Pyridine-functionalized graphene oxide, an efficient metal free electrocatalyst for oxygen reduction reaction

Electrochimica Acta 194 (2016) 95–103

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Pyridine-functionalized graphene oxide, an efficient metal free electrocatalyst for oxygen reduction reaction Ali A. Ensafi* , Mehdi Jafari-Asl, B. Rezaei Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 December 2015 Received in revised form 25 January 2016 Accepted 30 January 2016 Available online 1 February 2016

Here, a novel approach for the preparation of pyridine functionalized graphene nanosheets (Py-EGO) is developed. Using a simple thermal treatment, pyridine was linked to reduced graphene oxide using epoxy group or hydroxyl group that existing at a surface of the graphene. The electrochemical behavior of Py-EGO for the reduction of oxygen was studied. The results showed that the new nanocomposite has a powerful potential as an electrocatalyst for oxygen reduction reaction (ORR) especially compare to N-doped graphene oxide, in alkaline solution. In addition, the Py-EGO electrocatalyst exhibited tolerance to methanol crossover effect. The results of our studies indicate that Py-EGO has a better durability, much higher selectivity, much better electrochemical stability and high catalytic activity towards the ORR than that of commercial Pt/C electrocatalyst. The Pt-free electrocatalyst improves catalytic activity and reduce the cost of electrocatalyst for ORR. The proposed simple method for synthesize Py-EGO electrocatalyst promising its further application in fuel cells. ã 2016 Elsevier Ltd. All rights reserved.

Keywords: Graphene nanosheets Pyridine functionalized graphene nanosheet Oxygen reduction reaction Electrocatalyst

1. Introduction Increasing demands for clean energy has stimulated extensive research on the development of technologies those can effectively convert chemical energy into electricity at low cost and with high efficiency. In oxygen reduction reaction (ORR) slow cathodic kinetic and the demands of the complex 4-electron ORR pathway pose major barriers to reduce the Pt loading to fabricate electrocatalyst. Besides, degradation of the electrocatalyst caused, e.g., by poisoning, particle detachment, agglomeration and/or carbon corrosion also needs to be tackled to obtain reliable systems. On the other hand, catalysts for ORR are at the heart of key electrochemical technologies for direct methanol fuel cells and polymer electrolyte membrane fuel cells [1–5]. Recently, consideration effort has been devoted to explore Pt-free and even metal-free catalysts to improve catalytic activity and reduce cost of electrocatalysts [6–8]. Doped carbon and particularly N-doped carbon has attracted increasing attention as a promising metal-free electrocatalyst for ORR due to its excellent electrocatalytic performance and low cost. Recent studies have showed that different carbon materials including mono-doped or co-doped by nonmetallic heteroatoms such as N, S, P and I, have

* Corresponding author. Tel.: +98 31 33912351; fax: +98 31 33912350. E-mail addresses: Ensafi@cc.iut.ac.ir, aaensafi@gmail.com, ensafi@yahoo.com (A.A. Ensafi). http://dx.doi.org/10.1016/j.electacta.2016.01.221 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.

electrocatalytic activity compare to Pt and exhibit superior selectivity and durability [9–12]. One of effective method to modify materials, such as change in electronic properties and improve surface chemistry, is chemical doping of them with foreign atoms [13–15]. For the N-doped carbon, some researchers have explained that the metallic-like catalytic property results from the increase in electron density at the Fermi level, allowing the electrons to reach the conduction band [8]. Graphene with its unique specific surface area, good thermal, mechanical and chemical properties [10] has shown its interesting applications in methanol and proton exchange membrane fuel cells. Graphene acts as an electron mediator, provides numerous reaction sites and induces the adsorption of pollutants via p–p interactions [16]. Ionic liquids (ILs) functionalized graphene sheets have been synthesized in various solvents, with good dispersibility and long-term stability. Based on previous studies, use of ILs can increase intrinsic activity of ORR electrocatalysts about 2 to 3 times higher intrinsic activity vs. ORR electrocatalysts without IL [17,18]. Herein, we report a successful fabrication of pyridine functionalized reduced graphene oxide (ionic liquid functionalize exfoliated graphene oxide), Py-EGO, as a powerful electrocatalyst for oxygen reduction reaction. The synthesized catalyst exhibited high electrocatalytic activity toward ORR at lower overpotential with improved current density in comparison with N-doped graphene oxide. The Py-EGO showed a close onset potential, limiting current density and half-wave potential vs. Pt-rGO catalyst. In addition, the Py-EGO electrocatalyst also exhibited tolerance to methanol

