WO2000002215A1

WO2000002215A1 - A hybrid capacitor

Info

Publication number: WO2000002215A1
Application number: PCT/EP1999/002109
Authority: WO
Inventors: Anders Olof Lundblad; Viktor Petrovitj Kuznetsov; Roustam Aminovich Mirzoev
Original assignee: Alfar International Ltd
Current assignee: Alfar International Ltd
Priority date: 1998-07-03
Filing date: 1999-03-26
Publication date: 2000-01-13
Anticipated expiration: 2001-01-03
Also published as: AU3926399A; RU2145132C1

Abstract

The present invention relates to a hybrid capacitor including a positive (2) and a negative electrode (5), separated by a separator (3), the electrodes and the separator being saturated by aqueous electrolyte. According to the invention the negative electrode (5) is made of nanoporous carbon, essentially made from carbide precursor, the majority of the nanopores having a size less than 5 nm, preferably less than 2 nm.

Description

A hybrid capacitor

TECHNICAL FIELD

This invention relates to an electrochemical capacitor.

BACKGROUND

Double layer capacitors, pseudo capacitors and hybrid capacitors represent a new group of energy storage devices which are often called supercapacitors. One important application for supercapacitors is pulse power. Efforts in today's development of supercapacitor technology are, for electrode materials and electrolytes, mainly directed towards increasing energy and power of the devices.

Double layer capacitors are characterised by the charges being stored in the electrochemical double-layer of the solid/liquid interface of the electrodes. In a pseudo capacitor the charges are stored through a reversible electrochemical reduction/oxidation reaction (redox). A device using one double-layer electrode and one redox electrode is hereinafter called a hybrid capacitor.

The most commonly used material for double layer capacitor electrodes are activated carbon fiber cloths. Such materials has high specific surface areas, thus, providing a large interface to the electrolyte. The drawbacks of activated carbon fiber cloths are that they, due to their open woven structure, have a high porosity and thereby not so high specific capacitance per volume, and that the pore size distribution is generally rather wide and thereby not optimised for any electrolyte used.

WO 97/20333 discloses a double layer capacitor comprising at least two electrodes made from a nanoporous skeleton carbon. The nanoporous carbon described in this patent has an even higher specific surface area per volume (1200 m 2 /cm 3 ) than carbon fiber cloth, with a very narrow pore size distribution around 8 A. This feature gives the capacitors, using this material, a specific capacitance per volume which is one of the

3 highest reported in the world today for aqueous electrolytes ( up to 40 F/cm ). The disadvantage of these capacitors is, however, the drastic decrease of capacitance at higher charge/discharge currents. The explanation for this behaviour is the ion transport limitations set by the very narrow pores of the nanoporous carbon.

A number of patents and scientific publications disclosing hybrid capacitors can be found in the literature. A hybrid capacitor is dependent on the performance of both the negative double-layer electrode and the positive redox electrode. Typically, however, the charge/discharge performance is higher of the redox electrode. This is due to the fact that the density of the storable charges is usually higher in a redox electrode than in double-layer electrodes. This enables the redox electrode to be made thinner than the carbon double-layer electrode and thereby the ionic mass transport resistance of the redox electrode becomes smaller. The design of state-of-the-art hybrid capacitor is such that the capacitance of the positive redox electrode is much higher than that of the negative double-layer electrode. The consequence is usually that both the energy and power capability of the device are limited by the negative carbon electrode. Thus, further development of hybrid capacitors are mainly dependent on finding better carbon materials.

US 5,429,893 "Electrochemical capacitors having dissimilar electrodes" disclose a capacitor with a first electrode, made of activated carbon materials, using a double layer mechanism and a second electrode using an electrochemical redox reaction to store charges. The main objective of this patent is to increase the energy storage capacity (energy density) of the device, through the fact that the working voltage of a hybrid capacitor using activated carbon fiber cloth and porous nickel can be increased from less than 1 V to 1.4 V compared to a double-layer capacitor.

However, due to the low charge density of the carbon materials used in this patent, the energy density of the capacitors made accordingly cannot be very high.

The object of the present invention is to obtain a supercapacitor having a high energy storage density and a high extractable power. SUMMARY OF THE INVENTION

This object is accomplished with a hybrid capacitor including a positive and a negative electrode, separated by a separator, the electrodes and the separator being saturated by an aqueous electrolyte, characterized by a negative electrode made of nanoporous carbon, essentially made from carbide precursor, the majority of nanopores having a size less than 5 nm, preferably 2 nm. It has surprisingly been found that the ionic mass transport resistance of a nanoporous carbon decreases with decreasing starting potential which makes the use of electrodes of nanoporous carbon as negative electrodes advantageous.

