MOF-templated N-doped Carbon-coated CoSe2 Nanorods Supported

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MOF-templated N-doped Carbon-coated CoSe2 Nanorods Supported on Porous CNT Microspheres with Excellent Sodium-ion Storage and Electrocatalytic Properties Seung-Keun Park, and Yun Chan Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03607 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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ACS Applied Materials & Interfaces

MOF-templated N-doped Carbon-coated CoSe2 Nanorods Supported on Porous CNT Microspheres with Excellent Sodium-ion Storage and Electrocatalytic Properties

Seung-Keun Park, and Yun Chan Kang*

Department of Materials Science and Engineering, Korea University, Anam-Dong, SeongbukGu, Seoul 136-713, Republic of Korea

* Corresponding author: [email protected] (Y.C. Kang)

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ABSTRACT Three-dimensional (3D) porous microspheres composed of CoSe2@N-doped carbon nanoroddeposited carbon nanotube (CNT) building blocks (CoSe2@NC-NR/CNT) can be successfully synthesized using CNT/Co-based metal-organic framework (ZIF-67) porous microspheres as a precursor. This strategy involves the homogeneous coating of ZIF-67 polyhedrons onto porous CNT microspheres prepared by spray pyrolysis and further selenization of the composites under an Ar/H2 atmosphere. During the selenization process, the ZIF-67 polyhedrons on the CNT backbone are transformed into N-doped carbon-coated CoSe2 nanorods by a directional recrystallization process, resulting in homogeneous deposition of CoSe2@NC nanorods on the porous CNT microspheres. Such a unique structure of CoSe2@NC-NR/CNT microspheres facilitates the transport of ions, electrons, and mass and provides

a

conductive

pathway

for

electrons

during

electrochemical

reactions.

Correspondingly, the composite exhibits superior dual functionality as both an electrocatalyst for the hydrogen evolution reaction (HER) and an electrode for sodium-ion batteries (SIBs). The CoSe2@NC-NR/CNT microspheres exhibit a small Tafel slope (49.8 mV dec-1) and superior stability for HER. Furthermore, the composite delivers a high discharge capacity of 555 mA h g-1 after 100 cycles at a current density of 0.2 A g-1 and good rate capability for SIBs.

KEYWORDS: sodium ion battery, hydrogen evolution reaction, cobalt selenides, CNT microspheres, spray-pyrolysis, metal-organic framework


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1.

Introduction

Recently, transition metal chalcogenides (TMCs) including metal sulfides and selenides have received increasing interest because of their structural characteristics and unusual chemical, physical, or electronic properties; thus, they are promising materials for a wide range of electrochemical applications.1-9 Correspondingly, many differently structured TMCs, such as nanowires, nanosheets, nanocubes, and urchin-like structures, have shown strong potential as electrocatalysts for splitting water and as anode materials for Li- or Na-ion batteries.10-18 However, the intrinsic low electroconductivity and surface area of TMCs result in a substantial degradation in performance when TMCs are utilized for practical applications. Combining various structured TMCs with conductive carbonaceous materials such as graphite, graphene and carbon nanotubes (CNTs) is one of the most attractive strategies for improving their electrochemical properties.19-24 Among these, CNT-based hybrid structures are promising because of the high chemical stability, accessible surface area, electrical conductivity, and unique morphological characteristics of CNT.20,21,25-27 In particular, three-dimensional (3D) porous structures composed of 1D CNT building blocks have attracted considerable attention because they provide a highly conductive pathway for electrons and an interconnected network of pores for easy access of electrolytes. For example, Han et al. proposed a simple method for preparing core-shell structured CNT@MoS2 with 3D porous network from a CNT sponge template. As a lithium-ion battery anode, the 3D sponge, characterized by open channels and densely packed nanocrystallites, delivered high specific capacity with excellent cycling stability.27 Zhu et al. synthesized sponge-like NixP/CNT hybrid electrodes that consist of a 3D porous CNT support and electrodeposited amorphous NixP catalysts. These



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catalysts exhibited superior electrocatalytic activity for the hydrogen evolution reaction (HER) over the entire pH range.28 Recently, metal–organic frameworks (MOFs), consisting of metal ions and organic ligands, have been intensely studied as useful templates/precursors for preparing variously structured TMCs-based composites.29-34 Yu et al. facilely synthesized hollow Co9S8 nanospheres within graphitic carbon frames derived from a Co-based MOF (ZIF67) via a top-down method.35 Cao et al. successfully prepared N-doped, yolk-shell structured CoSe/C dodecahedra using Co-based MOFs as sacrificial templates.36 Furthermore, some research groups have recently incorporated MOF-derived TMCs with CNTs to improve the electrochemical properties. However, the synthesis of 3D porous structured composites composed of 1D CNT/TMCs building blocks remains a challenge because of the limitations of the preparation method. Herein, we present the successful synthesis of 3D porous composite microspheres composed of CoSe2@N-doped carbon nanorod-deposited CNT building blocks (CoSe2@NC-NR/CNT) via selenization of ZIF-67/CNT microspheres. 3D porous CNT microspheres prepared by ultrasonic spray pyrolysis were used as the backbone for the homogeneous growth of polyhedral-shaped ZIF-67 nanoparticles. During the subsequent selenization process, the ZIF-67 nanoparticles on the CNT backbone were transformed

