Porous N-doped carbon material derived from prolific chitosan biomass as a high-performance electrode for energy storage

Abstract

Although a wide variety of biomass, such as human hair, chicken eggshells and ox horns, have been used to prepare carbon electrode materials for energy storage, most of them have very limited production, which restricts their large-scale application. Herein, the very prolific biomass of chitosan is employed as an abundant raw material to successfully prepare one porous N-doped carbon material (PNCM). Structural characterizations demonstrate that this PNCM is hierarchically porous with abundant macro/micropores and 4.19% N-doping. The electrochemical properties of the PNCM as electrode materials for both supercapacitors and lithium ion batteries are also studied. When used in a supercapacitor, the optimized PNCM synthesized at 700 °C can store electrical energy with a specific capacitance of up to 220 F g⁻¹ in 1 mol L⁻¹ H₂SO₄ electrolyte, exhibit excellent cycle stability with only 1.3% capacitance decay over 11 000 cycles, and deliver high power and energy densities in both aqueous and organic electrolytes. In addition to supercapacitors, the PNCM also exhibits excellent Li-storage properties in terms of high specific capacity (above 460 mA h g⁻¹ at 50 mA g⁻¹) and superior cycle stability (without any capacity decay even after 1100 cycles) when used as an anode material for lithium ion batteries.

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1. Introduction

With the increasing depletion of fossil fuel and aggravation of greenhouse gas emission, the development of high-efficiency energy-storage technologies for intermittent renewable energy resources, as well as consumer electronics, is receiving considerable attention.^1–3 Presently, supercapacitors (SCs) and lithium-ion batteries (LIBs) are two key devices for energy storage.^4,5 While the former stores electrical energy only related to interfacial electrochemical processes between high-surface electrode materials and an electrolyte solution, the latter mainly involves changes in crystalline lattices due to the insertion/extraction of lithium ions. Hence, SCs and LIBs represent two significant developing directions for energy-storage devices with high-power and high-energy densities, respectively. In order to assemble superior SCs and LIBs, one of the top priorities is to develop advanced electrode materials, because their performances intimately depend on the properties of electrode materials. Among the electrode materials for SCs and LIBs, carbonaceous materials are the most important ones, because they can be used as electrodes for both the devices when they exist in different forms.⁶ For example, activated carbon is the main electrode material for commercial SCs due to its numerous merits such as ultra-high specific surface area (SSA), excellent electrochemical stability, and low cost.^7,8 In LIBs, (partly) graphitized carbon occupies the vast majority (higher than 95%) of the market of anode materials. In addition, recent studies have disclosed that several nanostructured carbon, such as graphene,^9–13 carbon nanotubes,^9,13–15 carbon nanosheets,^16,17 and carbide-derived porous carbon,^18,19 exhibited excellent electrochemical properties as electrodes for both SCs and LIBs, thus making carbon materials a very popular research area for energy storage.

Recently, researchers have become more and more interested in porous carbon materials from cheap and renewable precursors. As one class of efficient carbon-containing resource, biomass has been extensively investigated to prepare electrode materials for both SCs and LIBs because of its abundance, low cost, and environmental friendliness compared to other resources.^20–22 In LIBs, biomass-derived carbonaceous materials, particularly nanostructured porous materials, are of particular interest in providing high Li-storage capacities and excellent cycling stability because the porous nanostructure can not only shorten the lithium transport distance but also offer a large electrode/electrolyte interface for facilitating charge transformation. Moreover, in SCs, specific capacitance is mainly influenced by the degree of crystallinity, SSA and pore size distribution of the electrode materials. In addition, surface functionalities, which can be created by doping hetero-atoms (such as nitrogen, oxygen and sulfur) on the surface, are significantly efficient to further enhance specific capacitance due to surface redox reactions and improve surface wettability to optimize the interfacial characteristics between the electrode and electrolyte.^15,22–25 Therefore, carbonaceous materials that contain both porous nanostructures and surface functionalization are of very significance for both SCs and LIBs.

