A recent patent claims that adding a magnetic field to an electric double-layer capacitor increases its performance. How would this work?
There is an extensive history of claims for magnetic fields having special powers, and a great many devices or gadgets have been sold on that basis, usually for very specific purposes. Some claim to remove impurities from water by being placed around a pipe. Others wrapped around a car’s gas line allegedly increase its gas mileage. Some patents describe adding a magnetic field to improve the performance of batteries. It seems logical on this basis that adding magnetic fields to double-layer capacitors might be a means to improve their performance.
How this works is in many cases, however, a mystery. It is important to keep in mind that a charge is not influenced by a magnetic field unless it is moving. During charging and discharging a magnetic field can indeed have some influence on the motion of charges. But a second and perhaps more basic consideration is that adding a magnet to a device means adding mass and volume. Any improvement at all that might occur must produce benefits either equal to or greater than the combined costs of the extra mass and volume that adding the magnet to the system entails.
Despite the many claims made, to my knowledge no products on the market now or in the past have produced any improvement in the performance of electrochemical energy storage devices through the use of permanent magnets.
Fuel cell technology is sometimes used in the same applications that use electrochemical capacitors. How do these two technologies differ?
A fuel cell is a device that generates electricity. Fuel cells burn fuel, hydrogen for instance, and by oxidizing the fuel create electricity. So long as there is fuel, they continue to do so. Working best at a given rate, they generally provide a constant output. They can also be quite efficient in the conversion process when compared to other devices.
In contrast, an electrochemical capacitor is a storage device. It does not generate its own energy but is rather charged and discharged. This is a process that can be very highly reversible, such that the efficiency of the charge/discharge process can be quite high. A capacitor will deliver power until its charge is depleted. The major difference between the two technologies is simply that one generates electricity and the other stores it.
There are other differences as well with regard to applications where either of these may be used to best advantage. Fuel cells, for one, cannot capture regenerative energy, running backwards, as it were, to store energy generated by a stopping event in a hybrid vehicle. A capacitor, however, can both deliver and absorb energy, potentially quite efficient at capturing regenerative energy from stopping events.
A further difference to note is that a fuel cell cannot generally “follow” a load. That is, if the power requirement is increased in a step fashion for a transient, generally the fuel cell is unable to follow that transient and deliver energy to meet peak power requirements. An electrochemical capacitor, on the other hand, quite readily follows transience in the load and can both deliver many times the average power for short times and capture energy during short transients. The capacitor can load follow, whereas the fuel cell cannot. The response times of the two technologies are also very obviously different.
One more point of contrast is that fuels cell need maintenance, to have their fuel replenished. Electrochemical capacitors, however, use no fuels and are essentially maintenance free, being charged and discharged by their two electrical terminals. Maintenance is generally neither required nor, given their physical design, likely not even possible to perform.
One last item of interest is that in a given application for a particular purpose a fuel cell and an electrochemical capacitor will often be combined. One example would be a fuel-cell-powered forklift. The power required by the forklift is not constant but increases and decreases as loads are lifted or lowered and the forklift accelerates and decelerates, turns, and so on. The design of such a power system will have the fuel cell tasked with providing the average power required and the capacitor tasked both with providing the transients and absorbing regenerative energy to increase system efficiency. In such a combination both technologies complement each other, the fuel cell with its efficiency in generating power and the capacitor with its ability to follow the load and absorb regenerative energy.
It is well known that electrochemical capacitors cannot be used to filter rectified 60 Hz power. How is it, then, that they can be used to provide even higher frequency operation, as in a GSM phone operated with 217 Hz pulse modulation?
Electrochemical capacitors have a range of response times, on the order of a second in the most popular devices. That fully charging or discharging one will take as much as a second makes it impractical to use an electrochemical capacitor to remove ripple voltage on a DC bus that is generated by rectified 60 Hz AC current. These capacitors simply do not have sufficient response time for that kind of filtering. One used for that purpose will merely dissipate energy by becoming hot but will do very little, if any, filtering.
Electrochemical capacitors specifically designed for use in wireless transmission applications do generally have a response time on the order of 10-30 ms, still very long in consideration of the 217 Hz pulse modulation mentioned. Such capacitors are nonetheless very capable of being charged over a period of ≈4.5 ms and discharged during the 0.5 ms pulse, essentially DC charging and DC discharging. But in this application the capacitor is actually delivering power rather than performing a filtering operation. Capacitors have in fact been designed to fit precisely these needs in pulse modulation communications.
