Influence of Electrode Roughness on Double Layer Formation in Ionic

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On the Influence of Electrode Roughness on Double Layer Formation in Ionic Liquids Thomas Jänsch, Jens Wallauer, and Bernhard Roling J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 13 Feb 2015 Downloaded from http://pubs.acs.org on February 13, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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1

On the Influence of Electrode Roughness on Double

2

Layer Formation in Ionic Liquids

3

Thomas Jänsch*, Jens Wallauer, Bernhard Roling

4

Department of Chemistry, University of Marburg, Hans-Meerwein-Strasse 4, D – 35032

5

Marburg, Germany.

6

We have used broadband electrochemical impedance spectroscopy for characterizing double

7

layer formation at the interface between the ionic liquid 1-butyl-1-methylpyrrolidinium

8

bis(trifluoromethane)sulfonimide [Py1,4]TFSI and three gold electrodes with different surface

9

structure and roughness. Two alternative approaches for analyzing the data were compared: a) A

10

fit in the complex impedance plane using a resistor and a constant phase element (CPE)

11

connected in series; b) A fit in the complex capacitance plane using a Cole-Cole function. In the

12

complex capacitance plane, a high-frequency semicircle due to double layer formation could be

13

clearly distinguished from other capacitive or Faradaic processes detected at lower frequencies.

14

The Cole-Cole fit of the high-frequency semicircle revealed that this semicircle is almost

15

unsuppressed with  values close to unity, even for the rough polycrystalline Au electrode. In

16

contrast, the CPE exponent depends much more strongly on electrode potential and electrode

17

roughness. We show that this strong dependence is closely related to the existence of slower

18

capacitive or Faradaic processes, and is not caused by non-ideal double layer formation.


1


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19

1. Introduction

20

In recent years, the electrified interface between electrodes and ionic liquids has gained much

21

attention. A variety of different experimental1-9 and theoretical methods10-16 has been applied for

22

improving the understanding of the structure and dynamics of the interfacial double layers. A

23

better understanding is important for many applications of ionic liquids in electrochemistry, e.g.

24

for electrodeposition of ignoble metals from ionic liquids and for using ionic liquids as battery

25

and supercapacitor electrolytes.17-20 One important property of interfacial double layers, which

26

has been extensively studied, is its potential-dependent differential capacitance. Since ionic

27

liquids can be seen as highly concentrated electrolytes, the differential capacitance cannot be

28

described by the classical Stern model, but more sophisticated theoretical approaches are needed,

29

which take into account the finite volume of ions as well as specific interactions between the ions

30

and the electrodes.21-26

31

One of the most powerful methods for measuring the differential capacitance is broadband

32

electrochemical impedance spectroscopy (EIS).27-34 Although this method has been extensively

33

used, the analysis and interpretation of impedance data of metal/ionic liquid interfaces is still

34

being controversially discussed. Different approaches have yielded distinct, and sometimes even

35

contradictory, results for the dependence of the differential capacitance on electrode potential,

36

purity of the ionic liquid, and temperature.33,35,36

37

Recently, some theoretical work has been published on the influence of electrode roughness on

38

the double layer formation in ionic liquids.37,38 Singh and Kant used a model for the electrical

39

double layer at rough electrodes, which is based on the Debye-Falkenhagen equation.38 The

40

results show that the electrode roughness causes a non-ideal low-frequency double layer

41

impedance, which can be described by a constant-phase element. The usage of empirical


2

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42

constant-phase elements is also quite common in the analysis of experimental impedance

43

spectra.39,40 In the case of rough electrodes, CPE exponents considerably smaller than unity have

44

been found.40-43

45

An alternative approach is the analysis of the data in the complex capacitance plane and the fit

46

of semicircles in this plane by means of the empirical Cole-Cole function.44 In the complex

47

capacitance plane, a high-frequency semicircle due to “fast double layer formation” can often be

48

clearly distinguished from slower capacitive or Faradaic process.45 By “fast double layer

49

formation” we mean that the double layer formation time   Rbulk  CDL is determined by the bulk

50

resistance of the ionic liquids, Rbulk , whereas other capacitive or Faradaic processes are slower

51

due to the existence of additional barriers at the interface.46 In our view, the distinction between

52

fast double layer formation and slower processes is very important for the understanding of the

53

interfacial dynamics.

