Three-Dimensional Boron- and Nitrogen-Codoped Graphene Aerogel

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Three-Dimensional Boron- and Nitrogen-Codoped Graphene Aerogel Supported Pt Nanoparticles as Highly Active Electrocatalysts for Methanol Oxidation Reaction Miaomiao Li, Quanguo Jiang, Minmin Yan, Yujie Wei, Jianbo Zong, Jianfeng Zhang, Yuping Wu, and Huajie Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00425 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Sustainable Chemistry & Engineering

Three-Dimensional Boron- and Nitrogen-Codoped Graphene Aerogel Supported Pt Nanoparticles as Highly Active Electrocatalysts for Methanol Oxidation Reaction Miaomiao Li,# Quanguo Jiang,# Minmin Yan, Yujie Wei, Jianbo Zong, Jianfeng Zhang, Yuping Wu, and Huajie Huang* College of Mechanics and Materials, Hohai University, Nanjing 210098, China *Corresponding Author. E-mail: [email protected]

Full Mailing Address: College of Mechanics and Materials, Hohai University, No. 1 Xikang Road, Nanjing, Jiangsu Province, P. R. China 210098 (Miaomiao Li, Quanguo Jiang, Minmin Yan, Yujie Wei, Jianbo Zong, Jianfeng Zhang, Yuping Wu, and Huajie Huang)

ACS Paragon Plus Environment

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ABSTRACT Although direct methanol fuel cells have long been regarded as promising “green” power generators for portable electronics, their further development has been largely hampered by the high cost as well as the insufficient activity of the current Pt-based anode catalysts. Herein, we present a bottom-up approach to the large-scale fabrication of three-dimensional (3D) hybrid materials made from ultrafine Pt nanoparticles as well as B- and N-codoped graphene aerogels (BN-GA). The resulting Pt/BN-GA catalyst possesses 3D cross-linked porous networks, large specific surface areas, numerous B and N active sites, homogeneous Pt dispersion, and good electrical conductivity, which are all desirable for the anode catalytic system in direct methanol fuel cells. As a consequence, outstanding electrochemical properties in terms of high catalytic activity, reliable long-term durability, and strong anti-toxic ability are achieved for the Pt/BNGA catalyst, significantly superior to those for the conventional Pt/carbon black, Pt/graphene, and Pt/graphene aerogel catalysts. Density functional theory (DFT) calculations further reveal that the dramatically enhanced catalytic performance is attributed to the strong interaction between Pt and the BN-GA support, which generates remarkable synergetic coupling effects that enable rapid kinetics for methanol oxidation reaction.

KEYWORDS: Platinum particles, Graphene aerogel, Codoping, Electrocatalyst, Fuel cells


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INTRODUCTION With the increasing severity of global energy crisis and environmental pollution, fuel cells as “green” energy-conversion systems have gained considerable and persistent attentions.1,2 When compared with the conventional heat engines, fuel cells offer remarkably higher energy utilization efficiency but emit much less pollution, which have huge application potentials in many fields, such as aeronautics and astronautics, electric vehicles, and portable electronic devices.3,4 Among various types of fuel cells, direct methanol fuel cells (DMFCs) possesses a series of distinctive advantages, including simple power-system integration, wide operating temperature ranges, as well as easy storage and transport of liquid methanol.5,6 Nevertheless, some challenging issues, such as the sluggish methanol oxidation kinetics and the toxic effects caused by the byproducts (mainly CO), are still the principal barriers to the large-scale commercialization of DMFC technology.7-9 To circumvent the above issues, one effective strategy is to design and construct advanced catalyst materials, which can largely accelerate the methanol electrooxidation process and at the same time restrain the formation of CO species.10-14 State-of-the-art anode catalysts commonly contain Pt nanoparticles (NPs) deposited on various carbonaceous supporting materials, including carbon black,15,16 carbon nanofiber,17 carbon nanotube,18-20 graphene,21-24 and heteroatom-doped carbon.25-27 In particular, heteroatom doping into the graphene structures have recently become a hot topic because it substantially improves the electronic structures and chemical reactivity of graphene, thus leading to much enhanced catalytic performance.28,29 Moreover, the presence of heteroatoms (boron, nitrogen, sulfur etc.) in the graphene network could effectively strengthen the noble metal-support interaction and meanwhile guarantee a uniform Pt dispersion, which are conducive to realizing a high Pt utilization.30,31 In addition,


