14/7/2016 1

Development of an artificial photosynthesis device: from nano to macro-scale.

The BiVO4 example.

Simelys Hernández,*,1

Guido Saracco,1,2 Nunzio Russo.1

* E-mail: simelys.hernandez@polito.it

1 DISAT, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino (Italy)

2 Center for Sustainable Futures, CSF@POLITO, Istituto Italiano di Tecnologia, C.so Trento 21, 10129 Torino (Italy)

2H2O O2 + “2H2”

Solar EnergyLight reaction

“2H2” + CO2 (CH2O)Organic

molecules

Dark reacion

Two basic reactions of photosynthesis

UV

H++

Solar Fuels via Artificial Photosynthesis The “mimicking nature” approach:separation of the functions of light

collection and conversion from catalysis (e.g. Transition metal oxides: IrO2, Co3O4, RuO2, Mn2O3,… or Ru/ Co-

based molecular catalyst)

Many semiconductor materials (e.g.oxides, oxynitrides, oxysulfides,

oxyfluorides, and oxyfluoronitride) have been studyed to meet specific

requirements.

The direct water photo-electrolysis:

Why BiVO4 is interesting for water splitting ?

+ Good solar light absorption (band gap = 2.4 eV)

+ Conduction and Valence band edges straddling the red-ox potentials of water.

+ High chemical stability around neutral pHs

+ Earth abundant components

- Poor e- transport and limited superficial transfer properties that can lead to recombination of photo-exited charges.

Strategies to increase photo-activity of BiVO4

Study of Reaction kinetics of Water Splitting Catalysts

Evaluation of photoelectrodes

performance

9 cm2200 cm2

2500 cm2

Implementation of PEM Photo-electrolyzerfrom lab- to pilot-scale

1.6 m2

MJ-PV cell

H2 evolving PEC lab-reactor

Our work towards the Artificial Leaf…

Outline

1 cm2

Development of a system and a mathematical model to measure the actual activity of Water Oxidation Catalysts and Photo-catalysts (semiconductors) in powder form, for the water splitting reaction

Solar simulated light sourceBubbling Reactor

Clark-sensor & PH-meter

Mass Flow Controllers

Our research

Hernández S. et .al., Chem. Eng. J. 2014, 238, 17-267

Study of the kinetics for water oxidation half reactionof powder photo-catalysts

Vg

VLRO2

a)

b)

pO2

QAr+ ΦO2

pO2,analyzer

c)

d)

QAr

O2 CO2

Ag+1 Ag0

2H2O

BiVO4 Ar Bubble

hvh+

e-

4h+ + 2H2O → O2 +4H+ + 4e-

4Ag+1+ 4e- → 4Ag0

Water Oxidation

Silver Nitrate Reduction

8

RORO,O

LOR V)ε1(VH

akV)ε1(22

2

2−+⋅

−⋅=− RC

pC b

RTQV

Hak

RT εV ,O

RO,O

L,O

R2

2

22 bbb pC

pp⋅−⋅

−⋅−=

−⋅=

RTRTQ

RT V 222 O,OO

g

ppp b

)()(22 O,O ttptp analyzer ∆−=

Mathematical Model: O2 balance into the Bubbling reactor

- In the liquid phase:

- In the gaseous bubble phase:

- In the gas phase in the headspace above liquid

- In the gas phase at the analyzer:

Oxygen is formed on the catalyst surface

Actual rate of O2 evolution over time: RO2Hernández S. et .al., Chem. Eng. J. 2014, 238, 17-26

Strategies to increase photo-activity of BiVO4

BiVO4 in Powder form

14/7/2016 11

1μm

BiVO4 morphology control by Hydrothermal Synthesis

Thalluri S-M, et. al. Chem. Eng. J. 2014, 245, 124-132.

pH of the synthesis

media

Precursors: Bi(NO3)3.5H2O; NH4VO3 and (NH4)2CO3

0

2

4 - 6

8 - 10

14/7/2016 12

1μm

Thalluri M. et . al., Chem. Eng. J. 2014, 245, 124-132

BiVO4 morphology control by hydrothermal synthesis

14/7/2016 13Thalluri M. et. al., Ind. & Eng. Chem. Res. 2013, 52, 19, 17414-17418.

Optimization of calcination temperature of BiVO4 powders made at pH 0

Metal-doped BiVO4 powders by hydrothermal synthesis

Thalluri, S. M.; Hernández, S.; et. al. Appl. Catal. B: Environm. 2016, 180, 630-636.

