Hydrogen Storage for Transportation Applications

John J. Vajo, Ping Liu, Adam F. Gross, John J. Vajo, Ping Liu, Adam F. Gross, Sky L. Van Atta, Tina T. Salguero, Wen Li, Robert E. DotySky L. Van Atta, Tina T. Salguero, Wen Li, Robert E. Doty

HRL Laboratories, LLC HRL Laboratories, LLC Malibu, CA Malibu, CA

© 2008 HRL Laboratories, LLC. All Rights Reserved

2

OutlineOutline

• Introduction to PEM fuel cells and hydrogen storage needs

• Overview of hydrogen storage approaches

• Solid state methods - advantages and challenges

• Destabilized hydrides (addresses “thermodynamics challenge”)

• Nanoengineering (addresses “kinetics challenge”)

• Summary

3Source: U.S. DOE Energy Efficiency and Renewable Energy Office

Proton Exchange Membrane Fuel Cell

Solid polymer electrolyte sandwiched between two porous carbon electrodes containing catalyst

• H2 gas flows to anode– dissociates into protons and electrons

• Membrane only allows protons to pass

• Electrons follow external circuit to the cathode (e.g., powers motor)

• Electrons combine with oxygen from air and protons to form water (exhaust)

Each cell produces < 1 V cells stacked in series to produce usable amounts of electrical energy

Hydrogen must be available in quantities sufficient for fuel cell

operation

4

Requirements for Hydrogen Storage Material System

• High storage capacity2010 targets:

System weight: >6 % hydrogen; System volume: >45 g/L hydrogen

• Low energy investment to store and remove hydrogenTemperature for H2 release from storage material must be compatible with fuel cell operation (~80°C)

• Fast release and refueling times< 5 min refill time; H2 supply to fuel cell must not be limited by H2 release rate from hydride

• Material cost consistent with low overall storage system cost 2010 target: $133/kg-H2; 2015 target: $67/kg-H2

• Durability (to maintain 80% capacity):

240,000 km

5

Hydrogen Storage Options

REVERSIBLE

CRYO-ADSORPTION

LIQUID HYDROGEN

COMPRESSEDGAS

PHYSICAL STORAGE Molecular

REVERSIBLE

CHEMICAL STORAGE Dissociated

COMPLEX METALHYDRIDES

CONVENTIONALMETAL HYDRIDES

LIGHT ELEMENTSYSTEMS

NON-REVERSIBLE

NANO STRUCTURE ADSORPTION

DESTABILIZED LIGHT ELEMENT SYSTEMS

• Carbon

• Metal Organic Frameworks

• La Ni5

• Ti Fe

• LiAlH4

• NaAlH4

• LiBH4

• Mg(BH4)2

• MgH2

• Mg Alloys

• LiH + Si

• MgH2 + Al

• LiBH4 + MgH2

DECOMPOSED FUEL

HYDROLYZED FUEL

REFORMED FUEL

6

0

50

100

150

200

250

Gasoline LiBH4 LaNi5H6.5 Liquid-H2 700 bar-H2

Sto

rag

e M

ater

ial

Vo

lum

e (L

iter

s)Volume of 8 kg Hydrogen in Different Storage Media

(Compared with Gasoline)

8 kg hydrogen 300 mi range in GM Sequel

(Assumes ICE 2x less efficient than fuel cell)

7

0

50

100

150

200

250

Gasoline LiBH4 LaNi5H6.5 Liquid-H2 700 bar-H2

(Assumes ICE 2x less efficient than fuel cell)

Total Hydride Material Weight:59 kg 570 kg

Too Heavy

Research

Underway

8 kg hydrogen 300 mi range in Sequel

Volume of 8 kg Hydrogen in Different Storage Media (Compared with Gasoline)

Sto

rag

e M

ater

ial

Vo

lum

e (L

iter

s)

8

Recycle

To satisfy requirements, materials composed of light metal elements are needed

Energy to remove hydrogen (high heat)

HydrogenMaterial

withno hydrogen

Material hydride with

hydrogen stored

Material with

no hydrogen

HydrogenReleased

Solid State Hydrogen Storage Process

9

Potential for high weight (> 6 wt.%) hydrogen storage Enables 400 km driving range

Light Metal Hydrides are Promising Candidates for On-Board H-Storage

10

• Strong covalent/ionic chemical bonds in hydride High temperatures (>200°C) needed for hydrogen release

thermodynamics challenge

• Bonding is highly directional Large barriers for atomic diffusion Leads to prohibitively slow reaction rates (slow hydrogen

uptake and release)

kinetics challenge

These are the principal issues being addressed in the HRL hydrogen storage program

… But Challenges Exist

11

14

12

10

8

6

4

2

0

H2 C

apacit

y (

wt.

