Design and Analysis of MEMSbased direct methanol fuel cell

Transcription

1 Presented at the COMSOL Conference 2010 China Design and Analysis of MEMSbased direct methanol fuel cell Yuan Zhenyu Harbin Institute of Technology

2 Contents Background Principle Application of COMSOL Summary

3 Background Due to the unique advantages, micro direct methanol fuel cells (µdmfcs) have been considered as the most promising candidates for the micro power sources of mobile devices. µdmfc for speaker and gadget charger, Sony (Japan), 2009 µdmfc for cell phone, Motorola (USA), 2008 µdmfc for notebook, Ultracell (USA), 2007 µdmfc for PDA, Hitachi (Japan), 2005

4 MEMS Center, HIT 0.64cm 2,open circuit voltag:520mv,power density:5.9mw/cm 2 Open voltage:650mv power density:15.9mw/cm 2 fuel cell stack,open voltage:2.75v, Powered LED to work for 5h power density:6.8mw

5 contents Background Principle Application of COMSOL Summary

6 Principle O 2 anode: cathode: overall: CH3OH H2O CO26H 6e O H e H O 3 CH3OH O2CO 22H2O 2 (1)current collector material Methanol H + e - CO 2 H 2 O (2)structure design (3)mass transfer of MEA (4)theoretical analysis Anode MEA Cathode COMSOL Multiphysics

7 contents Background Principle Application of COMSOL Summary

8 3.Application of COMSOL 3.1Two-dimensional,two-phase mass transport mass transport,structure of DMFC 3.2 µdmfc Three-dimensional model A novel cathode structure,stability A air-breathing cathode flow field water-flooding Anode structure optimize structure

9 3.1A two-dimensional two-phase mass transport model :

10 continuous equation Mathematical model two-phase mass transport in the diffusion layer momentum transport equation Mass transport equation ( u ) S l l l ( u ) S u u l g g g g Kk l Kk g rl rg p l p g krl krg D C C u S eff il, il, il, l il, D C C S s 3 1s 3 eff ig, ig, ig, g ig, pressure difference p p p cos / K 0.5 Js c g l c D D s eff il, il, eff 1.5 u D D s 1.5 ig, ig, 1 Leverette function J s s s s s2.120s 1.263s c o c o o

11 Mathematical model two-phase mass transport in anode channel lul momentum transport equation luu l lpl lullg t 3 Cd CO 2 velocity u g u l u slip l slip slip l 4 d u u p C 16 d b Reb ( l u l ) 0 continuous equation 1 CO 2 1CO u 0 2 g t Mass transport of methanol D eff C C 0 two-phase mass transport in cathode channel MeOH MeOH MeOH u eff 1.5 l DMeOH DMeOH momentum transport equation 1 gug p t 1 uu 1 u 1 g g g g g g g g H 2 O velocity (1 ug u g u g ) 0 l HO 2 continuous equation HOu 0 2 l t eff Mass transport of O 2 D C C 0 O O O g eff u D D 1 O O

12 mass transport in PEM Mathematical model Mass transport of methanol (Concentration and electro-osmotic) eff m i ip NMeOH, cross DMeOH, memcmeoh, mem nd F 6 F Mass transport of H 2 O i NHOcross 2, nd F The transportations of electron and proton Electrochemical kinetics seff, s0 meff m 2, 0 ref Cmacl, Anode af 0 Cmacl, C i im s exp( a) ref Cm RTacl 1 Cmacl, C C ref O F 2 Cathode i i 1 s exp( ) Current density i p ic i Heat transfer, ccl c c O 2 ref c CO RTccl H a Ga Anode catalyst layer qacl ia ( a ) 6F H G H G Cathode catalyst layer qccl ic ( c ) ( ic ia ) 4F 6F ref m ref m ref Cm 0.1 mol/ L c c a a

13 Comparison of the modeling results and experimental results

14 Model result effect of different methanol concentrations diffusion mosis electroos mosis eff m p NMeOH, cross Dm, memcm, mem nd F 6 F i i Effect of the operating temperature

