Download E Z1 Characteristic Curve Of The Fuel Cell

E z1
Characteristic Curve
of the Fuel Cell
Materials required:
2
Dismantable fuel cell with 0.3 mg/cm Pt
membrane, hydrogen and oxygen end
plates, mounted according to assembly
instructions
Components from the Solar Hydrogen
Technology Science Kit:
Solar module
Electrolyser
Instructions:
Load measurement box
4 hook-up cables
2 long tubes
2 short tubes
2 tubing stoppers
Additional components:
Lamp 100-150 Watt
Distilled water
Please follow the operating instructions!
Wear protective goggles and keep ignition sources at a distance when experimenting!!!!
Fig. z1a (Purging):
Load measurement box
Solar module
A
Lamp
+ 0
ml
ml
2
4
6
6
O2
-
+
0
4
10
R
+ -
2
8
+
V
8
10
O2
H2
H2
Electrolyser
Fuel cell
© Copyright 2002 heliocentris
-
1. Set up the apparatus as shown in Fig. z1a. Check the polarity of the electrolyser!
2. Check that the gas tubes between the electrolyser and the fuel cell are correctly connected.
Adjust the rotary switch on the load measurement box to “OPEN”.
3. Make sure both of the gas storage cylinders of the electrolyser are filled with distilled water up to
the 0 ml mark. Using the illuminated solar module, set a constant current to the electrolyser of
between 200 and 300 mA. The solar module must be positioned towards the light source in such a
way that gas production can be clearly observed.
4. Purge the complete system (consisting of the electrolyser, fuel cell and tubes) for 5 minutes with
the gases produced. Then set the rotary switch on the load measurement box to 3 • for 3 minutes.
The ammeter of the load measurement box should now already show a current. Purge the system
again with the rotary switch in the “OPEN” position for 3 minutes.
Fig. z1b (Storing)
Oxygen
from electrolyser
Hydrogen
from electrolyser
Fuel cell
O2
H2
Tubing stoppers
5. Stop the power supply to the electrolyser for a short time and use the stoppers to close the two
short tubes at the gas outlets of the fuel cell (see Fig. z1b).
6. Reconnect the solar module to the electrolyser and store the gases in the gas storage cylinders of
the electrolyser. Interrupt the power supply when the hydrogen side of the electrolyser has reached
the 10 ml mark.
7. Remove the cables between the solar module and the electrolyser and use them to connect the
voltmeter of the load measurement box to the fuel cell (see Fig. z1c).
8. Record the characteristic curve of the fuel cell by varying the measurement resistance (rotary
switch of the load measurement box). Start at position “OPEN” (off-load voltage), then decrease
the resistance step by step by turning the rotary switch to the right. Record the voltage and current
for each resistance. Wait for 30 seconds each time before taking the measurements. Enter the
figures in the table of measurements. Finally measure the figures for the lamp and the electric
motor.
9. After recording the characteristic curve, reset the rotary switch of the load measurement box to
“OPEN” and remove the fuel cell stoppers.
© Copyright 2002 heliocentris
Fig. z1c (Recording the characteristic curve):
Load measurement box
A
+
+ 0
ml
ml
2
4
6
6
O2
-
+
-
0
4
10
R
+ -
2
8
V
O2
8
10
H2
H2
Electrolyser
Fuel cell
Table of Measurements:
Resistance (Ω)
Voltage (V)
Current (mA)
Evaluation:
1. Draw the VI characteristic curve of the fuel cell.
2. Interpret the characteristic curve.
3. Enter the motor's and the lamp’s operating voltages and currents in the VI characteristic
curve.
4. Draw a PI diagram.
5. Calculate the motor's and the lamp’s power consumption and enter the values into the P I
diagram.
© Copyright 2002 heliocentris
Interpretation/Notes:
Characteristic Curve of the Fuel Cell
1
Electric motor
Voltage (V)
0.9
Lamp
0.8
0.7
0.6
0.5
0
100
200
300
400
500
Current (mA)
In order to understand the characteristic curve of a fuel cell, recall the characteristic curve of the
electrolyser (see experiment e1 for the hydro-Genius Solar Hydrogen Technology Science Kit). The
processes in the fuel cell are the reverse of those that take place in electrolysis. In the electrolysis of
water, at least 1.23 volts must be applied before the water begins to split; as a rule the voltage is
higher (overpotential).
In the case of a fuel cell (a galvanic cell), less voltage is generated for the same reasons. Here, too,
the characteristic curve is affected by the materials used for the electrodes (catalysis), the internal
resistance, the temperature and the volume of hydrogen and oxygen being supplied.
