Enhanced electrocatalytic activity of oxygen reduction by cobalt-porphyrin functionalized with graphene oxide in an alkaline solution

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Enhanced electrocatalytic activity of oxygen reduction by cobalt-porphyrin functionalized with graphene oxide in an alkaline solution Jung-Min You, Hyoung Soon Han, Hyo Kyoung Lee, Sung Cho, Seungwon Jeon* Department of Chemistry and Institute of Basic Science Chonnam National University, Gwangju 500-757, Republic of Korea

article info

abstract

Article history:

Cobalt[5,15-(p-aminophenyl)-10,20-(pentafluorophenyl)porphyrin] (CoAPFP) functionalized

Received 8 November 2013

with graphene oxide (GO) by N-(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydro-

Received in revised form

chloride (EDC hydrochloride) was used as an amide-coupling reagent (GO-CoAPFP). Elec-

29 December 2013

trochemical investigation revealed that ERGO-CoAPFP exhibited much better catalysis and

Accepted 16 January 2014

better-improved oxygen reduction reaction (ORR) than GO-CoAPFP. ERGO-CoAPFP is a

Available online 13 February 2014

simple and facile method of improving electrocatalytic activity. The ERGO-CoAPFP catalysts were tested in ORR using electrochemical techniques such as cyclic voltammetry (CV)

Keywords:

and rotating-ring-disk-electrode (RRDE) hydrodynamic voltammetry to quantitatively

Oxygen reduction

obtain the ORR kinetic constants and the reaction mechanisms on a glassy carbon elec-

Cobalt-porphyrin

trode (GCE) in a 0.1 M KOH solution. The electrocatalytic oxygen reduction reaction of RRDE

Electrochemically reduced graphene

modified with ERGO-CoAPFP established a pathway of four-electron transfer reactions.

oxide

Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

Non-noble metal catalyst

reserved.

Fuel cells

1.

Introduction

Many researchers have focused of late on the development of non-noble metals or metal-free catalysts to replace Pt for catalyzing the oxygen reduction reaction (ORR), and on the development of low-cost energy conversion catalysts [1e8]. Although Pt nanomaterials have been regarded as the best electrocatalysts for ORR in fuel cells (FCs), they still have many problems [9e11]. Besides, the high cost of the Pt catalysts and their limited reserves in nature precluded the large-scale commercialization of FCs [12e14]. Therefore, the

development of non-noble metals or a metal-free ORR catalyst has generated the commercialization of the FC system. Researchers have confirmed that functionalized graphene (a metal-free or non-noble metal) [15e18] is a promising candidate for replacing the Pt-based catalysts for FCs because graphene has excellent physical and chemical properties such as high electrical conductivity [19] and high surface area [20]. Moreover, it not only exhibits high catalytic activity, long-term stability, and excellent physical and chemical properties but also possesses the advantages of low cost and environment-friendliness.

* Corresponding author. Tel.: þ82 625303380. E-mail address: [email protected] (S. Jeon). 0360-3199/$ e see front matter Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2014.01.107

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Scheme 1 e Schematic representation of Go-Co-APFP.

The presence of oxygen-containing groups in graphene oxide renders it useful provides for modification of GO sheets via organic molecules functionalization. The organic materials combined with GO were recovered to an sp2 carbon-network that enhanced electrocatalytic activity. In the authors’ previous experiment on the development of ORR electrocatalysts, cobalt-[tetrakis(o-aminophenyl) porphyrin] (CoTAPP), which was covalently attached to the carbon nanomaterials, was introduced [21]. The electrodes modified with CoTAPP attachment to GO showed enhanced electrocatalytic performance. This enhancement is believed to have originated from the modification of GO by functionalities with cobaltporphyrin; as a result, the electron transfer improved even after the modification [22]. Therefore, in this study, cobalt[5,15-(p-aminophenyl)10,20(pentafluorophenyl)-porphyrin] (CoAPFP) were used to functionalize GO (GO-CoAPFP) and to fabricate electrochemically reduced GO-CoAPFP (ERGO-CoAPFP). These nanomaterials were characterized via UVeVis absorption, time-resolved spectroscopy, infrared (IR), Raman atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), and the electrocatalytic behaviours of ERGO-CoAPFP and its precursor’s catalyst-supported glassy carbon electrode (GCE) O2 reduction were investigated by employing cyclic voltammetry (CV) and rotating-disk-electrode (RDE) and rotating-ring-disk-electrode (RRDE) configurations in an aqueous 0.1 M KOH alkaline solution. The KouteckyeLevich (K-L) equation and plots were used to calculate the electron number, and the results confirmed that the O2 reduction was close to a four-electron transfer reaction.

