Sea Urchin-Like MnO 2 /Biomass Carbon Composite Electrode Material for High-Performance Supercapacitors

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In order to solve the problem of MnO 2 used as an SC electrode, the electrochemical performance of composite carbon materials has been proven to work efectively [33,34].Terefore, MnO 2 used for electrode materials must be combined with conductive carbon black [35], carbon nanotubes [36], graphene [37], activated carbon [38], or biocarbon [39] to improve the electrochemical energy.Biomass carbon (BC) based on pomelo peel is now a promising carbon material, which has good applications in the feld of electrochemistry, catalysis, and other aspects [40][41][42][43][44][45][46].Using the special structure of biomass, the porous carbon material with a high-specifc surface area, long cycle stability, large porosity, and good electrical conductivity can be obtained by simple carbonization and subsequent treatment.Te porous carbon material combined with manganese oxide is expected to improve the electrochemical performance of manganese oxide [44][45][46][47][48][49][50].Te introduction of biomass carbon with strong electrical conductivity can greatly increase the electrical conductivity of the whole system.
Herein, we used a simple hydrothermal method to compound the highly conductive biomass carbon prepared from pomelo skin with MnO 2 nanorods to form a unique sea urchinlike MnO 2 /BC nanocomposite, in which the biomass carbon material with a larger specifc surface area and porous structure further enables the electrochemical performance of the obtained electrode materials to meet the expectations.Te specifc capacitance of MnO 2 /BC composites' electrode can reach 205.5 F•g −1 when the charge-discharge current density is 0.5 A•g −1 .In addition, the asymmetric supercapacitor constructed by using MnO 2 /BC composite as the positive electrode and BC as the negative electrode maintains 105% of the initial capacitance after 4000 long cycles, showing good long-cycle stability.Terefore, the MnO 2 /BC composite electrode as an excellent candidate provided a promising route for highperformance SCs.

Preparation of Biomass Carbon.
Remove the inner fesh of grapefruit peel and keep the outer skin and put it in an oven at 90 °C until drying.Ten, the pomelo peel is cut into smaller pieces, which are crushed in a shredder to get pomelo peel powder.A certain amount of dried pomelo peel powder was taken in a porcelain boat, and then the frst precarbonization was carried out in a tubular furnace for 2 h at 700 °C with a heating rate of 2 °C•min −1 .High-purity N 2 was passed through the tube furnace at a fow rate of 50 ml•min −1 .Te precarbonized powder was washed repeatedly with dilute hydrochloric acid and water to remove impurity ions until pH � 7. Te above centrifuged product was dried at 60 °C for 10 hours.According to the mass ratio of 1 : 2, we mix a certain mass of the above samples and potassium hydroxide, and after mixing, we grind with a mortar and then mechanical ball mill for 3 hours.After ball-milling, 2 g of the mixture was placed in a porcelain boat and recarbonized at 800 °C for 2 h in a tube furnace.Te carbonized black powder was washed repeatedly with dilute hydrochloric acid and water to remove impurity ions until pH � 7. Te above centrifuged products were dried at 60 °C for 10 hours to obtain biomass carbon (BC)./BC Composite.Firstly, 150 mg BC was placed in 15 mL deionized water, and then a certain amount of dilute hydrochloric acid was added to BC for about 30 minutes.Ten, we add 0.298 g of potassium permanganate to the above solution and stir it with a glass rod for 10 minutes until it fully dissolved.Finally, the mixture was transferred to a 20 mL reaction kettle resistant to high pressure and kept at 150 °C for 12 hours.After the reaction was completed, the reaction products were removed, centrifuged, and washed with ultrapure water and ethanol at 6000 R/min, and the collected samples were placed in a drying oven and dried at 80 °C for 10 hours with the mass ratio of MnO 2 and BC≈1.1 : 1.In addition, we add pure manganese dioxide only without adding biomass carbon, and other steps are the same.Te weight of the electrodes was all about 2 mg.

Characterization.
Te synthesized samples were characterized by XRD (Bruker AXS D8 difractometer with Cu Kα radiation), SEM (JEOL JSM-840), the CHI660E electrochemical workstation, and the LAND CT2001.Te battery tester was employed to measure the system with Ag/ AgCl electrode and platinum sheet as the reference and counter electrode in the electrolyte of 6 M KOH aqueous solution, respectively.

