Article pubs.acs.org/JPCB
Mass and Charge Transport in the Polymer−Ionic-Liquid System PEO−EMImI: From Ionic-Liquid-in-Polymer to Polymer-in-IonicLiquid Electrolytes Johannes Kösters,† Monika Schönhoff,‡ and Nicolaas A. Stolwijk*,† †
Institut für Materialphysik, University of Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany Institut für Physikalische Chemie, University of Münster, Corrensstrasse 28/30, 48149 Münster, Germany
‡
ABSTRACT: Conventional polymer electrolytes based on inorganic salts are commonly characterized and utilized over a small salt-poor composition range because of phase transitions accompanied by loss of ion conductivity at high salt concentrations. By contrast, well-chosen polymer−ionic-liquid (IL) systems offer the possibility to vary the IL content from the IL-in-polymer to the polymer-in-IL domain. We have investigated the temperature-dependent ionic conductivity in PEOyEMImI systems consisting of poly(ethylene oxide) complexed with 1-ethyl-3-methylimidazolium iodide for y = EO/ IL ratios ranging from 0.6 to 60 and compared diffusivity data with that arising from 1H pulsed-field-gradient nuclear magnetic resonance for EMIm and 125I radiotracer diffusion for iodine. Surprisingly, the diffusivity of cations and anions vary at most by 50% at fixed temperatures over the entire composition range. The much larger changes in the charge diffusivity Dσ relate to ion pairing exhibiting a minimum near the intermediate composition y = 10. Altogether, the results are relevant to application in dye-sensitized solar cells and show that a high ion density is crucial to enhance the iodine transport capacity.
1. INTRODUCTION In principle, salt-in-polymer electrolytes offer the possibility to reconcile a sufficient ion conductivity with beneficial mechanical properties for applications such as batteries, smart windows, and electrochemical solar cells.1 However, classical polymer electrolytes based on inorganic salts are limited to low salt concentrations because the concomitant increases in glass transition temperature Tg and/or the formation of crystalline phases lead to reduced mobilities,2,3 which overcompensates the seeming advantage of a higher ion density. In recent years, the availability of room-temperature ionic liquids (ILs) make it feasible to avoid these problems and thus to prepare fully miscible polymer−IL complexes covering the entire range from IL-in-polymer to polymer-in-IL electrolytes.4 So far, studies on ion transport in polymer−salt or polymer− IL systems that explore wide composition ranges have been sparse. Moreover, most of these studies focus on the ion conductivity and its correlation with Tg.5−9 The experimental data are often globally interpreted in terms of Angell’s decoupling index, referring to the ratio of characteristic times for ionic transport to mechanical processes (viscous flow).10,11 However, information on ion association and the individual mobility of cations and anions is largely missing. The present work deals with PEOyEMImI complexes of poly(ethylene oxide) (PEO) and 1-ethyl-3-methylimidazolium iodide (EMImI) with EO/EMIm ratios y ranging from 0.6 to 60. Apart from the ion conductivity, we also determine the diffusion coefficient of both cations and anions as a function of temperature in seven different compositions. The comparison © 2015 American Chemical Society
of mass and charge transport enables us to monitor compositional trends in the ion-pairing tendency and to discriminate between the contributions of positive and negative charge carriers to the conductivity. The results may be important to the optimization of electrolytes for dye-sensitized solar cells (DSSCs) based on the monoiodide/triiodide redox couple because in this case the cell efficiency crucially depends on electron transfer carried by unpaired anions.12−14
2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. The base materials were PEO, with a molecular weight of 8 × 106 g/mol and a purity of 98.4 wt %15 (Aldrich), and EMImI (238.07 g/mol, > 99 wt %, Iolitec). All substances were vacuum-dried before use. For the preparation of the polymer electrolytes, the same methods were used as described in earlier publications by our group.3,16−18 Great care was taken to remove traces of water and solvent (extra dry acetonitrile, Acros Organics) and to avoid contact with ambient atmosphere. In (PFG-)NMR analysis, no 1H resonance attributable to water could be detected. Considering that all preparation steps were carried out either in a nitrogenfilled glovebox (H2O = 1 ppm) or under continuous pumping in the HV range, the water contamination of the PEO−EMImI electrolytes is assumed to be at a negligibly low level. Received: February 3, 2015 Revised: April 7, 2015 Published: April 7, 2015 5693
DOI: 10.1021/acs.jpcb.5b01113 J. Phys. Chem. B 2015, 119, 5693−5700
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The Journal of Physical Chemistry B 2.2. Electrolyte Characterization. The mass density of the electrolytes was determined by hydrostatic weighing in dodecane and air, taking single-crystal silicon as a reference material. The density enters the calculation of the salt concentration per unit volume. Phase behavior was examined by differential scanning calorimetry (DSC) at different heating and cooling rates using PerkinElmer DSC-7 and Diamond-DSC devices.3 Tg values determined after ex situ quenching in a dewar with liquid nitrogen3 (first heating cycle) and in situ quenching in the calorimeter (second heating cycle) were mutually consistent. 2.3. Impedance Spectroscopy. The ionic conductivity was measured by impedance spectroscopy (EIS) in the frequency range of 5 Hz−13 MHz using an HP Agilent 4192A LF impedance analyzer.19 The reliability of the data was ensured by using at least two different samples for each composition and by running multiple heating and cooling cycles for each sample. Because ionic conduction mainly takes place in the amorphous phase of the electrolytes, the data covered the widest possible temperature range of this phase. The consistency of the results over measurement periods of several days for each sample points to a sufficient stability of the electrolytes in the T range of interest. 2.4. Pulsed-Field-Gradient Nuclear Magnetic Resonance. Diffusion coefficients of EMIm were measured on a 400 MHz FT-NMR spectrometer (Bruker, Avance) employing a probe head (Bruker, DIFF 30) with a RF-selective insert for protons and providing a gradient strength of up to 12 T/m. The stimulated echo was employed with a variation of the gradient strength g. The intensity I of the echo signal decreases with increasing strength of the magnetic field gradient g, i.e., I = I0 exp(−γ2g2δ2Dcat * (Δ − (δ/3))).20 Here, I0 denotes the zerogradient intensity, D*cat is the diffusion coefficient of the cation, γ is the gyromagnetic ratio, δ is the duration of the gradient pulse, and Δ is the observation time for the mean-square displacement. Of the six different EMIm-related 1H resonances, there are two resonances that interfere mutually and two others that overlap with the strong, hardly attenuated PEO signal. Thus, each of the three distinguishable peaks yields a value for Dcat * = DEMIm * at any selected temperature. The data presented below are the corresponding mean values and are further characterized by a standard deviation of about 10%. 2.5. Radiotracer Diffusion. The iodine diffusivity was measured by the 125I radiotracer technique, which is described in detail elsewhere.21 After placement of a radioactive polymer−electrolyte source film on top of a sample and subsequent isothermal annealing in an oil bath, the sample was sectioned with a rotary microtome at a suitable freezing temperature. Tracer depth profiles were obtained by counting the 125I radioactivity in each section. D*I = D*an was determined by fitting the measured penetration profiles with the appropriate solution of Fick’s second law.21 It should be emphasized that 125I concentrations are always very low fractions of the total iodine content, so that its diffusion does not involve compositional change. In contrast to PFG-NMR, the radiotracer diffusion (RTD) technique is destructive so that each measurement requires a new sample.
complexes are compiled in Table 1 including the number density Cs of salt molecules and the corresponding IL weight Table 1. Salt Weight Fraction (ws), Salt Concentration (Cs, Number Density), Mass Density (ρ), and (Onset) Melting a Temperatures (Tonset m ) of the PEOyEMImI Complexes complex PEO60EMImI PEO30EMImI PEO20EMImI PEO10EMImI PEO5.5EMImI PEO2.3EMImI PEO0.6EMImI a
ws
Cs (cm−3)
ρ (g cm−3)
Tonset (°C) m
0.0826 0.153 0.213 0.351 0.496 0.701 0.900
× × × × × × ×
1.26 1.28 1.30 1.31 1.46 1.92 1.94
58.3 56.7 56.6 54.7 53.0 49.9 44.8
2.63 4.95 6.97 1.16 1.84 3.39 4.41
20
10 1020 1020 1021 1021 1021 1021
Experimental error is about ±0.02 g cm−3 in ρ and ±2 °C in Tonset m
fraction ws ranging from 0.90 to 0.083. Mass densities ρ and (onset) melting temperatures Tonset m , also listed in Table 1, show the expected monotonic behavior as a function of the IL concentration. By contrast, the glass transition temperature Tg depicted in Figure 1 exhibits a pronounced maximum near the intermediate composition ws = 0.3 (y ≈ 10−20).
Figure 1. Glass transition temperature Tg of PEOyEMImI complexes as a function of salt concentration ws in weight percent. Solid line serves to guide the eye.
Figure 1 also includes the Tg value (−57 °C) of the employed high-molecular-weight pristine PEO taken from earlier work.3 The data show a clear compositional trend, as indicated by the solid curve intersecting all data points. Nevertheless, the Tg variation of the investigated electrolytes is restricted to a narrow range between −53.5 °C (y = 0.6) and −45.7 °C (y = 20), making up a maximum shift of only 8 °C. The extrapolation of the composition dependence in Figure 1 to ws = 1.0 suggests a Tg value of −55 °C for the pure EMImI IL. This may indicate that the reported estimate of −39 °C, which is based on the EMImI melting temperature, is only very approximate.22 However, the present extrapolation perfectly agrees with Tg = −55 °C detected for a very similar IL (1-ethyl3-ethylimidazolium iodide).12 We further note that DSC did not reveal features other than glass transition and melting, except for a weak and broad endothermic event near 123−135 °C (temperature at local maximum) for the three most salt-rich complexes. This reproducible event in multiple heating and cooling cycles may be tentatively explained by disordering or ion association phenomena in high-density ionic systems.
