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

Wen-Bin Li 0 1 Li Li 0 1 Qiu-Li Ren 0 1 Yi-Jing Wang 0 1 2 Li-Fang Jiao 0 1 Hua-Tang Yuan 0 1 0 L. Li, Q.-L. Ren, Y.-J. Wang 1 W.-B. Li The Energy Department of Chemical Engineering, Tianjin Bohai Vacational Technical College , Tianjin 300402, China 2 , L.-F. Jiao, H.-T. Yuan Institute of New Energy Material Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (MOE), Tianjin Key Laboratory of Metal and Molecule-Based Material Chemistry, Nankai University , Tianjin 300071, China By directly introducing Ni-B into NaAlH4 system using a facile two-step synthesis method, the effects of Ni-B on NaAlH4 were systematically investigated. NaAlH4 can be completely formed after 30 h milling under 1 MPa hydrogen pressure. In addition, the dehydrogenation kinetics of as-prepared NaAlH4 after different milling times were investigated. As the dehydrogenation temperature rises, both the hydrogen desorption capacity and dehydrogenation rate quickly increase. The apparent activation energy Ea for Ni-B-doped NaAlH4 is calculated to be 61.91 kJ mol-1 for the first dehydrogenation step. More importantly, the dehydrogenation temperature of as-prepared NaAlH4 nanocrystalline can be reduced to about 100 C. 1 Introduction 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 [310], sodium aluminum hydride (NaAlH4) was widely studied after the pioneering researches of Bogdanovic 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. NiB, 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 CoB or TiB2 [21, 22] catalysts. Based on the above considerations, NiB amorphous alloy was synthesized by ball milling and used firstly as catalyst for the synthesis of NaAlH4. Herein, NaAlH4 can be firstly synthesized by ballmilling the mixture of NaH/Al and NiB 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. 2 Experimental 2.1 Preparation of NiB NiB catalyst was prepared by mechanical ball-milling. Ni powder (99.5 wt%, 48 lm) and B powder (99 wt%) with Table 1 Samples preparation conditions a Hydrogen pressure b Milling time under H2 (e.g., S0.75a-30b, the mixture being milled under Ar atmosphere for 15 h, and then milled under 0.75 MPa hydrogen pressure for 30 h at ambient temperature) molar ratio of 1:1 were placed into a stainless steel container (100 ml) under an Ar atmosphere (purity of 99.99 wt%) in a glove box. The mixture milled for 110 h at 450 r min-1 using planetary ball-mill. The ball-to-powder ratio was about 20:1. 2.2 Preparation of NaAlH4 About 2.5 g mixture of NaH (97 %, Alfa Aesar)/Al (99.5 %, Alfa Aesar) powders (molar ratio 1:1) and 10 mol% NiB was introduced into a stainless-steel vessel with stainless-steel balls and milled in a planetary ball-mill. The ball-to-powder ratio was about 40:1, and the mixture was milled at 450 r min-1 under Ar atmosphere for 15 h in advance, then milled under 0.752.00 MPa hydrogen pressure for different time. All the operations were carried out in the glove box (Super 1220/750/ 900) under high-purified argon atmosphere (H2O:\10 9 106; O2: \10 9 106). For convenience, detailed preparation conditions for those samples are given in Table 1. 2.3 Sample measurements Structural characteristics of the samples were studied by X-ray diffraction (XRD, Rigaku D/Max PC2500, Cu Ka radiation). Temperature programmed desorption (TPD) of H2 was performed using in a home-made apparatus. About 70 mg sample was loaded into the reactor and heated in a 35 ml min-1 Ar flow at a ramping rate of 2 C min-1, while heating from 50 to 300 C. Hydrogen desorption was measured by isothermal dehydrogenation apparatus using a volumetric method. In the dehydrogenation experiment, the sample was quickly heated to and kept at a given temperature. The weight loss percentage of the samples was calculated according to the weight of NaAlH4 and NiB. 3 Results and discussion Figure 1 shows the XRD patterns of Samples S0.75-30, S1-10, S1-20, S1-30, and S2-40. Detailed preparation Fig. 2 Temperature programmed desorption profiles of H2 for Samples S1-10, S1-20, and S1-30 with heating rate of 2 C min-1 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 NiB-doped NaAlH4 hydrogen storage materials Fig. 3 Dehydrogenation kinetic curves of Sample S1-30 at different temperatures. Inset being Arrhenius plot for dehydriding kinetics of Sample S1-30 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 (...truncated)