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recent theoretical and experimental studies have verified that the codoping of graphene with two different elements can generate unique electronic structures with strong concerted effects between the dopants, making the resultant materials to be endowed with extra electrocatalytic properties.32-35 On the other hand, graphene-based materials with large external surfaces usually tend to reaggregate or re-stack owing to their van der Waals forces, which would cause the disappearance of the structural advantages related to the separated graphene nanosheets.36 Besides, during the above process, a large number of catalytically active sites would be covered and cannot directly contact with the reactants, resulting in an obvious deterioration in electrochemical activity.37,38 Within this context, tremendous efforts have been devoted to developing porous graphene structures that could prevent the carbon sheets from agglomeration.39-41 Among them, threedimensional (3D) graphene aerogel (GA) with macroporous features has emerged as a newgeneration supporting material, which can significantly alleviate the re-aggregation or restacking of graphene and simultaneously promote the deposition of noble metal nanoparticles.4244

Although some progress in GA-based electrocatalysts has been made, there is no report of the

controlled growth of metal nanoparticles on 3D codoped graphene frameworks as fuel cell catalysts. If that can be accomplished, then it is possible to achieve exceptional electrocatalytic performance because each component (noble metal NPs, heteroatoms and graphene) in the hybrid systems may fully implement its role in catalytic reactions. Herein, we present a facile and cost-effective method to the large-scale fabrication of ultrafine Pt NPs anchored onto 3D B- and N-codoped graphene aerogel (Pt/BN-GA) as anode catalysts for methanol oxidation reaction. As illustrated in Figure 1 and S1, graphene oxide synthesized by a modified Hummers' method was first ultrasonically dissolved in water to reach a concentration


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up to 2 mg mL-1.45 Afterwards, NH4BF4 as the B and N sources was introduced into above GO dispersion under vigorous magnetic stirring, and the obtained mixture was transferred into a 50 mL autoclave and then heated at 180 °C for 24 h. During this hydrothermal process, the GO sheets with abundant oxygenated functional groups were self-assembled to form a 3D monolithic graphene hydrogel, while the B and N atoms could gradually enter into the carbon lattices. Subsequently, the as-derived 3D B- and N-codoped graphene were able to provide numerous accessible active planes as well as sufficient anchoring sites, which ensure a homogeneous dispersion of Pt NPs in the hybrid system. Finally, the sample was further freeze-dried to maintain its porous configuration, giving rise to the 3D Pt/BN-GA architecture. Owing to its unique structural features, such as large surface area, 3D cross-linked porous network, abundance B and N active sites, uniform Pt dispersion, and high electrical conductivity, the resulting hybrid expresses outstanding electrocatalytic properties toward methanol oxidation reaction, superior to those for conventional Pt/carbon black (Pt/C), Pt/graphene (Pt/G), and Pt/undoped graphene aerogel (Pt/GA) catalysts.

