Electric double layer capacitor

1. An electric double layer capacitor comprising a polarizable electrode layer that contains an activated carbon, a conductive agent, and a binder, wherein an external specific surface area of said activated carbon (defined as a specific surface area calculated from a nitrogen absorption isotherm by a t-plot method by excluding micropores of pore diameter smaller than 20 angstroms), per unit volume of said polarizable electrode layer, lies within a range of 450 to 800 m 2 /cm 3 , and a volume based on inter-particle interstices per unit volume of said polarizable electrode layer lies within a range of 0.05 to 0.12 cm 3 /cm 3 .

2. An electric double layer capacitor electrode as claimed in claim 1 , wherein a relative particle quantity by volume in a mode diameter of a particle size distribution in said activated carbon lies within a range of 10 to 30%.

3. An electric double layer capacitor electrode as claimed in claim 2 , wherein particles contained within a range of ±30% of said mode diameter are 30 to 60% in terms of the relative particle quantity by volume.

4. An electric double layer capacitor electrode as claimed in claim 1 , wherein the external specific surface area of said activated carbon lies within a range of 1000 to 2000 m 2 /g.

5. An electric double layer capacitor electrode as claimed in claim 1 , wherein said activated carbon is formed from a plant-derived natural material.

6. An electric double layer capacitor electrode as claimed in claim 5 , wherein said plant-derived natural material is a coconut shell.

7. An electric double layer capacitor electrode as claimed in claim 1 , wherein said conductive agent is carbon black and is added in an amount not greater than 5% by mass with respect to the total mass of said polarizable electrode layer.

8. An electric double layer capacitor electrode as claimed in claim 1 , further comprising an electrolytic solution which contains polypropylene carbonate as a solvent in an amount not less than 50% by mass with respect to the mass of said electrolytic solution.

9. An electric double layer capacitor electrode as claimed in claim 1 , wherein capacitance measured under large-current discharge conditions is smaller by 35% or less than the capacitance measured under small-current discharge conditions that are the same as said large-current discharge conditions except that discharge current is 5 mA/cm 2 , said large-current discharge conditions being defined as follows:

Charge: Constant current, constant voltage method

Discharge: Constant current method

Charge current: 5 mA/cm 2

Discharge current: 200 mA/cm 2

Charge voltage: 2.7 V

Charge time: 60 minutes

Discharge voltage: 0 V

Discharge temperature: 25° C.

10. An electric double layer capacitor electrode as claimed in claim 1 , wherein capacitance measured under low-temperature discharge conditions is smaller by 10% or less than the capacitance measured under normal-temperature discharge conditions that are the same as said low-temperature discharge conditions except that discharge temperature is 25° C., said low-temperature discharge conditions being defined as follows:

Charge: Constant current, constant voltage method

Discharge: Constant current method

Charge current: 5 mA/cm 2

Discharge current: 5 mA/cm 2

Charge voltage: 2.7 V

Charge time: 60 minutes

Discharge voltage: 0 V

Discharge temperature: −30° C.