5. Materials for H2 Storage Flashcards
Metal hydride compounds: what is the major attractiveness to use them for hydrogen storage? Analyze advantages and disadvantages.
Major attractiveness to use metal hydride components as hydrogen storage:
- Very high Volumetric hydrogen capacity [kg_H2/m^3] – it is compact
o Metal hydrides = 80-160 = 3 times lower volume than compressed gas
- No safety problems (like for compressed gas cylinder and liquid hydrogen tanks). Without extremely high pressure, low temperatures, and cheaper (don’t need the big tanks (gas of H2))
- Can be stored in metal hydride tanks for a long time, have no expirery date. Long time storage for solar or wind energy
Disadvantages:
The main disadvantage so far is that we have no clear candidate for the best material to use, since they need to exhibit low desorption rate to be useful (so they can be run at low temperatures).
It is also a problem to remove exothermic heat when charging the hydrides at a high rate.
What are the typical bond types in metal hydrides? What are the optimum binding energies for H2-storage?
The typical bond types in metal hydrides are ionic hydrides, covalent polymeric hydrides, covalent hydrides and metallic hydrides.
Mostly covalent bonds (sharing of an electron pair), sharing of the outer valence electron, sometimes ionic but don’t want that because that is too stronger bonded.
The optimum binding energies for H2-storage is somewhere between 0.1-0.6 eV, or 10-60 kJ/mol. This is high enough to bind the hydrogen, but low enough to allow for easy desorption, so that it can be run at low temperatures.
Low affinity to hydrogen: ΔH > -40 kJ/mol[H2] = don’t like to form bonds with Hydrogen
High affinity with H2: ΔH < -40 kJ /mol[H2] = likes to form bonds with hydrogen. Very negative => strong bonds
What are the main steps in the process of hydrogenation? What is their impact for reversibility of the process?
Steps of hydrogenation:
1) H2 molecule approaches the metal surface
2) Interaction of the H2 molecule on metal surface by Van der Waals forces (physisorbed state), still a molecule
3) The H2 molecule dissolves into two H atoms and chemisorbs (formation of chemical bonds with the metal)
4) Occupation of subsurface sites and diffusion of H into bulk lattice sites of the metal.
The reaction is accompanied by lattice expansion and deformation, but without phase separation.
What are the typical strategies to improve metal hydride materials for hydrogen storage?
1) Making use of a hydrogen spillover mechanism by using a supported catalyst system. Here a metal catalyst for the absorption of hydrogen is used together with a support, where the hydrogen is dissociated and then later intercalated into the receptor. This lowers the energy barrier of absorption.
2) Using nanostructuring of intermetallic compounds. It is shown that for smaller particles of f.eks. NaAlH4, the desorption temperature is much lower. The same goes for Mg nanowires: the hydriding/dehydriding energies are lowered for smaller nanowires. Nanocrystalline Mg2Ni also show higher absorption rates than polycrystalline.
Some reasons the nanostructures are better is due to an increase in surface area and a decrease in diffusion length.
3) Mechanical activation / ball milling: Here the material is ground to small particles. Reasons for improved storage is here also higher surface area and decrease in diffusion length, but also breaking of the outer oxide layer and introduction of defects.
4) Composite materials: Mg nano-crystals are encapsulated by a selectively gas permeable polymer, PMMA. Only allows H2 to come in.
Name state-of-the-art and alternative non-metallic materials for hydrogen storage.
State of the art:
Highest volumetric hydrogen density in Mg2FeH6 (Magnesium iron hexahydride) (T = 265C).
Highest gravimetric hydrogen density in LiBH4 (T = 400C). Some of the materials closes to the DOE ultimate target are NH3BH3 and LiNH2BH3.
Magnesium hydride is also a very studied compound, but potential usage has been hindered due to bad desorption properties and bad reaction enthalpies.
Alternative:
Non-metallic materials: Carbon nanotubes, pillared graphene (theoretical), graphane, Clathrate compounds.
Supplement:
o MgH2 – (Mg very light, and very abundant 8 th most abundant element on earth, high reversible storage capacity of H2 (7.6 wt%), but high decomposition T ca 300 Celsius.
o LiH – ( high gravimetric capasity, but needs very high T to desorb the hydrogen, has a to high bonding energy.)
o AlH3 – (desired binding energy (between 10-60 kJ/molH2, so desorbed at a low T: 150 celsius, high gravimetric H2 capacity (10 wt%), Al is light weight. But AlH3 don’t have so good stability, drawback.
Aluminum hydride (AlH3) is a covalently bonded trihydride with a high gravimetric (10.1 wt%) and volumetric (148 kg·m−3) hydrogen capacity. AlH3 decomposes to Al and H2 rapidly at relatively low temperatures, indicating good hydrogen desorption kinetics at ambient temperature. Therefore, AlH3 is one of the most prospective candidates for high-capacity hydrogen storage materials.
Draw a plot of where the so-called Department of Energy Ultimate Target is placed.
Draw H content (wt%) vs. decomposition temperature. The ultimate target lies in the lower right area, with high H content and low decomposition temperature.
What is the main issue with hydrogen storage?
Hydrogen has a high gravimetric energy density, but a poor volumetric energy density. This means that we traditionally have used pressurized tanks to store gaseous hydrogen, or cooled the hydrogen down to below its sublimation temperature. This has posed safety risks.
Why making good hydrogen storage devices important?
Because one of the biggest challenges with the hydrogen economy is that we so far lack a good way to store and transport the hydrogen, especially for automotive applications. If we could “crack the code”, we could have cars which wouldn’t have to be retanked all the time. Fuel cells could be more used, and we could move torwards a cleaner future.
Compare the requirements for energy storage in a conventional diesel vehicle, a zero-emission vehicle run by fuel cell and a battery vehicle.
500 km:
Diesel: 43 kg, 46L
Fuel cell: 125 kg, 260 L
Battery: 1000 kg, 670 L (not sure if this is true?)
What are the three major competing technologies for hydrogen storage?
Compressed gas cylinders, liquid hydrogen tanks and metal hydrides.
Name some drawbacks using compressed hydrogen gas.
- Safety problems due to large pressures needed.
- large pressure drop during use
- hydrogen embrittlement of storage tanks.
Name some drawbacks using liquid hydrogen.
Large thermal losses, safety and cost of cooling the hydrogen.
What is hydrogen embrittlement? Why is this a problem?
The fact that some metals, such as high-strength steel or aluminium and titanium alloys become brittle and eventually crack under load following exposure to hydrogen. This poses problems for steel piping and compression tanks/cylinders.
What is the problem with high refueling rates?
Removal of exothermic heat.
Name the five most promising compositions for H2 storage in metal hydrides. Give one example of each. What is the hydrogen affinity in the different atoms?
A2B: Mg2Ni
AB: TiNi
AB2: LaNi2
AB3: LaCo3
AB5: LaNi5
A-atoms: high hydrogen affinity.
B-atoms: low hydrogen affinity.
