A Novel Type of BatteryâSupercapacitor Hybrid Device with Highly

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A Novel Type of Battery-supercapacitor Hybrid Device with Highly Switchable Dual Performances Based on Carbon Skeleton / Mg2Ni Free-standing Hydrogen Storage electrode Na Li, Yi Du, Qing-Ping Feng, Gui-Wen Huang, Hong-Mei Xiao, and Shao-Yun Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14271 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Applied Materials & Interfaces

A Novel Type of Battery-supercapacitor Hybrid Device with Highly Switchable Dual Performances Based on Carbon Skeleton / Mg2Ni Free-standing Hydrogen Storage electrode Na Li, †

†,§

Yi Du,

†,§

†

†

Qing-Ping Feng, *, Gui-Wen Huang, *, Hong-Mei Xiao,

†

and Shao-Yun Fu ‡

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190,

China ‡

College of Aerospace Engineering, Chongqing University, Chongqing 400044, China

§

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

__________________________________________________________________________________

ABSTRACT: The sharp proliferation of high power electronics and electrical vehicles has promoted growing demands for power sources with simultaneous performances of high energy and power densities. Under the circumstances, battery-supercapacitor hybrid devices are attracting considerable attention as they combine the advantages of both batteries and supercapacitors. Here, a novel type of hybrid device based on carbon skeleton / Mg2Ni free-standing electrode without the traditional nickel foam current collector is reported, which has been designed and fabricated through a dispersing-freeze drying method by employing the reduced graphene oxide (rGO) and multi-walled carbon nanotubes (MWCNTs) as hybrid skeleton. As a result, the Mg2Ni alloy is able to deliver a high discharge capacity of 644 mAh g-1 and more importantly, a high cycling stability with retention over 78% after 50 charge/discharge cycles has been achieved, which exceeds almost all the results ever reported on Mg2Ni alloy. Simultaneously, the electrode could also exhibit excellent supercapacitor performances including high specific capacities (296 F g-1) and outstanding cycling stability (100% retention after 100 cycles). Moreover, the hybrid device can switch between battery and supercapacitor modes immediately for need during application. These features make the C skeleton/alloy electrode a highly promising candidate for battery-supercapacitor hybrid devices with high power/energy density and favorable cycling stability. KEYWORDS:

Mg2Ni alloy, carbon nanotube, reduced graphene oxide, Nickel-Metal

Hydride battery, supercapacitor

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1. INTRODUCTION In the past several years, tremendous research efforts have been devoted to design and fabricate electrochemical energy storage systems as the energy storage has become a global concern. For the energy storage systems, high energy density and high power density are two desired performance targets that as known have been respectively achieved by rechargeable batteries and supercapacitors.

1-4

However, these

performances are often demanded simultaneously in complex applying situations but cannot be satisfied singly by either batteries or supercapacitors. Thus, combining the advantages of the two types of devices is a meaningful subject for the energy storage field. Under the circumstances, battery-supercapacitor hybrid device has been proposed and attracted growing attention with the expectation of achieving an energy source that owning high energy and power densities at the same time.

5

In general, the

battery-supercapacitor hybrid device could exceed the energy density of conventional supercapacitors and overcome the power density limitation of batteries due to the higher capacity of battery-type electrode and the introducing of capacitive electrode. Moreover, the advanced design of battery-type electrode could be utilized to ensure faster electrochemical kinetics.6, 7 Thus, it can be seen that the hybrid device has to be built based on a battery system. Up to now, many types of rechargeable batteries have been explored including lead (Pb)-acid, nickel-metal hydride (Ni/MH), nickel-cadmium (Ni/Cd), lithium (Li)-ion, lithium (Li) sulfur and lithium (Li)-ion polymer.

