Journal of The Electrochemical Society, 161 (1) A137-A141 (2014) A1370013-4651/2014/161(1)/A137/5/$31.00 © The Electrochemical Society

Study on the Electrochemical Kinetics of ManganeseDioxide/Multiwall Carbon Nanotube Composite by VoltammetricCharge AnalysisSeung-Beom Yoon,a Hyun-Kon Song,b Kwang Chul Roh,c,z and Kwang-Bum Kima,z

aDepartment of Material Science and Engineering, Yonsei University, Seoul 120-749, KoreabDepartment of Energy Engineering and Department of Chemical Engineering, UNIST, Ulsan 689-798, KoreacEnergy Efficient Materials Team, Energy & Environmental Division, Korea Institute of Ceramic Engineering andTechnology, Guemcheon-Gu, Seoul 153-801, Korea

Electrochemical properties of MnO2/multiwall carbon nanotube (MWCNT) composites were investigated by using cavity microelec-trodes (CME). Electrochemical kinetics of the MnO2/MWCNT composites were studied by analyzing the scan rate dependence ofvoltammetric charge, which was measured by cyclic voltammetry (CV) at various scan rates ranging from 20 mV s−1 to 1000 mV s−1.Based on several mathematical models, the relationship between voltammetric charge and scan rate was interpreted systematically.At slow scan rates, ion diffusion in MnO2 dominantly determined the rate of the overall electrochemical process. However, thefaradaic reaction of Mn3+/Mn4+ at the MnO2 surface competed with mass transfer in terms of kinetics when the potential scan ratewas higher than 400 mV s−1.© 2013 The Electrochemical Society. [DOI: 10.1149/2.070401jes] All rights reserved.

Manuscript submitted October 10, 2013; revised manuscript received November 18, 2013. Published November 23, 2013.

Cyclic voltammetry (CV), a powerful tool for the analysis of elec-trochemical systems, has been widely used to study electrochemi-cal kinetics and to monitor the surface electrochemical reactions oftransition-metal oxide electrodes.1–11 CV data are interpreted on thebasis of the relationship between current (I) and potential scan rate(ν) or between voltammetric charge (q*) and ν. In the former case, thecurrent response in the CVs can be deconvoluted into two terms: (1)the current attributed to the surface capacitive process (Is) and (2) thecurrent attributed to diffusion-controlled insertion processes (Id):5,9,10

I = Is + Id = k1ν + k2ν1/2 [1]

In the latter case, the charge obtained from voltammetric measure-ments includes components corresponding to outer (q*

outer, indepen-dent of ν) and inner voltammetric charge (q*

inner, proportionally relatedto ν−1/2):1–4,7,8,11

q∗ = q∗outer + q∗

inner = q∗outer + k3ν

−1/2 [2]

In both approaches, the total number of electrochemical reaction sitesis divided into surface and bulk reaction sites. In Eq. 1, the currentterms, which are proportional to ν and ν1/2, correspond to the constantand ν−1/2-dependent charge terms in Eq. 2, as the surface and bulkdescriptors, respectively.

While these relationships allow for a better understanding of theelectrochemical kinetics of electrochemical energy storage materials,they suffer from certain limitations. The relationship between I and ν1/2

and that between q* and ν−1/2 is not always linear in the desired scanrate range,3,6 making these dependences unsuitable for interpretingthe CV data measured over the entire potential scan rate range in agiven analytical method.

In this study, we analyzed the electrochemical kinetics of an elec-trode material at a low potential scan rate, for the diffusion of ionsin the active material (bulk electrochemical reaction sites). Moreover,we investigated the electrochemical kinetics at a high potential scanrate for a surface electrochemical reaction on the basis of a modifiedrelationship between q* and ν. A MnO2/multiwall carbon nanotube(MWCNT) composite was chosen as an electrode material to inves-tigate electrochemical kinetics at both low and high potential scanrates. The charge storage mechanism in MnO2 is based on ion inser-tion, similar to that of other electrochemical energy storage materials.However, the rate capability of MnO2 is superior to that of the con-ventional electrode materials used in lithium-ion batteries becauseMnO2 has a thinner effective diffusion layer. Moreover, introductionof the highly conductive MWCNT into MnO2 would improve the ratecapability of the resultant composites. Cavity microelectrodes (CME)

