Supercapacitors Based on Reduced Graphene Oxide Nanofibers

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Supercapacitors Based on Reduced Graphene Oxide Nanofibers Supported Ni(OH)2 Nanoplates with Enhanced Electrochemical Performance Chaoqi Zhang, Qidi Chen, and Hongbing Zhan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05255 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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ACS Applied Materials & Interfaces

Supercapacitors Based on Reduced Graphene Oxide Nanofibers Supported Ni(OH)2 Nanoplates with Enhanced Electrochemical Performance Chaoqi Zhang, Qidi Chen, Hongbing Zhan * College of Materials Science and Engineering, Fuzhou University, Fuzhou, Fujian, 350116, People’s Republic of China *E-mail: [email protected]

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Abstract Pseudocapacitive materials are critical to the development of supercapacitors but usually suffer from poor conductivity and bad cycling property. Here, we describe the production of novel graphene oxide nanofibers (GONFs) via a partial oxidization and exfoliation method and concurrently report that highly crystallized Ni(OH)2 nanoplates uniformly grow on reduced GONFs’ outer graphene nanosheets through the hydrothermal method. Because of their unique structure with high electric conductivity, the rGONF/Ni(OH)2 composite exhibits superior specific capacitance (SC), favorable rate capability and enhanced cycling stability relative to other composites or hybrids, e.g., 1433 F g-1 at 5 mV s-1 scan rate, 986 F g-1 at 40 mV s-1 and 90.5% capacitance retention after 2000 cycles, and as-fabricated rGONF/Ni(OH)2//active carbon asymmetric supercapacitor (ASC) exhibits a remarkable energy density and a 85.3% high retention (44.1 Wh kg-1 at 467 W kg-1 and 37.6 Wh kg-1 at 3185 W kg-1) with a wide potential window of 0-1.7 V. Therefore, this study shows that rGONFs offers an exciting opportunity as substrate materials for supercapacior applications and opens up a new pathway for design and manufacture of novel supercapacitor electrode materials.

Keywords: graphene oxide nanofiber, Ni(OH)2, composite, electrochemical performance, supercapacitor

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1.Introduction Because of the rapid consumption of global energy and the gradual exhaustion of traditional energy resources, the exploration of advanced, cost-efficient, and environmentally friendly energy-storage delivery devices able meet modern society’s needs is an urgent need.1-4 Among the different energy-storage delivery systems available, supercapacitors have drawn extensive research because of not only their remarkable power density and superior cycle life, but also the characteristic of excellent safety and environment friendliness.4-6 Supercapacitors can be generally categorized into two types: electric double-layer capacitors (EDLCs) and pseudocapacitors.7,8 EDLCs can provide high electrical power and high stability but have a low capacitance because they store charge by reversibly adsorbing electrolyte ions at the interface of electrode/electrolyte.9,10 However, pseudocapacitors store charge based on rapid redox reactions at or near the surface of electrode and deliver 10 times the capacitance and energy density of EDLCs. 7,11 Pseudocapacitive materials, mainly consist of transition metal oxides/hydroxides and conducting polymers, play critical roles in supercapacitive performance.12-15 Unlike conducting polymers, transition metal oxides/hydroxides exhibit outstanding capacitive performance and have attracted substantial attention from scientists and engineers.16 Ni(OH)2 is a current research focus account to its remarkable theoretical capacitance (2082 F g-1), superior chemical stability, cost efficient and simple preparation process.17-20 Unfortunately, pure Ni(OH)2 due to low electrical conductivity and large volume changes happening in the charging/discharging process, which was result in the bad rate capability, poor power performance and unsatisfied cycling stability of the electrode and significantly restricts its practical application in supercapacitors.5,20,21 Thus, considerable research has been performed in recent years to overcome this serious problem without 3



