Enhanced supercapacitor performance using an anodized stainless steel flexible electrode coated with an electron cyclotron resonance carbon film

Electrochemically anodized stainless steel (SS) shows promise as a free-standing electrode in flexible supercapacitors due to its low cost, eco-friendly nature, and binder-free characteristics. However, unsatisfactory cycling stability limits its practical use in wearable electronics. Herein, we introduce a conductive carbon film deposited on the surface of the anodized SS electrode as a protective layer via electron cyclotron resonance sputtering. By optimizing the deposition bias voltage and deposition time, the resulting flexible electrode exhibits a specific capacitance of 271.6 mF·cm-2 at a current density of 1 mA·cm-2, representing a 2.24-fold increase over the uncoated counterpart, with 88.7% capacitance retention after 8,000 cycles. The enhanced performance is closely related to the conductivity of the surface coating, which depends on the sp2/sp3 ratio (the relative proportion of graphitic to diamond-like carbon bonding). The carbon film-coated anodized SS electrode is combined with an activated carbon on carbon cloth electrode and a gel electrolyte to produce a flexible supercapacitor. The energy storage device exhibits a wide operating potential window of 1.8 V, a high energy density of 51.70 mWh·cm-3, and a power density of 0.50 W·cm-3, accompanied by robust flexibility and mechanical stability. These findings may pave the way for the development of high-performance, flexible, and cost-effective supercapacitors compatible with large-scale semiconductor device manufacturing.