of 30
In
-
situ Nanostructuring and Stabilization of Polycrystalline Copper by an Organic Salt
Additive Promotes Electrocatalytic CO
2
Reduction to Ethylene
Arnaud Thevenon, Alonso Rosas
-
Hernández
, Jonas
C.
Peters*, Theodor Agapie*
Division of Chemistry and Chem
ical Engineering and Joint Center for Artificial
Photosynthesis, California Institute of Technology, Pasadena, California 91125, United
States
2
Table of Contents
Materials.
................................
................................
................................
..............................
4
Synthetic Procedures
................................
................................
................................
............
4
Electrochemical Measurements.
................................
................................
..........................
5
X
-
ray photoelectron spectroscopy (XPS)
................................
................................
.............
6
Atomic Force Microscopy (AFM)
................................
................................
........................
6
Scanning Electron Microscopy (SEM) and Energy Dispersive X
-
ray (EDX)
....................
7
X
-
ray Diffract
ion (XRD)
.................................................................................
7
Supporting Figures and Tables
................................
................................
............................
7
Table S1.
Faradaic efficiency (%) for CO
2
RR products without and with 10 mM
1
-
Br
2
.
................
8
Figure S1.
1
H NMR spectrum of
1
-
Br
2
.
................................
................................
.........................
8
Figure S2.
GC
-
MS analyses
and
1
H NMR spectra products
collected
during CO
2
RR
with natural
abundance
and
13
C
-
e
nriched
CO
2
-
saturated KHCO
3
.
................................
................................
......
9
Figure S3.
Chronoamperograms of electrolysis
with
1
-
Br
2
at
different potentials
.
........................
10
Table S2.
Faradaic effi
ciency (%) for CO
2
RR products
with
1
-
Br
2
at different potentials.
............
10
Table S3.
Faradaic efficiency (%) for CO
2
RR products with
1
-
Br
2
at different concentration
......
.
1
1
Figure S4.
Chronoamperograms
of electrolysis
with
1
-
Br
2
at
different concentration
.
..................
11
Figure S5.
Plot
of the Faradaic efficiencies for gaseous product
s
over 10 h.
................................
.
12
Figure S6.
Chronoamperograms
for 10 h
of electrolysis
.
................................
..............................
12
Figure S7.
Plot
of the
Faradaic
efficien
cies for gaseous product
s
over 43
h.
................................
.
13
Figure S8.
Chronoamperograms
for 43 h
of electrolysis.
................................
..............................
13
Figure S
9
.
Normalized X
-
ray photoelectron
spectra
of Cu after electrolysis
.
................................
14
F
igure S1
0
.
Proposed structure
s
and
1
H NMR spectrum
of the dimers
.
................................
........
15
Figure S1
1
.
1
H
-
1
H COSY spectrum of
the dimers
.
................................
................................
.......
16
Table S4.
Faradaic efficiency (%) towards differe
nt products under different conditions.
.............
17
Figure
S
1
2
.
Rotating disk electrode results under N
2
................................
................................
...
18
Figure S13.
Rotating disk electrode results under CO
2
.....................................................18
Figure S14.
Ex
-
situ A
FM images before and after catalysis with and without
1
-
Br
2
..................18
Figure S15
.
Typical normalized XRD pattern of (a) an electropolished Cu electrode; post catalysis
Cu electrodes
(b) without additive; (c) with
1
-
Br
2
.
.....................................
...........
............................
19
Figure S16
.
Ex
-
situ AFM images of the same post catalysis Cu electrode after 43 h of
electroreduction at
-
1.07 V (a) before and (b) after the extraction of the organic film.
...................
19
Figure S1
7
.
Ex
-
situ AFM images
of copper electrode in presence of
KBr.
................................
...
20
Table S
5
.
Faradaic efficiency (%) for CO
2
RR products and hydrogen
with
KBr.
..........................
20
Table S
6
.
Faradaic efficiency
(%) for CO
2
RR products and hydrogen
with
1
-
X
2
.
.........................
21
Figu
re S18
.
Ex
-
situ
AFM images of Cu electrodes with (a)
1
-
Cl
2
; (b)
1
-
(OTf)
2
; (c)
1
-
I
2
; (d)
1
-
I
2
after extracting the organic film.
