Nitrogen-Doped Holey Graphene Film-Based Ultrafast

Jul 25, 2016 - The commercialized aluminum electrolytic capacitors (AECs) currently used for alternating current (AC) line-filtering are usually the l...
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Nitrogen-Doped Holey Graphene Film-Based Ultrafast Electrochemical Capacitors Qinqin Zhou,† Miao Zhang,† Ji Chen,† Jong-Dal Hong,‡ and Gaoquan Shi*,† †

Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China Department of Chemistry, Incheon National University, Incheon 406772, South Korea

‡

S Supporting Information *

ABSTRACT: The commercialized aluminum electrolytic capacitors (AECs) currently used for alternating current (AC) linefiltering are usually the largest components in the electronic circuits because of their low specific capacitances and bulky sizes. Herein, nitrogen-doped holey graphene (NHG) films were prepared by thermal annealing the composite films of polyvinylpyrrolidone (PVP), graphene oxide (GO), and ferric oxide (Fe2O3) nanorods followed by chemical etching with hydrochloride acid. The typical electrochemical capacitor with NHG electrodes exhibited high areal and volumetric specific capacitances of 478 μF cm−2 and 1.2 F cm−3 at 120 Hz, ultrafast frequency response with a phase angle of −81.2° and a resistor− capacitor time constant of 203 μs at 120 Hz, as well as excellent cycling stability. Thus, it is promising to replace conventional AEC for AC line-filtering in miniaturized electronics. KEYWORDS: nitrogen doping, holey graphene, meso-/macropores, ultrafast electrochemical capacitors, AC line-filtering

1. INTRODUCTION The rapid development of portable electronics requires miniaturized and compact energy storage devices with high energy densities and long lifetime.1,2 For most line-powered electronics, alternating current (AC) line-filters are used to smooth the leftover ripples on direct current voltage busses at 120 Hz (the double value of Sixty-Hz AC power, the United States standard).3,4 However, the low specific capacitances and bulky sizes of widely used commercial aluminum electrolytic capacitors (AECs) for AC line-filtering are supposed to hinder their practical applications in future electronics.3 Recently, ultrahigh-rate electrochemical capacitors (ECs) have been widely explored for replacing AECs because of their much higher specific capacitances.1,3−14 The pioneering works include the ECs based on three-dimensional (3D) vertical graphene electrodes prepared by chemically vapor deposition (CVD)3 or electrochemical reduction of graphene oxide (GO),6 exhibiting ultrahigh-rate performances. However, these electrode materials have low weight densities, making the corresponding ECs have small volumetric capacitances (CV). To address this issue, a variety of electrode materials have been developed such as highly stacked graphene sheets,7 mesoporous carbon,8 carbon black,5 carbon nanotubes (CNTs),10,11,14 conducting polymers,9,13,15 etc. However, most ECs based on these materials still showed insufficiently high-rate performances for AC linefiltering (|phase angle| < 80° at 120 Hz).5,7−10,13 Although several high-performance ECs with CNT electrodes were reported, their preparation usually involves expensive and complicated processes of CVD or vacuum filtration.11,14 © 2016 American Chemical Society

On the other hand, holey graphene-based ECs were reported to have higher CV values and improved rate-performances compared to those of the counterparts with pristine graphene materials.16−22 This is mainly because the nanopores on the basal planes of graphene sheets provide more edging sites to form electrical double layers and facilitate the diffusion of electrolyte.23,24 Nevertheless, holey graphene has not yet been applied to fabricate ultrafast EC for AC line-filtering. Herein, we report an ultrahigh-rate EC based on nitrogen-doped holey graphene (NHG) films for this purpose. The NHG films were prepared via doctor-blading of graphene oxide (GO)/ ferric oxide (Fe2O3) nanorods/polyvinylpyrrolidone (PVP) hybrid gel, followed by thermal annealing and acid treatment. In this case, Fe2O3 nanorods catalyzed the etching of carbon atoms to form pores and the decomposition of PVP led to dope N atoms into reduced graphene oxide (rGO) sheets. The hierarchical porous structure of NHG provided facile ion transport channels in the electrodes, and nitrogen doping enhanced the conductivity and hydrophilicity of holey graphene electrode.25−27 As a result, the typical NHG-based EC exhibited high areal and volumetric specific capacitances, ultrafast rate capability, and excellent electrochemical stability, promising for AC-line filtering. Received: May 10, 2016 Accepted: July 25, 2016 Published: July 25, 2016 20741

