MoSe2 Embedded CNT-Reduced Graphene Oxide Composite

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MoSe2 embedded CNT-reduced graphene oxide (rGO) composite microsphere with superior sodium ion storage and electrocatalytic hydrogen evolution performances Gi Dae Park, Jung-Hyun Kim, Seung-Keun Park, and Yun Chan Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00147 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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ACS Applied Materials & Interfaces

MoSe2 embedded CNT-Reduced Graphene Oxide (rGO) Composite Microsphere with Superior Sodium Ion Storage and Electrocatalytic Hydrogen Evolution Performances

Gi Dae Park, Jung Hyun Kim, Seung-Keun Park, and Yun Chan Kang*

Address: Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea. E-mail: [email protected]

*Corresponding author. E-mail : [email protected] (Yun Chan Kang, Fax: (+82) 2-9283584)

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ABSTRACT Highly porous MoSe2-reduced graphene oxide-carbon nanotube (MoSe2-rGO-CNT) powders were prepared by a spray pyrolysis process. The synergistic effect of CNTs and rGO resulted in powders containing ultrafine MoSe2 nanocrystals with a minimal degree of stacking. The initial discharge capacities of MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders for sodium ion storage were 501.6, 459.7, 460.2, and 364.0 mA h g-1, respectively, at 1.0 A g-1. The MoSe2-rGO-CNT composite powders had superior cycling and rate performances compared with the MoSe2-CNT, MoSe2-rGO composite and bare MoSe2 powders. The electrocatalytic activity of MoSe2-rGO-CNT in the hydrogen evolution reaction (HER) was also compared with that of MoSe2-CNT, MoSe2-rGO, and bare MoSe2. MoSe2-rGO-CNT composite powders exhibited an overpotential of 0.24 V at a current density of 10 mA cm‒2, which was less than that of MoSe2-CNT (0.26 V at 10 mA cm‒2), MoSe2-rGO (0.32 V at 10 mA cm‒2), and bare MoSe2 (0.33 V at 10 mA cm‒2). Tafel slopes for the MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders were 53, 76, 86, and 115 mV dec-1, respectively. Because a large electrochemical surface area and ultrafine MoSe2 nanocrystals, the MoSe2-rGO-CNT composite possesses more active sites than the MoSe2-CNT, MoSe2-rGO composite, and bare MoSe2 powders with extensive stacking and large crystalline size, which provide greater catalytic HER activity.

Keywords: sodium ion batteries; hydrogen evolution reaction; molybdenum diselenide; carbon nanotube; reduced graphene oxide

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INTRODUCTION Nanostructured molybdenum diselenide (MoSe2), whose crystal structure consists of covalently bonded Se–Mo–Se layers held together by graphite-like van der Waals forces, has attracted much attention in energy conversion as an electrocatalyst for the HER and oxygen reduction reaction (ORR) and an electrode material in lithium and sodium ion batteries for energy storage.1-15 Monolayer or few multilayer MoSe2 nanomaterials with abundant active sites and good electrical conductivity are effective electrocatalysts and electrode materials.6-8 However, an efficient synthetic procedure for large scale production of these materials has not been achieved. Nanostructured carbon materials such as graphene, amorphous carbon, and carbon nanotubes (CNTs) have been used to improve the characteristics of MoSe2 in energy conversion and storage by minimizing the stacking of MoSe2 layers.9-12 Choi and Kang synthesized porous three-dimensional (3D) CNT microspheres embedded with MoSe2 nanocrystals.13 The synergistic effect of ultrafine MoSe2 nanocrystals and CNT balls produced excellent sodium ion storage. Huang et al. synthesized a hierarchical nanostructure of MoSe2 nanosheets grown perpendicularly on carbon nanotubes by a one-step solvothermal reaction.14 Outstanding HER activity with a small Tafel slope of 58 mV dec-1 and excellent multicycle stability was achieved. Liu et al. produced high quality MoSe2/graphene hybrid nanostructures by synthesizing uniform 3D MoSe2 layers on graphene nanosheets.15 The MoSe2/graphene composite exhibited excellent electrocatalytic activity in the HER as evidenced by an overpotential of 125 mV and a Tafel slope of 67 mV dec-1. Nanostructured composite materials containing graphene and CNTs have been successfully applied in energy conversion and storage.16-23 The unique CoS2/reduced graphene oxide (rGO)-CNT nanostructure produced an HER activity among the greatest for non-noble metal electrocatalysts.20 Mo2C/CNT-graphene showed excellent HER activity with 3