54

In this paper, we present the results of EIS measurements on the interface between the ionic

55

liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethane)sulfonimide [Py1,4]TFSI and Au

56

electrodes with different surface structure and roughness. The impedance data were analyzed in

57

the complex impedance plane using a CPE-based equivalent circuit and also in the complex

58

capacitance plane using the Cole-Cole function. For all electrodes, the data in the complex

59

capacitance plane featured a high-frequency semicircle due to fast double layer formation and

60

additional capacitive or Faradaic contributions at lower frequencies. We found that the high-

61

frequency semicircle was always close to an ideal, i.e. unsuppressed semicircle, resulting in 

62

values in the Cole-Cole fit close to unity, even for the roughest electrode. In contrast, the CPE

63

exponents showed a much stronger dependence on electrode potential and on electrode

64

roughness. Our results reveal that this stronger dependence of the CPE exponent is mainly


3


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65

caused by the existence of the slower processes, which are strongly influenced by the electrode

66

potential and by the electrode roughness. The nature of these slower processes is not yet

67

completely understood. Evidence has been found that electrode surface reconstruction

68

phenomena as well as ordering effects of ions in the innermost ion layers are slower than fast

69

double layer formation.8,36,47

70 71

2. Experimental Section

72

The ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethane)sulfonimide [Py1,4]TFSI

73

was synthesized and purified by Passeriniet al. by means of a method described in 48,49 and dried

74

subsequently at a pressure of 10-6 mbar at elevated temperatures of 50-80°C.The impurity level is

75

below 100 ppm (weight) for Cl, Si, Mo, Al and Na, below 40 ppm (weight) for Fe and below

76

2 ppm (weight) for Li.48 The dried IL was handled and stored under Argon atmosphere in a glove

77

box (LABStar, MBraun GmbH). Water and oxygen content of the glove box were kept below 1

78

ppm. The water content of the IL was measured by using a Karl Fischer titrator (E 547 KF,

79

Metrohm) and was below the detection limit of 100 wt ppm.5 We note that other groups reported

80

water contents below 1 ppm after drying at elevated temperatures in vacuum.50

81

Three different gold working electrodes were used in this study: (i) A commercial thin gold

82

layer with (111) surface on a mica substrate (Phasis, Switzerland). The Au layer was prepared by

83

means of RF magnetron sputtering. In the following, “RF (111)” will be used as acronym for this

84

electrode. (ii) A thin gold layer with (111) surface on a mica substrate prepared in our lab by

85

means of DC sputtering. Acronym: “DC (111)”. (iii) A polycrystalline Au disc cut from a sputter

86

target. Acronym: “Polycryst”. The electrodes RF (111) and DC (111) were used without further

87

mechanical treatment of the surface, while the Polycryst electrode was polished using diamond


4

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88

polish (0.025 µm, Kemet). All electrodes were stored inside a glove box when not in use or

89

freshly prepared directly prior to use.

90 91 92 93

AFM measurements were carried out on a Cypher AFM (Asylum Research). The temperature was set to 30°C (± 0.1°C). The tip in use was a SSS-FMR (Nanosensors) in AC mode. SEM images were recorded using a JSM-7500F (JEOL) scanning electron microscope operating at 5.0 keV or 10 keV.

