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Cite this: DOI: 10.1039/c4nr07508k Received 19th December 2014, Accepted 4th March 2015 DOI: 10.1039/c4nr07508k
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Facile electrodeposition of reduced graphene oxide hydrogels for high-performance supercapacitors† Viet Hung Pham,a Tesfaye Gebreb and James H. Dickerson*a
www.rsc.org/nanoscale
We report both a facile, scalable method to prepare reduced graphene oxide hydrogels through the electrodeposition of graphene oxide and its use as an electrode for high-performance supercapacitors. Such systems exhibited specific capacitances of 147 and 223 F g−1 at a current density of 10 A g−1 when using H2SO4 and H2SO4 + hydroquinone redox electrolytes, respectively.
Graphene, a one-atom-thick 2D single layer of sp2-bonded carbon, has emerged as a promising candidate for use as a supercapacitor electrode material due to its extremely large effective surface area, extraordinarily high electrical conductivity, good chemical stability, and high mechanical strength.1–3 However, due to intersheet van der Waals interactions, the aggregation or restacking of graphene sheets inevitably occurs during the synthesis and electrode preparation procedures, resulting in a loss in the effective surface area.4,5 Several approaches have been developed to prevent the restacking of graphene during processing, including the fabrication of highly corrugated and crumpled graphene sheets, the use of guest materials as spacers, and the controlled assembly of graphene sheets into three-dimensional porous structures, namely graphene hydrogels.6–14 Among them, the later approach has been widely explored to fabricate high-performance supercapacitors.10–14 Different methods have been used to prepare graphene hydrogels, such as hydrothermal reduction, chemical reduction (in static condition), templatedirected reduction and electrochemical reduction of graphene oxide (GO), and vacuum-assisted filtration of reduced graphene oxide (RGO).10–17 Hydrothermal reduction of GO is commonly used to prepared RGO hydrogels due to the absence of a post-treatment for removing residual chemical reductants,
a Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA. E-mail: [email protected]; Fax: +1-631-344-3093; Tel: +1-631-344-8812 b Department of Physics, Florida A&M University, Tallahassee, Florida 32307, USA † Electronic supplementary information (ESI) available: GO synthesis, characterization, fabrication of ERGO supercapacitor and electrochemical measurement, elemental composition, TGA and XRD of GO and ERGO. See DOI: 10.1039/ c4nr07508k
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whereas electrochemical reduction is the most convenient method since RGO hydrogels are directly deposited onto the electrodes’ surfaces and, thus, can be directly used in electrochemical devices.10,12,14,16,17 RGO hydrogels contain large amounts of water, usually more than 98.0 wt%.14,15 For supercapacitor applications, RGO hydrogels are commonly freezedried or supercritical fluid-dried to remove entrapped water, followed by slicing into thin sheets and subsequently using them as free-binder electrodes.12–14 Recently, Li et al. reported that the performance of a supercapacitor that employed RGO hydrogels directly as the electrode material was outstanding compared with the performance of their freeze-dried RGO counterpart, especially at high charge–discharge rates.5,18 Entrapped water in the RGO hydrogel acts as spacer, preventing the restacking of the RGO sheets; this yields a highly open and porous structure that creates continuous ion transport networks and allows electrolyte solutions to access to the surface of individual RGO sheets easily. Further, this allows RGObased hydrogel supercapacitors to operate at ultrafast charge– discharge rates. However, RGO hydrogels have very low mechanical strength due to their large water content, which is easily altered during electrode processing. This challenge can be solved by directly growing RGO hydrogels onto the current collector. Electrochemical deposition has been explored to prepare RGO thin films and RGO hydrogels on the surface of electrodes for different applications.16,17,19–22 In the typical process, GO sheets are directly electrochemically reduced from an aqueous colloidal suspension in the presence of a buffer or supporting electrolytes to produce the RGO thin films on an electrode’s surface.21 However, for supercapacitor applications, the buffer and supporting electrolytes, such as phosphate buffer solution, sodium chloride, sodium nitrate and sodium perchlorate (which are entrapped in RGO hydrogels), are not desirable and require a post-treatment step for removal.16 Herein, we demonstrate one-step, facile approach to prepare RGO hydrogel film deposited directly on stainless steel mesh substrate by electrodeposition of acidic GO solutions, subsequently using said substrates directly as electrodes for
