Journal of Electroanalytical Chemistry 722-723 (2014) 32–37

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Journal of Electroanalytical Chemistry

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Electrogeneration of hydrogen peroxide in gas diffusion electrodes:Application of iron (II) phthalocyanine as a modifier of carbon black

http://dx.doi.org/10.1016/j.jelechem.2014.03.0071572-6657/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +55 16 33738659; fax: +55 16 33739903.E-mail address: marcoslanza@iqsc.usp.br (M.R.V. Lanza).

Fernando L. Silva, Rafael M. Reis, Willyam R.P. Barros, Robson S. Rocha, Marcos R.V. Lanza ⇑Instituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São Carlense 400, São Carlos 13566-590, SP, Brazil

a r t i c l e i n f o

Article history:Received 13 December 2013Received in revised form 5 March 2014Accepted 6 March 2014Available online 15 March 2014

Keywords:Hydrogen peroxideGas diffusion electrodesIron (II) phthalocyanineElectrochemical reduction of oxygen

a b s t r a c t

Hydrogen peroxide (H2O2) is commonly produced by redox reactions involving organic compounds inorganic medium, but such processes present several limitations including the need to extract and concen-trate a highly active product. Hydrogen peroxide can be generated in situ by the electrochemical reduc-tion of oxygen in aqueous medium, and the process is particularly efficient when gas diffusion electrodes(GDEs) are employed. A key challenge in the development of such electrodes is the choice of the catalyticparticles. This paper describes the evaluation of iron (II) phthalocyanine as a modifier of pigment carbonblack used in the construction of GDEs. After 90 min of electrolysis at constant potential, a GDE contain-ing 5% of modifier generated 240 mg L�1 of H2O2 (rate constant 7 mg L�1 min�1; energy consumption165 kW h kg�1 H2O2) while the unmodified GDE produced only 175 mg L�1 of H2O2 (3 mg L�1 min�1;300 kW h kg�1 H2O2) under the same experimental conditions.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Hydrogen peroxide (H2O2) is a potent oxidizing agent and isused widely in organic synthesis, in bleaching paper and in thetreatment of effluents containing organic contaminants [1–8].Hydrogen peroxide may be synthesized on an industrial scale bya process involving autoxidation of a substituted anthrahydroqui-none in organic medium, followed by reduction of the anthraqui-none so-formed by hydrogen gas in the presence of a catalyst.While the anthraquinone method accounts for approximately85% of commercially produced H2O2 [9], the process is subject toa number of restrictions since it involves extraction of the organicphase followed by concentration and purification of the activeagent. A number of alternative methods for the industrial synthesisof H2O2 have been proposed, such as direct synthesis from molec-ular O2 catalyzed by Au/Ti [10] or Pd/Au [11] and the application ofmicrobial fuel cells [12]. However, these methods suffer from var-ious limitations including poor production efficiencies, elevatedcosts and low concentrations of H2O2 formed. Furthermore, manu-facture of H2O2 at a site that is distant from its point of use neces-sitates the storage and transportation of a somewhat unstable andhighly active agent [9,13,14].

In this context, electrochemical technology offers the possibil-ity of generating H2O2 in situ, in aqueous medium at a range of

temperatures and pH values, and at concentrations reaching upto hundreds of milligrams per liter [15–21]. The primary reactantin the electrochemical process is O2 and this should be dissolvedin the reaction medium [22]. The rate of the electrochemicalprocess depends on the efficient replacement of O2 in the electro-lyte present at the electrode surface. However, the solubility of O2

in water is low and decreases with increasing temperature of themedium, a factor that determines the operational limit for thesynthesis of high concentrations of H2O2 when conventional elec-trodes are employed. On the other hand, gas diffusion electrodes(GDEs) comprise a porous and hydrophobic structure that enablesO2 to be supplied directly to the electrode/electrolyte interfacewithout limit [23–27]. In this manner, the GDE removes the limita-tions of mass transportation imposed by the low solubility of O2

[27], and allows high yields of H2O2 to be generated in both acidic[28] and alkaline [29] media.