94

All electrochemical measurements were carried out using an Alpha high-performance

95

frequency analyzer (Novocontrol Technologies GmbH & Co. KG) connected to a POTGAL

96

10V/15A or to a POTGAL 30V/2A potentiostat. As electrochemical cell we used a TSC surface

97

cell (rhd instruments GmbH & Co. KG) with a polycrystalline Au counter electrode and a Ag

98

wire quasireference electrode. The cell was mounted onto a Microcell HC platform (rhd

99

instruments) and kept at 25°C. The measurement protocol was as follows: The cell was

100

assembled inside the glove box and closed air tight. The open circuit potential of the working

101

electrode was measured, and subsequently, anodic and cathodicDC overpotentials were applied

102

to the working electrode in steps of 10 mV or 20 mV. After setting a newoverpotential, the

103

system was given 10 min to equilibrate, before the impedance measurement was started. The

104

impedance was measured by applying AC voltages of 10 mV (rms) at frequencies ranging from

105

100 kHz to 0,1 Hz. Once the measurement over the entire dc overpotential range was finished, a

106

ferrocene solution in [Py1,4]TFSI (10 mmol/l) was added, and the potential of the quasi-

107

reference electrode was calibrated by means of cyclic voltammetry.

108 109

3. Results and Discussion

110

a) Electrode surface structure and roughness


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111

XRD patterns of the three electrodes are shown in Fig. 1. The XRD patterns of RF (111) and

112

DC (111) exhibit one Bragg peak originating from the Au (111) plane, but no peaks from other

113

Au lattice planes. All other peaks can be attributed to the mica substrate. In contrast, the XRD

114

pattern of Polycryst exhibits peaks originating from various Au lattice planes.

115

116 117

a)

118 119

b)


6

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120 121

c)

122

Figure 1: XRD patterns of the three electrodes: a) RF (111), b) DC (111) and c) Polycryst. In a)

123

and b), the Bragg peak of the Au (111) plane is marked by an asterisk. The unmarked peaks can

124

be attributed to mica substrate. In c), all crystallographic planes of polycrystalline gold can be

125

identified, major reflexes are indexed with Miller indices. Smaller reflexes at higher angles are

126

higher-order reflexes of the same planes.

127 128

The surface roughness of the electrodes DC (111) and Polycryst were analyzed by means of

129

atomic force microscopy (AFM), see Fig. 2. AFM-based roughness data of the electrode RF

130

(111) were communicated by Phasis.51 The surface of RF (111) shows the characteristic terrace

131

structure of the ideal Au(111) surface and exhibits an rms roughness of 1.14 nm. Fig. 2 a) shows

132

the surface topography of DC (111). A terraces structure could not be detected. The

133

rmsroughness value was 3.6 nm. As seen from Fig. 2 b, the Polycryst surface is much rougher

134

with arms value of 23.3 nm.

135


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136 137

a)

138 139

b)

140

Figure 2:a) AFM topography and height histograms of a) the DC (111) electrode surface and of

141

b) the Polycryst electrode surface. The rms values of the roughness are a) 3.6 and b) 23.3 nm,

142

respectively.

143 144

In Fig. 3, we show SEM images of the DC (111) and of the Polycryst electrode. As seen from

145

Fig. 3 a), the surface of the DC (111) is smooth and shows no special features. The surface of the

146

Polycryst electrode shown in Fig. 3 b) exhibits rather large indentation lines caused by the

147

diamond used as polish. The width of the indentation lines is very close to the size of the


8

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148

diamond particles (0.025 µm). Residual diamond particles impressed into the surface are clearly

149

visible.

150 151

a)

152

153 154 155

b) Figure 3: a) SEM images of the DC (111) electrode surface. b) SEM images of the Polycryst

156

electrode surface at two different magnifications.