into

N-doped

carbon-coated CoSe2 nanorods

by

a

directional

recrystallization process. As a result, CoSe2@NC nanorods were homogeneously deposited on the 3D porous CNT microspheres. Such a unique structure of CoSe2@NCNR/CNT microspheres enables easy access of electrolyte and provides a highly conductive pathway for electrons during electrochemical reactions. Moreover, the flexible and robust CNT backbone enhances the structural integrity of composites. Thus, CoSe2@NCNR/CNT microspheres exhibit high capacity and superior rate capability as an anode 4 ACS Paragon Plus Environment

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material for sodium-ion batteries (SIBs). Furthermore, when investigating the electrocatalytic activities for the HER, the composite porous microspheres showed a low Tafel slope and excellent stability in acidic conditions.

2.

RESULTS AND DISCUSSION

Figure 1. Formation mechanism of CoSe2@NC-NR/CNT microsphere; (a) a droplet including acid-treated CNT and PS nanobeads, (b) a filled composite microsphere composed of CNTs and PS nanobeads, and (c) porous CNT microsphere.

The synthetic procedure for CoSe2@NC-NR/CNT microspheres is depicted in Figure 1. First, the 3D porous CNT microspheres were prepared using a one-step spray process, which was described in previous reports.37-39 A droplet (Figure 1-a) containing acidtreated CNTs and polystyrene (PS) nanobeads was generated via an ultrasonic nebulizer 5 ACS Paragon Plus Environment


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and then was carried into a thermal reactor by N2 gas. In the reactor, the droplet was first dried, resulting in a filled composite microsphere composed of CNTs and PS nanobeads (Figure 1-b). At that time, CNTs formed firm assembly through entanglement between them, yielding a structure similar to a ball of yarn. Thus, the unique structure of CNT microspheres retained well even after the decomposition of PS nanobeads. Further decomposition of the PS nanobeads at high temperature formed CNT microspheres with macrovoids

(Figure

1-c) and

their surfaces

were further functionalized

with

polyvinylpyrrolidone (PVP). Polyhedral ZIF-67 nanoparticles then were evenly grown on the PVP-functionalized CNT microspheres via a facile solution method. To confirm the effect of PVP treatment, we carried out a controlled experiment with untreatedporous CNT microspheres as precursor. From the SEM images (Figure S1), irregularly sized ZIF-67 polyhedrons were unevenly deposited on the porous CNT microspheres, implying that the PVP functionalization results in a homogeneous coating of ZIF-67 nanoparticles on the porous CNT microspheres. After selenization under Ar/H2, ZIF-67 nanoparticles were transformed into N-doped carbon-coated CoSe2 nanorods. During the selenization process, H2Se gas initially was generated from the combining H2/Ar with Se vapor and then, it reacted with the ZIF-67 polyhedrons, resulting in abundant CoSe2 crystal nuclei. Subsequently, owing to the high surface energy, the CoSe2 crystal nuclei rapidly aggregated, reducing their surface energy via the intrinsic crystal orientation.40 Finally, the CoSe2 crystals grew into CoSe2@NC nanorods via a directional recrystallization process. Some metal selenides/CNT composite microspheres were synthesized by spraypyrolysis method, followed by selenization process.37,38 The procedure is simple and scalable, but it is difficult to synthesize various structured nanomaterials. The further process is also required to coat conductive carbon on CoSe2. Compared to them, 3D CoSe2@NC-NR/CNT microspheres in this work were synthesized via the combination of the spray-pyrolysis and 6 ACS Paragon Plus Environment

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solution-based methods. From this new synthetic way, unique structured 1D CoSe2@Ndoped carbon nanorod/CNT microspheres were successfully synthesized. During the post-treatment, the formation of 1D CoSe2 nanorods and carbon coating on their surface were occurred at the same time, leading to a synergistic effect on the enhanced electrochemical performance.

