Pt Electrodes for Ionic Liquids

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Correlating the Ionic Drift under Pt/Pt Electrodes for Ionic Liquids Measured by Low-Voltage Electrophoretic NMR with Chronoamperometry Kikuko Hayamizu*,† and Yuichi Aihara‡ †

National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Center 5, Ibaraki 305-8565, Japan, and Samsung Yokohama Research Institute, 2-1-11 Senba-nishi, Minoshi, Osaka 562-0036, Japan

‡

ABSTRACT The current density under an electric field is usually measured by chronoamperometry. After the application of an electric field, electric double layers are immediately formed on the electrodes and the current spike decays quickly. The thickness of the electrical double layers is thin and composed of several ion layers. Ions in the bulk electrolyte remote from the electrodes are supposed to scarcely migrate. In this study, ion migration in the bulk for ionic liquids was measured by using a modified electrophoretic NMR (ENMR) or lowvoltage ENMR sequence in which a delay time is introduced after the application of the static electric field to induce the ion migration prior to the commencement of the pulsed gradient spin-echo NMR measurements. This is the first attempt to measure the ion migration in the bulk under the influence of small electric fields similar to those used in electric devices. SECTION Electron Transport, Optical and Electronic Devices, Hard Matter

oom-temperature ionic liquids (ILs) are composed of only anions and cations, have large ionic conductivity, and are stable over wide temperature ranges. ILs have been used in electrolytes of electric double layer capacitors and are important candidates for use as electrolytes in lithium batteries and solar cells.1 Pulsed gradient spin-echo (PGSE) NMR is a powerful analytical technique for probing the properties of ILs. The self-diffusion coefficients measured by PGSE NMR can be connected to ionic conductivities through the Nernst-Einstein (NE) equation. Experimentally, we have examined the NE relation for organic electrolytes near infinitesimal concentration2 and in neat ILs.3 Under an electric field X (V cm-1), the flux of the ions, (i.e., the current density J), is given as

R

J ¼

X

Ji ¼

X

i

i

zi Fci μi X ¼

X

zi Fci νi

ions in a lithium battery model system, lithium bis(trifluoromethanesulfonyl)amide (LiN(SO2CF3)2 or Li-TFSA) in an organic solvent under electric fields below 3 V cm-1 by using an NMR cell with Li/Li electrodes where the current density was on the order of 10-3 A cm-2 measured by chronoamperometry (CA).11 In this study, using an NMR cell with Pt/Pt blocking electrodes (Figure 1), the strength of the applied electric field was limited to less than 3 V cm-1 to prevent decomposition of the ILs. The present setup can serve as a model for electric double layer capacitors. Since the low electric fields induced negligible changes in the PGSE-NMR spectra by the ENMR pulse sequence, we modified the conventional ENMR pulse sequence to include a delay time, tdelay to allow sufficient ion migration as shown in Figure 2. Stable ILs were chosen to test the modified ENMR pulse sequence. 1-Ethyl-3-methylimidazolium TFSA (emimTFSA) is a typical and popular IL with relatively low viscosity (27.3 mPa s), ionic conductivity of 9.8 10-3 S cm-1, and self-diffusion coefficients of Demim = 6.2 10-11 m2 s-1 and DTFSA = 3.7 10-11 m2 s-1) at 30 °C. In addition, a higher viscosity IL (56.7 mPa s), N,N-diethyl-Nmethyl-N-(2-methoxyethyl)ammonium (DEME)-TFSAwith an ionic conductivity of 3.2 10-3 S cm-1 and self-diffusion coefficients of DDEME = 2.25 10-11 m2s-1 and DTFSA = 1.98 10-11 m2s-1 at 30 °C was also measured. Under the present experimental conditions, electric double layers are formed on the surface of electrodes, and the

