Introduction to Supercapacitors

Get a primer on the basics of supercapacitors, their functionality, and which applications they're best for.

The current trend of supercapacitors is to replace rechargeable batteries, offering a new method of storage for energy based on nanotechnology.

Unlike batteries, supercapacitors can recharge within seconds and withstand virtually unlimited charge cycles. Supercapacitors have a higher energy density than conventional capacitors, but a lower energy density than the standard batteries used in electronic products such as IoT devices.

 

Images of two KEMET supercapacitors, the FR0H224ZF (left) and the FYD0H223ZF (right), courtesy of Digi-Key 

 

 

It is theoretically possible to replace batteries with supercapacitors, but replacing an entire bank of batteries would require a great deal of volume. With steady progress, however, supercapacitors are gaining traction in many application markets such as the automotive sector, opening up new possibilities in emerging industries such as networked energy storage.

What Is a Supercapacitor?

Supercapacitors (sometimes referred to as SCs) are electrochemical devices capable of storing and supplying high-power electricity quickly and for a large number of cycles (up to millions of cycles) without showing performance decay.

The simplest supercapacitor consists mainly of two electrodes and an electrolyte interposed to that. The electrical charges are arranged in the electrode/electrolyte interface, and there are no chemical oxidation-reduction processes. Since the physical process of accumulation is limited, the materials must have a high surface area to accumulate many electric charges.

A supercapacitor is a double-layer capacitor with very high capacity but with low voltage limits. Supercapacitors, compared to capacitors, have a larger area for storing more charge, with capacitance into the farad (F) range, and they store more energy than electrolytic capacitors. They have a low leakage current and are suitable for many applications that can operate in the 1.8V - 2.5V range. The life of a supercapacitor is 10–20 years, though the capacity could be reduced from 100% to 80% after about 8–10 years.

Thanks to their low equivalent series resistance (ESR), supercapacitors provide high load currents and fast charging. Micro-supercapacitors are MEMS-like devices that tolerate repeated bending and thus are suitable for flexible applications. This is ideal for wearables and IoT applications. Flexible solid-state micro-supercapacitor glass, silicon, and paper substrates are being developed.

When a voltage is applied to a supercapacitor, two separate charge layers are produced on the surface with a separation distance that is smaller than those of conventional capacitors. This is why supercapacitors are often referred to as double-layer electrical capacitors or EDLCs.

 

Comparison of a supercapacitor with standard capacitors. Image from Fairprice Electronics, derived from Maxwell

What's the Difference Between a Supercapacitor and a Battery?

Batteries have been a dominant form of energy storage for a long time. How can they be overcome by a capacitor, even of the “super” variety?

First, batteries gradually lose the ability to be recharged, whereas capacitors offer virtually endless charge and discharge cycles.

Second, capacitors have a very low internal resistance compared to batteries. They can provide more instantaneous power than batteries.

For Internet of Things (IoT) applications with an energy supply mechanism, the ability to incorporate such powerful energy-storage devices into a chip is an essential requirement. Supercapacitors and micro-batteries are two tools that could fulfill these needs.

 

Power density and energy density of four energy-storage technologies. Image from International Journal of Scientific & Engineering Research, Volume 4, Issue 8, August-2013 583

 

Lithium-ion batteries power almost all modern portable electronic devices, in addition to almost all electric cars. With batteries, the process of charging and discharging is slow and can degrade chemical compounds within the battery over time, leading to lower power density and storage capacity.

A supercapacitor uses a different mechanism of energy storage. In supercapacitors, energy is stored electrostatically on the surface of the material, and chemical reactions are not involved. The primary deficiency of supercapacitors is their low energy density compared to batteries. Also, the cost of supercapacitor materials (such as graphene) often exceeds that of materials used in the manufacture of batteries.

Applications for Supercapacitors

Supercapacitors can be used in conjunction with energy collection solutions that are installed in tight spaces. When they are used as an auxiliary power source for peak output, you can reduce the size of the power supplies and improve overall performance.

Here are some possible applications for supercapacitors:

• Storage and backup of memory data in case of power failure: Supercapacitors can be integrated into consumer electronics, IT devices, and communication systems to protect memory content. A related application is internal backup power. Supercapacitors can act as a battery replacement or a short-term backup power supply.
• Electric vehicles: Electric battery vehicles are affected by limitations such as low power density, limited charge/discharge cycles, high-temperature dependence, and extended charging times. Supercapacitors overcome these limitations, though they have lower energy density and higher cost. A combination of storage devices could be the preferred solution. Peak load requirements associated with acceleration or effort on steep ascents can be met by high-power devices such as supercapacitor banks. Also, supercapacitors can be used in regenerative braking systems.
• Applications for renewable energy: In solar photovoltaic applications, it is necessary to replace the batteries every 3–7 years, as they tend to wear out. The use of supercapacitors could eliminate the need for frequent maintenance and replacement. Also, energy efficiency is a key aspect of producing energy in a renewable way, and supercapacitors demonstrate higher charge efficiency than batteries.

Conclusion

Supercapacitors are an emerging energy-storage technology that could become an important part of many electronic systems. Lithium-ion batteries have been very successful, but they will never be able to compete with supercapacitors when it comes to power density and number of charge/discharge cycles.

If you’d like to read more about how to incorporate supercapacitors into your low-voltage designs, check out our article on wireless RF energy harvesting using a Powercast.