Ni–B-doped NaAlH₄ hydrogen storage materials prepared by a facile two-step synthesis method

Hydrogen is the most ideal fuel in comprehensive clean energy. However, the most critical issues are the practical application of on-board hydrogen storage systems. In recent years, solid light metal complex hydrides [1, 2] drew intensive research interest due to their high hydrogen capacities and moderate operating conditions. Among various light metal complex hydrides [3‐10], sodium aluminum hydride (NaAlH₄) was widely studied after the pioneering researches of Bogdanović and Schwickardi [11]. Recently, many researches on exploring new kind of catalysts or modifying the microstructure to enhance the synthesis efficiency and dehydrogenation properties of NaAlH₄ were carried out. A lot of kinds of catalysts, such as TiCl₄ [12], TiF₃ [6, 13], Ti powder [14, 15], TiCl₃ [16], etc., are used as possible catalysts to synthesize NaAlH₄. However, higher hydrogen pressure retards the practical applications of NaAlH₄ system for hydrogen storage. Therefore, it is necessary to explore novel catalysts on the synthesis of NaAlH₄ from NaH and Al.

Ni–B, as an amorphous alloy catalyst, was used for the hydrogenation of benzene, cyclopentadiene, and acrylonitrile [17, 18]. In addition, nickel-based catalysts were also well known for catalytic hydrolysis of metal borohydride owing to its excellent catalytic activity [19, 20]. In our previous work, we successfully prepared NaAlH₄ by ball-milling NaH/Al mixtures with Co–B or TiB₂ [21, 22] catalysts. Based on the above considerations, Ni–B amorphous alloy was synthesized by ball milling and used firstly as catalyst for the synthesis of NaAlH₄.

Herein, NaAlH₄ can be firstly synthesized by ball-milling the mixture of NaH/Al and Ni–B catalyst under Ar atmosphere for 15 h and then in a low hydrogen pressure. More importantly, as-synthesized nanocrystalline NaAlH₄ can release hydrogen even at 100 °C.

Figure 1 shows the XRD patterns of Samples S0.75-30, S1-10, S1-20, S1-30, and S2-40. Detailed preparation conditions for those samples are given in Table 1. It demonstrates that Na₃AlH₆ is the main phase and NaAlH₄ peaks can be detected in S0.75-30. When the hydrogen pressure increases to 1 MPa, Na₃AlH₆ diffraction peaks are also detected in S1-10, and Al peaks are broad and weak. The intensity of Na₃AlH₆ gradually decreases and NaAlH₄ gradually increases with increasing the milling time to 20 h (S1-20). As the milling time increases to 30 h (S1-30), Na₃AlH₆ diffraction peaks completely disappear, suggesting that Na₃AlH₆ is completely hydrogenated to become NaAlH₄. Therefore, a higher hydrogen pressure is helpful for the conversion from Na₃AlH₆ to NaAlH₄. However, when the milling time increases to 40 h under 2 MPa hydrogen pressure (S2-40), Na₃AlH₆ peaks appear, indicating that NaAlH₄ can decompose into Na₃AlH₆ under higher hydrogen pressure and longer milling time.

Figure 2 displays thermal decomposition characteristics of S1-10, S1-20, and S1-30 samples. It shows that there are two plateau regions for the decomposition reactions, which are attributed to the dehydrogenation of the NaAlH₄ and Na₃AlH₆, respectively. It also can be seen that the onset dehydrogenation temperatures are lowered to about 100 and 165 °C, respectively. When the milling time is 10 h, a weight loss of about 0.82 wt% is observed, which is attributed to the dehydrogenation of synthesized Na₃AlH₆. The dehydrogenation capacity obviously increases to about 1.76 wt% in S1-20. For S1-30, the dehydrogenation capacities for the two dehydrogenation steps reach to about 1.20 wt% and 1.02 wt%, respectively. The results indicate that synthetic efficiency of NaAlH₄ can be enhanced by increasing the milling time.

Figure 3 shows the dehydrogenation rate of S1-30 at different temperatures. More interestingly, it is clearly observed from Fig. 3 that as-synthesized NaAlH₄ sample can release hydrogen even at a temperature as low as 100 °C. As dehydrogenation temperature increases from 90 to 120 °C, the dehydrogenation rate obviously increases due to the improvement in thermodynamic reactivity. The Arrhenius plot (inset in Fig. 3) of the dehydrogenation kinetic gives an activation energy value of about 61.91 kJ·mol⁻¹, which is lower than that of the Ti-doped NaAlH₄ reported before [23, 24]. The Arrhenius equation is as follows:where k is the maximum hydrogen generation rates, A is a temperature-independent coefficient, E _a is apparent activation energy for the decomposition of NaAlH₄, R is gas constant, and T is absolute temperature.

Figure 4 reveals that XRD patterns of S1-30 after dehydrogenation at different temperatures. The intensity of NaAlH₄ in Fig. 4 is intense, indicating that most of NaAlH₄ cannot decompose at 90 °C. We can also find that most of NaAlH₄ decomposes into Na₃AlH₆ at 100 °C, suggesting that only the first decomposition step of NaAlH₄ occurs at 100 °C. With the temperature elevation up to 120 °C, the sample fully decomposes (the first dehydrogenation step), reaching a dehydrogenation capacity of 1.36 wt%. It is also observed that Al–Ni diffraction peaks appear in Fig. 4. Results demonstrate that Ni–B is a promising catalyst for enhancing dehydrogenation properties of light-metal complex hydrides. Further investigations about the function of Ni–B in the synthesis and dehydrogenation of sodium alanate hydride are on the way.

Nanocrystalline NaAlH₄ was successfully synthesized with Ni–B catalyst under a low hydrogen pressure by a facile two-step synthesis method. The synthetic efficiency of NaAlH₄ can be enhanced by increasing the milling time and hydrogen pressure. The activation energy value for the first dehydrogenation step is 61.91 kJ·mol⁻¹. TPD measurements show that Ni–B catalyst markedly lowers the dehydrogenation temperature of NaAlH₄. More importantly, as-synthesized NaAlH₄ can release hydrogen even at 100 °C. Results demonstrate that synthesized Ni–B by ball-milling is a promising catalyst for enhancing the dehydrogenation performances of light-metal complex hydrides.