Hydrogen production from water with solar energy

Kazunari Domen

Chemical Systems Engineering

The University of Tokyo

2008.3.14

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

GIES2008

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

H2 + 1―2

O2 ΔG0 = 238 kJmol-1H2Ohν

Hydrogen Production from Water using Solar Energy

▪ Solar cell + Electrolysis: pilot plants

▪ Photoelectrochemical Cell: good efficiencies with tandem cells

Efficiency=12.4 %

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

Efficiency=18.3 %J. A. Turner et al., Science 1998, S. Licht et al., J. Phys. Chem. B 2000,

Two examples of photoelectrochemical cells

Photoelectrochemical cells + PV

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

solar energy conversion efficiency = ３～４％

M. Grätzel, Nature 2001, 414, 338–344.

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

▪ Solar cell + Electrolysis

▪ Photoelectrochemical Cell

H2 + 1―2

O2 ΔG0 = 238 kJmol-1H2Ohν

▪ Artificial Photosynthesis (Photocatalysis)Inorganic solid materialmetal complexorganic materialbiomaterial

▪ Photosynthesis

Available photons for water splitting

H2O → H2 + 1/2O2

ΔG0 = 237 kJ/mol= n×F ×E0

= 2×96500 C/mol×1.23 V

One electron energy = 1.23 eV

λ=1000 nm1.23 eV )(nmλ

ν 1238h (eV) E ==Photon energy

0 500 1000 1500 2000Wavelength λ/nm

0

1.5

3R

adia

tion

Pow

er k

Wm

-2μm

-1

UV Vis IR

Solar Energy Distribution

1.23 eV =1000 nm

Solar Energy Conversion Efficiency

Solar energy conversion efficiency(Q.E.=100 %)

33.5

29.4

25.1

20.6

16.2

11.9

7.95

4.29

1.67

Energy efficiency/ %

800

750

700

650

600

550

500

450

400

Wavelength/ nm

SEM Images of La-doped NaTaO3

0.1 µm

Prof. A. Kudo (Tokyo Univ. Sci.)

Overall water splitting on NiO(0.2wt%)/NaTaO3 :La

200W Xe-Hg Lamp

Layered photocatalyst

BG:4.1eVQY:50% (270nm)

Prof. A. Kudo (Tokyo Univ. Sci.)

Time Course of Overall Water Splitting on NiO(0.2wt%)/NaTaO3:La

Q.E. = 56 %

Challenge with Visible Light

H2 + 1―2

O2 ΔG0 = 238 kJmol-1H2O

Photocatalytic Materials with

・Sufficient Solar Energy (Visible Light) Absorption

・Potential for Overall Water Splitting・Enough Stability

?

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

GaN:ZnO solid solution

GaN GaN:ZnO ZnO

1 μm

RuO2 : 3.5 wt%

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

0 5 10 15 20 25 30 350

0.5

1.0

1.5

2.0

Am

ount

of e

volv

ed g

ases

/ m

mol

Reaction time / h

Evac.▼

H2

O2

N2

Magnetic stirrer

Water cooling

450 W high pressure Hg lamp

NaNO2 aq.

Catalyst: 0.3 gCocat.: Rh 1 wt%, Cr 1.5 wt%Reactant soln.: 370 mL (pH 4.5)Inner irradiation type cell with a 2 M NaNO2 aq. filter450 W Hg lamp

Catalyst: 0.3 gCocat.: Rh 1 wt%, Cr 1.5 wt%Reactant soln.: 370 mL (pH 4.5)Inner irradiation type cell with a 2 M NaNO2 aq. filter450 W Hg lamp

λ > 400 nmλ > 400 nm

Visible Light-Driven Overall Water Splitting on Rh-Cr oxide/GaN:ZnO

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

30 nm

SEM image TEM image

Rh-Cr mixed oxide

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

Electron micrographs of Rh-Cr oxide/GaN:ZnO

hνe-

h+

H+H2

H2OO2

Overall water splitting on Rh-Cr oxide/GaN:ZnO

λ > 300 nmλ > 300 nm

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

Q.E. of overall water splitting on Rh-Cr oxide/GaN:ZnO

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

300 350 400 450 5000

0.51.01.52.02.53.03.54.04.5

Q

uant

um e

ffici

ency

/ %

Wavelength / nmA

bsor

banc

e /

a.u.

Q.E.

