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Formation process of holey graphene and its assembled binder-free film electrode with high volumetric capacitance
Electrochimica Acta 187 (2016) 543–551
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Formation process of holey graphene and its assembled binder-free film electrode with high volumetric capacitance Yonglong Baia,b , Xiaofan Yanga,b , Yibo Hea,b , Jinyang Zhanga,b , Liping Kanga,b , Hua Xua,b , Feng Shia,b , Zhibin Leia,b , Zong-Huai Liua,b,* a b
Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, Xi’an 710062, PR China School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 October 2015 Received in revised form 4 November 2015 Accepted 17 November 2015 Available online 1 December 2015
Holey graphene with abundant in-plane nanopores is achieved through a mild defect-etching reaction between graphene oxide (GO) and hydrogen peroxide (H2O2) then followed by a reduction process in hydrazine solution. The porosity of the obtained holey graphene is systematically investigated by optimizing the reaction conditions between GO and H2O2. The optimum reaction conditions are that GO is hydrothermally treated in 0.4 mL H2O2 at 100 C for 10 h and the as-prepared holey graphene oxide (HGO) is refluxed in hydrazine solution at 100 C for 1 h, by which holey reduced graphene oxide (HRGO) suspension with good dispersity is obtained. By vacuum filtration of the HRGO suspension, the binderfree porous graphene film electrodes are successfully assembled. The obtained film electrodes exhibit high specific capacitance (251 F g 1 at a current density of 1 A g 1), high volumetric capacitance (up to 216 F cm 3), and enhanced rate capability (73% capacitance retention from 1–60 A g 1) due to their high packing density (0.86 g cm 3). The suitable porosity of the assembled porous graphene film keeps a balance between electrolyte ion diffusion rate and conductivity, which can bridge the gap between gravimetric capacitance and volumetric capacitance for the obtained electrode material. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: holey graphene hydrogen peroxide binder-free film electrode enhanced rate capability volumetric capacitance
1. Introduction Electrochemical capacitors (ECs) are one of the most effective and practical energy storage and conversion devices [1], which can bridge the critical performance gap between the high energy density of secondary battery and high power density of conventional dielectric capacitor, making it possible for the rapid energy storage and release [2]. The capacitive performance of the ECs lies in electrode materials, electrolyte, and assembled technology, with the most important factor being the electrode materials [3]. Among the electrode materials with different structures and morphologies, the flexible and binder-free film electrodes with high packing density have shown great promise in upcoming nextgeneration portable and flexible electronics such as roll-up displays, photovoltaic cells, and wearable devices [4–6]. Film architecture of the electrode materials can improve the electrolyte access, because they can show low ion diffusion length and
* Correspondence author at: School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, Shaanxi, PR China. Tel.: +86 29 81530716, fax: +86 29 81530702. E-mail address: [email protected] (Z.-H. Liu). http://dx.doi.org/10.1016/j.electacta.2015.11.090 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
reduced electron transfer resistance [7–9]. According to the available literature to date, most of the working electrodes are binder-enriched ones, which are prepared by the traditional slurry-coating technology [10–12]. Evidently, the added binder will decrease the electrical conductivity of the electrode materials, hindering their potential application in high performance supercapacitors [13]. In order to obtain the ideal electrochemical performance, it is also urgent to develop flexible and binder-free film electrode materials for supercapacitors. Graphene has attracted significant attention in recent years because of its extraordinary electrical conductivity, high surface area, chemical stability, and a distinct 2D nanostructure [14]. Graphene nanosheets can be acquired by delaminating their bulk materials with layered structure, and the obtained nanosheets are new classes of nanoscale materials, which can be used to assemble binder-free film electrodes with excellent flexibility and high packing density [15,16]. However, there are three key points remained to be solved when graphene nanosheets are used to assemble binder-free film electrodes. Firstly, the irreversible aggregation tendency of graphene nanosheets due to the strong p p stacking and van der Waals interactions between different graphene nanosheets should be prevented, because it will give rise to a significant deterioration of their properties including severely
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reduced specific surface area and much lower mass diffusion rate [17]. Secondly, the efficient ion transport pathway in the vertical direction of the film electrode is needed, which is favourable for improving the rate capability of the electrode materials [18]. Thirdly, the dispersity and processability of graphene nanosheets is needed for assembling the film electrode with high packing density [19]. Up to now, the preparation of graphene hydrogels with three-dimensional hierarchical porous network or holey graphene oxides with abundant in-plane nanopores are probably effective ways to solve the above challenges [20–22]. Porous graphene can be mainly classified into crumpled graphene, graphene foam, and graphene nanomesh [23]. The crumpled graphene with a wrinkled configuration is constructed with the bending and folding graphene nanosheets [24]. The graphene foam is a kind of 3D macroscopic graphene architecture via self-assembly of the 2D flexible graphene nanosheet or by chemical vapor deposition (CVD) [25,26]. The graphene nanomesh, also called holey graphene, is characterized as the existence of abundant in-plane pores with the pore sizes ranging from several angstroms to the nano-scale [27]. In comparison with the crumpled graphene and the graphene foam, the graphene nanomesh possesses higher surface area and much more active sites and edges for the ion diffusion shortcuts between different layers of graphene, thus they can be used to assemble outstanding electrode materials for supercapacitor [28]. The graphene nanomesh can be obtained by means of plasma etching [29,30] template direction [18,31], chemical activation [32,33] and catalytic oxidation [34,35], and so on. Although the graphene nanomesh with sufficient nanoscale periodic or quasi-periodic nanoholes can be prepared by above methods, the electrode materials assembled from these graphene nanomeshes show relatively low packing density, which generates low volumetric capacitance [1,32]. On the other hand, solution oxidation method is favourable for obtaining graphene nanomesh with good dispersity and processability, which would be able to conveniently process into holey graphene film with high packing density and bridge the gap between gravimetric capacitance and volumetric capacitance [22,36–38]. Shi and Liu groups have developed a solution approach for the production of graphene nanomesh with nitric acid, but the pollutant derived from nitric acid should be attracted more attention [37,39]. In virtue of the environmental-friendly merit of H2O2 used as oxidizing and etching reagent, Duan group has prepared the graphene nanomesh film with a packing density of 0.71 g cm 3 by mechanically compressing porous graphene hydrogel, and the prepared film shows simultaneously relatively high gravimetric and volumetric capacitances while retaining excellent rate capability [36]. However, the resultant holey graphene is in absence of dispersity and processability, which will preclude the integration of various pseudocapacitive materials into the holey graphene film to further boost its specific capacitance and energy density. From the foregoing, graphene nanomesh with abundant in-plane nanopores can be prepared by a convenient mild defectetching reaction, while its dispersity and processability should be further improved for the high packing density and high volumetric capacitance of the film electrode. In the present work, a mild defect-etching reaction is used to produce holey graphene with abundant in-plane nanopores by using H2O2 as oxidizer at relatively low temperature, and the porosity of the obtained graphene nanomesh is systematically investigated on the basis of optimizing the reaction conditions. By directly vacuum filtrating the graphene nanomesh with abundant in-plane nanopores and good dispersity, the binder-free graphene film electrodes with porous layered structure and high packing density are assembled, which exhibit enhanced volumetric capacitance, remarkable rate performance, and shortened time constant.
