1

Photo-Electrochemical Production of H2 Using Solar Energy

Anna Hankin, Geoff Kelsall, Chin Kin Ong and Fabien Petter

Department of Chemical Engineering, Imperial College London, SW7 2AZ, UK

Motivation for solar energy harvesting with storage of H2

Problems of photo-electrochemical reactor designs

Modelling of potential and current density distributions for reactor design

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Win

d, S

ola

r /

GW

Engl

and

+ W

ale

s El

ect

rici

ty D

em

and

/

GW

2 Source: www.nationalgrid.com/uk

mean

UK has only 27.6 GW h of pumped-hydro energy storage; fluctuations in electrical power demand and supply require more, especially with decreasing spare power generation capacity

Time Dependence of Electrical Power Demand + Renewable Supply in UK

Day

NightElectrical energy Chemical energy

3 3

Smooth dynamics of electrical power demands

Manage intermittency of renewable power sources

Storage of electrons reversibly in chemical bonds at grid scale

Chemical bonds High specific energies

Decarbonise power sources, especially for electric and hybrid vehicles

Technological Objectives

OECD Electricity Production (IEA)

Ca. 60 % Fossil Fuels

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Inte

gral

Po

wer

De

nsi

ty /

W m

-2

No

rmal

ised

Inte

gral

Po

wer

De

nsi

ty

H2O decomposition thermo-neutral potential Fe2O3 absorption edge

TiO2 absorption edge

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1.0

1.1

1.2 1.3

1.4

300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500

Wavelength, / nm

absorption bands H2O

H2O CO2

O2

H2O

H2O

(Power density of (air mass) AM1.5 light; ca. 1 kW m-2 1 Sun)

0.5 4.1 Energy (h.c / ) / eV 1.0 1.5 2.0 3.0 2.5

Spec

tral

Irra

dia

nce

/ W

m-2

nm

-1

Visible light

ASTM G173-03 Reference Solar Spectrum: Terrestrial Direct Normal + Circumsolar

92 A m-2 @ quantum yield = 1

Energy levels:

Conduction band energy below decomposition potential for H2O to H2

Valence band energy below

decomposition potential for H2O to O2

5

Photo-Electrochemical Reactors: Material Requirements

At pH 7.0

Absorption of hv:

Absorption spectrum well matched to solar spectrum

Physical Scale / eV

Electrode Potential vs. SHE / V

V . U

UU-e

E

SHE

SHE

444(vac)

vacSHE

0

0

ii

S. Trasatti, Pure & Appl. Chem., 58 (1986) 955

Fabrication:

Easily fabricated on large scale at low cost

Stability in electrolyte:

Stable to decomposition by e- and h+ over wide potential & pH range

Catalysis:

Catalysis of H2 and O2 evolution reactions, to minimise the rate of electron-hole recombination

Imperial College London (c)

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3.0

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Elec

tro

de

po

ten

tial

(SH

E) /

V

pH

Fe3+

Fe2+

Fe3O4

Fe(OH)2

Fe

FeO42-

HFeO4-

H3FeO4+ H2FeO4

Fe(

OH

) 4-

Fe2O3 (c)

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Potential-pH diagram of Fe-H2O System at 298 K; dissolved activity = 10-4

O2

H2O

H+

H2

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Ec

Ev

e-

h+

n-type Fe2O3 Semiconductor

Photo-anode

Catalysed

Metal

Cathode

e-

EF(SC)

Electron

Energy

Eg

e-

Ua

1.23 eV EFermi

esc(ph)

EF(h),s

EF(h),(x)

2 2F O H O

E

2F H O OH

E

| Aqueous

electrolyte

solution |

2 24 4 2VBOH h O H O

Ion-

permeable

Membrane

O2

O2 O2

Aqueous |

electrolyte

| solution

Ec,s

A

Ua

2O

2H

O2

Auxiliary Potential Source

2 24 4 2 4CBH O e H OH

H2

H2 H2

H2

H2

H2

H2

H2

H2

H2

H2

O2

O2

absorption

recombination

Semiconductor Semiconductor( , )VB CBh h e

Photo-Electrochemical Reactors: Photo-assisted Electrolysis

hv

Imperial College London (c)

