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Microporous Organic Polymers Derived Microporous Carbon Supported Pd Catalysts for Oxygen Reduction Reaction: Impact of Framework and Heteroatom Kunpeng Song, Zhijuan Zou, Deli Wang, Bien Tan, Jingyu Wang, Jian Chen, and Tao Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10358 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on January 17, 2016

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The Journal of Physical Chemistry

Microporous Organic Polymers Derived Microporous Carbon Supported Pd Catalysts for Oxygen Reduction Reaction: Impact of Framework and Heteroatom Kunpeng Song, Zhijuan Zou, Deli Wang, Bien Tan, Jingyu Wang*, Jian Chen and Tao Li* Key Laboratory for Large-Format Battery Materials and System (Ministry of Education), School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China E-mail: [email protected]; [email protected]

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Abstract

The usage of noble metal catalysts for oxygen reduction reaction (ORR) can be minimized by supporting them on porous carbon. Herein, a simple and industrially scalable approach for preparation of porous carbon supports is developed by directly pyrolyzing microporous organic polymers (MOPs). The rigid aromatic building blocks in MOPs function as self-templates to retain the high surface area and regular pore distribution during carbonization to the formation of high quality microporous carbon framework. The electrochemical performance of Pd catalysts supporting on MOPs-derived carbons are evaluated and compared with commercial products, indicating the superiority of MOPs as carbon precursor. Significant difference in ORR activity is observed among three types of MOPs due to the impact of framework and heteroatom. The highest performance of Pd/CKN catalyst is attributed to heteroatom-induced altering of electronic structures besides the stable skeleton.


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INTRODUCTION Electrochemical oxygen reduction reaction (ORR) plays a crucial role in renewable energy applications, such as fuel cells and metal–air batteries.1-2 For proton exchange membrane fuel cells (PEMFCs) and metal–air batteries, the typical electrocatalyst for ORR is the Pt-based precious metal catalyst. The high price and scarcity of Pt strongly limits the commercial application in terms of replacing fossil fuel-based energy conversion systems. The usage of expensive Pt-based catalysts can be minimized by supporting them on carbon materials. Plenty of literatures have focused on porous carbon, such as activated carbon,3 mesoporous carbon,4 carbon nanotubes,5-6 and graphene.7-8 Porous carbon materials have many specific features such as high surface area, thermal and chemical stability, and good electrical conductivities. Although many porous carbon materials have been achieved, most of them have disordered mesoporous or macroporous structure, which limits their performance as functional materials, especially for electrochemical materials.9 Some groups synthesized ordered mesoporous carbons from hard or soft templates.10-13 These techniques have great advantages for the formation of carbons with improved surface area and controlled pore texture, which are considered to be the key factors in optimizing the electrocatalytic performance. Also, there are some problems need to be improved, e.g., complicated steps for preparing hard templates, using hazardous chemicals for the removal of templates, and barely scalable productivity.14 Microporous organic polymers (MOPs), metal−organic frameworks (MOFs), and covalent organic frameworks (COFs) have been extensively studied in adsorption, gas storage, separation, catalysis, and drug delivery, owing to their large surface area, low skeleton density, regular pore structure, and controllable framework. Most recently, microporous carbon materials, with 2D or 3D pore structure suitable for small molecules to access, have been explored as efficient ORR


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catalysts, which could directly derive from the carbonization of MOFs and COFs.15-18 The first advantage of using polymers is that they can work as precursors as well as self-templates, which contribute to the formation of high quality microporous carbons.19 Secondly, the surface area, pore structure, and electronic property of the carbonized products can be easily regulated through designing the framework of polymers.20 More importantly, the incorporation of heteroatom like nitrogen (N) into the carbon framework is more feasible than carbon nanotubes or graphene because it can be achieved by directly pyrolyzing the N-containing group decorated polymer structures. Since Gong et al reported the high ORR activity of the N-doped carbon nanotubes in 2009,21 the N-doped carbon based catalysts have been widely investigated as ORR catalysts. The improved catalytic performance of ORR by N doping originates from the electronic properties tuned by the heteroatoms in the graphitic framework to facilitate the conductivity, oxidation stability and catalytic activity.22 Many of N-doped carbon materials present superior activity and stability to those of non-doped materials for catalyzing ORR.23-26 However, most of these materials present higher overpotential and relatively lower activity than commercial Pt/C catalysts.27 In this regard, supporting low proportion of metals on heteroatom-doped carbon materials emerges as a promising way to further improve the electrocatalytic efficiency.28 Based on theoretical and experimental studies, when using heteroatoms-doped carbon materials as metal support,29 the heteroatoms induced defects will favor the dispersibility of metal nanoparticles (NPs) and charge transfer between metal and supports.30-33 Pd is considered as a cheap alternative to the rare Pt catalyst because of its higher electrocatalytic activity and stability than that of other 3d transition metals (such as Ni, Co, and Fe) for ORR.34-38 For example, Jukk et al found that that Pd NPs decorated N-doped graphene catalyst showed


