Study on electrochemical supercapacitor performance of modified lavender biochar

In this study, we proposed a simple and low-cost method for the synthesis of heteroatom-doped porous carbons. Lavender waste residue as biomass precurso, using hydrothermal method synthesized N and S doped nano-layered porous carbon (NLPC-N/S) with high conductivity triumphantly. In the three-electrode system, NLPC-S showed an excellent capacitance (0.5A·g−1, 637.5 F·g−1) in 6 M KOH aqueous solution, which exceeded the specific capacitance of many reported biomass-derived porous carbons, and elucidated charge transfer mechanism and energy storage mechanism were both typical double-layer capacitance characteristics and pseudo-capacitance characteristics. The results of this study provide a cost-effective strategy for the recycling of biological waste, and that biomass carbon materials have high electrochemical performance potential in the field of energy storage.


Introduction
The massive consumption of non renewable fossil fuels has caused the current energy crisis and environmental pollution.Environmental protection is the premise of sustainable development.Green and low-carbon development has become the direction and trend of global economic development, and the key area of industrial and technological competition.To accommodate to global climate change and the implementation of China's "dual carbon" work of "carbon peaking and carbon neutrality", new energy electricity will play an important role, and the energy system and power system will usher in new changes.Renewable energy generation will account for up to 85.8% of the total generation, while the roportion of non-fossil energy generation will rise to 91%.And under the context of carbon neutrality becoming a global consensus, accelerating the development of electrochemical energy and reducing carbon emissions is one of the problems that China needs to overcome.Therefore, the development of new high-performance electrochemical energy storage devices with high energy density, high power density, and long lifespan has become an important development direction.
Supercapacitors have shown broad application prospects in novel electrochemical energy storage devices, smart grids, electric vehicles, and unmanned aerial vehicles due to their simple structure and fast energy conversion rate [1] .At present, electrode materials used in supercapacitors include carbon materials (graphene [2] , carbon nanotubes [3] ), metal oxides and hydroxides [4] , metal organic frameworks [5] .Biomass is favored by researchers because of its oversimplified structure, strong stability, and pollution-free.Xue et al. [6] illustrated co-pyrolyzed calcium gluconate (CG) and chitosan (CS), activated with KOH to adjust the micropore/mesopore ratio and bulk density of the resulting biochar.It exhibits balanced mass specific capacitance (386.3•F•g - ) and volume specific capacitance (320.6 F•cm -3 , 0.5 A•g -1 , 1.0 M H2SO4), respectively.Mian et al. [7] showed the potential of environment-friendly cellulose nanofibers (CNF) from plants as the adhesive for biochar based supercapacitors.The synthesized BN-Ac/CNF exhibits a capacitance of 268.4 F•g -1 at 5 A•g -1 .Sun et al. [8] proposed a simple hydrothermal reaction of Ti3C2Tx and reduced Graphite oxide (rGO), the unique structure generated is conducive to ion transport and reduces the stacking of Ti3C2Tx and rGO nanosheets, the optimal addition amount of Ti3C2Tx/rGO hydrogel showed capacitance of 403 F•g -1 at 2.0 mV•s -1 .Therefore, biochar have good application prospects in electrode materials of electrochemical capacitors.
In this study, we used lavender waste residue as raw material, urea and thiourea as dopants, undergo pyrolysis activation, and then hydrothermal synthesis of sulfur and nitrogen co-doped porous biomass based electrode materials.The electrochemical performance is tested through electrochemical workstation.This work provides research ideas for the efficient and clean utilization of electrode materials in supercapacitors.

Materials and chemicals
The experimental reagents used in this experiment include lavender waste residue, potassium hydroxide (KOH), hydrochloric acid (HCl), anhydrous ethanol (CH3CH2OH), urea (CO(NH)2) and thiourea (CH4N2S), which were purchased from Tianjin Shuisheng Fine Chemical Co., Ltd.All reagents were not further purified, and deionized water (H2O) was self-made in the laboratory.

Preparation of NLPC
Firstly, thoroughly cleaned the distilled lavender residue with deionized water several times to remove the scum.Dried it in an air drying oven at 70℃ for 6 h, and then ground it into particle powder smaller than 2 mm.Weighed 9 g of the above powder in a crucible, transferred it to a resistance furnace, controlled the temperature at 600℃ for 2 h, cooled it to room temperature, soaked it in 1.0 M HCl solution for 12 h, then filtered and washed with distilled water until the pH to neutral, and dried it to obtain biochar.
Secondly, ground the biochar with KOH fully in the mortar, put them into the crucible, and then programmed the temperature rose in a tubular furnace at 600℃ for 2 h at the rate of 5ºC•min -1 .After dropped to room temperature, soaked with 1 M HCl, washed for 5 times to dissolve, and washed with distilled water to neutral.Dried the prepared sample in a vacuum oven at 80℃ for 6 h, centrifuged wash, and finally dried in an air blast oven to remove moisture and obtain NLPC.
Added 0.5 g of CO(NH)2, 0.5 g of NLPC, and 50 mL of water, as well as 0.5 g of CH4N2S, 0.5 g of NLPC, and 50 mL of water to the reaction kettle, respectively.And conducted a hydrothermal reaction in 160°C drying oven for 8 h.Washed with filtered water near to neutral, and dried to obtain NLPC-N/NLPC-S.

