 1, above. The results are shown in Figure 1, which depicts the conductivity profile as function of the amount of HGA (in meq or mmol) removed from the solution (i.e., taken up or “loaded”) per liter of resin. The data in Figure 1 demonstrate that the mixed bed containing the WBA shows an immediate rise in conductivity compared to the mixed bed containing a 50V% SAC resins mixed with 50V% SBA resin in either OH or HCO3 form. In contrast, the conductivity of the stream exiting the mixed bed that contains OH form SBA begins rising when the total capacity loaded exceeds about 700 meq / L of HGA loading, and the conductivity of the stream exiting the mixed bed that contains HCO3 form SBA begins rising when the total capacity loaded exceeds about 800 meq / L of loading. These results demonstrate the limited benefit of using a WBA resin to treat a coolant stream contaminated with a common product of glycol degradation. Advantageously, the mixed resin beds described herein do not remove additives such as corrosion inhibitors and stabilizers from the coolant stream. In contrast, a strong acid cation / strong base anion (H / OH) mixed bed of the prior art removes the additives entirely or substantially entirely. In particular, the results depicted in Figure 2 demonstrate that a mixed bed of ion exchange resins as described herein (SAC resin in H form, SBA resin in HCO3form) does not fully remove stabilizers or reduce their concentration below operative levels after 7h or more of operation under the following conditions: • resin volume = 50mL mixed bed • test hydraulics = in loop circulation of 1L of aqueous Tolytriazole solution (concentration 1.5g / L) at a rate of 20BV / h (feed under agitation), hourly sampling for HPLC analysis • Coolant samples are analyzed by HPLC, using a WATERS™ e2695 separation module equipped with an XSelect HSS T35µm column (available from Waters Corp of Milton, MA) and operated at 25°C. The injection size was 10 ul, and UV detection was at 210 nm. The mobile phase was disodium hydrogen phosphate (Na2HPO4, 3.4g / L at pH 2.7, in a solution of 20% of acetonitrile by weight or volume, remainder water or DI water). In contrast, under the same conditions, the mixed bed of the prior art (SAC resin in H form, SBA resin in OH form) removes substantially all of the tolytriazole from the circulating solution after one hour. While certain of the preferred embodiments of this invention have been described and specifically exemplified above, it is not intended that the invention be limited to such embodiments. Various modifications may be made without departing from the scope and spirit of the invention, as set forth in the following claims.

Claims

CLAIMS 1. An apparatus for treating a liquid stream in an electrochemical system, wherein said electrochemical system is a fuel cell or a battery or a battery charger, said apparatus comprising a mixed bed of ion exchange resins, and said ion exchange resins comprising: a) a strong acid cation resin in H form; b) a strong base anion resin in OH form; and c) a strong base anion resin in HCO3 form; wherein, for a given volume of the mixed bed resin, the sum of the volumes of the strong base anion resins in OH and HCO3form is greater than the volume of the strong acid cation resin in H form; wherein the total ion exchange capacities of the strong base anion resins in OH and HCO3form are from 0.9 to 1.8 equiv / L, and the total ion exchange capacity of the strong acid cation resin in H form is from 1.0 to 2.65 equiv / L; and wherein said apparatus optionally further comprises an unmixed bed of ion exchange resin, said unmixed bed comprising a strong base anion exchange resin in OH form.

2. The apparatus of Claim 1, wherein the mixed bed, the unmixed bed, or both the unmixed bed and the mixed bed are characterized by one or more conditions selected from the group consisting of: a) in the mixed bed the volume of the strong base anion resin in OH form is greater than 33 % of the sum of the volume of the strong base anion resin in OH form and the volume of the strong base anion resin in HCO3 form; b) the volume of the strong base anion resin in OH form is less than 75 % of the sum of the volume of the strong base anion resin in OH form and the volume of the strong base anion resin in HCO3form; c) the combined volume of the strong base anion resins in OH form and HCO3 form in the mixed bed and in the unmixed bed is equal to or greater than the volume of the strong acid cation resin in H form;d) the volume of the strong base anion resin in OH form in the mixed bed and in the unmixed bed is greater than 33 % of the sum of the volume of the strong base anion resin in OH form and the volume of the strong base anion resin in HCO3 form in the mixed bed and in the unmixed bed; e) the sum of the volumes of the strong base anion resin in OH form in the mixed bed and in the unmixed bed is less than 75 % of the combined volumes of the strong base anion resin in OH form and the strong base anion resin in HCO3 form in the mixed bed and in the unmixed bed; f) the volume of the strong base anion resin in OH form in the mixed bed section is greater than 40% of the sum of the volume of the strong base anion resin in OH form in the unmixed bed section and in the mixed bed section; and g) the volume of the strong base anion resin in OH form in the mixed bed section is greater than 60% of the sum of the volumes of the strong base anion resin in OH form in the unmixed bed section and in the mixed bed section.

3. The apparatus according to claim 1 or claim 2, wherein said liquid stream comprises a mixture of glycol and water; wherein said glycol comprises ethylene glycol, propylene glycol, or a combination of ethylene glycol and propylene glycol; and optionally wherein said liquid stream further comprises one or more additives selected from the group consisting of thermal stabilizers, antifoam agents, and corrosion inhibitors.

4. The apparatus according to any preceding claim that comprises a coolant loop, and wherein said liquid stream is a coolant in the coolant loop.

5. The apparatus of any preceding claim wherein the unmixed bed section is above the mixed bed section.

6. The apparatus of any of claims 1 through 4 wherein the unmixed bed section is below the mixed bed section.

7. An apparatus for treating a liquid stream in an electrolyzer, said apparatus comprising a mixed bed of ion exchange resins, said mixed bed comprising: a strong acid cation resin in H form; and a strong base anion resin in OH form; and optionally a strong base anion resin in HCO3form; and wherein, for a given volume of the mixed bed resin, the volume of the strong base anion in OH form or the sum of the volumes of the strong base anion resins in OH and HCO3form is greater than the volume of the strong acid cation resin in H form; wherein the total ion exchange capacities of the strong base anion resin in OH form and of the optional strong base anion resin HCO3 form are from 0.9 to 1.8 equiv / L, and wherein the total ion exchange capacity of the strong acid cation resin in H form is from 1.0 to 2.65 equiv / L; and wherein said apparatus optionally further comprises an unmixed bed of ion exchange resin, said unmixed bed comprising a strong base anion exchange resin in OH form.

8. The apparatus of claim 7, wherein said liquid stream comprises water and at least one dissolved impurity selected from the group consisting of an anion rich mixture of materials classified as total organic carbon; one or more organic acids; and one or more ions selected from the group consisting of boron, silica, manganese, nickel, zinc, sodium, potassium, fluoride, chloride, bromide, nitrate, phosphate, and sulfate ions.

9. The apparatus of claim 7 or claim 8 that comprises a coolant loop, wherein said liquid stream is a coolant in the coolant loop.

10. The apparatus of claim 7, claim 8, or claim 9, wherein the volume of the strong base anion resin in OH form is equal to or greater than 3 times or preferably equal to or greater than 6 times the volume of the strong acid cation resin in H form.

11. A process for treating a liquid stream in an electrochemical system, wherein said electrochemical system is a fuel cell or battery, said process comprising the steps of: providing the apparatus of any of claims 1 through 6; and contacting the liquid stream with the mixed bed of ion exchange resins.

12. A process for treating a liquid stream in an electrolyzer, said method comprising the steps of: providing the apparatus of any of claims 7, 8, 9, or 10; and contacting the liquid stream with the mixed bed of ion exchange resins.

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