zE-mail: kbkim@yonsei.ac.kr; rkc@kicet.re.kr

were used for electrochemical measurements of the MnO2/MWCNTcomposites; these electrode materials have been widely used in thestudy of Li-ion batteries and electrochemical capacitors.10–18 Sincethe CME allows for the use of a potential scan rate of a few V s−1, it iswell-suited to investigate the electrochemical kinetics at much higherscan rates as compared to a conventional electrode.

Experimental

The MnO2/MWCNT composite was fabricated by the direct redoxreaction between MWCNTs and permanganate ions in an aqueous so-lution. Briefly, 1.0 g of MWCNTs (specific surface area = 200 m2 g−1;ILJIN Nanotech) dispersed in a 200 mL aqueous solution of 0.1 MKMnO4 (Aldrich) was heated and maintained at 70◦C. The suspen-sion was filtered and washed repeatedly with distilled water to affordMWCNTs coated with birnessite-type MnO2 including an amorphousphase. The detailed synthetic procedure and characterization is de-scribed elsewhere.19

Electrochemical measurements of all the MnO2/MWCNT com-posites were performed in a three-electrode electrochemical cell. ACME filled with the MnO2/MWCNT composite was used as theworking electrode, while a platinum plate and a Ag/AgCl/saturatedKCl electrode were used as the counter and reference electrodes,respectively. CV experiments were performed using a potentio-stat/galvanostat (Solartron 1470E, Cell Test System) in a 1 M Na2SO4

aqueous solution in the potential window between 0.0 and 0.8 V(vs. Ag/AgCl).

Results and Discussion

Figure 1a shows the cyclic voltammograms of the MnO2/MWCNTcomposite for various scan rates ranging from 20 mV s−1 to1000 mV s−1. The cyclic voltammograms of the composite inthe CME retained rectangular shapes up to scan rates as high as1000 mV s−1, indicating ideal capacitive behavior. However, despitea slight distortion in the voltammetric curves, the composite exhibiteda decrease in q*. As shown in Figure 1b, q* decreased drastically forscan rates below 200 mV s−1 and decreased gradually when the scanrate was increased up to 1000 mV s−1.

Gaberscek et al. studied the effect of electronic wiring topologyon the electrochemical kinetics of a given electrode. They suggestedthat the surface electrochemical reaction, which occurs at the site ofcontact of the electronic and ionic reservoirs with each wire, is fasterthan solid-state diffusion within bulk particles.20 According to thismodel (Figure 2a), total current is the sum of the currents ascribed tothe surface and bulk electrochemical reactions. The response of the

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A138 Journal of The Electrochemical Society, 161 (1) A137-A141 (2014)

Figure 1. (a) Cyclic voltammograms of the MnO2/MWNT composite in theCME, obtained from 20 mV s−1 to 1000 mV s−1, and (b) voltammetric chargeof the MnO2/MWNT composite in the CME as a function of potential scanrate.

current at a fixed potential is deconvoluted into the current attributed tothe surface capacitive effect and into the diffusion-controlled insertionprocesses:

I(V or t) = Isurface + Ibulk = AC(V or t)ν + AD(V or t)ν1/2 [3]

where C(V or t) and D(V or t) indicate the coefficients related to the surfacecapacitive effect and the solid-state diffusion at each potential, respec-tively. This equation generated from Eq. 1 suggests that the kineticsof the overall charge storage process is influenced by the diffusion ofions in the solid electrode materials. The electrochemically effectivesurface area, A (area of surface electrochemical reaction site), is givenby:

A = ns As [4]

where ns and As are the number of electrochemical reaction sites andthe area of each site, respectively. Effective surface area A, whichis the area over which charge transfer occurs, is not the Brunauer–Emmett–Teller area or the electrode area and hence is difficult orimpossible to be measured. The bulk electrochemical reaction site isinvolved in charge storage through the diffusion of cations insertedinto MnO2 following the surface electrochemical reaction; hence, thediffusion area in the bulk electrochemical reaction site might be similaror proportional to the area of the surface electrochemical reactionsite. When the current response was deconvoluted into contributionsfrom the surface and bulk electrochemical reactions, the current couldbe investigated at a fixed potential.9 When the scan rate increased,the current response also generally increased. However, the currentresponse sometimes decreased at a fixed potential despite an increasein the potential scan rate; this was due to the shift in the peak potentialresulting from a high overpotential. In that case, it is difficult to analyze

the electrochemical kinetics using relationship between the scan rateand current response. Therefore, we separate the contributions fromthe surface and bulk electrochemical reactions using the voltammetriccharge obtained by integrating the current:∫ t

t0

I(V or t)dt = Av

∫ t

t0

C(V or t)dt + Av12

∫ t

t0

D(V or t)dt = q∗s + q∗

b

[5]where q*

s and q*b indicate the voltammetric charges related to the

surface capacitive effect and solid state diffusion, respectively. In theplot of I vs. t, since the current is varied with potential and time,C(V or t) and D(V or t) also change with potential and time (Figure 2b).The voltammetric charge could be calculated from this plot, thoughC(V or t) and D(V or t) could not be calculated. In order to readily analyzethe relationship between the scan rate and voltammetric charge, meancoefficients related to the surface capacitive effect (C*) and solid-statediffusion (D*) were adopted. Since only the voltammetric charge (areaof the plot of I vs. t) was considered in our study, we assumed theplot of I vs. t (Figure 2c) with the same voltammetric charge and timerange using C* and D*.

Eq. 5 can be written as:

q∗ = AνC∗(t − t0) + Aν12 D∗(t − t0)

= AC∗(V − V0) + AD∗(V − V0)ν− 12 [6]

q∗

�V= AC∗ + AD∗ν− 1

2 = q∗s

�V+ q∗

b

�V[7]

where �V indicates the integrated potential range, and q* is obtainedby integrating the anodic current in the anodic scan and cathodiccurrent in the cathodic scan. Since the voltammograms are distortedat a high potential scan rate, the potential range decreases with anincrease in the potential scan rate; q* was divided by the integratedpotential range at each potential scan rate, and q*/�V was plotted asa function of ν−1/2 to analyze the electrochemical kinetics, as shownin Figure 2d. A linear relation between q*/�V and ν−1/2 was found atscan rates of 20 mV s−1 and 100 mV s−1, but deviated from linearityat higher scan rates, indicating that Eq. 7 is unsuitable for the analysisof electrochemical kinetics at high potential scan rates.

At low potential scan rates, the kinetics is influenced by only thesolid-state diffusion in MnO2, and the voltammetric charge per unitvoltage (q*

s/�V = AC*), related to the surface electrochemical reac-tion, remains constant but changes as the potential scan rate increases.The area (or number) of the surface electrochemical reaction siteswill be reduced by another kinetics limitation factor with an increasein the potential scan rate. Since the surface electrochemical reactionsite is connected to the electronic as well as ionic paths as shown inFigure 2a, the electrochemical kinetics of the surface electrochemicalreaction is supposed to be influenced by the supply of electrons andions, or the Mn3+/Mn4+redox reaction rate. The number of surfaceelectrochemical reaction sites (ns) might change with the feed rate ofelectrons and ions, or with the redox reaction rate at the MnO2 surface.

