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Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance
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Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance Effat Jokar a, Azam Iraji zad a,b, Saeed Shahrokhian a,c,* a
Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, Azadi Ave, Tehran, Iran Department of Physics, Sharif University of Technology, Azadi Ave, Tehran, Iran c Department of Chemistry, Sharif University of Technology, Azadi Ave, Tehran 11155-9516, Iran b
article info
abstract
Article history:
The high capacitance and cyclic stability of graphene nanosheets decorated with Co3O4
Received 16 August 2014
nanoparticles as a material for supercapacitor electrodes are reported here. Hydrothermal
Received in revised form
method is adopted to deposit cobalt oxides on the reduced graphene oxide (RGO) sheets in
30 September 2014
a mixture of water and dimethylformamide (DMF) as the solvent with different volume
Accepted 13 October 2014
ratios. The water volume ratio presents a crucial factor in the nucleation and growth
Available online xxx
process. In addition, it affects dispersion, particle size and the amount of nucleated cobalt oxide particle on the graphene sheets. By decreasing the water volume, the nucleation and
Keywords:
growth occur mainly on graphene rather than in solution. According to the obtained results
Cobalt oxide nanoparticles
from transmission electron microscopy, scanning electron microscopy and thermogravi-
Reduced graphene oxide
metric analysis, a model for growth of nanoparticles on graphene sheets is proposed. Based
Supercapacitors
on the obtained results, the presented model can also be used for the synthesis of other
Solvent effect
graphene-metal (oxide) composites. Electrochemical measurements indicate that water
Nucleation and growth
volume ratio in the mixture solvent influences on capacitance of the RGO/Co3O4 composite electrodes. The highest obtained specific capacitance is 440.4 F g1 with 50% volume ratio of water at current density of 5 A g1. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Supercapacitors are known as one of the most important energy storage devices that exhibit high power density, long lifetime and fast chargeedischarge rates [1,2]. Their energy density is higher than common capacitors but one order lower than batteries. Enhancement of the energy density, as well as
maintaining its power density and cycle life for their applications are very important factors. Supercapacitors are classified based on charge storage mechanism as electrochemical double layer capacitors (EDLC, carbon based electrodes) and the other class is called pseudocapacitors [3]. Pseudocapacitors are suitable candidates to increase the charge storage and energy density. Energy storage in pseudocapacitors is based on fast and reversible redox reactions [4,5]. Metal oxides such
* Corresponding author. Department of Chemistry, Sharif University of Technology, Azadi Ave, Tehran 11155-9516, Iran. Tel.: þ98 21 66164123; fax: þ98 21 66002983. E-mail address: [email protected] (S. Shahrokhian). http://dx.doi.org/10.1016/j.ijhydene.2014.10.061 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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as RuO2 [6], NiO [7,8], Fe3O4 [9], Mn3O4 [10], MnO2 [11] and Co3O4 [12,13] are examples of materials that have been widely studied in this area. Their specific capacitance is higher than carbon-based electrodes using physical adsorption of ions at the interface between electrode surface and electrolyte. Although pseudocapacitors have high energy density, they often suffer from shorter cycle life and low electrical conductivity in comparison with carbon materials. Synthesis of hybrid metal oxides and carbon materials can be useful to improve their properties [14e20]. Recently, graphene has been considered as a promising candidate for the supercapacitor electrode material, due to unique properties such as high surface area (theoretically 2620 m2 g1), mechanical stability (130 GPa), chemical stability, high electrical conductivity and electron mobility (charge carriers of 200,000 cm2 V1 s1), and abundant surface functional oxygen groups [15,16,21e26]. Therefore, fabrication of hybrid supercapacitors using graphene/metal oxide composites is considered as an efficient strategy to improve pseudocapacitance properties of these devises [14,15,19,27]. According to electrical conductivity of graphene, it can acts as a conductive template to enhance electrical properties of the metal oxides composites and provides a good electrical contact to the current collector [15,27,28]. Graphene diminishes the volume of expansion/contraction and aggregation of nanoparticles during charge/discharge cycles and improves life time of the electrodes. In addition, graphene is a good substrate for growing and anchoring the metal oxides with a well-defined size and shapes [23,29,30]. Oxygen functional groups on the graphene surface act as nucleation sites and provide desirable bonding, interfacial interactions and electrical contacts between the substrate and nanoparticles [29,31]. On the other hand, anchoring the nanoparticles on graphene acts as spacer between graphene sheets and reduces restacking of sheets [14,32], which helps better the diffusion process of electrolyte ion between the sheets to occur more simple [15]. Different composites of graphene/ metal oxide have been synthesized as supercapacitor electrodes: Graphene/MnO2 [26,27], graphene/NiO [23], graphene/ Fe2O3 [33], graphene/ZnO [34] and graphene/Co3O4 [35]. All of these studies have shown improvement of cyclability and electrochemical properties in comparison with metal oxides [32,36]. Different synthesis methods were applied to preparation of graphene/Co3O4 as supercapacitor electrodes including microwave (specific capacitance of 243.2 F g1 at 10 mV s1) [35], surfactant assistance (163.8 F g1 at 1 A g1) [37] and hydrothermal process (337.8 F g1 at 0.2 A g1) [38]. The present work studies nucleation and growth process of graphene/Co3O4 composite in hydrothermal condition in order to improve charge storage capacitance of the composite. For this reason, the effect of water volume ratios (30%, 50%, 70% and 100%) in the mixture with DMF is examined, which is an effective hydrolyzation parameter on the nucleation and growth processes. A model for nanoparticle growth on graphene sheets is obtained by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The obtained electrochemical measurements are represent that controlling the water volume ratio in the synthesis process can lead to composite with high specific capacitance. The
water volume ratio in the solvent mixture for hydrothermal process will reach but the sample with 50% water displays a better electrochemical activity to an optimum amount, which offers the highest capacitance to the resulted composite. The results of our work showed that the simplest way to control the growth condition is focusing on the solvent optimization. By this mean, composite based electrodes for supercapacitors with high performance can be achieved.
