Ti–V–Ni with graphene-mixing icosahedral quasicrystalline composites: Preparation, structure and its application in Ni–MH rechargeable batteries

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TieVeNi with graphene-mixing icosahedral quasicrystalline composites: Preparation, structure and its application in NieMH rechargeable batteries Jing Lin a,b, Chong Lu a,b, Lianshan Sun a,b, Fei Liang b,c, Zhanyi Cao a,**, Limin Wang b,c,* a

College of Materials Science and Engineering, Jilin University, Changchun 130022, China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China c Changzhou Institute of Energy Storage Materials and Devices, Changzhou 213000, China b

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abstract

Article history:

The TieVeNiquasicrystalline was prepared by arc-melting and subsequent melt-spinning

Received 22 July 2015

technique, and the Ti1.4V0.6Ni þ x graphene (x ¼ 3, 5, 7, 10 and 13, wt.%) composites were

Received in revised form

obtained by mechanical ball-milling method. The structures and electrochemical hydrogen

12 November 2015

storage properties of the composites were investigated. The results showed that, the

Accepted 13 November 2015

structures of the composites contained icosahedral quasicrystal, Ti2Ni-type, NiTi and

Available online 17 December 2015

graphene phases. The electrochemical hydrogen storage properties of the composites were improved with graphene addition. The cycling stabilities after 50 charging/discharging

Keywords:

cycles of Ti1.4V0.6Ni þ x graphene composites were improved obviously, especially the

TieVeNi quasicrystalline

Ti1.4V0.6Ni þ 10 graphene composite showed the best cycling stability of 70.3%, increased by

Graphene

6.5%, compared with the Ti1.4V0.6Ni electrode. The high-rate discharge abilities of the

Composites

composites were also increased appreciably. The improvement in the hydrogen storage

Hydrogen storage property

characteristics should own to the proper graphene addition and that could cause copacetic

NieMH batteries

electrocatalytic activity and anti-corrosion ability. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen storage materials have attracted increasing interest for designing future clean energy systems. Much of the research has examined the hydrogen storage properties of icosahedral quasicrystalline phase (I-phase) since it was first

observed in 1984 [1,2]. In particular, the Ti-based I-phase quasicrystals with hydrogen concentration reached 67 at% have been obtained [3e7]. Hu et al. firstly reported the electrochemical

performances

of

TieVeNiquasicrystallineas

active material of negative electrode for NieMH battery and the maximum discharge capacity could reach 271.3 mAh g1 [8]. Afterwards, considerable efforts have been devoted to

* Corresponding author. Tel.: þ86 431 8526 2447; fax: þ86 431 8526 2836. ** Corresponding author. E-mail addresses: [email protected] (Z. Cao), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.11.067 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 1 ( 2 0 1 6 ) 1 0 9 8 e1 1 0 3

enhance the overall electrochemical properties of TieVeNi quasicrystalline which have mainly involved partial element substitution [9,10], heat treatment [11], and composite formation [12,13]. Graphene is a newly discovered single layer graphite phase with a high specific area and electrical conductivity, which make it ideal as a dispersive phase designing the composite material [14]. Hydrogen is also adsorbed on graphene surfaces as sp3 CeH bonds, which enables the viability of graphene for hydrogen storage [15]. Additionally, graphene has a hydrophobic nature due to the non-polar covalent double bonds, which prevent hydrogen bonding with water and has potential as an ultrathin protective coating especially in protection of alloys from corrosion in marine environment [16]. Ghosh et al. has prepared graphene by the exfoliation of graphitic oxide and the hydrogen uptake at 100 atm and 298 K was found to be 3 wt.% [17]. Srinivas et al. gained graphene by the reduction of graphite oxide. At pressures of up to 10 bar, the hydrogen adsorption capacities were 1.2 wt.% and 0.1 wt.% at 77 K and 298 K, respectively [18]. Huang and coworkers studied the effects of graphene/Ag nanocomposite additives on the electrochemical properties of Mg-based alloy and found that the maximum capacity was higher than the alloy modification with single Ag [19]. Ouyang et al. further enhanced the rate discharge properties of AB3.0 alloy with graphene addition [20]. In our previous work, the crystallization, microstructure and hydrogen storage properties for various Ti-based I-phase composite materials were studied [7e13]. The I-phase structure was found to be the key factor to store hydrogen by electrochemistry method. However, little attention has been paid to the surface modification of the Ti-based quasicrystalline to improve the electrochemical performance. Novel TieVeNi and graphene composites were fabricated, and their electrochemical hydrogen storage properties for the NieMH rechargeable batteries were investigated.

