Dramatic
HER Su
p
pression on Ag Electrodes via Molecular Films for Highly Selective
CO
2
to CO Reduction
Arnaud Thevenon
‡
, Alonso Rosas
-
Hernandez
‡
, Alex M. Fontani Herreros, Theodor Agapie*
and Jonas C. Peters*
Joint Center for Artificial Photosynthesis
(JCAP) and Division of Chemistry and Chemical
Engineerin
g, California Institute of Tech
nology (Caltech), Pasadena, California 91125,
United States
‡
The authors contributed equally to this work.
S
2
Table of
Contents
Materials
and Methods
3
Electrochemical
Measurements
3
Electrochemical
Active surface Area
(ECSA)
and Partial Current Density E
xperiments
4
X
-
ray Photoelectron S
pectroscopy
(XPS)
5
Atomic Force Microscopy
(AFM)
5
Sca
nning Electron Microscopy
(SEM)
and Energy Dispersive
Spectroscopy
(EDS)
5
Physical Vapor Deposited
Silver Electrode Preparation
(PVD)
5
Supporting Figures and Tables
6
Table S1
.
Faradaic Efficiencies
(FE
s
) for CO
2
RR product
s
and H
2
on Ag at different potential
.
6
Fig.
S1
.
(a) F
E
s
;
(b)
current densities
for CO
2
RR
products
and
H
2
on Ag at different potential
.
7
Table S2
.
FE
s
for CO
2
RR product
s
and H
2
on
Ag
-
1
at different potential.
8
Fig.
S2
.
(a) FEs;
(b)
CO
2
RR
products
and H
2
current densities
on
Ag
-
1
at different potential
.
9
Table S3.
FE
s
for CO
2
RR product
s
and H
2
on
Ag
-
2
at different potential
.
1
0
Fig.
S3.
(a) FE
s;
(b) CO
2
RR
products
and H
2
current densities
on
Ag
-
2
at different potential
.
1
1
Table S4.
FE
s
for CO
2
RR products and H
2
on
Ag
-
1
and
Ag
-
2
with an additive free electrolyte
.
1
2
Table S5.
FEs for CO
2
RR products
and H
2
on
Ag
-
1
at different pH at
–
1.1 V.
1
2
Fig.
S4.
XPS of Ag,
Ag
-
1
and
Ag
-
2
.
1
3
Fig.
S5.
1
H NMR spectrum of the film extracted form
Ag
-
2
.
1
4
Fig.
S6.
1
H
-
1
H COSY NMR spectrum of the film extracted from
Ag
-
2
.
1
4
Fig.
S7.
1
H NMR spectra of the film
extracted
from
Ag
-
2
,
Cu
-
2
and from the reduction of
2
-
C
l
.
1
5
Fig.
S8.
1
H NMR spectrum of the film extracted from
Ag
-
1
.
1
6
Fig.
S9.
1
H
-
1
H COSY NMR spectrum of the film extracted from
Ag
-
1
.
1
6
Fig.
S10.
Stack of
1
H NMR spectra of the film extracted from
Ag
-
1
and
Cu
-
1
.
1
7
Fig. S11.
ESI
-
MS spectrum of the film extracted from
Ag
-
1
.
1
8
Fig.
S1
2
.
Ex
-
situ
SEM images of bare Ag before and after bulk electrocatalysis.
2
0
F
ig.
S1
3
.
Ex
-
situ
SEM images of
Ag
-
1
before and after extracting the organic film.
2
0
Fig.
S1
4
.
Ex
-
situ
SEM images of
Ag
-
1
after 1
h
of bulk electrocatalysis at
–
1.4 V.
2
0
Fig.
S1
5
.
Ex
-
situ
AFM images of bare Ag
,
Ag
-
1
and
Ag
-
2
before
and
after
electrocatalysis
.
2
1
Fig.
S1
6
.
Full Tafel plot.
2
2
Fig. S17.
Examples of voltam
m
ograms a
t
different scan rates between
–
0.1 V and
–
0.2 V.
2
3
Table S6.
Cathodic and anodic current
s
recorded by CVs at different scan rates for Ag.
2
3
Table S7.
Cathodic and anodic current
s
recorded by CVs at different scan rates for
Ag
-
2
.
2
4
Fig. S18.
Example of plot showing the experimentally determined ECSA for Ag and
Ag
-
2
.
2
4
Fig. S19.
Plo
t of
normalized
j
H2
and
j
CO
at
–
0.9 V
for different electrodeposition time of
2
-
Cl
.
2
5
Table S8
.
Summary of data
for EC
SA and
j
H2
at
–
0.9 V at different
electrodeposition time.
2
6
Table S9.
Summary of data for ECSA and
j
CO
at
–
0.9 V at different electrodeposition time.
2
6
Fig. S20.
LSV scans in N
2
-
saturated electrolyte.
2
7
Fig. S21.
LSV scans in CO
2
-
saturated electrolyte.
2
7
Fig. S22.
Plot of the dependence of the rotation rate of the Ag electrode and
j
H2
.
2
8
Fig. S23.
