Architecting a Mesoporous N-Doped Graphitic Carbon Framework

Subscriber access provided by University of Sussex Library

Article

Architecting Mesoporous N-Doped Graphitic Carbon Framework Encapsulating CoTe2 as Efficient Oxygen Evolution Electrocatalyst Ming Liu, Xiaoqing Lu, Chen Guo, Zhaojie Wang, Yanpeng Li, Yan Lin, Yan Zhou, Shutao Wang, and Jun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09897 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Architecting

Mesoporous

N-Doped

Graphitic

Carbon

Framework Encapsulating CoTe2 as Efficient Oxygen Evolution Electrocatalyst Ming Liu,† Xiaoqing Lu,‡ Chen Guo,‡ Zhaojie Wang,*†,‡ Yanpeng Li,† Yan Lin,† Yan Zhou,‡ Shutao Wang,‡ Jun Zhang*† † College of Chemical Engineering, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, 266580, P. R. China.

‡ College of Science, China University of Petroleum, Qingdao, 266580, P. R. China.

ABSTRACT To improve the efficiency of cobalt-based catalysts for water electrolysis, tremendous efforts have been dedicated to tuning the composition, morphology, size and structure of the materials. We report here a facile preparation of orthorhombic CoTe2 nanocrystals embedded in N-doped graphitic carbon matrix to form a 3D architecture with a size of ～500 nm and abundant mesopores of ～4 nm for the oxygen evolution reaction. The hybrid electrocatalyst delivers a small overpotential of 300 mV at 10 mA cm-2, which is much lower than that for pristine CoTe2 powder. After cycling for 2000 cycles or driving continual OER for 20 h, only a slight loss is observed. The mesoporous 3D architecture and the strong interaction between N-doped graphitic carbon and CoTe2 are responsible to the enhancement of the electrocatalytic performance.

KEYWORDS: Cobalt telluride, N-doped graphitic carbon, metal-organic framework, mesoporous architecture, oxygen evolution reaction

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. INTRODUCTION Electrochemical water splitting, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), has attracted increasing interests as a promising solution to develop sustainable and alternative energy.1-3 Compared to HER entailing the reduction of protons to form molecular hydrogen (2H++2e-→H2) via a single-electron process, OER is a kinetic bottleneck because it requires a multi-step proton-coupled electron process with high overpotentials associated with high activation energy for the formation of O=O chemical bond.4, 5 Although pioneering technologies on water splitting have been used currently, the large overpotentials for OER and the lack of robust and stable catalysts still hamper its widespread application.6, 7 Ruthenium and iridium are the ones well known to possess great catalytic activities for water oxidation, however, the high price and rare storage in earth of these noble metals have impeded their practical application in large scale.8 Therefore, a lot of efforts have been dedicated to develop advanced noble-metal-free electrocatalysts with low overpotentials and high activities.9-12 Cobalt as a typical transition metal with unique catalytic activity and earth-abundant nature has been greatly explored as a potential candidate to replace Ru and Ir.13-17 The rich variable valence feature endow various Co-based compounds with effective electrocatalytic OER activity and corrosion stability in alkaline.18 For example, porous nanowire arrays of Co3O4/C were successfully synthesized and delivered an onset potential as low as 1.47 V (vs.

2


Page 2 of 33

Page 3 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reversible hydrogen electrode, RHE) in 0.1 M KOH solution, which is comparable to IrO2/C (~1.45 V).19 Cobalt sulfide materials including CoS2, Co9S8, Co3S4 and amorphous CoSx were synthesized for OER as well.20-23 Structure vacancy and electrical conductivity were engineered to lower the OER overpotential in alkaline of CoSe2 nanosheets and Ag-doped CoSe2 nanobelts, respectively.24,

25

Yet Schottky barriers usually exist in these

catalysts due to low electrical conductivities, which is adverse to their catalysis reaction.26 Consequently, electrocatalysts with metallic properties are aimed to afford optimized electron states/transmittability.27 Tellurium, due to its intrinsic metallic feature different from O, S, and Se, is favorable to electrochemical processes such as energy storage electrodes (supercapacitors and sodium ion batteries) and electrocatalysis.28-31 Besides the component modification and nanostructure design, employing proper substrates is also important to improve the performance of OER electrocatalysts. It has been verified that one could employ conductive carbon as an outstanding substrate for supporting foreign catalyst.32-34 This modification can afford not only higher conductivity but also preventing catalysts from aggregation (exposing active sites more efficiently). Very recently, heteroatom doped carbon materials were utilized to optimize their electronic characteristics for more active sites and strong interaction with hosted catalysts.35 Metal-organic frameworks (MOFs) have excellent compatibility with metal ions

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 33

and are broadly used as precursors or templates in nanomaterials fabrication.36 They feature the final products with porosity, high surface area, and many novel structures in general.37 Moreover, they can derive into conductive carbon via carbonization treatment and endow better activity of the host materials for electrocatalytic reactions. Herein, we demonstrate a facile synthesis of 3D mesoporous carbon frameworks encapsulated CoTe2 nanocrystals from a MOF precursor. Different from the previously reported chemical transformation process to yield CoTe2,31 our approach combines tellurization and carbonization process together, yielding N-doped graphitic carbon and CoTe2 nanocomposites (CoTe2@N-GC) directly from Significantly,

the

CoTe2@N-GC

revealed

enhanced

OER

ZIF-67.

electrocatalytic

performances (an overpotential of 300 mV at a current density of 10 mA cm-2 and a Tafel slope value of 90 mV per decade) over the pristine CoTe2 powder and porous N-doped graphitic carbon powder. More than just acting as a conductive substrate in CoTe2@N-GC, the synergistic effect and N-doping would also facilitate the efficiency.

