An Artificial Electrode/Electrolyte Interface for CO2 Electroreduction by Cation Surfactant Self-Assembly

Article abstract of DOI:10.1002/anie.202005522

In this work, an artificial electrode/electrolyte (E/E) interface, made by coating the electrode surface with a quaternary ammonium cation (R₄N⁺) surfactant, was successfully developed, leading to a change in the CO₂ reduction reaction (CO₂RR) pathway. This artificial E/E interface, with high CO₂ permeability, promotes CO₂ transportation and hydrogenation, as well as suppresses the hydrogen evolution reaction (HER). Linear and branched surfactants facilitated formic acid and CO production, respectively. Molecular dynamics simulations show that the artificial interface provided a facile CO₂ diffusion pathway. Moreover, density-functional theory (DFT) calculations revealed the stabilization of the key intermediate, OCHO*, through interactions with R₄N⁺. This strategy might also be applicable to other electrocatalytic reactions where gas consumption is involved.

Full text of DOI:10.1002/anie.202005522

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: An Artificial Electrode/Electrolyte Interface for CO2
Electroreduction by Cation Surfactants Self-Assembly
Authors: Yang Zhong, Yan Xu, Jun Ma, Cheng Wang, Siyu Sheng,
Congtian Cheng, Mengxuan Li, Lu Han, Linlin Zhou, Zhao
Cai, Yun Kuang, Zheng Liang, and Xiaoming Sun
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202005522
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10.1002/anie.202005522
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RESEARCH ARTICLE
An Artificial Electrode/Electrolyte Interface for CO2
Electroreduction by Cation Surfactants Self-Assembly
Yang Zhong, Yan Xu, Jun Ma, Cheng Wang, Siyu Sheng, Congtian Cheng, Mengxuan Li, Lu Han,
Linlin Zhou, Zhao Cai, Yun Kuang,* Zheng Liang,* Xiaoming Sun*
[*]
Y. Zhong, J. Ma, S. Sheng, C. Cheng, M. Li, L. Han, L. Zhou, Prof. Y. Kuang, Prof. X. Sun
State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University
of Chemical Technology Beijing 100029 (China)
E-mail: kuangyun@mail.buct.edu.cn, sunxm@mail.buct.edu.cn
Dr. Y. Xu
Electrical Engineering and Automation, Shandong University of Science and Technology, Tsingtao, 266590, China
Dr. C. Wang
Institute of Water Environmental Research, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
Dr. Z. Cai
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China
Dr. Z. Liang
E-mail: lianzhen@lbl.gov
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA
Supporting information for this article is given via a link at the end of the document.
Abstract: The electrode/electrolyte (E/E) interface influences the
electrocatalytic CO2 reduction (CO2RR) behavior by controlling both
the diffusion kinetics of the reactants (CO2/H+) & products, and the
transporting dynamics of the electrons and intermediates.
Simultaneous manipulation of the CO2/H+ transportation and the
interfacial charge transfer would facilitate the CO2RR process. In this
work, we successfully developed an artificial E/E interface
construction strategy via coating the electrode surface with quaternary
ammonium cation surfactant. This artificial E/E interface with high CO2
permeability, leads to promoted CO2 transportation and
hydrogenation, as well as suppressed hydrogen evolution (HER).
Further study revealed that linear and branched surfactants facilitated
formic acid and CO production, respectively. Molecular Dynamics
simulations indicated that the artificial interface provided a facile CO2
As
a
prototypical gas consumption reaction, CO2
electroreduction involves multiple electron and proton
transportation, as well as multiple gas diffusion steps.[3] A good
electrode for CO2RR should have active catalytic centers that
facilitate the activation of CO2 molecules and enable unobstructed
CO2 diffusion pathway that favors efficient caption of CO2.
