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Abstract
In this dissertation, partial element substitution was used to improve the hydrogen storage properties and electrochemical cycle stability of AB3 type alloys. In addition composite method was proposed to enhance the cycle durability of AB3 type alloys. Meanwhile a critical review on the working principles, advantages and drawbacks, limitations of six categories of techniques commonly applied on the characterization of hydrogen storage materials was presented. These techniques include the Sieverts technique, gravimetry, secondary ion mass spectrometry, thermal desorption spectroscopy, neutron scattering and electrochemical technique.
The microstructure and hydrogen storage properties of cost-competitive La0.51-xY0.05+xCa1.03Mg1.32Ni9 (x=0, 0.15, 0.35) alloys were investigated. The change of Y content would alter the microstructure of La-Y-Ca-Mg-Ni alloy such as phase abundance, lattice parameters and cell volume. The Y abundant sample exhibited a higher absorption plateau pressure, lower hydrogen capacity and relatively inferior hydrogen absorption kinetics. In addition the hydride stability was reduced with the increase of Y content.
The electrochemical properties of La0.51-xY0.05+xCa1.03Mg1.32Ni9 (x=0, 0.15, 0.35) alloy powder were systematically investigated. The increase of Y content in sample alloy would result in an improvement of cycle stability, suppression of particle pulverization, reduction of formation of corrosion products and elevation of rate capability at higher discharge rate. However excessive Y in the alloy would lead to a decrease in maximum capacity. Overall the electrodes made by La0.42Y0.22Ca0.78Mg1.01Ni9 alloy exhibited the best performance; the cycle stability was improved without significant loss in maximum capacity.
The electrochemical performances of composite electrode composed of La0.51Y0.05Ca1.03Mg1.32Ni9 and x wt% La0.63Nd0.08Pr0.02Ce0.27(Al0.02Co0.14Mn0.06Ni0.78)5 (x =10, 30, 50) have also been studied. The electrode with 30 wt% AB5 electrode exhibited the best overall performance; both cycle durability and maximum discharge capacity were enhanced, although the rate capability was deteriorated. The linear polarization and potentiostatic discharge test indicated that rate capability of both La-Y-Ca-Mg-Ni and composite electrodes was dominant by charge transfer reaction at electrode surface at lower discharge rate, and the discharge performance became hydrogen diffusion dependent when the discharge current density rose.