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Electrochemistry Communications 9 (2007) 1924–1930

The effect of activation on the electrochemical behaviour of graphitefelt towards the Fe3+/Fe2+ redox electrode reaction

Victor Pupkevich, Vassili Glibin, Dimitre Karamanev *

Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada

Received 1 March 2007; received in revised form 18 April 2007; accepted 24 April 2007Available online 8 May 2007

Abstract

This paper is dedicated to the study of the effect of graphite felt activation by thermal oxidation in air on its electrocatalytic activitytowards Fe3+/Fe2+ redox electrode reaction. For the first time, the exchange current densities and electron transfer coefficients deter-mined from the Tafel equation were obtained within the wide range of burn-off levels (0–50%). The maximal catalytic activity wasobtained at the burn-off of 17%. The cathode having this burn-off level expressed almost three-fold enhancement in the galvanic cell per-formance (criterion for the performance evaluation in our case was a cell voltage at the current density of 300 mA cm�2) as compared tothat with the non-activated graphite felt, and allowed to obtain current densities up to 670 mA cm�2 at the cathode polarization as low as150 mV. The correlation between electrocatalytic activity and a surface oxide chemistry of graphite felt was established. The cell perfor-mance was found to be the best when the pH at a point of zero charge and the amount of surface quinoid groups per unit area wereminimal. The results obtained are of significant importance for practical applications, including the development of electrodes in redoxflow batteries.� 2007 Elsevier B.V. All rights reserved.

Keywords: Fe3+/Fe2+ redox electrode reaction; Exchange current; Electron transfer coefficient; Activated graphite felt; Surface oxide chemistry

1. Introduction

Redox flow batteries are very efficient alternative meansof energy storage possessing a number of advantages overconventional ones. However, the great potential of redoxbattery technology has not been fully revealed so far, anda recent commercialisation of three new redox systems [1]shows the potential of this very promising direction. Car-bon and graphite felts have been used extensively as elec-trode materials for redox flow batteries [1–5], inpreparative scale electrolysis [6], as well as for an impurityremoval and recovery of metals [7], among other applica-tions. The kinetics of the electrochemical processes on car-bon materials is quite sensitive to their physical, chemical,

1388-2481/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.elecom.2007.04.021

* Corresponding author. Tel.: +1 519 661 2111x88230; fax: +1 519 6613498.

E-mail addresses: vpupkevi@uwo.ca (V. Pupkevich), vglibin@uwo.ca(V. Glibin), dkaramanev@eng.uwo.ca (D. Karamanev).

and electrochemical pre-treatment such as polishing [8–11],heat treatment [11,12], argon sputtering treatment [10],laser activation [8], specific covalent bonding of organicmolecules to the carbon surfaces [8–11,13–16], intercalation[14,17], activation under various gasses [18–23], etc. Apartfrom the texture properties alteration (i.e. porosity, porevolume, pore size distribution, and surface area), the acti-vation of carbon and graphite fibres and other carbonmaterials leads to formation of functional groups on thesurface [8–23]. The detailed distribution of the oxygen-con-taining groups on the activated carbon (graphite) surfacesis not fully established yet, but carboxylic, lactone, phenol,carbonyl, anhydride, quinone and ether groups are thoughtto be predominant [12,15,17–20,24,25]. McCreery et al. [9]showed that increasing disorder during the activationallows for increasing the surface density of states at theFermi level, which facilitates a charge transfer, as well asnumber of specific chemical sites consequently resultingin a boost of electrochemical activity. Five types of

V. Pupkevich et al. / Electrochemistry Communications 9 (2007) 1924–1930 1925

influence of oxygen-containing functional groups on elec-trode reactions have been suggested in the literature: elect-rocatalytic effects, wetting, chemosorption, double layerand resistivity effects [8–12,14]. According to the McCreeryclassification [11,26], electrode kinetics of the Fe3+/Fe2+

redox system is greatly affected by C@O functional groupspresent on the electrode surface. However, as far as weknow, study of electrocatalytic activity of carbon fibrousmaterials with various burn-off levels towards the above-mentioned redox system has not been conducted yet. Inthis regard, the main aim of this work was an attempt tocorrelate the electrocatalytic activity of the activatedgraphite felt with the alteration of its surface oxide chemis-try depending on the burn-off level.

