Polyaniline-Reduced Graphene Oxide Hybrid Nanosheets with Nearly

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Polyaniline-Reduced Graphene Oxide Hybrid Nanosheets with Nearly Vertical Orientation Anchoring Palladium Nanoparticles for Highly Active and Stable Electrocatalysis Liming Yang, Yanhong Tang, Dafeng Yan, Tian Liu, Chengbin Liu, and Shenglian Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08022 • Publication Date (Web): 17 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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ACS Applied Materials & Interfaces

Polyaniline-Reduced Graphene Oxide Hybrid Nanosheets with Nearly Vertical Orientation Anchoring Palladium Nanoparticles for Highly Active and Stable Electrocatalysis

Liming Yang,†,‡ Yanhong Tang,*,†,| Dafeng Yan,‡ Tian Liu,‡ Chengbin Liu,‡ and Shenglian Luo‡,|

† College of Materials Science and Engineering, Hunan University, Changsha 410082, P. R. China ‡ State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, P. R. China | Key Laboratory of Jiangxi Province for Persistant Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, P. R. China

ABSTRACT: We report a nearly vertical reduced graphene oxide (VrGO) nanosheet coupled with polyaniline (PANI) for supporting palladium (Pd) nanoparticles. The PANI-coupled VrGO (PANI@VrGO) nanosheet is prepared by a simple one-step electrodeposition technique and Pd nanoparticles are anchored on the support of PANI@VrGO through the spontaneous redox reaction of PANI with a palladium salt. The designed PANI@VrGO nanosheet efficiently exposes the surface of rGO sheets and

stabilizes

metal

nanoparticles.

Consequently,

the

Pd/PANI@VrGO

electrocatalyst exhibits high catalytic activity and excellent durability for alcohol

1 ACS Paragon Plus Environment


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oxidation reaction. The proposed nanoarchitecture offers a new pathway to greatly promote the performances of rGO in various applications and moreover, this work provides a powerful and universal synthetic strategy for such an architecture. KEYWORDS: vertical graphene, polyaniline, one-step electrodeposition, palladium nanoparticles, electrocatalysis

INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) have currently received considerable attention as a new power source for electric vehicles and portable electronic devices because of its high energy conversion efficiency, low operating temperature, and reduced pollutant emission.1,2 Unfortunately, high cost and limited resource of the state-of-art platinum (Pt)-based electrocatalysts severely impedes the commercialization of the PEMFCs.3 Palladium (Pd) has aroused increasing interest due to its higher abundance, lower cost than Pt and the outstanding electrocatalytic oxidation of various alcohols in alkaline environment.4 Besides, catalyst support materials exhibit great influence on the cost, activity, and durability of PEMFCs.5 In recent years, reduced graphene oxide (rGO) has been shown to be a promising support due to its unique characteristics of large specific surface area, high electrical conductivity, and improved durability relative to traditional carbon black.6,7 Nevertheless, rGO supporting nanocatalysts still faces critical issues: (1) irreversibly restacking of rGO sheets resulting from their strong interactions, causing most catalyst nanoparticles embedded between rGO layers and unavailable to reactants so


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that the catalysts could not be fully utilized;8 (2) sintering and dissolution of catalyst nanoparticles because of their high surface energy and weak interactions with rGO supports, deteriorating the catalytic activity during fuel cell operation;9 and (3) carbon corrosion of rGO supports,10,11 especially in the anode of PEMFCs, disrupting the structural integrity of catalyst layer and leading to the detachment of catalyst nanoparticles. Various strategies towards solving these problems have been proposed, for example, inserting spacers between rGO sheets12,13 or self-assembly of rGO sheets into porous three-dimensional architectures to prevent rGO stacking14,15 and surface treatments of rGO to suppress its carbon corrosion and synchronously stabilize nanoparticles,16−18 however, the development a rGO-based catalyst with all of the above issues addressed simultaneously is a great challenge. Herein, we engineer ultrathin polyaniline (PANI) layers sandwiched reduced graphene oxide (rGO) nanosheets to align vertically on electrically conductive substrates

as

a

novel

support

platform

for

metal

nanoparticles.