Tremendous efforts have been devoted to the catalysts design,
where copper-based nanostructures have been studied
extensively due to the capability to produce both one-carbon
products (C1) and multi-carbon products (C2+). Strategies
including particles size tuning,[4] facet control,[5] bimetallic and
multi-metallic alloying,[6] and metal/metal compound hybrids
construction[7] were employed to enhance the intrinsic catalytic
activities. These strategies give deep insights into the interactions
between catalysts and key intermediates. However, the poor
solubility of CO2 in water (~34 mM at ambient conditions) leads to
rapid consumption of CO2 in the vicinity of the electrode surface,
resulting in altered local CO2/H+ supply and favored HER.[8]
According to classical catalytic theory, the property of the E/E
interface plays the key role in addition to the catalyst itself, which
controls the solid/liquid/gas contact, which dominates the kinetics
of gas-involved electrocatalytic reactions such as CO2RR.[9]
Electron and mass transfer at this interface are crucial to the
electrochemical CO2 reduction process. Previous report revealed
that larger alkali cations could alter the local electric properties of
electrode surface thus tuning the selectivity for CO, C2H4, and
C2H5OH,[10] while small organic molecule coating on the electrode
surface could alter the adsorption and diffusion behaviors of
CO2.[11] Inspired by these previous reports, herein, we developed
an artificial E/E interface enabled by quaternary ammonium cation
surfactants (Figure 1). In our study, a Cu-nanowire (Cu NW)
electrocatalyst was used as an example to demonstrate this
interface design since it has multiple reaction routes and reduction
products for CO2RR with selectivity towards various C1 and C2+
products, as well as the competing hydrogen evolution. With
surfactants coating, the selectivity is enhanced compared to
diffusion pathway.
Moreover, density functional theory (DFT)
calculations revealed the stabilization of the key intermediate OCHO*
via interacting with the R4N+ cations. This surfactant-based artificial
interface construction strategy might also be applicable to other
electrocatalytic reactions where gas consumption is involved.
Introduction
Electrode/electrolyte (E/E) interface is fundamentally
intriguing and essential due to its complex physical and chemical
properties. The unique local electric fields, different alkalinity, fast
charge and mass transfer all influence the electrochemical
processes.[1] Modification of E/E interface could enhance the
electrochemical catalytic activity via optimized interfacial electron
transfer dynamics and optimized adsorption energy of the
reaction intermediates. For gas involving reactions, modification
of the E/E interface via superwetting configuration has been
proved as an appealing route to drastically alter the transportation
efficiency of gas molecules and electrons.[2]
1
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Figure 1. Schematic of quaternary ammonium cation surfactant modification on a Cu nanowire array electrode surface. (A) Schematic of a Cu NW array. (B)
Schematic of the Cu NW array after surfactant modification. (C-D) Schematics showing the transportation behaviors of the CO2 (purple), and H+ (red) from bulk
electrolyte to electrode surface: (C) on pristine Cu NW and (D) on surfactant modified Cu NW. (E-F) The differential charge diagram of a CTAC modified Cu NW
surface: (E) top and (F) side view. Yellow region represents the electron accumulation area, blue region represents the electron loss area.
pristine Cu nanowires: Faradaic efficiency of formic acid is
increased from 17.8% to 41.8% under CTAC modification, while
Faradaic efficiency of CO greatly enhanced from 24.5% to
45.8% with Benzyltriethylammonium chloride (BTEAC)
modification. We also find that the Faradaic efficiency of HER is
suppressed to below 20%.
Figure 1D), leading to suppressed HER. In sharp comparison,
without surfactant modification, the direct contact between the
electrode and the electrolyte unavoidably results in fast proton
adsorption due to the hydrophilic nature of metal catalyst, which
causes severe HER (Figure 1C).
Among numerous electrocatalysts for CO2RR, Cu-based
materials could produce C1, C2+ products and hydrogen,
enabling the study of selectivity towards various products. Cu
NWs electrode was chosen as a model electrocatalyst because
of its high surface area and uniform structure that are beneficial
Results and Discussion
to
a uniform Cu/surfactant interface. The Cu NW was
synthesized via chemical etching process followed by annealing
and electrochemical reduction. The scanning electron
microscopy (SEM, Figure 2A) images show that the surface of
Cu foil was uniformly coated with Cu(OH)2 nanowire arrays after
chemical etching. The NWs were 100 nm in diameter and tens
of micrometers in length (Figure S1A). When annealed in air,
these NWs were transformed into CuO with rough surface
morphology (Figure S1B). XRD patterns before and after
annealing confirmed the phase transformation (Figure S2).
