Studies on Alkaline Iron Electrodes for Nickel-Iron Accumulators
Sundar Rajan, A
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A battery is a companion whose interests are world-wide and whose society has never-ending interests. Batteries have applications in cars to space and there is ever growing addiction to batteries. A battery consists of two electrodes, an anode and a cathode, and an electrolyte through which electrically charged particles but not electrons or reactants can move. Two chemical reactions take place at the same time. The reaction taking place at the anode is an oxidation reaction which results in generation of electrons while the chemical reaction taking place at the cathode is a reduction reaction which results in the depletion of electrons. Accordingly, the anode and cathode of the battery are also referred to as negative and positive plates. When the battery is connected to an external circuit, the excess electrons from the anode flow through the circuit and back to the cathode. As the electrons move through the circuit, they lose energy. This energy may be used to create heat or light as in an electrical heater or light bulb, or to do work as in a motor. The flow of electrons results in a current and by convention the direction of the current is opposite to the direction of flow of electrons. The energy which the electrons lose as they move through the circuit is called voltage. The product of the current and the voltage is power delivered to the circuit. When a battery delivers electric current to an external load, certain active materials in the battery are converted into other materials at lower energy states and the battery is eventually discharged. During recharge, a storage battery behaves like an electrolytic cell where the active electrode materials are retrieved. It is desirable that the energy delivered by a battery during discharge should be as high as possible. The energy output of a battery is dependent on the amount of active material present in the battery. Since the weight and volume of the battery are at premium for most of the applications, it is the energy density which has to be maximized. Engineers refer to the quantity of electricity stored per kilogram of the battery as the energy density; the speed of delivery or rate of discharge is called power density. For many applications, such as traction and automotive, it is also necessary to have a high power density. However, the energy density tends to decrease with an increase in power density because at high rates of discharge, a part of the energy is irreversibly lost as heat in the system. For an efficient delivery of charge from a battery, it is desirable that, the energy density be maximized at optimum required power. Between periods of use, a loss in the available energy of the battery occurs partly due to a leakage of charge between the electrodes and partly due to consumption of charge at the electrodes by the parasitic reactions. This is commonly referred to as self-discharge. This results in a decrease in both the effectiveness of the battery as a source of energy and also its reliability for a given application during storage. Structural integrity of the battery is another important characteristic since this confers immunity from mechanical stresses such as vibrations and shocks to which batteries are often subjected in practice. In short, a maximum energy at optimum power density, minimum internal resistance, maximum charge retention, mechanical strength and long cycle-life are the desirable characteristics of a battery.