Energy Storage Materials 1 (2015) 103–111
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Energy Storage Materials journal homepage: www.elsevier.com/locate/ensm
Tuning and understanding the supercapacitance of heteroatom-doped graphene Yingke Zhou a,n, Xiao Xu a, Bin Shan b, Yanwei Wen b, Tingting Jiang a, Jiming Lu a, Shaowei Zhang a, David P. Wilkinson c, Jiujun Zhang c,nn, Yunhui Huang b,nnn a
The State Key Laboratory of Refractories and Metallurgy, College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China c Department of Chemical and Biochemical Engineering, University of British Columbia, Vancouver, BC, V6T 1W5 Canada b
art ic l e i nf o
a b s t r a c t
Article history: Received 24 July 2015 Received in revised form 5 September 2015 Accepted 6 September 2015 Available online 25 September 2015
Carbon nanomaterials are promising for making high-performance supercapacitors. However, their specific capacitances and energy densities still need further improvement for many important and challenging applications. Here we report the superior capacitive performance of heteroatom-doped graphene synthesized by a thermal annealing method at low temperature (200 °C), and remarkably enhanced specific capacitance of 629 F g 1 at 0.2 A g 1, energy density of 43 Wh kg 1 at 140 W kg 1, and cycle life of 10,000 times are achieved. The mechanisms for the outstanding performance are analyzed, and a corresponding model connecting the dopant and capacitance is proposed and validated by the first-principle calculations. The thermal annealing temperature plays a critical role in the dopant configuration of heteroatom and hence significantly affects the capacitive properties of graphene. If annealing at low temperature, non-graphitic dopant configuration is dominant, inducing a large Faradaic pseudocapacitance; if annealing at high temperature, graphitic dopant configuration is dominant, giving rise to a relatively lower electrical double layer capacitance. These findings demonstrate that the supercapacitance of graphene can be purposely tuned by the rational doping of heteroatoms, which may open up new strategies for further design and application of advanced graphene-based materials for electrochemical supercapacitors. & 2015 Elsevier B.V. All rights reserved.
1. Introduction Supercapacitors, the so-called electrochemical capacitors or ultracapacitors, can give much higher electrical power density and much longer operating lifetime than the battery or fuel cell counterparts. Nevertheless, their energy density is still insufficient for many important applications [1–3]. In general, carbon-based materials are widely used in supercapacitors because of their high surface area, good electrical conductivity, and low cost [4–8]. Recently, graphene, a new type of carbon material composed of two dimensional sp2-hybridized carbon sheets, has been considered as a promising material for high-performance supercapacitors due to its high electric conductivity and, more importantly, its elevated surface area [9–11]. Several reports demonstrated improved performance of n
Corresponding author. Corresponding author. Corresponding author. E-mail addresses:
[email protected] (Y. Zhou),
[email protected] (J. Zhang),
[email protected] (Y. Huang). nn
nnn
http://dx.doi.org/10.1016/j.ensm.2015.09.002 2405-8297/& 2015 Elsevier B.V. All rights reserved.
