Doped Carbon Nanotube in Situ Generated from a

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Dec 29, 2016 - Electrocatalytic oxygen reduction reaction (ORR) is one of the ...... (38) Cerda, J. R.; Andres, P. L. d.; Cebollada, A.; Miranda, R.; Navas,.
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Electrocatalytic Cobalt Nanoparticles Interacting with NitrogenDoped Carbon Nanotube in Situ Generated from a Metal−Organic Framework for the Oxygen Reduction Reaction Haihong Zhong,† Yun Luo,‡ Shi He,† Pinggui Tang,† Dianqing Li,† Nicolas Alonso-Vante,§ and Yongjun Feng*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, No. 15 Beisanhuan East Road, Beijing 100029, China ‡ New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, Guangdong 510006, China § IC2MP, UMR-CNRS 7285, University of Poitiers, F-86022 Poitiers, France 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 at different temperature (700−900 °C) under nitrogen atmosphere. Crystallites size and microstrain 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 homemade samples, as attested to 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 that in 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 toward 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, and 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 multielectron reduction of molecular oxygen on CoNx active sites. In addition, the chronoamperometric response within 24 000 s showed a more stable current density at 0.69 V/RHE on MOFs-800 as compared with that of Pt/C. KEYWORDS: cobalt nanoparticles, nitrogen-doped carbon nanotube, metal−organic framework, oxygen reduction reaction, nonprecious metal electrocatalysts



INTRODUCTION

Usually, Co−Nx/C catalysts are prepared via pyrolysis of Cochelated macrocycles (such as porphyrin and its derivatives) complexes or Co salts mixed with carbon support and Nsource.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/

Electrocatalytic oxygen reduction reaction (ORR) is one of the key processes in fuel cells and metal-air batteries.1,2 To date, Ptbased materials show still the best electrocatalytic performance toward 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 nonprecious 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 (e.g., 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 toward ORR. © 2016 American Chemical Society

Received: November 21, 2016 Accepted: December 29, 2016 Published: December 29, 2016 2541

DOI: 10.1021/acsami.6b14942 ACS Appl. Mater. Interfaces 2017, 9, 2541−2549

Research Article

ACS Applied Materials & Interfaces

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 (gray lines, PDF No. 15−0806, JCPDS, 2004). (c) Williamson−Hall plots of Co (111), (200), and (220) peaks for MOFs-x (x = 700, 800, and 900) and refMOFs-800 sample; (d) stacking fault and microstrain values, calculated from c.

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.

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. More recently, carbon-supported nonprecious 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



EXPERIMENTAL SECTION

Synthesis of MOFs: Co-mela-BDC and Co-BDC. For Co-melaBDC, 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 of dimethylformamide (DMF, Sigma-Aldrich). It was simultaneously added into anhydrous ethanol (14 mL, SigmaAldrich) 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) first from room temperature to 250 °C with a ramp of 5 °C/min, remaining at 250 °C for 2 h. The temperature was then 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 4 h. Then, it was cooled down to room temperature under N2. The heat-treated powders were immersed into 10% HNO3 for 1 h, washed by ultrapure water (Milli-Q) 3 times, and then dried 2542

DOI: 10.1021/acsami.6b14942 ACS Appl. Mater. Interfaces 2017, 9, 2541−2549

Research Article

ACS Applied Materials & Interfaces 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°/2θ min−1 using Cu Kα (λ = 0.15406 nm) radiation at 40 kV and 30 mA. The diffraction peaks were first 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 highresolution 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. Lowtemperature 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 γalumina powder (5A) and successively ultrasonicated in water and ethanol for 10 min. The ink was prepared by dispersing 8.8 mg of catalyst powder in 250 μL of isopropanol, 1000 μL of ultrapure water, and 250 μL of Nafion (5 wt % in mixture of lower aliphatic alcohols and water, Dupont) mixed by ultrasound for 30 min. Then, 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. 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) eq 1:

1 1 1 1 1 = + = + 0 1/2 j jk jL jk BC ω

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

of MOFs-x (x = 700, 800, and 900, cf. Figure 1b) show two diffraction peaks at 26.38 and 42.2°/2θ, belonging to (002) and (100) facet of CNT (Powder Diffraction File (PDF) No. 26− 1076, Joint Committee on Powder Diffraction Standards (JCPDS), 2004). Three diffraction peaks at 44.37, 51.59, and 76.08°/2θ correspond to (111), (200), and (220) planes of face-centered cubic (fcc) Co (PDF No. 15−0806, JCPDS, 2004).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 MOFs700 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 (CoBDC) without mela ligand. From Figure 1b, the CNT (002) and (100) diffraction peaks can be located in ref-MOF-800 sample, suggesting that CNTs can also be obtained from CoBDC 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 eq 2:35−37