2. Experimental section 2.1. Samples preparation Graphite oxide (Nanjing XFNANO Material Tech. Co., Ltd) was dispersed into ultrapure water and treated by ultrasonication in a water bath for 4 h, and the homogeneous GO suspension (1 mg mL 1) was obtained. Then the GO suspension (36 mL, 1 mg mL 1) was mixed with 0.4 mL H2O2 (30%) in a 50 mL Telflon autoclave under continuously stirring for 30 min and the as-formed suspension was sealed and hydrothermally treated at 100 C for 10 h, through which the holey GO abbreviated as HGO was obtained. Afterward, 0.6 mL hydrazine solution (50%) was added into the as-obtained HGO suspension and refluxed at 100 C for 1 h, and then cooled naturally to room temperature, by which HGO was reduced into holey reduced graphene oxide and the obtained sample was abbreviated as HRGO. By changing the hydrothermal treatment time, HRGO with different porosity were prepared using the same procedure, and the obtained samples were abbreviated as HRGO-x, where x represented the hydrothermal treatment time. By using a cellulose membrane with a pore size of 0.45 mm, the HRGO film was finally prepared via vacuum filtration of the HRGO suspension. As a comparative experiment, the reduced graphene oxide (RGO) film was obtained under the same condition via replacing HGO with GO. 2.2. Material characterization The morphologies of the samples were observed on Tecnai G2 F20 S-Twin Field-emission transmission electron microscopy (FETEM) operated at an acceleration voltage of 200 kV. Raman spectra were measured and collected using a Renishaw inVia Raman microscope with an excitation wavelength of 532 nm. The conductivity of the obtained RGO and HRGO film was measured by a standard four-point-probe method. Nitrogen adsorption/desorption isotherms of the resultant products were measured at 77 K on a micromeritics ASAP 2420 sorptometer. Samples were degassed at 180 C for 12 h ahead of the measurement. The apparent surface area was calculated using the Brunauer-Emmett-Teller (BET) method with the adsorption data at the relative pressure (P/P0) range of 0.05–0.20. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of 0.99, whereas the micropore volume was analyzed from the Nonlocal Density Functional Theory (NLDFT). The pore size distribution (PSD) was also determined using NLDFT model assuming the cylinder pore geometry from the desorption data. 2.3. Electrochemical measurement The electrochemical performance of the obtained film electrodes was characterized by cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) on CHI660E electrochemical workstation (CH Instruments Inc. China) via using 6.0 M KOH as electrolyte. The working electrode was prepared by directly placing the obtained film (with a weight of about 1.4 mg and an area of 0.95 cm2) between two pieces of nickel foam without any other additives and then pressed under a pressure of 5 Mpa to make the electrode material adhere to the current collector more completely. The mass loading for each electrode is typically 1.5 mg cm 2 which yields an electrode with thickness about 16 mm and packing density of 0.86 g cm 3. In a three-electrode system, Pt foil and Ag/AgCl electrode were applied as the counter and reference electrodes, respectively. The gravimetric capacitance, Cwt (F g 1) of the electrode material was calculated from the galvanostatic discharge curves according to the following equation: Cwt = I Dt/(DV m), where I is the
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discharge current (A), Dt is the discharge time (s), DV represents the voltage change (V) excluding voltage drop (IR drop) in the discharge process, and m is the mass of the active material (g). The corresponding volumetric capacitance, Cvol (F cm 3) is calculated based on the following formula: Cvol = Cwt r, where r is the packing density of the prepared film. In a two electrode cell, two same films on separate nickel foam were directly used as electrodes and separated by an ion-porous separator (Celgard13501). The gravimetric capacitance was calculated using C = 4 I Dt/(DV m) where I,Dt, D V, and m are discharge current (A), discharge time (s), voltage change (V) excluding the IR drop during the discharge process, and the total mass of the active material (g), respectively. 3. Results and discussion The fabrication process of the HRGO film is illustrated in Fig. 1. By means of the low temperature hydrothermal reaction, GO is partially reduced and simultaneously etched by H2O2, and finally forms HGO nanosheets with porous structure and outstanding dispersity. Then the obtained HGO nanosheet is further reduced by hydrazine solution, and the resultant HRGO nanosheet suspension is filtrated directly to obtain the HRGO film with good flexibility as revealed in the digital photo. The HRGO film with good flexibility is suitable for binder-free electrode material and beneficial to enhance its volumetric capacitance. What's more, the flexible supercapacitor electrodes with higher performance can be prepared by integrating the pseudocapcacitive materials into the flexible HRGO film in virtue of its outstanding dispersity. GO nanosheets possess many active defective sites, around which carbon atoms can be partially oxidized and etched by suitable oxidants, leaving behind carbon vacancies, which will gradually extend into nanopores [40]. Because H2O2 molecule is an environmentally friendly oxidant, it can be selected as the suitable oxidant to introduce abundant in-plane pores into the graphene sheets. Duan group has prepared the holey graphene hydrogels via hydrothermally treating GO suspension in a controlled amount of H2O2 at 180 C for 6 h [36]. In order to further explain the pore formation process of in-plane pores, we have conducted a series of controlled experiments under relatively lower hydrothermal temperature. GO nanosheets suspension is firstly hydrothermally treated in 0.4 mL H2O2 at different temperatures for 10 h then
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followed by a reduction process in hydrazine solution, and the TEM images of the obtained samples are shown in Fig. 2. It can be seen that a smooth surface overlapped by graphene nanosheets is observed and no obvious in-plane pores are formed when the hydrothermal temperature is 90 C (Fig. 2a). As the hydrothermal temperature increases to 100 C, the obtained HRGO possesses appropriate porosity (Fig. 2b). However, relatively higher hydrothermal temperature (110 and 120 C) would lead to a more aggressive etching, enlarging the pore size of holey graphene and breaking them into small pieces (Fig. 2c, d). On account of these results, it can be concluded that the porosity of the as-obtained products is heavily dependent on the hydrothermal temperature and it increases with increasing the hydrothermal temperature. The optimum pore-forming hydrothermal temperature is 100 C. Furthermore, the relatively low temperature hydrothermal reaction is crucial to retain some certain amount of functional groups, which endows the obtained products exhibit superior dispersity and processibility. Effect of the amount of H2O2 on pore formation is investigated at 100 C for 10 h, and the corresponding TEM images are shown in Fig. 3. It turns out that the porosity of the obtained HRGO show slight increase with increasing the amount of H2O2. When the added amount of H2O2 is below 0.4 mL, the surface of graphene nanosheets are smooth and no obvious in-plane pore is found (Fig. 3a). On the other hand, excessive H2O2 not only increases the porosity of the obtained HRGO, but also causes the graphene sheets to break into small pieces (Fig. 3c, d). These experimental results indicate that a suitable amount of H2O2 in the reaction system is favourable for the formation of in-plane pores, and the optimum amount of H2O2 is about 0.4 mL. Subsequently, the systematic time-dependent experiments are carried out with 0.4 mL H2O2 at 100 C to investigate the evolution of pore structure (Fig. 4). In comparison with the TEM image of RGO (Fig. 4a), TEM images of the obtained products at different reaction time demonstrate that the in-plane pores are gradually formed by prolonging the reaction time, and a suitable hydrothermal treatment time is essential for the formation of in-plane pores. When the hydrothermal treatment time is 10 h, the irregular nanoholes with pore sizes of 0.5–4.0 nm and uniformly distributed in graphene nanosheets can be observed (Fig. 4c), while longer hydrothermal reaction time between GO and H2O2 causes an excessive etch of graphene sheet, making it break into small pieces
Fig. 1. Schematic preparation representation of HRGO film.
