Reduced Graphene Oxide Coating with Anti-corrosion and

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Reduced Graphene Oxide Coating with Anti-corrosion and Electrochemical Property Enhancing Effects Applied in Hydrogen Storage System Yi Du, Na Li, Tong-Ling Zhang, Qing-Ping Feng, Qian Du, Xing-Hua Wu, and Gui-Wen Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05809 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Applied Materials & Interfaces

Reduced

Graphene

Oxide

Coating

with

Anti-corrosion and Electrochemical Property Enhancing Effects Applied in Hydrogen Storage System Yi Du,†, ‡ Na Li,† Tong-Ling Zhang,†, ‡ Qing-Ping Feng,*,† Qian Du,† Xing-Hua Wu,†, ‡ Gui-Wen Huang*,† †

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, No. 29

Zhongguancun East Road, Beijing 100190, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: reduced graphene oxide, surface modification, anti-corrosion, hydrogen storage, magnesium-based alloy, electrochemical property ABSTRACT: Low capacity retention is the most prominent problem of magnesium nickel alloy (Mg2Ni) that blocks it from being commercially applied. Here, we propose a practical method for enhancing the cycle stability of Mg2Ni alloy. Reduced graphene oxide (rGO) possesses the graphene-based structure which could provide high-quality barriers that block the hydroxyl in the aqueous electrolyte and meanwhile it owns good hydrophilicity. rGO has been successfully coating on amorphous structured Mg2Ni alloy via electrostatic assembly to form the rGO encapsulated Mg2Ni alloy composite (rGO/Mg2Ni). The experimental results show that zeta potentials of rGO and modified Mg2Ni alloy are totally opposite in water, with values of -11.0 mV and +22.4 mV,

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respectively. The crumpled structure of rGO sheets and the contents of carbon element on the surface of alloy are measured through scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS). Tafel polarization test indicates the rGO/Mg2Ni system exhibiting a much higher anti-corrosion ability against the alkaline solution during charging/discharging. As a result, high capacity retentions of 94% (557 mAh g-1) at the 10th cycle and 60% (358 mAh g-1) at the 50th cycle have been achieved, which are much higher than the reported results on Mg2Ni capacity retention combined with the absolute value so far to our knowledge. Meanwhile, both the charge-transfer reaction rate and the hydrogen diffusion rate are proven to be boosted with the rGO encapsulating. Overall, this work demonstrates the effective anti-corrosion and electrochemical property enhancing effects of rGO coating and shows its applicability in Mg-based hydrogen storage system. ■ INTRODUCTION As an increasing number of energy resources are discovered by scientists, energy storage technology becomes more and more important in nowadays. Hydrogen is the most abundant element on Earth and considered one of the cleanest energy sources.1 As an universal available storage type of hydrogen energy, nickel-metal hydride (Ni–MH) secondary battery has been widely applied in mobile phones, portable computers, digital cameras and other personal electronic devices.2 The rare earth element based AB5-type alloys, AB2-type Laves phase alloys, V-based solid solution alloys, and Mg-based alloys are the major metal hydride negative electrodes, in which hydrogen atoms can be stored.3 Among them, Mg-based alloys have attracted great concern owing to its high specific capacity, low cost, and abundance. Especially, the Mg2Ni alloy exhibits a promising high


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theoretical capacity of 1000 mAh g-1, which is approximately 2.7 times higher than that of the commercial LaNi5 alloy (ca. 372 mAh g-1).4 However, the Mg-based alloys is still unsatisfactory for practical applications in Ni/MH batteries because of two significant disadvantages: (i) poor hydriding and dehydriding properties at ambient temperature, and (ii) a short charge/discharge cycle life. In the past several decades, a great deal of progresses have been made on Mg–Ni alloys. Kohno5 reported that by addition of Ni and mechanical grinding method, an amorphous-like state Mg2Ni shows a much higher discharge capacity (750 mAh g-1) at room temperature, compared with that of the crystalline Mg2Ni (nearly 0 mAh g-1). Subsequently, Iwakura6 found that by mechanical grinding of Mg2Ni together with 75% of Ni, an extremely high discharge capacity of 1082 mAh g-1 could be reached, which even exceeded the theoretically calculated value. Kohno7 also reported that the additional Ni may work as catalytic sites for the hydrogen dissociation, which improves the hydrogen storage properties. Therefore, it can be concluded that the addition of Ni can solve the problem of the poor hydriding and dehydriding properties of the Mg–Ni alloy at ambient temperature. Nevertheless, the alloy with such a high capacity can still not be applied in practical applications because its cycle lifetime is far from enough. The capacity degradation mechanism of Mg–Ni alloy during cycling is mainly associated with the following three aspects: (i) the formation of Mg(OH)2 layer on the surface of the alloy in alkaline electrolyte,8–11 which causes the loss of active materials and results in the reduction of hydrogen storage property. (ii) Cracking and pulverization caused by the volume variation during the charge/discharge processes.12 (iii) The hindering of the charge transfer reaction by the porous surface film of Mg(OH)2.13,14 In order to overcome


