NanoEngineering of Hybrid Carbon Nanotube Metal Composite Materials for Hydrogen Storage Anders Nilsson

Transcription

1 NanoEngineering of Hybrid Carbon Nanotube Metal Composite Materials for Hydrogen Storage Anders Nilsson Stanford Synchrotron Radiation Laboratory (SSRL) and Stockholm University

2 Coworkers and Ackowledgement B. Clemens 2, H. Dai 3, A. Nikitin 1, H. Ogasawara 1, D. Mann 3, Z. Zhang 4, X. Li 3, KJ Cho 4,, D. Friebel 1, S. Rajasekaran 1, R. Bhowmick 2, S. Beasley 2, Y. Lee 2 1) Stanford Synchrotron Radiation Laboratory 2) Department of Material Science and Engineering 3) Department of Chemistry, Stanford University 4) Department of Mechanical Engineering, Stanford University

3 Hydrogen Dream Photobiological H 2 Production Gas-Water Shift Solar H 2 Production Biomass Photoelectrochemical H 2 Production PV/PEC H 2 Conversion Fuel Cells Storage and Transport Carbon nanotubes Metal Hydrides

4 Current View of Hydrogen Current vision of the H as an energy carrier Production (electrolysis) 2H H 2 Hydrogen distribution system H 2 H 2 2H Vehicle Hydrogen transfer system to fuel cell Onboard storage (adsorption) 2H H 2 Use (fuel cell) H 2 H 2 2H 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

5 Chemisorption of H atoms on Carbon Nanotubes Idea: to store hydrogen in the chemisorbed form on the nanotube surface hydrogenation H-H bond energy 4.5 ev Adsorption Energy Decreases with diameter Pack et al, 2003

6 Investigation strategy Hydrogenation: in situ atomic hydrogen treatment Samples: "as grown" CVD SWCN films - elimination of H 2 dissociation step from hydrogenation process - well controlled environment (base pressure < 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

7 Probing tools X-ray photoelectron spectroscopy (XPS) C1s XPS spectra of n-octane and graphite (Weiss et al., 2003, Bennich et al., 1999) X-ray absorption spectroscopy (XAS) Carbon K-edge XAS spectra of graphite and diamond (Garo et al., 2001)

8 Hydrogenation induced changes in Graphite and SWCN XPS spectra Graphite Single wall Carbon Nanotube chemical shift of C1s peak due to C-H bond formation 0.65 ev Clean H-treated Nikitin et.al Surf. Sci. 602, 2575 (2008) Nikitin et.al Phys. Rev. Lett. 95, (2005)

9 The hydrogenation degree determination from XPS spectra Calculated C1s chemical shift values (MD and DFT) Decomposition of C1s XPS for hydrogenated SWCN film (n,m) D, nm Shift, ev (b-d) (a) (a) (10,0) (b) (12,0) (c) (15,0) (d) (22,0) peak 2 H C C peak 1 excited C atom H C C H atom Nikitin et.al Phys. Rev. Lett. 95, (2005) Hydrogenation = I peak 2 /(I peak 1 +I peak 2 )*100 at %

10 The influence of SWCN diameter on hydrogenation process Hydrogenation sequence of SWCN, type 1 Hydrogenation sequence of SWCN, type 2 constant C1s peak intensity with the H doze increase EtOH based growth the decrease of C1s peak intensity with the H doze increase CH 4 based growth Size distribution around 2.0nm Size distribution around 1.5nm Nikitin et.al Nano lett. 8, 162 (2008)

11 Structure of XPS C1s spectrum for highly hydrogenated SWCN Hydrogenation sequence of SWCN, type 1 Peak 1 Clean C atoms in SWCN walls C C peak 1 peak 2 Peak 2 ΔE ev C-H bonded C atoms in SWCN walls H C C peak ev H Peak 3 ΔE ev C-H bonded C atoms in SWCN walls with different electron screening mechanism in photoionization process 1.52 ev

