Performance <70 words. This paper tries to investigate the

Performance Optimization of
PEM Fuel Cell by Using Modified Bruggeman Correlation Model


Thapa1, Gye Choon Park*2, Sung Gi Kwon 3, Jin Lee4

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1,2,3,4 Department of Electrical Engineering, Mokpo
National University, Korea

[email protected],    [email protected], [email protected], [email protected]



Background/Objectives: In Proton Exchange
Membrane Fuel Cell (PEMFC), the modified Burggeman correlation is used
to estimate the effective conductivity and diffusivity of both catalyst and gas
diffusion of PEM fuel cell.


Methods/Statistical analysis: It should be <70 words. This paper tries to investigate the sensitivity of the gaseous diffusion to the various exponential value of tortuosity factor m and n. This model provides empirical correlation for the effective properties of composite system.   Findings: It should be <170 words. The effective conductivity and diffusivity in the catalyst layers (CLs) and gas diffusion layers (GDLs), is crucial for accurately predicting the fuel cell performance and optimizing design parameters in the numerical modeling/simulation. When the value of m and n are increased the normalized function's as well as saturation function's values are decreased. Which indicates that the distribution of pore size of porous medium and the water saturation has greater impact on the gas diffusion. Thus these parameter has greater impact on the performance optimization on the PEM fuel cell.   The simulation results shows that the performance of Polymer Exchange Membrane Fuel Cell (PEMFC) is varies with the m and n. At higher current density values, the value of m=n=1.5 give the better performance.   Improvements/Applications: In <30 words. For the better improvements of PEM fuel cell performance, conductivity and diffusivity of hydrogen gas must be increased. Conductivity can be increased by increasing the temperature of the gas as well as stack temperature whereas the diffusivity can be increased highly concentrated hydrogen and oxygen gas and more porous membrane.   Keywords: PEM Fuel Cell, Conductivity, Diffusivity, Bruggeman Correlation, Tortuosity Factors     1.   Introduction   A fuel cell is an electrochemical device that can convert chemical energy into electrical energy and produce heat and water as a byproduct through the electrochemical process. For the hydrogen gas generation either PEM water electrolyzer or alkaline electrolyzer is used but for the electricity generation we use PEM fuel cell stack. PEM fuel cell technology is one of the alternative resources which provides high quality power in distributed generation system 1. Due to smaller size, light weight, high power density, low operating temperature, safe construction, more efficient and fast start-up, PEM fuel cell is more preferable than other type of fuel cell 2. PEM fuel cell gives more than 40% efficiency in most of the stationary and transport applications. The schematic diagram of PEM fuel cell is shown in figure 1 3  




2. Proton
Exchange Membrane Fuel Cell (PEMFC)


Figure 1 shows
the geometry of a single fuel cell, which consists of a membrane, catalyst
layers, gas diffusion layers, gas channels, and two collector plates. The main
functions of these components are: collector plates with flow channels are used
for reactants and products transport, electron conduction and heat removal; gas
diffusion layers are for reactant distributions, electron conduction, and
liquid water removal; catalyst layers are used to promote electrochemical
reactions where reactants are consumed, and products and heat are generated;
and the membrane is used to conduct protons from the anode catalysts layer to
the cathode catalyst layer.

The electrochemical reaction occurring in the anode in which
hydrogen gas is consumed to produce protons and electrons 4, i.e.,


The produced electrons
are passes through an external circuit to the cathode by providing electrical
power, while the protons transport through the membrane to the cathode. At the
cathode catalyst layer, oxygen combines with the protons and electrons to
produce water, i.e.,


And overall cell
reaction occur during the electrochemical reaction process is given by,


                                       Water                                        (3)

 Above these reactions involve on border
between ionically conductive electrolyte and electrically conductive electrode.
For the better electrochemical reaction and the gases to arrive as well as
water to leave, the electrodes must be porous medium. Under steady state
conditions, the thickness of the cell is negligible compared to its other
isothermal approximation and the membrane is assumed to be fully hydrated.
Moreover, the anode reaction over potential is neglected in the present study,
because over potential due to the anode reaction to be negligible 5.
Therefore, overall cell potential is obtained by subtracting the losses from
reversible cell voltage which is given by the following expression;



Where,  is the reversible cell voltage and  is the activation loss,  is Ohmic loss,  is concentration loss and  is the diffusion loss.  is calculated from a modified version of the
Nernst equation, which is modified with an extra term to account for changes in
temperature from the standard reference temperature 6. Which is given by Eq.


