Confinement-enhanced Rapid Interlayer Diffusion within Graphene

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Functional Nanostructured Materials (including low-D carbon)

Confinement-enhanced Rapid Interlayer Diffusion within Graphene-supported Anisotropic ReSe Electrodes 2

Zhenjing Liu, Xuewu Ou, Minghao Zhuang, Jiadong Li, Md Delowar Hossain, Yao Ding, Hoilun Wong, Jiawen You, Yuting Cai, Irfan Haider Abidi, Abhishek Tyagi, Minhua Shao, Bin Yuan, and Zhengtang Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08157 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 4, 2019

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ACS Applied Materials & Interfaces

Confinement-enhanced Rapid Interlayer Diffusion within Graphene-supported Anisotropic ReSe2 Electrodes

Zhenjing Liu1§, Xuewu Ou1§, Minghao Zhuang1, Jiadong Li1, Md Delowar Hossain1, Yao Ding1, Hoilun Wong1, Jiawen You1, Yuting Cai1, Irfan Haider Abidi1, Abhishek Tyagi1, Minhua Shao1, Bin Yuan2* and Zhengtang Luo1*

1

Department of Chemical and Biological Engineering, William Mong Institute of Nano Science and

Technology and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, the Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 2

School of Materials Science and Engineering, South China University of Technology, Guangzhou,

Guangdong, China 510640

§ Z.L. and X.O. contributed to this work equally *E-mail: [email protected] (B. Y.) and [email protected] (Z.L.)

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Abstract To enhance interlayer lithium diffusion, we engineer electrodes consisting of epitaxial grown ReSe2 nanosheets by chemical vapor deposition (CVD), supported on three-dimensional (3D) graphene foam, taking advantage of its weak van der Waals coupling and anisotropic crystal structure. We further demonstrate its excellent performance as the anode for lithium ion battery (LIB) and catalyst for hydrogen evolution reaction (HER). Density functional theory (DFT) calculation reveals ReSe2 exhibits low energy barrier for lithium (Li) interlayer diffusion, because of negligible interlayer coupling and anisotropic structure with low symmetry that creates additional adsorption sites and leads to reduced diffusion barrier. Benefitting from these properties, 3D ReSe2/graphene foam electrode displays excellent cycling and rate performance with 99.6% capacity retention after 350 cycles and a capacity of 327 mAh g-1 at the current density of 1000 mA g-1. Additionally, it has exhibited a high activity for HER, in which an exchange current density of 277.8 μA cm-2 is obtained and only an overpotential of 106 mV is required to achieve a current density of -10 mA cm-2. Our work provides fundamental understanding into the interlayer diffusion of Li in TMDs materials, and new tool for TMDs-based catalyst design. Key words: Rhenium diselenides, anisotropic structure, DFT, lithium ion battery, hydrogen

evolution

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reaction

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Introduction Owning unique crystal and electronic structure, transition metal dichalcogenides (TMDs) have exhibited promising applications in energy area. In particular, they were explored as the anode for lithium ion battery (LIB) given their graphite-like layered structure, while graphite suffers from a low capacity.1 Such anodes also eliminate the huge volume change, commonly seen in alloyed anode such as Si and Sn.2-4 Recent research found that anodes made of TMDs such as MoS2 and MoSe2 exhibited impressive high capacity via intercalation and conversion mechanism in sequential steps.5-6 Additionally, their large interlayer spacing, and the resulted weak interlayer interaction, allow lithium ion (Li+) to diffuse effectively, while maintain minor volume change.7 Among TMDs family, Re-based materials are distinct from others with one order of magnitude weaker van der Waals coupling force (ReS2, 18 meV) than others like MoS2 (460 meV) due to their Peierls distortion of 1T structure.8 Besides, the distorted basal plane of Re-based TMDs provide more adsorption sites and create more unique diffusion paths for Li.9-11 Excellent stability at large current density of ReS2based anodes for alkali ion battery has been reported, however, its theoretical origin remain elusive.10-11 On the other hand, TMDs are also considered as the potential catalyst for hydrogen evolution reaction (HER), being alternative for expensive Pt-base electrodes. The distinct activity of MoS2 is confirmed with a low Gibbs free energy of hydrogen adsorption.12 To maximize the active sites in TMDs, different strategies such as hybridization with carbon support,13 morphology engineering,14 heteroatom doping and strain engineering,15 and phase engineering16 have been investigated. However, limited success is achieved for typical TMDs materials with ordinary structures, as compared with Pt. Therefore, it is also imperative to engineer new TMDs materials and explore their catalytic activity towards HER. For example, ReS2 has been grown on gold foil, but unsatisfied activity was obtained due to the bulk nature of ReS2 in this method.17 Herein, we reported the epitaxial growth of ultrathin ReSe2 nanosheets on graphene foam using chemical vapor deposition (CVD). This three-dimensional (3D) 3



