Yiping Zhao and Yuping He

Transcription

1 Yiping Zhao and Yuping He Department of Physics and Astronomy Nanoscale Science and Engineering Center The University of Georgia, Athens, GA April 29, 2010

2 Outline: Introduction Hydrogenation behaviors 1. Mg film versus Mg nanoblade array 2. V decorated Mg nanoblade array 3. Mg nanostructures with differently distributed V nanocatalysts Conclusions

3 Hydrogen Economy: Drivers Fossil Fuels Potential exhaustion Pollution: CO, CO 2 2, SO 2 2, NO x x, dust Global warming Hydrogen Abundant: 75% in universe; H 2 O on earth as raw materials Clean: H 2 +O 2 ->HO 2 +energy Only produces energy and water, zero emission Renewable energy

4 Hd Hydrogen Economy: Four Aspects Hydrogen production Today: Reform Fossil Fuels Tomorrow: Splitting H 2 O Hydrogen storage Today: Gas and liquid Tomorrow: Solid state Hydrogen transportation Hydrogen use Today: Combustion H 2 -> heat -> electricity Tomorrow: Fuel cells H 2 -> electricity

5 Hydrogen Storage Hydrogen storage is the bottleneck for hydrogen energy applications Pressurized gas Cryogenic liquid Solid-state storage Mtlhdid Metal hydrides Complex hydrides Carbon-based materials Solid-state storage is the safest and most efficient method.

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8 Mg-Based Hydrogen Storage MgH 2 is very promising for solid-state hydrogen storage Advantages (Mg): Low cost Lightweight Nontoxic High hydrogen storage capacity: 7.6 wt% in MgH 2 Problem Poor hydrogen sorption thermodynamics and kinetics (MgH 2 is very stable, T 300 C is needed to release H 2 ) Improvement? Tailor Mg into nanostructures? Add an appropriate transition metal catalyst

9 Tailor Mg into nanostructures Conventional method: mechanical alloying or ball milling

10 Fabrication of Mg Film and Mg Nanoblade Array Substrate Mg Film (nanoflakes) Mg nanoblade array Substrate Film Source Normal deposition i 200-nm Ti 200-nm Ti Hydrogenation (PCT Pro-2000): 20 bar H-pressure 2000 minutes 200 C, 250 C, 300 C, 350 C Source Oblique angle deposition (OAD) He et al., APL 2008, 93,

11 Fabrication and Characterization Systems

12 Hydrogenation at Different Temperatures Mg Thin Film Sample height (a) A Mg Nanoblades (d) B h 4μ μm h μm t 160 nm Blade thickness t h 6.2 μm η 55% Vl Volume expansion η (b) A300 (e) B300 h 7.3 μm t 220 nm η 41% (c) A350 (f) B350 h 7.7 μm t 250 nm η 69% 2 μm He et al., APL 2008, 93,

13 Hydrogenation at Different Temperatures ο Mg Ti MgH 2 ο ο Film Si sub bstrate (a) A ο Nanoblade array ο ο Si sub bstrate ο (e) B ο ο (b) A200 Δ TiH 1.5 TiH 2 MgO Δ Δ (f) B250 (c) A300 (g) B300 (d) A350 (h) B θ (degree) He et al., APL 2008, 93,

14 FRES (Forward REcoil Spectrometry) 9 (a) Film A200 A250 A300 6 A H conce entration (wt t%) (b) Nanoblade array B200 B250 B300 B350 2 TiH 1.5, MgH Deeper beneath surface Energy (kev) Surface He et al., APL 2008, 93,

15 FRES (Forward REcoil Spectrometry) content (wt% %) Total H Hydrogenated Mg film Hydrogenated Mg nanoblades T 250 C (blades) vs T 350 C (film) MgO small H amount in blades The unique nanoblade morphology and catalysis of fti layer nanoblade hydrogenation behavior Temperature ( C) He et al., APL 2008, 93,

16 Add an appropriate transition metal catalyst

17 Fabrication of V-Decorated Mg Nanoblade Arrays He et al., Phys Chem Chem Phys 2009, 11, 255 He et al., Nanotechnology 2009, 20,

18 Fabrication of V-Decorated Mg Nanoblade Arrays Mg α α = 70 Si (100) V α (a) α = 15 Joint Si (100) V coating Ti barrier Si (100) (b) () (c) He et al., Phys Chem Chem Phys 2009, 11, 255 He et al., Nanotechnology 2009, 20,

19 Characterizations: SEM and EDX (a) (b) Mg vapor (c) Scale bars: 5 μm h 50 μm t 65 nm Mg vapor Mg V Ti Si 0 Intensity (a.u.) V (a) (b) 50 μm Ti (c) Mg Si V/(Mg+V) 2.25 at.% He et al., Phys Chem Chem Phys 2009, 11, 255 He et al., Nanotechnology 2009, 20,

