Electric double layer capacitor

Details Electric double layer capacitor Electric double layer capacitor

2001-05-03

2002-10-15

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Double layer electrolytic capacitor

C361S503000

Reexamination Certificate

active

06466429

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
This invention relates to electrical engineering and to capacitor engineering in particular, and can be used for manufacturing of high capacitance capacitors utilizing the energy of the electric double layer (EDL). EDL capacitors have found their use as backup power sources in systems requiring uninterrupted electric power supply, such as computers, communication devices, digital programmable lathes, continuous production cycles; for electric starting of internal combustion engines, powering the engines of wheelchairs, golf carts, etc.
DESCRIPTION OF THE PRIOR ART
Several electric power storage devices exist in the form of electric double layer (EDL) capacitors, for example, as described in U.S. Pat. No. 4,313,084 and U.S. Pat. No. 4,562,511. Such capacitors include two porous polarizable electrodes with a porous separator made of a dielectric material between them and current collectors. A liquid electrolyte, which can be either non-aqueous or aqueous, including an aqueous sulfuric acid solution, is retained in the pores of the electrodes and the separator and in the free volume inside the capacitor case. The electric charge is accumulated in the pores on the interface between the electrode material and the electrolyte. Various porous carbon materials are normally used for manufacturing of polarizable electrodes. To increase the capacitance of the electric double layer capacitor, these carbon materials are subjected to prior activation for the purpose of increasing their specific surface area up to 300-3000 m
2
/g.
EDL capacitors have much higher capacitance than conventional electrostatic and electrolytic capacitors—up to tens or hundreds of farads per gram of active electrode material. However, a disadvantage of these capacitors is their rather low specific energy, not exceeding 3 Wh/l. This maximum value of specific energy for double-layer capacitors is set with non-aqueous electrolytes, where the maximum voltage values are in the range of 3 to 3.5 V. However, such capacitors permit very low discharge and charge currents due to the very low conductivity of non-aqueous electrolytes. Still lower specific energies, 0.5 to 2 Wh/l, have been achieved by double-layer capacitors using aqueous electrolytes with maximum voltage value of approximately 0.9 V. When such double-layer capacitors remain under charge for a prolonged period of time (which is often quite long) at voltages higher than 0.9 V, noticeable oxidation of the positive carbon electrode takes place.
An electric double layer capacitor having only one polarizable electrode made of a carbon material, is described in Patent of Japan, Accepted Application No. 2-11008. The other electrode is a non-polarizable (i.e., storage) one, made of lithium or lithium alloy, the electrolyte being nonaqueous. Such a capacitor has higher specific energy compared to the conventional double-layer capacitor with two polarizable electrodes. However, a drawback of this prototype is the very low practical charge and discharge current (0.1 to 1 mA/cm
2
) and, therefore, very low power density as a result of using a non-aqueous electrolyte. Another essential disadvantage of all rechargeable lithium devices, including the one in question, is the very low cyclability—about 100-200 cycles.
An EDL capacitor with only one polarizable electrode made of a fibrous carbon material is described in Patent WO 97/07518. The other electrode, made of nickel oxide, is non-polarizable. An aqueous solution of alkaline metal carbonate or hydroxide is used as electrolyte. Such a capacitor excels considerably the double-layer capacitors with two polarized electrodes in both specific energy (up to 12.5 Wh/l) and maximum voltage (1.4 V).
However, such a capacitor has a number of shortcomings: 1) Insufficiently high specific energy; and 2) High cost, due to the use of large amounts of nickel oxide.
From technological point of view and as effect achieved, closest to the present invention are EDL capacitors employing lead compounds as active ingredients of the non-polarizable electrode, which are disclosed in PCT/RU97/00353 and PCT/RU97/00411. In particular, lead dioxide is used in the case of PCT/RU97/00353 and lead sulfate in the case of PCT/RU97/00411. The device disclosed in PCT/RU97/00353 is considered as a prototype of the invention herein.
Still another drawback of the EDL capacitors is the gas generation on the electrodes at overcharge, e.g. of oxygen on the positive electrode and/or hydrogen on the negative electrode. This occurs when the evolution potentials of these gases on the corresponding electrodes are reached at overcharge. As a result, the pressure within the capacitor case increases, which can lead to its decompression and even explosion, unless it is equipped with a special pressure relief valve. But even such valves often are not reliable enough to prevent decompression or explosion: they can, for instance, become clogged with dirt, etc. On account of all this, EDL capacitors have a fundamental disadvantage: the possibility of their decompression and even explosion and need of special maintenance. In order to prevent decompression, the end-of-charge voltage is significantly reduced for reinsurance, thus reducing the initial discharge voltage as well. This, in its turn, leads to a considerable decrease in the EDL capacitor specific energy, which is directly proportional to the difference between the squares of the initial and final discharge voltages.
SUMMARY OF THE INVENTION
The objects of the present invention are to increase the specific energy of the capacitor and to reduce its cost.
Both of these objects are achieved by the invention described below. In accordance with the invention, a capacitor is provided, which comprises a polarizable electrode made of a porous carbon material, a nonpolarizable electrode made of a material including lead sulfate and lead dioxide as active components, and an aqueous solution of sulfuric acid as electrolyte. The polarizable carbon electrode is negative, and the PbO
2
/PbSO
4
electrode positive.
The preferable mass ratio of the lead sulfate to the lead dioxide ranges from about 0.1 to 99% to about 99 to 0.1%.
The following electrochemical reaction takes place during discharge and charge on the positive electrode:
PbO
2
+HSO
4
−
+3H
+
+2e⇄PbSO
4
+2H
2
O  (1)
Under the working conditions employed, the maximum equilibrium potential of this reversible reaction in the charged state is approximately 1.8 V.
During cycling, the EDL on the negative electrode is recharged. The charge-discharge process of the EDL can be described as follows:
(H
+
)
ad
/e+HSO
4
−
⇄H
+
+(HSO
4
−
)
ad
/−e+2e
−
  (2).
Here the designation (H
+
)
ad
/e refers to a proton adsorbed in the EDL on the negatively charged surface of the carbon electrode (for charged state of the capacitor), and (HSO
4
−
)
ad
/−e to the bisulfate ion adsorbed in the EDL on the positively charged surface of the carbon electrode (for discharged state of the capacitor). In our experiments, the potential of said electrode changed in the range of −0.2 to 1.0 V vs. normal hydrogen electrode in the same solution.
Combining reactions (1) and (2) give the overall equation of the electrochemical reaction taking place in the electrical double layer capacitor, described herein:
PbO
2
+2H
2
SO
4
+(H
+
)
ad
/e⇄PbSO
4
+2H
2
O+(HSO
4
−
)
ad
/−e  (3)
It should be noted that the active mass of the positive electrode contains both lead dioxide and lead sulfate at any degree of discharge. This has been demonstrated by the results of the chemical analysis. As such, at fully charged state the actual mass ratio of PbSO
4
to PbO
2
is generally not lower than about 0.1% PbSO
4
to about 99% PbO
2
, while at fully discharged state it is generally not higher than about 99% PbSO
4
to about 0.1% PbO
2
.
An essential characteristic of the capacitor in accordance with the pres

Affiliated with

Also associated with

Rating

Electric double layer capacitor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Electric double layer capacitor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electric double layer capacitor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2957818