Patent Application
20100230277
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.
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.
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
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.
As used herein, “effective capacitance” means dQ/dV for a membrane-electrode conjugate as determined by current interrupt as described herein.
Also as used herein, “durability” means hours until ion removal is less than 60% (under test conditions specified herein).
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.
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.
a is a cross sectional view of an assembled CDI cell according to an exemplary embodiment of the invention before compression.
b is a cross sectional view of an assembled CDI cell according to an exemplary embodiment of the invention after compression.
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 '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.
An exploded view of the inside of a CDI cell according to an exemplary embodiment of the present invention is illustrated schematically in
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.
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.
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.
A cross section of exemplary cell components as assembled in an exemplary cylindrical cell housing, (39), are shown in
In operation of this exemplary embodiment, as shown in
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
In some experiments it was considered useful to employ a Ag/AgCl reference electrode (see
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.
Cation Membrane was either NEOSEPTA CM1, obtained from ASTOM or GORE SELECT (GS018950-44us) produced by W.L. GORE & 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.
The spacer was a woven polyester screen, 0.004″ thick, 180 threads per inch, PETENYL, obtained from Tenyl Tecidos Técnicos Ltda, Brazil.
A test water made to simulate a “hard” tap water was formulated using the following recipe.
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.
4. The cell is closed and the materials compressed by means of the pneumatic cylinder.
In the example and comparative examples, the cation membrane was GORE SELECT and the Anion membrane was FUMATECH FAB.
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.
The experiment is conducted much like described above, except the cell is configured with a Ag/AgCl reference electrode, as shown in
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.
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
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)
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.
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.
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.
This application claims the benefit of commonly owned and co-pending U.S. Provisional Application No. 61/096,899 filed on Sep. 15, 2008.
| Number | Date | Country | |
|---|---|---|---|
| 61096899 | Sep 2008 | US |