96

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103

crossover effect. The simple method to synthesize Py-EGO electrocatalyst promising its further application in fuel cells. 2. Experimental 2.1. Reagents and apparatus Graphite, KOH, H2SO4, KNO3, KMnO4, K2PtCl6, commercial Pt/C and H2O2 solution (33%) were purchased from Merck. KOH solution (0.1 mol L
1) was prepared from KOH. The synthetic materials were characterized by Transmission Electron Microscopy (using a Philips CM120), Field Emission Scanning Electron Microscopy and energy-dispersive X-ray spectroscopy (EDS) (were made on a Philips XL-30 at an accelerating voltage of 20 kV), Atomic Force Microscopy images (operating in contact mode) were prepared in Bruker Nanosinstrument AFM (Germany). X-ray diffraction spectra were prepared on a Bruker

D8/Advance X-ray diffractometer (using Cu-Ka radiation). Moreover, X-ray photoelectron spectroscopy (XPS) measurements were performed using an ultra-high vacuum (UHV) set-up equipped with the concentric hemispherical analyzer. XR3E2 X-ray source was used as incident radiation. JASCO FT-IR (680 plus) spectrometer was used to record the Infrared spectra, using KBr pellets were used to characterize EGO. A potentiostat/galvanostat (Model PGSTAT-30, Eco-Chemie, the Netherlands) controlled by the GPES software, with a threeelectrode system, was used to characterize the electrochemical properties of the electrocatalysts. A glassy carbon-wire (as the counter electrode) and an Ag/AgCl electrode (KClsat’d) were used and a reference electrode. For perpetration of the working electrode, Py-EGO, rNGO, rGO, Pt-rGO and Pt/C electrocatalysts ink were prepared by dispersing the electrocatalysts powder (2.0 mg) into 1.0 mL of double distillate water (2.0 mg mL
1) using an ultrasonic bath at least 30 min. Then, 20 mL of each of the

Fig. 1. Survey XPS data for A): rNGO, B): Py-EGO; High resolution XPS spectra of N 1s for C): rNGO, and D): Py-EGO.

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103

electrocatalysts ink was dropped onto a glassy carbon electrode surface (GCE, 5 mm in diameter that previously polished with alumina, 0.05 mm) and dried at room temperature. 2.2. Synthesis of exfoliated graphene oxide and reduce graphene oxide Modified Hummers method was used to synthesis the exfoliated graphene oxide (EGO) from natural graphite powder. Briefly, graphite powder (2.00 g, 325 meshes) was put into a solution containing 20 mL of conc. H2SO4, 2.00 g of P2O5 and 2.00 g of K2S2O8. The mixture was kept at 80  C for 6 h. The solution was diluted with 1.0 L of deionized water and it left for 12 h. This preoxidized mixture was then subjected to oxidation by Hummers’ method described as follows. 100 mL conc. H2SO4 and 12.0 g KMnO4 were added to 2.00 g of the pre-oxidized graphite under stirring. The temperature of the mixture was kept to be below 25  C, using a water bath. Then, the mixture was stirred at 35  C for 2 h. After 2 h, the mixture was diluted with 600 mL of deionized water. Immediately, 5 mL of 33% H2O2 was added to the solution. To remove any metals ions, the result mixture was filtered and the filtrate was washed with 1:10HCl aqueous solution (2 L). Then, it was washed several times with water until the pH of the filtrate was neutral [19]. Reduction of EGO to rGO was performed as follows: 200 mg of EGO was mixed with 200 mL of water to prepare 1.0 mg mL
1 EGO suspension and the suspension temperature was kept at 0  C. Then, to reduce EGO to rGO, NaBH4 (five times ratio to rGO, w/w) was added to the mixture. The obtained product was filtered and washed several times with distilled water before it drying at room temperature. 2.3. Synthesis of pyridine-graphene oxide and nitrogen doped graphene oxide Pyridine-graphene was synthesized by an epoxy ring-opening reaction between exfoliated graphene oxide (EGO) and pyridine, according to previous report [20]. Briefly, a prepared suspension of EGO (5.0 mg mL
1) in pyridine was subjected to an ultrasonic bath for 5 min and then stirring for 48 h at 60 C. The Py-EGO was finally separated using filtration method, and the filtrate was washed with dry benzene and acetone several times, respectively. The solid product was dried in a vacuum desiccator at 60  C for 12 h. Nitrogen-doped rGOs were prepared by hydrothermal method. In a typical procedure, 70 mL of NH4OH aqueous solution was added to 0.070 g GO, which reaction was kept at 40  C with stirring

