稀有金属(英文版) 2015,34(08),569-579

收稿日期：28 December 2012

基金：financially supported by the National Natural Science Foundation of China (Nos. 50961009 and 51161015);the National High Technology Research and Development Program of China (No. 2011AA03A408);the National High Technology Research and Development Program of China (Nos. 2011ZD10 and 2010ZD05);

The electrochemical hydrogen storage performances of Si-added La–Mg–Ni–Co-based A₂B₇-type electrode alloys

Yang-Huan Zhang Li-Cui Chen Tai Yang Chao Xu Hui-Ping Ren Dong-Liang Zhao

Key Laboratory of Integrated Exploitation of Baiyun Obo MultiMetal Resources, Inner Mongolia University of Science and Technology

Department of Functional Material Research, Central Iron and Steel Research Institute

Abstract：

In order to improve the electrochemical cycle stability of the RE–Mg–Ni-based A2B7-type electrode alloys, a small amount of Si has been added into the alloys.The casting and annealing technologies were adopted to fabricate the La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) electrode alloys. The impacts of the addition of Si and annealing treatment on the structures and electrochemical performances of the alloys were investigated systematically. The results obtained by XRD and SEM show that all the as-cast and annealed alloys are of a multiphase structure, involving two main phases(La, Mg)2Ni7and La Ni5 as well as a residual phase La Ni3. Both adding Si and the annealing treatment lead to an evident change in the phase abundance and cell parameters of(La, Mg)2Ni7and La Ni5 major phases of the alloy without altering its main phase component. Moreover, the annealing treatment has the composition of the alloy distributed more homogeneously overall and simultaneously causes the grain of the alloy to be coarsened obviously. The electrochemical measurements indicate that adding Si and the annealing treatment give a significant rise to the influence on the electrochemical performances of the alloys. In brief, the cycle stability of the as-cast and annealed alloys evidently increases with the rising of Si content, while their discharge capacities obviously decrease under the same circumstances. Furthermore, the electrochemical kineticproperties of the electrode alloys, including the high rate discharge ability, the limiting current density(IL), hydrogen diffusion coefficient(D), and the charge-transfer resistance, first augment and then decline with the rising of Si content. Similarly, it is found that the above-mentioned electrochemical properties first mount up and then go down with the rising annealing temperature.

Keyword：

A2B7-type electrode alloy; Adding Si; Annealing treatment; Structure; Electrochemical performances;

Author： Yang-Huan Zhang,e-mail: zyh59@yahoo.com.cn;

Received： 28 December 2012

1 Introduction

The Ni/MH batteries, on account of their major advantages including high-energy density, excellent power density, and long cycle life, have been deemed to be the leading technology as the battery power source for electric vehicles (EVS) [1]. Factually, hybrid electric vehicles (HEV) with nickel metal hydride battery as the auxiliary power have been classified as a mature product for nationwide sale by the Ministry of Industry and Information Technology of China, which provides a golden opportunity for the development of the Ni/H battery. A series of metal hydride materials such as the rare earth-based AB₅-type alloys [2], the AB₂-type Laves phase alloys [3], the V-based solid solution alloys [4], and the Mg-based alloys [5, 6] are regarded as potential electrode materials and, in particular, the rare earth-based AB₅-type alloys have been industrialized on a large scale in China and Japan. However, there is still no perfect choice among the above-mentioned hydrogen storage materials to meet the transport applications owing to the limitation of their properties, for instance, the low discharge capacity of the AB₅-type electrode alloy, the poor activation capability of the AB₂type Laves phase and V-based solid solution electrode alloys, and the poor cycle stability of the Mg-based electrode alloy. Hence, further improvements of these performances especially the discharge capacity and electrochemical hydrogen storage kinetics are compulsory for Ni/MH batteries. In such a circumstance, La–Mg–Ni system A₂B₇-type alloys have been considered to be the most promising candidates as the negative electrode materials of Ni/MH rechargeable battery in the light of their higher discharge capacities (380–410 m A h g^-1) and lower production costs since Kadir et al. [7] and Kohno et al. [8] reported their research results. Moreover, The National High Technology Research and Development Program of China has provided significant financial support in order to promote the industrialization of these new-type alloys. However, the attempt was severely frustrated by the poor cycle stability of the new alloy. Thus, the production of the new-type alloys as the negative electrode in Ni/MH battery has not been found in China yet. But, an important breakthrough has been obtained through many scientific researchers’ studies as summarized by Liu et al. [9], which is encouraging.

