Enhanced oxygen reduction performance by novel pyridine substituent groups of iron (II) phthalocyanine with graphene composite

Journal of Power Sources 282 (2015) 9e18

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Enhanced oxygen reduction performance by novel pyridine substituent groups of iron (II) phthalocyanine with graphene composite Lili Cui, Guojun Lv, Xingquan He* Department of Chemistry and Chemical Engineering, Changchun University of Science and Technology, Changchun 130022, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


FeTPPc/Gr is synthesized through a simple solvothermal method.
FeTPPc/Gr exhibits better catalytic activity for the ORR than Pt/C in alkaline conditions.
FeTPPc/Gr displays better stability and tolerance to methanol crossover than Pt/C.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 November 2014 Received in revised form 4 February 2015 Accepted 5 February 2015 Available online 7 February 2015

In this paper, a novel iron (II) tetrapyridyloxyphthalocyanine decorated graphene (FeTPPc/Gr) is synthesized through a simple solvothermal method. The catalytic performance of the fabricated FeTPPc/Gr for the oxygen reduction reaction (ORR) is accessed by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and i-t chronoamperometry methods. The FeTPPc/Gr composite catalyst for the ORR displays an enhanced electrocatalytic activity compared with other FePc/Gr catalysts. More importantly, the proposed FeTPPc/Gr catalyst towards the ORR outperforms the commercial Pt/C catalyst in terms of higher diffusion-limiting current, more positive onset potential and half-wave potential, better stability and tolerance to methanol crossover. The improved ORR performance is attributed to the activity of peripheral pyridine substituents in the FePc, which facilitate O2 absorption and increase the additional active sites. Based on our experimental results, designing novel metal-N4 macrocycles and incorporating them into graphene or graphene derivatives, with both optimal activity and durability for the ORR, may hold great promise for application in alkaline direct methanol fuel cells (DMFCs). © 2015 Elsevier B.V. All rights reserved.

Keywords: Iron tetrapyridyloxyphthalocyanine Graphene Oxygen reduction reaction Direct methanol fuel cells

1. Introduction The oxygen reduction reaction (ORR) is an important process in electrochemical energy conversion, for example, fuel cells and metal-air batteries [1,2]. The cathode reaction for oxygen reduction

* Corresponding author. E-mail address: [email protected] (X. He). http://dx.doi.org/10.1016/j.jpowsour.2015.02.031 0378-7753/© 2015 Elsevier B.V. All rights reserved.

proceeds through a multistep electron transfer, leading to sluggish kinetics and a high overpotential. In fuel cells, Pt-based materials as effective electrocatalysts are needed to reduce the overpotential and expedite the ORR, through a direct four-electron transfer process. However, the disadvantages of Pt-based catalysts such as the high price, intolerance to fuel crossover, and instability have greatly hindered their broad commercialization. Therefore, recent researches of alternative non-noble ORR catalysts including metal

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chalcogenide [3], N-doped carbon nanotubes, graphene and graphene aerogel [4e10], and metal-N4 compounds [11e14], have been paid great attentions. Metal phthalocyanines (MPcs) are conjugated macrocyclic compounds and have unique plane structures, which exhibit excellent electronic properties, and are widely applied in sensors, photocatalysts, electrocatalysts and supercapacitors [15e21]. A high architectural flexibility in MPcs facilitates the tailoring of their physical and chemical parameters in a very broad range through changing the central metal and the peripheral substituents [22e26]. Since cobalt phthalocyanine was investigated as an ORR catalyst for fuel cells by Jasinski in 1964 [12], metal phthalocyanines have been extensively researched as potential catalysts for the ORR. The central metal atoms determinatively influence their catalytic activity, and their catalytic activity follow the order: FePc > CoPc > NiPc > CuPc > MnPc [27,28]. Moreover, the ORR performance is affected by different peripheral substituents of MPcs. Iron phthalocyanine alone exhibits low catalytic activity and poor stability for the ORR. To improve the catalytic performance of iron phthalocyanine, several research groups have found that carbon materials supported iron phthalocyanine (CM/FePc) composites exhibit excellent ORR performance [29e32]. Despite great efforts have been paid to develop CM/FePc as ORR catalysts, to date, rare CM/FePc composites display catalytic activity superior to commercial Pt/C [30]. Accordingly, it is necessary to explore new CM/FePc catalysts for the ORR. Graphene, an ultrathin two-dimensional network composed of sp2-hybridized carbon atoms, has become a very popular matrix for the ORR catalyst due to its large specific surface area, high thermal conductivity, excellent chemical and thermal stability. The some carbon atoms on the pristine graphene can be substituted by heteroatoms, such as N, B, P, S, the obtained heteroatom-doped graphene materials are considered to be high-efficient electrocatalysts for the ORR [33e36]. In heteroatom-doped graphene materials, nitrogen-doped graphene (NG) has drawn much attention as metal-free catalysts for the ORR because of their low price, excellent catalytic activity and rather good stability [37,38]. Several reports have shown that graphitic-N and pyridinic-N are the key catalytic active sites for the ORR in NG [39]. The carbon atoms neighboring pyridinic nitrogen play an important role in the ORR process and are the main active sites in the nitrogen-doped graphene [40]. Based on the above-mentioned results, the pyridinic nitrogen in the N-doped carbon materials can promote the ORR process. Inspired by the advantage of pyridine nitrogen in the N-doped carbon materials, a novel hybrid composed of iron (II) tetrapyridyloxyphthalocyanine and graphene (FeTPPc/Gr) is prepared through a simple solvothermal method. The structure of this novel iron phthalocyanine is shown in Scheme 1. The FeTPPc/Gr composite displays more positive onset potential, half-wave potential,

Scheme 1. Molecular structure of FeTPPc.

and higher diffusion-limiting current density than commercial Pt/C, respectively. The enhanced catalytic activity is mainly attributed to the effect of pyridine substituents in peripheral macrocycle of FePc, which increase the active sites towards the ORR.