Experimental Synthesis of graphene/cobalt oxide composite All chemicals were done by synthetic grade materials and used as received without further purification. Graphene oxide (GO) was synthesized by the modified Hummers method [39]. GO (11 mg) with a concentration of 0.5 mg mL1 was dispersed in a mixture of DMF and water with different volume ratios. A portion of 0.5 mmol Co(OAc)2.4H2O was dissolved in deionized (DI) water and the obtained cobalt solution slowly was added to GO solution under magnetic stirring. The solution was stirred for 1 h and then ammonia solution (5%) was added until pH of the solution reached to 9.5e10. The resultant solution was stirred for a 30 min and then was transferred to a teflon lined stainless steel autoclave and was subjected to heat at 130 C for 14 h. After cooling the autoclave to room temperature, black precipitate was separated by centrifugation and then was washed several times with DI water. Volume ratios of water in the solvent were chosen 100%, 70%, 50% and 30% so the obtained composites are named as GC-100%, GC-70%, GC-50% and GC-30% respectively. After that, the precipitate was dried by freeze-dryer and then was annealed at 300 C (with a rate of 2 C min1) for 4 h. The Co3O4 was synthesized by the mentioned method (with a water volume ratio of 50%) in the absence of GO.
Characterizations The X-ray diffraction (XRD) patterns were recorded using a STOE (STADI P) instrument operating with CuKa radiation ˚ ) at 40kV/30 mA. A SIGMA VP Field Emission (l ¼ 1.54178 A Scanning Electron Microscope (FESEM) and a FEI (Tecnai G2 F30) transmission electron microscope (TEM) were used to study the morphology and structure of the samples. Thermogravimetric (TG) analysis was carried out using a Mettler Tolledo (TG/DSC1) instrument in a temperature range of 25e650 C (heating rate of 10 C min1 in air). The electrochemical capacitive behaviors of electrodes were measured on a Potontiostat/Galvanostat (AUTOLAB 302). The working electrode was fabricated by mixing 80 wt % RGO/Co3O4 powder, 10 wt % carbon additive and 10 wt % poly (vinylidenedifluoride in N-methyl-2-pyrrolidone, PVdF as binder) and then was cast on the nickel foam (1 cm2) and was dried at 70 C. Finally, each working electrode containing about 1 mg of electroactive material was tested in 2 M KOH solution using Ag/AgCl as reference electrode and a platinum rod as counter electrode. The lifetime of the electrodes were measured by the battery tester Kimia pardaz Samaneh (Kimia State 126-Iran).
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Results and discussion Graphene/Co3O4 composites were synthesized by a hydrothermal method which is a suitable approach to apply for uniform and crystalline particles of metal oxide growth on graphene sheets [31,40]. In the initial stage of synthesis, Co2þ ions in water and DMF solution were interacted with oxygen functional groups on the surface of graphene oxide (GO) surface. Altering water volume in the solvent can be used to control the hydrolyzation process. Based on recent studies, decreasing the volume of water causes better dispersion of graphene sheets in solvent [29,41] and decreases the hydrolysis rate of metal ions [25,40,42]. This low water content lead to a selective nucleation of cobalt hydroxide on graphene sheets rather than nucleation in the solution [29,31,40,42]. However in the presence of higher water content, nucleation can occur also in the solution as well as on graphene oxide. Morphology studies of products with TEM and SEM can help to understand nucleation and growth steps. Fig. 1 presents SEM images of GC-100%, GC-70%, GC-50% and GC-30% (here % represents the present of water content in the solvent mixture). In all images, sheet structures can be observed that can be related to graphene sheets (Raman spectroscopy of samples are shown in Fig. S1) and all of sheets were covered by cobalt oxide nanoparticles. Many granular-shape species are observed among the sheets as is shown in Fig. 1a and b (that related to GC-100% and GC-70% samples, respectively). The presence of these particles may refer to freely growth of cobalt oxide nanoparticles in the phase solution, independently from graphene sheets, and precipitation together with the coated graphene sheets. By reducing the water volume which results to decline cobalt precursor hydrolyzation rate, the number of large granular-shaped particles decreased (Fig. 1c and d).This result reveals that cobalt ions preferably select
