Glycine-based ionic liquids as potential electrolyte for electrochemical studies of organometallic and organic redox couples

Electrochemistry Communications 13 (2011) 237–241

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Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m

Glycine-based ionic liquids as potential electrolyte for electrochemical studies of organometallic and organic redox couples Tzi-Yi Wu a,b, Shyh-Gang Su a, H. Paul Wang c, I-Wen Sun a,⁎ a b c

Department of Chemistry, National Cheng Kung University, Tainan, 70101, Taiwan Department of Polymer Materials, Kun Shan University, Tainan, 71003, Taiwan Department of Environmental Engineering, National Cheng Kung University, Tainan, 70101, Taiwan

a r t i c l e

i n f o

Article history: Received 6 December 2010 Received in revised form 18 December 2010 Accepted 21 December 2010 Available online 31 December 2010 Keywords: Ionic liquid Electrochemical window Diffusion coefficient Stokes–Einstein product

a b s t r a c t Ten ionic liquids (ILs) based on glycine cations with bis(trifluoromethanesulfonyl)amide or alkylsulfate anions as novel electrolytes are prepared and evaluated. The ILs exhibit wide electrochemical windows of 5.0 V and good ionic conductivity (up to 1.44 mS cm− 1 at 35 °C). The Stokes–Einstein products of two organometallic redox couples, Fc/Fc+ (ferrocene/ferrocenium) and Cc/Cc+ (cobaltocene/cobaltocenium), and an organic redox couple TMPD (N,N,N′,N′-tetramethyl-para-phenylenediamine) in the ILs are determined. The solvodynamic radii of Fc, Cc, and TMPD are found to be independent of the types of ILs. © 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Ionic liquids (ILs) have been employed in a wide range of electrochemical applications [1] due to their intrinsic conductivity and wide electrochemical windows. Although ILs with cations derived from imidazolium, pyridinium, quaternary ammonium, and pyrrolidinium have been extensively studied, the development of bio-renewable ILs based on amino acids and their derivatives to replace the above cations is another promising approach because amino acids and their derivatives are the most abundant natural source of quaternary nitrogen precursors. Although many amino acid ILs have been prepared for various applications [2,3], they have not been assessed for electrochemical applications. Amino acid ILs with ester groups have been reported to have low melting points as the result of reduced hydrogen bonding [4]. We thus prepared several glycine-based ILs containing an ester group on the cations, and evaluated their properties for use as electrolytes. Cyclic voltammetry measurements of reference systems, namely two organometallic redox couples, ferrocene/ferrocenium (Fc/Fc+) and cobaltocene/ cobaltocenium (Cc/Cc+), [5] and an organic redox couple, N,N,N′,N′-tetramethyl-para-phenylenediamine (TMPD), [6] in the ILs are studied and compared with other functional ILs reported in the literature.

The structures of the ILs studied in the present work are shown in Table 1. Glycine-based ILs were synthesized using two methods, a simple anion exchange reaction of the corresponding bromide to give bis (trifluoromethanesulfonyl)amide anion and a direct reaction of glycinebased ester with alkylsulfate to give halogen-free ILs [7,8]. All starting materials were purchased from Aldrich or Acros and used as received. The conductivity (σ) of ILs was measured with a conductivity meter LF 340 and a standard conductivity cell TetraCon 325 (WissenschaftlichTechnische Werkstätten GmbH, Germany). The cell constant was determined by calibration using an aqueous 0.01 M KCl solution. Densities of ILs were measured gravimetrically with a 1 mL volumetric flask. Values for the densities are ±0.01 g mL− 1. The viscosities (η) of the ILs were measured using a calibrated airtight Cannon-Fenske glass capillary viscometers (CFRU, 9721-A50) placed in a thermostatic water bath. The melting point of each IL was analyzed with a differential scanning calorimeter (DSC, Perkin-Elmer Pyris 1) in the temperature range of −140 °C to a predetermined temperature at a scan rate of 10 °C min− 1 under nitrogen. The thermal stabilities were measured using thermal gravimetric analysis (TGA, Perkin-Elmer, 7 Series Thermal Analysis System). The sample was heated at 20 °C min− 1 from room temperature to 800 °C under nitrogen. The water content of the dried ILs was detected using a Karl–Fischer moisture titrator (Metrohm 73KF coulometer); the values were less than 250 ppm. For measuring the selfdiffusion coefficient of the cations and anions, a portion of each IL sample was degassed and sealed in a cylindrical Pyrex tube under high vacuum at room temperature. The sealed sample tube was inserted into a standard 5 mm tube filled with an external lock solvent of D2O. NMR

