A comparison of TiF₃ and NbF₅ catalytic effects on hydrogen absorption and desorption kinetics of a ball-milled Mg₈₅Zn₅Ni₁₀ alloy

Abstract

In this investigation, the as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) composites were successfully produced via ball milling. The different influences between the catalysts TiF₃ and NbF₅ on the hydrogen storage behavior and microstructure of the composites were investigated by XRD, SEM, TEM and hydrogen absorption/desorption tests. The as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys contain the major phase Mg, the secondary phase Mg₂Ni, a small amount of MgZn₂, TiF₃ and NbF₅. After hydrogenation, MgH₂ and Mg₂NiH₄ are formed, which convert back into Mg and Mg₂Ni after dehydrogenation indicating that MgZn₂ and the catalysts TiF₃ and NbF₅ do not react with hydrogen. Compared with NbF₅ catalyzed alloy, the TiF₃ catalyzed alloy has a faster hydrogen absorption/desorption kinetics. On the basis of Arrhenius equation, the dehydrogenation activation energy values of the as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys are 75.514 and 82.367 kJ mol⁻¹ H₂, respectively, while the value of ball-milled Mg₈₅Zn₅Ni₁₀ alloy is 109.830 kJ mol⁻¹ H₂. As a result, both TiF₃ and NbF₅ can significantly ameliorate the hydrogen storage thermodynamics. TiF₃ shows better catalytic influence on hydrogen storage property of Mg₈₅Zn₅Ni₁₀ than NbF₅.

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Introduction

For nearly half a century, rapid economic development and energy crisis have threatened our existence because of excessive exploitation of non-renewable energy sources. Meanwhile, the consequent environmental problems arising from the excessive use of fossil fuels are becoming increasingly prominent. There is a growing demand for a new kind of renewable energy. Therefore, many scholars pay attention to hydrogen owing to its high efficiency, environment friendly nature, and renewable characteristics.^1–13 However, the problem that is severely restricting the application of hydrogen is availability of a safe and credible storage mode.^14–16 Because of the merits of high gravimetric storage density and safety, many researchers have great expectations from metal hydrides as a hydrogen storage material.^15–20 Most researchers believe that among the different metals which can form hydrides, Mg is the most potential candidate to be widely used as a hydrogen storage material because of its low price, excellent reversibility, abundance, and high gravimetric hydrogen storage densities (7.6 wt%).^21,22 However at the same time, the inherent shortcomings of Mg, such as the high decomposition temperature of its hydride and poor storage kinetics seriously impede its commercial development. Therefore, it is essential to ameliorate the kinetics and thermodynamics of the hydrogen storage alloys.

Some researches have confirmed that alloying Mg with transition metals such as Ni,^23–25 Zn,^26,27 Ti, Cr, Nb, Cu and Fe^28–31 can enhance the hydrogenation/dehydrogenation rates. Also, some rare earth elements,^32–34 intermetallic compounds,³⁵ metallic oxides,^36,37 and fluorides^38–40 are proven to have a good catalytic effect on accelerating the storage and release of hydrogen. Yavari et al.⁴¹ added FeF₃ into MgH₂ to manufacture nanostructured MgH₂ composites and found that hydrogenation/dehydrogenation rates were clearly enhanced. Lee et al.⁴² synthesized Mg–5NbF₅ and showed that Mg–5NbF₅ had faster initial hydrogen absorption and desorption and a larger effective storage capacity. Ma et al.³⁹ mechanically prepared a MgH₂ + TiF₃ system and proved that mechanical milling with 4 mol% TiF₃ could enhance the sorption kinetics markedly and reduce operation temperatures in particular. The hydrogenation could be accomplished in about 25 s even at 313–373 K. Recham et al.⁴³ showed that NbF₅ could reduce the dehydriding temperature while enhancing the kinetics of ball-milled MgH₂ with 2 mol% of NbF₅ as the optimum concentration. It is believed that the transition metal fluorides play a key role in enhancing the hydrogenation kinetics of Mg.⁴²

In this study, we designed a new kind of Mg based Mg₈₅Zn₅Ni₁₀ ternary alloy, and ball milled it with TiF₃ and NbF₅ as catalysts to ameliorate the hydrogen storage properties. We also thoroughly investigated the influence of doping TiF₃ and NbF₅ on the microstructure and hydrogen storage performances of experimental materials. The storing and releasing characteristics and activation energies were also analysed.