It was believed that the element substitution or addition is an effective method for improving the overall properties of the hydrogen storage alloys. In the case of La–Mg–Ni series hydrogen storage alloys, the partial replacement of Ni with Co, Fe, Mn, Al, and Cu [10, 11]; La with Ce, Pr, and Nd [12–14]; and Mg with Ca [15] was studied systematically. Furthermore, it was thought that the capacity deterioration of the La–Mg–Ni system alloy electrodes is mainly attributed to the pulverization of the alloy particles and the oxidation/corrosion of the elements Mg and La [16]. Therefore, we expect that the addition of Si can ameliorate the anti-oxidation and anti-corrosion abilities of the alloy because a dense silicon oxide film will form on the surface of the alloy electrode. To validate this, a systematical investigation of the effects of the Si content and the annealing temperature on the structures and electrochemical hydrogen storage kinetics of the La0.8Mg0.2Ni3.3Co0.2Six (x = 0–0.2) electrode alloys has been performed.

2 Experimental

The raw materials La, Ni, Co, Mg, and Si had a purity of 99.8 % at least and the chemical compositions of the electrode alloys were La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2). For convenience, the alloys were denoted with Si content as Si0, Si0.05, Si0.1, Si0.15and Si0.2. The alloy ingots were prepared using a vacuum induction furnace in helium atmosphere under the pressure of 0.04 MPa in order to prevent element Mg from volatilizing during the melting. A part of the alloys was annealed in vacuum at the temperatures of 900, 950, 1,000, and 1,050 °C, with thermal insulation for 8 h in order to stay the same as industrialized production.

The phase structures and compositions of the as-cast and annealed alloys were characterized by XRD (D/max/2400). The diffraction, with the experimental parameters of 160 m A, 40 k V, and 10(°) min^-1, was performed with Cu Ka1 radiation filtered by graphite. The morphologies of the as-cast and annealed alloys were examined by SEM (QUANTA 400).

The round electrode pellets with a 15 mm diameter were prepared by cold pressing a mixture of alloy powder and carbonyl nickel powder with the weight ratio of 1:4 under the pressure of 35 MPa. Then, the electrode pellets were immersed in a 6 mol L^-1KOH solution for 24 h in order to wet themselves fully before the electrochemical measurement.

The electrochemical measurements were performed at 30 °C using a tri-electrode open cell, consisting of a working electrode (the metal hydride electrode), a sintered Ni(OH)₂/Ni OOH counter electrode, and a Hg/Hg O reference electrode, which were also immersed in the electrolyte of 6 mol L^-1KOH, and the voltage between the negative electrode and the reference one was defined as the discharge voltage. In every cycle, the alloy electrode was first charged with a constant current density. After resting for 15 min, it was discharged at the same current density to cut-off voltage of -0.500 V.

The electrochemical impedance spectra (EIS) and the Tafel polarization curves of the alloys were measured by an electrochemical workstation (PARSTAT 2273). The fresh electrodes were fully charged and then rested for 2 h up to the stabilization of open circuit potential. For the EIS measurement, the frequency ranged from 1 9 10⁴to 5 9 10^-3Hz at 50 % depth of discharge (DOD), the amplitude of signal potentiostatic or galvanostatic measurements was 5 m V, and the number of points per decade of frequencies was 60. For the Tafel polarization curves, the potential range was from -1.2 to ?1.0 V (vs. Hg/Hg O) with a scan rate of 5 m V s^-1. For the potentiostatic discharge, the test electrodes in the fully charged state were discharged at 500 m V potential steps for 5,000 s on electrochemical workstation (PARSTAT 2273), using the electrochemistry corrosion software (Corr Ware).