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an overpotential of only 62 mV, a Tafel slope of 58 mV dec-1, and good stability in acid media.21 The CNT-graphene support is crucial in enhancing the activity of molybdenum compounds by preventing the aggregation of nanocrystals. Wang et al. synthesized CNTgraphene-Si composites with excellent performances in lithium ion batteries.22 Fang et al. constructed a Ge-graphene-CNT electrode consisting of germanium nanoparticles anchored on rGO intertwined with carbon nanotubes.23 The graphene sheets improved electrical conductivity and buffered against severe volume changes. The mechanical binding of CNT and Ge–rGO stabilized the conductive network of the Ge nanoparticles. The characteristics of graphene-CNT composite materials decorated with active materials such as a metal selenide are greatly affected by the preparation process. Spherical powders of finely sized composite materials can be usefully applied in commercial processes. However, an effective procedure has not been developed to prepare composite graphene-CNT powders containing active materials. Spray pyrolysis is a powerful and efficient method for synthesizing composites of metal compounds and carbon related materials such as rGO, CNTs, and amorphous carbon.24-27 The synthesis of graphene-CNT composite by spray pyrolysis has not been reported. We report here the successful preparation of highly porous MoSe2-rGO-CNT composite by spray pyrolysis. The stacking of MoSe2 layers and rGO is minimized by using CNTs in the spray solution. The characteristics of MoSe2-rGO-CNT as an anode in sodium ion batteries (SIBs) and electrocatalyst for HER are compared with those of MoSe2-rGO and bare MoSe2 powders prepared by the same procedure.

RESULTS AND DISCUSSION

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Scheme 1. Formation mechanism of highly porous MoSe2-rGO-CNT composite by spray pyrolysis process and subsequent one-step post-treatment.

The mechanism of forming highly porous MoSe2-rGO-CNT composite powders by spray pyrolysis is described in Scheme 1. A droplet (Scheme 1-1) containing ammonium molybdate, GO nanosheets, and CNTs is formed in the ultrasonic nebulizer. Drying of droplet in the front of the 500 °C reactor produces a porous composite powder (Scheme 1-2) of ammonium molybdate, GO nanosheets, and CNTs. The high-aspect-ratio CNTs form a porous, spherical backbone due to their low mobility during drying. Concurrently, deposition of GO nanosheets over the CNT backbone minimizes stacking of the GO nanosheets. The CNTs and GO nanosheets also minimize segregation of ammonium molybdate during the drying stage. Decomposition of ammonium molybdate to molybdenum oxide and thermal reduction of GO to rGO occur in the rear of the reactor to form a MoOx-rGO-CNT composite 5



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powder (Scheme 1-3). One-step post-treatment of the spray pyrolysis product at 300 °C under H2Se gas produces the porous MoSe2-rGO-CNT composite powder containing ultrafine MoSe2 nanocrystals (Scheme 1-4). Selenization of MoO3 to MoSe2 occurs during the posttreatment. The morphologies and XRD patterns of the MoOx-rGO-CNT, MoOx-CNT, MoOx-rGO, and MoO3 powders are shown in Figures S2 and S3, respectively. The SEM images in Figure S2 show that the MoOx-rGO-CNT and MoOx-rGO powders have a crumpled structure. However, the CNT backbone increases the sphericity of the composite powder. Bare MoO3 powders prepared from the ammonium molybdate solution have a spherical shape and smooth surface. The XRD patterns of the MoOx-rGO-CNT , MoSe2-CNT and MoOx-rGO composite did not show crystalline peaks of Mo oxides (Figure S3). The ultrafine MoOx nanocrystlas were formed due to CNTs and rGOs. However, the XRD pattern of the bare sample without CNTs and rGOs had sharp peaks of MoO3 phase without impurity phase.

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Figure 1. Characteristics of porous MoSe2-rGO-CNT composite powders: (a) and (b) SEM images, (c) and (d) TEM images, (e) HR-TEM image, (f) SAED pattern, and (g) elemental mapping images. 7



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Figure 2. XRD patterns of MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders.

