The Production, Purification and Characterization of Carbon Nanotubes for Hydrogen Storage

By

Jorge Ivan Salazar Gomez

Main Supervisor:

Prof. Peter J. Hall

Second Supervisor:

Dr. Len Belouis

INTRODUCTION

• The discovery of the fullerenes [1] and the carbon nanotubes [2] opened a new area of research in both the theoretical and the experimental field. Since the discovery of the single wall carbon nanotubes SWNTs, many chemical and physical properties have been predicted.

• Different techniques have been applied to produce carbon nanotubes: electric arc discharge [2,3,4], laser ablation [5,6], chemical vapor deposition (CVD) [7,8,9], being the last one the method that shows large scale production, but with the disadvantage of the formation of amorphous carbon and other impurities, which must be removed by physical or chemical methods. Therefore, the purification process [10,11] is a key step in the production of carbon nanotubes addressed toward hydrogen storage [12] or other applications.

• The characterization [13] becomes an important part of the process because it gives information about the catalyst and the carbon samples before and after purification, so giving information about the actual nature of the samples produced and how to control and improve them.

AIM

• The aim of this project focuses on achieving the best conditions for the production of carbon nanotubes by the CVD method, directed towards hydrogen storage. The storage of hydrogen by physical adsorption or by electrochemical methods is one of the most promising applications of carbon nanotubes due to their possible use in fuel cells, especially for the transportation sector, implying a clean process and therefore reducing the global contamination from CO2.

OBJECTIVES

1) To evaluate the effect of:

• Catalyst composition

• Temperature

• Time

• Flow rate of feed gas

• Particle size

2) To elucidate the impact of the processes of washing, graphitization and activation on the properties of the nanotubes.

3) To characterize the catalysts and the carbons using different techniques that permit to understand the structure and properties of these materials.

4) To evaluate the physical and electrochemical hydrogen storage.

EXPERIMENTAL

Catalyst preparation sol-gel method

Calcination Cu:Ni Calcination Cu:Ni:Mg

CVD (DEON) CVD (SP, CAT)

C2H4, 600oC C2H4, CH4,

600oC, 800oC

CATALYST REMOVAL

Washing with acids

Adding acid and stirring For minimum 4 h and filtering

Passing acid through the samples (filtering)

To rinse with deionized Water until pH 7

To dry in vacuum

GRAPHITIZATION

To heat at 10oC/min until 1500oC in inert atmosphere (Ar 100 ml/min)

To leave the samples 2h

To cool until room temperature

To put an amount of sample in an horizontal furnace and purge 30 min.

ACTIVATION (CO2)

To put an amount of the samples in an horizontal furnace

To purge 30 min. with Ar

To heat at 10oC/min from room temperature until 850oC

To change the gas by CO2 and to leave the sample for 1-4 h.

To cool until room temperature with Ar

CVD REACTION APPARATUS

RESULTS

0

1

2

3

4

5

6

7

8

0 20 40 60 80 100 120

Yield (g/g)

Yie

ld (

g/g

)

Weight % Ni

10

20

30

40

50

60

70

0 20 40 60 80 100

Yie

ld o

f C

arb

on

(g

/g)

Reaction Time (min)

DEON: Cu:Ni = 40:60

SP: Cu:Ni:Mg = 10:20:70

CAT: Cu:Ni:Mg = 0.3:0.7:3.0

No termination time observed

Catalyst still active

Constant Reaction rate

No diffusional effects

0

10

20

30

40

50

60

70

450 500 550 600

Yie

ld o

f C

arb

on

(g

/g)

Reaction Temperature (oC)

0

0.5

1

1.5

2

2.5

3

3.5

4

0.0011 0.00115 0.0012 0.00125 0.0013 0.00135 0.0014

y = 35.33 - 25490x R2= 0.99892

ln(k

)

1/T (1/K)

y = 14.36 - 9287.1x R2= 0.99964

SAMPLES PARTICLE SIZE (µm) YIELD (g/g)

