Cobalt Nanoparticles Encapsulated in Porous Carbons Derived from

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Cobalt nanoparticles encapsulated in porous carbons derived from coreshell ZIF67@ZIF8 as efficient electrocatalysts for oxygen evolution reaction Jujiao Zhao, Xie Quan, Shuo Chen, Yanming Liu, and Hongtao Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10138 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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ACS Applied Materials & Interfaces

Cobalt nanoparticles encapsulated in porous carbons derived from core-shell ZIF67@ZIF8 as efficient electrocatalysts for oxygen evolution reaction Jujiao Zhao, Xie Quan*, Shuo Chen, Yanming Liu, Hongtao Yu. Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China. KEYWORDS: core-shell structure; Co nanoparticles; oxygen evolution reaction; porous carbons; ZIF67@ZIF8.

ABSTRACT: The synthesis of electrocatalysts consisting of selectively functionalized parts is an effective strategy to prepare non-precious electrocatalysts with excellent performance for oxygen evolution reaction (OER). Herein, we synthesized core-shell structured ZIF67@ZIF8 and converted it into Co decorated porous carbons (CS-Co/Cs) consisting of the ZIF67 derived uniformly dispersed Co nanoparticles encapsulated in graphitic carbon as cores and the ZIF8 derived porous carbon as shells. Compared to individual ZIF67 derived samples (Co/Cs), the unique structure of CS-Co/Cs leads to the larger surface area and more hydrophilic surface, both

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of which facilitate the mass transfer, contributing to the enhanced OER performance. The optimized CS-Co/C sample presents the low overpotential of 290 mV to deliver 10 mA cm-2 towards OER in 1 M KOH, which is among the best of reported non-precious OER electrocatalysts. The CS-Co/C exhibits no obvious current attenuation at 1.53 V for 30000 s, demonstrating its robust stability.

1. Introduction Oxygen evolution reaction (OER) is an important half-cell reaction in the sustainable clean energy technologies, such as hydrogen fuel production (H2O → H2 + 1/2 O2) and rechargeable metal-air batteries (MxO2 → Mx + O2)1, 2. Due to the complex four-electron oxidation process, OER is kinetically sluggish, which results in the urgent demand for highly effective electrocatalysts3-5. Although Ir- and Ru- based materials present good performance for OER, the scarcity and high cost of these materials limit their large-scale applications6-8. Recently, the firstrow transition-metal (Fe, Co, Ni) and their derivatives have proven to be the active sites for OER. Thus, many efforts have been devoted into developing the transition-metal based electrocatalysts, such as layered double hydroxides (LDHs), metal/graphene, bimetallic alloy, porous metal oxides, metal phosphide and metal/porous carbons7,

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. Among them, Porous

carbons combined with metal species have stimulated a great deal of interest due to their lowcost, good conductivity and chemical stability19. The high surface area and hierarchical pores of porous carbon are favor to the interaction between active sties and solution, as well as the free diffusion of O2. The active metal species encapsulated in the porous carbons can be stabilized as nanoparticles and the uniform dispersion of the nanoparticles can be maintained during the operation due to the strong coupling between active sites and support carbons, leading to the


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stable exposure of active sites20, 21. Due to all of these advantages, the catalysts consisting of porous carbons and the metal nanoparticles could be attractive candidates for OER with superior performance22, 23. However, designing a rational synthetic strategy to prepare catalysts with such desirable structure still remains great challenges. Metal-organic frameworks (MOFs) constructed by metal and organic ligands with welldefined porous structure, are ideal precursors for producing metal decorated porous carbons via carbonization24-32. After the pyrolysis under inert atmosphere, the large surface area and high porosity of MOFs can be partially preserved and the metal nanoparticles can be dispersed uniformly in the resultant porous carbons33-36. Zeolitic imidazolate frameworks (ZIFs) are a promising subclass of MOFs to be used as precursors since they can be synthesized by a facile method at low cost37, 38. ZIF67 ([Co(MeIm)2]n) (MeIm = methylimidazole) has been used as precursor to produce OER electrocatalysts with excellent performance due to the highly active Co nanoparticles towards OER and the conductive graphitic carbon39-42. However the small surface area, the hydrophobic surface and relatively poor stability still limit the OER performance of these catalysts43, 44. ZIF8 ([Zn(MeIm)2]n), a Zn2+ based MOF with the same organic ligand to ZIF67, can cover the ZIF67 seeds through epitaxial growth to form a core-shell ZIF67@ZIF8 structure under mild conditions45, 46. Since the Zn will evaporate at high annealing temperature (>900 oC) while the Co will improve the graphitization degree of carbon under the same condition, the core-shell ZIF67@ZIF8 can be carbonized into selectively functionalized hybrid Co decorated porous carbons. The ZIF8 shell could be transformed into porous carbons without metal residue, which would lead to a larger surface area of Co decorated porous carbons derived from ZIF67@ZIF8 compared to that of ZIF67 derived composites. Meanwhile, because of the similar porous structure of ZIF67 and ZIF8, the core and shell of the hybrid materials


