NanoEngineering of Hybrid Carbon Nanotube Metal Composite Materials for

Hydrogen StorageAnders Nilsson

Stanford Synchrotron Radiation Laboratory (SSRL) and Stockholm University

Coworkers and Ackowledgement

B. Clemens2, H. Dai3, A. Nikitin1, H. Ogasawara1, D. Mann3, Z. Zhang4, X. Li3, KJ Cho4,, D. Friebel1, S. Rajasekaran1, R. Bhowmick2, S. Beasley2,

Y. Lee2

1) Stanford Synchrotron Radiation Laboratory2) Department of Material Science and Engineering3) Department of Chemistry, Stanford University4) Department of Mechanical Engineering, Stanford University

Hydrogen Dream

PhotobiologicalH2 Production

Biomass Gas-Water Shift PV/PEC

H2 ConversionFuel Cells

Storage andTransport

Carbon nanotubes Metal Hydrides

Solar H2Production

PhotoelectrochemicalH2 Production

Current View of Hydrogen

Hydrogendistribution

system

Production(electrolysis)

Onboardstorage

(adsorption)H2

H2 2H

Hydrogentransfer system

to fuel cell

Use(fuel cell)

Vehicle

2H H2H2 H2 2H

2H H2

Current vision of the H as an energy carrier

Molecular hydrogen at all steps of energy transfer from the

source to the consumer

- numerous problems and engineering challenges to handle gaseous or liquid hydrogen at production, distribution and utilization steps;- serious energy overhead to convert molecular to atomic hydrogen and back

hydrogenation

H-H bond energy 4.5 eV

Chemisorption of H atoms on Carbon Nanotubes

Adsorption Energy Decreases with diameter

Idea: to store hydrogen in the chemisorbed form on the nanotube surface

Pack et al, 2003

Investigation strategy

Hydrogenation: in situ atomic hydrogen treatment

Samples: "as grown" CVD SWCN films

- elimination of H2 dissociation step from hydrogenation process

- well controlled environment (base pressure < 1 10-9 Torr)

- low defect / amorphous carbon concentration (small D to G band intensity ration)

- low concentration of contamination (in situ annealing up to 900 C)

Probing tools: X-ray photoelectron spectroscopy (XPS) and X-ray adsorption spectroscopy (XAS)

- XPS and XAS allow to observe the formation of C-H bonds through the modification of the carbon nanotube electronic structure around specific carbon atoms

Probing toolsX-ray photoelectron spectroscopy (XPS)

X-ray absorption spectroscopy (XAS)

C1s XPS spectra of n-octane and graphite (Weiss et al., 2003, Bennich et al., 1999)

Carbon K-edge XAS spectra of graphite and diamond (Garo et al., 2001)

Hydrogenation induced changes in Graphite and SWCN XPS spectra

chemical shift of C1s peak due to C-H bond formation

Clean

H-treated

Graphite

0.65 eV

Single wall Carbon Nanotube

Nikitin et.al Surf. Sci. 602, 2575 (2008) Nikitin et.al Phys. Rev. Lett. 95, 225507 (2005)

The hydrogenation degree determination from XPS spectra

Calculated C1s chemical shift values (MD and DFT)

Decomposition of C1s XPS for hydrogenated SWCN film

excited C atom

H atom

(a)(b-d)

HC C

H C C

(a) (10,0) 0.78 0.65(b) (12,0) 0.94 0.77(c) (15,0) 1.16 0.74(d) (22,0) 1.72 0.76

(n,m) D, nm Shift, eV

peak 1peak 2

Hydrogenation = Ipeak 2/(Ipeak 1+Ipeak 2)*100 at %

Nikitin et.al Phys. Rev. Lett. 95, 225507 (2005)

The influence of SWCN diameter on hydrogenation process

Hydrogenation sequence of SWCN, type 2

constant C1s peak intensity with the H doze increase

Hydrogenation sequence of SWCN, type 1

the decrease of C1s peak intensity with the H doze

increase

Size distribution around 2.0nm Size distribution around 1.5nm

CH4 based growthEtOH based growth

Nikitin et.al Nano lett. 8, 162 (2008)

Structure of XPS C1s spectrum for highly hydrogenated SWCN

peak 1

peak 2

Hydrogenation sequence of SWCN, type 1

peak 3

Peak 1 C C

Clean C atoms in SWCN walls

Peak 2ΔE1-2 ≈ 0.7 eV

C-H bonded C atoms in SWCN walls

HC C

H

Peak 3ΔE1-3 ≈ 1.5 eV

C-H bonded C atoms in SWCN walls with different

electron screening mechanism in

photoionization process

0.66 eV

1.52 eV

The hydrogenation degree of SWCN

Ipeak 1:(Ipeak 2+Ipeak3) = 1:10which corresponds to ~7 wt % of

SWCN hydrogen capacity

HC C

H

C C

Decomposition of C1s XPS for hydrogenated SWCN film

Nikitin et.al Nano lett. 8, 162 (2008)

