Fabrication of g-C₃N₄ Quantum Dots/MnCO₃ Nanocomposite on Carbon Cloth for Flexible Supercapacitor Electrode

Abstract

In this study, the nanocomposite of g-C₃N₄ quantum dots/MnCO₃ on carbon cloth (q-MC//CC) is prepared via a simple hydrothermal method. The obtained q-MC//CC composite is employed for a flexible supercapacitor electrode. The g-C₃N₄ quantum dots could effectively improve the interface electrical conductivity and ion transportation of the MnCO₃ electrode, which results in superior electrochemical performance. The q-MC//CC electrode delivers a high specific capacity of 1001 F·g⁻¹ at a current density of 1 A·g⁻¹ and a good cycling performance of 96% capacity retention after 5000 cycles. Moreover, an asymmetric flexible supercapacitor (ASC) is assembled with q-MC//CC and carbon cloth as a positive and negative electrode, respectively, which exhibits a high energy density of 27.1 Wh·kg⁻¹ at a power density of 500 W·kg⁻¹. In addition, the fabricated ASC device demonstrates the ability to power the light-emitting diode effectively under mechanical bending.

1. Introduction

Supercapacitors have attracted considerable attention for the application of advanced energy storage devices of electronic vehicles and portable electronic devices due to their beneficial properties such as excellent power density, rapid charging time, and durability [1]. Electric double layer capacitors (EDLC) mainly use various carbon-based materials, such as activated carbon, carbon nanotubes (CNTs), and graphene. In EDLC, an accumulation of ionic charges occurs between the electrode and electrolyte of carbon-based electrodes, and carbon-based electrodes with large surface areas deliver high power density and long cycle stability. On the other hand, pseudocapacitors can exhibit substantially high specific capacitance and high energy density compared to EDLC, owing to the fast and reversible surface Faradaic redox reaction. In addition, transition metal oxide and carbonate materials have been explored as electrode materials of pseudocapacitors due to their high theoretical storage capacity and superior reversibility. Among them, Mn-based active materials have attracted a great deal of interest owing to the high theoretical capacitance, low cost, and natural abundance [2,3,4,5]. However, most research has been done with Mn-based oxides, and manganese carbonates (MnCO₃) are relatively new and popular active materials for pseudocapacitors. Recently, MnCO₃ has been extensively studied as a potential electrode of supercapacitors due to the low manganese valence state of +2 and the redox process to high manganese valence states (+3 or +4), which could show high electrochemical performance. The low manganese valence (+2) in MnCO₃ can exhibit a better hydroxyl ions insertion of alkaline electrolytes for the formation of MnOHCO₃ compared to high manganese valence states (+3 or +4) [5,6,7]. However, low electrical conductivity and poor cycle stability restrict the application of MnCO₃ as an active material in energy storage applications. To overcome these drawbacks, an effective approach is to hybridize MnCO₃ using conductive carbon-based additives such as carbon [8], CNTs [9,10], and graphene [11,12,13,14], which have proved to be effective to improve electrochemical performance.

Graphitic carbon nitride (g-C₃N₄) recently attracted a lot of interest as a promising electrode candidate for electrochemical devices due to its high nitrogen content, good structural stability, and low interfacial impedance. Thus, g-C₃N₄ was employed in the transition metal oxides or hydroxide-based electrodes for supercapacitors. Chang et al. [15] synthesized novel sandwich-like MnO₂/g-C₃N₄ composites for supercapacitor electrodes and showed the enhanced electrochemical performance, which was ascribed to various active sites and the existence of N of g-C₃N₄ for better wettability with electrolytes. Gao et al. [16] reported the endurable supercapacitance using MnO₂/protonated g-C₃N₄. It showed that the strong hydrogen bonding between MnO₂ and the g-C₃N₄ can restrain particle agglomeration in the cycling test, resulting in stable energy storage performance. Ding et al. [17] fabricated the honeycomb nanostructured g-C₃N₄@Ni(OH)₂ for an asymmetric supercapacitor, where the g-C₃N₄ showed a good charge transport with good chemical stability. However, there are limited reports studying the application of g-C₃N₄ quantum dots (CNQDs) in energy storage fields.

