Electronic Tutorial: Supercapacitor’s Basic Working Principle and Applications (related video)



Catalog

Article Core

Supercapacitors

Introduction

Basics

1. Supercapacitor Structure

2. Supercapacitor Materials

1) Acticarbon

2) Carbon Aerogel

3) Carbon Nano-tube

4) Activated Carbon Fiber

5) Graphene

6) Metallic Oxide

7) Conductive Polymer

3. Types and Working Principle

Supercapacitors Advantages and Disadvantages

Features of “Super”

Discharging Control

1) Discharging

2) Controlling Time

Selection Standard

1) Unit

2) Electrical Parameters Selection

Attention

Prospect of Supercapacitors

Application

1) Auto Field

2) Other Fields

Supplement

This video discuss the basic aspects of supercapacitors and how they compare to batteries.

Introduction

Terminology

A supercapacitor (also called a supercap, electrochemical capacitor, ultracapacitor or Goldcap) is a high-capacity capacitor with higher capacitance values but lower voltage limit much than other capacitors that bridge the gap between electrolytic capacitors and rechargeable batteries. It is an electrochemical element developed in the 1970s ~1980s to store energy through polarized electrolytes. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries.

It is different from the traditional chemical power supply, and is a power supply which has special performance between the traditional capacitor and the battery, and mainly relies on the electrostatic double- layer and the oxidation-reduction pseudocapacitance to store electric energy. But there is no chemical reaction in the process of its energy storage, and this type of energy storage is reversible.

Supercapacitor, as a new type of energy storage device, which has the advantages of fast charging and discharging speed, high efficiency, good stability, long service life and so on, and is a clean green energy source, which is a new type of green energy in the 21st century, it has great market potential. 

supercapacitor

Supercapacitor Basics

1) Structure

The structure details of supercapacitors depend on the application of it. In other words, these materials may vary slightly relying on the manufacturer or specific application requirements. All supercapacitors have basic common points: they contain a positive electrode, a negative electrode, and a diaphragm between the two electrodes, and the electrolyte fills in the two pores separated from the two electrodes and the diaphragm.

supercapacitor structure

The structure of the supercapacitor is as shown in the following, which is composed of a porous electrode material, a current collector, a porous battery separator and an electrolyte. The electrode material is closely connected to the current collector to reduce contact resistance; the diaphragm shall meet the conditions of having as high ionic conductance as possible and as low as possible electronic conductance, it is generally made from an electronic insulation material, such as a polypropylene film with fibrous structure. The type of electrolyte is selected depending on the characteristics of the electrode material.


1-PTFE carrier

2 and 4: active substance on the foamed nickel collector

3-Polypropylene cell diaphragm

Supercapacitor components can vary from product to product. This is determined by the geometric structure of the supercapacitor package. For prism or square packaging, the internal structure is based on the arrangement of the internal components, that is, the internal collector is extruded from the stack of each electrode, and these collector pads will be welded to the terminal to extend the current path outside the capacitor.

For circular or cylindrical packaging, electrodes are cut into reel configuration. Finally, the electrode foil is welded to the terminal to expand the external capacitance-current path.

2) Supercapacitor Materials

At present, carbon materials are mainly used as electrode materials of surpercapacitors. In the market they are mainly activated carbon materials, because of the lower cost and high specific surface area of activated carbon, this is the characteristic that supercapacitor electrode material must have. However, the conductivity of the activated carbon is general, and the microstructure is mainly in the form of micropores, so there will be a large resistance in the electrolyte, the process of immersion of the electrode in the electrolyte will be relatively slow, either the process of storing and transferring charges. But its cost is low, it can basically meet the requirements of the market, so it is used as the main material of capacitor on the market, although other carbon materials have sound performance, but their cost is higher, so they are not commercialized. Therefore, electrode materials with good performance and low cost is the mainstream in the field of supercapacitors now. In a word, it has great significance to improve supercapacitors with superior performance and low cost, in such a situation, supercapacitors can be widely used in the market.

