Flexible and thick carbon nanofiber (CNF) electrodes are developed for symmetric supercapacitor application. Free-standing CNF mats with diameter of 335 +/- 60 nm are produced via solution blowing technique and the microstructure and porosity are analyzed. The electrochemical performances of CNF are firstly investigated by determining the maximum operating voltage window in aqueous electrolytes with various pH. Then, CNF electrodes with high mass loading and thicknesses up to 1.2 mm are used for symmetric supercapacitor cells with 6 M KOH solution as an electrolyte. It is found that electrodes with mass loading of 25 mg/cm(2) can be cycled up to 10,000 cycles without capacity loss, at current of 2 A/g and 1.4 V cut-off voltage. While electrodes with higher mass loading retain 85% of their capacity. Post-mortem analysis shows that the capacity loss of the thicker electrodes is a result of decreased ion diffusion during cycling. The lower ion diffusion appears due to crystal-like products formed as a result of side reactions with the electrolyte.