The effects of Nafion loading in electrodes on the performance of supercapacitors have been investigated in this work. Electrodes based on carbon material were prepared with Nafion loading varying from 10 to 50 wt %, and the optimal loading was investigated by electrochemical impedance spectroscopy. The capacitors with lower Nafion content had higher conductivities and better capacitance performances. A maximum specific capacitance of 110 F/g (carbon of the electrode) was achieved for the capacitor with 10% Nafion loading in the electrode. Impedance spectroscopy was also used to evaluate separately the resistance of the Nafion electrolyte and of the carbon/Nafion layer of the electrodes. The results showed that the electrode with 50% Nafion gives a resistance about 50% higher with respect to that with 10% Nafion, even if the specific capacitance of the supercapacitor using the former electrode is only 20% lower. The similar performance exhibited by these all-solid supercapacitors compared to those of conventional devices, based on liquid electrolytes, is likely due to the high conductivity of Nafion and the contribution of the bound water surrounding the Nafion electrolyte in the electrode. A life cycling test carried out on a supercapacitor for 20,000 cycles, in cyclic voltammetry mode, showed variations of <3% of capacitance.