Characterization of NLPC
The morphology of the prepared materials was observed by Hitachi Regulus 8100 scanning electron microscope (SEM), the phase analysis was performed by D8 Advanced X-ray diffractometer (XRD), and the valence state and semi quantitative data of the elements were determined by Thermo Kalpha X-ray photoelectron spectroscopy (XPS).

Electrochemical measurements
Using pressing sheet method to manufacture working electrode, there 8 mg NLPC and 1 mg carbon black were mixed uniformly, polytetrafluoroethylene dispersion was used as bonding agent, dispersed in ethanol and ultrasonic dispersion, after drying, evenly divided into three parts, coated on nickel foam (3×1 cm 2 ) uniformly.Using electrochemical workstation carried out the electrochemical test was in 6 M KOH electrolyte and the platinum plate was the counter electrode, and the saturated calomel electrode was reference electrode, with the working electrode to form the three-electrode system.The cyclic voltammetry (CV) test, adjusted the potential and sensitivity, and then tested the CV performance at scanning speed of 5-60 mV•s -1 .Galvanostatic charge-discharge (GCD), calculated the current value at the current density of 0.5-6 A•g -1 , according to the sample mass, and set the high and low potential for testing.Electrochemical impedance spectroscopy (EIS) carried out in the frequency range of 0.1 Hz-100 KHz, and then analyzed its conductivity.
Using the following formula (2.1) to calculate the specific capacity: Where C (F g -1 ) is the mass specific capacitance of NLPC, △t (s) and I (A) are the discharge time and current, respectively, and m (g) and △V (V) are the mass of NLPC and difference of potential.

Characterization of NPLC
It can be seen from Figure 1 a, b and c that the surface of NLPC has a uniform and dense porous structure, and the three-dimensional spatial structure is complex, which is liable to form ion channel.After N doping as NLPC-N (Figure 1 d, e, f), the structure cracked into a porous membrane-structure, and broadened pores and reduced thickness, resulting in smoother ion transmission.After being co-doped with S and N, the structure transformed into a three-dimensional flower-like structure (Figure 1 h, i, j), with an increase in internal pores and a more distorted three-dimensional structure, increasing ions storage capacity.).These groups exhibit excellent hydrophilicity, greatly improve the specific surface area of the material, promote ion transfer, and thus improve electrochemical performance [9] .Thus.NLPC formed abundant functional groups in the pyrolysis process, which provides the conditions for electrochemical energy storage.In addition, from O 1s spectrum of NLPC-N (Figure 3b), it can be seen that O=N-C (530.28 eV), N-C=O (531.48 eV), C-O/C-ONO2 (533.18 eV), indicates that nitrogen has been successfully embed into the structure of carbon structures after carbonization treatment.The N 1s spectrum of NLPC-N from Figure 3b exhibits four peaks as follows, N-6 (pyridine nitrogen, 397.78 eV), N-5 (pyrrole nitrogen, 399.08 eV), N-Q (graphite nitrogen, 399.88 eV) and N-X (oxidized nitrogen, 401.08 eV), such as C-O-NO2 [10] .N-6 and N-Q bond providing one or two electrons for the π system and improving the conductivity of the material.Due to the existence of the five-membered rings in polypyrrole after carbonization at 500℃ and form N-5, which provide two electrons for the π system.Those groups can improve electrical storage performance through the Faraday reaction.According to N 1s spectrum analysis, the successful doping of nitrogen has been demonstrated.According to S 2p spectrum of NLPC-S in Figure 3c, peak at 170.28 eV corresponds to oxidized sulfur substances, such as sulfates (-C-SO4-C-) or sulfates (-C-SO3-C-).The other two peaks are located in 167.88 eV and 168.78 eV, respectively, corresponding to S 2p1/2 and S 2p3/2 of -C-SO3-C-covalent bond, which help to improve the pseudo-capacitance performance also [11]   .

