MetalâOrganic Framework Template Derived Porous CoSe2

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Metal Organic Framework Template Derived Porous CoSe2 Nanosheet Arrays for Energy Conversion and Storage Tian Chen, Songzhan Li, Jian Wen, Pengbin Gui, and Guojia Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12403 • Publication Date (Web): 28 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Applied Materials & Interfaces

Metal Organic Framework Template Derived Porous CoSe2 Nanosheet Arrays for Energy Conversion and Storage

Tian Chen, Songzhan Li, Jian Wen, Pengbin Gui, Guojia Fang*

Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, School of Physics and Technology, Wuhan University, Wuhan, 430072, P. R. China E-mail: [email protected]

ABSTRACT: Porous CoSe2 on carbon cloth is prepared from a cobalt-based metal organic framework template with etching and selenization reaction, which has both larger specific surface area and outstanding electrical conductivity. As the catalyst for oxygen evolution reaction, the porous CoSe2 achieves a lower onset potential of 1.48 V, versus the reversible hydrogen electrode (RHE), and a small potential of 1.52 V (vs RHE) at an anodic current density of 10 mA cm-2. Especially, the linear sweep voltammogram curve of the porous CoSe2 is in consist with the initial curve after durability test for 24 h. When tested as an electrode for supercapacitor, which can deliver a specific capacitance of 713.9 F g-1 at current density of 1 mA cm-2 and exhibit excellent cycling stability in that a capacitance retention of 92.4% can be maintained after 1 ACS Paragon Plus Environment


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5000 charge-discharge cycles at 5 mA cm-2. Our work presents a novel strategy for construction of electrochemical electrode.

KEYWORDS: porous CoSe2 arrays, metal organic framework template, etching

and

selenization

reaction,

oxygen

evolution

reaction,

supercapacitor

INTRODUCTION With the development of modern society, energy crisis and environment problem are those we must face now. Developing high effective electrocatalyst and supercapacitor are important method to solve these issues. However, transitional metal oxides, metal hydroxides and carbon material are difficultly satisfied with the requirement of electrocatalyst and supercapacitor. Compared with above transitional materials, metal selenides have attracted considerable attention with high electrochemical activity, good electronic conductivity and stability.1,

2

Among various metal selenides, CoSe2 have been proved possessing good electrochemical activity and stability.3,

4

Nevertheless, preparation of

higher effective electrocatalyst and supercapacitor electrode material is still urgent. The strategy of constructing porous architecture is one of the effective method to improve electrochemical performance. Metal organic frameworks (MOF) have attracted considerable 2 ACS Paragon Plus Environment

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attention in recent years with characteristics of high surface areas, high tunable porosities and rich reaction sites.5-8 They have been proved great promise applications for catalyst9, 10, gas storage11, 12, sensing13, 14 and lithium-ion batteries15,

16

. However, the bad electrical conductivity

restricts the application of MOF in above fields. The characteristics of designable framework structures for MOF can be combined with other kinds of porous active materials, such as carbonaceous materials, metal oxides, metal sulfide and phosphides.8, 17-20. Based on these characteristics of MOF, it can be served as template for synthesis electrode material of both electrocatalyst and supercapacitor. The strategy is well to provide rich reaction sites and short ion diffusion length as well as release the strain during the electrochemical reactions.21-26 Recently, metal selenides reported in the literatures have been shown excellent electrochemical performance in electrocatalyst.27-30 For example, Alshareef at al. demonstrated that edge Se atoms are intrinsic active sites in Ni0.33Co0.67Se2 electrocatalyst for hydrogen evolution reaction (HER) applications.31 And they further revealed that the (Ni, Co)0.85Se exhibits a low potential of 1.48 V (vs RHE) at 10 mA cm-2 for oxygen evolution reaction

(OER).32 Furthermore,

Zhang’s

group

transformed

the

NiCo-LDH into Ni1−xCoxSe2 by ions exchange, which also achieved high catalytic activities and excellent stabilities in all-PH conditions.2 These findings suggest that Se atoms introduced into electrode material could 3 ACS Paragon Plus Environment


