Improved Dehydrogenation Properties of LiAlH4 by Addition of Nanosized CoTiO3

Despite the application of lithium aluminium hydride (LiAlH4) being hindered by its sluggish desorption kinetics and unfavourable reversibility, LiAlH4 has received special attention as a promising solid-state hydrogen storage material due to its hydrogen storage capacity (10.5 wt.%). In this work, investigated for the first time was the effect of the nanosized cobalt titanate (CoTiO3) which was synthesised via a solid-state method on the desorption behaviour of LiAlH4. Superior desorption behaviour of LiAlH4 was attained with the presence of a CoTiO3 additive. By means of the addition of 5, 10, 15 and 20 wt.% of CoTiO3, the initial desorption temperature of LiAlH4 for the first stage was reduced to around 115−120 °C and the second desorption stage was reduced to around 144−150 °C, much lower than for undoped LiAlH4. The LiAlH4-CoTiO3 sample also presents outstanding desorption kinetics behaviour, desorbing hydrogen 30−35 times faster than undoped LiAlH4. The LiAlH4-CoTiO3 sample could desorb 3.0−3.5 wt.% H2 in 30 min, while the commercial and milled LiAlH4 desorbs <0.1 wt.% H2. The apparent activation energy of the LiAlH4-CoTiO3 sample based on the Kissinger analysis was decreased to 75.2 and 91.8 kJ/mol for the first and second desorption stage, respectively, lower by 28.0 and 24.9 kJ/mol than undoped LiAlH4. The LiAlH4-CoTiO3 sample presents uniform and smaller particle size distribution compared to undoped LiAlH4, which is irregular in shape with some agglomerations. The experimental results suggest that the CoTiO3 additive promoted notable advancements in the desorption performance of LiAlH4 through the in situ-formed AlTi and amorphous Co or Co-containing active species that were generated during the desorption process.

Influence of Nanosized CoTiO3 Synthesized via a Solid-State Method on the Hydrogen Storage Behavior of MgH2

Magnesium hydride (MgH₂) has received outstanding attention as a safe and efficient material to store hydrogen because of its 7.6 wt.% hydrogen content and excellent reversibility. Nevertheless, the application of MgH₂ is obstructed by its unfavorable thermodynamic stability and sluggish sorption kinetic. To overcome these drawbacks, ball milling MgH₂ is vital in reducing the particle size that contribute to the reduction of the decomposition temperature. However, the milling process would become inefficient in reducing particle sizes when equilibrium between cold-welding and fracturing is achieved. Therefore, to further ameliorate the performance of MgH₂, nanosized cobalt titanate (CoTiO₃) has been synthesized using a solid-state method and was introduced to the MgH₂ system. The different weight percentages of CoTiO₃ were doped to the MgH₂ system, and their catalytic function on the performance of MgH₂ was scrutinized in this study. The MgH₂ + 10 wt.% CoTiO₃ composite presents the most outstanding performance, where the initial decomposition temperature of MgH₂ can be downshifted to 275 °C. Moreover, the MgH₂ + 10 wt.% CoTiO₃ absorbed 6.4 wt.% H₂ at low temperature (200 °C) in only 10 min and rapidly releases 2.3 wt.% H₂ in the first 10 min, demonstrating a 23-times-faster desorption rate than as-milled MgH₂ at 300 °C. The desorption activation energy of the 10 wt.% CoTiO₃-doped MgH₂ sample was dramatically lowered by 30.4 kJ/mol compared to undoped MgH₂. The enhanced performance of the MgH₂-CoTiO₃ system is believed to be due to the in situ formation of MgTiO₃, CoMg₂, CoTi₂, and MgO during the heating process, which offer a notable impact on the behavior of MgH₂.

