Enhanced Supercapacitor Performance via Polyaniline‑Coated Nitrogen‑Doped Ordered Mesoporous Carbon Composites

Background

With escalating environmental concerns and finite resource availability, the pursuit of clean, efficient energy storage solutions has become paramount. Supercapacitors—known for their rapid charge‑discharge capability, high power density, long cycle life, and environmental friendliness—are emerging as attractive alternatives to conventional batteries. However, their relatively low energy density limits widespread adoption. The key to unlocking their full potential lies in the development of advanced electrode materials.

Polyaniline, a conductive polymer, offers high theoretical capacitance, low cost, and facile synthesis. Yet its practical performance is often hampered by volumetric changes during redox cycling, which compromise conductivity and stability. Combining PANI with robust, conductive carbon matrices is a proven strategy to mitigate these drawbacks. For instance, boron‑doped graphene supports have yielded high capacitance and excellent cycling stability in both acidic and alkaline media.

Mesoporous carbons, particularly those with ordered pore structures, are prized for their high surface area, tunable porosity, chemical resilience, mechanical strength, and conductivity. Doping these carbons with nitrogen and sulfur heteroatoms further enhances their electrochemical activity by introducing electron‑donating sites. In this work, we exploit nitrogen‑doped ordered mesoporous carbon (NOMC) as a scaffold for PANI deposition, producing a hybrid that synergistically combines the high pseudocapacitance of PANI with the structural and conductive advantages of NOMC.

Materials and Methods

Materials Synthesis

All reagents were of analytical grade and used without further purification. Resol was synthesized by the phenol‑formaldehyde condensation route: phenol (0.94 g) was heated to 42 °C, followed by the slow addition of 0.2 g 20 % NaOH solution while stirring. Subsequently, 1.62 g 37 % formaldehyde was added dropwise, and the mixture was stirred for 1 h at 70 °C. After cooling, the pH was adjusted to 7.0 with 0.1 M HCl, and the resol was obtained by vacuum drying at 50 °C.

For NOMC preparation, 0.33 g SBA‑15 was dissolved in 9 g ethanol, then 3 g resol (20 % in ethanol) and 0.3 g cyanamide were added and stirred for 8 h. The resulting yellow powder was precipitated by evaporating the solvent at 60 °C for 10 h, then calcined under N₂ at 800 °C for 3 h (10 °C min⁻¹ ramp). The product was etched in 10 % HF, filtered, washed with ethanol, and dried at 60 °C under vacuum for 12 h.

To fabricate PANI/NOMC‑x (where x denotes the initial PANI:NOMC mass ratio), 0.1 g NOMC was dispersed in 7.5 mL ethanol and 2.5 mL DMF by ultrasonication. 0.1 × g aniline was dissolved in this suspension under an ice‑water bath with stirring for 2 h. Then, ammonium persulfate and HCl (molar ratio 1:1:1 relative to aniline) were added, and the mixture was stirred for 10 h at 0 °C. The resulting solid was centrifuged at 8000 rpm for 20 min, washed with ethanol and deionized water, and dried at 50 °C under vacuum for 1 h.

Materials Characterization

Morphology was examined by TEM (Tecnai G2 F30) and SEM (Sirion 200). FT‑IR and XRD spectra confirmed the chemical structure of NOMC and PANI/NOMC‑x. XPS quantified the elemental composition (C, N, O) and revealed the bonding environments. BET analysis provided specific surface area and pore size distributions.

Electrochemical Measurement

Electrochemical performance was assessed using a CHI 660E workstation in 2 M KOH under ambient conditions with a three‑electrode cell (PANI/NOMC‑x as working electrode, Pt wire counter, SCE reference). The working electrode was prepared by mixing 85 % PANI/NOMC‑x, 10 % acetylene black, and 5 % PTFE (mass ratio) and casting onto a 1 cm² current collector, followed by pressing at 10 MPa and vacuum drying at 50 °C. Specific capacitance (C) was calculated from galvanostatic charge‑discharge curves: C = It/(ΔV m). Energy (E) and power (P) densities were derived from E = ½ C ΔV² and P = E/t respectively.

