Metal Organic Framework-Templated Synthesis of Bimetallic

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Metal Organic Framework-Templated Synthesis of Bimetallic Selenides with Rich Phase Boundaries for Sodium-Ion Storage and Oxygen Evolution Reaction Guozhao Fang, Qichen Wang, Jiang Zhou, Yongpeng Lei, Zixian Chen, Ziqing Wang, Anqiang Pan, and Shuquan Liang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00816 • Publication Date (Web): 25 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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TOC Figure

ACS Paragon Plus Environment

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Metal Organic Framework-Templated Synthesis of Bimetallic Selenides with Rich Phase Boundaries for Sodium-Ion Storage and Oxygen Evolution Reaction Guozhao Fang,a,1 Qichen Wang,b,d,1 Jiang Zhou,a,c,*, Yongpeng Lei,b,d,* Zixian Chen,a Ziqing Wang,a Anqiang Pan,a,c and Shuquan Lianga,c,* a

School of Materials Science and Engineering, Central South University, Changsha 410083, P. R.

China. b

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, 410083, P. R.

China. c

Key Laboratory of Electronic Packaging and Advanced Functional Materials of Hunan Province,

Central South University, Changsha 410083, Hunan, P.R. China d

Hunan Provincial Key Laboratory of Chemical Power Sources, College of Chemistry and

Chemical Engineering, Central South University, Changsha, 410083, P. R. China. 1

G. Fang and Q. Wang contribute equally to this work.

* E-mail address: [email protected], [email protected], [email protected].

ABSTRACT: Two-phase or multi-phase compounds have been explored to exhibit good electrochemical performance for energy applications, however, the mechanism insights into these materials, especially the performance improvement by engineering the high-active phase boundaries in bimetallic compounds, is still unveiled. Here, we report a bimetallic selenide heterostructure (CoSe2/ZnSe) and the fundamental mechanism behind their superior electrochemical performance. The charge redistribution at the phase boundaries of CoSe2/ZnSe was experimentally and theoretically proved. Benefiting from the abundant phase boundaries, CoSe2/ZnSe exerts low Na+ adsorption energy and fast diffusion kinetics for sodium-ion batteries, and high activity for oxygen 1


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evolution reaction. As expected, excellent sodium storage capability, specifically a superb cyclic stability up to 800 cycles for the Na3V2(PO4)3‖CoZn-Se full-cell, as well as efficient water oxidation with a small overpotential of 320 mV to reach 10 mA cm2 were obtained. This work demonstrates the importance of phase boundaries in bimetallic compounds to boost the performance in various fields. KEYWORDS: phase-boundary effect, synergistic effects, bimetallic selenides, sodium-ion battery, oxygen evolution reaction

Engineering the physical or chemical properties of functional materials can greatly promote their electrochemical performance which may address the present and future requirements for energy-related devices.1-3 Recent advances in this respect involve several important fields including rechargeable metal ion battery,4-6 electrochemical water splitting,7-9 rechargeable metal-air battery,10-12 and etc. Notably, constructing bimetallic heterostructure compounds have been explored to enhance electrochemical property due to their synergistic effects. For instance, these compounds as electrodes in battery exhibit richer redox reactions and higher electronic conductivity compared to the monometal phase.13-15 It is also reported in many cases that engineering heterogeneous structure can introduce phase boundaries, which were revealed as the reason for the performance improvement.7, 10, 16 In generally, phase boundaries are generally rich in lattice defects, distortions and dislocations, with distinguished electronic structures, which are long-range disorder and thermodynamic unstable, exerting a significant influence to electrochemical properties.17-19 Previous work has demonstrated that phase boundaries induced by hetero-interfacial structure could lower the activation barrier and hence accelerates reaction kinetics, thus enhance electrochemical 2


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performance.16 Unfortunately, relatively little knowledge is disclosed to know about how the synergistic effect between heterogeneous structure, especially the phase boundary with heterogeneous electronic states, alters electrochemical properties. Recently, the earth-abundant transition metal selenides (TMSs) have shown great potential in sodium-ion batteries (SIBs)20-23 and oxygen evolution reaction (OER)8,

24, 25

due to their high

theoretical capacity and intrinsic metallic property. As an alternative to graphite and hard carbon anodes, metal selenide could ensure better safety of the battery by avoiding the problem of sodium dendrite formation in the low reaction voltage range.26 In addition, compared to the metal oxides/sulfides, the bond of M-Se in TMSs are weaker, which could be kinetically favorable for conversion reactions. The discharge product (Na2Se) of TMSs also affords better conductivity than Na2O or Na2S. Many successful efforts have been devoted to promote the sodium storage performance of TMSs, such as by optimizing the electrolytes (e.g. ether-based electrolytes) to reduce the reaction energy barrier,20, 23 by employing delicate structures (e.g. urchin-like CoSe2, and hollow microsphere) to expose more active sites,21,

27

by constructing hybrid with carbon matrix

(e.g. MoSe2/N,P-doped carbon nanosheets, Co9Se8/rGO hybrid) to improve the conductivity.22,