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these disadvantages, lots of efforts have been done. The commonly conducted methods in recent years includes partly substituting Mg and/or Ni by certain elements,15–21 forming composite material by adding compounds or elements,22,23 and building amorphous or nanocrystalline structures.24,25 Compared to these methods, surface modifying is an effective way to improve the overall property of the alloy due to its multiple options and good operability. So far, by using the surface modifying methods, improvements of extended cycle life, reduced pulverization, and enhanced anti-corrosion ability of Mg–Ni alloy have been demonstrated by modifying with nano metal particles or conductive polymer.26–29 However, the property enhancements achieved are still far from satisfactory. Therefore, better material and process for the surface modification are still highly desired. Since discovered by Andre Geim, graphene has been gradually applied in almost all over the subjects because of its excellent performances. It is reported that graphene could provide high-quality barriers that block all gases and liquids.30 Meanwhile, it is proved that protons can transport in and out across the graphene.31 Figure S1 in Supporting Information schematically shows the electrochemical reactions on metal hydride during charge (a) and discharge (b). It can be seen that with the incorporation of water and OH–, protons form at the interface between the alloy and the electrolyte, and then diffusion in and out the alloy during charging or discharging.32 Thus, based on the information discussed above, graphene can be assumed to be an ideal material for the surface modifying of Mg–Ni alloy. Firstly, it serves as a barrier which separates the alloy and the alkaline electrolyte. So that the opportunity of the formation of Mg(OH)2 will be greatly reduced, and the active content in alloy can be well maintained to play the role of hydrogen storage. Meanwhile, due to the proton permeability of the graphene, hydrogen


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protons generated during the electrochemical reactions could freely diffuse across the barrier, which insures the reactivity of the system. Secondly, after tightly coated on the alloy surface, the two-dimensional graphene with excellent mechanical property helps to keep the mechanical integrality of the alloy, decreasing the pulverization effect during charging and discharging. Furthermore, several works have been done that applying the graphene-based material in hydrogen storage system. For instance, Huang28,33 synthesized graphene/Ag composite and reported its favorable effects on improving discharge capacity, cycle life, discharge potential and electrochemical kinetics of Mg–Ni– La-type hydrogen storage alloy.

Consequently, it is reasonable to expect exciting

performances of the Mg–Ni alloy modified by graphene. In this paper, we present a practical method to improve the overall property of Mg2Ni alloy by surface modifying with reduced graphene oxide (rGO). The rGO was chosen as the modifying material because compared to the bare graphene without functional groups, rGO owns much better hydrophilicity, which is favorable in the aqueous system of Ni– MH battery. At the same time, the rGO shows reasonable high electrical conductivity for performing the electron transmission during charging/discharging.34 The rGO was coated on the Mg2Ni alloy particles through a facile electrostatic adsorption method. In the method, amorphous Mg2Ni alloy with high initial discharge capacity was in advance fabricated by a melt-spinning and then mechanical milling process. The obtained alloy particles were then be modified by aminopropyltrimethoxysilane (APS) to get positive charges on the surface. Afterwards, the modified alloy particles were added into the rGO dispersing solution. As the rGO is negative in charge, they will be rapidly and closely adsorbed to the alloy particles in the mixed solution due to the electrostatic adsorption