This

sophisticated architecture provides a new direction towards solving the potential problems. First, vertical nanostructure can maximally facilitate the transport of both mass and electron, and meanwhile the supported catalyst particles can expose their as much active sites as possible to reaction molecules, ensuring the best utilization of the catalysts.19,20 Second, the chemically and electrochemically stable PANI overlays rGO, not only restraining the stacking interactions among rGO sheets, but also protecting rGO from direct exposure to the corrosive environment to promote durability.21−23 Third, compared to rGO, which usually shows incompatibility with



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inorganic materials, PANI is also an electrically conductive material and additionally it can provide abundant nitrogen atoms to bind with metal ions for facilitating the controllable growth of inorganic/metallic nanoparticles with enhanced anchoring effects.24 Therefore, the coupled PANI-vertically aligned rGO hybrid nanosheets supported catalysts can be expected to be both highly active and robust in PEMFCs. Vertically aligned graphene nanosheets have recently been achieved using chemical vapor deposition method at elevated temperature and high vacuum.25 Since the chemically-derived graphene, namely, rGO, can be obtained massively from low-cost graphene oxide (GO) under mild conditions and furthermore it can be easily hybridized with other functional materials to create synergistic effects, aligning rGO vertically on current collectors has been attractive for electrochemistry-related devices.26−28 However, general and effective strategies for vertically aligned rGO (VrGO) have been scarcely developed. Herein, we report a one-step electrochemical assembly strategy for preparing VrGO via PANI inhibiting the stacking interactions between rGO sheets, obtaining PANI-coupled VrGO (denoted as PANI@VrGO) hybrid nanosheets. By taking advantage of the unique redox chemistry of PANI, a series of noble metal nanoparticles (e.g., Pd, Ag, Au) could be decorated on PANI through the spontaneous redox reaction of PANI with selected metal salts.29−31

EXPERIMENTAL SECTION Materials and Chemicals. Aniline, methanol, ethanol and PdCl2 powder were all purchased from Sinopharm Chemical Reagent Co., Ltd., China. Natural flake graphite


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(SP grade) with an average particle size of about 50 µm was obtained from Shanghai Carbon Co., Ltd., China. Nafion 117 solutions and a commercial Pd/C catalyst (10 wt % Pd, Sigma-Aldrich 205699) were purchased from Shanghai Hesen Co., Ltd., China. All other chemicals were of analytical reagent grade and deionized water was used throughout. Preparation of Pd/PANI@VrGO Hybrid. Graphite oxide was prepared by the improved method with a minor modification.32,33 The graphite oxide powder was exfoliated in a 0.1 M pH = 6.0 citrate buffer solution by ultrasonication in ice water bath for 3 h to form a 0.3 g L−1 graphene oxide (GO) colloidal dispersion. For electrochemical preparation of PANI@VrGO, 0.2 M aniline monomer (twice distilled under reduced pressure) was added into the above GO suspension. The cyclic voltammetric electrolysis was performed in the GO-aniline solution under magnetic stirring on a CHI 660C electrochemical workstation (CH Instruments, Chenhua Co., Ltd., Shanghai, China). A conventional three-electrode system was used, including a glassy carbon electrode (0.07 cm2 in geometric area, Aida, Tianjin) as the working electrode, a saturated calomel electrode (SCE, Leici, Shanghai) as the reference electrode and a Pt foil (Leici, Shanghai) as the counter electrode. Glassy carbon electrode was polished with 0.5 µm alumina powders (Aida, Tianjin), then rinsed thoroughly, and finally dried by blowing N2. The scan was performed between −1.4 and 0.9 V at a rate of 50 mV s−1 for ten cycles. After electrodeposition, the electrode was immersed into a 2.0 mM Na2PdCl4 solution and allowed to react at 30 °C for 12 h to form the Pd/PANI@VrGO hybrid. Finally, the electrode was washed several