28

However, these metal selenides still suffer from severe volume expansion and sluggish Na+ diffusion kinetics during sodiation/desodiation, which can be improved by modifying the intrinsic property of the materials. Very recently, bimetallic selenides (heteroatom (Co1/3Fe2/3)Se2, (NiFe)Sex, Ni0.67Fe0.33Se2, etc. or heterostructure CoSe2/(NiCo)Se2, MoSe2@CoSe etc.) have been explored to exhibit good electrochemical performance for SIBs.29-33 In addition to SIBs, it is reported that the bimetallic selenides can also boost the OER activity.25, 34 None of them, however, deeply revealed the intrinsic mechanism for the performance improvement of these bimetallic selenides with 3


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heteroatoms or heterostructures. An in-depth insight into this related topic is a critical factor that accelerates the practical application of bimetallic selenide compounds, and it also provides an instructive reference for better exploitation and improvement of other bimetallic compounds. Herein, the phase-boundary effect was revealed via a bimetallic selenide heterostructure (CoSe2/ZnSe nanoflakes, termed as CoZn-Se). The phase boundaries with lattice distortion between the CoSe2 and ZnSe crystalline domains were verified. An experimental and theoretical analysis including multiple synchrotron-based X-ray spectroscopic characterizations and density functional theory (DFT) proves a redistribution of interfacial charge as the electrons transfer from the CoSe2 side to the ZnSe side at the hetero interfaces. It is further demonstrated by the calculation of Na+ adsorption energy that phase boundaries with high electron density in ZnSe side are more conducive to the adsorption of Na+ ions (as illustrated in Fig. 1), thus accelerates reaction kinetics. Moreover, in situ X-ray diffraction (XRD) and ex situ transmission electron microscopy (TEM) exhibit a multistep redox reaction mechanism in CoZn-Se toward SIB application, which could effectively relieve the stress of Na+ insertion, thereby enhance the reversibility of sodiation/desodiation. As expected, the CoZn-Se exhibits higher sodium diffusion coefficients compared to its counterparts. And a high rate capability and excellent cyclic stability up to 4000 cycles were obtained. More importantly, Na3V2(PO4)3‖CoZn-Se full cell delivers a high reversible capacity of 332 mA h g1 (based on the mass of CoZn-Se) at 0.1 A g1, and a superb cyclic stability with the capacity retention of 83% after 800 cycles. As another proof of concept, CoZn-Se also presents good OER activity with a small overpotential of 320 mV to reach 10 mA cm2 in 1 M KOH and excellent stability for 15 h or 2000 CV cycles.

4


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RESULTS AND DISCUSSION Materials characterization. As indicated in Fig. 2a, XRD pattern of CoZn-Se is composed of both cubic CoSe2 (space group Pa-3(205), PDF#88-1712) and hexagonal ZnSe (space group P63mc(186), PDF#80-0008), confirming the coexistence of CoSe2 and ZnSe in the hybrids. The ICP-AES result displays the atomic ratio of mZn: mCo is about 1.08:1 (Table S1, Supporting information (SI)), which is very close to the experimental value (mZn: mCo=1:1). CoZn-Se precursor was synthesized after oil-bath treatment, which shows the etched surface (Fig. S1, SI) because of the decomposition of organic matter from CoZn-MOFs nanoflakes and the ingress of selenium atoms. SEM images and TEM images show the well-maintained nanoflakes converted from CoZn-Se precursor after selenization (Fig. S2, SI). The organic matter was further decomposed in the calcining process to transform into the CoZn-Se nanoparticles and carbon, between which two categories of pore with an average pore diameter size is in 7.9 nm (inset in Fig. S2d, SI) were formed. The morphologies of CoSe2 and ZnSe are also porous nanosheet (Fig. S3, SI), but their surface areas (CoSe2: SLangmuir=59.6 m2 g-1, SBET=59.6 m2 g-1; ZnSe: SLangmuir=73.5 m2 g-1, SBET=17.7 m2 g-1) are both less than that of CoZn-Se (SLangmuir=118.2 m2 g-1, SBET=20.4 m2 g-1). As we known, the surface area is an important parameter for surface reaction. The larger surface area of CoZn-Se may provide more surface-active site for ion storage. Elemental (Co, Zn and Se) line scanning of CoZn-Se (Fig. 2b) shows that the signal peaks of Co and Zn elements with variation in width from ten to dozens of nanometers appear alternately, indicating the CoSe2 and ZnSe particles stacked each other, which may possess numerous phase boundaries in CoZn-Se. HRTEM measurement was performed to further reveal the two-phase property of the hybrid (Fig. 2c). The lattice fringes in the nano crystalline domains can be labeled by (002) plane for ZnSe and 5


