Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper

Article abstract of DOI:10.1002/anie.201912412

Highly alkaline electrolytes have been shown to improve the formation rate of C₂₊ products in the electrochemical reduction of carbon dioxide (CO₂) and carbon monoxide (CO) on copper surfaces, with the assumption that higher OH^? concentrations promote the C?C coupling chemistry. Herein, by systematically varying the concentration of Na⁺ and OH^? at the same absolute electrode potential, we demonstrate that higher concentrations of cations (Na⁺), rather than OH^?, exert the main promotional effect on the production of C₂₊ products. The impact of the nature and the concentration of cations on the electrochemical reduction of CO is supported by experiments in which a fraction or all of Na⁺ is chelated by a crown ether. Chelation of Na⁺ leads to drastic decrease in the formation rate of C₂₊ products. The promotional effect of OH^? determined at the same potential on the reversible hydrogen electrode scale is likely caused by larger overpotentials at higher electrolyte pH.

Full text of DOI:10.1002/anie.201912412

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Accepted Article
Title: Hydroxide is not a promoter of C2+ product formation in
electrochemical reduction of CO on Copper
Authors: Qi Lu, Jing Li, Donghuan Wu, Arnav S. Malkani, Xiaoxia
Chang, Mu-jeng Cheng, and Bingjun Xu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201912412
Angew. Chem. 10.1002/ange.201912412
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Angewandte Chemie International Edition
10.1002/anie.201912412
RESEARCH ARTICLE
Hydroxide is not a promoter of C2+ products formation in
electrochemical reduction of CO on Copper
Jing Li, Donghuan Wu, Arnav S. Malkani, Xiaoxia Chang, Mu-Jeng Cheng, Bingjun Xu,* and Qi Lu*
Abstract: Highly alkaline electrolytes have been shown to improve
Adapting this flow cell configuration, a recent study reports that
the C product formation in CO electrolysis using Cu catalyst can
the formation rate of C2+ products in the electrochemical reduction of
2
2
carbon dioxide (CO
2
) and carbon monoxide (CO) on copper surface,
be significantly improved by using a more concentrated alkaline
electrolyte[5a]. A similar phenomenon is also observed in the
-
with the assumption that higher OH concentrations promote the C-C
[7]
coupling chemistry. In this work, we demonstrate that higher
electroreduction of CO , a reduction reaction that is believed to
share the same pathway as CO electroreduction after CO is
converted to the surface adsorbed CO (COads) . While these
results seem to suggest a quite feasible way to promote CO
+
-
concentrations of cations (Na ), rather than OH , exert the main
promotional effect on the production of C2+ products by systematically
2
2
[
8]
+
-
varying the concentration of Na and OH at identical absolute
electrode potential. The impact of the nature and the concentration of
cations on the electrochemical reduction of CO is supported by
2
electroreduction towards more valuable products, i.e., to increase
the electrolyte pH, major inconsistencies exist between this
hypothesis and the current mechanistic understanding of the
formation of C2+ products[8e, 9]. As revealed by recent experimental
and computational investigations, the rate determining step
(RDS) of C2+ product formation is the dimerization of two COads
through Langmuir–Hinshelwood process which does not include
a proton transfer[9a, 9b]. Thus, the C2+ product formation rate is not
expected to show such a strong dependence on the electrolyte
pH.
+
experiments in which a fraction or all of Na is chelated by a crown
+
ether. Chelation of Na leads to drastic decrease in the formation rate
-
of C2+ products. The promotional effect of OH determined at the same
potential on the reversible hydrogen electrode scale is likely caused
by larger overpotentials at higher electrolyte pH.
Introduction
The increase of alkaline electrolyte concentration does not
merely increase the concentration of hydroxide, but also the
concentration of the balancing cations, which also could impact
the electrode-mediated reactions. Although the cation effect in
The electroreduction of CO
2
to energy-dense products in aqueous
solution is an attractive strategy to store the intermittent
[1]
renewable electricity in a sustainable fashion . A near-neutral
electrolyte is typically employed to conduct CO electrolysis
because (a) CO reacts with alkaline solution, and (b) many active
catalysts are thermodynamically not stable in acidic
electrocatalysis has been reported recently[10]
,
molecular
2
2
pathways through which cations exert impact remain debated.
