Supporting Information
Controllable Hydrocarbon Formation from the Electrochemical
Reduction of CO2 over Cu Nanowire Arrays
Ming Ma, Kristina Djanashvili, and Wilson A. Smith*
anie_201601282_sm_miscellaneous_information.pdf
Table of Contents
Materials …………………………………………………………………………………..…S3
Local pH ……………………………………………………………………………….…….S3
The schematic illustration of the electrochemical cell …………………………………...…..S4
Digital images of Cu(OH)2 nanowires ………………………………………………...……..S4
The measurement of Cu(OH)2 nanowire length …………………………………….……….S5
The density of nanowire arrays …………...…………………………………………...……..S7
CO2 reduction performance ……………………….....………………………………....…..S10
NMR analysis ………………………….....………………………………………...……...S14
Faradaic efficiency for products ………….....……………………………………...……....S15
S1
Materials
(NH4)2S2O8 (≥98.0%), NaOH (99.99%), KHCO3 (≥99.95%), KClO4 (≥99.99%) and
K2HPO4 (≥99.999%) were purchased from Sigma Aldrich. All chemicals were used in
this study without further purification. Copper foil (99.9999%) were purchased from Alfa
Aesar.
Local pH
CO2 is electrochemically reduced to a variety of major products in CO2 saturated-aqueous
solutions according to the reactions:
𝐶𝑂2 + 𝐻2 𝑂 + 2𝑒 − → 𝐶𝑂 + 2𝑂𝐻 −
(-0.11 V vs. RHE)
(1)
𝐶𝑂2 + 𝐻2 𝑂 + 2𝑒 − → 𝐻𝐶𝑂𝑂− + 𝑂𝐻 −
(-0.03 V vs. RHE)
(2)
2𝐶𝑂2 + 8𝐻2 𝑂 + 12𝑒 − → 𝐶2 𝐻4 + 12𝑂𝐻 −
(0.08 V vs. RHE)
(3)
H2 evolution is a competing reaction with CO2 reduction in CO2-saturated electrolytes. Thus,
water could be reduced to H2 on Cu according to the following reaction:
2𝐻2 𝑂 + 2𝑒 − → 𝐻2 + 2𝑂𝐻 −
(0 V vs. RHE)
(4)
OH– ions are released at the electrode in cathodic reactions according to the above reactions,
resulting in the fact that the pH rises locally at the electrode/electrolyte interface.[1,2] Thus, the
actual pH near the electrode is higher that of the bulk owing to the release of OH– in the
electrode reactions.[1,2]
S2
Figure S1. The schematic illustration of the electrochemical cell used for reduction of CO2
Figure S2. Digital images of Cu(OH)2 nanowires with synthesis time of 1, 2, 3, 5, 8, 15 min,
respectively.
S3
Figure S3. SEM images of Cu(OH)2 nanowires on Cu foils with synthesis time of 0.5 (a), 3
(b), 5 (c) and 8 min (d).
The measurement of Cu(OH)2 nanowire length
Cu(OH)2 nanowire was removed from the surface of Cu foil to carbon tapes (double sides
carbon tape) by pressing Cu foil (nanowires on the surface of Cu foil) on carbon tape, and the
nanowires stuck to the surface of carbon tapes. Thus, the length of Cu(OH)2 nanowire was
measured by using scanning electron microscope (SEM, JEOL JSM-6010LA). Figure S4
shows the nanowires on carbon tape substrates. Here, I measured more than 100 different
nanowires for each sample to get the length histograms of Cu(OH)2 nanowires, as presented in
Figure S5.
Figure S4. SEM image of Cu(OH)2 nanowires with synthesis time of 2 min (a) and 5 min (b).
S4
Figure S5. Length histograms of Cu(OH)2 nanowires in the samples with fabrication time of
(a) 2 min, (b) 3 min, (c) 5 min, (d) 8 min
The average length of nanowire can be calculated as follow:
𝑙 = ∑(𝐿𝑥 × 𝐹𝑥 )
(5)
where 𝑙, 𝐿𝑥 and 𝐹𝑥 are the average length of nanowire, the specific length of nanowire
measured from SEM and the percentage of the specific length of nanowire.
The standard deviation of the length of nanowire can be obtained from equation:
𝜕 = √∑ 𝐹𝑥 (𝐿𝑥 − 𝑙)2
(6)
S5
Table S1. The average length of Cu(OH)2 nanowire under different fabrication time.