heterostructure enables us to explore the potential application of ReSe2. Notably, this 3D free-standing structure here leads to high performance for both LIB and HER application, where binder material, such as Nafion and poly(vinylidene fluoride) (PVDF), is no longer necessary. This reduces the interfacial resistance introduced by binders, which would otherwise deteriorate the electrochemical performance.18 Moreover, the use of graphene foam enhances the conductivity and controls the size and distribution of active nanosheets. Density functional theory (DFT) calculation has shown that, unlike typical layered anode material such as graphite,19-20 MoS2,21-22 TiS223 and GeS,24 the diffusion barrier for lithium (Li) in the interlayer spacing of ReSe2 is smaller than that atop monolayer, arising from the weak interlayer coupling and confined anisotropic crystal structure of ReSe2. These properties avoid the increasement of diffusion barrier, but provide additional adsorption sites. The high interlayer mobility of Li coming from the small interlayer diffusion barrier, along with the large interlayer spacing of ReSe2, lead to the stable cycling performance and high rate capability of ReSe2/graphene foam electrode. After 350 cycles, a capacity of 479 mAh g-1 was maintained with a retention of 99.6%. When current density was set up to 1000 mA g-1, the capacity was around 327 mAh g-1. Besides, we also find ReSe2/graphene foam showed high activity for HER, with an overpotential of 106 mV to achieve a current density of -10 mA cm-2 and a large exchange current density of 277.8 μA cm-2, broadening the possibility of using TMDs materials as the catalyst for HER.

Experimental Section Chemical. The nickel foam, hydrochloric acid (HCl) (37%), iron (Ⅲ) chloride hexahydrate (FeCl3 ∙ 6H2O), ammonium perrhenate (NH4ReO4), selenium (Se) powder were purchased from Sigma-Aldrich without any pretreatment. Synthesis of 3D Graphene Foam. 3D nickel foam was selected as the catalyst and template for graphene growth based on previous work.25 The obtained graphene/nickel foam was immersed into the FeCl3 solution to remove nickel substrate, followed by the 4