Electrochemical performance of NLPC
As shown in Figure 4 a, b and c are CV curves of NLPC, NLPC-N, and NLPC-S at different scanning speed.NLPC are in the form of a quasi-rectangle (Figure 4a), the quasi-rectangular cycle of each curve shows a rapid electrochemical response [11] .It indicated that they show typical double-layer capacitance characteristics and weak pseudo-capacitance characteristics.However, with the doping of N, the CV curve gradually changes to a shuttle shape (Figure 4b), which is due to the existence of redox in the charge discharge process.Furthermore, the CV curves of NLPC-S (Figure 4c) demonstrate that voltage window show a wedge structure, for the introduction of sulfate containing substances provide partial pseudo-capacitance characteristics and charge storage capacity for the system [12] .In addition, at different scanning speeds of 5-60 mV•s -1 , there was no significant distortion in the rectangular curves of the three samples, further demonstrating their high-quality capacitive performance and enhanced ion transfer.
The enclosed area represents the specific capacitance, as shown in Figure 4d, the larger the area and the higher of specific capacitance.Under different scanning rates, it can be seen that the area enclosed curve is the largest at the corresponding 5 mV•s -1 , indicating its good layered porous structure, better charge storage capacity, and rate performance.In addition, comparing the CV curves of NLPC, NLPC-N, and NLPC-S at the same scanning rate, it can be seen that the CV curves of NLPC-S have the largest closed pattern area, indicated that NLPC-S has the highest specific capacitance, for such group N=C-O, N-C=O are reduced to NH-C-O, N-C-OH, and -C-SO4-C-to -C-SO3-C-, during charge and discharge process,which provide partial pseudo-capacitance characteristics and charge storage capacity significantly.As shown in Figure 5 a, b, and c, the relationship between specific capacitance values of NLPC, NLPC-N, and NLPC-S at different current densities shows the shape of a iequicrural triangle, which is a typical double-layer capacitance behavior [13] .It is proved that the specific capacity is almost constant under different current densities, which is because the material has good stability, rapid charge transfer and excellent electrical conductivity.In addition, the curves in Figure 5b and c are symmetrical triangles, and the slope gradually changes within the current density of 0.5-6 A•g -1 , indicating that the material has excellent electrochemical long-term cycling stability, and introduction of hybrid elements not only changed its structure, but also provided abundant active sites, which greatly promoted the increase of specific capacity.Therefore, the pyrolysis product of lavender waste residue is an ideal energy storage material.
In Figure 5d, at current density of 0.5 A•g -1 , and the specific capacitance values of NLPC, NLPC-N, and NLPC-S are 215 F•g -1 , 438.1 F•g -1 , and 637.5 F•g -1 , respectively.It is because doping causes changes in the material structure and contributes to a portion of the pseudo-capacitance characteristics.Figure 6 shows Electrochemical impedance spectroscopy (EIS) test of NLPC, NLPC-N, and NLPC-S.EIS can reflect the enhancement mechanism of capacitance [14] , which helps to better understand its internal resistance, charge transfer kinetics, and ion diffusion process.In the high-frequency range, a significant semicircle appears, which represents charge transfer resistance.This is attributed to the Faraday reaction and adsorption of the double-layer between the electrode interface and the electrolyte [15] .Here, the intercept of the real axis (Z') in the high frequency region is the electrode internal resistance (Rs), which consists of three parts, the contact resistance current collection at the carbon material/foam Ni interface, the inherent resistance of the carbon material, and KOH electrolyte.As is well known, vertical line features are key evidence of near ideal typical double-layer capacitance performance [16] .The slope between impedance and real axis in the low-frequency region is greater than the typical Warburg angle (45°), which supports their high capacitance.And NLPC-N, NLPC-S, and NLPC exhibit a straight line with an angle of nearly 90°, indicated that ions (or electrolytes) diffuse faster towards the electrode material.

NLPC storage charge mechanism
The above data analysis, it can be concluded that the pyrolysis of lavender waste residue has enhanced its specific surface area and electrical conductivity, and improved its charge storage capacity.Under the action of urea and thiourea, N and S elements are successfully doped in its structure, thus providing excellent pseudo-capacitance performance.The specific mechanism as follows: -

Conclusions
Ili Kazak Autonomous Prefecture is the largest lavender planting base in China, with a huge annual output of lavender.However, the utilization rate of lavender stems and leaves is low, making it difficult to treat, resulting in resource costs.This study provides a strategy for comprehensively and effectively utilizing sustainable biomass waste to achieve pollution-free preparation of high-performance carbon electrode materials.In the three-electrode system, NLPC-S shows a high specific capacitance (637.5 F•g -1 at 0.5 A•g -1 ), which is equivalent to or higher than most other biomass-derived carbon materials.This is because the doping of S and N elements promotes the structural change of the material, improves the charge storage capacity, and provides pseudo-capacitance characteristics, thus showing excellent electrochemical performance.Lavender based derived biochar prepared by chemical activation has a broad application prospect in low-cost and environmentally friendly super electrode materials, providing good guidance for the application of biochar.In addition, elemental doping and composite materials may be an effective way to improve material properties.

Figure 1 .
Figure 1.a, b, c are SEM images at different multiples of NLPC, d, e, f are SEM images at different multiples of NLPC-N, f, h, i are SEM images at different multiples of NLPC-S.

Figures 4 .
Figures 4. a, b and c are the CV curves of NLPC, NLPC-N, and NLPC-S at different scanning rates, d comparison of CV curves of NLPC, NLPC-N, and NLPC-S at scanning speed of 5 mV•s -1 .

Figure 5 .
Figure 5. a is GCD curve of NLPC-N, b is the GCD curve of b NLPC-S, c is the GCD curve of NLPC, d is comparison of GCD curves of NLPC, NLPC-N, and NLPC-S at current density of 0.5 A g -1 .