Palladium Nanoparticles Supported on Nitrogen ... - ACS Publications

Subscriber access provided by Weizmann Institute of Science

Article

Palladium Nanoparticles Supported on Nitrogen and Sulfur Dual-Doped Graphene as Highly Active Electrocatalysts for Formic Acid and Methanol Oxidation Xin Zhang, Jixin Zhu, Chandra Sekhar Tiwary, Zhongyuan Ma, Huajie Huang, Jianfeng Zhang, Zhiyong Lu, Wei Huang, and Yuping Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01580 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

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

ACS Applied Materials & Interfaces

Palladium Nanoparticles Supported on Nitrogen and Sulfur Dual-Doped Graphene as Highly Active Electrocatalysts for Formic Acid and Methanol Oxidation Xin Zhang,† Jixin Zhu,*,‡ Chandra Sekhar Tiwary,§ Zhongyuan Ma,‡ Huajie Huang,*,† Jianfeng Zhang,*,† Zhiyong Lu,† Wei Huang,‡ and Yuping Wu† †

College of Mechanics and Materials, Hohai University, Nanjing 210098, China. E-mail:

[email protected], [email protected] ‡

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzu Road, Nanjing 211816, China. E-mail: [email protected] §

Department of Materials Science and NanoEngineering, Rice University, Houston, Texas

77005, United States.

KEYWORDS: Palladium nanoparticles, Dual-doped graphene, Electrocatalyst, Fuel cells, Formic acid oxidation, Methanol oxidation

ACS Paragon Plus Environment

1


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 2 of 23

ABSTRACT: Optimized designing of highly active electrocatalysts has been regarded as a critical point to the development of portable fuel cell systems with high power density. Here we report a facile and cost-effective strategy to synthesis of ultrafine Pd nanoparticles (NPs) supported on N and S dual-doped graphene (NS-G) nanosheets as multifunctional electrocatalysts for both direct formic acid fuel cell and direct methanol fuel cell. The incorporation of N and S atoms into graphene frameworks is achieved by a thermal treatment process, followed by the controlled growth of Pd NPs via a solvothermal approach. Owning to the unique structural features as well as the strong synergistic effects, the resulting Pd/NS-G hybrid exhibits outstanding electrocatalytic performance toward both formic acid and methanol electrooxidation, such as higher anodic peak current densities and more exceptional catalytic stability than those of Pd/Vulcan XC-72R and Pd/undoped graphene catalysts. These findings open up new possibility in the construction of advanced Pd-based catalysts, which is conducive to solving the current bottlenecks of fuel cell technologies.


2

Page 3 of 23

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


1. INTRODUCTION Over the past few decades, fuel cell systems have received more and more considerations with increase of energy crisis and environmental pollution.1 Among various kinds of fuel cells, the direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) have been regarded as highly promising power sources owing to their facile device construction, higher-energy conversion efficiency, lower operating temperature, less hazards emission, and more convenient storage and transport of liquid fuels.2,3 As the core block of fuel cell systems, electrode catalyst plays a key role in achieving high energy densities.4,5 It is well known that platinum and platinum-based alloys are the most widely used anode electrocatalysts for both DFAFC and DMFC due to their unique catalytic properties.6,7 However, the limited resources and the poor antipoisoning ability of platinumrelated catalysts have largely impeded their large-scale commercial applications in fuel cells.8 In this regard, it is necessary for searching less expensive platinum alternatives with acceptable catalytic performance and strong poison tolerance. Recent theoretical and experimental investigations demonstrated that Pd-based anode catalysts with lower cost possess higher activity and better resistance to CO than those of Pt-based catalysts.9-12 Nevertheless, the low utilization efficiency and poor electrochemical stability commonly occurs in the Pd catalytic systems, resulting in a steep deterioration in electrocatalytic properties. To circumvent the above problems, one efficient strategy is to disperse Pd nanoparticles (NPs) onto a suitable support with large surface area and good electrical conductivity. In this regard, a variety of carbonaceous materials including carbon black,13 carbon nanofibers (CNFs),14 carbon nanotubes (CNTs)15,16 and graphene12,17-19 have been employed to control the sizes and morphology of Pd nanocrystals. Among them, graphene, a two-dimensional nanomaterial


3


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 4 of 23

consisting of sp2-hybridized carbon, has been conceived as a most promising matrix owing to its extremely high specific surface area, excellent electronic conductivity and good chemical stability.20,21 On the other hand, heteroatom (nitrogen, sulfur, boron etc.) doping into the graphene architectures could be benefit to improve their electronic structures and chemical reactivity, which are specifically expected to restrain the sizes of noble metal NPs as well as strengthen the interaction between metal and carbon supports.22,23 Moreover, recent studies indicated that the dual-doping of two different heteroatoms into graphene frameworks could provide much more electroactive sites due to the concerted effects, rendering further enhanced catalytic performance.24,25 On this basis, it is of great significance to explore high-performance hybrid electrode catalysts consisted of Pd NPs and dual-doped graphene sheets, which will offer great potential on developing advanced fuel cell systems with low cost. As far as we know, few studies on the preparation and fuel cell properties of the Pd/dual-doped graphene nanocomposites have been reported so far. In this work, we report a simple and cost-effective approach to the fabrication of Pd/N and S dual-doped graphene (Pd/NS-G) hybrid as a multifunctional electrocatalyst for both DFAFC and DMFC. As illustrated in Scheme 1, graphene oxide (GO) synthesized by an optimized Hummers’ method and 1,3,4-thiadiazole-2,5-dithol (C2H2N2S3, TDDT) as N and S precursor were first mixed together and sonicated into a homogenous dispersion. After removal of the water through freeze drying, the dried GO-TDDT mixture was heated to 700 °C under Ar atmosphere, which could allow the incorporation of both N and S atoms into the carbon skeletons as well as the reduction of GO. Subsequently, the as-obtained NS-G sheets could provide sufficient anchoring sites to grow highly uniform Pd NPs. Such distinctive structural features can offer (1) a large surface area for easy access of electrolytes, (2) a low charge-


