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A Superior Oxygen Reduction Reaction Electrocatalyst Based on Reduced Graphene Oxide and Iron (II) Phthalocyanine Supported Sub-2nm Platinum Nanoparticles Shuo Wang, Fan Li, Yan Wang, Dan Qiao, Chunwen Sun, and Jingbo Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00173 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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ACS Applied Nano Materials

A Superior Oxygen Reduction Reaction Electrocatalyst Based on Reduced Graphene Oxide and Iron (II) Phthalocyanine Supported Sub2nm Platinum Nanoparticles Shuo Wang, a Fan Li, a* Yan Wang, a Dan Qiao, a Chunwen Sunb,c* and Jingbo Liud,e* a.

Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China. b. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China. c. CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing, 100190, China. d. Department of Chemistry, Texas A&M University-Kingsville, Kingsville, TX 78363, [email protected] e. Department of Chemistry, Texas A&M University, College Station, TX, 77843 USA. *

Corresponding authors. Emails: [email protected] (F. Li), [email protected] (C. Sun) and [email protected] (J. Liu)

KEYWORDS: Suspension Unit for Reactive Evolution, Supported sub-2 nm platinum, Oxygen reduction reaction, Anti-methanol poisoning property, Long-term stability.

ABSTRACT: A robust triple-component electrocatalyst of sub-2 nm scale consisting of platinum nanoparticles (Pt NPs), Iron (II) phthalocyanine (FePc) and reduced graphene oxide (rGO) composite (Pt/FePc@rGO) was fabricated for oxygen reduction reaction (ORR) evaluation. The catalyst was prepared using a “Suspension Unit for Reactive Evolution (SURE)” approach. The merit of the SURE approach lies in the ease of formation of smaller Pt NPs with a size of 1.76 ± 0.26 nm and uniformity of dispersed on rGO surface by hydrogen reduction. The Pt/FePc@rGO catalysts show superior mass activities of 281.29 mA mgPt-1 at 0.86 V in an alkaline potassium hydroxide (KOH) electrolyte system. Compared with the commercial Pt on carbon (Pt/C) catalysts, the ORR current density of the Pt/FePc@rGO catalysts shows 2.8 times greater under the same operating conditions, while the average Pt loading is only about one-third of the commercial catalyst (Pt/C). The result of accelerated durability test (ADT) indicates the Pt/FePc@rGO catalysts show about 8 mV

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decrease in half-wave potential after 10,000 cycles 0-1.23V (vs. reversible hydrogen electrode, RHE) potential cycling; while the Pt/C catalyst was subjected much larger shift (~53 mV). Methanol tolerance study also indicates the Pt/FePc@rGO shows a 2 mV shift corresponding to a current density of 2.5 mA cm-2, while the commercial Pt/C shows 238 mV shift when an aliquot of 0.1 M methanol and 0.1 M KOH aqueous electrolyte was added. All these results demonstrate that the obtained catalyst exhibited enhanced electrocatalytic activity, excellent anti-methanol poisoning property, and long-term stability.


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1. Introduction Direct methanol fuel cells (DMFCs) have attracted significant attention for portable power applications due to their distinctive advantages of high energy density and energy conversion efficiency. Other advantages include safe storage and transportation of methanol as a liquid fuel, low pollutant emissions, and simple operation.1-3 Catalysts, especially used in oxygen cathode have caught a lot researcher’s eyesight. It is highly desired to design catalysts with high oxygen reduction reaction (ORR) activity, long-term stability, anti-poisoning of the crossover methanol from anode region, and cost-effective and ease of control during preparation. Platinum has been used as a catalyst for oxygen reduction reaction (ORR) at the cathode of DMFCs due to its high specific activity.4-6 In recent years, a series of new catalysts have been developed to enhance the performance of ORR, such as Pt atoms,7 designed Pt nanostructures,8-12 multicomponent Pt alloys,13,14 non-precious metal catalysts,15, metal-free catalysts19 and carbonaceous-supported Pt NPs.20,

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organometallic catalysts,17,18

