The benefits of using supercapacitors alongside traction batteries— lead-acid and lithium-ion— have been known for a long while. But its lack of use in electric vehicles is preventing the technology from realising its true potential. Frank Lev, whose company Tavrima Canada aims to commercialise advanced super-capacitor technology, explores how the technology can help OEMs win the electric vehicle war.
An unwinnable war
As I write this article on 25 February 2022, it’s the third day of the blitz that Russia’s president Vladimir Putin unleashed against Ukraine. With anguish and resentment, I follow the news about the bombing of the city of my youth. As a Canadian, I want to share my feelings and hopes for a better future, and to thank those giving my Ukrainian brethren support.
There is still a sense of novelty associated with supercapacitors, despite their commercial availability since the early nineties, which I believe is explained by their ambiguous positioning between capacitors and batteries. Indeed, these devices can be charged electrostatically as capacitors and can outperform some batteries, energy-wise. However, as almost 30 years of use in the field shows, this technology cannot replace batteries or capacitors despite their impressive capabilities.
Capacitors trace their ancestry to the good old Leyden jar, the first embodiment of an electrostatic storage device. Charged by an electrostatic generator, these condensers could produce an impressive blue spark akin to a lightning bolt.
The first practical application of the Leyden jar is described by Jules Verne in his book “Vingt mille lieues sous les mers (Twenty Thousand Leagues Under the Sea)” where a pneumatic gun discharged a small Leyden glass ball carrying a lethal electrical charge.
The principle that electric energy E = 1/2CV2— where C is capacitance in Farads and V is voltage in volts— can be stored in a charged capacitor has been known since 1745. The formula is universally applicable to all capacitors and supercapacitors alike.
Both the capacitor and the supercapacitor performance depend on specific capacitance, Csp (F/g), internal resistance known as equivalent series resistance (ESR), self-discharge, and life cycles. A capacitor (Fig 1) stores electrical charge on its two plates, and its capacity, expressed in Joules, depends on the value of capacitance and voltage. The capacitance value depends upon the surface area of the two conductive plates, the distance between the plates, and the permittivity of the dielectric separating the plates. The capacitance is expressed in fractions of the farad, such as pico-farads. The voltages could be as high as hundreds of volts. Despite high operating voltages, the capacitors have relatively low stored energy limiting their applications to the low energy discharge pulses.
Telling the whole story
In 1957, Becker et al. received a patent for “the energy storage utilising the charge held in the interfacial double layer at a porous carbon material perfused with an aqueous electrolyte”. The double-layer formed on all solid/electrolyte interfaces was electrically charged akin to a two-plate capacitor, albeit with an immensely higher area provided by the carbon material used for its plates.
The capacitance of a cell is proportional to the electrode surface area, and an activated carbon electrode may have a surface area exceeding 2,500 m2/g. The capacitance is also inversely proportional to the distance between the plates, whereby the carbon/electrolyte interface is only 0.3 to 0.5 nm. For this reason, supercapacitors are often referred to as electric double-layer capacitors or EDLCs (Fig 2). The activated carbon is uniquely suitable for the double-layer supercapacitors due to its extremely high specific area and the electrolyte-friendly porous structure.
The first supercapacitors were aqueous electrolyte devices with the cell voltages limited to 1-volt to prevent the decomposition of water due to electrolysis. Further development brought in an organic electrolyte (TEABF4 salt in acetonitrile) that enabled the 3-V cell voltages and became predominant.
Until now, we talked about the so-called symmetric supercapacitors with two similar electrodes and electrostatic ways of holding charge. There are supercapacitors with an asymmetric design whereby one electrode is activated carbon, and the other uses a battery-like active material. These configurations (architectures) have allowed doubling the energy output of such devices. But not without penalties though, as their cyclic life and ESR were traded off in favour of higher energy.
The activated carbons used for the supercapacitor electrodes have undergone steady development to increase the specific area from the initial 1,500 m2/g up to 3,000 m2/g enabling specific capacitance values as high as 200 F/g. Higher capacitance combined with higher voltage resulted in impressive specific capacities, almost in the same ballpark as lead-acid batteries.
