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216th ECS Meeting: October 8, 2009

Comparison of Inexpensive Photoanode Materials for Hydrogen Production Using

Solar Energy

N.Cook, R. Gallen S. Dennison, K. Hellgardt, G.H. Kelsall,

Department of Chemical EngineeringImperial College London, SW7 2AZ, UK

s.dennison@imperial.ac.uk

14 TW Energy Gap by 2050!

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H2 as sustainable energy carrier?

Nuclear Energy Non-Fossil Energy (Solar, Water, Wind) Fossil Energy

Heat

Mechanical Energy

Electricity

Electrolysis

Thermolysis

Biophotolysis

Fermentation

Biomass

Chemical Conversion

Carbon dioxideHydrogen

Photoelectrolysis

Nuclear Energy Non-Fossil Energy (Solar, Water, Wind) Fossil Energy

Heat

Electricity

Electrolysis

Thermolysis

Biophotolysis

Fermentation

Biomass

Chemical Conversion

Carbon dioxideHydrogen

Photoelectrolysis

Mechanical Energy

2 adapted and modified from J.A.Turner, Science 285, 687(1999)

Plugging the energy gap (14TW)

Combined area of black dots would provide total world energy demand

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Solar Hydrogen at Imperial

£4.2M EPSRC sponsored project (5 years)

Chemical Engineering, Chemistry, Biology, Earth

Science and Engineering

Approx. 20-25 researchers at any one time

2 strands: Biophotolysis and Photoelectrochemistry

Chemical Engineering to develop devices and

reactors and technology for scale-up and scale out

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Application

Targets: Biophotolytic H2: £5.00/kg; Photoelectrolytic H2: £2.50/kg

Fuel Cell Operation

Distributed Market

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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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Stability of Fe2O3

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-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

??

Stability of TiO2

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-1.6

-1.4-1.2

-1.0-0.8

-0.6-0.4

-0.20.0

0.20.4

0.60.8

1.01.2

1.4

-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

pH

Ele

ctro

de P

oten

tial

(SH

E)

/ V

TiO2 (hydrated)

Ti2O3 (hydrated)

TiO2+

H+/H2O

O2/H2O

Ti3+

Stability of WO3

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-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10 12 14

pH

Ele

ctro

de

Po

ten

tial

/ V

vs

SH

E

WO3

WO2

W

WO42-

H+/H2O

O2/H2O

Photoelectrolysis – Materials Evaluation

Photocurrent Spectroscopy Photo-electrochemical activity of photo-anodes based on transition metal oxides (Fe, W, Ti) Fe-based system needs bias but otherwise promising (& cheap)

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WO3: further investigations

• From H2WO4:

• Electrodeposition: potential cycling -0.4 - +0.8 V vs. SCE 1

• “Doctor blading”: using stabilised H2WO4 sol 2

Both annealed: 15 min at 550°C

1 Santato et al., J Amer Chem Soc, 2001, 123, 10639

2 Kulesza and Faulkner, J Electroanal Chem, 1988, 248, 305

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Measured band-edge potentials of WO3

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

0 2 4 6 8 10 12 14

pH

Ele

ctro

de

Po

ten

tial

/ V

vs

NH

E WO3

WO2

W

WO42-

H+/H2O

O2/H2O

EVB

ECB

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Ir/IrO2 Electrodeposition

• Ir:

• From “IrCl3,aq” : E0 = +0.86 V vs NHE 1

• Convert to IrO2 by electrochemical oxidation 2

• IrO2:

• From [IrCl6]3-/oxalate @ pH 10.5/galvanostatic deposition 3

1 Munoz and Lewerenz, J Electrochem Soc, 2009, 156, D1842 Elzanowska et al. Electrochim Acta, 2008, 53, 2706 3 Marzouk, Anal Chem, 2003, 75, 1258

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Ir Electrodeposition – Cycle 1

-15.00

-12.50

-10.00

-7.50

-5.00

-2.50

0.00

2.50

-1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25

Potential vs SCE / Volt

cd /

Am

-2

Vitreous carbon electrode:10 mM IrCl3/0.5 M KCl Sweep rate: 0.01 Vs-1 Ir nucleation

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Ir Electrodeposition – Selected Cycles

-20.0

-15.0

-10.0

-5.0

0.0

5.0

10.0

-1.25 -1.00 -0.75 -0.50 -0.25 0.00 0.25

Potential vs SCE / Volt

cd /

Am

-2

Cycle 1 Cycle 2 Cycle 5

Vitreous Carbon electrode10 mM IrCl3/0.5 M KClSweep rate: 0.01 Vs-1

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3 46 6IrCl e IrCl

0( ) ( )Ir II Ir I Ir

IrO2 Electrodeposition

-2.5

0.0

2.5

5.0

7.5

10.0

12.5

15.0

17.5

20.0

-0.25 0.00 0.25 0.50 0.75 1.00 1.25

Potential vs SCE / Volt

cd /

Am

-2

Cycle 1 Cycle 2 Cycle 3 Cycle 4

H2IrCl6 + (COOH)2 (pH 10.5, K2CO3)Sweep rate: 0.01Vs-1

2" ( )" ( ) " ( )"Ir IV COOH Ir III products

2" ( )"Ir III IrO e

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Effect of IrO2 on WO3 Photoresponse

-0.3

0.0

0.3

0.5

0.8

1.0

1.3

1.5

0.00 0.20 0.40 0.60 0.80 1.00

Potential vs SCE / Volt

cd /

Am

-2

"Bare" WO3 IrO2-coated

O2/H2O

1M H2SO4

Sweep rate: 0.01 Vs-1

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2 22 4 4VBH O h O H

Mott-Schottky analysis following IrO2-coating

0.0E+00

5.0E+14

1.0E+15

1.5E+15

2.0E+15

2.5E+15

3.0E+15

3.5E+15

4.0E+15

0.00 0.25 0.50 0.75 1.00 1.25 1.50

Potential vs SCE / Volt

Csc

-2 /

F-2

cm-4

1M H2SO4

Modulation frequency: 10 kHz

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Conclusions

• The electrodeposition of Ir and IrO2 is interesting!

• Deposition of Ir & IrO2 onto WO3 results in loss of photoelectrochemical O2 evolution activity.

• This is due to:

a) deposition of excessive quantities of Ir/IrO2

b) irreversible damage of the WO3 (MS data).

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Design and Development of a Photoelectrochemical Reactor

• Key criteria:

• Optimising illumination of photoelectrode

• Optimising fluid and current distributions

• Product separation

• Minimising bubble formation

• Materials (of construction) selection

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Photoelectrolytic Reactor Design

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Photoelectrolytic Reactor Design

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Photoelectrolytic Reactor Design

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Photoelectrolytic Reactor Design

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Photoelectrolytic Reactor Design

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Photoelectrolytic Reactor Performance

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Photoelectrolytic Reactor: conclusions

• Main contributing factors to response:

•Photoanode material quality

•Cathode gauze too coarse

•Large illumination losses (mirror, etc.)

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Future Work

• Materials fabrication: WO3 and Fe2O3

• Photoelectrochemical reactor:

• Photoanode material quality

• Reduce shading by cathode

• Hydrogen measurement and collection

• Fully develop reactor model

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