The SEM and TEM images in Figures 1a-d illustrate their porous structure. The CNTs forming the porous backbone are indicated by arrows in Figure 1d. The HR TEM image in Figure 1e shows lattice fringes separated by 0.67 nm, which correspond to the (002) crystal plane of the MoSe2 phase and indicate the high loading of ultrafine MoSe2 nanocrystals in the MoSe2-rGO-CNT composite. The ultrafine MoOx nanocrystals produced ultrafine MoSe2 nanocrystals less than 10 nm in size with few stacked layers as shown in Figure S4. The SAED and XRD patterns in Figure 1f and Figure 2, respectively, confirm the absence of impurities in the MoSe2-rGO-CNT composite powders. The elemental mapping images in Figure 1g show the MoSe2 nanocrystals to be uniformly distributed over the porous rGOCNT composite. 8


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Figure 3. XPS spectra of MoSe2-rGO-CNT composite powders: (a) wide-scan XPS spectrum, (b) Mo 3d, (c) Se 3d, and (d) C 1s.

The wide-scan XPS spectrum in Figure 3a reveals clear XPS peaks for Mo, Se, C, and O. The Mo 3d XPS spectrum displays peaks at approximately 228.5 and 231.6 eV, which are attributed to the Mo 3d5/2 and Mo 3d3/2 energies of MoSe2, respectively.28,29 The peak at 227.2 eV is assigned to Mo‒C bonds in the MoSe2-rGO-CNT composite, which indicates some formation of MoCx.30,31 The Se 3d spectrum in Figure 3c shows peaks at approximately 54.0 and 54.9 eV, which are assigned to the Se 3d5/2 and Se 3d3/2 binding energies of MoSe2, respectively.32 The intermediate fitting lines are associated with the Se oxidation state.33,34 The C 1s XPS spectrum in Figure 3d contains peaks corresponding to C=C (sp2), C–C (sp3), 9



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C–O, and O–C=O functionalities at 284.3, 285.3, 286.3, and 288.7 eV, respectively.35,36 The C=C (sp2) and C–C (sp1) peaks are strong, whereas the C–O and O–C=O peaks are weak, which indicates that GO has been thermally reduced to rGO during spray pyrolysis process and selenization in a reducing atmosphere.35,36

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Figure 4. Characteristics of MoSe2-rGO composite powders: (a) and (b) SEM images, (c) and (d) TEM images, (e) HR-TEM image, (f) SAED pattern, and (g) elemental mapping images. 11



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The morphologies of the MoSe2–rGO powders are shown in Figure 4. The SEM and TEM images confirm that the overall morphology of the MoOx–rGO powders is maintained after selenization. The TEM images in Figures 4c and 4d reveal a more filled-in structure than that of the MoSe2-rGO-CNT composites in Figures 1c and 1d. The uniformity of MoSe2 nanocrystals in MoSe2-rGO-CNT is greater than in MoSe2-rGO as confirmed by the high resolution TEM images in Figures 1e and 4e. The SAED and XRD patterns in Figures 4f and Figures 2, respectively, confirm the absence of impurities in the MoSe2-rGO powders. The elemental mapping images in Figure 4g show that MoSe2 nanocrystals are distributed over the entire filled-in structure of the spherical rGO matrix. The bare MoSe2 powders in Figure S5 are spherical in shape and non-aggregating. The compressed stacking of MoSe2 layers results in bare MoSe2 powders with a dense structure. The XRD patterns in Figure 2 confirm that samples formed after selenization adopt the crystal structure of a pure MoSe2 phase. The peak of the (002) phase is consistent with the stacking of MoSe2 layers observed by XRD.10,37 The peak intensity of the (002) phase decreased in the MoSe2-rGO composite and was negligible in the MoSe2-rGO-CNT composite. The synergistic effect of CNTs and rGO resulted in MoSe2-rGO-CNT composites containing ultrafine MoSe2 nanocrystals with minimal stacking. The MoSe2-CNT composite powders had similar morphologies to those of the MoSe2-rGO-CNT composite powders (Figures 1 and S6). The TG curves of MoSe2-rGO-CNT, MoSe2-rGO, and MoSe2-CNT are shown in Figure S7. The metal selenide components transform into MoO3 during TG analysis in air. The weight losses observed by TG between 300 and 500 oC are attributed to oxidation of MoSe2 to MoO3 and to combustion of the CNTs and rGO. The porous structure and low degree of stacking in MoSe2 nanocrystals and rGO sheets resulted in rapid weight loss during TG analysis of MoSe2-CNT-rGO as shown in Figure S7. The slight weight increase at around 300 oC in the 12


