Improvement of basic performances of a nickel-metal hydride battery

To develop a sealed-type nickel-metal hydride battery for use in portable equipment or in electric vehicles, investigations were conducted on negative electrodes using AB₅-type hydrogen storage alloy and positive electrodes. For the cycle life performance of the battery, alkaline treatment of the alloy and the substitution of more than 50% to the alloy with Co were effective. For the positive electrode, zinc as a solid solution in the nickel positive electrode obviously prevented γ-NiOOH from being formed in the charging process of β-Ni(OH)₂ and suppressed the migration of the electrolyte solution in the separator to the active material of the positive electrode. Also, hydrophobic treatment of the surface of the alloy was effective to prevent the elevation of the battery internal pressure of the battery in high rate charge.     

Influence of surface contaminations on the hydrogen storage behaviour of metal hydride alloys

Hydrogen storage in metal hydrides is a promising alternative to common storage methods. The surface of a metal hydride plays an important part in the absorption of hydrogen, since important partial reaction steps take place here. The development of surface contaminations and their influence on hydrogen absorption is examined by means of absorption experiments and surface analysis, using X-ray photoelectron spectroscopy (XPS), thermal desorption mass spectrometry (TDMS) and secondary neutral mass spectrometry (SNMS), in this work. All investigations were carried out on a modern AB₂ metal hydride alloy, namely Ti_0.96Zr_0.04Mn_1.43V_0.45Fe_0.08. Surface analysis (SNMS, XPS) shows that long-term air storage (several months) leads to oxide layers about 15 nm thick, with complete oxidation of all main alloy components. By means of in situ oxygen exposure at room temperature and XPS analysis, it can be shown that an oxygen dose of about 100 Langmuirs produces an oxide layer comparable to that after air storage. Manganese enrichment (segregation) is also clearly observed and is theoretically described here. This oxide layer hinders hydrogen absorption, so an activation procedure is necessary in order to use the full capacity of the metal hydride. This procedure consists of heating (T = 120° C) in vacuum and hydrogen flushing at pressures like p = 18 bar. During the activation process the alloy is pulverized to particles of ∼20 μm through lattice stretches. It is shown that this pulverization of the metal hydride (creating clean surface) during hydrogen flushing is essential for complete activation of the material. Re-activation of powder contaminated by small doses of air (p ≈ 0.1 bar) does not lead to full absorption capacity. In ultrahigh vacuum, hydrogen is only taken up by the alloy after sputtering of the surface (which is done in order to remove oxide layers from it), thus creating adsorption sites for the hydrogen. This is shown by TDMS measurements with and without sputtering and oxygen exposure. Figure Investigated metal hydride before and after activation     

Vapor phase reduction of cyclohexanone to cyclohexanol on CexZr1-xO2 solid solutions

Reduction of cyclohexanone to cyclohexanol using propane-2-ol as hydrogen donor has been carried out in vapor phase on Ce_xZr_1-xO₂ solid solutions synthesized by ombustion synthesized at 302°C. The solid solutions around 0.4 mol% cerium content show better catalytic activity compared to pure ZrO₂ and the selectivity to cyclohexanol is 98%. A moderate acid-base and good redox properties of Ce_xZr_1-xO₂ solid solutions are seen to be responsible for the catalytic activity. A possible mechanism of hydride transfer has been proposed with cerium ions as promoters. This revised version was published online in June 2006 with corrections to the Cover Date.     

Rare earth hydrogen storage alloy used in borohydride fuel cells

Fuel cell using the borohydride as the fuel has attracted much attentions because of high energy density and working potential. In this work, LaNi_4.5Al_0.5 hydrogen storage alloy used as the anodic material to replace noble metals has been investigated. Experimental results showed that H₂ evolution was unavoidable during discharge process because of the hydrolysis of , but the utilization of the fuel increased with the increasing current densities. At high discharge current, the alloy electrode showed the lowest hydrogen generation rate and higher utilization of the fuel because, the generated hydrogen was absorbed and oxidized to produce electric energy similar to the behavior of hydrogen storage alloy in nickel–metal hydride batteries. The reaction mechanism of borohydride on the surface of electrode made of hydrogen storage alloy also has been discussed. Hydrogen storage alloy would be a promising material as the anodic catalyst in borohydride fuel cell.     

