Electrocatalytic Cobalt Nanoparticles Interacting with Nitrogen-Doped

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Electrocatalytic cobalt nanoparticles interacting with nitrogendoped carbon nanotube in-situ generated from a metalorganic framework for the Oxygen Reduction Reaction Haihong Zhong, Yun Luo, Shi He, Pinggui Tang, Dianqing Li, Nicolas Alonso-Vante, and Yongjun Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14942 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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ACS Applied Materials & Interfaces

Electrocatalytic Cobalt Nanoparticles Interacting with Nitrogen-Doped Carbon Nanotube in-situ Generated from a Metal-Organic Framework for the Oxygen Reduction Reaction Haihong Zhong,1,‡ Yun Luo,2,‡ Shi He,1,‡ Pinggui Tang,1 Dianqing Li,1 Nicolas Alonso-Vante,3 Yongjun Feng1,* 1 State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Beijing, 100029, China. 2 New Energy Research Institute, School of Environment and Energy, South China University of

Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong 510006, China. 3 IC2MP, UMR-CNRS 7285, University of Poitiers, F-86022 Poitiers, France.

KEYWORDS. Cobalt nanoparticles; Nitrogen-doped carbon nano-tube; Metal-organic framework; Oxygen reduction reaction; Non-precious metal electrocatalysts.

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ABSTRACT: A metal organic framework (MOF), synthesized from cobalt salt, melamine (mela) and 1,4-dicarboxybezene (BDC), was used as precursor to prepare Co/CoNx/N-CNT/C electrocatalyst via heat-treatment (HT) at different temperature (700 – 900°C) under nitrogen atmosphere. Crystallites size and micro-strain in the 800°C heat-treated sample (MOFs-800) were the lowest, whereas the stacking fault value was the highest, among the rest of the home-made samples, as attested by the Williamson-Hall analysis, hence assessing that the structural or/and surface modification of Co nanoparticles (NPs), found in MOFs-800, was different from other samples. CNTs, in MOFs-800, interacting with Co NPs, were formed on the surface of the support, keeping the hexagonal shape of the initial MOF. Among the three homemade samples, the MOF-800 sample, with the best electrocatalytic performance towards oxygen reduction reaction (ORR) in 0.1 M KOH solution, showed the highest density of CNTs skin on the support, the lowest ID/IG ratio, the largest N atomic content in form of pyridinic-N, CoNx, pyrrolic-N, graphitic-N and oxidized-N species. Based on the binding energy shift toward lower energies, a strong interaction between the active site and the support was identified for MOFs-800 sample. The number of electron transfer was 3.8 on MOFs-800, close to the value of 4.0 determined on the Pt/C benchmark, thus implying a fast and efficient multi-electron reduction of molecular oxygen on CoNx active sites. In addition, the chronoamperometric response within 24000 s showed a more stable current density at 0.69 V/RHE on MOFs-800 as compared with Pt/C.

