View
221
Download
0
Category
Preview:
Citation preview
www.mvsystemsinc.com NHA conference May3-6, 2010
The Potential of Using a-SiC:H as the Photoelectrode for Water Splitting
Feng Zhua, Jian Hua, Ilvydas Matulionisa, Josh Gallona,
Todd Deutschb, Nicolas Gaillardc, Eric Millerc and Arun Madana
a. MVSystems, Inc., Golden, CO. USA
b. National Renewable Energy Laboratory, Golden, CO, USA.
c: Hawaii Natural Energy Institute, University of Hawaii at Manoa, HI, USA
Supported by the U.S. Department of Energy Program # DE-FC36-07GO17105
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA Focus of this talk
PEC - photoelectrochemicala-SiC – hydrogenated amorphous silicon carbide hybrid device- a-SiC integrated with a-Si/a-Si tandem solar cellSTH- solar-to-hydrogen
Introduction
A-SiC:H photoelectrode PEC characteristics
The challenges of a-SiC:H for water splitting
---Corrosion resistance in aqueous electrolyte
---The band-edge of the a-SiC:H
---Charge carrier extraction at the a-SiC:H/electrolyte interface
Pathway towards STH conversion efficiency>10%
Conclusion
Hydrogen production by conventional methods Greenhouse gases
PEC water splitting: Use electricity from sunlight
Why hydrogen production by PEC ?
Goals*:
* R. Garland et al, DOE Hydrogen Production Review Meeting, Arlington, VA, June 12, 2008.
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
Advantages of a-SiC:H*Lower Eg (band gap) allows more photocurrent
*Eg 1.9eV-2.3eV with varying carbon incorporation
*Uses the same deposition technique as the a-Si tandem junction, i.g. PECVD
Why amorphous SiC photoelectrode ?
Previously, 3.1% Solar-to-Hydrogen (STH) efficiency achieved in WO3+a-Si tandem PEC hybrid devices (from load-line analysis by HNEI) *
HER catalystStainless steel foil
a-Si nip/nip tandem solar cellsITO
WO3
light
electrolyte
a-Si/a-Si tandem solar cell 0
2
4
6
8
10
0.0 0.5 1.0 1.5 2.0
Voltage (V)
Cur
rent
(mA
/cm
2)
0
2
4
6
8
10
12
STH
effi
cien
cy (%
)
a-Si/a-Si tandem
a-Si/a-Si tandem, filtered by WO3 film
WO3 film
integrated HPE performance level
* MVS publication in Proc. SPIE, Vol. 6340, (2006)63400K.
Deposition system –cluster tool system M
VS
yste
ms,
Inc
.
G
olde
n, C
olor
ado,
US
A
RF power: 10-20 W
Excitation frequency: 13.56 MHz
Pressure: 300-550 mTorrr
SiH4 flow rate: 20 sccm
CH4 flow rate: 0-20 sccm
H2 flow rate 0-100 sccm
Substrate temperature 200°C
Sputtering chamberZnO, ITO, Al, Mo…..
PECVD chambers
LoadLock
Main deposition parameters:
All a-SiC:H films, photoelectrodes, solar cells and the PEC hybrid devices were fabricated in the cluster tool PECVD/Sputtering System, designed and manufactured by MVSystems, Inc.
www.mvsystemsinc.com
a-SiC material and its pin Solar Cell M
VS
yste
ms,
Inc
.
G
olde
n, C
olor
ado,
US
A
Device configuration and performance:
a-SiC(i) (~300 nm, Eg~ 2 eV)
a-Si(n+)
a-SiC(p+)SnO2 (Asahi U-type)
Ag
Glass
Light
For devices with thickness of ~ 100 nm, Jsc ~ 8.45 mA/cm2 achieved
0
2
4
6
8
10
12
14
16
0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 1
V (V)
J (m
A/cm
2 )
J sc=13. 34mA/ cm2Voc=0. 91VFF=0. 61effi . =7. 35%
As CH4 flow increases, the bandgap, Eg, increases
1.95
2
2.05
2.1
2.15
2.2
2.25
0 0.1 0.2 0.3 0.4 0.5 0.6
CH4/(CH4+SiH4)
Eg (
eV)
without H2
with H2
Eg vs. CH4/(CH4+SiH4)
* F. Zhu, J. Hu, I. Matulionis, Todd Deutsch, Nicolas Gaillard, A. Kunrath, E. Miller and A. Madan, "Amorphous Silicon Carbide Photoelectrode For Hydrogen Production Directly From Water Using Sunlight" Philosophic Magazine, 89:28,( 2009) 2723-2739
0
1
2
3
4
5
6
7
8
9
0.0 0.5 1.0 1.5 2.0
Voltage (V)
Cu
rre
nt
de
ns
ity
(m
A/c
m2 )
Performance of a-Si tandem device
Current vs. Voltage
Configuration of a hybrid PEC cell
a-SiC(i), ~100nm
SnO2Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si(i), 150nm
a-SiC p-i
a-Si(i), 500nm
SN Jsc Voc FF Eff. (mA/cm2) (V) (%)
#A 6.795 1.6 0.717 7.79 #B 8.70 1.66 0.67 9.62
#A
#BComparison of PV performance
#A -used in the first hybrid PEC cell.• #B- improved device. Shall incorporate into the hybrid PEC device.