According to the classical Gouy-Chapman-Stern (GCS) model,
cations reside at the outer Helmholtz plane (OHP) and the diffuse
layer[11]. Thus, a higher cation concentration of cations in the
electrolyte could increase the density of cations at the OHP, which
[
2]
environment . However, the use of near-neutral electrolyte poses
great challenges in achieving high energy efficiency in
2
commercial CO electrolyzers (e.g., in a membrane electrode
may induce a stronger electrostatic field at the interface[10a, 10d]
.
assembly configuration) due to the current lack of a high-
performance membrane and anode material, i.e., electrocatalyst
Strong field has recently been proposed to stabilize the key
[
3]
for the oxygen evolution reaction (OER), at neutral pH . The
intermediate of CO electroreduction and lower the energy barrier
2
[4]
of C2 products formation[10d]. However, recent work from Koper
and co-workers show, at least in a specific case, the strength of
microfluidic flow cell developed by Kenis’ group , as well as more
[
5]
recent advancements reported by others , enables the usage of
alkaline electrolyte for CO electrolysis, in which the gas-phase
CO is not directly purged into the electrolyte but is in contact with
2
electrostatic field does not change with the concentration of
protons[12]. Thus, the classical picture could be too simplistic to
2
the electrolyte at the catalyst surface, i.e., tripe-phase-boundary[4a, describe the complexity of the electrochemical interface. In
5b, 6]. This configuration effectively extends the pH range of the
electrolyte for CO electrolysis and also greatly enhances the
reaction rate by improving the mass transport[4-5]
addition, the formation rate of C2+ products at different electrolyte
pH should be compared at potentials referenced to the standard
hydrogen electrode (SHE) instead of the reversible hydrogen
electrode (RHE) because the RDS does not include any proton
transfer[8e, 9]. Otherwise the reactions performed at a higher
electrolyte pH are actually subjected to a larger overpotential by
2
.
[
*]
J. Li, D. Wu, Prof. Q. Lu
State Key Laboratory of Chemical Engineering, Department of
Chemical Engineering, Tsinghua University
Beijing 100084, China
E-mail: luqicheme@mail.tsinghua.edu.cn.
A. S. Malkani, X. Chang, Prof. B. Xu
Center for Catalytic Science and Technology, Department of
Chemical and Biomolecular Engineering, University of Delaware
Newark, Delaware 19716, United States
E-mail: bxu@udel.edu.
∆pH×59 mV at room temperature, i.e., 2.3kT∆pH, which would
lead to an increase in the reaction rate. Thus, systematic
investigations are highly desired to deconvolute the influence of
the electrolyte pH, the concentration of cations and the reference
scale with which overpotentials are calculated on the formation
rate of C2+ products in CO
Mechanistic studies of CO
electrolyte is challenging because the multitude of chemical
equilibria among CO , hydroxide, bicarbonate and carbonate
2
electroreduction.
2
electroreduction in an aqueous
Prof. M.-J. Cheng
Department of Chemistry, National Cheng Kung University
Tainan 701, Taiwan
2
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1

Angewandte Chemie International Edition
10.1002/anie.201912412
RESEARCH ARTICLE
+
make the isolation of roles of any specific ion in the reaction
the concentration of Na from 0.1 to 1 M at the same pH of 13.1.
However, the increase (ca. 2-fold) is less drastic than its
counterpart in pure NaOH electrolytes. Importantly, jCORR is
consistently lower in the less alkaline electrolyte with the same
Na+ concentration, which again appears to indicate the positive
difficult and in many cases lead to contradictory conclusions[13]
.
The electrochemical CO reduction reaction (CORR) is more
straightforward because CO does not participate in any
electrolyte reactions. The CORR is generally believed to share
the same reaction pathways and the RDS as the electroreduction
-
effect of the higher OH concentration on the CORR performance.
[
8]
2 2
CO reduction reaction (CO RR) on Cu surfaces , making it a
more appropriate probe reaction for mechanistic investigations.
Using polycrystalline Cu foil electrodes, Wang et al. performed
reactivity studies of CORR with an electrolyte pH of 7 and 13,
respectively, and reported that the onset potentials for C2+
products were similar[8c]. To rationalize these experimental data,
Liu et al. performed computational studies using a microkinetic
model to obtain further insights into the formation of C
However, the experimental results could be convoluted by the
2
products[8d]
.
sluggish mass-transport in their system due to the low solubility of
CO (~ 1 mM), making the kinetic analysis different than those
obtained in systems without mass-transport limitations[9a, 9b].
Herein, we use our recently developed polycrystalline Cu
electrode that employs a gas diffusion mechanism in a standard
three-electrode H-cell[9b] to systematically investigate the C2+
product formation in (a) electrolytes with an equal amount of
hydroxide and different amounts of cations and (b) electrolyte with
different amounts of hydroxide and an equal amount of cations at
both the RHE and the SHE scales. We demonstrate that the
Figure 1. CO electroreduction in various electrolytes. Partial current density
of the CORR in a series of x M NaOH electrolyte (x = 0.1, 0.25, 0.5, 0.75, 1) and
4
0.1 M NaOH + (x - 0.1) M NaClO electrolyte at -0.7 VRHE. The error bars
represent the standard deviation from at least three independent
measurements.
