Polyacrylamide-Mediated Silver Nanoparticles for Selectively Enhancing Electroreduction of CO2 towards CO in Water

Article abstract of DOI:10.1002/cssc.202002481

Conversion of the greenhouse gas CO₂ to value-added products is an important challenge for sustainable energy research. Here, a durably nanohybrid composed of Ag nanoparticles and polyacrylamide was constructed for the selectively electroreduction of CO₂ to CO. The nanohybrid exhibited an outstanding CO faradaic efficiency of 97.2±0.2 % at ?0.89 V_RHE (vs. the reversible hydrogen electrode) with a desirable CO partial current density of ?22.0±2.3 mA cm^?2 and maintained the CO faradaic efficiency above 95 % over a wide potential range (?0.79 to ?1.09 V_RHE), showing excellent stability during a 48 h prolonged electrolysis. The origins of selective enhancement of CO₂ reduction over the nanohybrid stemmed from the activation of CO₂ via hydrogen bond and the low basicity of the amide. DFT calculations implied that the synergy of Ag nanoparticles and amide could better stabilize the key intermediate (*COOH) and effectively lower the overpotential of CO₂ reduction. These results establish the synergistic effects of organic/inorganic hybrid as a complementary method for tuning selectivity in CO₂-to-fuels catalysis.

Full text of DOI:10.1002/cssc.202002481

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Polyacrylamide-Mediated Silver Nanoparticles for Selectively
Enhancing Electroreduction of CO2 towards CO in Water
Authors: Xiaofei Han, Lin Liu, Jiayi Yuan, Xinsheng Zhang, and
Dongfang Niu
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.202002481
Link to VoR: https://doi.org/10.1002/cssc.202002481
01/2020

10.1002/cssc.202002481
ChemSusChem
FULL PAPER
Polyacrylamide-Mediated Silver Nanoparticles for Selectively
Enhancing Electroreduction of CO2 towards CO in Water
Xiaofei Han, [a] [b] Lin Liu, [b] Jiayi Yuan, [b] Xinsheng Zhang, [a] [b] and Dongfang Niu*[a] [b]
[a]
X. Han, Prof. X. Zhang, Dr. D. Niu
State Key Laboratory of Chemical Engineering
School of Chemical Engineering
East China University of Science and Technology, Shanghai, 200237, China
X. Han, L. Liu, J. Yuan, Prof. X. Zhang, Dr. D. Niu
School of Chemical Engineering
[b]
East China University of Science and Technology, Shanghai, 200237, China
*Corresponding author: dfniu@ecust.edu.cn
Abstract: Conversion of the greenhouse gas CO2 to value-added
catalysts for CO2ER, including regulating the particle size,[25,26]
shape,[27,28] morphology[29,30] site[31,32]
and defect of
products is an important challenge for sustainable energy research.
Here we construct
a
durably nanohybrid composed of Ag
nanostructured Ag. Nevertheless, the activity and selectivity for
CO2ER are still unsatisfactory due to the weak adsorption of
CO2 (or intermediates) and the hydrophobicity on the surface of
these well-defined metal catalysts in aqueous electrolyte.
nanoparticles and polyacrylamide for the selectively electroreduction
of CO2 to CO. The nanohybrid exhibits outstanding CO Faradaic
efficiency of 97.2 ± 0.2 % at -0.89 VRHE with a desirable CO partial
current density of -22.0 ± 2.3 mA cm-2, and maintains the CO
Faradaic efficiency above 95% over a wide potential range (-0.79 to
-1.09 VRHE) and excellent stability during a 48 h prolong electrolysis.
We demonstrate that the origins of selective enhancement of CO2
reduction over the nanohybrid stem from the activation of CO2 via
hydrogen bond and the low basicity of amide. DFT calculations imply
that the synergy of Ag nanoparticles and amide could better stabilize
the key intermediate (*COOH) and effectively lower the overpotential
of CO2 reduction. These results establish the synergistic effects of
organic/inorganic hybrid as a complementary method for tuning
selectivity in CO2-to-fuels catalysis.