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excellent ORR performance in alkaline media.39 Vinayan et al reported that triangular shaped Pd NPs decorated N-doped graphene was a very good electrocatalyst with high stability and methanol tolerance for ORR in acidic media due to the N-doping induced strong binding between Pd NPs and graphene support.40 Therefore, incorporating Pd NPs into N-doped carbon support matrix with high surface area and regular pore texture can result in a promising fuel cell cathode catalyst material. Previously we have proposed a low-cost strategy to synthesize one type of high surface area microporous organic polymers, knitting aryl network polymers (KAPs), via one-step ‘knitting’ of rigid aromatic building blocks with an external crosslinker.41-43 In this work, we obtain high surface area microporous carbon materials from directly pyrolyzing such KAPs and use them to support Pd NPs. (Figure 1. see Supporting Information for experimental

Figure 1. Synthetic route of Pd/CKB, Pd/CKTB and Pd/CKN from different monomer. details) There have been no reports, to the best of our knowledge, involving microporous carbon supported Pd as ORR catalytsts. Further, the impact of framework and heteroatom on the ORR performance is systematically compared and analyzed through modulating the polymer skeleton.


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For this purpose, we have synthesized three types of KAPs (KAPs-Ph, KAPs-PhPh3, and KAPsNHC) from different monomers via a similar cross-linked reaction. These carbonized products can be used as typical metal supporting models for comparative investigation. Also, we examined Pd NPs loading on commercial Vulcan XC-72 to illustrate the superiority of such KAPs. EXPERIMENTAL SECTION Preparation of microporous carbon supports We synthesized three types of KAPs (KAPs-Ph, KAPs-PhPh3, and KAPs-NHC) from different monomers via a similar cross-linked reaction.41-43 The microporous carbon materials are obtained through directly pyrolyzing such KAPs at 800 oC for 4 h with a heating rate of 2 oC min-1 and then cooling down to room temperature under N2 flow. The corresponding black carbon products are labeled as CKB, CKTB and CKN; the yields of carbon products from KAPs-Ph, KAPsPhPh3, and KAPs-NHC is 37%, 48% and 55%, respectively. The products carbonized at different temperature are also obtained for comparison. Preparation of carbon supported Pd NPs Pd NPs are supported on the above carbonized products by a gas phase reduction method (Figure 1. b). In a typical process, 10 mg PdCl2 is dissolved in 5 ml water by the addition of 1 ml aqueous HCl (0.1 mol/ml) at room temperature with vigorous stirring. Then, 0.1 ml aqueous ammonia (37%, V/V) is mixed with PdCl2 solution, and 1 mol/L NaOH solution is dropwise added to adjust to pH 9. Then 110 mg CKB carbon powders are added to the above mixture and the slurry was stirred for 12 h. The water is removed from the slurry by rotary evaporator and dried at vacuum at 60 ºC for 12 h. The PdCl2 impregnated microporous carbon is heated in H2 flow to 200 oC at a heating rate of 0.6K min-1 and kept for 2 h to reduce Pd (II) to Pd (0), then


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cooled down to room temperature under N2 flow. The resulted microporous carbon supported Pd NPs catalyst is named as Pd/CKB. The theoretical loading amount of Pd is 5 wt% (weight proportion). Similarly, the Pd/CKTB, Pd/CKN, and Pd/XC-72 catalysts are prepared for comparison (Figure 1). Commercial Pt/C (Vulcan, 20 wt %) with much higher Pt content and commercial Pd/C (Aladdin, 5 wt %) with similar Pd content are also used for comparison. Characterizations Scanning electron microscope (FESEM) is investigated with a FEI Sirion 200 field emission. Transmission electron microscopy (TEM) is carried out on a FEI Tecnai G2 20 electron microscoperunning at 200 kV. The average Pd particle size is calculated by counting over 200 Pd NPs. The XRD patterns are obtained on a Bruker Advanced D8 diffractometer over a 2θ range of 5–90o with Cu Kα radiation. Elemental analyzer (EA) is performed on a Vario Micro cube Elemental Analyzer (Elementar, Germany). Pd content is analyzed by atomic absorption spectrometry (AAS) on a Perkin Elmer AA-300. X-ray photoelectron spectroscopy (XPS) analysis is carried out with a VG Multilab 2000 spectrometer using Al Kα radiation at a power of 300 W. The pass energy is set at 100 eV, and C1s line at 284.6 eV is used as a reference. All the samples are washed with acetone and dried in vacuum oven before XPS characterization. Electrochemical tests Electrochemical properties such as the ORR activities of the catalysts are investigated by a glassy carbon rotating disk electrode (RDE, Pine Research Instrumentation, USA) controlled with a CHI 760D electrochemical analysis instrument (CH Instruments, China) under ambient conditions. To simulate the alkaline environment of PEMFCs, 0.1M potassium hydroxide (KOH) is used as an electrolyte. A Pt wire is used as the counter electrode and an Ag/AgCl is used as the reference electrode. 4 mg of catalyst are dispersed in 0.8 ml 0.1wt% Nafion solutions (5wt%


