Electronic apparatus employing electrochemical capacitor and method for recovering capacitance of electrochemical capacitor

An electronic apparatus has an electric load, an electrochemical capacitor, and an applying section. The electric capacitor includes a positive electrode, a negative electrode and an electrolyte placed between the positive electrode and the negative electrode and supplies electric power to the electric load. The applying section opens an electrical connection between the electrochemical capacitor and the electric load, and applies a minus potential to the positive electrode and a plus potential to the negative electrode.

This Application is a U.S. National Phase Application of PCT International Application No. PCT/JP2005/010845 filed Jun. 14, 2005.

TECHNICAL FIELD

The present invention relates to a technology of recovering capacitance of an electrochemical capacitor in an electronic apparatus such as a vehicle driven by using a motor that is power-assisted by the electrochemical capacitor.

BACKGROUND ART

Recently, motor-driven vehicles have been manufactured and have received very much attention from the viewpoint of reduced environmental loading. In such a vehicle, basically, a fuel cell supplies electric power to a motor, thereby the motor is driven. At this time, since the fuel cell generates water without containing impurities, the emission of the water does not increase the environmental loading, which is greatly supported in the current environmental society.

When a motor is driven by a fuel cell, maximum electric power cannot be supplied to the motor from the fuel cell immediately after a switch is turned on. Consequently, with such a configuration, acceleration of the vehicle is very slow. Therefore, it is devised that a fuel cell and an electrochemical capacitor are coupled in parallel to a motor. That is to say, when electric power necessary for the motor is not sufficiently supplied from the fuel cell alone, for example, during acceleration of the vehicle, electric power is supplied from the electrochemical capacitor. Thus, acceleration performance can be improved.

In the case where the electrochemical capacitor is used in order to supplement the shortage of acceleration, however, capacitance of the electrochemical capacitor is reduced due to the long-term use. When the electrochemical capacitor is used for a long time, in a positive electrode and a negative electrode, ions approaching the positive and negative electrodes cause dielectric breakdown. Thereby, reaction products are generated on the surfaces of the positive and negative electrodes. When the reaction products are attached to the positive and negative electrodes, the surface areas of the positive and negative electrodes are reduced. Therefore, the number of ions in electrolyte attracted to the positive and negative electrodes is reduced, so that capacitance of the electrochemical capacitor is reduced. If such a reduction of capacitance is allowed to stand, the capacitance of the electrochemical capacitor is further reduced. When the capacitance of the electrochemical capacitor is reduced in this way, acceleration performance of the vehicle is deteriorated.

SUMMARY OF THE INVENTION

An electronic apparatus of the present invention includes an electric load, an electrochemical capacitor, and an applying section. The electrochemical capacitor has a positive electrode, a negative electrode and an electrolyte placed between the positive electrode and the negative electrode, and supplies electric power to the electric load. The applying section opens the electrical connection between the electrochemical capacitor and the electric load, and applies a minus potential to the positive electrode and a plus potential to the negative electrode. Thus, reduction of capacitance of the electrochemical capacitor can be suppressed. As a result, the deterioration of desired properties of an electronic apparatus to be used can be prevented. Furthermore, the present invention relates to a method for recovering capacitance of an electrochemical capacitor as mentioned above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1Ais a sectional view showing an electrochemical capacitor used in a vehicle in accordance with an exemplary embodiment of the present invention.FIG. 1Bis an exploded perspective view showing an internal structure of the electrochemical capacitor shown inFIG. 1A. In the exemplary embodiment, an electric double-layer capacitor is used as the electrochemical capacitor.

Electric double-layer capacitor (hereinafter, referred to as “capacitor”)8includes exterior case6in which element5is enclosed and which is sealed with sealing rubber7. As shown inFIG. 1B, element5includes band-like separator4, band-like positive electrode2and negative electrode3. Positive electrode2and negative electrode3are wound spirally on the front and rear surfaces of separator4. On the surfaces facing separator4of positive electrode2and negative electrode3, activated carbon is provided, respectively. Between positive electrode2and negative electrode3, an electrolyte, which is filled in exterior case6, is placed. Terminals1are coupled to positive electrode2and negative electrode3, respectively.

