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Superior catalytic activity of electrochemically reduced graphene oxide supported iron phthalocyanines toward oxygen reduction reaction Dong Liu, and Yi-Tao Long ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07068 • Publication Date (Web): 19 Oct 2015 Downloaded from http://pubs.acs.org on October 19, 2015

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ACS Applied Materials & Interfaces

Superior Catalytic Activity of Electrochemically Reduced

Graphene

Oxide

Supported

Iron

Phthalocyanines toward Oxygen Reduction Reaction Dong Liu, Yi-Tao Long* Key Laboratory for Advanced Materials & Department of Chemistry, East China University of Science and Technology, Shanghai, 200237, China KEYWORDS electrochemical reduction, graphene, iron phthalocyanine, self-supported structure, oxygen reduction

ABSTRACT

Structure and surface properties of supporting materials are of great importance for the catalytic performance of the catalysts. Herein, we prepared the iron phthalocyanine (FePc) functionalized electrochemically reduced graphene oxide (ERGO) by the electrochemical reduction of FePc/GO. The resultant FePc/ERGO exhibits higher catalytic activity toward ORR than that of FePc/graphene. More importantly, the onset potential for ORR at FePc/ERGO positively shifts by 45mV compared with commercial Pt/C in alkaline media. Besides, FePc/ERGO displays enhanced durability and selectivity toward ORR. The superior catalytic performance of

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FePc/ERGO for ORR are ascribed to the self-supported structure of ERGO, uniformly morphology and size of FePc nanoparticles.

1. INTRODUCTION The oxygen reduction reaction (ORR) is one of the major issues in the investigation of fuel cells that the sluggish kinetics of ORR at the cathode have seriously hampered the development of fuel cells.1 Platinum (Pt)-based catalysts are commonly utilized for the excellent catalytic activity toward ORR; however, the large-scale production is impeded by the high cost and limited reserves of Pt.2 As a result, the non-precious metal catalysts with highly efficient as well as durable low-cost are extremely desired. Currently, many efforts are devoted to the heteroatom (N, S, etc.) doped carbon nanomaterials due to the excellent catalytic performance for oxygen reduction.3-5 Generally, most of these catalysts are prepared via the pyrolysis of heteroatomcontaining precursors.6 Nevertheless, relatively complicated experiments are required to optimize the experimental parameters to obtain the high-performance catalysts.7 Recently, designing of ORR catalysts based on certain molecules has attracting extensively attentions for high oxygen reduction activity as well as pyrolysis-free synthesis methods.8 Of these molecules, iron phthalocyanine (FePc) has exhibited excellent catalytic activity toward ORR.9-12 However, the catalytic properties of FePc is seriously depressed due to the poor conductivity and aggregation.13 To mitigate this, FePc molecule has been immobilized on the surface of carbon nanotubes and graphene by π−π interaction and covalent functionalization.14-17 Meanwhile, the excellent conductivity of carbon materials could facilitate the electron transfer resulting in the enhanced catalytic activity of these catalyst.18 However, most investigations have focused on the preparation of powdery catalysts, and the insulating binder with an unwanted


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inner resistance is required in practical applications which may depress the catalytic ability.6,16 Then, retaining the intrinsic activity of high-performance catalysts in the applications seems to be another important issue to be solved. Herein, we synthesized FePc functionalized electrochemically reduced graphene oxide (FePc/ERGO) as high-performance electrocatalyst for ORR. For comparison, another two kinds of FePc functionalized reduced graphene oxide were also prepared. For FePc/ERGO, the immobilized FePc particles display a highly uniform morphology and size. More importantly, the resultant FePc/ERGO shows high catalytic activity toward ORR in alkaline media; meanwhile, it exhibits a unique self-supported structure free of any insulating binder in applications.

2. EXPERIMENTAL SECTION 2.1 Materials Graphene oxide was prepared by a modified Hummer’s method. Graphite (99.8%) is purchased from Alfa Aesar. Iron (II) phthalocyanine (FePc, 90%) is purchased from Aldrich. N, N-Dimethylformamide (DMF, ≥99.8%) is purchased from Sinopharm Chemical Reagent Co., Ltd (China, Shanghai). Potassium hydroxide (KOH, ≥85%) is obtained from Shanghai Ling Feng Chemical Reagent Co., Ltd (China, Shanghai). 2.2 Preparation of catalysts For the preparation of FePc functionalized graphene oxide (FePc/GO), 20mg FePc and 20mg graphene oxide was dispersed in DMF, then the mixture was centrifugated after stirring for 6hours. FePc functionalized reduced graphene oxide (FePc/ERGO) was prepared by reducing FePc/GO by cyclic voltammetry (CV) in the range of 0.2~-1.3V for 20cycles. The FePc functionalized reduced graphene oxide (FePc/rGO(1)) was prepared by heating 0.4mg/mL graphene oxide-DMF colloidal solution containing 0.4mg/mL FePc at 155 ℃ for 45min. FePc functionalized reduced graphene oxide (FePc/rGO(2)) was