我国金属氢化物化学研究

综述了我国金属氢化物化学的发展。我国是从５０年代中期开始研究离子型金属氢化物的合成、性能和应用的,发展了一些合成方法,获得了多项中国专利。储氢合金的化学研究是７０年代中期开始的。在储氢合金的化学合成、吸放氢热力学与动力学、储氢合金氢化催化和电化学方面都有较深入的研究,特别是储氢电极合金电化学及其在Ｎｉ／ＭＨ可逆电池中的应用研究,在国家８６３计划强有力的支持下,某些方面进入了国际先进行列。     

La-H化合物对LaMg2Ni合金中Mg2Ni相吸氢过程的影响

采用感应熔炼技术在Ar气氛保护下制备得到LaMg2Ni与Mg2Ni合金。X射线衍射(XRD)图表明LaMg2Ni合金在吸氢过程中分解为LaH3相和Mg2NiH4相,放氢过程中LaH3相转化为La3H7相。与Mg2Ni合金相比,LaMg2Ni合金显示出优良的吸氢动力学性能,这是由于镧氢化合物的存在及其在吸氢过程中所发生的相转变所造成的。LaMg2Ni合金280 s内吸氢即可达到最大储氢量的90%以上,而Mg2Ni合金则需要1200 s才能达到,且在相同温度下LaMg2Ni合金的吸氢反应速率常数大于Mg2Ni合金速率常数。镧氢化合物不仅有利于改善动力学性能,而且可以提高热力学性能。LaMg2Ni合金中的Mg2Ni相氢化反应焓与熵分别为-53.02 kJ.mol-1和84.96 J.K-1.mol-1(H2),这一数值小于单相Mg2Ni氢化反应焓与熵(-64.50 kJ.mol-1,-123.10 J.K-1.mol-1(H2))。压力-组成-温度(P-C-T)测试结果表明在603 K至523 K温度范围内,LaMg2Ni合金储氢容量保持稳定为1.95wt%左右,然而Mg2Ni合金的储氢容量则由4.09wt%衰减为3.13wt%,Mg2Ni合金的储氢容量在523K低温下仅为603 K时的76.5%,表明镧氢化合物能够改善Mg2Ni合金低温下的吸放氢性能。     

Investigation of the influence of nickel content on the correlation between the hydrogen equilibrium pressure for hydrogen absorbing alloys and the capacity of MH electrodes in open and closed cells

Two series of overstoichiometric AB_2.4 alloys [(Zr_0.35Ti_0.65)(V_1.33Cr_0.4Fe_0.27)_{2– x} Ni_{0.4+ x} and (Zr_0.5Ti_0.5)(V_0.8Mn_0.8Cr_0.4)_{2– x} Ni_{0.4+ x} ], differing in the Zr to Ti ratios in group A and the presence of Mn or Fe in group B, were examined to consider the influence of various amounts of nickel on the structural parameters following the sorption of hydrogen. To predict the electrochemical behaviour of the prepared alloys as negative electrodes for Ni-MH cells, the pressure-composition isotherms (PCT) determined for the gas/solid phase system were correlated with the electrochemical pressure-composition (EPC) isotherms estimated from the rest potential of the alloy electrode. For preliminary assessment of the practical usefulness of the prepared samples, the electrodes made of all the alloys were subjected to charge/discharge measurements in a half-cell in 6 M KOH solution and the discharge capacities were estimated. Of the alloys with Fe and Mn components, the samples (Zr_0.35Ti_0.65)(V_0.93Cr_0.28Fe_0.19Ni_1.0) and (Zr_0.5Ti_0.5)(V_0.68Mn_0.68Cr_0.34Ni_0.7) provided the highest capacities. These alloys were chosen for testing the charge/discharge cycleability in closed Ni-MH cells. The reversibility of the cell with the former sample decreased significantly around the 25th cycle, whereas the discharge capacity of the cell with the latter sample remained almost unchanged during 100 cycles of testing. Electronic Publication     

Novel 3‐hydroxypropyl‐bonded phase by direct hydrosilylation of allyl alcohol on amorphous hydride silica

