C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9

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Electron transfer kinetics of the VO2þ=VOþ2 – Reactionon multi-walled carbon nanotubes

0008-6223/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.carbon.2013.06.076

* Corresponding author: Fax: +65 68969950.E-mail address: ulrich.stimming@tum-create.edu.sg (U. Stimming).

Jochen Friedl a,b, Christoph M. Bauer a,b, Ali Rinaldi a, Ulrich Stimming a,b,c,*

a TUM CREATE, 1 CREATE Way, CREATE Tower, Singapore 138602, Singaporeb Department of Physics E19, Technische Universitat Munchen, James-Franck Str. 1, 85748 Garching, Germanyc Institute for Advanced Study (IAS) of the Technische Universitat Munchen, Lichtenbergstr. 2a, 85748 Garching, Germany

A R T I C L E I N F O A B S T R A C T

Article history:

Received 28 January 2013

Accepted 21 June 2013

Available online 29 June 2013

Multi-walled carbon nanotubes (MWCNTs) are suitable electrode materials for the all-vana-

dium redox flow battery. In addition to their high specific surface area, catalytic properties

for the VO2þ=VOþ2 redox reaction have been reported in literature. Electrochemical imped-

ance spectroscopy was employed to study the VO2þ=VOþ2 - and the Fe2+/Fe3+-reaction on

MWCNTs with varying amounts of surface functional groups. Our analysis method is based

on taking the large electrochemical interface area of the MWCNTs into account to obtain a

truly comparable value for the exchange current density. When evaluating the results for

Fe2+/Fe3+ it was found that the exchange current density on MWCNTs decorated with a

large amount of functional groups is more than 10 times larger than for thermally defunc-

tionalized MWCNTs. For the VO2þ=VOþ2 reaction, however, a decrease in activity for an

increase in amount of functional groups was observed. A possible reaction mechanism

and the influence of defects on MWCNTs are discussed.

This work distinguishes itself from previous publications, by showing the absence of a cat-

alytic effect of functional groups for the VO2þ=VOþ2 reaction. Therefore, a new discourse in

understanding the catalytic effect of MWCNTs and specifically of the surface functional

groups of carbon materials in electrochemical reactions is necessary.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The all-vanadium redox flow battery (VRB), invented and

developed by Skyllas-Kazakos and her co-workers at the Uni-

versity of New South Wales, represents one of the possible op-

tions for efficient energy storage [1,2]. As a means of energy

management, this could add significant value for the dissem-

ination of power generated from wind or sun [3,4]. However,

one of the contemporary challenges of VRBs is their low

power density [5,6] and therefore catalysis of the redox reac-

tions is desirable.

This work investigates the influence of surface functional

groups attached to MWCNTs on the heterogeneous electron

transfer kinetics of the VO2þ=VOþ2 -reaction. The redox-reac-

tion is of importance for the coming of age of the VRB which

employs vanadium(IV) and vanadium(V) ions in aqueous

solution on the high-potential half-cell. As this reaction is

rather sluggish, with reported electron transfer rate constants

as low as k0 = 3.0 · 10�7 cm s�1[7], considerable efforts have

been put into facilitating the electron transfer [8].

Zhu et al. fabricated electrodes for the cathode and anode of

a VRB out of graphite and carbon nanotubes (CNTs) composite

[9]. They stated that graphite improves kinetics whereas CNTs

increase currents due to their high electronic conductivity.

Employing cyclic voltammetry (CVs), Electrochemical

Impedance Spectroscopy (EIS) and charge–discharge tests,

Yue et al. investigated the performance of functionalized car-

bon fibers for the V2+/V3+ and VO2þ=VOþ2 electron transfer

C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9 229

reactions [10]. They describe a mechanism in which the –OH

groups seem to increase the electron transfer rate for both

reactions. However, the increase in activity is said to be more

significant for V2+/V3+ than for VO2þ=VOþ2 . In a similar fashion

Li et al. used MWCNTs with different kinds of functionaliza-

tion as electrode material [11]. They concluded that MWCNTs

with carboxyl-groups show a catalytic mechanism towards

the VOþ2 =VO2þ which is superior to MWCNTs with hydroxyl-

groups or pristine MWCNTs. Recently, Flox et al. investigated

monometallic Pt and bimetallic CuPt3 nanocatalysts sup-

ported on graphene and concluded that the latter greatly en-

hances the activity of the high potential electrode of a VRB

[12]. Similar to the previous studies, –OH groups are thought

to be partially responsible for the detected increase in

kinetics.

Since it is difficult to determine the charge transfer kinet-

ics on mesoporous structures such as MWCNTs from CVs

[13,14], a new method is proposed in this study. Employing

EIS the double layer capacitance CDL and charge transfer

resistance RCT for varying amounts of MWCNTs on a glassy

carbon (GC) disk electrode were determined. Both variables

are functions of the electrochemical interface area and a plot

of R�1CT vs:CDL is proportional to the exchange current density

j0. The activity of pristine Nanocyl 3100 MWCNT (NC 3100) is

compared to the performance of NC 3100 which were sub-

jected to various treatments which increase or reduce the

amount of surface functional groups. To validate the sensitiv-

ity of this method towards alteration of charge transfer kinet-

ics caused by surface functional groups a suitable redox

system, the ferrous/ferric electrode reaction [15–18], was also

investigated.

