Exceptional Electrocatalytic Activity and Selectivity of Platinum

Subscriber access provided by Kaohsiung Medical University

Energy, Environmental, and Catalysis Applications

Exceptional Electrocatalytic Activity and Selectivity of Platinum@NitrogenDoped Mesoporous Carbon Nanospheres for Alcohol Oxidation Lirui Nan, and Wenbo Yue ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06347 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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 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 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.

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 24 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

Exceptional Electrocatalytic Activity and Selectivity of Platinum@Nitrogen-Doped Mesoporous Carbon Nanospheres for Alcohol Oxidation Lirui Nan, and Wenbo Yue* Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China

Abstract: Porous carbon materials have attracted considerable attention for their various applications such as catalyst supports for fuel cells. However, few studies focus on the effect of carbon pore structure on different alcohols electrooxidation. In this work, platinum@nitrogen-doped carbon nanospheres with tailored mesopores (Pt@NMCs) are fabricated and exhibit outstanding electrocatalytic activity and durability for alcohol oxidation because of the structural advantages such as adjustable mesopores, Ndoped carbon and embedded catalysts. More importantly, the pore size of NMCs (or called the size of the windows connecting the neighboring spherical cavities), which can be tuned simply by adjusting the diameter of colloidal silica nanospheres, has a great effect on the electrocatalytic activity and selectivity of Pt catalysts towards oxidation of alcohols (methanol, ethanol and n-propanol). Accordingly, we can adopt optimal Pt@NMCs with appropriate pore size based on different requirements and applications. Keywords: mesoporous carbon nanospheres, nitrogen-doping, platinum, alcohol oxidation, shape selective catalysis 1. Introduction Direct alcohol fuel cells (DAFCs) based on methanol as fuel have attracted tremendous attention in 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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 24

recent years due to their high energy conversion efficiency, high energy density, low operating temperatures and environmental friendliness.1,2 The performance of fuel cells has a great relationship with the electrode catalysts. So far, Pt and Pt-based catalysts still display the highest electrocatalytic activity towards methanol oxidation.3-5 However, high cost and poor tolerance to carbon monoxide (CO) poisoning impede the practical application of Pt-based electrodes in DMFCs.6,7 Several strategies have been proposed to improve the electrocatalytic performance of Pt catalysts as well as catalyst utilization, such as fabrication of nanosized or nanostructured Pt-based catalysts to enlarge their active surface area,8,9 addition of catalyst promoters to improve the poison tolerance of Pt catalysts,10 or integration of Pt catalysts with carbon materials to enhance the electrical conductivity of electrodes.11 In particular, the component and structure design of carbon support has a great effect on the electrocatalytic performance of Pt catalysts. For instance, porous carbon can provide large surface area to highly disperse catalysts, and pore channels to facilitate mass diffusion.12-14 Nitrogen-doping can enhance the electric conductivity of carbon support and the dispersivity of Pt catalysts because of the synergistic interaction of Pt and N species.15-17 Nevertheless, Pt catalysts deposited on the surface of porous carbon supports have high surface energies and weak interactions with carbon supports, resulting in aggregation and detachment of Pt catalysts during cycling.18,19 There still remains substantial challenges to maintain the exceptional electrocatalytic activity of Pt catalysts after long-term cycling. In addition, few researches focus on the pore size effect of carbon supports on the electrocatalytic performance of Pt catalysts towards oxidation of methanol,20 not to speak of other alcohols (e.g., ethanol and n-propanol). In this work, we propose a novel strategy for the preparation of Pt@NMCs with remarkably wellcontrolled mesopores (7~22 nm). Pt@NMCs exhibit exceptional electrocatalytic activity and durability for methanol oxidation due to the structural advantages of NMCs. The mesoporous sturcture of NMCs can disperse Pt catalysts and protect them against aggregation and detachment, while the nitrogen doping can improve the electric conductivity of carbon support and immobilize Pt catalysts on NMCs through the synergistic interaction. Moreover, uniform Pt nanoparticles (3~4 nm) are embedded within the carbon walls of NMCs, which further improves the reusability and durability of Pt@NMCs. The ACS Paragon Plus Environment

2

Page 3 of 24 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


pore size of NMCs also has a great impact on the electrocatalytic activity and selectivity of Pt catalysts towards oxidation of alcohols (methanol, ethanol and n-propanol). Pt@NMCs with large pore size (~22 nm) exhibit higher electrocatalytic activity towards alcohol oxidation because larger pores permit more facile alcohol transport in the electrolyte within the pores.21 On the contrary, Pt@NMCs with small pore size (~7 nm) show good electrocatalytic selectivity towards oxidation of mixed alcohols because smaller pores impede the diffusion of large sized alcohols (e.g., n-propanol) within the pores. Accordingly, we can adopt optimal Pt@NMCs with appropriate pore size based on different requirements and applications.

2. Experimental Section 2.1 Chemicals Aniline and concentrated hydrochloric acid (37%, GR) were purchased from Beijing chemical factory. Colloidal silica nanospheres (AR), sodium borohydride (AR) and chloroplatinic acid hexahydrate (AR) were purchased from Sigma-Aldrich. Ammonium persulfate (AR) was purchased from Sinopharm chemical reagent co. LTD. Commercial Pt/C (20%) was purchased from Alfa Aesar (China).