Figure 1. Illustration of the synthetic processes for the 3D Pt/BN-GA architecture, which includes: (1) formation of 3D graphene hydrogel and codoping of B and N atoms in the carbon


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networks by solvothermal reaction; (2) controllable deposition of Pt NPs on B- and N-codoped graphene surface to obtain 3D Pt/BN-GA catalyst. EXPERIMENTAL SECTION Synthesis of 3D B- and N-codoped graphene aerogel supported Pt catalysts. GO nanosheets were produced from commercial graphite powders by a modified Hummers’ method.45 As to the incorporation of B and N atoms into the graphene sheets, 4 g of ammonium fluoroborate (NH4BF4) was added into 40 mL of as-synthesized GO solution (2 mg mL-1) with magnetic stirring for 20 minutes. Then, the resulting mixture was transferred into a 50 mL Telfon-lined stainless steel autoclave and heated at 180 °C for 20 h. During the hydrothermal process, the oxygen-containing groups on GO plane interconnected to afford 3D structure, while the B and N atoms could enter into the carbon networks, leading to the formation of B- and Ncodoped graphene hydrogels. Afterwards, the samples were converted to B- and N-codoped graphene aerogels (BN-GAs) by a freeze-drying approach to prevent the re-stacking of carbon sheets and maintain their 3D porous configuration. Finally, 20 mg of the obtained BN-GAs were exposed to an ethylene glycol solution containing 10.62 mg of K2PtCl4 (Alfa Aesar) and further heated at 120 °C for 10 h, giving birth to the 3D Pt/BN-GA hybrid. For comparison, Pt catalysts supported on conventional carbon black (Vulcan XC-72R), graphene, and undoped graphene aerogel were also prepared by a similar method, which were labled as Pt/C, Pt/G, and Pt/GA, respectively. The platinum contents for all above samples were kept at 20.0 wt%. Characterizations. The morphology, composition and structure of the 3D Pt/BN-GA hybrid were systematically studied by Field-emission scanning electron microscopy (FE-SEM, Sigma ZEISS), tansmission electron microscopy (TEM, JEOL JEM-2100F), Powder X-ray diffraction (XRD, Bruker D8 Advance diffractometer), Raman spectroscopy (Thermo DXR 532), and X-ray


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photoelectron

spectroscopy

(XPS,

PHI

Quantera

X-ray

photoelectron

spectrometer)

measurements. Nitrogen adsorption isotherms were recorded at 77 K by using a Micromeritics ASAP 2020 Plus system. Electrochemical measurements. All electrochemical tests were conducted on a CHI 760E electrochemical workstation using a standard three-electrode system, with a Pt wire serving as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and a glassy carbon disk (GC, 3 mm in diameter) coated with electrocatalysts as the working electrode. To fabricate the working electrode, 4 mg of catalyst powders were dispersed in a mixed solution (950 µL of water, 950 µL of ethanol, and 100 µL of 5% Nafion) by sonication for 1 h. Subsequently, 5 µL of the above suspension was carefully dropped onto the GC electrode surface and directly dried in air, and the metal Pt loadings on all electrodes were kept at 0.028 mg cm-2. The electrocatalytic performance of various catalysts was then investigated by cyclic voltammogram (CV), chronopotentiometry, chronoamperometry, and AC impedance techniques in N2-purged 0.5 M H2SO4 and 1 M methanol solution. RESULTS AND DISCUSSION The structure and morphology of as-synthesized 3D Pt/BN-GA catalyst were initially examined by means of FE-SEM and TEM. As presented in Figure 2A and Figure S2, the product shows a well-defined and interconnected 3D configuration with continuous pores ranging from several micrometers to tens of micrometers. On close inspection, small-sized Pt NPs were found to be well dispersed on both sides of the thin BN-G sheets (Figure 2B-C), evidencing a strong interaction between the metal and doped supports. The average diameter of these Pt particles was determined to be only ~2.5 nm, obviously smaller than those on traditional undoped graphene and carbon black materials (6.5-7.2 nm, Figure S3). This should be attributed not only to the


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multilevel accessible surface areas of 3D BN-GAs, but also to the large presence of B and N atoms in the hybrid system, which could efficiently optimize the growth behaviors of Pt NPs on graphene surface.26,46 High-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) images further disclose the typical lattice fringes for both Pt nanocrystals and NB-G layers (Figure 2D-E). The interplanar spacings for Pt and NB-G were determined to be 0.22 and 0.34 nm, corresponding to the exposed Pt(111) and C(002) facets, respectively. In addition, the element mapping and energy-dispersive X-ray (EDX) analysis identifies that the 3D Pt/BN-GA catalyst is made up of C, B, N, and Pt components (Figure 2F-I and Figure S4), and these four elements are distributed homogeneously throughout the whole sheets.