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(210) plane for CoSe2, respectively. Clear phase boundaries between the CoSe2 and ZnSe crystalline domains, confirm the large amount of lattice mismatch and distortion. Fig. S4a (SI) shows more phase boundaries between the two phase domains, which could create more crystal defects and more accessible active sites for Na+ storage and electrocatalysis.16 SAED pattern (Fig. S4b, SI) further confirmed the coexistence of CoSe2 and ZnSe in CoZn-Se. High-angle annular dark-field scanning TEM (HAADF-STEM) and elemental mapping images (Fig. 2d) reveals that C and N also distributed in the nanoflake, indicating the two-phase crystalline domains were coated by nitrogen-doped carbon, which was in situ derived from the CoZn-MOF precursor and also demonstrated by the XPS data (Fig. S5, SI).35 This coated nitrogen-doped carbon would provide the highway of the electron and ion transportation,36, 37 ensuring the fast reaction kinetic and good rate capability. XAFS measurements were further performed to verify the local structure of CoZn-Se, CoSe2 and ZnSe in detail. In the Co K-edge X-ray absorption near-edge structure (XANES) spectra (Fig. 2e), a shift of absorption edge to high energy is observed for CoZn-Se compared to CoSe2. Note that there is also a slight distinction between CoZn-Se and ZnSe in Zn K-edge XANES and EXAFS spectra (Fig. S6, SI). We also found that the white line intensity for CoZn-Se decreased in comparison to ZnSe, representing the oxidation of Zn species was lowered.38 It indicates that the electrons transfer from the CoSe2 side to the ZnSe side in CoZn-Se,39 thus causes the interfacial charge redistribution. The Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra of Co K-edge was also employed to investigate the geometry structure of CoZn-Se. As displayed in Fig. 2f, the Co-Se bonds of CoZn-Se sample shifted slightly to high-R side compared to that of CoSe2, which is ascribed to the coexistence of heterogeneous spin states in the phase 6


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boundaries and the mismatch in the degree of Jahn-Teller distortion between the Co-Se (for Co2+, 3d7, which splits to t2g6eg1 and the eg orbitals are unevenly occupied, yielding a strong Jahn-Teller effect) and Zn-Se (for Zn2+, 3d10, yielding no Jahn-Teller effect).40 The wavelet transform (WT) contour plots (Fig. 2g and 2h) and the fitting parameters of R-space in the first coordination shell of ZnSe, CoSe2 and CoZn-Se (Table S2) further demonstrate the different electronic structure and structural disorder in CoZn-Se. These phase boundaries interfacial charge redistribution and lattice distortion are expected to promote the sodium-ion storage and water oxidation. Na+ storage performance. The electrochemical Na-ion storage performance of CoZn-Se electrode was evaluated in a half cell. The first three cyclic voltammetry (CV) curves at a scan rate of 0.1 mV s1 exhibit multiple redox reaction for CoZn-Se electrode (Fig. S7a, SI). Two strong peaks at 0.81 and 0.31 V are observed during the initial cathodic process. In the subsequent scans, there are three main cathodic reduction peaks at 1.14, 0.61, and 0.4 V and two main anodic oxidation peaks at 1.05 and 1.79 V. In order to explain these redox couples in more detail, we compare carefully the CV curves of CoSe2 (Fig. S7b, SI) and ZnSe (Fig. S7c, SI). Two cathodic peaks at 0.76 and 0.50 V for CoSe2 during the initial process may due to the reduction of CoSe2 and the formation of solid electrolyte interphase (SEI). A pair of redox peaks at near 1.13/1.82 V in the subsequent CV curves of CoSe2 is ascribed to conversion reaction between CoSe2 and Co/Na2Se. For ZnSe electrode, one cathodic peaks at 0.16 V during the initial process may due to conversion/alloying reaction of ZnSe and the formation of SEI. In the subsequent CV curves, two cathodic peaks at around 0.64 and 0.40 V can be attributed to the reduction of ZnSe to Na2Se and Zn, and further alloying transformation of Zn to NaZn13, while only one main strong anodic peak at around 1.07 V may be due to the similar reaction dynamics of de-alloying transformation of NaZn13 7


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and oxidation of Zn to ZnSe. Fig. 3a clearly shows the second CV curves of CoZn-Se (red solid line), CoSe2 (cyan dotted line) and ZnSe (violet dotted line). It is obviously that the redox reactions in CoZn-Se are composed of the conversion reaction of CoSe2 (1.14/1.79 V) and the conversion/alloying reaction of ZnSe (0.61 and 0.40/1.05 V). The detailed reaction mechanism will be discussed later. Due to the different redox potentials of CoSe2 and ZnSe, the uniformly distribution of ZnSe compound in CoZn-Se can inhibit particle aggregation and volume expansion of CoSe2 during conversion reaction; On the other hand, the initially formed Na2Se matrix, supported by the electron conducting Co nano-network, also has a buffering effect for the conversion/alloying reaction of ZnSe. Therefore, CoSe2 and ZnSe in turn buffer the volume expansion of each other, beneficially combining the advantages of the bimetallic selenide. CoZn-Se electrode delivered a high initial discharge and charge capacity of 575 and 416 mA h g1 at 0.1 A g1. It is worth to be noted that except the initial cycle, the subsequent galvanostatic charge-discharge (GCD) curves are almost overlapped (Fig. 3b), suggesting good reversibility of reaction behavior and excellent stability of the electrode. A considerable retention of 93% after 100 cycles was obtained for CoZn-Se electrode, while the monometal selenides display rapid decay in capacity (Fig. 3c). The cycling performances of the samples obtained at different selenization temperature were also investigated. As shown in Fig. S8 (SI), sample synthesized at 500 °C delivered the rapidly fading capacity at 0.1 A g-1, which may due to the low crystallinity. The sample synthesized at 700 °C demonstrated the comparable capacity retention, but its capacity is lower than that of CoZn-Se, indicating that the optimized synthesis temperature is 600 °C. Moreover, the CoZn-Se electrode also delivers a better rate capability with 263 mA h g1 at 10 A g1 than those of monometal selenides (Fig. 3d), and it can fully restore its capacity when the 8


