Supporting Information Continuous Production of Ethylene from Carbon Dioxide and Water Using Intermittent Sunlight Dan Ren 1,2,, Nicholas Wei Xian Loo 1,, Luo Gong 1 and Boon Siang Yeo 1,2,* 1. Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 2. Solar Energy Research Institute of Singapore, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 These authors contributed equally to this work. * Correspondence should be addressed to B.S.Y. (email: chmyeos@nus.edu.sg) Table of Contents (21 Pages, 12 Figures and 7 Tables) S1 Preparation and characterization of the cathode... S2 S1.1 Preparation of the Cu 2 O cathode... S2 S1.2 Online gas chromatography for the quantification of gaseous products... S3 S1.3 Headspace gas chromatography and high performance liquid chromatography for the quantification of liquid products... S4 S1.4 CO 2 reduction products detected using a Cu 2 O-derived Cu cathode in a three-electrode configuration... S5 S1.5 Stability of ethylene formation during 4 hours electrolysis... S6 S2 Preparation and characterization of the anode... S7 S 2.1 Preparation of the IrO x anode... S7 S2.2 Atomic ratios of O:Ir in the anode before and after oxidation... S8 S2.3 Activity of IrO x towards the oxidation of ethylene... S9 S2.4 Activity of IrO x towards the oxidation of liquid products... S1 S3 Characteristics of the two-electrode electrolyzer... S11 S3.1 Geometry of the two-electrode electrolyzer... S11 S3.2 Effect of electrolyte concentrations and membrane on overall resistance... S12 S3.3 Major half reactions in the electrolyzer... S13 S3.4 CO 2 reduction products detected at various applied cathodic currents... S14 S4 Solar-Driven CO 2 reduction under solar simulator... S15 S4.1 Characterization of the solar panels... S15 S4.2 CO 2 reduction products detected using simulated one sun intensity... S16 S5 Continuous synthesis of C 2 H 4 from CO 2 and H 2 O under natural sunlight... S17 S5.1 Stability of ethylene formation in the electrolyzer... S17 S5.2 Characterization of the cathode after four hours electrolysis... S18 S5.3 Variation in the intensities of the incident solar flux during the tests.... S19 S5.4 Selection of solar panel and battery... S2 References... S21 S1
S1 Preparation and characterization of the cathode S1.1 Preparation of the Cu 2 O cathode A two-electrode set-up was used to deposit the Cu 2 O film. A geometric current density of -2.1 ma/cm 2 was applied for 6 seconds on the Cu disc (geometric surface area of electrode =.385 cm 2 ). A platinum wire was used as the counter electrode. The copper lactate deposition electrolyte was prepared as follows: a) 38 ml of lactic acid (VWR, >9%) was dissolved in 162 ml of H 2 O, resulting in 2.3 M of 2 ml lactic acid aqueous solution; b) CuSO 4 5H 2 O (GCE, >99%) was added (.3 M) and fully dissolved; c) NaOH (Chemicob, >99%) was slowly added (3.2 M) and the solution was stirred in an ice bath to prevent overheating, which might cause precipitation. 1 A representative chronopotentiogram measured during the deposition process is shown in Figure S1. -.2 Voltage (V) -.4 -.6 -.8 1 2 3 4 5 6 Time (s) Figure S1 Chronopotentiogram recorded during the deposition of the Cu 2 O film on a Cu substrate. S2
S1.2 Online gas chromatography for the quantification of gaseous products Gas chromatography (Agilent 789A) was used to quantify the gaseous products. Three detectors, including two flame ionization detectors (FIDs) and one thermal conductivity detector (TCD), were used. The first FID which was used for detecting hydrocarbons such as CH 4 and C 2 H 4 was equipped with Haysep Q and Haysep N columns. Haysep Q and Molsieve 5A columns were used for both the second FID and TCD. A methaniser was used for the conversion of CO to CH 4 before detection by the second FID. The carrier gases