Biomass Derived N-doped Porous Carbon Made from Reed Straw for Enhanced Supercapacitor

1. Introduction

Nowadays, the world is facing a series of problems caused by fossil fuel combustion [1]. It is urgently to develop the energy storage devices (EESs) by utilizing clean and sustainable energy (wind, solar, and tidal energy, etc) [2,3]. Electric double-layer capacitors (EDLCs) exhibit high specific power (up to 10 KW kg^-1), fast charge/discharge capability, and long cycle life (e.g. millions of cycles), has attracted much attention [4,5]. However, EDLCs contain porous carbon electrodes that presents the charge storage mechanism of ion adsorption at the electrolyte and electrode interface, and no electrochemical reactions are involved [6,7]. This is responsible for the EDLCs poor energy content [8]. Therefore, it is necessary to improve the porous carbon material to enhance the faradaic reaction for promoting the specific capacitance.

Biomass carbon material is a carbon-rich solid produced as a by-product from the thermochemical pyrolysis of biomass in an oxygen-limiting environment, which can serve as the electrode for supercapacitors (SCs) [9,10,11]. The chemical and physical properties of biomass carbon vary significantly with the feedstock and fabrication conditions [12,13]. Their carbonized network can provide a more surface area and give the ion diffusion pathway for charge storage. Momodu et al. synthesized activated carbon derived from bark through a simple activation and carbonization process, it showed a capacitance of 191 F g^-1 at 1 A g^-1 [14]. Kesavan et al. fabricated nitrogen-doped carbon nanosheets using peanut shell as a precursor. At a current density of 1 A g^-1, a capacitance of 195 F g^-1 can be achieved [15]. A Alpinia officinalis leaf derived porous carbon was reported by Taer et al., and it delivered a capacitance of 161 F g^-1 at 1 A g^-1 [16]. Kang et al. prepared hierarchical porous carbon materials using peanut bran as raw material through hydrothermal carbonization and KOH activation reaction, and a capacitance of 188 A g^-1 was reached at 0.04 A g^-1 [17]. Reed straw is a rich biomass material with rich cellulose, lignin, and hemicellulose [18]. The reed straw derived porous carbon is considered as the promising candidate for charge storage [19]. Such hollow structure can provide more interface of solid-liquid interface for EDLC.

Until now, various natural biomass materials such as bacterial cellulose [20], orange peels [21], rice stems [22], bamboo [23], and carb shell [24] have been reported as the precursors to prepared carbon anode for SCs. These biomass carbon materials based no EDLC charge storage mechanism can be improved by increasing their surface area. However, the increased surface area contributing EDLC is limited, and the capacitance and energy density of carbon materials usually remain poor [11]. Recently, the introduction of electrochemical-active functionalities (e.g., O, N, P, and S containing groups) on the carbon skeleton turned out to be a feasible approach to significantly boost the capacitance of carbon-based SCs [25,26,27,28], especially nitrogen doping of carbon, which can increase the surface polarity, electric conductivity, and electron-donor affinity for carbon network. The capacitance of nitrogen-doped carbons (NPCs) comes from not only EDLC but also the faradaic reaction at or near in the surface of carbon material [29]. Generally, the preparation of NPCs is achieved through the pyrolysis of biomass waste containing N element, and the post-treatment of carbon materials with ammonia and urea at high temperatures [30]. Therefore, we anticipated that pyrolyzing the reed straw with N-containing organic to fabricate the porous carbon materials with rich active sites, which can serve as the electrode for boosting SCs.

In this work, a reed straw derived biomass carbon (RSM) was prepared by directly pyrolyzing reed straw and melamine with KOH as the activator. The RSM as the electrode for SCs exhibited an enhanced electrochemical performance with rationally introducing melamine. In addition, the pyrolysis temperature was taken into considered. The optimized RSM-0.33-550 (the mass ratio of melamine to reed straw is 1 : 3 and the optimal activation temperature is 550 °C ) with a N content of 6.02% and a specific surface area of 547.1 m² g^-1 displayed a capacitance of 202.8 and 164 F g^-1 at a current density of 1 and 20 A g^-1, respectively. At a high current density of 20 A g^-1, it still remained a capacitance of 158 F g^-1. It also showed an outstanding stability with capacity retention of 96.3% at 20 A g^-1 after 5000 cycles.

