Supporting Information

Shape-controlled bismuth nanoflakes as highly selective catalysts for electrochemical carbon dioxide reduction to formate

Sungjoo Kim, Wan Jae Dong, Seungo Gim, Woonbae Sohn, Jae Yong Park, Chul Jong Yoo, Ho Won Jang*, Jong-Lam Lee*

Surface morphology of electrodeposited Bi films

Fig. S1. SEM images of electrodeposited Bi films formed by (a) PC-deposition applied 5 mA/cm2, (b) PC-deposition applied 10 mA/cm2, (c) PC-deposition applied 20 mA/cm2, and (d) DC-deposition 20 mA/cm2. PC-depositions were conducted with a duration of 10s pulse-on and 40s pulse-off. Nanoflake (PC-6c), Nanodot (DC-60s) and Nanodendrite (DC-120s) were selected for the three representative Bi nanostructures marked with red boxes.

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Density of nanoflakes

Fig. S2. SEM images were used to calculate the average density of nanoflakes according to the number of pulse-cycle. Nanoflakes of (a) PC-1c, (b) PC-3c and (c) PC-6c films were marked as black-, blue- and red-circles, respectively.

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Microstructure of PC-deposited Bi films

Fig. S3. XRD patterns of Cu substrate, PC-1c, PC-3c and PC-9c deposited Bi films on Cu substrate (with JCPDS profile of Bi).

Surface composition of Bi films

Fig. S4. X-ray photoelectron spectroscopy (XPS) of Bi 4f core spectra of DC-60s (Bi nanodot), PC-6c (Bi nanoflake) and DC-120s (Bi nanodendrite).

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CO2 reduction activity of PC-deposited Bi films

Fig. S5. Electrochemical CO2 reduction activities of PC deposited Bi on Cu films with different pulse-cycles. (a) Linear sweep voltammetry curves in N2-purged (dashed line) and CO2-purged (line) 0.1 M KHCO3 electrolyte at a scan rate of 50 mV/s. (b) Nyquist impedance plots of electrodes in CO2-purged 0.1 M KHCO3 solution in a frequency range from 1 MHz to 1 Hz with 10 mV amplitude at 0 V vs RHE.

4

Surface area of electrocatalysts

Fig. S6. Current density plots at various CV scan rates. (a) DC-60s, (b) DC-120s, (c) PC-1c, (d) PC-3c, (e) PC-6c, (f) PC-9c and (g) double layer capacitance plot of PC- deposited films with a different pulse cycles. All DC- and PC- deposited Bi films were fabricated in 20 mA/cm2

applying condition. The current densities were obtained from the double-layer charge/discharge curves at 0.34 VRHE in N2-purged 0.1 M KHCO3 electrolyte.

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To correlate the catalytic properties with the surface area, cyclic voltammograms (CVs) at

different scanning rates were collected in N2-purged 0.1 M KHCO3 (Fig. S5). The

electrochemical surface areas of the DC-60s film, DC-120s film, PC-1c film, PC-3c film, PC-6c

film and PC-9c film were determined by measuring the double layer capacitance. The

capacitance is 0.39 μF/cm2 for DC-60s film, 40.4 μF/cm2 for DC-120s film, 0.40 μF/cm2 for PC-

1c film, 1.46 μF/cm2 for PC-3c film, 82.2 μF/cm2 for PC-6c film and 13.1 μF/cm2 for PC-9c film.

The optimization was performed to maximize the surface area through 6 pulse-cycles. Especially

the capacitance of PC-6c film (Bi nanoflakes) is 200 times larger than that of DC-60s film (Bi

nanodots). This suggests that the PC-deposition method is effective for making the surface area

larger than the DC-electrodeposition method.