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washing process in dilute HCl solution and water respectively for several times. After freeze drying, we can get 3D porous graphene foam for the subsequent growth of ReSe2. Synthesis of ReSe2/graphene foam. Firstly, the selenium powder and ammonium perrhenate salt were loaded at the closed ends, respectively, with the 3D graphene foam as growth substrate loaded in the middle. Subsequently, the furnace was heated to 450 ℃ within 15 min then annealed for 60 min under a flow of gas mixture (Ar/H2=50/10sccm). Since the melting point of Se (220℃) is lower than the growth temperature, Se powder was put at the edge of the furnace. Synthesis of Bulk ReSe2 Particles. Selenium power and ammonium perrhenate salt were mixed together and put at the end of tube, which is put at the heating center. To ensure full selenization, the mole ratio of Se to Re in the precursors is larger than 2. Then the same CVD process as the synthesis of ReSe2/graphene foam was applied to obtain bulk ReSe2 particles. Materials Characterization. Back-scattering scanning electron microscopy (BS-SEM, JEOL 7100) was applied to study the morphology and structure. Notably, BS-SEM can provide the element distribution information as illustrating in Figure S1. Based on the SEM images, we used Image-J to obtain the size distribution of ReSe2 nanosheets growth on the graphene substrate (Figure S2). Transition electron microscopy (TEM, JEOL 2010F) was further used to investigate the morphology and crystal structure. Atomic force microscopy (AFM) was conducted to check the sample thickness. And X-ray photoelectron spectroscopy (XPS) was applied to check elemental information. Raman spectroscopy was obtained by Renishaw Raman RM3000 scope. Thermogravimetric analysis (TGA) is conducted in air from room temperature to 800 ℃ (heating rate: 5 ℃ min-1). Electrochemical Measurements for HER. The electrochemical test was conducted in an electrochemical workstation (CHI 760 E). We used standard three-electrode setup in 0.5 M H2SO4 with carbon bar and Ag/AgCl electrode as the counter and reference electrodes respectively while ReSe2/graphene foam is the working electrode. And the mass loading of graphene foam, bulk ReSe2 and ReSe2/graphene foam was designed to 5



be 0.83 mg cm-2. Linear sweep voltammetry (LSV) was applied with a potential sweep rate of 10 mV s-1. And cyclic voltammograms (CV) was acquired with different scan rates under the overpotential range of no faradaic reactions. Besides, electrochemical impedance spectroscopy (EIS) was measured from 100 K to 0.01 Hz. And the stability was tested between -0.2 to -0.5 V (vs. Ag/AgCl). Electrochemical Measurements for LIB. In the battery test, ReSe2/graphene foam can be applied as the working electrode directly without adding any binder material, and assembled into the coin cell (CR2025), in which the lithium metal worked as the anode with 1M LiPF6 (EC:EMC = 3:7 in volume) as the electrolyte. CV test was carried out in the electrochemical workstation CHI 760 E. And we conducted the galvanostatic charge and discharge measurement from 0.01 to 3.00 V (vs. Li/Li+) in Neware BTS4000. DFT Calculations. Vienna Ab Initio Simulation Package (VASP) is used for the simulation.26Here, we applied the generalized gradient approximation (GGA) exchange-correlation function of Perdew-Burke-Ernzerhof (PBE),27 and used projector augment wave (PAW) pseudopotential to describe the ion-electron interaction. For the diffusion process calculation, climbing-image nudged elastic band (CI-NEB) is used.28 For monolayer ReSe2, to determine the diffusion energy barrier of Li, 2*2 ReSe2 supercell was created as the adsorption substrate. Subsequently, we determined the stable adsorption site of Li atop ReSe2 (Figure S3), and calculated the binding energy, respectively. After that, two nearest stable adsorption sites were selected as the initial and final state. Ascribed to the anisotropic property of ReSe2, 3 different diffusion paths of Li were chose as shown in Figure S4. The binding energy value is calculated by previous method.29 For bilayer ReSe2, considering the van der Waals force between adjacent ReSe2 layers , DFT-D3 functional is used by setting IVDW=11, which have been demonstrated working effectively in dealing with 2D materials.30-31 Moreover, we have compared the ground state energy of two possible stacking mode for bilayer ReSe2 as shown in Figure S5, in which mode 1 displays lower energy and was selected to carry out the following simulation related to bilayer ReSe2. At last, to investigate the Li 6


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migration energy barrier within ReSe2, we have firstly optimized the adsorption structure and binding energy of Li at different sites, and then conducted the CI-NEB simulation with the diffusion paths similar to that of monolayer system (Figure S6). And we used VESTA to build up the 3D visualization models.32