2. Experimental

2.1. Materials and reagents

All chemicals were analytical grade and were used without

further purification. Ultra-pure type I water was obtained

from an ELGA Option-Q water purification system and was

used with sulfuric acid to prepare the supporting electrolyte.

The Nanocyl 3100 MWCNTs from Nanocyl S.A. were chosen

as they excel in terms of purity, size distribution and crystal-

linity [19]. The NC 3100 used throughout this study were from

the same batch.

2.2. Functionalization and purification of MWCNTs

HNO3 and a mixture of HNO3/H2SO4 were used as oxidizing

agents to create appreciable amounts of surface functional

groups on the caps and sidewalls of the MWCNTs [20].

The NC 3100 were ultrasonically dispersed in an oxidizing

agent for a well-defined time (see Table 1), followed by filtra-

tion and rinsing the filtrates with deionized water. After a dry-

ing process in air to remove water, the samples were further

dried at 150 �C in Argon atmosphere; later the temperature

was reduced to 30 �C while 5% v/v oxygen was added to the

sample in the furnace [21].

The samples under investigation in this study are listed in

Table 1. Untreated as-received NC 3100, in the following called

PRIST, are the starting point for the samples NITRIC, NIT-

SULF_3h, NITSULF_6h with added functional groups which

were treated using well known procedures [22,23]. In addition

a pristine NC 3100 sample was exposed to a temperature of

1000 �C for 3 h in Ar-atmosphere to remove present functional

groups. That sample is named DEFUNC.

2.3. Electrode preparation

As substrate for the MWCNTs GC was used. To ensure repro-

ducible results and to keep the surface roughness as small as

possible the surface was polished prior to usage [15]. The first

three polishing steps were performed with grit papers of suc-

cessively smaller grain sizes (P800, P2400, P4000) followed by

polishing with an alumina slurry (50 nm particles) and a silica

slurry (7 nm particles) on polishing cloth. In between the

steps the electrode was rinsed with ultra-pure water.

Variable amounts of MWCNTs were suspended in Hexane

and ultrasonically dispersed. Then 20 ll of this suspension

were dropped onto a GC rod with a diameter of 5 mm. Evapo-

ration of the organic solvent in an oven at 55 �C is the last step

in the coating process. Multiple iterations of this workflow led

to thicker layers of MWCNT on GC. The mass of MWCNTs on

the electrode, as given in Table 3, was calculated from the ini-

tial density of MWCNT in the solvent and the volume of sol-

vent cast onto the electrode. The obtained value is error-

prone, however, the error is of no importance, as the amount

of MWCNTs on the electrode enters the analysis via the value

for CDL.The mass is merely given to clarify the manufacturing

process.

In Fig. 1a–d Scanning Electron Microscopy (SEM) micro-

graphs of sample PRIST on GC are shown. In Fig. 1a and b

the plain GC surface can still be seen. Multiple iterations of

drop-casting MWCNTs onto the electrode surface will in-

crease the amount of MWCNTs on the surface.

2.4. Sample characterization

2.4.1. Microscopy and spectroscopyThe morphology of the MWCNT samples coated onto the GC

electrodes was investigated using a JEOL ‘‘7600F’’ Field emis-

sion SEM (FESEM). With a Fourier Transform Infrared (FTIR)

Spectrometer ‘‘Spectrum GC’’ from Perkin Elmer, the degree

and type of functionalization of the nanotube walls was

determined. The carbon samples were mixed with KBr to ob-

tain sufficient transmission. To study the decomposition

temperature of the surface functional groups a Thermogravi-

metric analysis coupled with mass spectroscopy (TGA–MS)

setup consisting of a ‘‘STARe TGA’’ from Mettler Toledo con-

nected to a ‘‘Thermostar GSD3200’’ from Pfeiffer Vacuum

was used. The samples were heated from room temperature

to 1000 �C applying a constant temperature ramp of 10 K/

min after dwelling at 100 �C for 2 h. To ensure an oxygen-

free atmosphere the furnace was purged with 100 ml of Ar-

gon per minute. Raman spectroscopy was performed on a

WITec ‘‘alpha 300SR’’ using an excitation wavelength of

k = 632.8 nm emitted from a HeNe-Laser driven at a power

of 60 mW.

Table 1 – MWCNT samples derived from as-received Nanocyl 3100 with their respective preparation method.

Sample name Treatment Exposure time Description

PRIST – – Pristine NC 3100DEFUNC 1000 �C in Argon for 3 h – Defunctionalized NC 3100NITRIC 3 M HNO3 18 h Mildly functionalized NC 3100NITSULF_3h Conc. H2SO4/conc. HNO3 vol. ratio 3:1 3 h Strongly functionalized NC 3100NITSULF_6h Conc. H2SO4/conc. HNO3 vol. ratio 3:1 6 h Heavily functionalized NC 3100

Fig. 1 – SEM images of the glassy carbon surface decorated with variable amounts of PRIST MWCNTs. The magnification is

30,000. (a) 1 lg of PRIST, (b) 2 lg of PRIST, (c) 10 lg of PRIST and (d) 20 lg of PRIST.