..............................................................................2
2
Table S7
Faradaic efficiency (%) for CO
2
RR products after extracting the film and submitting the
same electrode with no
1
-
Br
2
in the electrolyte.
................................
................................
............
2
2
Table S8
.
Faradaic efficiency (%) for CO
2
RR products with
1
-
Br
2
and submitting the same
electrode with no
1
-
Br
2
in the electrolyte.
................................
................................
....................
2
3
Dimerization Mechanism
................................
................................
................................
...
24
Fig
ure S1
9
.
Cyclic voltammogramms of
1
-
Br
2
under CO
2
.
................................
..........................
25
Figure S20
.
1
H NMR spectra
of the extracted films obtained after CV experiments.
.....................
26
Figure S21
.
Pictures of the
electrolyte containing
1
-
Br
2
befo
re and after catalysis
.
......................
26
Figure S
22
.
Stack of
1
H NMR spectra
of
electrolyte
after catalysis
, the organic precipitate extracted
from the counter electrode, and the organic film extracte
d from the
Cu electrode
.
.........................
27
3
Figure S
23
.
Cyclic voltammograms on a glassy carbon electrode of
1
-
Br
2
.
................................
..
27
Reactivity studies of 1
-
Br with CO
2
................................
................................
..................
28
Figure S2
4
.
Visible spectra of
1
-
Br
2
and
1
-
Br
in water.
................................
..............................
28
Figure S2
5
.
Visible spectra of
1
-
Br
under N
2
and under CO
2
in 0.1 M KHCO
3
.
..........................
28
Figure S2
6
.
EPR spectra of
1
-
Br
under CO
2
and N
2
.
................................
................................
...
29
References
................................
................................
................................
...........................
30
4
Materials
All solvents and reagents were obtained from commercial so
urces (Aldrich and Merck) and
used as received, unless stated otherwise.
Phenanthrolinium additives were
synthesized
according to previous literature procedures
,
[1
5]
and
re
crystallized from MeOH/Ether (1:5) prior
to use.
Copper foil (
product number 26674
4,
99.999% Cu, 25 mm
×
50 mm
×
1 mm), phosphoric acid
(85%, TraceSelect), potassium carbonate (99.995%), potassium hydroxide (semiconductor
grade, 99.99%
trace metals basis) and
13
CO
2
(99 atom %
13
C, <3 atom %
18
O) were purchased
from Sigma
-
Aldrich. Carbon
rods (99.999% C) were purchased from Strem Chemicals.
Natural
abundance
CO
2
(Research grade) was purchased from Airgas. Deuterium dioxide (D 99.96%),
d
-
chloroform (D 99.8%) and d
-
dimethylsulfoxide
(D 99.8%) were purchased from Cambridge
Isotope Laboratori
es.
Water was purified by a Nanopure Analytical Ultrapure Water System (Thermo Scientific) or
a Milli
-
Q Advantage A10 Water Purification System (Millipore) with specific resistance of
18.2 M
Ω·
cm at 25 °C.
1
H and
13
C NMR spectra were recorded on a Bruker 400 M
Hz instrument with a prodigy
broadband cryoprobe. Shifts were reported relative to the residual solvent peak.
Upon receiving
,
copper foil was polished to a mirror
-
like finish using alumina paste (0.05 μm,
Buehler) followed by rinsing and sonicating in wa
ter to remove residual alumina. Before each
experiment, the copper foil was electropolished using a method similar to the one employed by
Kuhl et al.
[6]
In a
n
85% phosphoric acid bath, +2.1 V versus a carbon rod counter electrode was
applied to the Cu foil
for 5 minutes and the foil was subsequently washed with ultra
-
pure water
and dried under a stream of nitrogen gas.
Potassium bicarbonate electrolyte (KHCO
3
(aq), 0.1 M)
was
prepared by sparging an aqueous
solution of potassium carbonate (K
2
CO
3
(aq), 0.05 M
) with CO
2
for at least 1 hour prior to
electrolysis.
This
process converts K
2
CO
3
into KHCO
3
and saturates the electrolyte solution
with CO
2
.
The proper organic salt
additive
was
added to the 0.1 M KHCO
3
(aq) catholyte
(unless
otherwise stated [additive] =
10 mM)
whereas 0.1 M KHCO
3
(aq) without any
additives
was
used as the anolyte.
