for water splitting yude su - the budker groupbudker.berkeley.edu/physics141_2013/physics141a...
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Nano-wire Photoelectrochemical cell For water splitting
Yude Su SID: 24045922
Peidong Yang’s group
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Outline
1. Motivation of PEC
2. Mechanism of PEC
3. Some examples
4. Future direction
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Motivation
• Renewable Energy replace Fossil Fuel 264 GW (2008)
70 GWe (2011)
Car running on Biomass (WW II) Car running on Solar (2012)
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Photochemical cell
Photovoltaic cell
H2O
Photocatalyst
H2 and O2
Motivation
η1>20%
Electricity
η2>80%
η3<<1%
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Overall picture of a PEC cell
PEC PV Device + Electrochemical Reaction
Chem. Rev. 1992, 92, 411-433
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qM
qS q
ECB
EVB
EFM
EFS
before contact
qM
qS
q
ECB
EVB
EFM EFS
after contact qVi
qVi qB
W A
i
qN
VW
2
+ + + +
- - -
-
Schottky contact: M/pS with M < S
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7
qM qS q
ECB
EVB
EFM
EFS
before contact
qM
qS
q
ECB
EVB
EFM EFS
after contact
+ + + +
- - -
-
Ohmic contact: M/nS with M > S
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light emission from semiconductors
- - - - - - + + +
+ + +
CB
VB
excitation
- - - - - + + +
+ + + + + +
- - - -
+
-
transient emission
excitation off
- - - - - - + + +
+ + + + +
+ + + +
- - - - - -
+
-
steady emission
after time
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Electrode Kinetics Butler-Volmer Equation
Current-Overpotential link
/ (1 ) /
0
1
0 0
[ ]
( 0) ( 0)
nF RT nF RTJ J e e
J k FC x C x
η: over-potential, defined by E-Eeq J0: exchange current density K0: the standard rate constant, defined by the activation energy when kO=kR
α: the transfer coefficient
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Cathode Counter Electrode O
R
R O
Mass transport
E1
E2
EEq
EC
Equivalent Circuit for a PEC cathode
V
E2
Butler-Volmer Term Behave as a Series Resistance in the PEC Equivalent Circuit!
( )/ ( )/2
/ ( 1) /
0
( 1) ( 1) ( ) /
[ ]
q V KT q V KT
s RG sh L
nF RT nF RT
I I e I e V R I
I e e
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How to define the efficiency of a PEC cell?
-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-20
-15
-10
-5
0
5
Voltage(V vs Solution)
Cu
rre
nt(
mA
)
Roughness factor=100
Red: Methyl Viologen
Purple: H2 evolution 100%Pt coating
Blue: H2 evolution 50%Pt coating
Green: H2 evolution 10%Pt coating
Cyan-blue: H2 evolution 1%Pt coating
Yellow: H2 evolution 0.1%Pt coating
Brown: H2 evolution 0.01%Pt coating
VOC: Open circuit voltage ISC: Short circuit current FF: Fill factor=
/ *Max OC SCP V I
/Max inputP P
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Why to choose Nanowire as the PEC electrode?
1. Larger surface area, more catalytic sites (Surface chemistry)
Y. J. Hwang, et. al., Nano Lett., 2009, 9, 410
2. Strong scattering increases light path (Optical absorption)
M. D. Kelzenberg, et. al., Nature Mater., 2010, 9, 239
3. Orthogonalize absorption and charge collection (Charge transport)
J. M. Foley, et. al., Energy Environ. Sci., 2012, 5, 5203
Resultant improvement of photoelectrode
K. Sivula, et. al., ChenSusChem, 2011, 4, 432
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Nanowire for PEC cathode Si
S. W. Boettcher, et. al., Science, 2010, 327, 185 M. D. Kelzenberg, et. al., Nature Mater., 2010, 9, 239
GaP
J. Sun, et. al., J. Am. Chem. Soc., 2011, 133, 19306
Nanowire for PEC anode
TiO2
B. Liu, et. al., J. Am. Chem. Soc., 2009, 131, 3985 I. S. Cho, et. al., Nano Lett., 2011, 11, 4978 M. Xu, et. al., Nano Lett., 2012, 12, 1503
ZnO
X. Yang, et. al., Nano Lett., 2009, 9, 2331 H. M. Chen, et. al., Angew. Chem. Int. Ed, 2010, 49, 5966
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Cathode and anode integration Z-scheme
Energy Diagram Current matching
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Future direction
1. Develop earth-abundant semiconductor material 2. Improve the efficiency of photo-anode 3. Further increase the light absorption 4. Decrease the synthesis price
Energy crisis can be solved!
Thanks for your attention