1.  Field of the Invention
The present invention relates to a supercapacitor and an electrochemical apparatus for water purification using the same, and more particularly, to a supercapacitor having high power output and energy density and an electrochemical apparatus that can be efficiently used for water purification using the same. 
2.  Description of the Related Art
Today, personal information as well as commercial information has been highly valued.  Accordingly, there is a need for highly reliable information communication systems.  Also, stable electrical energy sources are necessary.  Further, as markets for compact or portable electronic devices, such as notebook computers, and cellular phones have rapidly grown, conditions of the batteries for these electronic devices such as high-energy density, a long life, ultra slimness, lightweight, stability and ecological affinity have been strongly required.  In order to meet these requirements, energy storage type capacitors, which are an energy source system ensuring stable electrical energy supply, have recently been focused on. 
Energy storage type capacitors include a mechanism capable of functioning as a conventional capacitor for storing energy and are energy storing devices capable of crosslinking batteries and capacitors.  The energy storage type capacitors which have intermediate characteristics of both condensers and secondary batteries with regard to energy density and power density are capacitors having shorter charge time, longer lifetime, and higher power output compared to secondary batteries, and have an energy density at least 100 times that of conventional condensers. 
That is, the energy storage type capacitors have advantages of power characteristics of condensers and high energy storage characteristics of secondary batteries. 
In addition, positive and negative charges are distributed at very short intervals in an interface between a porous activated nano-carbon solid electrode and an electrolyte solution. 
When the porous activated nano-carbon solid electrode is positively charged, negative ions of the electrolyte solution are distributed to compensate for the positive charge.  On the other hand, when the porous activated nano-carbon solid electrode is negatively charged, positive ions of the electrolyte solution are distributed to compensate for the negative charge.  The arrangement of charges results in an electrical double layer, which is formed by a non-faradic reaction without electron transfer between the porous activated nano-carbon electrode and electrolyte ions. 
The energy storage type capacitor is an energy storage device that converts chemical reaction into electrical energy using electrostatic orientation (electrochemical double layer) of ions in the interface between the electrode and the electrolyte and stores the electrical energy.  Thus, capacitance (C) of conventional capacitors is proportional to a contact area, and is inversely proportional to a distance between positive charges and negative charges, i.e., the thickness of a dielectric layer.  Since the energy storage type capacitor uses nano-scale porous carbon as an electrode material on the surface thereof, the surface area of the energy storage type capacitor dramatically increases.  Further, while the thickness of the dielectric layer of conventional capacitors is measured in micrometers, the thickness of the dielectric layer of the energy storage type capacitor is only around 10 Å of ionic layer, and thus capacitance (C) is of a ultra high capacity.  Such an energy storage type capacitor is also referred to as a “supercapacitor”. 
The energy storage type capacitor is classified into two types according to its operating principle, an electrochemical double-layer capacitor (“EDLC”) which stores charges in an electrical double layer of an interface between an electrode and an electrolyte, and a pseudo capacitor which stores charges or electrons with changes in oxidation number of transition metal ions on the surface of a transition metal oxide.  However, although the energy storage type capacitor requires a wide specific surface area because of using activated carbon in the electrode, the available specific surface area for calculation of the actual capacitance (C) is only approximately 20 to 30% of the total specific surface area.  This is related to ion size of the electrolyte to be adhered to the activated carbon and adsorption degree. 
That is, porous activated carbon is classified into three types, microporous activated carbon, mesoporous activated carbon and macroporous activated carbon according to pore size.  The pore size of the macroporous activated carbon is too large for ions in the electrolyte to enter the pores.  Thus, the large number of macropores decreases the specific surface area which is an advantage of using activated carbon.  Thus, power density of the energy storage type capacitor can be improved only by maintaining a mesopores structure which is suitable for the ion size of the electrolyte. 
In conventional EDLCs, both of the cathode and anode use activated carbon, and the cathode adsorbs anions and the anode adsorbs cations.  In ELDC, two capacitors are connected in series, and thus only approximately half of the capacities of the cathode and the anode can be used.  To overcome those problems, a hybrid power system having maximized capacitor capacity has been developed and disclosed in EP 0864166, US 20030035982, and U.S. Pat.  No.  6,195,252. 
Conventional hybrid systems have serious corrosion problems since a KOH solution or an organic solvent is used as the electrolyte solution, and cannot be used to soften tap water, and thus characteristics thereof need to be improved.