INTRODUCTION Electrocatalytic oxygen reduction reaction (ORR) is one of the key processes in fuel cells, and metal-air batteries.1, 2 To date, Pt-based materials show still the best electrocatalytic performance towards ORR and are identified as the main barrier in the commercialization of fuel cells due to scarcity and high cost.3-5 Therefore, it is of great interest to explore suitable alternatives based on non-precious metals to replace or decrease the use of Pt-based materials. Indeed, in this sense, lots of cobalt based materials have been synthesized, such as CoNx,6-10 oxides (ex. CoO and Co3O4),11, 12 and chalcogenide (CoSe2).13-16 Among them, CoNx active centers have attracted increasing attention because of its high activity and durability in both acid and alkaline media.17 Various synthetic routes have been developed to prepare Co-Nx/C catalyst with the strong charge transfer between CoNx and carbon support.17-19 It remains, however, a big challenge to design and obtain CoNx/C catalyst with a high electrocatalytic performance towards ORR. Usually, Co-Nx/C catalysts are prepared via pyrolysis of Co-chelated macrocycles (such as porphyrin and its derivatives) complexes or Co salts mixed with carbon support, and N-source.18, 20, 21 The maximum ORR activity was achieved after heat-treatment of Co-chelated macrocyles complexes at 500°C.22 However, these organic moieties were decomposed, whereas metallic Co was formed when heating at > 850°C, showing an ORR activity in alkaline medium. Wu et al.23 reported that porous CoNx/C materials were in-situ formed, during a heat-treatment at 600 – 1000°C under inert gas (Ar or N2) by means of using Co salt precursor, and carbon mixed with a nitrogen source. In their work, the N-doped graphene in the composite was catalytically formed from MWCNT by Co species, and the precursors and the heating temperature play an important role in the in-situ synthesis of highly efficient CoNx/C catalyst. Highly graphitic carbon (e.g. carbon nanotube) supported metal nanoparticles (NPs) show enhanced ORR activity and stability because of an enhanced electron transport, and corrosion-resistance of highly graphitic carbon. Therefore, many efforts have been devoted to in-situ synthesize N-doped carbon nanotube (CNT) or graphene supported Co NPs for ORR.

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More recently, carbon supported non-precious metal electrocatalysts, derived from pyrolysis of metal-organic framework (MOF), have attracted increasing interest in the domain of energy conversion.24-27 The unique morphology of a nanocomposite, derived from MOF, favors electrocatalytic process. However, the decomposition of MOF usually leads to a porous low graphitic carbon support.28, 29 It remains great difficulty to prepare highly graphitic carbon support by in-situ pyrolysis of MOF compounds.27, 30-32 Xia et al.27 reported an in-situ formation of Co/N-doped CNTs (NCNTs) composite from the calcination of a MOF structure (ZIF-67, composed by Co ions and 2-methylimidazole ligands) under reducing atmosphere (Ar/H2, 90%/10% volume). Also Wu et al.23 reported that highly graphitic carbon could be catalytically formed to CNTs by Co species at > 600°C under the reducing atmosphere at 600-900°C, and no CNTs could be derived from calcination of MOF under inert gas.24 These authors suggest that the reducing atmosphere is essential for the formation of NCNTs. Herein, we reported the in-situ formation of NCNTs supported CoNx active centers via heat-treatment of a single MOF precursor (Co-mela-BDC). Co NPs embedded N-doped CNTs (NCNTs) were derived from calcination of MOF precursor under N2 atmosphere at 700-900°C, denoted as MOFs-x (x = heating temperature). The samples were carefully investigated for ORR in alkaline medium (activity and durability). The MOFs-800 sample shows the highest activity and the best durability for ORR among all the homemade samples, which is comparable with the commercial Pt/C (20 wt%) catalyst. EXPERIMENTAL SECTION Synthesis of MOFs: Co-mela-BDC and Co-BDC. For Co-mela-BDC, a solution containing C4H6CoO4•4(H2O) (0.998 g, 4 mmol, Sigma Aldrich), 1,4-dicarboxybenzene (BDC, 1.05 g, 6.33 mmol, Sigma Aldrich), and melamine (0.4730 g, 3.75 mmol, Sigma Aldrich) was prepared in 56 mL dimethylformamide (DMF, Sigma Aldrich). It was simultaneously added into anhydrous ethanol (14 mL, Sigma Aldrich) under ultrasound for 30 min. The mixed solution was then sealed in a Teflon-lined stainless steel autoclave and heated at 120°C for 12 h. The obtained pink powder was Co-mela-BDC, washed with DMF (50 mL × 3) and dried at 60°C under air overnight. The synthesis route of Co-BDC (without melamine) was the same. Synthesis of MOFs-x and Ref-MOFs-800 samples. The obtained Co-mela-BDC powder was heat-treated under N2 (99.99 %, Air Liquide) firstly from room temperature to 250°C with a ramp of 5°C / min, remaining at 250°C for 2h. The temperature was increased up to 700°C (MOFs-700), 800°C (MOFs-800) and 900°C (MOFs-900) with the same heating rate, remaining at the temperature for 4h. Then, it was cooled down to room temperature under N2. The heat-treated powders were immersed into 10 % HNO3 for 1h, washed by ultra-pure water (MilliQ) 3 times, and then dried under air at 60°C overnight. The Ref-MOFs-800 sample was prepared from Co-BDC precursor, following the same protocol. Physicochemical characterization. Powder X-ray diffraction (pXRD) patterns were collected on a Shimadzu XRD-6000 X-ray diffractometer with a scanning rate of 10°/2ɵ min-1 using Cu-Kα (λ= 0.15406 nm) radiation at 40