3. RESULTS AND ANALYSIS 3.1. General Characteristics and Glass Transition. We prepared seven PEOyEMImI complexes of different composition as indicated by their chosen EO/PMImI ratios y = 0.6, 2.3, 5.5, 10, 20, 30, and 60. General characteristics of these 5694
DOI: 10.1021/acs.jpcb.5b01113 J. Phys. Chem. B 2015, 119, 5693−5700
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The Journal of Physical Chemistry B 3.2. Ionic Conductivity and Charge Diffusivity. The direct current ionic conductivity σ of the PEOyEMImI electrolytes was determined over wide temperature ranges by EIS. Figure 2 displays representative data in the fully
unambiguous definition, Dσ is closely related to the molar conductivity given by Λ = σ/cs, where cs is the same salt concentration but in molar units. Thus, Dσ represents unbiased experimental data that can be compared with the individual diffusivities of cations and anions. 3.3. Ion-Specific Self-Diffusion Coefficients. The diffusion coefficient Dcat * of the EMIm cation was measured by PFG-NMR using the stimulated echo signal of the 1H nuclei.24 However, for the iodine anion no suitable PFG-NMR isotope exists. In this case, the 125I radiotracer technique based on the measurement of activity-depth profiles after diffusion annealing provides a powerful alternative to determine Dan * .21,24 For PEO0.6EMImI and PEO30EMImI, both types of diffusivity data are plotted in Figure 3 together with Dσ. The intermediate
Figure 2. Direct current conductivity of PEOyEMImI electrolytes with different EO/EMIm ratios y as a function of inverse temperature. Dashed line reproduces data for pure EMImI (y = 0) reported by Every et al.22
amorphous phase, which extends from ∼55 to ∼105 °C for the four salt-poor complexes (y ≥ 10) and from ∼45 to ∼145 °C for the three salt-rich complexes (y ≤ 5.5). At temperatures above ∼145 °C, the high fluidity of the salt-rich electrolytes caused leakage of the measuring cell. By contrast, the salt-poor electrolytes showed a (reversible) decrease of the conductivity upon heating above a critical temperature of ∼105 °C. This inversion of the σ-versus-1/T behavior is indicative of salt precipitation,23 as was confirmed by the appearance of a liquid film on the surface of a test sample during heating on a hot plate. At low temperatures, the σ data depicted in Figure 2 extend 5−12 °C below the onset crystallization temperature as measured by DSC (cf. Table 1), which is due to a composition-dependent supercooling effect. Upon further cooling, the onset of crystallization is characterized by an abrupt decrease of σ (not shown in Figure 2). It is seen in Figure 2 that the conductivity increases over about a factor of 50 from salt-poor to salt-rich electrolytes, roughly reflecting the difference in IL content. Moreover, the σ plots of different compositions run almost parallel, which may reflect some basic similarities in the underlying ion conduction processes. The present data also compare well to the conductivity of pure EMImI (y = 0) from Every et al.,22 as demonstrated by the dashed line in Figure 2. The ion conductivity may be converted into a quantity with the dimensions of a diffusion coefficient yielding the so-called charge diffusivity Dσ. This can be done with the aid of the Nernst−Einstein equation, i.e.,
Dσ =
kBT Cse 2
Figure 3. Measured diffusion coefficients of cations, anions, and charge (symbols) with their fits (solid lines) in (a) salt-rich and (b) salt-poor PEO−EMImI electrolyte as a function of inverse temperature. Upper solid line represents the sum of the cation and anion diffusivity.
composition PEO5.5EMImI is similarly displayed in Figure 4, whereas the PEO 60 EMImI data have been published previously.25 The low-temperature bound of the diffusivity measurements is 70 °C, which lies well above the melting temperature in all cases (cf. Table 1). The high-temperature bound depends on the composition-dependent onset of IL precipitation but is at least 110 °C. We note that the radiotracer technique demands macroscopic samples of sufficient mechanical stability21 so that low-viscosity complexes with y < 0.6 including the pure IL could not be analyzed. In six out of seven complexes, D*cat is larger than D*an over the temperature range investigated, which seems to be a general feature of both ILs26,27 and polymer−IL systems.28 Only for PEO20EMImI does D*an slightly exceed D*cat but not by more than 5%, which is within experimental error. For all compositions and temperatures, Dcat * and Dan * differ by less than 30%. These findings manifest themselves in the anion transport number given by
σ (1)
It should be emphasized that the denominator contains the known total salt or IL concentration Cs instead of the a priori unknown concentration of free (dissociated) ions. With this 5695
DOI: 10.1021/acs.jpcb.5b01113 J. Phys. Chem. B 2015, 119, 5693−5700
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value of 0.47 is a remarkable feature of the PEO−EMIm system. Not only the relative but also the absolute mass diffusion coefficients show little variation with IL content. Both Dcat * and D*an change by at most a factor of 1.5 when going at any fixed temperature from salt-poor (y = 30 or 60) to salt-rich (y = 0.6) compositions.25 Published Dcat * data of pure EMImI (y = 0) fit into this picture as they exceed the PEO0.6EMImI data by not more than 70%.22 However, the present shifts in Dσ amount to up to a factor of 4, as may be recognized from Figure 3. In particular, Figure 3a shows that Dσ runs between Dcat * and Dan * for PEO0.6EMImI, whereas in Figure 3b, Dσ of PEO30EMImI clearly falls below the two self-diffusivities. This trend is confirmed by inspection of the PEO60EMImI data published elsewhere.25 Small values of Dσ with regard to the individual ionic diffusivities may point to a high degree of ion association. This feature will be quantitatively analyzed in the following paragraphs. 3.4. Ion Association, Pair Components, and Transference Number. Ion association is commonly quantified by the Nernst−Einstein deviation parameter ΔNE, represented as25,29,30 ΔNE = 1 − Figure 4. (a) Measured diffusion coefficients of cations, anions, and charge (symbols) as a function of inverse temperature in PEO5.5EMImI. Solid lines represent the contribution of free cations and anions, as indicated by the different Deff annotations. Dashed line represents the contribution of cation−anion pairs. (b) Additional parameters derived from the measured diffusion coefficients comprising the anion transference number t−, the deviation parameter an ΔNE, and the fractional pair components f cat pair and f pair.
*= tan
(3)
where HR = (D*cat + D*an) /Dσ is the Haven ratio. Thus, a high fraction of associated ions occurs if Dσ is much smaller than the sum of the cation and anion diffusivity. The quantity Dcat * + Dan * is represented in Figure 3 by the upper solid line in either panel, and it was obtained by using best-fit curves (solid lines) through the diffusivity data. It is obvious from Figure 3 that ion association is much stronger in PEO30EMImI than in PEO0.6EMImI. Figure 4b shows the temperature dependence of ΔNE for PEO5.5EMImI. It is seen that ΔNE varies from 0.73 to 0.57 upon passing from the highest depicted temperature (144 °C) to the lowest (60 °C). In Figure 5, ΔNE is plotted as a function of the salt concentration for a fixed temperature of 90 °C. As a general trend, ΔNE decreases with increasing IL content, i.e., from 0.79 for PEO60EMImI to 0.47 for PEO0.6EMImI. However, a local minimum with ΔNE ≈ 0.57 is observed near the intermediate composition ws = 0.3 or y = 10, which is followed with increasing EMImI content by a local maximum appearing near ws = 0.5 or y = 20. Altogether, the resulting ΔNE values reveal that ion association in the PEO−EMImI system occurs to substantial extents. Apart from ΔNE, other quantities can be deduced from the experimental data, if the formation of neutral pairs may be assumed to be largely predominant in ion association. In this case, the data analysis can be based on the occurrence of only three ionic species, cat+, an−, and the cat+an− pair, which interact with each other according to the reversible reaction 31
* Dan * + Dan * Dcat
Dσ 1 =1− * * HR Dcat + Dan
(2)
Here, we prefer t*an to the related quantity t*cat = 1 − t*an because of the importance of the iodide anions for DSSCs. Figure 5 displays t*an (open triangles) as a function of composition for the intermediate temperature of 90 °C. The almost constant t*an
cat+ + an− ⇌ pair
(4)
Specifically, we have to solve the system of three coupled equations25 Figure 5. Deviation parameter ΔNE (solid triangles), fractional pair component of iodine diffusivity f an pair (open circles), anion transport * (open triangles), and anion transference number t− number tan (squares) of PEO y EMImI complexes as a function of salt concentration ws in weight percent for a fixed temperature of 90 °C. Solid lines serve to guide the eye. Dashed line is a fit to the t− data (solid squares), ignoring one outlier (open square). 5696
eff eff * = Dcat + + D Dcat pair
(5)
eff eff * = Dan − + D Dan pair
(6)
eff eff + + D − Dσ = Dcat an
(7) DOI: 10.1021/acs.jpcb.5b01113 J. Phys. Chem. B 2015, 119, 5693−5700