were purified helium (99.9995%) for the FIDs, and purified N 2 (99.9995%) for the TCD. The volumes of the sample loops of the two FID channels were 1 cm 3 each, and the one of the TCD channel was.5 cm 3. During the one-hour electrolysis, six gas aliquots (total volume injected: 2.5 cm 3 ) were flowed into the system and five products including hydrogen, methane, carbon monoxide, ethylene and ethane were detected and quantified. Representative chromatograms are shown in Figure S2. (A) (B) I (pa) 12 9 6 3 I (pa) 9 6 3 CH 4 CH 4 CO C 2 H 4 2 4 6 8 1 (C) Voltage (25 uv) 9 6 3 2 4 6 8 1 H 2 2 4 6 8 1 Figure S2 Gas chromatograms, with signals from (A) the first FID, (B) the second FID (equipped with a methaniser), and (C) TCD. Data were collected during the reduction of carbon dioxide on oxide-derived Cu cathode at -.95 V vs. RHE in.2 M KHCO 3 electrolyte. S3
S1.3 Headspace gas chromatography and high performance liquid chromatography for the quantification of liquid products The liquid products were analyzed after the electrolysis. Headspace gas chromatography with a flame ionization detector was used to quantify volatile liquid products including acetaldehyde, propionaldehyde, acetone, ethanol, n-propanol and allyl alcohol. 1 ml of the electrolyte was sealed in a 2 ml glass vial and heated in the oven for 3 minutes, after which 1 ml of the headspace was transferred to a DB-WAX column. High performance liquid chromatography with a variable wavelength detector was used to quantify non-volatile liquid products including formate and acetate. The mobile phase was aqueous.5 mm H 2 SO 4 solution and an Aminex HPX-87H column was used..5 ml of the electrolyte was injected. Representative chromatograms are shown in Figure S3. (A) I (pa) 3 2 1 Acetaldehyde Propionaldehyde Acetone Ethanol n-propanol Allyl alcohol 5 1 15 (B) mili Absorption Units 3 2 1 Formate 5 1 15 2 25 3 Figure S3 (A) Headspace gas chromatogram and (B) high performance liquid chromatogram of the catholyte after one hour reduction of carbon dioxide on oxide-derived Cu cathode at -.95 V vs. RHE in.2 M KHCO 3 electrolyte. The peak at 6 min in liquid chromatogram represents species from KHCO 3. S4
S1.4 CO 2 reduction products detected using a Cu 2 O-derived Cu cathode in a three-electrode configuration The faradaic efficiency of product X was calculated based on the following equation 2 : FE X = Charge used to produce product X Total charge passed through the system 1% For the gaseous products, six aliquots gave six sets of faradaic efficiencies for different products. Only the average of the 3 rd to 6 th aliquots was used for evaluation since the headspace takes ~2 minutes to homogenize. 1 The liquid products in the electrolyte were analyzed after electrolysis. For each potential, three separate measurements were averaged (Table S1). The total faradaic efficiency ranges from 89.2% to 13.7%. Table S1. Faradaic efficiencies and production rates of products detected during carbon dioxide reduction on oxidederived Cu in a three-electrode configuration. N.D.: not detectable. AD: acetaldehyde; AA: Allyl alcohol; PA: propionaldehyde; AT: Acetone; AC: Acetate. E (V) C 2H 4 C 2H 5OH C 3H 7OH HCOO - AD AA PA AT AC CH 4 C 2H 6 CO H 2 Total Faradaic efficiency (%) -1.1 4.26 2.26.21 1.23.9.12.2.9.18 2.23 N.D..3 92.93 13.65-1.5 14.27 6.41.57.8.28.22.6.8.6 3.71 N.D..7 74.14 1.67-1. 26.42 12.45 1.55 1.33.58.5.21.8 N.D. 1.35 N.D..12 56.2 1.79 -.95 29.7 14.33 2.51 2.19.64.72.3.13.32.87.6.22 41.75 93.74 -.9 26.23 11.96 3.91 5.43.76.97 1.15.13 N.D..59.23.43 37.38 89.17 -.85 21.57 8.39 5.87 1.9.78 1.13 2.4.17 N.D..14.49 1.22 39.76 92.82 Production rate (µmol cm -2 hr -1 ) -1.1 8.9 4.72.29 17.55.26.2.3.12.65 7.6 N.D..39 119.38 - -1.5 2.34 9.14.54 6.94.49.23.6.9.15 7.96 N.D..63 64.65 - -1. 28.71 13.51 1.11 8.77.76.41.18.6 N.D. 2.23 N.D..76 373.91 - -.95 22.92 11.3 1.29 1.25.61.42.17.8.37 1.2.4 1. 193.58 - -.9 15.2 6.91 1.5 18.68.53.42.49.5 N.D..52.11 1.48 129.75 - -.85 12.14 4.74 2.22 37.1.53.48 1.3.7 N.D..12.24 4.14 134.14 - S5