2.

Experiment

2.1.

Reagent

The graphite power (325 mesh, 99.999%), N(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDC hydrochloride), pyrrole, pentafluorobenzaldehyde, 4nitrobenzaldehyde, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and trifluoroacetic acid (TFA) were purchased from Aldrich. The membrane filters (pore size: 0.45 mm; diameter: 47 mm) were purchased from Tokyo Roshi Kaisha, Ltd. All the other reagents were of analytical grade and were used without further purification.

2.2.

Instruments

A three-electrode assembled cell in a grounded Faraday cage was used for voltammetric measurements. A GCE (3.0-mmdiameter) was made with a working electrode, a platinumwire counter electrode, and an Ag/AgCl reference electrode (manufactured by BASi, MF-2052, 3.0 M NaCl). A rotating electrode (GCE area: 0.196 cm2; disk-platinum ring area: 0.041 cm2) was employed as an RRDE electrode. An EG&G Model 636 RRDE system and a CHI 700C electrochemical workstation were used for hydrodynamic voltammetry. The UVeVis absorption spectra were recorded with an S-3100 spectrometer (Scinco, Ltd., Seoul, South Korea) and a Fourier transformed infrared (FTIR) spectroscope (ALPHA-T, Bruker). The Raman spectra were finally recorded on a LabRam HR800 (Horiba Jobin-Yvon, France). XPS was performed using a VG Multilab 2000 spectrometer (ThermoVG Scientific, Southendon-Sea, Essex, UK) in an ultra-high vacuum. The survey scan data were collected at 50 eV pass energy. Prior to measurement, the samples were treated with a mixture of HNO3, HF, and HBO3 to ensure complete dissolution.

2.3.

Preparation of GO-CoAPFP

The preparation and synthesis of CoAPFP was in accordance with the procedures in the literature [23e26]. GO with covalent bonding to CoAPFP was prepared through a one-step process, as illustrated in Scheme 1. First, GO was prepared using the Hummers method, with additional KMnO4 [27]. The GO (10 mg) was dispersed in anhydrous DMF 20 mL into roundbottom flasks and were then ultrasonicated for 1 h. After sonication, CoAPFP (2 mg) with EDC hydrochloride (4 mg) was added, and the mixture was stirred at room temperature (RT) for 12 h, under argon condition. The reacted mixture contained dark-purple materials, which were separated from the mixture through ultrasonication and centrifugation (4000 rpm); these were washed several times with DMF, resulting in GO-CoAPFP.

2.4.

Electrode modification with ERGO-CoAPFP

The electrode surfaces were well polished with alumina paste and were washed with distilled water then rinsed with methanol. After polishing, the electrodes were sonicated in a water bath for 5 min and were then coated with a GCE (5.0 mL)/

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Fig. 1 e (a) FTIR spectra of GO, CoAPFP, and GO-CoAPFP. (b) UVeVis absorption spectra of GO, CoAPFP, GO-CoAPFP, and the mixture (GO/CoAPFP). (c) Raman spectra of GO and GO-CoAPFP. (d) AFM image and height profile of GO-CoAPFP.

RRDE (16 mL) suspension of GO-CoAPFP (1 mg/1 mL DMF). For this evaluation, the mixture (Go/CoAPFP) was compared with 1 mg/1 mL DMF for the modified electrode at the same condition, to confirm the effect of GO-CoAPFP. The GO-CoAPFP coated onto the surface of GCE was electrochemically reduced by 30 successive cyclic voltammograms in an electrochemical cell containing a 0.05 M PBS (pH 5) solution over a potential range of 0 to 1.5 V at a scan rate of 50 mV/s [28]. All the experiments were carried out in an argon atmosphere at room temperature (RT). For comparison, a Pt/C electrode was prepared using 1 mg Pt/C powder in 1 mL ethanol in the presence of a 5% Nafion solution; this could effectively improve the dispersion of the Pt/C [29]. Also, Pt/C was coated onto the surface under the same condition.

