Biomass-Based Nitrogen-Doped Hollow Carbon Nanospheres

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Biomass-Based Nitrogen-Doped Hollow Carbon Nanospheres Derived Directly from Glucose and Glucosamine: Structural Evolution and Supercapacitor Properties Huixia Qu, Xiujuan Zhang, Jingjing Zhan, Weiqi Sun, Zhichen Si, and Hongkun Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04842 • Publication Date (Web): 26 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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ACS Sustainable Chemistry & Engineering

Biomass-Based Nitrogen-Doped Hollow Carbon Nanospheres Derived Directly from Glucose and Glucosamine: Structural Evolution and Supercapacitor Properties

Huixia Qua, Xiujuan Zhangb, Jingjing Zhana,*, Weiqi Suna, Zhichen Sia, Hongkun Chenc

a

Key Laboratory of Industrial Ecology and Environmental Engineering, MOE,

School of Food and Environment, Dalian University of Technology, Panjin, 124221, P. R. China b

School of Petroleum and Chemical Engineering, Dalian University of Technology,

Panjin, 124221, P. R. China c

State Key Laboratory of Petroleum Pollution Control, CNPC Research Institute of

Safety & Environment Technology, Beijing, 102206, P. R. China

Corresponding author. Tel: +86-427-2631789. Email: [email protected] Mailing address: 2 Dagong Road, Liaodongwan New District, Panjin, Liaoning, China, 124221

1

ACS Paragon Plus Environment

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Abstract Carbon-based materials generated from biomass have been studied extensively. However, to our best knowledge, there is no report of any hollow structured carbon nanospheres directly derived from biomass without the use of templates. In this research, a green route to directly convert biomass to nitrogen-doped hollow carbon nanospheres (N-HCSs) was reported, where only glucose and glucosamine were used as the precursors and an aerosol-assisted process was employed. In the process, amino groups of glucosamine triggered the co-assembly between glucose and glucosamine, resulting in the structural evolution of hollow structures. The aerosol-based technology ensures the obtained particles with the spherical morphology and in the nanoscale size range. The as-prepared materials have been thoroughly characterized by SEM, TEM, HAADF-STEM, EELS mapping, XPS, XRD, Raman and nitrogen adsorption. Owing to the unique structural and surface properties, the resultant N-HCSs exhibited excellent electrochemical properties for energy storage, including the high specific capacitance of 266 F g-1 at 0.2 A g-1, long cycling stability with 96.8% of capacitance retained after 3000 cycles and fast charge-discharge process. This discovery could be adapted to guide design and synthesis of a variety of hollow materials from biomass for wider applications in the environment and energy fields. Keywords: Aerosol-assisted, assembly, hollow spheres, biomass, carbon, N-doped

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Introduction Highly efficient energy storage and conversion devices have attracted increasing interests due to the growing demand for sustainable energy.1, 2 Supercapacitors with advantages of high power density, excellent cycle stability and fast charge/discharge speed have exhibited immense potential in the electrical energy storage technology.3, 4 It is well documented that the electrochemical performance of the supercapacitors mainly rely on the properties of the electrode materials.5 Hence, the development of advanced electrode materials through an economic and feasible method has been a research hotspot over the past few decades. Compared to common carbon materials, nitrogen doped hollow carbon spheres (N-HCSs) have been considered as a more promising candidate electrode for supercapacitors due to their excellent capacitive performance and rate capability.6 On the one hand, the hollow structure is beneficial for the electrolyte diffusion and electron migration to the active sites owing to its unique characteristics including high surface-to-volume ratios, short transport lengths for both mass and charge transport, thus

increasing

the

uptake

and

storage

of

electrostatic

charge

at

the

electrolyte-electrode interface.7-10 On the other hand, heteroatomic dopants, such as nitrogen,11 sulfur,12 boron13 and phosphorus14 are able to improve the energy storage ability by increasing electrical conductivity, surface polarity, surface basic sites and electron-donor affinity.15, 16 In addition, the doping of the heteroatoms on the carbon surface may induce a pseudocapacitive interaction between the heteroatom-containing functional groups and the electrolyte ions, leading to an extra capacitance increase. 3



