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

1. 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

2. 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.

3. 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.

4. 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.

5. 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.

6. EXPERIMENTAL Catalyst preparation sol-gel method Calcination Cu:Ni Calcination Cu:Ni:Mg CVD (DEON) CVD (SP, CAT) C2H4, 600oC C2H4, CH4, 600oC, 800oC

7. 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

8. GRAPHITIZATION To put an amount of sample in an horizontal furnace and purge 30 min. To heat at 10oC/min until 1500oC in inert atmosphere (Ar 100 ml/min) To leave the samples 2h To cool until room temperature

9. 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

10. CVD REACTION APPARATUS

11. RESULTS 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

12. Below 500oC Ea = 211.924 kJ.mol-1 Above 500oC Ea = 77.213 kJ.mol-1

13. a) b) c) BET Surface Area

14. a) b) X-Ray Diffraction nl = 2d sinq

15. TGA Graphitized more stable Raw carbon is more reactive Washed carbons have medium reactivity

17. ELECTROCHEMICAL STORAGE 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.

18. MS-TPD Adsorption at 10 bar for 24h at room temperature. Desorption at 20oC/min in He as carrier.

19. RAMAN SPECTROSCOPY 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 = 1.19 Metallic and Semiconducting

20. Small-Angle Neutron Scattering (SANS) Raw data for time-of-flight technique with corrections for instrument background and transmission.

21. TEM

22. 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.

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