Figure 2. FE-SEM and TEM analysis of 3D Pt/BN-GA catalyst. Typical (A and B) FE-SEM and (C) TEM images show that small Pt NPs are dispersed uniformly on 3D porous BN-GA supports. Inset in (C): Pt particle size distribution of Pt/BN-GA. (D) HRTEM image and (E) the selected area electron diffraction (SAED) pattern dislose the lattice fringes for both Pt NPs and BN-GA. The corresponding elemental mapping images of (F) C, (G) B, (H) N, and (F) Pt taken


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from the region marked in (A), revealing the homogeneous distribution of these four elements in Pt/BN-GA hybrid. XRD and Raman spectroscopy were then used to investigate the crystal and chemical structures of the Pt/BN-GA catalyst. As shown in Figure 3A, the sharp diffraction peak located at around 10.5° was observed in the GO pattern, which originated from the (002) reflections of the stacked GO layers. As for the Pt/GA and Pt/BN-GA materials, the above peak shifted to a higher angle of about 25.0°, suggesting that the transformation of GO to graphene occurred during the solvothermal reaction. Besides, other three characteristic peaks centered at ~39.7°, 46.3°, and 67.7° are indexed to the (111), (200), and (220) facets of the Pt crystals with face-center cubic (fcc) structure, respectively (JCPDS 87-0646). Notably, the intensities of these peaks in the Pt/BN-GA pattern are much higher than those in the Pt/GA pattern, implying that the evenly distributed B and N atoms in the carbon skeletons are beneficial to the growth and crystallization of supported Pt NPs. According to the Scherrer equation, the average Pt particle size for Pt/BNGA catalyst was estimated to be ~2.7 nm based on the Pt(200) peak, which is in good agreement with the TEM results. Figure 3B shows the Raman spectra of GO, Pt/GA and Pt/BN-GA, which are characterized by the well-known D and G bands appearing at ~1335 and 1584 cm-1, respectively. The former can be ascribed to the disordered defective graphite, while the latter is related to the ordered crystal graphite.47 Remarkably, the D/G intensity ratio (ID/IG) of Pt/BN-GA (1.19) is higher than those of GO (0.84) and Pt/GA (1.00), attesting its higher defective nature because of the B and N codoping. In addition, N2 adsorption–desorption measurement reveals that a high Brunauer–Emmett–Teller (BET) surface area of up to 369.2 m2 g-1 is achieved for the 3D porous Pt/BN-GA catalyst (Figure 3C-D), consistent with the values reported for high-quality 3D graphene-based materials.36,39


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Figure 3. Structural characteristics of 3D Pt/BN-GA catalyst. (A) XRD patterns and of GO, Pt/GA, and Pt/BN-GA, suggesting the efficient loading of Pt nanocrystals on BN-G sheets. (B) Raman spectra of GO, Pt/GA, and Pt/BN-GA. The much defective nature of Pt/BN-GA implies the successful doping of B and N atoms in the graphene structure. (C) N2 adsorption–desorption isotherm and (D) pore size distribution of Pt/BN-GA catalyst demonstrate its 3D porous configuration with a BET surface area of up to 369.2 m2 g-1. We further characterized the detailed elemental composition of the Pt/BN-GA catalyst by using X-ray photoelectron spectroscopy (XPS). The elemental analysis validates the co-existence of B, N, C, and Pt in the hybrid, and the corresponding B and N contents are determined to be 2.0 and 3.3 at%, which are similar to those of single-element-doped graphene materials.48,49 The complex B 1s spectrum manifests that there are three types of B bonding configurations (Figure 4A), consisting of B-C3/B-N and B-C-O at the binding energies of 191.8 and 192.8 eV,


