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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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