Metallic Cu array composed of nanocrystalline grains was
obtained via subsequent electrochemical reduction, as
evidenced by X-ray photoelectron spectroscopy (XPS) study of
the electrode surface (Figure S3). Transmission electron
microscope (TEM, Figure 2B) confirmed the rough surface of the
Cu NWs, which was composed of nanosized crystalline grains.
The lattice fringes measured from Fig. 2b with interplanar
spacings of 0.21 nm corresponded to (111) planes of metallic
Cu, with numerous grain boundaries observed.
The cation surfactants were coated on the electrode surface
by a drop-casting method (Figure 1A, 1B). The surfactant
molecules would assemble into a bilayer structure during the
drying process.[12] This cation surfactant interface plays multiple
roles in the CO2RR process: (1) CO2 molecules have easy
access to the catalysts surface through the bilayer region (Figure
1D), especially in use of long chain surfactant molecules. This is
because the long hydrocarbon chains provide multiple aerophilic
tunnels for CO2 transportation. Therefore, during continuous
bubbling in a H-type cell, the surfactant coated electrode should
be able to capture more CO2 bubbles (detailed discussion in
Figure. 3). In contrast to unmodified electrode, as the solubility
of CO2 was quite low, direct contact between the electrode and
the electrolyte results in slow CO2 transportation (Figure 1C). (2)
CO2RR intermediate is able to be stabilized by interacting with –
N+ of the adsorbed linear quaternary ammonium cations via
charge redistribution. Taking CTAC coating as an example,
presence of the surfactant molecules results in negatively-
charged surface of the Cu NM electrode (Figure 1E, 1F), thus
leading to varied adsorption energy between the electrode and
the OCHO* intermediate, a key intermediate towards formic acid
production. (3) The surfactant-modified interface weakened the
accessibility of protons to the catalyst surface due to
electrostatic repulsion between cation surfactants and protons
The Cu NW electrode was then modified by quaternary
ammonium cation surfactants through a drop-casting method as
described above, and the effectiveness of this modification was
confirmed by the XPS patterns. Using cetyltrimethylammonium
2
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same current densities (Figure 3A), indicating an improved mass
transfer. This current density increase confirmed that surfactant
coating provided unobstructed CO2 diffusion pathways.
Electrochemical impedance spectroscopy (EIS) was also
performed and showed that the contact resistance of the
electrode differed little after modification with CTAC (Figure
S10A). At a more negative potential, -0.8 V vs. RHE where
CO2RR could happen, the charge transfer resistance of CTAC-
modified electrode was lower than that of pristine Cu NW
electrode according to the EIS results (Figure S10B), implying a
favoured electron transfer for modified electrode during CO2RR.
This suggested that the interaction between Cu NW electrode
and cation surfactant could influence the electronic properties of
the E/E interface and surface electronic structure change always
results in varied electrocatalytic behaviour.
Figure 2. Structural characterization of a CTAC surfactant modified Cu NW
array electrode. (A) FESEM of the Cu NW array (Scale bar: 2 um). (B) TEM
Image (Scale bar: 50 nm) and HRTEM image of a CTAC surfactant modified
Cu NW (Scale bar: 2 nm). (C) N1s scan of the XPS patterns of the Cu NW
array electrode before and after CTAC modification. (D) XRD patterns of Cu
foil, Cu foil/CTAC self-assembly, and CTAC powders.
chloride (CTAC) as a typical example for the cation surfactant
(Figure 2C), a peak at around 400 eV in the N1s spectrum is
attributed to the -N+ group of CTAC, while no signal of nitrogen
species was observed for pristine Cu NW. In addition, this peak
still existed on the modified Cu NW after CO2RR (Figure S4),
and no NMR signal corresponding to -CH3 group was detected
in the electrolyte when chronoamperomtery test was carried out
in N2-saturated 0.1 M KHCO3 electrolyte for one hour at -0.8 V
vs. RHE. This indicated that the CTAC was stable on the Cu NW
electrode surface during the electrocatalysis process (Figure S5).