To date, commercial carbon-

supported Pt NPs (Pt/C ) are still the most widely used catalyst for ORR in DMFCs, however, its high cost, poor durability, and variable tolerance to methanol over prolonged operation have to be circumvented before wide spread commercialization of DMFCs can be realized.22 The catalysts composed of graphene supported Pt (reduced amount) NPs were proved to offer considerable improvements due to their large surface area, high electrical conductivity,23 and unique electron transfer and charge-carrier mobility.24 A newly discovered member of the carbon family, graphene is an attractive candidate for fuel cell application. Graphene is densely packed in a honeycomb crystal lattice with a planar geometry. It is a building block for carbon materials of all other dimensionalities, e.g. 0 dimensional (D) buckyball, 1 D nanotubes, 2 D sheets, and 3 D graphite. Importantly, the graphene sheets with sp2 hybridization and 2 D planar geometry will further facilitate electron transfer, due to lowering the activation energy related to the ORR.25 Compared with carbon nanotubes and other carbon supports, the graphene displays high electrical conductivity (106 S cm-1), thermal conductivity (5,000 W m-1 k-1), fracture strength (124 GPa), specific surface area (2,630 m2 g-1) and charge mobility (105 cm2 V-1 s-1), and low density (1 g cm-3), which ensure graphene as a more effective catalyst support to improve electrochemical reactivity of DMFCs.26, 27 However, the susceptibility to oxidative environments, the limited capacitance, and toxic qualities hindered applications of the pure graphene sheets despite the above physical property advantages.28, 29 It was found that graphene nanocomposites 3 ACS Paragon Plus Environment


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are more practical for diversified applications, such as solar energy and fuel cells, due to lower material usage, limiting potential toxicity and oxidation.30,

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Recent studies indicated that

graphene can provide a platform to produced nanocomposites as catalysts by controllable functionalization.32-37 Different approaches have been investigated to functionalize graphene and to create building blocks with unique structures. These methods include chemical covalent and noncovalent modification, galvanic replacement, electroless deposition, hydrothermal and solvothermal growth to tune the functionality of graphene sheets in reduced form, which can be utilized based on the end-user application.38-41 N4-chelate complex with transition metals was first reported as catalysts to be used in oxygen electrochemical reduction in 196442. Many works focus on the M-N4 towards their ORR catalytic property43,44. The ferrous Iron (II) phthalocyanine (FePc) was believed with the best performance among these M-N4 ORR catalyst

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. Although effort has been made for FePc in the usage of

ORR catalyst, they cannot reach the level of Pt-based catalyst both in reactivity and durability 46. The metal phthalocyanine (MPc) was chosen due to its property of tolerance to methanol and availability of an electron sink due to bond in the conjugated ring system. The aggregation of FePc molecular and the poor electron conductivity hinder its application in ORR

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. Different

carbon materials, for example, Vulcan XC-7248 and carbon nanotubes45,49 were used as a support for MPc. Reduced graphene oxide (rGO) was selected as support and MPc as a mediator agent to construction graphene building blocks.50,51 The conjugation between delocalized π-electrons within MPc and p-electron within the carbon atom from rGO will be one of the important factors to determine the redox properties of these composite. The characteristic of MPc, such as aromatic macrocyclic with nitrogenous functional groups allows for its conjugation with graphite surface structure of rGO.48 It was found that phthalocyanines

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act as planar tetradentate di-

anionic ligands to coordinatively bind with metal centers through “four inwardly projecting nitrogen centers”, leading to the common term “transition metal N4 macrocycles”. 53 The ferrous Iron (II) phthalocyanine (FePc) is a widely studied porphyrin-like molecule, which displays a strong tendency to axial ligation.54,55 The ferrous phthalocyanine (FePc) among the MPc series was implemented due to its flexibility and ability of electron transfer from Fe (II) d-orbital to the ligands. It is hypothesized that the phthalocyanine as an aromatic organic molecule can strongly interact with graphene via π-π stacking.56 rGO or N doped rGO supported FePc are normally used as the ORR catalysts in an alkaline system.57,58,59 Graphene oxide (GO) supported FePc 4 ACS Paragon Plus Environment

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could be applied in the aqueous system close to neutral

