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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

s.dennison@imperial.ac.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

4

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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