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TG curve of the MoSe2-CNT is attributed to the formation of molybdenum selenite by partial oxidation of MoSe2. The weight percent MoSe2 in MoSe2-CNT-rGO, MoSe2-rGO, and MoSe2-CNT determined by thermogravimetry was 70.6, 67.5, and 85.0 %, respectively. In this study, rGO and CNT contents of 29.4% were estimated from the TG analysis of the MoSe2-rGO-CNT composite powders. To confirm the accurate amount of each rGO and CNT in the MoSe2-rGO-CNT composite powders, the MoSe2-CNT composite powders without rGO was newly prepared under identical preparation conditions. From the TG analysis of the MoSe2-CNT composite powders without rGO, the CNT content in the sample was about 15.0 %. Therefore, rGO and CNT contents in the MoSe2-rGO-CNT composite powders estimated from the TG analysis were 16.9 and 12.5%, respectively. The N2 gas adsorption and desorption isotherms are presented in Figure S8. The BET surface areas of the MoSe2-rGO-CNT, MoSe2-rGO, and bare MoSe2 powders were 7.4, 8.8, and 7.9 m2 g‒1, respectively.

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Figure 5. Electrochemical properties of MoSe2-rGO-CNT, MoSe2-rGO, MoSe2-CNT, and bare MoSe2 powders: (a) initial charge/discharge curves at 1.0 A g-1, (b) CV curves, (c) cycling performances, (d) rate performances, and (e) long-term cycling performance of MoSe2-rGO-CNT.

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The sodium ion storage performances of the MoSe2-rGO-CNT composite were compared with those of the MoSe2-rGO, MoSe2-CNT, and bare MoSe2 powders. MoSe2-rGO-CNT, MoSe2-rGO, and MoSe2-CNT display overlapping initial discharge curves with plateaus at about 1.2 and 0.6 V (Figure 5a). The 1.2 and 0.6 V plateaus are attributed, respectively, to irreversible reaction between the sodium ions and the surface functional groups of rGO-CNT and to the decomposition of MoSe2 to metallic Mo and hexagonal Na2Se upon further Na+ insertion.38-42 The plateau at 1.2 V was not observed in the initial discharge curve of bare MoSe2 powders. The initial discharge capacities of the MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and MoSe2 powders were 501.6, 459.7, 460.2, and 364.0 mA h g-1, respectively. The dense structure of bare MoSe2 powders resulted in their low initial discharge capacity. CV curves of the four samples at a scan rate of 0.1 mV s-1 are shown in Figures 5b and S9. MoSe2-rGO-CNT, MoSe2-CNT, and MoSe2-rGO exhibit reduction peaks near 1.25 and 0.5 V in the initial cathodic sweep. The reduction peak at around 1.25 V is not observed in the initial cathodic sweep of the bare MoSe2 in Figure S9d. The CV curves of rGO-CNT obtained by selectively etching the MoOx by diluted hydrogen peroxide in the MoOx-rGO-CNT composite are also shown in Figure S9c. The reduction peak observed at around 1.25 V in the initial cathodic scan of the rGO-CNT disappears in the subsequent cycles. Therefore, the reduction peak observed at around 1.25 V in the initial cathodic scan of the MoSe2-rGO-CNT, MoSe2-rGO, and MoSe2-CNT is attributed to irreversible reaction between the sodium ions and the surface functional groups of rGO-CNT. The CV peaks overlap substantially after the first cathodic and anodic scan, which suggests the occurrence of a highly reversible Na+ insertion/desertion process in MoSe2-rGO-CNT and MoSe2-rGO. MoSe2-rGO-CNT and MoSe2-CNT display superior cycling performance compared with MoSe2-rGO and bare MoSe2 (Figures 5c). The discharge capacities of MoSe2-rGO-CNT and 15