DOI: 10.1021/acsami.6b05601 ACS Appl. Mater. Interfaces 2016, 8, 20741−20747

Research Article

ACS Applied Materials & Interfaces C′ = − Z″ /(2πf |Z|2 s);

2. EXPERIMENTAL SECTION 2.1. Electrode Fabrication. 2.1.1. NHG Electrodes. GO was prepared by using a modified Hummers’ method according to literature.28 The procedures of fabricating NHG electrodes are described as follows. First, an aqueous GO dispersion (2 mL, 5 mg mL−1) was mixed with the dispersion of Fe2O3 nanorods (70 μL, 10 wt %, 20 nm in diameter and 100 nm in length, De Ke Dao Jin Technology Co. Ltd., Beijing, China). Successively, this mixed dispersion was converted into a hydrogel by adding an aqueous solution of PVP (molar mass = 55 000 g, 0.3 mL, 20 mg mL−1). The weight ratio of GO, Fe2O3, and PVP in the composite was optimized to be 10:7:6. Second, the composite hydrogel described above was casted into a film with the thickness of 400, 600, or 800 μm on the surface of a polytetrafluoroethylene (PTFE) plate by doctor-blading and then dried at room temperature. Finally, the NHG electrodes were obtained by thermal annealing of the hybrid film at 850 °C for 2 h, followed by immersing in hydrochloric acid (HCl, 6 M) to remove Fe2O3 nanorods, and washed with deionized water until pH = 7.0. The thicknesses of the resultant NHG electrodes were measured to be 1.0, 1.5, and 2.0 μm, respectively. The NHG electrodes with different thicknesses are nominated as NHGx (x represents the thickness of the film in micrometer, μm). 2.1.2. Holey Graphene (HG) Electrodes. A 0.76 mL aliquot of a dispersion of GO (5 mg mL−1) was mixed with 26.7 μL of a dispersion of Fe2O3 nanorods (10 wt % aqueous dispersion). However, this mixture cannot be processed into a uniform film by doctor-blading. Thus, it was diluted to 7.9 mL with deionized water and then filtrated into a composite film with an area of 11.3 cm2 by using a 0.22 μm porous PTFE membrane. This film was dried and treated by thermal annealing, etching with acid, and washing with water under the same conditions described above. The resulting HG electrode with a thickness of about 2 μm has a mass loading of graphene sheets the same to that of NHG2 film. 2.1.3. Nitrogen-Doped Graphene (NG) Electrodes and Graphene (G) Electrodes. NG or G films were prepared by filtering the dispersion of GO with or without PVP, and then treated by thermal annealing at 850 °C for 2 h. The mass loadings of graphene sheets in G and NG films were also controlled to be nearly the same to that of NHG2 film. The thicknesses of G and NG films were measured to be about 1 μm. 2.2. Characterizations. Scanning and transmission electron micrographs (SEM and TEM) were taken out on a Sirion 200 fieldemission scanning electron microscope (FEI, USA) and a FEI TECNAI TF20 transmission electron microscope. X-ray photoelectron spectra (XPS) were obtained using a Escalab 250 photoelectron spectrometer (ThermoFisher Scientific, USA). Raman spectra were carried on a LabRAM HR Evolution (HORIBA Jobin Yvon, France) Raman spectrometer with a 532 nm laser. Electrical conductivity was measured by using a four-probe meter (KDY-1, Kunde Science and Technology Co. Ltd., China). 2.3. EC Fabrication and Electrochemical Characterizations. An EC was fabricated by assembling two identical electrodes (each has an area of 0.5 × 0.5 cm2) in a sandwiched manner, using a filtering paper as separator and 6 M KOH as electrolyte. Cyclic voltammetry (CV), galvanostatic charge−discharge tests (GCD), and electrochemical impedance spectroscopy (EIS) were performed on a CHI 660D Potentiostat (CH Instruments, Inc.). The electrochemical window for CV and GCD measurements was 0−0.8 V. EIS tests were carried out in the frequency range of 105−1 Hz at a 5 mV amplitude. The specific areal capacitance (CA), specific volumetric capacitance (CV), resistor-capacitor time constant (τRC) and the real or imaginary specific areal capacitance(C′ or C″) derived from EIS spectra were calculated by eqs 1−4, respectively.