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high-performance supercapacitors. The pH of the GO solution was controlled by titrating sulfuric acid into the solution; the acid also acted as an electrolyte for the supercapacitor. Thus, no post-treatment step was needed. The electrodeposited RGO hydrogels (ERGO) exhibited excellent electrochemical capacitive properties with a specific capacitance of 147 F g−1 at a current density of 10 A g−1 and a capacitance retention of 93.1% after 10 000 cycles. More interestingly, the specific capacitance of ERGO was improved by more than 50% by using hydroquinone (HQ) as a redox-active electrolyte. GO was prepared by the modified Hummers method using expanded graphite as the starting material.15 The assynthesized GO was diluted to a concentration of 2.0 mg mL−1, and the pH of the GO solution was adjusted to 1.35 using 1 M H2SO4 solution. Electrodeposition of RGO hydrogel was performed using constant potential mode, illustrated in Fig. 1(a). The carbon fiber paper counter electrode and stainless steel mesh (grade 304) working electrode of identical size (2.0 × 4.0 cm) were placed parallel to each other with a separation of 10 mm and were dipped 2.0 cm into the GO solution. A dc constant potential of 5.0 V was applied for 60 min to deposit the ERGO hydrogel. The electrochemical deposition of ERGO hydrogel can be carried out at 3.0 V but the deposition rate was quite slow. However, increase of the applied potential to 10.0 V did not increase the deposition rate. Fig. 1(b) shows a photograph of the ERGO hydrogels on stainless steel meshes for different electrodeposition times. Evident from the images is that ERGO first deposited onto and strongly wrapped around the stainless steel warp and weft wires of the mesh. The deposit extended throughout the pores of the mesh, creating a continuous, three-dimensional ERGO hydrogel on the substrate. The thickness of the ERGO hydrogel was around 2.0 mm and the yield of the electrodeposition was approximately 0.55 mg cm−2 h−1. The ERGO hydrogels contain large quantities of entrapped water, approximately 98.8 wt%. Fig. 1(c and d) show electron scanning microscopy (SEM) images of the freeze-dried ERGO, revealing highly porous, three-dimensionally
Fig. 1 (a) Schematic of electrodeposition of ERGO. (b) Photographs of ERGO hydrogel on stainless steel mesh. (c, d) SEM images of freezedried ERGO.
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interconnected graphene frameworks with the pore sizes ranging from submicron to several microns. The pore walls were very thin consisting of crumpled, stacked graphene sheets. The electrochemical reduction of ERGO was characterized by UV-Vis spectra, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, as shown in Fig. 2. The UV-vis spectrum of GO displays a strong absorption peak at 231 nm, which is attributed to π→π* transitions of aromatic C–C bonds, and a shoulder at ∼ 300 nm that is assigned to n→π* transitions of CvO bonds (Fig. 2(a)).23 After electrodeposition, the absorption peak of ERGO red-shifted to 270 nm, and the absorption in the whole spectra region greatly increased, indicating the restoration of conjugated CvC bonds.4 The survey XPS spectra of GO and ERGO show that the intensity of O 1s peak of ERGO greatly decreases compared to that of GO, indicating that a substantial quantity of oxygen functional groups has been removed during electrodeposition (Fig. 2(b)). The deconvolution of C 1s XPS spectrum of ERGO, depicted in Fig. 2(c), shows that most of the oxygen functional groups have been removed. The elemental composition (specifically the atomic % of oxygen) of GO juxtaposed with that of ERGO, as determined by XPS and shown in Table S1,† is 30.4% for GO versus 13.4% for ERGO. This corresponds to an increase of the C/O atomic ratio from 2.2 of GO to 6.5 of ERGO, indicating a satisfactory extent to the reduction of ERGO, favourable for supercapacitor applications. Raman spectroscopy provided additional characterization of the structural changes of GO during the electrodeposition process. Fig. 2(d) shows the Raman spectrum of GO, which has two prominent peaks at 1346 and 1594 cm−1 that correspond to the D and G bands,24 respectively. The ratio of the intensities of the D and G band peaks, I(D)/I(G), is approximately 1.12. After electrodeposition, the G band of ERGO red-shifted to 1582 cm−1, and the I(D)/I(G) ratio greatly increased to 1.78. Both indicate the recovery of sp2 domains in the graphitic structure.24
Fig. 2 (a) UV-Vis spectra, (b) XPS spectra, (c) Raman and (d) TGA of GO and ERGO.
