Hannover, April 2, 1995
The direct production of electricity from chemical reactions in an economically viable process is a goal shared by thousands of researchers around the world. All previous methods of producing electricity, with the exception of solar cells, require two or more stages. In the first stage, heat is given off as the result of a chemical reaction - combustion, for example. This heat must subsequently be converted into electricity. For example, it is used to heat water, thereby producing steam which then drives a turbine. The disadvantages of this method are well known: the multi-stage process produces considerably less electric current than is theoretically possible. Technicians speak of "low efficiency" in the production of electricity.For this reason, when it comes to mobile applications, electrical energy has, up to now, had little chance of competing against internal combustion engines. Electrical energy is not only uneconomical to produce, it must also be stored in heavy batteries. Such electric drives are hardly a serious alternative to the combustion engine, which can deliver its energy directly to the transmission. With the fuel cell as its energy source, the electric motor could now get a real chance as a means of propulsion.
The basic principle of the fuel cell
Electrolysis, the chemical splitting of water into its constituent elements hydrogen and oxygen, along with the reverse procedure, the explosive burning of hydrogen and oxygen ("oxyhydrogen gas reaction") - are part of the standard repertoire of any high school chemistry class. The "cold" variant of this reaction, however, is less well known: hydrogen and oxygen combine to form water in the fuel cell, whereby the resulting energy is not wasted in a flash of light, but is directly transformed into electric current.
How is this accomplished? The two gases are prevented from coming into direct contact with one another by an electrolytic layer which permits only one of the two gases to pass through it, and even then, only in its electrically charged state, i.e. in the form of ions. In the proton-conducting "Proton Exchange Membrane Fuel Cell," these positively charged hydrogen ions (protons) each leave behind an electron, thus causing a negative charge to be built up on the hydrogen side of the electrolyte and a positive charge on the oxygen side. The result is in an electrical voltage. The energy for this "charge pump" comes from the bonding of the hydrogen ions with oxygen to form water.
The electrolyte of the PEM cell consists of an ultra-thin polymer foil a mere tenth of a millimeter thick. The foil is coated on both sides with a platiniferous catalyst. The catalyst facilitates both the ionization of the hydrogen and the reaction between the hydrogen ions and the oxygen. So-called bipolar plates seal off the cell on both sides. The plates possess a fine system of channels which pass hydrogen and air over the respective catalytic surfaces of the foil. In addition, the plates also conduct away the heat given off by the reaction and establish the electrical connection to the next cell. Each cell of the type used by Daimler-Benz produces a voltage of 0.6 volts and a power of approximately 250 watts. Enough energy to operate a vehicle is generated by connecting several cells together in so-called stacks. By regulating the supply of hydrogen - or in future models, methanol - the required energy is produced directly and in precisely measured quantities.
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© 1995 Daimler-Benz