Abstract:
A capacitive deionization cell comprising a first electrode, an anion membrane adjacent said first electrode forming an anion membrane-electrode conjugate, a cation membrane in spacial relation with said anion membrane defining a fluid flow path between said anion membrane and said cation membrane, and a second electrode adjacent said cation membrane forming a cation membrane-electrode conjugate, wherein said anion membrane-electrode conjugate and said cation membrane-electrode conjugate each have an effective capacitance, and the ratio of the effective capacitance of said cation membrane-electrode conjugate to the effective capacitance of said anion membrane-electrode conjugate is a non-zero number less than 2.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of commonly owned and co-pending U.S. Provisional Application No. 61/096,899 filed on Sep. 15, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Capacitive deionization (CDI) cells are known for purifying or otherwise deionizing liquids such as water. For example, U.S. Pat. No. 5,954,937 discloses an electrically regeneratable electrochemical cell for capacitive deionization and electrochemical purification and regeneration of electrodes including two end plates, one at each end of the cell. Two end electrodes are arranged one at each end of the cell, adjacent to the end plates. An insulator layer is interposed between each end plate and the adjacent end electrode. Each end electrode includes a single sheet of conductive material having a high specific surface area and sorption capacity. In one embodiment of this disclosure, the sheet of conductive material is formed of carbon aerogel composite. The cell further includes a plurality of generally identical double-sided intermediate electrodes that are equidistally separated from each other, between the two end electrodes. As the electrolyte enters the cell, it flows through a continuous open serpentine channel defined by the electrodes, substantially parallel to the surfaces of the electrodes. By polarizing the cell, ions are removed from the electrolyte and are held in the electric double layers formed at the carbon aerogel surfaces of the electrodes. As the cell is saturated with the removed ions, the cell is regenerated electrically, thus minimizing secondary wastes. 
         [0003]    U.S. Pat. No. 6,709,560 discloses flow-through capacitors that are provided with one or more charge barrier layers. Ions trapped in the pore volume of flow-through capacitors cause inefficiencies as these ions are expelled during the charge cycle into the purification path. A charge barrier layer holds these pore volume ions to one side of a desired flow stream, thereby increasing the efficiency with which the flow-through capacitor purifies or concentrates ions. 
         [0004]    U.S. Pat. No. 6,798,639 discloses flow through capacitors in which, in general, asymmetric flowthrough capacitors are formed, increasing overall capacitance. The asymmetry is accomplished by utilizing electrodes of different materials, different dimensions, or the same materials with different capacitance properties. Referring to  FIG. 2  of the &#39;639 patent, a schematic of an asymmetrical flow-through capacitor is depicted. The flow-through capacitor  200  includes a positive electrode  210  having a capacitance value C+′ and a negative electrode  220  having a capacitance value C− (i.e., the same capacities as electrode  120 ). If the value of C+ is sufficiently increased, the expression 1/C+ in equation 1 (of the patent) approaches zero, and thus the overall capacitance C.sub.T′ can be expressed in Equation 2 (of the patent). The capacitance value C.sub.T may be sufficiently increased by using an asymmetrical configuration, wherein one electrode has a much higher capacitance than the other. Thus, as can be seen from Equation 2 (of the patent), the overall capacitance C.sub.T′ is approximately double that of the symmetrical cell with the same capacitance values for the negative electrode. Specifically, the reference states that the relative values of the negative and positive electrodes may be such that the capacitance of one is at least double that of the other. The term “substantially greater” as used in that patent is defined, therefore, as being at least double the value. That is, the reference teaches that to maximize capacitance, one electrode should have a capacitance at least twice that of the other, and that either electrode can be the larger one. 
         [0005]    These references all produce useful CDI cells, but a CDI cell that performs better is still needed. The desirable CDI cell has a large capacitance to remove ions from a liquid stream, and is durable enough to be able to run over a longer period of time. 
         [0006]    As used herein, “effective capacitance” means dQ/dV for a membrane-electrode conjugate as determined by current interrupt as described herein. 