supercapacitors if graphene-based materials were employed in the electrodes [9–12]. However, the resultant specific capacitance values of such graphene-based materials were still rather lower than those theoretically expected. When both sides are exposed, the surface area of monolayer graphene can be as high as 2630 m2 g 1, based on which, one could expect a theoretical capacitance 4550 F g 1, which is far greater than that has been achieved for graphene to date (300 F g 1) [9–12]. To improve the supercapacitive properties of graphene, several strategies have been proposed recently, among which doping graphene with heteroatoms such as boron, nitrogen, phosphorus and sulfur, has shown great potential in producing highly efficient graphene for supercapacitor applications [13–23]. Enhanced capacitance has been obtained by adopting N-doped graphene materials, which can be attributed to the increased charge carrier density resulted from the n-type N-doping [15–19]. In the case of p-type B-doping, the charge carrier density is normally lower than that of n-type doping, but interestingly, some improved capacitive performances have also been observed [13,14,17,18]. Some researchers suggested that the heteroatom doping induced quantum capacitance could be
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contributed significantly to the interfacial capacitance of graphene [16]; some proposed that the capacitance of heteroatom-doped graphene might be dominated by structural defects regardless of the doping type and level, similar to the pure carbon-containing well-ordered sp2 materials [18]. Actually, the mechanisms for the improved capacitance in the heteroatom-containing graphene electrodes have not been fully understood, though many efforts have been devoted to designing and optimizing high energy density graphene-based electrode materials. In this work, we designed several different heteroatom-doped graphene materials, optimized and compared their capacitive performances. Among them, the B-doped graphene exhibits a specific capacitance of 629 F g 1 at a current density of 0.2 A g 1, while the N-doped one 472 F g 1, both of which have high energy/ power density and long cycle life. In order to understand the possible mechanisms, the correlation of supercapacitance component of graphene with heteroatom doping and structure defect was carefully investigated based on a proposed model validated by first-principle calculations and recent reported experimental observations. The results indicate that the heteroatom doping type/configuration and defect level of graphene can be tuned by suitable doping strategy, bringing significantly enhanced capacitive performance.
A carbon rod and an Ag/AgCl electrode (3.5 mol L 1 KCl) were used as the counter electrode and reference electrode, respectively, while the working electrode was a glassy carbon electrode coated with the as-prepared sample. To fabricate the working electrode, the sample slurry was firstly prepared by dispersing 2.5 mg powder material in a mixture of 200 μL pure ethanol and s 50 μL Nafion solution (DUPONT, 5 wt%) by ultrasonication for about 30 min. 5 μL suspension was then dropped onto the surface of the pre-polished glassy carbon electrode by a TopPette pipettor. Finally, the working electrode was dried at 60 °C for 20 min and cooled down naturally. The electrochemical measurements were carried out in 6 mol L 1 KOH electrolyte which was pre-purified with nitrogen gas for about 20 min. CVs were tested with a potential range from 1.2 to 0.2 V at scan rate of 10, 20, 50, 100 and 200 mV s 1. Galvanostatic charge/discharge curves were measured at current densities of 0.2, 0.5, 1, 2, 5 and 10 A g 1, respectively, with a potential window of 1.2 and 0.2 V. The specific capacitances of the electrodes were calculated from the galvanostatic charge/discharge curves according to the equation of Cm ¼I Δt (m ΔV) 1, where Cm is the specific capacitance (F g 1), I is the charge/discharge current (A), Δt is the discharge time (s), m is the mass of active material in the electrode (g), and ΔV is the voltage change after a full charge or discharge [22]. 2.4. Density functional theory computation
2. Experimental procedures 2.1. Preparation of heteroatom-doped graphene GO was synthesized from the purified flaky graphite powder by the modified Hummers method [24]. The heteroatom-doped graphene was prepared by thermal annealing reaction of GO with dopant precursors. Typically, 100 mg dried GO was initially mixed with 300 mg dopant precursor (boric acid, urea, triphenylphosphine or benzyl disulfide) in an agate mortar. The resultant mixture was put into a quartz boat, heated in an Ar-protected tube furnace at a heating rate of 5 °C min 1 to the desired temperature (200, 400 or 700 °C), and held for 2 h. After furnace-cooling down to room temperature, the reacted black solids were washed with alcohol and excess distilled water by vacuum filtration. Finally, the sample was dried in a vacuum oven for 24 h at room temperature. The sample was designated as XGT, where X represents the elemental symbol of the corresponding heteroatom (B, N, P or S) and T denotes the annealing temperature. For instance, NG700 means the N-doped graphene synthesized at 700 °C. For comparison, the undoped graphene (G200) was also synthesized by exactly the same procedure at 200 °C but without addition of any dopant. 2.2. Morphology and structure characterization The morphology and microstructure of heteroatom-doped graphene materials were examined by transmission electron microscopy (TEM, JEM-2000 UHR SETM/EDS) with an acceleration voltage of 200 kV. FTIR and UV–vis measurements were conducted on NEXUS 670 and Cary 50, respectively. Raman spectra were recorded at room temperature on IVNIA with an argon ion laser operating at 514 nm to identify the structural defect of graphene. XPS test and analysis were performed on a VG Multilab 2000 apparatus to obtain quantitative information of the compositions and functional groups of the samples. 2.3. Electrochemical measurements The electrochemical measurements of the heteroatom-doped and undoped graphene samples were performed in a standard three-electrode cell with a CHI 660D electrochemical workstation.