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deviation parameter ΔNE is between the two fractional pair components. Indeed, it can be shown that ΔNE is the weighted cat 25 mean of f an Although the pair components appear pair and f pair. to increase with increasing temperature, the transference number t− is temperature independent within experimental error. The nearly constant t− value of 0.36 ± 0.04 implies that charge transport by the iodine anions is substantial, whereas the cation contribution is about twice as effective. The composition dependence of f an pair and t− is illustrated in Figure 5. It is observed that f an (open circles) follows the pair rollercoaster behavior exhibited by the deviation parameter ΔNE (triangles) but with slightly higher values. A similar up-anddown course is found for f cat pair (not shown in Figure 5), which runs below the ΔNE curve. By contrast, t− proves to be surprisingly constant over the entire IL weight-fraction range. Only the PEO20EMImI value (open square) clearly deviates from the dashed line given by t− = 0.37 ± 0.02, which results from fitting to the other t− data (solid squares). Again, the obtained t− value signifies that I− and EMIm+ do not greatly differ in their efficiency for charge transport and that very similar conditions are maintained over the entire composition range. 3.5. Partial Conductivity of Anions. As indicated above, effective transport of iodide species is important for DSSC electrolytes. The Nernst−Einstein equation (eq 1) states that σ = (e2/(kBT))CsDσ, so that
where on the left-hand side of each equation, a measured diffusivity appears. The effective diffusion coefficients on the right-hand sides are associated with the individual ionic or pair species. Here, Deff pair stands for rpairDpair with the pair fraction rpair = Cpair/Cs and the true (or inherent32) pair diffusivity Dpair. The latter quantity is closely related to the pair mobility. Similarly, Deff cat+ is the product of the unpaired ionic fraction 1 − rpair and − the true cation diffusivity Dcat+, whereas Deff an− = (1 − rpair)Dan holds true as well. However, the present evaluation will be restricted to the level of effective diffusivities; a further decomposition combined with a determination of rpair and the true diffusion coefficients as done in earlier work33,34 is not pursued in this paper. Solving eqs 5−7 yields expressions for the three a priori unknown effective diffusivities in terms of the three measured diffusion coefficients, i.e, eff + = Dcat
1 * * + Dσ ) (Dcat − Dan 2
(8)
eff − = Dan
1 * * + Dσ ) (Dan − Dcat 2
(9)
eff Dpair =
1 * * − Dσ ) (Dan + Dcat 2
(10)
Thus, the effective diffusivity of each ionic species can be obtained from the (interpolated and smoothed)35 experimental data by simple algebraic addition and subtraction operations. This is shown in Figure 4a for PEO5.5EMImI by the solid and dashed curves. Obviously, the pair contribution Deff pair to D* cat and Dan * is distinctly larger than the respective single-ion eff contributions. Furthermore, Deff cat+ exceeds Dan− by roughly a factor of 2 over the whole temperature range. We note that the + sum of the dashed curve (Deff pair) and upper solid curve (Dcat ) accurately reproduces the D*cat data according to eq 5. Similar consistency checks can be made for the other diffusion coefficients. The information contained in the effective diffusivities and their temperature dependence can be presented in a clear and useful manner by the anion transference number t − , represented by t− =
eff − Dan D eff− = eff an eff Dσ Dcat+ + Dan−
σ = σ+ + σ − =
e 2Cs eff eff −) (Dcat+ + Dan kBT
(14)
Thus, the partial conductivity σ− is technologically relevant and essentially given by the product of Cs and Deff an−. Corresponding results for the PEO−EMImI system are plotted in Figure 6 for
(11)
an and the fractional pair components f cat pair and f pair, defined as
cat f pair =
eff eff Dpair Dpair = eff eff * Dcat Dcat+ + Dpair
(12)
an f pair =
eff eff Dpair Dpair = eff eff * Dan Dan− + Dpair
(13)
Figure 6. Partial contribution of anions σ− to the total ionic conductivity σ of PEOyEMImI complexes as a function of salt concentration ws in weight percent for three selected temperatures, as indicated by the solid symbols. Open circles represent total anion number density Cs (right axis). Solid and dashed line serve to guide the eye.
Again, we prefer the DSSC-relevant quantity t− to its cationrelated counterpart t+ = 1 − t−. Thus, t− is a fractional component characterizing the share of an− (i.e., I−) in the an overall charge transport.36 Likewise, f cat pair and f pair may be viewed as pair-related transport numbers of cations and anions, respectively. Figure 4b displays the above fractional components or transport/transference numbers as a function of temperature cat for PEO5.5EMImI. The finding that f an pair is larger than f pair reflects the fact that Dan * is smaller than Dcat * . Moreover, the
three chosen temperatures, i.e., 75, 90, and 105 °C. The data show a monotonic increase of σ− with increasing salt concentration, i.e., for 90 °C from 1.3 × 10−4 to 6.0 × 10−3 S cm−1. This represents a gain in the iodide transport capacity of about a factor of 50. Moreover, the steep salt-poor regime and the less steep salt-rich regime are connected by a plateaulike intermediate σ− range with an inflection near y = 10. This phenomenon appears within the same narrow composition 5697
DOI: 10.1021/acs.jpcb.5b01113 J. Phys. Chem. B 2015, 119, 5693−5700
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The Journal of Physical Chemistry B interval also showing salient features in Tg and ΔNE (cf. Figures 1 and 5). We note that σ− can also be calculated from the wellknown expression σ− = t−σ, yielding identical results. eff Furthermore, decomposition of Dan into its constituting − quantities leads to the Nernst−Einstein-type relation σ− = (e2/(kBT))Can−Dan−, with the concentration of unpaired anions Can− and the true diffusivity Dan−.
relate to the changeover from IL-in-polymer to polymer-in-IL and the occurrence of a pronounced intermediate-range order37 in this domain. Altogether, it may be stated that ion association leads to more than 50% loss of the potential (an)ionic charge transport capacity in PEOyEMImI complexes. Another factor influencing electrolyte performance is the transference number of interest. In the present context, this is the iodine-related transference number t−. Our results in Figures 4b and 5 show that t− attains only 37% of its maximum value, virtually independent of temperature and composition. So, in the PEO−EMImI system there is no potential gain by optimizing the anionic transference number. We further note * by about 20% that t− is smaller than the transport number tan (cf. Figure 5), which is due to ion pairing. The present t− values are distinctly lower than those commonly observed in polymer electrolytes based on alkali metal cations. For these classical systems, Li+ or Na+ transference numbers on the order of 0.3 or less have been reported,1,17,33 which implies that t− amounts to 0.7 or more. However, the amount of alkali metal salt is strongly limited in these cases, as typically indicated by composition parameters y > 10. Beyond the corresponding salt concentration, the precipitation of salt-rich crystallites leads to a strong decrease of the conductivity. Thus, the relatively low t− value in the present PEO−IL system is overcompensated by the lack of other, more severe disadvantages. Altogether, the crucial parameter for maximizing transport of negative charge carriers in PEO−EMImI is the salt concentration. Increasing the EMImI fraction leads to an increase of the anion density comprising both charged and uncharged ionic species. Fortunately, because Tg, f an pair, and t− do not greatly change upon EMImI addition, there is virtually no penalty on salt addition, and every extra IL molecule improves electrolyte performance. This favorable feature is borne out by the monotonic increase of σ− with ws in Figure 6. Remarkably, σ− increases more strongly than Cs, which is also depicted in Figure 6. Because σ− is proportional to Cs and Deff an−, this effect an must be attributed to Deff an (cf. eq 13). Indeed, an−= (1 − f pair)D* close inspection reveals that the decrease of f an pair from 0.83 to 0.54 (cf. Figure 5) fully explains the additional enhancement of 1 − f an pair, and hence σ−, by a factor of ∼3, whereas D* an remains almost constant as mentioned above. In DSSCs, charge transfer is accomplished by electron transport via the I−/I−3 redox couple.12−14 This involves the net reaction I−3 + 2e → 3I− at the back electrode and the reverse reaction near the front electrode in order to promote electrons to the ionized dye molecules. Hence, opposite fluxes of I− and I−3 occur during solar-cell operation. To our knowledge, there is little published work providing a reliable separate determination of DI− and DI−3 . (Ion pairing is neglected so that no distinction is made between effective and true diffusivities.) Available data indicate that DI− and DI−3 do not differ by more than a factor of 2 in either direction.39 Commonly, it is assumed either that the overall process is rate limited by the larger I−3 anion with its lower abundance of at least a factor of 4 compared to I− or that both diffusion coefficients are equal.40 Regardless, it may be expected that the magnitude of DI−3 is closely related to that of DI−, whereas there is evidence that the bulky I−3 anion has a lower tendency to pair formation.17,18 We believe therefore that the present results are relevant to the development of electrolytes for DSSCs.
