DOI: 10.1002/cssc.201501303
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Nitrogen and Phosphorous Co-Doped Graphene Monolith for Supercapacitors Yangyang Wen,[a] Thomas E. Rufford,[b] Denisa Hulicova-Jurcakova+,[a, b] and Lianzhou Wang*[a, b] The co-doping of heteroatoms has been regarded as a promising approach to improve the energy-storage performance of graphene-based materials because of the synergetic effect of the heteroatom dopants. In this work, a single precursor melamine phosphate was used for the first time to synthesise nitrogen/phosphorus co-doped graphene (N/P-G) monoliths by a facile hydrothermal method. The nitrogen contents of 4.27– 6.58 at % and phosphorus levels of 1.03–3.00 at % could be
controlled by tuning the mass ratio of melamine phosphate to graphene oxide in the precursors. The N/P-G monoliths exhibited excellent electrochemical performances as electrodes for supercapacitors with a high specific capacitance of 183 F g¢1 at a current density of 0.05 A g¢1, good rate performance and excellent cycling performance. Additionally, the N/P-G electrode was stable at 1.6 V in 1 m H2SO4 aqueous electrolyte and delivered a high energy density of 11.33 Wh kg¢1 at 1.6 V.
Introduction tion reaction,[7] hydrogen evolution reaction,[9] supercapacitors[8, 10] and batteries.[11] However, two separate dopants are generally required in the co-doped graphene and complicated synthesis processes, high cost or toxic precursors are normally needed for co-doping,[7, 9–11] which limits the large-scale preparation of co-doped graphene for practical applications. Herein, we report a facile hydrothermal method to prepare nitrogen and phosphorus co-doped graphene (designated as N/P-G) monoliths by simply using a single precursor melamine phosphate (MP; C3H6N6·H3PO4). MP, which is a low-cost, nontoxic material used commonly as a flame retardant that includes both heteroatoms, has not been reported to date as a precursor for the preparation of doped graphene materials. N/P-G exhibited a superior electrochemical performance compared to annealed graphene in 1 m H2SO4 electrolyte.
Graphene has been reported widely as a promising candidate for use as electrodes for supercapacitors because of its unique two-dimensional (2 D) structure, large specific surface area and high electrical conductivity.[1] However, the practical capacitance of graphene electrodes is typically much lower than its theoretical value (550 F g¢1).[2] Generally, there are three main strategies to improve electrochemical performance of graphene: the generation of porous graphene with a high surface area,[3] the incorporation of heteroatoms to graphene[4] and the synthesis of graphene-based composites.[1a] The incorporation of heteroatoms enhances the electrochemical performances of carbons significantly.[4a, 5] For instance, after doping with N atoms, the electrical conductivity and chemical reactivity of the carbon matrix, which are essential for the electrochemical energy storage, can be modified drastically.[2, 4a,c, 6] The doping with N atoms and other foreign atoms (S, P, B) in carbons has been investigated widely to improve the performance in energy-storage systems.[2, 4a, 5a,e, 7] It is believed that co-doping could further modify and improve the properties of single-heteroatom-doped graphene.[8] The enhanced electrochemical performances of co-doped graphene have been explored in the fields of the oxygen reduc-
Results and Discussion The synthesis of N/P-G monoliths is illustrated in Scheme 1. N/ P-G was prepared from graphene oxide (GO) and MP in the presence of polyvinylpyrrolidone (PVP) by a hydrothermal, freeze-drying and pyrolysis process. We denote the samples as N/P-G-x, in which x is the MP/GO mass ratio, and we produced N/P-G-1, N/P-G-3, N/P-G-10 and N/P-G-30. In addition, an undoped graphene sample (un-G) and MP-derived carbon (MP-C) were synthesised. The XRD patterns of GO, un-G and N/P-G are shown in Figure 1 a. The peak at 2 q = 10.78 for GO is at a lower angle than peaks observed in the other samples and corresponds to an inter-layer distance of 8.24 æ, which results from the introduction of oxygenated functional groups in the matrix or edges of the GO nanosheets during the oxidation of graphite.[5e, 12] The reduction and annealing of GO to produce un-G or precursors to N/P-G remove these oxygenated functional groups, and the
[a] Y. Wen, Dr. D. Hulicova-Jurcakova,+ Prof. L. Wang Nanomaterials Centre Australian Institute for Bioengineering and Nanotechnology The University of Queensland Corner College and Cooper Road, St Lucia, 4072 Qld (Australia) E-mail: [email protected]
[b] Dr. T. E. Rufford, Dr. D. Hulicova-Jurcakova,+ Prof. L. Wang School of Chemical Engineering The University of Queensland Corner College and Cooper Road, St Lucia, 4072 Qld (Australia) [+] Deceased October 2014. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201501303.
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Scheme 1. Schematic illustration of the preparation of N/P-G monoliths.
Figure 1. (a) XRD patterns of GO, un-doped graphene and N/P co-doped graphene. (b) Schematic model of N- and P-containing functional groups on carbon. High-resolution XPS (c) N 1s and (d) P 2p spectra of N/P co-doped graphene.
XRD peaks shift to 2 q = 26.48 in un-G (d spacing of 3.37 æ) and 2 q = 24.88 (d spacing of 3.58 æ) in N/P-G-3. The larger interlayer spacing observed in N/P-G-3 than un-G may be explained by the expansion of the graphene layers by the introduction of N and P heteroatoms[8, 13] and changes in the surface morphology induced by the addition of MP. Raman spectra (Figure S1) show two clear peaks near n˜ = 1360 and 1600 cm¢1 that correspond to the D and G bands, respectively, for all the samples. The intensity ratio (ID/IG ratio) differences indicate the changes of the structural defects upon doping. These structural observations are discussed further with the TEM observations below. The successful doping of N and P heteroatoms into the graphene nanosheets was verified by X-ray photoelectron spectroscopy (XPS; Figures S2, S3 and S4, Table 1). As expected, the N and P surface concentrations increased with the MP/GO mass ratio in the series N/P-G-1 to N/P-G-30, with the amount of N from 4.27 to 6.58 at %, and the amount of P from 1.03 to 3.00 at %. The N doping concentrations in all N/P-G samples are higher than the P concentrations; this is related to the ratio of the N to P atoms in melamine phosphate (N/P 6:1) and the additional N in the PVP. Likewise, the concentrations of ChemSusChem 2016, 9, 513 – 520
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8.55 at % N and 4.20 at % P in MP-C are higher than that in N/P-G because of the N/P/C ratios in the precursor mixture, which does not contain GO for MP-C. The schematic model shows the N- and P-containing functional groups that were observed in the high-resolution XPS of N/P-G (Figure 1 b). The highresolution N 1s spectra were resolved into four components centred at binding energies (BEs) of (398.2 0.1), (399.8 0.1), (401.1 0.1) and (402.8 0.1) eV, which correspond to pyridinic N (N-I), pyrrolic/pyridone N (N-II), quaternary N (N-III) and pyridine-Noxide (N-IV), respectively (Figure 1 c and Figure S3),[5a, 14] whereas the P 2p spectra were fitted to P¢C (BE = 130.0 0.3 eV) and P¢O (BE = 133.1 0.3 eV) bonding (Figure 1 d and Figure S4).[5c, e] The dominant P functional group seen in the XPS data is a phosphate-like P¢O configuration, which is consistent with the oxygen concentrations shown in Table 1. Further evidence of this P¢O configuration is provided by 31P NMR and FTIR spectra (Figures S5 and S6). The SEM images of the as-prepared N/P-G monolith indicate that N/P-G features a 2 D sheetlike structure (Figure 2). The lateral sheet size in N/P-G is in
Table 1. C/O/N/P concentrations [at %] and the N/P configurations.[a]
Sample
C
O
un-G N/P-G-1 N/P-G-3 N/P-G-10 N/P-G-30 MP-C
96.86 3.14 90.98 3.72 88.78 4.55 85.84 7.33 82.27 8.15 71.96 15.29
N
P
N-I
N-II
N-III N-IV P¢C P¢O
– 4.27 4.51 4.60 6.58 8.55
– 1.03 2.16 2.23 3.00 4.20
– 32.4 29.4 39.7 40.8 35.4
– 20.0 26.0 26.6 30.2 23.8
– 39.8 36.0 26.8 25.3 33.4
– 7.8 8.6 6.9 3.7 7.4
– 26.1 27.9 7.2 8.2 0.9
– 73.9 72.1 92.8 91.8 99.1
[a] Pyridinic N (N-I, BE = 398.2 0.1 eV), pyrrolic/pyridone N (N-II, BE = 399.8 0.1 eV), quaternary N (N-III, BE = 401.1 0.1 eV) and pyridine-Noxide (N-IV, BE = 402.8 0.1 eV); P¢C bonding (BE = 130.0 0.3 eV) and P¢O bonding (BE = 133.1 0.3 eV).