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To date, most of N-HCSs are prepared via a multistep template method,17, 18 where a spherical template was firstly coated with a polymerizable N-containing carbon precursor, including dopamine,19 pyrrol,20 acetonitrile ,21 ethylenediamine22 and so on,23-25 followed by the carbonization and subsequent template removal processes. Most recently, a template-free method has been reported by Sun and co-workers to prepare N-HCSs through the carbonization of the poly (amic acid) vesicles and cross-linked melamine under nitrogen atmosphere.26 Obviously, all above-mentioned methods involving tedious procedures and organic toxic chemicals are complicated, time-consuming and expensive, not suitable for scale-up production and practical applications. Therefore, it is highly desirable to develop a facile sustainable approach to fabricate the high-value energy materials (N-HCSs), accelerating their practical applications. Currently, carbon-based materials generated from biomass have attracted much attention, because biomass is easily accessible, low-cost and available in natural abundance. Previous studies have demonstrated that direct pyrolysis and hydrothermal carbonization are two kinds of typical methods to convert biomass such as corncobs, 27

rapeseed shell,

28

wheat,29 chicken feather,30 and bagasse31 into carbon materials.

The former method only produced carbon materials with irregular shape, while the latter method may generate uniform carbon particles with spherical morphology when the carbohydrate sources such as sugar32 and cyclodextrins33 are used as the precursors. Meanwhile, N-doped carbon materials could be easily obtained when the used biomass contains nitrogen element, such as chitosan34 and okara35. However, to 4


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our knowledge, there is no report that hollow structured carbon nanospheres could be directly produced from biomass in the absence of any templates. In this paper, we firstly report a green route to directly convert biomass to nitrogen-doped carbon materials with spherical morphology and unique hollow structure, via an extremely fast (~10 s) aerosol-assisted process followed by a subsequent carbonization. In the procedure, the biomass model compounds glucose and glucosamine were used as precursors, and water was used as the solvent, avoiding the utilization of any templates and organic toxic solvents. Herein, we demonstrated that the amino-groups in biomass glucosamine triggered the co-assembly between glucose and glucosamine, directed the structural evolution of carbon nanospheres from solid or reticular to hollow. Comprehensive material characterizations have clearly revealed unique structural characteristics of the resultant materials including spherical morphology, nitrogen dopant and hollow structure. Importantly, benefiting from the synergistic effects from the hollow structure and nitrogen dopant, the as obtained N-HCSs directly from biomass exhibited a high capacitance of 266 F g−1 at a current density of 0.2 A g-1 as well as a long cycling stability.

Experimental section Chemicals Glucose and acetone were purchased from Tianjin Kermel Reagent Co., Ltd. Glucosamine sulfate (AR, 99%) was supplied from Xiya Reagent. Nafion (5%) was bought from Shanghai Hesen Electric Co., Ltd. Sulfuric acid (H2SO4, AR) was 5



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purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemical reagents were used without any further purification. Deionized (DI) water was used in all experiments. Material synthesis The aerosol-based process as shown in Fig.1a was applied for the material synthesis, while the synthesis route of N-HCSs is briefly described in Fig. 1b. In the process, only two kinds of common biomass including glucose and glucosamine sulfate were employed as precursors. In detail, 1.5 g of glucose and 4.5 g of glucosamine sulfate were first dissolved in 30 mL of water to form a clear solution, then 400 µL of concentrated H2SO4 was added as a dehydration catalyst. In the experiment, the precursor solution was first nebulized by an atomizer (HRH WAG-3, Beijing Huironghe Company), causing the formation of tiny droplets. Driven by flowing nitrogen gas, these droplets passed through a heating zone where the temperature was held at 400℃, leading to the occurrence of solvent evaporation and carbohydrate dehydration. The resulting particles were then collected by a filter maintained at 100℃, followed by repeated centrifugation and washing in ethanol. Considering that the residence time in the aerosol reactor is just of the order of 10 seconds, it is highly unlikely that there is complete dehydration and carbonization for glucose and glucosamine. Finally, N-HCSs were obtained after the collected powder was further pyrolyzed in a tube furnace at 800℃ for 3 h under inert atmosphere. Material characterization Scanning electron microscopy (SEM) was conducted using a FEI Nova NanoSEM 6


