Jungik Kim

1
In-situ Measurement of Temperature and Water
Content of the Fuel Cell Membrane Using Laser
Interferometry

• Jungik Kim
• Advisor Prof. Shao-Horn, Yang

2
How PEM Fuel Cell Works

• Current collector
• Electron conductor
• Gas flow channel
• ex) Graphite
• Gas diffusion electrode (GDE)
• Porous
• Contains catalyst
• Electron conductor
• ex) Platinum particle coated carbon cloth or
  paper
• Proton exchange membrane
• Proton conductor
• Chemically and mechanically stable
• ex) Nafion

H2
e-
Anode H2?2e-2H
H
Electric Load
Cathode 2e-2H1/2O2?H2O
e-
O2
H2O
Overall H21/2O2?H2O Ideal output voltage
1.17V _at_80C
3
Performance of Fuel Cell

• Activation loss, DVact
• Electrochemical reaction rate on electrodes
• Ohmic loss, DVohm
• Electron conductivity
• Proton conductivity
• Mass transport loss, DVmass
• Supply of reactants

Ideal V 1.17V
Power density W/cm2
Cell voltage V
Current density A/cm2
Cell voltage Videal - DVact - DVohm - DVmass
4
Reduction of Voltage Loss

• Activation loss
• Higher operating temperature
• Higher reactant gas pressure
• Effective GDE design (catalyst, larger active
  area)
• Ohmic loss
• Proper choice of material and cell design
• Proper water management, when Nafion is used
• Mass transport loss
• Sufficient supply of reactants

Increase
Proton conductivity of Nafion as a function of RH
Conductivity S cm-1
Relative Humidity
A. V. Anantaraman, C. L. Gardner / Journal of
Electrochemical Chemistry 414 (1996) 115-120
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Fuel Cell Testing Station
MFC
e-
H2
V-I measure
H
MFC
e-
H2O
O2

• Cell temperature
• Gas flow
• Mass flow rate
• Humidifier temperature, on / off (bypass)
• Back pressure

6
Effect of Temperature and Pressure

• Higher operating temperature and pressure result
  in better performance
• However, 70C / 0psi case (green line) show worse
  performance than 40C / 0psi case (red line)
• Due to drying out of the fuel cell
• Operating temperature and pressure have effects
  on water management

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Effect of Water Management
Anode RH
Cathode RH
,stoichiometry
More External Humidification
,extent of external humidification

• Better performance with more external
  humidification
• Higher proton conductivity at higher RH

8
Degradation of Fuel Cell

• Under chemical, mechanical, and thermal stresses
• GDE degradation
• Catalyst particle ripening / Loss of Catalyst
• Gas diffusion layer compaction
• Membrane degradation
• Permanent loss of proton conductivity
• Pinhole formation
• However, detailed degradation mechanism is not
  well understood
• Degradation can be accelerated by non-uniformity
  and cycling
• Intrinsic non-uniformity of material
• Non-uniformity during fuel cell operation
• Flow condition changes depending on the position
• Shut down / start up

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Research Objective

• Localized and time-resolved measurement of
  temperature and water content will be an
  effective way to
• Find optimized operating conditions
• Understand degradation mechanism
• Monitor degradation
• Find better fuel cell design
• Optical method can be utilized
• Nafion membrane is transparent
• Refractive index varies as a function of
  temperature and water content of the medium
• For polymers, dn/dT -10-4 / C
• From 1.346 at 20 relative humidity to 1.358 at
  50 relative humidity (_at_room temperature)
• Refractive index change can be measured by
  interferometry

Polymer data handbook Weiss, M. N., Srivastava,
R. Groger, H., Electronics Letters 1996, 32, 842
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Experimental Setup and Procedure

• Record fringe patterns of Nafion membrane during
  fuel cell operation by using an interferometer
• Reconstruct refractive index distribution within
  the membrane
• Convert refractive index information into
  temperature or water content

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Numerical Simulation
Temperature
Refractive Index
Reconstructed Refractive Index
dn/dT -1.510-4 / C
Fringe

• Non-uniform refractive index field generated from
  temperature distribution
• Fringe patterns observed
• Reconstruction of refractive index field shows
  poor resolution from lack of measurement angles

12
Preliminary Experiment
Sample
w
w
Glass ng
t
Dried Nafion115 np

• Refractive index vs. Temperature change
• Sample cooled from an elevated temperature by
  natural convection at room temperature
• During the cooling process, fringe pattern
  recorded

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Preliminary Result

• Intensity oscillation frequency decreases with
  time due to exponential decrease of the sample
  temperature

14
Analysis

• Assuming constant dn/dT and thermal expansion
  coefficient, measured intensity can be expressed
  as
• From exponential decrease model,

By curve fitting, ?T058C and t 47sec
15
Conclusion

• Verified importance of temperature, pressure, and
  water management in maximizing the fuel cell
  performance
• Localized and time-resolved monitoring technique
  is necessary for better performance and longer
  life time of the fuel cell
• Proposed an optical method to measure local
  temperature and water content of the membrane
  during the fuel cell operation
• Simulation results showed this technique is
  applicable to a real fuel cell setup
• Preliminary experiment demonstrated localized and
  time-resolved measurement of temperature using
  interferometry

16
Future Work

• Through-plane direction fringe pattern, very
  complicated
• Image processing and optical modeling required to
  reconstruct refractive index
• Data base on refractive index of Nafion membrane
  as a function of temperature, water content

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PEM Fuel Cell

• Fuel Cell
• Directly converts chemical energy into electrical
  energy
• Proton Exchange Membrane Fuel Cell
• Hydrogen as fuel
• Oxygen as oxidizer
• Platinum as catalyst
• Polymer membrane as electrolyte
• Low temperature operation
• Generates water only
• Applicable to portable electric devices,
  transportation, and small power plant

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Difficulties in Optimizing Fuel Cell

• Assembling pressure
• If too high, GDE compaction and poor mass
  transport
• If too low, leakage and higher contact resistance
• Water management
• If too low, poor proton conductivity of Nafion
• If too high, flooding in GDE and poor mass
  transport
• Temperature
• If too high, drying of Nafion and poor proton
  conductivity
• If too low, large activation voltage loss
• Reactant gas pressure
• Related to water management
• If too high, fuel crossover or leakage
• Reactant gas flow rate
• If too high, drying of Nafion and poor proton
  conductivity
• Even when the operating conditions are optimized,
  fuel cells are not free from performance loss
  over a prolonged period, Degradation