+
-
increase in concentration of Na , rather than OH , promotes the
formation rate of C2+ products when the CORR is conducted at
the same absolute potential.
Comparing the rate of C2+ products in the CORR at the same
absolute potential, i.e., on the SHE scale, reveals that the
+
-
concentration of Na , rather than OH , plays the dominant role in
promoting the reactivity. At -0.7 VRHE, the formation rates for all
Results and Discussion
2+ products are clearly enhanced by the higher Na+
concentration when x increases from 0 to 0.9 in 0.1 M NaOH +
xM NaClO (Figure 2 a), indicating a promotional effect of the
cation. Again, the increase is more pronounced in 1 M NaOH than
that in 0.1 M NaOH + 0.9 M NaClO , which seems to imply that
C
Differentiating the Impact from Hydroxide and Cation on
4
CORR. Reactivity studies based on the linear sweep voltammetry
(LSV) curves (Supporting Information, Figure S3) and
4
chronoamperometry measurements on the RHE scale show that
the CORR rate increases with the Na+ concentration and the
alkalinity of electrolytes. The CORR rate on micron-size Cu
particles supported on carbon paper at -0.7 VRHE increases almost
linearly with the concentration of NaOH in the electrolyte from 0.1
to 0.5 M, and then more gradually when the NaOH concentration
increases to 1 M (blue trace in Figure 1; Supporting Information,
Figure S4). An approximately 3-fold increase in the total current
density for the CORR (jCORR) is observed as NaOH concentration
is increased from 0.1 M to 1 M. The change in the resistance of
the electrolyte at different ion strengths has been accounted for in
the reactivity studies. This is consistent with several literature
the cation concentration is not solely responsible for the increase
of the formation rate of C2+ products in more concentrated alkaline
electrolytes. Recent reports have shown that the rate determining
step (RDS) is the formation of C2+ products is the C-C coupling of
adsorbed CO on Cu,[9a, 9b] which is independent of the H+ or OH-
concentration. Thus, the choice of comparing the cation effect in
the formation of C2+ product in the CORR at different pH with the
identical potential on the RHE scale is not justified, because the
driving force in the C-C coupling chemistry is the absolute
potential of the electrode. Strikingly, formation rates for all C2+
products are nearly identical at the same absolute potential (-1.47
+
-
V vs. SHE) with 1 M of Na regardless of the OH concentration in
the electrolyte (Figure 2 a). Similar results are obtained at
reports that CORR is more favored at higher electrolyte pH[7, 8d]
.
+
To isolate the impact of the concentration of Na on the CORR,
reactivity studies are also performed on the same catalyst and
4
intermediate concentrations of NaClO , i.e., x = 0.1 and 0.4, in the
NaOH + xM NaClO
4
electrolyte when comparing with the pure
potential in 0.1 M NaOH + (x-0.1) M NaClO
4
electrolyte (x = 0.1,
+
NaOH electrolyte with the identical Na concentration (Supporting
0
.25, 0.5, 0.75, 1; Supporting Information, Figure S5). NaClO
4
is
-
selected for matching the cation concentration because the ClO
4
anion does not change the electrolyte pH and is known not to
adsorb on the electrode surface[13a, 14]. Further, the high solubility
of NaClO
4
in water (17.1 M at room temperature) affords a wide
+
range of Na concentration. jCORR increases roughly linearly with
2
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Angewandte Chemie International Edition
10.1002/anie.201912412
RESEARCH ARTICLE
the identical potential on the
RHE
scale
is
more
appropriate
at
different
electrolyte pH. The promotion
of CH and H production is
4
2
likely due to the increase of
hydrogen binding energy
(
HBE) at higher electrolyte
pH[15]. The increased binding
of Hads on Cu surface can
improve the energy barrier of
the RDS of CH
provide more optimal HBE for
HER[16]
At -0.7 VRHE, the
4
formation and
.
Figure 2. CO electroreduction potential at SHE scale. Partial current densities of C2+ products, including ethylene,
acetate, ethanol, acetate and n-propanol. a) Comparison of CO electroreduction conducted in 0.1 M NaOH, 0.1 M NaOH
concentrated
NaOH
a clearly
+
4
0.9 M NaClO and 1 M NaOH, respectively. The reference potentials are chosen to be the same in RHE scale (-0.7 VRHE)
electrolyte shows
and SHE scale (-1.47 VSHE), separately. b) Hydroxide ion concentration dependence with a fixed 1.0 M cation concentration
at -1.5 VSHE. The error bars represent the standard deviation from at least three independent measurements.
higher CH
than those in both cation
compensated and
uncompensated electrolytes (Figure 3 a-c).