2.2 Synthesis of Pt@NMCs 1 mL of colloidal silica nanospheres (40%) with a diameter of 7 nm (or 12 nm or 22 nm) was uniformly dispersed in 10 mL of 0.5 M HCl under vigorous stirring for 30 min. 0.4 g of aniline and 0.15 g of H2PtCl6·6H2O were then added to the above suspension and the mixed suspension was stirred for another 30 min. 1 g of ammonium persulfate and 1 mL of hydrochloric acid was added dropwise into the above suspension under vigorous stirring in ice bath to initiate the polymerization of aniline. The dark green powder was collected by centrifugation of the suspension, washed with distilled water three times, and dried at 60 °C. The powder was transferred into a quartz tube furnace and heated at 900 °C for 2 h under a N2 gas flow. After cooling down to room temperature, the black powder was added into dilute hydrofluoric acid solution to remove the silica template. Finally, Pt@NMCs-X (X=7, 12, 22) was ACS Paragon Plus Environment

3


Page 4 of 24

collected by centrifugation of the suspension and washed 3 times with distilled water. For comparison, Pt@NMCs-0 and NMC-7 were synthesized by the same procedure except that colloidal silica nanospheres or Pt precursor were not used.

2.3 Synthesis of Pt/NMCs 0.14 g of NMC-7 and 0.9 g of NH4Cl were added into 60 mL of distilled water under vigorous stirring for 1h. The suspension was then heated in a water bath at 70 °C under a N2 gas flow. After 30 min, 0.04 g of H2PtCl6·6H2O and 0.3 g of NaBH4 was then added into the suspension with stirring for 20 min. Finally, Pt/NMCs-7 was collected by centrifugation of the suspension and washed 3 times with distilled water.

2.4 Characterization and Electrocatalytic Measurement Characterization and measurement methods may refer to our recent reports.13,14 Details are given in Supporting Information.

3. Results and Discussion Figure 1a shows the overall synthetic procedure of Pt@NMCs. Aniline was first mixed with Pt precursor (H2PtCl6) and silica template (colloidal silica nanospheres), and then in-situ polymerized on the surface of self-assembled silica by using ammonium persulfate as oxidizer (i), affording core-shell structured Pt/SiO2@PANI composites. Subsequently, N-doped carbon with abundant N atoms was synthesized by carbonization of PANI (ii). The in situ doping of nitrogen derived from PANI ensures uniform distribution of N species on carbon, which is conducive to the electron transfer and Pt dispersion. Meanwhile, Pt nanoparticles are formed and embedded within NMCs during the carbonization process, which protect Pt catalysts against aggregation and detachment. Pt@NMCs was finally obtained after removal of silica template in HF solution (iii). The pore size of Pt@NMCs can be tuned simply by adjusting the diameter of colloidal silica nanospheres,22 making it possible to study the ACS Paragon Plus Environment

4

Page 5 of 24 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


relationship between pore structure of NMCs and the electrocatalytic activity and selectivity of Pt catalysts towards alcohol oxidation.

Figure 1. (a) Schematic illustration of the synthesis route to Pt@NMCs. SEM images of (b, e) Pt@NMC-7, (c, f) Pt@NMC-12 and (d, g) Pt@NMC-22 at different magnifications.

To study the pore size effect of NMCs on the electrocatalytic performance of Pt@NMCs, colloidal silica nanospheres with a diameter of 7, 12 or 22 nm were selected as template to prepare Pt@NMCs (denoted as Pt@NMC-X, X=7, 12, or 22). For comparison, Pt@NMC-0 was prepared without using colloidal silica nanospheres as template. The morphologies and structural properties of Pt@NMCs were first investigated by SEM. As shown in Figure 1b-d, Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 are presented in the form of uniform dispersed spherical-like particles with homogeneous mesopores on the ACS Paragon Plus Environment

5


Page 6 of 24

surface. The particle size of Pt@NMCs increases from ~300 to ~500 nm with increasing the diameter of colloidal silica nanospheres (7 to 22 nm), suggesting that the particle size of silica template not only determines the pore size of Pt@NMCs, but also has an influence on the particle size of Pt@NMCs. The uniform mesopores of Pt@NMCs are easily identified in the SEM images at high magnification (Figure 1e-g). Moreover, Pt nanoparticles are hardly observed on the surface of Pt@NMCs, implying most of Pt nanoparticles are located inside the mesopores of Pt@NMCs. In contrast, Pt@NMC-0 has an irregular blocky shape without any pores (Figure S1a), indicating that silica template plays an important role in promoting the formation of separated particles with spherical-like morphology in addition to creating mesopores of Pt@NMCs. The mesoporous structure of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 was further studied by TEM. The spherical cavities of NMCs were distinctly observed in the TEM images (Figure 2a-c), especially at the edge of NMCs. We believe that the cavities are connected by small windows throughout NMCs, which offers three-dimensional pathway for mass transport. Pt nanoparticles are highly dispersed inside the mesopores of NMCs without any aggregation. STEM-EDX mapping images (Figure S2) further confirm the dispersion of Pt nanoparticles within N-doped carbon support based on the distribution of C, N and Pt elements. The Si peak is barely detected in the EDX spectrum (Figure S2b), validating the removal of silica template. The particle size distributions of Pt nanoparticles are shown in Figure S3. It is interesting that the average particle sizes of Pt nanoparticles are very similar within these NMCs, i.e., ~3.5 nm for Pt@NMC-7, ~3.5 nm for Pt@NMC-12 and ~3.6 nm for Pt@NMC-22. On the contrary, although Pt nanoparticles are still well-dispersed within NMC-0 (Figure S1b), they have a wide particle size distribution in the range of 2~9 nm. A possible explanation is that the growth of Pt nanocrystals within NMCs is gravely restricted due to the lack of Pt precursors in the mesopores.