A novel 3‐hydroxypropyl (propanol)‐bonded silica phase has been prepared by hydrosilylation of allyl alcohol on a hydride silica intermediate, in the presence of platinum (0)‐divinyltetramethyldisiloxane (Karstedt's catalyst). The regio‐selectivity of this synthetic approach had been correctly predicted by previous reports involving octakis(dimethylsiloxy)octasilsesquioxane (Q₈M₈^H) and hydrogen silsesquioxane (T₈H₈), as molecular analogs of hydride amorphous silica. Thus, C‐silylation predominated (～94%) over O‐silylation, and high surface coverages of propanol groups (5 ± 1 μmol/m²) were typically obtained in this work. The propanol‐bonded phase was characterized by spectroscopic (infrared (IR) and solid‐state NMR on silica microparticles), contact angle (on fused‐silica wafers) and CE (on fused‐silica tubes) techniques. CE studies of the migration behavior of pyridine, caffeine, Tris(2,2′‐bipyridine)Ru(II) chloride and lysozyme on propanol‐modified capillaries were carried out. The adsorption properties of these select silanol‐sensitive solutes were compared to those on the unmodified and hydride‐modified tubes. It was found that hydrolysis of the SiH species underlying the immobilized propanol moieties leads mainly to strong ion‐exchange‐based interactions with the basic solutes at pH 4, particularly with lysozyme. Interestingly, and in agreement with water contact angle and electroosmotic mobility figures, the silanol–probe interactions on the buffer‐exposed (hydrolyzed) hydride surface are quite different from those of the original unmodified tube.     

Pseudo-Binary Phase Diagram of LiNH2-MH (M = Na,K) Eutectic Mixture

The hunt for a cleaner energy carrier leads us to consider a source that produces no toxic byproducts. One of the targeted alternatives in this approach is hydrogen energy, which, unfortunately, suffers from a lack of efficient storage media. Solid-state hydrogen absorption systems, such as lithium amide (LiNH₂) systems, may store up to 6.5 weight percent hydrogen. However, the temperature of hydrogenation and dehydrogenation is too high for practical use. Various molar ratios of LiNH₂ with sodium hydride (NaH) and potassium hydride (KH) have been explored in this paper. The temperature of hydrogenation for LiNH₂ combined with KH and NaH was found to be substantially lower than the temperature of individual LiNH₂. This lower temperature operation of both LiNH₂-NaH and LiNH₂-KH systems was investigated in depth, and the eutectic melting phenomenon was observed. Systematic thermal studies of this amide-hydride system in different compositions were carried out, which enabled the plotting of a pseudo-binary phase diagram. The occurrence of eutectic interaction increased atomic mobility, which resulted in the kinetic modification followed by an increase in the reactivity of two materials. For these eutectic compositions, i.e., 0.15LiNH₂-0.85NaH and 0.25LiNH₂-0.75KH, the lowest melting temperature was found to be 307 °C and 235 °C, respectively. Morphological studies were used to investigate and present the detailed mechanism linked with this phenomenon.     

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.     

Investigation of some new hydride systems

Recent investigations of hydrogen absorption in AB₂ intermetallic compounds with the MoSi₂-type structure, the Ti₂Ni-type structure (including oxygen-stabilized phases) and the AlB₂-type structure are reviewed. Hydrogen absorption is shown to depend strongly on the particular intermetallic compound, the hydrogen pressure, the reaction temperature and kinetic factors, and may lead to

• 1.(1) solid solutions of hydrogen in the intermetallic compound,
• 2.(2) formation of an intermetallic hydride phase or
• 3.(3) disproportionation of the intermetallic compound.

Structural information, obtained from X-ray and neutron diffraction measurements as well as recent nuclear magnetic resonance experiments, is also reviewed and discussed.     

Synthesis and Crystal Structure of BaLaSi2H0.80

The polyanionic compound BaLaSi₂ featuring cis-trans silicon chains takes up hydrogen to form a hydride BaLaSi₂H_0.80. The crystal structure of the parent intermetallic compound is largely retained upon hydrogenation with the same space group type, a unit cell volume increase of 3.29 % and very similar atomic positions in the hydride. Hydrogen could be located in the crystal structure by neutron diffraction on the deuteride. Deuterium atoms occupy a tetrahedral Ba₃La interstitial with 40.6(2) % occupation (Cmcm, a = 464.43(4) pm, b = 1526.7(1) pm, c = 676.30(6) pm). BaLaSi₂H_0.80 is thus an interstitial Zintl phase hydride like LaSiH_1–x, but unlike BaSiH_2–x does not feature any covalent Si–H bonds. Si–Si distances within the polyanion increase upon hydrogenation from 240.1(6) and 242.9(5) pm to 244.7(2) pm and 245.5(2) pm. This is probably due to oxidation of the polyanion by hydrogen, which leads to the formation of hydride ions and the depopulation of the polyanion's antibonding π* states. Interatomic Ba–D [260.9(4) pm, 295.7(5) pm] and La–D distances [241.2(7) pm] are in the typical range of ionic hydrides.     