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increase the active edge sites and improve electrochemical activity. Therefore, considered about above reports and designable characteristics of MOF, introducing the Se atoms into MOF is significant. Herein, we report a facile approach to synthesis porous CoSe2 nanosheet arrays on carbon cloth (CC). The process of synthesizing porous CoSe2 nanosheet on CC involves in three steps: the MOF-Co nanowall arrays on CC, etching reaction process in deionized water/ethanol solution followed with a thermal treatment and selenation reaction. The synthesized porous CoSe2 nanosheet possess both larger specific surface area and higher electrical conductivity. Furthermore, the porous CoSe2 nanosheet was grown in conductive carbon cloth without any binder and additives, it can be directly used as electrode. Above advantages of porous CoSe2 nanosheets are good at improving the performance of energy conversion and energy storage. When tested as a catalyst for OER, which achieves a lower onset potential of 1.48 V (vs RHE) and a small potential of 1.52 V (vs RHE) at an anodic current density of 10 mA cm-2 and outstanding durability. Moreover, as for supercapacitor electrode, the porous CoSe2 electrode can deliver a specific capacitance of 713.9 F g-1 at current density of 1 mA cm-2, which is about three times as that of CoSe2 electrode (254.4 F g-1) at the same current density. The excellent electrochemical performance of porous CoSe2 is attributed to the porous nanostructure, high electrochemical 4 ACS Paragon Plus Environment

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activity and great electrical conductivity.

EXPERIMENTAL SECTION Preparation of MOF-Co on CC: An aqueous solution containing 2-methylimidazole (C4H6N2, 40 mL, 0.4 M) was quickly added into the aqueous solution of Co(NO3)2·6H2O (40 mL, 50 mM), after which a piece of clean CC substrate (1 × 1.5 cm2, the front of the carbon cloth was protected by glass slides ) was immersed into the mixture solution. After reaction for 4 h, the sample was then taken out and cleaned with deionized water, finally dried in vacuum overnight. The mass loading is ~0.50 mg cm-2. Preparation of porous CoSe2 on CC: First, a piece of MOF-Co on CC (1 × 1.5 cm2) was etched by an etching reaction process in deionized water/ethanol solution followed with a thermal treatment for 4 min. Then, a piece of treated MOF-Co (1 × 1.5 cm2) was placed in the middle of tube furnace. Then, 0.6 g Se powder was put in a crucible, and put in the side of a quartz tube inlet (the distance of MOF-Co and crucible is 16 cm). The tube furnace was heated to 450 °C at a fast ramping rate of 10 °C min-1 and hold for 2 h with argon flowing at 100 sccm. After cooling down to room temperature, porous CoSe2 was successfully prepared. The mass loading of porous CoSe2 is ~ 0.53 mg cm-2. Preparation of CoSe2 on CC: A piece of MOF-Co was directly 5 ACS Paragon Plus Environment


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annealed in Se atmosphere in a tube furnace, the same as that of synthesis porous CoSe2. The mass loading of CoSe2 is ~ 0.57 mg cm-2. Material characterization: The morphology, microstructure and chemical composition of the samples were characterized by field emission scanning electron microscopy (FESEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL, JEM-2100), X-ray diffraction (XRD, Bruker Axs, D8), X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCLAB 250Xi), Brunauer-Emmett-Teller (BET, JWGB, BK112T). Electrochemical

measurements:

(a)

OER

test:

all

the

electrochemical tests were carried out in a three-electrode testing system (CHI 660E electrochemical workstation, Chenhua, Shanghai). The as-prepared sample on CC and pure CC

were directly used as the

working electrode without any metal support or other current collector. A standard calomel electrode (SCE) is used as the reference electrode and the counter electrode is a Pt foil. 1 M KOH solution (PH=13.6) was used as the electrolyte during the electrochemical testing. The electrochemical impedance spectroscopy (EIS) tests were measured in the frequency between 0.01 and 100 kHz. The polarization curves were measured at 2 mV s-1 and iR corrected. All the measured potentials of electrocatalyst are referred to reversible hydrogen electrode

using the following equation:

E(RHE)=E(SCE)+0.059*PH+0.241. The electrochemical double-layer 6 ACS Paragon Plus Environment

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capacitance (Cdl) was determined from the CV curves measured in a potential range without redox process by: Cdl=I/ν, where I is the charging current (mA cm-2), and ν is the scan rate (mV s-1). (b) Supercapacitor test: the tests were carried out in a three-electrode testing system in 3 M KOH. The as-prepared sample and carbon cloth are used as the working electrode. The working electrode area is 1 cm × 1 cm. A SCE served as the reference electrode and the counter electrode is a Pt foil. Cyclic voltammetry (CV) measurements are carried out ranging from -0.1 to 0.5 V. The galvanostatic charge-discharge (GCD) measurements are conducted on a LAND battery testing system (LAND CT-2001A).

RESULTS AND DISCUSSION Materials Fabrication and Characterizations The synthesis procedure of porous CoSe2 on CC from MOF-Co nanowall is displayed in Figure 1 (see the Experimental Section for details). First, the MOF-Co nanowall was firstly obtained by a facile hydrothermal reaction.33, 34 The MOF-Co consists of many organic bond. Among these organic bond, the C-C bond can maintain essential structure. N element as a kind of natural active site, the MOF consists of it and can serve as the nitrogen source. The designable structure and triangular nanowell arrays of MOF-Co provide a possible to construct the porous CoSe2 nanosheet arrays. Then porous CoSe2 was subsequently 7 ACS Paragon Plus Environment


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synthesized using the solution etching treatment and selenization in Se atmosphere. It can be directly used as an electrode for both electrocatalyst and supercapacitor. Also, the schematic illustration of synthesis procedure for porous CoSe2 in tube furnace is shown in Figure S1 (Supporting Information). X-ray diffraction (XRD) experiment was firstly carried out to study the as-prepared materials. The XRD pattern of the crystalline MOF-Co precursor in Figure S2 is well matched with the literature reports.8, 33 After MOF-Co nanowall was etched and annealed in Se atmosphere, the porous CoSe2 has been successfully synthesized. As shown in Figure 2a, the diffraction angles of porous CoSe2 are loaded at 2θ = 30.48°, 34.19°, 37.62°, 43.69°, 51.75°, 54.23°, 58.85°, 63.44°, 71.96° and 74.00° correspond to the (200), (210), (211), (220), (311), (230), (321), (400), (420) and (421) crystal planes of CoSe2 (PDF#09-0234), respectively, and the crystal structure of porous CoSe2 is cubic. The result indicates the Se atoms have been successfully introduced into etched MOF-Co. Obviously, there are the peaks of C, which are attributed to carbon cloth and the residual C deriving from the MOF. Furthermore, the XRD pattern of CoSe2 is also explored, which is shown in Figure S2. Especially, in the XRD pattern of CoSe2, there are several weak peaks located at 15° to 20°, which are attributed to the MOF. And some main peaks located at 2θ = 30.8°, 34.7°, 36.3° and 47.8° are corresponding to the (110), (111), (012) 8 ACS Paragon Plus Environment