ACS Paragon Plus Environment

6

Page 7 of 24 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 2. TEM and HRTEM images of (a, d) Pt@NMC-7, (b, e) Pt@NMC-12 and (c, f) Pt@NMC-22.

More detailed information of Pt@NMCs is disclosed by HRTEM. HRTEM images of Pt@NMC-7, Pt@NMC-12, Pt@NMC-22 (Figure 2d-f) and Pt@NMC-0 (Figure S1c) demonstrate well-defined nanoparticles with a lattice spacing of ~0.226 nm, which is well-matched with the d-spacings of (111) plane of Pt crystal. According to HRTEM observation, some Pt nanocrystals are located on the surface of carbon walls of NMCs, while others are embedded within the carbon walls of NMCs. In view of the very thin pore walls (2~4 nm) of NMCs,22 these embedded Pt nanocrystals are still exposed to electrolyte and thereby activated for alcohol electrooxidation. Furthermore, the embedded nanophase and the interaction between Pt and N species on carbon can protect Pt nanoparticles against aggregation and detachment, further improving the electrocatalytic performance of Pt catalysts.18,19 The mesopores of NMCs generated by removal of the self-assembly of colloidal silica nanospheres are also distinguished in HRTEM images (white circle). It is reasonable that there are small windows connecting the neighboring spherical cavities, and the size of windows increases with increasing the diameter of colloidal silica nanospheres. ACS Paragon Plus Environment

7


Page 8 of 24

N2 adsorption-desorption technique was carried out to further analyze the porosities of Pt@NMCs. The N2 adsorption-desorption isotherms of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 (Figure 3a) exhibit type IV curves with H2 hysteresis loops, demonstrating that Pt@NMCs are typical mesoporous materials with uniform spherical pores. The pore size distributions (Figure 3b) indicate that the average pore sizes of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 are 6.6, 11.7 and 21.9 nm respectively, which are highly consistent with the diameters of silica templates (7, 12 and 22 nm, respectively). The pore volumes of Pt@NMCs also increase from 0.67 to 1.21 cm3/g with increasing the diameter of silica template, but the surface areas decrease slightly from 566 to 447 m2/g. The detailed structural parameters of Pt@NMCs are summarized in Table 1. In contrast, Pt@NMC-0 shows very small surface area (20 m2/g) due to its non-porous structure.

Figure 3. (a) N2 adsorption/desorption isotherms and (b) pore size distributions of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22.

Table 1. Structural parameters of Pt@NMC-7, Pt@NMC-12, Pt@NMC-22, Pt@NMC-0 and Pt/NMC7. Sample

Pore size / nm

Pore volume / cm3 g‒1

Surface area / m2 g ‒1

Pt@NMC-7

6.6

0.67

566

Pt@NMC-12

11.7

0.90

507


8

Page 9 of 24 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


Pt@NMC-22

21.9

1.21

447

Pt@NMC-0

N/A

N/A

20

Pt/NMC-7

6.6

0.58

532

XRD patterns of Pt@NMC-7, Pt@NMC-12, Pt@NMC-22 (Figure 4a) and Pt@NMC-0 (Figure S1d) show three well-resolved diffraction peaks, which are assigned to the (111), (200) and (220) planes of cubic Pt structure. A broad XRD peak at ~24º corresponds to the amorphous carbon phase of NMCs with some graphitic domains, which can also be observed by HRTEM (Figure 2e).22 The graphitic parts can strengthen the interactions between Pt and carbon, and also alleviate those issues such as aggregation and detachment of Pt catalysts on carbon supports.18 FT-IR spectra of Pt@NMCs (Figure 4b) show a characteristic absorption band at 1650 cm−1, which is ascribed to the skeletal vibration of C=C from unoxidized graphitic domains. The absorption bands corresponding to the stretching vibrations of C−N (1383 cm−1) and C=N (1558 cm−1) are also detected, illustrating the presence of nitrogen atoms in NMCs.23,24 These results provide strong evidence that the carbon supports are partially graphitized and doped with plenty of nitrogen, which is favorable for the electrical conductivity enhancement of Pt catalysts. XPS measurement was performed to analyze the surface composition of Pt@NMCs. Four elements (C, O, N and Pt) are detected in the XPS survey spectra (Figure 4c), confirming the presence of Pt and N species in Pt@NMCs. The high-resolution N 1s spectra (Figure 4d) reveal three types of nitrogen species in NMCs: pyridinic N (398.4 eV), pyrrolic N (399.8 eV) and quaternary N (400.9 eV).25 The pyridinic N and pyrrolic N serve the functions of dispersing and immobilizing Pt nanoparticles by providing the nucleation and active sites for the deposition of Pt nanoparticles, while the quaternary N, which is the main peak in N 1s spectra, remarkably enhances the electrical conductivity of carbon supports.26 Apparently, the electrocatalytic performance of Pt catalysts can be highly improved by doping carbon supports with nitrogen. The high-resolution C 1s spectra (Figure S4a) are deconvoluted into four peaks corresponding to C−C (284.6 eV), C−N (285.6 eV), C−O (286.6 eV) and C=O/C=N ACS Paragon Plus Environment

