Li3N-LiNH2体系的循环放氢性能

以等物质的量比的Li3N和LiNH2为起始原料,采用高能球磨法制得了Li-N-H体系,并研究了该体系循环放氢性能衰减的主要原因。XRD及FTIR结果表明,Li3N-LiNH2体系经首周吸氢后转变为LiNH2与LiH,在之后的吸放氢循环中,可逆的吸放氢过程发生在LiNH2与Li2NH之间相的转变。放氢动力学结果表明,Li3N-LiNH2体系在280℃下首周放氢量达5.6wt%,100 min内完成总放氢量的86%。但在循环3周后,100 min内的放氢量衰减至初始的36%。SEM及BET结果表明,放氢量的衰减主要是由于样品的烧结所致,但可通过再次球磨使其循环放氢性能恢复。     

3NaBH4/ErF3复合储氢材料的制备及吸放氢特性

采用高能球磨法制备了3NaBH4/ErF3复合储氢材料, 并研究了其相结构和储氢性能. X射线衍射(XRD)显示, NaBH4和ErF3在球磨过程中未发生反应; 同步热分析(TG-DSC)测试结果表明, 3NaBH4/ErF3体系在420℃开始放氢, 比相同测试条件下纯NaBH4的放氢温度降低了约100℃, 放氢量为3.06%(质量分数). 压力-成分-温度(Pressure-Composition-Temperature, PCT)性能测试结果显示, 3NaBH4/ErF3复合储氢材料在较低的温度(355~413℃)及平台氢压(<1 MPa)下即拥有良好的可逆吸放氢性能, 最高可逆吸氢量可达到2.78%(质量分数), 吸氢后体系重新生成了NaBH4相. 计算得吸氢焓变仅为-36.8 kJ/mol H2; 而放氢焓变为-180.8 kJ/mol H2. NaBH4在ErF3的作用下提高了热动力学性能, 并实现了可逆吸放氢.     

机械球磨固相化学反应制备AlH3及其放氢性能

以LiAlH4和AlCl3为原料,采用机械球磨固相化学反应方法制备了铝氢化合物,通过X射线衍射(XRD)、热分析(TG-DSC)和质谱(MS)分析等方法对反应产物进行分析和表征,研究了不同球磨时间(4、8、15和20 h)对LiAlH4+AlCl体系的固相反应转变规律合成产物和放氢性能的影响.研究结果表明,随球磨时间的增加,球磨固相反应按3LiAlH4+AlCl3→4AlH3+3LiCl方向进行,形成了非晶态铝氢化合物AlH3,球磨20 h时反应基本完全.球磨产物的放氢动力学特性随球磨时间增加而改善,其放氢起始温度均低于100℃,最大放氢量达到2.6%-3.6%(H2)(w),接近反应体系的理论储氢量4.85%(H2)(w).球磨过程中反应产物形成LiCl·H2O以及少量AlH3发生分解是影响球磨产物最大放氢量的主要因素.     

机械球磨固相化学反应制备AlH3及其放氢性能

以LiAlH4和AlCl3为原料, 采用机械球磨固相化学反应方法制备了铝氢化合物, 通过X射线衍射(XRD)、热分析(TG-DSC)和质谱(MS)分析等方法对反应产物进行分析和表征, 研究了不同球磨时间(4、8、15和20 h)对LiAlH4+AlCl体系的固相反应转变规律﹑合成产物和放氢性能的影响. 研究结果表明, 随球磨时间的增加, 球磨固相反应按3LiAlH4+AlCl3→4AlH3+3LiCl方向进行, 形成了非晶态铝氢化合物AlH3, 球磨20 h时反应基本完全. 球磨产物的放氢动力学特性随球磨时间增加而改善, 其放氢起始温度均低于100 ℃, 最大放氢量达到2.6%-3.6%(H2)(w), 接近反应体系的理论储氢量4.85%(H2)(w). 球磨过程中反应产物形成LiCl·H2O以及少量AlH3发生分解是影响球磨产物最大放氢量的主要因素.     

