View Article Online View Journal
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: H. Huang, G. Luo, L. Xu, C. Lei, Y. Tang, S. Tang and Y. du, Nanoscale, 2014, DOI: 10.1039/C4NR05776G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/nanoscale
Page 1 of 9
Journal Name
Nanoscale
Dynamic Article Links ►
Cite this: DOI: 10.1039/c0xx00000x
View Article Online
DOI: 10.1039/C4NR05776G
ARTICLE TYPE
www.rsc.org/xxxxxx
NH3 assisted Photoreduction and N-doping of Graphene Oxide as High Performance of Electrode Material for Supercapacitors Application Haifu Huang, a Guangsheng Luo,a, b Lianqiang Xu,a Chenglong Lei,a Yanmei Tang, a,c Shaolong Tang *a and Youwei Dua
10
15
20
25
30
35
40
45
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Nitrogen-doped graphene was synthesized by simple photoreduction of graphene oxide (GO) deposited on nickel foam under NH3 atmosphere. The combination of photoreduction and NH3 not only make the GO to be reduced in a short time but induce nitrogen doping easily. The nitrogen doped content of NrGO@NF is high up to 5.99 at% under irradiation time for only 15 min. The nitrogen-doped graphene deposited on Ni foam (N-rGO@NF) can be directly used as electrode for supercapacitors without any conductive agents and polymer binders. In the electrochemical measurement, N-rGO@NF displays remarkable electrochemical performance. In particular, the N-rGO@NF irradiated for 45 min at a high current density of 92.3 A g-1 maintained about 77% retention (190.4 F g-1) of its initial specific capacitance (247.1 F g-1 at 0.31 A g-1). Furthermore, the stable voltage window can be extended to 2.0 and 1.5 V by using the Li2SO4 and a mixed Li2SO4/KOH electrolyte, and the maximum energy density was high up to 32.6 and 21.2 Wh kg-1, respectively. But as compared to the Li2SO4 electrolyte, a mixed electrolyte (Li2SO4/KOH) is very well to balance the relationship between the high energy densities and high power densities.
1. Introduction With the growing demand for advanced, low-cost, and environmentally friendly energy conversion and storage devices, supercapacitors have attracted much interest as a new type of energy storage device.1-4 Compared to the batteries, supercapacitors can provide unique electrochemical properties such as high power density, fast charging-discharging rate, excellent cycle stability and low maintenance cost, due to their mechanism of energy storage that store and release energy through reversible adsorption/desorption of ion or highly reversible Faradic redox reaction at the interface between the electrode and the electrolyte.5 Although supercapacitors have relatively low energy densities, they play a very important role for those needing high power and excellent cycle stability, such as mobile electronic devices, electric/hybrid vehicles, uninterruptible power, and military devices. 2, 6 In order to improve the energy densities of supercapacitors, considerable efforts have been devoted to increasing the specific capacitance of electrode materials or the working voltage in currently. 7-10 But it is enormous challenges for supercapacitors to improve the energy density without sacrificing the performance of the high power densities. To achieve this goal, the high performance electrode materials are critical for supercapacitors with high energy densities.4, 7, 11-12 Graphene has attracted much interest for their superior electrical conductivity, exceptionally high surface area, and
This journal is © The Royal Society of Chemistry [year]
50
55
60
65
70
excellent mechanical flexibility, thus has been recognised as ideal candidates for high performance electrode materials.12-16 However, cost-effective and large-scale synthesis of graphene is a big problem for potential applications. Fortunately, reduced graphene oxide (rGO) can be obtained using the graphene oxide (GO) synthesized from inexpensive graphite powders as a precursor at low cost. At present, graphene oxide is often reduced to form reduced graphene oxide with the help of the chemical reducing agents or the thermal treatment at high temperatures. After the chemical reducing agents or the thermal treatment, a large portion of oxygen-containing functional groups are removed effectively. However, there are some disadvantages for chemical reductions and thermal treatments. For example, chemical reductions involved toxic agents (such as hydrazine and NaBH4), which could potentially bring some risks about environmental issues and safety of production as well as introduce impurities in the reduced graphene oxide;[17] thermal treatments require high temperatures lead to waste more energy and also is not suitable for GO film grown the substrate with low melting point.18 Thus, it is urgent to develop a simpler, mild and environmentally friendly method for extending the applications of GO. In the past few years, photoreduction of GO has emerged as a very important branch of GO reduction methodology. Various of photoreduction have been developed including photothermal reduction, 19-20 photochemical reduction 21-23 and laser reduction. 24-28 As compared to the chemical reductions and thermal treatments, photoreduction approaches have been attracted many attentions due to the advantages of high efficiency, [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
5
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
clean, low cost, and tuneable reduction degree via irradiation. It is noteworthy that it has more advantages for photoreduction on the fabrication and integration design of graphene based microdevices such as direct patterning for microdevice fabrication, flexible electronic devices. 19, 24-25, 27 In this work, the combination of photoreduction and NH3 gas was applied to prepare nitrogen-doped graphene deposited on nickel foam as electrode for supercapacitors. To our knowledge, there are few reports about the photochemical doping of graphene with nitrogen. Firstly, GO was easy coated on nickel foam framework without any binder by taking advantage of Van der Waals attractions between the GO and nickel foam. Then, the GO deposited on nickel foam (GO@NF) was irradiated by a highpressure Hg lamp under NH3 atmosphere. This method not only makes the GO to be reduced in a short time but induce nitrogen doping easily. The nitrogen doped content is high up to 5.99 at% under irradiated 15 min. Furthermore, N-rGO@NF displays remarkable electrochemical performance.
50
55
60
65
2. Experimental 20
2.1. Preparation of N-rGO@NF eleltrode. Graphite oxide (GO) was prepared by a modified Hummers′ method. Ni foam was carefully cleaned treated with acetone and hydrochloric acid to remove contaminants, and then washed in sequence with deionized water and absolute ethanol.
70
75
80
Page 2 of 9
All the electrochemical measurements were performed in a two-electrode cell configuration. Two identical N-rGO@NF View Article Online electrodes (1.0 cm×1.0 cm each) were separated by a thick DOI: 10.1039/C4NR05776G -1 separator (NKK TF45, 40 μm). 6 mol L KOH, 1 mol L-1 Li2SO4, and 1 mol L-1 mixed Li2SO4/KOH aqueous solution (consist of 1 mol L-1 Li2SO4 and 1 mol L-1 KOH) were used as the electrolyte. Cyclic voltammetry (CV), galvanostatic charge−discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical workstation (CHI 660D, Chenhua Instruments, China). All electrodes are without any binder or conduct additive. The gravimetric specific capacitance was obtained from the discharge process according to the following equation: Cs=4I / ( m dV/dt) where I is the applied current (A), m is the total graphene mass of the two electrodes (g), and dV/dt is the slope of the discharge curve(V s-1). The energy density (E) and power density (P) in the Ragone plots were calculated using the following equations: E=Cs∆V2/8 P=E/∆t ∆V is the sweep potential window, and Δt is the discharge time. 2.3 Material Characterization The morphology of N-rGO@NF was characterized by scanning electron microscope (SEM, JSM-6510, JEOL Co. Ltd. Japan). X-ray diffraction (XRD) patterns were recorded using on X-ray diffractometer (Bruker D8, Bruker, Germany) with Cu–Kα radiation in the range of 5–80°. The X-ray photoelectron spectroscopy (XPS) experiments were carried out using Phi 5000 VersaProbe Scanning ESCA Microprobe (Ulvac-Phi, Inc., Japan). Electrochemical performance measurements were carried out using an electrochemical workstation (CHI 660D, Chenhua Instruments, China).
3. Results and discussion 25
Fig. 1 Schematic illustration of the N-rGO@NF photoreduction system
30
35
40
45
The N-rGO@NF eleltrode can be easily prepared using by a modified technique according to our previous work.23 The preparation of N-rGO@NF eletrode mainly consists of two steps. First, the GO coated on Ni foam was carried out by simple centrifuged spin coating process that Ni foam was put into a centrifugal tube containing GO dispersions (4 mg/mL) and then centrifuged at 500 rpm. Second, NH3 assisted photoreduction Ndoped process is illustrated in Fig. 1. A GO@NF sheet (1.0×2.5cm) was placed inside a quartz tube, and irradiated for different time with light from a high-pressure Hg lamp (500 W) in NH3 (99%) atmosphere. For comparison, rGO@NF sheet without N-doping was prepared by irradiating for 30 min with light from a high-pressure Hg lamp in Ar atmosphere. In order to accurately measure the mass of N-rGO, NrGO@NF sheet was put into 1 M FeCl3 at room temperature for 72 h to completely remove the Ni foam and then gently in several baths of deionized water for obtain a whole N-rGO sheet without Ni foam support (Fig S1). To ensure no any residuary of Ni, NrGO sheet also can be further dipped in the 1M HCl for 24h, and finally rinsed with deionized water and ethanol. 2.2 Electrochemical Measurement 2 | Journal Name, [year], [vol], 00–00
85
Fig. 2 SEM images at different magnifications: (a)-(c) nickel foam, (d)-(f) N-rGO@NF irradiated for 30 min and (g)-(i) N-rGO@NF irradiated for 120 min
3.1 Morphology and Chemical Structure The surface morphologies of Ni foam and N-rGO@NF were observed by SEM as shown in Fig. 2. It can be seen from the Fig. 2a that bare Ni foam has a 3D continuous and interconnected This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
Nanoscale
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
20
Nanoscale
network structure. The Fig. 2b and 2c further show that the surface of Ni skeleton is relatively rough with reticular patterns. The SEM images of N-rGO@NF irradiated for 30 and 60 min under NH3 atmosphere (Fig. 2d-i) show that N-rGO film has been successfully deposited on the micropores of Ni foam by comparing to the bare nickel foam (Fig. 2a-c). Although there have some cracks (Fig. 2e and 2h), a smooth translucent N-rGO film with some wrinkles was still tightly coated on the surface of Ni skeleton (shown in the Fig. 2f and 2i), suggesting good contact between N-rGO sheets and metal substrate. Therefore, the strategy that 2D ultrathin film was adhered to the skeleton of nickel foam tightly by Van der Waals attractions can be overcome the fragile features of photoreduction graphene oxide film, which hinder its post-processing as an electrode material (see Fig. S2). As compared to the photoreduction graphene oxide film with fragile features, network structure of N-rGO@NF is similar to Ni foam with a continuous and interconnected 3D network, which is beneficial for the contact between electrolyte and electrode material that facilitate the efficient access of electrolyte ions to the graphene surface and shorten the ion diffusion path, and for good contact between N-rGO sheets and metal current collector which greatly reduce contact resistance.