9


Page 10 of 24

(288.8 eV).27 The high-resolution Pt 4f spectra (Figure S4b) are divided into two pairs of peaks, which are assigned to Pt0 (70.8 eV and 74.0 eV) and Pt2+ (72.2 eV and 75.4 eV), respectively. The atomic contents of C, O, N and Pt are listed in Table S1. Raman spectra of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 were shown in Figure S5. The D band at ~1340 cm−1 corresponds to disordered graphitic carbon, while the G band at ~1594 cm−1 is related to a graphitic carbon phase. Therefore, the high ID/IG ratios of Pt@NMC-7 (1.03), Pt@NMC-12 (1.01) and Pt@NMC-22 (1.06) reflect the high density of defects due to the nitrogen doping as well as partial graphitization of NMCs.17,22

Figure 4. (a) XRD patterns of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22. (b) FT-IR, (c) XPS survey and (d) high-resolution N 1s spectra of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22.

ICP analysis is carried out to determine the exact amount of Pt in composites, which is ~21.9 wt% for ACS Paragon Plus Environment

10

Page 11 of 24 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


Pt@NMC-7, ~23.7 wt% for Pt@NMC-12, ~22.0 wt% for Pt@NMC-22 and ~21.9 wt% for Pt@NMC-0 (Table S2). The very similar loadings of Pt allow us to focus on the study of pore size effect of NMCs on the electrocatalytic performances of Pt@NMCs without considering the influence of Pt contents. Cyclic voltammetry (CV) was performed in 0.5 M H2SO4 solution to evaluate the electrochemically active surface areas (ECSAs) of samples. The ECSAs of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 calculated from the area of the hydrogen adsorption region (Figure 5a) are 24.9, 31.0 and 44.4 m2 g‒1, respectively. Note that the ECSA of Pt@NMCs increases with increasing the pore size of NMCs. One reason is that larger pores provide more space to disperse Pt nanoparticles, which may increase the proportion of fully exposed Pt nanoparticles and the number of electrochemically available active sites. More importantly, enlarging the pore channels of NMCs facilitates mass diffusion, resulting in more facile and effective contact between nanocatalysts and alcohols. The electrocatalytic activity and durability of Pt@NMCs towards oxidation of methanol were measured in solution of 1.0 M CH3OH and 0.5 M H2SO4. The CV curves (Figure 5b) show that the forward peak current density (If) of Pt@NMC-22 is 5.67 mA cm‒2, much higher than those of Pt@NMC-7 (3.02 mA cm‒2) and Pt@NMC-12 (3.85 mA cm‒2). Moreover, the catalyst tolerance to CO poisoning is usually evaluated by the ratio of the forward peak current to the backward peak current (If/Ib).13,14 Pt@NMC-22 also exhibits higher If/Ib ratio (8.5) than Pt@NMC-7 (6.7) and Pt@NMC-12 (7.5). The superior methanol oxidation reaction (MOR) activity and stronger poison tolerance of Pt@NMC-22 can be attributed to the enhanced mass transport by increasing the pore size of NMCs. The electrocatalytic durability of Pt@NMCs for methanol oxidation is also compared (Figure 5c,d). Pt@NMC-22 exhibits 84% catalytic activity retention after 500 cycles, higher than Pt@NMC-7 (63%) and Pt@NMC-12 (76%). As a result, the electrocatalytic activity and durability of Pt@NMCs towards methanol oxidation are enhanced with increasing the pore size (or window size) of NMCs. The MOR performance comparison of Pt@NMC-22 with reported (N-doped) mesoporous carbon-supported Pt hybrids is listed in Table S3. NMCs inherit the advantages of porous carbon and N-doped carbon, such as dispersing and protecting Pt catalysts by mesostructure, and improving the electric conductivity by ACS Paragon Plus Environment

11


Page 12 of 24

the nitrogen doping. Thus Pt@NMC-22 exhibits higher electrocatalytic activity and stronger poison tolerance than porous carbon or N-doped carbon supported Pt catalysts. Even more remarkably, Pt catalysts are embedded within the carbon walls of NMCs, giving rise to excellent electrocatalytic durability of Pt@NMC-22 for methanol oxidation. TEM image of Pt@NMC-22 after 500 cycles (Figure S6) reveals that Pt nanoparticles are still dispersed on the surface of carbon walls of NMCs, and Pt sintering is not observed. Furthermore, the content of Pt nanoparticles slightly decreases from 22.0 wt% to 20.4 wt% after 500 cycles, indicating the high reusability and durability of Pt@NMCs.


