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NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-20287-w ARTICLE Fig. 6 SFB performance in comparison with representative previous works14,15,17,18,53,54,60,65–67. The number in the circle and the circle radius represent the demonstrated continuous cycling time (in an hour) and their corresponding range, respectively. The fill color of the circle shows the electrolyte pH range. The solar cell structure of each work is marked by the symbols of the red triangle (for single-junction) and green pentagon (for tandem junction), individually. The photoelectrodes, redox couples, and the corresponding energy capacity of SFB are displayed near each work. Then, a 500 nm of n+-GaAs buffer layer (for lattice matching to the active junc- tion)61,62 and a 40 nm of p++-AlGaAs tunnel junction were grown on the sub- strate, subsequently. Next, a 100 nm of p+-GaInP as a back surface field, and then GaAs SJ n-on-p configuration followed by an (emitter)-on-p (base) active junction, 35-nm-thick n+-AlGaAs window and 500-nm-thick n+-GaAs contact layers were grown by low-pressure MOCVD. The group III precursor sources were tri- methylgallium, trimethylaluminum, and trimethylindium, and the group V pre- cursor sources were arsine and phosphine. Carbon tetrabromide and dimethyl telluride were used as precursors for p- and n-type dopants, respectively. The growth was carried out at a low pressure of 40 torr with a hydrogen flow rate of 28,000 sccm. Ni/Au (50/150 nm) finger grides was deposited on the n+-GaAs contact layer as the front electrode. To fabricate the GaAs photoelectrode assembly, the GaAs solar cell was affixed onto a custom-made graphite plate, which has an open window of 9 mm × 9 mm at the center. Epoxy resin (Hysol 9460) was used to seal around the GaAs cell on the open window of the graphite, which can prevent electrolyte leakage and direct electrical contact between the GaAs cell and graphite plate. The light-harvesting surface and photoelectrode/electrolyte contact area were exposed without applying the epoxy. The active area of the SJ-GaAs photoelectrodes was calculated using calibrated digital images (as shown in Supplementary Fig. 3) in Photoshop. This configuration allows the GaAs photoanode to absorb light from one side (n+ window side) and form direct contact with liquid electrolyte on the other side (n+ substrate side). The Ni/Au electrode grids that act as carrier collector on the light-harvesting side were connected to a Cu foil using Ga/In eutectic alloy (Sigma-Aldrich) and then silver paste and sealed by epoxy resin. Solid-state and PEC characterization of SJ-GaAs cell. Solid-state J–V perfor- mance of the GaAs cells was measured in a two-electrode configuration63. The LSV measurements were carried out using a Bio-Logic SP-200 potentiostat with a scan rate of 100 mV s−1 under AM 1.5 G one Sun (100 mW cm−2) illumination by a Newport Model 91191 Xenon arc lamp solar simulator. The illumination intensity of the solar simulator was calibrated by a Si photodiode (Thorlabs) before LSV measurements. The PEC characteristics of the GaAs photoanode were measured using the integrated SFB device under solar cell mode in an N2 flush box by a Bio-Logic BP- 300 potentiostat in a two-electrode configuration under one Sun illumination. The LSV measurements were performed with a scan rate of 100 mV s−1. The simulated solar illumination was provided by a Newport Model 67011 quartz tungsten halogen (QTH) solar simulator and guided by a branched flexible silica light guild (Taiopto Mems International Co., Ltd.) fed through an N2 flush box. The QTH solar simulator was calibrated by the same Si photodiode calibration cell to generate the same value of current intensity as that measured under one Sun AM1.5 G illumination by the Newport 91191 simulator. EQE spectral measurements. The EQE spectra were measured using a spectral response system (Enli Technology Co., Ltd. R3011). The GaAs solid-state solar cell was measured under monochromatic illumination, with wavelength ranging from 300 to 890 nm, at a spot area of 2 × 2 mm and a chopping frequency of 230 Hz, in a two-electrode configuration. The EQE signal was recorded at an applied potential of 0 V (short-circuit). The reflection spectrum was measured by a UV–vis–NIR spectrometer (JASCO ARN-733) and scanned from the wavelength of 300–890 nm with an integrating sphere at a noise level of 0.002%. Electrochemical measurements of the redox couples. CV measurements were performed using a Bio-Logic SP-200 potentiostat. A Pt coil electrode (0.5 mm diameter, BASi) and a saturated calomel electrode (SCE, CH Instruments) was used as the counter and a reference electrode, respectively. The working electrode was a 3 mm diameter glassy carbon disk electrode (MF-2012, BASi), which was polished using 0.3 and 0.05 μm alumina slurry and washed by deionized water (Milli-Q, 18.2 MΩ cm) and methanol. An electrochemical cleaning procedure with 1 M Na2SO4 aqueous solution (with 1 mM potassium ferrocyanide as internal refer- ence) was used to further clean the surface of the glassy carbon electrode, which was performed by sweeping the potential of glassy carbon working electrode between −1.0 and 1.5 V vs. SCE at 100 mV s−1 until the peak separation of fer- rocyanide/ferricyanide redox couple reaches ca. 60 mV. Then 5 mM of bis((3- trimethylammonio)propyl)ferrocene dichloride (BTMAP-Fc), 5 mM of bis(3-tri- methylammonio)propyl viologen tetrachloride (BTMAP-Vi), and 5 mM of 4- trimethylammoinium-TEMPO (NMe-TEMPO) were both with 1.0 M NaCl, were used as the supporting electrolytes, which were purged with argon for 10 min before the CV measurements. CV was scanned at various scan rates of 10, 100, 200, 400, 600, 1000, and 2000 mV s−1. Fabrication of RFB and SFB device. A custom-made zero-gap device was used for both RFB and SFB measurements15. Graphite plates (1/8-in. thickness, MWI) were used as the current collector for RFB devices and the cathode side of SFB devices. The modified graphite plates in the GaAs photoanode assemblies were used as the current collector at the anode side of SFB devices. Graphite felt electrodes (GFD 3 EA, SIGRACELL®) (20 × 20 mm) was heated at 400 °C in the air for 24 h before being used as inert electrodes on both sides of the cell. A 25 × 25 mm Selemion DSV membrane (Ashahi Glass Co., Ltd.,) was pretreated by soaking in 1.0 M NaCl for 24 h before being used as the anion-exchange membrane in the cell. Four pieces of custom-made PTFE sheets (0.04-in. thickness) were used as gaskets. These NATURE COMMUNICATIONS | (2021)12:156 | https://doi.org/10.1038/s41467-020-20287-w | www.nature.com/naturecommunications 9PDF Image | flow battery enabled single-junction GaAs photoelectrode
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