the range of ten to several hundred micrometers, which is much larger than that of the un-doped graphene sheets (Figure 2 and Figure S8 a). Our un-G shows an interconnected framework of ultrathin nanosheets that create a porous structure (Figure S8 a).[15] The large sheet structure was observed in all N/P-G samples, but N/P-G-30 sheets were smaller and more wrinkled than the N/P-G materials prepared with lower MP/GO 514
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Full Papers The pore structures of N/P-G, un-G and MP-C were characterised by N2 sorption analysis at ¢196 8C and CO2 sorption analysis at 0 8C.[16] All six samples exhibit Type IV isotherms for N2 sorption (Figure 4 a) with well-defined H2 (un-G and MP-C) or H3 (N/P-G-x samples) hysteresis loops at relative pressures of approximately 0.4.[17] These hysteresis loops are attributed to capillary condensation in the mesopores created by the stacked nanosheet structures. The pore size distributions (PSD) shown in Figure 4 b show that all of the carbons feature some mesopores (from the N2 isotherm), and the N/P-G carbons have larger-width mesopores, broader mesopore size distributions and some narrow micropores (in the PSD derived from the CO2 isotherm). These narrow micropores, especially in the size range of 0.7–1 nm are critical in energy-storage applications.[18] The textural parameters of samples are summarised in Table 2. The specific surface areas (SBET) of all N/P-G samples are less than 50 m2 g¢1, which is much lower than that of un-G and MP-C (158 and 280 m2 g¢1, respectively). The relatively low SBET of all N/P-G monoliths is consistent with a large, stacked and thick sheet structure (Figure 2 a–d). The electrochemical performances of un-G and N/P-G were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge (GC) techniques by using three-electrode cells in 1 m H2SO4 aqueous electrolyte. The quasi-rectangular CV curves and the symmetrical GC profiles of both the N/P-G samples and un-G (Figure 5) are characteristic of an electrode material in which electric double layer capacitance (EDLC) is the dominant charge-storage mechanism. However, a pair of humps is observed at around 0.4 V vs. Ag/AgCl in the CV profiles of N/P-G-3 and N/P-G-10; these features are typically related to the surface oxygen functionalities[19] and contribute pseudo-capacitance to the charge storage. The GC profiles of the N/P-G-3 and N/P-G-10 electrodes show a very small IR drop whereas un-G has a much larger IR drop (around 0.1 V), which demonstrates that the N/P-G samples have faster charge propagation than un-G. Further examples of the good electrochemical performance of N/P-G-3 are provided in Figure 5 c, which shows GC curves measured at different current densities from 0.05 to 20 A g¢1. The gravimetric capacitances of the five electrode materials calculated from the GC data are shown in Figure 5 d. For example, N/P-G-3 gives a specific capacitance of 183 F g¢1 at a current density of 0.05 A g¢1, which is much higher than the value for un-doped graphene of 87 F g¢1 under the same conditions. Several conclusions can be drawn from the results shown in Figure 5: First, the higher capacitances of N/P-G-3 and N/P-G10 suggest a significant effect of N/P doping on the electrochemical performance. Second, there is an optimum MP/GO ratio required to prepare N/P-G with enhanced electrochemical properties as the capacitance of N/P-G-1 and N/P-G-30 were lower than that of un-G. The low capacitance of N/P-G-1 is related to the ultra-low specific surface area (1.6 m2 g¢1), and the poor performance of N/P-G-30 is attributed to its poor conductivity caused by the high loading of MP, which will be discussed in relation to the Nyquist plot (Figure 6). Third, the N/PG electrodes show better capacitance retention at high current loads than the un-G electrodes. For example, N/P-G-3 retained
Figure 2. SEM images of N/P co-doped graphene monoliths: (a) N/P-G-1, (b) N/P-G-3, (c) N/P-G-10, (d) N/P-G-30, (e) un-G and (f) MP-C.
ratios, which indicates the possible effect of functional groups on the doping process (Figure 2 a–d). TEM images illustrate that the thin, transparent individual nanosheets of un-G are retained in the MP-doped samples (Figure 3), which suggests that the larger sheet structure of N/ P-G observed by SEM (Figure 2 b) are aggregates of the ultrathin graphene nanosheets seen in Figure 3 b. The other difference observed by TEM in N/P-G-30 was a thin layer of porous amorphous carbon on the surface of the graphene nanosheets (Figure S10 d), but this amorphous carbon layer was not observed for MP/GO ratios less than or equal to 10. This observation of amorphous carbon in N/P-G-30 is consistent with the XRD patterns (Figure 1 a) in which the d spacing of N/P-G-30 layers (3.85 æ, 2 q = 23.18) was larger than that of the other NP/G samples. We postulate that the thin layer of amorphous carbon on the surface of N/P-G-30 is a product, such as MP-C, formed after the annealing process.
Figure 3. TEM images of (a) un-G and (b) N/P-G-3.
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Figure 4. (a) N2 adsorption–desorption isotherms of un-doped graphene and N/P co-doped graphene samples. (b) Pore size distribution from N2 and CO2 sorption. The pore size range of 0.3–1.07 nm is calculated from CO2 sorption, and the pore size range > 1.48 nm is obtained from N2 sorption (Figure S11).
was only 72 F g¢1 at 20 A g¢1; whereas N-G-3 retained 75 % of its capacitance at 0.05 A g¢1 if the current density was increased to 20 A g¢1. Therefore, this indicates that the enhanced specific capacitance of graphene is obtained from both N and P doping, and the capacitance retention is related mainly to the N doping effect.[20] The Nyquist plots of N/P-G, un-G and MP-C are shown in Figure 6. All electrodes exhibited two distinct regions in the impendence spectra with a semicircle in the high-frequency region and a nearly vertical line in the low-frequency region. The nearly vertical line of all samples in the low-frequency region validates the conclusion that the dominant charge-storage mechanism is EDLC. The smaller semicircle in the high-frequency region implies a low charge transfer resistance and high electrical conductivity of the electrodes.[21] The charge transfer resistance (the radius of the semicircle in the high-frequency region) of N/P-G-3 was only 0.9 W. The charge transfer resistances of the other samples with greater resistances were approximately 4.3 W for N/P-G-10 and un-G, 7.4 W for N/P-G1 and 7.5 W for N/P-G-30. The greater charge transfer resistance of N/P-G-10 explains why this electrode had a lower capacitance retention at high current loads than the N/P-G-3 electrode. The thick amorphous carbon layer deposited on the graphene surfaces in N/P-G-30 (Figure 2 d and Figure S10 d) will affect the conductivity properties of this electrode material, and the additional resistance is observed in the Nyquist plots as a large semicircle (radius of 7.5 W). A similar semicircle (radius of 7.4 W) was detected for N/P-G-1. No amorphous carbon layer was observed in the N/P-G-1 sample (Figure S10 a), and a possible explanation for this result is that the bulk electrode has a poor conductivity because N/P-G-1 has very narrow micropores as observed by the high surface area of CO2-accessible pores (167 m2 g¢1; Figure S11 d) that were not accessible to N2 in the gas sorption measurements. The ultrasmall micropores (< 0.4 nm) in N/P-G-1 that were not accessible to N2 at ¢196 8C are also likely to be too small for electrolyte ions to enter, and thus N/P-G-1 exhibited a poor electrochemical performance (Figure 5 d). The knee, the junction of the right-hand side of the semicircle and the start of the straight line, indicates the frequency at which the supercapacitor cells begin to exhibit capacitance behaviour.[22] This fre-
Table 2. Porous structure calculated from N2 and CO2 sorption isotherms.[a]
Samples
Surface area [m2 g¢1] SBET SCO2
Vmic
un-G N/P-G-1 N/P-G-3 N/P-G-10 N/P-G-30 MP-C
158 1.6 50 45 40 280
0.028 0.002 0.019 0.012 0.003 0.001
219 167 204 286 228 306
Volume [cm3 g¢1] Vtotal VCO2 0.159 0.005 0.040 0.051 0.082 0.333
0.110 0.119 0.090 0.143 0.160 0.203
[a] The specific surface areas (SBET) obtained using the BET method, the total pore volumes (Vtotal) at the N2 relative pressure equal to 0.98 and the micropore volumes (Vmic) from a t-plot method (carbon black STSA reference) were obtained from the N2 isotherms. Micropore surface areas (SCO2 ) and micropore volumes (VCO2 ) were determined from CO2 isotherms using the Dubinin–Astakhov method.