2. Experimental section 2.1. Materials Graphite powder and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were purchased from Sinopharm Chemical Reagent Co., Ltd. Poly(sodium-p-styrenesulfonate) (PSS) was obtained from SigmaeAldrich. 4-nitrophthalonitrile and 4-hydroxypyridine were bought from Sinopharm Chemical Reagent Co., Ltd. Pt/C (20 wt % Pt on Vulcan XC-72) was purchased from Alfa Aesar. All other reagents were analytical grade, and used without further purification, including n-pentanol, hydrazine hydrate solution, ethanol, N, Ndimethyl formamide (DMF), NaNO3, KMnO4, K2CO3 and H2O2. Distilled water was used to prepare the solutions.

2.2. Preparation of FeTPPc/Gr composite 4-(4-pyridyloxy)phthalonitrile was synthesized by adopting a similar strategy reported by Mariana's group [41]. A solution of 4nitrophthalonitrile (1.73 g, 12 mmol) in dry DMF (50 mL) under N2 was added 4-hydroxypyridine (1.04 g, 10.91 mmol). After stirring for 15 min, the temperature was raised to 50  C and finely ground anhydrous K2CO3 (2.47 g, 18 mmol) was added in portions during 2 h with efficient stirring. After the reaction mixture had been stirred for 48 h at 80  C, it was cooled to room temperature and poured into water (300 mL) with stirring 30 min. Then the precipitate was filtered off and washed with distilled water until the filtrate was neutral. Then, it was washed with ethyl acetate to get the product. FeTPPc and poly(sodium-p-styrenesulfonate) modified graphene (PSS-Gr) were synthesized according to the method reported previously [42,31]. FeTPPc was synthesized as follows: a mixture of ferrous chloride tetrahydrate (0.25 g, 1.25 mmol), 4-(4-pyridyloxy)phthalonitrile (0.56 g, 2.50 mmol), npentanol (60 mL) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, 2 mL) was stirred at 160  C for 6 h under nitrogen atmosphere. After cooling to room temperature, the solution was poured into 200 mL distilled water. The mixture was filtered and the precipitate was washed with distilled water for several times followed by ethanol and diethylether to gain the final product. PSS-Gr was prepared by the following procedure: 0.10 g GO was dispersed in 100 mL DMF by ultrasonic vibration for 2 h at a concentration of 1 mg/mL, then 0.50 g PSS was added. The above suspension was stirred for 12 h at room temperature and then 1 mL hydrazine hydrate was added. Following, the mixture was heated to 100  C and stirred for 24 h to reduce graphene oxide to graphene. After cooling to room temperature, the mixture was separated by centrifugation and washed with distilled water and DMF, respectively. The residual was dispersed in DMF at a concentration of 2 mg/mL to use. FeTPPc/Gr composite was fabricated by the following procedure: 0.01 g graphene (5 mL dispersion solution in DMF), varying amounts of FeTPPc/Gr, (0.01 g, 0.02 g, 0.03 g, 0.04 g, mass ratio to graphene is 1:1, 2:1, 3:1 and 4:1, respectively) were dispersed in 15 mL DMF under ultrasonic vibration for 30 min. Following, the dispersion solution was sealed in a 50 mL Teflon-lined autoclave and maintained at 100  C for 12 h. Afterwards, its temperature was naturally cooled to the room degree to allow for the powder separation. The product was then centrifuged and washed with ethanol for three times.