2. EXPERIMENTAL SECTION 2.1 Preparation of samples All the chemicals were used as received without further purification. Synthesis of ZIF-67 template. ZIF-67 was synthesized by the method of reported works before.38 Briefly, 1.64 g 2-methylimidazole was dissolved in 40 ml methanol to form a clear solution. After introducing 40 mL of methanol containing 1.45 g Co(NO3)2•6H2O, the mixed solution was stirred at room temperature for 24 h. The

4


Page 5 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


final product ZIF-67 were collected by centrifugation, washed with ethanol for three times and dried at 60°C in vacuum for 24 h. Synthesis of CoTe2@N-GC and CoTe2 powder. For the synthesis of CoTe2@N-GC, 100 mg ZIF-67 and 300 mg Te power were placed at two separate porcelain boats in a tube furnace, and were heated at 450 °C for 6 h under a combination atmosphere of 5% H2 and 95% Ar. For the synthesis of CoTe2 powder, 100 mg ZIF-67 was first annealed at 450 °C for 6 h under an air atmosphere to get the Co3O4 precursor, before undergoing the same chemical conversion process with Te power at separate porcelain boats. Synthesis of N-GC. Firstly, 100 mg ZIF-67 was placed in a porcelain boat before being heated to 450 °C for 6 h under Ar atmosphere to get Co@N-GC. Then, it was etched in excessive muriatic acid to removal Co element completely. The product of N-GC was collected by centrifugation and washed with deionized water and ethanol for three times before being dried at 60°C in vacuum for 24 h. 2.2 Materials Characterizations The X-ray diffraction (XRD) data were recorded on a Philips X'Pert diffractometer with Cu Kα radiation (λ = 0.15418 nm) and operated at 40 kV. The morphologies of the as-obtained samples were studied using a Hitachi S-4800 field emission scanning electronic microscope (FE-SEM) equipped with an energy dispersive X-ray spectrometer. Compositional mapping images were recorded on an EPMA-1600 electron probe X-ray Microanalyzer equipped with an OXFORD-INCA energy

spectrometer.

The

transmission

electron

microscope

5


(TEM)

and


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 33

high-resolution TEM images were collected on a JEM-2100UHR transmission electron microscope operating at 200 kV. The chemical information study was carried out by X-ray photoelectronic spectra (XPS) on an ESCALAB250 analyzer (Thermo, America) with aluminum Kα radiation. The Brunauer–Emmett–Teller (BET) surface area was measured with a Micromeritics ASAP 2020 nitrogen adsorption apparatus by N2 physisorption at 77 K. Thermogravimetric analysis (TGA, NETZSCH STA 449F3) was tested under an air flow from 50 to 800℃ at a heating rate of 10℃ min-1. 2.3 Electrochemical characterization All

Electrocatalytic

measurements

were

performed

on

a

CHI

760E

electrochemical workstation (CH Instruments, Inc., Shanghai) with a standard three-electrode system. A graphite rod and an Hg/HgO electrode were used as the counter electrode and reference electrode, respectively. To obtain working electrodes, the catalyst ink was prepared by dispersing 5 mg catalyst and 40 µL Nafion solution (5.0 wt %) into a mixture of 960 µL ethanol/H2O (volume ratio of 1:3). Then, the catalyst ink was dropped onto a piece of carbon paper (CP) to cover an area of 1 cm-2 before drying naturally at room temperature. The catalyst loading was controlled to be 1 mg cm-2. The electrochemical measurement was performed in O2 saturated KOH solution (1M, PH=14.04). Electrochemical properties are evaluated through linear sweep voltammetry (LSV) kept at 5 mV s-1, cyclic voltammetry (CV), chronoamperometric (i-t) and electrochemical surface areas (ECSAs). All the potentials in polarization curves were manually iR-corrected and converted to RHE scale by calibrating as reported previously.39

6


Page 7 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Faradaic efficiency of CoTe2@N-GC during OER is evaluated by the comparison in amount of oxygen generation from experiment to that calculated in theoretical considerations. Conveniently, a typical H-type reactor with two cavities was used to carry out the water electrolysis for O2 and H2 separately. The working electrode (1 cm-2) and the counter electrode (Pt net electrode) immersed in 80 mL of electrolyte in each cavity. The reactor was sealed in order to allow measurements of gas products. The O2 generated at the working electrode was collected by a water drainage method and its amount (in mol) was calculated according to the ideal gas law. The ideal amount in theory was determined by assuming that 100% of the current output took part in the OER reaction at the working electrode. The theoretical amount of O2 was computed according to the Faraday law.

3. RESULTS AND DISCUSSION The fabrication of CoTe2@N-GC sample is schematically illustrated in Figure 1a. To begin with, ZIF-67 with dodecahedral morphology was synthesized and separated directly from the precipitation solution of cobalt nitrite and 2-methyl-imidazole. As presented in Figure S1, all the diffraction peaks in the XRD pattern of the prepared ZIF-67 are consistent with those of the simulated results. The CoTe2@N-GC polyhedra were obtained by thermal annealing of the as-synthesized ZIF-67 polyhedra with tellurium under H2 atmosphere. The organic ligands were carbonization via pyrolysis in inert gases. Simultaneously, the upstream tellurium powder reacted with H2 in the mixed gas flow forming active H2Te gas at 450℃, which tellurized cobalt into CoTe2 nanocrystals. The morphology and structural information of the precursor

7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. a) Schematic illustration of the preparation process of the fabrication of CoTe2@N-GC. SEM images and magnified SEM images of b, d) ZIF-67 and c, e) CoTe2@N-GC. f) XRD pattern of CoTe2@N-GC with the inset showing crystal structure of CoTe2.