So far, carbon materials used to study and research electrode materials for supercapacitors include activated carbon, carbon aerogel, carbon nano-tubes, glass-carbon, graphene, carbon fiber and carbon / carbon composites. Due to the price of carbon raw material is low, surface area is large, it is suitable for mass production. However, pure carbon electrode materials do not have high specific capacitance, which need to be modified.

ultracapacitors

1. Acticarbon

For activated carbon materials, different processing methods will produce different activated carbons with different specific surface areas, which can be as high as 1000 ~ 3000m2/g, and have different voids with wide pore size range, simple production process and low cost. It can be obtained from asphalt, plant shells, petroleum coke, rubber and other raw materials. And it is a commercialized electrode material for supercapacitors. Activated carbon materials can be activated by various ways, and physical activation and chemical activation are mainly used today.

2. Carbon Aerogel

Carbon aerogel is a cross-linked reticulated carbon material with porous properties. Its advantages include good conductivity, large surface area, high porosity and wide pore size distribution. Carbon aerogel is the only aerogel with high conductivity. For example, nano-carbon materials, one of carbon materials with large density span, good porosity and light weight, and meanwhile, its pore size distribution and particle size can be controlled by adjusting the process parameters.

3. Carbon Nano-tube 

The carbon nano-tube is a hexagonal carbon material which is similar to graphite. A multi-layer tube, from micro perspective, its two ends are closed, and its diameter is several tens of nanometers, and the spacing of the layers is slightly larger than that of the graphite layer. For the requirement of the electrode material of the super capacitor, the carbon nano-tube material is very suitable for use as the electrode material because the structure of the carbon nanotube is hollow tube, the surface area is large, especially the carbon nano-tube with very thin walls, the specific surface area is more larger, which is very beneficial to the storage of the electric double-layer capacitor. If that carbon nano tube is made into an electrode, a special hole is also provided, the structure of holes formed between the tubes, the holes are connected to each other, and there is no plugging situation, which is very important for the flow of electrolyte when it is used as an electrode. And this kind of holes from the tube winding each other will not be too small, generally belong to the mesopore, this will make the electrode internal resistance is very low, which are required by the super capacitor electrode. At present, the research on carbon nano-tube as electrode materials for supercapacitors is mainly focused on the direct application of carbon nanotubes to supercapacitors, or the combination of carbon nanotubes and other materials as supercapacitors.

4. Activated Carbon Fiber

Activated carbon fiber (ACF) is a kind of environment-friendly material, which has better adsorption performance than activated carbon. Activated carbon fiber cloth with large specific surface area obtained from ACF has been successfully used in commercial electrode materials.

5. Graphene

Many researchers all over the world have studied graphene for a long time. Because graphene has many characteristics that other materials do not have. Its main advantages include good electrical conductivity, large surface area, low density, special thermal conductivity and optical properties,  high mechanical properties and so on, these are electrode material requirements of ideal supercapacitor. There are many methods for the preparation of graphene, including tape stripping, SiC decomposition, oxidized graphite reduction and so on. The redox method is the most widely used at present. Graphene made from this method has the advantages of high yield, good quality and relatively simple process. However, this method also has disadvantages, for example, the oxide graphene after strong oxidant is not easy to be completely reduced, what’s more, the reductant may usually have highly toxic.

6. Metallic Oxide 

Metal oxide material is a kind of material used on supercapacitor besides carbon material. Its storage is different from that of carbon material. It mainly adopts Faraday quasi-capacitance principle, which is much larger than double-layer capacitor. Faraday quasi-capacitance is explaining a highly reversible chemical adsorption-desorption reaction or redox reaction, which can occur at the interface between the electrodes or inside the electrodes. The capacitance values of capacitors electrode made from this material is larger than the double-layer capacitors.

7. Conductive Polymer 

Conductive polymer materials are another category of electrode materials for supercapacitors, and their capacitors theory are mainly from Faraday capacitor principle. After this kind of materials are made into electrodes, a small part of the double layer capacitance occurs at the interface of the electrode solution, more is the highly reversible the redox reaction based on Faraday principle. Because conductive polymer has plasticity, it is easy to make thin layer electrode with low internal resistance and low cost.