3.

Results and discussion

3.1.

Characterization of GO-CoAPFP

The preparation of GO-CoAPFP was first investigated using FTIR spectroscopy, followed by UVeVis absorption, timeresolved spectroscopic, Raman, and XPS spectroscopy as well

as atomic force microscopy (AFM). The changes in the samples’ functionalized surfaces were observed using FTIR spectroscopy (Fig. 1(a)). Signals attributable to the stretching of the carbonyl bonds (C]O) in the GO appeared at 1734 cm1, OeH deformation at 1396 cm1, CeOH stretching vibration at 1230 cm1, and CeOeC stretching vibration at 1061 cm1. A 1625-cm1 band was also shown, corresponding to the stretching vibration C]C bond or skeletal vibration of the aromatic ring [30,31]. In the GO-CoAPFP spectrum, the peak at 1734 cm1 almost disappeared, and a new peak appeared at 1648 cm1, which corresponds to the C]O characteristic stretching band of the amide group [32]. The stretching band of the amide CeN peak also appeared at 1255 cm1. These results clearly show that CoAPFP had been covalently bonded to GO with the amide linkage. Fig. 1(b) shows the UVeVis absorption spectra of GO, CoAPFP, GO-CoAPFP, and the mixture (GO/ CoAPFP) samples in DMF. GO showed an absorption peak at 260 nm, which agrees with the literature [33]. CoAPFP showed a strong Soret band at 420 nm, together with weaker Q-bands between 500 and 700 nm. GO-CoAPFP showed a Soret band redshifted to 438 nm, and shifted Q-band. No obvious shift was observed in the mixture (GO/CoAPFP) sample. These results indicate that GO-CoAPFP interaction is not physisorbed state,

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Fig. 2 e Survey XPS spectra of (a) GO (Black line), GO-CoAPFP (Blue line) and ERGO-CoAPFP (Red line). Core-level C 1s XPS spectra of (b) GO, (c) GO-CoAPFP and (d) ERGO-CoAPFP.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the GO-CoAPFP is strong electronic communication between CoAPFP and GO surface through the amide bridge [34,35]. Also we carried out time-resolved spectroscopic experiment to confirm the electronic communication between GO and CoAPFP moieties in GO-CoAPFP sample. It is the evidence of the strong electronic communication between GO and CoAPFP, which is well matched with the red-shifted absorption band of GO-CoAPFP compared to that of the mixture (GO/CoAPFP) (Supporting information Fig. S1). Raman spectroscopy is a powerful instrument used to determine significant structural changes. Therefore, Raman measurements were carried out on GO and GO-CoAPFP, as shown in Fig. 1(c). In a typical Raman spectrum, the G-band is assigned to the E2g phonon of the sp2 hybridization carbons and D-band corresponding to the breathing mode of the k-point phonon of the A19 symmetry. The intensity ratio (ID/IG) of GO was 0.87 while the ID/IG of GO-CoAPFP was 0.98. The increase in the ID/IG indicated the conversion of the sp2 carbons to sp3 carbons on the graphene surface [36e38], which suggests that porphyrin was

successfully attached onto the GO surface. The atomic force microscopic (AFM) image was used to characterize the CoAPFP covalent bonds to the GO nanosheets. When GO was covalently bonded to CoAPFP, the average thickness of GO-CoAPFP was significantly greater than that of GO. The measured thickness of the GO layer was around 0.830 nm (Supporting information Fig. S2); which is consistent with the previous reports [39]. As shown in Fig. 1(d), the thickness of GO-CoAPFP was 1.376 nm, which was higher than that of the GO nanosheet. This result suggests that GO has homogeneously covalent bonding to both sides of the CoAPFP amine moiety. The XPS spectra of the GO, GO-CoAPFP and ERGO-CoAPFP are shown in Fig. 2(a). Pristine GO showed distinct C and O 1s peaks with no other elements detected. Compared with GO, the GO-CoAPFP resulted in the emergence of Co 2p (780.41 eV), F 1s (686.671 eV), and N 1s (398.81 eV) signals in the spectra (Fig. 2(a)). Its Co content was ca. 0.74 atomic %. Moreover, a new peak corresponding to the CeN (285.9 eV) group suggests that CoAPFP is an amide attachment on the GO (Fig. 2(c)). The

