Imperial College London © 1

Enhancing Photoelectrode Performance with Nanoparticulate

Electrocatalysts P. Bumroongsakulsawat, S. Dennison,

K. Hellgardt, G. Kelsall

Dept of Chemical Engineering,Imperial College,

LONDON SW7 2AZ

e: s.dennison@imperial.ac.uk

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 1.23 V (equivalent to a photon of wavelength ~1000 nm)

Ef

Energy Requirements for Photoelectrolysis

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

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)

Stability of WO3 in aqueous media

-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

Fabrication of WO3 thin films

• From H2WO4:

– “Electrodeposition”:

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

– “Doctor blading”:

using stabilised H2WO4 sol 2

Both annealed: 15 min at 550°C

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

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

Dark electrochemistry of WO3 (1)

-2.50

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

-0.25 0.00 0.25 0.50 0.75 1.00 1.25

Potential vs SCE / Volt

cd /

Am

-2

O2/H2O

3 3x xWO xH xe H WO

3 3x xH WO WO H e

Dark electrochemistry of WO3 (2):Impedance (Mott-Schottky)

0.0E+00

2.5E+15

5.0E+15

7.5E+15

1.0E+16

-0.75 -0.50 -0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50

Potential vs SCE / Volt

Csc

-2 /

F-2

cm-4

Efb ~ 0.1 V vs. SCE

1M H2SO4

Modulation frequency: 10 kHz

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

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

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

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

Ir Electrodeposition – Cycle 5

-30.0

-25.0

-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 qre / Volt

cd /

Am

-2

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

3 46 6IrCl e IrCl

0( ) ( )Ir II Ir I Ir

0 ( ) ( )Ir Ir I Ir II

*

22 2H e H

4 36 6IrCl IrCl e

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

Stability of IrO2

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

0 2 4 6 8 10 12 14

pH

Ele

ctro

de

Po

ten

tial

/ V

vs

NH

E

H+/H2O

O2/H2O

IrCl62-

IrCl63-

IrO2

Ir

Electrodeposition Conditions

• Ir:

– Nucleation: 5 ms at -0.90 V vs. SCE

– Deposition: at -0.5 ± 0.05 V vs. SCE

• IrO2:

– Deposition: at +0.80 V vs SCE

(Electrodeposition or electrophoretic deposition?)

• On 5-layer, doctor-bladed WO3

Ir Electrodeposition on WO3

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0 10 20 30 40 50 60 70

t / s

cd /

Am

-2

WO3 electrode:

10 mM IrCl3/0.5 M KCl, pH ~2.5

-0.50V vs SCE for 60 s

Ir: Effect of deposition time

• 100 s: charge equiv. to ~15 monolayers for

growth @ -0.48 V vs SCE

• 60 s: charge equiv. to ~12 monolayers for

growth @ -0.50 V vs SCE

• 0.1 s: ~10 monolayers.

IrO2 Electrodeposition on WO3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 50 100 150 200 250 300 350

t / s

cd /

Am

-2

WO3 electrode:

H2IrCl3/(COOH)2/K2CO3, pH 10.5

+0.80 V vs SCE for 300 s

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

Capacitance-Potential: IrO2-coated WO3

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

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) irreversible damage of the WO3 (MS data).

b) deposition of excessive quantities of Ir/IrO2 (?)

Future Work

• Determine whether this is this a viable approach:

– Investigate deposition of sub-monolayer Ir/IrO2

– Investigate control of Ir solution species

– Further characterisation of Ir/IrO2 deposits