8

Hotplate

Spray head

Syringe

pump

Refill port

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3

4 1

5

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Films produced by

nebulising ethanolic

solutions of metal salts

onto heated substrate

1. Compressed Air 2. Precursor reservoir 3. Syringe pump 4. Quartz nebuliser 5. CNC machine 6. Substrate 7. Clamping block 8. Hotplate

Hematite Photo-anode Fabrication by Spray Pyrolysis on FTO or Ti Substrates

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200 nm thick SnIV-doped Fe2O3 Photo-Anode Spray Pyrolysed onto Fluorine-Doped Tin Oxide Coated Glass

SEM image

C. Carver, Z. Ulissi, C.K. Ong, S. Dennison, G.H. Kelsall, and K. Hellgardt, (2011). Modelling and development of photoelectrochemical reactor for H2 production. Int.J.Hydrogen Energy, 37(3), 2012, 2911–2923.

Potential / current distribution problems with transparent substrates (ITO, FTO etc.)

1E-5

1E-4

1E-3

1E-2

1E-1

1E+0

1E+1

1E+2

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| C

urr

ent

Den

sity

/ A

m-2

|

Electrode Potential (HgO|Hg) / V 10

Light

Dark

2 22 4 4H O O H e

2

0

O OHE 0.303 V HgO/Hg

2 2

0H O HE 0.926 V HgO/Hg

→Oxidation

←Reduction ( 1.2 cm3 H2 m-2 s-1)

2 24 4 2 4H O e H OH

Effect of Potential and (49 W m-2 Xe) White Light Illumination on Glass | F-doped SnO2 | 400 nm Spray Pyrolysed SnIV-doped Fe2O3 | 1 M NaOH

2 22 44VBOHOOHh

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Photo-Electrochemical Reactor Design: Scale-up Options

Mesh cathode

Anode (side facing cathode masked)

hν

Possible Compromise

Mesh cathode

Anode (hematite film facing cathode)

hν

Mesh cathode

hν

× Non-uniform current distribution

Mini cavities in photo-anode to improve current distribution

Fe2O3 film facing away from cathode

Even illumination

× ½ Fe2O3 film area in dark

Fe2O3 film facing towards cathode

More uniform current distribution

Symmetrical Vertical - Lateral Scale-up

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Photo-Electrochemical Reactor Design: Scale-up Options

Symmetrical Vertical - Lateral Scale-up Non-heat treated

Fe2O3 | TiHeat treated

Fe2O3 | Ti

100 mm × 100 mm

~

Electrolyte: 1M NaOH

Mirror

Front side illumination of anode

Pt/Ti mesh cathode

Nafionmembrane

Ti | SnO2-Fe2O3

Photo-anode

Solar simulator

380 W m-2

H2

Ong C.K., Dennison S., Fearn S., Hellgardt K., Kelsall G.H., Electrochim. Acta 2014, 125, 266-274

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Cu

rren

t, I

/ A

Applied electrode potential, E (SHE) / V

1.00 0.75

0.50 0.25

Fraction of exposed photo-anode area:

Effect of anode potential on O2 evolution current at 0.1 × 0.1 m2 Ti |SnIV-Fe2O3 anode

in 1 M NaOH solution (pH ca. 13.7) with parts of surface area masked; 50 mV s-1

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2 22 4 4 VBO H O OH h

Exposed area: 1.0 Exposed area: 0.75

Covered area

Exposed area: 0.5

Covered area

Exposed area: 0.25

Covered area

Imperial College London (c)

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Predicted Spatial Distribution of O2 Overpotential at 0.1×0.1 m2 Ti |SnIV-Fe2O3

(Back-side) Anode in 1 M NaOH solution (pH ca. 13.7); Ti | Pt cathode

Ti / SnIV-doped Fe2O3 | 1 M NaOH | Pt / Ti @ U = 1.5 V

Dis

tan

ce

O2 O

verp

ote

nti

al /

V

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Predicted Spatial Distribution of O2 Current Density at 0.1×0.1 m2 Ti |SnIV-

