Achieving convenient CO2 electroreduction and photovoltage in tandem using potential-insensitive disordered Ag nanoparticles

This paper describes the rational design of potential insensitive disordered Ag, which could achieve more than 90% faradaic efficiency for CO within a wide voltage range of ∼1.1 V in a photovoltaic-electrochemical systems for CO2 system.

support, the pH of activated carbon was adjusted by 10% NaOH. In a standard synthesis process of disordered Ag, 10 mL of EG, 0.002 g of cysteine, and 0.02 g of prepared activated carbon were sonicated for one hours. 0.02 g of AgNO 3 was dissolved in 10 ml of EG without stirring and this solution was heated to 50 °C in 5 min and last for 20 min with stirring. Then the prepared carbon solution was transferred to the silver precursor solution at 50 °C, and the mixed solution was kept at 50 °C for 10 min prior to heating to 160 °C in 30 min and last for 3 hours. When the solution cooled to room temperature naturally, the final samples were collected by centrifugation, washed several times with ethanol. For 3 nm and 5 nm Ag were synthesized by adding 0.02 g of AgNO 3 to 10 mL of EG with vigorous stirring and slowly heating to 50 °C for 20 min. The other experimental conditions were just like disordered Ag, only for one and three hours at 160 °C, respectively. For 11 nm Ag, 20 mg of Ag(acac) was dissolved in 10 mL ODE under nitrogen flow with vigorous stirring and 10 mL of ODE, 0.002g of cysteine, and 0.02g of prepared activated carbon were sonicated for one hours. The procedure was the same as the method above only for three hours at 200 °C. For distinction, the disordered particle size Ag nanoparticles were called disordered Ag and the average size of uniform particle diameter Ag in each sample was used in the sample name, for instance, 3 nm Ag indicated that Ag nanoparticles in the Ag catalyst had an average particle size of 3 nm.

Electrochemical Test
Electrochemical measurements were carried out in a homemade H-type cell with a cathodic and anodic compartment separated by a Nafion membrane, using a three-electrode configuration, with the glassy carbon electrode as WE, a platinum foil (2 × 2 cm 2 ) as CE, and saturated Ag/AgCl as the RE. A glassy carbon electrode with a diameter of 8 mm was used on which 40 μL of the ink formulation solution (catalysts 20mg were dissolved in the disordered 2 mL of ethanol and 20 μL of 2 vol% Nafion solution) was dropped. An electrochemical workstation (IVIUM CompactStat.e20250, Netherlands) was used to measure the current and provide voltages. A CO 2 saturated 0.1 M potassium bicarbonate (KHCO 3 ) solution (pH 6.8) were used as the electrolyte. In order to achieve solar-driven CO 2 RR cell ( Figure S2), the glassy carbon electrode as cathode, platinum foil as anode and a six-section a-Si solar cell (5 × 5 cm 2 ) as driving force. The simulated solar illumination was obtained by passing light from a 300 W Xenon arc lamp (Beijing Perfectlight Technilogy Co. Ltd., Microsolar 300 UV) equipped with an AM 1.5 filter, and the power intensity of the incident light was calibrated to 100 mW/cm 2 at the surface of solar cell. At the same time, in order to detect the potential of cathode, a saturated Ag/AgCl was used as RE and two electrochemical workstations were used to measure the current and the potential of cathode, respectively. Before the reaction, the cathode and anode compartment were filled with 70 mL and 80 mL of the electrolyte, respectively. The cathode was connected to a gas circulation system with a ten-port value (VICI) for on-line sampling to a gas chromatograph (Ruimin GC 2060, Shanghai). The gas circulation system was primarily made of stainless steel tubing and a home-made gas pump for the circulation of the gas. A mechanical pump was connected into the system to exhaust the carrier gas of the gas chromatograph when switch back the ten-port value. A pressure gauge was also connected into the system to monitor the gas pressure. The total volume of the gas in the circulation system after filling the reactor with electrolyte was 150 mL. Before the start of the reaction, both the electrolyte and circulation system were purged with CO 2 (≥99.995%) for 30 minutes to achieve CO 2 saturation and remove air. Then the system was sealed and the initial CO 2 pressure was kept atmospheric. There was no more CO 2 purged into the closed system during the reaction. The gases in the closed circulation system were continuously circulated through the electrolyte for the entire reaction period. All the potentials vs. Ag/AgCl were converted to the reversible hydrogen electrode (RHE) using E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 V + 0.0591 × pH.

