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Probing Ion Exchange in the Triflic Acid-Guanidinium Triflate System: a Solid- State Nuclear Magnetic Resonance Study

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Probing Ion Exchange in the Triflic Acid-Guanidinium Triflate System: a Solid- State Nuclear Magnetic Resonance Study Probing ion exchange in the triflic acid-guanidinium triflate system: a solid- state nuclear magnetic resonance study Citation: Zhu, Haijin, MacFarlane, Douglas and Forsyth, Maria 2014, Probing ion exchange in the triflic acid-guanidinium triflate system: a solid-state nuclear magnetic resonance study, Journal of Physical Chemistry C, vol. 118, no. 49, pp. 28520-28526. This is the accepted manuscript. ©2014, American Chemical Society This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry C, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see http://pubs.acs.org/doi/abs/10.1021/jp5101472 Downloaded from DRO: http://hdl.handle.net/10536/DRO/DU:30070195 DRO Deakin Research Online, Deakin University’s Research Repository Deakin University CRICOS Provider Code: 00113B Page 1 of 19 The Journal of Physical Chemistry 1 2 3 4 Probing Ion Exchange in the Triflic Acid - Guanidinium Triflate System: a 5 6 Solid-State NMR Study 7 8 9 Haijin Zhu,*,a Douglas MacFarlaneb and Maria Forsytha 10 11 a Institute for Frontier Materials and the ARC Centre of Excellence for Electromaterials 12 Science, Deakin University, Geelong, VIC 3216, Australia. E-mail: [email protected]; 13 Fax: 61 (03) 92446868; Tel: +61 (03) 52273696 14 15 b Department of Chemistry and the ARC Centre of Excellence for Electromaterials 16 Science, Monash University, Clayton, VIC 3800, Australia 17 18 19 * Corresponding author, Tel: 61 (03) 52273696; Fax: 61 (03) 92446868; Email: 20 [email protected] (H. Zhu) 21 22 ABSTRACT: Knowledge of ion exchange and transport behaviour in electrolyte materials is 23 24 crucial for designing and developing novel electrolytes for electrochemical device applications 25 26 such as fuel cells or batteries. In the present study, we show that, upon the addition of triflic 27 28 acid (HTf) to the guanidinium triflate (GTf) solid state matrix, several orders of magnitude 29 1 19 30 enhancement in the proton conductivity can be achieved. The static H and F solid-state NMR 31 results show that the addition of HTf has no apparent effect on the local molecular mobility of 32 33 the GTf matrix at room temperature. At higher temperatures, however, the HTf exhibits fast 34 35 ion exchange with the GTf matrix. The exchange rate, as quantified by our continuum T2 fitting 36 37 analysis, increases with increasing temperature. The activation energy for the chemical 38 exchange process was estimated to be 58.4 kJ/mol. It is anticipated that the solid-state NMR 39 40 techniques used in this study may be also applied to other organic solid state electrolyte systems 41 42 to investigate their ion exchange processes. 43 44 Keywords: organic ionic plastic crystal, T2 continuum fitting, proton conducting, 45 46 transport, activation energy. 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 2 of 19 1 2 3 4 1. Introduction 5 6 The rapidly growing demands for clean and sustainable energy sources to replace fossil energy 7 8 has necessitated the research and development on new powerful battery and fuel cell 9 technologies.1 As a vital part of fuel cells, proton-conducting electrolytes are currently 10 11 attracting significant interest from both industry and academic circles. The electrical 12 13 performance of a proton-conducting electrolyte is generally limited by both the transference 14 2 15 number and transport efficiency of the protons in the material. The transference number is 16 influenced not only by the doping concentration of the Brønsted acid, but also by the proton 17 18 dissociation and solvation. Yu et al. have recently studied the proton conductivity of 19 20 bis(trifluoromethanesulfonyl) imide (HTFSI) solutions in 1-butyl-3-methylimidazolium 21 22 bis(trifluoromethanesulfonyl) imide (BMITFSI) ionic liquid at various concentration from ~ 23 3 24 0.1 to as high as ~ 1.0 mol/L. At low HTFSI concentrations, the proton conductivity of the 25 solution was found to initially increase with increasing HTFSI, with a maximum appearing at 26 27 a concentration of approximately 0.1 mol/L at room temperature. Further increase of the acid 28 29 concentration leads to a fast drop of the conductivity. This was attributed to the higher proton 30 - + 31 association with TFSI and therefore lower solvated H concentration at high HTFSI 32 concentrations. Similar trends for the HTFSI/1-(1-Butyl-3-imidazolio)propane-3-sulfonate 33 34 (BIm3S) system and HTFSI/1-ethyl-3methylimidazolium bis-(triflluoromethanesulfonyl) 35 36 (EMImTFSI) have been observed by Yoshizawa et al. in 2004 and Jia et