Abstract: Supercapacitors have established a new design concept for transportation electric vehicles with their excellent performance, low cost, and zero emissions. This article summarizes the basic principles and characteristics of supercapacitors, and introduces the application and development of supercapacitors in pure electric vehicles and hybrid vehicles.

Keywords: supercapacitor; Pure electric vehicles; Hybrid electric vehicle; application

Nowadays, cars account for over 50% of urban pollutant emissions, and countries around the world are searching for alternative fuels for cars. Due to the increasingly severe shortage of oil, people are gradually realizing the importance of developing new types of cars, which aim to minimize exhaust emissions while using oil and other energy sources.

Supercapacitors have high power density, short charging and discharging time, good high current charging and discharging characteristics, long lifespan, and better low-temperature characteristics than batteries. These excellent properties make them have great application prospects in electric vehicles. Buses operating in urban areas have a route of less than 20 kilometers, and electric vehicles powered solely by supercapacitors can reach a range of over 20 kilometers on a single charge. They will have broad application prospects in urban buses.

Electric vehicles belong to the category of new energy vehicles, including three types: Battery Electric Vehicles (BEVs), Hybrid Electric Vehicles (HEVs), and Fuel Cell Electric Vehicles (FCEVs). It integrates the latest technologies from various disciplines such as optics, mechanics, electronics, and chemistry, and is an integrated product of the latest achievements in engineering technologies such as automotive, electric drive, power electronics, intelligent control, chemical power supply, computer, new energy, and new materials. There is not much difference in appearance between electric vehicles and traditional cars, the main difference between them is the power drive system.

Electric vehicles use battery packs as energy storage power sources to provide electrical energy to the motor drive system, drive the electric motor, and propel the wheels forward. Although electric vehicles do not have the same climbing slope and speed as traditional cars, they do not emit pollution during driving, have low thermal radiation, low noise, do not consume gasoline, have a simple structure, and are easy to use and maintain. They are a new type of transportation and are known as the "stars of tomorrow", favored by countries around the world.

Introduction to Supercapacitors

Supercapacitors, also known as electrochemical capacitors, are a new product that emerged in the late 1970s with a capacitance of up to Farad level. In terms of the electrode materials used, there are currently three main types: high specific surface area carbon material supercapacitors, metal oxide supercapacitors, and conductive polymer supercapacitors.

1.1 Basic principles

According to the different mechanisms of storing electrical energy, electrochemical capacitors can be divided into electric double layer capacitors (EDLCs) and pseudocapacitors (Pesudocapacitors). The mechanism of energy storage in carbon based supercapacitors mainly relies on the double layer formed near the carbon surface, which is commonly referred to as double layer capacitance; Metal oxides and conductive polymers mainly rely on pseudocapacitance generated by redox reactions.

The basic principle of double-layer capacitors is a new type of electronic component that uses the interface double layer formed between electrodes and electrolytes to store energy. When the electrode comes into contact with the electrolyte, stable and opposite sign charges appear at the solid-liquid interface due to Coulomb force, intermolecular force, or interatomic force, known as the interface double layer. The size of double-layer capacitance is related to the electrode potential and surface area. The electrodes of double-layer capacitors are usually composed of porous carbon materials with high specific surface area. Carbon materials have excellent thermal and electrical conductivity, low density, good chemical corrosion resistance, and low coefficient of thermal expansion. They can be produced in various forms such as powders, particles, blocks, fibers, fabrics, and felt through different methods.

Pseudo capacitance refers to the underpotential deposition of electroactive materials on the surface of electrodes or in the two-dimensional or quasi two-dimensional space of the bulk phase, resulting in highly reversible chemical adsorption/desorption or oxidation/reduction reactions, and the generation of capacitance related to the electrode charging potential. Due to the fact that pseudocapacitance not only occurs on the surface but can also penetrate deep into the interior, it can achieve higher capacitance and energy density than double-layer capacitors. Under the same electrode area, the pseudocapacitance can be 10-100 times the capacitance of the double layer. At present, the main materials for pseudocapacitive electrodes are some metal oxides and conductive polymers.