Construction of Supercapacitors.
Te half-cell test of MnO 2 /BC composite material and BC electrode material in the test was completed in the system with Ag/AgCl electrode as the reference electrode, platinum sheet as the counter electrode, and active material as the working electrode.Active material, acetylene black, and tefon were mixed in an 8 : 1 : 1 ratio.Ten, it spread on nickel foam as the working electrode and was pressed for 30 seconds under the pressure condition of 10 MPa and transferred to the oven after drying.In the two-electrode test, MnO 2 /BC composite material was used as the positive electrode and BC synthesized from pomelo skin was used as the negative electrode.Te positive and negative electrode materials follow the charge balance principle q + � q − .Te electrolyte used in the two-electrode test and three-electrode test was 1 M Na 2 SO 4 .

Results and Discussion
To synthesize composite electrodes, we frst dried grapefruit peels and converted them into biomass carbon by carbonization.Te biomass carbon obtained from grapefruit peel was activated by adding dilute hydrochloric acid and then mixed with potassium permanganate and heated hydrothermally at 150 °C for 12 hours to obtain the manganese dioxide/biomass carbon (MnO 2 /BC) composite.In addition, pure MnO 2 electrodes just do not have biomass carbon added, and all other steps are the same (Figure 1(a)).Te morphologies of MnO 2 and MnO 2 /BC composites were signifcantly diferent based on their TEM, which shows a great diference in the microstructure of the two electrode materials (Figures 1(b) and 1(c)).Te pure MnO 2 has a 2dimensional wire-like structure with an average length of about 1-2 microns (Figure 1(b)).However, after compositing with biomass carbon, MnO 2 /BC showed a sea urchinlike structure, which is consistent with the results of 2 Journal of Chemistry transmission observation.Te EDS test shows that manganese and oxygen elements are uniformly distributed in the prepared MnO 2 /BC composite, indicating that the MnO 2 nanorods are densely distributed on the surface of BC, which may be attributed to the fact that the loose and porous carbon materials provide abundant sites for manganese oxides (Figures S1 and 1(d)).
In order to characterize the accurate structure of the MnO 2 /BC composite electrodes and the pure MnO 2 electrodes, we provide the XRD of the electrode material before and after adding the biomass carbon (Figure 1(b)).Firstly, the XRD of MnO 2 corresponds to the difraction peak of the standard card of MnO 2 (JCPDS No. 44-0141), which represents its high purity.Te XRD of the MnO 2 /BC composites shows difraction peaks at 12.8 °, 28.8 °, 37.5 °, 41.9 °, 49.8 °, and 60.3 °correspond to (110), (310), ( 211), (301), (411), and (521) crystal planes, respectively, indicating that MnO 2 /BC composites have been successfully prepared [45].Both electrode materials show sharp peak shapes without excess miscellaneous peaks, indicating the high purity and crystallinity of the MnO 2 /BC electrode materials synthesized in our study.In addition, compared with pure MnO 2 , there is no obvious peak of biomass carbon in the difraction peak of the composite material because the synthesized biomass carbon is amorphous.
To explore the element composition of the composite electrode material, the EDS test of MnO 2 /BC was conducted, which shows that manganese, oxygen, and carbon elements are uniformly distributed in the prepared MnO 2 /BC composite (Figure S1a).Besides, the EDS test of MnO 2 shows that manganese and oxygen elements are uniformly distributed in the prepared MnO 2 composite (Figure S1b).Typical N 2 adsorption-desorption isotherms were supplied to further evaluate the specifc surface area of the prepared MnO 2 /BC and pure MnO 2 electrode materials [24,29].Te data for the specifc surface area P/P 0 in the range of 0.45-0.9show that (Figures S2a and S2b 139.291 m 2 •g −1 and 77.301 m 2 •g −1 , respectively, which are the typical characteristics of mesoporous materials (Table S1).
Te results show that in the presence of biomass carbon, the composite has a larger specifc surface area than the pure MnO 2 composite.Te larger specifc surface area of the composite can provide more active sites for electrochemical reactions.In the process of electrochemical testing, more pore structures can provide a good ion transfer pathway to supply electrolyte ions, which makes sufcient preparation for the improvement of the electrochemical performance of SC [42][43][44][45][46][47][48][49][50][51][52][53][54][55].
Te CV curves of MnO 2 /BC composite and pure MnO 2 at a scanning rate of 20 mV•s −1 in the potential range 0-1 V are shown in Figure 2(a), in which both electrode materials are approximately rectangular indicating that both materials rely on the double-layer efect to store charge.In addition, the MnO 2 /BC electrode material exhibits a larger capacitance compared with pure MnO 2 at the same scanning rate.In order to better understand the electrochemical properties exhibited by MnO 2 /BC, CV at diferent canning rates, tests were carried out under a voltage window of 0-1 V. that there is no obvious deformation of the curve as the sweep speed increases to 100 mV•s −1 , indicating that the MnO 2 /BC electrode material has good electrochemical reversibility and stability (Figure 2(b)).Te contribution of capacitance and difusion in the MnO 2 /biomass carbon composite electrode is further analyzed in Figure S3.Te peak current values are calculated by deriving the peak current values at diferent voltage scan rates after the following equation: Te b-value obtained after ftting the peak data is 0.8284 (Figure S3).Terefore, the MnO 2 /biomass carbon composite electrode material exhibits battery properties and pseudocapacitive properties [56,57].
In order to further understand the electrochemical properties of the MnO 2 /BC electrode material, we examined the specifc capacitance obtained at 0.5-10 A•g −1 (Figure 2(c)).Te symmetrical charge-discharge curves indicate typical double-layer behavior.Besides, the curve still does not appear obvious voltage drop, which indicates that the resistance of the material is very small even at large current.With the current density at 0.5, 1, 2, 3, 5, 8, and 10 A•g −1 , the specifc capacitance of the composite material can be 205.5, 193, 176, 166, 150, and 144, respectively, while the electrochemical performance of pure manganese dioxide at the same current density is 115, 100, 90, 84, 75, 64, and 60 F•g −1 (Figure 2(d)).Te MnO 2 /BC electrode material with biomass carbon composite exhibits better rate performance (63.2%), which retains more initial capacitance than pure manganese dioxide (52.2%).
