Production of Hydrogen Using Titania Based Photocatalysts Tit i B d Ph t t l t Wonyong Choi School of Environmental Science and Engineering Dept. of Chemical Engineering D f Ch i l E i i Pohang University of Science and Technology (POSTECH) Pohang, KOREA
Solar Energy Based Hydrogen Economy Solar Energy Based Hydrogen Economy
H 2 production CO 2 -producing CO 2 -neutral CO 2 -free H 2 from fossil fuels H 2 from biomass Water splitting Photoelectrolysis Thermo- Thermo- Electrolysis Biological Photocatalysis chemical chemical Oil Oil Coal Coal Natural Natural ( (alkaline, PEM , process process (PEC semiconductor (PEC, semiconductor cycles electrolyzer) process powder/colloid) gas (anaerobic (S-I cycle, etc) (gasification, digestion, pyrolysis, Gasification fermentation) reforming) High-T Steam Solar Nuclear Storage of PV+ PV+ Wind Wind electrolysis electrolysis thermal th l th thermal l electricity l i i reforming/ +EL EL (solar, as H 2 nuclear) water gas shift Proven + CO 2 H 2 technology but costly but costly process + H 2 CO 2 Current Current industrial Ideal solar-to- process hydrogen conversion process but far from commercial i l C Capture & Storage & S realization (CCS)
Solar Energy Solar Energy 1.2 x 10 5 TW 5 (10,000 x Current world demands) • Abundant • Environment-friendly energy source • Safe and Clean f Earth ~ 0.1% of the Earth’s surface (5 times as big as South Korea) Global need + 13 TW 13 TW ~ 10% conversion efficiency 10% i ffi i
Photocatalysis as a mean of solar energy conversion e - A/A - e - H O/H H 2 O/H 2 h ν Δ G 0 (H O Δ G 0 (H 2 O → H 2 + 1/2O 2 ) H + 1/2O ) =56.7 kcal/mol H 2 O/O 2 e - e - D/D + Photocatalyst (usually semiconductors)
Water Splitting on a Photocatalyst Particle 1. Photon absorption 3. Reduction Generation of e - and h + for H 2 evolution with sufficient potentials for with sufficient potentials for 2 Photon H 2 water splitting (Band Engineering) H O H 2 O e - e - h + h + + + 2. Charge Separation and migration to surface x x reaction sites reaction sites H 2 O 4. Oxidation O 2 2 for O 2 evolution
Band Gap Positions in Various Semiconductors eV E vs. NHE Vacuum level 0 -3.0 -1.0 -4.0 1.1 -4.5 H + /H 2 0 3.0 2.3 2.5 -5.0 2 Si +1.0 3.0 2.8 3.4 2.2 3.2 -6.0 3.2 3.2 3.8 GaP GaP +2 0 +2.0 SiC -7.0 CdS +3.0 Fe 2 O 3 -8 0 -8.0 TiO TiO 2 Z O ZnO 2 TiO 2 iO SrTiO 3 Anatase WO 3 Rutile Nb 2 O 5 SnO SnO 2 @ pH 0
Common Strategies for D Developing Visible Light Photocatalysts l i Vi ibl Li ht Ph t t l t 1. Impurity Doping in Wide Band-gap Oxide Semiconductors - transition metal ions (cations) - nitrogen, carbon (anions) 2. Sensitization of Wide Band-gap Oxide Semiconductors - organometallic complexes ( e.g., ruthenium bipyridyl derivatives) - organic dyes organic dyes - inorganic quantum dots ( e.g., CdS) 3 3. Nanohybrid Systems Nanohybrid Systems (metal oxides & chalcogenides, metal nanoparticles, organic & inorganic sensitizers, polymers, etc.)