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Figure 4. XPS analysis of the Pt/BN-GA catalyst. High-resolution (A) B 1s, (B) N1s, (C) C1s, and (D) Pt 4f XPS spectra of Pt/BN-GA, confirming that B, N, C, and Pt components co-exist in the Pt/BN-GA hybrid. respectively.39 Meanwhile, the high-resolution N 1s spectrum can be resolved into four peaks located at 398.3, 399.2, 399.8 and 401.2 eV (Figure 4B), which is linked to pyridinic N, C-N-B, pyrrolic N, and graphitic N, respectively.50 According to the previous theoretical and experimental study, the introduction of B-C, pyridinic N and pyrrolic N functionalities are able to facilitate the deposition of noble metal NPs on carbon matrixes,18,51 thus the high proportion of these species in Pt/BN-GA could render to a uniform Pt dispersion. Figure 4C presents the highresolution C 1s spectrum of Pt/BN-GA, which was deconvoluted into four different signals, belonging to sp2 C–C (284.6 eV), sp2 C–N/C-O (285.4 eV), sp3 C–N (287.6 eV) and C=O/C-B


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(289.9 eV).27,39 In comparison with the C 1s spectrum of GO (Figure S5), the much weaker peak intensities for the oxygen-containing groups in the case of Pt/BN-GA unravel that GO can be successfully converted to graphene during the solvothermal process. Furthermore, the Pt 4f spectrum in Figure 4D shows two pairs of doublets: the two intensive peaks at 71.4 and 74.7 eV are due to metallic Pt, while the other two peaks at 72.0 and 76.0 eV arise from a small amount of PtO in the hybrid. The electrocatalytic performance of the 3D Pt/BN-GA catalyst was first evaluated by cyclic voltammograms (CVs) in N2-saturated 0.5 M H2SO4 solution. As displayed in Figure 5A, the hydrogen adsorption/desorption peaks are clearly observed in all CV curves between -0.2 to 0.1 V, which can be used to calculate the electrochemically active surface areas (ECSAs) for different catalysts. Strikingly, the Pt/BN-GA catalyst was found to have an ECSA value of up to 106.0 m2 g-1, almost 1.9, 2.2, and 4.7 times larger than that for Pt/GA, Pt/G, and Pt/C catalysts, respectively (Figure 5C), indicating that the Pt NPs dispersed on 3D BN-GA support are electrochemically more accessible. Methanol oxidation tests were then performed in 0.5 M H2SO4 and 1 M methanol solution. It can be seen from Figure 5B and 5D that both high mass activity (1184.5 mA mg-1) and high ECSA-normalized specific activity (1.12 mA cm-2) are achieved on Pt/BN-GA electrode, which are significantly superior to those on the reference electrodes (126.2-406.7 mA mg-1 and 0.35-0.69 mA cm-2, respectively). Meanwhile, the electrocatalytic properties of our Pt/BN-GA catalyst are also more competitive than those of recent state of-the-art Pt-based electrocatalysts, such as Pt/CNT,52,53 Pt/graphene,21,23,24,54 Pt/doped graphene,27,29-31,46 Pt/porous carbon,44,55-57 Pt nanowires,11,58,59 and Pt-based bimetallic catalysts60-62 (Table 1). Additionally, linear-sweep voltammogram (LSV) measurements reveal that the electrode potential for Pt/BN-GA is apparently lower than that for other materials at a


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given current density (Figure S6), implying that the use of Pt/BN-GA catalyst enables the methanol oxidation reaction to take place more easily.