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sacrificing the advantages of Ni(OH)2. A viable strategy to enhance supercapacitive performance is to combine the target material with a carbon material with high conductivity and a flexible structure to simultaneously reduce the resistance and accommodate volume changes during the charge/discharge process. 19-26 Among the many types of carbon materials available, graphene and carbon nanofibers (CNFs) have recently attracted increasing attention as advanced substrate materials for improving the performance of Ni(OH)2 because of their high surface area, remarkable conductivity and exceptional chemical stability.26-30 However, these substrates have several limitations. In general, two-dimensional graphene tends to restack to form a graphite-like substance because of the intense van der Waals interactions between graphene sheets,31,32 and three-dimensional interconnected CNFs have lower specific surface area than graphene. Therefore, graphene nanofibers that combine the structure of graphene and CNFs and have high electrical conductivity and excellent flexibility were expected to overcome the limitations and display superior performance compared either material alone.33,34 Several reports in the literature address graphene nanofibers prepared by a coating process in graphene oxide solution.35,36 However, these materials were not ideal graphene nanofibers because ideal graphene nanofibers should maintain the internal nanofiber-like structure with graphene attached to the external structure. These methods link CNFs to large graphene sheets, resulting in mismatched structures. Therefore, further research to develop graphene nanofibers with more ideal structures as substrates for supercapacitor applications is a great but potentially rewarding challenge. In this work, we synthesized graphene oxide nanofibers (GONFs) with a unique structure through the selective partial oxidization and exfoliation of the outer-wall of CNFs via simple and controllable processes, and followed that highly crystallized Ni(OH)2 nanoplates are uniformly 4


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grown on the reduced graphene oxide nanofibers’ (rGONFs) outer graphene sheets. Because of its superior electrical conductivity, good flexibility and uniform interconnected architecture, the rGONF/Ni(OH)2 composite reveals a much higher SC and better rate capability than the CNF/Ni(OH)2 composite, reduced graphene oxide nanoribbon/Ni(OH)2 composite and the physical mixture of rGONF and Ni(OH)2 electrode. Furthermore, as-fabricated rGONF/Ni(OH)2//active carbon ASC exhibits remarkable energy density, high power density, indicating that GONFs would be excellent substrates and have enormous potential for applications in supercapacitors.

2. Experimental 2.1 Preparation of GONFs and GONRs GONFs were prepared using CNFs (PR-24-XT-HHT, Sigma-Aldrich Co, Inc, USA.) as the starting material with method adapted from the Hummers’ method. Typically, 1.0 g of sodium nitrate and 0.2 g of CNF were mixed in 40 ml of concentrated sulfuric acid at 0 ºC. Next, 6 g of potassium permanganate was added slowly with modest stir. The mixture was placed in an ice bath for 90 min, follow that put in an 35 ºC water bath for 120 min, and added into 250 ml of deionized water, followed that 10 ml of 30% hydrogen peroxide was added. After stirring about 10 min, solid GONF was obtained through vacuum filtration with a filter membrane (220 nm) and washed several times with water until pH neutral. Dispersion of the GONF in water was subjected under vigorous ultrasonic processing for 30 min, and then collected by freeze-drying. The graphene nanoribbon (GONR)40,41 were prepared by the same method as for GONF, except the time at 35 ºC change to 24h.

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2.2 Synthesis of Electrode Materials The rGONF (CNF or rGONR)/Ni(OH)2 composites were obtained according to the following procedures: 0.2 mL of N2H4‧H2O (35 wt.% in water), 0.4 mL of NH3‧H2O (28 wt.% in water) and 4 mL of Ni(NO3)2 (0.4 M in water) were added successively into 25 mL of 2 mg mL -1 GONF (CNF or GONR) aqueous dispersion (sonicated for 30 min). The mixture was sonicated for 3 min and next placed in a 50 mL Teflon-lined autoclave and heated at 180 ºC for 120 min. And then the autoclave was natural cooling. If the concentration of Ni(NO3)2 was changed to 0.1 M, 0.2 M, 0.4 M or 0.8 M, the obtained composites were named rGONF/0.1Ni(OH)2, rGONF/0.2Ni(OH)2, rGONF/0.4Ni(OH)2, rGONF/0.8Ni(OH)2. With the same hydrothermal method, the pure Ni(OH)2 nanoplates and rGONF were obtained, except no GONF or Ni(NO3)2, respectively. The physical mixture of rGONF and Ni(OH)2 was obtained by ultrosonic treatment of the pure Ni(OH)2 and rGONF in a mass ratio of ~71:29 in water, and then collected by freeze-drying. The characterization, electrochemical measurements and calculation details are described in supporting information.