CA = − 1/(2πfZ″s)

(1)

C V = − 1/(2πf Z″v)

(2)

τRC = − Z′/2πf Z″

(3)

C″ = − Z′/(2πf |Z|2 s)

(4)

where f is frequency, Z′ or Z″ is the real or imaginary impedance, s is the area of electrode, v is the volume of the two electrodes, v = 2ds, and d is the thickness of a single electrode. The CA of the device was also calculated from GCD discharging curve by using eq 5

CA = I Δt /sΔV

(5)

where I is the constant discharge current, Δt is the discharge time, and ΔV is the discharge potential drop (excluding IR drop).

3. RESULTS AND DISCUSSION The preparation of a NHG film is schematically illustrated in Scheme 1. Aqueous dispersions of GO (5 mg mL−1) and Fe2O3 Scheme 1. Schematic Illustration of the Preparation of a NHG Film

nanorods (20 nm in diameter and 100 nm in length, Figure S1a) were homogeneously mixed. Then, this mixture was turned into a hydrogel upon the addition of an aqueous solution of PVP (MW = 55 000, Figure S1b). This highly viscous hydrogel (Figure S1b) was doctor-bladed on a PTFE plate and then dried in air to form a GO/Fe2O3/PVP composite film. Successively, this film was thermally annealed at 850 °C for 2 h. During thermal annealing, the GO was reduced to rGO, Fe2O3 nanorods catalyzed the etching of carbon atoms to form pores29−31 and the decomposition of PVP led to dope N atoms into rGO sheets. Finally, NHG was obtained by removing Fe2O3 nanorods from the thermally annealed composite film with HCl and washed with water. Accordingly, this process can be readily scaled up because of using an industrially compatible doctor-blading method (Figure S2). To evaluate the performance of NHG in EC, we also prepared NG, HG and G films for comparison. SEM (Figure 1a) and TEM (Figure 1b) images of NHG electrode indicate its pore sizes range from several to hundreds nanometers, suggesting a hierarchical porous structure consisting of mesoand macropores. The nitrogen adsorption−desorption isotherm of the typical NHG2 film has a shape of Type-IV with a hysteresis loop (Figure S3a), also indicating the presence of meso- and macropores. On the basis of Brunauer−Emmett− Teller (BET) method, its specific surface area was measured to be 79 m2 g−1. At a relative pressure (P/P0) of about 0.95, the 20742

DOI: 10.1021/acsami.6b05601 ACS Appl. Mater. Interfaces 2016, 8, 20741−20747

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ACS Applied Materials & Interfaces

Figure 1. (a, b) SEM (a, inset, a magnified image) and TEM (b) images of NHG electrode; (c−f) STEM image of NHG electrode (c) and its carbon (d), nitrogen (e), and oxygen (f) element mapping images; (g, h) XPS (g) and Raman (h) spectra of NHG, NG, HG, and G film.

(4.64%), NG (5.87%), HG (6.87%), and G (7.43%) films are much smaller than that of GO (30.78%), indicating that the GO precursor was effectively reduced upon thermal annealing.24 This result is in a good agreement with that of high resolution C 1s XPS spectra (Figure S5b−f). The Raman spectrum of an NHG, NG, HG, or G film displays a D-band at ∼1350 and a G-band at ∼1590 cm−1 (Figure 1h), and they are attributed to graphene edges/structural defects and graphite lattice of carbon materials.24 The intensity ratio of D-/G-bands (ID/IG) of HG film (0.99) is almost identical to that (1.02) of G film, revealing that the pores generated in the basal planes of graphene sheets did not induce a significant increase of structural defects, and the intrinsic crystalline structure of graphene was unchanged.18,30,32,33 Furthermore, the Raman spectrum of NHG or NG film displays a higher ID/IG (about 1.13) compared to that of HG or G film, probably caused by the doping of N atoms.25,34,35 EC cells with a symmetric two-electrode configuration were employed to simulate the practical device behavior. EIS was an efficient method to characterize the electrochemical performance of an EC for AC line-filtering, and the phase angle at 120 Hz is a “factor of merit’’.4 As shown in Figure 2a, typically, the