In a preferred embodiment the carbon content in the negative electrode exceeds 95 %wt and the nanoporous carbon is a nanoporous skeleton carbon. The amount of transport pores in the negative electrode can be varied between 10-55 % of the total volume of the electrode and the amount of nanopores can be varied between 15-55 %. The nanopores in the negative electrode can be of the same or different types, each type having a narrow size distribution. The positive electrode is a porous Ni electrode made by thermochemical treatment of a TiNi-alloy. The positive electrode can also be a porous metal made by chemical and electrochemical treatment of a metal or metal-alloy, for example Ni, Co, Pb. Alternatively, the positive electrode can be a porous Ni electrode made by chemical and electrochemical treatment of a NiAl-alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the enclosed Figures, of which;

FIG. 1 is a schematic sectional view of a hybrid capacitor,

FIG. 2 is a principal drawing of an experimental microelectrode set-up,

FIG. 3 gives the initial currents vs. starting potential obtained in the potential step microelectrode experiments, and FIG. 4 shows a charge-discharge curve of an electrochemical cell having a porous nickel cathode and an anode made of nanoporous carbon made according to an _^, . embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

DEFINITIONS:

A hybrid capacitor is a supercapacitor where the positive electrode is using a red-ox reaction for charge storage (e.g. porous Ni electrodes), and the negative electrode stores charges in the electrochemical double layer.

Nanopores - pores smaller than 10 nm.

Nanoporosity - system of nanopores.

Transport pores- pores essentially greater than 100 nm, that are essentially intended for transport of ions into and out of the electrode.

With nanoporous carbons is meant, in this text, a carbon electrode material, essentially made from carbide precursor, containing nanopores in more than 15 % of the total pore volume, preferably more than 30 % (e.g. a carbon material essentially produced by chemically extracting the non-carbon material out of a carbide material, such as SiC, TiC, etc). A nanoporous carbon may for instance be a nanoporous skeleton carbon.

With nanoporous skeleton carbon is meant, in this text, a rigid skeleton network of carbon particles stemming from carbides, which are bound together only by carbon.

Internal resistance is in this patent defined in a wide sense, including not only the electrical resistance of the solid phases and the liquid phases, and the contact resistance but also the resistance related to ionic mass transport hindrance in the electrolyte phase (in the pore system of the electrodes etc). Specific capacitance, power density and energy density are in this patent calculated or estimated on the basis of the active materials of the capacitor (e.g. specific capacitance

3 (F/cm ) is calculated by dividing the capacitance with the aggregate volume of the positive and negative electrode and the separator).

The present invention relates to a mass-transport phenomenon which has been observed on small nanoporous carbon particles (size: 35-290 μm) by means of a microelectrode technique.

The microelectrode measurements were made using a potential step technique and measuring the generated initial current maximum created by exposing the particles to a potential step. The measurements have suprisingly shown that the initial current obtained (which also is the maximum current), when exposing a nanoporous carbon particle to a potential step of 100 mV, depends on the starting potential of the particle (i.e. its state-of-charge). If for example, the particle is charged to a starting potential of- 500 mV then the initial current is more than 5 times higher than when the starting potential is +500 mV.

The effect is believed to be related to the transport of ions, diffusion and migration, in the nanometer sized channels of the nanoporous carbons. One possible explanation for this phenomena is that there is chemisorption of hydroxide ions reacting with active sites at more positive potentials. In this way hydroxyl groups are fixed on the nanopore wall and they are partially blocking the pores for further diffusion. At more negative potentials the pores will become less blocked.

The practical effect of the above described phenomenon is that nanoporous carbons work better as negative double-layer electrodes in aqueous basic solutions, more specifically potassium hydroxide solutions. A negatively charged nanoporous carbon electrode has a higher effective diffusion coefficient and can thereby support/provide higher charge/discharge currents than a positively charged nanoporous carbon electrode, The charge/discharge rate difference between the positive and the negative electrode is expected to increase with smaller nanopore size. The effect should, thus, be greater when using a carbon essentially made from SiC or TiC which results in the majority of the nanopore volume pores being smaller than 2 nm, than when using a carbon essentially made from other carbides which results in the majority of the nanppore volume pores being greater than 2 nm. For nanopores having a size larger than 5 nm, the effect is no longer significant.

