WO2002035564A1

WO2002035564A1 - Electrochemical capacitor

Info

Publication number: WO2002035564A1
Application number: PCT/DE2001/003969
Authority: WO
Inventors: Werner Scherber; Cornelius Haas; Mathias BÖHMISCH
Original assignee: Dornier Gmbh
Priority date: 2000-10-27
Filing date: 2001-10-17
Publication date: 2002-05-02
Also published as: DE10053276C1

Abstract

The invention relates to an electrochemical capacitor consisting of a single cell or a stack of single cells, each single cell comprising an electrode (10) and a counter-electrode (20) consisting of an electroconductive or semiconductive material, and an electrolyte (13). The inventive capacitor has the following features: the electrode (10) is made up of a nanostructured film, nanostructured, discreet, needle-shaped elements (12) being electroconductively fixed on a surface (11); the electrolyte (13) is configured in the form of a thin film electrolyte which covers the electrode (10) in the form of a layer and prevents electronic contact between the electrode (10) and the counter electrode (20); the discreet, needle-shaped elements (12) that are coated with the electrolyte (13) are embedded in the counter electrode (20). The invention also relates to a method for producing the inventive capacitor.

Description

Electrochemical capacitor

The invention relates to an electrochemical capacitor according to the preamble of patent claim 1.

Electrochemical capacitors, also referred to in the literature as double-layer capacitors or supercapacitors, are electrochemical energy stores which are distinguished by a significantly higher power density compared to batteries and by an order of magnitude higher energy density than conventional capacitors. They are based on the potential-controlled formation of Helmholtz double layers and / or electrochemical redox reactions with high charge capacity and reversibility on electrically conductive electrode surfaces in suitable electrolytes. Priority potential areas of application with particular economic importance lie, for example, in the areas of electrical traction (motor vehicles) and telecommunications. By intercepting power peaks, the nominal power of the primary energy source can be reduced, the service life and range can be extended and the economy of the overall system can thus be significantly improved.

For the production of the supercapacitor electrodes, the material concepts based on active carbon have prevailed (BET surfaces up to 2000m ² / g), which, in combination with organic electrolytes, currently have the greatest market potential in terms of performance data and costs. There are first products in the small series stage that achieve energy densities of about 3 Wh / kg, for example WO 98/15962 A1. Furthermore, there are a large number of concepts for producing these activated carbon supercapacitors or their electrodes, for example EP 0 712 143 A2, DE 197 24 712 A1. Typically, a maximum of 50 to 100 farads capacity per gram of the active electrode material can be achieved here. With these elements, the ratio of useful energy and storage weight is often still too small for use as peak load storage in order to be used economically in real applications. The optimization the performance data can take place via the capacitor structure (stack design) as well as the actual capacitor electrodes (surface structures and materials).

1 shows the basic structure of such a supercapacitor according to the prior art with the activated carbon electrodes 1, 2, the porous separator 3 and the electronic contacts 4, 5. The entire system is filled with a liquid electrolyte. 2 shows the associated equivalent circuit diagram. It utilizes the formation of Helmholtz capacity at a large geometric surface area of the two activated carbon electrodes (up to 2000m ² / g), so the total capacitance Cg that _it consists of the series connection of the individual electrode capacitances C ^ and C ₂ is obtained. The cutoff frequency of the capacitor is substantially _saturated by the product of C and determines the sum of the design-dependent ohmic resistance loss.

The contact resistance of the electrodes to the current collector of the capacitor housing is given by R _c , the electronic conductivity of the electrode itself by R _E |. The electrolyte resistance R _E s is determined by the conductivity of the electrolyte in the pores of the electrodes, while the separator resistance R _SE P represents the electrolyte conductivity in the separator area and is essentially a function of the porosity and the thickness of the separator. To minimize the loss resistances, an electrode material with good electronic conductivity and connectivity, an electrolyte with good ionic conductivity and a thin separator with high porosity are required.

Activated carbon materials have an extremely high porous surface, but the distribution of pore sizes is very wide and extends down to the range of «1 nm. Since typical Helmholtz layer thicknesses are even up to 2 nm, the Helmholtz storage layer can be used with this electrode material are not completely formed on the surface actually present.

The typically rather sponge-like geometry of activated carbon materials also has a disadvantageous effect on the frequency behavior of the capacitance. Because for the electrolyte it is synonymous with relatively long and narrow paths and therefore inevitably linked to a relatively high electrolyte resistance R _E L. This leads to a reduction in the cutoff frequency of the overall component, ie even at moderate frequencies (typically around 1 Hz), only a fraction of the electrode capacity available with direct voltage can be used.

