graphene sandwich-structured composites with enhanced oxygen reduction catalytic performance

Accepted Manuscript Intercalation assembly of Li3VO4 nanoribbons/graphene sandwich-structured composites with enhanced oxygen reduction catalytic performance K. Huang, Q.N. Ling, C.H. Huang, K. Bi, W.J. Wang, T.Z. Yang, Y.K. Lu, J. Liu, R. Zhang, D.Y. Fan, Y.G. Wang, Ming Lei PII:

S0925-8388(15)01483-8

DOI:

10.1016/j.jallcom.2015.05.156

Reference:

JALCOM 34278

To appear in:

Journal of Alloys and Compounds

Received Date: 20 April 2015 Revised Date:

7 May 2015

Accepted Date: 17 May 2015

Please cite this article as: K. Huang, Q.N. Ling, C.H. Huang, K. Bi, W.J. Wang, T.Z. Yang, Y.K. Lu, J. Liu, R. Zhang, D.Y. Fan, Y.G. Wang, M. Lei, Intercalation assembly of Li3VO4 nanoribbons/graphene sandwich-structured composites with enhanced oxygen reduction catalytic performance, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.05.156. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Intercalation assembly of Li3VO4 nanoribbons/graphene sandwich-structured composites with enhanced oxygen reduction catalytic performance

Liuc,*, R. Zhanga, D.Y. Fana, Y.G. Wanga, Ming Leia,* a

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K. Huanga, Q.N. Linga, C.H. Huanga, K. Bia, W.J. Wangb, T.Z. Yangb, Y.K.Luc, J.

State Key Laboratory of Information Photonics and Optical Communications &

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School of Science, Beijing University of Posts and Telecommunications, Beijing

b

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100876, China

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics,

Chinese Academy of Sciences, Beijing 100190, China c

School of Materials Science and Engineering, Central South University, Changsha,

Abstract

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Hunan, 410083, China

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Novel sandwich-like nanocomposites of alternative stacked ultrathin Li3VO4

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nanoribbons and graphene sheets (LVO-G) were successfully developed by a facile intercalation assembly method with a post heating treatment. The characterization results demonstrate that the average size of the Li3VO4 nanoribbons with a non-layered crystal structure is a few micrometers in length, 50-100 nanometers in width and a few atomic layers in height. The addition of graphene sheets can modify the preferred orientation of the Li3VO4 nanoribbons from (110) to (011) plane and restrict the growth of impurity phase at the same time. In addition, EIS analysis has

ACCEPTED MANUSCRIPT also verified the reduced resistance and thus the enhance conductivity of LVO-G nanocomposites compared with bare Li3VO4 nanoribbons. What’s more, the electrocatalytic performances of these novel LVO-G nanocomposites for oxygen

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reduction reaction (ORR) in alkaline solution are further investigated by cyclic voltammetry (CV), rotating disk electrode (RDE) and chronoamperometry test. It is found that the enhanced activity and stability of LVO-G can be attributed to the

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synergistic effect between the Li3VO4 nanoribbons and graphene sheets with a larger

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reduction current density and a smaller onset potential value for LVO-G25 compared with LVO-G50 due to the change of components.

Keywords: Sandwich-like, Li3VO4 nanoribbons, Graphene, Nanocomposites, ORR

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* Corresponding author. Tel./Fax.: + 86 10 62282242.

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E-mail address: [email protected] (J. Liu); [email protected] (M. Lei)

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1. Introduction

Because of their fundamental interest, as well as potential applications in

nanostructure-based photonics and electronics, considerable attention has been focused on the syntheses and properties of semiconductor nanostructures [1-25]. Recently, a great scientific fervor has been brought about in the field of two-dimensional (2D) materials since the first discover of graphene by Novoselov and his co-workers in 2004 due to the superior electrical conductivity and mechanical

ACCEPTED MANUSCRIPT properties [26-28]. For example, other 2D “graphene-like” nanosheets or nanoribbons with a thickness of less than 5 nm and lateral dimension of submicron to micrometers such as layered transition metal chalcogenides (MoS2, WS2, TiSe2, Bi2Se3) [29-31],