230 C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9

2.4.2. Electrochemical characterizationThe samples listed in Table 1, supported on GC, were used as

working electrodes in a three-electrode setup. In a custom-

built glass cell an Au-wire served as counter electrode while

a Mercury/Mercourus Sulfate electrode in 1 M H2SO4 (MSE,

0.674 V vs. normal hydrogen electrode (NHE)) was used as ref-

erence electrode. All potentials reported are recalculated to

NHE. Both the VO2þ=VOþ2 and the Fe2+/Fe3+ redox reaction

were investigated in 1 M H2SO4 to which 50 mM of each spe-

cies of the respective couple was added. VO2+ was derived

from VOSO4, VO2+ from V2O5, the ferrous/ferric couple from

FeCl2 and Fe(NO3)3. Prior to each measurement the electrolyte

was purged for 20 min with argon to remove dissolved oxygen

from the solution.

A Bio-Logic SP-300 potentiostat was used for potential con-

trol and data acquisition.

2.4.3. Evaluation of electron transfer kineticsCVs obtained on porous electrodes have limited suitability to

derive quantitative data about the kinetics of a redox reac-

tion [13,14]. The potential difference DU between oxidation

and reduction peak in a CV decreases with increasing sur-

face area for mesoporous electrodes. This is due to the inter-

play of higher current for increased reaction sites and

hindered mass transport within porous structures. On the

other hand the surface area which is actually wetted by

the electrolyte is not an easily accessible variable for com-

plex porous carbons such as MWCNTs [24]. Surface area val-

ues obtained from the Brunauer–Emmett–Teller (BET)

equation are based on adsorption of N2 molecules at their

boiling point. The surface area accessible for these mole-

cules might be larger than for fairly large hydrated ions

[25]. Therefore, the charge transfer resistance RCT for porous

electrodes obtained from EIS cannot easily be converted to

an exchange current density j0, although Eq. (1), which is ob-

tained from the Taylor-series of the Butler–Volmer equation,

is straightforward.

Here, a method is proposed which makes use of the fact

that the inverse charge transfer resistance R�1CT as well as CDL

are directly proportional to the actual area of the electrode/

electrolyte interface A:

R�1CT ¼

nFj0A

RTð1Þ

C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9 231

CDL ¼ 2p�r�0l

ln rþtDLr

� � � 2p�r�0l

ln tDLr

� � ¼ �r�0AtDL

ð2Þ

Eq. (2) represents the formula for a cylindrical capacitor

which simplifies to that of a plate capacitor if one assumes

that the thickness of the double layer tDL is much smaller

than the radius r of one MWCNT. All the other symbols have

their usual meaning. Eqs. (1) and (2) can be combined to:

R�1CT ¼

nFtDL

RT�r�0j0CDL ð3Þ

R�1CT should therefore be a linear function of CDL, depending

only on constants and the reaction-specific j0, allowing for

analysis of reaction kinetics without explicit knowledge of

A. To exploit relation (3) variable amounts of MWCNTs were

coated onto the GC working electrodes, thus varying the

interface area A, which results in a range of different double

layer capacities and charge transfer resistances. Different

coverage of MWCNTs on GC was achieved as depicted in

Fig. 1a–d.

3. Results

3.1. Spectroscopic analysis

Employing the spectroscopic methods described in Sec-

tion 2.4.1 it was confirmed that the amount of functional

groups on the MWCNTs was successfully varied.

FTIR spectra for the five samples under investigation are

shown in Fig. 2. An absorption peak at U�¼ 1716 cm�1 indicates

the presence of carboxylic surface groups [10,22,26,27]. That

Fig. 2 – FTIR spectra for the MWCNTs based on the Nanocyl

3100. The curves are normalized to their transmission at

U�¼ 400 cm�1 and shifted for clarity.

Table 2 – Weight loss due to thermal removal of functional grou

PRIST DEFUNC

Weight loss (%) 2.2 0.7

peak is only evident for samples NITSULF_3h and NITSULF_6h.

Aromatic groups and quinones may result in absorption of

light at U�¼ 1558 cm�1[26,28,29]. From U

�¼ 1169 cm�1 to lower

wavenumbers, many different surface functional groups, i.e.

carboxylic, lactone, anhydride and phenol groups, can absorb

light [28,30]. While all the samples show absorption at

U�¼ 1558and U

�¼ 1169 cm�1 the signal is considerably in-

creased for MWCNTs treated with strong oxidizing agents.

Sample DEFUNC shows the lowest overall absorption.

