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3.3 Warm-up Procedure
Before performing any experiment, you should run the stack through a warm-up
procedure to ensure that the stack is ready to give good performance. Perform a warm-up
staircase as follow:
1.) Set air flow rate to 200 mL/min and hydrogen flow rate at 60 mL/min.
2.) Perform a current staircase from 0 to 2.5 amps with 0.5 amps increments and hold
each stair for 2 minutes.
3.4 Experiment Guide
Following is a list of four basic experiments to analyze PEMFC performance. In all of
these experiments you will collect the basic data of current and potential. The correlation
between the current and potential is the key information in analyzing PEMFC
performance. You will also need to keep a record of temperatures, gas flow rates, and
pressures.
Remember that a fuel cell system is similar to traditional chemical reaction experiments
your laboratory might have. It is a reaction system and you can study the kinetic,
transport, and design parameters that govern its performance. The reaction rates will be
affected by reactant and product concentrations, temperature, and catalyst just as in a
traditional chemical reactor. The difference in operation of the PEMFC is that there is
one more driving force, potential, and you will have direct control over the reaction rate,
which will be measured in current. Always remember that the measured current is your
reaction rate. Current is a measure of the rate of flow of electrons, which is directly
dependent on the reaction rate. A detailed discussion of the data analysis may be seen in
Chapter 4.
3.4.1 Experiment I: Flow Rates
The first experiment is the study of reactant gas flow rates. The flow rates of gas will
affect the concentration and mass transport of the system. For Experiment I, you will
perform several current staircases at varied flow rates. After performing the designated
warm-up procedures, setup the stack temperature at 60°C, the hydrogen humidifier at
75°C, and the air humidifier at room temperature.
Air Flowrate:
1.) Set Air Flow rate at 200 mL/min
2.) Set H Flow rate at 160 mL/min
3.) Perform 2 current staircase from 0 to 3.0 Amps with 1 minute stairs and 0.5 amp
increments.
4.) Change Air Flow rate to 400 mL/min
5.) Perform 2 current staircases from 0 to 5.0 Amps with 1 minute stairs and 0.5 amp
increments
6.) Change Air Flow rate to 600 mL/min
7.) Perform 2 current staircases from 0 to 5.0 Amps with 1 minute stairs and 0.5 amp
increments
8.) Set current at 3.0 Amps
9.) Set air flow rate at 200 mL/min and hold for 10 minutes
10.) Increase air flow rate to 300 mL/min and hold for 10 minutes
11.) Increase air flow rate to 400 mL/min and hold for 10 minutes
12.) Analyze the stack and cell voltages and discuss the results.
Hydrogen Flowrate:
1.) Set Air Flow rate at 600 mL/min
2.) Set H Flow rate at 120 mL/min
3.) Perform 2 current staircases from 0 to 5.0 Amps with 1 minute stairs and 0.5 amp
increments
4.) Change H flow rate to 160 mL/min
5.) Perform 2 current staircases from 0 to 6.0 Amps with 1 minute stairs and 0.5 amp
increments
6.) Change H Flow rate to 200 mL/min
7.) Perform 2 current staircases from 0 to 6.0 Amps with 1 minute stairs and 0.5 amp
increments
8.) Analyze the stack and cell voltages and discuss the results.
Once you are finished, follow the shut down procedures at the end of this chapter
Table 3.1 provides the reactant gases needed in order to generate respective current in 1:1
stoichiometric ratio. In reality, excess reactant gases are always needed. Hydrogen flow
may be controlled at 1.2 to 1.5 times the stoichiometric ratio. The air flow is usually well
above the stoichiometric requirement, use at least two times as much. In addition, for a
stack of three cells, multiply by 3 for reactant gases needed,
Table 3.1 Stoichiometric reactant flow rate required for current generation at STP.