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graphene sheets for nucleation, thus a selectivity growth feature occurs on the surface of graphene sheets”. A more detailed study was done by TEM technique (Fig. 2). It's clearly confirmed that Co3O4 nanoparticles are present in the composite structure as well as sheet structure of graphene. In TEM images of GC-100% sample (Fig. 2a and b), not only graphene sheet structures are shown, but also, Co3O4 nanoparticles in the size of 100 nm and 5 nm exist. The granular-shaped nanoparticles shown in Fig. 2a are the same particles among the flake-like structure in SEM images (Fig. 1a). Comparing TEM and SEM results concludes that large particles (100 nm) have not grown on the graphene surface despite of small particles (average size of 5.6 nm). When the volume content of water in the synthesis procedure was decreased (the sample of GC-50%), number of large granularshaped particles is reduced (Fig. 2c). In addition, the particles grown on graphene sheets are densely accumulated (Fig. 2d). The size of particles varies from 2 nm to 5 nm (average size of 3.9 nm). In Fig. 2e and f that are related to GC-30% sample, larger particles (100 nm) can't be seen. Furthermore, small Co3O4 particles about of 2e3 nm (average size of 3.3 nm) have grown more uniform and more densely accumulated on graphene sheets. In fact in this condition, oxygenated groups on the graphene sheets are the only candidates for nucleation of metal oxides nanoparticles. XRD data is applied to study the crystal phase of the synthesized nanocomposites (Fig. 3). The diffraction peaks at 19.0, 31.5, 36.6, 44.8, 59.6 and 65.3 can be indexed as the (111), (220), (311), (400), (511) and (440) crystalline planes of Co3O4 nanoparticles, respectively (JPDS No.43-1003). The diffraction peaks of the composites are similar to those of pure spinel Co3O4. The weak diffraction peaks indicate that the size of Co3O4 particles is relatively small [43]. In addition, it is clearly seen that by increasing the amount of DMF, the intensity peak gradually drops, which indicates whether that amorphous
Fig. 1 e SEM images of nanocomposites with different water content in mixture of solvent; (a) 100%, (b) 70%, (c) 50% and (d) 30%. Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Fig. 2 e TEM images of samples with different water content of solvent; (a,b) GC-100%, (c,d) GC-50% and (e,f) GC-30%.
phases are forming in the composites or the size of particles is decreasing. Considering SEM and TEM results, decrease of particle's size by water diminution can be confirmed. Beside the size of nanoparticles (2e5 nm), the poly-crystallinity of the samples (as also shown in the SAED pattern in HRTEM analysis for samples of GC-30%) can be stated as another reason of weak diffraction peaks. As mentioned above, SEM and TEM images presented large granulated particles in the samples GC-50% and GC-100% that were attributed to Co3O4. It seems that the amount of Co is not similar in all samples. This fact can be deduced from the pink color of the solution above the black precipitation in the
autoclave (Fig. S2), which is a sign for not reacted cobalt ions. To measure the amount of Co3O4 in each composite and compare the samples together, TGA technique was performed [43e45]. Fig. 4 shows the representative TGA of RGO-Co3O4 composites. RGO is thermally unstable and starts to lose mass upon heating above 350 C. It is clear that each sample shows different mass percentage losses in TGA result. GC Composites with 100% and 70% water show approximately 20 wt% loss during calcinations in air from 20 to 600 C. Decreasing volume of water in the synthesis process led to higher mass loss based on TGA result. Graphene composites of GC-50% and GC-30% have the mass loss of 36% and 44%, respectively. Since the
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Fig. 3 e XRD patterns of samples with different water content of solvent (a) GC-100%, (b) GC-70%, (c) GC-50%, (d) GC-30% and (e) XRD pattern of Co3O4 from literature.
initial amount of graphene oxide is equal in all of the samples at the beginningof synthesis and according to TGA results, the amount of Co3O4 in samples with 100%, 70%, 50% and 30% water can be estimated as 80, 80, 64 and 56%, respectively. Presence of lower amounts of cobalt oxide in GC-30% sample the pink color of the above solution in the autoclave indicates that there is some amount of cobalt salt which does not participate during the reaction. According to the results, a model for nucleation and growth of nanoparticles on graphene sheets is suggested in Scheme 1. When the volume of water is low, the rate of hydrolyzation is low and some of Co(OAC)2converts to Co2þ ions and interacts with oxygen groups of graphene oxide
Fig. 4 e Thermogravimetric analysis of different nanocomposites.