4
formation rate
Information, Figures S6 and S7). These results strongly indicate
that the improvement of 2+ product formation in
C
Impact of Cation Chelation by Crown Ether on CORR. The
impact of Na+ on the formation of C2+ products in the CORR is
further highlighted by its systematic removal through chelation
concentrated alkaline electrolytes could be largely, if not
completely, attributed to the high concentration of cations
-
-
rather than OH .
with a crown ether while maintaining the OH concentration in the
To confirm the decisive role of the Na+ concentration in the
electrolyte. 15-Crown-5 is a crown ether known to effectively
and completely chelate an equimolar Na+ to form a bulky
organic cation[17], referred to as C-Na+ in this work, which is
employed to change the nature of the cation in the electrolyte
formation of C2+ products, reactivity studies are conducted at the
+
identical Na concentration (1 M) while systematically varying the
-
OH concentration in a series of xM NaOH + (1-x)M NaClO
4
electrolytes (x = 0.1, 0.25, 0.5, 0.75, and 1) . At -1.5 VSHE, all C2+
(Figure 4 a). jCORR in 1 M NaOH at -1.53 VSHE (or -0.7 VRHE
)
+
products exhibit nearly identical formation rates as the electrolyte
decreases substantially with the increase of the fraction of Na in
-
+
2
[
OH ] is increased from 0.1 to 1 M (Figure 2 b; Supporting
the electrolyte chelated to C-Na , from 16.6 mA/cm without
2
+
Information, Figure S8), which provides further evidence that
hydroxide has a very weak, if at all, promotional effect on the
formation of C2+ products in the CORR. In contrast, the
formation rates of methane and hydrogen do increase quite
adding any crown ether to 4.1 mA/cm with all Na chelated to C-
+
Na (Figure 4 b). Further, the FE for C products decreases from
2+
+
62.3 % to 31.3 % with the increasing fraction of Na chelated. It is
worth noting that the potential on neither the SHE or the RHE
does not change when the increasing amount crown introduced
+
-
substantially with the increase of either Na or OH concentration
Figure 3). The RDS of CH formation the has been confirmed to
-
(
4
to the electrolyte, as the OH concentration is not affected by the
Figure 3. pH dependence of CH
NaClO and 0.1 + x M NaOH electrolyte (x = 0.9 a), 0.4 b) and 0.1 c)) at -0.7 VRHE. The error bars represent the standard deviation from at least three independent
measurements.
4 2
and H formation rate. Partial current densities of hydrogen and methane production in 0.1 M NaOH, 0.1 M NaOH + x M
4
be the hydrogenation of COads by Hads through a Langmuir–
Hinshelwood type mechanism[9a, 9b], and thus rate comparison at
chelation. This demonstrates that the nature of cations could have
a major impact on the reactivity and selectivity for electrode
3
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
has been demonstrated by
anions[11, 19], but no direct
experimental evidence has
been reported for specific
cation adsorption[11, 19-20].
Janik and Koper claimed
specific adsorption of alkali
cations on Pt in the potential
range relevant for the
underpotential deposited
H
based on indirect evidence
from cyclic voltammetric and
computational
investigations[21]
.
However,
cation specific adsorption is
unlikely in the present case.
+
Na should be preferred in the
specific adsorption, if it occurs
at all, over C-Na+ due to its
bulkiness. Thus, no change in
the CORR rate and product
distribution are expected with
C-Na+ in addition to Na+ if
specific
adsorption
is
Figure 4. Cation chelation with crown ether. a) A schematic shows the decrease of cations in OHP due to the chelation
of 15-Crown-5. IHP and OHP represent inner Helmholtz plane and outer Helmholtz plane, respectively. b) CO
electroreduction current densities and Faradaic efficiencies compared at 1 M NaOH electrolyte chelated with different
amount of 15-Crown-5 at -0.7 VRHE/-1.53 VSHE. c) CO electroreduction current densities and Faradaic efficiencies
compared in 0.5 M NaOH and in 1 M NaOH chelated with 0.5 M 15-Crown-5 at -1.53 VSHE. The error bars represent the
standard deviation from at least two independent measurements.
responsible for the cation
effect. This is in contradiction
to the data presented in
Figure 4 c. We note that the
lack of specific adsorption of
+
C-Na is confirmed by the lack
of Stark tuning of any bands
surface mediated reactions. This is consistent with previous
associated with the crown ether (Supporting Information, Figure
S10). Another commonly invoked mechanism to explain the
reports showing that different alkali metal cations lead to
significantly different rates and product distributions in CO
2
RR[10b,
cation effect is modification of the interfacial electric field due to
the different cation sizes[10a, 10d, 18]. However, we do not observe
10d].