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157 158

b) Impedance spectroscopic results for the double layer formation

159

In the following, we present the results of the impedance spectroscopic measurements in two

160

different representations, namely in the complex impedance plane (Nyquist plot) and in the

161

complex capacitance plane. Complex impedance Z and complex capacitance C are related via:

^

^

162

1 Cˆ  i Zˆ

163

As an example, Fig. 4 a) shows impedance data for the DC (111) electrode at two different

164

electrode potentials plotted in the Z plane. Since the low-frequency behavior is not ideally

165

capacitive, such data are typically fitted using an equivalent circuit shown in Fig. 4 c). The

166

resistor represents the bulk resistance ofthe electrolyte, Rbulk , while the constant phase element

167

connected in series represents the non-ideal capacitive behavior of the electrode/electrolyte

168

interface. The impedance of this equivalent circuit is given by:52

169

(1)

^

Zˆ  Rbulk 

1 Q  (i ) p

(2)

170

with Q and p denoting the CPE prefactor and exponent, respectively. As seen from Fig. 4 a), the

171

data at an electrode potential of 0.3 V vs. Ag can be fitted with a CPE exponent p  0.96 , while

172

the fit of the data at 0.16 V yields a lower exponent p  0.91.

173


10

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174 175

a)

176 177

b)

178 179

c)

180

Figure 4: a) Impedance data of the DC (111) / [Py1,4]TFSI interface in the complex impedance

181

plane together with a fit using the equivalent circuit shown in c). The fit parameter p and phf are

182

explained in the text. b) Zoom into the high-frequency range of the complex impedance plane.

183


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184

In Fig. 5, the same data are shown in the complex capacitance plane. At 0.3 V, a high-

185

frequency semicircle due to fast double layer formation is clearly seen. The high-frequency limit

186

of this semicircle is the bulk capacitance of the ionic liquid, while the low-frequency limit is the

187

double layer capacitance.5,30,31,36 The double layer capacitance CDL is about 10 µF/cm², which is

188

a typical value for metal / ionic liquid interfaces.31,53 The double layer formation time  can be

189

obtained

190

  / (2 max )  (2   .5Hz)1  2.92 ms . In the case of fast double layer formation,  should

191

be determined by the bulk resistance of the IL, and thus the relation   Rbulk  CDL should hold.

192

This is indeed the case, since the double layer capacitance obtained from this relation:

193

CDL 

194

of the semicircle. Here, Rbulk was determined from a fit of the impedance data to Eq. (2).

195 196

197

from

 Rbulk



the

2.92 ms 293.9 cm2

frequency

of

the

semicircle

maximum,

 max

via

 9.93 μFcm2 is in perfect agreement with the low-frequency limit

The semicircle is almost an ideal semicircle. Therefore, it can be fitted by a Cole-Cole function:

C  Cbulk Cˆ  Cbulk  DL 1  (i )

(3)

198

with an  value very close to unity, here:   0.98 . For the Cole-Cole fit, only the data points

199

from high-frequencies to frequencies just beyond the maximum of the semicircle were used,

200

since the data at lower frequencies are influenced by slower processes, which are visible as a

201

low-frequency spike. Such low-frequency processes can be capacitive or Faradaic. As a second

202

method for determining the non-ideality of the semicircle, the suppression parameter  

203

calculated, with b and a denoting the imaginary and the real part of the capacitance at  max (see

b was a


12

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204

Fig. 5 a)). In this case, we obtain   0.98 , also proving an almost ideal, i.e., almost

205

unsuppressed semicircle.

206

In the cathodic regime, we often observe that the low-frequency spike overlaps with the double

207

layer semicircle. An example is the complex capacitance plot at E=-0.46 V shown in Fig. 5 b).

208

In54, we have shown that herringbone-type structural changes of Au(111) electrodes take place in

209

the cathodic regime, which are closely related to the existence of a slow capacitive process. If the

210

overlap between double layer semicircle and low-frequency spike is strong, it is not possible to

211

obtain precise values for the parameters  and  . Therefore, in the following, we show

212

exclusively  and  values for electrode potentials, at which the values can be obtained with

213

high precision.