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Fig. 2. TEM images of the samples obtained by hydrothermal treating GO at different temperatures with 0.4 mL H2O2 for 10 h: (a) 90 C, (b) 100 C, (c) 110 C, and (d) 120 C.
(Fig. 4d). These results reveal that a large number of in-plane pores, with the pore sizes ranging from several angstroms to the nanoscale, can be successfully formed on the surface of graphene nanosheets, and the optimum pore formation conditions are GO is hydrothermally treated with 0.4 mL H2O2 at 100 C for 10 h. Raman spectroscopy has been widely used to reveal the defect structure of graphene material, and the change of G and D bands is an effective method to provide an evidence for the pore formation on graphene nanosheets. Therefore, Raman spectra of GO, RGO and HRGO-x are tested and the experimental results are shown in Fig. 5. It can be seen that both the G and D bands are clearly existed and the intensity ratio of D and G bands (ID/IG) is different from each other. It is acknowledged that the G band at 1600 cm 1 is associated with the first order scattering of the E2g mode of sp2 carbon atoms in the hexagonal carbon framework, while the D band at 1350 cm 1 is attributed to the sp3-hybridized carbon atoms at the edges or defects on graphene basal plane [41]. Because the nanopore formation on graphene nanosheets results in an increase of defects and disorders, the intensity ratio of D and G band (ID/IG) can be used to evaluate the porosity of the obtained HRGO film. The ID/IG ratio is 1.22 for GO sample, which is decreased to 0.69 for RGO sample due to the removal of the oxygen-containing functional groups. Moreover, the ID/IG ratio of the HRGO samples increases with prolonging the hydrothermal treatment time, and it increases from 0.69 for RGO to 0.82 for HRGO-8 and 0.91 for HRGO-12 (Table 1), suggesting the increase of the defects and disorders due to the formation of in-plane pores by oxidizing the carbon atoms with H2O2. The Raman spectrum results are well agreed with TEM images, clearly supporting the formation of in-plane pores on graphene nanosheets.
The porous structure of the obtained HRGO samples is further determined by N2 adsorption/desorption test. It can be seen that all the isotherms of the obtained samples belong to type IV with a pronounced hysteresis loop (Fig. 6), indicating the presence of mesopores derived from the assembling of the graphene nanosheets [42]. It is known that the hysteresis loop observed in the adsorption-desorption isotherm is attributed to the capillary condensation at high relative pressure, while the smaller pores will possess a significant decay of the capillary condensation, leading to the hysteresis loop occurred at a lower relative pressure [43]. Therefore, the formation of in-plane pores on graphene nanosheets will make the pore size decrease. In comparison with RGO, the specific surface area of the obtained HRGO samples obviously increases due to the uniformly distributed nanopores in the basal plane of graphene nanosheets, suggesting that H2O2 etching can develop additional nanopores with a broad pore size distribution ranging from 0.6–5.0 nm. Moreover, Vmicro/Vtotal ratio of the obtained HRGO samples also gradually increases with prolonging the hydrothermal treatment time, and it increases from 0.083 for RGO to 0.167 for HRGO-12 due to the continuous increase of micropore volume and the decrease of total pore volume (Table 1). These results suggest that the formation of in-plane pores not only increase the specific surface area of the obtained HRGO samples, but also lead to the increase of micropore volume. As imagined that the porous structure of HRGO will break the conjugated structure and localized p-electrons, which results in the decrease of its conductivity. Consequently, the conductivity of RGO and HRGO films assembled by vacuum filtration of the corresponding suspension is directly measured by a standard four point probe method. In comparison with RGO film, the
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Fig. 3. TEM images of the samples obtained by hydrothermal treating GO at 100 C for 10 h with different amounts of H2O2: (a) 0.2 mL, (b) 0.4 mL, (c) 0.6 mL, and (d) 0.8 mL.
conductivity of the obtained HRGO film decreases gradually with the increase of hydrothermal treatment time (Table 1). As a consequence, the balance between porosity and conductivity is of significance to ensure the desirable electrochemical properties of the obtained film electrodes. As expected, the unique porous structure of HRGO film is beneficial for high performance supercapacitor and its electrochemical property was firstly evaluated by a three electrode cell with 6.0 M KOH as aqueous electrolyte. In comparison with RGO film, the CV curves of HRGO-x films within the potential range from 1.1 to 0.2 V are much closer to rectangular at a high scan rate of 200 mV s 1, indicating a more excellent electrical double-layer capacitive behavior and lower contact resistance (Fig. 7a). Moreover, the triangular charge-discharge profiles of the HRGOx films also reflect the ideal electrical double-layer capacitance arising from the electro-adsorption of ions on the electrode surface (Fig. 7b). The specific capacitances of RGO and HRGO-x film electrodes calculated from their discharge curves at a current density of 1 A g 1 gradually increase with prolonging the hydrothermal treatment time, and a maximum value of 251 F g 1 is achieved for HRGO-10 film due to its suitable porosity (Table 1). In addition, according to the high packing density of 0.86 g cm 3, HRGO-10 film electrode has a high volumetric capacitance up to 216 F cm 3, which is considerably higher than values reported for graphene electrode materials in aqueous electrolytes (Table S1) [19,44–46], suggesting that HRGO-10 film electrode possesses good performance for energy storage devices in practical applications. It is expected to improve the rate performance of the obtained HRGO film electrodes by introducing in-plane pores into graphene
nanosheets. When the current density increases from 1 to 60 A g 1, the capacitance retention is 63%, 68%, 73% and 68% for RGO, HRGO-8, HRGO-10, and HRGO-12 film electrodes, respectively (Fig. 7c), suggesting that the formation of in-plane pores on graphene nanosheets facilitates the rapid diffusion of electrolyte ions. The ion transport properties within the electrodes are also investigated by electrochemical impedance spectroscopy (EIS) in the frequency range of 0.01 Hz to 100 kHz. It can be seen that the Nyquist plots of the RGO and HRGO-x film electrodes are similar to one another, which are composed of an arc at high frequency region and a straight line at low frequency region (Fig. 7d). By contrast with RGO film electrode, the Nyquist plots of HRGO-x film electrodes show shorter 45 Warburg region followed by the semicircle with smaller diameter at high frequency region (inset in Fig. 7d), which is ascribed to the formation of in-plane pores on graphene nanosheets. Moreover, the shortest 45 Warburg region and the smallest semi-circle of HRGO-10 film electrode clearly demonstrate that the optimum porosity of the obtained film is pivotal to guarantee the easy access of electrolyte ions within the electrode. In order to further understand the charge transfer reaction at the interface between electrolyte and electrode, the semicircle at high frequency range is amplified and the charge transfer resistance (Rct) is calculated by extrapolation of the semicircle on the real impedance axis. After fitting EIS spectra using the equivalent circuit diagram, the charge transfer resistances are 5.58, 4.51, 1.15 and 2.59 V for RGO, HRGO-8, HRGO-10, and HRGO-12 film electrodes, respectively. HRGO-10 film electrode shows the smallest Rct value, suggesting that the rate performance can be improved by forming in-plane pores on graphene nanosheets due to the accelerated ion diffusion rate across the entire
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Fig. 4. TEM images of (a) RGO and samples obtained by hydrothermal treating GO at 100 C with 0.4 mL H2O2 for different reaction time: (b) 8 h, (c) 10 h, and (d) 12 h.
