Supercapacitance behavior of porous oxide layer grown on 302 type stainless steel substrate

Nano-porous oxide layers on the surface of 302-type stainless steel were synthesized by galvanostatic anodization process carried out at two current densities, 7 and 15 mA cm−2 for the time periods of 15 and 30 min. A relatively thinner (∼300 nm) and compact oxide layer formed when anodization was performed at a current density of 7 mA cm−2. However, with increase in anodization current density, the oxide layer not only grew thicker but also changed in morphology. It was found to be about 0.7 and 1.2 μm after 15 and 30 minutes of anodization at 15 mA cm−2, respectively. The XRD analysis of the oxide layer suggested the formation of hematite ((Fe, Cr)2O3) and spinel ((Ni, Cr, Fe)3O4) phases. The cyclic voltammetry and galvanostatic charge-discharge studies (1 M Li2SO4) suggested that the nature of the capacitance of the anodized stainless steel electrodes were of predominantly electric double layer (EDL) type together with some pseudocapacitance like behavior. The specific capacitance obtained was higher for the sample anodized at higher current density (15 mA cm−2) than that of the sample anodized at lower current density (7 mA cm−2). The results were also complimented by electrochemical impedance spectroscopic (EIS) analysis. ► Synthesis of porous oxide layer on 302 type stainless steel substrate by electrochemical anodization method. ► Variations of oxide layer morphology based on the anodization current density and time. ► Specific electrochemical capacitance of as high as 112 mF cm−2 at galvanostatic charge–discharge current density of 1 mA cm−2. ► More than 60% capacitance retention even after 500 cycles of continuous charge–discharge.