steve dennison & chris carver chemical engineering imperial college

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