S1.5 Stability of ethylene formation during 4 hours electrolysis The stability of the Cu 2 O-derived Cu cathode towards the reduction of CO 2 to ethylene was tested over 4 hours at -.95 V in.2 M KHCO 3 electrolyte. The faradaic efficiency of ethylene was relatively stable at ~32% (Figure S4). 5 Faradaic efficiency (%) 4 3 2 1 6 12 18 24 Figure S4 Faradaic efficiency of ethylene during 4 hours CO 2 reduction at.95 V vs RHE in.2 M KHCO 3 electrolyte. S6
S2 Preparation and characterization of the anode S 2.1 Preparation of the IrO x anode A three-electrode setup was used to deposit the IrO x. A Ag/AgCl (Pine, Saturated KCl) and Pt wire was used respectively as reference and counter electrode. The IrO x was electrodeposited using cyclic voltammetry scans from.32 to 1.48 V vs RHE for 1 cycles. The deposition electrolyte was prepared as follows 3 : a) 75 mg IrCl 3 xh 2 O (reagent grade, Sigma Aldrich) was dissolved into 5 ml water and the solution was stirred for 1 minutes; b) 178.5 mg oxalic acid (99.9%, Merck) was added and the solution was stirred for 1 minutes; c).5 ml hydrogen peroxide solution (3% in H 2 O, Scharlau) was added and the solution was stirred for 15 minutes; d) K 2 CO 3 (GCE, 99%) was slowly added to adjust the ph to 1.5; e) the solution was left to stabilize for 3 days before usage. A representative cyclic voltammogram is shown in Figure S5. 6 2nd scan 5th scan 1th scan 4 j (ma/cm 2 ) 2-2.4.6.8 1 1.2 1.4 E (V) vs RHE Figure S5 Cyclic voltammograms collected during the deposition of IrO x on a Ti foil. S7
S2.2 Atomic ratios of O:Ir in the anode before and after oxidation The IrO x particles were characterized using energy dispersive X-ray spectroscopy (via transmission electron microscopy) before and after linear sweep voltammetry to OER-relevant potentials. The O:Ir ratios were determined as 1.8 and 2.1 respectively (Figure S6). The carbon and gold signal were from the underlying TEM grids. (A) 2, Atomic concentration Ir: 6.5 ±.2% (B) 4, C Atomic concentration Ir: 1.1 ±.1% 1,5 O: 11.8 ± 1.1% 3, O: 2.3 ±.2% Counts 1, Counts 2, 5 C Ir Ir O Ir Ir Ir Au Ir A 4 8 12 Energy (kev) 1, O Ir Ir Ir Ir Ir Ir Au 4 8 12 Energy (kev) Figure S6 Energy dispersive X-ray spectra of the anode (A) before and (B) after linear sweep voltammetry. S8
S2.3 Activity of IrO x towards the oxidation of ethylene To verify if ethylene could be oxidized at the IrO x anode, linear sweep voltammetry (without ir compensation) was carried out in.2 M KHCO 3, with the continuous bubbling of ethylene or N 2. The LSV curves overlap well with each other (Figure S7A). Chronopotentiometry measurements in a two-electrode setup were also performed. IrO x and a graphite disc were used as the anode and cathode respectively..2 M KHCO 3 electrolyte was used. ~2 mol ppm C 2 H 4 (balanced N 2 ) was flowed in the electrolyte at 2 cm 3 /min in lieu of CO 2. ma/cm 2 and 1.92 ma/cm 2 were applied to the IrO x anode for one hour respectively. Online gas chromatography (GC) measurement demonstrates that ethylene was not oxidized at the anode during one hour chronopotentiometry. Thus, both the LSV and chronopotentiometry/online GC measurements indicate the inertness of IrO x towards the oxidation of C 2 H 4. (A) 8 N 2 ~2 mol ppm of C 2H 4 in N 2 (B) 2 ma/cm 2 2 1.92 ma/cm 2 j (ma/cm 2 ) 6 4 C2H4 (mol ppm) 15 1 C2H4 (mol ppm) 15 1 2 5 5 1.3 1.4 1.5 1.6 1.7 1.8 1.9 E (V) vs RHE 1 2 3 4 5 6 1 2 3 4 5 6 Figure S7 (A) Linear sweep voltammogram of the anode in.2 M KHCO 3 with/without C 2 H 4 and (B) the amount of ethylene quantified by online gas chromatography during chronopotentiometry at ma/cm 2 and 1.92 ma/cm 2. S9