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2.3. Instrumentation Morphologies of the FeTPPc/Gr composites were studied by scanning electron microscopy (SEM, JEOL JSM-6701F electron microscope, operating at 100 kV) and transmission electron microscopy (TEM, JEOL JEM-2100F, operating at 200 kV). The UVevis spectra were operated on a mini UV-1240 spectrophotometer, and analysis of the X-ray photoelectron spectra (XPS) was performed on an ESCLAB 250 spectrometer using AlKa as the exciting source. Cyclic voltammetry (CV) was performed on a CHI 660D electrochemical workstation (Shanghai CHENHUA company) in a conventional three-electrode cell using the modified GC electrode (d ¼ 3 mm) as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as reference at a scan rate of 100 mV s1 in a N2- or O2-saturated 0.1 M NaOH solution. The rotating disk electrode (RDE, d ¼ 5 mm), current-time chronoamperometric response (iet) and rotating ring disk electrode (RRDE, ddisk ¼ 5.61 mm) measurements were operated on a Pine Instrument Company AF-MSRCE modulator speed rotator on a CHI660E electrochemical workstation (CH Instruments, Shanghai CHENHUA company) with a standard three-electrode system at a scan rate of 10 mV s1 in an O2-saturated 0.1 M NaOH solution. The polarization of Pt ring was regulated at 0.6 V vs SCE to detect the production of hydrogen peroxide species in a RRDE configuration. The collection efficiency of platinum ring is 37%. All electrochemical experiments were performed at room temperature and all potentials were reported against the saturated calomel electrode (SCE). 2.4. Electrode modification Before modification, the surface of glassy carbon electrode (GCE) was sequentially polished with 1.0, 0.3 and 0.05 mm a-Al2O3 powder slurry, and ultrasonically washed with distilled water and absolute ethanol for a few minutes, respectively. Following, the GCE was blow-dried with N2. In order to modify the GCE, 1.0 mg of the total catalyst and 1.0 mL ethanol were mixed ultrasonically to obtain a homogeneous ink. Then, 15 mL of the catalyst ink was coated on the freshly polished GCE surface, and then dried in an infrared lamp for 15 min. For comparison, the GCE coated with FeTPPc/Gr, Gr and Pt/C (20 wt % Pt on Vulcan XC-72) were also fabricated with the same procedure and catalyst loading. 3. Results and discussion Fig. 1 shows SEM and TEM images of Gr and FeTPPc/Gr composites. Graphene (Gr) presents typical crumpled nanosheets structure, seen from Fig. 1(A). In Fig. 1(B), aggregated nanoparticles can be clearly seen for FeTPPc alone. In FeTPPc/Gr composites, FeTPPc nanocrystals aggregates anchored on the surface of the graphene can be observed. With an increase in the amount of FeTPPc, FeTPPc nanocrystals gradually closely stack on graphene surface and the graphene surface is totally encapsulated by FeTPPc when the ratio of FeTPPc to graphene reaches to more than 2:1, as shown in Fig. 1(C)e(F). The TEM images of the as-obtained graphene and FeTPPc/Gr are also shown in Fig. 1(G)e(I). As can been seen from Fig. 1(G), graphene shows transparent sheets structure with wrinkled and folded features. In Fig. 1(H), the FeTPPc nanoaggregates coated on graphene are observed, in accord with SEM results. A high resolution transmission electron microscopy (HRTEM) image of FeTPPc/Gr reveals typical regular lattice fringes corresponding to the FeTPPc, demonstrating FeTPPc nanocrystals have a good crystalline structure. In an attempt to better understand the interaction between FeTPPc and graphene, UVevis spectra are employed. Fig. 2(a)

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displays UVevis spectra of graphene, FeTPPc and FeTPPc/Gr composite in DMF, respectively. As can be seen, graphene shows an absorption band at 273 nm due to the characteristic p-plasmon absorption. To FeTPPc, there are three peaks of Q band at 607, 640 and 668 nm, respectively. While the FeTPPc/Gr composite exhibits two peaks of Q band (671 nm, 744 nm), which correspond to characteristic absorption peaks of FeTPPc. Apparent red shift compared to FeTPPc alone indicates there is a strong p-p interaction between FeTPPc and Gr. The interaction between FeTPPc and Gr is further confirmed by XPS, as shown in Fig. 2(b)e(d). The broad and asymmetric N1s XPS spectras of FeTPPc and FeTPPc/Gr show that there are different binding configurations (Fig. 2(c) and (d)). From deconvolution of N1s XPS spectrum of FeTPPc (Fig. 2(c)), the spectrum is well-fitted to three peaks with binding energies at 398.53 eV (pyridinic N), 399.11 eV (CoeN) and 400.90 eV (pyrrolic N) [43,44]. While for FeTPPc/Gr, the corresponding N1s peaks shift to higher binding energies by 0.37 eV, 0.69 eV and 0.70 eV compared with those for pure FeTPPc, respectively, as shown in Fig. 2(d). Such higher binding energy shifts in the fabricated composite clearly indicate the charge transfers from FeTPPc to Gr. The Fe 2p core level spectra are shown in Fig. 2(e) and (f). In contrast with single FeTPPc, the binding energies of Fe 2p3/2 and Fe 2p1/2 in the FeTPPc/Gr shift to higher values, indicating that the interaction of FeTPPc and graphene results in more positive iron ion in the composite [30,45]. The electrocatalytic properties of FeTPPc/Gr with different mass ratio are firstly characterized by cyclic voltammetry (CV) in 0.1 M NaOH solution saturated with N2 or O2 at a scan rate of 100 mVs1. As shown in Fig. 3(a), only a small pair of redox peak within the potential range from 0.6 to 0.4 V vs SCE is observed for FeTPPc/Gr (1:1) materials in the nitrogen saturated solution, which is attributed to the intrinsic redox peak of FeTPPc [30,31]. The oxygen reduction peak potentials of FeTPPc/Gr composites are almost invariable with increasing the FeTPPc mass in the oxygen saturated solution, and the peak current density achieves the maximum when the mass ratio of FeTPPc to graphene gets to 1:1. Therefore, the FeTPPc/Gr composite with 1:1 mass ratio of FeTPPc to graphene is employed in the following study. In contrast, when the electrolyte is saturated with oxygen, FeTPPc/Gr exhibits a substantial reduction process with a remarkable reduction peak situated at 0.118 V vs SCE shown in Fig. 3(b), which is even more positive than that of commercial Pt/C (0.212 V vs SCE). The peak current density on the FeTPPc/Gr composite is about 2 times as large as that on graphene or FeTPPc, and similar to that on Pt/C catalyst with the same catalyst loading. The markedly improved ORR activity of FeTPPc/Gr composite compared with the single component should be attributed to the synergistic effect between FeTPPc and Gr. In order to gain better insight into the ORR activity of FeTPPc/Gr, RDE measurements of FeTPPc/Gr and Pt/C are investigated shown in Fig. 4(a) and (c). These polarization curves show typical increasing current density with the increase of rotation speeds, which could be explained by shortened diffusion distance at high rotation speeds [30]. The number of electrons transferred (n) for the ORR on FeTPPc/Gr and Pt/C can be estimated from the slope of K-L (J1 vs u1/2) plot according to RDE test (Fig. 4(b) and (d)). The kinetic parameters can be analyzed on the basis of the KouteckyeLevich (KeL) equation given below:

 1=J ¼ 1=JK þ 1=JL ¼ 1 Bu1=2 þ 1=JK 2=3

B ¼ 0:62nFC0 D0 y1=6 JK ¼ nFkC0 where J is the measured current density, JK and JL are the kinetic and diffusion-limiting current densities, u is the angular velocity of the

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Fig. 1. SEM and TEM images of: (A) Gr, (B) FeTPPc, (C) the FeTPPc/Gr composite (1:2), (D) the FeTPPc/Gr composite (1:1), (E) the FeTPPc/Gr composite (2:1) and (F) the FeTPPc/Gr composite (3:1) respectively, TEM images of: (G) Gr, (H) FeTPPc/Gr composite (1:1), (I) HRTEM of FeTPPc/Gr composite (1:1).

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Fig. 2. (a) UVevis absorption spectra of Gr, FeTPPc and FeTPPc/Gr (1:1) composite in DMF, respectively; (b) XPS spectra of Gr, FeTPPc and FeTPPc/Gr composite in the survey; highresolution N1s XPS spectra of: FeTPPc (c) and FeTPPc/Gr (d); Fe 2p XPS spectra of: FeTPPc (e) and FeTPPc/Gr (f).

Fig. 3. Electrocatalytic ORR performance of samples: (a) CV curves of different samples in O2 saturated 0.1 M NaOH; (b) CV curves of FeTPPc/Gr composite with different mass ratio in an O2 or N2 saturated 0.1 M NaOH.

disk (u ¼ 2pN, N is the linear rotation speed), n is the overall number of electrons transferred in oxygen reduction, F is the Faraday constant (F ¼ 96485 C mol1), C0 is the bulk concentration of O2, n is the kinematic viscosity of the electrolyte, and k is the

electron transfer rate constant. As shown in Fig. 4(b) and (d), the number of electrons transferred (n) and JK can be obtained from the slope and intercept of the KeL plots, respectively, and by using parameters C0 ¼ 1.38  106 mol L1, D0 ¼ 1.9  105 cm s1, and

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Fig. 4. Rotating disk electrode linear sweep voltammograms tests obtained for FeTPPc/Gr (a) and Pt/C (c) under various rotation speeds; KouteckyeLevich plots for FeTPPc/Gr (b) and Pt/C (d) (according to Fig. 4(a) and (c)) at fixed potentials of 0.3, 0.4, 0.5, 0.6 and 0.7 V vs SCE, respectively. (e) LSVs of different samples: Gr, FeTPPc, FeTPPc/Gr and Pt/C. (f) The electrons transfer number on the potential for FeTPPc/Gr and Pt/C at potentials ranging from 0.3 to 0.7 V vs SCE, respectively. (g) Tafel plots of FeTPPc/Gr and Pt/C at the ration rate of 1600 rpm. All the LSV curves are tested in O2-saturated 0.1 M NaOH electrolyte at a scan rate of 10 mVs1.

n ¼ 0.01 cm2 s1 in 0.1 M NaOH. As is well known, the reduction of O2 is a multi electron reaction in alkaline media that has two main possible pathways: one proceeding to the 2-electron product, the peroxide anion (HO 2 ), and the other, a direct 4-electron reduction pathway where O2 is reduced to H2O. As shown in Fig. 4(b) and (d), the corresponding K-L plots (J1 vs

u1/2) at various electrode potentials exhibit good linear relation,

and almost overlapped linear fitting lines suggest the ORR on the FeTPPc/Gr catalyst follows a first-order reaction kinetics toward the concentration of dissolved O2 and has a similar electron transfer number for the ORR in the studied potential range. Fig 4(f) shows the dependence of the electron transfer number on the potential for various catalysts. For FeTPPc/Gr, the electrocatalytic process for

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Fig. 5. (a) RRDE voltammograms of FeTPPc/Gr and Pt/C electrodes for the ORR in O2 saturated 0.1 M NaOH at a scan rate of 10 mV s1. The electrode rotation rate was 1600 rpm and  the Pt ring electrode was polarized at 0.6 V vs SCE. (b) HO 2 yield (HO2 %) and (c) the electron transfer number dependence of FeTPPc/Gr and Pt/C.

