Direct methanol fuel cells with other energy

The fuel cell has a proven history with solid oxide based systems. Recent needs for miniaturization have generated new ideas of fuel sources and structures. These include direct methanol fuel cells (DMFCs) and polymer electrolyte membrane fuel cells (PEMFCs). These fuel cells have advantages over others for several reasons. A key quality is the potential energy that the fuel sources provide with respect to the fuel costs. 

Although liquid methanol, ethanol, and gasoline all possess volumetric energy densities that are 1.9,2.5, and 3.7 times of that of the DOE target based on the LHV, only methanol can be directly oxidized in a fuel cell. In conjunction with other advantages of methanol, such as being biodegradable and easy to produce, carry, and transport, a direct methanol fuel cell (DMFC) becomes the mainstream choice for portable applications. 

A key element of the automotive fuel cell membrane electrode assembly is the proton exchange membrane (PEM), also referred to as the polymer electrolyte membrane (PEM), which is composed of a thermoplastic elastomer coated with a platinum catalyst. U.S. car-makers expect to have fuel cell-powered cars on the market by 2004. Polymer selection depends on, among other criteria, fuel selection such as Direct Methanol Fuel Cell (DMFC) or Direct Hydrogen Fuel Cell (DHFC). One prototype fuel cell vehicle is the product of the Partnership for a New Generation of Vehicles (PNGV), comprising U.S. automotive companies and the U.S. Department of Energy (DOE). ... 

In the case of direct methanol fuel cells, compared with oxygen reduction, methanol oxidation accounts for the main activation loss because this process involves six-electron transfer per methanol molecule and catalyst self-poison when Pt alone was used from the adsorbed intermediate products such as COads-From the thermodynamic point of view, methanol electrooxidation is driven due to the negative Gibbs free energy change in the fuel cell. On the other hand, in the real operation conditions, its rate is obviously limited by the sluggish reaction kinetics. In order to speed up the anode reaction rate, it is necessary to develop an effective electrocatalyst with a high activity to methanol electrooxidation. Carbon-supported (XC-72C, Cabot Corp.) PtRu, PtPd, PtW, and PtSn were prepared by the modified polyol method as already described [58]. Pt content in all the catalysts was 20 wt%. 

Fuel cells using directly liquid fuels are advantageous in this aspect. Methanol, formaldehyde (water solution), formic acid (water solution) and hydrazine are among fuels relatively easy to oxidize electrochemically. Alcohol and hydrocarbon with larger molecular weight are much harder to oxidize completely to C02- Other qualifications to be considered are price, availability, safety, energy density and ease of handling. 

DMFCs have potential near-term applications mainly in the portable power source market, as they are smaller, lighter, simpler, and cleaner than conventional batteries. Liquid methanol is consumed directly in a DMFC, which implies a higher energy density of the fuel cell system. But the power densities achievable with state-of-the-art DMFCs are still very small in comparison to hydrogen-fuelled PEMFCs. One of the major problems lies in the use of liquid methanol solution on the anode of the DMFC, which, on the one hand, keeps the ionomeric membrane water saturated (and thus no humidification is needed) but, on the other hand, does not keep fuel (methanol or any other organic fuel, e.g., formic acid, ethanol) and water from permeating to the cathode side, since the basic PFSA membranes are permeable to both methanol and water. - The fuel and water crossover from anode to cathode hampers the performance of the air cathode. 

The utilization of methanol directly in a fuel cell to power an electric vehicle is a particularly attractive long term goal. While devotees of hydrogen energy would not necessarily accept methanol as a substitute fuel, it must be admitted that from a resource and pollution aspect it is more desirable than petroleum, while as a road transport fuel it is much easier to visualize than hydrogen. It may well have a role to play, along with other alcohols (ethanol and isopropyl alcohol) in diversifying the fuel base of road transport in the next century. 

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