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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
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