A key challenge in developing GDEs specifically for the genera-tion of H2O2 is the choice of catalytic particles. Typically, GDEs areconstructed with pigment carbon black, the particles of which havea graphite-type crystalline structure. In order to maximize the effi-ciency of H2O2 generation and to achieve higher concentrations ofthe peroxide, particularly at less negative potentials, it is necessaryto incorporate modifiers into the graphitic material. Studies havebeen conducted with various types of modifiers including organic[25] and organometallic [24] compounds and rare earth oxide nano-particles [26]. Such investigations are particularly important sincethe results obtained can provide an indication of the mechanism

F.L. Silva et al. / Journal of Electroanalytical Chemistry 722-723 (2014) 32–37 33

of the oxygen reduction reaction (ORR), which may involve a2-electron transfer for the formation of H2O2 [30,31] or a4-electron transfer with application in fuel cells [32,33]. In thisresearch review article, we describe an investigation of theformation of H2O2 in GDEs prepared using carbon black modifiedwith different amounts of iron (II) phthalocyanine (FePc; CASnumber 132-16-1).

2. Experimental

2.1. Evaluation of percentage modifier required for the efficientgeneration of H2O2

An initial study of the efficiency of H2O2 generation on carbonblack with or without modifier was performed using the micropo-rous layer technique and a rotating ring-disc electrode (RRDE). APine Instruments model E7R9-GC/Pt RRDE, comprising a centraldisc of glassy carbon and a Pt ring (coefficient of collectionN = 0.37), formed the working electrode, with Pt as the counterelectrode and Ag/AgCl as the reference electrode. A portion(3 mg) of a mixture containing Printex 6L carbon (Evonik Co.),hereinafter referred to as carbon black, and 0, 0.1%, 1.0%, 3.0%,5.0% or 10.0% (w/w) of FePc (Aldrich # 379549; dye content � 90%)was suspended in 3 mL of water in an ultrasonic bath, and a 20 lLaliquot transferred by pipette onto the glassy carbon disc. Aconstant flow of N2 was applied to the disc in an open environmentin order to expedite evaporation and formation of a singlemicroporous layer of FePc on the surface of the disc. An aqueousalcoholic solution of Nafion� perfluorinated resin (Aldrich #510211; resin content 5% by weight) was diluted 1:100 with water,and a 20 lL aliquot was placed on the microporous layer and driedunder a N2 flow as described above. Samples containing carbonblack with and without modifier were characterized physicallyusing an X-ray fluorescence (XRF) spectrometer (PANalytical BV)equipped with a Rh tube.

The electrochemical experiments were performed using anAutoLab 302N PGSTAT bi-potentiostat controller coupled to a PineInstruments AFMSRCE rotator control box. Carbon black withoutmodifier was employed as reference for the 2-electron transferORR, while carbon supported Pt/C electrocatalyst (E-TEK) servedas reference for ORR by a 4-electron transfer [18,20,24–26]. In eachcase, the electrode system was characterized electrochemically bylinear voltammetry (LV) in an aqueous electrolyte containingH2SO4 (0.1 mol L�1) and K2SO4 (0.1 mol L�1). The electrolyte wasbubbled with N2 for 20 min prior to the initial determination ofLV, following which O2 was bubbled through the electrolyte for40 min and LV was performed again with the O2 flow maintained.All measurements were made at room temperature (25 �C) with ascan rate of 5 mV s�1 and an RRDE rotation of 1600 rpm.

Fig. 1. (A) X-ray fluorescence spectra of carbon black: (a) without modifier( ), (b) with 0.1% FePc ( ), (c) with 1.0% FePc ( ), (d) with3.0% FePc ( ), (e) with 5.0% FePc ( ), or (f) with 10.0% FePc ( ).(B) Area of Ka peak as a function of the percentage of FePc in carbon black.

2.2. Generation of H2O2 at the modified gas diffusion electrode

Based on the results obtained by LV in the RRDE experiments, aquantitative study was performed of H2O2 generation during elec-trolysis at constant potential with GDEs comprising unmodifiedcarbon black and carbon black modified with 5.0% of FePc. TheGDEs were prepared by the hot pressing (320 �C e 7.5 ton) methodusing catalytic masses consisting of carbon black, with or withoutFePc, and 20% (w/w) of a PTFE fluoropolymer dispersion TE-3893(Dupont; 60% dispersion PTFE.). Preparation of the catalytic massesand construction of the GDEs followed published procedures[7,23–27].

