Their metal electrodes are covered in very porous activated carbon traditionally made from coconut shells but increasingly made from aerogel carbon, other nano-carbon or even graphene or carbon nanotubes
. In between these electrodes is a porous separator to keep the electrodes apart when wound into a spiral. This is soaked in an electrolyte, some advanced forms of supercapacitor having a solid electrolyte. The sister product is the Asymmetric Electrochemical Double Layer Capacitors AEDLC better known as supercabatteries
. These have one battery
-like electrode, so some of the good features of batteries are exhibited and the capacitance
is more because there are no longer two double layers in series. The simplicity of these sandwich constructions belies their increasingly massive impact on most forms of electronics and electrical engineering.
Where supercapacitors fit in the marketplace
They replace batteries in uninterruptible power supplies right up to trucks. For example up to three lead-acid batteries in a truck are replaced by identical-looking supercapacitors to give superb cold starting instead of the all-too-familiar no starting at all on a cold day.
They make the bus doors work when the power goes down: they make the regenerative brakes work when a hybrid car loses its electrics. In the USA, Russia and China, there are buses with no traction batteries, all the work being done by supercapacitors.
has developed a pickup truck with a supercapacitor replacing the battery and something similar has occurred in some missiles, toys and power tools. Trials in Australia showed supercapacitors to be superior to lead-acid batteries in wind turbines. All that was achieved without the energy density of supercapacitors approaching that of lead-acid batteries.
It is now obvious that supercapacitors will be one of the very big nails in the coffin of lead acid batteries over the next few years, yet that is only part of the story as their parameters are being improved faster than the competition. Suppliers such as Elbit Systems
, Graphene Energy, Nanotech Instruments and Skeleton Technologies
now claim to meet or exceed the energy density of lead-acid batteries with their supercapacitors and supercabatteries, some of which can theoretically match lithium-ion energy density one day while retaining vastly superior characteristics in other respects.
Yet supercapacitors are an aspect of electronics and electrical engineering that is curiously neglected or talked down by the press, investors, potential suppliers and others living in the past despite a multi-billion dollar market rapidly emerging. For example, for land, water and airborne electric vehicles, there are about 200 serious traction motor manufacturers and 110 serious traction battery suppliers compared to just a few supercapacitor manufacturers making the types needed. In all, there are no more than 66 significant supercapacitor manufacturers in the world (we profile them) with most concentrating on the easier small ones for consumer electronics such as power backup that are not the largest part of the value market.
Like most other components, supercapacitors have a useful lifetime that decreases with increasing operating temperature, humidity, applied voltage, current and tougher duty cycle but, in this case, often only minimally.
Supercapacitor advantages over rechargeable batteries include:
- Low impedance (ESR) enhances pulse current and load handling when in parallel connection with a battery
- Very high cycle count - discharge takes milliseconds up to several minutes and can be charged in seconds to minutes
- Reduces voltage drop compared to battery-operated device with no supercapacitor
- Hecto- and kilofarad range capacitors are now available and multifarad versions are commonplace
- High efficiency at 97-98% and the DC-DC round-trip efficiency is 80%-95% in most applications
- In a hybrid electric vehicle, round trip efficiency is about 10% more than a battery
- Useful across fuel cells in vehicles without interfacing circuitry
- Functions well in a very wide temperature range, typically from -40C to +70C but can be -50C to +85C and there are special versions going up to 125C
- Capacity increases as temperature decreases, unlike a battery
- Little heat released during charge and discharge
Physical features and life
- Long calendar and cycle life time with high reliability which reduces maintenance costs
- Little degradation over hundreds of thousands of cycles and lasts up to 20 million cycles
- They lose no more than 20% of their storage capacity after 10 years, with a lifetime estimated to be 20 years or more
- Not subject to the wear and aging experienced by electro-chemical batteries
- Not affected by deep discharges unlike batteries
Safety, environmental and handling
- Improved safety over batteries and electrolytic capacitors - no danger of overcharging or exploding
- Can be brought down to zero volts for safe transport and maintenance
- They do not release any hazardous substances that can damage the environment
- No disposable parts during the whole operating life of the device
- At end of life, there are no hazardous materials for disposal unlike many batteries
- Meets environmental standards so no difficult disposal or recycling
- Not banned or restricted for air travel and use in aircraft unlike lithium-ion batteries
- Fit and forget - useful in locations where maintenance and replacement are impracticable
- Prices have dropped faster than those of most capacitors and batteries - a 3000F supercapacitor that cost $5000 in 2000 only cost $50 in 2011
- Low cost per cycle
- Low cost per farad (unit of capacitance)
- Cost-effective energy storage over (long) life
- Provides lowest cost-over-life in an increasing number of applications
There are disadvantages shown below, but supercapacitors and supercabatteries are being improved faster than either batteries or conventional capacitors, so there is a tendency for publicity to be out of date and unduly negative.
Supercapacitor disadvantages over rechargeable batteries include:
- Low energy density - typically holds 1/5th to 1/10th the energy of an electrochemical battery
- Linear discharge - inability to use the full energy spectrum and, depending on the application, not all energy is available (though this is also true of some batteries)
- As with batteries, cells have low voltages so serial connections are needed to obtain higher voltages and voltage balancing is required for more than three capacitors
- Self-discharge often higher than a battery (though already improved from one day to many months)
- When fully charged it takes some time to settle down to a low current
- Initial capacitance fade - care needed to ascertain which value is claimed
- Loss of capacitance at high frequency limits use for filtering in electronic circuits
- The voltage varies with the energy stored - to effectively store and recover energy requires sophisticated electronic control and switching equipment
- Has the highest dielectric absorption of all types of capacitors
- Upper temperature of use is usually 70C or less and rarely above 85C for the basic device
- Storage above 80C destroys the device unless specially constructed
Physical features and life
- Most of them contain a liquid electrolyte making downsizing for eg. MEMS very challenging
Safety, environmental and handling
- Care needed to avoid inadvertent fast discharge
- Expensive per unit of energy stored
- High cost per watt power delivery
- High up front cost
Now watch most of those limitations disappear like snow in summer. Many more types of battery will be replaced or at least downsized as the supercapacitor and supercabattery take over more of the work.