Herein, we report a simple hydrothermal process to prepare the nanostructured MnCO₃ and CNQDs nanocomposites on carbon cloth (denoted as q-MC//CC). The active material of MnCO₃ and CNQDs nanocomposites is synthesized directly on the carbon cloth as a flexible conductive substrate, which is then also used for an electrode of a solid-state flexible asymmetric supercapacitor (ASC). MnCO₃ and CNQDs nanocomposites exhibit excellent electrochemical performance, including good specific capacitance and rate capability, which is attributed to the enhanced electrical conductivity and shorter ions pathway. Moreover, CNQDs have generally large surface areas and accessible edges that can function as active sites of electrochemical reaction, which would improve the electrochemical performance. The q-MC//CC electrode delivers a high specific capacitance of 1001.4 F∙g⁻¹ under a current density of 1 A∙g⁻¹ as well as good cycling performance (96% capacity retention). Furthermore, a solid-state binder-free flexible ASC using q-MC//CC electrode is demonstrated, showing a high energy density of 27.1 Wh∙kg⁻¹ at a power density of 500 W∙kg⁻¹ and good mechanical stability.

2. Materials and Methods

2.1. Preparation of g-C₃N₄ Quantum Dots

First, 5 g of melamine were calcinated in a muffle furnace at 550 °C for 4 h to obtain bulk g-C₃N₄. Then, 2 g of bulk g-C₃N₄ were treated in a mixture solution of sulfuric acid (40 mL) and nitric acid (40 mL) for about 2 h at room temperature. Then, the mixture was rinsed several times with deionized (DI) water, and the as-obtained white product was dispersed in the concentrated NH₃·H₂O. The mixture solution was transferred into a Teflon-lined stainless-steel autoclave with heating at 180 °C for 12 h. After the hydrothermal process, the precipitate was rinsed with DI water to remove residual NH₃ molecules. Subsequently, the treated g-C₃N₄ nanosheets were dispersed and sonicated in 100 mL water for 6 h, and the as-obtained aqueous suspension was centrifuged at 7000 rpm and dialyzed in a dialysis bag (3000 Da, Spectrum Lab. Inc) for 48 h to remove large particles. Finally, the CNQDs solution was obtained.

2.2. Synthesis of q-MC//CC Nanocomposites

First, carbon cloth was treated by immersing in a mixture acid solution (H₂SO₄:HNO₃ = 1:3) at room temperature for 24 h and then washed with DI water. For the typical synthesis of q-MC//CC nanocomposites (Figure 1), MnSO₄∙H₂O (3.38 g) was dissolved in DI water (134 mL), and NH₄F (0.6 g) was added to the above solution with stirring to form a transparent solution. After several minutes, urea (2.4 g) and 5 mg of CNQDs was added to the above homogeneous solution with 30 min stirring at room temperature. Then, the treated carbon cloth (3 × 1 cm²) was immersed into the mixture solution and treated with the hydrothermal process, maintaining at 120 °C for 4 h. The obtained q-MC//CC nanocomposites were washed with DI water to remove the residual reactants and dried at 60 °C. As a control sample, the MnCO₃ on carbon cloth sample was also synthesized by following the same procedure without adding CNQDs (denoted as MC//CC).

2.3. Characterization

A transmission electron microscope (TEM, H-8100, Hitachi, Tokyo, Japan) and a field-emission scanning electron microscope (FE-SEM, JEOL-JSM820, JEOL Ltd., Akishima-shi, Japan) were used to investigate the morphology of the obtained samples. X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Fisher, Waltham, MA, USA) measurements were carried out using monochromatic AlKα radiation (hν = 1486.6 eV). Contact angle measurements with electrolyte (1.0 M Na₂SO₄) droplets were performed to investigate the wettability of the q-MC//CC electrode using an Attension Theta Optical Tensiometer (Biolin Scientific AB, Stockholm, Sweden). Fourier transform infrared (FTIR) spectra were collected using a Nicolet IR 200 FT-IR spectrometer (Thermo Scientific, Waltham, MA, USA), and Raman spectroscopy (DXR Raman spectrometer, Thermo Scientific, Madison, WI, USA) was performed with a 532 nm excitation source.