Results and Discussion Figure 1a illustrates the schematic preparation of ReSe2 nanosheets on 3D graphene foam. Here, we designed a semi-closed chamber for ReSe2 growth using CVD method. After growth, the ReSe2 loading ~ 68wt% in ReSe2/graphene foam is confirmed by TGA (Figure S7). The morphology is first characterized by SEM. ReSe2/graphene foam exhibits porous structure with the pore size ranging from 100 μm to 200 μm (Figure S8). Besides, backscattered electrons from sample are reflected or backscattered, where high atomic number element backscatters electrons more strongly than the light elements.33 As a result, based on the back-scattering SEM images, we found that the ReSe2 nanosheets (brighter part on Figure 1b and 1c) with the size of 191 ± 65 nm in Figure 1d were uniformly deposited on the graphene surface. The low magnified TEM image in Figure 1e illustrates the uniform loading of ReSe2 nanosheets on graphene foam. And the energy-dispersive X-ray spectroscopy (EDS) reveals the signal coming from Re, Se and C (Figure S 9), in which the distribution of Re element is overlapping with that of Se and the atomic ratio of Se to Re is ~2, in consistent with stoichiometric ratio in ReSe2. Additionally, the high resolution image in Figure 1f reveals that ReSe2 is well contacted with the graphene substrate, and the selected area electron diffraction (SAED) of graphene and ReSe2 with the same orientation indicates the epitaxial growth of ReSe2 crystals on graphene surface. The inter-planar spacing of 0.577 nm in Figure 1g confirms the (100) crystal plane of ReSe2. To elucidate the chemical nature of ReSe2 nanosheets on graphene surface, Raman and XPS were used. Raman spectra (Figure 2a) shows three strong peaks at ~1340 cm-1, ~1582 cm-1 and ~2688 cm-1from graphene foam,25 while the peaks appear at 100~300 cm-1 in Figure 2b typically originated from ReSe2, i.e. ~125 cm-1 (E1g), ~159 cm-1 (A1g) 7



and ~172 cm-1 (A2g).34 Figure 2c and 2d are the high resolution XPS spectra of Re 4f and Se 3d in ReSe2. Briefly, two peaks at 41.5 and 43.9 eV, related to the 4f7/2 and 4f5/2 of Re respectively, are detected with no significant peak shift, which usually results from higher oxidation state, confirming +4 oxidation state only in it. Meanwhile, the distinct peaks located at 54.6 and 55.4 eV correspond to 3d5/2 and 3d3/2 peaks of Se, indicating the -2 oxidation state of Se.35 Subsequently, AFM was applied to check the thickness. Figure 3a presents an overall two-dimensional (2D) view of ReSe2/graphene foam surface, where the brighter dots in view are ReSe2 nanosheets, and the darker part is the flat graphene surface, with a 3D profile shown in Figure 3b. The average thickness of ReSe2 nanosheets is less than 3 nm, implying 1~3 atomic layered ReSe2. The detailed height profiles are shown in Figure 3c, showing thickness of ~ 2 nm, with size ~200 nm, consistent with statistical results of Figure 1d. To show its potential application in energy storage, we used ReSe2/graphene foam as the free-standing anode material for LIB. Here, DFT calculation was first conducted to understand the Li diffusion process. Figure 4a shows the top and side views of ReSe2 unit cell with the optimized parameters of OX= 6.663 Å, OY= 6.784 Å and ∠XOY= 61.16o. The crystal structure of 1T'-ReSe2 is quite different from other 2H phase or 1T phase TMDs, exhibiting low symmetry, where every 4 Re atoms will group into Re4 and form the Re4 chains.34 Such anisotropic ReSe2 structure provides several adsorption sites for Li (numbered in Figure 4b), with all negative adsorption energy (Figure 4c), indicating a spontaneous adsorption. The favorable adsorption sites are identified as site 1, 2 and 4 with the binding energy of -1.586, -1.708, and -1.477 eV respectively. Here, we compared two diffusion processes, i.e. atop monolayer ReSe2 and between the interlayer spacing of bilayer ReSe2, in which Li would migrate from one favorable adsorption site to its counterparts in the nearby unit cell.9, 21 For atop monolayer ReSe2, the optimal diffusion paths for site 1 (red arrows), site 2 (blue arrows) and site 4 (magenta arrows) in a 2*2 supercell are shown in Figure 4b with the energy barrier of 0.427 (Figure 4d), 0.315 (Figure 4e) and 0.299 eV (Figure 4f) respectively. In contrast, the corresponding diffusion barriers for the interlayer diffusion for the above8