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metal carbides (Ti3C2,Ti2C, Ta4C3) [32], nitride (BN) [33], oxides (MoO3, V2O5, MnO2) [34-36] and double hydroxides systems (Ni-Fe, Co-Mn, Co-Al LDHs) [37-39], have been widely adopted in the important fields of sensing, catalysis, and energy

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storage applications [40-42]. Generally, although the bulk counterparts of the

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aforementioned nanoribbons or nanosheets have the typical layer structures, the exfoliation procedure always brought about an extremely low product yield due to the limitations of the fabrication method. Moreover, the preparation of novel two-dimensional nanosheets or nanoribbons by exploiting other materials without

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layer crystal structures still remains to be a significant challenge for both scientific researches and industrial applications.

Compared with the layer structured two-dimensional materials of V2O5 and LiVO2,

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Li3VO4 has an orthorhombic structure which is built up of oxygen atoms in

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approximate hexagonal close packing with ordered tetrahedral sites occupied by cations and empty and interconnected octahedral sites along the c axis. Due to this unique crystal structure, Li3VO4 has a high ionic conductivity which can be used as an ionic conductor but a quite low electronic conductivity which may results in a large resistance polarization. Many efforts have been made to improve the electrochemical performance by reducing the particle size and hybridization with electronically conductive carbon materials especially with graphene, which results in enhanced

ACCEPTED MANUSCRIPT performance compared with bare Li3VO4 anode for lithium-ion batteries indeed [43-46]. Meanwhile, some two-dimensional layer-like oxygen reduction reaction (ORR) catalysts such as N-doped single-layer graphene [47], B and N co-doped

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graphite layers [48], transition metal di-chalcogenides (MoSe2, WS2, WSe2) [49], iron polyphthalocyanine [50] and Co0.85Se/graphene hybrid nanosheets [51] have been widely investigated in the past five years. Even the layered SiC sheets have also been

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calculated to be a potential effective catalysts for ORR based on the density functional

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theory [52]. Thus, it is of great interest to develop and investigate the characteristics and electrochemical performance of two-dimensional Li3VO4 nanostuctures as well as their hybrids with graphene sheets.

In this work, we successfully synthesized ultrathin Li3VO4 nanoribbons with a

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thickness of a few atomic layers by immersing V2O5 nanosheets in LiNO3 solution, and further developed the sandwich-like LVO-G nanocomposites by electrostatic bonding with positively charged graphene oxide nanosheets using a facile

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intercalation assembly method with a post annealing under inert atmosphere. The

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resultant LVO-G nanocomposites show enhanced ORR activity and stability compared with bare Li3VO4 nanoribbons, which are believed to be resulted from the enhanced conductivity and the existence of abundant interspace among different layers.

2. Experimental 2.1. Preparation of LVO-G nanocomposites

ACCEPTED MANUSCRIPT Graphene oxide (GO) was first prepared by the modified hummers’ method [53], then dispersed in aqueous solution (ethanol: water = 3:1) with the aid of ultrasonication for 30 min to form a uniform solution (2 g/L) and further positively

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charged with 0.6 mg aminopropyltrimeth-oxysilane (APS) under vigorous stirring for another 30 min. Meanwhile, Li3VO4 nanoribbons were synthesized by adding LiNO3 solution dropwise into the sol-gel of V2O5 nanobelts according to the previous work

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by Li et al. [54] with a vigorous stirring for 30 min. The weights of LiNO3 and V2O5

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were controlled to be of 4.1 and 0.5 mmol, respectively. As for LVO-G nanocomposites, 23 or 69 mL dispersion of APS modified GO solution were added into the mixture above with a continuous stirring. The resulting solution was dried overnight in an oven at 90 oC for 12 h, following by heating at 550 oC under Ar

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atmosphere with a flow rate of 400 sccm for 2 h and suddenly cooled by taking off from the furnace. The obtained samples were labeled as LVO-G25 and LVO-G50,

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respectively.

