The rapid proliferation of portable electronics such as cell phones into the consumer marketplace has placed an urgent need for high energy density, reliable, and inexpensive energy generation and storage systems suitable for such device applications. Among the competing technologies under development for the portable market segment, direct methanol fuel cells (DMFC's) seem to have the biggest edge due to their high energy density and storage capability, simplicity of operation and maintenance, small size, and cheep and ready availability of methanol fuel. Direct methanol fuel cells offer five times the energy density of lithium ion batteries. Methanol is a cheap and widely available fuel with an extremely high volumetric energy density of 18,000 MJ (LHV)/m3, compared to only 10.9 MJ (LHV)/m3 for hydrogen. Furthermore, methanol's capability for easy delivery and instant charging as well as its safety are very desirable features for consumer acceptability.
However, this technology is not without problems. One of the most serious is the fact that polymer exchange membranes (PEM's) made from polyperfluorosulfonic acid (also known as perfluorinated membranes, which are available as Nafion membrane by DuPont, USA) that are commonly employed in DMFC's pose severe limitations in cost, chemical stability, water management, and chemical shorting due to transport of unreacted methanol through the membrane (termed “methanol crossover”) from the anode compartment to the cathode compartment where it is oxidized to produce CO2. This “chemical short circuit” causes a significant reduction in fuel utilization, adversely affects the cathodic reduction of oxygen, and polarizes the cathode resulting in increased cathodic overpotential. The two competing reactions of methanol oxidation and oxygen reduction give rise to mixed potential at the cathode and lower the open circuit voltage of the cell. This situation is exacerbated by dilution of the methanol fuel to 3-10% (about 1 to 3 M Methanol) in water to minimize crossover. All of these factors naturally have an adverse impact on cell efficiency. Another noteworthy problem with DMFC is the need for highly active and stable anode catalyst for the sustained direct oxidation of methanol.
Methanol also gives rise to chemical degradation of the membrane by corrosion. Hence, low concentration (typically 3-15 wt. % methanol in water) methanol solutions are employed in order to mitigate the effects of methanol crossover and membrane degradation problems. However, such dilution of the methanol fuel further lowers the cell potential, and reduces conversion efficiency. Employing high methanol concentrations is not an effective solution either, because methanol crossover due to electro-osmotic drag of protons then becomes significant at high concentrations.
Furthermore, to acquire sufficient proton conductivity, perfluorinated membranes require the presence of water to maintain at high hydration level. Even though water is one of the reaction products of methanol oxidation, it is not trivial to maintain the membrane properly soaked in water. Hence, water management is also an issue with practical consequences in these cells.
Low activity of anode catalyst for methanol oxidation is another concern. The theoretical value of the open circuit potential for methanol oxidation to CO2 and H2O is 1.2 V. Unfortunately, methanol oxidation proceeds slowly on the Pt anode because it is inhibited by the strongly adsorbed CO and other C1-intermediates, which block catalytically active surface sites. Surface kinetics are somewhat improved by using bimetallic catalysts such as PtRu, or even trimetallic systems, with the addition of Sn, W, Mo etc. But the cost of noble metal catalysts and anode fabrication are still high.
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