At very small or zero current drain, the voltage across the fuel cell amounts to approx. 0.9 volts. This
voltage is called off-load voltage (by analogy to a battery). In the case of the fuel cell, it is very
dependent on the volume and purity of the input gases. The more current is drawn from the fuel cell,
the smaller the voltage becomes. Accordingly, there is an exponential increase in the current as the
voltage declines.
If the operating point of the electric motor is entered in the P I diagram, it can be seen that the motor
does not run at the optimum point, i.e. hydrogen is being lost. More power can therefore be drawn
from the fuel cell.
In practice, efforts are made to draw as much current as possible from the fuel cell (i.e. to operate it
at maximum output). However, the efficiency of the fuel cell declines at high current values (see
experiment z2), so that the task here is to find an optimum operating point (high efficiency, high
output).
For a more precise explanation of characteristic curves of a fuel cell, please consult experiment
instructions z6.
© Copyright 2002 heliocentris
Power Curve of the Fuel Cell
300
Power (mW)
250
200
150
100
Lampe
50
Electric motor
0
0
100
200
300
Current (mA)
© Copyright 2002 heliocentris
400
500
E z2
FARADAY Efficiency and
Energy Efficiency
of the Fuel Cell
Materials required:
Dismantable fuel cell with 0.3 mg/cm 2 Pt
membrane, hydrogen and oxygen end
plates, mounted according to assembly
instructions
Components from the Solar Hydrogen
Technology Science Kit:
Solar module
Electrolyser
Load measurement box
Stop watch
4 hook-up cables
2 long tubes
2 short tubes
2 tubing stoppers
Additional components:
Lamp 100-150 Watt
Distilled water
Instructions:
Please follow the operating instructions!
Wear protective goggles and keep ignition sources at a distance when experimenting!!!!
Fig. z2a (Purging):
Load measurement box
Solar module
A
Lamp
+ 0
2
ml
ml
2
4
6
O2
-
+
0
6
10
R
+ -
4
8
+
V
8
10
O2
H2
H2
Electrolyseur
Fuel cell
© Copyright 2002 heliocentris
-
1. 1. Set up the apparatus as shown in Fig. z2a. Check the polarity of the electrolyser!
2. Check that the gas tubes between the electrolyser and the fuel cell are correctly connected.
Adjust the rotary switch on the load measurement box to "OPEN".
3. Make sure both of the gas storage cylinders of the electrolyser are filled with distilled water up to
the 0 ml mark. Using the illuminated solar module, set a constant current to the electrolyser of
between 200 and 300 mA. The solar module must be positioned towards the light source in such
a way that gas production can be clearly observed.
4. Purge the complete system for 5 minutes with the gases produced in the electrolyser. Then set
the rotary switch on the load measurement box to 3 • for 3 minutes. You should now already be
able to measure a current on the ammeter of the load measurement box. Purge the system again
with the rotary switch in the "OPEN" position for 3 minutes.
Fig. z2b (Storing):
Oxygen
from electrolyser
Hydrogen
from electrolyser
Fuel cell
O2
H2
Tubing stoppers
5. 5. Interrupt the connection between the solar module and the electrolyser and use the stoppers
to close the two short tubes at the gas outlets of the fuel cell (see Fig. z2b).
6. Reconnect the solar module to the electrolyser and store the gases in the gas storage cylinders of
the electrolyser. Interrupt the power supply when the hydrogen side of the electrolyser has
reached the 10 ml mark.
7. Since the whole system always shows a certain leakage rate because of its tubes and seals, an
idle measurement must be made first. Record the loss of hydrogen from the hydrogen storage
cylinder without load (position "OPEN") over a period of 5 minutes. Determine the leakage rate in
ml of hydrogen per minute.
8. Reconnect the electrolyser to the solar module and refill the storage cylinder up to the 10 ml
mark. Then interrupt the power supply to the electrolyser again.
© Copyright 2002 heliocentris
9. Remove the cables between the solar module and the electrolyser and use them to connect the
voltmeter of the load measurement box to the fuel cell (see Fig. z2c). Adjust the resistance to 3
•. Record the volume of hydrogen consumed by the fuel cell from the electrolyser's hydrogen
storage cylinder in 180 sec. Also measure and note the voltage and current at the fuel cell. Switch
back to "OPEN" after 180 sec.
10. Repeat steps 8 and 9 twice and calculate the average values for the amount of hydrogen
consumed by the fuel cell. After making the measurements, set the rotary switch to "OPEN" and
remove the stoppers from the tubes of the fuel cell.