1.2 Differences from traditional capacitors and batteries [4-6]

The operating mechanisms of electrochemical capacitors and batteries are fundamentally different. For double-layer supercapacitors, charge storage is a non Faraday process, which ideally does not involve electron transfer through the electrode interface, and the storage of charge and energy is electrostatic. For batteries, essentially a Faraday process occurs, which involves electron migration through the bilayer, resulting in changes in oxidation states and chemical properties of electroactive materials. Overall, there are important differences in the charge storage process as follows:

(1) For the illegal Faraday process, the accumulation of charges is achieved through electrostatic means, with positive and negative charges located at two separate interfaces, with a vacuum or molecular insulator in between, such as double-layer, mica film, air layer or oxide film in electrolytic capacitors.

(2) For the Faraday process, the storage of charge is completed by electron transfer, and the electroactive material undergoes chemical or oxidation state changes, which comply with Faraday's law and are related to the electrode potential. Under certain circumstances, quasi capacitance can be generated. The storage of this energy is indirect.

Supercapacitors are located between batteries and traditional capacitors in terms of specific energy and specific power performance parameters, with much higher cycle life and charge discharge efficiency than batteries. Due to their long service life, supercapacitors usually exceed the lifespan of the equipment they are used for. Therefore, supercapacitors do not require maintenance for life, and after use, they have relaxed environmental requirements and are pollution-free. Therefore, they are also known as green energy.

1.3 Advantages of Supercapacitors for Vehicle Energy Storage Devices [7] (1) Supercapacitors are green energy sources that do not pollute the environment; Chemical batteries cause two instances of environmental pollution.

(2) Long cycle life (about 100000 cycles); The cycle life of chemical batteries is short (200~1000 times) and prone to damage.

(3) Fast charging speed (0.3s~15min); The charging time of chemical batteries is long, usually 3-10 hours.

(4) High charging and discharging efficiency (98%); The charging and discharging efficiency of chemical batteries is low (70%).

(5) High power density (1000~10000W/Kg); The power density of chemical batteries is low (300W/Kg).

(6) Supercapacitors are completely maintenance free, with a wide operating temperature range (-40~+70 ℃) and minimal capacity changes; Lead acid battery electric vehicles reduce their driving range by 90% at -40 ℃, while supercapacitors only reduce it by 10%.

(7) The supercapacitor electric bus has a high regenerative energy recovery efficiency for braking, with up to 70% recovered during conventional braking, while the energy recovery efficiency of the chemical battery system is only 5%.

(8) Relatively low cost. The price of supercapacitors is twice as high as lead-acid batteries, but due to their lifespan being 10-100 times longer than chemical batteries, the overall operating cost of supercapacitor electric vehicles is significantly lower than that of chemical batteries.

Application of 2 Supercapacitors in Electric Vehicles

Approximately 500 billion transportation vehicles are deployed on fixed routes through the public transportation system worldwide each year, with buses still being the most commonly used mode of transportation. The sales volume in 2000 was 183000 vehicles, and in the next five years, the annual sales will reach 220000 vehicles. The United States has 47800 vehicles. It is estimated that the number of public transportation vehicles will reach 650000 by 2010. If so many vehicles are not retrofitted and still use diesel or gasoline, the amount of fuel required will become a heavy burden, causing significant air pollution.