         [0007]    Also as used herein, “durability” means hours until ion removal is less than 60% (under test conditions specified herein). 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a capacitive deionization cell comprising a first electrode, an anion membrane adjacent said first electrode forming an anion membrane-electrode conjugate, a cation membrane in spacial relation with said anion membrane defining a fluid flow path between said anion membrane and said cation membrane, and a second electrode adjacent said cation membrane forming a cation membrane-electrode conjugate, wherein said anion membrane-electrode conjugate and said cation membrane-electrode conjugate each have an effective capacitance, and the ratio of the effective capacitance of said cation membrane-electrode conjugate to the effective capacitance of said anion membrane-electrode conjugate is a non-zero number less than 2, preferably about 1. The first electrode and the second electrode each have a thickness, and a ratio of the thickness of said first electrode to the thickness of said second electrode is greater than 1, preferably greater than 2, 3, 4, and 5, respectively. The first electrode and second electrode may comprise different materials. A porous spacer is preferably disposed in said fluid flow path. 
         [0009]    In an alternative embodiment, the comprising a plurality of said anion membrane-electrode conjugates and said cation membrane-electrode conjugates, and wherein said ratio is the ratio of the effective capacitance of all of said cation membrane-electrode conjugates to the effective capacitance of all of said anion membrane-electrode conjugates. Preferably, the capacitive deionization cell as defined in claim  1  wherein the cell has a durability of greater than 200 hours. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an exploded view of an exemplary embodiment of the invention. 
           [0011]      FIG. 2   a  is a cross sectional view of an assembled CDI cell according to an exemplary embodiment of the invention before compression. 
           [0012]      FIG. 2   b  is a cross sectional view of an assembled CDI cell according to an exemplary embodiment of the invention after compression. 
           [0013]      FIG. 3  is a schematic of the test apparatus used for CDI testing. 
           [0014]      FIG. 4  is a graph of an Example test cycle illustrating TDS variation during the cycle. 
           [0015]      FIG. 5  is a cross section of an exemplary CDI test cell showing the location of the reference electrode, ( 70 ). 
           [0016]      FIG. 6  (reserved) 
           [0017]      FIG. 7  is an illustration of Ion Capacity—defined as the integrated area between the incoming TDS and the measured TDS at the outlet of the cell. 
           [0018]      FIG. 8  is a graph of Capacitance from current interrupt data. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Applicants have discovered that using an anion electrode that is thicker than a cation electrode, but substantially similar in effective capacitance (such that the effective capacitance of neither electrode is more than two times the capacitance of the other), provides a CDI cell with greater performance characteristics. Surprisingly, and contrary to conventional wisdom as expressed in the &#39;639 patent mentioned above, the phenomenon is not observed in reverse; that is, if the anion electrode is much thicker than the cation electrode. This concept is illustrated with reference to the attached figures. 
         [0020]    An exploded view of the inside of a CDI cell according to an exemplary embodiment of the present invention is illustrated schematically in  FIG. 1 . The cell consists of a stack of discs, consisting in order, of an anion electrode,  12 , an anion selective membrane,  13 , a woven spacer,  14 , that serves as a fluid flow path, a cation selective membrane,  15 , and a cation electrode,  16 . The stack of materials is compressed between two conductive graphite carbon blocks (POCO Graphite, Inc.),  11  and  17 , which serve as electrical contacts to the electrodes. During the charging, or purification cycle, the anion electrode contacting graphite carbon block,  11 , is electrically connected to the positive terminal of the power supply. The cation electrode contacting graphite carbon block,  17  is connected to the negative terminal of the power supply. A plurality of such cells may be used, in series or in parallel, in alternative embodiments of the invention. 
         [0021]    The anion and cation electrodes, ( 12 ) and ( 16 ) are cut from sheets, composed of activated carbon, conductive carbon black and a PTFE binder. Electrodes of this type are widely used in electric double layer capacitors. In these tests, electrodes of varying thickness were obtained from Japan Gore-Tex, Inc., Okayama, Japan. The dimensions of the electrodes in the cell of this embodiment are 3″ in diameter, and have a 0.5″ diameter hole ( 18 ) in the center to allow the treated water to pass out of the cell. 
         [0022]    The anion membrane ( 13 ) is cut from sheets of NEOSEPTA AM1 (Amerida/ASTOM). The dimensions are 3″ OD with a 0.5″ ID. The cation membrane ( 15 ) is cut from sheets of NEOSEPTA CM1 (Amerida/ASTOM). The spacer,  14 , is a 3.25″ OD×0.5″ ID disc cut from a 0.004″ woven polyester screen. 
