www.sciencemag.org/content/345/6204/1593/suppl/DC1

Supplementary Material for

Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts

Jingshan Luo, Jeong-Hyeok Im, Matthew T. Mayer, Marcel Schreier, Mohammad Khaja Nazeeruddin, Nam-Gyu Park, S. David Tilley, Hong Jin Fan, Michael Grätzel*

*Corresponding author. E-mail: michael.graetzel@epfl.ch

Published 26 September 2014, Science 345, 1593 (2014)

DOI: 10.1126/science.1258307

This PDF file includes:

Materials and Methods

Figs. S1 to S6

Full Reference List Other Supplementary Material for this manuscript includes the following: (available at www.sciencemag.org/content/345/6204/1593/suppl/DC1)

Movies S1 and S2

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Materials and Methods Chemicals

All chemicals were used as received without any further purification unless noted. All solutions were prepared with 18.2 MΩ∙cm deionized water from a Millipore deionized water system. Nickel nitrate hexahydrate (Ni(NO3)2•6H2O, ≥99%, Fluka), iron nitrate nonahydrate (Fe(NO3)3•9H2O, >99.99%, Sigma-Aldrich), urea (CO(NH2)2, 99.8% Sigma-Aldrich), ammonium fluoride (NH4F, >96%, AlliedSignal), sodium hydroxide (NaOH, REACTOLAB SA), titanium diisopropoxide bis(acetylacetonate) (75 wt.% in isopropanol, Aldrich), 1-butanol (99.8%, Aldrich), TiCl4 (>98%, Aldrich), PbI2 (99%, Aldrich), N,N-dimethylformamide (DMF) (99.8%, Sigma-Aldrich), lithium bis (trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg LI-TSFI in 1 mL acetonitrile (Sigma-Aldrich, 99.8%)), acetonitrile (Sigma-Aldrich, 99.8%), nickel foam ( >99.5%, 1.5 mm, Taiyuan Yingze Lizhiyuan Battery). The 1.5 mm thick nickel foam was compressed to ~1 mm using a hydraulic press, which helped increase the adhesion of the catalysts and to define the geometric area.

Perovskite solar cell fabrication

FTO glass (Nippon Sheet Glass) was cleaned in an ultrasonic bath containing ethanol for 30 min and then treated in UV/ozone for 30 min. The TiO2 blocking layer (BL) was spin-coated onto FTO at 2000 rpm for 20 s using a solution of 0.15 M titanium diisopropoxide bis(acetylacetonate) in 1-butanol, followed by heating at 125 °C for 5 min. After cooling to room temperature, the TiO2 paste (ca. 40 nm TiO2 particles) was spin-coated on top of the BL at 2000 rpm for 10 s, where the pristine paste had been diluted in ethanol (0.1 g mL-1). (44) After drying at 100 °C for 5 min, the film was annealed at 550 °C for 30 min, which led to a thickness of about 100 nm. The mesoporous TiO2 film was immersed in 0.02 M aqueous TiCl4 (>98%, Aldrich) solution at 70 °C for 30 min. After washing with DI water and drying, the film was heated at 500 °C for 30 min. CH3NH3PbI3 was formed using a two-step spin-coating procedure. A 1 M PbI2 solution was prepared by dissolving 462 mg PbI2 in 1 mL N,N-dimethylformamide (DMF) under stirring at 70 °C . 20 μL of PbI2 solution was spin-coated on the mesoporous TiO2 film at 3000 rpm for 5 s and 6000 rpm for 5 s (no loading time). After spinning, the film was dried at 100 °C for 10 min. After cooling to room temperature, 200 μL of 0.044 M CH3NH3I solution in 2-propanol was loaded on the PbI2-coated substrate and allowed to stand for 20 s (loading time) before spinning at 4000 rpm for 20 s and then drying at 100 °C for 5 min. 20 μL of (2,2`,7,7`-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene) (spiro-MeOTAD) solution was spin-coated on the CH3NH3PbI3 perovskite layer at 4000 rpm for 30 s. The spiro-MeOTAD solution was prepared by dissolving 72.3 mg of spiro-MeOTAD in 1 mL of chlorobenzene, to which 28.8 μL of 4-tert-butyl pyridine and 17.5 μL of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg LI-TSFI in 1 mL acetonitrile) were added. Finally, 80 nm of gold was thermally evaporated on the spiro-MeOTAD coated film.

Pt nanoparticles on Ni foam electrode fabrication

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The 3.5 nm Pt nanoparticles were coated by sputtering at room temperature onto both sides of the Ni foam using an Alliance-Concept DP 650-Sputtering system.

