A traditional approach is conducted for the preparation of polyaniline/graphene oxide (PANI/GO) nanostructures, where aniline monomer followed by ammonium persulfate (APS) were added to an acidic solution of graphene oxide (GO). Herein, we investigate a condition in which firstly GO is dispersed in acidic media containing APS. The aniline monomer is then added in order to the in-situ polymerization of aniline. An investigation is also carried out on the effect of the acid type including sulfuric acid (H2SO4), hydrogen chloride (HCl), perchloric acid (HClO4), and p-toluenesulfonic acid (PTSA) on the morphology and electrochemical behavior of resultant PANI/GO nanostructures. The characterizations reveal well-defined nanoarrays for PANI/GO-H2SO4 electrode with the highest porosity and specific surface area. This electrode exhibits a highest current density in cyclic voltammetry studies and the most specific capacitance (727 F g(-1)) in charge/discharge studies among other as-prepared electrodes. Utilizing this PANI/GO-H2SO4 nanocomposite as the electrodes in a symmetric configuration resulted in the development of devices with remarkable performance including a maximum energy density of 40 Wh kg(-1) and a power density up to 15.3 kW kg(-1) with an excellent cycle life (only 4.3% loss after 5000 cycles). (c) 2018 Elsevier Ltd. All rights reserved.