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

[1] L.Z. Liu, X.L. Wu, J.Q. Xu, T.H. Li, J.C. Shen, P.K. Chu, Oxygen-vacancy and depth-dependent violet double-peak photoluminescence from ultrathin cuboid SnO2 nanocrystals, Appl. Phy. Lett., 100 (2012) 121903.[2] C.W. Li, J. Ciston, M.W. Kanan, Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper, Nature, 508 (2014) 504-507.[3] L. Fan, Z. Xia, M. Xu, Y. Lu, Z. Li, 1D SnO2 with wire-in-tube architectures for highly selective electrochemical reduction of CO2 to C1 products, Adv. Funct. Mater., 28 (2018) 1706289.[4] V.A. Bijandra Kumar, J. Patrick Brian, Sudesh Kumari, Tu Quang Nguyen, Mahendra Sunkara, and Joshua M. Spurgeon, Reduced SnO2 porous nanowires with a high density of grain boundaries as catalysts for efficient electrochemical CO2-into-HCOOH conversion, Angew. Chem. Int. Ed., 56 (2017) 3645-3649.[5] R. Daiyan, X. Lu, W.H. Saputera, Y.H. Ng, R. Amal, Highly selective reduction of CO2 to formate at low overpotentials achieved by a mesoporous tin oxide electrocatalyst, ACS Sustain. Chem. Eng., 6 (2018) 1670-1679.[6] S. Zhang, P. Kang, T.J. Meyer, Nanostructured tin catalysts for selective electrochemical reduction of carbon dioxide to formate, J. Am. Chem. Soc., 136 (2014) 1734-1737.[7] H. Hu, L. Gui, W. Zhou, J. Sun, J. Xu, Q. Wang, B. He, L. Zhao, Partially reduced Sn/SnO2 porous hollow fiber: A highly selective, efficient and robust electrocatalyst towards carbon dioxide reduction, Electrochimica Acta, 285 (2018) 70-77.[8] J. Gu, F. Heroguel, J. Luterbacher, X. Hu, Densely packed, ultra small SnO nanoparticles for enhanced activity and selectivity in electrochemical CO2 reduction, Angew. Chem. Int. Ed., 57 (2018) 2943-2947.[9] Y. Zhao, J. Liang, C. Wang, J. Ma, G.G. Wallace, Tunable and efficient tin modified nitrogen-doped carbon nanofibers for electrochemical reduction of aqueous carbon dioxide, Adv. Energy Mater., 8 (2018) 1702524.[10] F. Li, L. Chen, G.P. Knowles, D.R. MacFarlane, J. Zhang, Hierarchical mesoporous SnO2 nanosheets on carbon cloth: A robust and flexible electrocatalyst for CO2 reduction with high efficiency and selectivity, Angew. Chem. Int. Ed., 56 (2017) 505-509.