Solar Water Splitting cells Artificial Photosynthesis Verena Schendel 14/03/2012.
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Transcript of Solar Water Splitting cells Artificial Photosynthesis Verena Schendel 14/03/2012.
Solar Water Splitting cells
Artificial Photosynthesis
http://images.sciencedaily.com/2008/03/080325104519-large.jpg
Verena Schendel 14/03/2012
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Overview
•Motivation• Photosynthesis • Artificial Photosynthesis• Photoelectrolysis• Devices • „Artificial leaf“• Outlook http://www.wissenschaft-aktuell.de/onTEAM/grafik/31286779743.jpg
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Solar is only 0.1 % of the market
Availability: Can run a society only when sun shines
Will have difficulty in penetrating a market until it can be stored
Material costs, prizes, efficiency……
4http://www.klima-suchtschutz.de/uploads/pics/windenergie-anlage.jpg
http://www.bike-components.de/images/logos/batterien.jpg
Energy density poor….
Fluctuations, Storage problems, costly,….
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Why fuel….?
OC H HO
O
ENERGY
High amount of energy stored in chemical bonds….
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Motivation“Finding a cost-effective way to produce fuels, as plants do, by combining sunlight, water, and carbon dioxide, would be a transformational advance in carbon-neutral energy technology.”(JCAP, Joint Center for artificial photosynthesis)
StorageAvailability
Eco-friendly
Sustainable
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What nature does….
O2 + „H2“ = NADPH Sugar
CO2
Most of the energy storage is been done in water splitting…..not in CO2 fixation !!!
OEC
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OC H HO
O
ENERGY
H HO
H HO
H H
H H
Solar input to make low energy bonds to high energy bonds
„fuel“ with highestenergy output relative to molecular weight
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Photoelectrolysis
H2O H2 + ½ O2 ΔG=237.2 kJ/molΔE°= 1.23 V /e(at least….overpotentials)
Solar spectrum absorbation of water poor
Photoconverter
H2O O2 + 2H+ + 2e-
2 H+ + 2 e- H2
Oxidation (Anode)
Reduction (Cathode)
-II 0
0+I
10http://vlex.physik.uni-oldenburg.de/elb_stromquellen_html_m295c2c2b.jpg
Electrolysis
use of voltage to drive reaction
Unefficient, costly…..
11M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010, 6446-6473.
Photoconverters - Semiconductors
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Dual band gap configuration Single band gap device
Vincent Artero et al. ,“Light-driven bioinspired water splitting: Recent developments in photoelectrode materials“, C. R. Chimie 14 (2011) 799–810.
13http://nsl.caltech.edu/_media/research:lizwirepicture.png?cache=&w=316&h=368 (pic taken at 2012/3/9)
Photoanode for Water Oxidation
Photocathode for Hydrogen Evolution
Water-Splitting Membrane
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N-type SC: Electric field generated by band bending directs holes towards solution
Photoanodes for Water Splitting
M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010, 6446-6473.
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Photoanodes for Water Splitting
M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010, 6446-6473.
Recombination pathways for photoexcited carriers
Jbr= recombination on the balk (radiative or non-radiative)Jdr= depletion region recombinationJss= surface recombination due to defectsJt= tunneling currentJet= e- overcome inferfacial barrier (thermoionic emission)Jss= get trapped in defects
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Crucial requirement:Stable under water oxidization conditions
Photoanodes -Materials
Mostly Metal-oxides(TiO3 also with Ba and Sr….)
Catalysts for TiO2: K
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Membranes
http://nsl.caltech.edu/_media/research:membrane:membrane1.jpg?cache=
Impermeable to H2 and O2
• Wires are grown by vapour-liquid-solid (VLS) growth on Si(111) at 1000°C• Diameter: 1.5µm-2µm, lenth: 100µm
Right:http://nsl.caltech.edu/_media/research:membrane:membrane3.jpg?cache=
Top:Plass et al, Flexible Polymer-Embedded Si Wire Arays, Avd. Mat., 2009
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Si wire arrays embedded in thin Nafion films
2 µm20 µm
100 µm 100 µm
http://nsl.caltech.edu/_detail/research:membrane:membrane2.jpg?id=research%3Amembrane
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Photocathodes for Hydrogen Evolution
Fermi level (SC) equilibration with electrochemical potential of the liquid by transferring charge across interface Photoexcitation injects e- from solid to solution
Acidic environment:
2H+ + 2e- H2 (low pH)2H2O + 2e- H2 + 2OH- (high pH)
P-type semiconductor
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GaP drawback:Small carrier diffusion length relativeto absorption depth of light
InP:Scarcity and high demand makes limits availability
P-Si: stable in acidic environmentEfficiency enhances by Pt-nanoparticles
22A. Heller et al.,“Transparent” Metals: Preparation and Characterizationof Light-Transmitting Platinum Films, J. Phys. Chem. 1985, 89, 4444-4452
• Kinetics of HER limits efficiencys
• Requires overpotentials
• Calalyst on surface can improve kinetics
• Metal cat: particles are smaller than wavelenght of photons
• Metal film „optically transparent“
• Does not change light absorption properties of SC
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Efficiencies
Theoretical efficiency:
S
J convexg
Jg= absorbed photon fluxµex= excess chemical potential generated by light absorptionΦconv= quantum yield for absorbet photonsS= total incident solar irradiance (mW/cm2)
Theoretical values
Single SC cell (S2) : 30%
Dual band gap (D4), tandem configuration: 41 %
In praxis: < 10%
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Ongoing research….
• Materials with high absorbance in the visible solar spectrum
• Suitable for both oxygen and hydrogen evolution• Stable under acidic enironment (cathodes)• Stable under permanent illumination (CdS and
CdSe are instable for instance)• Promising materials: nitride or oxynitride
compounds, composite oxides like In1-xNxTiO4
• Catalysts based on non-nobel metals
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Artificial Leaf
http://images.sciencedaily.com/2008/03/080325104519-large.jpg
Mimicking Photosynthesis:
H2 and O2 generated with inorganic materials using catalysts interfaced withlight harvesting SC
Storage mechanism for sunlight!!!
Use of earth-abundant metals andcobalt as catalysts
Electrode: a-Si
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• Co-OEC similar to OEC in PSII
• Co-OEC depostited on a Indium Tin Oxide (ITO) layer
• H2 evolving catalyst: NiMoZn
• Efficiencies: 2.5 % (wireless) 4.7% (wired)
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Blue trace: 0.5 M KBi + 1.5 M KNO3
(126 mS/cm)
Red trace: 1 M Kbi (26 mS/cm)
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MissionJCAP will develop and demonstrate a manufacturable solar-fuels generator, made of Earth-abundant elements, that will take sunlight, water and carbon dioxide as inputs, and robustly produce fuel from the sun 10 times more efficiently than typical current crops.
MembersJCAP partners include the California Institute of Technology, Lawrence Berkeley National Laboratory, the SLAC National Accelerator Laboratory, UC Berkeley, UC Santa Barbara, UC Irvine, and UC San Diego.
http://solarfuelshub.org/
Amount$122 million over five years, subject to Congressional appropriations.
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„….. That‘s where the future is, it‘s not that bad [….]
it‘s a message of hope, we just have to deal with
water and sun and you‘ll be fine“
Daniel Nocera, Talk: Personalized Energy, 2010