Supporting Information

Bifunctional NiFe Inverse Opal Electrocatalysts with Heterojunction Si Solar Cells for 9.54%-

Efficient Unassisted Solar Water Splitting

Hakhyeon Songa‡, Seungtaeg Oha‡, Hyun Yoonb, Ka-Hyun Kimb*, Sangwoo Ryua*, and Jihun

Oha ,c*

a Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea

Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

b Advanced Center for Energy, Korea Institute of Energy Research, Ulsan 44919, Republic of

Korea

cKAIST Institute for NanoCentury, Korea Advanced Institute of Science and Technology

(KAIST), Daejeon 34141, Republic of Korea

E-mail: ka-hyun.kim@kier.re.kr, sangwoo.ryu@kaist.ac.kr, jihun.oh@kaist.ac.kr

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S1. Fabrication of the NiFe inverse opal structures

Fig S1. Schematic of the fabrication process for NiFe inverse opal (IO) structures

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S2. Calculation of increased surface area by using the Eagleton and

Searson method

Calculation of increased thickness layers and surface area of inverse opals (IO) is performed by using

equations (1) and (2).

Thickness of N layers = [ (n−1 ) √3+2]×r (1)Surface area of inverse opals = V × π

2 d=[ (n−1 ) √3+2]×0.785 (2)

n, r, V, and d are number of layers, radius of polystyrene (PS), estimated volume of PS (via

multiplication of the base area and the height), and diameter of PS (600 nm), respectively.

Table S1 summarizes the increased surface area, presented as ratios of IO and planar film

areas, for different numbers of thickness layers.

Table S1. Ratio of the surface area of IO to planar

Number of thickness layers

Area of inverse opalAreaof planar film

1 1.57

2 2.93

3 4.3

4 5.6

5 7.0

6 8.4

7 9.7

8 11.1

9 12.4

10 14

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S3. Characterization of the NiFe inverse opal structures

Fig. S2. X-ray diffraction (XRD) patterns for NiFe IO structure, NiFe planar film, and Ni

planar films. Dotted lines are guides for eyes.

Fig. S3. Fe composition of NiFe IO structures with different numbers of thickness layers

obtained via X-ray photoemission spectroscopy (XPS) and energy-dispersive spectroscopy

(EDS).

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S4. Benchmark of NiFe electrocatalysts for water splitting performance

Table S2. Benchmark table for HER, OER and water splitting reactions of NiFe catalysts

OER

Catalysts Electrolyte Overpotentialat 10 mA cm-2 Reference

NiFe Inverse Opal

1 M KOH or NaOH

280 mV This WorkDendritic NiFe 300 mV [1]

NiFe electrodeposited with an inhibitor 253 mV [2]

NiFe(OH)x

nanoparticles in carbon nanotubes

260 mV [3]

Ni2/3Fe1/3-rGO (Exfoliated NiFe LDH on reduced graphene

oxide)

210 mV [4]

Ni3FeN Nanopaticles derived from NiFe

LDH280 mV [5]

Flower-like Ni-Fe LDH 344 mV [6]

NiFe@NC (NiFe nanoparticles @

nitrogen-doped carbon)

350 mV [7]

NiFe LDH/Ni Foam 240 mV [8]Exfoliated NiFe LDH 300 mV [9]

HER

Catalysts Electrolyte Overpotentialat -10 mA cm-2 Reference

NiFe Inverse Opal

1 M KOH or NaOH

234 mV This WorkNi2/3Fe1/3-rGO

(Exfoliated NiFe LDH on reduced graphene

oxide)

560 mV [4]

EG/Co0.85Se/NiFe-LDH (Exfoliated

graphene/CoSe/NiFe LDH)

260 mV [10]

Ni3FeN Nanopaticles derived from NiFe

LDH158 mV [5]

NiFe@NC (NiFe nanoparticles @

nitrogen-doped carbon)

~210 mV [7]

NiFe LDH/Ni Foam 210 mV [8]

5

Two Electrode Water Splitting

Catalysts Electrolyte Overpotentialat 10 mA cm-2 Reference

NiFe Inverse Opal

1 M KOH or NaOH

530 mV This WorkEG/Co0.85Se/NiFe-

LDH 440 mV [10]

NiFe@NC 580 mV [7]NiFe LDH/Ni Foam 470 mV [8]

S5. Calculation of the number of active sites obtained by integrating the

reduction current density

Fig. S4. Cyclic voltammograms (CV) of NiFe planar and IO (2.5, 5 and 10 thickness layers)

in 1 M NaOH with a 10 mV s-1 scan rate.

Table S3. Redox charge density and relative numbers of active sites of NiFe inverse opal structures.

Electrode Charge Density(mC cm-2)

Relative Numbers of Active Sites

Planar Film 26.2531 1

NiFe IO (2.5 Layers) 427.101 16.27

NiFe IO (5 Layers) 703.566 26.80

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NiFe IO (10 Layers) 1354.43 51.60

Fig. S5. XPS spectra of NiFe inverse opal structures with 5 thickness layers before and after

OER.

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S6. Possible causes of the phenomenon of OER limitation

(i) Blockage of active sites by generated bubbles

Fig. S6. Oxygen bubbles generated during OER remain on the electrode surface and block

the active sites.

Fig. S7. A multi-step chronoamperometry of NiFe inverse opal with five layers at 1.55, 1.65

and 1.75 V in 1 M NaOH.

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(ii) A comparison of diffusion length of reactants and products and IO structures

Fig. S8. Comparison of diffusion length of OH- in water between NiFe planar and inverse

opal structure.

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S7. Optimization of distance between NiFe IO electrodes for water

electrolysis

Fig. S9. Comparison of water-splitting performance at different distances between IO

electrodes in a two electrode measurement configuration (geometric area: 1.26 cm2).

In order to reduce solution resistance, the IO electrodes at distances of 1, 1.5, and 3

cm were set facing each other. The 1.5 cm separation distance shows the best performance in

the appropriate current range for PV-EZ (i.e., 50 mA). A shorter distance may suppress

transport of the reactants and/or products by generated bubbles between the cathode and

anode, whereas a longer distance may cause high solution resistance.

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S8. Characterization of series-connected Si heterojunction solar cells

Fig. S10. (a) The structure of a Si heterojunction (SHJ) solar cell and (b) current density-

potential (j-V) curves of SHJ solar cells under one-sun illumination. Black, red, and blue

correspond to 1 cell (1 cm2), 3 cells (3.4 cm2), and 4 cells (4.84 cm2), respectively.

Table S4. Characteristics of SHJ solar cell mini-modules

ItemsModule

1 Cell (1 cm2) 3 Cell (3.42 cm2) 4 Cell (4.84 cm2)

Voc (V) 0.691 2.074 2.751

Isc (mA) 43.59 42.29 39.9

Jsc (mA/cm2) 43.59 12.36 8.23

Fill Factor 59.67 63.28 59.31

Jmax (mA/cm2) 36.53 10.81 7.121

Vmax (V) 0.492 1.501 2.034

Pmax (mW/cm2) 17.97 16.22 14.49

Efficiency 17.97 16.22 14.49

References

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