A self-regulated passive fuel-feed system for passive direct methanol fuel cells

Unlike active direct methanol fuel cells (DMFCs) that require liquid pumps and gas compressors to supply reactants, the design of passive DMFCs eliminates these ancillary devices. As a result, the unique feature of passive DMFCs is that they have a rather simple and compact structure. More importantly, the parasitic power losses associated with ancillary devices are removed in this type of fuel cell. For these reasons, passive DMFCs have been projected as the most promising candidate to replace conventional batteries for powering portable electronic devices, such as cellular phones and laptop computers. Operating passive DMFCs with high methanol concentration is desired because this increases the energy density of the fuel cell system and hence results in a longer runtime. However, the increase in methanol concentration is limited by the adverse effect of methanol crossover in conventional design. To overcome this problem, we propose a new self-regulated passive fuel-feed system that not only enables the passive DMFC to operate with high-concentration methanol solution without serious methanol crossover, but also allows a self-regulation of the feed rate of methanol solution in response to discharging current. The experimental results showed that with this fuel-feed system, the fuel cell fed with high methanol concentration of 12.0 M yielded the same performance as that of the conventional DMFC running with 4.0-M methanol solution. Moreover, as a result of the increased energy density, the runtime of the cell with this new system was as long as 10.1 hours, doubling that of the conventional design (4.4 hours) at a given fuel tank volume. It was also demonstrated that this passive fuel-feed system could successfully self-regulate the fuel feed rate in response to the change in discharging currents. In addition, a passive direct methanol fuel cell (DMFC) stack that consisted of six unit cells was designed, fabricated, and tested. The stack was tested with different methanol concentrations under ambient conditions. It was found that the stack performance increased as the methanol concentration inside the fuel tank was increased from 2.0 to 6.0 M. The improved performance is primarily due to the increased cell temperature as a result of the exothermic reaction between the permeated methanol and oxygen on the cathode. Moreover, the increased cell temperature enhanced the water evaporation rate on the air-breathing cathode, which significantly reduced water flooding on the cathode and further improved the stack performance. This passive DMFC stack, providing 350 mW at 1.8 V, was successfully applied to power a seagull display kit. The seagull display kit can continuously run for about 4 hours on a single charge of 25 cm3 4.0-M-methanol solution. Keywords: Passive DMFC; DMFC stack; Methanol crossover; Portable power source; Temperature; Passive fuel-feed syste