1 Introduction

As urgent energy demands from various electrical vehicles to signature computers, various energy storage devices are invented such as fuel cell [1], lithium ion battery [2] and electrochemical capacitors [3]. Among them, electrochemical capacitors have attracted numerous scientists’ attention because of their unique properties including high power density. Therefore, great efforts have been devoted to the study focusing on electrochemical capacitors [4,5,6]. There are two mechanisms of storage energy for electrochemical capacitors: one is charge storage mechanism [electrical double-layer capacitors (EDLCs)] which is mostly used in carbon-based active materials, the carbonaceous materials are very popular owing to their very well properties such as their unique properties such as high surface area, better chemical stabilities and high conductivity [7, 8]; the other is using fast and reversible surface or near-surface reactions (giving rise to pseudocapacitance) based on transition metal oxides or conductive polymers which will generate high capacitive value [9,10,11]. However, either carbonaceous materials or metal oxides will not display dual EDLC and pseudocapacitance roles for energy storage.

Recently, composite materials have been developed to improve the performances of electrochemical capacitors [12, 13]. MnO2 is considered as a low-cost, environmental friendly and excellent pseudocapacitance (as high as 1370 F/g theoretical value). However, MnO2 suffers some shortages such as low conductivity and poor diffusivity which would sheet resistance and the charge-transfer resistance [14]. Therefore, various strategies such as wrapped by porous carbons or space-confined strategy have been developed [15, 16]. Hence, the conductive framework increases the electrical conductivity of the MnO2 system.

Lots of industrial wastes have been used to synthesize nanoporous carbons and carbon-based composites for their applications in various applications [17,18,19,20,21]. In addition to the economic and environmental problems associated with the generation of scrap tyres which are increasing due to the enormous generation of such wastes, all these issuses are posing a serious economic and environmental problem. Pyrolysis is considered as an optimal and environmentally friendly method for the thermo-chemical conversion of wastes such as tyres [22].

(A process chart is given in Scheme 1).

Scheme 1
scheme 1

Process of preparation of samples

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As highlighted here, our strategy is using modified rubber, followed by etching and dipping method.

2 Experimental procedure

2.1 Chemicals

NaOH and ethanol were purchased from Tianjin Chem Corp. All the chemicals were used as received without further purification.

2.2 Preparation of C–MnO2

The modified nitrile butadiene rubber (NBR) was prepared as previously reported [23]. The typical synthetic process of C proceeds as follows: first, the different NBRs with various amounts of SiO2 were directly put into the carbonization furnace under the nitrogen atmosphere at 800 °C for 3 h, and the samples obtained designed C/SiO2 composites. The samples were named according to the amount of SiO2 when preparing the modified NBR (C–SiO2: 2 g, 3 g, 4 g and 5 g, respectively). Then put the C–SiO2 composites into the 30 wt% NaOH solution with 50 °C thermostatic water bath, stirring overnight, and after filtration, washing with deionized water, drying in the oven to get nanoporous carbons (C-3, 5, which is consistent with the amount of SiO2 when preparing the modified NBR). To further improve the capacitive performance of materials, C is dipped with manganese nitrate solution concentrations, and calcination was carried out to obtain C–MnO2−X–Y; here the X and Y indicate the amount of SiO2 when preparing the modified NBR and different ratios of Mn(NO3)2/C = 1:1, 3:1, Y = 1, 2, respectively.

2.3 Characterization

X-ray diffraction (XRD) was recorded using TD-3700. SEM measure was investigated by the KYKY-3800. TEM measurement was conducted using a JEM-2100F. The samples were prepared by dispersing the products in ethanol with an ultrasonic bath for 30 min and then a few drops of the resulting suspension were placed on a copper grid. Nitrogen adsorption–desorption isotherms were performed at 77 K on a JW-BK122W volumetric adsorption system. The specific surface area was calculated from the adsorption data in the relative pressure interval from 0.01 to 0.2 using the Brunauer–Emmett–Teller (BET) method. The pore size distribution curve was obtained from adsorption branch using Barrett–Joyner–Halenda (BJH) method. The total pore volume (Vtotal) was calculated at the relative pressure of 0.99. The micropore volume (Vmicro) was determined by t plot model, and the mesoporous volume (Vmeso) was calculated by the difference of Vtotal and Vmicro.