97

for 1 h and sonicated for 30 min. Then, to complete the hydrothermal reaction, the mixture was transferred to a 120 mL autoclave at 210–220  C and stored during 24 h. This hydrothermal step also reduced NGO to rNGO. The resulted product was collected using centrifugation and then it was washed with water. Finally, the obtained precipitate was dried at room temperature [21,22]. 2.4. Decoration of Pt-nanoparticle on exfoliated graphene oxide Pt-nanoparticles were decorated on EGO sheets using chemical reduction of potassium hexachloroplatinate (K2PtCl6) in ethylene glycol water (2/1, v/v) solution. In a typical procedure, 100 mg of EGO plus 80 mg of K2PtCl6 was dispersed into a 100 mL of distillated water. It was then added into 200 mL of ethylene glycol in a 350-mL flask. To ensure EGO being uniformly dispersed in ethylene glycol-water, the mixture was treated in an ultrasonic bath for 1 h. The reaction mixture was kept at 100  C for 12 h under constant stirring. Moreover, in this process of decoration of Pt, EGO convert to reduced graphene oxide. The Pt nanoparticles were decorated on reduced graphene oxide (Pt-rGO). Finally, the nanoparticles were filtered, washed with water several times and dried in a vacuum desiccator at 60  C for 12 h [23]. 3. Results and discussion 3.1. Characterization of nitrogen doped graphene and pyridinegraphene Several techniques have been used to study the composition of the catalysts including FT-IR, XPS, FE-SEM, XRD and TEM. The XPS pattern of rNGO and Py-EGO shows characteristic peaks corresponding to C1s, O1s, and N1s (Fig. 1A and B). XPS of N-doped graphene and pyridine functionalized EGO are preformed to characterize the elemental composition and bonding configuration in the oxygen reduction electrocatalyst. The XPS spectra of N-doped graphene and pyridine functionalized EGO is shown in Fig. 4A and 4B, respectively. These figures confirm the presence of carbon (C1s = 285.3 eV), nitrogen (N1s = 401.4 eV) and oxygen (O1s = 533.6 eV) in rNGO and Py-EGO. Generally, the C1s peak of span-new graphene is at 285.3 eV, and the up-shift of binding energy is an evidence for the existence of N-doped carbon materials. Moreover, these results showed that the ratio of carbon/ oxygen in rNGO is higher than Py-EGO, whereas the amounts of O species in Py-EGO is higher than rNGO. Therefore, Py-EGO has N and O doped graphene in comparison with rNGO and has high

Fig. 2. XRD patterns of a): Py-EGO, and b): rGO products. The patterns for graphite and GO are included for comparison. (B) FT-IR spectra of a): Py-EGO and b): rGO.

98

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103

performance catalytic activity in oxygen reduction reaction. The deconvoluted high-resolution N1s and the XPS spectra of rNGO clearly show four different types of nitrogen configurations on the rNGO those are pyridinic-N (399.91 eV), pyrrolic-N (402.8 eV), graphitic-N (402.9 eV) and N-oxide (405.0 eV) (Fig. 1C). In addition, in Py-EGO, N1s has two different type of picks, which are pyridinicN (398.1 eV) and N-oxide (402.8 eV) (Fig. 1D). Pyridinic-N refers to nitrogen atoms in pyridine molecule, which can p-p interaction with graphene nanosheet. In addition, Py-EGO showed N1s from N-oxide, which produced from reaction between the epoxy functional group and pyridine as shown in the XPS spectrum [24,25]. XRD was used to characterize the stru cture of the graphite, EGO, rGO and Py-EGO. The XRD patterns (Fig. 2) confirms a clear structural change when reduced the GO sheets by pyridine. The XRD pattern of the GO shows strong (002) peak at 2u = 10.3 , corresponding to interlayers spacing of 8.3 Å. This large interlayer distance is due to the presence of carboxyl, epoxy and hydroxyl groups. During the reduction process these functional groups were removed, therefore the interlayers will get closer to each other. The interlayer distance for Py-EGO, that prepared without additional of reducing agent, was found as 4.8 Å (2u = 23.57 ). A higher interlayer distance is found for rGO (d = 5.6 Å, 2u = 26.2 ), that prepared using NaBH4 as an additional reducing agent. Interlayer spacing for both products were larger than natural graphite (d = 3.4 Å, 2u = 26.5 ). This is suggesting the existence of functional groups in Py-EGO and rGO. The smaller interlayer spacing observed in Py-EGO indicates that by the stronger reducer, more functional groups were removed from the EGO sheets and thus the resulting rGO sheets underwent a larger degree of reduction [26,27] FT-IR spectroscopy is implementing out to illuminate the formation of Py-EGO. Fig. 2B shows FT-IR spectra of rGO and PyEGO. IR spectra of rGO shows several peaks at 1067, 1400, 1628, 1702 and 3432 cm
1 due to C

O stretching of the epoxides (C

O

C), C¼C stretching, carbonyl (C¼O) stretching and O

H stretching, respectively. In the Py-EGO spectra, the broadness band between 3000 and 3500 cm
1 is due to both

NH and

OH groups of the hydrazo- keto form. The peak appeared in the region

of 1620–1653 cm
1 is due to C¼N (from the pyridine) and C¼C skeletal vibrations and the peaks between 1278 and 1251 cm
1 prove C