S2.4 Activity of IrO x towards the oxidation of liquid products The activity of the anode towards the oxidation of different liquid products was also assessed by chronopotentiometry using the same two-electrode setup described in Section S2.3..2 M KHCO 3, which contains 1 mm of ethanol, n-propanol, formate, acetaldehyde, allyl alcohol and propionaldehyde, was prepared as the electrolyte. After 3 minutes chronopotentiometry at 1.92 ma/cm 2 with the continuous flow of CO 2, the compounds were quantified by headspace gas chromatography and high performance liquid chromatography. A control experiment was done similarly except that no current was applied. No significant oxidation was observed for all products (Table S2). The decrease in the concentrations of acetaldehyde and propionaldehyde in both tests can be attributed to their evaporation under the bubbling of CO 2. Table S2 Concentration of different compounds after 3 minutes chronopotentiometry. Conc. (mm) Ethanol n-propanol Formate Allyl alcohol Acetaldehyde Propionaldehyde Control 1.4 ±.3.98 ±.3.96 ±.4 1.1 ±.3.82 ±.16.75 ±.16 After electrolysis 1.4 ±.5 1. ±.6 1.5 ±.5.98 ±.3.75 ±.18.67 ±.18 S1
S3 Characteristics of the two-electrode electrolyzer S3.1 Geometry of the two-electrode electrolyzer A custom-made Teflon electrochemical cell was used as the two-electrode electrolyzer (Figure S8). The cathode and anode have surface areas of.385 and 5 cm 2 respectively. The total volume of the cell was 15 cm 3 and the two electrodes were spaced ~ 5 mm apart. The volume of electrolyte used was 12.5 cm 3 and the headspace was ~ 2.5 cm 3. Gas outlet 25 mm IrO 2 (SA = 5 cm 2 ) 15 mm 4 mm Cu 2 O (SA =.385 cm 2 ) CO 2 Figure S8 The geometry of the two-electrode electrolyzer (not drawn to scale). S11
S3.2 Effect of electrolyte concentrations and membrane on overall resistance Many CO 2 reduction studies on Cu-based catalysts have been performed in aqueous.1 M KHCO 3 electrolyte. 4 However, we found that the use of this electrolyte will result in an overall resistance between cathode and anode of 155 Ω (Table S3). Such a high overall resistance will drastically decrease the energetic efficiency of the electrolyzer. By using.2 M KHCO 3 solution, we decrease the overall resistance to 87 Ω. An even more concentrated electrolyte could lower the solution resistance further, but the selectivity of the cathode towards ethylene formation would then suffer drastically. 5 Anion exchange membrane (AMV) would introduce an extra ~15 Ω overall resistance. Thus, the membrane was excluded in our system. Table S3 Overall resistance between cathode and anode at different conditions. Electrolyte concentration (mol/l) Anion exchange membrane (AMV) Overall Resistance (Ω).1 Without AMV 155. ±.7.2 Without AMV 87.2 ± 1.6.2 With AMV 12.3 ±.1 S12
S3.3 Major half reactions in the electrolyzer The thermodynamic potentials of possible reactions occurring in our electrolyzer are listed in Table S4. Table S4 Major half reactions and thermodynamic reaction potentials at standard conditions in the PV-electrolyzer system. 6, 7 Cathode Product Ethylene Ethanol Formate n-propanol Propionaldehyde Allyl Alcohol Acetaldehyde Hydrogen Reaction 2CO 2 + 12e - + 8H 2O C 2H 4 + 12OH - (E o =.8 V vs RHE) 2CO 2 + 12e - + 9H 2O C 2H 5OH + 12OH - (E o =.9 V vs RHE) CO 2 + 2e - + H 2O HCOO - + OH - (E o = -.23 V vs RHE) 3CO 2 + 18e - + 13H 2O C 3H 7OH + 18OH - (E o =.21 V vs RHE) 3CO 2 + 16e - + 11H 2O C 2H 5COH + 16OH - (E o =.14 V vs RHE) 3CO 2 + 16e - + 11H 2O C 3H 5OH + 16OH - (E o =.11 V vs RHE) 2CO 2 + 1e - + 7H 2O CH 3COH + 1OH - (E o =.5 V vs RHE) 2H 2O + 2e - H 2 + 2OH - (E o = V vs RHE) Anode Oxygen 2H 2O O 2 + 4H + + 4e - (E o = 1.23 V vs RHE) S13