4. DISCUSSION In discussing the present results from a more general viewpoint, we may distinguish between different factors promoting suitable ionic transport properties in electrolyte systems. These factors include a high glass transition temperature, a high salt concentration, little ion association, and a high transference number. The present paper provides, so far known for the first time, reliable data of all these crucial quantities for a polymer−IL system over an extremely wide composition range as a function of temperature. This was achieved by measuring Tg and the diffusion coefficients Dσ, Dcat * , and Dan * of PEO− EMImI complexes of variable IL content. The measured T g values are near −50 °C for all compositions, with deviations of about 4 °C in both directions (cf. Figure 1). This narrow interval for a polymer−IL system contrasts with the wide Tg ranges found for related polymer− inorganic-salt systems.5,10 In all these cases, a distinct maximum of Tg for some intermediate composition is observed. Such Tg maxima are often accompanied by minima in the compositiondependent conductivity at fixed temperatures,5,10 because ionic mobilities are inversely correlated with Tg. In this work we do not see such pronounced Tg-related effects (cf. Figures 2 and 6), which may be understood from the small Tg variation on the whole. However, a better measure of ionic mobility is the corresponding diffusivity because diffusion coefficients, unlike σ or σ−, do not involve the ionic concentration (volume density). As indicated before, Dcat * and Dan * show a very weak composition dependence, also reflected by virtually constant t*an data (Figure 5), whereas the change of Dσ with ws is moderate (by a factor of 4 or less, depending on temperature) and almost monotonic. Altogether, the weak Tg dependence on IL content complies with the observed diffusion behavior. Given a certain salt concentration, the potential charge transport capacity may be strongly reduced because of ion association, predominantly pair formation.37,38 In PEO− EMImI complexes, the ion-pairing effect leads to σ losses varying by about 50−80% depending on ws, as indicated by ΔNE in Figure 5. A separate consideration for the anions yields, through f an pair in Figure 5, a slightly worse figure of merit regarding ion pairing. It should be emphasized that ΔNE and f an pair take into account the combined effect of pair abundance (rpair) and pair mobility (Dpair). In salt-in-polymer electrolytes, the fractional pair components commonly increase both with increasing T and decreasing Cs.29,33 These reverse trends in temperature and salt concentration are demonstrated by Figure 4b and by the left-hand side of Figure 5, respectively. Reversed ion-association behavior in salt-poor electrolytes relates to a negative entropy of mixing caused by constraints in polymer conformation.4,33 We now find that the reverse f an pair trend with T for fixed Cs (or ws) is maintained over the entire composition range (not shown here). However, the reverse f an pair trend with Cs for fixed T is interrupted by the normal (weak electrolyte) composition dependence, as visible at the right-hand slope of the minimum in Figure 5. The reasons for this normal behavior at intermediate compositions is presently not clear, but it may 5698
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(9) Tiyapiboonchaiya, C.; MacFarlane, D. R.; Sun, J.; Forsyth, M. Polymer-in-Ionic-Liquid Electrolytes. Macromol. Chem. Phys. 2002, 203, 1906−1911. (10) Angell, C. A.; Liu, C.; Sanchez, E. Rubbery Solid Electrolytes with Dominant Cationic Transport and High Ambient Conductivity. Nature 1993, 362, 137−139. (11) Forsyth, M.; MacFarlane, D. R.; Hill, A. J. Glass Transition and Free Volume Behaviour of Poly(acrylonitrile)/LiCF3SO3 Polymer-inSalt Electrolytes Compared to Poly(ether urethane)/LiClO4 Solid Polymer Electrolytes. Electrochim. Acta 2000, 45, 1243−1247. (12) Fei, Z.; Kuang, D.; Zhao, D.; Klein, C.; Ang, W. H.; Zakeeruddin, S. M.; Grätzel, M.; Dyson, P. J. A Supercooled Imidazolium Iodide Ionic Liquid as Low-Viscosity Electrolyte for Dye-Sensitized Solar Cells. Inorg. Chem. 2006, 45, 10407−10409. (13) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. High-Performance Dye-Sensitized Solar Cells Based on Solvent-Free Electrolytes Produced from Eutectic Melts. Nat. Mater. 2008, 7, 626−630. (14) Song, D.; Cho, W.; Lee, J. H.; Kang, Y. S. Toward Higher Energy Conversion Efficiency for Solid Polymer Electrolyte DyeSensitized Solar Cells: Ionic Conductivity and TiO2 Pore-Filling. J. Phys. Chem. Lett. 2014, 5, 1249−1258. (15) According to the lot specifications, the PEO base material typically contained 1.4 wt % SiO2 and 0.2 wt % CaO. These contaminations are inherent to the manufacturing process and present in the form of small particles. (16) Obeidi, S.; Zazoum, B.; Stolwijk, N. A. Ionic Tracer Diffusion and DC Conductivity in the Polymer Electrolyte PEO30NaI. Solid State Ionics 2004, 173, 77−82. (17) Call, F.; Stolwijk, N. A. Impact of I2 Additions on Iodide Transport in Polymer Electrolytes for Dye-Sensitized Solar Cells: Reduced Pair Formation versus a Grotthuss-Like Mechanism. J. Phys. Chem. Lett. 2010, 1, 2088−2093. (18) Call, F.; Stolwijk, N. A. Correction to “Impact of I2 Additions on Iodide Transport in Polymer Electrolytes for Dye-Sensitized Solar Cells: Reduced Pair Formation versus a Grotthuss-Like Mechanism”. J. Phys. Chem. Lett. 2010, 1, 3213−3213. (19) Stolwijk, N. A.; Wiencierz, M.; Heddier, C.; Kösters, J. What Can We Learn from Ionic Conductivity Measurements in Polymer Electrolytes? - A Case Study on Poly(ethylene oxide) (PEO)-NaI and PEO-LiTFSI. J. Phys. Chem. B 2012, 116, 3065−3074. (20) Tanner, J. E. Use of the Stimulated Echo in NMR Diffusion Studies. J. Chem. Phys. 1970, 52, 2523−2526. (21) Stolwijk, N. A.; Wiencierz, M.; Fögeling, J.; Bastek, J.; Obeidi, S. The Use of Radiotracer Diffusion to Investigate Ionic Transport in Polymer Electrolytes: Examples, Effects, and Their Evaluation. Z. Phys. Chem. (Muenchen, Ger.) 2010, 224, 1707−1733. (22) Every, H. A.; Bishop, A. G.; MacFarlane, D. R.; Orädd, G.; Forsyth, M. Transport Properties in a Family of Dialkylimidazolium Ionic Liquids. Phys. Chem. Chem. Phys. 2004, 6, 1758−1765. (23) Bastek, J.; Stolwijk, N. A.; Köster, T. K.-J.; van Wüllen, L. Systematics of Salt Precipitation in Complexes of Polyethylene Oxide and Alkali Metal Salts. Electrochim. Acta 2010, 55, 1289−1297. (24) Fögeling, J.; Kunze, M.; Schönhoff, M.; Stolwijk, N. A. ForeignIon and Self-Ion Diffusion in a Crosslinked Salt-in-Polymer Electrolyte. Phys. Chem. Chem. Phys. 2010, 12, 7148−7161. (25) Stolwijk, N. A.; Kösters, J.; Wiencierz, M.; Schönhoff, M. On the Extraction of Ion Association Data and Transference Numbers from Ionic Diffusivity and Conductivity Data in Polymer Electrolytes. Electrochim. Acta 2013, 102, 451−458. (26) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species. J. Phys. Chem. B 2004, 108, 16593−16600. (27) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in Imidazolium Cation. J. Phys. Chem. B 2004, 109, 6103−6110.
5. CONCLUSIONS We have investigated mass and charge transport in the polymer−IL system PEOyEMImI as a function of temperature over the wide composition range from 8.3 (%) to 90 wt % IL. The overall ionic conductivity was compared with the individual diffusivities of cations and anions using a unique combination of methods involving impedance spectroscopy, PFG-NMR, and radiotracer diffusion. Remarkably, we found that the charge diffusivity shows a much stronger variation with concentration than the ionic diffusion coefficients, which can be explained by cation−anion association. A maximum in the glass transition temperature and a local minimum in the ion-pairing tendency are observed at intermediate compositions (y ≈ 10 and ws ≈ 0.3 wt %), which seems to characterize a transition from the IL-in-polymer to the polymer-in-IL regime. Surprisingly, the anion transference number remains constant at a value of 0.37 for all compositions. By contrast, the partial conductivity of iodine anions monotonically increases with increasing IL concentration over almost 2 orders of magnitude, which is even stronger than the increase of the anion density. These results are relevant to the application of polymer−IL electrolytes in dye-sensitized solar cells based on the I−/I−3 redox couple.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +49 (0)251 8339013. Fax: +49 (0)251 8338346. Email: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft. REFERENCES
(1) Gray, F. M. Solid Polymer Electrolytes; VCH: New York, 1991. (2) Lascaud, S.; Perrier, M.; Vallee, A.; Besner, S.; Prud’homme, J.; Armand, M. Phase Diagrams and Conductivity Behavior of Poly(ethylene oxide)-Molten Salt Rubbery Electrolytes. Macromolecules 1994, 27, 7469−7477. (3) Stolwijk, N. A.; Heddier, C.; Reschke, M.; Wiencierz, M.; Bokeloh, J.; Wilde, G. Salt-Concentration Dependence of the Glass Transition Temperature in PEO-NaI and PEO-LiTFSI Polymer Electrolytes. Macromolecules 2013, 46, 8580−8588. (4) Lee, H.-N.; Lodge, T. P. Lower Critical Solution Temperature (LCST) Phase Behavior of Poly(ethylene oxide) in Ionic Liquids. J. Phys. Chem. Lett. 2010, 1, 1962−1966. (5) Xu, W.; Wang, L.-M.; Angell, C. A. “PolyMOB”-Lithium Salt Complexes: from Salt-in-Polymer to Polymer-in-Salt Electrolytes. Electrochim. Acta 2002, 48, 2037−2045. (6) Lasinska, A.; Dygas, J. R.; Krok, F.; Marzantowicz, M.; Florjanczyk, A.; Tomaszewska, A.; Zygadlo-Monikowska, E. Ionic Conductivity of Polymer Electrolytes Comprising Acrylonitrile-Butyl Acrylate Copolymer and a Lithium Salt. Adv. Mater. Sci. (Warsaw, Poland) 2006, 24, 187−194. (7) Yang, Y.; Zhang, J.; Zhou, C.; Wu, S.; Xu, S.; Liu, W.; Han, H.; Chen, B.; Zhong Zhao, X. Effect of Lithium Iodide Addition on Poly(ethylene oxide)-Poly(vinylidene fluoride) Polymer-Blend Electrolyte for Dye-Sensitized Nanocrystalline Solar Cell. J. Phys. Chem. B 2008, 112, 6594−6602. (8) Chaurasia, S. K.; Singh, R. K.; Chandra, S. Structural and Transport Studies on Polymeric Membranes of PEO Containing Ionic Liquid, EMIM-TY: Evidence of Complexation. Solid State Ionics 2011, 183, 32−39. 5699
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The Journal of Physical Chemistry B (28) Gorlov, M.; Kloo, L. Ionic Liquid Electrolytes for DyeSensitized Solar Cells. Dalton Trans. 2008, 2655−2666. (29) Boden, N.; Leng, S. A.; Ward, I. M. Ionic Conductivity and Diffusivity in Polyethylene Oxide/Electrolyte Solutions as Models for Polymer Electrolytes. Solid State Ionics 1991, 45, 261−270. (30) Lonergan, M. C.; Shriver, D. F.; Ratner, M. A. Polymer Electrolytes: The Importance of Ion-Ion Interactions in Diffusion Dominated Behavior. Electrochim. Acta 1995, 40, 2041−2048. (31) Murch, G. E.; Dyre, J. C. Correlation Effects in Ionic Conductivity. Crit. Rev. Solid State Mater. Sci. 1989, 15, 345−365. (32) Kataoka, H.; Saito, Y. New Approaches for Determining the Degree of Dissociation of a Salt by Measurements of Dynamic Properties of Lithium Ion Batteries. J. Phys. Chem. B 2002, 106, 13064−13068. (33) Wiencierz, M.; Stolwijk, N. A. Systematics of Ionic Transport and Pair Formation in amorphous PEO-NaI Polymer Electrolytes. Solid State Ionics 2012, 212, 88−99. (34) Eschen, T.; Kösters, J.; Schönhoff, M.; Stolwijk, N. A. Ionic Transport in Polymer Electrolytes Based on PEO and the PMImI Ionic Liquid: Effects of Salt Concentration and Iodine Addition. J. Phys. Chem. B 2012, 116, 8290−8298. (35) Interpolation and smoothing of the diffusion data was done by fitting with the Vogel−Tammann−Fulcher equation. (36) It should be noted that eq 11 reduces to a similar expression in terms of the corresponding true diffusivities t− = Dan−/(Dcat+ + Dan−) eff because the joint factor 1 − rpair of Deff cat+ and Dan− (see also eq 7 and below) cancels in numerator and denominator. (37) Costa, L. T.; Ribeiro, M. C. C. Molecular Dynamics Simulation of Polymer Electrolytes Based on Poly(ethylene oxide) and Ionic Liquids. I. Structural Properties. J. Chem. Phys. 2006, 124, 184902. (38) Costa, L. T.; Ribeiro, M. C. C. Molecular Dynamics Simulation of Polymer Electrolytes Based on Poly(ethylene oxide) and Ionic Liquids. II. Dynamical Properties. J. Chem. Phys. 2007, 127, 164901. (39) Zakeeruddin, S. M.; Grätzel, M. Solvent-Free Ionic Liquid Electrolytes for Mesoscopic Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2009, 19, 2187−2202. (40) Kawano, R.; Watanabe, M. Equilibrium Potentials and Charge Transport of an I−/I3− Redox Couple in an Ionic Liquid. Chem. Commun. 2003, 330−331.