Synthesis time (min)
Average Length (µm)
Standard Deviation (µm)
0.5
2.0
1
1
2.4
0.56
2
3
0.87
3
5
1.68
5
7.3
1.3
8
8.1
1.3
15
8.5
1.1
The density of nanowire arrays
CuO nanowires were fabricated by annealing the Cu(OH)2 nanowire arrays at 150 ºC for 2
hours in air.[3] The resulting CuO nanowire arrays were directly used in electrochemical
reduction of CO2, and were electrochemically reduced to Cu nanowires during electrolysis.[3]
The charge used for the reduction of CuO to Cu can be calculated by equation:
𝑚
𝑄 = 2𝐹 × ( )
𝑀
(7)
where, F, M and m are Faraday constant, the molar mass and the mass of CuO, respectively.
This equation (7) can be rewritten as:
𝜌×𝑛×𝑙×𝐴
𝑄 = 2𝐹 × (
)
𝑀
(8)
Where, ρ is mass density of CuO. The n is the total number of nanowire involved in
electrolysis (geometric surface area involved in electrolysis is constant). The 𝑙 and A are the
length and cross-section area of CuO nanowires, respectively. Here, Cu nanowire length is
identical to the corresponding Cu(OH)2 nanowire length and we assume that the A is constant
for all different samples.
The electrodes exhibited an initially high current density as the CuO nanowires were reduced
to Cu nanowires, and subsequently a stable current density during electrolysis, as shown in
S6
Figure S6. Thus, we can get the total charge used for reducing CuO to Cu according to the
initial high current in Figure S5. Q3µm and Q7.3µm are 0.71 C and 1.93 C, respectively. From
the equation (8), we can get
𝑄7.3µm 𝑛7.3µm 𝑙7.3µm
=
𝑄3µm
𝑛3µm 𝑙3µm
(9)
According to the equation (9), n7.3µm ⁄n3µm is 1.12
𝑛8.1µm 𝑙7.3µm 𝑄8.1µm 7.3µm 2.28 C
=
×
=
×
𝑛7.3µm 𝑙8.1µm 𝑄7.3µm 8.1µm 1.93 C
(10)
Here, 𝑛8.1µm ⁄𝑛7.1µm is 1.06.
𝑛8.5µ𝑚 𝑙8.1µ𝑚
𝑄8.5µ𝑚
8.1µ𝑚 3.03 𝐶
=
×
=
×
𝑛8.1µ𝑚 𝑛8.5µ𝑚
𝑄8.1µ𝑚
8.5µ𝑚 2.28 𝐶
(11)
Thus, 𝑛8.5µm ⁄𝑛8.1µm is 1.27.
In this way, we can get the nanowire density as a function of synthesis time, as shown in
Figure S7. The color of the sample is getting darker blue with increasing synthesis time of
nanowires (Figure S2), which is consistent with the fact that the higher density and longer
length of nanowire with increased synthesis time.
Figure S6. CO2 reduction current as a function of time on CuO-derived Cu nanowires with
length of (a) 3 µm and (b) 7.3 µm at -1.1 V vs. RHE, respectively.
S7
Figure S7. Cu(OH)2 nanowire length and density as a function of synthesis time.
S8
CO2 reduction performance
Figure S8. CO2 reduction performance of Cu nanowires. CO2 reduction activity of
polycrystalline Cu (a) and Cu nanowires with increased length (b-f) at -1.1 V vs. RHE in
CO2-saturated 0.1 M KHCO3 electrolytes. The geometric current density is shown on the left
axis and the faradaic efficiency is shown on the right axis (
efficiency for C2H4 and C2H6 respectively).
S9
and
represent faradaic
Figure S9. Faradaic efficiency for C2H4 (a) and H2 (b) on Cu nanowire arrays as a function of
nanowire length at -1.1 V vs. RHE in CO2-saturated 0.1 M KHCO3 electrolytes.
S10
Figure S10. CO2 reduction performance of 8.1-µm-length Cu nanowire arrays at -1.1 V vs.
RHE in CO2-saturated 0.1 M K2HPO4 (a), CO2-saturated 0.1 M KHCO3 (b) and CO2-saturated
0.1 M KClO4 (c), respectively. The geometric current density is shown on the left axis and the
faradaic efficiency is shown on the right axis ( and
and C2H6 respectively).