It is estimated that fuel cells will not be able to achieve large-scale production in the next decade. Leaving aside the expensive fuel cells, the projects of ethanol gasoline and natural gas vehicles that are already in use or about to be promoted in China cannot escape the problem of high costs. Due to the higher production cost of fuel ethanol compared to gasoline, relevant national departments are formulating subsidy plans to make the price of ethanol gasoline for vehicles equal to that of gasoline of the same grade;  

And supercapacitors precisely solve this problem. Supercapacitors have sufficient capacity, low cost, and no pollution to the environment. High power supercapacitors are of great significance for the starting, acceleration, and uphill driving of electric vehicles: they quickly provide high-power current during car starting and uphill climbing; Fast charging of the battery during normal driving of the car; Quickly storing the high current generated by the generator during car braking can reduce the limitations of electric vehicles on charging the battery with high current, greatly extending the service life of the battery and improving the practicality of electric vehicles. Given the importance of electrochemical supercapacitors, developed industrial countries have attached great importance to them and made them a key strategic research and development project for each country.

2.1 Application and Development in Pure Electric Vehicles

The impact of supercapacitors on the overall power performance of the vehicle mainly lies in their influence on the driving range. The performance parameters of supercapacitors, such as capacity, energy density, discharge depth, and power density, all affect the energy consumption and driving range of vehicles.

The Institute of Electromagnetic and Electronic Technology at Harbin Institute of Technology has developed an electric bus that uses supercapacitors as energy storage devices. This is an electric bus that can travel continuously for 25 kilometers with a top speed of 52 kilometers per hour in just 15 minutes of charging. It is reported that the three major projects undertaken by the institute, including the "Electric Vehicle Powered by Capacitors" project, have been appraised by the Provincial Department of Science and Technology. This research has reached the international advanced level in terms of the driving range and maximum speed of electric vehicles powered by capacitors. The development of this supercapacitor electric bus is the first of its kind in China, and its performance indicators have reached the advanced level of similar international products. This project has achieved breakthroughs and innovations in vehicle control technology, electric drive technology, and capacitor management balancing technology. It is understood that electric vehicles with low pollution and energy-saving have attracted considerable attention internationally. Among the components of electric vehicles, supercapacitors have become one of the important directions for the development of electric vehicles due to their long service life and strong safety. This electric bus powered by capacitors has good characteristics of no pollution, zero emissions, and low temperature, and is suitable for public transportation in northern cities. It has good market prospects and social benefits

Applying supercapacitors to electric buses has become a hot topic. Due to the fixed and unchanging bus route stops, the charging time of supercapacitors is very short, which can be completed within one minute. Therefore, it is possible to use the time when the bus enters the station to charge, which does not affect the passenger's travel time and does not require two "braids" on the roof like current trams. This also saves the cost of setting up tram tracks and looks more beautiful. One disadvantage of supercapacitors is their low energy density. They can only run 20-25km on a single charge, but they have a fast charging speed and can continue running after being fully charged. Compared with lead-acid batteries, this is much better. A lead-acid battery takes 5-8 hours to charge once, so as long as a supercapacitor electric bus charging station is built in a suitable place on the line, it is enough. The cost of investing in building such a charging station is much smaller than building a gas station, and it is also cheaper than building a similarly sized gas station or lead-acid battery charging station.

2.2 Application in Hybrid Vehicles

Although pure electric vehicles have the above advantages, due to the limitation of battery capacity, the vehicle's driving range, climbing and acceleration performance are not as good as typical cars. Although people have made various efforts in the research and development of batteries, it is still difficult to achieve the mileage of 400-500 kilometers that can be driven on a full tank like a typical sedan [9]. To fully satisfy users' desires, it is currently difficult to achieve solely based on the performance of existing power storage devices, hence the emergence of hybrid electric vehicles.