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Mass and Charge Transport in the Polymer−Ionic-Liquid System PEO−EMImI: From Ionic-Liquid-in-Polymer to Polymer-in-IonicLiquid Electrolytes Johannes Kösters,† Monika Schönhoff,‡ and Nicolaas A. Stolwijk*,† †
Institut für Materialphysik, University of Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany Institut für Physikalische Chemie, University of Münster, Corrensstrasse 28/30, 48149 Münster, Germany
‡
ABSTRACT: Conventional polymer electrolytes based on inorganic salts are commonly characterized and utilized over a small salt-poor composition range because of phase transitions accompanied by loss of ion conductivity at high salt concentrations. By contrast, well-chosen polymer−ionic-liquid (IL) systems offer the possibility to vary the IL content from the IL-in-polymer to the polymer-in-IL domain. We have investigated the temperature-dependent ionic conductivity in PEOyEMImI systems consisting of poly(ethylene oxide) complexed with 1-ethyl-3-methylimidazolium iodide for y = EO/ IL ratios ranging from 0.6 to 60 and compared diffusivity data with that arising from 1H pulsed-field-gradient nuclear magnetic resonance for EMIm and 125I radiotracer diffusion for iodine. Surprisingly, the diffusivity of cations and anions vary at most by 50% at fixed temperatures over the entire composition range. The much larger changes in the charge diffusivity Dσ relate to ion pairing exhibiting a minimum near the intermediate composition y = 10. Altogether, the results are relevant to application in dye-sensitized solar cells and show that a high ion density is crucial to enhance the iodine transport capacity.
1. INTRODUCTION In principle, salt-in-polymer electrolytes offer the possibility to reconcile a sufficient ion conductivity with beneficial mechanical properties for applications such as batteries, smart windows, and electrochemical solar cells.1 However, classical polymer electrolytes based on inorganic salts are limited to low salt concentrations because the concomitant increases in glass transition temperature Tg and/or the formation of crystalline phases lead to reduced mobilities,2,3 which overcompensates the seeming advantage of a higher ion density. In recent years, the availability of room-temperature ionic liquids (ILs) make it feasible to avoid these problems and thus to prepare fully miscible polymer−IL complexes covering the entire range from IL-in-polymer to polymer-in-IL electrolytes.4 So far, studies on ion transport in polymer−salt or polymer− IL systems that explore wide composition ranges have been sparse. Moreover, most of these studies focus on the ion conductivity and its correlation with Tg.5−9 The experimental data are often globally interpreted in terms of Angell’s decoupling index, referring to the ratio of characteristic times for ionic transport to mechanical processes (viscous flow).10,11 However, information on ion association and the individual mobility of cations and anions is largely missing. The present work deals with PEOyEMImI complexes of poly(ethylene oxide) (PEO) and 1-ethyl-3-methylimidazolium iodide (EMImI) with EO/EMIm ratios y ranging from 0.6 to 60. Apart from the ion conductivity, we also determine the diffusion coefficient of both cations and anions as a function of temperature in seven different compositions. The comparison © 2015 American Chemical Society
of mass and charge transport enables us to monitor compositional trends in the ion-pairing tendency and to discriminate between the contributions of positive and negative charge carriers to the conductivity. The results may be important to the optimization of electrolytes for dye-sensitized solar cells (DSSCs) based on the monoiodide/triiodide redox couple because in this case the cell efficiency crucially depends on electron transfer carried by unpaired anions.12−14
2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. The base materials were PEO, with a molecular weight of 8 × 106 g/mol and a purity of 98.4 wt %15 (Aldrich), and EMImI (238.07 g/mol, > 99 wt %, Iolitec). All substances were vacuum-dried before use. For the preparation of the polymer electrolytes, the same methods were used as described in earlier publications by our group.3,16−18 Great care was taken to remove traces of water and solvent (extra dry acetonitrile, Acros Organics) and to avoid contact with ambient atmosphere. In (PFG-)NMR analysis, no 1H resonance attributable to water could be detected. Considering that all preparation steps were carried out either in a nitrogenfilled glovebox (H2O = 1 ppm) or under continuous pumping in the HV range, the water contamination of the PEO−EMImI electrolytes is assumed to be at a negligibly low level. Received: February 3, 2015 Revised: April 7, 2015 Published: April 7, 2015 5693
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The Journal of Physical Chemistry B 2.2. Electrolyte Characterization. The mass density of the electrolytes was determined by hydrostatic weighing in dodecane and air, taking single-crystal silicon as a reference material. The density enters the calculation of the salt concentration per unit volume. Phase behavior was examined by differential scanning calorimetry (DSC) at different heating and cooling rates using PerkinElmer DSC-7 and Diamond-DSC devices.3 Tg values determined after ex situ quenching in a dewar with liquid nitrogen3 (first heating cycle) and in situ quenching in the calorimeter (second heating cycle) were mutually consistent. 2.3. Impedance Spectroscopy. The ionic conductivity was measured by impedance spectroscopy (EIS) in the frequency range of 5 Hz−13 MHz using an HP Agilent 4192A LF impedance analyzer.19 The reliability of the data was ensured by using at least two different samples for each composition and by running multiple heating and cooling cycles for each sample. Because ionic conduction mainly takes place in the amorphous phase of the electrolytes, the data covered the widest possible temperature range of this phase. The consistency of the results over measurement periods of several days for each sample points to a sufficient stability of the electrolytes in the T range of interest. 2.4. Pulsed-Field-Gradient Nuclear Magnetic Resonance. Diffusion coefficients of EMIm were measured on a 400 MHz FT-NMR spectrometer (Bruker, Avance) employing a probe head (Bruker, DIFF 30) with a RF-selective insert for protons and providing a gradient strength of up to 12 T/m. The stimulated echo was employed with a variation of the gradient strength g. The intensity I of the echo signal decreases with increasing strength of the magnetic field gradient g, i.e., I = I0 exp(−γ2g2δ2Dcat * (Δ − (δ/3))).20 Here, I0 denotes the zerogradient intensity, D*cat is the diffusion coefficient of the cation, γ is the gyromagnetic ratio, δ is the duration of the gradient pulse, and Δ is the observation time for the mean-square displacement. Of the six different EMIm-related 1H resonances, there are two resonances that interfere mutually and two others that overlap with the strong, hardly attenuated PEO signal. Thus, each of the three distinguishable peaks yields a value for Dcat * = DEMIm * at any selected temperature. The data presented below are the corresponding mean values and are further characterized by a standard deviation of about 10%. 2.5. Radiotracer Diffusion. The iodine diffusivity was measured by the 125I radiotracer technique, which is described in detail elsewhere.21 After placement of a radioactive polymer−electrolyte source film on top of a sample and subsequent isothermal annealing in an oil bath, the sample was sectioned with a rotary microtome at a suitable freezing temperature. Tracer depth profiles were obtained by counting the 125I radioactivity in each section. D*I = D*an was determined by fitting the measured penetration profiles with the appropriate solution of Fick’s second law.21 It should be emphasized that 125I concentrations are always very low fractions of the total iodine content, so that its diffusion does not involve compositional change. In contrast to PFG-NMR, the radiotracer diffusion (RTD) technique is destructive so that each measurement requires a new sample.
