By Christophe Jehoulet, Saft Advanced Technologies, Bordeaux, France, and Anthony Green, Saft Industrial Battery Group, Paris, France.
The ‘electronic age’ demands high quality power, free from surges and harmonics. Supercapacitors can meet these needs, and have advantages over batteries and other power quality devices.
In the 140 years since Gaston Plante demonstrated the first lead-acid battery there has been nothing yet to rival the rechargeable battery in all its forms – through nickel-cadmium and nickel-metal-hydride to the latest high-tech Lithium-Ion chemistries – as a method of storing electrical energy for later usage. That is what batteries are particularly good at. It’s getting the electrical energy out of a battery fast enough that can sometimes create difficulties.
At relatively low rates of discharge, say around five hours, a battery can be totally discharged and 100 per cent of the stored energy can be obtained. However, power quality applications usually require a short, high power burst – just a few seconds at most – to support a temporary disturbance in the supply voltage. And at this very high discharge rate only a few per cent of the stored capacity is available. In fact, a lead-acid battery is so inefficient in these circumstances that the current available over one second is no better than that available at one minute. So although a battery can perform this type of duty, and regularly does, it requires an electrical capacity, and therefore weight, volume and cost, which is much higher than the electrical output actually required.
Supercapacitors are the power sources for which some applications have been waiting. They combine the storage capabilities of batteries with the discharge characteristics of capacitors
Capacitors exist at the opposite end of the scale. They store power as static electricity rather than in the reversible, faradaic chemical reaction found in a battery. And they can deliver all their stored energy virtually instantaneously – within a few thousandths of a second. But the amount available is actually tiny, just enough to pop a flashbulb.
Now, with the recent emergence of supercapacitors, there seems to be the possibility of an ideal compromise which combines some of the storage capabilities of batteries and some of the power discharge characteristics of capacitors in a device capable of storing useful quantities of electricity which can be discharged very quickly.
To understand exactly why supercapacitors are so special it is useful to consider the classic capacitor, a device for storing electric charge, which actually dates right back to the ‘Leyden Jar’ of the mid-eighteenth century. At its simplest, a capacitor is constructed by taking two metal plates and sandwiching them close together with an insulator (a vacuum, paper or other insulating material) in between them and then connecting one plate to the positive pole of a power source and the other to the negative pole.
The greater the surface area of the plates and the closer that they are brought together then the higher the charge storing capability, or ‘capacitance’ of the capacitor. Capacitance is measured in Farads (F) – a 1 F capacitor charged to 1 V can supply 1 A of current for 1 s. Typical modern capacitors usually feature a rolled construction using a metallized plastic film to store the charge, even so the highest value capacitors available do not even approach 1 F and are actually measured in microFarads, which of course is all that is required for most electronic applications.
The use of capacitors as practical energy storage devices only became possible with the emergence over the past 20 years of supercapacitors, which are sometimes also known as ‘ultracapacitors’ – the terms are interchangeable. In contrast to ordinary capacitors, their capacitance is measured in whole Farads (F), and now even kiloFarads (kF), which means that they can store a million times the electrical charge.
The first type of supercapacitor replaces the flat metal sheets with activated carbon electrodes. The microscopic pores in the carbon (typically just a few nanometres in size) have the effect of providing an enormously increased internal surface area, greater than 1000 m2/g. In order to utilise this additional active surface area it is necessary to use an electrically conductive liquid electrolyte. It is important to remember that the two electrodes are not directly analogous to the two plates of a conventional capacitor. Instead, on each of the electrodes an electrical double layer of charge is formed between the carbon and the electrolyte ions. In effect, you have a complete capacitor at each electrode, which makes the complete device two capacitors connected in series, and a supercapacitor based on this principle is sometimes referred to as an electrochemical double-layer (EDL) capacitor.
A variety of electrolytes can be used in supercapacitors, including water based solutions such as potassium hydroxide or sulphuric acid. However, since water tends to break down into hydrogen and oxygen at voltages over 1 V, these supercapacitors can only be used up to 1 V. But if a water-free organic solvent is utilised for the electrolyte then we can create a supercapacitor useable up to 2.5 V or higher.
The second type of supercapacitor uses transition metal oxides (ruthenium or iridium) as electrode material with an aqueous electrolyte. In that case, the operating mode is not exclusively electrostatic but relies mainly on highly reversible surface faradaic reactions.
A third type of supercapacitor is based on the use of electronic conducting polymers such as polypyrrole, polythiophene or polyaniline. These polymers go from the insulating state to the conducting state by anion doping (p-doping process). The first step corresponds to a faradaic process characterized by a standard potential. Anions can be added in a second step by overdoping without any change in the conducting properties. The amount of injected charge is proportional to the applied voltage (capacitive behaviour).
Considering the technical maturity, the manufacturing processes, the expected performances and costs, activated carbon based supercapacitors are the most interesting and promising devices for industrial applications. This is the approach taken in the design of Saft’s own range of supercapacitors, manufactured in a new facility in Bordeaux, France.
The manufacture of the Saft supercapacitors draws upon many of the production techniques already developed for Lithium-Ion rechargeable batteries, including the use of electrodes manufactured using carbon powder and a classical ink coating process in a spiral wound configuration and housed in an aluminium container.
In order to achieve the target capacitance of 3500 F, two 7 m-long electrodes with an activated carbon loading of 5 mg/cm2 are employed, these are calandered (rolled) down to a thickness of 100 μm and 70 per cent porosity. A high conductivity organic electrolyte is also used.
