NOVEL STRUCTURE FOR PASSIVE CO2 DEGASSING IN μDMFC Reporter: sang-chung yang Advisor: Prof. C.H. Liu C. Litterst1, S. Eccarius2, C. Hebling2, R. Zengerle1, and P. Koltay1 1University of Freiburg – IMTEK, Laboratory for MEMS Applications, Georges-Koehler-Allee 106, D Freiburg, Germany MEMS2006 Page:102

Sang-chung Micro system and control laboratory Introduction Flowfield Concept Simulation Experimental Verification Conclusions And Outlook Outline

Sang-chung Micro system and control laboratory Introduction Fundamental Structure of μDMFC Fig1.darft of the fuel cell assembly(not to scale)

Sang-chung Micro system and control laboratory Introduction 總反應式 :CH 3 OH+1.5O 2  CO 2 +2H 2 O 陽極反應式 :CH 3 OH+H 2 O  CO 2 +6H + +6e - 陰極反應式 :1.5O 2 +6H + +6e -  3H 2 O 直接甲醇燃料電池： 1. 水的管理 2. 陽極二氧化碳排除 3. 甲醇滲透（ cross-over)4. 高活性化時極化損失觸媒負載 Fig2.Photograph of one of the ransparen test samples

Sang-chung Micro system and control laboratory Flowfield Concept Work Principle 1.The flow field layout is based on the non-uniform capillary pressure P cap =σ(R x -1 +R y -1 ) 2.The pressure difference forces the bubble o move towards the wider channel part until both capillary pressures are in equilibrium Fig3.darft of gas bubble movement in a tapered channel

Sang-chung Micro system and control laboratory Fig5.one side of the T-shaped parallel with their inclination angles CHIC-type(channel in channel) Enhance bubble transport and avoid clogging in capillary systems (Ref:Mobility of Gas Bubbles in CHIC-type flow channels,ACTUATOR 2004) Fig4.Mobility of gas bubbles in CHIC SUPPLY

Sang-chung Micro system and control laboratory Simulation Fig6.Simulation sequence with distributed bubble sources and bubble removal due to capillaryforces. 1.Use CFD simulation to verify the working principle. 2.Ten areas are randomly chosen by an automatic script 3.Current density of 100mA cm -2 generates 0.26 ml min -1 cm -2 of CO 2

Sang-chung Micro system and control laboratory Experimental Verification 1.Use transparent PMMA Fig7.Picture sequence of gas bubbles developing and their movement inside one test sample with a 2M methanol solution

Sang-chung Micro system and control laboratory Fig8.4M methanol solution at two pumped flow rates and passive system setup 1.With decreasing flowrate the efficiency of the fuel cell increases as the increase of power densities.

Sang-chung Micro system and control laboratory 3 different type degas method comparison Fig9.long time measure with a 4M methanol type1.Continuously pumped type2.Pumped at intervals:23 min pause,15s pumping type3.passive:open cartridge without any pump 1.Performane for type2 and type3 increases until the depletion of methanol leads to a decline of the current density. 2.In type3,a small flow refuels the system continuously due to a small difference in hydrostatic height(about 10 mm) of cartridge and cell

Sang-chung Micro system and control laboratory Conclusions And Outlook 1.A μDMFC exhibited a higher efficiency when operated passively compared to the situation when methanol was supplied continuously or at intervals. 2.Long term measurements lead to the conclusion that an open system that would be assisted by pumping at intervals could perform best. 3.Could we find another way to degas?