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Fig. 3 e Electrochemical ORR catalytic performance of the CV curves on (a) GO-CoAPFP, (b) the mixture (GO/CoAPFP), and (c) ERGO-CoAPFP in an O2-saturated 0.1 M KOH solution at a scan rate of 50 mv/s. LSVs of the different samples at 1600-rpm RDE electrodes for ORR at a scan rate of 10 mv/s in an O2-saturated 0.1 M KOH solution.

high-resolution C 1s spectrum of GO indicated the presence of different types of carbon bonds: CeC/C]C (284.99 eV), CeO (287.29 eV), and C]O (288.88 eV). The high-resolution of the C 1s XPS spectra of GO-CoAPFP shows a significant decrease of oxygen-containing groups compared to GO (Fig. 2(c)). In this result, GO-CoAPFP shows that the hydroxide (-OH) of the GO ion can be bonded/replaced with CoAPFP-activated amine. Finally, the oxygen content in the XPS spectra of the fabricated ERGO-CoAPFP was dramatically decreased, owing to the sp2 carbon-network recovered by electrochemically reducing the GO sheet (Fig. 2(d)) [28]. Also, the relative surface atomic compositions were estimated from the corresponding peak areas and corrected with the tabulated sensitivity factors. The carbon and oxygen atomic ratio (C/O) increased from GO (0.44), to GO-CoAPFP (0.96). The ERGO-CoAPFP C/O ratio increased to 1.94; indicating that the oxygen-containing groups are recovered after electrochemically reducing.

3.2.

Electrocatalytic O2 reduction on ERGO-CoAPFP

To investigate the catalytic effects, CV experiments on oxygen reductions were carried out with the GO-CoAPFP, mixture

(GO/CoAPFP), and ERGO-CoAPFP cathodes in an O2-saturated 0.1 M KOH solution in the potential range of 0.2 to 1.0 V at a scan rate of 50 mV/s (Fig. 3(a)e(c)). The peak potentials of the O2 reduction curve that appeared for the half-wave potentials (E1/2) were those of GO-CoAPFP (0.30 V), mixture (GO/CoAPFP) (0.39 V) and the ERGO-CoAPFP (0.19 V) In addition we observed that the O2 reduction curves from GO-CoAPFP and ERGO-CoAPFP were show a more positive half-wave potential than the GO-APFP (0.32 V) and ERGO-APFP (0.25 V) (Supporting information Fig. S3(a) and (b)). To the previous report, these enhanced catalytic activity indicating that cobalt as a support indeed leads to a significant enhancement for the ORR catalytic effect [1,5,18,40]. Also, the ERGO sheet were more improved electron transfer reaction than that of GO [28,41]. Fig. 3(d) shows the linear sweep voltammograms (LSV) at 1600 rpm for ORR on an RDE electrode modified with Pt/C, GOCoAPFP, the mixture (GO/CoAPFP), and ERGO-CoAPFP in an O2-saturated 0.1 M KOH solution at a scan rate of 10 mV/s. Specifically, the electrocatalytic effect was optimized with the highest positive potential and largest scale from the ERGOCoAPFP modified electrode. The ERGO-CoAPFP of the O2 reduction onset potential at approximately 0.054 V and the

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Fig. 4 e RDE voltammograms for ORR on the (a) GO-CoAPFP and (b) ERGO-CoAPFP modified electrodes at various rotation speeds (100, 400, 900, 1600, 2500, and 3600 rpm) at a scan rate of 10 mV/s in an O2-saturated 0.1 M KOH solution. (c) Comparative K-L plots for the ORR catalysis and (d) kinetic-limiting current of the corresponding electron transfer numbers at L0.45 V (the inset shows the electron transfer number at various potentials).