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derived from ZIF67@ZIF8 would have the similar porous structure, which could facilitate the exposure of active sites to the electrolyte and the mass transfer during the OER. Herein, we synthesized the Co nanoparticles encapsulated in porous carbons by carbonizing the core-shell ZIF67@ZIF8 under Ar atmosphere at different temperatures. The electrochemical activity and stability of the as-prepared samples derived from ZIF67@ZIF8 towards OER were evaluated. Besides, the samples derived from ZIF67 were also prepared and evaluated as counterparts. Based on the electrochemical measurements and characterizations, the effect of the core-shell structure of ZIF67@ZIF8 precursors was further discussed and the mechanism was investigated. 2. Experimental 2.1. Materials Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%), Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%), methanol, 2-Methylimidazole (purity 99%) and IrO2 were purchased from Aladin Ltd. (Shanghai, China). Nafion solution (Dupont Nafion PFSA Polymer Dispersion D-520) was obtained from Dupont. All chemicals were used without further purification in this study. 2.2. Preparation of core-shell ZIF67@ZIF8, ZIF67 and ZIF8. In a typical process, Co(NO3)2·6H2O (2.184 g), Zn(NO3)2·6H2O (2.232 g) and 2Methylimidazole (2.464 g) were dissolved in 30 mL methanol, 30 mL methanol and 60 mL methanol, respectively. Then, the homogeneous solution with Co(NO3)2·6H2O was slowly dropped into 2-Methylimidazole solution under ultrasound and stirring for 5 min. After that, Zn(NO3)2·6H2O in methanol solution was added in the above-mentioned solution in the same


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speed and the resulting suspension was kept under ultrasound for 1 h. The ZIF67@ZIF8 was obtained after centrifugation at 10000 rpm with methanol for several times. ZIF67 was synthesized by the same procedure without adding of Zn(NO3)2·6H2O in methanol solution. ZIF8 was synthesized by the same procedure without adding of Co(NO3)2·6H2O in methanol solution. 2.3. Preparation of Co encapsulated in porous carbons derived from core-shell ZIF67@ZIF8 (CS-Co/Cs) In a typical process, the core-shell ZIF67@ZIF8 was thermally converted into CS-Co/C through carbonization under the Ar atmosphere at 900 oC for 6 h with a heating rate of 5 oC min1

.The samples annealed at 900 oC, 1000 oC and 1100 oC were prepared and named as CS-Co/C-

900, CS-Co/C-1000 and CS-Co/C-1100, respectively. As a control experiment, the ZIF67 was also annealed under the same condition at 900 oC, 1000 oC and 1100 oC and the samples were named as Co/C-900, Co/C-1000 and Co/C-1100, respectively. 2.4. Physicochemical characterization Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4800 microscope. Energy dispersive X-ray spectrometer (EDS) elemental mapping scan were recorded on an Iridium Ultra Premium EDS System (A550I, IXRF, USA). Transmission electron microscopy (TEM) images were collected on a FEI-Tecnai G2 20. X-ray diffraction (XRD) patterns were measured on a Shimadzu LabX XRD-6000 using monochromated Cu Kα radiation (40 kV, 40 mA) at a scanning rate of 5°·min−1. Raman spectroscopy was recorded using a Renishaw Micro-Raman system 2000 with He-Ne laser excitation (wavelength 532 nm). X-ray


