Tripotassium citrate monohydrate derived carbon nanosheets as a competent assistant to manganese dioxide with remarkable performance in the supercapacitor

Abstract

Production cost, capacitance, and electrode materials safety are the key factors to be concerned about for supercapacitors. In this work, a type of carbon nanosheets was produced through the carbonization of tripotassium citrate monohydrate and nitric acidification. Subsequently, a well-designed manganese dioxide/carbon nanosheets composite was synthesized through hydrothermal treating. The carbon nanosheets served as the substrate for growing the manganese dioxide, regulating its distribution, and preventing it from inhomogeneous dimensions and severe agglomeration. Many manganese dioxide nanosheets grew vertically on the numerous functional groups generated on the surface of the carbon nanosheets during acidification. The synergistic combination of carbon nanosheets and manganese dioxide tailors the electrochemical performance of the composite, which benefits from the excellent conductivity and stability of carbon nanosheets. The carbon nanosheets derived from tripotassium citrate monohydrate are conducive to the remarkable performance of manganese dioxide/carbon nanosheets electrode. Finally, an asymmetric supercapacitor with active carbon as the cathode and manganese dioxide/carbon nanosheets as the anode was assembled, achieving an outstanding energy density of 54.68 Wh·kg^–1 and remarkable power density of 6399.2 W·kg^–1 superior to conventional lead-acid batteries. After 10000 charge-discharge cycles, the device retained 75.3% of the initial capacitance, showing good cycle stability. Two assembled asymmetric supercapacitors in series charged for 3 min could power a yellow light emitting diode with an operating voltage of 2 V for 2 min. This study may provide valuable insights for applying carbon materials and manganese dioxide in the energy storage field.

Key words： carbon nanosheets; manganese dioxide; asymmetric supercapacitors; energy density; power density

1 Introduction

The development of energy storage devices, such as batteries and supercapacitors, tends towards miniaturization, portability, and transparency. However, the energy density and the power density of energy storage devices must be considered for practical applications. Conventional lead-acid batteries could meet the challenges of low energy density and power density applications [1]. Lithium-ion batteries offer higher capacitance, energy density, and power density than lead-acid batteries. Nevertheless, dendrites on the surface of electrodes could cause the lithium-ion batteries to explode [2]. Supercapacitors have been extensively studied due to their remarkable portability, acceptable production cost, and outstanding stability.

The energy density and the power density of energy storage devices could be calculated with the following equations [3]:

(1)E={\displaystyle \frac{\mathrm{1}}{\mathrm{2}}}C{(\mathrm{\Delta }V)}^{\mathrm{2}},

(2)P=E/T,

where the energy density E is determined by the specific capacitance C and operating potential window ∆V of the electrode materials, and power density P is determined by energy density and discharge period T. Both E and P could be increased with transition metal oxides including RuO₂, Fe₂O₃, Co₃O₄, and NiO₂ as the supercapacitor electrodes because of their excellent pseudo-capacitance via fast and reversible Faradic redox processes [4–6]. The production cost of RuO₂ electrodes is high though they hold the highest capacitance. Meanwhile, magnetic substances, such as Fe₂O₃, Co₃O_4, and NiO₂, could contaminate the instruments when they are characterized. As one of the pseudocapacitive materials, MnO₂ could be an appropriate electrode material offering prominent specific capacitance and various crystal structures [7], ensuring an effective ion diffusion essential for high energy density and power density. Wang et al. prepared a series of supercapacitor electrodes based on MnO₂ composites, mainly from biomass materials [8–12]. Nevertheless, many reported MnO₂ electrodes have nonuniform dimensions and severe agglomeration that weaken their electrochemical performance [13]. Meanwhile, the cycle stability of MnO₂ electrodes can not meet the demand of practical applications due to ordinary electrical conductivity and the expansion/contraction of crystal lattice during cycle testing [14,15].