the ORR is almost a one-step four-electron pathway throughout the potential range studied. Fig 4(g) presents the mass transportcorrected Tafel plots of FeTPPc/Gr and Pt/C for the oxygen reduction kinetics derived from the LSVs at 1600 rpm (Fig. 4(a) and (c)). Two distinct Tafel slopes in low and high overpotential regions are observed for each catalyst, which indicate that oxygen reduction occurred by different mechanisms in these two regions. At low overpotentials, the Tafel slopes for FeTPPc/Gr and Pt/C are 41 and 68 mV dec1, this reveals the transfer of the first electron on both of these two catalysts is the rate determining step under Temkin conditions for the adsorption of intermediates; while at high overpotentials, the corresponding slopes are 133 and 145 mV dec1, respectively. The result is attributed to a change in the mechanism of the ORR from Temkin to Langmuir adsorption conditions [46,47]. From a mechanistic point of view,

this imply the ORR mechanisms on FeTPPc/Gr and Pt/C catalysts are similar in an alkaline medium [48]. In addition, the smaller Tafel slopes of FeTPPc/Gr than Pt/C in both low and high potential regions reveal that the overpotential increases slowly with current density, leading to better ORR activity of FeTPPc/Gr [49,50]. To quantify the ORR catalytic pathway of the FeTPPc/Gr catalyst, another efficient measurement to estimate the electron transfer number (n) is the rotating ring-disk electrode (RRDE) technique, which is carried out to monitor the formation of peroxide species during the ORR process. Fig. 5(a) exhibits the disk and ring current for the FeTPPc/Gr catalyst, and the results of Pt/C catalyst are also given for comparison. The percentage of HO 2 and the electron transfer number (n) are calculated from the equation given below:

HO 2 % ¼ 200 

n¼4

iR =N iD þ iR =N

iD iD þ iR =N

where iD is the disk current, iR is the ring current, and N is the current collection efficiency of the Pt ring, which is consistent with the manufacturer's value (0.37). Calculated from Fig. 5(a), the HO 2 percentage produced by the FeTPPc/Gr is below 2%, even lower

Table 1 Electrochemical parameters for the ORR estimated from RDE polarization.

Fig. 6. Rotating disk electrode linear sweep voltammograms tests obtained for FePc(CP)4, FeTAPc, Pt/C and FeTPPc/Gr under rotation speed of 1600 rpm.

Electrocatalysts

JL [mA cm2]

Eonset [V vs SCE]

E1/2 [V vs SCE]

FePc(CP)4/Gr FePc(TA)4/Gr Pt/C FePc(TP)4/Gr

4.961 5.119 5.096 5.798

0.069 0.018 0.026 0.020

0.238 0.149 0.183 0.113

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Fig. 7. Chronoamperometric responses for the ORR on FeTPPc/Gr and Pt/C catalysts in O2 saturated 0.1 M NaOH solution for 12 h at 0.26 V vs SCE. Rotation speed: 1600 rpm.

than that of the Pt/C over the potential range of 0.2 to 0.6 V (shown in Fig. 5(b)). The electron transfer number of FeTPPc/Gr is 3.92 (exhibited in Fig. 5(c)). The RRDE test agrees well with the result calculated from the Kouteckyelevich equation that the ORR electron transfer number for FeTPPc/Gr is close to 4. To gain further insight into the role of pyridine substituent groups of FeTPPc during the ORR process, we compare catalytic properties on FeTPPc/Gr with those on commercial Pt/C, and other FePc/Gr composites fabricated by our group, including FePc(CP)4/Gr [31] and FeTAPc/Gr [32], with the same catalyst loading (210.1 mg cm2), using linear sweep voltammetry in an aqueous solution of oxygen saturated 0.1 M NaOH at a scan rate of 10 mVs1. As shown in Fig. 6, the FeTPPc/Gr catalyst exhibits the higher diffusion-limiting current, more positive onset potential and halfwave potential than other FePc/Gr composites, even better than Pt/C. To understand the catalytic activity directly, the electrochemical parameters are shown in Table 1. The improved catalytic activity of FeTPPc/Gr is related to the pyridine substituents, where N atoms are located in the out ring of the FeTPPc, thus providing additional sites for O2 adsorption. So the pyridine nitrogen in the FeTPPc/Gr can facilitate O2 absorption and increase catalytic active sites compared with other FePc/Gr composites. Therefore, the FeTPPc/Gr has better catalytic activity than other MPc/Gr composites. In a word, pyridine-N plays critical roles in determining the ORR activity. In addition, the durability of the FeTPPc/Gr composites is evaluated by a long-term chronoamperometric experiments, since it is also a major concern in fuel cell technology. The test is performed at a constant potential of 0.26 V vs SCE in 0.1 M NaOH solution

