Promising hydrogen storage properties of cost-competitive La(Y)–Mg–Ca–Ni AB₃-type alloys for stationary applications

Abstract

In this paper, La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ (x = 0, 0.05, 0.20, 0.40, 0.60) alloys were prepared by an induction melting method, and the effects of Y substitution on the hydrogen storage properties were systematically investigated. The results showed that the hydrogen sorption plateau pressures could be easily tailored by altering the Y amount, and the effective hydrogen desorption capacities at 0.1 MPa H₂ below 80 °C could be greatly promoted with the optimal amount of Y substitution. In addition, the cost of the prepared alloys is quite low due to the absence of high price rare earth metals. Therefore, the cost-competitive La(Y)–Mg–Ca–Ni AB₃-type alloys prepared in this study successfully meet the demands of proton-exchange membrane fuel cells and other stationary apparatus, showing broad application potential.

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1. Introduction

Due to the serious environmental pollution and fossil energy depletion, the research and development of efficient, clean and renewable energy sources is becoming particularly urgent. However, most renewable energy production is intermittent, unable to satisfy the demands of practical application. Therefore, energy storage media should be explored to fill the energy supply-demand gap. One of the most efficient solutions is to convert the intermittent energy sources, such as the solar energy, wind energy, and geothermal energy to hydrogen for later use.^1,2 Compared with the gas cylinders, metal hydrides have attracted lots of attention around the world due to the advantages of much higher volume density (70–150 kg m⁻³), more satisfying safety performance, much higher hydrogen purity, and much lower moderate operating conditions.^3–12

In recent years, the La–Mg–Ni based hydrogen storage alloys, especially the alloys with superlattice structures of AB₃-, A₂B₇- and A₅B₁₉ type, have been a focus of studies.^13–28 These structures possess characteristic layered structure consisting of one MgZn₂-type (A₂B₄) cell and one to three CaCu₅-type (AB₅) cells stacking along c-axis, which are considered as the promising candidates to replace current AB₅-type alloys as negative electrodes of MH–Ni batteries.

In 1997, Kadir et al. discovered a type of hydrogen storage alloys, with the formula of RMg₂Ni₉ (R = La, Ce, Pr, Nd, Sm and Gd).^29,30 Their results indicated that the hydrogen storage capacity of LaMg₂Ni₉ was quite small, only 0.33 wt%. By reducing the amount of La element as well as adding Mg and Ca, they successfully prepared an AB₃-type (La_0.65Ca_0.35)(Mg_1.32Ca_0.68)Ni₉ alloy, the hydrogen storage capacity of which was much larger than that of AB₅-type alloys. However, the studies of AB₃-type alloys are mostly focused on the electrochemical properties, while the gaseous hydrogen storage properties have seldom been reported so far.

In order to improve the performance of La–Mg–Ca–Ni AB₃-type alloy, Lim et al. investigated the effects of partial substitutions of Ce and Al on the hydrogenation properties of La_(0.65−x)Ce_xCa_1.03Mg_1.32Ni_9−yAl_y alloy.³¹ Their results indicated that the hydrogen storage capacity significantly decreased after Ce and Al substitution. Although the plateau pressures increased when La was partially replaced by Ce, the slopes of the PCT curves became much larger. Therefore, the effective hydrogen storage capacity was rather small, hindering the practical applications in fuel cells.

Aiming to promote the effective hydrogen storage properties of cost-competitive AB₃-type alloys, we also chose (La_0.65Ca_0.35)(Mg_1.32Ca_0.68)Ni₉ as the starting alloy, and systematically investigated the effects of Y element partial substitution on its overall hydrogen storage properties. Our results demonstrated that the effective hydrogen storage capacity of La(Y)–Mg–Ca–Ni alloy at 0.1 MPa H₂ was quite large (∼1.60 wt%) at 25 °C, 40 °C and 60 °C, and the hydrogen sorption plateau pressures could be easily tailored between 0.1–1 MPa by altering Y amount, which exactly falls into the ideal pressure ranges for practical applications. In addition, the cost of the alloys was quite low, due to the absence of expensive rare earth metals. Therefore, the cost-competitive La(Y)–Mg–Ca–Ni AB₃-type alloys prepared in our study successfully meet the operating conditions of typical PEM fuel cells and the requirements for other stationary applications.

