for water splitting yude su - the budker groupbudker.berkeley.edu/physics141_2013/physics141a...

Nano-wire Photoelectrochemical cell For water splitting

Yude Su SID: 24045922

Peidong Yang’s group

Outline

1. Motivation of PEC

2. Mechanism of PEC

3. Some examples

4. Future direction

Motivation

• Renewable Energy replace Fossil Fuel 264 GW (2008)

70 GWe (2011)

Car running on Biomass (WW II) Car running on Solar (2012)

Photochemical cell

Photovoltaic cell

H2O

Photocatalyst

H2 and O2

Motivation

η1>20%

Electricity

η2>80%

η3<<1%

Overall picture of a PEC cell

PEC PV Device + Electrochemical Reaction

Chem. Rev. 1992, 92, 411-433

6

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

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

8

light emission from semiconductors

- - - - - - + + +

+ + +

CB

VB

excitation

- - - - - + + +

+ + + + + +

- - - -

+

-

transient emission

excitation off

- - - - - - + + +

+ + + + +

+ + + +

- - - - - -

+

-

steady emission

after time

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

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

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

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

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

Cathode and anode integration Z-scheme

Energy Diagram Current matching

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