To prepare such porous and doped carbonaceous materials, a wide variety of biomass, including human hair,²² chicken eggshells,²⁶ dead leaves,²⁷ regenerated silk proteins,²⁸ waste coffee beans,²⁹ bacterial cellulose,³⁰ coconut shells,³¹ wheat straw,³² banana peels,³³ rice husk,³⁴ cherry stones,³⁵ and ox horns,³⁶ have been employed. However, most of them have very limited production, which means that they cannot be widely used in the fields of LIBs and SCs. Chitosan, which is a cellulose-like and nitrogen-containing biopolymer, is one of the most prolific biomass, which can be large-scale produced via the chemical treatment of renewable and inexpensive chitin, such as the exoskeleton of marine animals, including shrimp, crabs, and lobsters. Presently, chitosan has been extensively used in the food industry, medical applications and environmental protection with an annual production of up to 10 billion tons per year.³⁷ Hence, chitosan will become an attractive and large-scale producible carbonaceous source if its produced carbon materials could exhibit superior electrochemical properties when used as electrode materials for SCs and LIBs.

In this study, we used chitosan as a carbonaceous precursor and successfully prepared porous N-doped carbon materials (PNCMs) though a simple ZnCl₂-assisted carbonization and activation process. The as-prepared PNCMs exhibit higher SSA compared to nonporous N-doped materials (abbreviated as NCMs). In the PNCMs, the pores are hierarchical macro/micropores, which could ensure much improved electrochemical properties when used as electrode materials for SCs and LIBs. Electrochemical tests in SCs demonstrated that the specific capacitance of the PNCMs can reach up to 220 F g⁻¹ at a scan rate of 1 mV s⁻¹ in 1 mol L⁻¹ H₂SO₄ electrolyte and also show excellent cycle life with 98.7% capacitance retention after 11 000 cycles at the current density of 4 A g⁻¹. Furthermore, the capacitance characteristics of the as-prepared PNCMs were also evaluated in an organic electrolyte system, and they exhibited excellent electrochemical performance. In addition to the studies in a SC system, we also investigated the PNCMs' Li-storage properties. The test results show that the PNCMs can deliver a high specific capacity (above 460 mA h g⁻¹ at the current density of 50 mA g⁻¹) and exhibit excellent cycling stability (without obvious capacity decay after 1100 cycles) when used as an anode for LIBs. It can be rationally deduced that the present PNCMs can be widely used for SCs and LIBs due to their large-producibility, low-cost, greenness and superior electrochemical properties.^{32,34,35,38–40}

2. Experimental section

2.1. Preparation of PNCMs and NCMs

Chitosan was purchased from Sinopharm Chemical Reagent Co., Ltd. In a typical preparation for PNCMs, chitosan was firstly mixed with ZnCl₂ at a weight ratio of 1 : 2 and then carbonized at a given temperature, which ranged from 500 to 800 °C for 3 h under a nitrogen atmosphere (40 sccm min⁻¹) with a heating rate of 5 °C min⁻¹. The obtained dark solid was ground into a powder, washed with 1 mol L⁻¹ HCl solution and then 1 mol L⁻¹ HF solution, and finally thoroughly washed with distilled water. The residual carbon material was collected and dried at 60 °C for 10 h in a vacuum. Note that the optimal heating temperature is 700 °C, hence the following PNCMs refers specifically to the sample fabricated at 700 °C. In order to prepare NCMs, chitosan was carbonized directly at 700 °C.

2.2. Characterization of composition and structure

To characterize the porous texture and the porosity of the as-obtained carbon materials, N₂ absorption/desorption at 77 K was carried out using an ASAP 2020 instrument. The morphologies were characterized via scanning electron microscopy (SEM, Quanta 250 FEG) and field-emission transmission electron microscopy (TEM, JEOL-2100F). X-ray powder diffraction (XRD) was performed using a Bruker D8 ADVANCE diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. All samples were dried at 80 °C in a vacuum over overnight before the experiment.

