Polyaniline (PANI) synthesized by a potentiostatic method at 4°C in 1 M HNO3 with the polymerization charge density equal to/less than 0.45 C cm−2 was demonstrated to exhibit ideally capacitive characteristics (i.e. high reversibility and high-power property) with a high specific capacitance of 210 F g−1 for the application of electrochemical supercapacitors in NaNO3. Influences of the polymerization charge density (i.e. the polymer loading) and the polymerization temperature on the capacitive characteristics of PANI films compared by both cyclic voltammetry and charge–discharge technique were reasonably correlated with their structural properties examined by X-ray photoelectron spectroscopy (XPS). The highest specific capacitance of a PANI film polymerized at 4°C was attributed to its lowest density of structure defects. The surface morphology of these PANI films was examined by a scanning electron microscope (SEM).