3 Results and discussion

3.1 Structural characteristics

As shown in Fig. 1, the XRD profiles of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys reveal that all the as-cast and annealed alloys are of a multiphase structure, containing two major phases (La, Mg)₂Ni₇and La Ni₅as well as a residual phase La Ni₃. Significantly, adding Si and the annealing treatment can hardly affect their phase component, but can change their phase abundance to a large extent. Listed in Tables 1 and 2 are the lattice parameters together with the abundance of the (La, Mg)₂Ni₇and La Ni₅major phases in the alloys, which were calculated by Jade 6.0 software based on the XRD data. It is found that the addition of Si brings on an obvious decrease in the (La, Mg)₂Ni₇phase and an increase in the La Ni₅phase. Moreover, it can be clearly seen that the addition of Si causes the lattice constants and cell volumes of the two major phases to enlarge visibly, which justifies the successful alloying of Si with (La, Mg)₂Ni₇and La Ni₅. Adding Si also causes the lattice constants of the La Ni₅phase to increase much more than those of the (La, Mg)₂Ni₇phase, implying that Si atom prefers to form solid solution with the La Ni₅phase. Furthermore, Table 2 exhibits that the annealing treatment gives rise to a viewable increase in the lattice parameters and cell volume of the major phases in the alloys. The abundance of the (La, Mg)₂Ni₇phase first grows and then declines with the rising of the annealing temperature, while that of La Ni₅phase shows a completely opposite trend.

The SEM morphologies of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Si0.2alloys display a dendrite structure just as shown in Fig. 2. The analysis of SEM equipped with energy dispersive spectrometry (EDS) detects that all the experimental alloys have a multiphase structure, containing (La, Mg)₂Ni₇(denoted as A), La Ni₅(denoted as B), as well as La Ni₃(denoted as C) phases. It also can be found from the EDS patterns that the Si concentration in the La Ni₅phase is clearly higher than that in (La, Mg)₂Ni₇phase, which corresponds well with the results of the XRD observation. Furthermore, we discover that the annealing treatment has the alloy composition distributed more homogeneously and simultaneously causes the grain of the alloy to be coarsened obviously with the annealing temperature rising.

3.2 Electrochemical hydrogen storage performances

3.2.1 Activation capability and discharge capacity

The activation capability of an alloy electrode was evaluated by the number of charging–discharging cycles which were required for attaining the greatest discharge capacity at a constant current density of 60 m A g^-1. The fewer the number of charging–discharging cycles, the better the activation property. The variations of the discharge capacities of the as-cast and annealed La0.8Mg0.2Ni3.3Co_0.2Si_x(x = 0–0.2) alloys with the cycle number are depicted in Fig. 3, from which it is found that almost all the alloys can attain their maximum discharge capacities at first charging–discharging cycle. Neither adding Si nor the annealing treatment affects the activation capability of the alloy. Generally, the activation capability of an alloy electrode is related to the change of the internal energy of the hydride system before and after hydrogen absorption. The larger the added internal energy, which involves the surface energy originating from an oxidative film forming on the electrode alloy surface and the strain energy produced by a hydrogen atom entering the interstitial of the tetrahedron or octahedron of the alloy lattice, the poorer the activation performance of the alloy [17]. The superior activation performances of the as-cast and annealed alloys originate from their multiphase structures because the phase boundary not only decreases the lattice distortion and the strain energy produced in the process of hydrogen absorption but also provides good diffusion tunnels for hydrogen atoms, improving the activation performance of the alloy greatly. Additionally, the enlarged cell volume produced by adding Si reduces the expansion/contraction ratio of the alloy in the process of the hydrogen absorption/ desorption, lowering the strain energy and the diffusion activation energy.

