An electro-chemical double layer capacitor comprises positive and negative electrodes, where the carbon material that is incorporated into the positive electrode is halogenated carbon material, while the carbon material that is incorporated into the negative electrode is un-halogenated carbon material. Further, the carbon material incorporated into each respective electrode can have a distinct pore size distribution. A pore volume ratio of the carbon material incorporated into the positive electrode is greater than a pore volume ratio of the carbon material incorporated into the negative electrode. The pore volume ratio R is defined as R=V1/V, where V1 is a total volume of pores having a pore size of less than 1 nm, and V is a total volume of pores having a pore size greater than 1 nm.