Current (A)
H
J(cc/min) [02T j(cc/min) Air
(cc/mm) Current (A)
H
(cc/mm) 02
0.5
3.5
1.7
8.3
10.5
73.1
36.6 174.1
1
7.0
3.5
16.6
11
1.5
10.4 5.2
24.9
11.5
80.1
40.0 190.7
2
13.9 7.0
33.2
12
2.5
17.4 8.7
41.5
12.5
87.1
43.5 207.3
3
20.9 10.4 49.7
13
3.5
24.4 12.2 58.0
13.5
94.0
47.0 223.9
4
27.9 13.9 66.3
14
4.5
31.3 15.7 74.6
14.5
101.0
50.5 240.4
5
34.8 17.4 82.9
15
5.5
38.3 19.2 91.2
15.5
108.0
54.0 257.0
6
41.8 20.9 99.5
16
6.5
45.3 22.6 107.8
16.5
114.9
57.5 273.6
7
48.8 24.4 116.1
17
7.5
52.2 26.1 124.4
17.5
121.9
60.9 290.2
8
55.7 27.9 132.7
18
8.5
59.2 29.6 141.0
18.5
128.8
64.4 306.8
9
62.7 31.3 149.2
19
9.5
66.2 33.1 157.5
19.5
135.8
67.9 323.4
10
69.6 34.8 165.8
3.4.2 Experiment II: Stack Temperatures
Among different types of fuel cells, the proton exchange membrane (PEM) fuel cells are
most suitable for low temperature operation. PEMFCs are generally operated from 50°C
to 100°C. Higher temperature is favored by the reaction kinetics, but can also lead to
membrane dehydration, which will hinder ionic conductivity of the membrane.
Therefore, the stack operating temperature is a key parameter to study. You will start by
completing the warm-up procedure.
Temperature Experiment
1.) Set Air flow rate at 600 mL/min
2.) Set H flow rate at 160 mL/min
3.) Set hydrogen humidifier temperature to 75°C and leave that of oxygen at room
temperature
4.) Set cell temperature to 40°C
5.) Perform 2 current staircases from 0 to 6.0 Amps with 1 minute stairs
6.) Change cell temperature to 60°C
7.) Perform 2 current staircases from 0 to 6.0 Amps with 1 minute stairs
8.) Change cell temperature to 70°C
9.) Perform 2 current staircases from 0 to 6.0 Amps with 1 minute stairs
10.) Analyze the stack and cell voltages and discuss the results.
3.4.3 Experiment III: Pressure
Increasing the operating pressure of the fuel cell will improve performance by increasing
the concentration of the reactant gases. The back pressure (that is the pressure at the
effluent) of the fuel cell may be adjusted by using the back-pressure regulators and
monitored by the pressure gauges. We do not recommend a fuel cell test at differential
pressure (that is the pressure difference between hydrogen and air flow streams) because
the membrane is only 50 micrometer thick and the high pressure will increase the chance
of gas crossover and membrane rupture. Please be attentive when operating above
ambient pressures. You will compare the fuel cell performance at atmospheric pressure
and with pressures controlled at 5 and 10 psig. Turn the regulator clockwise to increase
the pressure. The pressure reading is displayed on the gauge right above the regulator.
Note that the flow rate reading on the flow meter will change when the backpressure
changes, this is because the flow meter is calibrated for room temperature and
atmospheric pressure.
Pressure
1.) Set Air flow rates at 600 mL/min and H flow rate at 160 mL/min
2.) Set hydrogen humidifier temperature to 75°C and cell temperature to 60°C
3.) Leave the Air humidifier at room temperature.
20
4.) Perform 2 current staircases from 0 to 6 Amps with 1 minute stairs and 0.5 amp
increments
5.) Slowly increase both Air and H back pressures to 5 psig
6.) Adjust the Air and H flow rate readings to account for the increase in pressure.
