Effect of Chloride Anions on the Synthesis and Enhanced Catalytic Activity of Silver Nanocoral Electrodes for CO2 Electroreduction

Article abstract of DOI:10.1021/acscatal.5b01235

Metallic silver (Ag) is known as an efficient electrocatalyst for the conversion of carbon dioxide (CO₂) to carbon monoxide (CO) in aqueous or nonaqueous electrolytes. However, polycrystalline silver electrocatalysts require significant overpotentials in order to achieve high selectivity toward CO₂ reduction, as compared to the side reaction of hydrogen evolution. Here we report a high-surface-area Ag nanocoral catalyst, fabricated by an oxidation-reduction method in the presence of chloride anions in an aqueous medium, for the electro-reduction of CO₂ to CO with a current efficiency of 95% at the low overpotential of 0.37 V and the current density of 2 mA cm^-2. A lower limit of TOF of 0.4 s^-1 and TON > 8.8 × 10⁴ (over 72 h) was estimated for the Ag nanocoral catalyst at an overpotential of 0.49 V. The Ag nanocoral catalyst demonstrated a 32-fold enhancement in surface-area-normalized activity, at an overpotential of 0.49 V, as compared to Ag foil. We found that, in addition to the effect on nanomorphology, the adsorbed chloride anions play a critical role in the observed enhanced activity and selectivity of the Ag nanocoral electrocatalyst toward CO₂ reduction. Synchrotron X-ray photoelectron spectroscopy (XPS) studies along with a series of control experiments suggest that the chloride anions, remaining adsorbed on the catalyst surface under electrocatalytic conditions, can effectively inhibit the side reaction of hydrogen evolution and enhance the catalytic performance for CO₂ reduction.

Full text of DOI:10.1021/acscatal.5b01235

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The Effect of Chloride Anions on the Synthesis and
Enhanced Catalytic Activity of Silver NanoꢀCoral
Electrodes for CO2 Electroreduction
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YuꢀChi Hsieh, Sanjaya D. Senanayake, Yu Zhang, Wenqian Xu†, and Dmitry E. Polyansky*
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973ꢀ5000, United
States
ABSTRACT
Metallic silver (Ag) is known as an efficient electrocatalyst for the conversion of carbon
dioxide (CO2) to carbon monoxide (CO) in aqueous or nonꢀaqueous electrolytes. However,
polycrystalline silver electrocatalysts require significant overpotentials in order to achieve high
selectivity toward CO2 reduction, as compared to the side reaction of hydrogen evolution. Here
we report a highꢀsurfaceꢀarea Ag nanoꢀcoral catalyst, fabricated by an oxidationꢀreduction
method in the presence of chloride anions in an aqueous medium, for the electroꢀreduction of
CO2 to CO with a current efficiency of 95% at the low overpotential of 0.37 V and the current
density of 2 mA cm−2. A lower limit of TOF of 0.4 s−1 and TON > 8.8 × 104 (over 72 h) was
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estimated for the Ag nanoꢀcoral catalyst at an overpotential of 0.49 V. The Ag nanoꢀcoral
catalyst demonstrated a 32ꢀfold enhancement in surfaceꢀareaꢀnormalized activity, at an
overpotential of 0.49 V, as compared to Ag foil. We found that, in addition to the effect on nanoꢀ
morphology, the adsorbed chloride anions play a critical role in the observed enhanced activity
and selectivity of the Ag nanoꢀcoral electrocatalyst toward CO2 reduction. Synchrotron Xꢀray
photoelectron spectroscopy (XPS) studies along with a series of control experiments suggest that
the chloride anions, remaining adsorbed on the catalyst surface under electrocatalytic conditions,
can effectively inhibit the side reaction of hydrogen evolution and enhance the catalytic
performance for CO2 reduction.
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KEYWORDS. Nanoꢀporous Ag, chloride modification, carbon dioxide reduction,
electrocatalysis, high selectivity.
INTRODUCTION
Decreasing the accumulation of carbon dioxide (CO2) in Earth’s atmosphere is one of the
most important issues for human society on the path to develop sustainable technology and
protect the environment.1,2 Electrochemical reduction of CO2 has received wide attention and is
considered as a promising method for synthesizing carbonꢀcontaining fuels and achieving
sustainable energy conversion and storage.3ꢀ14 However, one of the major drawbacks of
electrochemical CO2 reduction in aqueous solution is the high overpotential required to achieve
substantial reaction rates and selectivity.11,15ꢀ17 The origin of high overpotential was proposed by
•–
Bockris et al.15,16 to be the formation of CO2 as the first reaction intermediate. The very
•–
negative reduction potential of the CO2/CO2 couple, E° = −1.9 V vs. standard hydrogen
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electrode (SHE)17,18, creates a significant thermodynamic and kinetic bottleneck for the overall
CO2 reduction reaction. In addition to the associated energy loss, the use of more negative
potentials increases the efficiency of other side reactions, for example the hydrogen evolution
reaction (HER), and thus decreases the current efficiency of the CO2 reduction reaction.