Where P and T
represent the effective pressure and temperature respectively. Activation
losses can calculated by using empirical equation 7;



Where  are empirical coefficient and I is the cell
current and T is the absolute temperature.  is the undissolved oxygen concentration which
can be expressed as 8;


Ohmic Overpotential

Due to membrane
resistance (Ionic Resistance)

The voltage drop due
to the membrane resistance to the flow of ions produce the ohmic overpotential
loss in the fuel cell.

is the ionic resistance as a function of membrane conductivity,  is the membrane height and  is the ionic conductivity of membrane with
water content and temperature 9.

Where  be the degree of membrane humidification and is the cell temperature.


Electronic Resistance

The potential loss due
to the electronic bipolar plates and electrodes current collectors is called
electronic resistance losses and is given by,

But the ohmic
resistance of the electronic materials is given by,

Where,  is the material resistivity, l is the length
and A is the cross-sectional area of the conductor.

The ohmic
overpotential due to electronic and ionic resistance is 10 given by,


The resistance proton
is calculated by the following expression,

The resistivity of the
membrane depends on the water activity and cell temperature. Empirical formula for  is express as follows 11.


J is the current
density with in the cell. The value of l can be fitted for a particular cell.
To obtain the value of l we can use the Sharifi model, which is given by 7;



3. Gas
Diffusion and Catalyst Layer


Both gas diffusion and
catalyst layers are porous media, the diffusion of oxygen gas at the cathode
terminal is given by 12;

The diffusion of
hydrogen gas at the anode terminal is given by 13;


The production of
water at the cathode terminal side is given by 14;


4. Bruggeman Correlation

According to the
Flick’s law, the gaseous diffusion in gas diffusion layer (GDL) and catalyst
layers (Cl) of PEM fuel cell is given by,


Where,  is the porosity of GDL,  be the concentration and  be the effective diffusivity of the reactants

Similarly, according
to the Bruggemann correlation, the effective diffusivity in a porous structure
can be expressed as 15,


Where,  is the tortuosity factor of porous medium. The
tortuosity ( is defined as the ratio of the actual flow
path length and the thickness of the porous medium along the flow direction.

When the impact of
liquid water saturation is taking into account then above Bruggeman correlation
equation become,


Where,  are normalized functions.

Similarly, tortuosity
factor for Burggeman exponents m=0.5 and saturation exponents, n=1.5. Thus
above equation 7 becomes;

Then normalized
function for modified Bruggeman correlation 16 with tortuosity factor (m=n=1)
is given by the following equation;


Where, be the effective conductivity and is the concentration of the liquid.




Simulation Results

The figure 2 and 3 shows the simulated
polarization curve by using different value of m and n. At higher current
density the PEMFC has significant effect of the tortuosity factor m and n.




The normalized function g(s) Vs average
porosity value  and average water saturation function g(s) Vs
average saturation   for
various values of m and n are shown in figure 5 and 6.When the value of m and n
are increased the normalized function’s as well as saturation function’s values
are decreased. Which indicates that the distribution of pore size of porous
medium and the water saturation has greater impact on the gas diffusion. Thus
these parameter has greater impact on the performance optimization on the PEM
fuel cell.






The tortuosity factor for Burggeman
exponent’s m and n has significant effect on the performance of PEM fuel cell
at current density. So for the PEMFC design process the value of m and n must
be equal to 1.5. Above this value the fuel cell performance will be decreased.
The sensitivity of the fuel cell performance to the value of n in GDL and CL is
neglected as compared to that in the cathode GDL and CL but the sensitivity in
cathode CL is stronger than that in the cathode GDL. So for the performance
optimization of fuel cell, concentration of the gas should be increased and
water removal from the cathode GDL and CL during gaseous diffusion, must be


7. Acknowledgement

This work was supported by KEPCO Research
Institute grant funded by Korea Electric Power Corporation (R16DA11) and
Business for Cooperative R&D between Industry, Academy and Research
Institute funded by Korea Small and Medium Business Administration (C0442952).



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