Hydrocarbon Fuel Cell Membranes Containing Perfluorosulfonic Acid Group

Transcription

1 Hydrocarbon Fuel Cell Membranes Containing Perfluorosulfonic Acid Group Ying Chang and Chulsung Bae Department of Chemistry & Chemical Biology Rensselaer Polytechnic Institute, Troy, NY Collaborators Guiseppe F. Brunello,, Jeffrey Fuller, Seung Soon Jang (Georgia Tech) Marilyn Hawley, Yu Seung Kim (Los Alamos National Laboratory) Melanie Disabb-Miller, Michael A. Hickner (Pennsylvania State University) Contact: Center for Future Energy System Annual Conference (01/25/2013)

2 Fuel Cell Applications 2

3 Fuel Cells: PEMFC and AEMFC Reaction at Anode: H 2 2 H e - H OH - 2 H 2 O + 2 e - Reaction at Cathode: ½ O H e - H 2 O ½ O 2 + H 2 O + 2 e - 2 OH - Overall Reaction: H 2 + ½ O 2 electricity + H 2 O H 2 + ½ O 2 electricity + H 2 O Higher power density Automobile applications PEM: insufficient conductivity at low RH high cost of Nafion Catalyst: expensive Pt Lower power density than PEMFC Portable power applications AEM: insufficient conductivity poor stability against OH - Catalyst: non-noble metals, Ni, Co. etc 3

4 Proton Exchange Membrane (PEM) Fuel Cells Anode Cathode Key component in fuel cells Transport H + (and H 2 O) Acid-containing (-SO 3 H) polymer materials Nafion from Du Pont Separate H 2 and O 2 Ideal PEM Acidic (sulfonated) polymers that are Easy to synthesize Inexpensive Highly proton-conductive (even at low RH) Stable (chemical, thermal, mechanical) Low swelling in water Broad temp range (-20 to 120 o C) PV power capacity 4

5 Nafion vs. Sulfonated Hydrocarbon PEMs Perfluorinated flexible backbone Perfluorinated side-chain (15 mol% SO 3 H) IEC = 0.9 mmol/g Very high acidity (Superacid, pka = -14) -SO 3 H at flexible side chain Distinct nano-scale phase separation High 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 main chain Phase separation difficult (random copolymer) Low water diffusion Easy to modify structure & property Low cost Low proton conductivity at low RH 5

6 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 6

7 Strategy to Improve Proton Conductivity at Low RH via Hydrophilic-Hydrophobic Multi-Block Copolymers Proton conductivity depends on [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 7

8 Strategy to Improve Proton Conductivity at Low RH via Fluoroalkyl Sulfonic Acid (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 8

9 PEMs with Different Sulfonic Acid Groups: Acidity Effect on PEM Properties Nafion as benchmark PEM Nano-scale phase separation Multiblock HC ionomers Strong acidity (superacidic)??? Higher acidity Acidity Acidity Flexibility Objectives Role of acidity in proton conductivity? Any relationship among acidity, morphology, water property? Molecular level understanding of structure-property relation Develop low-cost high-performance hydrocarbon PEM 9

10 Traditional Functionalizations of Arene Electronic property of substituent already present in the aromatic ring determines reactivity and selectivity FG 1 = Alkyl, ortho-, para-directing activating FG 1 = NO 2, meta-directing deactivating 2010 Nobel Prize in Chemistry Efficient biaryl C-C bond formation Good functional group tolerance 10

11 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, J. Am. Chem. Soc. 2002, 124, 390 Smith, J. Am. Chem. Soc. 2000, 122,

12 Synthetic Applications of Borylated Arene: Intermediate for Functionalized Arenes Smith, J. Am. Chem. Soc. 2003, 125, 7792 Miyaura, Tetrahedron 2008, 64, 4967 Hartwig, Org. Lett. 2007, 9,

13 New Sulfonation Method of Aromatic Polymers via Borylated Polymer Entry [B 2 pin 2 ]/ [sps] sps-bpin M n PDI (M w /M n ) Bpin (%) C-H Borylation: control of sulfonate concentration Suzuki coupling: change of sulfonate structures Characterization of mol% (Bpin, SO 3 H) by 1 H NMR Macromolecules 2007, 40, 8600; Macromolecules 2011, 44,

14 Proton Conductivity, Water Uptake, Hydration Number vs. RH 80 o C IEC (mmol/g) mol% sulfonated Water uptake and hydration number by Michael Hickner at Penn State Macromolecules 2011, 44,

15 Origin of Improved Proton Conductivity of Superacid PEM: Pair Correlation Function & AFM Morphology S (sulfonate) O (hydronium) 34.3A S (sulfonate) O (water) 34.2A 40-sPS-S 1 -(CF 2 ) 2 SO 3 H (20 wt% water) 40-sPS-S 3 -(CH 2 ) 3 SO 3 H (20 wt% water) -CF 2 SO 3 H Better dissociation sulfonate to H 3 O + Acidity effect Collaboration with Seung Soon Jang (Georgia Tech) -CF 2 SO 3 H More aggregation of H 2 O near sulfonate Solvation effect Macromolecules 2011, 44, 8458 Yu Seung Kim (LANL) 15

16 Extension of Aromatic C-H Borylation to Polysulfone FG Sulfone part FG Bisphenol A part Excellent Stabilities (1) Good mechanical properties due to the rigid polymer chain (2) Excellent resistance of water, acid, base, and oxidants (3) Good thermal and hydrolytic stability (T g = 190 o C) Controlled incorporation of functional group into polysulfone will broaden its membrane application in a variety of different fields (Fuel Cells, Water Purification, Gas Separation, etc) Functionalized polysulfone is highly desired 16

17 Sulfonated Polysulfone for High Temp PEMs J. Am. Chem. Soc. 2009, 131, %-SO 3 H 160%-SO 3 H 200%-SO 3 H Polym. Chem. 2013, 4,

18 Water Absorption Properties & Proton Conductivity of Sulfonated Polysulfones IEC M.A. 100 o C 18

19 Morphology Study of Sulfonated PSUs with TEM & SAXS TEM by L. Ma Experimental SAXS (dry) by M. A. Hickner 2.7 nm 2.0 nm 2.8 nm 2.9 nm 2.3 nm No obvious difference in morphology among sulfonated PSUs Calculated SAXS (20% water) by S. S. Jang 19

20 Summary Ion-conducting aromatic polymers with different sulfonate structures synthesized by combination of C-H borylation & Suzuki coupling: PSU-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) Convenient controls of structure & concentration of sulfonic acid group Hydrophobic ponytail side chains reduce water uptake: PSU-S 1 & -S 3 < -S 2 Different sulfonic acids induce different proton conductivities at low RH: PSU-S 1 > -S 2, -S 3 Morphology and molecular dynamics studies suggests that superacidic character of PSU-S 1 induces better dissociated hydronium ions than PSU- S 3 and enhanced proton conductivity at low RH 20

21 Acknowledgment $$$ NSF (CAREER Award) Ministry of Knowledge & Economy (S. Korea) Rensselaer Polytechnic Institute ACS-PRF Nevada Renewable Energy Consortium Collaborators Georgia Tech: Seung Soon Jang (Molecular Dynamics) Los Alamos National Lab: Yu Seung Kim, Marilyn Hawley (AFM, Morphology) Penn State: Michael Hickner (Water Absorption, SAXS/SANS) Current and Past Group Members Dr. Ying Chang Jihoon Shin (Ph.D) Angela Adams-Mohanty Se HyeKim (MS) Bhagyashree Date Sarah Park 21