2. Experimental

2.1. Materials

The graphite felt SIGRATHERM� KFA-5 used in theexperiments was purchased from SGL Carbon Corp.(USA). The hydrogen electrode GDE LT 120E-W and cat-ion-exchange membrane Selemion HSF, used for separat-ing cathodic and anodic compartments, were purchasedfrom Fuel Cell Store (USA) and Asahi Glass Co. Ltd.(Japan), respectively. The solution of 0.45 M Fe2(SO4)3

was prepared by dissolving Fe2(SO4)3 (analytical grade,Merck) in deionized water and adjusting pH to 1.0 withsulphuric acid (Caledon). Series of aqueous solutions ofheptanoic acid with concentrations of 10�2, 7 · 10�3,5 · 10�3, 3 · 10�3, 10�3 M were prepared by sequentialdilution of a 10�2 M stock solution prepared from pureheptanoic acid (Sigma–Aldrich). The solution of 0.015 MTiCl3 was prepared by dilution of 20% TiCl3 in 3% HCl(Alfa Aesar). The compressed hydrogen was of UHP gradeand was used for carrying out the experiments with gal-vanic cell.

2.2. Graphite felt activation technique

The graphite felt activation was performed via thermaloxidation at 450 ± 2 �C in air. For this purpose20 · 20 cm specimens of the felt were placed into a muffleoven (Vulcan A-550) and weighed after certain periods oftime until a desired burn-off level was reached. The treat-ment temperature was found to be optimal both in termsof the oxidation rate and the possibility of getting uniformburn-off levels across the entire felt surface. Uniformity ofoxidation was estimated by measuring the electrical resis-tance of samples cut from different spots of activatedgraphite felt.

2.3. Electrical resistance measurement

For measuring the electrical resistance, 12.5 · 178 mmH-shaped samples were cut from the graphite felt sheet.The width of the felt strips at both ends was twice as wide

as the tested sample in order to reduce losses caused by thecontact resistance. The wide ends of the felt were pressedbetween two copper plates with similar size by screwclamps. The resistance was measured using a digital mul-timeter Mastercraft with a precision of ±1%. The term‘‘apparent specific resistance’’ represents the electrical resis-tance calculated per unit geometrical (visible) cross-sectional area of the sample studied.

2.4. Determination of the specific surface area

The surface area of the studied samples was determinedusing the method of heptanoic acid adsorption [27,28].Graphite felt samples of known mass (between 0.2 and0.5 g) were equilibrated with aqueous solutions of hepta-noic acid (125 mL) with various concentrations (10�2,7 · 10�3, 5 · 10�3, 3 · 10�3 M) for 48 h, and then the sur-face tension (r, dyn cm�2) of the solution was measuredby the maximum bubble air pressure using an apparatusdescribed in [28]. The equilibrium acid concentrations (C,mol l�1) were calculated by the modified Shishkowskiequation [28]:

Dr ¼ 2:303a log bþ 2:303a log C ð1Þwhere Dr is the difference between the surface tension ofdistilled water and that of the solution after adsorption;a and b are constants, equal to 14.1 and 955.0, respectively,in the case of heptanoic acid. Having assumed that the hep-tanoic acid adsorption (A, mol g�1) on the activated felt isdescribed by the Langmuir isotherm (monolayer equilib-rium adsorption), the specific surface area of the samples(S, m2 g�1) was calculated as follows:

S ¼ A1N ASm ¼A1C1

ð2Þ

where A1 is the limiting adsorption of heptanoic acid atthe graphite felt/solution interface, mol m�2, NA is Avoga-dro’s number, Sm is the area occupied by one molecule ofheptanoic acid, m2, and C1 is the limiting adsorption ofheptanoic acid at the solution/air interface, which is equalto a number of moles per unit area, mol m�2. The value ofthe limiting adsorption (A1, mol g�1) of heptanoic acid atthe solid/liquid interface was obtained from the slope of thelinear dependence of C

A versus C. The value of C1 was cal-culated by the following formula:

C1 ¼a

RTð3Þ

where R is the universal gas constant, 8.314 J mol�1 K�1, T

is temperature, K.The area occupied by one molecule of heptanoic acid

(Sm, m2) could be determined as:

Sm ¼1

C1NA

ð4Þ

In particular, C1 for the heptanoic acid was calculated as14:1 � 10�3

8:314 � 295¼ 5:7 � 10�6 mol m�2 and Sm was found to be

2.91 · 10�19 m2.