RuO2-loaded(Q.E. 0.23%)

5.9 %

Two approaches for overall water splitting

H+ / H2

O2 / H2O

Red

Oxhν

H+

H2

H2O

O2h+

e–

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

One-step excitation

e–

h+

h+

e–

H2 evolution

O2 evolution

H+

H2

H2O

O2

hν

hν

Two-step excitation(Z-scheme)

Oxide

BaTaO

2 N

CaTaO

2 N

SrTaO2 N

Ta3 N

5

LaTaON

2

CaLaTiO

N

LaTiO2 N

Li2 LaTa2 O

6 N

Various Oxynitrides

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

e-

e- H+

H2

h+

h+

I-

IO3-

e-

e-

e-

e-

H2O

O2

hν

hν

Pt/WO3

Pt/TaON

Two-step overall water splitting under visible light

(Reaction conditions)Pt/TaON: 0.2 g + Pt/WO3: 0.2 g5 mM-NaI aqueous solution: 250 mL (pH = 6.5)300 W Xe-lamp, L-42 cut-off filter

200

150

100

50

02520151050

H2

O2

N2

Chem. Commun. 2005, 3829

Time / hA

mou

nt o

f evo

lved

gas

es / μm

ol

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

(λ < 450 nm)(λ < 500 nm)

70

60

50

40

30

20

10

020151050

Time / h

Am

ount

of e

volv

ed g

ases

/ μm

ol

H2

O2

N2

λ > 420nm

Two-step overall water splitting on Pt/WO3+Pt/BaTaO2N

e-

e- H+

H2

h+

h+

I–

IO3–

e-

e-

e-

e-

H2O

O2

hν

hν

Pt/BaTaO2N

Pt/WO3

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

(λ < 450 nm)

(λ < 670 nm)

4

3

2

1

0

K-M

func

tion

(a.u

.)

800700600500400300Wavelength / nm

Ta3N5(O2 evolution)BaTaO2N(H2 evolution)

BaNbO2N

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

Available materials for two-step overall water splitting

Application of oxynitrides to PEC cell

(H+/H2)0

1.23(O2/H2O)

V.B.O2p + N2p

C.B. Ta5d

E.G. = 2.5 eV

+2.2

-0.3-

+

Potential / V vs NHE at pH0

OTaN

oxynitride TaON

K. Domen et al., J. Phys. Chem. B, 2003, 107, 1798

R. Abe et al., Chem. Commun., 2005, 3829

TaON electrode

e–

Visible lightH2 O2

Pt electrode

薄膜電極化

阿部（北海道大学）

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Cur

rent

den

sity

/ m

A c

m-2

1.00.80.60.40.20.0-0.2-0.4Potential / V vs. Ag/AgCl

(a) as-prepared TaON (b) treated with TaCl5 (c) treated with NH3

e-

e-

e-

e-

e-

e-

TaCl5-calcination

Ta2O5 fine particles

TaON fine particles

NH3-treatment

phase boundaries

[Conditions]Supporting electrolyte: 0.1 mol-L-1 of Na2SO4Potentiostat: PARSTAT 2263Rate of scan: 50 mV / sLight source: Xe illuminator (300 W, 300 < λ < 500 nm)

[Conditions]Supporting electrolyte: 0.1 mol-L-1 of Na2SO4Potentiostat: PARSTAT 2263Rate of scan: 50 mV / sLight source: Xe illuminator (300 W, 300 < λ < 500 nm)

Potential(V vs. NHE)

o- 0.3

+ 0.5

Ta 5d

W 5d

O2p + N2p

O2p

2.6 eV

2.5 eV

1.5

1.0

0.5

0.0

Cur

rent

den

sity

/ m

A c

m-2

0.80.40.0-0.4Potential / V vs. Ag/AgCl

WO3 TaON

Cell 1Rh-Cr/GaN:ZnO

Cell 2Pt/WO3

20 mmol L-1 NaI aq. 500 mL300 W Xe lamp (λ > 300 nm)

1200

1000

800

600

400

200

0403020100

H2 O2

300

250

200

150

100

50

0403020100

H2 O2

Time / h

Cell 1

Cell 2

Am

ount

of g

as e

volv

ed / μm

olA

mou

nt o

f gas

evo

lved

/ μm

ol

IO3−

IO3−

I−H2O

OO22

e− h+h+e− I−

HH22

HH++

IO3−

WO3TaON

Two-room overall water splitting

Overall water splitting with sacrificial reagents

H2O H2 + 1/2O2

2H2O + Na2S H2 + S + 2NaOH

H2O + CH3OH 3H2 + CO2

6H2O + C6H12O6 12H2 + 6CO2

ΔG0 = 238 kJmol-1

TEM image of stratified CdSTEM image of stratified CdS

CdS capsules (30 nm) consist of nanoparticles (5nm)

Prof. K. Tohji (Tohoku Univ.)

H2 production on stratified CdS from Na2S solution

Prof. K. Tohji (Tohoku Univ.)

Chemical System EngineerinChemical System EngineerinThe University of Tokyo

▪ Solar cell + Electrolysis

▪ Photoelectrochemical Cellvs

▪ Artificial Photosynthesis (Photocatalysis)

materials

Key IssuesNew materials

Application to wide area; Reactor designHybridization; structure

Hydrogen Production from Water using Solar Energy