saturated with oxygen. As shown in Fig. 7, a high relative current of 74.7% for FeTPPc/Gr is preserved after 12 h test, whereas the relative current of Pt/C catalyst decreases to 37.5%. These results strongly suggest the FeTPPc/Gr composite has much better stability than the Pt/C catalyst in an alkaline solution. The crossover effect is a serious problem for the overall performance of fuel cells. Thus, a good electrocatalyst must be inert to small-molecule organic fuels, such as methanol, ethanol, because the fuel molecules in the anode sometimes permeate through the polymer membrane to the cathode, seriously compromising the whole fuel cell performance. FeTPPc/Gr and commercial Pt/C is further compared by separately introducing oxygen and methanol into the electrolyte to examine their possible selectivity and the crossover effect through CV measurements. When 3 M methanol is added to the oxygen saturated electrolyte, the peak potential and peak current density on FeTPPc/Gr have negligible changes shown in Fig. 8(a). In contrast, a peak on the Pt/C appears at about 0.0 V vs SCE related to methanol oxidation, whereas the cathodic peak of the ORR at around 0.20 V vs SCE has absolutely decreased. The results imply the FeTPPc/Gr composite has better selectivity than the Pt/C catalyst for ORR when methanol coexists. Previous studies on N-doped graphene suggest pyridinic-N located at the edge and defect sites of graphene layers is one of the main catalytic active sites [37]. It is believed that pyridinic N, which possesses one lone pair of electrons in addition to the electron donated to the conjugated p bond, facilitates O2 adsorption and therefore may be catalytic active sites. For our fabricated FeTPPc/Gr catalyst, there are four pyridine substituents in the peripheral ring of each phthalocyanine molecule, thus increasing the reactive sites of O2 adsorption. Therefore, compared to Pt/C catalyst, higher catalytic activity of FeTPPc/Gr for the ORR is attributed to more active sites due to the existence of pyridine substituents in FeTPPc, as well as the synergistic effect between FeTPPc and graphene. 4. Conclusions In summary, a novel iron phthalocyanine and graphene composite catalyst (FeTPPc/Gr) as a non-noble catalyst for the ORR is prepared by a solvothermally assisted p-p assembling method, which has outstanding performance for the ORR in an alkaline medium. The ORR on the integrated catalyst follows an apparent four-electron transfer pathway. Our proposed FeTPPc/Gr catalyst for the ORR displays more positive onset potential, half-wave potential, higher limiting-diffusion current than commercial Pt/C catalyst as well as better selectivity in the presence of methanol and higher stability in alkaline condition. The enhanced ORR

Fig. 8. CVs for (a) FeTPPc/Gr and Pt/C (b) at a scan rate of 100 mVs1. Respectively in O2 saturated 0.1 M NaOH solution and O2 saturated 0.1 M NaOH solution with 3 M methanol.

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performance is related to the synergistic effect between FeTPPc and graphene. More importantly, the existence of the pyridine substituents in the out ring of the FeTPPc facilitates O2 absorption and therefore provides additional catalytic active sites. The superior electrocatalytic performance of FeTPPc/Gr towards the ORR makes it an ideal candidate for the replacement of commercial Pt/C for application in alkaline direct methanol fuel cells. Acknowledgements The work has been supported by the Natural Science Foundation of China (No. 21273024) and Natural Science Foundation of Jilin Province, China (No. 201215135). References [1] B.C.H. Steele, A. Heinzel, Materials for fuel-cell technologies, Nature 414 (2001) 345e352. [2] E.M. Erickson, M.S. Thorum, R. Vasic, N.S. Marinkovic, A.I. Frenkel, A.A. Gewirth, R.G. Nuzzo, In situ electrochemical X-ray absorption spectroscopy of oxygen reduction electrocatalysis with high oxygen flux, J. Am. Chem. Soc. 134 (2012) 197e200. [3] M.R. Gao, J. Jiang, S.H. Yu, Solution-based synthesis and design of late transition metal chalcogenide materials for oxygen reduction reaction (ORR), Small 8 (2012) 13e27. [4] M. Borghei, P. Kanninen, M. Lundahl, T. Susi, J. Sainio, I. Anoshkin, A. Nasibulin, T. Kallio, K. Tammeveski, E. Kauppinen, V. Ruiz, High oxygen reduction activity of few-walled carbon nanotubes with low nitrogen content, Appl. Catal. B Environ. 