3. Results and discussion 3.1. Characterization of the materials The GONFs were obtained by the modified Hummers method through the selective partial oxidization and exfoliation of CNFs’ outer walls via simple and controllable processes. 37-39 The morphologies of the CNFs, GONFs and GONRs are shown in Fig. 1a, b, c and d. Fig. 1a presents the TEM image of the hollow structure of the CNFs with the 150-200 nm outer diameter and 50-70 nm

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inner diameter, and the cylindrical fiber composed of highly crystalline, graphite basal planes stacked at certain degrees from the fiber’s longitudinal axis (Fig. 1a, inset). This architecture includes extra edge planes that are reactive relative to the basal plane of graphite and easily exfoliated to form graphene sheets while maintaining their connection through the inner graphite basal planes during the oxidation process. After the oxidation reaction, as shown by Fig. 1b, the outer walls of the CNFs were exfoliated to form several graphene slices of various sizes around the fiber; the hollow inner walls can also be observed, and the GONFs increased in size to 500 nm, which is much larger than the CNFs. The SEM image of the GONFs (Fig. 1c) reveals a curly morphology with a thin, wrinkled outer wall, which is a typical feature of graphene layers. The morphology of GONRs is shown in Fig. 1d, and they appear to consist of aggregated graphene oxides and to have thoroughly lost the fiber-like structure. The crystal structure of CNFs and GONFs were analyzed via XRD, as displayed in Fig. 1e. The CNFs have an intensively characteristic graphitic (002) peak at 26.4° and a low-intensity (100) peak at 42.3°, indicating that the graphite microchip layers of the CNFs are compactly arranged.42 After the reaction, the GONFs exhibited a characteristic peak (001) belong to GO at 9.9°, and a small 2θ peak was also observed at 26°, indicating that the GONFs had been partly unzipped and exfoliated and retained a fraction of the graphite structure, further confirming the TEM results (Fig. 1b). Fig. S1 shows that the GONR exhibits only the characteristic peak (001) of GO without any graphitic peak, indicating that the GONR was completely exfoliated and oxidized. Fig. 1f presents the Raman spectra of the CNFs and GONFs, which clearly reveal that the GONFs have two broad peaks at 1360 and 1590 cm-1; smaller and more intense peaks are observed for the CNFs at the same Raman shifts, which are assigned to the D and G bands of graphene, respectively.43 The intensity of D band correspond to the number of defects, dangling bonds and curved sheets in carbon 7



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Figure 1. (a) TEM images of CNF. (b) TEM image of GONF. (c) SEM image of GONF. (d) SEM image of GONR. (e) XRD patterns, (f) Raman spectra, (g) XPS patterns of CNF and GONF. (h) DFT pore-size distribution of GONF. 8


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structures,44 and thus, the GONFs possess a large quantity of defects and irregular surfaces. Furthermore, the peak intensity ratio of the D and G bands (ID/IG) was obviously increased in the CNFs relative to the GONFs. This is caused by strong oxidation processes, and the ratio is associated with the degree of disorder and the average size of the sp2 domains of the graphite materials. Moreover, compared with the CNFs, the 2D band of the GONFs at 2700 cm-1 became weaker and broader, suggesting that the compact multilayer graphitic structure of the CNFs had been disrupted, which is consistent with the TEM results (Fig. 1b). XPS can be used to illustrate the particle surface composition. Fig. S2 reveals that the GONFs mainly consist of carbon and oxygen. Whereas CNFs contain 99.02% carbon, the carbon content of GONFs (75.12%) constitutes 33.36 wt % at 284.6 eV, which is characteristic of sp2 hybridized carbon atoms (C-C bond); these values were 32.96 and 9 wt % at 286.4 and 288.2 eV, respectively, corresponding to the C-O, C=O and O-C=O bands, indicating abundant defects and functionalized sites (Fig. 1f).45 The O/C ratio is 0.33, which is much larger than that of the CNFs. Brunauer-Emmett-Teller (BET) and density functional theory (DFT) analysis shows that the GONF has a specific surface area of 197 m2 g-1 with micro- and meso-porous microstructures (Fig. S3 and 1h), this value is much higher than the CNF (14 m2 g-1), and pore diameters of GONF between microporous to mesoporous (Fig.1h), which could provide a high specific surface and favorable wettability for the next hydrothermal reaction. In order to evaluate the characteristic of substrate, we adopted the hydrothermal method to obtain the rGONF without nickel source during the reduction process, and the Raman and XPS spectra were shown in Fig. S4 and S5, it can be seen that a higher ID/IG ratio in Raman spectra(Fig. S4) compared with the GONF and CNF(Fig. 1f), indicating that the rGONF was highly reduced and formed a greatly disordered structure during the hydrothermal reduced process. And the XPS further 9