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sheets. After adding ammonia (NH3) to the solution, cobalt hydroxide forms both on the graphene surface and in the solution. In the hydrothermal process (under the high pressure and temperature), in addition to growth of nucleated particles, some of the cobalt precursors (green spheres) in low water volume condition hydrolyze and interact with oxygencontaining functional groups of the graphene oxide sheets. Therefore, smaller and more densely accumulated particles can be obtained. In high water volume condition, most of cobalt ions interact with graphene and the surface diffusion produces larger particles in the hydrothermal condition. According to the model, in GC-50% sample both large (~5 nm) and small (~2 nm) particles are seen, which means that the nucleation and the growth occur simultaneously in this condition. Cyclic voltammetry (CV) was used to study the electrochemical behavior of the prepared composites. The CV tests of the synthesized composites were measured in 2.0 M KOH aqueous electrolyte with the applied potential range of 0.0e0.5V at scan rate of 10 mV s1. As shown in Fig. 5a, it can be clearly observed in all of the samples that there is a distinct pair of redox peaks during the anodic and cathodic sweeps, with the presence of a broad redox background. It can be concluded that the prepared samples demonstrate pseudocapacitive properties that originate from the reversible and continuous electrochemical reactions of Co3O4 involved in the chargeedischarge processes [46,47]. As shown in Fig. 5a, GC samples with various water volumes from 30% to 100% show nearly same electrochemical behaviors but the sample with 50% water display a better electrochemical activity. The cyclic voltammetry of Co3O4 nanoparticles, (its SEM image is shown in Fig. S3), is compared with GC-50% (Fig. S4). A Shift in cathodic and anodic peak potentials as well as increasing in corresponding cathodic and anodic peak currents is observed. This phenomenon is related to the electrocatalytic role of graphene sheets in the composite samples. Also, we mixed pre-synthesized Co3O4 powder with reduced graphene oxide physically at a ratio of 0.64:0.36 (Fig. S5) and compared it with GC-50% water. The results show the effect of direct growth of Co3O4 nanoparticles on graphene sheets on the electrochemical properties of the prepared composite samples. Co3O4 nanoparticles, which were grown directly on the surface of graphene sheets, could have both covalent chemical bonding and van der Waals interactions with oxygen-containing defect sites and pristine regions of the graphene [25] respectively (Fig. S1). Less electrochemical activity of the mixture can be related to less cordial contact between the per-made Co3O4 and graphene sheets. Observation of a positive shift in the cathodic and anodic peak potentials confirms the role of graphene as catalyst for improvement of the electrochemical activity. According to these results, direct growth of Co3O4 on the graphene sheets is considered as an important parameter in improving the electrochemical behavior of the samples [31]. Galvanostatic methods are applied to calculate the specific capacitance of the prepared electrodes. The chargeedischarge properties of different samples at the current density of 5 A g1 are shown in Fig. 5b. The GC-30%, GC-70%, and GC-100% samples have the same chargeedischarge behavior that this similarity was also observed in the CV curves (in Fig. 5a).
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Scheme 1 e Summary of the synthesis process of nucleation and growth of nanoparticles on graphene sheets in hydrothermal step.
Calculations of the specific capacitance at the current density of 5 A g1 for each sample are shown in Fig. 5c. Specific capacities of 331.8, 311.3, 440.4 and 320.7 F g1 are obtained for GC samples with 100%, 70%, 50% and 30% water, respectively. Rate capability of different samples is presented at Fig. 5d. In
different current densities, GC-50% has highest specific capacitance in comparison with other samples and proper rate capability. An important result for specific capacitance is achieved for GC containing 30% water. The TGA results for this sample previously approved that the amount of Co3O4 is lower
Fig. 5 e Electrochemical results of different graphene-Co3O4nanocomposites; (a) Cyclic voltammetry at 10 mV s¡1, (b) chargeedischarge at 5 A g¡1, (c) calculated capacitance of nanocomposites with different water contents in mixture of solvent 5 A g¡1, (d) dependence of specific capacitance of different nanocomposites on current density. Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Co3O4 on graphene sheets can be altered. By controlling nucleation and growth on the graphene sheets rather than in solution, the electrochemical properties can be improved. This improvement can be related to the presence of graphene as a conductive support. Based on the electrochemical results, an optimum water ratio of 50% is selected for preparation of the more efficient electrochemical capacitors. This condition doesn't mean that the growth in solution has been eliminated; however, it states a balance condition between the amount of cobalt hydroxide grown in solution and simultaneously on the graphene sheets.
Acknowledgment Fig. 6 e Cycle stability of different nanocomposites electrode at current density 3 A g¡1.
than the other samples, but this sample has the approximately same specific capacitance as GC samples with 100% or 70% water. It can be related to the synergic effect of graphene and Co3O4 nanoparticles and size of Co3O4 nanoparticles. The effect of the presence of graphene as a 2D support provides a good contact between OH ion of the electrolyte and cobalt oxide nanoparticles [31,32]. In addition, graphene can improve the electron transition from Co3O4 nanoparticles to Ni foam as the current collector. In fact, in GC with 30% water all of Co3O4 nanoparticles grow directly on graphene sheets and cause an electrochemical behavior better than other samples. Beside of above reason the size of nanoparticles can be important. The size of nanoparticles in GC-30% is lower than GC-100%, with decreasing of particles size, surface area of them increase then the amount of charge transfer at interface of electrode and electrolyte increase. High cycle stability is an important factor for practical applications of supercapacitor electrodes. To evaluate the stability during the chargeedischarge cycle, the values of specific capacitances with respect to chargeedischarge cycle number (up to 2000 cycles) at 3 A g1 were illustrated in Fig. 6. The maintenance of the initial capacitance over 2000 cycles for GC-100%, GC-70%, GC-50% and GC-30% are 97%, 93%, 94% and 95%, respectively. The results demonstrate an excellent electrochemical stability of these composites as electrode materials for supercapacitors preparation. By controlling the nucleation and growth steps of metal oxide nanoparticles on graphene, we can obtain high capacitance in comparison with other works that used hydrothermal methods and graphene oxide as initial material in synthesis process (291 F g1 at 1A g1 [43] or 472 F g1at 2 mV s1 [48]).
Conclusion In this work we optimized synthesis conditions of Co3O4/ graphene composite by controlling hydrolyzation solvent strength with alteration of water/DMF volume ratio as the solvent mixture. With different ratios of water in the synthesis process, amount, dispersion and crystallite size of
Authors would like to thank Miss Dr. Somayeh Fardindoost and Prof. Hilmi Volkan Demir and Devices and Sensors Research Group led by him for helping in the TEM analysis performed at UNAM (Institute of Material Science and Nanotechnology) Bilkent University.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.10.061.
