A supercapacitor or ultracapacitor, is a capacitor with a higher capacitance value than regular capacitors, but what does this imply?

Supercapacitors serve the same function as ordinary capacitors and batteries: charging and discharging energy. The difference is how much energy they can store and at what voltages they operate. These variations are caused by the differences in how these systems work and how they are designed.

Background theory: Main Supercapacitor Types

Within electrochemical capacitors, the electrolyte is the conductive connection between the two electrodes, distinguishing them from electrolytic capacitors, in which the electrolyte only forms the cathode, the second electrode. Supercapacitors are polarized and must operate with correct polarity. Polarity is controlled by design with asymmetric electrodes, or, for symmetric electrodes, by a potential applied during the manufacturing process.

Supercapacitor is a generic term for three main types of electrochemical capacitors, as can be seen in Figure 1.

Figure 1. Overview over the most commonly used fixed capacitors in electronic equipment. Link.

Unlike ordinary capacitors, supercapacitors do not use the conventional solid dielectric, but rather, they use electrostatic double-layer capacitance (storage is electrostatic in origin) and electrochemical pseudocapacitance (faradaic in origin.), both of which contribute to the total capacitance of the capacitor, with a few differences:

• Electrostatic double-layer capacitors (EDLCs) use carbon electrodes or derivatives with much higher electrostatic double-layer capacitance than electrochemical pseudocapacitance, achieving separation of charge in a Helmholtz double layer at the interface between the surface of a conductive electrode and an electrolyte. The separation of charge is of the order of a few ångströms (0.3–0.8 nm), much smaller than in a conventional capacitor.
• Electrochemical pseudocapacitors use metal oxide or conducting polymer electrodes with a high amount of electrochemical pseudocapacitance additional to the double-layer capacitance. Pseudocapacitance is achieved by Faradaic electron charge-transfer with redox reactions, intercalation, or electrosorption.
• Hybrid capacitors, such as the lithium-ion capacitor, use electrodes with differing characteristics: one exhibiting mostly electrostatic capacitance and the other mostly electrochemical capacitance.

What's super about them?: Comparison with other storage technologies

With a capacitance value much higher than other capacitors, but with lower voltage limits, supercapacitors bridge the gap between electrolytic capacitors and rechargeable batteries, as can be seen in Figure 2.

Figure 2. Bridging the Gap, Energy vs Power Density. Link.

There are four main differences between supercapacitors and batteries:

• 1. Energy Density: Batteries store charge chemically, while capacitors store charge electrically. Chemical reactions have the capability to store much more energy than electrical storage, which is what contributes to batteries being used more often in applications that require higher storage.
• 2. Power Density: Supercapacitors have faster charge and discharge rates than batteries because the chemical reactions that take place within batteries take longer to release electrons than the electrical discharge in supercapacitors.
• 3. Cycles & Lifetime: Chemical reactions are the limiting factor for the lifetime of batteries. In batteries, once this electrolyte is used up, no chemical reactions can occur, and the battery stops working because it cannot store or discharge any longer. The number of cycles is much smaller than that of supercapacitors because capacitors do not rely on chemical reactions to store energy.

Table 1. Performance parameters of supercapacitors compared with other storage technologies. Link.

Long story short, here are the main benefits of supercapacitors:

• Can store 10 to 100 times higher energy density than electrolytic capacitors.
• Can accept and deliver charge much faster than batteries (in the order of seconds/minutes as opposed to hours).
• Thus, though their energy density is ~10% of a conventional battery, their power density is 10 to 100 times greater than batteries.
• The greater power density results in shorter charge/discharge cycles than a battery is capable, and a greater tolerance for numerous charge/discharge cycles, leading to a longer lifetime.
• This makes them well-suited for parallel connection with batteries, and may improve battery performance in terms of power density.
• Can handle temperatures down to -40°C (-40° F), something which drops battery power outputs to near uselessness in many cases.

Applications

Supercapacitors bridge the gap between electrolytic capacitors and rechargeable batteries. Although supercapacitors have enormous power densities compared to batteries, batteries (so far) have absolutely superior energy densities. A supercapacitor might kick an EV off in record fashion, but it might not even come close to finishing the quarter mile. A large battery pack would not necessarily provide the initial acceleration, but its energy would last for many miles at a reasonable speed. There is not the power for acceleration because the battery cannot charge and discharge its stored energy as quickly as a supercapacitor, but within its performance range, can dispense its energy over a longer time.

Supercapacitors today are used in applications requiring many rapid charge/discharge cycles, rather than long-term compact energy storage such as:

• Systems that require regenerative braking (buses, trains, elevators, etc.),
• Power electronics that require very short, high current (as in the KERSsystem in Formula 1 cars),
• Short-term energy storage, or
• Burst-mode power delivery.

From an application standpoint, there may be some applications that currently use batteries where supercapacitors can replace them, but most of those applications require batteries or a hybrid configuration, which utilizes both technologies. And this is where the potential of supercapacitors in the EV industry and Energy Storage industry lies: on hybrid energy systems. In this hybrid configurations, the supercapacitors provide the quick burst of energy for an application, while the batteries handle the long-term energy needs.

This also makes sense in terms of costs and reducing LCOE, supercapacitors have a much higher up-front cost than batteries, which causes many designs to use batteries instead. Given the differences in lifetime of supercapacitors and batteries, the long-term cost of supercapacitors may be a cheaper option even with the higher initial cost. With hybrid systems, that economic equation can be handled as needed depending on the lifetime needed for the specific application.

The hybrid supercapacitor-battery energy storage system will demonstrate multiple service applications with extended operational life and rapid response, such as:

• Power tools
• Electric Mobility
• EV chargers
• Stationary Energy Storage that require fast response

On June 30, through a Darcy Live Event we will be discussing the capabilities of Supercapacitors in stationary energy storage systems and their potential when combining supercapacitors with Long-duration energy storage (LDES) technologies. We will also have a special guest from Salt River Project (SRP) to share their thoughts on their strategy towards LDES. Save the date!

References

• Bueno, Paulo R. Nanoscale origins of super-capacitance phenomena. Harvard University. February 2019. Link.
• Z. Tehrani, D.J. Thomas, T. Korochkina, C.O. Phillips, D. Lupo, S. Lehtimäki, J. O'Mahony, D.T. Gethin, David Gethin Orcid Logo, Christopher Phillips Orcid Logo, Zari Tehrani Orcid Logo, Daniel Thomas. Large-area printed supercapacitor technology for low-cost domestic green energy storage. Cronfa, Swansea University's Research Repository. 2016. Link.
• B. E. Conway. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. 1999.
• Sigler, Dean. Are Ultracapacitors Ready for Prime Time?. BlogCafe Foundation. 2011. Link.