12

Page 13 of 24 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. CV curves of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 in (a) 0.5 M H2SO4 and (b) 0.5 M H2SO4 + 1.0 M CH3OH at a scan rate of 100 mV s−1. (c) The forward peak current densities of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 as a function of the cycle number for the methanol oxidation, and (d) percentage representation. (e) Electrochemical impedance spectra of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 in 0.5 M H2SO4 + 1.0 M CH3OH solution, and (f) the equivalent circuit used to fit the impedance spectra. ACS Paragon Plus Environment

13


Page 14 of 24

The large pores of NMCs are also conducive to the electron transfer through Pt@NMCs, which is confirmed by electrochemical impedance spectroscopy (EIS) measurements. Nyquist plots (Figure 5e) show a semicircle in the high frequency range, which is assigned to the charge transfer resistance (Rct).13,14 The diameter of the semicircle follows the sequence of Pt@NMC-7 > Pt@NMC-12 > Pt@NMC-22, demonstrating that Pt@NMC-22 has the lowest charge transfer resistance among these three samples. The Rct value simulated via the equivalent circuit model (Figure 5f) is 8.4 Ω for Pt@NMC-22, smaller than that for Pt@NMC-7 (26.1 Ω) and Pt@NMC-12 (21.3 Ω). The fast electron transfer also contribute to the outstanding electrocatalytic performance of Pt@NMC-22. All parameters of Pt@NMCs are summarized in Table S2 for convenient comparison. Although the MOR performance of Pt@NMC-0 is inferior to those of Pt@NMCs owing to its nonporous structure, Pt@NMC-0 still exhibits better electrocatalytic performance than commercial Pt/C (Figure S7), such as higher ECSA and If values (14.3 m2 g‒1 and 2.47 mA cm‒2), stronger poison tolerance (If/Ib=4.7), better electrocatalytic durability (45% catalytic activity retention) and lower charge transfer resistance (Rct=38.0 Ω, Figure S8a). The results indicate that the N species on carbon support also play an important role in improving the MOR performance of Pt catalysts. In addition to the superiority of NMCs such as mesostructure and nitrogen-doping, the exceptional MOR performance of Pt@NMCs is attributed to the embedded nanophase. To prove this conjecture, Pt/NMC-7 was synthesized by introduction of Pt catalysts into the mesopores of NMC-7. SEM and TEM images (Figure 6a-c) reveal that Pt nanoparticles are well-dispersed within the mesopores of NMC-7. However, compared to Pt@NMC-7, Pt nanoparticles in Pt/NMC-7 have larger particle size (3~8 nm), and are partially formed outside the mesopores of NMC-7. HRTEM image (Figure 6d) and XRD pattern (Figure S9a) verify the successful formation of Pt nanocrystals in Pt/NMC-7. The mesostructure of NMC-7 is preserved after loading of Pt catalysts (Figure S9b). However, the surface area and pore volume of Pt/NMC-7 decrease in comparison with Pt@NMC-7 (Table 1).


14

Page 15 of 24 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 6. (a, b) SEM, (c) TEM and (d) HRTEM images of Pt/NMC-7.

The electrocatalytic performance of Pt/NMC-7 was measured and compared with Pt@NMC-7 (Figure 7). CV curves shows that Pt/NMC-7 has lower ECSA and If values (19.7 m2 g‒1 and 2.70 mA cm‒2), lower catalytic activity retention (51%), and higher charge transfer resistance (Rct=34.7 Ω, Figure S8b) in comparison with Pt@NMC-7, though there is not much difference in the catalyst tolerance (If/Ib=6.6). The embedded nanophase is favorable for immobilization of Pt catalysts on carbon support and electron transfer from carbon support to Pt catalysts, thereby improving the MOR performance of Pt@NMCs. The structure and morphology of Pt/NMC-7 and Pt@NMC-7 after 500 cycles were examined using TEM. TEM image of Pt/NMC-7 (Figure S10b) reveals that Pt nanoparticles aggregate and grow into large particles after long-term cycling, whereas uniformly dispersed Pt nanoparticles are 15 ACS Paragon Plus Environment


Page 16 of 24

preserved within NMC-7 (Figure S10a), indicative of the excellent stability of Pt@NMC-7. The performance comparison of Pt/C, Pt@NMC-0, Pt/NMC-7 and Pt@NMC-7 is summarized in Table S4.

Figure 7. CV curves of Pt/NMC-7 in (a) 0.5 M H2SO4 and (b) 0.5 M H2SO4 + 1.0 M CH3OH at a scan rate of 100 mV s−1. (c) The forward peak current densities of Pt/NMC-7 as a function of the cycle number for the methanol oxidation, and (d) percentage representation.