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times with deionized water to remove the excess ions and dried at ambient condition. For a comparison, using the same procedures, Pd/VrGO hybrid with vertical structure was prepared from 0.2 g L−1 GO and 4.0 mM Na2PdCl4 solution. For the preparation of a commercial Pd/C electrode, typically, 2.0 mg catalysts were added into a mixture containing 1.0 mL ethanol and 50 µL Nafion (5 wt %) and the mixture was sonicated for 30 min. Then 20 µL of this mixture was dropped by a pipettor onto the surface of a newly polished glassy carbon electrode. All the experiments were carried out at room temperature. Characterization. The as-prepared samples were characterized by field-emission scanning electron microscopy (SEM, Hitachi, S-4800), transmission electron microscopy (TEM, JEM-3010 operating at 200 kV), high angle annular dark field-scanning TEM (HAADF-STEM, FEI Tecnai G2 F20 S-TWIN) equipped with an energy dispersive X-ray spectrometer (EDX), Raman spectroscopy (Labram-010 with a 632.8 nm laser), Fourier transform infrared spectroscopy (FTIR, FD-5DX), and X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-Alpha 1063 system with a monochromatic Al Kα source, 1486.6 eV, all the binding energies were corrected with reference to the C 1s peak at 284.8 eV). Electrochemical Measurements. All electrochemical measurements were carried out with a CHI 660C electrochemical workstation using a three-electrode test cell as same as the above. Cyclic voltammograms were recorded between -0.70 and 0.40 V for methanol oxidation reaction (MOR), and between -1.0 and 0.40 V for ethanol oxidation reaction (EOR) at a scan rate of 50 mV s−1. Chronoamperograms for MOR


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and EOR were measured at a given potential of -0.23 V and -0.20 V for 2000 seconds, respectively. The electrolyte solution was 1.0 M KOH solution containing 1.0 M CH3OH or C2H5OH. Prior to all the experiments, the electrolyte solution was purged with high purity N2 for 20 min to remove dissolved oxygen.

RESULTS AND DISCUSSION Synthesis and Morphological Characterization The overall synthetic strategy is illustrated in Scheme 1. A stable mixed solution of aniline and GO was subjected to cyclic voltametric (CV) electrolysis. During this electrochemical process, the reduction of GO occurred simultaneously with the polymerization of aniline on the electrode,34,35 and the resultant rGO and PANI self-assembled into PANI-sandwiched rGO hybrid nanosheets aligned vertically to the conductive substrate by tuning the concentration ratios between aniline and GO. Afterwards the PANI@VrGO electrode was kept in a Na2PdCl4 solution for some time to in situ grow Pd nanoparticles on the PANI@VrGO (Pd/PANI@VrGO), where PANI functions as both reducing agent and stabilizer. Figure 1a shows the typical SEM image of PANI@VrGO on electrode surface, and it can be seen that all the nanosheets are nearly standing on the substrate. The TEM image in Figure 1b reveals that the nanosheet is translucent without any aggregations, indicating that PANI forms ultrathin film and homogeneously coats the rGO sheets. PANI should be responsible for rGO standing upright since rGO nanosheets electrochemically prepared from only GO tend to horizontally stack on substrate



