Polymeric Proton hydronium Conductors

1
Polymeric Proton (hydronium) Conductors
teflon-like back-bone
sulfonated side-group
high fluorine content ? chemical stability, but
high cost
2
Microstructure of Nafion
Small angle X-ray scattering
Molecular dynamics simulation
?(CF2)n ?
?? SO3- (H2O)nH
Jang, SS., Molinero, V., Cagin, T., Merinov BV.,
Goddard III WA. Solid State Ionics
Kreuer, J Membr Sci 2 (2001) 185.
Phase separation between hyrophobic and
hydrophyllic regions Both regions form
continuous, percolated networks
3
Water Management
H(H2O)n
Anode side conductivity decreases
O2
H2
membrane
H2O
Conductivity / Scm-1
dries out
floods
Mole ratio H2O/SO3H
cathode side catalyst particles become
inaccessible
4
Direct Methanol Fuel Cell
JPL/Guiner demo

• Operation
• liquid feed, no pressure
• fuel 3-4 MeOH in H2O
• Reactions
• Anode CH3OH H2O ? CO2 6H 6e-
• Cathode 1.5O2 6H 6e- ? 3H2O
• --------------------------------------------------
  
• Cell CH3OH 1.5O2 ? CO2 2H2O
• Challenges
• MeOH permeation thru electrolyte
• anode catalyst Pt-Ru 8 mg/cm2
• low power densities 100 mW/cm2
• Ru transport across membrane!!

5
Desired Membrane Properties

• High conductivity
• resistivity lt 10 Wcm
• thickness 50-100 mm
• resistance lt 0.1 Wcm2
• resistivitythickness
• electro-osmotic drag H2O/H lt 3, even 1
• Impermeable to H2, O2
• Impermeable to MeOH

• flexible tough (at 50-100mm)
• pore-free
• thermally stable
• chemically stable
• inexpensive

thinner polymer membranes generally not
considered plausible - H2 and O2 gas permeation
become a problem - generally fabricated as
free-standing membranes - sets high standard for
required conductivity
6
Polymer Electrolyte Research Directions

• Decouple conductivity from water transport
• Lower water content and relieve H2O management
• Improve mechanical properties
• Lower methanol cross-over
• Inexplicably linked with decreased catalytic
  activity
• Increase operating temperature
• Increase catalysis rates
• Lower the size of automotive radiator
• Generate recoverable waste heat
• Lower cost by lowering extent of fluorination
• Challenges with electrolyte stability

7
Polymers Examples

• Nafion (Du Pont) Dow Experimental
• main chain -(CF2-CF2)n-
• side groups
• -O-(CF2-CF-O)m-CF2-CF2-SO3-
• CF2
• counter cation Na ? H
• saturate with H2O

• Raymion Permion
• main chain
• -(CH2-CH2-CF2-CF2)n- R
• -(CF2-CF2-CF2-CF2)n- P
• side groups
• -(CF2-CF)m-CF2-CF2
• SO3- SO3-

Permion F ? H
8
Polymers Examples

• a polyphosphazene

• poly(aryl ether ether ketone) PEEK

n
poly(aryl ether sulfone) PSU or PES
sulfonated and possibly cross-linked
9
Liquid Acids

• Proton transport relies on dissociation
• 5H3PO4 ? 2H4PO4 H3O H2PO4- H2P2O72-
• 6H2SO4 ? H3SO4 H3O HSO4- HS2O7- H2S2O7
  H2O
• Also of high conductivity
• aqueous H3PW12O40nH2O

• Disadvantages
• Corrosive
• Containment/Matrix

• Sluggish electrodes
• Low power densities

10
New Arrivals

• Acidified poly(benzimidazole) (PBI)
• Replace H2O in polymer matrix with H3PO4
• Several analogous combinations explored
• Achieves
• High temperature operation
• Challenges
• Acid leaching, sluggish cathode kinetics, Pt
  coarsening
• Ionic liquid infused Nafion
• Replace H2O with non-volatile heterocyclic amines
• Achieves
• High temperature operation
• Challenges
• Ionic liquid loss,
• Lower catalytic activity, why??

imidazole
11
Electrolyte Conductivity
12
Oxide Ion Conductors

• Trivalent dopant
• So long as solubility limit is not exceeded
• Conductivity

MO2 - Fluorite Structure
concentration
M Zr, Ce Tb, U
s ez n m
mobility