4

Page 5 of 23

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


transfer resistance for ultrafast electron transport, (3) numerous Pd electroactive sites with high catalytic activity, and (4) abundant hydroxyl species (produced by N and S atoms) for oxidation of CO-like poisoning intermediates, all of which are very beneficial in promoting the kinetics of catalytic reactions. As a consequence, the resulting Pd/NS-G hybrid exhibits outstanding electrocatalytic performance, superior to those of commercial Pd/Vulcan XC-72R (Pd/C) and Pd/undoped graphene (Pd/G) catalysts.

Scheme 1. Schematic of the synthetic procedures for the Pd/NS-G catalyst. 2. EXPERIMENTAL SECTION Materials. Commercial graphite powder was obtained from Qingdao Zhongtian Company (Qingdao, China) and has particle size around 40 µm. Vulcan XC-72R was bought from Cabot Corporation. The commercial K2PdCl4 (Alfa Aesar) and Nafion solution (117, 5%, Dupont) were obtained for direct use without further purification. The chemical reagents are all analytical grade and directly used as supplied. All aqueous solutions were prepared with deionized water. Preparation of NS-G nanosheets. First, graphite oxide was prepared from commercial graphite powder by an improved Hummers’ method as described previously.26 As to the dualdoping of N and S atoms into the carbon frameworks, 100 mL as-synthesized graphite oxide (0.5 mg mL-1) aqueous dispersion was mixed with 100 mg TDDT by sonication for 2 h. Afterwards,


5


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 6 of 23

the mixture was freeze-dried at -50 °C for 12 h to remove the water, and then the resulting GOTDDT samples were placed in a tube furnace and heated at 700 °C in Ar. During the thermal annealing process, both N and S atoms can be interacted with the functional groups and doped into the lattices of carbon layers, and simultaneously GO was reduced to graphene, forming the desired NS-G nanosheets. Preparation of Pd/NS-G, Pd/G and Pd/C catalysts. In a typical synthesis of Pd/NS-G, 20 mg of as-obtained NS-G sheets was ultrasonically dispersed in 40 ml ethylene glycol and 40 ml deionized water to form a black aqueous solution. Subsequently, 0.47 ml 0.1 M K2PdCl4 was mixed with the above as-prepared aqueous solution and stirred for 15 min. After that, the reaction mixture was moved to a 100 ml Teflon-lined stainless steel autoclave and then maintained at 120 °C for 15 h. The final resultant product (named as Pd/NS-G) was centrifuged, washed with ethylene and deionized water, and then collected after a freeze-dried step overnight. For comparison, Pd NPs supported on undoped graphene (denoted as Pd/G) and commercial Vulcan XC-72R (denoted as Pd/C) were also prepared via the same procedure. The palladium content of all above catalysts is 20 wt.%. Characterization. The morphology and nanostructures of the as-prepared samples were investigated by Field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). The crystalline structures of the obtained samples were studied by Powder X-ray diffraction (XRD) analyses (Bruker D8 coupled with Cu Kα radiation). Raman spectra were obtained by a LabRAM Aramis Raman Microprobe (HORIBA JOBIN YVON S.A.S) at a wavelength of 532 nm. X-ray photoelectron spectra (XPS) of the samples were recorded via a Thermo ESCALAB 250 XPS Microprobe.


6

Page 7 of 23

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


Electrocatalytic measurements. The electrocatalytic performance of as-prepared Pd/NS-G, Pd/G and Pd/C samples were tested at a CHI 760E electrochemical workstation. For both formic acid and methanol oxidation measurements, we used a Pt wire as the counter electrode and the saturated calomel electrode (SCE) as the reference electrode. The procedure for the manufacture of the working electrode has been described in our previous study.27 The loading amount of Pd on all electrodes was 0.0283 mg·cm-2. The electrolytes used in DFAFC and DMFC measurements were solutions of 0.5 M H2SO4 with 0.5 M HCOOH and 0.5 M NaOH with 1 M methanol, respectively. 3. RESULTS AND DISCUSSION The morphology and microstructure of the as-synthesized Pd/NS-G hybrid were investigated by FE-SEM and TEM. As shown in Figure 1a, the lamellar structure features of NS-G sheets can be clearly observed, which is similar to the previously reported GO or graphene materials.25 Under close inspection, the N and S dual-doped carbon sheets are decorated uniformly by numerous small Pd NPs with sizes below 10 nm (Figure 1b-c). Both large surface area of graphene and coexistence of N and S atoms in the carbon frameworks are considered to dramatically strengthen the metal-support binding and benefit to grow small and uniform Pd NPs. Selected area electronic diffraction (SAED) pattern and high resolution TEM (HRTEM) images of Pd/NS-G further reveal the typical atomic lattices for both NS-G plane and face-centered cubic (fcc) Pd crystals (the inset of Figure 1c and d-e), and the observed spacings of 0.34 and 0.22 nm for the adjacent fringes are in good accord with the known data.28 Moreover, the element mapping analysis convincingly demonstrates the well dispersion of C, N, S and Pd in the hybrid (Figure 1f-i). For comparison, undoped graphene sheets and commercial carbon black were also employed as supporting materials to disperse Pd NPs by the similar synthetic route. As can be