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kV and 30 mA. The diffraction peaks were firstly corrected by standard LaB6 obtained under the same experimental conditions. All the patterns were fitted by Pearson VII function using the Fityk free software. The morphology and structure of the samples were examined using a Zeiss Supra 55 scanning electron microscope (SEM). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM, a line resolution of 0.19 nm) were carried out on a JEOL JEM-2010 electron microscope at 200 kV. Species for TEM and HRTEM were covered with an additional coating to reduce magnetism. X-ray photoelectron spectroscopy (XPS) was measured on VG ESCALAB 2201 XL spectrometer equipped with an Al Kα anode. Low-temperature nitrogen adsorption-desorption experiments were performed on a Quantachrome Autosorb-1C-VP analyzer. The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) method based on the adsorption isotherm. The Barrett-Joyner-Halenda (BJH) method was used to calculate the pore volume, and the pore size distribution. Raman spectra of these samples were obtained using Nanofinder 3.0 Raman spectrometer (Tokyo Instrument) with a visible laser beam of 532 nm. Electrochemical measurement. All the measurements were carried out in a standard three-electrode cell with 0.1 M KOH as electrolyte, using a 1 cm2 Pt plate as counter electrode, and saturated calomel electrode (SCE = 0.99 V vs. RHE (reversible hydrogen electrode) in 0.1 M KOH) as reference electrode. In this work, all potentials are quoted versus RHE. The current density on the disk was calculated based on the geometric area. Before use, the glassy carbon disk electrode with a geometric area of 0.07 cm2 was polished with γ-alumina powder (5A) and successively ultrasonicated in water and ethanol for 10 min. The ink was prepared by dispersing 8.8 mg catalyst powder in 250 μL isopropanol, 1000 μL ultra-pure water and 250 μL Nafion (5 wt % in mixture of lower aliphatic alcohols and water, Dupont) mixed by ultrasound for 30 min. 4 μL of ink was deposited on the working electrode surface (the mass loadings of MOFs-x and 20 wt% Pt/C were 335 μg∙cm-2 for C0/CoNx/N-CNT/C and 40 μg∙cm-2 for Pt, respectively). Prior to electrochemical measurements on a Pine Instruments device, the electrolyte was saturated with Ar or O2. Then cyclic voltammograms were recorded at a scan rate of 50 mV s-1. In O2 saturated electrolyte, ORR curves were recorded using linear sweep voltammograms (LSVs) at a rate of 5 mV s-1 on RDE at different rotating speeds from 900 to 2500 rpm. The kinetics was analyzed using the Koutecky–Levich (K–L) Equation (1): (1) where j is the current density, jk the kinetic current density, jL the limiting current density, C0 the concentration of molecular oxygen (1.14

10-6 mol cm-3),33 ω the rotating rate. Given, n is the number of electron transfer, F

Faraday constant (96500 C), D diffusion coefficient of O2 (1.73

10-5 cm s-1),33 and v kinetic viscosity (0.01 cm2

s-1),33 the linear fit of equation (1) the slope, B, (B = 0.62nFD2/3v-1/6), and jk were derived. The durability of the MOFs-800 compared with commercial 20wt% Pt/C catalyst for the ORR can be evaluated by current-time (I-t) chronoamperometric response at 1600 rpm at 0.69 V in O2-saturated 0.1 M KOH solutions for 24000 s.