In order to characterize the electrical conductivity of the composite material after the composite of biomass carbon, we conducted the EIS of MnO 2 /BC electrode material in the frequency ranging from 10 −2 Hz to 10 5 Hz (Figure 2(e)).Both the composite material and the corresponding pure MnO 2 impedance map contain two parts: a semicircle in the high-frequency region and a straight line in the low-frequency region.Local amplifcation of illustrations is further shown in Figure 2(e).Te composite materials in the high-frequency region of the semicircle have a smaller and more steep low-frequency region of the straight line, which shows that the MnO 2 /BC electrode has smaller load transfer resistance, faster charge transport capacitance, and better ion difusion ability due to a shorter ion difusion pathway.In addition, the equivalent circuit diagram could be obtained by careful simulation of the resistance curves (the inset of Figure 2(e)), where Rs and Rct are used to represent the internal resistance and charge transfer resistance of SC, W signifes Warburg impedance, representing ion difusion in the low-frequency region, and CPE is a constant phase angle element to simulate the capacitance properties of the electrode.Notably, the MnO 2 /BC electrode exhibits a higher slope line, signifying the lower difusion resistance, indicating a faster reaction kinetic rate compared with a pure MnO 2 electrode.After ftting data, the Rs and Rct values of the MnO 2 /BC composite electrode material were 2.23 Ω and 6.29 Ω, both smaller than those of the pure MnO 2 electrode material (Table S2), which were consistent with the electrochemical experimental data.Terefore, sea urchin-like MnO 2 /BC composite has enhanced electrical conductivity compared with pure MnO 2 .To evaluate the suitability of supercapacitor materials for further practical applications, we conducted the long cycle performance test.As shown in Figure 2(f ), the specifc capacitance of MnO 2 /BC composite can still maintain 96.1% of the initial specifc capacitance, which is very competitive compared with 90.4% of the pure MnO 2 material at 0.5 A•g −1 .Te high cyclic stability indicates that MnO 2 enhances the stability of the material after composite porous BC, making the electrode material more stable during continuous charging and discharging.
We used the synthesized porous biomass carbon as an anode material for supercapacitors and further characterized its morphology and electrochemical properties.Te SEM diagram of biomass carbon BC shows a loose 3D network structure and large loose porous bulk structures with ordered channels, where carbon elements were evenly distributed (Figures 3(a) and 3(b)).Te phase composition biomass carbon is characterized by XRD (Figure S4a), which shows two peaks, one centered around 24 °with a broad and strong peak and the other around 43 °with a narrow and weak peak, corresponding to the (002) and (100) crystal planes of the graphitic phase carbon of a disordered structure.Te Raman test shows two distinct characteristic peaks between MnO 2 /BC and BC at 1309 cm −1 and 1570 cm −1 , which are the characteristic D and G bands, respectively (Figure S4b).In the low-frequency range, the characteristic peak centered at 638 cm −1 is the Mn-O bond vibration, confrming the presence of MnO 2 in the composite.In Raman spectra, the I D /I G intensity ratio refects the degree of the amorphous nature of the synthetic material.Te higher g-band (IG) intensity in BC indicates its higher graphitized carbon, ensuring the low resistance of BC during charge and discharge [49].
Te synthesized porous biomass carbon was tested electrochemically in 1 M Na 2 SO 4 at a potential of −1-0 V., and the GCD curve of BC shows a typical double-layer    3(e)).In addition, after continuous charging and discharging at 3 A/g, we found that the synthesized biomass carbon material could still maintain 100% of the initial capacitance, indicating the excellent longcycle stability of the synthesized biomass carbon material (Figure 3(f )).
In order to evaluate the efect of MnO 2 /BC composite in practical application, asymmetric supercapacitors was assembled with MnO 2 /BC composite as the positive electrode and biomass carbon (BC) synthesized from grapefruit skin as the corresponding negative electrode.In order to determine the voltage range of the whole cell, CV tests were performed for the positive and negative electrode materials, respectively (Figure 4(a)).When the scan speed was 20 mV•s −1 , the voltage range of BC and MnO 2 /BC composite was −1-0 V and 0-1 V, respectively.Terefore, the voltage window range of the assembled whole cell was determined to be 0-2 V. To examine the enhanced electrochemical performance after the composite biomass carbon material, CV tests performed on the whole cell composed of two electrode materials at 20 mV•s −1 .Both electrode materials exhibit a rectangular shape at 20 mV•s −1 .However, the full cell composed of MnO 2 /BC combined with biomass carbon exhibits a larger capacitance relative to pure MnO 2 (Figure 4(b)).Te CV test of the two electrode materials at 45-100 mV•s −1 (Figures S5a and S5b), which also presents intuitively that MnO 2 /BC composite has a larger capacitance than pure manganese dioxide.To detect its actual capacitance, the GCD test was carried out on the whole battery constructed by two diferent materials (Figure S5).When the asymmetric SCs constructed by composite materials were 0.2, 0.5, 1, 2, and 3 A•g −1 , the capacities displayed were 60, 53.75, 39, 30, and 27 F•g −1 , respectively.Even when the current density reaches 25 times the initial value, the specifc capacitance of the whole cell can still reach 25 F•g −1 , indicating the great potential of this all-cell device in practical applications (Figure 4(c)).
To distinguish the electrical conductivity of the device that can be enhanced after compound BC in the electrochemical test process, the EIS test is conducted (Figure 4(d)).Te asymmetric SCs constructed by the MnO 2 /BC composite electrode material and BC show smaller semicircles and steeper straight lines, indicating its better ionic and electronic conductivity.To express the relationship between the energy density (E) and power density (P) of asymmetric SCs, the Ragone curve is used (Figure 4(e)).It shows that at any power density, asymmetric SCs assembled by positive and negative materials with active materials of composite BC show greater energy density.Te maximum E value can reach 33.3 W•h•kg −1 , even when the P value is 5000 W•kg −1 , and the energy density can also reach 13.9 W•h•kg −1 .Such a high energy density is due to its high capacitance and 2.0 V large voltage window according to the following equation: where E represents the energy density of the asymmetric supercapacitor device, C is used to represent the capacitance that can be stored by the device, and V represents the voltage window of the entire device in the drainage electrolyte.At the same time, the power density can be calculated based on the equation (2) as follows:

Journal of Chemistry 7
P � E t . (3) Here, P stands for power density and T stands for time.Under 0.5 A•G −1 , the coulomb efciency remains 100% even after 4000 cycles (the inset of Figure 4(f )).Besides, it can be observed that after 4000 times of continuous charging and discharging, the whole cell constructed by composite materials and biomass carbon shows better cycle stability, which can reach 105% of the initial capacitance, indicating that the electrode material obtained by composite BC shows better electrochemical performance, as compared to the recently reported supercapacitor device [58][59][60][61][62][63][64][65][66][67][68][69] (Figure 4(f )).
In order to understand the performance of the full battery in practical applications, two 2.0 V asymmetric supercapacitors were connected in series and examined.Te device after series still shows excellent electrochemical performance under the voltage window of 0-4 V, and the CV tests at diferent sweep speeds show excellent magnifcation performance (Figure S7a).Te constant current charge and discharge test shows that the discharge time of two devices in series is twice that of one device, indicating a small capacitance loss after series (Figure S7b).Two full-cell selfassembled devices with a 4.0 V large voltage window were used to test their performance in practical applications (Figure 4(g)).It charged continuously three times at 1 A•g −1 and then connected to a red LED.Te light-emitting process of the diode from bright to dark lasted for 20 minutes.Besides, we used two series of asymmetric supercapacitors to light up the pure bromoperovskite light-emitting device, which shows that the series of 4.0 V full battery could make the pure bromoperovskite from colorless to black (Figure S8).Since the asymmetric supercapacitor can combine the voltage window of the positive and negative electrodes, there are a large voltage window of 2.0 V and its own large specifc capacitance.Te constructed asymmetric supercapacitor shows a high energy density of 33.3 Wh•kg −1 (power density of 200 W•kg −1 ), which is also the reason that it can light a small bulb for 20 minutes.Tree 2.0 V asymmetric supercapacitors were connected in series and used to light up a yellow electrochemical light-emitting device with Rubrene (RUB) as the light-emitting active layer, and excellent light-emitting efects were found.Te excellent performance in the feld of supercapacitor energy storage and perovskite discoloration gives us a reason to believe that the composite of carbon materials with high conductivity can still be an excellent solution to the problem of poor conductivity of Mn oxides.