complexes are compiled in Table 1 including the number density Cs of salt molecules and the corresponding IL weight Table 1. Salt Weight Fraction (ws), Salt Concentration (Cs, Number Density), Mass Density (ρ), and (Onset) Melting a Temperatures (Tonset m ) of the PEOyEMImI Complexes complex PEO60EMImI PEO30EMImI PEO20EMImI PEO10EMImI PEO5.5EMImI PEO2.3EMImI PEO0.6EMImI a
ws
Cs (cm−3)
ρ (g cm−3)
Tonset (°C) m
0.0826 0.153 0.213 0.351 0.496 0.701 0.900
× × × × × × ×
1.26 1.28 1.30 1.31 1.46 1.92 1.94
58.3 56.7 56.6 54.7 53.0 49.9 44.8
2.63 4.95 6.97 1.16 1.84 3.39 4.41
20
10 1020 1020 1021 1021 1021 1021
Experimental error is about ±0.02 g cm−3 in ρ and ±2 °C in Tonset m
fraction ws ranging from 0.90 to 0.083. Mass densities ρ and (onset) melting temperatures Tonset m , also listed in Table 1, show the expected monotonic behavior as a function of the IL concentration. By contrast, the glass transition temperature Tg depicted in Figure 1 exhibits a pronounced maximum near the intermediate composition ws = 0.3 (y ≈ 10−20).
Figure 1. Glass transition temperature Tg of PEOyEMImI complexes as a function of salt concentration ws in weight percent. Solid line serves to guide the eye.
Figure 1 also includes the Tg value (−57 °C) of the employed high-molecular-weight pristine PEO taken from earlier work.3 The data show a clear compositional trend, as indicated by the solid curve intersecting all data points. Nevertheless, the Tg variation of the investigated electrolytes is restricted to a narrow range between −53.5 °C (y = 0.6) and −45.7 °C (y = 20), making up a maximum shift of only 8 °C. The extrapolation of the composition dependence in Figure 1 to ws = 1.0 suggests a Tg value of −55 °C for the pure EMImI IL. This may indicate that the reported estimate of −39 °C, which is based on the EMImI melting temperature, is only very approximate.22 However, the present extrapolation perfectly agrees with Tg = −55 °C detected for a very similar IL (1-ethyl3-ethylimidazolium iodide).12 We further note that DSC did not reveal features other than glass transition and melting, except for a weak and broad endothermic event near 123−135 °C (temperature at local maximum) for the three most salt-rich complexes. This reproducible event in multiple heating and cooling cycles may be tentatively explained by disordering or ion association phenomena in high-density ionic systems.
3. RESULTS AND ANALYSIS 3.1. General Characteristics and Glass Transition. We prepared seven PEOyEMImI complexes of different composition as indicated by their chosen EO/PMImI ratios y = 0.6, 2.3, 5.5, 10, 20, 30, and 60. General characteristics of these 5694
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The Journal of Physical Chemistry B 3.2. Ionic Conductivity and Charge Diffusivity. The direct current ionic conductivity σ of the PEOyEMImI electrolytes was determined over wide temperature ranges by EIS. Figure 2 displays representative data in the fully
unambiguous definition, Dσ is closely related to the molar conductivity given by Λ = σ/cs, where cs is the same salt concentration but in molar units. Thus, Dσ represents unbiased experimental data that can be compared with the individual diffusivities of cations and anions. 3.3. Ion-Specific Self-Diffusion Coefficients. The diffusion coefficient Dcat * of the EMIm cation was measured by PFG-NMR using the stimulated echo signal of the 1H nuclei.24 However, for the iodine anion no suitable PFG-NMR isotope exists. In this case, the 125I radiotracer technique based on the measurement of activity-depth profiles after diffusion annealing provides a powerful alternative to determine Dan * .21,24 For PEO0.6EMImI and PEO30EMImI, both types of diffusivity data are plotted in Figure 3 together with Dσ. The intermediate
Figure 2. Direct current conductivity of PEOyEMImI electrolytes with different EO/EMIm ratios y as a function of inverse temperature. Dashed line reproduces data for pure EMImI (y = 0) reported by Every et al.22
amorphous phase, which extends from ∼55 to ∼105 °C for the four salt-poor complexes (y ≥ 10) and from ∼45 to ∼145 °C for the three salt-rich complexes (y ≤ 5.5). At temperatures above ∼145 °C, the high fluidity of the salt-rich electrolytes caused leakage of the measuring cell. By contrast, the salt-poor electrolytes showed a (reversible) decrease of the conductivity upon heating above a critical temperature of ∼105 °C. This inversion of the σ-versus-1/T behavior is indicative of salt precipitation,23 as was confirmed by the appearance of a liquid film on the surface of a test sample during heating on a hot plate. At low temperatures, the σ data depicted in Figure 2 extend 5−12 °C below the onset crystallization temperature as measured by DSC (cf. Table 1), which is due to a composition-dependent supercooling effect. Upon further cooling, the onset of crystallization is characterized by an abrupt decrease of σ (not shown in Figure 2). It is seen in Figure 2 that the conductivity increases over about a factor of 50 from salt-poor to salt-rich electrolytes, roughly reflecting the difference in IL content. Moreover, the σ plots of different compositions run almost parallel, which may reflect some basic similarities in the underlying ion conduction processes. The present data also compare well to the conductivity of pure EMImI (y = 0) from Every et al.,22 as demonstrated by the dashed line in Figure 2. The ion conductivity may be converted into a quantity with the dimensions of a diffusion coefficient yielding the so-called charge diffusivity Dσ. This can be done with the aid of the Nernst−Einstein equation, i.e.,
Dσ =
kBT Cse 2
Figure 3. Measured diffusion coefficients of cations, anions, and charge (symbols) with their fits (solid lines) in (a) salt-rich and (b) salt-poor PEO−EMImI electrolyte as a function of inverse temperature. Upper solid line represents the sum of the cation and anion diffusivity.
composition PEO5.5EMImI is similarly displayed in Figure 4, whereas the PEO 60 EMImI data have been published previously.25 The low-temperature bound of the diffusivity measurements is 70 °C, which lies well above the melting temperature in all cases (cf. Table 1). The high-temperature bound depends on the composition-dependent onset of IL precipitation but is at least 110 °C. We note that the radiotracer technique demands macroscopic samples of sufficient mechanical stability21 so that low-viscosity complexes with y < 0.6 including the pure IL could not be analyzed. In six out of seven complexes, D*cat is larger than D*an over the temperature range investigated, which seems to be a general feature of both ILs26,27 and polymer−IL systems.28 Only for PEO20EMImI does D*an slightly exceed D*cat but not by more than 5%, which is within experimental error. For all compositions and temperatures, Dcat * and Dan * differ by less than 30%. These findings manifest themselves in the anion transport number given by
σ (1)
It should be emphasized that the denominator contains the known total salt or IL concentration Cs instead of the a priori unknown concentration of free (dissociated) ions. With this 5695
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value of 0.47 is a remarkable feature of the PEO−EMIm system. Not only the relative but also the absolute mass diffusion coefficients show little variation with IL content. Both Dcat * and D*an change by at most a factor of 1.5 when going at any fixed temperature from salt-poor (y = 30 or 60) to salt-rich (y = 0.6) compositions.25 Published Dcat * data of pure EMImI (y = 0) fit into this picture as they exceed the PEO0.6EMImI data by not more than 70%.22 However, the present shifts in Dσ amount to up to a factor of 4, as may be recognized from Figure 3. In particular, Figure 3a shows that Dσ runs between Dcat * and Dan * for PEO0.6EMImI, whereas in Figure 3b, Dσ of PEO30EMImI clearly falls below the two self-diffusivities. This trend is confirmed by inspection of the PEO60EMImI data published elsewhere.25 Small values of Dσ with regard to the individual ionic diffusivities may point to a high degree of ion association. This feature will be quantitatively analyzed in the following paragraphs. 3.4. Ion Association, Pair Components, and Transference Number. Ion association is commonly quantified by the Nernst−Einstein deviation parameter ΔNE, represented as25,29,30 ΔNE = 1 − Figure 4. (a) Measured diffusion coefficients of cations, anions, and charge (symbols) as a function of inverse temperature in PEO5.5EMImI. Solid lines represent the contribution of free cations and anions, as indicated by the different Deff annotations. Dashed line represents the contribution of cation−anion pairs. (b) Additional parameters derived from the measured diffusion coefficients comprising the anion transference number t−, the deviation parameter an ΔNE, and the fractional pair components f cat pair and f pair.