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Effect of pulse-cycle control on Bi nanostructures

Fig. S7. (a) Faradaic efficiency and (b) production rate of PC-deposited Bi films obtained at -0.8 VRHE in CO2-purged 0.1 MKHCO3 electrolyte. The insets (b) show the SEM images of PC- deposited Bi films fabricated with different pulse-cycles at 20 mA/cm2. (Scale bar: 300 nm)

To find out the effect of low-dimensional nanostructure on selectivity CO2 reduction, we

measured the formate faradaic efficiencies (FE) and the production rates of PC-deposited Bi

films fabricated at different pulse-cycles (Fig. S7a,b). All the samples were measured in CO2-

purged 0.1 M KHCO3 at -0.8 VRHE. The samples fabricated between 1 and 3 pulse-cycles were

not completely converted to Bi nanoflakes, and they showed FE of 89 and 90%, respectively.

After 6 pulse-cycles were applied, the flake-shaped Bi nanostructures were formed, leading to

increased FE over 100% and production rate of 58.8 μmol/cm2hr. However, when we grew the

tripod-shaped Bi nanostructures by applying 9 pulse-cycles, decreased FE and production rate

were obtained. It suggests that additional Bi layers covering the edges led to worsened

performance in CO2 reduction. From these results, it can be concluded that Bi nanostructures

having large number of edge and corner sites facilitate higher FE and production rate.

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Effect of pulse-current amplitude on Bi nanostructures

Fig. S8. (a) Faradaic efficiency and (b) production rate of PC-deposited Bi films obtained at -0.8 VRHE in CO2-purged 0.1 MKHCO3 electrolyte. The inset (a) is a schematic graph of applied current as function of time for one pulse, and the insets (b) show the SEM images of PC- deposited Bi films fabricated with different pulse-current amplitudes in 6 pulse-cycles. (Scale bar: 300 nm)

In order to determine the effect of pulse-current amplitudes on the production of Bi

nanostructures for CO2 reduction, we measured the formate faradaic efficiencies (FE) and the

production rates of PC-deposited Bi films fabricated with different pulse-current amplitudes in 6

pulse-cycles (Fig. S8a,b). The higher current applied to pulse-electrodeposition, the higher

faraday efficiency and production rate. Samples synthesized below 20 mA/cm2 pulse-current

amplitude were not completely converted to Bi nanoflake, and the sample prepared at 5 mA/cm2

exhibited low faraday efficiency by exposing the Cu substrate. This result indicates that high

applied currents in PC- deposition assist in the production of sharp Bi nanostructures and thus

are effective for CO2 reduction.

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Shape of Bi nanoflakes

Fig. S9. (a) PC-6c deposited Bi film was observed by angle-tilted SEM measurement. (b) Images were obtained by rotating the area measured in (a) by 90 degrees horizontally; yellow boxes indicate the identical region. (c) Schematic images of Bi nanoflakes, a number of edge and corner sites based on thin hexagonal nanostructures.

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Simulated electric field distribution

Fig. S10. Simulated electric field distribution in two-dimensional Bi nanostructures; strong electric field is formed as the shape is sharper. Even at low applied potentials, the electric field of a sharp structure (corner angle 10o) is 1.7 times stronger than that of a blunt structure (Corner angle 60o). Moreover, a blunt structure requires 0.7 V applied potential to achieve 6.0 × 105 V/m, but a sharp nanostructure can produce the same amount of electric field with a small applied potential of 0.4V

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Table S1. Capacitance values and surface roughness factors measured using CV for selected electrodes discussed in this report. The surface roughness factor for DC-60s is defined to be 1.

Electrode preparation Capacitance Surface Roughness Factor

Direct current (60 sec): DC-60s 0.398 μF/cm2 1

Direct current (120 sec): DC-120s 40.4 μF/cm2 101

Pulse current (1 cycle): PC-1c 0.404 μF/cm2 1.01

Pulse current (3 cycles): PC-3c 1.46 μF/cm2 3.67

Pulse current (6 cycles): PC-6c 82.2 μF/cm2 206

Pulse current (9 cycles): PC-9c 13.1 μF/cm2 32.9

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Table S2. List of electrocatalysts that show current densities based on the geometric surface area (nominal area).