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photoelectron spectra (XPS) were obtained with a VG ESCALAB 250 spectrometer using a nonmonochromatized A1 Kα X-ray source (1486.6 eV). N2 adsorption-desorption isotherms were measured at 77K with a Quadrasorb instrument. The contact angle was measured by the KINO SL 200KB. The Inductively coupled plasma-atomic emission spectrum (ICP-AES) was recorded by an Optima2000DV, Perkinelmer, USA. 2.5. Electrochemical characterization The electrochemical measurements were performed on an electrochemical workstation (CHI 750E CH Instruments) in a three-electrode cell. The samples were mixed with 0.5% Nafion solution to obtain a black ink. Glassy carbon electrode with a loading mass 0.2 mg cm-2 was used as the working electrode for electrochemical characterizations. The saturated Hg/HgO electrode (S.C.E.) and platinum wire were reference electrode and counter electrode, respectively. A flow of O2 was maintained over the electrolyte (1 M KOH) during electrochemical measurements in order to ensure the O2/H2O equilibrium at 1.23 V vs. R. H. E (ESCE=ERHE - 0.059 × pH 0.2415). Linear sweep voltammograms (LSVs) were measured with the scan rate of 5 mV s-1. The overpotential was calculated as follows: η = ERHE – 1.23 = ESCE + 0.059 × pH + 0.2415 – 1.23. The Tafel slope was calculated according to Tafel equation: η = b·logj. η is the overpotential, b is the Tafel slope and j is the current density (refers to the geometric surface area). The stability of the samples was investigated by cyclic voltammetry (CV) and chronoamperometric response. All the electrochemical curves are tested without compensation. 3. Results and discussion


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Figure 1: a, b) SEM images of ZIF67@ZIF8. c) TEM ZIF67@ZIF8. d) XRD patterns of ZIF67@ZIF8, ZIF67 and ZIF8, respectively. The morphology of the core-shell ZIF67@ZIF8 was firstly characterized by Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM), as shown in Figure 1. The ZIF67@ZIF8 shows the well-defined rhombic dodecahedral structure, which could be due to the similar unit cell parameters of ZIF67 and ZIF8, with a diameter among 300~500 nm. X-ray diffraction (XRD) patterns of ZIF67@ZIF8, ZIF67 and ZIF8 (Figure 1d) indicate the similar unit cell parameters of the three samples. To further identify the core-shell structure of ZIF67@ZIF8, the energy dispersive X-ray spectrometer (EDS) elemental mapping scan was employed and the images could be seen in Figure 2. It should be noticed that the Co is located at the center while the Zn is distributed among the entire sample, indicating the successful formation of the coreshell structure of ZIF67@ZIF8.


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Figure 2: a) TEM image of ZIF67@ZIF8. b) the corresponding elemental mapping of Co. c) the corresponding elemental mapping of Zn. d) the layered image of a), b) and c).

Scheme 1. The graph of brief synthesis process of CS-Co/Cs.


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Figure 3: a) XRD patterns of CS-Co/C-900, CS-Co/C-1000 and CS-Co/C-1100, respectively. b) Raman spectra of CS-Co/C-900, CS-Co/C-1000 and CS-Co/C-1100, respectively. CS-Co/C and Co/C samples were prepared by carbonizing ZIF67@ZIF8 and ZIF67 at different temperatures, respectively. The Scheme 1 illustrates the brief synthesis process. The XRD patterns of CS-Co/Cs and Co/Cs are shown in Figure 3a and Figure S1. Two intensive peaks located at 44° and 51° corresponding to the (111) facet and (200) facet of face-centeredcubic (fcc) Co crystal can be observed in the XRD patterns of all samples. None of the peaks relating to other Co species (CoOx, Co-N, Co-C) could be found, suggesting the metallic Co is the main Co species in all samples. The broad diffraction peak centered at 26° corresponds to the carbon (002) facet, indicating the formation of porous carbon which is transformed from the


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organic ligand of ZIFs. Remarkably, the carbon (002) peak becomes shaper and narrower with the temperature increasing, demonstrating the higher graphitization degree of the sample annealed at a higher temperature. Raman spectra of the CS-Co/Cs are shown in Figure 3b. The Raman spectra of all samples reveal two peaks located at 1340 and 1580 cm-1 corresponding to the D band and the G band, respectively. Since the D band is caused by the disordered carbon atoms and the D band could be ascribed to the sp2 hybrid carbon atoms, the intensity ratio of D band and G band (ID/IG) is generally used to evaluate the graphitization degree of carbon materials. As shown in Figure 3b, the ID/IG values for CS-Co/C-900, CS-Co/C-1000 and CSCo/C-1100 are 1.08, 0.95 and 0.39, respectively. The lowest ID/IG value for CS-Co/C-1100 indicates the highest graphitization degree of CS-Co/C-1100, in accordance with the XRD results.