volume of adsorbed nitrogen increased dramatically, reflecting that NHG2 also has large pores. Because the large pores are beyond the range accessible to gas adsorption,32 we can survey only the size distribution of mesopores from the N 2 adsorption−desorption isotherm. The mesopores of NHG2 have a broad distribution of 2−30 nm (Figure S3b). These results indicate that NHG2 film has both mesopores and macropores, which is in consistent with the morphological study described above. The meso- and macropores of NHG film can provide accessible channels for ion transport across the basal planes of rGO sheets. HG film shows a similar porous morphology, while NG and G films do not have observable pores (Figure S4). Furthermore, elemental mappings of NHG demonstrate the uniform distributions of C, N, and O elements in NHG film (Figure 1c−f). The X-ray photoelectron spectrum (XPS) of NHG (or NG) film shows a N-peak at about 400 eV, and its atomic content of N was measured to be 3.82% (or 5.28%) (Figure 1g). The highresolution N 1s XPS of NHG film can be divided into four peaks, corresponding to pyridinic N (398.3 eV), pyrrolic N (399.2 eV), quaternary N (401.1 eV), and oxidized N (404.0 eV) (Figure S5a). The atomic contents of oxygen in NHG 20743

DOI: 10.1021/acsami.6b05601 ACS Appl. Mater. Interfaces 2016, 8, 20741−20747

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ACS Applied Materials & Interfaces

Figure 2. EIS performance of NHG2-based EC. (a) Plot of phase angle versus frequency. (b) Nyquist plot (inset: the enlarged view at high frequencies). (c) Plot of specific areal capacitance (CA) as a function of frequency. (d) Plot of the real or imaginary specific areal capacitance (C′ or C″) versus frequency.

phase angle of NHG2-based EC at a frequency of 120 Hz is −81.2°, which is among the best results of reported ultrafast ECs3−6,8,9,11,13,14,36 (Table S1) and comparable to that of AEC (Figure S6). The Nyquist plot of NHG2-based EC (Figure 2b) exhibits a nearly vertical line at low frequencies, suggesting a pure double layer capacitive (DLC) behavior. The absence of semi-circle and 45° line at high frequencies reflects the fast electron transfer and ion diffusion within NHG electrode. Furthermore, the extraordinarily small equivalent series resistance (ESR, 0.22 Ω) implies an excellent interfacial contact between NHG electrode and Au current collector. A series-RC circuit model was established to analyze the capacitive and resistive elements of the capacitor.3 The CA of NHG2-based EC was calculated to be 478 μF cm−2 at 120 Hz (Figure 2c). The bare Au current collector-based EC exhibited a CA of only 9 μF cm−2 at 120 Hz (Figure S7). Thus, the CA of NHG2-based EC (478 μF cm−2) is mainly attributed to the porous NHG electrodes. To our knowledge, the performance of NHG2-based EC (478 μF cm−2 with a phase angle of −81.2° at 120 Hz) is superior to most of reported ECs for AC line-filtering, such as the ECs based on chemically vapor deposited vertically oriented graphene nanosheets (VOGN, 87.5 μF cm−2, − 82.0°),3 graphene/carbon nanotube composites (G/CNTs, 230 μF cm−2, − 81.5°)4 and CNTs (282 μF cm−2, − 82.2°).14 The CV at 120 Hz of NHG2-based EC was calculated to be 1.2 F cm−3. This value is 1.6−17 times higher than that (0.07 to 0.73 F cm−3) of most reported carboneous material-based ECs with phase angles larger than −80° at 120 Hz (Table S1).3,4,6,36 Moreover, the τRC of NHG2-based EC was calculated to be 203 μs. This time is also among the shortest values of ultrafast ECs, and far smaller than the required 8.3 ms for 120 Hz.3,4,11,14 The C′ or C″ calculated from eq 4 was plotted as a function of frequency in Figure 2d. Relaxation time constant (τ0) derived from the frequency (f 0) at the maximum C″ (τ0 = 1/f 0) is the minimum time needed to discharge the whole energy at an efficiency larger than 50 %.37 NHG2-based EC showed a small τ0 of 819 μs, indicating the fast ion diffusion within electrodes. The electrochemical performance of NHG2-based EC was also evaluated using CV tests (Figure 3a-e and Figure S8). The

