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. Nos. 4,313,084 and 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 m2/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/cm2) 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 non-polarizable 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 PbO2/PbSO4 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:
PbO2+HSO4 −+3H++2⇄PbSO4+2H2O (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 +HSO4 −⇄H++(HSO4 −)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 (HSO4 −)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:
PbO2+2H2SO4+(H+)ad/e ⇄PbSO4+2H2O+(HSO4 −)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 PbSO4 to PbO2 is generally not lower than about 0.1% PbSO4 to about 99% PbO2, while at fully discharged state it is generally not higher than about 99% PbSO4 to about 0.1% PbO2.
An essential characteristic of the capacitor in accordance with the present invention is the effect of the lead electrode on the surface condition and characteristics of the activated carbon electrode. It is caused by transfer of lead from the positive electrode to the negative one at 500 to 2000 mg/cm2 (as determined the methods of inductively coupled plasma, atomic absorption and spectral analysis). As a result, the specific capacitance of the activated carbon electrodes increases by 150-300 F/g.
In a preferred embodiment a capacitor cell or capacitor battery stack is compressed between the power covers of the housing. This significantly reduces the capacitor internal resistance, especially when using negative electrodes made of carbon fabric or felt, and prevents shedding of the active mass of the positive electrode, which is one of the factors limiting the cycle life of EDL capacitors.
The technological approaches described above provide significant increase in the specific energy of the capacitor, reducing its cost at the same time. Increase in specific energy is achieved by rise of the initial discharge voltage up to about 2.0-2.1 V, as well as by a 50% increase in the conductivity of the sulfuric acid electrolyte in comparison with an alkaline one. The cost of the EDL capacitor manufactured according to the present invention is reduced due to use of lead sulfate and lead dioxide electrode, which are considerably cheaper than nickel oxide.
One of the goals of the present invention is to provide completely sealed and maintenance-free capacitor. This is achieved by establishing in the capacitor a system of interconnected gas pores in both electrodes and the separator. The following technological approaches have been proposed:
The volume of the electrolyte in the capacitor is lower than the total pore volume in the electrodes and the separator.
The electrolyte-free portion of pore volume in the electrodes and the separator is in the range of 10 to 50%.
The internal space of the capacitor is under vacuum.
The technological approaches described above allow the oxygen evolved on the positive electrode at end of charge and overcharge to be fully consumed on the negative electrode. The ionization (electroreduction) reaction is favored by both its extremely high polarization (Ep>1 V) and excellent catalytic ability of the activated carbon for this process. On the other hand, gaseous hydrogen, which can be evolved on the negative (carbon) electrode at overcharge can also in principle be fully consumed on the positive electrode by ionization (electrooxidation) reaction as a result of the extremely high polarization of this reaction (Ep>1 V).
The gas pore system assures very fast transport of the gaseous oxygen and hydrogen generated at charge and overcharge to the opposite electrodes, where their ionization takes place. This fast transport within the gas pore system is guaranteed by gas diffusion coefficient in gaseous phase, which is by 3 to 4 orders of magnitude higher than the corresponding diffusion coefficients for gases dissolved in a liquid electrolyte, and by filtration transport mechanism, which is still much faster than the diffusion one.
Thus, the invention presented herein provides a several-times increase in the specific capacitance and about 10 times reduction of the cost in comparison to the EDL capacitor known in WO 97/07518. The capacitor disclosed herein allows both series and parallel element connections and thus assembly of various capacitor batteries on its basis.
Furthermore, using the invention described herein, a completely sealed and maintenance-free capacitor can be assembled. With the possibility of decompression excluded, the end-of-charge value, and by this means the specific energy, could raise. Sealed design also permits considerable increase of the charge current and, therefore, corresponding decrease in the charge time, which is critical for many practical applications. Another beneficial consequence of the technological approach proposed herein, namely, the electrolyte being hold only in a part of the pore volume of the electrodes and the separator and absence of any free electrolyte, is that performance and working characteristics of EDL capacitor become independent of its position in space horizontal, vertical, etc. By the same token, such a capacitor should function normally in objects moving at high accelerations, such as automobiles, aircraft, spacecraft, etc.