Tunable Na-doped MnO₂ Nanowires/g-C₃N₄ Composites for Efficient Energy Storage in Supercapacitors

Experimental

In this study, Na-MnO₂ nanowires were hydrothermally synthesized and subsequently integrated with exfoliated g-C₃N₄ sheets to form a conductive composite electrode.

MnO₂ and Na-MnO₂ synthesis

A facile hydrothermal technique was employed to produce MnO₂ NWs, with and without sodium doping.^[32] First, 4 g of KMnO4 were dissolved in DI water to create a homogeneous solution. Subsequently, 2 g of (NH₄)₂S₂O₈ were added, followed by 1 mL of HNO₃. The mixture was stirred for 30 min to ensure proper mixing. Once well mixed, the solution was poured into a 100 mL Teflon-lined autoclave heated to 180°C. The autoclave was then placed at this temperature for 16 h. After the reaction was complete, the autoclave was cooled to ambient temperature, and the precipitates were carefully recovered. These precipitates were completely free of impurities, neutralized, and dried at 100 °C. Na-MnO₂ NWs were prepared using the same preparation method by adding 340 mg of Na₂SO₄. The reaction conditions (180°C and 16 h) were selected based on earlier research and preliminary experiments that demonstrated that lower temperatures or shorter durations resulted in poor crystallinity and incomplete nanowire growth, while higher temperatures led to nanowire agglomeration.^[33]

g-C₃N₄ sheets synthesis

g-C₃N₄ was synthesized by annealing melamine for 2 h at 550 °C in a muffle furnace. The thereby formed g-C₃N₄ was finely ground in a mortar and pestle after cooling. The ground product was recognized as graphitic carbon nitride (g-C₃N₄). To prepare g-C₃N₄ sheets, the material was ultrasonicated for 5 h. The sonicated product was then dried at 80°C in an oven.^[34]

Na-MnO₂/g-C₃N₄ synthesis

To synthesize the Na-MnO₂/g-C₃N₄ composite, 20 mg of g-C₃N₄ was dissolved in DI water and sonicated for 30 min to achieve enough dispersion. The composite was prepared by adding 80 mg of Na-MnO2 to the dispersed g-C3N4 solution and sonicating for 45 min. After the composite was thoroughly mixed, it was dried at 80°C. A schematic illustration of the synthesis of the Na-MnO2 NWs/g-C₃N₄ composite is shown in Figure 1.

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Figure 1:

Schematic illustration for the synthesis of Na-MnO₂/g-C₃N₄ composite.

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Electrode preparation

Electrodes of MnO₂, Na-MnO₂, and Na-MnO₂/g-C₃N₄ composites were fabricated for electrochemical performance evaluation without using any binder. The geometric area of each electrode was 1 cm² and was loaded with a mass of 6 mg cm^-2. The electrochemical performance of these electrodes was explored in 1 M Na2SO4 aqueous electrolyte after deposition on nickel foam (NF). The active ingredient was sonicated for an hour in DI water to prepare a slurry. Afterwards, the paste was transferred to NF. After deposition, the drying was carried out at 60°C.

Characterization

A Philips X’Pert Pro X-ray diffractometer was employed for structural investigation, while an IR Affinity-1S spectrophotometer was utilized to determine functional groups. FEI Inspect S50 scanning electron microscope was used for morphological analysis, while elemental analysis was executed on JEOL JCM-6000 Plus SEM. Micromeritics ASAP 2020 Physisorption analyzer was utilized to measure the surface area. The Gamry interface 5000E was utilized for electrochemical performance evaluation. The BET surface area values were obtained from the MS Excel datasheet, and the pore size distribution was determined by plotting the relevant data in Origin. To extract equivalent circuit characteristics, data from electrochemical impedance spectroscopy (EIS) were fitted using EC-Lab software.

Electrochemical measurements

An electrochemical study was performed in a half-cell configuration. Ag/AgCl was used as the reference electrode, and a Pt wire was used as the counter electrode. The NF was used as a current collector. All measurements were performed in an aqueous solution of 1 M Na2SO4 as the electrolyte. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements were performed in the potential range of 0-0.6 V. EIS measurements were made in the frequency range of 0.1 Hz to 0.1 MHz.