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effect, forming the rGO encapsulated Mg2Ni structure (rGO/Mg2Ni). The obtained composites show high cycling stability, with capacity retention of 94% (557 mAh g-1) at the 10th cycle and 60% (358 mAh g-1) at the 50th cycle, which are much higher than the reported results on Mg2Ni alloy so far. The electrochemical kinetics of the system is also improved by the rGO coating. Overall, the rGO modifying method proposed in this work shows outstanding effects in enhancing the overall property of Mg2Ni alloy, which makes a significant step forward to the practical application of Mg-based hydrogen storage alloy. ■ EXPERIMENTAL SECTION Preparation of amorphous structured Mg2Ni alloy. The Mg2Ni alloy belt was prepared by a melt-spinning process with spinning speed of 25 m s-1 and then broken into powder in an agate mortar. The melt-spinning process was choose because compared to other preparing methods including mechanical alloying, sintering process or hydriding combustion, it is high efficient and products with refined grain can be obtained. The resulting Mg2Ni alloy powder was mixed with Ni powder in a molar ratio of 1:27 and then added into a stainless steel vessel (with total volume of 70 ml) for mechanical milling. The milling was conducted under argon atmosphere at room temperature using a planetary-type ball milling machine at a speed of 350 r min-1 with the ball to powder mass ratio of 20:1. The ball milling duration was selected to be 100 h by milling for 30 min in the clockwise direction then cooling for 15 min and then milling for 30 min in the reverse direction in order to prevent overheat of the vessels and to obtain better homogeneity. The milling vessels were vacuumed and refilled with high purity Ar gas for three times before milling and opened every 10 h during milling procedure in order to


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crush the aggregation on the inside wall and bottom of the vessels. All the above mentioned operations were performed in glovebox filled with dry Ar gas in order to prevent from oxidation. Preparation of rGO/Mg2Ni alloy composite. The obtained 1 g Mg2Ni alloy powder was dispersed into 100 ml dry toluene solution via stirring. After 30 min, 1 ml of APS was instilled into the above solution and refluxed for 24 h to obtain APS modified Mg2Ni alloy powder. The rGO aqueous dispersing solution with concentration of 1.02 wt.% was purchased from Chengdu Organic Chemicals Co., Ltd., China. In a typical process, 0.6 g APS modified Mg2Ni alloy powder was added into 30 ml solution (1 ml rGO aqueous dispersing solution and 29 ml distilled water) and shook for several minutes. The amount of rGO used was based on the approximate calculation, under consideration of the total surface area of the alloy particles and the density of rGO. The rGO/Mg2Ni then precipitated after standing for a minute. Finally, the rGO/Mg2Ni powders were obtained after suction filtration and drying. Characterization. For electrochemical tests, the alloy electrode was made by mixing 0.2 g sample powder with 0.3 g Ni powder and then pressed into a foam nickel sheet (7 cm × 25 mm) under a pressure of 26 MPa. Electrochemical 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. In each testing unit, 150 ml of this electrolyte was used and the separator applied was PP/PE blended film. The NiOOH/Ni(OH)2 electrode was employed as the counter electrode, which had exceeded capacity than that of the test electrode. The discharge capacity and cycle life were determined by the galvanostatic method on a CT2001A Land battery testing system and capacities are calculated based on


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the active substance. 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. To investigate the high rate dischargeability (HRD) of the alloy electrodes, discharge capacities at different current densities (300, 600, 900, 1200 mA g-1) were measured. Linear polarization and electrochemical impedance spectroscopy (EIS) were performed at the stage of 50% depth of discharge (DOD). Linear polarization measurements were measured at a scanning rate of 0.1 mV s-1 from -5 to 5 mV (vs. open circuit potential). For EIS measurement, the frequency ranged from 100 KHz to 5 mHz with an AC amplitude of 5 mV under the open circuit condition. Tafel polarization curves were measured at a scanning rate of 1 mV s-1 from -300 to 300 mV (vs. open circuit potential) at 100% DOD. For the potentiostatic discharge, the electrodes were discharged at +600 mV (vs. open circuit potential) potential step and 100% depth of charge (DOC) for 3600 s. 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 fully activated by four charge/discharge cycles. The X-ray diffraction (XRD) measurements were carried out using a Bruker D8 Focus diffractometer with Cu Kα radiation. The morphologies and microstructures were observed by a HITACHI S-4300 scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) and a JEM-2100 transmission electron microscopy (TEM). Zeta potential measurements were performed using a Zetasizer Nano ZS. Fourier


