Investigation on capacitive behaviors of porous Ni electrodes in ionic liquids

We previously reported some findings on the applicability of porous Ni materials to the electrodes of electric double layer capacitors (EDLCs) [1]. It was found that the porous Ni material prepared via alkali-leaching of Ni-Al alloys and dry process without heating and air-contact provides pseudocapacitance as well as electric double-layer (EDL) capacitance in organic electrolyte solution, TEA·BF4/PC. The pseudocapacitance is ascribed to the electrochemically active surface and bulk state of the porous Ni with low crystallinity. In TEA·BF4/PC, porous Ni materials were found to provide lower volumetric total capacitance than the values of the commercial activated carbons, due to the large difference of specific surface areas (i.e., porous Ni: 43 m2/g, the activated carbons examined: 1508-2164 m2/g). However, a significant point was the high value of EDL capacitance normalized by the surface areas (CSA), i.e., 10.2 μF/cm2, which was beyond 3.6-6.6 μF/cm2 of the activated carbons. In this study, the volumetric total capacitance and CSA of porous Ni materials have been further enhanced by using ionic liquids as electrolytes. The volumetric total capacitance has reached 67.4 F/cm3 (three-electrode evaluation) in EMIm·BF4 ionic liquid, which approaches 79.3 F/cm3 of a high-capacitance-type activated carbon, MSP-20. The total capacitance is affected by the class of ionic liquids due to the difference of the viscosity and conductivity, whereas the pure EDL capacitance is dependent on the ion sizes rather than the physical properties of ionic liquids. Furthermore, the difference of either anions or cations affects the capacitive behaviors in both positive and negative electrodes. Significantly, the CSA value of porous Ni electrodes has increased from 10.2 μF/cm2 in TEA·BF4/PC to 16.6 μF/cm2 in EMIm·BF4, which is much higher than 7.0 μF/cm2 of MSP-20. Furthermore, the electrochemical stabilization of porous Ni materials has been achieved by heat treatment under vacuum, resulting in an excellent cycle performance caused by the exclusion of pseudocapacitance. More noteworthy is that the high CSA can be retained even after the stabilization. The results of this study further emphasize the potential of porous Ni materials as EDLC electrodes.