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confirm this result, the carbon content of rGONF increase to 91.9 % (Fig. S5a), the C 1s spectrum of rGONF (Fig. S5b) shows that it constitutes 75.83 wt % of C-C bond, and the others were C-O, C=O and O-C=O bands, indicating considerable oxygen-containing functional group lose after hydrothermal process, which is confirmed the Raman results. The XRD patterns recorded the pure Ni(OH)2, the rGONF/Ni(OH)2 composite and the reduced rGONFs obtained using the same hydrothermal methods without Ni(NO3)2 (Fig. 2a). Compared with the standard data from hexagonal-phased β-Ni(OH)2 (JCPDS no. 14-0117), the XRD pattern of the pure Ni(OH) 2 exhibited the characteristic diffraction peaks of hexagonal-phased β-Ni(OH) 2 , 2 θ =19 . 2 6 ° , 38 . 54 ° and 52 . 10 ° , bel ong to the (001) , (101 ) and (102) lat t i ce pl a nes, respectively.48 The rGONF/Ni(OH)2 composite has characteristic diffraction peaks that are almost identical to those of pure Ni(OH)2, indicating the similar structure information. Moreover, the broad diffraction peaks of the rGONFs (2θ≈25°) can also be observed, in the pattern of the rGONF/Ni(OH)2 composite, though faintly, implying that the rGONFs are an appropriate substrate and are successfully loading Ni(OH)2 nanoplates. Further confirming the XRD results, SEM and TEM images reveal a regular hexagonal shape with a particle size of 100-200 nm; these particles appear to stack easily and could be a result of the presence of fewer nucleation sites during the growth process. Fig. 2c and 2d show the SEM and TEM images of the CNF/Ni(OH)2 composite. It can be seen that the Ni(OH)2 nanoplates are clearly scattered around the CNFs and do not compounded well with the CNFs. Additionally, the CNFs have a smooth surface and a chemically inert outer wall with a diameter of approximately 150 nm, similar to the Ni(OH)2 nanoplates, indicating that CNFs are not an appropriate substrate for Ni(OH)2 nanoplates. Because of its poor specific surface area and inferior composite structure, the CNF/Ni(OH)2 did not exhibit outstanding 10


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Fig 2. (a) XRD patterns of samples. (b) TEM image of the pure Ni(OH)2. (c) SEM and (d) TEM image of CNF/Ni(OH)2. (e) SEM and (f) TEM image of rGONF/Ni(OH)2. (g) SEM and (h) TEM image of rGONR/Ni(OH)2. 11



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electrochemical performance, such as uperior SC, favorable rate capability and good cycling stability. The morphology of the rGONF/Ni(OH)2 composite is illustrated in Fig. 2e, 2f and S6. Compared to the GONFs that did not undergo a hydrothermal reaction (Fig. 1 b), the rGONFs in the composite retained a similar morphology without serious breakage or separation, indicating that the GONFs can provide great mechanical strength and structural stability. Fig. 2e and 2f clearly show Ni(OH)2 nanoplate growth on the crinkled surfaces of the rGONFs and confirm that they are interconnected through the rGONFs’ outer graphene sheets, thereby increasing the electrical conductivity and enhancing the electrochemical performance of the Ni(OH)2 nanoplates. In addition, the outer graphene sheet can extend independently without agglomeration because of the surface oxygen-containing groups (Fig. S5), suggesting that the composite could exhibit good wetting properties and high accessibility for electrolyte ions. The Ni(OH)2 nanoplates in rGONF/Ni(OH)2 exhibited a better distribution and stronger attachment with irregular shapes compared with CNF/Ni(OH)2 composite, indicating that the GONFs with large surface areas and abundant active sites influenced the growth and nucleation of the Ni(OH)2 nanoplates. Fig. S7 shows the morphology and electrochemical performance of NiO obtained from pure Ni(OH)2 and rGONF/Ni(OH)2 through annealing in air condition, reveals the effects of interface between rGONF and Ni(OH)2. The morphology of rGONR/Ni(OH)2 is displayed in Fig. 2g, 2h. In this case, the fiber-like structure was destroyed, and the graphene nanoribbon was loosely connected. Although the Ni(OH)2 nanoplates were well distributed, their structural strength may have been lost. As discussed above, compared with the CNFs and rGONRs, the rGONFs were the best substrate compound for Ni(OH)2, and the composite exhibited a unique morphology and favorable structure. These effects play an critical role in increasing the electrical conductivity of the Ni(OH)2 nanoplates and facilitating electrolyte ion 12


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diffusion and insertion. Indeed, the good distribution ensures the highly efficient utilization of the specific surface area and sufficient faradaic reaction during the charging/discharging process. Consequently, the rGONF/Ni(OH)2 composite is expected to have superior electrochemical performance to the CNF/Ni(OH)2 and rGONR/Ni(OH)2 composites.