If the supply of electrons is the kinetics limitation factor in thesurface electrochemical reaction, the area for this reaction (the num-ber of surface electrochemical reaction sites) can be determined bythe number of electrons reaching the MnO2 surface. According toOhm’s law, the voltammetric charge is proportional to ν−1, as shown inFig. S3. We assumed that electrical resistance of electrode materials(R) is a constant value. The relationship between the number of sur-face electrochemical reaction sites (or voltammetric charge) and thepotential scan rate is as follows:

ns = q∗e

F= �V 2

2RFν−1 [8]

where q*e and F are the amount of charge carried by the electrons

and the Faraday constant, respectively. The area of the surface elec-trochemical reaction site can be expressed as:

A = As�V 2

2RFν−1 [9]

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Journal of The Electrochemical Society, 161 (1) A137-A141 (2014) A139

Figure 2. (a) Schematics of the surface electrochemical reac-tion site connected to the electronic and ionic reservoir witheach wire and bulk electrochemical reaction site associatedwith solid-state diffusion; (b) voltammetric charge calculatedfrom I–t (or V) curve with peak current; (c) voltammetric chargecalculated from I–t (or V) curve by adopting mean coefficientsrelated to the surface capacitive effect (C*) and solid-state diffu-sion (D*); (d) plot of voltammetric charge per voltage (q*/�V)of the MnO2/MWNT composite in the CME at various scanrates (ν) as a function of v−1/2.

Then, on the basis of Eq. 9, Eq. 7 can be re-written as:

q∗

�V= As

�V 2

2RFν−1C∗ + As

�V 2

2RFν−1 D∗ν− 1

2 [10]

q∗ν�V 3

= As

2RFC∗ + As

2RFD∗ν− 1

2 [11]

As shown in Figure 3a, there exists a linear relationship betweenq* ν/�V3 and ν−1/2 at a high potential scan rate, but a negative slopeis observed despite As, R, F, D*, and �V being positive values. Thissuggests that the feed rate of electrons is not the kinetics limitationfactor in the surface electrochemical reaction.

If we assume that the kinetics limitation factor in the surface elec-trochemical reaction is governed by ion diffusion in the electrolyte,the area of the surface electrochemical reaction would be determinedby the number of ions diffusing to the MnO2 surface. The voltam-metric charge governed by ion diffusion is proportional to ν−1/2. Thedetails about establishing the equation is shown in the SI. 4. The rela-tionship between the number of surface electrochemical reaction sitesand potential scan rate is considered to be:

ns = q∗i

F= D∗

s �V

Fν− 1

2 [12]

where q*i and D*

s are the amount of charge carried by the ions anda coefficient related to the diffusion of ions in the electrolyte, respec-tively. The area of the surface electrochemical reaction sites can be

written as:

A = As D∗s �V

Fν− 1

2 [13]

Using Eq. 13, Eq. 7 can be written as:

q∗

�V= As D∗

s �V

Fν− 1

2 C∗ + As D∗s �V

Fν− 1

2 D∗ν− 12 [14]

q∗ν12

�V 2= As D∗

s

FC∗ + As D∗

s

FD∗ν− 1

2 [15]

In this case, also, a linear relationship is observed between q*ν1/2�V−2

and ν−1/2 at a high potential scan rate (Figure 3b), but despite thepositive D*

s value, the slope is negative. Moreover, ion diffusion in theelectrolyte is not the rate-limiting step in the surface electrochemicalreaction.

Finally, we assumed that the rate of the Mn3+/Mn4+ redox reactionat the MnO2 surface is the kinetics limitation factor in the surfaceelectrochemical reaction, and that this redox reaction is a first-orderreaction. As shown in S5, the relationship between the number ofsurface electrochemical reaction sites and the potential scan rate is:

ns = C0(1 − e− k�Vν ) [16]

where C0 and k are the total number of Mn atoms on the surface ofthe active material and the rate constant, respectively. The area of the

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A140 Journal of The Electrochemical Society, 161 (1) A137-A141 (2014)

Figure 3. (a) Plot of q∗ν/�V 3 of the MnO2/MWNT composite in the CMEat various scan rates (ν) as a function of v−1/2; (b) plot of q∗ν1/2/�V 2 of theMnO2/MWNT composite in the CME at various scan rates (ν) as a function ofv−1/2; (c) plot of q∗/�V [1 − exp(−k�V/ν)] of the MnO2/MWNT compositein CME at various scan rates (ν) as a function of v−1/2.

surface electrochemical reaction site can be expressed as:

A = AsC0(1 − e− k�Vν ) [17]