Experiments were carried out in a cylindrical, single compart-ment, polypropylene electrochemical cell with a total capacity of450 mL. The working electrode was a GDE with exposed reactional

area of 20 cm2, while a pure Pt screen (30.0 mm Ø; constructedwith 0.25 mm Ø wires spaced 4.0 mm apart) formed the counterelectrode and Ag/AgCl was the reference electrode [24–26]. Elec-trolyses were performed with an aqueous electrolyte (400 mL)containing H2SO4 (0.1 mol L�1) and K2SO4 (0.1 mol L�1). Samples(0.5 mL) of electrolyte were collected during electrolysis and addedto aliquots (4 mL) of a solution of (NH4)6Mo7O24 (2.4 � 10�3 -mol L�1) in H2SO4 (0.5 mol L�1) to form yellow solutions, the colorintensities of which were proportional to the amount of H2O2 pres-ent. Quantitative analysis of H2O2 was performed by comparison ofthe absorbance of the electrolyte at 350 nm (measured in a VarianCary-50 UV–Vis spectrometer) with values from a standard cali-bration curve [24–26].

3. Results and discussion

3.1. Characterization of rotating ring-disc electrodes

With the aim of determining the percentage of FePc that wouldbe most appropriate as GDE modifier for the efficient electrogener-ation of H2O2, RRDEs were prepared with carbon black containingdifferent amounts of additive. Samples of the preparations wereanalyzed by XRF spectroscopy to determine the effectiveness ofthe modifier in dispersing the carbon black. A shown in Fig. 1A,the characteristic Ka and Kb emission lines of the Fe atom (at 6.4and 7.1 keV, respectively) were readily detectable in the samples,and the signals were proportional to the FePc present (Fig. 1A in-sert). A plot of peak area of the Ka signal vs. percentage modifieradded to the carbon black revealed a linear relationship (Fig. 1B),a finding that is consistent with the sample preparation process.The additional signals in the region between 18 and 21 keV thatwere present in all of the spectra (Fig. 1A) were related to emissionsof the metals Ru and Rh. These signals, which derived from theemission tube of the spectrometer, were not completely retainedby the filter employed in the analysis but, because of their greaterenergy, they did not interfere with the detection of Fe.

34 F.L. Silva et al. / Journal of Electroanalytical Chemistry 722-723 (2014) 32–37

RRDE discs bearing microporous layers of carbon black contain-ing different amounts of FePc were characterized electrochemicallyby LV analysis. The initial LV measurements were recorded whilethe electrolyte was bubbled with N2 in order to determine the cur-rent in the acidic electrolyte. Subsequently, the total current of theexperiment, including the contribution to ORR, was measuredwhile the electrolyte was bubbled with O2. This sequence of exper-iments allowed the current associated with ORR to be determinedas the difference between the total current and the currents ofredox reactions of water and H+. All of the LV data presented in thisarticle relate to values obtained in the presence of O2 minus theinitial values measured in the presence of N2. [18,20,24,25,34,35].

In the RRDE experiments, the Pt ring was polarized at a poten-tial of +1.0 V (vs. Ag/AgCl) and, under this condition, the H2O2

detected at the ring derived by diffusion from the glassy carbondisc [18,21,24,25]. It should be possible, therefore, to correlatethe current values at the ring (referring to the formation of H2O2)with those at the disc (referring to ORR) provided that the follow-ing assumptions are valid: (i) all species that are produced at thedisc by ORR pass to the surface of the ring by virtue of the rotationapplied to the electrode, (ii) there is a direct relationship betweenthe active areas of the ring and the disc as indicated by the coeffi-cient of collection (N = 0.37) determined by the manufacturer ofthe electrode, and (iii) the current detected at the ring originatesonly from the oxidation of H2O2 [33,35,36]. Based on the above,and considering the hydrodynamic modeling of the RRDE, thecurrent observed at the ring polarized at +1.0 V vs. SCE should berepresentative of the amount of H2O2 formed at the disc.

