J. Cent. South Univ. Technol. (2009) 16: 0876-0880

DOI: 10.1007/s11771-009-0145-9

Hydrogen sorption properties of nanocrystalline Mg₂FeH6-based

complex and catalytic effect of TiO₂

LIU Yi(刘 燚)^{1, 2}, TANG Sheng-long(汤盛龙)¹, FANG Yu-hu(方于虎)¹, LIU Huai-fei(刘怀菲)¹,

CUI Jian-min(崔建民)³, LI Song-lin(李松林)¹

(1. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China;

2. Shenzhen Jinzhou Precision Technology Corporation Ltd, Shenzhen 518116, China;

3. Powder Metallurgy Corporation Ltd, Laiwu Iron and Steel Group, Laiwu 271105, China)

Abstract: The diversities of hydrogen sorption properties of Mg₂FeH₆-based complexes with and without TiO₂ were investigated. Mg₂FeH₆-based complexes with and without TiO₂ were synthesized respectively by reactive mechanical alloying, and hydrogen sorption properties of the complexes were examined by Sieverts-type apparatus. The results show that the sample without TiO₂ releases 4.43% (mass fraction) hydrogen in 1.5 ks at 653K under 0.1MPa H₂ pressure and absorbs 90% of the total 4.43% (mass fraction) hydrogen absorbed in 85s at 623K under 4.0MPa H₂ pressure. But for the sample with TiO₂ addition under the same condition, it only needs 400s to release all of the stored hydrogen and 60s to absorb 90% of the total hydrogen absorbed. The activation energies for desorption process of the samples with and without TiO₂ are determined to be 71.2 and 80.3kJ/(mol·K), respectively. The improvement in hydrogen sorption rate and and reduction in activation energy can be attributed to the addition of TiO₂.

Key words: Mg-based hydrogen storage materials; reactive mechanical alloying; hydrogen sorption properties; kinetics; activation energy

1 Introduction

One of the key roadblocks to the widespread use of hydrogen as a renewable fuel on board of passenger vehicles is hydrogen storage[1]. The hydrogen storage alloy is the key material of dynamical battery and fuel cell system that are used in vehicles[2]. Mg and Mg-based alloys are a group of attractive materials for hydrogen storage due to its low density and cost,  abundance in the earth and high hydrogen capacity. Dimagnesium iron hydride, Mg₂FeH₆, has one of the highest volumetric hydrogen density of 150 kg/m³, which is double of the volumetric hydrogen density of liquid hydrogen, and a gravimetric hydrogen density of 5.66% (mass fraction), which is far larger than that of conventional hydrogen storage materials such as LaNi₅ or TiFe. So Mg₂FeH₆ has become a hot topic in recent years.

Mg₂FeH₆ is more difficult to synthesize than the conventional transition metal hydride, Mg₂NiH₄, mainly due to the absence of intermetallic compound of the type Mg₂Fe in the Mg-Fe binary system[3-4]. The traditional way of preparing Mg₂FeH₆ is by sintering the raw powder at high temperature (723-793 K) under high hydrogen pressure (2-12 MPa)[5]. More recently, progress has been made by mechanical alloying. The aim is to obtain a maximum yield of Mg₂FeH₆ and to minimize the contents of Fe, MgH₂, Mg and MgO. SAI et al[3] obtained a yield of 63% (mass fraction) Mg₂FeH₆ by ball milling 2Mg-Fe mixture under 1 MPa H₂ pressure. Shang et al[4] synthesized Mg₂FeH₆ by milling 3MgH₂-Fe mixture under Ar atmosphere for 60 h, leading to a yield of 79% (mass fraction) of Mg₂FeH₆ which co-existed with 16% (mass fraction) of MgO. Puszkiel et al[6] synthesized Mg₂FeH₆ utilizing mechanical milling of 2Mg-Fe mixture, followed by heating at 673 K under 6 MPa H₂ pressure and the obtained yield of Mg₂FeH₆ was about 50%. In Refs.[7-8], effects of ball milling method and starting powder mole ratio of Mg to Fe on the synthesis and hydrogen storage properties of nanocrystalline Mg₂FeH₆ were investigated. So far, high sorption temperatures and slow kinetics are still the main drawbacks for practical application of Mg₂FeH₆. In recent years, many studies have shown that the addition of transition metal oxides such as V₂O₅, Cr₂O₃, Fe₃O₄, TiO₂ and Fe₂O₃ in Mg-based hydrogen storage materials can obviously improve their hydrogen sorption properties[9-11]. WANG et al[12] found that TiO₂ provided a diffusion pathway for hydrogen atoms. TiO₂ improves the hydrogen sorption property of Mg-TiO₂ mixture at low temperature (473 K). WANG et al[13] synthesized Mg-based hydrogen storage materials with good sorption kinetics by the addition of transition metal chloride or oxide in Mg-Ni powders. SHANG et al[4] and HERRICH et al[14] added Cu and Pt respectively to Mg₂FeH₆ and found that there was no catalytic effect on Mg₂FeH₆ hydrogen sorption properties for these additives. But little work has been done interiorly on the effect of additives on Mg₂FeH₆.