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2.2. Characterization

Powder X-ray diffraction (XRD) patterns were obtained using an X'pert PRO

X-ray diffractometer (Panalytical, Netherlands) from 15° to 75° with CuKα as radiation source (λ = 0.15406 nm). Morphological analysis was performed with a field-emission scanning electron microscope (FESEM, Hitachi, S-4800), an atomic force microscope (AFM, Picoplus) and a JEOL-JEM-2100F transmission electron microscope (TEM) operating at a 200 kV accelerating voltage.

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2.3. Electrochemical measurement

The electrochemical tests such as cyclic voltammetry (CV), Rotating disk

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electrode (RDE) measurements and chronoamperometry (i-t curve) were performed on a CHI660E electrochemical workstation with a three-electrode system in 0.1 M

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KOH electrolyte. Typically, 0.5 mg cm-2 of catalysts loaded on a glassy carbon electrode (5 mm in diameter) as the working electrode, a Pt foil as the counter

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electrode and an Hg/HgO electrode as the reference electrode which was calibrated with respect to reversible hydrogen electrode (RHE) by E(RHE) = E(Hg/HgO) + 0.92 V. For the detailed procedure of the electrode preparation, 5 mg of as-prepared samples were dispersed in the mixture of 700 µl of ultrapure water, 300 µl of iso-propanol and

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20 µl of Nafion solution (5.0 wt.%) by sonication for 30 min in ice-bath to form the homogeneous ink, then 20 µl of the catalysts ink was loaded onto the glassy carbon

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electrode at three steps and dried under ambient temperature. Specially, the bare LVO catalyst was made of mixing Li3VO4 with Vulcan XC-72R carbon powder at a ratio of

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3:1. EIS spectra were further acquired in the constant voltage mode with the frequency ranging from 100 kHz to 1 Hz using a freguency response analyzer controlled by a potentiostat (Autolab PGSTAT-204) at the open circuit potential.

3. Results and discussion As the XRD patterns shown in Fig. 1, a series of sharp and high intensity peaks can be found which indicates the well crystallization of all samples. Most diffraction

ACCEPTED MANUSCRIPT peaks of all samples can be indexed to the orthorhombic Li3VO4 phase (JCPDS no. 38-1247) with few peaks belonging to small amount of LiVO3 phase (JCPDS no. 25-1100) which was also reported in previous works by different methods [55,56].

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Meanwhile, it is found that the intensity of LiVO3 phase in the LVO-G nanocomposite is much weaker than that in the pure Li3VO4 nanoribbons which suggests the addition of graphene sheets can inhibit the growth of LiVO3. What’s more, the relative

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intensity of (011) plane of LVO-G is quite stronger than that of Li3VO4 and other

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peaks which indicates the anisotropic growth of the nanostructure as well as the preferred orientation (011) plane of the Li3VO4 nanoribbons on the sheets of graphene sheets. The differences between LVO and LVO-G in XRD patterns can be attributed to the charge interaction between electronegativity Li3VO4 nanoribbons and positively

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charged graphene sheets, which will not only prevent the Li3VO4 growing along the original direction of (110) plane and increase the explosion of (011) plane, but also facilitate the insertion of Li+ to enhance the purity of Li3VO4 phase. In addition, the

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absence of diffraction peaks for graphene may result from the destroyed regular stacks

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of few graphene layers by Li3VO4 nanoribbons [57]. Morphology analysises characterized by field-emission scanning electron

microscope (FE-SEM) and transmission electron microscope (TEM) of LVO-G nanocomposites are shown in Fig. 2. It is obvious that LVO-G nanocomposites exhibit a typical sandwich-like structure layer-by-layer with plenty of interspace (Fig. 2A). TEM and HR-TEM images further reveal that LVO-G nanocomposites are composed of Li3VO4 nanoribbons and graphene nanosheets. We can clearly see that plentiful

ACCEPTED MANUSCRIPT Li3VO4 nanoribbons which is almost completely transparent to the electron beam distribute disorderly among the wrinkled graphene layers (Fig. 2B). Moreover, lattice fringes of Li3VO4 nanoribbons with an interfringe spacing of 0.389 nm corresponding

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to the d-spacing of (011) plane of Li3VO4 can be identified in Fig. 2C, the selected area electron diffraction (SAED) pattern (Fig. 2D) for Li3VO4 nanoribbons also exhibits well-arranged polycrystal rings, which stands for (011), (101), (200) crystal

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planes from inner to exterior respectively. All the TEM results are consistent with the

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XRD analysis in Fig. 1. AFM image (Fig. 2E) demonstrates the thickness of the Li3VO4 nanoribbons is about 3 nm which corresponds to a few atomic layers in height. Therefore, the average size of the Li3VO4 nanoribbons can be defined as a few micrometers in length, 50-100 nanometers in width and a few atomic layers in height.

