Ultra‑Fast, Ultra‑Stable Charge Storage in Hierarchically Macroporous Graphitic Nanowebs

Abstract

The macro‑ and micro‑architecture of carbon‑based electrodes is a decisive factor in supercapacitor performance. Here we report hierarchically macroporous graphitic nanowebs (HM‑GNWs) synthesized from bacterial cellulose by high‑temperature pyrolysis at 2400 °C. The resulting network consists of high‑aspect‑ratio graphitic nanofibers entangled into a 3‑D nanoweb. This unique morphology delivers outstanding charge‑storage: area capacitances of 8.9 mF cm⁻² at 5 V s⁻¹ down to 3.8 mF cm⁻² at 100 V s⁻¹, and ~97 % capacitance retention after more than 1,000,000 cycles.

Background

Multidimensional carbon‑based nanomaterials (MCNs) are highly attractive for energy storage owing to their large specific surface area, high conductivity, and exceptional thermal/mechanical stability [1,2,3]. Their inexpensive, abundant precursors make them suitable for scalable supercapacitor electrodes [4,5]. Supercapacitors excel in power delivery because their charge‑storage mechanism relies on surface adsorption/desorption rather than solid‑state diffusion [6]. Power density scales with the square of the operating voltage (P_max = V_i²/(4R)) and energy density with the square of the voltage (E = ½ C V²) [7,8]. Achieving high cell voltages (≥3 V) is possible with ionic liquid electrolytes (ILEs), but bulky organic ions can impede ion transport [7,8,9], underscoring the need for advanced MCN architectures that combine macroporosity with graphitic conductivity.

Macroporous, hierarchically open structures assembled from nanometer‑scale carbon building blocks promote rapid ion transfer, as demonstrated in recent studies [10–13]. Graphitic sp² carbon offers superior conductivity compared to amorphous carbon; however, high‑temperature graphitization often collapses nanoporous architectures. Therefore, designing graphitic nanomaterials that retain open porosity while achieving high conductivity remains a key challenge.

Bacterial cellulose (BC), produced by Acetobacter xylinum, is a sustainable nanofibrous polymer with exceptional purity, crystallinity, and mechanical strength [14–15]. Prior work has shown that BC pellicles can be carbonized while preserving their pore structure [14,16,17], and further graphitized at 2400 °C [17]. The resulting free‑standing carbon nanowebs can serve as binder‑free electrodes for energy storage.

In this study, we prepared hierarchically macroporous graphitic nanowebs (HM‑GNWs) and carbon nanowebs (HM‑CNWs) from BC pellicles via 2400 °C and 800 °C treatments, respectively. HM‑GNWs exhibit well‑ordered graphitic microstructures and superior electrochemical performance over a 3 V window under ILE, achieving a capacitance of 3.8 mF cm⁻² at a ultra‑high sweep rate of 100 V s⁻¹ and excellent cycling stability over 1,000,000 cycles.

Experimental

Preparation of HM‑GNWs and HM‑CNWs

BC pellicles were cultured with Acetobacter xylinum BRC 5 in Hestrin and Schramm medium for 14 days. The hydrogel was purified in 0.25 M NaOH, rinsed with deionized water, and soaked in tert‑butanol at 60 °C for 12 h. After freeze‑drying at –20 °C and lyophilization at –45 °C, 4.5 Pa for 3 days, the cryogels were thermally treated at 800 °C or 2400 °C in an argon atmosphere (5 °C min⁻¹). The resulting HM‑GNWs or HM‑CNWs were stored in a vacuum oven at 30 °C.

Electrochemical Characterization

Cyclic voltammetry (CV), chronopotentiometry, and electrochemical impedance spectroscopy (EIS) were performed with an Autolab PGSTAT302N. Ag/AgCl and Pt wire served as reference and counter electrodes. The electrolyte was a 1:1 (w/w) mixture of 1‑ethyl‑3‑methylimidazolium hexafluorophosphate (EMIM·PF₆) and acetonitrile (ACN). A three‑electrode cell in a beaker was used. Working electrodes were punched from HM‑GNWs (diameter = 0.5 in) with an active mass loading of ~4–5 mg. Specific capacitance (C) was calculated from galvanostatic discharge as C = 4 I_cons / (m dV/dt) (1), where I_cons is the constant current, m is the total mass of both electrodes, and dV/dt is the discharge slope.