7.) Perform 2 current staircases from 0 to 7 Amps with 1 minute stairs and 0.5 amp
increments
8.) Slowly increase pressures to 10 psig
9.) Adjust the Air and 112 flow rate readings to account for the increase in pressure.
10.) Perform 2 current staircases from 0 to 8 Amps with 1 minute stairs and 0.5 amp
increments.
11.) Analyze the stack and cell voltages and discuss the results.
When you are finished, reduce the pressures back to ambient pressure and follow the shut
down procedures.
3.4.4 Experiment IV: Anode and Cathode Humidification
During operating water is dragged from the anode to the cathode by electro-osmosis.
(Protons exist in the membrane as hydrated protons. So when they move from the anode
to the cathode they bring water along with them.) Consequently, to prevent membrane
dehydration at the anode water is often added to the anode stream. The common approach
of adding water to the anode side of the membrane is to use the hydrogen gas to carry it
in. This can be accomplished by humidifying the hydrogen gas stream prior to feeding it
to the fuel cells. Anode humidification is done in this fuel cell setup by bubbling the
hydrogen gas stream through a column of DI liquid water. DI water is used to prevent
contamination of the membrane.
On the other hand, at the cathode water is generated both by the oxygen reduction
reaction and electro-osmotic drag from anode. Depending on the operating conditions
(current, gas flow rate and temperature) the cathode could be operating at flooding
condition, neutral hydration state or dehydration. The cathode becomes flooded when the
water generation rate exceeds the water removal rate by evaporation, water vapor
diffusion and wicking of liquid water. When these processes are equal the cathode is not
flooded and the membrane on the cathode is at its hydrated state. However, when the
water removal rate exceeds the water generation rate (for example, at high temperature
and high air flow rate) the membrane on the cathode side can become dehydrated.
In this experiment, you will study the effects of anode and cathode humidification on the
fuel cell performance. Remember that the amount of water you can add to a gas stream is
a function of the water vapor pressure (which depends on the temperature of the
humidifier system), the gas flow rate and the total pressure. Similarly, the water removal
rate by evaporation depends on the system temperature which controls the water vapor
pressure, the humidity level and the flow rate of the feed gas stream, and the total
pressure.
Anode Humidification Experiment
1.) Set Air flow rate at 600 mL/min and H flow rate at 160 mL/min
2.) Leave Air humidifier at room temperature.
3.) Set hydrogen humidifier temperature to 50°C
4.) Set cell temperature to 70°C
5.) Perform 2 current staircases from 0 to 5.5 Amps with 1 minute stairs
6.) Set hydrogen humidifier temperature to 65°C
7.) Perform 2 current staircases from 0 to 5.5 Amps with 1 minute stairs
8.) Set hydrogen humidifier temperature to 80°C
9.) Perform 2 current staircases from 0 to 5.5 Amps with 1 minute stairs
10.) Analyze the stack and cell voltages and discuss the results.
Cathode Humidification Experiment
1.) Set Air flow rate at 600 mL/min and H flow rate at 160 mL/min
2.) Set hydrogen humidifier temperature to 80°C
3.) Set Air humidifier temperature to 25°C
4.) Set cell temperature to 70°C
5.) Perform 2 current staircases from 0 to 5.5 Amps with 1 minute stairs
6.) Set Air humidifier temperature to 40°C
7.) Perform 2 current staircases from 0 to 5.5 Amps with 1 minute stairs
8.) Set Air humidifier temperature to 55°C
9.) Perform 2 current staircases from 0 to 5.5 Amps with 1 minute stairs
10.) Analyze the stack and cell voltages and discuss the results.
1.2
I
E
0.8
0.6
U)
a
0.4
0.
U)
>
0.2
0
120
Figure 3.2 Saturated Water Vapor Pressure with Respect to Temperature.
