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
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
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.
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
Keywords: PEM Fuel Cell, Conductivity,
Diffusivity, Bruggeman Correlation, Tortuosity Factors
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
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,
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;
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.
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.
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;