The practical importance of the effect is of course also depending on the design of the electrode, for example, using a small precursor particles size and a large electrode thickness, will lead to that the ion transport limitations of the electrode are determined by the transport pores, where no effect of the potential should be detected.

The essence of the present invention is a hybrid electrochemical capacitor, schematically shown in Figure 1, with a combined mechanism of charge storage, comprising at least two electrodes 2, 5 separated by an ion conducting separator 3. These elements are covered by a two-piece metal cap 7, the two pieces being sealed by sealing insulator 4. Current collector layers 6 are extended between the electrodes and the respective inner wall of the metal cap 7.

The negative electrode 5 performs the charge storage in the electrochemical double layer at the solid/liquid interface and is made of a nanoporous carbon material.

The positive electrode 2 realising the charge storage mechanism through a reversible electrochemical redox reaction taking place in the mono- or poly-molecular layers of products resulting from interaction of electrode material with electrolyte, is made out of a porous material, comprising at least a metal from the following group: nickel (Ni), cobalt (Co), lead (Pb).

Said porous metal can be made in several different ways. One preferable way being the thermochemical treatment of a NiTi alloy in chlorine gas at elevated temperatures.

Another preferable way being the electrochemical treatment of a mckel aluminium alloy (formed by annealing of a bi- or tri-metal foil) in a basic solution of potassium hydroxide. Yet another preferable way being firstly an oxidation of a metal or metal-alloy substrate providing a porous surface oxide layer and secondly, reducing the porous surface -oxide layer to a porous metal layer.

Compared to the use of nanoporous carbon electrodes in a double-layer capacitor, the replacement of the positive electrode by a redox electrode provides a system where:

1) ion transport hindrance and thereby internal resistance of the device will be decreased. Thus, retaining one nanoporous carbon electrode with its high energy density, while lessening the effects of the high internal resistance of the nanoporous carbons.

2) the working voltage of the device can be increased compared to a double-layer capacitor,

3) the asymmetric electrochemical double-layer behaviour of nanoporous carbons leads to that they are better as negative double-layer electrodes than as positive ones.

By designing a hybrid capacitor with the capacitance of the positive electrode being much higher than that of the negative double-layer electrode, a capacitor device can be obtained which has a energy storage density more than 4 times higher and a maximum extractable power density which is more than 2 times higher than a what is obtained from a similar double-layer device made of two similar nanoporous carbon electrodes.

A preferred way of producing said nanoporous skeleton carbon is by:

1) forming a work-piece with transport porosity using particles of a carbide or carbides of elements from III, IV, V and VI groups of the Mendeleyev's Periodic System, in the form of a rigid carbonaceous skeleton network containing in its structure particles of a carbide or carbides selected from the said groups and arranged in a predetermined order providing formation in the subsequent steps desired transport porosity and nanoporosity by sizes, volume and distribution of pores throughout the volume of the article.

2) formation of nanoporosity throughout the volume of the work-piece obtained in the first step by thermochemical treatment of the said work-piece in gaseous halogens, such as chlorine, at elevated temperatures in the range of 350 to 1200°C, preferably 500-1100°C. -

In the first step particles of chosen carbide or carbide powders are formed into an intermediate body with a porosity in the range of 30-70% by any known method, e.g. by pressing with or without a temporary binder, slip casting, tape casting. The size and distribution of the transport pores can be controlled by selecting appropriate particle sizes and particle distribution. The degree of packing due to the forming process will of course also influence the porosity of the work-piece.

The subsequent step of forming which results in the production of a work-piece with high mechanical strength and a desired transport porosity, can be treating of the intermediate body in a medium of gaseous hydrocarbon or hydrocarbon mixtures at a temperature above their decomposition temperature.

It is possible to use natural gas and/or at least a hydrocarbon selected from the group comprising acetylene, methane, ethane, propane, pentane, hexane, benzene and their derivates.

Duration of treatment in said medium is controlled by measuring the mass of the article. When the mass has changed by at least 3%, the strength is already sufficient for use of the article as a capacitor electrode.

Another way of forming the work piece when using SiC as a starting material, is by infiltrating the pyrocarbon deposited intermediate body with liquid silicon at 1500 - 1700 °C, thus, converting also the pyrocarbon into SiC.

In order to form nanoporosity the obtained workpiece is subjected to thermochemical treatment by chlorine at 500-1100°C. Nanoporosity is formed at removal of volatile chlorides of carbide-forming elements in accordance with reaction:

E]_iCf+ (km/2n) Cl2 → k/n Ε_nC\_m +f (1) Where E^Cf - primary carbide; k,f n, m - stoichiometric coefficients. The treatment is carried out until the mass change of the work-piece has stopped.