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RESULTS AND DISCUSSION From the PXRD patterns, In Fig. 1(a), one observes three intense diffraction peaks at 9.0˚, 10.78˚ and 17.96˚/2θ for the MOF structure synthesized from Co2+, mela, and BDC. The SEM image shows the morphology of micro-crystalline powder of Co-mela-BDC: typical hexagonal prism with diameter of ca. 2.5 μm, cf. Fig. 1(a) inset. After pyrolysis, the PXRD patterns of MOFs-x (x=700, 800, 900), cf. Figure 1(b), show two diffraction peaks at 26.38˚ and 42.2˚/2θ, belonging to (002) and (100) facet of CNT (JCPDS No.26-1076). Three diffraction peaks at 44.37˚, 51.59˚ and 76.08˚/2θ correspond to (111), (200) and (220) plane of face centered cubic (fcc) Co (JCPDS No.15-0806).34 One can further observe that MOFs-800 sample exhibits an intense diffraction peak (002) indicating additional CNTs obtained in MOFs-800 sample, with respect to MOFs-700 and MOFs-900. This phenomenon assesses that the heating temperature is crucial for the CNTs formation. In order to verify the mechanism of CNTs formation, Ref-MOF-800 was prepared, via calcination at 800°C under N2 of MOF (Co-BDC) without mela ligand. From Fig. 1(b), the CNT (002) and (100) diffraction peaks can be located in Ref-MOF-800 sample, suggesting that CNTs can also be obtained from Co-BDC at 800°C, as supported by the SEM/TEM images in the following section. With the Williamson-Hall analysis, Co fcc lattice parameter (afcc), crystallite size (Lv), stacking fault (α), and microstrain (ε) were estimated via Equation (2): 35-37 (2) where is β is the full-width-at-half-maximum of the Co diffraction peak, λ the wavelength of X-ray source, θ in radians, k the Scherrer constant, Khkl the constant regarding miller’s indices. From Fig. 1(c), βcosθ value changes with 4sinθ one, indicating that the peak broadening is affected by Lv, α, and ε. From calculation, the afcc values for all the homemade catalysts are close to that of the bulk fcc from the literature38 (ca. 0.3548 nm), namely, ca. 0.3547 nm for MOFs-x (x = 700 and 900) and ca. 0.3548 nm for MOFs-800 and Ref-MOFs-800. The MOFs-800 sample has is the lowest Lv value of ca. 14.9 nm among the prepared samples, for example, ca. 16.3 nm for MOFs-700, ca. 23.2 nm for MOFs-900, and ca. 22.8 nm for Ref-MOFs-800. This observation points out that the heat-treatment at 800°C favors the formation of well-dispersed Co NPs, leading to larger active surface. The MOFs-800 has the highest α value, and the lowest ε value among the homemade catalysts, cf. Fig. 1(d). This demonstrates a different structural change of Co or/and a surface modification in MOFs-800, possibly associated to morphology and strong metal-support interaction, so-called SMSI effect. These factors may affect the ORR electrocatalytic process on active sites.

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Figure 1. pXRD patterns for (a) Co-mela-BDC with (inset) SEM image, (b) MOFs-x (x = 700, 800 and 900), Ref-MOFs-800 and Co (grey lines, JCPDS No.15-0806). (c) Williamson-Hall plots of Co (111), (200) and (220) peaks for MOFs-x (x = 700, 800 and 900) and Ref-MOFs-800 sample; (d) Stacking fault and microstrain values, calculated from (c).