TGA–MS measurements showed that with increased

functionalization the weight loss increases significantly

due to thermal removal of functional groups. The exact val-

ues for the measured weight loss are given in Table 2. Two

of these measurements, which were obtained with a tem-

perature program as described in Section 2.4.1, are shown

in Fig. 3. Decomposition temperature and the type of gas(es)

released indicate the type of functional groups available on

the carbon surface [26,30–32]. Strong COþ2 signals at 250–

300 �C indicate a significant presence of carboxylic groups

on samples NITSULF_3h and NITSULF_6h. For PRIST and NI-

TRIC a smaller signal is observed while DEFUNC shows no

gas evolution in that temperature range. Decomposition of

lactones and phenols leads to a COþ2 signal at temperatures

from 400 to 500 �C and a CO+ signal within the temperature

range of 650–750 �C respectively. However, for these surface

groups the intensity is similar for samples PRIST, NITRIC,

NITSULF_3h and NITSULF_6h while again absent for

DEFUNC.

The gain in weight of roughly 0.05% for sample DEFUNC

(see Fig. 3a) is caused by a slight mismatch between the cru-

cible holding the sample and the empty reference crucible.

FTIR and TGA–MS analysis clearly demonstrate that the

acid treatment increases the amount of functional groups

while the defunctionalization process lowers the amount of

functional groups as compared to the PRIST sample. Samples

which were exposed to concentrated sulfuric and nitric acid,

NITSULF_3h and NITSULF_6h, contain a significant amount of

carboxylic groups. Changes in the graphitic character of the

MWCNTs, induced by the functionalization process, were

examined with Raman Spectroscopy. The ratio of the D-band

and G-band intensity reflects the graphitic character of the

carbon samples [15,33,34]. Fig. 4 depicts the Raman spectra

for the five samples under investigation and gives the ID/IG ra-

tios. The lower this ratio the less defective is the MWCNT. To

determine the peak height Lorentz curve-fitting was

performed.

Treatment with 3 M HNO3 had little effect on the gra-

phitic character of bulk MWCNTs sample while the treat-

ment in concentrated H2SO4 and HNO3 deteriorates the

graphitic structure and introduces defects. Heat treatment,

as performed for the sample DEFUNC, heals lattice defects

[35].

ps during the temperature ramp described in Section 2.4.1.

NITRIC NITSULF_3h NITSULF_6h

4.5 7.0 19.0

Fig. 4 – (a) Comparison of the Raman spectra for the MWCNT samples. (b) Detail of the D-band peak.

Fig. 5 – Cyclic voltammograms for (a) 50 mM VO2+ and 50 mM VO2+, (b) 50 mM Fe2+ and 50 mM Fe3+ in 1 M H2SO4 at a GC

electrode decorated with sample DEFUNC. Scan Speed was 20 mV/s.

Fig. 3 – Two exemplary TGA–MS spectra for the samples (a) DEFUNC and (b) NITSULF_3h.

232 C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9

3.2. Electrochemical analysis

The electrochemical performance of the drop cast MWCNT-

electrodes was tested in a three-electrode setup as described

in Section 2.4.2. Fig. 5 shows CVs of the two investigated re-

dox-systems, the left graph (a) depicts the VO2þ=VOþ2 redox

reaction, while the Fe2+/Fe3+ redox couple is shown on the

right (b). Both graphs were taken with sample DEFUNC as

Fig. 6 – Impedance data for the MWCNT samples based on Nanocyl 3100. The left side shows the Nyquist

representation while the right side gives the Bode representation for one selected mass of MWCNT on the electrode. For the

VO2þ=VOþ2 -system: (a): PRIST, (b) DEFUNC, (c) NITSULF_3h. For the Fe2+/Fe3+-system: (d) PRIST. The data was fitted to the

Randles equivalent circuit, which is shown in the graphs, and the simulated curves are shown as solid lines.

C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9 233

Table 3 – Values for double layer capacitance CDL and charge transfer resistance RCT for the different MWCNT samples in thetwo different electrolytes.

Mass (lg) RCT (O) CDL (lF) Mass (lg) RCT (O) CDL (lF)

VO2þ=VOþ2 PRIST Fe2+/Fe3+ PRIST1 1915 91 1 95 502 1330 119 2 25 9010 692 230 3 16 10820 319 387 4 17 133VO2þ=VOþ2 DEFUNC Fe2+/Fe3+ DEFUNC16 308 237 1 652 924 144 467 2 532 2332 120 674 3 61 11240 63 935 4 146 49VO2þ=VOþ2 NITRIC8 1424 10916 650 21724 324 42232 132 691VO2þ=VOþ2 NITSULF_3h Fe2+/Fe3+ NITSULF_3h8 1595 114 1 90 3416 666 246 2 41 4524 336 474 3 8 10132 346 484 4 6 132VO2þ=VOþ2 NITSULF_6h Fe2+/Fe3+ blank glassy carbon substrate16 1134 264 n/a 348 3124 364 559 VO2þ=VOþ2 blank glassy carbon substrate32 324 646 n/a 5471 3740 267 987

Errors are not shown, they are approximately ±20% for CDL, ±20% for the mass and ±5% for RCT.

Table 4 – Extracted slopes and electron transfer constants k0 for the VO2þ=VOþ2 redox reaction on the MWCNT samples.