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A catalyst’s performance can be evaluated using three key factors: its activity, selectivity
and durability. High activity is characteristic to catalysts that operate at low overpotential and
exhibit large current for the electrocatalytic reaction. Selectivity refers to the yield of the main
product over that of side products. Under the reaction conditions for CO2 electroꢀreduction in
aqueous solutions, the major side reaction is the HER. Finally, the durability characterizes a
catalyst’s ability to maintain its initial activity over an extended operation time with little or no
degradation. Among others, three transition metals: Cu,5,13,19ꢀ23 Ag,24ꢀ29 and Au19,30ꢀ32 have been
most extensively studied as electrocatalysts for the CO2 reduction reaction. Hori reported that
Cu, Ag, and Au had high activities for CO2 reduction at potentials of −1.04 V, −0.97 V, and
−0.74 V (vs. reversible hydrogen electrode, RHE) respectively,17 with a current density of −5
mA cm−2 in 0.1 M KHCO3. However, Cu demonstrated poor selectivity, resulting in the
formation of a variety of products. While Au was reported to operate at the lowest overpotential
among these three metals and showed a high selectivity for CO2 reduction to carbon monoxide
(CO),30,32 its substantial cost may limit potential industrial applications.
The electroreduction of CO2 on flat Ag surfaces was shown to produce CO with a
reasonably high selectivity (~90%), but at substantially high overpotentials,24,33 with respect to
the standard reduction potential E° = −0.106 V (vs. SHE) of the reaction CO2 + 2H+ + 2e− = CO
+ H2O.11 In order to further enhance the CO2 reduction activity of Ag, nanostructured Agꢀbased
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catalysts with increased surface area have been developed.26ꢀ28 A tenfold increase in the intrinsic
activity was achieved by 5 nm Ag nanoparticles compared to bulk Ag.28 Nanoporous Ag
prepared by the deꢀalloying method showed 92% selectivity for CO2 reduction to CO at a
moderate overpotential of 0.49 V, and a 20 times higher intrinsic activity than bulk Ag.26 The
enhanced activity of the nanostructured Ag catalysts was attributed to the greater stabilization of
CO2•– on the nanoporous Ag.26
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In this work, we fabricated a nanostructured coralꢀlike Ag electrocatalyst, via an
oxidationꢀreduction process using chloride anions. The Ag nanoꢀcoral catalyst exhibited a high
activity for CO2 reduction with a low overpotential of 0.37 V, and a high selectivity with a CO
current efficiency of 95%. The Ag nanoꢀcorals displayed a 660ꢀfold enhancement in CO partial
current density compared to bulk Ag, which corresponds to a 32ꢀfold improvement in intrinsic
activity, along with a 20ꢀfold increase of electrochemical surface area. We found that the effect
of chloride anions was more than facilitating the fabrication of the highꢀsurfaceꢀarea nanoꢀ
structured Ag catalyst. More importantly, the presence of chloride anions on the catalyst surface
plays a crucial role in the performance enhancement for the CO2 reduction reaction, by
enhancing the intrinsic activity for CO2 reduction, and by improving the selectivity for the
product CO by suppressing the side reaction HER. The longꢀterm durability of the Ag nanoꢀcoral
catalyst was evaluated, and the possible reason for the performance degradation is discussed.
EXPERIMENTAL SECTION
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Materials. Silver foil (99.9985%, 0.5 mm thick, Alfa Aesar) was used following the
cleaning procedure described below. Potassium bicarbonate (KHCO3, 99.7%, Baker Analyzed)
and potassium chloride (KCl, 99.1%, Baker Analyzed) were used to prepare the electrolytes. The
0.1 M KHCO3 solution was purified by preꢀelectrolysis, where the solution was saturated with
Ar, and Pt foil (24.6 cm2) was used as the working electrode at an applied potential to maintain a
constant current of −10 mA for 12 h. The 0.1 M KCl solution was also preꢀelectrolyzed using a
similar method. Deionized water (purified by a MilliꢀQ system) was used to prepare all solutions
and to rinse samples and glassware.
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Sample preparation. The Ag foil was cleaned by immersion and sonication for 15 min
sequentially in hexane (99.9%), ethanol, and then deionized water. The AgCl sample was
prepared by oxidizing Ag foil in 0.1 M Arꢀsaturated preꢀelectrolyzed KCl solution at 0.3 V (vs.
Ag/AgCl, 3 M KCl) for 12 h and then rinsing. The Ag nanoꢀcoral sample was prepared by
reducing the asꢀprepared AgCl in 0.1 M CO2ꢀsaturated preꢀelectrolyzed KHCO3 solution at −1.2
V (vs. Ag/AgCl, 3 M KCl) for 30 min and then rinsing. After the reduction process, the
electrochemical cell was thoroughly rinsed, and filled with fresh 0.1 M preꢀelectrolyzed KHCO3
solution purged with CO2 to perform the CO2 reduction reaction.
Synthesis of Ag nanoparticles (NPs). 1g of AgNO3 was dissolved in 100 mL of
deionized water, and 500 mg of sodium citrate was dissolved in another 100 mL of deionizedꢀ
water. The two solutions were mixed together and stirred for 30 min. A solution of 1 g of NaBH4
in 50 ml of deionized water was quickly added into the reaction solution, and the mixture was
stirred for 2 h. Then the solution was filtered and the precipitate was rinsed repeatedly with
deionized water to remove any remaining byproducts, until the pH of the filtered liquid reached 7.