film. It is noteworthy to assert that the porous structure of HRGO-x film electrodes not only provide an open network for efficient electrolyte ion diffusion, but also functions as a conductive network for electrical transport to enable superior capacitive performance. Consequently, the balance between electrolyte ion diffusion rate and conductivity can be successfully controlled by varying the porosity of the obtained film. In order to further investigate the reliable capacitive performance of HRGO-10 film electrode, it is employed to assemble a
Fig. 5. Raman spectra of samples: (a) GO, (b) RGO, (c) HRGO-8, (d) HRGO-10, and (e) HRGO-12.
symmetric EC (HRGO-10 EC) with 6.0 M KOH as aqueous electrolyte. By contrast, we have also assembled the RGO film based symmetric EC (RGO EC). In comparison with RGO EC, CV curve of HRGO-10 EC at a scan rate of 200 mV s 1 within a potential window of 0–1.0 V shows a remarkable enhanced current response and a rectangular CV shape (Fig. 8a), indicating more energy can be stored in HRGO-10 film electrode through a fast ion adsorption mechanism. Interestingly, there is a slight distortion of RGO EC as compared with HRGO-10 EC, suggesting the latter one has a faster charge/discharge stability and more excellent rate capability. The gravimetric capacitances of RGO and HRGO-10 film electrodes at various current densities from 0.5 to 30 A g 1 are tested and the experimental results are shown in Fig. 8b. It can be seen that the specific capacitance of HRGO-10 film electrode still remains as high as 148 F g 1 even at high current density of 30 A g 1 with an excellent capacitance retention of 78%, which is significantly better than RGO film electrode (64%), suggesting that the unique porous structure is favorable for enhancing the ion kinetics. It is recognized that the improved capacitance retention is intrinsically determined by the internal resistance of the supercapacitor, which can be manifested in the voltage drop (IR drop) at the beginning of discharge curves. The HRGO-10 EC shows a smaller voltage drop (0.04 V) than the RGO EC (0.12 V) at the current density of 10 A g 1 (Fig. 8c), implying a lower equivalent series resistance (ESR) for HRGO-10 EC due to the considerable advantages of the porous structure of HRGO-10 film. Furthermore, the electrochemical impedance spectroscopy (EIS) is used to investigate the ion diffusion dynamics within HRGO-10 EC and RGO EC. It can be seen that the Nyquist plots obtained over a frequency range from 100 kHz to 0.01 Hz show a
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Table 1 Structural parameters and electrochemical properties of RGO and HRGO-x. Samples RGO HRGO-8 HRGO-10 HRGO-12
SBET (m2 g 552 744 763 729
Vtotal
Vmicro/Vtotal
ID/IG
Conductivity (S m 1)
Gravimetric capacitance (1 A g 1, F g 1)
Volumetric capacitance (F cm 3)
Capacitance Retention (%) (1–60 A g 1)
0.44 0.47 0.45 0.42
0.083 0.128 0.156 0.167
0.69 0.82 0.87 0.91
3287 2130 1786 1319
188 213 251 205
162 183 216 176
63 68 73 68
1
)
Fig. 6. N2 adsorption/desorption isotherms of RGO, HRGO-8, HRGO-10 and HRGO12; the DFT pore size distribution of HRGO-10 sample (inserted).
vertical line in the low-frequency regime, indicating a nearly ideal capacitive property for HRGO-10 and RGO ECs. A close-up view of the high-frequency regime of the Nyquist plots reveals a semicircle with smaller diameter and shorter 45 Warburg region for HRGO10 EC, suggesting a lower charge transfer resistance and more rapid
ion diffusion within the HRGO-10 EC due to the porous structure of HRGO-10 film (Fig. 8d). By extrapolating the semicircle to the real axis, the ESR is about 2.09 V for HRGO-10 EC, and it is smaller than that of RGO EC (5.89 V) which is consistent with the results of galvanostatic charge/discharge studies. Moreover, the discharge speed of supercapacitor can also be evaluated by a relaxation time constant t 0, and it is defined by t 0 = 1/f0 at a phase angle of 45 , where the resistive and capacitive impedances are equal [47]. The t 0 value is 0.67 s for HRGO-10 EC, which is smaller than that of RGO EC (1.51 s), further supporting the significantly enhanced ion transport rate within the HRGO-10 film electrode (Fig. 8e). The t 0 value is also smaller than those of RGO reduced by urea based EC (4.2 s) [18] and graphene aerogel based EC (0.73 s) [33] in the same aqueous electrolyte, suggesting that the in-plane nanopores dramatically promote the kinetic diffusion of electrolyte ions in the interior of the electrodes. Cycling stability of HRGO-10 EC tested by a consecutive charge/discharge technique at a constant current of 5 A g 1 shows that a capacitance retention of 94% is achieved after 6000 cycles, which is comparable to the expected value for an ideal electrical double-layer capacitor (Fig. 8f). From the above experimental results, it can be concluded that HRGO10 film not only delivers much higher specific capacitance than RGO film, but also exhibits remarkable rate performance and excellent cycling stability, which is satisfactory for binder-free film electrode materials with high performance for supercapacitor.
Fig. 7. Electrochemical characterization of RGO, HRGO-8, HRGO-10, and HRGO-12 film electrodes: (a) CV curves at a scan rate of 200 mV s 1, (b) Galvanostatic charge/ discharge curves at a current density of 1 A g 1, (c) Plots of specific capacitances at various current densities, and (d) The electrochemical impedance spectra measured at frequency range from 100 kHz to 0.01 Hz (inserted: the close-up view of the high-frequency region).
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Fig. 8. Electrochemical characterization of HRGO-10 and RGO ECs with 6.0 M KOH as aqueous electrolyte in a potential window of 0–1.0 V: (a) CV curves at 200 mV s 1, (b) specific capacitance at different current densities, (c) galvanostatic charge/discharge curves at 10 A g 1, (d) Nyquist plots and the inset shows the close-up view of the high-frequency region, (e) bode plots of phase angle versus frequency, and (f) cycling stability of HRGO-10 EC at a current density of 5 A g 1.
4. Conclusions
References
A mild defect-etching method is further improved to prepare holey graphene with abundant in-plane nanopores, and its porosity connects with the hydrothermal reaction conditions between GO and H2O2. The optimum pore forming conditions are that GO is hydrothermally treated in 0.4 mL H2O2 at 100 C for 10 h and followed by the reduction reaction in hydrazine solution. By means of the simple vacuum filtration of the obtained well-dispersed HRGO suspension, the binder-free graphene film electrodes with porous layered structure can be successfully assembled. The obtained film electrodes not only exhibit enhanced gravimetric capacitance and improved rate capability, but also show high packing density and high volumetric capacitance. The optimum porosity plays a key role between efficient electrolyte ion diffusion rate and conductivity of the obtained film electrode, which sheds a promising light on the configuration of graphene film electrodes with high volumetric capacitance. In summary, this is an efficient method to introduce in-plane pores into graphene nanosheets without sacrificing dispersity and processibility of the resultant graphene nanomesh.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (51172137, 21471093), the Program for Key Science & Technology Innovation Team of Shaanxi Province (2012KCT-21), the 111 Project, and the Fundamental Research Funds for the Central Universities (GK201301002 and GK201501007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2015. 11.090.