In addition to the negative effect on frequency behavior already described, a large ohmic resistance also causes dissipative losses, which further restrict the possible uses due to the associated thermal load on the component or, with appropriate design measures (cooling plates, etc.), the usable mass and volume-related Drastically reduce energy storage density.

The object of the invention is to provide an electrochemical capacitor which enables a significant reduction in the totality of the loss resistances.

This object is achieved with the subject matter of patent claim 1. Advantageous embodiments of the invention and a method for producing the capacitor according to the invention are the subject of further claims.

The electrochemical capacitor according to the invention has an electrically conductive or semiconducting electrode which is formed from a nanostructured film in which nanostructured discrete, needle-shaped elements are anchored in an electrically conductive manner on a surface. Nanostructured element in the sense of the present invention refers to a material structure with dimensions of at least one structural dimension in the nanometer range (<1 μm).

The electrolyte is in the form of a thin-film electrolyte which coats the surface of the nanostructured electrode, in particular the surface of the needle-shaped elements.

The discrete, needle-shaped elements coated with the electrolyte are embedded in the counter electrode. The capacitor according to the invention thus has an interdigital structure. Electrode and counter electrode interlock. The thin film electrolyte fills the entire space between the electrode and the counter electrode. A supercapacitor with geometrically significantly shorter charging paths and reduced ohmic loss resistances is thus made possible.

The electrolyte acts as a geometric separator and at the same time prevents electronic contact between the electrode and the counter electrode. The mechanical porous separator according to the known capacitors mentioned above can thus be dispensed with.

The electrolyte is preferably designed as a gel-like or solid thin film. In advantageous embodiments, the layer thickness of the electrolyte is not greater than 1 μm, preferably not greater than 100 nm, in particular not greater than 50 nm.

The thin-film electrolyte must on the one hand electronically separate the two capacitor electrodes from one another (electronic insulator) on the other hand must have a high ionic conductivity and be doped with suitable mobile ionic charge carriers which are required to form Helmholtz double layers on the electrode surfaces.

Special organic or inorganic ultra-thin layers can be used for this, which have become a focus in the development of nanostructured materials in recent years. Often simple self-assembly processes (e.g. self-assembled

Monolayers, SAMs) are used, which means that these monolayers are applied by simply immersing the substrate in the corresponding solution.

An important extension of this self-organization concept consists in the separation of cationic or anionic polyelectrolytes (polymer chains with ionically dissociable groups, lonic soap assembling monolayers, ISAMs) from aqueous solutions [G. Decher, Science 277, 1232 (1997)]. In the case of SAMs, the molecules are bound via chemisorption (e.g. thiol group substrate, covalent bond), this mechanism is advantageous here through physisorption, thus determined purely electrostatic interactions. This approach thus allows decisive advantages in the construction of multilayer systems (alternation of positive and negative polyelectrolytes, so-called layer-by-layer technology) and represents the more general concept for the synthesis of supramolecular architectures. For example, in G. Decher, Adv. Mater. 9, 61 (1997) showed how, from aqueous solution, negatively charge-stabilized gold colloids can be applied to a cationic polyelectrolyte layer by electrostatic interaction and then covered with a cationic polyelectrolyte layer (sandwich multilayer structure).

The layers are characterized by an extremely low defect density, and the layer thicknesses in the range of a few nanometers can be easily controlled via concentration, chain lengths or activities of the end groups.

In recent studies of the physical and chemical properties of these polyelectrolyte layer systems [Han et al, Electrochimica Acta 45, 845 (1999)], very good ionic conductivity and dopability with ionic charge carriers (electrostatic interaction) as well as vanishing electronic conductivity have been demonstrated. The layer system can be applied by simply immersing it in corresponding aqueous solutions and is therefore a preferred exemplary embodiment of a thin-film electrolyte in the electrochemical capacitor according to the invention.

The feasibility of SAMs as only a few nanometers thick dielectric for capacitors is first described in Rampi et al, Appl. Phys. Lett. 72, 1781 (1998). Two monolayers are placed as a dielectric (thickness approx. 9nm) between two drops of mercury as capacitor electrodes, the dielectric of this material and the breakdown field strength are determined and the absence of defects (i.e. freedom from short circuits) is impressively demonstrated.

At this point it should be clearly pointed out that the polyelectrolyte layer system described above does not play the role of an electronically insulating dielectric cum in the dielectric capacitor, but the role of an ionically doped thin film electrolyte in an electrochemical capacitor.