A finished electrode produced by the described method has a predetermined shape and size, and its structure is a porous carbon skeleton with a transport porosity of 10-55% obtained in the step of forming of the work-piece and a nanoporosity of 15-55%. The electrode comprises one or several types of nanopores and each type is being characterised with narrow distribution by size. The type of nanopore depends on the type of carbide used for the particles forming the workpiece. The carbon content in the electrode is more than 95%wt, preferably 99%wt, i.e., the obtained article consists practically of pure carbon and has considerable strength and high electrical conductivity allowing to increase its life-time and also to decrease the amount of leakage currents occurring in the capacitor due to electrode impurities.

Such a way of producing a nanoporous skeleton carbon is known from WO 97/20233, WO 98/54111 and US 5,876,787.

Other methods of producing nanoporous carbon can of course be used but the above mentioned method is preferred due to that it yields a nanoporous skeleton carbon with high purity and high electrical conductivity.

In one preferred embodiment the positive electrode is a porous nickel electrode made by thermochemical treatment in halogen gas of a nickel alloy. In accordance with the patent application PCT/EP98/06106 the production steps are:

1. Producing of an intermediate body of nickel and modifier. As modifier, elements from III, rv, or V group of the Mendeleyev's Periodic System of Elements, and more preferably Ti, B, Si and P are used. An intermediate body is produced by metallurgical, chemical and physico-chemical methods (alloying, chemical interaction, spraying, diffusion processes, etc.) Shaping of the intermediate body can be made by methods of rolling, deformation, etc. 2. Treating of the intermediate body in a flow of halogen (or of mixtures with inert gas) at elevated temperatures. In this case, conditions of the process are chosen so that to provide high forming rate of gaseous halogens during interaction between halogen and modifier, however rate of interaction between halogen and nickel is very small. The duration of the process depends on the type of modifier and also on the goal. For example, when the process is short in time it is possible to produce a porous nickel material only on the surface of an intermediate body and the content of the intermediate body in the centre of the sample practically remains the same. Thus, a material containing both porous and nonporous parts is produced. A similar material also can be produced if a layer of nickel-modifier alloy is deposited on the surface of nickel. In this case, porosity will be formed only in those parts of the intermediate body, which contain modifier.

3. In a series of cases, for removal of halogenide impurities out of the final body a step of heat treatment in a reducing (air) or inert (argon, etc.) medium or in vacuum can be used.

Following the sequence of the stated technological steps a porous nickel material is produced in the parts of the intermediate body where halogenization took place.

The produced porous metallic body has good physico-chemical properties, in particular high electrical capacitance in electrolyte solutions, and hence such bodies can be effectively used as electrodes for charge accumulation and storage.

For example, as intermediate body the following ones can be used: Foils prepared from Ni-modifier alloys Ni foil with sprayed Ni-modifier alloys

Ni foil + introduced Si (or B, etc.) to produce NiSi (or NiB., etc.) film on the surface Ni-modifier compound films produced by electrochemical deposition

One of the advantages of this method is that in a bipolar design concept, the carbon electrode and the nickel electrode can be exposed to thermochemical treatment in a halogen gas, such as chlorine, at the same time. In another preferred embodiment the positive electrode is a porous nickel (Ni) or c bajt (Co) electrode made in accordance with the Russian patent application (appl. No. 9811373). The electrode consists of a backing layer made of a material chemically and electrochemically inactive in the electrolyte and an electrochemically active layer being obtained by means of chemical and/or electrochemical treatment in solutions of acids, salts or alkalis of a Ni-alloy being deposited or formed on the backing layer. Said alloy can be formed on one or both sides of said backing layer. Said alloy satisfying the formula:

l(l-χ)M₂(_x) (2)

Where: 0.4 < x < 0.97

Mi - loosening metal from the group: aluminium (Al), zinc (Zn), tin (Sn), alkali and alkali-earth metals, or their combinations,

M2 - a metal from the group: nickel (Ni), cobalt (Co) or their alloys, or an alloy of at least one metal out of this group with one or several additional alloying elements from the group: silver (Ag), lanthanum (La) or lanthanides, molybdenum (Mo), tungsten (W), manganese (Mn), vanadium (V), titanium (Ti), bismuth (Bi), antimony (Sb), iron (Fe).