         [0023]    The flow of water into the cell is radial, with water entering the cell from the outside edge of the spacer, ( 14 ), and flowing out the center exit tube, ( 30 ). Holes ( 31 ) are positioned in the center exit tube to enable water to flow from the spacer into the tube. 
         [0024]    A cross section of exemplary cell components as assembled in an exemplary cylindrical cell housing, ( 39 ), are shown in  FIG. 2   a . The housing consists of a top half ( 40 ) and a bottom half ( 41 ), joined by means of 4 bolts ( 46 ). The cation contacting graphite carbon block, ( 17 ) is mounted to a pneumatically actuated air cylinder ( 47 ). The cell components,  12 - 16  are stacked on top of the carbon block ( 17 ), and around the exit tube ( 30 ). The anion contacting carbon block ( 11 ), is rigidly mounted to the top half to the housing ( 40 ). Electrical leads  44  and  45  connect the anion contacting carbon block ( 11 ) and the cation contacting carbon block ( 17 ) to the power supply. Water is brought into the cell through the water inlet ( 43 ) and fills the circular cavity ( 51 ) surrounding the cell components ( 12 - 16 ). The water flows radially through the spacer ( 14 ) and exits the cell via holes ( 31 ) in the exit tube ( 30 ) and the cell water outlet ( 42 ). The pneumatic cylinder is mounted to a base ( 49 ), which is attached to the bottom half of the housing ( 41 ) by means of bolts ( 50 ). The air cylinder piston ( 48 ) is mounted to the cation contacting carbon block  17 . When the air cylinder is activated the air cylinder piston is extended from the air cylinder, raising ( 17 ) and compressing the cell assembly as shown in  FIG. 2   b.    
         [0025]    In operation of this exemplary embodiment, as shown in  FIG. 3 , water is pumped from a reservoir, ( 61 ), via a peristaltic pump ( 62 ) into the cell ( 39 ). Treated water is analyzed with a conductivity probe ( 63 ). The output of the conductivity probe is converted to total dissolved solids (TDS), based on a NaCl calibration. Power is applied to the cell by means of an programmable battery cycle tester ( 64 ) (ARBIN BT2000). Potential, current and conductivity are recorded as a function of time on a computer ( 65 ). The inlet pressure to the cell is monitored by an inlet pressure transducer ( 66 ), whose output can optionally be included in the ARBIN ( 64 ). 
         [0026]    The cell TDS can be utilized as a set point by the battery cycle tester in the controlling charge and discharge cycles. Inlet water TDS is nominally 480 ppm. At the beginning of the charge cycle, the TDS rapidly declines to some minimum value (see  FIG. 4 ). After reaching the minimum value, TDS increases slowly. Typically charge cycles are conducted until the product TDS reaches 320 ppm, at which point the polarity of the potential is reversed, causing the cell to discharge. There is a rapid increase in current and TDS on discharge. After reaching a peak, the TDS decreases and the discharge is typically allowed to proceed until the product TDS falls to 580 ppm. 
       EXAMPLES 
       [0027]    In some experiments it was considered useful to employ a Ag/AgCl reference electrode (see  FIG. 5 ) ( 70 ) to determine how the potential split between the two electrodes. The position of the reference electrode is shown in  FIG. 5 . Positioned in the circular cavity ( 51 ) surrounding the cell components, the solution potential should be constant. The chloride activity of the test water was estimated to be 0.00356 M using Debye-Huckle approximations for the activity coefficient. From this activity, the potential of the reference electrode was determined to be 0.367V vs. the standard hydrogen electrode. Protocols could be programmed that enabled a short open circuit condition, or a so called current interrupt. This protocol enabled in-situ determination of the potential of each electrode, free of cell IR. 
       Electrodes 
       [0028]    Activated Carbon Electrodes in thicknesses of 250 micron, 600 micron, 800 micron and 1000 micron, were obtained from Japan Gore-Tex. These electrodes are marketed commercially for electrolytic double layer capacitor, and particularly for coin cell applications. 
       Membranes 
       [0029]    Cation Membrane was either NEOSEPTA CM1, obtained from ASTOM or GORE SELECT (GS018950-44us) produced by W.L. GORE &amp; Associates, Inc. Anion membrane was either NEOSEPTA AM1, also obtained from ASTOM or FUMASEP FAB 30 um non-brominated (lot MI0507-140), obtained from FUMATECH GmbH. 
       Spacer 
       [0030]    The spacer was a woven polyester screen, 0.004″ thick, 180 threads per inch, PETENYL, obtained from Tenyl Tecidos Técnicos Ltda, Brazil. 
       Test Water 
       [0031]    A test water made to simulate a “hard” tap water was formulated using the following recipe. 
         [0000]                                                Calcium chloride dehydrate   293.6 mg/L           (CaCl2•2H2O)           Sodium bicarbonate (NaHCO3)   310.7 mg/L           Magnesium sulfate heptahydrate   246.5 mg/L           (MgSO4•7H2O)                        
The resulting water had a total hardness of 300 mg CaCO3/L, calcium hardness of 200 mg/L, alkalinity 185 mg CaCO3/L and a pH of approximately 8.0.
 