3) Types and Working Principle

supercapacitor types

For supercapacitors, there are different classification methods depending on the different contents.

Firstly, according to different energy storage mechanism, supercapacitors can be divided into double electric layer capacitors and Faraday quasi capacitors. Among them, double electric layer capacitors are mainly generated by pure electrostatic charge adsorbing on the electrode surface to generate storage energy. The Capacitance of Faraday quasi capacitors are mainly produced by reversible redox reactions on and near the surface of active electrode materials (such as transition metal oxides and high polymer materials), to achieve the storage and conversion of energy.

Secondly, according to the type of the electrolyte, it can be divided into two types: the water system supercapacitor and the organic supercapacitor.

In addition, according to the type of active materials, it can be divided into symmetric supercapacitors and asymmetric supercapacitors.

Finally, according to the state of electrolyte, it can be divided into solid-electrolyte supercapacitors and liquid-electrolyte supercapacitors.

The working principle of supercapacitors is to use the double electric-layer structure composed of activated carbon porous electrode and electrolyte to obtain the super capacity.

The working principle of the double electric-layer capacitor and the Faraday quasi-capacitor will be explained below.

1. Double electric-layer capacitor: it is produced by charge confrontation at the electrode /solution interface through the directional alignment of electrons or ions. For an electrode /solution system, a double electric-layer is formed at the interface between the electron-conducting electrode and the ionic conductive electrolyte solution. Specifically, when the electric field is applied on the two electrodes, the anion and cations in the solution migrate to the positive and negative electrodes respectively, then a double electric layer is formed on the electrode surface. After the electric field is removed, the positive and negative charge on the electrode is attracted to the opposite charge ion in the solution, and the double layer is stabilized, resulting in a relatively stable potential difference between the positive and negative electrodes. At this point, for a certain electrode, within a certain distance (dispersion layer), the heterosexual ion charge is produced in the equal amount as the charge on the electrode to keep electric neutrality. When the two poles are connected with the external circuit, the charge transfer on the electrode leads to the generation of electric current in the external circuit, and the ions in the solution migrate reflect electric neutrality, which is the charge and discharge principle of the double electric-layer capacitor. 

electric double layer capacitor

2. Faraday quasi capacitor: the theory was first proposed by Conway. On and near the surface of the electrode or in two-dimensional or quasi-two-dimensional space in the bulk phase, electroactive substances have a underpotential deposition, through highly reversible chemical adsorption-desorption and redox reaction occur, resulting in capacitors related to the charge potential of the electrode. For Faraday quasi capacitors, the process of charge storage includes not only includes the storage on the double electric layer, but also contains the redox reaction between electrolyte ions and electrode active substances. When ions in electrolyte (such as H+, OH-,K+ or Li+) from solution spread to electrode / solution interface under the action of external electric field, a large amount of charge is stored in the electrode by redox reaction. During discharge, these ions will return to the electrolyte through the reversible redox reaction and the stored charge will be released through the external circuit. This is the charging and discharging mechanism of the Faraday quasi capacitor.

Supercapacitors Advantages and Disadvantages

Advantages

1) The speed of charging is fast, and the charge can reach 95% of its rated capacity in 10 seconds ~ 10 minutes.

2) Long cycle life, cycle times of full charge and discharge can up to 1 ~ 500000 times.

3) Strong discharge capacity in large current , high energy conversion, small loss, large current energy cycle efficiency ≥ 90%.

4) High power density, up to 300W/ KG ~5000W/ KG, is equivalent to 5-10 times of the battery.

5) No pollution in product raw material composition, production, use, storage and disassembly processes, supercapacitor is the ideal and echo electric copmponent.

6) Simple charging and discharging circuit, opposite to the rechargeable battery, very safe, no maintenance for long term use.