The electrocatalytic activity of Pt@NMCs towards ethanol (or n-propanol) oxidation was also measured to investigate the electrocatalytic selectivity of Pt@NMCs for alcohol oxidation. On one hand, the electrocatalytic activity of Pt@NMCs for ethanol or n-propanol oxidation increases with increasing the pore size of NMCs, which is consistent with the MOR performance of Pt@NMCs. On the other hand, the electrocatalytic activity of Pt@NMCs for alcohol oxidation follows the sequence of methanol


16

Page 17 of 24 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


> ethanol > n-propanol. Although ethanol (or n-propanol) has less toxicity and a higher specific energy than methanol, the ethanol (or n-propanol) oxidation reaction performance of Pt is limited by the relatively sluggish kinetics and the difficulty of C–C cleavage.28,29 For instance, differing from methanol oxidation, complete oxidation of ethanol to CO2 requires the breaking of the C–C bond in ethanol or the acetaldehyde intermediate. Therefore, ethanol can be selectively oxidized to acetaldehyde and acetic acid, the latter of which cannot be further oxidized to CO2. Besides, it is remarkable that the transport of alcohols with large shape and size (e.g., n-propanol) would be hampered by reducing the pore size (or window size) of NMCs, especially to NMC-7. As shown in Figure 8a,b, the If value of Pt@NMC-7 for n-propanol oxidation is only 0.24 mA cm‒2 (8.0% If value of methanol oxidation), much lower than those of Pt@NMC-12 (1.17 mA cm‒2, 30.4%) and Pt@NMC-22 (2.32 mA cm‒2, 40.9%). When the ambient temperature is increased to 40 ℃, the If values of Pt@NMC-12 and Pt@NMC-22 increase to 1.51 and 3.24 mA cm‒2 (39.2 and 57.1%), respectively, because higher operating temperature not only enhances the mass transfer of n-propanol through the mesopores of NMCs, but also accelerates the kinetics of methanol oxidation.30 However, the If value of Pt@NMC-7 does not change much (0.26 mA cm‒2, 8.7%) at an operating temperature of 40 ℃, implying the mass transfer of n-propanol through the mesopores of NMC-7 is very difficult (Figure 8c). As a result, Pt@NMCs with large mesopores are beneficial to improving the electrocatalytic activity and durability towards alcohol oxidation, while Pt@NMCs with small mesopores exhibit good electrocatalytic selectivity towards alcohol oxidation. The electrocatalytic activity of nonporous Pt/C towards methanol, ethanol and n-propanol oxidation was shown in Figure S11. In view of the nonporous structure of Pt/C, Pt/C shows low selectivity for alcolhol oxidation, similar to Pt@NMC-22 with large mesopores.


17


Page 18 of 24

Figure 8. (a) The forward peak current densities of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22 as a function of types of alcohols (methanol, ethanol and n-propanol), and (b) percentage representation. (c) Schematic illustration of alcohol transport through the windows between mesopores of NMCs.

4. Conclusions In summary, we described a simple method for the fabrication of Pt@NMCs with tailored mesopores. Pt@NMCs have several advantages such as adjustable mesopores that facilitate the mass diffusion throughout NMCs, nitrogen-doping that enhances the electric conductivity of carbon support and the dispersivity of Pt catalysts, and the embedded nanophase that protects Pt nanoparticles against aggregation and detachment. All above factors are beneficial to improving the electrocatalytic activity and durability of Pt@NMCs for alcohol oxidation. Moreover, the pore size (or window size) of NMCs has a great effect on the electrocatalytic activity and selectivity of Pt catalysts towards alcohol oxidation. ACS Paragon Plus Environment

18

Page 19 of 24 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


Enlarging the pore size of NMCs can enhance the electrocatalytic performance of Pt@NMCs towards alcohol oxidation, whereas reducing the pore size of NMCs can improve the electrocatalytic selectivity of Pt@NMCs towards alcohol oxidation. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami. STEM-EDX mapping images of Pt@NMC-7. Size distribution histograms of Pt nanoparticles. High-resolution XPS and Raman spectra of Pt@NMC-7, Pt@NMC-12 and Pt@NMC-22. SEM, TEM, HRTEM images and XRD pattern of Pt@NMC-0. CV curves, cycle performance and electrochemical impedance spectra of Pt@NMC-0 and Pt/C. XRD pattern and N2 adsorption/desorption isotherm of Pt/NMC-7. Performance summary and comparison. AUTHOR INFORMATION Corresponding Author *Tel: 86-10-58804229. Fax: 86-10-58892075. E-mail: [email protected] (W. Y.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by National Natural Science Foundation of China (21573023). REFERENCES (1) Fan, J. C.; Qi, K.; Zhang, L.; Zhang, H. Y.; Yu, S. S.; Cui, X. Q. Engineering Pt/Pd Interfacial Electronic Structures for Highly Efficient Hydrogen Evolution and Alcohol Oxidation. ACS Appl. Mater. Interfaces 2017, 9, 18008−18014. (2) Munjewar, S. S.; Thombre, S. B.; Mallick, R. K. Renew. Approaches to overcome the barrier issues of passive direct methanol fuel cell-Review. Sust. Energ. Rev. 2017, 67, 1087–1104.


