Reduced Graphene Oxide Coating with Anticorrosion and

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Aug 4, 2017 - Reduced Graphene Oxide Coating with Anticorrosion and. Electrochemical Property-Enhancing Effects Applied in Hydrogen. Storage System.
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Reduced Graphene Oxide Coating with Anticorrosion 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*,† †

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 S Supporting Information *

ABSTRACT: Low-capacity retention is the most prominent problem of the magnesium nickel alloy (Mg2Ni), which prevents it from being commercially applied. Here, we propose a practical method for enhancing the cycle stability of the Mg2Ni alloy. Reduced graphene oxide (rGO) possesses a graphene-based structure, which could provide high-quality barriers that block the hydroxyl in the aqueous electrolyte; it also possesses good hydrophilicity. rGO has been successfully coated on the amorphous-structured Mg2Ni alloy via electrostatic assembly to form the rGO-encapsulated Mg2Ni alloy composite (rGO/Mg2Ni). The experimental results show that ζ potentials of rGO and the modified Mg2Ni alloy are totally opposite in water, with values of −11.0 and +22.4 mV, respectively. The crumpled structure of rGO sheets and the contents of the carbon element on the surface of the alloy are measured using scanning electron microscopy, transmission electron microscopy, and energy dispersive spectrometry. The Tafel polarization test indicates that the rGO/Mg2Ni system exhibits a much higher anticorrosion 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 results on Mg2Ni capacity retention combined with the absolute value reported so far to our knowledge. In addition, both the charge-transfer reaction rate and the hydrogen diffusion rate are proven to be boosted with the rGO encapsulation. Overall, this work demonstrates the effective anticorrosion and electrochemical property-enhancing effects of rGO coating and shows its applicability in the Mg-based hydrogen storage system. KEYWORDS: reduced graphene oxide, surface modification, anticorrosion, hydrogen storage, magnesium-based alloy, electrochemical property



g−1).4 However, the Mg-based alloys are 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 progress has been made in developing Mg−Ni alloys. Kohno5 reported that by the addition of Ni and a mechanical grinding method, an amorphous-like Mg2Ni shows a much higher discharge capacity (750 mAh g−1) at room temperature, compared to 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 exceeded even the theoretically

INTRODUCTION As an increasing number of energy resources have been discovered by scientists, energy storage technology has become more and more important nowadays. Hydrogen is the most abundant element on Earth and is considered one of the cleanest energy sources.1 As a universally available storage type of hydrogen energy, the 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 interest owing to their high specific capacity, low cost, and abundance. Especially, the Mg2Ni alloy exhibits a promising high theoretical capacity of 1000 mAh g−1, which is approximately 2.7 times higher than that of the commercial LaNi5 alloy (ca. 372 mAh © 2017 American Chemical Society

Received: April 25, 2017 Accepted: August 4, 2017 Published: August 4, 2017 28980

