Degradation Characters of La-Mg-Ni-Based Metal Hydride Alloys: Corrosion and Pulverization Behaviors

doi:10.1007/s40195-017-0696-y

Abstract

Abstract:

Degradation behaviors of three typical La-Mg-Ni alloys, La₂MgNi₉, La_1.5Mg_0.5Ni₇ and La₄MgNi₁₉, were studied. La_1.5Mg_0.5Ni₇ with (La,Mg)₂Ni₇ as main phase presents better discharge capacity and cycling stability. The three alloys suffer severe pulverization and corrosion after electrochemical cycles, which are considered to be the significant factor attributing to the capacity deterioration. However, the overall corrosion extent of the three cycled alloys aggravates successively, which is inconsistent with the result that La₂MgNi₉ presented poor cycling stability and also the assumption that alloy with high Mg content is easy to be corroded. The intrinsic anti-corrosion and anti-pulverization characteristics of the three alloys are mainly focused in this work. Immersion corrosion experiments demonstrate that the Mg-rich phases are more easily to be corroded. The corrosion resistance of the three alloys presents an improved trend which is inversely proportional to abundance of the Mg-rich phases. However, the anti-pulverization abilities present an inverse trend, which is closely related to the mechanical property of various phase structures. LaNi₅ with the highest hardness is easy to crack, but the soft (La,Mg)Ni₂ is more resistant to crack formation and spreading. Thus, the weaker corrosion of La₂MgNi₉ after electrochemical cycling is attributed to the better intrinsic anti-pulverization capability though the anti-corrosion is poor. As La₄MgNi₁₉ possesses excellent corrosion resistance, enhancement of the anti-pulverization ability is urgent for improvement in the cycling stability.

Key words: La-Mg-Ni-based alloys, Degradation behaviors, Corrosion, Pulverization

Yi-Ming Li, Yang-Huan Zhang, Hui-Ping Ren. Degradation Characters of La-Mg-Ni-Based Metal Hydride Alloys: Corrosion and Pulverization Behaviors[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(7): 723-734.

Figures/Tables 19

Fig. 1 Microstructures of annealed La2MgNi9 a, d; La1.5Mg0.5Ni7 b, e; La4MgNi19 c, f at low a-c; high d-f magnification

Table 1 EDS results of annealed alloys corresponding to areas marked in Fig. 1

Alloy	Location	La	Mg	Ni	Ni/(La + Mg)	Phase
La₂MgNi₉	1	16.15	20.69	63.16	1.71	(La,Mg)Ni₂
	2	17.93	9.18	72.89	2.69	(La,Mg)Ni₃
	3	19.35	5.16	76.50	3.26	(La,Mg)₂Ni₇
	4	17.17	0.00	82.83	4.82	LaNi₅
La_1.5Mg_0.5Ni₇	1	16.31	10.32	73.01	2.63	(La,Mg)Ni₃
	2	18.91	4.51	76.58	3.27	(La,Mg)₂Ni₇
	3	17.87	0.00	83.13	4.96	LaNi₅
	4	17.74	3.30	78.95	3.75	(La,Mg)₅Ni₁₉
La₄MgNi₁₉	1	18.31	4.37	77.32	3.41	(La,Mg)₂Ni₇
	2	19.16	2.73	78.11	3.57	(La,Mg)₅Ni₁₉
	3	17.35	0.00	82.65	4.76	LaNi₅