⁎ Corresponding author. E-mail address: [email protected] (I.-W. Sun). 1388-2481/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2010.12.022

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Table 1 Physicochemical and thermal properties of glycine-based ionic liquids. Tg/°C

Ts-s/°C

Tm/°C

Tda/°C

ρb/g cm−3

C/mol dm−3

η/mPa s

σ/mS cm−1

Dcation/10-7 cm2 s−1

Danion/10−7 cm2 s−1

Λimp/S cm2 mol−1

ΛNMR/S cm2 mol−1

Λimp/ ΛNMR

[MGlyA][TFSI]

452.39

− 72

–

26

337

1.4152

3.128

93.3

1.439

1.025

1.456

0.490

0.902

0.543

[MGlyP][TFSI]

482.46

− 65

− 36

− 17

334

1.2706

2.634

144

0.694

0.593

0.648

0.263

0.451

0.584

[MGlyH][TFSI]

496.49

− 64

–

–

354

1.3242

2.667

133

0.768

0.608

0.637

0.280

0.453

0.618

[EGlyA][TFSI]

466.42

− 65

–

− 28

302

1.4206

3.046

150.3

1.071

0.735

0.667

0.364

0.509

0.714

[MGlyM][MeSO4]

257.31

− 70

–

38

306

–

–

–

–

–

–

–

–

–

[MGlyE][EtSO4]

285.36

− 75

− 26

21

251

1.1773

4.126

657

0.283

–

–

0.0653

–

–

[MGlyB][BuSO4]

341.47

− 71

–

–

215

1.1051

3.236

1446

0.064

–

–

0.0191

–

–

[EGlyM][MeSO4]

271.33

− 73

–

− 31

251

1.2406

4.572

920

0.296

–

–

0.0646

–

–

[EGlyE][EtSO4]

299.39

− 65

–

− 30

249

1.1024

3.682

1530

0.167

–

–

0.0453

–

–

[EGlyB][BuSO4]

355.49

− 60

–

− 33

228

1.1276

3.172

2294

0.048

–

–

0.0152

–

–

a b

Structure

Decomposition temperature of 10% weight loss. Density (ρ), concentration (C), viscosity (η), conductivity (σ), self-diffusion coefficient (Dcation and Danion), and molar conductivity (Λimp and ΛNMR) were measured at 35 °C.