DOI: 10.1021/acsami.7b05809 ACS Appl. Mater. Interfaces 2017, 9, 28980−28989

Research Article

ACS Applied Materials & Interfaces calculated value. Kohno7 also reported that the additional Ni may work as catalytic sites for 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 the Mg−Ni alloy during cycling is mainly associated with the following three aspects: (i) the formation of then Mg(OH)2 layer on the surface of the alloy in an alkaline electrolyte,8−11 which causes the loss of active materials and results in the reduction of the 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 To overcome these disadvantages, lots of efforts have been made. The commonly conducted methods in recent years include partly substituting Mg and/or Ni by certain elements,15−21 forming a composite material by adding compounds or elements,22,23 and building amorphous or nanocrystalline structures.24,25 Compared to these methods, surface modification 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 such as extended cycle life, reduced pulverization, and enhanced anticorrosion ability of the Mg−Ni alloy have been demonstrated by modification with nanometal particles or a conductive polymer.26−29 However, the property enhancements achieved are still far from satisfactory. Therefore, better materials and processes for surface modification are still highly desired. Since its discovery by Andre Geim, graphene has been gradually applied in almost all fields because of its excellent properties. It is reported that graphene can provide high-quality barriers that block all gases and liquids.30 Moreover, it is proved that protons can transport in and out across graphene.31 Figure S1 in the Supporting Information schematically shows the electrochemical reactions on a metal hydride during charge (a) and discharge (b). It can be seen that with the incorporation of water and OH−, protons are formed at the interface between the alloy and the electrolyte and then diffusion occurs in and out of the alloy during charging or discharging.32 Thus, on the basis of the information discussed above, graphene can be assumed to be an ideal material for surface modification of the Mg−Ni alloy. First, it serves as a barrier that separates the alloy and the alkaline electrolyte. Thus, the opportunity of the formation of Mg(OH)2 is greatly reduced, and the active content in the alloy is well maintained to play the role of hydrogen storage. Moreover, due to the proton permeability of the graphene, hydrogen protons generated during the electrochemical reactions freely diffuse across the barrier, which ensures the reactivity of the system. Second, after getting tightly coated on the alloy surface, the two-dimensional graphene with excellent mechanical property helps keep the mechanical integrality of the alloy, decreasing the pulverization effect during charging and discharging. Furthermore, several works have been carried out applying graphene-based materials in hydrogen storage systems. For instance, Huang28,33 synthesized a graphene/Ag composite and reported its favorable effects on improving discharge capacity, cycle life, discharge potential, and electrochemical kinetics of the 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 article, we present a practical method to improve the overall property of the Mg2Ni alloy by surface modification with reduced graphene oxide (rGO). rGO was chosen as the modifying material because compared with bare graphene without functional groups, rGO owns much better hydrophilicity, which is favorable in the aqueous system of a Ni−MH battery. At the same time, the rGO shows reasonably high electrical conductivity for performing the electron transmission during charging/discharging.34 rGO was coated on the Mg2Ni alloy particles through a facile electrostatic adsorption method. In this method, the amorphous Mg2Ni alloy with high initial discharge capacity was fabricated in advance by a melt-spinning and then a mechanical milling process. The obtained alloy particles were then modified by aminopropyltrimethoxysilane (APS) to get positive charges on the surface. Afterward, the modified alloy particles were added into the rGO dispersing solution. As rGO is negative in charge, they rapidly and closely adsorb onto the alloy particles in the mixed solution due to the electrostatic adsorption effect, forming the rGO-encapsulated Mg2Ni structure (rGO/Mg2Ni). The obtained composites show high cycling stability, with a 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 the 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 the Mg2Ni alloy, which is a significant step forward to the practical application of Mg-based hydrogen storage alloys.



EXPERIMENTAL SECTION

Preparation of Amorphous-Structured Mg2Ni Alloy. The Mg2Ni alloy belt was prepared by a melt-spinning process with a spinning speed of 25 m s−1 and then broken into powder in an agate mortar. The melt-spinning process was chosen because compared with other preparation methods, including mechanical alloying, sintering process, or hydriding combustion, it is highly 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 a total volume of 70 mL) for mechanical milling. The milling was conducted under an argon atmosphere at room temperature using a planetary-type ball milling machine at a speed of 350 rpm 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 to prevent overheating of the vessels and to obtain better homogeneity. The milling vessels were vacuumed and refilled with high-purity Ar gas three times before milling and were opened every 10 h during the milling procedure to crush the aggregation on the inside wall and the bottom of the vessels. All of the above-mentioned operations were performed in a glovebox filled with dry Ar gas to prevent oxidation. Preparation of rGO/Mg2Ni Alloy Composite. Briefly, 1 g of the obtained Mg2Ni alloy powder was dispersed in 100 mL of 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 an APS-modified Mg2Ni alloy powder. The rGO aqueous dispersion solution with a concentration of 1.02 wt % was purchased from Chengdu Organic Chemicals Co., Ltd., China. In a typical process, 0.6 g of the APSmodified Mg2Ni alloy powder is added into 30 mL of solution (1 mL of rGO aqueous dispersion solution and 29 mL of distilled water) and shaken 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 28981

DOI: 10.1021/acsami.7b05809 ACS Appl. Mater. Interfaces 2017, 9, 28980−28989

Research Article

ACS Applied Materials & Interfaces precipitated after allowing to stand 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 of the sample powder with 0.3 g of 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 a 6 mol L−1 KOH solution containing 20 g L−1 LiOH as the electrolyte at room temperature. In each testing unit, 150 mL of this electrolyte was used, and the separator applied was a 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 were calculated on the basis of 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 cutoff potential of 0.9 V after a 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 made 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 a +600 mV (vs open circuit potential) potential step and 100% depth of charge (DOC) for 3600 s. The above electrochemical tests except the EIS tests were conducted on a CHI660E electrochemical workstation. The EIS tests were measured under the same condition by analytical devices including Potentiostat/Galvanostat (model 263 A) and Frequency Response Detector (model FRD 100) purchased from Princeton Applied Research. Before performing 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 S4300 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) and a JEM-2100 transmission electron microscope (TEM). ζ Potential measurements were performed using a Zetasizer Nano ZS. Fourier 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.