In addition, the results combined with N1s XPS spectrum (Figure
S4) indicated that the CTAC is not being consumed during
CO2RR process, and all carbonaceous products originate from
CO2 reduction. The adsorption of quaternary ammonium cations
on the electrode surface was further confirmed by Fourier
transform infrared spectroscopy (FT-IR). Two typical peaks at
about 2920 and 2850 cm-1 indexed to the stretching vibration of
-CH3 and -CH2- bond were observed (Figure S6). Energy-
dispersive X-ray spectrum (EDX) further confirmed uniform
distribution of CTAC on electrode surface (Figure S7). X-ray
diffraction (XRD) was utilized to characterize the assembly
structures (Fig 2D). Two diffraction peaks at small angels (3.5
degree and 6.9 degree), corresponding to the bilayer and single
layer diffraction pattern. In order to further evidence the two
peaks were the diffraction of the assembly, we also took the XRD
profile of the CTAC powder, in which CTAC molecules would
form smectic layer structure stacked along the c-axis [12b]. Two
peaks at the same degrees were observed, suggesting the
assembly structure and evidencing the bilayer structure of the
CTAC assembly on the copper electrode surface. Differential
scanning calorimetry (DSC) and Zetapotential analysis further
support the bilayer structure(Figure S8, S9).
Figure 3. CO2 electroreduction performances of the Cu NW electrodes with
and without surfactant modification. (A) Linear sweep voltammetric curves of
the Cu NW electrodes with and without CTAC modification in the CO2-
saturated 0.1 M KHCO3 aqueous solution. With CTAC modification, the
overpotential of the electrode showed ~80 mV positive shift at the same
current density. (B) Faradaic efficiencies at various potentials of the Cu NW
electrodes with and without CTAC. (C) CO2RR products distributions of the Cu
NW electrode modified by different types of liner quaternary ammonium
surfactants, data were collected at -0.8 V vs. RHE. (D) FE of formic acid for
the Cu electrode modified with different linear surfactants. Data were collected
at -0.8 V vs. RHE. Numbers in orange circles show the enhancement factors
of FEHCOOH, i.e. FE ratios for formic acid of pristine Cu NW electrodes to
modified Cu NW electrode.(E) CO2RR products distributions of the Cu NW
electrodes modified with different types of branched quaternary ammonium
surfactants , data were collected at -0.8 V vs. RHE. (F) FE of CO for the Cu
NW electrodes modified with different types of branched surfactants. Data
were collected at -0.8 V vs. RHE. Numbers in orange circles show the
enhancement factors of FECO, i.e. FE ratios for CO of pristine Cu NW electrode
to modified Cu NW electrode.
To unveil the underlying charge transfer mechanism, DFT
calculations were performed (Figure S11). For electrodes with
linear quaternary ammonium chain length lead to larger negative
charged area on the Cu surface, with CTAC modification
reaching a maximum value. Specifically, the interaction shows a
donation of 0.337 electrons from CTAC to Cu, while 0.250, 0.249,
0.291 for TMAC, OTAC and DTAC, respectively. Additional long
chains (ODAC) could not further contribute to electrons donation,
Linear sweep voltammetry (LSV) was performed to
investigate the influence of CTAC on the electrocatalytic activity
of Cu NW catalyst in CO2-purged electrolyte (0.1 M KHCO3, pH
6.8). It is obvious that the electrode modified with CTAC shows
largest current density increase relative to pristine Cu NW at the
3
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which agreed with the experimental observation that the FE for
formic acid was not further enhanced. In addition, the quaternary
ammonium cation surfactants modification facilitated the
adsorption and activation of CO2 molecules. In the CO2RR
process, generating * 퐶푂2− (radical anion) via one electron
transfer from the electrode to CO2 molecule is believed to be the
rate-determining step on most catalysts due to the high energy
barrier required [13]. Adsorption of 푂퐻− as a surrogate for *퐶푂2−
to identify the binding affinity to *퐶푂2− was carried out under a
N2-bubbled 0.1 M NaOH electrolyte via cyclic voltammetry on
the electrode with and without surfactant modification (Figure
S12).[13] It was discovered that the peak area for * 퐶푂2−
adsorption on the CTAC modified electrode was higher than that
of pristine Cu NW. This suggests that the CTAC modification
enhanced CO2 adsorption to form *퐶푂2−. The stronger ion pairing
resulted in higher intrinsic activity (i.e. 78 mV more positive onset
potential of CO2RR) as compared with pristine Cu NW, which is
consistent with the observation from LSV curve (Figure 3A).
