Investigation on the Structure and Electrochemical Properties of La-Ce-Mg-Al-Ni Hydrogen Storage Alloy

Structure and electrochemical characteristics of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 hydrogen storage alloy have been investigated. X-ray diffraction analyses reveal that the La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 hydrogen storage alloy consisted of a (La, Mg)Ni 3 phase with the rhombohedral PuNi 3 -type structure and a LaNi 5 phase with the hexagonal CaCu 5 -type structure. TEM shows that the alloy is multicrystal with a lattice space 0.187 nm. EDS analyse shows that the content of Mg is 3.48% (atom) which coincide well with the designed composition of the electrode alloy. Electrochemical investigations show that the maximum discharge capacity of the alloy electrode is 325mAh g. The alloy electrode has higher discharge capacity within the discharge current density span from 60mAg to 300mAg. Electrochemical impedance spectroscopy measurements indicate that the charge transfer resistance RT on the alloy electrode surface and the calculated exchange current density I 0 are 0.135Ω and 1298mAg, respectively; the better eletrochemical reaction kinetic of the alloy electrode may be responsible for the better high-rate dischargeability.


Introduction
La-Mg-Ni system alloys are promising materials owing to their higher electrochemical capacity compared to AB 5 -type alloys; however, the poor cycling stability and HRD of the La-Mg-Ni hydrogen storage alloys have prevented it from being practically used as electrode alloys for Ni-MH battery [1][2][3][4][5][6][7][8][9][10][11][12][13][14].According to the literature [15], the capacity degradation during cycling of La-Mg-Ni based hydrogen storage electrode alloy is influenced mainly by two factors: the passivation due to the formation of La(OH) 3 and Mg(OH) 2 on the alloy surface and the accelerated corrosion rate of the active components due to the large molar volume of hydrogen  H of the alloy hydrides.As the alloy electrode with a large  H undergoes a large cell volume expansion/contraction during charge/discharge cycle, a higher degree of pulverization resulted and thus more surface area is exposed to the corrosive electrolyte, which would lead to a lower cycling stability [16][17][18].
The partial substitution of Al for Ni in La-Mg-Ni based alloy can improve the cycling stability due to the noticeable decrease in the cell volume expansion rate (Δ/) on hydriding [19,20].According to the literature [21], Al was helpful to the formation of LaNi 5 phase in La 0.67 Mg 0.33 Ni 3.0− Al  ( = 0, 0.1, 0.2, 0.3) hydrogen storage alloys.LaNi 5 phase with CaCu 5 type appeared when Al was added.The increase of Al content leads to an increase of content of LaNi 5 phase, and the main phase of the alloy is LaNi 5 phase with  = 0.3.According to the literature [22,23], rare earth elements (such as Dy and Pr) are helpful to decrease the cell volume of LaNi 5 phase.For -based solid solution alloy [24], the addition of Ce can improve the dynamic performance, which makes the charge transfer resistance ( T ) decrease and the exchange current density ( 0 ) increase markedly.Just for those considerations, Al has been used to partly substitution for Ni, Ce has been used to partly substitution for La, and the structure and electrochemical properties of the La-Ce-Mg-Al-Ni hydrogen storage electrode alloy have been investigated in this paper.

Experimental
La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 hydrogen storage electrode alloy was prepared by induction melting under argon and then remelted to ensure for homogeneity.The purity of the individual starting metal was higher than 99.5 mass%.The prepared electrode alloy was mechanically crushed to particles and then pulverized to a fine powder of about 300 mesh, and the sample powder was used for XRD, TEM, and electrochemical characteristics.Crystal structure of La-Ce-Mg-Al-Ni hydrogen storage electrode alloy was investigated by XRD (CuK, Si internal standard) on Rigaku D/max 2500pc X-ray diffraction meter using JAD5 software and by TEM on JEM-2010.
The metal hydride electrode was prepared by pressing the mixture of alloy powder with carbonyl nickel powder in a weight ratio of 1 : 5 into a tablet with a diameter about 10 mm and a thickness about 1.5 mm.The electrochemical properties were performed on a DC-5 battery testing instrument using a half-cell system which consists of a metal hydride electrode as the negative electrode and a sintered Ni(OH) 2 /NiOOH electrode with excess capacity in 6 mol L −1 KOH electrolyte solution; the cut-off voltage for discharge was 0.8 V.
Electrochemical impedance spectroscopy (EIS) was used to clarify the kinetic properties of dehydrating action, such as the charge-transfer resistance ( T ) and the exchange current density ( 0 ).After the test electrodes were completely activated, EIS measurements were conducted at 50% depth of discharge (DOD) using a Solartron SI1187 electrochemical interface with ZPLOT electrochemical impedance software and 1255B frequency response analyzer.The EIS spectra of the electrodes were obtained in the frequency range from 100 kHz to 10 −2 Hz with alternating current amplitude of 5 mV under open-circuit conditions.According to the analysis model proposed by Kuriyama et al. [26], an equivalent circuit for the alloy electrode was used and the parameters in the equivalent circuit were fitted using least-square method with ZVIEW electrochemical impedance software;  0 is calculated from the following formulation ( 0 = / T ).