Material Characterization

Morphology was examined by field‑emission SEM (FE‑SEM, Hitachi S‑4300) and TEM (FE‑TEM, JEOL JEM2100F). Raman spectra were recorded with a 514.5 nm laser (2.41 eV, 16 mW) focused by a ×100 objective. X‑ray diffraction (XRD, Rigaku DMAX 2500) used Cu‑Kα radiation (λ = 0.154 nm). X‑ray photoelectron spectroscopy (XPS, PHI 5700) assessed surface chemistry. Nitrogen adsorption/desorption isotherms (Micromeritics ASAP 2020) quantified surface area and pore volume at –196 °C. Electrical conductivity was measured by a four‑probe method with silver paint contacts and a Loresta GP meter (0.01 mA step, ±1 mA sweep).

Results and Discussion

FE‑SEM images (Fig. 1a,b) reveal that both HM‑CNWs and HM‑GNWs form macroporous nanowebs composed of highly entangled nanofibers with aspect ratios > 100. The fibers (~20 nm diameter) exhibit distinct microstructures: HM‑CNWs are amorphous, while HM‑GNWs display well‑ordered graphitic layers (Fig. 1c,d).

Raman spectra (Fig. 2a) show distinct D (1352 cm⁻¹) and G (1582 cm⁻¹) bands for HM‑GNWs, indicating a well‑ordered sp² network; a sharp 2D band at 2701 cm⁻¹ confirms three‑dimensional graphitic ordering. HM‑CNWs exhibit broad, merged D and G peaks with no 2D band, reflecting defective, amorphous carbon. XRD (Fig. 2b) corroborates these findings: a sharp (002) peak at 25.7° for HM‑GNWs versus a broad 24.0° peak for HM‑CNWs.

XPS C 1s spectra (Fig. 2c) reveal sp² carbon at 284.4 eV and minimal oxygen (< 1 %). The C/O ratios are 23.4 (HM‑CNWs) and 110.1 (HM‑GNWs), confirming the high graphitic purity of the latter.

Nitrogen adsorption isotherms (Fig. 2d) display type‑I/II hybrid shapes, indicating the coexistence of micropores and abundant macropores (tens of nm to several µm). Specific surface areas are 158.5 m² g⁻¹ (HM‑CNWs) and 138.7 m² g⁻¹ (HM‑GNWs); pore volumes are 0.346 and 0.310 cm³ g⁻¹, respectively.

Electrochemical performance in 1:1 EMIM·PF₆/ACN (0–3 V) is shown in Fig. 3. CV curves at 5 V s⁻¹ (Fig. 3a) are rectangular for HM‑GNWs, evidencing ideal double‑layer behavior; the shape remains intact up to 100 V s⁻¹ (0.04 s). HM‑CNWs exhibit dented curves and lower area, reflecting poorer rate capability.

Nyquist plots (Fig. 3c,d) show vertical lines at low frequency, confirming capacitive behavior. RC semicircles at ~420 Hz (HM‑GNWs) and ~425 Hz (HM‑CNWs) correspond to resistances of ~2.0 Ω and ~3.3 Ω, respectively; ESRs are 2.3 Ω (HM‑GNWs) and 3.7 Ω (HM‑CNWs). HM‑GNWs therefore possess substantially lower internal resistance, driven by two orders of magnitude higher conductivity (~130 S cm⁻¹) versus ~3.7 S cm⁻¹ for HM‑CNWs.

Specific area capacitance of HM‑GNWs is 8.9 mF cm⁻² at 5 V s⁻¹, decreasing linearly to 3.8 mF cm⁻² at 100 V s⁻¹ (Fig. 3e). HM‑CNWs start at 6.7 mF cm⁻² and drop more sharply, retaining only ~50 % at 25 V s⁻¹ and 1.1 mF g⁻¹ at 100 V s⁻¹. The superior rate performance of HM‑GNWs stems from their high conductivity and open architecture.

Long‑term cycling (1,000,000 CV cycles at 20 V s⁻¹, Fig. 3f) shows only a 3 % loss in capacitance, underscoring the reversibility and durability of the HM‑GNWs. These results surpass many reported carbon‑based supercapacitor electrodes [18–25].

Conclusions

Hierarchically macroporous graphitic nanowebs (HM‑GNWs) and carbon nanowebs (HM‑CNWs) were fabricated from bacterial cellulose by pyrolysis at 2400 °C and 800 °C, respectively. Both exhibit entangled, high‑aspect‑ratio nanofibers and macroporous networks. HM‑CNWs are amorphous, whereas HM‑GNWs possess well‑ordered graphitic microstructures. This microstructural difference translates into a pronounced gap in electrochemical performance: HM‑GNWs deliver ultra‑fast charge storage (8.9 mF cm⁻² at 5 V s⁻¹, 3.8 mF cm⁻² at 100 V s⁻¹) and exceptional cycling stability (> 97 % retention after 1,000,000 cycles).