X-ray diffraction studies (XRD)

XRD was employed to investigate the crystal structure of g-C₃N₄, MnO₂ NWs, Na-MnO₂ NWs, and Na-MnO₂ NWs/g-C₃N₄. Figure 2 depicts the XRD profile of all the samples, with 2θ at 12.53°, 17.89°, 25.56°, 28.35°, 37.42°, 41.62°, 49.53°, 56.60°, and 69.09°. According to ICDD card No. 00-044-0141,^[35] these peaks are ascribed to the (110), (200), (220), (310), (211), (301), (411), (600), (521), and (541) diffraction planes, respectively. The XRD profile confirmed that all MnO₂ samples are grown in the I4/m space group with a tetragonal structure. As displayed in Figure 2(d), a change in peak position and reduction in intensity for Na-MnO₂ and Na-MnO₂/g-C₃N₄ were observed, which demonstrates the successful synthesis of doped and composite samples. This XRD peak shift towards lower 2θ values is associated with the larger ionic radii of the dopant, which expand the crystal lattice.^[36]

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Figure 2:

(a) XRD profile of all the samples, (b) Demonstration of XRD peak shifting with doping.

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The crystallite size of bare and Na-doped MnO₂ NWs was calculated using the Debye-Scherrer equation.

The crystallite size of MnO₂ and Na-MnO₂ was calculated to be 23 nm and 20 nm, respectively. The crystallite size of Na-doped MnO₂ NWs is reduced with respect to bare MnO₂ NWs. The greater ionic radii of the doped Na⁺ ions lead the crystallite lattice to expand, resulting in a decrease. Lattice parameters for MnO₂ and Na-MnO₂ NWs are given in Table 1.

Fourier transform infrared spectroscopy (FTIR)

FTIR spectra of bare MnO₂ NWs and Na-MnO₂ NWs were recorded between 4000-400 cm^-1 and are shown in Figure 3. Five absorption bands have been found in bare MnO₂ NWs. The bands at 455 cm^-1, 538 cm^-1, and 734 cm^-1 correspond to Mn-O bonds of MnO₂, illustrating the successful synthesis of MnO₆ octahedra.^[32] The OH functional group from water adsorbed on the MnO₂ surface or within the tunnels accounts for the absorption band at 1634 cm^-1, which stabilizes the α-phase. A wide band at 3425 cm^-1 is ascribed to the O-H stretching vibration.^[37] The FTIR spectrum of Na-MnO₂ NWs demonstrates similar absorption bands to bare MnO₂ NWs. However, with doping, the metal-oxygen (Mn-O) bands shift to lower wavenumbers, suggesting a weakening of the bond due to the stress induced in the crystal by Na doping. Na-MnO₂/g-C₃N₄ exhibited all characteristic absorption bands of Na-MnO₂, along with additional bands between 1175 cm^-1 and 1470 cm^-1 due to C-N, C=N, and C-O of g-C₃N₄.^[38] FTIR peak positions and their corresponding vibration modes are given in Table 2.

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Figure 3:

FTIR profile of the samples.

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Scanning electron microscopy (SEM)

SEM has examined the morphologies of MnO₂, Na-MnO₂, g-C₃N₄, and Na-MnO₂/g-C₃N₄ composites. As shown in Figure 4(a), the MnO₂ sample has grown into a 1-D NW morphology. The 1-D nano-architecture has an aspect ratio greater than 50 nm, making it best recognized as a nanowire. The majority of MnO₂ NWs are elongated, needle-like structures that are randomly interconnected, forming an irregular, mesh-like configuration. As shown in the Na-MnO₂ micrograph (Figure 4b), the doped NWs are uniformly dispersed and look randomly interwoven, demonstrating the fungal filaments’ structure. SEM images of Na-MnO₂/g-C₃N₄ (Figure 4c) discovered that the synthesized Na-MnO₂ NWs have been incorporated in g-C₃N₄ sheets, confirming the preparation of the Na-MnO₂/g-C₃N₄ composite.