To verify the effect of the quaternary ammonium cation
surfactants modification on CO2RR activity and selectivity, the
product distributions of Cu NW electrodes with and without
surfactants modification were analyzed (Figure 3B). The
Faraday efficiencies (FE) of carbonaceous products on pristine
Cu NW electrode was quite low due to severe hydrogen
evolution reaction in competition (HER). After the quaternary
ammonium cation surfactants modification, the HER was
suppressed obviously and the FEs of carbonaceous products
were enhanced greatly. Setting the reduction potential to -0.8V
vs. RHE as a standard condition (the same hereafter if not
mentioned), the FE for HER decreased from 40.3% to 19.0%
after CTAC surfactant modification, while the total FE for
carbonaceous products increased from 56.9% to 80.7% (Figure
3B). Six-hour stability test at -0.8V vs RHE showed that the
CTAC-modified Cu NW electrode maintained a stable current
density (Figure S13). The adsorbed CTAC surfactant was also
very stable on the electrode as could be detected by XPS after
6h stability test (Figure S4). Thus the presence of CTAC could
protect the Cu NW from damage (Figure S14C). However, the
current density decayed for the pristine Cu NW electrode (Figure
S13), along with structural change after 6h CO2RR (Figure
S14D). Such recrystallization due to electrochemical sinter was
quite common during CO2 electroreduction which has often been
observed on a hydrophilic Cu surface.[14] It should also be
pointed out that the electrochemical active surface area are
similar after quaternary ammonium cation surfactants adsorption
compared with that of pristine Cu NW (Figure S15), suggesting
the selectivity change mainly originated from surfactant
modification. In order to further exclude other external influences
such as halide anions, which could be adsorbed on the electrode
surface and improve CO2RR selectivity, hexadecyltrimethyl-
ammonium hydroxide (CTAH) was used to replace CTAC as a
surface modification surfactant. Similar selectivity was found by
CTAH modification compared with that of CTAC modification
(Figure S16), further evidencing the cation part of the surfactant,
CTA+, played the essential role on selectivity regulation. It is
noticeable here that only a small amount of surfactant were used,
therefore the influence of surface pH is negligible when
considering the diffusion of OH- from CTAH into bulk electrolyte.
In addition, another typical anionic surfactant, sodium dodecyl
sulfate (SDS) was used to modify the electrode and its
corresponding CO2RR performance was also tested in 0.1 M
KHCO3. However almost no selectivity change was found for
SDS-modified Cu NW electrode in comparison to pristine Cu NW
electrode (Figure S17). Therefore, it is concluded that
quaternary ammonium cations are responsible for the enhanced
selectivity for CO2RR and suppression of HER.
Interestingly, the selectivity for C1 product, especially the
formic acid, varied when electrodes were modified by cation
surfactants with different types of hydrocarbon chains.
Increasing the linear hydrocarbon chains from 1-C
tetramethylammonium
octyltrimethylammonium chloride (OTAC), 12-C dodecyl
trimethyl ammonium chloride (DTAC), and to 16-C
chloride
(TMAC),
to
8-C
cetyltrimethylammonium chloride (CTAC), FE of formic acid
gradually increased from 17.8% to 41.8%. An optimal FEHCOOH
was achieved under CTAC modification (Figure 3C, 3D, S18).
The enhancement factor (the ratio of FEHCOOH of pristine
electrodes to that of surfactant-modified electrode) grew from
1.58 tetramethylammonium chloride (TMAC) to 2.34 (CTAC)
after long chain surfactants modification (Figure 3D). With the
hydrocarbon chain further increasing to 18-C octadecyl dimethyl
ammonium chloride (ODAC), the FEHCOOH decreased a little
(Figure 3D, S18D), indicating that additional long chains could
not contribute to the selectivity regulation. However, further
increase of the hydrocarbon chains leads to the decrease of
current density due to increased resistance.