7


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 8 of 23

seen from Figure S1, Pd particles grew randomly on these matrixes, and the average particle sizes for Pd/G (7.9 nm) and Pd/C (6.8 nm) samples are obviously larger than that for Pd/NS-G (4.6 nm), which should be resulted from the insufficient anchor sites on unmodified graphene and carbon black surfaces.

Figure 1. Typical (a-b) FE-SEM, (c) TEM and (d-e) HRTEM images of the Pd/NS-G hybrid. The insets in (c) are corresponding SAED pattern and Pd particle size distribution of Pd/NS-G. The corresponding elemental mapping images of (f) C, (g) N, (h) S, (i) Pd elements taken from the square region marked in (b). The structural information of the as-prepared Pd/NS-G hybrid was further examined by X-ray diffraction (XRD) and Raman spectroscopy. Figure 2a displays the typical XRD patterns of Pd/NS-G, Pd/G, Pd/C and GO samples. As seen from the GO diffraction pattern, the sharp diffraction peak located at 2θ=10.5° is associated with the reflection on (001) plane, and the much larger basal spacing (0.86 nm) compared with that of natural graphite (0.34 nm) is mainly due to the presence of oxygenated functional groups. In the cases of Pd/NS-G and Pd/G, the above characteristic peak shifts to a much higher angle (2θ=25.0°) with a broad profile,


8

Page 9 of 23

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


indicative of the deoxygenation of GO sheets during the synthesis process. Notably, the intensity of this (001) peak for Pd/NS-G is stronger relative to that of Pd/G, which should be linked to the higher graphitic degree of NS-G layers originated from the thermal annealing. Moreover, the other three peaks observed at 39.8°, 46.2° and 67.7° in Pd/NS-G and Pd/G can be indexed to the (111) plane, (200) plane, and (220) plane of metal Pd with cubic structure, respectively (JCPDS no. 46-1043), implying the high crystallinity of supported Pd NPs in the composites. The Raman spectra of GO, Pd/G and as-prepared Pd/NS-G are displayed in Figure 2b. Apparently, two prominent scattering peaks at ~1340 and 1580 cm-1 are evident in all spectra of the three samples, which are commonly ascribed to D and G bands of distorted graphitic carbon materials. The former is usually induced by the topological defects in the carbon sheets, while the latter is resulted from the crystalline graphitic domains.29 Remarkably, the D/G intensity ratio (ID/IG) for Pd/NS-G (1.15) is much higher than those for Pd/G (0.89) and GO (0.79), demonstrating a higher defect density in NS-G sheets because of the N and S dopants. Besides, it is found that the G band of Pd/NS-G depicts an apparent redshift in comparison with those of Pd/G and GO, which is consistent with the previously reported results for heteroatom doped carbon materials,30 further proving the successful doping of N and S atoms.


9


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 10 of 23

Figure 2. (a) XRD patterns of Pd/NS-G, Pd/C, Pd/G, and GO; (b) Raman spectra of GO, Pd/G and Pd/NS-G. XPS measurements and elemental analysis were also conducted to gain more insights into the chemical composition of the Pd/NS-G hybrid. As shown in the survey scan spectrum of Pd/NS-G (Figure S2), five characteristic peaks located at the binding energies of ca. 285.0, 400.0, 165.0, 530.0, and 340.0 eV are clearly observed, belonging to the C 1s, N 1s, S 2p, O 1s and Pd 3d signals, respectively. The N and S contents are determined to be 6.6 and 7.5 wt.%, respectively, which are comparable to those single-heteroatom-doped graphene materials.31 The high magnification N 1s profile (Figure 3a) validates the presence of three types of N functionalities in the composite, including pyridine N (N1, 398.2 eV), pyrrolic N (N2, 399.6 eV) and graphitic N (N3, 401.1 eV).32 Since the former two N functionalities are responsible for the interaction with palladium,33 the high N1 and N2 contents (~79.0% total) within graphene layers could effectively enhance the structural stability of Pd NPs and prevent them from aggregation. The major peaks of S 2p3/2 and S 2p1/2 in the complex S 2p spectrum of Pd/NS-G are centered at 163.9 and 165.2 eV (Figure 3b), implying the doped S atoms could combine with their neighboring C atoms to form thiophene-like structures.24 Other sulfur species, such as chemically inactive SOx groups appeared at higher binding energies (167.0-172.0 eV), were not detected in the spectrum. Meanwhile, the peak fitting of the C 1s spectrum (Figure 3c) reveals different C components, which are sp2 C-C (284.7 eV), C-N-C/C-S-C (285.4 eV) and C-OH (286.5 eV).34 Notably, the relative peak area of sp2 C-C in Pd/NS-G is much larger than that in GO (Figure S3), confirming that GO was reduced to graphene with less residual oxygen functional groups after thermal annealing process. Furthermore, there are two chemical states presented in Pd (Figure 3d): the two lower energy split peaks located at 335.6 eV and 340.9 eV are related to the metallic


