Charge Transport in Nonstoichiometric 2-Fluoropyridinium Triflate

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C: Energy Conversion and Storage; Energy and Charge Transport

Charge Transport in Nonstoichiometric 2Fluoropyridinium Triflate Protic Ionic Liquids Patric Jannasch, Junaiz Rehmen, Drew R. Evans, and Christoffer Karlsson J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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The Journal of Physical Chemistry

Charge Transport in Nonstoichiometric 2Fluoropyridinium Triflate Protic Ionic Liquids Patric Jannasch,a Junaiz Rehmen,b Drew Evans,b Christoffer Karlssona* a

Centre for Analysis and Synthesis, Department of Chemistry, Lund University, SE-221 00 Lund, Sweden

b

Thin Film Coating Group, Future industries Institute, University of South Australia, Adelaide 5001 SA, Australia * [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Pyridinium triflates are highly dissociated protic ionic liquids, and nonstoichiometric compositions have been used in protic energy devices such as the all-organic proton battery. Herein, we use a combination of pulsed field gradient NMR spectroscopy and electrochemical impedance spectroscopy to investigate the charge transport properties of the nonstoichiometric protic ionic liquid 2-fluoropyridinium triflate and the variation with acid doping level. While all diffusion coefficients decreased with the amount of acid doping, the room temperature conductivity increased due to the concurrent increase in charge carrier concentration. The maximum room temperature conductivity was 7.33 mS/cm, obtained when 14% of the pyridine was protonated with triflic acid, while higher acid doping levels lead to liquid/solid mixtures with low conductivity. PEDOT supercapacitor cells with this electrolyte demonstrated very high capacitance (83.9 F/g) and charge storage capacity (23.3 mAh/g). In addition, we predict that using a lower acid doping level than previously will result in superior electrolyte performance in proton batteries due to improvements in conductivity, processability and electrochemical stability.


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INTRODUCTION As opposed to stoichiometric protic ionic liquids, which are pure salts obtained by the proton transfer from a strong acid to an organic base, nonstoichiometric protic ionic liquids contain an excess of the base (Figure 1).1-2 They therefore contain both proton donors and acceptors in the form of a conjugate acid/base pair, yielding very low hysteresis between the oxidation and reduction processes of proton coupled redox reactions, minimizing the energy losses in energy storage devices.2-3 Such electrolytes also have well-controlled proton activity, determined by the pKa of the conjugate organic acid, that can be tuned by the proper choice of substituents.2, 4-5 In the so-called “all-organic proton battery”, the nonstoichiometric protic ionic liquid 2fluoropyridinium triflate (FPT) with an equimolar ratio of base/salt was used (Figure 1).3 In that device, a well-controlled proton activity is crucial since it controls the potentials of the electrodes as well as the reaction kinetics.2-3 Using a stoichiometric protic ionic liquid as electrolyte on the other hand might lead to slow kinetics of the reduction reaction due to the lack of proton acceptors. Furthermore, the benefits of nonstoichiometric protic ionic liquid electrolytes is now increasingly discussed for a broader range of protic electrochemical devices.2, 6-9 For example, it has recently been shown that stoichiometric protic ionic liquids cannot be efficiently used as fuel cell electrolytes, and that proton shuttles in the form of the conjugate organic base are needed.6 Nonstoichiometric protic ionic liquids have also yielded superior performance as electrolytes in carbon supercapacitors1,

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as well as in a range of other fields including catalysis,10-11 gel

formation,12 and pharmaceutical formulations.1, 13


3


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Figure 1. a) Structure of the nonstoichiometric protic ionic liquid FPT with a certain fraction (x) of the pyridine units protonated by triflic acid (atom numbering of the 2-fluoropyridine ring also shown), and b) picture of FPT with varying composition (x) at 22 °C.

In a seminal work by Angell et al., the importance of ΔpKa on the stability, ionicity and conductivity of protic ionic liquids is detailed.14 This is one of very few studies into nonstoichiometric pyridinium protic ionic liquids. Among the investigated ionic liquids, 2methylpyridinium triflate was highlighted as the most favorable one, primarily because of its negligible vapor pressure resulting from the very high boiling point, and that it demonstrates a nearly “ideal” Walden behavior (high degree of dissociation), even more so than some aprotic ionic liquids.14 Pyridinium triflate ionic liquids can thus exhibit high conductivities without a volatile component due to the large difference in pKa of pyridinium and triflic acid.14 Employing their model in which ΔpKa in used to predict the rise in boiling point (relative to the average of the respective boiling points for the acid and the base), the boiling point of stoichiometric FPT can be predicted to 352 °C, well above its decomposition temperature.14 Despite these promising results, there have been no reports on the charge transport mechanisms of any nonstoichiometric protic pyridinium ionic liquids to the best of our knowledge.