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Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance Effat Jokar a, Azam Iraji zad a,b, Saeed Shahrokhian a,c,* a
Institute for Nanoscience and Nanotechnology (INST), Sharif University of Technology, Azadi Ave, Tehran, Iran Department of Physics, Sharif University of Technology, Azadi Ave, Tehran, Iran c Department of Chemistry, Sharif University of Technology, Azadi Ave, Tehran 11155-9516, Iran b
article info
abstract
Article history:
The high capacitance and cyclic stability of graphene nanosheets decorated with Co3O4
Received 16 August 2014
nanoparticles as a material for supercapacitor electrodes are reported here. Hydrothermal
Received in revised form
method is adopted to deposit cobalt oxides on the reduced graphene oxide (RGO) sheets in
30 September 2014
a mixture of water and dimethylformamide (DMF) as the solvent with different volume
Accepted 13 October 2014
ratios. The water volume ratio presents a crucial factor in the nucleation and growth
Available online xxx
process. In addition, it affects dispersion, particle size and the amount of nucleated cobalt oxide particle on the graphene sheets. By decreasing the water volume, the nucleation and
Keywords:
growth occur mainly on graphene rather than in solution. According to the obtained results
Cobalt oxide nanoparticles
from transmission electron microscopy, scanning electron microscopy and thermogravi-
Reduced graphene oxide
metric analysis, a model for growth of nanoparticles on graphene sheets is proposed. Based
Supercapacitors
on the obtained results, the presented model can also be used for the synthesis of other
Solvent effect
graphene-metal (oxide) composites. Electrochemical measurements indicate that water
Nucleation and growth
volume ratio in the mixture solvent influences on capacitance of the RGO/Co3O4 composite electrodes. The highest obtained specific capacitance is 440.4 F g1 with 50% volume ratio of water at current density of 5 A g1. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Supercapacitors are known as one of the most important energy storage devices that exhibit high power density, long lifetime and fast chargeedischarge rates [1,2]. Their energy density is higher than common capacitors but one order lower than batteries. Enhancement of the energy density, as well as
maintaining its power density and cycle life for their applications are very important factors. Supercapacitors are classified based on charge storage mechanism as electrochemical double layer capacitors (EDLC, carbon based electrodes) and the other class is called pseudocapacitors [3]. Pseudocapacitors are suitable candidates to increase the charge storage and energy density. Energy storage in pseudocapacitors is based on fast and reversible redox reactions [4,5]. Metal oxides such
* Corresponding author. Department of Chemistry, Sharif University of Technology, Azadi Ave, Tehran 11155-9516, Iran. Tel.: þ98 21 66164123; fax: þ98 21 66002983. E-mail address: [email protected] (S. Shahrokhian). http://dx.doi.org/10.1016/j.ijhydene.2014.10.061 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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as RuO2 [6], NiO [7,8], Fe3O4 [9], Mn3O4 [10], MnO2 [11] and Co3O4 [12,13] are examples of materials that have been widely studied in this area. Their specific capacitance is higher than carbon-based electrodes using physical adsorption of ions at the interface between electrode surface and electrolyte. Although pseudocapacitors have high energy density, they often suffer from shorter cycle life and low electrical conductivity in comparison with carbon materials. Synthesis of hybrid metal oxides and carbon materials can be useful to improve their properties [14e20]. Recently, graphene has been considered as a promising candidate for the supercapacitor electrode material, due to unique properties such as high surface area (theoretically 2620 m2 g1), mechanical stability (130 GPa), chemical stability, high electrical conductivity and electron mobility (charge carriers of 200,000 cm2 V1 s1), and abundant surface functional oxygen groups [15,16,21e26]. Therefore, fabrication of hybrid supercapacitors using graphene/metal oxide composites is considered as an efficient strategy to improve pseudocapacitance properties of these devises [14,15,19,27]. According to electrical conductivity of graphene, it can acts as a conductive template to enhance electrical properties of the metal oxides composites and provides a good electrical contact to the current collector [15,27,28]. Graphene diminishes the volume of expansion/contraction and aggregation of nanoparticles during charge/discharge cycles and improves life time of the electrodes. In addition, graphene is a good substrate for growing and anchoring the metal oxides with a well-defined size and shapes [23,29,30]. Oxygen functional groups on the graphene surface act as nucleation sites and provide desirable bonding, interfacial interactions and electrical contacts between the substrate and nanoparticles [29,31]. On the other hand, anchoring the nanoparticles on graphene acts as spacer between graphene sheets and reduces restacking of sheets [14,32], which helps better the diffusion process of electrolyte ion between the sheets to occur more simple [15]. Different composites of graphene/ metal oxide have been synthesized as supercapacitor electrodes: Graphene/MnO2 [26,27], graphene/NiO [23], graphene/ Fe2O3 [33], graphene/ZnO [34] and graphene/Co3O4 [35]. All of these studies have shown improvement of cyclability and electrochemical properties in comparison with metal oxides [32,36]. Different synthesis methods were applied to preparation of graphene/Co3O4 as supercapacitor electrodes including microwave (specific capacitance of 243.2 F g1 at 10 mV s1) [35], surfactant assistance (163.8 F g1 at 1 A g1) [37] and hydrothermal process (337.8 F g1 at 0.2 A g1) [38]. The present work studies nucleation and growth process of graphene/Co3O4 composite in hydrothermal condition in order to improve charge storage capacitance of the composite. For this reason, the effect of water volume ratios (30%, 50%, 70% and 100%) in the mixture with DMF is examined, which is an effective hydrolyzation parameter on the nucleation and growth processes. A model for nanoparticle growth on graphene sheets is obtained by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The obtained electrochemical measurements are represent that controlling the water volume ratio in the synthesis process can lead to composite with high specific capacitance. The
water volume ratio in the solvent mixture for hydrothermal process will reach but the sample with 50% water displays a better electrochemical activity to an optimum amount, which offers the highest capacitance to the resulted composite. The results of our work showed that the simplest way to control the growth condition is focusing on the solvent optimization. By this mean, composite based electrodes for supercapacitors with high performance can be achieved.