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electrodes,36,37 due to the π−π and Van der Waals interactions between rGO.38 Moreover, aniline concentration has a decisive influence on the assembly of PANI and rGO, because we have observed the evolution of PANI/rGO hybrid nanosheets from horizontal to vertical alignment as increasing the concentration ratios between aniline and GO (Figure S1, Supporting Information), similar to our previous work,39 which shows that the CV electrodeposition technique can control metal nanoparticles/rGO hybrid nanosheets from horizontally stacking to nearly vertical orientation just by increasing the concentration ratios between precursors for metal nanoparticles and GO. Although the exact growth mechanism has not been completely understood, we speculate that during the electrodeposition process, the function groups of rGO provide nucleation sites for the growth of PANI, and when rGO surfaces are fully covered by PANI, which are electropolymerized at higher aniline concentrations, the stacking attractions between rGO are thought to be inhibited, so that the resultant PANI/rGO/PANI sandwich-like nanosheets become relatively independent units that tend to stand on electrodes. Otherwise, the residual π−π and Van der Waals interactions between the rGO sheets that are not fully covered by PANI due to the lower aniline concentrations would still draw the rGO sheets stacking together to lie on electrodes. We further confirmed the generality of this strategy by using polypyrrole (PPy) conducting polymer instead of PANI (Figure S2, Supporting Information), or even using inorganic materials (Figure S3, Supporting Information), indicating that a variety of species (including metals, polymers, and inorganics) that can co-electrodeposit and interact with rGO can potentially lead to


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the vertical orientation of rGO sheets via inhibiting the stacking attractions between rGO. Figure 1c and d are the SEM images of Pd/PANI@VrGO obtained through the spontaneous redox reactivity of PANI with Na2PdCl4 in solution, and it can be seen that loading Pd nanoparticles onto PANI@VrGO did not change the morphology of PANI@VrGO one bit. It should be noted that the Pd loading using this in-situ method can be controlled to low amounts by decreasing the reaction time and moreover the Na2PdCl4 reaction solution can be reused, greatly reducing the catalyst cost. The surfaces of the PANI/VrGO nanosheets are entirely exposed and hierarchical void spaces forms between the interlaced nanosheets (Figure 1c and d), enabling high accessibility of the anchored catalyst nanoparticles to reactants and thus efficient catalyst utilization. Additionally, the conductive PANI/VrGO nanosheets in direct contact with the substrate electrode would minimize electronic resistances. Further TEM observation reveals that ultrafine Pd nanoparticles form fractal clusters of about 12 nm in diameter to uniformly disperse on the PANI/VrGO nanosheets (Figure 1e). The high-resolution TEM (HR-TEM) image shows that the nanoparticles have a d spacing of 0.23 nm, which corresponds to the (111) plane in face-centered cubic Pd metal (Figure 1f). Composition and Structure Characterization The chemical composition and structure of Pd/PANI@VrGO were further investigated by HAADF-STEM equipped with EDX, Raman spectroscopy, FTIR, and XPS. The EDX analysis verifies the presence of C, N, O, and Pd elements in the



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Pd/PANI@VrGO hybrid (Figure 2), and the chemical maps demonstrate that all the four elements are homogeneously distributed alike, meaning the uniform coating of Pd and PANI on rGO sheets. The Raman spectrum of Pd/PANI@VrGO hybrid exhibits characteristic D-band and G-band of graphene (Figure 3a), whereas the intensity ratio of the ID/IG value increases from 1.05 for GO to 1.14 for rGO in Pd/PANI@VrGO for the formation of more sp2 domains in rGO after GO reduction.39 FTIR spectra disclose the predominant adsorptions of oxygen-containing groups on GO, e.g., 3200–3800 cm−1 (O−H stretching), 1650 cm−1 (C=O stretching), 1450 cm−1 (O−H deformation), and 1285 cm−1 (C−O−C vibration) significantly decrease after it was incorporated in Pd/PANI@VrGO (Figure 3b), further confirming the electrochemical reduction of GO. The Raman and FTIR spectra show that the typical peaks for PANI (e.g., C−N+•/C=C stretching in Raman spectrum and N−H/C=N/C−N stretching in FTIR spectrum)40 are not notable in Pd/PANI@VrGO due to the presence of rGO, so XPS characterization was additionally performed to identify the PANI component in Pd/PANI@VrGO. As demonstrated in Figure 4a, besides the C， O and Pd element peaks, the XPS full spectrum of Pd/PANI@VrGO manifests the N 1s signal at 400.0 eV attributed to PANI. The high-resolution XPS spectrum of the N 1s region further indicates four different electronic states (Figure 4b), with three of the binding energies centered at 400.72, 400.01, 399.50 eV being assigned to nitrogen cationic radical (N+•), benzenoid amine (-NH-), and quinoid imine (=N-), respectively, characteristic of PANI.35,41 The fourth peak at 398.85 eV is ascribed to the nitrogen atoms of PANI bonded with Pd nanoparticles,41,42 suggesting the


