MetalâOrganic Framework Template Derived Porous CoSe2

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 35927-35935

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* Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, School of Physics and Technology, Wuhan University, Wuhan 430072, PR China

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S Supporting Information *

ABSTRACT: Porous CoSe2 on carbon cloth is prepared from a cobalt-based metal organic framework template with etching and selenization reaction, which has both a 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, it 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 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

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INTRODUCTION With the development of modern society, the energy crisis and environmental problems are those we must face now. Developing highly effective electrocatalyst and supercapacitor are important method to solve these issues. However, transitional metal oxides, metal hydroxides, and carbon material difficultly satisfy 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 proved to possess 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 attention in recent years with characteristics of high surface areas, high tunable porosities, and rich reaction sites.5−8 They have been proved to have great promise applications for catalyst,9,10 gas storage,11,12 sensing,13,14 and lithium-ion batteries.15,16 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 On the basis of these characteristics of MOF, it can be served as template for synthesis electrode material of both electrocatalyst and supercapacitor. The strategy is to © 2017 American Chemical Society

provide rich reaction sites and short ion diffusion length as well as to release the strain during the electrochemical reactions.21−26 Recently, metal selenides reported in the literature 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 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 three steps: MOF-Co nanowall arrays on CC, etching reaction process in deionized water/ethanol solution, followed by a thermal treatment and selenation reaction. The synthesized Received: August 17, 2017 Accepted: September 28, 2017 Published: September 28, 2017 35927

DOI: 10.1021/acsami.7b12403 ACS Appl. Mater. Interfaces 2017, 9, 35927−35935

Research Article

ACS Applied Materials & Interfaces

A SCE served as the reference electrode and the counter electrode is a Pt foil. 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).

porous CoSe2 nanosheet possess both larger specific surface area and higher electrical conductivity. Furthermore, the porous CoSe2 nanosheet was grown in conductive carbon cloth (CC) without any binder and additives, so 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, it 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 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 activity and great electrical conductivity.

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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

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 CC 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, and 0.6 g of 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 held 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 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. OER Test. All the electrochemical tests were carried out in a three-electrode testing system (CHI 660E electrochemical workstation, Chenhua, Shanghai). The asprepared 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. KOH solution (1 M, 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 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). Supercapacitor Test. The tests were carried out in a three-electrode testing system in 3 M KOH. The as-prepared sample and CC are used as the working electrode. The working electrode area is 1 cm × 1 cm.

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

for details). First, the MOF-Co nanowall was first obtained by a facile hydrothermal reaction.33,34 MOF-Co consists of many organic bonds. Among these organic bonds, the C−C bond can maintain the essential structure. The MOF consists of N as a kind of natural active site, and it 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 synthesized using the solution etching treatment and selenization in Se atmosphere. It can be directly used as an electrode for both electrocatalyst and supercapacitor. X-ray diffraction (XRD) experiment was first carried out to study the as-prepared materials. The XRD pattern of the crystalline MOF-Co precursor is well-matched with the literature reports.8,33 After MOF-Co nanowall was etched and annealed in Se atmosphere, the porous CoSe2 was successfully synthesized. As shown in Figure 2a, the diffraction angles of porous CoSe2 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 (Powder Diffraction File (PDF) no. 09−0234, Joint Committee on Powder Diffraction Standards (JCPDS), [year]), respectively, and the crystal structure of porous CoSe2 is cubic. The result indicates the Se atoms have been successfully introduced into etched MOFCo. 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. Especially, in the XRD pattern of CoSe2, there are several weak peaks located at 15−20°, which are attributed to the MOF, and some main peaks located at 2θ = 30.8, 34.7, 36.3, and 47.8° corresponding to the (110), (111), (012), and (121) crystal planes of CoSe2 (PDF no. 10−0408, JCPDS, [year]), respectively. 35928