Fig. z2c (Determination of efficiency):
Load measurement box
A
+
+ 0
2
ml
ml
2
4
6
O2
-
+
0
6
10
R
+ -
4
8
V
8
10
O2
H2
H2
Electrolyser
Fuel cell
© Copyright 2002 heliocentris
-
Table of measurements:
Fuel cell without load – idle measurement:
t = 300 s = 5 min
Volume loss of hydrogen from storage:
Leakage rate of system
V=
V/t =
ml
ml/min
Fuel Cell with load:
R=
Ω
t=
s
V1 =
ml
U=
V
V2 =
ml
mA
V3 =
ml
I=
Evaluation:
1. Calculate the respective volumes of hydrogen.
2. Determine the FARADAY efficiency of the fuel cell.
3. Determine the energy efficiency of the fuel cell.
© Copyright 2002 heliocentris
Vaverage=
ml
(consumed hydrogen)
Interpretation/Notes:
Example measurement:
Idle measurement
t
= 5 min
V
=1.5 ml
Leakage rate: V/t = 0.3 ml/min
Fuel cell with load:
R
=3Ω
t
= 180 s
U
= 0.74 V
V1 = 6.5 ml
V2 = 6.6 ml
I
= 233 mA
V3 = 6.4 ml
Vaverage = 6.5 ml
(consumed hydrogen from storage)
• The fuel cell consumes 5.6 (6.5-0.9) ml of hydrogen to supply a current of 233 mA.
Determination of the FARADAY efficiency of the fuel cell
The FARADAY efficiency is the ratio between the theoretical volume of hydrogen consumed by the
load at a certain current flow and the experimentally determined consumption of hydrogen.
η = V H2 theoretical / VH2 experimental
The FARADAY efficiency should come to 1 (100%).
nd
The expected theoretical consumption of hydrogen can be calculated using FARADAY's 2 law.
I·t=n·z·F
n = V / Vm
VH2 theoretical = I · t · V m / z · F
-1
VH2 theoretical = 233 mA · 180 s · 24 l mol / 2 · 96 484 C mol
VH2 theoretical = 5.2 ml
See description of experiment e3 for measurement units.
© Copyright 2002 heliocentris
-1
Interpretation/Notes:
Example measurement:
Idle measurement
t
= 5 min
V
=1.5 ml
Leakage rate: V/t = 0.3 ml/min
Fuel cell with load:
R
=3Ω
t
= 180 s
U
= 0.74 V
V1 = 6.5 ml
V2 = 6.6 ml
I
= 233 mA
V3 = 6.4 ml
Vaverage = 6.5 ml
(consumed hydrogen from storage)
• The fuel cell consumes 5.6 (6.5-0.9) ml of hydrogen to supply a current of 233 mA.
Determination of the FARADAY efficiency of the fuel cell
The FARADAY efficiency is the ratio between the theoretical volume of hydrogen consumed by the
load at a certain current flow and the experimentally determined consumption of hydrogen.
η = V H2 theoretical / VH2 experimental
The FARADAY efficiency should come to 1 (100%).
nd
The expected theoretical consumption of hydrogen can be calculated using FARADAY's 2 law.
I·t=n·z·F
n = V / Vm
VH2 theoretical = I · t · V m / z · F
-1
VH2 theoretical = 233 mA · 180 s · 24 l mol / 2 · 96 484 C mol
VH2 theoretical = 5.2 ml
See description of experiment e3 for measurement units.
© Copyright 2002 heliocentris
-1
Experiment Variations:
Determine the energy efficiency as a function of the current flowing through the fuel cell.
Set the fuel cell to currents between 100 and 500 mA by varying the resistance.
Do not exceed 500 mA!
Determine the current-dependent efficiency and interpret the result.
See our teaching material on solar hydrogen technology for ideas on interpretation.
© Copyright 2002 heliocentris
Impact of Catalyst
Concentration on the
Characteristic Curve of the
Fuel Cell
E z3
Materials required:
Dismantable fuel cell with 0.3 mg/cm 2 Pt
membrane, hydrogen and oxygen end
plates, assembled according to assembly
Electrolyser
Load measurement box
4 hook-up cables
instructions (standard set-up)
2
Additional 0.1 mg/cm Pt membrane
(membrane is marked)
Hexagonal socket screw key (Allen key)
Spanner
2 long tubes
2 short tubes
2 tubing stoppers
Components from the Solar Hydrogen
Technology Science Kit:
Solar module
Additional components:
Lamp 100-150 Watt
Distilled water
Instructions:
Please follow the operating instructions!
Wear protective goggles and keep ignition sources at a distance when experimenting!!!!
Fig. z3a (Purging):
Load measurement box
Solar module
A
Lamp
+ 0
2
ml
4
6
8
10
O2
ml
+
V
R
-
+
+ 0
2
4
6
8
10
O2
H2
H2
Electrolyseur
Fuel cell
© Copyright 2002 heliocentris
-
1. First carry out the measurements using the membrane coated with 0.3 mg of catalyst per cm2.
Make sure that the two end plates are 7 mm apart.
Set up the apparatus as shown in Fig. z3a. Check the polarity of the electrolyser!