10

Page 11 of 23

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


Pd, and the two split peaks located at higher energy of 336.4 eV and 342.4 eV are due to the Pd2+ specie.35 All above results from structural characterizations are in conformity with each other, convincingly demonstrating the efficient anchoring of Pd particles on the NS-G surface.

Figure 3. High-resolution (a) N 1s, (b) S 2p, (c) C 1s and (d) Pd 3d core-level XPS spectra of the Pd/NS-G hybrid. To inverstigate the electrocatalytic activity of as-prepared Pd/NS-G hybrid, cyclic voltammetry (CV) measurements were first carried out in a N2-saturated solution of 0.5 M H2SO4 at 50 mV·s-1. As shown in Figure 4a, during the cathodic scan, there are two obvious adsorption peaks at the potential regions of 0 to -0.1 V and -0.1 to -0.2 V for each CV curve, which arise from the hydrogen atoms adsorbed on Pd sites.36 Correspondingly, the representative current peaks for desorption process appearing in the anodic scan are also visible. Through integration of the hydrogen adsorption areas, Pd/NS-G was found to possess an electrochemically active surface


11


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 12 of 23

area (ECSA) value of 83.4 m2·g-1 with a good repeatability (inset in Figure 4a and Table 1), which is much larger than those of Pd/C (21.2 m2·g-1), Pd/G (34.3 m2·g-1) and other Pd-based nanostructures reported recently, e.g. Pd-Pt nanodendrites (48.5 m2·g-1),37 Pd/CNT (37.6-55.3 m2·g-1),15,38 Pd/N-doped graphene (30.2 m2·g-1),39 Pd/polypyrrole-modified graphene (63.6 m2·g1 40

) et al. Meanwhile, the CV curves were also recorded in 0.5 M NaOH solution at 50 mV·s-1 to

examine the catalytic activity of different catalysts in alkaline medium. As seen from the Figure 4b, the obvious cathodic peak between -0.6 and -0.3 V is assigned to the reduction of palladium oxide, which could also provide the ECSA values of the different catalysts.41 Similar to the results from hydrogen adsorption/desorption measurements, Pd/NS-G is found to have a significantly increased ECSA value of 103.6 m2·g-1 in comparison with the Pd/G (42.8 m2·g-1) and Pd/C (32.3 m2·g-1) catalysts. This value is also more competitive than that of reported Pdbased electrocatalysts, including Pd/polypyrrole-modified graphene (41.8 m2·g-1),42 Pd/Co3O4modified carbon (48.0 m2·g-1),43 Pd-Pt/C (51.4 m2·g-1),44 Pd/graphene nanoplate (58.6 m2·g-1),45 Pd/MnO2-modified graphene (82.6 m2·g-1),27 revealing the outstanding catalytic ability of the asprepared Pd/NS-G sample in alkaline medium.

Figure 4. CVs of the Pd/NS-G, Pd/C and Pd/G electrodes in (a) 0.5 M H2SO4 and (b) 0.5 M NaOH at 50 mV·s-1. Insets in (a, b): bar plots showing the corresponding ECSA values for these three catalysts.


12

Page 13 of 23

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


Table 1. Electrochemical behavior on Pd/C, Pd/G and Pd/NS-G electrodes in DFAFC and DMFC condition.

Electrode

Forward peak current density (IF)

ECSA (m2·g-1)

Specific activity (mA·cm-2)

Mass activity (mA·mg-1)

DFAFC

DMFC

DFAFC

DMFC

DFAFC

DMFC

Pd/C

21.2

32.3

4.3

5.4

151.9

190.8

Pd/G

34.3

42.8

7.0

6.6

247.3

233.2

Pd/NS-G

83.4

103.6

14.2

11.3

501.8

399.3

Inspired by its abundant catalytically active regions, Pd/NS-G hybrid was further tested as an anode electrocatalyst for electrooxidation of liquid fuels. Figure 5a presents the CV curves of Pd/NS-G, Pd/C and Pd/G catalysts recorded in 0.5 M H2SO4 with 0.5 M HCOOH solution at 50 mV·s-1. It is discerned that the CV curves display two peaks occurring at around 0.1 and 0.5 V during the anodic scan, which are ascribed to the oxidation of HCOOH (“direct path”) and the oxidation of the intermediate CO (“CO path”), respectively.16 Remarkably, a maximum current density of 14.2 mA·cm-2 is achieved for the Pd/NS-G electrode, which is over 2 times higher than that for Pd/G (7.0 mA·cm-2) and 3 times higher than that for Pd/C (4.3 mA·cm-2), suggesting that the use of NS-G nanosheets as matrixes is conducive to elevating the electrocatalytic kinetics for the formic acid electrooxidation. Moreover, in the electrolyte of 0.5 M NaOH with 0.5 M methanol, all CV curves are characterized by a strong anodic peak and a relatively weak cathodic peak (Figure 5b). The former comes from the oxidation of chemisorbed methanol, while the latter is correlated with the elimination of the intermediate carbonaceous species.14,41 As expected, Pd/NS-G exhibits the highest activity (11.3 mA·cm-2) among the three catalysts, further confirming that the novel structural design of Pd/NS-G is very favorable for