In this work, Mg₂FeH₆-TiO₂ nanocrystalline hydrogen storage material was synthesized at mole ratio of Mg to Fe of 3?1 and 2.5% (mole fraction) TiO₂ via reactive mechanical alloying. The structure and sorption kinetic properties of the Mg₂FeH₆-based complexes were examined by an X-ray diffractometry (XRD) and a hydrogen sorption apparatus, respectively. The effect of TiO₂ additive was investigated.

2 Experimental

Elemental powders of Mg (purity＞99%, 0.841 mm) and Fe (purity＞99%) were mixed with mole ratio of 3?1, and 2.5% TiO₂ was used as an additive. The sample handling was performed in a glove-box fulfilled with Ar (purity 99.99%). The powders were aggregately milled for 150 h under reactive H₂ atmosphere. The samples with and without addition of TiO₂ were designated as 3MFHTOL150 and 3MFH150, respectively. The details of the processing parameters are shown in Table 1.

Table 1 Processing parameters for reactive mechanical alloying of samples 3MFH150 and 3MFHTOL150

The phase compositions were tested by Rigaku D/max2550 diffractometer using Cu K_α radiation (λ=0.154 18 nm). The grain size of phases was calculated using Scherrer equation. Hydrogen sorption properties of the milled powders were measured by a Sieverts-type apparatus. The activation energy for desorption process was estimated from Arrhenius plot of reaction rate constant K with temperature [15-20]:

K=K₀exp[-Q/(RT)]                            (1)

where  K is the reaction rate constant; K₀ is the coefficient; Q is the activation energy; R is the gas constant; and T is the temperature. K is determined by fitting the desorption curves with the Johnson-Mehl-Avrami (JMA) equation:

α=1-exp[-(Kt)ⁿ]                              (2)

where  n is the reaction exponent; and α is the desorption fraction at time t.

3 Results and discussion

3.1 Synthesis of nanocrystalline Mg₂FeH₆-TiO₂ complex

XRD patterns of powders with TiO₂ as a function of reactive milling time are shown in Fig.1. At the initial step, Mg reacts with H₂ to form MgH₂, and then Mg₂FeH₆ is formed according to the following reaction[21-22]:

3MgH₂+FeMg₂FeH₆+Mg                    (3)

Weak MgO diffraction peak appears after milling for 105 h, and the peaks of Mg₂FeH₆ and Fe on the XRD patterns are obviously broadened after milling for 150 h. There is no phase corresponding to TiO₂ peaks observed, which is due to the small amount of TiO₂ addition in the starting milling powder and the rapid reduction of TiO₂ grain size.