Figure 4: a, b, c) SEM images of CS-Co/C-900, CS-Co/C-1000 and CS-Co/C-1100, respectively. d, e) TEM and HRTEM images of CS-Co/C-1000. f) Co particle-size distribution of CS-Co/C-1000 based on Figure S4.


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Figure 4a-c show the SEM images of CS-Co/C-900, CS-Co/C-1000 and CS-Co/C-1100, respectively. It could be noticed that CS-Co/C-900 and CS-Co/C-1000 retain the rhombic dodecahedron-like structure from the core-shell ZIF67@ZIF8 precursor. Remarkably, the CSCo/C-1100 presents the amorphous morphology and the obvious agglomeration of Co particles is revealed, both of which could be due to the high annealing temperature. The SEM images of Co/C-900, Co/C-1000 and Co/C-1100 are also shown in Figure S2. Compared to CS-Co/C-1000, Co/C-1000 derived from ZIF67 without the ZIF8 shell shrinks significantly and presents a cubiclike morphology which is resulting from the distortion of the surface. This could be attributed to the catalytic activity of Co for the formation of graphitic carbon during the annealing process while the ZIF8 shell contains Zn as the metal species, which will be evaporated at 1000 oC. The morphology difference of the samples is further indicated by TEM images (Figure 4d, Figure S3). The results reveal that the diameter of Co particles in CS-Co/C-1100 is even larger than 200 nm while the Co particles in CS-Co/C-1000 are dispersed uniformly with a diameter of 5-20 nm. High-resolution TEM (HRTEM) was further employed to investigate the crystal information. As shown in Figure 4e, the Co nanoparticles exhibit the d-spacing of 0.22 nm, which corresponds to the (111) facet of fcc Co, in accordance with the results of XRD. The carbons surrounding the Co particles present the d-spacing of 0.338 nm, which is the typical value of graphite, indicating the transformation from amorphous carbon to graphitic carbon through catalytic graphitization by Co nanoparticles. The Co nanoparticle diameter distribution in CS-Co/C-1000 was calculated and shown in Figure 4f. It could be noticed that most of the Co nanoparticles in CS-Co/C-1000 exhibit a diameter of 5-15 nm with the uniformly dispersion (Figure S4).


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Figure 5: a) N2 adsorption-desorption isotherms of CS-Co/Cs and Co/C-1000. b) pore-size distribution of CS-Co/Cs and Co/C-1000. c) high-resolution XPS Co 2p spectrum of CS-Co/C1000. d) high-resolution XPS O 1s spectrum of CS-Co/C-1000. The N2 adsorption−desorption isotherms of CS-Co/Cs were measured to investigate the Brunauer−Emmett−Teller (BET) surface areas and the pore-size distributions (Figure 5a-b). Co/C-1000, the sample derived from ZIF67 at the same annealing temperature as CS-Co/C-1000, was also tested to evaluate the influence of the ZIF8 shell. The BET surface areas of CS-Co/C900, CS-Co/C-1000, CS-Co/C-1100 and Co/C-1000 are 282, 306, 144 and 190 m2 g-1, respectively. Remarkably, the surface area of CS-Co/C-1000 is 1.6 times larger than that of Co/C-1000. The increased surface area of CS-Co/C-1000 could be attributed to the core-shell


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structure of ZIF67@ZIF8 precursors since the ZIF8 shell could be transformed into porous carbons during the carbonization, which has the larger surface area than the composite derived from ZIF67. Additionally, the surface area of CS-Co/C-1100 is significantly smaller than that of CS-Co/C-1000, indicating that the high annealing temperature (>1100 oC) will lead to the decreased surface area. As shown in Figure 5a, the sharp uptakes at low relative pressure (

Cobalt Nanoparticles Encapsulated in Porous Carbons Derived from

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