S11
represent faradaic efficiency for C2H4
Figure S11. Faradaic efficiency for HCOOH, Ethanol and n-Propanol on Cu nanowire arrays
(8.1 ± 1.3 µm) at -1.1 V vs. RHE in CO2-saturated 0.1 M K2HPO4 (pH= 6.5), CO2-saturated
0.1 M KHCO3 (pH= 6.8) and CO2-saturated 0.1 M KClO4 (pH= 5.9) electrolytes, respectively.
Figure S12. The geometric current density as a function of potential in CO2-saturated 0.1 M
KHCO3 (pH= 6.8).
S12
NMR analysis
NMR spectra were obtained at Agilent MR400DD2 NMR spectrometer operating at 400 MHz.
For the preparation of the samples, 50 μL of D2O containing a known concentration of
internal reference t-BuOH were added to 450 μL the electrolyte solutions. Water suppression
hard pulse sequence was applied during the acquisition of the 1H NMR spectra with 2 s
saturation delay, 2 s relaxation delay, 2.5 s acquisition time, and spectral window of 6400 Hz.
After collection of 8 scans, peak areas were integrated and the concentration of the solutes can
be calculated by considering the difference in the number of protons in the reference
compound and that of the product. Chemical shifts (δ) are reported in ppm with respect to
internal standard set at 1.2 ppm. Thus, the 1H NMR spectra (Figure S12) allows for the
identification of products.
Table S2. NMR data used for the calculation of concentration of the products.
Compound
Number of H
Chemical Shift
(n)
(δ)
t-BuOH (ref)
9
1.2
singlet
-
Formic Acid
1 (HCO)
8.45
singlet
-
Ethanol
3 (CH3)
1.13
triplet
7.24
2 (CH2-O)
3.52
quartet
7.24
3 (CH3)
0.84
triplet
7.76
2 (CH2)
1.41
multiplet
7.44
2 (CH2-O)
3.43
triplet
6.66
n-Propanol
Multiplicity
Coupling
constant (Hz)
The concentration of products can be calculated by equation:
9 × 𝐶𝑅𝑒𝑓
𝑛 × 𝐶𝑝𝑟𝑜𝑑𝑢𝑐𝑡
=
(𝑃𝑒𝑎𝑘 𝑎𝑟𝑒𝑎)𝑅𝑒𝑓 (𝑃𝑒𝑎𝑘 𝑎𝑟𝑒𝑎)𝑝𝑟𝑜𝑑𝑢𝑐𝑡
(12)
where Cproduct and n are the concentration of product and the number of corresponding protons
in the of related product (Table 2S), respectively. CRef is the concentration of reference tBuOH (0.0825 mM). (Peak area)Ref and (Peak area)product represent the integrals for t-BuOH
(reference) and product in the 1H NMR spectra, respectively. Thus, n will be 9 (3xCH3) for
tBuOH and 1 (COH), 3 (CH3), and 3 (CH3) at chemical shifts of 8.45, 1.13 and 0.84,
respectively.
S13
Figure S13. 1H NMR spectra of electrolytes after reduction of CO2 on 7.3-µm-length Cu
nanowires in CO2-saturated 0.1 M KHCO3 electrolytes.
Faradaic efficiency for products
Table S3. Faradaic efficiency for C2H4, C2H6, CO, HCOOH, Ethanol, n-Propanol and H2 on
Cu nanowire arrays with different lengths at -1.1 V vs. RHE in CO2-saturated 0.1 M KHCO3
electrolytes (0 µm nanowire represents Cu foil).
Average Length
(µm)
Faradaic efficiency (%)
C2H4
C2H6
CO
HCOOH
Ethanol
n-Propanol
H2
0 (flat Cu)
2
6
12.7
83
2.0
5
9.9
18.4
63.7
2.4
10.4
10
19
5.8
52
3
14.2
10
20.7
7.8
48.4
5
14.4
7
23.5
8
46
7.3
16.6
2
8.5
15.5
5
9
44.3
8.1
17.4
2.4
7.6
17.5
3.8
7.8
44.2
S14
REFERENCES
[1]
Y. Hori, in Electrochemical CO2 Reduction on Metal Electrodes (Ed.: E. Vayenas, C.
G., White, R. E., Gamboa-Aldeco, M. E.), Springer New York, 2008, p. Vol. 42, p 89.
[2]
Y. Hori, A. Murata, R. Takahashi, Journal of the Chemical Society, Faraday
Transactions 1: Physical Chemistry in Condensed Phases 1989, 85, 2309.
[3]
M. Ma, K. Djanashvili, W. A. Smith, Phys. Chem. Chem. Phys. 2015, 17, 20861–
20867.
S15