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MoSe2-CNT increased during cycles due to the formation of SEI layer. The discharge capacity of MoSe2-rGO decreased steadily throughout 200 cycles. On the other hand, the discharge capacity of bare MoSe2 increased steadily during the first 80 cycles and then sharply decreased during the subsequent 120 cycles. Increased contact with the liquid electrolyte arising from partial structural degradation increased the capacity of bare MoSe2 powders during the first 80 cycles. However, severe structural decomposition upon further cycling resulted in destruction of the electrode. The discharge capacities of MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders at the 200th cycle were 393, 355, 312, and 92 mA h g-1, and their retained capacities calculated relative to the second cycle were 111, 102, 92 and 31 %, respectively. As shown in Figure 5d, MoSe2-rGO-CNT and MoSe2-CNT exhibited superior rate performance compared with MoSe2-rGO and bare MoSe2 powders. MoSe2-rGO-CNT produced final discharge capacities of 411, 371, 341, 314, 275, 242, 225, 208, 190, and 173 mA h g-1 at current densities of 0.2, 0.5, 1, 2, 5, 10, 15, 20, 25, and 30 A g-1, respectively. In addition, MoSe2-rGO-CNT recovered a discharge capacity of 453 mA h g-1, when the current density was restored to 0.2 A g-1 after cycling at 30 A g-1. In comparison, MoSe2-rGO exhibited final discharge capacities of 397, 355, 323, 293, 248, 202, 171, 144, 119, and 96 mA h g-1 at current densities of 0.2, 0.5, 1, 2, 5, 10, 15, 20, 25, and 30 A g-1, respectively. The long-term cycling performance of MoSe2-rGO-CNT at 1 A g-1 is shown in Figure 5e. The discharge capacities at the 2nd and 400th cycle were 352 and 335 mA h g-1, respectively, and the capacity retention calculated from the second cycle was 95 %.

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Figure 6. Nyquist impedance plots of MoSe2-rGO-CNT, MoSe2-rGO, and bare MoSe2 powders for sodium-ion storage: (a) before cycling, (b) after 1 cycle, (c) after 100 cycle, and (d) after 200 cycle.

The superior sodium ion storage performance of MoSe2-rGO-CNT composite powders is supported by electrochemical impedance spectroscopy (EIS) analysis, as shown in Figure 6. EIS measurements were performed before and after 1, 100, and 200 cycles. The Nyquist plots contain compressed semicircles in the medium frequency range, which represent the charge transfer resistance (Rct) of the electrode.43-45 The Rct of the MoSe2-rGO-CNT composite is

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550 Ω, which is greater than that of the MoSe2-rGO composite (429 Ω) and bare MoSe2 (430 Ω before cycling), as shown in Figure 6a. After the first cycle, the Rct of bare MoSe2 is less than that of MoSe2-rGO-CNT and MoSe2-rGO until the 100th cycle. However, the Rct of bare MoSe2 exceeds that of MoSe2-rGO-CNT and MoSe2-rGO after the 200th cycle. The significant increase in Rct of bare MoSe2 is due to structural decomposition caused by repeated sodium ion insertion and removal after 100 cycles.46,47 The Rct of the MoSe2-rGOCNT composite was 65.5, 78.5, and 59.5 Ω after 1, 100, and 200 cycles, respectively. The formation of ultrafine MoSe2 nanocrystals during the first Na+ insertion and extraction processes decreased the Rct from 550 to 65.5 Ω. The low Rct of the MoSe2-rGO-CNT composite was maintained even after 200 cycles. The EIS results attest to the structural stability of MoSe2-rGO-CNT during cycling. The morphologies of the MoSe2-rGO-CNT and MoSe2-rGO composite were well maintained even after 200 cycles (Figures S10a-d). However, the spherical shape of the bare MoSe2 powders was destroyed and a hierarchical structure was formed during cycling, as shown in Figures S10e and S10f. The structural stability during cycling resulted in superior cycling performances for the MoSe2-rGO-CNT and MoSe2-rGO composite for sodium-ion storage.

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Figure 7. Electrocatalytic performances of MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders: (a) polarization curves, (b) Tafel plots, (c) Nyquist plots, and (d) stability of the MoSe2-rGO-CNT composite.

We also compared the electrocatalytic HER activity of the MoSe2-rGO-CNT, MoSe2-CNT, and MoSe2-rGO composite powders with that of bare MoSe2 powders. Electrochemical measurements were conducted on these samples and on commercial Pt/C in 0.5 M H2SO4 using a three electrode cell with iR compensation. Figure 7a shows extremely high HER activity and near zero overpotential for the Pt/C catalyst. The as-prepared MoSe2-rGO-CNT powders contain many active edges for H2 evolution and exhibit greater catalytic activity than the MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders. The high degree of stacking and large size of MoSe2 nanocrystals results in poor catalytic activity for MoSe2-CNT, MoSe219



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rGO, and bare MoSe2. The MoSe2-rGO-CNT composite powders exhibited an overpotential of only 0.24 V at a current density of 10 mA cm‒2, which was less than that of MoSe2-CNT