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prepared by a two-step method. Briefly, the reduced graphene oxide (rGO) was prepared by heating 0.4mg/mL graphene oxide-DMF colloidal solution at 155 ℃ for 45min, and then 0.4mg/mL FePc was mixed with the obtained rGO to stir for 6 hours before centrifugation. 2.3 Electrochemical measurements FePc/GO modified electrode was prepared by casting 4µL 0.4mg/mL FePc/GO-DMF colloid solution on 3mm glassy carbon electrode (GCE). Then, the FePc/GO was reduced via CVs between 0.2V and -1.3V for 20cycles in the 0.1M nitrogensaturated PBS solution to prepared FePc/ERGO modified electrode. For the preparation of FePc/rGO(1) and FePc/rGO(2) modified electrode, 4µL 0.4mg/mL colloid solution was casted on the GCE, then 2µL 0.5wt.% Nafion solution was dropped on the electrode. All the electrochemical experiments were performed on a CHI 660C electrochemical workstation using a three-electrode system comprised by a saturated calomel electrode (SCE) as reference electrode, a Pt wire counter electrode and a modified glassy carbon electrode as working electrode. The electrochemical impedance spectroscopy (EIS) was obtained on a ZAHNER IM6ex electrochemical workstation. Rotating disk electrode (RDE) and rotating ring−disk electrode (RRDE) measurements were conducted employing a GCE disk electrode (Pine Instruments) and a GCE disk/platinum ring electrode (Pine Instruments) as working electrode. 2.4 Characterization The morphology and composition of the samples were characterized by transmission electron microscopy (TEM, JEOL JEM-2100), X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250) and Raman spectroscopy (Renishaw inVia Reflex).

3. RESULTS AND DISCUSSION


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Figure 1. TEM images of (A) FePc/GO, (B) FePc/ERGO, (C) FePc/rGO(1) and (D) FePc/rGO(2). The inset of Figure 1B is the optical photograph of self-supported FePc/ERGO film on the indium-tin-oxide (ITO) glass. The immobilization of FePc on the graphene is based on the π−π stacking interaction between FePc and graphene without damaging the intrinsic properties of FePc. The resultant samples could be uniformly dispersed in DMF which exhibits a black, brown and dark green color for FePc/rGO(2), FePc/rGO(1) and FePc/GO (Figure S1A). The morphology of these samples are characterized by TEM as shown in Figure 1. As for FePc/GO, FePc aggregates with an average size below 1nm are loaded on the surface of GO; however, after the electrochemical reduction of GO, uniformly-dispersed FePc NPs with an average size of 3.5nm are observed on ERGO for FePc/ERGO (Figure 1A and 1B). Few particles could be distinguished on the surface of graphene which may result from the relatively small size of FePc NPs for FePc/rGO(1) (Figure 1C). As for FePc/rGO(2), FePc NPs with a mean size of 5.4nm are immobilized on the surface of


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graphene sheets as shown in Figure 1D. Noteworthy, FePc NPs on ERGO exhibit more uniform morphology and size compared with that for FePc/rGO(2). Thus, FePc NPs could be uniformly immobilized on ERGO without aggregation. Besides, FePc/ERGO could be simply coated on the conductive substrates without using any insulting binder which is of great importance to retain the catalytic activity of FePc as well as facilitate the applications (inset of Figure 1B).19

Figure 2. (A) XPS survey spectra and (B) Raman spectra of FePc/GO, FePc/ERGO, FePc/rGO(1) and FePc/rGO(2). The XPS spectra are performed to investigate the composition of the samples. Four elements including Fe, N, C, O are found in these catalysts as shown in Figure 2A. Of these elements, Fe is derived from the π-conjugated FePc molecules, while N element may be from the FePc molecules.20 The peaks at 286.6eV accounting for the C-OH group in C 1s XPS spectra significantly decreases after the reduction of FePc/GO, implying the reduction of GO (Figure S2A).21,22 According to XPS experiments, the content of N for FePc/GO, FePc/ERGO, FePc/rGO(1) and FePc/rGO(2) is 4.3, 4.0, 5.3 and 8.3 at.%, respectively. The peaks at 398.6 and 400.3eV observed in N 1s XPS spectra of samples are assigned to the pyridinic-N and pyrrolic-N (Figure S2B).23


