A Combinatorial and Distributed Approach to Solving the Problem of Splitting Water with Sunlight or. GCEP Lecture Stanford University March 8, 2007

Transcription

1 A Combinatorial and Distributed Approach to Solving the Problem of Splitting Water with Sunlight or The Search for the Holy Grail Bruce Parkinson Department of Chemistry Colorado State University GCEP Lecture Stanford University March 8, 2007

2 The Hydrogen Economy We have to go there sooner or later - so why not sooner? 1. Non-precious metal fuel cell catalysts 2. Better hydrogen storage 3. Renewable source of hydrogen (water) We aim to replace two expensive components with one (hopefully) cheaper photoelectrolysis system. 2

3 Materials for Photoelectrolysis of Water Must: Have a band gap between ~1.0 and 2.0 ev Be stable for many years under illumination in aqueous electrolytes Have conduction band and valence band positions that straddle water oxidation and reduction potentials Have some catalytic activity for hydrogen or oxygen evolution from water Be cheaper than a solid state solar cell connected to an electrolyzer

4 Single Photoelectrode Scheme h Tex t Electrolyt e Semiconducto r Already done with large bandgap (>3.0 ev) metal oxides that only use small fraction of solar spectrum

5 p-n Photoelectrolysis Scheme Two photon process uses more of solar spectrum Voltage provided by carrier recombination at junction Requires discovery of two lower bandgap materials

6 Metal Oxides Can Meet All the Criteria But Which Oxide(s)??? > 60 elemental metal oxides (most have been investigated already) For ternary > 200,000 (1:1:1) For quaternary > 10,000,000 Extraordinary properties may require many elements High T c oxides are multicomponent - HgBa 2 CaCu 2 O 6+ Hg 0.8 Tl 0.2 Ba 2 Ca 2 Cu 3 O 8.33

7 Our approach: Therefore a Combinatorial Search is Necessary Ink jet print overlapping patterns of metal oxide precursors Very flexible, cheap and versatile Metal nitrate salts Sol gel chemistry, oxometallates, nanoparticles Use conductive glass as substrate - pyrolysis at ~500 C Screen by laser scanning in solution and look for photocurrent generation: In acid, base and neutral electrolytes At positive and negative biases Stability with higher power and extended illumination

8 Scanning Laser Screening Lock-In Potentiostat Y-scan X-scan Cell 45 mm Laser Chopper Laser Scanner

9 Determine the Optimum Film Thickness Print a thickness gradient with constant composition 0.25 M Fe(NO 3 ) 3 Printed on F doped SnO 2 False Color Photocurrent Map 1X 1X 2X 2X Pattern 3X Fe2O3 3X Photocurrent (AU) Photo Scanned in 0.1M NaOH at 0.5 V Distance (pixels) Average response

10 Why Thickness Matters Silicon Indirect Bandgap Low absorption GaAs Direct Gap High absorption Transition metal oxide Indirect bandgap Low absoprtion 10

11 One Printing Scheme Internal Standards Zoom Four elements three at a time method of Mallouk et al Colors represent different element oxide precursors Gradients can be designed to screen low doping of elements Zoom-In for promising area to refine composition

12 Internal Standards Cu Fe Cs Nd A Compositional Zoom 4 metals 3 at a time Printed and pyrolyzed plate Scanned at 0.5V bias Fe Internal Standard False color photocurrent image Zoom in

13 Narrow Search if Possible Choose Metals From Different Groups Structural Metals Ti, W, Zr, Ta, Si, Mo, Nb, Hf, In, Sn, Y, Sc, Al, Zn Light Absorbers Fe, Cr, V, Co, Mn, Ni, Cu, U, Ce, Nd some other rare earths Catalytic Ru, Rh, Pd, Pt, Ir, Os, Re Charge Compensators Ca, Sr, Ba, Mg, Li, Na, K, Rb, Cs, some rare earths Except Try Not To Use Hg, Pb, Tl, Cd