The reason for the interest in supercapacitors soon becomes apparent from Table 2 which compares their performance with accumulators.
In considering the specific energy/energy density, which relates energy storage capacity in relation to weight (Wh/kg) it is clear that even though the extremely high capacitance of supercapacitors enables them to store a useable amount of energy, this is still only around one twelfth of what can be stored in an unsophisticated lead-acid battery and nearly 30 times less than a Lithium-Ion battery (5 Wh/kg for supercapacitor and 140 Wh/kg for Li-ion). But when considering specific power/power density (in kW/kg), which measures how fast a supercapacitor or battery can deliver its energy in relation to its weight and volume, then the supercapacitor can vastly outperform virtually any battery.
Another important advantage of supercapacitors is their longevity and low temperature dependence. Since a battery depends on a chemical reaction between its electrolyte and electrodes each charge-discharge cycle will cause both the active materials and the electrolyte to deteriorate, which means that its useful lifetime, when the full battery capacity is employed, is normally measured in thousands of cycles (although Ni-MH or Li-ion batteries can achieve several hundred thousand cycles when used at a depth of discharge of just a few per cent of capacity). During their lifetime, some batteries, such as vented lead-acid and nickel-cadmium, will also require routine maintenance to top up the electrolyte.
In contrast, a carbon type supercapacitor is in theory a pure electrostatic device which stores energy with no physical changes taking place, which means that we might expect it to last forever! In practice, in the same way that chemical batteries consume water over time due to electrolysis of the solvent, the same process can occur with the supercapacitor’s organic electrolyte, albeit to a much smaller degree. In addition, over a long period of time there will be some limitations linked to side reactions. Even so, if designed and used correctly, a supercapacitor can be expected to achieve a typical life of several million cycles, with no maintenance required.
Since the rate of the chemical reactions taking place in a battery are temperature dependent then a battery’s performance will deteriorate at extremes of temperature. For example, a lead-acid car battery can be expected to lose approximately one per cent of its capacity and cranking power for every degree drop in temperature. Again, since there are no complicated chemical reactions taking place in a supercapacitor during normal operation and combined with the choice of a highly conductive organic electrolyte, its performance remains completely unaffected over a very wide range of temperatures from +70°C down to -20°C. Below this temperature the conductivity of the organic electrolyte will start to limit the performance of the supercapacitor. But even at -40°C half the power is still available.
Individual supercapacitors can be coupled together to provide a unit with a required voltage in exactly the same way that electrochemical cells are assembled in series to create a battery. But this does have an important effect on the total capacitance of the unit. So if say five 2.5 V supercapacitors are connected in series to create a nominal 12.5 V unit then the capacitance of the assembly actually becomes one fifth of the original capacitance of a single supercapacitor. Power and energy, on the other hand, are additive in the same way that they are with conventional batteries.
A hybrid future
Not surprisingly, supercapacitors found their first application in military projects, such as starting diesel engines in battle tanks or submarines, or replacing batteries on board missiles. But they are now starting to find their way into many civil applications requiring a lightweight, maintenance-free source of energy where the major need is a high peak discharge for less than 30 s. This includes engine starting, switch tripping, power quality and UPS systems where supercapacitors can provide the emergency power for the vital time needed to maintain an electrical supply network until the back-up generator comes on line.
The electrochemical double-layer (EDL) capacitor
Saft believes that one of the potentially most interesting areas of application for supercapacitors is when they are used in combination with other energy storage devices such as batteries or fuel cells. This ‘hybrid’ format enables a designer to effectively enjoy the ‘best of both worlds’ by combining a high energy density power source with the high power density of a supercapacitor.
To see how this might work in practice, imagine a supercapacitor connected in parallel with a battery so that it can meet the power requirements of an expected power surge – such as starting a generator or fire pump. When the power surge is over then the supercapacitor will automatically be recharged by the battery so that it is ready for the next power surge. This ensures that the battery does not have to be over-dimensioned in order to meet a high power demand, while the supercapacitor does not need to store large amounts of energy.
An important area where supercapacitors are expected to make a significant impact is in the realm of the power quality of the electric supply. This has been a factor ever since the conception of electricity, but only over the last two decades, due to the advances in technology, has it become a serious problem. The widespread use of computers in the 1980s and the network revolution in the 1990s brought the potential for more interference and created many new problems. This has led to a requirement for the utilities to produce a ‘clean’ electrical supply free from surges and harmonics.
Sectional view of a supercapacitor
The supercapacitor is easily able to provide the few seconds or milliseconds of discharge required to smooth out these aberrations, but there are of course a number of competing short time power storage systems, most notably flywheels and superconductive electromagnetic energy storage (SMES). However, aside from their ‘fit and forget’ maintenance-free nature, supercapacitors have the advantage of being both simple and compact – a typical flywheel system can store 630 W per litre of space occupied, while supercapacitors can store 2 kW per litre.
Anthony Green and Christophe Jehoulet, “The Non-battery Battery – The Potential Role of Supercapacitors in Standby Power Applications”, BATCON 2002, Florida, USA, April 2002.
B.E. Conway, J. Electrochem. Soc., 138, (1991), 1539.
A.F. Burke, M. Miller and J.T. Guerin, “Recent Test Results for Aqueous and Organic Electrolyte Ultracapacitors”, Sixth International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Florida, USA, Dec. 1996, and references therein.
A. Rudge, J. Davey, I. Raistrick and S. Gottesfeld, “Conducting Polymers as Potential active materials in electrochemical supercapacitors”, Second International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Florida, USA, Dec. 1992, and references therein.