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In this study, we used a combination of pulsed field gradient NMR spectroscopy and electrochemical impedance spectroscopy to investigate the charge transport properties of FPT and how it varies with composition. As the electrolyte of choice for the all-organic proton battery,3 we demonstrate that significantly improved conductivity, processability and electrochemical stability can be obtained by optimising the composition. We demonstrate this point by utilizing FPT as electrolyte in supercapacitor cells, in which FPT with lower acid doping had superior performance.

MATERIALS AND METHODS Synthesis f 2-Fluoropyridinium toriflate Electrolyte. FPT samples with varying compositions were prepared by mixing appropriate amounts of 2-fluoropyridine (98%, Sigma Aldrich) and trifluoromethanesulfonic acid (“triflic acid”, 98%, Sigma Aldrich) at room temperature. The resulting electrolytes were pale yellow liquids or solid/liquid mixtures. Throughout the paper, the composition will be given as the fraction of the total amount of 2fluoropyridine that has been protonated (x, Figure 1). Thermal Analysis. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a TGA Q500 and a DSC Q2000 (TA Instruments), respectively, with a heating/cooling rate of 10 °C/min. Samples were cooled from room temperature to -70 °C and then heated to 50 °C (except for FPT with x = 100% which was heated to 150 °C). The melting point was taken as the end of the broad melting peak in the DSC thermogram. NMR Spectroscopy. All NMR experiments were performed on neat samples without locking onto a deuterium signal, and shimming on a 1H signal (so called “no D shimming” using the Bruker Topspin topshim function). The samples were characterized with 1H, 13C, and 19F NMR


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spectroscopy on a Bruker 500 MHz NMR spectrometer. A small amount of tetramethylsilane was added as a reference for 1H chemical shifts, and Ξ referencing was used for

13C

and

19F

chemical shifts.15 Reference experiments without tetramethylsilane showed that it did not influence the measured diffusion coefficients. COSY, HSQC and HMBC spectroscopy were also performed on 2-fluoropyridine in a capillary with D2O in the outer compartment in order to assign all signals.2 Pulsed field gradient 1H and

19F

NMR (gradient strength 48.2 G/cm)

experiments were performed at 25 °C, as well as at 20 and 30 °C for FPT with x = 14%. A stimulated echo sequence using bipolar gradients was used,16 and the intensity of the stimulated echo (I) as a function of gradient strength (g) was fitted to the equation: 𝐼 = 𝐼0ⅇ

(

𝛿

)

―𝐷𝛾2𝑔2𝛿2 𝛥 ― 3

(1),

where I0 is the reference intensity, D is the diffusion coefficient, γ is the gyromagnetic ratio, and Δ and δ are parameters of the pulse program (diffusion time and twice the gradient pulse, respectively), using the Bruker Topspin t1/t2 relaxation module. The standard deviation for the fits was below 1% of I0 in all cases. The apparent diffusion coefficient obtained is this way for each NMR signal was the same for all CH protons (DCH) as well as the pyridine fluorine signal (DFPyr-F). This diffusion coefficient corresponds to the composition weighted average of the vehicle diffusion of the neutral pyridine (DFPyr) and the protonated pyridinium cation (DFPyrH+): 𝐷𝐶𝐻 = 𝐷𝐹𝑃𝑦𝑟 ― 𝐹 = 𝑥𝐷𝐹𝑃𝑦𝑟𝐻 + + (1 ― 𝑥)𝐷𝐹𝑃𝑦𝑟

(2).

The diffusion coefficient of the NH signal (DNH) corresponds to the diffusion of mobile protons both through vehicle transport and proton hopping (Dh): (3).

𝐷𝑁𝐻 = 𝐷𝐹𝑃𝑦𝑟𝐻 + + 𝑥𝐷ℎ


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However, proton hopping is apparently negligible in FPT since the NH signals feature slower diffusion than the vehicle transport (DCH, see below). The diffusion of the neutral pyridine can thus be calculated as: 𝐷𝐹𝑃𝑦𝑟 =

𝐷𝐶𝐻 ― 𝑥𝐷𝑁𝐻

(4).