3.2. Electrochemical performance To verify the above conjecture, CV curves and galvanostatic charging/discharging curves were measured to compare the CNF and GONF, as shown in Fig. S8. It reveals a greatly improved electrochemical performance for GONF in comparison with CNF. The GONF shows nearly rectangular CV curves at 40 mV s-1 and the almost triangular charging/discharging curves at 1 A g-1, indicating the typical EDLC behavior. The SC of the GONF was 102 F g-1 at 5 mV s-1, while the CNF only has 4.8 F g-1. The CNF/Ni(OH)2, rGONF+Ni(OH)2 physical mixture, rGONR/Ni(OH)2, and rGONF/Ni(OH)2 composite were tested with same methods at a 40 mV s-1 or 1 A g-1 in 6 M KOH electrolyte, respectively. The comparison between the bare Ni foam and the composite CV curves is shown in Fig. S9a, revealing the negligible contribution from the Ni foam. In order to obtain the accurate and credible value of the SC, the contribution of Ni foam was deducted in equation (Supporting Information). Fig. S9b recorded the CV curves of Ni foam at various scan rates. For the different composites, in Fig. 3a, the CV curves clearly exhibit the capacitive behavior typical of Ni(OH)2/NiOOH, including a couple of distinct redox peaks, which is assigned to the reversible faradaic reaction of Ni(Ⅱ)↔Ni(III) (Ni(OH)2 + OH-↔ NiOOH + H2O + e-),18,19,21 and the peak current density of the rGONF/Ni(OH)2 composite is much larger than that of the mixture of the other 13



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Fig 3. (a) CV curves at 40 mV s-1 scan rate, (b) SC of CNF/Ni(OH)2 composite, rGONF+Ni(OH)2 physical mixture, rGONR/Ni(OH)2 composite and rGONF/Ni(OH)2 composite at various scan rates. (c) Nyquist plots of three samples: rGONF/Ni(OH)2, rGONR/Ni(OH)2 and physical mixture of rGONF+Ni(OH)2 .

composites. The peak current density of the rGONF+Ni(OH)2 physical mixture and the rGONR/Ni(OH)2 composite are similar, but the shapes of the CV curves are not, most likely because of differences in the composition and chemical bonds in the electrode.The difference of galvanostatic charging/discharge curves of the composites and the mixture is shown in Fig. S10. It can be observed that the discharging time of the rGONF/Ni(OH)2 composite is much longer than those of the others materials, indicating that it has a higher SC. Fig. 3b shows the SC of the composites and mixture derived from CV curves at various scan rates. The SC of the rGONF/Ni(OH)2 composite was 1433 F g-1 at a 5 mV s-1, which is larger than those of the CNF/Ni(OH)2 composite (560 F g-1), rGONF+Ni(OH)2 physical mixture (687 F g-1) and rGONR/Ni(OH)2 composite (735 F g-1) at the same scan rate. When the scan rate increased to 40 mV s-1, the rate capability of the rGONF/Ni(OH)2 composite was 68.8% retention (986 F g-1), which is little higher than the physical mixture of rGONF+Ni(OH)2 (61.3% retention, 421 F g-1), but much higher than the CNF/Ni(OH)2 composite (51.8% retention, 290 F g-1) and rGONR/Ni(OH)2 composite (45.3% retention, 333 F g-1). 14


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Additionally, the rate performance of the rGONR/Ni(OH)2 composite decreased significantly, most likely because of the GNR’s unstable structure and inferior electric conductivity resulting from over oxidation during the preparation process. To further evaluate the electrochemical performances of the samples, electrochemical impedance spectroscopy (EIS) was conducted in the frequency range of 0.1 Hz to 100 kHz. In Fig. 3c, the Nyquist plots are consisted of a depressed arc in the high-frequency range and, typically, a straight line in the low-frequency range. The former contributes to the charge-transfer resistance of the redox reaction at the electrode/electrolyte interface, which is related to the porous structure of the electrodes.49 And the latter corresponds to the capacitor behavior in supercapacitance.50 Because ion diffusion occurred across the electrolyte-to-electrode interface and the surface layers of the electrochemically active materials, the Nyquist plot shows an oblique line instead of the theoretically predicted vertical line relative to the axis.22 Therefore, the composite had better ion diffusion and a steeper curve in the low-frequency region. In general, rGONF/Ni(OH)2 with a smaller depressed arc and a more vertical line than the other electrode materials, suggests a smaller charge-transfer resistance and a faster ion-diffusion rate, respectively. From Fig. 4 and as discussed above, the rGONF/Ni(OH)2 composite has better electrochemical performance than other samples due to its unique structure and a uniform distribution of Ni(OH)2 nanoplates, while the CNF/Ni(OH)2 composite has a low specific surface area and lower SC, the physical mixture of rGONF+Ni(OH)2 with phase separation has a large charge-transfer resistance, and the rGONR/Ni(OH)2 composite with over oxidation and structural instability, has poor rate capability and a slower ion-diffusion rate. Therefore, as expected, the unique morphology and favorable structure of the GONFs offers large surface areas and abundant active sites, facilitating the uniform distribution of the Ni(OH)2 15