and product were studied and shown in Figure 1b-f. The as-prepared ZIF-67 crystal presented a conventional rhombic dodecahedral morphology with a size of 0.5～1 µm. CoTe2@N-GC derived from ZIF-67 preserved the original polyhedron morphology well even after the chemical conversion with Te powder at high temperature (Figure

8


Page 8 of 33

Page 9 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1c). There was no slight change in size can be distinguished for CoTe2@N-GC after a suitable annealing progress. According to the magnified SEM images of ZIF-67 (Figure 1d) and CoTe2@N-GC (Figure 1e), CoTe2@N-GC showed a much rougher surface with numerous wrinkles, compared to a smooth surface of ZIF-67 before chemical conversion. As shown in Figure 1f, the XRD pattern of CoTe2@N-GC exhibits high crystallinity and all the diffraction peaks can be indexed to orthorhombic CoTe2 (JCPDS 89-2091), indicating the successful tellurization. The sample of CoTe2@N-GC was further characterized by TEM and SEM elemental mapping. The TEM image (Figure 2a) of CoTe2@N-GC outlines of the polyhedron shape inherited from ZIF-67, which was consistent with the observation in the SEM images. Higher resolution images in Figure 2b and c reveal that CoTe2 nanocrystals with a diameter of 10～30 nm were confined and homogeneously dispersed in the 3D matrix of carbon. The marked lattice spacing of 0.282 and 0.208 nm were assigned to the (111) and (211) plane of CoTe2 nanocrystals while that of 0.343 nm were attributed to (002) plane of graphitic carbon, respectively. The elemental mappings in Figure 2d reveal that Co, Te, and N elements disperse uniformly in CoTe2@N-GC, which was further confirmed by the compositional mapping with the EPMA analysis in Figure S2. Moreover, the Co:Te atomic ratio is close to a 1:2 as shown in Figure S3. In comparison, the pyrolysis of ZIF-67 in Ar atmosphere without Te powder was also conducted and resulted in the formation of metal cobalt nanoparticle-embedded in mesoporous carbon (denoted as Co@N-GC), as confirmed by TEM image and XRD patterns (Figure S4a and b). The porous

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

carbon matrix presented in Figure S5 for a control study was obtained by etching Co@N-GC in dilute hydrochloric acid to release Co. For another, we annealed pristine ZIF-67 precursor in air at 450 ℃ to decompose 2-methyl-imidazole completely before being tellurized to form pristine CoTe2 powders. As shown in Figure S6, great changes in morphology occurred during the tellurization process, indicating that the N-GC matrix is crucial to preserve the polyhedral structure and form CoTe2 nanocrystals in small sizes.

Figure 2. TEM images of CoTe2@N-GC at a) low magnification and b) high magnification. c) HRTEM image of CoTe2@N-GC. d) SEM elemental mapping images of CoTe2@N-GC composites.

XPS was further conducted to get insight into the composition and valence states of CoTe2@N-GC electrocatalyst. The elements of Co, Te, C, O and N were detected

10


Page 10 of 33

Page 11 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


according to their distinctive peaks (Figure 3a). The high-resolution XPS peaks of Co 2p (Figure 3b) centered at 780.9 and 796.9 eV can be assigned to Co 2p3/2 and 2p1/2, respectively.40 The satellite peaks centered at 786.5 eV and 802.7 eV can be attributed to the surface oxidized CoOx which can be further verified by the presence of O in the survey scan.13, 27 As we know, minority CoOx sites formed on the surface where the charge and OH- anions can reach would provide more necessary active catalytic sites to promote OER process.41-44 For the high-resolution Te 3d XPS spectrum (Figure 3c), the two main peaks at 576.1 and 586.5 eV in high-resolution Te 3d correspond to Te 3d 5/2 and Te 3d 3/2, respectively.31 The remaining two weak peaks at 572.9 and 583.3 eV indicating the existence of zero valent Te.45Small amount of H2Te deposited on the surface of the product at high temperature and finally settled down as zero valent Te during the cooling down process. The coexistence of N-doped carbon matrix was confirmed by XPS as well (Figure 3d-e). The C 1s XPS spectrum can be deconvoluted into two peaks at 284.7 and 285.7, which corresponded to C-C, and C-N/C=N bonds, respectively. N 1s positioned at 398.6, 400.0 and 400.9 eV were highly in agreement with the binding energy values of pyridinic N, pyrrolic N and graphitic N, respectively. Moreover, the peak centered at 399.2 eV corresponding to Co-Nx, indicating the presence of chemical bonding of between Co and N.46, 47 Here, the total nitrogen content (the total amount of N element in atom ratio) was calculated as 6.8%, which was close to the typical level (2%-6%) in the ZIF-derived carbon matrix via pyrolysis. The enhanced nitrogen doping efficiency is due to the strong interaction between metal and nitrogen (e.g. forming metal-nitrogen bonds).47 In

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

addition, the major nitrogen component in CoTe2@N-GC is pyridinic N (~48.5%, Figure 3f), which is considered as efficient active sites for electrocatalysis.48, 49

Figure 3. a) XPS survey of CoTe2@N-GC and high-resolution spectra of b) Co 2P, c) Te 3d, d) C1s and e) N1s. f) Different N dopant contents.

12


Page 12 of 33

Page 13 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. a) TGA curves and b) N2 sorption isotherms curve of CoTe2@N-GC.