1926 V. Pupkevich et al. / Electrochemistry Communications 9 (2007) 1924–1930

The reliability of the technique used was tested by mea-suring the specific area of an activated KYNOL felt sam-ples with known surface area. The mean error indetermining the surface area by this technique was 13%.

Fig. 1. Schematic of the galvanic cell.

2.5. Analysis of surface groups

Recently, the most common technique, used for charac-terization of a chemical composition of surfaces, hasbecome X-ray photoelectron spectroscopy (XPS) [29,30].The XPS method has received a wide spread because ofits capability of determining different types of oxygenatedspecies present on the surface of various materials. How-ever, it is not always easy to single out and assign a certainpeak to a corresponding functional group [31]. Applicationof the Fourier transform at analysing spectra allowed toachieve fairly good precision in a peaks assignment[16,17,19,20]. However, in spite of its above advantages itmainly allows to do a surface elemental analysis (i.e. O/Catomic ratio) and relative surface concentrations (%) ofgiven functional groups rather than determine an actualconcentration (mmol g�1 or mmol cm�2). For this reasonthe functionality of carbon felt surfaces in this work wasdetermined applying chemical techniques. The content ofacidic functional groups on the carbon felt surface wasdetermined using the Bohem’s methods [24]. The samplesof graphite felt (�0.2–0.5 g) were placed into 125 mL Ehr-lenmeyer flasks, and then 30 mL of bases of differentstrength (0.05 M NaOH or 0.025 M Na2CO3) were intro-duced and allowed to equilibrate for 48 h. The excess ofthe base was titrated with 0.05 M HCl. The phenolic groupcontent was determined as a difference between the resultsof both titrations.

The amount of basic groups on the carbon surface wasdetermined by similar to the above technique with the onlydifference that the studied samples were equilibrated with0.05 M HCl solution and then titrated with 0.05 M NaOH.

The pH values at the point of zero charge (pHpzc), orisoelectric point, which could be the measure of a total sur-face acidity/basicity were obtained by the mass titrationmethod [32]. For this purpose, known mass (�0.01 g) ofthe activated graphite felt sample was added to 25 mL of0.1 M NaCl solution and the equilibrium pH was mea-sured. New portions of the felt were added until the pH sta-bilized. That pH reading was taken as a pH of the point ofzero charge.

The number of surface quinoid functional groups wasdetermined via their reduction with titanium(III) chloride.The samples of 0.2–0.6 g were equilibrated with the solu-tion of 0.015 M TiCl3 (60 mL). The residual titanium chlo-ride, left after the reaction with quinoid groups, wastitrated, in a strongly acidic medium, with 0.025 MK2CrO4 and phenylantranylic acid as an indicator. Inorder to prevent oxidation by air of the titanium (III) insolution, the entire procedure was carried out in inertatmosphere.

2.6. Determination of the electrocatalytic activity

The electrocatalytic activity of samples having differentburn-off levels was estimated on the basis of exchange cur-rent density of the Fe3+/Fe2+ redox couple as well as by theelectron transfer coefficient determined from the Tafelequation. The exchange current density is the product ofthe electrochemical specific rate constant and a concentra-tion term, and, in this respect, is useful in comparing thecatalytic activities of different electrode materials for givenreaction. For this reason, polarization curves of a galvaniccell with various flow-through cathodes were recorded.Commercially available hydrogen electrode was used asthe anode. Both electrodes had geometric (visible) area of17 cm2. The anodic and cathodic compartments were sepa-rated by cation-exchange membrane Selemion HSF(Fig. 1). This kind of a cell was used to create conditionsas close to those in redox flow batteries as possible. Theoverall reaction taking place in the cell was

Fe2(SO4)3 + H2! 2FeSO4 + H2SO4. ð5Þ

The catholyte, 0.45 M Fe2(SO4)3, was pumped through thecathode in a direction parallel to the membrane with theflow rate of 70 mL min�1. Electrode polarization curveswere measured by the steady-state galvanostatic methodusing an electronic load system Chroma 63103. All theexperiments were carried out at 21.5 �C and hydrogen pres-sure of 1 atm (abs).

