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<p>Metal hydrides hydrogen storage materials have the ability to reversibly absorb and release large amounts of hydrogen at low temperature and pressure. In this study, metal hydride materialsemployed as negative electrodes in Ni-MH batteries are investigated. Attention is on AB5 alloys due to their intermediate thermodynamic properties. However, AB5 alloys a have&nbsp / tendency of forming oxide film on their surface which inhibits hydrogen dissociation and penetration into interstitial sites leading to reduced capacity. To redeem this, the materials were micro-encapsulated by electroless deposition with immersion in Pd and Pt baths. PGMs were found to increase activation, electrochemical activity and H2 sorption kinetics of the MH alloys. Between the two catalysts the one which displayed better performance was chosen. The materials were characterized by X-ray difractommetry, and the alloys presented hexagonal CaCu5 &ndash / type&nbsp / structure of symmetry P6/mmm. No extra phases were found, all the modified electrodes displayed the same behavior as the parent material. No shift or change in peaks which corresponded to Pd or Pt were observed. Scanning Electron Microscopy showed surface morphology of the materials modified with Pd and Pt particles, the effect of using different reducing agents (i.e., N2H4 and NaH2PO2), and alloys functionalized with &gamma / -aminosopropyltrietheosilane solution prior to Pd deposition. From all the surface modified alloys, Pt and Pd particles were observed on the&nbsp / surface of the AB5 alloys. Surface modification without pre-functionalization had non-uniform coatings, but the prefunctionalized exhibited more uniform coatings. Energy dispersive X-ray Spectroscopy and Atomic Absorption Spectroscopy determined loading of the Pt and Pd on the surface of all the alloys, and the results were as follows: EDS ( Pt 13.41 and Pd 31.08wt%), AAS (Pt 0.11 and Pd 0.78wt%). Checking effect of using different reducing agents N2H4 and NaH2PO2 for electroless Pd plating the results were as follows: EDS (AB5_N2H4_Pd- 7.57 and AB5_NaH2PO2_Pd- 31.08wt%), AAS (AB5_N2H4_Pd- 11.27 and AB5_NaH2PO2_Pd- 0.78wt%). For the AB5 alloys pre-functionalized with &gamma / -APTES, the results were: EDS (10.24wt%) and AAS (0.34wt%). Electrochemical characterization was carried out by charge/discharge cycling controlled via potential to test the AB5 alloy. Overpotential for unmodified, Pt and Pd modified&nbsp / electrodes were -1.1V, -1.24V, and -1.60V, respectively. Both modified electrodes showed discharge overpotentials at lower values implying higher specific power for the battery in comparison with the unmodified electrodes. However, Pd electrode exhibited higher specific power than Pt. To check the effect of the reducing agent the results were as follows: AB5_ N2H4_Pd (0.4V) and AB5_NaH2PO2_Pd (-0.2V), sodium hypophosphite based alloy showing lower overpotential values, implying it had higher specific power than hydrazine based bath. Alloy prefunctionalized with &gamma / -APTES, the overpotential was (0.28V), which was higher than -0.2V of the alloy without pre-functionalization, which means pre-functionalization with &gamma / -APTES did not improve the performance of the alloy electrode. Polarization resistance of the electrodes was investigated with Electrochemical Impedance Spectroscopy. The unmodified alloy showed high resistance of&nbsp / 21.6884 while, both Pt and Pd modified electrodes exhibited decrease 14.7397 and 12.1061 respectively, showing increase in charge transfer for the modified electrodes. Investigating the effect of the reducing agent, the alloys exhibited the following results: (N2H4 97.8619 and NaH2PO2 12.1061) based bath. Alloy pre-functionalized with &gamma / -APTES displayed the&nbsp / resistance of 9.3128. Cyclic Voltammetry was also used to study the electrochemical activity of the alloy electrodes. The voltammograms obtained displayed the anodic current peak at -0.64V&nbsp / o -0.65V for the Pt and Pd modified electrodes, respectively. Furthermore, the electrode which was not coated with Pt or Pd the current peak occurred at -0.59V. The Pd and Pt coated&nbsp / alloy electrodes represented lower discharge overpotentials, which are important to improve the battery performance. Similar results were also observed with alloy electrodes Pd modified&nbsp / using N2H4 (-0.64V) and NaH2PO2 (-0.65V). For the electrode modified with and without &gamma / -APTES the over potentials were the same (-0.65V). PGM deposition has shown to significantly&nbsp / improve activation and hydrogen sorption performance and increased the electro-catalytic activity of these alloy electrodes. Modified electrodes gave better performance than the unmodified&nbsp / electrodes. As a result, Pd was chosen as the better catalyst for the modification of AB5 alloy. Based on the results, it was concluded that Pd electroless plated using NaH2PO2 reducing agent&nbsp / had better performance than electroless plating using N2H4 as the reducing agent. Alloy electrode pre-functionalized with &gamma / -APTES gave inconsistent results, and this phenomenon needs to&nbsp / be further investigated. In conclusion, the alloy modified with Pd employing NaH2PO2 based electroless plating bath exhibited consistent results, and was found to be suitable candidate for&nbsp / use in Ni-MH batteries.</p>

No description available.