Synthetic Procedures
Synthesis of N,N’
-
ethyl
ene
-
phenanthrolinium dibromide
(1
-
Br
2
)
In a round bottom flask charged with a magnetic stir bar,
phenant
h
roline (
500
m
g, 2.8
mmol, 1
equiv.
) was di
ssolved
in di
bromoethane (5 mL, 67.4 mmol, > 24 equiv.)
and the final mixture
was heated to
110 °C for 18 h
. The precipitate formed was collected by filtration and washed
with hexane (3 x 10 mL) and acetone (3 x 10 mL) to
afford the final pr
oduct.
Yield: 970
m
g
(94
%,
2.6
mmol).
1
H and
13
C NMR spectra were in accordance with reported
values
.
[1
3]
Synthesis of
N,N’
-
ethylene
-
phenanthrolinium ditriflate
[1
-
(OTf
)
2
]
In a
flame dried
Schlenk
flask charged with a magnetic stir bar
,
under N
2
and in
absence of
light
, phenant
h
rolinium dibromide (200 mg, 0.5 mmol, 1 equiv.) was dissolved in dry
acetonitrile (10 mL). Silver triflate (280 mg, 1
.0
mmol, 2 equiv.)
was added and
the final
5
mixture was
stirred
for 18 h
, at 25 °C
. The
AgBr precipitate was disc
arded by filtration. The
solvent was evaporated yielding a brown powder. Yield: 212 mg (84 %, 0.4
mmol).
1
H and
13
C
NMR spectra were in accordance with reported
values
.
[4]
Synthesis of
N,N’
-
ethylene
-
phenanthrolinium dichloride
Phenanthrolinium dibromide (
100
mg,
0.25 mmol, 1
equiv.) was dissolve
d
in
1.0
mL of water
and el
uted several times through an
ion exchange resin (Amberlite IRA
-
400 chloride form)
.
Yield:
70 mg (100 %, 0.25 mmol)
.
1
H and
13
C NMR spectra were in accordance with reported
literatures
.
[5]
Synthesis of N,N’
-
ethyl
ene
-
phenanthrolinium
diiodide
Phenanthrolinium dibromide (
100
mg,
0.27
mmol) was dissolve
d
in
10
mL of water and eluted
several times through a pre
-
washed ion exchange resin (Amberlite IRA
-
400 chloride form) with
HI.
After removal
of
the water under vacu
um
, 1
12
mg of a red solid were obtained
(
91
%
y
ield
)
.
1
H NMR (400 MHz, D
2
O)
δ
(ppm): 8.78 (dd,
2
J
H
-
H
= 4.5 Hz,
3
J
H
-
H
= 1.7 Hz, 2H)
,
8.10 (dd,
2
J
H
-
H
= 8.31 Hz,
3
J
H
-
H
= 1.7 Hz, 2H), 7.53 (m, 2H), 7.45 (s, 2H), 4.42 (m, 1H), 3.88 (m, 1H), 3.68
(m, 2H), 3.29 (m, 2H).
13
C NMR (101 MHz, D
2
O)
δ
(ppm): 148.2 (s), 141.1 (s), 137.7 (s), 127
.9
(s), 125.9 (s), 123.6 (s), 46.3 (s), 44.1 (s), 32.9 (s).
Electrochemical Measurements
Chronoamperometry measurements were carried out in a custom
-
made PEEK flow cell setup
similar to the one reported by Ager et al.
[7]
using a copper foil as
the
workin
g electrode and a
platinum foil as
the
counter electrode. The cathode compartment was separated from the anode
compartment by a Selemion AMV anion
-
exchange membrane (AGC Engineering Co.). All
potentials were measured versus a leakless Ag/AgCl reference ele
ctrode (Innovative
Instrume
nts) with an outer diameter of 5
mm that was inserted into the cathode compartment.
The reference electrode was calibrated against ferrocenecarboxylicacid in a 0.2 M phosphate
buffer solution at pH 7.0 (+0.239 V vs. Ag/AgCl).
All
electrochemical measurements were
carried out using a Biologic VMP3 multichannel potentiostat.