8-14

Among

which, the Ni/MH and Li-ion batteries are focused topics due to their high energy density and good environmental compatibility. Although the Li-ion batteries have been successfully applied in extensive fields, Ni-MH batteries still show advantages in safety, low-temperature performance and relatively low cost.15 Due to the aqueous electrolyte, Ni-MH battery possesses inherent ability of preventing explosion, which makes it applicable in fields that having strict safety requirements, such as large public buses, trucks, or military vehicles.16 On the other hand, benefitting from its good low-temperature performance, Ni-MH battery can be used in systems that service in extreme cold area where the Li-ion batteries cannot work normally.17, 18 In addition, with the development of the new type of Mg2Ni hydrogen storage alloys owing high ACS Paragon Plus Environment

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theoretical capacity of 1000 mAh g-1, it is reasonable to expect the come of performance enhanced Ni-MH battery in the near future. However, to achieve this goal, it has firstly to solve the prominent problems of the Mg2Ni alloys: poor hydriding and dehydriding properties and the short charge/discharge cycle life.9, 19, 20 Lots of efforts have been done to improve the properties of Mg2Ni alloys, including building amorphous or nanocrystalline structures,

9, 21

forming composites by

partly substituting Mg and/or Ni with certain elements 22-24 and surface modifying with functional materials.

23, 25, 26

In these methods, surface modifying is an effective way to

improve the overall property of the alloy due to its multiple options and good operability. It was reported that the carbon-based additives display beneficial effects on improving the de/hydrogenation of the alloys or active materials.

27-30

Furthermore, it has been

demonstrated in our previous work that the discharge capacity and cycle stability of the Mg2Ni alloy can be greatly improved by surface modifying with carbon material.31 Meanwhile, the C-based materials have been demonstrated to be promising candidates in fabricating supercapacitors with high performances. Therefore, introducing C-based materials into the Ni-MH battery system delivers the double benefits: on one hand, the de/hydrogenation and cycling properties of the Mg2Ni alloy can be greatly enhanced; on the other hand, the C-based materials could bring the system excellent capacitive performance, which exactly fits the original intention of this work: to build a new type of high performance battery-supercapacitor hybrid device. Consequently, in this work, by utilizing a dispersing-freeze drying method, a free standing “C skeleton/ Mg2Ni alloys” electrode without the traditional nickel foam current collector was obtained. Reduced graphene oxide (rGO) and multiwalled carbon nanotubes (MWCNTs) hybrid system was chosen as the C skeleton because of their good hydrophilicity and the mixed-effects of the one-dimensional and two-dimensional materials in fabrication of high-performance supercapacitors.32, 33 After processing, the rGO and MWCNTs were promiscuously coated and twisted around the Mg2Ni alloy as schematically shown in Figure 1a. This configuration allows an easy diffusion of hydrogen, an excellent anti-corrosion ability and a high kinetics of charge transfer, which have been demonstrated by a series of electrochemical measurements. As a result, ACS Paragon Plus Environment


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the high discharge capacity and cycling stability as battery and excellent capacitive properties as supercapacitor can be simultaneously achieved (as described in Figure 1b). What worth noting is, different from the reported battery-supercapacitor hybrid devices that usually composed of one battery-type electrode and one capacitor-type electrode, 5, 34

the one we fabricated is assembled with electrodes that each containing both

capacitive and battery-type materials, giving the better synergistic performances. The reversible switch ability of the device between battery and supercapacitor modes have been demonstrated, stating the feasibility of this kind of battery-supercapacitor hybrid device. Our results demonstrate the effectiveness of combining C-based materials with alloys to construct the battery-supercapacitor system and the facility of the process compared to the present methods which have suffered the disadvantage of complicated synthesis process.7 We believe the attempts done here could be meaningful for the development of the battery-supercapacitor hybrid energy storage technology.

2. EXPERIMENTAL 2.1

Materials. The pristine Mg2Ni alloy was prepared by a melt-spinning process

through the method reported in our previous work.31 The -COOH functionalized reduced graphene oxide (rGO) and the multi-walled carbon nanotubes (MWCNTs) were purchased from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. Potassium hydroxide (KOH) and lithium hydroxide (LiOH) were purchased from Lan Yi Co. Ltd., Beijing, China. α-Ni(OH)2 were synthesized through the method reported in our previous work.35 All chemicals are analytical grade and were directly used without

further


purification.