12 The hydrogenation degree of SWCN Decomposition of C1s XPS for hydrogenated SWCN film H C C H I peak 1 :(I peak 2 +I peak3 ) = 1:10 which corresponds to ~7 wt % of SWCN hydrogen capacity C C Nikitin et.al Nano lett. 8, 162 (2008)

13 Hydrogen desorption temperature C1s XPS spectra measured during annealing of H treated SWCN, type 1 C1s peak shape change due to C-H bond cleavage was observed Hydrogen desorption is observed in the range from 300 o C to 600 o C Nikitin et.al Nano lett. 8, 162 (2008)

14 Hydrogenation cycling C1s XPS spectra of SWCN, type 1 exposed to two cycles of hydrogenation / dehydrogenation chemical shift of C1s peak due to C-H bond formation Hydrogenation of SWCN film can be cycled Nikitin et.al Nano lett. 8, 162 (2008)

15 Activated Desorption Graphite 1nm SWCN Nikitin et.al Nano lett. 8, 162 (2008)

16 Energetics of C-H bond 2 energy (ev) graphite (,0) (28,0) (24,0) (20,0) (19,0) (16,0) (13,0) (15,0) (10,0) (12,0) (14,0) (17,0) (18,0) (11,0) (9,0) (8,0) (7,0) (6,0) Interesting region Energy neutral? -2 C2C3 (3k,0) C2C3 (3k+1,0,0) C2C3 (3k+2,0) C2C5 (3k,0) C2C5 (3k+1,0) C2C5 (3k+2,0) With Free Energy Around 1nm /d (nm ) Niktin et. Al submitted Nano lett.

17 Catalyst Metals as Atomic Hydrogen Source A. Nikitin et al. PRL 95, (2005) E p Heat of dissociation E ch Spontaneous dissociation path H 2 H Catalyst metal Potential energy of hydrogen with separation distance Reversible hydrogen storage in SWNT, 5.1±1.2 wt% (65±12 at%) using an atomic hydrogen source dissociation spillover surface diffusion Hydrogen molecules Metal catalyst Atomic hydrogen pump (spontaneous dissociation, subsequent spillover and diffusion) H 2 H Catalyst metals: Ni, Pd, Pt, etc. H 2 gas cylinder sample chamber

18 Catalyst Metal-Doped Carbon Nanotubes I. Accurate measurement of hydrogen uptake of CNTs Sieverts type apparatus for volumetric analysis Specially-designed setup for small size samples II. Increase hydrogen storage capacity 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 + Pt 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)

19 Uptake Enhancement in Pt-Doped SWNT Hydrogen uptake in CNT corrected by using H Pt =0 Larger enhancement for Sp Pt composite ( fpt ) HCNT H = f H + 1 total Pt Pt H CNT = H 1 total f Pt No hydrogen stored in Pt Sample H total [wt%] f Pt H CNT [wt%] α SWNT EC Pt (5.23 at%) 2.59 Sp Pt (7.02 at%) 3.47 W. Lee et.al. to be published

20 Sp Pt Composites w/ Different Pt Thickness 0.2 nm 0.5 nm 1.0 nm 3.0 nm Nanoparticle Pt particle agglomeration H-saturated region R ~ 1 nm Increasing Pt particle density

21 Hydrogen Uptake of CNT-Pd Composites Significant increase in uptake for both composites Increase by a factor of three (EC Pd), or four (Sp Pd) Hydrogen stored in both SWNT (H CNT ) and Pd NP s (H Pd ) total Pd Pd ( fpd) HCNT H = f H + 1 Sample size [mg] P H 1/2 [Bar] total at 30 bar [wt%] SW NT EC Pd Sp Pd Uptake kinetics during isotherm measurements 30.0 Bar atm 3.0 MPa psi

22 Spillover Process hydrogenation of SWCN with molecular hydrogen Hydrogenation via spillover mechanism (A.Lueking, 2002, A.Lueking, 2004)

23 Conclusions SWCN with different diameters can reach different hydrogenation degree before "unzipping" and etching for specific SWCN it is possible to hydrogenate almost 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