Potentio
static
e
lectrochemical impedance
spectroscopy (PEIS)
measurements were carried out
prior to
each
electrolysis experiment to determine the Ohmic resista
nce of the flow cell. The
impedance measurements were carried out at frequencies ranging from 200 kHz to 100 MHz
to measure the solution resistance. A Nyquist plot was plotted and in the high
-
frequency part a
linear fit was performed and the axis intersect
ion was calculated. The value of this intersection
represents the Ohmic resistance of the cell. An average of 3 measurements was taken to
calculate the value of R. Typically, small resistances were measured, rang
ing from 40 to 60
Ω
.
All chronoamperometric experiments were performed for
65 min
at 25 °C
using CO
2
-
saturated
0.1 M KHCO
3
as electrolyte
. The
potentiostat was set to compensate for 85 % of the Ohmic
drop, with the remaining 15 % being compensated for af
ter the measurements.
The effluent gas
stream coming from the flow cell (5
mL
/min) was flowed into the sample loops of a gas
chromatograph (GC
-
FID/TCD, SRI 8610C, in Multi Gas 5 configuration) equipped with
HayeSep D
and Molsieve 5A columns. Methane, ethyl
ene, ethane and carbon monoxide were
detected by a methanizer
-
flame ionization detector (FID) and the hydrogen was detected by a
thermal conductivity detector (TCD).
Every 15 minutes,
2 mL of gas was sampled to determine
the concentration of gaseous produc
ts. After electrolysis, the liquid products in both catholytes
and anolytes were quantified by both HPLC (Thermo Scientific Ultimate 3000) and
1
H NMR.
6
For
1
H NMR, solutions containing 90% electrolyte and 10% D
2
O (v/v) with internal standard
(N,N
-
dimethylfo
rmamide or dimethylsulfoxide) were prepared and measured using a water
suppression technique on a Bruker 400 MHz NMR spectrometer.
All potentials were converted from the Ag/AgCl scale to the reversible hydrogen electrode
(RHE) scale by using V
RHE
= V
(Ag/A
gCl)measured
-
0.197
-
0.059
×
pH, where V
RHE
, V
(Ag/AgCl)measured
and pH are potential vs RHE, measured potential vs Ag/AgCl reference electrode and pH of the
electrolyte (6.8).
Cyclic
voltammetry
(CV) measurements were recorded at 25
o
C using a one
-
compa
rtment cell
with a Cu
or a glassy carbon
disk working electrode
s
(diameter 3 mm), Pt counter electrode,
and a Ag/AgCl reference elect
rode. The electrolyte solutions were
either CO
2
or N
2
saturated
0.1 M KHCO
3
in H
2
O
and were stirred during all measurements
.
For
isotopic
labeling experiments the same experimental configurations as described above
were employed except KH
13
CO
3
(aq) solution and
13
CO
2
were used as the electrolyte and CO
2
source, respectively. To prepare the 0.1 M KH
13
CO
3
(aq) solution, 50 mL of
nanopure water
was sparged with nitrogen for 1 h and was added to potassium hydroxide (0.32 g containing
12.6% water) in a Schlenk flask under nitrogen atmosphere. The headspace was evacuated for
a few seconds, and
13
CO
2
was introduced. The solution was st
irred vigorously for 5 h and an
aliquot was extracted to make sure the pH was ~7. The solution was then added to a pre
-
evacuated
4 mL vial containing
1
-
Br
2
to yield the final electrolyte solution of 0.1 M
KH
13
CO
3
(aq) and 10 mM
1
-
Br
2
. During the electrolysi
s,
13
CO
2
was introduced from the bottom
of the flow cell at 5 ml/min. The outlet was connected to the inlet of the sample loop of the GC
-
FID/TCD for quantitative analyses every 15 min. To collect the gaseous products for GC
-
MS
and NMR analyses, the outlet
of the GC sample loops was connected to a syringe with rubber
plunger pulled by a syringe pump set to the same rate as the gas flow. GC
-
MS analyses was
performed using an Agilent 7820A GC coupled with a 5977E MS with a heated cold quadrupole
detector and a
capillary CarbonPLOT column for identification of the mass fragmentation of
ethylene.
The background signal was subtracted.
