Article pubs.acs.org/JPCC
Electrochemical, Transport, and Spectroscopic Properties of 1‑Ethyl3-methylimidazolium Ionic Liquid Electrolytes Containing Zinc Dicyanamide T. J. Simons,*,† P. C. Howlett,† A. A. J. Torriero,† D. R. MacFarlane,‡ and M. Forsyth† †
The Australian Research Council (ARC) Centre of Excellence for Electromaterials Science (ACES), Institute for Frontier Materials (IFM), Deakin University Burwood Campus, Burwood 3125, Australia ‡ The Australian Research Council (ARC) Centre of Excellence for Electromaterials Science (ACES), School of Chemistry, Monash University, Clayton 3800, Victoria, Australia ABSTRACT: The electrochemical, physical, and transport properties of 1-ethyl-3methylimidazolium dicyanamide [emim][dca] containing varying levels of Zn2+ and H2O have been investigated to evaluate the suitability of dicyanamide (dca−) ionic liquids (ILs) for applications in secondary zinc−air batteries. Electrochemical experiments indicate that [emim][dca] containing Zn(dca)2 can support high current density Zn0/Zn2+ deposition and stripping, at Zn(dca)2 concentrations up to 40 mol % and H2O concentrations of 10 wt %. Physical property experiments show that good conductivities can be obtained in these IL/Zn2+ systems. The [emim][dca] containing 30 mol % Zn(dca)2 electrolyte displayed higher conductivities (15 mS·cm−1 at 25 °C) than the same IL containing 30 mol % ZnCl2 (12 mS·cm−1 at 25 °C). A significant difference in the solvation environments of the Zn2+ in the two IL/zinc salt mixtures has been suggested to be the cause of the significant improvement in physical and electrochemical properties of [emim][dca] containing Zn(dca)2 over the more extensively studied ZnCl2.
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solar cells.8 ILs can have extremely low vapor pressuresdue to the fact they are composed entirely of charged species; this potentially eliminates the evaporation issues that are experienced with aqueous electrolytes.9 ILs are conductive, display relatively good electrochemical stability in many cases, and have proven to be excellent media for the electrodeposition of metals such as zinc.10 ILs containing the Lewis base anion dicyanamide have been a major focus of recent studies involving Zn2+ electrochemistry11 due to the fact that dicyanamide ILs display some of the lowest viscosities and highest conductivities of any of the major families of aprotic ILs.12 Deng et al.11b have shown that quasi-reversible Zn2+/Zn0 electrochemistry could be supported by 1-butyl-1-methylpyrrolidinium dicyanamide ([C4mpyr][dca]) when ZnCl2 was dissolved into the IL; however, only modest current densities were obtained when zinc was deposited on a magnesium alloy (AZ91D) electrode at potentials as negative as −2.3 V vs Fc0/+. Similar results have been previously observed for ZnCl2 dissolved in [emim][dca] on a glassy carbon electrode.11a Recently, we have demonstrated that the nature of the anion in the Zn2+ salt added to [emim][dca]shown in Figure 1 had a dramatic effect on the Zn2+ electrochemistry; when Zn(dca)2 was used in place of ZnCl2, higher currents and more
INTRODUCTION Intermittent renewable power sources are becoming increasingly prominent in global energy generation, increasing the need for the development of an efficient, affordable, and highcapacity rechargeable energy storage device for both mediumand large-scale applications.1 Current lithium-ion batteriesas well as all other commercially available secondary battery technologieshave generally been identified as not meeting the economic requirements of large-scale energy storage due to high material costs or environmental concerns.2 Zn−air secondary batteries are one of a number of battery technologies under investigation that have the potential to meet these demands. Currently limited to primary devices, commercial Zn−air batteries display high energy density (500 W·h·kg−1 vs 250 W·h·kg−1 for commercial Li-ion),3 utilize inexpensive materials (zinc metal and oxygen from the atmosphere), and have a relatively low cost per unit of energy stored.2 Currently, these low-voltage batteries (1.2 V) are not available as a rechargeable technology due to a number of issues encountered during cell recharge, including dendritic zinc deposition, loss of the aqueous electrolyte through evaporation, and the reversibility of the oxygen reduction reaction (ORR).4 Recent studies have investigated the possibility of replacing the current highly basic aqueous electrolytes (14 M KOH) with ionic liquid (IL) electrolytes.5 ILs have previously been applied in a variety of electrochemical devices such as lithium and lithium-ion batteries,6 magnesium cells,7 and dye sensitized © 2013 American Chemical Society