The thermogravimetric analysis of CoTe2@N-GC, CoTe2 powder and N-GC were performed under an air flow from 50 to 800℃. As shown in Fig 4a, the weight of CoTe2@N-GC and CoTe2 powder increased due to the oxidation of CoTe2. However, the weight increase for CoTe2@N-GC was smaller than pure CoTe2 powder. This can be explained by the fact that the combustion of C and N at high temperature is similar to the decomposition behavior of N-GC. It is estimated that the CoTe2@N-GC contains approximately 9 wt. % of C and N. The porous structure and surface area of CoTe2@N-GC was studied by N2 adsorption-desorption isotherm. It appeared to give normal Type IV full adsorption-desorption isotherms with the well-defined hysteresis loop at high N2 pressures (from 0.45 to 1.0), indicating the mesoporous characteristic (Figure 4b). The specific area calculated by Brunauer-Emmett-Teller (BET) equation of CoTe2@N-GC was 38.4 m2 g-1. The pore size distribution based on Barrett-Joyner-Halenda (BJH) model demonstrated that the mesopores were concentrated at ~4 nm in size, and the pore volume was calculated as 0.11 cm3 g-1. The large surface area and 3D porous texture inherited from ZIF-67 is expected to

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

promote the mass and charge-transport as well as providing abundant surface-active sites for electrocatalysis performance.

Figure 5. The electrocatalytic OER activities of CoTe2@N-GC, CoTe2 Power, N-GC, ZIF-67 and CP. a) LSV polarization curves, b) Tafel plots, c) Plots of the current density. d) Polarization curves of CoTe2@N-GC before and after 2000 cycles with the inset showing the time dependence of the current density.

The electrocatalytic OER activities of CoTe2@N-GC were evaluated in a three-electrode system. For comparison, the LSV curves of the bare carbon paper, ZIF-67 precursor, N-GC, and CoTe2 powder were recorded at a scan rate of 5 mV s-1 and illustrated in Figure 5a. Apparently, CoTe2@N-GC presents much higher current density and earlier onset of catalytic current contrast to the others during the anodic scan from 0.924 V to 1.724 V vs. RHE. By adoption of a popular evaluation method,

14


Page 14 of 33

Page 15 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


we compared the overpotentials at the current density of 10 mA cm-2 of the various samples. As illustrated in Figure 5a, CoTe2@N-GC can afford it at a small overpotential of 300 mV and that is lower than 360 and 410 mV for pristine CoTe2 powder and ZIF-67 precursor, respectively. The catalytic performance of our CoTe2@N-GC was found to be comparable to that of precious RuO2@N-GC (Figure S8). In addition, CoTe2@N-GC exhibits an OER current as high as 78.5 mA cm-2 at an overpotential of 400 mV, which is twice higher than that of CoTe2 powder and even tens of times higher than the others. The greater current density in the featured LSV curve indicates superior OER activity of CoTe2@N-GC, as the current value is in direct proportion to the yield of oxygen. The Tafel plots are also important to probe the reaction kinetics. Figure 5b shows the Tafel slopes of various samples derived from the polarization curves. The resulting Tafel slope estimated for CoTe2@N-GC is 90 mV dec-1, which is smaller than those powder of pristine CoTe2 (102 mV dec-1), N-GC (240 mV dec-1), ZIF-67 (212 mV dec-1) and bare CP (223 mV dec-1). The small value reveals the favorable OER kinetics of CoTe2@N-GC for applications in electrocatalysis. To get insight into the high OER catalytic activity of CoTe2@N-GC, electrochemical surface areas (ECSAs) of the samples were measured. Here the electrochemical double-layer capacitance (Cdl) was utilized to represent ECSA typically. CV curve of each sample recorde at the potential window ranging from 1.144 to 1.244 V (vs. RHE) is shown in Figure S7. The current densities valued from the anodic scan and cathodic scan at the middle potential were plotted in Figure 5c

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

against the scan rates. As known, the linear slope, which is equivalent to twice of Cdl, can reflect the ECSA due to its direct proportional relationship to Cdl. The Cdl value of CoTe2@N-GC was calculated to be 56.4 mF cm-2, and is much higher than that of bare CP (14.9 mF cm-2), N-GC matrix (24.2 mF cm-2), ZIF-67 (18.3 mF cm-2), and even CoTe2 powder (39.8 mF cm-2). These results reveal that CoTe2@N-GC has the largest active surface area and is coincidence with the superior OER catalytic performance.

Figure 6. a) TEM image and b) XRD pattern of CoTe2@N-GC after 2000 cycles.

The long-term durability is another critical criterion for practical application of an ideal OER electrocatalysts. To probe the stability of our CoTe2@N-GC, we first conducted a continuous CV scan with a potential window from 1.45 to 1.65 V (vs. RHE) at a scan rate of 50 mV s-1. As shown in Figure 5d, the LSV curve exhibits slight loss after 2000 potential sweeps. There is a 15 mV increment observed for the overpotential at the current density of 20 mA cm-2. Furthermore, the continuous electrolysis at a fixed potential of 1.55 V (overpotential of 320 mV) was monitored over 20 h. Notably, the time-resolved current density plotted in the inset of Figure 5d

