Wir schaffen Wissen heute für morgen

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1 Wir schaffen Wissen heute für morgen Paul Scherrer Institut L. Gubler, A. Albert, Y. Buchmüller, O. Nibel, L. Bonorand Radiation Grafting: Tailored Ion-conducting Membranes for Electrochemical Applications

2 Content Introduction Radiation grafted membranes Introducing Antioxidants Polymer-bound phenolic antioxidants Accelerated stress tests in the fuel cell Regeneration strategy PFSA vs. hydrocarbon membranes? Beyond Fuel Cells Conclusion 2

3 Acknowledgement Technical support: A. Albert PhD student L. Bonorand senior engineer Y. Buchmüller former PhD student O. Nibel PhD student Jürg Thut Funding: 3

4 Radiation Grafted Membranes 4

5 Radiation Grafted Membranes base polymer 2 dissimilar polymer constituents: - hydrophobic base polymer - polyelectrolyte connected via covalent bonds SO 3 H base polymer anion exchange membranes OH + N base polymer H 3 PO 4 N membranes for HT-PEFC (phosphoric acid doped) L. Gubler, Adv. Energy Mater. 4 (2014)

6 Performance & Durability of Grafted Membranes Cell Voltage / V T = 80 C, H 2 / O 2, p = 2.5 bar a Nafion 212 optimized grafted membrane Current Density / A/cm HF Resistance / Ohm.cm 2 ETFE L. Gubler, L. Bonorand, ECS Transactions 58 (2013),1, 149 SO 3 H C N durability (accelerated): grafted membrane Naf. 212 Naf. Nafion XL Operation time / h 2/3 membranes intact after 2'400 h 6

7 HO Radical Attack PFSA HO k 10 6 M -1 s -1 τ(ho ) 1 µs main chain & side chain degradation SO 3 H HO k = M -1 s -1 τ(ho ) = 10 ns follow-up reactions, chain fragmentation L. Gubler et al., J. Electrochem. Soc. 158 (2011) B755 S.M. Dockheer et al., PCCP 15 (2013)

8 Introduction of Polymer-bound Antioxidants 8

9 Introducing Antioxidant (HO Scavenger) known mechanism*: OH OH OH acid catalyzed water elimination H + k = s -1 + HO k PhOH k PhOH s -1 O O phenoxyl radical phenoxyl radical: relatively stable unlikely to attack polymer *Z. Rappoport, The Chemistry of Phenols, John Wiley & Sons,

10 Introduction of Antioxidant Functionality covalent tethering of phenol type antioxidant (AO) to graft copolymer: ETFE 1. attach AO to linker 2. sulfonation ETFE linker NH 2 SO 3 H OH Cl vinylbenzyl chloride (VBC) O O O glydidyl methacrylate (GMA) OH AO: tyramine Y. Buchmüller et al., J. Mater. Chem. A 2 (2014)

11 Grafted Antioxidant ETFE VBC linker O C O OH GMA linker O HO S O NH SO 3 H HN OH VBC(Tyr) OH GMA(Tyr) poor co-grafting kinetics low yield of tyramination of 33% (side reactions) poor fuel cell performance superior co-grafting kinetics yield of tyramination: 56% good fuel cell performance (better than pure styrene based) 11

12 Grafted Antioxidant in situ accelerated degradation stress test (H 2 /O 2, 80 C, OCV) Voltage loss 1 A/cm w/ antioxidant: only with linker: GMA(Tyr) GMA(diol) Voltage loss (mv) IEC loss after in situ accelerated test (%) sulfonated PSSA only styrene (no co-grafted linker) concept works IEC loss (%) Y. Buchmüller et al., J. Mater. Chem. A 2 (2014)

13 Extended Accelerated Chemical Stress Test Cell Voltage (V) T = 80 C, H 2 / O 2, p = 2.5 bar a GMA(Tyr) "fresh" GMA(Tyr) "stored" PSSA (unstabilized)? HF Resistance (Ohm cm 2 ) "fresh" membrane: tested immediately after synthesis of the membrane "stored" membrane: kept in fridge (no light) for 7 days before being tested; sudden increase of resistance suggests depletion of antioxidant (?) deactivation of the antioxidant Time at OCV (h) Y. Buchmüller et al., RSC Adv. 4 (2014)

14 Antioxidant Strategies PFSA incorporate transition metal redox couples (Mn 2+ /Mn 3+, Ce 3+ /Ce 4+ ) or corresponding oxides to scavenge HO : e.g.: Ce 3+ + HO + H + Ce 4+ + H 2 O k = M -1 s -1, lifetime of HO in PFSA ~µs regeneration of Ce 3+ : Ce 4+ + H 2 O 2 Ce 3+ + HOO + H + with [H 2 O 2 ] = 0.5 mm: τ 1 ms catalytic HO scavenging Hydrocarbon based HO reacts rapidly with aromatic units (τ ns), need much more effective scavenger, e.g., phenol type H-donor (k M -1 s -1 ): PhOH + HO PhO + H 2 O PhOH is depleted over time. Could it be regenerated? Regeneration by H 2 O 2 unlikely for energetic reasons (thermodynamics, kinetics) Repair by reductive power of the anode (~0-50 mv)? J. Electrochem. Soc. 159 (2012) B211 14