The nominal numbers did not tell the whole story as the supercapacitors were more efficient than the batteries at higher than C2 discharges due to their lower internal resistance. Apart from the activated carbon, other materials such as graphene, metal carbide-derived carbon, aerogels and carbon composites had been evaluated in numerous projects as active materials for the supercapacitor electrodes. The activated carbon remains the material of choice for many supercapacitor manufacturers. The preferable precursors for the activated carbons are renewable resources such as coconut shells, bamboo, lignin and biowastes.
Back in the day, when the supercapacitors were only trying to gain traction in the markets, the scientific and manufacturing communities made a considerable effort to break through the market impasse and expedite the commercialisation of supercapacitors worldwide.
Annual Florida seminars launched by the former Duracell executives Dr Shep Wolsky and Dr Nick Marincic were routinely attended by such supercapacitor celebrities as professor Brian Conway from UC Ottawa, Dr Andy Burke from UC Davis, Dr John Miller from JME, Dr CJ Farahmandi from Maxwell Technologies and many others.
These seminars were winter affairs, and the attendees combined business with pleasure, delivering enticing presentations and enjoying the Florida sun. The technology was not mature yet, and further development was vigorously pursued in Japan, South Korea, Germany, France, and Russia. However, the American firm Maxwell’s supercapacitors were ahead of the competition.
Despite some concerns about the safety aspect of the organic electrolyte, the supercapacitors continued their advancement. The flammability issue is still a concern, just as with lithium-ion batteries. However, spontaneous combustion is much less likely with the supercapacitors, as there is no dendritic growth in their carbon-based active materials.
So, given all the constraints, where should the supercapacitors be applied? Generally speaking, in any cyclic application where high power is discharged relatively quickly. In other words, for high-efficiency applications at high C-rates and short RCs— especially where millions of cycles are required with minimal degradation in performance.
Supercapacitors provide high power density and high load currents and are quickly recharged. Their temperature performance is also good. However, we should not forget supercapacitors are voltage-dependent storage systems with a gradual voltage loss. While the batteries provide a near-constant voltage output to a designated depth of discharge, the voltage output of the supercapacitors declines linearly with their charge. Professor Conway, a renowned authority on supercapacitors, said that: “The energy of charging the supercapacitor to a terminal voltage Vb with charge Q is half that needed for charging the battery with the same charge also to Vb from which follows that the supercapacitor energy Ec = ∫ VcdQ = 1/2 Eb = ∫ VbdQ.”
The batteries owe their high energy density to the Faradaic redox reactions, and in this regard, they outperform supercapacitors. Still, their power density and cyclic life are much lower. Table 1 shows the fundamental differences between the batteries and the supercapacitors.
The table unequivocally shows that batteries are better suited for prolonged energy applications, such as electric vehicles, due to their much higher energy density and lower cost per kWh. For example, that is why Tesla electric vehicles run on lithium-ion batteries rather than on supercapacitors.
However, supercapacitors are beyond the competition in people-transporters running on short routes since they can be quickly recharged at the end stations. There are other numerous applications requiring high power charge/discharge cycles for short-term power needs where the long cycle life of the supercapacitors offsets their high cost. Here is a list of some of the most prevalent applications of the supercapacitors:
- Voltage stabilisation in start/stop systems
- Power supply for electrically actuated door locks
- Energy recuperating applications for various lifting devices
- Regenerative braking systems in racing vehicles
- Stiffening of batteries in high power density applications
- Medical devices such as X-ray machines
- Green energy harvesting applications such as windmills and solar batteries
- Consumer electronics
- Military applications such as lasers and rail guns
Table 2 compares relevant data for the several 3000 Farads commercial brands of supercapacitors.
Skeleton supercapacitors demonstrate the highest specific power at 27 kW/kg due to their low ESR. The company produces a patented active material— curved graphene— to achieve the most advanced in its league power and energy at the highest efficiency. Eaton’s cell has a higher specific energy rating and cell voltage at the most affordable price, making it a worthy contender to Skeleton.
Real-life applications require higher system voltages attainable by connecting cells in series strings; such energy storage systems are called modules. The numerous available supercapacitor modules come in rated voltages from 12V to 220V and higher. For example, the Maxwell Technologies 125V Heavy Transportation series (Fig 3) of supercapacitor modules are high-performance energy storage systems for hybrid bus, truck, trolley, light rail, mining, construction and seaport crane applications.
