Chiang Mai J. Sci. 2017; 44(4) : 1676-1685http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Effect of Cell Temperatures and Flow-field Patterns of Bipolar Plate Electrodes on the Performance of Proton Exchange Membrane Fuel Cell by Computational SimulationLirada Saraihom [a], Kridsanapan Srimongkon [b], Chesta Ruttanapun [d,e,f], Apishok Tangtrakarn [b], Narit Faibut [b], Pikaned Uppachai [b], Madsakorn Towannang [b] and Vittaya Amornkitbamrung*[b,c][a] Materials Science and Nanotechnology Program, Faculty of Science, Khon Kaen University, Khon Kaen

40002, Thailand.[b] Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand.[c] Integrated Nanotechnology Research Center (INRC), Department of Physics, Faculty of Science, Khon

Kaen University, Khon Kaen 40002, Thailand.[d] Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang,

Ladkrabang, Bangkok 10520, Thailand.[e] Functional Phosphate Materials and Alternative Fuel Energies Unit (FPM-AFE), Faculty of Science, King

Monkut’s Institute of Technology Ladkrabang, Ladkrabang, Bangkok 10520, Thailand.[f] Advanced Energy Material and Application Research Laboratory, Department of Physics, Faculty of

Science, King Mongkut’s Institute of Technology Ladkrabang, Ladkrabang, Bangkok 10520, Thailand.*Author for correspondence; e-mail: vittaya2206@gmail.com and vittaya@kku.ac.th

Received: 29 July 2015Accepted: 17 December 2015

ABSTRACT Theperformancesof fuelcellemployingabipolarplatewithdifferentgas-flow-field

patterns for proton exchange membrane fuel cell (PEMFC) were simulated using higher-order polynomials(h-p)finiteelementmethod(h-p FEM). The patterns of each model were as follows: thestraightpipeonbothsides(Model1),theserpentineflow-fieldforanodeandthestraightpipe for cathode (Model 2), the slotted serpentine for anode and the straight pipe for cathode (Model 3), and the serpentine on both sides (Model 4). It was found that as the cell temperature increased,thediffusionvelocityof reactantgasesandMaxwell-Stefan-diffusioncoefficientof proton dramatically increased. The performance of PEMFC reached the highest value as the flowvelocityof reactantgasesandthediffusioncoefficientof protonthroughmembranewereoptimized at the temperature of 80 oC.Themostefficientflow-fieldpatterninthisstudyisModel 2.

Key words: PEMfuelcell,mathematicalmodeling,flow-fieldpattern,diffusionvelocityof gas

1. INTRODUCTIONProton exchange membrane fuel cell

(PEMFC) receives much attention due to their high power density of power generation. It is considered as one of the cleanest energy

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sources because it can convert the chemical energy of fuel and oxygen into electrical energy, water and heat. Typically, the fuel especially hydrogen gas is fed onto the anode side whilst oxygen is supplied onto cathode. These gases are carried through channels (or sometimes called, “pipe”) patterned on a bipolar plate which functions as an electrode as well. The bipolar plate is in contact with conductive gas diffusion layer (GDL) which is adjacent to the catalyst loaded (CL) layer where oxygen reductions on cathode side as well as hydrogen oxidation on anode side simultaneously occur. It is desirable to maximize the delivery of gas onto the electrochemical active area by widening gasflowchannelspatternedonthebipolarplatebut this strategy also reduce the reactant gas flowefficiencyduetochannel-to-channelgascrossover[1].Theconfigurationof thepatternon the bipolar plate also affects how pressure and the heat [2-5] are distributed along the GDL. The optimization of heat is important for speeding up the electrochemical reaction and to reduce the water stagnation [6-12] in the proton exchange membrane parts. The ion conductivity of gases [13-14] through the membrane of the fuel cell, transport of gases species [15-16] in the system and the effect of channel and rib [17-21] on the performance of PEMFC were studied.

The performances of PEMFC rely on three processes: the diffusion of gases in the electrode channel and GDL, the reduction/oxidation of reactants in the CL layer and the diffusion of proton through the membrane. However, the diffusivity of gases along the channel also plays an important role to the performance of the fuel cell. The differences in pattern of the flow channel of bipolar plate make the difference in velocity of gas, building-up pressure, and the distribution of heat, the consequence is that all of which also affect the performance of PEMFC. Especially, the velocity of gas affects the rate of chemical reaction, the higher velocity of gas, the lower

reaction rate of reactant gases is. That is why it is important to optimize the velocity of reactant gases before achieving the highest reaction rate.