Fig.1 XRD patterns of as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six (x = 0–0.2) alloys: a as-annealed (1,000 °C) and b La0.8Mg0.2Ni3.3Co0.2Si0.2alloy

Table 1 Lattice parameters and abundance of (La, Mg)₂Ni₇and La Ni₅major phases of asannealed (1,000 °C) alloys  下载原图

Table 1 Lattice parameters and abundance of (La, Mg)₂Ni₇and La Ni₅major phases of asannealed (1,000 °C) alloys

Table 2 Lattice parameters and abundance of (La, Mg)₂Ni₇and La Ni₅major phases of the La_0.8Mg_0.2Ni_3.3Co_0.2Si_0.2alloy annealed at different temperature  下载原图

Table 2 Lattice parameters and abundance of (La, Mg)₂Ni₇and La Ni₅major phases of the La_0.8Mg_0.2Ni_3.3Co_0.2Si_0.2alloy annealed at different temperature

Figure 4 demonstrates the variations of the discharge capacity of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Si_x(x = 0–0.2) alloys with the Si content and the annealing temperature. Apparently, the discharge capacity of the asannealed (1,000 °C) alloys shows an almost linear decline, being reduced from 398.4 to 343.3 m A h g^-1with Si content increasing from 0 to 0.2. We can also find that La0.8Mg0.2Ni3.3Co0.2Si0.2alloy has a maximum discharge capacity of 354.1 m A h g^-1corresponding to the annealing temperature of 900 °C. We consider that the decreased discharge capacity resulting from adding Si is ascribed to two aspects. Firstly, the addition of Si is harmful to the discharge capacity of La Ni₅phase, which is universally accepted [18, 19]. Secondly, the reduction of the (La, Mg)₂Ni₇phase engendered by adding Si is detrimental to the discharge capacity of the alloy since the (La, Mg)₂Ni₇phase possesses much higher electrochemical capacity than that of the La Ni5phase. The reason why La0.8Mg0.2Ni3.3Co_0.2Si_0.2alloy can have a maximum discharge capacity is because the structure of the alloy has changed after annealing treatment. When the annealing temperature is 900 °C, the abundance of (La, Mg)₂Ni₇phase increases and the cell volume becomes enlarged, which facilitate the improvement of the discharge capacity.

3.2.2 Cycle stability

The cycle life of the electrode alloy is viewed as a decisive factor for the application of the Ni/MH battery, which is characterized by the cycle number when the discharge capacity is reduced to 60 % of the maximum capacity. Evidently, the more the number of the charging– discharging cycle, the better the cycle life. The variation of discharge capacity of the as-cast and annealed La_0.8Mg_0.2Ni_3.3Co0.2Si_x(x = 0–0.2) alloys with cycle number is described in Fig. 5, from which the process of the capacity degradation can be seen clearly. The slopes of the curves qualitatively reflect the degradation rate of the discharge capacity during the charge–discharge cycle; that is to say, the smaller the slope of the curve, the better the cycle life of the alloy. With respect to the degradation rate of the discharge capacity, it visibly falls with the Si content growing, indicating that the addition of Si plays a positive impact on the cycle life of the alloy electrode. Simultaneously, it can be found from Fig. 5b that the degradation rate of the discharge capacity of the La0.8Mg0.2Ni3.3Co0.2Si0.2alloy first goes down and then mounts up with the annealing temperature increasing, suggesting the existence of an optimal annealing temperature. In order to establish the relationship between the cycle stability of the alloy and Si content as well as the cycle stability and annealing temperature, the capacityretaining rate (S₁₅₀) is introduced to accurately evaluate the cycle stability of the alloy and it is defined as S₁₅₀= C₁₅₀/ C_max9 100 %, where C_maxis the maximum discharge capacity and C₁₅₀is the discharge capacity at the 150th cycle with a current density of 300 m A g^-1. Here, the evolutions of the S150values of the La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys with Si content as well as annealing temperature are also inserted in Fig. 5. Figure 5 displays that the S₁₅₀values of the as-annealed (1,000 °C) alloys dramatically grow with the rising of Si content, being enhanced from 77.1 % to 92.4 % by increasing Si content from 0 to 0.2. Similarly, the S₁₅₀value of the La0.8Mg0.2Ni3.3Co0.2Si0.2alloy is also clearly augmented with the annealing temperature rising to 1,000 °C, but when the temperature reaches 1,050 °C, an undesired decline appears, namely the S₁₅₀value first rising from 66.8 % to 92.4 % and then decreasing to 81.6 % at last.