2. Experimental

La(Y)–Mg–Ca–Ni AB₃-type alloys were prepared by induction melting method. La, Y, Mg, Ca, Ni metals were supplied by Grirem Advanced Materials Co., Ltd. Due to the different melting points of the metals, the alloy preparation process was divided into two steps. Firstly, La, Y, Ni metals were melted together to form La(Y)–Ni alloys. Then, Mg and Ca metals were added simultaneously to form La(Y)–Mg–Ca–Ni alloys. To compensate for the volatility weight losses during the melting process, Mg and Ca were excessively added, respectively. The ingot was turn over and remelted three times to achieve homogeneity. Then, all the ingots were annealed at 900 °C for 20 h under Ar atmosphere. After that, the ingots were mechanically crushed and grinded into fine powders (80 mesh samples for hydrogen storage measurements and 500 mesh samples for XRD characterization). The as-received powder samples were directly used for the structural characterizations and property measurements without any further purifications.

The chemical compositions of the alloys were identified by inductively coupled plasma spectrometry (ICP). The results are listed in Table 1. For simplicity, the alloys are denoted as 0# (x = 0), 1# (x = 0.05), 2# (x = 0.20), 3# (x = 0.40), and 4# (x = 0.60), respectively. The crystal structures of different samples were studied by step-wise powder X-ray diffraction (XRD) (PANalytical Xpert Pro X-ray diffractometer) using monochromated Cu Kα radiation and θ–2θ scan operating at 40 kV × 300 mA. The XRD patterns were analyzed by Jade 6 and refined by FullProf Suite ToolBar software. The surface morphologies were examined by scanning electron microscopy (SEM) measurements (Hitachi S4800) coupled with an energy dispersive X-ray spectrometer (EDS) and a backscattered electron (BSE) detector. Besides, the accurate phase compositions were further observed by electron microprobe analysis (EPMA) (JEOLJXA 8230).

Table 1 Chemical compositions of the alloys obtained by ICP analysis

Samples	Designed composition	Observed composition
0# (x = 0)	La0.65Mg1.32Ca1.03Ni9	La0.65Mg1.36Ca1.01Ni9
1# (x = 0.05)	La0.60Y0.05Mg1.32Ca1.03Ni9	La0.54Y0.05Mg1.39Ca1.13Ni9
2# (x = 0.20)	La0.45Y0.20Mg1.32Ca1.03Ni9	La0.47Y0.24Mg1.13Ca1.11Ni9
3# (x = 0.40)	La0.25Y0.40Mg1.32Ca1.03Ni9	La0.25Y0.41Mg1.21Ca1.08Ni9
4# (x = 0.60)	La0.05Y0.60Mg1.32Ca1.03Ni9	La0.10Y0.66Mg1.26Ca0.98Ni9

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The kinetics properties and pressure-composition-temperature (PCT) curves were measured by a custom designed Sievert apparatus, using 99.999% ultrahigh purity hydrogen. Before PCT measurements, all the alloys were activated for three times. During the activation process, the alloys were firstly degassed in vacuum at 300 °C for 2 h, and then hydrogenated at ∼2.5 MPa H₂ for 30 min at 25 °C.

3. Results and discussion

3.1 Phase structures and morphologies

The X-ray diffraction (XRD) patterns of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys are shown in Fig. 1. The diffraction patterns indicate that all the alloys were composed of multi-phases, which is consistent with most of the results reported by previous researchers.^32,33 For the alloy without Y substitution (0# sample), three phases were observed: AB₃ phase with a PuNi₃-type rhombohedral structure, AB₅ phase with a CaCu₅-type hexagonal structure and trance amount of Ni impurity phase. After Y element substitution, the Ni impurity phase almost disappeared, and only two main phases were detected: AB₃ phase and AB₅ phase.

Fig. 1 XRD patterns of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ (x = 0, 0.05, 0.20, 0.40, 0.60) alloys.

The detailed Rietveld refinement results are presented in Table 2. It indicates that 0# sample mainly consisted of 73 wt% of AB₃ phase and 21 wt% of AB₅ phase. It is worth noting that after Y substitution, the abundance of AB₃ phase decreased and the abundance of AB₅ phase increased. It can be concluded that the addition of Y element inhibited the formation of AB₃ phase and promoted the formation of AB₅ phase, which is in agreement with the results from Lim et al.³¹ When x reached to 0.60 (4# sample), AB₅ phase turned to the dominate phase (52 wt%) and AB₃ phase became the secondary phase (48 wt%). It can be observed that the cell volumes of both AB₃ and AB₅ phases decreased with the increase of Y element substitution. The reduction of both lattice constants and cell volumes can be ascribed to the smaller atomic radius of Y (181 ppm) than that of La (188 ppm).