(1)

where j is the current density, jk is the kinetic current density, jL is the limiting current density, C0 is the concentration of molecular oxygen (1.14 × 10−6 mol cm−3),33 and ω is the rotating rate. Given n the number of electron transfer, F the Faraday constant (96 500 C), D the diffusion coefficient of O2 (1.73 × 10−5 cm s−1),33 and v the kinetic viscosity (0.01 cm2 s−1),33 for the linear fit of eq 1, the slope B (B = 0.62nFD2/3v−1/6) and jk were derived. The durability of the MOFs-800 compared with commercial 20 wt % 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 24 000 s.

K β cos θ k 4 sin θ = + hkl α + ε λ λ Lv afcc

(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 Figure 1c, β cos θ value changes with 4 sin θ 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 MOFs800 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



RESULTS AND DISCUSSION From the PXRD patterns, In Figure 1a, 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 microcrystalline powder of Co-mela-BDC: typical hexagonal prism with diameter of ca. 2.5 μm (cf. Figure 1a, inset). After pyrolysis, the PXRD patterns 2543

DOI: 10.1021/acsami.6b14942 ACS Appl. Mater. Interfaces 2017, 9, 2541−2549

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Figure 3. 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.

active surface. The MOFs-800 has the highest α value and the lowest ε value among the homemade catalysts (cf. Figure 1d). This demonstrates a different structural change of Co or/and a surface modification in MOFs-800, possibly associated with morphology and strong metal−support interaction, the socalled SMSI effect. These factors may affect the ORR electrocatalytic process on active sites. For all lab-made samples, one sees that the shape of MOF is maintained after heat treatment (Figure 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 (Figure 2a−d). Nevertheless, it is obvious that the density of CNTs at the surface is much higher on MOFs-800 compared with that on MOFs-700, and MOFs-900. On ref-MOFs-800, however, it is hard to observe the presence of CNTs on the surface (Figure 2e,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 Figure 3. The average crystallite size was individually ca. 19.5, 15.2, 23.6, and 22.6 nm for MOFs-700, MOFs-800, MOFs-900, and refMOFs-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, a smaller amount of CNTs is detected on MOFs700 and MOFs-900 as compared with that on MOFs-800 (Figure 3a−c). It is clear that Co NPs are embedded into CNTs, as in Figure 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 reported by Xia et al.27 A possible explanation is the reducing effect of H-rich ligands in the Co-mela-BDC precursor as described in the literature.25 The ref-MOFs-800 sample (Figure 3e,f) displays 2544

DOI: 10.1021/acsami.6b14942 ACS Appl. Mater. Interfaces 2017, 9, 2541−2549

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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.

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, given in Table 1, was determined based on the ID/IG ratio via eq 3:45,46

Table 1. G-Band Position, In-Plane Crystallite Size (La), and Full Width at Half-Maximum for D-Band (ω1/2D) from Raman Spectraa sample refMOFs800 MOFs900 MOFs800 MOFs700

G-band position (cm−1)

La (nm)

ω1/2D (cm−1)

BET surface (m2 g−1)

pore diameter (nm)

1578.5

11.2

94

190.5

3.9

1596.9

10.6

91

103.1

3.6

1599.4

12.6

146

138.9

3.8

1593.6

9.6

121

126.3

3.6

4 La = 2.4 × 10−10 λlaser (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. Figure 4b 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 m2 g−1) is higher than that of MOFs-700 (ca. 126.3 m2 g−1) and MOFs-900 (ca. 103.1 m2 g−1), but lower than that of ref-MOFs-800 (ca. 190.5 m2 g−1, see Table 1). The inset of Figure 4b shows the pore size: ca. 3.8 nm for MOFs800, which is larger than MOFs-700 (ca. 3.6 nm) and MOFs900 (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 Figures 2 and 3). ref-MOFs-800 is obviously more porous than MOFs-800, possibly associated with hollow carbon nanospheres (cf. TEM images in Figure 3). Figure 5 demonstrates high-resolution X-ray photoelectron spectra of MOFs-x to investigate the temperature effect toward 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 Figure 5a. 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, the

a

BET surface and pore width based on N2 adsorption/desorption isotherms.

