Photoelectrochemical water splitting systemfor biocatalytic hydrogen production
Jun Tae Song, Masaki Nobukuni, Yuta Itagoe,
Motonori Watanabe, Atsushi Takagaki, Tatsumi Ishihara
Department of Applied Chemistry, Kyushu University
International Institute of Carbon-Neutral Energy Research (I2CNER), Kyushu University

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Inorganic-bio hybrid photocatalytic H2 production
Escherichia.coli (E. coli)
biocatalyst

TiO2/E.coli w/ MV2+
TiO2/E.coli
w/o MV2+

TiO2
Redox mediator: methyl viologen (MV)w/ 300 W Xe lamp••

E.coli cell: H2-forming site in inorganic-bio hybrid photocatalytic system
Redox mediator (MV) boosts hydrogen evolution→ Redox mediator should be key factor for biocatalytic H2 production !
Honda et al., Angew. Chem. 2016, 128, 8177

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Inorganic-bio hybrid photocatalytic H2 production
Escherichia.coli (E. coli)
biocatalyst

TiO2/E.coli w/ MV2+TiO2/E.coli
w/o MV2+

TiO2
Redox mediator: methyl viologen (MV)O2・–••

O2

w/ 300 W Xe lamp

E.coli cell: H2-forming site in inorganic-bio hybrid photocatalytic system
Redox mediator (MV) boosts hydrogen evolution→ Redox mediator should be key factor for biocatalytic H2 production !

Reversible reaction of MV+ by oxygen → Obstacle for overall water splitting
Honda et al., Angew. Chem. 2016, 128, 8177

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Photoelectrochemical (PEC) system approach
External potential
e-O2

Cathodes2H2 4H+MV+

Photoelectrode

Biocataylst

MV2+membrane

PEC configuration separates H2 & O2 evolution sites:: Enables overall water splitting

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Photoelectrochemical (PEC) system approach
External potential
e-O2

Cathodes2H2 4H+MV+

PhotoelectrodeBiocataylst

MV2+membrane

PEC configuration separates H2 & O2 evolution sites:: Enables overall water splitting
Research issues & purpose
1. The behavior of electrochemical reduction of MV2+
- Investigation of cathode materials
2. Light-driven PEC cell’s operation- PEC demonstration with TiO2 model electrode5

Experimental Method
Gas chromatography
Potentiostat

In situ UV-Vis(Photo)Electrochemical evaluation condition (three-electrode configuration)• Working electrode (WE) :- Cathode materials: Au, Ag, Pt, Ni,- Anode: TiO2 (coated on FTO by electrophoresis method)• Counter electrode (CE) : Pt wire
• Reference electrode (RE) : Ag/AgCl in 3.0M NaCl
• Electrolyte: 1.0M KOH +1.0M KH2PO4 (pH=8) + MV

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Electrochemical behavior of MV
MV+/MV0 MV2+/MV+

Current density (mA/cm 2)

0.0

-0.1

-0.2

0 µM
200 µM
500 µM
1000 µM

-0.3

-0.4
-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential (V vs RHE)

MV concentration dependence with Ag cathode• Reduction potentials for MV2+/MV+ & MV+/MV0 are -0.08 V & -0.33 V.
• Higher concentration of MV increases current density.
7Electrochemical behavior of MV
MV+/MV0 MV2+/MV+

Current density (mA/cm 2)

0.0-0.1

Cathode investigation !
at -0.08 V-0.2

0 µM
200 µM
500 µM
1000 µM

-0.3

-0.4
-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential (V vs RHE)

MV concentration dependence with Ag cathode• Reduction potentials for MV2+/MV+ & MV+/MV0 are -0.08 V & -0.33 V.
• Higher concentration of MV increases current density.
8Cathode materials property on MV2+ reduction
ü The effect of cathode material on MV2+ reduction
100-0.3

0.00-0.6-0.01

NiAu
Pt
Ag-0.02

-0.9

-0.03
0-1.2

10-1.5-1.820@ -0.08 V vs RHE
0102030

Faradaic Efficiency (%)

Current Density (mA/cm2)

0.0

H2MV

80
60
40
200Ag

Au

Ni

Pt

Time (min)• Ag, Au, Ni: High Faradaic efficiency for MV reduction, low current density• Pt: the highest current density due to superior H2 evolution activity

9Cathode materials property on MV2+ reduction
ü The effect of cathode material on MV2+ reduction
100-0.3

0.00-0.6-0.01

Ni
Au
Pt
Ag-0.02-0.9-0.03
0-1.2

10-1.5-1.8

20@ -0.08 V vs RHE
0102030

Faradaic Efficiency (%)

Current Density (mA/cm2)

0.0

H2MV

80
60
40
200Ag

Au

Ni

Pt

Time (min)• Ag, Au, Ni: High Faradaic efficiency for MV reduction, low current density• Pt: the highest current density due to superior H2 evolution activity
Partial current density: JMV2+/MV+ = total current density (J) x Faradaic efficiency

JMV2+/MV+ for Pt = 0.17 > JMV2+/MV+ for Ni = 0.09

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Fabrication of TiO2 anode3.50 µm
Power150
V, supply
10 secglass FTO P25FTOSi2 µm

SnSi

2 µm

Ti

8 mm
TiO2, I2, Acetone

Electrophoretic deposition

2 µm

2 µm

SEM & EDX mapping images of P25 TiO2 electrode• Via electrophoresis, we prepared ~3.5 𝜇m TiO2 anode on FTO substrate.
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Characteristic of TiO2 anodeü XRD patterns

ü UV-Vis spectra
1.5P25Anatase

P25Anatase
Rutile1.2AbsorbanceIntensity (a.u.)