2.3. Electrochemical measurements

2.3.1 SCs tests and evaluation in 1 mol L⁻¹ H₂SO₄. The capacitive performance of all the prepared carbon materials was firstly evaluated using a two-electrode and three-electrode system in 1 mol L⁻¹ H₂SO₄ aqueous electrolyte solution. To prepare the working electrode, 80 wt% prepared carbon materials, 10 wt% carbon black, and 10 wt% polytetrafluoroethylene (PTFE) as binder were mixed and pressed onto a 316 L stainless steel net at 8 MPa. The fabricated electrodes (with a thickness of 50–100 μm and loading quantity of 5–8 mg cm⁻²) were soaked in 1 mol L⁻¹ H₂SO₄ electrolyte for 10 h before experiment. In a three-electrode system, the active-material-loaded stainless steel net, 1 × 1 cm² Pt plate and saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) curves were obtained in the three-electrode system over a voltage range of −0.1–0.9 V vs. SCE. Constant charge–discharge measurements were made from 0 to 1.1 V at different densities using the two-electrode system, in which two active-material-loaded stainless steel nets with almost equivalent amounts were used as both working and counter electrodes, respectively. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range from 10⁶ Hz to 10⁻² Hz with an alternate current amplitude of 5 mV. The specific capacitance of the three electrode system was calculated as follows:where C is the specific capacitance (F g⁻¹), I is the discharge current (A), Δt is the discharge time (s), m is the mass of the active material on the working electrode (g) and Δv is the potential change (V) in the discharge process. The gravimetric capacitance for a single electrode in two electrodes was calculated using the following equation:where C_S is the specific capacitance (F g⁻¹), I is the current (A), m is the active mass and Δv (V) refers to the potential change within the discharge time Δt (s). Furthermore, the energy and power densities were calculated using the following formula:where S_E is the specific energy density, S_P is the specific power density, C_S is the specific capacitance of the active material (F g⁻¹), and Δv (V) refers to the potential change within the discharge time Δt (s).

2.3.2 SCs tests and evaluation in organic electrolyte. A slurry of 80 wt% as-prepared carbon materials, 10 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) binder in N-methyl pyrrolidone (NMP) was coated onto a copper foil and then dried at 120 °C overnight in a vacuum oven. The resulting active material had a mass loading of 1–3 mg cm⁻². Two symmetrical electrodes were separated by a porous inorganic separator (Whatman GF/D) in CR2032-type stainless steel coin cells, which were assembled in an argon-filled glove box with both oxygen and water contents below 1 ppm. The organic electrolyte was 1 mol L⁻¹ tetraethylammonium tetrafluoroborate (Et₄NBF₄) in acetonitrile (AN). The calculation processes for the electrochemical parameters are similar to the abovementioned methods.

2.3.3 Measurement of lithium-storage performance. The lithium-storage performance of the PNCMs as a LIB anode was investigated in 1 mol L⁻¹ LiPF₆ in an ethylene carbonate–diethyl carbonate (EC–DEC, 1 : 1 in volume) electrolyte. All tests were conducted on CR2032-type coin cells. A lithium slice with a diameter of 1.6 cm was used as the counter electrode and a piece of glass fiber (Whatman GF/D) was used as the separator. The assembly of all the cells was carried out in an argon-filled glove box with both oxygen and water contents below 1 ppm. Galvanostatic charge–discharge measurements and cycle performance tests were performed on the LAND CT2001A cell test apparatus.

3. Results and discussion

In order to optimize the electrochemical properties of the as-prepared carbon materials, we firstly adjusted the preparation parameters, including activation temperature and mass ratio, between chitosan and ZnCl₂. As shown in Fig. S1,† the best activation temperature is 700 °C, and the optimal mass ratio between chitosan and ZnCl₂ is 1 : 2. Therefore, we will focus the structural and property studies on the PNCMs prepared at this optimized condition, and also compare them with those of the nonporous NCMs in the following section.