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Nafion solution dissolved in isopropanol) by at least 30min sonication to form a homogeneous ink. Then 6 µl of the ink is dropped and dried to form a thin film on a glassy carbon disk electrode with a 0.19625cm2 geometric surface area. Before the measurement, electrochemical cleaning was performed by sweeping potentials between 0.2 and -0.8 V at a scan rate of 20mVs-1 for 10 cycles in O2-saturated 0.1M KOH. Cyclic voltammetry (CV) was performed in the potential range from 0.2 to -0.8 V at a scan rate of 20 mV/s in O2-staturated 0.1M KOH. The ORR activity evaluation was carried out by linear sweep voltammetry (LSV) in the anodic direction from 0.2 to -0.8V with rotating speeds of 900 rpm at a scan rate of 5 mV/s in O2saturated 0.1 M KOH. The accelerated durability test (ADT) of the catalysts was performed to investigate the electrochemical durability of the catalysts. The potential was cycled in O2saturated 0.1 M KOH between 0.2 and -0.8 V at a scan rate of 50 mV/s for 1000 cycles. Durability of catalysts are evaluated from the i–t chronoamperometric responses of as-prepared Pd loading on carbon in O2-staturated 0.1 M KOH at 0.7 V (vs. Ag/AgCl) for 5000s with a rotation rate of 900 rpm. The methanol-crossover effects are measured by recording i-t chronoamperometric responses for ORR after adding 1.5M methanol into the O2-saturated 0.1M KOH solution. The Koutecky-Levich plots were obtained by RDE measurements at different rotating speeds from 400 to 2500 rpm. The exact kinetic parameters including electron transfer number (n) and kinetic current density (Jd) were analyzed using Koutecky–Levich equations. Methanol-tolerant experiments were conducted by recording the difference in LSV of electrocatalysts in O2-saturated 0.1 M KOH solution without methanol and with adding 1.5 mol/L methanol. In addition, n was determined using an alternative method, which involved measurement of rotating ring-disk electrode (RRDE) voltammograms (Pine Research Instrumentation, USA). The disk electrode was scanned at a rate of 5 mV·s-1 and a rotation rate


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of 900 rpm in O2 saturated 0.1 M KOH. Results and Discussion The surface areas and pore distributions of products are determined by nitrogen physisorption analysis (Figure S1 and Figure 2, Table 1). The N2 adsorption-desorption isotherms of these

Figure 2. Nitrogen sorption isotherms of MOPs-derived carbon supported Pd NPs, Pd/CKB, Pd/CKTB and Pd/CKN at 77.3 K (a), and pore size distributions (b and c). samples are typical type I according to the IUPAC classification, indicating their microporous properties (Figure S1 and Figure 2a). The hysteresis loops at the high relative pressure (P/P0=0.8-1) are

attributed to capillary condensation-evaporation from the mesopores and

macropores. Among three carbonized products, Pd/CKTB shows a less pronounced hysteresis loop at this relative pressure range, revealing more mesopores in Pd/CKB and Pd/CKN. The results were further confirmed by the pore size distribution curves, which are shown in Figure 2 (b and c). The three catalysts all show a sharp peak at approximately 0.6 nm, which corresponds to primary micropores. Pd/CKB and Pd/CKN exhibit a less intensity peak in the microporous region, while in mesoporous region of about 3.5 nm, the intensity of Pd/CKTB is the least. The Pd/CKB has a broad peak in the region of 10-40 nm, showing the existence of a large amount of


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Table 1. Surface area and porosity of the as-prepared microporous organic polymers and carbonized products. SBETa)

SLb)

P.V.c)

M.A.d)

M.A.e)

Pd

Sample

[m2g-1]

[m2g-1]

[cm3g-1]

[m2g-1]

[%]

[wt%]

