An ultracapacitor (UCap) based on carbon xerogel electrodes and sodium sulfate electrolyte was investigated in the voltage range between 0 and 1.8 V. Notwithstanding the high value of maximum voltage (1.8 V) the UCap exhibited excellent stability during 20000 of cycling test. Moreover, the achievement of this high voltage made possible to obtain high value of specific energy. The stability was possible because the potential limits of electrode–electrolyte decomposition at positive and negative electrodes were never achieved. This is because an asymmetric UCap with different amounts of carbon xerogel in the electrodes was used. The UCap with the carbon xerogel of BET specific surface area of 3100 m2 g−1 demonstrated a specific energy of 17.5 Wh kg−1 and a specific capacitance of 156 F g−1 and, retained 91% of initial capacitance after 20000 cycles of duration test.