李金華 劉寶忠,3 韓樹民,,* 扈 琳 朱惜林 王明智

(1燕山大學(xué)亞穩(wěn)材料制備技術(shù)與科學(xué)國家重點實驗室,河北秦皇島066004; 2燕山大學(xué)環(huán)境與化學(xué)工程學(xué)院,河北秦皇島066004;3河南理工大學(xué)材料科學(xué)與工程學(xué)院,河南焦作454000)

1 Introduction

The industrial application of hydrogen as a fuel depends on the development of materials that can store hydrogen in a dense form.Magnesium alloys have been widely investigated due to high hydrogen storage capacity and low cost.The simplest hydride,MgH2,has a hydrogen storage capacity of 7.67% (mass fraction,the same below).1However,high hydrogen desorption temperature(623 K)and relatively slow kinetics of hydrogen adsorption/desorption make them still far from practical application.In order to improve hydrogen storage applicability of Mg-based alloys,several methods,such as adding transition metals and rare-earth elements,2modifying surface properties,3employing novel preparing technique,4etc.,have been used to modify the reaction thermodynamics and to enhance H-sorption kinetics.5-8

A lot of investigations have been carried out by alloying with rare earth(RE)and nickel to form intermetallics,such as Mg2Ni,La2Mg17,LaMg2Ni,etc.Mg2Ni phase exhibits relative high hydrogen storage capacity and can catalyze the hydrogen sorption of Mg.The kinetics of Mg2Ni alloy is still slow.9,10La2Mg17has been widely investigated due to high hydrogen storage capacity.But the hydrogen desorption conditions are not satisfying.11Ouyang et al.12pointed out that LaMg2Ni alloy exhibited excellent hydrogen sorption properties and low decomposition enthalpy.However,the hydrogen storage capacity of LaMg2Ni alloy(1.96%)was very low.Hu et al.13have reported that multiphase LaMg2Cu alloy was easily activated and exhibited satisfying sorption kinetics.The multiphase structure in Mg20-xLaxNi alloys was beneficial to the activation and hydrogen sorption kinetics.14Multiphase structure was beneficial to the activation and hydrogen sorption kinetics because interface area in the multiphase structure provided favourable diffusion channels for hydrogen atoms and buffer area for the release of the distortion and stress of the crystal lattice.14,15It is reasonable to assume that the multiphase alloy containing La2Mg17,LaMg2Ni,and Mg2Ni phases would exhibit excellent hydrogen storage properties.In this article,LaMg8.40Ni2.34alloy was selected according to phase diagram of La-Mg-Ni,and microstructure and hydrogen storage properties were investigated.

2 Experimental

LaMg8.40Ni2.34alloy ingot was prepared by inductive melting La,Mg,and Ni(purity more than 99.9%)in an alumina crucible under argon atmosphere(purity more than 99.999%).After melting,the as-cast alloy was annealed at 723 K for 10 h under the argon atmosphere with the pressure of 0.08 MPa.Crystal structure was measured by D/max-2500/PC X-ray diffractometer with Cu Kαradiation.Field emission scanning electron microscopy(FE-SEM)images were obtained by HITACHI S-4800 scanning electron microscope with an energy dispersive X-ray spectrometer(EDS).Hydrogen absorption property measurements were carried out using pressure-compositiontemperature (PCT)characteristic measurementequipment (made by Suzuki Shokan in Japan).The measurement conditions were set as:delay time 300 s and maximum pressure 3 MPa.To calculate enthalpy and entropy,hydrogen pressure at midpoint of adsorption pressure plateau of PCT curves was taken.The hydriding and dehydriding kinetics of the as-cast alloy were also tested with initial hydrogen pressure of 5 and 0.01 MPa,respectively.

3 Results and discussion

3.1 Crystal structure and morphology

XRD pattern of LaMg8.40Ni2.34alloy is presented in Fig.1. LaMg8.40Ni2.34alloy is composed of La2Mg17,LaMg2Ni,and Mg2Ni phases.SEM experiment was performed to investigate the phase structure and results are showed in Fig.2.Clearly the distribution of chemical composition is non-homogeneous.It is composed of grey block(denoted by A),white little block(denoted by B),and dark area(denoted by C).EDS results in Table 1 indicate that A,B,and C areas are considered to the Mg2Ni,LaMg2Ni,and La2Mg17phases,respectively.