70 % of the gravimetric capacitance if the current density was increased from 0.05 to 20 A g¢1, whereas the un-doped graphene retained only 55 % of the capacitance measured at 0.05 A g¢1. From these results, we postulate that the improved capacitances of N/P-G-3 and N/P-10 result from contributions from both (i) pseudo-capacitance provided by N and P heteroatoms and (ii) the development of hierarchical mesoporous and macroporous structures as an effect of N/P doping (Figure 4). Although the MP-C sample had a porous structure (SBET = 280 m2 g¢1; Figure S8), this benchmark electrode material exhibited a poorer performance (Figure S12) than the N/P-G electrodes with capacitances of 49 F g¢1 at 0.05 A g¢1 and only 1 F g¢1 if the current density was increased to 2 A g¢1. We further examined the effect of the co-doping in N/P-G by measurement of the electrochemical performances in 1 m H2SO4 aqueous electrolyte of electrode carbons doped with 3.47 at % N (N-G-3) or with 2.64 at % P (P-G-3). Details of these samples are provided in the Supporting Information as SEM images in Figure S9 and electrochemical measurements in Figure S13. The specific capacitances measured for these samples at 0.05 A g¢1 were 133 F g¢1 for N-G-3 and 166 F g¢1 for P-G-3, which are both lower than the capacitance of co-doped graphene (N/P-G-3, 183 F g¢1). The capacitance retention of P-G-3 ChemSusChem 2016, 9, 513 – 520
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Figure 5. (a) Cyclic voltammograms at 5 mV s¢1. (b) Galvanostatic charge–discharge curves at 1 A g¢1. (c) Galvanostatic charge–discharge curves of N/P-G-3 at a series of current densities. (d) Variation of gravimetric capacitances with a series of current densities. All profiles were recorded in 1 m H2SO4 electrolyte by using three-electrode cells.
powders that had been ground by hand in a mortar and pestle. This electrode construction was used (i) to allow easier comparison with other reported results measured by this commonly used method and (ii) because the N/P-G monolith structures shown in Scheme 1 were not strong. The grinding of the N/P-G monolith structures did disrupt the macropore structure created between doped nanosheets (Figure S16). However, to evaluate the performance of the monoliths and compare these results to the powder electrode materials, we also constructed an electrode directly from a N/P-G-3 monolith, without any black carbon or polyvinylidene fluoride (PVDF) binder. At a current density of 0.05 A g¢1, the monolith electrode exhibited a higher specific capacitance (213 F g¢1) than the electrode prepared from a powder of N/P-G-3 (183 F g¢1 at 0.05 A g¢1). Further details of the measurements with the N/P-G-3 monolith are included in Figures S14–S18. These results confirm the excellent potential electrochemical properties of the N/P-G monolith but a future challenge is to improve the mechanical strength of the nanosheet monoliths. Previous studies have concluded that P-rich/doped carbons are stable in aqueous electrolytes at potential windows beyond the decomposition potential of water (1.23 V),[4d, 5b, 23] thus we tested the stability of N/P-G-3 in 1 m H2SO4 by using a two-electrode cell above a potential of 1.0 V by step-wise increases in the positive potential range. The CV curves recorded at a scan rate of 50 mV s¢1 and cell voltages from 1 to 1.6 V are shown in Figure 7 b. It is clear that the N/P-G-3 electrode provided an almost quasi-rectangular shape up to 1.6 V. Furthermore, the GC curves shown in Figure 7 c retained triangular shapes up to a high potential window of 1.6 V at a current
Figure 6. Nyquist plot from data obtained by using a three-electrode cell, and the inset shows the high-frequency part.
quency is 10 Hz for N/P-G-3, which is the highest value in all samples and is consistent with the GC results that show that N/P-G-3 has the best EDLC performance at high charge rates (Figure 5 d). The cycling stability of the N/P-G-3 electrode was measured using GC cycling at a current density of 5 A g¢1 by using a twoelectrode setup (Figure 7 a). After 10 000 consecutive charge– discharge cycles N/P-G-3 retained 94.0 % of the capacitance from the first cycle, which represents an excellent cycling performance. The electrochemical measurements reported above were performed by using electrodes prepared from slurries of N/P-G ChemSusChem 2016, 9, 513 – 520
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Figure 7. (a) Cycling profiles of N/P-G-3 measured at a current density of 5 A g¢1 by using a two-electrode cell. (b) Cyclic voltammograms recorded at 50 mV s¢1 and (c) galvanostatic charge–discharge curves at 1 A g¢1 at different potential windows in two-electrode cells. (d) Ragone plot. The black line shows the energy/powder densities from the current density of 0.05–20 A g¢1. The other three points are the energy/powder densities from a current density of 1 A g¢1 at 1.2, 1.4 and 1.6 V, respectively.
Conclusions
density of 1 A g¢1, which is considerably higher than the theoretical decomposition potential of water (1.23 V). The key challenge for supercapacitors in practical, largescale applications is to develop devices with a high energy density.[1b, 24] Compared to other energy-storage technologies such as batteries, the limitation of supercapacitors is the low energy density, which is generally 5–10 Wh kg¢1 in an aqueous electrolyte based on a reported N-rich graphene (Table S2). One route to increase the energy density is the development of electrode materials that can operate in wider voltage potential ranges than the current limitation of 1.0 V for aqueous electrolytes, and the Ragone plot for N/P-G-3 (Figure 7 d) highlights the advantages that could be gained with a N/P-doped carbon electrode able to operate at potential windows of up to 1.6 V. The energy/power densities of N/P-G-3 at 1.2, 1.4 and 1.6 V at a current density of 1 A g¢1 are 6.30 Wh kg¢1/ 571 W kg¢1, 8.46 Wh kg¢1/659 W kg¢1 and 11.33 Wh kg¢1/ 745 W kg¢1, respectively, which are almost 2–3 times higher than the value at 1.0 V (4.53 Wh kg¢1/482 W kg¢1). Currently, the mechanism of the wide operating potential is still not fully understood, but it is clear that P-containing functional groups (phosphate groups) in the graphene matrix play an important role. A possible mechanism to explain the widening of the potential window is the enhanced oxidation stability of P-doped graphene at positive potentials because of the blocking of active oxidation sites by P-containing functional groups and the adsorption of hydrogen at negative potentials.[4d, 5b, 23, 25]
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We demonstrated that melamine phosphate is a promising precursor to synthesise N/P co-doped graphene (N/P-G) monolith. The N- and P-doping levels in the N/P-G monolith are estimated to be 4.27–6.58 and 1.03–3.00 at %, respectively. The N/ P-G monolith is an excellent electrode material for supercapacitors that has a high specific capacitance of 183 F g¢1 at a current density of 0.05 A g¢1, a good rate performance (70 % of the gravimetric capacitance was retained if the current density was increased from 0.05 to 20 A g¢1) and excellent cycling performance (94.0 % of the original value was maintained after 10 000 consecutive cycles). Interestingly, the presence of P-containing functional groups makes N/P-G an excellent supercapacitor electrode, which is stable over a wide potential window of 1.6 V in aqueous electrolyte and thus delivers high energy/ power densities of 11.33 Wh kg¢1/745 W kg¢1.
