Intro to Fuel Cells

1

A General Discussion on PEMFC Performance
Evaluation And Testing Resources at the Wright
Fuel Cell Group Mirko F Antloga Wright Fuel
Cell Group
2
Fuel Cell Testing(emphasizing PEM)

• Fuel Cell Structure
• Basic Performance Tests
• Polarization Curves and Whats Going on in the
  Fuel Cell
• Component-by-component breakdown

methods are generally applicable but most
developed for PEM
3
Schematic of Fuel Cell Single Cell Used in
Experimental Studies
4
Fuel Cell Stacks

• The Unit Cell shown in the preceding slide is
  stacked in series to provide a high voltage
  device
• Note that flowfield plates are bipolar
• Must be electronically conductive but impermeable
  to gases

5
The Basics

• Need to measure cell performance curve,
• The polarization curve usually V vs. J (current
  density, A/cm2)
• Should simultaneously measure cell resistance
  (best expressed as ASR, areal specific
  resistance, ohm-cm2)
• From there untangle sources of voltage loss
• Many variables to answer specific questions
• Gas total and partial pressure
• Electrolyte resistance

6
What cells to test on?Different information from
different tests

• Small single cells
• Well-controlled, uniform conditions
• Use for ideal case studies and for testing
  individual components (catalysts, new
  electrolytes, layer compositions etc.)
• Large single cells
• Small cell information complicated by flow
  distribution and reactant utilization
• Short stacks
• Introduce manifolds and bipolar plates issues
• Full stacks
• the real McCoy

7
Fuel Cell Performance CharacterizationPolarizati
on Curves--the workhorse
kinetic region
resistance region
mass transport region
8
Kinetic Region

• Dominated by electrode kinetics
• Cathode is limiting for hydrogen-fed cells
• Anode has large polarization for reformate,
  methanol cells
• High voltage, low current density
• Little water production
• High voltage efficiency, low power
• ??????Vcell/Vtheor

9
Ohmic Region

• This nomenclature is a misnomer
• Held over from days when membrane resistance was
  very high
• Observed behavior results from combination of
  membrane resistance and mass transport losses in
  catalyst layer
• Water production significant
• Typically, max power is in this region

10
Mass Transport Region

• Limiting current, typically caused by failure to
  provide reagents sufficiently rapidly
• Associated with gas diffusion layer and/or
  flooding of catalyst layer to block gas access

11
So whats happening during operation?

• Were feeding in gases (say, hydrogen and air) at
  some flow rate
• Fuel or oxygen utilization 1/stoichiometric
  ratio (usually called stoich)
• ratio of the moles used per unit time (related to
  current density by some conversion factors) to
  the incoming flow rate
• Gases may be humidified (esp. for single cells)

12
So whats happening during operation? II

• Were generating current and product water (that
  we may have to get rid of!!!) at cathode
• Were generating heat (that we may have to get
  rid of!!!)
• Fuel converted to protons (go through membranes)
  and electrons (go through external circuit) at
  anode
• Possibly also produce gas (e.g. DMFC)

13
Measurements Needed to Characterize Fuel Cells
Membranes

• Ex situ tests
• Water uptake vs. RH
• Conductivity vs. RH
• Water diffusion coefficient vs. RH
• Electro-osmotic drag vs. RH
• In situ tests
• Resistance, measured at each point of pol curve
• Needed for IR correction,
• Essential for interpretation
• Usually high frequency impedance or current
  interrupt method
• Gas cross-over
• Potentiostat cell, determine arrival rate of gas
  across membrane

14
Measurements Needed to Characterize Fuel Cells
Electrodes

• Ex situ
• intrinsic catalyst activity
• RDE or RRDE method catalyst ink deposited on
  catalytically inactive substrate such as GC
• In situ
• electrochemically accessible surface area,
  utilization, catalyst activity
• Surface area from CV, based on charge from
  hydrogen or CO
• Mass transfer losses in catalyst layer coupled
  oxygen diffusion, proton conduction processes
  produce reaction zone
• Tafel slope from pol curve Poisoning
  processes lead to extra polarization
• Hydrogen pumping cell

15
Measurements Needed to Characterize Fuel Cells
GDLs

• Ex situ wettability, permeability (in-plane,
  through-plane)
• In situ mass transport resistance
• From pol curve

16
Measurements Needed to Characterize Fuel Cells
Bipolar Plates

• Ex situ
• Permeability (in-plane, through-plane)
• Electronic conductivity
• Wettability
• In situ
• Contact resistance
• Impurity release

17
Fuel Cell Performance CharacterizationPolarizati
on Curves--the workhorse
kinetic region
resistance region
mass transport region
18
Testing Strategy

• Vary test parameters to attempt to isolate a
  specific process
• Choose appropriate cell size, type to probe
  specific effect
• Lots of tests needed to gather statistics
• Life-tests
• Accelerated tests critical to know what you are
  accelerating
• Statistical analysis is important

19
Advanced Diagnostics

• AC impedance for separating simultaneous
  processes by rate
• Segmented Cell for Down-the-Channel Effects
• Using Multi-decker Sandwiches to deduce
  transport effects

20
AC Impedance Methods

• Set voltage at some DC voltage apply small
  (e.g. 25 mV) ac perturbation measure current as
  a fcn. of frequency of perturbation
• Processes occurring at the given frequency will
  be reflected in impedance
• relative importance of different processes vary
  with cell voltage.
• Analyze by combination of theory and experiment
• NOTE the analysis is complicated!!!