50
55
60
65
70
75
80
25
30
35
40
45
Fig. 3 XRD patterns of GO and N-rGO@NF irradiated under NH3 atmosphere for from 15 to 60 min
The XRD spectrum is used to characterize the structure of the prepared N-rGO. XRD spectrum in Fig. 3 shows that a typical diffraction peak of GO at 10.4° completely disappears after irradiation under NH3 atmosphere from 15 to 60 min and a weak broad peak relating to N-rGO appeared at around 24.5°, which is in accord with the feature peak of graphite. The peak with weaker intensity and broad shape indicates it could be related to the removal of oxygen containing groups from the GO during the photoreduction N-doped process. The result suggests the GO had been reduced effectively even if UV light cannot be transmitted in to the inside of nickel foams, due to the existence of both photochemical reduction and photothermal reduction or thermal reduction simultaneously. XPS spectrum is further used to characterize the elemental composition of samples irradiated from 15 to 90 min under NH3 atmosphere (shown in Fig. 4). The GO shows obvious C1s and O1s peak but no N signal in the XPS full scan spectrum (Fig. 4a). However, it is visible that there are clear N signals in the XPS spectrum of the N-rGO@NF irradiated from 15 to 90 min under NH3 atmosphere (Fig. 4a), indicating the introduction of N to NThis journal is © The Royal Society of Chemistry [year]
85
90
95
100
rGO@NF by the simple photochemical doping treatment using NH3 gas as nitrogen source. The C/O atomic ratio for NView Article Online rGO@NF irradiated from 15 to 90 min DOI: increases from 4.64 to 10.1039/C4NR05776G 7.26 according the XPS analysis (Fig. 4b), which is much higher than that of pristine GO (2.3). The increase of C/O atomic ratio indicates the de-oxygenation or reduction of GO after the light irradiation treatment, which agrees with the results of XRD analysis. The nitrogen doping content calculated from XPS analysis are 5.99, 6.20, 7.97, and 7.80 at% under irradiation time of 15, 30, 60, and 90 min, higher than that of N-doped graphene treated with NH3 at high temperatures. 29-32 It suggests that the high nitrogen doping content could be carried out at a very short time by the simple photochemical doped treatment, due to the fact that the enriched oxygen functional groups in GO favor the reactions with NH3 and C–N bond formation.23,32 To understand the role of NH3 during photoreduction process, as-prepared GO@NF sheet had been irradiated for 30 min under Ar atmosphere for comparison. The comparison of XPS full scan spectrum under NH3 and Ar atmosphere is shown in the Fig. 4c. It had confirmed by XPS spectrum that GO@NF has been effectively reduced by irradiation under Ar atmosphere, and the C/O atomic ratio is 4.51 according the XPS analysis. But under NH3 atmosphere, the C/O atomic ratio of N-rGO@NF can reach high up to 5.80 at the same irradiation time (30 min). Moreover, both reduction of GO and nitrogen doping occur simultaneously. The results indicates that NH3 gas and light irradiation promote each other in the nitrogen doping and reduction of GO, which NH3 accelerate light irradiation reduction of GO and light irradiation can induce nitrogen doping easily. The high-resolution C1s XPS spectrum of the GO (Fig. 4d) shows two separated peaks consisted two main components, which can be fitted to four components including C=C/C–C (284.6 eV) and C–O (hydroxyl and epoxy, 286.7 eV)species and two minor components from C=O (carbonyl, 287.7 eV) and O– C=O (carboxyl, 288.9 eV) species. After photo-irradiation under NH3 atmosphere, the high-resolution C1s XPS spectrum of the NrGO@NF (Fig. 4e) shows a typical single peak with a small tail in the higher binding energy region. The C=C/C–C bonds become dominant and the hydroxyl and epoxy species of C-O, and C=O reduced significantly, indicating that most of the hydroxyl and epoxy functional groups on the GO nanosheets were successfully removed. Moreover, a new peak appears at 285.8 eV (attributed to the C-N bond), due to the doping of nitrogen atoms. In order to determine the possible bonding configurations of N-atoms in NrGO@NF, the high-resolution of N1s spectrum (Fig. 4f) was employed. From the N1s XPS spectrum, the four kinds of nitrogen were possible doped, located at 398.4, 399.5, 400.4,and 401.3 eV, which can be assigned to pyridine-like(N-6), aminolike (N-A), pyrrolic-like (N-5), and graphitic-like nitrogen (N-Q), respectively. 23, 33-34 3.2 Electrical measurements and electrochemical properties The electrochemical performance of N-rGO@NF was evaluated using a symmetric two-electrode configuration at 6 mol L-1 KOH electrolytes with in a range of 0-1.0 V, as shown in the Fig. 5-7. Fig. 5a shows the influences of irradiation time under NH3 atmosphere for performance of N-rGO@NF. When irradiated for only 15 min, CV curve of N-rGO@NF exhibites a bad quasi-rectangular shape. However, the time of light irrradiation increases, CV curves exhibite a good quasiJournal Name, [year], [vol], 00–00 | 3
Nanoscale Accepted Manuscript
Page 3 of 9
Nanoscale
Page 4 of 9
View Article Online
5
10
15
20
Fig. 4 (a) XPS full scan spectrum of GO and N-rGO@NF irradiated for from 15 to 120 min. (b) The C/O atomic ratio. (c) The comparison of XPS full scan spectrum under NH3 and Ar atmosphere, respectively. (d) High resolution C1s XPS spectra of GO. (e) High resolution C1s XPS spectra of NrGO@NF. (f) High resolution N1s XPS spectra of N-rGO@NF.
Fig. 5. (a) CV curves of N-rGO@NF irradiated for 15, 30, 45, and 60 min under NH3 atmosphere at scan rate of 50 mV s-1, (b) the comparison of CV curves at scan rate of 50 mV s-1 for the N-rGO@NF with N-doping and rGO@NF without N-doping irradiated for 30 min under NH3 and Ar atmosphere, respectively. (c) and (d) CV curves of N-rGO@NF irradiated for 30, 45, and 60 min under NH3 atmosphere at scan rate of 500 and 1000 mV s-1, respectively. (e) and (f) CV curves of N-rGO@NF irradiated for 30 and 45 min under NH3 atmosphere at scan rate of 2000 mV s-1, respectively.
rectangular shape, corresponding to a nearly ideal double-layer capacitor response, own to the improvement of conductivity when N-rGO irradiated with longer time. In addition, with irradiation time increasing from 30 to 60 min, the area of CV curve has small change, suggesting that enough irradiating time ( ≥ 30 min) is necessary for the good electrochemical performance of N-rGO@NF but the effect of irradiation with longer time is limited for improvement of the specific capacitances. For comparison, rGO@NF sheet without N-doping was 4 | Journal Name, [year], [vol], 00–00
25
30
prepared by irradiating for 30 min under Ar atmosphere. Notably, there is an obvious difference in the shape of the CV curves with scan rate of 50 mV s-1 for the as-prepared GO@NF irradiated under Ar and NH3 atmosphere with the same time (30 min), as shown in Fig. 5b. The CV curve (irradiated under Ar atmosphere) obviously deviates from the quasi-rectangular shape compared with that of the N-rGO@NF (irradiated under NH3 atmosphere), suggesting more effective reduction of GO by irradiating in NH3 than in Ar with the same time, consistent with XPS data. The area enclosed by the CV curve of N-rGO@NF was sufficiently larger This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
DOI: 10.1039/C4NR05776G
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
20
25
Nanoscale
than rGO@NF irradiated under Ar atmosphere, indcating the NrGO@NF have higher specific capacitances than the rGO@NF. The improved electrochemical performance of N-rGO@NF may be attributed to the introduction of N-enriched functional groups into the carbon network like previous work reported.29-30, 35 Nitrogen doping can improve the electrical conductivity, enrich the free charge-carrier density and incerease holes on graphene sheets, which facilitate electrolyte penetration and ion transport and increase the pseudocapacitance effect. Further, the CV curves of N-rGO@NF irradiated different time under scan rates of 500 and 1000 mV s-1 (Fig. 5c and 5d) still keep a typical quasirectangular shape with small distortions, suggesting good chargetransfer efficiency at electrode/electrolyte interfaces following the mechanism of double-layer capacitors (EDLCs). Even at a high scan rate of 2000 mV s-1, the CV curves of N-rGO@NF (Fig. 5e and 5f) are still close to rectangular. These results indicate that NrGO@NF has a high rate capability and a small equivalent series resistance (ESR) of electrodes. Fig. 6 shows the galvanostatic charge–discharge (GCD) curves of the N-rGO@NF (irradiated for 15, 30, 45, and 60 min, respectively) measured in a range of 0-1 V at different current densities. The shapes of all curves are approximate to isosceles triangle, which is characteristic of the mechanism of double-layer capacitors (EDLCs). The specific capacitances of N-rGO@NF irradiated for 30, 45, and 60 min are high up to 236.6, 239.2, and
30
35
40
45
50
55
60
65
70
Fig. 6 GCD curves at different current densities of N-rGO@NF irradiated for 15 min (a), 30 min (b), 45 min (c) and 60 min (d) under NH3 atmosphere, respectively.
