l-cadmium cells have a nickel hydroxidecathode and a cadmium anode emersed in a potassium hydroxide electrolyte,all assembled in a steel container. No porous separators are used; instead, theelectrodes are held in place by plastic or rubber moldings that prevent themfrom touching each other.More modern "sealed" nickel-cadmium cells contain the same substances buthave a very small quantity of electrolyte, porous separators, and design featuresthat permit gas under pressure within the cell to escape but preventatmospheric air from drying out the electrolyte. These features make themparticularly useful in portable devices. The fuel cell is a special kind of battery. Its operating principles were firstdiscovered in 1839 by the British physicist Sir William Grove, but the deviceremained just a laboratory curiosity for many years. In the 1960s scientistsrediscovered the fuel cell and used it to make electricity for spacecraft.The fuel cell resembles a conventional battery in that it has a positiveelectrode, a negative electrode, and an electrolyte. It works quite differently,however, combining oxygen and some hydrogen-containing fuelelectrochemically to produce electricity. The fuel (it might be hydrogen gas,methanol, petroleum, natural gas, or propane) is passed across the fuel(positive) electrode, where it dissociates into hydrogen ions and electrons. Theions enter the electrolyte and move to the oxygen (negative) electrode. Theelectrons move through an external circuit, producing a current. At the negativeelectrode, the ions, electrons, and oxygen combine to form water.The characteristics of fuel cells are affected by their electrolyte, operatingtemperature, oxidant, and fuel. Although numerous combinations of these arepossible, only a few are practical. Common electrolytes include phosphoricacid, potassium hydroxide, and sulfuric acid. Operating temperature can rangefrom 120o F (50o C) to more than 1,800o F (1,000o C), depending upon the...