Nontoxic nano-scale carbon materials with excellent electrical conductivity, such as reduced graphene oxide (rGO) [16], carbon nanotubes (CNTs) [17], carbon nanofibers [18], and carbon quantum dots (CQDs) [19], have been explored to solve the problems with MnO₂ electrodes. Zhou et al. [16] synthesized an asymmetric supercapacitor (ASC) using rGO/MnO₂/polypyrrole ternary film, which exhibited a fascinating potential window of 1.7 V with outstanding cycle stability of 93% after 8000 cycles. Prasath et al. [19] prepared a CQD@MnO₂ electrode through the hydrothermal method and carbonization, which showed an extraordinary specific capacitance with an excellent rate capability due to the CQD networks’ great specific surface area and effective influence on electroconductivity. Compared with other carbon electrodes, carbon nanosheets (CNSs) are suitable for synthesizing MnO₂ due to their low cost and simplicity [20]. CNS is two-dimensional compared to particulate (0D) or linear (1D) electrode materials [21], which could facilitate large-quantity MnO₂ formation on the electrode and enhance the performance of MnO₂ composites.

In this work, a low-cost, novel electrode material based on MnO₂/CNS was prepared. After the carbonization of tripotassium citrate monohydrate and HNO₃ acidification, an active CNS was produced for the uniform growth of MnO₂ on its surface, eliminating MnO₂ granules with nonuniform dimensions and severe agglomeration. The activation of HNO₃ produced large quantities of functional groups for the combination of CNS and MnO₂ nanosheets, which provided plenty of transferring and reacting sites for ions from the electrolyte and promoted the electrochemical performance of the composite. The carbon materials offered excellent conductivity and stability under strong current; thus, the composite showed much better rate capability. An ASC was successfully assembled with purchased active carbon (AC) as the cathode and MnO₂/CNS as the anode, which showed excellent energy density and power density superior to conventional lead-acid batteries. The supercapacitor exhibited good cycle stability after 10000 charge-discharge cycles. Two of the assembled ASCs in series could power a yellow light emitting diode (LED) with an operating voltage of 2 V.

2 Experimental

2.1 Materials

Tripotassium citrate monohydrate (K₃C₆H₅O₇∙H₂O), KMnO₄(AC), KOH (AC), hydrochloric acid (38%), C₂H₅OH (95%), and HNO₃ (68%) were purchased from Zhenjiang Yinhai Co. Ltd. and Sinopharm Chemical Reagent Co., Ltd. Deionized water was produced in the laboratory.

2.2 CNS preparation

CNS was produced through carbonization. In a nitrogen atmosphere, 5 g of K₃C₆H₅O₇·H₂O was gradually heated to 800 °C at 5 °C∙min^–1 and maintained at 800 °C for 2 h. The obtained powder was purified 4 to 5 times in a centrifuge with 30 mL HCl (1 mol∙L^–1) solution to remove impurities. Then, the powder was put into a beaker containing 50 mL HNO₃ (5 mol∙L^–1) and stirred for 12 h at 75 °C to produce functional groups on the surface. Finally, the products were rinsed three times with 20 mL ethanol and another three times with 20 mL deionized water before dried in a vacuum oven at 60 °C over 12 h for the subsequent synthesis of the MnO₂/CNS composite.

2.3 MnO₂/CNS composite synthesis

The MnO₂/CNS composite was prepared by the hydrothermal method from a previous study [22]. First, 20 mg of CNS was immersed into 30 mL deionized water and dispersed through ultrasonic treatment for 20 min. Second, 0.316 g (0.002 mol) KMnO₄ and 1 mL (2 mol∙L^–1) HCl were alternately added into the solution and stirred at room temperature for another 1 h. Subsequently, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept at 90 °C for 2 h. Then, the products were rinsed several times in a centrifuging with 10 mL ethanol and several times with 10 mL deionized water. After drying in a vacuum oven at 65 °C for 10 h, the MnO₂/CNS composite was obtained. The schematic diagram for the preparation of MnO₂/CNS is presented in Fig. 1. According to a similar method, pure MnO₂ was synthesized without the introduction of CNS.

Fig.1 Schematic diagram for the preparation of MnO₂/CNS.

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2.4 Characterization

The crystal structure of the obtained materials was observed via X-ray diffraction (XRD, D8 ADVANCE) with Cu Kα radiation (λ = 0.153 nm) scanning from 15° to 75° at 4°·min^–1. A Raman microscope (LabRam HR Evolution, wavelength of 531 nm) was used to produce the Raman spectra of the obtained products. The valence states of the composite elements were explored via X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) with the excitation of Al Kα radiation at 1486.8 eV. Scanning electron microscopy (SEM, Zeiss Gemini 300), transmission electron microscopy (TEM, FEI Talos F200X), atomic force microscope (AFM, Bruker Dimension ICON), and energy-dispersive X-ray spectroscopy (EDS) were used to observe the thickness of CNS, the morphologies of the products, and the distributions of elements in the composite, respectively.