Fig. 2 displays the LVs obtained using electrodes bearing micro-porous layers of carbon black containing different amounts of FePc.It is of note that for RRDEs prepared using lower percentages ofmodifier, i.e. 0.1%, 1.0% and 3.0%, the values of the ring currentwere close to those recorded with unmodified carbon black.Increasing the amount of FePc to 5.0% resulted in a higher ring cur-rent although the potential at the start of H2O2 detection remainedunchanged. Interestingly, further augmentation of modifier to 10%produced a decrease in ring current and a shift in ORR potential of

Fig. 2. Linear voltammograms (adjusted for the current in the acidic electrolyte)obtained with RRDE using discs onto which had been deposited microporous layersof carbon black: (a) without modifier ( ), (b) with 0.1% FePc ( ), (c)with 1.0% FePc ( ), (d) with 3.0% FePc ( ), (e) with 5.0% FePc ( ),or (f) with 10.0% FePc ( ). The aqueous electrolyte contained H2SO4

(0.1 mol L�1) and K2SO4 (0.1 mol L�1); measurements were made at room temper-ature (25 �C) with a scan rate of 5 mV s�1, an RRDE rotation of 1600 rpm, and apotential range of 0 to �1.0 V vs. Ag/AgCl.

approximately 70 mV to more negative values compared withunmodified carbon black, This behavior may be associated withincreased resistivity of the microlayer with the increase in theamount of FePc. With regard to the disc current, however,increases in the amount of modifier gave rise to only minoralterations such that the disc current profiles were close to themaximum values and the ORR potential was little changed. Thisdifference in behavior of disc and ring currents was clearly associ-ated with the activity of the modifier. Thus, while FePc did notchange the total current of ORR significantly, it appeared to favorORR current for H2O2 production leading to a higher current atthe disc. Similar results have been observed in other studies inwhich disc currents remained constant in the presence of modifi-ers, but ring currents exhibited significant variation. In these cases,the authors associated this differential behavior with the presenceof modifier in the carbon, which favored the formation of H2O2 bythe ORR [18,20,21,24–26,29].

The current efficiency for the generation of H2O2 [I(H2O2)%]and the number of electrons involved in the reaction (nt) can becalculated from the equations [18]:

IðH2O2Þ% ¼200� Ir=NIdþ Ir=N

ð1Þ

nt ¼4jIdjjIdj þ Ir

N

ð2Þ

where Ir is the ring current, Id is the disc current and N is the coef-ficient of collection for the RRDE. The values of these parameterswere determined for RRDEs prepared with carbon black containingdifferent amounts of FePc modifier (Table 1). The results reveal thatthe formation of H2O2 from ORR generally remained unchanged,irrespective of the percentage of modifier present in the carbonblack, except for the electrode modified with 5% FePc, whichshowed a 10% increase in H2O2 formation compared with unmodi-fied carbon black. Additionally, 2.4 electrons were exchanged in theORR when 5% of modifier was present in the carbon black whereasfor all other modified electrodes, and for unmodified carbon blackitself, 2.5 electrons were transferred, these values show that themodification in the carbon did not cause significant changes inthe total number of exchanged electrons, it can be associated withthe mechanism of O2 reduction, where the FePc is electrochemicallyreduced and then back to the oxidized state chemically reducingthe O2, thus the chemical O2 reduction does not promote changein the current system and not promoting changes in the numberof electrons.

Based on the results presented in Table 1, it can be proposedthat the formation of H2O2 follows a mechanism that tendstowards a 2-electron exchange, and that the addition of FePc doesnot promote significant modifications in the process of electrontransfer. According to Wang and Hu [36], when the ring/disccurrent relation approaches 100% efficiency (i.e. N � 1) the ORRfollows a 2-electron mechanism with negligible contribution from4-electron exchange. In the present study, although the addition ofmodifier exerted a small influence on H2O2 formation (increasingfrom 71.9% in the absence of modifier to 78.2% with 5.0% of FePc),

Table 1Values of current efficiency of H2O2 formation [I(H2O2)%] and total number ofelectrons transferred (nt) both calculated from the disc currents at �0.5 V vs. Ag/AgClin the LV (Fig. 2).

Parameter Carbon black Carbon black with FePc modifier Pt/C

0.1% 1.0% 3.0% 5.0% 10%

I(H2O2)% 71.9 71.3 72.4 72.0 78.2 71.2 1.5nt 2.5 2.5 2.5 2.5 2.4 2.5 3.9

F.L. Silva et al. / Journal of Electroanalytical Chemistry 722-723 (2014) 32–37 35

the total number of electrons exchanged in the ORR did not changemarkedly.