2. Experimental Section

2.1. Materials and Methods

Reed straw powder (60 mesh) purchased from Shaanxi Jinhe Agricultural Technology Co., Ltd. Melamine and potassium hydroxide (KOH) were purchased from Shanghai Aladdin Biochemical Technology Co. (Shanghai, China). Polyvinylidene fluoride (PVDF), acetylene black, concentrated hydrochloric acid (HCl), N-methylpyrrolidone (NMP), and other chemical reagents were purchased from Chengdu Kelong Chemical Reagent Co. (Chengdu, China). Nitrogen was purchased from Guangdu Gas Business Department (Chengdu, China). All reagents were analytical grade and used as received without any further purification.

2.2. Preparation of RSM-y-550

2 g of reed straw powder was mixed with varying weights of melamine (0 g, 0.66 g, 2 g, and 6 g), and stirred continuously after adding 50 mL deionized water, then transferred to a 75 mL Teflon stainless steel autoclave and reacted in an oven at 120 °C for 2 h. When cooled to room temperature, a yellowish powder (RSM) is obtained. Subsequently, 1 g of RSM powder and 1 g of KOH were added in 10 mL deionized water with continuously stirring for 4 h, and then dried at 80 °C for 12 h. The mixture is transferred in a porcelain boat and putted in a tubular furnace for further activation. The activation operation was conducted at 550 °C for 2 h, with the temperature raise rate of 5 °C min^-1 under nitrogen atmosphere to obtain a porous carbon. Then it was cleaned with 1 M hydrochloric acid and washed with deionized water for several times, and dried by vacuum at 60 °C for 12 h. The obtained sample was denoted as RSM-y-550, where y represents the mass ratio of melamine to reed straw (y = 0, 0.33, 1, and 3).

2.3. Characterization

XRD patterns were collected on an X-ray diffractometer (Philip Company, Pw1730) equipped with a Cu Kα radiation (λ = 1.5418 Å). XPS spectra were measured on an X-ray photoelectron spectroscopy spectrometer (XPS, a Kratos Axis Ultra spectrometer, Kratos Analytical Inc. Manchester, UK). SEM images were captured on a Hitachi S-4800 field emission scanning electron microscope (Hitachi High-Tech Co., Tokyo, Japan) at an accelerating voltage of 20 kV. Raman spectra were measured on a DXR Raman microscope (Thermo Fisher Scientific Inc., Waltham, MA, USA) with an excitation wavelength of 455 nm. Brunauer-Emmett-Teller (BET) surface area was collected on a Micromeritics TRISTAR II3020 surface area analyzer (Micromeritics Instrument, Norcross, GA, USA).

2.4. Electrochemical Measurement

To prepare the RSM-y-550 working electrode, the prepared mixture of active carbon material (80 wt%), polyvinylidene fluoride (10 wt%), and acetylene black (10 wt%) were coated on nickel foam (1.0 × 1.0 cm²) and dried at 80 °C for 24 h. The three-electrode system was employed to estimate the electrochemical performance of all electrodes. RSM-y-550 was used as the working electrode (anode), with platinum as the opposite electrode, saturated calomel electrode (SCE) as the reference electrode, and 6 M KOH solution as the electrolyte. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed at CHI600E electrochemical workstation (Chenhua Instrument Co., Shanghai, China). The GCD was employed at a current density of 1–10 A g^-1. The CV was conducted at a scan rate of 10–100 mV s^-1. The frequency range of EIS was 0.01–100 kHz, and the potential window was 5 mV. According to the results of GCD curve, the capacitance is calculated as follows: C = I × ∆t / (m × ∆V), where ∆V (V) is the potential window, m (g) is the mass load of the active substance, ∆t (s) is the discharge time, I (A) is the current density, C (F g^-1) is the specific capacitance.