2. Check that the gas tubes between the electrolyser and the fuel cell are correctly connected.
Adjust the rotary switch on the load measurement box to “OPEN”.
3. Make sure both of the gas storage cylinders of the electrolyser are filled with distilled water up to
the 0 ml mark. Using the illuminated solar module, set a constant current to the electrolyser of
between 250 and 300 mA. The solar module must be positioned towards the light source in such
a way that gas production can be clearly observed.
4. Purge the complete system (consisting of the electrolyser, fuel cell and tubes) for 5 minutes with
the gases produced. Then set the rotary switch on the load measurement box to 3 • for 3
minutes. The ammeter of the load measurement box should now already show a current. Purge
the system again with the rotary switch in the "OPEN" position for 3 minutes.
Fig. z3b (Purging):
Oxygen
from electrolyser
Hydrogen
from electrolyser
Fuel cell
O2
H2
Tubing stoppers
5. Stop the power supply to the electrolyser for a short time and use the stoppers to close the two
short tubes at the gas outlets of the fuel cell (see Fig. z3b).
6. Reconnect the solar module to the electrolyser and store the gases in the gas storage cylinders of
the electrolyser. Interrupt the power supply when the hydrogen side of the electrolyser has
reached the 10 ml mark.
7. Remove the cables between the solar module and the electrolyser and use them to connect the
voltmeter of the load measurement box to the fuel cell (see Fig. z3c).
8. Record the characteristic curve of the fuel cell by varying the measurement resistance (rotary
switch of the load measurement box). Start at position "OPEN" (off-load voltage), then decrease
the resistance step by step by turning the rotary switch to the right. Record the voltage and
current for each resistance. Wait for 30 seconds each time before taking the measurements.
Enter the figures in the table of measurements.
9. After recording the characteristic curve, reset the rotary switch of the load measurement box to
"OPEN" and remove the fuel cell stoppers.
10. Dismantle the fuel cell according to the assembly instructions and replace the membrane by the
one coated with 0.1 mg of catalyst per cm 2. Make sure two end plates are 7 mm apart.
11. Measure the characteristic curve as described in points 1. - 9..
© Copyright 2002 heliocentris
12. Dismantle the fuel cell after recording the measurements and reassemble it with using the
membrane coated with 0.3 mg of catalyst per cm 2 (standard set-up).
Fig. z3c (Recording the characteristic curve):
Load measurement box
A
+
+ 0
2
ml
ml
R
-
+
-
+ 0
2
4
4
6
6
8
O2
8
10
10
O2
V
H2
H2
Electrolyser
Fuel cell
Table of Measurements:
Membrane coated with 0.1 mg of
catalyst per cm 2
Resistance (Ω)
Voltage (V)
Current (mA)
Membrane coated with 0.3 mg of
catalyst per cm 2
Voltage (V)
Current (mA)
Evaluation:
1. Draw the VI characteristic curve of the fuel cell for both catalyst concentrations.
2. Draw the P I diagrams.
3. Interpret the results.
© Copyright 2002 heliocentris
Interpretation/Notes:
Characteristic Curve of the Fuel Cell as a Function of
Catalyst Loading
1
0.9
0.8
Voltage (V)
0.7
0.6
0.5
0.4
0.3
0.2
Catalyst loading 0.1 mg
0.1
Catalyst loading 0.3 mg
0
0
100
200
300
400
500
Current (mA)
Power Output of the Fuel Cell as a Function of
Catalyst Loading
300
250
Power (mW)
200
150
100
50
Catalyst loading 0.1 mg
Catalyst loading 0.3 mg
0
0
100
200
300
400
Current (mA)
An interpretation of the results is summarised in the experiment instructions z6.
© Copyright 2002 heliocentris
500
Impact of Gas Input on the
Characteristic Curve
of the Fuel Cell
E z4
Materials required:
Dismantable fuel cell with 0.3 mg/cm 2 Pt
membrane, hydrogen and oxygen end
Electrolyser
Load measurement box
plates, assembled according to assembly
instructions (standard set-up)
End plate with ventilation slits
Hexagonal socket screw key (Allen key)
Spanner
4 hook-up cables
2 long tubes
2 short tubes
2 tubing stoppers
Components from Solar Hydrogen
Technology Science Kit:
Additional components:
Lamp 100-150 Watt
Distilled water
Solar module
Instructions:
Please follow the operating instructions!
Wear protective goggles and keep ignition sources at a distance when experimenting!!!!
Fig. z4a (Purging, measurement with oxygen):
Load measurement box
Solar module
A
Lamp
+ 0
ml
ml
2
4
6
6
O2
-
+
0
4
10
R
+ -
2
8
+
V
8
10
O2
H2
H2
Electrolyseur
Fuel cell
© Copyright 2002 heliocentris
-
1. Carry out the measurements using the membrane coated with 0.3 mg of catalyst per
cm2. Make sure that the distance between the two plates is 7 mm.
