Comparing the metallic catalysts used in producing magnesium hydride for PowerPaste production
The first step of producing PowerPaste is the production of magnesium hydride. Magnesium hydride is considered a promising hydrogen storage material because of its efficient cost, lightweight, excellent reversibility, and natural abundance.
Generally, doping transition metals and their alloys into magnesium hydride is regarded as one of the most practicable means of accelerating magnesium hydride sorption kinetics. Many recent studies have investigated and developed many transition metals as well as their alloys.
This article discusses these catalysts and their similarities and differences. The benefits and shortcomings of each catalyst are also discussed. Most importantly, the effects of the catalysts on the properties of magnesium hydroxide for hydrogen storage are discussed.
Transition metals have a great catalytic effect on enhancing the hydrogen storage properties of magnesium hydride. Unlike iron and titanium, nickel is the most used catalyst for magnesium hydride.
Earlier studies combined nickel and magnesium hydride powder via ball milling to produce a magnesium hydride-nano nickel composite. The desorption mechanism of the composite was reduced to 260 ℃. This value is lower than 370 ℃ obtained for pure magnesium hydride. The particle size and the amount of catalyst affect the catalytic property of nickel.
Magnesium hydride nanoparticles doped with 10 wt% of nickel nanoparticles can absorb up to 6.1 wt% of hydrogen at 250 ℃ in 600 seconds. The rate of desorption of magnesium hydride-nano nickel composite increases with an increase in catalyst amount. Exceeding the nickel catalyst amount above a certain amount may not lower the activation energy of desorption. Maintaining the hydrogen storage capacity and reducing the nickel catalyst amount simultaneously can improve the catalytic impact of nickel.
The Fraunhofer Institute for Manufacturing Technology and Advanced Material (IFAM) in Germany developed PowerPaste by combining magnesium hydride and mineral oil. IFAM explained that, in contact with water, PowerPaste releases about 10 wt% of hydrogen in a fuel cell.
The desorption of hydrogen by magnesium hydride is largely affected by the size of nickel particles. A recent study mixed magnesium hydride with 2 wt% of different fine nickel particle sizes to form a magnesium hydride-nickel composite. The composite at 90 nm of nickel desorbed about 6.5 wt% of hydrogen from 280 ℃.
Comparing this with the hydrogen desorption capacity of 100 nm and 200 nm nickel particles at the same 2 wt% catalyst amount, the 90 nm particle size has a desorption temperature 10 ℃ lower than that of the 100 nm and 200 nm particle sizes. The nickel catalyst site density over the magnesium hydride particle is the main factor that improves the kinetics of hydrogen adsorption of magnesium hydride and not just the particle size.
Titanium demonstrates better catalytic properties for magnesium hydride compared to nickel. Earlier studies on the mechanism of the catalysis of titanium in enhancing the hydrogen storage properties of mechanically milled magnesium hydride indicated complete hydrogen desorption at 250 ℃ in 1000 seconds. Under the same conditions, no hydrogen was released with ball-milled magnesium hydride.
The dehydrogenation properties of magnesium hydride-titanium composite are better than that of pure magnesium hydride. A recent study was conducted on the microstructure and dehydrogenation/hydrogenation kinetics of magnesium hydride-titanium composite by ball milling. The study showed that the initial dehydrogenation temperature of the composite is 51 ℃ lower than that of pure magnesium hydride.
When titanium is used as a catalyst, the capacity of hydrogen could reach 6.18 wt%. Unlike the desorption kinetics of pure magnesium hydride, which is rather sluggish, the magnesium hydride-titanium composite has an Ea of 103.9 kJ/mol. This is lower than the Ea of pure magnesium hydride by 38.5%.
Doping magnesium hydride with elemental titanium produces titanium hydride during ball milling. Titanium hydride acts as an active specie throughout the desorption process. Catalyzing magnesium hydride using titanium nanoparticles shows that the dehydrogenated sample can absorb up to 6 wt% at 257 ℃ in less than 3600 seconds. This can be compared with commercial magnesium hydride, which needs nearly 10800 seconds to absorb 6 wt% of hydrogen at 350 ℃.
Iron is one of the most common metallic elements used in numerous applications. Many recent studies have investigated the use of iron with magnesium hydride to examine the catalytic impact of iron. Most studies use the ball milling method to produce magnesium hydride.
A recent study concluded that the optimum concentration of iron catalyst is about 10 wt%. Concentrations lower than 10 wt% are considered insufficient and might result in several poorly catalyzed regions. Research indicates that using iron catalysts could release around 5 wt% of hydrogen at 300 ℃ in 600 seconds.
Exploring the cycling properties of iron-catalyzed magnesium hydroxide nanocomposites at 300 ℃ shows that the sorption rate and maximum storage capacity are stable after the first 10 cycles. The main impact of cycling on particle morphology is the continuous magnesium extraction from the magnesium oxide shell. A recent study indicates that the capacity of hydrogen of magnesium hydride doped with 5 wt% of nano iron composites can be maintained at 5 wt% after 50 cycles.
A more recent study introduced iron nanosheets produced via wet-chemical ball milling into magnesium hydride. The study added 5 wt% of nano iron composite into magnesium hydride releasing and absorbing hydrogen at 75 ℃. Then the study released and absorbed hydrogen into magnesium hydride at 182.1 ℃. The magnesium hydride doped with iron nano-composites absorbed 6 wt% of hydrogen at 200 ℃ in 600 seconds.
One of the advantages of using iron nanoparticles as catalysts is the weakening of the interaction strength of magnesium and hydrogen in magnesium hydride. The result is the facilitation of dehydrogenation of magnesium.
Future research direction
The realization of the practical application of hydrogen for energy generation still requires several future research. In general, the transition metals mentioned in this article have very good catalytic effects on magnesium hydride’s hydrogen storage properties.
Further research directions to improve the hydrogen storage properties of magnesium hydride include investigating transition metal alloys. The components of these alloys can be regulated to have the best catalytic effect on magnesium hydride.
Further investigation can be carried out to reduce the particle size of the alloys into smaller sizes. Also, carbon materials can be investigated and combined with transition metal alloys to enhance the properties of magnesium hydride for hydrogen storage.