Beyond Combustion: Fuel Cell Cars for the 21st Century

As the number of cars and the amount of driving increase, it becomes necessary to reduce air pollutant emissions per car in order to meet air quality goals. Yet it is increasingly difficult and costly to meet these goals by ever tighter regulations on tailpipe emissions of internal combustion engine cars. Moreover, concerns about climate change may lead to substantial constraints on emissions of greenhouse gases. It is wholly impractical to capture from tailpipes the principal greenhouse gas, carbon dioxide, which is created when hydrocarbon fuel is burned in an internal combustion engine car. But the fuel cell, an energy conversion device that is a candidate to replace the internal combustion engine in cars, and hydrogen, the energy carrier that will ultimately replace hydrocarbon fuel in fuel cell vehicles, can effectively address both of these seemingly intractable challenges. Together, they can provide the next generation of automotive propulsion cleanly and with low life-cycle emissions of greenhouse gases - and perhaps at competitive cost. Fuel Cell Technology Fuel cells convert fuel energy directly into electricity without first burning the fuel to produce heat. All practical fuel cells are fueled by hydrogen. The fuel delivered to a fuel cell is either hydrogen or an energy carrier such as gasoline or methanol that is converted at the point of use into a hydrogen-rich gas that the fuel cell can use. In some respects the fuel cell is similar to an internal combustion engine and in other respects it is like a storage battery. The internal combustion engine takes in reactants from outside the engine - fuel from the fuel tank and air from the atmosphere - and combines them in combustion. The resulting hot, high-pressure, combustion-product gases push pistons that turn the engine crankshaft. Batteries, by contrast, store their reactants within the battery enclosure where they are converted directly into electricity without combustion, but the supply of reactants is limited by the restricted storage capacity of the battery enclosure. The fuel cell combines the best features of both. Like the battery, it produces electricity directly from the reactants - the fuel and oxidizer - but like the internal combustion engine, it converts the energy stored in a separate fuel tank and is able to continue producing electricity as long as sufficient oxidizer and fuel are supplied to it. The main features of the fuel cell system are a fuel supply, an oxidant - typically oxygen from the air - and two electrodes with an ionically conducting but electronically insulating electrolyte sandwiched between them. The electrodes are connected through a load, such as an electric motor, by an external circuit. At the anode, fuel molecules give up electrons to the external circuit and thereby become hydrogen ions; at the cathode, oxygen ions are formed by oxygen molecules from the air plus electrons from the external circuit. The buildup of electrons at the anode compared to the cathode creates a voltage that drives current through the external load. The electrical circuit is completed by the flow of ions through the electrolyte. The net effect of the reactions at the electrodes is that fuel and oxidant combine to form water and, in some types of fuel cells, carbon dioxide also. Fuel Cells Rediscovered Fuel cells were discovered more than a century and a half ago by British lawyer and judge Sir William Grove. While experimenting with electrolysis - using electricity to split water into hydrogen and oxygen - Grove reasoned that the reverse process should also work: that hydrogen and oxygen should combine in such a way as to generate electricity. His reasoning proved correct, and he demonstrated the first fuel cell in 1839. Little research was done on fuel cells for nearly a century. In the 1960s, however, the fuel cell was rediscovered by scientists working in the aerospace industry. …