2.4. Electrochemical Measurements

Electrochemical performance was investigated by a cyclic voltammetry (CV) test, galvanostatic charge/discharge (GCD) test, and electrochemical impedance spectroscopy (EIS) test. All electrochemical characterizations were carried out using a BioLogic VSP electrochemical workstation at room temperature. A three-electrode system was used to test the electrochemical performance of the prepared electrodes with platinum wire as a counter electrode and Ag/AgCl electrodes as a reference electrode. The obtained products such as MC//CC and q-MC//CC are used as a binder-free working electrode. The mass loading of active materials was about 2.5 mg in this work. All electrochemical characterizations were measured in an aqueous electrolyte of 1 M Na₂SO₄ at room temperature. EIS was performed at open circuit potential in a frequency range from 10 to 0.01 Hz with an AC amplitude of 5 mV. The solid-state flexible ASC was prepared using the q-MC//CC as a positive electrode and carbon cloth as a negative electrode, which were assembled with polyvinyl alcohol (PVA)/Na₂SO₄ gel electrolyte and a separator. The prepared two electrodes were wetted with the PVA/Na₂SO₄ gel electrolyte for 10 min and then assembled face to face with a separator. The specific capacitance (Cs) was calculated from the CV curves and galvanostatic charge–discharge curves using the following equations, respectively:

$$C_s=\frac{{{\displaystyle \int }}^{ }I\left(V\right) dV}{v\times m\times \Delta V}$$

$$C\mathrm{s}=\frac{I\times \Delta t}{m\times \Delta V}$$

where ${{\displaystyle \int }}^{ }I\left(V\right) dV$ is the area enclosed by the CV curves, ΔV (V) is the potential window, m (g) is the mass of electrode, ν (mV s⁻¹) is the potential scan rate, and Δt (s) is the discharge time.

3. Results and Discussion

Figure 2a,b show FE-SEM images of as-prepared q-MC//CC nanocomposites. The pristine carbon cloth shows a smooth surface consisting of aligned carbon fiber bundles. After the hydrothermal reaction of the synthesis of q-MC//CC, it is observed that clusters of spindle-shaped petals of MnCO₃ were densely grown on the surface of carbon fibers. In addition, the prepared q-MC//CC sample shows good flexibility, as shown in the inset of Figure 2a. The morphology of MC//CC (Figure S1) has a similar nanostructure to that of q-MC//CC, showing that the addition of CNQDs has no effect on the morphological formation of MnCO₃. The TEM image (Figure 2c) of the q-MC//CC confirms that the CNQDs are uniformly embedded in MnCO₃, and the lattice fringe of CNQDs is found in the high resolution TEM (HRTEM) image (Figure 2d), where a lattice distance is 0.336 nm, corresponding to the (002) planes of g-C₃N₄. The SEM image of q-MC//CC and its elemental mapping from energy-dispersive X-ray spectroscopy (EDS) reveal the existence of the expected atomic elements (Figure 2e), which clearly confirms that Mn, C, and N elements are uniformly distributed on the MnCO₃ surface and the successful formation of q-MC//CC nanocomposite. In addition, the obtained q-MC//CC has a larger surface area (182 m²/g) than the pristine carbon cloth (69 m²/g) due to its unique nanostructure (Figure S2).