Preferably, the backing layer of said positive electrode should be within 5 to 150 μm, more preferably from 10 to 50 μm, and the thickness of the electrochemically active layer should preferably be within 5 to 100 μm.

Said backing and alloy layer can preferably be formed by annealing a bi- or tri-metal foil in an inert gas at elevated temperatures.

Finally, said positive electrode is obtained by means of chemical and/or electrochemical treatment of said backing and alloy layer in aqueous solutions of sulphuric or phosphoric acids of 1 to 30% mass, or by means of aqueous solution of potassium, sodium or ammonium salts of chemical and/or electrochemical treatment of said backing and alloy layer in aqueous solutions of potassium, sodium or ammonium salts of both organic and inorganic acids, or by means of chemical and/or electrochemical treatment of said backing and alloy layer in aqueous or mixed aqueous-organic solutions of alkalis of 1 to 70% mass. The treatment leads to that the loosening metal is completely or partially removed.

In yet another preferred embodiment the positive redox electrode is obtained by a method of forming a porous metal layer of open porosity from an electrically conducting substrate, consisting in that on the electrically conducting substrate first a surface layer of intermediate oxides is formed, which then is converted to a layer of the final composition in the form of a porous metal layer of open porosity. Formation of the porous intermediate oxide surface layer on the electrically conducting substrate is made by porous oxidation (producing pores) of the substrate initial surface layer material, the initial surface layer being a dense or porous metal or metal-alloy. Finally, the porous intermediate oxide surface layer obtained on the substrate is being reduced to metal at temperature which is below the sintering temperature for the pore structure of the porous layer.

In some instances of using the claimed method, the porous oxidation of the substrate initial surface layer material is carried out electrochemically in electrolyte solution, plasma-elecfrochemically in electrolyte solution, electrochemically in a molten electrolyte, as well as chemically in the solution. The thereby received intermediate surface layer of a porous oxide is then being reduced to metal chemically in a solution, containing a reducing agent in its composition, chemically in gas medium containing a reducing gas, electrochemically cathodically in a electrolyte solution, as well as electrochemically cathodically in a molten electrolyte. As a substrate it is possible to use a sheet or a foil of: dense metal, bimetallic element made of dense metals, an element made of porous metal as well as an element of a dense metal with surface layer made of porous metal. As for the substrate materials it is possible to use metals from the following group: zinc (Zn), lead (Pb), copper (Cu), silver (Ag), iron (Fe), nickel (Ni), cobalt (Co), cadmium (Cd), bismuth (Bi), antimony (Sb), tin (Sn), titanium (Ti) and their alloys. Thus, formed on a substrate the surface layer of a porous oxide contains in its composition one or several metals out of group : zinc (Zn), lead (Pb), copper (Cu), silver (Ag), iron (Fe), nickel (Ni), cobalt (Co), cadmium (Cd), bismuth (Bi), antimony (Sb), tin (Sn), titanium (Ti). Practically, the porous oxidation procedure is carried out, for example, by dipping the substrate in electrolyte (solution or melt) of appropriate composition at a defined temperature and connecting it to the positive pole of power source, to the negative pole of which the auxiliary electrode - cathode is connected (an example of electrochemical oxidation). It is possible to carry out that porous oxidation using alternating current or current of complex form, as well as without current at all (an example of chemical oxidation).

The second operation - the operation of conversion of a porous oxide layer into a porous metal layer - presents in itself a technological operation of reducing the porous oxide to metal which is carried out chemically or electrochemically.

Practically, the operation of electrochemical conversion of a porous oxide layer to porous metal layer is carried out, for example, by means of electrochemical cathodic reduction of porous oxide in an electrolyte solution or melt. Electrochemical cathodic reduction of porous oxide in electrolyte solution is carried out, for example, by dipping the oxidised substrate into an aqueous electrolyte solution at a defined temperature and connecting it to a negative pole of the power source. The positive pole of the power source is connected to the auxiliary electrode - anode. Similarly, the electrochemical cathodic reduction of porous oxide in electrolyte melt is carried out. Chemical reduction is carried out by simply exposing the oxidised substrate to a solution, a melt, a gas medium containing an appropriate reducing agent, which is capable to reduce the oxide to metallic state. It is essential that the temperature during the reduction is below the temperature of sintering the oxide and metal in order that during the reduction, no sintering of the oxide microstructure take place and , correspondingly, of porous metal.

