3 Solar is only 0.1 % of the market Material costs, prizes, efficiency……Availability: Can run a society only when sun shinesWill 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)AvailabilityStorageEco-friendlySustainable
7 What nature does…. O2 + „H2“ = NADPH Sugar OEC CO2 Most of the energy storage is beendone 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 bondsH HOHH„fuel“ with highestenergy output relativeto molecular weight
9 Photoelectrolysis H2O H2 + ½ O2 Photoconverter H2O O2 + 2H+ + 2e- ΔG=237.2 kJ/molΔE°= 1.23 V /e(at least….overpotentials)H2O H2 + ½ O2-IIH2O O H e-2 H e H2Oxidation (Anode)+IReduction (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 poorPhotoconverter
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 deviceDual band gap configurationVincent 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 MembranePhotocathodefor 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 solutionM.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010,
15 Photoanodes for Water Splitting Recombination pathways for photoexcited carriersJbr= 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 defectsM.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010,
16 Photoanodes -Materials Crucial requirement:Stable under water oxidization conditionsMostly Metal-oxides(TiO3 also with Ba and Sr….)Catalysts for TiO2: K
18 Diameter: 1.5µm-2µm, lenth: 100µm Wires are grown by vapour-liquid-solid (VLS) growth on Si(111) at 1000°CDiameter: 1.5µm-2µm, lenth: 100µmTop:Plass et al, Flexible Polymer-Embedded Si Wire Arays, Avd. Mat., 2009Right:
20 Photocathodes for Hydrogen Evolution Acidic environment:2H+ + 2e H (low pH)2H2O + 2e H OH (high pH)P-type semiconductorFermi level (SC) equilibration with electrochemical potential of the liquid by transferring charge across interfacePhotoexcitation injects e- from solid to solution
21 P-Si:stable in acidic environmentEfficiency enhances by Pt-nanoparticlesInP:Scarcity and high demand makeslimits availabilityGaP drawback:Small carrier diffusion length relativeto absorption depth of light
22 Kinetics of HER limits efficiencys Requires overpotentials Calalyst on surface can improve kineticsMetal cat: particles are smaller than wavelenght of photonsMetal film „optically transparent“Does not change light absorption properties of SCA. Heller et al.,“Transparent” Metals: Preparation and Characterizationof Light-Transmitting Platinum Films, J. Phys. Chem. 1985, 89,
23 Efficiencies Theoretical efficiency: Theoretical values Jg= absorbed photon fluxµex= excess chemical potential generated by light absorptionΦconv= quantum yield for absorbet photonsS= total incident solar irradiance (mW/cm2)Theoretical valuesSingle 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 spectrumSuitable for both oxygen and hydrogen evolutionStable 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-xNxTiO4Catalysts based on non-nobel metals
25 Artificial Leaf Mimicking Photosynthesis: H2 and O2 generated with inorganicmaterials using catalysts interfaced withlight harvesting SCUse of earth-abundant metals andcobalt as catalystsElectrode: a-SiStorage mechanism for sunlight!!!
26 Co-OEC depostited on a Indium Tin Oxide (ITO) layer Co-OEC similar to OEC in PSIICo-OEC depostited on a IndiumTin Oxide (ITO) layerH2 evolving catalyst: NiMoZnEfficiencies: 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 withwater and sun and you‘ll be fine“Daniel Nocera, Talk: Personalized Energy, 2010
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