NANOSTRUCTURED AND NANOCOMPOSITE LIGHT-METAL BASED COMPOUNDS FOR HYDROGEN STORAGE

R.A. Varin1), L.Guo1), S. Li1*), Ch. Chiu1), A. Calka2)

1)Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

*) On leave of absence from Powder Metallurgy Research Academy, Central South University, Changsha, P.R. China

2) Department of Materials Science and Engineering, University of Wollongong, Wollongong, NSW 2522, Australia

NATO ARW - METALLIC MATERIALS WITH HIGH STRUCTURAL EFFICIENCYKyiv, Ukraine, 7-13 September 2003

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HYDROGEN – THE FUEL OF THE FUTURE

With the concerns of the global climate warming it is absolutely indispensable for the mankind to develop new clean energy sources other than fossil fuels. The consensus of opinion is setting on hydrogen for either supplying fuel cells or internal combustion engines as the way forward. According to many in the scientific community we are now at the verge of a new hydrogen age. Extensive research efforts are lying down the foundation of the next industrial revolution in the application of hydrogen as the fuel of the future.

HYDROGEN STORAGE FOR PROTON MEMBRANE EXCHANGE (PEM) FUEL CELLS FOR VEHICULAR (MOBILE) APPLICATIONS

Excerpts from Ritter et al, Materials Today, September 2003, pp.18-23

“In recent years, months, weeks, and even days, it has become increasingly clear that hydrogen as an energy carrier is “in “ and carbonaceous fuels are “out”. The hydrogen economy is coming with the impetus to transform our fossil energy-based society, which inevitably will cease to exist, into a renewable energy-based one.

However, hydrogen storage is proving to be one of the most important issues and potentially biggest roadblock for the implementation of a hydrogen economy. Of the three options that exist for storing hydrogen, in a solid, liquid and gaseous state, the former is becoming accepted as the only method potentially able to meet the gravimetric and volumetric densities of the recently announced FreedomCar goals; and of all known hydrogen storage materials, complex hydrides may be the only hope”.

HYDROGEN STORAGE FOR PEM FUEL CELL-POWERED VEHICLES

None~80-150 Solid metal/intermetallic hydrides

Large thermal losses (open system!)

~71 Liquid hydrogen at cryogenic tank at -2520C (21K):

Safety problems (enormous pressures required)

~40 Compressed hydrogen gas under 80 MPa pressure

DrawbacksVolumetric density (kgH2 m-3)

Storage system

The highest volumetric density required

HIGH VOLUMETRIC HYDROGEN DENSITY FOR VEHICLES

~4 kg of hydrogen range ~480 km (300 miles)

Volume of 4 kg of hydrogen compacted in different ways, with size relative to the size of a car

Toyota press, 33rd Tokyo Motor Show, 1999; L.Schlapbach and A. Zűttel, Nature,414, 353-358 (2001)

HYDROGEN FOR PEM FUEL CELLS-Gravimetric density

The highest gravimetric density: light metal-based hydrides

350 (?)2.704.5Mg2CoH5Mg-Co-H

2802.305.2Mg3MnH7Mg-Mn-H

3202.725.4Mg2FeH6Mg-Fe-H

4001.0710.6NaBH4Na-B-H

300-800 (?)0.9915.3Mg(BH4)2 or MgB2H8

Mg-B-H

3800.6718.4LiBH4Li-B-H

Decomposition temperature (0C)

Density of hydride(g/cm3)

Theoretical hydrogen

capacity (wt%)

HydrideMetal-hydrogen

system

HYDROGEN FOR PEM FUEL CELLS-Requirements for metal/intermetallic hydrides

• World Energy Network (Japan):

Hydrogen capacity > 3wt%; desorption temp. ~1000C;5000 cycles life

• Department of Energy (USA):

Hydrogen capacity > 6wt%

• International Energy Agency:

Hydrogen capacity > 5wt%; desorption temp. < 1500C; 1000 cycles life

HYDROGEN FOR PEM FUEL CELLS-Metal/intermetallic hydrides-Conclusions

• All light metal-based hydrides have excellent hydrogen storage capacities sometimes exceeding those required by various agencies for vehicular applications• All of them have a fatal drawback: too high desorption temperature!

• Their desorption kinetics are slow for polycrystalline alloys

HOW CAN WE IMPROVE KINETICS AND DESORPTION TEMPERATURE ?!