       Experimental Procedure 
       [0000]    
       
         
           
             1. Electrodes and membranes are cut to 3″OD by 0.5″ ID. Spacer is cut slightly oversized 3.25″ OD by 0.5″ ID. 
             2. All materials are soaked in a solution of 1000 ppm NaCI for a minimum of 1 hour. 
             3. The materials are assembled in the cell as shown in  FIGS. 1-2 . 
           
         
       
     
         [0035]    4. The cell is closed and the materials compressed by means of the pneumatic cylinder.
       5. A flow of 7 ml/min of test water is initiated by means of a peristaltic pump.   6. Potential was applied by an ARBIN battery cycler. The test protocol consisted of the application of 1.3 V across the cell (Also called charging or purify). TDS was recorded as a function of time as illustrated in  FIG. 4 .   7. Upon applying a potential, ions are removed and TDS drops. This continues until the cell becomes saturated. At this point the TDS exiting the cell begins to increase.   8. When the TDS level reaches 320 ppm, the polarity of the voltage is reversed (−1.3 volts) to fully discharge the cell (also called discharge or waste). This discharge step is continued until the outlet TDS reaches a value of 580 ppm. At that point the cycle is repeated.   9. Average TDS is calculated by averaging the TDS measurements over the course of an entire charge (purify) cycle. A test is stopped when the Average TDS reaches 60% of the incoming TDS or an average TDS of approximately 290 ppm.   10. One can also define a cell capacity as the integrated TDS over the duration of a charge cycle as illustrated in  FIG. 7 . The difference between TDS in and TDS out is measured at each point, multiplied by the flow rate and the time interval. This is integrated over all the peaks, to produce an integrated ion capacity.       
 