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Transmission electron microscope (TEM) was used to obtain selected-area electron diffraction patterns of Ti1.4V0.6Ni ribbon.

Electrochemical measurement The electrode was constructed through mixing alloy powders with nickel powders in a weight ratio of 1:5 under 15 MPa pressure to form a pellet of 10 mm in diameter and 1.5 mm in thickness. Electrochemical hydrogen storage properties were measured at room temperature in a standard three electrode NieMH cell system consisting of a working electrode, a sintered Ni(OH)2/NiOOH counter electrode and an Hg/HgO reference electrode immersed in 6 M KOH as the electrolyte. The electrodes were fully charged at current density of 60 mA g1 for 6 h and discharged at 30 mA g1 to 0.6 V versus the reference electrode. The cycling stabilities after 50 charging/ discharging cycles were represented by S50 (%), which was calculated by S50 (%) ¼ C50/Cmax, where C50was the discharge capacity at the fiftieth cycle, Cmaxwas the maximum discharge capacity of the alloy electrode. Then capacity retention CR (%) was obtained by CR (%) ¼ (1  2Cb/(Ca þ Cc)), where Cb was the discharge capacity after the open-circuit condition for 24 h, and Ca and Cc was the capacity before and after the measurement of Cb, respectively. Electrochemical impedance spectroscopy (EIS) analysis and potential-step measurement were tested by the electrochemical workstation (Biologic Inc.). EIS was conducted at 50% depth of discharge in the frequency range of 102 e 105 Hz. For potentiostatic discharge, the electrodes in the fully charged state were discharged with þ500 mV potential-step for 3600 s.

Results and discussion Phase structure

Experimental Materials and preparation

The phases of Ti1.4V0.6Ni þ x graphene composites are identified in Fig. 1. The diffraction peaks of Ti1.4V0.6Ni include Iphase, Ti2Ni-type FCC phase and NiTi with a simple cubic

Titanium (>99.7 wt.%), Vanadium(>99.9 wt.%), Nickel (>99.7 wt.%), and purchased graphene powders (1e10 layers). The Ti1.4V0.6Ni ingot was prepared by arc-melting in a watercooled copper hearth under argon atmosphere, and then melt-spun onto a copper wheel rotating at 34 m s1. The obtained ribbons were ground to powder of 200e300 mesh and mixed with graphene by mechanical milled in high energy ball mill for 15 min under argon gas. Stainless steel vial and balls at the ball to powder weight ratio of 10:1 were used. Weighting, filling, and handling of the powders were performed in a glove box filled with argon gas.

Characterization The phase structures of the composites were analyzed by a Bruker D8 focus powder X-ray diffractometer using Cu Ka radiation at a scan rate of 4 min1 (XRD). The morphology and elemental analysis were characterized by scanning electron microscopy (SEM)at an accelerating voltage of 10 kV.

Fig. 1 e XRD patterns of Ti1.4V0.6Ni þ x graphene composites.

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structure. After mechanical ball-milling of graphene, the peaks of graphene are demonstrated evidently as well as the intensity of diffraction peaks corresponding to graphene addition increases with growing x from 3 to 13. Simultaneously, the peaks of Ti2Ni-type FCC phase become slightly broaden due to the high-energy impact of the milling balls. Fig. 2(a) is the bright-field TEM image of Ti1.4V0.6Ni alloy displaying the typical growth morphology of I-phase nodules. In diffraction studies of the point group symmetry of I-phase, the expected 5-fold symmetry pattern is manifested in Fig. 2(b).

Discharge capacity and cycling stability The electrochemical discharge capacities and cycle performances of Ti1.4V0.6Ni þ x graphene electrodes are displayed in Fig. 3. In the experimental NieMH batteries, graphene addition has little effect on the maximum discharge capacities of the batteries, among them, the maximum discharge capacities of Ti1.4V0.6Ni þ 7 graphene and Ti1.4V0.6Ni þ 10 graphene are higher than the mother alloy, and the Ti1.4V0.6Ni þ 10 graphene electrode shows the highest maximum discharge capacity of 273.2 mAh g1. However, the maximum discharge capacities of Ti1.4V0.6Ni þ x graphene composites increase at first and then decrease with increasing the amount of graphene from 3 to 13. Therefore, the enhanced discharge capabilities of samples probably are due to the addition of appropriate proportion of graphene powder. Mixed graphene may lead to the improvement of electrocatalytic activity and kinetics properties of the electrodes. All the electrodes can be activated at two cycles, then the discharge capacities of graphene-mixing composites decay on the same trend with Ti1.4V0.6Ni. After activation, the discharge capacities of graphene-mixing composites deteriorate more seriously than that of the mother alloy until the 25th cycle, subsequently, the velocity of capacity fading is slower clearly than Ti1.4V0.6Ni. And in consequence, after 50 consecutive cycles, the S50 of Ti1.4V0.6Ni þ graphene is better than Ti1.4V0.6Ni and the Ti1.4V0.6N þ 10 graphene shows the best