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and even the binuclear iron (III)

phthalocyanine graphene nanosheets could be used in acidic solution.61 In this work, we aim to develop a simple and feasible synthetic route to produce cost-effective and robust electrocatalysts with tunable structure and excellent stability. A “Suspension Unit for Reactive Evolution (SURE)” method was developed to prepare Pt/FePc@rGO catalysts, which showed uniform nanostructure and superior electrochemical performance. Even some researchers have investigated the FePc/rGO electrocatalysis properties,56 but the low current density can still not reach the level of Pt base catalysts. Although some attempts to prepare Pt NPs and GO sheet composite, most of them only focus on the electrocatalysis properties in acidic solutions,62 a few of them show a light on the basic solution systems,63 let alone studying the effects in methanol and alkaline system. The catalyst prepared by SURE method shows good properties in alkaline system and methanol tolerant property as well. Due to the high electrical conductivity of rGO, the d electron of Fe2+ with the Pc unit electrons (ē) transfer to rGO is thermodynamically favourable when FePc is combined with rGO. Therefore, the FePc will be ‘electron deficient’ to attract ē from Pt in PtCl42- (using the K2PtCl4 salt) and then the PtCl42- will be reduced to Pt by H2 to achieve a uniform loading of a metal catalyst. Pt/FePc@rGO catalyst shows a high ORR activity, long time stability, and anti-poisoning property towards the crossover of methanol in an alkaline system.

2. EXPERIMENTAL METHODS Experiments were designed as Scheme 1. Typically, reduced graphene oxide dispersed in a suspension system, reacted with the dissolved potassium tetra chloroplatinate and iron phthalocyanine under the hydrogen gas flow to produce the Pt/FePc@rGO composite catalyst (loading 6.6wt% Pt). The physical characterization and electrochemical test were carried on the as-synthesized catalyst.

Scheme 1 Synthesis and Characterization of the Pt/FePc@rGO composite catalyst 5 ACS Paragon Plus Environment


2.1

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Chemicals

All chemicals were purchased from Companies within China unless otherwise noted. Reduced graphene oxide (rGO, dry black powder) was purchased from Graphene-Supermarket, Calverton, USA. Potassium tetra chloroplatinate (II) (K2PtCl4, MW=415.09) was purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai. Iron (II) phthalocyanine (FePc, C32H16FeN8, MW=568.38) and Nafion solution (C9HF17O5S, 5 wt %), platinum, nominally 20 mass % on carbon Black (Pt/C) were purchased from Alfa Aesar, Tokyo, Japan. Potassium hydroxide (KOH) and ethanol (C2H6O) were purchased from Beijing Chemical Works, Beijing. N, Ndimethylformamide (DMF, C3H7NO) and n-butanol (C4H10O) were purchased from Tianjin Fuchen Chemical Reagents Factory, Tianjin. All the chemicals are used directly without any further purify. Milli-Q grade water was used in all the experiments and was produced within the laboratory water purification system. 2.2

Synthesis of Pt/FePc@rGO Composites

10 mg of reduced graphene oxide (rGO) was added to 10 mL of n-butyl alcohol solution to prepare 1 mg/mL suspension (defined as suspension A). The iron (II) phthalocyanine (FePc, 5 mg) was added to 10 mL of N, N-dimethylformamide (DMF) to prepare 0.5 mg mL-1 (solution B). n-butanol and DMF exhibit a higher solubility and dispersity of the precursors among the selected several media and solvents (supporting information, Figure S1, and Table S1). The potassium tetra chloroplatinate (II) (K2PtCl4, 8 mg) was added to Milli-Q water (5 mL) to prepare a 1.6 mg mL-1 (solution C). The suspension A, solution B, and C were mixed by magnetically stirring at room temperature. Prior to mixing, each was individually sonicated for 30 min to dissolve and disperse the individual components. After aging the above obtained mixed suspension for 6 days to build an equilibrium of stacking and adsorptions, the mixture was then reduced by H2 (99.99 ％, 200 mL min-1) for 5 h and continuously magnetically stirring at ambient temperature until it was used to prepare the catalyst. The resulting dark black suspension was purified by centrifugation and washed with Milli-Q Water, ethanol and then dried under the ambient condition to obtain a black powder, labeled as Pt/FePc@rGO composite. The schematic illustration of the SURE procedures is shown in Figure S2. 2.3