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Figure 4:

SEM micrographs of (a) MnO₂ (×30,000; inset: HRSEM image), (b) Na-MnO₂,(×50,000; inset: HRSEM image), (c) Na-MnO₂/g-C₃N₄ (×24,000).

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Elemental analysis

EDX has analyzed the prepared materials to determine their chemical purity and elemental composition. The EDX spectrum of the MnO₂ NWs (Figure 5a) demonstrated only peaks for oxygen and manganese, presenting the material’s purity. The energy loss associated with electronic transitions from a higher-energy level to a lower-energy level (L/K shell) is measured in keV, with the K shell having a higher energy loss than the L shell. The EDX spectrum of the Na-MnO₂ NWs (Figure 5b) presented extra peaks around 1 keV, which corresponded to the electronic de-excitation of the Na atom. The EDX pattern for Na-MnO₂/g-C₃N₄ (Figure 5c) displayed the existence of both Na-MnO₂ (Na, Mn, O) and g-C₃N₄ (C, N) components, indicating that the composite has been successfully synthesized. Elemental composition (atomic %) of all the samples is given in Table 3.

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Figure 5:

EDX profile of (a) MnO₂, (b)Na-MnO₂, (c) Na-MnO₂/g-C₃N₄.

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Surface area analysis

BET analysis was employed to explore Na-MnO₂ and Na-MnO₂ reinforced with g-C₃N₄, with a focus on the effect of g-C₃N₄ reinforcement on specific surface area. Figure 6 shows the N₂ adsorption/desorption isotherms of the synthesized materials. Based on IUPAC standards, the isotherms for both samples are of type III, with H3 hysteresis loops. At elevated relative pressure, a rapid rise in adsorption across both profiles suggests small pores of the synthesized materials. Even though the adsorption behavior appears similar, the doped and composite samples demonstrated much greater gas adsorption than Na-MnO₂, indicating that metal doping and composite formation enhance surface area. The BET analysis showed specific surface areas of 55.2 m² g^-1 for Na-MnO₂ and 85 m² g^-1 for the g-C₃N₄-reinforced composite. Na-MnO₂ demonstrated a narrow mesoporous distribution positioned at 18-22 nm, generated by voids between MnO₂ nanowires (Figure 6a inset). The Na-MnO₂/g-C₃N₄ composite demonstrated a wider mesoporous structure with bigger pores (25-35 nm), thanks to the spacer effect and interfacial coupling of g-C₃N₄ nanosheets Figure 6b, inset). This improved pore network enhances electrolyte penetration and ion-diffusion kinetics, leading to superior capacitance and rate performance for the composite electrode.

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Figure 6:

BET adsorption-desorption isotherm of (a) Na-MnO₂ (inset shows the pore size distribution), and (b) Na-MnO₂/g-C₃N₄ (inset shows the pore size distribution).

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Electrochemical performance analysis

The electrochemical performance of MnO₂, Na-MnO₂, and the Na-MnO₂/g-C₃N₄ composite was evaluated by coating them onto NF to fabricate working electrodes. Key experiments, including CV, GCD, and Electrochemical Impedance Spectroscopy (EIS), were conducted at room temperature to assess the potential of the fabricated electrodes for supercapacitor applications. Figure 7(a) demonstrates the CV profiles for MnO₂ NWs, Na-MnO₂ NWs, and Na-MnO₂ NWs/g-C₃N₄ on NF at a scan rate of 50mV s^-1. The appearance of oxidation and reduction peaks in the CV curves of bare and Na-MnO₂ electrodes specifies the presence of Faradaic redox processes. The CV curve area for Na-MnO₂ is larger than that of MnO2, indicating a higher specific capacitance. Additionally, the CV profile of the Na-MnO₂/g-C₃N₄ electrode is relatively flat and symmetrical, indicating higher capacitance and a hybrid charge-storage mechanism.^[42] It is ascribed to the synergistic effect of Na-MnO₂ and g-C₃N_4, which has two components contributing to charge storage: electric double-layer capacitance (EDLC) from g-C₃N₄ and pseudocapacitive from Na-MnO₂. The predominant charge-storage mechanism in g-CN is EDLC.^[43]

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Figure 7:

(a) CV profile of all the samples @ 50 mV s^-1, (b) CV profile of Na-MnO₂/g-C₃N₄ @ various scan rates, (c) Power law plot to determine prominent charge storage process, (d) Capacitive-controlled and diffusion-controlled capacitance contribution towards total capacitance.