Experimental

Mg₈₅Zn₅Ni₁₀ alloy was prepared using a vacuum induction furnace under 0.04 MPa helium atmosphere. Extra 10 wt% Mg was added to reduce the volatilization of magnesium during melting. The as-cast alloy ingots were mechanically crushed into powders (particle size < 75 μm). Then the as-cast alloy powder was mixed with 4 wt% TiF₃ and NbF₅ and transferred to a mechanical ball mill. A planetary-type mill was used under an Ar atmosphere for milling. The weight ratio of the alloy powder and Cr–Ni stainless steel balls was 1 : 40. The milling speed was 350 rpm. During ball milling, in order to prevent excess heating, the mill was set to rest for 1 h after every 1 h of working. The samples were milled for a total of 5 h. The as-milled Mg₈₅Zn₅Ni₁₀ + 4 wt% C (C = TiF₃ and NbF₅) in this research are named as Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅). Phase structures, morphologies and crystalline states of the ball-milled Mg₈₅Zn₅Ni₁₀ + 4 wt% C (C = TiF₃ and NbF₅) alloys were investigated and observed by X-ray diffraction (XRD) (D/max/2400), SEM and high resolution transmission electron microscopy (HRTEM) (JEM-2100F).

A Sieverts-type apparatus was used to measure the pressure–composition isotherms (P–C-I) and hydrogen absorption/desorption kinetics. First, 6 hydrogen absorption and desorption cycles were performed at 360 °C to completely activate the samples. PCI curves were tested at 360, 340 and 320 °C. The hydrogen absorption was conducted at a hydrogen pressure of 3 MPa, at 360, 340, 320, 300, 280, 260, 240, 220, 200 and 150 °C respectively. The hydrogen desorption was tested at a pressure of 1 × 10⁻⁴ MPa at 360, 340, 320, 300 and 280 °C.

The dehydrogenation process of the ball-milled Mg₈₅Zn₅Ni₁₀ + 4 wt% C (C = TiF₃ and NbF₅) alloys was studied by differential scanning calorimetry (DSC) on a NETZSCH, STA 449F3 instrument. The flow rate of argon was 50 mL min⁻¹. The hydrogen saturated-samples were gradually heated from room temperature to 500 °C with an increase of 5 °C min⁻¹.

Results and discussion

Phase and microstructural characteristics

X-ray diffraction (XRD) has been utilized in studying the phase compositions and structures of ball-milled Mg₈₅Zn₅Ni₁₀ and Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) before and after hydrogen absorption/desorption, as described in Fig. 1. We can see that after 5 h ball milling, the broadened diffraction peaks reveal that mechanical ball milling causes the reduction of grain size and amorphization. According to Fig. 1, the ball-milled Mg₈₅Zn₅Ni₁₀ contains three phases: Mg as the major phase, the secondary phase Mg₂Ni, and a small MgZn₂ phase. Meanwhile, after adding TiF₃ and NbF₅, no new phase appeared in the XRD curves, indicating no reaction occurred between the catalysts TiF₃ and NbF₅ and Mg₈₅Zn₅Ni₁₀ during ball milling. Mg and Mg₂Ni are still the major and the secondary phase in Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys, respectively. After hydrogen absorption, it is clear that the Mg phase is converted into MgH₂. Simultaneously, Mg₂Ni converts to Mg₂NiH₄. It is worth noting that among all three samples, only MgZn₂ existed after hydrogen absorption, showing that it does not react with hydrogen. After hydrogen desorption, MgH₂ and Mg₂NiH₄ change into Mg and Mg₂Ni, respectively. MgZn₂ is also found in the dehydrogenated alloys. Based on the XRD patterns, there are two reversible reactions for hydrogenation and dehydrogenation of the alloys. The reaction path ways can be inferred as follows:

Mg + H₂ ↔ MgH₂

Mg₂Ni + H₂ ↔ Mg₂NiH₄

Fig. 1 XRD patterns of the ball-milled Mg₈₅Zn₅Ni₁₀ and Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys before and after 30 hydrogen absorption and desorption cycles: (a) ball-milled, (b) C = TiF₃, (c) C = NbF₅.

The Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloy samples after hydrogen absorption and desorption were detected by HRTEM and ED, as described in Fig. 2. After ball milling with the catalysts and hydrogen absorption and desorption, partial nanocrystalline and amorphous phases form in the Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys. Evidently, after hydrogen absorption, Mg₈₅Zn₅Ni₁₀–4TiF₃ contains MgH₂, Mg₂NiH₄, MgZn₂ and TiF₃ phases, and Mg₈₅Zn₅Ni₁₀–4NbF₅ possesses MgH₂, Mg₂NiH₄, MgZn₂ and NbF₅ phases, which are sustained through ED patterns and also consistent with the XRD test. According to the structural analysis and index of ED rings, MgH₂ and Mg₂NiH₄ convert into Mg and Mg₂Ni respectively. MgZn₂, TiF₃ and NbF₅ are found after hydrogen desorption, suggesting absence of reaction among MgZn₂, TiF₃, NbF₅ and hydrogen. The catalysts TiF₃ and NbF₅ exist stably in the Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) composites after ball milling, even after 30 hydrogenation and dehydrogenation cycles. The results of HRTEM and ED coincide with the XRD results.

Fig. 2 HRTEM micrographs and SAED patterns of the Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys after hydrogen absorption and desorption: (a) hydrogenated C = TiF₃, (b) dehydrogenated C = TiF₃, (c) hydrogenated C = NbF₅, (d) dehydrogenated C = NbF₅.

Fig. 3 and 4 present the bright field FETEM images and EDS mapping of the Mg₈₅Zn₅Ni₁₀–4TiF₃ and Mg₈₅Zn₅Ni₁₀–4NbF₅ alloys after dehydrogenation cycles, respectively. The different colours express the distributions of different elements. As can be seen from Fig. 3(a) and (b), Mg is the major component covering the main matrix of a single Mg₈₅Zn₅Ni₁₀–4TiF₃ alloy particle. In Fig. 3(c) and (d), the Ni and Zn elements are mainly distributed in the bright regions of the alloy particle illustrating that Mg–Ni and Mg–Zn metal compounds are distributed on the surface of the alloy particle. The Ti and F elements are relatively well-distributed around the alloy particle indicating uniform inlaying of TiF₃ on Mg matrix which is corroborated by XRD and TEM results. Similarly, according to Fig. 4, Nb and F elements are also well-distributed on the Mg₈₅Zn₅Ni₁₀–4NbF₅ alloy particles. Based on XRD, TEM and EDS mapping, the catalysts TiF₃ and NbF₅ are both present after hydrogenation and dehydrogenation cycles suggesting that they do not react with hydrogen or decompose after hydriding and dehydriding cycles.

Fig. 3 FETEM micrographs and EDS mapping of the Mg₈₅Zn₅Ni₁₀–4TiF₃ alloy after hydrogen desorption cycles.

Fig. 4 FETEM micrographs and EDS mapping of the Mg₈₅Zn₅Ni₁₀–4NbF₅ alloy after hydrogen desorption cycles.

Hydrogenation and dehydrogenation kinetics

With the aim to study the influence of different catalysts on hydrogenation kinetics of Mg₈₅Zn₅Ni₁₀, the hydrogen absorptions of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys were measured at a pressure of 3 MPa and at 360, 340, 320, 300, 280, 260, 240, 220, 200 and 150 °C, as represented in Fig. 5. We can observe that at the beginning of the hydrogenation process, the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys have a very fast hydrogenation rate and the alloys need relatively long time to get saturated. This is because the hydride layer that is formed swiftly covers the particle surfaces and blocks the hydrogen diffusion into the alloys. For the convenience of comparison, the hydrogen absorption capacity at 600 s has been taken as the reference. According to Fig. 5, the C = TiF₃ alloy can absorb 4.404, 4.33, 4.238, 4.153, 4.117 wt% at 360, 340, 320, 300, 280 °C, respectively in 600 s. Simultaneously, the C = NbF₅ alloy can absorb 4.313, 4.244, 4.235, 4.197, 4.06 wt% at the same condition. Distinctly, the hydrogenation absorption rate within 600 s of C = TiF₃ alloy is a little faster than the C = NbF₅ alloy, also at relatively low temperatures. From Fig. 5(c) and (d), even at 150 °C, the C = TiF₃ alloy shows a faster absorption rate comparing with C = NbF₅ alloy. This explains that TiF₃ has more effective catalytic effect than NbF₅. The ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys show very fast hydrogenation rate. This is probably due to the TiF₃ and NbF₅ catalysts promoting H₂ molecules to dissociate into H atoms and distribute on the surfaces of the Mg particles. Also, after ball milling with TiF₃ and NbF₅, the uniform mixing powders generated defects and cracks during the hydriding/dehydriding process, as illustrated in Fig. 6. These defects and cracks provide hydrogen diffusion channels and nucleation positions to form magnesium hydrides. As a comparison, the cracks formed on the C = TiF₃ alloy particles are evidently more than those on the C = NbF₅ alloy. This is the reason why the C = TiF₃ alloy has faster hydrogen absorption kinetics, even at relatively low temperatures.