Fig.2 SEM images of as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Si0.2alloy together with typical EDS spectra of sections A, B, and C in Fig.2b: a as-cast, b 900 °C, c 1,000 °C, and d 1,050 °C

Fig.3 Evolution of discharge capacity of La0.8Mg0.2Ni3.3Co0.2Six (x = 0–0.2) alloys with cycle number: a as-annealed (1,000 °C) and b La0.8Mg0.2Ni3.3Co0.2Si0.2alloy

Some elucidations can be provided as reasons for the cycle stability changing by adding Si and annealing treatment. Commonly, the invalidation of an electrode is characterized by the decay of discharge capacity and the drop of discharge voltage [20]. It is well known that the degradation of the discharge capacity of the A₂B₇-type alloy electrode principally originates from the forming and ever thickening of Mg(OH)₂and La(OH)₃surface layers which hinder the hydrogen atoms from diffusing in or out in alkaline solutions [21]. Besides, the hydrogen storage alloy suffers from a volume change during the charge– discharge process which inevitably aggravates the cracking and pulverization of the alloy, consequently making the surface of the alloy facilitate oxidized. Generally, the beneficial aspect of the Si additive on the cycle stability of the alloy is affiliated with the following several factors. Firstly, the addition of Si facilitates forming a compact silicon oxide layer on the surface of the alloy electrode [2, 22] which prevents it from being corroded effectively. Secondly, the enlarged cell volume caused by adding Si reduces the ratios of expansion/contraction in the process of hydrogen absorption/desorption, enhancing the antipulverization capability. What is more, the La Ni₅phase increased by adding Si plays a beneficial role in the cycle stability due to an inarguable fact that the cycle stability of the La Ni₅phase is superior to that of the (La, Mg)₂Ni₇phase. Then, we take a look at the effect of annealing treatment. It can be derived from Fig. 5b that the annealing treatment engenders beneficial and harmful impacts on the cycle stability of the alloy simultaneously. The beneficial aspect exhibits that the cell volumes are enlarged and the composition distribution becomes more homogeneous, which facilitates to retard the pulverization of the alloy, improving the cycle stability of the alloy. The harmful side is that the grains are coarsened after annealing. As considered by Sakai et al. [23], a passivation layer forming on the grain boundaries could produce an effectively protective action until the alloy was pulverized below the grain size. Hence, it is demonstrated that the alloy with the smaller grain size has a better cycle stability.

Fig.4 Evolution of discharge capacity of as-cast and annealed alloys with Si content a and annealing temperature b

Fig.5 Evolution of the discharge capacity of La0.8Mg0.2Ni3.3Co0.2Six (x = 0–0.2) alloys with cycle number: a as-annealed (1,000 °C) and b La0.8Mg0.2Ni3.3Co0.2Si0.2alloy

3.2.3 Electrochemical hydrogen storage kinetics

The electrochemical hydrogen storage kinetics of an alloy electrode is evaluated by its high rate discharge ability (HRD), which is defined as HRD = Ci,max/C60,max9 100 %, where Ci,maxand C60,maxare the maximum discharge capacities of the alloy electrode charged–discharged at the current densities of i and 60 m A g^-1, respectively. The evolutions of HRD values of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys with the discharge current density are demonstrated in Fig. 6. It is found that whatever the current density is, the HRD values of the alloys first mount up and then go down with the rising of the Si content and the annealing temperature. In order to describe the influences of Si content and annealing temperature on the HRD values, we establish the relationships between the HRD value and Si content as well as the HRD value and annealing temperature for a fixed current density of i = 300 m A g^-1, as depicted in Fig. 6. Evidently, the as-annealed (1,000 °C) alloy yields a maximum HRD value of 93.2 % when Si content x = 0.05, and the La0.8Mg0.2Ni3.3Co0.2Si0.2alloy has a maximum HRD value of 93.6 % when annealed at 900 °C.