75
229.2 F g-1 at a low current density of 0.57, 0.62, and 0.62 A gaccording to the discharging curves, respectively, which are View Article Online higher than that of photocatalytically reduced oxide (220 DOI: graphite 10.1039/C4NR05776G -1 22 F g ) and laser-scribed graphene thin film (202 F g-1). 28 This also is high to or comparable with values reported in the literature for graphene deposited on nickel foam, 36-37 N-doped graphene (using ammonia gas as nitrogen sources), 29-30 microwaveexfoliated graphite oxide,38-39 and thermal exfoliation graphene nanosheets. 18 More calculation of the specific capacitances under different current densities is provided in Fig. 7a. It can see that the NrGO@NF irradiated for 30 min had the highest specific capacitance with 252.3 F g-1 at the current density of 0.29 A g-1. When the charge/discharge current density increases, NrGO@NFs irradiated for 30, 45, and 60 min show excellent rate capability. In particular, the specific capacitance of N-rGO@NF irradiated for 45 min still reach up to 190.4 F g-1 at a high current density of 92.3 A g-1 and maintains about 77% retention of its initial specific capacitance (247.1 F g-1, 0.31 A g-1). The excellent electrochemical performances of N-rGO@NFs can be attributed to their continuous and interconnected 3D network similar to Ni foam, high electronic conductivity of nitrogen-doped graphene, and good contact that N-rGO sheets was directly attach to the skeleton of nickel foam by Van der Waals attractions without any binder which greatly reduce contact resistance. Although the nitrogen doping content of N-rGO@NF irradiated for 15 min is about 5.99 at% and only slightly smaller than that of other N-rGO@NF irradiated for longer time, its specific capacitance declines obviously with the increase of current density, due to bad reduction effect of GO at a short irradiation time that exits high oxygen levels leading to poor conductivity. However, the nitrogen doping content increases slowly with longer irradiation time, the specific capacitances and rate capability have been improved greatly. At the same time, we also see that that performance of the N-rGO@NF irradiated for 30, 45, and 60 min with different nitrogen doping only has slightly changed. Even the specific capacitance of the NrGO@NF irradiated for 60 min has slight smaller than that of those irradiated for 30 and 45 min due to the more expansion of N-rGO film when irradiated for longer time (as shown in Fig. 2 h), which cause some graphene sheet to separate from metal substrate and thus induce loose contact between N-rGO sheets 1
Fig.7 (a) Gravimetric specific capacitance of N-rGO@NF irradiated for 15, 30, 45, and 60 min at different current densities. (b) Nyquist plots of NrGO@NF irradiated for 15, 30, 45, and 60 min. Inset: High-frequency region of the plots. (c) Cycle stability tests for N-rGO@NF at a current density of 7.14 A g-1. Inset: CV curves of the first, the 1,000th and 3,000th cycles for the device, respectively.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5
Nanoscale Accepted Manuscript
Page 5 of 9
Nanoscale
Page 6 of 9
View Article Online
5
10
15
20
25
30
35
Fig.8 The electrochemical measurement of N-rGO@NF at 1 mol L-1 Li2SO4 electrolyte: (a) CV curves at different working voltage ranging from 1.0 to 2.0 V at a scan rate of 50 mV s-1, (b) GCD curves at different working voltage ranging from 1.0 to 2.0 V at 0.83 A g-1, (c) CV curves in the voltage range of 0–2.0 V at different scan rates from 100 to 500 mV s-1, (d) GCD curves in the voltage range of 0– 2.0 V at different current densities from 0.50 to 8.33 A g-1. and metal current collector. The results suggest that the increases of nitrogen doping content after radiation for 30 min has small improvement for the performance of N-rGO@NF, which would be affected by other factors such as oxygen levels, and contact between N-rGO and nickel foam. Therefore, the high performance graphene materials can be fabricated by the combination of photoreduction and NH3, and the optimization of UV light radiation time may be about 45 min. The electrochemical impedance spectroscopy (EIS) of the supercapacitors was further measured at open-circuit potential and frequencies ranging from 0.01Hz to 100 kHz with a 5 mV ac amplitude in 6 mol L-1 KOH, and the corresponding Nyquist plots are shown in Fig. 7b. All Nyquist plots show a near-vertical curve at the low frequency region, indicating good capacitor behaviour. At the high frequency region, N-rGO@NF irradiated for 15 min shows a large semicircle, a characteristic of charge transfer resistance (Rct). However, there is no obvious semicircle for the N-rGO@NF (irradiated for 30, 45, and 60 min, respectively), indicating an ideal capacitor behaviour and low ion-transport resistance. Furthermore, the first intersection of the semicircle with the real axis at the high frequency region corresponds to the equivalent series resistance (ESR), which comprises of the resistance of aqueous electrolyte, the intrinsic resistance of the composite material and the contact resistance at the electrode interface. The RESR of N-RGO@NF irradiated for 15, 30, 45, and 60 min at the real axis was about 0.93, 0.95, 0.76, and 0.86 Ω, respectively. Moreover, N-rGO@NF material exhibits excellent cycle stability, which is an important requirement for practical applications. Typically, the N-rGO@NF irradiated for 30 min 6 | Journal Name, [year], [vol], 00–00
40
45
50
55
60
65
was chosen to evaluate the cycle stability by repeating the GCD test at a current density of 7.14 A g-1, as shown in Fig. 7c. The specific capacitance retained at a high level with a specific capacitance of 208 F g-1 after 3,000 cycles (the capacitance retention was about 98.5%), and CV curves of 1th and 3,000th cycle inserted in Fig. 7c had only a small change, demonstrating the excellent cycle stability of N-rGO@NF. According to the equation E=1/2(CV2), the energy density of a supercapacitor can be improved by increasing the specific capacitance of electrode materials (C) or the working voltage (V). We try to improve the specific capacitance and energy density of of N-rGO@NF by using the neutral electrolytes. As compared to the acidic and alkaline electrolytes, neutral electrolytes are relative cheap, environmentally friendly, non-corrosive and suitable for various of current collectors. What is more important is that the neutral electrolyte can provide a much higher working voltage up to 1.6-2.2 V to improve the energy density of a supercapacitor. 9, 40-41 Therefore, the electrochemical performance of N-rGO@NF irradiated for 60 min was investigated using a symmetric two-electrode configuration at 1 mol L-1 Li2SO4 electrolyte and a mixed electrolyte (1 mol L-1 Li2SO4/KOH) shown in the Fig. 8 and 9, respectively. Fig. 8a shows the CV curves (with a scan rate of 50 mV s-1) measured using 1 mol L-1 Li2SO4 electrolyte at various voltage ranges. It can be obviously found that all CV curves exhibit a rectangular shape and the stable voltage window can be extended to 2.0 V. It is also further confirmed by the galvanostatic charge– discharge (GCD) curves at various voltage ranges shown in Fig. 8b. The CV curves at different scan rates ranging from 100 to 500 mV s-1 and GCD curves at different current densities ranging This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
DOI: 10.1039/C4NR05776G
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
Nanoscale
from 0.50 to 8.33 A g-1 between 0 and 2 V are shown in Fig. 8c and 8d, respectively. The CV curves exhibit typical quasirectangular shape and the GCD curves look nearly symmetric, indicating good charge propagations at the electrode interfaces following the mechanism of electric double-layer capacitors. The specific capacitances are reaching up to 235.0, 217.3, 192.0 and 179.3 F g-1 at a current density of 0.50, 0.83, 2.5 and 5.0 A g-1, respectively. More calculation of the specific capacitances at different current densities is shown in Fig. 10a. It is obviously that the value of specific capacitances measured at 1 mol L-1 Li2SO4 electrolyte is lower than that of the specific capacitances measured at 6 mol L-1 KOH electrolytes but the voltage window can be extended to 2.0 V, which energy density is improved effectively. In addition, specific capacitances measured at 1 mol L-1 Li2SO4 electrolyte decline obviously with increase of the current densities, indicating a bad rate capability at high current density.
20
25
30
In order to balance the energy density and high rate capability of N-rGO@NF, a mixed electrolyte (1 mol L-1 Li2SO4/KOH) was View Article Online used as a compromise solution. The stableDOI: voltage window can be 10.1039/C4NR05776G extended to 1.5 V according to the CV and GCD curves at various voltage ranges shown in the Fig. 9a and 9b. The high voltage window above water decomposition voltages (1.23V) may be attributed to the existence of Li2SO4 in mixed electrolyte. In pure neutral electrolyte Li2SO4, because H+ and OH- are in equilibrium at neutral pH, which do not favour hydrogen or oxygen evolution, and more important that both alkali metal cations (Li+) and sulfate anions have strong solvation, lead to the formation of the high over-potential for di-hydrogen evolution, so it is possible that the voltage window can extend to 2 V.9, 41 Similarly, there also exits strong solvation of ions in mixed electrolyte, the voltage window should larger than water decomposition voltages (1.23V), but less than 2V, which may be due to the KOH electrolyte that weakens the solvation of ions.
35
Fig. 9 The electrochemical measurement of N-rGO@NF at a mixed electrolyte (1 mol L-1 Li2SO4 /KOH): (a) CV curves at different working voltage ranging from 1.0 to 1.5 V at a scan rate of 50 mV s-1, (b) GCD curves at different working voltage ranging from 1.0 to 1.5 V at 0.77 A g-1, (c) CV curves in the voltage range of 0–1.5 V at different scan rates from 100 to 500 mV s-1, (d) GCD curves in the voltage range of 0–1.5 V at different current densities from 0.46 to 9.23 A g-1
40
Fig.10 The comparison of the electrochemical measurement at 1 mol L-1 Li2SO4 and a mixed electrolyte (1 mol L-1 Li2SO4/KOH): (a) Gravimetric specific capacitance of N-rGO@NF at different current densities; (b) Nyquist plots of N-rGO@NF. Inset: High-frequency region of the plots. (c) The Ragone plots of N-rGO@NF measured at 6 mol L-1 KOH, 1 mol L-1 Li2SO4 and a mixed electrolyte (1 mol L-1 Li2SO4/KOH).
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7
Nanoscale Accepted Manuscript
Page 7 of 9
Nanoscale
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
20
25
30
35
40
45
60
65
70
high power densities, high rate capability performance and View Article Online excellent cycle stability. The maximum of specific capacitance is DOI: 10.1039/C4NR05776G -1 -1 up to 252.3 F g at the current density of 0.29 A g in the KOH electrolyte. The results of the electrochemical measurement showed that (i) enough irradiating time ( ≥ 30 min) is necessary for the good electrochemical performance but the effect of irradiation with more long time is limited for improvement of the specific capacitances, (ii) To achieve the best rate capability of NrGO@NF, the best irradiation time could be 45 min. (iii) As compared to the KOH electrolyte, the stable voltage window can be extended to 2.0 and 1.5 V by using the Li2SO4 and a mixed Li2SO4/KOH electrolyte, and the maximum energy density was high up to 32.6 and 21.2 Wh kg-1, respectively.
Acknowledgements
75
This work was supported by the National Key Project of Fundamental Research of China (Grant No. 2012CB932304), the Innovation Program for Doctoral Research of Jiangsu Province (grant no KYLX_0026) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Notes and references 80
85
90
a Jiangsu Key Laboratory for Nanotechnology, Nanjing National Laboratory of Microstructures and Department of Physics, Nanjing University, Nanjing, 210093, P. R. China. Fax: 025-83595535; Tel: 02583593817; E-mail: [email protected] b Academy of Space Technolgy, Nanchang University, Nanchang, 330031, People’s Republic of China c College of Physics and Technology, Guangxi Normal University, Guilin, 541004, People’s Republic of China †Electronic Supplementary Information (ESI) available: Digital photographs of nickel foam, GO deposited in nickel foam, N-rGO@NF, and N-rGO@NF electrodes (1.0×1.0 cm); Digital photographs: (1) NrGO@NF sheet was put into 1 M FeCl3 at room temperature dissolve the Ni metal and (2) a whole N-rGO sheet without Ni foam support after nickel etching; Photograph of film with fragile features after irradiated by a high-pressure Hg lamp (500 W) in Ar and NH3 atmosphere.. See DOI: 10.1039/b000000x/
95
1. 2. 100
105
3. 4. 5. 6. 7. 8.