15 Model result CO 2 content in anode flow field CO 2 content increase with the current density and temperature The anode flow rates have the effect of the removal rate of the CO 2 in accord with the results of Yang [1] and Liao [2] [1] H. Yang, T. S. Zhao, Q. Ye. J. Power Sources, 2005, 139: [2] Q. Liao, X. Zhu, X. Y. Zheng, et al. J. Power Sources, 2007, 171: Current density Flow rate Temperature

16 Simulation Result Temperature distribution in the µdmfc with different current collector materials : (a) silicon; (b) stainless steel; (c) PMMA. excellent qualities of the silicon and stainless steel in heat transfer, uniform temperature distributions were achieved in their corresponding cells stainless steel mesh smaller methanol concentration in the anode catalyst layer, obtain the better performance G Vcell ia thvoltfuel 100% H E i cell c Journal of Power Sources 195 (2010) (IF=3.792)

17 3.2 µdmfc three-dimensional Model 3.2.1An air-breathing direct methanol fuel cell with a novel cathode shutter current collector Current collector Inlet Diffusion layer PEM

18 MEMS Center, Harbin Institute of Technology Harbin, , China

19 The velocity simulation results in diffusion layer with two types of collectors. The mass fraction of oxygen simulation results with the two types of collector.

20 Test result All experiments of the stack were performed with air self-breathing at room temperature on the cathode. Flow rates of 2 ml/min for 2 M methanol solution were supplied bya peristaltic pump. international journal of hydrogen energy 35 ( 2010 ) (IF=3.945)

21 3.2.2a three-dimensional two-phase-flow model for self-breathing µdmfcs In order to ensure that the oxygen from air could enter the electrochemical reaction sufficiently and is well-distributed, the intension of this paper is to design a self-breathing μdmfc with a new cathode spoke structure structure a similar back interface (air-contacting interface) structure to conventional perforated cathode, its front interface (electrode-contacting interface) is equipped with a certain number of blade-form channels which are connected with self-breathing circular openings to form a spoke unit.

22 Schematic of simulation domains of the three-dimensional model Governing equations electronic current Multicomponent mixed gas diffusion ( leff, l) Si ac ( s, eff s) Si a c MD w n w w M w 3 ij j i { g i ( j ) i j1 M j M (Maxwell-Stefan)

23 Governing equations oxygen concentration ( DcO) Ru c 2 g O2 mass transport equation of the liquid-phase l Kkrl 0.5 ( cos( c)( ) J ( s)) Rw 0 M K l l The gas flow in the cathode flow channel is expressed by N-S equation: ug T g g( ug ( ug) ) g( ug ) ug pc 0 t Kkrg The gas velocity in the diffusion layer u g p g g Butler-Volmer equation is used for explaining the relation between current and potential: i c i 0 c 2 ( )exp( ) c 2 F( ) RT o c S l o, ref 0

24 Result and discussion compare of different cahtode increases the efficiency of oxygen mass transport make the concentration more equal promotes the water transfer to PEM declining the inner resistance among PEM Enchancing Electrocatalytic Activity Promoting the air vaporization

25 compare of different cahtode The latter shows a lower content than the former indicating a higher discharge capability

26 different numbers of blades Model result the eight-blade cathode shows the highest The distributions of the four-bladeand the eight-blade are relatively uniform

27 cathode structure Test reult Performance comparison number of blade two-blade 13.33mW/cm 2 four-blade 14.79mW/cm 2 eight-blade 14.01mW/cm 2 Durability Performance comparison

28 Methanol concentration distributions at the catalyst layers under different flow fields are 和 mol/m 3 single serpentine field flow has best performance

29 Current density distributions at the catalyst layers along x direction under the conditions of different open ratios. Current density distributions at the catalyst layers along x direction under the conditions of different channel depth.

30 Comparison of the line pressure drop at the interfaces between the diffusion layers and two single-serpentine flow fields Methanol concentration distributions

31 contents Background Principle Application of COMSOL Summary

32 4.summary outlook: Methanol crossover Analyses of the fuel cell stack assembly pressure

33 THANKS!