S3.4 CO 2 reduction products detected at various applied cathodic currents To optimize the current/voltage of the electrolyzer (two-electrode system) towards ethylene formation, chronopotentiometry at different currents was carried out. It was found that ~7.7 ma with the voltage of ~3.3 V were the optimized conditions for the ethylene formation, with the faradaic efficiency of ethylene ~34% (Table S5). The voltage represents the potential difference between cathode and anode, which includes ir drop. Table S5 Average faradaic efficiencies and production rates of products detected during carbon dioxide reduction on the oxide-derived Cu in a two-electrode configuration using chronopotentiometry. N.D.: not detectable. AD: acetaldehyde; AA: Allyl alcohol; PA: propionaldehyde; AT: Acetone; AC: Acetate. I (ma) E (V) C 2H 4 C 2H 5OH C 3H 7OH HCOO - AD AA PA AT AC CH 4 C 2H 6 CO H 2 Faradaic efficiency (%) -6.5-3.18 13.67 5.44 1.91 13.82.33.52.39.6 N.D..89 N.D..41 55.7-7. -3.26 8.6 3.23 2.26 2.77.31.78.64.8 N.D. 2.1 N.D..67 36.83-7.5-3.22 17.57 9.4 2.4 4.59.32.43.23.9 N.D..35 N.D..14 45.35-7.7-3.33 33.69 8.71 3.79 9.46.44.88.85.8.38.75.3.72 47.27-8. -3.3 24.91 9.83 3.23 7.5.44.74.55.1 N.D..65.14.27 45.24-8.3-3.31 24.72 7.73 4.74 9.75.62 1.15 1.85.13.22.52.19.58 46.82-8.5-3.6 21.39 9.2 1.49 1.54.28.36.16.6 N.D..21.3.1 47.9 Production rate (µmol cm -2 hr -1 ) -6.5-3.18 7.18 2.86.67 43.53.21.2.15.2 N.D..7 N.D. 1.29 175.44-7. -3.26 4.86 1.83.85 7.45.21.33.27.3 N.D. 1.7 N.D. 2.27 124.93-7.5-3.22 1.64 5.69.82 16.68.23.2.1.4 N.D..32 N.D..51 164.81-7.7-3.33 2.95 5.42 1.57 35.3.33.41.4.4.35.7.16 2.69 176.37-8. -3.3 16.9 6.35 1.39 27.33.34.36.27.5 N.D..63.8 1.5 175.37-8.3-3.31 16.57 5.18 2.12 39.21.5.58.93.7.22.52.11 2.33 188.32-8.5-3.6 14.68 6.32.68 6.34.23.19.8.3 N.D..22.2.41 193.95 S14
S4 Solar-Driven CO 2 reduction under solar simulator S4.1 Characterization of the solar panels The as-received IXYS solar panel consists of eight individual monocrystalline silicon solar cells. Each solar cell has the size of.24 cm.5 cm =.12 cm 2. Hence, each solar panel has the effective illuminated area of 8.12 cm 2 =.96 cm 2. The panel is then characterized under AM1. one sun irradiation and the measured I-V curve is shown in Figure S9A. The data sheet is available at https://drive.google.com/file/d/b-tkxk51_xefrjdnhrhblhzvm8/view?usp=sharing. Two solar panels connected in parallel were characterized, and its I-V curve was shown in Figure S9B. There are energy losses due to the resistance of the wires (1 m) used to connect the solar panel and the electrochemical cell. This energy loss is taken into account during our calculations for the solar to electricity efficiency (Figure 2a). (A) Current (ma) 5 4 3 2 1 Current Power 16 12 8 4 Power (mw) (B) Current (ma) 1 8 6 4 2 Current Power 3 25 2 15 1 5 Power (mw) 1 2 3 4 5 Voltage (V) 1 2 3 4 5 Voltage (V) Figure S9 Characteristics of (A) the individual panel and (B) two parallel-connected panels. S15
S4.2 CO 2 reduction products detected using simulated one sun intensity During the solar-driven CO 2 reduction test, two solar panels connected in parallel were illuminated under AM 1. one sun (1 mw/cm 2 ). The solar panels were connected to our electrolyzer using wires. A potentiostat (Gamry Reference 6) was used to record the current during one-hour electrolysis. Three separate measurements were carried out and the faradaic efficiencies and the production rates of all the products are presented in Table S6. Table S6 Average faradaic efficiencies and production rates of products detected during solar-driven carbon dioxide reduction under 1 sun solar flux (three reproduced sets). N.D.: not detectable. AD: acetaldehyde; AA: Allyl alcohol; PA: propionaldehyde; AT: Acetone; AC: Acetate. j (ma/cm 2 ) C 2H 4 C 2H 5OH C 3H 7OH HCOO - AD AA PA AT CH 4 C 2H 6 CO H 2 Total Faradaic efficiency (%) -2.17 32.29 9.29 4.9 8.46.78 1.32 2.34.19 1.5 N.D..75 3.63 92.45-2.4 32.36 9.75 5.21 7.89.73 1.29 2.38.19.89.19.81 31.9 92.78-19.96 31.2 9.76 5.5 9.95.89 1.44 3.1.21.88.24 1.3 26.51 9.44 Production rate (µmol cm -2 hr -1 ) -2.17 2.25 5.83 2.5 31.83.59.62 1.1.9 1.41 N.D. 2.82 115.26 - -2.4 2.16 6.8 2.16 29.5.55.6 1.11.12.83.1 3.3 116.23 - -19.96 19.25 6.6 2.28 37.5.66.67 1.4.13.82.13 3.84 98.71 - S16