4. Conclusion
50
55
In summary, nitrogen-doped graphene (N-rGO@NF) was synthesized by irradiation of GO deposited on nickel foam with the assistance of NH3 gas for different time from 15 to 120 min. NH3 gas and light irradiation promote each other in the nitrogen doped and reduction of GO, which NH3 accelerate light irradiation reduction of GO and light irradiation can induce nitrogen doped easily. In just 15 minutes, the nitrogen doped content and C/O atomic ratio are high up to 5.99 at% and 4.64. The N-rGO@NF irradiated for different time as electrodes have been studied for supercapacitors applications in detail, and exhibited remarkable electrochemical performance including 8 | Journal Name, [year], [vol], 00–00
9. 10. 110
11. 12. 13. 115
14. 15. 16.
J. R. Miller and P. Simon, Science, 2008, 321, 651-652. J. R. Miller and A. F. Burke, Electrochem. Soc. Interface, 2008, 17, 53-57. Y. Gogotsi and P. Simon, Science, 2011, 334, 917-918. P. Simon and Y. Gogotsi, Nat. mater., 2008, 7, 845-854. B. E. Conway, J. Electrochem. Soc., 1991, 138, 1539-1548. J. R. Miller, Science, 2012, 335, 1312-1313. E. Frackowiak, Phys. Chem. Chem. Phys., 2007, 9, 1774-1785. X. Yang, C. Cheng, Y. Wang, L. Qiu and D. Li, Science, 2013, 341, 534-537. K. Fic, G. Lota, M. Meller and E. Frackowiak, Energy Environ. Sci., 2012, 5, 5842-5850. H. Wei, X. Yan, S. Wu, Z. Luo, S. Wei and Z. Guo, J. Phys. Chem. C, 2012, 116, 25052-25064. G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 797828. Y. Sun, Q. Wu and G. Shi, Energy Environ. Sci., 2011, 4, 1113-1132. M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498-3502. Y. Huang, J. Liang and Y. Chen, Small, 2012, 8, 1805-1834. Y. B. Tan and J.-M. Lee, J. Mater. Chem., 2013, 1, 14814-14843. H. Huang, L. Xu, Y. Tang, S. Tang and Y. Du, Nanoscale, 2014, 6, 2426-2433.
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
5
Furthermore, Fig. 9c shows that the CV curves keep a good quasi-rectangular shape at different scan rates ranging from 100 to 500 mV s-1. To our surprise, the specific capacitances estimated from the discharging curves (Fig. 9d) are high up to 272.7, 253.4, 236.6 and 224.8 F g-1 at a current density of 0.46, 0.92, 2.31 and 4.62 A g-1, which higher than those measured at pure Li2SO4 or KOH electrolyte. The gravimetric specific capacitance at different current densities ranging from 0.46 to 38.5 A g-1 are shown in Fig. 10a. As expected, the rate capability at high current density has been improved significantly by using the mixed Li2SO4/KOH electrolyte replace the Li2SO4 electrolyte. The result is also supported by the electrochemical impedance spectroscopy (EIS) measured at open-circuit potential. Fig. 10b shows the Nyquist plot based on a frequency response analysis of frequencies ranging from 0.01 Hz to 100 kHz with a 5 mV ac amplitude in 1 mol L-1 Li2SO4 electrolyte and 1 mol L-1 Li2SO4/KOH mixed electrolyte, respectively. The RESR of NrGO@NF measured at 1 mol L-1 Li2SO4 electrolyte and 1 mol L-1 Li2SO4/KOH mixed electrolyte was about 1.56 and 2.03 Ω, respectively. However, N-rGO@NF shows a larger semicircle measured at 1 mol L-1 Li2SO4 electrolyte than 1 mol L-1 Li2SO4/KOH mixed electrolyte, indicating there exists a large charge transfer resistance (Rct) using 1 mol L-1 Li2SO4 solution as electrolyte that lead to a bad rate capability. The energy and power densities of N-rGO@NF measured at different electrolytes were calculated according to the discharge curves at different current densities. Their relationship was described by the Ragone plots (Fig. 10c). It is obviously that NrGO@NF measured at Li2SO4 electrolyte and a mixed electrolyte (Li2SO4/KOH) exhibit higher energy densities at different current densities than that measured at KOH electrolyte. The maximum energy density was 8.22, 32.6 and 21.2 Wh kg-1 at a power density of 0.16, 0.54 and 0.41 kW kg-1 for the N-rGO@NF measured at KOH, Li2SO4 and a mixed Li2SO4/KOH electrolyte, respectively. The N-rGO@NF measured at KOH, Li2SO4 electrolyte and a mixed electrolyte (Li2SO4/KOH) also maintain 78.2, 66.6 and 72.4 % of their energy density with a value of 6.43, 21.72 and 15.35 Wh kg-1 as the power density increases up to maximum with 58.3, 24.7 and 47.7 kW kg-1 at current density of 76.9, 16.7 and 38.5 A g-1, respectively. These results demonstrate that Li2SO4 electrolyte can improve the energy densities but sacrifice some high rate capability and high power densities, and a mixed electrolyte (Li2SO4/KOH) is very well to balance the relationship between the high energy densities and high power densities.
Page 8 of 9
5
10
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
15
20
25
30
65
Nanoscale
17. S. Park, J. An, J. R. Potts, A. Velamakanni, S. Murali and R. S. Ruoff, Carbon, 2011, 49, 3019-3023. 18. X. Du, P. Guo, H. Song and X. Chen, Electrochim. Acta, 2010, 55, 4812-4819. 19. L. J. Cote, R. Cruz-Silva and J. Huang, J. Am. Chem. Soc., 2009, 131, 11027-11032. 20. R. Mukherjee, A. V. Thomas, A. Krishnamurthy and N. Koratkar, ACS Nano, 2012, 6, 7867-7878. 21. G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 14871491. 22. H.-C. Huang, C.-W. Huang, C.-T. Hsieh, P.-L. Kuo, J.-M. Ting and H. Teng, J. Phys. Chem. C, 2011, 115, 20689-20695. 23. F. Liu, N. Tang, T. Tang, Y. Liu, Q. Feng, W. Zhong and Y. Du, Appl. Phys. Lett., 2013, 103, 123108. 24. Y. Zhang, L. Guo, S. Wei, Y. He, H. Xia, Q. Chen, H.-B. Sun and F.S. Xiao, Nano Today, 2010, 5, 15-20. 25. W. Gao, N. Singh, L. Song, Z. Liu, A. L. M. Reddy, L. Ci, R. Vajtai, Q. Zhang, B. Wei and P. M. Ajayan, Nat. Nanotech., 2011, 6, 496500. 26. L. Huang, Y. Liu, L.-C. Ji, Y.-Q. Xie, T. Wang and W.-Z. Shi, Carbon, 2011, 49, 2431-2436. 27. V. Strong, S. Dubin, M. F. El-Kady, A. Lech, Y. Wang, B. H. Weiller and R. B. Kaner, ACS Nano, 2012, 6, 1395-1403. 28. M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science, 2012, 335, 1326-1330. 29. C.-M. Chen, Q. Zhang, X.-C. Zhao, B. Zhang, Q.-Q. Kong, M.-G. Yang, Q.-H. Yang, M.-Z. Wang, Y.-G. Yang, R. Schlögl and D. S. Su, J. Mater. Chem., 2012, 22, 14076-14084. 30. L. L. Zhang, X. Zhao, H. Ji, M. D. Stoller, L. Lai, S. Murali, S. McDonnell, B. Cleveger, R. M. Wallace and R. S. Ruoff, Energy Environ. Sci., 2012, 5, 9618-9625. 31. M. Li, N. Tang, W. Ren, H. Cheng, W. Wu, W. Zhong and Y. Du, Appl. Phys. Lett., 2012, 100, 233112.
35
40
45
50
32. X. Li, H. Wang, J. T. Robinson, H. Sanchez, G. Diankov and H. Dai, J. Am. Chem. Soc., 2009, 131, 15939-15944. 33. J. Casanovas, J. M. Ricart, J. Rubio, F. Illas and J. M.Article JiménezView Online DOI: 10.1039/C4NR05776G Mateos, J. Am. Chem. Soc., 1996, 118, 8071-8076. 34. L. S. Zhang, X. Q. Liang, W. G. Song and Z. Y. Wu, Phys. Chem. Chem. Phys., 2010, 12, 12055-12059. 35. G. Luo, L. Liu, J. Zhang, G. Li, B. Wang and J. Zhao, ACS Appl. Mater. Interfaces, 2013, 5, 11184-11193. 36. J. Chen, K. Sheng, P. Luo, C. Li and G. Shi, Adv. Mater., 2012, 24, 4569-4573. 37. H. Huang, Y. Tang, L. Xu, S. Tang and Y. Du, ACS Appl. Mater. Interfaces, 2014, 13, 10248-10257. 38. Y. Zhu, S. Murali, M. D. Stoller, A. Velamakanni, R. D. Piner and R. S. Ruoff, Carbon, 2010, 48, 2118-2122. 39. 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 and R. S. Ruoff, Science, 2011, 332, 1537-1541. 40. X. Yang, Y.-S. He, G. Jiang, X.-Z. Liao and Z.-F. Ma, Electrochem. Commun., 2011, 13, 1166-1169. 41. Q. Gao, L. Demarconnay, E. Raymundo-Piñero and F. Béguin, Energy Environ. Sci., 2012, 5, 9611-9617.