KAPs-Ph

957

1167

1.4

518

54.1

-

KAPs-PhPh3

1068

1155

0.56

912

85.4

-

KAPs-NHC

581

710

1.1

356

61.3

-

Pd/CKB

625

727

1.3

251

40.2

4.32

Pd/CKTB

933

991

0.8

736

78.9

4.40

Pd/CKN

493

546

0.9

281

57.0

4.63

a) Surface area calculated from N2 adsorption/desorption isotherms at 77.3 K using BET equation; b) Surface area calculated from N2 adsorption/desorption isotherms at 77.3 K using Langmuir equation; c) Pore volume calculated from N2 isotherm at P/P0 = 0.995, 77.3 K; d) t Plot micropore area; e) t-Plot micropore area/BET surface area *100%. macropores. The different type of porosity in the resulting carbons depends on the structure of organic monomer and the skeleton of polymer precursor. In the hypercrosslinked aromatic compoundbased polymers, the Friedel-Crafts reaction site is uncontrolled, so that the methylene bridge formed between the benzene rings is random. Compared to Ph, the PhPh3 monomer has a sterichindrance effect promoting the reaction response in para-position. Moreover, PhPh3 is a threedimensional configuration, which caused the resulting polymer (KAPs-PhPh3) to possess higher surface area, stronger skeleton strength, and narrower pore distribution. The structural properties of NHC-derived polymer precursor (KAPs-NHC) are between KAPs-Ph and KAPs-PhPh3. Detailed characterizations and discussions can be found in our previous studies.41-43 All of three as-prepared KAPs show a high surface area, and the KAPs-PhPh3 can reach the highest BET


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value of 1068 m2g-1 with a 85% microporosity. High-temperature carbonization causes the obvious decrease in surface area and microporosity: (1) surface area: 35% for KAPs- Ph, 13% for KAPs-PhPh3, and 15% for KAPs-NHC; (2) microporosity: 26% for KAPs-Ph, 8% for KAPsPhPh3, and 7% for KAPs-NHC. Although KAPs-Ph possesses an approximate value of SBET to KAPs-PhPh3, many of the micropores disappears during high-temperature treatment, resulting in the dramatical decrease of surface area. As a result, we suppose that using PhPh3 and NHC as monomers benefits the formation of rigid polymer frameworks, which can keep the stable skeleton without collapse even after annealing at 800oC. The results indicate that the carbon derived from rigid polymer skeleton exhibits higher surface area and narrower pore distribution. Raman spectroscopy is adopted to study the change from polymer framework to carbon skeleton by examining the bonding state of carbon atoms (Figure 3a and b). The D band and G band are located at around 1330 and 1580 cm-1, respectively. It has been found that the G band arise from the bond stretching of all sp2-bonded pairs, including C-C and N-C bond, while the D band is associated with the sp3 defect sites.44 A small broad peak at 2500-3910 cm-1 in Pd/CKN can be

Figure 3. Raman spectra of as-prepared samples. (a) Pd loading on carbonized products from three types of MOPs at 800 ºC. (b) Pd/CKN samples at different temperature. (c) XRD patterns of Pd NPs supporting on different carbon materials.


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assigned to a combination of D+D and D+G bands. The relatively high intensity of D band in Pd/CKN sample, with respect to those of broad D bands in Pd/CKB and Pd/CKTB samples (Figure 3a), can be attributed to the change of electronic structure by introducing N- dopant heteroatoms into the graphitic networks.45 For carbon-based materials, ID/IG is an interesting indicator of the level of defects in the prepared carbon materials. Take in Pd/CKN as example, the influence of annealing temperature is given in Figure 3b. With the increase of annealing temperature, the ID/IG

of the product increases. The gradually increased values (0.9–1.1) of

Pd/CKN-X (X=600–900) could be attributive to the transitional stage from amorphous carbon to nanocrystalline graphite, indicating the formation of nanocrystalline graphite with increasing the carbonization temperature.46-47 Meanwhile, the surface area and N proportion gradually decrease at the elevated temperature (Table 2), which is consistent with other reports.48-49 The slight change in surface area with varied temperature also suggests the stable polymer framework being kept up to 800oC. The decline in N content is mainly caused by the reduction effect and “selfrepairing” of the graphite structure at higher temperature.50 The broadening D band of Pd/CKN900 is very similar to Pd/CKTB, which might be ascribed to the decreased N content being not enough to induce obvious disorder of graphitic networks. The broad diffraction peaks at around 25o in XRD patterns further confirm the increase of graphitization degree with the carbonization temperature, which is in good agreement with Raman spectra (Figure S2). Each catalyst shows four broad peaks at around 40.1, 46.7, 68.2 and 82.2o, which are indexed to the (111), (200), (222) and (311) planes of the face-centered-cubic structure of Pd (JCPDS 03-065-2867). Although the temperature for reducing Pd2+ is same, the dramatically sharpening of Pd diffraction peaks at the carbonization temperature of 900 oC suggests the aggregation of Pd NPs because of the decrease


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Table 2. BET surface area and element analysis of Pd/CKN from different temperature. NHC

Pd/CKN -600

Pd/CKN -700

Pd/CKN -800

Pd/CKN -900

SBET(m2 g-1)

581

529

499

493

447

N/C (weight ratio)