The nanostructured electrode according to the invention made of a film with needle-shaped elements has a large effective surface for forming the Helmholtz storage layer. Their size on a flat metallic surface is typically about 40μF / cm ² in the aqueous electrolyte. The areal density of the nanostructured needle-shaped elements is preferably in the range of 1-500 per μm ² , the diameter of which is preferably in the range of 15-500nm. B. the necessary material stability is guaranteed for metallic structures. The aspect ratio (ratio between height and average diameter) of the nanostructured needle-shaped elements is advantageously larger than 20. The nanostructured discrete needle-shaped elements can either be a solid cylinder, a hollow cylinder (tube) or a solid cylinder with an inner sponge-like porosity for an additional surface enlargement available.

A further advantage of the supercapacitor according to the invention is that the nanostructured electrode film can be produced from any semiconducting or conductive materials such as metals, noble metals, galvanomains (galvanically depositable metals), in particular nickel, gold or conductive polymers, using suitable manufacturing processes.

The production of the carrier foil of the electrode and the growth of the nanostructured elements thereupon can be carried out in one work step when using electrochemical deposition. The thickness of the carrier film is advantageously set between 1 and 20 μm. This guarantees the electrical conductivity, contactability and also the mechanical stability for the construction of a supercapacitor single cell or stack.

In contrast to the known sponge-like electrode structures, the discrete - preferably regular - arrangement of the nanostructured elements of the electrode allows the Helmholtz layers to be formed more quickly and completely on the existing surface and thus a significant improvement in the performance characteristics. Some metal oxides (e.g. Ru0 ₂ ) or conductive polymers allow energy storage in suitable electrolytes through surface redox reactions. The recharging of such redox systems on the electrode surface leads to the formation of the Helmholtz double layer and an additional electrode capacity (pseudo capacity). This property can be achieved in the electrode of the present invention either by a thin (<10 nm) coating with an appropriate redox system (eg Ru0 ₂ ) or by direct formation of the nanostructured elements from precisely this material.

To build up the stack, the individual cells are stacked on top of one another. This creates a series connection of individual capacitor elements via the conductive electrode films without additional contacting steps.

The invention is explained in more detail using exemplary embodiments with reference to drawings. Show it:

1: the basic structure of a classic supercapacitor according to the prior art, as explained in the introduction to the description; 2: the equivalent circuit diagram of a supercapacitor, as explained in the introduction to the description;

3: the schematic diagram of a regularly nanostructured metallic thin-film electrode of a supercapacitor according to the invention;

4: SEM image of an embodiment of a regularly nanostructured metallic thin-film electrode of a supercapacitor according to the invention;

5: schematic diagram for the production of a supercapacitor according to the invention.

Fig. 3 shows an electrode as it is used in the supercapacitor according to the invention. In this embodiment, it consists of a self-supporting film 11 and nanostructured, discrete elements 12 anchored thereon, which are needle-shaped. Discrete in the sense of the present invention means that they are separate elements, each with its own structure, ie not around interconnected elements, as is the case, for example, with a sponge-like structure.

4 shows the SEM image of an electrode for the supercapacitor according to the invention. It consists of a self-supporting metal foil 11 and nanostructured metallic elements 12 anchored thereon. The nanostructured needle-shaped elements are oriented in this embodiment essentially perpendicular to the surface of the foil and evenly distributed over the surface of the foil.

5 shows a schematic representation of the production of a supercapacitor according to the invention with an interdigital structure. The starting point is a nanostructured, in particular metallic, electrode 10 with discrete, needle-shaped elements 12, which are preferably arranged regularly. The thin-film electrolyte 13 is applied to the surface of the electrode 10, in particular to the needle-shaped elements 12, by means of dip coating. As electrolyte 13 e.g. a polyelectrolyte can be used. The distance between the needle-shaped, nanostructured elements 12 is set such that after the polyelectrolyte coating, the remaining spaces can be filled with a metal, so that a conductive, coherent counterelectrode 20 results. The counter electrode thus completely fills the spaces between the needle-shaped elements 12. The needle-shaped elements 12 extend into the material of the counterelectrode and are surrounded on all sides (with the exception of its base surface). Electrode 10 and counter electrode 20 are contacted via corresponding current-conducting contacts 15, 16. 5, the electrolyte layer 13 occupies the entire space between the electrode and the counter electrode. Compared to the capacitors according to the prior art, there are significantly reduced charging paths. An additional mechanical separator is not required.