Figure 2. Morphologies and crystalline phase of ZIF-67/CNT microspheres: (a) SEM and (bd) TEM images, (e) SAED pattern, and (f) EDX mapping images. 7 ACS Paragon Plus Environment


The formation mechanism of CoSe2@NC-NR/CNT microspheres was examined through the morphological and structural changes which were confirmed by various tools. As displayed in Figure S2 and S3, the morphologies and crystalline phase of the porous CNT microspheres are similar to those of previous studies.37-39 The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Figure S2a–d show the porous and spherical morphology of the microspheres, which originate from the presence of hydrophobic PS nanobeads within the interconnected CNT network. After direct ZIF-67 growth, the porous CNT microspheres still have a spherical shape but show an uneven surface comprised of polyhedral ZIF-67 nanoparticles, as shown in Figure 2a–d. The inner CNT backbone can be clearly identified from some microspheres with exposed CNTs. TEM images reveal that ZIF-67 nanoparticles with a diameter of 100 nm were densely distributed over the microspheres (Figure 2b and c). The X-ray diffraction (XRD) pattern shows that the CNT/ZIF-67 porous microspheres have an XRD pattern similar to that of bare ZIF-67 nanoparticles (Figure S3a), and the selected area electron diffraction (SAED) pattern reveals that the composite microspheres are polycrystalline (Figure 2e). The successful growth of ZIF-67 nanoparticles on the Porous CNT microspheres is further verified by energy dispersive X-ray spectroscopy (EDX) analysis (Figure 2f).


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Figure 3. Morphologies and crystalline phase of CoSe2@NC-NR/CNT microsphere: (a) SEM, (b, c) TEM and (d) HR-TEM images, (e) SAED pattern, and (f) EDX mapping images. The morphologies and crystal structure of CoSe2@NC-NR/CNT microspheres formed by selenization of CNT/ZIF-67 porous microspheres are shown in Figure 3, S3b and S4, respectively. As shown in Figure 3a–c and S4, the polyhedral ZIF-67 nanoparticles on the porous CNT microspheres were completely transformed into rod-shaped CoSe2 after selenization owing to a directional recrystallization process, but the overall morphologies of the microspheres were maintained well. As shown in Figure S4, the CNT backbone and CoSe2 nanorods deposited on the internal and external surfaces of porous CNT microspheres were obviously confirmed from the SEM and TEM images. The 9 ACS Paragon Plus Environment


high-resolution (HR)-TEM image reveals that the CoSe2 nanorods with a diameter of ~5 nm were coated well by the amorphous carbon layer and shows a evident lattice space of 0.26 nm, correlated to the (111) plane of CoSe2 (Figure 3d). Owing to the strong interaction between CNT backbone and ZIF-67 polyhedrons, the overgrowth of CoSe2 nanorods was restricted during the selenization process, leading to the formation of CoSe2 nanorods with small diameter. The XRD and SAED patterns (Figure S3b and 3e) indicate that the composite is composed of dominant orthorhombic CoSe2 and minor cubic CoSe2 without other impurities. The EDX mapping images show the homogeneous distribution of the CoSe2 nanorods (Figure 3f). In addition, the presence of the element N verifies that the carbonization of N-containing organic ligands formed N-doped carbon layers.

Figure 4. XPS spectra of CoSe2@NC-NR/CNT microsphere: (a) C 1s, (b) N 1s, (c) Se 3d, and (d) Co 2p. 10 ACS Paragon Plus Environment

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To investigate the chemical characteristics of CoSe2@NC-NR/CNT microspheres, Xray photoelectron spectroscopy (XPS) analysis was conducted (Figure 4 and S5). As shown in Figure S5, the XPS survey scan reveals the presence of C, N, Co, O and Se. The spectrum of C 1s was deconvoluted into different two peaks located at 284.6 and 285.6 eV (Figure 4a), which correspond to sp2-bonded C–C and N–C/sp3-bonded C–C groups, respectively. The presence of the N–C peak verifies that nitrogen was doped well in the carbon layers.41 The N 1s spectrum shows three peaks at 398.8, 399.9, and 400.9 eV, which are assigned to pyridinic N, pyrrolic N, and graphitic N, respectively (Figure 4b).42 The total content of N dopant in the composites was approximately 6.2 %. According to previous reports, nitrogen incorporated into the carbon network enhances its electron donating ability and also enables fast ion diffusion during electrochemical evaluation.42,43 The Se 3d spectrum (Figure 4c) shows two deconvoluted peaks located at 54.9 and 55.7 eV, corresponding to Se 3d5/2 and Se 3d3/2, respectively, which are in agreement with those of previous reports.37,44 The peaks in the range of 58–62 eV are attributed to Co 3p and Se–O bonding at the surface of composites.44,45 As shown in Figure 4d, the Co 2p spectrum shows two major peaks with each shoulder located at 780.8 and 797.1 eV, respectively corresponding to Co 2p3/2 and 2p5/2, which are attributed to the Co2+ in CoSe2.30 Owing to partial surface oxidation in air, the Co3+ peak was found in the Co 2p spectrum. Additionally, two shake-up satellites were attributed to the anti-bonding orbital between cobalt and selenium atoms.30,45