13


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 14 of 23

electrocatalysis of the methanol oxidation. The above data have also been normalized to the Pd loading on each electrode, as shown in Table 1. The mass activities of Pd/NS-G electrode were calculated to be 501.8 and 399.3 mA·mg-1 in acid and alkaline mediums, respectively, significantly outperforming the Pd/G and Pd/C electrodes, which indicates a high Pd utilization efficiency in Pd/NS-G hybrid. In addition, based on the linear sweep voltammetry (LSV) analysis (Figure S4), we can recognize that the potential of Pd/NS-G is apparently lower than those of Pd/G and Pd/C at a given oxidation current density, which discloses that the catalysis processes take place more easily with the help of Pd/NS-G catalyst.

Figure 5. CVs of the Pd/NS-G, Pd/C and Pd/G electrodes in (a) mixture of 0.5 M H2SO4 + 0.5 M HCOOH solution; (b) mixture of 0.5 M NaOH + 1 M methanol solution at a scan rate of 50 mV·s-1. Insets in (a, b): bar plots showing the corresponding forward peak current densities for these three catalysts. On the other hand, the long-term electrocatalytic stability of Pd systems is also key indicator for their practical usage in portable fuel cells. In this respect, chronoamperometry measurements were performed to study the durability of the three types of electrocatalysts. As displayed in Figure 6a, the current densities of all samples gradually decrease with the test progresses owing to the unavoidable accumulation of poisoning by-products on Pd sites. However, the current


14

Page 15 of 23

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


decay rate of the Pd/NS-G electrode was much slower than those of the other two electrodes over the whole time range, verifying its duarable catalytic stability for formic acid oxidation. As for methanol electrooxidation (Figure 6b), Pd/NS-G hybrid also manifested the slowest current degradation and retained the highest catalytic activity among the three as-prepared catalysts, which is consistent with the current variation trend observed in formic acid system. Additionally, the durability of Pd NPs on various supporting materials was also evaluated by the continuous CV tests (Figure 6c-d). It is impressive that the initial activity of Pd/NS-G decreased by only 48.9 % in acid medium after 50 cycles and 58.4 % in alkaline medium after 500 cycles, in contrast to those of Pd/G (56.8 and 90.8 % loss, respectively) and Pd/C (83.7 and 78.9 % loss, respectively) samples. This further proves the superior durability of the newly-developed Pd/NSG catalyst toward electrooxidation of formic acid and methanol. It is worth mentioning that all catalysts investigated in this study could afford more stable properties for methanol oxidation than those for formic acid oxidation, which correlates well with the previously reported results.45


15


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 16 of 23

Figure 6. Chronoamperometric curves recorded in (a) mixture of 0.5 M H2SO4 + 0.5 M HCOOH solution at the working potential of 0.15 V (vs. SCE); (b) mixture of 0.5 M NaOH + 1 M methanol solution at the working potential of 0.2 V (vs. SCE) for the Pd/NS-G, Pd/G and Pd/C electrodes. Cycling stability of the Pd/NS-G, Pd/G and Pd/C electrodes in (c) mixture of 0.5 M H2SO4 + 0.5 M HCOOH solution; (d) mixture of 0.5 M NaOH + 1 M methanol solution. Based on the above analysis, it is safe for us to derive that the combination of Pd and NS-G nanosheets can adequately merge their respective functions to achieve extremely high electrocatalytic efficiency. The outstanding electrochemical properties of Pd/NS-G are attributed to four aspects as follows: (i) the well-dispersive Pd NPs with small sizes in the hybrid offer large amounts of active centers, which enhance the catalytic activity to a great extent; (ii) the graphene sheets with a 2D configuration not only provide high surface areas for an effective mass transfer of reactants, but also act as the unique “superhighways” for rapid electron diffusions in the catalytic system. As indicated in Figure S5, electrochemical impedance spectroscopy (EIS) tests show that the charge-transfer resistances in Pd/NS-G (Rct = 7.4 Ω) and Pd/G (Rct = 39.9 Ω) are apparently lower than that in Pd/C (Rct = 1098.5 Ω), implying the faster electrochemical kinetics and larger populations of the triple-phase boundaries on graphene based electrode surfaces; (iii) the introduction of N and S atoms significantly changes the electronic structures of graphene, which could increase the spin density and activate the neighboring carbon atoms to elevate the catalytic ability of the supporting material; (iv) the coexistence of N and S atoms in graphene networks also ensures a strong interaction between Pd and substrates, and meanwhile play a vital role in the elimination of the carbonaceous intermediates during the catalytic process. As a consequence, our novel Pd/NS-G catalyst is endowed with exceptional