7) Characteristic is good in the ultra-low temperature, the temperature range is -40 ℃ ~+70 ℃.

8) Easy to detect and the remaining quantity of electricity can read out directly.

9) Capacity range is usually 0.1F-1000F.

10) A very small volume can reaches the capacity of the Farah class.

11) No need for special charging and discharge control circuits.

12) Overcharging does not have a negative impact on the life of the supercapacitor compared with batteries.

13) Supercapacitors can be welded to solve the unstable problem that batteries had have.

Disadvantages

1) If it is not used properly, it will cause electrolyte leakage, etc.

2) Compared with aluminum electrolytic capacitor, its internal resistance is larger, so it can not be used in AC circuit.

"Super" Means What?

supercapacitor

1. Supercapacitors can be considered as two non-reactive but active porous electrode plates suspended in the electrolyte. Charging on the electrode plate, positive plate attracts the negative ions in the electrolyte, and the negative plate attracts the positive ion, which actually forms two capacitive storage layers. The separated positive ion is near the negative plate and the negative ion is near the positive plate.

2. The supercapacitor stores energy in the separated charge. The larger the area used to store the charge, the denser the charge, the larger the capacitance.

3. The area of the traditional capacitor is the area of the conductor plate. In order to obtain the larger capacity, the conductor material is curled up very long, sometimes with special structure to increase its surface area. In addition, traditional capacitors use insulating materials to separate its polar plates, generally plastic film, paper, and so on, these materials usually require as thin as possible.

4. The area of supercapacitor is based on the porous carbon material. The porous structure of the supercapacitor allows its area to reach 2000m2/g, which can achieve a larger surface area through some measures. The distance from which the supercapacitor charges are separated is determined by the size of the electrolyte ion attracted to the charged electrode. Further more, Supercapacitors can achieve a smaller distance compared with conventional capacitor film materials.

5. The large surface area and the very small charge separation distance make supercapacitors have an amazing electrostatic capacity compared with conventional capacitors.

Discharging Control of Supercapacitor

Ultracapacitors

1) Discharging

The resistance of the supercapacitor blocks its rapid discharge. The time constant τ of the supercapacitor is 1 ~2s, and the complete discharge of the resistance-capacitance circuit requires about 5τ. That is, if the short-circuit discharge takes about 5 ~10s (because of the special structure of the electrode, they actually take several hours to remove the remaining charge completely).

2) Controlling Time

Supercapacitors can charge and discharge quickly, and peak currents are limited by their internal resistance, even short circuits are not the big deal. It actually depends on the volume of the capacitor unit. For matching loads, the small unit can be placed at 10A, and the large cell can be placed at 1,000A. Another limiting factor of discharge rate is heat. Repeated discharge frequently  will raise the temperature of the capacitor and eventually lead to an open circuit.

Supercapacitor Selection

Power requirement, discharging time and systematic voltage change play a decisive role in selecting supercapacitors. The output voltage drop of the supercapacitor consists of two parts, one is the supercapacitor releasing energy, another is caused by the supercapacitor internal resistance. In addition, two part who account for most depending on time. In the case of very fast pulse, the internal resistance is the main part, whereas in the long time discharge, the internal resistance is the main part.

Unit

Unit: F

1F=1C/1V

1C=1A·S

The 12V/14A battery discharge =14*3600*1/12=4200F, (Note: the 12V/14A battery is connected in series by six 2V/14A batteries. If these batteries were parallel connection, it will be equal to 2V/84A.

Electrical Parameters Selection

The following basic parameters determine the size of the capacitor selected:

1)maximum operating voltage 

2)working cutoff voltage

3)average discharging current

4)discharging time

Attention

Supercapacitors image

1. Supercapacitors have fixed polarity, which should be confirmed before use.

2. It should be used at nominal voltage. When the capacitor voltage exceeds the nominal voltage, it will lead to the decomposition of electrolyte, meanwhile, the capacitor will heat up, the capacity will decrease, and the internal resistance will increase, the life will be shortened, and in some cases, the capacitor performance will faulty.