*= tan
(3)
where HR = (D*cat + D*an) /Dσ is the Haven ratio. Thus, a high fraction of associated ions occurs if Dσ is much smaller than the sum of the cation and anion diffusivity. The quantity Dcat * + Dan * is represented in Figure 3 by the upper solid line in either panel, and it was obtained by using best-fit curves (solid lines) through the diffusivity data. It is obvious from Figure 3 that ion association is much stronger in PEO30EMImI than in PEO0.6EMImI. Figure 4b shows the temperature dependence of ΔNE for PEO5.5EMImI. It is seen that ΔNE varies from 0.73 to 0.57 upon passing from the highest depicted temperature (144 °C) to the lowest (60 °C). In Figure 5, ΔNE is plotted as a function of the salt concentration for a fixed temperature of 90 °C. As a general trend, ΔNE decreases with increasing IL content, i.e., from 0.79 for PEO60EMImI to 0.47 for PEO0.6EMImI. However, a local minimum with ΔNE ≈ 0.57 is observed near the intermediate composition ws = 0.3 or y = 10, which is followed with increasing EMImI content by a local maximum appearing near ws = 0.5 or y = 20. Altogether, the resulting ΔNE values reveal that ion association in the PEO−EMImI system occurs to substantial extents. Apart from ΔNE, other quantities can be deduced from the experimental data, if the formation of neutral pairs may be assumed to be largely predominant in ion association. In this case, the data analysis can be based on the occurrence of only three ionic species, cat+, an−, and the cat+an− pair, which interact with each other according to the reversible reaction 31
* Dan * + Dan * Dcat
Dσ 1 =1− * * HR Dcat + Dan
(2)
Here, we prefer t*an to the related quantity t*cat = 1 − t*an because of the importance of the iodide anions for DSSCs. Figure 5 displays t*an (open triangles) as a function of composition for the intermediate temperature of 90 °C. The almost constant t*an
cat+ + an− ⇌ pair
(4)
Specifically, we have to solve the system of three coupled equations25 Figure 5. Deviation parameter ΔNE (solid triangles), fractional pair component of iodine diffusivity f an pair (open circles), anion transport * (open triangles), and anion transference number t− number tan (squares) of PEO y EMImI complexes as a function of salt concentration ws in weight percent for a fixed temperature of 90 °C. Solid lines serve to guide the eye. Dashed line is a fit to the t− data (solid squares), ignoring one outlier (open square). 5696
eff eff * = Dcat + + D Dcat pair
(5)
eff eff * = Dan − + D Dan pair
(6)
eff eff + + D − Dσ = Dcat an
(7) DOI: 10.1021/acs.jpcb.5b01113 J. Phys. Chem. B 2015, 119, 5693−5700
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The Journal of Physical Chemistry B
deviation parameter ΔNE is between the two fractional pair components. Indeed, it can be shown that ΔNE is the weighted cat 25 mean of f an Although the pair components appear pair and f pair. to increase with increasing temperature, the transference number t− is temperature independent within experimental error. The nearly constant t− value of 0.36 ± 0.04 implies that charge transport by the iodine anions is substantial, whereas the cation contribution is about twice as effective. The composition dependence of f an pair and t− is illustrated in Figure 5. It is observed that f an (open circles) follows the pair rollercoaster behavior exhibited by the deviation parameter ΔNE (triangles) but with slightly higher values. A similar up-anddown course is found for f cat pair (not shown in Figure 5), which runs below the ΔNE curve. By contrast, t− proves to be surprisingly constant over the entire IL weight-fraction range. Only the PEO20EMImI value (open square) clearly deviates from the dashed line given by t− = 0.37 ± 0.02, which results from fitting to the other t− data (solid squares). Again, the obtained t− value signifies that I− and EMIm+ do not greatly differ in their efficiency for charge transport and that very similar conditions are maintained over the entire composition range. 3.5. Partial Conductivity of Anions. As indicated above, effective transport of iodide species is important for DSSC electrolytes. The Nernst−Einstein equation (eq 1) states that σ = (e2/(kBT))CsDσ, so that
where on the left-hand side of each equation, a measured diffusivity appears. The effective diffusion coefficients on the right-hand sides are associated with the individual ionic or pair species. Here, Deff pair stands for rpairDpair with the pair fraction rpair = Cpair/Cs and the true (or inherent32) pair diffusivity Dpair. The latter quantity is closely related to the pair mobility. Similarly, Deff cat+ is the product of the unpaired ionic fraction 1 − rpair and − the true cation diffusivity Dcat+, whereas Deff an− = (1 − rpair)Dan holds true as well. However, the present evaluation will be restricted to the level of effective diffusivities; a further decomposition combined with a determination of rpair and the true diffusion coefficients as done in earlier work33,34 is not pursued in this paper. Solving eqs 5−7 yields expressions for the three a priori unknown effective diffusivities in terms of the three measured diffusion coefficients, i.e, eff + = Dcat
1 * * + Dσ ) (Dcat − Dan 2
(8)
eff − = Dan
1 * * + Dσ ) (Dan − Dcat 2
(9)
eff Dpair =
1 * * − Dσ ) (Dan + Dcat 2
(10)
Thus, the effective diffusivity of each ionic species can be obtained from the (interpolated and smoothed)35 experimental data by simple algebraic addition and subtraction operations. This is shown in Figure 4a for PEO5.5EMImI by the solid and dashed curves. Obviously, the pair contribution Deff pair to D* cat and Dan * is distinctly larger than the respective single-ion eff contributions. Furthermore, Deff cat+ exceeds Dan− by roughly a factor of 2 over the whole temperature range. We note that the + sum of the dashed curve (Deff pair) and upper solid curve (Dcat ) accurately reproduces the D*cat data according to eq 5. Similar consistency checks can be made for the other diffusion coefficients. The information contained in the effective diffusivities and their temperature dependence can be presented in a clear and useful manner by the anion transference number t − , represented by t− =
eff − Dan D eff− = eff an eff Dσ Dcat+ + Dan−
σ = σ+ + σ − =
e 2Cs eff eff −) (Dcat+ + Dan kBT
(14)
Thus, the partial conductivity σ− is technologically relevant and essentially given by the product of Cs and Deff an−. Corresponding results for the PEO−EMImI system are plotted in Figure 6 for
(11)
an and the fractional pair components f cat pair and f pair, defined as
cat f pair =
eff eff Dpair Dpair = eff eff * Dcat Dcat+ + Dpair
(12)
an f pair =
eff eff Dpair Dpair = eff eff * Dan Dan− + Dpair
(13)
Figure 6. Partial contribution of anions σ− to the total ionic conductivity σ of PEOyEMImI complexes as a function of salt concentration ws in weight percent for three selected temperatures, as indicated by the solid symbols. Open circles represent total anion number density Cs (right axis). Solid and dashed line serve to guide the eye.
Again, we prefer the DSSC-relevant quantity t− to its cationrelated counterpart t+ = 1 − t−. Thus, t− is a fractional component characterizing the share of an− (i.e., I−) in the an overall charge transport.36 Likewise, f cat pair and f pair may be viewed as pair-related transport numbers of cations and anions, respectively. Figure 4b displays the above fractional components or transport/transference numbers as a function of temperature cat for PEO5.5EMImI. The finding that f an pair is larger than f pair reflects the fact that Dan * is smaller than Dcat * . Moreover, the
three chosen temperatures, i.e., 75, 90, and 105 °C. The data show a monotonic increase of σ− with increasing salt concentration, i.e., for 90 °C from 1.3 × 10−4 to 6.0 × 10−3 S cm−1. This represents a gain in the iodide transport capacity of about a factor of 50. Moreover, the steep salt-poor regime and the less steep salt-rich regime are connected by a plateaulike intermediate σ− range with an inflection near y = 10. This phenomenon appears within the same narrow composition 5697
DOI: 10.1021/acs.jpcb.5b01113 J. Phys. Chem. B 2015, 119, 5693−5700
Article
The Journal of Physical Chemistry B interval also showing salient features in Tg and ΔNE (cf. Figures 1 and 5). We note that σ− can also be calculated from the wellknown expression σ− = t−σ, yielding identical results. eff Furthermore, decomposition of Dan into its constituting − quantities leads to the Nernst−Einstein-type relation σ− = (e2/(kBT))Can−Dan−, with the concentration of unpaired anions Can− and the true diffusivity Dan−.
relate to the changeover from IL-in-polymer to polymer-in-IL and the occurrence of a pronounced intermediate-range order37 in this domain. Altogether, it may be stated that ion association leads to more than 50% loss of the potential (an)ionic charge transport capacity in PEOyEMImI complexes. Another factor influencing electrolyte performance is the transference number of interest. In the present context, this is the iodine-related transference number t−. Our results in Figures 4b and 5 show that t− attains only 37% of its maximum value, virtually independent of temperature and composition. So, in the PEO−EMImI system there is no potential gain by optimizing the anionic transference number. We further note * by about 20% that t− is smaller than the transport number tan (cf. Figure 5), which is due to ion pairing. The present t− values are distinctly lower than those commonly observed in polymer electrolytes based on alkali metal cations. For these classical systems, Li+ or Na+ transference numbers on the order of 0.3 or less have been reported,1,17,33 which implies that t− amounts to 0.7 or more. However, the amount of alkali metal salt is strongly limited in these cases, as typically indicated by composition parameters y > 10. Beyond the corresponding salt concentration, the precipitation of salt-rich crystallites leads to a strong decrease of the conductivity. Thus, the relatively low t− value in the present PEO−IL system is overcompensated by the lack of other, more severe disadvantages. Altogether, the crucial parameter for maximizing transport of negative charge carriers in PEO−EMImI is the salt concentration. Increasing the EMImI fraction leads to an increase of the anion density comprising both charged and uncharged ionic species. Fortunately, because Tg, f an pair, and t− do not greatly change upon EMImI addition, there is virtually no penalty on salt addition, and every extra IL molecule improves electrolyte performance. This favorable feature is borne out by the monotonic increase of σ− with ws in Figure 6. Remarkably, σ− increases more strongly than Cs, which is also depicted in Figure 6. Because σ− is proportional to Cs and Deff an−, this effect an must be attributed to Deff an (cf. eq 13). Indeed, an−= (1 − f pair)D* close inspection reveals that the decrease of f an pair from 0.83 to 0.54 (cf. Figure 5) fully explains the additional enhancement of 1 − f an pair, and hence σ−, by a factor of ∼3, whereas D* an remains almost constant as mentioned above. In DSSCs, charge transfer is accomplished by electron transport via the I−/I−3 redox couple.12−14 This involves the net reaction I−3 + 2e → 3I− at the back electrode and the reverse reaction near the front electrode in order to promote electrons to the ionized dye molecules. Hence, opposite fluxes of I− and I−3 occur during solar-cell operation. To our knowledge, there is little published work providing a reliable separate determination of DI− and DI−3 . (Ion pairing is neglected so that no distinction is made between effective and true diffusivities.) Available data indicate that DI− and DI−3 do not differ by more than a factor of 2 in either direction.39 Commonly, it is assumed either that the overall process is rate limited by the larger I−3 anion with its lower abundance of at least a factor of 4 compared to I− or that both diffusion coefficients are equal.40 Regardless, it may be expected that the magnitude of DI−3 is closely related to that of DI−, whereas there is evidence that the bulky I−3 anion has a lower tendency to pair formation.17,18 We believe therefore that the present results are relevant to the development of electrolytes for DSSCs.