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Thermogravimetric analysis (TGA) of the GO and ERGO, performed in a nitrogen atmosphere, can be found in Fig. S3.† The TGA curve for GO exhibits an approximately 5% mass loss at 100 °C, which can be attributed to the evaporation of absorbed water. The primary mass loss that occurred from 150–250 °C can be attributed to the decomposition of labile oxygen functional groups, such as hydroxyl, epoxy, and carbonyl groups.25 ERGO was much more thermally stable than GO with only about a 15 wt% mass loss at 600 °C in comparison to a 55 wt% mass loss for GO. The mass loss of ERGO at 250 °C was only 3.8 wt%, implying that most of the labile oxygen functional groups were removed via electrochemical reduction during the deposition. The electrochemical capacitive properties of ERGO were characterized by cyclic voltammetry (CV), galvanostatic charge– discharge (GCD), and electrochemical impedance spectroscopy (EIS) in two-electrode full cell supercapacitors using 1 M H2SO4 and 1 M H2SO4 + 0.2 M HQ electrolytes. Use of HQ as a redox-active additive in combination with H2SO4 electrolyte has been widely applied to improve the capacitance of carbonbased supercapacitors.26–29 Fig. 3(a) shows the CV curves of an ERGO supercapacitor in 1 M H2SO4 electrolyte at different scan rates in the potential window of 0–1.0 V; the curves were nearly rectangular even at high scan rate of 200 mV s−1, implying favourable electrical double layer capacitive behaviour. The broad redox peaks in the range of 0.0–0.7 V can be attributed to the reversible redox reaction of remained oxygen-containing functional groups of ERGO hydrogel such as hydroxyl, carbonyl and carboxyl groups.30 The GDC curves were symmetrically triangular with a small IR drop, suggesting the low internal resistance within the electrode (Fig. 3(b)). By incorporating HQ into H2SO4 electrolyte as a redox-active additive, the shape of CV curves became quasi-rectangular, indicating their pseudocapacitive behaviour. The current density values of the CV curves of the ERGO supercapacitor using H2SO4 + HQ electro-
Fig. 3 (a, b) CV and GCD curves of ERGO supercapacitor using H2SO4 electrolyte, and (c, d) H2SO4 + HQ redox-active electrolytes.
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lyte were significantly higher than those of the ERGO electrode using the H2SO4 electrolyte at the same scan rate, implying a higher specific capacitance due to the contribution of the pseudocapacitance of the redox-active HQ. Note, however, that the ERGO supercapacitor that uses the H2SO4 + HQ redoxactive electrolyte only operated well in the potential window of 0–0.9 V, narrower than that of the ERGO supercapacitor that used the H2SO4 electrolyte alone. According to Blanco et al., the addition of HQ to the electrolyte of a supercapacitor leads to important changes in the energy storage mechanism of both electrodes of the device. The anode becomes a batterytype electrode due to the development of the quinone/hydroquinone redox reaction on its surface while the cathode retains a capacitor-type behaviour, resulting in the asymmetric redistribution of the voltage between the electrodes after the addition of HQ.27 The specific capacitances of ERGO supercapacitors using H2SO4 electrolyte were 165.5 and 147.2 F g−1 at current densities of 1.0 and 10.0 A g−1, respectively, both of which are comparable to previous specific capacitances of graphene hydrogel.5,12,14,31 Note that ERGO exhibited excellent rate performance with a capacitance retention of 88.9% as the current density increased from 1.0 to 10 A g−1. With the addition of HQ into the electrolyte, the specific capacitances of ERGO increased to 252.6 and 223.2 F g−1 at current densities of 1.0 and 10.0 A g−1, respectively. This corresponds to 52.6 and 51.6% enhancement of the capacitance. Moreover, EGRO exhibited excellent cycling stability with 93.1% capacitance retention after 10 000 cycles. With the addition of HQ into the electrolyte, the capacitance retention decreased to 80.7% after 10 000 cycles, which could be explained by the slightly irreversible redox reaction of HQ.26 EIS analysis was used to characterize the electrochemical behaviour of ERGO. The Nyquist plots in Fig. 4(d) display a
Fig. 4 (a) Specific capacitance at different current densities, (b) cycling stability at a current density of 2.0 A g−1, (c) GCD curves at different cycles, and (d) Nyquist plots of ERGO supercapacitors using H2SO4 and H2SO4 + HQ electrolytes.