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Figure 1. (a) The schematic of synthesis and structure of C skeleton/Mg2Ni (b) Diagram showing the battery and capacitive performance at the same time during discharging process. 2.2 Sample preparation. The prepared Mg2Ni alloy was broken into powder in an agate mortar and mixed with Ni powder in a mass ratio of 1:1,36 and then the mixture was loaded into a stainless steel vessel (70 mL in volume) for mechanical milling. The vessel was vacuumed and refilled with high purity Ar gas for three times before milling and opened every 10 h during milling procedure in order to crush the aggregation on the inside wall and bottom of the vessel. The milling process was conducted by utilizing a planetary-type ball milling machine with the ball to powder mass ratio of 20:1 at a ACS Paragon Plus Environment


speed of 350 r min-1 for 100 h at room temperature. In order to prevent overheating and obtain better homogeneity, it was set to mill for 30 min in the clockwise direction while cooling for 15 min then milled for 30 min in the reverse direction. All the above mentioned operations were performed in glovebox filled with dry Ar gas in order to prevent from oxidation.31 The obtained Mg2Ni-Ni powder mixture was weighed for 0.2 g for further use. In a typical process, 0.15 g rGO was dispersed into 4.8 mL deionized water with stirring and sonicated for 3 min. Then, 0.15 g MWCNTs was added into the rGO dispersion and stirred incessantly for 10 min to get homogeneous rGO and MWCNTs mixture. Afterwards, the weighed Mg2Ni-Ni was added into the above mixture with another 10 min stirring. The obtained sample was transferred into a square mold immediately for freeze drying. As a comparison, a normal drying method also has been employed to fabricate the C skeleton/Mg2Ni in the same ratio as described above. The typical fabrication process of battery device is as follows: When finished freeze drying, a certain amount of the C skeleton/Mg2Ni was tableted into wafer (12 mm in diameter) using a tablet machine with 20 MPa pressure for 30 seconds. One piece of foam nickel sheet (7 cm × 25 mm) with a circular hole (12 mm in diameter) in the top center was employed for the electrode preparation. The obtained C skeleton/Mg2Ni wafer was pressed into the hole of the foam nickel sheet using the table machine with 10 MPa pressure for 30 seconds. The NiOOH/Ni(OH)2 electrode was employed as the counter electrode, which had exceeded capacity than that of the test electrode. The measurements were performed in a two-electrode cell using 6 mol/L KOH solution containing 20 g L-1 LiOH as electrolyte at room temperature. For a typical preparation process of supercapacitor device, a given mass of the C skeleton/Mg2Ni was pressed onto the foam nickel wafer (10 mm in diameter) using the tablet machine with 20 MPa pressure for 30 seconds as the testing electrode. While for the counter electrode, the mass of α-Ni(OH)2 was four times of Mg2Ni to ensure the exceeded capacity. When the α-Ni(OH)2 was weighed, rGO and MWCNTs with the equal mass which is quarter of α-Ni(OH)2 respectively were added into α-Ni(OH)2 powder in an agate mortar for well-mixed. By employing the same procedure with ACS Paragon Plus Environment