Therefore, some cocatalysts[33-35] have been employed to
modify the metal catalysts to enhance the interaction between
the metal catalyst and CO2 or the key intermediates and lower
the energy barrier of CO2ER. For example, Gao, et al.
constructed an Au-CeOx interface to stabilize the key
intermediates COOH* and thus facilitated CO2ER.[36] However,
the introduction of metal oxide significantly reduces the electrical
conductivity of the electrode and the hydrophobicity remains
unsolved. Zhang, et al. improved CO2ER performance by
coating the N-doped carbon nanotubes (CNTs) with an overlayer
film of polyethylenimine (PEI), which can concentrate CO2 on the
electrode surface.[6] On the other hand, the key intermediate
CO2•-* was presumably stabilized by the N-doped CNTs and the
co-catalyst PEI, lowering the activation energy of CO2 reduction
into formic acid. Unfortunately, HER was also promoted to a
great extent probably due to the protonation of PEI in the near-
neutral aqueous electrolyte.[37]
Inspired by the synergistic effect in enhancing CO2 adsorption
and activation, we propose herein a strategy to selectively
facilitate CO2ER by fabricating a durably organic/inorganic
nanohybrid composed of polyacrylamide (PAM) and Ag
nanoparticles (NPs). As for the amide group in PAM, the
carbonyl group is conjugated with the p orbital occupied by the
lone pair on the nitrogen atom, decreasing the electron cloud
density on the nitrogen atom and increasing the polarity of the
N-H bond. Consequently, the amide group would not be
protonated in near-neutral electrolyte and the reactive H atom
may form hydrogen bond with the O atom in CO2, thus
enhancing the adsorption of CO2 and activating CO2. Meanwhile,
the introduction of PAM appropriately improves the hydrophilicity
of the electrode surface, which may facilitate the timely diffusion
of CO2 dissolved in the aqueous solution to the electrode
surface, thereby contributing to an enhanced CO2ER controlled
by reactant diffusion at high overpotentials. Furthermore, we
tried to prepare Ag NPs with large sizes of ~100 nm to alleviate
the sensitivity of the catalytic activity to particle size, aiming at
enhancing the durability of the hybrid catalyst. Density functional
theory (DFT) calculations reveal that the PAM-modified Ag NPs
indeed could stabilize the key intermediate *COOH and
effectively lower the overpotential of CO2ER.
Introduction
With the rapid development of the global economy, the
enormous fossil fuel consumption has resulted in a sharp
increase in the concentration of carbon dioxide (CO2) in the
atmosphere, causing progressively severe environmental
problems. How to take full advantage of CO2 has become a hot
spot in the scientific research.[1] Superior to emission reduction
and CO2 storage, electrochemical conversion of CO2 is
considered as an environmental-friendly and promising route
since it can not only reduce CO2, but also convert it into valuable
low-carbon fuels such as CO,[2-4] formic acid,[5,6] alcohols,[7,8] and
hydrocarbons,[9-12] etc. However, there are still many challenges
to overcome, such as the large overpotential, low durability of
electrocatalyst and the competitive hydrogen evolution reaction
(HER) in aqueous environment.[13,14] Therefore, designing an
efficient electrocatalyst with outstanding activity, selectivity and
stability is still desperate.