0.075

0.05

0.037

0.033

0.029

Samples

in specific surface area as well as N content of carbon support. The crystallite size of Pd NPs on different carbon supports are calculated to be 18.9 nm for CKB, 7.62 nm for CKTB, 9.52 nm for CKN, 14.7 nm for XC-72 using Scherrer`s equation (Figure 3c). Since the physicochemical characters of carbon support (e.g. surface area, pore structure, and electronic property) can influence the metal nucleation and growth, which will determine the catalytic activity and durability of catalysts.51-52 The size and morphology of Pd NPs loading on three carbonized products are investigated by TEM observation. The Pd content is controlled to be low enough to circumvent the particle size effect on the ORR activity and durability of catalyst. In this sense, the Pd loading is adjusted to 5%, and the well-dispersed Pd NPs are supported on the surface of different kinds of carbon materials, as shown in Figure 4 and Figure S3. The lattice spacing of d = 0.22 nm in HRTEM image can be ascribed to the Pd(111) planes. Different brightness of Pd NPs in STEM image indicates that they are loaded on the surface of support as well as encapsulated between the carbon layers (Figure S4). The average sizes of the Pd NPs, using approximate 200 particles, are measured to be 8.1, 7.1, 3.0, 3.3, and 4.4 nm for Pd/XC-72, Pd/CKB, Pd/CKTB, Pd/CKN, and commercial Pd/C, respectively (Figure 4a-e, 4i-k, and S3a-d). It was found that there are differences in size between TEM measurement and XRD calculations. The extensive regions of TEM images of Pd/CKN are collected, as shown in Figure


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Figure 4. TEM images of Pd/CKN (a-c), Pd/CKTB (d), and commercial Pd/C (e) before ADT; TEM images of Pd/CKN (f), Pd/CKTB (g), and commercial Pd/C (h) after ADT; particle size distributions of Pd/CKN (i), Pd/CKTB (j), and commercial Pd/C (k) before ADT; particle size distributions of Pd/CKN (l), Pd/CKTB (m), and commercial Pd/C (n) after ADT. The samples after ADT were obtained by ultrasonic treatment of electrode in ethanol. S5. Among abundant Pd NPs, there are a few larger Pd NPs with size distribution of 10~20 nm (indicated by red circles). These larger crystals may contribute to the strong diffraction peaks of XRD patterns. Moreover, the Pd NPs on CKTB and CKN have much narrower size distribution than those on other supports. The CKTB system implies that the high surface area, regular microporous structure, and stable carbon skeleton all facilitate the dispersibility of Pd NPs. For CKN support, besides the stable carbon skeleton, N-dopant induced defects can act as trapping sites for anchoring Pd NPs to prevent the aggregation.53 As shown in TEM images and size distributions after ADT, the average sizes of the Pd NPs grow to 3.9, 3.4, and 5.7 nm for


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Pd/CKN, Pd/CKTB, and commercial Pd/C, respectively (Figure 4f-h and 4l-n). The changes in size distributions are in accordance with the results of electrocatalytic measurements discussed below. In order to confirm the composition of the as-obtained samples and the chemical states of the heteroatom, XPS characterization is performed (Figure S6 and Figure 5). The deconvoluted high resolution C1s spectrum can be separated into four components centered at the binding energies of 284.5 (C-C), 285.7 (C-OH), 287.4 (C-N) and 290.7 eV (oxidized C) which also reveal the incorporation of N within carbon networks (Figure S6).54 The N1s signal in KAPs-NHC indicates two types of nitrogen species, pyridinic N (397.9 eV) and pyrrolic N (399.6 eV) (Figure 5a). The N1s signal in the carbonized product (Pd/CKN) can be deconvoluted into four different types of nitrogen species, i.e., pyridinic N (398.1 eV), pyrrolic N (399.7 eV), quaternary N (400.8 eV) and oxidized N (402.3 eV) (Figure 5a).55-58 The peak of pyridinic N also includes a contribution from N bound to the metal (Pd-N) due to the small difference in binding energies.59,60 The high-temperature pyrolysis induces decomposition and reconstruction of N

Figure 5. (a) High-resolution N1s XPS spectra of N-containing samples, i.e. before carbonization (KAPs-NHC) and after carbonization (Pd/CKN). (b) High-resolution Pd 3d XPS spectra of Pd/CKB, Pd/CKTB, and Pd/CKN.