3.2 Hydrogen storage properties

3.2.1 Activation property

Fig.1 XRD of LaMg8.4Ni2.34alloy and the first dehydriding alloy

Fig.2 SEM image of LaMg8.4Ni2.34alloy

Absorption curves at the first and second hydriding processes at 588 K and initial hydrogen pressure of 5 MPa are shown in Fig.3.It shows the hydrogen absorption curves at the first and second cycles.It can be seen that the hydrogen capacity reached to 3.85%at 588 K at the first absorption,which is close to the theoretical capacity(4.10%)based on the formation of MgH2and LaH3.The first dehydriding process was in vacuum for 2 h in order to desorb the hydrogen.In the second hydrogen absorption process,99%of maximum hydrogen capacity can be obtained in 450 s.It is reasonable to assume that the alloy has been activated.The good activation of LaMg8.40Ni2.34alloy may be attributed to multiphase structure. The multiphase structure increased the interface area in the alloy.Xie et al.15have investigated the activation property of Mg2-xNdxNi alloys and pointed out that interface areas in multiphase structure provided favourable diffusion channels for hydrogen atoms and buffer areas for the release of the distortion and stress of the crystal lattice.

Table 1 Results of EDS analysis for LaMg8.4Ni2.34alloy

Fig.3 Absorption curves at the first and second hydriding processes at 588 K and initial hydrogen pressure of 5 MPa

XRD pattern of alloy after the first dehydriding was tested and shown in Fig.1.It can be seen that the alloy after dehydriding process consists of LaH2.3,Mg,and Mg2Ni phases.The peaks corresponding to La2Mg17and LaMg2Ni phases disappear.This implies that La2Mg17and LaMg2Ni phases transfer to LaH2.3,Mg,and Mg2Ni after the first hydriding/dehydriding process.Mg2Ni and Mg become the main hydrogen storage phases in following hydrogen adsorption and desorption cycles because the LaH2.3is too stable to desorp hydogen.It is believed that La2Mg17and LaMg2Ni decompose into MgH2, Mg2NiH4and thermally stable LaH2.3by irreversible disproportion reaction with H2in the first hydrogen absorption process. The irreversible reaction can be written as follows:

These results are in agreement with literature.12,15The disproportion reaction between initial phases and hydrogen also contributes to the activation process.

3.2.2 Thermodynamic property

Fig.4 PCT curves of LaMg8.4Ni2.34alloy at different temperatures

Fig.4 shows the PCT curves of LaMg8.40Ni2.34alloy measured at 558,573,and 588 K.As seen from the PCT curves,reversible hydrogen storage capacity of the alloy is 3.01%at 558 K, which is higher than that of LaMg2Ni alloy(1.96%).9The hydrogenation reaction proceeded in two steps,confirmed by two plateaus in hydriding curves,which correspond to the formation of MgH2and Mg2NiH4.It is deduced that the plateau at the low pressure is related to the formation of MgH2and the plateau at the high pressure to the Mg2NiH4formation.However, only one plateau can be observed in dehydriding curves,which may be ascribed to synergetic effect of hydrogen desorption between MgH2and Mg2NiH4.Since desorption pressure of Mg2NiH4is higher than that of MgH2at the same temperature, Mg2NiH4first desorbs hydrogen and exhibits a significant volume contraction,causing a significant contraction strain of MgH2,the dehydrogenation of Mg2NiH4is slightly depressed because it can be stabilized by excessive MgH2.16,17

In order to obtain the thermodynamic parameters of the hydriding reaction of Mg and Mg2Ni in the alloy,the plateau pressure(hydriding process)and the temperature are plotted according to the van′t Hoff equation(Eq.(3)).The van′t Hoff plot for hydriding process is demonstrated in Fig.5.According to the van′t Hoff equation

the values of enthalpy and entropy were obtained.The enthalpy and entropy for hydriding reaction of Mg in LaMg8.4Ni2.34alloy were(-76.0±0.6)kJ·mol-1and(-116.0±2.4)J·mol-1·K-1, and the enthalpy and entropy for hydriding reaction of Mg2Ni in LaMg8.4Ni2.34alloy were(-54.9±0.1)kJ·mol-1and(-88.9± 1.1)J·mol-1·K-1.The ΔH and ΔS for hydriding reaction of pure Mg were-74.5 kJ·mol-1and-135 J·mol-1·K-1,18for hydriding reaction of pure Mg2Ni were-64.4 kJ·mol-1and-121 J·mol-1·K-1.Ouyang et al.19pointed out that the Mg-H in the PrMg3alloy(ΔH=-79.9 kJ·mol-1)was more stable than Mg-H in pure Mg.The Mg or Mg hydride in LaMg8.40Ni2.34alloy is transferred from the La2Mg17.So the ΔH for hydriding Mg in LaMg8.40Ni2.34alloy is higher than that for hydriding pure Mg-H.The absolute value of ΔH for hydriding reaction of Mg2Ni in LaMg8.40Ni2.34alloy is lower than that for hydriding reaction of pure Mg2Ni and higher than that for hydriding reaction of Mg2Ni(-51 kJ·mol-1)in the LaMg2Ni alloy.This may be ascribed to the fact that partial Mg2Ni in LaMg8.40Ni2.34alloy is transferred from LaMg2Ni phase.Thus,it is assumed that ΔH values of Mg and Mg2Ni in LaMg8.40Ni2.34alloy are related to the initial La2Mg17and LaMg2Ni phases.