ð1Þ

i

where i is the ionic species, z is the charge, F is the Faraday constant (= 95485 C mol-1), c is the ion concentration (mol cm-3), μ is the mobility (cm2 s-1 V-1), and ν is the drift velocity (cm s-1) in the direction of the applied electric field.4 Electrophoretic NMR (ENMR) has been used to measure ionic drift in predominantly aqueous systems under large electric fields from 10 to 100 V cm-1 by using electrodes such as Pt/Pt.5-7 In such measurements, gases are generated near electrodes due to the electrolysis of water. So far, the current densities estimated from ENMR data and eq 1 are quite large on the order of amperes per square centimeter and may be relevant to various fuel cells used in high power electrochemical devices.8-10 Previously, we observed the acceleration of lithium

r 2010 American Chemical Society

Received Date: May 10, 2010 Accepted Date: June 15, 2010 Published on Web Date: June 18, 2010

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DOI: 10.1021/jz100595f |J. Phys. Chem. Lett. 2010, 1, 2055–2058

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Figure 3. Diffusion plots of TFSA (19F NMR) of emimTFSA with increasing electric field from 0 to 3 V at 30 °C. The tdelay was 100 ms, g = 0.96 T m-1, δ = 0.5-4 ms, Δ = 50 ms. The apparent diffusion constants were 5.1, 5.3, 6.9, and 9.5 10-10 m2 s-1 under an electric field of 0, 1, 2, and 3 V cm-1, respectively. The inset shows the voltage dependence of the apparent diffusion constant.

Figure 1. NMR cell with Pt/Pt electrodes. The target area of the NMR measurements is the bulk without including the electric double layers of about 1-100 nm thickness.

upon acquisition of only the real component, will appear as a cosine modulation.5-7 EX ¼ cosðγgνδΔÞ EX ¼ 0

PGSE attenuation profiles obtained in the presence of increasing electric field strength are shown in Figure 3. In contrast to the predictions of eq 2, no complex phase modulation of the echo signals was observed. Thus, the effects of the low-voltage electric field appear as an enhanced diffusion constant rather than an expected drift velocity. If the data in Figure 3 were analyzed assuming that the applied electric field results in a drift velocity using eq 3, the ν was determined to be 10-5 to 10-6 ms-1, which is in agreement with literature values for ILs.12 Substitution of these values into eq 1 results in a current density JNMR about 103 larger than the CA-derived current density JCA. The large discrepancy may originate from the usage of eq 3. To investigate the Δ dependence of the apparent enhanced diffusion induced by the electric field, the tdelay was set to 0 (as in conventional ENMR), and the attenuation of the echo signals was plotted following eq 2 without inclusion of the drift term, since the experimental echo signals did not show any phase distortion. The effect of the applied potential on the signal attenuation was obtained as the difference of the apparent diffusion constants with and without the electric field. The apparent enhanced diffusion constants of the cation DEME and the anion TFSA by varying Δ from 20 to 600 ms (longer Δ resulted in small signal-to-noise (S/N) ratios that prevented proper PGSE measurements) are shown in Figure 4. As Δ increased up to about 300 ms, both the anion and cation migrations increased and then slowed down. On the other hand, JCA reached a maximum in less than 1 ms after the application of electric field and then decayed quickly due to the formation of electric double layers. Clearly a time interval (tdelay in the pulse sequence) is necessary to initiate the ion

Figure 2. The pulse sequence for the low-voltage ENMR including a delay time after application of electric field prior to the PGSENMR measurement. In the conventional ENMR pulse sequence, tdelay = 0.

CA-determined current density decays very quickly. In this situation, eq 1 is not always valid. An apparent enhancement of the diffusion constants depending on the delay time was observed in the PGSE-NMR experiment. It is difficult to connect the phenomena under low-voltage electric field to drift velocity in the direction of the applied electric field, because the induced flux includes diffusion, forced convection, and ion migration exchanging with all the electrolytes. Here we assume a pseudomobility as the sum of the potential-enhanced diffusion constants of the cation and anion under low-voltage electric fields and show its time dependence. Including the effects of drift and diffusion, the attenuation of the echo-signal is given by ! δ expðiγgvδΔÞ E ¼ exp - γ δ g D Δ 3 2 2 2

ð2Þ

where γ is the gyromagnetic ratio. The first term states that the diffusive motion will lead to single exponential attenuation of the signal, whereas the drift (i.e., the second term) leads to a complex modulation of the signal amplitude, which,