Results and Discussion

The synthesis route is illustrated in Fig. 1a. SBA‑15 templates were impregnated with resol and cyanamide, followed by carbonization at 800 °C. Post‑etching with HF removed the template, yielding NOMC. PANI was then polymerized in situ on the NOMC surface, producing PANI/NOMC‑x composites. SEM images (Fig. 1b–f) reveal that both NOMC and PANI/NOMC‑0.5 consist of ~1 µm cylindrical particles; the presence of a coating layer on the latter confirms successful PANI deposition. TEM (Fig. 1d, g) shows that the ordered 3 nm stripe‑like pore structure of NOMC is preserved after PANI coating.

FT‑IR spectra (Fig. 1h) display characteristic PANI peaks at 1120, 1300, and 1496 cm⁻¹, whose intensities increase with higher PANI loading, confirming successful functionalization. XRD patterns (Fig. 1i) indicate that both NOMC and PANI/NOMC‑0.5 retain an amorphous carbon structure, implying that PANI deposition does not disrupt the mesoporous framework.

XPS analysis (Fig. 2 and Table 1) shows the presence of pyridinic, pyrrolic, and graphitic nitrogen species, as well as C=O functionalities. Increasing the PANI mass ratio reduces the C=C fraction and increases C=O content, reflecting the integration of PANI chains.

BET measurements (Fig. 3) reveal that NOMC possesses a high surface area of 1051 m² g⁻¹ with an average pore size of 2.82 nm. Coating with PANI progressively reduces the surface area (e.g., PANI/NOMC‑0.5: 209 m² g⁻¹) and enlarges the average pore size, consistent with pore blockage by the polymer shell.

Electrochemical evaluation (Fig. 4) shows that NOMC exhibits a nearly rectangular CV profile, characteristic of double‑layer capacitance. PANI/NOMC‑x displays distinct redox peaks attributable to the leucoemeraldine/emeraldine/pernigraniline transitions, indicating pseudocapacitance contributions. At 1 A g⁻¹, the specific capacitances of PANI/NOMC‑0.2, ‑0.5, ‑1, ‑2, and ‑4 are 211.2, 258.9, 244.5, 143.6, and 53.0 F g⁻¹, respectively. The optimal 0.5 ratio delivers the highest capacitance (258.9 F g⁻¹) and maintains 81 % of its capacitance when the current density increases 25‑fold (0.2 to 5 A g⁻¹). Nyquist plots (Fig. 4c) confirm low charge‑transfer resistance for all hybrids, while the linear low‑frequency region indicates excellent capacitive behavior.

Cycling tests (Fig. 5c) demonstrate that NOMC retains 95 % of its capacitance after 5000 cycles, whereas PANI/NOMC‑0.5 retains ~80 %—a testament to the robust composite structure. Ragone plots (Fig. 5d) reveal that PANI/NOMC‑0.5 achieves an energy density of 38.4 Wh kg⁻¹ at 200 W kg⁻¹, with minimal decline across higher power densities, a phenomenon rarely reported for similar systems.

Conclusion

The polyaniline‑coated nitrogen‑doped ordered mesoporous carbon hybrid has been successfully synthesized via a hard‑template and in‑situ polymerization approach. By marrying the high theoretical pseudocapacitance of PANI with the structural stability and conductivity of NOMC, the composite exhibits a superior specific capacitance (up to 276.1 F g⁻¹), excellent rate capability, and long‑term cycling durability (≈ 80 % retention after 5000 cycles). These attributes position PANI/NOMC as a compelling candidate for next‑generation supercapacitors, especially in flexible and wearable energy‑storage applications.

Abbreviations

DMF:

Dimethylformamide

NOMC:

Nitrogen‑doped ordered mesoporous carbon

OMC:

Ordered mesoporous carbon

PANI:

Polyaniline

PANI/NOMC-x:

Composites of nitrogen‑doped ordered mesoporous carbon and polyaniline with different mass ratios

SEM:

Scanning electron microscope

TEM:

Transmission electron microscopy

XPS:

X‑ray photoelectron spectroscopy

XRD:

X‑ray powder diffraction