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and (121) crystal planes of CoSe2 (PDF#10-0408), respectively. The X-ray photoelectron spectroscopy (XPS) measurement was further employed to investigate the chemical composition of the materials. After directly selenization the MOF-Co, the XPS spectrum survey of CoSe2 indicates the presence of Se, Co, O, N and C without any other impurities (Figure S3a). The fitting peaks at binding energies of Co 2p for CoSe2 (Figure S3b) at 780.3 eV and 796.4 eV are attributed to Co2+ and corresponded shakeup satellite, while the other two fitting peaks at 778.1 eV and 793.1 eV are ascribed to Co0.35-38 Figure S3c reveals two distinct binding energies of 54.4 eV and 55.2 eV, which corresponds well with previous reported results of Se 3d5/2 and Se 3d3/2.27, 29, 30 In addition, the binding energy of Se 3d are located at 57.0 eV~ 62.0 eV are attributed to SeOx.39, 40 As shown in Figure S3d, the XPS spectrum of C 1s contain three main peaks, the C-C bond (284.4 eV), C-N & C-O (285.8 eV) and C-O-C & C-N (287.5 eV).38, 41 The XPS results of O1s for CoSe2 is shown in Figure S3e, from which the O1s spectrum can be resolved into two components located at 530.4 eV and 531.2 eV, respectively. The low binding energy of 530.4 eV is attributed to the typical metal-oxygen bonds and another peak is 530.9 eV assigning to chemisorbed oxygen and hydroxyls.8, 42 The results of C 1s and O 1s for CoSe2 indicate that the organic ligand still exist in the sample, which will have a negative influence on that of the electrochemical performance. The N1s spectrum 9 ACS Paragon Plus Environment


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of CoSe2 (Figure S3f) can be decomposed into two peaks, which are belong to pyridinic N and pyrollic N.43, 44 Frequently, pyridinic N and pyrrolic N are generally considered as the efficient active sites for electrochemical.45 Furthermore, the MOF-Co was etched and annealed in Se atmosphere, the synthesized sample was further measured by XPS. The XPS survey spectrum of porous CoSe2 (Figure 2b) reveals that of the chemical composition contain Co, Se, O, N and C without any other impurities. In Figure 2c and Figure 2d, the peaks position of Co 2p and Se 3d for porous CoSe2 are similar to that of CoSe2. Observably, the XPS spectrum of C1s (Figure S4a) for porous CoSe2 is different from CoSe2, there is only peak at 284.6 eV (C-C). In addition, the O 1s XPS spectrum (Figure S4b) exhibits two peaks at 530.9 eV and 532.8 eV, corresponding to cobalt oxides and C=O.38, 41, 46 The results of C 1s and O 1s for porous CoSe2 are different from CoSe2, indicating that the etching and annealing reaction can remove from some organic ligand. Especially, in Figure S4c, we can see that pyridinic N and pyrrolic N still exist in porous CoSe2. At the same time, the pyridinic N and pyrrolic N can be retained in the etching process. Above results of XRD and XPS for final sample proved that of CoSe2 has been synthesized. The morphology of the as-prepared sample are explored with scanning electron microscopy (SEM). Figure 3a shows that the MOF-Co 10 ACS Paragon Plus Environment

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nanowalls were homogeneously aligned on CC surface. As shown in high magnification SEM image (Figure 3b), the solid nature of these Co-based nanowalls present triangular. Furthermore, the higher magnified SEM image reveals that the MOF-Co possess smooth surfaces (Figure 3c). In addition, the etched MOF-Co nanowall still exhibits triangular shape and were homogeneously aligned on CC surface (shown in Figure S5). After directly selenization MOF-Co, the CoSe2 nanowalls (Figure 3d) are also uniform arranged on CC. In Figure 3e, the basic structure of these nanowalls have been not changed after selenization, the triangular shape was still maintained. From an enlarged view in Figure 3f, we can see that many crystal particles exist in the surface of CoSe2 nanowall. The high magnification SEM image of CoSe2 is shown in Figure S6a, the diameters of crystal particle is 10 ~ 70 nm. A porous nanowall structure has been formed. Taking consideration the structure of the MOF can be rebuilt, the very weak acid solution (deionized water/ethanol) was used to etch the MOF-Co. The solution of deionized water/ethanol shows faintly acid, which could remove from a part of organic bond, decrease thickness of MOF-Co and ensure full selenization of precursor. After selenization etched MOF-Co (Figure 3f-i and Figure S6b), many nanoparticles connected each other to form a porous nanostructure and the triangular shape can be maintained. The porous nanostructure feature of porous CoSe2 can provide abundant electrode/electrolyte contact interfaces and 11 ACS Paragon Plus Environment