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straight line in the low-frequency region and a typical arc in the high-frequency region. The straight lines in the lowfrequency were almost vertical; indicating a nearly ideal capacitive behaviour.16 The ERGO supercapacitor using the H2SO4 electrolyte exhibited a shorter Warburg-type line (the slope of 45° portion of the curve) and smaller diameter arc than those of the ERGO supercapacitor using the H2SO4 + HQ electrolyte, indicating a lower charge transfer resistance and a more efficient electrolyte diffusion.5 The equivalent series resistances (ESR), determined by extrapolating the vertical portion of the plot to the real axis, were only 1.4 Ω and 2.6 Ω for ERGO supercapacitors that used H2SO4 and H2SO4 + HQ electrolyte, respectively. In summary, we have reported a one-step electrodeposition approach toward the preparation of ERGO hydrogels on stainless steel meshes, which can be used directly as high-performance supercapacitor electrodes without post-treatment. The ERGO exhibited excellent specific capacitance, rate performance, and cycling stability. More interestingly, a >50% enhancement of the specific capacitance % was achieved by using HQ as redox-active electrolyte. In total, our results have demonstrated the great potential of ERGO hydrogels as a promising electrode material for high-performance supercapacitors.
Acknowledgements This work was performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract no. DE-AC02-98CH10886.
Notes and references 1 Y. Huang, J. Liang and Y. Chen, Small, 2012, 8, 1805. 2 J. Chen, C. Li and G. Si, J. Phys. Chem. Lett., 2013, 4, 1244. 3 H.-J. Choi, S.-M. Jung, J.-M. Seo, D. W. Chang, L. Dai and J.-B. Baek, Nano Energy, 2012, 1, 534. 4 D. Li, M. B. Müller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101. 5 X. Yang, J. Zhu, L. Qui and D. Li, Adv. Mater., 2011, 23, 2833. 6 J. Yan, J. Liu, Z. Fan, T. Wei and L. Zhang, Carbon, 2012, 50, 2179. 7 J. Luo, H. D. Jang and J. Huang, ACS Nano, 2013, 7, 1464.
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8 J. Li and M. Östling, Crystals, 2013, 3, 163. 9 S. Nardecchia, D. Carriazo, M. L. Ferrer, M. C. Gutierrez and F. D. Monte, Chem. Soc. Rev., 2013, 42, 794. 10 C. Li and G. Shi, Adv. Mater., 2014, 26, 3992. 11 V. Chabot, D. Higgins, A. Yu, X. Xiao, Z. Chen and J. Zhang, Energy Environ. Sci., 2014, 7, 1564. 12 L. Zhang and G. Shi, J. Phys. Chem. C, 2011, 115, 17206. 13 J. Chen, K. Sheng, P. Luo, C. Li and G. Shi, Adv. Mater., 2012, 24, 4569. 14 Y. Xu, K. Sheng, C. Li and G. Shi, ACS Nano, 2010, 4, 4324. 15 H. D. Pham, V. H. Pham, T. V. Cuong, Th.-D. Nguyen-Phan, J. S. Chung, E. W. Shin and S. Kim, Chem. Commun., 2011, 47, 9672. 16 K. Sheng, Y. Sun, C. Li, W. Yuan and G. Shi, Sci. Rep., 2012, 2, 247. 17 K. Chen, L. Chen, Y. Chen, H. Bai and L. Li, J. Mater. Chem., 2012, 22, 20968. 18 X. Yang, C. Cheng, Y. Wang, L. Qiu and D. Li, Science, 2013, 341, 534. 19 M. Hilder, B. Winther-Jensen, D. Li, M. Forsythc and D. R. MacFarlane, Phys. Chem. Chem. Phys., 2011, 13, 9187. 20 L. Chen, Y. Tang, K. Wang, C. Liu and S. Luo, Electrochem. Commun., 2011, 13, 133. 21 S. Y. Toh, K. S. Loh, S. K. Kamarudin and W. R. W. Daud, Chem. Eng. J., 2014, 251, 422. 22 S. A. Hasan, J. L. Rigueur, R. R. Harl, A. J. Krejci, I. Gonzalo-Juan, B. R. Rogers and J. H. Dickerson, ACS Nano, 2010, 4, 7367. 23 J. I. Paredes, S. Villar-Rodil, A. Martinez-Alonso and J. M. D. Tascón, Langmuir, 2008, 24, 10560. 24 I. K. Moon, J. Lee, R. S. Ruoff and H. Lee, Nat. Commun., 2010, 1, 73. 25 S. Villar-Rodil, J. I. Paredes, A. Martínez-Alonso and M. D. Tascón, J. Mater. Chem., 2009, 19, 3591. 26 S. Roldán, C. Blanco, M. Granda, R. Menéndez and R. Santamaría, Angew. Chem., Int. Ed., 2011, 50, 1699. 27 S. Roldán, M. Granda, R. Menéndez, R. Santamaría and C. Blanco, J. Phys. Chem. C, 2011, 115, 17606. 28 W. Chen, R. B. Rakhi and H. N. Alshareef, J. Phys. Chem. C, 2013, 117, 15009. 29 W. Chen, R. B. Rakhi and H. N. Alshareef, Nanoscale, 2013, 5, 4134. 30 J. Yan, Q. Wang, T. Wei, L. Jiang, M. Zhang, X. Jing and Z. Fan, ACS Nano, 2014, 8, 4720. 31 X. Feng, W. Chen and L. Yan, Nanoscale, 2015, 7, 3712.