镨、钕氢化物催化剂对NaH/Al复合物吸放氢性能的影响

采用NaH和Al为合成原料,镨、钕氢化物为催化剂,通过机械球磨(NaH/Al+6%(摩尔分数)RE-H)(RE=Pr,Nd)复合物的方法并加氢合成NaAlH4络合氢化物,系统研究了催化剂对其吸放氢性能的影响。结果表明,加入PrH2.92和NdH2.27能明显改善NaH/Al复合物的吸放氢动力学性能,有效降低NaAlH4的脱氢温度。(NaH/Al+6%PrH2.92)和(NaH/Al+6%NdH2.27)复合物的120℃吸氢容量分别为3.57%和3.61%(质量分数),170℃放氢容量分别为2.57%和2.95%;且两者均具有较好的吸放氢循环稳定性,但吸(放)氢后样品中均存在少量Na3AlH6相,表明样品的吸(放)氢反应进行得并不彻底,使得其实际吸放氢容量低于理论可逆储氢容量。研究表明,PrH2.92和NdH2.27在球磨、吸/放氢过程中始终稳态存在,起着催化储氢作用;(NaH/Al+6%PrH2.92)复合物的放氢活化能稍低于(NaH/Al+6%NdH2.27)复合物。     

Ti-Zr催化剂对NaH/Al复合物可逆储氢特性的影响

采用机械球磨(NaH/Al+Ti)和(NaH/Al+Ti-Zr)复合物的方法加氢制备了NaAlH4配位氢化物, 系统研究了Ti、Ti-Zr催化剂以及不同加氢条件对其可逆储氢行为的影响. 结果表明, 对于NaH/Al体系的吸放氢性能, 共掺金属Ti粉/Zr粉的催化作用比单独掺金属Ti粉的催化作用要好. 随着加氢温度从85 ℃上升到140 ℃, 体系的吸氢容量先增后减, 并在120 ℃时达到最大值; 同时, 发现共掺Ti-Zr催化剂的复合物具有最佳的储氢性能, 在120和85 ℃时的吸氢量分别为4.61%和3.52%(w), 比仅掺Ti 催化剂的复合物分别高出0.40%和0.70%(w)的吸氢量. 随着加氢压力的增大, (NaH/Al+Ti-Zr)复合物的吸氢性能随之提高. XRD和DSC分析结果表明, NaAlH4体系的放氢过程明显发生两步分解反应, 共掺Ti-Zr催化剂的复合物储氢性能优于单独掺Ti 催化剂的原因是, 共掺催化剂能有效改善NaAlH4体系吸放氢反应的动力学性能,并降低体系的放氢温度.     

络合氢化物Ti-NaAlH4的制备与储氢特性

采用Ti粉为催化剂前驱体、预处理Al粉和NaH为合成原料, 通过机械球磨-加氢方法合成出络合氢化物Ti-NaAlH4, 系统研究了球磨保护气氛、球磨时间和氢化加氢压力等制备参数对其储氢性能的影响. 结果表明, 制备方法对Ti-NaAlH4储氢特性有很大影响. 与氩气保护气氛相比, 在氢气气氛中球磨制备的复合物具有更高的吸放氢性能. 在氢气保护气氛下, 随着球磨时间从6 h增至24 h, 复合物的吸氢容量和吸氢速率先增后减, 12 h时达到最佳值, 而复合物的放氢容量和放氢速率则逐渐增高; 进一步延长球磨时间会使颗粒发生团聚, 从而导致吸氢性能下降. 随着氢化加氢压力从7.5 MPa升至13.5 MPa, 复合物的吸氢容量(质量分数)由2.83%逐渐增至4.21%. 复合物球磨后出现的Na3AlH6中间氢化物相表明, 在氢气下掺Ti球磨对NaH和Al的氢化反应起到很好的促进作用.     

MAlH4(M=Li,Na)储氢材料*

氢能是一种新型的清洁能源,有望替代碳经济,而氢的储存是氢能应用的关键。近年来,研究集中在具有储氢容量高和可逆性好等优点的固态储氢材料上。许多新型储氢材料不断出现,其中以MAlH₄(M=Li, Na)为代表的金属复合氢化物体系被认为是最有前景的储氢材料之一。本文综述了MAlH₄(M=Li, Na)作为可逆储氢材料的研究现状,主要从吸放氢反应、储氢性能、反应机理、理论计算和存在的问题等方面进行了讨论,并指出其相关发展趋势。     

AlH3的制备及放氢特性

以LiAlH₄, LiBH₄和AlCl₃为原料, 采用有机合成法制备了单一相的α-AlH₃和γ-AlH₃, 并对其放氢性能进行了研究.结果表明, 两种晶型AlH₃的放氢量均可达8.3%~8.5%(质量分数), 放氢温度范围在120~160℃之间, 且γ-AlH₃的放氢峰值温度比α-AlH₃低8.2℃; α-AlH₃和γ-AlH₃的放氢反应表观活化能分别为94.6和86.3 kJ/mol; 加热过程中α-AlH₃直接发生放氢反应, γ-AlH₃在放氢前先发生向α-AlH₃的相变, 这一相变过程使得AlH₃的晶格得到活化, 从而促进放氢反应的进行.     