Sample name VO2þ=VOþ2 Fe2+/Fe3+

Slope (F�1 O�1) k0 (cm s�1) Slope (F�1 O�1) k0 (cm s�1)

PRIST 8.1 ± 1.5 (17.6 ± 3.3) · 10�7 (8.1 ± 3.8) · 102 (17.8 ± 8.4) · 10�5

DEFUNC 17.5 ± 2.8 (38.0 ± 6.1) · 10�7 (1.5 ± 0.26) · 102 (32.3 ± 5.6) · 10�6

NITRIC 7.8 ± 1.2 (17.0 ± 2.6) · 10�7

NITSULF_3h 6.2 ± 1.2 (13.6 ± 2.0) · 10�7 (18.3 ± 5.9) · 102 (4.0 ± 1.3) · 10�4

NITSULF_6h 4.2 ± 1.1 (9.1 ± 2.3) · 10�7

234 C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9

electrode material. The potential was scanned at 20 mV/s,

and for each MWCNT sample and redox system multiple

measurements are shown, corresponding to different

amounts of MWCNTs on the electrode. Equilibrium potentials

U0 for the observed redox reactions were determined from the

potential position of the respective oxidation peak ðUoxpeakÞ and

reduction peak ðUredpeakÞ according to:

U0 ¼Uox

peak þ Uredpeak

2ð4Þ

In Fig. 5a, the VO2þ=VOþ2 redox reaction is clearly observa-

ble with an equilibrium potential of U0 = 1.05 V vs. NHE. At

higher potentials the oxygen evolution reaction (OER) takes

place. At lower potentials, at U = �0.25 V vs. NHE, a current

peak for the oxidation of V2+ to V3+ can be seen. The current

of the associated reduction reaction is superimposed onto

the hydrogen evolution reaction (HER). For the Fe2+/Fe3+ re-

dox reaction (see Fig. 5b) a smaller potential window for

the CVs was chosen. The equilibrium potential can be iden-

tified at U0 = 0.69 V vs. NHE. This considerable shift from the

thermodynamic equilibrium potential of UH = 0.77 V vs. NHE

is due to the fact that the supporting electrolyte is 1 M

H2SO4[36,37]. The sulfate anions of the supporting electrolyte

cause an anion effect resulting in a bridge assisted complex

[38].

The plotted CVs make it immediately apparent why this

experimental method is not suitable to obtain information

about the kinetics of a redox reaction at porous electrodes.

For the same system of redox couple and electrode material

the apparent onsets of the electron transfer and also the po-

tential of the current peak changes with the amount of

MWCNTs on the working electrode. A smaller peak separa-

tion between oxidation and reduction peak is not related to

faster kinetics but merely to interplay between higher cur-

rents due to more accessible electrode surface and hindered

mass transport in the mesoporous MWCNT structure. CVs

for the other samples were recorded but are not shown here

as they resemble the ones in Fig. 5.

Fig. 7 – Inverse charge transfer resistance R�1CT over double

layer capacitance CDL for all the investigated samples

obtained from EIS on a double logarithmic scale. The data

points are fit linearly, the values for the slopes with their

errors are given in Table 4. (a) Fe2+/Fe3+, (b) VO2þ=VOþ2 .

C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9 235

For the investigation of the VO2þ=VOþ2 redox reaction, each

of the MWCNT samples was measured with EIS; an amplitude

of 10 mV around the equilibrium potential U0 was applied.

Only the three samples PRIST, DEFUNC and NITSULF_3h were

measured for the Fe2+/Fe3+ redox couple at the respective

equilibrium potential U0 because the ferrous/ferric-system is

selected to act as a benchmark system to validate the sensi-

tivity of the method towards catalysis by surface functional

groups. Representative Nyquist plots for both redox systems

are shown in Fig. 6. A Bode plot for one electrode is given as

a representative.

To obtain quantitative results for RCT and CDL the imped-

ance data was fitted to an equivalent circuit consisting of

the Randles circuit with a constant phase element (CPE) in-

stead of a capacitor [39]. The value of the CPE was converted

to CDL[40]. The solution resistance Rsol varied with electrode

but was always smaller than 10 X. The values for RCT and

CDL for different amounts of MWCNT on the electrode are gi-

ven in Table 3. Errors originate from the fits and reading

accuracy.

Porous structures, such as the MWCNT modified GC elec-

trode we investigate, show a distribution of pore sizes leading

to a dispersed concentration of active species. This is often

represented by an equivalent circuit with multiple serial

and parallel impedances. This scheme incorporates the vari-

able distances from the pore orifice [40]. In our case this

was not necessary and the simple Randles circuit sufficed.

This might be due to the fact that we chose to coat only small

amounts of MWCNTs on the electrode in the case of Fe2+/Fe3+

and, in the case of VO2þ=VOþ2 , due to the slow electron trans-

fer kinetics.