3. Results and discussion

The kinetic curve of the thermal oxidation of graphitefelt in air at 450 �C and the effect of the burn-off level onthe electrical resistance are shown in Fig. 2. It can be seen

Fig. 2. Plots of the burn-off level versus time (a) and the specific apparent resistance versus burn-off (b) for the SIGRATHERM� KFA-5 graphite felt.

V. Pupkevich et al. / Electrochemistry Communications 9 (2007) 1924–1930 1927

that at the very beginning of the thermal oxidation process,there is a period (�5 h) where the burn-off rate of the sam-ple is gradually increasing (Fig. 2a). Beyond that initial per-iod, the activation intensifies and the burn-off becomesalmost linear function of time (within the burn-off rangeof 15–50%). Apparently, such behaviour points out thatthe oxidation follows an autocatalytic reaction mechanism.Similarly, the dependence of the apparent specific resis-tance (Fig. 2b) can be described by two different curves.Initially, it increases almost linearly, by about 45% at theburn-off of 21.8% compared to untreated graphite felt,while the further oxidation leads to an exponential growthof resistance with the burn-off level.

To evaluate the electrocatalytic activity of the felt, thegraphite felt cathodes were tested in the above-mentionedgalvanic cell. As shown in Fig. 3, the galvanic cell with acti-vated cathode (at 17.4% of burn-off level) showed muchhigher voltage at the same current density as comparedto the untreated graphite felt. The treated electrode allowedto obtain more than twice as high current density at shortcircuit as that obtainable with the non-activated graphitefelt. Such a significant enhancement in performance is alsoconfirmed by the exchange current densities (Fig. 4). Inparticular, the samples with 17.4% burn-off had a nine-foldincrease in the exchange current density compared to thosewith no activation. These data once again show that thesample burn-off level in the vicinity of 17% is optimum interms of electrocatalytic activity towards the Fe3+/Fe2+

redox reaction and facilitates the electron transfer(a = 0.47 at 17.4% burn-off). It should also be noted thatthe potential of the untreated graphite felt cathode at opencircuit was by 100 mV lower as compared to that of acti-vated, which could point out that the irreversible potentialwas imposed on the electrode.

In order to find out the origin of this phenomenon, themain surface chemistry characteristics of the graphite feltsamples with different burn-off percentages were deter-mined (Table 1, Fig. 5). As shown in Table 1, the amountsof the individual surface functional groups graduallyincrease as the function of a sample burn-off level and aspecific surface area. In contrast to the dependence of a sur-face group concentration on the burn-off level, relationshipbetween pH of the point of zero charge, characterizing thecolligative acid-base properties of a surface, and the burn-off level has the minimum at 21.8% (Fig. 5), and is equal to6.01.

The surface chemistry data obtained are qualitatively ingood agreement with studies by XPS method, whichrevealed presence of the same types of functional surfacegroups on the pre-treated carbon and graphitized carbonfibres [19,21–23]. It was previously shown that the sampletreatment in various gaseous atmospheres, such as CO2 orO2 [19,21], N2 and O2 mixtures [22], O3 [23] etc., results inincreasing the amount of different types of functionalgroups. However, so far there is not too much consistencybetween XPS results on the ratios of surface functionalgroups available in the literature. In particular, Blythet al. [19] determined the highest concentration of phenolicgroups, and less but almost equal amounts of carbonyl andcarboxylic groups. In contrast, Jin et al. [23] reported thatthe major surface group is a carbonyl one, while carboxylicand phenolic are present in fairly equal numbers, but aretwice as less as for carbonyl. Apparently, surface func-tional group composition is highly sensitive to the pre-treatment method as well as carbon material origin, which,in bundle, account for a significant difference in surfacegroup concentrations. Similarly to Jin’s et al. work [23],our experimental data (Table 1) points out that the

Fig. 3. Polarization curves and cell voltage for a galvanic cell with non-treated and activated (17.4% burn-off) graphite felt cathode at 21.5 �C.

Fig. 4. Plots of the exchange current density and electron transfer coefficient versus sample burn-off level.

