Hydrogen Generation Using a
Photoelectrochemical Reactor:
Materials Assessment
and Reactor Development
Steve Dennison & Chris Carver
Chemical Engineering
Imperial College
Photoelectrolysis of water
absorption


Semiconductor  h 
 Semiconductor (eCB
, hVB
)

2H2O  4hVB

O2  4H 

2H2O  2eCB

 H2  2OH 
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
Redox
Electroyte
Conduction
Band
EF
EF
Valence
Band
Semiconductor
Metal
Valence
Band
Semiconductor
Metal
The semiconductor-electrolyte interface 2
Band
Bending
eSeparation between
Fermi energy and
Conduction band edge
H+ / H2
Ef
Thermodynamic
Potential of Water:
h
O2 / H2O
h+
Overpotential
for O2 evolution
e-
The semiconductor-electrolyte interface 3
0.4V
0.3V
Ef
An ideal semiconductor
for water-splitting has
band gap of: ~2.6eV
H+ / H2
1.23V
O2 / H2O
0.4V
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
WO3
Fe2O3
Fe2O3: typical photoresponse
3.5E-06
Normalised Photocurrent
3.0E-06
2.5E-06
2.0E-06
1.5E-06
1.0E-06
5.0E-07
0.0E+00
300
350
400
450
500
Wavelength / nm
550
600
650
700
Fe2O3: voltammetry under illumination
2.000E-05
5.0E-05
1.500E-05
1.000E-05
cd / Acm -2
Water-MeOH
5.000E-06
0.000E+00
4.8
4.0E-05
4.9
5
5.1
5.2
5.3
5.4
5.5
-5.000E-06
Water
-1.000E-05
-1.500E-05
t/s
Water
Water/MeOH
3.0E-05
1.00E-05
6.00E-06
4.00E-06
2.0E-05
2.00E-06
cd / Acm -2
0.00E+00
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
-2.00E-06
-4.00E-06
-6.00E-06
-8.00E-06
1.0E-05
-1.00E-05
t/s
Water
Water-MeOH
2.000E-05
1.500E-05
0.0E+00
1.000E-05
cd / Acm -2
cd / Acm
-2
8.00E-06
5.000E-06
0.000E+00
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
-5.000E-06
-1.0E-05
-0.5
-1.000E-05
-0.25
0
0.25
0.5
-1.500E-05
0.75
Potential vs qre / Volt
1
t/s
Water
Water/MeOH
Fe2O3: photocurrent transients @ +0.6V
2.50E-05
cd / Acm
-2
2.00E-05
1.50E-05
1.00E-05
5.00E-06
0.00E+00
-5.00E-06
4.8
4.9
5
5.1
5.2
t/s
Water
Water/MeOH
5.3
5.4
5.5
Fe2O3: photocurrent transients @ +0.6V
2.50E-05
2.00E-05
cd / Acm
-2
1.50E-05
1.00E-05
5.00E-06
0.00E+00
-5.00E-06
4.80
4.90
5.00
5.10
5.20
5.30
Time / s
Water/MeOH
Water
5.40
5.50
5.60
5.70
Fe2O3: photocurrent transients @ +0.1V
8.0E-06
6.0E-06
cd / Acm
-2
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
t/s
Water
Water-MeOH
5.3
5.4
5.5
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
 Hydrogen production
experiments
-
10 x 10cm photoanode
-
Photon absorption
-
Semiconductor material
-
Quantum efficiency
-
Electrode configuration
-
High mass transfer rate
coefficients
-
Separate hydrogen and oxygen
titanium
Good absorption
Stable in alkali
Recombination
Stable in
acid/alkali
UV absorption
only
Good efficiency
Stable in acid
PVDF
quartz
Current density (A/m2)
Butler-Volmer equation
Kinetic
control
Increasing mass
transport rate
Transport control
Overpotential (V)
PC
SOLAR SIMULATOR
PEC
REACTOR
POTENTIOMETER
PUMP
RESERVOIR / H2 COLLECTION
quartz window
PHOTOANODE
membrane
cathode
PHOTOANODE
electrolyte
Photo-Response of Fe2O3
90
80
70
Current Density (A/m2)
60
50
Dark
40
Illum
30
20
0.5 V
1.0 V
1.5 V
2.0 V
2.5 V
3.0 V
10
0
0
-10
20
40
60
80
100
Time (s)
120
140
160
180
200
Averaged Photo-Response of Fe2O3
90
80
70
Current Density (A/m2)
60
50
Dark
40
Illum
30
20
0.5 V
1.0 V
1.5 V
2.0 V
2.5 V
3.0 V
10
0
0
-10
20
40
60
80
100
Time (s)
120
140
160
180
200
Photoresponse of Fe2O3: Configuration A
2.5
Current Density (mA/cm2)
2
1.5
1
0.5
0
0.8
-0.5
0.9
1
1.1
1.2
Voltage (V)
1.3
1.4
1.5
1.6
Photoresponse of Fe2O3: Configuration B
2.5
Current Density (mA/cm2)
2
1.5
1
0.5
0
0.8
-0.5
0.9
1
1.1
1.2
Voltage (V)
1.3
1.4
1.5
1.6
quartz window
membrane
cathode
PHOTOANODE
electrolyte
Set Up Reactor
Experiments
Analyse Results
Electrical
connections
Semiconductors /
electrolyte
Quantify H2
production
Hydrogen extraction
system
Electrode
configuration
Optimise reactor
design
Evaluate reactor
design
Multiple electrodes
Temperature
Deliver report
QUESTIONS?
Electrolyte Flow
(with H2 or O2)
Electrolyte Inflow
(2)
Diffusion
(3)
Kinetics
(1)
Fluid Flow+
Diffusion
(4?)
Diffusion
(1)
Fluid Flow+
Diffusion
H2
Electrolyte Flow
(if laminar)
2H+
Membrane
O2 +
(3)
Mesh Cathode
(Conductor)
2e
2H+
H2O
Kinetics
Absorption,
(2) Diffusion (2),
Band Bending
+
Absorption, α
Photo-Anode
h+
e-
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
5.0E-05
Dark
450 nm
4.0E-05
cd / Acm
-2
3.0E-05
2.0E-05
1.0E-05
0.0E+00
-1.0E-05
-0.5
-0.25
0
0.25
Potential vs qre / Volts
0.5
0.75
1
Fe2O3: NaOH-H2O/MeOH (80:20)
5.0E-05
Dark
450 nm
4.0E-05
cd / Acm
-2
3.0E-05
2.0E-05
1.0E-05
0.0E+00
-1.0E-05
-0.50
-0.25
0.00
0.25
Potential vs qre/ Volt
0.50
0.75
1.00