2.5 Electrochemical analysis

A three-electrode system was adopted on a CHI760E electrochemical workstation to evaluate the obtained products’ electrochemical performance. Cyclic voltammetry testing (CV), galvanostatic charge/discharge testing (GCD), and electrochemical impedance spectroscopy (EIS) testing were performed. In the three-electrode system, the working electrode was the obtained product, the reference electrode was a cylindrical Ag/AgCl electrode, and the counter electrode was a 1 cm × 1 cm platinum flake. The electrolyte was 3 mol∙L^–1 KOH, and the working electrode was soaked in the electrolyte for 5 h before testing for sufficient ion infiltration. The specific capacitance of the working electrode can be calculated with the following equation [22]:

(3)C={\displaystyle \frac{Q}{\mathrm{\Delta }V}},

where Q represents the total discharging charge of the working electrode, and ΔV represents the potential window in the charging and discharging process of the working electrode. The specific capacitance can be calculated with the following equation [22]:

(4)C_M={\displaystyle \frac{I}{M}}{\displaystyle \frac{\mathrm{\Delta }t}{\mathrm{\Delta }V}}\mathrm{\hspace{0.17em}}\mathrm{or}\mathrm{\hspace{0.17em}}{C}_S={\displaystyle \frac{I}{S}}{\displaystyle \frac{\mathrm{\Delta }t}{\mathrm{\Delta }V}},

where I is the current magnitude of the charging and discharging process, Δt represents the discharging period of the working electrode, ΔV is the potential window in the charging and discharging process, and M and S are the loading and coverage of the working electrode, respectively. The mass loading of CNS, MnO₂, and MnO₂/CNS were 1.85, 1.77, and 2.02 mg·cm^–2, respectively.

3 Results and discussion

3.1 Morphologies and compositions

Figure 2 presents the results of XRD, Raman spectra, and XPS for CNS and MnO₂/CNS. Two diffraction peaks at 28.01° and 42.02° in Fig. 2(a) are verified as the (002) and (004) planes of the graphitic carbon [23]. For MnO₂/CNS, six peaks at 21.81°, 36.84°, 42.05°, 55.29°, 59.93°, and 66.17° are indexed to the (101), (210), (211), (212), (312), and (412) planes of MnO₂, which is birnessite-type (JCPDS No.39-0375) [24,25]. Due to the intensive peaks of MnO₂ in the composite, the CNS peaks are too slight to find. Raman and XPS evaluations were performed to verify the existence of CNS peaks and the combination between CNS and MnO₂. Two strong peaks at 1347 and 1576 cm^–1, representing the D-band and G-band for graphitic carbons, respectively [26], are emerged in CNS and MnO₂/CNS, which prove the existence of CNS in the composite. Three major peaks at 493, 551, and 645 cm^–1 of MnO₂ can be observed in MnO₂/CNS. The peak at 645 cm^–1 corresponds to the crystal structure of MnO₂ formed orderly in a broad area [27]. Figures 2(c)–2(f) exhibit the XPS results of MnO₂/CNS for evaluating the valence states of different elements in the composite. Figure 2(c) shows four distinct peaks from 284.04 to 653.01 eV, indicating the Mn, O, and C elements in the composite. As shown in Fig. 2(d), the peak at 284.08 eV signifies the C–C and C=C bonds of CNS [28]. Two weak peaks at 285.28 and 287.88 eV represent the C–O and C=O bonds, respectively, derived from HNO₃ acidification during CNS synthesis [28]. The slight peak at 282.78 eV can be ascribed to the C–Mn bond formed in the hydrothermal process. The peak at 529.08 eV in Fig. 2(e) represents the Mn–O–Mn bond. Two minor peaks at 530.48 and 532.18 eV represent the C–O–C and C=O bond [28]. The characteristic valence state of the manganese is shown in Fig. 2(f). The binding energy space between Mn2p_3/2 at 641.58 eV and Mn2p_1/2 at 653.18 eV is 11.5 eV, prominently demonstrating that the original manganese valence state in the composite is Mn⁴⁺ [29]. The other peaks of Mn at 644.38, 648.58, and 656.48 eV correspond to a small quantity of Mn³⁺ and Mn²⁺ originated from the hydrothermal reaction.

