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Supporting Information
Wavy SnO2 catalyzed simultaneous reinforcement of
carbon dioxide adsorption and activation towards
electrochemical conversion of CO2 to HCOOH
Zhou Chena,b, Tingting Fanb, Ya-Qian Zhang a, Jing Xiaoa, Minrui Gaoa, Nanqi Duana,
Jiawei Zhanga,b, Jianhui Lib, Qingxia Liua, Xiaodong Yib* and Jing-Li Luoa*
* a Department of Chemical and Materials Engineering, University of Alberta,
Edmonton, Alberta T6G 1H9, Canada.
* b National Engineering Laboratory for Green Chemical Productions of Alcohols,
Ethers and Esters, College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen 361005, P. R. China.
*Corresponding author: xdyi@xmu.edu.cn (X.-D. Yi) and jingli.luo@ualberta.ca (J.-
L. Luo).
Experimental section
Chemicals and gases: The synthesis and evaluation system includes tin (IV)
chloride pentahydrate (SnCl4·5H2O), urea, hydrochloric acid (HCl), isopropanol
(IPA), commercial nanoparticle SnO2 (NP-SnO2), carbon black, potassium
bicarbonate (KHCO3, ≥ 99.99%) and potassium hydroxide (KOH). Nafion
perfluorinated ion-exchange resin solution (5 wt% in mixture of lower aliphatic
alcohol & H2O), Toray carbon paper (Toray TGP-H-060) and Nafion® N-117
membrane (0.18 mm thick) were purchased from Sigma-Aldrich, Toray
industries Inc and Alfa Aesar, respectively. Deionized water was produced and
taken from a Millipore Autopure system. All the chemicals were used as
received without further purification. Hydrogen (H2, 99.999%), argon (Ar,
99.999%), compressed air (extra dry) and carbon dioxide (CO2, 99.999%) were
purchased from Prexair.
Preparation:
Wavy SnO2 (Denoted as NW-SnO2) was prepared using a hydrothermal
reaction method reported by previous literature[1]. In a typical synthesis of
NW-SnO2, 0.08 g SnCl4·5H2O and 0.8 g urea were firstly dissolved in 32 mL
deionized water. Subsequently, 1.6 mL of hydrochloric acid was added under
ultrasonic treatment. The whole solution was then transferred into a 50 mL
Teflon-lined stainless-steel autoclave to react at 90 °C for 15 h. The autoclave
was cooled down to room temperature naturally after the reaction. For the
synthesis of electrocatalyst of carbon black loaded wavy SnO2 (NW-
SnO2/Carbon black), 25 mg carbon black was added during the synthesis
process of nanowire SnO2. The product was washed three times by water in
order to remove the residual reactant.
For comparison, commercial NP-SnO2/carbon black electrocatalyst was obtained
by physically mixing commercial nanoparticle SnO2 (Denoted as NP-SnO2) with
carbon black (with mass ratio of 6:4, in terms of the NP-SnO2/carbon black).
Electrochemical measurements
To prepare the NW-SnO2 working electrode, a suspension containing 10 mg
NW-SnO2/Carbon black, 500 µL IPA, 450 µL water and 50 µL of 5 wt% Nafion
(Aldrich) was sonicated to obtain a homogeneous dispersion. 100 µL of the
suspension was painted onto one side of a Toray carbon paper (2 cm*1 cm) to achieve
a NW-SnO2/Carbon black electrocatalyst loading of 1 mg cm-2. According to the
results of TG, the mass percentage of NW-SnO2 was 57.6 wt%, suggesting the
NW-SnO2 loading amount on our catalysts was 0.576 mg cm-2. The commercial
NP-SnO2 working electrode was fabricated in the same way with the NW-SnO2
electrode. NP-SnO2 loading amount on our catalysts was controlled at 0.6 mg
cm-2.
The electrochemical performance measurements were carried out on Autolab
electrochemical workstation. The electrochemical reduction was performed in a
home-made electrochemical cell with a standard three-electrode system where the
saturated calomel electrode (SCE), a platinum gauze and the catalyst loaded on
carbon paper serving as the reference electrode, the counter electrode and the working
electrode, respectively. CO2 was bubbled into the electrolyte (0.5M KHCO3 solution)
at a flow rate of 20 mL/min for at least 30 mins before each experiment in order to
remove all the oxygen from the electrolyte and achieve a saturated CO2 condition. All
the potentials versus SCE were recorded and converted the reversible hydrogen
electrode (RHE) reference scale by the equation:
ERHE=ESCE+0.241+0.0592*pH
The pH value for CO2 saturated 0.5 M KHCO3 solution was 7.2.
Product analysis on the three-electrode setup for CO2RR.