3. It can’t be applied to a high-frequency charging and discharging circuit. A high frequency of rapid charge and discharge will result in internal heat generation in the capacitor, an decrease in capacity, an increase in internal resistance, which in some cases lead to a breakdown of the normal operation.

4. The external ambient temperature also has an important influence on the service life. The capacitor should be as far away as possible from the heat source.

5. Exist voltage drop when used as a backup power source. Because of the large internal resistance of supercapacitors, there is voltage drop Δ V=IR, at the moment of discharging.

6. It is not in a place with a relative humidity of more than 85% or toxic gases. These conditions will contribute to corrosion of the lead and the capacitor shell, leading to an open circuit.

7. It can not be placed in a high-temperature and high-humidity environment. It should be carried out at a temperature of -30℃~+50℃, and a relative humidity of less than 60% to avoid a sudden temperature change, which may result in component damage.

8. When used on double-sided circuit boards, connections are not allowed to pass through capacitors. Because of the supercapacitor installation, once surcapacitor near the wiring contracts will cause the short-circuit phenomenon.

9. When the capacitor is welded to the circuit board, the capacitor shell shall not be contacted to the circuit board. Otherwise, the solder will seep into the perforating hole of the capacitor, which will affect the performance of the capacitor.

10. After installing supercapacitor, don’t tilt or twist capacitor at random. Otherwise, it will lead to loosening of capacitor wires, then cause deterioration of performance.

11. Avoid overheating the capacitor during welding. If the capacitor is overheated during welding, the service life of the capacitor will be reduced. For example, if using a printed circuit board with thickness of 1.6mm, the temperature of welding process shall be 260℃ and the soldering time of one contract shall not exceed 5s.

12. After the capacitor is welded, the circuit board and the capacitor need to be cleaned, because some impurities may cause short-circuit to the capacitor.

13. Supercapacitor should adopt series connection. For technical reasons, the rated operating voltage of a unipolar supercapacitor is generally around 2.8 V, so it is necessary to use it in series in most cases. Since it is difficult to guarantee the same capacity of each unit in the series circuit, either the leak of each unit, which will lead to the different charge voltage of each cell in the series loop, resulting in the overvoltage damage of the capacitor ultimately. Therefore, the supercapacitor in series must attach the equalizer circuit. When supercapacitors are used in series, there is a problem of voltage balance between units, a simple series of supercapacitors can cause one or more single capacitors to overvoltage, leading to damage to these capacitors, and the overall performance is affected. So in series use of capacitors, it is necessary to get technical support from capacitor manufacturers.

Prospect of Supercapacitors

The most promising future of supercapacitors is the combination of a double-layer charging interface with existing energy-storage technologies. By adding EC technology to fuel-cell applications, companies have been successful in rapidly improving the charge/discharge cycle performance of hybrid- and electric-vehicle applications. Many cities using hybrid technologies for public transit have also seen an improvement in overall energy storage and charge cycles when coupling their energy systems with things like supercapacitor-based engine starters and charging stations.

The closest future application for supercapacitors is in energy storage and rapid charging. Many applications of this type have already hit the market, and are changing how we think about energy storage.

The realization of a commercially viable, standalone supercapacitor battery may be further off into the future. Still, supercapacitor applications that have been achieved are an exciting realization of part of an age-old technology that is only getting better with time.

Once the scientists made a breakthrough in the supercapacitor technique, the new energy industry can be greatly driven by the development of supercapacitors.

Application

1)Auto Field

In auto industry, the application of intelligent starting and stopping control system (light hybrid power system) provides a broad stage for supercapacitors, especially in plug-in hybrid vehicles. The discharge process of battery varies greatly due to the frequent starting and stopping of electric vehicle. And during normal driving, the average power of electric vehicle from battery is very low, but the peak value of acceleration and climbing is quite high.

Under the existing battery technology of electric vehicle, the battery must balance the specific energy and specific power consumption, and cycle life. But it is difficult to reach high specific energy, high specific power and long service life in a set of energy system at the same time.