Ni(OH)2 on Ni foam catalyst electrode fabrication

The electrode comprising Ni(OH)2 nanosheets on a three dimensional Ni foam substrate was fabricated by a facile hydrothermal method. Briefly, 0.6 g Ni(NO3)2, 0.6 g urea and 0.15 g NH4F were dissolved into 70 ml deionized water. After stirring, the mixture was poured into a 100 ml autoclave with a piece of Ni foam leaned against the wall. The growth was carried out at 105 °C in an electric oven for 6h. After allowing the autoclave to cool naturally to room temperature, the samples were removed, washed with deionized water, and then blown dry under a stream of compressed air.

NiFe LDH on Ni foam catalyst electrode fabrication

The NiFe LDH was fabricated by hydrothermal growth according to previous reports but with a modified recipe. (44) Briefly, 0.3 g Ni(NO3)2, 0.4 g Fe(NO3)2 and 0.3 g urea were mixed in 80 ml deionized water. After dissolution, the solution was poured into a 100 ml autoclave with a piece of Ni foam leaned against the wall. The growth was carried out at 120°C in an electric oven for 12 h. After allowing the autoclave to cool naturally to room temperature, the samples were removed, washed with deionized water, and then blown dry under a stream of compressed air.

Perovskite tandem solar cell fabrication

Two single perovskite solar cells were assembled as depicted schematically in Figure 3A. Kapton tape was used to hold the two cells together side by side. The cathode of one cell was directly soldered to the anode of the second cell to wire them together in a series connection. Copper wires were then soldered to the other contacts to make a connection to the water splitting electrodes. The cells were then encapsulated by hot glue with a hot glue gun.

Material characterization

For the XRD characterization, in order to avoid the interference of nickel from the substrate, Ni(OH)2 and NiFe LDH samples were grown on glass microscope slides. The XRD patterns were acquired with a Bruker D8 Discover diffractometer in the Bragg–Brentano mode, using Cu K α radiation (1.540598 Å) and a Ni β-filter. Spectra were acquired with a linear silicon strip ‘Lynx Eye’ detector from 2θ = 10°– 80° at a scan rate of 1° min−1, step width of 0.02° and a source slit width of 4 mm. Reflection patterns were matched to the PDF-4+ database (ICDD). The morphology of the films was characterized using a high-resolution scanning electron microscope (ZEISS Merlin) and a transmission electron microscope (Philips, FEI CM12).

Electrochemical characterization of the catalyst electrodes

The electrochemical performance of the catalyst electrodes was evaluated both in a three-electrode and two-electrode configuration with an Ivium Potentiostat. Ag/AgCl (saturated KCl) with a glass frit was used as the reference electrode in a three-electrode configuration, and Pt mesh was used as the counter electrode (as a control against Pt contamination from the counter electrode, the catalyst electrodes were also tested against

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a Ti counter electrode which gave identical results). The electrolyte used was 1M NaOH (aqueous). The OER and HER reactions were characterized by linear sweep voltammetry at a scan rate of 1 mV s-1, and the scan direction was from positive to negative potential and from negative to positive potential, respectively, on the RHE scale. For the two-electrode water splitting measurement, two of the same kind of catalyst electrode were used for both the OER and HER, and the overall water splitting reaction was characterized by linear sweep voltammetry at a scan rate of 1 mV s-1 from 2 V to 0 V. The stability of the NiFe DLH catalyst electrodes were characterized by chronoamperometry applying 1.8V external bias for 10h. Gas measurements

Product gas quantification was carried out by gas chromatography. Two catalyst samples contained in ca. 30 mL of 1 M aqueous NaOH were hermetically sealed in a 50 mL three-neck round-bottom flask. The contents of the flask were vigorously stirred and helium (99.99990%, Carbagas Switzerland) was sparged at a constant rate of 20.13 mL min-1 using a mass flow controller (Bronkhorst EL-Flow, Netherlands). A galvanostatic current of 6 mA cm-2 was applied to the catalyst samples using a potentiostat (Gamry Interface 1000, Gamry Instruments). The outflowing gas was collected and passed into the sampling loop of a gas chromatography apparatus (Trace GC Ultra, Thermo Scientific) equipped with a Shincarbon ST column (Restek) and a pulse discharge detector (VICI AG). Gas samples were periodically injected onto the column. Calibration was carried out using a similar setup but with two cleaned Pt wires as electrodes. Various galvanostatic currents within the investigated range were applied, resulting in a linear calibration for hydrogen and oxygen concentration. The measured concentration of oxygen was corrected for the presence of residual air, the concentration of which was verified before and after each measurement. The standard deviation of the gas measurements depicted in Figure 2D was approximately 2%. Perovskite solar cell characterization

For single perovskite solar cells, the current-voltage characteristics were recorded by applying an external potential bias to the cell while recording the generated photocurrent with a digital source meter (Keithley Model 2400). The light source was a 450 W xenon lamp (Oriel) equipped with a Schott K113 Tempax sunlight filter (Praezisions Glas & Optik GmbH) to match the emission spectrum of the lamp to the AM1.5G standard. Before each measurement, the exact light intensity was determined using a calibrated Si reference diode equipped with an infrared cut-off filter (KG-3, Schott). IPCE spectra were recorded as a function of wavelength under a constant white light bias of approximately 5 mW cm-2 supplied by an array of white light-emitting diodes. The excitation beam coming from a 300-W xenon lamp (ILC Technology) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd) and chopped at approximately 2 Hz. The signal was recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems). All measurements were conducted using a non-reflective metal aperture of 0.159 cm2 to define the active area of the device and avoid light scattering through the sides.