T.-Y. Wu et al. / Electrochemistry Communications 13 (2011) 237–241

Mw/g mol−1

Ionic liquids

T.-Y. Wu et al. / Electrochemistry Communications 13 (2011) 237–241

239

measurements were made on a Varian Inova 300 spectrometer at 299.9 MHz for 1H and at 282.1 MHz for 19F with a 5 mm pulsed-field gradient probe. The signals of 1H in cations and of 19F in anions were used for the determination of self-diffusion coefficients of the cation and anion species, respectively. Cyclic voltammetry measurements were performed at 25 °C using an electrochemical workstation (CHI, model 750A). The electrochemical cell consisted of a glassy carbon working electrode, a Pt wire counter electrode, and a Pt quasi-reference electrode. The limiting potentials of electrochemical window are taken at a current density of 0.2 mA cm− 2. 3. Results and discussion Table 1 summarizes the fundamental thermal, physicochemical, and transport properties of glycine-based ILs. The TFSI-based ILs have higher thermal decomposition temperatures (Td) than those of alkylsulfate-based ILs, and ethylsulfate-based ILs have higher Td values than those of butylsulfate-based ILs. During the re-heating step in the DSC thermograms, all the prepared ILs exhibited a heat capacity change corresponding to the glass transition, Tg. Among them, [MGlyP][TFSI] and [MGlyE][EtSO4] exhibited a solid–solid transition, Ts–s, prior to a melting transition, Tm. In agreement with previous reports, ILs with TFSI anions exhibited higher densities than those of alkylsulfate-based ILs [7,8]. The viscosity of ILs is influenced by the cation–anion interaction, hydrogen bonding, and the coordinating ability and symmetry of the ions. ILs with TFSI anions exhibited lower viscosities than those of alkylsulfate-based ILs, which can be attributed to the anion conformation difference between TFSI and alkylsulfate [9]. Moreover, methylsulfate-based ILs exhibited lower viscosities than those of butylsulfate-based ILs, allowing the smaller methylsulfate anions to move faster under a given molecular motion. It is expected that TFSI-based ILs will have higher conductivities than alkylsulfate-based ILs, and methylsulfate-based ILs will have higher conductivities than butylsulfate-based ILs due to their greater disassociation of ion pairs and lower viscosities. The self-diffusion coefficients of the cations (Dcation) and anions (Danion) for the ILs were measured using the pulsed-field gradient spin-echo (PGSE) NMR technique. Allyl substitution in IL cations leads to a larger selfdiffusion coefficient, compared to those of pentyl and hexyl substitutes, not only for Dcation but also for Danion, which can be attributed to the influence of geometrical shape and free space around ionic species. Moreover, allyl group has π-bonds, it is not ordinary alkyl groups. The Λimp/ΛNMR ratio can be defined as the ionicity,

Fig. 2. Cyclic voltammogram of ILs relative to Pt reference electrode, with glassy carbon working electrode and Pt counter electrode. Potential was calibrated using the redox potential of ferrocene/ferricenium (Fc/Fc+) redox couple measured in each ionic liquid.

indicating the percentage of ion pair disassociation in the ILs [10]. [MGlyH][TFSI] and [MGlyP][TFSI] have larger ionicities than that of [MGlyA][TFSI], suggesting that long alkyl substitution at nitrogen atom promotes the dissociation of anions and cations in ILs. A Walden plot is shown in Fig. 1 to clarify the relationship between our results and those reported in the literature regarding conductivity and viscosity. The ionic conductivity and viscosity can be correlated using the qualitative approach given by Angell et al. based on the classical Walden rule [11]: Λimp η = C

ð1Þ

where C is a temperature-dependent constant. A log–log plot of Λ vs. η predicts a straight line (0.01 M KCl) that passes through the origin, in which the ions are completely dissociated, as a reference ‘ideal’ line for the plot. The ILs examined via Walden plots are significantly below the ideal line; deviations from the ideal line suggest increased electrostatic interaction between cations and anions. Typical cyclic voltammograms of the ILs are shown in Fig. 2. The electrochemical windows determined from Fig. 2 are listed in Table 2. The potential window lies between approximately −3.1 and +2.3 V vs. Fc/Fc+ for TFSI-based ILs, and between approximately − 3.1 and +1.8 V vs. Fc/Fc+ for alkylsulfate-based ILs, indicating quite high electrochemical stability, which is desired for various electrochemical applications. Fc/Fc+ is a conventional reference electrode in electrochemistry because the redox reaction is a standard reversible one-electron transfer process. The Cc/Cc+ process provides a more useful reference Table 2 Electrochemical windows of ionic liquids.

Fig. 1. Walden plots for glycine-based ILs, where Λ is the equivalent conductivity and η−1 is the fluidity. The ideal line that runs from corner to corner is generated from data obtained in an aqueous 0.01 M KCl solution.