N stretching vibration. The peak at 1022 cm
1 is due to the C

OH vibration. Decreasing in the C

O stretching of the epoxides (C

O

C) at 1067 cm
1 confirms that during the interaction between pyridine and EGO, the epoxy bonds are broken. All of the peaks in the IR spectra, in addition to the peaks in the region of 800–650 cm
1, confirm the formation of the Py-EGO [28,29]. The morphologies and structures of rGO and Py-EGO are investigated by FE-SEM and TEM, as shown in Fig. 3. The FE-SEM images show that after functionalization of EGO with pyridine, the inter plane spacing of the prepared Py-EGO nanosheets increases. The TEM results also confirm the SEM results that after functionalization of the rGO with pyridine, the inter plane spacing of the prepared Py-EGO nanosheets increase. 3.2. Characterization of Pt nanoparticle decorated on graphene oxide Cyclic voltammetry was used to check the synthesis of Pt-rGO. The cyclic voltammetric responses of Pt-rGO/GCE in 0.5 mol L
1 H2SO4 is shown in Fig. 4A. It is clearly observed that Pt-rGO shows hydrogen adsorption/desorption peak between
0.32 and 0.00 V (vs. Ag/AgCl). The anodic and cathodic cycles show the oxidation of Pt to Pt4+ and reduction of Pt4+ to Pt , respectively confirming the synthesis of Pt nanoparticle on the rGO. XRD patterns of Pt-rGO is shown in Fig. 4B. The typical diffraction peak (0 0 2) of EGO, after reduction by ethylene glycol, could observe at 2u = 26.23. Moreover, when ethylene glycol is used as a reducing agent, decoration of Pt-nanoparticles on EGO is assumed to play a crucial role in the catalytic reduction of EGO. About Pt nanoparticle, strong diffraction peaks at 2u = 39.8, 46.3 and 67.5 can be assigned to the characteristic (1 1 1), (2 0 0) and (2 2 0) as a result of crystalline cubic of Pt. The sharpest diffraction peak of Pt is used to estimate the Pt particle sizes by the Scherrer’s equation: D = 0.89k/(b.cosu). Here, the wavelength k is equal to 0.15418 nm, and b is the peak width at the half-maximum (FWHM). The average particles size of Pt on Pt-rGO sheets is

Fig. 3. A): Typical FE-SEM images of a): Py-EGO and b): rGO; B): TEM images of a): Py-EGO, and b): rGO.

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103

99

Fig. 4. A): Cyclic voltammograms of Pt-rGO modified GCE in 0.5 mol L
1 H2SO4 solution; B): XRD spectrum of Pt-rGO; C): TEM images of Pt-rGO; and D): AFM images of PtrGO.

calculated as 40 nm. The EDS (result not shown) reveals the presence of 58.16% (w/w) C, 17.34% Pt and 24.50% O. These results demonstrated that Pt-rGO was successfully synthesized. The morphology of Pt-rGO is characterized by AFM and TEM. Fig. 4C shows typical TEM images of Pt-rGO. It is clear that the nanosized Pt uniformly decorated at rGO particles with few aggregations, confirming a strong interaction between the Pt nanoparticles and the graphene as a support. In addition, AFM (Fig. 4D) results confirm the TEM results for Pt-rGO. On the other hand, the particle size determined by AFM consistent with the XRD results.

the carbon-nitrogen or carbon-oxygen-nitrogen interface in Py-EGO. Therefore, this charge transfer may affect the behavior of Py-EGO as a good electrocatalyst toward the ORR. Moreover, in ionic liquid functionalized graphene, carbon-oxygen-nitrogen+O2
was formed in alkaline solution and therefore, increase the electrocatalytic effect [30]. This process is confirmed by high electrocatalytic activity of Py-EGO (high current density and lower onset potential) in comparison with N-doped graphene. There are

3.3. Electrocatalytic activities of Py-EGO and rNGO for oxygen reduction The electrocatalytic activities of Py-EGO, rNGO, rGO, Pt-rGO and Pt/C toward the ORR were studied using linear sweep voltammetry (LSV) in N2 and O2 saturated 0.1 mol L
1 KOH solution, from +0.10 V to
0.60 V vs. Ag/AgCl and at a scan rate of 10 mV s
1. Py-EGO, rNGO, rGO, Pt-rGO and Pt/C catalysts displayed featureless curves in N2-saturated solution (Fig. 5). On the other hand, the LSV curves present a well-defined ORR peak in O2-saturated solution for all prepared electrocatalysts, which demonstrate the good electrocatalytic activity of Py-EGO in comparison with rNGO catalyst toward the ORR. Quaternary nitrogen and pyridinic nitrogen are usual nitrogen functional group those provide active sites and are contributes to the effective electrocatalytic activity of rNGO toward the ORR. In quaternary nitrogen and pyridinic nitrogen, neighboring N atoms could activate the p electrons in C; therefore, C has partial positive charge. Thus, oxygen could reduce on the positively charged C atoms [30]. In addition, due to the presence of Fermi level difference between the carbon materials and nitrogen, because of carbon-nitrogen spacing charge transfer exists across

Fig. 5. LSV curves of the ORR at various electrodes at a scan rate of 10 mV s
1 for a): unmodified GCE, b): rGO modified GCE, c): rNGO modified GCE, d): Py-EGO modified GCE, e): Pt-rGO modified GCE, and f): Pt/C modified GCE, in O2-saturated 0.1 mol L
1 KOH solution.

