Hydrogen Generation Using a Photoelectrochemical Reactor:
Materials Assessment and Reactor Development
Steve Dennison & Chris Carver
Chemical Engineering
Imperial College
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: thermodynamic value from G0 = -237 kJmol-1. Equivalent to a photon of wavelength ~1000 nm
The semiconductor-electrolyte interface
Conduction Band
Valence Band
Redox Electroyte
Metal Semiconductor
EF
Conduction Band
Valence Band
Metal Semiconductor
EF
Ef
The semiconductor-electrolyte interface 2
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
The semiconductor-electrolyte interface 3
An ideal semiconductor for water-splitting has band gap of: ~2.6eV
H+ / H2
O2 / H2O
1.23V
0.3V
0.4V
0.4V
Ef
Choosing the semiconductor
• It must be an OXIDE
– Stability/insolubility in aggressive media
– Stability under conditions of oxygen 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)
Match to Solar Spectrum
TiO2 Fe2O3WO3
Fe2O3: typical photoresponse
0.0E+00
5.0E-07
1.0E-06
1.5E-06
2.0E-06
2.5E-06
3.0E-06
3.5E-06
300 350 400 450 500 550 600 650 700
Wavelength / nm
No
rmal
ised
Ph
oto
curr
ent
Fe2O3: voltammetry under illumination
-1.0E-05
0.0E+00
1.0E-05
2.0E-05
3.0E-05
4.0E-05
5.0E-05
-0.5 -0.25 0 0.25 0.5 0.75 1
Potential vs qre / Volt
cd /
Acm
-2
Water-MeOH
Water-1.500E-05
-1.000E-05
-5.000E-06
0.000E+00
5.000E-06
1.000E-05
1.500E-05
2.000E-05
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5
t / s
cd /
Acm
-2
Water Water/MeOH
-1.00E-05
-8.00E-06
-6.00E-06
-4.00E-06
-2.00E-06
0.00E+00
2.00E-06
4.00E-06
6.00E-06
8.00E-06
1.00E-05
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5
t / s
cd /
Acm
-2
Water Water-MeOH
-1.500E-05
-1.000E-05
-5.000E-06
0.000E+00
5.000E-06
1.000E-05
1.500E-05
2.000E-05
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5
t / s
cd /
Acm
-2
Water Water/MeOH
Fe2O3: photocurrent transients @ +0.6V
-5.00E-06
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5
t / s
cd
/ A
cm
-2
Water Water/MeOH
Fe2O3: photocurrent transients @ +0.6V
-5.00E-06
0.00E+00
5.00E-06
1.00E-05
1.50E-05
2.00E-05
2.50E-05
4.80 4.90 5.00 5.10 5.20 5.30 5.40 5.50 5.60 5.70
Time / s
cd /
Acm
-2
Water/MeOH Water
Fe2O3: photocurrent transients @ +0.1V
-8.0E-06
-6.0E-06
-4.0E-06
-2.0E-06
0.0E+00
2.0E-06
4.0E-06
6.0E-06
8.0E-06
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5
t / s
cd
/ A
cm
-2
Water Water-MeOH
Findings for Fe2O3
• Preliminary (and not concluded yet)
– In the absence of MeOH see cathodic “dark” current, even at 0.6 V.
– As applied potential is decreased, the photocurrent becomes more transient
– As applied potential is decreased the cathodic “dark” current increases (relative to the photocurrent)
WO3: recent work
• Photocurrent observed (poor efficiency)
• Enhancement with oxygen evolution catalyst (electrodeposited IrO2) not realised
• Further detailed electrochemical analysis underway (plus SEM/TEM, XRD, etc.)
Christopher Carver
Dr Klaus Hellgardt
Design flexible test-bed reactor
- 10 x 10cm photoanode
- Photon absorption
- Quantum efficiency
- High mass transfer rate
coefficients
- Separate hydrogen and oxygen
Hydrogen production
experiments
- Semiconductor material
- Electrode configuration
Good absorptionStable in alkaliRecombination
Stable in acid/alkaliUV absorption only
Good efficiencyStable in acid
Fe2O3
TiO2 WO3
titanium PVDF
quartz
Butler-Volmer equation
Overpotential (V)
Kinetic control
Transport control
Increasing mass transport rate
POTENTIOMETER
SOLAR SIMULATOR
PUMP RESERVOIR / H2 COLLECTION
PEC REACTOR
PC
PHOTOANODE
PHOTOANODE
quartz window
membrane
cathode
electrolyte
PHOTOANODE
quartz window
membrane
cathode
electrolyte
QUESTIONS?
Mesh Cathode
Electrolyte Inflow
Electrolyte Flow(with H2 or O2)
Photo-Anode
MembraneFluid ChamberFluid Chamber
Mesh Cathode(Conductor)
Membrane
Photo-Anode
Absorption, α e-h+
2e
2H+
H2
+
Diffusion
Kinetics
Fluid Flow+Diffusion
Diffusion
Fluid Flow+Diffusion
H2O+ 2H+O2
Kinetics
Absorption,Diffusion (2),Band Bending
Electrolyte Flow(if laminar)(1)
(1)
(2)
(2)
(3)
(3)
(4?)
Choosing the semiconductor
• Absolute levels of the electronic levels in the semiconductor:
– Defined by the electron affinity
– Require EA ~ 3.7
eV
Fe2O3: NaOH-H2O
-1.0E-05
0.0E+00
1.0E-05
2.0E-05
3.0E-05
4.0E-05
5.0E-05
-0.5 -0.25 0 0.25 0.5 0.75 1
Potential vs qre / Volts
cd /
Acm
-2
Dark 450 nm
Fe2O3: NaOH-H2O/MeOH (80:20)
-1.0E-05
0.0E+00
1.0E-05
2.0E-05
3.0E-05
4.0E-05
5.0E-05
-0.50 -0.25 0.00 0.25 0.50 0.75 1.00
Potential vs qre/ Volt
cd /
Acm
-2
Dark 450 nm