S1

Electronic Supplementary Information

Photoelectrochemical Water Splitting in an Organic Artificial Leaf

Serkan Esiner, Robin E. M. Willems, Alice Furlan, Weiwei Li, Martijn M. Wienk, and René A. J. Janssen Contents 1. A larger area organic leaf with earth-abundant catalysts S2 2. Catalyst stability S3 3. Catalyst characterization S4 3.1 RuO2 on Ti metal S4 3.2 Co3O4 nanoparticles on ITO S4 3.3 NiMoZn on Ni foil S7 4. References S8

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2015

S2

1. A larger area organic leaf with earth-abundant catalysts

PEC water splitting with a large area solar cell was also performed with earth-abundant catalysts. A ~1.2 cm2 triple junction cell was integrated with Co3O4/NiMoZn catalysts (~1.0 cm2 each) in 0.1 M KBi for oxygen and hydrogen evolution reactions. This solar cell had an efficiency of 5.7%, which reduced during the 20 min water splitting experiment (Fig. S1a). The operating voltage for this device was 1.83 V as shown in Fig. S1b. A low Jop of 1.03 mA cm-2 was observed during water splitting, which resulted in a solar to hydrogen conversion efficiency of 1.27%. The reason for low ηSTH is partially due to reduced fill factor by degradation of the solar cell and partially due to increased current density on the catalyst surfaces which increased the overpotentials and the operating voltage (Fig. S2 - ).

-0.4 0.0 0.4 0.8 1.2 1.6 2.0

-6

-5

-4

-3

-2

-1

0

1

2

Jsc

= 5.64 mA cm-2

Voc

= 2.03 V

FF = 0.50P

max = 5.72 mW cm-2

before after during water splitting

Cur

rent

den

sity

, J

(mA

cm

-2)

Voltage (V)

(a)

0 2 4 6 8 10 12 14 16 18 20 220.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Voperating

= 1.83 V

Joperating

= 1.03 mA cm-2

STH

= 1.27%

Op

era

ting

vo

ltag

e [

V]

Time [min]

(b)

0

1

2

3

4

Fig. S1 (a) JV curves of a ~1.2 cm2 triple junction solar cell connected to Co3O4 – NiMoZn catalysts in 0.1 M KBi, before, during and after water splitting measurement of 20 min. (b) Simultaneous measurement of operating voltage and current density of the solar cell during photoelectrochemical water splitting.

-6 -5 -4 -3 -2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

1.7 cm2

0.0676 cm2

RuO2 for H

2

NiMoZn for H2

RuO2 for O

2

Ove

rpo

tent

ial (

V)

Current density [log (A cm-2)]

Co3O

4 for O

2

1.7 cm2

1.2 cm2

Fig. S2 Tafel plots of RuO2 in 1.0 M of KOH and of Co3O4 and NiMoZn in 0.1 M potassium borate (KBi) at pH 9.2. The markers indicate the expected overpotentials during the operation of the small scale RuO2/RuO2 PEC cell (),the small scale Co3O4/NiMoZn PEC cell (□), the large scale RuO2/RuO2 PEC cell (○), and the large scale Co3O4/NiMoZn PEC cell ().

S3

2. Catalyst stability Stabilities of the catalysts used in the PEC water splitting devices were separately tested with two-electrode measurements. Fig. S3a shows the stability of two RuO2 catalysts deposited on a Ti plate for oxygen and hydrogen evolution reactions in 1.0 M KOH. Fig. S3b shows data for earth abundant Co3O4 and NiMoZn catalysts for oxygen and hydrogen evolution in 0.1 M KBi. The two-electrode measurements were performed at applied potentials of 1.65 V and 1.85 V for 20 min. The applied potentials were selected with respect to the operating potentials of large area artificial leafs. Especially after 15 min, current flow in both electrochemical cells significantly stabilizes. Nevertheless, RuO2 catalyst is known to be not completely stable for oxygen evolution reaction as it may oxidize further to one of its other states which are soluble.1 We have also observed that if a RuO2 catalyst is continuously kept in 1.0 M KOH for two days without operating, noticeable decrease in its catalytic activity takes place. However if the catalyst is stored in air it quickly restores to its original performance level when contacted again with the electrolyte and enables stable operation up to at least 2 h (Fig. S3c). A detailed study on the stability of RuO2 for hydrogen and oxygen evolution reactions in acid and alkaline media has been reported recently.2

0 2 4 6 8 10 12 14 16 18 20 220.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Curr

ent

(m

A)

Time (min)

RuO2 - RuO

2 catalysts in 1.0 M KOH at 1.65 V

(a)

0 2 4 6 8 10 12 14 16 18 20 220.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Cur

rent (

mA

)

Time (min)

Co3O

4 - NiMoZn catalysts in 0.1 M KBi at 1.85 V

(b)

0 20 40 60 80 100 1200.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Cu

rren

t (m

A)

Time (min)

RuO2 - RuO

2 catalysts in 1.0 M KOH at 1.65 V

(c)

Fig. S3 Stability of (a, c) RuO2/RuO2 catalysts in 1.0 M KOH and (b) Co3O4/NiMoZn catalysts in 0.1 M KBi for oxygen and hydrogen evolution reactions. Two-electrode water splitting test potentials of 1.65 V (a, c) and 1.85 V (b) were applied for 20 min. (a, b) or 2 h (c). The catalyst used in (c) were stored in air and reached the same activity as freshly prepared catalyst (a) after a short time.

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3. Catalyst characterization

3.1 RuO2 on Ti metal The RuO2 catalyst was characterized using X-ray diffraction (XRD) (Fig. S4) which confirms the formation of RuO2 on Ti metal.