1―𝑥

The triflate anion diffusion (DOTf-) can be obtained directly from its fluorine signal (DOTf-F): (5).

𝐷𝑂𝑇𝑓 ― = 𝐷𝑂𝑇𝑓 ― 𝐹

The contribution of each ion’s diffusion (Di) to the molar conductivity (Λi) can be obtained by the Nernst-Einstein equation: 𝛬𝑖 =

𝐹2𝑧2𝑖 𝐷𝑖

(6),

𝑅𝑇

where F is the Faraday constant, zi is the valence of the ion, R is the gas constant, and T is the temperature. The conductivity as predicted by the diffusion observed by NMR spectroscopy (σNMR) can then be calculated as: 𝜎𝑁𝑀𝑅 = ∑𝑖𝛬𝑖 ⋅ 𝑐𝑖

(7),

where ci is the molar concentration of each ion. Electrochemical impedance spectroscopy Electrochemical impedance spectroscopy (EIS) was performed on a Novocontrol high resolution dielectric analyzer V1.01S using a two-probe setup in the frequency range 10-1 – 107 Hz with an amplitude of 50 mV. Two brass disc electrodes sandwiched the samples with a Teflon ring spacer (250 µm thickness, 12 mm inner diameter). Impedance was measured while cooling the sample from 30 to -30 °C. The ionic conductivity (σ) was taken as the extrapolation to 100 Hz of the plateau of the absolute conductivity at intermediate frequencies. The relative standard deviation of lg σ was below 0.2% in all cases. These conductivity values, as well as those predicted by NMR spectroscopy, were fitted to the Arrhenius equation:


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𝐸𝑎

𝜎 = 𝜎∞ⅇ

― 𝑅𝑇

(8).

Supercapacitor Cells. Poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes were made by vapor phase polymerization according to a previously published procedure, without any triblock copolymer additive.17-18 The electrodes consisted of carbon paper coated with PEDOT (20 µg/cm2), with neglible contribution to the observed capacitance from the carbon paper substrate.17 Squares of roughly 6 mm × 6 mm were cut out and soaked in the FPT electrolyte, as was a filter paper separator (200 µm thickness). Supercapacitor cells were assembled by sandwiching the separator between two PEDOT electrodes, which were contacted with Pt foil current collectors. The supercapacitor cells were cycled at different constant currents up to either 1.0 V (x = 14%) or 0.8 V (x = 50%) cell potential. Specific current, charge (capacity), capacitance and energy was calculated based on the total mass of PEDOT on both electrodes in the cell. For reference, the specific electrode current is thus double that stated for the cell, and the specific electrode charge a factor four times the stated cell value.17

RESULTS AND DISCUSSION Stoichiometric FPT (x = 100%) is a protic ionic liquid with a thermal decomposition temperature of Td,95% = 163 °C (Figure S1) and a melting point just above room temperature (26 °C, Figure S2). When 2-fluoropyridine was added to FPT in equimolar amounts (producing FPT with x = 50%) it became noticeably more crystalline, reflected in its higher melting point (39 °C, Figure 1 and S2), while the melting point was again lower at x < 50%. This behavior is contrary to many other nonstoichiometric protic ionic liquid systems that feature a lowered melting point of intermediate compositions compared to the pure base or salt.2, pyridine/pyridinium

triflate

mixtures

form

stable

14, 19-20

monoprotonated

It is known that

bispyridine

cation


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complexes,21 which seem to crystallize easily in the case of FPT, yielding the higher observed melting point. Thus, nonstoichiometric compositions were only completely liquid at rather low levels of acid doping, namely x ≤ 14% at room temperature (22 °C, see Figure 1), and compositions with higher x contained a solid component. Due to their liquid state at room temperature, only compositions of x ≤ 14% were examined by NMR spectroscopy in the neat state (Figure S3 – S8). The 1H chemical shift of the CH protons increased linearly with x (Figure S3 and S4), reflecting the decreased electron density on the aromatic ring as the pyridine was increasingly protonated, similarly to reported for imidazolium and 1,2,4-triazolium bistriflimide nonstoichiometric protic ionic liquids.2, 19 The NH signal on the other hand had large variations that did not correlate with x, possibly due to the high sensitivity to trace amounts of water which makes it difficult to make any assertions based on this chemical shift. As expected, both the 13C and 19F chemical shifts of triflate, as well as the pyridine carbons ortho to the nitrogen (C2, C6, see atom numbering in Figure 1a), were relatively constant with x. The 13C chemical shifts of the carbons in meta and para positions (C3, C4, C5) were shifted downfield (increasing δC) by protonation, analogous to the 1H chemical shifts (Figure S5 and S6). Conversely, the