For exterior case6, a collector of positive electrode2and a collector of negative electrode3, aluminum is used from the viewpoint of weight and conductivity. Exterior case6may be formed of stainless steel or nickel-plated iron by giving priority to strength. The collectors of positive electrode2and negative electrode3may be formed of nickel, and the like. Sealing rubber7includes a material unaffected by an electrolyte, for example, ethylene propylene rubber, and the like. Separator4is composed of nonwoven fabric or microporous membrane of cellulose, polyethylene, polypropylene, and the like.

Capacitor8is manufactured as follows. Element5shown inFIG. 1Bis subjected to vacuum drying at 110° C. for 12 hours, and then inserted into exterior case6under atmosphere at a dew point of −40° C. or less. Then, an electrolyte is filled in exterior case6, followed by carrying out vacuum pressure impregnation and sealing with sealing rubber7. The electrolyte is prepared by mixing tetraethylammonium tetrafluoroborate into a propylene carbonate solvent at a concentration of 0.69 mol/L.

In this configuration, a voltage is applied between positive electrode2and negative electrode3via terminals1, and thereby, electric charges are accumulated. When electric charges are accumulated in this way, a voltage is generated between terminals1. Then, electric power by capacitor8is supplied to an electric load. As mentioned above, an electric double-layer capacitor using an organic electrolyte as an electrolyte can be used at voltage ranging from 2.0V to 2.7V.

Next, the change over time of capacitance in the case where a voltage is applied to capacitor8is described.FIG. 2Ashows the change of capacitance in an acceleration test in which 2.5V of voltage is continuously applied between positive electrode2and negative electrode3for 2000 hours and the atmospheric temperature is kept at 60° C.

The capacitance is calculated from the below-mentioned equation (1) as follows. That is to say, in a discharge curve as shown inFIG. 2Bshowing the change over time of voltage between positive electrode2and negative electrode3when constant-current discharge is carried out, the discharge curve when the voltage between the positive and negative electrodes is changed from 80% to 60% is linearly approximated.
C=I×(t0.6−t0.8)/(0.8V0−0.6V0)  (1)
In equation (1), C denotes capacitance, I denotes current at the time of discharge, V0denotes charging voltage, t0.6denotes a time at 0.6V0, and t0.8denotes a time at 0.8V0.

As shown inFIG. 2A, as time passes, capacitance reduces, and the capacitance is reduced by about 20% at 2000 hours as compared with the capacitance at the time when the measurement is started.

After an electric double-layer capacitor whose capacitance has been reduced is fully discharged, a minus potential is applied to positive electrode2and a plus potential is applied to negative electrode3. Herein, in general charge and discharge, an electrode showing relatively plus potential is positive electrode2and electrode3showing a minus potential is a negative electrode. Hereinafter, “a minus potential is applied to positive electrode2and a plus potential is applied to negative electrode3” is referred to as “a reversed polarity voltage is applied.”

FIG. 3shows the change of capacitance when 1.5V of a reversed polarity voltage is applied between positive electrode2and negative electrode3and when 2.5 V of a reversed polarity voltage is applied therebetween. As is apparent fromFIG. 3, in both cases, when the reversed polarity voltage is applied, the reduced capacitance is recovered.FIG. 4shows the results ofFIG. 2Aand point9that shows a state in which the capacitance is recovered by applying a reversed polarity voltage. In this way, by applying a reversed polarity voltage to an electric double-layer capacitor whose capacitance has been reduced, the capacitance is recovered by 10% at the maximum.

It is thought that when a reversed polarity voltage is applied, ions remaining in the activated carbon of positive electrode2and negative electrode3are diffused. Thereby, the reaction products attached to the activated carbon surfaces of positive electrode2and negative electrode3are thought to be detached, the surface area of positive electrode2and negative electrode3is thought to be recovered, and the number of ions attracted by positive electrode2and negative electrode3is thought to be recovered. Thus, capacitance is recovered.

Furthermore, the recovered amount of capacitance is dependent upon a voltage to be applied. As the voltage is increased, the recovered amount is increased. This is thought that when the voltage to be applied is higher, energy for diffusing ions existing in an electrolyte is increased, so that the reaction products attached to positive electrode2and negative electrode3are thought to be actively detached. However, when a reversed polarity voltage larger than the normal-rated voltage of capacitor8is applied, deterioration may be promoted contrarily. Therefore, it is preferable that the reversed polarity voltage is not more than the normal-rated voltage of capacitor8.