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nanoplates. Furthermore, the better contact between the graphene and the Ni(OH)2 nanoplates may improve the charge-transfer kinetics and accelerate ion diffusion and faradic reaction, leading to superior rate performance and electrochemical activity.51 We also evaluate the influence of the Ni(OH)2 nanoplate mass fraction on the electrochemical performance and determine the optimal ratio for supercapacitive electrodes, Thermogram curves were obtained for different ratios of the rGONF/Ni(OH)2 composite, respectively. The composites were named according to their proportion of Ni(NO3)2: rGONF/0.1Ni(OH)2, rGONF/0.2Ni(OH)2, rGONF/0.4Ni(OH)2, and rGONF/0.8Ni(OH)2, and the mass fraction of Ni(OH)2 were calculated from TG curves. As shown in Fig. 4a, because the rGONF decomposed totally after the temperature up to 600 ºC , the residue content of the composite is mainly contributed by NiO. The mass fraction of NiO are 31.1%, 45.5%, 57.9%, 67.5%, corresponding to 38.5%, 56.4%, 71.8%, 83.7% mass fraction of

Ni(OH)2 within

rGONF/0.1Ni(OH)2, rGONF/0.2Ni(OH)2,

rGONF/0.4Ni(OH)2,

rGONF/0.8Ni(OH)2, respectively. And the main weight loss observed at 250-300 ºC and 400-450 ºC, the former contributed by the chemical reaction of Ni(OH)2 change to NiO, and the latter due to the

Fig 4. (a) Thermogram of rGONF, rGONF/0.1Ni(OH)2, rGONF/0.2Ni(OH)2, rGONF/0.4Ni(OH)2, rGONF/0.8Ni(OH)2 taken under the flow of air during the temperature ramp. (b) SC of rGONF/0.1Ni(OH)2, rGONF/0.2Ni(OH)2, rGONF/0.4Ni(OH)2, rGONF/0.8Ni(OH)2 and pure Ni(OH)2 at various scan rates. (c) SC at different mass fraction of Ni(OH)2. 16


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decomposition of rGONF. The curves of the composites with various proportions of Ni(OH)2 measured at a 1 A g-1 current density are displayed in Fig. S11, and the rGONF/0.4Ni(OH)2 composite exhibited the longest discharging time, suggesting that it had the best SC. Fig. 4b shows that the SC of samples containing 38.5 wt. %, 56.4 wt. %, 71.8 wt. %, 83.7 wt. % and 100wt. % Ni(OH)2 were 761, 1206, 1433, 1092, and 615 F g-1, respectively, at 5 mV s-1. When the scan rate was increased to 40 mV s-1, the SC values were reduced to 608, 879, 986, 679, and 320 F g-1, with corresponding rate capabilities of 79.9% retention, 72.9% retention, 68.8% retention, 62.2% retentionand and 52% retention, respectively. The “bell-shaped” curve (Fig. 5c) clearly indicates the altered trend of the SC. Initially, the capacitive performance increased as Ni(OH)2 increased because the behavior of capacitor was mainly dependent on Ni(OH)2, and the well-dispersed Ni(OH)2 particles on the rGONF substrate could be sufficiently utilized, simultaneously increasing the overall capacitance and maintaining the outstanding rate capability. However, above a certain concentration, too many Ni(OH)2 nanoplates grew on the rGONFs, and the rate capability and capacitive performance decreased significantly. This is most likely because the high mass ratio of the Ni(OH)2 reduced the electrical conductivity and surface area of the composites and confined the electrolyte ion diffusion to the electrochemically active sites on the electrode with large charge-transfer resistance. Consequently, the influence of the Ni(OH)2 nanoplate mass fraction must be evaluated. In this research, we found that the rGONF/0.4Ni(OH)2 composite containing 71.8 wt. % Ni(OH)2 was the optimal material and exhibited enhanced capacitive property. The rGONF/Ni(OH)2, with best substrate and optimal proportion, was evaluated in detail. Fig. 5a displays the typical CV curves collected at different scan rates. The peak current response improved as the scan rate increased, and shape-keeping well, demonstrating its good rate capability.52 17