100

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103

two reaction pathways for the electrocatalytic activity of Py-EGO (high current density and lower onset potential) in comparison with the N-doped graphene [31]. Based on the dual-site mechanism, N

C type catalysts have two classes of active surface. The first active site is C

N+ in Py-EGO that has an electrostatic interaction with oxygen species, and the second active site is the p-p interaction between EGO and pyridine (the hetero atom) and thus ORR is occurs via serial electron transfer pathways as follows: O2 + 2H2O + 4e
! 4OH


(1)

O2 + H2O +2e
! HO2
+ OH


(2)

The kinetics of the ORR activity of the best proficiency Py-EGO was further survey at different rotating rates (from 250 rpm to 2500 rpm). As shown in Fig. 6A, the current density increases with the increment of the rotation rate. The Koutecky–Levich (K-L) plots (I
1 vs. v
1/2) at different potentials (from linear sweep voltammetry) at different rotating rates are shown in Fig. 6B. It is clear that the K-L plots present fine linearity. The results show that over different potential (from
0.30 to
0.80 V) the slopes remain nearly constant, which confirms that at the different electrode potentials, numbers of the electron transfer for the reduction of oxygen are analogous. The parallel and linearity of the K-L plots are commonly regarded as a symptom of the first-order kinetics regarding to the dissolved O2 concentration. Koutecky– Levich equation was used to analyze the kinetic parameters as follow [32]. 1 1 1 ¼ þ I Ik 0:62nFAn
1=6 D2=3 cv1=2

ð3Þ

where I is the measured current density, IK is the kinetic current, n is the overall number of electrons transferred in the oxygen reduction pathway, v is the rotating disk angular velocity (v = 2pN, N is the linear rotation speed), F is the Faraday constant

Table 1 Number of the electrons and peaks potential for several reported ORR electrocatalysts. Catalyst

Peak Potential

Number of electron

Reference

N-CNT1 N, P, S-tridoped graphene ON-CNW2 N-Graphene-C3 N-CNW4 PEDOT:PSS/rGO5 NS-MC6 N-doped Graphene Py-EGO


0.28
0.31
0.28
0.28 –
0.27
0.35
0.31
0.28

3.42 2.99 3.7 3.55 3.05 3.55 3.6 3.28 3.74

33 34 35 36 37 38 39 This work This work

1

N-doped carbon nanotubes. O- and N-doped carbon nanoweb. N-doped graphene citric acid. 4 N-doped carbon nanoweb. 5 poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/reduced graphene oxide. 6 Nitrogen and sulfur dual-doped hierarchical micro/mesoporous carbon foams. 2 3

(F = 96485C mol
1), c is the bulk concentration of O2,D is the electron transfer rate constant and n is the kinematic viscosity of the electrolyte. IK and the number of transferred electrons (n) can be obtained from the intercept and the slope of the KouteckyLevich plots (Fig. 6C), respectively. Based on the given parameters as C = 1.2  10
3 mol L
1, D = 1.9  10
5 cm2 s
1, and n = 0.01 cm2 s
1 in 0.1 mol L
1 KOH [26,32], ORR occurs either via direct four electrons pathway (O2 is reduced to H2O) or 2-electron reduction pathway (where it is reduced to hydrogen peroxide). In fuel cell processes, 4-electron direct pathway is preferred. Based on the K–L equation, the calculated n was 3.74 for Py-EGO (closing to 4e
pathway) and 3.27 for rNGO, as shown in Fig. 6C. These results confirm that the ORR activity of Py-EGO follows the 4-electron transfer pathway. Table 1 compare the performance of these modified electrodes with other metal free oxygen reduction electrocatalyst reported elsewhere. From Table 1, it is clear that the

Fig. 6. A): LSV curves of ORR at Py-EGO modified GCE obtained at different rotating rates; B): K-L plots of the current reciprocal (-i
1) versus v
1/2 at different potential at PyEGO modified GCE; C): The transferred electron number (n) per oxygen molecule at different potential.

