Peanut shells were subjected to pyrolysis and chemical activation to produce activated carbon with high specific surface area. MnO2 particles were synthesized on the activated carbon surface. Supercapacitors were fabricated by using activated carbon electrodes and tested by constant current charge-discharge, self-discharge, and life-cycle tests. MnO2 loading led to a significant decrease in specific surface area. The pore volume distribution calculations revealed that the MnO2 particles were in nanometer size. Because of the reduction of MnO4- Ions to MnO2 over the activated carbon surface, the amount of oxygen-containing surface functional groups, changed. Although the MnO2 loading caused a decrease in surface area, the specific capacitance increased from 49 F/g to 68 F/g.