% -

mate

rial b

asis

)Comparison Of Selected Hydrides with

DOE System Requirements

LiH

ZrNiH3

Mg2NiH4

LiBH4

VH2

MgH2

DOE 2010 System Target

30% system penalty

0% system penalty

NaAlH4

ZrMn2H3.6

20100200300400500

Temperature (°C)

LaNi5H6.5

• Existing hydrides do not meet DOE requirements

• Need either new material or method for altering existing hydrides

Conventional (transition-metal) hydrides

Light-metal hydrides

12

Strong Bonds in Light Metal Hydrides– Bond breaking (H2 release) requires high temperature –

M

MM

M

MM

M

M

M

H H+

M

MM

M

H H

H

H H

Metal Hydride (MH)

Metal (M)

Hydrogen Gas

High Temperature

Conventional hydrides

EN

ER

GY

(H

eat)

MH

M + H2

Dehydrogenated State

Hydrogenated State

High energypath

13

Hydride “Destabilization” by Alloy Formation Reduces Temperature for H2 Release

M

MM

M

H H

H

H H

A

A

A

A

+

M AA

MM A

M AA

H H+

Metal HydrideDestabilizing

Agent

Alloy

Hydrogen Gas

Reduced Temperature

Destabilized hydrides

EN

ER

GY

MH + xA

M + H2

MAx+ H2

Dehydrogenated State

Alloy State

Hydrogenated State

Lower energypath

• Alloy gives tightly bound metal hydride a lower energy path to release H2

• Reduced energy demand means lower temperature for hydrogen release

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LiH + B + H2

LiH + MgB2 + H2

EN

ER

GY

LiBH4 + MgH2

T=225°C

T=400°C

Ref: J. J. Vajo, S. L. Skeith, F. Mertens “Reversible Storage of Hydrogen in Destabilized LiBH4”, J. Phys. Chem. B, vol. 109 (2005) pp. 3719-3722.

2LiBH4 + MgH2 2LiH + MgB2 + 4H2Lithium

borohydrideMagnesium

hydrideLithium hydride

Magnesiumboride

Hydrogen

(System with very high storage capacity (11.4 wt.%, 95 g/L)

• System has been tested: 10 wt.% capacity demonstrated

• Temperature for H2 release lowered 175°C by alloying with MgH2

LiBH4/MgH2 Destabilized System

– a promising candidate –

15

14

12

10

8

6

4

2

0

H2 C

apacit

y (

wt.

% -

mate

rial b

asis

)Destabilization of LiBH4 by Alloying with

MgH2 Reduces Temperature

LiH

ZrNiH3

Mg2NiH4

LiBH4

VH2

MgH2

DOE 2010 System Target

30% system penalty

0% system penalty

NaAlH4

ZrMn2H3.6

20100200300400500

Temperature (°C)

LaNi5H6.5

Significant reduction in H2 release temperature with only small decrease in capacity (13.6 wt.%11.4 wt.%)

LiBH4/MgH2

Conventional (transition-metal) hydrides

Light-metal hydrides

Destabilized light-metal hydride

16

14

12

10

8

6

4

2

0

H2 C

apacit

y (

wt.

% -

mate

rial basis

)

LiH

ZrNiH3

Mg2NiH4

LiBH4

VH2

MgH2

DOE 2010 System Target

30% system penalty

0% system penalty

NaAlH4

ZrMn2H3.6

20100200300400500

Temperature (°C)

Summary of Destabilized Systems and Comparison with Known Hydrides

• Hydride destabilization is a versatile approach for reducing temperature

• However; reaction rates are much too slow for practical use

LaNi5H6.5

Calculated

Demonstrated

Conventional (transition-metal) hydrides

Light-metal hydrides

Destabilized light-metal hydrides

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<100 nm

Long diffusion distances in bulk material:

slow H-exchange rate

Enhanced Reaction Rates Using Nano-engineering

Increase Hydrogen exchange rate by decreasing particle size

Short diffusion distances in nanoparticles:

fast hydrogen exchange rate

Issues:Need efficient, low cost method for producing nanoparticles

Sintering during hydrogen uptake and release can increase particle size – could be a big problem

Bulk Alloy Material Nanoparticles

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• Inter-penetrating network of carbon nanopores (10-30 nm pore size)

• “Scaffold” serves as structure-directing agent for forming nano-scale hydrides

Carbon Aerogels

Carbon Aerogel “Scaffold” Hosts for Nanoscale Hydrides

C-aerogel cubes

Mix aerogel and LiBH4 under N2

Melt LiBH4

(T=290 °C)Aerogel

absorbs LiBH4

Scrape to remove surface material

Incorporate molten LiBH4 into aerogel by “wicking” process

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Faster Hydrogen Release from LiBH4 in Nanoporous Carbon Scaffold

8

6

4

2

0

Des

orb

ed H

2

(wt

% L

iBH

4)

1.00.80.60.40.20.0

Time (hr)

LiBH4 LiH + B + 1.5H2(13.6 wt %) Pore size distributions

13 nm

25 nm

Graphite

• Rate for 13 nm aerogel ~60X rate for control sample• Rate faster for smaller pore aerogel

0.10

0.08

0.06

0.04

0.02

0.00P

ore

Volu

me

(cm3/g

-nm

)

35302520151050

Pore Size (nm)

13 nm

25 nm300 °C

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Summary

• Hydrogen storage – a key hurdle in creating a hydrogen–based transportation system

• Sufficient hydrogen can be stored on a vehicle to meet customer desires for range by either:

Changing the vehicle architecture to allow more room for fuel storage Improving the capacity of the storage system

• Light-metal hydrides are promising candidates for high capacity, on-board storage of hydrogen, but no existing material meets targets

High temperatures needed for hydrogen release Release/uptake rates slow

• Hydride destabilization being used to address the high temperature problem

• Nano-engineering approaches are providing solutions to slow release/uptake

Research efforts in these critical technology areas are on-going at HRL Labs in two projects sponsored by GM and U.S. DOE