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Cite this: DOI: 10.1039/c4nr07508k Received 19th December 2014, Accepted 4th March 2015 DOI: 10.1039/c4nr07508k
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Facile electrodeposition of reduced graphene oxide hydrogels for high-performance supercapacitors† Viet Hung Pham,a Tesfaye Gebreb and James H. Dickerson*a
www.rsc.org/nanoscale
We report both a facile, scalable method to prepare reduced graphene oxide hydrogels through the electrodeposition of graphene oxide and its use as an electrode for high-performance supercapacitors. Such systems exhibited specific capacitances of 147 and 223 F g−1 at a current density of 10 A g−1 when using H2SO4 and H2SO4 + hydroquinone redox electrolytes, respectively.
Graphene, a one-atom-thick 2D single layer of sp2-bonded carbon, has emerged as a promising candidate for use as a supercapacitor electrode material due to its extremely large effective surface area, extraordinarily high electrical conductivity, good chemical stability, and high mechanical strength.1–3 However, due to intersheet van der Waals interactions, the aggregation or restacking of graphene sheets inevitably occurs during the synthesis and electrode preparation procedures, resulting in a loss in the effective surface area.4,5 Several approaches have been developed to prevent the restacking of graphene during processing, including the fabrication of highly corrugated and crumpled graphene sheets, the use of guest materials as spacers, and the controlled assembly of graphene sheets into three-dimensional porous structures, namely graphene hydrogels.6–14 Among them, the later approach has been widely explored to fabricate high-performance supercapacitors.10–14 Different methods have been used to prepare graphene hydrogels, such as hydrothermal reduction, chemical reduction (in static condition), templatedirected reduction and electrochemical reduction of graphene oxide (GO), and vacuum-assisted filtration of reduced graphene oxide (RGO).10–17 Hydrothermal reduction of GO is commonly used to prepared RGO hydrogels due to the absence of a post-treatment for removing residual chemical reductants,
a Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA. E-mail: [email protected]; Fax: +1-631-344-3093; Tel: +1-631-344-8812 b Department of Physics, Florida A&M University, Tallahassee, Florida 32307, USA † Electronic supplementary information (ESI) available: GO synthesis, characterization, fabrication of ERGO supercapacitor and electrochemical measurement, elemental composition, TGA and XRD of GO and ERGO. See DOI: 10.1039/ c4nr07508k
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whereas electrochemical reduction is the most convenient method since RGO hydrogels are directly deposited onto the electrodes’ surfaces and, thus, can be directly used in electrochemical devices.10,12,14,16,17 RGO hydrogels contain large amounts of water, usually more than 98.0 wt%.14,15 For supercapacitor applications, RGO hydrogels are commonly freezedried or supercritical fluid-dried to remove entrapped water, followed by slicing into thin sheets and subsequently using them as free-binder electrodes.12–14 Recently, Li et al. reported that the performance of a supercapacitor that employed RGO hydrogels directly as the electrode material was outstanding compared with the performance of their freeze-dried RGO counterpart, especially at high charge–discharge rates.5,18 Entrapped water in the RGO hydrogel acts as spacer, preventing the