Fig. 5 Hydrogen absorption kinetic curves of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys at different temperatures: (a) and (c) C = TiF₃, (b) and (d) C = NbF₅.

Fig. 6 SEM images of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys: (a) C = TiF₃ alloy after ball milling, (b) C = TiF₃ alloy after hydrogenation, (c) C = TiF₃ alloy after dehydrogenation, (d) C = NbF₅ alloy after ball milling, (e) C = NbF₅ alloy after hydrogenation, (f) C = NbF₅ alloy after dehydrogenation.

The hydrogen desorptions of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys were tested to investigate the different catalytic effects between TiF₃ and NbF₅ on the hydrogen desorption kinetics, as depicted in Fig. 7. The hydrogen desorption from the alloys was tested at 360, 340, 320, 300 and 280 °C. Evidently, the temperature significantly affects the hydrogen desorption kinetics. The ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys have very fast desorption kinetics over 320 °C. The ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys could release all the hydrogen in less than 300 s at 360 °C, but took more than 30 minutes at 280 °C. According to Fig. 7, the time taken for releasing 4 wt% H₂ at 360, 340, 320, 300 and 280 °C is 102, 156, 318, 726 and 1836 s, respectively for the C = TiF₃ alloy, and 150, 222, 348, 756 and 1860 s, respectively for the C = NbF₅ alloy. This indicates the C = TiF₃ alloy has better desorption kinetics than C = NbF₅ alloy owing to the defects and cracks (Fig. 6(c) and (f)) formed on the particles which promote the hydrogen diffusion. Certainly, both catalysts have significant effects on the hydrogenation kinetics compared with the milled MgH₂, which hardly decomposes at 300 °C.⁴⁴

Fig. 7 Hydrogen desorption kinetic curves of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys at different temperatures: (a) C = TiF₃, (b) C = NbF₅.

Hydrogen desorption property

To further study the different catalytic effects between TiF₃ and NbF₅ on hydrogen desorption property of Mg₈₅Zn₅Ni₁₀, the composites were characterized by DSC measurement as shown in Fig. 8. There is a big difference between the DSC curves of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys, as seen in Fig. 8. There are two endothermic peaks in the curve of C = NbF₅ alloy at 228.84 °C and 349.93 °C. On the basis of XRD and TEM results, there are two reversible reactions in every hydrogenation and dehydrogenation reactions. The dehydrogenation reactions are MgH₂ → Mg + H₂ and Mg₂NiH₄ → Mg₂Ni + H₂. Therefore, the peak at 228.84 °C belongs to the endothermic peak of Mg₂NiH₄ and the peak at 349.93 °C corresponds to MgH₂.^43,45–47 However, for the C = TiF₃ alloy, there seems to be only one endothermic peak at 256.88 °C in the DSC curve. This indicates that the catalyst TiF₃ strongly decreases the endothermic peak temperature of MgH₂, and even causes the two peaks of Mg₂NiH₄ and MgH₂ to coincide. It illustrates that the reaction for MgH₂ decomposition becomes more easy by adding TiF₃. Conversely, there is no such significant catalytic effect by NbF₅. This also proves that TiF₃ improves the hydrogen desorption property of Mg₈₅Zn₅Ni₁₀ more effectively than NbF₅. This result is in line with previous discussions of hydrogenation and dehydrogenation kinetics. Moreover, the onset temperatures of dehydrogenations are 218.63 and 210.08 °C of C = TiF₃ and NbF₅ alloys, respectively.

Fig. 8 DSC curves of the desorption process of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys.