AB5 hydrogen storage alloys have been intensively studied due to its superior ability to store hydrogen and release at ambient conditions. It is also a major component in the negative electrode of Ni-MH batteries. However, it has poor high rate capability and cycle life stability. Carbon nanotubes (CNTs) were found to store a tremendous amount of hydrogen, owing to the fact that they possess very large surface areas. It is because the hydrogen storage capacity is in general highly dependent on the surface area of the storing materials. The aim of this project has been to investigate the effect on electrochemical behaviours of Ab5 negative electrode in Ni-MH batteries by adding carbon nanotubes. The research also studied the influence of the ball milling treatments applied to both the Ab5 and CNTs. La0.59Ce0.27Nd0.08Pr0.06 (Ni0.76Mn0.08Al0.01Co0.15)5 AB5 alloy powder was used as active material in the negative electrode in the Ni-MH batteries, CNTs were used as additive, nickel powers as conductor in a three-electrode cell. Electrodes with compositions of AB5 + x wt.% CNTs (x=0, 5, 10) were studied. Activation, high rate capability and cycle life stability were investigated. The three-electrode cell in an open container with 6 M of KOH as electrolyte was connected to charge/discharge machine where galvanostatically charging and discharging took place. Hydrogenation of ball milled and as-received AB5 alloy powders were examined by conventional volumetric method. Morphology of AB5 and CNTs was examined by scanning electron microscopy (SEM) and transition electron microscopy (TEM), respectively. The phase identification and crystal lattice parameters were analysed by multi-purpose X-ray diffraction before and after ball milling treatments for both materials. The chemical composition of Ab5 alloy powders was tested using ICP chemical method. The results show the addition of CNTs in negative electrode in a Ni-MH battery enhanced the specific discharge capacity remarkably. A maximum discharge capacity of 252 mAh/g was observed for electrode with low energy ball-milled (LEBM) Ab5 with 5 wt.% of CNTs. This was due to the superior properties and great surface area of CNTs which allow more hydrogen to be stored and diffused onto the surface. Not only CNTs could act as a hydrogen reservoir in the negative electrode, it also acted as a conductor by building a conductive network between active material and nickel powders, and hence an increase in discharge capacity. However, the milling on CNTs alone will not improve the electrochemical properties of the electrode. In contrary, the activation profiles, high rate capability and cycle stability have been enhanced significantly when Ab5 alloy powders were ball-milled. The possible explanation is the smaller particle size and rough surface (and hence large surface area) obtained after ball milling induces a better hydrogen diffusion between the particles, as a result of shorter distance between particles after ball milling. Ball milling treatments on AB5 alloy powders did not improve the hydrogen absorption capacity. A highest value of 1.27 wt.% was observed for LEBM alloy powders. Ball milled samples have a slightly lower plateau pressure as compared with that of as-received alloy powders. In addition, only 4% of the maximum absorption capacity was lost after 10 repeated absorption and desorption cycles due to pulverisation of the particle over cycling. It can be concluded that LEBM Ab5 with addition of 5 wt.% CNTs, can significantly improve the electrochemical properties of negative electrode in Ni-MH batteries.

Thesis (M.S.)--University of Nevada, Reno, 2007. / "December 2007." Includes bibliographical references (leaves 68-70). Online version available on the World Wide Web.

Thesis (M.S.)--University of Texas at El Paso, 2009. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis--Stockholm. / Extra t.p., with thesis statement, inserted. Bibliography: p. at end.

The homogeneous hydrogenation of arenes with functional groups was studied with allylcobalt complexes containing the bulky chelating diphosphines dippp (1,3-bis(diisopropylphosphino)propane and dippcyp (trans-(±)-l,2-bis(diisopropylphos-phino)cyclopentane). The results indicated that these catalyst precursors were unsuitable for the hydrogenation reactions, being too sensitive to the nature of the substrate. From these hydrogenation reactions, the intermediates (η⁵-cyclohexadienyl)Co(dippcyp) (10) and (η⁴-2-methoxynaphthalene)Co(H)(dippcyp) (11) were isolated and structurally characterized, thus providing some insight into the mechanism of the hydrogenation reaction. The production of binuclear hydrides such as [(dippp)CoH₂]₂ (4) and [(dippcyp)CoH₂]₂ (9) was observed to lead to the end of the catalysis. An X-ray structural characterization of the blue hydride [(dippp)CoH₂]₂ (4) showed that in the solid state it is binuclear. Although the complex is diamagnetic in the solid state (6-280 K), in solution its paramagnetic behaviour could only be attributed to an equilibrium with a second species proposed to be mononuclear, (dippp)CoH₂. In addition, a cyclic voltammogram of the complex in solution indicated that the predominant species still was the binuclear compound [(dippp)CoH₂]₂ (4). One of the syntheses of [(dippp)CoH₂]₂ (4) gave a product identified as (dippp)CoH₃ (5). Based on variable-temperature spin-lattice relaxation time (T₁) measurements and an electrochemical study, this red hydride complex appeared to contain an η²-H₂ ligand. The relationship of this complex with the blue hydride apparently involves the mononuclear species, (dippp)CoH₂. Independent pathways led to the formation of both the blue and red hydrides, and these pathways are discussed in terms of possible mechanisms. / Science, Faculty of / Chemistry, Department of / Graduate