Various electrocatalysts have been investigated for CO2
electrochemical reduction reaction (CO2ER), and among them,
metal catalysts have stood out thanks to their good electrical
conductivity and stability in aqueous environment.[15-20]
Furthermore, those metal electrocatalysts selective for CO
production are more attractive as they exhibit highest energetic
efficiencies. Ag is considered as the more promising
electrocatalyst relative to Au since it is low-cost and has a
comparable CO selectivity.[21-24] Tremendous efforts have been
dedicated to developing and improving the performance of Ag
1
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10.1002/cssc.202002481
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Results and Discussion
agrees well with the XRD result. Additionally, the identical FT-IR
spectral fingerprints of AgNPs@PAM and PAM (Figure 1f)
further confirm the successful introduction of PAM to the surface
of Ag NPs. Specifically, peaks observed at ~3412 cm-1 are
ascribed to the stretching vibration of N–H.[38] The doublet peaks
at 2975 cm-1 and 2924 cm-1 show the antisymmetric stretching
vibration of methyl and methylene and the peaks at 1650 cm-1
and 1572 cm-1 are assigned to the C=O stretching vibration and
the N-H bending vibration, respectively. The surface composition
of Ag NPs and AgNPs@PAM was further scrutinized by means
of the XPS technique. The as-prepared samples show typical Ag
metal spectra with peak separation of 6 eV and no Ag2O
residuals (Figure S1). The presence of the N 1s peak at around
400.0 eV (Figure S2) indicates the existence of amide N on the
surface of Ag NPs.[39]
The structure and morphology of the PAM-modified Ag NPs
(AgNPs@PAM) powder were firstly characterized by SEM and
are shown in Figure 1. The large-scale SEM image (Figure 1a)
indicates that the hybrid powder has
a honeycomb-like
macroporous morphology. The partially enlarged SEM image
(Figure 1b) further displays that the size of Ag NPs coated with
PAM is 100 ± 20 nm. The elemental mapping images (Figure 1d)
confirm that the elemental carbon, nitrogen and oxygen in PAM
indeed exist in the hybrid catalyst. The XRD patterns (Figure 1e)
indicate that Ag NPs in the as-prepared hybrid are the face-
centered cubic (fcc) structure since Ag (111), (200), (220), (311)
and (222) crystal planes are detected respectively and Ag (111)
is the major plane in the crystalline structure. The HR-TEM
image (Figure 1c) shows that the lattice distance of Ag NPs is
0.23 nm, which corresponds to the Ag (111) crystal plane and
Figure 1. (a) SEM image, (b) partially enlarged image and (c) HR-TEM image of AgNPs@PAM; (d) The elemental mapping images of silver (green), carbon (red),
nitrogen (cyan) and oxygen (purple) of AgNPs@PAM; (e) XRD patterns of AgNPs@PAM; (f) FT-IR spectra of PAM and AgNPs@PAM.
The CO2 adsorption isotherms of Ag NPs, PAM and
AgNPs@PAM were measured to determine the CO2 adsorption
ability. As seen in Figure 2a, Ag NPs shows the worst CO2
adsorption ability (0.2141 cm3 g-1, 1 atm), which is far less than
that of PAM (0.5118 cm3 g-1, 1 atm). The much higher adsorption
capacity for the linear polymer implies that PAM is likely to
adsorb CO2 chemically. Under the accumulative effect of Ag
NPs and PAM, AgNPs@PAM shows the best CO2 adsorption
capacity (0.5442 cm3 g-1, 1 atm), which is more than 2.5 times
larger than that of Ag NPs. Further, the contact angles of a drop
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of water (3 μL) on the surface of Ag NPs and AgNPs@PAM
were measured to evaluate the hydrophilicity of the catalysts
(Figure 2b, c). In contrast to the hydrophobic surface of Ag NPs
(70.35°), AgNPs@PAM exhibits an appropriate hydrophilic
surface property (26.85°) thanks to the introduction of amide
groups. Obviously, the hydrophilic surface of AgNPs@PAM is
more favorable for CO2ER since it can not only ensure a
sufficient contact area between the metal electrode and the
electrolyte to facilitate the mass transfer,[34,40] but also accelerate
the adsorbed gaseous products to escape the surface of metal
electrode.[41-43]
Figure 2. (a) CO2 adsorption isotherms of Ag NPs, PAM and AgNPs@PAM; Contact angle measurements of (b) Ag NPs and (c) AgNPs@PAM.
