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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

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Solar Water Splitting cells

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

  2. Overview • Motivation • Photosynthesis • ArtificialPhotosynthesis • Photoelectrolysis • Devices • „Artificialleaf“ • Outlook http://www.wissenschaft-aktuell.de/onTEAM/grafik/31286779743.jpg

  3. Solar isonly 0.1 % ofthemarket Material costs, prizes, efficiency…… Availability:Can run a societyonlywhensunshines Will havedifficulty in penetrating a marketuntilitcanbestored

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

  5. Whyfuel….? ENERGY O H H C O O High amountofenergystored in chemicalbonds….

  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) Availability Storage Eco-friendly Sustainable

  7. Whatnaturedoes…. OEC O2+ „H2“ = NADPH Sugar CO2 Most oftheenergystorageisbeen done in watersplitting….. not in CO2fixation !!!

  8. ENERGY Solar inputtomake lowenergybondsto high energybonds H H O H H O H H C O O O H H H H „fuel“ withhighest energyoutputrelative tomolecularweight

  9. Photoelectrolysis ΔG=237.2 kJ/mol ΔE°= 1.23 V /e (at least….overpotentials) H2O H2 + ½ O2 0 -II H2O O2 + 2H+ + 2e- 2 H+ + 2 e- H2 Oxidation (Anode) 0 +I Reduction (Cathode) Solar spectrumabsorbationofwaterpoor Photoconverter

  10. Electrolysis useofvoltagetodrivereaction Unefficient, costly….. http://vlex.physik.uni-oldenburg.de/elb_stromquellen_html_m295c2c2b.jpg

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

  12. Dual band gapconfiguration Single band gapdevice Vincent Artero et al. ,“Light-driven bioinspired water splitting: Recent developments in photoelectrodematerials“, C. R. Chimie 14 (2011) 799–810.

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

  14. Photoanodes for Water Splitting N-type SC: Electricfieldgeneratedby band bendingdirectsholestowardssolution M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010, 6446-6473.

  15. PhotoanodesforWater Splitting Recombinationpathwaysforphotoexcitedcarriers Jbr= recombination on thebalk (radiativeor non-radiative) Jdr= depletionregionrecombination Jss= surfacerecombination due todefects Jt= tunnelingcurrent Jet= e-overcomeinferfacialbarrier (thermoionicemission) Jss= gettrapped in defects M.G. Walter et al., Solar Water Splitting Cells, Chem. Rev. 2010, 6446-6473.

  16. Photoanodes-Materials Crucialrequirement: Stableunderwateroxidizationconditions MostlyMetal-oxides (TiO3 also withBaandSr….) Catalystsfor TiO2: K

  17. Membranes Impermeable to H2and O2 http://nsl.caltech.edu/_media/research:membrane:membrane1.jpg?cache=

  18. Wiresaregrownbyvapour-liquid-solid (VLS) growth on Si(111) at 1000°C • Diameter: 1.5µm-2µm, lenth: 100µm Top: Plass et al, Flexible Polymer-Embedded Si WireArays, Avd. Mat., 2009 Right: http://nsl.caltech.edu/_media/research:membrane:membrane3.jpg?cache=

  19. Si wirearraysembedded in thinNafionfilms 100 µm 100 µm 20 µm 2 µm http://nsl.caltech.edu/_detail/research:membrane:membrane2.jpg?id=research%3Amembrane

  20. Photocathodesfor Hydrogen Evolution Acidicenvironment: 2H+ + 2e- H2 (low pH) 2H2O + 2e- H2 + 2OH- (high pH) P-type semiconductor Fermi level(SC) equilibrationwithelectrochemicalpotential ofthe liquid by transferring chargeacrossinterface Photoexcitationinjects e-from solid tosolution

  21. P-Si: stable in acidicenvironment Efficiency enhancesbyPt-nanoparticles InP: Scarcityandhighdemandmakes limitsavailability GaPdrawback: Small carrierdiffusionlength relative toabsorptiondepthoflight

  22. Kineticsof HER limitsefficiencys • Requiresoverpotentials • Calalyst on surfacecanimprovekinetics • Metalcat: particlesaresmallerthanwavelenghtofphotons • Metal film „optically transparent“ • Does not change light absorptionpropertiesof SC A. Heller et al.,“Transparent” Metals: Preparation and Characterization of Light-Transmitting Platinum Films, J. Phys. Chem. 1985, 89, 4444-4452

  23. Efficiencies Theoreticalefficiency: Jg= absorbedphotonflux µex= excesschemical potential generatedbylightabsorption Φconv= quantumyieldforabsorbetphotons S= total incident solar irradiance (mW/cm2) Theoreticalvalues Single SC cell (S2) : 30% Dual band gap (D4), tandemconfiguration: 41 % In praxis: < 10%

  24. Ongoingresearch…. • Materials with high absorbance in thevisible solar spectrum • Suitableforbothoxygenand hydrogen evolution • Stableunderacidicenironment (cathodes) • Stableunder permanent illumination (CdSandCdSeareinstableforinstance) • Promising materials: nitrideoroxynitridecompounds, compositeoxideslike In1-xNxTiO4 • Catalystsbased on non-nobel metals

  25. ArtificialLeaf MimickingPhotosynthesis: H2and O2generatedwithinorganic materialsusingcatalystsinterfacedwith light harvesting SC Useofearth-abundant metalsand cobaltascatalysts Electrode: a-Si http://images.sciencedaily.com/2008/03/080325104519-large.jpg Storage mechanismforsunlight!!!

  26. Co-OEC similarto OEC in PSII • Co-OEC depostited on a Indium • Tin Oxide (ITO) layer • H2evolvingcatalyst: NiMoZn • Efficiencies: 2.5 % (wireless) • 4.7% (wired)

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

  28. http://solarfuelshub.org/ 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 partnersincludetheCalifornia 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.

  29. „….. That‘swherethefutureis, it‘s not thatbad [….] it‘s a messageofhope, we just haveto deal with waterandsunandyou‘llbefine“ Daniel Nocera, Talk: PersonalizedEnergy, 2010

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