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The filtered Ag nanoparticles were then added into 250 mL of deionized water, and the mixture
was sonicated for 2 h. After that, the suspension was centrifuged at 3000 rpm for 1 h, and the
clear supernatant with a yellowꢀgreen color was collected in order to remove the larger
nanoparticles. Then the yellowꢀgreen supernatant was centrifuged at 9000 rpm for 1 h, and the
solid sediment was collected in order to separate from smaller nanoparticles. The collected Ag
nanoparticles were reꢀdispersed in water, and then deposited onto clean pure Ag foil. After
drying, the Ag foil with Ag nanoparticles was baked at 200℃ for 1 h, and then immersed in acid,
in order to remove impurities on the nanoparticle surface. The nanoparticle loading for
electrochemical measurements was 1.2 mg cm−2 Ag. The average particle size of the Ag NPs
(50±24 nm) was measured by SEM (Figure S1) using ImageꢀPro Plus software.
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Electrochemical methods. An ECꢀepsilon BASi potentiostat was used for all
electrochemical experiments. Pt gauze was used as the counter electrode. The reference electrode
was a Ag/AgCl (3 M KCl, BASi) electrode unless otherwise noted, with the potential deviation
of 0.13 mV.34 The reference electrode was periodically calibrated against Hydrogen Reference
Electrode (Hydroflex, eDAQ) and the environmental temperature variations (± 5 °C) resulted in
uncertainties in reference potentials of ca. ± 1 mV. The potentials were measured against the
reference electrode, and converted to the reversible hydrogen electrode (RHE) reference scale by
ERHE = EAg/AgCl + 0.197 + 0.059 × pHelectrolyte. All the potentials were reported versus RHE unless
otherwise noted. Electrochemical impedance spectroscopy was used to determine the high
frequency resistance (HFR) as the uncompensated solution resistance (Ru) at AC frequency
ranging from 10 MHz to 1 Hz. The HFR was measured at −0.3 V and −1.5 V where its values at
the two potentials differ by only 0.1 ꢀ, and the average value was used. The potential was
corrected using automatic iR compensation function of the potentiostat by applying 85% of the
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Ru value measured using impedance spectrometry. The use of this value insured the stable
operation of the potentiostat.35 The remainder 15% of the Ru was mathematically corrected, using
the equation:24
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E100% iRꢀcorrected = E85% iRꢀcorrected – 15% × average Ru × average i,
where E100% iRꢀcorrected is the final reported potential; E85% iRꢀcorrected is the potential used in the
controlled potential electrolysis experiment with the iR compensation function using 85% of Ru;
average i is the average current attained by averaging the total electric charge over the duration
of the controlled potential electrolysis experiment.
The electrochemical surface area was determined using lead (Pb) underpotential
deposition (UPD) in 1 mM Pb(acetate)2 + 1 mM HClO4 + 0.5 M NaClO4 solution, at 10 mV s−1.
A saturated calomel electrode (SCE) was used as the reference electrode. As shown in Figure S2,
the Pb UPD desorption peak at −0.35 to −0.2 V (vs. SCE) was integrated to calculate the
electrochemical surface area, assuming 420 ꢁC cmec−2 for Agꢀbased samples.36,37
CO2 reduction measurements and product analyses. The customꢀdesigned
electrochemical cell consisting of three gasꢀtight compartments (for the working, counter, and
reference electrodes respectively) was used in electrocatalytic experiments. The working and
counter compartments contained 12 mL electrolyte each with 14.8 mL headspace. The
compartment for the working electrode was separated from the counter electrode compartment
by a glass frit with porosity sufficient to prevent the diffusion of Pt species produced by
dissolution of the counter electrode under the experimental conditions, as well as to prevent the
diffusion of products produced at the working electrode to the counter electrode compartment.
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The reference electrode compartment was connected to the working electrode compartment
through a capillary.
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The CO2 reduction reaction was carried out in CO2ꢀsaturated 0.1 M preꢀelectrolyzed
KHCO3 (pH = 6.8, measured by an Accumet AB15 pH meter) at different potentials for 30 min,
while the electrolyte was stirred at 870 rpm. Upon the completion of electrolysis, a 60 ꢁL gas
sample was removed through a septum port via a gasꢀtight syringe, and was injected into a gas
chromatograph (GC) instrument (Agilent 6890N) to quantify the amount of CO and H2. For the
asꢀprepared AgCl sample (Supporting Information Figure S3), the catalytic performance was
measured in CO2ꢀsaturated 0.1 M preꢀelectrolyzed KHCO3 (pH = 6.8) at −0.6 V for 2 h, and a 60
μL gas sample was taken every 15 min into the GC for product analyses. The solutionꢀphase
products were analyzed by ion chromatography (ThermoꢀFisher Dionex ICSꢀ1600).
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The durability test of CO2 reduction was carried out in CO2ꢀsaturated 0.1 M preꢀ
electrolyzed KHCO3 at −0.6 V for 72 h. After the 72 h durability test, the electrochemical cell
was thoroughly rinsed, and fresh 0.1 M preꢀelectrolyzed KHCO3 solution was filled and purged
with CO2 to perform the CO2 reduction reaction at −0.6 V for 30 min.