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mentioned three diffusion paths are reduced to 0.325, 0.292 and 0.084 eV (diffusion paths are shown in Figure S6). This phenomenon is different from other layered anode material such as graphite,19-20 MoS2,21-22 TiS223 and GeS,24 in which the interlayer diffusion energy is higher due to the interlayer space confinement and coupling force. In the case of ReSe2, on the one hand, the extremely weak van der Waals coupling force is insignificant and has minimal effect on diffusion barrier. In addition, compared monolayer surface, due to the lower structure symmetry, for bilayer system the interlayer spacing could provide more stable adsorption sites from both two layers and thus further reduce the diffusion barrier. Since the diffusion constant at a certain temperature T is proportional to exp (-Eb/kT), where Eb is the diffusion energy barrier and k the Boltzmann constant, ReSe2-based anode materials are expected to have higher interlayer mobility of Li+, and lead to high charge/discharge rate.20, 23 For the electrochemical test of ReSe2/graphene foam electrode, we conducted CV measurements. Figure 5a illustrates the three initial CV cycles measured with the range from 0.01 to 3 V (vs. Li/Li+) with the scan rate of 0.1 mV s-1. During the first discharging process, the peaks at 1.59 and 0.97 V, are associated with the intercalation of Li+ into the interlayer of ReSe2 and formation of LixReSe2 compounds as shown in reaction (1), followed by the conversion into Li2Se and Re metal according to reaction (2), accompanied with the formation of solid electrolyte interface (SEI).10 By contrast, the first anodic sweep only shows one distinct peak at ~2V, ascribed to the oxidation of Re back to ReSe2.36 In the 2nd and 3rd CV cycles, the peaks at 1.59 and 0.97 V disappeared and were replaced by a sharp peak at 1.9 V, corresponding to conversion from ReSe2 to Li2Se.10 The cathodic peak at 0.17 V and anodic peak at 0.2 V arise from the intercalation process between Li+ and graphene substrate, revealing the capacity contribution from graphene.37-38 Additionally, the highly overlap of all anodic sweep curves during these cycles indicates the reversibility of the reaction. 𝑅𝑒𝑆𝑒2 +𝑥𝐿𝑖 + +𝑥𝑒 ― →𝐿𝑖𝑥𝑅𝑒𝑆𝑒2

(1)

𝐿𝑖𝑥𝑅𝑒𝑆𝑒2 + (4 ― 𝑥)𝐿𝑖 + + (4 ― 𝑥)𝑒 ― →𝑅𝑒 + 2 𝐿𝑖2𝑆𝑒

(2)

The results of galvanostatic charge/discharge measurement are consistent with the 9