Ionic liquid

Cathodic limiting potential V vs. Fc/Fc+

Anodic limiting potential V vs. Fc/Fc+

Electrochemical window V

[MGlyA][TFSI] [MGlyP][TFSI] [MGlyH][TFSI] [EGlyA][TFSI] [MGlyE][EtSO4] [MGlyB][BuSO4] [EGlyM][MeSO4] [EGlyE][EtSO4] [EGlyB][BuSO4]

− 3.08 − 3.16 − 3.26 − 3.22 − 3.05 − 3.04 − 3.12 − 3.20 − 3.19

2.32 2.38 2.34 2.26 1.88 1.84 1.74 1.70 1.75

5.40 5.54 5.60 5.48 4.93 4.88 4.86 4.90 4.94

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T.-Y. Wu et al. / Electrochemistry Communications 13 (2011) 237–241

Table 3 Calculated Stokes–Einstein products (ηD T−1) of organometallic and organic redox couples in ILs. Spices

Ionic liquid

Method

107 D/cm2 s−1

η/cP

Temp/K

1010 ηD T−1/g cm s−2 K−1

Reference

Fc

[C2mim][NTf2] [C4mim][NTf2] [C4mpyrr][NTf2] [C4mim][OTf] [C4mim][BF4] [C4mim][PF6] [emim][NTf2] [mimSBu][NTf2] [TEtA][Ac] [TEtA][Of] [MGlyA][TFSI] [MGlyA][TFSI] [MGlyP][TFSI] [MGlyP][TFSI] [MGlyH][TFSI] [MGlyH][TFSI] [EGlyA][TFSI] [EGlyA][TFSI] [C2mim][NTf2] [C4mim][NTf2] [C4mpyrr][NTf2] [C4mim][PF6] [C4mim][BF4] [MGlyA][TFSI] [MGlyA][TFSI] [MGlyP][TFSI] [MGlyP][TFSI] [MGlyH][TFSI] [MGlyH][TFSI] [EGlyA][TFSI] [EGlyA][TFSI] [C2mim][NTf2] [C4mim][NTf2] [C4mim][BF4] [EMIM][N(Tf)2] [BMIM][N(Tf)2] [C8MIM][N(Tf)2] [C10MIM][N(Tf)2] [N6222][N(Tf)2] [MGlyA][TFSI] [MGlyP][TFSI] [MGlyH][TFSI] [EGlyA][TFSI]

CA CA CA CA CA CA CV CV CV CV CV CA CV CA CV CA CV CA CA CA CA CA CA CV CA CV CA CV CA CV CA CA CA CA CA CA CA CA CA CV CV CV CV

5.34 3.77 2.31 2.36 1.83 0.59 5.09 1.35 8.1 8.7 1.61 1.76 1.08 1.21 1.25 1.37 1.11 1.22 3.16 2.22 1.45 0.47 1.11 1.01 1.09 0.68 0.76 0.73 0.81 0.63 0.69 4.87 2.05 1.74 2.62 1.87 1.12 1.13 0.7 1.39 0.93 0.97 0.96

34 52 89 90 112 371 34.7 98.5 11 10 93.3 93.3 144 144 133 133 150.3 150.3 34 52 89 371 112 93.3 93.3 144 144 133 133 150.3 150.3 34 89 112 32.1 57.6 119.3 152.8 220 93.3 144 133 150.3

299 299 299 299 299 299 294 294 298 298 308 308 308 308 308 308 308 308 299 299 299 299 299 308 308 308 308 308 308 308 308 303 303 303 293 293 293 293 293 308 308 308 308

6.07 6.56 6.88 7.10 6.85 7.32 6.01 4.52 2.99 2.92 4.88 5.33 5.05 5.66 5.40 5.92 5.42 5.95 3.59 3.86 4.32 5.83 4.16 3.06 3.30 3.18 3.55 3.15 3.50 3.07 3.37 5.46 6.02 6.43 2.87 3.68 4.56 5.89 5.26 4.21 4.35 4.19 4.68