First-principle calculations were performed using the Vienna Ab Initio Simulation Package [25] (VASP) within the projector augmented-wave approach and the local density approximation (LDA) exchange-correlation function was adopted. The cutoff of the kinetic energy for plane waves was set to 400 eV for all calculations. A supercell of 4 4 1 graphene unit cell was used for N-doped model to avoid the interaction between the neighboring doped N atoms. The k-points were sampled on a Γ-centered Monkhorst–Pack grid of 5 5 1 and the geometry was allowed to relax until Hellmann–Feynman force on each atom was less than 0.05 eV Å 1.
3. Results and discussion The heteroatom-doped graphene materials (doped with B, N, P and S, respectively) were prepared via a thermal annealing reaction between graphene oxide (GO) and the dopant precursors at low temperature (200 °C). The preparation conditions for all doped materials were kept identical for a fair comparison. During the reaction, GO was reduced, and meanwhile, the reduced graphene was doped with the corresponding dopants. The resultant doped graphenes are named in terms of the dopant and annealing temperature. For example, We refer NG200 to the N-doped graphene annealed at 200 °C, and BG400 to the B-doped one at 400 °C. Morphologies of the as-prepared GO, G200, BG200, NG200, PG200 and SG200 examined by transmission electron microscopy (TEM) are shown in Fig. 1 and Supplementary Fig. S1–S3. Typical curved and wrinkled sheet-like structures with several layers are observed in all samples. The interlayer spacing in NG200, based on the high-resolution TEM, is around 0.38 nm (Fig. 1b), which is larger than that of graphite (0.34 nm). This is due to the presence of Ncontaining functional groups [24]. The selected area of electronic diffraction patterns displays two weak diffraction rings corresponding to the (101) and (110) planes of layered graphene (Fig. 1c, Fig. S1c and d) [26]. The elemental mappings reveal that the doped heteroatoms are distributed homogeneously on the reduced graphene nanosheets. These observations indicate that similar morphology and structure can be obtained for all the doped graphene materials.
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Fig. 2a shows Fourier transform infrared spectroscopy (FTIR) for GO, G200, BG200, NG200, PG200 and SG200. All exhibit a strong and wide peak at 3430 cm 1, arised from the O–H stretching vibrations of absorbed water molecules and the structural OH
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groups [27]. For GO, the peaks at 1726, 1627, 1385 and 1051 cm 1 are assigned to the vibration modes of C ¼O stretching, C ¼C stretching plus O–H bending, tertiary C–OH, and C–O stretching of GO nanosheets, respectively [27,28]. For G200, the peak at
Fig. 1. (a, b) TEM and high-resolution TEM images of NG200. (c) Selected area electronic diffraction pattern of NG200. (d, e, f) TEM and corresponding elemental mapping images of C and N of NG200.
(a) Absorbance / a.u.
(b)
Wave number / cm
-1
Wavelength / nm
(c)
Intensity / a.u.
Intensity / a.u.
(d)
-1
Raman shift / cm
Binding energy / eV
Fig. 2. (a) FTIR spectra, (b) UV–vis spectra, (c) First-order Raman spectra of GO, G200, BG200, NG200, PG200 and SG200. (d) Survey XPS spectrum of NG200.