Experimental Section Synthesis of N/P-G monolith GO was prepared from graphite powder using the Hummers’ method.[4d, 26] The four-step synthesis procedure for N/P-G monolith is: (1) polyvinylpyrrolidone (PVP, 120 mg, Sigma–Aldrich) was dissolved in GO solution (2 mg mL¢1, 30 mL), melamine phosphate (BOC Sciences, USA) was added and the mixture was stirred at 80 8C for 1 h. (2) The mixture was transferred to an autoclave and treated hydrothermally at 170 8C for 12 h, after which a black hy-
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Full Papers drogel was obtained. (3) The hydrogel was freeze-dried (¢80 8C, < 1 mbar for 2 days). (4) The “green” hydrogel was annealed at 800 8C under Ar at a heating rate of 5 8C min¢1 to obtain the N/P-G monolith. The synthesis procedure of the N/P-G monolith is illustrated in Scheme 1. The addition of PVP to the solution was critical to prevent the precipitation of GO after the addition of the MP powder; without the PVP the hydrogel did not form. The effect of the MP/GO mass ratio on the N/P-G monolith properties was investigated at MP/GO ratios of (1, 3, 10 and 30) to produce samples N/ P-G-1, N/P-G-3, N/P-G-10 and N/P-G-30. As the counterparts, single N-doped graphene (N-G-3) and P-doped graphene (P-G-3) were synthesised under same conditions using melamine or phosphoric acid as the single dopant at a weight ratio of single dopant/GO = 3. In addition, an un-doped graphene sample was prepared by hydrothermal treatment, freeze-drying and annealing process (un-G), and MP-C was synthesised by the pyrolysis of MP powder at 800 8C in Ar.
in which I is the current load, Dt is discharge time from the discharge branch of the galvanostatic curves, DU is the potential difference in the discharge process of galvanostatic charge–discharge curves after the IR drop was excluded, and m is the weight of active material in the working electrode. If we used the two-electrode setup, the specific capacitances of electrodes were calculated by multiplying the cell capacitance (Cgcell) by 4 according to Equations (2) and (3):[5b] C g cell ðF g¢1 Þ ¼
C g2E ðF g¢1 Þ ¼ 4 C g cell
ð3Þ
The energy density (E) and power density (P) were calculated according to Equations (4) and (5): E ðWh kg¢1 Þ ¼
The carbon products were characterised by SEM (Schottky Field Emission, JEOL 7100), TEM (JEOL, 1010), XRD (Bruker D8 Advance X-ray diffractometer using CuKa radiation), XPS (VG Scientific, ESCALAB220i-XL using a mono-chromated AlKa excitation source), NMR (Bruker AMX 300 spectrometer at a 31P frequency of 242 MHz) and FTIR spectroscopy (Thermo Scientific Nicolet 6700 spectrometer). To investigate the textural properties, sorption isotherms of N2 at ¢196 8C and CO2 at 0 8C were measured by using a Micromeritics Tristar II 3020 instrument. The specific surface areas (SBET) were obtained using the BET method, the total pore volumes (Vtotal) at a N2 relative pressure of 0.98 and the micropore volumes (Vmic) from a tplot method (carbon black STSA reference) were obtained from the N2 isotherms. Micropore surface areas (SCO2 ) and micropore volumes (VCO2 ) were determined from CO2 isotherms using a Dubinin– Astakhov method. The pore size distributions from N2 and CO2 were obtained using the density functional theory model for carbon slit pores (TriStarII 3020 V1.03).
PðW kg¢1 Þ ¼
ð5Þ
Keywords: carbon · doping · electrochemistry · graphene · phosphorus [1] a) C. H. Xu, B. H. Xu, Y. Gu, Z. G. Xiong, J. Sun, X. S. Zhao, Energy Environ. Sci. 2013, 6, 1388 – 1414; b) L. L. Zhang, X. S. Zhao, Chem. Soc. Rev. 2009, 38, 2520 – 2531; c) X. Huang, X. Y. Qi, F. Boey, H. Zhang, Chem. Soc. Rev. 2012, 41, 666 – 686; d) C. Lee, X. Wei, J. W. Kysar, J. Hone, Science 2008, 321, 385 – 388. [2] H. Cao, X. Zhou, Z. Qin, Z. Liu, Carbon 2013, 56, 218 – 223. [3] Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach, R. S. Ruoff, Science 2011, 332, 1537 – 1541. [4] a) J. H. Lee, N. Park, B. G. Kim, D. S. Jung, K. Im, J. Hur, J. W. Choi, ACS Nano 2013, 7, 9366 – 9374; b) N. Xiao, D. M. Lau, W. H. Shi, J. X. Zhu, X. C. Dong, H. H. Hng, Q. Y. Yan, Carbon 2013, 57, 184 – 190; c) B. Zheng, T. W. Chen, F. N. Xiao, W. J. Bao, X. H. Xia, J. Solid State Electrochem. 2013, 17, 1809 – 1814; d) Y. Wen, B. Wang, C. Huang, L. Wang, D. Hulicova-Jurcakova, Chem. Eur. J. 2015, 21, 80 – 85. [5] a) D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z. H. Zhu, G. Q. Lu, Adv. Funct. Mater. 2009, 19, 1800 – 1809; b) D. Hulicova-Jurcakova, A. M. Puziy, O. I. Poddubnaya, F. Suarez-Garcia, J. M. D. Tascon, G. Q. Lu, J. Am. Chem. Soc. 2009, 131, 5026 – 5027; c) M. LatorreSnchez, A. Primo, H. Garcia, Angew. Chem. Int. Ed. 2013, 52, 11813 – 11816; Angew. Chem. 2013, 125, 12029 – 12032; d) S. Some, J. Kim, K. Lee, A. Kulkarni, Y. Yoon, S. Lee, T. Kim, H. Lee, Adv. Mater. 2012, 24, 5481 – 5486; e) C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Adv. Mater. 2013, 25, 4932 – 4937; f) X. Fan, C. Yu, J. Yang, Z. Ling, J. Qiu, Carbon 2014, 70, 130 – 141. [6] Z. Li, G. Wu, S. Deng, S. Wang, Y. Wang, J. Zhou, S. Liu, W. Wu, M. Wu, Chem. Eng. J. 2016, 283, 1435 – 1442.
Calculation of specific capacitances, energy density and power density The specific capacitances of electrodes in the three-electrode setups were calculated according to Equation (1): ð1Þ
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IðAÞ DUðVÞ 2 mcell ðkgÞ
ð4Þ
The authors acknowledge the Australian Research Council for its financial support through ARC DP and LP programs and the facilities and technical assistance of the Australian Microscopy&Microanalysis Research facility at the Centre for Microscopy&Microanalysis at the University of Queensland.
A carbon electrode slurry was prepared by mixing 90 wt % carbons (N/P-G or un-G), 5 wt % carbon black and 5 wt % polyvinylidene fluoride dissolved in N-methyl pyrrolidone. This slurry was painted on a titanium foil (current collector) with 1 Õ 1 cm2 of carbons. Electrodes were dried at 110 8C in vacuum overnight. The two-electrode cells were constructed from two carbon electrodes that faced each other separated by a glassy fibre paper. Three-electrode cells were assembled by assigning one carbon electrode as a working electrode, the other carbon electrode as a counter electrode and a Ag/AgCl reference electrode was inserted into the electrolyte. Both cyclic voltammetry and galvanostatic charge–discharge curves were measured in 1 m H2SO4 aqueous electrolyte by using a Solartron 1480 Multistat station.
IðAÞ Dt ðsÞ mðgÞ DUðVÞ
1 C DU2 ðVÞ 2 3:6 g cell
Acknowledgements
Electrode preparation and electrochemical measurements
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ð2Þ
in which mcell is the total weight of active material in both electrodes.
Characterisation
C g3E ðF g¢1 Þ ¼
IðAÞ Dt ðsÞ mcell ðgÞ DUðVÞ
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Full Papers [7] C. H. Choi, M. W. Chung, H. C. Kwon, S. H. Park, S. I. Woo, J. Mater. Chem. A 2013, 1, 3694 – 3699. [8] T. Wang, L. X. Wang, D. L. Wu, W. Xia, D. Z. Jia, Sci. Rep. 2015, 5, 9591 – 9591. [9] Y. Zheng, Y. Jiao, L. H. Li, T. Xing, Y. Chen, M. Jaroniec, S. Z. Qiao, ACS Nano 2014, 8, 5290 – 5296. [10] P. Wang, H. He, X. Xu, Y. Jin, ACS Appl. Mater. Interfaces 2014, 6, 1563 – 1568. [11] X. Ma, G. Ning, C. Qi, C. Xu, J. Gao, ACS Appl. Mater. Interfaces 2014, 6, 14415 – 14422. [12] T. F. Yeh, J. M. Syu, C. Cheng, T. H. Chang, H. S. Teng, Adv. Funct. Mater. 2010, 20, 2255 – 2262. [13] F. M. Hassan, V. Chabot, J. Li, B. K. Kim, L. Ricardez-Sandoval, A. Yu, J. Mater. Chem. A 2013, 1, 2904 – 2912. [14] a) J. R. Pels, F. Kapteijn, J. A. Moulijn, Q. Zhu, K. M. Thomas, Carbon 1995, 33, 1641 – 1653; b) S. Biniak, G. Szymanski, J. Siedlewski, A. Swiatkowski, Carbon 1997, 35, 1799 – 1810; c) D. Hulicova, J. Yamashita, Y. Soneda, H. Hatori, M. Kodama, Chem. Mater. 2005, 17, 1241 – 1247. [15] Y. X. Xu, K. X. Sheng, C. Li, G. Q. Shi, ACS Nano 2010, 4, 4324 – 4330. [16] T. X. Nguyen, S. K. Bhatia, J. Phys. Chem. C 2007, 111, 2212 – 2222. [17] M. Kruk, M. Jaroniec, Chem. Mater. 2001, 13, 3169 – 3183.