Figure 5. Electrocatalytic activity of the Pt/BN-GA catalyst. Cyclic voltammograms of Pt/BNGA, Pt/GA, Pt/G, and Pt/C catalysts in (A) 0.5 M H2SO4 and (B) 0.5 M H2SO4 with 1 M methanol solution at 20 mV s-1. (C) Specific ECSA values and (D) mass activities as well as specific activities (normalized by ECSA) for these four catalysts, proving the highest electrocatalytic activity of Pt/BN-GA. Table 1. Comparison of methanol oxidation activity for the 3D Pt/BN-GA hybrid and recent state-of-the-art Pt-based catalysts. Catalyst

ECSA

Mass activity

Scan rate

(m2 g-1)

(mA mg-1)

(mV s-1)

3D Pt/BN-GA

106.0

1184.5

20

Pt/[BMIM]BF4/CNT

N.A.

155.0

50


Electrolyte

Reference

0.5 M H2SO4 and 1 M CH3OH 0.5 M H2SO4 and 1 M CH3OH

This work 52

13


Pt/imidazolium-salt ionic liquid/CNT Pt/low-defect graphene

67.6

~410.0

50

63.0

203.8

20

Pt/graphene

101.3

333.3

50

Pt/thermal exfoliated graphene Pt/CNT/graphene

51.0

~300.0

50

95.6

617.9

20

Pt/N-doped graphene

N.A.

~400.0

200

Pt/N-doped graphene nanoribbon PtAu/N-doped graphene

64.6

~390.0

20

60.9

417.0

50

Pt/B-doped graphene

58.8

~410.0

50

PtRu/N-doped CNT/ graphene 3D Pt/C3N4/graphene

N.A.

500.5

10

69.0

612.8

20

Pt/mesoporous carbon

N.A.

~450.0

20

Pt/macroporous carbon

N.A.

81.6

50

Pt/N-doped porous carbon PtCo nanowires

24.6

343.0

50

52.1

1020.0

50

FePtPd nanowires

N.A.

488.7

50

AuPtCu nanowires

N.A.

~500.0

50

PtPd dendrites

N.A.

490.0

50

dendrites/

81.6

647.2

50

dendrites/

100.8

365.0

50

PtPd graphene PtAu graphene

0.5 M H2SO4 and 0.5 M CH3OH 1 M H2SO4 and 2 M CH3OH 0.5 M H2SO4 and 0.5 M CH3OH 0.5 M H2SO4 and 2 M CH3OH 1 M H2SO4 and 2 M CH3OH 0.5 M H2SO4 and 1 M CH3OH 1 M H2SO4 and 2 M CH3OH 0.5 M H2SO4 and 0.5 M CH3OH 0.5 M H2SO4 and 0.5 M CH3OH 0.5 M H2SO4 and 1 M CH3OH 1 M H2SO4 and 2 M CH3OH 0.5 M H2SO4 and 1 M CH3OH 0.5 M H2SO4 and 0.5 M CH3OH 0.5 M H2SO4 and 1 M CH3OH 0.1 M HClO4 and 0.2 M CH3OH 0.1 M HClO4 and 0.2 M CH3OH 0.1 M HClO4 and 1 M CH3OH 0.5 M H2SO4 and 1 M CH3OH 0.5 M H2SO4 and 1 M CH3OH 0.5 M H2SO4 and 1 M CH3OH

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To get more insights into the practicability and reliability of the Pt/BN-GA catalyst, we also employed chronoamperometric technique to study its long-term durability. As seen from Figure 6A, owing to the accumulation of intermediate carbonaceous species as well as the unavoidable