Table 2 Refinement results of major hydrogen absorption phases in the alloys

Samples	Phases	Lattice parameters			Cell volumes	Abundance (wt%)
		a/Å	c/Å	c/a	V/Å^3
0#	(La, Mg, Ca)3Ni9	4.961	23.926	4.823	510.521	73%
	(La, Ca)Ni5	4.992	3.968	0.795	85.947	21%
	Ni	3.540	3.540	1.000	44.362	6%
1#	(La, Y, Mg, Ca)3Ni9	4.955	23.935	4.830	510.372	70%
	(La, Y, Ca)Ni5	4.987	3.981	0.798	85.364	30%
2#	(La, Y, Mg, Ca)3Ni9	4.948	23.942	4.839	508.659	66%
	(La, Y, Ca)Ni5	4.963	3.995	0.805	84.527	34%
3#	(La, Y, Mg, Ca)3Ni9	4.935	23.958	4.855	506.834	59%
	(La, Y, Ca)Ni5	4.947	4.048	0.818	83.926	41%
4#	(La, Y, Mg, Ca)3Ni9	4.921	23.964	4.869	504.467	48%
	(La, Y, Ca)Ni5	4.926	4.137	0.839	82.638	52%

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Fig. 2 shows the typical BSE images of the alloys and Table 3 exhibits the element compositions of different phases. It can be seen that all the La(Y)–Mg–Ca–Ni alloys consisted of two main phases (i.e. AB₃ and AB₅ phases). The darker phase composition has a lower average atomic number and vice versa for the lighter phase. According to the EDS results, it indicates that the darker phase was rich in Mg (AB₃ phase) and the lighter phase was lack of Mg (AB₅ phase), which is in accordance with the literatures that Mg tends not to reside in AB₅ phase.^20,34 The EDS mappings of different alloys are shown in Fig. 3, and it can be observed that La, Y, Ca, Ni elements were uniformly distributed across the sample surface. While Mg element was rich in certain domains and lack in other sites, confirming that AB₃ and AB₅ phases coexisted in all the samples.

Fig. 2 SEM images of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys: (a) x = 0; (b) x = 0.05; (c) x = 0.20; (d) x = 0.40; (e) x = 0.60.

Table 3 Element compositions of different regions in the alloys

Samples	Regions	Atomic ratios of different elements (at%)					Phases
		La	Y	Mg	Ca	Ni
0#	A	5.07	0	15.03	5.94	73.96	AB3
	B	7.27	0	2.84	6.65	83.24	AB5
1#	A	9.87	1.61	9.25	4.54	74.73	AB3
	B	7.13	4.73	1.87	4.46	81.82	AB5
2#	A	6.37	2.95	10.65	4.89	75.14	AB3
	B	5.07	4.04	2.40	5.03	83.46	AB5
3#	A	2.58	3.16	11.9	6.08	76.28	AB3
	B	1.46	2.17	5.51	6.47	84.39	AB5
4#	A	1.98	3.87	13.34	5.57	75.24	AB3
	B	0.38	5.48	3.47	4.28	86.38	AB5

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Fig. 3 EDS mappings of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys: (a) x = 0; (b) x = 0.05; (c) x = 0.20; (d) x = 0.40.

In order to further identify the phase structures, the electron microprobe analysis (EMPA) of all the samples were conducted, as shown in Fig. 4. Obviously, the EMPA results were basically identical with the BSE images, consisting of two separate areas with different gray levels. The elemental analysis proved the co-existence of AB₃ (region A) and AB₅ (region B) phases, which powerfully confirmed the XRD refinement results and the SEM analysis.

Fig. 4 EMPA images of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys: (a) x = 0; (b) x = 0.05; (c) x = 0.20; (d) x = 0.40; (e) x = 0.60. (Region A: AB₃ phase, region B: AB₅ phase).

3.2 Hydrogen storage properties

The pressure-composition-temperature (PCT) curves of different La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys at room temperature are shown in Fig. 5. From the PCT curves, it is easy to see that the hydrogen storage capacity of 0# sample was extremely large. Approximately 1.831 wt% hydrogen was absorbed under 10 MPa H₂, and 93% of the absorbed H₂ (∼1.701 wt%) could be released when evacuating the system to the vacuum at room temperature. These results basically agree well with the consequences obtained by Kadir et al. with the same alloy composition, who reported that 1.87 wt% of hydrogen storage capacity was achieved for La_0.65Mg_1.32Ca_1.03Ni₉ alloy at 10 °C.³⁰ After Y substitution, the hydrogen storage capacity slightly decreased. When the Y addition amount was quite small (1# and 2# samples), only a slight decrease of the hydrogen storage capacity was observed. Approximately 1.784 wt% and 1.753 wt% hydrogen could be absorbed for 1# and 2# samples at room temperature, respectively. However, the hydrogen storage capacity seriously decreased when further increasing the Y addition amount, and only 1.292 wt% of hydrogen was absorbed for 4# sample under 10 MPa H₂ at room temperature, which might be ascribed to the gradually diminishing AB₃ phase abundance. It must be mentioned that the reversibility of all the samples was really satisfying and only a little hysteresis was detected, with merely 0.10 wt% of hydrogen retaining in the lattice under vacuum conditions.