In this study, the three-dimensional of gasflow-fieldpatternsweresimulatedinorderto compare the diffusivity of gas and fuel cell performance resulted from each model as a function of temperatures. The simulation deal with three parts for each model: the bipolar plate electrodes, CLs and membranes. The parameters kept constants were pressure, gas flowrate,andrelativehumidity.Thetemperaturewas varied as 30, 40, 50, 60, 70 and 80 °C and the pressure was kept constant at 1.1 atm while the relative humidity was kept at 100% and gas flowrateswerealways150sccm.

2. MATERIALS AND MOTHODS2.1 Mathematical Modeling

The calculation utilized COMSOL Multiphysics software version 4.2a in order to simulate the results of the models. There were4flow-fieldpatterns:(1)straightpipeonboth sides (Model 1), (2) serpentine for anode with straight pipe for cathode (Model 2), (3) slotted serpentine for anode with straight pipe for cathode (Model 3), and (4) serpentine on both sides (Model 4), as shown in Figure1.

Theappropriateflow-fieldpatternaffectto accuracy simulation result. For this case the flow-fielddistancewasconsidered,becausethe distance magnitude is an indicator of the chemical reaction rate per unit area. So the longflow-fieldof gasaffectsincreasingof chemical reaction rate. The high chemical rate can also increase amount of mobile electron carries,resultinginhigherfuelcellefficiency.If gasflow-fieldpatternissinuousrouteorvariousfetchroundroute,thegasflowvelocitywilldecreaseduetothechangingof gasflowdirection. In this research, we used straight pipe line because it did not change the gas movement directions, so the gas did not lost energy for flowontothecell.However,thestraightpipewasshortrangeforgasflowaffectedtothe

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a)

b)

c)

d)

a)

b)

c)

d)

a)

b)

c)

d)

a)

b)

c)

d)

Figure1. Flow patterns of fuel cell a) Model 1 b) Model 2 c) Model 3 and d) Model 4.

d)

small area for chemical reaction, therefore we try to increase the area of chemical reaction by using serpentine and slotted serpentine flowpattern.

The dimension of the PEMFC was 10×10×4.5 cm3 for electrode size and 5×5×0.1 cm3 for membrane size, electrode layer was 5×5×0.15 cm3 and catalyst layer was 5×1.5×0.0019 cm3. The input parameters for calculations were temperature, pressure and relative humidity. Hydrogen gas was fed onto anode channel and air was fed onto cathode channel. Instead of using pure oxygen, air was used in this study. The chemical reaction between hydrogen and oxygen is expressed as Eq. 1

2 2 22   2H O H O heat+ + (1) The assumptions of the simulation used

in developing the model for the fuel cell were shown as follows:

(1) Steady state conditions on the single cell(2) Ideal gas mixtures assumption(3)Incompressibleandlaminarflow(4)Workingfluidshavebeenconsideredas

H2, H2O and air (5) Constant pressure 1.1 atm(6) Constant relative humidity at 100 percent

The hydrogen gas is split into hydrogen proton and electron and then hydrogen protons move through membrane to cathode side to interact with reduced oxygen to form water. At the same time, the electrons move through an external circuit and come to the cathode, socurrentflowsduetotheflowof electrons.Typically, the principle of fuel cell involves the mechanisms such as moving of gas, mass transport, proton transferring through membrane and thermal energy of the system. This research studied the performance of PEMFC which set tobelaminarflow.Diffusionfluxof currentdependsonconcentrationfluxandtemperatureas shown in the following equation:

2 2 2 2 2, H H H m H T H

TJ D Y D

T∇

=−ρ ∇ − (2)

fordiffusionfluxof hydrogengas,and

2 2 2 2 2, O O O m O T O

TJ D Y D

T∇

=−ρ ∇ − (3)

fordiffusionfluxof oxygengas.Where 2HJ isdiffusionfluxof hydrogengas, 2OJ isdiffusionfluxof oxygengas,ρ is density of gas,

2H mD isdiffusivecoefficientof hydrogengas in mixture,

2O mD isdiffusivecoefficientof oxygen gas in mixture,

2,T HD is thermal diffusion of hydrogen gas,

2,T OD is thermal diffusion of oxygen gas,

2HY is mass fraction of hydrogen

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gas and 2OY is mass fraction of oxygen gas.