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

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

and C−N (287.5 eV).38,41 The XPS results of O 1s for CoSe2 can be resolved into two components located at 530.4 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, assigned to chemisorbed oxygen and hydroxyls.8,42 The results of C 1s and O 1s for CoSe2 indicate that the organic ligand still exists in the sample, which will have a negative influence on that of the electrochemical performance. The N 1s spectrum of CoSe2 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 efficient active sites for electrochemical.45 Furthermore, MOF-Co was etched and annealed in Se atmosphere; the synthesized sample was further measured by

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. The fitting peaks at binding energies of Co 2p for CoSe2 at 780.3 and 796.4 eV are attributed to Co2+ and corresponded shakeup satellite, while the other two fitting peaks at 778.1 and 793.1 eV are ascribed to Co0.35−38 XPS reveals two distinct binding energies of 54.4 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 57.0−62.0 eV, attributed to SeOx.39,40 The XPS spectrum of C 1s contain three main peaks, the C−C bond (284.4 eV), C−N and C−O (285.8 eV), and C−O−C 35929


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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.

CoSe2 and CoSe2 were recorded to measure the specific area. The porous CoSe2 displays a specific Brunauer−Emmett− Teller (BET) surface area (34.1 m2 g−1) higher 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 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 does not have any porous morphology. Figure 4b shows 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 the corresponding insert image, a set of lattice fringes with interplanar spacing of 0.289 nm can be observed, which corresponds well to the (110) direction of CoSe2 (PDF no. 10−0408, JCPDS, [year]), and the result is consistent with the XRD results. The structure of 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 the structure of final sample being different from that of CoSe2, the smooth face of connecting many nanoparticles has disappeared, and the porous structure has formed, which is good at improving specific surface area of the materials. The detailed 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 no. 09−0234, JCPDS, [year]). 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. 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 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 electrodes, respectively. In Figure 5a, the linear sweep voltammogram

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,d, the peak positions of Co 2p and Se 3d for porous CoSe2 are similar to those of CoSe2. Observably, the XPS spectrum of C 1s for porous CoSe2 is different from that of CoSe2; there is only peak at 284.6 eV (C−C). In addition, the O 1s XPS spectrum exhibits two peaks at 530.9 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, 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. The above results of XRD and XPS for final samples proved that of CoSe2 has been synthesized. The morphologies of the as-prepared samples are explored with scanning electron microscopy (SEM). Figure 3a shows that the MOF-Co 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. After direct selenization of MOF-Co, the CoSe2 nanowalls (Figure 3d) are also uniformly arranged on CC. In Figure 3e, the basic structure of these nanowalls has been not changed after selenization, and 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 show that the diameter of crystal particle is 10−70 nm. A porous nanowall structure has been formed. Taking into consideration that the structure of the MOF can be rebuilt, the very weak acid solution (deionized water/ethanol) was used to etch the MOFCo. The solution of deionized water/ethanol is faintly acid, which could remove a part of the organic bond, decrease thickness of MOF-Co, and ensure full selenization of precursor. After selenization etched MOF-Co (Figure 3f−i), 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 reduces the ion diffusion path for this fast electrochemical kinetic process. Moreover, the N2 adsorption isotherms of porous 35930


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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.

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 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) 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 bulk resistance smaller than that of CoSe2, which can be attributed to the following two aspects: (i) In XRD, the porous CoSe2 shows better crystallinity than that of CoSe2, and thus less recombination and better carrier transfer. (ii) The porous structure enables close contact with electrolyte and thus allows more efficient utilization of active sites. The nanopores can provide rich reaction sites and short ion diffusion length as well as releasing the strain during the electrochemical reactions and be good at release of gas during OER catalysis. (iii) In the XRD pattern of dense CoSe2, there are several weak peaks located at 15−20°, which are attributed to the MOF, which is not good at ion diffusion. The diameter of the semicircle indicating charge transfer resistance is significantly smaller in the porous CoSe2 electrode (0.71 Ω) when compared to that of the CoSe2 (1.21 Ω), MOF-Co (0.93 Ω), and CC electrode (6.19 Ω). In the lower-frequency region, the slope of the porous CoSe2 electrode is larger than that of other electrodes, indicating its

