Practical Models of DMFCs and Their Features

A further problem in DMFC operation is due to the evolution of gaseous CO2 at the anode (Ye et al 2005b). Then gas bubbles that can locally interfere with the flow of the aqueous methanol solntion may form in the flow field on the anodic side of the bipolar plates. This leads to a nonuniform distribntion of the reaction (and thus current) across the MEA snrface. This effect is particularly noticeable when the solution is snpplied passively (e.g., by free flow from a tank above). To overcome it, one shonld nse an active reactant supply at flow rates several times in excess of the stoichiometric requirements (Cowart, 2005). This raises the question of how to dimension the means of pumping the solution through (and what energy they wonld consnme). The dimensions would depend on the pressure drop within the flow field channels between solution input and outlet (Yang et al., 2005). 

In parallel with the large amount of work done to study the mechanism and operating features of methanol fuel cells with proton-conducting membranes, operating models of such fuel cells started to appear in the mid-1990s, first as laboratory-type small single-element fuel cells and finally, in the form of multielement batteries of relatively large power. 

Some of the earliest work (Sarumpudi et al., 1994) was done in the Jet Propulsion Laboratory at the University of California in Pasadena. Laboratory samples of cells with 5x5 cm electrodes were examined. It was seen that for a cell with a 2 M methanol solution working at a current density of 100 mA/cm, the working voltage rose from 0.35 V to 0.55 V when the temperature was raised 

In somewhat later work at the same laboratory (Ren et al., 2000), data were reported for a five-cell battery that was tested at a temperature of 100°C with 1 M methanol solution and air at a pressure of about 2 bar. At a working voltage of 0.4 V (per cell), this battery provided a current density of 500 mA/cm.  

At the Korea Instimte for Science and Technology, Kim et al. (2006b) studied the operation of a six-cell battery that had a total power of 50 W. The battery was fed with a 2 M methanol solution. The battery was supposed to work at ambient temperature, but internal heat production raised its temperature to above 80°C. Using oxygen, a maximum power of 254 mW/cm was attained with air, this value was 85 mW/cm.  

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