Physical Characterization


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The powdered X-ray diffraction (PXRD) patterns were recorded with an X-ray diffractometer (Bruker D8 Advance, Germany) with Cu Kα1 radiation (40 kV, 40 mA) at a step of 0.02 ° and a scan speed of 6 °/min. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) images were obtained using an FEI Tecnai G2 F20 TEM (United States) at an of 200 kV. The samples for TEM measurements were suspended in ethanol and deposited onto carbon-coated Cu grids. The surface properties and electronic structure of the catalysts were evaluated by X-ray photoelectron spectrometer (XPS, Kratos Analytical Ltd Axis Ultra, United Kingdom) with Al Kα radiation. The binding energy was calibrated with the C 1s position of the carbon support at 284.80 eV. The composition of the catalysts was determined by an inductively coupled plasma optical emission spectrometry (ICP-OES, Perkin-Elmer optimal 8300, United States of America). 2.4

Electrochemical Test

The electrochemical tests, cyclic voltammetry (CV) and linear sweep voltammograms (LSVs) were measured using an electrochemical analyzer system (Bio-logic VMP3, France) under ambient conditions. All the curves were directly obtained from the measurement without any potential compensation of the Ohmic resistance loss. A three-electrode system was used in this study. A glassy carbon electrode (5 mm in diameter), a graphite rod and a mercuric oxide electrode (Hg/HgO/OH-, 0.165V vs. a reversible hydrogen electrode (RHE) 25 °C) were used as the working, the counter, and the reference electrodes, respectively. The method of preparing the working electrode is referred to our early report.64 In brief, the sealed glass carbon substrate was polished and washed, about 2.25 mg of the catalyst was dispersed in 1000 µL of ethanol, and then mixed with 500 µL of a 0.2 wt % Nafion solution (in ethanol) by ultrasonicated treatment about 30 min at room temperature to obtain a uniform catalyst ink. An aliquot of the ink (20 µL) was injected onto a glassy carbon (GC) substrate. The loading of catalysts on the catalyst-modified GC electrodes was controlled at 30 µg. The 0.1 M KOH solution was used as the electrolyte. The CV measurements were conducted under N2 saturated conditions over a potential range of 0-1.23 V (vs. RHE) with a scan rate of 50 mV s-1. The rotating disk electrode (RDE) technique was employed to study the ORR activity and kinetics at a rotating speed ranging from 625-2500 revolutions-per-minute (rpm) and a scan rate of 10 mV s-1 under O2 saturated conditions in the potential range of 0.00-1.23 V (vs. RHE). The preparation procedures of RDE was described in SI (supporting information) and the 7 ACS Paragon Plus Environment


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calculation of Pt content in catalysts ink and loading on RDE were listed in Table S2. In order to trace the byproduct H2O2 during the ORR process, the rotating disk electrode (RRDE) technique was also used; the description was provided in the supporting information. The activity of methanol-tolerant oxygen reduction (MTOR) was also evaluated in the solution composed of CH3OH and KOH mixtures with different concentrations under O2 saturated conditions. The compositions of the test electrolyte solution are 0.1 M CH3OH + 0.1 M KOH, 0.5 M CH3OH + 0.1 M KOH, 1.0 M CH3OH + 0.1 M KOH, and 3.0 M CH3OH + 0.1 M KOH, respectively.

3. RESULTS AND DISCUSSION 3.1

Physical characterization

3.1.1 Phase characterization The XRD pattern of the synthesized Pt/FePc@rGO composite indicates that the ultrafine crystalline NPs of Pt (face-centered cubic) and graphene were obtained (Figure S3). A wide and smooth peak around 26.6 ° can be indexed to the (002) plane of the graphene support.65 The other two widened diffraction peaks at 39.8 °and 46.2 ° can be indexed as the (111) and (200) planes of the Pt (JCPDS 87-0642, a=3.923, α=90°), respectively. The peak broadening results from the ultra-fine crystallite size and different defects of Pt NPs. Compared with the XRD patterns of FePc and rGO, two prominent peaks of Pt NPs were detected, indicating the synthesis of the catalyst support (Figure S3). There is no obvious diffraction peak of the FePc in the XRD pattern of the Pt/FePc@rGO composite, indicating that FePc is conjugating oriented layer upon rGO as confirmed by UV-vis spectroscopy in the later section. 3.1.2 Morphological characterization TEM was used to evaluate surface morphology and uniform dispersion of the Pt in the Pt/FePc@rGO composites obtained by the SURE synthesis procedures. From the TEM images (Figure 1a), the micrographs demonstrate that the synthesized Pt NPs are ultrafine in diameter and uniformly dispersed on the rGO support. Figures S4-S5 show the dispersion was homogeneous across the rGO surface. In comparison with the unmodified rGO (Figure S4a) the TEM micrographs of modified Pt/FePc@rGO, show ultrafine pseudo-spheres uniformly dispersed on the support film as Pt nanocatalysts are loaded on the substrates of rGO. As shown in different magnifications (Figure 1a and Figures S4b - S5), no aggregation of catalyst NPs was observed, and its average diameter of the dispersed particles was determined to be 1.76 ± 0.26 8 ACS Paragon Plus Environment

