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 OH−adsorption, LSV curves were obtained from 0.1 to 0.6
VRHE after the electrolysis in Ar-saturated 0.1 M KOH
electrolyte.