To preliminarily evaluate the catalytic performance of
AgNPs@PAM, linear sweep voltammetry (LSV) tests were first
conducted in N2- and CO2-saturated 0.1 M KHCO3 electrolyte
with pH 6.8 at a scan rate of 10 mV s-1 (Figure S3). A clear
positive shift of onset potential and enhanced cathodic current
are found in the atmosphere of CO2, indicating that CO2 is
electrochemically reduced over AgNPs@PAM. Electrocatalytic
performance of the hybrid catalyst for CO2ER was further
investigated by the controlled potential electrolysis in 0.1 M
KHCO3 electrolyte saturated with CO2. Only CO and H2 were
observed during the overall electrolysis and no liquid phase
reduction products was found by 1H NMR spectroscopy. The
contents of PAM in the Ag NPs hybrid catalysts quantified by
TGA (Figure S4, Table S1) was first evaluated to determine the
optimal ratio. As expected, the CO partial current density (jco) is
boosted significantly when the content of PAM continues to
increase to 5.34 wt.%. However, both CO2ER and HER (Figure
S5) are suppressed (especially for the former) with further
increasing the content of PAM to 6.35 wt.% probably due to the
deteriorated electrical conductivity of hybrid catalyst (Table S2).
Additionally, it should be noted that the excessively enhanced
hydrophilicity with further increasing the content of PAM tends to
cause the Ag NPs hybrid to be flooded (Figure S6). Taking
CO2ER performance and structure stability of catalyst into
account, the Ag NPs hybrid with 3.74 wt.% PAM (AgNPs@PAM)
was chosen to further explore the role of PAM in selectively
enhancing electrochemical conversion of CO2 to CO. As shown
in Figure 3a, the onset potential of CO2ER over AgNPs@PAM (-
0.39 V) shifts positively by 100 mV with a CO Faradaic efficiency
(FE) of 35.3% compared with Ag NPs without PAM modification
(-0.49 V), implying an improved intrinsic activity. With the
overpotential increasing, both CO FE and jco (Figure 3a, b) rise
rapidly. The AgNPs@PAM hybrid exhibits the maximum CO FE
of 97.2% at -0.89 V with a jco of -22.0 mA cm-2, while Ag NPs in
the absence of PAM only delivers a CO FE of 48% with a jco of -
2.1 mA cm-2 at the same potential. Most impressively, the H2
partial current density (jH2) over AgNPs@PAM is only -5.5 mA
cm-2 when the jco reaches -58.5 mA cm-2 at -1.29 V, providing an
attractive catalyst system for the electrochemical conversion of
CO2 into valuable fuels. To make clear the origin of the
excellently selective CO2ER over AgNPs@PAM, the catalytic
performance of Ag NPs with a similar size of ~ 100 nm (denoted
as AgNPs-100, Figure S7) prepared with the aid of surfactant
sodium dodecyl sulfate was also evaluated. As seen from Figure
3a, b, the jco over AgNPs-100 is indeed remarkably enhanced at
each applied potential compared with that over Ag NPs with the
size of ~ 500 nm, but it still lags far behind that over
AgNPs@PAM, especially at the high overpotentials. On the
contrary, it is interesting to note that the jH2 over AgNPs-100 is
much higher than that over AgNPs@PAM at potentials ≥ −0.79
V. Accordingly, the CO FE over AgNPs-100 reaches the
maximum of 90.0% at -0.69 V with a poor jco of -5.8 mA cm-2,
and then drops rapidly as the potential shifts negatively. By
contrast, the CO FE over AgNPs@PAM can be maintained
above 95% in the wide potential range from -0.79 to -1.09 V, and
it is still above 90% when the applied potential increases to -1.29
V. These comparisons fully reveal that PAM plays a crucial role
in enabling AgNPs@PAM to simultaneously possess high
activity and selectivity at high overpotential. In addition, the
maximum CO FEs of the previously reported Ag-based
electrocatalysts are summarized in Table S3. These results
demonstrate that AgNPs@PAM is a very competitive catalyst for
CO2ER in aqueous solution.