158e159 (2014) 233e241. [5] S. Ni, Z. Li, J.L. Yang, Oxygen molecule dissociation on carbon nanostructures with different types of nitrogen doping, Nanoscale 4 (2012) 1184e1189. [6] H.P. Cong, P. Wang, M. Gong, S.H. Yu, Facile synthesis of mesoporous nitrogendoped graphene: an efficient methanol-tolerant cathodic catalyst for oxygen reduction reaction, Nano Energy 3 (2014) 55e63. [7] C.Y. He, Z.S. Li, M.L. Cai, M. Cai, J.Q. Wang, Z.Q. Tian, X. Zhang, P.K. Shen, A strategy for mass production of self-assembled nitrogen-doped graphene as catalytic materials, J. Mater. Chem. A 1 (2013) 1401e1406. [8] Y. Sun, C. Li, G. Shi, Nanoporous nitrogen doped carbon modified graphene as electrocatalyst for oxygen reduction reaction, J. Mater. Chem. 22 (2012) 12810e12816. [9] C.H. Choi, S.H. Park, S.I. Woo, Facile growth of N-doped CNTs on Vulcan carbon and the effects of iron content on electrochemical activity for oxygen reduction reaction, Int. J. Hydrogen Energy 37 (2012) 4563e4570. [10] Y.Z. Su, Y. Zhang, X.D. Zhuang, S. Li, D.Q. Wu, F. Zhang, X.L. Feng, Low-temperature synthesis of nitrogen/sulfur co-doped three-dimensional graphene frameworks as efficient metal-free electrocatalyst for oxygen reduction reaction, Carbon 62 (2013) 296e301. [11] H.C. Kong, X.X. Yuan, X.Y. Xia, Z.F. Ma, Effects of preparation on electrochemical properties of CoTMPP/C as catalyst for oxygen reduction reaction in acid media, Int. J. Hydrogen Energy 37 (2012) 13082e13087. [12] R. Jasinski, A new fuel cell cathode catalyst, Nature 201 (1964) 1212e1213. [13] W. Li, A. Yu, D.C. Higgins, B.G. Llanos, Z. Chen, Biologically inspired highly durable iron phthalocyanine catalysts for oxygen reduction reaction in polymer electrolyte membrane fuel cells, J. Am. Chem. Soc. 132 (2010) 17056e17058. [14] R.L. Liu, C.V. Malotki, L. Arnold, N. Koshino, H. Higashimura, M. Baumgarten, K. Mullen, Triangular trinuclear metal-N4 complexes with high electrocatalytic activity for oxygen reduction, J. Am. Chem. Soc. 133 (2011) 10372e10375. [15] K.I. Ozoemena, T. Nyokong, Novel amperometric glucose biosensor based on an ether-linked cobalt (II) phthalocyanine-cobalt (II) tetraphenylporphyrin pentamer as a redox mediator, Electrochim. Acta 51 (2006) 5131e5136. [16] B. Agboola, K.I. Ozoemena, T. Nyokong, Comparative efficiency of immobilized non-transition metal phthalocyanine photosensitizers for the visible light transformation of chlorophenols, J. Mol. Catal. A Chem. 248 (2006) 84e92. [17] K.I. Ozoemena, T. Nyokong, Surface electrochemistry of iron phthalocyanine axially ligated to 4-mercaptopyridine self-assembled monolayers at gold electrode: applications to electrocatalytic oxidation and detection of thiocyanate, J. Electroanal. Chem. 579 (2005) 283e289. [18] K.I. Ozoemena, J. Pillay, T. Nyokong, Preferential electrosorption of cobalt (II) tetra-aminophthalocyanine at single-wall carbon nanotubes immobilized on a basal plane pyrolytic graphite electrode, Electrochem. Commun. 8 (2006) 1391e1396. [19] K.I. Ozoemena, D. Nkosi, J. Pillay, Influence of solution pH on the electron transport of the self-assembled nanoarrays of single-walled carbon nanotubecobalt tetra-aminophthalocyanine on gold electrodes: Electrocatalytic detection of epinephrine, Electrochim. Acta 53 (2008) 2844e2851. [20] A.T. Chidembo, K.I. Ozoemena, B.O. Agboola, V. Gupta, G.G. Wildgoose, R.G. Compton, Nickel (II) tetra-aminophthalocyanine modified MWCNTs as potential nanocomposite materials for the development of supercapacitors,