16

Page 17 of 23

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


electrocatalytic properties, holding a great promising application in advanced energy conversion devices. 4. CONCLUSIONS In conclusion, we have presented a combined thermal annealing and hydrothermal method for the deposition of ultrafine Pd NPs on N and S dual-doped graphene sheets. Thanks to the distinct structural advantages, such as large surface area, coexistence of N and S atoms, homogeneous dispersion of small Pd particles and good electrical conductivity, the resulting Pd/NS-G expresses

extraordinary

catalytic

activity

and

unusual

long-term

durability

toward

electrochemical oxidation of formic acid and methanol, more competitive than those of the traditional Pd/G and Pd/C catalysts. In addition to the use in fuel cells, we believed that this kind of nanomaterials should also be attractive candidates for other applications including sensors, photocatalysis and batteries.

ASSOCIATED CONTENT Supporting Information TEM images; particle size distribution; XPS spectra; linear sweep voltammetrys; Nyquist plots of EIS. This material is available free of charge via the Internet at http://pubs.acs.org. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


17


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 18 of 23

This work was financially supported by the Ministry of Education of the People's Republic of China through the Fundamental Research Funds for the Central Universities (No. 2014B13514, 2015B01914), Ministry of Science and Technology of the People's Republic of China through National 973 Plan Project (No. 2015CB057803) and National Natural Science Foundation of China (No. 51301059, 21501091). J. Zhang also acknowledges the financial support from the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL201401SIC). REFERENCES 1.

Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Van Schalkwijk, W. Nanostructured Materials for Advanced Energy Conversion and Storage Devices. Nat. Mater. 2005, 4, 366377.

2.

Wang, X. X.; Yang, J. D.; Yin, H. J.; Song, R.; Tang, Z. Y. "Raisin Bun"-Like Nanocomposites of Palladium Clusters and Porphyrin for Superior Formic Acid Oxidation. Adv. Mater. 2013, 25, 2728-2732.

3.

Zhao, X.; Yin, M.; Ma, L.; Liang, L.; Liu, C. P.; Liao, J. H.; Lu, T. H.; Xing, W. Recent Advances in Catalysts for Direct Methanol Fuel Cells. Energy Environ. Sci. 2011, 4, 27362753.

4.

Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51.

5.

Kakati, N.; Maiti, J.; Lee, S. H.; Jee, S. H.; Viswanathan, B.; Yoon, Y. S. Anode Catalysts for Direct Methanol Fuel Cells in Acidic Media: Do We Have Any Alternative for Pt or PtRu? Chem. Rev. 2014, 114, 12397-12429.

6.

Kuo, P. L.; Hsu, C. H. Stabilization of Embedded Pt Nanoparticles in the Novel Nanostructure Carbon Materials. ACS Appl. Mater. Interfaces 2010, 3, 115-118.

7.

Zhang, S.; Shao, Y. Y.; Yin, G. P.; Lin, Y. H., Electrostatic Self-Assembly of a Pt-aroundAu Nanocomposite with High Activity towards Formic Acid Oxidation. Angew. Chem. Int. Ed. 2010, 49, 2211-2214.


18

Page 19 of 23

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


8.

Huang, H. J.; Wang, X., Recent Progress on Carbon-Based Support Materials for Electrocatalysts of Direct Methanol Fuel Cells. J. Mater. Chem. A 2014, 2, 6266-6291.

9.

Qu, K.; Wu, L.; Ren, J.; Qu, X. Natural DNA-Modified Graphene/Pd Nanoparticles as Highly Active Catalyst for Formic Acid Electro-Oxidation and for the Suzuki Reaction. ACS Appl. Mater. Interfaces 2012, 4, 5001-5009.

10. Huang, Y. X.; Xie, J. F.; Zhang, X.; Xiong, L.; Yu, H. Q. Reduced Graphene Oxide Supported Palladium Nanoparticles via Photoassisted Citrate Reduction for Enhanced Electrocatalytic Activities. ACS Appl. Mater. Interfaces 2014, 6, 15795-15801. 11. Wang, H. N.; Lu, S. F.; Zhang, Y. W.; Lan, F.; Lu, X.; Xiang, Y. Pristine Graphene Dispersion in Solvents and Its Application as A Catalyst Support: A Combined Theoretical and Experimental Study. J. Mater. Chem. A 2015, 3, 6282-6285. 12. Chen, X. M.; Wu, G. H.; Chen, J. M.; Chen, X.; Xie, Z. X.; Wang, X. R. Synthesis of "Clean" and Well-Dispersive Pd Nanoparticles with Excellent Electrocatalytic Property on Graphene Oxide. J. Am. Chem. Soc. 2011, 133, 3693-3695. 13. Qian, H. Y.; Huang, H. J.; Wang, X. Design and Synthesis of Palladium/Graphitic Carbon Nitride/Carbon Black Hybrids as High-Performance Catalysts for Formic Acid and Methanol Electrooxidation. J. Power Sources 2015, 275, 734-741. 14. Qin, Y. H.; Jia, Y. B.; Jiang, Y.; Niu, D. F.; Zhang, X. S.; Zhou, X. G.; Niu, L.; Yuan, W. K. Controllable Synthesis of Carbon Nanofiber Supported Pd Catalyst for Formic Acid Electrooxidation. Int. J. Hydrogen Energy 2012, 37, 7373-7377. 15. Bai, Z. Y.; Guo, Y. M.; Yang, L.; Li, L.; Li, W. J.; Xu, P. L.; Hu, C. G.; Wang, K. Highly Dispersed Pd Nanoparticles Supported on 1,10-Phenanthroline-Functionalized Multi-Walled Carbon Nanotubes for Electrooxidation of Formic Acid. J. Power Sources 2011, 196, 62326237. 16. Cui, Z. M.; Kulesza, P. J.; Li, C. M.; Xing, W.; Jiang, S. P. Pd Nanoparticles Supported on HPMo-PDDA-MWCNT and Their Activity for Formic Acid Oxidation Reaction of Fuel Cells. Int. J. Hydrogen Energy 2011, 36, 8508-8517. 17. Yang, J.; Tian, C. G.; Wang, L.; Fu, H. G. An Effective Strategy for Small-Sized and Highly-Dispersed Palladium Nanoparticles Supported on Graphene with Excellent Performance for Formic Acid Oxidation. J. Mater. Chem. 2011, 21, 3384-3390.