Dye- Dye -Sensitized TiO Sensitized TiO 2 Solar Cell Solar Cell Schematics of DSSC Performance of DSSC R J sc : short circuit current V oc : open circuit voltage - e - e ff : fill factor Dye* E CB ff = J ( ) (-) J sc × V oc V y (eV) Pt A - A - P max Energy Dye + / 0 Dye + / 0 - e - e A E g (TiO 2 ) Current ≈ 3.2 eV J sc = 18mA/cm 2 J sc - ↔ - + 2e - + 2e - ↔ 3I - ↔ 3I V oc = 0.74V I 3 - + 2e - 3I - - I I 3 E VB 3 3 3 C VB 3 (+ ) (+ ) ff = 73% 0 Semiconductor Solution V oc 0 Voltage
H 2 Production on Dye Production on Dye- -Sensitized TiO Sensitized TiO 2 e - e - + / S S + / S * CB CB H 2 0 Pt Pt + OH - 1/ 2H 1/ 2H 2 + OH 1/ 2H 1/ 2H + OH + OH D/ D + D/ D + / S S + / S / VB VB 2H + + 2e - (1) H 2 ( E 0 = -0.41 V) 2H 2 O O 2 + 4H + + 4e - ( E 0 = + 0.82 V) (2) Δ Δ (3) (3) H O H 2 O H 2 + 1/2O 2 ( H + 1/2O ( G G = 1.23 V) 1 23 V)
Hydrogen Production with Dye-Sensitized TiO 2 Controlling/Modifying Interfacial Properties : • Sensitizer anchoring mode • Ion-exchange resin coating • Barrier layer coating • Hybridization with carbon nanotubes • Non-Ruthenium Dye sensitized systems
Anchoring Group Anchoring Group Different ways of anchoring molecules on surfaces Different ways of anchoring molecules on surfaces
HOOC c-Ru II L 3 COOH COOH 3 HOOC N N N Ru COOH N N 20 c-RuL 3 N mol g -1 ) p-RuL 3 HOOC 15 uL 3 ] ad ( μ m COOH 10 Tris(4,4’-dicarboxy-2,2’-bipyridyl) Ruthenium(II) [Ru 5 5 p-Ru II L 3 N N 0 2 4 6 8 10 12 pH H Ru N N N N pH-dependent adsorption of the Ru-sensitizer on TiO 2 ([TiO 2 ] = 0.5 g/L, [RuL 3 ] = 10 μ M) P O P O OH HO HO OH (B (Bpy) 2 (4,4’-bis(phosphonato)-2,2’-bipyridyl) Ruthenium(II) ) (4 4’ bi ( h h t ) 2 2’ bi id l) R th i (II) Bae et al., J. Phys. Chem. B 2004 , 108, 14903
Anchoring Groups in Ru-Sensitizers Carboxyl OH OH OH OH OH O O O O O OH N N N N N N Ru N O O O N Ru O N O N Ru N N N N N N N N HO OH HO OH O O O O C4 C6 C2 OH OH OH OH O OH OH OH O O Phosphonic p OH O O P P P P P OH OH P P OH N N O N N O N N O O O Ru N N Ru Ru OH N N N N P OH HO P P HO P N HO P N N N N N OH OH OH OH OH OH O P P O OH OH OH OH P2 P4 P6
A Anchoring Group Effect: pH-dependent Hydrogen Production h i G Eff t H d d t H d P d ti on Ru II /Pt-TiO 2 under Visible-light Illumination 30 160 140 25 120 P2 P4 20 100 P6 ol) l) H 2 ( μ mo C2 C2 H 2 ( μ mo 15 C4 80 C6 60 10 40 5 20 0 0 2 2 4 4 6 6 8 8 10 10 2 2 4 4 6 6 8 8 10 10 pH pH [Pt/TiO 2 ] = 0.5 g/L; [RuL x ] i = 10 μ M; [EDTA] = 10 mM; λ > 420 nm 10 μ M; [EDTA] 10 mM; λ [Pt/TiO 2 ] 0.5 g/L; [RuL x ] i 420 nm (Bae and Choi, J. Phys. Chem. B 2006 , 110 , 14792)