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103

101

Fig. 7. A): Chronoamperometry carried out at a): rNGO), b): Py-EGO, c): Pt-rGO, and d): Pt/C (30 wt%) modified GCEs in O2-saturated 0.1 mol L
1 KOH solution at 1800 rpm and at a potential of –0.27 V (vs. Ag/AgCl) after 1500 s, methanol (1.0 mol L
1) was added to the KOH solution in order to assess the crossover effects. B): LSVs of commercial Pt/C (30 wt%) modified GCE (a): in an O2-saturated 0.1 mol L
1 KOH solution and (b): in an O2-saturated 0.1 mol L
1 KOH solution upon addition of methanol (1.0 mol L
1) at a scan rate of 10 mV s
1; and (C): Py-EGO modified GCEs (a): in an O2-saturated 0.1 mol L
1 KOH solution and (b): in an O2-saturated 0.1 mol L
1 KOH solution upon addition of methanol (1.0 mol L
1) at a scan rate of 10 mV s
1.

number of electrons and peak potential of the Py-EGO are comparable and even better than those obtained by the other metal free ORR [33–40]. Stability and electrochemical crossover effect are two important factors to evaluate the performance of an electrocatalyst in fuel cells. The electrocatalytic activity of Py-EGO in ORR vs. the electrooxidation of methanol was studied using hydrodynamic amperometry in an O2-saturated 0.10 mol L
1 KOH solution in the presence of 1.0 mol L
1 methanol. The commercial Pt/C and Pt-rGO

were used as a control and were tested under the same conditions. The results showed that with applied potential of
0.26 V (vs. Ag/ AgCl), the cathodic signals for ORR were observed for Pt/C, Pt-rGO, rNGO and Py-EGO (Fig. 7A,). After addition of methanol into the O2saturated 0.10 mol L
1 KOH solution, for Pt/C and Pt-rGO a large current from the oxidation of methanol was observed. On the other hand, with Py-EGO and rNGO electrodes at the same conditions, no significant difference between the peak currents can be observed, with and without methanol (Fig. 7A). From these results

Fig. 8. ORR polarization curves of A): Commercial Pt/C (30 wt%) modified GCE before (a), and after 10000 s (b) chronoamperometry in the presence of O2 and methanol. B): PyEGO-GCE before (a), and after 10000 s (b) chronoamperometry in the presence of O2-saturated 0.1 mol L
1 KOH solution. Condition: Scan rate, 10 mV s
1 at a fixed rotating speed of 1800 rpm from 0.20 to –1.00 V vs. Ag/AgCl.

102

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103

suggesting that Py-EGO is free from methanol poisoning. To confirm the amperometric results, linear sweep voltammetry was also used. For this purpose, LSV for Pt/C and Py-EGO in an O2saturated solution containing 0.10 KOH and 1.0 mol L
1 methanol were recorded. As presented in Figs. 7B and 7C, the cathodic signals toward the ORR was appeared at about
0.28 V for Py-EGO (Fig. 7C) and
0.19 V for Pt/C (Fig. 7B), after addition of methanol into the O2-saturated 0.10 mol L
1 KOH solution. At Pt/C electrode the current intensity corresponds to methanol oxidation increases significantly, whereas no significant difference was observed between the obtained peaks current of Py-EGO electrode with and without methanol. The linear sweep voltammetry results confirm the amperometric results. These obtained results clearly demonstrate that the metal-free Py-EGO has a high electrocatalytic activity towards ORR, with much higher selectivity toward ORR than the commercial Pt/C. The durability of Py-EGO and commercial Pt/C electrocatalysts were also studied and are compared with them. Liner sweep voltammograms were obtained from Py-EGO and commercial Pt/C modified GCEs in O2-saturated 0.10 mol L
1 KOH at a scan rate of 10 mV s
1 from 0.10 to
0.60 V. For the commercial Pt/C and PyEGO and (Fig. 8A and B), LSV before applied hydrodynamic amperometry and after 10000 s hydrodynamic amperometry were compared (in the presence of O2-saturated and 1.0 mol L
1 KOH). Fig. 8B shows the LSV of Py-EGO modified electrode before and after hydrodynamic amperometry. A negligible change in the oxygen reduction peaks were observed upon an extended cycling, whereas a clear decay in the oxygen reduction peaks can be observed for the commercial Pt/C (Fig. 8A). These results indicate that Py-EGO has a much better durability and stability than the commercial Pt/C. 4. Conclusion A simple thermal treatment method was used to synthesize PyEGO electrocatalyst. The pyridine was linked to reduced graphene oxide by epoxy group or hydroxyl group that existing at a surface of the graphene. The synthesized catalyst exhibited high electrocatalytic activity toward ORR at lower overpotential with improved current density in comparison with N-doped graphene oxide. In particular, the Py-EGO showed a close onset potential, limiting current density and half-wave potential vs. Pt-rGO catalyst. In addition, Py-EGO electrocatalyst also exhibited tolerance to the methanol crossover effect. The simple method to synthesize of PyEGO electrocatalyst promising its further application in fuel cells. Acknowledgements The authors wish to thank the Iran National Science Foundation and National Elites Foundation, for their support. References [1] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345–352. [2] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature 486 (2012) 43–51. [3] H.A. Gasteiger, N.M. Markovic, Just a dream or future reality? Science 324 (2009) 48–49. [4] G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, Lithium-air battery: promise and challenges, J. Phys. Chem. Lett. 1 (2010) 2193–2203. [5] J.S. Lee, S.T. Kim, R. Cao, N.S. Choi, M. Liu, K.T. Lee, J. Cho, Metal-air batteries with high energy density: Li-Air versus Zn-Air, Adv Energy Mater. 1 (2011) 34– 50. [6] Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, A review on non-precious metal electro- catalysts for PEM fuel cells, Energy Environ. Sci. 4 (2011) 3167–3192. [7] C. Domínguez, F.J.P. Alonso, M.A. Salam, Sh A.A. Thabaiti, M.A. Peña, F.J.G. García, L. Barrio, S. Rojas, Repercussion of the carbon matrix for the activity and stability of Fe/N/C electrocatalysts for the oxygen reduction reaction, App. Cat. B: Environmental 183 (2016) 185–196.