Fig. 3 e Cycle performances of Ti1.4V0.6Ni þ x graphene electrodes.

cycling stability of 70.3%, increased 6.5% compared with the Ti1.4V0.6Ni electrode. SEM image depicting the surface morphology of Ti1.4V0.6Ni þ 10 graphene composite is shown in Fig. 4(a). It can be seen that there are some holes in the particles which are beneficial to hydrogen diffusion implying that the positive hydrogen diffusion process is one of reasons for the outstanding discharge capacity. The appropriate proportion of graphene can prevent the agglomeration of alloy particles during the mechanical ball-milling progress, which is more beneficial to generate porous. The elemental mapping images revealed in Fig. 4(bee) exhibit that a slight amount of C is distributed in the surface of the Ti1.4V0.6Ni alloy particles, illustrated the alloy particles are encapsulated by graphene. The moderate amount of graphene encapsulated outside of the alloy can restrain the corrosion and pulverization of alloy electrodes in the electrolyte, reducing volume expansion and strengthen the stress to improve the electrochemical reaction. However, excess graphene is easy to separate from the surface of Ti1.4V0.6Ni during the chargeedischarge cycles, which leads to the more serious capacity fading. In addition, the excess graphene can decrease the porous during the mechanical ballmilling progress, going against the hydrogen diffusion.

High-rate discharge ability and kinetic properties

Fig. 2 e (a) TEM image of Ti1.4V0.6Ni alloy with its (b) 5-fold symmetry of I-phase.

These electrode performances at high discharge current density are investigated. Fig. 5 shows the discharge capacities of the Ti1.4V0.6N þ x graphene electrodes at different current densities from 30 mA g1 to 240 mA g1. As expected, the Ti1.4V0.6N þ 10 graphene electrode displays excellent high-rate performance and the discharge capacities of Ti1.4V0.6N þ x graphene composites are all higher than the mother alloy at the current density of 240 mA g1. The HRD of the metal hydride electrodes is influenced mainly by the electrochemical hydriding/dehydriding process, which is composed of the charge-transfer reaction at the electrode/electrolyte interface and the hydrogen diffusion within the bulk of alloy. Thus, the preferable rate performance may be attributed to the

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Fig. 4 e SEM image (a) and elemental analysis (bee) of Ti1.4V0.6Ni þ 10 graphene composite.

Fig. 5 e High-rate discharge abilities of Ti1.4V0.6Ni þ x graphene composites.

graphene which can enhance the electron transfer through the active materials during charging and discharging. In order to investigate the properties of charge-transfer reaction, EIS experiment are performed on the alloy electrodes. The EIS of all the electrodes and the equivalent circuit are illustrated in Fig. 6. It indicated that all the EIS curves consist of two semicircles followed with a straight line related to Warburg impedance. According to the analysis model proposed by Kuriyama et al., relatively larger semicircle in the medium frequency region represents the charge-transfer resistance (Rct) for the chargeedischarge process of hydrogen atoms over the entire alloy surface, wherein R1 is the electrolyte resistance between

Fig. 6 e EIS of Ti1.4V0.6Ni þ x graphene electrodes and the equivalent circuit.

the working and the reference electrode and R2 and C1 characterize the contact resistance and the contact capacitance between the current collector and the alloy pellet, respectively [21]. The contact resistance and the contact capacitance between alloy powders in the electrode pellet are described by R3 and C2, respectively. Rct and C3 depict the charge-transfer resistance and the double-layer capacitance on alloy particles, respectively. W is the Warburg resistance. On the basis of the circuit the charge-transfer resistances, Rct is obtained by means of fitting program zview. Table 1 gives the values of Rct and exchange current density I0 which is used to character the electrochemical activity for charge transfer at the metal/ electrolyte interface obtained by the following formula [21]:

Table 1 e Electrochemical behavior of Ti1.4V0.6Ni þ x graphene composites. Samples

CR/%

Rct/U

I0/mA g1

C240/C30 (%)

S50 (%)