ð3Þ

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migration in the bulk, which could otherwise only be observed with the longer Δ values in ENMR measurements. It is certain that we are measuring the time-dependent transient effects in

the apparently enhanced diffusion constants. To ensure measurement of the same motional process, Δ was fixed at 100 ms, and the tdelay was systematically changed from 0 to 2 s to measure the time dependence of ion migration after the application of the electric field. The enhanced values of the anions and cations versus tdelay showed similar behaviors under the same measuring conditions. Hence we assume that the potential-enhanced apparent diffusion constants (m2 s-1) may be connected to the mobility μ (m2 s-1 V-1) in eq 1. However, as shown in Figure 3, the apparent diffusion constants did not change linearly with the applied electric field X (V m-1), which implies that the drift velocity ν (m s-1) does not equal μX and thus eq 1 cannot be used. This might be caused by changes in potential distribution arising from the polarization process in bulk electrolytes. Inserting the NMRderived mobilities into eq 1 gave current densities comparable to those observed with the CA. Phenomenologically, we assigned the sum of the enhanced diffusion constants of the cation and anion to “pseudomobility” under a given electric field. In Figure 5, the pseudomobility is plotted against the delay time under 2 and 3 V cm-1, and for reference the CA current density is also shown for the two ILs. The peak maximum data are summarized in Table 1. The calculation method of the estimated current density based on NMR will be published elsewhere. The estimated maximum current densities for emimTFSA were 9 and 13.8 mAcm-2 under 2 and 3 V cm-1, respectively, and the electrochemical values of DEME-TFSA were 3.2 and 4.9 mA cm-2 soon after the application of electric field (∼ 0.2 ms). The CA current densities in emimTFSA decayed very quickly and were only 1.1 mA cm-2 at 300 ms under 3 V cm-1. Although the current densities derived from the NMR measurements are a little lower than those derived from CA, the trends in the CA and NMR data are consistent with

Figure 4. (a) The Δ dependence of the apparent enhanced diffusion constants of DEME and TFSA in DEME-TFSA at 40 °C under 2 V cm-1 with tdelay = 0. (b) The current density for DEME-TFSA measured by CA from 1 to 3 V cm-1 at 30 °C.

Figure 5. Pseudomobility (upper) and the CA current density (lower) plotted versus time under the electric field of 2 (squares) and 3 (circles) V cm-1 at 30 °C for (a) emimTFSA and (b) DEME-TFSA.


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Table 1. Current Density and Delay Time of the Peak Maxima Measured by CA and Low-Voltage ENMR CA

low-voltage ENMR

sample ionic liquid

electric field V/cm

J (mA cm-2)

time at peak (ms)

estimated J (mA cm-2)

pseudomobility (10-11 m2 s-1)

time at maxium (ms)

emimTFSA

3 2

17.1 14.2

0.5 0.3

49 33

34 6.8

∼ 50 ∼ 50

DEME-TFSA

3 2

7.7 5.0

0.3 0.3

19 13

3.2 2.0

∼ 50 ∼ 50

each other. It must be noted that, when tdelay is 0 and Δ is 20-30 ms, the pseudomobility is negligible. The pseudomobility in Figure 5 reflects the potential-induced ion migration in the bulk. The ion migration is accelerated slowly in the bulk after the formation of the electric double layers with the maxima appearing after about 50 ms. The much smaller CA current densities and the slower decay in the bulk for DEME-TFSA can be explained by the lower viscosity. It is noted that the cosine term (real part) in eq 2, i.e., cos(γgνδΔ) in eq 3, is 1 because of the small ν. Conclusively, attainment of ion migration in the bulk under a direct electric field (less than 3 V cm-1) is not instantaneous. The acceleration of the ion migration under an applied electric field was measured using low-voltage ENMR including a delay time. The estimated current density from the NMR measurements was of comparable magnitude to that derived from the electrochemical current density measurements. The ion migration in the bulk lags after the application of the electric field. The concentration gradients induced at the surface of the electrodes move to the bulk to bring the enhancement in the diffusion measured by NMR and decay slowly depending on the viscosity of the electrolytes. The NMR cell with Pt/Pt electrodes (Figure 1) was supplied by JEOL, Tokyo. Using a JEOL NMR probe equipped with pulsed field gradient coils, a current amplifier, and an apparatus to apply an electric field, NMR spectra were measured by using a Tecmag Apollo at 270.17 MHz for cations (1H) and 254.18 MHz for anions (19F) from 30 to 50 °C. By using the sequence in Figure 2, the PGSE measurements were performed by varying the gradient pulse duration (δ) from 0.1 to 2 ms. The gradient amplitude (g) was set from 0.5 to 1 T m-1, the duration between the leading edges of the gradient pulses, Δ, was set to a value in the range of 20-600 ms, and tdelay was varied from 0 to 2 s. CA was measured using an AUTOLAB PGSTAT30 controlled by a personal computer with the NMR cells contained in a temperature chamber (ESPEC TH-241) at 30 °C. At least 1 h was allowed for temperature equilibration prior to measurement. Potential differences from 1.0 to 3.0 V in 0.5 V increments were applied across the two platinum electrodes in the cell (Figure 1), and the current profiles were recorded. Sufficient time to achieve the rest potential was allowed in each measurement for the electric double layer, which was formed by the application of the electric field to relax and thereby avoiding oxidative/reductive decomposition of the samples.