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species, resulting in the transformation of pyridinic N and pyrrolic N to quaternary N.55 The pyridinic type is the N atoms doping at the edges of the graphitic carbon layers, while the quaternary N is defined as doping inside the graphitic carbon plane.56 That is, N atoms are successfully doped into C networks to replace the C atoms which are located at the edges and inside of the graphitic carbon layers by directly pyrolyzing N-containing MOPs. It has been reported that the quaternary N within carbon layer could enhance the electric conductivity, which is considered to be favorable for ORR.57 Meanwhile, the pyridinic N and pyrrolic N may provide the active sites for enhancing metal anchoring and effectively avoiding agglomeration, which further improve the catalytic activity and stability of metal catalysts.58,61-63 Hence all of these N species are expected to play a crucial role in the ORR process except the uncertain contribution of the oxidized N.57,60 Figure 5b displays high resolution spectra of Pd 3d peaks of Pd/CKB, Pd/CKTB and Pd/CKN, respectivity. For Pd/CKN catalyst, the binding energy of the main spinorbit split doublet (Pd 3d5/2 and Pd 3d3/2) appears at 335.9 and 341.1 eV with a spin-orbital doublet splitting (∆ = Pd 3d3/2 - Pd 3d5/2) of 5.2 eV, confirming the presence of metallic Pd.64 The higher binding energy than the standard value (1.1~1.3 eV) reveals that the strong interactions between Pd and carbon supports lead to a decrease in the electron intensity on the Pd atom.65-66 Compared to N-free carbon supports, slight shift (~0.2 eV) of Pd 3d peaks can be observed when supporting on CKN, indicating the additional electronic interactions of Pd nanoparticles with N species. An additional doublet at about 336.9 and 342.0 eV is attributed to a higher oxidation state of Pd, suggesting the formation of Pd-O bond or Pd-N bond.67-68 Since the probability for the formation of Pd-O bond is less possible in the present synthesis, this higher oxidation state may be mainly because of the formation of Pd-N bond.40


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The influences of framework and heteroatom of MOPs on the ORR catalytic activity of microporous carbonized products as well as carbonized products supported Pd composites are investigated using a RDE in O2-saturated 0.1 M KOH solution (Figure 6a and b). In comparison to commercial carbon (Vulcan XC-72) and CKB, the MOPs derived microporous carbon catalysts (CKN and CKTB) show obviously better performance in onset potential, indicating the ORR process occurred much more easily on CKN and CKTB. The higher reaction current density in CKTB system than that in CKB system suggests the efficient reactant transfer through the regular microporous-rich structure. Among them, the CKN possesses the most positive halfwave potential (E½) and highest reaction current density because N-dopant altered the electronic property of carbon (Figure 6a). The electrical conductivity of CKN (0.83 S cm-1) is much higher than that of XC-72 (0.20 S cm-1), CKB (0.25 S cm-1), and CKTB (0.27 S cm-1). After introducing Pd NPs, the electrocatalytic activities for ORR of all carbon based materials are enhanced.

Figure 6. ORR polarization curves of catalysts at a rotation rate of 900 rpm and scan rate of 5 mV s-1. (a) Several carbonized products from three types of MOPs at 800 ºC. (b) Comparisons of as-prepared microporous carbon supported Pd catalysts to XC-72 supported Pd, commercial Pd/C and Pt/C catalysts. The sample Pd/C refers to commercial Pd/C containing 5 wt% Pd. The sample Pt/C refers to commercial Pt/C containing 20 wt% Pt. (c) Pd loading on CKN that derived from KAPs-NHC at different temperature.


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The ORR performance of Pd loading on MOPs derived microporous carbon follows the similar order: Pd/CKN>Pd/CKTB>Pd/CKB (Figure 6b and Table S1). The smaller metal particles with narrower size distribution usually have high surface energy and active surface area, so they possess high ORR catalytic activity. Moreover, the narrower size distribution implies the close interaction between Pd NPs and microporous carbon supports, which facilitate the charge transfer in ORR system. Therefore, the higher catalytic performance in Pd/CKTB than Pd/CKB is ascribed to the stable CKTB framework with high surface area and regular microporosity besides smaller Pd NPs with narrower size distribution. The highest catalytic activity is achieved in Pd/C system using CKN as support when compared with other MOPs derived microporous carbon supports (CKB, CKTB) and XC-72. The performance of Pd/CKN catalyst function better for ORR than commercial Pd/C (Aladdin, 5 wt %) with similar Pd content in terms of onset potential, reaction current density, and E½. It is noted that the E½ of Pd/CKN catalyst even exhibits 25 mV positive than commercial Pt/C (Vulcan, 20 wt %) with much higher Pt content (Table S1). The improvement of reactivity in Pd/CKN catalyst may be ascribed to the influence of N heteroatom on the electronic structure of carbon framework as well as that of metal NPs through metal/support interfacial interactions. Meanwhile, N-doping will facilitate the charge transfer and electronic interactions between supports and metal NPs.31,69 Figure 6c displays the dependence of ORR activity of Pd/CKN catalysts on the carbonization temperature of KAPsNHC. It could be observed that CKN obtained at 800 oC is the most excellect support for loading metal catalysts probably due to the optimized composition of carbon and heteroatom configurations at this temperature. The same tendency is also observed in Pd/CKTB system (Figure S7). The low-temperature carbonization cannot lead to a high graphitization degree to ensure a good conductivity, while high-temperature could destroy the conjugated structure of the


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Figure 7. CV and ORR polarization curves before and after ADT. (a,b) Pd/CKB; (c,d) Pd/CKTB; (e,f) Pd/CKN; (g,h) commercial 5% Pd/C. (i,j) commercial 20% Pt/C. ADT was performed in O2-saturated 0.1 M KOH electrolyte at a scan rate of 50 mVs-1.