Considering the fact that the influence of quaternary
ammonium cations on CO2RR selectivity may be related to the
coverage and binding ability to the Cu NW surface (geometric
and electronic effects), a series of quaternary ammonium
cations with varied branch chain structures, including
tetraethylammonium chloride (TEAC), tetrabutylammonium
chloride (TBAC), tetraoctylammonium chloride (TTAC),
benzyltrimethylammonium
chloride
(BTMAC)
and
benzyltriethylammonium chloride (BTEAC) were also
investigated. Total C1 products increased while C2+ products
decreased when quaternary ammonium surfactants with
branched chains were introduced. It was also observed that FE
of HER was suppressed from 40.3% to 19.4% after surfactant
modification, accompanied by increased FE for total
carbonaceous products (Figure 3E). Different from long chain
surfactants modification that facilitated formic acid production,
modification with branched surfactants increased the selectivity
for CO. The enhancement factor (the ratio of FECO of pristine
electrode to that of surfactant-modified electrode) grew from
Tetraethylammonium chloride 1.15 (TEAC) to Benzyltriethyl-
ammonium chloride 2.16 (BTEAC) after different surfactant
modification (Figure 3F, S19). Specifically, when all the
branches of quaternary ammonium cations were replaced from
methyl to ethyl (TEAC), butyl (TBAC) or octyl (TTAC), the FEs
for CO increased from 24.5% to 42.7%, with the trend that the
longer the carbon chains of the branches, the higher the FE for
CO. Quaternary ammonium surfactants with a benzene ring in
the branches (benzyltrimethylammonium chloride, BTMAC)
were also employed and it was found that the benzene ring had
similar effect to long hydrocarbon branches (FE for CO
increased to 41.6% at -0.8 V). Furthermore, when combining
both long hydrocarbon branches with benzene ring in the same
quaternary ammonium surfactant (Benzyltriethylammonium
chloride, BTEAC), the FE for CO was further enhanced (FE for
CO increased to 45.8% at -0.8 V).
4
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Figure 4. MD simulations of CO2 diffusion behaviors on the Cu NW surface. Insertion of CO2 molecules into E/E interface. (A-B) The time sequence of typical
snapshots of CO2 diffusion dynamics on (A) CTAC-modified Cu electrode and (B) pristine Cu electrode. (C) Energy evolution of the system composed of the CO2
molecules and the Cu electrodes with or without surfactant modification. (D) CO2 molecule distribution relative to the distance from the electrode surface.
CO2 reduction usually suffers from slow kinetics due to low
local concentration of CO2 around the electrode, thus competing
HER reaction always dominates the electrochemical reduction
process. It has been founded in the above section that
quaternary ammonium cation surfactants could suppress HER,
and that long linear chains of quaternary ammonium cations
favor formic acid production while long branched chains prefer
CO formation. Based on these two findings, molecular dynamics
(MD) simulations and DFT calculations were further utilized to
understand the selectivity change. MD simulations provided
insights into the molecular-level mechanism of how the CO2
diffuse into E/E interface from bulk aqueous solution. Time
sequence of typical snap-shots of how CO2 diffuse into E/E
interface (Figure 4A and Figure S20) revealed that Cu NW
electrode decorated with CTAC was able to capture CO2
molecules and facilitate CO2 diffusion, leading to accumulation
of CO2 molecules at E/E interface. However, CO2 molecules
could not easily spread to the Cu NW surface. Instead, most of
them stayed at the bulk solution with a slight perturbation (Figure
4B). As for the CTAC-modified electrode surface, the MD
simulations revealed that CTAC molecules well covered the Cu
NW surface with bilayer assembly structure, which agreed with
the XRD (Figure 2D), DSC and Zeta potential characterization
(Figure S8-S9). It is worthwhile mentioning that the bilayer CTAC
orientation provides an unobstructed CO2 diffusion pathway (t <
6 ns) to minimize the system energy due to hydrophobic (i.e.
aerophilic) interaction (Figure 4C).
behavior. CO2 molecule distribution relative to distance from the
electrode surface further confirmed that CO2 molecules tend to
easily accumulate at the electrode surface when quaternary
ammonium cations are present (Figure 4D). MD of other linear
quaternary ammonium cation surfactants was also investigated
(Figures S22-24). CO2 molecules also tend to accumulate on the
modified electrode surface, and CTAC is the optimal choice due
to its fastest diffusion for CO2 and strongest capturing ability of
CO2 molecules on the electrode surface.
