Current distribution in PEMFC: I-Validation step by ex-situ and - - PowerPoint PPT Presentation

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Current distribution in PEMFC: I-Validation step by ex-situ and - - PowerPoint PPT Presentation

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Current distribution in PEMFC: I-Validation step by ex-situ and in-situ electrical characterization PhD student: Samir RACHIDI PhD Director : Sergue Martemianov CEA tutors: Ludovic Rouillon, Jol Pauchet PhD program: October 2008 to

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Current distribution in PEMFC:

I-Validation step by ex-situ and in-situ electrical characterization

PhD student: Samir RACHIDI

PhD Director : Sergueï Martemianov CEA tutors: Ludovic Rouillon, Joël Pauchet PhD program: October 2008 to Septembre 2011

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Plan

  • Introduction
  • Why Current Density?
  • Current Density measurements, State Of Art
  • Reverse method approach
  • Methodology
  • Wires’

instrumentation

  • Electrical Model
  • Preliminary results and validation
  • Preliminary results
  • Model sensitivity
  • Potential measurements’

validation

  • Conclusions & perspectives
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Why the current density?

  • Key output of a PEMFC:
  • Globally: «

Visualize » the cell performance

  • Locally: understand the non uniformity of the electrochemical reaction (rib/channel effect,

flooding/drought aspect,…) Contribute in understanding local transfer phenomena

  • Feed/validate multi-physics models in our lab
  • Rib/channel scale: polarization curves not enough
  • All transfer phenomena into account

Improve modeling predictability

JG Pharoah et al., 2006, JPS, 161

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Current density measurements, State of Art

Partial Catalytic Deposit

L.Wang et al, JPS 180 (2008)

Segmented Electrodes Magnetic field Method Wire approach

Stefan A. Freunberger et al, (2006)

  • D. Candusso

et al,. J. Appl. Phys. 25, 67–74 (2004).

  • R. Eckl, et al., JPS 154 (2006)

Spatial resolution of measurements evolved from centimeters to a sub-millimeter scale

J.Stumper et al, Electrochimica Acta,1998.

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Plan

  • Introduction
  • Why Current Density?
  • Current Density measurements, State Of Art
  • Reverse method approach
  • Methodology
  • Wires’

instrumentation

  • Electrical Model
  • Preliminary results and validation
  • Preliminary results
  • Model sensitivity
  • Potential measurements’

validation

  • Conclusions & perspectives
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1/ Potential measurement between each wire and monopolar plate 2/ Implementation of the potential profile as a boundary condition in an electrical model 3/ Determination of local current density thanks to the model via Laplace Equation: Reverse Method : Potential Current density

Methodology

GDL

GDL

Canal Microporous Layer Catalyst Layer GDL

GDL

Channel Wires=Potential probes

V

Rib (Monopolar Plate)

) . .(     V 

Modeling

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Wires’ Instrumentation

  • Potential Probes:
  • Tungsten (W) wires insulated by a polyimide layer
  • Diameter: 25 µm of tungsten + 5µm of polyimide
  • Insulating layer removed from the measurement zone
  • Minimal achievable distance between two wires : 115µm

Improvement of the spatial resolution of potential measurements (500µm until now)

2mm

GDL MPL CL

Tungsten wire

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Rib Channel

  • Software: Comsol

Multiphysics

  • Boundary conditions:
  • Rib : Contact Resistance
  • MPL outer boundary: Measured potential profile
  • Model Inputs :
  • Electrical conductivity tensor (measured in-house under stress by 4-points sensors)
  • Electrical contact resistance (in-house values)
  • Computing of the electrical potential field “V”
  • Current density calculation in a post processing step: local Ohm’s law

“J= ”

Electrical Model

Rib Half Channel Half Channel GDL MPL

        

  

