Materials Development Drives New Momentum for Supercaps
If you forgot how useful supercapacitors (aka ultracapacitors and electric double-layer capacitors [EDLCs]) can be let me throw some big numbers at you:
- IDTechEx estimates that the high-power energy storage market is expected to grow almost tenfold to $2 billion a year by 2026, up from about $240 million currently. The research firm estimates that supercapacitors could capture about $800 million to $1 billion of that potential market opportunity.
- Markets and Markets predicts the supercapacitor market will reach $2.18 billion by 2022 at a CAGR of 20.7 % between last year and 2022.
- Research and Markets projects that in China alone the supercapacitor market is roughly RMB 4.96 billion (about US$718 million) and the figure by 2020 will swell to RMB 13.84 billion.(about US$2 billion).
Here’s a bit of background: A supercapacitor stores and releases energy in an electric field by physically separating positive and negative charges (unlike batteries which use a chemical reaction). Supercapacitors typically have higher energy density than other capacitors and higher power density than batteries. A supercapacitor is able to respond almost instantly and can routinely operate for more than 15 years with very little, if any, need for maintenance.
Conventional capacitors consist of two conductive metal plates separated by a dielectric layer. Unlike ceramic capacitors or electrolytic capacitors a typical supercapacitor doesn’t have a dielectric between the positive and negative electrodes. Instead, an electrolyte which has positive ions and negative ions is filled between the two electrodes. The electrostatic capacitance of the supercapacitor is directly proportional to the surface area of its electrodes. One way to achieve high capacitance is to use tiny nano-sized materials for the electrodes such as activated carbon powders. These materials possess a huge specific surface area, up to 2000 m2/g.
With regard to both technology and new applications there is plenty of wind in the sails of supercapacitor developers, as the following notable achievements demonstrate.
The colder it gets, the more challenging it is to start (crank) an engine. The power necessary is dependent on the equivalent series resistance (ESR) of the starter motor and a battery or of its Engine Starter Module.(ESM). For batteries, one of the key standards established for starting is Cold Cranking Amps (CCA), a measure of the battery’s ability to start the engine in cold temperatures. It is defined as the current that a fully charged battery can deliver for 30 seconds at −18°C, maintaining voltage at 7.2 V or higher (for a nominal 12V battery).
Unlike a battery, however, supercapacitors are not sensitive to extremely cold or warm temperatures in the same way batteries are. So supercapacitor-based ESMs can deliver hundreds of thousands of cranks in temperatures down to -40°F. What is more, using supercapacitors for engine starting means you can crank the engine within the first few seconds, so for supercapacitor-based ESM standards something other than CCA will be necessary to evaluate engine cranking performance.
Recently, Skeleton Technologies of Großröhrsdorf, Germany employed ESM technology based on curved graphene. Curved graphene differs significantly from regular activated carbon, which uses organic pre-cursor materials, mostly carbon made from coconut shells. Skeleton uses an inorganic pre-cursor and has patented the synthesis process for the proprietary material. Compared to lead acid batteries, the Skeleton ESM is claimed to offer much lower ESR, thus cutting the total series resistance of the system almost in half. This allows the supercapacitor-based ESM to crank the engine nearly instantaneously. The company’s latest offering, SkelStart Engine Start Module version 2.0, is available in 24V and 12V varieties and is claimed to provide the highest power and energy density on the market. It is rated at 1218 Cold Cranking Amps. Typical CCA readings for a car range from 350 to 600A (more for trucks).
Not all supercapacitors are big. A Rice University group led by Dr. James M. Tour plans to commercialize laser-induced graphene (or LiG), a process in which a computer-controlled laser operating at room temperature burns through a polymer to create flexible, patterned sheets of multilayer graphene. Among the applications for LiG include so-called “microsupercapacitors”, which are miniaturized energy-storage components for on-chip electronics. The microsupercapacitors' capacitance of 934 microfarads per square centimeter and energy density of 3.2 milliwatts per cubic centimeter rival that of commercial lithium thin-film batteries. Dr. Tour has said that these materials can lead to the world's fastest charging supercapacitors, as charging speed can reach 500 V/sec.
To test the viability of supercapacitors in high temperature apps such as either in the automobile or truck engine compartment or with power electronics placed near circuit board hot spots, Yunasko (London, UK) recently completed independent tests of a proprietary high-temperature supercapacitor system with a -25°C to +100°C operating temperature range. Test results confirmed the stable performance of the high-temperature supercapacitors that successfully passed endurance tests at +100°C for 2,000 hours, which normally corresponds to a cycle life as long as a million charge-discharge cycles. Another advantage of the Yunasko high-temperature supercapacitors is said to be their high power output at both +25°C (up to 25 kW/kg) and +100°C (up to 34 kW/kg).
Supercapacitors today are generally made from carbon. They use carbon nanotubes, graphene, activated carbon in all shapes and forms, but its always carbon. That may all be changing. Researchers at MIT developed a supercapacitor that uses no conductive carbon at all. Instead, a class of materials called metal-organic frameworks (MOFs), which are extremely porous, sponge-like structures. For their size these materials have a larger surface area than carbon materials do. That is an essential characteristic for supercapacitors, whose performance depends on their surface area. Called Ni3(hexaiminotriphenylene)2, the new material allowed researchers to create the first electric double-layer supercapacitor without conductive carbon.
The team's findings were reported in the journal Nature Materials, in a paper by Mircea Dincă, an MIT associate professor of chemistry, Yang Shao-Horn, the W.M. Keck Professor of Energy and four colleagues: former MIT post-doctoral student Dennis Sheberla (now at Harvard University), MIT graduate student John Bachman, Joseph Elias and Cheng-Jun Sun of Argonne National Laboratory.
The advantage, according to the researchers, is that the new material can be made under much less harsh conditions than those needed for the carbon-based materials, which require very high temperatures (above 800°C) and strong reagent chemicals for pretreatment.