Fig.5 van′t Hoff plot of hydriding Mg(a)and Mg2Ni(b)in LaMg8.4Ni2.34alloy

3.2.3 Kinetic property

The hydrogen sorption kinetic properties of LaMg8.40Ni2.34alloy were measured at 558,573,and 588 K with initial hydrogen pressure of 5 MPa.Typical hydriding and dehydriding curves are given in Fig.6.Clearly the uptake time for hydrogen content to reach 99%of the maximum hydrogen capacity is less than 450 s at experimental conditions.And 2.51%hydrogen can be released within 2000 s.

To further investigate the kinetics properties of LaMg8.4Ni2.34alloy,the reaction mechanics of hydrogen sorption was analyzed by comparing the hydriding/dehydriding curves with the rate equation derived form different processes.20,21The rate equations in the literature20,22were used to compare with the experimental data.The hydriding/dehydriding curves at different temperatures can be fitted by Jander diffusion model(JDM) and shown in Fig.7.The rate equation isis the initial pressure of the reaction.ptand peqare the pressures at time t and final equilibrium,respectively,and k is temperature-dependent rate constants.g(α)is defined as follows:

Fig.6 Typical hydriding(a)and dehydriding(b)curves at different temperatures

Fig.7 g(α)vs time for hydriding(a)and dehydriding(b)processes at different temperatures

This indicates that hydriding/dehydriding processes can be described by the three-dimensional diffusion mechanism.The rate constant of the hydrogen diffusion in the alloy,23and the temperature-dependent rate constants(k)were obtained from the slope of each of the straight lines obtained from Fig.7.The rate constant of any thermally activated process is usually described by an Arrhenius relationship.Fig.8 shows the Arrhenius plots of lnk vs 1/T for LaMg8.40Ni2.34alloy.The slope of the straight line gives the ratio of activation energy to the universal gas constant(-Ea/R).The activation energies of LaMg8.40Ni2.34alloys are(52.4±0.4)and(59.2±0.1)kJ·mol-1for hydriding and dehydriding,respectively,which are lower than that of Mg2Ni alloy(64 kJ·mol-1).20This should be attributed to the following reasons.Firstly,La hydride forms in the activation according to the XRD.It has been reported that RE hydride can catalyze the sorption reaction of Mg and Mg2Ni.9-11The formation of La hydride is beneficial to hydrogen sorption kinetics.Secondly,the existence of Mg2NiH4plays a catalytic role for the hydrogen sorption of Mg.24,25Thirdly,multiphase structure can provide lots of phase boundary,which can be used as the hydrogen diffusion channel and then improve the sorption kinetics.

Fig.8 Arrhenius plots of hydriding(a)and dehydriding(b)reaction rate constants

4 Conclusions

LaMg8.40Ni2.34alloy was prepared by inductive melting.XRD and SEM showed the alloy was composed of La2Mg17, LaMg2Ni,and Mg2Ni phases.The alloy can be activated easily in the first hydriding and dehydriding cycle.Reversible hydrogen storage capacity is 3.01%at 558 K.Because of the synergetic effect between MgH2and Mg2NiH4,the PCT curves of hydrogen desorption show only one plateau.ΔH and ΔS for hydriding Mg and Mg2Ni in LaMg8.40Ni2.34alloy are different from those for hydriding pure Mg and Mg2Ni alloy,which implies the thermodynamic properties of Mg and Mg2Ni phases in the multiphase are related to the initial phase.The activation energy values of LaMg8.40Ni2.34alloy are lower than that of Mg2Ni alloy.Compared with Mg2Ni alloy,the LaMg8.40Ni2.34alloy has higher hydrogen storage capacity,better activation property and kinetic property,which is attributed to the multiphase structure of alloy.

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