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Effect of reduced graphene oxide-supported copper addition on electrochemical properties of La0.7Mg0.3Ni2.8Co0.5 electrodes
Journal Pre-proof Effect of Reduced Graphene Oxide-supported Copper Addition on Electrochemical Properties of La0.7Mg0.3Ni2.8Co0.5 Electrodes Xiantun Huang, Zhiqiang Lan, Jiacheng Li, Hao Zhang, Jin Guo PII:
S1002-0721(19)30244-3
DOI:
https://doi.org/10.1016/j.jre.2019.06.003
Reference:
JRE 585
To appear in:
Journal of Rare Earths
Received Date: 27 March 2019 Revised Date:
7 June 2019
Accepted Date: 10 June 2019
Please cite this article as: Huang X, Lan Z, Li J, Zhang H, Guo J, Effect of Reduced Graphene Oxidesupported Copper Addition on Electrochemical Properties of La0.7Mg0.3Ni2.8Co0.5 Electrodes, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.06.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Effect of Reduced Graphene Oxide-supported Copper Addition on Electrochemical Properties of La0.7Mg0.3Ni2.8Co0.5 Electrodes Xiantun Huanga, b, Zhiqiang Lanb,*, Jiacheng Lib, Hao Zhangb, Jin Guob a
Department of Materials Science and Engineering, Baise College, Baise 533000, China Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related Technology, College of Physics Science and Technology, Guangxi University, Nanning 530004, China b
Abstract: The reduced graphene oxide-supported copper (Cu@rGO) nanocomposite was introduced to improve the electrochemical properties of La0.7Mg0.3Ni2.8Co0.5alloy electrodes. Experimental results show that adding Cu@rGO nanocomposite with mass fractions of x wt% (x=0, 3, 6 and 9) to the alloy electrodes provides electrodes with maximum discharge capacities of 368.9 mAh/g (x=0), 373.2 mAh/g (x=3), 407.3 mAh/g (x =6) and 398.6 mAh/g (x=9), and high-rate dischargeabilities at a discharge current density of 1200 mA/g of 40.5% (x=0), 64.0% (x=3), 82.0% (x=6) and 76.0% (x=9). The addition of Cu@rGO nanocomposite also provides alloy electrodes with hydrogen diffusion coefficients of 3.7×10−10cm2/s (x=0), 4.1×10−10cm2/s (x=3), 4.2×10−10cm2/s (x=6) and 4.0×10−10cm2/s (x=9). Clearly, the addition of 6 wt% Cu@rGO nanocomposite not only increases the electrochemical capacity of La0.7Mg0.3Ni2.8Co0.5 alloy electrodes, but also improves their electrochemical kinetic properties. Keywords: Kinetic properties; alloy electrode; electrochemical properties; grapheme 1. Introduction Lanthanum-magnesium-nickel hydrogen storage alloys have attracted considerable attention for use as active materials in the negative electrodes of Ni/MH batteries due to their high hydrogen storage capacity and reduced environmental impact [1, 2]. However, the electrochemical cycling stability and electrochemical kinetics of La-Mg-Ni alloy electrodes must be improved to meet the requirements of practical applications. A number of approaches have been commonly employed to improve the overall electrochemical properties of La-Mg-Ni alloy electrodes, such as the addition or substitution of elements [3-9], heat treatment [10, 11], and surface modification [12, 13]. Among these approaches, the addition or substitution of elements is considered to be one of the most effective. For example, the addition of Al has been demonstrated to be advantageous for suppressing the expansion of the (La, Mg)2Ni7 unit cell volume in La-Mg-Ni alloy electrodes during hydrogen absorption [3].Moreover, in an alkaline solution, Al readily forms a dense Al2O3 film covering the surface of the electrode, which can effectively improve its corrosion resistance, and significantly improve its cycle life. However, the Al2O3 film also inhibits the diffusion of hydrogen atoms in the alloy electrodes, resulting in decreased kinetic properties. Additionally, Wu et al reported [4] that the cycling stability of the La-Mg-Ni alloy electrodes can be enhanced by replacing partial La with Ce, and the La0.65Ce0.1Mg0.25Ni3Co0.5 alloy electrode exhibits the best electrochemical properties among the La0.75-xCexMg0.25Ni3Co0.5 (x=0, 0.05, 0.10, 0.15, 0.2 at.%) samples. And Co substitution for Ni can regulate the phase composition in La-Mg-Ni alloy also had been studied by Wu et al [6]. The studied results show that the amount of (LaMg)2Ni7 phase *
Corresponding author: Lan zhiqiang; E-mail:[email protected],Tel:0771-3237386 Acknowledgement: The National Natural Science Foundation of China (51571065), the Natural Science Foundation of Guangxi Province (017GXNSFAA198337, 2018GXNSFAA294125, 2018GXNSFAA281308) and the Innovation-Driven Development Foundation of Guangxi Province (AA17204063).
decreased with Co content increasing. Zhang et al [7] reported that the cycle stability of the La-Mg-Ni can be enhanced by replacing partial La with Sm, but it was not conducive to improve the high-rate discharge ability of the alloy. Substituting a portion of the Ni with Fe can increase the electrochemical capacity of La-Mg-Ni electrodes by increasing the volume of the primary alloy phase in the electrodes, and thereby provides a greater number of storage sites for the hydrogen atoms [8]. However, Fe readily forms an oxide covering the surfaces of the electrodes in an alkaline electrolyte, which limits the diffusion of hydrogen atoms, and correspondingly degrades the kinetic properties of the electrodes. The partial substitution of Ni with Cu can also improve the cycling stability of La-Mg-Ni alloy electrodes [9]. However, as is obtained under the partial substitution of Ni with Fe, the kinetic properties of the La-Mg-Ni alloy electrodes are degraded after the addition of Cu. While these past studies indicate that the addition of metal elements such as Al, Fe, and Cu can improve the electrochemical cycling stability of La-Mg-Ni alloy electrodes, the development metal oxide films inhibits the diffusion of hydrogen atoms in the alloy electrodes, resulting in decreased kinetic properties. In recent years, the discovery of graphene has made it possible to improve the overall performance of La-Mg-Ni alloy electrodes. Graphene is a special two-dimensional carbon nanomaterial composed of carbon atoms with sp² hybrid orbitals forming a hexagonal honeycomb lattice and includes a number of advantageous properties such as a large specific surface area (theoretical value of 2630 m2/g), high catalytic activity and high electron mobility (1.5×105 cm2/(V·s) at room temperature) [14, 15]. Currently, graphene has been widely used to excellent effect in numerous applications such as semiconductor materials [16, 17], super capacitors [18, 19] and hydrogen storage materials [20, 21]. For example, Ouyang et al [22] found that the addition of 1 wt% graphene to an AB3 hydrogen storage alloy increased the high-rate dischargeability (HRD) of the alloy electrodes from 53.7% to 89.6% at a large discharge current density of 1750 mA/g. Huang et al [23] found that the addition of nanocomposites formed of graphene and Ag nanoparticles greatly improved the maximum discharge capacity and HRD of Mg65Ni27La8 alloy electrodes. In particular, adding 20 wt% graphene/Ag nanocomposites to the alloy attained a maximum discharge capacity of 814.8 mAh/g. In addition, our previous research work [24] demonstrated that the addition of graphene-supported Ni to La-Mg-Ni alloy electrodes could not only improve their electrochemical capacity, but also enhance their kinetic properties under the synergistic effect of reduced graphene oxide and Ni. A2B7 alloy with superstructure exhibits excellent electrochemical performance compared to AB5 alloys. It was reported that the La0.7Mg0.3Ni2.8Co0.5 alloy was mainly consisted of A2B7 phase, and the electrochemical maximum discharge capacity was around 410 mAh/g, which was 1.3 times higher discharge capacity than that of AB5 alloy [25]. Thus, La0.7Mg0.3Ni2.8Co0.5 series alloys had been considered to be an ideal candidate material to fabricate the negative electrode for Ni/MH battery. The present work capitalizes on the success of past efforts to prepare La0.7Mg0.3Ni2.8Co0.5 hydrogen storage alloy electrodes with reduced graphene oxide-supported Cu (Cu@rGO) nanocomposite prepared via hydrothermal synthesis as a catalyst additive. This work not only makes effective use of the good electrical conductivity of metallic Cu, but also benefits from its relatively low price. The effect of Cu@rGO addition on the electrochemical properties of the La0.7Mg0.3Ni2.8Co0.5 alloy electrodes is experimentally investigated, and the results demonstrate that Cu@rGO addition can substantially increase the maximum discharge capacity, HRD and hydrogen diffusion of the electrodes. 2. Experimental methods 2.1 Preparation of reduced graphene-supported copper nanocomposite
