www.sciencemag.org/cgi/content/full/science.1209816/DC1

Supporting Online Material for Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts Steven Y. Reece,* Jonathan A. Hamel, Kimberly Sung, Thomas D. Jarvi,* Arthur J. Esswein, Joep J. H. Pijpers, Daniel G. Nocera* *To whom correspondence should be addressed. E-mail: [email protected] (D.G.N.); [email protected] (S.Y.R.); [email protected] (T.D.J.) Published 29 September 2011 on Science Express DOI: 10.1126/science.1209816

This PDF file includes: Materials and Methods Figs. S1 to S5 Other Supporting Online Material for this manuscript includes the following: (available at www.sciencemag.org/cgi/content/full/science.1209816/DC1) Movie S1

Supporting Information

An Artificial Leaf Comprising Earth-Abundant Materials Steven Y. Reece,1,* Jonathan A. Hamel,1 Kimberly Sung,1 Thomas D. Jarvi,1,* Arthur J. Esswein,1 Joep J. H. Pijpers,2,3 and Daniel G. Nocera2,* 1

Sun Catalytix, 200 Technology Sq., Cambridge, MA 02139.

2

Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139-4307.

3

FOM Institute for Atomic and Molecular Physics, Science Park 104, 1098 XG, Amsterdam, The Netherlands

[email protected], [email protected], [email protected]

Index

Page

Methods

S2-S4

Fig. S1. EDX analysis of the 3jn a-Si and Co-OEC | 3jn a-Si cells

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Fig. S2. Current-voltage plots of dry 3jn-a-Si cells

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Fig. S3. Tafel plots of Ni and NiMoZn in 1 M KBi (pH 9.2)

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Fig. S4. Faradaic efficiency for O2 production

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Fig. S5. Stability assessment by mass spectrometry

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Movie S1. Description of the movie showing the real time operation of the artificial leaf

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S10

Methods Materials. Cobalt nitrate hexahydrate (Co(NO3)2 •6H2O, Alfa Aesar), hydrochloric acid (HCl, BDH), potassium hydroxide (KOH, BDH), and boric acid (H3BO3, BDH) were purchased from the indicated supplier and used as received. Water (>18 MΩ cm resistivity, <3 ppb total organic carbon) was provided by a Barnstead NANOpure Diamond water purification system. Triple junction a-Si solar cells on stainless steel substrates were obtained from Xunlight Corp. (Toledo, OH). Cells were cut with scissors to the appropriate dimensions and etched at the edges with 1 M HCl. General methods. Simulated (AM 1.5) sunlight was provided by a Sol 2A solar simulator (Newport Corp.). The intensity was confirmed using a calibrated Si photodiode (Model 91150V, Newport Corp.). Scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) was performed with a Leo 435 VP SEM instrument by MicroVision Laboratories, Inc (Chelmsford, MA). Gas chromatography experiments were performed with an HP5980 Series II instrument equipped with a thermal conductivity detector (column: HayeSep DB 100/120 mesh; carrier gas: N2; flow rate: 62 mL/min @ 90 psi; oven temp: 50 oC; detector temp: 100 oC). Mass spectrometry (MS) experiments were performed with an Agilent Technologies 5975C instrument, using Ar as the carrier gas. For calibration purposes, Ar containing 0.1% O2 was used. The flowrate of the gas through the headspace of the electrochemical cell was controlled at 18 mL/min using a mass flow controller (Aalborg). NiMoZn cathode. The NiMoZn cathode was electrodeposited from a solution of nickel(II) chloride hexahydrate (9.51 g L–1), sodium molybdate dihydrate (4.84 g L–1), anhydrous zinc chloride (0.0409 g L–1), tetrabasic sodium pyrophosphate (34.57 g L–1) and sodium bicarbonate (74.77 g L–1; VWR). Hydrazine hydrate (1.21 mL L–1; Alfa Aesar) was added immediately before plating. NiMoZn was deposited onto a Ni mesh substrate (Ni 200; 100 × 100 mesh; 0.002´´ wire diameter), which had been pre-treated at –2 V vs. Ag/AgCl in 0.5 M H2SO4 for 3 min. The NiMoZn alloy was deposited at a current density of 0.0775 A cm–2 for 30 min. The deposit was left to leach FOR 16+ hours in 10 M KOH. Successful leaching was indicated by bubbles evolving from the electrode surface. After leaching, the deposit became slightly darker in appearance. Crystalline Si electrodes. The crystalline Si electrodes were fabricated following ref 38 in the main manuscript. FTO films were deposited on a crystalline Si pn-junction by spray pyrolysis. In a typical spray deposition, 60 mL ethanol containing 4.2 g SnCl4 •5H2O and 0.7 mL saturated NH4F solution was used to coat a 3´´ Si wafer with FTO. During spray deposition, the Si wafer S2

