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The Effect of Cathode Stoichiometric Ratio on PEMFC
Performance in Cold Operating Conditions
东 方 (Carl Cayabyab), Oakland University蓝 天 (Jonathan Guidoboni), The George Washington University
任 龙 (Holden Ranz), Lafayette College
National Science FoundationInternational Research Experience for
Students 2011
快乐队
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Background Cold-start conditions and Stoichiometric ratio Project Design Methods and Procedures Equipment and Calibration Results Analysis Conclusions
Outline
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Background
Proton exchange membrane fuel cell (PEMFC)
Chemical energy Electrical energy Hydrogen ions + Oxygen Water Voltage difference makes current flow
GDL – conductive carbon cloth or paper. Porous material, gases pass through to catalyst layer
CCL – catalyst layer, encourage separation of inlet gas atoms into ions / electrons for easier and faster reaction
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Interest
Advantages: high power density, extremely low pollutant emissions, and low operating temperature
Vehicle applications
Reduce dependence oil
Lower harmful emissions
Hydrogen very abundant resource
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Cold-start Conditions
PEMFC normally operate at 70 to 80°C
Cold climates, sub-freezing temperatures
Ice blocks pores in membrane and GDL suppresses FC performance
More difficult to start, especially on its own
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Actual / Theoretical Ratio of air cathode StC
Ratio of hydrogen anode StA
Stoichiometric Ratio
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Initially Relative Humidity Manipulated Variables
Heating jacket voltage (V) relative humidity (%) Flow rate (L/min) Applied current load (A)
Controlled Variable Output cell voltage (V)
Perform trials in both cold and room temp conditions Trend between relative humidity and cell performance Difference between cold and room temp conditions
Project Design
Hygrometer
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Project Design - RedirectedStoichiometry and Temperature Manipulated Variables
Flow rate (L/min) Stoichiometric ratio Temperature Applied current load (A)
Controlled Variable Output cell voltage (V)
Objectives trends between FC performance, StC, and temperature conditions Analyze behavior of the membrane and GDL at sub-freezing
temperatures.
Project Schematic
1. Open nitrogen tank to 1MPa2. Turn on water heater to 45°C3. Check for leaks using soapy
water4. Run water pump for 5 minutes5. Turn off nitrogen gas. Hook
up air and hydrogen tanks.
Procedures
6. Turn on multimeter and computer program.
7. Open hydrogen and air tank to 1MPa
8. Adjust flow rates to an Stc of 2.5
9. Gather temperature readings every 30 seconds
10. Record voltage every 3 minutes and increase current density. Change flow rates as well.
Procedures
11. Stop fuel cell once voltage change is small.12. Save data. Flush out water with air for 5
minutes.13. Repeat this for Stc ratios of 3 and 4
14. Repeat process for 0°C and -3°C.
Procedures
Polyurethane Tubing (6mm OD) Fan Water Heater Insulation PEMFC Humidifier
Equipment
Freezer (FYL-YS-108L) Rubber Plug 2 Thermocouples Digital Thermometer Soapy Water
Equipment
Digital thermometer accurate 0.05°C Accuracy
Calibrated 2 thermocouples Air inlet Hydrogen inlet
Calibration
Methods of Calibration Water bath with varying temp Take temp readings at the same point Steady state Plot curves to find factor
Calibration
Calibration
25 30 35 40 45 50 55 60 65 70 750
10
20
30
40
50
60
70
80
f(x) = 0.987641858423725 x − 0.844592921186269R² = 0.999771752535319
Thermocouple A
Measured Thermocouple Temp (°C)
Accura
te D
igit
al Tem
p (
°C)
20 30 40 50 60 70 800
10
20
30
40
50
60
70
80
f(x) = 0.984437921197337 x + 0.297763787459076R² = 0.999880689487375
Thermocouple B
Thermocouple C
Measured Thermocouple Temp (°C)
Accura
te D
igit
al Tem
p (
°C)
Thermocouple A:Y=0.9876X-0.8446
Thermocouple B:Y=0.9844X+0.2978
Ideal Flow Rate Calculations
where = molar flow rate of reactant (mol/s), = moles of electrons per mole of reactant (mol e-/mol), = Faraday’s constant = 96485 C/mol. = = current load on the fuel cell (A),
= current density (A/cm2),
and = active area of the fuel cell = 32 cm2 (FC1), 16 cm2 (FC2)
H2 = 2
H2 = 4
Flow Rates for Stc valuesAir M = 0.0288479 kg/mol
ρ = 1.205 kg/m3Ideal Q (L/min) Actual Q (L/min)
i (A/cm2) I (A) v (mol/s) ṁ (kg/s) 1 MEA 2 MEA Stc = 2.5 Stc = 3 Stc = 4
0.05 1.6 1.974E-05 5.695E-07 0.028 0.057 0.142 0.170 0.227
0.1 3.2 3.948E-05 1.139E-06 0.057 0.113 0.284 0.340 0.4540.15 4.8 5.922E-05 1.709E-06 0.085 0.170 0.425 0.510 0.681
