Christian Litterst

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 (μDMFCs) 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 (α) and along their axis (β). 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 α = 5° and β = 1.5°. The experimental verification was done by using a transparent flowfield to show the passive bubble removal as well as with a fully operational μDMFC. 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.

Fully passive degassing and fuel supply in direct methanol fuel cells

In this paper a micro direct methanol fuel cell (muDMFC) is presented, which is operated in a completely passive way, i.e. the cell does not require an external pump for fuel supply. The surface energy of deformed CO2 bubbles, generated as a reaction product during DMFC operation, is employed to supply methanol to the anode. In contrast to a digital valve based approach presented earlier by Meng et. al. [1], a tapered channel is applied to achieve a pumping mechanism. This way the pump rates can be adapted to the requirements of a specific cell. The presented study reveals that this concept is able to maintain the supply for all typical DMFC operation conditions. Experimental results are presented that demonstrate the continuous operation of a passive muDMFC for more than 15 hours.

Novel Structure for Passive CO 2 Degassing in μDMFC

A new flowfield design enabling fully passive CO2-gas bubble removal from the anode of a micro direct methanol fuel cell (µ DMFC) is presented. The flowfields channels have an upside down T-shaped cross-section with defined opening angles along their axis and cross-section. The angles create an intrinsic transport mechanism removing growing gas bubbles from the electrode by capillary forces only. Applicable opening angles α and β are 5 ° and 1.5 ° respectively. The experimental verification has been done based on a transparent flowfield and by a fully operational µ DMFC delivering a power density output of up to 8 mW/cm2without the need of external pumps. Although the fuel cell supply can be operated both passively and pump assisted, the passive operation shows the highest efficiency.