Fig.2 (a) XRD patterns and (b) Raman spectra of CNS and MnO₂/CNS; (c–f) XPS patterns of MnO₂/CNS: (c) full scans of C, O, and Mn, (d) scan of C1s, (e) O1s, and (f) Mn2p.

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Figure 3 shows the SEM images of acidless-treated CNS, active CNS, MnO₂ granules, and MnO₂/CNS. As shown in Figs. 3(a) and 3(b), CNS produced through the carbonization of K₃C₆H₅O₇·H₂O and sufficient HNO₃ acidification is more dispersive and rough than those produced through the direct carbonization of K₃C₆H₅O₇·H₂O. Unlike the MnO₂ granules (Fig. 3(c)) with ununiform dimensions that are synthesized without active CNS, the MnO₂ nanosheets are vertically assembled on CNS (Fig. 3(d)), demonstrating a more homogeneous distribution in the composite ascribed to the active CNS advantages in the formation and growth of MnO₂ nanosheets.

Fig.3 SEM images of (a) acidless-treated CNS, (b) active CNS, (c) MnO₂, and (d) MnO₂/CNS.

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Figure 4 shows the TEM images of active CNS and MnO₂/CNS, the 3D AFM image of active CNS, and the thickness curve of active CNS. Figure 4(a) illustrates that CNS is a two-dimensional material. Compared to raw CNS, significantly more MnO₂ nanosheets synthesized through hydrothermal treatment have adhered to active CNS (Fig. 4(b)). Furthermore, the combination of 2D CNS and 2D MnO₂ nanosheets (Figs. 3(d) and 4(b)) endows the composite more transferring sites for ions on the electrode than those in the MnO₂ granules, notably promoting the electrochemical performance of the composite. Figures 4(c) and 4(d) indicate that the active CNS achieves an ultrathin thickness of 1.6 nm.

Fig.4 TEM images of (a) active CNS, and (b) MnO₂/CNS; AFM image of (c) active CNS, and (d) thickness curve for active CNS.

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As shown in Fig. 5, the element distribution in MnO₂/CNS is evaluated with EDS. Three spectra of C, O, and Mn in Figs. 5(b)–5(d) exhibit their homogeneous distribution in MnO₂/CNS.

Fig.5 (a) SEM image of MnO₂/CNS; EDS mappings of (b) C, (c) O, and (d) Mn.

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3.2 Electrochemical performance of electrodes

As shown in Fig. 6, CV, GCD, and EIS were conducted on CNS, MnO₂, and MnO₂/CNS. The CV estimations at a voltage range from –1 to 0.2 V were adopted in the electrolyte of 3 mol∙L^–1 KOH. Figure 6(a) shows the cyclic voltammetry curves of CNS, MnO₂, and MnO₂/CNS at the scan rate of 100 mV·s^–1, where both oxidation and reduction peaks appear in MnO₂ and MnO₂/CNS at –0.2 V, indicating the apparent reversibility of Faradic reaction in the electrolyte for pseudocapacitor material [30]. Due to the fast insertion and extraction of K-ion that comes from the electrolyte, large quantities of Faradaic reactions take place in the interior and surface of the MnO₂ electrode as follows [31]:

(5){\mathrm{MnO}}_{\mathrm{2}}{\mathrm{+K}}^{\mathrm{+}}{\mathrm{+e}}^{\mathrm{-}}\mathrm{=MnOOK}

Fig.6 (a) CV curves of CNS, MnO₂, and MnO₂/CNS at the scan rate of 100 mV·s^–1; (b) GCD curves of CNS, MnO₂ and MnO₂/CNS at the current density of 1 A·g^–1; (c) rate capabilities and (d) Nyquist plots of CNS, MnO₂ and MnO₂/CNS.

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MnO₂/CNS possesses a larger curve area than MnO₂, facilitating a much higher electrochemical performance. Figure 6(b) is the GCD curves ranging from –1 to 0.2 V of CNS, MnO₂, and MnO₂/CNS under a basic current density of 1 A·g^–1. Due to the faradaic reactions of pseudocapacitor material, the curves of MnO₂ and MnO₂/CNS show irregular triangles [32]. The mass-specific capacitance of MnO₂/CNS is higher at 395.5 F·g^–1 compared with that of MnO₂ (190 F·g^–1). The composition of CNS significantly enhances the performance of MnO₂, which is attributed to the refined dimensions of MnO₂ when growing on the surface of CNS, the amplified specific surface area, and the prevented agglomeration. When the current density of GCD tests increases to 20 A·g^–1 (Fig. 6(c)), the mass-specific capacitance of MnO₂/CNS remains 158.2 F·g^–1, showing a superior rate capability (40%, 158.2/395.5) to that of MnO₂ (11%, 22/190) thanks to the active CNS (44%, 53.7/122 F·g^–1) as carbon materials always maintain excellent stability under high current density.

