Ultracapacitors - Review of Literature

Currently, ultracapacitors find uses in certain specialized applications that require quick bursts of power. These include high-end car stereos, solar tiles, electric-hydraulic pump in mechanical braking system of hybrid cars, UPS (Uninterrupted Power Supply) systems etc. At present, commercially available ultracapacitors fall short in energy density compared to the battery counterparts, which limits its use as alternate energy storage device in electric/hybrid cars. However, it is only a matter of time before the energy density will improve and match that of batteries. At that point, ultracapacitors will take over batteries with its many advantages like, longer life, low maintenance, and lesser environmental hazards (batteries consists of heavy metals like lead, nickel, cadmium, and mercury).

Batteries, the other independent variable, when compared to capacitors have high energy densities, but they offer very low power output. This makes them popular and convenient with small electronic devices, but their use for heavy equipments and automobiles are not satisfactory. Batteries consist of two electrodes that are submerged in an ion-conducting medium (electrolyte) (Cieslak, 2007, para.1). They create electricity by using an electro-chemical reaction. The reaction requires the transfer of electrons between the two electrodes. The positive electrode, the cathode, gains electrons, and the negative electrode, the anode, loses electrons. The electrolyte material is usually a liquid paste, but some solid-electrolyte batteries use a conductive polymer. Batteries are often referred to as cells, but really a battery is made up of energy cells. A single battery, depending upon its size may have many energy cells, but each cell in a battery has a given voltage. For example, an alkaline battery cell holds about 1.5 volts. Therefore, a 9-volt alkaline battery would have 6 cells.

After the invention of the first electrochemical battery by Alessandro Volta, batteries fell into two main categories: primary and secondary. Primary batteries, once used, are hard to recharge and have to be disposed. On the other hand, secondary batteries are rechargeable and can be used many times. Zinc, lithium, and alkaline are good primary batteries, and probably are the most commonly used. Zinc batteries, are cheap, widely available, and have great electro-chemical properties. A zinc battery cell provides 1.5 volts. Alkaline batteries are comparatively expensive, but function better in low-temperatures, and have better power output and shelf life. They have similar cell voltages to zinc batteries. Lithium batteries, the most expensive, offer a high voltage (2.6-3.6 Volts), function better, and have a longer shelf life than zinc or alkaline batteries. Secondary batteries, have lower energy densities and lower charge retention than primary batteries, but have the attractive feature of being rechargeable. The lead-acid battery, most commonly used with cars, has a 2-volt cell. Nickel-cadmium, another type of rechargeable battery, has a 1.2-volt cell and is used in many household appliances. Lithium-ion, has the highest voltage, and produces the most power. Unfortunately, they can overheat and even burst if not handled properly.