The results shown in Table 1 suggest that the mechanism ofH2O2 formation in the presence of FePc-modified carbon black iselectrochemical/chemical, in which the electrochemical reductionof Fe2+ in phthalocyanine is followed by auto-oxidation to Fe2+ withchemical reduction of O2 to H2O2. Such a mechanism would con-tribute to the increased formation of H2O2 (the chemical step), asobserved in the present study, but should not influence the numberof electrons exchanged because the electrochemical reduction ofFePc likely involves a 2-electron transfer [18,20,26,34,36,37].

The value for nt calculated according to Eq. (2) relates to the glo-bal number of electrons transferred during electrolysis rather thanto the exact number involved in the formation of H2O2, since addi-tional reactions occur in parallel to ORR. Therefore, it is not possi-ble to determine the exact contribution of each of the mechanismsinvolved. An alternative evaluation of the number of electronsexchanged can be performed using the Koutecky–Levich (KL)equation (Eq. (3)), which relates the current in the disc (i) to therotation of the electrode (x):

1i¼ 1

ikþ 1

0:6nFAD23v�1

6C0x12

ð3Þ

where n is the number of electrons transferred in the half reaction, Fis the Faraday constant, A is the electrode area, D is the diffusioncoefficient, t is the kinematic viscosity and C0 is the concentrationof analyte [18,34,37].

Fig. 3 shows KL plots of the ORR for C/Pt and for carbon blackwith different amounts of FePc. Unmodified carbon black, whichis a reference material for the reduction of O2 involving a 2-elec-tron transfer [20], produced a linear KL plot with a slope of 53,while C/Pt, the reference for the reduction of O2 involving a 4-elec-tron transfer [21], gave a KL plot with a slope of 33. These resultsare as expected and predicted in the literature [20,21,37,38]. Thestraight lines in Fig. 3 relating to carbon black modified with FePcexhibit slopes that are close to 51 ± 2.4. This behavior provides fur-ther evidence that the addition of FePc to carbon black maintainsthe tendency toward a 2-electron transfer mechanism for ORR[21,27,38]. Another factor to be noted in the KL plots relates tothe contributions of the kinetic and diffusion currents. All of the

0.02 0.04 0.06 0.08 0.10

1

2

3

4

5

6

7

8

i-1 /

mA

ω-1/2

Fig. 3. Koutecky–Levich plots for ORR with RRDE using discs onto which had beendeposited microporous layers of C/Pt ( ) or carbon black: without modifier( ), with 0.1% FePc ( ), with 1.0% FePc ( ), with 3.0% FePc ( ), with5.0% FePc ( ), or with 10.0% FePc ( ). The aqueous electrolyte containedH2SO4 (0.1 mol L�1) and K2SO4 (0.1 mol L�1). Koutecky–Levich plots was calculatedfrom the currents at �0.5 V vs. Ag/AgCl in the LV (Fig. 2).

materials studied showed intercepts on the y-axis close to zero,indicating that the major contribution was from the diffusioncurrent while that of the kinetic current, although present, wassmall and could be ignored for the purpose of evaluating tenden-cies of the ORR mechanism. In the case of carbon black modifiedwith 5% FePc, for example, the total current in the disc was237 lA at a potential of �0.4 V (vs. Ag/AgCl), while the kineticcurrent obtained from the KL equation was 0.7 lA. This small con-tribution of kinetic current to O2 reduction suggests that electrontransfer (charge transfer) was very fast while the reposition of O2

to the surface of electrode (mass transport) was slow, therebylimiting the reduction reactions [18,20].