MgH_2+20%(w)MgTiO_3复合材料的吸/放氢性能(英文)

为了降低MgH2的吸放氢温度,提高其吸放氢动力学性能,本文通过球磨方法制备了MgH2+20%(w)MgTiO3复合储氢材料,并研究了其储氢性能.X射线衍射(XRD)结果表明,MgTiO3在与MgH2球磨过程中生成Mg2TiO4和TiO2,并且Mg2TiO4和TiO2在体系的吸放氢过程中保持稳定,能够对MgH2的吸放氢过程产生催化作用.程序升温脱附和吸/放氢动力学测试结果表明,添加MgTiO3后MgH2的初始放氢温度从389°C降至249°C.150°C下的吸氢量从0.977%(w)提高到2.902%(w),350°C下的放氢量从2.319%(w)提高到3.653%(w).同时,MgH2放氢反应的活化能从116kJ·mol-1降至95.7kJ·mol-1.与MgH2相比,MgH2+20%(w)MgTiO3复合材料的热力学与动力学性能均有显著提高,这主要是由于球磨和放氢过程中原位生成的TiO2和Mg2TiO4具有良好的催化活性.     

Dehydrogenation of a combined LiAlH4/LiNH2 system

Although there have been numerous materials systems studied as potential candidates for hydrogen storage applications, none of the materials known to date has demonstrated enough hydrogen capacity or efficiency at required operating temperature ranges. There are still considerable opportunities for discovery of new materials or material systems that could lead to advances in science as well as commercial technologies in this area. LiAlH(4) is one of the most promising materials owing to its high hydrogen content. In the present work, we investigated dehydrogenation properties of the combined system of LiAlH(4) and LiNH(2) under atmospheric argon. Thermogravimetric analysis (TGA) of 2LiAlH(4)/LiNH(2) mixtures without any catalysts indicated that a large amount of hydrogen (approximately 8.1 wt %) can be released between 85 and 320 degrees C under a heating rate of 2 degrees C/min in three dehydrogenation reaction steps. It is found that LiNH(2) effectively destabilizes LiAlH(4) by reacting with LiH during the dehydrogenation process of LiAlH(4).     

In situ formation of TiB2 and TiH2 catalyzed Li/Mg based dual-cation borohydride with a low onset dehydrogenation temperature below 100 °C

Chemical complex borohydride is a promising hydrogen storage material due to its large gravimetric and volumetric hydrogen capacities. However, the high dehydrogenation temperature and sluggish kinetics still place strong restrictions on its practical application in the hydrogen storage field. In this work, a synergetic approach of partial cation substitution and catalysis is developed to enhance the hydrogen storage properties of LiBH₄. The Li/Mg based dual-cation borohydride (LiMg₂(BH₄)₅, LMBH) was successfully synthesized by wet chemical ball milling of LiBH₄ and MgCl₂. The optimal (LMBH (4.5:1) sample, LiBH₄ and MgCl₂ in molar ratios of 4.5:1, possesses a maximum hydrogen desorption capacity (11.27 wt%) and the outstanding initial decomposition temperature (～250 °C). Importantly, the LMBH (4.5:1) doped with TiF₃ shows a remarkable onset dehydrogenation temperature as low as 97.2 °C, which is about 190 °C lower than that of pristine LiBH₄. The LMBH (4.5:1) doped with TiF₃ system releases 7.98 wt% H₂ within 170 min below 350 °C. And the dehydrogenation product of doped composite can reversibly absorb ～4.72 wt% H₂ at a relatively moderate temperature of 280 °C, which is substantially lower than the reversible hydrogen absorption temperature of previous modified borohydride systems. Based on the structural characteristic analyses, the TiF₃ reacts with LMBH (4.5:1) to in-situ form actual catalytic components of TiB₂ and TiH₂ as the actual catalysts for LMBH (4.5:1), resulting in the improved hydrogen re/dehydrogenation properties. The synergetic modification of Li/Mg dual-cation substitution and TiB₂/TiH₂ catalysis may lead to the development of light-metal borohydrides with outstanding hydrogen storage properties.     

A new Li-Al-N-H system for reversible hydrogen storage

Complex metal hydrides are considered as a class of candidate materials for hydrogen storage. Lithium-based complex hydrides including lithium alanates (LiAlH(4) and Li(3)AlH(6)) are among the most promising materials owing to its high hydrogen content. In the present work, we investigated dehydrogenation/rehydrogenation reactions of a combined system of Li(3)AlH(6) and LiNH(2). Thermogravimetric analysis (TGA) of Li(3)AlH(6)/3LiNH(2)/4 wt % TiCl(3)-(1)/(3)AlCl(3) mixtures indicated that a large amount of hydrogen (approximately 7.1 wt %) can be released between 150 degrees C and 300 degrees C under a heating rate of 5 degrees C/min in two dehydrogenation reaction steps. The results also show that the dehydrogenation reaction of the new material system is nearly 100% reversible under 2000 psi pressure hydrogen at 300 degrees C. Further, a short-cycle experiment has demonstrated that the new combined material system of alanates and amides can maintain its hydrogen storage capacity upon cycling of the dehydrogenation/rehydrogenation reactions.     