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testing electrode, the counter electrode was prepared with the nickel foam wafer. The measurements were performed in a three-electrode cell system consisting of a C skeleton/Mg2Ni electrode, a C skeleton /α-Ni(OH)2 electrode and an Hg/HgO reference electrode, using 6 mol/L KOH solution containing 20 g L-1 LiOH as electrolyte at room temperature. During the whole process, in order to control the optimal content of Mg2Ni in the composite electrodes, the composites were firstly uniformly mixed. After freeze-drying, the obtained free-standing composite was totally used as electrode without tailoring to avoid loss of active material. In this way, the content of the Mg2Ni in the alloy composite electrodes can be well controlled. 2.3 Sample characterization. The morphologies and microstructures of the Mg2Ni alloy and C skeleton/Mg2Ni were observed by a HITACHI S-4800 scanning electron microscopy (SEM) in secondary electron scattering mode at 10 kV equipped with an energy dispersive spectrometer (EDS) and a JEM-2100 transmission electron microscopy (TEM). The phase of the products was characterized by an X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.154 nm) between 2θ =10° and 2θ =90° using a Bruker D8 Focus diffractometer. The Brunauer–Emmett–Teller (BET) specific surface area of the samples were evaluated by physisorption of nitrogen at 77 K by a volumetric adsorption apparatus (Quadrasorb SI-MP). 2.4 Electrochemical measurements. The galvanostatic method was employed by using a CT2001A L battery testing system to determine the discharge capacity of the electrode, which was evaluated by the amount of active substance of Mg2Ni. The electrode was charged at 100 mA g-1 for 10 h and then discharged at 50 mA g-1 to a cut-off potential of 0.9 V after 5 min rest. The high rate dischargeability (HRD) of the alloy electrodes was investigated through measuring the discharge capacities at different current densities (600, 1200, 1800, 3000 and 6000 mA g-1). To study the high rate chargeability, charge capacities at different current densities (300, 600, 1200, 1800, 3000 and 6000 mA g-1) were tested. To study the tafel polarization and linear polarization curves, a scanning rate of 1 mV s-1 ranging from -300 to 300 mV (vs. open circuit potential) and 0.1 mV s-1 from -5 ACS Paragon Plus Environment


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to 5 mV (vs. open circuit potential) were performed, respectively. The depth of discharge (DOD) was 100% for tafel polarization measurement, and was 50% for linear polarization. Potentiostatic discharge curves was measured at +600 mV (vs. open circuit potential) potential step for 3600 s after 100% depth of charge (DOC).9 For electrochemical impedance spectroscopy (EIS) measurement, the frequency ranged from 100 KHz to 5 mHz with an AC amplitude of 5 mV under the open circuit condition. The cyclic voltammetry (CV) response of the electrodes was measured at different scan rates varying from 0.5 mV s-1 to 10 mV s-1. The galvanostatic charge– discharge curves was conducted with the voltage window of 1.45 to 0.9 V. The above electrochemical tests except EIS test were conducted on a CHI660E electrochemical workstation. EIS tests were measured at the same condition by analysis devices including Potentiostat/Galvanostat (Model 263 A) and Frequency Response Detector (Model FRD 100) purchased from Princeton Applied Research. Before these measurements, the electrodes were all fully activated by several charge/discharge cycles. Through calculating from the CV curves according to the following equation, the specific capacitance of the electrode can be obtained: C=

(1)

Where the I (A g-1 ), V, υ(V s-1 ) and m (g) refer to the response current density, the potential, the potential scan rate, and the active materials mass in the electrodes, respectively.

3. RESULTS AND DISCUSSION 3.1 Characterizations of the C skeleton/Mg2Ni. The schematic structure of the products have been shown in Figure 1a. The C skeleton/Mg2Ni alloys have been fabricated through a dispersing-freeze drying method which could guarantee the well dispersed of Mg2Ni in the rGO and MWCNTs and meanwhile a free-standing structure could be obtained without the traditional nickel foam current collector. It can be seen by adding the appropriate amount of rGO and MWCNTs, the Mg2Ni would be surrounded


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and twisted by C skeleton. This special structure plays important role in anti-corrosion and anti-pulverization. Figure 2a and c show the SEM images of the Mg2Ni milled with nickel for 100 h. It can be seen that the bare Mg2Ni particles are in scattered distribution with particle-size varying from a few microns to more than 10 microns (Figure 2a). The milled powder are irregular in shape but almost spheroidal, and the surface of Mg2Ni particles becomes very rough as shown in the TEM images in Figure 2e. After encapsulating and twisting with C skeleton, the diameter of Mg2Ni alloy have no significantly change (Figure 2b). The MWCNTs was obviously tangled outside the Mg2Ni as shown in Figure 2d while the rGO could distinctly catch the sight through the TEM image (Figure 2f) which was uniformly covered on the surface of Mg2Ni particles. The results suggest that such an efficient freeze drying approach can make the three ingredients undergo a relatively adequate interfacial interaction which have been rare reported before.