Electrocatalysts Electrolyte Ref.

Reduced Cu2O film 0.5 M NaHCO3 [1]

Sn / SnOx film on Ti substrate 0.5M NaHCO3 [2]

Monodisperse Au nanoparticles 0.5 M KHCO3 [3]

Sn nanoparticles 0.1 M NaHCO3 [4]

Electrodeposited Sn dendrite 0.1 M KHCO3 [5]

High-index facets of Au 0.5 M KHCO3 [6]

Oxide-derived Pb nanoparticles 0.5 M NaHCO3 [7]

Electrodeposited Cu2O film 0.1 M KHCO3 [8]

Graphene / Sn quantum / Graphene 0.1 M NaHCO3 [9]

Sn / Carbon fiber of the gas diffusion electrode 0.5 M NaHCO3 [10]

Bi nanodendrite / Carbon paper 0.5 M NaHCO3 [11]

Electrodeposited hexagonal-Zn 0.5 M KHCO3 [12]

Sn/CuxO nanowire 0.5 M KHCO3 [13]

Ag3Sn core / SnOx shell 0.5 M NaHCO3 [14]

Cu composite wires 1 M KHCO3 [15]

There were many reports on catalysts with nanostructures normalizing the current density with

the geometrical surface area instead of ECSA (Table S2). For instance, M. W. Kanan et al.

reported that there was no linear correlation between ECSA of the nanostructured electrode and

the current density. They fabricated Cu nanowires by thermal oxidation and measured

electrochemical surface area. [1] The roughness factor of Cu nanowire was 480 times larger than

that of the Cu foil. Despite of the increase of ECSA by 480, the current density only increased by

30 times due to the current preferentially flow through the active sites such as grain boundaries.

They concluded that the number of active site is more important factor for CO 2 reduction rather

than the increase of surface area through the formation of porosities. From this, it is concluded

that the geometric surface area is more appropriate than ECSA in determining the current density

in the plot with the potential.

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Table S3. Comparison of flat catalysts (foil) with nanostructured catalysts.

Electrocatalyst Electrolyte potential (VRHE)

Faradaic efficiency (%) Ref.

Sn foilSn dendrite 0.1 M KHCO3 -1.36 HCOO- 49.1%

HCOO- 71.6% [5]

Pb foilNanostructured PbOx

0.5 M NaHCO3 -0.8 HCOO- 58%HCOO- 98% [7]

Zn foilZn dendrite 0.5 M NaHCO3 -1.1 CO 32%

CO 79% [16]

Zn foilHexagonal Zn 0.5 M KHCO3 -0.95 CO 25%

CO 85% [12]

Polycrystalline AgNanoporous Ag 0.5 M KHCO3 -0.6 CO 1.1%

CO 92% [17]

Au particleAu nano-needle 0.5 M KHCO3 -0.35 CO 5%

CO 95% [18]

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Table S4. Electrochemical CO2 reduction properties of electrocatalysts for formate production.

Electrocatalysts Electrolyte Potential (VRHE)

Formate Faradaic efficiency

Ref.

Sn Sn/SnOx thin film 0.5 M NaHCO3 -0.70 40% [19]

SnOx/graphene 0.1 M NaHCO3 -1.16 94% [4]

Electroplated Sn/Cu foam 0.1 M KHCO3 -1.19 84% [20]

Electroplated Sn/gas diffusion electrode 0.5 M NaHCO3 -1.10 71% [10]

Electrodeposited Sn powder 0.5 M KHCO3 -1.5 74% [21]

Electrodeposited Sn dendrite 0.1 M KHCO3 -1.3 83% [7]

Nanoporous Sn foam 0.1 M NaHCO3 -1.4 90% [22]

Etched Sn foil 0.1 M KHCO3 -1.7 85% [23]

Sn nanoparticles 0.45 M KHCO3 -1.5 70% [24]