We’ve improved the state-of-the-art photo-activity

(RO2o ~ 200 to 400 μmol/gcat.h) of

Mo & W-doped BiVO4

Development of a Photo-electrochemical Device for water splitting

BiVO4 electrodes from powders: By doctor-blade

PEC tests in Na2SO4 (0,1M) pH=6.5

Max. photo-current density at 1 sun & 1.23VRHE = 0.04

mA/cm2

Preparation of electrodes from BiVO4 powders

Max. photo-current density at 1 sun & 1.23VRHE = 0.6 mA/cm2

Dip-coating:

PEC tests in Na2SO4 (0,1M) pH=6.5

Preparation of BiVO4 electrodes by in-situ synthesis

S. Hernández, et.al., Applied Catalysis A: General, 2015, 504, 266-271.

Modeling of transport properties

S. Hernández, et. al., Applied Catalysis A: General, 2015, 504, 266-271.

Limitation of use dip-coating technique with intermediate calcination steps: Decrease of activity after a certain thickness (~160nm)

trapping of charges in the surface states

charge transfer between the electrolyte and the surface states

Rct: direct charge transfer

at the semiconductor/

electrolyteinterface

BV-3: larger impedance than the other two films -> reduced quantity of the deposited material.

BV-10 and BV-15: an additional process with time constant at very large frequencies (around 5 kHz) occurs, suggesting that a charge transfer mechanism occur via surface states -> due to an imperfect interconnection between adjacent layers in the thicker films.

Rct: 540 Ω

Rct + Rtrap = 580 Ω

Procedure 1 Precursors: Bi(NO3)3 ∙5H2O - NH4VO3

C1=50 mmol L-1 (in HNO3 1 M)Calcination: T1=773 K per 2 h.2 steps spin-coating on FTO:

500 rpm-10 s/ 2000 rpm- 15 s

Procedure 3Precursors: Bi(NO3)3 ∙5H2O -

VO(AcAc)2C3 =33 mmol L-1

(in Acetic Acid & Acetilacetone)Calcination: T3=673 K per 2 h.

1 step spin-coating on FTO:500 rpm-10 s

Procedure 2Precursors: Bi(NO3)3 ∙5H2O -

NH4VO3C2=200 mmol L-1 (in HNO3 2 M)Calcination: T2=673 K per 2 h.2 steps spin-coating on FTO:

500 rpm-10 s/ 2000 rpm- 15 s

Higher stability of the solution

Spin-coating

1.23V vs. RHE under solar light illumination (100

mW/cm2) in Na-phosphate Buffer (pH=7), active area:

4cm2

(Used for dip-coating)

Preparation of BiVO4 electrodes by in-situ synthesis

S. Hernández, et. al., Applied Catalysis B: Environm., 2016, 190, 66-74.

Strategies to increase the activity of BiVO4

BiVO4 photo-electrodes in FTO-glass

0

5

10

15

20

25

30

300 400 500

IPCE

(%

)

Wavelenght (nm)

IPCE at 1.23 V vs. RHEBiVO4 (6L)_WO3Mo-BiVO4BiVO4 (6L)WO3 (2)

WO3

Mo-BiVO4

BiVO4 / WO3Modification of high porous BiVO4 electrodes to better

exploits the high surface area

Preparation of BiVO4 electrodes by in-situ synthesis

S. Hernández et. al., Applied Catalysis B: Environm., 2016, 190, 66-74.D. Valerini et. al. Materials Science in Semiconductor Process. 2016, 42, Part 1, 150-154.

+ CoPi

0.4 0.6 0.8 1.0 1.2 1.40.0

0.5

1.0

1.5

2.0

Cur

rent

den

sity

(mA/

cm2 )

Potential vs. RHE (V)

WO3 (2)_BiVO4 (6L) 3%Mo-BiVO4 (6L) BiVO4 (6L) WO3 (2)

0.4 0.6 0.8 1.0 1.2 1.40.0

0.5

1.0

1.5

2.0

Cur

rent

den

sity

(mA/

cm2 )

Potential vs. RHE (V)

WO3 (2)_BiVO4 (6L)+CoPi 3%MoBiVO4_6L+CoPi

0 1 2 3 4 5 6 7 8 9 100.00.20.40.60.81.01.21.41.61.82.02.2

Cur

rent

den

sity

(mA/

cm2 )

time (min)

WO3 (2)_BiVO4 (6L) WO3 (2)_BiVO4+CoPi 3%Mo-BiVO4 (6L) 3%MoBiVO4+CoPi BiVO4 (6L) WO3 (2)

Mo doping enhances BiVO4activity by 6.5 fold.

