Thought

Abstract

 

The electric vehicle (EV) is one of the most promising innovations of modern times, offering several benefits over gasoline vehicles, including low maintenance and lack of emission. Nonetheless, many drivers favor the use of gas-powered vehicles due to their long travel range. Regenerative braking has been shown to improve the range of EVs recovering 60-70% of kinetic energy back to storage. However, the high-power transient charging that occurs with regenerative braking is problematic for lithium-ion batteries, as the high current flow puts the batteries at risk of damaging. Using supercapacitors as assisting device to store energy from braking has been shown to increase efficiency (up to 88%) due to their electrochemical properties of low internal resistance. Not only that, but the supercapacitor module can also be used during the acceleration of the electric vehicle, where the current peaks are the highest and the overheating of the battery is most common. By assisting in regenerative braking and vehicle acceleration, supercapacitors may be the component missing in modern electric vehicles.

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Supercapacitors are energy storage element with high energy-density compare to capacitors and high-power density compare to batteries. Its research has been going on for decades, due to supercapacitor’s superiority over batteries in low internal resistance, allowing the device to charge and discharge at a higher current value. Because the improvement of the capacitance and the operational voltage is so dependent on the material or the chemistry of supercapacitors, those fields are where most researches are happening. However, when asked about the applications to supercapacitors, the answers are not always confidential. Supercapacitors cannot be used as an extensive power device, because of its low energy-density compare to conventional batteries and most of applications where a quick power is needed can be done using capacitors, which are much more affordable.

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Regenerative braking recovers partial kinetic energy to storable energy, which increases fuel efficiency by up to 26%. However, researches are finding that lithium-ion batteries are not the most ideal recovery method for this braking method since it damages the cell, due to the battery’s strict operational voltage limitation and limits of high charging current. Current demand for EVs calls for batteries with higher energy densities, such as lithium-sulfur and metal-air. These batteries provide up to 500 Wh/kg (Tesla Model 3 has estimated 168 Wh/kg) but have a lower power density (time rate of energy transfer) than current lithium-ion batteries. This is a problem because high power density is favored for vehicle acceleration and regenerative braking system. By having a supercapacitor module in the electric vehicle, it’s high power-density can support acceleration and regenerative braking, which would increase the lifetime of the battery pack.

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Other researchers found that supercapacitors also have benefits outside regenerative braking and acceleration assistance. Andrew Burke (2011) has done extensive research in supercapacitor-assisted electric vehicles in past years and found various benefits compared to a battery-only system. The vehicles were able to operate until the state of charge was 30% on highways; fuel economy was 50 to 100% higher; and most importantly, the average currents and the peak currents from the batteries were lowered by factors of 2-3.   

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I decided to test the supercapacitor combination in electric vehicle by fabricating a supercapacitor and testing them to a model vehicle that I have built. Most supercapacitor construction process requires expensive equipment, but I develop a method that would be accessible to most people. The supercapacitor is made with 2 electrodes, an electrolyte, and a separator. The electrolytes need high conductively and good chemical characteristic, as electrolyte determines the operating voltage. Usually, aqueous electrolytes have higher energy-density, and ionic electrolytes have higher power-density. Because of the purpose of this research is assisting acceleration, ionic electrolyte’s high power-density is preferred. The electrode needs to be conductive and have high surface areas, so electrolyte ions can transport with good access to the pores.

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Method

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Conductive ink will be used as the electrode agent. 35g of 5 wt.% graphene dispersion (XFNano) is sonicated in 20 kHz for 15 minutes. 30g of activated carbon (XFNano) is then added to the dispersion, and the mixture is moved to magnetic stirrer for 2 minutes at 300 RPM. Commercial graphite felt (Zibo Ouzheng Carbon Co.) is submerged in the conductive ink mixture until all the felt pores are saturated.  To minimize leaking, the electrode is held in the air for 5 minutes, giving drying time to the outer layer of the electrode. Next, the electrode is rested in the heatbed at 60°C for 20 minutes.

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The deep eutectic solvent electrolyte is made with choline chloride and ethylene glycol with the mole ratio of 1:2, respectively. The solution was heated on 80°C until they dissolved and formed a uniform solution. Fabrication of the electrode composite is completed after 10 ml of electrolyte is coated on both electrodes and rested for 3 hours for better distribution of electrolytes in the electrode. When the conductive ink is prepared in advanced, the entire electrode construction takes 3 minutes of physical fabrication by hands and 55 minutes of drying time. Then, two electrodes are sandwiched between a separator. Copper wire is attached to the outer layer of the electrode, and the supercapacitor is sealed with vacuum packing.

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The fabricated supercapacitor has a specific of about 14 Farads per gram, which is about four times better than commercially available supercapacitors (Maxwell). If you have read any supercapacitor research, you might be wondering why the specific capacitance is so low compared to other studies. Usually, if the values are over a 100, then the particular capacitance only measure the electrode (electrode capacitance for this research is around 170 Farads per gram). Please note that it is incredibly easy to outperform many commercial supercapacitors by making your own. The problem is scalability and storage. I store my supercapacitors in a vacuum seal, which lasts longer than having them exposed in air.

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I built the model vehicle with metal sheets and Lego parts, which resembles a typical electric car chassis (fig. 2). The car will only test the acceleration assisting, as regenerative braking shifts the focus too much to engineering aspects. The battery pack and the supercapacitor modules were connected in series to two DC motors (775 DC Motor 12V). The vehicle is controlled using a simple Arduino configuration with a switch. Based on the connections, when I turn on the switch, I can run the battery, supercapacitor, or both. The four supercapacitors supporting the vehicle acceleration provided 70 Farad at 4.2 Volts, for both motors (35 Farad for each side) and the total weight of the supercapacitors was at 35 grams. When tested, the supercapacitor gave power for about 15 seconds of acceleration starting from 4.2 Volts to 0 Volts linearly with time. By assisting the vehicle acceleration, the acceleration time and the peak current from the battery was lowered compare to a battery-only configuration.

There is and will be various studies on supercapacitor integrated vehicles. It is found that in all case, including my test, the performance using the supercapacitor is generally more efficient than those using batteries. There is an obvious drawback from using supercapacitor, which is the added cost and weight. The question is whether the benefit of using supercapacitor in regenerative braking and acceleration outweigh the inconvenience of adding weight and price, which are the two main reason why there are currently no electric vehicles in the market that embraces supercapacitors.

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Reference

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[1] M. Halper, J. Ellenbogen, Supercapacitors: A Brief Overview. MITRES (2006)

[2] G. Xu, An Intelligent Regenerative Braking Strategy for Electric Vehicles. Energies, vol. 4, no. 9, pp. 1461–1477 (2011).

[3] A. Burke, H. Zhao, Applications of Supercapacitors in Electric and Hybrid Vehicles. UCDAVIS ITS (2015)

[4] A. Burke & M. Miller, The power capability of ultracapacitors and lithium batteries for electric and hybrid vehicle applications. Journal of Power Sources, 196(1), 514–522. (2011)

[5] Zhong, Cheng, Yida, Deng, Hu, Wenbin, Qiao, Jinli, Zhang, Lei, Zhang, Jiujun, A review of electrolyte materials and compositions for electrochemical supercapacitors. Chemical Society reviews. 44. 7431-7920 (2015)

[6] ZS. Iro, A Bried Review on Electrode Materials for Supercapacitor. Electro Chem Sci (2016)