19


Page 20 of 24

(3) Shen, Y.; Zhang, Z. H.; Long, R. R.; Xiao, K. J.; Xi, J. Y. Synthesis of Ultrafine Pt Nanoparticles Stabilized by Pristine Graphene Nanosheets for Electro-oxidation of Methanol. ACS Appl. Mater. Interfaces 2014, 6, 15162−15170. (4) Cui, Z. M.; Chen, H.; Zhao, M. T.; Marshall, D.; Yu, Y. C.; Abruña, H.; J DiSalvo F. Synthesis of Structurally Ordered Pt3Ti and Pt3V Nanoparticles as Methanol Oxidation Catalysts. J. Am. Chem. Soc. 2014, 136, 10206−10209. (5) Ma, Y. X.; Yin, L. S.; Yang, T.; Huang, Q. L.; He, M. S.; Zhao, H.; Zhang, D. G.; Wang, M. Y.; Tong, Z. W. One-Pot Synthesis of Concave Platinum−Cobalt Nanocrystals and Their Superior Catalytic Performances for Methanol Electrochemical Oxidation and Oxygen Electrochemical Reduction. ACS Appl. Mater. Interfaces 2017, 9, 36164−36172. (6) Suntivich, J.; Xu, Z. C.; Carlton, C. E.; Kim, J.; Han, B. H.; Lee, S. W.; Bonnet, N.; Marzari, N.; Allard, L. F.; Gasteiger, H. A.; Hamad-Schifferli, K.; Shao-Horn, Y. Surface Composition Tuning of Au-Pt Bimetallic Nanoparticles for Enhanced Carbon Monoxide and Methanol Electro-oxidation. J. Am. Chem. Soc. 2013, 135, 7985−7991. (7) Chen, D. -J.; Tong, Y. J. Irrelevance of Carbon Monoxide Poisoning in the Methanol Oxidation Reaction on a PtRu Electrocatalyst. Angew. Chem. Int. Ed. 2015, 54, 9394–9398. (8) Li, L. Y.; Liu, H. Q.; Qin, C.; Liang, Z. X.; Scida, A.; Yue, S. Y.; Tong, X.; Adzic, R. R.; Wong, S. S. Ultrathin PtxSn1−x Nanowires for Methanol and Ethanol Oxidation Reactions: Tuning Performance by Varying Chemical Composition. ACS Appl. Nano Mater. 2018, 1, 1104−1115. (9) Wang, C. Z.; Zhang, Y.; Zhang, Y. J.; Xu, P.; Feng, C. M.; Chen, T.; Guo, T.; Yang, F. N.; Wang, Q.; Wang, J. X.; Shi, M. T.; Fan, L. Z.; Chen, S. W. Highly Ordered Hierarchical Pt and PtNi Nanowire Arrays for Enhanced Electrocatalytic Activity toward Methanol Oxidation. ACS Appl. Mater. Interfaces 2018, 10, 9444−9450.


20

Page 21 of 24 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


(10) Chu, Y. -Y.; Wang, Z. -B.; Jiang, Z. -Z.; Gu, D. -M.; Yin, G. -P. A Novel Structural Design of a Pt/C-CeO2 Catalyst with Improved Performance for Methanol Electro-Oxidation by beta-Cyclodextrin Carbonization. Adv. Mater. 2011, 23, 3100–3104. (11) Huang, M. H.; Zhang, J. S.; Wu, C. X.; Guan, L. H. Pt Nanoparticles Densely Coated on SnO2℃Covered Multiwalled Carbon Nanotubes with Excellent Electrocatalytic Activity and Stability for Methanol Oxidation. ACS Appl. Mater. Interfaces 2017, 9, 26921−26927. (12) Zhang, C. W.; Xu, L. B.; Shan, N. N.; Sun, T. T.; Chen, J. F.; Yan, Y. S. Enhanced Electrocatalytic Activity and Durability of Pt Particles Supported on Ordered Mesoporous Carbon Spheres. ACS Catal. 2014, 4, 1926−1930. (13) Nan, L. R.; Yue, W. B.; Jiang, Y. Fabrication of graphene-porous carbon-Pt nanocomposites with high electrocatalytic activity and durability for methanol oxidation. J. Mater. Chem. A 2015, 3, 22170– 22175. (14) Nan, L. R.; Fan, Z. T.; Yue, W. B.; Dong, Q.; Zhu, L. S.; Yang, L.; Fan, L. Z. Graphene-based porous carbon-Pd/SnO2 nanocomposites with enhanced electrocatalytic activity and durability for methanol oxidation. J. Mater. Chem. A 2016, 4, 8898–8904. (15) Zhang, L. -M.; Wang, Z. -B.; Zhang, J. -J.; Sui, X. -L.; Zhao, L.; Gu, D. -M. Honeycomb-like mesoporous nitrogen-doped carbon supported Pt catalyst for methanol electrooxidation. Carbon 2015, 93, 1050–1058. (16) Zhang, L. -M.; Wang, Z. -B.; Sui, X. -L.; Li, C. -Z.; Zhao, L.; Gu, D. -M. Nitrogen-doped carbon with mesoporous structure as high surface area catalyst support for methanol oxidation reaction. RSC Adv. 2016, 6, 39310–39316.