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Figure 2. SEM images (a, c) and TEM image (e) of the bare Mg2Ni alloy, and the C skeleton/Mg2Ni composite (b, d and f) under different magnifications. XRD was employed to characterize the phase structure of the rGO, MWCNT, pristine Mg2Ni, 100 h milled Mg2Ni and C skeleton/Mg2Ni as shown in Figure 3a. It can be seen that the pristine Mg2Ni shows a typical crystalline structure. After mechanically milled for 100 h, the diffraction peaks are flattened or even vanished, indicating the transformation from crystalline structure to amorphous structure.37 While for the XRD patterns of the C skeleton/Mg2Ni, except for the above diffraction peaks for 100 h milled Mg2Ni, the new significant diffraction peak at 2 θ= 26.4° and 43° matches well with that of rGO and MWCNTs (the bottom two lines), ACS Paragon Plus Environment

38, 39

which

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demonstrated the existence of C skeleton. To further confirm the homogeneity of C skeleton encapsulating, surface EDS analyses have been conducted on different samples of the C skeleton/Mg2Ni surface and the results are shown in Figure 3b. Results show that the contents of carbon element for different samples in weight are basically around 45–55% that is in accordance with the initial mass ratio of C to Mg2Ni. Also, it indicates the uniformly surrounding of C skeleton on the alloy particle.

Figure 3. (a) XRD patterns of the rGO, MWCNT, pristine Mg2Ni alloy, alloy milled for 100 h, and C skeleton-Mg2Ni, (b) EDS analysis on the different particles of the C skeleton-Mg2Ni samples (insets show the weight and atomic percentage of elements). 3.2 Battery performance of the C skeleton/Mg2Ni electrode. We first unveil the discharge capacity of the Mg2Ni with and without C skeleton as a function of cycle number. As shown in Figure 4a, the discharge capacity was significantly improved by twisting and surrounding with C skeleton. In details, the Mg2Ni without C reaches its highest value of 586.67 mAh g-1 after two cycles, and the discharge capacity declines sharply to 289.48 mAh g-1 after 50 cycles, which is similar to the previous reports.40, 41 The discharge capacity at the maximum and 50th cycle are listed in Table 1, and the retention is 49% after 50 cycles for pure Mg2Ni. As comparison, the discharge capacity of the Mg2Ni with C reaches the maximum value after about ten cycles and maintains high (>500 mAh g-1) even after 50 cycles (Table 1) with the retention of 78%. This can



be further demonstrated from the charge-discharge curves as shown in Figure 4b. For the Mg2Ni with C (solid line), no obvious degradation can be observed from 10th to 30th cycles, and high discharge capacity still can be remained even after 50th cycles, which was almost equal to the one without C at the 10th cycle. Moreover, the highest discharge capacity is as high as 644 mAh g-1 which is an outstanding value among C/Mg2Ni composite and Mg2Ni related alloy.42, 43 These achievements exceed the reported results on Mg2Ni capacity retention combined with the absolute value in literatures so far to our knowledge,9 which pioneers a promising approach for the practical application of Mg2Ni alloy. The slight delay of activation of the Mg2Ni with C could be attributed to the more complex proton paths needed to be stablished in the C skeleton/Mg2Ni system. By contrast, the dramatic performance degradation of the Mg2Ni without C mainly results from the corrosion by electrolyte and the formation of Mg(OH)2 layer on the surface by reacting with the hydroxyl in electrolyte which will increase the resistance of the inter-particle.44 Thus, it can be concluded the introduction of C skeleton have resolved the trouble effectively. On one hand, the rGO encapsulating layer offers a selective barrier to the hydroxyl.45 As a result, the corrosion of the alloy can be largely reduced and the pulverization effect during charging and discharging would be obvious decreased, which will lead to an increase in discharge cycling ability. The improvements in corrosion and pulverization effect can be further demonstrated by the comparative SEM images of the pure Mg2Ni and C skeleton/Mg2Ni electrodes before and after cycling measurements showing in Figure S1 of the Supporting Information. On the other hand, the twisting MWCNTs on the surface of the Mg2Ni alloys supply enough paths for proton but no other ions, which will guarantee the excellent discharge capacity.46-48 Besides, the C skeleton/Mg2Ni free-standing electrode exhibited much better discharge capacity compared to the one fabricated through the normal drying method as shown in Figure S2 of Supporting Information, which demonstrated the effectiveness of the dispersing-freeze drying method.


