1 hydrogen storage with carbon nanotubes andrew musser

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Hydrogen Storage with Carbon Nanotubes

Andrew Musser

"Energy & Nano" - Top Master in Nanoscience Symposium 17 June 2009

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Outline

The hydrogen economy

Storage options

What are carbon nanotubes?

Promising initial results

Simulations of storage

Recent experimental results

Prospects


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The Hydrogen Economy

Most abundant element on Earth, almost entirely within water

Production of hydrogen: break down hydrocarbons or water

Efficient consumption: fuel cells


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The Storage Problem

Highest chemical energy mass density

of any chemical fuel: 142 MJ/kg

4 kg of H2 compared to standard vehicle size

US Dept. of Energy baselines for lightweight, energy-efficient

storage:

6.0 wt% and 0.20-0.70 eV/H2 binding energy by 2010

9.0 wt% by 2015

Extremely poor

volumetric mass

density


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Storage Options

One of the most promising to date: Carbon NanotubesUS Dept. of Energy, www.eere.energy.gov

carbon nanotubes

metal hydrides


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(0,n) zig-zag

(n,n) armchair

What are Carbon Nanotubes?

Single-walled nanotubes

(SWNT): rolling graphene

Multi-walled nanotubes

(MWNT): concentric SWNTs


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Chirality (n,m)

Physical and electronic properties vary widely with the vectors that determine rolling

n

m


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Why CNTs?

Stable, lightweight, inexpensive

Large active surface area

Large internal volume if it can be accessed

+


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How Are They Produced?

Decomposition of hydrocarbons

soots

Arc discharge soots and fibers

Laser ablation catalytic control

of nanotube type

Chemical vapor deposition

catalytic control of CNT diameter

Consistency between batches

can be problematicLiu et al., Science 1999


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How Are They Purified?

Removal of catalyst particles and hydrocarbon contaminants acid treatment and UHV baking Opens tube ends, acid damage

to side walls

Limited ability to separate CNTs by diameter and/or chirality Needed for future applications

Breakdown of fibers and bundles into individual CNTs surfactants and intense sonication


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First and simplest approach: Physisorption

Van der Waals interaction between H2 and CNT wall

Internal or external

No energy barrier to overcome, but relatively weak binding low temperatures

Negligible effect on CNT electronic and physical structure

How Can They Store Hydrogen?


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Dillon et al., Nature 1997

A Remarkable Capacity?The 1st Observation of H2 Storage in CNTs (1997)

Arc discharge soots containing 0.1-0.2% narrow SWNT bundles

Low H2 pressure at low T

Mass spectrometry of desorbed gases upon reheating in UHV

Total soot storage capacity: 0.01 wt%, attributed to SWNTs

Unclear where in SWNT H2 is

stored

Extrapolated pure SWNT capacity: 5-10 wt%

Markedly lower capacity found in later studies


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Nanotube DopingImproved Capacity with Alkali Metals (1999)

Large MWNTs produced by catalytic decomposition of hydrocarbons, purified to 90%

Tubes doped with Li or K via solid-state reactions Alkali to carbon ratio: 1/15

Weight changes monitored during heating and cooling cycles in pure H2 stream at ambient pressure

Chen et al., Science 1999

3.2

3.1

3.0

2.9

2.8

5.1

4.9

4.7

4.5

Li-doped

K-doped

Sam

ple

Wei

ght

(mg

)

Temperature (K)

+15%

+14%

270 370 470 570 670 770 870

270 370 470 570 670 770 870


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Nanotube DopingImproved Capacity with Alkali Metals (1999)

Li-doped: peak adsorption of 15-20 wt% at 673 K stable in ambient conditions

K-doped: peak adsorption of 14 wt% at 298 K highly unstable in ambient conditions

Storage attributed to tube exterior surface

Later studies suggested hydroxide and water formation

3.2

3.1

3.0

2.9

2.8

5.1

4.9

4.7

4.5

Li-doped

K-doped

Sam

ple

Wei

ght

(mg

)