The specific reduced graphene-supported Cu nanocomposite preparation process employed is illustrated in Fig.1. Here, graphite oxide powder (purity ≥ 99.85%, Nanjing XFNANO Materials Tech Co., Ltd.) and deionized water were thoroughly mixed at a mass ratio of 1:1000 and subjected to ultrasonic excitation for 30 min. Copper acetate (analytic reagent) dissolved in water was added, and the produced Cu2+ ions combined with the graphite oxide particles by electrostatic forces in the water under stirring for 1 h using a magnetic stirrer and allowing the mixture to stand for 24 h. Subsequently, aqueous ammonia (25%–28%) was slowly dropped into the resulting suspension, and, after filtration, the precipitate was collected, excess ions were washed away with pure water, and the solid was dried for use. The as-prepared product was subjected to a reduction reaction in a hydrogen-argon mixed atmosphere at a temperature of 1023 K for 4 h to obtain the Cu@rGO nanocomposite. 2.2 Preparation of alloy samples and alloy electrodes The purity of the La, Mg, Ni, and Co raw materials used in the preparation of alloy samples were all above 99%, and the samples were prepared according to the stoichiometric ratio of La0.7Mg0.3Ni2.8Co0.5. The alloy was prepared by smelting the raw materials in a magnetic levitation high frequency induction furnace under the protection of an Ar atmosphere. To compensate for the loss of Mg during the smelting process due to volatilization, the mass of Mg used in the starting material was increased by 10 wt% based on the desired stoichiometric ratio. The alloy was repeatedly inverted and smelted three times during the smelting process to ensure a uniform composition and reduced segregation of components. The obtained alloy ingot was ground and crushed, passed through a 200-mesh sieve, and divided into four equal portions that served as alloy samples. Finally, the Cu@rGO nanocomposite was added in mass fractions of x wt% (x=0, 3, 6 and 9) to the alloy samples, and the mixture was subjected to mechanical ball milling at a milling speed of 300 r/min, a ball-to-batch ratio of 20:1, and a milling time of 10 min. To form an alloy electrode, an appropriate amount of an alloy powder and carbonyl Ni powder were thoroughly mixed at a mass ratio of 1:4, and then cold pressed for 10 min under a pressure of 20 MPa using a powder tablet press machine to obtain a circular alloy electrode sheet with a diameter of 10 mm and a thickness of about 3 mm. 2.3 Characterization The phase compositions and structures of the alloys were analyzed by X-ray diffraction (XRD) using a Rigaku Miniflex 600 diffractometer. The light source was Cu target Kα X-rays, and measurements were conducted with a 2θ scan range of 10° to 80° and a scan speed of 10°/min. The surface morphology and elemental concentrations of the Cu@rGO nanocomposite were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) using a JEOL JSM-6510A microscope. The alloy electrodes were tested in an open three-electrode test system with a Ni(OH)2/NiOOH electrode as the positive electrode and a Hg/HgO electrode as the reference electrode. A KOH solution with a concentration of 6 mol/L was used as the electrolyte. In cycling stability testing, the charge current density was 100 mA/g, the discharge current density was 80 mA/g, and the discharge cut-off voltage was 500 mV. The electrodes were subjected to cyclic voltammetry testing with a voltage scan range of −1.2 to 0 V and a scan rate of 5 mV/s, Tafel polarization curve testing with a scan voltage of −0.3 to 1.0 V and a scan rate of 10 mV/min, and electrochemical impedance spectroscopy (EIS) testing with a starting frequency of 50 kHz and an endpoint frequency of 0.003 Hz using a Gamry Instruments Reference-1000 electrochemical workstation. 3. Results and discussion 3.1 Phase composition and microstructure
The XRD patterns of the Cu@rGO nanocomposite and the base La0.7Mg0.3Ni2.8Co0.5 alloy are shown in Fig.2(a) and (b), respectively. The diffraction peaks of metallic Cu are observed at 2θ=43°, 50° and 74° in Fig.2(a), indicating that the Cu2+ ions have been successfully reduced to metallic Cu. As can be seen from Fig.2(b), the alloy mainly consisted of a La2Ni7 phase of the Ce2Ni7 type, a LaNi3 phase of the PuNi3 type, and a LaNi5 phase of the CaCu5 type. The Ce2Ni7-type phase can be described by stacking along c axis of CaCu5-type and Laves-type subunit in the rate of 2:1[26]. Additionally, the CaCu5-type La-Ni alloys can convert to the Ce2Ni7-type super-stacking phases via a peritectic reaction. Han et al [27] demonstrated that the phase abundance of the Ce2Ni7-type in the La-Mg-Ni alloy increased with increasing the annealing temperature. Additionally, it had been proved that the Ce2Ni7-type phase ultimately stayed stable in a temperature range of 1223K-1243K. The distribution of elemental Cu on the reduced graphene oxide substrate of the Cu@rGO nanocomposite was investigated using SEM and EDS, and the results are presented in Fig.3. As can be seen from Fig.3(a) and (b), Cu particles with diameters of 50 nm to 100 nm were dispersed and distributed on the reduced graphene oxide substrate, forming a disc-shaped structure. From Fig.3(c) and (d), we note that the distribution of Cu and C were relatively uniform, indicating that a Cu@rGO composite has been successfully prepared by the hydrothermal method. 3.2 Electrochemical performance Fig.4(a) presents the discharge curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0,3,6 and 9) alloy electrodes after being sufficiently activated by 2-3 times charge-discharge cycles. We note that relatively flat discharge curves were obtained for all of the alloy electrodes evaluated. However, the electrodes corresponding to x=6 and x=9 exhibit discharge curve plateaus at significantly higher voltages than the electrodes corresponding to x=3 and x=0, indicating that the addition of Cu@rGO nanocomposite can effectively increase the discharge power of the alloy electrodes. The maximum discharge capacity (Cmax) of each alloy electrode was calculated from the data presented in Fig.4(a), and the results are listed in Table 1. The results in Table 1 show that the value of Cmax increases with increasing x up to x=6, and then decreases slightly for x=9. This indicates that the addition of the Cu@rGO nanocomposite can increase the electrochemical capacity of the La0.7Mg0.3Ni2.8Co0.5 alloy. Since the carbon materials have good electrical conductivity and unique structure, which is beneficial to the diffusion of hydrogen atoms, adding an appropriate amount of carbon materials to the Ni-MH batteries had proved to be an effective way to enhance the electrochemical capacity of the materials [28, 29]. Therefore, the improvement in the discharge capacity for the La0.7Mg0.3Ni2.8Co0.5 electrodes may be due to the fact that the reduced graphene oxide-copper framework with a loose porous structure provides more channel for hydrogen atom diffusion. The discharge potential of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes at 50% depth of discharge was –0.891, –0.881, –0.891 and –0.882 V, respectively. The discharge platform for the electrodes broaden as the x value increasing to x=6 and x=9. It was evident that the addition of Cu@rGO nanocomposite had a positive effect on the discharge capacity of the electrodes. Fig.4(b) shows the cycle life of La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. As shown, after 30 times charge-discharge cycles, the capacity retention ratio for the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) electrodes was 83.8%, 82.2%, 80.1% and 78.8%, respectively. Obviously, the addition of the Cu@rGO nanocomposite cannot be improved the cycle life of the La0.7Mg0.3Ni2.8Co0.5 electrode, but it can enhance the discharge capacity slightly. Generally, during the charge process, the hydrogen atoms are first adsorbed on the surface of a hydrogen storage electrode, and then diffused from the surface toward the interior of the electrode. And in the discharge process, the diffusion path of the hydrogen atom is exactly the opposite.