Characterization. The soft Xꢀray photoelectron spectroscopy (sXPS) experiments were
conducted at the National Synchrotron Light Source (NSLS, BNL) at the U12A beamline. The
endꢀstation at this beamline is an ultrahighꢀvacuum (UHV) chamber (1 × 10−10 Torr) equipped
with a hemispherical analyzer for photoemission and is described in detail elsewhere.38 The
incident photon energy for all measurements was 600 eV at an energy resolution of 0.3 eV.
Energy calibration was performed for all sXPS spectra using the corresponding position of the
adventitious carbon feature in the C 1s peak at 284.6 eV, with cross reference with the Fermi
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edge of the Cu sample mount (0 eV). The Xꢀray diffraction (XRD) experiments were conducted
at the National Synchrotron Light Source (NSLS, BNL) at the X7B beamline. The wavelength
was 0.3196 Å. The Rietveld refinements were performed using the GSAS software to estimate
the grain size (Supporting Information Figure S4 and Table S1). Scanning electron microscopy
(SEM) images were obtained using a Hitachi 4800 scanning electron microscope at the Center
for Functional Nanomaterials (CFN) at BNL. The atomic ratio of Ag to Cl was determined by Xꢀ
ray photoelectron spectroscopy (XPS) measurements using an RHK Technology UHV 7500 with
monochromatized Al Kα radiation (1486.7 eV) at the Center for Functional Nanomaterials
(CFN) at BNL, and by energyꢀdispersive Xꢀray spectroscopy (EDS) measurements using an
Ametek EDAX silicon drift detector (SDD) equipped on a Hitachi Sꢀ3400N SEM.
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RESULTS AND DISCUSSION
Synthesis and characterization of Ag nano-corals. Nanoꢀstructured coralꢀlike Ag
samples were fabricated by subjecting a Ag foil to an oxidationꢀreduction process in the presence
of Cl–. A schematic illustration of the synthetic process is depicted in Figure 1a, and the SEM
images (Figures 1bꢀ1d) show the corresponding changes to the surface morphology. First, during
the oxidation in Arꢀsaturated 0.1 M KCl, an anodic potential is applied to pure Ag foil (Figure 1b)
which oxidizes Ag surface atoms to form Ag+ in the solution phase, followed by the interaction
of Ag+ with Cl– to form solid AgCl (Figure 1c) on the Ag surface.39,40 With prolonged oxidation
of the Ag foil, the AgCl formed on the Ag surface exhibits a dense structure with microꢀchannels
(as shown schematically in Figure 1a). In the next step, the surface layer of AgCl is reduced in
CO2ꢀsaturated 0.1 M KHCO3. Under cathodic potential, AgCl decomposes into equal amounts of
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Ag+ and Cl– near the electrode surface. The Ag+ cations are reduced to Ag atoms, and
subsequently the Ag atoms nucleate and are deposited back onto the substrate, while Cl– anions
diffuse into the electrolyte.40,41 These reactions convert the dense AgCl (Figures 1c, 1e) into a
coralꢀlike nanoporous Ag sample (denoted as “Ag nanoꢀcorals”, Figures. 1d, 1f) with the pore
size depending on the Ag nucleation rate determined by the reduction current density.40
Synchrotron XRD measurements were carried out to investigate the crystal structure of these Agꢀ
based samples (Figure S4, Table S1). Ag foil showed a faceꢀcentered cubic (fcc, space group 225,
Fmꢀ3m) structure. The asꢀprepared AgCl sample exhibited two phases, including the AgCl(fcc)
phase lying on the surface and metallic Ag(fcc) phase due to the substrate, and therefore two sets
of fcc peaks from Ag and AgCl can be found in the diffractogram. In contrast, the Ag nanoꢀ
corals displayed only one set of Ag(fcc) peaks, and the absence AgCl(fcc) peaks was probably
due to the very small amount of Cl– left on the catalyst surface after the oxidationꢀreduction
process (vide infra).
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Figure 1. (a) Schematic illustration of the formation of asꢀprepared AgCl and Ag nanoꢀcorals. SEM
images of (b) untreated Ag foil, (c) asꢀprepared AgCl after 12 h oxidation, (d) Ag nanoꢀcorals. Highꢀ
magnification SEM images of (e) asꢀprepared AgCl after 12 h oxidation, and (f) Ag nanoꢀcorals.