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reduces ion diffusion path for this fast electrochemical kinetic process. Moreover, the N2 adsorption isotherms of porous CoSe2 and CoSe2 were recorded to measure the specific area. In Figure S7a and Figure S7b, the porous CoSe2 displays a higher specific Brunauer–Emmett–Teller (BET) surface area of 34.1 m2 g-1 than that of CoSe2 (17.6 m2 g-1). The rich porous channel and high surface area of porous CoSe2 may result from the porous structure of etching MOF-Co template. The method of etching and annealing in Se atmosphere, which not only can exclude the poor electronic conductivity of MOF, but also improve larger specific surface area and enrich active sites. The morphology and crystal structure of the as-prepared sample are further measured by a transmission electron microscopy (TEM). As shown in Figure 4a, we can see that the surface of the MOF-Co is smooth and don’t have any porous morphology. Figure 4b shows that a novel structure has been formed after directly annealing MOF-Co in Se atmosphere, from which we can see that the triangular nanowalls consist of many small crystalline particles. The high resolution electron microscopy (HRTEM) of CoSe2 is further explored the crystal structure of crystalline particles. In Figure 4c and corresponding insert image, a set of lattice fringes with inter planar spacing of 0.289 nm can be observed, which correspond well to the (110) direction of CoSe2 (PDF#10-0408) and the result is consist with the XRD resuts. Especially, the structure of 12 ACS Paragon Plus Environment

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the final sample has a great change after etching and selenization of Co-based MOF and corresponding TEM image is shown in Figure 4d. From which the structure of final sample is different from CoSe2, the smooth face of connecting many nanoparticles have been disappeared and the porous structure have formed, which is good at improving specific surface area of the materials. The detail crystal structure and morphology is further investigated by a HRTEM analysis. As shown in the HRTEM image of Figure 4e, the lattice spacing of 0.239 nm corresponds to the (221) planes of porous CoSe2 (PDF#09-0234). The corresponding selected area electron diffraction (SAED) pattern (shown in Figure 4f) further confirms the crystalline structure of CoSe2 and grows along the (211), (311) and (321) directions.

3.2 Oxygen Evolution Reaction As the cobalt selenide material is a good oxygen evolution catalyst for water splitting, the OER performance of the as-prepared sample and carbon cloth (CC) are also examined in 1 M KOH solutions. The catalytic activity is tested in a standard three-electrode system in 1 M KOH (PH=13.6) solution, using a Pt plate and a standard calomel electrode (SCE) as the counter and reference electrode. In Figure 5a, the linear sweep voltammogram (LSV) is first used to measure the catalytic activity of as-prepared sample and CC at 2 mV s-1, from which the onset potential of porous CoSe2 electrode is only 1.48 V (vs RHE) and it is lower than 13 ACS Paragon Plus Environment


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that of CoSe2 (1.52 V vs RHE), as well as that of MOF-Co (1.70 V vs RHE). In addition, the OER performance of CC electrode is also measured, one can see that it has little contribution to the catalytic process. Furthermore, the porous CoSe2 shows higher current density at the same potential than that of other as-prepared electrode, indicating a higher catalyst activity of the porous selenization nanosheet. The anodic current densities of 10 mA cm-2 is usually evaluate the catalytic activity in OER. The porous CoSe2 electrode can achieve the current density of 10 mA cm-2 at a lower potential of 1.52 V (vs RHE). By contrast, the CoSe2 electrode needs 1.59 V (vs RHE) to deliver a significant O2 evolution and MOF-Co is 1.61 V, which are larger than that of porous CoSe2 electrode. Also, the lower potential of 1.52 V for porous CoSe2 at 10 mA cm-2 is comparable with that of other many Co-based electrocatalysts: CoP hollow polyhedron (1.62 V vs RHE),47 CoSe2 (1.63 V vs RHE),4 Co9S8/S-C (1.56 V vs RHE),48 Co@Co3O4/NC (1.64 V vs RHE),37 Co3O4 Hollow Polyhedrons (1.62 V vs RHE),49 Co3O4/rGO (1.57 V vs RHE)50. Moreover, Tafel plot calculated from LSV data is a much more straight forward way to estimate the OER kinetics of the catalysts. The Tafel slope can be obtained by linear fitting base on the Tafel equation as follow: η= b logJ + a, where J is the current density and b is the Tafel slope. The Tafel slope of porous CoSe2 is 114.7 mV dec-1, which is lower than that of CoSe2 (124.3 mV dec-1), MOF-Co (122.1 mV dec-1) and CC 14 ACS Paragon Plus Environment