Meanwhile,thediffusioncoefficientof masstransportontoflow-fieldscanbeexplainedbyMaxwell-Stefan multi-component diffusion as shown below [22]

( )1

NT

i i ij j j j j i i ij j

M M p Tw w D w w x w w u D R

t M M p T=

∂ ∇ ∇ ∇ρ +∇ −ρ ∇ + + − + ρ + =

∂

∑

( )1

NT

i i ij j j j j i i ij j

M M p Tw w D w w x w w u D R

t M M p T=

∂ ∇ ∇ ∇ρ +∇ −ρ ∇ + + − + ρ + =

∂

∑ (4)

where iw is mass fraction of solution i, Ri is

excess reaction product rate of i component, ρ is density of gas, u is the velocity of gas, p is pressure, ijD is the Maxwell-Stefan diffusivities,

TiD isthermaldiffusioncoefficientof i, M is

molecular mass of gas Mj is molecular mass of solution j and xj is mole fraction of solution i. Fromthediffusionfluxof massandMaxwell-Stefan multi-component diffusion, it can be seen that the temperature affects mass transport activity and thus affects fuel cell performance.

3. RESULTS AND DISCUSSION3.1 The Effect of Temperature on Diffusive Flux Magnitude of H2 and O2 Gases

The calculation was performed for diffusive fluxmagnitudeof hydrogengas,thediffusivefluxmagnitudeof hydrogengasinModel1increases with increasing cell temperature and thediffusivefluxmagnitudeof hydrogengasis shown in Figure 2a(left), whereas Figure 2b(left),2c(left) and 2d(left) illustrates the diffusive fluxmagnitudeof Model2,3and4.Incaseof Model2,thediffusivefluxmagnitudeof hydrogen gas is also affected by temperature. However, In case of Model 3, the diffusive flux magnitude of hydrogen gas does not increase with increasing temperature due to theslottedserpentinecharacteristicof theflowpattern. The slotted serpentine characteristic affects the decomposition capability and the movementof gasonflowpatternsothediffusion of gas is unavoidably hindered due to the nature of flow pattern. The

diffusivefluxmagnitudeof hydrogengasalso increases with increasing temperature in Model 4.Figure2 (right). shows the diffusive fluxmagnitudeof O2 for Model 1,Model 2 and Model 3 which are equal, stright line, becauseof thesameflowfieldof gasasdirect tube. However, in case of model 4, thediffusivefluxmagnitudeof oxygenhasmuch lower value than the other because of serpentine characteristic affected to lossenergyof diffusionwhentheflowingdirection of gas is changed.

3.2 The Effect of Temperature and Flow-field Patterns on Diffusive Velocity of H2 and O2 Gases

Figure 3 shows the diffusion velocity of H2 and O2 of all models at 30 °C on the same conditions. In Figure 4a, the diffusion velocity of H2 of Model 1 increases with increasing of thegasflow-fielddistanceonthestraightpipeflow.However,thediffusionvelocityof H2 and O2 are the difference because of the property of each gas. Considering the Model 1 (Figure 3a), Model 2 (Figure 3b) and Model 3 (Figure 3c), the difference of H2gasflowpatterns(anodeside)affecttothe diffusion velocity. Model 1 shows the highest diffusion velocity. Model 2 has less diffusion velocity than Model 1 because theserpentineflowpatternontheModel2 makes the length of diffusion velocity of gas longer. Moreover, the changing of gas directions affect to energy loss, resulting in the decrease of diffusion velocity of gas. Model 3 (Figure 3c) has less diffusion velocity of H2 than Model 1 and Model 2 because theflowpatternobstructsthegasflow.Considering the Model 2 and Model 4 (Figure 3b and 3d), the diffusion velocity of H2 on the Model 2 is higher than Model 4 because the difference of the diffusion velocity of O2 on both models affected to the production

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a)

b)

c)

d)

H2 O2

H2

O2 H2

b)

H2 O2

H2 O2

d)

Figure2. Diffusivefluxmagnitudeof H2 (left) and O2 (right) gases as a function of temperatures, a) Model 1, and b) Model 2, Model 3 and Model 4.

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a)

b)

c)

d)

Figure 3. Diffusion velocity of H2 (left) and O2 (right) gases a) Model 1 b) Model 2 c) Model 3 and d) Model 4.

a)

b)

c)

d)

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of water within a cell. Therefore, the velocities of electron which moves from the anode side to the cathode side of these two models are different; resulting in the changing of the fuel cellefficiencyforeachmodel.

