Nanoarchitectonics with highly porous, thick, stable anodic films on 304 stainless steel for high- performance supercapacitors

The nanoporous oxide layer formed on stainless steel (SS) foils through constant current anodization typically faces the challenge of partial detachment of the anodic film during prolonged anodization, which limits the significant increase in film thickness and thus restricts its specific surface area capacitance. In this study, we propose a method to prepare ultra-thick nanoporous oxide films through long-term constant voltage anodization, effectively mitigating the detachment effect. By adjusting the anodization voltage, the porosity, thickness, and conductivity of the oxide film can be precisely controlled. Specifically, the oxide film with a thickness of 26.9 μm shows a high porosity and low conductivity at 50 V. As the voltage increases to 75 V, the oxide exhibits a thicker thickness (32.2 μm), and the porosity decreases. Both oxide films exhibit a gradient distribution of pore size along the thickness direction, contributing to more stable contact between the anodic film and the substrate. The thinner oxide film achieves a record-breaking area capacitance value of 215 mF cm⁻² at 1 mA cm⁻². The stable anodic film, with its tunable porosity, thickness, and conductivity, has great potential for applications in high-performance supercapacitors based on stainless steel foils.