TEAM PROJECT REPORT
Project 2: Synthesis and Characterization of Graphene for Energy Storage Devices
Submitted To
The RET Site
For
“Challenge-Based Learning and Engineering Design Process Enhanced Research Experiences for Middle and High School In-Service Teachers”
Sponsored By
The National Science Foundation
Grant ID No.: EEC-1404766
College of Engineering and Applied Science
University Of Cincinnati, Cincinnati, Ohio
Prepared By
Participant # 1: John J. D’Alessandro, Physics, 11-12th Grade, St. Xavier High School, Cincinnati, OH
Participant # 2: Michael P. Day, Sr., Mathematics/Engineering, 9th-12th Grade, Reading High School, Reading, OH
Approved By
(Signature of Faculty Mentor)
Dr. Noe Alvarez
Departed Name of Faculty Mentor
College of Engineering and Applied Science
University of Cincinnati
Reporting Period: June 13 – July 28, 2016
Abstract
The group synthesized three-dimensional graphene foam (3DGF), in the form of sheets called three-dimensional graphene paper (3DGP) using chemical vapor deposition (CVD), milled it using a laser milling system to create interdigitated patterns (IP) with digits of thickness 25 µm and interdigital spacing, d, also of width 25 µm, and coated the IP with electrolyte solutions to form flexible micro-supercapacitors (µSC). 3DGF has been shown to be a good material for electrodes. This research focused on increasing the energy-density of µSCs to that of or above high-quality electrolytic storage devices (batteries) while maintaining the power-density and reusability of more traditional capacitors. Past techniques on the manufacture and characterization of supercapacitors were applied to reduced-scale IPs to manufacture and characterize µSCs. µSCs have noticeably better capacitance and energy-density, with minimal added internal resistance, than more traditional capacitors. Capacitors are limited to capacitances on the order of 100 pF and specific energies of no more than 0.1 Wh/L. µSCscan exceed each of those by 100 times. This research should help enhance the performance of µSCsfurther. The group’s µSCswill lead to general use electrical devices that are smaller, charge faster, and are more flexible than using any conventional technology. The porosity and flexibility of 3DGP scales down well to 25 µm and by modifying the electrolyte used and doping the carbon will yield further advantages in power- and energy-densities. The group will present in written, video, and oral form to the Research Experience for Teachers (RET) 2016 cohort, assistance, mentors, and administrators as well as publish it in journals.
Key Words
Micro-supercapacitor, Graphene, Laser, Three-dimensional, Chemical Vapor Deposition
Main Body
1.INTRODUCTION
Cell phones, computers, tablets and other portable electronic devices are common place today. Society is truly dependent upon these devices working. And since plugging them in all the time is not an option, the need for energy storage devices that will have the necessary power and long cycle life are required
Batteries have decreased in size, but have too short a cycle life to adequately handle the demands. Micro-supercapacitors (µSCs) have shown to demonstrate much greatercycle life and competitive energy density to batteries. They also happen to have flexibility as a characteristic, which makes them even more useful.
Graphene has been introduced as the electrode material for the µSCs. Graphene is highly conductive and when manufacturing graphene, chemical vapor deposition (CVD) allows the graphene to form as crumpled sheets inthree dimensions.This allows for more surface area and so more energy storage.
When it comes to energy storage devices, the demands of society are incredible. Micro-supercapacitors made with 3D graphene should help society meet both the energy storage and the power usage demands in a package that can be bent to match the shape of most devices, even those that should be flexible.
2.LITERATURE REVIEW
Our society is awash in tiny electronics that can fit in a tight jean pocket and large, semi-autonomous devices like robots and cars that demand tremendous energy storage with increasingly urgency on reducing the time to charge the devices. One can find lists of specific devices throughout the literature, including papers by Chmiola, et. al., Niu, et. al., El-Kady and Kaner, Nery and Kubota, and others. Industry standard thin-film and polymer batteries serve the role of energy storage device for many of these devices, but they have short cycle life, are slow to charge, and have issues relating to overheating and starting fires (Long, et. al., 2004). There are further challenges integrating the batteries with the electronic circuits and miniaturizing the collective system (El-Kady and Kaner, 2013). In-plane µSCs show potential for replacing batteries in many instances as they may make integration and manufacture simpler as well as supply higher power densities due to the speed of capacitive charge transfer rates (Pech, et. al., 2010 and Beidaghi, et. al., 2012).