In order to investigate the surface composition and chemical states of the Ag nanoꢀcorals,
the surfaceꢀsensitive soft Xꢀray photoelectron spectroscopy (sXPS) measurements were
performed at a synchrotron beamline. First, an XPS survey in a wide energy range (Figure 2a)
was carried out, and the peaks of Ag, Cl and C were identified. The adventitious C 1s peak was
used for energy scale calibration. A higher resolution scan of the Ag 3d region is presented in
Figure 2b. The two bands at 368.1 eV and 374 eV can be attributed to the Ag 3d5/2 and Ag 3d3/2
binding energies, respectively. According to the literature values, the Ag metallic state (Ag0)
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exhibit two peaks at 368.2 eV and 374.2 eV and AgCl sample (Ag+) has peaks at 367.2 eV and
373.2 eV, respectively.42ꢀ44 The Ag nanoꢀcoral sample (red) has a similar Ag 3d band structure to
the Ag foil (black). Figure 2c shows the Cl 2p region including the 2p1/2 and 2p3/2 features. In the
AgCl sample (olive green), Cl and Ag are chemically bonded with a 1:1 atomic ratio (Figures
2c,d), and form an ordered rockꢀsalt crystal structure (Figure S4). For the Ag nanoꢀcoral sample
(red), Cl forms a stable chemically bonded thin layer on the Ag surface in the Cl– state, instead of
entering the Ag lattice.45 The Cl 2p peaks of the Ag nanoꢀcorals (red) are red shifted by 0.8~1.1
eV compared to those of the AgCl (olive green) due to an imbalanced surface charge. The
bonding between the surface Cl− species and the Ag surface atoms is very stable for Ag nanoꢀ
corals,39,46 as demonstrated by the high desorption temperature of 700 K for the surface Cl−
species on Ag.46
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Figure 2. Synchrotron sXPS spectra of (a) survey, (b) Ag 3d, (c) Cl 2p, and (d) Ag MVV for Ag foil
(black), asꢀprepared AgCl (olive green), and Ag nanoꢀcorals (red).
The Ag foil and Ag nanoꢀcorals show similar Ag 3d structure, but their corresponding Ag
MVV auger transitions exhibit different peak positions and densities (Figure 2d). The Ag MVV
spectra are shown using a binding energy scale, but in the numeric direction opposite to
convention in order to facilitate the comparison with literature results. MVV energies were
calibrated using the adventitious C 1s at 284.6 eV, since the surface work function (ψ) was not
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accurately accessible. The energy shift of the Ag M4VV peak for asꢀprepared AgCl compared to
that for Ag foil was 2.3 eV (Table 1), which is consistent with the reported value of a 2.5 eV
shift between these two samples.47,48 It should be noted that for the Ag nanoꢀcoral sample after
the oxidationꢀreduction process in presence of the chloride anions, both of its Ag MVV peaks
shifted by 0.3 eV, compared with Ag foil, suggesting that the Ag nanoꢀcoral sample contained a
small amount of Ag+ on the Ag surface. The analyses of Ag MVV and Cl 2p peaks of the Ag
nanoꢀcoral sample suggest that a small amount of Ag+ bound to Cl– is retained on the catalyst
surface, which may be the active species contributing to the high activity and selectivity of the
Ag nanoꢀcorals toward CO2 reduction.
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Table 1. Summary of XPS measurements of various Ag electrocatalysts studied in this work.
Ag M5VV
Ag M4VV
Ag 3d5/2 Ag 3d3/2
Cl 2p3/2 Cl 2p1/2
(B.E.)
244.4
246.3
244.7
241.5
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238.4
240.7
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235.7
Ag foil
368.1
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/
/
Asꢀprepared AgCl
Ag nanoꢀcorals
198.2
197.4
197.8
199.8
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199.3
Ag nanoꢀcorals, 72 h CO2 reduction 367.4
Performance for CO2 reduction. The CO2 reduction activity was evaluated at constant
potentials in CO2ꢀsaturated 0.1 M KHCO3. The gaseous products and the solutionꢀphase products
were analyzed by gas chromatography (GC), and by ion chromatography (IC), respectively.
First, we assessed the activity of asꢀprepared AgCl at −0.6 V (vs. RHE) as shown in Figure S3.
Within 25 minutes from the beginning of electrolysis, high current densities were observed,
although the current efficiencies for both CO and H2 were close to zero, indicating that the
observed currents were dominated by the reductive decomposition of AgCl. After 45 minutes of
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electrolysis, the CO current efficiency increased rapidly and reached 95%, while the H2 current
efficiency stabilized below 1.5%, and the only solutionꢀphase product that could be accurately
detected was formate (HCOO–) with 1~2% current efficiency. It is informative to compare the
CO2 reduction activity of the Ag nanoꢀcoral sample, pure Ag foil and Ag nanoparticles (Ag NPs)
as shown in Figure 3. The Ag foil exhibited very low CO current efficiency (2~12%) and high
H2 current efficiency (60~90%) at potentials ranging from −0.5 to −0.8 V.24ꢀ26,33 In stark contrast,
the Ag nanoꢀcoral sample demonstrated very high CO current efficiencies reaching 95%, and
very low H2 current efficiency (< 2%), at potentials below −0.6 V, indicating excellent
selectivity for CO2 reduction to CO. The Ag NPs (50±24 nm) showed higher CO2 reduction
activity compared to Ag foil as a result of the nanoꢀsize effect which provided an enhanced
surface area and higher intrinsic activity. The CO partial current densities for the Ag nanoꢀcorals
and Ag NPs were 660 times and 40 times, respectively, higher as compared to Ag foil.
