Flow-through capacitor and method

Details Flow-through capacitor and method Flow-through capacitor and method

2002-01-30

2004-08-17

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S307000, C361S503000, C361S517000, C361S530000, C361S532000

Reexamination Certificate

active

06778378

ABSTRACT:

BACKGROUND OF THE INVENTION
Flow-through capacitors utilizing high surface area electrodes have proven commercially useful for water purification and are well-represented in patents, for example, Andelman U.S. Pat. Nos. 5,192,432, issued Mar. 9, 1993; 5,196,115; issued Mar. 23, 1993; 5,200,068, issued Apr. 6, 1993; 5,360,540; issued Nov. 1, 1994; 5,415,768; issued May 16, 1995; 5,547,581; issued Aug. 20, 1996; 5,620,597; issued Apr. 15, 1997; 5,748,437; issued May 5, 1998; 5,779,891; issued Jul. 14, 1998; Otowa U.S. Pat. No. 5,538,611, issued Jul. 23, 1996; Farmer U.S. Pat. No. 5,425,858, issued Jun. 20, 1995; and Benak U.S. Pat. No. 3,658,674, issued Apr. 25, 1972, which all describe the use of high surface area materials. These materials are often of a high surface area formed of carbon, including active powdered carbon, active carbon cloth, and aerogel. For example, Otowa U.S. Pat. No. 5,538,611 claims the use of active carbon with surface areas generally greater than about 1000 B.E.T. (see column 6, lines 29-33), and Andelman U.S. Pat. No. 5,620,597 describes the use of 2000 square meters per gram active carbon in a flow-through capacitor (see column 14, lines 8-10), and U.S. Pat. No. 5,620,597 describes flow-through capacitor electrodes made from virtually any corrosion-resistant, high surface area material (see column 4, line 51 to column 5, line 16). U.S. Pat. Nos. 5,196,115; 5,192,432, 5,200,068 all describe the use of high surface area materials. U.S. Pat. No. 5,547,581 states high surface area materials greater than about 1000 square meters per gram (see column 14, line 7). U.S. Pat. No. 5,425,858 specifies the use of a high specific surface area aerogel in flow-through capacitor construction (see column 5, lines 66-69) and describes the use of exceptionally high, specific surface area aerogel electrodes, for example, 400 to 1000 square meters per gram (see column 5, line 66 to column 6, line 1).
There are also many examples of flow-through capacitors utilizing low surface area, yet high resistance electrodes. This may be either due to the use of intrinsically nonconductive materials or due to the incorporation of enough nonconductive inert materials or binders to adversely affect bulk conductivity properties. For example, the publication by Danny D. Caudel et al, entitled
Electrochemical Demineralization of Water with Carbon Electrodes
, U.S. Department of the Interior Research and Development Progress Report No. 188, May 1966, describes the use of flow-through capacitors with carbon electrodes. Table III from this publication describes demineralization as a function of surface area. This work generally describes the use of carbon black electrodes calendared into backing materials, which add considerable nonconductive material to the electrode, and therefore, impart a high degree of resistance. All of the carbons used were of low surface area, i.e., below 1000 square meters per gram but were combined with inert high resistance material. All anode-cathode pairs listed contained at least one with a low surface area material, i.e., below 310 square meters per gram.
However, the through resistance that is listed in Tables I through III is high, i.e., over 0.5 ohms, and generally, over 1 ohm. Even though the Caudle publication uses highly conductive carbon blacks and graphite in the electrodes, the fabricated electrodes have a high resistance, due to the large amount of inert binder material used to fabricate a monolithic electrode from powdered conductive carbon black materials, for example, Caudle describes the use of a calendared DACRON® (a registered trademark of E.I. du Pont de Nemours and Company of Wilmington, Del.) combined with the conductive carbon blacks (see page 66).
SUMMARY OF THE INVENTION
The invention relates to flow-through capacitors, electrodes employed in such capacitors, and a method of fabricating and operating the flow-through capacitors.
The invention relates to a flow-through capacitor for the purification of a fluid, which capacitor comprises: a housing; an inlet in the housing for the introduction of a fluid to be purified; and an outlet in the housing for the withdrawal of a purified fluid. The invention also comprises a plurality of anode-cathode electrode layers composed of electrode material to form one or more cells for the purification of the fluid. These layers are arranged and constructed to provide for the flow of the fluid between the layers and through the cells; with a capacitor having a series resistance per individual cell of about 0.03 ohms or less; and an electrode material having a surface area of between about 10 and 1000 square meters per gram B.E.T. (Brunauer Emmett Teller method) and having a capacitance of above about 1 farad per gram capacitance.
One object of the invention is to take advantage of low electrical resistance, low surface area, high capacitance materials in order to provide flow-through capacitors with improved flow rates, better energy efficiency and more corrosion resistance.
An additional object of the invention is to provide flow-through capacitors with low surface area and highly conductive materials with a series resistance of less than about 0.3 ohms.
High resistance limits the amount of electric current that can flow into a capacitor. By limiting the electric current, high resistance also limits the flow rate of water that may be purified in a flow-through capacitor. Low resistance allows a more energy efficient, flow-through capacitor, with a faster flow rate per gram utilization of capacitor material. The flow rate which is required in order to achieve 90 percent purification should be above 1 ml per minute per gram of low surface area material, preferably, above 4 ml per minute per gram low surface area material. This should apply across a broad range of concentrations, up to at least 0.02 N. Many low surface area materials are intrinsically conductive, so they do not require current collectors or compression contacts to their current collectors, thereby eliminating an element of resistance.
The high surface area materials of the prior art are used because of the increased capacitance intrinsic to these materials. The high surface area materials of the prior art need to be highly porous or highly textured, which lowers the bulk conductivity of these materials. However, high porosity intrinsic to high surface area materials increases bulk resistivity. Therefore, flow rate and energy efficiency, per given amount of material, is also decreased. Flow-through capacitors made from high surface area materials generally require higher operating voltages to compensate for voltage drops, due to the high resistance of these materials. This stresses the materials and decreases electrode life, due to oxidation or corrosion of the high surface area materials. Generally, while low surface area materials may also be useful when operated at over one volt, low surface area materials charge faster and can utilize higher amps per gram of material, and therefore, allow operation of the flow-through capacitor.
Therefore, since there is less resistance-caused voltage drop during the charge cycle, the average measured operating voltage in the capacitor itself, as opposed to the power supply terminals, will be lower. For example, a power supply terminal may supply 2 volts, but the voltage experienced inside the capacitor itself, as averaged during charging period, may optimally be less than 1 volt. The remaining voltage drop is in the wire leads to the capacitor and is not available to oxidize or adversely affect the surface area electrode materials. The low average operating charge cycle voltage, thus provided, increases electrode life, due to decreased oxidation and corrosion. Low average operating cycle voltage of the invention also allows better energy usage.
Where corrosion is not a concern, the low resistance, low surface area materials are also able to operate under more amps, or power, and therefore, charge faster and offer a faster flow rate purification, of about above 1 ml per minute per gram, and preferably

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