EXAMPLES

Example 1

The advantage of using nanoporous carbon as a negative electrode has been demonstrated by a microelectrode experiment. The nanoporous carbon particles used in this experiment were made from SiC powder (producer: Norton-Lillesand AS, Norway) that had been chlorine treated by a process described in WO 97/20333 to become carbon. This carbon has a very narrow pore size distribution of about 8A.

Description of the experimental set-up - FIG. 2 shows a principal drawing of an experimental microelectrode set up, where 8 is a microscope, 9 is the particle, 10 the separator, 11 the carbon fibre, 12 is the counter electrode, 13 the micro manipulator, and 14 is the reference electrode. The experimental cell was surrounded by a sealed plastic cover and a slight overpressure of a protection gas (N2) was applied. The counter electrode 12 was a piece of activated carbon fibre cloth. The potential of the counter electrode 12 was between 0 and -200 mV (normally around -100 mV) versus the reference electrode 14 (Hg/HgO in 6M KOH). Before an experiment was started the potential of the counter electrode 12 was allowed to equilibrate for at least 24 hours. For experimental simplicity the counter electrode 12 served as a reference during the experiments. The charging/discharging of the nanoporous carbon particle 9 did not have any significant influence on the potential of the counter electrode 12. The electrolyte was 6M KOH and the experiments were all conducted at room temperature.

Both the current and the potential of the particle 9 were probed through the carbon fibre 11 contact. The current was determined as a potential drop over a 1 kΩ resistor (R\). The current source was a standard 1,5 V R20 battery. The data was collected by a digital multimeter (PREMA 5017) connected with a PC. The resolution of the multimeter was 10 nV at a minimum sampling time of 2 seconds. The resistance of the carbon fibre 11 (0,5-3 kΩ) was measured ex-situ, before starting each experiment.

The experimental set-up can be regarded as a double layer capacitor with the nanoporous carbon particle 9 being much smaller than the carbon fibre counter electrode 12. The cell was used to conduct potential step experiments where the potential is instantaneously changed and the current response is monitored. The potential step experiments were conducted by opening for charging or closing for discharging the contact (Ci). Before recording the nanoporous carbon particle 9 was cycled and allowed to equilibrate at least twice. A set of experiments were conducted where a 70 μm particle was subsequently charged: +500 to +600 mV, discharged: +600 to +500 mV, charged: 0 to +100 mV, discharged: +100 to 0 mV, charged: 0 to -100 mV, discharged: -100 to 0 mV, charged: - 500 to -600 mV, and discharged: -600 to -500 mV. The multimeter was first set to a sampling time of 2 s, but for the negative side the experiments were repeated with a sampling time of 0.2 s, the particle being charged: -500 to -600 mV, discharged: -600 to -500 mV, charged: 0 to -100 mV, discharged -100 to 0 mV. The whole set of experiments were conducted within 4h (see Fig. 3).

Fig. 3. shows how the initial current, i₀, increases as the starting potential decreases. It can be concluded that a more than 5 times higher current can be obtained by a 100 mV potential step at a potential of -500 mV compared to at +500 mV.

Example 2.

A sample of porous nickel is produced according a method described in patent application PCT/EP98/06106. The intermediate body is produced by cold rolling of NiTi alloy containing 45 %wt Ti and 55 %wt Ni. A foil of thickness 200 microns and size 25 x 20 mm were prepared by the described way. The foil was chlorinated in a chlorine flow (0.3 1 min). Heat treatment in chlorine was carried out at temperature of

400 C until the decrease of mass was 22 wt %. The produced sample was used as a cathode of 020mm (positive electrode) of a hybrid capacitor cell.

As anode an electrode of 020mm, h=lmm of carbon nanoporous material produced according to US 5,876,787 was applied.

Electrolyte in the cell was 25 % aqueous solution of KOH. The cell was charged by DC 10 mA and then discharged through external resistor 100 Ohm. The obtained charging- discharging curve is presented in Fig. 4. As the figure shows the electrode with porous nickel yields high efficiency for energy accumulation. The exhibited electrical capacitance of the cell is 21 F, while a capacitance of the same cell with carbon cathode produced according to US 5,876,787 is 10 F. Example 3.

One positive porous nickel electrode was made according to a procedure described in a Russian patent with appl. No. 9811373. The starting material was a bi-metal foil of Ni (0.05 mm) and Al (0.05 mm). The foil was annealed in argon atmosphere at 625°C during 3.5 hours. Finally, it was subjected to an electrochemical anode treatment in 30%wt aqueous solution of KOH, at a current density of 25 mA/cm² and a temperature of 80°C during 80 minutes, during which the Al was removed. The formed Ni-electrode consists of a dense backing layer (-0.03 mm) and a porous active layer (-0.05 mm).