Set up the apparatus as shown in Fig. z4a. Check the polarity of the electrolyser!
2. Check that the gas tubes between the electrolyser and the fuel cell are correctly
connected.
Adjust the rotary switch on the load measurement box to “OPEN”.
3. Make sure both of the gas storage cylinders of the electrolyser are filled with distilled
water up to the 0 ml mark. Using the illuminated solar module, set a constant current to
the electrolyser of between 250 and 300 mA. The solar module must be positioned
towards the light source in such a way that gas production can be clearly observed.
4. Purge the complete system (consisting of the electrolyser, fuel cell and tubes) for 5
minutes with the gases produced. Then set the rotary switch on the load measurement
box to 3 • for 3 minutes. The ammeter of the load measurement box should now already
show a current. Purge the system again with the rotary switch in the “open” position for 3
minutes.
Fig. z4b (Storing, measurement with oxygen):
Oxygen
from electrolyser
Hydrogen
from electrolyser
Fuel cell
O2
H2
Tubing stoppers
5. Stop the power supply to the electrolyser for a short time and use the stoppers to close the two
short tubes at the gas outlets of the fuel cell (see Fig. z4b).
6. Reconnect the solar module to the electrolyser and store the gases in the gas storage cylinders of
the electrolyser. Interrupt the power supply when the hydrogen side of the electrolyser has reached
the 10 ml mark.
7. Remove the cables between the solar module and the electrolyser and use them to connect the
voltmeter of the load measurement box to the fuel cell (see Fig. z4c).
8. Record the characteristic curve of the fuel cell by varying the measurement resistance (rotary
switch of the load measurement box). Start at position “open” (off-load voltage), then decrease the
resistance step by step by turning the rotary switch to the right. Record the voltage and current for
each resistance. Wait for 30 seconds each time before taking the measurements. Enter the figures
in the table of measurements.
9. After recording the characteristic curve, reset the rotary switch of the load measurement box to
“open” and remove the fuel cell stoppers.
© Copyright 2002 heliocentris
Fig. z4c (Recording the characteristic curve with oxygen):
Load measurement box
A
+
V
ml
ml
0
2
4
6
6
10
O2
-
+ -
4
8
-
+
+ 0
2
R
O2
8
10
H2
H2
Electrolyser
Fuel cell
10. Now remove the tubes from the oxygen side of the fuel cell. The fuel cell is now supplied with
oxygen from the ambient air, which can flow in through the gas nozzles. Repeat the experiment
as described under 1.-9. In this case only hydrogen is taken from the gas storage cylinder of the
electrolyser (see Fig. z4d).
Fig z4d: (Recording the characteristic curve/air through gas nozzles)
Fuel cell
O2
Hydrogen
from electrolyser
H2
Tubing stopper
11. Carry out the measurements as described under 8., but wait for 2 minutes at each measurement
point before reading the values for current and voltage, since the system takes longer to reach
equilibrium than in oxygen operation.
© Copyright 2002 heliocentris
12. Subsequently dismantle the fuel cell and replace the oxygen end plate by the end plate with the
ventilation slits. Make sure two the end plates are 7 mm apart when screwing the fuel cell back
together.
13. The fuel cell is now being supplied by oxygen from ambient air, which can flow in through the
ventilation slits. Repeat the experiment as described under 1.-9. Again, only hydrogen is taken
from the gas storage cylinder of the electrolyser (see Fig. z4e). Wait for about 1 minute at each
measurement point before recording the characteristic curve.
Fig. z4d: (Recording the characteristic curve/air through ventilation slits)
Fuel cell
O2
Hydrogen
from electrolyser
H2
Tubing stopper
14. Dismantle the fuel cell after recording the measurements and reassemble it with using the oxygen
end plate (standard set-up).
Table of measurements:
Oxygen
Resistance (Ω)
Voltage (V)
Current (mA)
Air through gas nozzles
Air through ventilation slits
Voltage
(V)
Voltage (V)
Current (mA)
Current (mA)
Evaluation:
1. Draw the three VI characteristic curves of the fuel cell for the different types of oxygen input.
2. Also draw the P I diagrams.
6. Interpret the results.
© Copyright 2002 heliocentris
Interpretation/Notes:
Characteristic curve as a function of oxygen supply
1
0.9
0.8
Voltage (V)
0.7
0.6
0.5
0.4
0.3
Air/gas nozzles
0.2
Air/gas end plate
0.1
Oxygen
0
0
100
200
300
400
500
600
Current (mA)
Power output as a function of oxygen supply
300
250
Power (mW)
200
150
100
Air/gas nozzles
50
Air/gas end plate
Oxygen
0
0
100
200
300
400
500
600
Current (mA)
For further information on interpretation of the results, please consult the experiment instruction z6.