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transform infrared (FTIR) spectra were recorded on an Excalibur 3100 spectrometer in the range of 4000–400 cm−1. The X-ray photoelectron spectroscopy (XPS) was tested on ESCALAB 250Xi. The thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out with NETZSCH STA 409 PC. ■ RESULTS AND DISCUSSION The assembly strategy of rGO/Mg2Ni has been schematically depicted in Figure 1. The synthetic procedure of rGO/Mg2Ni involves two steps: hydroxyl groups on the Mg2Ni surface modified by APS and electrostatic self-assembly in the aqueous rGO dispersing solution. The existing of hydroxyl groups on Mg2Ni surface may be owing to the partly oxidation during processing.35 The surface complex model theory36 also points out that metal hydroxyl groups exist on the surface of many metal oxides, which can be detected by FTIR spectroscopy.37 FTIR tests of Mg2Ni alloy and rGO were then performed and presented in Figure 2a and 2b, which proves the existing of hydroxyl groups on the surface of Mg2Ni alloy and figures out the chemical structure of rGO. Firstly the hydroxyl groups are modified by grafting of APS to render the Mg2Ni surface positively charged. Then rGO encapsulated Mg2Ni alloy is fabricated via the electrostatic interaction between modified Mg2Ni alloy with positive charge and rGO with negative charge in aqueous solution. The macroscopic electrostatic adsorption effect is demonstrated in Figure S2 of Supporting Information. In Figure S2a, the left vial shows the pristine rGO dispersing solution, which owns good uniformity and high stability. After the addition of APS modified Mg2Ni powder, as shown in the right vial, rGO are rapidly and completely absorbed onto the alloy particles and precipitate with the powder. Compared with the gray powder of bare Mg2Ni (Figure S2b), the rGO/Mg2Ni powder


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(Figure S2c) are black in color, demonstrating the successfully coating of rGO on the alloy power. For further proving the spontaneity of the electrostatic interaction, the zeta potentials of APS modified Mg2Ni and graphene in aqueous solution (PH = 7) have been tested. Results (Figure S3 in Supporting Information) show that the APS modified Mg2Ni alloy powders are positively charged (+22.4 mV), and in contrast the rGO are negatively charged (-11.0 mV). The opposite charges between the alloy and the rGO make the electrostatic assembly spontaneously take place in the mixed solution. After assembly, the rGO are electrostatically coated on the Mg2Ni particles without forming chemical bond according to the XPS results shown in Figure S4 in Supporting Information.


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Figure 1. The synthetic procedure of rGO/Mg2Ni: (a) modification with APS, (b) electrostatic self-assembly.


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Figure 2. FTIR spectra of Mg2Ni (a) and rGO (b), XRD patterns (c) of the pristine Mg2Ni alloy, alloy milled for 100 h, rGO and rGO/Mg2Ni, the HRTEM and the inserted corresponding SAED images (d) of the 100 h milled Mg2Ni alloy. As shown in Figure 2a, the broad band at 3411 cm−1 is attributed to the O–H stretching vibration, and the band at 1627 cm−1 can be assigned to the bending vibration of water molecules which may come from KBr.39 A strong band appearing at 433 cm−1 is due to the stretching vibrations of Ni–O.40 Another strong peak at 526 cm−1 can be attributed to the vibrational mode of Mg–O bonding and the band at 863 cm−1 indicates the pattern of cubic phase of periclase MgO.41 The band at 1235 cm−1 can be assigned to the bending vibration of hydroxyl group (M–OH) on Mg2Ni alloy oxides,39,42 which proves the existing of hydroxyl group on Mg2Ni. In Figure 2b, FTIR spectrum shows the characteristic peaks for rGO, which illustrates the presence of various oxygen containing


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groups onto its surface. The peaks of rGO that positioned at 3429, 1640, 1432, 1216 and 1085 cm−1 can be attributed to O–H stretching vibration of interlayer water, C=O stretching of carbonyl and carboxyl groups at edges of the rGO networks, O–H stretching vibration of carboxyl, C–O stretching vibration of epoxide, and C–O stretching vibration from alkoxy groups, respectively.43-47 Figure 2c presents the XRD patterns of the pristine Mg2Ni alloy, alloy milled for 100 h, rGO and rGO/Mg2Ni, respectively. As can be seen in Figure 2c(a), the pristine Mg2Ni shows a typical crystalline structure. After mechanical milling for 100 h, diffraction peaks of the alloy are flattened or even vanished (Figure 2c(b)), indicating the transformation from crystalline structure to amorphous structure. Figure 2c(c) presents the XRD pattern of the rGO, as expected, with no obvious diffraction peaks.38 Due to the small amount of rGO used for the modification, the XRD pattern of the assembled rGO/Mg2Ni is only slightly changed in the range of 20°–30° compared to the bared Mg2Ni alloy. In the Figure 2d, the high resolution transmission electron microscopy (HRTEM) image of milled Mg2Ni alloy shows many disorderly stripes and the selected area electron diffraction (SAED) image exhibits a broaden ring, which further indicates the disappearance of the long-range ordered structure in the alloy after 100 h ball milling, namely the formation of the ambiguous structure.