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current density is reset to 0.1 A g1. The abundant phase boundaries induced by two-phase construction may be responsible for the outstanding cyclability and high Na+ storage capability of CoZn-Se. For further demonstration of the phase-boundary effect, a physically mixed CoSe2/ZnSe (molar ratio of 1:1) electrode was also prepared (Fig. S9, SI). However, its electrochemical behavior (Fig. S10, SI, Fig. 3b and c) only exhibited the physical superposition of that of CoSe2 and ZnSe, which could not maintain stability and endure high-rate discharge/charge. The rapid fading of capacity in CoSe2 and ZnSe may be due to large volume changes and voltage polarization of monometallic selenide caused by the insertion and release of Na+ ions, leading to the pulverization of electrode material in the repeated cycle.26 What makes the CoZn-Se electrode more attractive is its ultrastable cycle life with the capacity retention of 83% and 84% after 4000 cycles at 8 and 10 A g1, respectively (Fig. 3e). A magnified view of coulombic efficiencies of CoZn-Se at 8 A g-1 and 10 A g-1 in Fig. S11 (SI). We note that the capacities and coulombic efficiencies during cycling are fluctuant, which may be due to the following reasons: 1) At the large current densities, it is difficult for the electrode to completely overcome the rapid expansion and contraction of the volume. Besides, large current causes greater voltage hysteresis, which directly affects the capacity and coulomb efficiency; 2) Many side reactions may occur in the Na+ insertion and extraction process for metal selenides; 3) It is hardly to fully control constant test conditions (e.g. the test temperature) during the long-time test. Therefore, more works are need to study this issue in-depthly. A full-cell with CoZn-Se anode and Na3V2(PO4)3 cathode (Fig. 3f) was assembled to further evaluate the performance of bimetallic selenides. The full-cell delivers a reversible capacity of 332 mA h g1 (based on the mass of CoZn-Se) at 0.1 A g1 with a discharge plateau above 2.0 V (Fig. S12, SI). Good capacity retention of 83% after 800 cycles was obtained even at a high current density of 1 A 9


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g1, suggesting the excellent cyclic stability of the full-cell (Fig. 3g). We also compared CoZn-Se electrode with other transition metal chalcogenides (Table S3, SI), rendering that the CoZn-Se is among the top level of those reported previously. Experimental and theoretical analysis of reaction kinetics in SIBs. The sodium reaction kinetics of CoZn-Se was in-depth analyzed by GITT (see details in Fig. S13, SI). The GITT curves also reveal that the multistep sodiation/desodiation of CoZn-Se is the superposition of that of two monometal phases (Fig. 4a and b). The Na+ diffusion coefficients of all three samples were calculated and varied at the course of discharge/charge. Note that CoZn-Se electrode shows higher Na+ diffusion coefficient and minor fluctuant than that of CoSe2 and ZnSe, suggesting that the abundant phase boundaries in two-phase bimetallic selenides create crystal defects and introduce active sites, which is helpful for the diffusion of Na+. Furthermore, separate multistep sodiation/desodiation of two phases can effectively relieve the stress during the reaction. In addition, a CV experiment features the partial capacitive contribution to sodium-ion storage (Fig. 4c and Fig. S14, SI), which is higher than that of CoSe2 and ZnSe (Fig. S15, SI), further suggesting the fast reaction kinetics of CoZn-Se.41 In addition, the electrochemical impedance spectra (EIS) were conducted to study the resistance changes during cycling. As shown in Fig. S16 (SI), the charge-transfer resistances (RCT) of CoSe2 (73 Ω) and CoZn-Se (79 Ω) at 1st cycle were comparable, which was lower than that of ZnSe (214 Ω). RCT of CoZn-Se (69 Ω) at 10th cycle became smaller and lower than that of CoSe2 and ZnSe, suggesting the improvement in the reaction kinetic of CoZn-Se during cycling, which contributes to the excellent cyclic performance. To gain insight into the effect of phase boundaries on reaction kinetics, the DFT calculations were conducted to understand the interfacial charge behavior of the CoZn-Se heterostructure. 10