S5 Continuous synthesis of C 2 H 4 from CO 2 and H 2 O under natural sunlight S5.1 Stability of ethylene formation in the electrolyzer The stability of the electrolyzer was investigated using chronopotentiometry with a cathodic current of 2 ma/cm 2 (7.7 ma) for four hours and the faradaic efficiency of ethylene decreased from ~3 to ~15% (Figure S1A). Concomitantly, the appearance of the cathode changed from shiny and reflective to blackish (Figure S11). We hypothesize that the inevitable dissolution of the Ir anode during the electrolysis could lead to solvated Ir ions reducing onto the negatively-biased cathode, and hence deactivating its selectivity towards the production of multicarbon products. 8 SEM and XPS confirmed this hypothesis by the detection of Ir on the cathode s surface after four hours electrolysis (Figure S11). Once the chelating agent (Chelex 1) was introduced, the stability improved greatly (Figure S1B). The surface remained shiny after four hours electrolysis, and SEM and XPS analyses indicated that no Ir was deposited on the surface (Figure S11). (A) 6 (B) 6 Faradaic efficiency (%) 5 4 3 2 Without Chelex Faradaic efficiency (%) 5 4 3 2 With Chelex 1 1 6 12 18 24 6 12 18 24 Figure S1 Faradaic efficiency of ethylene over four hours electrolysis (A) without and (B) with Chelex 1. Insert of B shows the structure of Chelex 1 at ph 7.1 (CO 2 saturated.2 M KHCO 3 ) and ϕ represents the polymer chain. S17
S5.2 Characterization of the cathode after four hours electrolysis (A) (B) 5 nm 5 nm (C) (D) Cu 3s With Chelex Intensity (a.u.) Ir 4f 7/2 Ir 4f 5/2 Intensity (a.u.) Without Chelex 58 6 62 64 66 68 Binding Energy (ev) 118 12 122 124 126 128 Binding Energy (ev) Figure S11 SEM images of the Cu 2 O-derived Cu cathode after four hours electrolysis (A) without and (B) with Chelex 1. The insets show respective photographs of the films. X-ray photoelectron spectra of (C) Ir 4f and (D) Cu 3s of the cathode after four hours electrolysis without and with Chelex 1. S18
S5.3 Variation in the intensities of the incident solar flux during the tests. 25 2 Intensity (W/m 2 ) 15 1 5 Test 1 Test 2 Test 3 3 6 9 12 Figure S12 Variation of the incident solar flux during three independent tests. The lines drawn to connect the data points are for guiding the eye. S19
S5.4 Selection of solar panel and battery The equation to determine the minimum area of solar panel (A min ) and capacitance of the battery (Q min ) for a given system to operate day and night without interruption is given as: A!"# I!"#$% η!"#$% t!"#$% η!"#!$"% = P!"## 24 hrs Q!"# = I!"## (24 hrs t!"#$% ) The meaning of the different symbols and representative values for our test is given in Table S7. The calculated minimum area of solar panel is: A min = 58 cm 2 ; Q min = 125 mah. Based on these numbers, we chose the commercially available solar panel with a total area of 7 cm 2 and battery with the electric charge of 5 mah. Table S7 Parameters used to determine the requirements for the solar panel and battery. Parameter Meaning Representative value I solar Average solar intensity 15 mw/cm 2 η solar Efficiency of the solar panel 1% t solar Solar flux time during one day 8 hrs η circuit Circuit efficiency 9% P cell Power requirement of the solar cell 26 mw I cell Required current of the cell 7.8 ma S2
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