The rise in use of portable electronic devices has been increasing steadily in the United States and abroad over the past few years and most likely will continue to increase over the years to come as the population becomes more dependent on multifunctional portable electronics. Harvesting energy from renewable resources has become an important focus in order to eliminate our dependency on oil and other nonrenewable resources necessary as primary power sources. It is well known that industrialized nations are the highest energy consumers and that there is a correlation between energy consumption and status of economic and technological development [9]. About 65% of the world’s primary energy was consumed in 1992 by industrialized countries and some of the more populated

Applications and Notes

Used in space vehicles, e. g., Apollo, shuttle

vehicles and mobile applications, and for lower-power systems

Suitable for portable electronics systems of low power, running or long times

Large numbers of 200-kW systems in use

Suitable for medium — to large-scale systems, up to MW capacity

Suitable for all sizes of CHP systems, 2-kW to multi-MW

countries, while developing areas consume more biomass energy such as wood and wood wastes.

The demand for energy is slowly increasing with developing technology, which in turn explains the extreme situation of the United States. The United States has only 5% of the world’s population and yet consumes about one quarter of the total global primary energy. The sources for energy production in the United States are usually obtained from coal, natural gas, and oil where oil is most common [9]. In order to minimize our dependency on oil, researchers are attempt­ing to harvest energy from renewable resources, such as alcohols, sugars, fats, and other biologically derived materials.

Biofuel cells are electrochemical devices in which energy derived from bio­chemical reactions is converted to electrical energy by means of the catalytic activity of microorganisms and/or their enzymes. Unlike metal catalysts, biocat­alysts are derived from biomatter, which is a renewable resource. Recent biofuel cell research has explored using enzymes as biocatalysts due to their availability and specificity. Enzymes are functional proteins whose purpose is to catalyze specific biochemical reactions by lowering the activation energy of the reaction, without undergoing a permanent chemical change itself. Enzymes can be manip­ulated and produced by genetic engineering or harvested and extracted from living organisms. Both means of acquiring enzymes are more cost effective than mining precious metals used as traditional catalysts. Biofuel cell catalysts are more environmentally friendly compared to heavy metal batteries due to the fact they naturally biodegrade. Another advantage of enzyme employment in biofuel cells is the enzyme specificity that pushes the fuel cell technology one step further. Specificity of the enzyme’s fuel utilization eliminates the need for employment of a salt bridge and therefore simplifies the fuel cell system [4].

The first biofuel cell was demonstrated by Potter in 1912 by employing glucose and yeast to obtain electrical energy [10]. This concept inspired scientists to investigate the metabolic pathways of power production [10]. Early biofuel cells employed microorganisms to oxidize the fuel for electricity generation; however, due to the slow mass transport of fuel across the cell wall, power densities are too low for practical applications. State-of-the-art microbial fuel cells developed by Lovley have shown greater than 40-day lifetimes, but power densities of 0.0074 mA/cm2 [11].

More recently, enzyme-based fuel cells were constructed employing enzymes in the solution. These fuel cells had higher power densities due to the elimination of cell walls that slowed the mass transport; however, their lifetime only extended from hours to a few days because of the enzyme’s stability. In contrast, higher power densities have been obtained with enzymatic fuel cells reaching up to 0.28 mW/cm2 for a glucose/oxygen membraneless biofuel cell at room temperature [12] and 0.69 mW/cm2 for a methanol/oxygen biofuel cell with a polymer elec­trolyte membrane [13]; however, enzymatic fuel cells are plagued with low lifetimes ranging from two hours [14] to seven days [15]. Table 12.2 depicts a brief history of biofuel cell technology.

Enzymes have been shown to be effective biocatalysts for biofuel cells com­pared to microbial biofuel cells. However, enzymes are very delicate catalysts. The optimal activity of enzymes depends on their three-dimensional configura­tion, which can be denatured with slight changes in pH or temperature. Therefore, it is necessary to develop an immobilization technique that will keep the enzyme active at the electrode surface in its optimal working conditions.