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THE ALKALINE FUEL CELL

Of the four acidic fuel cells, the phosphoric acid fuel cell is the only one that is now in commercial use, with units installed for fixed power generation. It has also be used experimentally with large vehicles, such as buses.

The PAFC uses a phosphoric acid (H3PO4) electrolyte. Most acids operate in solution, which means that a fuel cell using them must operate below the boiling point of water, reducing efficiency. Concentrated phosphoric acid does not need to be in solution and can operate at higher temperature.

The phosphoric acid is contained in a matrix of silicon carbide and Teflon and sandwiched by the anode and cathode, which are built as thin plates of porous graphite. Platinum catalyst laid down on these electrodes helps accelerate the electrochemical reactions. The PAFC operates at 175 to 200 degrees Celsius (350 to 400 degrees Fahrenheit). Higher temperatures of course help accelerate the reaction, but above 220 degrees Celsius (428 degrees Fahrenheit), the phosphoric acid tends to attack the catalyst.

The proton exchange fuel cell, sometimes known as the polymer electrolyte fuel cell, was originally developed by GE in the late 1950s, but still is not in commercial use.

However, there has been considerable work on its use as an automotive power source due to its relatively light weight and low operating temperature, and even some work on using it to replace batteries in portable electronic equipment such as laptop computers. One of the advantages of focusing on such applications is that both electric vehicles and portable electronics equipment run on DC electricity, reducing the requirements for power conditioning.

The operating principles of the PEM fuel cell are very similar to those of the PAFC, the main difference being that uses a polymer film, based on sulfonic acid, for an electrolyte rather than phosphoric acid. The membrane-electrode assembly of a PEM fuel cell is very thin, on the order of a few millimeters.

The PEM fuel cell operates at low temperatures, similar to those of the alkaline fuel cell, in the range of 80 to 95 degrees Celsius (175 to 200 degrees Fahrenheit). Also like the alkaline fuel cell, it uses platinum catalyst to increase the reverse electrolysis reaction rate. Much work has been done on reducing the amount of platinum required, and in current fuel cells small atomic clusters of platinum are deposited on fine carbon particles.

The two remaining acidic fuel cell types, the molten carbonate and solid oxide fuel cells, remain generally experimental devices. They are being considered for fixed site power generation systems much like the PAFC systems now in use.

The MCFC uses a mix of molten lithium, sodium, and potassium carbonate (K2CO3). It operates at 540 to 650 degrees Celsius (1,000 to 1,200 degrees Fahrenheit), which is hot enough to keep the electrolyte molten. The high operating temperature allow the MCFC to convert hydrocarbon fuel into hydrogen without a separate reformer.

The carbonate electrolyte is contained in a porous board of lithium aluminate. The anode is made of nickel and the cathode is nickel oxide, to which silver is sometimes added. The nickel and silver act as catalysts. The operating temperature of the MCFC is between 600 and 700 degrees Celsius, hot enough to keep the electrolyte molten. The major problem with the MCFC is that the molten carbonate electrolyte tends to attack the electrodes.

The SOFC is attractive because its electrolyte will not leak and is not corrosive. The electrolyte consists of solid zirconium oxide, stabilized with yttrium oxide. The SOFC operates at 980 degrees Celsius (1,800 degrees Fahrenheit) and uses titanium based perskovite crystals for a catalyst. Like the MCFC, its high operating temperature eliminates the need for a separate fuel reformer subsystem. However, the electrolyte materials are expensive.

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