The electrochemically active surface areas (ECSA) of
AgNPs@PAM and Ag NPs were measured through the Cu
underpotential deposition (UPD) method[44] and their specific
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activities were obtained via normalizing the jco by the
corresponding ECSA. As seen in Figure S8, AgNPs@PAM
indeed exhibits an extremely higher specific jco than that of Ag
NPs at each operating potential, further confirming that
AgNPs@PAM is intrinsically more active than Ag NPs. Tafel
analysis was performed to gain insight into the reaction kinetics
of CO2ER over AgNPs@PAM, (Figure S9). The measured Tafel
slope for AgNPs@PAM is 133 mV dec-1, which is comparable to
the 118 mV dec-1 expected for rate-limiting single-electron
transfer from the adsorbed *CO2 to form the surface-adsorbed
*CO2-.[45] However, the smaller Tafel slop for AgNPs@PAM
relative to our previously reported values for Ag NPs with sizes
of ~ 5 nm[46] also suggests that the introduction of PAM may
contribute to the enhancement of the intrinsic activity of Ag NPs.
In addition, the electrochemical stability of AgNPs@PAM was
measured at -0.79 V by prolonging the electrolysis. As shown in
Figure 3c, the total current density remains stable at ~ -11 mA
cm-2 and the CO FE only slightly declines by ~ 2% after the 48 h
electrolysis. This outstanding electrochemical durability could be
reasonably attributed to the following two aspects. First, the
utilization of Ag NPs with large size could exert a mitigated effect
of particle size on CO2ER activity, circumventing a significant
decrease in CO production rate due to the slight agglomeration
of Ag NPs during electrolysis (Figure S10). Second, the
appropriate hydrophilicity of AgNPs@PAM could accelerate the
desorption of CO from the electrode surface, preventing the
hybrid catalyst from being poisoned effectively.[47]
Figure 3. (a) CO FE, (b) jco and (c) jH2 over Ag NPs, AgNPs-100 and AgNPs@PAM versus the applied potential; (d) Stability test for CO2ER over AgNPs@PAM at
−0.79 V.
To shed further light on the origin of selectively enhancing
CO2ER over AgNPs@PAM, a set of comparative experiments
were conducted in CO2-saturated 0.1 M KHCO3 electrolyte
without and with cocatalysts by using the same Ag NPs as
working electrode. As seen in Figure 4a, b, the maximum CO FE
is only 58.0% over Ag NPs at -0.79 V, while it can be
substantially increased to 86.3% at a more positive potential of -
0.69 V after addition of 6 mg PAM to the catholyte. Meanwhile,
the jco in the presence of PAM is also boosted significantly and
such enhancement effect for CO2ER is more pronounced at
higher overpotentials. Interestingly, HER almost shows an
opposite variation trend to CO2ER, implying that the introduction
of a proper amount of PAM is capable of promoting CO2ER
exclusively, and the resultantly increased CO2 concentration on
the surface of Ag NPs could simultaneously suppress HER to
some extent (Figure 4c, d). These results further evidently
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indicate that the synergistic effects of Ag NPs and PAM does
play a preponderant role in selectively enhancing CO2ER, which
enables the AgNPs@PAM hybrid to show a very competitive CO
FE of 97.2% at -0.89 V even though the average size of Ag NPs
water molecules (pKa = 15.8) to release H2, leading to an
improved HER kinetics. In addition, most of the imine groups (its
conjugate acid pKa = 7) in PEI are not protonated and would
play the same role as amide groups to promote CO2ER. All in all,
is as high as ~ 100 nm. For comparison, polyethyleneimine (PEI), the synergistic effect of PEI and Ag NPs can only deliver an
[6,48]
a widely investigated cocatalyst for CO2ER
was also
increased CO production rate at high overpotentials, but
contributes little to the improvement of CO FE. Furthermore, the
ability of PEI to boost the jco is obviously inferior to that of PAM
at each applied potential. From the results of these comparative
experiments, it can be reasonably concluded that the
performance of PAM over PEI in facilitating CO2ER, especially in
terms of selectivity, is closely associated to the p-π conjugate
system of amide group. First, such a unique electron structure
makes the conjugate acid of amide have a pKa of only around -
0.5. This lack of basicity ensures that PAM will not be protonated
to accelerate the H2 evolution rate in the catholyte with a pH of
6.8. Second, the enhanced reactivity of N-H hydrogen atoms
may be more effective in activating CO2 or stabilizing the key
intermediates formed in CO2ER through the formation of
hydrogen bonds compared with those of imine counterpart.
