Polymer Electrolyte for Fuel Cells-an Overview Dr. S. P. Vernekar Emeritus Scientist Polymer Science & Engg. Division National Chemical Laboratory Pune-8
What is a Fuel Cell? An electrochemical device that converts chemical energy into electrical energy directly Advantages Applications • More efficient than heat Engines • Automobiles • Environment friendly Portable electronic devices • • Produces no noxious emissions • Mobile & Stationary power stations • Operates quietly (less sound) • Fuel flexibility • Compact design
Fuel Cell Types Fuel cell type/ Electrolyte/ Operating Efficiency Applications Mobile ion temp. ºC Fuel used % Alkaline (AFC) KOH/ 50-100 45-60 Space vehicles: OH - (low) ~10 kW H 2 & O 2 Proton exchange PEM/ 50-130 40-60 Small and mobile (low) applications: membrane H + 0.01-100 kW (PEMFC)/DMFC H 2 / MeOH- O 2 /air Phosphoric acid H 3 PO 4 / 180-240 35-40 Medium applications: (PAFC) H 2, H + (medium) 100-1000 kW Natural gas-air Molten Molten ~650 45-60 Medium and large carbonate applications: carbonate (high) 2- (MCFC)/ CO 3 0.1-10 MW Natural gas-air Solid oxide Ceramic/ 500-1000 50-65 Wide scale applications: (high) O 2- 1 kW-10 MW (SOFC) Natural gas-air
Major Components of PEMFC Membrane Major Components � Polymer Electrolyte Membrane Oxidizer Fuel � Electrocatalyst ( Pt ) Cathode Anode � Bipolar plates ( Graphite/ Polymer) Electrodes Basic Components of PEMFC
Membrane Electrode Assembly (MEA)
Electrochemical Reactions in PEMFC Cathode: 1/2 O 2 + H 2 O 2OH - +2e Anode: H 2 2H + + 2 e - ` Cathode: 1/2 O 2 + 2H + +2 e - Anode: 2OH - + H 2 + 2e H 2 O H 2 O Overall: H 2 + 1/2 O 2 H 2 O + E. E + H. E Overall: H 2 + 1/2 O 2 H 2 O + E. E + H. E
Polymer Electrolyte Membrane (MEA) Desired Properties • Good Mechanical strength (operating conditions) • Good Thermal stability • High Stability in oxidative & reductive environment • Good Chemical & Electrochemical Stability • Good Barrier property for reactant species • Good Processability for MEA preparation • Low electro-osmotic drag • Zero Electronic conductivity • High Proton conductivity (>0.1S/cm) • Long life above 100 0 C temperature (Operational conditions) • Low Cost
NAFION * CF 2 CF 2 CF 2 CF * * CF 2 --CF 2 CF 2 - CF * x n x n O CF 2 -CF-CF 3 O CF 2 -CF 2 O-CF 2 -CF 2 SO 3 H SO 3 H Disadvantages: Advantages: • High cost • Good mechanical strength • Need to maintain humidity • High proton conductivity High electro-osmotic drag • • Good chemical resistance • Poor mechanical strength at • Low solvent solubility high water uptake High water uptake • • High methanol crossover • Proven durability(>60,000h ) • Catalyst poisoning in DMFC • Only material used in fuel Low operating temperature • cell today (~80 0 C )
Advantages of High temperature (> 100 0 C) PEM Kinetics of both electrode reactions will be enhanced • (especially) for DMFC • Water is in single vapor phase ( management easy) • Cooling system will be simpler ( larger temp. gradient between coolant & stack) Heat can be recovered as steam ( can be used for reforming • MeOH) CO tolerance can be enhanced ( 10-20 ppm at 80 0 C; 1000 ppm • at 130 0 C; 100000 ppm at 200 0 C. Pure H 2 is not required. H 2 from reformer can be used at 200 0 C. • Development of prototype PEM fuel cell operating at high temp.(>100 0 C)
Polymers used as Polymer Electrolytes Polymers used as Polymer Electrolytes Fluorinated Polymers Fluorinated Polymers Heterocyclic Polymers Heterocyclic Polymers • Sulfonated Ionomers (Nafion Type) • Sulfonated Ionomers (Nafion Type) • Polybenzimidazoles • Polybenzimidazoles • Sulfonated Poly(trifluorostyrene) • Sulfonated Poly(trifluorostyrene) • Polyoxadiazoles • Polyoxadiazoles • Graft fluorinated Polymers • Graft fluorinated Polymers • Polytriazoles • Polytriazoles Sulfonated Aromatic Polymers Sulfonated Hydrocarbon Polymers Sulfonated Aromatic Polymers Sulfonated Hydrocarbon Polymers • Phenol Formaldehyde • Styrene Propylene Block Copolymer • Phenol Formaldehyde • Styrene Propylene Block Copolymer • Polystyrenes • Styrene Butadiene Block Copolymer • Polystyrenes • Styrene Butadiene Block Copolymer • Polyphosphazenes (PPZ) • Styrene Ethylene Propylene Triblock • Polyphosphazenes (PPZ) • Styrene Ethylene Propylene Triblock • Polyphenylenequinoxaline (PPQ) Polymer • Polyphenylenequinoxaline (PPQ) Polymer • Polyphenylene oxides (PPO) • Polyphenylene oxides (PPO) • Polysulfones (PES) • Polysulfones (PES) • Polyetheretherketones (PEEK) • Polyetheretherketones (PEEK) • Polyphenylenesulfides • Polyphenylenesulfides • Polyimides (Polyimides) • Polyimides (Polyimides)
Fluorinated Polymers * CF CF 2 CF CF 2 * n * CF * CF CF 2 CF CF 2 CF CF 2 * CF 2 * n n SO 2 SO 3 H SO 3 H SO 3 H R * CF CF 2 * • High proton conductivity • Resistant to oxidative degradation • Cross-linking improves flexibility, dimensional stability and swelling • Stable upto 15000 h at 50 0 C ( BAM3G)
Sulfonated Polyphenylene Oxides OH O O n n HO 3 S • Sulfonated by ClSO 3 H at back bone aryl group, • Deactivation by Br gives sulfonation at peripheral phenyl ring • Proton conductivity of sPPO (IEQ =2.63) is 0.012 S/cm at RT • Life is 450 h
Sulfonated Polyphenylenequinoxaline SO 3 H H 2 N NH 2 N n n + H 2 SO 4 O O C C O SO 3 H H 2 N NH 2 N O O C Ar C Ar N N Ar = ; O SO 3 H n • It is sulfonated by H 2 SO 4 /oleum at 125 0 C or by heating H 2 SO 4 doped film at 300 0 C • It has high Tg of 220 0 C • Stable upto 300 0 C • Proton conductivity is 0.1 S/cm at 80 0 C ( Nafion) • It has limited life of 350 h at 70 0 C for H 2 /O 2 FC.