D/  1011 cm2 s1

Ti1.4V0.6Ni x¼3 x¼5 x¼7 x ¼ 10 x ¼ 13

86.4 86.1 86.9 87.2 88.7 87.0

0.213 0.143 0.119 0.113 0.096 0.115

803.7 1197.1 1438.6 1514.9 1783.2 1457.4

77.3 78.5 79.4 79.5 80.8 78.9

63.8 65.0 64.8 68.7 70.3 69.3

6.45 7.61 8.07 8.22 8.88 8.14

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I0 ¼ RT=mFRct

(1)

where R, T, m and F are the gas constant, the absolute temperature, the effective mass of material and the Faraday constant, respectively. It is clear that the Rct decreases at first and then increases with increasing the amount of graphene from 3 to 13. The fitting results manifest that the Rct decreases from 0.143 U to 0.096 U and then increases to 0.115 U, accordingly, the I0 increases at first and decreases subsequently. The I0 rises from 1197.1 mA g1 to 1783.2 mA g1, then reduced to 1457.4 mA g1. Results indicate proper graphene addition is beneficial to the charge-transfer reaction at the electrode/ electrolyte interface and electrical conductivity. When graphene content is climbed from 3 to 10 wt.%, larger proportion of electrode surface is coated by graphene layer, meaning fewer V and Ti atoms experience corrosion degradation and higher electrical conductivity of the electrodes. Bearing this in mind, for composites containing graphene amount within 3e10 wt.%, the surface reaction kinetics enhanced with growing x. Meanwhile, it is found that proper graphene addition has a positive effect on the anode's self-discharge performance. After 24 h relaxation, the capacity retention of all the graphene-mixing TieVeNi I-phase electrodes preponderates over the graphene-free electrode. However, When the graphene increases to 13 wt.%, the Rct increases appreciably, and the I0 as well as the CR decrease correspondently. And this reason can also be come down to the desquamating of graphene. The diffusion coefficient of hydrogen in the bulk of alloy electrode is determined by the potential-step method. Fig. 7(a) gives semi-logarithmic plots of the anodic current versus the time response of as-prepared electrodes. Clearly from the spectra, the current-time response can be divided into two time domains after the application of over potential. According to the model of Zheng et al. [22], the diffusion coefficient in the bulk electrode, which is used to characterize the diffusion rate of hydrogen, estimated through the slope of the linear region of the corresponding plots by following formula:

Log i ¼ Log 6FD ðC0  Cs Þ = da2






 p2 = 2:303 D = a2 t

(2)

wherein D is the diffusion coefficient of hydrogen over a defined concentration range (cm2 s1); a, the radius of the spherical particle (cm); i, the diffusion current density (A g1); C0, the initial hydrogen concentration in the bulk electrode (mol cm3); Cs, the hydrogen concentration on the surface of alloy particles (mol cm3); d, the density of the hydrogen storage materials (g cm3); and t, the discharge time. Assuming that the alloys have a similar particle distribution with an average particle radius of 15 mm, the hydrogen diffusion coefficient D in the bulk electrodes is estimated by Eq. (2) and also listed in Table 1. Owning to the doped graphene, the D of Ti1.4V0.6Ni þ graphene is higher than the mother alloy. With the increase of graphene content from 3 to 13 wt.%, the D goes up from 7.61  1011 cm2 s1 to 8.88  1011 cm2 s1 and then decreases to 8.14  1011 cm2 s1, which implies appropriate amount of graphene content is favorable to hydrogen diffusion in the bulk of the alloy. It also indicates that the change of D value is consistent with that of high-rate of discharge ability. This suggests that the hydrogen diffusion process plays an important rolein controlling the electrochemical reaction.

Conclusions In summary, a series of graphene-mixing Ti-based icosahedral composites have been successfully synthesized. As negative electrode materials for NieMH battery, the electrochemical hydrogen storage properties of the electrodes have been improved obviously with high electrical conductivity graphene addition. Especially, the Ti1.4V0.6Ni þ 10 graphene composite displays the best cycling stability of 70.3%after 50 charging/discharging cycles, increased by 6.5%, compared with the mother alloy. Likewise, the remarkable high-rate discharge ability and positive charge retention are also acquired. Moreover, proper amount of graphene addition to Ti1.4V0.6Ni alloy leads to the charge-transfer resistance Rct decrease and D increase. It is thought that moderate amount of graphene prevents the corrosion and pulverization of alloy electrodes, and is beneficial to hydrogen diffusion and chargetransfer reaction, thus the advantageous electrochemical performances are expected. It is believed that the graphenemixing Ti-based quasicrystalline will greatly expand the range of anode choices and assist a long term effort in developing favourable discharge capacity electrodes for NieMH batteries.

Acknowledgements

Fig. 7 e Anodic current-time responses of Ti1.4V0.6Ni þ x graphene electrodes.

This work is financially supported by the National Natural Science Foundation of China (21373198, 21221061), Natural Science Foundation of Jiangsu Province (BK20141174) and Natural Science Foundation of Changzhou (cj20140016).

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