Development of a novel method for the fabrication of nanostructured complex oxide Zr(x)Ni(y) catalyst for improving the hydrogen storage properties of MgH2

Abstract

The present study involves the development of a novel method for the fabrication of zirconium/nickel-based nanocatalysts to improve the hydrogen storage properties of the Magnesium/Magnesium Hydride (Mg/MgH2) system. Three novel methods were implemented for fabricating the catalysts. Method 1: Step 1 - “Synthesis of ZrO2/Ni Nanocatalyst” Step 1 – High-Pressure Reactor and Catalyst Activation. The catalyst ZrO2/Ni was synthesized using a high-pressure reactor and calcinated under the flow of hydrogen before being mechanically milled with the MgH2 for 20 hours under Argon (Ar) in a SPEX High-Energy Ball Mill. Step 2 – Ball Milling - SPEX High-Energy Ball Mill. The synthesized catalyst was activated under hydrogen before being mechanically milled with the MgH2 for 20 hours under Ar in a SPEX High-Energy Ball Mill. Method 2: : “High-Pressure Reactor, Catalyst Activation and Ball Milling - Planetary Ball Mill PM 100”.The catalyst ZrO2/Ni was synthesized using the high-pressure reactor utilized in method 1, calcinated under the flow of hydrogen, and activated under hydrogen before being mechanically milled with the MgH2 for 5 hours under Ar in a Planetary Ball Mill PM 100. Method 3: “Synthesis of ZrO2/Ni Vs ZrO2/NiCl2 Nanocatalyst. High-pressure reactor, Catalyst Activation, and Ball Milling - Planetary Ball Mill PM 100”. The catalyst prepared in Method 2 (ZrO2/Ni) was compared with a new catalyst, ZrO2/NiCl2.The ZrO2/NiCl2 was synthesized via the high-pressure reactor and calcinated under the flow of 95%Ar /5%H2 before being mechanically milled with the MgH2 for 5 hours under Ar in a Planetary Ball Mill PM 100. For all the 3 methods, the hydrogenation and dehydrogenation properties of hydrogen storage materials in MgH2 were measured from the plots of the PCT (Pressure-Composition Temperature) with the help of a Manometric Gas Sorption Analyser. The synthesized catalysts and composites were characterized by the XRD (X-Ray Diffraction), FE-HRTEM (Field Emission High-Resolution Transmission Electron Microscopy), and SEM-EDX (Scanning Electron Microscope) to determine the microstructure of the prepared powder. The desorption characteristic of the nanocomposite was determined via TPD (Temperature-Programmed Desorption). The outcome of the experiment from Method 1 shows that the milling of the powder via the SPEX ball mills resulted in particle agglomeration of the nanostructured composite MgH2/10wt%ZrO2/Ni mainly in the range of 2000 nm to 4000nm, as well as in a high level of impurities, and oxide formation, which significantly affected the absorption property of the MgH2, 4.4wt.% at 250°C after 1 minute. On the other hand, Method 2 shows that the nanostructured MgH2 powder milled with 10 wt.% of ZrO2/Ni-based nanocatalyst resulted in a wide range of MgH2 particle size distribution below 1000 nm. A faster hydrogen release of 5.9 wt.% at onset temperature 210 °C/peak temperature 232 °C was recorded. Lastly, Method 3 shows that the nanocomposites MgH2 powder milled with 10 wt.% of ZrO2/Ni have a high percentage of hydrogen 5.4 wt.% at 250ºC with a fast kinetic of hydrogen uptake at 1 minute, and a maximum absorption capacity of 7.3 wt.% was achieved in less than 10 minutes. Whereas the composite MgH2/10wt.% ZrO2/NiCl2 shows moderate improvement of hydrogen uptake, 4.0 wt.% at 250ºC, with a maximum uptake of 5.3wt.%, which may be due the presence of a high percentage of chloride catalyst on the surface of the Mg/MgH2. The PCT plot of the MgH2/10 wt.% of ZrO2/Ni resulted in significant thermodynamic improvement, with the enthalpy value of -57.17KJ/mol which is approximately 20% when compared to the commercial MgH2. Alloying ZrO2 and Ni significantly enhanced the hydrogenation and dehydrogenation properties of the MgH2.The observed significant improvement in the hydrogen sorption properties may be due to the impact of the highly dispersed catalyst on the surface of the Mg/MgH2 system as well as due to the reduction in particle size during the ball milling process and the formation of Mg0.996Zr0.004 phase during the milling process. Furthermore, the significant reduction in particle size and homogenous distribution of the catalyst in the MgH2 was probable the result of the catalyst activation prior to the milling process with the MgH2.