15 Repair of Spent Phenol Type Antioxidant by H 2 O 2? PhO + H 2 O 2 PhOH + HOO (mild oxidant) [H 2 O 2 ] in operating fuel cell ~0.5 mm 1 E (ph 0) (HO /H 2 O) (RO /ROH) (Ce 4+ /Ce 3+ ) (Mn 3+ /Mn 2+ ) (HOO /H 2 O 2 ) (PhO /PhOH) X quenching e - e - regenerate E (PhO,H + /PhOH) < E (HOO,H + /H 2 O 2 ) regeneration by H 2 O 2 unlikely Increase E (PhO ) by substitution? R OH R' e withdrawing group yet kinetics expected to be slow 1 W. Liu, D. Zuckerbrod, J. Electrochem. Soc. 152 (2005) A

16 Repair through e Provided by the Anode? anode membrane H 2 O HO PhOH PhO H 2 O e - e - HO PhOH PhO e - PhO / PhOH: 1. radical scavenging 2. regeneration (reduction), electron supply via conducting polymer (φ anode < φ redox ) conducting polymer (PPy) surface near layer with regenerative antioxidants 16

17 Polymerization of Pyrrole (Py) into Membrane N PPy surface near conducting polymer layer membrane mixed Nafion / PPy layer Nafion 212 Buchmüller et al., ChemElectroChem (2015) in press (doi: /celc ) 17

18 Electrochemical Response fuel cell configuration: cyclic voltammetry, using hydroquinone (H 2 Q) as redox probe: CE, RE WE 5 % H 2 / N 2 N 2 issues: Pt/C C only PPy layer membrane resistance increases >2x electronic conductivity in PPy phase poor, high transport losses PhO undergoes fast follow-up reactions Buchmüller et al., ChemElectroChem (2015) in press 18

19 Use Alternative Antioxidant Chemistry? use hindered phenol: butylated hydroxytoluene (BHT) well known in plastics industry hindered amine light stabilizers (HALS)*? R [o] Denisov cycle N X N O N O R target H 2 O 2 decomposition (τ >> 1 s)? 19 non-radical products ROO *J.L. Hodgson, M.L. Coote, Macromolecules 43 (2010) 4573

20 Antioxidant Mechanism and Strategy fuel cell: HO HOO RO (+ HO ) 1 R-H (polymer) M (z+1)+ H 2 O R Cycle 2 R ROOH 4 M z+ H 2 O 2 1 O 2 3 Cycle 1 R-H ROO radical scavengers H-donors hydroperoxide decomposers metal chelating agents 20

21 Beyond Fuel Cells 21

for high purity H 2 production H 2 for fuel

electricity ( power-to-gas ) Flow batteries

energy and power rating Lithium batteries

22 Electrochemical Devices with Polymer Electrolytes Electrolysis water electrolysis for high purity H 2 production H 2 for fuel cell vehicles renewables: storage of excess electricity ( power-to-gas ) Flow batteries grid-scale storage of electricity decoupled energy and power rating Lithium batteries consumer electronics electromobility load leveling, peak shaving 22

23 Membranes for Water Electrolysis 6 80 C, 100 % r.h., 2.5 bar a Figure of merit: 1 RΩ i x (in 10-3 /V) H 2 Crossover (ma/cm 2 ) E(C)TFE Nafion membranes SO 3 H C N grafted membranes Nafion : 5.8 ± 1.3 Grafted membranes: 12.6 ± 3.7 low-cost (5-10 x cheaper than state-of-the-art) low H 2 and O 2 crossover through crosslinked polymer architecture mechanically robust to 100 C and creep resistant (semicrystalline polymer, crosslinked graft component) Area Resistance (mohm cm 2 ) EU-Project NOVEL 23

24 Vanadium Barrier in VRB Vanadium redox flow battery (VRB) U cell 1.2 V + V 2+ H + V 3+ VO 2+ VO 2 + H 2 SO 4 (aq) ETFE ion-exchange membrane negative electrode: V 3+ + e V 2+ positive electrode: VO H + + e VO 2+ + H 2 O Poster by Olga Nibel grafted membrane with V-barrier: V(IV) Diffusivity (cm 2 /s) Ohmic Resistance (Ohm cm 2 ) V-complexing group 1e-7 8e-8 6e-8 4e-8 2e baeline w/ V-barrier Nafion 212 effect of vanadium barrier European Patent Application EP

Li-metal loss of active material Polysulfide Shuttle in the

porous membrane Li + 2 S 3..8 S 8 Li 2 S 1.

exchange layer (~ 1 µm) porous separator PP polysulfide

lithium polysulfide diffusion test grafted separator

25 Li-metal loss of active material Polysulfide Shuttle in the Li-S Battery + porous support layer (~ 25 µm) asymmetric porous membrane Li + 2 S 3..8 S 8 Li 2 S 1..8 sulfur electrode polysulfides dense skin = cation exchange layer (~ 1 µm) porous separator PP polysulfide rejection SO 3 - Li + grafted cation exchange groups mobile lithium polysulfide diffusion test grafted separator pristine separator 25 unmodified side grafted sided J. Conder et al., J. Mater. Chem. A, in prep.

26 Conclusion Radiation grafted membranes can reach promising performance / durability attributes compared to PFSA membranes Membranes with polymer-bound antioxidants show considerably improved stability However, the phanol type antioxidants are depleted. What antioxidant strategies need to be adopted for hydrocarbon membranes? Through adapted design, membranes with improved barrier properties can be synthesized for the water electrolyzer, redox flow cell, and lithium-sulfur battery 26

Batteries (Electrochemical Power Sources)

Batteries (Electrochemical Power Sources) 1. Primary (single-discharge) batteries. => finite quantity of the reactants 2. Secondary or rechargeable batteries => regeneration of the original reactants by