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450. Transmission electron microscopy (TEM) and high angle annular dark field (HAADF) images were collected on a Tecnai G220S-Twin instrument with an element energy-dispersive X-ray photoelectron spectroscopy. Surface structure of the samples was determined by X-ray photoelectron spectroscopy (XPS) on a Thermo ESCALAB 250Xi, using Al Kα (1486.6 eV, 15 kV, 10.8 mA) as X-ray source for excitation. The X-ray diffraction (XRD) patterns of samples were recorded using Shimadzu XRD-7000S X-ray diffraction system with Cu Kα radiation of wavelength λ =0.154 nm. Raman spectrum was performed by a Renishaw inVia Raman microscope. The nitrogen adsorption/desorption isotherms were collected on an AUTOSORB-1 nitrogen adsorption apparatus. The pore size distribution (PSD) was calculated using the BJH (Barret-Joyner-Halenda) model. Elemental analysis including C, H, N, O was taken on VARIO EL cube (Elementar Analysensysteme, Germany). Electrochemical Measurement All

electrochemical

measurements

were

carried

out

using

a

RST5200F

electrochemical workstation (Suzhou Risetest Electronic Co., Ltd) in a standard three-electrode cell containing 1 M H2SO4 aqueous solution at room temperature. A platinum wire and saturated calomel (Hg/Hg2Cl2) electrode were used as the counter electrode and reference electrode, respectively. To prepare the working electrode, 4 mg of the specific carbon material were ultrasonically dispersed into 1 mL of a mixed solution containing DI water, acetone, Nafion (5 wt%) with the volume ratio at 8:1:1. Then, 5 µL of the resulting suspension was dropped onto a glassy carbon electrode 7



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surface (CHI 104, 3mm diameter) and dried at room temperature. Hence, 20 µg of carbon spheres were coated on the glass carbon working electrode. Cyclic voltammetry (CV) curves were recorded at potentials between -0.200 and 0.800 V in the 1 M H2SO4 electrolyte at sweep rates of 5–200 mV s-1. Galvanostatic charge–discharge (GCD) curves were obtained at current densities ranging from 0.1 to 10 A g-1 in the potential range between -0.200 and 0.800 V. Electrochemical impedance spectroscopy (EIS) were recorded at open circuit potential in a frequency range of 1 mHz to 100 kHz with an AC amplitude of 5 mV. The electrochemical stability was evaluated by the capacitance retention after 3000 cycles at a current density of 1 A g-1. In the three-electrode system, the specific capacitance was calculated from the galvanostatic test using the following equation: Cs =

ூ△௧ ௠△௏

(1)

Where Cs is the specific capacitance (F g-1), I is the charge–discharge current (A), m is the mass of material on the electrode (g), ∆t is the discharge time (s) and ∆V is the potential window (V). Results and Discussion Morphology and chemical structure of N-HCSs The morphology and microstructure of the obtained materials were analyzed through scanning and transmission electron microscope observations. The low magnification SEM image in Fig. 2a reveals that the obtained particles show spherical morphology and a broad size range of 30-500 nm, which are typical characteristics of particles 8


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synthesized by the aerosol-based process. The SEM image of an individual particle (Fig. 2b) indicates that the particle has an uneven and rough surface attached by some tiny particles. In addition, TEM image in Fig. 2c provides a panoramic view of multiple particles showing that every particle presents hollow structure with non-uniform size. The high-resolution TEM image in Fig. 2d illustrates a large number of disordered mesopores on the wall of the hollow spheres, which were supposedly created from glucose decomposition. Furthermore, the typical high-angle annular dark-field scanning TEM (HAADF-STEM) image and the corresponding electron energy-loss spectroscopy (EELS) mapping images are shown in Fig 2e. It is clearly observed that C, O and N elements are uniformly distributed along the outer ring of the spheres, which is more illustrative in clarifying the presence of the void inside the core and the dopant of element N in the as obtained particles. Hence, we hereafter use nitrogen-doped hollow carbon spheres (N-HCSs) to depict the obtained particles. In order to prove the aerosol process is the vital procedure to obtain the particles with a hollow and spherical structure, SEM and TEM images of materials only with the aerosol-assisted process but without carbonization were provided in Fig. 2f and g. The chemical composition and the surface electronic state of N-HCSs were examined by XPS. As shown in Fig. 3a, the peaks at 284.9, 401.0 and 532.0 eV can be ascribed to the binding energies of C 1s, N 1s and O 1s. In the evaluation of the nitrogen bonding configurations, the high resolution N 1s spectrum was proved to be fitted into four component peaks (Fig. 3b), which are ascribed to pyridinic nitrogen 9



