Synthesis of Highly Ion-Conductive Polymers for Fuel Cells (for H + and OH )

Transcription

1 Synthesis of Highly Ion-Conductive Polymers for Fuel Cells (for H + and OH ) Chulsung Bae Department of Chemistry & Chemical Biology Rensselaer Polytechnic Institute Collaborators: Michael A. Hickner (Penn State) Seung Soon Jang (Georgia Tech) Yu Seung Kim (Los Alamos National Laboratory) Chang Y. Ryu (Rensselaer Polytechnic Institute) Asilomar Fuel Cell Symposium contact: baec@rpi.edu 1

2 Nafion vs. Sulfonated Hydrocarbon PEMs Perfluorinated hydrophobic backbone Perfluorinated sulfonated side-chain (15 mol%) IEC = 0.9 mmol/g Very high acidity (Superacid, pka = 14) -SO 3 H at flexible side chain Distinct nano-scale phase separation Fast water diffusion Difficult to modify structure & property High cost Strong acidity & Favorable Morphology Guiver, Holdcroft, Ding, Adv. Funct. Mater. 2006, 16, 1814 Stiff aromatic main-chain backbone No side-chain IEC = mmol/g for good conductivity Strong acidity (pka = -6.5) -SO 3 H at rigid aromatic backbone chain Low level phase separation (random copolymer) Slow water diffusion Excessive swelling on hydration Easy to modify structure & property Low cost Low proton conductivity at low RH 2

3 Morphology Difference of Nafion & HC PEMs Kreuer, K. D. J. Membr. Sci. 2001, 185, 29 Hickner, Pivovar Fuel Cells 2005, 5, 213 Close packing of ionic groups Wide channels & good connectivity Good phase-separated morphology Promotes loosely bound water Good water (& H 3 O + ) transport Narrow hydrophilic domain channels Highly branched & dead-end channels Lower degree of phase separation More tightly bound water Decreased water (& H 3 O + ) transport 3

4 Strategy to Improve Proton Conductivity at Low RH via Morphology Control Hydrophilic-Hydrophobic Sulfonated Block Copolymers Proton conductivity depends on concentration and diffusion of H 3 O + [H 2 O] x [H + ] 1. To improve transport of [H 2 O] Create favorable morphology via Hydrophobic-hydrophilic sulfonated block copolymers Continuous H 3 O + /H 2 O pathways via self-assembled microstructures Better transport of H 2 O at low RH, but still high one-dimensional swelling Sulfonated random copolymer IEC = 1.53 meq/g Kim, McGrath, Guiver, Pivovar Chem. Mater. 2008, 20, 5636 Sulfonated multiblock copolymer IEC = 1.51 meq/g 4

5 Strategy to Improve Proton Conductivity at Low RH via Strong Acidity Fluoroalkyl Sulfonic Acid Side Chain (Superacid) 2. To increase [H + ] More SO 3 H to PEM Higher IEC, Higher WU, Excessive swelling Mechanical failure of PEM Increase acidity Introduce F to SO 3 H -CF 2 CF 2 SO 3 H Better dissociation to increase [H + ] Reduce water uptake Prevent excessive swelling Synthetic challenge a Estimated by the H o method, In Superacids; G. A. Olah, G. K. S. Prakash, J. Sommer b In Advanced Organic Chemistry; 5 th Ed.; M. B. Smith and J. March c In Organic Chemistry; 7 th Ed.; J. McMurry 5

6 Molecular Engineering of PEM Design: Different Sulfonate Groups & Polymer Structures Nafion as benchmark PEM Nano-scale phase separation Multiblock HC ionomers Strong acidity (superacidic) Higher acidity Acidity Acidity Flexibility Approach: Molecular level understanding of structure-property relationships Side Chain (ion-conducting moiety) Acidity effect of side chain on proton conductivity Relationship among acidity strength, morphology, ion transport Polymer Backbone (mechanical, physical property) Hydrophobic vs. hydrophilic Secondary polymer structure (morphology) Random, block, or graft copolymers Discover the optimized polymer structures for high-performance hydrocarbon PEMs 6

7 Synthesis of Sulfonated Aromatic Polymers 1. Electrophilic post-sulfonation Nunes et al, J. Appl. Polym. Sci. 2002, 86, Polymerization of sulfonated monomer McGrath et al. J. Membr. Sci. 2002, 197, 231 7

8 Direct Borylation of Aromatic C-H Bonds Iridium-catalyzed aromatic C-H bond activation/borylation Boron substitutes only aromatic C H bonds selectively Mixture of meta and para-borylated products Miyaura & Hartwig, JACS 2002, 124, 390 Smith, JACS 2000, 122,

9 Synthetic Applications of Borylated Arene: Intermediate for Functionalized Arenes Smith, JACS 2003, 125, 7792 Miyaura, Tetrahedron 2008, 64, 4967 Hartwig, Org. Lett. 2007, 9, 761 9