In this exemplary embodiment, an electric double-layer capacitor using an organic electrolyte as an electrolyte is described. Other than this, this recovering method is also effective to an electric double-layer capacitor using ambient temperature molten salt, and the like, as an electrolyte when a deterioration mechanism of capacitance is the same.

Furthermore, as shown inFIG. 3, a capacitance recovering effect in this exemplary embodiment can be exhibited by only one second of application, which shows the recovery corresponding to 98% or more of that exhibited after 6 minutes of application. After 30 seconds of application, the recovery corresponds to 99% or more of that exhibited after 350 seconds application. The effect is saturated in one to two minutes. That is to say, in this exemplary embodiment, the reversed polarity voltage is preferably applied for one second or more, and further preferably, for 30 seconds or more. The capacitance is rapidly recovered immediately after a reversed polarity voltage is applied. When the capacitance is recovered to some extent, even if the application time is increased, the capacitance is not recovered more. Energy stored in capacitor8by the application of an arbitrary reversed polarity voltage is saturated when a certain time has passed. Therefore, it is thought that diffusion of ions is gradually reduced, and that the recovered amount of the surface area of positive electrode2and negative electrode3is saturated. Consequently, the reversed polarity voltage exhibits the effect immediately after it is applied and a voltage may not be applied for a time longer than necessary. That is to say, it is not necessary to apply a reversed polarity voltage for longer than two minutes.

Such a technology of applying a reversed polarity voltage to an electric double-layer capacitor is disclosed in, for example, Japanese Patent Unexamined Publication No. 2002-142369. However, the technology disclosed in this publication is different from the present invention in that an object of the invention of the publication is to equalize a voltage of each cell in a capacitor unit in which single cells are connected in series. Furthermore, this publication discloses the content that is similar to the experiment described with reference toFIG. 2andFIG. 4. However, a reversed polarity voltage is continued to be applied to the capacitor unit for five days. Thus, the above-mentioned publication and the present invention are remarkably different from each other in the time necessary to exhibit effects.

As mentioned above, capacitance of capacitor8is recovered by applying a reversed polarity voltage to capacitor8. In this exemplary embodiment, an applying section for applying such a reversed polarity voltage is provided in an electronic apparatus such as a vehicle driven by a motor, deterioration of desired properties of the electronic apparatus is prevented.

FIG. 5is a schematic view showing vehicle10driven by motor11that is an electric load, andFIG. 6is a conceptual diagram showing a drive system of vehicle10. Vehicle10includes car body13, fuel cell stack (hereinafter, referred to as “fuel cell”)12that is a power source disposed in car body13, and electric double-layer capacitor module (hereinafter, referred to as “module”)19. In module19, a plurality of capacitors8are coupled. Module19is coupled in parallel to fuel cell12via control circuit18.

To fuel cell12, hydrogen as a fuel is supplied from hydrogen supply source20, and fuel cell12generates electric power by using this hydrogen and oxygen in the air. Control circuit18monitors voltages of fuel cell12and module19and controls supply of electric power to motor11. Furthermore, control circuit18also controls charge of electric power from fuel cell12to module19.

In a car formed by car body13, driver's sheet14is arranged and steering17is disposed in front of it. Steering17is coupled to front wheel15that is a steering wheel. Motor11is coupled to rear wheel16that is a driving wheel. Control circuit18is electrically connected to motor11.

Motor11rotates reversely and generates electricity during deceleration of vehicle10. The electric power generated at this time is charged to module19via control circuit18. Note here that a generator coupled to a driving body of motor11may be additionally provided, and electric power generated by this generator may be charged to module19via control circuit18. Furthermore, control circuit18includes applying section21for applying a reversed polarity voltage to capacitor8. Alternatively, applying section21may be provided in module19.

FIG. 7shows a relation between electric power necessary for motor11and electric power supplied from fuel cell12and module19in various running patterns of vehicle10. For understanding easily, electric power necessary for motor11is shown in the upper side when motor11is electrically driven and in the lower side when motor11reversely rotates and generates electricity. Furthermore, electric power supplied from fuel cell12and module19is shown in the upper side at the time of discharge (supply) and in the lower side at the time of charge.