DEON 011(i)a x < 150 40.36

DEON 011(ii)a 212 > x > 150 31.01

DEON 011(iii)a x > 212 33.89

DEON 011(i)b x < 150 41.14

Below 500oC Ea = 211.924 kJ.mol-1

Above 500oC Ea = 77.213 kJ.mol-1

BET Surface Area

0

50

100

150

200

250

300

350

0 0.2 0.4 0.6 0.8 1

raw

washed

Graphitised

Ad

sorp

tio

n a

mo

un

t (c

c/g

)

Relative pressure

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

010a(600oC-80ml/min)010a(Activated)

010e(600oC-80ml/min)010e(Washed)

010f(600oC-80ml/min)010f(Graphitized)

Ad

sorp

tio

n A

mo

un

t (c

c/g

)

Relative Pressure

0

50

100

150

200

250

0 0.2 0.4 0.6 0.8 1

09e(500oC-10ml/min)

09d(600oC-80ml/min)

09c(600oC-57ml/min)

09c(Graphitized)

09b(600oC-33ml/min)

09a(600oC-10ml/min)

Relative pressure

Ad

sorp

tio

n a

mo

un

t (c

c/g

)

a) b) c)

1

00

1

00

0 11

11

1n

nn

m

PPCP

PC

PPnP

Pn

PP

C

W

W

X-Ray Diffraction

0

200

400

600

800

1000

1200

20 30 40 50 60 70 80 90

DEON 01a (Raw)

DEON 01a (W)

DEON01a (G)

Co

un

ts

2 THETA

0

500

1000

1500

2000

2500

3000

3500

4000

10 20 30 40 50 60 70 80 90

Cat. SP1Cat. SP2SP1aSP2aSP2b

Inte

ns

ity/

a.u

.

2 Theta

a) b)

nd sinSAMPLE

TEMPERATURE(oC)

C2H4 FLOW RATE(ml/min)

d-SPACING(nm)

DEON 09a 600 10 0.3437DEON 09b 600 33 0.3405DEON 09c 600 57 0.3418DEON 09d 600 80 0.3405DEON 09e 500 10 0.3445DEON 09f 700 10 0.3406, 0.3427SP1a 600 80 3.441SP2a 600 80 3.426SP2b 700 80 3.422

TGA

Graphitized more stable

Raw carbon is more reactive

Washed carbons have medium reactivity

0

20

40

60

80

100

0

200

400

600

800

1000

0 10 20 30 40 50 60

SP1a (600oC-90 min)

SP2a (600oC-90 min)

SP2d (700oC-90 min)

SP2e (700oC-90 min)

SP2f (600oC-30 min)

SP2g (700oC-30 min)

Temperature (oC)

Temperature (oC)

Temperature (oC)

Temperature (oC)

Temperature (oC)

Temperature (oC)

Wei

gh

t lo

ss (

%) T

emp

eratu

re (

oC)

Time (min)

SAMPLEVOLATILES

(%)PURE CARBON

(%)ASHES (%)

SP1a-2 5.01 93.06 1.95SP2a-1 6.48 93.01 0.58SP2d-1 6.90 92.17 0.82SP2e-1 5.06 94.52 0.39SP2f-1 17.36 79.10 3.57SP2g-1 9.63 89.76 0.56

ELECTROCHEMICAL STORAGE

CYCLEDENSITY

DELOADING(mA/g)

CAPACITY(mAh/g)

H:Cratio

WEIGHT(%)

DEON 09d-1 100 34.94 1:64 0.130

DEON 09d-2 50 10.27 1:124 0.067

DEON 09d-3 100 15.07 1:104 0.080

DEON 09d-4 50 14.09 1:158 0.053

DEON 09d-5 5 8.82 1:160 0.052

DEON 09d-6 20 13.64 1:172 0.048

DEON 09d-7 20 13.99 1:171 0.049

DEON 09d-8 50 19.60 1:114 0.073

DEON 09d-9 23 13.93 1:163 0.051

DEON 09d-10 100 20.53 1:109 0.076

DEON 09d-11 50 38.39 1:125 0.066

DEON 09d-12 23 8.24 1:129 0.065

CA-01 100 57.33 1:39 0.210

CA-02 200 69.05 1:32 0.260

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

100 110 120 130 140

DEON 09d

Cycle 10

Cycle1

PO

TE

NT

IAL

(V

)

CAPACITY (mAh/g)

Discharging process after 1 and 10 cycles for the sample DEON 09d previously loaded electrochemically with hydrogen at a current density of 1000 mA/g for 1h in a 6M KOH solution.