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Calculated density of states (DOS) of CoSe2 and ZnSe show no band gap for CoSe2 and a band gap of 2.13 eV for ZnSe (Fig. 4d), indicating CoSe2 possesses intrinsic metallic property, while ZnSe is a direct-gap semiconductor.42 A combination of DOS of CoSe2 and ZnSe was observed for CoZn-Se, which is due to the strong interfacial interaction between each other. Differential charge density in phase boundaries of CoZn-Se and the bader charge analysis (0.09 electrons gather at the ZnSe side) show that electrons would migrate from CoSe2 side to ZnSe side (Fig. 4e). Planar averaged electrostatic potential (cyan line) and charge density difference (red line) across the interface of CoZn-Se are presented in Fig. 4f. Clearly, the averaged electrostatic potential of ZnSe is much higher than that of CoSe2, and the difference value of the two individuals is as big as 4.94 eV. This indicates a strong electrostatic field across the interface, making electrons transfer from CoSe2 side to ZnSe side. As a result, ZnSe surface would accumulate electrons, inducing the interfacial charge redistribution, which is also consistent with Co K-edge and Zn K-edge XAFS spectra as discussed above. This electron accumulation would strongly attract Na+ ions on the phase boundaries close to ZnSe side. The calculations of Na+ adsorption energy in Fig. 4g indicate that the Na+ adsorption energy of CoSe2 (-1.23 eV) is calculated to be lower than that of ZnSe (-0.28 eV), which indicates that ZnSe itself does not adsorb Na+ well, but CoSe2 is better. By constructing the heterojunction, the Na+ adsorption energy of CoZn-Se (-2.74 eV) is much lower than that of CoSe2 and ZnSe. It further supports that the interface of CoZn-Se is favorable for the adsorption of Na+ ions, thus enabling fast reaction kinetics. In addition, the ab initio molecular dynamics (AIMD) were performed for the CoZn-Se heterostructure with Na+ adsorption at 300 K for 3 ps. The animation of AIMD process (Movies S1) clearly demonstrates the change process of heterojunction interface and the adsorption process of Na+ ions. While the temperature and total energy fluctuates 11


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with time, the CoZn-Se heterostructure interface framework still maintains (Fig. S17, SI). Based on the above AIMD simulation and formation energy, it is confirmed CoZn-Se heterostructure with Na+ adsorption is kinetically stable at 300 K. Electrochemical reaction mechanism in SIBs. The excellent electrochemical performance of CoZn-Se also inspires us to explore its sodium reaction mechanisms, which will play a guiding role in designing new electrode materials for SIBs. In-situ XRD was conducted to visualize the overall evolution of the CoZn-Se electrode (Fig. 5a and b). The expanded views of the selected XRD patterns at point 1 to 6 are shown in Fig. 5c. During the first discharge process, the gradual disappearance of the diffraction peaks of CoSe2 occurred in the first discharge plateau (0.81 V), while the three diffraction peaks of ZnSe became weaker and weaker until they completely disappeared during the discharge plateau (0.31 V), suggesting CoSe2 in CoZn-Se was firstly reduced. Two diffraction peaks at 22.8o and 37.5o are indexed to the (111) and (220) crystal planes of cubic Na2Se (PDF#47-1699). The ex-situ SAED (Fig. 5d) and HRTEM image (Fig. 5e) were performed to confirm the formation of Co (PDF#15-0806) and NaZn13 (PDF#03-1008) at the 1st full discharge state. Therefore, during the sodiation process, CoZn-Se was reduced to Na2Se, Co and NaZn13. The diffraction peaks of Na2Se gradully disappeared and the phase of CoSe2 and ZnSe were detected again during the charging process. The ex-situ SAED pattern (Fig. 5f) and HRTEM image (Fig. 5g) at the 1st full charge state further demonstrated the reversibility of CoSe2 and ZnSe phases in CoZn-Se. Furthermore, the elemental (Co, Zn and Se) line scanning (Fig. 5h) and HRTEM image of CoZn-Se after 50 cycles (Fig. 5i) shows that the phases of CoSe2 and ZnSe are adjacent, indicating that the two-phase hybrids show stability and the phase boundaries still exist in CoZn-Se after cycling. According to the above discussion, the related reaction mechanism of 12


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CoZn-Se could be described as: (I) CoSe2 + 4Na + + 4e - ↔Co + 2Na2Se (II) ZnSe + 2Na + + 2e - ↔Zn + Na2Se (III) 13Zn + Na + + e - ↔NaZn13 Based on the reaction mechanism of CoZn-Se, we have calculated its theoretical capacity via 𝑛∙𝐹