Therefore, the kinetic properties of an electrode are mainly controlled by its surface charge transfer rate and internal charge diffusion rate [30, 31]. The kinetic properties of an electrode can be characterized by its high rate dischargeability (HRD), where an increasing HRD indicates better kinetic properties. The HRD of an electrode discharged at a relatively large current density of In can be calculated as
HRDn =
Cn ×100% Cn + C60
(1)
where Cn represents the discharge capacity at In, and C60 represents the residual discharge capacity of the electrode when discharged to a cut-off potential at a small current density of 60 mA/g after being discharged at In to a cut-off potential. Fig.5 shows the HRD curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes at different In (i.e., 300, 600, 900 and 1200 mA/g). It can be seen from Fig.5 that the HRD of the alloys decreased with increasing In, but that the HRD of the alloy electrodes decreased less under the catalytic action of the Cu@rGO nanocomposite. At small In, the HRD values of the alloy electrodes were essentially equivalent. For example, the HRD at In=300 (i.e.,HRD300) of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes were 96%, 96%, 97% and 96%, respectively. However, the values of HRD1200 obtained for the alloy electrodes corresponding to x=0, 3, 6 and 9 decreased to 40.5%, 64.0%, 82.0% and 76.0%, respectively. The addition of the Cu@rGO nanocomposite obviously increased the HRD of the alloy electrodes, and the highest HRD was obtained for the alloy electrode corresponding to x=6. It was reported that the electrochemical kinetic properties of the La-Mg-Ni alloy electrodes was mainly controlled by the charge transfer rate on the surface of the electrodes and the hydrogen diffusion rate in the bulk electrodes [30, 31]. The Cu@rGO catalyst includes metallic Cu embedded in a reduced graphene oxide (rGO) matrix, which forms an rGO-Cu framework. The ultra-high specific surface area and good electrical conductivity of the rGO coupled with the good electrical conductivity of Cu ensures that the rGO-Cu framework of the added Cu@rGO composite improves the internal contact of the La0.7Mg0.3Ni2.8Co0.5 alloy electrode. As shown in Fig.6, the straight slope corresponding to x=6 alloy is the smallest, suggesting that adding 6wt.% of Cu@rGO composite can decreased the polarization resistance of the alloy. The charge transfer rate on the electrodes surfaces can be characterized according to the exchange current density I0, because the exchange current density I0 increases with increasing charge transfer rate on the electrodes surface. The value of I0 can be calculated using the Butler-Volmer equation, as follows [32]:
I0 =
IRT RT = Fη FRP
(2)
Here, I0 is the current density (mA/g), R is the molar gas constant (8.314 J·mol/K), T is the testing temperature (298 K), F is the Faraday constant (96.487 C/mol), η is the polarization voltage (mV), and Rp is the polarization resistance of the alloy electrodes (mΩ). Normally, Rp can be calculated from linear polarization curves. Fig.6 shows the linear polarization curves obtained with a depth of discharge (DOD) of 50% for the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. The values of I0 calculated according to Equation (2) with the values of Rp obtained from the linear polarization curves in Fig.6 are listed in Table 1. As can be seen from Table 1, the value of I0 was greatest for the alloy electrode corresponding to x=6. We can accordingly conclude that the surface charge transfer rate of this alloy electrode was the largest, which indicates that the addition of an appropriate amount of Cu@rGO composite helped to accelerate the transfer of hydrogen atoms on the surface of the alloy electrode. Fig.7 presents the EIS graphs (DOD = 50%)
of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. It can be seen from the figure that the EIS graphs of all alloy electrodes are composed of a large semicircle in the middle to low frequency regions and a diagonal line in the low frequency region. Generally, the EIS graph of an La-Mg-Ni alloy electrode consists of a small semicircle in the high frequency region, a large semicircle in the middle to low frequency region, and a diagonal portion in the low frequency region. These three sections correspond to the contact resistance (Rcp) between the alloy electrode and the current collector, the electrochemical reaction charge transfer impedance (Rct) on the surface of the alloy electrode, and the Warburg diffusion impedance (W1) [24, 33]. This equivalent circuit is illustrated in the inset of Fig.7. The value of Rct was calculated from the equivalent circuit in conjunction with Fig.7, and the calculated results are listed in Table 1. We note that the value of Rct for the alloy electrodes obviously decreased under the beneficial action of the rGO-Cu framework. Cyclic voltammetry is another important method employed for characterizing the kinetic properties of alloy electrodes. Fig.8 shows the cyclic voltammetry curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. It can be seen from the figure that the cyclic voltammetry curves of the alloy electrodes with added Cu@rGO exhibit two anodic peaks PO1 and PO2 and three cathodic peaks PR1, PR2, and PR3, while only a single anodic peak PO1 and a single cathodic peak PR3 are observed for the base La0.7Mg0.3Ni2.8Co0.5 alloy electrode. As shown in Fig.8, the PO1 peaks of the alloy electrodes appear in the oxidation process within the potential range of 500 mV to 650 mV, which correspond to hydrogen absorption and oxidation on the surfaces of the alloy electrodes, and the PO2 peaks correspond to the oxidation of Cu to form CuO. In the reduction process, the corresponding CuO is converted to Cu2O, and then Cu2O is converted into Cu [34]. Therefore, the following equation can be used to describe the reduction process. 2CuO + H 2 O + 2e − → Cu 2 O + 2OH − Cu 2 O + H 2 O + 2e − → 2Cu + 2OH −
(3)
The peak current density Ip of the hydrogen absorption corresponding to oxidation process of the alloy electrodes was obtained from Fig.8, and the results are listed in Table 1. We note that the value of Ip increased with increasing x up to x=6, and then decreased slightly for x=9. Increasing values of Ip indicate better surface activities for alloy electrodes [35, 36]. As such, these results indicate that the addition of Cu@rGO significantly improved the electrochemical activity of the base La0.7Mg0.3Ni2.8Co0.5 alloy electrode. The fact that the highest electrochemical activity was obtained for the alloy electrode corresponding to x=6 is consistent with the HRD test results. In addition, the area of the PO1 peak corresponds to the electrochemical capacity of the electrode, where the electrochemical capacity increases with increasing peak area [37]. From Fig.8, we note that the PO1 peak area is largest for x=6, and the PO1 peak area for x=9 was only slightly less, which is consistent with the results obtained from Fig.4. Finally, the reversibility of the alloy electrodes can be reflected by the difference between the oxidation peak potentials and the reduction peak potentials of the cyclic voltammetry curves in Fig.8, where the reversibility increases as this difference decreases. As shown in Fig.8, the voltage difference between the oxidation peak potentials (PO1) and the reduction peak potentials (PR3) for the alloy electrodes is decreased when the Cu@rGO was used as catalyst, suggesting that addition of Cu@rGO was obviously beneficial for improving the electrochemical reversibility of the alloy electrodes. Fig.9 shows the Tafel polarization curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes, which can be employed to study the diffusion of hydrogen atoms in the alloy electrodes. As can be seen from Fig. 9, the active region and the passivation region appeared