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Figure 7(b) displays the CV curves of Na-MnO₂/g-C₃N₄ obtained at scan rates from 5-50 mV s^-1. The material’s excellent stability and reversible nature are demonstrated by the flatter CV curves at greater scan rates. This trend is apparent in the widening gap between the anodic and cathodic scans, along with the enlarged current responsiveness. Significantly, despite these variations, the overall shape of the CV curves remains consistent, confirming the stability of the Na-MnO₂/g-C₃N₄ material.

To identify the primary charge storage process (EDLC/Pseudocapacitive) in Na-MnO₂/g-C₃N₄ composite, a power law relationship i=av^b was employed.^[44,45] A plot of the log of current density vs the log of scan rate, as shown in Figure 7(c), was drawn to estimate the slope and intercept by linear fitting. A slope value of 0.5 indicates a diffusion-controlled process, whereas a slope value of 1 indicates a surface-controlled charge-transfer mechanism.^[46,47] In this study, the slope is 0.95, signifying that the surface-controlled process is dominant in Na-MnO₂/g-C₃N₄.

We employed Dunn’s method to calculate the capacitive and diffusion-controlled capacitance (percentage) to total capacitance in Na-MnO₂/g-C₃N₄.^[48-50] The results are illustrated in Figure 7(d). Across all feasible sweep rates, diffusion-controlled mechanisms made a much lesser contribution than surface-controlled mechanisms. At 5 mV s^-1, the diffusion-controlled mechanism had the highest contribution (35%), as slower sweep rates allow ions to permeate the active material’s bulk or core. However, at 100 mV s^-1, the diffusion-controlled mechanism contributed only 13% to the total capacitance. At higher sweep rates, there is little time for deep electrolyte ion penetration, making the surface-dominated charge storage mechanism more prominent.^[32,51] The redox peaks of Na-MnO₂ and Na-MnO₂/g-C₃N₄ are due to reversible Mn³⁺/Mn⁴⁺ redox processes at the surface and near-surface regions. Na⁺ ions pre-intercalation enhances ion accessibility and subsurface penetration. Nevertheless, charge storage is still mostly through swift pseudocapacitive processes, not bulk intercalation. The charge storage process can be described as.

${\text{MnO}}_{\text{2}}+{\text{Na}}^{+}+{\text{e}}^{-}\to \text{MnOONa}$

GCD behavior was explored on bare, sodium-doped, and g-C₃N₄-reinforced electrodes to obtain more accurate capacitance values. Figure 8(a) compares the GCD profiles of three electrodes at a current density of 1 A g^-1. Among these, the discharge time for Na-MnO₂/g-C₃N₄ (561 s) was longer than that of MnO₂ (392 s) and Na-MnO₂ (451 s). The accurate capacitance value for each electrode was calculated using Equation 2^[52] by using current density (I m^-1), discharging time ($\Delta t$) and potential window ($\Delta V$).

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Figure 8:

(a) GCD profile of all the samples, (b) GCD profile of Na-MnO₂/g-C₃N₄ @ various current densities, (c) Error bars for 5 different electrodes of Na-MnO₂/g-C₃N₄, (d) Cyclic performance of Na-MnO₂/g-C₃N₄ over 6000 cycles @ 12 A/g (inset: first 5 cycles and last 5 cycles, (e) EIS profile of all the samples (Inset shows the Randles circuit), (f) Graphical illustration of performance enhancement mechanism of Na-MnO₂ NWs/g-C₃N₄ nanosheets.

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The Na-MnO₂/g-C₃N₄ electrode delivered a capacitance of 935 F g^-1, higher than the MnO₂ (653 F g^-1) and the Na-MnO₂ (751 F g^-1). As depicted in Figure 8(a), the MnO₂ NWs endured an IR drop (or voltage drop) at the beginning of the discharging curve. The IR drop indicates the electrode’s internal resistance.^[53] Sodium doping of MnO₂ reduced this IR loss, and the introduction of g-C₃N₄ practically eliminated it. This improvement is attributed to the synergistic effects of Na doping and g-C₃N₄ reinforcement, which enhance MnO₂ conductivity. The specific capacitance of all the samples is given in Table 4.