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[18] a) C. Lin, J. A. Ritter, B. N. Popov, J. Electrochem. Soc. 1999, 146, 3639 – 3643; b) E. Raymundo-PiÇero, K. Kierzek, J. Machnikowski, F. Beguin, Carbon 2006, 44, 2498 – 2507. [19] J. E. Z. Unda, E. Roduner, Phys. Chem. Chem. Phys. 2012, 14, 3816 – 3824. [20] X. Zhao, A. Wang, J. Yan, G. Sun, L. Sun, T. Zhang, Chem. Mater. 2010, 22, 5463 – 5473. [21] Y. Gu, Z. G. Xiong, W. Al Abdulla, G. H. Chen, X. S. Zhao, Chem. Commun. 2014, 50, 14824 – 14827. [22] D. W. Wang, F. Li, H. T. Fang, M. Liu, G. Q. Lu, H. M. Cheng, J. Phys. Chem. B 2006, 110, 8570 – 8575. [23] C. Huang, T. Sun, D. Hulicova-Jurcakova, ChemSusChem 2013, 6, 2330 – 2339. [24] Y. Huang, J. J. Liang, Y. S. Chen, Small 2012, 8, 1805 – 1834. [25] a) M. P. Bichat, E. Raymundo-Pinero, F. Beguin, Carbon 2010, 48, 4351 – 4361; b) C. Vix-Guterl, E. Frackowiak, K. Jurewicz, M. Friebe, J. Parmentier, F. Beguin, Carbon 2005, 43, 1293 – 1302. [26] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339 – 1339. Received: September 25, 2015 Revised: November 9, 2015 Published online on February 2, 2016
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Nitrogen and Phosphorous Co-Doped Graphene Monolith for Supercapacitors Yangyang Wen,[a] Thomas E. Rufford,[b] Denisa Hulicova-Jurcakova+,[a, b] and Lianzhou Wang*[a, b] The co-doping of heteroatoms has been regarded as a promising approach to improve the energy-storage performance of graphene-based materials because of the synergetic effect of the heteroatom dopants. In this work, a single precursor melamine phosphate was used for the first time to synthesise nitrogen/phosphorus co-doped graphene (N/P-G) monoliths by a facile hydrothermal method. The nitrogen contents of 4.27– 6.58 at % and phosphorus levels of 1.03–3.00 at % could be
controlled by tuning the mass ratio of melamine phosphate to graphene oxide in the precursors. The N/P-G monoliths exhibited excellent electrochemical performances as electrodes for supercapacitors with a high specific capacitance of 183 F g¢1 at a current density of 0.05 A g¢1, good rate performance and excellent cycling performance. Additionally, the N/P-G electrode was stable at 1.6 V in 1 m H2SO4 aqueous electrolyte and delivered a high energy density of 11.33 Wh kg¢1 at 1.6 V.
Introduction tion reaction,[7] hydrogen evolution reaction,[9] supercapacitors[8, 10] and batteries.[11] However, two separate dopants are generally required in the co-doped graphene and complicated synthesis processes, high cost or toxic precursors are normally needed for co-doping,[7, 9–11] which limits the large-scale preparation of co-doped graphene for practical applications. Herein, we report a facile hydrothermal method to prepare nitrogen and phosphorus co-doped graphene (designated as N/P-G) monoliths by simply using a single precursor melamine phosphate (MP; C3H6N6·H3PO4). MP, which is a low-cost, nontoxic material used commonly as a flame retardant that includes both heteroatoms, has not been reported to date as a precursor for the preparation of doped graphene materials. N/P-G exhibited a superior electrochemical performance compared to annealed graphene in 1 m H2SO4 electrolyte.
Graphene has been reported widely as a promising candidate for use as electrodes for supercapacitors because of its unique two-dimensional (2 D) structure, large specific surface area and high electrical conductivity.[1] However, the practical capacitance of graphene electrodes is typically much lower than its theoretical value (550 F g¢1).[2] Generally, there are three main strategies to improve electrochemical performance of graphene: the generation of porous graphene with a high surface area,[3] the incorporation of heteroatoms to graphene[4] and the synthesis of graphene-based composites.[1a] The incorporation of heteroatoms enhances the electrochemical performances of carbons significantly.[4a, 5] For instance, after doping with N atoms, the electrical conductivity and chemical reactivity of the carbon matrix, which are essential for the electrochemical energy storage, can be modified drastically.[2, 4a,c, 6] The doping with N atoms and other foreign atoms (S, P, B) in carbons has been investigated widely to improve the performance in energy-storage systems.[2, 4a, 5a,e, 7] It is believed that co-doping could further modify and improve the properties of single-heteroatom-doped graphene.[8] The enhanced electrochemical performances of co-doped graphene have been explored in the fields of the oxygen reduc-
Results and Discussion The synthesis of N/P-G monoliths is illustrated in Scheme 1. N/ P-G was prepared from graphene oxide (GO) and MP in the presence of polyvinylpyrrolidone (PVP) by a hydrothermal, freeze-drying and pyrolysis process. We denote the samples as N/P-G-x, in which x is the MP/GO mass ratio, and we produced N/P-G-1, N/P-G-3, N/P-G-10 and N/P-G-30. In addition, an undoped graphene sample (un-G) and MP-derived carbon (MP-C) were synthesised. The XRD patterns of GO, un-G and N/P-G are shown in Figure 1 a. The peak at 2 q = 10.78 for GO is at a lower angle than peaks observed in the other samples and corresponds to an inter-layer distance of 8.24 æ, which results from the introduction of oxygenated functional groups in the matrix or edges of the GO nanosheets during the oxidation of graphite.[5e, 12] The reduction and annealing of GO to produce un-G or precursors to N/P-G remove these oxygenated functional groups, and the
[a] Y. Wen, Dr. D. Hulicova-Jurcakova,+ Prof. L. Wang Nanomaterials Centre Australian Institute for Bioengineering and Nanotechnology The University of Queensland Corner College and Cooper Road, St Lucia, 4072 Qld (Australia) E-mail: [email protected]
[b] Dr. T. E. Rufford, Dr. D. Hulicova-Jurcakova,+ Prof. L. Wang School of Chemical Engineering The University of Queensland Corner College and Cooper Road, St Lucia, 4072 Qld (Australia) [+] Deceased October 2014. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201501303.
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Scheme 1. Schematic illustration of the preparation of N/P-G monoliths.
Figure 1. (a) XRD patterns of GO, un-doped graphene and N/P co-doped graphene. (b) Schematic model of N- and P-containing functional groups on carbon. High-resolution XPS (c) N 1s and (d) P 2p spectra of N/P co-doped graphene.