in sequence during the anodic polarization process as the scanning potential increased. In the active region, the anodic current density increases to a maximum value then decreases with the increasing of potential, the limiting current density which corresponding to the maximum point is defined as the limiting current density (IL). An oxidation reaction occurred and forming the oxidation film which covering on the surface of the electrodes in this process. Obviously, the oxidation film was not benefit for the hydrogen atom diffusion. So the limiting current density (IL) should be considered as the critical current density for the electrodes [38]. The values of IL for the alloy electrodes were obtained from Fig.9, and the results are listed in Table 1. Here, we note that the addition of 3 wt%Cu@rGO negatively affected IL, while IL was significantly increased for an addition of 6 wt%Cu@rGO, (i.e., increased by 63.4%), and IL was only slightly less for x=9. The value of IL for the alloy electrodes mainly reflects the diffusion rate of hydrogen atoms in the alloys [39]. Therefore, the alloy electrode corresponding to x=6 in particular exhibited an increased diffusion rate for hydrogen atoms in the electrode. Combining the results obtained for I0 and IL indicates that the electrochemical kinetic properties of the alloy electrodes were mainly affected by the synergistic effect of the mass transfer resistance on the surface of the alloy electrodes and the diffusion rate of hydrogen atoms inside the electrodes. Fig.10 presents logarithmic curves of the anodic currents of La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes with respect to time, which can be used to further study the diffusion of hydrogen atoms inside the alloy electrodes. It is clear that the anodic current density (A/g) versus time (s) (lg(i)-t)) response curves of the alloy electrodes can be divided into two phases [40]. In the first phase, the oxidation current of hydrogen decreased rapidly in the beginning due to the rapid consumption of hydrogen on the surface of the alloy. In the second phase, the current decreased, and the values of lg(i) and t were basically in a linear relationship, where the current of the alloy electrode was mainly controlled by the diffusion of hydrogen in the alloy. The diffusion rate D of hydrogen atoms in the alloy electrodes can be calculated from the slopes of the linear portions of the curves given in Fig.10 by fitting to the following equation [41]: ி
lg݅ = lg ൬
ௗ
ሺܥ − ܥ௦ ሻ൰ − మ
గమ
ଶ.ଷଷమ
ݐ
(4)
where C0 and Cs are the initial concentration of hydrogen in an alloy electrode and the concentration of hydrogen on the surface of the alloy electrode, respectively, and α and d are the average alloy particle radius and the density of the alloy, respectively. The values of D calculated for the alloy electrodes are listed in Table 1. Again, it can be seen from Table 1 that the value of D increased with increasing x up to x=6, and decreased slightly for x=9. Therefore, the addition of the Cu@rGO nanocomposite was beneficial for enhancing the diffusion of hydrogen atoms in the alloy electrodes. This may due to the unique properties of the rGO-Cu framework, and, in particular, its high porosity, which facilitated the diffusion of hydrogen atoms in the alloy electrodes by providing more convenient channels for its propagation. 4. Conclusions In this paper, reduced graphene oxide-supported Cu (Cu@rGO) nanocomposite was prepared by a hydrothermal method, and its effects on the electrochemical hydrogen storage performance of La0.7Mg0.3Ni2.8Co0.5 alloy electrodes with x wt%Cu@rGO (x=0, 3, 6 and 9) were studied. The addition of Cu@rGO nanocomposite has a positive effect on the electrochemical hydrogen storage performance of La0.7Mg0.3Ni2.8Co0.5 alloy electrodes. The maximum discharge capacity of the alloy electrodes increased from 368.9 mAh/g at x=0 to 373.2 mAh/g at x=3 to 407.3 mAh/g at x=6, and then decreased slightly to 398.6 mAh/g at x=9. The addition of Cu@rGO nanocomposite promotes the diffusion of hydrogen atoms in the alloy electrode bulks, thereby improving the electrochemical
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Graphical Abstract
The electrochemical kinetics of the La0.7Mg0.3Ni2.8Co0.5 alloy electrodes can be enhanced by adding Cu@rGO nanocomposite, and the La0.7Mg0.3Ni2.8Co0.5+6wt%Cu@rGO nanocomposite shows an excellent comprehensive electrochemical properties.
Figures and tables
Fig.1 Preparation process of the reduced graphene-supported Cu nanocomposite
Fig.2 XRD patterns of the Cu@rGO nanocomposite (a) and the La0.7Mg0.3Ni2.8Co0.5 alloy (b)
Fig.3 SEM images (a–b) and EDS (c–d) results of the Cu@rGO nanocomposite
Fig.4 Discharge curves (a) and Cycle life (b) of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.5 High-rate discharge curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.6 Linear polarization curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.7 Electrochemical impedance spectroscopy graphs of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x= 0, 3, 6 and 9) alloy electrodes
Fig.8 Cyclic voltammetry curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.9 Tafel polarization curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.10 Logarithmic curves of the changes in anodic current with respect to time for the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Table 1 Parameters of the electrochemical properties for the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Sample
Cmax (mAh/g)
I0 (mA/g)
Rct (mΩ)
Ip (mA/g)
IL (mA/g)
R0 (mΩ)
D×10−10 (cm2/s)
x=0
368.9
61.2
200.2
1808
1558
236
3.7
x=3
373.2
57.1
86.9
2218
1258
221
4.1
x=6
407.3
81.3
109.9
2983
2547
318.2
4.2
x=9
398.6
61.5
118.6
2503
2160
421.1
4.0
S1002-0721(19)30244-3
DOI:
https://doi.org/10.1016/j.jre.2019.06.003
Reference:
JRE 585
To appear in:
Journal of Rare Earths
Received Date: 27 March 2019 Revised Date:
7 June 2019
Accepted Date: 10 June 2019
Please cite this article as: Huang X, Lan Z, Li J, Zhang H, Guo J, Effect of Reduced Graphene Oxidesupported Copper Addition on Electrochemical Properties of La0.7Mg0.3Ni2.8Co0.5 Electrodes, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.06.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.