Fig. S2a and S2b† firstly compare the morphology and size of the PNCMs and NCMs through SEM observation. As shown in Fig. S2a,† the nonporous NCMs were composed of smooth and ellipsoid-like particles (tens of micrometres) without obvious macropores in/on them. Interestingly, the ZnCl₂-assisted activation process made the carbon particles rougher and facilitated the formation of many macropores with a diameter of 0.3–1 μm (also observed by TEM in Fig. 1a).³² As shown in Fig. 1b, the PNCMs were further characterized using high resolution TEM (HRTEM). Large quantities of micropores and channels can be clearly seen in the HRTEM image. The nanoscale pores formed a three-dimensional connected porous structure, which could offer an effective path for the penetration and transportation of electrolyte ions to improve the electrochemical properties of SCs. Furthermore, the PNCMs shown the typical amorphous structure of carbon materials, which is consistent with previous reports on hard carbon derived from the pyrolysis of a biomass precursor.^25,36,41

Fig. 1 (a) TEM and (b) HRTEM images of the PNCMs.

To further study the pore size and distribution in the as-prepared carbon materials derived from chitosan, nitrogen absorption/desorption isotherms were obtained and the corresponding pore size distribution curves were determined using the Horvath–Kawazoe (HK) method. Fig. 2a compares the nitrogen absorption/desorption isotherm of the PNCMs and NCMs. While the former is the typical flat type I isotherm with no hysteresis loop, which suggests the existence of abundant micropores (<2 nm) and absence of mesopores (2–50 nm) in the PNCMs, the latter exhibits an extremely low adsorption amount of nitrogen. The Brunauer–Emmett–Teller (BET) specific surface area of the PNCMs is as high as 1785 m² g⁻¹, which is markedly higher than the 7 m² g⁻¹ for the NCMs (Table 1). The abundant microporous feature of the PNCMs can also be supported by the HK pore size distribution (PSD), as illustrated in Fig. 2b and Table 1. The total micropore volume of the PNCMs is 0.13 m³ g⁻¹ with the hierarchical distribution centred at 0.46, 0.52, 0.88, 1.10 and 1.30 nm and that of the NCMs can be almost negligible (0.002 m³ g⁻¹). These demonstrate that the porosity and specific surface area of the chitosan-derived carbon materials can be significantly increased during the ZnCl₂-assisted activation processes, which result in the formation of abundant microporous textures that are very beneficial for charge accommodation to improve the electrochemical properties as demonstrated below.

Fig. 2 (a) Comparison of the N₂ adsorption–desorption isotherms of the NCMs and PNCMs. (b) The corresponding pore size distribution of the PNCMs calculated using the HK method.

Table 1 Pore characteristics of the porous carbon materials

Samples	SBET (m^2 g^−1)	Smicro (m^2 g^−1)	SLangmuir (m^2 g^−1)	Vpore (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Daver (nm)
PNCMs	1785	331	2446	0.54	0.13	2.6
NCMs	7	5	10	0.0023	0.002	14.6

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The disordered structure of the PNCMs and NCMs was further confirmed by XRD and Raman spectroscopy. Both XRD patterns show two broad peaks of (002) at 24.3° and (101) at 43.84°, which imply their highly disordered feature (Fig. 3a and S3a†).⁴² This amorphous structure will be helpful for lithium ion intercalation/deintercalation due to the larger Li-transport channel compared to the graphitized structure.³⁶ In the Raman spectrum, the two obvious peaks at around 1330 and 1580 cm⁻¹ could be assigned to the D (defects and disorder) and G (graphitic) bands. The D/G ratio of the PNCMs was around 1.19. The strong D-band peak indicates that the PNCMs have a low degree of graphitization and they contain a large number of disordered sections and defects, which is well consistent with the HRTEM image in Fig. 1b. At the same time, the XRD pattern and Raman spectrum of the NCMs were also obtained (Fig. S3†). Their disordered structure is similar to the PNCMs.

