Destabilization of Mg-based hydrogen storage materials: literature review (Dr. Liudas Pranevicius)

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Outline of the presentation

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MgH2 as hydrogen storage material.

Literature review on destabilization of Mg/MgH2 with (Ti, V, Ni) doping.

Oxidation/catalysis problems.

Summary and Outlook.

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DOE FreedomCar Technical Targets On-Board Hydrogen Storage System (300 mile range - 5.6 kg H 2 ) Property

Hydrogen Density (gravimetric) Energy Efficiency Energy Density (volumetric) Hydrogen Density (volumetric) Specific Energy Cost Operating Temperature Start-Up Time to Full Flow Hydrogen Loss Cycle Life Refueling Time Recoverable Usable Amount

Units

wt.% H % W-h/L kg H 2 /m 3 W-h/kg $/kW-h ($/kg H 2 ) ˚C sec scc/hr/L cycles min %

Target

6 97 1100 33 2000 5 (167) -40 - +50 15 1.0

500 <5 90

MgH2 ( Y. Song, Z.X. Guo, R. Yang.2004)

Formation of MgH

2 : 7.6 % wt H by weight - quite high

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Problems: The hydrogen is too strongly bound (hydrogen gas at 1 bar is released at around 350 o C).

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Binding energy of 0.38 eV per H-atom, should be 0.20 eV for the gas to be released at 100 o C.

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Diffusion of hydrogen through the hydride is very slow, and becomes slower when the hydrogen pressure is raised

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MgH2 ( Y. Song, Z.X. Guo, R. Yang.2004, D. Noreus 2004)

If the hydrogenation temperature is reduced to 200 C or lower, the hydrogen storage capacity drops down dramatically (up to 1.4 % wt).

It has been reported experimentally that mixing magnesium with catalytic transition elements, such as Ti, V, Fe, Co or Ni improves the hydriding and dehydriding kinetics of magnesium at lower temperature.

Preliminary results from FP5 HYSTORY show that systems such as Mg7TiH~16 and Mg6VH~14 form crystalline compounds with a rather ordered structure but including some vacancies in the magnesium sites.

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Mg

7

TiH

x Kyoi et al. (including Toyoto and Dag) 2001

Desorption Reaction

Mg 7 TiH 16 TiH 2 -> 7Mg+ TiH 2 +7H 2 -> Ti + H 2

@605 K @825 K

4a –› Experimental

lattice const: 9.532(2) Å weight load: 5.5%

4b –› 24d–– ›

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MgH2 ( Y. Song, Z.X. Guo, R. Yang.2004, D. Noreus 2004)

Mg7TiH~16 and Mg6VH~14 hydrides can store about 6 wt% of hydrogen and release it at temperatures about 150 C lower than ordinary magnesium hydride.

The problem is that they have to be synthesised at extremely high pressures and when they desorb hydrogen, the metal atom structure collapses and the metals segregate.

Magnesium is also known not to form any alloys with these metals.

The hydrogen atoms in these systems help to hold them together.

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Mg-Ti ( P. Vermeulen et al. 2006)

The most dominant reflection is that of the (002) oriented hexagonal a Mg structure. The corresponding peak position is shifted with respect to that of pure Mg (34.4 2h) due to the incorporation of Ti in the Mg lattice 8

Mg-Ti - H2 (FP5 HYSTORY, D. Noreus 2004)

Recently has been shown that very thin magnesium-TM-films, with a similar composition deposited and firmly supported on a quarts substrate, can be electrochemically charged and discharged with the same high hydrogen storage capacity.

These films are more amorphous in character, than the high pressure synthesized hydrides.

But on the other hand they seem not to disintegrate when being discharged.

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Doped MgH2: Oxidation/catalysis problems (P.Selvam; 1990; G. Liang, J. Houst 1999)

Ti or Ni or V oxide Mg (Ti, Ni, V) Substrate Ti and V are much better catalyst for H2 dissociation than Ni.

If extract Mg thin films doped with Ti or Ni or V to atmosphere air after deposition – formation of Ti or Ni or V oxide observed.

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Doped MgH2: Oxidation/catalysis problems (P.Selvam; 1990; G. Liang, J. Houst 1999)

TiO2/TiH2 Mg -Ti Substrate Mechanical milling of MgH2 with Ti and V leads to titanium and vanadium hydrides, which could protect Ti and V from oxidation, and therefore the catalytic effect towards hydrogen is preserved 11

Doped MgH2: Oxidation/catalysis problems (P.Selvam; 1990; G. Liang, J. Houst 1999)

Ni – NiO - Ni Mg -Ti Substrate Ti and V have very strong affinity to oxygen, and their oxides cannot be reduced by hydrogen under normal conditions, while NiO can be readily reduced by hydrogen to form nickel clusters on the surface.

Proposal for experiments: use catalytic layer in order to prevent Mg-Ti thin film from oxidation and use Ni catalyst instead of Pd 12

Summary & Outlook

• Additives can be used to tune (destabilize) strongly bound (high hydrogen density) Mg based hydrides.

• This approach holds promise for finding a reversible hydrogen storage material with the required hydrogen density, > 6 wt % - system, and operating conditions, T(1 bar) < 150 °C.

• A major remaining obstacle is likely to be the kinetics. For destabilized materials at T < 150 °C, the kinetics of solid-state diffusion and phase nucleation and growth are very slow.

• The behavior of nanoscale systems is being explored in order to address this kinetic obstacle.

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