Effect of Reduced Graphene Oxide-supported Copper Addition on Electrochemical Properties of La0.7Mg0.3Ni2.8Co0.5 Electrodes Xiantun Huanga, b, Zhiqiang Lanb,*, Jiacheng Lib, Hao Zhangb, Jin Guob a
Department of Materials Science and Engineering, Baise College, Baise 533000, China Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related Technology, College of Physics Science and Technology, Guangxi University, Nanning 530004, China b
Abstract: The reduced graphene oxide-supported copper (Cu@rGO) nanocomposite was introduced to improve the electrochemical properties of La0.7Mg0.3Ni2.8Co0.5alloy electrodes. Experimental results show that adding Cu@rGO nanocomposite with mass fractions of x wt% (x=0, 3, 6 and 9) to the alloy electrodes provides electrodes with maximum discharge capacities of 368.9 mAh/g (x=0), 373.2 mAh/g (x=3), 407.3 mAh/g (x =6) and 398.6 mAh/g (x=9), and high-rate dischargeabilities at a discharge current density of 1200 mA/g of 40.5% (x=0), 64.0% (x=3), 82.0% (x=6) and 76.0% (x=9). The addition of Cu@rGO nanocomposite also provides alloy electrodes with hydrogen diffusion coefficients of 3.7×10−10cm2/s (x=0), 4.1×10−10cm2/s (x=3), 4.2×10−10cm2/s (x=6) and 4.0×10−10cm2/s (x=9). Clearly, the addition of 6 wt% Cu@rGO nanocomposite not only increases the electrochemical capacity of La0.7Mg0.3Ni2.8Co0.5 alloy electrodes, but also improves their electrochemical kinetic properties. Keywords: Kinetic properties; alloy electrode; electrochemical properties; grapheme 1. Introduction Lanthanum-magnesium-nickel hydrogen storage alloys have attracted considerable attention for use as active materials in the negative electrodes of Ni/MH batteries due to their high hydrogen storage capacity and reduced environmental impact [1, 2]. However, the electrochemical cycling stability and electrochemical kinetics of La-Mg-Ni alloy electrodes must be improved to meet the requirements of practical applications. A number of approaches have been commonly employed to improve the overall electrochemical properties of La-Mg-Ni alloy electrodes, such as the addition or substitution of elements [3-9], heat treatment [10, 11], and surface modification [12, 13]. Among these approaches, the addition or substitution of elements is considered to be one of the most effective. For example, the addition of Al has been demonstrated to be advantageous for suppressing the expansion of the (La, Mg)2Ni7 unit cell volume in La-Mg-Ni alloy electrodes during hydrogen absorption [3].Moreover, in an alkaline solution, Al readily forms a dense Al2O3 film covering the surface of the electrode, which can effectively improve its corrosion resistance, and significantly improve its cycle life. However, the Al2O3 film also inhibits the diffusion of hydrogen atoms in the alloy electrodes, resulting in decreased kinetic properties. Additionally, Wu et al reported [4] that the cycling stability of the La-Mg-Ni alloy electrodes can be enhanced by replacing partial La with Ce, and the La0.65Ce0.1Mg0.25Ni3Co0.5 alloy electrode exhibits the best electrochemical properties among the La0.75-xCexMg0.25Ni3Co0.5 (x=0, 0.05, 0.10, 0.15, 0.2 at.%) samples. And Co substitution for Ni can regulate the phase composition in La-Mg-Ni alloy also had been studied by Wu et al [6]. The studied results show that the amount of (LaMg)2Ni7 phase *
Corresponding author: Lan zhiqiang; E-mail:[email protected],Tel:0771-3237386 Acknowledgement: The National Natural Science Foundation of China (51571065), the Natural Science Foundation of Guangxi Province (017GXNSFAA198337, 2018GXNSFAA294125, 2018GXNSFAA281308) and the Innovation-Driven Development Foundation of Guangxi Province (AA17204063).
decreased with Co content increasing. Zhang et al [7] reported that the cycle stability of the La-Mg-Ni can be enhanced by replacing partial La with Sm, but it was not conducive to improve the high-rate discharge ability of the alloy. Substituting a portion of the Ni with Fe can increase the electrochemical capacity of La-Mg-Ni electrodes by increasing the volume of the primary alloy phase in the electrodes, and thereby provides a greater number of storage sites for the hydrogen atoms [8]. However, Fe readily forms an oxide covering the surfaces of the electrodes in an alkaline electrolyte, which limits the diffusion of hydrogen atoms, and correspondingly degrades the kinetic properties of the electrodes. The partial substitution of Ni with Cu can also improve the cycling stability of La-Mg-Ni alloy electrodes [9]. However, as is obtained under the partial substitution of Ni with Fe, the kinetic properties of the La-Mg-Ni alloy electrodes are degraded after the addition of Cu. While these past studies indicate that the addition of metal elements such as Al, Fe, and Cu can improve the electrochemical cycling stability of La-Mg-Ni alloy electrodes, the development metal oxide films inhibits the diffusion of hydrogen atoms in the alloy electrodes, resulting in decreased kinetic properties. In recent years, the discovery of graphene has made it possible to improve the overall performance of La-Mg-Ni alloy electrodes. Graphene is a special two-dimensional carbon nanomaterial composed of carbon atoms with sp² hybrid orbitals forming a hexagonal honeycomb lattice and includes a number of advantageous properties such as a large specific surface area (theoretical value of 2630 m2/g), high catalytic activity and high electron mobility (1.5×105 cm2/(V·s) at room temperature) [14, 15]. Currently, graphene has been widely used to excellent effect in numerous applications such as semiconductor materials [16, 17], super capacitors [18, 19] and hydrogen storage materials [20, 21]. For example, Ouyang et al [22] found that the addition of 1 wt% graphene to an AB3 hydrogen storage alloy increased the high-rate dischargeability (HRD) of the alloy electrodes from 53.7% to 89.6% at a large discharge current density of 1750 mA/g. Huang et al [23] found that the addition of nanocomposites formed of graphene and Ag nanoparticles greatly improved the maximum discharge capacity and HRD of Mg65Ni27La8 alloy electrodes. In particular, adding 20 wt% graphene/Ag nanocomposites to the alloy attained a maximum discharge capacity of 814.8 mAh/g. In addition, our previous research work [24] demonstrated that the addition of graphene-supported Ni to La-Mg-Ni alloy electrodes could not only improve their electrochemical capacity, but also enhance their kinetic properties under the synergistic effect of reduced graphene oxide and Ni. A2B7 alloy with superstructure exhibits excellent electrochemical performance compared to AB5 alloys. It was reported that the La0.7Mg0.3Ni2.8Co0.5 alloy was mainly consisted of A2B7 phase, and the electrochemical maximum discharge capacity was around 410 mAh/g, which was 1.3 times higher discharge capacity than that of AB5 alloy [25]. Thus, La0.7Mg0.3Ni2.8Co0.5 series alloys had been considered to be an ideal candidate material to fabricate the negative electrode for Ni/MH battery. The present work capitalizes on the success of past efforts to prepare La0.7Mg0.3Ni2.8Co0.5 hydrogen storage alloy electrodes with reduced graphene oxide-supported Cu (Cu@rGO) nanocomposite prepared via hydrothermal synthesis as a catalyst additive. This work not only makes effective use of the good electrical conductivity of metallic Cu, but also benefits from its relatively low price. The effect of Cu@rGO addition on the electrochemical properties of the La0.7Mg0.3Ni2.8Co0.5 alloy electrodes is experimentally investigated, and the results demonstrate that Cu@rGO addition can substantially increase the maximum discharge capacity, HRD and hydrogen diffusion of the electrodes. 2. Experimental methods 2.1 Preparation of reduced graphene-supported copper nanocomposite