MS-TPD

0

5 10-10

1 10-9

1.5 10-9

2 10-9

2.5 10-9

3 10-9

3.5 10-9

4 10-9

50 100 150 200 250 300 350 400 450

DEON 09d He (2nd cycle)

Mass 1Mass 2Mass 16Mass 17

Partia

l P

res

su

re (

To

rr)

Temperature (oC)

Adsorption at 10 bar for 24h at room temperature.

Desorption at 20oC/min in He as carrier.

RAMAN SPECTROSCOPY

0

500

1000

1500

2000

2500

800 1200 1600 2000 2400 2800 3200

Inte

nsi

ty/a

.u.

Raman Shift/cm-1

406 nm

785 nm

457 nm

011(ii)a-1011(ii)a-3011(ii)a-3

0

1000

2000

3000

4000

5000

6000

400 800 1200 1600 2000 2400 2800

406 nm517nm632 nm

Inte

nsi

ty/a

.u.

Raman Shift/cm-1

CAT 3B

G-Band at 1575 cm-1

D-Band at 1312 cm-1

G/D = 3.87

Mainly Semiconducting

G-Band at 1589 cm-1 (shoulder at 1546 cm-1)D-Band at 1351 cm-1

G/D = 1.19Metallic and Semiconducting

Small-Angle Neutron Scattering (SANS)

0.01

0.1

1

10

100

1000

0.001 0.01 0.1 1

PJH1414-1434m-SP2a-Dry

SP2a DrySP2a CM

d

d

Q)(

cm-1

)

Q (Å-1)

0.1

1

10

100

1000

0.001 0.01 0.1 1

PJH1416-1436m-09d-Dry

DEON 09d DryDEON 09d CM

d

d

Q)(

cm-1

)

Q (Å-1)

Raw data for time-of-flight technique with corrections for instrument background and transmission.

TEM

250nm

5nm

100nm 5nm

CONCLUSIONS

•The best catalyst composition for the synthesis of the carbon nanofibers is Cu:Ni=40:60 and the optimum temperature is 600oC, at higher temperature catalyst deactivation appears.

• The washing process with nitric acid was effective in the removal of catalyst particles and induced some ordering. It had little effect on the surface area.

• The graphitization process enhances the chemical stability of the nanofibres and induces more order (formation of bundles and reduction of aggregated pores) and enhances crystallinity, but it decreases the capacity of adsorption.

• The activation process with CO2 opens some of the tubes, but it does not

apparently remove the amorphous carbon. The selectivity though appears to be better than oxygen.

•The Raman results indicate that the nanotubes formed are mainly semiconducting, but a high proportion of nanofibers and impurities are present.

• The capacity of adsorption for hydrogen is very low for the raw samples, but higher uptakes are expected for purified samples.

REFERENCES

[1] H.W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, R. E. Smalley, Nature 318 (1985) 162.[2] S. Iijima, Nature 354 (1991) 56.[3] S. Iijima, T. Ichihashi, Nature 363 (1993) 603.[4] D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605.[5] A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C. Xu, Y. H. Lee, S. G. Kim, A. G. Rinzler, D. T. Colbert, G. E. Scuseria, D. Tománek, J. E. Fischer, R. E. Smalley, Science 273 (1996) 483.[6] F. Kokai, K. Takahashi, M. Yudasaka, S. Iijima, J. Phys. Chem. B 104 (2000) 6777.[7] L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen, M. Meyyappan, Chem. Phys. Lett. 348 (2001) 368.[8] W. Z. Li, D. Z. Wang, S. X. Yang, J. G. Wen, Z. F. Ren, Chem. Phys. Lett. 335 (2001) 141.[9] X. Chen, S. Motojima, Carbon 37 (1999) 1817.