19F

chemical shift of the pyridine fluorine signal decreased with x (Figure S7 and S8), since the protonated 2-fluoropyridinium ion has a lower

19F

chemical shift than the neutral species.22

Although the origins of 19F chemical shifts are still not completely understood, this “abnormal” behavior has also been observed and investigated for some fluoroimidazole systems.23 It was concluded that when fluorine is adjacent to an unprotonated nitrogen in the aromatic heterocycle, all fluorine valence electrons are delocalized, leaving it without any lone pairs and thus deshielded.23 Protonation of the nitrogen, however, changes the electronic structure so that the fluorine lone pairs localize and shields the fluorine nucleus, shifting the signal upfield


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(decreasing δF).23 The linear decrease of δF vs x observed here supports that this is also true for FPT in the absence of solvent. Diffusion coefficients for the neutral pyridine, the pyridinium cation and the triflate anion are shown in Figure 2 as functions of x at 25 °C (see Figure S9 for different temperatures). The fact that the pyridinium cation diffused slower than the neutral species is again an indication that the cations exist as bispyridine complexes, since the pyridinium cation by itself would otherwise be expected to have a diffusion coefficient very similar to neutral pyridine, due to the similar size and shape.2 All diffusion coefficients decreased with x, likely due to the increase in viscosity. Nevertheless, the room temperature conductivity increased with x in this range, as can be seen in Figure 3 (open symbols show conductivities predicted by NMR spectroscopy, while closed symbols are conductivities measured by EIS, both of which increased with x at 25 °C, 0% ≤ x ≤ 14%). The decrease in diffusion rate was hence more than compensated for by the increase in charge carrier concentration.

Figure 2. Diffusion coefficients at 25 °C of neutral 2-fluoropyridine (red squares), the 2fluoropyridinium cation (purple circles) and the triflate anion (yellow triangles), for FPT with varying composition (x), including linear fits (r2 = 0.99, 0.94 and 0.98, respectively). The right yaxis shows the corresponding contributions of the ions to the molar conductivity.


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Figure 3. Conductivity of FPT at varying temperature as measured by EIS (solid symbols) and predicted by NMR spectroscopy (open symbols). Inset shows the conductivity at room temperature as a function of the composition (x). The ionic conductivity of FPT (Figure 3) followed Arrhenius behavior in the liquid state, and decreased rapidly below the melting point (cf. DSC in Figure S2). The conductivity of 2fluoropyridine (x = 0%, Figure S10) was below 10-10 S/cm at all temperatures since it lacks any charge carriers. The activation energy for the conduction process, obtained by fitting the linear region of the conductivity to Equation 8, was in the range 60 – 120 meV (Figure S11). This is similar to other triflate nonstoichiometric protic ionic liquid systems.2, 20 As can be seen in the inset of Figure 3, the room temperature conductivity at very low acid doping (x = 1%) was limited by the low number of charge carriers. Only moderate doping levels were needed, however, to achieve increased conductivity, and was fairly constant for 5% ≤ x ≤ 14%. At higher acid doping levels, the conductivity decreased again due to the solid component at x > 14%. The maximum conductivity of FPT at 25 °C was 7.33 mS/cm, obtained for x = 14%. For comparison, the conductivity for FPT with x = 50%, which was used in the recently presented all-organic


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proton battery, was only 1.08 mS/cm.3 At lower temperatures the conductivity for that composition decreased very quickly and was almost two orders of magnitude lower than for x = 5% at 0 °C. The conductivity predicted by NMR spectroscopy was almost an order of magnitude larger than the actual EIS conductivity (Figure 3), indicating a high degree of ion pairing or larger ion aggregates that cause correlated motion of cations and anions.2,