Fig.1 XRD patterns of powders with TiO₂ (3MFHTOL150) as function of reactive milling time

Table 2 certifies that there is an obvious reduction of grain size of each phase with increasing ball-milling

Table 2 Grain size (d) of each phase in milled powders after ball-milling for different time calculated from XRD patterns

time. After milling for 2 h, the grain sizes of Mg and Fe are in the range of 20-30 nm, respectively. After milling for 20 h the grain sizes of MgH₂ and Mg₂FeH₆ are 6.1 and 8.7 nm, respectively. The grain size of Mg₂FeH₆ becomes 5.5 nm after milling for 150 h. The nano- structure is favorable to the hydrogen sorption properties of the hydrogen storage material.

3.2 Hydrogen sorption properties of nanocrystalline Mg₂FeH₆-TiO₂ and Mg₂FeH₆-based complex

The nanocrystalline Mg₂FeH₆-based complex synthesized by reactive mechanical alloying can desorb/absorb hydrogen directly without any activation, because the nanocrystalline Mg₂FeH₆-based complex has more crystal boundaries that can effectively increase the diffusion passageway for hydrogen atoms[23]. The hydrogen absorption and desorption curves of Mg₂FeH₆-based complex at different temperatures are shown in Fig.2.

Fig.2(a) shows that the as-synthesized Mg₂FeH₆- based complex possesses good hydrogen absorption performance. Only 85 s is needed for the accomplish- ment of 90% of hydrogen absorption at 623 K under 4.0 MPa H₂ pressure. The synthesized powder can absorb 4.43% (mass fraction) hydrogen at 623 K under 4.0 MPa H₂ pressure. It also takes only 90 s to accomplish 90% of hydrogen absorption and aggregately absorb 4.13%

Fig.2 Hydrogen absorption—desorption curves of sample 3MFH150 from Sieverts-type apparatus at different temperatures: (a) Absorption curves (Initial H₂ pressure 4.0 MPa); (b) Desorption curves (Desorption under 0.1 MPa H₂ pressure)

hydrogen even at 593 K under 4.0 MPa H₂ pressure.

The hydrogen desorption curves of the 3MFH150 sample are shown in Fig.2(b). Hydrogen desorption to 90% capacity of sample 3MFH150 can be accomplished in 1 ks and it completely releases 4.43% hydrogen at  653 K under 0.1 MPa H₂ pressure in 1.5 ks. However, the hydrogen amount and desorption rate decrease remarkably with the decrease of desorption temperature. Sample 3MFH150 only releases 2.57% hydrogen at  623 K in 7.2 ks and 1.13% hydrogen at 593 K.

Hydrogen absorption curves of sample 3MFHTOL150 at different temperatures are shown in Fig.3(a). It can be seen that sample 3MFHTOL150 possesses good hydrogen absorption kinetics. More than 90% of hydrogen absorption is accomplished in 60 s at 653 K under 4.0 MPa H₂ pressure, and finally hydrogen capacity reaches 3.12%. Compared with sample 3MFH150, it only takes 60 s to accomplish 90% of hydrogen absorption for sample 3MFHTOL150. Even though the temperature drops to 593 K, sample 3MFHTOL150 also displays higher hydrogen absorption rate, but the hydrogen capacity falls to 2.34%.

Sample 3MFHTOL150 also exhibits good hydrogen desorption kinetics (Fig.3(b)). The hydrogen desorption is accomplished in 400 s at 653 K and completely releases 2.58% hydrogen. The hydrogen desorption rate

Fig.3 Hydrogen absorption—desorption curves of sample 3MFHTOL150 at different temperatures: (a) Absorption curves (Initial hydrogen pressure 4.0 MPa); (b) Desorption curves (Desorption under 0.1 MPa H₂ pressure)

decreases with the decrease of temperature, but the amount of hydrogen desorption is less affected by the temperature when the temperature exceeds 623 K.

