Cells are DC voltage sources in which chemical energy is converted into electrical energy- a process that is essentially the reversal of electrolysis. The term battery originally described an assembly of several cells, although today it is often used merely as another word for a cell.
A primary cell gradually uses up the chemical fuel it contains and cannot be recharged. The original Daniel cell, for instance. Consists of a plate of copper and another of zinc immersed in an electrolyte of dilute sulfuric of zinc immerse in an electrolyte of dilute sulfuric acid. The zinc slowly dissolves in the acid, forming zinc ions and leaving behind electrons on the zinc plate. Which therefore becomes negatively charged. At the copper late, positively-charged hydrogen ions (present in the acid) are neutralized by gaining electrons form the copper. The hydrogen is liberated as bubbles of hydrogen gas, and the copper is left positively charged.
Other types include the western cadmium cell, important for standardizing other voltage sources, and mercury and silver oxide cells. They all work on the same principle. At one electrode, chemical oxidation takes place and as a result a negative charge builds up. At the other electrode, chemical reduction takes place, which uses up negative charges and leaves the electrode with a net positive charge. An electromotive force (EMF) develops between the electrodes, and if they are connected by a wire a current flows. All such cells have a limited lifetime because the oxidation or reduction process reaches completion. In the simple cell, for example, the reaction must stop when all the zinc has dissolved. The total energy the cell is able to deliver—its capacity-is therefore limited. Standard cells are never used as sources of current; their voltage is “standard” only at low current drain.
The EMF of a cell depends on the chemical composition of the electrodes and electrolyte, not on how large it is. The size of a cell does, however, affect its capacity. Large electrodes and a large volume of electrolyte give a cell a large capacity—it will continue to produce current for a longer time. The current it is able to deliver depends partly on its internal resistance, which in turn depends crucially on the size and separation of the electrodes. A cell with large electrodes close together has a low internal resistance and is consequently capable of delivering a larger current.
In the simple cell , the zinc electrode dissolves away, zinc ions appear in the electrolyte, and hydrogen is lost from the cell in the form of gas (at the copper electrode). If it were possible to make good the loss of hydrogen, to re move the zinc ions form solution, and to re-lace the zinc electrode as the cell operates, the cell would not become exhausted. This is the principle of a fuel cell – it is a primary cell that is continuously supplied with a chemical fuel as it is being used.
In a fuel cell, hydrogen or a liquid fuel is fed to one electrode, and oxygen or air is supplied to the other , in the hydrogen –oxygen type (which has electrodes of porous nickel) hydrogen enters the electrolyte as positive hydrogen ions, leaving behind negatively-charged electrons on the electrode—which therefore becomes the cathode. The ions move through the electrolyte to the other electrode, which is supplied with oxygen, where they combine with the oxygen to produce water. In doing so, they take electrons from this electrode, leaving it with a net positive charge and making it the anode of the cell. A hydrogen –oxygen fuel cell essentially re-verses the electrolysis of water (in which and electric current splits it into hydrogen and oxygen gases).
With certain combinations of electrodes and electrolyte, it is possible to rejuvenate the cell when it is exhausted and return it to its original condition by forcing a current through it in the reverse direction. An external voltage source, such as a battery charger, is used to pass current in at the positive terminal and out at the negative one.
A cell that can be recharged in this way is called a secondary cell, or accumulator. Two particularly important types are the lead acid accumulator and the nickel-iron alkali accumulator.
The lead –acid type of accumulator has electrodes consisting of lead oxide and lead plates immersed in sulfuric acid; the plates are large and close together, so as to give a very low internal resistance. The EMF is about 2 volts. A set of 6 cells arranged in serried comprises the familiar 12-volt car battery, which is capable of delivering a current of up to 200 amps for a short period. The nickel-iron alkali type uses a nickel hydroxide cathode, an anode of iron, and electrolyte of potassium hydroxide solution. There are also small nickel-cadmium rechargeable cells, with and alkaline electrolyte. Alkali cells have EMFs of only 1.2 to 1.5 volts, but they have a longer lifetime than do lead –acid cells. They are also lighter and more robust.
Efficiency and cost
Cells (and batteries ) convert chemical energy to electrical energy with an efficiency of about 90 per cent-much higher than any other method of electricity production, including the burning of fossil fuels (such as coal and oil) in conventional power stations. But the cost of electricity from cells is much higher because the chemicals are expensive research continues into way of using fossil fuels or their derivatives to generate electricity chemically in fuel cells.