For all homemade samples, one sees that the shape of MOF is maintained after heat-treatment, see Fig. 2, whereas the structure of the coordination polymer is decomposed to carbon, and cobalt. On the surface of the hexagonal prism, CNTs are observed for all the MOFs-x samples, Fig. 2(a-d). Nevertheless, it is obvious that the density of CNTs at the surface is much higher on MOFs-800 compared with MOFs-700, and MOFs-900. On Ref-MOFs-800, however, it is hard to observe the presence of CNTs on the surface, Figure 2(e-f ). Instead, it seems that the surface is covered by carbon nanospheres. Co NPs were obtained after pyrolysis of MOFs for all the homemade samples as revealed by TEM in Fig. 3. The average crystallite size was individually ca. 19.5 nm, 15.2 nm, 23.6 nm and 22.6 nm for MOFs-700, MOFs-800, MOFs-900 and Ref-MOFs-800, which were in agreement with the calculated Lv value from the XRD patterns. In addition, all the samples show open voids, implying the formation of porous matrix. Similar to SEM results, less amount of CNTs are detected on MOFs-700, and MOFs-900 as compared with MOFs-800, Fig. 3(a-c). It is clear that Co NPs are embedded into CNTs, Fig. 3d, which possibly are attributed to the catalytic effect of Co on CNTs growth.39, 40 In our work, however, CNTs are in-situ obtained by pyrolysis under inert gas, which is not under reducing atmosphere as

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reported by Xia et al.27 A possible explanation is the reducing effect of H atoms rich ligands in the Co-mela-BDC precursor as described in the literature.25 In the Ref-MOFs-800 sample, Fig. 3(e-f ) displays hollow carbon spheres mixed with Co embedded CNTs. Taking the SEM images depicted in Fig. 2(e-f ) into account, the morphology of Ref-MOFs-800 should be CNTs formed beneath carbon nanosphere surface. Based on the results from both SEM and TEM, the heating temperature (e.g. 800°C) is an optimum temperature to obtain large amount of CNTs, and mela ligands affect the surface growth of CNTs.

Figure 2. The SEM images of (a) MOFs-700, (b) MOFs-900, (c-d) MOFs-800 and (e-f) Ref -MOFs-800.

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Figure 3. The TEM images of (a) MOFs-700, (b) MOFs-900, (c-d) MOFs-800 and (e-f) Ref -MOFs-800. The insets display size distribution histograms.

Also, Fig. 4a shows Raman spectra of three calcined samples with overlapping bands. After deconvolution, four bands are identified in the spectra: (1) 1190-1200 cm-1, (2) 1350 cm-1, (3) 1500 cm-1, and (4) 1579-1599 cm-1. The band (1) is related to sp3 rich phase, hexagonal diamond and nanocrystalline diamond,41, 42 and the band (3) is associated to amorphous sp2 phase.43 It is well known that the band (2) and (4) are D and G mode, respectively. The D band is dominant for all the homemade samples, suggesting that disorder (e.g. porous carbon matrix)

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coexists with the graphitic CNTs. The ID/IG ratio, derived from the integrated area of each band, allows us to evaluate the graphitization degree of the homemade samples. This ratio decreases from ca. 2.00 for MOFs-700 to ca. 1.52 for MOFs-800, then, increases to ca. 1.81 for MOFs-900. Such a fact show that MOFs-800 is more graphitized related to MOFs-700 and MOFs-900, in good agreement with SEM/TEM results (cf. Figs. 2-3). The ID/IG ratio in Ref-MOFs-800 sample is ca. 1.72 with less graphitic phase (CNTs) compared with MOFs-800. Moreover, as listed in Table 1, the G-band position in MOFs-x (x = 700, 800 and 900) is centered at higher wave numbers with respect to Ref-MOFs-800, probably suggesting that MOFs-x carbon materials are more disordered.44 The in-plane crystallite size, La, Table 1, was determined based on the ID/IG ratio via Equation (3): 45, 46

