Processing, modification and characterisations of carbon-based and carbon/AB5 composite hydrogen storage materials

摘要

Nickel powder is a prevalent conductive additive in negative electrode of conventional Ni/MH batteries due to its superior electrical conductivity. Nevertheless, it is physically heavy and economically expensive. Searching for an alternative substitute is thus essential for improving mobility and affordability of batteries. In this dissertation, possibility of employing multi-walled nanotubes (MWNTs) in conjunction with AB5 hydrogen-storage alloy as the substitute for Ni powder, enhancing the hydrogen storage behaviours of pristine MWNTs with gamma- and microwave-irradiations, and utilising and fabricating MWNTs film, which is also known as buckypaper, as a substitute for metal substrate for electrochemical and gaseous hydrogen storage were explored. The results show that electrochemical properties of 5% pristine MWNTs-contained electrode are comparable to that of its Ni counterpart. The cycle stability of the former, however, is inferior in comparison with that of the latter (26 and 75% of the maximum discharge capacity remained after 150 cycles for CNTs- and Ni-contained electrodes, respectively), due to the hydrophobic nature of the pristine MWNTs that decreases the packing density upon long-term cycling. The discharge capacity decays rapidly as a corollary of disintegration and the shedding of the anode materials from the electrode. Such deficiency can be compensated by modifying MWNTs with gamma-irradiation. This can be attributed to a hydrophobic-hydrophilic transition of the MWNTs that improves the packing density of the electrode, and facilitates the rate of charge transfer between the active material and electrolyte, consequently resulting in better cycle stability. The hydrogen storage capacity of pristine MWNTs with various diameters and gamma- and microwave-induced defects has been systematically investigated. The experimental results reveal that MWNTs with smaller diameters have higher hydrogen uptake and faster adsorption kinetics than those with larger diameters. Microwave-irradiation appears to be a more destructive approach to modify MWNTs, while gamma-irradiated MWNTs with diameter of 10-20 nm at 100 kGy possess a hydrogen uptake of 0.166 wt.%. Excessive dose demonstrates adverse effect on hydrogen storage capacity. The optimum dose for microwaving MWNTs with 20-40 nm and 60-100 nm in diameter was found to be 10 and 14 minutes, with corresponding hydrogen storage capacity of 0.87 and 0.79 wt.%, respectively.Conventional filtration technique, which is a widely adapted technique for fabricating single-walled carbon nanotubes (SWNTs) buckypapers, has been modified for the synthesis of MWNTs buckypapers. This is the first work which successfully utilises filtration technique for the making of flexible, low-cost and conductive MWNTs buckypaper. This is also the pioneering work for incorporating as-synthesised buckypapers as substrate for negative electrode in Ni/MH batteries. AB5 hydrogen storage alloy has been sputtered onto both sides of buckypapers by magnetron sputtering. The electrochemical performance of the films as a function of film thickness was investigated. Thinner films exhibit higher maximum discharge capacity (276 mAh/g) than that of the thicker films (168 mAh/g), whereas the latter possess better durability and chargeability (45% and 28% of its maximum discharge capacity after 200 cycles for BM-3 and BM-1 electrodes, respectively). Both films show exceptional high efficiencies at high discharge rates, with approximately 45% of its maximum discharge capacity retained at 5 C. Attenuation of the capacity is caused by the formation of La(OH)3 nanoparticles, which impedes the hydrogen from diffusing into the lattice of AB5 alloy.The hydrogenation behaviours of the composite films have been examined by volumetric approach under ambient conditions. The maximum hydrogen uptake of 0.63 and 0.27 wt.% of the mass of the entire film (i.e., AB5 and buckypaper) was acquired for films sputtered for 3 hours using MWNTs and SWNTs buckypapers as substrates, respectively. In addition, hydrogen storage capacity augments with increasing film thickness. Material shedding and pulverisation have been efficaciously suppressed with the introduction of flexible buckypapers as substrates. This work successfully demonstrated that the replacement of conventional heavy metal substrates by light-weight low-cost buckypapers is highly feasible for its applications in energy devices.