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1385 cm 1 disappears, but a new peak of C–O bond appears at 1200 cm 1. In addition, the peaks attributed to C ¼O and C ¼C stretching shift to lower wave number with respect to GO, indicating that the O functional groups are partially reduced and the sp2 networks of graphene sheets restored during heat treatment [27]. BG200 has a broad and strong peak from 900 to 1300 cm 1, contributed by a combination of single bond stretches between C, O and B atoms, such as asymmetric B–O stretch of B–O–B bonds and B–C stretching vibration, while the sharp peak at around 660 cm 1 is attributed to the vibration of O–B–O bonds [13,29]. For NG200, the existence of N functional groups can be confirmed by the peaks at 3450, 1620 and 1380 cm 1, which arise from N–H, C ¼N and N–H, and C–N stretching vibrations, respectively [30]. For PG200, the broad peak between 900 and 1300 cm 1 can be assigned to the stretching vibrations of P ¼O, P–O–P and P–O–C bonds, while the peaks at 670 and 800 cm 1 to C–P bonding [31]. For SG200, the peak at 3100 cm 1 is attributed to the aromatic C– H stretching vibrations, while the characteristic peaks of S functional groups are located at 900–1300 cm 1 (C ¼S, S ¼O), 670 cm 1 (S–O, C–S), and 550 cm 1 (S–S), respectively [32,33]. These results confirm the successful incorporation of the heteroatom-containing functional groups into the reduced graphene sheets during the thermal annealing process. The UV–vis absorption spectra of GO, G200, BG200, NG200, PG200 and SG200 are shown in Fig. 2b. The spectrum of GO exhibits a strong band at 240 nm and a shoulder peak at around 300 nm. They are attributed respectively to the π–π* electron transfer of C¼C bonds, and the n–π* electron transfer of C¼O bonds [28]. For the spectrum of G200, the π–π* absorption band red-shifts to 275 nm and the band at 300 nm nearly disappears, while for BG200 and NG200, both absorbance bands shift to 260 nm with a significant increase in intensity in comparison with that of G200. The spectrum of PG200 shows a weak absorption band at 275 nm, and it red-shifts to 304 nm in the case of SG200 along with increase in intensity. These results indicate that the electronic conjugation within graphene network might be restored and modified during the thermal annealing reaction. Upon introduction of various heteroatomcontaining function groups and heteroatoms with electronegativities (B: 2.04; N: 3.04; P: 2.19; S: 2.58) different from that of C atoms (C: 2.55), different charge densities on the adjacent C atoms can be induced [34]. Generally, the band gap energy can be estimated according to the Kubela–Munk function, (αhυ)2 ¼A(hυ Eg), which shows the relationship between the absorption coefficient (α) and the incident photo energy [28]. The corresponding plots are shown in Fig. S4. From the intercepts of the curves' tangents, the direct band gap energies of GO, G200, BG200, NG200, PG200 and SG200 are determined as 4.0, 1.8, 2.6, 3.1, 2.7 and 3.15 eV, respectively. Upon increasing the annealing temperature from 400 to 700 °C, band gap energy decreases respectively to 2.4 and 2.6 eV for BG400, BG700, NG400 and NG700, in comparison to BG200 and NG200. The structure defects of graphene are identified by Raman spectroscopy, as shown in Fig. 2c and Fig. S5. The G bands at 1590 cm 1 correspond to E2g vibration modes of the ordered sp2 carbon domains and the D bands at 1340 cm 1 are associated with structural defects and disorders [24]. The intensity ratio of D peak and G peak (ID/IG) is generally used to evaluate the disorder degree of graphene [35]. Compared to GO, the G band of G200 red-shifts by 6 cm 1 and the ID/IG ratio decreases from 1 to 0.85, suggesting a recovery of sp2 domain after the removal of oxygen functional groups from the original GO. Compared to G200, the BG200, NG200, PG200 and SG200 samples display some increased ID/IG ratios as well as slightly shifted D peaks and G peaks, revealing the increased structure disorder and modified electronic structure with the introduction of the heteroatom-containing functional groups [30,35]. The heteroatom doped materials exhibit similar ID/IG ratios, suggesting their similar structure disorders. Upon increasing annealing
