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

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Presentation on theme: "Solar Water Splitting cells Artificial Photosynthesis Verena Schendel 14/03/2012."— Presentation transcript:

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

2 Overview Motivation Photosynthesis Artificial Photosynthesis Photoelectrolysis Devices Artificial leaf Outlook 2

3 3 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……

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

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

6 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) 6 Storage Availability Eco-friendly Sustainable

7 What nature does…. O 2 + H 2 = NADPH Sugar CO 2 Most of the energy storage is been done in water splitting….. not in CO 2 fixation !!! 7 OEC

8 8 OC H H O O ENERGY H H O H H O H Solar input to make low energy bonds to high energy bonds fuel with highest energy output relative to molecular weight

9 Photoelectrolysis H 2 O H 2 + ½ O 2 ΔG=237.2 kJ/mol ΔE°= 1.23 V /e (at least….overpotentials) Solar spectrum absorbation of water poor Photoconverter H 2 O O 2 + 2H + + 2e - 2 H e - H 2 Oxidation (Anode) Reduction (Cathode) -II 0 0 +I 9

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

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

12 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–

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

14 14 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,

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

16 16 Crucial requirement: Stable under water oxidization conditions Photoanodes -Materials Mostly Metal-oxides (TiO 3 also with Ba and Sr….) Catalysts for TiO 2 : K

17 Membranes 17 Impermeable to H 2 and O 2

18 Wires are grown by vapour-liquid-solid (VLS) growth on Si(111) at 1000°C Diameter: 1.5µm-2µm, lenth: 100µm Right: arch:membrane:membrane3.jpg?ca che= Top: Plass et al, Flexible Polymer- Embedded Si Wire Arays, Avd. Mat., 2009

19 19 Si wire arrays embedded in thin Nafion films 2 µm 20 µm 100 µm

20 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 20 Acidic environment: 2H + + 2e - H 2 (low pH) 2H 2 O + 2e - H 2 + 2OH - (high pH) P-type semiconductor

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

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

23 Efficiencies 23 Theoretical efficiency: J g = absorbed photon flux µ ex = excess chemical potential generated by light absorption Φ conv = quantum yield for absorbet photons S= total incident solar irradiance (mW/cm 2 ) 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 In 1-x N x TiO 4 Catalysts based on non-nobel metals 24

25 Artificial Leaf 25 large.jpg Mimicking Photosynthesis: H 2 and O 2 generated with inorganic materials using catalysts interfaced with light harvesting SC Storage mechanism for sunlight!!! Use of earth-abundant metals and cobalt as catalysts Electrode: a-Si

26 26 Co-OEC similar to OEC in PSII Co-OEC depostited on a Indium Tin Oxide (ITO) layer H 2 evolving catalyst: NiMoZn Efficiencies: 2.5 % (wireless) 4.7% (wired)

27 27

28 28 Blue trace: 0.5 M KBi M KNO 3 (126 mS/cm) Red trace: 1 M Kbi (26 mS/cm)

29 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 30 ….. Thats where the future is, its not that bad [….] its a message of hope, we just have to deal with water and sun and youll be fine Daniel Nocera, Talk: Personalized Energy, 2010

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