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Figure 3. SEM and TEM images of the bare Mg2Ni alloy and the rGO/Mg2Ni composite under different magnifications: (a, c, e) the bare alloy; (b, d, f) the rGO encapsulated alloy. Figure 3 shows the morphology of the 100 h milled Mg2Ni before and after rGO encapsulating. From the Figure 3a, 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. After rGO encapsulating, it can be seem from Figure 3b that the particles are


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relatively concentrated and the surface of Mg2Ni is wrapped in a layer of rGO. With higher magnification, by comparing Figure 3c and Figure 3d, one can distinctly catch the sight of rGO sheets uniformly covered on the surface of Mg2Ni particles, suggesting that such an efficient electrostatic self-assembly approach can make the two ingredients undergo a relatively adequate interfacial interaction. Figure 3e and Figure 3f display the TEM images of the bare alloy and rGO/Mg2Ni, respectively. It can be observed from the image of rGO/Mg2Ni that there are several sheets of rGO covering on the surface of Mg2Ni, where the crumpled structure of the rGO can be clearly seen. Combined with the SEM and TEM results, it is demonstrated that rGO sheets are successfully encapsulated on the surface of alloy particles by electrostatic adsorption. To further confirm the homogeneity of rGO encapsulation, contrastive surface EDS analyses have been conducted on the APS modified Mg2Ni and rGO/Mg2Ni surface and the results are shown in Figure S5–7 and Table S1–3 in Supporting Information. The results adequately indicate the uniformly encapsulating of rGO sheets on the alloy particles by comparing the atomic percent of carbon in the contrastive samples. Figure S8 in Supporting Information shows the TGA/DSC curves of bare Mg2Ni and typical rGO/Mg2Ni testing under air atmosphere with a heating rate of 10°C min-1. Compared to the TGA curve of the bare Mg2Ni, it is clear that the rGO coating in rGO/Mg2Ni provides a protective layer which slows down the oxidation process.


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Figure 4. Discharge capacities as functions of cycle number for the bare Mg2Ni alloy and rGO/Mg2Ni composite electrodes. The effect of rGO encapsulating on the discharge behaviors of Mg2Ni alloy is shown in Figure 4. The discharge capacity of the bare alloy reaches its highest value of 583 mAh g-1 after two cycles. While the capacity of the rGO/Mg2Ni reaches the highest value of 594 mAh g-1 after four cycles. The slight delay of activation may attributed to the build of more complex proton paths in the rGO/Mg2Ni system. However, for the bare alloy, the discharge capacity decreases sharply after activation as the similar trance of reported results,10,48–57 the capacity retention almost droping to 79% (459 mAh g-1) after only 10 cycles. By contrast, the rGO/Mg2Ni performs a greatly enhanced capacity maintaining property. A high capacity retention of 94% (557 mAh g-1) is achieved at the 10th cycle, and over 60% (358 mAh g-1) of the initial capacity can be maintained after 50 cycles.


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These achievements beyond the reported results on Mg2Ni capacity retention combined with the absolute value in literatures so far to our knowledge,10,48–57 which pioneers a promising approach for the practical application of Mg2Ni alloy. The dramatic property degradation of the bare alloy mainly comes from the corrosion and the formation of Mg(OH)2 layer on the surface by reacting with the hydroxyl in electrolyte.8–11 Once the insulating Mg(OH)2 layer are formed, the inter-particle resistance will definitely increase,13,14 and hydroxyl will further concentrate in the passivating film because of its porosity and permeability. The introduced rGO encapsulating layer offers a selective barrier to the hydroxyl, so the corrosion of the alloy can be largely reduced. Figure 5 presents the SEM images of the rGO/Mg2Ni before and after 50 charging-discharging cycles. It can be seen that the morphology of the rGO/Mg2Ni particles were well maintained after cycling, which proves the good stability of the rGO coating in the electrolyte. In addition, the excellent mechanical property of rGO helps to keep integrality of the alloy structure and decrease the pulverization effect during cycling, which also leads to an increase in cycle life.