Fig.6 Evolution of HRD values of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys with the discharge current density: a as-annealed (1,000 °C) and b La0.8Mg0.2Ni3.3Co0.2Si0.2alloy

Undoubtedly, a clear understanding of the mechanism of electrochemical hydriding/dehydriding is very important to better understand the influences of adding Si and the annealing treatment on the electrochemical hydrogen storage kinetics. The electrochemical hydriding/dehydriding reaction taking place at the hydrogen storage electrode in an alkaline solution during charging and discharging process can be summarized as follows:

where M is the hydrogen storage alloy. It can be seen from the above equation that when the alloy electrode is charging in KOH solution, hydrogen atoms originating from electrolyzing H₂O diffuse from the interface between the alloy and electrolyte into the bulk alloy and then store themselves in the metallic lattice in the form of hydride. In the process of discharging, the hydrogen stored in the bulk alloy diffuses toward the surface where it is oxidized. The hydrogen atoms adhering to the grain surface of the alloy electrode have two possible destinations, forming hydrogen molecule or diffusing into the bulk alloy where they exist in the form of hydride. This means that the diffusion rate of hydrogen atom on the surface layer of alloy is just the ratio of the diffusion current to the imposed current, which is a vital factor to determining the utilization of charging current. That is to say, the electrochemical hydrogen storage kinetics of the alloy electrode is determined by the chargetransfer rate on the surface of an alloy electrode and the hydrogen diffusion capability in the alloy bulk [24]. Thereby, it is necessary to investigate the hydrogen diffusion coefficient and the charge-transfer rate.

The diffusion coefficients of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys, which can be derived by measuring the semilogarithmic curves of anodic current versus working duration of an alloy, are presented in Fig. 7. According to the model founded by White and coworkers [25], the diffusion coefficient of the hydrogen atoms in the bulk of the alloy could be calculated through the slope of the linear region of the corresponding plots by the following Eqs.:

where i is the diffusion current density (A g^-1), F is the Faraday constant, D is the hydrogen diffusion coefficient (cm²s^-1), C₀is the initial hydrogen concentration in the bulk of the alloy (mol cm^-3), C_sis the hydrogen concentration on the surface of the alloy particles (mol cm^-3), a is the alloy particle radius (cm), d is the density of the hydrogen storage alloy (g cm^-3), and t is the ll to the results in Fig. 6.discharge time (s). In Eq. (3), is the slope of the linearregion in Fig. 7. The variations of the D values of the ascast and annealed alloys with the Si content and the annealing temperature are also provided in Fig. 7. It is evident that the D values of the alloys first increase and then decline with the rising of the Si content and the annealing temperature, which conforms well to the results in Fig. 6.

With respect to the charge-transfer ability on the surface of an alloy electrode, it can be qualitatively evaluated by its electrochemical impedance spectrum (EIS), as interpreted and modeled by Kuriyama et al. [26]. The EIS curves of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys are displayed in Fig. 8 which shows that each EIS spectrum contains two distorted capacitive loops at high and middle frequencies as well as an almost straight line at low frequency. The smaller semicircle in the high frequency region reflects the contact resistance between the alloy powder and the conductive material, and the larger one in the middle frequency region represents the chargetransfer resistance on the alloy surface, while the line in low frequency relates to the atomic hydrogen diffusion in the alloy. Hence, it seems to be self-evident that the larger the radius of the semicircle in the middle frequency region, the higher the charge-transfer resistance of the alloy electrode. Clearly, the radii of the large semicircles of the ascast and annealed alloys in the middle frequency first shrink and then expand with the rising of Si content and the annealing temperature.

Fig.7 Semilogarithmic curves of anodic current versus time responses of the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys: a as-annealed (1,000 °C) and b La0.8Mg0.2Ni3.3Co0.2Si0.2alloy

Another important electrochemical kinetic characteristic of an alloy electrode, limiting current density (I_L), can be obtained by measuring the Tafel polarization curve, which is illustrated in Fig. 9 for the as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys. It can be discovered that in all cases, there is an obvious point of inflection in each anodic polarization curve, implying the presence of critical value which is termed as limiting current density (I_L). It suggests that an oxidation reaction takes place on the surface of the alloy electrode, and the oxidation layer hinders hydrogen atoms from further penetrating [27]. Hence, the limiting current density (I_L) can be regarded as a critical current density of passivation which is basically dominated by the diffusion rate of hydrogen in alloy electrode [24]. In order to exhibit the influences of the Si content and the annealing temperature on the I_Lvalue, Fig. 9 also shows the variations of the I_Lvalues of the alloys with the Si content and the annealing temperature. Clearly, the I_Lvalues of the alloys first mount up and then go down with the growing of the Si content and the annealing temperature. Summarily, based on the abovementioned results, we can conclude that the HRD of the experimental alloys is dominated by the charge-transfer rate and the hydrogen diffusion capability jointly.