hollow carbon spheres mixed with Co embedded CNTs. Taking the SEM images depicted in Figure 2e,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. Also, Figure 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. Band (1) is related to sp3-rich phase, hexagonal diamond, and nanocrystalline diamond,41,42 and band (3) is associated with amorphous sp2 phase.43 It is well-known that bands (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) 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 shows that MOFs-800 is more graphitized related to MOFs-700 and MOFs-900, in good agreement with SEM/ TEM results (cf. Figures 2 and 3). The ID/IG ratio in ref-MOFs800 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 2545

DOI: 10.1021/acsami.6b14942 ACS Appl. Mater. Interfaces 2017, 9, 2541−2549

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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 for comparison purpose.

presence of side-peaks is possible associated with N-dopant atoms.53,55 The surface electrochemistry of MOFs-x, ref-MOFs-800, and the benchmark (Pt/C, 20 wt %, ETEK) were evaluated by cyclic voltammetry (CV) in 0.1 M 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 Figure 6a. 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-MOFs800 (0.77 V). The ORR curves were recorded on the rotating disk electrode (RDE) in O2-saturated solution. Figure 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 that on MOFs-x (x = 700 and 900) and ref-MOFs-800 (cf. Table 3 and Figure 6b). In terms of the halfwave 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 toward 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 MOFs900, 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 Figure 6c. Apparently, the ORR mechanism on CoNx in MOFs-800 can be associated with the Co surface/ structural modification by the support (N and C). Compared

Table 2. XPS Spectra Analysis for MOFs-x Samples of Co 2p Signala

a

sample

Co

Co2+

CoNx

MOFs-900 MOFs-800 MOFs-700

778.1 eV (48%) 778.1 eV (17%) 778.5 eV (26%)

779.6 eV (25%) 779.3 eV (38%) 779.8 eV (40%)

781.7 eV (19%) 781.5 eV (30%) 782.3 eV (24%)

Peak position (eV) and atomic percentage.

following trend is obtained: MOFs-800 < MOFs-900 < MOFs700 (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 SMSI49,50 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 Figure 5b, one recognizes five types of nitrogen species: N1 for pyridinic-N (∼398.2 eV), N2 for CoNx (∼399.1 eV), N3 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 Figure 5d, the total N atomic content in MOFs-800 is higher than that 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. Figure 5c). For MOFs-x, the graphitic sp2 is dominant on the surface, and the 2546

DOI: 10.1021/acsami.6b14942 ACS Appl. Mater. Interfaces 2017, 9, 2541−2549

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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 Ar-saturated (dashed line) 0.1 M KOH at a sweep rate of 50 mV s−1. (b) Linear-sweep voltammograms in O2-saturated 0.1 M KOH at a scan rate of 5 mV s−1 at electroderotation 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 at 1600 rpm.

well-dispersed Co embedded N-doped CNTs. Among the homemade samples, MOFs-800 showed the best ORR activity in alkaline media because of the 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 microstrain 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; and (4) the highest N content. It is noteworthy that under the same conditions MOFs-800 showed a higher stability as compared to the Pt/C benchmark.

Table 3. ORR Onset Potential (Eonset), Limiting Current Density (jL), Half-Wave Potential (E1/2), and Number of Electron Transfer (n) sample

Eonset (V vs RHE)

MOFs-700 MOFs-800 MOFs-900 ref-MOFs-800 Pt/C

0.90 0.90 0.90 0.89 0.93

jL (mA cm−2 −3.47 −3.84 −2.71 −3.78 −4.51

geo)

E1/2 (V vs RHE)

n

0.79 0.80 0.77 0.77 0.80

3.15 3.77 2.80 3.57 4.02



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)18 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 24 000 s was ca. 87.1% on MOFs-800 and ca. 61.5% on Pt/C, respectively; see Figure 6d. Such a result shows that MOFs-800 is more stable compared with Pt/ C benchmark.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86 10 6444 8071. Fax: +86 10 6442 5385. ORCID

Nicolas Alonso-Vante: 0000-0002-6311-9258 Yongjun Feng: 0000-0001-9254-6219



Author Contributions

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

H.Z., Y.L., and S.H. contributed equally to the creation of this work. Notes

The authors declare no competing financial interest. 2547

DOI: 10.1021/acsami.6b14942 ACS Appl. Mater. Interfaces 2017, 9, 2541−2549

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ACS Applied Materials & Interfaces



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ACKNOWLEDGMENTS 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−China under the frame of PHC Xu-Guangqi 2016 Program (Project 36488YD).



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