The introduction of CNS improves the electrical conductivity of MnO₂/CNS, resulting in much better performance than MnO₂, which is also demonstrated with EIS in Fig. 6(d). The Nyquist plot with frequency ranges from 10^–1 to 10⁵ Hz for CNS, MnO₂, and MnO₂/CNS. The Nyquist plot usually consists of the resistive component (Z′) of the impedance and the imitate component (Z″), in which the frequency decreases as the profile shifts from the bottom left section to the top right [33]. The Nyquist plot intersects the (Z′) axis at the inherent resistance of the electrode, R_S. The smaller the R_S, the better the conductivity. The slope of the oblique line on the plot refers to the resistance derived from the ions transferring between electrode and electrolyte, which is represented by the Warburg element (Z_W). The bigger the Z_W, the better the conductivity [33]. As shown in Fig. 6(d), the intrinsic R_S of MnO₂/CNS is 0.94 Ω, lower than that of MnO₂ (1.04 Ω). The Z_W of MnO₂/CNS is 1.523, which is higher than that of MnO₂ (1.39), indicating the superior conductivity of MnO₂/CNS to raw MnO₂ thanks to the superior conductivity of CNS.

The storage mechanism is investigated to explain the excellent performance of the composite, which is presented in Fig. 7. As shown in Fig. 7(a), a group of CV tests was conducted on MnO₂/CNS with different scan rates ranging from 10 to 100 mV·s^–1. The relationship between the scan rate (v) and the imported current (i) can be expressed with the appointed power-law [34]:

(6)i=a{v}^b,

where the slope of the plot, log (v)–log (i), can be written as b. When b approaches 0.5, the electrode charge storage behavior is a battery-type, which emerges with the phase transition during the charging and discharging process. When b is close to 1.0, the charge storage behavior is a double layer capacitor-type, in which the capacitance on the electrode surface relies merely on the physical interactions between electrode and electrolyte [34]. Figure 7(b) is the fitting output originating from the scan rates and current density peaks of anode and cathode. The b values of anode and cathode are 0.524 and 0.852, respectively, proving that the charge storage behavior of MnO₂/CNS is a Faradaic pseudocapacitance-type, which can be generated both on the surface (capacitive behavior) and interior (diffusion behavior) of the electrode. The pseudocapacitor has superior capacitance to the double-layer capacitor. The crystal structure is much more stable than the battery-type materials. i_c = k₁v+ k₂v^0.5 can be utilized to the capacitance generated by interior diffusion behavior of MnO₂/CNS at different scan rates (v) can be calculated with the following equation [34]:

(7)i_{\mathrm{c}}=k_{\mathrm{1}}v+k_{\mathrm{2}}{v}^{\mathrm{0.5}},

where i represents the definite total current that corresponds to the tested scan rate (v), and k₁v and k₂v^0.5 are the surface capacitive current and diffusion-controlled current, respectively [34]. As shown in Fig. 7(c), the capacitance of surface behavior at 10 mV·s^–1 remains 54% of the total value. The proportion of interior diffusion behavior ascends steeply with the increasing scan rate (Fig. 7(d)), accounting for 78% of the whole capacitance at a higher scan rate of 100 mV·s^–1.

Fig.7 (a) CV curves of MnO₂/CNS at different scan rates; (b) plots of log (scan rate) versus log (peak current); (c) surface capacitive CV curves of 10 mV·s^–1; (d) contribution ratios of surface capacitive and diffusion-controlled process at different scan rates.