Fig. 3 (a) XRD pattern and (b) Raman spectrum of the PNCMs.

XPS was also carried out to analyse the chemical composition of the PNCMs. The results show that the PNCMs consist of 86.58% C, 4.19% N and 9.23% O. Fig. 4b–d show the high-resolution XPS spectra of C 1s, N 1s, and O 1s. The C 1s spectrum ranges from 294.0 to 284.0 eV, which can be approximately fitted into three peaks centred at 288, 285.7, and 284.7 eV, which are attributed to the bonds of C=O, C–O (C–N), and C–C, respectively. The N 1s spectrum in Fig. 4c can be fitted into two peaks located at ca. 400.3 and 398.2 eV, which correspond to the pyrrolic N (N-5) and pyridinic N (N-6). In the O 1s spectrum, the fitted two peaks of 534.2 and 532.7 eV correspond to the C–O and C=O bonds, respectively.

Fig. 4 XPS spectra of PNCMs: (a) XPS survey, (b) C 1s, (c) N 1s, (d) O 1s.

The electrochemical performance of the as-obtained carbon materials was firstly evaluated as electrodes of SCs in 1 mol L⁻¹ H₂SO₄ electrolyte at room temperature (Fig. 5). Fig. 5a shows the CV patterns of the PNCMs at various scan rates from 5 to 50 mV s⁻¹ in a three-electrode system. All the curves exhibit quasi-rectangular shapes, which indicate their ideal double-layer capacitor nature in the charge/discharge processes. Furthermore, it is obvious that faradaic humps are observed at around 0.4 V in these curves at various scan rates, which may be due to the redox reactions of doped heteroatoms such as pyrrolic and pyridinic nitrogen species. From the CV curves in Fig. 5a, it can also be seen that the capacitive behaviours are maintained well even at the high scan rate of 50 mV s⁻¹.

Fig. 5 Electrochemical properties of the PNCMs and NCMs in 1 mol L⁻¹ H₂SO₄ electrolyte. (a) CV curves of the PNCMs at various scan rates of 5, 10, 20, 50 mV s⁻¹ measured in a three-electrode system (b) GCD curves of the PNCMs at different current densities measured in a two-electrode system. The comparison of (c) rate and (d) cycling performances.

In order to further investigate the electrochemical performances of the PNCMs, their galvanostatic charge/discharge (GCD) behaviour was further measured at different current densities in a two-electrode system in 1 mol L⁻¹ H₂SO₄ (Fig. 5b). All the GCD curves of the PNCMs at various current densities are quasi-triangular and symmetrical, which indicate that the PNCMs possess typical electrical double layer capacitive behaviour and superior charge–discharge reversibility when used as electrodes for SCs. Fig. 5c compares the specific capacitance of the PNCMs with the NCMs in a very wide range of current densities from 0.1 A g⁻¹ to 20 A g⁻¹. As is well known to all, specific capacitance will decrease with the increase of current density, because the ions in the electrolyte do have not enough time to reach all the pore surface of the as-prepared carbon materials due to the diffusion limitation since the current density is too high. As shown in Fig. 5c, the PNCMs exhibit excellent capacitance retention in comparison to the NCMs. Even at the very high current density of 20 A g⁻¹, the specific capacitance of the PNCMs is still 134 F g⁻¹ with a capacitance retention of about 67% according to the specific capacitance of 200.2 F g⁻¹ at 0.1 A g⁻¹. All these data are significantly higher than those of the NCMs. To examine the electrochemical stability of the as-prepared carbon materials, galvanostatic cycling was performed, as shown in Fig. 5d. The capacitance retention of the PNCMs is as high as 98.7% after 11 000 cycles, which is significantly higher than 82.07% for the NCMs after 10 000 cycles. Hence, the cycle stability of the PNCMs is much better than the inactivated NCMs. The much enhanced cycle life for the PNCMs should be attributed to its unique structure and composition of hierarchical pores and doped N and O atoms incorporated into the carbon framework.