In a similar application now becoming popular, LEDs are used for the flash feature on both digital and cell phone cameras. This requires considerably less total energy than earlier flashlamps and can be driven by electrochemical capacitors designed to provide the pulse energy to power the LEDs. This is a short pulse application in an emerging market that electrochemical capacitors are particularly well suited for. They store considerably more energy than conventional capacitors and have a response time to deliver power as required.
Can electrochemical capacitor technology be used to enhance “green” energy?
But of course. “Green” energy is by and large energy that was not created by the burning of fossil fuels. It should involve little if any carbon emissions and, consequently, little if any pollution. Examples of green energy include geothermal, wind, solar, hydroelectric, and sometimes even nuclear power. The greenness of every one of these, particularly those like wind and solar that involves renewable energy, can be enhanced tremendously by energy storage. Solar, for example, generates only during the day and even then quite variably depending upon rain and cloud cover. A storage system can make originally solar-generated power available at night or on days when the sun does not shine. Likewise for wind, which can be strongest at those times when demand is lowest. Storage makes it possible for that energy to be available when demand increases.
Electrochemical capacitors can indeed be used in applications like these to increase the green value of energy. They can readily be expected to be a vital constituent of the shift in energy supply and distribution that appears to be a necessary feature of our future. The characteristic advantages of electrochemical capacitors in regard to their high cycle life, ability to operate over a wide range of temperatures, and near absence of any maintenance requirements makes them quite attractive specifically for applications like these.
Asymmetric electrochemical capacitors claim to offer more cycle life than a battery. How is this possible when one of the electrodes in such capacitors is the same electrode used in a battery?
The name “asymmetric” when applied to an electrochemical capacitor came about because of the asymmetry in capacity between the two electrodes in the device. The asymmetric electrochemical capacitor is a hybrid device constructed with one electrode from a battery and the other from an electrochemical capacitor. As a design it is quite interesting, although not without its challenges. If designed improperly it is capable of rolling the low cycle life of the battery and the low energy density of the capacitor all into one very undesirable device. But when properly designed, it can combine the high cycle life of the capacitor and the high energy density of the battery with great effectiveness. The asymmetry in capacity is what allows the device to have a high cycle life and contributes to the higher energy density that can be obtained than from a capacitor in its standard symmetric form. Because of the oversized battery electrode, an asymmetric capacitor can have reasonably good power performance, in fact generally much better than a battery that would use that same battery electrode. Owing to the fact that the current density on that electrode is generally much less when used in a capacitor than when used in a battery, it is not generally rate limiting for the device.
One interesting feature of this asymmetry in the electrodes is the much larger capacity of the battery electrode compared to that of the capacitor electrode. Thus, when the device is discharged the battery electrode is only partially discharged. For example, if there is a 3-1 capacity ratio between the battery electrode and the capacitor electrode, discharging the device leaves the battery electrode remaining discharged only 30%, even though the capacitor electrode has been fully discharged. This allows for a much higher cycle life than could be obtained from a battery with the same electrode.
Also very notable about asymmetric capacitors is their generally lower cost. The carbon for the electrodes is one of the most expensive ingredients in an electrochemical capacitor. But in an asymmetric capacitor only one of the electrodes is carbon, and thus only one-half the amount of carbon is necessary. And since the battery electrode in the asymmetric electrochemical capacitor remains essentially at a constant potential at all times, the capacitor electrode has twice the value of the capacitance, i.e., its voltage change is one-half the amount of a device with two carbon electrodes. A device with one-half the amount of carbon and twice the capacitance is in fact one that “works” the carbon four-times harder than a symmetric device can, making it a lower cost energy storage device both in fact and in principle.
Every improvement of this sort comes, however, at a price. An asymmetric electrochemical capacitor will generally not have as short a response time as its symmetric counterpart. Some commercial units have response times on the order even of several seconds, very much longer than the one second of the most popular symmetric devices. Cycle life will also be shorter. The most likely wearout mechanism for an asymmetric capacitor will be associated with the battery electrode, despite the fact that it will have been subjected to only shallow discharges. Electron transfer does occur, along with volume change during operation, and both will cause the battery electrode to fail earlier than the activated carbon electrode, on which charge is stored only physically. Despite the longer time response, the lower power, and the shorter cycle life, however, for some applications the lower cost and higher energy density of the asymmetric capacitors will make them a better choice than the symmetrics.
But if, as you just claimed, an asymmetric capacitor gets its improved performance by shallow discharges on the battery electrode, why can’t I just have shallow discharges on a battery and achieve the same high cycle life?