La = 2.4 10-10

(ID/IG)-1

(3)

where λ is the laser source wavelength (532 nm). The La value varies from 9.6 to 12.6 nm, Table 1. It reveals that the carbon crystallite size of MOFs-800 is slightly larger than that in other homemade samples, probably leading to a reduced resistivity.47 From the full-width-at-half-maximum of D band (ω1/2D in Table 1), the Ref-MOFs-800 (ca. 94 cm-1), and MOFs-900 (ca. 91 cm-1) have a narrower in-plane crystallite size distribution related to MOFs-700 (ca. 121 cm-1). MOFs-800 (ca. 146 cm-1) displays the broadest distribution among all the catalysts. The surface area of carbon was determined by Brunauer-Emmett-Teller (BET) measurements. Fig. 4(b) depicts the N2 adsorption-desorption isotherms of type III with a hysteresis loop for all the samples. The BET surface area of MOFs-800 (ca. 138.9 m2g-1) is higher than that of MOFs-700 (ca. 126.3 m2g-1) and MOFs-900 (ca. 103.1 m2g-1), but lower than Ref-MOFs-800 (ca. 190.5 m2g-1), see Table 1. The inset of Fig. 4(b) shows the pore size: ca. 3.8 nm for MOFs-800, which is larger than MOFs-700 (ca. 3.6 nm), and MOFs-900 (ca. 3.6 nm) but lower than Ref-MOFs-800 (ca. 3.9 nm). These result asses a different carbon matrix in MOFs-800 with respect to MOFs-700 and MOFs-900, attributed to a greater amount of CNTs (cf. SEM/TEM images in Fig. 2-3). Ref-MOFs-800 is obviously more porous than MOFs-800, possibly associated to hollow carbon nanospheres (cf. TEM images in Fig. 3).

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Figure 4. (a) Raman spectra, and (b) N2 adsorption-desorption isotherms with (inset) pore width distribution (using BJH method), for MOFs-700, MOFs-800, MOFs-900 and Ref-MOFs-800.

Table 1. The G-band position, in-plane crystallite size (La), full width at half maximum for D-band (ω1/2D) from Raman spectra. BET surface and pore width based on N2 adsorption/desorption isotherms. La (nm)

ω1/2D -1 (cm )

BET surface 2 -1 (m g )

Pore diameter (nm)

1578.5

11.2

94

190.5

3.9

MOFs-900

1596.9

10.6

91

103.1

3.6

MOFs-800

1599.4

12.6

146

138.9

3.8

MOFs-700

1593.6

9.6

121

126.3

3.6

Sample

G-band -1 (cm )

Ref-MOFs-800

position

Fig. 5 demonstrates high-resolution X-ray photoelectron spectra of MOFs-x to investigate the temperature effect towards the in-situ formation of CoNx/C composite. The Co 2p spectra present three chemical signal, Co (~778.5 eV), Co2+ (~780.5 eV) and CoNx (~782.5 eV) in Fig. 5(a). As summarized in Table 2, Co2+ representing CoOx or/and CoCxNy species is dominant for MOFs-700 and MOF-800, whereas metallic Co is dominant in MOFs-900. The content of CoNx moieties, known as the most active sites for ORR,48 is the highest in MOFs-800 (ca. 30 %), suggesting that the optimized heating temperature is 800°C for the formation of CoNx active sites. Moreover, concerning the peak position of Co, Co2+ and CoNx, a following trend is obtained: MOFs-800 < MOFs-900 < MOFs-700, cf. Table 2. This fact suggests that the electron transfer between Co species and the support in MOFs-800 is stronger than that in MOFs-700, and MOFs-900. Such an effect, known as SMSI,49, 50 and widely present in metal/support nanocomposites, may be at the origin of an enhanced ORR activity and stability.51,

52

Such a result is actually with good agreement of PXRD analysis (cf. Figure 1d), that the

surface/structural modification of Co NPs in MOFs-800 should be related to SMSI. From N 1s spectra in Fig. 5(b), one recognizes five types of nitrogen species: N1 for pyridinic-N (~398.2 eV), N2 for CoNx (~399.1 eV), N3