In order to solve the contradiction between driving mileage and accelerating climbing performance of electric vehicle, two energy systems should be adopted. The main energy can improve the optimum driving range, and the auxiliary energy can provide the short auxiliary power when accelerating and climbing. The auxiliary energy system can directly take its own energy, or it can also recover the renewable kinetic energy when the electric vehicle brakes and downhill, and selects the super capacitor as the auxiliary energy.

In a short term, specific energy of supercapacitor makes it impossible to be used as an electric vehicle energy system alone, but it has significant advantages as an auxiliary energy source. The best combination used in electric vehicles is the hybrid-energy system of battery-supercapacitor, but this system requires separate specific energy and specific power of the battery.

The supercapacitor has the function of load balancing. The discharge current of the battery reduces improve the available energy efficient of the battery and its service life significantly. Compared with the battery, the supercapacitor can quickly and efficiently absorb the regenerative kinetic energy generated by the braking of electric vehicle. The load balance of supercapacitors and the recovery of energy greatly improve the driving mileage of the vehicle. However, to accomplish this goal, the system should make comprehensive control and optimization matching for battery, supercapacitor, motor and power inverter, in addition, the design of power converter and its controller should fully consider the matching between motor and supercapacitor.

2)Other Fields

In the development of supercapacitors for dozens of years, micro supercapacitors have been widely used in small mechanical devices, such as computer memory systems, cameras, audio equipment and intermittent electrical auxiliary facilities. Large-sized columnar supercapacitors are mostly used in automotive and natural energy collection industry. So supercapacitors will be an important part of the transportation industry and natural energy collection in the future.

1) Large size supercapacitors (125V) can be used in braking systems of trains and subways, and can also provide energy for material handling vehicles; medium-sized supercapacitors (75V) can be used in solar energy collection because they have the ability to work at high temperatures; 48V supercapacitors are used in cars; small-sized supercapacitors (within 2.7V) contribute significantly to the continuous power supply of communications facilities and the storage of backup power in computer memory systems.

2) The low impedance of supercapacitors is essential for many high-power applications today. For fast charging and discharging, the supercapacitor's lower ESR means more power output: a few seconds to charge and a few minutes to discharge. For example, it is suitable for electric tools and toys.

3) In UPS systems, supercapacitors provide instantaneous power output as a supplement to the motor engine or other uninterrupted systems devices.

4) As power support when the bus switches from one power source to another.

5) With the features of small current, long time continuous discharge, which can used as computer memory backup power supply.

6) Transient power pulse applications, important storage, short-time power support of memory system.

In the field of natural energy collection, wind power generation can’t without hydraulic system or battery. Because each time the fan of the generator stops, the internal turbine adjusts the blade to a specified position, this process is called the variable pitch wind control system in wind power generation, and the electricity required during the operation is supplied by the hydraulic system or battery. For batteries, intermittent work intensity and perennial load, will lead to their own service life reduced greatly. Due to this reason, every few years, every wind turbine needed detection, so the maintenance and replacement of the batteries is costly. For high-power supercapacitors, having the characteristics of fast charge and discharge and long cycle life, it can replace the battery to do that work. Although the cost is high in the earlier period, the cost of frequent maintenance and replacement of the battery will be reduced, and the working intensity of devices can be reduced compared with batteries.

Supplement of Supercapacitors

“Can Supercapacitors Surpass Batteries for Energy Storage?”

Supercapacitors have many advantages:

A. Low ESR and power density is more than ten times of that of the lithium ion battery, which is suitable for high current discharge. (a 4.7F capacitor can release the instantaneous current of more than 18A).

B. The ultra-long service life, charge and discharge more than 500000 times, is 500 times that of Li-Ion battery, is 1000 times that of Ni-MH and Ni-Cd battery. If the supercapacitor is charged and discharged 20 times a day, it can be used continuously for 68 years.