al. in 2009, 37 4-5 38 respectively. Besides the transference number, the proton transport behaviour is another 39 40 important and controlling factor for the conductivity of the electrolyte material. There have 41 been numerous efforts dedicated to elucidating the specific proton conduction mechanisms at 42 43 a molecular level.1-3, 5-8 So far, there are two plausible models for the proton transport 44 45 mechanism: the vehicle-type model where the protons migrate through the medium along with 46 47 a ‘vehicle’ or proton solvent such as H3O+ etc., and the Grotthuss-type model where protons 48 move via local hopping from a proton donating site to a proton accepting site through the 49 50 breaking and formation of hydrogen bonds.1, 9-11 In many cases, both mechanisms contribute 51 52 cooperatively to the overall proton conductivities. For example, in the sulfonic acid containing 53 54 polymer electrolyte membranes (PEMs), the vehicle-type mechanism is dominant in the 55 presence of water, whereas the Grotthus-type mechanism also contributes to the proton 56 57 conductivity through the forming and breaking of the hydrogen bonds among the water 58 59 molecules.9 60 2 ACS Paragon Plus Environment Page 3 of 19 The Journal of Physical Chemistry 1 2 3 4 Room-temperature ionic liquids (ILs) have been shown to be promising candidates as the 5 proton-conducting electrolyte materials due to their remarkable properties, such as high ionic 6 7 conductivity, high thermal stability, low vapour pressures, non-flammability, good solvation 8 12 9 of many organic and inorganic chemicals, and ability to be used under anhydrous conditions. 10 11 Many families of ionic liquids display rotatory phases and/or plastic crystalline phases in their 12 13-14 solid state. Rotator phases are those where one or more of the ions can rotate on its 13 14 crystallographic site, but without any significant translational motion.15 As well as resulting in 15 16 the plasticity of the materials, this local mobility is highly desirable because it is believed to 17 16 18 result in the creation of vacancies that facilitate fast proton transport in the materials. 19 20 Protic ionic liquids (PILs) represent a proton-conducting sub-class of the ionic liquids family 21 22 and have attracted much attention as next-generation proton conductors for fuel cell 23 + 24 applications. The guanidinium cation (C(NH2)3 ), for example, is formed by protonation of the 25 imine group on the basic guanidine molecule, with subsequent redistribution of the electron 26 27 density of the double bond to yield three equivalent C–N bonds and a resonance stabilization 28 29 of the whole entity as an ion with the three-fold rotational symmetry. It is relatively stable and 30 - 31 can readily form salts, and some ILs, with stable anions such as triflate (Tf, CF3SO3 ), 32 - - 6 dicyanamide (DCA, N(CN)2 ), and thiocyanate (SCN ), etc. The six dissociable protons per 33 34 cation make it an ideal candidate for a proton-conducting electrolyte in both the solid and liquid 35 36 states. 37 38 In our previous work, we have investigated the proton conductivity in the aprotic versions of 39 40 organic ionic plastic crystals (OIPCs) by adding acids of various strengths and compositions.17- 41 18 42 High proton conductivity was achieved in the plastic crystal phase of various acid-containing 43 44 OIPCs. In line with the emergence of these new proton-conducting materials, there is an 45 increasing need to fully understand the nature and mechanisms of the ionic transport and related 46 47 molecular dynamics in these materials. In our most recent work related to this study,19 we have 48 49 investigated the solid-state dynamics of a protic organic solid which we hypothesised may 50 51 display OIPC behaviour, guanidinium triflate (Figure 1), and its mixtures with triflic acid. It 52 was initially hypothesised that diffusion of the additional proton from the doped acid would 53 54 benefit from the six dissociable protons on the guanidinium cations, although we have shown 55 56 that, at room temperature, this behaviour is not evident in this material despite achieving a very 57 58 high conductivity with relatively low acid concentrations. In this study, we will show the 59 - 60 evidence of Tf anions from the doped triflic acid exchanging with the guanidinium triflate 3 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 4 of 19 1 2 3 4 matrix at higher temperatures. The rate of exchange increases with increasing temperature as 5 suggested by NMR T2 continuum fitting analysis. 6 7 8 9 10 2. Experimental 11 12 2.1. Sample Preparation. The guanidinium triflate (GTf) was synthesized by the reaction of 13 14 triflic acid (HTf) with guanidinium carbonate. The detailed sample preparation procedures 15 19 16 have been described elsewhere.
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