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Raman spectra are employed to estimate the structural characteristics of obtained catalysts as shown in Figure 2B. Generally, the Raman spectrum of carbon materials displays two band peak, a D band peak located at 1350 cm−1 assigned to the vibrations of sp3-bonded carbon atoms with a disordered structure, and a G band peak located at 1600 cm−1 indicating the vibration of sp2 carbon atoms.24 The ratio of the D and G bands (ID /IG) could be used to estimate the number of disordered structures in carbons.25 FePc/ERGO exhibits a smaller ID/IG ratio of 0.919 than that of FePc/GO (0.926) which may result from the decrease of the oxygen-containing group after the reduction of FePc/GO.26 On the other hand, compared with FePc/rGO(1) or FePc/rGO(2), the relatively larger ID/IG ratio for FePc/ERGO confirms the presence of abundant edge-plane-like sites/defects.

Figure 3. CVs of (A) FePc/GO, (B) FePc/ERGO, and (C) FePc/rGO(1) and (D) FePc/rGO(2) modified glassy carbon electrodes in 0.1M KOH solution. Scan rate: 50 mV s-1.


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The electrochemically accessible surface area (EASA) and conductivity of samples are measured by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively. The EASA of FePc/rGO(2) calculated by the Randles-Sevcik equation is 0.11cm2, larger than that of FePc/GO (0.06cm2), FePc/ERGO (0.09cm2), and FePc/rGO(1) (0.08cm2), which could be ascribed to the higher reduction degree of graphene in FePc/rGO(2) (Figure S3).27 Due to the high affinity with the electrode surface and the absence of insulating binder, FePc/ERGO displays the smallest charge transfer resistance among these samples according to the EIS measurements (Figure S4).28 We applied the resultant samples to catalyze the oxygen reduction in alkaline media. In contrast to the poor catalytic activity of rGO, GO and ERGO, FePc-loaded products show significantly enhanced catalytic activity for ORR (Figure S5 and Figure 3). The peak potential of ORR at FePc/GO, FePc/ERGO, FePc/rGO(1) and FePc/rGO(2) is -0.32, -0.15, -0.23 and -0.18V, respectively. Obviously, the FePc/ERGO shows a most positive ORR peak potential, suggesting fast electron transfer kinetics and enhanced catalytic activity. On the other hand, the FePc/rGO(2) provides a larger charging current density compared with that at FePc/GO, FePc/ERGO or FePc/rGO(1) due to the large EASA; however, the response current density for oxygen reduction at FePc/rGO(2) is relatively small (Figure 3D). Compared with FePc/rGO(2), FePc/ERGO with a smaller charging current density and higher catalytic activity could be more desirable for applications, such as biosensors.


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Figure 4. Linear sweep voltammetric curves of FePc/ERGO, FePc/rGO(1), FePc/rGO(2), and commercial Pt/C modified glassy carbon electrodes in 0.1M O2-saturated KOH solution. Scan rate: 10 mV s-1. We further evaluated the catalytic activity of these samples and commercial 20% Pt/C catalysts toward ORR by the rotating disk electrode (RDE) measurements. As shown in Figure 4, the RDE polarization curves confirm the superior catalytic activity of FePc/ERGO with the onset potential of -0.015V, while the onset potential of ORR for FePc/rGO(1) and FePc/rGO(2) is -0.08, -0.04V. Furthermore, the onset potential for FePc/ERGO is positively shifted by 45mV in comparison with that for commercial Pt/C. In terms of catalytic activity for ORR, the as-obtained FePc/ERGO could be a potential alternative to the commercial Pt/C. It is of great importance to investigate the reaction mechanism to give a comprehensive evaluation of the catalysts. Generally, the ORR in basic solution has two possible pathways, including a 2e- reduction pathway with H2O2 as an intermediate product and a 4e- pathway to directly produce H2O which is more desirable for high power output.3 In this work, rotating ring-