Received: December 3, 2012 Revised: January 20, 2013 Published: January 22, 2013 2662
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platinum wire (APS, 99.95%) counter electrode, and a reference electrode consisting of a silver wire immersed in a solution of the IL containing 100 mM silver triflate (AgOTf), separated from the bulk solution by a glass frit. However, all potentials quoted are relative to the ferrocene/ferrocinium potential scale (approximately 0.45 V vs Ag/AgOTf (100 mM)). All voltages discussed will be referenced to this internal standard. Prior to each experiment, the working electrodes were polished with 0.05 μm alumina (Buehler, Lake Bluff, IL) on a clean polishing cloth (Buehler), sequentially rinsed with distilled water and acetone and dried with a stream of dry N2 gas. Viscosity. Viscosity measurements were performed on an A&D SV1-A Sine-wave Vibro viscometer using the tuning fork vibration method (30 Hz), which was calibrated to a standard of deionized water. Temperature was monitored by a TH5 Digital Thermoregulator and maintained by a RT7 recirculating tank feeding into a water jacket, which maintained the temperature in the range of 20−70 °C. A dry N2 gas stream was blown over the sample to minimize water absorption. Conductivity. Ionic conductivity was measured by ac impedance techniques using a SP-200 Impedance/Frequency Response Analyzer. The sample size of approximately 2 mL was measured over a temperature range of 25−70 °C at 5 °C intervals for the frequency range 0.1 Hz−1 MHz using an alternating voltage of 0.1 V amplitude. Samples were contained in a custom-built dip-cell probe containing two platinum wires sheathed in glass (Monash Scientific, Australia), sealed with a rubber O-ring, and placed into a cavity in a brass block. The temperature was ramped up at a steady rate of 0.2 °C/min under the control of a Eurotherm 2204E temperature controller to reach the desired isothermal temperature (measured with a Type T thermocouple inserted in the brass block adjacent to the cell). The cell constant was determined using a solution of 0.01 M KCl at 25 °C. Data acquisition commenced following isothermal equilibration for 20 min at each temperature point, and the cell resistance was recorded as a function of frequency. The resistance (Ω) was determined from the value of the real axis touchdown of the Nyquist plot (plotting real impedance (Z′) vs the imaginary impedance (Z″)), from which the conductivity (S·cm−1) was calculated. Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR). ATR-IR experiments were performed on a FT-IR Spectrometer (Varian 3100) with an ATR attachment (Pike, MiRacle) utilizing a diamond prism. Data was collected on Resolutions Pro software. The light path was purged with dry N2 gas for 1 h (2.0 L·min−1) prior to and during the experiments (50 mL·min−1) to remove any CO2 or moisture present. The measurements were averaged over 82 times at a resolution of 2 cm−1. Measurements were made by placing 300 μL of the ionic liquid sample into a Teflon well placed around the ATR crystal using a 1000 μL autopipet (Lab Co.), ensuring a bead was formed uniformly over the crystal, and hence the acquired spectra were not normalized.
Figure 1. Molecular structure of [emim][dca].
positive potentials for overpotential deposition (OPD) could be observed.13 Furthermore, the water content in this Zn2+/IL system was shown to influence the electrochemistry and the deposition morphology, but the Zn(dca)2 based electrolyte still showed the best electrochemical characteristics. One of the chief determinants of electrochemical behavior of a metal ion in a solventfor example, Zn2+ in [emim][dca]is the solvation or coordination environment of the electrochemically active species. It has been postulated that upon addition of ZnCl2 to [C4mpyr][dca] IL, Zn2+ is solvated as a complex anion such as Zn(dca)3−.11b Given the difference in electrochemical behavior observed in the presence of chloride,11b,13 it is likely that the Zn2+ species will in fact be different when only the dca anion is present. To the best of our knowledge, the speciation of Zn2+ with dca− in dicyanamide ILs and its effect on Zn2+ electrochemistry are yet to be explored. This paper investigates the influence of water and zinc salt concentration on the speciation in [emim][dca] electrolyte, containing both ZnCl2 and Zn(dca)2 to better understand why these Zn2+/IL mixtures give such contrasting electrochemical performance.