24

For x = 14% at 25 °C, the

ionicity (as calculated by σEIS/σNMR) was 0.18, meaning that only a small fraction of all ions are free to migrate under an imposed electric field.24-25 Vehicle diffusion can therefore be concluded to be the main mode of charge transport, rather than structure diffusion (Grotthuss mechanism) or proton hopping (see above). The activation energies for the diffusion was higher than that of the conductivity (180 – 270 meV, Figure S11), as observed previously, and similar to those of the nonstoichiometric protic ionic liquid 1-methyl-1,2,4-triazolium bistriflimide.2 This has been attributed to its low viscosity, which is also exhibited by FPT.2 PEDOT supercapacitor cells with FPT electrolytes were subjected to constant current charge and discharge cycles with different specific currents (Figure 4). PEDOT manufactured by vapor phase polymerization has recently been showed to exhibit very high specific charge due to its ability to achieve high doping levels before the onset of oxidative degradation reactions, so called overoxidation.17 The cells were herein charged to either 1.0 or 0.8 V cell potential, depending on the electrolyte, in order to avoid overoxidation on the positive electrode. Cells with x = 50% exhibited overoxidation at a rather low potential, leading to the limited potential window of 0.8 V being used, while cells with x = 14% could be safely charged up to a cell potential of 1.0 V. Both specific charge and energy decreased with the current, mainly due to the electrolyte resistance. For x = 14%, a specific charge of 23.3 mAh/g was obtained for the slowest


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current used (5 A/g). This is similar to what was obtained for similar cells with an aprotic ionic liquid electrolyte, and the specific capacitance was even higher than previously reported (83.9 F/g).17 Cells with x = 50% performed worse (lower specific charge and energy) due to the larger electrolyte resistance and limited cell potential. Although the electrolyte proton activity is not crucial for PEDOT supercapacitors, they can be considered as model systems for conducting redox polymer based proton batteries.3, 17, 26 The electrodes in these proton batteries are based on PEDOT (or sometimes another conducting polymer backbone) functionalized with redox active moieties such as benzoquinone.3, 26 The simpler redox processes of the PEDOT electrodes used here (only doping/dedoping) allows for a clear evaluation of the impact of the electrolyte properties on device performance. Since the previously presented PEDOT based proton batteries have used an “equimolar slurry” (i.e. x = 50%) of FPT,3 we predict that using a lower acid doping level (x = 14% or below) will yield superior performance in such devices due to the higher conductivity, better processability (being liquid rather than “wet salt”), higher electrochemical stability, and superior performance when employed in PEDOT supercapacitor cells.


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Figure 4. a) Constant current charge/discharge curves for PEDOT supercapacitor cells with FPT electrolyte (black: x = 14%, 5 A/g; red: x = 50%, 10 A/g) b) Specific charge (corresponding to the x-axis in a) and energy (corresponding to the shaded area under the curves in a) for the 5th discharge of the cells at varying specific current.

CONCLUSIONS The diffusion coefficients of both ions and the neutral species in nonstoichiometric FPT decreased with the amount of acid doping, x, due to the increase in viscosity. The lower diffusion coefficients were however more than compensated for by the increase in charge carrier concentration, leading to an increase in room temperature conductivity with x up to 14% acid doping. The ionic conductivity for all compositions followed Arrhenius behavior above the respective melting points, and decreased quicker below. The activation energy for the conductivity was 60 – 120 meV while that of the diffusion coefficients was 180 – 270 meV. The maximum conductivity at 25 °C was 7.33 mS/cm, obtained for x = 14%. At very low acid doping (x = 1%), the conductivity was limited by the low number of charge carriers, but at slightly higher doping levels (x = 5%) the conductivity was almost as high as at x = 14%. At higher acid doping levels, however, the conductivity decreased due to the solid component, and at x = 50% the conductivity was only 1.08 mS/cm. The conductivity predicted by NMR spectroscopy was almost an order of magnitude larger than the actual conductivity, indicating a high degree of ion pairing, and at x = 14%, the ionicity was only 0.18 at 25 °C. The main mode of charge transport is thus vehicle diffusion, and we observe no structure diffusion or proton hopping. PEDOT supercapacitor cells with FPT electrolyte (x = 14%) had a specific capacitance of 83.9 F/g, a specific charge of 23.3 mAh/g and a specific energy of 7.2 mWh/g. Cells with x = 50%


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performed worse due to the larger electrolyte resistance and limited cell potential imposed by the lower electrochemical stability. Considering the PEDOT supercapacitors as model systems for conducting redox polymer based proton batteries (in which only x = 50% has been used so far), we predict that using a lower acid doping level (x = 14% or lower) will yield superior performance in such devices due to the higher conductivity, better processability, higher electrochemical stability.

