2018/19 Research Paper 

Abstract

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Supercapacitor cost and production variables should be looked upon equal as to the performance of the supercapacitor. In this report, the fabrication of symmetric supercapacitor using activated carbon and deep eutectic solvent solution infused graphite felt as the positive and negative electrode is built, which allows for a single-step fabrication that requires less than 5 minutes. Fiber-made electrode allows for flexibility without material damage or performance loss, which makes a more excellent range of applications in supercapacitors. A single supercapacitor with a 6 cm^2 area shows a voltage range between 1.8~2.1 V and delivers a capacity of 100 Farads or 17 mAh and an energy density of 13 mWh in battery terms. The supercapacitor also shows excellent voltage stability when the device is bent, which shows that 80% of the voltage remained at 40 degrees bent without any protecting case. The cost of the supercapacitor lower than an average commercial supercapacitor, which was shown to be 50 cents per cube centimeter. 

 

Introduction

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As devices increase in needs for alternative energy sources other than batteries, supercapacitors have been in development since its beginning of discovery. Thanks to wonder-materials such as carbon nanotube and graphene, the researches have further advanced the technologies behind making supercapacitors with high energy density. Some of the research of supercapacitors with the most advanced materials and production methods had been built to surpass the capacity of both lead-acid and lithium-ion batteries, which means it could replace the slow charging process that batteries suffer. [1] However, most of the researched supercapacitors cannot be produced for commercial use because of the production methodology and the cost. This is because materials such as graphene cost exceedingly high for consumer products and the assembling method is tedious and difficult. [1] [2] This means that the supercapacitor must not only be superior in its capacitance but also the production method and its cost must be on the low side. The logical approach to the research was simple. First, the physical and chemical characteristics of each component of the supercapacitors were found. A basic supercapacitor consists of two electrodes, two carbon materials, electrolytes, and a separator. [3] [4] Each component is required to achieve a specific characteristic in order for the supercapacitor to achieve high capacitance. An example would be carbon-material requiring a large surface area, which allows more ion storage and electrode requiring to be conductive. After the characteristics were found, the materials were then researched. The cost was a significant factor during material research because the objective was to build a cheap supercapacitor. 

 

Method

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Exploring the suitable electrode came early due to the low capacitance the previous electrodes such as graphite foil and copper foil. The biggest challenge came from this phase, since active material, which determines the ion charge storage, was not part of the research. This meant, the only method of increasing the ion charge area is to increase the physical surface area of the electrode. Porous 3D electrodes have been increasingly used for making batteries and fuel cells due to the characteristics of porosity and conductivity. [5] Porosity was an important factor in choosing the electrode because it increases the surface area of the electrode. A material called graphite felt was found. This material is manufactured by the carbonization and graphitization of carbon felt. [6] It is not a novel material related to electrochemistry, since it has been used for the electrode in energy storage devices like battery and fuel cell. However, the conductivity is relatively low compared to metal 3-D electrode, and research continues to increase the conductivity. The reason why graphite felt was used instead of other 3-D electrode materials was due to its porosity and its cost. It has been evaluated that graphite felt electrodes has a typical surface area that is 800 times greater than those of pyrolytic carbon. [6] The cost was relatively low compared to other materials costing $9.45 per 600 mm^2. The principal precursor used to manufacture the GF is polyacrylonitrile (PAN), which is a form of acrylic fiber and this type of precursor is stronger than other type of carbon fiber. The carbonization occurs at 700-1300 °C followed by graphitization at 1600-1800 °C. Overall, the use of graphite felt as the positive and negative electrode showed good conductivity, flexibility, and porosity. A simple demonstration was done with the material comparison between graphite foil, a metal electrode and graphite felt by building a simple capacitor. It showed that the graphite felt showed about 2.5 times the capacity rate compares to the metal electrodes

 

Experiments

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Electrolyte development is the most important factor in the research because the voltage window is directly related to the theoretical energy density, which is E = 1/2 CV^2. [7] Organic electrolytes and ionic liquids were found and researched due to its high voltage window, however, due to its toxicity and its high cost, alternative electrolytes were focused. [7] Deep eutectic solvent (DES) was found to be the alternative electrolyte. The eutectic mixture is a unique composition of two phase-immiscible solid components that undergoes a complete change of phase to liquid at a precise temperature. [8] Deep eutectic solvent can be considered as organic salt in the liquid state. During the research, cheap biocompatible deep eutectic solvents were developed using choline chloride and propylene glycol, which could be produced under 2 dollars per 100 milliliters. As the DES electrolyte was applied to the supercapacitor, it showed a starting voltage of 2 Volts. Comparing to the previous electrolyte, sodium sulfate, which showed a starting voltage of 0.5 to 1 volt, the DES electrolyte gave an excellent starting voltage and increased the energy density four times.

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Herein, a flexible symmetric supercapacitor with excellent electrochemical performance was assembled using low-cost pan-based graphite and deep eutectic solvent. Due to the combination of the two materials, the production time decreased from 60 minutes to 5 minutes. However, due to its foam-like electrode, the supercapacitor has a larger height than average devices. [1] Moreover, it is a significant challenge to perfect the supercapacitor to have the same capacitance when it is mass-produced since most of the materials used during the research are not for scientific use. Despite the drawbacks, the supercapacitor can easily compete in the current market, thanks to its advantage of low-cost, flexibility, fast charging, and safe production method.

 

Painting Method

 

There are two steps in the electrode and carbon material assembly. First, the AC and DES solution is poured evenly through the graphite felt surface. (5 grams per 6 cm^3) The second step is embedding activated carbon on top of the wet surface for maximum surface area. With the new fabrication method, which was possible from DES and GF, the building time reduced from 50 minutes (painting) to 5 minutes. Also, the novel method did not require a binder substance, which is used for an adhesive purpose, which reduces the conductivity for traditional supercapacitors. With the novel process, the electrode became more conductive, and the surface area increased.