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stabilizing effect of PANI on Pd nanoparticles. In additional, the Pd 3d XPS spectrum of Pd/PANI@VrGO was also collected (Figure 4c). Both the Pd 3d5/2 (~337 eV) and the Pd 3d3/2 (~343 eV) peaks, from the spin-orbital splitting of Pd 3d, can be deconvoluted into doublets assigned to Pd0 and Pd2+. The presence of Pd2+ means a part of the Pd atoms are bonding with their support. Compared to the Pd/VrGO hybrid without PANI, Pd/PANI@VrGO shows significantly positive shifts of the Pd peaks. Such chemical shifts towards higher binding energies reflect enhanced interactions of the Pd nanoparticles with the PANI@VrGO support in Pd/[email protected],41 Electrocatalytic Performance Electrochemically active surface area (ECSA) concerning the number of electrochemically active sites per gram of the Pd/PANI@VrGO catalyst is investigated by cyclic voltammetry in N2-saturated 0.5 M H2SO4 solution (Figure 5). The

cyclic

voltammogram

exhibits

characteristic

peaks

of

hydrogen

adsorption/desorption on Pd, and palladium oxide formation/reduction (redox pairs b/b'), as well as characteristic leucoemeraldine/emeraldine (redox pairs a/a') and emeraldine/pernigraniline (redox pairs c/c′) conversions of PANI.43 The ECSA can be calculated quantitatively basing on hydrogen desorption according to the Equation (1):44

ECSA =

Q mPd * 212 µC ⋅ cm −2

(1)

where Q represents the charge for the hydrogen desorption (µC), 212 µC cm−2 is the charge density required to oxidize a complete hydrogen monolayer on Pd surface, and 11 ACS Paragon Plus Environment


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mPd is the Pd loading (g) on the glassy carbon electrode. The Pd loading on the electrode was evaluated by inductively coupled plasma-atomic emission spectrometry (ICP-AES, PS-6, Baird, USA) after dissolution with aqua regia. Commercial Pd/C was investigated for comparison. Based on the integrated area under the desorption peak between –0.2 and 0.05 V in the CV curves, the ECSA of Pd/PANI@VrGO catalyst is calculated to be 0.132 m2 g−1, which is much higher than 0.026 m2 g−1 of commercial Pd/C catalyst. The distinctly high ECSA for the Pd/PANI@VrGO catalyst is mainly due to the vertically aligned PANI@VrGO support that leads to full exposure of Pd nanoparticle active sites, implying that the Pd/PANI@VrGO catalyst is electrochemically more accessible for electrocatalytic reactions. Nevertheless, without the introduction of PANI, Pd/VrGO shows the ECSA to be 0.164 m2 g−1, slightly larger than that of Pd/PANI@VrGO. The electrocatalytic activity of Pd/PANI@VrGO was evaluated by the methanol and ethanol oxidation reactions in alkaline media. Commercial Pd/C was also included. Figure 6a shows the mass activity for the MOR on Pd/PANI@VrGO is 375.6 A g−1, which is 3.6 times that of commercial Pd/C (105.2 A g−1). Compared to the ECSA value, this performance is slightly poorer, possibly due to the difference in the Pd loading, crystallinity and the catalyst morphology. In addition, the onset potential for the MOR on Pd/PANI@VrGO is ca. –0.4 V, which is 100 mV more negative than the ca. –0.3 V observed on commercial Pd/C. The higher mass activity and the more negative onset potential mean enhanced electrocatalytic activity of Pd/PANI@VrGO for the MOR. Similarly, Pd/PANI@VrGO exhibits higher