A-Si tandem solar cell
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
Main Features of a-SiC p/i Photoelectrode: Behaves like a p-type photoelectrode
Saturated photocurrent: up to 8 mA/cm2
Flatband voltage: +0.26V (vs Ag/AgCl)
The flatband voltage of the a-SiC p/i photoelectrode is above the water oxidation half-reaction potential which means external voltage is required to initiate water splitting.
Flatband voltage Vfb vs. pH for an a-SiC p/i photoelectrode and a hybrid PEC device
a-SiC(p) (20 nm)
Glass
a-SiC(i) (200nm, 2.0eV)
textured SnO2
light
a-SiC p/i photoelectrode
The challenges of a-SiC:H for water splitting -the band-edge M
VS
yste
ms,
Inc
.
G
olde
n, C
olor
ado,
US
A
[ Data measured by NREL and HENI]
• Vfb determined by the illuminated open-circuit voltage (IOCP).
• For the p-type photoelectrode, Vfb needs to be below H2O/O2
redox potential for water splitting.
• In the hybrid PEC device, the Vfb shifts by ~+1.6V or +0.97V
below H2O/O2 half-reaction potential at pH2
Flatband potential vs. pH
[ Data measured by NREL and HENI]
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5-1 4 9 14
pH
V v
s. A
g/A
gCl
hydrogen
oxygen
a-SiC p-i
hybriddevice
textured SnO2
hybrid PEC cell
a-SiC (i), 100nm
Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
light
The challenges of a-SiC:H for water splitting -the band-edge
-7
-6
-5
-4
-3
-2
-1
0
-2 -1. 8 -1. 6 -1. 4 -1. 2 -1 -0. 8 -0. 6 -0. 4 -0. 2 0 0. 2potenti al (Vs. Ag/ AgCl )
phot
o cu
rrent
den
sit
y(mA
/cm
2)
a- Si C photoel ect rodehybr i d devi ce
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2
Potential (v)
Cu
rre
nt
(mA
/cm
2 )
a-SiC photoelectrode
hybrid device
Photocurrent is low
charge carrier extraction problem at a-SiC/electrolyte interface
a-SiC(p) (20 nm)
Glass
a-SiC(i) (200nm, 2.0eV)
textured SnO2
light
textured SnO2
hybrid PEC cell
a-SiC (i), 100nm
Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
light
The challenges of a-SiC:H for water splitting - Charge carrier extraction
3-electrode setup 2-electrode setup
0.33 mA/cm2
After HF etch for 30s, photocurrent increases and its onset shift anodically Photocurrent degrades and reverts to its initial value when the a-SiC photoelectrode is exposed to the air XPS measurement also verify this, please see the paper published in 2009*
Effect of surface SiOx on photocurrent M
VS
yste
ms,
Inc
.
G
olde
n, C
olor
ado,
US
A
Effects of SiOx on a-SiC photoelectrode
-8
-7
-6
-5
-4
-3
-2
-1
0
1
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
Potential (V-Ag/AgCl)
Cu
rren
t (m
A/c
m2)
pre - HF dip
post - HF dip
67hr later (in air)
* HF etch time: 30 sec
[ Data measured by NREL]
* F. Zhu, J. Hu, I. Matulionis, Todd Deutsch, Nicolas Gaillard, A. Kunrath, E. Miller and A. Madan, "Amorphous Silicon Carbide Photoelectrode For Hydrogen Production Directly From Water Using Sunlight" Philosophic Magazine, 89:28,( 2009) 2723-2739
Test conditions:
Sample tested: hybrid PEC cell with ZnO/Ag back reflector Setup: 2-Electrode Counter electrode: RuO2
Electrolyte: buffer pH2 HF dip: 5% HF for 30 sec
Textured SnO2 coated with silver and ZnO
1.3 mA/cm2 achieved by removal of silicon oxide from a-SiC with HF etch.