temperature, slightly increased but very close ID/IG ratios can be observed for the B and N doped samples (Fig. S5a). The second-order Raman spectra of 2D bands, S3 bands and 2D' bands (Fig. S5b and c) further verify the existence of structure defects, as analyzed from the first-order spectra. Typical X-ray photoelectron spectroscopy (XPS) spectra of BG200, NG200, PG200 and SG200 are shown in Fig. 2d and Fig. 3, and the corresponding components and fitting results are summarized in Table S1. The survey spectrum of NG200 displays the composition of C, O and N elements (Fig. 2d). The deconvolution analysis of B 1s spectrum in Fig. 3a results in two components of BC2O and BCO2 located at 190.7 and 192.1 eV, respectively [27], and the N 1s spectrum of NG200 (Fig. 3b) is composed of pyridinic, pyrrolic and graphitic N at 398.4, 399.9 and 401.1 eV, respectively [36]. The P 2p peak in Fig. 3c is fitted with three peaks at 131.9 (C3–P), 132.7 (C2–PO2), and 133.7 eV (C–O–PO3) [21], while the S 2p spectrum of SG200 (Fig. 3d) is deconvoluted into three peaks at 163.5 (C–S–C), 164.4 (–S¼C–) and 165.3 eV (sulfoxide) [22,34]. These results verify the successful doping and introduction of heteroatom containing functional groups into graphene. Furthermore, only a small quantity of substitution of C atoms in graphene networks are doped with heteroatoms at low annealing temperature (200 °C), which is consistent with the previous reports [37]. Actually, the doping of heteroatom into graphene can be classified into two types: one is the graphitic doping (substitution of C atom in graphene network), and the other is the nongraphitic doping. With increasing temperature, a gradual transformation of heteroatom doping style occurs. For example, for the Ndoping case, as shown in Table S1, the non-graphitic doping is dominant at 200 °C (totally 96.5% of pyridinic N and pyrrolic N), and the graphitic N component increases with temperature (32.3% at 400 °C) and becomes dominant at 700 °C (54%). The case for B-doping is similar. These results indicate that the graphitic substitution of C atoms with heteroatoms preferentially occurs at high temperatures. The morphology and structure characterizations described above demonstrate that the reduction of GO has been realized during the thermal annealing procedure, and the added dopant precursors can react with GO to introduce heteroatom functional groups into the reduced graphene frameworks. During the thermal annealing process at low temperature, the dopant precursor may melt and decompose into small organic molecules (e.g., amide and/or ammonia in the case of urea for nitrogen doping), and react with the defects and/or edges sites to form the non-graphitic heteroatom doping. With increasing annealing temperature, the non-graphitic doping configurations gradually convert to graphitic configurations due to the thermal stability [38]. As observed, the incorporated heteroatom functional groups could modify the electronic structure and energy level of the reduced graphene, while the doping levels and defects could be tailored by the dopant type and the pyrolysis temperature. Cyclic voltammetry (CV) was used to characterize the capacitances of supercapacitor electrode materials. Fig. 4a presents several typical CV curves of various electrodes at a potential scan rate of 50 mV s 1. It can be seen that the pristine GO displays a capacitive curve with the smallest current response. The G200, BG200, NG200, PG200 and SG200 samples display the increased currents with clear humps, indicating that the capacitances of these doped materials come from the combination of electrical double layer capacitance (Cdl) and pseudocapacitance (Cp), which is further proved by the CVs at various scan rates (Fig. S6a and b). The specific capacitance (Cm) and rate capability of the resultant samples as electrode materials for supercapacitors are further evaluated by galvanostatic charge/discharge tests. Fig. 4b, as an example, presents typical results at a current density of 0.2 A g 1. It can be seen that BG200, NG200 and PG200 display the specific capacitances of 629, 472 and 391 F g 1, respectively, much higher than 269 F g 1 of G200 and 150 F g 1 of GO, suggesting that B, N and P doping introduced into graphene could remarkably enhance the specific capacitance. The capacitances of
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Intensity / a.u.