2.4 Electrochemical measurements

Cyclic voltammetry (CV) and galvanostatic charge/discharge measurements were conducted in an electrochemical workstation (CHI 660E) using a conventional three-electrode electrochemical cell under the potential window of − 1.0 to 0 V for Na2SO4 (1 M) and 0.9 to − 0.1 V for KOH (6 M), respectively. A Pt sheet and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The specific gravimetric capacitance C (F/g) was calculated from the discharge process of the charge–discharge curve under different loading current density according to C = I t/m V, where I is the constant discharge current density (A/g), t is the discharging time (s), m is the mass of electrode (mg) and V is the potential window (V).

3 Results and discussion

The crystalline of all the samples is revealed by XRD analysis as exhibited in Fig. 1. As shown in Fig. 1a, the C–SiO2-3 composite exhibits several peaks of inorganic salts which are mostly composed of silica oxide and aluminum oxide after calcination. Figure 1b presents the XRD pattern of the sample after etching, and the peaks of inorganic salts almost disappeared suggesting that inorganic salts were dissolved. To further improve the capacitive performance of C-3, C-3 is dipped in manganese nitrate solution, and calcination was carried out to obtain C–MnO2. It can be noticed that the peak of C–MnO2-3-1, 2 can hardly be seen owing to their amount which is too little to be tested or highly dispersed in the porous carbon framework as demonstrated in Fig. 1c, d.

Fig. 1
figure 1

XRD patterns of a C–SiO2-3, b C-3, c C–MnO2-3-1 and d C–MnO2-3-2

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Figure 2 shows the SEM images of the obtained samples. As shown in Fig. 2a, C–SiO2-3 is composed of solid and smooth surface, and almost no porosity on the surface of C–silica. After etching, the C displays fluffy particle morphology and also existing porosity between the particles. After coating the MnO2, the lamellar structure can be found in the composites especially when there is increase in the amount of MnO2 for the C–MnO2-3-2.

Fig. 2
figure 2

SEM images of a C–SiO2-3, b C-3, c C–MnO2-3-1 and d C–MnO2-3-2

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The porous characteristics of all the samples are verified by N2 adsorption/desorption isotherms and pore size distributions are shown in Fig. 3a, b, respectively. According to the classification of porous materials by the International Union of Pure and Applied Chemistry (IUPAC), C–silica composites generated a type IV isotherm, characteristic of mesoporous materials with gas affinities which is consistent with pore size distribution in Fig. 3b [24]. Upon etching overnight with aqueous NaOH solution, it is obvious that porosity of C surface area increased from 146 to 200 m2/g possibly because of the removal of inorganic salts. However, the surface area became as low as 26 and 48 m2/g, and small pore volume was 0.08 and 0.09 cm3/g for C–MnO2-3-1 and C–MnO2-3-2, respectively. The BET surface area of C–MnO2-3-1 can be attributed to the MnO2 run into the pore of the C. C–MnO2-3-2 has the higher surface compared to that of C–MnO2-3-1 maybe because the manganese nitrate would emit NO2 during the carbonized process, which would further have expanded the porosity of the C-3. C–MnO2-3-2 presents much better specific capacity despite their low surface area, and this will be discussed later.

Fig. 3
figure 3

N2 adsorption/desorption isotherms (a) and BJH pore size distribution curve (b) of the samples

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Table 1 shows the pore parameter of the samples after thermal treatment and etching. It can be found that the C–SiO2-3, 5 present higher surface areas as 146 and 145 cm2/g, respectively, compared to the other two. Then the two samples were selected to be etched in KOH aqueous solution, and the C-3 exhibits the higher surface area as high as 200 compared to the 170 m2/g of the C-5. Therefore, the C-3 was chose to be doped by the MnO2 in the later experiment.