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disk electrode (RRDE) measurements were performed to calculate the number of electrons transferred (n) per oxygen molecule as well as the percentage of peroxide species (%H2O2) produced during the oxygen reduction process (Figure S6). The n and %H2O2 could be obtained according to the following equation (1) and (2).29 n = 4ID / (ID + (IR / N))

(1)

%H2O2 = 100 (4-n) / 2

(2)

ID and IR is the faradic-disk and -ring current density, and the N is the collection efficiency given a constant value of 0.37. The calculated results are showed in Figure 5. The oxygen reduction reaction catalyzed at FePc/ERGO follows predominantly a 4e‒ pathway that the electron transfer numbers is 3.94, 3.93, 3.92, 3.92 at -0.3, -0.4, -0.5, -0.6V, respectively. Meanwhile, the percentage of peroxide species (%H2O2) produced at FePc/ERGO is significantly decreased compared with that at FePc/rGO(1), FePc/rGO(2), and Pt/C (Figure 5B).

Figure 5. The calculated (A) electron transfer numbers (n) and (B) produced percentage of peroxide species (%H2O2) at FePc/ERGO, FePc/rGO(1), FePc/rGO(2) and Pt/C during the oxygen reduction based on the RRDE measurements.


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The above results demonstrate the superior catalytic activity of FePc/ERGO; on the other hand, the stability and anti-poisoning ability of catalysts are also the important issues for applications. The long-time stability of FePc/ERGO is accessed at a potential of -0.26V in the O2-saturated 0.1M KOH solution with a rotation speed of 1600rpm. As shown in Figure 6, a relatively slower attenuation of current density could be observed at FePc/ERGO compared with commercial Pt/C. The ORR current density at FePc/ERGO remains 88% after a continuous measurement of 8000s; however, only 62% of the current density is retained at commercial Pt/C. Thus, FePc/ERGO displays a significantly enhanced durability than commercial Pt/C, which is of great importance for catalysts in applications.

Figure 6. Chronoamperometric response at FePc/ERGO and commercial 20% Pt/C in O2saturated 0.1M KOH solution with a potential of -0.26V. Rotation rate: 1600rpm. To estimate the possible crossover at the catalysts for ORR, the catalytic behaviors of FePc/ERGO and commercial Pt/C were studied in a 0.1M KOH solution containing 1M methanol.30 As shown in Figure 7, the CV curves for FePc/ERGO remains unchanged after the addition of 1M methanol; however, obvious catalytic behavior toward the methanol oxidation could be observed at Pt/C in the presence of methanol with the vanish of charactertic peak for


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oxygen reduction. Then, FePc/ERGO exhibits better anti-poisoning ability than commercial Pt/C catalysts.

Figure 7. CVs of (A) FePc/ERGO and (B) commercial Pt/C in O2-saturated 0.1M KOH solution upon the addition of 1M CH3OH. Scan rate: 50mV s-1. Based on the above measurements, the FePc/ERGO exhibits the superior catalytic performance toward ORR, which are ascribed to the uniformly anchored FePc particles, high conductivity and self-supported structure of ERGO.

4. CONCLUSION We have prepared several Pt-free electrocatalysts for oxygen reduction reaction based on the FePc functionalized graphene materials. Of these catalysts, FePc/ERGO with a self-supported structure could be simply coated on the conductive substrates free of any insulting binder. Meanwhile, the FePc particles could more uniformly anchored on the ERGO with small particle size. As a low-cost Pt-free electrocatalyst, it shows superior catalytic activity, enhanced longtime stability and anti-poisoning ability compared with commercial Pt/C catalyst which could be a candidate material for Pt-based catalysts in fuel cells. Our work emphasizes the importance to


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tailor the interface properties of supporting materials for catalysts which may promote investigation for the alternative catalysts for Pt. ASSOCIATED CONTENT Supporting Information. The photograph of FePc/GO, FePc/rGO(1) and FePc/rGO(2)-DMF colloid solution; nyquist diagrams of FePc/GO, FePc/ERGO FePc/rGO(1) and FePc/rGO(2) FePc/ERGO modified glassy carbon electrodes; RRDE polarization curves for oxygen reduction at FePc/ERGO, FePc/rGO(1), (C) FePc/rGO(2) and Pt/C. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Corresponding Author: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21421004) and China Postdoctoral Science Foundation (No. 2014M561421). REFERENCES [1] Dai, L. M.; Xue, Y. H.; Qu, L. T.; Choi, H. J. ; Baek, J. B. Metal-Free Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823–4892.


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