An XPS study was performed to characterize the chemical compositions of the q-MC//CC. The XPS survey scans in Figure 3a show that both q-MC//CC and MC//CC samples contain the Mn, O, and C atoms, while the N peak is only found in the q-MC//CC nanocomposites due to CNQDs. The deconvolution of the Mn 2p spectrum of q-MC//CC (Figure 3b) displays two peaks located at 655.8 eV and 643.8 eV respectively, assigning to Mn 2p_1/2 and Mn 2p_3/2 with an energy separation of 12.0 eV, which coincides with Mn²⁺ [18,19]. In the high-resolution C1s spectrum of q-MC//CC (Figure 3c) and MC//CC (Figure S3), two typical peaks of the C-C bond at 284.6 eV and C=O bond at 288.5 eV are observed, relating to the surface carbon of the carbon cloth and carbon element in carbonate ions [20]. In addition, two additional peaks at 286.0 and 292.7 eV are only observed for q-MC//CC, which correspond to the C-N bond and carboxylate bond of CNQDs, respectively. In addition, the high-resolution N 1s spectrum of q-MC//CC can be fitted into three peaks, as shown in Figure 3d, corresponding to =N- (398.4 eV), C=N-C (399.1 eV), and N-(C)₃ (400.4 eV) [21], indicating the presence of CNQDs in the q-MC//CC nanocomposite. The contact angle measurements with 1 M Na₂SO₄ electrolyte were performed to study the effect of CNQDs on the surface wettability of the prepared q-MC//CC composite. Figure S4 illustrates the images of electrolyte droplets on the prepared MC//CC and q-MC//CC composites, respectively. The q-MC//CC composite shows a lower contact angle with Na₂SO₄ electrolyte than MC//CC, which is due to the more ion-accessible surfaces by CNQDs. This enhanced surface wettability of the q-MC//CC composite facilitates the accessibility of electrolyte, leading to better electrochemical performance compared to the MC//CC composite. In addition, the FTIR spectra of the as-prepared q-MC//CC and MC//CC are shown in Figure S5. The characteristic peak of the Mn-O-Mn bond is observed at 727 cm⁻¹ for both samples, while that of the characteristic peaks corresponding to the C-N (1033 cm⁻¹) and C=N bonds (1780cm⁻¹) are only found for q-MC//CC due to the existence of CNQDs.

The electrochemical performances of q-MC//CC composites as binder-free electrodes of supercapacitors are studied using CV and GCD characterization in 1 M Na₂SO₄ aqueous electrolyte using a three-electrode system. Figure 4a shows the CV curves of q-MC//CC and MC//CC measured at a scan rate of 5 mV·s⁻¹ with potentials ranging from −0.2 to 0.8 V. CV curves of both electrodes show rectangular shapes in a potential window, indicating excellent capacitive behavior. The larger integral area of the CV curve for q-MC//CC electrode suggests higher specific capacitance than the MC//CC electrode, which represents its superior electrochemical property due to the synergistic effects of CNQDs and MnCO₃. The CV curves of q-MC//CC measured at different scan rates from 5 mV·s⁻¹ to 100 mV·s⁻¹ are shown in Figure 4b. The CV curve shapes are maintained upon increasing the scan rate, revealing fast ionic and electronic transportation. The current response also simultaneously increases along with the scan rate, showing a diffusion-controlled phenomenon. The GCD measurement of the q-MC//CC electrode was performed at different current density (Figure 4c). All GCD curves show some linear and symmetric behavior, indicating capacitive characteristics and good reversibility of the q-MC//CC electrode. In addition, a noticeable potential drop at the discharge plot is not observed, suggesting the low internal resistance of the q-MC//CC electrode. The specific capacitance decreases with increasing current densities, due to slow ionic diffusion at high current. Figure 4d compares the calculated specific capacitance of q-MC//CC and MC//CC electrodes at current densities. It demonstrates the superior capacitive performance of q-MC//CC compared to the MC//CC electrode by the incorporation of CNQDs. The calculated specific capacitances of the q-MC//CC electrode are 1001.4, 609.3, 469.8, and 322.6 F·g⁻¹ at 1, 2, 3, and 5 A·g⁻¹, respectively. The charge storage mechanism of MnCO₃ involves the intercalation/de-intercalation of Na⁺ with the redox reactions of the Mn ions. In addition, a surface process of the adsorption/desorption of Na⁺ ions occurs. The redox reaction of Mn (II) ↔ Mn (I) occurs in Na₂SO₄ electrolyte: MnCO₃+ Na⁺+ e⁻ ↔ NaMnCO₃. CNQDs on MnCO₃ can facilitate the intercalation/de-intercalation of Na+ ions and the redox reaction of Mn ions. This excellent electrochemical performance of the q-MC//CC electrode is highly comparable with previously reported MnCO₃-based electrodes (Table S1).

