CO2 capture activation and recycle

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Fe2O3 Photoanodesfor Hydrogen Production Using
Solar Energy
S. Dennison, K. Hellgardt, G.H. Kelsall,
Department of Chemical EngineeringImperial
College London, SW7 2AZ, UK s.dennison_at_imperial.ac
.uk
216th ECS Meeting October 8, 2009
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Project Objectives

• Solar-powered hydrogen generation systems
• Biophotolysis
• Photoelectrolysis
• Assessment of materials for photoelectrodes

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Photoelectrolysis of water
Requires gt 1.5 V (? ? lt ca. 830 nm)
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Energy requirements for Photoelectrolysis of water
Band Bending
e-

Separation between Fermi energy and Conduction
band edge
e-

H / H2
Thermodynamic Potential of Water

h?
O2 / H2O
h
Overpotential for O2 evolution

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Energy requirement for Photoelectrolysis of water




0.4V


0.
3
V

E

f

H
/ H

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An ideal semiconductor for water-splitting has
band gap of ca. 2.6eV
1.5
V


O
/ H
O

2
2
0.4V

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Candidate Materials

• TiO2 Eg 3.0-3.2 eV (410-385 nm)
• Fe2O3 Eg 2.2 eV (gt565 nm)
• WO3 Eg 2.6 eV (475 nm)

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Fe2O3 range of stability
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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
0.1M NaOH/Water 0.01 Vs-1 Black dark Red
illuminated _at_ 450nm
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Fe2O3 electrochemistry
0.1M NaOH/Water-MeOH 8020 Scan rate 0.01 Vs-1
Black dark Red illuminated _at_ 450nm
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Impedance analysis

• Impedance analysis in the dark (Mott-Schottky)
• Plot of CSC-2 vs. electrode potential
• gradient proportional to donor density (ND)
• intercept flatband potential

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Fe2O3 electrochemistry
Modulation frequency 10KHz Vmod 0.005 V
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Impedance analysis

• From Mott-Schottky plots
• ND gt 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
Chopped Illum (87 Hz) _at_ 450nm Scan rate 0.01
Vs-1 0.1M NaOH Red Water Blue Water-MeOH
8020
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Fe2O3 electrochemistry photocurrent transients
Water 450nm Chop _at_ 3 Hz Potential 0.6 V
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Fe2O3 electrochemistry photocurrent transients
Water-MeOH 8020 450nm Chop _at_ 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
  

• Oxidation of Fe2O3 to FeO42- is possible
• This reaction would generate a net cathodic
  current
• CH3OH would suppress formation of FeO42-

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Fe2O3 range of stability including CH3OH
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Fe2O3 photoelectrochemistry summary
Surface state (reduced by CH3OH?)
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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
hn
kmin
kmaj
k0
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Modelling Fe2O3 Photoresponse

• Gärtner photoresponse

• Steady-state photocurrent given by

Peter et al., J Electroanal Chem, 1984, 165, 29
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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 eV

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Initial modelling results
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Depletion Layer Model for Fe2O3
hn
kmin
kS
kmaj
k0
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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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