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EXPERIMENTAL SECTION Materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%, Sigma Aldrich), silver trifluoromethanesulfonate (AgOTf, 99.95%), and sodium dicyanamide (Na(dca), 96%, Alfa Aesar) were used as received. Zinc chloride (ZnCl2, 99.5%) was purchased from Merck chemicals and dried to constant mass in a drying oven at 110 °C for 48 h prior to use. Deionized water was prepared using a Millipore Q3 Reverse Osmosis system. 1-Ethyl-3-methylimidazolium dicyanamide [emim][dca] (99%) was purchased from Merck Chemicals and purified by dissolving in dry dichloromethane under nitrogen gas and filtering the resulting cloudy solution through a 0.2 μm Teflon syringe filter. Zn(dca)2 was synthesized according to literature methods14 by combining aqueous solutions of Zn(NO3)2·6H2O and Na(dca), filtering the resulting white precipitate, washing with distilled water, and drying under high vacuum for 2 days at 40 °C. Solution Preparation. Solutions were prepared as reported previously.13 Samples were prepared by adding 10−40 mol % of dry Zn2+ salt (chloride or dicyanamide) to [emim][dca] IL containing 2000 ppm H2O (0.02 wt %). Accounting for the water content of the IL, the volume of deionized water required to adjust the total H2O content to the desired content was calculated and added using a 10 or 100 μL autopipet (Lab Co.). Water Content. Water content was measured using a Model 756 Karl Fischer Coulometer (Metrohm) using Hydranal Coulomat AG as the titrant. Electrochemistry. Voltammetric experiments were performed on a Biologic VMP3/Z multichannel potentiostat. Uncompensated resistance was measured in a potential region where no faradaic reaction occurs, using the RC time constant method available with the instrument, which was compensated by 85%. Cyclic voltammograms were obtained using a conventional three-electrode arrangement, consisting of a 1 mm glassy carbon (GC, ALS, Japan) working electrode, a
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RESULTS AND DISCUSSION Cyclic Voltammetry. Figure 2 shows voltammograms obtained with the Zn2+/[emim][dca] system containing 3 wt % H2O and different initial concentrations of Zn(dca)2. As previously reported, [emim][dca] containing 10 mol % Zn(dca)2 supports extremely high current densities for zinc deposition and stripping on glassy carbon substrates.13 In the 2663
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different potentials. It could also be possible that this second process (P2)which occurs at potentials approaching the electrochemical limit of the solvent (−2.4 V vs Fc0/+)is in some part due to the reduction of imidazolium cations in the solution. While the experiment containing no Zn2+ salt shows electrochemical stability to −2.4 V vs Fc0/+, the kinetics of solvent reduction may be altered when Zn2+ salts are added to the IL, as has been observed previously.16 When the concentration of Zn2+ is raised to 40 mol %, Zn2+ reduction peaks are broadened with onset at an even more positive potential, R = −1.75 V vs Fc0/+. In the negative scan, two broad reduction peaks can again be observed (P1 = −2.02 V vs Fc0/+ and P2 = −2.15 V vs Fc0/+), which again may be attributed to different zinc morphologies being deposited on the electrode surface or minor decomposition of the solvent during the zinc reduction process. The CV for [emim][dca] containing 3 wt % H2O and 10 mol % Zn(dca)2 indicates that this mixture has electrochemical properties that are favorable, including a reduction onset potential (R = −1.80 V vs Fc0/+) more positive than expected. The single reduction peak (P1 = −1.92 V vs Fc0/+) on the negative scan reaches a current density of 45 mA·cm−2. The scan was reversed at potentials more positive than the other CVs presented as no additional peaks corresponding to zinc reduction were observed. The onset of reduction in 10 mol % Zn(dca)2 (R = −1.80 V vs Fc0/+) does not align with the trend of the other concentrations as it has a more positive onset potential than 20 mol % (R = −1.87 V vs Fc0/+). In all of the CVs presented, oxidation of the reduced zinc metal commences at −1.6 V vs Fc0/+ (O). The magnitude of the peak oxidation current increases in size with increasing concentration of Zn2+ just as it does for the reduction peak, suggesting that the zinc deposits