Kutecky
-
Levich plot of
j
H2
.
2
8
Fig. S24.
p
CO
2
dependence of
j
CO
at
–
1.1 V.
29
Fig. S25.
p
CO
2
dependence of
j
CO
at
–
0.9 V (a) before, (b) after correction with ECSA.
29
Fig. S26.
Voltamogram of Ag in CO
2
-
saturated 0.1 M KHCO
3
electrolyte
.
3
0
Fig. S27.
Voltamogram of Ag in CO
2
-
saturated 0.1 M KHCO
3
electrolyte with
1
-
Br
2
.
3
0
Fig. S28.
Voltamogram of Ag in CO
2
-
saturated 0.1 M KHCO
3
electrolyte with
2
-
Cl
.
3
0
Fig. S2
9
.
Plot of
the
EC
SA
against
the electrodeposition time
under N
2
.
3
1
Fig. S
30
.
Plot of
the
j
H2
at
–
1.1 V
against the electrodeposition time
under N
2
.
3
2
Table S
10
.
Summary of data
for ECSA
and
j
H2
at
–
1.1 V
against the
electrodeposition time.
3
2
Table
S11
.
Summary of data for
EC
SA and
j
CO
at
–
1.1 V
against the
electrodeposition time.
3
3
Fig. S
31
.
Plot of the
EC
SA
against
the electrodeposition time
under CO
2
.
3
3
Fig. S
32
.
Plost of
j
CO
at
–
1.1 V and
the electrodeposition time
under CO
2
.
3
4
Fig. S3
3
.
Evolution of FEs for CO and H
2
, and total current density over 24 h.
3
4
References
.
3
5
S
3
Materials
and Methods
All solvents and reagents were obtained from commercial sources (Aldrich and Merck) and
used as received, unless stated otherwise.
The N,N’
-
ethyl
-
1,10
-
phenanthrolinium dibromide
(phenanthrolinium,
1
-
Br
2
) and tolyl
-
pyridinium
chloride
(
2
-
Cl
)
additives were
synthesized
according to previous literature procedures
(1, 2)
and
re
crystallized from MeOH/Ether (1:5)
prior to use.
S
ilver
foil (
99.998
% Ag
, 25 mm
×
25 mm
×
0.1 mm) w
ere
purchased from Alfa
Aesa
r and the
potassium carbonate (99.995%) from Sigma
-
Aldrich. Natural abundance
CO
2
(Research grade) was purchased from Airgas. Deuterium dioxide (D 99.96%), d
-
chloroform (D
99.8%) and d
6
-
dimethylsulfoxide
(D 99.8%) were purchased from Cambridge Isotope
Laboratories.
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
MHz instrument with a prodigy broadband cryoprobe. Shifts were reported relative to the
residual solvent peak.
Upon receiving
,
silver
foil was polished to a mirror
-
like finish using
alumina paste
(0.05 μm, Buehler) followed by rinsing and sonicating in water to remove
residual alumina. Before each experiment, the
silver
foil was
manually polished
using alumina
paste (0.05 μm, Buehler), rinsed with 500 mL of ultra
-
pure water, sonicated for 5 min in
ultra
-
purewater,
subsequently washed with
500 mL of
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) wit
h 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.
Electrochemical Measurements
Chronoamperometry measurements
were carried out in a custom
-
made PEEK flow cell setup
similar to the one reported by Ager et al.
(3)
using a
silver
fo
il as
the
working 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/Ag
Cl reference electrode (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 v
s. 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 t
he Ohmic resistance 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 th
e axis intersection 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, ranging from 40 to 60
Ω
.
All c
hronoamperometric 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 after the measurements.
The ef
fluent 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, and carbon monoxide were detected
by a
S
4
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 products. After electrolysis, the liquid products i
n both catholytes and
anolytes were quantified by both HPLC (Thermo Scientific Ultimate 3000) and
1
H NMR. For
1
H NMR, solutions containing 90% electrolyte and 10% D
2
O (v/v) with internal standard (N,N
-
dimethylformamide 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/AgCl)measured
–
0.197
–
0.059
×
pH, where V
RH
E
,
V
(Ag/AgCl)measured
and pH are potential vs RHE, measured potential vs Ag/AgCl reference
electrod
e and pH of the electrolyte (6.8
).
Linear sweep voltammetry (LSV) and c
yclic
voltammetry
(CV) measurements were recorded
at 25
o
C using a
two
compartment
H
-
cell
, separated with a Selemion
-
AMV anion
-
exchange
membrane,
with a
Ag
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
and contained 10
mM of additive when needed
.
Rotation disk electrode experiments were done in a two
compartments
H
-
cell
using a Ag disc
electrode, a Pt fo
il as a counter electrode and a
Ag/AgCl reference electrode using N
2
-
saturated
KHCO
3
electrolyte
with 10 mM of additive
. The cathodic chamber was separated from the
anodic chamber with an Selemion AMV anion
-
exchange membrane.