TiO 2 Surface Modification w ith Nafion TiO Surface Modification w ith Nafion (CF 2 CF 2 ) x ( 2 ) x (CFCF 2 ) ( 2 ) 2 O O – SO - (CF 2 CFO)CF 2 CF 2 O O – CF 3 – – Monomer unit of nafion – – – Cation-exchanger TiO 2 – Stable against photocatalytic oxidation – – 5.0 nm – – hydrophobic – zone Interface 1.0 nm 4.0 nm Nafion-Coated TiO 2 Particle aqueous zone zone ( H P k ( H. Park and W. Choi, J. Phys. Chem. B d W Ch i J Ph Ch B 2005 , 109 , 11667 ) sulfonic fluorocarbon anion chain
2+ /Nafion 2+ Ru(dcbpy) Ru(dcbpy) 3 -TiO TiO 2 vs. vs. Ru(bpy) Ru(bpy) 3 Nafion/TiO /TiO 2 OH OH O O Pt Pt H + H + (a) h v e - N N Ru N O O N O Ti N N L 2 'Ru II (bpy) H 2 H 2 HO L 2 Ru (bpy) OH C C Ti O TiO 2 O O O O (b) h v Ru II (dcbpy) 3 [H + ] Nf > [H + ] aq H + - - e - H + 2+ 2+ RuL 3 H + - - - - H 2 H + - - - N N H + RuL 2+ 2 Ru Ru N N RuL 3 H + H + N N H + TiO 2 - - - H + N N Nafion layer Nafion layer Solution TiO 2 Ru II (bpy) 3 2+
Photo-sensitized H 2 Production in tw o anchoring systems tw o anchoring systems 30 20 2+ + TiO 2 Ru(bpy) 3 Ru(dcbpy) + TiO 2 25 25 ex] ads ( μ M) ) 2+ + Nf-TiO 2 Ru(bpy) 3 15 Pt/P25 mol h -1 ) 20 Nf-Pt/P25 [H 2 ] ( μ m 15 15 Ru-comple 10 10 10 5 [R 5 5 0 0 2 3 4 5 6 7 8 2 4 6 8 10 pH pH ( H. Park and W. Choi, Langmuir 2006 , 22 , 2906 )
Photoelectrochemical Hydrogen Production Carbon Nanotube Assisted Generation of Hydrogen in Dye-Sensitized Photoelectrochemical Cell under Visible Light e - e - e - e - (-0.62 V NHE ) (-0.62 V NHE ) dye * /dye + dye * /dye + h v h v TiO 2 CB TiO 2 CB TiO 2 CB TiO 2 CB ½ H 2 ½ H 2 CNT CNT (-0.5 V NHE ) (-0.5 V NHE ) (~ 0 V NHE ) (~ 0 V NHE ) h v h v dye dye H + H + e - e - e - e - e - e - e - e - nafion nafion fi fi D D matrix matrix dye/dye + dye/dye + FTO FTO FTO FTO Pt Pt e - e - e - e - D + D + dye dye TiO 2 /Nafion/CNT/Dye electrode ( J P k ( J. Park and W. Choi, J. Phys. Chem. C d W Ch i J Ph Ch C 2009 , 113 , 20974) 1 µm
Photoelectrochemical Hydrogen Production 1.2 1.2 14 E1 (current) 0.30 {CNT+TiO 2 }/Nf 1.0 E2 (current) (mA) 2+ 12 {CNT+TiO 2 }/Nf/Ru(bpy) 3 E1 (H 2 ) {TiO 2 }/Nf 0.8 0 8 l) E2 (H 2 ) E2 (H ) 10 10 Δ H 2 ( μ mol tocurrent (1-R) 2 /2R 0.20 8 0.6 6 CNT CNT CNT CNT 0.4 0 4 Δ Phot 0.10 4 0.2 2 0 00 0.00 0.0 0 0 0 0 0 10 20 30 40 50 60 300 400 500 600 700 Wavelength (nm) Irradiation time (min) E1: TiO 2 /Nf/RuL 3 with CNT E2: without CNT
Dye-Sensitized TiO 2 with Thin Overcoat of Al 2 O 3 Dye-Sensitized TiO 2 with Thin Overcoat of Al 2 O 3 100 80 ) [H 2 ] ( μ mol 60 40 Al 2 O 3 /TiO 2 /Pt TiO 2 /Pt 20 0 0 50 100 150 200 Al 2p Al 2p Al O /TiO /Pt Al 2 O 3 /TiO 2 /Pt Intensity (a TiO 2 /Pt 0.25 nm arb. unit) Al 2 O 3 80 78 76 74 72 70 68 (W. Kim et al., J. Phys. Chem. C 2009 , 113, 10603) XPS Binding Energy (eV)
Organic Dye Organic Dye Donor Acceptor Strong Intra-molecular Charge Transfer O Organic Dye g y O O (LUMO) NC N COOH S O S O O CB e − h ν h ν TiO 2 VB VB (HOMO) (Y. Park et al., Chem. Commun. 2010 , 46 , 2477) 22
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