Sulfonated Polyphosphazenes SO 3 H Cl N Cl O O Cl P P S NaOAr H 2 SO 4 Cl Cl P N N P N P N P N 285 o C Oleum Cl O O n Cl Cl n n SO 3 H 2) HCl HO 3 S ONa 1) O CH 3 H 3 C N P CH 3 O n SO 3 H sPPZ prepared by condensing phenoxide with dichloroPZ • followed by sulfonation By condensing sulfonated phenoxide with dichloroPZ •
Sulfonated Polyphosphazenes ( Cont.) O O Br O O P(OH) 2 Me O Me N P N P N P N P O Me O Me O Me O Me n n m m • Copolymers are prepared by condensing two different phenoxides • 30% sulfonated polymer is soluble in water & S.P is 76 0 C • Photo cross-linking ( Water uptake reduces 19 to 13) • Proton conductivity is 0.04-0.08 S/cm at 30-60 0 C & RH 100% ( IEC 1.4 meq/g) • Proton conductivity of phosphonic acid substituted (IEC 1.43 meq/g) PSZ is ~0.05 S/cm at RT ( low MeOH permeability 12 times lower than Nafion) It may be useful for DMFC
Sulfonated polysulfones CH 3 CH 3 O O H 2 SO 4 O C O S O C O S n n O CH 3 CH 3 O SO 3 H SO 3 H O HO R OH + Cl S Cl O HO 3 S K 2 CO 3 190 o C NMP ~30h Toluene SO 3 H O * S O R O * n O HO 3 S CF 3 CH 3 C C , R = CF 3 CH 3
Sulfonated polysulfones (cont.) CH 3 CH 3 O O O C O S O C O S n n O O CH 3 CH 3 SO 3 H SO 3 H • Sulfonation by H 2 SO 4 or ClSO 3 H leads to degradation SO 3 in DCM is preferred • Sulfonation at o- phenol, (SO 3 H group on sulfone moiety more stable) • sPES with IEC- 2.5-3.0 has proton conductivity similar to Nafion, but high swelling. • Cross-linked by diamine, reduces IEC • Condensation polymers preferred. sPES with IEC 0.41-2.2 has 0.01-0.16 S/cm at 30 0 C conductivity.
Sulfonated PEEK O O O n + F C F n HO OH O O O O C C n n O O H 2 SO 4 O O O O C C n n SO 3 H SO 3 H O O O n + F C F n HO OH O O O O C C n n SO 3 H SO 3 H • PEEK is synthesized by condensing bisphenol with difluorobenzophenone
Sulfonated PEEK (cont.) SO 3 H O O O O C n SO 3 H C O sPEEK n sPPBP • PEEK is sulfonated by oleum/H 2 SO 4 ( o-ether group) • Time & Temp decide extent of sulfonation 90% sulfonated-water soluble-proton conductivity Nafion • • Cross-linked by diamine or heating • Solvent for film casting affects proton conductivity. (NMP-10 -2 ; DMF 10 -5 S/cm • Decompose at 240-300 0 C • Proton conductivity mechanism is similar to Nafion • sPPBP has higher proton conductivity(9x10 -2 S/cm) than sPEEK • Life time of 5000 h
Sulfonated Block Copolymer SEBS CH 2 CH * CH 2 CH * * CH 2 CH 2 * CH CH 2 CH * * CH 2 * * CH 2 CH * * y p q x v u CH 2 n CH 3 SO 3 H SO 3 H • Sulfonation is by acetyl sulfate or SO 3 in DCM • Polymer with 60% sulfonated phenyl group has proton conductivity more than Nafion • Polymer degrade at higher temp. • Life time 2500 h at 60 0 C and 4000h at RT
Sulfonated polyimides O O SO 3 H O O + H 2 N NH 2 H 2 N Ar NH 2 + O O HO 3 S O O O O SO 3 H N N N N Ar O O O O HO 3 S p m sPI are synthesized by condensing dianhydride with sulfonated • diamine • Properties can be adjusted by copolymerization • Length of ionic block in copolymer has significant effect on proton conductivity • 5 membered imide ring is hydrolytically unstable. Six membered imide ring is stable.
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