Chemical Systems Engineering The University of Tokyo
Chemical System Engineerin The University of Tokyo
Hydrogen Production from Water using Solar Energy
H2O
hν
H2
+
1 ― O2 2
ΔG0 = 238 kJmol-1
▪ Solar cell + Electrolysis : pilot plants
▪ Photoelectrochemical Cell : good efficiencies with tandem cells
Chemical System Engineerin The University of Tokyo
Two examples of photoelectrochemical cells
Efficiency=12.4 % J. A. Turner et al., Science 1998,
Efficiency=18.3 % S. Licht et al., J. Phys. Chem. B 2000,
Chemical System Engineerin The University of Tokyo
Photoelectrochemical cells + PV
M. Grätzel, Nature 2001, 414, 338–344.
solar energy conversion efficiency = ３～４％
Chemical System Engineerin The University of Tokyo
H2O
hν
H2
+
1 ― O2 2
ΔG0 = 238 kJmol-1
▪ Solar cell + Electrolysis ▪ Photoelectrochemical Cell ▪ Artificial Photosynthesis (Photocatalysis) Inorganic solid material metal complex organic material biomaterial
▪ Photosynthesis
Chemical System Engineerin The University of Tokyo
Available photons for water splitting H2O → H2 + 1/2O2 ΔG0 = 237 kJ/mol = n×F ×E0 = 2×96500 C/mol×1.23 V One electron energy = 1.23 eV Photon energy 1238 E (eV) = hν = λ (nm ) 1.23 eV
λ=1000 nm
Solar Energy Distribution Radiation Power kWm-2μm-1
3 UV
Vis
IR
1.23 eV =1000 nm
1.5
0 0
500
1000
Wavelength λ/nm
1500
2000
Solar Energy Conversion Efficiency
Solar energy conversion efficiency (Q.E.=100 %) Wavelength / nm
Energy efficiency /%
400
1.67
450
4.29
500
7.95
550
11.9
600
16.2
650
20.6
700
25.1
750
29.4
800
33.5
SEM Images of La-doped NaTaO3
0.1 µm
Prof. A. Kudo (Tokyo Univ. Sci.)
Overall water splitting on NiO(0.2wt%)/NaTaO3 :La
Layered photocatalyst
200W Xe-Hg Lamp
BG:4.1eV QY:50% (270nm)
Prof. A. Kudo (Tokyo Univ. Sci.)
Time Course of Overall Water Splitting on NiO(0.2wt%)/NaTaO3:La
Q.E. = 56 %
Challenge with Visible Light H2
H2O
+
1 ― O2 2
ΔG0 = 238 kJmol-1
Photocatalytic Materials with
?
・Sufficient Solar Energy (Visible Light) Absorption ・Potential for Overall Water Splitting ・Enough Stability
Chemical System Engineerin The University of Tokyo
GaN:ZnO solid solution RuO2 : 3.5 wt%
1 μm
GaN
GaN:ZnO
ZnO
Chemical System Engineerin The University of Tokyo
Visible Light-Driven Overall Water Splitting on Rh-Cr oxide/GaN:ZnO Amount of evolved gases / mmol
Evac. ▼
2.0
λ 400 nm nm λ >> 400 H2
1.5
Catalyst: Catalyst:0.3 0.3gg Cocat.: Cocat.:Rh Rh11wt%, wt%, Cr Cr1.5 1.5wt% wt% Reactant soln.: 370 mL (pH Reactant soln.: 370 mL (pH4.5) 4.5) Inner irradiation type cell Inner irradiation type cell with withaa22MMNaNO NaNO22aq. aq.filter filter 450 450W WHg Hglamp lamp
1.0 O2 450 W high pressure Hg lamp
0.5
0 0
5
10 15 20 25 Reaction time / h
30
N2
Water cooling
35
NaNO2 aq. Magnetic stirrer
Chemical System Engineerin The University of Tokyo
Electron micrographs of Rh-Cr oxide/GaN:ZnO
TEM image
SEM image
O2 H 2 O H2
H+
Rh-Cr mixed oxide 30 nm
eh+
hν
Chemical System Engineerin The University of Tokyo
Overall water splitting on Rh-Cr oxide/GaN:ZnO λλ >> 300 300 nm nm
Chemical System Engineerin The University of Tokyo
Q.E. of overall water splitting on Rh-Cr oxide/GaN:ZnO 5.9 %
4.5 3.5
Absorbance / a.u.
Quantum efficiency / %
4.0 3.0
Q.E.