Experimental Synthesis of graphene/cobalt oxide composite All chemicals were done by synthetic grade materials and used as received without further purification. Graphene oxide (GO) was synthesized by the modified Hummers method [39]. GO (11 mg) with a concentration of 0.5 mg mL1 was dispersed in a mixture of DMF and water with different volume ratios. A portion of 0.5 mmol Co(OAc)2.4H2O was dissolved in deionized (DI) water and the obtained cobalt solution slowly was added to GO solution under magnetic stirring. The solution was stirred for 1 h and then ammonia solution (5%) was added until pH of the solution reached to 9.5e10. The resultant solution was stirred for a 30 min and then was transferred to a teflon lined stainless steel autoclave and was subjected to heat at 130 C for 14 h. After cooling the autoclave to room temperature, black precipitate was separated by centrifugation and then was washed several times with DI water. Volume ratios of water in the solvent were chosen 100%, 70%, 50% and 30% so the obtained composites are named as GC-100%, GC-70%, GC-50% and GC-30% respectively. After that, the precipitate was dried by freeze-dryer and then was annealed at 300 C (with a rate of 2 C min1) for 4 h. The Co3O4 was synthesized by the mentioned method (with a water volume ratio of 50%) in the absence of GO.
Characterizations The X-ray diffraction (XRD) patterns were recorded using a STOE (STADI P) instrument operating with CuKa radiation ˚ ) at 40kV/30 mA. A SIGMA VP Field Emission (l ¼ 1.54178 A Scanning Electron Microscope (FESEM) and a FEI (Tecnai G2 F30) transmission electron microscope (TEM) were used to study the morphology and structure of the samples. Thermogravimetric (TG) analysis was carried out using a Mettler Tolledo (TG/DSC1) instrument in a temperature range of 25e650 C (heating rate of 10 C min1 in air). The electrochemical capacitive behaviors of electrodes were measured on a Potontiostat/Galvanostat (AUTOLAB 302). The working electrode was fabricated by mixing 80 wt % RGO/Co3O4 powder, 10 wt % carbon additive and 10 wt % poly (vinylidenedifluoride in N-methyl-2-pyrrolidone, PVdF as binder) and then was cast on the nickel foam (1 cm2) and was dried at 70 C. Finally, each working electrode containing about 1 mg of electroactive material was tested in 2 M KOH solution using Ag/AgCl as reference electrode and a platinum rod as counter electrode. The lifetime of the electrodes were measured by the battery tester Kimia pardaz Samaneh (Kimia State 126-Iran).
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Results and discussion Graphene/Co3O4 composites were synthesized by a hydrothermal method which is a suitable approach to apply for uniform and crystalline particles of metal oxide growth on graphene sheets [31,40]. In the initial stage of synthesis, Co2þ ions in water and DMF solution were interacted with oxygen functional groups on the surface of graphene oxide (GO) surface. Altering water volume in the solvent can be used to control the hydrolyzation process. Based on recent studies, decreasing the volume of water causes better dispersion of graphene sheets in solvent [29,41] and decreases the hydrolysis rate of metal ions [25,40,42]. This low water content lead to a selective nucleation of cobalt hydroxide on graphene sheets rather than nucleation in the solution [29,31,40,42]. However in the presence of higher water content, nucleation can occur also in the solution as well as on graphene oxide. Morphology studies of products with TEM and SEM can help to understand nucleation and growth steps. Fig. 1 presents SEM images of GC-100%, GC-70%, GC-50% and GC-30% (here % represents the present of water content in the solvent mixture). In all images, sheet structures can be observed that can be related to graphene sheets (Raman spectroscopy of samples are shown in Fig. S1) and all of sheets were covered by cobalt oxide nanoparticles. Many granular-shape species are observed among the sheets as is shown in Fig. 1a and b (that related to GC-100% and GC-70% samples, respectively). The presence of these particles may refer to freely growth of cobalt oxide nanoparticles in the phase solution, independently from graphene sheets, and precipitation together with the coated graphene sheets. By reducing the water volume which results to decline cobalt precursor hydrolyzation rate, the number of large granular-shaped particles decreased (Fig. 1c and d).This result reveals that cobalt ions preferably select
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graphene sheets for nucleation, thus a selectivity growth feature occurs on the surface of graphene sheets”. A more detailed study was done by TEM technique (Fig. 2). It's clearly confirmed that Co3O4 nanoparticles are present in the composite structure as well as sheet structure of graphene. In TEM images of GC-100% sample (Fig. 2a and b), not only graphene sheet structures are shown, but also, Co3O4 nanoparticles in the size of 100 nm and 5 nm exist. The granular-shaped nanoparticles shown in Fig. 2a are the same particles among the flake-like structure in SEM images (Fig. 1a). Comparing TEM and SEM results concludes that large particles (100 nm) have not grown on the graphene surface despite of small particles (average size of 5.6 nm). When the volume content of water in the synthesis procedure was decreased (the sample of GC-50%), number of large granularshaped particles is reduced (Fig. 2c). In addition, the particles grown on graphene sheets are densely accumulated (Fig. 2d). The size of particles varies from 2 nm to 5 nm (average size of 3.9 nm). In Fig. 2e and f that are related to GC-30% sample, larger particles (100 nm) can't be seen. Furthermore, small Co3O4 particles about of 2e3 nm (average size of 3.3 nm) have grown more uniform and more densely accumulated on graphene sheets. In fact in this condition, oxygenated groups on the graphene sheets are the only candidates for nucleation of metal oxides nanoparticles. XRD data is applied to study the crystal phase of the synthesized nanocomposites (Fig. 3). The diffraction peaks at 19.0, 31.5, 36.6, 44.8, 59.6 and 65.3 can be indexed as the (111), (220), (311), (400), (511) and (440) crystalline planes of Co3O4 nanoparticles, respectively (JPDS No.43-1003). The diffraction peaks of the composites are similar to those of pure spinel Co3O4. The weak diffraction peaks indicate that the size of Co3O4 particles is relatively small [43]. In addition, it is clearly seen that by increasing the amount of DMF, the intensity peak gradually drops, which indicates whether that amorphous