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3.3 Electrochemical performance of the ASC

An ASC was assembled, of which the anode was the MnO₂/CNS composite, and the cathode was AC. The loading of AC is calculated based on the charge balance between cathode and anode as follows [35]:

(8){\displaystyle \frac{M_{+}}{M_{-}}}={\displaystyle \frac{C_{-}\mathrm{\Delta }{V}_{-}}{C_{+}\mathrm{\Delta }{V}_{+}}},

where ∆V₊, C₊, and M₊ are the operating voltage range, the capacitance, and the loading of the anode, respectively, and ∆V_–, C_–, and M_– represent the operating voltage range, the capacitance, and the loading of the cathode, respectively [35]. According to Eq. (8), AC mass loading was calculated to be 3.51 mg·cm^–2.

As presented in Fig. 8, to evaluate the performance of the ASC of MnO₂/CNS//AC (Fig. 8(a)), CV and GCD were performed. Figure 8(b) presents the CV testing profiles of the supercapacitor under the same scan rate of 100 mV·s^–1, ranging from 1.0 to 1.8 V, all of which manifested similar shapes, significantly overlapped from 0 to 1.0 V. The asymmetric profiles of CV testing are attributed to the asymmetric structure of the anode and the cathode. Figure 8(c) shows the GCD curves of the supercapacitor under the basic current density of 1 A∙g^‒1, ranging from 1.0 to 1.6 V. As shown in Figs. 8(b) and 8(c), the capacitance of the device increases significantly with the voltage, presenting an effective peak of 114.35 F·g^–1 at 1.6 V. Meanwhile, the effects of two supercapacitors in series and in paralleled (Fig. 8(d)) on the current and potential window were evaluated. The CV curves are shown in Figs. 8(e) and 8(f). Two supercapacitors in series showed double potential window than the single one (Fig. 8(e)), and two supercapacitors in paralleled showed much greater current than the single one (Fig. 8(f)). The results above are consistent with the electrical regulations in which supercapacitors in series amplify the working potential, and supercapacitors in paralleled broaden the working current, authenticating the feasibility of the supercapacitor [36–38].

Fig.8 (a) Schematic diagrams of ASC; (b) CV curves of ASC from 1.0 to 1.8 V at the scan rate of 100 mV·s^–1; (c) GCD profiles of ASC from 1.0 to 1.6 V at the current density of 1 A·g^–1; (d) schematic diagrams of two devices in series and in paralleled; CV curves of one device and two devices (e) in series and (f) in paralleled.

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CV and GCD tests were also performed to investigate the property of one single supercapacitor, as shown in Fig. 9. The CV curves (Fig. 9(a)) at the increasing scan rates from 10 to 100 mV·s^–1 showed similar trends with the scan rate. Significantly, the curve at 100 mV·s^–1 had much more area than those at other scan rates. Different current densities were adopted in the GCD tests to evaluate the supercapacitor rate capability, as shown in Figs. 9(b) and 9(c). The supercapacitor specific capacitances are 114.35, 140.8, 150.9, 153.8, 84.75, and 73.5 F·g^–1 at the current density of 1, 2, 3, 4, 5, 10 A·g^–1, respectively. Activated in the durative testing from the previous current densities, the ions on the electrode have the best reaction kinetics at 4 A·g^–1, facilitating the optimum specific capacitance of 153.8 F·g^–1. The supercapacitor retains 64% of the specific capacitance calculated at 1 A·g^–1, while the current density increases to 10 A·g^–1, showing outstanding rate capability. The ion transfer rate increases as the current density increases, promoting the energy storing and releasing efficiency of the charging and discharging processes. The supercapacitor coulombic efficiency (Fig. 9(c)) reaches up to 97% at the current density of 10 A·g^–1.

Fig.9 (a) CV curves of ASC at different scan rates; (b) GCD profiles of ASC at different current densities; (c) rate capability and coulombic effciency of ASC.

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Finally, Fig. 10 presents the practical application of the supercapacitor. The Ragone plot was adopted as the criteria for evaluating its energy density and power density. Dots situated in the top right corner of the chart illustrate the outstanding electrochemical performance [39,40]. The energy density and power density of a supercapacitor can be calculated with the following equations [41]:

(9)E={\displaystyle \frac{C\times \mathrm{\Delta }{V}^{\mathrm{2}}}{\mathrm{2}\times \mathrm{3.6}}},

(10)P={\displaystyle \frac{E\times \mathrm{3600}}{T}},

where C refers to the specific capacitance (F/g), ∆V represents the operating potential window (V), and T is the discharging period (s). As presented in Fig. 10(a), the optimal energy density is 54.68 Wh·kg^–1 at the current density of 4 A·g^–1, while the power density is 2559.8 W·kg^–1. As the current density increases, the power density reaches the peak of 6399.2 W·kg^–1 at 10 A·g^–1, while the energy density maintains at 26.13 Wh·kg^–1, showing the superiority to conventional lead-acid batteries and better electrochemical performance than the those based on MnO₂ (Table 1) [42–49]. Compared with particulate and linear electrode materials, two-dimensional MnO₂ nanosheets are planar, endowing much more space for ion movement and energy storage, protecting the electrode against structure collapse, and improving the energy density and power density of supercapacitors.