(b)
Intensity / a.u.
(a)
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Binding energy / eV
Binding energy / eV
(d) Intensity / a.u.
Intensity / a.u.
(c)
Binding energy / eV
Binding energy / eV
Fig. 3. Core level XPS spectra and fitting peaks of (a) B 1s of BG200, (b) N 1s of NG200, (c) P 2p of PG200, and (d) S 2p of SG200.
(b)
i/Ag
-1
E vs. (Ag/AgCl) / V
(a)
t/s
Specific current / A g
-1
(d)
Specific capacitance / F g
Specific capacitance / F g
-1
(c)
-1
E vs. (Ag/AgCl) / V
Cycle Number
1
Fig. 4. (a) CVs of the samples at 50 mV s . (b) Galvanostatic charge/discharge curves at a current density of 0.2 A g 1. (c) Specific capacitances as a function of the discharge current densities. (d) Cycle performance measured at a currents density of 10 A g 1 for 1000 cycles.
BG200 and NG200 are higher than the B, N-doped graphene aerogels (239 F g 1) [17], the three-dimensional strutted graphene (250 F g 1) [39], and the graphene/polyaniline composite paper and film (210–
233 F g 1) [40,41], The Cm value of SG200 is 222 F g 1 at 0.2 A g 1, lower than that of G200 but still higher than 150 F g 1 of GO. The charge/discharge tests were also performed at various current
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densities, and the results are shown in Fig. 4c and Fig. S7a and b. At 1 A g 1, the capacitances of BG200 and NG200 are also higher than those previously reported for the doped carbon materials (Table S2). It is notable that the Cm values are still as high as 228 and 134 F g 1 for NG200 and BG200 even at a high current density of 10 A g 1, indicative of excellent rate capability [35]. The corresponding Cm values at 0.2 A g 1 are 316, 242, 412 and 259 F g 1 for BG400, BG700, NG400 and NG700, respectively (Figure S7c), implying that the capacitance decreases with increasing the annealing temperature for both B and N-doped graphene, induced by the gradual conversion of doping style affected by increasing temperature. To evaluate the cyclic stability of the prepared electrodes, galvanostatic charge–discharge tests were also performed at a high rate of 10 A g 1 for 1000 times, as displayed in Fig. 4d and Fig. S8a. The pristine GO shows a Cm value lower than 50 F g 1 with a retention of 93% after 1000 cycles, whereas it is 60 F g 1 for G200 and retains almost unchanged after cycling. The NG200 electrode exhibits an outstanding cycling stability and the specific capacitance can be maintained as high as 206 F g 1 even after 1000 cycles, which is 90.3% of its initial capacity. For the BG200 sample, the Cm value is initially 134 F g 1 and becomes 122 F g 1 after 1000 cycles with a retention of 91%. The cycling stability was also investigated, as shown in Fig. 4d and Fig. S8a. Stable cycling capabilities of both BG200 and NG200 are further verified from the prolonged 10,000 times of continuous charge/discharge cycling (Fig. S8b). As recognized, the energy density and power density are two important parameters to characterize the supercapacitive performance [42]. Fig. 5a and Fig. S9 present the Ragone plots, which correlates the energy density with the power density (normalized in a two-electrode configuration) of the doped