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

negative charge in an aqueous solution. The macroscopic electrostatic adsorption effect is demonstrated in Figure S2 of the Supporting Information. In Figure S2a, the left vial shows the pristine rGO dispersion solution, which possesses good uniformity and high stability. After the addition of APSmodified Mg2Ni powder, as shown in the right vial, rGO gets rapidly and completely absorbed onto the alloy particles and precipitates with the powder. Compared to the gray powder of bare Mg2Ni (Figure S2b), the rGO/Mg2Ni powder (Figure S2c) is black in color, demonstrating the successful coating of rGO on the alloy powder. For further proof of the spontaneity of the electrostatic interaction, the ζ potentials of APS-modified Mg2Ni and graphene in aqueous solution (pH = 7) have been tested. Results (Figure S3 in the Supporting Information) show that the APS-modified Mg2Ni alloy powders are positively charged (+22.4 mV), whereas the rGO is negatively charged (−11.0 mV). The opposite charges between the alloy and the rGO make the electrostatic assembly take place spontaneously in the mixed solution. After assembly, the rGO is electrostatically coated on the Mg2Ni particles without forming a chemical bond, according to the XPS results shown in Figure S4 of the Supporting Information. 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 be attributed to 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 the cubic phase of periclase MgO.41 The band at 1235 cm−1 can be assigned to the bending vibration of the hydroxyl group (M−OH) on Mg2Ni alloy oxides,39,42 which proves the existence of hydroxyl groups on Mg2Ni. In Figure 2b, the FTIR spectrum shows the characteristic peaks for rGO, which illustrates the presence of various oxygen-containing groups on its surface. The peaks of rGO 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

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: modification of the hydroxyl groups on the Mg2Ni surface by APS and electrostatic self-assembly in the aqueous rGO dispersion solution. The existence of hydroxyl groups on the Mg2Ni surface may be due to the partial 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 the Mg2Ni alloy and rGO were performed and are presented in Figure 2a,b, which proves the existence of hydroxyl groups on the surface of the Mg2Ni alloy and depicts the chemical structure of rGO. First, the hydroxyl groups are modified by grafting of APS to render the Mg2Ni surface positively charged. Then, the rGO-encapsulated Mg2Ni alloy is fabricated via the electrostatic interaction between the modified Mg2Ni alloy with a positive charge and rGO with a 28982

DOI: 10.1021/acsami.7b05809 ACS Appl. Mater. Interfaces 2017, 9, 28980−28989

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ACS Applied Materials & Interfaces

Figure 2. FTIR spectra of Mg2Ni (a) and rGO (b). (c) XRD patterns of the pristine Mg2Ni alloy, milled for 100 h, rGO, and rGO/Mg2Ni. (d) HRTEM and the inserted corresponding selected area electron diffraction (SAED) images (d) of the 100 h milled Mg2Ni alloy.

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 a crystalline structure to an amorphous structure. Figure 2c(c) presents the XRD pattern of the rGO, as expected, with no obvious diffraction peaks.38 Owing 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 that of the bare Mg2Ni alloy. In Figure 2d, the high-resolution transmission electron microscopy (HRTEM) image of the milled Mg2Ni alloy shows many disorderly stripes, and the selected area electron diffraction (SAED) image exhibits a broad ring, which further indicates the disappearance of the long-range ordered structure in the alloy after 100 h of ball milling, namely the formation of the amorphous structure. Figure 3 shows the morphology of the 100 h milled Mg2Ni before and after rGO encapsulation. From Figure 3a, it can be seen that the bare Mg2Ni particles are scattered in distribution with particle sizes varying from a few microns to more than 10 μm. After rGO encapsulation, it can be seen from Figure 3b that the particles are relatively concentrated and the surface of Mg2Ni is wrapped in a layer of rGO. With higher magnification, by comparing Figure 3c,d, one can distinctly catch the sight of

rGO sheets uniformly covered on the surface of Mg2Ni particles, suggesting that such an efficient electrostatic selfassembly approach can make the two ingredients undergo a relatively adequate interfacial interaction. Figure 3e,f displays 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 the surface of Mg2Ni, wherein the crumpled structure of 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 Figures S5−S7 and Tables S1−S3 in the Supporting Information. The results adequately indicate the uniform encapsulation of rGO sheets on the alloy particles by comparing the atomic percent of carbon in the contrastive samples. Figure S8 in the 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 that of the TGA curve of the bare Mg2Ni, it is clear that the rGO coating in rGO/Mg2Ni provides a protective layer that slows down the oxidation process. The effect of rGO encapsulation on the discharge behaviors of the Mg2Ni alloy is shown in Figure 4. The discharge capacity of the bare alloy reaches its highest value of 583 mAh g−1 after 28983