XRD peaks shift to 2 q = 26.48 in un-G (d spacing of 3.37 æ) and 2 q = 24.88 (d spacing of 3.58 æ) in N/P-G-3. The larger interlayer spacing observed in N/P-G-3 than un-G may be explained by the expansion of the graphene layers by the introduction of N and P heteroatoms[8, 13] and changes in the surface morphology induced by the addition of MP. Raman spectra (Figure S1) show two clear peaks near n˜ = 1360 and 1600 cm¢1 that correspond to the D and G bands, respectively, for all the samples. The intensity ratio (ID/IG ratio) differences indicate the changes of the structural defects upon doping. These structural observations are discussed further with the TEM observations below. The successful doping of N and P heteroatoms into the graphene nanosheets was verified by X-ray photoelectron spectroscopy (XPS; Figures S2, S3 and S4, Table 1). As expected, the N and P surface concentrations increased with the MP/GO mass ratio in the series N/P-G-1 to N/P-G-30, with the amount of N from 4.27 to 6.58 at %, and the amount of P from 1.03 to 3.00 at %. The N doping concentrations in all N/P-G samples are higher than the P concentrations; this is related to the ratio of the N to P atoms in melamine phosphate (N/P 6:1) and the additional N in the PVP. Likewise, the concentrations of ChemSusChem 2016, 9, 513 – 520
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8.55 at % N and 4.20 at % P in MP-C are higher than that in N/P-G because of the N/P/C ratios in the precursor mixture, which does not contain GO for MP-C. The schematic model shows the N- and P-containing functional groups that were observed in the high-resolution XPS of N/P-G (Figure 1 b). The highresolution N 1s spectra were resolved into four components centred at binding energies (BEs) of (398.2 0.1), (399.8 0.1), (401.1 0.1) and (402.8 0.1) eV, which correspond to pyridinic N (N-I), pyrrolic/pyridone N (N-II), quaternary N (N-III) and pyridine-Noxide (N-IV), respectively (Figure 1 c and Figure S3),[5a, 14] whereas the P 2p spectra were fitted to P¢C (BE = 130.0 0.3 eV) and P¢O (BE = 133.1 0.3 eV) bonding (Figure 1 d and Figure S4).[5c, e] The dominant P functional group seen in the XPS data is a phosphate-like P¢O configuration, which is consistent with the oxygen concentrations shown in Table 1. Further evidence of this P¢O configuration is provided by 31P NMR and FTIR spectra (Figures S5 and S6). The SEM images of the as-prepared N/P-G monolith indicate that N/P-G features a 2 D sheetlike structure (Figure 2). The lateral sheet size in N/P-G is in
Table 1. C/O/N/P concentrations [at %] and the N/P configurations.[a]
Sample
C
O
un-G N/P-G-1 N/P-G-3 N/P-G-10 N/P-G-30 MP-C
96.86 3.14 90.98 3.72 88.78 4.55 85.84 7.33 82.27 8.15 71.96 15.29
N
P
N-I
N-II
N-III N-IV P¢C P¢O
– 4.27 4.51 4.60 6.58 8.55
– 1.03 2.16 2.23 3.00 4.20
– 32.4 29.4 39.7 40.8 35.4
– 20.0 26.0 26.6 30.2 23.8
– 39.8 36.0 26.8 25.3 33.4
– 7.8 8.6 6.9 3.7 7.4
– 26.1 27.9 7.2 8.2 0.9
– 73.9 72.1 92.8 91.8 99.1
[a] Pyridinic N (N-I, BE = 398.2 0.1 eV), pyrrolic/pyridone N (N-II, BE = 399.8 0.1 eV), quaternary N (N-III, BE = 401.1 0.1 eV) and pyridine-Noxide (N-IV, BE = 402.8 0.1 eV); P¢C bonding (BE = 130.0 0.3 eV) and P¢O bonding (BE = 133.1 0.3 eV).
the range of ten to several hundred micrometers, which is much larger than that of the un-doped graphene sheets (Figure 2 and Figure S8 a). Our un-G shows an interconnected framework of ultrathin nanosheets that create a porous structure (Figure S8 a).[15] The large sheet structure was observed in all N/P-G samples, but N/P-G-30 sheets were smaller and more wrinkled than the N/P-G materials prepared with lower MP/GO 514
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Full Papers The pore structures of N/P-G, un-G and MP-C were characterised by N2 sorption analysis at ¢196 8C and CO2 sorption analysis at 0 8C.[16] All six samples exhibit Type IV isotherms for N2 sorption (Figure 4 a) with well-defined H2 (un-G and MP-C) or H3 (N/P-G-x samples) hysteresis loops at relative pressures of approximately 0.4.[17] These hysteresis loops are attributed to capillary condensation in the mesopores created by the stacked nanosheet structures. The pore size distributions (PSD) shown in Figure 4 b show that all of the carbons feature some mesopores (from the N2 isotherm), and the N/P-G carbons have larger-width mesopores, broader mesopore size distributions and some narrow micropores (in the PSD derived from the CO2 isotherm). These narrow micropores, especially in the size range of 0.7–1 nm are critical in energy-storage applications.[18] The textural parameters of samples are summarised in Table 2. The specific surface areas (SBET) of all N/P-G samples are less than 50 m2 g¢1, which is much lower than that of un-G and MP-C (158 and 280 m2 g¢1, respectively). The relatively low SBET of all N/P-G monoliths is consistent with a large, stacked and thick sheet structure (Figure 2 a–d). The electrochemical performances of un-G and N/P-G were evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge (GC) techniques by using three-electrode cells in 1 m H2SO4 aqueous electrolyte. The quasi-rectangular CV curves and the symmetrical GC profiles of both the N/P-G samples and un-G (Figure 5) are characteristic of an electrode material in which electric double layer capacitance (EDLC) is the dominant charge-storage mechanism. However, a pair of humps is observed at around 0.4 V vs. Ag/AgCl in the CV profiles of N/P-G-3 and N/P-G-10; these features are typically related to the surface oxygen functionalities[19] and contribute pseudo-capacitance to the charge storage. The GC profiles of the N/P-G-3 and N/P-G-10 electrodes show a very small IR drop whereas un-G has a much larger IR drop (around 0.1 V), which demonstrates that the N/P-G samples have faster charge propagation than un-G. Further examples of the good electrochemical performance of N/P-G-3 are provided in Figure 5 c, which shows GC curves measured at different current densities from 0.05 to 20 A g¢1. The gravimetric capacitances of the five electrode materials calculated from the GC data are shown in Figure 5 d. For example, N/P-G-3 gives a specific capacitance of 183 F g¢1 at a current density of 0.05 A g¢1, which is much higher than the value for un-doped graphene of 87 F g¢1 under the same conditions. Several conclusions can be drawn from the results shown in Figure 5: First, the higher capacitances of N/P-G-3 and N/P-G10 suggest a significant effect of N/P doping on the electrochemical performance. Second, there is an optimum MP/GO ratio required to prepare N/P-G with enhanced electrochemical properties as the capacitance of N/P-G-1 and N/P-G-30 were lower than that of un-G. The low capacitance of N/P-G-1 is related to the ultra-low specific surface area (1.6 m2 g¢1), and the poor performance of N/P-G-30 is attributed to its poor conductivity caused by the high loading of MP, which will be discussed in relation to the Nyquist plot (Figure 6). Third, the N/PG electrodes show better capacitance retention at high current loads than the un-G electrodes. For example, N/P-G-3 retained
Figure 2. SEM images of N/P co-doped graphene monoliths: (a) N/P-G-1, (b) N/P-G-3, (c) N/P-G-10, (d) N/P-G-30, (e) un-G and (f) MP-C.
ratios, which indicates the possible effect of functional groups on the doping process (Figure 2 a–d). TEM images illustrate that the thin, transparent individual nanosheets of un-G are retained in the MP-doped samples (Figure 3), which suggests that the larger sheet structure of N/ P-G observed by SEM (Figure 2 b) are aggregates of the ultrathin graphene nanosheets seen in Figure 3 b. The other difference observed by TEM in N/P-G-30 was a thin layer of porous amorphous carbon on the surface of the graphene nanosheets (Figure S10 d), but this amorphous carbon layer was not observed for MP/GO ratios less than or equal to 10. This observation of amorphous carbon in N/P-G-30 is consistent with the XRD patterns (Figure 1 a) in which the d spacing of N/P-G-30 layers (3.85 æ, 2 q = 23.18) was larger than that of the other NP/G samples. We postulate that the thin layer of amorphous carbon on the surface of N/P-G-30 is a product, such as MP-C, formed after the annealing process.
Figure 3. TEM images of (a) un-G and (b) N/P-G-3.
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Figure 4. (a) N2 adsorption–desorption isotherms of un-doped graphene and N/P co-doped graphene samples. (b) Pore size distribution from N2 and CO2 sorption. The pore size range of 0.3–1.07 nm is calculated from CO2 sorption, and the pore size range > 1.48 nm is obtained from N2 sorption (Figure S11).