ACKNOWLEDGMENT The authors heartily thank W. S. Price for sincere discussion on the phase modulation and apparent enhanced diffusion under the electric field.

REFERENCES (1)

Ohno, H., Ed. Electrochemical Aspects of Ionic Liquids; John Wiley & Sons, Inc.: Hoboken, NJ, 2005. (2) Aihara, Y.; Sugimoto, K.; Price, W. S.; Hayamizu, K. Ionic Conduction and Self-Diffusion near Infinitesimal Concentration in Lithium Salt-Organic Solvent Electrolytes. J. Chem. Phys. 2000, 113, 1981–1991. (3) Tokuda, H.; Tsuzuki, S.; Susan, M. A. B. H.; Hayamizu, K.; Watanabe, M. How Ionic are Room-Temperature Ionic Liquids? An Indicator of the Physicochemical Properties. J. Phys. Chem. B 2006, 110, 19593–19600. (4) Bockris, J. O'M.; Reddy, A. K. N. Modern Electrochemistry 1, Ionics; Plenum Press: New York, 1998; Vol. 1. (5) Holz, M., Field-Assisted Diffusion Studied by Electrophoretic NMR. In Diffusion in Condensed Matter; Heitjans, P., K€ arger, J., Eds.; Springer: Berlin, 2005. (6) Griffiths, P. C. Electrophoretic NMR-Ions, Molecules, Mixtures, Pores and Complexes. Annu. Rep. NMR Spectrosc. 2009, 65, 139–159. (7) Hallberg, F.; Fur o, I.; Yushmanov, P. V.; Stilbs, P. Sensitive and Robust Electrophoretic NMR: Instrumentation and Experiments. J. Magn. Reson. 2008, 192, 69–77. (8) Lee, W. Y.; Wee, D.; Ghoniem, A. F. An Improved OneDimensional Membrane-Electrode Assembly Model to Predict the Performance of Solid Oxide Fuel Cell Including the Limiting the Current Density. J. Power Sources 2009, 186, 417–427. (9) Baker, D. R.; Caulk, D. A.; Neyerlin, K. C.; Murphy, M. W. Measurement of Oxygen Transport Resistance in PEM Fuel Cells by Limiting Current Methods. J. Electrochem. Soc. 2009, 156, B901–B1003. (10) Scott, K.; Taama, W. M.; Argyropoulos, P.; Sundmacher, K. The Impact of Mass Transport and Methanol Crossover on the Direct Methanol Fuel Cell. J. Power Sources 1999, 83, 204–216. (11) Hayamizu, K.; Seki, S.; Miyashiro, H.; Kobayashi, Y. Direct In Situ Observation of Dynamic Transport for Electrolyte Components by NMR Combined with Electrochemical Measurements. J. Phys Chem. B 2006, 110, 22302–22305. (12) Umecky, T; Saito, Y; Matsumoto, H. Direct Measurements of Ionic Mobility of Ionic Liquids using the Electric Field Applying Pulsed Gradient Spin-Echo NMR. J. Phys. Chem. B 2009, 113, 8466–8468.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: hayamizu.k@ aist.go.jp.


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Pt Electrodes for Ionic Liquids

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