55
60
A graphic entry for the Table of Contents (TOC)
The nitrogen-doped graphene deposited on nickel foam was prepared by combining the photo-irradiation and ammonia, and as high performance of electrode for supercapacitors application. 70
75
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 9
Nanoscale Accepted Manuscript
Page 9 of 9
Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: H. Huang, G. Luo, L. Xu, C. Lei, Y. Tang, S. Tang and Y. du, Nanoscale, 2014, DOI: 10.1039/C4NR05776G.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/nanoscale
Page 1 of 9
Journal Name
Nanoscale
Dynamic Article Links ►
Cite this: DOI: 10.1039/c0xx00000x
View Article Online
DOI: 10.1039/C4NR05776G
ARTICLE TYPE
www.rsc.org/xxxxxx
NH3 assisted Photoreduction and N-doping of Graphene Oxide as High Performance of Electrode Material for Supercapacitors Application Haifu Huang, a Guangsheng Luo,a, b Lianqiang Xu,a Chenglong Lei,a Yanmei Tang, a,c Shaolong Tang *a and Youwei Dua
10
15
20
25
30
35
40
45
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Nitrogen-doped graphene was synthesized by simple photoreduction of graphene oxide (GO) deposited on nickel foam under NH3 atmosphere. The combination of photoreduction and NH3 not only make the GO to be reduced in a short time but induce nitrogen doping easily. The nitrogen doped content of NrGO@NF is high up to 5.99 at% under irradiation time for only 15 min. The nitrogen-doped graphene deposited on Ni foam (N-rGO@NF) can be directly used as electrode for supercapacitors without any conductive agents and polymer binders. In the electrochemical measurement, N-rGO@NF displays remarkable electrochemical performance. In particular, the N-rGO@NF irradiated for 45 min at a high current density of 92.3 A g-1 maintained about 77% retention (190.4 F g-1) of its initial specific capacitance (247.1 F g-1 at 0.31 A g-1). Furthermore, the stable voltage window can be extended to 2.0 and 1.5 V by using the Li2SO4 and a mixed Li2SO4/KOH electrolyte, and the maximum energy density was high up to 32.6 and 21.2 Wh kg-1, respectively. But as compared to the Li2SO4 electrolyte, a mixed electrolyte (Li2SO4/KOH) is very well to balance the relationship between the high energy densities and high power densities.
1. Introduction With the growing demand for advanced, low-cost, and environmentally friendly energy conversion and storage devices, supercapacitors have attracted much interest as a new type of energy storage device.1-4 Compared to the batteries, supercapacitors can provide unique electrochemical properties such as high power density, fast charging-discharging rate, excellent cycle stability and low maintenance cost, due to their mechanism of energy storage that store and release energy through reversible adsorption/desorption of ion or highly reversible Faradic redox reaction at the interface between the electrode and the electrolyte.5 Although supercapacitors have relatively low energy densities, they play a very important role for those needing high power and excellent cycle stability, such as mobile electronic devices, electric/hybrid vehicles, uninterruptible power, and military devices. 2, 6 In order to improve the energy densities of supercapacitors, considerable efforts have been devoted to increasing the specific capacitance of electrode materials or the working voltage in currently. 7-10 But it is enormous challenges for supercapacitors to improve the energy density without sacrificing the performance of the high power densities. To achieve this goal, the high performance electrode materials are critical for supercapacitors with high energy densities.4, 7, 11-12 Graphene has attracted much interest for their superior electrical conductivity, exceptionally high surface area, and
This journal is © The Royal Society of Chemistry [year]
50
55
60
65
70
excellent mechanical flexibility, thus has been recognised as ideal candidates for high performance electrode materials.12-16 However, cost-effective and large-scale synthesis of graphene is a big problem for potential applications. Fortunately, reduced graphene oxide (rGO) can be obtained using the graphene oxide (GO) synthesized from inexpensive graphite powders as a precursor at low cost. At present, graphene oxide is often reduced to form reduced graphene oxide with the help of the chemical reducing agents or the thermal treatment at high temperatures. After the chemical reducing agents or the thermal treatment, a large portion of oxygen-containing functional groups are removed effectively. However, there are some disadvantages for chemical reductions and thermal treatments. For example, chemical reductions involved toxic agents (such as hydrazine and NaBH4), which could potentially bring some risks about environmental issues and safety of production as well as introduce impurities in the reduced graphene oxide;[17] thermal treatments require high temperatures lead to waste more energy and also is not suitable for GO film grown the substrate with low melting point.18 Thus, it is urgent to develop a simpler, mild and environmentally friendly method for extending the applications of GO. In the past few years, photoreduction of GO has emerged as a very important branch of GO reduction methodology. Various of photoreduction have been developed including photothermal reduction, 19-20 photochemical reduction 21-23 and laser reduction. 24-28 As compared to the chemical reductions and thermal treatments, photoreduction approaches have been attracted many attentions due to the advantages of high efficiency, [journal], [year], [vol], 00–00 | 1
Nanoscale Accepted Manuscript
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
5
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
clean, low cost, and tuneable reduction degree via irradiation. It is noteworthy that it has more advantages for photoreduction on the fabrication and integration design of graphene based microdevices such as direct patterning for microdevice fabrication, flexible electronic devices. 19, 24-25, 27 In this work, the combination of photoreduction and NH3 gas was applied to prepare nitrogen-doped graphene deposited on nickel foam as electrode for supercapacitors. To our knowledge, there are few reports about the photochemical doping of graphene with nitrogen. Firstly, GO was easy coated on nickel foam framework without any binder by taking advantage of Van der Waals attractions between the GO and nickel foam. Then, the GO deposited on nickel foam (GO@NF) was irradiated by a highpressure Hg lamp under NH3 atmosphere. This method not only makes the GO to be reduced in a short time but induce nitrogen doping easily. The nitrogen doped content is high up to 5.99 at% under irradiated 15 min. Furthermore, N-rGO@NF displays remarkable electrochemical performance.
50
55
60
65
2. Experimental 20
2.1. Preparation of N-rGO@NF eleltrode. Graphite oxide (GO) was prepared by a modified Hummers′ method. Ni foam was carefully cleaned treated with acetone and hydrochloric acid to remove contaminants, and then washed in sequence with deionized water and absolute ethanol.
70
75
80
Page 2 of 9
All the electrochemical measurements were performed in a two-electrode cell configuration. Two identical N-rGO@NF View Article Online electrodes (1.0 cm×1.0 cm each) were separated by a thick DOI: 10.1039/C4NR05776G -1 separator (NKK TF45, 40 μm). 6 mol L KOH, 1 mol L-1 Li2SO4, and 1 mol L-1 mixed Li2SO4/KOH aqueous solution (consist of 1 mol L-1 Li2SO4 and 1 mol L-1 KOH) were used as the electrolyte. Cyclic voltammetry (CV), galvanostatic charge−discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS) measurements were performed using an electrochemical workstation (CHI 660D, Chenhua Instruments, China). All electrodes are without any binder or conduct additive. The gravimetric specific capacitance was obtained from the discharge process according to the following equation: Cs=4I / ( m dV/dt) where I is the applied current (A), m is the total graphene mass of the two electrodes (g), and dV/dt is the slope of the discharge curve(V s-1). The energy density (E) and power density (P) in the Ragone plots were calculated using the following equations: E=Cs∆V2/8 P=E/∆t ∆V is the sweep potential window, and Δt is the discharge time. 2.3 Material Characterization The morphology of N-rGO@NF was characterized by scanning electron microscope (SEM, JSM-6510, JEOL Co. Ltd. Japan). X-ray diffraction (XRD) patterns were recorded using on X-ray diffractometer (Bruker D8, Bruker, Germany) with Cu–Kα radiation in the range of 5–80°. The X-ray photoelectron spectroscopy (XPS) experiments were carried out using Phi 5000 VersaProbe Scanning ESCA Microprobe (Ulvac-Phi, Inc., Japan). Electrochemical performance measurements were carried out using an electrochemical workstation (CHI 660D, Chenhua Instruments, China).
3. Results and discussion 25
Fig. 1 Schematic illustration of the N-rGO@NF photoreduction system
30
35
40
45
The N-rGO@NF eleltrode can be easily prepared using by a modified technique according to our previous work.23 The preparation of N-rGO@NF eletrode mainly consists of two steps. First, the GO coated on Ni foam was carried out by simple centrifuged spin coating process that Ni foam was put into a centrifugal tube containing GO dispersions (4 mg/mL) and then centrifuged at 500 rpm. Second, NH3 assisted photoreduction Ndoped process is illustrated in Fig. 1. A GO@NF sheet (1.0×2.5cm) was placed inside a quartz tube, and irradiated for different time with light from a high-pressure Hg lamp (500 W) in NH3 (99%) atmosphere. For comparison, rGO@NF sheet without N-doping was prepared by irradiating for 30 min with light from a high-pressure Hg lamp in Ar atmosphere. In order to accurately measure the mass of N-rGO, NrGO@NF sheet was put into 1 M FeCl3 at room temperature for 72 h to completely remove the Ni foam and then gently in several baths of deionized water for obtain a whole N-rGO sheet without Ni foam support (Fig S1). To ensure no any residuary of Ni, NrGO sheet also can be further dipped in the 1M HCl for 24h, and finally rinsed with deionized water and ethanol. 2.2 Electrochemical Measurement 2 | Journal Name, [year], [vol], 00–00
85
Fig. 2 SEM images at different magnifications: (a)-(c) nickel foam, (d)-(f) N-rGO@NF irradiated for 30 min and (g)-(i) N-rGO@NF irradiated for 120 min
3.1 Morphology and Chemical Structure The surface morphologies of Ni foam and N-rGO@NF were observed by SEM as shown in Fig. 2. It can be seen from the Fig. 2a that bare Ni foam has a 3D continuous and interconnected This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
Nanoscale
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
20
Nanoscale
network structure. The Fig. 2b and 2c further show that the surface of Ni skeleton is relatively rough with reticular patterns. The SEM images of N-rGO@NF irradiated for 30 and 60 min under NH3 atmosphere (Fig. 2d-i) show that N-rGO film has been successfully deposited on the micropores of Ni foam by comparing to the bare nickel foam (Fig. 2a-c). Although there have some cracks (Fig. 2e and 2h), a smooth translucent N-rGO film with some wrinkles was still tightly coated on the surface of Ni skeleton (shown in the Fig. 2f and 2i), suggesting good contact between N-rGO sheets and metal substrate. Therefore, the strategy that 2D ultrathin film was adhered to the skeleton of nickel foam tightly by Van der Waals attractions can be overcome the fragile features of photoreduction graphene oxide film, which hinder its post-processing as an electrode material (see Fig. S2). As compared to the photoreduction graphene oxide film with fragile features, network structure of N-rGO@NF is similar to Ni foam with a continuous and interconnected 3D network, which is beneficial for the contact between electrolyte and electrode material that facilitate the efficient access of electrolyte ions to the graphene surface and shorten the ion diffusion path, and for good contact between N-rGO sheets and metal current collector which greatly reduce contact resistance.
