Solar Water Splitting cells Artificial Photosynthesis Solar Water Splitting cells Verena Schendel 14/03/2012 http://images.sciencedaily.com/2008/03/080325104519-large.jpg

Overview Motivation Photosynthesis Artificial Photosynthesis Photoelectrolysis Devices „Artificial leaf“ Outlook http://www.wissenschaft-aktuell.de/onTEAM/grafik/31286779743.jpg

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

Fluctuations, Storage problems, costly,…. Energy density poor…. http://www.bike-components.de/images/logos/batterien.jpg Fluctuations, Storage problems, costly,…. http://www.klima-suchtschutz.de/uploads/pics/windenergie-anlage.jpg

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

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

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

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

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 O2 + 2H+ + 2e- 2 H+ + 2 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

Electrolysis use of voltage to drive reaction Unefficient, costly….. http://vlex.physik.uni-oldenburg.de/elb_stromquellen_html_m295c2c2b.jpg

Photoconverters - Semiconductors M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010, 6446-6473.

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.

Photoanode for Water Oxidation Water-Splitting Membrane Photocathode for Hydrogen Evolution http://nsl.caltech.edu/_media/research:lizwirepicture.png?cache=&w=316&h=368 (pic taken at 2012/3/9)

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, 6446-6473.

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, 6446-6473.

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

Membranes Impermeable to H2 and O2 http://nsl.caltech.edu/_media/research:membrane:membrane1.jpg?cache=

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: http://nsl.caltech.edu/_media/research:membrane:membrane3.jpg?cache=

Si wire arrays embedded in thin Nafion films http://nsl.caltech.edu/_detail/research:membrane:membrane2.jpg?id=research%3Amembrane

Photocathodes for Hydrogen Evolution Acidic environment: 2H+ + 2e- H2 (low pH) 2H2O + 2e- H2 + 2OH- (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

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

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, 4444-4452

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%

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

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 http://images.sciencedaily.com/2008/03/080325104519-large.jpg Storage mechanism for sunlight!!!

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)

Blue trace: 0.5 M KBi + 1.5 M KNO3 (126 mS/cm) Red trace: 1 M Kbi (26 mS/cm)

http://solarfuelshub.org/ 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.

„….. 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