C. High current charging, short charge and discharge time, simple charging circuit, no memory effect.

D. Maintenance-free, sealed

E. The temperature range of supercapacitor is -40 ℃~+70 ℃, and the common battery is -20 ℃ ~+60 ℃.

F. Supercapacitors can be composed of series and parallel into a supercapacitor set, which can withstand voltage and store higher capacity

Supercapacitors sample

Advances in supercapacitors are delivering better-than-ever energy-storage options. In some cases, they can compete against more-popular batteries to develop a more wider-range markets.

1) Supercapacitors are superior to batteries in some applications. It is a better way to combine the power characteristic of capacitor with the high energy storage of battery.

2) The super capacitor can be charged to any potential within its rated voltage range and can be fully discharged. while the battery is limited to a narrower voltage range by its own chemical reaction, and if overdischarge may cause permanent damage.

3) The charge state (SOC) of the supercapacitor and the voltage constitute a simple function, but the charge state of the battery includes various complex conversion.

4) Supercapacitors can store more energy than conventional capacitors, and batteries can store more energy than supercapacitors with same volume. But in some applications where power determines the size of energy storage devices, supercapacitors may be a better option.

5) Supercapacitors can transmit energy pulses repeatedly without any bad effect, on the contrary, if the batteries repeatedly do it, their lives are greatly reduced.

6) Supercapacitors can be charged quickly, but batteries can be damaged by quick charging.

7) Supercapacitors can be charged and discharged hundreds of thousands of times, while battery has only a few hundred of cycles.

Supercapacitors-Related News

ultra capacitors

Devices called supercapacitors have recently become attractive forms of energy storage: They recharge in seconds, have very long lifespans, work with close to 100 percent efficiency, and are much lighter and less volatile than batteries. But they suffer from low energy-storage capacity and other drawbacks, meaning they mostly serve as backup power sources for things like electric cars, renewable energy technologies, and consumer devices.
But MIT spinout FastCAP Systems is developing 
ultra film capacitors, and supercapacitor-based systems, that offer greater energy density and other advancements. This technology has opened up new uses for the devices across a wide range of industries, including some that operate in extreme environments.

Based on MIT research, FastCAP's supercapacitors store up to 10 times the energy and achieve 10 times the power density of commercial counterparts. They're also the only commercial supercapacitors capable of withstanding temperatures reaching as high as 300 degrees Celsius and as low as minus 110 C, allowing them to endure conditions found in drilling wells and outer space. Most recently, the company developed a AA-battery-sized supercapacitor with the perks of its bigger models, so clients can put the devices in places where supercapacitors couldn't fit before.

Founded in 2008, FastCAP has already taken its technology to the oil and gas industry, and now has its sights set on aerospace and defense and, ultimately, electric, hybrid, and even fuel-cell vehicles. "In our long-term product market, we hope that we can make an impact on transportation, for increased energy efficiency," says co-founder John Cooley PhD '11, who is now president and chief technology officer of FastCAP.

FastCAP's co-founders and technology co-inventors are MIT alumnus Riccardo Signorelli PhD '09 and Joel Schindall, the Bernard Gordon Professor of the Practice in the Department of Electrical Engineering and Computer Science.

A "hairbrush" of carbon nanotubes

Supercapacitors use electric fields to move ions to and from the surfaces of positive and negative electrode plates, which are usually coated with a porous material called activated carbon. Ions cling to the electrodes and let go quickly, allowing for quick cycling, but the small surface area limits the number of ions that cling, restricting energy storage. Traditional supercapacitors can, for instance, hold about 5 percent of the energy that lithium ion batteries of the same size can.

In the late 2000s, the FastCAP founding team had a breakthrough: They discovered that a tightly packed array of carbon nanotubes vertically aligned on the electrode provided much more surface area. The array was also uniform, whereas the porous material was irregular and difficult for ions to move in and out of. "A way to look at it is the industry standard looks like nanoscopic sponge, and the vertically aligned nanotube arrays look like a nanoscopic hairbrush" that provides the ions more efficient access to the electrode surface, Cooley says.