50
55
60
65
70
75
80
25
30
35
40
45
Fig. 3 XRD patterns of GO and N-rGO@NF irradiated under NH3 atmosphere for from 15 to 60 min
The XRD spectrum is used to characterize the structure of the prepared N-rGO. XRD spectrum in Fig. 3 shows that a typical diffraction peak of GO at 10.4° completely disappears after irradiation under NH3 atmosphere from 15 to 60 min and a weak broad peak relating to N-rGO appeared at around 24.5°, which is in accord with the feature peak of graphite. The peak with weaker intensity and broad shape indicates it could be related to the removal of oxygen containing groups from the GO during the photoreduction N-doped process. The result suggests the GO had been reduced effectively even if UV light cannot be transmitted in to the inside of nickel foams, due to the existence of both photochemical reduction and photothermal reduction or thermal reduction simultaneously. XPS spectrum is further used to characterize the elemental composition of samples irradiated from 15 to 90 min under NH3 atmosphere (shown in Fig. 4). The GO shows obvious C1s and O1s peak but no N signal in the XPS full scan spectrum (Fig. 4a). However, it is visible that there are clear N signals in the XPS spectrum of the N-rGO@NF irradiated from 15 to 90 min under NH3 atmosphere (Fig. 4a), indicating the introduction of N to NThis journal is © The Royal Society of Chemistry [year]
85
90
95
100
rGO@NF by the simple photochemical doping treatment using NH3 gas as nitrogen source. The C/O atomic ratio for NView Article Online rGO@NF irradiated from 15 to 90 min DOI: increases from 4.64 to 10.1039/C4NR05776G 7.26 according the XPS analysis (Fig. 4b), which is much higher than that of pristine GO (2.3). The increase of C/O atomic ratio indicates the de-oxygenation or reduction of GO after the light irradiation treatment, which agrees with the results of XRD analysis. The nitrogen doping content calculated from XPS analysis are 5.99, 6.20, 7.97, and 7.80 at% under irradiation time of 15, 30, 60, and 90 min, higher than that of N-doped graphene treated with NH3 at high temperatures. 29-32 It suggests that the high nitrogen doping content could be carried out at a very short time by the simple photochemical doped treatment, due to the fact that the enriched oxygen functional groups in GO favor the reactions with NH3 and C–N bond formation.23,32 To understand the role of NH3 during photoreduction process, as-prepared GO@NF sheet had been irradiated for 30 min under Ar atmosphere for comparison. The comparison of XPS full scan spectrum under NH3 and Ar atmosphere is shown in the Fig. 4c. It had confirmed by XPS spectrum that GO@NF has been effectively reduced by irradiation under Ar atmosphere, and the C/O atomic ratio is 4.51 according the XPS analysis. But under NH3 atmosphere, the C/O atomic ratio of N-rGO@NF can reach high up to 5.80 at the same irradiation time (30 min). Moreover, both reduction of GO and nitrogen doping occur simultaneously. The results indicates that NH3 gas and light irradiation promote each other in the nitrogen doping and reduction of GO, which NH3 accelerate light irradiation reduction of GO and light irradiation can induce nitrogen doping easily. The high-resolution C1s XPS spectrum of the GO (Fig. 4d) shows two separated peaks consisted two main components, which can be fitted to four components including C=C/C–C (284.6 eV) and C–O (hydroxyl and epoxy, 286.7 eV)species and two minor components from C=O (carbonyl, 287.7 eV) and O– C=O (carboxyl, 288.9 eV) species. After photo-irradiation under NH3 atmosphere, the high-resolution C1s XPS spectrum of the NrGO@NF (Fig. 4e) shows a typical single peak with a small tail in the higher binding energy region. The C=C/C–C bonds become dominant and the hydroxyl and epoxy species of C-O, and C=O reduced significantly, indicating that most of the hydroxyl and epoxy functional groups on the GO nanosheets were successfully removed. Moreover, a new peak appears at 285.8 eV (attributed to the C-N bond), due to the doping of nitrogen atoms. In order to determine the possible bonding configurations of N-atoms in NrGO@NF, the high-resolution of N1s spectrum (Fig. 4f) was employed. From the N1s XPS spectrum, the four kinds of nitrogen were possible doped, located at 398.4, 399.5, 400.4,and 401.3 eV, which can be assigned to pyridine-like(N-6), aminolike (N-A), pyrrolic-like (N-5), and graphitic-like nitrogen (N-Q), respectively. 23, 33-34 3.2 Electrical measurements and electrochemical properties The electrochemical performance of N-rGO@NF was evaluated using a symmetric two-electrode configuration at 6 mol L-1 KOH electrolytes with in a range of 0-1.0 V, as shown in the Fig. 5-7. Fig. 5a shows the influences of irradiation time under NH3 atmosphere for performance of N-rGO@NF. When irradiated for only 15 min, CV curve of N-rGO@NF exhibites a bad quasi-rectangular shape. However, the time of light irrradiation increases, CV curves exhibite a good quasiJournal Name, [year], [vol], 00–00 | 3
Nanoscale Accepted Manuscript
Page 3 of 9
Nanoscale
Page 4 of 9
View Article Online
5
10
15
20
Fig. 4 (a) XPS full scan spectrum of GO and N-rGO@NF irradiated for from 15 to 120 min. (b) The C/O atomic ratio. (c) The comparison of XPS full scan spectrum under NH3 and Ar atmosphere, respectively. (d) High resolution C1s XPS spectra of GO. (e) High resolution C1s XPS spectra of NrGO@NF. (f) High resolution N1s XPS spectra of N-rGO@NF.
Fig. 5. (a) CV curves of N-rGO@NF irradiated for 15, 30, 45, and 60 min under NH3 atmosphere at scan rate of 50 mV s-1, (b) the comparison of CV curves at scan rate of 50 mV s-1 for the N-rGO@NF with N-doping and rGO@NF without N-doping irradiated for 30 min under NH3 and Ar atmosphere, respectively. (c) and (d) CV curves of N-rGO@NF irradiated for 30, 45, and 60 min under NH3 atmosphere at scan rate of 500 and 1000 mV s-1, respectively. (e) and (f) CV curves of N-rGO@NF irradiated for 30 and 45 min under NH3 atmosphere at scan rate of 2000 mV s-1, respectively.
rectangular shape, corresponding to a nearly ideal double-layer capacitor response, own to the improvement of conductivity when N-rGO irradiated with longer time. In addition, with irradiation time increasing from 30 to 60 min, the area of CV curve has small change, suggesting that enough irradiating time ( ≥ 30 min) is necessary for the good electrochemical performance of N-rGO@NF but the effect of irradiation with longer time is limited for improvement of the specific capacitances. For comparison, rGO@NF sheet without N-doping was 4 | Journal Name, [year], [vol], 00–00
25
30
prepared by irradiating for 30 min under Ar atmosphere. Notably, there is an obvious difference in the shape of the CV curves with scan rate of 50 mV s-1 for the as-prepared GO@NF irradiated under Ar and NH3 atmosphere with the same time (30 min), as shown in Fig. 5b. The CV curve (irradiated under Ar atmosphere) obviously deviates from the quasi-rectangular shape compared with that of the N-rGO@NF (irradiated under NH3 atmosphere), suggesting more effective reduction of GO by irradiating in NH3 than in Ar with the same time, consistent with XPS data. The area enclosed by the CV curve of N-rGO@NF was sufficiently larger This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
DOI: 10.1039/C4NR05776G
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
20
25
Nanoscale
than rGO@NF irradiated under Ar atmosphere, indcating the NrGO@NF have higher specific capacitances than the rGO@NF. The improved electrochemical performance of N-rGO@NF may be attributed to the introduction of N-enriched functional groups into the carbon network like previous work reported.29-30, 35 Nitrogen doping can improve the electrical conductivity, enrich the free charge-carrier density and incerease holes on graphene sheets, which facilitate electrolyte penetration and ion transport and increase the pseudocapacitance effect. Further, the CV curves of N-rGO@NF irradiated different time under scan rates of 500 and 1000 mV s-1 (Fig. 5c and 5d) still keep a typical quasirectangular shape with small distortions, suggesting good chargetransfer efficiency at electrode/electrolyte interfaces following the mechanism of double-layer capacitors (EDLCs). Even at a high scan rate of 2000 mV s-1, the CV curves of N-rGO@NF (Fig. 5e and 5f) are still close to rectangular. These results indicate that NrGO@NF has a high rate capability and a small equivalent series resistance (ESR) of electrodes. Fig. 6 shows the galvanostatic charge–discharge (GCD) curves of the N-rGO@NF (irradiated for 15, 30, 45, and 60 min, respectively) measured in a range of 0-1 V at different current densities. The shapes of all curves are approximate to isosceles triangle, which is characteristic of the mechanism of double-layer capacitors (EDLCs). The specific capacitances of N-rGO@NF irradiated for 30, 45, and 60 min are high up to 236.6, 239.2, and
30
35
40
45
50
55
60
65
70
Fig. 6 GCD curves at different current densities of N-rGO@NF irradiated for 15 min (a), 30 min (b), 45 min (c) and 60 min (d) under NH3 atmosphere, respectively.