This current density corresponds to 1.6% solar-to-hydrogen efficiency
[ Data measured by HNEI ]
-4.0
-3.0
-2.0
-1.0
0.0
-2.0 -1.5 -1.0 -0.5 0.0
No HF etch, Eg = 2.06 eVNo HF etch, Eg = 1.98 eVAfter HF etch, Eg = 2.06 eVAfter HF etch, Eg = 1.98 eV
Potential (V)
Cu
rre
nt
(m
A/c
m2 )
two-electrode test
[ Data measured by HNEI ]
textured SnO2
hybrid PEC cell
a-SiC (i), 100nm
Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
light
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
Effect of surface SiOx on photocurrent of the hybrid PEC device
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
Comparison with a Solid-State Configuration
a-SiC (i), 100nm
SnO2Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
a-SiC (i), 100nm
Glass
a-Si (i), 150nm
a-Si (i), 500nm
ITOExposed to electrolyte
-4
-3
-2
-1
0
-1.5 -1.0 -0.5 0.0
Potential (V)
Ph
oto
cu
rre
nt
(m
A/c
m2)
two-electrode test
1.3 mA/cm2
0
1
2
3
4
5
6
0 1 2
Cu
rre
nt
(mA
/cm2 )
Voltage (V)
glass/SnO2/p-i-n/p-i-n/p/a-SiC/ITO
Jsc>5.0 mA/cm2
light
light
The solid state version (right) shows the STH efficiency of hybrid PEC cell should be >6% Low current in hybrid PEC cell (left), indicating the charge carrier extraction problem at the a-SiC/electrolyte interface Surface modification is underway to improve the interface (The progress will be reported in SPIE conference @ San Diego, August, 2010 )
Test conditions: Sample tested: hybrid PEC cell Counter electrode: Pt Electrolyte: buffer pH2 (sulphamic acid solution with added potassium biphthalate) Current bias: 1.6 mA/cm2
Current vs. potential (before and after test)
Before testing
After 200-hr testing
H2 production throughout the test No degradation during durability/corrosion test for 200 hours (So far)
[ Data measured by NREL ]
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-2 -1 0 1 2
Potential (V vs. Ag/AgCl)
Ph
oto
curr
ent
(mA
/cm
2)
Initial
148 hrs
172 hrs
200 hrs
three-electrode test
textured SnO2
hybrid PEC cell
a-SiC (i), 100nm
Glass
a-SiC(p)
a-Si p-i-n(top cell)
a-Si p-i-n(bottom cell)
a-SiC(p)
a-Si(n)
a-SiC(p)
a-Si(n)
a-Si (i), 150nm
a-SiC p-i
a-Si (i), 500nm
light
The challenges of a-SiC for water splitting –corrosion resistance
Road map towards STH efficiency > 10% M
VS
yste
ms,
Inc
.
G
olde
n, C
olor
ado,
US
A
Expected photocurrent (Jph) ~8.2 mA/cm2. Solar-to-hydrogen (STH) efficiency: ~10.08%
PEC (a-SiC:H) PV cellSTH (%)Eg Jph (mA/cm2) configuration
Jph after filtered by a-SiC:H
(mA/cm2)
2 8.55 a-si/a-Si 5.5 6.77
2.3 8.2 nc-Si/a-Si 8.35 10.08
a-Si/a-Si
a-Si/nc-SiSolar-to-hydrogen conversion efficiency:
n)irradiatiosolar 1.5 AM(for 23.1area collection over thesunlight in theenergy
device PEC ain producedhydrogen in energy chemical phJ
Summary M
VS
yste
ms,
Inc
.
G
olde
n, C
olor
ado,
US
A
Photocurrent of the hybrid PEC device has been improved from 0.33 to ~1.3 mA/cm2, nearly STH of 1.6%
The hybrid PEC cell exhibits excellent durability in pH2 electrolyte for up to ~200 hours (so far tested).
In a solid state version, the device has achieved a current density of > 5 mA/cm2 (possible STH efficiency >6%).
Surface modification is being underway to improve charge carrier extraction problem at the a-SiC/electrolyte interface
The simulation has shown the pathway toward STH>10%
And thanks Ed Valentich for assistance in sample preparation.
MV
Sys
tem
s, I
nc.
Gol
den,
Col
orad
o, U
SA
Recommended