X
-
ray photoelectron spectroscopy (XPS)
X
-
ray photoelectron spectroscopy (XPS) data were collected using a Surface Science
Instru
ments M
-
Probe ESCA controlled by Hawk Data Collection software (Service Physics,
Bend OR; V7.04.04). The monochromatic X
-
ray source was the Al K
α
line at 1486.6 eV,
directed at 35° to the sample surface (55° off normal). Emitted photoelectrons were collect
ed
at an angle of 35° with respect to the sample surface (55° off normal) by a hemispherical
analyzer. The angle between the electron collection lens and X
-
ray source is 71°. Low
-
resolution survey spectra were acquired between binding energies of 1
-
1000 eV
. Higher
-
resolution detailed scans, with a resolution of ~0.8 eV, were collected on individual XPS lines
of interest. The sample chamber was maintained at < 2
×
10
-
9
Torr. The XPS data were analyzed
using the CasaXPS software. Copper foils after electropol
ishing or electrolysis were rinsed with
copious amount of water, dried under a stream of nitrogen and immediately transferred to a
nitrogen glove box before XPS measurements.
Atomic Force Microscopy (
AFM
)
7
All AFM images were recorded on a
Bruker Dimensi
on Icon
using the ScanAssyst
mode
. A
scanassyst
-
air canteliver was used with a spring
constant
of 0.4 N/m and a resonant frequency
of 70 KHz.
AFM images were acquired at a scan rate of 0.977 Hz applying a
peak
force of
1.2
nm
over 10
μ
m
with 512 samples pe
r line
.
Scanning Electron Microscopy (SEM)
and Energy Dispersive X
-
ray
(EDX)
All SEM images were recorded on a ZEISS 1550VP FESEM instrument, equipped with in
-
lens
SE, below
-
lens SE, variable pressure SE and Robinson
-
type BSE detectors. EDX
measurements
were done on an Oxford X
-
Max SDD X
-
ray Energy Dispersive Spectrometer
(EDS) system
.
X
-
ray Diffraction (XRD)
The crystal structures were determined through XRD measurements using a Bruker
DISCOVER D8 diffractometer with Cu K
α
radiation from a Bruker ImS
source (50 kV voltage
and 1000
μ
A current). With a 0.3 mm collimator and 6
°
incident angle, a two theta scan mode
was used and the effective thin film measurement foot print was approximately 3 mm. The
grazing x
-
ray diffraction was measured using the same
two theta scan mode but with a 0.1 mm
collimator and 0.5
°
incident angle. Diffraction images were collected using a two
-
dimensional
V
Å
NTEC
-
500
detector
and
integrated
into
one
-
dimensional
patterns
using
DIFFRAC.SUITE
EVA software.
Supporting Figures
and Tables
8
Table S
1
.
Faradaic efficiency (%) for CO
2
RR products and hydrogen obtained during CO
2
RR without
and with 10 mM
1
-
Br
2
in a CO
2
saturated 0.1 M KHCO
3
electrolyte at
-
1.07 V.
Additive
Faradaic Efficienc
y (%)
j
(mA
/
cm
2
)
Run
H
2
CO
HCOOH
CH
4
C
2
H
4
C
2
H
5
OH
C
3
H
7
OH
C
2
Total
-
1
36.4
1.1
3.4
22.0
15.8
8.2
3.2
29.7
92.6
-
5.0
2
49.4
2.9
2.7
14.0
11.1
7.6
2.7
25.2
97.1
-
4.2
3
42.5
2.6
2.6
24.7
10.1
5.9
2.6
23.0
96.6
-
4.2
Average
42.8
2.2
2.9
20.2
12.3
7.2
2.8
26.0
95.4
-
4.5
1
-
Br
2
1
17.8
0.3
6.2
0.0
45.2
14.7
3.1
63.1
87.3
-
3.9
2
13.6
1.0
6.1
0.1
45.6
15.4
3.8
64.7
85.5
-
3.5
3
15.2
0.7
6.3
0.0
45.4
13.8
3.8
62.9
85.1
-
3.8
Average
15.5
0.7
6.2
0.1
45.4
14.6
3.6
63.6
86.0
-
3.
8
Figure
S
1
.
1
H NMR spectrum of
1
-
Br
2
(D
2
O, 298 K).
0
.
0
0
.
5
1
.
0
1
.
5
2
.