The specific reduced graphene-supported Cu nanocomposite preparation process employed is illustrated in Fig.1. Here, graphite oxide powder (purity ≥ 99.85%, Nanjing XFNANO Materials Tech Co., Ltd.) and deionized water were thoroughly mixed at a mass ratio of 1:1000 and subjected to ultrasonic excitation for 30 min. Copper acetate (analytic reagent) dissolved in water was added, and the produced Cu2+ ions combined with the graphite oxide particles by electrostatic forces in the water under stirring for 1 h using a magnetic stirrer and allowing the mixture to stand for 24 h. Subsequently, aqueous ammonia (25%–28%) was slowly dropped into the resulting suspension, and, after filtration, the precipitate was collected, excess ions were washed away with pure water, and the solid was dried for use. The as-prepared product was subjected to a reduction reaction in a hydrogen-argon mixed atmosphere at a temperature of 1023 K for 4 h to obtain the Cu@rGO nanocomposite. 2.2 Preparation of alloy samples and alloy electrodes The purity of the La, Mg, Ni, and Co raw materials used in the preparation of alloy samples were all above 99%, and the samples were prepared according to the stoichiometric ratio of La0.7Mg0.3Ni2.8Co0.5. The alloy was prepared by smelting the raw materials in a magnetic levitation high frequency induction furnace under the protection of an Ar atmosphere. To compensate for the loss of Mg during the smelting process due to volatilization, the mass of Mg used in the starting material was increased by 10 wt% based on the desired stoichiometric ratio. The alloy was repeatedly inverted and smelted three times during the smelting process to ensure a uniform composition and reduced segregation of components. The obtained alloy ingot was ground and crushed, passed through a 200-mesh sieve, and divided into four equal portions that served as alloy samples. Finally, the Cu@rGO nanocomposite was added in mass fractions of x wt% (x=0, 3, 6 and 9) to the alloy samples, and the mixture was subjected to mechanical ball milling at a milling speed of 300 r/min, a ball-to-batch ratio of 20:1, and a milling time of 10 min. To form an alloy electrode, an appropriate amount of an alloy powder and carbonyl Ni powder were thoroughly mixed at a mass ratio of 1:4, and then cold pressed for 10 min under a pressure of 20 MPa using a powder tablet press machine to obtain a circular alloy electrode sheet with a diameter of 10 mm and a thickness of about 3 mm. 2.3 Characterization The phase compositions and structures of the alloys were analyzed by X-ray diffraction (XRD) using a Rigaku Miniflex 600 diffractometer. The light source was Cu target Kα X-rays, and measurements were conducted with a 2θ scan range of 10° to 80° and a scan speed of 10°/min. The surface morphology and elemental concentrations of the Cu@rGO nanocomposite were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) using a JEOL JSM-6510A microscope. The alloy electrodes were tested in an open three-electrode test system with a Ni(OH)2/NiOOH electrode as the positive electrode and a Hg/HgO electrode as the reference electrode. A KOH solution with a concentration of 6 mol/L was used as the electrolyte. In cycling stability testing, the charge current density was 100 mA/g, the discharge current density was 80 mA/g, and the discharge cut-off voltage was 500 mV. The electrodes were subjected to cyclic voltammetry testing with a voltage scan range of −1.2 to 0 V and a scan rate of 5 mV/s, Tafel polarization curve testing with a scan voltage of −0.3 to 1.0 V and a scan rate of 10 mV/min, and electrochemical impedance spectroscopy (EIS) testing with a starting frequency of 50 kHz and an endpoint frequency of 0.003 Hz using a Gamry Instruments Reference-1000 electrochemical workstation. 3. Results and discussion 3.1 Phase composition and microstructure
The XRD patterns of the Cu@rGO nanocomposite and the base La0.7Mg0.3Ni2.8Co0.5 alloy are shown in Fig.2(a) and (b), respectively. The diffraction peaks of metallic Cu are observed at 2θ=43°, 50° and 74° in Fig.2(a), indicating that the Cu2+ ions have been successfully reduced to metallic Cu. As can be seen from Fig.2(b), the alloy mainly consisted of a La2Ni7 phase of the Ce2Ni7 type, a LaNi3 phase of the PuNi3 type, and a LaNi5 phase of the CaCu5 type. The Ce2Ni7-type phase can be described by stacking along c axis of CaCu5-type and Laves-type subunit in the rate of 2:1[26]. Additionally, the CaCu5-type La-Ni alloys can convert to the Ce2Ni7-type super-stacking phases via a peritectic reaction. Han et al [27] demonstrated that the phase abundance of the Ce2Ni7-type in the La-Mg-Ni alloy increased with increasing the annealing temperature. Additionally, it had been proved that the Ce2Ni7-type phase ultimately stayed stable in a temperature range of 1223K-1243K. The distribution of elemental Cu on the reduced graphene oxide substrate of the Cu@rGO nanocomposite was investigated using SEM and EDS, and the results are presented in Fig.3. As can be seen from Fig.3(a) and (b), Cu particles with diameters of 50 nm to 100 nm were dispersed and distributed on the reduced graphene oxide substrate, forming a disc-shaped structure. From Fig.3(c) and (d), we note that the distribution of Cu and C were relatively uniform, indicating that a Cu@rGO composite has been successfully prepared by the hydrothermal method. 3.2 Electrochemical performance Fig.4(a) presents the discharge curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0,3,6 and 9) alloy electrodes after being sufficiently activated by 2-3 times charge-discharge cycles. We note that relatively flat discharge curves were obtained for all of the alloy electrodes evaluated. However, the electrodes corresponding to x=6 and x=9 exhibit discharge curve plateaus at significantly higher voltages than the electrodes corresponding to x=3 and x=0, indicating that the addition of Cu@rGO nanocomposite can effectively increase the discharge power of the alloy electrodes. The maximum discharge capacity (Cmax) of each alloy electrode was calculated from the data presented in Fig.4(a), and the results are listed in Table 1. The results in Table 1 show that the value of Cmax increases with increasing x up to x=6, and then decreases slightly for x=9. This indicates that the addition of the Cu@rGO nanocomposite can increase the electrochemical capacity of the La0.7Mg0.3Ni2.8Co0.5 alloy. Since the carbon materials have good electrical conductivity and unique structure, which is beneficial to the diffusion of hydrogen atoms, adding an appropriate amount of carbon materials to the Ni-MH batteries had proved to be an effective way to enhance the electrochemical capacity of the materials [28, 29]. Therefore, the improvement in the discharge capacity for the La0.7Mg0.3Ni2.8Co0.5 electrodes may be due to the fact that the reduced graphene oxide-copper framework with a loose porous structure provides more channel for hydrogen atom diffusion. The discharge potential of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes at 50% depth of discharge was –0.891, –0.881, –0.891 and –0.882 V, respectively. The discharge platform for the electrodes broaden as the x value increasing to x=6 and x=9. It was evident that the addition of Cu@rGO nanocomposite had a positive effect on the discharge capacity of the electrodes. Fig.4(b) shows the cycle life of La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. As shown, after 30 times charge-discharge cycles, the capacity retention ratio for the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) electrodes was 83.8%, 82.2%, 80.1% and 78.8%, respectively. Obviously, the addition of the Cu@rGO nanocomposite cannot be improved the cycle life of the La0.7Mg0.3Ni2.8Co0.5 electrode, but it can enhance the discharge capacity slightly. Generally, during the charge process, the hydrogen atoms are first adsorbed on the surface of a hydrogen storage electrode, and then diffused from the surface toward the interior of the electrode. And in the discharge process, the diffusion path of the hydrogen atom is exactly the opposite.
