1

Destabilization of Mg-based hydrogen storage materials: literature review

(Dr. Liudas Pranevicius)

2

Outline of the presentationOutline of the presentation

1.1. MgH2 as hydrogen storage material.MgH2 as hydrogen storage material.

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

3.3. Oxidation/catalysis problems.Oxidation/catalysis problems.

4.4. Summary and OutlookSummary and Outlook..

3

DOE FreedomCar Technical TargetsOn-Board Hydrogen Storage System(300 mile range - 5.6 kg H2)

Property Units Target

Hydrogen Density (gravimetric) wt.% H 6Energy Efficiency % 97Energy Density (volumetric) W-h/L 1100Hydrogen Density (volumetric) kg H2/m3 33Specific Energy W-h/kg 2000Cost $/kW-h 5

($/kg H2) (167)Operating Temperature ˚C -40 - +50Start-Up Time to Full Flow sec 15Hydrogen Loss scc/hr/L 1.0Cycle Life cycles 500Refueling Time min <5Recoverable Usable Amount % 90

US DOE

4

Formation of MgH2 : 7.6 % wt H by weight - quite high

Problems: - The hydrogen is too strongly bound (hydrogen gas at 1 bar is released at around 350 oC). -Binding energy of 0.38 eV per H-atom, should be 0.20 eV for the gas to be released at 100 oC.

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

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

5

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

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

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

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

6

Mg7TiHx Kyoi et al. (including Toyoto and Dag)Kyoi et al. (including Toyoto and Dag) 2001 2001

Desorption ReactionMg7TiH16 -> 7Mg+ TiH2 +7H2 @605 K

TiH2 -> Ti + H2 @825 K

Experimental lattice const: 9.532(2) Å

weight load: 5.5%

4a–›

4b–›

24d––›

7

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

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

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

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

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

8

MgMg-Ti-Ti ( (P. Vermeulen et al. P. Vermeulen et al. 20020066))

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

9

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

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

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

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

10

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

Substrate

Mg (Ti, Ni, V)

Ti or Ni or V oxide

Ti and V are much better catalyst for H2 dissociation than Ni.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 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.after deposition – formation of Ti or Ni or V oxide observed.

11

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

Substrate

Mg -Ti

TiO2/TiH2

Mechanical millingMechanical milling of MgH2 with Ti and V leads to titanium and of MgH2 with Ti and V leads to titanium and vanadiumvanadium hydrides, which could protect Ti and V from oxidation, andhydrides, which could protect Ti and V from oxidation, and therefore the catalytic effect towards hydrogen is preservedtherefore the catalytic effect towards hydrogen is preserved

12

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

Substrate

Mg -Ti

Ni – NiO - Ni

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

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

13

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.