16


Page 16 of 33

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exhibits a slight variation over the entire time, indicating the outstanding stability. We carried out the TEM and XRD measurements after the stability test (Figure 6). No obvious morphology and structure changes were observed after long-term electrolysis. In addition, the Faradaic efficiency over CoTe2@N-GC was examined to be nearly 100% as shown in Figure S9, indicating that the electrocatalytic currents flew through the CoTe2@N-GC electrode are exclusive to pure OER. Considering the unique structure and hybrid composition of CoTe2@N-GC, the enhanced catalytic activity of CoTe2@N-GC, compared to pristine CoTe2 and N-GC, might be due to both the intrinsic activity and synergistic contribution of CoTe2 and N-doped graphitic carbon. As previously reported, Co-based compounds, such as Co3O4, CoxSy, Co(OH)2 etc., have been widely developed to be superior alternatives of Pt, Ir and Ru for OER. The unique 3d electron number and special eg orbitals assume the importance in catalysis.50 As is accepted, the oxidization process of Co(II) in Co-based electrocatalysts to Co(III) and Co(IV) is the key step for OER catalytsis.51 In our study, we have chosen CoTe2 as the main active center during OER catalysis, due to its higher conductivity and charge transfer capability, both of which are crucial for OER electroactivity. The combination of tellurization and carbonization of ZIF-67 in one step is the key to the formation of CoTe2 nanocrystals uniformly embedded in mesoporous carbon with high specific surface area and preservation of its original polyhedron shape. During the heat treatment, the unique MOF structure of ZIF-67 prevented CoTe2 nanocrystals from agglomeration while forming N-doped graphitic carbon encapsulated CoTe2 polyhedron matrix with mesoporous structure,

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

which was further confirmed by the comparison to 2D CoTe2@N-rGO in Figure S10 and S11. Since the electrocatalytic oxygen evolution take place at the catalyst-electrolyte-O2 triple-phase interface, the construction of mesoporous cavity inside polyhedron can provide extra triple-phase interface to accelerate OER.52, 53 Due to the electronegativity of N atoms in N-GC, the strong interaction between N-GC and CoTe2 can lead to more active sites for OH- adsorption and activation.54 Simultaneously, the edge Co atom near the interaction of Co and N between the interface was verified to be more active than the others by chemisorption of the *O intermediates during OER.55 The electron donation from N-GC to CoTe2 could weaken the interaction of surface-oxygen to produce moderate bond strength as well.27 In the meantime, the existing N-GC could provide highly conductive network that afford fast electron transport and buffer the stress and volume changes during the long-term electrolysis.47

4. CONCLUSION In summary, we have successfully prepared a mesoporous polyhedral architecture of N-doped graphitic carbon encapsulating CoTe2 nanocrystals from ZIF-67 by a simple heat treatment including tellurization and carbonization simultaneously. The hybrid texture affords for a facile path for OER with both high activity and robust stability. By comparison, a lower overpotential of 300 mV is required at 10 mA cm-2 in 1 M KOH for CoTe2@N-GC. The N-doped graphitic carbon matrix offers not only excellent electrical conductivity and richly accessible active sites as a supporter, but also interacts with confined CoTe2 nanocrystals intensively for boosting OER. This

18


Page 18 of 33

Page 19 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


facile synthesis could be extended to develop other metal tellurides embedded in advanced mesoporous carbon matrix with the adoption of suitable MOFs precursors for broader energy and environment applications.

ASSOCIATED CONTENT Supporting Information Available: XRD patterns, TEM images, EPMA analysis, EDS analysis, CV curves, LSV curves and Tafel plots of CoTe2@N-GC and other control samples. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] ORCID Zhaojie Wang: 0000-0002-6476-6449 Jun Zhang: 0000-0002-7068-5135 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support by National Natural Science Foundation of China (51402362, 21471160 and 21303266), PetroChina Innovation Foundation

(2016D-5007-0401),

Shandong

Natural

19


Science

Foundation


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

(ZR2017MA024, ZR2017QB015), the Fundamental Research Funds for the Central Universities (16CX05016A, 15CX05045A, 15CX05050A, 14CX02214A, and 15CX08010A).

REFERENCES (1) Dresselhaus, M.; Thomas, I. Alternative Energy Technologies. Nature 2001, 414, 332-337. (2) Vojvodic, A.; Nørskov, J. Optimizing Perovskites for the Water-Splitting Reaction.

Science 2011, 334, 1355-1356. (3)

Zhou,

M.;

Lou,

X.

W.;

Xie,

Y.

Two-dimensional

Nanosheets

for

Photoelectrochemical Water Splitting: Possibilities and Opportunities. Nano

Today 2013, 8, 598-618. (4) Xia, C.; Jiang, Q.; Zhao, C.; Hedhili, M. N.; Alshareef, H. N. Selenide-Based Electrocatalysts and Scaffolds for Water Oxidation Applications. Adv. Mater.

2016, 28, 77-85. (5) Lu, X. F.; Gu, L. F.; Wang, J. W.; Wu, J. X.; Liao, P. Q.; Li, G. R. Bimetal-Organic Framework

Derived

CoFe2O4/C

Porous

Hybrid

Nanorod

Arrays

as

High-Performance Electrocatalysts for Oxygen Evolution Reaction. Adv. Mater.

2017, 29, 1604437. (6) Liu, Y.; Li, Q.; Si, R.; Li, G. D.; Li, W.; Liu, D. P.; Wang, D.; Sun, L.; Zhang, Y.; Zou, X. Coupling Sub-Nanometric Copper Clusters with Quasi-Amorphous Cobalt Sulfide Yields Efficient and Robust Electrocatalysts for Water Splitting Reaction. Adv. Mater. 2017, 29, 1606200. (7) Subbaraman, R.; Tripkovic, D.; Chang, K. C.; Strmcnik, D.; Paulikas, A. P.; Hirunsit, P.; Chan, M.; Greeley, J.; Stamenkovic, V.; Markovic, N. M. Trends in Activity for the Water Electrolyser Reactions on 3d M(Ni,Co,Fe,Mn) Hydr(oxy)oxide Catalysts. Nat. Mater. 2012, 11, 550-557. (8) Moller, F. M.; Kriegel, F.; Kiess, M.; Sojo, V.; Braun, D. Steep pH Gradients and Directed Colloid Transport in a Microfluidic Alkaline Hydrothermal Pore. Angew.

