Novel approaches to store hydrogen in high capacity hydride

Abstract

Currently, there is no practical method that satisfies the requirement of high capacity hydrogen storage under ambient conditions for on-board application. In this thesis, size effects of metal hydrides and gas-phase electrochemical charging/discharging methods were investigated to achieve a near-room-temperature hydrogen storage in high capacity metal hydride with LaNi5 as a model and Mg as the targeted material. Firstly, size effects were studied on LaNi5. LaNi5 nanoparticles by combustion-reduction method showed greatly improved kinetics but no significant changes in thermodynamics compared with bulk LaNi5. The study was then extended to Mg nanoparticles from solution reduction method and results indicated that the decrease in particle sizes made hydrogen absorption near 100 °C possible because of the improved absorption kinetics. However, the desorption kinetics were slowed down compared to ball-milled MgH2 due to the inactive surface, although Mg nanoparticles showed a trend of smaller particles possessing faster kinetics. Ni coating on Mg surface could activate the surface and lead to fast desorption kinetics. Furthermore, size reduction of Mg particles decreased the reaction entropy but enthalpy-entropy compensation phenomena caused the desorption temperature not to be decreased as low as expected. To finely tune the thermodynamics, elemental coating is a potential method, but further work is required to lower the desorption temperature to near room-temperature. Since size effects were proved not as effective as expected to reduce the desorption temperature of Mg, research was transited to the development of gas-phase electrochemical charging/discharging method, which uses proton conductive membrane as solid electrolyte and gaseous hydrogen as H source. This novel method was proved successful with reversibly charging 1.56 wt% hydrogen in LaNi5 with humidified Nafion as solid electrolyte. The extension of its use to Mg showed that hydrogen can be reversibly charged into Mg but moisture caused oxidation of Mg/MgH2, which prevented further charging/discharging. The use of anhydrous membrane, fluorite-treated Mg/MgH2 plus improved proton conductivity with LaNi5 addition avoided the oxidation and 2.53 wt% was charged into Mg. This novel method avoids the use of corrosive alkaline electrolyte and shows great potential to charge/discharge high capacity hydride near room temperature for practical on-board hydrogen storage.