the fundamental thermodynamic equation as follows:43 Q = 3.6 ∙ 𝑀𝑤; where Q is the specifc capacity (mA h g-1), n is the transferred electron number (mol), F is the Faraday constant (~96485 C mol-1), and Mw is the per unit molar mass (g). Here, we assume that 1 mol CoZn-Se contains 0.5 mol CoSe2 and 0.5 mol ZnSe. The theoretical capacity of CoZn-Se is calculated to be about 451 mA h g-1, which is much higher than that of graphite and hard carbons (close to 300 mA h g-1). The CoZn-Se can deliver a reversible capacity of 416 mA h g-1 at the current density of 0.1 A g1, which is up to 92% capacity utilization compared to the theoretical capacity. The loss of partial capacity may be due to the formation of solid electrolyte interphase (SEI) during the initial cycle that consumes part of the active material.44 It should be noted that the electrochemical performance of the CoZn-Se is still far from practical applications, more efforts should be devoted to enhancing their performances. OER electrochemical performance. The rational construction of phase boundaries in CoZn-Se is expected to make a great contribution to electrocatalytic performance due to its unique electronic structure. As an illustration, we further investigated the OER activity of CoZn-Se, CoSe2 and ZnSe for water oxidation in 1 M KOH, with commercial RuO2/C catalyst for comparison. Fig. 6a exhibits the polarization curves of the different samples collected at a scan rate of 2 mV s−1 after iR correction. The CoZn-Se shows a very small overpotential of 320 mV to reach 10 mA cm2 (η10), 13


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which is much lower than that of ZnSe (408 mV) and CoSe2 (424 mV), and even the same to that of commercial RuO2/C (315 mV). Assuming that all of the Co and Zn species act as active sites, CoZn-Se possess a high turnover frequency (TOF) value of 0.022 s1 at η = 320 mV.10 The low overpotential of CoZn-Se is also comparable with other advanced OER catalysts reported previously (Fig. 6b).7,

11, 45-53

The OER kinetic behaviors of studied selenides were further

confirmed by the Tafel plots (Fig. 6c) and EIS (Fig. S18, SI). As seen, CoSe2 presents a lowest Tafel slope and charge transfer resistance, suggesting its metallic character.54 In contrast, the Tafel slope for CoZn-Se hybrids was determined to be 66 mV dec-1. This phenomenon indirectly verified that the phase-boundary engineering could significantly promote OER reactivity. As noted above, the interfacial charge redistribution caused by electron transfer from CoSe2 to ZnSe at interfaces would induce the increase of positive charges on CoSe2 side, thus facilitate the adsorption of OHand enable the conversion to O2 molecules with high-efficiency.48 In addition, the double-layer capacitances (Cdl) were evaluated by cyclic voltammetry (CV) method (Fig. 6d and Fig. S19, SI) to measure the electrochemical surface area (ECSA). As known, the larger ECSA represent the more effective utilization of catalytic active sites. The Cdl of CoZn-Se is determined to be 11.8 mF cm-2 (Fig. 6e), higher than that of ZnSe (4.7 mF cm-2) and CoSe2 (4.9 mF cm-2), indicating that the CoZn-Se have the highest accessible active surface areas than that of ZnSe and CoSe2. The increased ESCA of CoZn-Se may be attributed to the CoSe2/ZnSe interfacial structure, which provides more accessible active sites.55-57 Besides, as discussed above, the morphologies of CoSe2 and ZnSe are also porous nanosheet, but their surface areas are both less than that of CoZn-Se. As known, the larger surface area is very beneficial for OER due to more surface-active site. In addition, the OER current density only displayed a small decay of 13.8% after 14


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15 h operation assessed by chronoamperometric response at 1.55 V (Fig. 6f). Specially, only a little increase of 17 mV in η10 observed over 2000 cycles between 1.10 and 1.65 V at 100 mV s-1 suggests an excellent OER stability of CoZn-Se. The robust stability of nanoflake nanosheet and the existence of abundant phase boundaries are verified by ex-situ TEM and HRTEM even after 2000 CV cycles (Fig. S20, SI), showing its promising potential for future applications.

CONCLUSIONS In summary, we have demonstrated that the enhanced SIB and OER performance of CoZn-Se heteronanoflake were derived from the abundant phase boundaries induced by two-phase construction. Both experimental and theoretical analysis confirmed the interfacial charge redistribution at the hetero interfaces. As for SIBs, the calculation of Na+ adsorption energy demonstrated that phase boundaries with high electron density in ZnSe side are more conducive to the adsorption of Na+ ions, thus accelerates reaction kinetics. Furthermore, the multistep redox reaction mechanism in CoZn-Se heterostructure could effectively relieve the stress of Na+ insertion. As expected, the CoZn-Se shows superior sodium-ion storage capability than its counterparts, especially the excellent cyclic stability up to 4000 cycles in half cell and 800 cycles in full cell. As another proof of concept, CoZn-Se also presents good OER activity. It is expected that such a strategy for engineering abundant phase boundary in bimetallic selenides could be extended to other functional materials for various applications.