3.2. Controlled potential experiments with GDE

In order to conduct a more extensive study of ORR in H2O2 gen-eration, it was necessary to scale up the process so that the forma-tion of H2O2 could be analyzed quantitatively. To meet thisrequirement, GDEs were constructed with unmodified and modi-fied carbon black, and the reduction of O2 to H2O2 was quantifiedby UV–Vis spectrophotometry. The results obtained with RRDEusing microporous layers revealed that carbon black modified with5% of FePc produced the highest current in the ring (Fig. 2) and thehighest value of current efficiency for H2O2. According to Barros[24], tendencies observed in experiments with microporous layersat the RRDE can be extrapolated to the generation and quantifica-tion of H2O2 in GDEs. On this basis, GDEs comprising carbon blackand carbon black containing 5% FePc were subjected to LV in therange �0.4 V to �4.0 V (vs. Ag/AgCl). The LV plots shown in Fig. 4were obtained by subtracting the currents measured with theGDE under N2 pressure from those measured under O2 pressure,as described by Barros [24], in order to display current profiles ad-justed for the component of current in the electrolyte. The varia-tion of ORR current in the GDE with 5% FePc displayed twosections with different profiles in which a larger increase in ORRcurrent could be observed in the region up to �2.0 V. These regionsmay represent two distinct sectors for the reduction of O2, asdescribed in the literature in general, with the preferential forma-tion of H2O2 (2-electron mechanism) at less negative potentials,and the preferential formation of H2O (4-electron mechanism) atmore negative potentials [18,20,26,34,36,37].

For both unmodified and modified GDEs, the current continuedto increase with applied potential (Fig. 4), and it was not possible

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

i / A

E vs Ag/AgCl / V

Fig. 4. Linear voltammograms (adjusted for the current in the acidic electrolyte)obtained with GDEs constructed with carbon black ( ) and carbon black with5% FePc ( ). The aqueous electrolyte contained H2SO4 (0.1 mol L�1) andK2SO4 (0.1 mol L�1): measurements were made at room temperature (25 �C) with ascan rate of 20 mV s�1, and a potential range of �0.4 to �4.0 V vs. Ag/AgCl.

Fig. 5. Production of H2O2 as a function of electrolysis time with GDEs constructedwith (A) unmodified carbon black, and (B) carbon black with 5.0% FePc underconstant applied potentials of �0.4 V ( ), �0.5 V ( ), �0.6 V ( ), �0.7 V( ), �0.8 V ( ), �0.9 V ( ), �1.0 V ( ), �1.1 V ( ), �1.2 V ( ),�1.3 V ( ), and �1.4 V ( ). Panel (C) shows the final concentration ofH2O2 as a function of the potential applied to GDEs constructed with carbon black( ) and carbon black with 5% FePc ( ).

Fig. 6. (A) Global kinetic constants for the formation of H2O2 (determined duringthe first 30 min of electrolysis) plotted as a function of the potential applied to GDEsconstructed with carbon black ( ) and carbon black with 5% FePc ( ).(B) Energy consumption (kW h kg�1) for the final concentration of H2O2 after90 min of electrolysis plotted as a function of the potential applied to GDEsconstructed with carbon black ( ) and carbon black with 5% FePc ( ).

36 F.L. Silva et al. / Journal of Electroanalytical Chemistry 722-723 (2014) 32–37

to determine the best potential for H2O2 generation. Therefore,separate experiments were performed at constant potentials inthe range �0.4 V to �1.4 V (vs. Ag/AgCl), and the concentrationof H2O2 generated at each potential was evaluated as a functionof time of electrolysis. For the GDE with unmodified carbon black

(Fig. 5A), the variation in H2O2 generation was almost linear withrespect to time over the 90 min of the experiment for all appliedpotentials. However, for the GDE comprising carbon black with5% FePc (Fig. 5B), the variation in H2O2 concentration was linearover the full 90 min for potentials up to �0.9 V (vs. Ag/AgCl), butfor more negative potentials linearity occurred only within the first60 min, after which H2O2 concentration either stabilized or showeda decrease with respect to time. This change in profile was associ-ated with the geometry of the electrochemical cell employed inwhich H2O2 generated in the GDE could be oxidized at the anodeor reduced at the cathode. Thus, the concentration of H2O2 ob-served in the last 30 min of electrolysis reflected the balance be-tween formation of the peroxide and its degradation at theanode and cathode. In Fig. 5C, the final concentrations of H2O2 gen-erated after 90 min of electrolysis are plotted as functions of thepotential applied to the GDEs studied. The GDE without modifiershowed a tendency to increase the final H2O2 concentration withincreasing applied potential up to �1.0 V (maximum H2O2 concen-tration 175 mg L�1), while at more negative potentials the finalH2O2 concentration decreased. In the case of the GDE modifiedwith FePc, the final H2O2 concentration increased with increasingapplied potential up to �1.0 V (maximum H2O2 concentration240 mg L�1), following which H2O2 generation stabilized such thatthat increases in the applied potential did not give rise to signifi-cant increases in the final concentration of H2O2.