Nickel hydrogen gas batteries: From aerospace to grid-scale energy storage applications

The challenging requirements of high safety, low-cost, all-climate and long lifespan restrict most battery technologies for grid-scale energy storage. Historically, owing to stable electrode reactions and robust battery chemistry, aqueous nickel–hydrogen gas (Ni–H₂) batteries with outstanding durability and safety have been served in aerospace and satellite systems for over three decades ever since their first development in the 1970s. Despite their satisfactory performances, this technology has difficulty to be applied for grid-scale energy storage primarily because of their high cost resulting from the utilization of expensive platinum as anode hydrogen catalyst. In recent years, with the extensive exploration of inexpensive hydrogen evolution/oxidation reaction catalysts, advanced Ni–H₂ batteries have been revived as promising battery chemistry for grid-scale energy storage applications. This mini-review provides an overview of the development activities of Ni–H₂ batteries and highlights the recent advances in the application of advanced Ni–H₂ batteries for grid-scale energy storage. New cost-effective hydrogen evolution/oxidation reactions catalysts, novel cathode materials, and advanced Ni–H₂ battery designs toward further development of Ni–H₂ batteries are discussed. The renaissance of advanced Ni–H₂ battery technology is particularly attractive for future grid-scale energy storage applications.     

Metal hydrides as bi-functional catalysts for hydrogen generation and oxidation in reversible MH-air fuel cells

Two kinds of metal hydride alloys as the bi-functional catalyst concept for hydrogen generation and oxidation in hydrogen-diffusion electrodes were investigated. The AB₅-type hydride electrode shows much higher catalytic activities than the Zr-based AB₂-type hydride electrode. However, the activity of Zr-based hydride electrodes can be improved only after removal of zirconium oxides on surface by a 1.0 M HF solution. The experiments demonstrated that the both metal-hydride hydrogen-diffusion electrodes for cycles of hydrogen generation (12 h) and oxidation (12 h) had good stability under the current densities of 100 and 50 mA/cm², respectively. The results also showed that small amounts of oxygen below 500 ppm and moisture up to 145,000 ppm in the hydrogen gas have little effect on the activity. It indicated that the hydride alloys as the non-noble-metal bi-functional catalysts in a reversible MH-air fuel cell have potential applications.     

The use of optical emission spectrometry with microwave induced plasma (MIP) discharges in a surfatron combined to different types of hydride generation for the determination of arsenic

The stability and analytical figures of merit of argon microwave induced plasma (MIP) discharges in a surfatron as sources for optical emission spectrometry (OES) are described. These MIPs have been used for the determination of arsenic after hydride generation. They could cope with the excess of hydrogen developed during the hydride generation step and thus not necessitated an isolation of the hydrides before releasing them into the MIP. Two methods for the generation of the volatile AsH₃ were applied. First a micro method was used with solid NaBH₄ on which 10 1 of the acidified sample solution is transferred. Its capabilities were compared to those of continuous hydride generation using a 5% (w/w) NaBH₄-solution and continuous liquid removal in a flow cell. Both methods were optimized for an argon MIP operated at a power of 120–160 W and gas flows of 20 l/h Ar. In the case of solid NaBH₄ the detection limit for As has been found to be 1.0 g/ml (10 ng) and with the flow cell hydride generation 50 ng/ml. The calibration curves are linear over three orders of magnitude. Interferences caused by Sb, Fe, Sn and NaCl were investigated. No interferences occurred for Sb up to an interferent concentration of 250 g/ml. The presence of Fe causes a significant depression of the As signal whereas an increase of the As signal was observed in the case of Sn. High NaCl concentrations did not influence the As signals when using continuous hydride generation, but had a great influence when using solid NaBH₄.     