3.4.5 Other Experiments
The RU-2 100 is not limited to these five experiments. An infinite number of experiments
may be performed. You will find the features of TVN’s ControlWolf very useful in
designing and running custom experiments. You are encouraged to design you own
experiment to examine other parameters of interest. Particularly you can consider
different flow patterns, different flow plate designs, different number of cells, MEAs with
different catalyst loadings, MEAs with different GDL, MEAs with different active areas,
etc. Contact TVN for any accessories you require.
3.5 Shutdown Procedure
1.) Disengage the load (15)
2.) Disconnect FC (2)
3.) Change all set point temperature to 0 °C (20-22)
4.) Disconnect all heaters (25,35,36)
5.) Reduce flow rate using needle valve (7,8), but do no tighten.
6.) Close gas shutoff valves (3,4); handles perpendicular to flow tubes.
7.) Disconnect all gas lines to fuel cell
8.) Disconnect line from H2 humidifier to the inside panel (33)
0
20
40
60
80
100
Temperature (°C)
9.) Turn off main power switch (1)
10.) For additional safety, unplug power cable and gas lines
3.5.1 Emergency Shutdown Procedures
In case of any emergencies, follow these immediate procedures
1.) Immediately close shutoff valve (3,4)
2.) Turn off main power switch (1)
3.) Disengage FC (2)
Note: If the system will be idle for long periods of time, disconnect all lines to the fuel
cell stack and cap the inlet and outlet fittings.
CHAPTER 4: DATA ANALYSIS
4.1 Graphical Approach
A fuel cell’s performance is often displayed by its Cell-Potential-versus-Current- Density
curve, also known as the polarization curve. See Figure 4.1 below. The current density
(A/cm can be obtained by dividing the total current, I (A), by the electrode area (16 cm
Thermodynamically, the standard potential for a hydrogen/oxygen fuel cell is about 1.23
V. However, due to the formation of hydrogen peroxide as a side reaction and the
crossover of the hydrogen to the oxygen side and likewise for the oxygen the fuel cell
open-circuit potential is never equal to the thermodynamic potential. The gas crossover
rate is a function of the membrane dehydration. A more hydrated membrane acts as a
better gas separator. During operation, you will notice that as the membrane becomes
more hydrated, the gas crossover rates are reduced giving rise to higher open circuit
voltages.
1.25
Open-circuit voltage is lower than thermodynamic voltage due to gas crossover and
peroxide formation
5 C.)
Voltage Loss due to Activation Resistance
1.15
1.05
0.95
0.85
0.75
0.65
0.55
0.45
0.35
0.25
0.15
Voltage Loss due to Ohmic
Resistance and Concentration Polarization
Voltage Loss due to Mass Transport Limitation
0.00
0.50
1.00
1.50
2.00
Current Density (A/cm
2.50
Figure 4.1. A polarization curve of a H PEM fuel cell.
The performance of a fuel cell can be divided into three main regions, each controlled by
different physical and chemical phenomena. The sharp voltage drop of the first region is
associated with the activation resistance in the cell. This resistance is attributed to the
type of catalyst and the catalyst surface area that is both in contact with the electrolyte
and the electrical network in the electrode and accessible to the reacting gases. Lowering
this resistance will raise the whole polarization curve.
The gradual drop in voltage of the second region is attributed to the ohmic resistance in
the cell and the depletion of the reactive gas at the catalyst surface. The ohmic resistance
comes from ionic resistance of the membrane, electronic and contact resistance of the
electrodes, bipolar plates and current collectors. Lowering this resistance will result in a
lower slope in the E-I curve and consequently higher power densities at higher energy
efficiencies.
The sharp voltage drop in the third region is attributed to the mass transport limitation,
which occurs when the transport of the reactant to the reaction surface fails to keep up
with the reaction. This phenomenon is especially severe at the cathode of the fuel cell
where oxygen is the reactant because of the presence of liquid water within the porous
structure of the electrode and on the catalyst membrane surface. This liquid water, which
is the product of the cathodic reaction and proton transport from the anode, acts as an
additional barrier to the transport of oxygen to the reaction sites. Minimizing this
resistance will allow the ohmic region to be extended and result in much higher power
density operation.
