Hydrogen fuel cells show promise for providing portable electric power generation for electric vehicles, if certain inherent problems can be overcome (Wald, Scientific American, May 2004). While ethanol has been proposed as a hydrogen source for such a fuel cell, it too has problems that make it impractical for use in vehicles. Other organic hydrogen sources are being investigated for use in fuel cells.
The pure hydrogen-oxygen fuel cell has problems with storing hydrogen gas (H2). Storing a large enough volume for practical use requires storing a highly flammable gas at high pressures, adding weight and safety problems. Using ethanol as a storage medium for hydrogen requires reforming hydrogen using catalysts, steam and high temperatures. This causes safety problems for automobiles as well.
Microbe-Organic Fuel Cell
In a recent review article (Environmental Science and Technology, 2006) Logan describes microbial fuel cells (MFC) where a bacterial culture is used to oxidize organic molecules to produce CO2 and H2. The temperatures required for the reaction are mild, ranging between 30 C and 60 C. These temperatures are well within the range associated with automobile engines.
These systems are not very efficient, however, because the bacteria us a portion of the available energy for cell growth and replication. Power output varies over time due to changes in sugar and cell concentration. These limitations make this process more suited to electrical cogeneration production at wastewater treatment plants than portable electrical generation.
Microbe-Cellulose Fuel Cell
While this is essentially the same thing as the Microbe-Organic Fuel Cell, it deserves special mention due to the specific use of cellulose as the hydrogen source instead of sugar. Cellulose is the component of biomass that is the most difficult to use for energy production except for by directly burning the material.
The Penn State Hydrogen Center (Renn, Environ. Sci. Technol., 2007) has developed an MFC that uses a dual microbe system to digest cellulose and then oxidize that product to produce H2 and CO2. Due to the complex requirements to maintain the bacterial cultures, such a system is more suited to the centralized production of hydrogen or electricity than for use in portable devices.
Enzyme-Starch Fuel Cell
An approach developed at Virginia Tech (Zhang, PLOS One, May 2007) uses a starch-water solution and a collection of thirteen enzymes to produce hydrogen. The reaction conditions are mild with the operating temperature at about 30 C. The system is relatively easy to maintain as long a nearly pure source of starch or sugar solution is feed into the cell.
The enzymes drive the reaction to completion without requiring the application of outside energy. The starch is first reduced to its component sugars and then a complex series of enzyme mediated reactions results in near complete conversion to H2 and CO2 (12 moles of H2 and 6 moles of CO2 per mole of sugar). The excess hydrogen and oxygen comes from the water in which the starch is dissolved.
The fuel cells reported in the literature rely on food-sourced starch or sugars, but enzyme systems for the conversion of biomass waste molecules like cellulose or lignin could be developed. The processes for producing a relatively clean solution or suspension of cellulose or lignin in water should not be as energy intensive as that required for producing ethanol.
Conclusion
The enzyme driven fuel cell seems to have the most potential for providing fuel cell technology for portable electronic devices and would be suitable for powering electric vehicles. The development of the Enzyme-Cellulose Fuel Cell may provide the solution to the problem of portable electrical generation for vehicles.