Consequently, these two properties and the appropriate
hydrophilicity mentioned above underlie the AgNPs@PAM
hybrid for selectively enhancing CO2ER.
examined to evaluate its ability to tune the reactivity of Ag NPs.
As shown in Figure 4d, the jH2 appreciably and systematically
rises across all applied potentials examined after addition of 6
mg PEI into the catholyte. By contrast, the jco first follows the
opposite variation trend to the jH2 when the Ag NPs catalyst is
not sufficiently activated at low overpotentials (E ≥ -0.59 V).
Moreover, it can be observed that the onset potential of CO
production also shifts negatively from -0.49 V to -0.59 V. Then
with the overpotential increase, the Ag NPs catalyst is further
activated and PEI starts to exhibit a positive effect on CO2ER.
Accordingly, the change in the jco shows synchronous with that
of jH2, contributing to an almost PEI-independent CO FE owing to
their similar growth rate. In fact, PEI is positively charged in the
CO2-saturated 0.1 M KHCO3 electrolyte (pH = 6.8) because of
partial protonation at the primary amine N’s positions (its
conjugate acid pKa = 9.5) (Inset in Figure 4a). Clearly, these
protonated amine groups in PEI are more easily reduced than
Figure 4. (a) CO FE, (b) jco, (c) H2 FE and (d) jH2 over Ag NPs electrode in 0.1 M KHCO3 electrolyte without and with cocatalysts. Inset: partial protonation of PEI.
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Subsequently, the in-situ ATR-IR and UV-vis spectroscopy
were used to verify the ability of PAM to chemically activate CO2.
As shown in Figure 5a, a broad infrared absorption peak
centered at ~ 837 cm-1 can be found over AgNPs@PAM in CO2
atmosphere, which is very close to that reported infrared
characteristic absorption peak (~ 839 cm-1) of the reactive
intermediate formed by CO2 bonding with -NH2,[49] and the peak
at 837 cm-1 will disappear in the absence of CO2 or PAM. These
two observations further confirm the chemisorbing CO2 taking
place over AgNPs@PAM during the electrolysis, and the
activation of CO2 is likely to be achieved through the formation of
hydrogen bonds between the active hydrogen atoms in the
amide group and the oxygen atoms in CO2.[50,51] In addition, the
UV-vis spectra of 300 mg L-1 PAM aqueous solution in N2 and
CO2 atmosphere were measured to further examine the
formation of hydrogen bond between PAM and CO2. As shown
in Figure 5b, the distinct absorption peaks observed around 200
nm in N2 atmosphere can be assigned to the n→π* electronic
transition resulted from the p-π conjugate effect of amide group
in PAM.[52] The peak wavelength underwent a 3 nm blue shift
after bubbling CO2 into the PAM solution, which can be
attributed to the formation of hydrogen bond between PAM and
CO2. Specifically, the
O
atom in CO2 with
a
strong
electronegativity (3.44) causes an inductive effect toward the
amide group through the formation of hydrogen bond, which
increases the electron cloud density on the nitrogen atom and
reduces the liquidity of π electron in the amide group, thereby
elevating the energy level of π* orbit.[53] Therefore, more energy
for electron migration of n→π* is needed, leading to the blue
shift of the peak wavelength.
Figure 5. (a) In-situ ATR-IR spectra of Ag NPs and AgNPs@PAM purged with N2 or CO2 at -0.89 V; (b) UV-vis spectra of PAM aqueous solution in N2 and CO2
atmosphere.
To sum up, our results have demonstrated that amide group
indeed facilitates the adsorption and the activation of CO2, which
are achieved through the hydrogen bond between the highly
active H atom in amide group and the O atom in CO2. Similar to
the common phenomenon that the adsorption energies of
reaction intermediates tend to follow linear scaling relations in
heterogeneous catalysis, we envision that the key intermediate
COOH* could behave as CO2 to form hydrogen bond with amide
group to lower the overpotential of CO2ER. To this end, DFT
calculations were conducted with Ag (111) and Ag
(111)@C4H9NO models to calculate the free energies of the
following reaction mechanistic steps [Equation 1–3] ,[54] where *
denotes an adsorption site and e- is an electron. For the
convenience of calculation, isobutanamide (C4H9NO), which is
similar in structure to the PAM unit, was chosen to model with
Ag (111) to describe the Ag NPs-PAM interface.