was only 72 F g¢1 at 20 A g¢1; whereas N-G-3 retained 75 % of its capacitance at 0.05 A g¢1 if the current density was increased to 20 A g¢1. Therefore, this indicates that the enhanced specific capacitance of graphene is obtained from both N and P doping, and the capacitance retention is related mainly to the N doping effect.[20] The Nyquist plots of N/P-G, un-G and MP-C are shown in Figure 6. All electrodes exhibited two distinct regions in the impendence spectra with a semicircle in the high-frequency region and a nearly vertical line in the low-frequency region. The nearly vertical line of all samples in the low-frequency region validates the conclusion that the dominant charge-storage mechanism is EDLC. The smaller semicircle in the high-frequency region implies a low charge transfer resistance and high electrical conductivity of the electrodes.[21] The charge transfer resistance (the radius of the semicircle in the high-frequency region) of N/P-G-3 was only 0.9 W. The charge transfer resistances of the other samples with greater resistances were approximately 4.3 W for N/P-G-10 and un-G, 7.4 W for N/P-G1 and 7.5 W for N/P-G-30. The greater charge transfer resistance of N/P-G-10 explains why this electrode had a lower capacitance retention at high current loads than the N/P-G-3 electrode. The thick amorphous carbon layer deposited on the graphene surfaces in N/P-G-30 (Figure 2 d and Figure S10 d) will affect the conductivity properties of this electrode material, and the additional resistance is observed in the Nyquist plots as a large semicircle (radius of 7.5 W). A similar semicircle (radius of 7.4 W) was detected for N/P-G-1. No amorphous carbon layer was observed in the N/P-G-1 sample (Figure S10 a), and a possible explanation for this result is that the bulk electrode has a poor conductivity because N/P-G-1 has very narrow micropores as observed by the high surface area of CO2-accessible pores (167 m2 g¢1; Figure S11 d) that were not accessible to N2 in the gas sorption measurements. The ultrasmall micropores (< 0.4 nm) in N/P-G-1 that were not accessible to N2 at ¢196 8C are also likely to be too small for electrolyte ions to enter, and thus N/P-G-1 exhibited a poor electrochemical performance (Figure 5 d). The knee, the junction of the right-hand side of the semicircle and the start of the straight line, indicates the frequency at which the supercapacitor cells begin to exhibit capacitance behaviour.[22] This fre-
Table 2. Porous structure calculated from N2 and CO2 sorption isotherms.[a]
Samples
Surface area [m2 g¢1] SBET SCO2
Vmic
un-G N/P-G-1 N/P-G-3 N/P-G-10 N/P-G-30 MP-C
158 1.6 50 45 40 280
0.028 0.002 0.019 0.012 0.003 0.001
219 167 204 286 228 306
Volume [cm3 g¢1] Vtotal VCO2 0.159 0.005 0.040 0.051 0.082 0.333
0.110 0.119 0.090 0.143 0.160 0.203
[a] The specific surface areas (SBET) obtained using the BET method, the total pore volumes (Vtotal) at the N2 relative pressure equal to 0.98 and the micropore volumes (Vmic) from a t-plot method (carbon black STSA reference) were obtained from the N2 isotherms. Micropore surface areas (SCO2 ) and micropore volumes (VCO2 ) were determined from CO2 isotherms using the Dubinin–Astakhov method.
70 % of the gravimetric capacitance if the current density was increased from 0.05 to 20 A g¢1, whereas the un-doped graphene retained only 55 % of the capacitance measured at 0.05 A g¢1. From these results, we postulate that the improved capacitances of N/P-G-3 and N/P-10 result from contributions from both (i) pseudo-capacitance provided by N and P heteroatoms and (ii) the development of hierarchical mesoporous and macroporous structures as an effect of N/P doping (Figure 4). Although the MP-C sample had a porous structure (SBET = 280 m2 g¢1; Figure S8), this benchmark electrode material exhibited a poorer performance (Figure S12) than the N/P-G electrodes with capacitances of 49 F g¢1 at 0.05 A g¢1 and only 1 F g¢1 if the current density was increased to 2 A g¢1. We further examined the effect of the co-doping in N/P-G by measurement of the electrochemical performances in 1 m H2SO4 aqueous electrolyte of electrode carbons doped with 3.47 at % N (N-G-3) or with 2.64 at % P (P-G-3). Details of these samples are provided in the Supporting Information as SEM images in Figure S9 and electrochemical measurements in Figure S13. The specific capacitances measured for these samples at 0.05 A g¢1 were 133 F g¢1 for N-G-3 and 166 F g¢1 for P-G-3, which are both lower than the capacitance of co-doped graphene (N/P-G-3, 183 F g¢1). The capacitance retention of P-G-3 ChemSusChem 2016, 9, 513 – 520
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Figure 5. (a) Cyclic voltammograms at 5 mV s¢1. (b) Galvanostatic charge–discharge curves at 1 A g¢1. (c) Galvanostatic charge–discharge curves of N/P-G-3 at a series of current densities. (d) Variation of gravimetric capacitances with a series of current densities. All profiles were recorded in 1 m H2SO4 electrolyte by using three-electrode cells.
powders that had been ground by hand in a mortar and pestle. This electrode construction was used (i) to allow easier comparison with other reported results measured by this commonly used method and (ii) because the N/P-G monolith structures shown in Scheme 1 were not strong. The grinding of the N/P-G monolith structures did disrupt the macropore structure created between doped nanosheets (Figure S16). However, to evaluate the performance of the monoliths and compare these results to the powder electrode materials, we also constructed an electrode directly from a N/P-G-3 monolith, without any black carbon or polyvinylidene fluoride (PVDF) binder. At a current density of 0.05 A g¢1, the monolith electrode exhibited a higher specific capacitance (213 F g¢1) than the electrode prepared from a powder of N/P-G-3 (183 F g¢1 at 0.05 A g¢1). Further details of the measurements with the N/P-G-3 monolith are included in Figures S14–S18. These results confirm the excellent potential electrochemical properties of the N/P-G monolith but a future challenge is to improve the mechanical strength of the nanosheet monoliths. Previous studies have concluded that P-rich/doped carbons are stable in aqueous electrolytes at potential windows beyond the decomposition potential of water (1.23 V),[4d, 5b, 23] thus we tested the stability of N/P-G-3 in 1 m H2SO4 by using a two-electrode cell above a potential of 1.0 V by step-wise increases in the positive potential range. The CV curves recorded at a scan rate of 50 mV s¢1 and cell voltages from 1 to 1.6 V are shown in Figure 7 b. It is clear that the N/P-G-3 electrode provided an almost quasi-rectangular shape up to 1.6 V. Furthermore, the GC curves shown in Figure 7 c retained triangular shapes up to a high potential window of 1.6 V at a current
Figure 6. Nyquist plot from data obtained by using a three-electrode cell, and the inset shows the high-frequency part.
quency is 10 Hz for N/P-G-3, which is the highest value in all samples and is consistent with the GC results that show that N/P-G-3 has the best EDLC performance at high charge rates (Figure 5 d). The cycling stability of the N/P-G-3 electrode was measured using GC cycling at a current density of 5 A g¢1 by using a twoelectrode setup (Figure 7 a). After 10 000 consecutive charge– discharge cycles N/P-G-3 retained 94.0 % of the capacitance from the first cycle, which represents an excellent cycling performance. The electrochemical measurements reported above were performed by using electrodes prepared from slurries of N/P-G ChemSusChem 2016, 9, 513 – 520
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Figure 7. (a) Cycling profiles of N/P-G-3 measured at a current density of 5 A g¢1 by using a two-electrode cell. (b) Cyclic voltammograms recorded at 50 mV s¢1 and (c) galvanostatic charge–discharge curves at 1 A g¢1 at different potential windows in two-electrode cells. (d) Ragone plot. The black line shows the energy/powder densities from the current density of 0.05–20 A g¢1. The other three points are the energy/powder densities from a current density of 1 A g¢1 at 1.2, 1.4 and 1.6 V, respectively.
Conclusions
density of 1 A g¢1, which is considerably higher than the theoretical decomposition potential of water (1.23 V). The key challenge for supercapacitors in practical, largescale applications is to develop devices with a high energy density.[1b, 24] Compared to other energy-storage technologies such as batteries, the limitation of supercapacitors is the low energy density, which is generally 5–10 Wh kg¢1 in an aqueous electrolyte based on a reported N-rich graphene (Table S2). One route to increase the energy density is the development of electrode materials that can operate in wider voltage potential ranges than the current limitation of 1.0 V for aqueous electrolytes, and the Ragone plot for N/P-G-3 (Figure 7 d) highlights the advantages that could be gained with a N/P-doped carbon electrode able to operate at potential windows of up to 1.6 V. The energy/power densities of N/P-G-3 at 1.2, 1.4 and 1.6 V at a current density of 1 A g¢1 are 6.30 Wh kg¢1/ 571 W kg¢1, 8.46 Wh kg¢1/659 W kg¢1 and 11.33 Wh kg¢1/ 745 W kg¢1, respectively, which are almost 2–3 times higher than the value at 1.0 V (4.53 Wh kg¢1/482 W kg¢1). Currently, the mechanism of the wide operating potential is still not fully understood, but it is clear that P-containing functional groups (phosphate groups) in the graphene matrix play an important role. A possible mechanism to explain the widening of the potential window is the enhanced oxidation stability of P-doped graphene at positive potentials because of the blocking of active oxidation sites by P-containing functional groups and the adsorption of hydrogen at negative potentials.[4d, 5b, 23, 25]
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We demonstrated that melamine phosphate is a promising precursor to synthesise N/P co-doped graphene (N/P-G) monolith. The N- and P-doping levels in the N/P-G monolith are estimated to be 4.27–6.58 and 1.03–3.00 at %, respectively. The N/ P-G monolith is an excellent electrode material for supercapacitors that has a high specific capacitance of 183 F g¢1 at a current density of 0.05 A g¢1, a good rate performance (70 % of the gravimetric capacitance was retained if the current density was increased from 0.05 to 20 A g¢1) and excellent cycling performance (94.0 % of the original value was maintained after 10 000 consecutive cycles). Interestingly, the presence of P-containing functional groups makes N/P-G an excellent supercapacitor electrode, which is stable over a wide potential window of 1.6 V in aqueous electrolyte and thus delivers high energy/ power densities of 11.33 Wh kg¢1/745 W kg¢1.