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Considering the existence of abundant interspace in this novel sandwich-like structure which will provide adequate contact between the LVO-G nanocomposite and electrolyte, the ORR electrocatalytic performances were further investigated. Fig. 3

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shows the CV curves of LVO, LVO-G25 and LVO-G50 electrodes in both

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N2-saturated and O2-saturated 0.1 M KOH at a scan rate of 50 mV s-1. It is found that all LVO-based electrodes are featureless for ORR in the N2-saturated KOH solution and the current density increases with the addition of graphene sheets, while the oxygen reduction current density of LVO-G25 is the largest in the O2-saturated electrolyte. The possible reasons may be attributed to the enhanced conductivity of LVO-G and reduced interspace of LVO-G50 with respect to LVO-G25. Furthermore, the ORR polarization curves at 1600 rpm for all above electrodes are exhibited in Fig.

ACCEPTED MANUSCRIPT 4A. An obvious increase of onset potential related to the addition amount of graphene sheets can be found as 0.802 V for LVO, 0.837 V for LVO-G25 and 0.854 V for LVO-G50. However, the highest current density below 0.6 V is obtained from

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LVO-G25 electrode which is consistent with the result of CV tests in O2-saturated KOH electrolyte. To further illustrate the enhanced performance of LVO-G25, another important ORR kinetic parameters j0 (the exchange current density) can be calculated

η = a + b lg | j | , where a = −

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according to the Tafel equations:

2.3RT 2.3 RT lg | j0 | , b = α nF α nF

(1)

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According to the plots of applied electrode potential ( η ) as a function of lg (j) shown in Fig. 4B, the slopes obtained are 150.7, 80.9 and 80.7 mV dev-1 while the intercepts are 0.860, 0.854 and 0.873 V for LVO, LVO-G25 and LVO-G50, respectively.

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Obviously, the corresponding exchange current densities can be calculated to be about

1.965 ×10−3 , 2.778 ×10−8 and 1.521×10−8 mA cm-2. It is worth pointing out that the advantage of LVO-G25 than LVO-G50 for ORR may result from the higher value of

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j0 with the almost same slopes (thus the same transfer coefficient α ). In addition, it is

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clear that the ORR current density increases with the acceleration of rotate rate (Fig. 4C), thus the transferred electrons numbers at different potentials for various electrodes during the ORR process are further given in Table 1, which are worked out based on the Koutechy-Levich theory referring to the previous reports [58,59]. Both LVO-G25 and LVO-50 exhibit enlarged number compared with bare LVO sample due to the introduction of highly conductive graphene sheets. EIS spectra in Fig. 5 are further obtained to investigate the conductivity

ACCEPTED MANUSCRIPT enhancement where the high-frequency region can be associated with the charge-transfer process as well as the properties of electrochemical reaction resistance, and the low frequency straight lines relate to the properties of the diffusion process.

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The equivalent circuit model for the impedance spectra is also shown in the inset, where R is the combination of solution resistance and the film resistance, Rct is represented of charge-transfer resistance while CPE and W are the capacitance of the

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double layer capacitance and the Warburg impedance, respectively. As shown in Table

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2, the Rct values of LVO-G are much smaller than that of bare LVO electrode while the R value of LVO-25 is the lowest which corresponds to the lowest film resistance among all LVO-based electrodes and responds for the enhanced performance of LVO-G25.

