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Introduction/Background
Introduction
Presently the majority of the electrical energy that we utilize is produced through thermal or combustion processes that are both inefficient and environmentally damaging (Yue et al., 1986). Since the quantity of electrical energy used has also been rising year after year, scientists have been researching alternative methods for producing electrical energy. An attractive alternative comes in the form of fuel cells, which are expected to have a higher fuel to electricity efficiency with minimal environmental interference as compared to currently used processes (Hirschenhofer et al., 1994).
Background
Conventional Fuel Cells
 Figure 1.Schematic representation of a conventional electrochemical fuel cell
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In a conventional electrochemical fuel cell, there is a constant supply of fuel and oxidant fed into the anode and cathode compartment respectively. The gases flow pass the surface of the electrode that is located in their compartment, which oxidizes the fuel and reduces the oxidant. In doing this, an electric current is produced as the electrons obtained from the oxidation reaction travel through an external circuit to the cathode where they are used to perform the reduction reaction. The ionic charge produced through the electrochemical reactions are then conducted between the anode and cathode through the electrolyte, which is located between the electrodes (Figure 1). Through this process, electrical energy and heat are produced by directly converting the chemical energy of reaction associated with the production of water from hydrogen and oxygen.
There are currently several types of electrochemical fuel cells being researched such as the polymer electrolyte (PEFC), alkaline (AFC), phosphoric acid (PAFC), molten carbonated (MCFC), and solid oxide (SOFC) fuel cells. Characterization of these fuel cells is dependent on the type of electrolyte used in their construction. Since the electrolyte usually limits the characteristics of the fuel cell, each fuel cell will have its own advantages and disadvantages. For example, the solid electrolyte in SOFC eliminates concerns such as corrosion and electrolyte movement. At its high operating temperature of 1000 °C, catalysts are not needed to reform the carbon monoxide fuel, but the selection of construction materials is limited and problems concerning mismatched thermal expansion coefficients become evident. The advantages and disadvantages of each fuel cell are being considered to determine their appropriate application whether it is in a power plant or vehicle (Hirschenhofer et al., 1994).
Ethanol Fuel Cell
Table 1. Electrochemical Reactions in an Ethanol Fuel Cell
Anode: C2H5OH + 2OH- ® CH3CHO + 2H2O + 2e-
Cathode: 4e- + O2 +2H2O ® 4OH-
Overall:
2C2H5OH + O2 ® 2CH3CHO + 2H2O
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In an electrochemical ethanol fuel cell, the electrons obtained from the oxidation of the ethanol are used to reduce the oxygen in the cathode compartment. In the anode, the ethanol is oxidized by hydroxide, which is obtained from the potassium hydroxide electrolyte solution. This produces acetaldehyde, water and two electrons. In the anode the four electrons that are produced in the anode are used to reduce oxygen and water to produce hydroxide (Table 1).
Methanol Fuel Cell
Table 2. Electrochemical Reactions in a Methanol Fuel Cell
Anode:
CH3OH + 2OH- ® HCHO + 2H2O + 2e-
HCHO + 2OH- ® HCOOH + H2O + 2e-
HCOOH + 2OH- ® CO2 + 2H2O + 2e-
Cathode:
Cathode: 4e- + O2 +2H2O ® 4OH-
Overall:
CH3OH + 1.5O2 ® CO2 + 2H2O
Side Reaction:
CO2 + 2KOH -> K2CO3 + H2O
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Unlike the ethanol conventional fuel cell, methanol is oxidized in three stages.
In the first stage methanol is oxidized by hydroxide to form formaldehyde, water and two electrons. The formaldehyde is then oxidized by hydroxide to form formic acid, water and two electrons. After that formic acid electrochemically reacts with hydroxide to form carbon monoxide, water, and two electrons. The reaction seen in the cathode of the ethanol fuel cell is also observed in the cathode of the methanol fuel cell (Table 3).