With funding from the Ford-MIT Alliance and MIT Energy Initiative, the researchers built a fingernail-sized prototype that stored twice the energy and delivered seven to 15 times more power than traditional ultracapacitors.

In 2008, the three researchers launched FastCAP, and Cooley and Signorelli brought the business idea to Course 15.366 (Energy Ventures), where they designed a three-step approach to a market. The idea was to first focus on building a product for an early market: oil and gas. Once they gained momentum, they'd focus on two additional markets, which turned out to be aerospace and defense, and then automotive and stationary storage, such as server farms and grids. "One of the paradigms of Energy Ventures was that steppingstone approach that helped the company succeed," Cooley says.

FastCAP then earned a finalist spot in the 2009 MIT Clean Energy Prize (CEP), which came with some additional perks. "The value there was in the diligence effort we did on the business plan, and in the marketing effect that it had on the company," Cooley says.

Based on their CEP business plan, that year FastCAP won a $5 million U.S. Department of Energy (DOE) Advanced Research Projects Agency-Energy grant to design supercapacitors for its target markets in automotive and stationary storage. FastCAP also earned a 2012 DOE Geothermal Technologies Program grant to develop very high-temperature energy storage for geothermal well drilling, where temperatures far exceed what available energy-storage devices can tolerate. Still under development, these supercapacitors have proven to perform from minus 5 C to over 250 C.

From underground to outer space

Over the years, FastCAP made several innovations that have helped the ultracapacitors survive in the harsh conditions. In 2012, FastCAP designed its first-generation product, for the oil and gas market: a high-temperature ultracapacitor that could withstand temperatures of 150 C and posed no risk of explosion when crushed or damaged. "That was an interesting market for us, because it's a very harsh environment with [tough] engineering challenges, but it was a high-margin, low-volume first-entry market," Cooley says. "We learned a lot there."

In 2014, FastCAP deployed its first commercial product. The Ulysses Power System is an supercapacitor-powered telemetry device, a long antenna-like system that communicates with drilling equipment. This replaces the battery-powered systems that are volatile and less efficient. It also amplifies the device's signal strength by 10 times, meaning it can be sent thousands of feet underground and through subsurface formations that were never thought penetrable in this way before.

After a few more years of research and development, the company is now ready to break into aerospace and defense. In 2015, FastCAP completed two grant programs with NASA to design ultracapacitors for deep space missions (involving very low temperatures) and for Venus missions (involving very high temperatures).

In May 2016, FastCAP continued its relationship with NASA to design an ultracapacitor-powered module for components on planetary balloons, which float to the edge of Earth's atmosphere to observe comets. The company is also developing an ultracapacitor-based energy-storage system to increase the performance of the miniature satellites known as CubeSats. There are other aerospace applications too, Cooley says: "There are actuators systems for stage separation devices in launch vehicles, and other things in satellites and spacecraft systems, where onboard systems require high power and the usual power source can't handle that."

A longtime goal has been to bring supercapacitors to electric and hybrid vehicles, providing high-power capabilities for stop-start and engine starting, torque assist, and longer battery life. In March, FastCAP penned a deal with electric-vehicle manufacturer Mullen Technologies. The idea is to use the supercapacitors to augment the batteries in the drivetrain, drastically improving the range and performance of the vehicles. Based on their wide temperature capabilities, FastCAP's ultracapacitors could be placed under the hood, or in various places in the vehicle's frame, where they were never located before and could last longer than traditional supercapacitors.

The devices could also be an enabling component in fuel-cell vehicles, which convert chemical energy from hydrogen gas into electricity that is then stored in a battery. These zero-emissions vehicles have difficulty handling surges of power—and that's where FastCAP's supercapacitors can come in, Cooley says.

"The supercapacitors can sort of take ownership of the power and variations of power demanded by the load that the fuel cell is not good at handling," Cooley says. "People can get the range they want for a fuel-cell vehicle that they're anxious about with battery-powered electric vehicles. So there are a lot of good things we are enabling by providing the right supercapacitor technology to the right application."

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