The focus of this thesis has been to determine the usable voltage range of carbon-based supercapacitors (SC). Supercapacitors are a relatively new type of capacitors with a vast increase in capacitance compared to capacitors which utilize a dielectric as charge separator. A SC consists of two electrodes and an electrolyte separating the electrodes. The charges are stored by electrostatic forces in the interface between the electrode and the electrolyte, forming the so called electrochemical double-layer (EDL). With porous electrodes the effective surface area of the interfacial zone can be made very large, giving SCs a large storage capacity. The limiting factors of a SC is the decomposition potential of the electrolyte and the decomposition of the electrodes. For commercially manufactured SCs the electrolyte is usually an organic solvent, which has a decomposition potential of up to 2.7-2.8 V. Compared to aqueous electrolytes with a thermodynamic limit of 1.23 V. The drawback of using non-aqueous electrolytes is that they are not environmentally friendly, and they increase the production cost. It is claimed that the voltage range can be up to 1.9 V using aqueous electrolytes. Some researchers have focused on aqueous electrolytes for these reasons. In this thesis two different electrolytes were tested to determine if the voltage range could be extended. The experiments were conducted using a three electrode cell and performing cyclic voltammogram measurements (CV). The carbon electrodes were made of  two different sources of grahite, battery graphite or exfoliated graphite, and nano fibrilated cellulose was added to increase the mechanical stability. The results show that the oxidation potential of the carbon electrode was the positive limit. A usable potential of about 1 V was shown. However, when cycling the electrodes to potentials below the decomposition limit, for hydrogen evolution, interesting effects were seen. A decrease in reaction kinetics, indicating a type of conditioning of the electrode was observed. An increase in charge storage capacitance was also observed when comparing the initial measurements with the final, probably corresponding to an increase in porosity. / KEPS projekt Sundsvall Mitt Universitet