NOVELTY - The capacitor (10) has a positive electrode (14) having a halogenated first carbon material and a negative electrode (16) having an unhalogenated second carbon material. The halogenated first carbon material includes fluorinated carbon and comprises up to about 15 wt.% halogen. The un-halogenated second carbon material comprises less than 5 wt.% oxygen. The first carbon material and the second carbon material comprise activated carbon. A thickness of the negative electrode is greater than a thickness of the positive electrode.    USE - High-voltage electro-chemical double layer capacitor e.g. ultracapacitor used in e.g. cellphones and hybrid vehicles.    ADVANTAGE - The electro-chemical double layer capacitor has electrode materials tailored for more efficient interaction with particular anions and cations and radicals within the electrolyte to provide high specific capacitance as well as improved capacitive aging and long term stability at high voltages. Faradaic reactions with halogen atom-containing ions such as tetrafluoroborate ions that would otherwise react with the carbon or with surface-terminating species on the carbon are avoided by halogenating the carbon material to effectively passivate the carbon material and minimize the reactivity of the carbon toward other halogen-containing species.    DETAILED DESCRIPTION - The positive electrode comprises a mixture of the first carbon material, conductive carbon and binder, and the negative electrode comprises a mixture of the second carbon material, conductive carbon and binder. The electrolyte solution comprises a salt dissolved in a solvent, the salt being selected from the group consisting of quaternary ammonium salts. The electrolyte solution further comprises tetraethyl ammonium tetrafluoroborate and acetonitrile. An INDEPENDENT CLAIM is also included for a method of making an electro-chemical double layer capacitor.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic cross-sectional view of a portion of an electro-chemical double layer capacitor.    Ultracapacitor (10)    Positive electrode (14)    Negative electrode (16)    Current collectors (22,24)    Electrical leads (26,28)