The gaseous products were detected by a gas chromatograph (GC; Agilent 6890N)
with a thermal conductivity detector (TCD) for H2 and a flame ionization detector
(FID) for CO and hydrocarbons. A continuous sample analysis system was built up in
order to analyze the composition of gas product. The liquid product was quantified by
1H nuclear magnetic resonance (NMR) spectrometer (Bruker Advance 500 MHz).Ⅲ
500 μL electrolyte was mixed with 100 μL DMSO (interinal standard diluted 100 ppm
(v/v) by deuterated).
Electrochemical impedance spectroscopy (EIS) measurements were carried out at
0.6 vs VRHE in CO2 saturated 0.5 M KHCO3 aqueous solution with a three-electrode
configuration over the frequency range 0.1 - 105 HZ. Double layer capacities studies
were carried out between -0.1 V and -0.2 V vs SCE at different scan rates (between 10
to 120 mV s-1). ECSA of the electrodes was calculated from the double layer
capacitance according to the equation as follows: ECSA = Cdl/Cs, where Cdl value is
half of the slope and Cs is the specific capacitance. In our calculation, we used
general specific capacitances of Cs = 0.029 mF based on typically reported value.[2]
Calculation of Faradaic Efficiency (FE)
The Faradaic efficiency (FE) of gas phase product was calculated at a given potential
based on the equation as follows:
FE=ZFVvP0/RT0Itotal ×100%
FE: FE for gas product;
Z: charge transfer of per mole of gas product;
F: Faradaic constant, 96485 C mol-1;
V (mL/min): gas flow rate measured by a flow meter;
v (Vol%): volume concentration of gas product in the exhaust gas from GC;
Itotal (A): total current density at each applied potential;
R=8.314 J mol-1 K-1;
P0=1.01 × 105 Pa;
T0=298.15 K.
The Faradaic efficiency (FE) of liquid phase products can be calculated as follows:
FE=2F × n(HCOOH)/Q=2F ×(HCOOH)/(Itotal × t), where F is the Faraday constant.
Calculation of Energy Efficiency (EE)
EEM¿EM
0 × F EM❑
EM0 +η
×100%,
where EM0 is the equilibrium cell potential for a certain product.
FEM is the Faraday efficiency of the aiming product M.
𝜂 is the overpotential.
Characterization
The X-ray diffraction (XRD) measurements were performed on a Rigaku
Rotaflex X-ray diffractormeter using Cu Kα radiation at 40 kV and 44 mA to
determine the phase structure of samples. The microstructure of the samples was
observed on a scanning electron microscope (SEM, Zeiss EVO MA 15) and
transmission electron microscope (TEM Hitachi H9500). X-ray photoelectron
spectroscopy (XPS) measurements were carried out using Kratos AXIS to determine
the surface chemistry of the samples. The XPS spectra were referenced to the C 1s
bonding energy (284.6 eV). Nitrogen physisorption was measured by a Micromeritics
ASAP 3020 apparatus. Raman spectra were obtained from an Xploar PLUS Confocal
Raman Microscope with a 514 nm laser source.
The photoluminescence (PL) spectroscopy measurements were performed using
a Hitachi F-7000 fluorescence spectrophotometer and the electron spin resonance
(ESR) measurements were carried out with Bruker ESR300E at room temperature in
the air. CO2-temperature programed desorption (CO2-TPD) was performed on a
Micromeritics AutoChemII 2920 instrument. Typically, 100 mg sample was placed in
a quartz U-tube, degassed under He atmosphere at 150 oC for 2 h, purged by 10%
CO2/He and then kept for 1 h. After that, the gas flow was switched to He to remove
the free CO2 in tube for 2 h. Subsequently, TPD was performed in He flow by raising
the temperature to 700 oC with a speed of 10 oC/min and the chemically adsorbed CO2
was detected with a mass spectroscopy (MS) with a signal of m/z=44.
Figure S1. (a, b) Low resolution TEM images and (c, d) high resolution TEM images
of commercial nanoparticle SnO2 (NP-SnO2).
Figure S2. SEM images of carbon back. The size of carbon black is around 100 nm.
Figure S3. TGA curve of NW-SnO2/Carbon black in air atmosphere. The loading of
SnO2 in the composites was determined by this curve, assuming SnO2 was the only
pyrolysis product at the end of TGA experiment.
Figure S4. (a, b) TEM images of NW-SnO2 loading on carbon black by one-pot urea
assisted hydrothermal method (NW-SnO2/Carbon black).
Figure S5. Chronoamperometry curves of (a) NP-SnO2 and (b) NW-SnO2
electrocatalysts at all applied potential in CO2 saturated 0.5 M KHCO3 electrolyte.
The applied potential shown in picture was relative RHE.
Figure S6. Histograms of partial current density of HCOOH for NP-SnO2 and NW-
SnO2 electrocatalysts at -1.0 vs VRHE.
Figure S7. XRD patterns of carbon paper, fresh electrode, carbon paper, the fresh
electrode, -0.8 VRHE electrode (the used electrode after the stability test at -0.8 VRHE)
and -1.0 VRHE electrode (the used electrode tested at -1.0 VRHE). The XRD analysis of
the catalysts after electrolysis indicated that the wavy SnO2 was partially reduced to
metallic Sn in the CO2 electroreduction.