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for pyrrolic-N (~400.3 eV), N4 for graphitic-N (~401.1 eV) and N5 for oxidized-N (~402.5 eV)

48, 53, 54

on all

MOFs-x. As summarized in Fig. 5(d), the total N atomic content in MOFs-800 is higher than in MOFs-700 and MOFs-900, suggesting that pyrolysis at 800°C favors the N-doping. In addition, the N2, N3, N4, and N5 species are the highest in MOFs-800, while N1 specie is similar in MOFs-800 and MOFs-900. All N species, except for the N5 one are active sites for the ORR.48 As for the C 1s spectra analysis, four carbon species are present, namely, graphitic sp2 (~248.4 eV), trigonal sp2 (~285.4 eV), ternary alcohols (~286.5 eV) and carboxylic (~289.6 eV), cf. Fig. 5(c). For MOFs-x, the graphitic sp2 is dominant on the surface, and the presence of side-peaks is possible associated to N-dopant atoms.53, 55

Figure 5. (a) Co 2p3/2, (b) N 1s, (c) C 1s photoemission spectra for MOFs-700, MOFs-800 and MOFs-900 and (d) the content of nitrogen calculated from N 1s spectra. The C 1s signal of Ref-MOFs-800 is contrasted for comparison purpose.

Table 2. XPS spectra analysis for MOFs-x samples of Co 2p signal: peak position (eV) and atomic percentage. 2+

Sample

Co

Co

CoNx

MOFs-900

778.1 eV

779.6 eV

781.7 eV

48 %

25 %

19 %

MOFs-800

778.1 eV

779.3 eV

781.5 eV

17 %

38 %

30 %

MOFs-700

778.5 eV

779.8 eV

782.3 eV

26 %

40 %

24 %

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The surface electrochemistry of MOFs-x, Ref-MOFs-800 and the benchmark (Pt/C, 20 wt%, ETEK) were evaluated by cyclic voltammetry (CV) in 0.1M KOH solution saturated with Ar and O2 gas at room temperature. In the potential interval of 0.04 - 0.99 V, all the homemade catalysts show a similar electrochemical behavior in Ar-saturated solution, see Fig. 6(a). The Pt/C depicts the typical CV curve. In the O2-saturated solution, MOFs-800 shows a well-defined oxygen reduction peak at 0.82 V, rather similar to that of MOFs-700 (0.81 V), but more positive than that of MOFs-900 (0.79 V), and Ref-MOFs-800 (0.77 V). The ORR curves were recorded on the rotating disk electrode (RDE) in O2-saturated solution. Fig. 6b displays the ORR curves recorded at 1600 rpm. The onset potential, Eonset, see Table 3, of MOFs-x are slightly positive than that on Ref-MOFs-800, and negative related to that of Pt/C. Additionally, the limiting current density (jL) on MOFs-800 is much higher than on MOFs-x (x = 700, 900), and Ref-MOFs-800, cf. Table 3, and Fig. 6(b). In terms of the half-wave potential, E1/2, see Table 3, that of MOFs-800 is the most positive among the homemade catalysts, and similar to that of Pt/C. That is to say, MOFs-800 have the highest electrocatalytic activity towards ORR among the homemade samples. The number of electron transfer (n) at 0.4 V in Table 3, increases from ca. 3.15 for MOFs-700, ca. 2.80 for MOFs-900, and ca. 3.57 for Ref-MOFs-800 to ca. 3.77 for MOFs-800, close to that of Pt/C (4.02). The Tafel slope of MOFs-800 is lower than MOFs-700 and MOFs-900 samples, and differing to that of Pt/C, see Fig. 6(c). Apparently, the ORR mechanism on CoNx in MOFs-800 can be associated to the Co surface/structural modification by the support (N and C). Compared with Co-Nx/C reported in the literature, the enhanced ORR activity on MOFs-800, based on Eonset (0.90 V in this work vs. 0.80 V in literature), and E1/2 value (0.80 V in this work vs. 0.75 V in literature)7a is remarkable. Besides the ORR activity, the ORR stability was also determined by chronoamperometric measurements at 0.69 V in comparison with Pt/C. The remaining current, after 24000 s, was ca. 87.1 % on MOFs-800 and ca. 61.5 % on Pt/C, respectively, see Fig. 6(d). Such a result shows that MOFs-800 is more stable compared with Pt/C benchmark.

