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