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216th ECS Meeting: October 8, 2009
Fe2O3 Photoanodes
for Hydrogen Production Using Solar Energy
S. Dennison, K. Hellgardt, G.H. Kelsall,
Department of Chemical EngineeringImperial College London, SW7 2AZ, UK
Project Objectives
• Solar-powered hydrogen generation systems:
BiophotolysisPhotoelectrolysis
Assessment of materials for photoelectrodes
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Photoelectrolysis of water
( , )absorptionCB VBSemiconductor h Semiconductor e h
2 22 4 4VBH O h O H
2 22 2 2CBH O e H OH
Requires > 1.5 V ( < ca. 830 nm)
2
Ef
Energy requirements for Photoelectrolysis of water
H+ / H2
O2 / H2O
Thermodynamic Potential of Water:
h
e-
h+
e-
Separation between Fermi energy and Conduction band edge
Band Bending
Overpotential for O2 evolution 3
Energy requirement for Photoelectrolysis of water
An ideal semiconductor for water-splitting has band gap of ca. 2.6eV
H+ / H2
O2 / H2O
1.5 V
0.3V
0.4V
0.4V
Ef
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Candidate Materials
•TiO2: Eg ~ 3.0-3.2 eV (410-385 nm)
•Fe2O3: Eg ~ 2.2 eV (>565 nm)
•WO3: Eg ~ 2.6 eV (475 nm)
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Fe2O3: range of stability
-1.2
-1.0-0.8
-0.6
-0.4-0.2
0.0
0.20.4
0.60.8
1.0
1.21.4
1.6
1.82.0
2.2
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
pH
Ele
ctro
de p
oten
tial
(S
HE
) / V
Fe3+
Fe2O3
Fe(OH)2
O2
H2
H+ H2O
Fe
FeO42-
HFeO4-
H2FeO4
Fe2+
hVB+
Fe3O4
H3FeO4+
eCB-
Potential-pH diagram of Fe-H2O System at 298 K; activity = 10-4
??
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Production of Fe2O3 Photoelectrodes
• CVD:
• Fe(CO)5 + tetraethoxysilane (Si-dopant)
• Spray pyrolysis: • FeCl2 + SnCl4
• Ultrasonic spray pyrolysis:
• Fe(acac)3 + ~1% Nb
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Fe2O3 electrochemistry
-0.25
0.00
0.25
0.50
0.75
-0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Potential vs qre / Volt
cd /
Am
-2
0.1M NaOH/Water; 0.01 Vs-1;
Black: dark; Red: illuminated @ 450nm
2 22 4 4VBH O h O H
2 22 4 4H O O H e
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Fe2O3 electrochemistry
-0.25
0.00
0.25
0.50
0.75
-0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Potential vs qre / Volt
cd /
Am
-2
0.1M NaOH/Water-MeOH 80:20; Scan rate: 0.01 Vs-1
Black: dark; Red: illuminated @ 450nm
2 22 4 4VBH O h O H
3 2 26 6 5CH OH OH h CO H O
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Impedance analysis
20 0 0
1 2 BFB
SC D
TE E
C e N e
• Impedance analysis in the dark (Mott-Schottky)
• Plot of CSC-2 vs. electrode potential:
• gradient proportional to donor density (ND)
• intercept = flatband potential10
Fe2O3 electrochemistry
0.0E+00
2.0E+10
4.0E+10
6.0E+10
8.0E+10
1.0E+11
1.2E+11
-1.00 -0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25
Potential vs SCE / Volts
Csc
-2 /
F-2
0.1M NaOH/Water
0.1M NaOH/Water-MeOH 80:20
Modulation frequency: 10KHz
Vmod = 0.005 V
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Impedance analysis
• From Mott-Schottky plots:
• ND > 5 x1019 cm-3
• EFB = -0.55 V vs SCE (water)
= -0.35 V vs SCE (water-methanol)
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Fe2O3 electrochemistry: illuminated
-0.05
0.00
0.05
0.10
0.15
0.20
-0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Potential vs qre / Volt
Ph
oto
curr
ent
den
sity
/ A
m-2
Chopped Illum (87 Hz) @ 450nm
Scan rate: 0.01 Vs-1; 0.1M NaOH
Red: Water
Blue: Water-MeOH 80:20
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Fe2O3 electrochemistry: photocurrent transients
-5.0E-02
0.0E+00
5.0E-02
1.0E-01
1.5E-01
4.80 4.90 5.00 5.10 5.20 5.30 5.40 5.50
Time / s
cd /
Am
-2
Water
450nm;
Chop @ 3 Hz
Potential: 0.6 V
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Fe2O3 electrochemistry: photocurrent transients
0.0E+00
5.0E-02
1.0E-01
1.5E-01
2.0E-01
4.80 4.90 5.00 5.10 5.20 5.30 5.40 5.50
Time / s
cd /
Am
-2
Water-MeOH 80:20
450nm
Chop @ 3 Hz
Potential: 0.6 V
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Source of apparent dark reduction reaction
• From photochemically generated FeO42-
FeO42- is unstable and decomposes according to:
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24 2 3 22 10 5 6FeO H Fe O H O e
Oxidation of Fe2O3 to FeO42- is possible
This reaction would generate a net cathodic currentCH3OH would suppress formation of FeO4
2-
Fe2O3: range of stability – including CH3OH
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Fe2O3 photoelectrochemistry: summary
-0.05
0.00
0.05
0.10
0.15
0.20
-0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Potential vs qre / Volt
Ph
oto
curr
ent
den
sity
/ A
m-2
Surface state (reduced by CH3OH?)
2 22 4 4H O O H e
3 2 26 6 5CH OH OH h CO H O
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Possible nature of surface state
• Derives from surface Fe3O4
Formed by reduction of Fe2O3
Reactive Fe3+ at the surface:
Reduced chemically or electrochemically
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Modelling Fe2O3 Photoresponse
kmaj
kmin
k0
h
+
-
Es
EV
B
EC
B EF
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Modelling Fe2O3 Photoresponse
• Gärtner photoresponse:
0
exp( )1
1 d
Wg I
L
• Steady-state photocurrent given by:
0photo n S Sj g k n N f f
Peter et al., J Electroanal Chem, 1984, 165, 2921
Data input to model
ND = 1020 cm-3 = 2.2 x 105 cm-1 I0 = 1014 cm-2 = 50 kp = 10-6 cm-2 s-1 kn = 2 x 10-8 cm-2 s-1
k0 = 103 cm s-1
n0 = 1021 cm-3
Ns = 1012 cm-2
Es = 0.7 eV22
Initial modelling results
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40
Potential / Volt vs flatband
Q E
ff
Water Water-MeOH 80:20 Model
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Depletion Layer Model for Fe2O3
kmaj
kmin
k0
h
+
-
Es
EV
B
EC
B EF kS
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Conclusions
• Spray pyrolysed Fe2O3 demonstrates:
• Poor efficiency (Vonset ca. 0.7 V from Vfb)
• Surface states from photoelectrochemically generated
»FeO42-
»Fe3O4
• Modelling approximates some observed behaviour
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Future Work
• Develop Fe2O3 deposition methods
• Refine model
• Add surface state mediated charge transfer
• Apply to Fe2O3 from other deposition methods
• Improvements to Fe2O3: surface catalysis?
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