Figure 5. Morphologies and crystalline phase of CoSe2@NC polyhedrons: (a) SEM, (b, c) TEM and (d) HR-TEM images, (e) SAED pattern, and (f) EDX mapping images. The morphologies and crystal phase of CoSe2@NC polyhedrons formed by the selenization of bare ZIF-67 polyhedrons are shown in Figure 5 and S3b. SEM and TEM images of CoSe2@NC polyhedrons show that the polyhedral morphology of ZIF-67 was nearly preserved after the selenization, although a few damaged particles appeared (Figure S6). Unlike the clean surface of ZIF-67, the surface of CoSe2@NC polyhedrons was comprised of primary CoSe2 nanoparticles with a diameter of 20–30 nm and interstices. From the TEM image (Figure 5c), some rod-shaped CoSe2 nanoparticles can be identified at the surface of polyhedrons. In the HR-TEM image displayed in Figure 5d, primary CoSe2 particles coated by amorphous carbon 12 ACS Paragon Plus Environment

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can be seen clearly, and they show a 0.26 nm crystal lattice fringe, which is matched with the (111) plane of orthorhombic CoSe2. The XRD and SAED results shown in Figure S3 and 5e, respectively, demonstrate that CoSe2@NC polyhedrons had the dominant crystal structure of orthorhombic CoSe2 with a partial cubic CoSe2 phase. The EDX mapping images in Figure 5f reveal an even distribution of the C, Co, N, and Se components. The thermogravimetric (TG) analysis curve of CoSe2@NC polyhedrons, shown in Figure S7a, indicates an initial weight increase (~320 °C) and a subsequent weight loss, corresponding to the partial oxidation of CoSe2 to CoSeO4 and SeO2 and the complete oxidation into Co3O4, respectively.37 As for the TG curve of CoSe2@NC-NR/CNT microspheres, the first weight increase was not found owing to the combustion of CNTs that occurred at around 300 oC. Actually, as confirmed by Figure S7a, the weight loss of CNT porous microspheres occurred ranging from 250-400 oC, and they were completely combusted at temperatures over 400 oC. Based on the TG results, the estimated carbon contents in CoSe2@NC-NR/CNT microspheres and CoSe2@NC polyhedrons were 18.5 and 5.5 %, respectively, on the assumption that the CoSe2 totally transformed into Co3O4 after the test. XRD pattern of the products obtained after TG analysis was well matched with that of pure phase Co3O4 (Figure S8). The structural characteristics of carbon in the composites were investigated by Raman spectroscopy.46 The Raman spectra of CoSe2@NC-NR/CNT microspheres and CoSe2@NC polyhedrons, shown in Figure S7b, indicate two dominant peaks at ~1340 and ~1610 cm-1, which are attributed to the D and G bands of the defected graphitic carbon materials, respectively.46,47 The peak intensity ratio (ID/IG) of the D to G bands used to confirm the degree of graphitization is approximately 0.92 for CoSe2@NC-NR/CNT microspheres and 0.98 for CoSe2@NC polyhedrons. The lower ID/IG value of CoSe2@NC-



NR/CNT microspheres was contributed by the highly graphitic CNT backbone in the composite. The Brunauer–Emmett–Teller (BET) surface area and the pore structure of the two composites were characterized from N2-sorption measurements (Figure S7c, d). As shown in Figure S7c, the BET surface areas of CoSe2@NC-NR/CNT microspheres and CoSe2@NC polyhedrons were 76.8 and 55.4 m2 g-1, respectively. The pore size distribution data indicated that CoSe2@NC-NR/CNT microspheres have a hierarchical porous structure comprised of mesopores (< 50 nm) and macropores (> 50 nm), which may be attributed to the interstices between CoSe2 nanorods and macroporous CNT microspheres, respectively.

Figure 6. Electrochemical properties of CoSe2@NC-NR/CNT microsphere and CoSe2@NC polyhedron: (a) and (b) CV graphs, and (c) and (d) charge-discharge curves at a current density of 0.2 A g-1. 14 ACS Paragon Plus Environment

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The electrochemical properties of CoSe2@NC-NR/CNT microspheres, when used in SIBs, were investigated to compare with those of CoSe2@NC polyhedrons. Figure 6a and b show the cycling voltammetry graphs of the two electrodes for the first two cycles. During the 1st discharging process, a sharp cathodic peak located at about 0.8 V, which corresponds to the electrochemical reaction between the Na+ and CoSe2 resulting in the formation of metallic Co nanograins and Na2Se, was observed in both electrodes.30 Two distinct peaks at 1.86 and 1.95 V in the 1st anodic sweep were attributed to the restoration of metallic Co and Na2Se into CoSe2 nanocrystals. From the 2nd cycle onward, the first two cathodic peaks that newly appeared at 1.40 and 1.07 V could be ascribed to the insertion of Na+ into CoSe2, whereas the peak at 0.65 V corresponded to the conversion reaction with further Na+. The charge and discharge curves of both electrodes for the 1st, 2nd, and 5th cycles at a current density of 0.2 A g-1 are shown in Figure 6c and d. In the 1st discharge curve of CoSe2@NC-NR/CNT microspheres, a clear plateau, matched with the sharp reduction peaks in the CV data, was observed at about 0.91 V, but the plateau of CoSe2@NC polyhedrons was found at a lower potential owing to electrode resistance. CoSe2@NC-NR/CNT microspheres and CoSe2@NC polyhedrons exhibited 1st discharge capacities of 926 and 699 mA h g-1 and their initial Coulombic efficiencies were 65.8 and 70.5 %, respectively. The large capacity loss and low Coulombic efficiency at the initial cycle were primarily contributed by electrolyte decomposition and the formation of a solid-electrolyte interphase (SEI) layer on the surface of electrode.48 The higher surface area and high carbon content of CoSe2@NC-NR/CNT microspheres result in a lower initial Coulombic efficiency compared to that of CoSe2@NC polyhedrons.