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Figure 3. (a) Total current density, (b) CO current efficiency, and (c) H2 current efficiency, measured in
CO2ꢀsaturated 0.1 M KHCO3 at different potentials (iRꢀcorrected), for Ag foil (hollow black square), Ag
nanoparticles (NPs, hollow blue triangle), and Ag nanoꢀcorals (red spheres).
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The CO partial current density was normalized by the electrochemical surface area,
determined by the Pb underpotential deposition (UPD) method (shown in Figure S2), to calculate
the specific activity (SA in Table 2). Specific activity excludes the influence of different surface
areas, and therefore it can be used to represent the intrinsic activity of catalysts. Ag nanoꢀcorals
demonstrated the highest specific activity of 162 ꢁA cm−2, which was 32 times higher than that
of Ag foil (5 ꢁA cm−2), and was 7 times greater than the specific activity of the Ag nanoparticles
(21.6 ꢁA cm−2). Despite the larger particle size, Ag nanoꢀcorals (~150 nm, Table S1) exhibit
substantially higher specific activity as compared to Ag nanoparticles (50±24 nm, shown in
Figure S1), suggesting that the effect of nanoꢀstructured morphology is not the sole contributor
to the CO2 reduction performance. We propose that the surface bound chloride ions (or
associated surface silver ions) are responsible for the higher activity and selectivity of AgClꢀ
derived Ag nanoꢀcorals towards electroꢀreduction of CO2 to CO.
Table 2. CO2 reduction performance on Ag foil, Ag nanoꢀcorals, and Ag nanoparticles (Ag NPs). jCO is
the CO partial current density calculated by multiplying the CO current efficiency by the measured
current density (normalized by the electrode geometric area, Ageo = 1 cm2). Aec is the electrochemical
surface area measured by the Pb UPD method (Figure S2). SA is the specific activity where SA = jCO
Ageo / Aec. The jCO and CO current efficiency were measured at −0.6 V (vs. RHE, with iRꢀcorrection).
×
jCO
Aec
SA
CO current efficiency
(%)
−2
2
−2
(ꢁA cmgeo
)
(cmec ) (ꢁA cmec )
Ag foil
Ag nanoꢀcorals
Ag NPs
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2.2
6620
400
40.8
18.5
162
21.6
95.0
45.4
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Furthermore, Tafel analysis was performed to gain mechanistic insights49 for the CO2
reduction on the Agꢀbased catalysts’ surface. It is commonly considered that the initial step in
the overall twoꢀelectron reduction of CO2 to CO on most metal surfaces is a oneꢀelectron transfer
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•–
step forming adsorbed CO2 :
•–
ꢂ ꢅꢆꢇ
ꢀꢁꢂ + ꢃꢄ → CO
(1)
The standard reduction potential for the CO2/CO2ads couple is very negative (E0 = −1.9 V vs.
SHE),17,18 but can be shifted toward more positive values due to specific interactions between the
adsorbed CO2•– radical and the surface atoms of a metal electrode.50
•–
•–
Next, the reaction intermediate CO2ads reacts with two protons, and gains another
electron to form CO:17,51
•–
CO
+ ꢋꢀꢁꢌꢄ → ꢀꢁꢁꢋꢈꢉꢊ + ꢀꢁꢌꢂꢄ
(2)
(3)
(4)
ꢂ ꢈꢉꢊ
ꢀꢁꢁꢋꢈꢉꢊ + ꢋꢀꢁꢌꢄ + ꢃꢄ → ꢀꢁꢈꢉꢊ + ꢋꢂꢁ + ꢀꢁꢌꢂꢄ
ꢀꢁꢈꢉꢊ → ꢀꢁ
The step (2) is a chemical step involving protonation of CO2ads•– to form COOHads intermediate.52
−
Under experimental conditions similar to those described in this work, HCO3 is the most likely
source of protons, since the pKa of bicarbonate is much lower as compared to that of water.51 The
step (3) is an electrochemical step coupled to a chemical reaction involving proton and electron
transfer, and the step (4) is the desorption of CO product.
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As shown in Figure 4, the polycrystalline Ag foil exhibited a Tafel slope of 137 mV
dec−1, which is close to 140 mV dec−1 for polycrystalline Ag under similar experimental
conditions,24 and comparable to 132 mV dec−1 measured in 0.5 M KHCO3.26 The Tafel slope of
137 mV dec−1 for polycrystalline Ag foil in our experiments was also close to the value of 130
mV dec−1 for polycrystalline Au.17 These results indicate that polycrystalline Ag and Au could
have the same rateꢀdetermining step, which was assigned previously to the oneꢀelectron
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•–
reduction of CO2 to form CO2ads (eq. 1) with the theoretical Tafel slope of 118 mV
dec−1.17,24,26,30 In contrast, in our experiments Ag nanoparticles showed a Tafel slope of 87.9 mV
dec−1, indicating that the reaction kinetics is faster as compared to Ag foil, and the rate
determining step is not pure electron transfer.26,51
Figure 4. Tafel plot at different overpotentials η (iRꢀcorrected) as a function with the CO partial current
density jCO, on Ag foil (hollow black square), Ag NPs (hollow blue triangle), and Ag nanoꢀcorals (red
ball).
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Interestingly, Ag nanoꢀcoral sample displayed the lowest Tafel slope of 58.5 mV dec−1,
indicating that the reaction kinetics on Ag nanoꢀcorals is the fastest among these samples.