Voltage loss in the first region can be reduced by using catalysts with lower activation
resistance and increasing the catalyst surface available for reaction per unit volume of
electrode. Currently, platinum is the best catalyst available. Note that the voltage loss by
gas crossover can be minimized by using a thicker membrane and keeping the membrane
well hydrated. However, any reduction in voltage loss by gas crossover by using a thicker
membrane must be considered against the additional ohmic voltage loss of a thicker
membrane.
Voltage loss in the second region can be reduced by employing thinner membranes and
membranes with lower ionic and water transport resistance and humidifying the anode
gas stream (and the cathode gas if air is used). The last voltage loss region, associated
with mass transport limitation, can be reduced by using flow fields that can remove liquid
water from the cathode more effective, like the interdigitated flow fields.
As described above, the performance of a fuel cell can be analyzed by plotting its cell
potential versus the current density and analyzing its three voltage loss regions.
Performance in these three regions can be used to determine the performance of a fuel
cell at various operating conditions and to compare one fuel cell design to another.
Voltage losses in each region are indicative of how well or poorly a fuel cell performs.
Lower voltages losses lead to higher power densities and therefore better performance.
Figure 4.2 below illustrates how the cell potential and power density change as the
individual resistance is reduced.
1.15
N
—C
C
o
0.
0
=
I
U
0
a.
Figure 4.2 Effects of Lower Kinetic and Ohmic Resistance on Cell Potential and Power
Density
4.1.1 Points for Consideration:
1. The activation resistance can be reduced by using higher catalyst loading, higher
catalyst utilization, better catalyst distribution and higher oxygen and hydrogen
concentrations (pressures). Devise an experiment to test these parameters beyond those
given in Chapter 3.
2. The ohmic resistance can be reduced by using thinner membranes, membranes with
higher hydration and therefore conductivity, and materials with lower electronic and
contact resistance. Devise an experiment to test these parameters beyond those given in
Chapter 3.
3. The mass transport limitation region can be extended if higher transport rate of
reactants and products to and from the catalyst surface is achieved. Devise an experiment
to test these parameters beyond those given in Chapter 3.
4. Conversion efficiency is defined as the ratio between the amount of electrical energy
generated by the fuel cell and the amount of energy (enthalpy) generated if the same
amount of hydrogen and oxygen were reacted at the same pressure and temperature.
Perform this calculation throughout E-I range. Compare how the efficiency changes with
decreasing potential. Compare this to the efficiency of a thermal cycle.
1.05
0.95
0.85
0.75
0.65
0.55
0.45
0.35
0.25
0.15
0.00
0.50
1.00
1.50
2.00
2.50
Current Density (A/cm
0.00
0.50
1.00
1.50
2.00
2.50
Current Density (A/cm
5. Note that the power peaks at around 0.4 V to 0.45 V. Discuss at what potential it is
best to operate the fuel cell at in terms of fuel efficiency and economics (system cost
versus operating cost). Does it make sense to operate beyond the peak
power?
Example 1: For a fuel cell generating 6 A at 0.6 V per cell:
1) Current = 6 (Amps) = 6 (C/s) = 6.22E-5 (moles of electrons/s) =
3.11E-5 (moles of H and 1.55E-5 (moles of 0 consumed.
(Note: Faraday’s Constant = 96487 C/mole electrons, H2 releases
2 electrons per mole, 02 consumes 4 electrons per mole)
2) Power generated = (6 A)(0.6 V) = 3.6 W 4 3.6 J/s per 3.1 1E-5 moles/s of H = 116
kJ/mole of H consumed.
3) When the value in (2) is compared to the rate of energy (-AH at 50°C = 284 kJ/mole )
that is released when 1 mole of H is
reacted (combusted) to form H the energy conversion efficiency is about 41%.