restacking of the RGO sheets; this yields a highly open and porous structure that creates continuous ion transport networks and allows electrolyte solutions to access to the surface of individual RGO sheets easily. Further, this allows RGObased hydrogel supercapacitors to operate at ultrafast charge– discharge rates. However, RGO hydrogels have very low mechanical strength due to their large water content, which is easily altered during electrode processing. This challenge can be solved by directly growing RGO hydrogels onto the current collector. Electrochemical deposition has been explored to prepare RGO thin films and RGO hydrogels on the surface of electrodes for different applications.16,17,19–22 In the typical process, GO sheets are directly electrochemically reduced from an aqueous colloidal suspension in the presence of a buffer or supporting electrolytes to produce the RGO thin films on an electrode’s surface.21 However, for supercapacitor applications, the buffer and supporting electrolytes, such as phosphate buffer solution, sodium chloride, sodium nitrate and sodium perchlorate (which are entrapped in RGO hydrogels), are not desirable and require a post-treatment step for removal.16 Herein, we demonstrate one-step, facile approach to prepare RGO hydrogel film deposited directly on stainless steel mesh substrate by electrodeposition of acidic GO solutions, subsequently using said substrates directly as electrodes for
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high-performance supercapacitors. The pH of the GO solution was controlled by titrating sulfuric acid into the solution; the acid also acted as an electrolyte for the supercapacitor. Thus, no post-treatment step was needed. The electrodeposited RGO hydrogels (ERGO) exhibited excellent electrochemical capacitive properties with a specific capacitance of 147 F g−1 at a current density of 10 A g−1 and a capacitance retention of 93.1% after 10 000 cycles. More interestingly, the specific capacitance of ERGO was improved by more than 50% by using hydroquinone (HQ) as a redox-active electrolyte. GO was prepared by the modified Hummers method using expanded graphite as the starting material.15 The assynthesized GO was diluted to a concentration of 2.0 mg mL−1, and the pH of the GO solution was adjusted to 1.35 using 1 M H2SO4 solution. Electrodeposition of RGO hydrogel was performed using constant potential mode, illustrated in Fig. 1(a). The carbon fiber paper counter electrode and stainless steel mesh (grade 304) working electrode of identical size (2.0 × 4.0 cm) were placed parallel to each other with a separation of 10 mm and were dipped 2.0 cm into the GO solution. A dc constant potential of 5.0 V was applied for 60 min to deposit the ERGO hydrogel. The electrochemical deposition of ERGO hydrogel can be carried out at 3.0 V but the deposition rate was quite slow. However, increase of the applied potential to 10.0 V did not increase the deposition rate. Fig. 1(b) shows a photograph of the ERGO hydrogels on stainless steel meshes for different electrodeposition times. Evident from the images is that ERGO first deposited onto and strongly wrapped around the stainless steel warp and weft wires of the mesh. The deposit extended throughout the pores of the mesh, creating a continuous, three-dimensional ERGO hydrogel on the substrate. The thickness of the ERGO hydrogel was around 2.0 mm and the yield of the electrodeposition was approximately 0.55 mg cm−2 h−1. The ERGO hydrogels contain large quantities of entrapped water, approximately 98.8 wt%. Fig. 1(c and d) show electron scanning microscopy (SEM) images of the freeze-dried ERGO, revealing highly porous, three-dimensionally
Fig. 1 (a) Schematic of electrodeposition of ERGO. (b) Photographs of ERGO hydrogel on stainless steel mesh. (c, d) SEM images of freezedried ERGO.