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(N-6, 398.13 eV), pyrrolic nitrogen (N-5, 399.31 eV), quaternary nitrogen (N-Q, 400.95 eV) and oxidized nitrogen (N-X, 401.95 eV).36 Hence, all possible functional groups in the carbon framework are schemed in Fig.3c based on the above XPS results. The XRD pattern of N-HCSs is presented in Fig. 3d. Two diffraction peaks at 2θ =23° and 43° are indexed to the (002) and (100) plane reflections of carbon materials, respectively. Meanwhile, the peak at 2θ =23° shift to lower diffraction angle compared to the ideal angle for the ordered graphite at 26°, indicating that N-HCSs exhibit amorphous carbon structure.37, 38 In addition, the Raman spectrum of N-HCSs in Fig. 3e reveals two characteristic peaks at around 1340 and 1580 cm-1, corresponding to the D and G bands of carbon. The D band is a typical characteristic of disordered carbon, while the G band is a result of sp2 electronic configuration in the graphitic carbon. Therefore, the relative intensity ratio of the D to G band (ID/IG) is widely adopted to evaluate the graphitization degree of carbon material.39 From Fig. 3e, the ID/IG value at 0.91 implies a low graphitized degree of N-HCSs, in agreement with prior XRD results. The specific surface area and the total pore volume of N-HCSs (Fig. 3f and the inset) were measured to be 327 m2 g-1 and 0.13 cm3 g-1, respectively. The type-IV isotherm demonstrates the existence of a large amount of the mesopores within the shell of N-HCSs, ensuring a large contact area and continuous electron transport. Structural Evolution of N-HCSs In the experiments, we found that N-HCSs were obtained only when the mass ratio between two kinds of biomass was set at the appropriate value. Hence, we fixed the 10


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total amount of biomass precursors at 6 g, i.e., Mglucose + Mglucosamine = 6 g, but tuned the mass ratio (Mglucose:Mglucosamine) to the different values including 6:0, 4.5:1.5, 3:3, 1.5:4.5 and 0:6, in order to investigate the structural evolution of the hollow structure. The corresponding particles were designated as CSs (6:0, glucose only), N-CSs-1 (4.5:1.5), N-CSs-2 (3:3), N-HCSs (1.5:4.5) and N-CSs-3 (0:6, glucosamine only), respectively. As shown by the TEM images in Fig. 4a, particles synthesized with pure glucose (CSs) only exhibit a solid spherical structure and are nothing special, as well as N-CSs-1 and NCSs-2. However, when pure glucosamine was used, a reticulated structure was formed which has been reported in our previous work.40 Only when Mglucose:Mglucosamine equals to 1.5:4.5, the hollow structure with a void in the core appears. SEM images (Fig. 4b) also display the difference among all kinds of the obtained particles. It is observed that CSs have a smooth and round surface, while the surfaces of other particles are not perfectly even but stuck with few tiny particles. This phenomenon is more obvious for N-HCSs and N-CSs-3 particles, where a large number of humps are located on their surface. The low magnification TEM and SEM images of all biomass-derived carbon nanospheres are shown in Fig. 4c and 4d. It can be seen that the microstructure of biomass-derived nitrogen-doped carbon materials was controlled by the ratio of precursors, which implies that the co-assembly of glucose and glucosamine play an important role in the formation of hollow structure in N-HCSs. Hence, we proposed the formation mechanism as depicted in the schematic of Fig. 5. Presumably, when the aerosol droplet was passing 11