DOI: 10.1021/acsami.7b05809 ACS Appl. Mater. Interfaces 2017, 9, 28980−28989

Research Article

ACS Applied Materials & Interfaces

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.

(358 mAh g−1) of the initial capacity can be maintained after 50 cycles. These achievements in our work 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 the Mg2Ni alloy. The dramatic property degradation of the bare alloy mainly comes from the corrosion and the formation of the Mg(OH)2 layer on the surface by reaction with the hydroxyl in the electrolyte.8−11 Once the insulating Mg(OH)2 layer is formed, the interparticle 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 was 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 keep the integrality of the alloy structure and decrease the pulverization effect during cycling, which also leads to an increase in cycle life. For further understanding of the selective barrier effect of the rGO, Figure 6 shows a schematic representation of electrochemical hydrogen absorption and desorption in the charging/ discharging process. During the discharge, as illustrated in Figure 6a, hydrogen atoms are desorbed first from the amorphous Mg2Ni and diffused to the two-phase (Mg2Ni and rGO) interface. To combine with the hydroxyl in the electrolyte and release electrons, the hydrogen atoms can

Figure 4. Discharge capacities as functions of cycle number for the bare Mg2Ni alloy and rGO/Mg2Ni composite electrodes.

two cycles, whereas the capacity of the rGO/Mg2Ni reaches the highest value of 594 mAh g−1 after four cycles. The slight delay of activation may be attributed to the building of more complex proton paths in the rGO/Mg2Ni system. However, for the bare alloy, the discharge capacity decreases sharply after activation, similar to previously reported results,10,48−57 with the capacity retention dropping to almost 79% (459 mAh g−1) after only 10 cycles. In contrast, the rGO/Mg2Ni exhibits a greatly enhanced capacity-maintenance property. A high capacity retention of 94% (557 mAh g−1) is achieved at the 10th cycle, and over 60% 28984

DOI: 10.1021/acsami.7b05809 ACS Appl. Mater. Interfaces 2017, 9, 28980−28989

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ACS Applied Materials & Interfaces

Figure 5. SEM images of the rGO/Mg2Ni composite under different magnifications: (a, c) 0 cycle; (b, d) 50 cycles.

Figure 6. Schematic of electrochemical hydrogen absorption and desorption in discharging (a) and charging (b) processes.

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. The process of charging is exactly the opposite of that of discharging, as described in Figure 6b. Because the graphene-based structure could provide highquality barriers that block all gases and liquids, the hydroxyl in the aqueous electrolyte cannot contact with the alloy directly during the reaction. Thus, the alloy can be protected from being corroded by the alkaline electrolyte. Consequently, the encapsulating rGO simultaneously offers corrosion resistance and fast pathways for the protons and the electrons. To investigate the anticorrosion ability of the alloy electrodes, the Tafel polarization test was performed. The

polarization curves for the bare Mg2Ni alloy and rGO/Mg2Ni are shown in Figure 7, and 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 positive direction and the ic is lower than that of the bare Mg2Ni alloy, suggesting that the rGO encapsulation can enhance the anticorrosion ability of the bare Mg2Ni alloy. The HRD data of the bare Mg2Ni alloy and the rGO/Mg2Ni composite are shown in Figure 8a. The HRD is a comprehensive index reflecting the kinetic properties of hydrogen storage alloy electrodes, which is defined by the following equation 28985

DOI: 10.1021/acsami.7b05809 ACS Appl. Mater. Interfaces 2017, 9, 28980−28989

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ACS Applied Materials & Interfaces I0 =

Cd × 100% Cd + C100

(2)

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 the rGO/Mg2Ni composite. On the basis of the measured curves, the exchange current densities of the bare Mg2Ni alloy and the 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 increases from 92.7 to 225.9 mA g−1, owing to the addition of rGO. This change can be attributed to the increase in interparticle ionic conductivity, which is favorable for the charge-transfer process during cycling. The hydrogen diffusion rate in the bulk of the alloy 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 (