20


Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Chem. Int. Ed. 2017, 56, 2340-2344. (9) Zhang, T.; He, C.; Sun, F.; Ding, Y.; Wang, M.; Peng, L.; Wang, J.; Lin, Y. Co3O4 Nanoparticles Anchored on Nitrogen-doped Reduced Graphene Oxide as a Multifunctional Catalyst for H2O2 Reduction, Oxygen Reduction and Evolution Reaction. Sci. Rep. 2017, 7, 43638. (10) Zhang, J.; Hu, Y.; Liu, D.; Yu, Y.; Zhang, B. Enhancing Oxygen Evolution Reaction at High Current Densities on Amorphous-Like Ni-Fe-S Ultrathin Nanosheets via Oxygen Incorporation and Electrochemical Tuning. Adv. Sci.

2017, 4, 1600343. (11) Zhang, C.; Shao, M.; Zhou, L.; Li, Z.; Xiao, K.; Wei, M. Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres with Highly-Efficient Behavior toward Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2016,

8, 33697-33703. (12) Geng, J.; Kuai, L.; Kan, E.; Sang, Y.; Geng, B. Hydrothermal Synthesis of a rGO Nanosheet Enwrapped NiFe Nanoalloy for Superior Electrocatalytic Oxygen Evolution Reactions. Chemistry 2016, 22, 14480-14483. (13) Lu, W.; Liu, T.; Xie, L.; Tang, C.; Liu, D.; Hao, S.; Qu, F.; Du, G.; Ma, Y.; Asiri, A. M.; Sun, X. In Situ Derived CoB Nanoarray: A High-Efficiency and Durable 3D Bifunctional Electrocatalyst for Overall Alkaline Water Splitting. Small 2017,

13, 1700805. (14) Xie, L.; Tang, C.; Wang, K.; Du, G.; Asiri, A. M.; Sun, X. Cu(OH)2@CoCO3(OH)2·nH2O Core-Shell Heterostructure Nanowire Array: An Efficient

3D

Anodic

Catalyst

for

Oxygen

Evolution

and

Methanol

Electrooxidation. Small 2017, 13, 1602755. (15) Cui, L.; Qu, F.; Liu, J.; Du, G.; Asiri, A. M.; Sun, X. Interconnected Network of Core-Shell CoP@CoBiPi for Efficient Water Oxidation Electrocatalysis under Near Neutral Conditions. ChemSusChem 2017, 10, 1370-1374. (16) Ji, X.; Hao, S.; Qu, F.; Liu, J.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X. Core-shell CoFe2O4@Co-Fe-Bi Nanoarray: a Surface-amorphization Water Oxidation Catalyst Operating at Near-Neutral pH. Nanoscale 2017, 9, 7714-7718. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 33

(17) Cui, L.; Liu, D.; Hao, S.; Qu, F.; Du, G.; Liu, J.; Asiri, A. M.; Sun, X. In Situ Electrochemical

Surface

Derivation

of

Cobalt

Phosphate

From

a

Co(CO3)0.5(OH)·0.11H2O Nanoarray for Efficient Water Oxidation in Neutral Aqueous Solution. Nanoscale 2017, 9, 3752-3756. (18) Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A.; Sun, X. Recent Progress in Cobalt-based Heterogeneous Catalysts for Electrochemical Water Splitting. Adv.

Mater. 2016, 28, 215-230. (19) Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Metal-organic Framework Derived Hybrid Co3O4-carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925-13931. (20) Fang, W.; Liu, D.; Lu, Q.; Sun, X.; Asiri, A. M. Nickel Promoted Cobalt Disulfide Nanowire Array Supported on Carbon Cloth: An Efficient and Stable Bifunctional Electrocatalyst for Full Water Splitting. Electrochem. Commun.

2016, 63, 60-64. (21) Ganesan, P.; Prabu, M.; Sanetuntikul, J.; Shanmugam, S. Cobalt Sulfide Nanoparticles Grown on Nitrogen and Sulfur Codoped Graphene Oxide: An Efficient Electrocatalyst for Oxygen Reduction and Evolution Reactions. ACS

Catal. 2015, 5, 3625-3637. (22) Liu, Y.; Xiao, C.; Lyu, M.; Lin, Y.; Cai, W.; Huang, P.; Tong, W.; Zou, Y.; Xie, Y. Ultrathin Co3S4 Nanosheets that Synergistically Engineer Spin States and Exposed Polyhedra that Promote Water Oxidation Under Neutral Conditions.

Angew. Chem. Int. Ed. 2015, 54, 11231-11235. (23) Cai, P.; Huang, J.; Chen, J.; Wen, Z. Oxygen-Containing Amorphous Cobalt Sulfide Porous Nanocubes as High-Activity Electrocatalysts for the Oxygen Evolution Reaction in an Alkaline/Neutral Medium. Angew. Chem. Int. Ed. 2017,

56, 4858-4861. (24) Liu, Y.; Cheng, H.; Lyu, M.; Fan, S.; Liu, Q.; Zhang, W.; Zhi, Y.; Wang, C.; Xiao, C.; Wei, S.; Ye, B.; Xie, Y. Low Overpotential in Vacancy-rich Ultrathin CoSe2 Nanosheets for Water Oxidation. J. Am. Chem. Soc. 2014, 136, 15670-15675. (25) Zhao, X.; Zhang, H.; Yan, Y.; Cao, J.; Li, X.; Zhou, S.; Peng, Z.; Zeng, J. 22


Page 23 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Engineering the Electrical Conductivity of Lamellar Silver-Doped Cobalt (II) Selenide Nanobelts for Enhanced Oxygen Evolution. Angew. Chem. Int. Ed.