         [0042]    In the example and comparative examples, the cation membrane was GORE SELECT and the Anion membrane was FUMATECH FAB. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Cation 
                 Anion 
                   
                 Hours until 
               
               
                   
                 Electrode 
                 Electrode 
                 Hours until 
                 capacity is 
               
               
                   
                 Thickness 
                 Thickness 
                 average 
                 &lt;50% of 
               
               
                 Run 
                 (microns) 
                 (microns) 
                 TDS is 290 ppm 
                 initial 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Comparative 
                 250 
                 250 
                 21 
                 15 
               
               
                 Example 1 
               
               
                 Comparative 
                 600 
                 600 
                 58 
                 70 
               
               
                 Example 2 
               
               
                 Comparative 
                 800 
                 800 
                 67 
                 92 
               
               
                 Example 3 
               
               
                 Example 1 
                 250 
                 800 
                 &gt;245 
                 &gt;245 
               
               
                 Comparative 
                 800 
                 250 
                 22 
                 13 
               
               
                 Example 4 
               
               
                   
               
             
          
         
       
     
       Current Interrupt Experiments 
       [0043]    Current interrupt experiments were conducted to determine the in-situ capacitance of the electrodes in operation. It is well known that double layer capacitance is a function of both voltage and concentration, so measurement of the actual capacitance during operation must be determined by current interrupt techniques. 
         [0044]    The experiment is conducted much like described above, except the cell is configured with a Ag/AgCl reference electrode, as shown in  FIG. 5 , above. At periodic intervals an open circuit condition is created, which generates an interruption in current. The cell potential observed immediately after current interrupt is defined as the IR free potential of the cell. Total cell potential and the potential of the anion electrode (relative to the reference electrode) are obtained experimentally. The potential of the cation electrode is taken by difference. The battery cycler also records the integrated charge over the course of each cycle. Capacitance, dQ/dV, can then be calculated as shown in  FIG. 8 . Since three potential differences are available, total potential, anion potential and cation potential, three capacitances can also be obtained: Ccell, Canion and Ccation. 
         [0045]    Some of the experiments from the Table 1 were repeated using the current interrupt protocol. In these experiments, NEOSEPTA membranes were utilized. In most cases only a few cycles were conducted and capacitance was averaged over the last few cycles after capacitance had stabilized. 
       Comparative Example 1 
     250 micron Cation Electrode—250 micron Anion Electrode 
       [0046]    Although nominally identical electrodes, the current interrupt data suggests that there is a very large difference in capacitance between the cation and anion electrodes. The anion electrode has approximately 1/10 the capacitance of the cation electrode. See Table 2 
       Comparative Example 3 
     800 micron Cation Electrode—800 micron Anion Electrode 
       [0047]    Once again, nominally identical electrodes had quite different capacitance when measured by means of current interrupt protocols. The average capacitance of the anion electrode was once again 1/10 of the cation electrode. Total cell capacitance increased due to the extra capacity available from the thicker electrode. (See Table 2) 
       Example 1 
     250 micron Cation Electrode—800 micron Anion Electrode 
       [0048]    By utilizing a 250 micron cation electrode in conjunction with a 800 micron anode electrode, the capacitance was balance so the difference in in-situ capacitance of the cation and anion electrodes was only about a factor of ½. As shown in Table 1, above, achieving this balance unexpectedly resulted in a significant improvement in cell durability. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Summary of Current Interrupt Data 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Anion 
                 Cation 
                   
               
               
                   
                   
                   
                   
                 Total 
                 effective 
                 effective 
               
               
                   
                 Cation 
                 Anion 
                   
                 Capacitance 
                 capacitance 
                 Capacitance 
               
               
                 Run 
                 electrode 
                 electrode 
                 cycle 
                 (F) 
                 (F) 
                 (F) 
                 Check 
               
               
                   
               
             
          
           
               
                 Comparative 
                 JGI 250 
                 JGI 250 
                 1 
                 108 
                 143 
                 444 
                 108 
               
               
                 Example 1 
               
               
                   
                   
                   
                 2 
                 99 
                 127 
                 446 
                 99 
               
               
                 weight(g) 
                 0.577 
                 0.577 
                 3 
                 102 
                 124 
                 567 
                 102 
               