Example 2: For the same fuel cell generating 1 A at 0.85 V per cell
1) Current = 1 (Amps) = 1 (C/s) = 1 .04E-5 (moles of electrons/s) =
5.18E-6 (moles of H and 2.59E-6 (moles of 02/S) consumed.
2) Power generated = (1 A)(0.85 V) = 0.85 W 4 0.85 J/s per 5.18E6 moles/s of H = 164 kJ/mole of H consumed.
3) When the value in (2) is compared to the rate of energy (-AH at 50°C = 284 kJ/mole )
that is released when 1 mole of H is
reacted (combusted) to form H the energy conversion efficiency is about 58%.
4.2 Numerical Approach
The reactions occurring in a fuel cell are heterogeneous electrochemical reactions that
work similarly to those in heterogeneous chemical reactions. The main difference is in
the driving force. Electrical potential is used to overcome the activation barrier in
electrochemical reactions while temperature is used in chemical reactions. Next, we will
develop the analogy between the two systems and show you how the kinetic and transport
properties of a fuel cell can be characterized.
Since the kinetic rate of the hydrogen reaction at the anode is orders of magnitude faster
than that of the oxygen reaction at the cathode, the performance of a proton exchangemembrane fuel cell is controlled mostly by the oxygen reaction. Consequently, the kinetic
expression for the oxygen reaction can be used to analyze the performance of a PEM fuel
cell. Table 4.1 below gives the analogy between the kinetics of a fuel cell and that of a
catalytic reactor.
Table 4.1. Analogy between the kinetics of a fuel cell and that of a catalytic reactor
where
i = P exp{fl
i = Current density (C/s/area) = kinetic parameter
jg =
where 4
= Exchange or spec current
density (A/area= C/s/area)
‘ = Reference oxygen pressure (atm)
e = Effectiveness factor (dimensionless) = Oxygen partial pressure in the fuel cell
(atm)
= aF/RT = 1/Tafel slope where a = Transfer coefficient =kinetic
parameter
F = Faraday ‘s constant (C/mo!)
R = Ideal gas constant (J/mol/K)
T = Temperature (K)
A V = Voltage difference needed to drive the reaction (J9 = Vthermo — Vcat
where Vthermo = Thermodynamic voltage (V) V = Cell voltage at catalyst
surface (T’7 = V +
AV
where V = Voltage measured at fuel cell terminals
A V Voltage difference between external point and catalyst surface = iRa where R is the
resistance (ohm-cm between the two points
*
b
I E’
Rate =k exp1— cat
where
Rate* = Reaction rate (mol O2/mcatalys/s) I = Spec rate (mol O2/mcatalys/s/atm
02) = kinetic parameter
e = Effectiveness factor
= Oxygen partial pressure in the reactor (atm)
E* =E/R(K)
where E = Activation energy (J/mol) = kinetic parameter
R = Ideal gas constant (J/mol/K)
Tcat = Temperature at the catalyst surface
= Texternal + AT
where AT = Temperature difference between external surface and catalyst surface
Fuel Cells
(Electrochemical Systems)
Catalytic Reactors
(Chemical Systems)
The cartoon above is used to show that the driving force is the difference between the
thermodynamic potential and potential at the catalyst surface. However, the voltage that
we measure at the fuel cell terminals is not equal to that at the catalyst surface.
The cartoon above is used to show that the driving force is the temperature at the catalyst
surface. However, the temperature that we measure at the outer surface of the reactor is
not equal to that at the catalyst surface.
Kinetic Parameters of Interests:
• a (kinetic) = f(reaction mechanism)
• 4 (kinetic) = f(catalyst area, utilization and type)
Note: ln(i) = ln(i P - flA V
Kinetic Parameters of Interests:
• E (kinetic) = f(reaction mechanism)
• k (kinetic) = f(catalyst area, utilization, type)
Note: ln(Rate *) = ln(k,e F — E*/Tcag
Note that i is a function of temperature and its temperature dependence is expected to be
exponential. You may want to verify this.