Fe2O3 (Back-side) Anode in 1 M NaOH solution (pH ca. 13.7); Ti | Pt cathode

Ti / SnIV-doped Fe2O3 | 1 M NaOH | Pt / Ti @ U = 1.5 V

Dis

tan

ce

Cu

rren

t d

ensi

ty /

A m

-2

Perforated Fe2O3 Photo-anode with back-side illumination (h)

Cathode

In-active side

i j

Potential () distribution in electrolyte solution; conductivity current density j

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(Normalised) Primary Potential Distribution in Photo-assisted Electrolyser

02

2

2

2

2

22

zyx

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Predicted Spatial Distribution of O2 Overpotential at 0.1×0.1 m2 Ti |SnIV-Fe2O3

(Back-side) Anode in 1 M NaOH solution (pH ca. 13.7); Ti | Pt cathode

Ti / SnIV-doped Fe2O3 | 1 M NaOH | Pt / Ti @ U = 1.5 V

Dis

tan

ce

O2 O

verp

ote

nti

al /

V

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Predicted Spatial Distribution of O2 Secondary Current Density at 0.1×0.1 m2 Ti |SnIV-

Fe2O3 (Back-side) Anode in 1 M NaOH solution (pH ca. 13.7); Ti | Pt cathode D

ista

nce

Ti / SnIV-doped Fe2O3 | 1 M NaOH | Pt / Ti @ U = 1.5 V

Cu

rren

t d

ensi

ty /

A m

-2

ld

lh

ld

lx

ld/2

Unit cell of photo-anode

Geometry of Perforated Ti / SnIV-doped Fe2O3 Photo-anode

19 Imperial College London (c)

Is there an optimal geometry for perforations? Ionic current paths homogenise current distribution, but lose photo-active area

𝑙𝑑𝑙ℎ

= 5

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Cu

rren

t d

ensi

ty /

A m

-2

𝑙𝑑𝑙ℎ

= 2

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Cu

rren

t d

ensi

ty /

A m

-2

𝑙𝑑𝑙ℎ

= 1.2

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Cu

rren

t d

ensi

ty /

A m

-2

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Cu

rren

t d

ensi

ty /

A m

-2

Ratio of Hole centre-to-centre distance, ld to Hole diameter, lh

Effect of Ratio of Hole Centre-to-centre Distance to Hole Diameter on (Geometric) Anode Current Density

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Ti / SnIV-doped Fe2O3 | 1 M NaOH | Pt / Ti @ U = 1.5 V

Summary

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(SnIV-doped) Fe2O3 photo-anodes produced on F-doped SnO2–coated glass

or metal substrates by spray-pyrolysis.

Fe2O3 photo-anode | anolyte| membrane | catholyte | cathode photo-

assisted electrolyser produced current densities of 1-10 A m-2

( (0.5 – 5)10-5 mol H2 m-2 s-1).

Current densities of photo-electrochemical reactor with Fe2O3 photoanode

increased linearly with photon flux.

Models well predict distributions of potential, current densities, dissolved

gas, and photocurrent densities as f(light intensity).

25 Imperial College London (c)

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Incident Solar Power

• 125,000 TW at earth’s surface

• 36,000 TW on land

10 213

2

-2photonphoton

8.2 10 m1.4 10 WPhoto-electrochemical Area, / m

170 W m

photon ee

A

Global mean irradiance Photon yield

Power Challenge

World power consumption of ca. 14 TW in 2010 predicted to double by 2050

Photo-electrode requirements: • Efficient • Cheap • robust + easily fabricated on large scale

Any 2 of 3 properties possible at present

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pH 0 2 4 6 8 10 12 14 16

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Fe3+

Fe(OH)3 (aq)

Fe(OH)4-

Fe2O3 (c)

FeOH2+

Fe(OH)2+

Fe2(OH)24+

log

( Fe

III a

ctiv

ity)

Solubility (activity-pH) diagram for Fe2O3 (c)

in equilibrium with aqueous FeIII species at 298 K