© Copyright 2002 heliocentris
Impact of Total Resistance
on the Characteristic Curve
of the Fuel Cell
E z5
Materials required:
Dismantable fuel cell with 0.3 mg/cm2
Pt membrane, hydrogen and oxygen
end plates, assembled according to
assembly instructions (standard set-up)
Plug-in resistance 0.47 •
Components from Solar Hydrogen
Technology Science Kit:
Solar module
Electrolyser
Load measurement box
4 hook-up cables
2 long tubes
2 short tubes
2 tubing stoppers
Additional components:
Lamp 100-150 Watt
Distilled water
Instructions:
Please follow the operating instructions!
Wear protective goggles and keep ignition sources at a distance when
experimenting!!!!
Fig. z5a (Purging):
Load measurement box
Solar module
A
Lamp
+ 0
ml
ml
2
4
6
6
8
10
O2
-
+
0
4
10
R
+ -
2
8
+
V
O2
H2
H2
Electrolyser
Fuel cell
© Copyright 2002 heliocentris
-
1. Carry out the measurements using the membrane coated with 0.3 mg of catalyst per
cm2. Make sure that the distance between the two plates is 7 mm.
Set up the apparatus as shown in Fig. z5a. Check the polarity of the electrolyser!
2. Check that the gas tubes between the electrolyser and the fuel cell are correctly
connected.
Adjust the rotary switch on the load measurement box to “OPEN”.
3. Make sure both of the gas storage cylinders of the electrolyser are filled with distilled
water up to the 0 ml mark. Using the illuminated solar module, set a constant current to
the electrolyser of between 250 and 300 mA. The solar module must be positioned
towards the light source in such a way that gas production can be clearly observed.
4. Purge the complete system (consisting of the electrolyser, fuel cell and tubes) for 5
minutes with the gases produced. Then set the rotary switch on the load measurement
box to 3 • for 3 minutes. The ammeter of the load measurement box should now already
show a current. Purge the system again with the rotary switch in the “open” position for 3
minutes.
Fig. z1b (Storing):
Oxygen
from electrolyser
Hydrogen
from electrolyser
Fuel cell
O2
H2
Tubing stoppers
5. Stop the power supply to the electrolyser for a short time and use the stoppers to close
the two short tubes at the gas outlets of the fuel cell (see Fig. z5b).
6. Reconnect the solar module to the electrolyser and store the gases in the gas storage
cylinders of the electrolyser. Interrupt the power supply when the hydrogen side of the
electrolyser has reached the 10 ml mark.
7. Remove the cables between the solar module and the electrolyser and use them to
connect the voltmeter of the load measurement box to the fuel cell (see Fig. z5c).
8. Record the characteristic curve of the fuel cell by varying the measurement resistance
(rotary switch of the load measurement box). Start at position “open” (off-load voltage),
then decrease the resistance step by step by turning the rotary switch to the right.
Record the voltage and current for each resistance. Wait for 30 seconds each time
before taking the measurements. Enter the figures in the table of measurements.
© Copyright 2002 heliocentris
Fig. z5c (Recording the characteristic curve):
Load measurement box
A
+
V
ml
ml
-
+
+ 0
R
-
+ 0
2
2
4
4
6
6
8
O2
8
10
10
H2
H2
O2
Electrolyser
Fuel cell
Fig. z5d (Recording the characteristic curve with plug-in resistance):
Load measurement box
A
+
+ 0
ml
ml
2
4
6
6
O2
-
+
0,47 Ω
0
4
10
R
+ -
2
8
V
8
10
O2
H2
H2
Electrolyser
Fuel cell
© Copyright 2002 heliocentris
-
9. After recording the characteristic curve, reset the rotary switch of the load measurement
box to “open” and remove the fuel cell stoppers.
10. Now plug the 0.47 • plug-in resistance into one of the fuel cell tip jacks (see Fig. z5d)
and
re-measure the characteristic curve as described under 1.-9.
Table of measurements:
Characteristic curve
without resistance
Resistance
(Ω)
Voltage (V)
Current (mA)
Characteristic curve
with plug-in resistance
Voltage (V)
Current (mA)
Evaluation:
1. Draw the VI characteristic curves of the fuel cell with and without the plug -in
resistance.
2. Also draw the P I diagrams.
7. Interpret the results.
© Copyright 2002 heliocentris
Interpretation/Notes:
Characteristic curve as a function of cell resistance
1
0.9
0.8
Voltage (V)
0.7
0.6
0.5
0.4
0.3
0.2
without resistance
0.47Ω resistance
0.1
0
0
100
200
300
400
500
Current (mA)
Power output as a function of fuel cell resistance
300
250
Power (mW)
200
150
100
without resistance
50
0.47Ω resistance
0
0
100
200
300
400
500
Current (mA)
The interpretation of the results is summarised in the experiment instructions z6.