EXPERIMENTAL SECTION Material synthesis. CoZn-MOFs nanoflakes were synthesized as our previous reports.58, 15


59

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Two steps were conducted to synthesize CoZn-Se. First, 40 mL hydrazine hydrate containing 100 mg CoZn-MOFs nanoflakes and 100 mg selenium powder were placed in oil-bath device at 90 oC for 20 min; Secondly, the powders were collected and thermally treated at 600 °C under an Ar flow for 2 h with a heating rate of 2 °C/min. The corresponding monometallic selenides were converted from monometallic Co-MOF and Zn-MOF nanosheets. Na3V2(PO4)3 was prepared through a similar method of our group.60 Structure Characterization. The X-ray diffraction (XRD) data were collected using a Rigaku D/max2500 powder diffractometer (Cu Kα, λ = 0.15405 nm). For in situ XRD testing, an electrochemical cell module with a beryllium window was used, while the slurry was directly cast on the beryllium window. Inductively coupled plasma atomic emission spectrometry (ICP-AES) (icp6300) was performed to determine the content of Zn, Co and Se element. Scanning electron microscopy (SEM; FEI Nova NanoSEM 230, 10 kV) was used to reveal the morphologies of the samples. Titan G2 60-300 transmission electron microscopy (TEM) was performed to scan high-resolution transmission electron microscopy (HRTEM) images, selected area electron diffraction (SAED) patterns and energy dispersive spectrometer (EDS) mapping. X-ray photoelectron spectroscopy (XPS) spectra were performed on an ESCALAB 250Xi X-ray photoelectron spectrometer (Thermo Fisher). The X-ray absorption fine structure (XAFS) spectra (Zn/Co K-edge) were collected at the 1W1B station in the Beijing Synchrotron Radiation Facility (BSRF; the storage rings were operated at 2.5 GeV with a maximum current of 250 mA). The data were collected at room temperature in transmission mode using a N2-filled ionization chamber (Si (111) monochromator for Zn/Co K-edge). All samples were pelletized as disks 13 mm in diameter using graphite powder as a binder (ground thoroughly with a mortar and pestle). The acquired 16


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Fourier-transformed extended X-ray absorption fine structure (EXAFS) data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software package. The EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Electrochemical measurements of SIBs. The electrochemical performances of the samples were carried out via stainless-steel coin cells (CR 2016). Working electrodes were prepared by coating

a

slurry

mixed

70%

active

materials,

20%

super

P,

and

10%

sodium

carboxymethylcellulose (CMC) in deionized water onto a copper foil and dried at 100 °C in vacuum overnight. Glass fiber filter paper was used as a separator. The electrolyte was 1 M NaClO4 EC/DEC (1:1 w%) with 5% FEC. Metallic Na film was used as both reference and counter electrodes for CR2016 coin half cells and Na3V2(PO4)3 was used to assemble CR2032 coin full cells. For full cells, the CoZn-Se electrode was firstly discharged to 0.01 V at 100 mA g-1 in half-cell to compensate the loss of sodium during the initial cycle. CV curves (0.01V-3.0 V vs. Na+/Na) were recorded using an electrochemical workstation (MULTI AUTOLAB M204, Metrohm). The galvanostatic charge/discharge experiments were studied in a potential range of 0.01 V-3.0 V (vs. Na+/Na) using a multichannel battery testing system (Land CT 2001A). Electrochemical measurements of OER. The OER was measured on an electrochemical workstation (CHI 660E, Chenhua China) in a three-electrode cell, which a Pt wire and a SCE electrode (saturated KCl-filled) as a counter electrode and the reference electrode, respectively. And O2-saturated 1 M KOH solution was used as electrolyte. All the potentials were calibrated to the reversible hydrogen electrode (RHE) according to Nernst equation. In brief, to prepare the working electrode, 5.0 mg of sample was dispersed in 1 ml of water-isopropanol solution (V: V = 3:1) 17


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containing 40 μL Nafion solution, then sonicating for at least 1 h to obtain the catalyst ink. 20 mL of the ink was dropped onto the glassy carbon (GC) electrode (8.0 mm inner diameter) and dried at 50 oC. Before test, the cyclic voltammetry curves (CVs) were performed to activate the working electrode. The linear sweep voltammetry curves (LSVs) were collected at a scan rate of 2 mV s-1. The cyclic voltammetry curves (CVs) were measured in O2-satuated 1 M KOH at scan rates of 5, 10, 15, 20, 25 and 30 mV s-1, separately. The capacitive current measured at 1.20 V vs. RHE was plotted as a function of scan rate. The stability test was performed by i-t chronoamperometric response of the CoZn-Se hybrids during a constant potential of 1.55 V for 15 h. The electrochemical impedance spectroscopic analysis (EIS) datas were collected with frequency range of 0.1–100 kHz using an electrochemical workstation (MULTIAUTOLAB M204, Metrohm) at a constant overpotential of 550 mV (vs RHE). The TOF values were calculated by the following equation: TOF = jS/4nF; where j (mA cm-2) is the current density at η = 320 mV, S is the surface area of the working electrode, the number 4 means 4 electrons mol−1 of O2 , F is Faraday’s constant (~96485 C mol−1), and n is the moles of coated metal atom on the lectrode. DFT calculations. The calculations were based on the density functional theory (DFT) combined with the projector-augmented-wave (PAW) potential as implemented by the Vienna ab initio simulation package (VASP). To accurately describe the weak van der Waals interactions, the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA) with the approach of Grimme (DFT-D3) was adopted. The cutoff energy for the plane wave basis set is 500 eV, and the first Brillouin zone was sampled by the Monkhorst-Pack method with a 3×3×1 k-point grid. The bimetallic selenides structure was fully relaxed until the frce is less than 0.01 eV/Å, and the convergence threshold for the total energy is 10-6 eV. The Na+ adsorption was studied to 18