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electrocatalytic activity than commercial Pd/C towards the EOR (Figure 6b), also evidenced by the significantly higher anodic oxidation current and the lower onset potential. Moreover, in both cases, much higher If/Ib (If and Ib is the forward and backward current, respectively) ratios are observed for Pd/PANI@VrGO than commercial Pd/C. This indicates that the alcohol molecules can be effectively oxidized by the Pd/PANI@VrGO catalyst with relatively less poisoning species yielded.45,46

Figure

6c

and

d

show

the

chronoamperometric

results

of

Pd/PANI@VrGO and commercial Pd/C for the MOR and EOR, respectively. Commercial Pd/C demonstrates fast current decays to zero while Pd/PANI@VrGO retains high oxidation current over the whole time in both cases, revealing better electrocatalytic stability of Pd/PANI@VrGO. The stability of Pd/PANI@VrGO was further evaluated using the CV technique. Figure 7a shows the cyclic voltammograms of Pd/PANI@VrGO for the MOR at the 5th, 25th, 50th, 250th and 500th cycles. The peak current initially increases till at about the 50th cycle, and afterwards, the peak current is almost constant, indicating a good durability of Pd/PANI@VrGO. In contrast, the prepared Pd/VrGO without PANI and commercial Pd/C show obvious current deterioration during the CV scanning (Figure S4, Supporting Information). The forward anodic peak current of the MOR as a function of cycle number for Pd/PANI@VrGO, Pd/VrGO, and commercial Pd/C are summarized in Figure 7b. Again commercial Pd/C demonstrates rapid current decays to zero maybe due to the dropped catalyst coating easily suffering from peeling off during the electrochemical process, while the electrodeposited Pd/VrGO is relatively



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stable, and especially, after the incorporation of PANI, the resulting Pd/PANI@VrGO exhibits high durability, manifesting outstanding stabilization of Pd nanoparticles by PANI. PANI acting as an efficient stabilizer for Pd nanoparticles was confirmed by the unchanged SEM morphology of Pd/PANI@VrGO before and after the 500 CV cycles, where not only the Pd nanopartilces but also the PANI film maintained their original arrangements on the rGO sheets (Figure 8a and b). On the contrary, Pd/VrGO, with the Pd nanoparticles uniformly dispersed on the rGO sheets before the CV cycles (Figure 8c), shows serious aggregation and even loss of the Pd nanoparticles after the 500 CV cycles, leaving most surfaces of the rGO sheets uncovered (Figure 8d). In addition, the degradation of commercial Pd/C was also investigated, and the Pd nanoparticles in the catalyst grow increasingly larger and tend to form aggregates (Figure 8e and f). CO stripping experiments were performed to evaluate the tolerance of the catalysts towards poisoning species. CO adsorption was conducted by exposing the catalysts to CO gas at a potential of −1.0 V for a period of 20 min. Figure 9 shows the CO stripping CV curves of Pd/PANI@VrGO and commercial Pd/C catalyst in N2-saturated 1.0 M KOH solution at a scan rate of 50 mV s−1. Strong CO oxidation peaks are present for the both catalysts in the first scan and no CO oxidation is monitored in the second scan, meaning their abilities in removal of the adsorbed CO species. Nevertheless, the distinctly lower onset potential of CO electrooxidation on Pd/PANI@VrGO (−0.73 V) than commercial Pd/C (−0.35 V) together with the


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significantly higher oxidation current density on Pd/PANI@VrGO suggests that the Pd/PANI@VrGO catalyst possesses better CO electrooxidation capability, so it has less opportunity to be poisoned by adsorbed CO species during the alcohol oxidation, also contributing to improving the durability. It should be noted that the synthetic route for the Pd/PANI@VrGO catalyst can be potentially applied to the preparation of other metals (or even alloys) on PANI@VrGO. As shown in Figure S5, Supporting Information, Ag/PANI@VrGO exhibits its structure similar to Pd/PANI@VrGO, with Ag nanoparticles well distributed on the PANI@VrGO sheets.