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generation of Pt oxides, the oxidation currents are found to gradually decrease for all catalytic systems as time goes on. Impressively, the initial current of Pt/BN-GA catalyst only decreased by ~39% after 2000 s, which is lower than that of Pt/GA (~78% loss), Pt/G (~93% loss), Pt/C (~95% loss), and other previously reported Pt nanostructures (~50-80% loss after 2000 s),12,16,44,52,60 giving an exceptional electrocatalytic stability. Meanwhile, FE-SEM analysis shows that the microstructure and morphology of Pt/BN-GA catalyst were almost unchanged after the durability test (Figure S7), mainly due to the firm interfacial adhesion between Pt and BN-G sheets. Moreover, the anti-toxic abilities of different catalysts were further investigated by chronopotentiometric tests. At a constant current density, the electrocatalytic activity of the catalyst would gradually decay due to the generation of CO species, and thus the electrode potential needs to be increased to a higher level where more H2O could be decomposed. As depicted in Figure 6B, Pt/BN-GA can keep a low voltage platform for the longest time (~900 s) among these four catalysts, which reflects its highest poison tolerance. This indicates that the 3D porous structure and the incorporation of B and N atoms are very favorable for the removal of CO species. It is worth mentioning that the B and N components may play different roles in eliminating the byproducts on Pt sites. The B atoms could weaken the adsorption energy between Pt and CO-like species,35 while N atoms with larger electron affinity could provide abundance OH sources to oxidize COads.29 In addition, comparison of the AC impedance spectra shows that the charge-transfer resistance for Pt/BN-GA (3.6 Ω) is significantly lower than that for Pt/G (5.3 Ω) and Pt/C (2718.0 Ω) catalyst (Figure 6C-D and S8), confirming that the existence of 3D B and N codoped graphene networks could allow a higher electrical conductivity during the methanol oxidation process.


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Figure 6. Long-term stability, poison tolerance and electrical conductivity of the Pt/BN-GA catalyst. (A) Chronoamperometric (at 0.5 V) and (C) chronopotentiometric curves of Pt/BN-GA, Pt/GA, Pt/G, and Pt/C catalysts in 0.5 M H2SO4 with 1 M methanol solution, indicating that Pt/BN-GA has the best electrochemical stability and strongest anti-toxic ability. The applied currents for chronopotentiometric tests were the same as those at 0.5 V on the forward scan of the corresponding CVs. (C) AC impedance spectra of Pt/BN-GA, Pt/G, and Pt/C catalysts in 0.5 M H2SO4 with 1 M methanol solution measured at open circuit potentials. The amplitude of alternate voltage is 10 mV. (D) The local enlargement of Pt/BN-GA and Pt/G spectra. To shed light on the unusual electrocatalytic ability of the Pt/BN-GA catalyst, spin-polarized density functional theory (DFT) calculations were conducted to investigate the stability of a single Pt atom on the intrinsic, N-doped and B, N-codoped graphene surfaces. Figure S9 shows the most stable configuration for Pt atom adsorbed on the intrinsic graphene, where the Pt atom


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is located at the bridge site between two adjacent C atoms with an adsorption energy Ead = -1.83 eV. For graphene doped with graphite-like N (Ngraphite), the Pt atom prefers to bind with the C atoms neighboring the N site with an adsorption energy Ead = -2.33 eV as shown in Figure 7A. Since the N atom in Ngraphite-doped graphene uses three valence electrons to form σ bands, one valence electron to form a π bond, and places the remaining electron in the higher energy π* state, which activates the neighboring C atoms and enhances interaction between neighboring C and Pt atoms. Considering the codoping of Bgraphite and Ngraphite, the formation of B-N pair makes graphene even more stable than the pristine graphene (Figure 7D), where the Pt atom is located at the bridge site between the B and C atoms with an adsorption energy Ead = -1.81 eV. For the B-N pair in Figure 7D, lone-pair electrons from N atom are neutralized by the vacant orbital of B atom, resulting in lower interaction between C and Pt atoms than that of Ngraphite-doped graphene. Nevertheless, owing to its low content in the BN-GA frameworks (Figure 4B), the graphitic N atom should not have significant impact on the growth behavior of Pt NPs. The stability of Pt on graphene doped with pyridine-like N (Npyridine) was then studied in Figure 7B. Contrary to the Ngraphite-doped graphene, the Pt atom is bound with N atom in Npyridinedoped graphene with an adsorption energy Ead = -2.62 eV. The Npyridine atom uses two valence electrons to form σ bonds, one to form a π bond, and the remaining two valence electrons to form a π-like lone pair state. Hybridization between the N-π orbital and Pt-d orbital enhances the adsorption energy of Pt on Npyridine-doped graphene compared to that in Ngraphite-doped graphene. The most stable configuration for codoping of Bgraphite and Npyridine in graphene is shown in Figure 7E, where Pt atom is bound with the Npyridine atom with adsorption energy Ead = -2.99 eV. One of the π-like lone pair electrons from Npyridine atom are neutralized by the vacant orbital of B atom, and the remaining π-like electron enhances the adsorption energy of Pt on BNpyridine-


