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Page 2 of 17 Zhang et al. Soft Sci. 2026, 6, 16

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 , and a power density of 0.50 W·cm , accompanied by robust flexibility and mechanical
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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.

INTRODUCTION
Owing to the rapid development of the Internet of Things (IoT) and artificial intelligence (AI), wearable and
portable electronic devices have attracted enormous attention for applications such as energy storage devices
and sensors [1-2] . Flexible supercapacitors are particularly promising due to their fast charge-discharge
capability, high power density, safe operation, and long cycle life . For high-performance flexible
[3]
supercapacitors, free-standing electrodes - including conductive substrates combined with active materials in
situ or ex situ - are highly preferred, offering advantages such as excellent portability, low volatility, ease of
packaging and molding, and high safety . Recently, we reported the fabrication and performance of
[4]
Electrochemically anodized stainless steel (SS) as a free-standing electrode . Compared with widely reported
[5]
alternatives such as carbon cloth (CC) , cellulose paper and metallic mesh , the anodized SS electrode
[7]
[8]
[6]
offers advantages including easy preparation, moderate cost, non-toxicity, binder-free characteristics, and a
reliable capacitance of up to 100 mF·cm . However, due to the large crystal size of ferric oxide, which induces
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significant volume changes during the charge-discharge process, the anodized SS electrode suffers from poor
cycling stability, with retention dropping to 40% , severely limiting its practical application.
[9]
The most widely used route to enhance the cycling stability of anodized SS electrodes was to increase their
conductivity by material modification. Ahmed et al. grew Co O nanowires on an anodized SS substrate
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using a hydrothermal method. The heterojunction exhibited a specific capacitance of 529.0 F·g , with a
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retention of 76.0% after 1,000 cycles . Feng et al. fabricated nitrogen-doped anodized SS electrodes using an
[10]
ion implantation method, achieving a specific capacitance of 332.4 mF·cm and retention of 76.7% after
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8,000 cycles . Chai et al. reported a vanadium-doped anodized SS free-standing electrode with a capacitance
[11]
of 320.9 mF·cm and 88.4% capacitance retention after 8,000 cycles . Although significant improvements
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[12]
had been achieved through material modification, methods involving wet processing or costly fabrication
were not fully compatible with semiconductor device manufacturing in the electronics industry, particularly
for wearable electronics .
[13]
Considering the drawbacks mentioned above, another possible route was to build a protection layer on the
surface of electrodes. Carbon film is one of the most promising candidates due to its stable chemical
properties, good mechanical strength, and highly controllable deposition process . Moreover, the
[14]
combination of a pseudocapacitance material (anodized SS) with a double-layer material (carbon film) could
lead to improved capacitance and cycling stability theoretically . Unfortunately, most conventional
[15]
methods can only deposit amorphous carbon films with relatively low conductivity. As a protective layer, the
nearly insulating carbon film could hinder charge transport between the electrode and the electrolyte,
resulting in a significant decline in capacitance. In 2011, Wang and Diao reported a carbon film embedded
with graphene sheets inside by electron cyclotron resonance (ECR) sputtering . This kind of film exhibits
[16]
significantly improved conductivity due to a high proportion of sp2 carbon. Beyond the general tribology
application and recent uses in photodetectors and electrocatalytic sensors [17,18] , the film was expected to be
adopted as a protection layer on the surface of the anodized SS flexible electrode.

Herein, to enhance the cycling stability of anodized SS electrodes, a series of carbon films were deposited for
protection via ECR sputtering. The electrochemical performance was optimized by adjusting the deposition