Fig.10 (a) Ragone plot of ASC; (b) cyclic testing of ASC; (c) Nyquist plots of ASC before and after cycle testing; (d) two devices linked in series light a LED of 2 V.

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Tab.1 Comparison of ASCs based on MnO₂/CNS//AC with the reported ASCs based on MnO₂ in terms of the maximum for energy density (E) and power density (P) [42–49].

Electrode Material	Electrolyte	Potential window	E/(Wh∙kg–1)	P/(W∙kg–1)	Ref.
MnO2/CNS//AC	3 mol∙L–1 KOH	0–1.6 V	54.68	6399.2	This work
MnO2/CNTs//AC	1 mol∙L–1 Na2SO4	0–1.5 V	13.3	600	[42]
MnO2@N-APC//NAPC	1 mol∙L–1 Na2SO4	0–2 V	28	560	[43]
HCNF-MnO2//HCNF-MnO2	PVA-Na2SO4	0–1.6 V	30.5	3090	[44]
MnO2@PANI@CNF//CNF	1 mol∙L–1 H2SO4	–0.5–1.6 V	43	1650	[45]
MnO2@CQDs@GA//MnO2@CQDs@GA	1 mol∙L–1 Na2SO4	0–2 V	38.2	1000	[46]
MnO2@CNT@NCT//CNT@NCT	PVA-Na2SO4	0–1.8 V	5.5	3600	[47]
MnO2//MnO2	PVA-Na2SO4	0–1.6 V	32	1390	[48]
PPy-MnO2//PPy-MnO2	PVA-Na2SO4	0–1.2 V	37.63	830	[49]

Figure 10(b) is the cycling test result of the supercapacitor performed at 4 A·g^–1. The supercapacitor retains 75.3% of the initial specific capacitance after 10000 cycles, showing outstanding cycling performance. A significant quantity of ions gradually gathered on the electrode surface during the cycling tests, increasing the R_S and decreasing the Z_w of the supercapacitor (Fig. 10(c)), resulting in inferior properties. Two of the assembled ASCs in series could power a yellow LED with an operating voltage of 2 V for 2 min charging for 3 min (Fig. 10(d)). The application of carbon materials and MnO₂ in the energy storage field is promising.

4 Conclusions

In this work, a low-cost, high capacitance electrode based on MnO₂/CNS was prepared. After the carbonization of tripotassium citrate monohydrate and HNO₃ acidification, CNS was produced, which regulated the magnitude of MnO₂ nanosheets grown on its surface while avoiding MnO₂ inhomogeneous dimensions and severe agglomeration. HNO₃ activation provided the large quantities of functional groups for the combination of CNS and MnO₂ nanosheets and promoted the reproduction of MnO₂ on it. The corporation of 2D CNS and 2D MnO₂ nanosheets endowed the composite with plenty of transferring and reacting sites for the ions from the electrolyte, improving the composite electrochemical performance. With excellent conductivity and stability of CNS under strong current, the composite showed a higher rate capability than MnO₂. The K₃C₆H₅O₇·H₂O derived ultrathin CNS is conducive to the MnO₂/CNS electrode performance. Lastly, an ASC with AC as the cathode and MnO₂/CNS as the anode was successfully assembled, reaching an energy density of 54.68 Wh·kg^–1 and a power density of 6399.2 W·kg^–1 superior to conventional lead-acid batteries. After 10000 cycles, the supercapacitor retained 75.3% of the initial capacitance. Successfully powering a LED demonstrated the practical application of the supercapacitor.

Acknowledgements

This work is financially supported by Six Talents Peak Project in Jiangsu Province (Grant No. 2011-ZBZZ045), Key R&D Program of Zhenjiang (Grant No. GY2018016), and Innovative Training Program in Jiangsu University (Grant No. Y18A017).

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