In terms of hydrogen diffusion, it is accepted that the diffusion coefficient of hydrogen atoms in the metallic lattices depends on the strength of the metal–hydrogen interaction as well as the structure of the alloy [28]. As concluded by Cui and Luo [29], the increase of the lattice constants and cell volumes facilitates the reduction of the diffusion activation energy of hydrogen atoms, thus enhancing hydrogen diffusion. Also, the diffusion ability of hydrogen atoms is very sensitive to grain size [30] owing to the fact that grain boundaries can provide many sites with low diffusing activation energy, aiding the diffusion of hydrogen atom in alloys. The measurements of the HRD, EIS, limiting current density (I_L), and hydrogen diffusion coefficient (D) show that the electrochemical kinetics of the alloys first increases and then falls with the rising of the Si content and the annealing temperature, implying that both the adding of Si and the annealing treatment give a rise to positive and negative impacts on the electrode hydrogen storage kinetics. Here, we will first discuss the actions of the addition of Si. The positive contribution has two aspects: On the one hand, the enlarged cell volume incurred by adding Si reduces diffusion activating energy of hydrogen atoms; on the other hand, the La Ni₅phase increased by adding Si ameliorates the electrocatalytic activity of the alloy electrodes dramatically. Contrarily, the compact silicon oxide layer created by adding Si not only severely impairs the charge-transfer rate on the alloy surface but also hinders the hydrogen diffusion from inner part of the bulk to the surface, consequently impairing the electrochemical kinetic property. As regards the actions of the annealing treatment on the electrochemical kinetics, the positive contribution of the annealing treatment to the electrochemical kinetic is inarguably ascribed to the increased cell volumes induced by the annealing, and its opposite action is credibly attributed to the coarsened grains resulting from the annealing.

Fig.8 Electrochemical impedance spectra (EIS) of as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys: a as-annealed(1,000 °C) and b La0.8Mg0.2Ni3.3Co0.2Si0.2alloy

Fig.9 Tafel polarization curves of as-cast and annealed La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) alloys: a as-annealed (1,000 °C) and b La0.8Mg0.2Ni3.3Co0.2Si0.2alloy

4 Conclusion

Both adding Si and annealing treatment have been ascertained to produce significant impacts on the structures and electrochemical hydrogen storage performances of the La0.8Mg0.2Ni3.3Co0.2Six(x = 0–0.2) electrode alloys.

The addition of Si gives rise to an increase in the La Ni₅phase and a decrease in the (La, Mg)₂Ni₇phase without changing the major phase structures of the alloys. Furthermore, such addition brings on a visible increment in the lattice constants and cell volumes of the alloys. With respect to the electrochemical performances of the alloys, it is found that adding Si reduces the discharge capacity obviously, but it enhances cycle stability considerably. The HRD values of the as-cast and annealed alloys first increase then decrease with the rising of Si content, to which a rather similar variation tendency is obtained by measuring the hydrogen diffusion coefficient (D), limiting current density (I_L), and EIS, which confirms that the HRD of the experimental alloys is dominated by the charge-transfer rate together with the hydrogen diffusion capability.

The annealing treatment leads to a clear enlargement of the lattice constants and cell volumes of the alloy. Simultaneously, it causes the grains of the alloys to coarsen obviously. Also, the abundance of the (La, Ni)₂Ni₇phase first grows and then declines with the annealing temperature increasing, but the situation of La Ni₅phase is quite the contrary. The electrochemical performances of the alloys, including cycle stability, discharge capacity, and HRD, first increase and then decrease with the annealing temperature rising.