75
229.2 F g-1 at a low current density of 0.57, 0.62, and 0.62 A gaccording to the discharging curves, respectively, which are View Article Online higher than that of photocatalytically reduced oxide (220 DOI: graphite 10.1039/C4NR05776G -1 22 F g ) and laser-scribed graphene thin film (202 F g-1). 28 This also is high to or comparable with values reported in the literature for graphene deposited on nickel foam, 36-37 N-doped graphene (using ammonia gas as nitrogen sources), 29-30 microwaveexfoliated graphite oxide,38-39 and thermal exfoliation graphene nanosheets. 18 More calculation of the specific capacitances under different current densities is provided in Fig. 7a. It can see that the NrGO@NF irradiated for 30 min had the highest specific capacitance with 252.3 F g-1 at the current density of 0.29 A g-1. When the charge/discharge current density increases, NrGO@NFs irradiated for 30, 45, and 60 min show excellent rate capability. In particular, the specific capacitance of N-rGO@NF irradiated for 45 min still reach up to 190.4 F g-1 at a high current density of 92.3 A g-1 and maintains about 77% retention of its initial specific capacitance (247.1 F g-1, 0.31 A g-1). The excellent electrochemical performances of N-rGO@NFs can be attributed to their continuous and interconnected 3D network similar to Ni foam, high electronic conductivity of nitrogen-doped graphene, and good contact that N-rGO sheets was directly attach to the skeleton of nickel foam by Van der Waals attractions without any binder which greatly reduce contact resistance. Although the nitrogen doping content of N-rGO@NF irradiated for 15 min is about 5.99 at% and only slightly smaller than that of other N-rGO@NF irradiated for longer time, its specific capacitance declines obviously with the increase of current density, due to bad reduction effect of GO at a short irradiation time that exits high oxygen levels leading to poor conductivity. However, the nitrogen doping content increases slowly with longer irradiation time, the specific capacitances and rate capability have been improved greatly. At the same time, we also see that that performance of the N-rGO@NF irradiated for 30, 45, and 60 min with different nitrogen doping only has slightly changed. Even the specific capacitance of the NrGO@NF irradiated for 60 min has slight smaller than that of those irradiated for 30 and 45 min due to the more expansion of N-rGO film when irradiated for longer time (as shown in Fig. 2 h), which cause some graphene sheet to separate from metal substrate and thus induce loose contact between N-rGO sheets 1
Fig.7 (a) Gravimetric specific capacitance of N-rGO@NF irradiated for 15, 30, 45, and 60 min at different current densities. (b) Nyquist plots of NrGO@NF irradiated for 15, 30, 45, and 60 min. Inset: High-frequency region of the plots. (c) Cycle stability tests for N-rGO@NF at a current density of 7.14 A g-1. Inset: CV curves of the first, the 1,000th and 3,000th cycles for the device, respectively.
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5
Nanoscale Accepted Manuscript
Page 5 of 9
Nanoscale
Page 6 of 9
View Article Online
5
10
15
20
25
30
35
Fig.8 The electrochemical measurement of N-rGO@NF at 1 mol L-1 Li2SO4 electrolyte: (a) CV curves at different working voltage ranging from 1.0 to 2.0 V at a scan rate of 50 mV s-1, (b) GCD curves at different working voltage ranging from 1.0 to 2.0 V at 0.83 A g-1, (c) CV curves in the voltage range of 0–2.0 V at different scan rates from 100 to 500 mV s-1, (d) GCD curves in the voltage range of 0– 2.0 V at different current densities from 0.50 to 8.33 A g-1. and metal current collector. The results suggest that the increases of nitrogen doping content after radiation for 30 min has small improvement for the performance of N-rGO@NF, which would be affected by other factors such as oxygen levels, and contact between N-rGO and nickel foam. Therefore, the high performance graphene materials can be fabricated by the combination of photoreduction and NH3, and the optimization of UV light radiation time may be about 45 min. The electrochemical impedance spectroscopy (EIS) of the supercapacitors was further measured at open-circuit potential and frequencies ranging from 0.01Hz to 100 kHz with a 5 mV ac amplitude in 6 mol L-1 KOH, and the corresponding Nyquist plots are shown in Fig. 7b. All Nyquist plots show a near-vertical curve at the low frequency region, indicating good capacitor behaviour. At the high frequency region, N-rGO@NF irradiated for 15 min shows a large semicircle, a characteristic of charge transfer resistance (Rct). However, there is no obvious semicircle for the N-rGO@NF (irradiated for 30, 45, and 60 min, respectively), indicating an ideal capacitor behaviour and low ion-transport resistance. Furthermore, the first intersection of the semicircle with the real axis at the high frequency region corresponds to the equivalent series resistance (ESR), which comprises of the resistance of aqueous electrolyte, the intrinsic resistance of the composite material and the contact resistance at the electrode interface. The RESR of N-RGO@NF irradiated for 15, 30, 45, and 60 min at the real axis was about 0.93, 0.95, 0.76, and 0.86 Ω, respectively. Moreover, N-rGO@NF material exhibits excellent cycle stability, which is an important requirement for practical applications. Typically, the N-rGO@NF irradiated for 30 min 6 | Journal Name, [year], [vol], 00–00
40
45
50
55
60
65
was chosen to evaluate the cycle stability by repeating the GCD test at a current density of 7.14 A g-1, as shown in Fig. 7c. The specific capacitance retained at a high level with a specific capacitance of 208 F g-1 after 3,000 cycles (the capacitance retention was about 98.5%), and CV curves of 1th and 3,000th cycle inserted in Fig. 7c had only a small change, demonstrating the excellent cycle stability of N-rGO@NF. According to the equation E=1/2(CV2), the energy density of a supercapacitor can be improved by increasing the specific capacitance of electrode materials (C) or the working voltage (V). We try to improve the specific capacitance and energy density of of N-rGO@NF by using the neutral electrolytes. As compared to the acidic and alkaline electrolytes, neutral electrolytes are relative cheap, environmentally friendly, non-corrosive and suitable for various of current collectors. What is more important is that the neutral electrolyte can provide a much higher working voltage up to 1.6-2.2 V to improve the energy density of a supercapacitor. 9, 40-41 Therefore, the electrochemical performance of N-rGO@NF irradiated for 60 min was investigated using a symmetric two-electrode configuration at 1 mol L-1 Li2SO4 electrolyte and a mixed electrolyte (1 mol L-1 Li2SO4/KOH) shown in the Fig. 8 and 9, respectively. Fig. 8a shows the CV curves (with a scan rate of 50 mV s-1) measured using 1 mol L-1 Li2SO4 electrolyte at various voltage ranges. It can be obviously found that all CV curves exhibit a rectangular shape and the stable voltage window can be extended to 2.0 V. It is also further confirmed by the galvanostatic charge– discharge (GCD) curves at various voltage ranges shown in Fig. 8b. The CV curves at different scan rates ranging from 100 to 500 mV s-1 and GCD curves at different current densities ranging This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
DOI: 10.1039/C4NR05776G
5
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
Nanoscale
from 0.50 to 8.33 A g-1 between 0 and 2 V are shown in Fig. 8c and 8d, respectively. The CV curves exhibit typical quasirectangular shape and the GCD curves look nearly symmetric, indicating good charge propagations at the electrode interfaces following the mechanism of electric double-layer capacitors. The specific capacitances are reaching up to 235.0, 217.3, 192.0 and 179.3 F g-1 at a current density of 0.50, 0.83, 2.5 and 5.0 A g-1, respectively. More calculation of the specific capacitances at different current densities is shown in Fig. 10a. It is obviously that the value of specific capacitances measured at 1 mol L-1 Li2SO4 electrolyte is lower than that of the specific capacitances measured at 6 mol L-1 KOH electrolytes but the voltage window can be extended to 2.0 V, which energy density is improved effectively. In addition, specific capacitances measured at 1 mol L-1 Li2SO4 electrolyte decline obviously with increase of the current densities, indicating a bad rate capability at high current density.
20
25
30
In order to balance the energy density and high rate capability of N-rGO@NF, a mixed electrolyte (1 mol L-1 Li2SO4/KOH) was View Article Online used as a compromise solution. The stableDOI: voltage window can be 10.1039/C4NR05776G extended to 1.5 V according to the CV and GCD curves at various voltage ranges shown in the Fig. 9a and 9b. The high voltage window above water decomposition voltages (1.23V) may be attributed to the existence of Li2SO4 in mixed electrolyte. In pure neutral electrolyte Li2SO4, because H+ and OH- are in equilibrium at neutral pH, which do not favour hydrogen or oxygen evolution, and more important that both alkali metal cations (Li+) and sulfate anions have strong solvation, lead to the formation of the high over-potential for di-hydrogen evolution, so it is possible that the voltage window can extend to 2 V.9, 41 Similarly, there also exits strong solvation of ions in mixed electrolyte, the voltage window should larger than water decomposition voltages (1.23V), but less than 2V, which may be due to the KOH electrolyte that weakens the solvation of ions.
35
Fig. 9 The electrochemical measurement of N-rGO@NF at a mixed electrolyte (1 mol L-1 Li2SO4 /KOH): (a) CV curves at different working voltage ranging from 1.0 to 1.5 V at a scan rate of 50 mV s-1, (b) GCD curves at different working voltage ranging from 1.0 to 1.5 V at 0.77 A g-1, (c) CV curves in the voltage range of 0–1.5 V at different scan rates from 100 to 500 mV s-1, (d) GCD curves in the voltage range of 0–1.5 V at different current densities from 0.46 to 9.23 A g-1
40
Fig.10 The comparison of the electrochemical measurement at 1 mol L-1 Li2SO4 and a mixed electrolyte (1 mol L-1 Li2SO4/KOH): (a) Gravimetric specific capacitance of N-rGO@NF at different current densities; (b) Nyquist plots of N-rGO@NF. Inset: High-frequency region of the plots. (c) The Ragone plots of N-rGO@NF measured at 6 mol L-1 KOH, 1 mol L-1 Li2SO4 and a mixed electrolyte (1 mol L-1 Li2SO4/KOH).
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7
Nanoscale Accepted Manuscript
Page 7 of 9
Nanoscale
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
10
15
20
25
30
35
40
45
60
65
70
high power densities, high rate capability performance and View Article Online excellent cycle stability. The maximum of specific capacitance is DOI: 10.1039/C4NR05776G -1 -1 up to 252.3 F g at the current density of 0.29 A g in the KOH electrolyte. The results of the electrochemical measurement showed that (i) enough irradiating time ( ≥ 30 min) is necessary for the good electrochemical performance but the effect of irradiation with more long time is limited for improvement of the specific capacitances, (ii) To achieve the best rate capability of NrGO@NF, the best irradiation time could be 45 min. (iii) As compared to the KOH electrolyte, the stable voltage window can be extended to 2.0 and 1.5 V by using the Li2SO4 and a mixed Li2SO4/KOH electrolyte, and the maximum energy density was high up to 32.6 and 21.2 Wh kg-1, respectively.
Acknowledgements
75
This work was supported by the National Key Project of Fundamental Research of China (Grant No. 2012CB932304), the Innovation Program for Doctoral Research of Jiangsu Province (grant no KYLX_0026) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Notes and references 80
85
90
a Jiangsu Key Laboratory for Nanotechnology, Nanjing National Laboratory of Microstructures and Department of Physics, Nanjing University, Nanjing, 210093, P. R. China. Fax: 025-83595535; Tel: 02583593817; E-mail: [email protected] b Academy of Space Technolgy, Nanchang University, Nanchang, 330031, People’s Republic of China c College of Physics and Technology, Guangxi Normal University, Guilin, 541004, People’s Republic of China †Electronic Supplementary Information (ESI) available: Digital photographs of nickel foam, GO deposited in nickel foam, N-rGO@NF, and N-rGO@NF electrodes (1.0×1.0 cm); Digital photographs: (1) NrGO@NF sheet was put into 1 M FeCl3 at room temperature dissolve the Ni metal and (2) a whole N-rGO sheet without Ni foam support after nickel etching; Photograph of film with fragile features after irradiated by a high-pressure Hg lamp (500 W) in Ar and NH3 atmosphere.. See DOI: 10.1039/b000000x/
95
1. 2. 100
105
3. 4. 5. 6. 7. 8.