For perovskite tandem cells, the current-voltage characteristics were recorded by an Ivium Potentiostat with the linear sweep voltammetry method. The light source was a 450

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W xenon lamp (Oriel) equipped with an AM 1.5G filter (Lot-Oriel), and the intensity was calibrated to 100 mW cm-2 by a standard Si reference diode equipped with an infrared cut-off filter (KG-3, Schott).

Integrated water splitting cell characterization

The current of the water splitting cell was recorded by chronoamperometry without applying an external bias for different time periods under chopped AM 1.5G illumination. All measurements were conducted using two non-reflective metal apertures of 0.159 cm2 on each single perovskite solar cell to define the active area of the device and avoid light scattering through the sides. For the device in Figure 3C, D and Figure S5, the geometric catalyst electrode area was ~5 cm2. The larger area (relative to the illuminated area of the perovskite cells) was used to match the maximum photocurrent of the perovskite tandem cell. For the device in Figure S5, the area of each catalyst electrode was 1 cm2.

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Fig. S1. Three consecutive chronoamperometry experiments of the NiFe LDH/Ni foam electrodes in a two-electrode configuration. A bias of 1.8 V was applied during 10h, and the rest period between each test was ~8h.

0 1 2 3 4 5 6 7 8 9 100

10

20

30

40 NiFe LDH/Ni foam 1st 10h test NiFe LDH/Ni foam 2nd 10h test NiFe LDH/Ni foam 3rd 10h test

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nt d

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ty (m

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Fig. S2. XRD characteristics of the Ni(OH)2 and NiFe LDH grown on glass slides. The NiFe XRD pattern is in good agreement with previous reports. (39-40)

10 20 30 40 50 60 70 80

NiFe LDH

Ni(OH)2

α-Ni(OH)2 JCPDS card No. 38-0715

Inte

nsity

(a.u

.)

2θ (degree)

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A B

C D

Ni(OH)2/Ni foam HER Ni(OH)2/Ni foam OER

NiFe LDH/Ni foam HER NiFe LDH/Ni foam OER

Fig. S3. Photos of the catalyst electrodes during water splitting in a two electrode configuration in 1 M NaOH with 2V external bias (A) Ni(OH)2/Ni foam electrode for HER. (B) Ni(OH)2/Ni foam electrode for OER. (C) NiFe LDH/Ni foam electrode for HER. (D) NiFe LDH/Ni foam electrode for OER.

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100 µm

10 µm

Ni(OH)2/Ni foam

A

100 nm

Ni(OH)2/Ni foam D

20 µm

NiFe LDH/Ni foam

C

100 nm

NiFe LDH/Ni foam F

10 µm

BNi foam Ni foam

E

Fig. S4. SEM characterization of the catalyst electrodes. (A, B) Ni foam at different levels of magnification. (C, D) Ni(OH)2/Ni foam electrode image at different levels of magnification. (E, F) NiFe LDH/Ni foam electrode image at different levels of magnification.

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Fig. S5. Chronoamperometry experiment of the integrated water splitting device without external bias under chopped simulated AM 1.5G 100 mW cm-2 illumination for 2h.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0-2

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Perovskite dark Perovskite light before Perovskite light after NiFe LDH/Ni foam catalyst

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Fig. S6. Photoelectrochemical characterization of a 2nd integrated water splitting device (A) J-V curves of the perovskite tandem cell under simulated AM 1.5G 100 mW cm-2 illumination and in the dark before and after 4 h measurement, and the J-V curve of the NiFe/Ni foam electrodes under two electrode configuration. (B) Current density-time curve of the integrated water splitting device without external bias under chopped simulated AM 1.5G 100 mW cm-2 illumination during the initial 800 s. (C) Current density-time curve of the integrated water splitting device without external bias under chopped simulated AM 1.5G 100 mW cm-2 illumination during 4 h.

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Movie S1 Bubble evolution by applying 1.8 V and 2 V to the NiFe LDH/Ni foam catalyst electrodes.

Movie S2 The water splitting device combining the perovskite tandem cell and NiFe LDH/Ni foam catalyst electrodes under dark and simulated AM1.5G solar illumination.

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