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MOPs-derived porous carbon frameworks with a concomitant reduce in conductivity.18 Cyclic voltammograms of microporous carbon supported Pd NPs, commercial Pd/C, and commercial Pt/C before and after ADT are measured using an O2-saturated 0.1 M KOH electrolyte in the potential range from 0.2 to -0.8V at a scan rate of 50 mV s-1 (Figure 7). In the cathodic scan, the CV shows a peak at approximately -0.2 V, which corresponds to the reduction of the surface oxide on the Pd nanoparticles. The cycling condition of ADT usually causes the corrosion of carbon support, which concomitantly accelerates the agglomeration and Ostwald ripening of metal nanoparticles.70-71 Significant changes in CV and ORR activity (E0, E1/2, and limited current density) are observed on commercial Pd/C and Pt/C catalyst after 1000 cylcles. In comparison, the MOPs-derived porous carbon supported Pd NPs catalysts show slightly different voltammograms during durability test, indicating slight decrease in ORR mass activity after 1000 cycles. The LSV results further confirm the much better stability of metal loading on MOPsderived porous carbon than commercial Pd/C or Pt/C catalyst, which is in consistent with TEM observation (Figure 4).

Figure 8. (a) Durability evaluation from the i–t chronoamperometric responses of as-prepared Pd loading on carbons in O2-staturated 0.1 M KOH at 0.7 V (vs. Ag/AgCl) for 5000 s with a rotation rate of 900 rpm. Also included is the commercial Pd/C and Pt/C catalyst for comparison. (b) Methanol poison effect evaluation on i-t chronoamperometric responses for ORR.


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The current-time (i-t) chronoamperometric measurements are also carried out to investigate the durability of the catalysts, which is performed at a constant potential of 0.7 V (vs Ag/AgCl) for 5000 secends in O2-saturated 0.1 M KOH solution with a rotating speed of 900 rpm (Figure 8). Under the samilar reduction condition for Pd NPs, the catalysts show distinct difference in durability with the order of Pd/CKN>Pd/CKTB>Pd/CKB>Pd/C≈Pt/C>Pd/XC-72 (Figure 8a). That is, metal NPs loading on all MOPs-derived porous carbon supports are more durable than those on commercial XC-72 carbon and commercial Pd/C or Pt/C. Among MOPs-derived carbon supports, the difference mainly originates from the framework and elemental composition. Noteworthy, i-t curves of Pd/CKN and Pd/CKTB exhibit relatively weak current decay as compared to commercial Pd/C with similar Pd content. In addition, the methanol -crossover effects are measured by adding 1.5 M methanol into the O2-saturated 0.1 M KOH solution. As shown in Figure 8b, although Pd/CKN electrode display a further decrease in current, but it still has better methanol tolerance than other catalysts. These results clearly indicate that the catalytical active sites on the CKN are much more stable than those on the commercial Pd/C and have high potential application in methanol base and alkaline fuel cells. The durable Pd/CKTB catalyst is attributable to the efficient stabilization of small metal NPs on the stable carbon skeleton with high surface area and regular pore distribution, which may prevent metal from migrating/agglomerating or detaching from the support. Among these carbon supported Pd NPs, Pd/CKN exhibits the best performance from the aspects of ORR catalytic activity, durability, and methanol tolerance. Besides the carbon skeleton, the superiority in Pd/CKN catalyst may be ascribed to the influence of N heteroatom from the following three factors: (1) acting as trapping sites for anchoring the metal nanoparticles (NPs) to improve the dispersibility on the support


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Figure 9. (a) LSVs of Pd/CKN at different rotating speeds. (b) The Koutecky–Levich plots for Pd/CKN at different potentials. (c) RRDE voltammograms of Pd/CKN at a rotating speed of 900 rpm; (d) The electron-transfer number n and H2O2 yield for Pd/CKN catalyst. RRDE voltammograms was performed in O2-saturated 0.1 M KOH solution with a scanning rate of 5 mV s-1. surface and prevent the aggregation during reaction; (2) facilitating the charge transfer and electronic interactions between supports and metal NPs; (3) altering the electronic structure of carbon framework as well as metal NPs through metal/support interfacial interactions to influence their activity and stability for specific reactions.30-31, 39, 61-63, 69 To explore the kinetic aspects of the Pd/CKN and Pd/CKTB catalysts for the ORR, their respective Koutecky-Levich plots were obtained from the electrode current densities at different rotating speeds (Figure 9 and S8). Figure 9a and Figure S8a shows the RDE polarization curves