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electrode (184.7 mV dec-1). The result of Tafel slope shows that the porous CoSe2 possess better OER kinetics than other catalyst. Electrochemical impedance spectroscopy (EIS) measurement is carried out and the corresponding higher frequency Nyquist plots is displayed in Figure 5c. The intercept of X axis for porous CoSe2 is 1.52 Ω, which is smaller than that of CoSe2 (1.60 Ω), MOF-Co (1.76 Ω) and CC (1.84 Ω) electrode, showing that the bulk resistance of the porous electrode is smaller. The porous CoSe2 shows smaller bulk resistance than CoSe2 can be attributed to following two aspects: i) In XRD part, the porous CoSe2 shows better crystallinity than that of CoSe2, therefore less recombination and better carrier transfer. ii) The porous structure enables close contact with electrolyte thus allows more efficient utilization of active sites. The nanopores can provide rich reaction sites and short ion diffusion length as well as release the strain during the electrochemical reactions and be good at releasing of gas during the OER catalysis. iii) In the XRD pattern of dense CoSe2 (Figure S2a), there are several weak peaks located at 15° to 20°, which are attributed to the MOF. And MOF is not good at ion diffusion. And the diameter of the semicircle indicating charge transfer resistance is significantly smaller in the porous CoSe2 electrode (0.71 Ω), when compared to the CoSe2 (1.21 Ω), MOF-Co (0.93 Ω) and CC electrode (6.19 Ω). In the lower-frequency region (shown in Figure S8), the slope of the porous CoSe2 electrode is large than that of other 15 ACS Paragon Plus Environment


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electrodes, indicating its lower ion diffusion resistance. These results exhibit that porous CoSe2 electrode has better ion diffusion and charge transfer properties. To further explore the intrinsic activity of the as-prepared sample, the double-layer capacitances (Cdl) was measured to illustrate the electrochemical active surface areas (Figure S9). As shown in Figure 5d, the calculated electrochemical double layer capacitance of porous CoSe2 is ~6.30 mF cm-2, which is much bigger than that of the other catalysts. The results of the Cdl indicate that the porous CoSe2 possess larger activity surface area, which is mainly attributed to the porous structure and more active sites. The

stability is another important

aspect to evaluate the

electrochemical performance of catalyst (Figure 5e). The porous CoSe2 and CoSe2 electrode are directly as the electrode, which are tested under a current density of 10 mA cm-2 for 24 h. In Figure 5e, it can be seen that the curves of porous CoSe2 and CoSe2 electrode present a flat straight line. The durability electrochemical performance of porous CoSe2 and CoSe2 are attributed to the selenation reaction, indicating that the Se atom introduce into the materials is good at improving the durability of the catalysts. In Figure 5f, the digital photograph for porous CoSe2 shows the process of OER. The porous CoSe2 electrode is further measured after 24 h stability test. In Figure 6a-b, the LSV and EIS curves are basically superposing compared with the initial curves after 24 h stability 16 ACS Paragon Plus Environment

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test. In addition, the CdI of the porous CoSe2 electrode (Figure S10a and Figure 6c) is measured after 24 h durability test with the value is ~5.78 mF cm-2, which is little consistent with the initial electrode. To further demonstrate the stability of the porous CoSe2 electrode, the SEM images of tested porous CoSe2 is shown in Figure 6d and Figure S10b. One can see that the nanosheets are still uniform arranged on CC and triangular shape of the porous nanosheets can still be remained. Above studies demonstrate excellent stability of the porous CoSe2 in OER.

