Fuel Sources

In most researched fuel cell applications, hydrogen fuel must be supplied by extremely pure hydrogen gas in pressurized tanks. See Hydrogen Storage Possibilities. If hydrogen resources are impure, they run the high risk of poisoning the metallic catalyst. See Catalysts.

Fossil fuels like natural gas, as well as cleaner fuels like methanol include hydrogen in their molecular structure. A fuel processor could be used to remove hydrogen from these fuels to power fuel cells.

Another way to fuel vehicles would be by the steam reformation of methanol. This reaction can be run at a relatively low temperatures (ideal for zero-emission vehicle design) and would eliminate the problem of transporting and storing hydrogen gas. Methane can be stored and distributed much like gasoline, and many buildings are already connected via pipeline to natural gas sources, so supplying these fuels in vehicular and residential applications, respectively, would not be that difficult.

Methanol Reformation

The process of reforming methanol begins with the vaporization of liquid methanol and water, which is then passed through a heated chamber containing a catalyst. When the methanol molecules hit the catalyst, they split into carbon monoxide (CO) and hydrogen gas. Water splits into hydrogen and oxygen gas, and the oxygen combines with CO to form carbon dioxide, reducing the amount of carbon monoxide that is released into the atmosphere.

On-board methanol reformers, however, make the fuel cell reaction procedure much more complex, and research is inconclusive as to whether reformation will contribute to the problem of catalyst poisoning. Methanol is also a highly corrosive material, which would make replacing fuel tanks a constant and expensive problem.

Natural Gas Reformation

The processing of natural gas is similar to that of methanol. Methane is reacted with water vapor over a catalyst to form carbon monoxide and hydrogen gas. Oxygen gas supplied from the catalyzed splitting of water can be combined with CO to form carbon dioxide. Most of today's hydrogen is produced by steam reformation of natural gas and other fossil fuels.

Disadvantages of Fuel Reformation

The reformation reactions can never go to one hundred percent completion, so some of the natural gas and methanol will inevitably make it through the chamber without reacting. These molecules can contribute to the poisoning of the fuel cell catalyst and pollute the atmosphere.

While fuel processors produce much less pollution than a traditional internal combustion engine, they still produce a significant amount of carbon dioxide, which can contribute to global warming.

Another downside of the fuel processor is that it decreases the overall efficiency of the fuel-cell car. The fuel processor uses heat and pressure to aid the reactions that split out the hydrogen, but heat capture can never be a perfect process.

Methanol is a highly corrosive material, so the utilization of the fuel stored in tanks on-board could lead to expensive maintenance and frequent replacement of methanol storage tanks.

Electrolysis

Although electolysis is the simplest way to produce hydrogen and releases no carbon dioxide into the atmosphere, it is only employed on a small scale. The electrical demands of electrolysis simply make it too expensive to produce hydrogen on an industrial scale. Some proponents of of renewable energy sources such as solar, wind, and geothermal believe that electricity will soon be abundant and cheap enough to make electrolysis the best choice for producing hydrogen. Electrolysis also produces pure hydrogen, so carbon monoxide poisoning of the catalyst a non-issue.

Image courtesy of Carleton Comprehensive High School.

Oxygen

In some fuel cell research laboratories, oxygen is supplied via tanks of pure oxygen gas to achieve maximum efficiency. In commerical applications, however, oxygen will most likely be provided from atmospheric sources (i.e., air). Atmospheric oxygen does not contain carbon monoxide in significant quantities, so catalyst poisoning is not a major concern. Since air only contains 21% oxygen, the air flow rate through a fuel cell stack would have to be about five times greater than if pure oxygen were used. This can introduce mass transport problems and other inefficiencies.

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