10 20 30 40 50 60 70 800.0

5.0x103

1.0x104

1.5x104

2.0x104

202

301

002

220 11

0

002

211

101

Inte

nsi

ty

2(degree)

RuO2

Ti

110

101

102

103

112

112

310

Fig. S4 X-Ray diffraction pattern of RuO2 on Ti metal. The XRD peaks were assigned to RuO2 in a tetragonal crystallographic structure (JCPDS, No.40-1290)3,4 and Ti metal (JCPDS, No. 44-1294).5 3.2 Co3O4 nanoparticles on ITO Cobalt oxide nanoparticles were synthesized from a cobalt(II) acetate precursor and obtained as a solution in methanol. To estimate the size of the nanoparticles, the UV-vis spectrum of the nanoparticle solution was measured (Fig. S5). Transitions were observed at 371 and 668 nm. When compared with the transitions at 375 and 675 nm reported in literature for 3 nm Co3O4 nanoparticles, the absorption is slightly blue-shifted indicating that average size of the particles is slightly less than 3 nm.6 The two transitions have been assigned to O2−→ Co2+ and O2−→ Co3+ transitions.7,8

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300 400 500 600 700 800 9000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

O2- to Co3+Abso

rptio

n (

O.D

.)

(nm)

O2- to Co2+

Fig. S5 Absorption spectrum of the Co3O4 nanoparticles in methanol.

Transmission electron microscopy (TEM) measurements were carried out to study the geometry and size of the nanoparticles (Fig. S6). TEM shows that the nanoparticles were found to be 3 -5 nm in size in fair agreement with the UV-vis results. The particle size distribution was narrow.

Fig. S6 TEM images of the prepared Co3O4 nanoparticles.

The nanoparticles were deposited onto an ITO coated glass slide to be able to use the nanoparticles as oxygen evolving electrode. To improve the binding to the substrate, the particles were annealed in hot air of 400 to 500 °C for 1 to 2 min. The structure of the nanoparticles at the electrode surface was studied with scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and XRD. SEM showed that the Co3O4 layer on ITO is rough (Fig. S7). AFM revealed height differences of ~100 nm.

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Fig. S7 Scanning electron microscopy images of a Co3O4 nanoparticle catalyst layer.

In the accompanying EDX spectrum (Fig. S8), recorded during the SEM measurements, peaks of silicon, indium, tin, and cobalt, which were expected to be in the spectrum. Also potassium was present, which originates from the KBi electrolyte in which the electrode was tested.

Fig. S8 EDX spectrum of a Co3O4 nanoparticle catalyst layer on an ITO coated glass slide as substrate. XRD measurements of the powder were consistent with the formation Co3O4 on cubic In2O3

(Fig. S9). The diffraction peaks of Co3O4 are not very clear. The most intense (311) diffraction 36.7° of Co3O4 is broad, consistent with the small size of the particles. Of the other expected diffraction peaks the (220) reflection can be identified, but others are not clear.

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10 20 30 40 50 60 70 800.0

2.0x103

4.0x103

6.0x103

8.0x103

1.0x104

(440

)

(511

)

(422

)

(400

)(4

31)

(22

0)

(62

2)

(04

4)

(004

)

(22

2)

Inte

nsity

2(degee)

Co3O

4

ITO

(211

)

(311

)

Fig. S9 X-Ray diffraction pattern of Co3O4 on ITO (Sn doped In2O3). The XRD peaks were assigned to In2O3 in a cubic crystallographic structure (JCPDS, No. 65-3170)9,10 and Co3O4 (JCPDS, No. 42-1467, 43-1003). The arrows indicate expected positions for Co3O4 for which no clear reflection is observed. Note that the 36.7° (311) peak is the most intense. 3.3 NiMoZn on Ni foil

The successful preparation of a NiMoZn electrode is see from the significant drop of the overpotential of the electrochemically deposited NiMoZn layer on Ni foil by comparing the Tafel plots of NiMoZn and the Ni foil itself.

-5 -4 -3 -2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ove

rpot

ent

ial f

or H

2[V

]

Current denisty [log(A cm2)]

Ni foil NiMoZn on Ni foil

Fig. S10 Tafel plots of NiMoZn and Ni in 0.1 M potassium borate (KBi) at pH 9.2.

S8

4. References

1 K. Juodkazis, J. Juodkazytė, R. Vilkauskaitė, B. Šebeka and V. Jasulaitienė, Chemija., 2008, 19, 1.

2 S. Cherevko, S. Geiger, O. Kasian, N. Kulyk, J.-P. Grote, A. Savan, B. R. Shrestha, S. Merzlikin, B. Breitbach, A. Ludwig and K. J. J. Mayrhofer, Catal. Today, 2015, doi: 10.1016/j.cattod.2015.08.014.

3 D. Rochefort and D. Guay, J. Alloys Compd., 2005, 400, 257. 4 J. C. Cruz, V. Baglio, S. Siracusano, V. Antonucci, A. S. Aricò, R. Ornelas, L. Ortiz-Frade,

G. Osorio-Monreal, S. M. Durón-Torres and L. G. Arriaga, Int. J. Electrochem. Sci., 2011, 6, 6607.

5 A. W. Hull, Phys. Rev., 1921, 18, 88. 6 M. Grzelczak, J. Zhang, J. Pfrommer, J. Hartmann, M. Driess, M. Antonietti and X. Wang,

ACS Catal., 2013, 3, 383. 7 J. Pal and P. Chauhan, Mater. Charact., 2010, 61, 575. 8 R. Xu and H. C. Zeng, Langmuir, 2004, 20, 9780. 9 M. Marezio, Acta Crystallogr., 1966, 20, 723. 10 Y. Shigesato, Y. Hayashi and T. Haranoh, Appl. Phys. Lett., 1992, 61, 73.