ASSOCIATED CONTENT Supporting Information. Supplementary TGA, DSC, NMR and EIS characterization of FPT, as well as activation energies. AUTHOR INFORMATION Corresponding Author * [email protected] Acknowledgements This work was funded by the Swedish Energy Agency (Grant No. 42894-1). DE acknowledges the support of the Australian Research Council (FT160100300). JR acknowledges the support of the Commonwealth Government of Australia through the Research Training Program.


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17. Karlsson, C.; Nicholas, J.; Evans, D.; Forsyth, M.; Strømme, M.; Sjödin, M.; Howlett, P. C.;

Pozo-Gonzalo,

C.

Stable

Deep

Doping

of

Vapor-Phase

Polymerized

Poly(3,4‐ethylenedioxythiophene)/Ionic Liquid Supercapacitors. ChemSusChem 2016, 9 (16), 2112-2121. 18. Mueller, M.; Fabretto, M.; Evans, D.; Hojati-Talemi, P.; Gruber, C.; Murphy, P. Vacuum vapour phase polymerization of high conductivity PEDOT: Role of PEG-PPG-PEG, the origin of water, and choice of oxidant. Polymer 2012, 53 (11), 2146-2151. 19. Noda, A.; Susan, M. A. B. H.; Kudo, K.; Mitsushima, S.; Hayamizu, K.; Watanabe, M. Brønsted Acid−Base Ionic Liquids as Proton-Conducting Nonaqueous Electrolytes. J. Phys. Chem. B 2003, 107 (17), 4024-4033. 20. Luo, J.; Hu, J.; Saak, W.; Beckhaus, R.; Wittstock, G.; Vankelecom, I. F. J.; Agert, C.; Conrad, O. Protic ionic liquid and ionic melts prepared from methanesulfonic acid and 1H-1,2,4triazole as high temperature PEMFC electrolytes. J. Mater. Chem. 2011, 21 (28), 10426-10436. 21. Carlsson, A.-C. C.; Uhrbom, M.; Karim, A.; Brath, U.; Gräfenstein, J.; Erdélyi, M. Solvent effects on halogen bond symmetry. CrystEngComm 2013, 15 (16), 3087-3092. 22. Giam, C. S.; Lyle, J. L. Medium effects on the fluorine-19 magnetic resonance spectra of fluoropyridines. J. Am. Chem. Soc. 1973, 95 (10), 3235-3239. 23. Kasireddy, C.; Bann, J. G.; Mitchell-Koch, K. R. Demystifying fluorine chemical shifts: electronic structure calculations address origins of seemingly anomalous 19F-NMR spectra of fluorohistidine isomers and analogues. Phys. Chem. Chem. Phys. 2015, 17 (45), 30606-30612. 24. Greaves, T. L.; Drummond, C. J. Protic Ionic Liquids: Properties and Applications. Chem. Rev. 2008, 108 (1), 206-237.


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25. Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. How Ionic Are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties. J. Phys. Chem. B 2006, 110 (39), 19593-19600. 26. Karlsson, C.; Huang, H.; Strømme, M.; Gogoll, A.; Sjödin, M. Quinone pendant group kinetics in poly(pyrrol-3-ylhydroquinone). J. Electroanal. Chem. 2014, 735, 95-98.


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TOC graphic 60x44mm (300 x 300 DPI)



Figure 1. a) Structure of the nonstoichiometric protic ionic liquid FPT with a certain fraction (x) of the pyridine units protonated by triflic acid (atom numbering of the 2-fluoropyridine ring also shown), and b) picture of FPT with varying composition (x) at 22 °C.


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Figure 2. Diffusion coefficients at 25 °C of neutral 2-fluoropyridine (red squares), the 2-fluoropyridinium cation (purple circles) and the triflate anion (yellow triangles), for FPT with varying composition (x), including linear fits (r2 = 0.99, 0.94 and 0.98, respectively). The right y-axis shows the corresponding contributions of the ions to the molar conductivity.



Figure 3. Conductivity of FPT at varying temperature as measured by EIS (solid symbols) and predicted by NMR spectroscopy (open symbols). Inset shows the conductivity at room temperature as a function of the composition (x).


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Figure 4. a) Constant current charge/discharge curves for PEDOT supercapacitor cells with FPT electrolyte (black: x = 14%, 5 A/g; red: x = 50%, 10 A/g) b) Specific charge (corresponding to the x-axis in a) and energy (corresponding to the shaded area under the curves in a) for the 5th discharge of the cells at varying specific current.


Charge Transport in Nonstoichiometric 2-Fluoropyridinium Triflate

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