17

Energy Environ. Sci. 3 (2010) 228e236. [21] B.O. Agboola, K.I. Ozoemena, Synergistic enhancement of supercapacitance upon integration of nickel (II) octa [(3,5-biscarboxylate)-phenoxy] phthalocyanine with SWCNT-phenylamine, J. Power Sources 195 (2010) 3841e3848. [22] M. Hanack, M. Lang, Conducting stacked metallophthalocyanines and related compounds, Adv. Mater. 6 (1994) 819e833. [23] G. De La Torre, P. Vazquez, F. Agullo-Lopez, T. Torres, Phthalocyanines and related compounds: organic targets for nonlinear optical applications, J. Mater. Chem. 8 (1998) 1671e1683. [24] G. De La Torre, P. Vazquez, F. Agullo-Lopez, T. Torres, Role of structural factors in the nonlinear optical properties of phthalocyanines and related compounds, Chem. Rev. 104 (2004) 3723e3750. [25] G. De La Torre, C.G. Claessens, T. Torres, Phthalocyanines: The need for selective synthetic approaches, Eur. J. Org. Chem. 2000 (2000) 2821e2830. [26] T. Nyokong, Effects of substituents on the photochemical and photophysical properties of main group metal phthalocyanines, Coord. Chem. Rev. 251 (2007) 1707e1722. [27] K. Wiesener, D. Ohms, V. Neumann, N4 macrocycles as electrocatalysts for the cathodic reduction of oxygen, Mater. Chem. Phys. 22 (1989) 457e475. [28] R.R. Chen, H.X. Li, D.Y. Chu, G.F. Wang, Unraveling oxygen reduction reaction mechanisms on carbon-supported Fe-phthalocyanine and Co-phthalocyanine catalysts in alkaline solutions, J. Phys. Chem. C 113 (2009) 20689e20697. [29] G.F. Dong, M. H Huang, L.H. Guan, Iron phthalocyanine coated on singlewalled carbon nanotubes composite for the oxygen reduction reaction in alkaline media, Phys. Chem. Chem. Phys. 14 (2012) 2557e2559. [30] Y. Jiang, Y. Lu, X. Lv, D. Han, Q. Zhang, L. Niu, W. Chen, Enhanced catalytic performance of Pt-Free iron phthalocyanine by graphene support for efficient oxygen reduction reaction, ACS Catal. 3 (2013) 1263e1271. [31] L.L. Cui, G.J. Lv, Z.Y. Dou, X.Q. He, Fabrication of iron phthalocyanine/graphene micro/nanocomposite by solvothermally assisted pep assembling method and its application for oxygen reduction reaction, Electrochim. Acta 106 (2013) 272e278. [32] L.L. Cui, Y. Liu, X.Q. He, Iron (II) tetraaminophthalocyanine functionalized graphene: synthesis, characterization and their application in direct methanol fuel cell, J. Electroanal. Chem. 727 (2014) 91e98. [33] M. Vikkisk, I. Kruusenberg, U. Joost, E. Shulga, I. Kink, K. Tammeveski, Electrocatalytic oxygen reduction on nitrogen-doped graphene in alkaline media original, Appl. Catal. B: Environ. 147 (2014) 369e376. [34] Z. Yang, Zhen Yao, G.F. Li, G.Y. Fang, H.G. Nie, Z. Liu, X.M. Zhou, X.A. Chen, S.M. Huang, Sulfur-doped graphene as an efficient metal-free cathode catalyst for oxygen reduction, ACS Nano 6 (2012) 205e211. [35] Z.H. Sheng, H.L. Gao, W.J. Bao, F.B. Wang, X.H. Xia, Synthesis of boron doped graphene for oxygen reduction reaction in fuel cells, J. Mater. Chem. 22 (2012) 390e395. [36] R. Li, Z.D. Wei, X.L. Gou, W. Xu, Phosphorus-doped graphene nanosheets as efficient metal-free oxygen reduction electrocatalysts, RSC Adv. 3 (2013) 9978e9984. [37] L.T. Qu, Y. Liu, J.B. Baek, L.M. Dai, Nitrogen-doped graphene as efficient metalfree electrocatalyst for oxygen reduction in fuel cells, ACS Nano 4 (2010) 1321e1326. [38] D.S. Geng, Y. Chen, Y.G. Chen, Y.L. Li, R.Y. Li, X.L. Sun, S.Y. Ye, S. Knights, High oxygen-reduction activity and durability of nitrogen-doped graphene, Energy Environ. Sci. 4 (2011) 760e764. [39] Z.Y. Lin, G.H. Waller, Y. Liu, M.L. Liu, C.P. Wong, Simple preparation of nanoporous few-layer nitrogen-doped graphene for use as an efficient electrocatalyst for oxygen reduction and oxygen evolution reactions, Carbon 53 (2013) 130e136. [40] T. Xing, Y. Zheng, L.H. Li, Bruce C.C. Cowie, D. Gunzelmann, S.Z. Qiao, S.M. Huang, Y. Chen, Observation of active sites for oxygen reduction reaction on nitrogen-doped multilayer graphene, ACS Nano 8 (2014) 6856e6862. [41] B.S. Mariana, N.D. Edgardo, Synthesis and antibacterial photosensitizing properties of a novel tricationic subphthalocyanine derivative, Dyes Pigment. 77 (2008) 229e237. [42] S. Arslan, I. Yilmaz, A new water-soluble metal-free phthalocyanine substituted with naphthoxy-4-sulfonic acid sodium salt. Synthesis, aggregation, electrochemistry and in situ spectroelectrochemistry, Polyhedron 26 (2007) 2387e2394. [43] X.G. Fu, J.T. Jin, Y.R. Liu, Q. Liu, K.X. Niu, J.Y. Zhang, X.P. Cao, Graphene-xerogelbased non-precious metal catalyst for oxygen reduction reaction, Electrochem. Commun. 28 (2013) 5e8. [44] X.F. Dai, M.F. Zhen, P. Xu, J.J. Shi, C.Y. Ma, J.L. Qiao, Electrochemical behavior of pyridine-doped carbon-supported co-phthalocyanine (Py-CoPc/C) for oxygen reduction reaction and its application to fuel cell, Acta. Phys. Chim. Sin. 29 (2013) 1753e1761. [45] L. Lin, M. Li, L.Q. Jiang, Y.F. Li, D.J. Liu, X.Q. He, L.L. Cui, A novel iron (II) polyphthalocyanine catalyst assembled on graphene with significantly enhanced performance for oxygen reduction reaction in alkaline medium, J. Power Sources 268 (2014) 269e278. [46] F.H.B. Lima, M.L. Calegaro, E.A. Ticianelli, Investigations of the catalytic properties of manganese oxides for the oxygen reduction reaction in alkaline media, J. Electroanal. Chem. 590 (2006) 152e160. [47] N.M. Markovíc, T.J. Schmidt, V. Stamenkovíc, P.N. Ross, Oxygen reduction reaction on Pt and Pt bimetallic surfaces: a selective review, Fuel Cells 1 (2001) 105e116.

18

L. Cui et al. / Journal of Power Sources 282 (2015) 9e18

[48] S.K. Bikkarolla, F.J. Yu, W.Z. Zhou, P. Joseph, P. Cumpsond, P. Papakonstantinou, A three-dimensional Mn3O4 network supported on a nitrogenated graphene electrocatalyst for efficient oxygen reduction reaction in alkaline media, J. Mater. Chem. A 2 (2014) 14493e14501. [49] G. Ma, R. Jia, J. Zhao, Z. Wang, C. Song, S. Jia, Z. Zhu, Nitrogen-doped hollow

carbon nanoparticles with excellent oxygen reduction performances and their electrocatalytic kinetics, J. Phys. Chem. C 115 (2011) 25148e25154. [50] J. Liu, X.J. Sun, P. Song, Y.W. Zhang, W. Xing, W.L. Xu, High-performance oxygen reduction electrocatalysts based on cheap carbon black, nitrogen, and trace iron, Adv. Mater. 25 (2013) 6879e6883.