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EPRI Proprietary Licensed Material capacitor than for a typical lead-acid battery design since they have minimal chemical reactions for charge storage. Capacitors generally can be charged at any rate provided overheating does not occur. This means that higher power chargers can be effectively used for capacitors since they can be charged in seconds to minutes, not hours. Similarly, their discharge rate can be high and is only limited by the series resistance of the capacitor. However, high-rate charge and discharge, particularly with cycling, can lead to internal heating of the capacitor, which without dissipation, can lead to overtemperature conditions and system failure as described previously. Shorting an electrochemical capacitor generally does not cause damage provided maximum temperatures are not exceeded. Type III and IV capacitors generally cannot be left in a shorted state without damage. Also they have a minimum operating voltage before damage may occur. Health, Safety, and Environmental Issues Safety issues can be grouped into several categories. One relates to electrical, a second to chemical, and a third to fire and explosion hazards. Electrical hazards are similar to those of batteries, not any better and not any worse. Hazards from chemical burns and chemical exposures can be similar to some batteries. Fire hazard is essentially nonexistent for type I and III products, which have aqueous electrolyte. For type II capacitors, fire hazard should be similar to some organic electrolyte batteries. An unknown safety related issue arises because acetonitrile is contained in the electrolyte of some large type II capacitors (see discussion under Chemical Hazards about acetonitrile). This situation has not been fully evaluated for potential problems it may create in larger scale utility or automotive applications. To consider these issues, it is helpful to identify the exact materials used in each type of capacitor. Large type I capacitors use potassium hydroxide electrolyte, carbon electrodes, and generally nickel or steel current collectors or conductive polymer bipolar plates. Packages are generally steel or epoxy. The Elit and the ECOND companies make capacitors using this construction. Type II electrochemical capacitors use carbon electrodes, paper or polymer separators, aluminum current collectors, and usually an acetonitrile solvent containing an ammonium salt for the electrolyte. Manufacturers of large type II capacitors include Maxwell, Panasonic, NESS, and EPCOS. Type III electrochemical capacitors use nickel-oxyhydroxide positive electrodes, carbon negative electrodes, potassium hydroxide electrolyte, polyethylene case, polymer separator, and module packages generally of steel or a polymer. ESMA is the manufacturer of commercial products of this type. Type IV devices are under development. They use carbon for one electrode and various types of battery electrodes for the second electrode. Electrolytes typically are various salt-containing organic solvents including acetonitrile-based solutions in some cases. Electrical Hazards Series-strings of the electrochemical capacitor cells often have voltages at lethal levels. These systems are similar to any voltage source with respect to electrical operating safety. Electrochemical capacitor systems are capable of delivering very high currents, higher than comparable lead-acid battery systems for instance, which can cause severe Electrochemical Capacitors 20
EPRI Proprietary Licensed Material electrical burns from inadvertent short circuit. Safe operation procedures are exactly like those for battery systems of the same voltage and capacity. Chemical Hazards Aqueous electrolyte type electrochemical capacitors contain potassium hydroxide solutions at approximately 30-wt % concentration. This is similar to the electrolyte used in nickel metal hydride and nickel cadmium batteries, and in primary alkali cells. It is a common electrolyte, but it can cause chemical burns if contacted to bare skin as well as eye injuries. Safe operating procedures are similar to those for battery systems with the same electrolyte. Some of the large type II capacitors contain acetonitrile solvent in their electrolyte. The synonym for the chemical acetonitrile is methyl cyanide. This chemical can create severe health problems from exposure due to respiration, ingestion, or skin contact. The amount of acetonitrile used in the electrolyte varies. The material specification data sheets (MSDS) will state percentages. Some type IV products under development also are reported to contain acetonitrile solvent. Fire and Explosion Hazards Whenever there is a concentrated quantity of stored energy, the possibility always exists of creating high temperatures that can lead to combustion. Type I and III products generally do not have fire hazard problems because they use an aqueous electrolyte. Type II products, with organic electrolytes may present a potential fire hazard problem. For example acetonitrile solvent is highly volatile and has flammability like kerosene and depending on the application may be classified as a fire hazard. All commercial electrochemical capacitors should be designed so that they are safe and will not explode under any operating or use condition. Type I devices having aqueous electrolyte will become hot and vent steam under extreme conditions, but they should not explode. Type II products usually have a hermetic package. If they have a functioning safety pressure release valve, then they should vent before package rupturing. Type III products are expected to use water-based electrolytes and to be packaged in plastic containers with a resealable pressure release valve. Thus they present little hazard from explosion. Type IV products are presently in the research and development stage so it is not possible to comment on their safety. The issues of fire and explosion will be based on product designs and materials, which are not in their final form. Disposal and/or Recycling There are presently no recycling programs for electrochemical capacitors. There is no motivation to recycle some symmetric capacitors because they contain little high-value material. Proper disposal may be an issue for type II products containing acetonitrile because this solvent is classified as a toxic material for waste reporting purposes. Type III products contain high value and reclaimable nickel, very much like the nickel used in nickel metal hydride and nickel cadmium batteries. Nickel current collectors are used in some type I products. There are well-established programs for recycling these nickelcontaining batteries. It is possible that recycling of the battery-like electrode and nickel collectors could be accommodated into these programs, once such capacitor products come into general use. The carbon electrodes and aqueous electrolyte in these capacitors present no specified disposal issues. Electrochemical Capacitors 21
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