//

.  V 

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Plan

  • Introduction
  • Why Current Density?
  • Current Density measurements, State Of Art
  • Reverse method approach
  • Methodology
  • Wires’

instrumentation

  • Electrical Model
  • Preliminary results and validation
  • Preliminary results
  • Model sensitivity
  • Potential measurements’

validation

  • Conclusions & perspectives
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Preliminary results

J

0.005 0.01 0.015 0.02 0.025 0.03 0.035 Measured potential (V) 1 2 3 4 5 6 7 8 Wires

rib channel

A/m²

Potential profile

  • Electrical potential higher under the channel in both studies
  • The same order of magnitude of potential difference between the wires

encountered in the PSI study (some mV)

  • Two operating phases:
  • At low loads : current density higher under the rib
  • At high Loads: current density higher under the channel
  • Interesting technique: understand local transfer phenomena

Current density distribution

0.7 A/cm² 0 A/cm²

Stefan A. Freunberger et al. ECS, (2006)

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  • Our approach is based on experimental measurements that feed an electrical model

Need to evaluate the model sensitivity towards measurements’ uncertainties

  • Four measured parameters:
  • Electrical potential measured locally :Vmeas

; [0; 34 mV]

  • Through plane electrical conductivity : σ┴

; [70; 200 S/m]

  • In plane electrical conductivity σ//

; [8400; 10600 S/m]

  • Contact Resistance between the BPP and the GDL: arround

Rc = 2.10-7ohm.m²

  • We vary each measured parameter separately and we observe the relative change in current

density profile (∆J/J ) Electrical model strongly depends on the electrical contact resistance In plane conductivity σ// isn’t a sensitive parameter

Model Sensitivity

Parameter ∆J/J < 10% ∆J/J < 5% Vmeas (µV) +/-100 +/-10 σ┴ (S/m) +/-10 +/-1 σ// (S/m) +/-1000 +/-100 Rc (ohm.m²) +/-0.1*10-7 +/-0.01*10-7 Half Channel Rib Half Channel

J

Increasing

Vmeas

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  • Why?:

Small potential difference between the wires + Model sensitivity towards the measured potential Need to validate the in-situ potential measurements

  • Idea:

Verify electrical conductivity of some known materials via potential measurements

  • HOW?:

confront the experimental and the theoretical potential profiles

  • Case1: electrical conducting liquids
  • Isotropy
  • Homogeneity
  • Environment continuity at the scale of tungsten wires (25µm)
  • The choice of the liquid
  • High electrical conductivity
  • Wettability
  • Liquids used: Aqueous solutions e.g. (K+;Cl-); Ionic liquids

Potential measurements’ validation (1/2)

Experiments and results’ exploitation in progress

liquid Rib Channel Bi Polar Plates DC

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Potential measurements’ validation (2/2)

  • Case 2: Through plane conductivity of a GDL, σ┴
  • Confronting theoretical and experimental potential profiles
  • Potential Profiles’

fitting

Satisfying conductivity values with a good approximation The wire system can be used as a 4-points sensor

(see J. Kleemann, F. Finsterwalder, W. Tillmetz Journal of Power Sources 190 (2009) 92–102)

Vbip VBipo_O VBipo_Ba GDL Bipolar Plates Laminated shim GDL Wires Potential (V)

30 mm 1 mm Increasing

σ┴

Potential (V)

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Conclusions

  • A very interesting approach to understand local transfer phenomena in the

PEMFC’s core

  • Efficient tool in the future for on-line diagnosis of an operating stack
  • A reverse method has been set up to determine current density distribution
  • The sensitivity of the electrical model towards measured parameters used

was studied

  • Improvement of the spatial resolution of the in-situ potential measurements

115µm instead of 500µm

  • A validation procedure was initiated in order to verify the potential

measurements’ quality

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Perspectives

  • The reverse method will be used to determine a local current density

distribution in a PEMFC

  • Finalize the validation step
  • Implementing wires in an operating cell
  • Results and model exploitation
  • Coupling local thermal measurements
  • Tests on an instrumented stack