Figure 3. (a−e) CV curves of NHG2-based EC at different scan rates. (f) Plot of discharging current density as a function of scan rate.

CV curves have a rectangular shape at a high scan rate of 100 V s−1, indicating the ultrafast ion diffusion and electron transfer (Figure 3c). Even at 500 V s−1, the CV curve still remains a quasi-rectangular shape (Figure 3e). The discharge current density as a function of scan rate is plotted in Figure 3f. It displays a perfect linear relationship in the range of scan rates from 1 to 500 V s−1. As the scan rate increases to 800 V s−1, the discharge current slightly deviates from the linear relationship. The up-limit of scan rate for linearity (500 V s−1) is higher than that of most reported ultrafast ECs, for example, ErGO (350 V s−1) and onion-like carbon (100 V s−1).1,6,11 The rate-capability of NHG2-based EC has also been studied by using GCD technique. As shown in Figure 4a, the GCD

Figure 4. (a) Galvanostatic charge−discharge test (GCD) curves of NHG2-based EC at different current densities (mA cm−2). (b) Cycling stability test of NHG2-based EC at 1 mA cm−2.

curves have a perfect triangular shapes within the range of discharging current density (id) of 0.5 to 10 mA cm−2, indicating a pure electrochemical DLC behavior. The negligible voltage drops at the beginnings of the discharge curves reflect the extremely small ESR of NHG2-based EC. Furthermore, the capacitance derived from GCD discharging curves (CA) remained 93% as id increased from 0.5 to 10 mA cm−2, indicating the ultrahigh rate-performance (Figure S9). The CA 20744

DOI: 10.1021/acsami.6b05601 ACS Appl. Mater. Interfaces 2016, 8, 20741−20747

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ACS Applied Materials & Interfaces

Figure 5. Plot of (a) phase angle, (b) CA, or (c) C″ as a function of frequency for NHGx (x = 1, 1.5, or 2) based ECs.

CV curves of NHG1- or NHG1.5-based ECs still keep quasirectangular shape even at an ultrahigh scan rate of 1000 or 800 V s−1 (Figure S15). As described above, the rate-performance and capacitance shows an opposite changing tendency with the increasing thickness of electrode. Thus, NHG2-based EC is the most suitable device for AC line-filtering considering its overall performances.