Hybrid vehicles are specifically designed and developed for urban public transportation, which can be powered by both electricity and oil. They are the most realistic industrial product for electric vehicles in the short term. Compared with traditional cars of the same type, this type of car can reduce exhaust emissions by 50% to 70% and fuel consumption by more than 30%. It can meet increasingly strict environmental requirements and has both the energy-saving and low emission characteristics of electric vehicles and the convenience of fuel vehicles. Hybrid electric vehicles are mainly divided into two types based on energy synthesis: series hybrid electric vehicles (SHEVs) and parallel hybrid electric vehicles (PHEVs). In a series hybrid system, the engine drives the generator, and the generated electrical energy is used to drive the wheels by the electric motor. All the kinetic energy emitted by the engine needs to be converted into electrical energy first, and this electrical energy is used to drive the vehicle. The parallel hybrid power system uses an engine and an electric motor to drive the wheels, and applies these two power sources according to the situation. Since the power sources are parallel, it is called a parallel hybrid power system. In addition, there is also a hybrid type, also known as series parallel type, which can maximize the advantages of both series and parallel types [13].

From the perspective of hybrid electric vehicles currently manufactured, their power system is mainly powered by a fuel engine, and their electric energy storage system is usually a secondary power source. However, the secondary power source currently used has many shortcomings that need to be greatly improved. These problems can be solved by replacing them with supercapacitors. The use of ultra large capacity capacitors as auxiliary starting devices in the electric starting system of internal combustion locomotives has shown outstanding advantages, which are reflected in

Due to the increase in starting power, the starting time of the diesel generator set has been shortened. The increase in rotational acceleration of the diesel engine improves the ignition quality of the fuel.

2. It reduces the maximum current load of the battery pack during startup, which helps to extend the service life of the battery.

3. It ensures the reliability of starting, especially in low temperatures and when the battery pack is depleted or parameters deteriorate.

4. Under the current state of battery technology, the battery capacity can be effectively reduced.

But supercapacitors cannot completely replace batteries because their energy density is relatively low. The working voltage of a supercapacitor unit is relatively low, so it is necessary to connect multiple capacitor units in series to obtain a higher working voltage. However, connecting multiple units in series requires high uniformity of the units, and the capacity of the system will be reduced exponentially after being connected in series. Many processes in this area are still under development.

The characteristics of supercapacitors perfectly meet the special requirements of hybrid electric vehicles. By utilizing the instantaneous high power characteristics of supercapacitors, the special requirements of frequent engine starting and instantaneous high power provided by the battery are avoided. At the same time, the braking energy can be recovered and utilized, which can save energy and reduce emissions and pollution, especially suitable for hybrid electric vehicles that frequently drive in cities. In terms of recovering braking energy, at least 30% of the energy consumed by cars during driving is due to heat dissipation and braking, especially in urban driving where red lights are often encountered. This not only causes energy waste but also increases environmental pollution.

If the energy consumed by braking can be recovered for car starting and acceleration, it can be said to kill two birds with one stone. Due to the fact that battery charging is completed through chemical reactions, it takes a long time, but the braking time is short, resulting in poor energy recovery efficiency. The flywheel battery currently under research is difficult to enter the practical stage in a short period of time due to high precision requirements and manufacturing difficulties. The unique characteristics of supercapacitors are very suitable for energy recovery during braking processes, and the cost is low, with broad application prospects.

In terms of providing instantaneous high power for engine cold start, the cold start of the engine places special requirements on the battery, which must provide instantaneous high power for the engine to start. However, general batteries do not possess this characteristic unless starting ignition batteries are used. However, starting ignition batteries are not suitable for long-term low current working environments and often fail at low temperatures, so they are also not suitable. Research has found that if supercapacitors and batteries are combined in the engine starting system to utilize the unique characteristics of supercapacitors and form a new type of starting system, this problem can be easily solved

As a new type of energy storage component, supercapacitors have filled the gap between traditional electrostatic capacitors and chemical power sources. With the advantages of low cost and high performance, coupled with their pollution-free nature, people are paying more and more attention to them. With the deepening of research on electric vehicles, the advantages of using supercapacitors in this field are becoming increasingly apparent. The high performance of supercapacitors determines their broad market prospects, while low cost determines their significant economic benefits. Although supercapacitors have the drawback of low specific capacity, we believe that through improvement, they will definitely promote a qualitative leap in the automotive industry.