19


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 20 of 23

18. Tsang, C. H. A.; Hui, K. N.; Hui, K. S.; Ren, L. Deposition of Pd/Graphene Aerogel on Nickel Foam as A Binder-Free Electrode for Direct Electro-Oxidation of Methanol and Ethanol. J. Mater. Chem. A 2014, 2, 17986-17993. 19. Lim, E. J.; Kim, Y.; Choi, S. M.; Lee, S.; Noh, Y.; Kim, W. B. Binary PdM Catalysts (M = Ru, Sn, or Ir) Over A Reduced Graphene Oxide Support for Electro-Oxidation of Primary Alcohols (Methanol, Ethanol, 1-Propanol) under Alkaline Conditions. J. Mater. Chem. A 2015, 3, 5491-5500. 20. Liu, M. M.; Zhang, R. Z.; Chen, W. Graphene-Supported Nanoelectrocatalysts for Fuel Cells: Synthesis, Properties, and Applications. Chem. Rev. 2014, 114, 5117-5160. 21. Xia, B. Y.; Yan, Y.; Wang, X.; Lou, X. W. Recent Progress on Graphene-Based Hybrid Electrocatalysts. Mater. Horiz. 2014, 1, 379-399. 22. Zhao, S. L.; Yin, H. J.; Du, L.; Yin, G. P.; Tang, Z. Y.; Liu, S. Q. Three Dimensional NDoped Graphene/PtRu Nanoparticle Hybrids as High Performance Anode for Direct Methanol Fuel Cells. J. Mater. Chem. A 2014, 2, 3719-3724. 23. Zhang, W. Y.; Huang, H. J.; Li, F.; Deng, K. M.; Wang, X. Palladium Nanoparticles Supported on Graphitic Carbon Nitride-Modified Reduced Graphene Oxide as Highly Efficient Catalysts for Formic Acid and Methanol Electrooxidation. J. Mater. Chem. A 2014, 2, 19084-19094. 24. Liang, J.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Sulfur and Nitrogen Dual-Doped Mesoporous Graphene

Electrocatalyst

for

Oxygen

Reduction

with

Synergistically

Enhanced

Performance. Angew. Chem. Int. Ed. 2012, 51, 11496-11500. 25. Ai, W.; Luo, Z. M.; Jiang, J.; Zhu, J. H.; Du, Z. Z.; Fan, Z. X.; Xie, L. H.; Zhang, H.; Huang, W.; Yu, T. Nitrogen and Sulfur Codoped Graphene: Multifunctional Electrode Materials for High-Performance Li-Ion Batteries and Oxygen Reduction Reaction. Adv. Mater. 2014, 26, 6186-6192. 26. Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Layer-By-Layer Assembly of Ultrathin Composite Films From Micron-Sized Graphite Oxide Sheets and Polycations. Chem. Mater. 1999, 11, 771-778. 27. Huang, H. J.; Wang, X. Design and Synthesis of Pd-MnO2 Nanolamella-Graphene Composite as A High-Performance Multifunctional Electrocatalyst towards Formic Acid and Methanol Oxidation. Phys. Chem. Chem. Phys. 2013, 15, 10367-10375.


20

Page 21 of 23

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


28. Huang, X.; Zeng, Z. Y.; Bao, S. Y.; Wang, M. F.; Qi, X. Y.; Fan, Z. X.; Zhang, H. SolutionPhase Epitaxial Growth of Noble Metal Nanostructures on Dispersible Single-Layer Molybdenum Disulfide Nanosheets. Nat. Commun. 2013, 4, 1444. 29. Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401. 30. Wei, D. C.; Liu, Y. Q.; Wang, Y.; Zhang, H. L.; Huang, L. P.; Yu, G. Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties. Nano Lett. 2009, 9, 1752-1758. 31. Yang, S. B.; Zhi, L. J.; Tang, K.; Feng, X. L.; Maier, J.; Mullen, K. Efficient Synthesis of Heteroatom (N or S)-Doped Graphene Based on Ultrathin Graphene Oxide-Porous Silica Sheets for Oxygen Reduction Reactions. Adv. Funct. Mater. 2012, 22, 3634-3640. 32. Sheng, Z. H.; Shao, L.; Chen, J. J.; Bao, W. J.; Wang, F. B.; Xia, X. H. Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and Its Excellent Electrocatalysis. ACS Nano 2011, 5, 4350-4358. 33. Pan, C. T.; Qiu, L. H.; Peng, Y. J.; Yan, F. Facile Synthesis of Nitrogen-Doped Carbon-Pt Nanoparticle