3. Results and Discussion

Figure 1 shows the schematic illustration of the preparation process of the RSM-y-5. The reed straw powder and melamine were firstly mixed as the carbon and the nitrogen resource, respectively. Subsequently, the mixture was further pyrolysis under N₂ atmosphere with the KOH as the activator. Finally, the N-doped porous carbon was collected by after washing with HCl to remove the impurity. Figure 2 shows the SEM images of different RSM-y-550 samples (y = 0, 0.33, 1, and 3). The low-magnification SEM images of all samples shows irregular particle morphology, implying the defected structure of carbon material after pyrolysis and activation process. Figure 2a,b show the SEM images of RSM-0-550 sample without melamine addition. After introduction of melamine, no obvious change of morphology is observed (Figure 2c–h). Notably, some macropore structure can be found from the high-magnification SEM images for the RSM-0.33-550 (Figure S1a) and RSM-1-550 (Figure S2a) samples, and such macropore is conductive to the ion transport in electrolyte. In addition, the effect of activation temperature on the morphology of carbon is also investigated. For simplification, the as-prepared carbon materials were name as the RSM-1-x, and the x presents the value of the pyrolysis temperature of 400, 500, 550, 600, and 650 °C. As shown in Figure S2, the morphology of carbon material shows a more disorder and porous structure with the increased of activation temperature, which implies that higher pyrolysis temperature is favorable to the formation of porous structure.

Figure 3a clearly shows the XRD pattern of RSM-y-550. It can be observed that all the XRD patterns show a broad peak at 25 °, which corresponds to the typical feature of amorphous carbon [31]. Additionally, no characteristic peak of C₃N₄ derived from melamine is found [32,33]. Figure 3b shows the Raman spectra of a series of RSM-y-550 samples. Two obvious peaks are observed at 1363 and 1596 cm^-1, corresponding to G and D bands, respectively [34]. D band is related to the graphitic defective and disordered carbon structure, while G band is associated with to the E_2g symmetry between graphitic layers, which indicates ordered carbon structure [35]. The intensity ratio of G to D band (I_G/I_D) for all samples are illustrated as shown in Figure 3b. With the increased melamine addition, I_G/I_D decreased from 1.310 to 1.232, indicating a more disorder of the carbon structure. This could be conducive to the diffusion of electrolyte ions and the improvement of EDLC. It is worth noting that the value of I_G/I_D is 1.324 for RSM-3-550 sample, which suggests that the excessive addition of melamine increased the degree of carbon graphitization. This could be due to the formation of C₃N₄ after excessive melamine introduction. In addition, Figure S3a shows the XRD patterns of RSM-1-x obtained at different pyrolysis temperature, and similar results are observed. The Raman spectra of RSM-1-x are displayed in Figure S3b. Notably, with the increased activation temperature from 400 to 600 °C, the value of I_G/I_D decreased from 1.466 to 1.185. This indicates the higher temperature is conductive to the activation process, thus providing more interface of electrode and electrolyte. However, the increased value of I_G/I_D of 1.213 is observed for RSM-1-650 sample, which could be attributed to the enhanced degree of graphitization at a higher pyrolysis temperature.