4. DISCUSSION In discussing the present results from a more general viewpoint, we may distinguish between different factors promoting suitable ionic transport properties in electrolyte systems. These factors include a high glass transition temperature, a high salt concentration, little ion association, and a high transference number. The present paper provides, so far known for the first time, reliable data of all these crucial quantities for a polymer−IL system over an extremely wide composition range as a function of temperature. This was achieved by measuring Tg and the diffusion coefficients Dσ, Dcat * , and Dan * of PEO− EMImI complexes of variable IL content. The measured T g values are near −50 °C for all compositions, with deviations of about 4 °C in both directions (cf. Figure 1). This narrow interval for a polymer−IL system contrasts with the wide Tg ranges found for related polymer− inorganic-salt systems.5,10 In all these cases, a distinct maximum of Tg for some intermediate composition is observed. Such Tg maxima are often accompanied by minima in the compositiondependent conductivity at fixed temperatures,5,10 because ionic mobilities are inversely correlated with Tg. In this work we do not see such pronounced Tg-related effects (cf. Figures 2 and 6), which may be understood from the small Tg variation on the whole. However, a better measure of ionic mobility is the corresponding diffusivity because diffusion coefficients, unlike σ or σ−, do not involve the ionic concentration (volume density). As indicated before, Dcat * and Dan * show a very weak composition dependence, also reflected by virtually constant t*an data (Figure 5), whereas the change of Dσ with ws is moderate (by a factor of 4 or less, depending on temperature) and almost monotonic. Altogether, the weak Tg dependence on IL content complies with the observed diffusion behavior. Given a certain salt concentration, the potential charge transport capacity may be strongly reduced because of ion association, predominantly pair formation.37,38 In PEO− EMImI complexes, the ion-pairing effect leads to σ losses varying by about 50−80% depending on ws, as indicated by ΔNE in Figure 5. A separate consideration for the anions yields, through f an pair in Figure 5, a slightly worse figure of merit regarding ion pairing. It should be emphasized that ΔNE and f an pair take into account the combined effect of pair abundance (rpair) and pair mobility (Dpair). In salt-in-polymer electrolytes, the fractional pair components commonly increase both with increasing T and decreasing Cs.29,33 These reverse trends in temperature and salt concentration are demonstrated by Figure 4b and by the left-hand side of Figure 5, respectively. Reversed ion-association behavior in salt-poor electrolytes relates to a negative entropy of mixing caused by constraints in polymer conformation.4,33 We now find that the reverse f an pair trend with T for fixed Cs (or ws) is maintained over the entire composition range (not shown here). However, the reverse f an pair trend with Cs for fixed T is interrupted by the normal (weak electrolyte) composition dependence, as visible at the right-hand slope of the minimum in Figure 5. The reasons for this normal behavior at intermediate compositions is presently not clear, but it may 5698
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(9) Tiyapiboonchaiya, C.; MacFarlane, D. R.; Sun, J.; Forsyth, M. Polymer-in-Ionic-Liquid Electrolytes. Macromol. Chem. Phys. 2002, 203, 1906−1911. (10) Angell, C. A.; Liu, C.; Sanchez, E. Rubbery Solid Electrolytes with Dominant Cationic Transport and High Ambient Conductivity. Nature 1993, 362, 137−139. (11) Forsyth, M.; MacFarlane, D. R.; Hill, A. J. Glass Transition and Free Volume Behaviour of Poly(acrylonitrile)/LiCF3SO3 Polymer-inSalt Electrolytes Compared to Poly(ether urethane)/LiClO4 Solid Polymer Electrolytes. Electrochim. Acta 2000, 45, 1243−1247. (12) Fei, Z.; Kuang, D.; Zhao, D.; Klein, C.; Ang, W. H.; Zakeeruddin, S. M.; Grätzel, M.; Dyson, P. J. A Supercooled Imidazolium Iodide Ionic Liquid as Low-Viscosity Electrolyte for Dye-Sensitized Solar Cells. Inorg. Chem. 2006, 45, 10407−10409. (13) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Grätzel, M. High-Performance Dye-Sensitized Solar Cells Based on Solvent-Free Electrolytes Produced from Eutectic Melts. Nat. Mater. 2008, 7, 626−630. (14) Song, D.; Cho, W.; Lee, J. H.; Kang, Y. S. Toward Higher Energy Conversion Efficiency for Solid Polymer Electrolyte DyeSensitized Solar Cells: Ionic Conductivity and TiO2 Pore-Filling. J. Phys. Chem. Lett. 2014, 5, 1249−1258. (15) According to the lot specifications, the PEO base material typically contained 1.4 wt % SiO2 and 0.2 wt % CaO. These contaminations are inherent to the manufacturing process and present in the form of small particles. (16) Obeidi, S.; Zazoum, B.; Stolwijk, N. A. Ionic Tracer Diffusion and DC Conductivity in the Polymer Electrolyte PEO30NaI. Solid State Ionics 2004, 173, 77−82. (17) Call, F.; Stolwijk, N. A. Impact of I2 Additions on Iodide Transport in Polymer Electrolytes for Dye-Sensitized Solar Cells: Reduced Pair Formation versus a Grotthuss-Like Mechanism. J. Phys. Chem. Lett. 2010, 1, 2088−2093. (18) Call, F.; Stolwijk, N. A. Correction to “Impact of I2 Additions on Iodide Transport in Polymer Electrolytes for Dye-Sensitized Solar Cells: Reduced Pair Formation versus a Grotthuss-Like Mechanism”. J. Phys. Chem. Lett. 2010, 1, 3213−3213. (19) Stolwijk, N. A.; Wiencierz, M.; Heddier, C.; Kösters, J. What Can We Learn from Ionic Conductivity Measurements in Polymer Electrolytes? - A Case Study on Poly(ethylene oxide) (PEO)-NaI and PEO-LiTFSI. J. Phys. Chem. B 2012, 116, 3065−3074. (20) Tanner, J. E. Use of the Stimulated Echo in NMR Diffusion Studies. J. Chem. Phys. 1970, 52, 2523−2526. (21) Stolwijk, N. A.; Wiencierz, M.; Fögeling, J.; Bastek, J.; Obeidi, S. The Use of Radiotracer Diffusion to Investigate Ionic Transport in Polymer Electrolytes: Examples, Effects, and Their Evaluation. Z. Phys. Chem. (Muenchen, Ger.) 2010, 224, 1707−1733. (22) Every, H. A.; Bishop, A. G.; MacFarlane, D. R.; Orädd, G.; Forsyth, M. Transport Properties in a Family of Dialkylimidazolium Ionic Liquids. Phys. Chem. Chem. Phys. 2004, 6, 1758−1765. (23) Bastek, J.; Stolwijk, N. A.; Köster, T. K.-J.; van Wüllen, L. Systematics of Salt Precipitation in Complexes of Polyethylene Oxide and Alkali Metal Salts. Electrochim. Acta 2010, 55, 1289−1297. (24) Fögeling, J.; Kunze, M.; Schönhoff, M.; Stolwijk, N. A. ForeignIon and Self-Ion Diffusion in a Crosslinked Salt-in-Polymer Electrolyte. Phys. Chem. Chem. Phys. 2010, 12, 7148−7161. (25) Stolwijk, N. A.; Kösters, J.; Wiencierz, M.; Schönhoff, M. On the Extraction of Ion Association Data and Transference Numbers from Ionic Diffusivity and Conductivity Data in Polymer Electrolytes. Electrochim. Acta 2013, 102, 451−458. (26) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 1. Variation of Anionic Species. J. Phys. Chem. B 2004, 108, 16593−16600. (27) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. Physicochemical Properties and Structures of Room Temperature Ionic Liquids. 2. Variation of Alkyl Chain Length in Imidazolium Cation. J. Phys. Chem. B 2004, 109, 6103−6110.
5. CONCLUSIONS We have investigated mass and charge transport in the polymer−IL system PEOyEMImI as a function of temperature over the wide composition range from 8.3 (%) to 90 wt % IL. The overall ionic conductivity was compared with the individual diffusivities of cations and anions using a unique combination of methods involving impedance spectroscopy, PFG-NMR, and radiotracer diffusion. Remarkably, we found that the charge diffusivity shows a much stronger variation with concentration than the ionic diffusion coefficients, which can be explained by cation−anion association. A maximum in the glass transition temperature and a local minimum in the ion-pairing tendency are observed at intermediate compositions (y ≈ 10 and ws ≈ 0.3 wt %), which seems to characterize a transition from the IL-in-polymer to the polymer-in-IL regime. Surprisingly, the anion transference number remains constant at a value of 0.37 for all compositions. By contrast, the partial conductivity of iodine anions monotonically increases with increasing IL concentration over almost 2 orders of magnitude, which is even stronger than the increase of the anion density. These results are relevant to the application of polymer−IL electrolytes in dye-sensitized solar cells based on the I−/I−3 redox couple.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +49 (0)251 8339013. Fax: +49 (0)251 8338346. Email: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft. REFERENCES
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