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fuel is fed to the cell or within the stack itself. Current selective oxidation technology can reduce CO levels to <10 ppm. Another approach involves the use of a selective oxidation catalyst that is placed between the fuel stream inlet and the anode catalyst. Since the stack cannot tolerate even 10 ppm, air is usually bled into the anode directly to manage CO. Research to find approaches and materials that better tolerate impurities in the fuel continues today. These unit operations add weight and volume, and reduce the efficiency of the system (fuel processor “efficiencies” typically range from 75 to 90 percent, but similar losses occur in the production of hydrogen from fossil fuels). In addition to the unit operations however, it is important to realize that their presence also impacts the size, performance, and cost of the fuel cell stack: • The hydrogen in the anode feed of reformate-based systems is typically diluted with CO2 and (in case of POX or ATR) nitrogen. As a consequence, the hydrogen mole fraction at the anode inlet is rarely higher than 0.3 (vs. 75 percent in the case of a direct hydrogen system). This decreases the ideal potential of the cells and increases the concentration-related losses. • The presence of trace CO and sulfur and large quantities of CO2 affects the performance of the anode electro-catalyst. As a consequence, more platinum must be used (typically 0.4 to 1 mg/cm2 more), and even then the power density is typically 30 to 40 percent lower than with hydrogen-based systems. The choice between a direct hydrogen and a reformate-based system depend on the application. For light duty vehicles, most experts now prefer direct hydrogen systems (hence the focus of the U.S. DOE program), while for stationary applications natural gas reformer-based PEFC systems are favored. 3.3.3 DirectMethanolFuelCellSystems Specially optimized PEFCs can be fed with methanol (or fuels with similar chemical structure), creating a so-called direct methanol fuel cell (DMFC). Conceptually, this could lead to a very simple system with a fuel that has a relatively high energy density and is a liquid under ambient conditions. Performance levels achieved with a DMFC using air is now in the range of 180 to 250 mA/cm2 (29) but because cell voltages typically range between 0.25 to 0.4 V, the power density ranges between 40 to 100 mW/cm2. This low cell voltage is caused by a few common problems with the DMFC, several of which result from the cross-over of neutral methanol from the anode to the cathode side: • High anode overpotential has been shown to be caused by absorption of partial de- composition products of methanol (e.g. CO) • High cathode overpotential, caused by poisoning of the cathode electro-catalyst by cross-over methanol and its decomposition products This performance still requires platinum loadings that are almost ten times higher (around 3 to 5 mg/cm2) than needed in high-performance direct hydrogen PEFC. When feeding concentrated methanol directly, the cross-over can be as high as 30 to 50 percent compared with the amount oxidized electrochemically. If the concentration is reduced, the cross-over is reduced but so is the current density (due to reduced activity of the reactants). Obviously, the methanol crossed over is 3-19PDF Image | Fuel Cell Handbook (Seventh Edition)
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