The wide scan XPS spectrum of RSM-0-550 (Figure 4a) shows the existence of C, N, and O elements, and the content was measured to be 81.1%, 5.2% and 13.6%, respectively. The existence of N element is ascribed to the protein from the reed straw [36]. After melamine addition, the obtained RSM-0.33-550 presents a N content of 6.1%, suggesting the N introduction (Figure 4d) [37]. Figure 4b and 4e compare the C 1s core-level XPS spectra of the RSM-0-550 and RSM-0.33-550 samples. Four peaks at 284.8, 286.1, 287.6, and 289.2 eV are observed, corresponding to the C-C/C=C, C-N/C-O, C=O, and O=C-O, respectively [38,39]. Notably, a little decline content of C-C and a small increase of C-N are observed for RSM-0.33-550 compared with RSM-0-550, implying that a part of the C-N bond replaced the C-C bond after melamine introducing. The N 1s XPS spectra of RSM-0-550 (Figure 4c) and RSM-0.33-550 (Figure 4f) display the four peaks at 398.6, 400.1, 401.3 and 405.6 eV, corresponding to the Pyridinic-N (N-6), Pyrrolic-N (N-5), Graphitic-N (G-N), and N-Oxide (N-O), respectively [11,40]. The relative densities of these N species can be illustrated from the proportional areas of corresponding peaks as displayed in Figure S4. It is well known that the N-6 presents more active compared with the other types of N, due to it can give more active sties and sufficient defects, and is more energetically favorable towards promoting the interaction between the electrode and electrolyte ions (alkali metal ions) [41]. Clearly, the content of N-6 increased from 15.4% to 24.8% after melamine introducing, indicating the increased active sites for charge storage. Moreover, the decline content of G-N from 14.9% to 10.3% after melamine introducing implies the more exposed edge sites, which can be due to the pyrolysis and activation process of melamine [42]. The N doping can considerably boost the performance of SCs by tunning the electronic conductivity, surface accessibility, and faradaic reaction. In the carbon framework, the N and C atoms with multi-graded elecronegativities give a polarized/accessible electrode surface with enhanced elecrosorption active sites for improved electrolyte-electrode interaction. Additionally, the boosted adsorption of alkali metal ions can be achieved on the N faradaic-active sites, resulting in good pseudocapacitance by electrochemical redox reactions [43,44].

To study the porosity of the carbon materials, BET measurement was conducted. As displayed in Figure 5a, N₂ adsorption-desorption isotherms of RSM-0.33-550 displays the sharply rise adsorption quantity at P/P₀ < 0.05, indicating the type I (IUPAC classification) feature for microporous materials [45]. Clearly, the obvious hysteresis loop at P/P₀ = 0.45–0.9 and sharply increased at P/P₀ = 0.9–1.0 are found, indicates the existence of both meso- and microporous stricture in RSM-0.33-550 [42]. Figure 5b shows the corresponding pore size distribution, and the mesopores and micropores with the diameters of around 1.0 and 2.3 nm are achieved, respectively. The mesoporous pores have a positive effect on the charge storage, which is important for reaching high capacitance [35]. The detailed parameters are shown in Table S1, and it evidently shows that the surface area of RSM-0.33-550 is 547.1 m² g^-1 with a pore volume of 0.159 cm³ g^-1. Therefore, this hierarchical porosity of such RSM-0.33-550 can give an abundant surface area and rapid electrolyte ions transfer for energy storage.

The electrochemical performance of all samples was evaluated by a three-electrode system, in which 6 M KOH was employed as the electrolyte due to its a higher ionic conductivity and a smaller radius of the hydration sphere (3.00 Å for OH^- and 3.31 Å for K⁺). Figure S5 shows the GCD curves of RSM-1-x prepared by the different pyrolysis temperature. Clearly, the RSM-1-400, RSM-1-500, RSM-1-550, RSM-1-600, and RSM-1-650 electrode delivered a capacitance of 45.6, 145.4, 164.7, 146.3, and 120.1 F g^-1, respectively. Based on the GCD data, the related rate performance was illustrated in Figure S6. In addition, Figure S7 illustrates the CV curves of the RSM-1-y electrode, similar results were found. As shown in Figure S7f, the capacitance of 42.3, 131.2, 153.4, 142, and 114.9 F g^-1 was achieved for RSM-1-400, RSM-1-500, RSM-1-550, RSM-1-600, and RSM-1-650 electrode, respectively. Notably, the CV curves of RSM-1-550, RSM-1-600, and RSM-1-650 electrodes (Figure S7c,d) show a relative quasi-rectangle shape compared with the RSM-1-400 and RSM-1-500, which is ascribed to that a higher pyrolysis temperature can provide a sufficient activation process, thus offering capacitive behavior for charge storage. This indicates that the activation temperature has a great influence on the electrochemical performance, which is manly ascribed to the change of the structure and surface chemistry of carbon materials.

