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Zhang et al. Soft Sci. 2026, 6, 16 Page 11 of 17

The specific contributions of the total capacitance from capacitive and diffusion-controlled processes were
calculated using a modified Power-law model as follows :
[46]

( ) = 1 + 2 1/2 (8)

where i(V) is the total current under a fixed potential V, v denotes the scan rate, and k and k represent the
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constants of the capacitive and diffusion process. Figure 4I shows the capacitance contribution of the C-3
electrode at 10 mV·s in brown, and the detailed contribution proportions at various scan rates are
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summarized in Figure 4J. As the scan rate increased from 10, 50, 100, 150 to 200 mV·s , the corresponding
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capacitance contribution ratios increased to 35%, 47%, 58%, 65%, and 73%, respectively. This phenomenon
can be attributed to the fact that at higher scan rates, the charge storage and release processes are accelerated,
making the capacitive-controlled process more dominant, while the diffusion-controlled process becomes
limited. Moreover, the carbon film coating enhanced the material’s electrical conductivity, thereby
promoting rapid storage of capacitive charges . The relatively high contribution ratio indicates that the C-3
[11]
electrode can withstand higher current densities, demonstrating superior rate capability. Supplementary
Figure 6 displays the CV, GCD, cycling stability, and capacitance contribution curves of the uncoated C-0
electrode for comparison. As mentioned earlier, the C-3 electrode exhibited a 2.24-fold increase in areal
capacitance compared with C-0 at a current density of 1 mA·cm . A 48.9% improvement in capacitance
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retention after 8,000 cycles was achieved through surface carbon film deposition. Since the retention values
of C-1, C-2, and C-3 were nearly the same, this enhancement may be related to the stabilization effect of the
carbon film on the fragile sidewalls of the anodized nanoporous structure. To further confirm this, the
surface morphology of the C-3 electrode after 8,000 GCD cycles was observed by SEM. No obvious breakage
was found when comparing Figure 2E and Supplementary Figure 2D, demonstrating the protective effect of
the carbon film. In addition, the proportion of capacitive contribution for C-3 was much higher than that of
the pristine C-0 electrode, which can be attributed to carbon film-induced resistance reduction, leading to
faster charge transfer at the electrode surface and superior capacitive behavior .
[47]

As a result, both the capacitance and cycling stability of the anodized SS flexible electrode were significantly
enhanced by a 50 nm-thick ECR carbon film coating. The underlying mechanism can be summarized as
follows. Regarding capacitance improvement, the ECR carbon film exhibits much higher conductivity than
Fe O or Fe O , enabling efficient electronic transport during charging and discharging, which facilitates
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rapid redox reactions and effective utilization of the entire electrode . Moreover, the ECR carbon film
[48]
contains abundant surface hydroxyl and carboxyl groups, enhancing hydrophilicity and providing additional
redox-active sites, which promote the migration of electrolyte ions into the electrode and contribute to
higher specific capacitance [49,50] . In terms of cycling stability, the carbon film covers the anodized SS surface,
mitigating volumetric changes of Fe O and Fe O during charge-discharge cycles. This protection reduces
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active material loss due to dissolution or detachment, thereby improving capacitance retention .
[51]
Table 3 summarizes the electrochemical performance of other ferric oxide-based electrodes. While these
electrodes exhibited high specific capacitance, their cycle life retention was limited. Our ECR carbon film
coating approach addresses this limitation by preserving capacitance, making it a promising strategy for
commercial applications due to its high processing compatibility.

Performances of flexible supercapacitor based on carbon film-coated nanoporous structure
The carbon film-coated nanoporous structure on SS substrate was considered to have broad commercial
potential as a flexible supercapacitor electrode. As a step toward practical application, an all-solid-state
flexible supercapacitor was assembled using the optimized C-3 electrodes. CV tests were performed on the
device to determine a suitable voltage window, with applied voltages ranging from 1.2 to 2.0 V. As shown in