Fig. 2 XRD profiles of annealed La2MgNi9 a; La1.5Mg0.5Ni7 b; La4MgNi19 c

Table 2 Crystal structure, cell parameters and phase content of annealed alloys

Alloy	Phase type	a (nm)	c (nm)	Content (wt%)
La₂MgNi₉	MgCu₄Sn	0.71660	-	4.1
	PuNi₃	0.50358	2.43612	61.4
	CaCu₅	0.50182	0.39806	0.5
	Ce₂Ni₇	0.50387	2.42605	13.6
	Gd₂Co₇	0.50350	3.63793	20.4
La_1.5Mg_0.5Ni₇	PuNi₃	0.50339	2.42432	11.1
	Ce₂Ni₇	0.50334	2.42635	32.6
	Gd₂Co₇	0.50341	3.43493	21.9
	Ce₅Ni₁₉	0.50319	4.84459	16.3
	Pr₅Co₁₉	0.50347	3.22350	13.5
	CaCu₅	0.50182	0.39806	4.6
La₄MgNi₁₉	Ce₂Ni₇	0.50341	2.42416	22.8
	Gd₂Co₇	0.50298	3.63639	10.7
	Ce₅Ni₁₉	0.50323	4.83377	27.5
	Pr₅Co₁₉	0.50328	3.22454	15.3
	CaCu₅	0.50212	0.39792	23.7

Fig. 3 P-C-T curves of three alloys (Abs and Des are the abbreviation for the absorption and desorption, respectively)

Table 3 Hydrogen storage performances of three alloys

Alloy	MGC (wt%)	RGC (wt%)	DC (mA h g^-1)	S₁₀₀ (%)
La₂MgNi₉	1.594	1.15	350.8	70.3
La_1.5Mg_0.5Ni₇	1.619	1.275	365.5	75.4
La₄MgNi₁₉	1.572	1.443	332.1	70.1

Fig. 4 Morphologies of original a; electrochemical cycled particles at low b; high c magnification, and EDS analysis of La2MgNi9 alloy by cycling d

Fig. 5 XRD profiles of La2MgNi9, La1.5Mg0.5Ni7 and La4MgNi19 after electrochemical cycling

Fig. 6 TEM images of alloy particles at a low; b, c high magnification of La2MgNi9 alloy after electrochemical cycling

Table 4 Oxygen contents and size retention of alloys

Alloy	Oxygen content after electrochemical cycling (wt%)	Oxygen content (wt%)^after immersion	Size retention after gaseous cycling (%)
La₂MgNi₉	2.68	4.66	84.6
La_1.5Mg_0.5Ni₇	3.21	4.38	68.5
La₄MgNi₁₉	3.62	4.14	66.5

Fig. 7 Morphologies at low a; high b; magnification and EDS spectra c of La2MgNi9 particles after immersion

Fig. 8 XRD profiles of La2MgNi9, La1.5Mg0.5Ni7 and La4MgNi19 alloys after immersion

Fig. 9 SEM images at low a; high b; magnification, EDS spectra of different points c in annealed La1.5Mg0.5Ni7 alloy after immersion

Fig. 10 EDS mapping a; elements La b; Mg c; O d distribution of annealed La1.5Mg0.5Ni7 alloy after immersion

Fig. 11 Morphologies of the La2MgNi9 particles before a, b; after c, d; gaseous cycling at low a, c; and high b, d magnification

Table 5 Volume expansion of various La-Mg-Ni phases by hydrogenation in the reported works

Alloy	La_2.3Mg_0.7Ni₉	La₂MgNi₉	LaMg₂Ni₉	La_1.5Mg_0.5Ni₇	La_1.63Mg_0.37Ni₇	La₄MgNi₁₉	LaNi₅
Type	AB₃	AB₃	AB₃	A₂B₇	A₂B₇	A₅B₁₉	AB₅
?V/V	27.1	26.7	23	25.2	25.6	25.3	24
References	[28]	[28]	[29]	[30]	[30]	[31]	[11]

Fig. 12 Original optical images of La2MgNi9 a, b; La1.5Mg0.5Ni7 c, d; alloys, and indentation morphologies of La2MgNi9 e, f; La1.5Mg0.5Ni7 g, h; alloys e, g; f, h were tested under 10 and 25 g, f, respectively)

Fig. 13 Micro-hardness of various phases

Fig. 14 Morphologies of as-cast La2MgNi9 alloy in different regions a, b after being partially charged

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