(LSV) is first used to measure the catalytic activity of asprepared 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 is lower than that of CoSe2 (1.52 V vs RHE) as well as that of MOF-Co (1.70 V vs RHE). The OER performance of CC electrode is also measured, and one can see that it has little contribution to the catalytic process. Furthermore, the porous CoSe2 shows higher current density at the same potential as that of the other as-prepared electrode, indicating a higher catalyst activity of the porous selenization nanosheet. The anodic current densities of 10 mA cm−2 is used to 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 that needed by MOF-Co is 1.61 V, both of which are larger than that needed by porous CoSe2 electrode. 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 and Co3O4/rGO (1.57 V vs RHE).50 Moreover, Tafel plot calculated from LSV data is a much more straightforward way to estimate the OER kinetics of the catalysts. The Tafel slope can be obtained by linear fitting base on the Tafel equation as follows: η = b log J + a, where J is the current density and b is the Tafel slope. The Tafel slope of 35931


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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.

Figure 7. (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.

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 superposed compared with the initial curves after 24 h stability test. In addition, the Cdl of the porous CoSe2 electrode (Figure 6c) is measured after 24 h durability test with a value of ∼5.78 mF cm−2, which is little consistent with the initial electrode. To

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 asprepared sample, the double-layer capacitances (Cdl) were measured to illustrate the electrochemical active surface areas. 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 used as 35932


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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 releasing the strain during the electrochemical reactions and be good at releasing of gas during the OER catalysis. (iii) Pyridinic N and 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.

further demonstrate the stability of the porous CoSe2 electrode, the SEM images of tested porous CoSe2 is shown in Figure 6d. One can see that the nanosheets are still uniformly arranged on CC and that triangular shape of the porous nanosheets can still be retained. The above studies demonstrate excellent stability of the porous CoSe2 in OER. Supercapacitive Property. Considering the excellent performance of porous CoSe 2 in OER, the potential applications for energy storage are worth exploring. 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 a scan rate of 5 mV s−1, 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), while the second pair of redox peaks belong to the typical conversion between Co3+ and Co4+ (O2 and R2). On the basis of the CoSe2 electrode being activated through the following reaction before the CV measurement and two pairs of redox peaks of CV curves, the electrode reaction equation may be as follows:

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CONCLUSIONS The porous CoSe2 nanosheet has been successfully prepared on a carbon cloth substrate by three steps: hydrothermal 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 enhancing 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 others.

CoSe2 + H 2O + (1 + x)/2O2 ↔ Co(OH)2 + SeOx

Co(OH)2 + OH− ↔ COOH + H 2O + e− COOH + OH− ↔ CoO2 + H 2O + e−

It is apparent that the enclosed area of the porous CoSe2 electrode is much larger than that of 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 capacitance of ∼713.2 F g−1 under current densities of 1 mA cm−2, 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 materials, 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. The specific capacitance of porous CoSe2 electrode of about 535.8 F g−1 could be achieved at a higher current density of 20 mA cm−2. 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. The CV curves of porous CoSe2 electrode exhibit two symmetrical pairs of redox peaks. With increasing scan rate, 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, we can see that the curves exhibits better symmetry at different current density, indicating the porous CoSe2 electrode possess good 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

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b12403. 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, image of EIS for synthesized sample in lowerfrequency region, CV curves of as-prepared sample and higher multiplying SEM image of porous CoSe2 after 24 h stability test, CV and GCD curves of porous CoSe2 for supercapacitor (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guojia Fang: 0000-0002-3880-9943 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National High Technology Research and Development Program (2015AA050601), the 35933


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National Natural Science Foundation of China (11674252, 61376013, and 91433203).

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