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nm in Figure 1b. The energy dispersive X-ray analysis (EDX) result of the Pt/FePc@rGO composite shows elements C, N, Pt, Fe, and O uniformly co-exist in the sample (Figure 1c). The peaks of Cu may arise from the Cu grid support. The results of XRD (Figure S3) and XPS (Figure S6) collectively indicated that Pt, FePc, and rGO are homogeneously co-existing in the Pt/FePc@rGO composite with the one phase orientation of FePc and rGO. The unchained rings in Figure 1c were indexed by the selected area electron diffraction (SAED) pattern of rGO thin film. This electron diffraction pattern is close to normal incidence from two layers graphene (or rGO).66 Usually, the rGO shows the planar morphology, while Pt NPs show spherical morphology. However, there is no diffraction ring in this image of SAED could be indexed to the NPs. It might be attributed to the small content (6.6wt% Pt) and well dispersed of Pt NPs in Pt/FePc@rGO, the diffraction rings would be weak and unchained, which are submerged in the background. High-resolution electron microscopy (HRTEM) was applied to investigate these ultrafine particles. The HRTEM images shown in Figures 1d and 1f demonstrate that the NPs show high intensity which is attributed to Pt NPs. Different Pt NPs exhibits different crystal planes. The Pt NPs (Figure 1d) and its inverse fast Fourier-filtered FT (IFFT) image (Figure 1e) shows that the lattice spacing d (about 2.26 Å) corresponds to the (111) plane of face-centered cubic (FCC) of Pt. The other particles are shown in Figure 1f and their inverse fast Fourier-filtered FT (IFFT) image Figure 1g showed the lattice spacing d (about 1.96 Å) corresponds to the (200) plane of the FCC Pt. The elements N and Fe were traced in the composite based on the EDS and XPS results. The FePc was found to be in parallel with rGO, but its planar orientation within the 111 and 200 planes requires further analyses, using different spectroscopic approaches to better understand the interaction between Pt source, FePc, and rGO, during the SURE synthesis of the Pt/FePc@rGO composite nanocatalyst.



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Figure 1. TEM image of the Pt/FePc@rGO composite (a) and (b) shows an image of the corresponding particle size distribution obtained by counting 200 particles. (c) The corresponding EDX pattern of the Pt/FePc@rGO composite, the SAED pattern of the same region shown the inset in (c). (d) and (f) High-Resolution TEM images of the different region of the Pt/FePc@rGO composite; the inset of (d) shows a Fourier transformed (FT) pattern of the marked red box regions in (d). (e) inverse fast Fourier-filtered FT (IFFT) image of the red box regions in (d). (f) the inset of (f) shows a Fourier transformed (FT) pattern of the marked red box regions in (f). (g) inverse fast Fourier-filtered FT (IFFT) image of the red box regions in (f).

3.2

Electrochemical performance

3.2.1

Oxygen Reduction Reaction (ORR) properties

3.2.1.1 The effect of the crystal face of Pt NPs on ORR property From the XRD, SAED, and TEM characterization, the loaded FCC Pt NPs were found to be on the surface of FePc/rGO. This FCC single crystal unit Pt possesses the surface energies associated with the low-index crystallographic planes, in the order of γ(111) < γ(100) < γ(110). The Pt NPs, which were enclosed by the (111) planes in the spherical shape are often observed.38,


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These observations were consistent with the HRTEM results reported previously (Figures 1d

and 1f). Compared with the Pt bulk single crystals, the nano-sized Pt offers several advantages, including larger surface to volume ratio,67 increased Pt utilization, and a greater number of available catalytic sites for electrochemical catalytic activity.68 The ORR activity of the Pt lowindex single-crystal surfaces increases in the order of Pt(100)

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