According to Eq. (1)RCT decreases linearly with increasing

exposed interface area A, as indicated by the Nyquist plots

in Fig. 6a–d. Higher amounts of MWCNTs on the electrode in-

crease the actual area A. Values for CDL lower than the capac-

itance of the polished GC electrode were obtained for small

amounts of sample DEFUNC. This can be explained by the

non-polar and therefore hydrophobic character of those

MWCNTs which reduces the effective surface area.

In Fig. 7a values of R�1CT vs:CDL obtained from curve fitting

for samples PRIST, DEFUNC, NITSULF_3h and the polished

GC electrode immersed in Fe2+/Fe3+ containing electrolyte

are shown on a double logarithmic scale. All data points, ex-

cept the one with the lowest CDL for sample DEFUNC, can be

fitted linearly, which indicates that the two observables are

inversely proportional as predicted in Eq. (3). The deviation

for the one data point of sample DEFUNC might result from

a systematic error caused by gas bubbles on the hydrophobic

sample. This data point is marked with parenthesis and is ex-

cluded for the linear fit.

Mean values and absolute errors for the slopes are ob-

tained by fitting lines with maximal and minimal slopes.

The slope for sample NITSULF_3h, MWCNTs with a large

amount of functional groups, especially carboxylic groups,

is (18.3 ± 5.9) · 102 (O F)�1. For sample PRIST, the as-received

NC 3100, a slope of (8.1 ± 3.8) · 102 (O F)�1 was determined,

and for the thermally defunctionalized DEFUNC an even low-

er slope of (1.50 ± 0.26) · 102 (O F)�1 was observed. Therefore,

it can be concluded that this analytical method yields more

than ten times faster kinetics for MWCNTs with oxygen con-

taining surface groups than for MWCNTs devoid of almost all

functional groups. Such a behavior is consistent with earlier

studies which state that the ferrous/ferric redox reaction on

carbon materials is surface sensitive and affected by oxygen

containing groups [15–18]. The CDL and RCT of the polished

glassy carbon substrate in Fe2+/Fe3+containing electrolyte

are given in Table 3 and an empty, black circle denotes the

respective CDL and R�1CT in Fig. 7a. This data point coincides

with the linear fit for sample DEFUNC.

An entirely different behavior can be observed for the

VO2þ=VOþ2 reaction, as depicted in Fig. 7b. In general, all

slopes are significantly lower than for the Fe2+/Fe3+-reaction,

which reflects the slower reaction kinetics for the vanadium

redox reaction (k0 = 3.0 · 10�7 cm s�1[7]). Even if the true elec-

tron transfer constant k0 for the ferrous/ferric redox electrode

reaction is not easy to find, as very small traces of chloride

Fig. 8 – (a) CVs for samples PRIST and NITSULF_3h in 1 M H2SO4, scan-speed was 1 V/s. (b) Double layer capacitance CDL over

mass of MWCNT on the glassy carbon electrode for samples PRIST and NITSULF measured in 1 M H2SO4 and 1 M H2SO4 with

50 mM VO2+ and 50 mM VO2+. (c) Double layer capacitance CDL over mass of MWCNT on the glassy carbon electrode for all

samples in 1 M H2SO4 with 50 mM VO2+ and 50 mM VO2+ obtained from EIS.

236 C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9

have a considerable accelerating effect, the lowest reported

values in chloride free solution are around k0 = 2 · 10�5-

cm s�1[18]. For the VO2þ=VOþ2 reaction the sequence of the

reaction speeds for the individual samples seems to be in-

verted in comparison to Fe2+/Fe3+. This time the reaction

shows the fastest kinetics on electrodes coated with sample

DEFUNC, featuring a slope of (17.6 ± 3.3) (O F)�1. Samples

PRIST and NITRIC show intermediate slopes with (8.1 ± 1.5)

(O F)�1 and (7.8 ± 1.2) (O F)�1 respectively. The two samples

with the most functional groups, NITSULF_3h and NIT-

SULF_6h, have the lowest slopes, (6.2 ± 1.2) (O F)�1 and

(4.2 ± 1.1) (O F)�1. Again, a double-logarithmic scale for the

representation of Eq. (3) was chosen to separate the data

points for enhanced clarity. Two data points, one of sample

DEFUNC and one of sample NITRIC deviate from the linear

behavior and are marked with parenthesis and were not con-

sidered in the linear fit. The observed order of reaction speeds

contradicts earlier reports [10,11] and indicates that the

VO2þ=VOþ2 reaction is not catalyzed in the presence of func-

tional groups, like Fe2+/Fe3+.

To ensure that the different MWCNT samples can be com-

pared in the way we propose, we investigated their specific

double layer capacitance. For this purpose additional elec-

trodes coated with samples PRIST and NITSULF_3h were pre-

pared and tested in 1 M H2SO4 and in 1 M H2SO4 with 50 mM

VO2+ and 50 mM VO2+. To determine the CDL in 1 M H2SO4

without redox couple CVs at a scan-speed of dU/dt = 1 V/s

were recorded, as seen in Fig. 8a. The vertex potentials were

�0.025 V and 0.375 V vs. NHE and CDL was then calculated

according to:

CDL ¼Iox � Ired

2dUdt

� ��1

ð5Þ

Iox and Ired are the anodic and cathodic current respec-

tively and were measured at three different potentials and

then averaged. The shape of the CV of sample NITSULF_3h

deviates from the rectangular shape of an ideal capacitor

more than the CV of sample PRIST. This is due to the heavier

functionalization of NITSULF_3h and therefore a more pro-

nounced contribution of faradaic reactions to the current

in the CV. The values for samples PRIST, NITSULF_3h and

the GC electrode in 1 M H2SO4 and 50 mM VO2+ and 50 mM

VO2+ were determined from EIS, as described earlier.