CO2 (g) + * + H+ (aq) + e- → *COOH
*COOH + H+ (aq) + e- → *CO + H2O (l)
*CO → CO + *
(1)
(2)
(3)
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Figure 6. The DFT calculated free energy changes of CO formation without isobutanamide (black line) and with isobutanamide (red line)
Figure 6 shows the optimized structures and calculated free
0.89 V with a desirable jco of -22.0 ± 2.3 mA cm-2, and the CO FE
energy diagram of CO2ER over Ag (111) and Ag (111)@C4H9NO. can even be maintained above 95% over a wide potential
Both Ag (111) and Ag (111)@C4H9NO show a largest free
energy for the formation of carboxyl (*COOH) species by the
initial proton-coupled electron transfer [Equation (1)].
Encouragingly, the corresponding energy cost over Ag
(111)@C4H9NO is 0.23 eV lower than that over Ag (111).
Subsequently, the protonated decomposition of *COOH to *CO
[Equation (2)] and the desorption of *CO [Equation (3)] are also
facilitated under the synergetic effect of Ag (111) and amide
group owing to the spontaneous process. These calculations
demonstrate that the overpotential of CO2ER is indeed reduced
via the formation of hydrogen bond between amide group and
the key intermediate *COOH, leading to an enhanced CO2ER
performance.
window of −0.79 to −1.09 V. Meanwhile, this organic/inorganic
hybrid catalyst also displayed an outstanding electrochemical
durability during a 48 h prolonged electrolysis thanks to the large
size of Ag NPs, showing great potential for practical application
in the large-scale production of CO from CO2 electrolysis. We
anticipate that this combined nanoscale and molecular strategy
can be extended to other important electrocatalytic reactions
and provide a new route for the design of hybrid materials with
synergistic catalytic effects.
Experimental Section
Catalyst preparation
The hybrid catalyst composed of Ag NPs and PAM was synthesized by
one-pot reaction in water phase. Firstly, 0.03 g PAM and 1.275 g AgNO3
were dissolved into 100 mL deionized water successively. The above
solution was then quickly added into a freshly prepared solution of
NaBH4 (0.275 g in 100 mL H2O) under vigorous stirring. After stirring the
mixture for 2 h at 20 ± 3 ℃, the black precipitate was collected by high
speed centrifugation of 8000 r∙min-1, washed thoroughly with deionized
water and dried in a vacuum oven at 60 ℃ overnight, and then the hybrid
catalyst was obtained and denoted as AgNPs@PAM. Notably, the
amount of PAM was easily adjustable, and another three contents of
0.015 g, 0.045 g and 0.06 g were also adopted in this work, the as-
prepared hybrids were named as AgNPs@PAM-15, AgNPs@PAM-45,
AgNPs@PAM-60, respectively. Ag NPs (~ 500 nm diameter) without any
modifications and Ag NPs (~ 100 nm diameter) prepared with the aid of
sodium dodecyl sulfate were synthesized according to our early work.[55]
Conclusion
In summary, we successfully constructed a durably PAM-
mediated Ag NPs electrocatalyst for selectively enhancing the
electroreduction of CO2 to CO in water. The adsorption
isotherms of CO2 and the contact angle measurements show
that amide group can endow this hybrid catalyst excellent CO2
adsorption capacity and favorable hydrophilicity. The
comparative experiments suggest that the exclusive CO2ER
enhancement over Ag NPs after introduction of PAM could be
attributed to the highly active hydrogen atom and the appropriate
pKa value of amide group. The in-situ ATR-IR and UV-vis
spectra confirm that the activation of CO2 is achieved by forming
hydrogen bond with the amide group. Further, DFT calculations
imply that the overpotential of CO2ER could be further reduced
via the same reaction mode between the key intermediate
*COOH and amide group. Benefiting from these merits
mentioned above, the synergistic catalysis of Ag NPs and amide
group contributes an outstanding CO FE of 97.2% ± 0.2 % at -
Characterizations