Table 1 Pore parameters of the samples
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FTIR spectrum of C–SiO2-3, C-3, C–MnO2-3-1 and C–MnO2-3-2 is demonstrated in Fig. 4. As for C–silica composite, the peak located at 1039 cm−1 matches typical Si–O–Si of C–silica composite. After etching, the peak of 1039 cm−1 receded considerably indicating that most of the silica was etched for the other three samples.

Fig. 4
figure 4

FTIR spectrum of (a) C–SiO2-3, (b) C-3, (c) C–MnO2-3-1 and (d) C–MnO2-3-2

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The microstructure of C–MnO2-3-2 was investigated by TEM. Figure 5 gives the TEM images of typical foam-like C–MnO2-3-2 with different magnifications, exhibiting foam-like structure. The MnO2 can hardly be found maybe because the amount of MnO2 is too little or dispersed even in the porous carbon framework.

Fig. 5
figure 5

TEM images of typical foam-like C–MnO2-3-2 (a–c) with different magnifications

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Figure 6 displays the voltammograms recorded for the samples in the range of − 1 to 0 V for C–SiO2-3 and C-3, 0.9–0.1 V for C–MnO2-3-1 and C–MnO2-3-2, respectively. As can be seen, the contribution of C–SiO2-3 and C-3 for capacitance is relatively low. The CV curve recorded for C–MnO2-3-2 exhibits more curved rectangular shape, indicating that the improved capacitance for C–MnO2-3-2 electrode. The improved specific capacitance is probably because of the pseudocapacitance of MnO2.

Fig. 6
figure 6

CV curves with 10 mV/s in 6 M KOH (for a, b) and 1.0 M Na2SO4 (for c, d), (a) CV curve of C–SiO2-3, (b) CV curve of C-3, (c) CV curve of C–MnO2-3-1 and (d) CV curve of C–MnO2-3-2

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Figure 7 presents the constant charge–discharge curves at current density of 0.5 A/g of C–SiO2-3, C-3, C–MnO2-3-1 and C–MnO2-3-2. The specific capacitance of C–SiO2-3 is as low as 13.1 F/g. After etching, the C-3 exhibits 30.1 F. However, the C–MnO2-3-1 gives only 22.2 F/g, even lower than that of C-3, which is possibly because the doped MnO2 could not compensate for the loss of the surface area. The C–MnO2-3-2 can present specific capacitance as high as 109 F/g, which is probably owing to the synergistic effect of porous carbon frame and the pseudocapacitance of MnO2.

Fig. 7
figure 7

The constant current charge–discharge curves at current density of 0.5 A/g of (a) C–SiO2-3, (b) C-3, (c) C–MnO2-3-1 and (d) C–MnO2-3-2

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The Nyquist curves of the samples were measured to assess the electrochemical reaction kinetics of the supercapacitors in Fig. 8. The equivalent series resistance (ESR) in the high-frequency region for all the samples as shown in Fig. 8. The C-3 and C/SiO2-3 showed the similar and smaller ESR compared to that of the other two samples. The semicircle diameter is related to the faradaic charge-transfer resistance (Rct). Rct increased as the MnO2 was introduced, indicating that MnO2 decreased the conductivity of the materials.

Fig. 8
figure 8

AC impedance diagrams in the frequency range 100 kHz to 0.01 Hz of (a) C–SiO2-3 composite, (b) C-3, (c) C–MnO2-3-1 and (d) C–MnO2-3-2

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4 Conclusion

The C–MnO2 composites were prepared by a facile thermal treatment and dipping method. The amount of MnO2 can be tuned by the concentration the Mn(NO3)2 aqueous solution. The specific capacity of C/MnO2 electrode material for supercapacitor can be as high as 109 F/g under the current density of 0.5 A/g. The superior electrochemical properties originated from the synergic effect of pseudocapacitive MnO2 particles with the C matrix which functions as a conductive support.