Hybrids

via

Carbonization

of

Poly(Bvim

Br-co-acrylonitrile)

for

Electrocatalytic Oxidation of Methanol. J. Mater. Chem. 2012, 22, 13578-13584. 34. Dong, Y. Q.; Pang, H. C.; Yang, H. B.; Guo, C. X.; Shao, J. W.; Chi, Y. W.; Li, C. M.; Yu, T. Carbon-Based Dots Co-doped with Nitrogen and Sulfur for High Quantum Yield and Excitation-Independent Emission. Angew. Chem. Int. Ed. 2013, 52, 7800-7804. 35. Priolkar, K. R.; Bera, P.; Sarode, P. R.; Hegde, M. S.; Emura, S.; Kumashiro, R.; Lalla, N. P. Formation of Ce1-xPdxO2-δ Solid Solution in Combustion-Synthesized Pd/CeO2 Catalyst: XRD, XPS, and EXAFS Investigation. Chem. Mater. 2002, 14, 2120-2128. 36. Qiu, J. D.; Wang, G. C.; Liang, R. P.; Xia, X. H.; Yu, H. W. Controllable Deposition of Platinum Nanoparticles on Graphene as An Electrocatalyst for Direct Methanol Fuel Cells. J. Phys. Chem. C 2011, 115, 15639-15645. 37. Lim, B.; Jiang, M. J.; Camargo, P. H. C.; Cho, E. C.; Tao, J.; Lu, X. M.; Zhu, Y. M.; Xia, Y. N. Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science 2009, 324, 1302-1305.


21


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 22 of 23

38. Nassr, A.; Quetschke, A.; Koslowski, E.; Bron, M. Electrocatalytic oxidation of formic acid on Pd/MWCNTs nanocatalysts prepared by the polyol method. Electrochim. Acta 2013, 102, 202-211. 39. Wu, P.; Huang, Y. Y.; Zhou, L. Q.; Wang, Y. B.; Bu, Y. K.; Yao, J. N. Nitrogen-Doped Graphene Supported Highly Dispersed Palladium-Lead Nanoparticles for Synergetic Enhancement of Ethanol Electrooxidation in Alkaline Medium. Electrochim. Acta 2015, 152, 68-74. 40. Yang, S. D.; Shen, C. M.; Liang, Y. Y.; Tong, H.; He, W.; Shi, X. Z.; Zhang, X. G.; Gao, H. J. Graphene Nanosheets-Polypyrrole Hybrid Material as A Highly Active Catalyst Support for Formic Acid Electro-Oxidation. Nanoscale 2011, 3, 3277-3284. 41. Singh, R. N.; Singh, A.; Anindita. Electrocatalytic Activity of Binary and Ternary Composite Films of Pd, MWCNT and Ni, Part II: Methanol Electrooxidation in 1 M KOH. Int. J. Hydrogen Energy 2009, 34, 2052-2057. 42. Zhao, Y. C.; Zhan, L.; Tian, J. N.; Nie, S. L.; Ning, Z., Enhanced Electrocatalytic Oxidation of Methanol on Pd/Polypyrrole-Graphene in Alkaline Medium. Electrochim. Acta 2011, 56, 1967-1972. 43. Xu, C. W.; Tian, Z. Q.; Shen, P. K.; Jiang, S. P. Oxide (CeO2, NiO, Co3O4and Mn3O4)Promoted Pd/C Electrocatalysts for Alcohol Electrooxidation in Alkaline Media. Electrochim. Acta 2008, 53, 2610-2618. 44. Mahapatra, S. S.; Dutta, A.; Datta, J. Temperature Dependence on Methanol Oxidation and Product Formation on Pt and Pd Modified Pt Electrodes in Alkaline Medium. Int. J. Hydrogen Energy 2011, 36, 14873-14883. 45. Huang, H. J.; Wang, X. Pd Nanoparticles Supported on Low-Defect Graphene Sheets: for Use as High-Performance Electrocatalysts for Formic Acid and Methanol Oxidation. J. Mater. Chem. 2012, 22, 22533-22541.


22

Page 23 of 23

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


Table of Contents Graphic

A combined thermal annealing and solvothermal approach is developed to homogeneously disperse nanosized Pd particles onto the surface of nitrogen and sulfur dual-doped graphene (NSG) nanosheets. Because of the attractive structural features, such as large surface area, uniform nitrogen and sulfur distribution, extremely small particle size and good electrical conductivity, the resulting Pd/NS-G hybrid possesses superior electrocatalytic ability toward both formic acid and methanol oxidation reactions.


23

Palladium Nanoparticles Supported on Nitrogen ... - ACS Publications

Recommend Documents