Experimental Section Synthesis of N/P-G monolith GO was prepared from graphite powder using the Hummers’ method.[4d, 26] The four-step synthesis procedure for N/P-G monolith is: (1) polyvinylpyrrolidone (PVP, 120 mg, Sigma–Aldrich) was dissolved in GO solution (2 mg mL¢1, 30 mL), melamine phosphate (BOC Sciences, USA) was added and the mixture was stirred at 80 8C for 1 h. (2) The mixture was transferred to an autoclave and treated hydrothermally at 170 8C for 12 h, after which a black hy-
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Full Papers drogel was obtained. (3) The hydrogel was freeze-dried (¢80 8C, < 1 mbar for 2 days). (4) The “green” hydrogel was annealed at 800 8C under Ar at a heating rate of 5 8C min¢1 to obtain the N/P-G monolith. The synthesis procedure of the N/P-G monolith is illustrated in Scheme 1. The addition of PVP to the solution was critical to prevent the precipitation of GO after the addition of the MP powder; without the PVP the hydrogel did not form. The effect of the MP/GO mass ratio on the N/P-G monolith properties was investigated at MP/GO ratios of (1, 3, 10 and 30) to produce samples N/ P-G-1, N/P-G-3, N/P-G-10 and N/P-G-30. As the counterparts, single N-doped graphene (N-G-3) and P-doped graphene (P-G-3) were synthesised under same conditions using melamine or phosphoric acid as the single dopant at a weight ratio of single dopant/GO = 3. In addition, an un-doped graphene sample was prepared by hydrothermal treatment, freeze-drying and annealing process (un-G), and MP-C was synthesised by the pyrolysis of MP powder at 800 8C in Ar.
in which I is the current load, Dt is discharge time from the discharge branch of the galvanostatic curves, DU is the potential difference in the discharge process of galvanostatic charge–discharge curves after the IR drop was excluded, and m is the weight of active material in the working electrode. If we used the two-electrode setup, the specific capacitances of electrodes were calculated by multiplying the cell capacitance (Cgcell) by 4 according to Equations (2) and (3):[5b] C g cell ðF g¢1 Þ ¼
C g2E ðF g¢1 Þ ¼ 4 C g cell
ð3Þ
The energy density (E) and power density (P) were calculated according to Equations (4) and (5): E ðWh kg¢1 Þ ¼
The carbon products were characterised by SEM (Schottky Field Emission, JEOL 7100), TEM (JEOL, 1010), XRD (Bruker D8 Advance X-ray diffractometer using CuKa radiation), XPS (VG Scientific, ESCALAB220i-XL using a mono-chromated AlKa excitation source), NMR (Bruker AMX 300 spectrometer at a 31P frequency of 242 MHz) and FTIR spectroscopy (Thermo Scientific Nicolet 6700 spectrometer). To investigate the textural properties, sorption isotherms of N2 at ¢196 8C and CO2 at 0 8C were measured by using a Micromeritics Tristar II 3020 instrument. The specific surface areas (SBET) were obtained using the BET method, the total pore volumes (Vtotal) at a N2 relative pressure of 0.98 and the micropore volumes (Vmic) from a tplot method (carbon black STSA reference) were obtained from the N2 isotherms. Micropore surface areas (SCO2 ) and micropore volumes (VCO2 ) were determined from CO2 isotherms using a Dubinin– Astakhov method. The pore size distributions from N2 and CO2 were obtained using the density functional theory model for carbon slit pores (TriStarII 3020 V1.03).
PðW kg¢1 Þ ¼
ð5Þ
Keywords: carbon · doping · electrochemistry · graphene · phosphorus [1] a) C. H. Xu, B. H. Xu, Y. Gu, Z. G. Xiong, J. Sun, X. S. Zhao, Energy Environ. Sci. 2013, 6, 1388 – 1414; b) L. L. Zhang, X. S. Zhao, Chem. Soc. Rev. 2009, 38, 2520 – 2531; c) X. Huang, X. Y. Qi, F. Boey, H. Zhang, Chem. Soc. Rev. 2012, 41, 666 – 686; d) C. Lee, X. Wei, J. W. Kysar, J. Hone, Science 2008, 321, 385 – 388. [2] H. Cao, X. Zhou, Z. Qin, Z. Liu, Carbon 2013, 56, 218 – 223. [3] Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach, R. S. Ruoff, Science 2011, 332, 1537 – 1541. [4] a) J. H. Lee, N. Park, B. G. Kim, D. S. Jung, K. Im, J. Hur, J. W. Choi, ACS Nano 2013, 7, 9366 – 9374; b) N. Xiao, D. M. Lau, W. H. Shi, J. X. Zhu, X. C. Dong, H. H. Hng, Q. Y. Yan, Carbon 2013, 57, 184 – 190; c) B. Zheng, T. W. Chen, F. N. Xiao, W. J. Bao, X. H. Xia, J. Solid State Electrochem. 2013, 17, 1809 – 1814; d) Y. Wen, B. Wang, C. Huang, L. Wang, D. Hulicova-Jurcakova, Chem. Eur. J. 2015, 21, 80 – 85. [5] a) D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z. H. Zhu, G. Q. Lu, Adv. Funct. Mater. 2009, 19, 1800 – 1809; b) D. Hulicova-Jurcakova, A. M. Puziy, O. I. Poddubnaya, F. Suarez-Garcia, J. M. D. Tascon, G. Q. Lu, J. Am. Chem. Soc. 2009, 131, 5026 – 5027; c) M. LatorreSnchez, A. Primo, H. Garcia, Angew. Chem. Int. Ed. 2013, 52, 11813 – 11816; Angew. Chem. 2013, 125, 12029 – 12032; d) S. Some, J. Kim, K. Lee, A. Kulkarni, Y. Yoon, S. Lee, T. Kim, H. Lee, Adv. Mater. 2012, 24, 5481 – 5486; e) C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Adv. Mater. 2013, 25, 4932 – 4937; f) X. Fan, C. Yu, J. Yang, Z. Ling, J. Qiu, Carbon 2014, 70, 130 – 141. [6] Z. Li, G. Wu, S. Deng, S. Wang, Y. Wang, J. Zhou, S. Liu, W. Wu, M. Wu, Chem. Eng. J. 2016, 283, 1435 – 1442.
Calculation of specific capacitances, energy density and power density The specific capacitances of electrodes in the three-electrode setups were calculated according to Equation (1): ð1Þ
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IðAÞ DUðVÞ 2 mcell ðkgÞ
ð4Þ
The authors acknowledge the Australian Research Council for its financial support through ARC DP and LP programs and the facilities and technical assistance of the Australian Microscopy&Microanalysis Research facility at the Centre for Microscopy&Microanalysis at the University of Queensland.
A carbon electrode slurry was prepared by mixing 90 wt % carbons (N/P-G or un-G), 5 wt % carbon black and 5 wt % polyvinylidene fluoride dissolved in N-methyl pyrrolidone. This slurry was painted on a titanium foil (current collector) with 1 Õ 1 cm2 of carbons. Electrodes were dried at 110 8C in vacuum overnight. The two-electrode cells were constructed from two carbon electrodes that faced each other separated by a glassy fibre paper. Three-electrode cells were assembled by assigning one carbon electrode as a working electrode, the other carbon electrode as a counter electrode and a Ag/AgCl reference electrode was inserted into the electrolyte. Both cyclic voltammetry and galvanostatic charge–discharge curves were measured in 1 m H2SO4 aqueous electrolyte by using a Solartron 1480 Multistat station.
IðAÞ Dt ðsÞ mðgÞ DUðVÞ
1 C DU2 ðVÞ 2 3:6 g cell
Acknowledgements
Electrode preparation and electrochemical measurements
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ð2Þ
in which mcell is the total weight of active material in both electrodes.
Characterisation
C g3E ðF g¢1 Þ ¼
IðAÞ Dt ðsÞ mcell ðgÞ DUðVÞ
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