We report a new class of electrochemical capacitors by utilizing vertically aligned carbon nanotubes as the electrodes and environmentally friendly ionic liquids (ILs) as the electrolytes. With their vertically aligned structures and well spacing, aligned carbon nanotubes showed a strong capacitive behavior in the ionic liquid electrolyte. Plasma etching played an important role in opening the end tips of nanotubes and in introducing defects and oxygenated functionalization to the nanotubes, further enhancing the capacitive behavior of carbon nanotubes. With the combined contribution from double-layer capacitance and redox pseudocapacitance, carbon nanotubes showed a remarkable capacitance in ionic liquid electrolyte. Combining the highly capacitive behavior of carbon nanotube electrodes with the large electrochemical window of ionic liquid electrolytes, the resultant capacitors showed a high cell voltage, high energy density, and high power density, potentially outperforming the current electrochemical capacitor technology. The device configuration incorporating vertically aligned nanostructured electrodes and inherently safe electrolytes would be useful for improving performances for new energy storage technologies. (c) 2009 Elsevier B.V. All rights reserved.