We further examined the composite-based electrode’s behavior by calculating the specific capacitance of Na-MnO₂/g-C₃N₄ at various current densities. As demonstrated in Figure 8(b), even when the current density progressively increases, the shape of the GCD curves remains fundamentally the same, despite longer discharging times. This behavior determines the electrode material’s astonishing stability through the charging/discharging cycles. The specific capacitance of Na-MnO₂/g-C₃N₄ was estimated by Equation 2, and the values are provided in Table 4. The Na-MnO₂/g-C₃N₄ delivered a specific capacitance of 953 F g^-1 at 1 A g^-1 and retained 89.83% of this value when tested at 9 A g^-1. The finding complies well with the anticipated rate performance. The decrease in capacitance at 9 A g^-1 is likely due to slower redox processes occurring at higher current densities. Particularly, at higher current densities, electrolyte ions endure less time to percolate into the material’s bulk, causing them to reside on the surface, minimizing total capacitance predominantly. On the other hand, at a low current density of 1 A g^-1, the electrolyte ions have sufficient time to percolate deeper into the electrode material, utilizing the bulk for charge storage and thereby enhancing capacitance. Specific capacitance of Na-MnO₂/g-C₃N₄ at different current densities is given in Table 5. Figure 8(c) shows the error bar profiles of specific capacitance for 5 different electrodes of the Na-MnO₂/g-C₃N₄ composite. One of the most important criteria determining how often a supercapacitor can be utilized is the cyclic stability of its electrode material. This measurement indicates a supercapacitor’s potential to retain its original capacity after numerous charging/discharging cycles. The stability of the Na-MnO₂/g-C₃N₄ was assessed over 6000 GCD cycles at a current density of 12 A g^-1. As demonstrated in Figure 8(d), the Na-MnO₂/g-C₃N₄ possesses outstanding cycling stability by keeping 96.2% of its capacitance after 6000 cycles.

Na⁺ ions occupy the tunnels of the MnO₂ lattice, which leads to lattice expansion and deformation, generating extra defect sites and oxygen vacancies. These modifications enhance ionic conductivity through rapid Na+ ion diffusion and electrical conductivity by increasing the charge-carrier density. Additionally, Na⁺ doping strengthens MnO2, thereby enhancing electrochemical stability. The introduction of g-C₃N₄ enhances charge transfer by serving as a conductive, chemically stable 2D support. The π-conjugated structure of g-C₃N₄ facilitates electronic transport and establishes a close interfacial contact with Na-MnO₂, thereby decreasing charge-transfer resistance. Thus, combining these two distinctive features in a single electrode material has improved overall supercapacitor performance. A graphical illustration of the performance enhancement mechanism of Na-MnO₂ NWs/g-C₃N₄ nanosheets is shown in Figure 8(f). A comparison of the specific capacitance of Na-MnO₂/g-C₃N₄ with previously reported values is shown in Table 7.

EIS analysis provides information on the various obstacles an electrode encounters when an AC flows through it. The Nyquist plot is drawn by plotting the real and imaginary components along the x- and y-axes for bare MnOO₂, Na-MnO₂, and Na-MnO₂/g-C₃N₄ composite electrodes, respectively. The Nyquist plots for all three samples are illustrated in Figure 8e. Three sections in a Nyquist plot are usually observed: (i) the x-intercept; (ii) a semicircular diameter; and (iii) a diagonal line.^[54] The Nyquist plot for the bare MnO₂ shows a broader semicircle with a larger x-intercept, indicating higher charge transfer resistance (Rct) and solution resistance (Rs). The Na-MnO₂ shows a smaller semicircle and a smaller x-intercept, indicating improved electrical conductivity due to Na doping. The Na-MnO₂/g-C₃N₄ exhibits the smallest semicircle and the x-intercept closest to the origin, indicating low resistance and efficient charge and mass transfer. The resistance values of all the samples are given in Table 6.