Supercapacitors

What is a supercapacitor?

Electrochemical Double Layer Capacitors (EDLCs) – also called supercapacitors (SC) - are electrochemical capacitors that have high capacitance and high energy density when compared to common capacitors, and higher power density when compared to batteries.

Most EDLC's are constructed from two carbon based electrodes (mostly activated carbon with a very high surface area), an electrolyte (aqueous or organic) and a separator (that allows the transfer of ions, but provides electronic insulation between the electrodes). As voltage is applied, ions in the electrolyte solution diffuse across the separator into the pores of the electrode of opposite charge. Charge accumulates at the interface between the electrodes and the electrolyte (the double layer phenomenon that occurs between a conductive solid and a liquid solution interface), and forms two charged layers with a separation of several angstroms – the distance from the electrode surface to the center of the ion layer (d in Fig. 1). The double layer capacitance is the result of charge separation in the interface.  Since capacitance is proportional to the surface area and the reciprocal of the distance between the two layers, high capacitance values are achieved.

schematic of edcl

Fig. 1: Schematic of EDLC

EDLC's store electrical charge electrostatically, and almost no reaction occurs between the electrodes and the electrolyte. Consequently, electrochemical capacitors can undergo hundreds of thousands of charge and discharge cycles.

The first electrical device using double layer charge storage was reported in 1957 by General Electric. This was followed by the development from the Standard Oil Company of Ohio (SOHIO), who patented a device that shaped the format commonly used today. In 1978, NEC was the first to introduce supercapacitors commercially as low voltage devices with high ESR, primarily designed for backup applications. Since then, EDLC's have evolved: high voltage and very high capacitance (thousands of Farads) on the one hand, and low ESR pulse supercapacitors with a capacitance range of few mili-Farads, up to 1- 2 Farads, on the other hand.

Significant improvements in materials, design and the process of production of pulse supercapacitors has lead to low profile prismatic devices that can supply high peak currents of up to 2 – 3 A. These devicesare suited to meet the peak power demands of many battery-powered electronics and other consumer and industrial devices with current-limited energy sources.

Why use supercapacitors for energy storage?

Supercapacitors are unique electrical storage devices that can store much more energy than conventional capacitors, and offer higher power density than batteries.

Batteries are widely used for energy storage in industrial and consumer electronics devices because of their high energy density, but are limited in their power density. With its limited power the battery often cannot supply the required power while still retaining its open circuit voltage. The larger the voltage drop of the battery the larger the load on the battery. Often, when a battery needs to supply high power at short pulse widths, the voltage drop may be too large, causing lower voltage than required by the end product. The large load decreases the energy stored in the battery, harming it and shortening its life-span.

When high power is required in battery operated devices (i.e. in pulse applications), the combination of the supercapacitor connected in parallel to the battery gives  the advantages of both, enhancing the performance of the battery and extendingits life, exploiting the batteries to its maximum potential. The supercapacitor connected to the battery in parallel produces a voltage damping effect - low impedance supercapacitors can be charged in seconds with a low current during standby times between high current pulses. Adding the supercapcitor to the battery in parallel decreases the voltage drop, leading to:

  • Better energy and power management
  • Battery life and operational range extension
  • Superior energy density in the battery
  • Power to be produced by both the EDLC and the battery, each supplying power inversely to its own ESR
  • Fewer battery replacements in some of the applications (or use of smaller batteries)

Batteries are very inefficient at low temperatures. Their internal resistance increases due to the slower kinetics of the chemical reaction within the battery. The internal resistance is reduced by incorporating a supercapacitor into the system with the battery. The internal resistance of pulse type supercapacitors is much lower than that of batteries, even at low temperatures down to -40C.