Valence state of hydrogen in hydrides of intermetallic compounds

The experimental data on the mechanism of hydride dispersion of intermetallic compounds of the LaNi₅ type and the crystal structures of hydride phases based on these compounds were analyzed. A new approach was suggested and substantiated, which allows one to consider hydride dispersion as a result of a redox process associated with the formation of H^δ− hydride ions at concentrations of hydrogen in the solid hydrideC _H>-C _{H cr}. The value ofC _{H cr} is determined by the redox potential of the reaction H^δ++M^δ−⇌H^δ′−+M^δ′+. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 214–217, February, 1998.     

Ti0.17Zr0.08V0.34Nb0.01Cr0.1Ni0.3氢化物电极合金微观结构及电化学性能研究

采用XRD、FESEM-EDS、ICP及EIS等方法研究了Ti_0.17Zr_0.08V_0.34Nb_0.01Cr_0.1Ni_0.3氢化物电极合金微观结构和电化学性能。X射线衍射分析表明：该合金由体心立方结构(bcc)的V基固溶体主相和少量六方结构的C14型Laves相组成；FESEM及EDS分析表明：V基固溶体主相形成树枝晶，C14型Laves相呈网格状围绕着树枝晶的晶界，元素在两相中的分布呈现镜像关系。电化学性能测试结果表明：该合金的氢化物电极在303~343 K较宽的温度区间内，表现出较高的电化学容量，在303 K和343 K时，电化学容量分别为337.0 mAh·g^-1和327.9 mAh·g^-1。在303 K循环100周后，容量为282.7 mAh·g^-1。ICP分析结果表明，氢化物电极在充放电循环过程中，V及Zr元素向KOH电解质中的溶出较为严重。EIS研究表明，金属氢化物电极表面电化学反应的电荷转移电阻(R_T)随循环次数的增加而增加，相应的交换电流密度则随循环次数的增加而降低。氢化物电极循环过程中RT的增大以及V和Zr元素的溶解，可能是导致电极容量衰减的主要原因。     

Hydrogen sorption and electrochemical properties of alloys: Systems Zr-Ti-Ni-V-Mn with laves phase structures

Using scanning electron microscopy and x-ray phase analysis techniques, it is shown that the C14-type Laves phase with a wide homogeneity reaching the AB_2.3 stoichiometry is the main phase in the Zr_0.5Ti_0.5Ni_yV_0.5Mn _x alloys, where y = 0.8–1.4 and x = 0.1–1.7. With the increase in the nickel and manganese contents and in the stoichiometric ratio B/A, the hydrogen capacity decreases from 2 to 1.6 wt %, the equilibrium pressure of hydride phases increases, and the hydrides become less stable. The highest discharge capacity is reached for stoichiometric ratios AB_1.7–2.3, where the maximum discharge capacity at a discharge current density of 100 mA/g is 300 (mA h)/g. Alloys that contain ferrovanadium in place of vanadium are also considered.     

Reaction Pathways Determined by Mechanical Milling Process for Dehydrogenation/Hydrogenation of the LiNH2/MgH2 System

The dehydrogenation/hydrogenation processes of the LiNH₂/MgH₂ (1:1) system were systematically investigated with respect to balller milling and the subsequent heating process. The reaction pathways for hydrogen desorption/absorption of the LiNH₂/MgH₂ (1:1) system were found to depend strongly on the milling duration due to the presence of two competing reactions in different stages (i.e., the reaction between Mg(NH₂)₂ and MgH₂ and that between Mg(NH₂)₂ and LiH), caused by a metathesis reaction between LiNH₂ and MgH₂, which exhibits more the nature of solid–solid reactions. The study provides us with a new approach for the design of novel hydrogen storage systems and the improvement of hydrogen‐storage performance of the amide/hydride systems.     

Molecular Magnesium Hydrides

Solid magnesium hydride [MgH₂]_∞ has been pursued as a potential hydrogen‐storage material. Organic chemists were rather interested in soluble magnesium hydride reagents from mid‐20th century. It was only in the last two decades that molecular magnesium hydride chemistry received a major boost from organometallic chemists with a series of structurally well‐characterized examples that continues to build a whole new class of compounds. More than 40 such species have been isolated, ranging from mononuclear terminal hydrides to large hydride clusters with more than 10 magnesium atoms. They provide not only insights into the structure and bonding of Mg?H motifs, but also serve as models for hydrogen‐storage materials. Some of them are also recognized to participate in catalytic transformations, such as hydroelementation. Herein, an overview of these molecular magnesium hydrides is given, focusing on their synthesis and structural characterization.