current response were significantly higher than those of the GO-CoAPFP (0.16 V) and mixture (GO/CoAPFP) (0.24 V) electrodes. The RDE curve from ERGO-APFP was showed clearly a twostep ORR mechanism and more negative onset potential than that of ERGO-CoAPFP (Supporting information Fig. S3(c)). The more positive onset potential and the higher current density with one-step reaction on ERGO-CoAPFP clearly indicate that this material will have a better catalytic performance, which also supports the proposed synergistic effect caused by the ERGO sheet and cobalt-porphyrin. The RDE hydrodynamic voltammetry experiments were useful in confirming the electrocatalytic reduction pathway of O2. The disk current curves on GO-CoAPFP and ERGO-CoAPFP were obtained for ORR at the different rotation speeds of 100e3600 rpm in a 0.1 M KOH electrolyte saturated with O2. As shown in Fig. 4(b), at various rpm values, the reduction current of ORR in the ERGO-CoAPFP modified electrode was always higher than that in the GO-CoAPFP (Fig. 4(a)), Pt/C and mixture (GO/CoAPFP) (Supporting information Fig. S4) modified

electrodes. The KouteckyeLevich (K-L) equation was used for the calculation of the n value, as follows [42,43]:
1=J ¼ Jk þ 1= Bu1=2 ; and B ¼ 0:2nFCO2 DO2


2=3

v1=6 ;

(1) (2)

where J is the measured current density, Jk is the kinetic current density of the ORR, u is the angular velocity of the disk (u ¼ where rpm is the linear rotation speed), 0.2 is adopted when the rotation speed is expressed in rpm, n is the overall number of electrons transferred during the oxygen reduction, F is the Faraday constant (96,485 C mol1), C is the bulk concentration of O2 (1.2  106 mol cm3), D is the diffusion coefficient of O2 (1.9  105 cm2 s1), n is the kinetic viscosity of the electrolyte (1  102 cm2 s1), and B could be determined from the slope of the K-L plots. To qualify the ORR process on this novel catalyst, the K-L plots (J1 vs. u1/2) were obtained for each sample from the reaction currents at 0.45 V on the LSVs at various rotating speeds (Fig. 4(c)); the K-L plots were

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Fig. 5 e Corresponding H2O2 production potentials of GOCoAPFP and ERGO-CoAPFP from the RRDE analysis at 2500 rpm.

Significantly, the Jk value of ERGO-CoAPFP exceeded that of the commercial Pt/C at 0.45 V and was considerably superior to those of GO-CoAPFP and the mixture (GO/CoAPFP) over the entire potential range. Moreover, the Jk value of ERGO-CoAPFP sharply increased from 10.31 to 14.9 mA cm2 as the potential varied from 0.3 to 0.7 V (Supporting information Fig. S4), which is a good indication that the potential current can be improved even further through structural modification. Hydrogen peroxide was produced when O2 was reduced through two-electron transfer. The formation of H2O2 intermediate can be calculated from the ring and disk currents via RRDE electrode. So we were used to evaluate the ORR performance of GO-CoAPFP and ERGO-CoAPFP. Supporting information Fig. S6 shows the disk and ring currents for the GO-CoAPFP and ERGO-CoAPFP electrodes, respectively, at a rotation speed of 2500 rpm and a scan rate of 10 mV/s in an O2saturated 0.1 M KOH solution. The percentage of hydrogen peroxide (% H2O2) during the ORR can be calculated from the previous report [43,44]: H2 O2 % ¼ 200Ir =ðN  Id þ IRÞ

also obtained at other potentials, as illustrated in Supporting information Fig. S5. The K-L plots of all the samples showed good linearity, and the ERGO-CoAPFP sample showed a noticeably high ORR current, which was very close to that of the commercial Pt/C catalyst and was significantly higher than that of GO-CoAPFP. The electron transfer numbers (n) of all the samples at different potentials were calculated according to the slopes of the linearly fitted K-L plots, based on the K-L equation. The dependence of n on the potentials in the case of the various samples is shown in the inset of Fig. 4(d), which shows that the electron transferred at ERGO-CoAPFP was always higher than those of the other electrodes. The four-electron (n > 3.8) ORR reaction commenced at around 0.3 to 0.7 V, indicating that ERGO-CoAPFP is a more efficient ORR electrocatalyst compared to the other electrodes. To more clearly determine the potential suitability of ERGO-CoAPFP as an ORR catalyst, the kinetic-limiting current (Jk) was acquired from the intercept of the linearly fitted K-L plots at 0.45 V (Fig. 4(d)) and at other potential shows supporting information Fig. S5.