Study of the Hydrogen Storage Properties and Catalytic Mechanism of a MgH2-Na3AlH6 System Incorporating FeCl3

In this work, the catalytic effects of FeCl₃ toward the hydrogen storage properties of the MgH₂-Na₃AlH₆ composite were investigated for the first time. The temperature-programed desorption results indicated that the onset temperature of the hydrogen release of a 10 wt % FeCl₃-doped MgH₂-Na₃AlH₆ composite was ∼30 °C lower than that of the undoped MgH₂-Na₃AlH₆ composite. The addition of FeCl₃ into the MgH₂-Na₃AlH₆ composite resulted in improved absorption and desorption kinetics performance. The absorption/desorption kinetics measurements at 320 °C (under 33 and 1 atm hydrogen pressure, respectively) indicated that within 10 min, the doped sample absorbed ∼4.0 wt % and desorbed ∼1.5 wt % hydrogen. By comparison, the undoped sample absorbed only ∼2.1 wt % and desorbed only ∼0.6 wt % hydrogen under the same conditions and time. Comparably, the apparent activation energy value of the doped composite is 128 kJ/mol, which is 12 kJ/mol lower than that of the undoped composite (140 kJ/mol). The formation of the new species of MgCl₂ and Fe in the doped composite was detected from X-ray diffraction analysis after heating and absorption processes. These two components were believed to play a vital role in reducing the decomposition temperature and kinetics enhancement of the MgH₂-Na₃AlH₆ composite.

Influence of K2NbF7 Catalyst on the Desorption Behavior of LiAlH4

In this study, the modification of the desorption behavior of LiAlH4 by the addition of K2NbF7 was explored for the first time. The addition of K2NbF7 causes a notable improvement in the desorption behavior of LiAlH4. Upon the addition of 10 wt.% of K2NbF7, the desorption temperature of LiAlH4 was significantly lowered. The desorption temperature of the LiAlH4 + 10 wt.% K2NbF7 sample was lowered to 90°C (first-stage reaction) and 149°C (second-stage reaction). Enhancement of the desorption kinetics performance with the LiAlH4 + 10 wt.% K2NbF7 sample was substantiated, with the composite sample being able to desorb hydrogen 30 times faster than did pure LiAlH4. Furthermore, with the presence of 10 wt.% K2NbF7, the calculated activation energy values for the first two desorption stages were significantly reduced to 80 and 86 kJ/mol; 24 and 26 kJ/mol lower than the as-milled LiAlH4. After analysis of the X-ray diffraction result, it is believed that the in situ formation of NbF4, LiF, and K or K-containing phases that appeared during the heating process promoted the amelioration of the desorption behavior of LiAlH4 with the addition of K2NbF7.

Dehydrogenation Properties and Catalytic Mechanism of the K2NiF6-Doped NaAlH4 System

The K₂NiF₆ catalytic effect on the NaAlH₄ dehydrogenation properties was studied in this work. The desorption temperature was studied using temperature-programmed desorption and exhibited a lower onset hydrogen release after doped with different wt % of K₂NiF₆ (5, 10, 15 and 20 wt %). It was found that the NaAlH₄ doped with 5 wt % K₂NiF₆ showed the optimal value that can reduce the onset desorption temperature of about 160 °C compared to 190 °C for the milled NaAlH₄. The NaAlH₄ + 5 wt % K₂NiF₆ sample showed faster desorption kinetics where 1.5 wt % of hydrogen was released in 30 min at 150 °C. In contrast, the milled NaAlH₄ only released about 0.2 wt % within the same time and temperature. From the Kissinger analysis, the apparent activation energy was 114.7 kJ/mol for the milled NaAlH₄ and 89.9 kJ/mol for the NaAlH₄-doped 5 wt % K₂NiF₆, indicating that the addition of K₂NiF₆ reduced the activation energy for hydrogen desorption of NaAlH₄. It is deduced that the new phases of AlNi, NaF, and KH that were formed in situ during the dehydrogenation process are the key factors for the improvement of dehydrogenation properties of NaAlH₄.