To further investigate the kinetics of ion and charge transfer, an EIS test was performed, and the Nyquist plots of q-MC//CC and MC//CC are presented in Figure 5a. The plots consist of a depressed circle at the high-frequency region and a straight line at the low-frequency region. The depressed is attributed to the charge transfer process of the Faradaic reaction at the interface of the electrode and electrolyte, and it belongs to the kinetic control, while the low-frequency region belongs to the mass transfer control. The formation of a depressed circle due to the roughness of the electrode/electrolyte surface results in the change of capacitance in double layer. In addition, the high slope of observed straight lines at the low-frequency region represents a small Warburg resistance. At the low-frequency region, the linear shape suggests the fast ion diffusion in electrolyte and the adsorption process at the electrode surface. The solution resistances (Rs) of q-MC//CC and MC//CC are 2.1 Ω and 2.5 Ω; the as-prepared q-MC//CC enhances electron transport from electrode to current collector, implying the decrease in equivalent series resistance by the strong synergistic effect between MnCO₃ and CNQDs. The diameter of the depressed circle denotes the charge transfer resistance (R_ct). The q-MC//CC electrode exhibits a much lower R_ct value (10.8 Ω) than the MC//CC electrode (18.0 Ω), clearly revealing that the highly conductive interface between CNQDs and MnCO₃ significantly improves the charge transfer kinetics. The long-term cycle stability of q-MC//CC and MC//CC electrodes was examined by CV measurement for 5000 cycles at a scan rate of 50 mV·s⁻¹. As shown in Figure 5b, the q-MC//CC electrode exhibits superior cycle performance with a 96% retention of its initial specific capacitance compared to the MC//CC electrode (71%). These results may be attributed to the distribution of CNQDs on MnCO₃, which can suppress the deformation and damage of the structure. As shown in the insert images in Figure 5b, the q-MC//CC electrode retains its initial microstructure after cycling, while the MC//CC electrode shows a swollen structure, revealing the excellent cycle stability of the q-MC//CC electrode.

To study the practical application of q-MC//CC as a binder-free electrode, an all-solid-state asymmetric supercapacitor (ASC) device was constructed. The q-MC//CC electrode was the positive electrode and a carbon cloth was the negative electrode, which were then assembled with a polyvinyl alcohol (PVA)/Na₂SO₄ gel electrolyte and separator (Figure 6a). The CV curves measured at different scan rates are shown in Figure 6b. All CV curves present typical quasi-rectangular shapes without any obvious redox peaks, indicating the ideal capacitive behavior of the prepared ASC with the q-MC//CC electrode. Figure 6c presents the GCD curves at different current densities from 1 to 5 A·g⁻¹. The nearly symmetric charge–discharge curves imply the high reversibility of the q-MC//CC electrode. The specific capacitances of ASC using the q-MC//CC electrode obtained from the curves are 195, 168, 147, and 130 F·g⁻¹ at current densities of 1, 2, 3, and 5 A·g⁻¹, respectively. The energy density of the assembled ASC is calculated and compared with other previous Mn-based ASC devices in the Ragone plot, as shown in Figure 6d. The prepared q-MC//CC electrode ASC delivers an energy density of 27.1 Wh·kg⁻¹ at a power density of 500 W·kg⁻¹ and retains 18.1 Wh·kg⁻¹ even at a high power density of 2500 W·kg⁻¹, which is superior to other systems reported [22,23,24,25,26,27]. To explore mechanical stability, the CV curves of the all-solid-state ASC using q-MC//CC are measured under different bending angles at a scan rate of 50 mV·s⁻¹. The results in Figure 6e reveal that the CV curve shapes are well maintained with no remarkable changes, even at a bending angle of 180^o, which reveals the good mechanical stability of the a-MC//CC electrode during bending. In addition, the application potential of the assembled ASC is further explored. Figure 6f shows that the ASC device under a flat and folded state lights up the green light-emitting diode (LED).

4. Conclusions

In summary, q-MC//CC for a binder-free electrode of supercapacitor was successfully prepared via a simple hydrothermal method. The synergistic effects of CNQDs embedded on MnCO₃ can improve the interface electrical conductivity and shorten the ion diffusion paths for the utilization of MnCO₃. The binder-free q-MC//CC electrode shows enhanced supercapacitive performance, including a high specific capacitance of 1001.4 F·g⁻¹ at a current density 1 A·g⁻¹ and a superior cycling performance of 96% retention after 5000 cycles. In addition, the q-MC//CC electrode-based ASC was assembled with a gel electrolyte, which exhibited a high energy density of 27.1 Wh·kg⁻¹ with a power density of 500 W·kg⁻¹ with good mechanical stability. These results provide a promising alternative method to prepare a binder-free electrode for high-performance flexible supercapacitors.