Fig. 5 Pressure-composition-temperature (PCT) curves of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ (x = 0, 0.05, 0.20, 0.40, 0.60) alloys at room temperature.

Without Y element substitution, the hydrogen absorption and desorption plateau pressures of 0# sample mainly located in the range of 0.01–0.1 MPa H₂ at room temperature. After Y substitution, the hydrogen absorption and desorption plateau pressures significantly increased due to the contracted lattice parameters and cell volumes. For 1# and 2# samples, both the absorption and desorption plateau pressures perfectly falls into the range of 0.1–1 MPa at room temperature, which is the operating pressure region of PEM fuel cells, demonstrating promising potential applications. When the Y addition amount was too larger (3# and 4#), the plateau pressures were beyond the operating pressure ranges of PEM fuel cells, and the PCT curves could not be completely measured due to the pressure limitations of the instrument. From the above results we can conclude that the hydrogen sorption plateau pressures of La–Mg–Ca–Ni AB₃-type alloys can be easily tailored by optimal amount of Y substitution to meet the operating pressure demands of PEM fuel cells, without sacrificing the overall hydrogen storage capacity.

In order to evaluate the thermodynamic properties of different La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys, the PCT curves of all the samples at 40 °C, 60 °C and 80 °C were also measured, as indicated in Fig. 6. It is obvious that the hydrogen sorption plateau pressures notably improved and the hydrogen storage capacities declined when increasing the temperatures. The hydrogen storage capacities of the alloys at different temperatures are summarized in Table 4. After optimal amount of Y substitution, the effective hydrogen storage capacities significantly increased. Compared with 0# sample, 1# sample exhibited much larger hydrogen desorption capacities both in vacuum and 0.1 MPa hydrogen. At 0.1 MPa H₂, the hydrogen desorption capacities of 1# sample were approximately 1.624 wt%, 1.616 wt%, 1.610 wt% at 25 °C, 40 °C and 60 °C, respectively. According to Table 4, 0#, 1# and 2# samples are promising candidates to couple with the PEM fuel cells, due to the large effective hydrogen desorption capacities at 0.1 MPa H₂ below 80 °C.

Fig. 6 PCT curves of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys at 25 °C, 40 °C, 60 °C and 80 °C: (a) x = 0; (b) x = 0.05; (c) x = 0.20; (d) x = 0.40; (e) x = 0.60.

Table 4 The hydrogen storage capacities of the alloys at different temperatures

Systems	Hydrogen absorption capacity (wt%)	Hydrogen desorption capacity (wt%)
	10 MPa H2	Vacuum	0.1 MPa H2
La–Mg–Ca–Ni (0#)	∼1.831 (25 °C)	∼1.701 (25 °C)	∼1.417 (25 °C)
	∼1.806 (40 °C)	∼1.676 (40 °C)	∼1.482 (40 °C)
	∼1.787 (60 °C)	∼1.644 (60 °C)	∼1.527 (60 °C)
	∼1.615 (80 °C)	∼1.485 (80 °C)	∼1.406 (80 °C)
La–Y–Mg–Ca–Ni (1#)	∼1.784 (25 °C)	∼1.694 (25 °C)	∼1.624 (25 °C)
	∼1.780 (40 °C)	∼1.688 (40 °C)	∼1.616 (40 °C)
	∼1.778 (60 °C)	∼1.677 (60 °C)	∼1.610 (60 °C)
	∼1.489 (80 °C)	∼1.409 (80 °C)	∼1.349 (80 °C)
La–Y–Mg–Ca–Ni (2#)	∼1.753 (25 °C)	∼1.663 (25 °C)	∼1.593 (25 °C)
	∼1.643 (40 °C)	∼1.523 (40 °C)	∼1.463 (40 °C)
	∼1.428 (60 °C)	∼1.318 (60 °C)	∼1.278 (60 °C)
	∼1.406 (80 °C)	∼1.306 (80 °C)	∼1.266 (80 °C)
La–Y–Mg–Ca–Ni (3#)	∼1.642 (25 °C)	∼1.522 (25 °C)	∼1.442 (25 °C)
	∼1.524 (40 °C)	∼1.377 (40 °C)	∼1.264 (40 °C)
	∼1.461 (60 °C)	∼1.321 (60 °C)	∼1.221 (60 °C)
	∼1.175 (80 °C)	∼1.045 (80 °C)	∼0.945 (80 °C)
La–Y–Mg–Ca–Ni (4#)	∼1.292 (25 °C)	∼1.202 (25 °C)	∼1.108 (25 °C)
	∼1.186 (40 °C)	∼1.116 (40 °C)	∼1.032 (40 °C)
	∼1.032 (60 °C)	∼0.965 (60 °C)	∼0.905 (60 °C)
	∼0.859 (80 °C)	∼0.802 (80 °C)	∼0.752 (80 °C)

