Activated carbon was produced from waste coffee grounds by treatment with ZnCl2. Supercapacitor electrodes prepared from this coffee grounds carbon exhibited energy densities up to 20 Wh kg(-1) in 1 M H2SO4, and excellent stability at high charge-discharge rates. In a two-electrode cell a specific capacitance as high as 368 F g(-1) was observed, with rectangular cyclic voltammetry curves and stable performance over 10,000 cycles at a cell potential of 1.2 V and current load of 5 A g(-1). The good electrochemical performance of the coffee grounds carbon was attributed to a well developed porosity, with a distribution of micropores and mesopores 2-4 nm wide, and the presence of electrochemically active quinone oxygen groups and nitrogen functional groups. This work highlights the potential to utilize waste biomass to produce electrode materials for cost-effective energy storage systems. (C) 2008 Elsevier B.V. All rights reserved.