20 Hydrogen Storage Performance H-abso orption (wt t.%) 6 4 (a) under 10 bar H K 521 K 546 K K Kinetic curves under vacuum Time (hour) (b) H-deso orption (w wt.%) He et al., Phys Chem Chem Phys 2009, 11, 255 He et al., Nanotechnology 2009, 20,

21 Hydrogen Storage Performance Arrhenius plots (c) ln k = -E a /RT + ln (A) H-absorption H-desorption k (h -1 ) E a a = ± 1.2 kj/mol H 2 a E a d = 65.0 ± 0 Pure Mg film or powder: E ad kj/mol H 2 MgH 2 layer Mg layer ± 0.3 kj/mol H /T (K -1 ) Ti barrier Si (100) Spherical particle (radius R) model C C tt C H = khch + D 2 H { C C H ( r < R, t = 0) = C C ( r = R, t > 0) = H C s H m 2 2 C0 2R ( 1) m r DH m = π π 1+ sin exp[ ( + k ) t ] 2 H C πr m R R H + s 0 m= 1 0 Initial condition boundary condition The larger k H & D H, and smaller R, the faster the kinetics The V coating The unique nanoblade morphology: 1. large surface area 2. small H-diffusion length. He et al., Phys Chem Chem Phys 2009, 11, 255 He et al., Nanotechnology 2009, 20,

22 Mg nanostructures with differently distributed V nanocatalysts He et al., Int J Hydrogen Energy 2010, 35, 4162

23 Fabrication of V-Decorated Mg Nanoblade Arrays He et al., Phys Chem Chem Phys 2009, 11, 255 He et al., Nanotechnology 2009, 20,

24 Fabrication of V-Incorporated Mg Nanostructures θ = 70 θ Mg Si (100) 8 V coating θ = 70, 50, 10 Step 1 θ Mg nanoblade Mg θ V θ = Ti barrier Si (100) Step 2 (a) Si (100) Step 3 Si (100) (b) V V decorated Mg at 70 V doped Mg at 10, 50, 70 He et al., Int J Hydrogen Energy 2010, 35, 4162

25 Morphological Characterizations V decorated Mg at 70 As-prepared V doped Mg at 70 As-prepared V doped Mg at 50 As-prepared 2 μm 2 μm 2 μm V-Mg (a) 10 μm (b) 2 μm (c) 2 μm Ti rods Ti film Si 2 μm 2 μm 2 μm (a ) 10 μm (b ) 2 μm (d) 2 μm As-prepared After 14 cycles After 21 cycles V doped Mg at at% V He et al., Int J Hydrogen Energy 2010, 35, 4162

26 TEM and ED (As-Prepared Samples) (a) (b) V decorated Mg at 70 : V crystals in Mg matrix Rings (V) + spots (Mg) in ED 10 nm 5 nm -1 (c) (d) V doped Mg at 70 : No V signal is detected Only Mg diffraction pots in ED 20 nm 5 nm -1 He et al., Int J Hydrogen Energy 2010, 35, 4162

27 Hydrogen Storage Performance T (K) 0.0 H-release fraction S dec S dop 50 S dop 484 K S K 499 K S dop K 455 K 499 K d T c 514 K H-uptake fraction T c a 441 K 457 K (a) (b) E a E a a = 35.3 ± 0.9 kj/mol H 2 (h -1 ) H 2 E d a = 64.8 ± 0.4 kj/mol H 2 E a d = 38.9 ± 0.3 kj/mol H E d a = 69.4 ± 0.6 kj/mol H 2 E a a = 50.6 ± 0.9 kj/mol H E d a = 63.2 ± 0.4 kj/mol H H 2 a E a = 46 ± 1 kj/mol H 2 k (h -1 ) k (c) (d) 7.4 E a a = 52 ± 1 kj/mol H 2 2 H /T (K -1 ) He et al., Int J Hydrogen Energy 2010, 35, 4162

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29 Conclusions The H-storage performance of Mg can be improved by 1. Tailoring Mg into nanoblades 2. Coating a layer of V on the surface of individual Mg nanoblades The H-storage performance of V-Mg depends strongly on g p g p g y the distribution of V nanocatalyst

30 Acknowledgements Prof. Jin Z. Zhang Prof. Matthew D. McCluskey Prof. Howard Wang Prof. Russell llj. Composto Leo Sabellos Nicole Richards Win Maw Hlaing Oo Chris Hoffman Robert Blackwell UGA DOE NSF

31 XRD V peak a e b Mg(101) a b c e a b d e f g θ (degree) He et al., Int J Hydrogen Energy 2010, 35, 4162