graphene materials developed in this work. It can be seen that when the power density increases from 140 to 7000 W kg 1, the energy density decreases from 43 to 9 Wh kg 1 for BG200, and from 32 to 16 Wh kg 1 for NG200. Noticeably, ultrahigh energy densities of 43 Wh kg 1 for BG200 and 32 Wh kg 1 for NG200 are obtained at the power density of 140 W kg 1, which are higher than the B, N-doped graphene aerogels (8.5 Wh kg 1) [17], 3D strutted graphene (9 Wh kg 1) [39], and also comparable to the commercially available lead-acid batteries (around 30 Wh kg 1) [12]. At the power density of 5000 W kg 1, the power densities of BG200 and NG200 are also superior to the doped carbon materials previously reported (see Table S2). These results indicate that considerable improvements in energy density have been achieved for the heteroatom-doped graphene materials even at high charge/discharge rates, which would be very promising for the flash charging of portable electronics and quick startup of electric vehicles [39]. The superior specific capacitance, energy density and power density of the heteroatom doped graphene electrode materials annealed at low temperature (e.g. 200 °C) can be explained by the
pseudocapacitance effects brought by heteroatom function groups. From the previous XPS analysis, it can be seen that upon increasing the annealing temperature (from 200 to 400 then to 700 °C, for both B-doped and N-doped cases), the C-substituted graphitic heteroatoms increase, while the non-graphitic heteroatom functional groups decrease. To analyze the capacitive properties of the doped graphene electrode materials, the CV curves are differentiated into two components of the electrical double layer capacitance (Cdl), which is the central rectangular area, and the pseudocapacitance (Cp), which is the hump areas [43], as shown in Fig. S10. The differentiation results are summarized in Table S3. Compared to G200, the percentage of Cp in the total capacitance (Ct) increases significantly for all the heteroatom doped graphene materials. With increasing the temperature from NG200 to NG400 then to NG700, or from BG200 to BG400 then to
Non-graphitic heteroatom Carbon atom
Graphitic heteroatom Cdl Ct
Cp Fig. 6. Schematical illustration of the effects of heteroatoms on capacitance of graphene. The graphitic heteroatoms and carbon atoms contribute to electric double layer capacitance (Cdl), and the non-graphitic heteroatoms contribute to pseudocapacitance (Cp), Cdl and Cp connect in parallel to form the total capacitance (Ct).
(b) (Cp/Ct) / %
Energy density / Wh Kg
-1
(a)
Power density / W Kg
-1
Ratio of non-graphitic heteroatom / %
Fig. 5. (a) Ragone plots of energy density versus power density of GO, G200, BG200, NG200, PG200 and SG200 samples. (b) Percentage of Cp as a function of the ratio of the non-graphitic heteroatom of the B and N-doped samples.
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Fig. 7. Schematic presentation, calculated band structure and density of states (DOS) of (a) Pristine graphene, (b) Graphitic N-doped graphene, (c) Non-graphitic pyridinic N-doped graphene, and (d) Non-graphitic pyrrolic N-doped graphene.