are readily reoxidized into the solution. The observed current densities of up to 100 mA·cm−2 for 20 mol % Zn(dca)2 suggest that if these electrolytes were applied to zinc anode type batteries, high discharge rates would be possible. In a real device open to the air, H2O from the atmosphere may be absorbed by hygroscopic electrolytes. For this reason the effect of water concentration (1, 3, or 10 wt % H2O) on Zn2+/Zn0 electrochemistry in [emim][dca] + 10 mol % Zn(dca)2 has been investigated, and the voltammetry is presented in Figure 3. As the potential is scanned in the negative direction, all three voltammograms show a single clear Zn2+ reduction peak, regardless of the amount of water added
Figure 2. CVs of [emim][dca] + 3 wt % H2O containing varying concentrations of Zn(dca)2. Scan rate: 100 mV·s−1. Substrate: glassy carbon.
CVs presented, the onset potential of reduction is labeled as R. P1 and P2 denote the first and second reduction peak potentials during the negative going potential scan, both of which are observed for the 20 and 40 mol % samples. However, P2 is not clearly observed in the 5 and 10 mol % samples, indicating a difference in the electrochemical activity of zinc at different Zn(dca)2 concentrations. P3 denotes the reduction peak potential for a given scan after the potential scan direction has been reversed, as shown for the 5 mol % sample. Finally, O refers to the onset of oxidation current during the positive potential scan. It can be seen that at a zinc concentration of 5 mol % the onset potential of zinc reduction occurs at R = −2.12 V vs Fc0/+, suggesting that a large overpotential is required to initiate zinc deposition on the glassy carbon substrate. No clear reduction peak is observed on the negative potential scan; however, a broad current is observed. The presence of a broad reduction process at −2.0 V vs Fc0/+marked by P3upon reversal of the scan direction is indicative that zinc continues to be reduced onto the freshly generated zinc metal nucleation sites at lower overpotentials. However, it has also been previously postulated13,15 that this phenomenon may be due to the arrangement of IL anions and cations adjacent to the electrode in a compact layer that can influence the kinetics of oxidation and reduction of zinc. Currently, it is not well understood whether either of these factors are responsible for this “negative peak” behavior. Further work in this area is currently being conducted in our laboratories. When the concentration of Zn2+ is raised to 20 mol %, the onset potential of zinc reduction appears at a more positive potential, R = −1.90 V vs Fc0/+, due to the higher concentration of Zn2+ cations at the electrode surface available for reduction. This availability of Zn2+ is also the cause of the significantly higher observed current densities for both the deposition and stripping processes. Two distinct reduction processes can be observed when the concentration is 20 mol %, during the negative potential scanP1 = −2.02 V vs Fc0/+ and P2 = −2.20 V vs Fc0/+suggesting that two different morphologies or structures of zinc metal are being deposited onto the glassy carbon surface. These different structures may also be indicative of the presence of different chemical environments of Zn2+ in the electrolyte adjacent to the electrode, which are reduced at
Figure 3. CVs of [emim][dca] + 10 mol % Zn(dca)2 containing varying concentrations of H2O. Scan rate: 100 mVs−1. Substrate: glassy carbon. 2664
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(1, 3, or 10 wt %). The onset potential of this Zn2+ reduction process displays a very minor amount of drift, occurring at −1.80 ± 0.02 V vs Fc0/+, which has been attributed to small changes in the surface morphology of the hand polished glassy carbon working electrode. As the potential is scanned back in the positive direction, the onset of zinc oxidation occurs at O = −1.60 V vs Fc0/+ for all three samples. The size of this oxidation peak is similar for the samples containing 1 and 3 wt %. When the H2O concentration is increased to 10 wt %, the oxidation peak is significantly reduced in magnitude, suggesting that the reoxidation of the deposit has been hindered in some way. Despite this, the Zn2+ reduction process on the negative scan seems to be unaffected by large concentrations of water. These