3.3 Supercapacitive Property Consideration the excellence performance of porous CoSe2 in OER, the potential applications for energy storage is worth to explore. Electrochemical

properties

of

the

porous

CoSe2 electrode

for

supercapacitor was tested in a three-electrode system in 3 M KOH electrolyte. For comparison, the CoSe2 electrode was also tested. In Figure 7a, in the CV test at scan rate of 5 mV s-1, the both porous CoSe2 and CoSe2 electrode exhibit two pairs of redox peaks. The first pair of redox peaks are attributed to Co2+/Co3+ (O1 and R1), the second pair of redox peaks belong to the typical conversion between Co3+ and Co4+ (O2 and R2). Based on the CoSe2 electrode has an activated process through the following reaction before the CV measurement and two pairs of redox peaks of CV curves, the electrode reaction equation maybe: CoSe2 + H2O + (1+x)/2O2

Co(OH)2 + SeOx

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Co(OH)2 + OH-

COOH + H2O + e-

COOH + OH-

CoO2 + H2O + e-

It is apparent that the enclosed area of the porous CoSe2 electrode is much larger than that of the CoSe2, suggesting the porous CoSe2 electrode has much larger capacitance than the CoSe2. The GCD is measured energy storage property of the as-prepared sample at 1 mA cm-2 (Figure 7b) and the discharge-specific capacitances are calculated. The porous CoSe2 electrode exhibits a high specific capacitances ~713.2 F g-1 under current densities of 1 mA cm-2, which is larger than that of CoSe2 electrode (~254.4 F g-1 at 1 mA cm-2). Also, this specific capacitance of porous CoSe2 electrode is comparable with those from other Co-based supercapacitor material, such as Co2P nanoflower (416 F g-1 at 1 A g-1),51 Co0.85Se nanowire (674 F g-1 at 1.48 A g-1),52 Co0.85Se nanosheet (287 F g-1 at 0.5 A g-1),53 CoO nanoparticle (600 F g-1 at 0.5 A g-1),54 CoS1.097 (764 F g-1 at 0.5 A g-1)55 and Co3O4 (739 F g-1 at 1 A g-1)56. In addition, the discharge-specific capacitances with different current densities are displayed in Figure 7c. A specific capacitances of porous CoSe2 electrode about 535.8 F g-1 could be achieved at a higher current density of 20 mA cm-2. And this electrode exhibits good rate of the capability with 75.1 % of the capacitance retained when the current density increases from 1 to 20 mA cm-2, which is a little larger 17.2 % than that of CoSe2 electrode. In detail, CV and GCD curves of porous CoSe2 are shown in Figure S9. 18 ACS Paragon Plus Environment

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In Figure S11a, the CV curves of porous CoSe2 electrode exhibits two symmetrical pairs of redox peaks. With the scan rate increasing, the redox peaks is still maintained, the peak of cathode is shifting to higher potential and the peak of anode is transforming to lower potential, which shows the ability of rapid oxidation reduction reaction for porous CoSe2 electrode. In addition, From the GCD curves (Figure S11b), we can see that the curves exhibits better symmetry at different current density, indicating the porous CoSe2 electrode possess well coulombic efficiency. The cycling stability test of porous CoSe2 electrode is further curried out. As shown in Figure 7d, after 5000 charge-discharge cycles at a current density of 5 mA cm-2, the porous CoSe2 electrode maintains about 92.4% of the initial capacitance. The insert SEM image of tested porous CoSe2 is further measured, the basic structure can be maintained and shows excellent stability. The excellent electrochemical performance of the porous CoSe2 could be attributed to the following aspects: i) The Se substitution alters the electronic structure and improves electrical conductivity, which favors fast electron transport. ii) The porous structure enables close contact with electrolyte thus allows more efficient utilization of active sites. The nanopores can provide rich reaction sites and short ion diffusion length as well as release the strain during the electrochemical reactions and be good at releasing of gas during the OER catalysis. iii) Pyridinic N and 19 ACS Paragon Plus Environment