Sn-Pb alloy 0.5 M KHCO3 -1.4 80% [25]

Graphene/Sn/Graphene 0.1 M NaHCO3 -1.8 89% [9]

Ag3Sn nanoparticles 0.5 M NaHCO3 -0.9 87% [14]

In Anodized In 0.5 M KHCO3 -1.5 79% [26]

Co Partially oxidized Co nanoparticles 0.1 M Na2SO4 -0.2 45% [27]

Pb Oxide-derived Pb 0.5 M NaHCO3 -0.8 98% [5]

Roughened Pb 0.1 M KHCO3 -1.0 89% [28]

Pd Pd-Pt nanoparticle 0.1 M KH2PO4 -0.4 88% [29]

Electrodeposited Pd nanoparticles 0.1 M KHCO3 -0.4 55% [30]

Pd nanoneedle 0.5 M KHCO3 -0.2 91% [18]

Pd nanoparticle/ Carbon 0.5 M NaHCO3 -0.25 94% [31]

Bi Bi nanodendrite / Carbon paper 0.5 M NaHCO3 -1.56 96% [11]

Bi nanoparticle/Cu foil 0.1 M KHCO3 -1.46 95% [32]

Bi flake 0.1 M KHCO3 -0.6 99% This work

Table S5. Electrochemical CO2 reduction properties of electrocatalysts prepared by

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electrodeposition method.

Electrocatalysts Electrolyte Potential (VRHE)

Faradaic efficiency Ref.

Sn Electroplated Sn/Cu foam 0.1 M KHCO3 -1.19 HCOO- 84% [20]

Electroplated Sn/GDE 0.5 M NaHCO3 -1.1 HCOO- 71% [10]

Electrodeposited Sn powder 0.5 M KHCO3 -1.5 HCOO- 74% [21]

Electrodeposited Sn dendrite 0.1 M KHCO3 -1.3 HCOO- 83% [7]

Cu-Sn alloy 0.1 M KHCO3 -0.6 CO 95% [33]

Sn nanoparticles on CuO nanowire 0.1 M KHCO3 -0.7 CO 90% [13]

Zn Zn dendrites 0.5 M NaHCO3 -1.1 CO 79% [16]

Hexagonal Zn 0.5 M KHCO3 -1.1 CO 85% [12]

In Cu-In alloy 0.1 M KHCO3 -0.5 CO 90% [34]

Ag Ag nanobranch 0.5 M KHCO3 -0.9 CO 61% [35]

Porous Ag nanofoam 0.5 M KHCO3 -1.1 CO 93.4% [36]

Nanoporous Ag 0.5 M KHCO3 -0.5 CO 90% [17]

Ag3Sn nanoparticle 0.5 M NaHCO3 -0.9 HCOO- 87% [14]

Au Nanoporous Au 0.1 M KHCO3 -0.4 CO 75% [37]

Au nanoneedle 0.5 M KHCO3 -0.4 CO 98% [38]

Cu Cu2O nanoparticle 0.1M KHCO3 -1.1 C2H4 35% [39]

Cu2O film 0.1 M KHCO3 -1.0 C2H4 41 % [8]

Cu composite wire 1 M KHCO3 -0.5 C2H4 40% [15]

Cu mesocrystals 0.1 M KHCO3 -1.0 C2H4 81% [40]

Pd Electrodeposited Pd nanoparticles 0.1 M KHCO3 -0.4 HCOO- 55% [30]

Pd nanoneedle 0.5 M KHCO3 -0.2 HCOO- 91% [18]

Cu/Pd nanoparticle 0.5 M NaHCO3 -0.15 HCOO- 84% [41]

Pd chloride/Cu2O 0.1 M KHCO3 -1.0 C2H6 30% [42]

Bi Bi nanodendrite / Carbon paper 0.5 M NaHCO3 -1.56 HCOO- 96% [11]

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Bi flake 0.1 M KHCO3 -0.6 HCOO- 99% This work

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