© Copyright 2002 heliocentris
Theoretical Principles
behind Fuel Cells
z6
1
Principles of operation, general characteristics
Fuel cells are highly efficient electrochemical electricity generators. The principle of the fuel cell is
much simpler than that of conventional power generation, since it converts the energy carrier directly
into electrical energy.
Fuel-cell power station
Electrical energy
Chemical energy
Heat
Mechanical energy
Conventional power station
Fig. 1: The fuel cell
compared to conventional
power-generation
processes
The basic principle behind the fuel cell is the direct generation of electricity using a fuel (e.g.
hydrogen) and an oxidant (oxygen) in an electrochemical process.
© Copyright 2002 heliocentris
A fuel cell consists of two electrodes and the
electrolyte. The anode is supplied with the fuel
and the cathode with the oxidant; the electrolyte
connects the two electrodes.
The fuel is oxidised at the anode (negative pole).
The electrons released during this process flow
via the external circuit to the cathode (positive
pole). Here the oxidant is reduced by absorbing
electrons. The flow of electrons through the
external circuit can be used to perform work.
The charge transfer within the fuel cell is
effected by the movement of the ions through
the electrolyte.
Electric load
e-
e-
Fuel
e- e- Air / Oxygen
Ions
Electrolyte
Anode
Cathode
Fig. 2: Principle of operation of the fuel cell.
Hence, like a battery or an accumulator, a fuel cell supplies energy from an electrochemical process.
The essential difference is, however, that the electrodes of the fuel cell are not converted, i.e. the fuel
cell cannot be discharged.
Comparison between the battery, the accumulator and the fuel cell
Similarities:
They all generate electrical energy from chemical energy by an
electrochemical reaction.
Differences:
Battery:
becomes discharged once all the reactants are used up.
Accumulator:
the electrochemical reaction is reversible; it can be re-charged after
becoming discharged.
Fuel cell:
can be used without discharging, can be refuelled with reactants as
required.
© Copyright 2002 heliocentris
2
Principles of operation of the "dismantable fuel cell"
The dismantable fuel cell uses polymer electrolyte membrane technology. The term polymer
electrolyte membrane fuel cell (PEMFC) refers to the proton-conductive polymer membrane that
serves as the electrolyte. The term PEM stands for "proton exchange membrane".
The PEM fuel cell operates with hydrogen and oxygen. The electrochemical conversion of energy
inside the PEM fuel cell is practically the reversal of water electrolysis.
Electric load
-
Hydrogen
At the anode, hydrogen molecules are
oxidised to positively charged hydrogen
ions, releasing electrons. The hydrogen
ions diffuse through the ion-conducting
polymer electrolyte membrane
+
H+
Oxygen
or air
(electrolyte) to the cathode.
Water
If the anode and cathode are connected
H+
Anode
At the cathode, the hydrogen ions react
with oxygen and the electrons supplied
via the electric conductor, forming water.
with an electric conductor (e.g. an electric
motor), the electrons flow from the anode
to the cathode (electric current).
Cathode
Polymer electrolyte
Fig. 3: Principle of operation of a PEM fuel cell
→
Anode:
2 H2
Cathode:
+
O2 + 4 H + 4 e →
Overall reaction:
2 H2 + O 2
→
+
4H +4e
oxidation (release of electrons)
2 H 2O
reduction (absorption of electrons)
2 H 2O
∆ G = -237 kJ/mol (at 25°C)
A single cell's maximum theoretical voltage depends on the thermodynamic data of the reaction
between hydrogen and oxygen to water. Under standard conditions, the figure for an individual cell is
1.23 volts.
1.23 V = •G
zF
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Current conduction losses (overvoltage) occur during operation, e.g. as a result of reaction inhibition
(experiment z3), internal resistance (experiment z5) or insufficient gas diffusion (experiment z4). In
practice, this leads to lower cell voltages, which generally lie between 0.4 and 0.9 volts for an
individual cell.
The membrane electrode assembly (MEA) is the heart of a PEM fuel cell. The membrane is coated
with a finely distributed platinum catalyst (approx. 0.1 – 0.5 mg of platinum per cm 2). The membranes
coated in this way are subsequently press-bonded with porous carbon electrodes in the fuel cell. This
creates an electric/electrical contact.
By contact pressure, the polymer electrolyte membrane partially extends into the porous electrode
structures, forming the gas/catalyst/electrolyte interface. The catalyst must have simultaneous
contact with the gas, the proton conductors (polymer electrolyte membrane) and the electron
conductors (electrodes). This is where the electrochemical reactions take place (Fig. 4, right).
