Experiments
Chemicals and Materials . All chemicals were used without further purification. Deionized water (DI water) with a resistivity of 18.2 MΩ·cm (Millipore) was used. Copper chloride dihydrate (CuCl2∙2H2O, 99.0%), aqueous hydrogen peroxide (H2O2, 30% w/w in H2O), potassium hydrogen carbonate (KHCO3, 99.7%), 5% Nafion solution, isopropanol, ethanol, ethylene glycol (anhydrous, 99%), dimethyl sulfoxide (anhydrous, 99.5%), and phenol (≥99%) were supplied by Sigma Aldrich. Copper nitrate trihydrate (CuNO3∙3H2O, 99.99%) and sodium hydroxide (NaOH, 99.99%) were purchased from Aladdin and VWR Chemicals, respectively. D2O (99.9% atom%D) was purchased from J&K Chemicals.
Synthesis of OD-Cu NS . The pristine CuO nanosheets (NSs) were synthesized by following a procedure previously reported by Zhang et al.29 85.2 mg of CuCl2∙2H2O was dispersed into 10 ml of 3 M NaOH solution by vigorous stirring, then transferred into the Teflon-liner stainless steel autoclave. After kept at 100 ℃ for 12 hours in the oven and then cooled down to room temperature, the obtained product was washed by DI water and ethanol for four times respectively and dried at 80 ℃ under the vacuum oven for overnight. To prepare the OD-Cu NS, 20 mg of the as-synthesized CuO NS with 30 wt% (Cu-30) or 100 wt% (Cu-100) Cu(NO3)∙3H2O was suspended into 10 ml of ethylene glycol, then transferred into a Pyrex glass bottle. The mixture solution was ultrasonicated in ice water for 1 hour, then moved into the digestive microwave (Milestone, Ethos One) with the power of 1050 W for 210 s under an ambient atmosphere. The dark brownish colored precipitate was cooled to room temperature and then collected by centrifugation. Subsequently, the dark brownish precipitate was collected by a centrifuge process at 15000 rpm for 10 mins, and washed four times by ethanol. After dried under a vacuum oven at 60 ℃ overnight, the obtained OD-Cu NS were collected for further usage.
Physical Characterizations . Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2010F (JEOL, Tokyo, Japan). Aberration-corrected TEM was operated on JEOL JEM-ARM200F at 200kV acceleration voltage. The samples used for TEM characterization were dispersed in absolute ethanol firstly, then dropped onto carbon coated Cu or Ni grid and dried completely at room temperature. The X-ray diffraction (XRD) patterns were recorded at room temperature with an X’pert Pro K-Alpha diffractometer (PANalytical, Almelo, Netherlands), using Cu Kα radiation (λ=1.5406 Å). The XRD samples were prepared by dropping the as-prepared nanoparticles (NPs) suspensions onto a glass slide. The X-ray photoelectron spectroscopy (XPS) was measured by a Physical Electronics 5600 with an Al Kα source (hν = 1486.6 eV), and the fresh samples were used for the XPS measurements to avoid over-oxidation issue.
Electrochemical Measurement . To prepare the working electrode, 2 mg of each catalyst (commercial Cu NP, pristine CuO NS, and OD-Cu NSs) was suspended into 100 μL of absolute ethanol with 5 μL of 5% Nafion solution (Anhydrous, Sigma Aldrich) by ultrasonication. After that, 5 μL of the dispersion was loaded onto a cleaned glassy carbon electrode with a diameter of 5 mm, and dried in ambient atmosphere. For the electrochemical CO2RR measurement, a two-component H-type cell was assembled with a proton exchange Nafion 117 membrane (Aldrich) in the middle. Then each component was filled with 50 ml of 0.1 M KHCO3 electrolyte, afterwards CO2gas (99.999%, Asia Pacific Gas Enterprise Co., LTD) was purged into the solution for 1 hour in advance. Platinum (Pt) mesh (20 × 20 mm2) and an Ag/AgCl (saturated into 3.0 M KCl, BASi) electrode were used as a counter and a reference electrode, respectively. The reference electrode was calibrated with 0.1 M HClO4 solution (pH 1.1) by reversible hydrogen electrode (RHE) and the potential E was converted to the RHE reference scale in terms of the equation:
E (versus RHE) = E (versus Ag/AgCl) + 0.197 V + 0.059 V × pH.
As connecting the gas-tight H-cell to the potentiostat (CHI 627E, CH Instruments, Inc.), iR compensation was conducted automatically at a level of 85%, whereas the rest of 15 % IR drop was manually compensated during data processing. CO2 was continuously fed into both parts of the electrolyte via mass flow controllers (Sevenstar, Beijing) at a rate of 30 SCCM. Since trapping air bubbles on the working electrode could cause high resistance, the component containing the working electrode was stirred magnetically at 1500 rpm. The gas products were analyzed by a gas chromatograph (GC2060, Ramiin, Shanghai) equipped with a thermal conductivity detector (TCD) for hydrogen (H2) quantification and a flame ionization detector (FID) for carbon monoxide (CO), methane (CH4), and ethylene (C2H4). The calibration curve was obtained by a series of standard gas mixtures (H2, CO, CH4, and C2H4 balanced in Ar, Shanghai Haizhou Special Gas Co., LTD). Finally, four gas samples at 15, 28, 41, and 54 min were collected and quantified in average for 1 hour electrolysis. Liquid products were collected after the measurement and analyzed by using a Varian 500 MHz 1H nuclear magnetic resonance (NMR) spectra. The NMR samples were prepared by mixing 450 μL of electrolyte with 50 μL home-made internal standard solution containing 500 × 10−6 M phenol, and 100 × 10−6M dimethyl sulfoxide in D2O. The calibration curves of internal standard solution were established by several standard solutions (0, 50, and 100 μM of formate, methanol, ethanol, acetate, and iso-propanol in 0.1 M KHCO3 solution).
The electrochemical activity was examined by linear sweep voltammetry (LSV) from −0.1 VRHE to −1.8 VRHE in CO2 or Ar-saturated 0.1 M KHCO3electrolyte and the electrochemical surface area (ECSA) was measured by cyclic voltammetry (CV) between 0 and 0.4 VRHE with different scan rates in CO2-saturated 0.1 M KHCO3 electrolyte after 1 hour electrolysis by each catalyst. To compare the binding energy of OHadsorption, LSV curves were obtained from 0.1 to 0.6 VRHE after the electrolysis in Ar-saturated 0.1 M KOH electrolyte.