Figure 7. Electrochemical properties of CoSe2@NC-NR/CNT microsphere and CoSe2@NC polyhedron: (a) cycling stability at a current density of 0.2 A g-1 for 100 cycles, (b) rate performances at various current densities along with Coulombic efficiencies, and EIS spectra of (c) fresh cells, and (d, e) cycled cells.

The cycling performances of the both electrodes at a current density of 0.2 A g-1, along with their Coulombic efficiencies, are shown in Figure 7a. Although the capacity of CoSe2@NC polyhedrons remained stable during early cycles, it decreased rapidly after 60 cycles, which could be due to pulverization of the polyhedrons. In contrast, CoSe2@NC-NR/CNT microspheres exhibited excellent cycle stability for 100 cycles. Indeed, as confirmed in Figure S9, the unique spherical structure of CoSe2@NCNR/CNT microspheres was retained without any destruction after 100 cycles. HR-TEM and EDX mapping images also revealed the presence of crystalline CoSe2 on the CNT microspheres. In contrast, the morphology and structure of CoSe2@NC polyhedrons were 16 ACS Paragon Plus Environment

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hardly observed owing to the pulverization of electrodes (Figure S10a and b), suggesting that the robust CNT backbone in the composite is responsible for the excellent cycling stability of the electrode. The chemical information of CoSe2@NC-NR/CNT microspheres after cycling was investigated by XPS analysis (Figure S11). The Co 2p spectrum after cycling are barely changed compared to those before cycling, but the proportion of Co3+ species increase a little bit. These indicate that some Co2+ species are transformed into trivalent ones after cycling due to side reactions. Thus, CoSe2@NCNR/CNT microspheres and CoSe2@NC polyhedrons delivered specific discharge capacities of 555 and 312 mA h g-1 after 100 cycles, respectively, and their capacity retentions calculated from the 3rd cycle were 85 and 61 %, respectively. The Coulombic efficiencies of CoSe2@NC-NR/CNT microspheres and CoSe2@NC polyhedrons reached more than 96 % within the 10th cycle. After 100 cycles, however, CoSe2@NC polyhedrons had low Coulombic efficiencies below 95 %. Furthermore, it is notable that CoSe2@NCNR/CNT electrode exhibited a discharge capacity of 484 mA h g-1 after 100 cycles even at a high current density of 5 A g-1, suggesting the stable cycling performance at high rate (Figure S12). To examine the effect of carbon content on the electrochemical performance, porous CNT microsphere electrodes were evaluated by using a galvanostatic charge/discharge method. As shown in Figure S13, the electrodes exhibited excellent cycling stability but a low specific capacity of ~ 150 mA h g-1 at a current density of 0.2 A g-1. Thus, if the carbon content were increased in composites, the specific capacities of composites could decrease due to the low specific capacity of carbon. However, despite of their higher carbon contents, CoSe2@NCNR/CNT microspheres delivered higher specific capacities than CoSe2@NC polyhedrons, which could be ascribed to the robustness and conductive properties of CNT backbone. According to the previous report, metal selenide-based composites exhibited much better 17 ACS Paragon Plus Environment