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Figure 4. (a) Discharge capacities as functions of cycle number and (b) Charge– discharge curves of the 10th, 30th and 50th cycles between 0.9-1.45 V (For (a) and (b), the electrode was charged at 100 mA g-1 for 10 h and then discharged at 50 mA g-1 to a cut-off potential of 0.9 V after 5 min rest), (c) High rate dischargeability (HRD) of the Mg2Ni with and without C and (d) Charge–discharge curves under the current density of 2C (1200 mA g-1), 3C (1800 mA g-1) and 10C (6000 mA g-1) between 0.9-1.45 V (inset shows the one without C at 10C, which has been destroyed due to the high current density). (For (c) and (d), the electrode was charged at 100 mA g-1 for 10 h and then discharged at Id to a cut-off potential of 0.9 V after 5 min rest) In order to investigate the kinetic properties of hydrogen storage of the Mg2Ni with and without C skeleton, the high rate dischargeability (HRD) have been measured and shown in Figure 4c. The values of HRD is calculated according to the following equation: HRD =

Cd Cd + C300

× 100%

(2)

where Cd is the discharge capacity at Id (600, 1200, 1800, 3000 and 6000 mA g-1 in this work) current density to a cut-off potential of 0.9 V, C300 is the residual discharge capacity measured at current density of 300 mA g-1 (0.5C) after the tested alloy ACS Paragon Plus Environment


electrode was discharged at Id to the same cut-off potential and rested for 5 min. It can be found that at relative low discharge current densities, the HRD values of the Mg2Ni with C are higher than that of the Mg2Ni without C and the gaps of HRD data between them are enlarged with increasing the discharge current density. For instance, the HRD at discharge current density of 1800 mA g-1 increase from 26% to 59% when encapsulated with C skeleton, suggesting that the C skeleton/Mg2Ni has better electrochemical kinetics than the bare Mg2Ni alloy. Also, by comparing to other work, the HRD of the C skeleton/Mg2Ni is much higher even at relative high discharge current density (49% for 3000 mA g-1 and 34% for 6000 mA g-1).9, 49 However, the sample of the Mg2Ni without C has been destroyed at high (3000 and 6000 mA g-1) discharge current density (imaginary line of the red curve). Further demonstration has been shown in the charge-discharge curves (Figure 4d), it can be seen that the higher discharge current density, the more residual discharge capacity (C300) would be observed. For the one without C, there was almost no C300 remained when the Id was 1800 mA g-1 (3C), and when increased to 6000 mA g-1 (10C), the sample has been destroyed and the voltage was decreased to negative value even at the charge stage (inset of Figure 4d). This result effectively illustrated the role of C skeleton in protecting the Mg2Ni from corrosion by electrolyte. Moreover, the two-phase interface of the C skeleton and the Mg2Ni alloy would offer a diffusion channel for the hydrogen.9 To verify the deduction, tafel polarization, linear polarization, potentiostatic discharge and EIS measurements were further performed.