To understand this, it is best to make a concrete comparison of a lead acid battery to a lead oxide/ activated carbon asymmetric capacitor. The lead acid battery has ≈25 Wh/kg of specific energy. Discharging it to 30% depth corresponds to ≈7.5 Wh/kg. That 30% depth of discharge should, of course, offer much longer cycle life than the standard 80% depth of discharge, although by no means as long as would be the case with an asymmetric capacitor with a 3-1 capacity ratio. More importantly, the battery cannot compete with regard to specific energy: asymmetric capacitors, using lead oxide positive electrodes and activated carbon negative electrodes, have been reported in the literature to offer 12-19 Wh/kg, depending on design. This is higher than the 7.5 Wh/kg at 30% depth of discharge on the standard lead acid battery. In response to the question, therefore, whether the cycle life of a lead acid battery can be increased by decreasing its depth of discharge, the answer is yes, but not in any way that makes it competitive with the asymmetric electrochemical capacitor. One other advantage of the asymmetric capacitor over a battery operated with only shallow discharges has to do with efficiency, particularly in charging. While batteries are generally limited in their charge rate, electrochemical capacitors can be charged at much higher rates, response times typically measured in several seconds.
Although we could get higher cycle life from batteries by shallow depths of discharge, they would still not be competitive with electrochemical storage devices of the asymmetric design.
Exactly what are the performance differences between an electrochemical capacitor and a standard aluminum electrolytic capacitor?
Comparing electrolytic and electrochemical capacitors, the most obvious difference between the two is the very high capacitance of electrochemical capacitors, a capacitance often measured in units simply unheard for electrolytic capacitors. A second, and contrasting difference, is that electrochemical capacitors all have very low voltage, the highest-rated cell being at only 2.7 V. Electrolytic capacitors, on the other hand, may have much lower capacitance but operate at voltages ranging from only a few volts to more than six hundred. Stark differences in capacitance and voltage are what stand out immediately between the two technologies.
Furthering this comparison is helped by looking more deeply at the published specifications for both. Specifications for electrolytic capacitors will often include a working voltage, a maximum ripple current, an operating temperature range, a measure of capacitance (usually set at 120 Hz), and, quite often, an equivalent series resistance (ESR). In some cases a dissipation factor will appear, listed at, again, 120 Hz. Specifications for electrochemical capacitors will be markedly different, usually including a capacitance, an ESR value, a temperature range, sometimes the cycle life, and perhaps a leakage current, but nothing at all about either ripple current or dissipation factor.
Looking beyond specifications to the operational conditions of the two technologies, a still more striking difference appears in that while some electrolytic capacitors are able to operate effectively at 125 C, the maximum rated temperature for electrochemical capacitors will be at ~70 C. In regard to the low-temperature end, both technologies operate well at very low temperatures, some devices of course better than others, but all generally down to ~40 C.
Understanding how electrolytic and electrochemical capacitors operate helps to highlight differences and similarities between the two technologies. Both physically store charge, and both have relatively high capacitance compared to that obtainable on a planer surface. The electrochemical capacitor electrode gain is generally much higher than that of an electrolytic capacitor. This really is what explains why their frequency responses are so different, why electrochemical capacitor performance is so “sluggish” compared to that of an electrolytic capacitor. Added to this are energy density differences that arise directly from the thickness of the dielectric and its surface area. The electrochemical capacitor provides very high capacitance, generally 100-times or even higher capacitance per unit volume than available from electrolytic capacitors of comparable voltage.
For most electrochemical capacitor devices, the logarithm of the leakage current is proportional to the applied voltage. There is thus a limit on the voltage at which an electrochemical capacitor can be operated. This will generally be well below the breakdown potential of the electrolyte, due to the leakage current limitations. This can be quite a practical matter, in that electron transfer often results in gas generation, a potentially life-limiting event in a sealed device. Accordingly, electrochemical capacitors will generally have some published upper limit on their operating voltage, usually derived from life considerations. But because of their solid aluminum oxide dielectric, electrolytic capacitors generally do not have leakage current issues. So the temperature limitations that differentiate the two technologies can be explained simply by breakdown differences between liquid and solid dielectrics.
Electrolytic capacitors come in a substantial range of different designs. Some are designed for high volume efficiency like in photo-flash applications, while others designed to effectively shed heat are directed specifically at high ripple current filtering applications. The same considerable range of optimizations made in electrolytic capacitors has likewise been developed for electrochemical capacitors. Some devices have been optimized for very low-rate applications and so have very minimal leakage current. Others have been optimized for high cycle efficiency, with correspondingly lower energy density, not unlike the high ripple current electrolytic capacitors. Such a range of optimizations makes it necessary to look as carefully as possible not only at the specifications for each product but also at how it is constructed. Devices intended to handle high ripple current will generally have a form factor allowing heat to be dissipated much more efficiently. Electrochemical capacitors may sometimes be structured with terminals on each end while others have both terminals on only one end, each configuration optimized for different uses.