results would suggest that while zinc electrochemistry is still possible when high concentrations of water are present, the coulombic efficiency of this process would be reduced. Transport PropertiesViscosity and Conductivity. Two parameters that provide valuable information about the suitability of a candidate electrolyte system for battery applications are viscosity and conductivity, which are closely linked to the transport properties of the anions and cations in the solution. The effect of water content on the viscosity and conductivity of [emim][dca] + 10 mol % Zn(dca)2 is shown in Figure 4. Due to the fact that the viscosity for the [emim][dca] ionic liquid is already very lowreported values are around 21 mPa·s (298 K)12 for the pure ionic liquidthe drastic changes in viscosity observed as a result of water addition to the more viscous ILs such as those based on the phosphonium cation are not observed here.17 Moving from 0.5 to 10 wt % H2O leads to lowering of the viscosity from 33 to 17 mPa·s at 25 °C when the concentration of Zn(dca)2 is 10 mol %, a change of less than half an order of magnitude. Overall, each of the IL solutions obey Arrhenius behavior at ambient temperatures, with deviations from the linear relationship observed at high temperatures, which is consistent with behavior observed in ILs previously.18 The effects of H2O concentration on the conductivity are shown in Figure 4b. At 25 °C, an H2O content of 0.5 wt % yields a conductivity of 21.4 mS·cm−1, whereas when H2O content is increased to 10 wt %, the conductivity increases to 34 mS·cm−1. These changes are most likely related to the increase in fluidity of the solution, rather than a drastic change in speciation. This relationship can be seen clearly in Figure 4c, which graphs these changes as a function of H2O concentration. Figure 5 shows the effects of Zn(dca)2 concentration on the viscosity and density of [emim][dca] containing 3 wt % H2O. In Figure 5a it can be seen that increasing the Zn 2+ concentration leads to an increase in the dynamic viscosity of the solution, from 14.9 mPa·s at 0 mol % Zn(dca)2 to 90.3 mPa·s at 40 mol % Zn(dca)2 at 25 °C. Of particular note is the sharp increase in viscosity when the concentration is raised from 30 to 40 mol % Zn(dca)2, which suggests that between these values there is a change in the intermolecular interactions that are determining the viscosity. In Figure 5b, it can be seen that the increases in viscosity are paired with corresponding decreases in the conductivity. At 25 °C, the conductivity is decreased from 36 mS·cm−1 at 0 mol % to 8.1 mS·cm−1 at 40 mol % Zn(dca)2. It is interesting to note the fact that upon addition of Zn(dca)2 to [emim][dca] there is a significant gain in the ion concentration, yet the conductivity is decreasing. This will be in a large part due to the increase in viscosity. The viscosity itself increases due to the strong interionic associations present when the divalent Zn2+ is added
Figure 4. Transport properties of [emim][dca] + 10 mol % Zn(dca)2 + varying concentrations of H2O: (a) Arrhenius plot of the dynamic viscosity (mPa·s) vs 1000/temperature (K−1); (b) Arrhenius plot of conductivity (mS·cm−1) vs 1000/temperature (K−1) ; (c) dynamic viscosity (mPa·s) and conductivity (mS·cm−1) as a function of H2O concentration (wt %).
to the IL. This association (or complexation) would have an added effect of reducing the effective number of ions in the electrolyte. Furthermore, if a new Zn2+ species is being formed by the solvation of Zn(dca)2 by [emim][dca] it would have a molecular radius larger than the ionic liquid ions. This would also result in slower diffusion through the IL thereby reducing the conductivity. The transport property trends above suggest that while water plays no major role in the speciation of Zn2+ in Zn(dca)2/ [emim][dca] mixtures, the role of the ionic liquid in solvating Zn(dca)2 is of interest and importance to the resulting physical 2665
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Figure 6. Transport properties of [emim][dca] + 3 wt % H2O + different Zn2+ salts: (a) Arrhenius plot of the dynamic viscosity (mPa·s) vs 1000/temperature (K−1) where [Zn2+] = 10 mol % of Zn(dca)2 or ZnCl2: (b) Arrhenius plot of conductivity (mS·cm−1) vs 1000/temperature (K−1) where [Zn2+] = 30 mol % of Zn(dca)2 or ZnCl2.