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As the durability of catalysts is an another important issue, Fig. 6 shows the chronoamperometric responses of LVO, LVO-G25 and LVO-G50 electrodes at 0.65 V in O2-saturated 0.1 M KOH at 1600 rpm within 10000 s. The relative current densities

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of LVO-G25 and LVO-G50 remain at a level of 83.2 % and 81.5% while the LVO

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electrode lose 32.5 % of initial values. It is clear that the LVO-G nanocomposites catalysts own superior stability compared with bare LVO electrodes. As demonstrated by Jian et al. that LiVO3 phase is an intercalation materials which has a very small effect on the performance of a Li3VO4 electrode [60], the enhanced durability of LVO-G can be attributed to the chemical interaction among the different layers of Li3VO4 nanoribbons and graphene sheets which can decrease the corrosion of surface V atoms by alkaline solution [61].

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4. Conclusion In summary, novel sandwich-like nanocomposites of LVO-G exhibiting a unique

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hierarchical morphology with the ultrathin Li3VO4 nanoribbons combined with graphene sheets layer-by-layer alternatively have been revealed. The introduction of graphene has been found to crucial for the inhibition of impurity LiVO3 phase and the

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modification of preferred orientation of the Li3VO4. Moreover, enhanced activity and

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stability of LVO-G electrodes for oxygen reduction reaction electrocatalysis compared with bare LVO electrode have also been obtained due to the improved conductivity and abundant interspace which can act as the pathway for electrochemical reactant and product. Considering the fascinating structural feature and enhanced

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electrochemical performance of LVO-G, our work will provide a prospect of exploiting other graphene based sandwich-like composites for catalysis and energy

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storage applications.

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Acknowledgments

This work was financially supported by The National Basic Research Program of

China (Grant No. 2013CB932901), Program for New Century Excellent Talents in University (NCET-13-0684), Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications, P. R. China), and National Natural Science Foundation of China (Grant nos. 61376018, 61377097, 51102019, 61177085, 51272031, 51472033).

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ACCEPTED MANUSCRIPT Figure captions Fig. 1. XRD patterns of as-prepared Li3VO4 nanoribbon and LVO/G anocomposite. Fig. 2. (A) SEM image of the LVO/G25 nanocomposite; (B, C and D) TEM,

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HR-TEM and SAED images of the LVO/G25 nanocomposite; (E) AFM image with the corresponding height profile for ultrathin Li3VO4 nanoribbons.

Fig. 3. Cyclic voltammetry curves using LVO, LVO-G25 and LVO-50 samples as

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ORR catalysts in N2-saturated (A) and O2-saturated (B) 0.1 M KOH solution.

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Fig. 4. ORR polarization curves (A) and Tafel plots (B) of LVO, LVO-G25 and LVO-G50 catalysts at 1600 rpm and (C) ORR polarization curves of LVO-25 at different rotate speeds in O2-saturated 0.1M KOH solution: Scan rate of 5 mV s-1. Fig. 5. The chronoamperometric responses of LVO, LVO-G25 and LVO-G50

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electrodes at 0.65 V in O2-saturated 0.1 M KOH at 1600 rpm for 10000 s. Fig. 6. Nyquist plots corresponding to LVO, LVO-G25 and LVO-G50 electrodes in 0.1 M KOH solution from 100 kHz-1 Hz. In the insets the regions of low impedance

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and the equivalent circuit are shown.

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Figures

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Fig. 1

Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

Fig. 6

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0.2 V 2.05 3.05 3.08

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Table 1 The transferred electrons numbers at different potentials. Sample 0.6 V 0.5 V 0.4 V 0.3 V LVO 1.92 1.91 1.89 1.98 LVO-G25 2.30 2.29 2.27 2.39 LVO-G50 2.34 2.25 2.21 2.37

54.549 45.879 52.955

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LVO LVO-G25 LVO-G50

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Table 2 The fitting parameters of Nyquist Plots Sample R /Ω (CPE /F) 0.857 0.869 0.815

(W/uF) 3.3 5.5 3.4

(Rct /k Ω ) 4.073 1.175 0.355

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Novel sandwich-structured LVO-G by a facile intercalation assembly method.

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Addition of G sheets can modify the preferred orientation of Li3VO4 nanoribbon.

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▲ Enhanced ORR activity and stability due to synergistic effect are demonstrated.