Fuel cells are electrochemical devices which directly convert hydrogen, or hydrogen-rich fuels into electricity without combustion. This process is much more efficient than traditional thermal power plants, converting up to 80% of the chemical energy in the fuel into electricity (compared to a maximum of 40% for conventional power plants). Compared to traditional energy sources, fuel cells require low working temperatures, produce little pollution and noise during operation, and are capable of responding rapidly to changes in the power demand.
Based on the physical characteristics of the fuel supply, fuel cells may be roughly divided into two families: gas-type fuel cells and liquid-type fuel cells. Direct alcohol fuel cells (DAFC) are a relatively new member of the liquid-type fuel cell family. In a DAFC, an alcohol fuel such as methanol, is oxidized at an anode catalyst layer to produce protons and carbon dioxide (CO2). The protons migrate through an electrolyte from the anode to the cathode. At a cathode catalyst layer, oxygen reacts with the protons to form water. The anode and cathode reactions in this type of direct methanol fuel cell are shown in the following equations:Anode reaction (fuel side): CH3OH+H2O→6H++CO2+6e−  1.Cathode reaction (air side): 3/2 O2+6H++6e−→3H2O  2.Net: CH3OH+3/2 O2→2H2O+CO2  3.
The electrolyte can be an acidic or an alkaline solution, or a solid polymer ion-exchange membrane (PEM) characterized by a high ionic conductivity. Ideally, all the electro-oxidation should occur at the anode catalyst layer. However, since DAFCs use basically the same catalyst for both anode and cathode, a water soluble alcohol fuel may permeate through the electrolyte and combine with oxygen on the surface of the cathode catalyst. This phenomenon is termed “fuel crossover”. Fuel crossover lowers the operating potential of the oxygen electrode and results in consumption of fuel without producing useful electrical energy. In general, fuel crossover is a parasitic reaction which lowers efficiency, reduces performance and generates heat in the fuel cell. It is therefore desirable to minimize the rate of fuel crossover.
There are a number of approaches to reduce fuel crossover. The rate of crossover is proportional to the permeability of the fuel through the solid electrolyte membrane and increases with increasing fuel concentration and temperature. The permeability of the PEM to the liquid fuel can be reduced by choosing a PEM with low water content, or by coating the PEM with a material that is permeable to hydrogen but not permeable to the fuel. In addition, the concentration of the liquid fuel can be lowered to reduce the crossover rate.
In direct methanol fuel cells (DMFC), fuel crossover is typically controlled by using diluted methanol fuel that contains 3-5 wt % methanol and 97-95 wt % water. The diluted fuel, however, reduces not only the fuel crossover but also the fuel efficiency of the fuel cell. Since one water molecule (molecular weight=18) is consumed with each methanol molecule (molecular weight=34) in the electrochemical reaction, only about 2.5 wt % water will be consumed with methanol in a fuel composition containing only 5 wt % methanol, the other 92.5 wt % of water becomes “dead weight”. Therefore, the real “consumable fuel” in the diluted methanol fuel accounts to less than 8% of the total fuel composition.
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