The invention relates to an electrical capacitor. Such capacitors are known in the art and typically comprise a first and a second electrode of a conducting material, e.g., a metal, an electrolyte or carbon, both electrodes being interconnected by an insulating medium. In the state of the art normally the medium is a thin polymer, an oxide formed on the metal, or the electrochemical double-layer, separating both electrodes.

Capacitors with very high capacitance per mass or volume are typically realized by forming an electrochemical double-layer, called the Helmholtz double-layer near the electrodes. Since in a capacitor of this kind the double-layer is only formed in the interface region of electrode and electrolyte, the capacity of such capacitors can only be increased by increasing the surface area of the electrodes.

Accordingly it is an object of the invention to overcome the drawbacks of such capacitors and to provide a capacitor for storing electric charge that has a higher capacity compared to the known construction, while avoiding liquids and using fewer, more robust and inexpensive constituents.

According to the invention this object is solved in a new capacitor in which the medium that interconnects the two electrodes is formed by nanoparticles of a semiconducting material.

It has been found by the inventors that surprisingly such a construction leads to a significant increase of capacity. Capacitors may be produced having a capacity of 50 to 100 Farad per gram of nanoparticles used for interconnecting the electrodes. It is expected that the increase of capacity is due to electron-transfer-reaction in the huge surface area of the nanoparticles. According to investigations until now it is the medium and its structure / porosity that provides the high capacitance instead of the electrodes.

In a preferred embodiment the size of the nanoparticles within the medium is less than 200 nm, preferably less than 100 nm, in particular in the range of 2 to 30 nm.

Preferably the semiconducting nanoparticular material is chosen from a doped semiconductor or insulator, in particular silicon, silicon-containing alloys or compounds or a silicide of a transition element. According to initial experimental results, FeSi ₂ is most preferred.

A medium comprising such a material, in particular exclusively only one single material, may be formed as the residue of a dispersion after total evaporation of a solvent, the dispersion comprising the nanoparticular material and the solvent.

Accordingly a method for manufacturing a capacitor may be provided according to which a dispersion is first produced, the dispersion comprising nanoparticles of a semiconducting material (as mentioned before) in a solvent. The dispersion is then applied to at least one of two electrodes and the two electrodes are interconnected by means / via the dispersion.

For example the electrodes are covered simultaneously in case they are

positioned side by side. In case the medium is to be positioned between to electrodes for example only one of the electrodes is covered with the dispersion and the second electrode is brought into contact with the covered one. It would be also possible to cover both electrodes with the dispersion and to put the electrodes together by contacting their covered sides. This can be done in a non-parallel arrangement of the electrode in order to assure escape of air.

After applying the dispersion it is dried, in particular by evaporating the solvent. This may be done in ambient temperature or in increased temperature to speed up drying. Accordingly after drying a solid-state bulk or porous layer of nanoparticles is formed that interconnects the two electrodes. After this drying step the capacitor is ready for use.

In a possible embodiment the electrodes and/or the dispersion may be applied by printing, for example inkjet printing or any other suitable printing technique in case the dispersion is diluted to be printable in this manner. In another example the dispersion may be applied with a doctor blade, in particular in case the dispersion has a paste like consistency.

Another possibility may be pressing a nanoparticle powder down onto one or between the two electrodes.

A capacitor according to the invention has pores or free spaces between the nanoparticles of the chosen material. It has been found by the inventors that it is advisable to fill the free spaced with vapor of a suitable compound, in particular with a mixture of air and vapor of a polar molecule, especially water, preferably to achieve a relative vapor pressure of 100%. In such a case the capacity may be increased significantly.

In a possible embodiment this can be done by arranging the two electrodes and the nanoparticles in a watertight housing and positioning a reservoir of water in the housing. Depending on the temperature more or less water will be evaporated, but preferably in any case 100 % relative humidity is realized.

According to this fact it is also possible to use a capacitor of the invention in case that it is not encapsulated for measuring the humidity in the surroundings of the capacitor since its capacity is significantly dependent on the humidity. Due to this dependency its instant capacity / capacitive response is a measure for humidity.

The electrodes of the capacitor may have different possible configurations. In a first preferred embodiment the two electrodes are plate electrodes being positioned in a distance to each other and the nanoparticles being positioned between the electrodes. Particularly in such a construction the distance of the electrodes may be less than 100 micrometers, in particular in the range of 1 to 50 micrometers.

In a second preferred embodiment the two electrodes are positioned on the same substrate in a distance, in particular in the same plane of a planar substrate, the nanoparticles covering the two electrodes. The nanoparticles may be forming a layer on top of the two electrodes, particularly with a thickness of less than the electrode spacing, in particular in the range of half of the electrode spacing.

In order to provide a large overlap between the electrodes itself, each electrode may comprise several parallel spaced stripes. In such a case the stripes of the two electrodes may be interdigitated, in particular by positioning a stripe of one electrode in the space between two stripes of the other electrode. This forms a configuration of two interlocking electrode combs.

In a third preferred embodiment (3D equivalent of second embodiment), the electrodes consist of two interlocking stacks of parallel plates.

Two embodiments of the capacitor are shown in the figures.

According to figure 1 two electrodes e1 and e2 are provided on the same substrate t. The electrode may be composed of a conducting material, in particular metal. The electrode may be deposited on the substrate by vapor deposition or may be formed by etching a closed conducting layer or by a suitable printing technique.

Both electrodes e1 , e2 comprise several stripes st. The stripes of each electrode are all connected and are spaced parallel to each other. A comb-like shape of each electrode e1 and e2 is given. The two electrodes are positioned side by side in such a manner that a single stripe of one electrode is positioned between two stripes of the other electrode.

The so formed surface area of the two electrodes is covered with a layer s of nanoparticles of FeS ^' i2 in a preferred embodiment or a layer s of another semiconducting or doped insulating material. The layer s may have a thickness of less than the electrode spacing, particularly half of the electrode spacing. The distance between two stripes of the two electrodes is preferably in the range of a few microns to a few hundred microns.

Figure 2 shows another embodiment according to which the plate-like electrodes e1 and e2 are positioned in a parallel configuration. The entire space between the plates is filled with nanoparticles of FeSi2 in a preferred embodiment or with another semiconducting material. The distance between the plates / electrodes is preferably in the range of a few microns to a few hundred microns.