The produced sample was used as a cathode of 020mm (positive electrode) of a hybrid capacitor cell. As anode an electrode of 020mm, h=lmm of carbon nanoporous material produced according to US 5,876,787 was applied. Electrolyte in the cell was 25 % aqueous solution of KOH.

In a comparative experiment two nanoporous skeleton carbon electrodes, similar to the one used in the hybrid, were assembled as a double-layer capacitor. The carbon electrodes were coated with nickel (up to 7 μm thick layer) on one side to decrease the contact resistance. The discharging was conducted through an external resistor.

The double-layer capacitor and the hybrid capacitor were charged and discharged between 0.4-0.8 V and 0.7-1.4 V, respectively, the charging capacitances are presented in Table 1.

Table 1. Comparison table

The capacitors of Example 2 and Example 3 were electrochemically tested using a Russian test equipment named IEK-5. The IEK-5 instrument is designed, for measurement of capacitance at charging with constant current at any part of the charging curve.

Example 4

The advantage of using a nanoporous carbon over an activated carbon cloth in a hybrid capacitor, is illustrated by the following example.

A porous nickel foil was prepared according to example 3. Two circular electrodes of 3 cm² area were cut from the nickel foil.

A nanoporous carbon electrode was prepared from a SiC precursor powder of about 1 μm particle size, pressed to a work-piece tablet of 1.4 mm thickness. Pyrocarbon was deposited on the work-piece using natural gas at 820°C for 5h. Chlorination was conducted at 1000°C for 3h. The pyrocarbon content of the final electrode was about 25%wt, the total porosity was about 65% and the nanoporosity was about 30%.

An electrode of activated carbon fiber cloth was cut out from a commercial carbon fiber cloth (Institute of Sci. & Tech., Electrostal of Moscow, type: TSA) of 0.5 mm thickness. Two hybrid capacitor cells of 3 cm² were made using the one nickel foil as positive electrode and the two different carbons as negative electrodes in each cell. The carbon electrodes were coated with nickel (up to 7 μm thick layer) on one side to decrease the contact resistance. The separator was 0.1 mm thick.

The capacitors were tested at 10 mA. The capacitances and other characteristics of the capacitors are given in Table 1. Table 2. Characteristics of capacitor cells.

The higher specific capacitance of the hybrid using a nanoporous carbon is due to the higher specific capacitance of the nanoporous electrode.

Examples 5-7 presents different redox electrodes suitable as positive electrodes in the present invention. In these examples the roughness coefficient C_R of the obtained porous metal layers is defined by means of correlation of cyclic voltammagrams obtained of these layers, with voltammagrams obtained on a smooth (mechanically or electromechanically polished) sample of metal which is of the same composition, as that of the porous layers. The electrolyte and the range of cycling potentials for each metal were selected in such way, that the reversible redox reactions of formation by reduction of the thin, surface mono or polymolecular oxide(hydrooxide), layers took place in the selected potential range. Oxidation and reduction current of these layers depends on the rate of potential scanning and on the value of true surface of a sample. With fixed scanning rate (usually 10 mV/s) the redox current is propotional to the true surface of a sample. Assuming that a polished sample has the true surface which coincides with geometrical one, we receive the formula for defining the roughness coefficient C _R.

C _R = i_R / i _s , ( 3 )

Whe i_R - regular current density (for example, in maximum or minimum) of cyclic re: voltammagram of volume-porous layer, is - corresponding current density of voltammagram of smooth sample.

Further calculation of specific surface Ss_P doesn't present great difficulty :

S_sp [m²/ m³] = C _R / δ [μm], (4 ) where: δ [μm] - thickness of porous metal layer, μm. Example 5. _tr

A 0,5 mm thick foil of a cobalt alloy (87% mass) with tungsten (13% mass) was electrochemically oxidated in a molten mixture of alkalies: KOH - NaOH (ratio 1 : 1 mol.) at temperature 250 °C during 60 min at voltage of 1,2 V. The cathode is a plate out of steel X18H10T. Current density was decreasing from 10 mA/cm² to 3 mA/cm² during 20 min, further it stayed approximately constant, equal to (2,5 - 3) mA/cm². As a result of the oxidation 25 - 27 μm thick porous oxide layers were formed on both sides of the foil.