Figure 6a shows the viscosities of [emim][dca] + 3 wt % H2O containing either 10 or 30 mol % of ZnCl2 or Zn(dca)2. Initially, upon addition of 10 mol % of either salt, the viscosities are very similar across the whole temperature range. This would suggest that either the Zn2+ speciation in each case is identical or that the level of Zn2+ in the solution is too low to differentiate any significant difference between the mixtures. When the concentration of Zn2+ is increased to 30 mol %, it can clearly be seen that [emim][dca] containing ZnCl2 reaches viscosities higher than the Zn(dca)2 mixtures, suggesting a significant difference in the species that exist in the solution. When the corresponding conductivities are considered shown in Figure 6ba trend is observed where at 10 mol % nearly no difference is observed, but at 30 mol % ZnCl2 containing [emim][dca] shows a significantly lower conductivity, consistent with the higher viscosity and the likely different Zn2+ speciation present. FT-IR Spectroscopy. In order to further understand the effect of the added salt on the speciation of Zn2+ in the IL, FTIR (ATR) spectroscopy was used to investigate the IL/Zn2+/ H2O mixtures discussed above. The ATR spectra of [emim][dca] containing 3 wt % H2O and increasing concentrations of Zn(dca)2 in the region 3500−550 cm−1 are displayed in Figure 7. Assignments of the peaks can be found in Table 1, which were allocated with guidance from the studies of Jensen et al.,14 MacFarlane et al.,12 and Hvastijova et al.19
Figure 5. Transport properties of [emim][dca] + 3 wt % H2O + varying concentrations of Zn(dca)2: (a) Arrhenius plot of the dynamic viscosity (mPa·s) vs 1000/temperature (K−1); (b) Arrhenius plot of conductivity (mS·cm−1) vs 1000/temperature (K−1); (c) dynamic viscosity (mPa·s) and conductivity (mS·cm−1) as a function of Zn2+ concentration (mol %).
properties and electrochemical properties. It has been previously reported by the authors that the nature of the Zn2+ salt added to [emim][dca] has a significant effect on the Zn2+/Zn0 reduction onset potential, presumably due to a change in the speciation of Zn2+ in the IL mixture. It has been proposed in the literature11b that upon addition of ZnCl2 to dca− based ionic liquids, Cl− is replaced by three dca− anions as the solvating ligands. In order to investigate this further, the transport properties of [emim][dca] containing 3 wt % H2O and either Zn(dca)2 or ZnCl2 salts have been presented in Figure 6. 2666
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Figure 7. FT-IR (ATR) spectra of [emim][dca] + 3 wt % H2O + varying concentrations of Zn2+ in the range 3500−550 cm−1.
Table 1. Peak Assignments for [emim][dca] + 3 wt % H2O from FT-IR (ATR) Experiments wavenumber (cm−1) ± 2
intensity
assignment
3493 3151 3104 2361 2230 2130 2102 1570 1308 1168 903 845 802 753 650 622 579
w w w w s vs m m m s m m w m m s s
H2O CC−H [emim] C−H [emim] ν(CN) [emim] ν(CN) [dca] νs(CN) [dca] νas(CN) [dca] ν (CC) [emim] νas(C−N) [dca] ν(C−N) [emim] νs(C−N) [dca] C−H [emim] C−H [emim] C−H [emim] C−H [emim] δs(NCN) [dca] δas(NCN) [dca]
Figure 8. FT-IR (ATR) spectra of the ν(CN) and ν(C−N) peaks of [emim][dca] + 3 wt % H2O + varying concentrations of Zn(dca)2 in the range (a) 1450−1250 cm−1; (b) 2400−2050 cm−1.