0
2
.
5
3
.
0
3
.
5
4
.
0
4
.
5
5
.
0
5
.
5
6
.
0
6
.
5
7
.
0
7
.
5
8
.
0
8
.
5
9
.
0
9
.
5
f
1
(
p
p
m
)
N
,
N
'
-
e
t
h
y
l
-
p
h
e
n
a
n
t
r
o
l
i
n
i
u
m
(
a
d
d
i
t
i
v
e
5
)
2
.
0
8
0
.
9
6
0
.
9
9
0
.
9
8
1
.
0
0
4
.
7
0
D
2
O
5
.
6
1
8
.
5
2
8
.
6
1
9
.
4
4
9
.
5
5
9
Figure
S
2
.
(a) GC
-
MS analyses of ethylene
generated during CO
2
RR at
-
1.07 V with
10 mM of
1
-
Br
2
with natural abundance (blue) and
13
C
-
enriched
(red)
CO
2
-
sat
urated KHCO
3
(0.1 M).
1
H NMR spectra
(H
2
O:D
2
O = 9:1, 298 K) of ethanol
produced
under the same electrocatalytic conditions with
1
3
C
-
enriche
d
CO
2
(b) and with natural abundance CO
2
-
saturated KHCO
3
(c).
10
Figure S
3
.
Chronoamperog
rams of electrolysis on a
Cu
electrode in a
CO
2
-
saturated 0.1 M KHCO
3
electrolyte with 10 mM of
1
-
Br
2
at
different potentials.
Table S
2
. Faradaic efficienc
ies
for CO
2
RR
products and hydrogen obtained during
catalytic runs in the
presence of
10 mM
1
-
Br
2
at
different potentials
.
The results are from the average of three runs
.
Potential (V)
Faradaic Efficiency (%)
j
(mA cm
-
2
)
H
2
CO
HCOOH
CH
4
C
2
H
4
C
2
H
5
OH
C
3
H
7
OH
C
2
Total
-
1.19
40.5
0.2
0.1
0.8
28.0
19.0
1.2
48.2
90.7
-
7.4
-
1.15
2
5.2
0.4
1.5
0.4
39.7
20.7
1.8
62.2
89.8
-
6.4
-
1.11
19.1
0.6
2.5
0.3
44.0
17.9
2.0
64.0
86.5
-
6.9
-
1.07
15.5
0.7
6.2
0.1
45.4
14.6
3.6
63.6
86.0
-
3.8
-
0.99
21.6
4.7
21.8
n.d.
26.9
n.d.
n.d.
26.9
75.0
-
1.6
-
0.90
22.3
14.9
35.1
n.d.
11.0
n.d.
n.d.
11.0
83
.2
-
0.6
11
Table
S
3
.
Faradaic efficiency (%)
for
CO
2
RR products and hydrogen
obtained
at
-
1.
07
V
us
ing
different concentration of
1
-
Br
2
.
Figure
S
4
.
Chronoamperograms of electrolysis on a
Cu
el
ectrode in a
CO
2
-
saturated 0.1 M KHCO
3
electrolyte with
different concentrations
of
1
-
Br
2
at
-
1.
07
V
.
Concentration
Faradaic Efficiency (%)
j
(mA cm
-
2
)
H
2
CO
HCOOH
CH
4
C
2
H
4
C
2
H
5
OH
C
3
H
7
OH
C
2
Tot
al
10 mM
15.5
0.7
6.2
0.1
45.4
14.6
3.6
63.6
86.0
-
3.8
1 mM
25.0
1.1
5.9
0.1
41.6
16.5
8.1
66.2
97.2
-
3.2
0.1 mM
28.9
0.8
7.4
0.4
34.3
13.8
6.3
54.4
91.9
-
3.7
12
Figure S
5
.
Plot
of the Faradaic efficiencies for gaseous product
s
over 10
h on a
Cu
electrode in a
CO
2
-
saturated 0.1 M KHCO
3
electrolyte with 10 mM of
1
-
Br
2
at
-
1.
07
V.
Figure S
6
.
Chronoamperograms of electrolysis on a
Cu
electrode in a
CO
2
-
saturated 0.1 M KHCO
3
electrolyte with 10 mM of
1
-
Br
2
for 10 h.