CONCLUSIONS We have proposed a novel strategy for constructing highly active and robust graphene-based electrocatalysts by employing conducting polymer PANI coupled nearly vertical aligned rGO sheets to anchor catalytic metal nanoparticles. The nearly vertical orientation of rGO nanosheets was achieved by a simple cyclic voltammetric electrodeposition technique with the help of PANI that inhibits the stacking interactions of rGO. PANI, sandwiching rGO sheets, not only assist rGO to stand up, but also is thought to protect rGO from carbon corrosion, and additionally functions as reducing agent and

stabilizer for metal nanoparticles. The prepared

Pd/PANI@VrGO catalyst, as expected, shows excellent stability, and also high electrocatalytic activity towards alcohol oxidation because vertical supporting materials can ensure fast transport of both electron and mass and moreover fully



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available active sites of catalysts. Furthermore, this work provides a universal strategy to control the upstanding of rGO nanosheets, opening opportunities for fabricating advanced VrGO-based hybrids for applications such as energy conversion, energy storage, and sensing.

ASSOCIATED CONTENT Supporting Information Available: SEM images of PANI/rGO electrodeposited from 0.3 g L−1 GO and aniline with varied concentrations; SEM images of PPy/rGO electrodeposited from 0.3 g L−1 GO and pyrrole with varied concentrations; SEM image of FeS2/VrGO hybrid; Cyclic voltammograms of Pd/VrGO and commercial Pd/C at the 5th, 50th, 250th and 500th cycles; SEM image of Ag/PANI@VrGO hybrid. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS This work was supported by Hunan Provincial Natural Science Foundation of China (14JJ1015 and 2015JJ2030), the National Natural Science Foundation of China (51572077 and 514781714), Program for Innovation Research Team in University (IRT1238), and Hunan Provincial Innovation Foundation for Postgraduate (521293157).

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected] (Tang Y. H.) Phone: 86-731-88823805. Fax: 86-731-88823805. Notes The authors declare no competing financial interest.

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Na2PdCl4

one-step electrodeposition aniline

GO

PANI@VrGO

Pd/PANI@VrGO

Scheme 1. Preparation of the PANI@VrGO supporting material and the Pd/PANI@VrGO catalyst.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

1 µm

20 nm (d)

(c)

1 µm

200 nm

(e)

(f)

20 nm

1 nm

Figure 1. (a) SEM image of PANI@VrGO, (b) TEM image of PANI@VrGO, (c, d) SEM images of Pd/PANI@VrGO, (e) TEM image of Pd/PANI@VrGO, and (f) HR-TEM image of Pd nanoparticles.



(b)

(a)

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

Pd/PANI@VrGO

Pd CN

O Energy / eV

C

N

Pd

O

Figure 2. (a) HAADF-STEM image and corresponding elemental mapping for Pd/PANI@VrGO, (b) EDX spectrum of the selected area in (a).


Page 27 of 34

(a)

D

G

(b)

Pd/PANI@VrGO

νC=N

νC-N+•

-1

νC=C

1340 cm 1600 cm-1

1000

1200

Reflection

νC-N

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PANI

νO-H νC-O-C

Pd/PANI@VrGO PANI GO νC=O

GO

1400 1600 1800 -1 Raman Shift / cm

νN-H

2000

δO-H

4000 3500 3000 2500 2000 1500 1000 -1 Wave Number / cm

Figure 3. (a) Raman spectra and (b) FTIR spectra for Pd/PANI@VrGO, PANI and GO.



(a)

Pd/PANI@VrGO

Intensity / a.u.

Pd 3d

(c)

Pd 3d5/2

Pd/PANI@VrGO

O 1s Pd 3p

0

Pd

Pd 3d3/2

N 1s 2+

C 1s

Pd

300 400 500 Binding Energy / eV

(b)

396

600

N 1s

Intensity / a.u.