14 Wavelength and Bias Scans Co, Fe Cs, Al V Bias V Bias 532 nm 532 nm 633 nm 633 nm Promising composition

15 Photocurrent Spectroscopy IPCE Photocurrent spectrum of a promising p-type Co-Fe-Al Oxide Band gap ~ 1.58 ev

16 Quantitative Printing to Optimize Research Printer Allows Control of: 1. Drop density 2. Substrate T 3. Piezo V, t 16

17 Another Screening Method Internal Standards A=Fe B=Co C=Al

18 Refinement of Composition 18

19 Bandgap Measurements of Different Printings 532 nm 0.1-M NaOH 477 nm 0.1-M NaOH 532 nm 0.1-M HClO nm 0.1-M NaOH n-fe 2 O 3 (sign flipped) 19

20 Printing Near Optimum Proportions Single Cartridge of Mixed Nitrates Co2.52Fe0.3Al0.18O4 Nominal Stoichiometry 532 nm 0.1-M NaOH 0-V Bias 20 Fourth component can be added

21 Discovery and Optimization Protocol Gradient Ink Jet Printing of New Compositions (Over 450 compositions screened to date) Discovery Discrete Ink Jet Printing to Optimize Composition, Doping, Catalysts... Deployment Spray Pyrolysis to Produce Large Area Films 21

22 Distributed Screening (Outsource) Recruit 1000s of Researchers Distributed Computing Model SETI or Protein folding but we will engage hands and minds not just spare computing power Develop inexpensive screening kits to distribute to undergrad and high school students Students have their future at stake Learn about the energy problem and chemistry Create on-line data base and bulletin board Recruit labs to help with characterization of 22 promising compositions

23 Distributed Screening Progress and plans so far: Lego Mindstorms based scanning station Can use for both printing and scanning Diode laser (laser pointers) USB port powered electronics and software Hewlett Packard simple versatile ink jetter Inexpensive and easy to fill cartridges Teaching research class to freshman students at CSU this semester No funding yet for this outreach effort 23

24 TIPS (Thermal Ink Jet Pico-Fluidic System) HP Beta Product TIPS System Allows Flexibility and Ease of Use for Ink 24

25 Lego Mindstorms Based Printing and Scanning Array printed with Lego/TIPS system Used for both printing and screening Inexpensive consumer item User customization 25

26 Cost Comparison Research System Cost Distributed System Cost Modulated Laser $500 Laser Pointer $40 Scanning Mirrors $500 Lego Scanner $200 Lock-in Amp $2500 Software Potentiostat $2000 USB or D/A +A/D $100 Furnace $3000 Fresnel Solar $35 Printer $80 TIPS + Lego??? Total ~$9000 ~$500 26

27 When Promising Material is Identified - How to Engineer an Efficient Device? Use Grätzel Approach with Nanocrystallites A - Nanoparticles of new material B - Coat TiO 2 nanoparticles C -Two layer coating on TiO 2 Nanocrystalline film can absorb all the light All light is absorbed near interface - low d-band mobility Low microscopic current density reduces overpotential losses

28 Films Can Be Nanocrystalline F Doped SnO2 -Fe 2 O 3 on FTO 28

29 A Potentially Efficient Configuration Nanoparticle films Separate hydrogen and oxygen compartments Photons used more effectively

30 The PEC Hydrogen Production Lament (From Gary Hodes ~1980) If it is cheap, it will be inefficient If it is efficient, it will be expensive If it is cheap and efficient, it will be unstable If it is cheap, efficient, and stable, then: Just before you collect your first royalty check, someone will invent something better than hydrogen, or You ll die, in which case: In Heaven, hydrogen will be a useless substance, or, more likely: In Hell, hydrogen will be unstable under ambient conditions. 30

31 Acknowledgements Aaron Wolfe Mike Woodhouse Michelle Romanishan Jeff Head Greg Hermann Simon Dodd Hewlett Packard Bill Buckley DOE-BES