2.5 2.0 1.5 1.0
RuO2-loaded (Q.E. 0.23%)
0.5 0
300
350
400
450
500
Wavelength / nm
Chemical System Engineerin The University of Tokyo
Two approaches for overall water splitting One-step excitation
Two-step excitation (Z-scheme) e–
e–
H2
H+
H+
H+ / H2
e–
hν
Red
hν
Ox
hν
O2 / H2O H2O H2O
h+
O2
H2
O2
h+
H2 evolution h+
O2 evolution
Chemical System Engineerin The University of Tokyo
Various Oxynitrides
BaTaO2N
Ta3N5
LaTaON2
LaTiO2N
SrTaO2N
CaTaO2N
Li2LaTa2O6N
CaLaTiON
Oxide
Chemical System Engineerin The University of Tokyo
Two-step overall water splitting under visible light e-
eIIO3H2O
H2 H+
e-
hν e-
hν
O2 e
h+ h+
Pt/WO3 (λ < 450 nm)
H2 O2 N2
200
Pt/TaON (λ < 500 nm)
Amount of evolved gases / μmol
e-
150
100
50
0 0
5
10
15
20
25
Time / h (Reaction conditions) Pt/TaON: 0.2 g + Pt/WO3: 0.2 g 5 mM-NaI aqueous solution: 250 mL (pH = 6.5) 300 W Xe-lamp, L-42 cut-off filter
Chem. Commun. 2005, 3829
Chemical System Engineerin The University of Tokyo
eeee-
H+
I– IO3–
hν
H2O O2
H2
hν eh+
e-
Pt/BaTaO2N
h+
(λ < 670 nm)
Pt/WO3
(λ < 450 nm)
Amount of evolved gases / μmol
Two-step overall water splitting on Pt/WO3+Pt/BaTaO2N λ > 420nm 70 60
H2 50 40
O2
30 20 10
N2
0 0
5
10
Time / h
15
20
Chemical System Engineerin The University of Tokyo
Available materials for two-step overall water splitting Ta3N5(O2 evolution) BaTaO2N(H2 evolution)
K-M function (a.u.)
4
BaNbO2N
3
2
1
0 300
400
500 600 Wavelength / nm
700
800
Chemical System Engineerin The University of Tokyo
Application of oxynitrides to PEC cell
N Ta O
Visible light
H2
e–
O2
oxynitride TaON 薄膜電極化 Potential / V vs NHE at pH0
-
(H+/H2)
C.B. Ta5d
-0.3
0
Pt electrode
TaON electrode
E.G. = 2.5 eV 1.23 (O2/H2O)
+2.2 V.B. O2p + N2p + K. Domen et al., J. Phys. Chem. B, 2003, 107, 1798 R. Abe et al., Chem. Commun., 2005, 3829
阿部（北海道大学）
1.2
(a) as-prepared TaON (b) treated with TaCl5 (c) treated with NH3
-2
1.0
Current density / mA cm
ephase boundaries
0.8
e-
0.6
TaCl5-calcination e-
0.4
Ta2O5 fine particles 0.2
e0.0 -0.4
-0.2
0.0 0.2 0.4 0.6 Potential / V vs. Ag/AgCl
0.8
NH3-treatment
1.0
e[Conditions] Supporting electrolyte: 0.1 mol-L-1 of Na2SO4 Potentiostat: PARSTAT 2263 Rate of scan: 50 mV / s Light source: Xe illuminator (300 W, 300 < λ < 500 nm)
TaON fine particles
e-
1.5
-2
Current density / mA cm
Potential (V vs. NHE)
WO3 TaON 1.0
- 0.3
Ta 5d
o
0.5
W 5d
+ 0.5 2.5 eV
0.0
2.6 eV -0.4
0.0 0.4 Potential / V vs. Ag/AgCl
0.8
[Conditions] Supporting electrolyte: 0.1 mol-L-1 of Na2SO4 Potentiostat: PARSTAT 2263 Rate of scan: 50 mV / s Light source: Xe illuminator (300 W, 300 < λ < 500 nm)
O2p + N2p
O2p
Two-room overall water splitting Amount of gas evolved / μmol
1200
Cell 2 Pt/WO3
20 mmol L NaI aq. 500 mL 300 W Xe lamp (λ > 300 nm)
I−
−
e
H+
h+
I− e− h+
IO3−
H2O O2
IO3−
IO3− TaON
800 600 400 200 0
10
300
-1
H2
H2 O2
0
WO3
Amount of gas evolved / μmol
Cell 1 Rh-Cr/GaN:ZnO
Cell 1 1000
20 Time / h
30
40
30
40
Cell 2 250
H2 O2
200 150 100 50 0 0
10
20
Overall water splitting with sacrificial reagents H2O
H2 + 1/2O2 ΔG0 = 238 kJmol-1
2H2O + Na2S H2O + CH3OH 6H2O + C6H12O6
H2 + S + 2NaOH 3H2 + CO2 12H2 + 6CO2
TEM TEM image image of of stratified stratified CdS CdS
CdS capsules (30 nm) consist of nanoparticles (5nm)
Prof. K. Tohji (Tohoku Univ.)
H2 production on stratified CdS from Na2S solution
Prof. K. Tohji (Tohoku Univ.)
Hydrogen Production from Water using Solar Energy ▪ Solar cell + Electrolysis vs
▪ Photoelectrochemical Cell materials
▪ Artificial Photosynthesis (Photocatalysis) Key Issues New materials Hybridization; structure Application to wide area; Reactor design
Chemical System Engineerin The University of Tokyo