Solar to Cathode Product Conversion (STC) Efficiency Calculation [8]
Considering the CO 2 RR current of 1.5 mA ( Figure S3), corresponds to an electric charge of 1.5 mC (9.36 * 10 15 electrons for one second). These charges are used to produce H 2 and CO which both need two electrons to be formed. Over PV-EC system driven by a six-section a-Si solar cell, an average faradaic efficiency of 90.1% for the CO production and 9.7% for the H 2 production were measured during one hour. Thus, we can calculate that: 0.901 * 9.36 * 10 15 / 2 = 4.22 * 10 15 molecules of CO (7.01 * 10 -9 moles) 0.097 * 9.36 * 10 15 / 2 = 4.54 * 10 14 molecules of H 2 (7.57 * 10 -10 moles).
Considering the respective molar masses, we obtain 0.196 µg of CO and 0.0015 µg of H 2 .
Taking into account the lower heat value and higher heat value for CO and H 2 of 283.5 kJ/mol and 286 kJ/mol, respectively, the specific power of the produced products can be obtained as 10125 kW/kg for CO and 143330 kW/kg for H 2 .
Therefore, η STC is given by η C = (0.196 µ × 10125 / + 0.0015 µ 2 × 143330 / ) / where the input power (for 25 cm 2 illuminated solar cell area) is the incident light intensity (2500 mW). Thus, the solar to cathode products efficiency of our device was 0.09 %, which was smaller than most reported work [8][9] owing to excessive resistance and low current density caused by multiple batteries in series.

ATR-SEIRAS Test
Au film deposition. According to Miyake method, Au film electrodes were deposited directly on a Si prism used in ATR-SEIRAS. [S1] To get a flat reflector, the Si crystal was polished with a 0.05 µm Al 2 O 3 polishing powder. Then, in order to clean the organic contaminants on crystal, the Si crystal was immersed in Piranha solution (a 3:1 by volume solution H 2 SO 4 and H 2 O 2 ) for 1 hour. Before deposition, in order to improve adhesion of the Au film, we immersed the reflecting plane of the crystal in 40% NH 4 F solution for 90s to remove the oxide layer and generate a hydrogen-terminated surface. [S2] The reflecting plane of Si crystal was then immersed in a mixture of 68 μL of 40% HF and 4 mL of Au plating solution preparing by constant volume 0.1050 g NaOH and 0.2276 g HAuCl 4 , 0.1337 g NH 4 Cl, 0.98453 g Na 2 SO 3 , 0.6205 g and Na 2 S 2 O 3 ·5H 2 O to 100 mL for 5 min at 55 °C.
Different Ag electrodes were made by droping 100μL of the ink formulation solution (catalysts 20 mg were dissolved in the disordered 2 mL of ethanol and 20 of 2 vol% Nafion solution) on Si prism with Au film. All reagents were purchased from Alfa Aesar without further purifying.

In ATR-SEIRAS experiments. The Si prism with incidence of a 60° angle, Pt foil and an
Ag/AgCl electrode were served as the working electrode, counter electrode and reference electrode. A H-type electrochemical cell, separated by Nafion membrane, was designed to accommodate the Si prism and to avoid any possible cross-contamination from the counter electrode. A Nicolet iS50 FT-IR spectrometer equipped with MCT detector and a Pike Technologies VeeMAX Ⅲ ATR accessory were employed for the electrochemical ATR-SEIRAS. Before performing experiment, 5 L/min N 2 was used to purge the optical path system for one hour to reduce the influence of CO 2 and H 2 O(g) in air. All spectra were collected with a 4 cm -1 resolution and 8 scans. The electrolyte used was 0.1 M KCl due to its invisibility for infrared radiation, which was purged with CO 2 for 30 min (pH=3.94) before test. All the back ground was collected without applied potential in CO 2 saturated 0.1 M KCl and spectrum was collected with a fixed potential at 10 min. There will exist an inverted peak because of SiO 2 reduction at cathod. [S3] Results and Discussion Figure S1. XRD data of 3 nm, 5 nm, 11 nm and disordered Ag with only crystalline face centered cubic Ag peaks present.