Similar Tafel slopes of 56 ~ 58 mV dec−1 were also reported for oxideꢀderived Au30 and for
nanoporous Ag.26 The Tafel slope for Ag nanoꢀcorals is very close to the theoretical value of 59
mV dec−1, suggesting that a chemical step (such as in eq. 2) is the rateꢀdetermining, followed by
another electron transfer.51,53,54 The effects of halides present at the electrode surface or in the
electrolyte have been previously reported for CO2 electroreduction on copper surfaces55, and
substantial improvement in current efficiency and selectivity was observed. It appears that the
presence of chloride ions on the Ag electrode surface or in the electrolyte solution in proximity
to the electrode promotes selectivity and activity for the CO2 reduction reaction.
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In an attempt to further confirm the role of the surface chloride in CO2 electroreduction,
the AgClꢀderived nanoꢀcoral catalyst was subjected to a reductive treatment under hydrogen
atmosphere, in order to remove the adsorbed Cl from the Ag surface while retaining the porous
structure (see Supporting Information for details). The reduced sample shows a relatively low
CO current efficiency of 41% and high H2 current efficiency of 58%, as compared to a 95%
current efficiency for the nanoꢀcoral Ag catalyst under similar experimental conditions
(Supporting Information Figure S5). However, addition of Cl– to the electrolyte solution leads to
an increase in CO current efficiency even without the oxidation/reduction treatment; though only
up to ca. 63% (Supporting Information Figure S5c). In order to quantify the effect of the Cl
surface coverage on the electrocatalytic performance, the correlation between the amount of
surface chloride and the CO2 reduction activity was investigated. Ag NPs deposited on silver foil
supports were used as substrate material in order to attain controllable chloride concentration and
maintain the constant catalysts’ surface area. Ag NPs were immersed in HCl solution for
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different periods of time, and the atomic ratio of Cl:Ag on NPs’ surface was determined by XPS
measurements. With the prolonged immersion in 3.5 wt% HCl solution the Cl:Ag ratio increases,
together with an increase in CO partial current density (Supporting Information Figure S6a). The
Ag NPs immersed in HCl solution for 4 h exhibit the highest Cl:Ag ratio and therefore the best
CO2 reduction performance within the sample series. Compared to unmodified Ag NPs, the Ag
NPs immersed for 4 h demonstrate a 4.5ꢀfold enhancement in CO partial current density and a
4.7ꢀfold enhancement in specific activity, and the CO current efficiency rise from 43% to 74%,
while the surface morphology remains virtually unaffected (Supporting Information Figure S6b).
In summary, these experiments confirm that the presence of chloride on the electrode surface or
in the proximity of the solidꢀsolution interface beneficially impacts the selectivity and activity of
the CO2 electroreduction reaction.
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The longꢀterm stability of the Ag nanoꢀcorals under catalytic conditions was investigated by
performing CO2 reduction at −0.6 V for 72 h. It was found that upon completion of the test, the
surface morphology changed from a coralꢀlike to a dendriteꢀlike structure (Figure 5aꢀb). As a
result, both the electrochemical surface area and the current density dropped by 20% and 30%,
respectively, and the CO current efficiency decreased from 95% to 66%, while the H2 efficiency
increased from 1.7% to 29% after 72 h of CO2 reduction (striped gray, Figure 5c), compared to
that after 30 min of reduction (solid black, Figure 5c). Alternatively, the nanoꢀcoral catalyst’s
stability can be measured in terms of turnover number (TON). The TON of the Ag nanoꢀcoral
catalyst is estimated to be 720 over 30 minutes or 8.8 × 104 over 72 hours, with the turnover
frequency (TOF) estimated as 0.4 s−1, based on the ratio of the number of CO2 molecules
reduced per unit time to the number of surface silver atoms (see Supporting Information for
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details). These estimates represent a lower limit, due to the assumption that all surface Ag atoms
are catalytically active.
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We have also considered the possibility of contamination of working electrodes by
electrodeposited Pt ions, which can be produced through dissolution of oxidized Pt counter
electrode and Pt ion crossing to the working electrode compartment. It is known that the Pt
counter electrode can undergo significant oxidation and dissolution,56,57 generating Pt(II) and
Pt(IV) species in the electrolyte,58 which may subsequently diffuse to and be deposited at the
working electrode. Such Pt deposit could consequently cause a change in the activity of the
working electrode.49,59 In our CO2 reduction measurements, the working electrode compartments
and the counter electrode compartment were separated by a glass frit with an appropriate
porosity to prevent the diffusion of dissolved Pt species to the working electrode. This was
confirmed by placing solutions of various Pt species (e.g. PtCl62‒) in the counter electrode
compartment and pure water in the working electrode compartment and monitoring the UvꢀVis
absorbance of both solutions. No Pt ions crossover was observed over the time frame of ca. 72
hours. In addition, XPS spectra of Ag nanoꢀcoral samples exhibited no observable Pt signals (Pt
4f) either after 30 min or after 72 h of CO2 reduction (Supporting Information Figure S7). The
absence of Pt signals shows that the diffusion of dissolved Pt species has been effectively
blocked by the glass frit between working and counter electrode compartments.