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Figure 5. SEM images of the rGO/Mg2Ni composite under different magnifications: (a, c) 0 cycle; (b, d) 50 cycles. For further understanding of the selective barrier effect of the rGO, Figure 6 shows a schematic representation of electrochemical hydrogen absorption and desorption in charging/discharging process. During the discharging, as illustrated in Figure 6a, hydrogen atoms are desorbed first from the amorphous Mg2Ni and diffuse to the two phase (Mg2Ni and rGO) interface. To combine with the hydroxyl in electrolyte and release electrons, the hydrogen atoms can then diffuse across the rGO due to the high transport rates of rGO for protons.31 The electrons are then quickly transferred out through the sp2-hybridized structure of the two-dimensional carbon network. Charging is exactly the converse process of discharging, as described in Figure 6b. Because the graphene-based structure could provide high-quality barriers that block all gases and


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liquids, the hydroxyl in the aqueous electrolyte cannot contact with the alloy directly during reaction. Thus the alloy can be protected from being corroded by the alkaline electrolyte. Consequently, the encapsulating rGO simultaneously offers the corrosion resistance and the fast pathways for the protons and the electrons.

Figure 6. The schematic of electrochemical hydrogen absorption and desorption in discharging (a) and charging (b) process. To investigate the anti-corrosion ability of the alloy electrodes, Tafel polarization test was performed. The polarization curves for bare Mg2Ni alloy and rGO/Mg2Ni are shown in Figure 7 and the corrosion potential Ecorr and corrosion current density ic are listed in Table 1. From Figure 7 it can be found that the Ecorr of rGO/Mg2Ni shifts toward the


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positive direction and the ic is lower than that of bare Mg2Ni alloy, suggesting the rGO encapsulation can enhance the anti-corrosion ability of the bare Mg2Ni alloy. Table 1. The maximum discharge capacities (Cmax), the discharge capacities after 10 cycles (C10) and 50 cycles (C50), and Tafel fitting data of bare Mg2Ni alloy and rGO/Mg2Ni composite. Cmax

C10

C50

Ecorr

ic

(mAh g-1)

(mAh g-1)

(mAh g-1)

(V)

(mA cm2)

Mg2Ni

583

459

267

-0.933

0.891

rGO/Mg2Ni

594

557

358

-0.823

0.522

Samples

Figure 7. Tafel polarization curves of the bare Mg2Ni alloy and rGO/Mg2Ni composite electrodes (scan rate: 1 mV s-1).


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Figure 8. High rate dischargeability (a); linear polarization curves (b); potentiostatic discharge curves (c) and electrochemical impedance spectra and corresponding equivalent circuit (d) of the bare Mg2Ni alloy and the rGO/Mg2Ni composite electrode, respectively. The HRD of bare Mg2Ni alloy and rGO/Mg2Ni composite are shown in Figure 8a. The HRD is a comprehensive index reflecting kinetic properties of hydrogen storage alloy electrodes, which defined as following equation: HRD =

Cd Cd + C100

× 100%

(1)

where Cd is the discharge capacity at Id (300, 600, 900 and 1200 mA g-1 in this work) current density to a cut-off potential of 0.9 V, C100 is the residual discharge capacity


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measured at current density of 100 mA g-1 after the tested alloy electrodes were discharged at Id to the cut-off potential and rested for 30 min. It can be found that the HRD data of the bare Mg2Ni alloy are lower than that of the rGO/Mg2Ni composite at various discharge current densities and the gaps of HRD data between this two materials are enlarged with increase of the discharge current density. The HRD at discharge current density of 1200 mA g-1 increase from 61% to 67%, suggesting that the rGO/Mg2Ni composite has better electrochemical kinetics than the bare Mg2Ni alloy. The HRD of a metal-hydride electrode is mainly determined by two factors: charge-transfer rate at the electrode-electrolyte interface and hydrogen diffusion rate in the bulk of alloy.58 As the rGO owns high charge-transfer rate, the introduction of graphene is reasonable to facilitate the charge-transfer reaction of the system. On the other hand, the crumpled structure of graphene greatly increases the surface area of the bare Mg2Ni alloy which act as the absorption and desorption channels of hydrogen. As a result, the improvement of HRD by rGO coating Mg2Ni may be attributed to the combination effect of both increased inter-particle conductivity and specific area for hydrogen absorption and desorption. Besides the HRD, there are some other kinetic parameters, such as the exchange current density and the hydrogen diffusion rate, that determine the electrochemical reaction rate of a Ni/MH battery.59 In general, the exchange current density I0 represents the electrochemical reaction rate around the equilibrium potential. When the overpotential is changed within a small range (η < 10 mV), the exchange current density I0 can by calculated by the following equation: I0 =

RTId

(2)

