Graphene-based Electrochemical Supercapacitors Using Different Mass Ratios of FCB Spacer

Kailiang Chen, Bingcheng ZhouDepartment of Mechanical Engineering, Purdue School of Engineering and Technology

1. AbstractA test, which is based on cyclic voltammetry method[1], has been developed to find out the best mass ratio between graphene and functionalized carbon black (FCB) spacer for supercapacitor application. As one of the promising future materials, graphene has a very high theoretical specific surface area (2630m2/g) which is ideal material for supercapacitors that require porous structure[2]. However, the pure graphene sheets restack to each other and significantly reduce the specific surface area and lead to a bad performance of graphene as electrode material. To solve this problem, FCB spacers can be added to graphene sheets to form a sandwiched structure between the stacks [3]. And the best mass ratio between graphene and FCB Spacer to form the composite material turns out to be 1:1.

Mentors: Prof. Jian Xie, Department of Mechanical Engineering, Purdue School of Engineering and Technology, IUPUICo-mentors: Dr. Zhe-Fe Li, Postdoctoral Research Associate, Department of Mechanical Engineering, School of Engineering and Technology and Prof. Lei Li, Assistant Professor, Department of Chemistry, School of Science.

2. Introduction

One of the biggest and most urgent problems in our society today is the storage and efficient usage of energy. For over a century, fossil fuels are dominant energy consumption types, and dominant energy storage types for their relatively easy storage, high power density and cheap price. However, it also has fatal disadvantages as non-renewable, high environmental cost, and low efficiency. On the contrary, energy storage in electrochemical method has its own pros and cons. As a clean, renewable and efficient energy source, electrochemical storage such as battery and capacitor have different characteristic. Battery has relatively high energy density, stable discharging voltage, but a low charging and discharging rate, and a bad tolerance for charging and discharging cycles. Capacitors, which are widely used in controlling circuit, have good tolerance, high rate, but relatively low energy density, and unstable voltage. Their characteristics highly restrict their ability in replacing the fossil fuels as a clean energy source.Supercapacitors, which is high-capacity capacitors, get both the advantages of battery and capacitor for its high power density and high voltage[4]. On the other hand, graphene, which is a two-dimentional carbon based nanoparticle, has a large specific surface area (2630m2/g) and is rife with porous structures, that are perfect for adsorption of electric charges. Although graphene is a very promising material in electrochemical industry, other problems arouse. Pure graphene tends to restack to each other and significantly reduce the specific surface area and lead to a bad performance[3]. Consequently, researchers start to use spacers to isolate and separate the stacks between graphene sheets. As one of the spacer, functionalized carbon black (FCB) can insert to the graphene sheets to form a composite material as a sandwich. FCB has excellent electrical conductibility, a similar structure as graphene for their carbon based material characteristic, while its nanoscale solid can fit in the porous structure of graphene. So it makes FCB a good spacer in theory.In this project, graphene/FCB nanocomposites with different ratios were prepared and studied their application as supercapacitor. Since it’s very troublesome and high cost to confirm the structure of the composite material, a chemical test has been set up to figure out the best mass ratio for the graphene and FCB to form the electrode material.

3. Theory and Experimental Method

3.1 Preparation of FCB2 g of EC600 carbon black was treated with 100 mL 0.5M HCl solution.Add 3.5 g sulfanilic acid to the solution.Stir the dispersion for 30mins and then add 3.6g sodium nitrite.The resulting dispersion was stirred for 4 hours and then heated up to 70℃ for another 3 hours.The final dispersion was filtrated using suction filtration method with DI water.

3.2 Preparation of Graphene-FCB composite materialOne of the most time consuming experiment in this test is the preparation of graphene. The preparation of graphene include the process of pre-oxidation of graphite, turning from pre-oxidized graphite to graphene oxide, and the reduction of GO with FCB, as well as the final desiccation process.

(Materials list: Graphite flake, sulfuric acid, NaNO3, KMnO4, FCB, H2O)

3.2.1Pre-oxidation of graphitePrior to the Hummer's method, an additional graphite oxidation procedure was carried out.10 g of graphite flakes were mixed with 50 mL concentrated H2SO4, 10 g K2S2O8, and

10 g P2O5. The resulting mixture heated at 80 ºC and slowly cooled down for 4 h under constant

stirring. The mixture was then filtered and washed with DI water. The product was dried in an

oven at 80 ºC.

3.2.2Synthesis of graphene oxide by modified Hammer’s method[6].2g of pre-oxidized graphite and 1g of sodium nitrate was added into 46 mL of concentrated sulfuric acid. After stirring for 30 min, 6 g of KMnO4 was slowly added into the above suspension in an ice bath. Then the ice bath was removed and the suspension was maintained at 35 ℃ for another 30 mins. 92 mL of water was dropwise added into the suspension, and the temperature was increased to around 95-100 ℃. The suspension was kept stirring for 15 mins.Then the suspension was diluted by 280 mL of water and treated with 10 mL H2O2. The above suspension was washed a few times by centrifuge at 10000 rpm for 30 mins. Use a ultrasonic bath to treat the solid for 1 hour to get the pure graphene oxide.