Figure 4 shows the diffusion velocity of H2 gas as a function of temperature, the solid line illustrates the behavior of each model. In the case of Model 1 as well as Model 2 and 4, the diffusion velocities of gas increase with increasing temperature. Nevertheless, in the case of Model 3, the diffusion velocity of H2 gas behaves as same as those of Model 1, 2 and 4 from 30 to 40 °C but when the temperature is higher than 40 °C the diffusion velocity of H2 gas starts to decrease and stays constant after the temperature exceeds 50 °C. The diffusion velocity of O2 gas has the same velocity on the Model 1, Model 2 and Model 3 because the samecharacteristicof flowpatternasFigure4(right). Similarly to Figure3, the Model 4 has the lowest diffusion velocity.

3.3 The Effect of Temperature on Diffusion Through Membrane

Figure 5 shows the same diffusivity values (Model 1-4) of proton through membrane as a function of temperature based on Maxwell-Stefan diffusivity model. It’s clear that the diffusivity of proton increases with increasing temperature. Therefore, the higher temperature of the cell, the higher amount of proton through membrane diffuse; that also implies the amount of produced water in cathode side. Since the amount of proton diffusing through membrane is promoted, the higher performance of PEMFC will eventually be achieved.

3.4 The Effect of Temperature and Flow Pattern on Performance of PEMFC

The temperatures varied from 30 to 80 with incrementof 10°Cwhilethepressure,gasflowrate and relative humidity were 1.1 atm, 150 sccm and 100%, respectively. The calculated results reveal that the performances of fuel

cells have the same fashion and increases with increasing temperatures [2-5]. These results are consistent with the reference, explaining that the energy is increased upon increasing of temperature.

Figure 6 shows the relation between current density and voltage for all models increases. It is clearly seen that Model 2 exhibits the highest performance. In comparison, Model 2 shows higher performance than that of Model 1 and 3. This result could be explained by the fact that Model 2 has a larger catalytic region generating more hydrogen decomposition which results in the number of proton and electron populations. Furthermore, in comparison of Model 2 and Model4,becausetheflowpatternonthecathodeside of Model 4 is the serpentine which usually inhibitstheflowof reactantgasattheturningpoint of channels because of the turbulence of gases. Therefore performance of Model 4 is lower than that of Model 2.

Figure 7 shows the relationship between current density and power density for all models.Infact,flowdifferentpatternsaffectgas velocity. This mean that the distribution and diffusion of gas in each region of models that affect the performance of fuel cells are related totheflowpatterns[21].Incaseof Model1,ithas the higher diffusion velocity value than the others. Since the velocity of gases is in proper region the reaction rate is also promoted. In Model2theserpentineflowpatternareutilizedontotheanodesidewhilecathodeflowpatternis straight pipe, the performance of fuel cell hasbeenmoremodified.Model3hasslottedserpentineatanodesidewhilecathodeflowpatternisstraightpipe;theflowpatternhindersthegasflow.TheModel4hasserpentineinboth sides; therefore, it has higher performance than Model 1 and Model 3.

4. CONCLUSION Theefficiencyof fuelcellrelatingthe

proton exchange membrane fuel cell depends on the cell temperatures and the diffusion of

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a)

b)

c)

d)

H2 O2

H2 O2

H2 O2

c)

H2 O2

22

d)

Figure4. Diffusion velocity of H2 (left) and O2 (right) gases a) Model 1 b) Model 2 c) Model 3 and d) Model 4.

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Model 1,2,3,4

Figure5. Diffusivity values (Model 1-4) of proton through membrane as a function of temperatures based on Maxwell-Stefan diffusivity model.

Figure6. Relationship between current density and cell voltage of Model 1-4 cells.

Figure7. Relationship between current density and power density of Model 1-4 cells

gas in each region of the model. The increasing in the thermal energy of system promotes more energy into system and fuel performance also.Theserpentineflow-fieldforanodeandstraight pipe for cathode (Model 2) has the best performance of 86.04% which is due to theflow-fieldpatternthatdidnotretardflowof gas maintains good thermal distribution.

ACKNOWLEDGMENTSThisworkwasfinanciallysupportedby

the Higher Education Research Promotion and National Research University Project of Thailand through the research grant no.AFM-2553-M-08 and Computer Service Center of King Mongkut’s Institute of Technology Ladkrabang for COMSOL Multiphysics and computer facilities, Office of the Higher Education Commission through the Advanced Functional Materials Center of Khon Kaen University, and Thailand Center of Excellence in Physics (ThEP), Chiang Mai University.

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