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demonstrate theoretical proof for the electrocatalytic performance. Na+ adsorption energies were calculated relative to Na atom were defined as: ∆E=E(slab+Na)-E(slab)-E(Na).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website: Figures S1−S20 and Tables S1-S3. The authors declare no competing financial interest.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. G.Z.F. and Q.C.W. contributed equally.

ACKNOWLEDGMENTS This work was supported by National Key R&D Program of China (2018YFB0704100), National Natural Science Foundation of China (Grant no. 51802356, 51872334 and 51572299), Innovation-Driven Project of Central South University (No. 2018CX004). Y. Lei thanks the 19


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financial support from the Changsha Science and Technology Plan (kq1801065), the Hunan Provincial Science and Technology Plan Project (No. 2017TP1001). We also thank the 1W1B station for XAFS measurements in Beijing Synchrotron Radiation Facility (BSRF).

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Figures and Captions

Fig. 1 Scheme of phase-boundary effect in CoZn-Se.

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Fig. 2 Material characterization. (a) XRD patterns of CoZn-Se, ZnSe and CoSe2; (b) Elemental (Co, Zn and Se) line scanning, (c) HRTEM image and (d) Elemental mappings (Zn, Co, Se, C and N) of CoZn-Se; (e) Co K-edge XANES spectra and (f) Co K-edge EXAFS spectra of CoSe2 and CoZn-Se; Wavelet transforms for (g) The k2-weighted Zn K-edge EXAFS signals for ZnSe and CoZn-Se and (h) The k2-weighted Co K-edge EXAFS signals for CoSe2 and CoZn-Se.

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ACS Nano

Fig. 3 Na+ storage performance. (a) The second CV curves at 0.1 mV s-1 in the voltage range of 0.01-3.0 V vs Na+/Na of CoZn-Se, ZnSe and CoSe2; (b) Representative galvanostatic charge/discharge curves at 0.1 A g-1 of CoZn-Se; (c) Cycling performance at 0.1 A g-1 and (d) Rate capability from 0.1 A g-1 to 10 A g-1 of CoZn-Se, ZnSe, CoSe2 and CoSe2/ZnSe; (e) Long cycling performance at 8 A g-1 and 10 A g-1 of CoZn-Se; (f) Schematic of Na3V2(PO4)3‖CoZn-Se full cell; (g) Cycle performance at 1 A g-1 of full cell.

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Fig. 4 Experimental and theoretical analysis of reaction kinetics. GITT curves and the corresponding Na+ diffusion coefficient at (a) The 1st discharge process and (b) The 1st charge process; (c) CV curves at different scan rates, inset: log(i) vs. log(v) plots at specific peak currents; (d) Calculated density of states (DOS) of CoZn-Se, CoSe2 and ZnSe; (e) The computed differential charge density between CoSe2 and ZnSe in phase boundaries, red and yellow bubbles represent the electron accumulation and depletion, respectively; (f) Planar and macroscopic averaged electrostatic potential; (g) Calculations of Na+ adsorption energy of CoZn-Se, CoSe2 and ZnSe.

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Fig. 5 Electrochemical reaction mechanism. Operando XRD patterns and corresponding discharge/charge curve of (a) The 1st discharge process, (b) The 1st charge and 2nd discharge process and (c) Selected operando XRD patterns from Fig. 5a and b; (d and f) Ex situ SAED patterns and (e and g) Ex situ HRTEM images of (d and e) The 1st full discharge state and (f and g) The 1st full charge state; (h) Elemental (Co, Zn and Se) line scanning of CoZn-Se nanosheet after 50 cycles; (i) Ex situ HRTEM images of CoZn-Se electrodes at 50th full charge state.

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Fig. 6 (a) LSVs of CoZn-Se, ZnSe, CoSe2 and RuO2/C for OER. (b) The values of overpotential at 10 mA cm−2 for CoZn-Se and various OER catalysts reported. (c) Tafel plots of ZnSe, CoSe2 and CoZn-Se and RuO2/C. (d) CV curves of CoZn-Se at 5, 10, 15, 20, 25 and 30 mV s-1. (e) The capacitive current measured at 1.20 V vs RHE was plotted as a function of scan rate. (f) Chronoamperometric response at 1.55 V to assess the OER stability of CoZn-Se hybrids in 1 M KOH solution. The inset shows OER polarization curves of CoZn-Se before and after 2000 cycles.

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Metal Organic Framework-Templated Synthesis of Bimetallic

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