Fig. 8b shows the values for CDL over the mass of MWCNT

samples and also CDL of the polished GC electrode. Data

points obtained from sample PRIST in both electrolytes and

from sample NITSULF_3h in 1 M H2SO4 and 50 mM VO2+

and 50 mM VO2+ match well and can be extrapolated onto

the GC substrate at zero mass. Sample NITSULF_3h in 1 M

H2SO4 gives constantly higher values for CDL due to the con-

tribution of faradaic reactions to the CV. In Fig. 8c those data

points from Fig. 8b, that were obtained from EIS, and the

data points from Table 3 in 50 mM VO2+ and 50 mM VO2+

are shown. CDL increases with increasing mass of MWCNT

on the GC electrode and all samples follow the same trend.

The data points show some scattering; this might be due to

the fact that the determined value for the mass of the

MWCNTs on the GC electrode is prone to error, as stated

in Section 2.3. However, no trend in those deviations can

be observed. This indicates, that the functionalization of

the MWCNTs has no influence on measured CDL, if the value

is obtained from EIS at a potential at which the functional

groups are not redox active.

4. Discussion

As shown by the spectroscopic and thermal gravimetric

methods, MWCNT samples were modified with functional

groups. It is difficult to create a narrow distribution of func-

tional groups as the present reactions are too manifold [41].

While the samples were not decorated with only one specific

type of functional group, samples with a wide range of differ-

ent amounts of functional groups were fabricated, as can be

seen by the percentile weight loss in the TGA–MS (see Table 2).

In addition samples NITSULF_3h and NITSULF_6h are deco-

rated with more carboxylic groups than the other samples,

as demonstrated by the TGA–MS and FTIR experiments. In a

Fig. 9 – Electron transfer constants for the two redox

systems under investigation versus the weight loss

recorded for the three MWCNT samples DEFUNC, PRIST and

NITSULF_3h. Dashed lines are just a guide to the eye to

demonstrate the principal dependence.

C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9 237

previous study catalytic properties were ascribed especially to

carboxylic groups [11].

Fig. 5 clearly demonstrates that CV is not suitable for deter-

mining the kinetic properties of charge transfer reactions on

porous electrodes. The shape of the CVs changes considerably

with varying amounts of MWCNTs on the electrode. The peak

separation is determined by the interplay of surface area and

hindered mass transport in the mesoporous electrode struc-

ture rather than by a combination of Butler–Volmer kinetics

and linear mass transport diffusion limitation.

Our results are based on the fact that the specific double

layer capacitance for the different samples is the same. While

Li and Li stated that carbonyl, carboxyl and hydroxyl groups

change the specific capacitance of CNTs [38], we think that

this does not falsify our results because:

- Carbonyl and carboxyl groups contribute by adding

pseudo-capacitance due to redox-reactions of the surface

groups. However, the equilibrium potential of these reac-

tions is at roughly U0 � 0 V vs. SCE � 0.243 V vs. NHE.

Therefore, they do not transfer electrons during EIS mea-

surement of the VO2þ=VOþ2 redox reaction at

U = (1.05 ± 0.01) V vs. NHE or the Fe2+/Fe3+redox reaction

at U = (0.69 ± 0.01) V vs. NHE.

- Functional groups increase the hydrophilicity of the

MWCNTs and therefore lead to increased wetting and a

larger surface area for functionalized samples [42]. Still,

this does not refute the measurements either, because

the additional electrode area which contributes to the

electrochemical double layer capacitance can also partici-

pate in the redox reactions so that this effect is inherently

considered.

The above hypothesis is affirmed by our finding, that the

specific double layer capacitance for the different samples is

not affected by the amount of functional groups on the

MWCNTs when determined from EIS, as shown in Fig. 8c.

The most fundamental kinetic constant, the electron

transfer constant k0, can be calculated from the exchange cur-

rent density:

K0 ¼j0

nFc1�aox ca

red

ð6Þ

In this equation cox and cred are the bulk concentration of the

oxidized and reduced species, cox = cred = 5 · 10�5 mol cm�3.