C-Na+ is much bulkier than Na , and thus not expect to get
+
+
as close to the electrode surface as Na . This is confirmed by the
reduced Stark tuning rate of the adsorbed CO on polycrystalline
any detectable change in the Stark tuning rate in adsorbed CO on
Cu in the Na+ concentration range of 0.1 to 1 M (30-33 cm-1/V;
-
1
-1
Cu from 34 cm /V in 0.1 M NaOH to 25 cm /V in 0.1 M C-NaOH
determined with attenuated total reflectance surface enhanced
infrared absorption spectroscopy (ATR-SEIRAS) (Supporting
Information, Figure S9). A lower Stark tuning rate within the Gouy-
Chapman-Stern (GCS) model suggests that the distance between
the outer Helmholtz plane (OHP) and the electrode surface is
larger, presumably due to the larger cation size of C-Na+ as
compared to Na+[18]. It is worth noting that the presence of 0.5 M
Supporting Information, Figure S11). The lack of change in the
Stark tuning rate of adsorbed CO on Pt at different pH values, i.e.,
+
H concentrations, has been reported recently by Koper and co-
workers[12]. This is likely because that the concentration of cations
in neither the bulk nor the diffuse layer has any significant impact
on the Helmholtz capacitance, as suggested by
a recent
computational study[10a]. The average electric field between the
OHP and the electrode surface within the framework of the GCS
model does not change appreciably in the Na+ concentration
+
+
of C-Na in the presence of 0.5 M of free Na still exerts a major
impact on the reactions occurring on the electrode surface (Figure
range investigated in this work, and thus cannot explain the
enhanced production of C2+ products in the CORR at higher Na+
4
5
c), with jCORR being only roughly half as much in 1 M NaOH with
+
+
0% of Na chelated into C-Na as in 0.5 M NaOH (Figure 4 c).
concentrations. Modified local electric field, as suggested by Bell
and coworkers[10a, 10d], at higher cation concentrations could lead
The FE for C2+ products is also lower than the latter by 20.6 %.
This suggests the bulkier C-Na+ still have access to and impact
the surface mediated reactions.
to higher densities of “hot spots” that favor the formation of C2+
products. Another possibility is that interfacial water structure is
Potential Pathways for Cations to Affect Surface Mediated
CORR. Although multiple mechanisms through with cations could
influence the electrode surface mediated reactions have been
proposed in the literature, results presented in this work highlight
the drastic impact and complexity of the cation effect in
electrocatalysis. Site blocking during specific adsorption of ions
modified at higher concentrations of cations[18, 22], which could
facilitate the C-C coupling pathway by better solvating the
transition state complex. We stress that these speculative
rationalizations of the cation effect highlight the gap in the
molecular level understanding how the nature and concentration
of cations impact surface-mediated electrocatalytic reactions.
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
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Conclusion
By comparing the formation rate of C2+ products in two sets of
CORR experiments in electrolytes containing (a) an identical
-
+
concentration of OH and varying concentrations of Na ; and (b)
-
varying concentrations of OH and an identical concentration of
[
2]
+
Na by controlling the electrode potential at both the RHE and the
+
SHE scales, we show that higher concentrations of Na , rather
2008, pp. 89-189.
-
than that of OH , promote the formation of C2+ products in the
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2
assumption made in the CO RR and CORR community, it is
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adsorbed CO as the RDS entails the overpotential should be
referenced to the absolute potential, rather than on the RHE scale.
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[
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Angewandte Chemie International Edition
10.1002/anie.201912412
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
The nature and concentrations of
cations have drastic impact on the
electroreduction of CO. The
experimental measurements by
systematically varying the
Jing Li, Donghuan Wu, Arnav S.
Malkani, Xiaoxia Chang, Mu-Jeng
Cheng, Bingjun Xu,* and Qi Lu*
Page No. – Page No.
+
-
concentration of Na and OH
revealed that higher concentrations of
Hydroxide is not a promoter of C2+
products formation in electrochemical
reduction of CO on Copper
raphic here))
+
-
cations (Na ), rather than OH , exert
the main promotional effect on the
production of C2+ products at identical
absolute electrode potential.
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7