Porous Silicon as a Proton Exchange Membrane for Direct Methanol Fuel Cells
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Direct Methanol Fuel Cells (DMFCs) show the most potential in efficient chemical to electrical energy conversion having approximately half the specific energy compared to gasoline at 5.54 kW-hr/kg (19.9 MJ/kg). With a density of 0.792 kg/L, methanol?s energy density is 4.39 kW-hr/L. By designing a system to utilize methanol, the advantages from quick refills and the elimination of recharge times offer great motivation for further analysis on this topic. Furthermore, methanol is a relatively low cost alcohol/fuel with popular applications such as automobile windshield wiper and aircraft de-icing fluids. One major source of inefficiency within the DMFC is the electrolyte allowing fuel to cross over from the anode to cathode. Proprietary DuPont Nafion 117 has been the standard thus far for all meso-scale direct methanol power conversion systems and its shortcomings are primarily in the areas of slow anodic reaction rates and fuel crossover resulting in lower voltage generation or ?mixed potential.? Porous Silicon (P-Si) is traditionally used in photovoltaic and photoluminescence applications. Rarely is it used to function as a mechanical filter or membrane. The research deals with investigations into using P-Si as a functioning electrolyte to transfer ions from the anode to cathode of a DMFC. In addition, an effort to observe the consequences of stacking multiple layers of anodes is attempted. Porous silicon was fabricated in a standard Teflon cylindrical cell by an anodization process including varying the current density to etch and electro-polish the silicon membrane. The result was a silicon membrane with pore sizes of approximately 1.5 ?m when optically characterized by a scanning electron microscope. The porous membranes were then coated in approximately 0.2 mg/cm2 Pt-Ru catalyst with a 10% Nafion solution binding agent onto the anode. Voltage versus current data shows that an open circuit voltage (OCV) of 0.25V was achieved with one layer when operating at 20oC. When adding a second layer of porous silicon, the OCV was raised to approximately 0.32V under the same conditions. The experimental data suggests that the current collected also increases with an additional identical layer of anode prepared the same way. The only difference is that the air cathode side was surface treated to 0.1 mg of Pt black catalyst with a 10% Nafion binding agent to aid in the recombination of hydrogen atoms to form the water byproduct. Porous silicon endurance runs with 2ml of 3% by volume methanol (0.7425M) fuel dissolved in water show that an operating voltage was generated for approximately 3 hours before the level dropped to approximately 65% of the maximum voltage of 0.25V. Endurance runs with a second layer added extended the useful life of the cell by approximately 2 hours to 5 hours when tested under the same conditions. When tests were conducted for voltage generation by varying the methanol concentration, a linear relationship developed up until the point where methanol seepage through the porous membrane affected measurements. In an effort to quantify the results and confirm the usefulness of the addition of a second layer, Fourier Transform Infrared Spectrometry was conducted on a number of samples to verify the methanol concentration for each layer. Additionally, a pH test was conducted to measure the relative amounts of protons dissolved in solution between the layers.