2017, 56, 328-332. (26) Sun, Y.; Gao, S.; Lei, F.; Xie, Y. Atomically-thin Two-dimensional Sheets for Understanding Active Sites in Catalysis. Chem. Soc. Rev. 2015, 44, 623-636. (27) Yu, J.; Li, Q.; Xu, C.-Y.; Chen, N.; Li, Y.; Liu, H.; Zhen, L.; Dravid, V. P.; Wu, J. NiSe2 Pyramids Deposited on N-doped Graphene Encapsulated Ni Foam for High-performance Water Oxidation. J. Mater. Chem. A 2017, 5, 3981-3986. (28) Liu, M.; Wang, Z.; Liu, J.; Wei, G.; Du, J.; Li, Y.; An, C.; Zhang, J. Synthesis of Few-layer

1T’-MoTe2

Ultrathin

Nanosheets

for

High-performance

Pseudocapacitors. J. Mater. Chem. A 2017, 5, 1035-1042. (29) Cho, J. S.; Ju, H. S.; Lee, J.-K.; Kang, Y. C. Carbon/two-dimensional MoTe2 Core/shell-structured Microspheres as an Anode Material for Na-ion Batteries.

Nanoscale 2017, 9, 1942-1950. (30) Wang, K.; Ye, Z.; Liu, C.; Xi, D.; Zhou, C.; Shi, Z.; Xia, H.; Liu, G.; Qiao, G. Morphology-Controllable

Synthesis

of

Cobalt

Telluride

Branched

Nanostructures on Carbon Fiber Paper as Electrocatalysts for Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 2910-2916. (31) Gao, Q.; H, C.; Ju, Y.; Gao, M.; Liu, J.; An, D, Cui, C.; Zheng, Y.; Li, W.; Yu, S. Phase Selective Synthesis of Unique Cobalt Telluride Nanofleeces for Highly Efficient Oxygen Evolution Catalyst. Angew. Chem. Int. Ed. 2017, 56, 7769-7773. (32) Li, M.; Cheng, J.; Wang, J.; Liu, F.; Zhang, X. The Growth of Nickel-manganese and Cobalt-manganese Layered Double Hydroxides on Reduced Graphene Oxide for Supercapacitor. Electrochim. Acta 2016, 206, 108-115. (33) Tang, J.; Liu, W.; Wang, H.; Gomez, A. High Performance Metal Oxide-Graphene Hybrid Nanomaterials Synthesized via Opposite-Polarity Electrosprays. Adv. Mater. 2016, 28, 10298-10303. (34) Tang, H.; Yin, H.; Wang, J.; Yang, N.; Wang, D.; Tang, Z. Molecular Architecture of Cobalt Porphyrin Multilayers on Reduced Graphene Oxide Sheets for 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

High-performance Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2013, 52, 5585-5589. (35) He, G.; Qiao, M.; Li, W.; Lu, Y.; Zhao, T.; Zou, R.; Li, B.; Darr, J. A.; Hu, J.; Titirici, M. M.; Parkin, I. P. S, N-Co-Doped Graphene-Nickel Cobalt Sulfide Aerogel: Improved Energy Storage and Electrocatalytic Performance. Adv. Sci.

2017, 4, 1600214. (36) Wu, R.; Wang, D. P.; Zhou, K.; Srikanth, N.; Wei, J.; Chen, Z. Porous Cobalt Phosphide/graphitic Carbon Polyhedral Hybrid Composites for Efficient Oxygen Evolution Reactions. J. Mater. Chem. A 2016, 4, 13742-13745. (37) Liu, J.; Zhu, D.; Guo, C.; Vasileff, A.; Qiao, S.-Z. Design Strategies toward Advanced MOF-Derived Electrocatalysts for Energy-Conversion Reactions. Adv.

Energy Mater. 2017, 1700518. (38) Wu, R.; Wang, D. P.; Han, J.; Liu, H.; Zhou, K.; Huang, Y.; Xu, R.; Wei, J.; Chen, X.; Chen, Z. A General Approach towards Multi-faceted Hollow Oxide Composites Using Zeolitic Imidazolate Frameworks. Nanoscale 2015, 7, 965-974. (39) Stevens, M.; Enman, L.; Batchellor, A.; Cosby, M.; Vise, A.; Trang, C.; Boettcher, S. Measurement Techniques for the Study of Thin Film Heterogeneous Water Oxidation Electrocatalysts. Chem. Mater. 2017, 29, 120-140. (40) Ming, F.; Liang, H.; Shi, H.; Xu, X.; Mei, G.; Wang, Z. MOF-derived Co-doped Nickel Selenide/C Electrocatalysts Supported on Ni Foam for Overall Water Splitting. J. Mater. Chem. A 2016, 4, 15148-15155. (41) Xu, K.; Ding, H.; Jia, K.; Lu, X.; Chen, P.; Zhou, T.; Cheng, H.; Liu, S.; Wu, C.; Xie, Y. Solution-Liquid-Solid Synthesis of Hexagonal Nickel Selenide Nanowire Arrays with a Nonmetal Catalyst. Angew. Chem. Int. Ed. 2016, 55, 1710-1713. (42) Chen, P.; Xu, K.; Fang, Z.; Tong, Y.; Wu, J.; Lu, X.; Peng, X.; Ding, H.; Wu, C.; Xie, Y. Metallic Co4N Porous Nanowire Arrays Activated by Surface Oxidation as Electrocatalysts for the Oxygen Evolution Reaction. Angew. Chem. Int. Ed.