214

215 216

Figure 5: Complex capacitance data of the DC (111) / [Py1,4]TFSI interface at three different

217

electrode potentials together with fits using the Cole-Cole expression. b and a denote the

218

imaginary and the real part of the capacitance, respectively, at the maximum of the

219

semicircle.The solid line represents the fit of the data up to frequencies just beyond the

220

maximum, while the dotted line is the extrapolation of the fitted semicircle to low frequencies.


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221 222

In Fig. 6, we compare the CPE exponent p with the parameters  and  for all three Au

223

electrodes. Fig. 6a) shows the results for the RF (111) electrode. Precise values for  and 

224

could only be obtained in the anodic regime. In this regime, all  and  values are very close to

225

unity, i.e. the fast double layer formation causes an almost ideal, i.e. almost unsuppressed

226

semicircle. In contrast, the CPE exponent p is considerably smaller than unity. These lower

227

values are mainly caused by the existence of slower processes. This can be directly shown by an

228

additional fit of the complex impedance data to the equivalent circuit in Fig. 4 c) in a limited

229

frequency range. This limited frequency range extends from high frequencies to the same low-

230

frequency limit that was used in the Cole-Cole fit of the complex capacitance data (Fig. 5). In

231

this case, a considerably higher CPE exponent phf  0.98 is obtained, as illustrated in Fig. 4 b)

232

showing a zoom into the high-frequency regime of the complex impedance plane. In particular at

233

electrode potentials below 0.7 V, there is a strong overlap between the double layer semicircle

234

and capacitance contributions from slower processes, most likely related to herringbone-type

235

structural changes of the Au(111) electrode54,55, so that no precise values for  and  could be

236

obtained.

237

Fig. 6 b) illustrates the results obtained for the DC (111) electrode. Again,  and  values

238

very close to unity were obtained in the anodic regime, whereas no reliable values could be

239

obtained in the cathodic regime between 0.46 V and 0.84 V and below 0.92 V due to strong

240

capacitance contributions of slower processes, again most likely related to a herringbone-type

241

structural changes of the Au(111) surface.54,55 In the anodic regime, the p parameter is slightly

242

lower than the parameters  and  , but still rather close to unity. With decreasing electrode


14

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243

potential (cathodic direction), the p parameter drops strongly, see also Fig. 6 c). Again, this is

244

not related to a suppression ofthe double layer semicircle, but tothe existence of slower processes

245

in the cathodic regime. This becomes evident when comparing the values obtained for p and phf

246

.

247

Finally, we consider the results for the Polycryst electrode shown in Fig. 6 d). In this case, the

248

 and  parameters in the anodic regime are slightly lower as compared to other two electrodes,

249

but still quite close to unity (typically around 0.95). This provides strong indication that the

250

much higher roughness of the Polycryst electrode leads to a weak non-ideality (weak

251

suppression) ofthe double layer semicircle. The values of the parameter p are close to those of

252

the parameters  and  . When changing the electrode potential intocathodic direction, there is a

253

strong drop of the CPE parameter p , which is again caused by low-frequency processes, as seen

254

from a comparison of p and phf .

255 256

257 258

a)


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Page 38 of 45

259 260

b)

261 262

c)

263 264

d)

265

Figure 6: Cole-Cole parameter , suppression parameter , and CPE exponents p and phf

266

obtained a) for the RF (111) electrode, b) for the DC (111) electrode, c) for the DC (111)

267

electrode at electrode potentials above – 0.4 V, and d) for the Polycryst electrode.


16

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268 269

c) Discussion

270

The results of the XRD, AFM, and SEM studies show clearly that the RF (111) electrode

271

exhibits an ideal (111) surface with a typical terrace structure and a very low roughness. The DC

272

(111) electrode exhibits also a (111) surface, but the terrace structure does not seem to be well

273

established. Furthermore, the rms roughness is slightly higher as compared to the RF (111)

274

electrode. The Polycryst electrode does not exhibit any preferred crystal lattice plane surface and

275

is much rougher than the other two electrodes. Thus, there are strong differences between the

276

surface structure and surface roughness of the three electrode, which should have an influence on

277

the electrochemical properties.

