BACKGROUND Nanostructured metal oxides have shown capabilities in the application of energy conversion and storage. Among a variety of metal oxide semiconducting materials, molybdenum oxide recently became a promising candidate as an electrochemical supercapacitor for storing electrical energy. Supercapacitors, which are storage devices for electrical energy, benefit from the separation of opposite charges in electrochemical double-layer capacitors and charge transfer as a function of electrochemical potential in pseudocapacitors. RESULTS A thin film of nanostructured molybdenum oxide was prepared using potentiodynamic electrodeposition on the surface of a stainless-steel electrode. Its electrochemical capacitive behavior was investigated using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). To investigate the effects of the electrolyte ion type, two solutions were investigated in a comparative way: 0.005 mol L-1 H2SO4 and 0.005 mol L-1 H2SO4 + 0.095 mol L-1 Na2SO4 mixture where each solution has the same initial concentrations of hydronium ions but with different ionic strengths. The EIS studies gave rise to the specific capacitances of 340 and 160 F g(-1) for H2SO4 and mixed solutions, respectively. An appropriated electrical circuit was used to interpret these EIS results. CONCLUSIONS The present studies show that the electrolyte cations contribute to the capacitive behavior of the nanostructured molybdenum oxide in different ways. Sodium ions only participate in the process of charging the double layer from the outer Helmholtz and diffusion layers, whereas hydronium ions contribute to the Faradaic process of electrosorption by penetrating into the inner Helmholtz layer. (c) 2019 Society of Chemical Industry