Increasing µDMFC efficiency by passive CO2 bubble removal and discontinuous operation
A new concept that enables fully passive CO2 gas bubble removal in micro direct methanol fuel cells (mu DMFCs) is presented. The original concept behind the presented degassing structure (flowfield) is based on microchannels with a T-shaped cross section. These channels have defined tapering angles over their cross section (alpha) and along their axis (beta). The tapered channel design creates an intrinsic transport mechanism that removes the gas bubbles from the electrodes by capillary forces only. Computational fluid dynamic (CFD) simulations have been used to determine applicable opening angles of alpha = 5 degrees and beta = 1.5 degrees. The experimental verification was done by using a transparent flowfield to show the passive bubble removal as well as with a fully operational mu DMFC. During the operation, the fuel cell delivered an output of up to 8 mW cm(-2) without the need for external pumping in short-term measurements. During the long-term measurements, discontinuous pumping showed the highest fuel cell efficiency compared to the continuously pumped fuel supply.