BEHAVIOR OF NANOSTRUCTURED/NANOCOMPOSITE

HYDRIDESZaluska et al., Appl. Phys. A 72 (2001) 157-165 (review paper)

Absorption kinetics

poly

nano

Desorption temperature

DSC

METHODS OF SYNTHESIS OF NANOSTRUCTURED/NANOCOMPOSITE

HYDRIDESDefinition: Nanostructured/nanocomposite means that each phase present in the individual powder particle is in the form of grains with nanometer size; one particle is one nano-polycrystal

1.Two-step: mechanical alloying (MA) of elemental metal powders or milling (MM) of bulk alloys under protective gas (Ar, He) ; subsequent hydrogenation in a separate step under appropriate pressure of H2

2.One-step: mechanical alloying/milling of elemental metal powders/bulk alloys directly under hydrogen – Reactive Mechanical Alloying/Milling (RMA/RMM) – cost reduction and ease of hydride formation - preferable

METHODS OF SYNTHESIS OF NANOSTRUCTURED/NANOCOMPOSITE

HYDRIDES-contCommon milling techniques

Drawback: completely uncontrolled (chaotic) movement of grinding balls

CONTROLLED REACTIVE MECHANICAL ALLOYING/MILLING (CRMA/MM)

Magneto-mill Uni-Ball-Mill 5 for controlled milling - trajectories of milling balls are controlled by strong NdFeB magnetsCourtesy of A.O.C. Scientific Engineering, Australiaa=WD=working distance

Mg-M-H SYSTEMS SELECTED FOR SYNTHESIS BY CRMA

Mg(BH4)2 or MgB2H8Mg-2B (crystalline)(c)-H

Mg-2B (amorphous)(a)-H2Mg-Co-H Mg2CoH5

3Mg-Mn-H Mg3MnH7

2Mg-Fe-H Mg2FeH6

Complex metal hydrides: mixed ionic-covalent bonding between metal and hydrogen complex, e.g. (FeH6)4-

EXPERIMENTAL OUTLINE-Milling

1.Elemental powders of Mg, B (cryst&amorph.), Co, Mn and Fe.

3. Handling of powders in the glove bag filled with helium for environmental protection.

4. Milling in the magneto-mill Uni-Ball-Mill 5; ball-to-powder weight ratio (BPWR) was 10:1 for the 2Mg-Co and 3Mg-Mnmixtures and ~40:1 for the other mixtures.

3.Hydrogen pressure in the milling vial 400-500 kPa

5. Working distance WD= 10 to 3 mm depending on the specific alloy; it governs the force of the magnetic attraction exerted onto the steel balls.

EXPERIMENTAL OUTLINE-Microstructural and thermal studies

• High-resolution field emission SEM (FE SEM) LEO 1530 with integrated EDAX Pegasus 1200

•X-ray diffraction (XRD) using Philips PW 1730 and Siemens D500 diffractometers; CuKα radiation (λ=0.15418 nm)

• Differential scanning calorimetry (DSC) (Netzsch 404); heating rate 4 K/min; argon flow rate 16ml/min

• Thermogravimetric analysis (TGA)(TA Instruments); heating rate 10 K/min; helium flow

EXPERIMENTAL OUTLINE-Nanograin size calculations

22

2

16sintan

)2(tan

)2( eLK

+

=

θθθδλ

θθδ

From XRD peak broadening using linear regression procedure (Klug&Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, John Wiley & Sons, New York (1974).

L-nanograin (crystallite) size; e-lattice strain; λ-the wave length; θ-position of the analyzed XRD peak maximum; K-constant; δ(2θ)=B(1-b2/B2)(rad)-the instrumental broadening-corrected “pure” XRD peak profile breadth; B and b-FWHM (full width at half maximum) of analyzed and reference peak, respectively

RESULTS-Microstructure of powders

b) a)

Co Mg

Backscattered electron (BSE) images of the morphology of powders processed under shearing mode by CRMA under hydrogen. a) 2Mg-Co mixture milled for 30h using WD=10 mm and BPWR=10:1 and b) Mg-2B (crystalline (c) boron) mixture milled for 5h using WD=5 mm and BPWR=44:1. RPM=60 applied during milling.