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(3)

where Ir and Id are the ring- and disk-limiting currents and N is the RRDE collection efficiency. Ring currents corresponding to the formation of H2O2 were observed under such conditions. According to the equation, the H2O2 proportion at ERGOCoAPFP is much lower than that of the GO-CoAPFP electrodes (Fig. 5). This result could be due to the variation in the electron transfer number. Specific to the ORR catalysts, the crossover effect must be considered because these molecules that are fed to the anode sometimes permeate through the membrane to the cathode and seriously affect the performance of the cathode catalyst [1,18,45]. The methanol and ethanol crossover effect was evaluated on ERGO-CoAPFP and commercial Pt/C via the chronoamperometric method at 0.3 V (Fig. 6(a)). The current increased when oxygen was added to the argon-saturated 0.1 M KOH aqueous solution at about 500 s, indicating that ORR occurred on ERGO-CoAPFP and Pt/C. After the addition of 3 M methanol and ethanol at around 1500 and 2000 s, respectively, ERGO-CoAPFP exhibited excellent selectivity in ORR, with no visible response to methanol/ethanol oxidation.

Fig. 6 e (a) (j vs. t) Chronoamperometric responses of ERGO-CoAPFP and Pt/C in 0.1 M KOH at L0.3 V. The arrow indicates that O2, 3 M methanol, and ethanol were added at around 1500 and 2000 s. (b) Long-term stability of ORR in an O2-saturated 0.1 M KOH solution at 1600 rpm at L0.3 V.

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In contrast, Pt/C was methanol/ethanol oxidation reaction occurred, and a drastic current decrease also occurred with respect to time. These experiment results clearly reveal that ERGO-CoAPFP has a much better methanol and ethanol acceptance towards ORR than the commercial Pt/C, or a great acceptance of the possible crossover effect. The stability of ERGO-CoAPFP and Pt/C was also evaluated using a chronoamperometric method at 0.3 V (Fig. 6(b)). The current tolerance of all the electrodes decreased at the same rate, but ERGO-CoAPFP showed a slower decrease than the other electrode after 2.5 h, and a high relative current of approximately 80% still response, demonstrating the longer-term stability of ERO-CoAPFP over the Pt/C electrodes.

4.

Conclusions

In summary, GO-CoAPFP was synthetized by covalently bonding cobalt[5,15-(p-aminophenyl)-10,20-(pentafluorophenyl) porphyrin] (CoAPFP) onto a graphene oxide (GO) sheet via N(3-dimethylaminopropyl)-N0 -ethylcarbodiimide hydrochloride (EDC hydrochloride), which was used as an amide-coupling reagent (GO-CoAPFP) and to fabricate electrochemically reduced GO-CoAPFP (ERGO-CoAPFP). To determine the catalytic effects, the cyclic voltammetry (CV), ring-disk electrode (RDE), and rotating-ring-disk-electrode (RRDE) techniques were used for O2 reduction reaction (ORR) in a 0.1 M KOH solution. The voltammetry techniques’ results showed that the ERGO-CoAPFP catalyst had superior catalytic activity towards ORR compared to the other electrodes. Moreover, with respect to the commercial Pt/C catalysts, ERGO-CoAPFP showed a comparable electrocatalytic activity but better stability and increased acceptance of the crossover effect. The KouteckyeLevich equation and plots were used to determine the transferred electrons. The RDE results confirmed that the ORR was four-electron transfer. This work thus clearly indicates the application of ERGO with cobalt-porphyrin materials for the development of ORR catalysts for fuel cell applications.

Acknowledgement This research has supported by the Basic Science Research Program through a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science, and Technology (2010-0007864).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.01.107.

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