1. Solar Water Splitting cells
Artificial Photosynthesis
Solar Water Splitting cells
Verena Schendel
14/03/2012

2. Overview Motivation Photosynthesis Artificial Photosynthesis
Photoelectrolysis
Devices
„Artificial leaf“
Outlook

3. Solar is only 0.1 % of the market
Material costs, prizes, efficiency……
Availability: Can run a society only when sun shines
Will have difficulty in penetrating a market until it can be stored

4. Fluctuations, Storage problems, costly,….
Energy density poor….
Fluctuations, Storage problems, costly,….

5. Why fuel….? ENERGY High amount of energy stored in chemical bonds…. O

6. Motivation Storage Sustainable Availability Eco-friendly
“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)
Availability
Storage
Eco-friendly
Sustainable

7. What nature does…. O2 + „H2“ = NADPH Sugar OEC CO2
Most of the energy storage is been
done in water splitting…..
not in CO2 fixation !!!

8. ENERGY O C H H O H H Solar input to make low energy bonds to
high energy bonds
H H O H H
„fuel“ with highest
energy output relative
to molecular weight

9. Photoelectrolysis H2O H2 + ½ O2 Photoconverter H2O O2 + 2H+ + 2e-
ΔG=237.2 kJ/mol
ΔE°= 1.23 V /e
(at least….overpotentials)
H2O H2 + ½ O2
-II
H2O O H e-
2 H e H2
Oxidation (Anode) +I
Reduction (Cathode)
The photodriven conversion of liquid water tp gaseous H2 and O2 id goven by ……
Tht free energy change for the conversion of one molecule of H2O to H2 and O2 under standard condions is Delta_G=…..which corresponds according to the Nernst equ. To Delta_E= 1.23 V per e- tranferred.
However….
Since water itself does not absorb appreciable radiantion within the solar spectrum, so that a more radiant absorbing species (photoconverter) is required to transduce the radiant energy to chemical energy in form of e-/h+;
Solar spectrum absorbation of water poor
Photoconverter

10. Electrolysis use of voltage to drive reaction Unefficient, costly…..

11. Photoconverters - Semiconductors
M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010,

12. Dual band gap configuration
Single band gap device
Dual band gap configuration
Vincent Artero et al. ,“Light-driven bioinspired water splitting: Recent developments in photoelectrode materials“, C. R. Chimie 14 (2011) 799–810.

13. Photoanode for Water Oxidation
Water-Splitting Membrane
Photocathode
for Hydrogen Evolution
(pic taken at 2012/3/9)

14. Photoanodes for Water Splitting
N-type SC: Electric field generated by band bending directs holes towards solution
M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010,

15. Photoanodes for Water Splitting
Recombination pathways for photoexcited carriers
Jbr= recombination on the balk (radiative or non-radiative)
Jdr= depletion region recombination
Jss= surface recombination due to defects
Jt= tunneling current
Jet= e- overcome inferfacial barrier (thermoionic emission)
Jss= get trapped in defects
M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010,

16. Photoanodes -Materials
Crucial requirement:
Stable under water oxidization conditions
Mostly Metal-oxides
(TiO3 also with Ba and Sr….)
Catalysts for TiO2: K

17. Membranes Impermeable to H2 and O2

18. Diameter: 1.5µm-2µm, lenth: 100µm
Wires are grown by vapour-liquid-solid (VLS) growth on Si(111) at 1000°C
Diameter: 1.5µm-2µm, lenth: 100µm
Top:
Plass et al, Flexible Polymer-Embedded Si Wire Arays, Avd. Mat., 2009
Right:

19. Si wire arrays embedded in thin Nafion films

20. Photocathodes for Hydrogen Evolution
Acidic environment:
2H+ + 2e H (low pH)
2H2O + 2e H OH (high pH)
P-type semiconductor
Fermi level (SC) equilibration with electrochemical potential of the liquid by transferring charge across interface
Photoexcitation injects e- from solid to solution

21. P-Si:
stable in acidic environment
Efficiency enhances by Pt-nanoparticles
InP:
Scarcity and high demand makes
limits availability
GaP drawback:
Small carrier diffusion length relative
to absorption depth of light

22. 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
A. Heller et al.,“Transparent” Metals: Preparation and Characterization
of Light-Transmitting Platinum Films, J. Phys. Chem. 1985, 89,

23. Efficiencies Theoretical efficiency: Theoretical values
Jg= absorbed photon flux
µex= excess chemical potential generated by light absorption
Φconv= quantum yield for absorbet photons
S= total incident solar irradiance (mW/cm2)
Theoretical values
Single SC cell (S2) : 30%
Dual band gap (D4), tandem configuration: 41 %
In praxis: < 10%

24. 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

25. Artificial Leaf Mimicking Photosynthesis:
H2 and O2 generated with inorganic
materials using catalysts interfaced with
light harvesting SC
Use of earth-abundant metals and
cobalt as catalysts
Electrode: a-Si
Storage mechanism for sunlight!!!

26. Co-OEC depostited on a Indium Tin Oxide (ITO) layer
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)

28. Blue trace:
0.5 M KBi M KNO3
(126 mS/cm)
Red trace:
1 M Kbi (26 mS/cm)

29.
Mission JCAP 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.
Members JCAP 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.
Amount $122 million over five years, subject to Congressional appropriations.

30. „….. 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