Hydrogenation and dehydrogenation cyclic stability

Fig. 9 and 10 depict the hydrogen absorbing and desorbing cyclic stability curves of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys at 360 °C. According to Fig. 9 and 10, basically there is no change of hydrogen absorption and desorption capacities for both C = TiF₃ alloy and C = NbF₅ alloy after 15 cycles. It is remarkable that the hydrogenation rate of first cycle for the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys is quite slow owing to a long process of complete activation of the alloys. However, after the second cycle, the hydrogenation and dehydrogenation rates are significantly accelerated. Subsequently, with the increase of cycle times, the hydrogenation and dehydrogenation rates gradually improve and show good kinetics. This is because defects and cracks are generated during the hydrogenation and dehydrogenation cycles. These defects and cracks can provide more hydrogen diffusion channels and nucleation positions for Mg hydrides which was discussed previously. Therefore, the hydrogen absorption and desorption cyclic curves are almost identical. Also, there is no capacity loss even after 30 hydrogenation/dehydrogenation cycles. On the basis of Fig. 9 and 10, the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys reveal good hydrogenation and dehydrogenation cyclic stability. Furthermore, the capacity without any loss indicates that there is no other stable hydride formation during the hydrogenation/dehydrogenation process. Subsequently, the decomposition of TiF₃/NbF₅ and formation of TiH₂/NbH₂ is always accompanied by the loss of capacity owing to the formation of TiH₂. TiH₂/NbH₂ cannot release hydrogen during dehydrogenation of MgH₂.^48–50 Therefore, this also proves that both TiF₃ and NbF₅ exist stably in the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys.

Fig. 9 Hydrogenation/dehydrogenation cycling curves of Mg₈₅Zn₅Ni₁₀–4TiF₃ alloy at 360 °C: (a) hydrogenation cycles, (b) dehydrogenation cycles.

Fig. 10 Hydrogenation/dehydrogenation cycling curves of Mg₈₅Zn₅Ni₁₀–4NbF₅ alloy at 360 °C: (a) hydrogenation cycles, (b) dehydrogenation cycles.

Hydrogen desorption activation energy

In order to further study the catalytic mechanism of the differences between TiF₃ and NbF₅, the dehydriding activation energy of the Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys is estimated by Arrhenius method. As a general rule, in a gas–solid reaction, the total energy barrier which is to be crossed determines the activation energy. The energy barrier of H₂ released from MgH₂ is the leading cause which may explain the dehydrogenation rate.³⁴ During hydrogenation, the activation energy is considered to be relative to the total energy barrier. As we know, the dehydrogenation reaction is accomplished by nucleation and growth processes. In addition, the Johnson–Mehl–Avrami (JMA) model always simulates the nucleation and growth processes during hydrogen desorption, which is represented by the following equation:⁵¹

ln[−ln(1 − α)] = η ln k + η ln t (1)

where α, k and η represent the reaction fraction at time t, an effective kinetic parameter, and the Avrami exponent reaction order, respectively. Fig. 11 expresses the linear plots of ln[−ln(1 − α)] vs. the ln t. Evidently, the JMA plots described in Fig. 11 are nearly linear, illustrating that dehydriding reactions of the samples follow instantaneous nucleation followed by interface controlled-three-dimensional growth process.⁵² According to the slope and intercept of the linear fitting, η and η ln k values at each temperature can be calculated. On the basis of η and η ln k values, the rate constant k is calculated, then the activation energy (E_a) for dehydrogenation can be computed from the following Arrhenius equation:^53,54

k = A exp(−E_a/RT) (2)

where, A, R and T represent a temperature independent coefficient, universal gas constant and the absolute temperature, respectively. The Arrhenius plots for the dehydrogenation kinetics are described in Fig. 12. Accordingly, on the basis of the slopes of these plots, the activation energy, E_a (de), can be calculated. The E_a (de) values of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys are 75.514 and 82.367 kJ mol⁻¹ H₂, respectively, and are listed in Table 1. Clearly, the E_a (de) of C = TiF₃ alloy is much lower than that of the C = NbF₅ alloy. In this study, the E_a (de) value of ball-milled Mg₈₅Zn₅Ni₁₀ is calculated to be 109.83 kJ mol⁻¹ H₂, suggesting that both catalysts remarkably decreased the dehydriding activation energy when compared with the E_a (de) value of pure milled MgH₂ (158.5 kJ mol⁻¹ H₂).⁵⁵

Fig. 11 JMA graphs of the as-milled of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys at different temperatures: (a) C = TiF₃, (b) C =NbF₅.

Fig. 12 P–C–I curves of the as-milled of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys at different temperatures: (a) C = TiF₃, (b) C = NbF₅.