of NHG2-based EC kept almost unchanged after charging− discharging for 10 000 cycles at 1 mA cm−2, suggesting an excellent stability (Figure 4b). This result is also certified by the EIS spectra before and after this lifetime test (Figure S10). The excellent performance of NHG2-based EC is mainly attributed to the meso-/macroporous structures of graphene sheets and their N doping atoms. To provide an insight of the importance of holey structure, an EC with NG electrodes has also been tested for comparison. Actually, this EC showed a much lower CA (72 μF cm−2) with much worse rateperformance (phase angle = −67.4° and τRC = 542 μs at 120 Hz) compared with those of NHG2-based EC (Figures S9, S11, and S12a and b and Table S2). This is mainly due to that the highly stacked graphene sheets remarkably hindered the diffusion of ions within the electrodes. NHG2 film has a much looser microstructure than that of NG film (Figure S13). Furthermore, the in-plane pores in NHG2 film provided unobstructed channels for the diffusion of electrolyte ions, and increased the accessible surface area and edge sites for forming electrochemical double layers.19,38 On the other hand, in the case of using HG films as electrodes, the EC showed a slightly lower CA of 453 μF cm−2 and worse rate-performance (phase angle = −77.7° and τRC = 284 μs at 120 Hz) (Figures S9, S11, and S12c and d). This is mainly due to that N doping provides additional capacitive sites, and enhances the conductivity and hydrophilicity of electrodes, making the electrolyte ions be able to transport rapidly within the NHG electrode.25−27,34 Similarly, the CA of NG-based EC was measured to be higher than that of G-based EC (Table S2). Nitrogen-doped sites at basal planes was detailedly demonstrated to improve the capacitance in a previously reported paper.34 Indeed, NG film (443 S cm−1) has a higher conductivity compared with that of pristine G film (216 S cm−1), and the conductivity of NHG film (122 S cm−1) is also much higher than that of HG (37 S cm−1). This result can be explained by the existence of quaternary N atoms in NHG (Figure S5a).21,34 The water contact angle of a NHG film (34.6°) is smaller than that of a HG film (45.1°); similarly, a NG film (67.1°) also shows a smaller water contact angle compared with that of a G film (88.7°, Figure S14). Therefore, it is reasonable to conclude that the synergistic effect of the porous structure and nitrogen doping provided NHGbased EC with excellent performance. The capacitive performances of the ECs based on NHGx (x = 1−2) can be adjusted by the thickness of their electrodes. The phase angles of NHG1-, NHG1.5-, and NHG2-based ECs at 120 Hz were measured to be −83.8°, −83.6o, and −81.2°, respectively (Figure 5a). Their CAs at 120 Hz were calculated to be 210, 334, and 478 μF cm−2, with τRC of 141, 146, and 203 μs, correspondingly (Figure 5b). The τ0 increases from 372 to 819 μs as x increases from 1 to 2 (Figure 5c). Furthermore, the

4. CONCLUSION We developed a simple and scalable method to fabricate Ndoped graphene electrodes with meso-/macropores by doctorblading of GO/Fe2O3/PVP hybrid gel and followed by thermal annealing and acid treatment. The in-plane pores in NHG sheets facilitate the rapid diffusion of electrolyte ions within electrodes. Meanwhile, N doping derived from PVP decomposition provides additional capacitive sites, and improves the conductivity and hydrophilicity of electrodes, accelerating electrons transfer and ions diffusion. The typical EC exhibited high CA and CV of 478 μF cm−2 and 1.2 F cm−3, ultrafast frequency response characteristics (at 120 Hz, phase angle = −81.2°, τRC = 203 μs) and excellent electrochemical stability. Therefore, the NHG-based ECs have a great potential to replace AEC for miniaturized AC-line filtering application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05601. TEM image of Fe2O3 nanorods, viscosities at different shear rates of GO/Fe2O3 dispersion and GO/Fe2O3/ PVP hydrogel, photographs of a GO/Fe2O3/PVP hybrid film before and after thermal annealing, SEM images of HG, NG, and G films, N 1s XPS spectrum of an NHG film, C 1s XPS spectra of GO, NHG, NG, HG, and G films, plots of impedance phase angle versus frequency for NHG2-based EC and a commercial AEC, plot of CA versus frequency for a bare Au-based EC, plot of CA versus discharging current density for NHG2-based EC, the comparison of EIS parameters for an NHG2-based EC before and after charging−discharging for 10 000 cycles, the comparison of EIS parameters for NHG-, NG-, HG-, and G-based ECs, CV curves of NG- and HG-based ECs at different scan rates, plot of discharging current density versus scan rate for NG- or HG-based EC, cross-section SEM images of NHG and NG films, water contact angles of NHG, HG, NG, and G films, CV curves of NHG1- and NHG1.5-based ECs at different scan rates, and tables of the performance parameters (τRC, 20745

DOI: 10.1021/acsami.6b05601 ACS Appl. Mater. Interfaces 2016, 8, 20741−20747

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ACS Applied Materials & Interfaces

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phase angle, CA and CV at 120 Hz) of various ECs for AC-line filtering (PDF)

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Corresponding Author

*Tel: (+86) 10 62773743. Fax: (+86) 10 62771149. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2012CB933402, 2013CB933001), Research Grant of Incheon National University in 2014, and Natural Science Foundation of China (51433005).

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DOI: 10.1021/acsami.6b05601 ACS Appl. Mater. Interfaces 2016, 8, 20741−20747