During acceleration, since electric power from fuel cell12runs short, electric power is supplied from module19to motor11. When the running pattern shifts to a cruising state in which acceleration and deceleration are hardly carried out, electric power is supplied mainly from fuel cell12to motor11since electric power is started to be supplied from fuel cell12. During deceleration, since motor11functions as a generator, electric power generated at this time is accumulated in module19.

FIG. 8is a graph showing the change of voltage between the positive electrode and the negative electrode in capacitor8in module19when electric power is supplied to motor11. In a general operation, since electric power is supplied from module19to motor11during acceleration, the voltage of capacitor8is reduced. During deceleration, since module19is charged with electric power generated by motor11, the voltage of capacitor8is recovered. During cruising, since electric power is supplied mainly from fuel cell12to motor1, the voltage of capacitor8is not reduced. During idling, since electric power is not supplied from module19, the voltage of capacitor8is not changed. However, when the voltage of capacitor8is too low, charging may be carried out from fuel cell12in order to secure the electric power of module19, which is necessary for acceleration.

In order to prevent the reduction of capacitance of capacitor8used in an electronic apparatus such as vehicle10, it is desirable that a reversed polarity voltage is routinely applied to capacitor8. It is preferably that it is applied while the electronic apparatus is driven.

However, when the reversed polarity voltage is applied to positive electrode2and negative electrode3, it is not realistic in use to apply the reversed polarity voltage to all of the mounted capacitors8simultaneously. Then, in the case where the reversed polarity voltage is applied to capacitor8while an electronic apparatus is driven, it is necessary that capacitor8to be applied is separated from a main circuit that is coupled to an electric load. Then, capacitors8, the number of which is the number capable of supplying electric power necessary for acceleration to motor11, are maintained in a state in which a general operation can be carried out.

A method for realizing application of the reversed polarity voltage as mentioned above is described with reference to an electric double-layer capacitor module in which three capacitors8are coupled in series as an example.

FIG. 9is a diagram showing a circuit configuration for applying the reversed polarity voltage to each electric double-layer capacitor in the case where three capacitors8A,8B, and8C are coupled in series.FIG. 10is a flowchart showing a procedure for applying the reversed polarity voltage to capacitors8A,8B, and8C.

As a procedure for applying the reversed polarity voltage to an electric double-layer capacitor, firstly, an electric double-layer capacitor separated from a main circuit is coupled to load R, and electric charges accumulated during a general operation are discharged. For example, when capacitor8A is separated, as in step (1), by turning off switches S4and S7, electrical connection between capacitor8A and the main circuit coupled to motor11is opened. At the same time, switch S1is turned on. Then, switches S5, S6and S17are turned on.

After electric charges of capacitor8A are discharged, as in step (2), switch S17is turned off and switch S16is turned on so as to couple capacitor8A to power source E. Thus, a reversed polarity voltage is applied so as to recover capacitance. When the reversed polarity voltage is applied, the polarity of voltage between the positive and negative electrodes of capacitor8A is reversed with respect to a general operation. In order to return the capacitor to the main circuit, it is necessary that electric charges accumulated by the application of the reversed polarity voltage are discharged by coupling capacitor8A to loading R. To do so, as in step (3), switch S16is turned off and switch S17is turned on. Such a series of processing with respect to an electric double-layer capacitor for recovering capacitance is referred to as “refresh mode” hereinafter. The circuit for realizing a refresh mode shown inFIG. 9composes applying section21. Applying section21is included in, for example, control circuit18or module19.

In this exemplary embodiment, the refresh mode is executed with respect to capacitors8A,8B and8C, sequentially in this order. That is to say, as shown inFIG. 10, after the refresh mode is executed for capacitor8A as shown in steps (1) to (3), the refresh mode is similarly executed for capacitor8B as shown in steps (4) to (6), and then the refresh mode is similarly executed for capacitor8C as shown in steps (7) to (9). Finally, as shown in step (10), capacitor8C is coupled to the main circuit.

