Hydrogen Storage Performance Research Of Nanometer Magnesium Hydride Particles

Hydrogen is a clean,abundant and non-toxic renewable energy source.When compared to other hydrocarbon fuels,hydrogen contains a higher chemical energy per unit mass of 142 MJ,which is 4 times of that of gasoline.However,the energy per unit volume of hydrogen is very low,1/5 of that if gasoline.When compared to traditional gasoline fuel,hydrogen burns more quickly.Its energy density is 3.2 times less than natural gas,2700 times less than gasoline.Therefore,hydrogen is not only a source of energy,but also a carrier of energy.This means that it stores and provides energy in a usable form.The most attractive nature of hydrogen is its natural compatibility with fuel cells,which is more energy efficient than gasoline?22%?and diesel?45%?.The energy efficiency of hydrogen is high,reaching 60%,which greatly improves the utilization efficiency of future energy.The application of hydrogen energy makes the rapid development of safe and effective solid-state hydrogen storage methods possible.Magnesium-based hydrogen storage materials are widely used for hydrogen storage because of their wide range of sources,low cost,and high hydrogen storage capacity.However,due to their slow kinetics and high thermodynamic stability,the hydrogen releasing temperature is as high as 300 ° C,which seriously hinders their practical application,especially in transportation.It was found that the smaller the particle size of Mg-based hydrogen storage materials,the shorter the diffusion path of hydrogen gas would be,i.e.,the better the kinetic properties,namely,the rate of hydrogen absorption and desorption.In this paper,a mixture of MgH₂ and LiCl with extremely small particle size was prepared by high energy ball milling combined with displacement reaction.MgH₂ particles were evenly embedded in LiCl salt,which effectively prevented the growth of MgH₂ or Mg particles and improved the material's cycle life.The hydrogen storage properties of our materials were characterized by XRD,SEM and PCT.The effects of the particle size of hydrogen storage materials on their hydrogen absorption and desorption were systematically studied.Titanium doping of of our materials was also used to further improve their thermodynamic and kinetic properties.The results show that the particle size of as-prepared MgH₂ is about 20 nm.The hydrogen absorption and desorption rates of the MgH₂ materials can be effectively improved.Ther is no much change in the hydrogen storage capacity after 20 absorpition-desorption cycles.Doping 10at% Ti in MgH₂ by ball milling futher improves the hydrogen absorption and desorption rate.In addition,when compared to pure magnesium,the initial hydrogen desorption temperature of our 10at%Ti-MgH₂ is reduced by nearly 40?.These results suggest that it is difficult to improve the thermodynamics and kinetics of hydrogrn storage materials by only reducing their particle size.Most likely,this is because mannesium is active and contains a magnesium ocide layer.Since thermodynamic stability is a state function,it is necessary to lower the thermodynamic stability,and siumulataneously improve the dynamic performance.These require the change in the structure of materials or the use of catalysts to affect the reaction mechanism and rate,as well as to reduce the activation energy.