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Pyrrolic N still exist in porous CoSe2 after etching and annealing, which can increase the active sites. iv) The direct integration of porous CoSe2 on carbon cloth not only enables good mechanical adhesion and electrical connection but also avoids the use of extra binders.

CONCLUSIONS In summary, the porous CoSe2 nanosheet has been successfully prepared on a carbon cloth substrate by three steps: hydrothermally synthesis of Co-based MOF nanowall, etching with water/alcohol solution and selenization reaction. The prepared porous CoSe2 possesses higher electrical conductivity, porous nanostructure and larger specific surface area, which is good at improving the electrochemical performance. Our as-synthesized porous CoSe2 on carbon cloth achieved superior performance for both the OER and the supercapacitor. The results suggest that introducing Se atoms into electrode material could effectively alter the nanostructure, which can solve the problem of the lower electrical conductivity, thus dramatically enhance the catalytic activity and durability. This porous structure fabrication strategy for metal selenides may be generally applied to the materials for diversified applications such as electrocatalyst, supercapacitor and etc.

ASSOCIATED CONTENT 20 ACS Paragon Plus Environment

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Supporting Information Supporting document containing: The image of the synthesis procedure for porous CoSe2, XRD patterns of CoSe2 and MOF-Co, XPS spectra of porous CoSe2 and CoSe2, SEM images of Etched MOF-Co and higher multiplying SEM images porous CoSe2 and CoSe2, N2 adsorption– desorption isotherms of porous CoSe2 and CoSe2, the image of EIS for synthesized sample in lower-frequency region, CV curves of as-prepared sample, CV curves and Higher multiplying SEM image of porous CoSe2 after 24 h stability test, CV and GCD curves of porous CoSe2 for supercapacitor.

AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program (2015AA050601), the National Natural Science Foundation of China (11674252, 61376013, 91433203).



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Figure 1. Schematic illustration of the formation for porous CoSe2 and CoSe2 from MOF-Co nanowall.



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Figure 2. (a) The XRD pattern of the porous CoSe2. (b) The XPS survey spectrum, (c) Co 2p and (d) Se 3d spectra of porous CoSe2.


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Figure 3. SEM images of (a-c) MOF-Co nanowall, (d-f) CoSe2 and (g-i) porous CoSe2.



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Figure 4. (a) TEM image of MOF-Co; (b-c) TEM and HRTEM images of CoSe2; (d-f) TEM, HRTEM and corresponding SAED images of porous CoSe2.


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Figure 5. (a-b) LSV and corresponding Tafel plots curves of as-prepared sample in 1 M KOH; (c) EIS measurement of as-synthesized; (d) Plots show the extraction of the double-layer capacitances allows the estimation of the electrochemically active surface area; (e-f) Stability measurements at J = 10 mA cm-2 for as-prepared electrode and Visual illustration figure of porous CoSe2 in OER.



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Figure 6. (a-c) Comparison curves of LSV, EIS and Cdl for initial and tested porous CoSe2; (d) Corresponding image of porous CoSe2 after 24 h stability test.


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Figure 7. (a) A comparison of CV curves for porous CoSe2 and CoSe2 at scan rate of 5 mV s-1. (b) GCD curves of as-prepared electrode at current densities of 1 mA cm-2. (c) Specific capacitances of synthesized electrode materials at different current density (d) Cycling performance of synthesized electrode materials for 5000 cycles at a current density of 5 mA cm-2 and corresponding SEM image.



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TOC figure


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MetalâOrganic Framework Template Derived Porous CoSe2

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