was heated at 400 ºC. After all of the precursor solution had been sprayed, the FTO-coated wafer was annealed at 400 ºC in air for 1 h. Current-voltage, dry cell measurements. The performance of each 3jn-a-Si cell was determined prior to each photoelectrochemical measurement. A thin strip of copper tape (1 × 15 mm2) was affixed down the center of the cell (typically 7 × 12 mm2) on top of the ITO coating and served as the anode current collector. Similarly, copper tape was attached to the bottom, stainless steel substrate to serve as the cathode current collector. The cell was illuminated with AM 1.5 (1 sun) simulated sunlight and the current from the test cell was measured as a function of the voltage applied between the anode and the cathode. The voltage was swept from 0 to 2.4 V with a potentiostat (CH instruments) at a scan rate of 0.5 V/s. Cells with open circuit voltages (VOC) greater than 2.1 V and fill factors (FF = (Vmp • jmp) / (VOC • jSC)) greater than 0.5 were selected for experimentation. Wired three electrode PEC. Three electrode experiments were performed using a Ag/AgCl reference electrode and Pt wire counter electrode. The working electrode lead was attached to the 3jn-a-Si cell by means of adhesive Cu tape affixed to the back of the stainless steel substrate and covered with vinyl electrical tape to prevent contact of the Cu with the electrolyte. Wired two electrode PEC. Two electrode PEC experiments (Fig. 3A) were conducted by connecting the working electrode lead of the potentiostat to the stainless steel support of the 3jna-Si cell and the reference and auxiliary electrode leads of the potentiostat to a NiMoZn/Ni mesh (200 mesh size), which was positioned over the illuminated face of the 3jn-a-Si cell. In this way, the gap between the anode and the cathode could be minimized (~1 mm). The current in the cell was measured by performing bulk electrolysis with the potentiostat at 0V bias (in this configuration, the potentiostat serves as an ammeter). The Faradaic efficiency of the twoelectrode PEC (Fig. S4) was determined by measuring the concentration of oxygen in the headspace of the cell using a phosphorescence-based FOXY oxygen sensing probe (Ocean Optics). Wireless water-splitting solar cell. Wireless cells (Fig. 3B) were prepared by electrodepositing Co-OEC on the ITO and electrodepositing the NiMoZn cathode catalyst on the stainless steel substrate of the solar cell. Rubber gaskets were used to prevent contact of the NiMoZn plating and leaching solution with the rest of the cell during cathode preparation. The cell was operated in 1 M KBi, pH 9.2 electrolyte under AM 1.5 illumination. The electrolyte solution was stirred with a stirrer bar to prevent local pH drops at the Co-OEC/electrolyte interface. MS experiments

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were performed to quantify the SFE (Fig. 3B) and the stability of the wireless water-splitting cells (Fig. S5). FTO-coated crystalline Si. The Co-OEC functionalized, FTO-coated crystalline Si sample was measured in a three-electrode configuration using a Ag/AgCl reference electrode and Pt mesh counter electrode. The cell was operated in 1 M KBi, pH 9.2 electrolyte under AM 1.5 illumination. The sample was measured in a two-compartment electrochemical cell and the electrolyte in the working electrode compartment was stirred with a stirrer bar to prevent local pH drops at the Co-OEC/electrolyte interface. To obtain comparable current densities as those obtained in the wireless water-splitting solar cells, a potential of 0.55 V vs. AgCl was applied on the FTO-coated crystalline Si cell.

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Figure S1. Low energy EDX analysis of the ITO-coated surface of the 3jn-a-Si cell (A) and with 5 min (B) and 1 hr (C) deposition of the Co-OEC catalyst. The intensity of the Co-scattered xrays increases with Co-OEC deposition time (catalyst film thickness).

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Figure S2. Current-voltage plot of dry 3jn-a-Si cell under 1 sun irradiation in the presence (▬) and absence (▬) of the Co-OEC film.

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Figure S3. Tafel plots of Ni (▬) and NiMoZn (▬) in 1 M KBi (pH 9.2). Data were obtained with a rotating disk electrode operated at 3000 rpm. The NiMoZn alloy was electrodeposited on a polished Ni disk and leached in KOH solution.

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Figure S4. Percent O2 in the headspace of the cell as a function of electrolysis time. Oxygen concentrations were measured (▬) using a phosphorescent probe and calculated (▬) concentrations were derived from the amount of charge passed during photoelectrolysis.

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Figure S5. Results from mass spectrometry experiments assessing the stability of (A) wireless Co-OEC | 3jn-a-Si | NiMoZn cell (▬), and (B) a Co-OEC functionalized crystalline silicon cell passivated with FTO (▬). For the crystalline cell, a potential of 0.55 V vs. an AgCl reference electrode was applied to obtain a comparable current density as the wireless current in (A). The spikes in the data originate from sudden release of gas bubbles that were adhered to the cells, resulting in a temporary increase of the O2 concentration in the headspace.

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Movie S1. Time-course of a movie of the wireless cell (artificial leaf) in operation The movie at: 0-4 secs

The Co-OEC | 3jn-a-Si | NiMoZn wafer (Figure 3b) sitting in an open container in the dark. The solution is aqueous KBi electrolyte. The view is a far field shot of the photoanode.

4 sec

An AM1.5 (1 sun) lamp is turned on.

4-15 sec

Water splitting begins and bubbles begin to nucleate on the photoanode.

15-32 sec

Growth of bubbles is large enough to exhibit visible gas evolution from the photoanode.

32-47 sec

Close up of the photoanode and oxygen evolution.

47-102 sec

Close up of the NiMoZn cathode on the back side of cell. Evolution of hydrogen is clearly visible.

102-120 sec

A picture of the cell design (Figure 3b) in background with container and wafer in foreground.

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