Fη


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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). Id/η is the slope of these straight lines. Figure 8b presents the linear polarization curves of the bare Mg2Ni alloy and rGO/Mg2Ni composite. Based on the measured curves, the exchange current densities of bare Mg2Ni alloy and rGO/Mg2Ni composite are calculated and summarized in Table 2. In general, a high value of I0 corresponds to good kinetics for hydriding/dehydriding. It can be found that the exchange current density increase from 92.7 mA g-1 to 225.9 mA g-1, owing to the addition of rGO. This change can be attributed to the increase of inter-particle ionic conductivity which is favorable for the charge-transfer process during cycling. Table 2. The exchange current density (I0), contact resistance (Rcp), charge-transfer resistance (Rct) and hydrogen diffusion coefficient (D) for the bare Mg2Ni alloy and rGO/Mg2Ni composite I0

Rcp

Rct

D/a2

(mA g-1)

(mΩ)

(Ω)

(10-5 s-1)

Mg2Ni

92.7

684.9

34.4

3.7

rGO/Mg2Ni

225.9

423.7

22.1

5.5

Samples

The hydrogen diffusion rate in the bulk of alloy, which is an important parameter and usually determines the electrochemical kinetics. It can be represented by potentiostatic discharge curves. Figure 8c shows the potentiostatic discharge measurements of the bare Mg2Ni alloy and rGO/Mg2Ni composite. It can be observed that at the initial stage of discharging (< 500 s), the current declines sharply, and after a period about 1500 s, the current begins to decline linearly. In the linear region, the electrochemical reaction kinetic


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is dominated by the hydrogen diffusion rate in the bulk of alloy. The hydrogen diffusion coefficient parameter D in the bulk of alloy can be calculated by the following equation:60 log i = log ±

6FD da2

C0 – Cs –

π2

D

2.303 a2

t

(3)

where i (A g-1) is the current density, F is Faraday constant, D (cm2 s-1) is the hydrogen diffusion coefficient, d (g cm-3) is the density of the hydrogen storage alloy, a (cm) is the alloy particle radius, C0 (mol cm-3) is the initial hydrogen concentration in the bulk of the alloy, Cs (mol cm-3) is the hydrogen concentration on the surface of the alloy particles and t (s) is the discharge time. According to Eq. (3), it is reasonable to use D/a2 to simplify characterize the hydrogen diffusion kinetics in the bulk of alloy because of the similar particle radius. The D/a2 value were calculated and listed in Table 2. It can be seen that the calculated value of D/a2 for Mg2Ni alloy increase from 3.7 × 10-5 s-1 to 5.5 × 10-5 s-1 after rGO encapsulating, indicating that rGO facilitates the hydrogen diffusion which is in good agreement with the results of HRD measurement. To further investigate the charge-transfer properties, the charge-transfer resistance (Rct) of hydrogen storage electrode alloys are measured by the EIS.61 The EIS and corresponding equivalent circuit for bare Mg2Ni alloy and rGO/Mg2Ni composite are shown in Figure 8d which is obtained by using fitting software of “ZSimpWin”. The detailed fitted parameters and spectra are shown in Figure S9 and Table S4 in Supporting Information. Generally, a typical EIS of a hydrogen storage electrode alloy is composed of two semicircles and followed by a straight line in the low-frequency range. The small arc in the high-frequency region is assigned to contact resistance (Rcp) between the current collector (Ni foam) and the alloy particles. The large arc in the


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mid-low-frequency region is ascribed to the charge-transfer reaction resistance (Rct) on the alloy surface. The existing of the two processes are also supported by the two time constants in Bode plots shown as Figure S10 in Supporting Information. According to the equivalent circuit and fitted data, the values of Rct and Rcp are calculated and listed in Table 2. Rs, Zw, Q1 and Q2 in the Figure 8d represent the solution resistance, the Warburg impedance and the imperfect capacitors, respectively. Both of the Rcp and Rct of rGO/Mg2Ni composite electrode are lower than that of bare Mg2Ni electrode, showing the amelioration of the contact resistance between current collector and alloy particles and charge-transfer reaction resistance after the encapsulating of rGO. ■ CONCLUSIONS In order to improve the electrochemical properties of Mg2Ni hydrogen storage alloy, rGO is used to encapsulate the Mg2Ni alloy via electrostatic adsorption method. The effects of surface encapsulation on electrode performance is investigated in detail. Zeta potential, XRD results, and SEM/TEM observations reveal that the rGO has been successfully encapsulated on the surface of the Mg2Ni alloy particles. The rGO layer offers an effective barrier to protect the active materials from being corroded by the alkaline electrolyte. As a result, the capacity retention increases from 79% to 94% at the 10th cycle, and from 46% to 60% at the 50th cycle. Tafel polarization test shows the Ecorr increases from -0.933 V to -0.823 V and the ic reduces from 0.891 mA cm2 to 0.522 mA cm2 after rGO encapsulating. Meanwhile, from the tests of HRD, exchange current density (I0) and hydrogen diffusion coefficient (D), it can be concluded that the kinetics of the system have been enhanced by introducing the rGO encapsulating layer. Furthermore, the contact resistance (Rcp) and charge-transfer reaction resistance (Rct) are