Figure S8. TEM images of the -0.8 VRHE electrode, which was used in the stability
test at -0.8 VRHE. The morphology change of the used catalyst is negligible and the
lattice distance (0.34 nm) corresponding to the (110) plane of SnO2 can still be clearly
observed. After carefully examining the used catalyst, we found that some part of
surface of wavy SnO2 was reduced to metallic tin where the lattice distance (0.29 nm)
corresponding to the (200) plane of metallic Sn is observed in Fig. S8d.
Figure S9. (a) O 1s, (b) Sn 3d XPS spectra of the -0.8 VRHE electrode, which was used
in the stability test at -0.8 VRHE. After 18 hours of electrolysis at -0.8 VRHE, the O-
vacancies still existed but at a lower percentage (24.8%), which indicates a portion of
SnO2 was reduced to metallic tin. However, the electrode examined by Sn 3d XPS
exhibited a SnOx: Sn0 ratio of 89:11, indicating that the SnO2 catalyst is relatively
stable under the reduction conditions. This is in agreement with the results of XRD
and TEM.
Figure S10. Double layer capacitance obtained from CV measurements. CV curves of
(a) NP-SnO2 and (b) NW-SnO2 in CO2 saturated 0.5 M KHCO3 electrolyte between -
0.1 V and -0.2 V vs SCE at different scan rate (between 10 to 120 mV s-1).
Figure S11. N2 adsorption-desorption isotherms NP-SnO2 and NW-SnO2
electrocatalysts.
Table S1. The deconvolution result of O 1s and Sn 3d XPS spectra of NP-SnO2 and
NW-SnO2 samples.
NP-SnO2 NW-SnO2
O-
vacancies
(eV)
Lattic
e O
(eV)
Sn
3d5/2
(eV)
Sn
3d3/2
(eV)
O-
vacancies
(eV)
Lattic
e O
(eV)
Sn
3d5/2
(eV)
Sn
3d3/2
(eV)
Peak/
eV533.0 531.2 487.3 495.5 532.7 530.9
487.
1
495.
3
Ratio
(%)28.5 71.5 / 44.2 55.8 /
Table S2. The partial current density and normalized partial current density of
HCOOH of NP-SnO2 and NW-SnO2 samples at -1.0 vs VRHE.
CatalystsPartial current
density for HCOOH
Electrochemical
surface area
Normalized partial
current density
NP-SnO2 9.6 mA/cm2 91.0 mF/cm2 0.1 mA/mF
NW-SnO2 22.0 mA/cm2 134.1 mF/cm2 0.17 mA/mF
Table S3. Summary of our and previously SnO2 related electrocatalysts for
HCOOH production based on catalytic CO2RR.
CatalystsElectrochemical CO2RR
electrolyte
Highest
CO2RR
activity for
HCOOH
Partial
current
density for
HCOOH
Onset
overpotential
(mV)
Reference
SnO2 nanofiber[3] 0.1M KHCO3 solution63% at -0.99
V vs. RHE3.3 mA/cm2 400
Adv. Fun. Mater, 28, 2018,
1706289
SnO2-pNWs[4] 0.1M KHCO3 solution
80% at -
0.8V vs.
RHE
3.2 mA/cm2 350Angew. Chem. Int. Ed., 56,
2017, 3645-3649.
Mesoporous SnO2[5] 0.1M KHCO3 solution75% at -1.15
V vs. RHE
10.8
mA/cm2325
ACS Sustain. Chem. Eng, 6,
2018, 1670-1679.
Nano-SnO2[6] 0.1M NaHCO3 solution93% at -1.8
V vs. SCE
10.2
mA/cm2340
J. Am. Chem. Soc, 136,
2014, 1734-1737.
Sn/SnO2 PHF[7] 0.1M KHCO3 solution
82.1% at -
0.93 V vs
RHE
22.9
mA/cm2Not mention
Electro. Acta, 285, 2018,
70-77.
Ultra Small SnO2[8] 0.5M KHCO3 solution70% at -0.86
V vs. RHE8.5 mA/cm2 240
Angew. Chem. Int. Ed.
2018, 57, 2943-2947
Sn modified N-doped
carbon nanofiber[9]0.5M KHCO3 solution
62% at -0.8
V vs RHE11 mA/cm2 280
Adv. Energy Mater, 8, 2018,
1702524.
Hierarchical SnO2
nanosheets[10]0.5M NaHCO3 solution
87% at -
0.88V vs.
SHE
45 mA cm-2 230Angew. Chem. Int. Ed.
2017, 56, 505-509
NW-SnO2 0.5M KHCO3 solution
87.4% at -
1.0V vs.
RHE
22 mA/cm-2 190 This work
Reference
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