Electrodes for µSCs using electric dual-layer capacitance (EDLC) have been comprised of graphene oxide (GO) or reduced graphene oxide (rGO),as describedin papers by Niu, et. al. (2013), Y. Wang, et. al. (2013), L.L. Zhang, et. al.(2012), C. Zhang, et. al.(2013), and El-Kady, et. al.(2015), among others. Drawbacks to these methods include complex electrode structure and binders that decrease the power density (Luo, et. al., 2012 and Suboja, et. al., 2013). Using simpler, monolithic 3D electrode structures created using a binder-free process should simplify fabrication and integration of µSCs. Capacitance, C, obeys a relationship that is proportional to cross-sectional area, A, and inversely proportional to distance between electrode plates, d (the same as our interdigital distance).
(1)
So, having monolithic 3D electrodes made of graphene paper yields very large surface areas relative to the mass of carbon used and the surface area of the IP. At the same time, using the micro-milling device allows d on the order of 25 µm for capacitance and on the order of the size of the ions in the electrolyte for the EDLC. (Balakrishnan and Subramanian, 2014)
By using chemical vapor deposition (CVD), electrodes can be created from 3D graphene that have high electrical conductivity and are monolithic in structure (M. Zhang, et. al., 2014, Miller, et. al., 2010, and Beidaghi, et. al., 2014). Traditionally, these are made using non-flexible hard metals as catalysts or silicon as a substrate. There is an additional challenge in transferring the 3D graphene to a more suitable substrate for manufacture or flexible use. There are further challenges matching electrolyte to electrode polarity so that performance is not reduced.
In this work, nickel catalyst particles are bound in a polymer to prepare 3D graphene by CVD. This makes a monolithic form of carbon structure referred to as three-dimensional graphene paper (3DGP), as previously reported. This method should lead to scale-up methods for production, according to previous work by the group (L. Zhang, et. al., 2015 and L. Zhang, et. al., 2016). The obtained 3DGP has high porosity, complex surface features, and is easily transferable in processing to virtually any substrate due to its high Van der Waals adhesion. It shows excellent electrical conductivity and flexibility compared to GO and rGO components. Surface polarity of the material has been shown to be easily tuned using plasma treatment and introduction of oxygen functional groups on the out-layer surface of the graphene. This increases the electrochemical relativity to aqueous-based electrolyte but maintains the low-defect structure of the inner layer of graphene and its correlate fast electron transfer. This is a critical concern as the research will attempt to use water-based, organic, and possibly other ionic materials between IPs, and this will require appropriate physical behavior of the 3DGP to the substance. The use of the substance is to introduce EDLC, which greatly enhances performance of the supercapacitor. Some ionic liquid electrolytes can handle higher potential differences without undergoing phase-changing redox reactions. This is an advantage, as the energy stored in a capacitor, U, is proportional to the square of the electrical potential difference, V,applied, as well as proportional to the capacitance, C,of the device.
(2)
(Balakrishnan and Subramanian, 2014)
Even a simple doubling of electric potential difference allowed before breakdown of the electrolyte should then yield a four-fold increased energy density.
3.GOALS AND OBJECTIVES
The participants intended to perform in-lab research to create 3DGP µSCs, improve the performance of the devices, and develop units appropriate for their classes. To these ends, they expected to learn to synthesize, process, characterize graphene paper and micro-supercapacitors, write this report, disseminate the information to the RET collaborators, their classes, and peers through professional development.