[5] [5] [5] [5] [5] [5] [13] [13] [14] [14] This This This This This This This This [5] [5] [5] [5] [5] This This This This This This This This [15] [15] [15] [16] [16] [16] [16] [16] This This This This

Cc+a

TMPD

a

work work work work work work work work

work work work work work work work work

work work work work

Cc+: cobaltocenium hexafluorophosphate.

scale for ILs, since Cc+ salt typically has a higher thermal stability than that of Fc. TMPD shows two separate reversible one-electron oxidation waves, and the first one-electron oxidation process (TMPD/TMPD+) is used for diffusion coefficient measurement. The diffusion coefficient (D) of Fc, Cc+, and TMPD in ILs was estimated from cyclic voltammetry measurements using the Randles–Sevcik equation (Eq. 2) and chronoamperometry results using the Cottrell equation (Eq. 3) [12]:  ip = 0:4463nF

i=

nFAD1 = 2 C π1 = 2 t 1 = 2

nF RT

1 = 2

1=2 1=2

CAD

ν

ð2Þ

ð3Þ

where ip is the peak current (A), n is the number of electron equivalents exchanged during the reversible redox process, A is the active surface area of the working electrode (cm2), D is the diffusion coefficient (cm2 s−1), C is the bulk concentration of the diffusing species (mol cm−3), ν is the voltage scan rate (V·s−1), F is Faraday's constant, and R is the universal gas constant. The diffusion coefficients for Fc/Fc+, Cc+/Cc, and TMPD/TMPD+ in ILs follow the order: [MGlyA] [TFSI] N [MGlyH][TFSI] N [MGlyP][TFSI] N [EGlyA][TFSI]. The values

obtained from chronoamperometry are larger than those obtained from cyclic voltammetry. This is often observed in electrochemical studies. Transport properties of Fc, Cc+, and TMPD in ILs are generally interpreted on the basis of the Stokes–Einstein equation [10]:

D=

kB T 6πηα

ð4Þ

where kB is the Boltzmann constant, T is the temperature, α is the hydrodynamic radius of the diffusing species, and η is the viscosity of the solution. Table 3 summarizes the calculated Stokes–Einstein products, Dη/T, of Fc, Cc+, and TMPD in ILs. The Stokes–Einstein product of ferrocene in ILs is larger than that of cobaltocenium in ILs; these values are comparable to Dη/T values of Fc, Cc+, and TMPD in other functionalized ILs [5,13–16]. Since the Dη/T value of organometallic and organic redox couples is related to the hydrodynamic radius of the diffusing species, the comparable Dη/T values indicate that the radius of the diffusing entity is independent of the type of IL. Consequently, one can estimate the viscosity of a new IL from the experimentally determined D values and the Dη/T values of Fc, Cc+, and TMPD.

T.-Y. Wu et al. / Electrochemistry Communications 13 (2011) 237–241

4. Conclusions A series of glycine-based ILs were physicochemically and electrochemically characterized. The results are discussed in terms of the cationic and anionic characteristics, emphasizing the interactions that take place in the fluid. TFSI-based ILs show a wider potential windows (5.40–5.60 V) than those of alkylsulfate-based ILs (4.86–4.94 V). All the ILs show non-ideal behavior in the Walden plot, which can be explained in terms of associated ionic liquid interactions. The diffusion coefficients of three standard redox couples in the ILs agree well with those reported previously for other ILs. The results indicate that the prepared glycine-based ILs are suitable electrolytes. References [1] L.E. Barrosse-Antle, L. Aldous, C. Hardacre, A.M. Bond, R.G. Compton, J. Phys. Chem. C 113 (2009) 7750. [2] K. Fukumoto, M. Yoshizawa, H. Ohno, J. Am. Chem. Soc. 127 (2005) 2398. [3] H. Ohno, K. Fukumoto, Acc. Chem. Res. 40 (2007) 1122.

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