cycling stability when using ether-based electrolytes than carbonated-based electrolytes for SIBs, which is because the carbonated-based electrolytes easily react with the intermediates of metal selenide, resulting in prompt depletion of active materials.10 In light of these results, using ether-based electrolytes could further improve the electrochemical performances. In addition, the investigation on optimal carbon contents in composites could achieve better cycling stability. To examine the rate properties of two electrodes, electrochemical tests were conducted at various current densities, as displayed in Figure 7b. As the current density rose from 0.2 to 0.5, 1, 2, 3, and 5 A g-1, CoSe2@NC-NR/CNT microspheres exhibited reversible discharge capacities of 645, 606, 582, 559, 533, and 517 mA h g-1, respectively. Furthermore, when the current density reduced to 0.2 A g-1, the capacity increased to 625 mA h g-1, indicating that the sodium-ion storage performance of the electrode did not deteriorate at even high current densities. These high capacity values at various current densities are comparable or superior to those of previously reported cobalt selenide materials (as shown in Figure S14). In contrast, the specific capacity of CoSe2@NC polyhedrons declined rapidly at high current densities (from 2 A g-1), and when the current density decreased again, the capacity did not recover. Furthermore, CoSe2@NC-NR/CNT microspheres had high and stable Coulombic efficiencies over various current densities, whereas CoSe2@NC polyhedrons showed relatively low Coulombic efficiencies at high current densities. To further compare the electrochemical performances of two electrodes, charge-discharge profiles of CoSe2@NC-NR/CNT microsphere and CoSe2@NC polyhedron after 50 and 100 cycles at a current density of 0.2 A g-1, and at different current densities (from 0.2 to 5.0 A g-1) are presented in Figure S15. The change in the profile curves of CoSe2@NCNR/CNT microsphere is slighter than that of CoSe2@NC polyhedron, suggesting good cycling stability and excellent rate capability of CoSe2@NC-NR/CNT microsphere. Compared to 18 ACS Paragon Plus Environment

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previous reports,10,29,34,49 the unique structure of 3D porous composite microspheres enables easy access of electrolyte through numerous pores, leading to high utilization of CoSe2 nanorods. The nanorods with small diameter also provide shortened diffusion path and fast kinetics for ions. Moreover, the robust and conductive CNT backbone provides a highly conductive pathway for electrons during electrochemical reactions as well as enhances the structural integrity of the composites. Thus, CoSe2@NC-NR/CNT microspheres exhibited enhanced electrochemical properties such as long-term cycling and excellent rate capability. The electrochemical impedance spectroscopy (EIS) results of CoSe2@NC-NR/CNT microspheres and CoSe2@NC polyhedrons before and after 1, 10, and 100 cycles are shown in Figure 7c–e. In the medium-frequency area, the Nyquist plots indicated distinct semicircles, which are related to the charge-transfer resistance (Rct) of the electrode.50,51 In the Nyquist plots of the fresh state, CoSe2@NC-NR/CNT microspheres electrode had a lower Rct value than CoSe2@NC polyhedrons electrode, which may be due to the highly conductive CNT backbone in the composites. After the 1st cycling, the Rct of the two anodes were significantly decreased by virtue of the formation of ultrafine CoSe2 nanocrystals during cycling.52 Owing to an activation process during early cycles, the Rct value decreased after the 10th cycle, as shown in Figure 7d. In addition, the low Rct value of CoSe2@NC-NR/CNT microspheres electrode was retained even after 100 cycles. In contrast, the Rct value of CoSe2@NC polyhedrons electrode increased as the cycling proceeded, as shown in Figure 7e, which was attributed to a structural collapse, as confirmed in Figure S10.



Figure 8. Electrocatalytic properties of CoSe2@NC-NR/CNT microsphere, CoSe2@NC polyhedron, commerical Pt/C, and porous CNT microspheres: (a) polarization curves, (b) corresponding Tafel plots, (c) Nyquist plots, and (d) cycling stability of CoSe2@NC-NR/CNT microsphere.

To examine the electrocatalytic activities of the three samples and a commercial Pt/C composite for HER, electrochemical tests were performed in a 0.5 M H2SO4 solution using a conventional three-electrode system (Figure 8). In the linear sweep voltammograms (LSV, Figure 8a), the commercial Pt/C composite showed very high current density with small overpotential, which agrees with previous reports.29,53 In contrast, the porous CNT microsphere exhibited very low current density values, suggesting no electrocatalytic activity for HER. Meanwhile, CoSe2@NC-NR/CNT microspheres 20 ACS Paragon Plus Environment

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showed a larger current density value (23.5 mA cm‒2 at η (overpotential) = 200 mV vs. RHE) than CoSe2@NC polyhedrons (9.6 mA cm‒2 at η = 200 mV vs. RHE). In light of these results, the CoSe2@NC nanorods are responsible for the HER activity and also the porous CNT microspheres offer a conductive pathway for electrons during electrochemical reactions. The synergistic effect between CoSe2@NC-NR and CNT backbones results in enhanced electrocatalytic activity for HER. To evaluate the rate-limiting step, the Tafel slope was calculated from the linear region fitted with the Tafel equation in Tafel plots (Figure 8b). The Tafel slopes for CoSe2@NC-NR/CNT microspheres, CoSe2@NC polyhedrons, and Pt/C composites were 49.8, 70.8, and 30.0 mV dec‒1, respectively. A low Tafel slope means that the reaction rate would increase rapidly with increasing overpotential. From the EIS spectra of the two samples at −0.2 V (vs. RHE), we also confirmed that CoSe2@NC-NR/CNT microspheres had a lower Rct value than CoSe2@NC polyhedrons owing to a smaller ion-transfer resistance at the solid-liquid interface (Figure 8c). Moreover, in the cycle stability test, the LSV curve of CoSe2@NC-NR/CNT microspheres was negatively shifted slightly after 1,000 cycles, indicating superior stability for long-term cycling (Figure 8d). The excellent electrocatalytic properties of CoSe2@NC-NR/CNT microspheres, which are comparable to those of previous reports about CoSe2 catalysts. (Table S1).