Using Eq. 17, Eq. 7 can be written as:

q∗

�V (1 − e− k�Vν )

= AsC0C∗ + AsC0 D∗ν− 12 [18]

When k is 7 s−1, a linear relationship between q*/�V[1 − exp(−k�V/ν)] and ν−1/2 is observed in the high scan rate region, anda positive slope is obtained (Figure 3c). This result is in agreementwith the positive C0 value. However, the relationship generated fromthe rate equation will not apply in all cases. As shown in Figure 4,

Figure 4. Plots of q∗/�V [1 − exp(−k�V/ν)] and ν−1/2 at (a) k = 1 s−1,(b) k = 5 s−1, and (c) k = 10 s−1.

when the rate constant is below 5 s−1, the relationship expected fromEq. 18 is not observed, suggesting that the redox reaction should havea rate constant above 5 s−1.

From the above-mentioned analysis of the MnO2/MWCNT com-posite, we can say that the voltammetric charge is a function of thepotential scan rate. The voltammetric charge is governed by ion diffu-sion in MnO2 at low scan rates and by the redox reaction at the MnO2

surface as well as the ion diffusion in the active material for scan ratesexceeding 400 mV s−1. Figure 5 depicts the three different sites onthe MnO2 material: two electrochemical reaction sites and an electro-chemically unreactive site. The surface electrochemical reaction siteis accessible to both electrons and ions, while the bulk electrochem-ical reaction site is inaccessible to either electrons or ions at a giveninstance, or to both. The bulk electrochemical reaction site partici-pates in electrochemical reactions through an ion diffusion processin MnO2. The electrochemically unreactive site cannot participate inthe electrochemical reaction because of kinetic limitations. At a low

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Journal of The Electrochemical Society, 161 (1) A137-A141 (2014) A141

Figure 5. Schematics of change in the electrochemical reaction site as afunction of potential scan rate.

potential scan rate range (below 200 mV s−1), as the potential scanrate increases, the electrochemically unreactive sites increase only inthe bulk electrochemical reaction site. However, at a high potentialscan rate range (above 400 mV s−1), the electrochemically unreactivesites increase in both bulk and surface electrochemical reaction sites.

In summary, the voltammetric charge is composed of the chargefor the bulk electrochemical reaction site and the charge for thesurface electrochemical reaction site. At lower potential scan rates,the voltammetric charge shows a scan rate dependence of the bulkelectrochemical reaction due to the kinetic limitation of the ion diffu-sion process in the bulk, however, it shows no scan rate dependenceof the surface electrochemical reaction. At higher potential scan rates,the voltammetric charge shows not only a scan rate dependence of thebulk electrochemical reaction due to the kinetic limitation of the iondiffusion process in the bulk, but also a scan rate dependence of thesurface electrochemical reaction due to the kinetic limitation of thecharge transfer process in the surface electrochemical reaction. Thelatter gets more pronounced as the potential scan rate is increased.

Conclusions

Cyclic voltammograms of MnO2/MWCNT composites were ana-lyzed using the voltammetric charge equations to investigate the elec-trochemical kinetics of the MnO2/MWCNT composites as a functionof the potential scan rate. At a low potential scan rate, the voltam-metric charge, related to the surface electrochemical reaction site, is

constant and the loss in voltammetric charge is ascribed to the kineticlimitation of the ion diffusion process in the bulk electrochemical re-action site. When the potential scan rate exceeds 400 mV s−1, the lossin voltammetric charge is due to the kinetic limitation of the chargetransfer process in the surface electrochemical reaction and the iondiffusion process in the bulk electrochemical reaction.

Acknowledgment

This work was supported by an Energy Efficiency & Resourcesof the Korea Institute of Energy Technology Evaluation and Plan-ning (KETEP) grant funded by the Korea government Ministry ofKnowledge Economy (No. 20122010100090). This study was par-tially supported by a grant (B551179-10-01-00) from the cooperativeR&D Program funded by the Korea Research Council Industrial Sci-ence and Technology, Republic of Korea.

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