A novel anodic nanostructured stainless steel-304L oxide as an emerging electrode material for high energy density asymmetric supercapacitors: experimental and DFT insights

Abstract

The electrode material plays a significant role in the performance of supercapacitors. Therefore, the development of a novel electrode material for high energy density, high power density, and stable supercapacitors is highly desirable. The emergence of the stainless steel 304L (SUS-304L) oxide nanostructure as an electrode material provides an opportunity to explore its supercapacitive behavior. In the present study, a simple and facile technique of anodization was employed to synthesize nanostructured SUS-304L oxide (in powder form), which would be otherwise impossible to achieve using the existing synthetic routes. The structural and microscopic results confirm the formation of multiple phases (Fe₂O₃, Fe₃O₄, and NiCr₂O₄), the polycrystalline behavior, and a morphology in which the nanoparticles (NPs) overlap nanosheets (NSs). As an electrode, the developed material demonstrated excellent electrochemical performance, achieving a high specific capacitance of ∼1226 F g⁻¹ at 2 A g⁻¹ in a KOH electrolyte. The fabricated asymmetric supercapacitor device (SUS-304L oxide//activated carbon) exhibited a specific capacitance of ∼209 F g⁻¹ and an energy density of ∼29 Wh kg⁻¹ at a power density of ∼751 W kg⁻¹. Additionally, the device retained ∼89% of its initial capacitance over 8000 cycles. The outstanding performance is due to the synergetic effect of the multiple phases. To further elucidate the supercapacitive behavior, ab initio calculations based on density functional theory (DFT) were used to determine the quantum capacitance. The observed large capacitance is mainly contributed by Fe-oxides and can be ascribed to the large density of states of minority spin states of t_2g and e_g orbitals of Fe atoms at the octahedral sites. These findings demonstrate the potential of the prepared composite nanostructure (SUS-304L oxide) as a high-performance electrode material for practical asymmetric supercapacitor applications.