Methanol fuel acceptability

The electrooxidation of methanol has attracted tremendous attention over the last decades due to its potential use as the anode reaction in direct methanol fuel cells (DMFCs). A large body of literature exists and has been periodically reviewed [130,131,156], [173-199]. Unlike for formic acid, a generally accepted consensus on the specific mechanistic pathways of methanol electrooxidation is still elusive. 

In case other fuels are used, the fuel cell must be capable of accepting them (direct methanol fuel cell, etc.), or they must be converted to hydrogen, typically using a reformer (natural gas, gasoline, methanol, etc., cf. Chapter 2, Fig. 2.44). The store is now accommodating the fuel of choice. 

Carbon aerogels and xerogels have been used as supports for Pt and Pt-based electrocatalysts for proton-exchange membrane fuel cells (PEMFCs), also known as polymer-electrolyte fuel cells [56,58,83-90], These fuel cells are convenient and environmentally acceptable power sources for portable and stationary devices and electric vehicle applications [91], These PEMFC systems can use H2 or methanol as fuel. This last type of fuel cell is sometimes called a DMFC (direct methanol fuel cell). 

Wu et al. [106] prepared hybrid direct methanol fuel cell membranes by embedding organophosphorylated titania submicrospheres (OPTi) into a CS polymer matrix. The pristine monodispersed titania submicrospheres of controllable particle size are synthesized through a modified sol-gel method and then phosphorylated by amino trimethylene phosphonic acid (ATMP) via chemical adsorption. Compared to pure CS membrane, the hybrid membranes exhibit increased proton conductivity to an acceptable level of 0.01 S/cm for DMFC application and a reduced methanol permeability of 5 xlO cm /s at a 2 M methanol feed. 

Men, Y, Kolb, G., Zapf, R., Tiemann, D., Wichert, M., Hessel, V. and Lowe, H. (2006) A complete miniaturised microstructured methanol fuel processor/fuel cell system for low power applications. Int. J. Hydrogen Energ., accepted for pubbcation. 

In practice, the suitability of a reaction system is determined by the kinetics of the reaction, which depends on temperature, pressure of gases, electrode polarization, surface area of electrodes, and presence of a catalyst. A fuel cell that is thermodynamically and kinetically feasible must be considered from an econonuc viewpoint before it is accepted. Thus, since hydrogen, hydrazine, and methanol are too expensive for general application, their use in fuel cells has been limited to special cases. Hydrogen has been used for fuel cells in satellites and space vehicles, in which reliability and lightness are more important than cost. Hydrazine fuel cells have been used in portable-radio power supplies for the United States Army because of their truly silent operation. Methanol fuel cells have been used to power navigation buoys and remote alpine television repeater stations because such power systems are comparatively free from maintenance problems over periods of a year or more. The polarization at the electrodes of a fuel cell is the most important single factor that limits the usefulness of the cell. The various polarization characteristics for a typical fuel cell are plotted separately as a function of current density in Fig. 9.11. 

As is clear from the previous discussion, the use of gas as a fuel presents problems of electrode construction and storage, that are not there when the fuel is water soluble. Three substances have been seriously suggested as such fuels—methanol, hydrazine and ammonia. Of these, methanol will probably become more accepted than the other two, mainly for economic and safety reasons. Methanol, if it becomes widely used for fuel cell operation, is likely to drop in price markedly, while hydrazine and ammonia will probably not. This section will describe briefly the methanol fuel cell, although the technology of the hydrazine one is more advanced. 

However, considering the portable market, efficiency and cost are less important than system size, so these trade-offs are acceptable. The most developed DAFC is the direct methanol fuel cell (DMFC). Many prototype and nearly commercial DMFC systems exist for powering small electronics, laptop computers, cell phones, hand-held electronics, and other devices. Direct alcohol fuel cells are expected to occupy a growing market for portable power for years into the future. 

Although a preferred fuel for military applications would be logistic fuel (JP-8), because of difficulties in reforming this fuel, particularly on such a small size, the same fuels used for nonmilitary applications, such as hydrogen, metal hydrides, chemical hydrides methanol, are acceptable as long as they are supplied in closed canisters or cartridges and do not have to be dispensed. 

Fuel supply is usually from liquid hydrogen or pressurized gaseous hydrogen. For other fuels, such as methanol, a fuel processor is needed, which includes a reformer, water-gas shift reactors, and purification reactors (in order to decrease to an acceptable level—a few tens of ppm—the amount of CO, which would otherwise poison the platinum-based catalysts). This equipment is still heavy and bulky and limits the dynamic response of the fuel cell stack, particularly for the electric vehicle in some urban driving cycle. 

Methanol has advantages as a carrier of hydrogen because it is a liquid and stable at room temperature. Instead of manufacturing methanol from a fossil fuel such as methane or coal, it would be possible to extract C02 from the atmosphere (Section 15.3.2), produce H2 from solar-based water decomposition, and combine H2 and C02 to CH3OH. The C02 used up to do this would be re-injected to the atmosphere again upon the re-forming of methanol to H2 for the fuel cells, but there would be no net buildup of C02. Further, we would not have to worry about the exhaustion of fossil fuels, which, as far as oil at an acceptable price is concerned, will occur before 2040. 

California Energy Commission, Methanol as a Motor Fuel Review of the Issues Related to Air Quality, Demand, Supply, Cost, Consumer Acceptance and Health and Safety, Pub. P500-89-002, Sacramento, Calif., April 1989. 

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