RESULTS – XRD patterns vs. milling time

Mg

β-MgH2

Co2Mg-Co

100+50h

100+20h

100h

50h

10h

Inte

nsity

(arb

. uni

t)

Typical for

2Mg-Co

Mg-2B

3Mg-Mn

systems

RESULTS- XRD intensities vs. milling time

0

2000

4000

6000

8000

10000

12000

0 20 40 60 80 100 120 140 160Milling tim e (h)

Inte

rget

ed in

tens

ity (a

rb. u

nits

)

WD=10mmWD=3mm

(101) Mg

3Mg-Mn

0

1000

2000

3000

4000

5000

6000

7000

8000

0 20 40 60 80 100 120 140 160M illing tim e (h )

Inte

gret

ed in

tens

ity (a

rb. u

nits

)

WD=10mmWD=3mm

(110) β -M gH2

3M g-M n

0

50 0 0

10 0 0 0

150 0 0

2 0 0 0 0

2 50 0 0

3 0 0 0 0

3 50 0 0

0 2 0 4 0 6 0 8 0 10 0 12 0 14 0 16 0

M i l l i n g t i m e ( h )

W D=10 mmW D=3 mm

(101)M g2M g-Co

0

10 0 0

2 0 0 0

3 0 0 0

4 0 0 0

50 0 0

6 0 0 0

70 0 0

0 2 0 4 0 6 0 8 0 10 0 12 0 14 0 16 0

M i l l i n g t i m e ( h )

W D =10 mmW D =3 mm

(110)β -M gH2

2M g-CoMg

consumed almost

completely to form

β-MgH2

in

2Mg-Co3Mg-Mn

RESULTS-XRD intensities vs. milling time (cont.)• Mg-2B(a)-complete consumption of Mg to form β-MgH2 (at 50h)

0

0.2

0.4

0.6

0.8

1

1.2

0 25 50 75 100

Milling time (h)

Nor

mal

ized

inte

grat

ed in

tens

ity I/

I0

0

0.5

1

1.5

2

2.5

3

(101) Mg

(110) β− MgH 2

Mg-2B(a)

• Mg-2B(c)-incomplete consumption of Mg to form β-MgH2

• Partial amorphization (?) of β-MgH2after 50 and 20h of CRMA, respectively

100

1000

10000

100000

0 10 20 30 40 50 60 70

Milling time (h)

Inte

grat

ed in

tens

ity (a

rb.u

nits

)

(101)MgMg-2B(c)

0

500

1000

1500

2000

2500

3000

3500

0 10 20 30 40 50 60 70

Milling time (h)

Inte

grat

ed in

tens

ity (a

rb.u

nits

)

(110)β-MgH2

(101)β -MgH2

Mg-2B(c)

RESULTS-Nanostructured β-MgH2 hydride

Principal hydride in 2Mg-Co,3Mg-Mn and Mg-2B is nano-β-MgH2

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160Milling time (h)

Nan

ogra

in s

ize

(nm

)

WD=10mmWD=3mm

β-MgH2

3Mg-Mn

d)

1012

162122

25

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160

Milling time (h)

Nan

ogra

in s

ize

(nm

)

WD=10mmWD=3mm

β-MgH2

2Mg-Co

c)

10

14

24

28

1819

0

10

20

30

40

0 10 20 30 40 50 60 70

Milling time (h)

Nan

ogra

in s

ize

(nm

) B3HB4Hβ -MgH 2

38

14 15 125 8

a)

WD=8mm(B3H)5mm (B4H)

Mg-2B(c)

2522

9 75 5

89

0246810121416182022242628

0 50 100 150 200 250 300

Milling time (h)

Nan

ogra

in s

ize

(nm

) β -MgH 2 B5H

b)

Mg-2B(a)

WD=3mm

RESULTS-Nanostructured Mg

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160Milling time (h)

Nan

ogra

in s

ize

(nm

)

WD=10mmWD=3mm

3Mg-Mn

Mg

d)

Nanocomposite Mg+β-MgH2+ [Mg(BH4)2?]up to 50-60h of milling

Nanocomposite Mg+β-MgH2 + (remnant Co; Mn) up to 120h of milling

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70

Milling time (h)

Nan

ogra

in s

ize

(nm

) B3HB4HMg

Mg-2B(c)

a)

WD=8mm (B3H) 5mm(B4H) 0

10

20

30

40

50

60

0 10 20 30 40 50 60 70

Milling time (h)

Nan

ogra

in s

ize

(nm

) MgMg-2B(a)

WD=3mm

b)

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160

M illing tim e (h)

Nan

ogra

in s

ize

(nm

)

WD=10mmWD=3mm

M g2M g-Co

c)

RESULTS-Nanostructured Mg(BH4)2 in Mg-2B(c)system

shows the exact peak position for Mg(BH4)2according to JCPDS Powder Diffraction File No.26-1212

[similar peaks but weaker also in XRD from Mg-2B(a)]

Inte

nsity

(arb

. uni

t)

Mg β-MgH2

Mg(BH4)2

59h

40h

20h

9.5h

Mg-2B(c)

RESULTS - Nanostructured Mg(BH4)2 –XRD after thermal analysis

Thermally stable hydride?