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through the drying zone, the biomass glucose and glucosamine started to go through dehydration and condensation due to the elevated temperature, giving rise to amphiphilic oligomers with a hydrophobic aromatic and hydrophilic groups (hydroxyl groups –OH for glucose and amino groups –NH2 for glucosamine). In fact, the formation of these oligomers was very similar to the procedure took place in the hydrothermal treatment of biomass glucose, which also led to the amphiphilic intermediates.41, 42 Furthermore, in our understanding, when pure glucose was used as the precursor, the formed oligomers with –OH groups aligned irregularly, leading to a solid structure in the final carbon materials. However, when pure glucosamine was used as the precursor, the formed oligomers with –NH2 groups were able to display a reverse micelle-like structure, leading to a reticular structure in the final carbon materials.40 With the concentration of amphiphilic oligomers reached the critical ratio, these amphiphilic oligomers underwent a co-assembly to form a kind of vesicular structure, where the hydrophobic groups occupied the inner and the hydrophilic groups face the surface of the bilayer. In the following heating zone, water evaporation, continuous carbohydrate dehydration and sulfate decomposition took place, leading to the formation of hollow carbonaceous spheres. It is worth pointing out that, carbon materials with a hollow structure were normally obtained using surfactants or hard spheres as a template, however our work provided a novel approach by the temple-free method in which biomass molecules directly generated hollow structure. Electrochemical performances of N-HCSs 12


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The supercapacitor performances of the nitrogen-doped hollow carbon spheres directly obtained from biomass were evaluated systematically by CV, GCD and EIS. Figure 6 displays the CV curves of N-HCSs at a constant scan rate of 20 mV s-1 over the potential range of -0.200 to 0.800 V, where the curves of other materials including CSs, N-CSs-1, N-CSs-2 and N-CSs-3 are also shown for comparison. From the element analysis, the nitrogen content in CSs, N-CSs-1, N-CSs-2, N-HCSs and N-CSs-3 are 0, 2.92%, 3.44%, 4.94% and 6.4%, respectively. Obviously, the surrounded area by CV curves for N-HCSs is the largest, followed by N-CSs-3, N-CSs-2, N-CSs-1 and CSs. Considering that the specific capacitance is proportional to the integrated area in the CV curves, the specific capacitance of all as-prepared materials are qualitatively following the order of N-HCSs > N-CSs-3 > N-CSs-2 > N-CSs-1 > CSs. In addition, the CV curves of N-HCSs exhibit a roughly rectangular-like shape, suggesting that its capacitive behavior is mainly due to the formation of an electric double layer.43 A slight hump is observed from 0.1 to 0.3 V, which is the result of the faradic reaction between the protic ion and the doped nitrogen elements (pyridinic nitrogen and pyrrolic nitrogen) and the resulting small pseudo-capacitance behavior in the N-HCSs.44 It is worth noting that the specific capacitance of NCSs-3 is still less than that of N-HCSs although NCSs-3 particles hold the higher nitrogen content (4.94% to 6.40%). Hence, the excellent electrochemical performance of N-HCSs is attributed to not only the dopant of nitrogen but also the special hollow structure. The former enhanced the electrical conductivity, surface polarity, surface basic sites and electron-donor affinity, while the 13



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latter is beneficial for the electrolyte diffusion and electron migration to the active sites. In order to further investigate the electrochemical performance of N-HCSs, CV curves at different scan rates and GCD curves at various current densities were recorded. Herein, we chose the sample N-CSs-3 as a reference material considering that it has the highest N-content but without the hollow structure, in order to reveal the combined effects from the heteroatom doping and the unique structure. Figure 7a shows typical CV curves of N-HCSs at different scan rates of 5, 10, 20, 50, 100 and 200 mV s-1. It is observed that the CV curves maintain a quasi-rectangular shape at a high scan rate up to 200 mV s-1, indicating the ultrafast ion and electron transport in the charge–discharge process for N-HCSs. As shown in Fig. 7b, the GCD curves at different current densities (0.2 - 10 A g-1) exhibit almost linear and symmetrical, which indicates that the N-HCSs have good capacitor behavior and excellent electrochemical reversibility. Besides, there is no obvious ohmic drop from discharge curves, suggesting a low internal resistance and a high conductivity. From Figs. 7c and d, the similar trends and shapes were observed for N-CSs-3, but integrated areas in the CV curves and the discharge time are less than those of N-HCSs. The specific capacitance values of N-HCSs and N-CSs-3 were determined from the discharge curves. As shown in Fig.8a, N-HCSs exhibit an excellent specific capacitance of 266 F g-1 at a current density of 0.2 A g-1, which is comparable or better than most of N-doped carbon materials found in the literature (Fig.8b).45-53 Although the specific capacitance values of N-HCSs have a slight decrease with the 14


