Optimizing the performance of supercapacitors based on carbon electrodes and protic ionic liquids as electrolytes

Protic ionic liquids (PILs) were implemented as electrolytes for supercapacitors using activated carbons with various porous textures as electrode material. The carbon with the largest specific surface area and highest amount of narrow mesopores (pore diameter: 2-7 nm) was found to give the highest specific capacitance in pyrrolidinium nitrate (PyNO3) ionic liquid. However, it should be noted that when the pH value of this ionic liquid was adjusted around 11, higher specific capacitance was achieved, revealing a better electrochemical performance of carbon electrodes in basic media (i.e., capacitance values of 121 and 208 F g(-1) for an electrolyte based on PyNO3 with a pH value of 7 and 11, respectively). This ionic liquid contained a small amount of water, which restricted the maximum voltage of symmetric capacitors to a value of 1.2 V, even after PyNO3 had been partially dried (H2O content around 1110 ppm). Therefore, the triethylammonium bis(trifluoromethylsufonyl)imide - NEt3H TFSI - PIL was prepared in order to expand the potential window; after drying this PIL contained 200 ppm water. The results obtained with NEt3H TFSI suggest that maximum voltages as high as 2.5V can be achieved. This clearly shows that the presence of water in PILs has a negative effect on the performance of supercapacitors. (C) 2013 Elsevier Ltd. All rights reserved.