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Page 8 of 17 Zhang et al. Soft Sci. 2026, 6, 16

the ratio of D and G band intensities (I /I ) with the corresponding conductivity of the carbon-coated
D
G
samples. Since the I /I value is inversely related to the sp2 fraction , C-1 exhibited the highest resistivity,
[34]
D
G
followed by C-2, with C-3 showing the lowest, consistent with the TEM and XPS results. This observation
aligns with the elemental composition analysis described above: the sp2 fraction increased with increasing
substrate bias voltage from -5 V (C-1) to +20 V (C-2) and +100 V (C-3), resulting in reduced resistivity .
[35]
Electrochemical performance
The effect of carbon film deposition parameters, including substrate bias and deposition time, on the
supercapacitor performance of the composite electrode materials was investigated using a three-electrode
system with 1 M Na SO electrolyte. Figure 4A shows the CV curves of the uncoated electrode C-0 and the
2
4
coated electrodes C-1, C-2, and C-3, prepared under different substrate bias voltages, with a voltage window
of -0.8 to 0.0 V and a scan rate of 100 mV·s . All these curves displayed semi-rectangular shapes with no
-1
characteristic redox peaks observed, which was widely reported for Na SO electrolyte, demonstrating the
4
2
outstanding pseudocapacitive behavior . Moreover, it was observed that the enclosed area and current
[36]
response of the CV curves were directly proportional to the storage capacitance of the materials . Thus, the
[37]
carbon film-coated electrodes could have the largest capacitance evidently, and among them, C-3 electrode
performed the best. The detailed capacitance was calculated based on the GCD curves [Figure 4B] and
Equation (1). At a current density of 1 mA·cm , the capacitance of C-1, C-2 and C-3 electrodes was 161.5,
-2
178.5 and 271.6 mF·cm , respectively, much larger than that of the uncoated electrode (121.1 mF·cm ), and
-2
-2
increased with increasing deposition bias voltage. In addition, owing to the binder-free anodization method,
the internal resistance drops (IR-drops) of the four electrodes were below 0.05 V, reflecting a highly effective
charge transfer between active material and SS foil collector . Figure 4C shows the capacitance of all
[38]
electrodes at different current densities. The results indicate that the capacitance decreased with increasing
current density, while the relative order of the electrodes remained unchanged. Importantly, the coated
electrodes exhibited significantly improved initial capacitance retention: only 6.7% for the pristine sample
C-0, compared with 23.9%, 27.0%, and 51.8% for C-1, C-2, and C-3, respectively, as the current density
increased from 0.5 to 10 mA·cm . To understand the reaction kinetics of CV curves for the four electrodes,
-2
the diffusion coefficient was calculated using :
[39]

= (4)

log( ) = log( ) + log( ) (5)

where i is the peak current, v is the scan rate, b represents the diffusion coefficient and a is a constant. A b
value of 0.5 indicated the diffusion-controlled process, while the value of 1 demonstrated a surface-controlled
process.

The calculation results shown in Figure 4D indicated diffusion coefficients of 0.76, 0.87, 0.88, and 0.91 for
C-0, C-1, C-2, and C-3 electrodes, which could be attributed to a surface-controlled dominant process
combined with a diffusion-controlled process. The coated electrodes exhibited a significantly higher
diffusion coefficient compared with the uncoated electrode, indicating faster charge transfer at the electrode
surfaces and superior capacitive behavior due to the increased surface conductivity . To explore the
[40]
relationship between electrochemical performance and electrode resistance, EIS tests were conducted over a
frequency range of 100 kHz to 0.01 Hz. The Nyquist plots are shown in Figure 4E with the equivalent circuit
model inset, and the corresponding parameters are listed in Table 2. The interception of the curves with the
real axis represents the series resistance (R ) of the electrodes, with the coated samples exhibiting much lower
s
values that decreased with increasing substrate bias. These results indicate that the carbon film coating
effectively reduced the internal resistance, in good agreement with the four-probe measurement data. The