CV analysis. During the initial cycle (Figure 5b), two plateaus (one at ~1.6V and the other at ~1V) and one plateau at ~2V are observed for the discharging and charging process, respectively. In the second cycle, plateaus at ~1.9 V and 2 V are seen for discharge and charge process respectively, associated with the conversion between ReSe2 and Li2Se. The curves for 100th, 200th and 300th cycle are very similar to the curve obtained at 2nd cycle, indicating the high stability of ReSe2/graphene foam electrode, consistent with cycling performance (Figure 5c). Notably, at a current density of 200 mA g-1, our ReSe2/graphene foam electrode delivers the initial specific capacities of 638 and 448 mAh g-1 for discharge and charge respectively, with the initial Coulombic efficiency of 70%, and then gradually increases to 481 mAh g-1 (discharge) in 100th cycle. This phenomenon is associated with the stabilization of active material, which is also observed on other TMDs materials when applied as the anode for LIB.39-40 After 350 cycle, the discharge capacity can be maintained at 479 mAh g-1 with an extremely high capacity retention of 99.6% (compared to 100th cycle), and the Coulombic efficiency can be as high as 99%, which is comparable to the previous reports on TMDs-based anode (Table S1). Moreover, the morphology of cycled electrode has been checked by TEM, and we can observe isolated nanosheets on the surface of graphene, confirming the structural stability during cycling (Figure S11). The rate performance of ReSe2/graphene foam electrode is also investigated with current density ranging from 0.25C to 2.5C as depicted in Figure 5d (1C =400 mA g-1). ReSe2/graphene foam electrode can deliver average discharge capacities of 529, 498, 407, and 327 mAh g-1 at 0.25C, 0.5C, 1.25C and 2.5C respectively. The Coulombic efficiency can be as high as 99.5% at 2.5C in the 40th cycle with discharge and charge capacity of 323.9 and 322.3 mAh g-1 respectively. And when the current density returns to 0.25C, the discharge capacity is recovered to 539 mAh g-1. We repeat the stepwise setting of current density and observed the similar phenomenon (from 41st cycle to 80th cycle), in which the discharge and charge capacity change following the same trend. And the discharge capacity retention of ReSe2/graphene foam anode can be as high as 100% when current density is back to 0.25C for the second round. Here, the excellent 10


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rate performance of ReSe2/graphene foam confirms the prediction in previous theoretical study that the low diffusion barrier of Li in the interlayer spacing of ReSe2 could ensure the fast transportation of Li, rendering high capacity and Coulombic efficiency at high rate. We have also explored the performance of ReSe2/graphene foam structure for HER catalysis. As shown in Figure 6a, the ReSe2/graphene foam only needs 106 mV of overpotential to achieve current density of -10 mA cm-2, lower than pristine graphene foam and bulk ReSe2 (285 mV), and close to Pt/C (41 mV). Here, the obtained catalytic activity is better than most of TMDs-based catalysts (Table S2). The Figure 6b presents the corresponding Tafel slope derived from 6a, where the ReSe2/graphene foam exhibited a smaller value of 68 mV dec-1, as compared to 289 mV dec-1 for pristine graphene foam and 118 mV dec-1 for bulk ReSe2. Besides, ReSe2/graphene foam showed a large exchange current of 277.8 μA cm-2, calculated by extrapolating the Tafel plot, compared with that of graphene foam and bulk ReSe2 of 6.2 and 87.1 μA cm-2, respectively. Moreover, the performance of ReSe2/graphene foam is still stable after 1000 cycles with insignificant activity decay (Figure S12). CV and EIS were further applied to evaluate the electrochemical properties of ReSe2/graphene foam electrode. CV from 0.1374 V to 0.2374 V (vs. RHE) was conducted to measure catalyst’s double-layer capacitance (Cdl), through which the electrochemically active surface area (ECSA) can be estimated.41 The obtained Cdl values of ReSe2/graphene foam, bulk ReSe2 and graphene foam are 15.57, 2.14 and 0.96 mF cm-2 respectively (Figure S13b), revealing that ReSe2/graphene foam can expose more active sites compared to bulk ReSe2 and pure graphene foam. Besides, the charge-transfer resistance (Rct) was evaluated via EIS test (Figure S13c~d). The semicircles shown in the Nyquist plot can be used to obtain Rct values. At an overpotential of 100 mV, Rct value of ReSe2/graphene foam (165.7 Ω) is much smaller than that of bulk ReSe2 (569.8 Ω). Thus, the introduction of graphene foam facilitates the charge transfer process and make it more kinetically favorable. The full exposure of the active sites of ReSe2 and high conductivity of graphene contribute to the excellent 11



electrochemical performance of ReSe2/graphene foam electrode for HER.