The cathode reduction of porous oxide was carried out in 3% aqueous KOH solution at temperature of 450°C and current density of 5 mA/cm². As a result of the reduction a porous metal layer of 25 μm thickness was received, having the following composition: Co - 95%, W - 4,5%, O - 0,4%, H - 0,1% mass. Coefficient of roughness C _R = 920, specific surface S_sp = 37 m²/cm³.

Example 6.

A 0,5 mm thick foil of nickel having a 50 μm thick layer comprising of an alloy of : Ni - 75%, Mo - 10%, Al - 15% which was deposited by means of a plasma deposition technique, was electrochemically oxidated in aqueous solution of KOH (35% mass) at temperature 80 °C. Cathode is a plate out of steel X18H10T. Current density was maintained constant at 25 mA/cm until reaching a voltage of 1.6 V. The oxidation time was 200 min. In these conditions the nickel foil was not oxidated and the layer of the above mentioned alloy turned into a layer of porous oxide of 60 μm thickness.

By cathode reduction in aqueous solution of KOH (30% mass) at temperature of 40 °C, current density of 10 mA/cm² during 150 min the oxide layer was reduced to porous metal of 55 μm thickness. The porous metal composition is: Ni - 92%, Mo - 6%, Al - 1%, O - 0,8%, H - 0,2% mass. Coefficient of roughness C _R = 5700, specific surface S_sp = 104 m²/cm³. Example 7.

V A porous lead electrode, obtained by sintering a powder of particles with sizes (100 -

150) μm, 1 mm thick, with 40% porosity was anodically oxidised in an aqueous solution of NaOH (3% mass.) at room temperature and a current density of 50 mA/cm² during 1 hour. Auxiliary electrode - cathode - was a lead plate. As a result of oxidation each of the particle of the porous lead electrode was coated with a porous lead oxide layer PbO₂ • nH₂O of 10 -15 μm thickness.

After the cathodic reduction in aqueous solution of H₂SO (5% mass.) at 35 °C temperature, 2 V voltage during 2,5 hours (auxiliary electrode - anode- lead plate) the lead oxide porous layer of lead particles through the whole electrode thickness was transformed into metallic lead porous layer. As a result, a sample of porous lead with two-level porous structure was obtained : macrostructure, due to initial porosity of originally sintered electrode, and microstructure due to the porous metal layer, which envelops every metal particle of the macrostructure.

The roughness coefficient of the initial porous lead electrode was C _R = 150 and the specific surface S_sp = 0,3 m²/cm³. After oxidation and reduction C _R has increased almost 100 times, reaching the value of C _R = 14000, specific surface has increased up to value of S_sp = 28 m²/cm³.

Claims

CLAIMSV -

1. A hybrid capacitor capacitor including a positive (2) and a negative electrode (5), separated by a separator (3), the electrodes and the separator being saturated by aqueous electrolyte, characterized by a negative electrode (5) made of nanoporous carbon, essentially made from carbide precursor, the majority of the nanopores having a size less than 5 nm, preferably less than 2 nm.

2. The hybrid capacitor according to Claim 1, characterized in that the carbon content in the negative electrode (5) exceeds 95 %wt.

3. The hybrid capacitior according to Claim 2, characterized in the nanoporous carbon being a nanoporous skeleton carbon.

4. The hybrid capacitor according to Claim 3, characterized in that the amount of transport pores in the negative electrode (5) is 10-55 % of the total volume of the electrode and the amount of nanopores is 15-55 %.

5. The hybrid capacitor according to any one of Claims 1-4, characterized in that the nanopores in the negative electrode (5) are of the same type having a narrow size distribution.

6. The hybrid capacitor according to any one of Claims 1-4, characterized in that the nanopores in the negative electrode (5) are of different types, each type having a narrow size distribution.

7. The hybrid capacitor according to any one of Claims 1-6, characterized in that the positive electrode (2) is a porous Ni electrode made by thermochemical treatment of a TiNi-alloy.

8. The hybrid capacitor according to any one of Claims 1-6, characterized in that the positive electrode (2) is a porous metal made by chemical and electrochemical treatment of a metal or metal-alloy.

9. The hybrid capacitor according to Claim 8, characterized in that the positive elecfrode (2) is a porous metal or metal-alloy, comprising at least a metal from the following group: Ni, Co, Pb.

10. The hybrid capacitor according to any one of Claims 1-6, characterized in that the positive electrode (2) is a porous Ni electrode made by chemical and electrochemical treatment of a NiAl-alloy.