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increased current density, they still reached at 237, 205, 186 and 174 F g-1 under the conditions of 0.5, 1, 2, and 5 A g-1, respectively. Even when the current density was increased to 10 A g-1, the capacitance retained 163 F g-1, corresponding to 61.3% of capacitance measured at 0.2 A g-1. In contrast, the capacitance values of N-CSs-3 (220, 185, 155, 152, 146 and 134 F g-1 at the designated current density) are smaller than the corresponding values of N-HCSs although the N-content is higher for the former. Hence, the excellent supercapacitive properties of N-HCSs may be ascribed to the synergistic effects from the following: (1) the crumpled surface and hollow structure guarantee the fast ion transportation and electrolyte penetration within the electrode materials; (2) the dopant of N element increases the surface wetting character, facilitating the diffusion of the electrolyte; (3) the doped N-groups may enhance the electrical conductivity of carbon materials and introduce a small pseudo-capacitance, thereby further improving the capacitance property. The EIS data were analyzed using the Nyquist plot, which reflects the characteristic frequency response of the materials and is the plot of the imaginary component (Z’’) of the impedance against the real component (Z’). The Nyquist plots as shown in Fig. 8c display nearly vertical lines at the low frequency segment due to Warburg impedance, which reflects fast ions diffusion in the electrodes and the ideal capacitor behavior for both N-HCSs and N-CSs-3. Meanwhile, the unapparent arc-shaped curves (the inset in Fig. 8c) in the high frequency region indicate the extremely low charge transfer resistance, further implying the good capacitor behavior. The long-term cycle stabilities of N-HCSs and N-CSs-3 capacitors were 15



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evaluated by GCD measurements for 3000 cycles at a current density of 2 A g-1 (Fig. 8d and 8e). After 3000 cycles, the capacitances still retained 96.8% of its initial capacitance, which demonstrates the high stability and the good reversibility during the repeated discharge/charge process of N-HCSs. Meanwhile, the GCD curve in the 3000th cycle overlaps well with that in the 1st cycle as shown in the inset, further confirming the outstanding cycling stability and good charge propagation. Conclusions In summary, we have developed a one-pot strategy for directly converting biomass to hollow structured nitrogen-doped carbon nanospheres.

When glucose and

glucosamine (mass ratio=1.5:4.5) were exploited as precursors, N-HCSs were successfully obtained through the aerosol-assisted process without the use of any templates. In the process, glucosamine as a crucial component plays a triply role of the hollow structure controlling agent, the nitrogen source and partial carbon sources. From the characterizations by CV, GCD and EIS, the obtained N-HCSs manifest the high specific capacitance (266 F g-1 at 0.2 A g-1), excellent rate performance and good cycling stability (96.8% capacitance retained after 3000 cycles), attributed to their unique structural and surface properties. The innovative work reported here offers an effective yet extremely simple, low-cost and eco-friendly way for the scalable designed synthesis of hollow structured carbon nanospheres, which were directly derived from biomass. Acknowledgements This work was supported by the Fundamental Research Funds for the Central 16


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Universities (DUT16ZD226), PetroChina Innovation Foundation (2017D-5007-0609), the National Natural Science Foundation of China (NSFC, No. 31400840). We also wish to thank Dr. Xue Wang at Dalian University of Technology and Dr. Wen-Feng Lin at Loughborough University for the fruitful discussions.