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also reduced as proven by the EIS results. In conclusion, Mg2Ni alloy with greatly enhanced cycle stability in hydrogen storage and improved overall electrochemical properties has been achieved by encapsulating with rGO using a high efficient electrostatic assembly method. This work may have significant promoting effects on the practical application of Mg-based hydrogen storage alloys. ■ ASSOCIATED CONTENT Supporting Information Schematics of the charging/discharging between electrolyte and metal hydride, photographs of the self-assembly result of rGO and APS modified Mg2Ni, zeta potential data of rGO and APS modified Mg2Ni, high resolution XPS C1s of rGO/Mg2Ni, EDS analysis of the APS modified Mg2Ni and rGO/Mg2Ni alloy, tables of element and atomic percent of APS modified Mg2Ni and rGO/Mg2Ni alloy, TGA and DSC curves, EIS bode phase angle plot and fitted EIS spectra of Mg2Ni and rGO/Mg2Ni, table of fitting EIS results. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. ORCID Yi Du: 0000-0002-3233-5580 Qing-Ping Feng: 0000-0001-9697-9628 Gui-Wen Huang: 0000-0001-5186-1351 Notes


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The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS The authors gratefully acknowledge financial support received from Sino-tech Tenonscience & Technology Development Ltd. (2014110031009434). The authors would like to thank Dr. Yu Liu for the kind help in TGA/DSC tests. ■ REFERRENCES (1) Schlapbach, L.; Züttel, A. Hydrogen-storage Materials for Mobile Applications. Nature 2001, 414, 353-358. (2) Ruiz, F.; Castro, E.; Real, S.; Peretti, H.; Visintin, A.; Triaca, W. Electrochemical Characterization of AB2 Alloys Used for Negative Electrodes in Ni/MH Batteries. Int. J. Hydrogen Energy 2008, 33, 3576-3580. (3) Zhang, Y. H.; Yuan, Z. M.; Zhai, T. T.; Yang, T.; Bu, W. G.; Guo, S. H. Effects of Annealing Temperature on the Electrochemical Hydrogen Storage Behaviors of La-Mg-Ni-Based A2B7-Type Electrode Alloys. Metallurgical and Materials Transactions A 2015, 46, 2294-2303. (4) Yang, H. B.; Zhang, H. C.; Mo, W.; Zhou, Z. X. Mg1.8La0.2Ni–xNi Nanocomposites for Electrochemical Hydrogen Storage. J. Phys. Chem. B 2006, 110, 25769-25774. (5) Kohno, T.; Tsuruta, S.; Kanda, M. The Hydrogen Storage Properties of New Mg2Ni Alloy. J. Electrochem. Soc. 1996, 143, L198-L199. (6) Iwakura, C.; Inoue, H.; Zhang, S. G.; Nohara, S. Hydriding and Electrochemical Characteristics of a Homogeneous Amorphous Mg2Ni-Ni Composite. J. Alloys Compd. 1998, 270, 142-144. (7) Kohno, T.; Yamamoto, M.; Kanda, M. Electrochemical Properties of Mechanically Ground Mg2Ni Alloy. J. Alloys Compd. 1999, 293-295, 643-647. (8) Tian, Q. F.; Zhang, Y.; Chu, H. L.; Sun, L. X.; Xu, F.; Tan, Z. C.; Yuan, H. T.; Zhang, T. The Electrochemical Performances of Mg0.9Ti0.1Ni1−xPdx (x=0–0.15) Hydrogen Storage Electrode Alloys. J. Power Sources 2006, 159, 155-158. (9) Souza, E. C.; de Castro, J. F. R.; Ticianelli, E. A. A New Electrode Material for Nickel–metal Hydride Batteries: MgNiPt Alloy Prepared by Ball-milling. J. Power Sources 2006, 160, 1425-1430.


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Table of Contents:

The rGO coating provides anti-corrosion and electrochemical property enhancing effects in Mg-based hydrogen storage system


32

Reduced Graphene Oxide Coating with Anti-corrosion and

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