Hydrogen desorption temperatureC1s XPS spectra measured during annealing of H treated SWCN, type 1

Hydrogen desorption is observed in the range from 300 oC to 600 oC

C1s peak shape change due to C-H bond

cleavage was observed

Nikitin et.al Nano lett. 8, 162 (2008)

Hydrogenation cyclingC1s XPS spectra of SWCN, type 1 exposed to two cycles of hydroge-nation / dehydrogenation

Hydrogenation of SWCN film can be cycled

chemical shift of C1s peak due to C-H bond formation

Nikitin et.al Nano lett. 8, 162 (2008)

Activated Desorption

Nikitin et.al Nano lett. 8, 162 (2008)

Graphite

1nm SWCN

Energetics of C-H bond

-2

-1

0

1

2

2.01.51.00.50.0

C2C3 (3k,0)C2C3 (3k+1,0,0)C2C3 (3k+2,0)

C2C5 (3k,0)C2C5 (3k+1,0)C2C5 (3k+2,0)

ener

gy (e

V)

1/d (nm )-1

(6,0)

(7,0)(8,0)

(9,0)

(10,0)

(11,0)

(12,0)

(13,0)

(14,0)

(15,0)

(16,0)

(17,0)

(18,0)

(19,0)(20,0)

(24,0)

(28,0)

graphite (∞,0)

Niktin et. Al submitted Nano lett.

Interesting regionEnergy neutral?

With Free EnergyAround 1nm

• Hydrogen molecules Metal catalyst Atomic hydrogen “pump”

(spontaneous dissociation, subsequent spillover and diffusion)

• Catalyst metals: Ni, Pd, Pt, etc.

A. Nikitin et al. PRL 95, 225507 (2005)

• Reversible hydrogen storage in SWNT, 5.1±1.2 wt% (65±12 at%) using an atomic hydrogen sourcePotential energy of hydrogen with separation distance

Ech

Ep

Spontaneous dissociation path

Catalyst metalH

Heat of dissociation

H2

H2 gas cylinder

dissociation spillover surface diffusion

sample chamber

H

Catalyst Metals as Atomic Hydrogen Source

H2

I. Accurate measurement of hydrogen uptake of CNTs

II. Increase hydrogen storage capacity

Sieverts’ type apparatus for volumetric analysis

Specially-designed setup for small size samples

Modest hydrogen uptake by molecular hydrogen hydrogen dissociation to atomic hydrogen facilitated by metal catalysts

Synthesis of (CNT-Metal catalyst) composites for larger storage capacity

HiPco SWNT (Ref)

Pd-doped SWNT

Pt-doped SWNT

Electrochemical deposition (EC Pd)

UHV sputter deposition (Sp Pd)

E-beam evaporation (EE Pd)

Electrochemical deposition (EC Pt)

UHV sputter deposition (Sp Pt)

+ Pd

+ Pt

Catalyst Metal-Doped Carbon Nanotubes

Uptake Enhancement in Pt-Doped SWNT

0.13

0.08

-

fPt

2.590.44 (5.23 at%)0.40EC Pt

3.47

1.0

α

0.59 (7.02 at%)0.51Sp Pt

0.170.17SWNT

HCNT [wt%]Htotal[wt%]Sample

• Hydrogen uptake in CNT corrected by using HPt=0

• Larger enhancement for Sp Pt composite

( ) CNTPtPtPttotal HfHfH ×−+×= 1

No hydrogen stored in Pt

Pt

totalCNT f1

HH−

=∴

W. Lee et.al. to be published

Sp Pt Composites w/ Different Pt Thickness0.2 nm 0.5 nm 1.0 nm 3.0 nm

Pt particle agglomeration

Increasing Pt particle density

0.032

0.023R ~ 1 nm

Nanoparticle

H-saturated region

Hydrogen Uptake of CNT-Pd Composites

( ) CNTPdPdPdtotal HfHfH ×−+×= 1

• Significant increase in uptake for both composites

• Increase by a factor of three (EC Pd), or four (Sp Pd)

• Hydrogen stored in both SWNT (HCNT) and Pd NP’s (HPd)

Uptake kinetics during isotherm measurements

30.0 Bar 29.61 atm 3.0 MPa 435.0 psi

0.176.829

SWNT

0.720.53Htotal at 30 bar [wt%]

3.03.2P1/2 [Bar]1723Sample

size [mg]

Sp Pd

EC Pd

• hydrogenation of SWCN with molecular hydrogen

Hydrogenation via spillover mechanism (A.Lueking , 2002, A.Lueking , 2004)

Spillover Process

Conclusions• SWCN with different diameters can reach different hydrogenation degree before "unzipping" and etching

•for specific SWCN it is possible to hydrogenatealmost 100 at % of the carbon atoms in the walls to form C-H bonds which corresponds to 7.7 weight %of hydrogen capacity

• We can generate spillover processes using Catalyst for gas phase hydrogenation

•Maybe spillover process for electrochemical storage