(0.26 V at 10 mA cm‒2), MoSe2-rGO (0.32 V at 10 mA cm‒2), and bare MoSe2 (0.33 V at 10 mA cm‒2). These results indicate excellent HER catalytic activity for MoSe2-rGO-CNT. To further study the HER activity of the MoSe2-based samples, the Tafel slope, which is related to the rate limiting step of the HER, was determined from linear segments of the current density versus overpotential response fitted to the Tafel equation.48,49 In Figure 7b, the Tafel slope of the commercial Pt/C catalyst is ~30 mV dec-1, which is close to reported values.48,49 Tafel slopes for MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, bare MoSe2 powders are about 53, 76, 86, and 115 mV dec-1, respectively. The smaller Tafel slope of MoSe2-rGO-CNT indicates a greater increase in HER rate with increasing overpotential, which is advantageous in practical applications. Based on a Tafel slope of 53 mV dec-1, the HER of MoSe2-rGO-CNT may occur by either the Volmer-Tafel or Volmer-Heyrovsky mechanism with the Volmer reaction as the rate determining step 50,51 The MoSe2-rGO-CNT, MoSe2-CNT, MoSe2-rGO, and bare MoSe2 powders were investigated by EIS to determine the electrochemical behavior of the electrodes during hydrogen evolution. The semicircles at high frequencies in Figure 7c reflect the Rct at the electrode/electrolyte interface. Values of Rct for the MoSe2-rGO-CNT, MoSe2-CNT, MoSe2rGO, and bare MoSe2 powders were 42.7, 46.9, 64.4, and 159.1 Ω, respectively. The MoSe2rGO-CNT composite with high electrical conductivity had the lowest Rct. This small Rct value, which correlates with fast HER kinetics, indicates that the MoSe2-rGO-CNT composite powders should be an excellent HER catalyst.52,53 The long-term stability of MoSe2-rGOCNT was investigated by performing 1000 cycles from 0.0 to ‒0.3 V (vs. RHE) at 0.1 V s‒1 (Figure 7d). Despite a slight decay in current density, the MoSe2-rGO-CNT composite 20


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retained its low onset potential, which demonstrates the durability of the catalyst during long term cycling. The long-term stability of the MoSe2-rGO-CNT electrocatalyst is also evaluated by prolonged electrolysis at a static overpotential of 260 mV (Figure S12). The current density remains stable for 10000s with small degradation, suggesting the excellent stability of the MoSe2-rGO-CNT electrocatalyst in acidic environment.

Figure 8. Cyclic voltammetry curves of (a) MoSe2-rGO-CNT, (b) MoSe2-rGO, (c) bare MoSe2 in the region of 0.2-0.3 V vs. RHE with rates from 2 to 20 mV s−1, and (d) linear fitting of the capacitive current densities of MoSe2-rGO-CNT, MoSe2-rGO, and bare MoSe2 vs. scan rate.

To compare electrochemically active sites for three samples, electrochemical double-layer capacitance (EDLC) values were obtained from CV method. The CV shape of MoSe2-rGO21



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CNT seems to have rectangular shape compared to MoSe2-rGO and bare MoSe2. The rectangular CV implies facile electron and ion transport in the electrochemical reaction.54,55 As shown in Figure 8d, the MoSe2-rGO-CNT powders had larger EDLC value (7.9 mF cm-2) compared to the MoSe2-rGO (0.64 mF cm-2), and bare MoSe2 (0.20 mF cm-2). The porous structure consisting of GO nanosheets deposited CNT backbone increased the electrochemical surface area in composites. In other word, the CNT backbone in MoSe2-rGOCNT composite minimized the restacking of rGO sheets, resulting in improved electrochemical surface area. The presence of many active MoSe2 sites in MoSe2-rGO-CNT leads to greater catalytic HER activity than the MoSe2-rGO and bare MoSe2 powders. In addition, the porous structure of the MoSe2-rGO-CNT composite provides accessibility for electrolyte ions and pathways for escape of H2 gas.56-58 Finally, the rGO nanosheets and CNTs contribute to the high electrical conductivity of the material, which facilitates fast electron transfer by improving electrical contact between the active sites and the electrode. The synergistic effect of rGO sheets, CNTs, and MoSe2 nanocrystals and the unique porous structure of the material result in outstanding sodium ion storage and HER capabilities of the MoSe2-rGO-CNT composite powders. Electrocatalytic performances of the MoSe2 embedded CNT-reduced graphene oxide composite are compared with those of various nanostructured MoSe2 materials reported in the previous literatures (Table S1). As for electrocatalytic properties as hydrogen evolution materials, our sample shows the best Tafel slope value (53 mV/dec) compared with the previous reported MoSe2-based composites (Table S1) despite a little high onset potential. A small Tafel slope indicates a faster increase of hydrogen evolution reaction (HER) velocity as the increase of potential, catalysts with a small Tafel slope value are suitable to practical applications. The synergistic effect of CNTs and rGO resulted in powders containing 22