Table 1Surface properties of the SIGRATHERM� KFA-5 graphite felt

Burn-off(%)

Surface area(m2 g�1)

Number of groups (mmol g�1)

Carboxylic Phenolic Quinoid Basic

0 1–2 – – – –9.6 48 ± 16 0.248 0.034 0.64 0.016

17.4 180 ± 23 0.450 0.041 1.13 0.01921.8 300 ± 38 0.530 0.275 1.16 0.03634.6 420 ± 53 0.720 0.387 2.21 0.02249.7 540 ± 26 0.870 0.680 3.09 0.07162.0 370 ± 46 0.910 0.490 2.48 0.212

1928 V. Pupkevich et al. / Electrochemistry Communications 9 (2007) 1924–1930

predominant groups present on the surface are quinoid(carbonyl) ones.

In accordance with McCreery’s studies [9,26], thosemake the major contribution into electrocatalytic activityof different carbon materials. Mainly it occurs because offormation of inner sphere complexes between surface car-bonyl groups and aquated iron ions, which in turn facilitateelectron transfer. Beyond that, a comparison of stabilityconstants for the complexes of ferric and ferrous ions withdifferent oxygen-containing organic ligands in the bulkrevealed that the former are always more stable compared

Fig. 5. Plots of the pHpzc and the number of surface quinoid groups versus sample burn-off level.

V. Pupkevich et al. / Electrochemistry Communications 9 (2007) 1924–1930 1929

to the latter ones [33]. Having supposed that stability ofsurface complexes behaves in a similar manner, this factcan apparently be considered as another favourable factorfor carrying out the electrochemical reaction.

However, bridging mechanism model seems not toreflect the entire complexity of the electrochemical processand another factor, such as the low double layer capaci-tance, could favour the electron transfer as well. Fryszet al. [18] reported that carbon fibres with 17–22% burn-off possess the lowest capacitance, which is a result of theirmicrostructure, i.e. microholes, being formed at thermaloxidation in air, do not deeply penetrate into the interioryet.

In addition, we believe that there are other factors,which could significantly lower electrocatalytic activity.According to Kastening et al. [34] protons adsorbed onthe surface @C@O groups (quinoid groups in our particu-lar case) create holes (h+) by the following scheme:

ð6Þ

In turn, the holes formed are mobile charge carriers, whichaccount for a double layer capacity of the activated carbonelectrodes, and also can be considered as ‘‘traps’’ forelectrons.

Having studied chemical and surface properties of differ-ent carbon blacks, Fabish et al. [35] established that theirwork function has a non-linear dependence on a pH, andits minimum falls into the pH range of 5.5–6.0. It is inter-esting to note, that in our case, the minimum pHpzc valuewas found to be in the same pH range for the sample withthe burn-off of 17.4% (Fig. 5), which also exhibited thehighest electrocatalytic activity. According to Kinoshita[36] such a minimum is observed due to the least influence

of the basal plane electronic systems (or Cp). In this regard,it is reasonable to explain reduction in the electrochemicalactivity after the burn-off of 17% by a significant increase inthe amount of basal planes on the surface. This suggestionis indirectly supported by shifting the pHpzc into basicregion as the burn-off level increases (Fig. 5), since, it isthought, the basal plane system behaves as a Lewis baseand takes part in the following reaction [36]:

Cp + 2H2O = CpH3Oþ+ OH� ð7Þ

4. Conclusions

A highly efficient cathode material was obtained by ther-mal oxidation of the SIGRATHERM� KFA-5 graphite felt.Its electrocatalytic properties towards the Fe3+/Fe2+ redoxreaction allowed to obtain current densities up to670 mA cm�2 at low polarization of the cathode, whichmakes it very promising as an electrode in redox flow cells.The study of the surface properties of the activated graphitefelt showed that observed maximum of electrocatalyticactivity is a complex effect of: (a) the proper number of func-tional groups present on the surface, which in turn areresponsible for formation of inner sphere surface complexeswith iron ions, and in this way, facilitate electron transfer; (b)minimal amount of the surface @C@O groups per unit areaon the activated graphite felt, which account for a doublelayer capacitance and (c) low electrical resistance.

Acknowledgements

This work was supported financially by the Natural Sci-ence and Engineering Research Council of Canada and bythe Ontario Centres for Excellence.

1930 V. Pupkevich et al. / Electrochemistry Communications 9 (2007) 1924–1930

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