Result and Discussion
3.1.XRD. Figure 1 shows the XRD patterns of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy.It can be seen that the alloy consists of a (La, Mg)Ni 3 phase with the rhombohedral PuNi 3 -type structure (space group: R3 m (166)) and a LaNi 5 phase with the hexagonal CaCu 5 -type structure (space group: P6/m 3 (191)).This result coincides well with that reported by Zhang et al. [21] and Liao et al. [15]: Al was helpful to the formation of LaNi 5 phase with CaCu 5 type in La-Mg-Ni hydrogen storage alloys, and also the content of Al decides the content of LaNi 5 phase.According to the literature, about 2% Al has been used to substitute for Ni in the La-Mg-Ni alloy in this paper to form PuNi 3 -type structure and CaCu 5 -type structure.
The lattice parameters of , , and the cell volume of LaNi 5 phase are 0.5012 nm, 0.3984 nm, and 86.7 × 10 −3 nm 3 , respectively.Each one of those parameters is smaller than the relevant one of La-Mg-Al-Ni alloy [25] ( = 0.5058 nm,  = 0.4042 nm, cell volume = 89.5 × 10 −3 nm 3 ), respectively.According to the literature [22,23], rare earth elements (such as Dy and Pr) were helpful to decrease the cell volume of LaNi 5 phase, as shown in Table 1.The cell volume of LaNi 5 phase without rare earth element was larger than 87 × 10 −3 nm 3 , while it is smaller than 87 × 10 −3 nm 3 for the alloys with rare earth element.It should be pointed out that the smaller the cell volume of the main phase (LaNi 5 phase) would decrease the cell volume expansion rate (Δ/) on hydriding, and it is favorable for the alloy to decrease pulverization.

TEM.
Figure 2 shows the TEM and EDS micrographs of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 electrode alloy.It is obvious that the alloy is composed of multicrystal.The lattice space  can be calculated from the following formulation ( = /), where  means the length of the camera (0.2 m),  means the wavelength of the accelerated voltage (0.00370 nm), and  means the radius of the diffractive spot (0.593 cm).The result shows that the  is 0.187 nm.
The phase composition of the alloy has been semiquantitatively analyzed with EDS which is shown in Figure 2 and the results are shown in Table 2.It can be seen that electrode alloy is composed of La, Ce, Mg, Al, and Ni, and the content of Mg is 3.48% (atom), which coincide well with the designed composition of the electrode alloy.charge/discharge cycle process and reached maximum discharge capacity of 325 mAh g −1 at 4th cycles to be proved to have the higher electrochemical activity.According to the literature [21], Al was helpful to the formation of LaNi 5 phase in La-Mg-Ni hydrogen storage alloys, but the content of Al would lead to some decrease in discharge capacity.The result also shows that the discharge capacity at 30th cycles is 268 mAh g −1 , which is about 82.5% of the maximum discharge capacity (325 mAh g −1 ), and implies that cycle stability of the alloy electrode is acceptable.Figure 3(b) shows the typical discharge capacity-potential curves for La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode.It can be clearly seen that the alloy electrode has a wider discharge voltage plateau.It is about 4.5 h for the battery to hold the discharge voltage above 1.2 V with a wider discharge voltage plateau from 1.38 V to 1.20.It was reported by Iwakura et al. [27] that the curve of equilibrium potentialdischarge capacity and the curve of P-C isotherm were rather coordinate, especially for the tendency of the plateau region; that was to say, the plateau region of equilibrium potential for La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode has a good discharge characteristics.