It is clear from the plots shown in Fig. 5A and B that the changein H2O2 concentration was practically linear during the first min-utes of the experiment, probably indicating zero order kinetics.

F.L. Silva et al. / Journal of Electroanalytical Chemistry 722-723 (2014) 32–37 37

However, the change observed in H2O2 concentration with respectto time is determined by the cumulative effects of all of the reac-tions that occur in parallel at the cathode and the anode. Thus,the electrosynthesis of H2O2 in a GDE generally follows globalpseudo zero order kinetics [6,7,27]. Values of the kinetic rateconstant for the formation of H2O2 at different applied potentialswere calculated from the angular coefficients of plots of H2O2

concentration (mg L�1) vs. time (min), considering only the first60 min of each experiment (Fig. 6A). For the GDE without modifier,increasing the applied potential promoted an increase in therate of H2O2 formation, which attained 3 mg L�1 min�1 at �0.8 V(vs. Ag/AgCl): at more negative potentials the rate of H2O2 forma-tion was observed to decrease. For the GDE modified with 5% FePc,the maximum rate of formation of H2O2 was 7 mg L�1 min�1 andthis was achieved at �1.0 V (vs. Ag/AgCl): at more negative poten-tials the rate of H2O2 formation tended to stabilize. The observedrate constants are close to the values reported by Barros [24],who obtained 4.7 mg L�1 min�1 for the best modifier studied, andValim [25], who achieved 5.9 mg L�1 min�1 for a GDE modifiedwith tert-butyl anthraquinone in acidic medium.

A further important factor in the electrogeneration of H2O2 isthe energy consumption (EC) required to generate 1 kg of H2O2.In the present study, EC values were calculated, according to themethod described in the literature [23–25,27,28], on the basis ofthe final concentrations of H2O2 presented in Fig. 5C. As shownin Fig. 6, EC values increased as the applied potential increased,as expected, and were similar for unmodified and modified GDEsat both low and high applied potentials. However, at a potentialof �1.0 V (vs. Ag/AgCl), where the concentrations of H2O2 andthe rate constants for peroxide formation were maximal forboth electrodes, the unmodified GDE consumed 300 kW h kg�1

H2O2 whereas the GDE modified with 5% FePc consumed only165 kW h in the formation of 1 kg of H2O2. These results indicatethat the best potential for the generation of H2O2 in the GDE is�1.0 V (vs. Ag/AgCl), under which conditions the highest concen-trations of H2O2 were formed with the lowest energy consumption.

4. Conclusions

A study of O2 reduction in aqueous solution using RRDEs com-prising microporous layers of carbon black modified with FePcrevealed that the presence of modifier induced an increase in ringcurrent indicating an increased formation of H2O2. Moreover, cur-rent efficiency for the generation of H2O2 was 78.2% for carbon blackwith 5% FePc compared with 71.9% for carbon black without modi-fier, although the number of electrons exchanged in the ORR wasnot significantly altered by the presence of modifier. Quantitative

analysis of the electrogeneration of H2O2 under conditions of con-stant potential applied to a GDE comprising carbon black modifiedwith 5% FePc showed that H2O2 formation was linear with respectto time during the first 60 min of electrolysis, after which the ratedecreased with time. The maximum concentration of H2O2

(240 mg L�1) attained at the end of 90 min of electrolysis with a po-tential of �1.0 V (vs. Ag/AgCl) applied to the modified GDE wasgreater than that obtained with the unmodified GDE (175 mg L�1).Under these conditions, the rate constant (determined for the first60 min of electrolysis) for the formation of H2O2 in the modifiedGDE was 7 mg L�1 min�1, and the energy consumed in the genera-tion of 1 kg of H2O2 was 165 kW h. These results demonstrate thefeasibility of using GDEs modified with FePc in the efficient electro-generation of H2O2.

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