Figure 9. Electrochemical double-layer capacitance results of (a) CoSe2@NC-NR/CNT microsphere and (b) CoSe2@NC polyhedron, and (c) summary.

To confirm the electrochemically active surface area of the two samples, we calculated their double-layer capacitance from CV results at various scan rates from 0.2–0.3 V (vs. RHE, Figure 9). As shown in Figure 9a, all the CV curves of CoSe2@NC-NR/CNT microspheres had a nearly rectangular shape, which indicates an ideal capacitive behavior in the electrochemical reaction.54 However, the CV curves of CoSe2@NC polyhedrons were slightly tilted owing to their high internal resistance (Figure 9b). The differences between positive and negative current density at 0.25 V vs. RHE are plotted against the scan rate, as shown in Figure 9c. A linear trend was acquired in both samples. CoSe2@NC-NR/CNT microspheres had a larger capacitance value (Cdl) of 6.34 mF cm22 ACS Paragon Plus Environment

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2

compared to CoSe2@NC polyhedrons (2.31 mF cm-2), which could be attributed to the

abundant active sites and the effective electrochemically active area of CoSe2@NCNR/CNT microspheres.

3.

CONCLUSIONS

Three-dimensional porous microspheres comprising 1D CoSe2@NC-NR/CNT building blocks were synthesized via selenization of precursor CNT/ZIF-67 porous microspheres. The porous CNT microspheres, prepared by spray pyrolysis, were utilized as a backbone to grow CoSe2@NC nanorods. During the selenization process, ZIF-67 polyhedrons on a porous CNT backbone were transformed into CoSe2@NC nanorods by a directional recrystallization process. Because of the synergistic effect between the porous CNT backbones with high conductivity and the 1D structured CoSe2 nanorods with ample active sites, CoSe2@NC-NR/CNT microspheres exhibited superior electrochemical properties as both an electrocatalyst for the HER and an electrode for SIBs. This strategy may provide a universal method for fabrication of 3D porous composite microspheres for electrochemical applications.

4.

EXPERIMENTAL SECTION

4.1. Sample Preparation. Porous CNT microspheres were prepared via a one-pot spray pyrolysis process, which was described in detail in our previous reports.37,38 The spray solution was prepared by dispersing acid-treated CNT stock solution (10 mg mL-1) and PS nanobeads (~100 nm, 3.0 g) in distilled water (500 mL). The thermal reactor was maintained at a temperature of 700 °C. The flow rate of the carrier gas (N2) was 10 L min-1. The obtained porous CNT microspheres (0.1 g) were functionalized with PVP (2.5 g, Mw = 40,000) in 50 23 ACS Paragon Plus Environment


mL of ethanol for 12 h and then collected by centrifugation with ethanol several times. To synthesize the CNT/ZIF-67 porous microspheres, 0.05 g of PVP-functionalized porous CNT microspheres and 3 mmol of Co(NO3)2∙6H2O were dispersed in 100 mL of methanol by sonication, followed by rapid addition of 24 mmol of 2-methylimidazole in 100 mL methanol (2-MIM, Sigma-Aldrich). The mixture was then maintained at room temperature for 2 h without agitation and obtained by centrifugation. The 3D porous CoSe2@NC-NR/CNT microspheres were prepared by selenization of the as-obtained CNT/ZIF-67 porous microspheres with Se powder (mass ratio of 1:2) at 300 °C for 6 h in an Ar/H2 (10 %) atmosphere. As a reference, CoSe2@NC polyhedra were obtained by the selenization of polyhedral-shaped ZIF-67 nanoparticles. 4.2. Characterization Techniques. SEM and TEM images were acquired using a VEGA3 (TESCAN) and JEM-2100F (JEOL), respectively. The crystalline structures of samples were confirmed by XRD patterns obtained using an X’Pert PRO with Cu Kα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). Thermo Scientific K-Alpha XPS system was used to confirm the chemical information and electron valence of the samples. The surface areas were calculated by the BET method with high purity N2. The carbon contents of samples were estimated with a TG analyzer (Pyris 1, Perkin Elmer, Korea Basic Science Institute (Busan)) ranging from 25–650 °C with a heating speed of 10 °C min-1 under air atmosphere. The structural characteristics of carbon in the samples were investigated by Raman spectroscopy (Jobin Yvon LabRam HR800, excited by a 632.8-nm He/Ne laser) at room temperature. 4.3. Electrochemical Measurements. SIB Test. The electrochemical performances of samples as SIB anodes were tested by a 2032-type coin cell. The electrode was prepared by casting the mixture containing 70 wt % of 24 ACS Paragon Plus Environment