Sorption and desorption properties of a CaH2/MgB2/CaF2 reactive hydride composite as potential hydrogen storage material

The hydrogenation behavior of 3CaH₂+4MgB₂+CaF₂ composite was studied by manometric measurements, powder X-ray diffraction, differential scanning calorimetry and attenuated total reflection infrared spectroscopy. The maximum observed quantity of hydrogen loaded in the composite was 7.0 wt%. X-ray diffraction showed the formation of Ca(BH₄)₂ and MgH₂ after hydrogenation. The activation energy for the dehydrogenation reaction was evaluated by DSC measurements and turns out to be 162±15 kJ mol⁻¹ H₂. This value decreases due to cycling to 116±5 kJ mol⁻¹ H₂ for the third dehydrogenation step. A decrease of ca. 25–50 °C in dehydrogenation temperature was observed with cycling. Due to its high capacity and reversibility, this composite is a promising candidate as a potential hydrogen storage material.     

Physiochemical pathway for cyclic dehydrogenation and rehydrogenation of LiAlH4

A five-step physiochemical pathway for the cyclic dehydrogenation and rehydrogenation of LiAlH4 from Li3AlH6, LiH, and Al was developed. The LiAlH4 produced by this physiochemical route exhibited excellent dehydrogenation kinetics in the 80-100 degrees C range, providing about 4 wt % hydrogen. The decomposed LiAlH4 was also fully rehydrogenated through the physiochemical pathway using tetrahydrofuran (THF). The enthalpy change associated with the formation of a LiAlH4.4THF adduct in THF played the essential role in fostering this rehydrogenation from the Li3AlH6, LiH, and Al dehydrogenation products. The kinetics of rehydrogenation was also significantly improved by adding Ti as a catalyst and by mechanochemical treatment, with the decomposition products readily converting into LiAlH4 at ambient temperature and pressures of 4.5-97.5 bar.     

Hydrogen desorption mechanism in a Li-N-H system by means of the isotopic exchange technique

The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH2 --> Li2NH + NH3 and LiH + NH3 --> LiNH2 + H2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.     

Hydrogen absorption and desorption by the Li-Al-N-H system

Lithium hexahydridoaluminate Li(3)AlH(6) and lithium amide LiNH(2) with 1:2 molar ratio were mechanically milled, yielding a Li-Al-N-H system. LiNH(2) destabilized Li(3)AlH(6) during the dehydrogenation process of Li(3)AlH(6), because the dehydrogenation starting temperature of the Li-Al-N-H system was lower than that of Li(3)AlH(6). Temperature-programmed desorption scans of the Li-Al-N-H system indicated that a large amount of hydrogen (6.9 wt %) can be released between 370 and 773 K. After initial H(2) desorption, the H(2) absorption and the desorption capacities of the Li-Al-N-H system with a nano-Ni catalyst exhibited 3-4 wt % at 10-0.004 MPa and 473-573 K, while the capacities of the system without the catalyst were 1-2 wt %. The remarkably increased capacity was due to the fact that the kinetics was improved by addition of the nano-Ni catalyst.     

In situ hybridization of LiNH2-LiH-Mg(BH4)2 nano-composites: intermediate and optimized hydrogenation properties

Nano-composites of LiNH(2)-LiH-xMg(BH(4))(2) (0 ≤ x ≤ 2) were prepared by plasma metal reaction followed by a nucleation growth method. Highly reactive LiNH(2)-LiH hollow nanoparticles offered a favorable nucleus during a precipitation process of liquid Mg(BH(4))(2)·OEt(2). The electron microscopy results suggested that more than 90% of the obtained nano-composites were in the range 200-400 nm. Because of the short diffusion distance and ternary mixture self-catalyzing effect, this material possesses enhanced hydrogen (de)sorption attributes, including facile low-temperature kinetics, impure gases attenuation and partial reversibility. The optimal hydrogen storage properties were found at the composition of LiNH(2)-LiH-0.5Mg(BH(4))(2), which was tentatively attributed to a Li(4)(NH(2))(2)(BH(4))(2) intermediate. 5.3 wt% hydrogen desorption could be recorded at 150 °C, with the first 2.2 wt% release being reversible. This work suggests that controlled in situ hybridization combined with formula optimization can improve hydrogen storage properties.