Heterojunction BiVO4/WO3improves by 13 times BiVO4

performance

CoPi deposition increases de photocurrent of about 2 to 3

times.

Modification of high porous BiVO4 electrodes

Preparation of BiVO4 electrodes by in-situ synthesis

0.4 0.6 0.8 1.0 1.2 1.4 1.60.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0CoPi-BiVO4 photo-anode with Illuminated area of

0.785 cm2

6 cm2

Cur

rent

den

sity

( m

A/cm

2 )

Potential vs. RHE (V)

At higher O2 evolution rates the higher covering of electrodes by bubbles creates mass transfer

limitations

Increase of electrode size can induce formation of defects

acting as recombination centres

MJ-PV cellLab H2 evolving PEC

Issues on scaling-up of photo-electrodes

S. Hernández et. al., Applied Catalysis B: Environm., 2016, 190, 66-74.S. Hernandez et. al., Journal of Physical Chemistry C 2015, 119, 9916-9925.

e-

H2O

2H+ + ½O2

2H+ + 2e-

H2

BiVO4 active area:8x8 cm2

Photo-electrodes scaling-up

24

Upper Plate housing BiVO4 over FTO

Grooved plates containing electrolyte

Lower Plate holding setupGrooved plates containing electrolyte

Nafion membrane separating two chambersCo NPs/C-GDL adjacent to Nafion membrane

Cathode: Co NPs / GDL

Cathodic electrolyte: 0.5mM Co-nitrate + 0.5M Na-Phosphate (pH=7)

Anode: 3%Mo-BiVO4 (AR5)

Anodic electrolyte: 0.5mM Co-nitrate + 0.1M Na-Phosphate (pH=7)

Liquid flow rate: 30 rpm (300 ml/min)

Artiphyction device configuration

e-

2H++2e-

H2

H2O

2H++ ½O2

H+

0 50 100 150 200 250 30050

100

150

200

250

300

350

Curre

nt (m

A)

time (s)

Photocurrent over time for each PV-PEC couples

00.5

11.5

22.5

33.5

44.5

55.5

0 1 2 3 4 5

J (m

A/cm

2 )

V bias (V)

Efficiency (%)5

4

3

2

1

Trade-off of performance

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.00

4080

120160200240280320360400

current decrease

of 19%

2 PVs for each PEC305 mA => 2.2 mA/cm2

Although activity reduces by 25%

-> current will decrease only 5%

Curre

nt d

ensit

y (m

A)

Voltage (V)

PEC curve (Active area 64 cm2) Hyphotesis of 25% of loss in activity on PEC Performance of 1.2 PV for each PEC (Area=75 cm2) 2 PVs for each PEC

Working Point = 192 mA => 1.4 mA/cm2

Artiphyction device operation

Lab-scale results

1.6 m2

Prototype…27

1.6 m2

Prototype…

Conclusions

In order to develop an efficient device for sun driven water splitting more advances on different fields are necessary:

- Development of efficient and stable water splitting(photo)electrocatalysts prepared by using scalable techniques

- Design of optimum device configurations able to diminishmass transfer limitations

- Improve of compactness of the system, for instance byincorporating solar capture systems (ex. PV cells) in thephotoelectrochemical reactor design.

Diana Hidalgo, Carminna Ottone, Adriano Sacco, Angelica Chiodoni, Marco Fontana, Marzia Quaglio, Katarzyna Bejtka, Guido Saracco, Nunzio Russo, Samir Bensaid, Mouli Thalluri, Marco Armandi, Barbara Bonelli, Edoardo Garrone, Giovanni Barbero, Anca Ionescu, Domenico Mombello, Fabrizio Pirri, Elena tresso.

Dr. Antonella Rizzo and Dr. Gabriele Valerini.

Dr. Vincent Artero and Dr. Bruno Jousselme

The European commission is gratefully acknowledged for the financial support and the partners of the 7th Framework Programme projects:

Acknowledgments

FP7- NMP2012 Collaborative project nr.309701 (2013-2016)

FCH-JU Call 2011-1 ArtipHyctionProject nr.303435(2013-2015)