This behavior is mirrored by the ν(CN) peak of dca− (Figure 8b). Free dca− at 2130 cm−1 rapidly disappears as a new “bound” dca− peak appears at 2169 cm−1, again suggesting a net consumption of free dca− anions in the solution. In this region we have also observed the appearance of a new peak at 2303 cm−1 as the Zn2+ concentration increases. Unlike the previous two examples, this peak does not appear to develop in conjunction with the loss of intensity of any other characteristic peak. It is possible that this peak corresponds to dca− acting as a bridging ligand between two Zn2+ cations, suggesting the existence of larger complex ions of zinc. This feature will be explored further in future work utilizing 13C NMR and Raman spectroscopy to confirm its origins. In order to corroborate the previous data that suggests that H2O does not play a role in Zn2+ speciation in these mixtures, changes in the dca− peaks were monitored with increasing H2O concentrations, shown in Figure 9. In Figure 9a it can be seen that the ν(C−N) peak at 1308 cm−1 which corresponds to the free dca− in solutionundergoes a slight shift to larger wavenumbers, suggesting that it may be having a slight interaction with water molecules in the solution. It also becomes reduced in intensity as the H2O concentration increases, due to the dilution effect of adding H2O to the sample, which reduces the concentration of dca−. Second, the peak at 1369 cm−1which corresponds to dca− coordinated to Zn2+undergoes no major shift (unlike the peak at 1308 cm−1), suggesting the interactions of this dca− with H2O are minor in comparison to the binding interactions with Zn2+. Once again the dilution effects can be observed at 1369 cm−1 as a decrease in peak intensity.
The dicyanamide peaks occur in two prominent regions: the nitrile region of 2400−2100 cm−1 and the C−N region of 1450−1250 cm−1. As all of the possible solvation mechanisms involve dca− in some way, changes in these regions should be indicative of Zn2+ solvation by dca−. Close-ups of the appropriate regions in Figure 7 are shown in Figure 8. In an anionically pure IL/Zn2+ mixture, it is assumed that the only species that will coordinate Zn2+ are dca− and H2O. First, the effect of Zn2+ concentration on these characteristic peaks will be examined. Figure 8a shows that in [emim][dca] + 3 wt % H2O there is a single ν(C−N) peak in the region of 1450− 1250 cm−1, appearing at 1308 cm−1. However, upon addition of Zn(dca)2, a new peak gradually appears at 1369 cm−1. It has been previously shown that shifts to higher wavenumbers for the dca− vibrations generally correspond to binding of the anion to a metal center.19 Therefore, it is proposed here that the growth of this new peak at 1369 cm−1, coupled with the loss of intensity of the peak at 1308 cm−1, corresponding to free dca−, indicates that free dca− in solution is solvating Zn2+ to create a new complex anion. 2667
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polynuclear complexes that may occur via this association is unknown and is currently under investigation. However, it can be concluded that the ATR-IR data suggests that the active species in [emim][dca] + 3 wt % H2O + 10 mol % Zn(dca)2 that is responsible for the favorable observed electrochemistry is most likely a Zn2+/dca− complex ion. It has been postulated previously that this is also the species that is present in dca− ILs that contain ZnCl2, despite the fact it has shown relatively poor electrochemical performance.11 To explore this further, we have compared the ν(CN) peaks of dca− in [emim][dca] containing either Zn(dca)2 or ZnCl2 at 10 and 30 mol %, shown in Figure 10. Figure 10a shows the
Figure 9. FT-IR (ATR) spectra of the ν(CN) and ν(C−N) peaks of [emim][dca] + 10 mol % Zn(dca)2 + varying concentrations of H2O in the range (a) 1450−1250 cm−1; (b) 2400−2050 cm−1.