Fig. 1 e SEM images of nanocomposites with different water content in mixture of solvent; (a) 100%, (b) 70%, (c) 50% and (d) 30%. Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Fig. 2 e TEM images of samples with different water content of solvent; (a,b) GC-100%, (c,d) GC-50% and (e,f) GC-30%.
phases are forming in the composites or the size of particles is decreasing. Considering SEM and TEM results, decrease of particle's size by water diminution can be confirmed. Beside the size of nanoparticles (2e5 nm), the poly-crystallinity of the samples (as also shown in the SAED pattern in HRTEM analysis for samples of GC-30%) can be stated as another reason of weak diffraction peaks. As mentioned above, SEM and TEM images presented large granulated particles in the samples GC-50% and GC-100% that were attributed to Co3O4. It seems that the amount of Co is not similar in all samples. This fact can be deduced from the pink color of the solution above the black precipitation in the
autoclave (Fig. S2), which is a sign for not reacted cobalt ions. To measure the amount of Co3O4 in each composite and compare the samples together, TGA technique was performed [43e45]. Fig. 4 shows the representative TGA of RGO-Co3O4 composites. RGO is thermally unstable and starts to lose mass upon heating above 350 C. It is clear that each sample shows different mass percentage losses in TGA result. GC Composites with 100% and 70% water show approximately 20 wt% loss during calcinations in air from 20 to 600 C. Decreasing volume of water in the synthesis process led to higher mass loss based on TGA result. Graphene composites of GC-50% and GC-30% have the mass loss of 36% and 44%, respectively. Since the
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Fig. 3 e XRD patterns of samples with different water content of solvent (a) GC-100%, (b) GC-70%, (c) GC-50%, (d) GC-30% and (e) XRD pattern of Co3O4 from literature.
initial amount of graphene oxide is equal in all of the samples at the beginningof synthesis and according to TGA results, the amount of Co3O4 in samples with 100%, 70%, 50% and 30% water can be estimated as 80, 80, 64 and 56%, respectively. Presence of lower amounts of cobalt oxide in GC-30% sample the pink color of the above solution in the autoclave indicates that there is some amount of cobalt salt which does not participate during the reaction. According to the results, a model for nucleation and growth of nanoparticles on graphene sheets is suggested in Scheme 1. When the volume of water is low, the rate of hydrolyzation is low and some of Co(OAC)2converts to Co2þ ions and interacts with oxygen groups of graphene oxide
Fig. 4 e Thermogravimetric analysis of different nanocomposites.
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sheets. After adding ammonia (NH3) to the solution, cobalt hydroxide forms both on the graphene surface and in the solution. In the hydrothermal process (under the high pressure and temperature), in addition to growth of nucleated particles, some of the cobalt precursors (green spheres) in low water volume condition hydrolyze and interact with oxygencontaining functional groups of the graphene oxide sheets. Therefore, smaller and more densely accumulated particles can be obtained. In high water volume condition, most of cobalt ions interact with graphene and the surface diffusion produces larger particles in the hydrothermal condition. According to the model, in GC-50% sample both large (~5 nm) and small (~2 nm) particles are seen, which means that the nucleation and the growth occur simultaneously in this condition. Cyclic voltammetry (CV) was used to study the electrochemical behavior of the prepared composites. The CV tests of the synthesized composites were measured in 2.0 M KOH aqueous electrolyte with the applied potential range of 0.0e0.5V at scan rate of 10 mV s1. As shown in Fig. 5a, it can be clearly observed in all of the samples that there is a distinct pair of redox peaks during the anodic and cathodic sweeps, with the presence of a broad redox background. It can be concluded that the prepared samples demonstrate pseudocapacitive properties that originate from the reversible and continuous electrochemical reactions of Co3O4 involved in the chargeedischarge processes [46,47]. As shown in Fig. 5a, GC samples with various water volumes from 30% to 100% show nearly same electrochemical behaviors but the sample with 50% water display a better electrochemical activity. The cyclic voltammetry of Co3O4 nanoparticles, (its SEM image is shown in Fig. S3), is compared with GC-50% (Fig. S4). A Shift in cathodic and anodic peak potentials as well as increasing in corresponding cathodic and anodic peak currents is observed. This phenomenon is related to the electrocatalytic role of graphene sheets in the composite samples. Also, we mixed pre-synthesized Co3O4 powder with reduced graphene oxide physically at a ratio of 0.64:0.36 (Fig. S5) and compared it with GC-50% water. The results show the effect of direct growth of Co3O4 nanoparticles on graphene sheets on the electrochemical properties of the prepared composite samples. Co3O4 nanoparticles, which were grown directly on the surface of graphene sheets, could have both covalent chemical bonding and van der Waals interactions with oxygen-containing defect sites and pristine regions of the graphene [25] respectively (Fig. S1). Less electrochemical activity of the mixture can be related to less cordial contact between the per-made Co3O4 and graphene sheets. Observation of a positive shift in the cathodic and anodic peak potentials confirms the role of graphene as catalyst for improvement of the electrochemical activity. According to these results, direct growth of Co3O4 on the graphene sheets is considered as an important parameter in improving the electrochemical behavior of the samples [31]. Galvanostatic methods are applied to calculate the specific capacitance of the prepared electrodes. The chargeedischarge properties of different samples at the current density of 5 A g1 are shown in Fig. 5b. The GC-30%, GC-70%, and GC-100% samples have the same chargeedischarge behavior that this similarity was also observed in the CV curves (in Fig. 5a).
Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Scheme 1 e Summary of the synthesis process of nucleation and growth of nanoparticles on graphene sheets in hydrothermal step.
Calculations of the specific capacitance at the current density of 5 A g1 for each sample are shown in Fig. 5c. Specific capacities of 331.8, 311.3, 440.4 and 320.7 F g1 are obtained for GC samples with 100%, 70%, 50% and 30% water, respectively. Rate capability of different samples is presented at Fig. 5d. In
different current densities, GC-50% has highest specific capacitance in comparison with other samples and proper rate capability. An important result for specific capacitance is achieved for GC containing 30% water. The TGA results for this sample previously approved that the amount of Co3O4 is lower
Fig. 5 e Electrochemical results of different graphene-Co3O4nanocomposites; (a) Cyclic voltammetry at 10 mV s¡1, (b) chargeedischarge at 5 A g¡1, (c) calculated capacitance of nanocomposites with different water contents in mixture of solvent 5 A g¡1, (d) dependence of specific capacitance of different nanocomposites on current density. Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Co3O4 on graphene sheets can be altered. By controlling nucleation and growth on the graphene sheets rather than in solution, the electrochemical properties can be improved. This improvement can be related to the presence of graphene as a conductive support. Based on the electrochemical results, an optimum water ratio of 50% is selected for preparation of the more efficient electrochemical capacitors. This condition doesn't mean that the growth in solution has been eliminated; however, it states a balance condition between the amount of cobalt hydroxide grown in solution and simultaneously on the graphene sheets.
Acknowledgment Fig. 6 e Cycle stability of different nanocomposites electrode at current density 3 A g¡1.
than the other samples, but this sample has the approximately same specific capacitance as GC samples with 100% or 70% water. It can be related to the synergic effect of graphene and Co3O4 nanoparticles and size of Co3O4 nanoparticles. The effect of the presence of graphene as a 2D support provides a good contact between OH ion of the electrolyte and cobalt oxide nanoparticles [31,32]. In addition, graphene can improve the electron transition from Co3O4 nanoparticles to Ni foam as the current collector. In fact, in GC with 30% water all of Co3O4 nanoparticles grow directly on graphene sheets and cause an electrochemical behavior better than other samples. Beside of above reason the size of nanoparticles can be important. The size of nanoparticles in GC-30% is lower than GC-100%, with decreasing of particles size, surface area of them increase then the amount of charge transfer at interface of electrode and electrolyte increase. High cycle stability is an important factor for practical applications of supercapacitor electrodes. To evaluate the stability during the chargeedischarge cycle, the values of specific capacitances with respect to chargeedischarge cycle number (up to 2000 cycles) at 3 A g1 were illustrated in Fig. 6. The maintenance of the initial capacitance over 2000 cycles for GC-100%, GC-70%, GC-50% and GC-30% are 97%, 93%, 94% and 95%, respectively. The results demonstrate an excellent electrochemical stability of these composites as electrode materials for supercapacitors preparation. By controlling the nucleation and growth steps of metal oxide nanoparticles on graphene, we can obtain high capacitance in comparison with other works that used hydrothermal methods and graphene oxide as initial material in synthesis process (291 F g1 at 1A g1 [43] or 472 F g1at 2 mV s1 [48]).
Conclusion In this work we optimized synthesis conditions of Co3O4/ graphene composite by controlling hydrolyzation solvent strength with alteration of water/DMF volume ratio as the solvent mixture. With different ratios of water in the synthesis process, amount, dispersion and crystallite size of
Authors would like to thank Miss Dr. Somayeh Fardindoost and Prof. Hilmi Volkan Demir and Devices and Sensors Research Group led by him for helping in the TEM analysis performed at UNAM (Institute of Material Science and Nanotechnology) Bilkent University.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.10.061.
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Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061
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Please cite this article in press as: Jokar E, et al., Growth control of cobalt oxide nanoparticles on reduced graphene oxide for enhancement of electrochemical capacitance, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.10.061