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samples (CoSe2@NC-NR/CNT microsphere and CoSe2@NC polyhedron), 20 wt% of Super P, and 10 wt% of sodium carboxymethyl cellulose (CMC) in DI water onto the copper foil. The fabrication of the sodium-ion cell was conducted in Ar-filled glove box with a Na metal, electrolyte (1 M NaClO4 in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 v/v) with 5 wt% fluoroethylene carbonate (FEC)), a glass filter separator, and the as-made electrodes. The galvanostatic tests were performed in a battery test station (WonAtech) with a voltage window of 0.001–3 V, and CV test was carried out with a scan rate of 0.1 mV s-1. The average mass loading was about 1.3 mg cm-2. HER Test. A catalyst ink for HER test was prepared by adding 5 mg of samples to 1mL DI water/isopropyl alcohol (3:1 v/v %) mixed solvent containing 40 µL of Nafion solution (5 wt%, Sigma-Aldrich). To form homogenous ink, the above-mixture was sonicated for at least 1 hr. Then, A 10 μL of catalyst ink was loaded onto the glassy carbon electrode with a diameter of 5 mm. The same method was also employed to prepare Pt/carbon (20 wt%, Johnson Matthey) modified electrode. Electrocatalytic tests were fulfilled on a potentiostat (ZIVE SP-1) with a three-electrode cell which included a working electrode of glassy carbon electrode, a counter electrode of Pt foil, and a reference electrode of saturated calomel electrode. Linear sweep voltammetry (LSV) was carried out at a scanning rate of 5 mV s‒1 in 0.5 M H2SO4 solution. All potential values reported in this work are relative to the reversible hydrogen electrode (RHE). The EIS measurements were performed in the frequency range of 100 kHz to 0.1 Hz with the amplitude potential of 5 mV. All the data have been corrected for the iR drop based on EIS.

ASSOCIATED CONTENT Supporting Information 25 ACS Paragon Plus Environment


Page 26 of 35

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. SEM images of ZIF-67 polyhedrons grown on untreated porous CNT microspheres; SEM and TEM images of porous CNT microspheres; XRD patterns of CNT microspheres, ZIF-67 and ZIF-67/CNT microspheres, CoSe2@NC and CoSe2@NC-NR/CNT microspheres; SEM and TEM images of CoSe2@NC-NR/CNT microsphere; XPS survey of CoSe2@NC-NR/CNT microspheres; SEM and TEM images of ZIF-67 polyhedrons; TGA curves, Raman spectra, N2 adsorption and desorption isotherms, and BJH pore size distributions of CoSe2@NC-NR/CNT microspheres and CoSe2@NC polyhedrons; XRD pattern of the final product after TG analysis of CoSe2@NC-NR/CNT microspheres; SEM, TEM, HR-TEM and EDX mapping images of CoSe2@NC-NR/CNT microspheres, and SEM images of CoSe2@NC polyhedrons after 100 cycles; high-resolution XPS spectra of Co 2p of CoSe2@NC-NR/CNT microspheres obtained after 20 cycles; Cycling performance of CoSe2@NC-NR/CNT microspheres at a current density of 5 A g-1; charge-discharge profiles and cycling performance of porous CNT microspheres at current density of 0.2 A g-1; Rate comparison of CoSe2@NC-NR/CNT microspheres with various

nanostructured

CoSex-based

electrodes;

charge-discharge

profiles

of

CoSe2@NC-NR/CNT microsphere and CoSe2@NC polyhedron after 50 and 100 cycles at a current density of 0.2 A g-1, and at different current densities (from 0.2 to 5.0 A g-1); comparison table of electrocatalytic activity for HER of various nanostructured CoSex materials reported in the previous literatures

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] 26 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (NRF-2017R1A2B2008592 and NRF-2017R1A4A1014806). REFERENCES (1)

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ToC Page 35 of 35

ZIF-67 growth

Selenization

Porous CNT microsphere

ZIF-67/CNT microsphere

Electrode material 1000

CoSe2@NC-NR/CNT

800 600 400 200

Current density = 0.2 A g-1 0

0

10

20

30

40

50

60

Cycle number

70

80ACS Paragon 90 Plus 100 Environment

CoSe2@NC-NR/CNT microsphere Current density (mA cm-2)

Capacity (mA h g-1)

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


Electrocatalyst 0

CoSe2@NC-NR/CNT -10

Pt/C (20 wt%)

-20

-30 -0.5

-0.4

-0.3

-0.2

-0.1

Potential (V vs. RHE)

0.0

MOF-templated N-doped Carbon-coated CoSe2 Nanorods Supported

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