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Formation process of holey graphene and its assembled binder-free film electrode with high volumetric capacitance Yonglong Baia,b , Xiaofan Yanga,b , Yibo Hea,b , Jinyang Zhanga,b , Liping Kanga,b , Hua Xua,b , Feng Shia,b , Zhibin Leia,b , Zong-Huai Liua,b,* a b
Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, Xi’an 710062, PR China School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 October 2015 Received in revised form 4 November 2015 Accepted 17 November 2015 Available online 1 December 2015
Holey graphene with abundant in-plane nanopores is achieved through a mild defect-etching reaction between graphene oxide (GO) and hydrogen peroxide (H2O2) then followed by a reduction process in hydrazine solution. The porosity of the obtained holey graphene is systematically investigated by optimizing the reaction conditions between GO and H2O2. The optimum reaction conditions are that GO is hydrothermally treated in 0.4 mL H2O2 at 100 C for 10 h and the as-prepared holey graphene oxide (HGO) is refluxed in hydrazine solution at 100 C for 1 h, by which holey reduced graphene oxide (HRGO) suspension with good dispersity is obtained. By vacuum filtration of the HRGO suspension, the binderfree porous graphene film electrodes are successfully assembled. The obtained film electrodes exhibit high specific capacitance (251 F g 1 at a current density of 1 A g 1), high volumetric capacitance (up to 216 F cm 3), and enhanced rate capability (73% capacitance retention from 1–60 A g 1) due to their high packing density (0.86 g cm 3). The suitable porosity of the assembled porous graphene film keeps a balance between electrolyte ion diffusion rate and conductivity, which can bridge the gap between gravimetric capacitance and volumetric capacitance for the obtained electrode material. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: holey graphene hydrogen peroxide binder-free film electrode enhanced rate capability volumetric capacitance
1. Introduction Electrochemical capacitors (ECs) are one of the most effective and practical energy storage and conversion devices [1], which can bridge the critical performance gap between the high energy density of secondary battery and high power density of conventional dielectric capacitor, making it possible for the rapid energy storage and release [2]. The capacitive performance of the ECs lies in electrode materials, electrolyte, and assembled technology, with the most important factor being the electrode materials [3]. Among the electrode materials with different structures and morphologies, the flexible and binder-free film electrodes with high packing density have shown great promise in upcoming nextgeneration portable and flexible electronics such as roll-up displays, photovoltaic cells, and wearable devices [4–6]. Film architecture of the electrode materials can improve the electrolyte access, because they can show low ion diffusion length and
* Correspondence author at: School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, Shaanxi, PR China. Tel.: +86 29 81530716, fax: +86 29 81530702. E-mail address: [email protected] (Z.-H. Liu). http://dx.doi.org/10.1016/j.electacta.2015.11.090 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
reduced electron transfer resistance [7–9]. According to the available literature to date, most of the working electrodes are binder-enriched ones, which are prepared by the traditional slurry-coating technology [10–12]. Evidently, the added binder will decrease the electrical conductivity of the electrode materials, hindering their potential application in high performance supercapacitors [13]. In order to obtain the ideal electrochemical performance, it is also urgent to develop flexible and binder-free film electrode materials for supercapacitors. Graphene has attracted significant attention in recent years because of its extraordinary electrical conductivity, high surface area, chemical stability, and a distinct 2D nanostructure [14]. Graphene nanosheets can be acquired by delaminating their bulk materials with layered structure, and the obtained nanosheets are new classes of nanoscale materials, which can be used to assemble binder-free film electrodes with excellent flexibility and high packing density [15,16]. However, there are three key points remained to be solved when graphene nanosheets are used to assemble binder-free film electrodes. Firstly, the irreversible aggregation tendency of graphene nanosheets due to the strong p p stacking and van der Waals interactions between different graphene nanosheets should be prevented, because it will give rise to a significant deterioration of their properties including severely
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reduced specific surface area and much lower mass diffusion rate [17]. Secondly, the efficient ion transport pathway in the vertical direction of the film electrode is needed, which is favourable for improving the rate capability of the electrode materials [18]. Thirdly, the dispersity and processability of graphene nanosheets is needed for assembling the film electrode with high packing density [19]. Up to now, the preparation of graphene hydrogels with three-dimensional hierarchical porous network or holey graphene oxides with abundant in-plane nanopores are probably effective ways to solve the above challenges [20–22]. Porous graphene can be mainly classified into crumpled graphene, graphene foam, and graphene nanomesh [23]. The crumpled graphene with a wrinkled configuration is constructed with the bending and folding graphene nanosheets [24]. The graphene foam is a kind of 3D macroscopic graphene architecture via self-assembly of the 2D flexible graphene nanosheet or by chemical vapor deposition (CVD) [25,26]. The graphene nanomesh, also called holey graphene, is characterized as the existence of abundant in-plane pores with the pore sizes ranging from several angstroms to the nano-scale [27]. In comparison with the crumpled graphene and the graphene foam, the graphene nanomesh possesses higher surface area and much more active sites and edges for the ion diffusion shortcuts between different layers of graphene, thus they can be used to assemble outstanding electrode materials for supercapacitor [28]. The graphene nanomesh can be obtained by means of plasma etching [29,30] template direction [18,31], chemical activation [32,33] and catalytic oxidation [34,35], and so on. Although the graphene nanomesh with sufficient nanoscale periodic or quasi-periodic nanoholes can be prepared by above methods, the electrode materials assembled from these graphene nanomeshes show relatively low packing density, which generates low volumetric capacitance [1,32]. On the other hand, solution oxidation method is favourable for obtaining graphene nanomesh with good dispersity and processability, which would be able to conveniently process into holey graphene film with high packing density and bridge the gap between gravimetric capacitance and volumetric capacitance [22,36–38]. Shi and Liu groups have developed a solution approach for the production of graphene nanomesh with nitric acid, but the pollutant derived from nitric acid should be attracted more attention [37,39]. In virtue of the environmental-friendly merit of H2O2 used as oxidizing and etching reagent, Duan group has prepared the graphene nanomesh film with a packing density of 0.71 g cm 3 by mechanically compressing porous graphene hydrogel, and the prepared film shows simultaneously relatively high gravimetric and volumetric capacitances while retaining excellent rate capability [36]. However, the resultant holey graphene is in absence of dispersity and processability, which will preclude the integration of various pseudocapacitive materials into the holey graphene film to further boost its specific capacitance and energy density. From the foregoing, graphene nanomesh with abundant in-plane nanopores can be prepared by a convenient mild defectetching reaction, while its dispersity and processability should be further improved for the high packing density and high volumetric capacitance of the film electrode. In the present work, a mild defect-etching reaction is used to produce holey graphene with abundant in-plane nanopores by using H2O2 as oxidizer at relatively low temperature, and the porosity of the obtained graphene nanomesh is systematically investigated on the basis of optimizing the reaction conditions. By directly vacuum filtrating the graphene nanomesh with abundant in-plane nanopores and good dispersity, the binder-free graphene film electrodes with porous layered structure and high packing density are assembled, which exhibit enhanced volumetric capacitance, remarkable rate performance, and shortened time constant.