The following example describes the production of a capacitor according to the invention on a laboratory scale. In principle, similar methods can be used on an industrial scale. example

The anodic oxidation of an aluminum substrate creates a nanoporous oxide film with parallel, continuously cylindrical pores aligned perpendicular to the substrate surface. The pore diameter can be set in the range of 15-500 nm, the surface density of the pores from approx. 1 to 500 per μm ² , and the pore length up to 100 μm. The oxide film is detached from the aluminum substrate, so that a ceramic nanoporous filter membrane is created. This membrane is vapor-coated on one side with a metallic film as a contact electrode. The film thickness is chosen so that the oxide pores are closed. In order to produce nanostructured gold elements on a gold film, the vapor-deposited membrane is contacted and placed in a galvanic gold bath. In the case of galvanic deposition, on the one hand the oxide pores are filled from the vapor-deposited base electrode with the desired nanostructured elements, on the other hand the base electrode is thickened to a metallic film in the micrometer range. The oxide ceramic can then be selectively pickled using wet chemistry, so that the desired electrode film with conductively bonded nanostructured gold elements is produced.

The gold electrode is cleaned in hot ethanol / chloroform (1: 1), rinsed in water and dried and then coated by immersion in an aminoethanethiol / ethanol solution with a SAM, which carries a positive surface charge in neutral and aqueous solutions [Han et al, Electrochimica Acta 45, 845 (1999)]. Then, by alternately immersing in an aqueous Na-PSS solution (+ NaCI) rinsing and immersing in an aqueous PAH solution (+ NaCI), a polyelectrolyte layer system consisting of anionic and cationic polyelectrolyte layers is applied. The layer thickness can be adjusted to approx. 10 nm by the number of layers. For the further coating it is advantageous to finish with a cationic polyelectrolyte layer.

The thin film electrolyte thus formed is doped by electrostatic incorporation of suitable ions which can diffuse into the electrolyte layer from aqueous solution, e.g. B. Fe (CN) ₆ ⁴ (Han et al, see above) To form the counterelectrode, it is advisable to apply negatively charge-stabilized gold colloids (see above) to the last cationic polyelectrolyte layer. These colloids with a sufficiently small diameter (<10 nm) then serve as germ cells for the subsequent electroless metal deposition on the thin film electrolytes. The remaining space is filled with gold so that a coherent metallic counter electrode is created.

The example described in this way uses gold as the electrode material, but analogous methods are also possible for nickel and other metals that can be electrolessly deposited.

Claims

claims

1 . Electrochemical capacitor consisting of a single cell or a stack of single cells, each individual cell comprising an electrode (10) and a counter electrode (20) made of an electrically conductive or semiconducting material and an electrolyte (13), characterized in that the electrode (10) is made of a nanostructured film is formed, in which nanostructured discrete, needle-shaped elements (12) on a surface

(1 1) are anchored in an electrically conductive manner, the electrolyte (13) is designed as a thin-film electrolyte, which covers the electrode (10) in a layer-like manner and prevents electronic contact between the electrode (10) and counter-electrode (20); the discrete, needle-shaped elements coated with the electrolyte (13)

(12) are embedded in the counter electrode (20).

2. Electrochemical capacitor according to claim 1, characterized in that the layer thickness of the electrolyte (13) is not greater than 1 μm, preferably not greater than 100 nm, in particular not greater than 50 nm.

3. Electrochemical capacitor according to one of the preceding claims, characterized in that the electrolyte (13) consists of a gel-like ion conductor material.

4. Electrochemical capacitor according to one of the preceding claims, characterized in that the electrolyte (13) consists of a solid ion conductor material.

5. Electrochemical capacitor according to one of the preceding claims, characterized in that the electrolyte consists of a layer system anionic shear and / or cationic polyelectrolytes.

6. Electrochemical capacitor according to one of the preceding claims, characterized in that the electrolyte (13) is doped with movable ionic charge carriers.

7. Electrochemical capacitor according to one of the preceding claims, characterized in that the areal density of the nanostructured elements (12) of the electrode (10) is between 1 and 500 per μm ² .

8. Electrochemical capacitor according to one of the preceding claims, characterized in that the nanostructured elements (12) of the

Electrode (10) have an aspect ratio greater than 20.

9. Electrochemical capacitor according to one of the preceding claims, characterized in that the nanostructured elements (12) of the electrode (10) have a diameter between 15 and 500 nm.

10. Electrochemical capacitor according to one of the preceding claims, characterized in that the nanostructured elements (12) of the electrode (10) are designed as hollow cylinders or solid cylinders.

11. Electrochemical capacitor according to one of the preceding claims, characterized in that the nanostructured elements (12) of the electrode (10) have an internal porosity.

12. Electrochemical capacitor according to one of the preceding claims, characterized in that the nanostructured elements (12) of the electrode (10) are coated with a redox material or consist entirely of this material.

13. A method for producing an electrochemical capacitor according to one of the preceding claims, characterized by the following method steps: Applying the electrolyte layer (13) to the nanostructured electrode (10) by dip coating;

Filling the spaces between the discrete, needle-shaped elements (12) of the electrode (10) coated with the electrolyte (13), so that a coherent counter electrode (20) is formed.