Study of hydrogen storage and electrochemical properties of LANI5-based thin films and porous silicon thin films for mini-fuel cells and micro-batteries

Two thin film materials - intermetallic and porous silicon thin films, have been studied in this thesis. The first part focuses on the hydrogen storage and electrochemical properties of single layer LaNi5-based thin films fabricated by magnetron sputtering. The aim is to enhance their performance in mini hydrogen storage systems, and their application as electrodes in thin film Ni-MH micro-batteries. Such LaNi5-based thin films were fabricated by magnetron puttering. Using X-ray diffraction (XRD), these thin films revealed a crystalline structure with uniform chemical composition. Using AFM, SEM and TEM, they were found to have a unique microstructure: (1) Nanopores of approximately 15-40 nm which could possibly act as hydrogen reservoir (2) A dense, defect free cross sectional region which would ultimately improve the efficiency and lifetime of the thin film electrodes used in thin film battery.The hydrogen absorption/desorption behaviour of these thin films were determined by volumetric method. The maximum hydrogen content of the La-Ni-A1 film was found to be 1.45 wt% at 333 K which was very close to the theoretical capacity of 1.47 wt%; and higher than that of the La-Ni-AI powder materials (1.2 wt%). Electrochemical properties of the films were measured by simulated battery tests. When discharged at low current, the discharge capacity of the film was similar to that of powder materials - about 220 mAh/g for the first 30 cycles. When the thin film electrode was discharged at a high rate, 4C (current density of 100 mA/g), it could reach the maximum specific capacity of 200 mAh/g and maintained this capacity for 200 cycles; the value was not attainable for La-Ni-AI powder electrode. The presence of crack propagation in film during charge/discharge cycles would improve the electrochemical performance which was different to that of powder materials. Cyclic voltammetry reported that the efficiency of the film could maintain at 80% for the first 200 cycles and gradually decreased due to the formation of corrosion products on surface, which is consistent with the galvanostatic results. XPS (X-ray Photoelectron Spectroscopy) revealed that the corrosion products – A1203, La203 and La(OH)3 formed on the film surface after cyclic voltammetry.The second part reported the hydrogen absorption/desorption behaviour of porous silicon thin films. The hydrogen content was determined quantitatively by both volumetric method and thermogravimetric analysis (TGA) and found to be 15 wt% at 423 K under 15 atm of hydrogen pressure. This is an extraordinary amount of hydrogen absorption which supersedes the US Department of Energy's 2007 target of 6.5 wt%. Hydrogen depth profiles of the film after hydrogenation performed by Secondary Ion Mass Spectroscopy confirmed there was hydrogen within the film structure, this was an indication that hydrogen was not just physisorbed on the film surface, but chemisorbed into the porous Si lattice. X-ray diffraction found that there was a lattice contraction upon hydrogen insertion, again suggesting the hydrogen entered into the film structure by chemisorption