[8] Y. Gao, H. Zhao, D. Chen, C. Chen, F. Ciucci, In situ synthesis of mesoporous manganese oxide/sulfur-doped graphitized carbon as a bifunctional catalyst for oxygen evolution/reduction reactions, Carbon 94 (2015) 1028–1036. [9] M.R. Ayats, E. Herreros, G. García, M.A. Peña, M.V.M. Huerta, Promotion of oxygen reduction and water oxidation at Pt-based electrocatalysts by titanium carbonitride, App. Cat. B: Environmental 183 (2016) 53–60. [10] Y. Zheng, Y. Jiao, Y. Jin, M. Jaroniec, S.Z. Qiao, Nanostructured metal-free electrochemical catalysts for highly efficient oxygen reduction, Small 8 (2012) 3550–3566. [11] C. Huang, C. Li, G. Shi, Graphene based catalysts, Energy Environ. Sci. 5 (2012) 8848–8868. [12] S.G. Peera, A.K. Sahu, A. Arunchander, S.D. Bhat, J. Karthikeyan, P. Murugan, Nitrogen and fluorine co-doped graphite nanofibers as high durable oxygen reduction catalyst in acidic media for polymer electrolyte fuel cells, Carbon 93 (2015) 130–142. [13] L. Qu, Y. Liu, J.B. Baek, L. Dai, Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells, ACS Nano 4 (2010) 1321– 1326. [14] D. Geng, Y. Chen, L.Y. Chen, R. Li, X. Sun, S. Ye, S. Knights, High oxygen-reduction activity and durability of nitrogen-doped graphene, Energy Environ. Sci. 4 (2011) 11760–11764. [15] Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, Sulfurdoped graphene as an efficient metal-free cathode catalyst for oxygen reduction, ACS Nano 6 (2012) 205–211. [16] J. Kim, L.J. Cote, F. Kim, W. Yuan, K.R. Shull, J. Huang, Graphene oxide sheets at interfaces, J. Am. Chem. Soc. 132 (2010) 8180–8186. [17] L. Dai, Functionalization of graphene for efficient energy conversion and storage, Acc. Chem. Res. 46 (2013) 31–42. [18] G.R. Zhang, M. Munoz, B.J.M. Etzold, Boosting performance of low temperature fuel cell catalysts by subtle ionic liquid modification, ACS Appl. Mater. Interfaces 7 (2015) 3562–3570. [19] A.A. Ensafi, Z. Ahmadi, M. Jafari-Asl, B. Rezaei, Graphene nanosheets functionalized with Nile blue as a stable support for the oxidation of glucose and reduction of oxygen based on redox replacement of Pd-nanoparticles via nickel oxide, Electrochim. Acta 173 (2015) 619–629. [20] Y. Zhao, R. Nakamura, K. Kamiya, Sh. Nakanishi, K. Hashimoto, Anisotropic two-dimensional electron gas at the LaAlO3/SrTiO3 (110) interface, Nature Commun. 4 (2013) 1–7. [21] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nature Materials 10 (2011) 780–786. [22] E.Y. Choi, T.H. Han, J. Hong, J.E. Kim, S.H. Lee, H.W. Kim, S.O. Kim, Noncovalent functionalization of graphene with end-functional polymers, J. Mater. Chem. 20 (2010) 1907–1912. [23] A.A. Ensafi, M. Jafari-Asl, B. Rezaei, A new strategy for the synthesis of 3-D Pt nanoparticles on reduced graphene oxide through surface functionalization, Application for methanol oxidation and oxygen reduction, Electrochim. Acta 130 (2014) 397–405. [24] L. Lai, H. Yang, L. Wang, B.K. The, J. Zhong, H. Chou, L. Chen, W. Chen, Z. Shen, R. S. Ruoff, J. Lin, Preparation of supercapacitor electrodes through selection of graphene surface functionalities, ACS Nano 7 (2012) 5941–5951. [25] S. Liu, K. Chen, Y. Fu, S. Yu, Z. Bao, Reduced graphene oxide paper by supercritical ethanol treatment and its electrochemical properties, Appl. Surf. Sci. 258 (2012) 5299–5303. [26] Y. Zhou, C.H. Yen, Sh. Fu, G. Yang, Ch. Zhu, D. Du, P. Ch. Wo, X. Cheng, J. Yang, C. M. Waic, Y. Lin, One-pot synthesis of B-doped three-dimensional reduced graphene oxide via supercritical fluid for oxygen reduction reaction, Green Chem. 17 (2015) 3552–3560. [27] A. Gupta, Sh K. Saha, Emerging photoluminescence in azo-pyridine intercalated graphene oxide layers, Nanoscale 4 (2012) 6562–6567. [28] A.A. Ensafi, M. Jafari-Asl, B. Rezaei, Graphene nanosheets functionalized with 4-aminothiophenol as a stable support for the oxidation of formic acid based on self-supported Pd-nanoclusters via galvanic replacement from Cu2O nanocubes, J. Electroanal. Chem. 731 (2014) 20–27. [29] M. Sh Ahmed, S. Jeon, New functionalized graphene sheets for enhanced oxygen reduction as metal-free cathode electrocatalysts, J. Power Sources 218 (2012) 168–173. [30] C.A. Hancock, L.A. Ong, P.R. Slater, J.R. Varcoe, Development of CaMn1– xRuxO3–y (x = 0 and 0.15) oxygen reduction catalysts for use in low temperature electrochemical devices containing alkaline electrolytes: ex situ testing using the rotating ring-disk electrode voltammetry method, J. Mater. Chem. A 2 (2014) 3047–3056. [31] J. Li, Y. Zhang, X. Zhang, J. Han, Y. Wang, L. Gu, Z. Zhang, X. Wang, J. Jian, P. Xu, B. Song, Direct transformation from graphitic C3N4 to nitrogen-doped graphene: an efficient metal-free electrocatalyst for oxygen reduction reaction, ACS Appl. Mater. Interfaces 7 (2015) 19626–19634. [32] M. Jahan, Q. Bao, K.P. Loh, Electrocatalytically active graphene-porphyrin MOF composite for oxygen reduction reaction, J. Am. Chem. Soc. 134 (2012) 6707– 6713. [33] Q. Liu, Z. Pu, Ch. Tang, A.M. Asiri, A.H. Qusti, A.O. Al-Youbi, X. Sun, N-doped carbon nanotubes from functional tubular polypyrrole: A highly efficient electrocatalyst for oxygen reduction reaction, Electrochem. Commun. 36 (2013) 57–61. [34] A. Sh. Dou, Z. Shen, J. Ma, L. Wu, Wang Sh. Tao, N-, P- and S-tridoped graphene as metal-free electrocatalyst for oxygen reduction reaction, J. Electro, Anal. Chem 753 (2015) 21–27.