Therefore, the kinetic properties of an electrode are mainly controlled by its surface charge transfer rate and internal charge diffusion rate [30, 31]. The kinetic properties of an electrode can be characterized by its high rate dischargeability (HRD), where an increasing HRD indicates better kinetic properties. The HRD of an electrode discharged at a relatively large current density of In can be calculated as
HRDn =
Cn ×100% Cn + C60
(1)
where Cn represents the discharge capacity at In, and C60 represents the residual discharge capacity of the electrode when discharged to a cut-off potential at a small current density of 60 mA/g after being discharged at In to a cut-off potential. Fig.5 shows the HRD curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes at different In (i.e., 300, 600, 900 and 1200 mA/g). It can be seen from Fig.5 that the HRD of the alloys decreased with increasing In, but that the HRD of the alloy electrodes decreased less under the catalytic action of the Cu@rGO nanocomposite. At small In, the HRD values of the alloy electrodes were essentially equivalent. For example, the HRD at In=300 (i.e.,HRD300) of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes were 96%, 96%, 97% and 96%, respectively. However, the values of HRD1200 obtained for the alloy electrodes corresponding to x=0, 3, 6 and 9 decreased to 40.5%, 64.0%, 82.0% and 76.0%, respectively. The addition of the Cu@rGO nanocomposite obviously increased the HRD of the alloy electrodes, and the highest HRD was obtained for the alloy electrode corresponding to x=6. It was reported that the electrochemical kinetic properties of the La-Mg-Ni alloy electrodes was mainly controlled by the charge transfer rate on the surface of the electrodes and the hydrogen diffusion rate in the bulk electrodes [30, 31]. The Cu@rGO catalyst includes metallic Cu embedded in a reduced graphene oxide (rGO) matrix, which forms an rGO-Cu framework. The ultra-high specific surface area and good electrical conductivity of the rGO coupled with the good electrical conductivity of Cu ensures that the rGO-Cu framework of the added Cu@rGO composite improves the internal contact of the La0.7Mg0.3Ni2.8Co0.5 alloy electrode. As shown in Fig.6, the straight slope corresponding to x=6 alloy is the smallest, suggesting that adding 6wt.% of Cu@rGO composite can decreased the polarization resistance of the alloy. The charge transfer rate on the electrodes surfaces can be characterized according to the exchange current density I0, because the exchange current density I0 increases with increasing charge transfer rate on the electrodes surface. The value of I0 can be calculated using the Butler-Volmer equation, as follows [32]:
I0 =
IRT RT = Fη FRP
(2)
Here, I0 is the current density (mA/g), R is the molar gas constant (8.314 J·mol/K), T is the testing temperature (298 K), F is the Faraday constant (96.487 C/mol), η is the polarization voltage (mV), and Rp is the polarization resistance of the alloy electrodes (mΩ). Normally, Rp can be calculated from linear polarization curves. Fig.6 shows the linear polarization curves obtained with a depth of discharge (DOD) of 50% for the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. The values of I0 calculated according to Equation (2) with the values of Rp obtained from the linear polarization curves in Fig.6 are listed in Table 1. As can be seen from Table 1, the value of I0 was greatest for the alloy electrode corresponding to x=6. We can accordingly conclude that the surface charge transfer rate of this alloy electrode was the largest, which indicates that the addition of an appropriate amount of Cu@rGO composite helped to accelerate the transfer of hydrogen atoms on the surface of the alloy electrode. Fig.7 presents the EIS graphs (DOD = 50%)
of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. It can be seen from the figure that the EIS graphs of all alloy electrodes are composed of a large semicircle in the middle to low frequency regions and a diagonal line in the low frequency region. Generally, the EIS graph of an La-Mg-Ni alloy electrode consists of a small semicircle in the high frequency region, a large semicircle in the middle to low frequency region, and a diagonal portion in the low frequency region. These three sections correspond to the contact resistance (Rcp) between the alloy electrode and the current collector, the electrochemical reaction charge transfer impedance (Rct) on the surface of the alloy electrode, and the Warburg diffusion impedance (W1) [24, 33]. This equivalent circuit is illustrated in the inset of Fig.7. The value of Rct was calculated from the equivalent circuit in conjunction with Fig.7, and the calculated results are listed in Table 1. We note that the value of Rct for the alloy electrodes obviously decreased under the beneficial action of the rGO-Cu framework. Cyclic voltammetry is another important method employed for characterizing the kinetic properties of alloy electrodes. Fig.8 shows the cyclic voltammetry curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes. It can be seen from the figure that the cyclic voltammetry curves of the alloy electrodes with added Cu@rGO exhibit two anodic peaks PO1 and PO2 and three cathodic peaks PR1, PR2, and PR3, while only a single anodic peak PO1 and a single cathodic peak PR3 are observed for the base La0.7Mg0.3Ni2.8Co0.5 alloy electrode. As shown in Fig.8, the PO1 peaks of the alloy electrodes appear in the oxidation process within the potential range of 500 mV to 650 mV, which correspond to hydrogen absorption and oxidation on the surfaces of the alloy electrodes, and the PO2 peaks correspond to the oxidation of Cu to form CuO. In the reduction process, the corresponding CuO is converted to Cu2O, and then Cu2O is converted into Cu [34]. Therefore, the following equation can be used to describe the reduction process. 2CuO + H 2 O + 2e − → Cu 2 O + 2OH − Cu 2 O + H 2 O + 2e − → 2Cu + 2OH −
(3)
