3D Graphitic Carbon Nitride Nanowire Scaffold Boosts Flexibility and Capacitance in Solid‑State Supercapacitors

Abstract

This study reports a three‑dimensional composite electrode that couples a graphitic carbon nitride (g‑C₃N₄) nanowire scaffold with the conductive polymer PEDOT:PSS to create a flexible, solid‑state supercapacitor. By leveraging the high surface area of the nanowire network, the composite surpasses pure PEDOT:PSS, achieving a specific capacitance of 202 F g⁻¹. The device maintains 83.5 % of its initial capacitance after 5,000 charge‑discharge cycles, demonstrating exceptional cycling stability while preserving mechanical flexibility.

Background

Wearable energy storage, particularly flexible supercapacitors, has attracted significant interest due to their high power density and robust cycling performance [1,2,3,4]. Current electrode materials fall into three main categories: carbon‑based high‑surface‑area materials (e.g., activated carbon, graphene, carbon fibers), transition metal oxides (MOs), and conducting polymers (CPs) [5,6,7,8]. While carbon electrodes store charge through electrochemical double‑layer capacitance (EDLC), MOs and CPs operate via Faradaic pseudocapacitance [9,10,11]. MOs offer high theoretical capacities but suffer from poor conductivity, toxicity, instability, and cost; CPs overcome many of these issues yet are limited by relatively low mechanical and cycle durability and a modest specific surface area, which hampers their applicability in flexible devices.

To reconcile these trade‑offs, composite strategies that integrate 3D EDLC materials with MO or CP pseudocapacitive partners have emerged as promising solutions [12,13,14]. Among carbon candidates, graphitic carbon nitride (g‑C₃N₄)—a two‑dimensional graphene analogue—has recently been spotlighted for its favorable electronic properties, low cost, and environmental friendliness [15,16]. Although g‑C₃N₄ has been extensively studied for photocatalysis [17–22], its potential in energy storage remains underexplored, particularly in three‑dimensional morphologies [23–27]. PEDOT:PSS, a widely used conductive polymer, offers high conductivity and mechanical resilience, yet its limited surface area restricts capacitance. Enhancing its active area through a 3D scaffold is therefore a logical path forward.

In this work, we synthesize a 3D g‑C₃N₄ nanowire (GCNW) framework that serves as a structural backbone for PEDOT:PSS. The resulting composite delivers a specific capacitance of 202 F g⁻¹ in a three‑electrode configuration and 78 F g⁻¹ in a symmetric, all‑solid‑state device, achieving an energy density of 6.66 Wh kg⁻¹ at 200 W kg⁻¹. The study also examines how varying the g‑C₃N₄ ratio influences morphology and electrochemical performance.

Methods

Materials

Sodium hydroxide (NaOH) and urea were sourced from Beijing Chemical Corp. PEDOT:PSS (1.0 wt.% in H₂O, high‑conductivity grade) was purchased from Sigma‑Aldrich. No further purification was performed.

Synthesis of g‑C₃N₄

Urea (10 g) was calcined at 550 °C (10 °C min⁻¹) for 2 h in a muffle furnace to yield yellow g‑C₃N₄ powder.

Three‑Dimensional Fabrication of the GCNW

Five hundred milligrams of g‑C₃N₄ powder were dispersed in 20 mL of 3 M NaOH, stirred at 60 °C for 12 h, then sonicated for 2 h. Excess NaOH was removed by dialysis, and the resulting nanowire hydrogel was freeze‑dried to produce a self‑supporting aerogel.

Three‑Dimensional Preparation of GCNW/PEDOT:PSS Composite

The composite was assembled with four mass ratios of GCNW to PEDOT:PSS: 10 %, 20 %, 50 %, and 80 % (6 mg mL⁻¹ GCNW). After 12 h of stirring, the mixtures were freeze‑dried. A pure PEDOT:PSS thin film was fabricated by filtration for comparison.

Characterization

Morphology and structure were examined by field‑emission scanning electron microscopy (FESEM, JEOL 7610), transmission electron microscopy (TEM, Tecnai F20), and X‑ray diffraction (XRD, D‑MAX II A). Fourier transform infrared spectroscopy (FTIR, Nicolet‑6700) and X‑ray photoelectron spectroscopy (XPS, ESCALAB MK II) probed chemical composition.

