Giuseppe Felice Mangiatordi

Transcription

1 Charge transport in poly-imidazole membranes: a fresh appraisal of the Grotthuss mechanism Giuseppe Felice Mangiatordi Laboratoire d'electrochimie, Chimie des Interfaces et Modélisation, Chimie ParisTech (ESCP) SSPC16, sept 2012 Grenoble

2 Outline Context? I) Introduction State of the Art? Challenges? - DFT II) Proton conduction in P4VI - MD III) Conclusions 2

3 I) Introduction 3

4 PEMFC Solid electrolyte Fuel: H2 Operating temperature < 100 C Anode: 2H2 4H+ + 4e Cathode: O2 + 4H+ + 4e- 2 H2O Overall reaction: 2H2 + O2 2 H2O 4

5 PEMFC/ Solid electrolyte DESIRED PROPERTIES Mechanical, thermal and chemical stability Proton conductivity S/cm (1) < 100 C STATE OF THE ART afion Perfluorosulfonic acid membranes (1) A.M. Affoune et al., Journal of Power Sources 148 (2005)

6 PEMFC/ Solid electrolyte CHALLEGE Membranes working at higher temperatures (T > 100C ) Electrode reaction kinetics Heat management STRATEGY Imidazole: 10-3 S/cm ( C )(2) 1,2,3 Triazole: S/cm ( C )(3) leaching problems!!! immobilization in a polymeric matrix?? (2) Kawada, A. ; McGhie, A. R. ; Labes, M. M. J. Chem. Phys. 1970, 52, 3121 (3) Z. Zhou, S. Li, Y. Zhang, M. Liu and W. Li, J. Am. Chem. Soc., 127 (2005)

7 step 1 H H H H H H H H+ electrode (-) charge generation electrode (+) Goal of the work step 2 charge-carrier transport Grotthuss mechanism! step 3 reorientation H H H step 4 new charge generation H+ H H H The Bredas mechanism for poly[4-vinylimidazole](4) What is the geometrical constrain imposed by the polymeric backbone? (4) Zhou, Z.; Liu, R.; Wang, J.; Li, S.; Liu, M.; Brédas, J. L. J. Phys. Chem. A 2006, 110,

8 Density Functional Theory Total energy expressed in terms of the total one-electron density E [ρ] = T [ρ] + EJ [ρ] + E-e [ρ] + Exc [ρ [ ] form not known Selection of a method?? Selection of a functional! 8

9 II) Proton conduction in P4VI 9

10 Proton conduction in P4VI Poly(4-vinyl-imidazole)(6) 1 Two non-equivalent nitrogen atoms! Protocol BMK//B3LYP (5) (5) Mangiatordi G. F.; Hermet, J.; Adamo, C. J. Phys. Chem. A 2011, 115, (6) A. Bozkurt, W.H. Meyer: Proton conducting blends of poly(4-vinylimidazole) with phosphoric acid, Solid 10 State Ionics 138 (2001)

11 Proton conduction in P4VI Ebarrier (3-3) = 2.5 kcal/mol Ebarrier (1-1) = 23.5 kcal/mol Ebarrier (1-3) = 25.1 kcal/mol Ebarrier (3-1) = 21.2 kcal/mol Rotation of the protonated heterocycle Only 3-3 PT reactions 11

12 Proton conduction in P4VI BMK//B3LYP Energy barrier: 10.6 kcal/mol (Vs 20 kcal/mol) Imidazole Poly(4-vinyl-imidazole) Different mechanisms of conduction! 12

13 Proton conduction in P4VI Ab initio quantum chemistry methods Use of quantum physics QM Smallest particle : electron Calculation of the electronic structure Accuracy DFT Computational cost Molecular mechanic methods MM Use of classical physics Smallest particle : atom MD Calculation of the molecular structure (position of atoms) Accuracy Computational cost 13

14 Proton conduction in P4VI From DFT to MD Published parameters(6) Force costant of dihedral kcal/mol QM (BMK//B3LYP) GAFF Bond streching parameters kcal/mol Angular MM bending (SP ) parameters RESP procedure Dihedral twisting(hf/6-31g*) parameters F.F. Parameterization VdW parameters degrees degrees 14 A (6) Chen H.; Yan, T.; Voth, G. A. : «A computer simulation model for proton transport in liquid imidazole» J. Phys. Chem. 2009, 113,

15 Proton conduction in P4VI a b b c 3 -H3 ϕ Imb+ 40 3b H3b a c Imc ϕ( ) d( ) Ima..after a frustrated rotation! 1 0 φ t(ps) a-3b (3a-H3b) breaking of Ima-Imb H-bond 3b-3c (3c-H3b) formation of Imb-Imc H-bond 15

16 Proton conduction in P4VI τ = 5.8 ps φ = -33 τ = 6.0 ps φ = +35 rate limiting step < 6kcal/mol! 16

17 Proton conduction in P4VI 15 monomers time step = 1 fs φ VT (393 K) equilibration: 0.5 ns MD simulation: 4 ns Torsion angle Imb Time (ps) Ima φ oscillates in a range of about 80 Im c large flexibility of the imidazolium! 17

18 A new mechanism of conduction H H H step 1 charge generation charge generation n H+ H step 2 H electrode (-) H+ electrode (+) step 1 ew proposed mechanism electrode (-) electrode (+) Grotthuss mechanism H step 2 H H H H frustrated rotation chargecharge-carrier transport n H step 3 H H H H reorientation H H step 3 single proton transfer step 4 n H + H H H H new charge generation H H H step 4 new frustrated rotation n H H H H 18 (8) Mangiatordi G. F.; Laage, D.; Russo,.; Butera, V.; Adamo, C. : «Charge transport in poly-imidazole membranes: a fresh appraisal of the Grotthuss mechanism», Phys. Chem. Chem Phys, 2012, DOI: /c2cp23727j.

19 I) Conclusions 19

20 Conclusions Models including the polymeric matrix are necessary A new conductivity mechanism for P4VI (7) H bond network? Work in progress? Larger systems simulations Acid-blended systems (7) Mangiatordi G. F.; Laage, D.; Russo,.; Butera, V.; Adamo, C. : «Charge transport in poly-imidazole membranes: 20a fresh appraisal of the Grotthuss mechanism», Phys. Chem. Chem Phys, 2012, 14 (31)

21 Acknowledgements Prof. Carlo Adamo (ESCP) Dr. Damien Laage (École ormale Supérieure- Paris) Valeria Butera(University of Calabria) 21

22 22

23 From 4-vinyl to 2-vinyl systems P4VI no H-bond network along the chain P2VI H-bond network along the chain poly(2-vinyl-4,5-dicyanoimidazole) (7) Densmore C, G,; Rasmussen P. G.; Goward G. R., Macromolecules 2005, 38,