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interconnected graphene frameworks with the pore sizes ranging from submicron to several microns. The pore walls were very thin consisting of crumpled, stacked graphene sheets. The electrochemical reduction of ERGO was characterized by UV-Vis spectra, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy, as shown in Fig. 2. The UV-vis spectrum of GO displays a strong absorption peak at 231 nm, which is attributed to π→π* transitions of aromatic C–C bonds, and a shoulder at ∼ 300 nm that is assigned to n→π* transitions of CvO bonds (Fig. 2(a)).23 After electrodeposition, the absorption peak of ERGO red-shifted to 270 nm, and the absorption in the whole spectra region greatly increased, indicating the restoration of conjugated CvC bonds.4 The survey XPS spectra of GO and ERGO show that the intensity of O 1s peak of ERGO greatly decreases compared to that of GO, indicating that a substantial quantity of oxygen functional groups has been removed during electrodeposition (Fig. 2(b)). The deconvolution of C 1s XPS spectrum of ERGO, depicted in Fig. 2(c), shows that most of the oxygen functional groups have been removed. The elemental composition (specifically the atomic % of oxygen) of GO juxtaposed with that of ERGO, as determined by XPS and shown in Table S1,† is 30.4% for GO versus 13.4% for ERGO. This corresponds to an increase of the C/O atomic ratio from 2.2 of GO to 6.5 of ERGO, indicating a satisfactory extent to the reduction of ERGO, favourable for supercapacitor applications. Raman spectroscopy provided additional characterization of the structural changes of GO during the electrodeposition process. Fig. 2(d) shows the Raman spectrum of GO, which has two prominent peaks at 1346 and 1594 cm−1 that correspond to the D and G bands,24 respectively. The ratio of the intensities of the D and G band peaks, I(D)/I(G), is approximately 1.12. After electrodeposition, the G band of ERGO red-shifted to 1582 cm−1, and the I(D)/I(G) ratio greatly increased to 1.78. Both indicate the recovery of sp2 domains in the graphitic structure.24
Fig. 2 (a) UV-Vis spectra, (b) XPS spectra, (c) Raman and (d) TGA of GO and ERGO.
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Thermogravimetric analysis (TGA) of the GO and ERGO, performed in a nitrogen atmosphere, can be found in Fig. S3.† The TGA curve for GO exhibits an approximately 5% mass loss at 100 °C, which can be attributed to the evaporation of absorbed water. The primary mass loss that occurred from 150–250 °C can be attributed to the decomposition of labile oxygen functional groups, such as hydroxyl, epoxy, and carbonyl groups.25 ERGO was much more thermally stable than GO with only about a 15 wt% mass loss at 600 °C in comparison to a 55 wt% mass loss for GO. The mass loss of ERGO at 250 °C was only 3.8 wt%, implying that most of the labile oxygen functional groups were removed via electrochemical reduction during the deposition. The electrochemical capacitive properties of ERGO were characterized by cyclic voltammetry (CV), galvanostatic charge– discharge (GCD), and electrochemical impedance spectroscopy (EIS) in two-electrode full cell supercapacitors using 1 M H2SO4 and 1 M H2SO4 + 0.2 M HQ electrolytes. Use of HQ as a redox-active additive in combination with H2SO4 electrolyte has been widely applied to improve the capacitance of carbonbased supercapacitors.26–29 Fig. 3(a) shows the CV curves of an ERGO supercapacitor in 1 M H2SO4 electrolyte at different scan rates in the potential window of 0–1.0 V; the curves were nearly rectangular even at high scan rate of 200 mV s−1, implying favourable electrical double layer capacitive behaviour. The broad redox peaks in the range of 0.0–0.7 V can be attributed to the reversible redox reaction of remained oxygen-containing functional groups of ERGO hydrogel such as hydroxyl, carbonyl and carboxyl groups.30 The GDC curves were symmetrically triangular with a small IR drop, suggesting the low internal resistance within the electrode (Fig. 3(b)). By incorporating HQ into H2SO4 electrolyte as a redox-active additive, the shape of CV curves became quasi-rectangular, indicating their pseudocapacitive behaviour. The current density values of the CV curves of the ERGO supercapacitor using H2SO4 + HQ electro-
Fig. 3 (a, b) CV and GCD curves of ERGO supercapacitor using H2SO4 electrolyte, and (c, d) H2SO4 + HQ redox-active electrolytes.