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codoped graphene compared to that on Npyridine-doped graphene. Similarly, in the case of Bgraphite and Npyrrole-codoped graphene, the adsorption energy of Pt atom is increased to Ead = -3.40 eV.

Figure 7. The relaxed atomic structures of a Pt atom adsorbed on (A) graphite-like N-doped graphene, (B) pyridine-like N-doped graphene, (C) pyrrole-like N-doped graphene, (D) graphitelike BN-codoped graphene, (E) pyridine-like BN-codoped graphene, and (F) pyrrole-like BNcodoped graphene. The gray, blue, pink, green and white balls represent C, N, B, Pt and H atoms, respectively. The deformation charge density near the Pt atom for each adsorption configurations with the same isosurface value (0.05 e/bohr3) is also shown in the down panels, where the red and yellow isosurfaces correspond to the increase in the number of electrons and the depletion zone, respectively.


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The deformation charge density near the Pt atom for each adsorption configurations is also shown in the down panels in Figure 7, where electrons accumulate on the Pt_dxz +dyz orbital and deplete on the Pt_dz2 orbital. The number of electrons on the Pt_dz2 orbital decreases in Figure 7A than that in Figure 7D, suggesting a weak interaction between Pt and C atoms in graphene codoped with Bgraphite and Ngraphite. In sharp contrast, the electrons accumulation on the Pt_dxz +dyz orbital in Figure 7E and 7F decrease obviously than that in Figure 7B and 7C, respectively, further demonstrating the much larger adsorption energies of Pt on BNpyridine- and BNpyrrolecodoped graphene networks. Therefore, it is safe to derive that the high percentages of pyridinic N and pyrrolic N in the BN-GA architecture could lead to a stronger interaction between metal Pt and the support, which effectively restrain the overgrowth of Pt NPs and simultaneously fully exert the synergetic coupling effects. CONCLUSIONS In summary, we have demonstrated the successful construction of 3D B and N codoped graphene aerogel-supported Pt catalysts by a simple and scalable strategy. Thanks to its unique structural features, including interconnected macro- and mesoporous frameworks, large specific surface areas, uniform B and N distribution, small Pt particle sizes, as well as excellent electrical conductivity, the resulting Pt/BN-GA hybrid exhibits extraordinary electrocatalytic activity, reliable long-term stability, and high poison tolerance for methanol oxidation reaction, far outperforming the conventional Pt/GA, Pt/G, and Pt/C catalysts. In principle, this bottom-up synthetic approach can be easily extended to the fabrication of other 3D codoped nanocarbonbased materials for a broad range of applications, such as battery, supercapacitor, photocatalysis, and sensor.


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ASSOCIATED CONTENT Supporting Information. Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *H. Huang. E-mail: [email protected]. Author Contributions #

M. Li and Q. Jiang contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by Natural Science Foundation of Jiangsu Province (No. BK20160871), China Postdoctoral Science Foundation (No. 2015M580387 and 2016T90414), and Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1601026A). REFERENCES (1) Debe, M. K. Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012, 486, 43-51. (2) 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 Pt–Ru? Chem. Rev. 2014, 114, 12397-12429.


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Table of Contents Graphic

Synopsis: A bottom-up approach is developed to the fabrication of B- and N-codoped graphene aerogel supported Pt toward methanol oxidation.


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Three-Dimensional Boron- and Nitrogen-Codoped Graphene Aerogel

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