In contrary to the liner quaternary ammonium cations that
assembled into bilayer structure firmly coating on the Cu
electrode surface, branched molecules exhibited different
behaviours. Taking tetraoctylammonium chloride (TTAC) as a
typical example (Figure S25), TTAC with long branched chains
tends to aggregate to form spherical micelles, leading to weak
interaction between Cu and the cation surfactant, as evidenced
by the NMR signal in the electrolyte after electrolysis (Figure
S26). However, CO2 can still be transported to the electrode
surface via diffusion into the micelles, and CO2RR could still be
promoted by collisions of tetraoctylammonium chloride (TTAC)
micelle on electrode surface via week electrostatic interaction.
MD simulations of other branched quaternary ammonium cations
were also investigated (Figure S27, S28) and similar micelle-type
CO2 transportation behaviors were revealed.
Further, the Gibbs free energies for the CO2RR reaction on
the Cu surface with and without surfactant modification were
calculated. The Cu (110) surface (Figure S29) was employed to
model the experimental systems because it is the exposed facet
of the Cu NW. In order to simplify the calculations, TMAC with
the shortest hydrocarbon chain was used to model the surfactant
coated surface of Cu (110) facet. Since the surfactant was
surrounded by a large number of aqueous species, the VASPsol
approach was adopted to correct the solvation effect. As shown
in Figure S30A, the formation of OCHO* species, a typical
intermediate for formic acid formation,[15] is energetically
favorable after TMAC modification. Specifically, the ΔG for the
Similar trend was found on other long chain linear
quaternary ammonium surfactants, but not applicable to
tetramethylammonium chloride (TMAC). This is because TMAC
could not form bilayer structure (Figure S21), and thus it could
not provide hydrophobic (i.e. aerophilic) gas transportation
pathways. When CO2 molecules in contact with linear quaternary
ammonium cations, the capture of CO2 from bulk solution is
thermodynamically favorable, with a trend that the longer the
hydrocarbon chain, the superior CO2 capture and transportation
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OCHO*
species
formation
is
-0.32
eV
at
Similarly, DFT calculations were performed to evaluate the
strength between the branch quaternary ammonium cations and
the CO* species, which is the intermediate for CO production. It
was found that increasing the chain length of the branches or
introducing benzene ring into the branch decreased the affinity
to *CO species. The combination of long chain branches and
benzene ring in BTEAC maximized this trend and this surfactant
could even repel the *CO species, thus favors CO desorption,
which is the rate-limiting step for CO production (Figure 5C).
Therefore, the weaker the interaction between the branch
quaternary ammonium cations and the CO* species, the higher
the selectivity for CO production. In order to investigate the
universality of branched surfactant modification, we applied
TTAC modification on Zn foil, an electrocatalyst that favors CO
production intrinsically. Impressively, the FE for CO increased
from ~30% to above 90% over all the testing potentials (Figure
5D). From the results we have obtained, it can be concluded that
this interface modification strategy by cation surfactants provides
a new route in the development of advanced electrode for
electrocatalysis besides catalyst design.
Cu/tetramethylammonium chloride (TMAC) interfaces, lower
than that of 0.38 eV for the COOH* intermediate. Therefore,
surfactant modified Cu surface would stabilize OCHO* and
enhance the selectivity for formic acid. However, the COOH*
species, a key intermediate for CO pathway, is energetically
more favorable at bare Cu surface (Figure S30B), thus the CO
production was promoted when branched surfactants were used.
This is because branched surfactants could not be adsorbed on
the Cu surface (MD simulations results) but could increase the
solubility of CO2.