An undesired side reaction is observed in the methanol fuel cell that does not occur in the ethanol fuel cell. The carbon dioxide produced in the third stage of oxidation reacts with the potassium hydroxide solution thus decreasing the amount available for the oxidation reactions occurring in the anode.
Biological Fuel Cells
New technologies, especially in the medical field, have created a demand for power supplies that are capable of drawing fuel from natural resources. This requirement can be resolved through biological fuel cells. Biological fuel cells are capable of generating electricity by converting naturally available substrates into environmental friendly by-products. Unlike the conventional electrochemical fuel cells, biofuel cells use biocatalysts to convert chemical energy into electrical energy.
A biocatalyst can have two different functions in a biological fuel cell. It can either be used to generate the fuel substrates through metabolic processes or biocatalytic transformations, or it could partake in the electron transfer that occurs between the fuel substrate and the electrode's surface (Katz et al., 2003).
Conventional electrochemical fuel cells differ from biological fuel cells in other respects. In a biofuel cell, the electrolyte layer found in conventional fuel cells is replaced by a membrane, which still allows ion exchange. The operating conditions of biofuel cells are also milder since they usually operate at ambient temperature, atmospheric pressures, and neutral pH (Yue et al., 1986).
The two types of biofuel cells currently available are the microbial and enzymatic fuel cell.
 Figure 2. Microbial fuel cell consisting of a) a separate microbial reactor b) an integrated microbial reactor for the production of the fuel product (Katz et al., 2003).
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Microbial Biofuel cells
In a microbial biofuel cell, fuel is obtained through microorganisms in the form of metabolic intermediaries or as the final product of anaerobic respiration. This fuel is produced when the microorganisms, which act as a micro reactor, processes the primary substrate through oxidative degradation. This fuel can be delivered to the biofuel cell in one of two ways. It can be produced in a separate microbial reactor and transported to the biofuel cell or it can be produced in a microbial reactor that is integrated with the anode compartment of the biofuel cell (Figure 2). (Katz et al., 2003)
Once the fuel product is delivered or produced, the electrons obtained from the oxidized fuel are not immediately transported to the anode since the electron transfer rate would be too slow. To improve the electron transfer rate, an initially oxidized redox mediator is used to extract electrons. The electrons are then transferred to the anode and the mediator is once again oxidized (Kim et al., 2000).
Microbial fuel cells have been shown to produce a considerable amount of energy, but there are several difficulties associated with their maintenance. For example, certain conditions must be sustained in order for the microorganisms to survive. The microorganisms are also unable to tolerate direct electrochemical contact with the electrode support making it difficult to have the microbial reactor integrated with the biofuel cell (Katz et al., 2003).
Enzymatic Biofuel cell
In contrast to microbial biofuel cells, enzymatic biofuel cells utilize the redox enzymes rather than the whole microorganism as a biocatalyst. The redox enzyme, which are separated and purified from an organism, participates in the electron transfer chain that occurs between the substrate and the anode by oxidizing the fuel.
 Figure 3. Schematic of electron transfer from substrate to electrode through a mediator denoted as R.
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Redox enzymes are incapable of direct contact with the electrode since their redox centers are insulated from the conductive support by the protein matrices (Katz et al., 2003). In order to contact these enzymes with the electrode, mediators, which are dependent on the class of oxidative enzymes, are utilized (Figure 3). Several types of schemes have been developed to enable this electric contact.
Garza et al. (2003) recently proposed using a porphyrin sensitizer as an mediator to oxidize the reduced form of the nicotiamide redox couple, NAD(P)H/NAD(P)+ . The oxidized portion of the couple is once again reduced by the oxidation of the substrate, in this case reduced carbon fuel, by the dehydrogenase enzyme. A nanoparticulate SnO2 electrode was used as the anode. The results of this photoelectrochemical biofuel cell indicate that an open circuit current of 0.75 V was obtained without additional optimizations.