4. Conclusion
50
55
In summary, nitrogen-doped graphene (N-rGO@NF) was synthesized by irradiation of GO deposited on nickel foam with the assistance of NH3 gas for different time from 15 to 120 min. NH3 gas and light irradiation promote each other in the nitrogen doped and reduction of GO, which NH3 accelerate light irradiation reduction of GO and light irradiation can induce nitrogen doped easily. In just 15 minutes, the nitrogen doped content and C/O atomic ratio are high up to 5.99 at% and 4.64. The N-rGO@NF irradiated for different time as electrodes have been studied for supercapacitors applications in detail, and exhibited remarkable electrochemical performance including 8 | Journal Name, [year], [vol], 00–00
9. 10. 110
11. 12. 13. 115
14. 15. 16.
J. R. Miller and P. Simon, Science, 2008, 321, 651-652. J. R. Miller and A. F. Burke, Electrochem. Soc. Interface, 2008, 17, 53-57. Y. Gogotsi and P. Simon, Science, 2011, 334, 917-918. P. Simon and Y. Gogotsi, Nat. mater., 2008, 7, 845-854. B. E. Conway, J. Electrochem. Soc., 1991, 138, 1539-1548. J. R. Miller, Science, 2012, 335, 1312-1313. E. Frackowiak, Phys. Chem. Chem. Phys., 2007, 9, 1774-1785. X. Yang, C. Cheng, Y. Wang, L. Qiu and D. Li, Science, 2013, 341, 534-537. K. Fic, G. Lota, M. Meller and E. Frackowiak, Energy Environ. Sci., 2012, 5, 5842-5850. H. Wei, X. Yan, S. Wu, Z. Luo, S. Wei and Z. Guo, J. Phys. Chem. C, 2012, 116, 25052-25064. G. Wang, L. Zhang and J. Zhang, Chem. Soc. Rev., 2012, 41, 797828. Y. Sun, Q. Wu and G. Shi, Energy Environ. Sci., 2011, 4, 1113-1132. M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498-3502. Y. Huang, J. Liang and Y. Chen, Small, 2012, 8, 1805-1834. Y. B. Tan and J.-M. Lee, J. Mater. Chem., 2013, 1, 14814-14843. H. Huang, L. Xu, Y. Tang, S. Tang and Y. Du, Nanoscale, 2014, 6, 2426-2433.
This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
5
Furthermore, Fig. 9c shows that the CV curves keep a good quasi-rectangular shape at different scan rates ranging from 100 to 500 mV s-1. To our surprise, the specific capacitances estimated from the discharging curves (Fig. 9d) are high up to 272.7, 253.4, 236.6 and 224.8 F g-1 at a current density of 0.46, 0.92, 2.31 and 4.62 A g-1, which higher than those measured at pure Li2SO4 or KOH electrolyte. The gravimetric specific capacitance at different current densities ranging from 0.46 to 38.5 A g-1 are shown in Fig. 10a. As expected, the rate capability at high current density has been improved significantly by using the mixed Li2SO4/KOH electrolyte replace the Li2SO4 electrolyte. The result is also supported by the electrochemical impedance spectroscopy (EIS) measured at open-circuit potential. Fig. 10b shows the Nyquist plot based on a frequency response analysis of frequencies ranging from 0.01 Hz to 100 kHz with a 5 mV ac amplitude in 1 mol L-1 Li2SO4 electrolyte and 1 mol L-1 Li2SO4/KOH mixed electrolyte, respectively. The RESR of NrGO@NF measured at 1 mol L-1 Li2SO4 electrolyte and 1 mol L-1 Li2SO4/KOH mixed electrolyte was about 1.56 and 2.03 Ω, respectively. However, N-rGO@NF shows a larger semicircle measured at 1 mol L-1 Li2SO4 electrolyte than 1 mol L-1 Li2SO4/KOH mixed electrolyte, indicating there exists a large charge transfer resistance (Rct) using 1 mol L-1 Li2SO4 solution as electrolyte that lead to a bad rate capability. The energy and power densities of N-rGO@NF measured at different electrolytes were calculated according to the discharge curves at different current densities. Their relationship was described by the Ragone plots (Fig. 10c). It is obviously that NrGO@NF measured at Li2SO4 electrolyte and a mixed electrolyte (Li2SO4/KOH) exhibit higher energy densities at different current densities than that measured at KOH electrolyte. The maximum energy density was 8.22, 32.6 and 21.2 Wh kg-1 at a power density of 0.16, 0.54 and 0.41 kW kg-1 for the N-rGO@NF measured at KOH, Li2SO4 and a mixed Li2SO4/KOH electrolyte, respectively. The N-rGO@NF measured at KOH, Li2SO4 electrolyte and a mixed electrolyte (Li2SO4/KOH) also maintain 78.2, 66.6 and 72.4 % of their energy density with a value of 6.43, 21.72 and 15.35 Wh kg-1 as the power density increases up to maximum with 58.3, 24.7 and 47.7 kW kg-1 at current density of 76.9, 16.7 and 38.5 A g-1, respectively. These results demonstrate that Li2SO4 electrolyte can improve the energy densities but sacrifice some high rate capability and high power densities, and a mixed electrolyte (Li2SO4/KOH) is very well to balance the relationship between the high energy densities and high power densities.
Page 8 of 9
5
10
Published on 21 November 2014. Downloaded by University of Queensland on 23/11/2014 03:54:57.
15
20
25
30
65
Nanoscale
17. S. Park, J. An, J. R. Potts, A. Velamakanni, S. Murali and R. S. Ruoff, Carbon, 2011, 49, 3019-3023. 18. X. Du, P. Guo, H. Song and X. Chen, Electrochim. Acta, 2010, 55, 4812-4819. 19. L. J. Cote, R. Cruz-Silva and J. Huang, J. Am. Chem. Soc., 2009, 131, 11027-11032. 20. R. Mukherjee, A. V. Thomas, A. Krishnamurthy and N. Koratkar, ACS Nano, 2012, 6, 7867-7878. 21. G. Williams, B. Seger and P. V. Kamat, ACS Nano, 2008, 2, 14871491. 22. H.-C. Huang, C.-W. Huang, C.-T. Hsieh, P.-L. Kuo, J.-M. Ting and H. Teng, J. Phys. Chem. C, 2011, 115, 20689-20695. 23. F. Liu, N. Tang, T. Tang, Y. Liu, Q. Feng, W. Zhong and Y. Du, Appl. Phys. Lett., 2013, 103, 123108. 24. Y. Zhang, L. Guo, S. Wei, Y. He, H. Xia, Q. Chen, H.-B. Sun and F.S. Xiao, Nano Today, 2010, 5, 15-20. 25. W. Gao, N. Singh, L. Song, Z. Liu, A. L. M. Reddy, L. Ci, R. Vajtai, Q. Zhang, B. Wei and P. M. Ajayan, Nat. Nanotech., 2011, 6, 496500. 26. L. Huang, Y. Liu, L.-C. Ji, Y.-Q. Xie, T. Wang and W.-Z. Shi, Carbon, 2011, 49, 2431-2436. 27. V. Strong, S. Dubin, M. F. El-Kady, A. Lech, Y. Wang, B. H. Weiller and R. B. Kaner, ACS Nano, 2012, 6, 1395-1403. 28. M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science, 2012, 335, 1326-1330. 29. C.-M. Chen, Q. Zhang, X.-C. Zhao, B. Zhang, Q.-Q. Kong, M.-G. Yang, Q.-H. Yang, M.-Z. Wang, Y.-G. Yang, R. Schlögl and D. S. Su, J. Mater. Chem., 2012, 22, 14076-14084. 30. L. L. Zhang, X. Zhao, H. Ji, M. D. Stoller, L. Lai, S. Murali, S. McDonnell, B. Cleveger, R. M. Wallace and R. S. Ruoff, Energy Environ. Sci., 2012, 5, 9618-9625. 31. M. Li, N. Tang, W. Ren, H. Cheng, W. Wu, W. Zhong and Y. Du, Appl. Phys. Lett., 2012, 100, 233112.
35
40
45
50
32. X. Li, H. Wang, J. T. Robinson, H. Sanchez, G. Diankov and H. Dai, J. Am. Chem. Soc., 2009, 131, 15939-15944. 33. J. Casanovas, J. M. Ricart, J. Rubio, F. Illas and J. M.Article JiménezView Online DOI: 10.1039/C4NR05776G Mateos, J. Am. Chem. Soc., 1996, 118, 8071-8076. 34. L. S. Zhang, X. Q. Liang, W. G. Song and Z. Y. Wu, Phys. Chem. Chem. Phys., 2010, 12, 12055-12059. 35. G. Luo, L. Liu, J. Zhang, G. Li, B. Wang and J. Zhao, ACS Appl. Mater. Interfaces, 2013, 5, 11184-11193. 36. J. Chen, K. Sheng, P. Luo, C. Li and G. Shi, Adv. Mater., 2012, 24, 4569-4573. 37. H. Huang, Y. Tang, L. Xu, S. Tang and Y. Du, ACS Appl. Mater. Interfaces, 2014, 13, 10248-10257. 38. Y. Zhu, S. Murali, M. D. Stoller, A. Velamakanni, R. D. Piner and R. S. Ruoff, Carbon, 2010, 48, 2118-2122. 39. 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 and R. S. Ruoff, Science, 2011, 332, 1537-1541. 40. X. Yang, Y.-S. He, G. Jiang, X.-Z. Liao and Z.-F. Ma, Electrochem. Commun., 2011, 13, 1166-1169. 41. Q. Gao, L. Demarconnay, E. Raymundo-Piñero and F. Béguin, Energy Environ. Sci., 2012, 5, 9611-9617.
55
60
A graphic entry for the Table of Contents (TOC)
The nitrogen-doped graphene deposited on nickel foam was prepared by combining the photo-irradiation and ammonia, and as high performance of electrode for supercapacitors application. 70
75
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 9
Nanoscale Accepted Manuscript
Page 9 of 9