Electrochemical Measurement

Electrochemical performance was measured with a CHI 660E workstation. In a three‑electrode setup, a platinum foil served as counter and a saturated calomel electrode (SCE) as reference. Working electrodes were fabricated by pressing the composite onto carbon cloth (1 mg cm⁻² loading). The electrolyte was 1 M H₂SO₄. Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) were recorded over 0–1 V. Electrochemical impedance spectroscopy (EIS) spanned 1–10⁵ Hz with a 5 mV perturbation.

Symmetric devices were assembled by depositing 2 mg of active material on each carbon cloth electrode, separating them with a PVA/H₂SO₄ hydrogel soaked on non‑woven fabric, and then sandwiching the layers. Electrochemical tests were performed on these two‑electrode cells.

The specific capacitance of a single electrode (C_m) was calculated from CV data: \n\nC_m = \frac{1}{U\nu}\int_{U^{-}}^{U^{+}}i(U)dU\n\nwhere U is the voltage window and ν is the scan rate. Energy (E) and power (P) densities were derived from: \n\nE = \frac{1}{2}CU^{2}\n\nP = \frac{E}{\Delta t}\n

Results and Discussion

The fabrication workflow and device architecture are illustrated in Fig. 1. The g‑C₃N₄ nanowire content critically determines the 3D framework; a minimum of 20 % GCNW is required to preserve the scaffold, whereas higher PEDOT:PSS concentrations (90 %) collapse the structure. Sodium hydroxide concentration also dictates the micro‑architecture: <3 M fails to cleave the layers adequately, while >8 M over‑cuts the nanowires and collapses the aerogel. A 3 M NaOH treatment therefore yields the most robust 3D scaffold.

3D Graphitic Carbon Nitride Nanowire Scaffold Boosts Flexibility and Capacitance in Solid‑State Supercapacitors

SEM images (Fig. 2a–c) reveal the transformation from layered g‑C₃N₄ to nanowire and the maintenance of a 3D network in the 20 % composite. TEM confirms nanowire dimensions of ~10 nm width and several hundred nanometers length (Fig. 2d–e). The BET surface areas are 82.67 m² g⁻¹ for GCNW and 69.86 m² g⁻¹ for the 20 % composite, while pure PEDOT:PSS exhibits a negligible surface area.

XRD patterns (Fig. 3a) show sharp (100) and (200) peaks of g‑C₃N₄ at 13.84° and 27.81°, while a broad PEDOT:PSS feature between 15°–30° indicates the polymer’s amorphous nature. FTIR spectra (Fig. 3b) confirm the characteristic triazine ring (804 cm⁻¹) and C‑N heterocycles (1299–1605 cm⁻¹) of g‑C₃N₄, alongside PEDOT:PSS signatures. XPS (Fig. 3c–f) verifies the presence of C, N, O, S, and residual Na from NaOH, confirming that the composite is a physical mixture without chemical bonding.

Electrochemical evaluation (Fig. 4) shows that the 20 % composite delivers the largest CV area and the longest GCD times, confirming superior capacitance. At 5 mV s⁻¹, its specific capacitance is 202 F g⁻¹, a 46.9 % improvement over pure PEDOT:PSS. The composite also retains 83.5 % of its initial capacitance after 5,000 cycles (Fig. 5c). The enhanced performance is attributed to the 3D scaffold, which increases active sites and shortens ion diffusion pathways.

The symmetric supercapacitor, assembled with 20% GCNW/PEDOT:PSS on carbon cloth, exhibits a rectangular CV shape across 0–1 V (Fig. 5a) and an ESR of 5.41 Ω (Fig. 5b). After 5,000 cycles at 1 A g⁻¹, the device retains 83.5 % of its capacitance (Fig. 5c). Mechanical flexibility tests reveal that capacitance retention remains above 80 % after 2,000 bending cycles at 90° (Additional file 1: Figure S11). Energy density reaches 6.66 Wh kg⁻¹ at 200 W kg⁻¹ (Fig. 5d).

Conclusion

For the first time, a 3D g‑C₃N₄ nanowire/PEDOT:PSS composite has been fabricated and deployed as an electrode for flexible supercapacitors. The 20 % composite delivers a record specific capacitance of 202 F g⁻¹ in a three‑electrode test and 78 F g⁻¹ in a symmetric, all‑solid‑state device, yielding an energy density of 6.66 Wh kg⁻¹. The 3D architecture is pivotal in boosting electrochemical performance and maintaining mechanical resilience. Given its cost‑effective synthesis and superior performance, this composite holds promise for next‑generation flexible energy storage solutions and commercial applications.