0.2 6.4 7.897E-05 2.278E-06 0.113 0.227 0.567 0.681 0.9070.25 8 9.871E-05 2.848E-06 0.142 0.284 0.709 0.851 1.134
0.3 9.6 1.184E-04 3.417E-06 0.170 0.340 0.851 1.021 1.3610.35 11.2 1.382E-04 3.987E-06 0.198 0.397 0.992 1.191 1.588
0.4 12.8 1.579E-04 4.556E-06 0.227 0.454 1.134 1.361 1.815
Hydrogen M = 0.002 kg/molρ = 0.08375 kg/m3
Ideal Q (L/min) Actual Q (L/min)i (A/cm2) I (A) v (mol/s) ṁ (kg/s) 1 MEA 2 MEA Stc = 1.5
0.05 1.6 8.291E-06 1.658E-08 0.012 0.024 0.0360.1 3.2 1.658E-05 3.317E-08 0.024 0.048 0.071
0.15 4.8 2.487E-05 4.975E-08 0.036 0.071 0.1070.2 6.4 3.317E-05 6.633E-08 0.048 0.095 0.143
0.25 8 4.146E-05 8.291E-08 0.059 0.119 0.1780.3 9.6 4.975E-05 9.950E-08 0.071 0.143 0.214
0.35 11.2 5.804E-05 1.161E-07 0.083 0.166 0.2490.4 12.8 6.633E-05 1.327E-07 0.095 0.190 0.285
Considerations: Active area = 32 cm2, fuel cell has 2 membranes
Assumptions: Air is 79% N2 and 21% O2, gas is flowing at NTP (20°C and 1 atm), constant density
Data Collection
Output voltage at 0.1 A/cm2
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FC Performance Results
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(ii) Cold Start, 0°C
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(iii) Cold Start, -3°C
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(i) Room Temperature
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(ii) Cold Start, 0°C
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(iii) Cold Start, -3°C
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(i) Room Temperature
Stc = 2.5 Stc = 3 Stc = 4
Δ ≈ 0.1 V
Δ ≈ 0.08 V
Δ ≈ 0.9 V
Δ ≈ 0.08 V
Δ ≈ 0.07 V
Δ ≈ 0.03 V
Δ ≈ 0.06 V
Δ ≈ 0.06 V
Output voltage at 0.15 A/cm2
**Fluctuations most likely due to flow rate adjustments when switching between current loads
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Output voltages essentially the same for 0.1 and 0.15 A/cm2 at different Stc
FC Performance ResultsOutput voltage at 0.1 A/cm2 Output voltage at 0.15 A/cm20
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(ii) Cold Start, 0°C
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(iii) Cold Start, -3°C
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(i) Room Temperature
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(i) Room Temperature
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(ii) Cold Start, 0°C
Stc = 2.5 Stc = 3 Stc = 4
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 20 40 60 80 100 120 140 160
Volt
age
(V)
Time (s)
(iii) Cold Start, -3°C
Stc = 2.5 Stc = 3 Stc = 4
FC Performance – Polarization Curves
Initial loss due to activation of the fuel cell Linear loss due to resistance End loss due to concentration
0.0 0.2 0.4 0.6 0.8
0.2
0.4
0.6
0.8
1.0
Vo
lta
ge
(V
)
Current Density (A/cm2)
Experimental data Numerical resultsActivation
Loss
Ohmic Loss
Concentration
Loss
Experimental Polarization Curves
Polarization curves mostly linear Ohmic losses
At room temp and 0°C, 2.5<3<4 At -3°C curves nearly identical Approximate current densities
resulting in failure: Room temp – I ≈ 0.33 A/cm2
0°C – I ≈ 0.30 A/cm2
-3°C – I ≈ 0.31 A/cm2
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Ice Formation?
After Room Temp Trial-water present in the form of foggy condensate
After Cold Temp Trial
-small droplets and ice crystal formation
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Cathode stoichiometric ratio affects fuel cell performance at room temperature and cold conditions
Increase in Stc at room temp and 0°C corresponds to increase in output voltage
Current densities ≥0.1 A/cm2
FC performance hardly affected by Stc changes at -3°C Linear polarization curves indicate resistance losses Ice formation inside gas lines creates blockages
FC stops generating electricity
Conclusions
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Flow meter instability and inaccuracy Tendency to get stuck
Inability to maintain constant operating temperature of the PEMFC
Duration of membrane humidification prior to testing
Recommendations Calibrate glass flow meters using digital meters Use the heat exchange line in fuel cell to
cool/maintain operating temperature Use a hot water bath to try to control relative
humidity
Sources of Error
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Kang Mi et al. Experimental Study on Dynamic Characteristics of Proton Exchange Membrane Fuel Cells (PEMFC) under Subzero Temperatures.
Shanhai Ge et al. Characteristics of subzero startup and water/ice formation on the catalyst layer in a polymer electrolyte fuel cell.
Jer-Huan Jang et al. Effects of operating conditions on the performances of individual cell and stack of PEM fuel cell.
US Department of Energy, http://www.energy.gov/.
References
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