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for ORR on Pd/CKN and Pd/CKTB modified electrode with various rotation rates from 400 to 2500 rpm. The diffusion-limiting current densities increased proportionally with the rotating speed. ORR occurs under mixed kinetic-diffusion control at high potential regions, followed by a plateau of the diffusion limiting current, which increase with the rotation rate due to the increase of the oxygen diffusion through the electrode surface. The transferred electron was evaluated using the below Koutecky-Levich (K-L) equation: 1/J = 1/Jk + 1/(Bω1/2)

(1)

Where J represents the measured current density, Jk is the kinetic current density, and ω is the angular velocity of the electrode. B could be calculated from the slope of K-L plots based on the Levich equation as follows: B = 0.62nFCD2/3v-1/6

(2)

Where n is the number of electrons transferred, F is the Faraday constant (F = 96485C mol-1), C is the concentration of O2 gas in 0.1 M KOH solution (C = 1.2*10-6 mol cm-3), D is the diffusion coefficient of O2 gas (D=1.9*10-5cm2s-1), and v is the kinematic viscosity of 0.1M KOH solution (v=0.01 cm2s-1). Figure 9b and Figure S8b depicts the K-L plots of the Pd/CKN and Pd/CKTB catalysts at various potentials from 0.2 to -0.8V vs. Ag/AgCl. There is a good linearity between j-1 and ω-1/2 over the examined potential range. The number of transferred electrons as calculated from the slopes of catalysts are 4 for Pd/CKN and Pd/CKTB catalysts calculated by K-L methods, implying that the ORR of the catalysts followed a direct four-electron transfer mechanism. The ORR kinetics of the catalysts is also investigated by measuring the ring currents, as shown in Figure 9c-d and


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Figure S8c-d. The H2O2 yields during the ORRs were calculated from the measured ring currents, and the number of electrons transferred could be calculated from the following equation as well. n = 4 JD/(JD + JR/N)

(3)

% H2O2 = 200 JR/(JDN + JR)

(4)

Here, JD and JR are the disk and ring currents, respectively, and N is the ring collection efficiency (0.37). The n of 3.51 to 3.83 was achieved in the boltage range of 0.2~-0.8. The ORR over Pd/CKN and Pd/CKTB composite catalysts yields below 20% H2O2 over a wide potential range of -0.2~-0.8, corresponding to an n value of 3.5~3.8. The RRDE results show again the dominant four-electron ORR pathway with few peroxide intermediates, which is consistent with the RDE results. CONCLUSIONS Three types of MOPs are synthesized via one-step ‘knitting’ of rigid aromatic building blocks with an external crosslinker and explored as novel carbon precursors to produce micorpours carbon materials. Surface area and pore structure of MOPs are mostly kept in the carbonization process. To circumvent the size effect, Pd content is controlled to be as low as 5 wt%. Pd NPs are uniformly distributed on three types of MOPs-derived carbon supports. The catalytic performance and durability of MOPs-derived carbon supported Pd NPs follow an order of Pd/CKN>Pd/CKTB>Pd/CKB, most of which exhibit superior behaviors to those loading on commercial XC-72 carbon. First, the difference is ascribed to the characteristic of MOPs framework, such as stability of skeleton, high surface area and uniform pore distribution. For example, carbonization at 800 oC causes different degree of decrease: (1) surface area: 35% for


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KAPs-Ph, 13% for KAPs-PhPh3, and 15% for KAPs-NHC; (2) microporosity: 26% for KAPs-Ph, 8% for KAPs-PhPh3, and 7% for KAPs-NHC. Second, the incorporation of N heteroatom into carbon network affects the ORR process from the following three factors: (1) acting as trapping sites for anchoring the metal nanoparticles (NPs) to improve the dispersibility on the support surface and prevent the aggregation during reaction; (2) facilitating the charge transfer and electronic interactions between supports and metal NPs; (3) altering the electronic structure of carbon framework as well as metal NPs through metal/support interfacial interactions to influence their activity and stability for specific reactions. In addition, the carbonized products from different temperature are studied to present the optimal effects of graphitization degree and heteroatom content. Both Pd/CKN and Pd/CKTB mainly exhibit an efficient 4-electron pathway with excellent durability and methanol tolerance. Although the ORR performance of as-prepared MOPs-derived carbon supported Pd catalysts cannot come up to commercial Pt/C with much higher Pt content, they behaves much better than XC-72 supported Pd and commercial Pd/C with similar Pd content. Therefore, this work opens up new insights for the rational design and development of novel highly-efficient carbon for supporting metal or metal oxide for ORR by simply modulating the framework as well as electronic structure of such polymers. ACKNOWLEDGEMENTS We thank the Analysis and Testing Center, Huazhong University of Science and Technology for their assistance in characterization of materials. This work is supported by the National Natural Science Foundation of China (21571071 and 21473064), the Natural Science Foundation of Hubei Province of China (2015CFB313), the Fundamental Research Funds for the Central


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Microporous Organic Polymers Derived ... - ACS Publications

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