3.3 Hydrogen desorption kinetics of nanocrystalline Mg₂FeH₆-based complex and catalytical effect of  TiO₂

In order to further investigate the catalytical efficiency of TiO₂ in Mg₂FeH₆-TiO₂ complex, hydrogen desorption curves of samples 3MFH150 and 3MFHTOL150 were analyzed. The hydrogen desorption curves of samples 3MFH150 and 3MFHTOL150 at temperatures of 593, 623 and 653 K under 0.1 MPa H₂ pressure are shown in Fig.4. By comparing the hydrogen

Fig.4 Hydrogen desorption curves of composite powders desorbed at 653 K (a), 623 K (b) and 593 K (c) under 0.1 MPa H₂ pressure

desorption curves at different temperatures under 0.1 MPa H₂ pressure, it is found that sample 3MFHTOL150 has higher hydrogen desorption rate that releases 90% of hydrogen capacity in 300 s at 653 K. This sample still keeps high hydrogen desorption rate even when the temperature falls to 623 K.

The reaction exponent n in Eq.(2) for all samples was taken as 3[20]. From the Arrhenius plot of K with temperature (see Fig.5, where R² is the degree of fitting), the activation energies of desorption for samples 3MFH150 and 3MFHTOL150 are calculated to be 80.3 and 71.2 kJ/(mol·K), respectively. Czujko et al[20] estimated the activation energies of desorption for Mg+10% X (X=V, Y, Zr) submicrocrystalline composites from similar plot. By taking the data into Eqs.(1) and (2), the hydrogen desorption equations of samples 3MFH150 and 3MFHTOL150 are given respectively as follows:

α=1-exp{-[11 669texp(-9 652/T)]³}              (4)

α=1-exp{-[3 707texp(-8 562/T)]³}               (5)

The addition of TiO₂ obviously improves hydrogen absorption/desorption rate. Several explanations are available: (1) TiO₂ can accelerate the decomposition of molecular hydrogen and its c-axis is a passageway for fast diffusion of hydrogen atom; (2) TiO₂ nano-particles

Fig.5 Arrhenius plot curves for samples 3MFH150 (a) and 3MFHTOL150 (b)

distributed on the surface of matrix speed up hydro- genation rate at the beginning, and those distributed in matrix supply a freeway for hydrogen atom diffusing into matrix[11]; and (3) TiO₂ can destroy the layer of oxygenation.

4 Conclusions

(1) The as-synthesized sample 3MFH150 releases 4.43% hydrogen in 1.5 ks at 653 K under 0.1 MPa H₂ pressure and absorbs 90% of total 4.43% hydrogen stored in 85 s at 623 K under 4.0 MPa H₂ pressure. It also indicates that the hydrogen storage material Mg₂FeH₆-based complex synthesized by reactive mechanical alloying can release/absorb hydrogen directly without any activation.

(2) At 653 K under 0.1 MPa H₂ pressure, it only needs 400 s to release all of hydrogen for sample 3MFHTOL150. At 623 K under 4.0 MPa H₂ pressure, it only takes 60 s to absorb 90% of hydrogen stored. The addition of TiO₂ improves hydrogen absorption/ desorption rate of Mg₂FeH₆-based complex.

(3) The activation energies for desorption process of samples 3MFH150 and 3MFHTOL150 are 80.3 and  71.2 kJ/(mol·K), respectively. The addition of TiO₂ helps to the decrease of the activation energy for Mg₂FeH₆ desorption process, which gives the kinetical explanation for the higher desorption rate of Mg₂FeH₆-TiO₂ complex.

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(Edited by CHEN Wei-ping)

Foundation item: Project (50574105) supported by the National Natural Science Foundation of China; Project (NCET-06-0683) supported by the Program for the New Century Excellent Talents in University; Project (08-030239) supported by the Program for 121 Excellent Talents in Hunan Province; Project (07MX21) supported by Mittal Student Innovation Foundation of Central South University

Received date: 2009-01-17; Accepted date: 2009-04-10

Corresponding author: LI Song-lin, Professor; Tel: +86-731-88830614; E-mail: lisl@mail.csu.edu.cn