21
Typical Impedance Spectraas function of
IR-corrected cell voltage
22
4th Generation Segmented Cell

• Too much cross-talk between segments in directly
  probed cells
• Weve switched to Hall sensor detection
• Real time CVs, High frequency resistance,
  transients enabled for every segment
• Hardware is much more like standard hardware

23
Segmented Cell Hardware
24
Segmented Cell Measurement Setup
25
CO Poisoning of Segmented Cell 100 ppm CO, 1 min
poison time
CVs of CO stripping
CO Coverage vs. Time
26
Degradation Modes
27
Degradation of FC Performance Materials Effects

• Polymer
• Chemical degradation
• Creep
• Pinholes
• Mechanical Property Changes
• GDL
• Stress/Unstress--fiber exposure?

28
Degradation of FC Performance Materials Effects

• Catalysts and Catalyst Layers
• Dissolution
• Oxide Formation
• Agglomeration
• CL Structure Changes
• Interface Modification
• Support Corrosion

29
Degradation of FC Performance Second Order
Effects

• Catalyst Changes create hot spots
• Hot spots create local catalyst changes
• Thinned membrane
• Non-uniform reactant distribution
• Effects of start-up, shutdown
• Break-in

30
WFCG Provides

• Advanced Fuel Cell Testing Services
• Assess to experienced faculty researchers
• Connections to skilled industrial collaborators
• Joint venture projects and proposals
• Personal attention for start-ups established
  companies
• Detailed needs assessment to enable tactical
  big picture development plans
• In conjunction with OFCC, were bringing Ohios
  fuel cell community together

www.WFCG.org
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Fuel Cell Testing Center in WFCG Headquarters
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Wright Center Capabilities

• Test stands for various sizes
• 6 Stations 50cm2 , Small Stacks up to 600W
• 3 Stations 5cm2
• 1 Station 1Kw with Environmental Chamber
• Access to wide array of ex-situ tests

33
CWRU Capabilities

• Areas of Focus
• Electrocatalysis Nanoparticle and Fuel Cell
  Operation Aspects
• Electrode Structures
• Gas Diffusion Media
• Durability
• Modeling
• New Membranes
• Fuel Cell Testing

• Resources
• NMR for transport
• Membrane Conductivity
• Electrochemical Analysis
• Polymer Synthesis
• Permeation Apparatus
• Wettability
• Fuel Cell Modeling
• Fuel Cell Test Stands

34
University of Toledo Capabilities

• Areas of Focus
• To study fundamental and molecular level
  information, surface and bulk structures, surface
  reaction intermediates under in-situ conditions.
• To study surface of catalyst. In-situ
  pretreatment and analyze adsorbed species on the
  surface
• Measure BET surface area
• Synthesize low density and high surface area
  aerogel catalyst materials

• Resources
• In-situ Raman Spectroscopy
• FTIR Spectroscopy with Drift Cell
• Micromeritics
• Supercritical CO2 Pressure Reactor System
• GC/MS
• Total Organic Carbon Analyzer

35
OSU-CAR Capabilities

• Areas of Focus
• Modeling of fuel cell stacks
• Dynamics and control of PEM systems
• Component characterization
• Fuel cell system integration and optimization
• Hybridization and supervisory control of fuel
  cell vehicles

• Resources
• 2.5 kW PEM fuel cell stack laboratory
• Optically accessible single cell for water
  formation and transport studies
• 80 kW complete pressurized PEM fuel cell system
  laboratory
• Simulation Tools

36
University of Cincinnati Capabilities

• Areas of Focus
• Employs multilayer self-assembly to create
  heterogeneous, multi-functional coatings for PEM
  fuel cell components
• Bipolar plates
• Gas diffusion Media
• Membrane-electrode assemblies
• Surface chemical characterization of fuel cell
  components for failure analysis
• Colloids synthesis and characterization

• Resources
• Dynamic Light Scattering
• Particle size and zeta potential
• Quasi-elastic light scattering
• Particle size in concentrated dispersions
• Dynamic wetting balance
• Contact angle surface scans
• Contact resistance measurement
• EDS, ESEM, TOF-SIMS Surface analysis
• Fuel cell component processing and
  characterization

37
Kent State Capabilities

• Areas of Focus
• Low-humidity high temperature polymer membrane
  materials
• Ultralow Pt loading nanostructured catalytic
  layers
• X-ray synchrotron studies of fuel cell
  electrocatalysts

• Resources
• Membrane/powder conductivity testing
• Atomic layer deposition station
• 1H NMR diffusivity measurements
• 50W fuel cell testing station

38
University of Akron Capabilities

• Areas of Focus
• Synthesis of new membranes for high temperature
  fuel cells
• Polyelectrolyte fuel cell membranes
• New class of polymers for fuel cell applications
• Fuel Cell, Syngas, Synfuel and CO2 capture
• Biological production of hydrogen

39
Cleveland State University Capabilities

• Areas of Focus
• High Temperature PEM membranes
• Hydrogen storage
• Vibration testing

40
Thanks for your time!

• Coming soon
• Short courses (with CAPI, throughout next year )
• Cell 101
• Fuel Cell Testing (hands-on)
• AC impedance for Beginners
• Academic Fuel Cell Course (Case, Spring 08)