Supercapacitors have several advantages and disadvantages relative to batteries, as described below:

Advantages:

  • Long life time
    • Little degradation over hundreds of thousands of cycles
    • Not subject to the wear and aging experienced by electro-chemical batteries
  • Low impedance (ESR)
    • Enhances pulse current handling by parallel connection with an electro-chemical battery
  • Rapid charging and discharging
    • Low-impedance super capacitors charge in seconds (in contrast to secondary batteries)
  • Simple charge methods
    • No full-charge detection is needed; no danger of overcharge
  • Cost-effective energy storage
    • Low cost per cycle
    • Lower energy density compensated by a very high cycle count
  • One supercapacitor can replace many regular capacitors
  • Reduces voltage drop compared to battery operated device with no SuperCapacitor
  • Infinite charging cycles versus secondary battery
  • Extended operation temperature range
    • Allows use of batteries at very low temperatures
  • No chemical reaction occurs on the electrodes
    • Much slower ageing and degradation compared to batteries
    • Low heating levels
  • Meets environmental standards
  • Improved safety
    • Supercapacitors do not explode even if overcharged

Disadvantages:

  • Low energy density
    • Typically holds one-fifth to one-tenth the energy of an electrochemical battery
  • Low voltage
    • Serial connections are needed to obtain higher voltages
  • Linear discharge
    • Unability to use the full energy spectrum and depending on the application, not all energy is available
  • Higher self-discharge than that of an electrochemical battery

Battery-free devices are one of the emerging applications for supercapacitors. They consist of low power energy harvesters that supply power, while the supercapacitors store the energy and provide the high current pulses. Energy harvesting combined with energy storage can achieve extended life span, and eliminate the need for battery replacements in WSN (Wireless Sensor Networks).

What is the difference between aqueous and organic supercapacitors?

In most commercially available supercapacitors, the electrolyte is either aqueous or organic. Aqueous electrolytes are RoHS and REACH compliant (mostly sulfuric acid or KOH based), offer low internal resistance, but limit the voltage to roughly one volt per cell, whereas organic electrolytes are generally based on acetonitrile or propylene carbonate that allow higher voltage per cell, with higher internal resistance.  Salts, for example ammonium salts, are added to the electrolyte to provide the ions.

Aqueous

Organic

Voltage per cell

Maximum = 1V

Maximum = 2.7V, with limited range flexibility

Manufacture

Simple

Difficult

Cost

Low price

High price

Balancing circuit

Usually not required

Required

Leakage current

Quick stabilization

Lengthy stabilization required. Balancing circtui adds additional leakage current, which may be higher than the SC itself.

Environmentally friendly

Green product

Not a green product

 

 

 

In addition, the organic supercapacitors have higher specific resistance (higher ESR per capacitance) compared to aqueous ones, are water sensitive and require dry conditions during production (trace quantities of of water in the electrolyte can degrade the performance). The aqueous supercapacitors have higher ion mobility with higher conductance, which leads to faster charge/discharge.

Why should you use pulse supercapacitors based on aqueous electrolytes:

  • Product is friendlier to the environment
  • Aqueous electrolyte enables the building of products for 0.7, 1.4, 2.1, 2.8, 3.5 Volts and up, with greater voltage flexibility (compared to increments of 2.3 or 2.7 Volts of organic based supercapacitors)
  • Final leakage current stability value is obtained in less than 12 hours (for small capacitors in less than one hour) instead of the 72 hours needed for organic electrolytes
  • No balancing resistor requirement, which decreases leakage current
  • Improved safety due to non-toxic materials used (acetonitrile is used in organic supercapacitors, which is harmful and flammable)

Why choose Cellergy Super capacitors?

In addition to the benefits experienced when using supercapacitors, by choosing Cellergy Super -capacitors, additional benefits are enjoyed:

  • Cellergy’s efficient automated manufacturing process lowers the cost of each supercapacitor, making using supercapacitors affordable for many applications
  • Cellergy Super capacitor’s ESR performs well at low temperature down to - 40C
  • Better uniformity of products compared to competitors, due to the automatic manufacturing process
  • Environmentally friendly product (RoHS & REACH certified)
  • Smallest SC available commercially in the world
  • Balancing resistors are not required in most instances
  • No need to worry about polarity
  • Shape and dimension flexibility
  • Wide range of voltages, from 1.4V up to 18V
  • No dry or clean room needed during production
  • No need for derating procedures. What you see in the specification is what you get in reality
 
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