Fig. 6a compares the EIS Nyquist plots of the PNCMs and NCMs in 1 mol L⁻¹ H₂SO₄ electrolyte in the frequency range of 10⁶ Hz to 10⁻² Hz. The real axis intercept represents the equivalent series resistance (ESR), which is the sum of the ohmic and charge transfer resistances. The ohmic resistance is the combination of the resistance of the electrolyte, internal resistance of the active material and current collector as well as the contact resistance among all the materials.²⁹ As revealed in the inset of Fig. 6a, both prepared carbon materials possess similar ESRs. In the low-frequency region of the EIS Nyquist plot, the more vertical line of the PNCMs represents the better capacitive nature of their electric double layers in comparison to the NCMs, which should be attributed to the much easier accessibility of electrolyte ions onto the surface due to their abundant pores. The inset of Fig. 6a shows the magnified high-frequency region, which shows semicircles in the EIS Nyquist plots of the PNCMs and NCMs. Moreover, the Bode plots of the PNCMs and NCMs, which exhibit the frequency response of capacitance, are presented in Fig. 6b. The operating frequencies at the position of 50% capacitance are 0.62 Hz and 0.075 Hz for the PNCMs and NCMs, respectively, which further imply the better supercapacitive characteristics of the PNCMs.

Fig. 6 (a) EIS Nyquist plots in the frequency range of 10⁶ Hz to 10⁻²​ Hz and (b) the corresponding frequency responses of the PNCMs and NCMs with frequency.

The Ragone plots of the PNCMs and NCMs, which show the relationship between power and energy densities, are further compared in Fig. 7. As the red and green ball-lines disclose, the PNCMs possess much enhanced energy densities compared to the NCMs at various power densities, ranging from 100 to 10 000 W kg⁻¹, in the electrolyte of 1 mol L⁻¹ H₂SO₄. For instance, at the specific power density S_P of 110 W kg⁻¹, the specific energy density S_E of the PNCMs is ca. 34 W h kg⁻¹, which is about 7 times that (ca. 5 W h kg⁻¹) of the NCMs. In comparison to the NCMs, the much higher power densities of the PNCMs should be attributed to their higher specific surface area and more abundant micropores.

Fig. 7 Ragone plots of the PNCMs and NCMs were measured using a two-electrode symmetric supercapacitor in water solution and an organic system (in 1 mol L⁻¹ H₂SO₄ and Et₄NBF₄-AN). The energy and power densities were calculated from the charge–discharge curves at different current densities.

Nevertheless, the above energy densities are still somewhat low because of the limitation of the stable voltage window of the aqueous electrolyte. Hence, in order to increase the energy density, the PNCMs were further evaluated in the organic electrolyte of 1 mol L⁻¹ Et₄NBF₄ in AN, whose stable voltage window is up to 2.8 V. Fig. S4† shows the corresponding GCD curves at various current densities from 0.1 A g⁻¹ to 20 A g⁻¹, which disclose their good symmetrical and stable capacitance behaviour. More interestingly, both energy and power densities increased several times in comparison to those tested in the aqueous electrolyte (Fig. 7). This should be mainly due to the fact that the voltage window, Δv, of the organic electrolyte is 2.8 times that of the aqueous electrolyte and the S_E is proportional to the square of Δv. Specifically as disclosed in Fig. 7, the S_E will be up to about 150 W h kg⁻¹ at the S_P of 680 W kg⁻¹, which is significantly higher than that (32 W h kg⁻¹) in the aqueous electrolyte. Moreover, the S_P and S_E are found to be 6302 W kg⁻¹ and 105.2 W h kg⁻¹, respectively, at a drain time of 60 s. The above-mentioned advanced supercapacitive properties of the PNCMs should be attributed to its unique structural features, which include abundant nitrogen-doping and micropores.

