Magnesium-based materials supported on carbon for hydrogen storage

Abstract

The increasing CO2 content in the atmosphere due to combustion of fossil fuels is the main cause of enhanced “global warming”. Switching to renewable energy sources, such as solar or wind energy is promising for a sustainable future, but the produced electricity cannot be used easily for off-grid applications. Fuels are still required for mobile applications and can be produced for instance by electrolysis using the electricity from renewable energy sources. Hydrogen is promising in this context to be used as fuel in the future. A possible scenario is to store the hydrogen and regain the energy either by combustion or by using fuel cells. However, a suitable storage method has not yet been realized. Storage of hydrogen as atoms or ions in solid materials is interesting, because the volume is greatly reduced and it is generally safer than storage as compressed gas. Unfortunately, several drawbacks have to be overcome before solid-state hydrogen storage can be implemented. Although storage in solid materials is more compact than storage as a gas, the gravimetric density is quite low. The operation conditions for solid-state hydrogen storage materials are determined by the equilibrium and kinetic characteristics of hydrogen sorption. Due to the strong bond between the hydrogen and the storage material, high operation pressures and temperatures are often required for the charging and discharging processes. Also cycling stability is an important material property. The thesis discusses several methods to improve the hydrogen storage properties of Mg-based materials. Magnesium based materials are interesting due to the abundance and low costs of magnesium (~ 3 €/kg in 2013). However, the equilibrium temperature for hydrogen sorption in Mg(H2) is 300 oC at 1 bar H2 and both hydrogen absorption and desorption are slow at room temperature. In order to change the thermodynamic and kinetic properties, other elements were added to Mg either as catalyst or to form different Mg-compounds. One of the main challenges was to find new synthesis strategies to produce materials with particle sizes in the nanometer range. Small particle sizes are crucial to improve the kinetic properties and reversibility. A main synthesis technique to obtain nanoparticles involved using porous carbon supports in order to control the particle size. The use of a support also prevented particle growth. An important aim of the study was to relate structural properties of the carbon supported Mg-based nanomaterials to the hydrogen storage performance in order to understand the impact of particle size, composition and interaction with the carbon support.