Conclusion
In summary, surfactants modification was an efficient
strategy to construct an artificial interface for activity and
selectivity regulation of CO2 electroreduction. Introduction of
quaternary ammonium cation surfactants favors CO2
transportation while limits proton supply to the electrode surface,
enabling enhanced total FE for carbonaceous products. Further
study revealed that linear long chains of quaternary ammonium
cation surfactants boost formic acid formation while branched
long chains of quaternary ammonium cation surfactants favor
CO production. Both electronic effects and geometric effects
derived from quaternary ammonium cations contribute to the
selectivity variation of CO2RR. The unique E/E interface
structure, quaternary ammonium cations on the surface of Cu
endow the Cu-surfactant electrodes with high catalytic activities
and tunable selectivity. This study provides a new tool and deep
insight on how interface regulates charge and mass transfer, and
therefore, the strategy may be potentially applicable to other
electrocatalytic gas consumption systems, such as oxygen
reduction reaction (ORR), hydrogen oxidation reaction (HOR),
nitrogen/nitrogen-oxide reduction reaction (N2RR/NOxRR) etc.
Figure 5. CO2 electroreduction performances of Sn and Zn electrodes with
surfactant modification. (A) The calculated adsorption energy between *OCHO
intermediate and various liner quaternary ammonium surfactants. (B) FEs of
formic acid at various potentials between Sn foil electrodes with and without
CTAC modification (80 μL of 1 mM CTAC modification), data were collected in
CO2-saturated 0.1 M KHCO3 aqueous solution. (C) The calculated adsorption
energy between *CO intermediate and various branched quaternary
ammonium surfactants. (D) FEs of CO at various potentials between Zn foil
electrodes with and without TTAC modification (80 μL of 10 mM TTAC
modification), data were collected in CO2 saturated 0.5 M KHCO3 aqueous
solution.
To obtain a deeper understanding of the activity and
selectivity enhancement, density functional theory (DFT)
calculations were performed to evaluate the strength between
the quaternary ammonium cations and the OCHO* specie,[16]
which is the intermediate of the CO2 conversion to formic acid on
copper. Figure 5A represents calculated OCHO* adsorption
energy against different types of linear molecules. Previous work
showed that CO2RR to formic acid was proceeded via an initial
proton-coupled electron transfer to form an intermediate of
OCHO*. Here, via plotting the partial current density for formic
acid versus *OCHO binding energy, a volcano relationship
emerges (Figure S31). According to the classical Sabatier
principle, a neither too strong nor too weak binding energy for
OCHO* intermediate is essential to improve catalytic
performance. It was found that the binding energy for OCHO* on
CTAC modified E/E interface had the most suitable value, which
accounted for the best activity and selectivity for CO2RR to formic
acid. On the basis of above results, Sn foil, a typical formic acid
favored electrocatalyst, was chosen to confirm the enhancing
effect. It was observed that the FEs for formic acid increased
from 36.3% to 85.9% at -0.6 V after CTAC modification (Figure
5B). Similar enhancements were found on all other testing
potentials, suggesting CTAC modification was a general way to
facilitate CO2RR for formic acid production.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC), the National Key Research and
Development
2018YFA0702002), the Royal Society and the Newton Fund
through the Newton Advanced Fellowship award
Project
(No.
2018YFB1502401,
(NAF\R1\191294), the Program for Changjiang Scholars and
Innovation Research Team in the University (No. IRT1205), the
Fundamental Research Funds for the Central Universities, and
the long-term subsidy mechanism from the Ministry of Finance
and the Ministry of Education of PRC.
Keywords: CO2 reduction • electrode/electrolyte interface •
cation surfactant • gas diffusion
6
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10.1002/anie.202005522
Angewandte Chemie International Edition
RESEARCH
ARTICLE
Entry for the Table of Contents
An artificial quaternary ammonium cation surfactant E/E interface construction strategy change the CO2RR
pathway. MD simulations indicated that the artificial interface provided a facile CO2 diffusion pathway. DFT
calculations revealed the stabilization of the key intermediates via interacting with the R4N+ cations.
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