Display omitted •Preparing three PANI nanostructures in HCl, H2SO4, and HNO3 reaction media during a slow interfacial polymerization.•Obtaining nanofibrous, nanogranular and hollow ball–like morphologies in the order of oxidizability of acids.•Demonstrating high specific capacitance, good rate performance and long cycle life of PANI nanofibers. Three nanostructured polyanilines (PANIs) were prepared by interfacial polymerization using different inorganic acids including HCl, H2SO4, and HNO3 as reaction media, respectively. The morphology-dependent structure and properties of as-prepared PANIs were characterized by means of scanning electron microscope, ultraviolet-visible spectra, Fourier transform infrared spectroscopy and X-ray diffraction patterns. Meanwhile, the electrochemical performance of the fabricated electrodes was evaluated by cyclic voltammetry, galvanostatic charge/discharge measurement, and electrochemical impedance spectroscopy. It is found that the reaction medium plays a vital role in deciding the final morphology and structure of product when a slow reaction rate occurred in interfacial polymerization. The as-prepared PANIs exhibited nanofibrous, nanogranular and hollow ball-like morphologies according to the order of the relative oxidizing ability of HCl<H2SO4<HNO3, respectively. Furthermore, the capacitive properties of these composites as electrode materials highly depended not only on their morphologies but also on the conductivity, crystalline property and the inhibitory role of the lattice during the redox process, as well as the interparticle contact resistance. It is demonstrated that PANI nanofibers prepared in HCl medium exhibited high specific capacitance, good rate performance and long cycle life for the supercapacitor application.