To get the crystallographic information of the catalyst, powder X-ray
diffraction (XRD) patterns were obtained by using a Rigaku X-ray facility
with CuKα radiation (λ = 1.5406 Å) at 40 kV and 20 mA with the 2θ-angle
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from 5° to 90°. A field-emission scanning electron microscope (FESEM;
Nova Nanosem 450) equipped with energy dispersive X-ray
spectroscopy (EDX) was used to observe the morphology and elemental
composition of the catalyst. High-resolution electron microscopy
(HRTEM; Jeol, JEM-2100, 200 kV) was performed to determine the
crystal faces and lattice stripes of the catalyst. X-ray photoelectron
spectroscopic (XPS) characterizations were performed on a Thermos
Scientific Escalab 250Xi instrument with Al Kα X-ray to measure the
surface properties of the as-prepared hybrid catalysts. To determine the
Ag content of catalyst, the thermal gravimetric analysis (TGA) was
performed on a Netzsch STA449F3 at a temperature ramp of 5 °C min−1
under an air flow from room temperature to 900 °C. Fourier transform
infrared spectroscopy (FT-IR) data were obtained using a Nicolet Magna-
IR 550 spectrometer and the contact angles were recorded using a
DataPhysics OCA-30 instrument. The resistivity of as-prepared catalysts
was tested using a M-3 Mini type four-probe tester (Suzhou Jingge
Electronic Co., Ltd.). The adsorption isotherms of CO2 were obtained at
298 K using a Quadrasorb SI analyser and the catalysts were degassed
at 298ꢀK for 10ꢀh prior to the measurement.
correlation functional,[56,57] and the electron-ion interaction was treated
using the projector-augmented-wave (PAW) method.[34,58] Wave functions
of valence electrons were expanded using plane wave basis sets with a
kinetic energy cutoff of 400 eV. The (4×4) periodic slab models with four
atomic layers were combined with a 20 Å vacuum layer. Along the z axis,
two bottom layers were fixed during the geometry-optimization processes
whereas the upper two layers were allowed to relax. The closest-packed
faces which are (111) for the fcc structure were considered to be
representative reactive surfaces and the (1×1×1) k-point grid was
employed. To estimate the enthalpy changes during solvation process
implemented in the VASP program, a dielectric constant ε = 80 was used
for water.[59-61]
Acknowledgements
We acknowledge the National Natural Science Foundation of
China (No. 21972042), and the Student’s Platform for Innovation
and Entrepreneurship Training Program (No. S19005). We also
thank the Research Center of Analysis and Test of East China
University of Science and Technology for help with the
characterization.
The in-situ attenuated total reflectance infrared (ATR-IR) spectra were
collected by a PerkinElmer Spectrum 100 spectrometer equipped with a
liquid nitrogen cooled mercury cadmium telluride (MCT) detector (LingLu
Instruments, Electrochem IR) with unpolarized IR radiation at an
incidence angle of 55°. The catalysts were compressed into square
sheets and placed on the basal plane of a hemi-cylindrical Si prism
beveled at 60°. Typically, 64 scans were collected for each spectrum at a
resolution of 4 cm-1. An ultraviolet and visible spectrophotometer (UV-vis;
Shimadzu UV-2550) was used to collect the absorption curves of PAM
aqueous solution saturated with N2 and CO2, respectively. Using distilled
water as the blank, the different solutions were scanned in the
wavelength range of 190 ~ 800 nm.
Keywords: Nanohybrid • Amide • Ag nanoparticles • CO2
reduction • Synergistic catalysis
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Entry for the Table of Contents
The polyacrylamide-modified Ag nanoparticles with extraordinary CO2 adsorption capacity and favorable hydrophilicity were
constructed for the selective electroreduction of CO2 to CO in near-neutral aqueous solution.
10
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