Hydrogen and oxygen are catalytically converted during the reaction; the electrodes themselves
remain unaffected. The platinum particles function as catalytic centres whose effectiveness increases
with surface area.
Reaction centres
Membrane electrode
assembly
2e-
H2
Platinum
catalyst
H2
Individual Cell
2H+
Nafion
Interface
Gas (H2)/Catalyst (Pt)/Electrolyte (Nafion)
O2
H2O
Anode
H+
platinum
PlatinKatalysator
catalyst
Cathode
Polymer electrolyte
(Nafion)
4e-
O2
Nafion
Carbon mat
Carbon mat
4H+
Nafion
2H2O
Grenzfläche
Interface
Elektrolyt (Nafion)/
Electrolyte
(Nafion)/
Katalysator
Catalyst
(Pt)/(Pt)/
Gas (O2)
Fig. 4: Cross-section of a (polymer electrolyte) membrane electrode assembly, illustrating the
processes taking place during fuel-cell reactions
The electrolyte membrane functions like an ion exchanger. The protons of the acid groups present in
the membrane are mobile. When humidified, the membrane conducts protons between the anode
electrode and the cathode electrode.
The electric contact is made via the electricity conductors, in this case special-steel perforated plates.
In large-scale fuel cells, too, the electricity conductors must ensure gas supply and water removal, i.e.
they must be gas- and water-permeable.
© Copyright 2002 heliocentris
The electric current of a fuel cell is a function of the surface area of the electrodes and reaches levels
of up to 2 amp/cm 2.
The current/voltage characteristic curve of a fuel cell
The current/voltage characteristic curve and the power curve of the dismantable fuel cell can be
determined in the experiments. In addition, the influence of specific parameters on the shape of the
characteristic curve can also be examined in experiments z3-z5.
Current (mA)
Fig. 5: Current/voltage characteristic curve of a fuel cell divided into the three areas: catalysis,
resistance of the fuel cell and transport of the reactands
What does the shape of such a characteristic curve tell us?
U0 (H2/H3O+//H2O/O2) is the thermodynamically maximum voltage that a fuel cell can deliver. The
figure is taken from the electrochemical series and amounts to 1.23 volts. Actual cell voltages are
always lower than this. The difference between the measured cell voltage and the thermodynamic
voltage is termed overvoltage. The size of the overvoltage is the decisive parameter that determines
a fuel cell's efficiency.
© Copyright 2002 heliocentris
Overvoltage is made up of various contributors. The size of these contributors determines the shape
of the characteristic curve, as a function of current conduction. The individual contributors are as
follows:
(I) Penetration overvoltage – influence of the catalyst
At low currents and at voltages close to the thermodynamic voltage, the shape of the characteristic
curve is determined by the catalytic processes taking place at the electrodes. This is shown here by
an exponential increase in current in relation to the overvoltage. The decisive determinant of the level
of current is the speed of the catalytic conversion of the gases H2 and O 2, i.e. the speed with which
the electrons pass through the border between the Pt catalyst and the electrolyte. This elementary
process is depicted in Fig. 4 (right). The overvoltage involved is termed penetration overvoltage.
(II) Internal resistance – influence of the fuel cell's structure
Every fuel cell has an internal resistance (electrolyte, electricity conductors, interior structure, external
wiring) which is recorded as ohmic voltage drop at high currents. In this case, the voltage/current
characteristic curve is linear, i.e. voltage falls in proportion to the increase in current.
This resistance must be kept very small, particularly in large fuel cells, because it otherwise results in
excessive power losses. In experiment z5 this difference is simulated by a plug-in resistance, i.e. the
behaviour of the characteristic curve relative to total resistance is examined.
(III) Diffusion overvoltage – influence of the transport of material
At higher currents, the input of the gases through the porous electrode structure (Fig. 4, middle)
becomes decisive. Diffusion overvoltage occurs when the gases at the catalyst are used up more
quickly than they can diffuse to the catalyst. A typical symptom of diffusion overvoltage is when the
voltage/current characteristic curve suddenly dips. The fuel cell's voltage then declines very quickly
as the current rises, and the electrode is "starved" of gas.
The instructions for experiment z4 describe how to measure the characteristic curve in air operation
mode. It dips at approx. 40 mA, a typical example of diffusion overvoltage. When the air end plate is
installed, the voltage is almost as high as in oxygen operation, i.e. the fuel cell receives enough
oxygen from the ambient atmosphere.
The aim of all fuel-cell development is to minimise these three elements which contribute towards
overvoltage by using (I) better electrocatalysts, (II) highly conductive materials and contacts, and (III)
optimised electrode structures and gas ducts.
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