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By plotting the logarithm of plateau pressures with respect to the reciprocal of the temperatures, the enthalpy change values of different samples during hydrogen absorption and desorption processes can be calculated, as shown in Fig. 7. The calculated enthalpy changes of different samples were listed in Table 5. The hydrogen absorption and desorption enthalpy changes of 0# sample showed great similarity with the data of other La–Mg–Ni–H systems.^23,24,35 However, the absolute values of enthalpy changes diminished after the addition of Y elements, indicating that the stability of the hydride was remarkably reduced with Y substitution. It is confirmable that all the enthalpy change values are comparable with that of LaNi₅ system (∼30 kJ mol⁻¹),³⁶ indicating that La(Y)–Mg–Ca–Ni alloys are also good hosts for H₂.

Fig. 7 Van't Hoff plots of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys: (a) absorption; (b) desorption.

Table 5 The enthalpy changes of different systems

Systems	Enthalpy changes (kJ mol^−1 H2)
	Absorption	Desorption
La2MgNi9–H^23	−35.0	35.9
La3MgNi14–H^35	−31.4	33.6
La4MgNi19–H^24	−32.1	31.5
La–Mg–Ca–Ni (0#)	−34.4 ± 2.3	29.5 ± 1.0
La–Y–Mg–Ca–Ni–H (1#)	−27.9 ± 1.2	28.2 ± 2.7
La–Y–Mg–Ca–Ni–H (2#)	−25.3 ± 1.6	26.4 ± 3.9
La–Y–Mg–Ca–Ni–H (3#)	−21.4 ± 2.3	26.8 ± 0.3
La–Y–Mg–Ca–Ni–H (4#)	−22.1 ± 2.9	30.3 ± 2.7

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From the above results, we can conclude that 0#, 1# and 2# samples presented promising hydrogen storage properties under the conditions of low pressures (<10 MPa H₂) and mild operating temperatures (<80 °C). The poor hydrogen sorption properties of the samples with high Y substitution were mainly ascribed to the following reasons: firstly, AB₅ phase increased with the enhancement of Y substitution, and it became the dominate phase for 4# sample, resulting in the capacity reduction; secondly, the hydrogen sorption plateaus were quite high for 3# and 4# samples, and the PCT curves were incomplete at high temperatures due to the instrumental limitations (<10 MPa H₂). Therefore, although the samples with high Y substitution are not suitable for the applications of PEM fuel cells, they can still be widely used in other fields which require much higher hydrogen sorption plateau pressures.

4. Conclusions

In summary, the overall hydrogen storage properties of La_(0.65−x)Y_xMg_1.32Ca_1.03Ni₉ alloys were systematically investigated. The results indicated that the hydrogen sorption plateau pressures could be easily tailored by altering Y amount, and the effective hydrogen desorption capacities at 0.1 MPa hydrogen below 80 °C could be greatly promoted with optimal amount of Y substitution, which can be coupled with the PEM fuel cells. Moreover, the cost of the prepared alloys are quite low due to the absence of high price rare earth metals. Therefore, the cost-competitive La(Y)–Mg–Ca–Ni AB₃-type alloys prepared in this study successfully meet the demands of PEM fuel cells and other stationary apparatus, showing broad application potential.

Acknowledgements

The authors acknowledge the financial supports provided by the Rare Earth and Rare Metal New Materials Research and Development (R & D) and Industrialization Special Fund and the rare earth industry adjustment and upgrade special fund of Ministry of Industry and Information Technology (MIIT). The authors also want to acknowledge the Youth Science Fund of General Research Institute for Nonferrous Metals (GRINM) for the financial support.

Notes and references

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