4.2.1 Determination of Kinetic Parameters
A fuel cell’s performance is often displayed by its Voltage versus Current Density curve
as shown below in Figure 4.3 a. The equivalent curve for a catalytic reaction is given in
Figure lb. From an experimental run, one obtains for a given experimental condition a set
of Vexternal (V) versus I (A) data. The current density (A/cm can be obtained by
dividing the total current, 1(A), by the electrode area (16 cm
Based on the expressions given in Table 1, plotting ln(i) versus A V yields a straight line
when the effect of transport is minimal (e is close to one). The slope of this straight line is
-/1 and the intercept is ln( i Ps). Analogously, plotting ln(Rate*) versus 1/Tcag yields a
straight line with the slope being -E/R and the intercept being ln(k Ps). The deviation
of the non-linear curve from the extrapolated linear curve gives the value of the
effectiveness factor (e) which indicates how much the oxygen concentration (partial
Vcatalyst
Tcajaiy
eiV due to the ohmic and ionic resistances in the fuel cell
1
1
i.IT due to the
limited conductivity
[ J of the reactor bed
1
pressure equivalent) deviates from the bulk oxygen concentration in the flow channels of
the fuel cell.
Before this curve can be generated one needs to calculate A V from Vthermo and
Vexternal. Vthermo can be calculated using the equation given below:
“thermo (19=1.23(19 - O.0009(T(K) -298) + RT(K In (atm)P (atm)]
Note that due to the crossover of the hydrogen to the oxygen side and likewise for the
oxygen the fuel cell open-circuit potential is never equal to the thermodynamic voltage.
You will notice that as the membrane becomes more hydrated during operation, the gas
crossover rates are reduced giving rise to higher open circuit voltages.
To calculate A V one needs to know the value of the fuel cell’s internal resistance (Ra).
The fuel cell internal resistance can be measured using a milliohm meter or by the current
interruption technique. For simplicity, we will use a different approach which still yields
very reasonable values for R It is known that a good estimate for the internal resistance
can be obtained from the slope of the linear region of the Vexternal versus i curve. One
can start with this value and adjust it accordingly to get a good fit to the linear region of
the
Vexi
Figure 4.3 Fuel Cell Performance is usually plotted in terms of external cell voltage
versus current density, which is comparable to plotting l/T versus the reaction rate for a
thermal reactor
i
Rate
ln(i) versus A V curve. Since this slope includes the effect of oxygen depletion, the value
for R is typically around 0.7-0.75 times that of the slope value.
In summary, the procedure for determining the values of the kinetic and transport
parameters consists of:
1. Calculating i from I.
2. Plotting Vexternai versus i. From the magnitude of the slope of the linear region
determine Ra. Since this slope includes the effect of oxygen depletion, the value for R is
typically around 0.7-0.75 times that of the slope value.
3. Calculating 4 V from Vthermo, Vextemal and R
4. Plotting ln(i) versus A V. From the slope and intercept of the linear region calculate a
and i P
4.2.2 Points for Consideration:
1. Higher i means higher surface catalyst area, catalyst utilization or more effective
catalyst for oxygen reduction. Fuel cells with higher 4 should generate higher voltage and
power at a given current density.
2. Since a is only a function of reaction mechanism. If nitrogen has no effects on the
oxygen reaction mechanism, air and pure oxygen operation should yield the same value
for a
3. Lower internal resistance Ra means higher voltage at the external terminals and
consequently higher power at a given current density. Determine operating conditions
that will result in lower RQ. It is known that R is strongly affected by conductivity of the
membrane which in turn is strongly dependent of the membrane water content.
4. The cell voltage falls quickly when the electrodes become mass transport limited.
What operating variables (flow rate, pressure, temperature) are most effective in reducing
the mass transport effects?
31