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Figure 5. (a) Tafel polarization curves, (b) Linear polarization curves, (c) Potentiostatic discharge curves, (d) Electrochemical impedance spectra (the inset shows the high-frequency region of the plot), (e) Impedance phase angle versus frequency, (f) Discharge capacity with high rate charge current density (0.5C, 1C, 2C, 3C, 5C, 10C) and discharge current density (0.5C). The anti-corrosion ability of the electrodes has been investigated through the tafel polarization method. The polarization curves for Mg2Ni with and without C are shown in Figure 5a and the corrosion potential Ecorr together with the corrosion current density ic are listed in Table 1. It is suggested that the anti-corrosion should be judged from the ic first, and the Ecorr will be comparable only when the ic is approximate.22 Generally, the more negative Ecorr and the larger ic usually correspond to the faster corrosion rate while



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the more positive Ecorr and the smaller ic mean a slower corrosion process.50 For the C skeleton/ Mg2Ni, it can be seen that the ic is much lower than that of bare Mg2Ni alloy, suggesting favorable effect of the C skeleton’s encapsulating on enhancing the anti-corrosion ability of the bare Mg2Ni alloy. To investigate the kinetics of charge transfer on the alloy surface, the linear polarization measurements have been performed to obtain the exchange current density (I0) which is the key index for kinetics of charge transfer. It can be calculated through the following equation when the overpotential is changed within a small range (η < 10 mV):

I0 =

RTId

(3)

Fη

where I0 is the exchange current density (mA g-1), R is the gas constant, T is the absolute temperature (K), Id is the applied current density (mA g-1), F is the Faraday constant and η is the total overpotential (mV). The values of I0 can be estimated from the slope (Id/η) of the obtained curves which have been shown in Figure 5b, and the calculated I0 have been listed in Table 1. It can be found that the value of I0 increased from 168.4 mA g-1 to 251.6 mA g-1 for the Mg2Ni when encapsulated with C skeleton, which is favorable for the charge-transfer process during cycling. The potentiostatic discharge measurement is used to characterize the hydrogen diffusion rate of the electrodes. Figure 5c presents the potentiostatic discharge curves for Mg2Ni with and without C. It is suggested that the electrochemical reaction kinetic is dominated by the hydrogen diffusion rate in the bulk of alloy in the linear region.9 And the hydrogen diffusion coefficient parameter D in the bulk of alloy can be calculated by the following equation:51 log i = log ±

6FD da2

π2

C0 – Cs – 2.303 a2 t D

(4)

In the equation, the i, F, D, d and a refer to the current density (A g-1), Faraday constant, hydrogen diffusion coefficient (cm2 s-1), density (g cm-3) and particle radius (cm) of the hydrogen storage alloy, respectively. C0 and Cs (mol cm-3) are respectively the hydrogen concentrations in the bulk of the alloy in the beginning and that on the surface of the alloy particles after discharge. The t is the discharge time (s). When the


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average particle sizes are not identical for different samples, the value of D/a2 can be used to analyze the hydrogen diffusion kinetics,9 which were determined from the slopes of the curves and listed in Table 1. It can be seen that the calculated value for Mg2Ni alloy increases from 6.5 × 10-5 s-1 to 12.2 × 10-5 s-1 after C skeleton coating, which indicates the positive effect of C skeleton in improving the hydrogen diffusion. Also, it is in good agreement with the results of HRD measurement. Table 1. The maximum discharge capacities (Cmax), the discharge capacities after 50 cycles (C50), tafel fitting data (Ecorr and ic), the exchange current density (I0) and hydrogen diffusion coefficient (D) for the bare Mg2Ni alloy and C skeleton/Mg2Ni composite. Cmax

C50

Ecorr

ic

I0

D/a2

(mAh g-1)

(mAh g-1)

(V)

(mA cm2)

(mA g-1)

(10-5 s-1)

Mg2Ni

586

289

-1.43

4.653×10-3

168.4

6.5

C skeleton/Mg2Ni

644

505

-1.437

7.571×10-4

251.6

12.2

Samples

The EIS method was further conducted to demonstrate the above analysis. In the EIS spectrum, the intersection of the curve at the real part in the range of the high frequency indicates the equivalent series resistance (Rs).52 As shown in the inset of Figure 5d, in both case, they were relatively low (

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