2. Experimental section 2.1. Samples preparation Graphite oxide (Nanjing XFNANO Material Tech. Co., Ltd) was dispersed into ultrapure water and treated by ultrasonication in a water bath for 4 h, and the homogeneous GO suspension (1 mg mL 1) was obtained. Then the GO suspension (36 mL, 1 mg mL 1) was mixed with 0.4 mL H2O2 (30%) in a 50 mL Telflon autoclave under continuously stirring for 30 min and the as-formed suspension was sealed and hydrothermally treated at 100 C for 10 h, through which the holey GO abbreviated as HGO was obtained. Afterward, 0.6 mL hydrazine solution (50%) was added into the as-obtained HGO suspension and refluxed at 100 C for 1 h, and then cooled naturally to room temperature, by which HGO was reduced into holey reduced graphene oxide and the obtained sample was abbreviated as HRGO. By changing the hydrothermal treatment time, HRGO with different porosity were prepared using the same procedure, and the obtained samples were abbreviated as HRGO-x, where x represented the hydrothermal treatment time. By using a cellulose membrane with a pore size of 0.45 mm, the HRGO film was finally prepared via vacuum filtration of the HRGO suspension. As a comparative experiment, the reduced graphene oxide (RGO) film was obtained under the same condition via replacing HGO with GO. 2.2. Material characterization The morphologies of the samples were observed on Tecnai G2 F20 S-Twin Field-emission transmission electron microscopy (FETEM) operated at an acceleration voltage of 200 kV. Raman spectra were measured and collected using a Renishaw inVia Raman microscope with an excitation wavelength of 532 nm. The conductivity of the obtained RGO and HRGO film was measured by a standard four-point-probe method. Nitrogen adsorption/desorption isotherms of the resultant products were measured at 77 K on a micromeritics ASAP 2420 sorptometer. Samples were degassed at 180 C for 12 h ahead of the measurement. The apparent surface area was calculated using the Brunauer-Emmett-Teller (BET) method with the adsorption data at the relative pressure (P/P0) range of 0.05–0.20. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of 0.99, whereas the micropore volume was analyzed from the Nonlocal Density Functional Theory (NLDFT). The pore size distribution (PSD) was also determined using NLDFT model assuming the cylinder pore geometry from the desorption data. 2.3. Electrochemical measurement The electrochemical performance of the obtained film electrodes was characterized by cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) on CHI660E electrochemical workstation (CH Instruments Inc. China) via using 6.0 M KOH as electrolyte. The working electrode was prepared by directly placing the obtained film (with a weight of about 1.4 mg and an area of 0.95 cm2) between two pieces of nickel foam without any other additives and then pressed under a pressure of 5 Mpa to make the electrode material adhere to the current collector more completely. The mass loading for each electrode is typically 1.5 mg cm 2 which yields an electrode with thickness about 16 mm and packing density of 0.86 g cm 3. In a three-electrode system, Pt foil and Ag/AgCl electrode were applied as the counter and reference electrodes, respectively. The gravimetric capacitance, Cwt (F g 1) of the electrode material was calculated from the galvanostatic discharge curves according to the following equation: Cwt = I Dt/(DV m), where I is the
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discharge current (A), Dt is the discharge time (s), DV represents the voltage change (V) excluding voltage drop (IR drop) in the discharge process, and m is the mass of the active material (g). The corresponding volumetric capacitance, Cvol (F cm 3) is calculated based on the following formula: Cvol = Cwt r, where r is the packing density of the prepared film. In a two electrode cell, two same films on separate nickel foam were directly used as electrodes and separated by an ion-porous separator (Celgard13501). The gravimetric capacitance was calculated using C = 4 I Dt/(DV m) where I,Dt, D V, and m are discharge current (A), discharge time (s), voltage change (V) excluding the IR drop during the discharge process, and the total mass of the active material (g), respectively. 3. Results and discussion The fabrication process of the HRGO film is illustrated in Fig. 1. By means of the low temperature hydrothermal reaction, GO is partially reduced and simultaneously etched by H2O2, and finally forms HGO nanosheets with porous structure and outstanding dispersity. Then the obtained HGO nanosheet is further reduced by hydrazine solution, and the resultant HRGO nanosheet suspension is filtrated directly to obtain the HRGO film with good flexibility as revealed in the digital photo. The HRGO film with good flexibility is suitable for binder-free electrode material and beneficial to enhance its volumetric capacitance. What's more, the flexible supercapacitor electrodes with higher performance can be prepared by integrating the pseudocapcacitive materials into the flexible HRGO film in virtue of its outstanding dispersity. GO nanosheets possess many active defective sites, around which carbon atoms can be partially oxidized and etched by suitable oxidants, leaving behind carbon vacancies, which will gradually extend into nanopores [40]. Because H2O2 molecule is an environmentally friendly oxidant, it can be selected as the suitable oxidant to introduce abundant in-plane pores into the graphene sheets. Duan group has prepared the holey graphene hydrogels via hydrothermally treating GO suspension in a controlled amount of H2O2 at 180 C for 6 h [36]. In order to further explain the pore formation process of in-plane pores, we have conducted a series of controlled experiments under relatively lower hydrothermal temperature. GO nanosheets suspension is firstly hydrothermally treated in 0.4 mL H2O2 at different temperatures for 10 h then
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followed by a reduction process in hydrazine solution, and the TEM images of the obtained samples are shown in Fig. 2. It can be seen that a smooth surface overlapped by graphene nanosheets is observed and no obvious in-plane pores are formed when the hydrothermal temperature is 90 C (Fig. 2a). As the hydrothermal temperature increases to 100 C, the obtained HRGO possesses appropriate porosity (Fig. 2b). However, relatively higher hydrothermal temperature (110 and 120 C) would lead to a more aggressive etching, enlarging the pore size of holey graphene and breaking them into small pieces (Fig. 2c, d). On account of these results, it can be concluded that the porosity of the as-obtained products is heavily dependent on the hydrothermal temperature and it increases with increasing the hydrothermal temperature. The optimum pore-forming hydrothermal temperature is 100 C. Furthermore, the relatively low temperature hydrothermal reaction is crucial to retain some certain amount of functional groups, which endows the obtained products exhibit superior dispersity and processibility. Effect of the amount of H2O2 on pore formation is investigated at 100 C for 10 h, and the corresponding TEM images are shown in Fig. 3. It turns out that the porosity of the obtained HRGO show slight increase with increasing the amount of H2O2. When the added amount of H2O2 is below 0.4 mL, the surface of graphene nanosheets are smooth and no obvious in-plane pore is found (Fig. 3a). On the other hand, excessive H2O2 not only increases the porosity of the obtained HRGO, but also causes the graphene sheets to break into small pieces (Fig. 3c, d). These experimental results indicate that a suitable amount of H2O2 in the reaction system is favourable for the formation of in-plane pores, and the optimum amount of H2O2 is about 0.4 mL. Subsequently, the systematic time-dependent experiments are carried out with 0.4 mL H2O2 at 100 C to investigate the evolution of pore structure (Fig. 4). In comparison with the TEM image of RGO (Fig. 4a), TEM images of the obtained products at different reaction time demonstrate that the in-plane pores are gradually formed by prolonging the reaction time, and a suitable hydrothermal treatment time is essential for the formation of in-plane pores. When the hydrothermal treatment time is 10 h, the irregular nanoholes with pore sizes of 0.5–4.0 nm and uniformly distributed in graphene nanosheets can be observed (Fig. 4c), while longer hydrothermal reaction time between GO and H2O2 causes an excessive etch of graphene sheet, making it break into small pieces
Fig. 1. Schematic preparation representation of HRGO film.