Similar effects can be observed for the CN peaks shown in Figure 9b. The peak at 2130 cm−1which represents free dca− in solutionagain shows the characteristic shift to higher wavenumbers and dilution effects, while the peak corresponding to bound dca− to Zn2+ shows the same dilution effects but without the obvious interactions with surrounding H2O. These results suggest that H2O is not coordinating to the Zn2+ cations in the solutioneven at H2O concentrations up to 10 wt % as the ratio of bound to free dicyanamide is consistent with that seen in Figure 8b. However, it could be concluded that there are minor interactions involving water and the remaining labile dca− ions. This lack of coordination of the H2O with the Zn2+ cations is consistent with the findings of the cyclic voltammetry experiments, where no major change in electrochemistry was observed that could be indicative of a change in the electroactive Zn2+ species. This hypothesis is also supported by the reported transport properties, where it was shown that the observed increases in conductivity were due only to the corresponding decreases in viscosity caused by the increased water content, rather than any change in the number or nature of charged species. From the data above, it can be concluded that when Zn(dca)2 is added to [emim][dca] containing 3 wt % H2O, the solvation of the salt into the IL undergoes a coordination reaction between free dca− from the IL and the Zn2+ center. The nature of the possible mononuclear, dinuclear, and
Figure 10. FT-IR (ATR) spectra of the ν(CN) peak of [emim][dca] + 3 wt % H2O + Zn(dca)2 and ZnCl2 in the range 2400−2100 cm−1. (a) [Zn2+] = 10 mol %; inset: range 2200−2140 cm−1. (b) [Zn2+] = 30 mol %; inset: range 2200−2140 cm−1 .
resulting peaks at a concentration of 10 mol % Zn2+. Due to the relatively low sensitivity of the instrument, small changes in the magnitude of peaks cannot reliably be taken as an indication of a significant change in real concentration. However, changes in the wavenumber can give valuable information as to the environment of the dca−. At 10 mol %, the peak assigned to labile dca− (2130 cm−1) clearly shows no change in position, suggesting that labile dca− in both situations is unchanged due to the relatively high abundance and low concentration of dopant salt. However, when we examine the peak assigned to bound dca−, there is a significant change in both intensity and wavenumber. Logically, this is due to the fact that the addition of Zn2+ in the form of Zn(dca)2 effectively leads to more bound dca−, thereby increasing the concentration of this form of dca−. This peak also displays a shift of 5 cm−1 to lower wavenumbers when the zinc salt is changed from Zn(dca)2 to ZnCl2, 2668
dx.doi.org/10.1021/jp311886h | J. Phys. Chem. C 2013, 117, 2662−2669
The Journal of Physical Chemistry C
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suggesting that it does not exist in the same chemical environment. When the concentration of Zn2+ is raised to 30 mol % shown in Figure 10bthe labile dca− peak has remained very similar in intensity for both Zn(dca)2 and ZnCl2, suggesting a similar concentration of labile dca−. Equation 1 outlines the previously proposed11b reaction for dca− complexation of ZnCl2 salt in dca− ILs. − − ZnCl 2(S) + 3dca(IL) → Zn(dca)−3(IL) + 2Cl(IL)
(1)
If this were a true indication of the solvation mechanism, a significant proportion of the original dca− would be bound to the Zn2+. This does not appear to be the case as the amount of labile dca− remains similar in both ZnCl2 and Zn(dca)2 containing IL solutions, signified by the similar intensities for both peaks. While the exact composition of the Zn2+ complex ion is not obtainable from the ATR-IR spectra, the evidence from both the IR spectra and the observed electrochemistry support that it is significantly different from the active species that must exist when the dca− IL is doped with Zn(dca)2 instead of ZnCl2.
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CONCLUSIONS [emim][dca] has been used as a model system to evaluate dca− based ILs for their suitability for application in zinc−air secondary batteries. [emim][dca] containing Zn(dca)2 was found to display stable Zn/Zn2+ electrochemistry in high concentrations of H2O and Zn2+ as well as conductivities significantly higher than [emim][dca] containing ZnCl2. Spectroscopic data suggests that the favorable electrochemistry and transport properties are probably due to the formation of Zn/dca− complex anions in the IL/Zn(dca)2 solution. It was also shown that [emim][dca] containing the more common ZnCl2 salt did not display this behavior, with spectroscopic data suggesting the formation of a different complex anion. ILs containing the dca− anion have thus been shown to be viable candidates to further investigate in zinc−air secondary battery applications.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors thank the Australian Research Council for support of this work through the Centre of Excellence for Electromaterials Science (ACES). Professor Forsyth and Professor MacFarlane acknowledge the ARC for the funding of an Australian Laureate fellowship and Federation fellowship, respectively.
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REFERENCES
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