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Fig 5. (a) CV curves of rGONF/Ni(OH)2 at different scan rates, (b) SC at various scan rates. The inset are the liner relationship between the anodic peak current and the square root of scan rate(ν1/2). (c) Galvanostatic charging/discharging curves at various current densities. (d) Cycling test at 40 mV s-1. The inset are the initial CV curves and after 2000 cycles at 40 mV s-1.

Meanwhile, the redox peaks shifted to a more positive or negative position, because of internal diffusion resistance increasing in electrode.48,53 Fig. 5b shows the SC of rGONF/Ni(OH)2 at various scan rates. And the values were 1433 F g-1 at 5 mV s-1, and the 68.8% retention (986 F g-1) at 40 mV s-1, much better than the pure Ni(OH) 2. The SC decreased slowly as the scan rate increased, demonstrated the better rate capability. Overall, because of the unique morphology and favorable structure of rGONF/Ni(OH)2, which is benefited from its shortened ion pathway, accelerated ion diffusion and improved charge-transfer kinetics, the composite electrode exhibited a good rate capability and best charge-storage ability.26 The inset of Fig. 5b displays the relationship between the anodic peak currents and the square root of the scan rate. This curve clearly reveals a typical linear 18


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relationship between them, indicating the diffusion-controlled process is dominant for redox reactions in the composite.53-55 Fig. 5c shows the galvanostatic charging/discharging property of the rGONF/Ni(OH)2 composite at different current densities. Long-term cycling stability is another critical parameter for evaluating a supercapacitor’s electrochemical performance. Fig. 5d displays how the SC changed as the cycle number increased of the composite electrode. The SC was retained at approximately 90.5% of the initial SC after the 2000th cycle, which is much better than the pure Ni(OH)2 (79.3%, Fig S12). This improved cycling stability corresponds to the stable structure and unique morphology of the rGONF/Ni(OH)2 composite, Fig. S13 shows the SEM images of the composite after 2000 cycles, and it can be observed clearly that the morphology of rGONR was maintained well and without the Ni(OH)2 nanoplates’ obviously peeling off. The capacitance of the GNF/Ni(OH)2 composite electrode increased gradually during the first 150 cycles, which could benefit the activation of the electrode materials through gradual electrolyte infiltration into the near face of Ni(OH)2 and rGONF during testing process.17,56,57 The inset in Fig. 5d shows the CV curves change of the composite electrode before and after the cycling test. The curves reveal very similar shape with a couple of redox peaks, apart from the smaller area and the potential difference between the redox peaks and reduction peaks compared to the initial curve. The former indicates the slightly decrease of SC, the smaller value of latter suggests better reversibility.17 The two-electrode ASC was assembled by employing the rGONF/Ni(OH)2 composite as positive electrode and AC as negative electrode. The ASCs can take full advantage of extended potential windows of the different electrodes to reach a larger operating voltage for the whole system. This is a very important to promote energy density performance because the energy density is calculated as: E =1/2 C‧V2 ,where C and V is the SC and testing potential in two-electrode system, 19



