Solar Hydrogen Generation For Energy Storage. Laura Meda Istituto eni-donegani Via Fauser 4, Novara

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1 Solar Hydrogen Generation For Energy Storage Laura Meda Istituto eni-donegani Via Fauser 4, Novara Energia e idrogeno: L esperienza italiana nel programma europeo Idrogeno e celle a combustibile FAST 13 DIC 2012

2 Solar Energy Conversion Strategies Light Fuels Chemicals Electricity CO 2 Sugar O 2 e H 2 e H O 2 O 2 Photosynthesis sc H2O M Semiconductor/Liquid Junctions n p Photovoltaics [N.S. Lewis, D.G. Nocera, Powering the Planet, Proc. Nat. Acad. Sci. USA 103 (43) 2006, 15729]

3 Possible applications of photosplitting technology Electricity : solar H2 in fuel cell to generate electricity off-grid Energy Storage : accumulation of chemical energy in flow batteries Environment : photo-redox reactions of wastewater Fuels : reaction of H2 with CO2 to fuels Automotive : addition of H2 to CH4 in vehicles to decrease emissions 3

4 Water Splitting in a PEC cell SUN ENERGY CHEMICAL ENERGY Semiconductor materials can absorb sunlight; generate carriers; promote redox reactions with water O2 e - H 2 H 2 O 2 e - e - H 2 metal H 2 O h + OH - H +

5 Glass Conductive support Pt Counter-electrode Reactions and Energetic diagrams hν + semic. e - + h + 2 H 2 O + 4 h + O H e - 4 H e - 2 H hν + semic. + 2 H 2 O O H 2 G = kj/mol V bias e - e - e - 2H 2 O+2e - 2OH - +H 2 Light 1.23 ev h + 2OH - +2h + H 2 O+1/2O 2 Semiconductor Electrolyte 0 L b x

6 Semiconductor Candidates [M. Gratzel, Nature 414 (2001) 338] Low bandgap Good harvesting; Scarce stability High bandgap Scarce harvesting; Good stability ph 1 Thermodynamic request 1.23 ev Kinetic losses & overpotentials E g > 2 ev [K. Rajeshwar, J. Appl. Electrochem.37 (2007) 765]

7 Nanostructured Photo-electrodes WO3 solgel Xstal domains nm Area max. 100 cm 2 Fe2O3 + Ti spray Xstal domains nm Area max. 100 cm 2 TiO2 -anodic Xstal domains nm Area max. 150 cm 2 WO3 -anodic Xstal domains nm Area max. 150 cm 2

8 Sol-gel nanostructures AFM SEM 10 x 10 cm2 From tungstate salts to acidic solution + colloidal dispersion and gelification + blade deposition on conductive glass (FTO) + final calcination (450 C 650 C) [L. Meda, G. Tozzola, A.Tacca, G. Marra, et al, SOLMAT 94 (2010) 788]

9 Electro-anodized nanostructures 30 V Ti: EG + NH4F, RT + calc. 500 C W: H2O + NMF + NH4F, 40 C + calc. 500 C S. Caramori, V. Cristino, C.A. Bignozzi, L.Meda, Topics in Current Chem. Vol. 303 (2011) 215

10 Absorbance Absorbance Spray deposited Nanostructures Fe2O3 + Ti 5% Solutions sprayed on FTO at high temperature (> 400 C) Subtraction Result:*30202/ Subtraction Result:*30202/85 Subtraction Result:*30202/ Subtraction Result:*30202/81 Subtraction Result:*30202/80 Subtraction Result:*30202/87A 2.0 6, 10, 15, 20, 30 steps Photoanode on FTO Wavelength (nm) Wavelength

11 Current-voltage measurements and gas collection under solar simulated illumination N2 carrier Voltmeter Amperometer Counter O2 Counter H2 Gas cromatograph O2 H2 ABET Tech. solar simulator 550W AM 1.5G filtered 10x10 cm2 uniform area Photo anode Catode

12 J(mA/cm 2 ) Anodized WO M H 2 SO 4 1M H 2 SO 4 /MeOH 8/ ,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 V (V vs SCE) S. Caramori, V. Cristino, C.A. Bignozzi, L.Meda, Topics in Current Chem. Vol. 303 (2011), 215 V. Cristino, S. Caramori, CA. Bignozzi, L. Meda Langmuir 27(11) 2011, 7276 A. Tacca, L. Meda,G. Marra, A. Savoini, S. Caramori, V. Cristino, C. Bignozzi, S. Gimenez, J. Bisquert Chem PhysChem, 13(12) 2012,

13 IPCE % Efficiencies : Energy OUT / Energy IN Quantum Efficiency = electrons / photons (V nm) x J (ma/cm 2 ) IPCE % = P (mw/cm 2 ) x λ (nm) % UV IPCE SPECTRA OF WO 3 PHOTOANODES 40 % Vis anodically grown at 1V vs SCE anodically grown at 1V vs SCE colloidal at 1.5 V vs SCE (nm) Global Efficiency STH (energy gain) J x ( V - V bias ) STH % {A.M. 1.5} = x 100 P = 3,15 % J = 5 ma/cm 2 V = 1,23 V V bias = 0,6 V P (1 sun) = 100 mw/cm 2 Z. Chen et al., J. Mater. Res. 25 (2010) 3

14 Efficiency for a PEC I e Light on The pink area represents the power spent due to the applied bias. Dark The blue area represents the power gained and stored as hydrogen. The sum pink + blue areas represents the global stored power. I PEC The grey area is the power spent for electrolysis. V PEC 1,23 V EL Storage Efficiency (total power converted in stored chemical energy) J x 1,23 STO. EFF. % {A.M. 1.5} = x 100 = 6,20 % J = 5 ma/cm 2 P V = 1,23 V V bias = 0,6 V P (1 sun) = 100 mw/cm 2

15 From photocurrent density to evolved H2 J A-C [ A moli] [moli H2 ] = Fa [cm 2 coulomb] [ s cm 2 ] 1 Fa = [coulomb/mole] 1 mole = [litri] For J A-C = 1 10 [ma /cm 2 ] The evolved H 2 = [ml/min 100 cm 2 ] = 4 40 [ l/h m 2 ] 6 63 [ml/min 900 cm 2 ] To feed a Fuel Cell (1W) => 14 [ml/min] are required

16 Outdoor demonstrators DEMO1 DEMO2 PEC cell PHOTO-CURRENT density H2 production PEC cell COST SOLAR H2 COST [ma/cm 2 ] [NL/h/m 2 ] [ /m 2 ] [ /kg] DEMO1 - TiO DEMO2 - WO EU target (2020)for solar H2 = 9.9 /kg

17 Comparison between PV+EL and PEC - - PV + EL Si-PV absorbs 60 % of solar spectrum Efficiency STH: 14% x 45-70% = 6-10 % High Energy dissipation Voltage required : > 2 V Solar H2 Cost : 30 /kg Lifetime 5 years PEC WO3 absorbs 9 % of solar spectrum Efficiency STH: 3 % Lower Energy dissipation Voltage required : < 1 V - zero Solar H2 Cost : /kg Lifetime years (?) Improvements are in PV efficiency... there are continuous improvements and 10% is not impossible

18 Thanks to collaborators! I believe that water will one day be employed as fuel, that hydrogen and oxygen that constitute it will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable Water will be the coal of the future! From: L Ile mystérieuse by Jules Verne (1874) L. Abbondanza G. Bianchi R. Paglino R. Preda A. Romano F. Rubertelli F. Simone A. Tacca G. Tozzola