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ultrafine MoSe2 nanocrystals with a minimal degree of stacking and the MoSe2-rGO-CNT composite possesses numerous active sites, which provide greater electrocatalytic activity.

CONCLUSIONS A one-step post-treatment of MoOx-rGO-CNT powders obtained by spray pyrolysis produced porous MoSe2-rGO-CNT composite containing ultrafine MoSe2 nanocrystals. The high aspect ratio CNTs produced a porous, spherical backbone for the composite powders that minimized the stacking of rGO nanosheets. The CNTs and rGO nanosheets also minimized the growth of MoSe2 nanocrystals. When used as an anode material for sodium ion batteries and an electrocatalyst for the HER, the MoSe2-rGO-CNT composite powders exhibited superior properties compared with MoSe2-CNT, MoSe2-rGO and bare MoSe2 powders prepared by a similar synthetic procedure. The synergism of rGO sheets, CNTs, and MoSe2 nanocrystals and the unique porous structure of the material resulted in outstanding sodium ion storage and HER properties.

EXPERIMENTAL SECTION Sample preparation The procedure for preparation of MoSe2-rGO-CNT composite powders by spray pyrolysis used a solution containing ammonium molybdate (2.9 g L-1, Samchun), graphene oxide (GO) nanosheets (0.5 mg ml−1), and acid-treated multiwall CNTs (MWCNTs, 0.5 mg ml−1). The preparation procedures of GO nanosheets and oxidized MWCNTs are described in our previous reports.59,60 Schematic diagram of the spray pyrolysis system for MoOx-rGO-CNT, MoOx-CNT, MoOx-rGO, and bare MoO3 powders is shown in Figure S1.59,60 MoOx-rGOCNT powders were treated for 12 h at 300 °C in a 10% H2/Ar atmosphere to form MoSe223



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rGO-CNT composites.61 MoOx-CNT and vare MoOx powders powders were also prepared by spray pyrolysis. MoSe2-CNT, MoSe2-rGO composite, and bare MoSe2 powders were prepared by the above selenization procedure Characterization The powders were characterized by X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and thermal gravimetric analysis. The experimental procedures for electrochemcial test of the powders as anode materials for SIBs and electrocatalytic activities for HER are described in previous articles.61,62

■ ASSOCIATED CONTENT Supporting Information. Schematic diagram of the spray pyrolysis system, Morphologies and XRD patterns of of the precursor powders, HR-TEM images of the MoSe2-rGO-CNT, Morphologies of bare MoSe2 and MoSe2-CNT powders, TG curves, N2 adsorption and desorption isotherms and pore size distributions of the MoSe2‒rGO-CNT and MoSe2-rGO composite powders, CV curves of the MoSe2-rGO, MoSe2-CNT, bare MoSe2, tGO-CNT composite powders, Morphologies of MoSe2‒rGO-CNT, MoSe2-rGO and bare MoSe2 powders observed after 200 cycles by SIB measurements. Morphologies of MoSe2‒rGO-CNT and MoSe2-rGO composite powders observed after 1000 cycles by HER measurements, Time-dependent current density curve of MoSe2-rGO-CNT, Tables of electrocatalytic performances of various nanostructured MoSe2 materials reported in the previous literatures, Morphologies and sodium-ion storage performances of the rGO-CNT composite, Gravimetric and areal capacities of the MoSe2rGO-CNT composite at the different active materials mass loadings on the electrode, Areal capacity of various 2D-nanostructured materials in sodium ion batteries reported in the previous literature.s This material is available free of charge via the Internet at http://pubs.acs.org. 24


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AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (Yun Chan Kang, Fax: (+82) 2-928-3584)

ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2015R1A2A1A15056049). This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030091450).

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MoSe2 Embedded CNT-Reduced Graphene Oxide Composite

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