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respectively. 57 The electrochemical performance of AC in three-electrode system is shown in Fig. S14. Fig. 6a exhibits a variety of CV curves with different potential that can evaluate the best potential window of the two-electrode system. As can be seen, it shows that the combination of pseudocapacitor and EDLC,and the intensely redox peaks occurs from about 0.6 to the high potential, indicates the pseudocapacitive properties contributed from the positive electrode (rGONF/Ni(OH)2). In addition, when the potential windows increased to 1.7 V, more faradic reactions occurred. When the potential windows extended to 1.8 V, oxygen evolution reaction can be found. Thus, we choose 1.7 V as the cut-off potential for the next research. Fig. 6b displays SC of the ASC calculated from CV curves with different scan rates in the potential window of 0-1.7 V, and the inset is the corresponding CV curves. Consistent with the discussed above of the three-electrode system, the peak current response improved as the scan rate increased, the redox peaks transfered to a more positive or negative position, and the shapes of the CV curves preserved well. The SC values were 109.8 F g-1 at 5 mV s-1, and remained 93.7 F g-1 when the scan rate increased to 40 mV s-1. Remarkably, the rGONF/Ni(OH)2//AC ASC with 85% retention demonstrated the superior rate capability and best charge-storage ability. And the cycling test is shown in Fig S15, the SC was retained at approximately 77.4% of the initial SC after the 2000th cycle in a wide potential window of 0-1.7 V. Fig. 6c shows the galvanostatic charging/discharging performance of the the rGONF/Ni(OH)2//AC ASC at different current densities. Fig. 6d shows the Ragone plot (energy densitiy versus power density) of the rGONF/Ni(OH)2//AC ASC. The cell exhibits a promising energy density (44.1Wh kg-1) at power density of 467 W kg-1, and the energy density remains 37.6 Wh kg-1 at a high power density of 3185 W kg-1. Theoretically, an ideal energy storage device should reveal a parallel line to the axis of power 20


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Fig 6. (a) CV curves of the rGONF/Ni(OH) 2//AC ASC measured at 40 mV s-1 with different potential window. (b) The SC of tested with various scan rates at a potential window of 1-1.7V, and the inset is the corresponding CV curves. (c) Galvanostatic charging/discharging curves at different current densities. (d) Ragone plots of the rGONF/Ni(OH) 2//AC ASC, and the inserted picture is double rGONF/Ni(OH) 2//AC ASC with series connection to lighting up a blue LED (3.1-3.6V).

density in Ragone plot. Nevertheless, the plot shows a deviation from the parallel line normally due to the resistance and some other reasons. It can be seen that the line in the plot deviated slightly, demonstrated the energy density of the rGONF/Ni(OH)2//AC ASC decreasing slightly with the power density increasing. This values surpasses many previously reported Ni(OH)2-based ASC, including Ni(OH)2/UGF//a-MEGO

17,

AC//Ni(OH)2/AC/CNT

24,

21


CNT@Ni(OH)2//3DG

52,

coaxial


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CNT/Ni(OH)2//rGO 23, few-layered Ni(OH)2//AC, 53 and Ni(OH)2/CNT//AC 58, as well as some other type supercapacitor, including NiCo2S4//G/CS

15

, symmetric HGF-EC

, and CoO@PPy//AC

31

.

49

Due to the limited working potential window of the supercapacitor, assembling with serial method would be a facile way to solve this issue for wide range applications. The inset picture shows that the two ASC with serial assembly can light up a blue LED (working voltage 3.1–3.6 V) after charging to 3.4 V. These impressive consequence verify again the superior property of the rGONR/Ni(OH)2//AC ASC.

4. CONCLUSION In this study, we has been obtained a novel rGONF/Ni(OH)2 composite with a unique structure and researched their electrochemical supercapacitor performance. The GONFs were obtained through selective partial oxidization and exfoliation of CNFs’ outer walls, and highly crystallized Ni(OH)2 nanoplates were uniformly grown on rGONFs’ outer graphene sheet via hydrothermal processing. The rGONF/Ni(OH)2 composite displayed enhanced SC (1433 F g-1 at 5 mV s-1), smaller charge-transfer resistance and faster ion-diffusion rate than other composite as well as good cycling stability, indicating that rGONFs are a superior substrate material for supercapacitors because of their unique

structure

and

high

electrical

conductivity.

Attributing

to

those

merits,

the

rGONF/Ni(OH)2//active carbon ASC exhibited a remarkable energy density and a 85.3% high retention (44.1 Wh kg-1 at 467 W kg-1 and 37.6 Wh kg-1 at 3185 W kg-1) with a wide potential window of 0-1.7 V. This study illustrates that rGONFs possess excellent potential as a substrate material for supercapacitive applications and paves a new avenues for the design and fabrication of high performance supercapacitive electrode materials. 22


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Acknowledgments Thanks to Dr. Baihua Qu and Daoping Cai (Xiamen University) for useful discussions and suggestion during this work.

Associated Content Supporting Information The characterization, electrochemical measurements and calculation details. The additional figures are the XRD pattern, Raman spectra, XPS spectra, nitrogen adsorption and desorption isotherms of as-prepared carbon materials, the electrochemical performance of different composites, mixture and AC. The table is a summary of electrochemical data for previous reports. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding author Hongbing Zhan, E-mail: [email protected]

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TOC

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Supercapacitors Based on Reduced Graphene Oxide Nanofibers

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