An activated carbon is known as adsorbent, is in an important position for various industrial field such as food, medicine, housing, automobile, chemical industry. Also the activated carbon is considered as adsorbent for stocking gas mainly containing natural gas which is substitute for petroleum resource such as gasoline.
Natural gas mainly contains methane or ethane. Generally speaking, an activated carbon being larger in BET specific surface area and being larger in volume of micro pore having pore diameter of 1 nm or less advances more adsorption of gas having small molecular size such as methane, ethane, or other hydrocarbon gas having low molecular weight, or hydrogen. Adjustment of activated carbon structure such as pore diameter, pore volume, and pore configuration is researched to improve adsorption property of activated carbon, i.e. to enlarge selectively adsorption quantity for gas having set molecular size. However, activated carbon having larger specific surface area is lower in a packing property into tank.
JP-A-06-55067 discloses an adsorbent obtained by carrying metallic simple substance or metallic compound, which has chemical adsorption ability for methane, on an activated carbon having BET specific surface area of approximately 750 m2/g or more.
JP-A-07-155587 suggests an activated carbon having a pore volume of 2.5 to 4.0 cm3/g, an average pore diameter of 2.1 to 4.0 nm, and specific surface area of 1600 to 2500 m2/g. JP-A-07-155587 discloses the activated carbon can be obtained by activating pore-containing carbonaceous material being at least 100 m2/g in a specific surface area with metallic compound in the 8th group.
In JP-A-2001-122608 is described an activated carbon obtained by activating with alkali metallic compound. The activated carbon has volume of micro pore, which has pore diameter of approximately 0.7 to 1.2 nm, of 50% or more to the total pore volume.
An activated carbon is useful for material of a polarizable electrode in an electric double layer capacitor. An electric double layer capacitor is quickly chargable and dischargable, tough against overcharge or overdischarge, long in life for cause of no chemical reaction, workable under wide range of temperature, and easy on the environment for cause of heavy metal free, and has other property which battery does not have. An electric double layer capacitor is mainly employed for a memory backup power supply and so on. An electric double layer capacitor is researched in application to electric power storing system or engine assist for hybrid car by assembling with battery such as solar battery and fuel cell.
An electric double layer capacitor comprises a pair of polarizable electrodes as cathode and anode made from material containing an activated carbon, in which the pair of polarizable electrodes set facing each other in solution containing electrolytic ion, and a separator is interposed between the polarizable electrodes. Impressing of direct current voltage into the electrodes causes drawing anion in solution near cathode (positive electrode) and drawing cation in solution near anode (negative electrode), making an electric double layer at the interface between electrode and solution. And the electric double layer supplies electric energy.
Therefore, using of activated carbon having larger specific surface area have been researched to make larger electric double layer. The using of this activated carbon enlarges an electric capacity per mass (F/g), but density of electrode is lowered. Consequently an electric capacity per volume (F/cm3) is not as large as expectation.
There is suggestion in JP-A-11-317333, JP-A-2000-68164, and JP-A-2000-68165 that an activated carbon, which is made by activating with alkali metallic compound, having micro-crystalline structure similar to graphite structure, and having distance between layers of 0.365 nm to 0.385 nm, is used as material for polarizable electrode. An electric double layer capacitor, which the activated carbon is employed as material for polarizable electrode, has large electric capacity per volume (F/cm3) and thus the activated carbon may be good material in view of an electric capacity per volume.
In producing method for activated carbon comprising a step of activating coke with potassium hydroxide (KOH) to form pore, it is difficult to control suitable pore diameter of activated carbon for electric double layer capacitor workable under cryogenic temperatures.
JP-B-319563 or JP-A-2004-182511 produces an activated carbon having meso-pore being within 2 to 50 nm in a pore diameter obtained by activating raw material with steam, in which the raw material is obtained by mixing pitch having low softening point and metallic compound.
JP-A-2004-175660 filed by the applicant shows an activated carbon having BET specific surface area of 10 to 2000 m2/g containing alkaline earth metallic compound inside particle. It is shown that the activated carbon having a pore volume of 0.02 cm3/g or more, in which the pore volume is within 2.0 to 5.0 nm in a pore diameter under pore size distribution as calculated by BJH method, can enlarge an electric capacity of electric double layer capacitor.
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