A.A. Ensafi et al. / Electrochimica Acta 194 (2016) 95–103 [35] L. Li, S. Liu, A. Manthiram, Co3O4 nanocrystals coupled with O- and N-doped carbon nanoweb as a synergistic catalyst for hybrid Li-air batteries, Nano Energy 12 (2015) 852–860. [36] Y. Liao, Y. Gao, Sh. Zhu, J. Zheng, Z. Chen, Ch. Yin, X. Lou, D. Zhang, Facile fabrication of N-doped Graphene as efficient electrocatalyst for oxygen reduction reaction, ACS Appl, Mater. Interfaces 7 (2015) 19619–19625. [37] S. Liu, L. Li, H.S. Ahn, A. Manthiram, Delineating the roles of Co3O4 and Ndoped carbon nanoweb (CNW) in bifunctional Co3O4/CNW catalysts for

103

oxygen reduction and oxygen evolution reactions, J. Mater. Chem. A 3 (2015) 11615–11623. [38] M. Zhang, W. Yuan, B. Yao, Ch. Li, G. Shi, Solution-processed PEDOT:PSSgraphene composites as electrocatalyst for oxygen reduction reaction, ACS Appl, Mater. Interfaces 6 (2014) 3587–3593. [39] Sh. Jiang, Y. Suna, H. Dai, J. Hu, P. Ni, Y. Wang, Z. Lia, Facile synthesis of nitrogen and sulfur dual-doped hierarchical micro/mesoporous carbon foams as efficient metal-free electrocatalysts for oxygen reduction reaction, Electrochim. Acta 174 (2015) 826–836.