2015, 54, 14710-14714. (43) Wang, P.; Song, F.; Amal, R.; Ng, Y. H.; Hu, X. Efficient Water Splitting 24


Page 24 of 33

Page 25 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Catalyzed by Cobalt Phosphide-Based Nanoneedle Arrays Supported on Carbon Cloth. ChemSusChem 2016, 9, 472-477. (44) Wang, J.; Zhong, H. X.; Wang, Z. L.; Meng, F. L.; Zhang, X. B. Integrated Three-Dimensional Carbon Paper/Carbon Tubes/Cobalt-Sulfide Sheets as an Efficient Electrode for Overall Water Splitting. ACS Nano 2016, 10, 2342-2348. (45) Di Nardo, S.; Lozzi, L.; Passacantando, M.; Picozzi, P.; Santucci, S. Growth of Te Thin Films Deposited at Room Temperature on the Si (100) 2× 1 Surface. J.

Electron. Spectrosc. Relat. Phenom. 1995, 71, 39-45. (46) Kuang, M.; Wang, Q.; Han, P.; Zheng, G. Cu, Co-Embedded N-Enriched Mesoporous Carbon for Efficient Oxygen Reduction and Hydrogen Evolution Reactions. Adv. Energy Mater. 2017, 7, 1700193. (47) Wang, Q.; Shang, L.; Shi, R.; Zhang, X.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. NiFe Layered Double Hydroxide Nanoparticles on Co,N-Codoped Carbon Nanoframes as Efficient Bifunctional Catalysts for Rechargeable

Zinc-Air

Batteries.

Adv.

Energy

Mater.

2017,

DOI:

10.1002/aenm.201700467. (48) Cui, X.; Yang, S.; Yan, X.; Leng, J.; Shuang, S.; Ajayan, P. M.; Zhang, Z. Pyridinic-Nitrogen-Dominated Graphene Aerogels with Fe-N-C Coordination for Highly Efficient Oxygen Reduction Reaction. Adv. Funct. Mater. 2016, 26, 5708-5717. (49) Zhang, J.; Qu, L.; Shi, G.; Liu, J.; Chen, J.; Dai, L. N,P-Codoped Carbon Networks as Efficient Metal-free Bifunctional Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions. Angew. Chem. Int. Ed. 2016, 55, 2230-2234. (50) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J.; Nocera,

D.

G.

Wireless

Solar

Water

Splitting

Using

Silicon-based

Semiconductors and Earth-abundant Catalysts. Science 2011, 334, 645-648. (51) Tian, T.; Jiang, J.; Ai, L. In Situ Electrochemically Generated Composite-type CoOx/WOx in Self-activated Cobalt Tungstate Nanostructures: Implication for Highly Enhanced Electrocatalytic Oxygen Evolution. Electrochim. Acta 2017,

224, 551-560. 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(52) Liu, S.; Wang, Z.; Zhou, S.; Yu, F.; Yu, M.; Chiang, C. Y.; Zhou, W.; Zhao, J.; Qiu, J. Metal-Organic-Framework-Derived Hybrid Carbon Nanocages as a Bifunctional Electrocatalyst for Oxygen Reduction and Evolution. Adv. Mater.

2017, 29, 1700874. (53) Arnal, P. M.; Comotti, M.; Schüth, F. High-Temperature-Stable Catalysts by Hollow Sphere Encapsulation. Angew. Chem. Int. Ed. 2006, 118, 8404-8407. (54) Chen, S.; Qiao, S. Z. Hierarchically Porous Nitrogen-doped Graphene-NiCo2O4 Hybrid Paper as an Advanced Electrocatalytic Water-splitting Material. ACS

Nano 2013, 7, 10190-10196. (55) Jia, X.; Zhao, Y.; Chen, G.; Shang, L.; Shi, R.; Kang, X.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Ni3FeN Nanoparticles Derived from Ultrathin NiFe-Layered Double Hydroxide Nanosheets: An Efficient Overall Water Splitting Electrocatalyst. Adv. Energy Mater. 2016, 6, 1502585.

26


Page 26 of 33

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. a) Schematic illustration of the preparation process of the fabrication of CoTe2@N-GC. SEM images and magnified SEM images of b, d) ZIF-67 and c, e) CoTe2@N-GC. f) XRD pattern of CoTe2@N-GC with the inset showing crystal structure of CoTe2. 656x683mm (96 x 96 DPI)


Page 28 of 33

Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. TEM images of CoTe2@N-GC at a) low magnification and b) high magnification. c) HRTEM image of CoTe2@N-GC. d) SEM elemental mapping images of CoTe2@N-GC composites. 664x507mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. a) XPS survey of CoTe2@N-GC and high-resolution spectra of b) Co 2P, c) Te 3d, d) C1s and e) N1s. f) Different N dopant contents. 678x796mm (96 x 96 DPI)


Page 30 of 33

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. a) TGA curves and b) N2 sorption isotherms curve of CoTe2@N-GC. 717x266mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The electrocatalytic OER activities of CoTe2@N-GC, CoTe2 Power, N-GC, ZIF-67 and CP. a) LSV polarization curves, b) Tafel plots, c) Plots of the current density. d) Polarization curves of CoTe2@N-GC before and after 2000 cycles with the inset showing the time dependence of the current density. 1064x786mm (96 x 96 DPI)


Page 32 of 33

Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. a) TEM image and b) XRD pattern of CoTe2@N-GC after 2000 cycles. 746x280mm (96 x 96 DPI)


Architecting a Mesoporous N-Doped Graphitic Carbon Framework

Recommend Documents