1.Structure and Composition Analysis of MnO₂Nanobelt-Coated Flexible Cotton Carbon Fiber Felt for Supercapacitor Electrodes

1.1 Crystal Structure of MnO₂ Nanobelt-Coated Flexible Cotton Carbon Fiber Felt

High-resolution transmission electron microscopy (HRTEM) images of MnO₂/CDCC reveal the presence of birnessite-type MnO₂ in the supercapacitor electrodes, as shown in Figure 1(a). The lattice fringes with spacings of approximately 0.347 nm, 0.258 nm, and 0.226 nm are consistent with the lattice spacings of the (002), (201), and (201) planes of birnessite-type MnO₂, respectively. Furthermore, X-ray diffraction (XRD) analysis also confirmed the presence of birnessite-type MnO₂. As shown in Figure 1(b), the XRD peaks at 2-12.6°, 25.4°, 42.3°, and 65.7°correspond to the (001), (002), (112), and (312) crystal planes of birnessite-type MnO₂(K_xMn₂O₄·1.5H₂O, JCPDS No. 42-1317).

The formation mechanism of MnO₂ nanoribbons can be attributed to the “rolling and curling mechanism” of MnO₂ nanosheets. Graphitic potassium manganese oxides have a layered structure containing edge-sharing MnO₆ octahedral sheets. These sheets are separated by potassium ions and/or water and can be extracted from or inserted into the sheets in aqueous or organic phases. Therefore, they are ideal candidates for the preparation of many other manganese oxides. The MnO₆ octahedral cores are initially assembled into a one-dimensional material with a small atomic layer thickness, so the layered one-dimensional material tends to curl and re-stack to form porous nanosheets. By rolling the nanosheets, ribbon-like MnO₂ is ultimately produced. The formation mechanism of MnO₂ nanoribbons is shown in Figure 1(c).

Figure 1 (a) HRTEM image and (b) XRD pattern of MnO₂/CDCC. The base material is the standard JCPDS No. 42-1317 for birnessite-type MnO₂; (c) Schematic diagram of the formation mechanism of MnO₂ nanoribbons.

1.2 Chemical Composition of Flexible Cotton Carbon Fiber Felt Coated with MnO₂ Nanoribbons

X-ray photoelectron spectroscopy (XPS) is a powerful tool for characterizing elemental valence states and can provide useful information on the oxidation state and chemical bonding of manganese in MnO₂/CDCC used in supercapacitor electrodes. As shown in Figure 2(a), the full-scan XPS spectrum of MnO₂/CDCC detected Mn, C, and O. Furthermore, the Mn 2p core-level XPS spectrum of MnO₂/CDCC exhibits two peaks at 653.9 eV and 642.3 eV (Figure 2(b)), corresponding to Mn 2p_1/2 and Mn 2p_3/2, respectively.

Furthermore, the spin separation energy between the Mn _1/2 and Mn 2p_3/2 peaks is 11.6 eV, consistent with previously reported data for MnO₂, indicating a predominant oxidation state of +4. A high-resolution O 1s spectrum is shown in Figure 2(c), where the two peaks at 531.3 eV and 529.7 eV correspond to Mn-O-H and Mn-O-Mn, respectively. This result further confirms that the shell is composed of MnO₂. Thermogravimetric (TG) analysis was used to roughly determine the MnO₂ loading in MnO₂/CDCC by subtracting the residual CDCC content from the residual MnO₂/CDCC content at 750°C. As shown in Figure 2(d), the slight weight loss is likely due to the evaporation or decomposition of residual oxygen-containing species. The calculated MnO₂ content in MnO₂/CDCC is approximately 17.1% by mass, demonstrating uniform and stable loading in the material prepared by XUANSN.

Figure 2: (a) XPS full scan spectrum, (b) Mn 2p core level XPS spectrum, and (c) O 1s core level XPS spectrum of MnO₂/CDCC; (d) TG curves of CDCC and MnO₂/CDCC.