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Lists of Figures Fig.1 Schematic illustration of (a) the aerosol-based process and (b) the synthetic route for the preparation of the N-HCSs. Fig. 2 Morphology of the as-prepared N-HCSs: (a) the low magnification SEM image ; (b) SEM image of an individual particle; (c) the low magnification TEM image; (d) the high-resolution TEM image and (e) the HAADF-STEM image and EELS mapping images of C, N, O of N-HCSs. (f) and (g) are SEM and TEM images of the materials just after the aerosol process. Fig. 3 (a) XPS spectra and (b) N1s spectrum of N-HCSs; (c) schematic of the different nitrogen functionalities on carbon network; (d) XRD pattern and (e) Raman spectrum of N-HCSs; (f) The specific surface area of N-HCSs and the inset which shows the total pore volume. Fig. 4 The structural evolution of carbon nanospheres directly derived from biomass. The high and low magnification images of CSs, N-CSs-1, N-CSs-2, N-HCSs and N-CSs-3: (a, c) TEM and (b, d) SEM images. Fig. 5 Schematic diagrams of the formation mechanism of N-HCSs. Fig. 6 Cyclic voltammetry curves of CSs, N-CSs-1, N-CSs-2, N-CSs-3 and N-HCSs at a scan rate of 20 mV s-1. Fig. 7 Electrochemical capacitive behaviors: (a), (c) CV curves of N-HCSs and N-CSs-3 at different scan rates; (b), (d) GCD curves of N-HCSs and N-CSs-3 at different current densities. Fig. 8 (a) Specific capacitance of N-HCSs and N-CSs-3 at different current densities

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(0.2-10 A g-1); (b) Comparison of the specific capacitance value of the N-HCSs with those of nitrogen-doped carbons reported in the ref.45-53 at discharge current (1 A g-1) in H2SO4; (c) Nyquist impedance plots of N-HCSs and N-CSs-3, and the inset is the enlarged plot around high frequency region; (d) and (e) cycling stabilities of N-HCSs and N-CSs-3 at a current density of 2 A g-1.

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Fig.1 Schematic illustration of (a) the aerosol-based process and (b) the synthetic route for the preparation of the N-HCSs.

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Fig. 2 Morphology of the as-prepared N-HCSs: (a) the low magnification SEM image ; (b) SEM image of an individual particle; (c) the low magnification TEM image; (d) the high-resolution TEM image and (e) the HAADF-STEM image and EELS mapping images of C, N, O of N-HCSs. (f) and (g) are SEM and TEM images of the materials just after the aerosol process. 25



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Fig. 3 (a) XPS spectra and (b) N1s spectrum of N-HCSs; (c) schematic of the different nitrogen functionalities on carbon network; (d) XRD pattern and (e) Raman spectrum of N-HCSs; (f) The specific surface area of N-HCSs and the inset which shows the total pore volume. 26


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Fig. 4 The structural evolution of carbon nanospheres directly derived from biomass. The high and low magnification images of CSs, N-CSs-1, N-CSs-2, N-HCSs and N-CSs-3: (a, c) TEM and (b, d) SEM images.

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Fig. 5 Schematic diagrams of the formation mechanism of N-HCSs.

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Fig. 6 Cyclic voltammetry curves of CSs, N-CSs-1, N-CSs-2, N-CSs-3 and N-HCSs at a scan rate of 20 mV s-1.

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Fig. 7 Electrochemical capacitive behaviors: (a), (c) CV curves of N-HCSs and N-CSs-3 at different scan rates; (b), (d) GCD curves of N-HCSs and N-CSs-3 at different current densities.

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Fig. 8 (a) Specific capacitance of N-HCSs and N-CSs-3 at different current densities (0.2-10 A g-1); (b) Comparison of the specific capacitance value of the N-HCSs with those of nitrogen-doped carbons reported in the ref.45-53 at discharge current (1 A g-1) in H2SO4; (c) Nyquist impedance plots of N-HCSs and N-CSs-3, and the inset is the enlarged plot around high frequency region; (d) and (e) cycling stabilities of N-HCSs and N-CSs-3 at a current density of 2 A g-1. 31



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Graphical abstract

Brief synopsis: A sustainable approach from biomass glucose and glucosamine to nitrogen-doped hollow carbon spheres for supercapacitors

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Biomass-Based Nitrogen-Doped Hollow Carbon Nanospheres

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