Mg-2B(c)

DSC 5500C

4000C/1h/Ar

59h

Inte

nsity

(arb

. uni

t)

β-MgH2

Mg(BH4)2

Mg

RESULTS - Amorphization in the 2Mg-Fe-Hsystem - XRD pattern

30 40 50 60 70 80 90D egrees 2θ

M gM gH 2 Fe

5h

10h

15h 20h

40h

59h

Inte

nsity

(arb

. uni

t)

2M g-Fe

RESULTS - Amorphization in the 2Mg-Fe-H system - XRD intensities

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60 70

Milling time (h)

Inte

grat

ed in

tens

ity (a

rb.u

nit)

(110)β-MgH2

(101)β-MgH2

2Mg-Fe

1

10

100

1000

10000

100000

0 10 20 30 40 50 60 70

Milling time (h)

Inte

grat

ed in

tens

ity (a

rb.u

nit)

(101)Mg2Mg-Fe

Amorphization of Mg and β-MgH2!

RESULTS - Amorphization in the 2Mg-Fe-H system – Qualitative EDS

2Mg-Fe 20h

a)

2Mg-Fe 40h

b)

Mg peak clearly seen in EDS profile but absent in XRD:

Mg exists in theamorphous state

RESULTS - Amorphization in the 2Mg-Fe-H system - Nanograin size/nanocomposite

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40 50 60 70

Milling time (h)

Nan

ogra

in s

ize

(nm

)

FeMgMgH2

2Mg-Fe

Amorphization of Mg and β-MgH2

Oleszak&Shingu,Mater.Sci.Forum 235-238

(1997) 91-96

Critical average nanograin size

favorable to the formation of

amorphous phase is on the order of ~10 nm

RESULTS - Amorphous hydrides in the 2Mg-Fe-Hsystem – Thermal behavior-TGA

-7

-6

-5

-4

-3

-2

-1

0

0 100 200 300 400 500 600

Temperature (0C)

Wei

ght l

oss

(%)

2

1

2Mg-Fe 59h

-7

-6

-5

-4

-3

-2

-1

0

0 100 200 300 400 500 600

Temperature (0C)W

eigh

t los

s (%

)

1

2

59h

2Mg-Fe

Desorption from amorphous hydrides: range 2.21-4.16wt% between tangent lines 1 and 2 from all TGA runs

RESULTS - Amorphous hydrides in the 2Mg-Fe-Hsystem – Thermal behavior-DSC

2Mg-Fe 59h

endothermic

Peak temperature range: 320.7 -329.20C – agrees well with TGA

RESULTS - Amorphous hydride in the 2Mg-Fe-Hsystem – stoichiometric formula

Based on the results of desorbed hydrogen observed in TGA runs which was in the range 2.21-4.16wt% the stoichiometric formula of the amorphous hydride can be estimated as MgH0.6-1.1. The hydrogen-to-metal ratio in this formula is nearly 1 which implies that the amorphous hydride has a metallic character. This is in excellent agreement with Orimo et al [Acta mater. 45 (1997) 2271-2278] who reported that amorphous hydrides in various systems have the hydrogen-to-metal ratio ~1 and metallic character.

SUMMARY/CONCLUSIONS

1. A principal nanostructured hydride formed in2Mg-Co, 3Mg-Mn and Mg-2B mixtures is β-MgH2; no Mg2CoH5 and Mg3MnH7 complex hydrides have been formed during CRMA under shearing mode despite a profound nanostructurization of elemental species in the mixture (Question: why no formation of complex hydrides has occurred?)

2. XRD peaks close to the peaks from Mg(BH4)2 are observed on the scans from the Mg-2B(crystalline)mixture and on the scans (but weaker) from the Mg-2B(amorphous) mixture

SUMMARY/CONCLUSIONS (cont.)

3. In the 2Mg-Fe mixture there is initially a gradual nanostructurization of the Mg and β-MgH2 phases followed by amorphization of both phases with increasing milling time. Eventually the amorphous hydride, possibly with the stoichiometric formula MgH0.6-1.1, is being formed; no formation of complex hydride Mg2FeH6 is observed

(Question: why no formation of Mg2FeH6 has occurred (successfully synthesized by some other researchers) and instead an amorphous hydride has been formed ?

ACKNOWLEDGEMENT

This work was supported by a grant from the Natural Sciences and Engineering Research

Council of Canada which is gratefully acknowledged