Conclusions
In conclusion, we composited the porous, high-specifc surface area biomass carbon material (BC) prepared by the carbonization of grapefruit peel with MnO 2 and successfully prepared the anode material of high conductivity asymmetric SCs.Te sea urchin-like MnO 2 /BC composites were achieved by the direct growth of nanorods MnO 2 on the BC surface by using a simple hydrothermal method.Based on the unique sea urchin-like structure of the materials themselves, the MnO 2 /BC electrode material has a large specifc surface area and high conductivity carbon material.Te introduction of biomass carbon with strong electrical conductivity can greatly increase the electrical conductivity of the whole system.In addition, the structure combined with two-dimensional nanosheets provides more active surface, which is conducive to the enhancement of electrochemical performance.After the composite of BC, the MnO 2 /BC electrode material not only obtained a higher specifc capacitance value compared with the pure MnO 2 , but also the resistance of the material became smaller.In addition, the asymmetric supercapacitor constructed from MnO 2 /BC positive and BC negative materials showed good long cycling ability after 4000 continuous charging and discharging cycles.Te specifc capacitance of the MnO 2 /BC composites' electrode can reach 205.5 F•g −1 when the charge-discharge current density is 0.5 A•g −1 .Considering the simple synthesis of the biomass carbon material and the excellent electrochemical performance of the obtained composite electrode materials, the method of enhancing the electrochemical performance by composite carbon materials is still attractive in the future.

Figure 1 :
Figure 1: Schematic diagram of the process of preparing MnO 2 and MnO 2 /BC electrode materials (a); the transmission images of the synthesized MnO 2 (b) and MnO 2 /BC (c) are magnifed transmission images with scales of (b-c) 500 nm and insets of 200 nm; XRD patterns of MnO 2 and MnO 2 /BC (d).

Figure 2 :
Figure 2: Comparison of CV curves of pure MnO 2 and composite materials (a); CV (b) of the composite at 5-100 mV•s −1 ; GCD curve of MnO 2 /BC electrode material (c); magnifcation plot of two electrode materials (d); the EIS diagram of the prepared electrode material, and the inset is the magnifed diagram of part of the high frequency region and the low-frequency region and the simulated equivalent circuit diagram (e); long cycle curve of two materials (f ).

Figure 3 :
Figure 3: Scanning (a) and elemental analysis (b) of synthetic biomass carbon BC, the scale bar is 20 μm; the constant current chargedischarge curve of BC material (c); CV curve (d); impedance cardiography, and the inset is the simulated equivalent circuit diagram (e); cyclic performance (f ).

Figure 4 :
Figure 4: CV curves of BC and MnO 2 /BC at 20 mV•s −1 (a); CV curves of two whole cells at 20 mV•s −1 (b); the magnifcation plot of the two full cells (c); the EIS plot of the asymmetric supercapacitor constructed (d); Ragone (E-P) curve (e); long-cycle curve (f ); luminance photos of light-emitting diodes at diferent times (g); three 2.0 V asymmetric supercapacitors were connected in series and used to light up a yellow electrochemical light-emitting device with RUB as the light-emitting active layer (h). 2