Investigation on capacitive behaviors of porous Ni electrodes for electric double layer capacitors

The present study aims at assessing the applicability of porous metals to the electrodes of electric double layer capacitors (EDLC). Porous Ni materials for test electrodes were prepared from commercially available Ni–Al alloys through an alkali-leaching process and the subsequent dry and surface-treatment processes. The surface area and pore structure (mesoporous structure) have been found to depend on the class of the alloys. Furthermore, the porous Ni materials prepared via the dry processes with heating and with exposure to air have shown high electrochemical stability but quite low capacitance. In contrast, the dry process without heating and in no contact with air has made the surface and bulk sate electrochemically more active, which induces pseudocapacitance as well as double-layer capacitance. The pseudocapacitive reaction is responsible for the gradual modification of the active electrode to a high-resistance material with charge/discharge cycles. The volumetric total capacitance has reached as high as 31.9 F/cm3 (in a three-electrode system) irrespective of not so high specific surface areas (SSA) of 43 m2/g. At present, the capacitance is lower than the values, i.e., 40.3–78.0 F/cm3, of the commercial microporous active carbons with the high SSAs of 1508–2164 m2/g. However, the capacitance normalized by the surface areas has reached 10.2 μF/cm2, which is beyond 3.6–6.6 μF/cm2 of the active carbons. In addition, the mesoporous Ni electrodes have shown more favorable rate performance than the microporous active carbons due to the low ion-transfer resistance.