The film was generated
first without rotation for 15 min at
–
1.1 V. Chronoamperometry experiments were performed
at different
rotating rate (500 rpm, 1000 rpm, 2000 rpm, 3000 rpm and 5000 rpm) for 15 min
at
–
1.1 V. The average current density over 15 min was used to generate the plot of the
dependence of the rotation rate of the Ag electrode on the HER activity
.
The order in [HC
O
3
–
] was determined by performing
c
hronoamperometry experiments
in the
same flow cell
set
up as previously described
. Bulk electrolysis were done
at
–
0.9 V vs. NHE
,
at different concentration of KHCO
3
varying from 0.1 M to 1 M, with or without 10 mM of
1
-
Br
2
or
2
-
Cl
dissolved in the electrolyte.
As the pH (6.8
at 0.1 M and 7.6 at 1 M) and [K
+
] were
also affected, control experiments were performed. Similar results were obtained by the
addition of HClO
4
and/or KCl to the electrolyte to maintain the same pH and [K
+
], respectively,
across the entire range of [HCO
3
–
] studied. Similar results were also obtained at
–
0.9 V vs RHE.
The CO
2
partial pressure dependence
experiments
were performed
in the same flow
cell set
-
up
as previously described
.
C
hronoamperometry experiments
were performed
at
–
0.9 V vs. NHE,
at different partial pressure of CO
2
varying from 1 SCCM to 5 SCCM. N
2
was used as the
balance gas to keep a total gas flow at 5 SCCM.
As the pH was also
affected
, control
experiment
s
were performed. The trend in the partial current density of CO remained unaffected
by performing the reactions at
–
0.9 V vs RHE or by using HClO
4
to keep the pH constant.
Electrochemical
Active
S
urface Area
(ECSA)
and Partial
Current Density Experiments
All experiments were done in the same flow cell as previously described for bulk electrolysis
experiments.
All HER experiments were done in N
2
-
saturated and all CO
2
RR experiments were
done in CO
2
-
saturated 0.1 M KHCO
3
electrolyte.
We have intentionally investigated HER and
CO
2
RR separately to decouple the mass transfer properties of proton carriers and CO
2
, and thus,
to study the intrinsic effect of the molecular film on HER and CO
2
RR.
The
ECSA
was
determined by CV by
scanning a non
-
Faradaic region from
–
0.1 V to
–
0.2 V at different scan
S
5
rate
(from 10 mV/s to 250 mV/s). The anodic and cathodic current values at
–
0.15 V were
extracted
. The plot of
j
cathodic
and
j
anodic
against the scan rate
w
ere
generated and gave straig
ht
line
s
. The slope of
the curve
s
was used as a
n
estimate of
the number of
ECSA
area
.
The current
densit
ies
of
H
2
w
ere
determined by
b
ulk electrocatalysis either at
–
0.9 V or
–
1.1 V for 15 min
.
CO
partial current densit
ies
w
ere
determined by performing bulk electrocatalysis at
–
0.9 V or
–
1.1 V for 35 min.
The gas mixture was analyzed
by in
-
line GC
,
using
the same method as
previously
described
for bulk electrocatalysis experiments.
ECSA
area
and
bulk electrolysis
were always
per
formed in a fresh, additive
-
free, 0.1 M KHCO
3
electrolyte.
The following protocol was rigorously
followed
for each experiments to ensure data
reproducibility. The
ECSA
area and current densities for H
2
(for HER experiments) or for CO
(for CO
2
RR experiment
s)
were first measured and determined on a freshly polished silver
electrode. The cathodic electrolyte was then changed to a fresh electrolyte containing 10 mM
of
2
-
Cl
. The film was generated by bulk electrolysis at
–
0.7 V for different times (from 10 ms
t
o 1 h
). The cathodic electrolyte was changed for a fresh electrolyte containing no additive. The
ECSA
area and bulk electrolysis were recorded again. The obtained data w
ere
normalized
against each blank silver previously measured.
X
-
ray photoelectron
spectroscopy (XPS)
X
-
ray photoelectron spectroscopy (XPS) data were collected using a Surface Science
Instruments 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
li
ne at 1486.6 eV,
directed at 35° to the sample surface (55° off normal). Emitted photoelectrons were collected
at an angle of 35° with respect to the sample surface (55° off normal) by a hemispherical
analyzer. The angle between the electron collection len
s 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 maintai
ned at < 2
×
10
–
9
Torr. The XPS data were analyzed
using the CasaXPS software.
Post
-
catalysis silver
foils were rinsed with copious amount of
water, dried under a stream of nitrogen and immediately transferred to a nitrogen glove box
before XPS measurement
s.
Atomic Force Microscopy (
AFM
)
All AFM images were recorded on a
Bruker Dimension 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.
Scanning Electron Microscopy
(SEM)
and Energy Dispersive
Spectrometer (EDS
)
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
. EDS
measurements
were done on an Oxford X
-
Max SDD
X
-
ray
EDS
system
.
Physical
vapor deposited (PVD) silver electrode preparation
PVD electrode were used to perform cross sectional SEM imaging of the film. These electrodes
can cleanly be cut without disrupting the film deposited on the surface of the
electrode.