Temperature (K)

+15%

+14%

270 370 470 570 670 770 870

270 370 470 570 670 770 870

Chen et al., Science 1999


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Liu et al., Science 1999

Room-Temperature CapacityStorage with Bare SWNTs of Higher Purity (1999)

Arc discharge SWNT fibers of 50-60% purity in large scale

Relatively large SWNTs

High H2 pressure at ambient temperature

Weight changes monitored

Impure capacity of 4.2 wt%

Storage attributed to tube surface and curvature

Markedly lower capacity found in later studies


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A Theoretical Reexamination

Early experiments too variable and sample dependent new focus on calculations and MD simulations

More reactive species on CNT surface could physisorb and hold H2 more strongly, as in Chen et al.

Affinity of bare CNTs for H2 is too weak for RT storage


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Durgun et al., Phys Rev B 2008

Storage Inside and Outside Simulated functionalization

with light transition metals Sc, Ti and V on slightly larger SWNTs Sufficient interior space allows

functionalization of inner surface

Each metal atom, inside or outside, can physisorb up to 4 H2

At high coverage ~8 wt% storage should be possible with excellent binding energy

Trade-off: H2 binding energy versus clustering

Ti


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Raising the Affinity of Carbon

Liu et al., J Phys Chem C 2009

Problem with transition metals is material self-weight significantly heavier than carbon

Simulation of medium-sized SWNTs with Li adsorbates Stable against clustering

Charge transfer from Li activates carbon atoms the entire SWNT can physisorb H

2

At moderate Li coverage, 13.45 wt% storage capacity and binding energy close to benchmarks

Li


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H

A New Approach: Chemisorption Simulated systems difficult to achieve in practice: chirality

selection, clustering and controlled functionalization

Bare SWNT simulations find chemisorption more favorable

A fully hydrogenated SWNT could store 7.8 wt% hydrogen

Stability of hydrogenated SWNTs increases with diameter

Large kinetic barrier to chemisorbtion: dissociation of H2

Nikitin et al., Nano Lett 2008


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Avoiding the Dissociation BarrierHydrogen Storage in C–H Bonds (2008)

Hydrogen chemisorption studied on 2 types of high-purity CVD films of SWNT

Mean CNT diameters of 16Ǻ and 20Ǻ determined by AFM

To avoid dissociation barrier, charged films with beam of atomic H H2 cracked by W catalyst at high

temperature

16Ǻ

20Ǻ


500 1000 1500Frequency (cm-1)

Inte

nsity

(ar

b.)

diameter (nm)

diameter (nm)


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Avoiding the Dissociation BarrierHydrogen Storage in C–H Bonds (2008)

C-H bond formation monitored by in situ XPS

Small-diameter film degrades above 30% hydrogenation

Large-diameter film stable up to ~100% hydrogenation

C=C bonds

C-H bonds

Binding Energy

C=C bonds

C-H bonds

Degradation

Binding Energy

16Ǻ 20Ǻ

~7.0 wt% storage capacity, almost entirely on bundle surface

2/3 of H2 recovered at 200-300 C



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Summary

Early studies yielded promising but extremely controversial results Problems of inconsistent production, purification and characterization

Subsequent simulations suggest promise of physisorption on functionalized nanotubes Offers possibility of utilizing interior space of CNTs Systems difficult to synthesize

Chemisorption of atomic H can be thermodynamically favorable Significant kinetic barrier of hydrogen dissociation must be overcome High storage capacity through chemisorption shown to be feasible

with some SWNTs


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Prospects for CNT storage Synthesis of functionalized

SWNT systems to investigate the feasibility of storage through physisorption

Investigation of catalytic “spillover” mechanisms for a practical source of atomic hydrogen for chemisorption

Parallel studies with other carbon nanomaterials


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Thank you for your attention

Questions?

I would like to acknowledge Dr. Maria Loi for her guidance in reviewing the literature and preparing this presentation.

1 hydrogen storage with carbon nanotubes andrew musser

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