3.2.3Reduction of graphene oxide with different mass ratio of FCB[7](Procedure example of mass ratio between graphene and FCB equals to 1:1)Weigh 100 mg FCB in the round flask, mixed it with DI water, stir it for a while.Weigh 100 mg GO and mix it in the round flask with FCB, add DI water into a 100ml turbid liquid.Stir the solution using an ultrasonic bath for 1 hour.An amount of 0.5 μL hydrazine solution was added into the mixture.The mixture was stirred and heated treated with an oil field for 8 hours at about 90℃.The dispersion solution is ready for desiccation process.

Fig 1. (a)Demonstration of the experiment process, functionlized carbon black(FC in graph), graphene oxides(GO) to process into Graphene-FCB Composite; (b)Cryo-TEM image of the material before reduction. Green arrows point to the FC particles and yellow arrow to the graphene edge; (c)Cryo-TEM image of the material after reduction. Blue and red arrows point to sandwiched graphene layers, and green arrow point to FCB particles[3].

3.2.4Desiccation of graphene dispersion solution with other soluble impuritiesPreparing for a set of suction filtration equipment, using a filter paper with bore diameter of 400 nm.Wetting the filter paper with DI water, and turn on the air exhaust machine.Filtrate the dispersion solution, making sure the composite grains will not dry out.After the filtration of dispersion solution, add DI water for several times.After the filtration of DI water, collect the solid and put it in the freeze-dying for 24

hours.Heat-dying it for 1 hour using a vacuum at 80℃.The resulting solid becomes the composite material and ready for testing.

3.3 Cyclic Voltammetry(CV) MethodIn the cyclic voltammetry method, a series of time based controlled voltage with a specific rate are applied to the tested object, and the corresponding current quantities are measured. By the data collected, one could build up graphs in current vs. time, voltage vs. time, and current vs. voltage. The voltage is the driving force of different kinds of oxidation and reduction reaction, and the corresponding current value and graph shape can be used to determine the reversibility, performance of electrode material, adsorption rate, storing ability of material, etc[1]. In the experiment, all the electrode materials were tested in the aqueous 1 mol/L Na2 SO4 electrolyte.

We can use the CV method for performance test by calculation of average capacitance of the tested object[5]:

C=QV

=∫dq

V=∫ Idt

V= 1

V×∫ I

μ× dV= 1

Vμ∫ IdV

where C=capacitance, Q=total charges, V=voltage, I=current, μ=scan rate

Cm=1

2m∆ Vμ ∫V initial

V final

|I|dV

where Cm=specific capacitance, m=mass of one electrode, ∆ V =V final−V initial, μ=scan rate, V=voltage, I=current

4. Results and Conclusion

4.1 Graphene in comparison to activated carbon as an electrode materialOne of the characteristic properties of graphene is its relatively large surface area. This character and its high conductivity are our initiatives to study its application in supercapacitors. In this set of experiment, commercially available activated carbon, which has a loose structure, was used as the reference for the large surface area material. The electrochemical capacitance was collected by measuring the cells, which used nearly identical materials on both electrodes. All the cells used Ni foil as the current collector.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-0.2

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AC-cell-5mVs

G-cell-5mVs

Voltage(V)

Curr

ent(

A/g

)

a

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80Graphene

Activated carbon

Scan rate(mV/s)

F/g

b

Fig 3. Graphene and activated carbon performance in coin cell (Cyclic voltammograms of two samples obtained in 1 mol/L Na2 SO4at scan rate of 5mV/s (a) and overall specific

capacitance at different scan rates (b)). From the Fig 3, it can be observed at scan rate of 2 mV/s, the graphene have a higher specific capacitance (69.59 F/g) than the activated carbon (48.98 F/g). And at each scan rate, the graphene has better performance. This phenomenon may be explained from two reasons. On one hand, the electron in the activated carbon still need to overcome the gap between different band energy, thus its conductivity is smaller. On the other hand, a large part of the surface of the activated carbon may not be effectively used. The blocks inside its structure can prevent some of its area get in touch with the electrolyte. Thus, the graphene has the high potential in the cell industry as an electrode.

4.2 The ideal value for the graphene performanceTo find out the optimum performance of graphene, a RDE test is conducted in a three-electrode system in 1 M Na2SO4 using Ag/AgCl electrode as a reference electrode and platinum wire as the counter electrode. In the test, a thin graphene layer (20 μg) was deposited on the working electrode by drop casting method. Since the material is mostly presented in a uniform thin layer, the effect of the particle agglomeration can be excluded to some extent.

0 5 10 15 20 25 30 350

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Graphene RDEGraphene cellAC-RDE

Title

Tit

le

Fig 4 Specific capacitance at different scan rates for graphene and activated carbon in RDE device and graphene cell.

Fig 4 shows that the results from the RDE test are always larger than the values in the actual coin cells. This points out that in real world practice, several factors, like the quality of the ion exchange membrane, contact between the current collector and the electrode, as well as the electrolyte transport, can largely reduce the specific capacitance that one material can perform.