4.RESEARCH STUDY DETAILS
4.1 3D Graphene Paper Synthesis
4.1.1 Catalyst Preparation
Nickel powder and a poly-vinyl alcohol (PVA) solution were mixed, resulting in a wet slurry solution. This solution was allowed to dry for 24 hours and became a black flexible film-like substance. This substance was cut into strips that became the catalyst for this project.
4.1.2 Graphene Synthesis
The catalyst was placed into a tube chemical vapor deposition (CVD) furnace and heated to 1000 0C with Argon, Hydrogen and Methane gasses introduced during the heating process. Argon was used to create an oxygen free environment in the furnace. Hydrogen was used to reduce any metal catalyst oxide. Methane was the hydrocarbon that decomposed into carbon and hydrogen atoms, in which the carbon atoms were absorbed by the catalyst and formed a hexagonal graphene structure.
Then hydrochloric acid was used to remove the nickel, leaving a three-dimensional graphene paper(3DGP). The 3DGP was washed in alcohol to remove the acid residue.
4.1.3 3DGP on Flexible Substrate Preparation
3DGP was free-standing and could be transferred onto any substrate. Kapton was chosen because it was flexible and non-conductive, and therefore was used in our experiment. The 3DGP was fished out of the alcohol solution with the Kapton film and then dried at room temperature for 12 hours. A good adhesion was created between the 3DGP and the Kapton during the drying process thanks to Van Der Waals forces.
4.2 3DGP Characterization
4.2.1 Raman Spectroscopy
For testing the 3DGP on Kapton film, called “graphton,” Raman Spectroscopy was used. This procedure will give data that, when graphed, will show if the graphton had multi-layer structure. Raman also gives the information of the structural defects found in the graphene hexagonal structure.
4.2.2 Scanning Electron Microscopy (SEM)
SEM was used to study the morphology of the 3DGP. It yields information about the structure and the porosity of the 3DGP.
4.3 3DGP Micro-supercapacitor (µSC) Manufacturing
4.3.1 Laser Etching
Two-dimensional designs for interdigitated patterns (IP) were created using SolidWorks software. Two different designs were settled upon: one with 50-µm width digits and channels for testing different electrolytes and another with 25-µm width digits and channels for scaling down the size of the supercapacitors. Oxford Lasers Micro Machining System was then used to etch the patterns onto GP and to cut the parts free from the remaining Kapton film substrate.
4.3.2 Quality Control
Once the raw IP part was cut from the main Kapton sheet, it was characterized for quality using an optical microscope for visual inspection and a digital multimeter to insure there was no short across the channels in the IP.
4.3.3 3DGP Micro-supercapacitor Assembly
To complete the assembly of the µSC, electrolyte was applied to the IP. Because two different electrolytes, poly-vinyl alcoholand sulfuric acid mixture (PVA/H2SO4) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4), were to be tested, two different sets of supercapacitors were manufactured. After the electrolyte-coated components had dried, copper tape was applied to the contact parts of the GP device that did not include the IP using silver paint, and then epoxy resin was used to bond the copper tape to the remaining Kapton substrate below the µSC. The copper tape was used as the primary contact region for macroscopic attachment to testing equipment. These steps were used to protect the actual µSC device from damage while undergoing final characterization.
4.4 3DGP Micro-supercapacitor Characterization
A potentiostat system by Gamry Instruments was used to run characterization on the completed µSCs with attached copper contacts.
Cyclic Voltammetry (CV) tests vary the potential difference across a component while measuring the current the component drew. This was done repeatedlyto the µSCs, using set potential difference intervals, with first charging then discharging cycles. Using CV graphs, the total areal charge and energy densities of the µSCs could be calculated based on the area inside the CV curves and the cross-sectional area of the µSC’s IP.