278

The most important results of our impedance spectroscopic study is that the surface exerts a

279

strong influence on slow processes, which can be capacitive or Faradaic in nature, but only a

280

weak influence on the non-ideality of the fast double layer semicircle. In the case of the RF (111)

281

and DC (111) electrode, we obtain almost ideal, i.e. almost unsuppressed double layer

282

semicircles with  and  values very close to unity, whenever the  and  values can be

283

determined with high precision. We have never found  and  values below 0.95 for double

284

layer semicircles clearly separated from low-frequency processes. In the cathodic regime, the

285

capacitance contribution of such slow processes often becomes so large that the ideality ofthe

286

double layer semicircle cannot be analyzed anymore in a meaningful fashion. In the transition

287

regime from the anodic to cathodic regime, the  and  values are still quite close to unity, but

288

the CPE exponent p drops strongly in cathodic direction. A comparison ofthe CPE parameters

289

p obtained by a fit over the entire frequency with the CPE parameter phf obtained by a fit in the

290

same frequency window of the Cole-Cole fit reveals that the drop of p with decreasing


17


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Page 40 of 45

291

electrode potential (cathodic direction) is not related to an increasing suppression ofthe double

292

formation, but to stronger capacitance contributions of slower processes.

293

In the case ofthe Polycryst electrode, the  and  values in the anodic regime are slightly

294

lower than found forthe other two electrodes, but still quite close to unity with typical values

295

around 0.95. This provides strong indication that the much rougher surface ofthe Polycryst

296

electrode leads to a weak non-ideality of the double layer formation. Also the p parameter is in

297

the range of 0.95. When the electrode potential is changed in cathodic direction, the same effect

298

as forthe other two electrodes is observed, namely the drop of the p parameter is caused by

299

strong capacitance contributions of slower processes.

300

In summary, we can state that the surface structure and in particular the roughness of the

301

electrode exertonly a weak influence on the non-ideality ofthe double layer formation and thus

302

on the suppression of the double layer capacitance semicircle. For all electrodes, even for the

303

rough Polycryst electrode, we obtain  and  values between 0.95 and 1.0. The comparison

304

between the p and phf values provide strong indication that p parameters below 0.95 are

305

mainly caused by slow capacitive or Faradaic processes.

306 307

4. Conclusions

308

The interface between the ionic liquid [Py1,4]TFSI and Au electrodes with different surface

309

structure and roughness was characterized by means of impedance spectroscopy. The plot of the

310

impedance data in the complex capacitance plane revealed that the high-frequency semicircle

311

due to fast double layer formation is only weakly suppressed, even for the roughest electrode.

312

This implies that the electrode surface structure and roughness exert only a weak influence of the

313

non-ideality of the fast double layer formation. However, the electrode surface exerts a strong


18

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314

influence on slower processes, which can be capacitive or Faradaic in nature. The existence of

315

these slow processes lead to a strong dependence of CPE exponent p on electrode potential and

316

on electrode structure and roughness. In particular, we have shown that p values below 0.95 are

317

mainly caused by slow processes and are not related to a suppression of the double layer

318

semicircle.

319 320

Acknowledgements

321

This work was financially supported by the Federal State of Hessen within the LOEWE

322

program of excellence (project initiative STORE-E).

323

We thank Prof. Stefano Passerini(Helmholtz Institute for Electrochemical Energy Storage,

324

Ulm) for providing the ionic liquid [Py1,4]TFSI in high purity. Furthermore, we are grateful to

325

Michael Klues (Witte group, Marburg) and to Jens Stellhorn (Pilgrim group, Marburg) for X-ray

326

diffraction measurements, to Stephan Bradler (Roling group, Marburg) for AFM measurements

327

and to Michael Hellwig (Structure and Technology Research Laboratory, Marburg) for SEM

328

measurements.

329 330

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Influence of Electrode Roughness on Double Layer Formation in Ionic

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