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Fig. 2. TEM images of the samples obtained by hydrothermal treating GO at different temperatures with 0.4 mL H2O2 for 10 h: (a) 90 C, (b) 100 C, (c) 110 C, and (d) 120 C.
(Fig. 4d). These results reveal that a large number of in-plane pores, with the pore sizes ranging from several angstroms to the nanoscale, can be successfully formed on the surface of graphene nanosheets, and the optimum pore formation conditions are GO is hydrothermally treated with 0.4 mL H2O2 at 100 C for 10 h. Raman spectroscopy has been widely used to reveal the defect structure of graphene material, and the change of G and D bands is an effective method to provide an evidence for the pore formation on graphene nanosheets. Therefore, Raman spectra of GO, RGO and HRGO-x are tested and the experimental results are shown in Fig. 5. It can be seen that both the G and D bands are clearly existed and the intensity ratio of D and G bands (ID/IG) is different from each other. It is acknowledged that the G band at 1600 cm 1 is associated with the first order scattering of the E2g mode of sp2 carbon atoms in the hexagonal carbon framework, while the D band at 1350 cm 1 is attributed to the sp3-hybridized carbon atoms at the edges or defects on graphene basal plane [41]. Because the nanopore formation on graphene nanosheets results in an increase of defects and disorders, the intensity ratio of D and G band (ID/IG) can be used to evaluate the porosity of the obtained HRGO film. The ID/IG ratio is 1.22 for GO sample, which is decreased to 0.69 for RGO sample due to the removal of the oxygen-containing functional groups. Moreover, the ID/IG ratio of the HRGO samples increases with prolonging the hydrothermal treatment time, and it increases from 0.69 for RGO to 0.82 for HRGO-8 and 0.91 for HRGO-12 (Table 1), suggesting the increase of the defects and disorders due to the formation of in-plane pores by oxidizing the carbon atoms with H2O2. The Raman spectrum results are well agreed with TEM images, clearly supporting the formation of in-plane pores on graphene nanosheets.
The porous structure of the obtained HRGO samples is further determined by N2 adsorption/desorption test. It can be seen that all the isotherms of the obtained samples belong to type IV with a pronounced hysteresis loop (Fig. 6), indicating the presence of mesopores derived from the assembling of the graphene nanosheets [42]. It is known that the hysteresis loop observed in the adsorption-desorption isotherm is attributed to the capillary condensation at high relative pressure, while the smaller pores will possess a significant decay of the capillary condensation, leading to the hysteresis loop occurred at a lower relative pressure [43]. Therefore, the formation of in-plane pores on graphene nanosheets will make the pore size decrease. In comparison with RGO, the specific surface area of the obtained HRGO samples obviously increases due to the uniformly distributed nanopores in the basal plane of graphene nanosheets, suggesting that H2O2 etching can develop additional nanopores with a broad pore size distribution ranging from 0.6–5.0 nm. Moreover, Vmicro/Vtotal ratio of the obtained HRGO samples also gradually increases with prolonging the hydrothermal treatment time, and it increases from 0.083 for RGO to 0.167 for HRGO-12 due to the continuous increase of micropore volume and the decrease of total pore volume (Table 1). These results suggest that the formation of in-plane pores not only increase the specific surface area of the obtained HRGO samples, but also lead to the increase of micropore volume. As imagined that the porous structure of HRGO will break the conjugated structure and localized p-electrons, which results in the decrease of its conductivity. Consequently, the conductivity of RGO and HRGO films assembled by vacuum filtration of the corresponding suspension is directly measured by a standard four point probe method. In comparison with RGO film, the
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Fig. 3. TEM images of the samples obtained by hydrothermal treating GO at 100 C for 10 h with different amounts of H2O2: (a) 0.2 mL, (b) 0.4 mL, (c) 0.6 mL, and (d) 0.8 mL.
conductivity of the obtained HRGO film decreases gradually with the increase of hydrothermal treatment time (Table 1). As a consequence, the balance between porosity and conductivity is of significance to ensure the desirable electrochemical properties of the obtained film electrodes. As expected, the unique porous structure of HRGO film is beneficial for high performance supercapacitor and its electrochemical property was firstly evaluated by a three electrode cell with 6.0 M KOH as aqueous electrolyte. In comparison with RGO film, the CV curves of HRGO-x films within the potential range from 1.1 to 0.2 V are much closer to rectangular at a high scan rate of 200 mV s 1, indicating a more excellent electrical double-layer capacitive behavior and lower contact resistance (Fig. 7a). Moreover, the triangular charge-discharge profiles of the HRGOx films also reflect the ideal electrical double-layer capacitance arising from the electro-adsorption of ions on the electrode surface (Fig. 7b). The specific capacitances of RGO and HRGO-x film electrodes calculated from their discharge curves at a current density of 1 A g 1 gradually increase with prolonging the hydrothermal treatment time, and a maximum value of 251 F g 1 is achieved for HRGO-10 film due to its suitable porosity (Table 1). In addition, according to the high packing density of 0.86 g cm 3, HRGO-10 film electrode has a high volumetric capacitance up to 216 F cm 3, which is considerably higher than values reported for graphene electrode materials in aqueous electrolytes (Table S1) [19,44–46], suggesting that HRGO-10 film electrode possesses good performance for energy storage devices in practical applications. It is expected to improve the rate performance of the obtained HRGO film electrodes by introducing in-plane pores into graphene
nanosheets. When the current density increases from 1 to 60 A g 1, the capacitance retention is 63%, 68%, 73% and 68% for RGO, HRGO-8, HRGO-10, and HRGO-12 film electrodes, respectively (Fig. 7c), suggesting that the formation of in-plane pores on graphene nanosheets facilitates the rapid diffusion of electrolyte ions. The ion transport properties within the electrodes are also investigated by electrochemical impedance spectroscopy (EIS) in the frequency range of 0.01 Hz to 100 kHz. It can be seen that the Nyquist plots of the RGO and HRGO-x film electrodes are similar to one another, which are composed of an arc at high frequency region and a straight line at low frequency region (Fig. 7d). By contrast with RGO film electrode, the Nyquist plots of HRGO-x film electrodes show shorter 45 Warburg region followed by the semicircle with smaller diameter at high frequency region (inset in Fig. 7d), which is ascribed to the formation of in-plane pores on graphene nanosheets. Moreover, the shortest 45 Warburg region and the smallest semi-circle of HRGO-10 film electrode clearly demonstrate that the optimum porosity of the obtained film is pivotal to guarantee the easy access of electrolyte ions within the electrode. In order to further understand the charge transfer reaction at the interface between electrolyte and electrode, the semicircle at high frequency range is amplified and the charge transfer resistance (Rct) is calculated by extrapolation of the semicircle on the real impedance axis. After fitting EIS spectra using the equivalent circuit diagram, the charge transfer resistances are 5.58, 4.51, 1.15 and 2.59 V for RGO, HRGO-8, HRGO-10, and HRGO-12 film electrodes, respectively. HRGO-10 film electrode shows the smallest Rct value, suggesting that the rate performance can be improved by forming in-plane pores on graphene nanosheets due to the accelerated ion diffusion rate across the entire
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Fig. 4. TEM images of (a) RGO and samples obtained by hydrothermal treating GO at 100 C with 0.4 mL H2O2 for different reaction time: (b) 8 h, (c) 10 h, and (d) 12 h.