BG700, the Cp components largely decrease. The relationship between the percentage of Cp and that of the non-graphitic heteroatom functional groups is illustrated in Fig. 5b, showing nearly linear changes in both N-doped and B-doped cases. From these analyses, it could be concluded that for the heteroatom doped graphene electrode materials, the non-graphitic heteroatom functional groups participate the fast redox reactions and
contribute to the Faradaic pseudocapacitance, while the Csubstituted graphitic heteroatoms mainly contribute to the electrical double layer capacitance, and both capacitances are combined in parallel to form the total capacitance. As the relatively larger contribution of the Faradaic pseudocapacitance from the heteroatoms than the electrical double layer capacitance, the total capacitance decreased with the increase of temperature. A model
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is proposed to demonstrate the different capacitance contributions, as shown in Fig. 6. The model shown in Fig. 6 can be further discussed in terms of the unique electronic structure of graphene. Density functional theory calculations are preformed to investigate the N doping effect on the electronic properties of graphene. The band structures and density of states (DOS) of graphitic and non-graphitic N-doped graphene models are displayed in Fig. 7 and Fig. S11. Comparing to pristine graphene, heteroatom doping creates modifications to electronic structure of graphene, including Dirac point shifts, band gap opening, DOS increasing, etc., which will result in changes in the capacitance [16]. When the heteroatoms are graphitic, the pz electron of the substituted heteroatoms anticipates the large π conjugate bond system of graphene, and the px and py electrons sp2-hybridize with the s orbital and form σ bonds with the adjacent three carbon atoms, resulting in increased electrical double layer capacitance (Cdl). For non-graphitic doping, a sharp peak contributed by px and py electrons appears at about 0.7 V below the Fermi level (Fig. 7c and d), corresponding to the localized excess electron, which can be easily involved in the proton and electron transfer processes, resulting in increased Faradaic pseudocapacitance of graphene. As displayed in Table S3, compared to G200, significant increase in Cp components for the heteroatom doped samples can be clearly seen. Therefore, from the above model, the heteroatom doping configurations may be designed and tuned by the corresponding expermintal conditions to purposely control the Cdl and Cp components for optimizing the graphene-based supercapacitor electrode materials with high specific capacitance and energy density. Recently, it was reported that the capacitances of B and Ndoped graphene were dominated by the structural defects regardless of the dopant type and level [18], where the doped graphene materials were annealed at high temperatures (800 and 1000 °C), and the C-substituted graphitic heteroatoms were dominant. In this case, the capacitances were mainly contributed from the Cdl component (capacitance lower than 120 F g 1 at the current density of 0.1 A g 1), therefore the structure defects played a crucial role in affecting the specific capacitance of graphene, and the influence of doped heteroatoms was believed to be the secondary. Another report on the pyrolytically synthesized Bdoped graphene provided further support on this point, where capacitance of 173 F g 1 was obtained for the graphitic doping obtained at the pyrolytical temperature of 900 °C [35]. In the present work, when the doped graphene materials are synthesized at low temperature (200 °C), the non-graphitic heteroatom functional groups are dominant and mainly contribute to the pseudocapacitance (as high as 629 F g 1 at 0.2 A g 1), and the effects of structure defects seem to be less significant. This can be verified by the Raman spectra, where the ID/IG ratios of BG200, NG200, PG200 and SP200 are similar (Fig. 2c, around 1.1) as the identical annealing procedures are applied to obtain these doped graphene materials, which indicates the similar defect levels. However, significantly varied specific capacitances have been obtained for these materials (Fig. 4b and c), implying the remarkable effects of dopant type and configuration on capacitance of the doped graphene obtained at low temperature.
4. Conclusions In summary, the heteroatom doped graphene materials synthesized by the thermal annealing method at low temperature (200 °C) display excellent electrochemical performance when explored as electrode materials for supercapacitors, including superior specific capacitance, excellent energy density/power density, and long cycle life. The non-graphitic dopant
configurations induced large Faradaic pseudocapacitance components contribute mainly to the remarkable capacitance observed. When thermal annealing at higher temperature of 700 °C, the graphitic dopant configuration is dominant, it mainly contributes to the relatively smaller electrical double layer capacitance and on which the structure defects display clear influences. The model describing the relationship of dopants and capacitance in this work coincides with the previous reports and can be used to explain the origin of the previous observations in the literature. The doping styles and configurations of heteroatoms into graphene can be purposely designed and realized to tune the supercapacitance components, to optimize and remarkably improve its capacitance performance. These findings promote new opportunities for further design and application of advanced graphenebased materials for high-performance supercapacitors and other energy storage and conversion devices.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51372178), the PCSIRT (No. IRT14R18), and the Natural Science Foundation for Distinguished Young Scholars of Hubei Province of China (No. 2013CFA021).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.ensm.2015.09.002.
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