The peak current density Ip of the hydrogen absorption corresponding to oxidation process of the alloy electrodes was obtained from Fig.8, and the results are listed in Table 1. We note that the value of Ip increased with increasing x up to x=6, and then decreased slightly for x=9. Increasing values of Ip indicate better surface activities for alloy electrodes [35, 36]. As such, these results indicate that the addition of Cu@rGO significantly improved the electrochemical activity of the base La0.7Mg0.3Ni2.8Co0.5 alloy electrode. The fact that the highest electrochemical activity was obtained for the alloy electrode corresponding to x=6 is consistent with the HRD test results. In addition, the area of the PO1 peak corresponds to the electrochemical capacity of the electrode, where the electrochemical capacity increases with increasing peak area [37]. From Fig.8, we note that the PO1 peak area is largest for x=6, and the PO1 peak area for x=9 was only slightly less, which is consistent with the results obtained from Fig.4. Finally, the reversibility of the alloy electrodes can be reflected by the difference between the oxidation peak potentials and the reduction peak potentials of the cyclic voltammetry curves in Fig.8, where the reversibility increases as this difference decreases. As shown in Fig.8, the voltage difference between the oxidation peak potentials (PO1) and the reduction peak potentials (PR3) for the alloy electrodes is decreased when the Cu@rGO was used as catalyst, suggesting that addition of Cu@rGO was obviously beneficial for improving the electrochemical reversibility of the alloy electrodes. Fig.9 shows the Tafel polarization curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes, which can be employed to study the diffusion of hydrogen atoms in the alloy electrodes. As can be seen from Fig. 9, the active region and the passivation region appeared
in sequence during the anodic polarization process as the scanning potential increased. In the active region, the anodic current density increases to a maximum value then decreases with the increasing of potential, the limiting current density which corresponding to the maximum point is defined as the limiting current density (IL). An oxidation reaction occurred and forming the oxidation film which covering on the surface of the electrodes in this process. Obviously, the oxidation film was not benefit for the hydrogen atom diffusion. So the limiting current density (IL) should be considered as the critical current density for the electrodes [38]. The values of IL for the alloy electrodes were obtained from Fig.9, and the results are listed in Table 1. Here, we note that the addition of 3 wt%Cu@rGO negatively affected IL, while IL was significantly increased for an addition of 6 wt%Cu@rGO, (i.e., increased by 63.4%), and IL was only slightly less for x=9. The value of IL for the alloy electrodes mainly reflects the diffusion rate of hydrogen atoms in the alloys [39]. Therefore, the alloy electrode corresponding to x=6 in particular exhibited an increased diffusion rate for hydrogen atoms in the electrode. Combining the results obtained for I0 and IL indicates that the electrochemical kinetic properties of the alloy electrodes were mainly affected by the synergistic effect of the mass transfer resistance on the surface of the alloy electrodes and the diffusion rate of hydrogen atoms inside the electrodes. Fig.10 presents logarithmic curves of the anodic currents of La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes with respect to time, which can be used to further study the diffusion of hydrogen atoms inside the alloy electrodes. It is clear that the anodic current density (A/g) versus time (s) (lg(i)-t)) response curves of the alloy electrodes can be divided into two phases [40]. In the first phase, the oxidation current of hydrogen decreased rapidly in the beginning due to the rapid consumption of hydrogen on the surface of the alloy. In the second phase, the current decreased, and the values of lg(i) and t were basically in a linear relationship, where the current of the alloy electrode was mainly controlled by the diffusion of hydrogen in the alloy. The diffusion rate D of hydrogen atoms in the alloy electrodes can be calculated from the slopes of the linear portions of the curves given in Fig.10 by fitting to the following equation [41]: ி
lg݅ = lg ൬
ௗ
ሺܥ − ܥ௦ ሻ൰ − మ
గమ
ଶ.ଷଷమ
ݐ
(4)
where C0 and Cs are the initial concentration of hydrogen in an alloy electrode and the concentration of hydrogen on the surface of the alloy electrode, respectively, and α and d are the average alloy particle radius and the density of the alloy, respectively. The values of D calculated for the alloy electrodes are listed in Table 1. Again, it can be seen from Table 1 that the value of D increased with increasing x up to x=6, and decreased slightly for x=9. Therefore, the addition of the Cu@rGO nanocomposite was beneficial for enhancing the diffusion of hydrogen atoms in the alloy electrodes. This may due to the unique properties of the rGO-Cu framework, and, in particular, its high porosity, which facilitated the diffusion of hydrogen atoms in the alloy electrodes by providing more convenient channels for its propagation. 4. Conclusions In this paper, reduced graphene oxide-supported Cu (Cu@rGO) nanocomposite was prepared by a hydrothermal method, and its effects on the electrochemical hydrogen storage performance of La0.7Mg0.3Ni2.8Co0.5 alloy electrodes with x wt%Cu@rGO (x=0, 3, 6 and 9) were studied. The addition of Cu@rGO nanocomposite has a positive effect on the electrochemical hydrogen storage performance of La0.7Mg0.3Ni2.8Co0.5 alloy electrodes. The maximum discharge capacity of the alloy electrodes increased from 368.9 mAh/g at x=0 to 373.2 mAh/g at x=3 to 407.3 mAh/g at x=6, and then decreased slightly to 398.6 mAh/g at x=9. The addition of Cu@rGO nanocomposite promotes the diffusion of hydrogen atoms in the alloy electrode bulks, thereby improving the electrochemical
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Graphical Abstract
The electrochemical kinetics of the La0.7Mg0.3Ni2.8Co0.5 alloy electrodes can be enhanced by adding Cu@rGO nanocomposite, and the La0.7Mg0.3Ni2.8Co0.5+6wt%Cu@rGO nanocomposite shows an excellent comprehensive electrochemical properties.
Figures and tables
Fig.1 Preparation process of the reduced graphene-supported Cu nanocomposite
Fig.2 XRD patterns of the Cu@rGO nanocomposite (a) and the La0.7Mg0.3Ni2.8Co0.5 alloy (b)
Fig.3 SEM images (a–b) and EDS (c–d) results of the Cu@rGO nanocomposite
Fig.4 Discharge curves (a) and Cycle life (b) of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.5 High-rate discharge curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.6 Linear polarization curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.7 Electrochemical impedance spectroscopy graphs of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x= 0, 3, 6 and 9) alloy electrodes
Fig.8 Cyclic voltammetry curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.9 Tafel polarization curves of the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Fig.10 Logarithmic curves of the changes in anodic current with respect to time for the La0.7Mg0.3Ni2.8Co0.5 + x wt%Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Table 1 Parameters of the electrochemical properties for the La0.7Mg0.3Ni2.8Co0.5 + x wt% Cu@rGO (x=0, 3, 6 and 9) alloy electrodes
Sample
Cmax (mAh/g)
I0 (mA/g)
Rct (mΩ)
Ip (mA/g)
IL (mA/g)
R0 (mΩ)
D×10−10 (cm2/s)
x=0
368.9
61.2
200.2
1808
1558
236
3.7
x=3
373.2
57.1
86.9
2218
1258
221
4.1
x=6
407.3
81.3
109.9
2983
2547
318.2
4.2
x=9
398.6
61.5
118.6
2503
2160
421.1
4.0