FTO
Anatase
Rutile

0.9
0.6
0.3

Rutile102030 40 50 60
2 theta (degree)

70

80

0.0
300

400

500
600
700
Wavelength (nm)800

• P25, anatase and rutile type of TiO2 are deposited on FTO glasses.
• XRD & UV-Vis confirm each of TiO2 phases is successfully prepared.
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Electrochemical behavior of TiO2 anodeü I-V curves w/ light illumination
Current density (mA/cm2)

1.5

w/ 1mM MV

1.2

P25
P25 dark
Anatase
Rutile

0.9
0.6
0.3
0.0
-0.5

0.00.51.0
1.5
2.0
Potential (V vs RHE)

2.5

• P25 shows the highest PEC activity.

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Electrochemical behavior of TiO2 anodeü Open-circuit property

ü I-V curves w/ light illumination
Current density (mA/cm2)

1.5

w/ 1mM MV

1.2

P25
P25 dark
Anatase
Rutile

0.9
0.6
0.3
0.0
-0.5w/ 1mM MV

0.0

0.5
1.0
1.5
2.0
Potential (V vs RHE)

2.5

• P25 shows the highest PEC activity.
• Higher photovoltage for P25.

Light
① Open-circuit
potential
P-25-1.08 V

Anatase-1.07 V

Rutile-0.82 V14

Electrochemical behavior of TiO2 anodeü Open-circuit property

ü I-V curves w/ light illumination
Current density (mA/cm2)

1.5

w/ 1mM MV

Anatase
1.2 Voc = -0.14 VRHE

0.9

P25Voc = -0.09 VRHE

0.6

RutileVoc= 0.18 VRHE

0.3
0.0
-0.5

P25
P25 dark
Anatase
Rutilew/ 1mM MV

0.0

0.5
1.0
1.5
2.0
Potential (V vs RHE)

2.5

• P25 shows the highest PEC activity.
• Higher photovoltage for P25.

Light
① Open-circuit
potential
P-25-1.08 V

Anatase-1.07 VRutile-0.82 V15

PEC reduction of MV in open-circuit condition

Current density (mA/cm2)

0.25

w/ 1mM MV
No bias0.20

ü Amounts of MV+ production

P25 w/o MVP25
Anatase
Rutile0.15
0.092 mA/cm2

0.10

0.055 mA/cm2

0.05

20.026 mA/cm

20.00

0.004 mA/cm

0102030
Time (min)

4050

60

Amount of converted MV+ (µmol)

ü I-t graph w/ light illumination

10.0P25
Anatase
Rutile7.5
5.0
2.5
0.005101520
Time (min)2530• For P25, 3.5 times higher current density is obtained w/ MV than w/o MV.
• We confirmed MV+ production by reducing MV2+ without external bias.

16PEC reduction of MV in open-circuit condition
ü Production rate

0.3

w/ 1mM MV
No bias

+MV

0.2
0.1
0.0
P25 w/o MV P25

100H2

Anatase

RutileFaradaic efficiency (%)

Formation rate (µmol/min)

0.4

ü Products selectivityMV+
H27550250

P25 w/o MV P25

Anatase

Rutile• P25 shows the highest MV+ formation rate.
• MV reduction reduction is dominant under open-circuit condition.
→ MV reduction potential should be lower than hydrogen evolving potential.
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PEC reduction behavior of MV with external bias

Current density (mA/cm2)

2.0

ü Production rates comparison
7w/ 1mM MV

61.5541.0

320.5

10.0005101520Time (min)2530

Amount of converted MV+ (µmol)
Formation rate (µmol/min)ü Photoelectrolysis at -1.7 V vs RHE

1.0
0.8

MV+
H20.6
0.4

Decrease

0.2
0.0-0.09 V vs RHE≈ Voc

1.67 V vs RHE• Applied bias boosts H2 production while decreasing MV2+ reduction.
→ To selectively transfer electrons for MV reduction,controlling cathodic potential should be significant issue !
18Summary§ MV is key element for electron transfers to biocatalysts for H2 evolution.
§ We approach PEC system for inorganic-bio water splitting.
§ Electrochemical behavior of MV with Ag, Au, Ni, Pt has been investigated.
§ PEC cell with P25 TiO2 & Pt shows successful MV reduction without bias.
§ Applied bias just boosts H2 production while decreasing MV reduction
reaction.
§ Cathode potential should be properly controlled for preferential MV reduction
toward biocatalytic H2 production.

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Thank you for your kind attention !

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