10 Ir-Catalyzed C H Borylation of Polysulfone: Control of Degree of Functionalization THF M n = 25.2 kg/mol $0.7/g (Sigma-Aldrich) A: [IrCl(COD)] 2 B: [Ir(OMe)(COD)] 2 Entry B 2 pin 2 /PSU Bpin (mol %) Effic. (%) B 2 pin 2 /PSU Jo et al, JACS 2009, 131, 1656 Chang et al, Macromolecules 2013, 46,

11 Sulfonated Polysulfone for High Temp Low RH PEMs T. S. Jo et al, JACS 2009, 131, %-SO 3 H 160%-SO 3 H 200%-SO 3 H Chang et al., Polym. Chem. 2013, 4, Ying Chang 11

12 Water Absorption Properties & Proton Conductivity of Sulfonated Polysulfones M.A. Hickner IEC o C Ying Chang 12

13 Morphology Study of Sulfonated PSUs with TEM & SAXS TEM by L. Ma Experimental SAXS by M. A. Hickner 2.7 nm 2.0 nm 2.8 nm No obvious difference in morphology among sulfonated PSUs Ying Chang 13

14 Anion Exchange Membrane (AEM) for Fuel Cells Common polymer materials Inexpensive Commercially available Easy to modify Common cations AEM Easy to prepare Reasonably good stability Advantages: Metal catalysts: Fe, Co, Ni, Ag Fast cathode reaction rates Fuel flexibility / Low fuel crossover Issues of OH conducting polymers Low ion conductivity Poor chemical & mechanical stabilities in high ph and >80 o C 14

15 Chemical Degradation of AEM by OH - Attack OH - is an aggressive base/nu Degradation at Cation Group» S N 2 substitution Pivovar, J. Phy. Chem. C. 2010, 114, Degradation at Polymer Backbone» Chain cleavage at C O C» Ylide intermediate formation» E2 (β-hofmann) elimination In alkaline condition, degradation occurs at both Kim, J. Membr. Sci. 2012, , 438 Ramani, PNAS 2013, 110, 2490 cation group and polymer backbone simultaneously! Hickner, ACS Macro Lett. 2013, 2, 49 15

16 Molecular Engineering Approach to Improve AEM Stability Cations Polymer Backbone Increased steric hindrance Resonancestabilized Utilize polymer backbones made of only C C bond Hibbs et al. Macromolecules (2009) 42, 8316 Approach: decouple the stabilities of cation and polymer backbone 1. Investigate cation stability systematically using model QAs Sterically hindered cations, resonance-stabilized cations 2. Employ more stable polymer backbones No C O bonds, high molecular weights 3. Incorporate stable QA structures to non-degradable polymer Angela Mohanty 16

17 NMR Study of QA Model Compounds (1) Ion exchange & isolate QA in OH - form (2) Transfer to NMR tube & immerse in preheated oil bath for 1 mo. a a b c d 0 h 1 d 6 d 11 d D 2 O 18-crown-6 b c d Degradation product (3) Record NMR spectrum periodically 18 d 28 d A. Mohanty & C. Bae J. Mater. Chem. A. 2014, 2, Angela Mohanty 17

18 Quantitative Stability Comparison of Small Molecule QAs Angela Mohanty 18

19 Summary Ion-conducting aromatic polymers with different cation/anion structures synthesized by combination of C-H borylation & Suzuki coupling Convenient controls of structure & concentration of ionic groups Proton Exchange Membrane Poly-S 1 (-CF 2 CF 2 -SO 3 H), -S 2 (-C 6 H 4 -SO 3 H), -S 3 (-CH 2 CH 2 CH 2 SO 3 H) Hydrophobic ponytail side-chains reduced water uptake: Poly-S 1 & -S 3 < -S 2 Superacidic sulfonate improved proton conductivities at low RH without significantly affecting morphology: Poly-S 1 > -S 2, -S 3 Anion Exchange Membrane PF-, SEBS-QA (where QA = -CH 2 N + R 3 ) Good stability with OH conductivity Excellent fuel cell performance 19

20 $$$ Acknowledgment Collaborators Molecular Dynamics: Seung Soon Jang (Georgia Tech) AFM, Mechanical Property, MEAs: Yu Seung Kim (Los Alamos National Laboratory) Water Absorption, SAXS: Michael A. Hickner (Penn State) SAXS: Joel Morgan, Chang Y. Ryu (RPI) Ir and Pd catalysts: Sino Chemicals B 2 pin 2 : Frontier Scientific Current and Past Group Members Postdoc: Ying Chang, Woo-Hyung Lee Dongwon Shin Graduate student: Angela Mohanty, Bhagyashree Date, Sarah Park, Stefan Turan, Jihoon Shin, Se Hye Kim 20