film. It is noteworthy to assert that the porous structure of HRGO-x film electrodes not only provide an open network for efficient electrolyte ion diffusion, but also functions as a conductive network for electrical transport to enable superior capacitive performance. Consequently, the balance between electrolyte ion diffusion rate and conductivity can be successfully controlled by varying the porosity of the obtained film. In order to further investigate the reliable capacitive performance of HRGO-10 film electrode, it is employed to assemble a
Fig. 5. Raman spectra of samples: (a) GO, (b) RGO, (c) HRGO-8, (d) HRGO-10, and (e) HRGO-12.
symmetric EC (HRGO-10 EC) with 6.0 M KOH as aqueous electrolyte. By contrast, we have also assembled the RGO film based symmetric EC (RGO EC). In comparison with RGO EC, CV curve of HRGO-10 EC at a scan rate of 200 mV s 1 within a potential window of 0–1.0 V shows a remarkable enhanced current response and a rectangular CV shape (Fig. 8a), indicating more energy can be stored in HRGO-10 film electrode through a fast ion adsorption mechanism. Interestingly, there is a slight distortion of RGO EC as compared with HRGO-10 EC, suggesting the latter one has a faster charge/discharge stability and more excellent rate capability. The gravimetric capacitances of RGO and HRGO-10 film electrodes at various current densities from 0.5 to 30 A g 1 are tested and the experimental results are shown in Fig. 8b. It can be seen that the specific capacitance of HRGO-10 film electrode still remains as high as 148 F g 1 even at high current density of 30 A g 1 with an excellent capacitance retention of 78%, which is significantly better than RGO film electrode (64%), suggesting that the unique porous structure is favorable for enhancing the ion kinetics. It is recognized that the improved capacitance retention is intrinsically determined by the internal resistance of the supercapacitor, which can be manifested in the voltage drop (IR drop) at the beginning of discharge curves. The HRGO-10 EC shows a smaller voltage drop (0.04 V) than the RGO EC (0.12 V) at the current density of 10 A g 1 (Fig. 8c), implying a lower equivalent series resistance (ESR) for HRGO-10 EC due to the considerable advantages of the porous structure of HRGO-10 film. Furthermore, the electrochemical impedance spectroscopy (EIS) is used to investigate the ion diffusion dynamics within HRGO-10 EC and RGO EC. It can be seen that the Nyquist plots obtained over a frequency range from 100 kHz to 0.01 Hz show a
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Table 1 Structural parameters and electrochemical properties of RGO and HRGO-x. Samples RGO HRGO-8 HRGO-10 HRGO-12
SBET (m2 g 552 744 763 729
Vtotal
Vmicro/Vtotal
ID/IG
Conductivity (S m 1)
Gravimetric capacitance (1 A g 1, F g 1)
Volumetric capacitance (F cm 3)
Capacitance Retention (%) (1–60 A g 1)
0.44 0.47 0.45 0.42
0.083 0.128 0.156 0.167
0.69 0.82 0.87 0.91
3287 2130 1786 1319
188 213 251 205
162 183 216 176
63 68 73 68
1
)
Fig. 6. N2 adsorption/desorption isotherms of RGO, HRGO-8, HRGO-10 and HRGO12; the DFT pore size distribution of HRGO-10 sample (inserted).
vertical line in the low-frequency regime, indicating a nearly ideal capacitive property for HRGO-10 and RGO ECs. A close-up view of the high-frequency regime of the Nyquist plots reveals a semicircle with smaller diameter and shorter 45 Warburg region for HRGO10 EC, suggesting a lower charge transfer resistance and more rapid
ion diffusion within the HRGO-10 EC due to the porous structure of HRGO-10 film (Fig. 8d). By extrapolating the semicircle to the real axis, the ESR is about 2.09 V for HRGO-10 EC, and it is smaller than that of RGO EC (5.89 V) which is consistent with the results of galvanostatic charge/discharge studies. Moreover, the discharge speed of supercapacitor can also be evaluated by a relaxation time constant t 0, and it is defined by t 0 = 1/f0 at a phase angle of 45 , where the resistive and capacitive impedances are equal [47]. The t 0 value is 0.67 s for HRGO-10 EC, which is smaller than that of RGO EC (1.51 s), further supporting the significantly enhanced ion transport rate within the HRGO-10 film electrode (Fig. 8e). The t 0 value is also smaller than those of RGO reduced by urea based EC (4.2 s) [18] and graphene aerogel based EC (0.73 s) [33] in the same aqueous electrolyte, suggesting that the in-plane nanopores dramatically promote the kinetic diffusion of electrolyte ions in the interior of the electrodes. Cycling stability of HRGO-10 EC tested by a consecutive charge/discharge technique at a constant current of 5 A g 1 shows that a capacitance retention of 94% is achieved after 6000 cycles, which is comparable to the expected value for an ideal electrical double-layer capacitor (Fig. 8f). From the above experimental results, it can be concluded that HRGO10 film not only delivers much higher specific capacitance than RGO film, but also exhibits remarkable rate performance and excellent cycling stability, which is satisfactory for binder-free film electrode materials with high performance for supercapacitor.
Fig. 7. Electrochemical characterization of RGO, HRGO-8, HRGO-10, and HRGO-12 film electrodes: (a) CV curves at a scan rate of 200 mV s 1, (b) Galvanostatic charge/ discharge curves at a current density of 1 A g 1, (c) Plots of specific capacitances at various current densities, and (d) The electrochemical impedance spectra measured at frequency range from 100 kHz to 0.01 Hz (inserted: the close-up view of the high-frequency region).
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Fig. 8. Electrochemical characterization of HRGO-10 and RGO ECs with 6.0 M KOH as aqueous electrolyte in a potential window of 0–1.0 V: (a) CV curves at 200 mV s 1, (b) specific capacitance at different current densities, (c) galvanostatic charge/discharge curves at 10 A g 1, (d) Nyquist plots and the inset shows the close-up view of the high-frequency region, (e) bode plots of phase angle versus frequency, and (f) cycling stability of HRGO-10 EC at a current density of 5 A g 1.
4. Conclusions
References
A mild defect-etching method is further improved to prepare holey graphene with abundant in-plane nanopores, and its porosity connects with the hydrothermal reaction conditions between GO and H2O2. The optimum pore forming conditions are that GO is hydrothermally treated in 0.4 mL H2O2 at 100 C for 10 h and followed by the reduction reaction in hydrazine solution. By means of the simple vacuum filtration of the obtained well-dispersed HRGO suspension, the binder-free graphene film electrodes with porous layered structure can be successfully assembled. The obtained film electrodes not only exhibit enhanced gravimetric capacitance and improved rate capability, but also show high packing density and high volumetric capacitance. The optimum porosity plays a key role between efficient electrolyte ion diffusion rate and conductivity of the obtained film electrode, which sheds a promising light on the configuration of graphene film electrodes with high volumetric capacitance. In summary, this is an efficient method to introduce in-plane pores into graphene nanosheets without sacrificing dispersity and processibility of the resultant graphene nanomesh.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (51172137, 21471093), the Program for Key Science & Technology Innovation Team of Shaanxi Province (2012KCT-21), the 111 Project, and the Fundamental Research Funds for the Central Universities (GK201301002 and GK201501007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2015. 11.090.
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