Power from Ethanol Fuel Cells
A Biomass Derived Method of Hydrogen Storage
Using ethanol as the hydrogen storage medium for fuel cells could solve the safety problems associated with the storage of hydrogen gas.
The fuel cell, powered by hydrogen and oxygen, can provide a mobile source of electricity (Fuel Cell Technology Handbook, 2002). While the oxygen can be taken from the local atmosphere, safe onboard storage of hydrogen is still a problem. Ethanol fuel cells store molecularly bonded hydrogen; a safer storage medium than hydrogen gas.
Ethanol as a Hydrogen Source
Because hydrogen is so flammable and must be stored under great pressure, it is a dangerous fuel to carry. An alternative to storing hydrogen gas is to carry a chemical that is a liquid at room temperature that contains a significant amount of hydrogen. The hydrogen atoms could be stripped from that molecule and then reformed into H2 for use in the fuel cell.
A short chain alcohol like ethanol would seem well suited to this purpose. It is liquid at very low temperatures (melting point -114° C, Lange’s Handbook of Chemistry, 15th Ed, 1999) and it has limited and well understood toxicity. It is produced from renewable raw materials, and is becoming generally available as a fuel.
Steam Reforming
The classical method of reforming H2 from a hydrocarbon is known as steam reforming. The ethanol is heated to temperatures in excess of 600° C in the presence of steam and a catalyst. Under proper conditions the steam, ethanol and catalyst react to provide H2 and carbon monoxide (CO) (Haryanto, Energy & Fuels, 2005).
CO is a poison to most fuel cell catalysts so this must be further reacted with steam and a second catalyst system in a two-stage reaction to provide additional H2 (from the water) and CO2. This requires a complex system and the temperatures involved are probably not suitable for use in electric fuel cell vehicles or portable electronic devices.
Raney Nickel Reforming
David Morgenstern (Energy & Fuels, 2005) has developed a lower temperature reforming system using a Raney Nickel catalyst system. This system works at temperatures around 300° C and, in a multi-step process produces Methane (CH4), H2 and CO2. While not as efficient in H2 production this system does provide its own burnable fuel, CH4, to produce the heat necessary for the reformation. The lower temperature is probably feasible in a vehicle, but it is still too high for a portable electronic device.
Ethanol not as clean as Hydrogen
Both of these reforming systems produce CO2, a greenhouse gas. This means that an ethanol-powered fuel cell is not as clean as a pure hydrogen fuel cell, which only produces water. Ethanol, however, is produced from renewable resources. Thus the CO2 emissions are offset by the CO2 consumption of the plants grown to make the biomass used to ferment the ethanol. Thus the ethanol fuel cell is carbon neutral.
If compared to hydrogen produced by wind, solar or hydropower, there is also the energy cost of the biomass production and conversion systems that be taken into account when comparing the two fuel cell types. Both systems are still much cleaner than fossil fuel driven engines, with fewer and less toxic emissions.
Conclusion
Ethanol fuel cells still require additional development before they can be used in commercially successful electric vehicles. While they do have the potential for providing a clean and safe portable-fuel source for electric vehicles, they will still require temperatures too high for use in portable electronic devices.
