A Computational Study of Stoichiometry Sensitivity of a PEM Fuel Cell with

Multi-Parallel Flow ChannelsM. A. Rahman 1, J.M. Mora2, P. A. Chuang1

1. Thermal and Electrochemical Energy Laboratory (TEEL), Department of Mechanical Engineering,

University of California, Merced, CA, USA

2. University of Philippines – Diliman, Quezon City, Philippines

IntroductionEven though parallel channel flow fields are efficient in uniform reactant

distribution, they are also susceptible to liquid water accumulation and

blockage due to low pressure drop. In this study, the effects of

stoichiometry on water accumulation and fuel cell performance is

analyzed in a fuel cell with 17 straight-parallel channels based on Comsol

simulated results.

Computational MethodsThe fuel cell computational domains include inlet/exit manifolds, flow

channels, diffusion layers, electrodes, and membrane. “Reacting flow in

porous media” interface is used to solve both the Brinkman equation for

momentum transport and Maxwell-Stefan equation for species diffusion. To

solve for electrode kinetics, “Secondary current Distribution” interface is

used, which solves Ohm’s law to obtain the electric potential. More

equations and variables of our simulation method can be found in Table 2.

Results

ConclusionsA three dimensional isothermal steady-state simulation model was

performed using COMSOL Multiphysics 5.2. Our results demonstrate

that with the increase of stoichiometric flow rate, water accumulation in

channel was reduced significantly. At an operating voltage of 0.7V, the

level of saturated liquid water (RH > 100%) in the reference channel

(shown in Fig. 1) is reduced to 0% at the channel exit when the

stoichiometric flow rate is greater than 5.

References:1. E.U. Ubong et al. Three-Dimensional Modeling and

Experimental Study of a High Temperature PBI-Based PEM

Fuel Cell, Journal of The Electrochemical Society, 156 B1276-

B1282 (2009)Figure 2. User controlled mapped mesh in diffusion layers, electrodes, and membrane

Figure 1. Simulated flow field geometry

Figure 5. The distribution of relative humidity in cathode channels when the cell is operated at 0.7 V (a,b,c) and 0.4 V (d,e,f)

Figure 6. Distribution of relative humidity along the channel when the cell is operated at 0.7 V (left) and 0.4 V (right)

Anode inlet

Anode outlet

Cathode outlet

Cathode inlet

Stoich 5 Stoich 15

Physics Equation Variable

Electrochemistry

Ohm's Law∅_l Electrolyte potential and

∅_s electric potential

Charge

Conservation

∅_l Electrolyte potential and

∅_s electric potential

Linearized Butler-

VolmerHOR Overpotential

Cathodic Tafel ORR Overpotential

Fluid flow in porous

media

Continuity Velocity components of flow (u)

Brinkman Equation Velocity (u) and Pressure (P)

Mass TransportMaxwell-Stefan

DiffusionMass fraction (x)

MeshUser controlled structured mesh has been created on YZ plane in the

channel, which is then swept in X direction. In the inlet and outlet manifold,

physics controlled tetrahedral mesh has been used. Figure 2 shows the

generated mesh for simulation.

Table 2. Physics, equations, and solved variables used in our simulation model

Stoich 1

Operating Condition

Temperature 40°C

Pressure 150 kPa

Relative Humidity 50%

Table 1. Operating conditions studied in COMSOL

Figure 3. Polarization curve Figure 4. Power density curve

a b c

d e f

Ref.

Channel

XY

Z

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.4

0.5

0.6

0.7

0.8

0.9

Ce

ll V

olt

ag

e (

V)

Current Density (A/cm^2)

Stoich 5

Stoich 15

Stoich 1

Polarization Curve

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Po

we

r D

en

sit

y (

W/c

m^

2)

Current Density (A/cm^2)

P_Stoich 5

P_Stoich 15

P_Stoich 1

Power Density Curve

0.000 0.005 0.010 0.015 0.020

50

100

150

200

250

Rela

tive H

um

idit

y (

%)

Length of channel

Stoich 1

Stoich 5

Stoich 15

RH Distribution in channel at 0.7V

0.000 0.005 0.010 0.015 0.020

0

100

200

300

400

500

Re

lati

ve

Hu

mid

ity

(%

)

Length of Channel

Stoich 1

Stoich 5

Stoich 15

RH Distribution in channel at 0.4V

Excerpt from the Proceedings of the 2017 COMSOL Conference in Boston