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Fuel Cell

A fuel cell is a device in which the energy released in the oxidation of a conventional fuel is made directly available in the form of an electric current. It thus avoids the wasteful detour of the conventional thermal power stations, i.e., the generation of electricity via the inferior thermal energy. The principle of the fuel cell was formulated by W.Ostwald as long ago 1894, it is only in recent years that some success has been achieved in the construction of efficient cells of this kind.

The principle of the fuel cell was discovered by German scientist Christian Friedrich Schönbein in 1838 and published in the January 1839 edition of the Philosophical Magazine. Based on this work, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1845. United Technology Corp.'s UTC Power subsidiary was the first company to manufacture and commercialize a large, stationary fuel cell system for use as a co-generation power plant in hospitals, universities and large office buildings.

UTC Power continues to advertise this fuel cell as the PureCell 200, a 200 kW system. UTC Power continues to be the sole supplier of fuel cells to NASA for use in space vehicles, having supplied the Apollo missions, and currently the Space Shuttle program, and is developing fuel cells for automobiles, buses, and cell phone towers; the company has demonstrated the first fuel cell capable of starting under freezing conditions with its proton exchange membrane automotive fuel cell.

In the fuel cell constructed by Baurand Ehrenberg in 1911 (Fig 1) a carbon rod serves as the fuel. It functions as the anode, introducing C+++++ions into the solution. This necessitates an operating temperature of 1000°-1100°C. The electrolyte is molten soda. The cathode consists of molten silver, forms 0----- ions from the oxygen that is continuously injected.

According to the equation C+++++ + 20----- = CO2, the reaction product obtained is carbon dioxide, just as in ordinary combustion. For every carbon atom that is converted, four electrons are given off to the carbon rod and four electrons are withdrawn from the oxygen electrode. These electrons can produce a current in an external circuit.

According to this conception, a coal-burning stove is an internally short-circuited fuel cell. The major disadvantage of the fuel cell described above is the high temperature and consequently, the very short service life of the materials employed. Less service conditions can be achieved by using gases (hydrogen, in particular) as the fuel. Thus, the Bacon fuel cell (Fig.2) produces current densities of up to about 6 ½ amp. / in2. at a temperature of 240°C. The pressure of the aqueous electrolytes does, however, rise to 1000 lb / in2. and upwards.

The ionization of the gas fed to the cell is effected at diffusion electrodes of nickel. These are porous sintered components which on one side are connected to the gas supply and on the other side are in contact with the electrolyte. The active region is at the boundary of the three phase’s gas or electrode or electrolyte. To make this boundary as long as possible, all the pores must have the same optimum diameter, as is clarified by Fig.3 (principle of homoporosity).

In order completely to obviate the passage of unutilized gas through the pores, each electrode is provided with a fine-pored cover layer (double-layer electrode). As a result of the high catalytic activity of the electrodes employed, the cell can operate already at room temperature. The H2 O2 cell designed by Justi and Winsel (Fig.4), which is known as the dissolved fuel cell, also operates at ordinary temperatures.

In this cell the oxygen electrodes contain Raney silver and the hydrogen electrodes contain Raney nickel as the catalyst. Already at temperatures below 100°C this cell attains current density values almost as high as those of the Bacon cell. The fundamental voltage is over 90% of the theoretically attainable voltage of 1.23 volt.

The electrodes used are described as double-skeleton catalyst electrodes. Because of their great catalytic activity, they are able to dehydrate liquid organic fuels e.g. methanol. This results in the relatively simple constructional features of the dissolved fuel cell (Fig.5). The alcohol serving as fuel is mixed with the electrolyte potassium hydroxide solution.