Figure 6a summarizes the GCD curve of RSM-y-550 electrode at a current density of 1 A g^-1. All GCD curves present a relatively symmetrical profile, indicating the good coulombic efficiency and capacitive properties. Notably, RSM-0.33-550 electrode exhibited a longer discharge time, which implies its superior capacitance. The specific capacitance of RSM-0-550, RSM-0.33-550, RSM-1-550, and RSM-3-550 was calculated to be 181.6, 202.8, 164.7, and 159.3 F g^-1, respectively. This indicates that the rational melamine introduction is more beneficial to improve the electrochemical performance. The GCD cures of RSM-y-550 at different current densities are shown in Figure 6b and S8a–S8d. All GCD curves show the symmetrical triangle shape, indicating the good capacitive behavior. The corresponding rate performance based on GCD data is displayed in Figure 6c. Clearly, RSM-0.33-550 delivered a superior rate performance with a capacitance of 202.8, 194.4, 185.5, and 177, F g^-1 at 1, 2, 5, and 10 A g^-1. Even at a current density of 20 A g^-1, a capacitance of 164 mF g^-1 can be achieved, which is higher than that of RSM-0-550 (148 F g^-1), RSM-0-550 (120 F g^-1), and RSM-0-550 (98 F g^-1). In addition, the GCD curves of RSM-y-550 were also given in Figures 6d and S9. The capacitance of 171.2, 189.7, 153.4, and 146.3 F g^-1 was reached for RSM-0-550, RSM-0.33-550, RSM-1-550, and RSM-3-550, respectively. More comparisons are listed in Table S2, and this indicates the superior electrochemical performance of RSM-0.33-550. Furthermore, EIS measurement was employed as shown in Figure 6e. The Nyquist plots of the RSM-0-550, RSM-0.33-550, and RSM-1-550 displayed a small semi-circle in the high frequency region, indicating their low charge transport resistance [46]. On the contrary, the Nyquist plot of RSM-3-550 presents a larger semicircle at high frequency region. The increased charge transport resistance could be ascribed to the formation of C₃N₄ derived from melamine after introducing excessive N resource [47]. At the low frequency region, it can be seen that RSM-0.33-550 has a highest slope, indicating that the electrolyte ion diffusion of RSM-0.33-550 is the most active and can provide faster current response. Figure 6f shows the long cycling test of RSM-0.33-550 electrode. Notably, it exhibited a good cycle stability with a capacitance retention rate of 96.3% (158 F g^-1) at a current density of 20 A g^-1 after 5000 cycles.

4. Conclusions

In summary, we proposed an efficient route to prepare a N-doped carbon material by directly pyrolyzing reed straw and melamine. The pyrolysis temperature and the mount of activator were carefully investigated. The rational regulation of the activation temperature and the amount of N resource is significant for the electrochemical performance of reed-straw based electrode. The optimized RSM-0.33-550 prepared at 550 °C with a mass ratio of melamine to reed straw of 0.33 possessed a N content of 6.02% and a specific surface area of 547.1 m² g^-1. As the anode for SCs, such RSM-0.33-550 electrode delivered a higher capacitance of 202.8 F g^-1 at a current density of 1 A g^-1. EIS measurement exhibited a faster ion transfer for the RSM-0.33-550 electrode. Even at a current density of 20 A g^-1, the RSM-0.33-550 electrode can achieve a capacitance of 164 mF g^-1, which is higher than that of RSM-0-550 (148 F g^-1), RSM-0-550 (120 F g^-1), and RSM-0-550 (98 F g^-1). Notably, the RSM-0.33-550 electrode showed an excellent cyclic stability with 96.3% capacitance retention after 5000 cycles. This work offers us a new insight for rationally utilizing the biomass waste to prepare the functionalized carbon materials and the related application.