5.RESEARCH RESULTS
In order to study a micro-supercapacitor, they need to be manufactured. Nickel powder was used in our experiment as catalyst to synthesize graphene. To create a 3D catalyst template we use polymer as binders for Nickel powder (Figure 1a). Making the slurry with Nickel powder and a polymer is dangerous so masks had to be worn at all times. After the slurry dried, we transferred it into a CVD reactor to synthesize the graphene (Figure 1b). Heating the furnace to 1000°C was used to break the hydrocarbon into hydrogen and carbon atoms. The hydrogen atoms will form into hydrogen gas and escape into the atmosphere. The carbon atoms will be absorbed by the nickel catalyst and form graphene. After cooling, the graphene on nickel was transferred into an acid solution to remove the nickel (Figure 1c). Once the nickel was removed, the graphene paper was fished out with Kapton film (Figure 1d). The non-conductive Kapton film was used as a flexible substrate for the micro-supercapacitor.
Figure 1. (a) Nickel powder and polymer slurry; (b) CVD reactor for graphene synthesis; (c) Removing nickel using hydrochloride acid to get graphene; (d) Fishing graphene from water bath using Kapton film.
Figure 2. (a) Raman Spectroscopy of 3DGP sample; (b) SEM image of 3DGP sample; (c) IP laser-milled onto 3DGP sample.
Characterization of the graphene paper was an important step to understand the structure of the graphene before making into a micro-supercapacitor electrode. In our experiment, we used the Raman spectrum to study the quality of graphene paper. Figure 2a shows a typical Raman spectrum of graphene paper, in which the D peak (1300 cm-1) suggests defects in the structure and the ratio of the G peak (1580 cm-1) to the 2D peak (2700 cm-1) suggests the number of graphene layers. In our graphene paper we have a small D peak which means a high quality of graphene was created, that could potentially contribute to a high electrical conductivity. Based on the ratio of G to 2D, we have a crumpled, layered graphene structure in our graphene paper. We further studied the morphology of the graphene paper by using SEM. Figure 2b shows SEM image of our graphene paper suggesting a crumpled and porous graphene structure that could potentially contribute to a high specific surface area.
The two main areas of research that RET teachers did with this project were changing the formation of the interdigitation of the micro-supercapacitors (see Figure 2c) and the chemistry of the electrolyte that is applied to the interdigitation.
The first task for this project was to change the parameters to the final interdigitated pattern on the graphene paper. The graphene was cut interdigitally with the digits 25 µm in width, and the space between the digits also 25 µm. The parameters used were the percent power the laser would be cutting, the number of passes the laser would make and the cut speed of the laser. The pattern included the milling process to create the interdigitated pattern (IP) and the outside cutting process to cut through the substrate to free the graphene. (See Figure 3).
Figure 3. (a) Graphene paper on Kapton film before laser cutting; (b) Interdigitated pattern of graphene paper; (c) Applying PVA/H2SO4 electrolyte on graphene paper interdigitated pattern; (d) 3DGPµSC.
Table 1 shows the attempts that the RET teachers made so that the IP would have infinite resistance and could potentially contribute to a high capacitance for the µSC.
Table 1: Effect of Different Laser Parameters to the Final Interdigitated Pattern
Sample / InnerPower
(%) / # Passes / Cut Speed
(mm/s) / Outer
Power
(%) / Results
1 / 0.5 / 1 / 1 / 3 / IP was good, outer cut was not strong enough and outside digits were greatly affected.
2 / 0.4 / 2 / 1 / 5 / IP was good, outer cut was strong enough, but outside digits were greatly affected.
3 / 0.4 / 2 / 1 / 0.5, 5 / IP was good, outer cut was strong enough and outside digits were not greatly affected.
4 / 0.4 / 1 / 1 / 0.5, 5 / IP was not good, outer cut was strong enough and outside digits were not greatly affected.
5 / 1 / 1 / 1 / 5 / IP was not good, outer cut was good, and outside digits were greatly affected
Sample 3's process was the most successful in creating the pattern where the interdigitation was not touching each other, the outer digits were still intact, and the outer cut was through the substrate. While characterization using the Gamry demonstrated that energy density improved with 25 µm digit width and interdigital spacing over IPs designed at 50 µm, the improvement was below expectation.