In addition to the application of supercapacitors, porous carbon materials derived from biomass have also attracted great attention as promising anode materials for LIBs. Herein, the electrochemical properties of the prepared PNCMs and NCMs were further studied as an LIB anode in a half-cell configuration with metallic lithium as the counter electrode. Fig. 8a compares the rate performance of the PNCMs and NCMs at various current densities from 0.05 to 5 A g⁻¹. The reversible capacities of the PNCMs are 460.7, 421.9, 353, 298.5, 227.8, 180.5, 134.3, and 378.5 mA h g⁻¹ at the current densities of 0.05, 0.1, 0.2, 0.5, 1, 2, 5 and 0.05 A g⁻¹, respectively. When the current density is reset to 0.05 A g⁻¹, the specific capacity can recover to 378.5 mA h g⁻¹, which demonstrate their good cycling stability.⁴² In comparison to the NCMs, the PNCMs delivers higher specific capacities when the current densities are lower than 1 A g⁻¹. This enhancement of PNCMs should be attributed to its hierarchically macro/microporous structure, which could improve the electronic/ionic transport within the electrode and hence the electrode kinetics. This has also been confirmed by EIS measurements (Fig. 8b), which show that the charge-transfer resistance (R_ct) value of 28.5 Ω for the PNCMs is significantly lower than that (38.4 Ω) for the NCMs.

Fig. 8 Li-storage performances of the PNCMs and NCMs. Comparison of (a) rate performance and (b) EIS Nyquist plots. (c) GCD curves of PNCMs cycled at current rate of 0.2 A g⁻¹. (d) Cycling performance of the PNCMs.

Fig. 8c and d further assess the cycle stability of the PNCMs at a current density of 0.2 A g⁻¹ after the initial 5 cycles at 0.05 A g⁻¹. As disclosed in Fig. 8d, the specific capacities delivered by the PNCMs have no obvious attenuation even after 1100 cycles. In the cycling process, the slight activation phenomenon should be due to the gradual infiltration of electrolyte into the micropores.³⁴ Fig. 8c shows the GCD curves of the PNCMs anode at the 1st, 2nd, 100th, 400th, 800th and 1100th cycles at the current density of 0.2 A g⁻¹ between 0.02 V and 3.0 V vs. Li⁺/Li. Except for the increase of specific capacity during the initial cycles, which is in agreement with the above-mentioned activation process, there is no obvious change in the curve shape and specific capacities after activation. This also demonstrates the superior cycling performance of the PNCMs when used as an anode for LIBs.

4. Conclusions

In summary, a porous N-doped carbon material, PNCMs, was successfully prepared by using the prolific chitosan biomass as the raw material. In the preparation processes, the ZnCl₂-assisted activation and usage of chitosan are two key steps to create abundant micropores and nitrogen doping respectively. The PNCMs prepared at 700 °C exhibit the best electrochemical properties with a specific capacitance up to 220 F g⁻¹ in 1 mol L⁻¹ H₂SO₄ electrolyte at a scanning speed of 1 mV s⁻¹, excellent cycle stability over 11 000 cycles (only 1.3% capacitance decay) at the current density of 4 A g⁻¹, and high power and energy densities in both aqueous and organic electrolytes. In addition, the PNCMs can also exhibit outstanding Li-storage properties in terms of high specific capacity (above 460 mA h g⁻¹ at the current density of 50 mA g⁻¹) and excellent cycle stability (without obvious capacity decay after 1100 cycles) when used as an anode material for LIBs. The outstanding electrochemical properties of the PNCMs is attributed to its unique structural characteristics, which include a high specific surface area, appropriate and well developed porous texture of hierarchical macro/micropores and nitrogen doping.

Acknowledgements

This work was supported by the the Fundamental Research Funds for the Central Universities (14QNJJ014) and the Science Technology Program of Jilin Province (20140101087JC, 20150520027JH). XL thanks the support of the International Postdoctoral Exchange Fellowship Program.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20933a

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