FIG. 11shows states of switches S1to S17in steps (1) to (10) inFIG. 10. “ON” denotes a state in which a switch is closed, and “OFF” denotes a state in which a switch is open. Note here that step (10) shows a state of the switches in a general operation.

As mentioned above, when a refresh mode is executed for one electric double-layer capacitor, the other electric double-layer capacitors execute a general operation. With such a configuration, it is possible to execute the refresh mode even while electronic apparatus is driven.

In general, since a voltage necessary for motor11to drive vehicle10is as high as several hundreds V, when capacitor8has a low rated voltage, several tens to several hundreds capacitors8are needed. For example, when a driving voltage of motor11is 250 V, in the case where capacitor8having a rated voltage of 2.5V is used, it is necessary to connect at least 100 capacitors8in series. In the case where a large number of capacitors8are used in this way, a reversed polarity voltage may be applied to individual electric double-layer capacitors as mentioned above. Meanwhile, as shown inFIG. 12, the mounted electric double-layer capacitors may be divided into subunits SU1to SUn and a refresh mode may be executed with respect to each subunit, sequentially. Each subunit is also configured of two or more electric double-layer capacitors as shown in a section surrounded by a dotted line in the drawing. In this way, even if electrochemical capacitors are divided into a plurality of subunits, the procedure for executing a refresh mode is the same as that in the above-mentioned configuration in which three capacitors8are coupled.

Furthermore,FIG. 13shows a circuit configuration in which three electric double-layer capacitors are coupled in parallel, andFIG. 14shows a circuit configuration in which subunits SU1to SUn, in which a plurality of electric double-layer capacitors are coupled in series, are coupled in parallel. Even if electric double-layer capacitors or subunits are coupled in parallel as described above, the procedure for executing a refresh mode is the same as that in the above-mentioned configuration in which they are coupled in series. The refresh mode can be realized with a simple configuration of circuit shown inFIG. 13orFIG. 14.

A switching operation for refreshing capacitor8A is described with reference toFIG. 13as an example. In order to separate capacitor8A, switches S21and S23are turned off, respectively. Then, switches S22, S24and S26are turned on so as to couple capacitor8A to load R, and electric charges of capacitor8A are discharged. Then, after electric charges of capacitor8A are discharged, switch S26is turned off and switch S25is tuned on, thus coupling capacitor8A to power source E. Then, a reversed polarity voltage is applied so as to recover capacitance. When the reversed polarity voltage is applied, the polarity of voltage between the positive electrode and the negative electrode is reversed with respect to a general operation. In order to return to a main circuit, it is necessary that electric charges accumulated by the application of reversed polarity voltage are discharged by coupling capacitor8A to charge R. To do so, switch S25is turned off and switch S26is turned on. Thereafter, by turning switches S25and S26off, and turning S22and S24off and turning S21and S23on, capacitor8A is returned to the main circuit. Then, capacitors8B and8C are also subjected to the same operations.

As mentioned above, all the circuits for applying a reversed polarity voltage shown inFIGS. 9,12,13and14are applying sections, which can be provided regardless of methods to be employed for coupling electric double-layer capacitors mounted on an electronic apparatus.

Note here that the present invention is not necessarily limited to an electric double-layer capacitor and can be applied to an electrochemical capacitor in which capacitance is recovered by applying a reversed polarity voltage. For example, the present invention can be employed to a hybrid capacitor using activated carbon for a positive electrode and graphite for a negative electrode.

Furthermore, in this exemplary embodiment, a vehicle in which fuel cell12is mounted as a power source is described. Other than this, the present invention may be applied to a vehicle in which a rechargeable battery is mounted as a power source or a hybrid vehicle in which at least one of a fuel cell and a rechargeable battery is mounted as a power source so as to drive motor11and engine is also mounted so as to drive a driving wheel. The present invention may be applied to apparatuses other than a vehicle.

INDUSTRIAL APPLICABILITY

Since an electronic apparatus provided with an applying section for applying a reversed polarity voltage to an electrochemical capacitor of the present invention can suppress the deterioration of the electrochemical capacitor, it is possible to prevent the deterioration of desired properties of the electronic apparatus so as to improve the reliability. This configuration is useful for an electronic apparatus on which an electrochemical capacitor is mounted.