Table 1 Hydrogen desorption activation energy (E_a (de)), enthalpy change (ΔH) and entropy change (ΔS) of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅, none) alloy

Mg85Zn5Ni10–4C alloys	Ea (de) (kJ mol^−1)	ΔHab (kJ mol^−1)	ΔSab (J mol^−1 K^−1)	ΔHde (kJ mol^−1)	ΔSde (J mol^−1 K^−1)
C = TiF3	75.514	−70.100	−122.473	80.054	137.781
C = NbF5	82.367	−75.869	−128.945	79.005	132.683
C = none	109.830	−82.446	−140.628	86.187	146.114

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Hydrogen storage thermodynamics

The pressure–composition Isotherms (PCI) tests were measured at 360, 340 and 320 °C for the sake of studying the influence of different catalysts on the hydrogen storage thermodynamics of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloy. The PCI curves are represented in Fig. 13. Evidently, in each PCI curves there are two distinct plateaus. The lower pressure plateau belongs to MgH₂, and the higher one is the pressure plateau of Mg₂NiH₄ according to the reported results for Mg–10Ni–xMm alloy.⁵⁶ All the curves exhibit flat plateaus. On the basis of the plateau pressures in PCI curves, both enthalpy change ΔH and entropy change ΔS can be derived from the Van't Hoff equation:⁵⁷

ln[P(H₂)/P₀] = ΔH/(RT) − ΔS/R (3)

where, P(H₂), P₀, T and R represent the equilibrium hydrogen gas pressure, the standard atmospheric pressure, the sample temperature and the gas constant, respectively. Fig. 14 shows the Van't Hoff plots and the enthalpy value (ΔH) and entropy value (ΔS) of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloy. Therefore, the thermodynamic parameters can be easily calculated by intercepts and slopes of the Van't Hoff plots which are listed in Table 1. The calculations reveal that the hydrogen absorption/desorption ΔH and ΔS of the C = TiF₃ alloy are smaller than of the C = NbF₅ alloy indicating that TiF₃ is more effective than NbF₅ as a catalyst to improve the hydrogen storage thermodynamics of Mg₈₅Zn₅Ni₁₀. Meanwhile, the hydrogen absorption/desorption ΔH and ΔS of ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys are smaller than those of ball-milled Mg₈₅Zn₅Ni₁₀ alloy, suggesting both TiF₃ and NbF₅ are helpful to ameliorate the thermodynamics of Mg₈₅Zn₅Ni₁₀.

Fig. 13 P–C–I curves of the as-milled of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys at different temperatures: (a) C = TiF₃, (b) C = NbF₅.

Fig. 14 Van't Hoff plots of the as-milled of the ball-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys at different temperatures: (a) C = TiF₃, (b) C = NbF₅.

Conclusions

In this investigation, as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) specimens have been prepared successfully via ball milling. The different influences between the catalysts TiF₃ and NbF₅ on the microstructure and hydrogen storage properties were studied. The main conclusions drawn are as follows:

(1) After 5 h ball milling, and 30 hydrogenation/dehydrogenation cycles, the catalysts TiF₃ and NbF₅ always exist in the Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) composites, indicating that both TiF₃ and NbF₅ do not decompose and have a good stability.

(2) The C = TiF₃ alloy possesses a faster hydrogen absorption/desorption rate than the C = NbF₅ alloy. The dehydrogenation activation energy values calculated by Arrhenius equation of the as-milled Mg₈₅Zn₅Ni₁₀–4C (C = TiF₃, NbF₅) alloys are 75.514 and 82.367 kJ mol⁻¹ H₂, respectively, while the value of ball-milled Mg₈₅Zn₅Ni₁₀ alloy is 109.830 kJ mol⁻¹ H₂, indicating that the catalysts TiF₃ and NbF₅ visibly reduce the dehydrogenation activation energy and effectively improve the hydrogen absorption/desorption kinetics.

(3) Compared with the ball-milled Mg₈₅Zn₅Ni₁₀ alloy, the composites catalyzed by TiF₃ and NbF₅ possess lower thermodynamic parameters (ΔH and ΔS), suggesting that TiF₃ and NbF₅ can significantly ameliorate the hydrogen storage thermodynamics.

(4) TiF₃ has a better effect to generate more defects and cracks during the hydriding/dehydriding process than NbF₅, which is responsible for the faster hydrogen absorption/desorption rate. During hydrogenation and dehydrogenation, defects and cracks existing on alloy particles play an effective and important role in increasing the hydrogen absorption and desorption rates. TiF₃ has a better catalytic effect on the hydrogen storage property of Mg₈₅Zn₅Ni₁₀ than NbF₅.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study is financially supported by the National Natural Science Foundations of China (51761032, 51471054 and 51871125).

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