21


Page 22 of 24

(17) Zhang, Y. M.; Liu, Y.; Liu, W. H.; Li, X. Y.; Mao, L. Q. Synthesis of honeycomb-like mesoporous nitrogen-doped carbon nanospheres as Pt catalyst supports for methanol oxidation in alkaline media. Appl. Surf. Sci. 2017, 407, 64–71. (18) Wu, Z. X.; Lv, Y. Y.; Xia, Y. Y.; Webley, P. A.; Zhao, D. Y. Ordered Mesoporous Platinum@Graphitic Carbon Embedded Nanophase as a Highly Active, Stable, and Methanol-Tolerant Oxygen Reduction Electrocatalyst. J. Am. Chem. Soc. 2012, 134, 2236−2245. (19) Guo, L.; Jiang, W. -J.; Zhang, Y.; Hu, J. -S.; Wei, Z. -D.; Wan, L. -J. Embedding Pt Nanocrystals in N-Doped Porous Carbon/Carbon Nanotubes toward Highly Stable Electrocatalysts for the Oxygen Reduction Reaction. ACS Catal. 2015, 5, 2903−2909. (20) Su, F. B.; Zeng, J. H.; Bao, X. Y.; Yu, Y. S.; Lee, J. Y.; Zhao, X. S. Preparation and characterization of highly ordered graphitic mesoporous carbon as a Pt catalyst support for direct methanol fuel cells. Chem. Mater. 2005, 17, 3960−3967. (21) Ren, Y.; Armstrong, A. R.; Jiao, F.; Bruce, P. G. Influence of Size on the Rate of Mesoporous Electrodes for Lithium Batteries. J. Am. Chem. Soc. 2010, 132, 996–1004. (22) Wang, G.; Sun, Y. H.; Li, D. B.; Liang, H. -W.; Dong, R. H.; Feng, X. L.; Müllen, K. Controlled Synthesis of N-Doped Carbon Nanospheres with Tailored Mesopores through Self-Assembly of Colloidal Silica. Angew. Chem. Int. Ed. 2015, 54, 15191–15196. (23) Ma, F. W.; Zhao, H.; Sun, L. P.; Li, Q.; Huo, L. H.; Xia, T.; Gao, S.; Pang, G. S.; Shi, Z.; Feng, S. H. J. Mater. Chem. 2012, 22, 13464–13468. (24) Nie, R. F.; Jiang, H. Z.; Lu, X. H.; Zhou, D.; Xia, Q. H. Highly active electron-deficient Pd clusters on N-doped active carbon for aromatic ring hydrogenation. Catal. Sci. Technol. 2016, 6, 1913–1920. (25) Wang, S. H.; Shang, L.; Li, L. L.; Yu, Y. J.; Chi, C. W.; Wang, K.; Zhang, J.; Shi, R.; Shen, H. Y.; Waterhouse, G. I. N.; Liu, S. J.; Tian, J.; Zhang, T. R.; Liu, H. Y. Metal–Organic‐Framework‐ ACS Paragon Plus Environment

22

Page 23 of 24 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


Derived Mesoporous Carbon Nanospheres Containing Porphyrin‐Like Metal Centers for Conformal Phototherapy. Adv. Mater. 2016, 28, 8379–8387. (26) Su, F. B.; Tian, Z. Q.; Poh, C. K.; Wang, Z.; Lim, S. H.; Liu, Z. L.; Lin, J. Y. Pt Nanoparticles Supported on Nitrogen-Doped Porous Carbon Nanospheres as an Electrocatalyst for Fuel Cells. Chem. Mater. 2010, 22, 832–839. (27) Zhang, A. F.; Li, X. W.; He, Y. Platinum/nitrogen-doped carbon nanoparticles synthesized in nitrogen-doped carbon quantum dots aqueous solution for methanol electro-oxidation. Electrochim. Acta 2016, 213, 332–340. (28) Lee, C. -G.; Umeda, M.; Uchida, I. Cyclic voltammetric analysis of C-1-C-4 alcohol electrooxidations with Pt/C and Pt-Ru/C microporous electrodes. J. Power Sources 2006, 160, 78–89. (29) Mistry, H.; Varela, A. S.; Kühl, S.; Strasser, P.; Cuenya, B. R. Nanostructured electrocatalysts with tunable activity and selectivity. Nat. Rev. Mater. 2016, 1, 16009–16022. (30) Golikand, A. N.; Shahrokhian, S.; Asgari, M.; Maragheh, M. G.; Irannejad, L.; Khanchi, A. Electrocatalytic oxidation of methanol on a nickel electrode modified by nickel dimethylglyoxime complex in alkaline medium. J. of Power Sources 2005, 144, 21–27.


23


Page 24 of 24

Abstract Graphic


24

Exceptional Electrocatalytic Activity and Selectivity of Platinum

Recommend Documents