2 Electrochemical Performance and Energy Storage Mechanism of MnO₂ Nanobelt-Coated Flexible Cotton Carbon Fiber Felt

The electrochemical performance of CDCC and MnO₂/CDCC as supercapacitor electrodes was studied in a three-electrode system using 1 mol/L Na₂SO₄ solution as the electrolyte over the potential range of 0∽0.8 V. As shown in Figure 3(a), the cyclic voltammetry (CV) curves of MnO₂/CDCC at scan rates of 5∽100 mV/s exhibit quasi-rectangular shapes and near mirror symmetry. Even at a high scan rate of 100 mV/s, the terminal currents at 0 V and 0.8 V show good symmetry, reflecting ideal electrochemical capacitive behavior. Furthermore, the charge curves of MnO₂/CDCC are nearly symmetrical with the corresponding discharge curves (Figure 3(b)), further revealing the favorable capacitive behavior and the reversible Faradaic reaction between Na⁺ and MnO₂: MnO₂+Na⁺+e^– ⇌MnOONa. Figure 3(c) shows the areal and gravimetric capacitances of MnO₂/CDCC as a function of increasing current density. It can be observed that at a current density of 0.1 mA/cm^2, the electrode exhibits the highest areal capacitance of 202 mF/cm², which is higher than many previously reported MnO₂-based electrodes, such as MnO₂ nanowire/ZnO nanorod arrays (41.5 μF/cm²), agarose gel-encapsulated MnO₂ (52.55 μF/cm²), and carbon-modified MnO₂ nanosheet arrays (143 μF/cm²). Furthermore, the specific capacitance of MnO₂/CDCC decreases with increasing current density, likely due to the varying degrees of Na⁺ intercalation and deintercalation from the electrolyte into MnO₂. Higher current densities significantly reduce the effective interaction between MnO₂ and ions, leading to a significant decrease in the capacitance of MnO₂/CDCC.

Figure 3. CV curves of the MnO₂/CDCC electrode at (a) different scan rates, (b) GCD curves at different current densities, and (c) rate performance; CV curves of the CDCC electrode at (d) different scan rates, (e) GCD curves at different current densities, and (f) rate performance.

Unlike the rectangular CV curves of conventional high-purity carbon, the CV curves of CDCC deviate significantly from a rectangular shape (Figure 3(d)). This is likely due to the electrochemical Faradaic reactions that occur in CDCC, such as the reaction between carbonyl (C=0) and hydroxyl (C-OH), i.e., C=0+H₂O+e^–⇌C-OH+OH^–. Similar results can be found in other oxygen-containing carbon materials, such as HNO₃ activated biochar. The galvanostatic charge-discharge (GCD) curves of CDCC exhibit a nearly symmetrical shape with a slight curvature (Figure 3(e)), indicating the synergistic effect of double-layer capacitance and pseudocapacitance. Figure 3(f) shows the rate performance of CDCC. When the current density increases 20-fold (from 0.1 mA/cm² to 2 mA/cm²), the CDCC electrode retains only 2.5% of its initial capacitance (decreases from 119 mF/cm² to 3 mF/cm²). In contrast, the MnO₂/CDCC core-shell structure electrode of the supercapacitor electrodes exhibits a significantly higher capacitance retention of 37.3% (see Figure 3(c)), indicating its superior rate capability. The higher electrochemical performance of MnO₂/CDCC may be attributed to the following three main reasons:

(1) The presence of a large number of nano-scale MnO₂ carbon ribbons (acting as sheaths on the surface of cotton carbon fibers) helps to increase the electrochemical active area accessible to electrolyte ions, thereby promoting ion adsorption and shortening the ion migration path during charge and discharge.

(2) The adsorption of electrolytes by the hierarchical porous CDCC matrix helps to shorten the ion diffusion path and accelerate the effective penetration of electrolyte ions. In addition, the conductive 3D carbon fiber network ensures the rapid transmission of electrons. EIS analysis (Figure 4) confirms this speculation. With the addition of MnO₂, the semicircle diameter (reflecting the charge transfer resistance) in the high-frequency region increases. The results show that CDCC with good conductivity can reduce the charge transfer resistance of the overall composite material, thereby improving the power density of the composite electrode and even the assembled ASC.

Figure 4 Nyquist plot of MnO₂/CDCC of CDCC and supercapacitor electrodes, the inset shows the magnified image of the high-frequency region

(3) The two-step synthesized core-shell nanostructure facilitates electron transfer between MnO₂ nanoribbons and CDCC carbon fibers due to its good interfacial bonding. This self-supporting structure also avoids the use of polymer binders or conductive additives in the preparation of the electrode, which will increase the additional contact resistance. The core-shell structure material is provided by XUANSN, which ensures the high quality and electrochemical stability of the material.