200

397

Pd/VrGO 0

Pd

Pd−N =N− −NH− +• N

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

2+

Pd

398 399 400 401 Binding Energy / eV

402

332 334 336 338 340 342 344 346 348 Binding Energy / eV

Figure 4. (a) XPS full spectra of Pd/PANI@VrGO, (b) N 1s spectrum of Pd/PANI@VrGO, and (c) Pd 3d spectra of Pd/PANI@VrGO (top) and Pd/VrGO (bottom).


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Cyclic voltammograms of Pd/PANI@VrGO and commercial Pd/C catalyst in N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV s−1. (The currents were normalized to the geometric areas of glassy carbon electrodes.)



400

(a)

1000

Pd/PANI@VrGO Pd/C

If

-1

100

Ib

0

-0.6

-0.4

(c)

-0.2 0.0 E / V vs SCE

0.2

200

Ib

0 -400 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 E / V vs SCE

0.4

Pd/PANI@VrGO Pd/C -1

300

400

J / A g Pd

-1

400

-200

-100

400

Pd/PANI@VrGO Pd/C

If

600

J / A g Pd

-1

J / A g Pd

200

(b)

800

300

J / A g Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

200 100

(d)

0.2

0.4

Pd/PANI@VrGO Pd/C

300 200 100

0

0 0

500

1000 Time / s

1500

2000

0

500

1000 Time / s

1500

2000

Figure 6. Cyclic voltammograms of Pd/PANI@VrGO and commercial Pd/C for the MOR (a) and EOR (b) at a scan rate of 50 mV s−1. Chronoamperograms of Pd/PANI@VrGO and commercial Pd/C for the MOR at −0.23 V (c) and the EOR at −0.2 V (d). The electrolyte solution is N2-saturated 1.0 M KOH containing 1.0 M CH3OH or 1.0 M C2H5OH. (Pd loading in catalysts, Pd/PANI@VrGO: 0.023 mg cm−2, commercial Pd/C: 0.057 mg cm−2)


300

(a)

th

500 th 250 th 50 th 25 th 5

-1

200 J / A g Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 0 -100 -200

-0.6

-0.4

-0.2

0.0

0.2

0.4

Anode peak current (vs initial)

Page 31 of 34

E / V vs SCE

(b) 1.5 Pd/PANI@VrGO Pd/VrGO Pd/C

1.0 0.5 0.0 0

100

200

300

400

500

Number of CV Cycles

Figure 7. (a) Cyclic voltammograms of Pd/PANI@VrGO at the 5th, 25th, 50th, 250th and 500th cycles between −0.7 and 0.4 V at a scan rate of 50 mV s−1 in 1.0 M KOH + 1.0 M CH3OH solution. (b) Plots of anodic peak current as a function of cycle number for Pd/PANI@VrGO, Pd/VrGO, and commercial Pd/C.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

Pd PANI 20 nm

20 nm

(c)

(d)

50 nm

50 nm

(e)

(f)

50 nm

50 nm

Figure 8. Typical TEM images of Pd/PANI@VrGO (a, b), Pd/VrGO (c, d) and commercial Pd/C (e, f) catalyst before and after 500 consecutive cycle scans in 1.0 M KOH + 1.0 M CH3OH solution. 32 ACS Paragon Plus Environment

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300

1st scan 2nd scan

(a) Pd/PANI@VrGO

300

(b) Pd/C

1st scan 2nd scan

200

100

J / A g Pd

-1

-1

200 J / A g Pd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


−0.73 V

0

100

−0.35 V

0

-100 -100

-200

-0.9

-0.6

-0.3

0.0

0.3

0.6

-0.9

E / V vs SCE

-0.6

-0.3

0.0

0.3

0.6

E / V vs SCE

Figure 9. CO stripping voltammograms of (a) Pd/PANI@VrGO and (b) commercial Pd/C in N2-saturated 1.0 M KOH solution at a scan rate of 50 mV s−1.



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Table of Contents (TOC)


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Polyaniline-Reduced Graphene Oxide Hybrid Nanosheets with Nearly

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