Conclusion In summary, we have successfully obtained ReSe2/graphene foam through a CVD method, and used it as the free-standing anode for LIB and electrocatalyst for HER. By using graphene foam as the growth template, we obtain 1~3 layered ReSe2 nanosheets deposited on the graphene surface densely and uniformly in size of 191 ± 65 nm. The obtained ReSe2/graphene foam electrode shows excellent electrochemical performance in LIB, ascribing to the effects of weak van der Waals coupling and low structure symmetry on Li diffusion. Different from other layered anode material, the energy barrier of Li diffusion between ReSe2 interlayers is lower than that atop monolayer since the lower structure symmetry of the interlayer spacing provide additional adsorption sites. Along with the large interlayer spacing, Li can diffuse rapidly between the layers while maintain a stable volume, subsequently lead to stable cycling and excellent rate performance. In addition, resulting from the fully exposure of edge site of ReSe2 and the free-standing graphene foam support, the binder-free ReSe2/graphene foam exhibits excellent performance as catalyst for HER. Herein, such a series of study on ReSe2 from synthesis design to bifunctional applications will benefit the broader researches and applications of Re-based TMDs toward energy storage and conversion. Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:XXX. Additional characterization results of ReSe2/graphene foam, including SEM, TGA, EDS, XRD and TEM, electrochemical measurements, and computational details for DFT calculation.

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Acknowledgement This project was supported by the Research Grant Council of Hong Kong SAR (Project numbers 16204815), NSFC-RGC Joint Research Scheme (N_HKUST607/17), the Innovation and Technology Commission (ITC-CNERC14SC01), the Guangzhou Science & Technology (Project 201704030134) and Training Program of Major Basic Research Project of Provincial Natural Science Foundation of Guangdong (2017B030308001). Technical assistance from the Materials Characterization and Preparation Facilities of HKUST is greatly appreciated.

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Figure 1. Preparation of ReSe2 nanosheets on graphene foam (ReSe2/GF). (a) Schematics of the growth of ReSe2 on graphene foam (GF). (b-c) Back-scattering scanning electron microscope (SEM) images of ReSe2 nanosheets well-dispersed on the surface of GF. (d) size distribution derived from panel 1b. (e-g) Transmission electron microscope (TEM) results of ReSe2/GF. Insets in panel 1f are the selected area electron diffraction (SAED) patterns.

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Figure 2. Chemical characterization. (a) Raman spectrum of ReSe2/graphene foam. (b) zoomed-in range from 100 to 300 cm-1 from panel 2a. (c-d) Re 4f (c) X-ray photoelectron spectroscopy (XPS) spectrum of Re 4f (c) and Se 3d (d), respectively.

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Figure 3. Morphology study by Atomic force microscopy (AFM). (a) AFM image. (b) The 3D profile of the selected area in panel 3a, (x: 2.5 m, y: 2.5 m, z: 3 nm). (c) The height profiles of ReSe2 nanosheets on graphene foam surface, corresponding to the marked positions 1 to 3 in panel 3a.

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Figure 4. Density functional theory (DFT) calculations on lithium diffusion in ReSe2. (a) Top and side view of ReSe2 unit cell. (b) The adsorption sites of Li on ReSe2 and the diffusion paths for the most favorable adsorption sites. (c) The adsorption energy for different adsorption sites. (d~f) Diffusion energy barrier for the optimal diffusion paths for site 1(d), site 2(e) and site 4(f) atop monolayer ReSe2 (dark color)and in the interlayer spacing of ReSe2 (light color) respectively.

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Figure 5. Electrochemical performance of ReSe2/graphene foam as anode material for lithium ion battery (LIB). (a) First three cyclic voltammetries at a scan rate of 0.1 mV s-1. (b) Galvanostatic charge and discharge profiles from 1st cycle to 300th cycle at a current density of 200 mA g-1. (c) Long-life cycling performance at a current density of 200 mA g-1. (d) Rate performances at different current densities (1C = 400 mA g-1).

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Figure 6. ReSe2/graphene foam as catalyst for hydrogen evolution reaction (HER) in 0.5 M H2SO4. (a) The polarization curves of graphene foam (GF), bulk ReSe2, ReSe2/GF and 20wt% Pt/C in HER test, respectively. (b) Corresponding Tafel plots derived from 6a.

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Confinement-enhanced Rapid Interlayer Diffusion within Graphene

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