The modules include balancing, monitoring and thermal management to ensure high reliability and long operational life. The low ESR provides high power capability. The modules can be used independently or in conjunction with batteries to give high-efficiency recharge and high power. These modules can outlast and outperform a conventional SLI battery in some applications, compensating for their higher cost.
An excellent example of the supercapacitor application is Lamborghini Silan— an 800 horsepower hybrid supercar. Its drivetrain is equipped with a 48V e-motor motivated by a supercapacitor that is reported to be three times more powerful than a lithium-ion battery of the same weight. The 34 horsepower the supercapacitor produces is used to bridge the gaps in power delivery at the transmission gear changes. The supercapacitor is recharged during braking. A quick calculation shows the energy rating of such a supercapacitor does not exceed 6Wh.
If Skeleton’s supercapacitor were used in this application, it would only weigh 1kg. French company Nawa is building a Racer hybrid motorcycle that uses a supercapacitor to extend its range and increase acceleration. A 9kWh lithium battery gives the bike an about 176 km run, whereas the additional 0.1kWh supercapacitor extends the range to 280 km and shaves a second off the acceleration time.
Assuming the motor peak power is 90 kW, the bike would need only four seconds of the supercapacitor power to get to 100 km/h, thus confirming its supercapacitor’s 0.1kWh energy rating. It also prolongs the battery life.
In April 2000, the American industry/academic consortium, assembled by National Aeronautical Space Association’s Glenn Research Center, designed and built the first hybrid electric transit bus (HETB) to verify the efficiency of the supercapacitors versus batteries in a hybrid electric vehicle, and to evaluate the fuel economy and emission levels. My company, Tavrima Canada, supplied the 200V bipolar supercapacitors for this project.
At the time, the HETB was the largest known vehicle in the world to use the supercapacitors for propulsion. In those pre-lithium-ion battery times, energy storage was a problem for hybrid electric vehicles due to the limited quantity of cycles the lead-acid batteries could endure. The project demonstrated supercapacitors lasted longer than the batteries would have and improved the reliability of the drive train.
We measured the charging and discharging currents in two tests (Fig 4) with supercapacitors and, separately, with batteries. In each trial, the HETB was driven through two cycles at speeds between 0 and 24 km/h.
The test with the supercapacitors had shown a 20% higher output current (power) than the test with the batteries. It confirmed the supercapacitors were effective for both acceleration and deceleration. With supercapacitors, a diesel engine of a hybrid drive could be smaller and more economical. In 2003 commercial HETBs appeared in major California, US cities. Produced by the ISE company of San Diego, these vehicles used Maxwell supercapacitors in combination with various engines.
There were numerous HETBs with Maxwell supercapacitors made in China. And yet, despite all the efforts, automotive OEMs did not extensively use supercapacitors in their production vehicles. What is going to happen when internal combustion engines are phased out? Will supercapacitors be used in electric cars?
There is credible scientific and practical evidence that supercapacitors in parallel with traction batteries can improve vehicle dynamics, efficiency, and mileage. However, the existing electric vehicles use lithium batteries without supercapacitors. Why? The answer to this question may be as follows:
- High supercapacitor cost
- The acceptable power density of the lithium batteries— Tesla Model S accelerates from 0-100 km/h in 2.8 sec on batteries alone
- Supercapacitors add more cost, weight, and complexity
- EV benchmark OEMs such as Tesla do not use supercapacitors, believing they are unnecessary for electric vehicles.
Time to embrace the technology
Supercapacitor enthusiasts hope the automotive industry will embrace supercapacitors. Should that happen, the manufacturing cost would get dramatically reduced, contributing to the proliferation of supercapacitors.
In numerous projects, supercapacitors have demonstrated their viability for conventional engine starting and hybrid electric vehicles, but their acceptance by automotive OEMs was not unanimous. The cost and complexity were the barriers, and the ubiquitous SLI batteries did the job at a much lower price.
A new hope arose in 2019 when Tesla purchased Maxwell Technologies, which raised expectations for supercapacitors to win the EV markets. But Tesla wanted Maxwell’s dry electrode technology for its new 4680 cell design rather than the supercapacitor itself. Elon Musk said that supercapacitors were unnecessary for electric vehicles. On 21 July 2021, Tesla sold Maxwell Technologies.
Without winning the automotive markets, supercapacitors cannot win the war. Although they are not losing the battle, and their industrial demand is steadily growing, they need to find their way into every electric vehicle to win the war.