               
                 area(cm2) 
                 44.3   
                 44.3   
                 4 
                 96 
                 105 
                 1126 
                 96 
               
               
                   
                   
                   
                 5 
                 92 
                 108 
                 659 
                 92 
               
               
                   
                   
                   
                 6 
                 90 
                 98 
                 1074 
                 90 
               
               
                   
                   
                   
                 7 
                 90 
                 100 
                 971 
                 90 
               
               
                   
                   
                   
                 8 
                 87 
                 93 
                 1277 
                 87 
               
               
                   
                   
                   
                 9 
                 89 
                 95 
                 1327 
                 89 
               
               
                   
                   
                   
                 10  
                 87 
                 93 
                 1188 
                 87 
               
               
                   
                   
                   
                 Avg 
                 89 
                 96 
                 1167 
               
               
                   
                   
                   
                 last 5 
               
               
                   
                   
                   
                 cycles 
               
               
                 Comparative 
                 JGI 800 
                 JGI 800 
                 1 
                 250 
                 320 
                 1142 
                 250 
               
               
                 Example 3 
               
               
                   
                   
                   
                 2 
                 249 
                 367 
                 769 
                 249 
               
               
                 weight(g) 
                 1.738 
                 1.738 
                 3 
                 249 
                 347 
                 888 
                 249 
               
               
                 area(cm2) 
                 44.3   
                 44.3   
                 4 
                 253 
                 347 
                 928 
                 253 
               
               
                   
                   
                   
                 5 
                 239 
                 293 
                 1289 
                 239 
               
               
                   
                   
                   
                 6 
                 235 
                 274 
                 1621 
                 235 
               
               
                   
                   
                   
                 7 
                 225 
                 251 
                 2210 
                 225 
               
               
                   
                   
                   
                 8 
                 225 
                 252 
                 2122 
                 225 
               
               
                   
                   
                   
                 9 
                 214 
                 240 
                 1997 
                 214 
               
               
                   
                   
                   
                 10  
                 216 
                 226 
                 4547 
                 216 
               
               
                   
                   
                   
                 Avg 
                 223 
                 249 
                 2499 
               
               
                   
                   
                   
                 last 5 
               
               
                   
                   
                   
                 cycles 
               
               
                 Example 1 
                 JGI 250 
                 JGI 800 
                 1 
                 145 
                 208 
                 482 
                 145 
               
               
                   
                   
                   
                 2 
                 156 
                 221 
                 532 
                 156 
               
               
                 weight(g) 
                 0.577 
                 1.738 
                 3 
                 150 
                 211 
                 512 
                 150 
               
               
                 area(cm2) 
                 44.3   
                 44.3   
                 4 
                 142 
                 225 
                 387 
                 142 
               
               
                   
                   
                   
                 5 
                 137 
                 222 
                 360 
                 137 
               
               
                   
                   
                   
                 6 
                 138 
                 220 
                 372 
                 138 
               
               
                   
                   
                   
                 7 
                 141 
                 208 
                 437 
                 141 
               
               
                   
                   
                   
                 8 
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                 9 
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                 10  
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                 Avg 
                 142 
                 217 
                 413 
               
               
                   
                   
                   
                 last 5 
               
               
                   
                   
                   
                 cycles 
               
               
                   
               
             
          
         
       
     
       Comparative Example 4 
     800-250 Experiment 
       [0049]    In this experiment, an 800 micron cation and a 250 micron anion electrode were employed. In this case capacitance could not be calculated because the potential of the cation electrode became more positive, rather than more negative, over the course of a charging cycle. This indicates that some process other than electrostatic charging is taking place. It is interesting that this behavior coincides with the worst overall performance observed in Table 1. 
         [0050]    While particular embodiments of the present invention have been illustrated and described herein, the present invention should not be limited to such illustrations and descriptions. It should be apparent that changes and modifications may be incorporated and embodied as part of the present invention within the scope of the following claims.