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Figure 6. (a) CV curves for MOFs-700, MOFs-800, MOFs-900, Ref- MOFs-800 and Pt/C in O2-saturated (solid line) or -1 Ar-saturated (dashed line) 0.1 M KOH at a sweep rate of 50 mV s . (b) Linear-sweep voltammograms in O2-saturated 0.1 M KOH -1 at a scan rate of 5 mV s , at electrode-rotation speed of 1600 rpm. (c) Tafel plot derived from ORR curves. (d) Chronoamperometric responses (percentage of current retained vs. operation time) of MOFs-800 and Pt/C at 0.69 V vs. RHE in O2-saturated 0.1 M KOH, 1600 rpm.

Table 3. ORR onset potential (Eonset), limiting current density (jL), half-wave potential (E1/2), and number of electron transfer (n). Eonset (V vs. RHE)

jL (mA cm-2 geo)

E1/2 (V vs. RHE)

n

MOFs-700

0.90

-3.47

0.79

3.15

MOFs-800

0.90

-3.84

0.80

3.77

MOFs-900

0.90

-2.71

0.77

2.80

Ref-MOFs-800

0.89

-3.78

0.77

3.57

Pt/C

0.93

-4.51

0.80

4.02

Sample

CONCLUSIONS A highly active and stable Co/CoNx/CNT nanocomposite (MOFs-800) was successfully prepared from a MOF precursor (Co-mela-BDC) at 800°C under inert gas. The heat temperature at 800°C is the optimized condition to obtain a well-dispersed Co


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embedded N-doped CNTs. Among the homemade samples, MOFs-800 showed the best ORR activity in alkaline media because of the

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following reasons: (1) the highest active surface with smallest Co nanoparticles size; (2) surface/structural modification of Co NPs with most increased stacking fault and decreased micro-strain values, related to highest SMSI effect between Co-Nx and CNT; (3) the largest amount of graphical NCNTs in-situ formed on carbon support surface; (4) the highest N content.. It is worthy to notice that, under the same conditions, MOFs-800 showed a higher stability as compared to the Pt/C benchmark.

AUTHOR INFORMATION Corresponding Author *(Y.J.

Feng) E-mail: [email protected]; Telephone: +86 10 6444 8071; Fax: +86 10 6442 5385.

Author Contributions ‡These authors contributed equally. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (21571015, the Innovative Research Group Program), National Basic Research Program of China (973 program, 2014CB932104), Beijing Engineering Center for Hierarchical Catalysts, and Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1205). Further financial help was provided by the bilateral cooperation France-Chine under the frame of PHC Xu-Guangqi 2016 Program (Project 36488YD).

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Table of Contents (TOC)

H.H. Zhong, Y. Luo, S. He, P.G. Tang, D.Q. Li, N. Alonso-Vante, Y.J. Feng* ACS Appl. Mater. Inter. 200X, XX, XXXX Electrocatalytic cobalt nanoparticles interacting with nitrogen-doped carbon nanotube in-situ generated from a metal-organic framework for the Oxygen Reduction Reaction

A highly active and stable nanocomposite Co/CoNx/CNT (MOFs-800) was successfully prepared from a MOF precursor (Co-mela-BDC) at 800°C under inert gas. MOFs-800 showed the best ORR activity in alkaline media because of the smallest Co nanoparticles, highest N content in N-CNTs form, as well as an important SMSI effect, and higher stability as compared to the Pt/C benchmark in alkaline media.


Electrocatalytic Cobalt Nanoparticles Interacting with Nitrogen-Doped

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