As the concentrations are equal in the performed measure-

ments, the exact value of the transfer coefficient a is of no

importance. As seen in Eq. (3) the conversion of the slope to

an exchange current density requires the knowledge of the

ratio tDL/er. This ratio was estimated by plotting R�1CT vs:CDL

for two differently sized GC disc-electrodes in VO2þ=VOþ2 con-

taining electrolyte and inserting the obtained slope of 1.4

(O F)�1 into Eq. (3), assuming the electron transfer constant

on GC to be kGC0 ¼ 3:0� 10�7 cm s�1[7]. With these assump-

tions a value of tDL/er = 2.2 · 10�11 m is calculated. . This ratio

is fairly large when assuming that tDL is the inner Helmholtz

plane (IHP) extending for some A and the relative permittivity

being that of water er � 80. However, according to Bockris

et al. the dielectric constant within the first water layer is

much smaller, roughly �IHPr � 7[43]. This result is supported

by other findings [44,45]. Considering this, the value for tDL/er

is a valid estimation for the case of the MWCNTs in 1 M

H2SO4 and corresponds to a thickness the innermost water

layer of roughly tDL � 2 A.

Comparing the values of the electron transfer constant for

the MWCNT samples with those for GC the DEFUNC shows

the highest increase of kDEFUNC0 =kGC

0 ¼ 12:7, while on NIT-

SULF_6h the increase is lowest, kNITSULF 6h0 =kGC

0 ¼ 3:0. The

amount of surface functional groups has no catalytic effect

on the VO2þ=VOþ2 redox reaction, unlike assumed earlier

[10,11]. Fig. 9 shows the electron transfer constant k0 versus

the weight loss recorded during the TGA–MS measurements,

which is proportional to the amount of functional groups on

the sample. The Fe2+/Fe3+ redox couple exhibits faster elec-

trons transfer kinetics as seen on the left ordinate, the higher

the functionalization of the surface the higher is k0. It was re-

ported earlier that anions such as chloride [16–18] and surface

oxygen groups [15] can accelerate charge transfer kinetics via

a bridging mechanism; this is well confirmed by the results

shown.

For the VO2þ=VOþ2 redox couple the situation is diametrical

to Fe2+/Fe3+; increasing functionalization reduces the origi-

nally small rate constant even further. The latter result clearly

demonstrates that surface groups do not lead to any enhance-

ment of the charge transfer rate for the vanadium couple. The

effect is rather opposite, functionalization reduces the elec-

tron transfer constant k0 by more than a factor of four.

An influence of the type of carbon used and its method of

preparation on the VO2þ=VOþ2 redox reaction has been re-

ported [7,46]. The study presented here indicates that defunc-

tionalized MWCNT promotes the VO2+/VO2+ redox reaction

better than MWCNTs with surface oxygen groups.

One hypothesis would be that polar surface groups might

not function as reaction sites but instead hinder the move-

ment of the vanadium molecules. In 1 M H2SO4 the surface

functional groups are protonated [47,48] and hydrogen-bonds

might form between those protons and the oxygen-ions of

the hydrated VO2+ and the VO2+-molecules.

238 C A R B O N 6 3 ( 2 0 1 3 ) 2 2 8 – 2 3 9

As the V2+/V3+ redox reaction is surface sensitive and af-

fected by oxides, as the Fe2+/Fe3+ reaction [15], heavily func-

tionalized MWCNTs, like sample NITSULF_3h, can

effectively catalyze the reaction by a bridging mechanism en-

abling a faster electron transfer between ion and electrode.

From the slopes in Fig. 7a it can be concluded that the Fe2+/

Fe3+ reaction is approximately ten times faster on sample

NITSULF_3h, than it is on the thermally defunctionalized DE-

FUNC. Therefore electrodes for the low potential side of the

VRB could benefit from surface functional groups.

Eq. (6) relates the experimentally accessible exchange cur-

rent density j0 to the electron transfer constant, the concen-

trations of the active species and the surface area of the

electrode. Especially for porous electrodes, such as ones made

from MWCNTs, the electrolyte–electrode interface system is

complicated. It is unclear how much of the electronically con-

nected carbon material is accessible to the molecules of the

electrolyte, which renders the actual surface area A an un-

known variable. There might even be pockets entirely se-

cluded from the bulk material, leading to a dispersion of the

values for cox and cred. Furthermore, diffusion might not be

uniform but dictated by the structure of the pores. Therefore,

some of the generally assumed electrochemical equations do

not hold as they were derived for linear or semi-hemispheri-

cal diffusion.

The complexity of the problem makes it comprehensible

that catalytic properties for the VO2þ=VOþ2 were assigned to

functionalized, porous electrodes in the past.

5. Conclusions

We present a novel way to determine the charge transfer

kinetics of redox reactions on porous MWCNT electrodes

and detected a catalytic behavior of surface functional groups

towards the Fe2+/Fe3+ reaction. However, for the VO2þ=VOþ2 we

could detect no catalytic effect. We conclude that polar mole-

cules play no role in the redox reaction and propose that such

molecules actually slow down the reaction by decreasing the

mobility of the vanadium ions.

The described method makes it possible to compare the

suitability of various electrodes-materials for their applicabil-

ity in RFBs, without having to know the exact surface area,

electrode geometry and porosity.

Acknowledgements

The authors would like to thank Prof. Harry Hoster and Dr.

Denis Yu for useful discussions.

This publication is made possible with the financial sup-

port from the Singapore National Research Foundation under

its Campus for Research Excellence and Technological Enter-

prise (CREATE) program.

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