Temperature Effect and High-Rate Dischargeability.
Figure 4(a) shows the dependence of the discharge capacity of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode on different temperatures.As can be seen in the figure, the discharge capacity of the alloy electrode is decreased as the temperature increased from 303 K to 333 K.For example, the discharge capacity of the alloy electrode is 284 mAh g −1 and 263 mAh g −1 at 323 K and at 333 K, respectively.It implies that the discharge capacity of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode is sensitive to temperature within the temperature span.
HRD is an important property of electrode alloy used as negative electrode material in Ni-MH battery, especially   It can be clearly seen that the alloy electrode has higher discharge capacity within the discharge current density span from 60 mA g −1 to 300 mA g −1 , and the cycle stability of alloy electrode is accepted.It is worthy of notice that the discharge capacity of the alloy electrode is higher than 200 mAh g −1 at 600 mA g −1 and 100 mAh g −1 at 900 mA g −1 , respectively.This characteristic of the alloy is crucial for it to be used as a negative electrode material of Ni-MH batteries.is attributed to the charge transfer resistance  T on the alloy surface, and the fitted result shows that the  T is 0.135 Ω.

EIS.
The exchange current density  0 for the alloy electrode can be calculated by the following formulation:  0 = / T , where , , , and  T denote the gas constant, the absolute temperature, the Faraday constant, and charge transfer resistance on  the alloy surface, respectively.The calculated result shows that  0 is 1298 mA g −1 , which is much larger than that in -based solid solution electrode alloy [28].This may be responsible for the better high-rate dischargeability.
3.6.Self-Discharge.According to the literature [29], selfdischarge of the hydrogen storage alloy can be divided into two parts, the reversible ( 3 - 2 ) and irreversible ( 1 - 3 ), respectively.The self-discharge rate of the alloy electrode can be calculated with {1 − 2 2 /( 1 +  3 )} × 100% day. 1 ,  2 , and  3 have been determined with the tested alloy electrode standing at open circuit for 1, 2, and 4 days, respectively, as shown in Table 3.The results indicate that the self-discharge rate of the alloy electrode is increased as the time standing increased from 1 day to 4 days, also, the reversible part ( 3 - 2 ) which due to desorption of hydrogen is larger than that of irreversible ( 1 - 3 ) part which was due to deterioration of the hydrogen storage alloy.

Conclusion
The structure and electrochemical properties of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode have been studied.The conclusions can be drawn as follows.
(1) The hydrogen storage alloy is consisted of a (La, Mg)Ni 3 phase with the rhombohedral PuNi 3 -type structure and a LaNi 5 phase with the hexagonal CaCu 5 -type structure.
(2) The maximum discharge capacity 325 mAh g −1 and cyclic stability 82.5% after 30 cycles are obtained; the discharge capacity is decreased as the temperature increased from 303 K to 333 K; the alloy electrode has higher discharge capacity within the discharge current density span from 60 mA g −1 to 300 mA g −1 .
(3) The charge transfer resistance  T on the alloy surface is 0.135 Ω and the exchange current density  0 is 1298 mA g −1 , respectively.
(4) The self-discharge rate of the alloy electrode is increased as the time standing increases (increased), and also the reversible part ( 3 - 2 ) is larger than that of irreversible ( 1 - 3 ) part.

Figure 4 :
Figure 4: Effect of temperature and discharge current density on the discharge capacity for La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy.

Table 2 :
Phase composition and interplanar distance for La 0.24 Ce 0.01 Mg 0.04 Al 0.01 Ni 0.70 alloy.Figure 4(b) shows the relationship between the discharge capacity and the discharge current density of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode.
5 shows the EIS of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode at 50% DOD.EIS has been used for determining  T and  0 to clarify the kinetic properties of dehydrating action at 303 K.According to the equivalent circuits [26], the large semicircle in the low-frequency region EIS of La 0.96 Ce 0.04 Mg 0.15 Al 0.05 Ni 2.8 alloy electrode at 50% DOD.