4.3 The best proportionIt has been discovered that pure graphene has many disadvantages that limits its further application. Semiconductor transistors have a band gap: a transition point that allows an

asymmetric flow of charge through circuits. Pure graphene lacks a band gap[8], which needs to be addressed. Its susceptibility to oxidative environments also influences the usage in high voltage condition. Finally, the graphene works poorly in the aqueous environment. One of the solutions is to functionalize it. After adding some functional groups on its surface, the material is expected to have a higher performance. In this experiment, the samples contain graphene nanosheets mixed with different amounts of functionalized carbon. The sample is symbolled as in the pattern of “G/F=weight ratio”, in which “G” stands for graphene, “F” stands for functionalized carbon. All the following results in this section are collected from RDE test, which use Ag/AgCl electrode as a reference electrode. This is environmentally friendly way to judge the property of a single material because each cycle only cost a tiny amount of the ink to discover the ideal value.

4.4 Result Tables and GraphSpecific capacitances (Cm) obtained in 1 mol /L Na2 SO4 from CV method and capacitance retentions from 5mV/s to 30 mV/s for the samples.Sample name Cm(F /g) at different scan rate Capacitance

retention (%)2mV/s 5mV/s 10mV/s 20mV/s 30mV/s

G 97.75 88.34 82.66 77.79 76.85 78.62%G/F=5/1 115.34 107.09 103.21 96.15 93.47 66.47%G/F=3/1 145.94 127.55 118.77 115.38 110.54 75.74%G/F=2/1 166.37 138.27 130.12 124.32 118.98 71.52%G/F=1/1 178.48 158.37 149.09 142.84 134.33 75.26%G/F=1/2 70.46 64.21 61.14 61.91 56.95 80.83%F 61.46 55.54 52.54 50.80 49.34 80.26%

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6-0.5

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0

0.1

0.2

0.3

0.4

G G/F=5/1

G/F=1/2 F

Voltage(V)

Curr

ent(

A/g

)

Fig. 5 Graphene composite performance in coin cell (Cyclic voltammograms of all samples obtained in 1 mol/L Na2 SO4 at scan rate of 5 mV/s (1)

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6-1

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Voltage(V)

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ent(

A/g

)

Fig. 6 Graphene composite performance in coin cell (Cyclic voltammograms of all samples obtained in 1 mol/L Na2 SO4 at scan rate of 5 mV/s (2)

In general, it is shown that when graphene and FCB were mixed by half and half, the device showed the best outcome at each scan rate. The following CV diagrams give visualized support to the results. From the diagrams, the sample G/F=1/1 has the largest area in one

cycle. This give the largest value of ∫V ¿itial

V final

|i|dV , thus provide the largest Cm.

Furthermore, the trend with different mass ratio in this composite material are more obvious when taking out the data at a scan rate of 2 mV/s, the curve’s peak can be easily pointed out to 50% proportion of the graphene in the electrode material.

0.00% 10.00% 20.00% 30.00% 40.00% 50.00% 60.00% 70.00% 80.00% 90.00% 100.00%

0

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graphene porportion

F/g

Fig.7 Cm of samples at 2mV/s

Thus, the addition of the factionalized groups can efficiently improve the property of the graphene by 60 F/g at most. One possibility is that this can be largely owed to expansion of the surface. Even a single layer of the graphene is smooth at every point. It can fold and contains impurities. The FCB pull out the space between surfaces and allow more surface conduct electrons. But FCB itself has low conductivity compared to other electrical materials. So when the proportion of the FCB keeps getting higher, like the sample G/F=1/2, the FCB begins to conduct the most of the electrons instead of improving graphene’s performance, and it comes out that the Cm drops.

4. Using new composite in actual cells

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Graphene/FCB=1/1 Graphene/FCB=1:1 cell

Scan rate(mV/s)

F/g

Fig. 8 The comparison between the outcome of the G/F=1/1 composite from RDE device and coin cell

0 50 100 150 200 250-20

0

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140G/F=1/1

Z'(ohm)

Z''(

ohm

)

Fig. 9 Impedance plots measured from 1000 kHz to 0.1 Hz of the sample G/F=1/1 in 1 mol /L Na2 SO4

At low scan rates, the cell works in an ideal condition. But when the scan rate gets higher, the cell performance drops significantly. It is mainly due to the large internal resistance of the cell. The radius of our semicircle is about 1.5 ohm, which may cause some energy loss when charge and discharge in cycles.

6 7 8 9 100

0.5

1

1.5

Z'(ohm)

Z''(

ohm

)

5. Conclusion After a series of experiments, we have got to some results. Although both pure graphene

and activated carbon have a high specific surface area, the performance of pure graphene is about 20F/g (40%) better than the activated carbon in coin cell. Moreover, it is concluded that the RDE test will provide a better value than the real application in coin cells just as we expect. Finally, adding certain amount of FCB into the graphene will improve its performance. The improvement of the property can be observed in every sample from G/F=5/1(115.34F/g at scan rate 2mV/s) to G/F=1/1(178.48F/g at scan rate 2mV/s), and the best mass ratio between graphene and FCB is 1:1 according to our tests. All in all, graphene-spacer composite material groups (in our case: FCB) perform a lot better than activated carbon.

Reference

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