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lyte were significantly higher than those of the ERGO electrode using the H2SO4 electrolyte at the same scan rate, implying a higher specific capacitance due to the contribution of the pseudocapacitance of the redox-active HQ. Note, however, that the ERGO supercapacitor that uses the H2SO4 + HQ redoxactive electrolyte only operated well in the potential window of 0–0.9 V, narrower than that of the ERGO supercapacitor that used the H2SO4 electrolyte alone. According to Blanco et al., the addition of HQ to the electrolyte of a supercapacitor leads to important changes in the energy storage mechanism of both electrodes of the device. The anode becomes a batterytype electrode due to the development of the quinone/hydroquinone redox reaction on its surface while the cathode retains a capacitor-type behaviour, resulting in the asymmetric redistribution of the voltage between the electrodes after the addition of HQ.27 The specific capacitances of ERGO supercapacitors using H2SO4 electrolyte were 165.5 and 147.2 F g−1 at current densities of 1.0 and 10.0 A g−1, respectively, both of which are comparable to previous specific capacitances of graphene hydrogel.5,12,14,31 Note that ERGO exhibited excellent rate performance with a capacitance retention of 88.9% as the current density increased from 1.0 to 10 A g−1. With the addition of HQ into the electrolyte, the specific capacitances of ERGO increased to 252.6 and 223.2 F g−1 at current densities of 1.0 and 10.0 A g−1, respectively. This corresponds to 52.6 and 51.6% enhancement of the capacitance. Moreover, EGRO exhibited excellent cycling stability with 93.1% capacitance retention after 10 000 cycles. With the addition of HQ into the electrolyte, the capacitance retention decreased to 80.7% after 10 000 cycles, which could be explained by the slightly irreversible redox reaction of HQ.26 EIS analysis was used to characterize the electrochemical behaviour of ERGO. The Nyquist plots in Fig. 4(d) display a
Fig. 4 (a) Specific capacitance at different current densities, (b) cycling stability at a current density of 2.0 A g−1, (c) GCD curves at different cycles, and (d) Nyquist plots of ERGO supercapacitors using H2SO4 and H2SO4 + HQ electrolytes.
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straight line in the low-frequency region and a typical arc in the high-frequency region. The straight lines in the lowfrequency were almost vertical; indicating a nearly ideal capacitive behaviour.16 The ERGO supercapacitor using the H2SO4 electrolyte exhibited a shorter Warburg-type line (the slope of 45° portion of the curve) and smaller diameter arc than those of the ERGO supercapacitor using the H2SO4 + HQ electrolyte, indicating a lower charge transfer resistance and a more efficient electrolyte diffusion.5 The equivalent series resistances (ESR), determined by extrapolating the vertical portion of the plot to the real axis, were only 1.4 Ω and 2.6 Ω for ERGO supercapacitors that used H2SO4 and H2SO4 + HQ electrolyte, respectively. In summary, we have reported a one-step electrodeposition approach toward the preparation of ERGO hydrogels on stainless steel meshes, which can be used directly as high-performance supercapacitor electrodes without post-treatment. The ERGO exhibited excellent specific capacitance, rate performance, and cycling stability. More interestingly, a >50% enhancement of the specific capacitance % was achieved by using HQ as redox-active electrolyte. In total, our results have demonstrated the great potential of ERGO hydrogels as a promising electrode material for high-performance supercapacitors.
Acknowledgements This work was performed at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract no. DE-AC02-98CH10886.
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