Conclusions

Research into fuel cells began with hydrogen and oxygen as reactants in 1838, expanded during attempts to use coal as fuel, and flourished after 1950 when the technology found its first prominent application in space missions. The first fuel cells, studied by Grove and Schoenbein and called "gaseous voltaic batteries," demonstrated the electrochemical reaction of hydrogen and oxygen in a time when the chemical combination of the reactant gases was known to occur on platinum. Grove improved his design by increasing the surface area of the platinum electrodes, and another researcher, Lord Rayleigh, used platinum sponge. Mond and Langer identified the flooding of the catalyst as a problem, and to solve it used a diaphragm to contain the sulfuric acid electrolyte. This design allowed Mond and Langer to build a self-contained battery of cells, but its originality was challenged by Alder Wright and Thompson, who built a similar device that they called "double-aeration" plate cells.

With the success of fuel cells using hydrogen as fuel, researchers turned their attention to developing a fuel cell that used coal as fuel. Instead of using coal in a steam power plant, which was an inefficient process at the time, the coal would be used in a fuel cell to produce electricity directly in a more efficient and cleaner process. The first to build a supposed "direct coal" fuel cell was Jacques, who called it a "carbon electric generator," but Haber and Bruner showed that the reaction was between the coal and the electrolyte rather than between the coal and the oxidant, making it an "indirect" fuel cell. Another problem with this cell was that the alkaline electrolyte would degrade because of the carbon dioxide in the product of the oxidation reaction. Baur and Ehrenberg used hydroxide, carbonate, silicate, and borate as electrolytes; with carbonate electrolytes, the feeding of carbon dioxide to the cathode helped to reduce the concentration polarization, which Baur and Brunner discovered in 1935. Containing the molten carbonate electrolyte was difficult to manage, however, so Baur and Preis developed a fuel cell with a solid electrolyte using "Nernst-Mass," which was a mixture of zirconia and yttria compounds.

By this time, the four types of chemicals that are used today as electrolytes had been used in fuel cells: acid, alkaline, carbonate, and oxide. Although the first acid fuel cells used sulfuric acid, phosphoric acid was more stable at high temperatures and was used in the fuel cells developed during the TARGET program. In the 1960s, the General Electric Company developed a fuel cell that used a polymer with sulfonic acid functional groups as electrolyte. Alkaline fuel cells were developed by Bacon, and the technology was modified by Pratt & Whitney Aircraft for use in the Apollo space missions to produce electricity for on-board use. Different methods were devised to prevent the liquid potassium hydroxide electrolyte from flooding the electrodes, such as double-porosity electrodes, wetproofed electrodes, and an electrolyte matrix.

The modern development of the molten carbonate fuel cell electrolyte was influenced by Davtyan, who used mixtures that he presumed were solid but were shown by Broers and Ketelaar to have been a combination of molten and solid phases, including compounds of carbonate, phosphate, tungstates, and silicates, and a solid phase of rare earth oxides. Broers and Ketelaar chose carbonates over other compounds because they were compatible with the products of the reaction with hydrocarbon fuel. The Institute of Gas Technology and the General Electric Company continued research into molten carbonate fuel cells. The truly solid electrolytes of Baur and Preis were tested to determine mixtures that would be the most conductive, and Weissbart and Ruka at Westinghouse Electric Corporation chose zirconia and calcia.

The direct methanol fuel cell was a return to the hope of oxidizing fuel directly. Sulfuric acid was used as electrolyte, and, because the electrolyte was circulated through the fuel cell, the fuel could be delivered with the electrolyte. With this method of fuel delivery, methanol could also reach the cathode and react on it, decreasing the performance of the electrode and the cell. For the catalyst, platinum alloyed with ruthenium showed the best performance. After the development of the solid polymer fuel cell, researchers used the solid polymer membrane as electrolyte and revived the prospects of developing a practical direct methanol fuel cell.

The first solid polymer fuel cell of General Electric was difficult to operate because of the membrane. To maintain conductivity, the membrane had to contain water. Also, the service life of the membrane was short because the membrane degraded in the oxidative environment at the cathode electrode. With the Nafion membrane of the DuPont Company, the service life was extended because of the stable fluorine chemistry of the polymer. An improvement to the performance of the fuel cell was made by incorporating Nafion in the catalyst layer to give the electrolyte continuity between the catalyst and electrolyte, which increased the catalyst surface area of the electrode. Also, carbon supports for the catalyst, a technique developed for phosphoric acid fuel cells, improved the surface area for a given amount of catalyst. The flooding of the electrodes could be managed by wetproofing the electrodes with PTFE, as was done with alkaline cells.

The solid polymer fuel cell was deemed the most appropriate type for use in road vehicles because of its compatibility with the reaction products of hydrocarbon fuels, its low operating temperatures, and its high power densities. Prior to the development of the solid polymer fuel cell, the phosphoric acid fuel cell had been considered the technology best available for use in a bus. An alkaline fuel cell system was used by General Motors in a van to determine the feasibility of a fuel-cell-powered vehicle. The reactants were hydrogen and oxygen, stored on board, and the fuel cells were stored under the floor of the van — a design used today.

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