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Figure 5. (a) Lowꢀ and (b) highꢀ magnification SEM image of the Ag nanoꢀcorals after 72 h CO2
reduction. (c) A comparison of CO2 reduction activity for the Ag nanoꢀcorals after 30 min (solid black)
and 72 h (striped grey) CO2 reduction at −0.6 V (with iRꢀcorrection). Surface area and current density are
shown in relative percentage (%). XPS spectra of (d) survey, (e) K 2p, and (f) Cl 2p for the Ag nanoꢀ
corals after 72 h CO2 reduction (dark yellow).
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It is noteworthy that the XPS analysis of the sample after 72 h of CO2 reduction (Figure 5d)
indicated the presence of potassium (K) atoms on the surface. The K 2p signal is analyzed and
fitted with two peaks corresponding to 2p1/2 at 295.8 eV and 2p3/2 at 293 eV (Figure 5e). The Cl
2p signal is fitted with two peaks due to 2p1/2 at 199.7 eV and 2p3/2 at 198.1 eV (Figure 5f). The
observed signals reveal the presence of K+ and Cl−, respectively. It is feasible that during the
longꢀterm CO2 reduction in KHCO3 solution, an adlayer of K+ and Cl− forms at the Ag surface at
−0.4~0.8 V.45,60,61 This leads to gradual deposition of K+ on the Ag surface, possibly causing a
rearrangement of the surface Ag atoms62 and finally resulting in a decrease in the surface area
and in the CO2 reduction activity. Different electronic structures of Ag 3d and Ag MVV for the
Ag nanoꢀcoral sample before and after the 72 h CO2 reduction (Table 1) could also be explained
by the interaction between deposited K+ and Cl− with the surface Ag atoms. An additional
oxidationꢀreduction treatment in the presence of Cl– can restore the initial catalytic CO current
efficiency.
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CONCLUSION
In conclusion, we have demonstrated a simple preparation method for a chlorideꢀmodified
nanoporous silver (Ag nanoꢀcoral) electrocatalyst using an oxidationꢀreduction process in the
presence of chloride anions. The major product of CO2 reduction on the Ag nanoꢀcorals is CO
with a current efficiency of 95% at an overpotential as low as 0.37 V and a current density of 2
mA cm−2. A lower limit of TOF of 0.4 s−1 and TON > 8.8 × 104 (over 72 h) was estimated for the
Ag nanoꢀcoral catalyst at −0.6 V (vs. RHE) based on the assumption that all silver atoms on the
surface are catalytically active. A 32ꢀfold increase in specific activity at −0.6 V (vs. RHE) of the
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Ag nanoꢀcorals was observed, as compared to unmodified Ag foil, which is a greater
enhancement compared to the 4ꢀfold increase using 50 nm Ag nanoparticles. While the
enhancement in the catalytic activity was previously attributed solely to a nanostructured
morphology effect, hereby we propose new insights into the role of chloride anions on the
increased activity of the Ag nanoꢀcoral electrocatalyst. The chloride anions not only aid in the
fabrication of a high surface area nanoꢀstructured Ag electrode through an oxidationꢀreduction
process, but are also adsorbed on the Ag surface leading to the increased intrinsic activity of CO2
electroreduction and suppressed hydrogen evolution. Tafel analysis indicates the change in the
rateꢀdetermining step from the initial electron transfer for unmodified Ag foil to the subsequent
chemical step on Clꢀmodified Ag nanoꢀcorals. Detailed XPS studies of the Ag nanoꢀcoral
catalyst revealed the presence of Cl– anions, which are strongly bound to the surface Ag+ ions. In
addition, longꢀterm durability tests indicate that the activity and selectivity of the Ag nanoꢀcoral
catalyst decreases by only 30% over 72 hours of continuous electrolysis and the original activity
can be restored by an additional oxidationꢀreduction treatment in the presence of Cl−. The
performance degradation is attributed to a change in the morphology of the nanoporous structure
and the formation of a K+ꢀCl− surface layer.
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A similar oxidativeꢀreductive treatment of silver electrodes with iodide and bromide anions
results in the formation of an active surface layer, but with a different morphology than the
samples produced using chloride treatment. Nevertheless, the specific activity of these samples is
comparable to that of the chlorideꢀderived Ag nanoꢀcoral electrocatalyst. Detailed mechanistic
studies of the CO2 reduction reaction using Ag electrodes modified by different halides are
currently underway.
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ASSOCIATED CONTENT
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Supporting Information. XRD analysis of Ag foil, asꢀprepared AgCl and Ag nanoꢀcorals; CO2
reduction performance of asꢀprepared AgCl; SEM of Ag nanoparticles; Pb UPD of Ag electrodes
treated by different methods; CO2 reduction performance in electrolytes with different Cl−
concentrations for Ag nanoꢀcorals after annealing in H2; CO2 reduction performance of Ag
nanoparticles with different chloride concentration; details of TON and TOF calculations for the
Ag nanoꢀcoral catalyst. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: dep@bnl.gov.
Notes: The authors declare no competing financial interest.
Present Address
† Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL
60439.
ACKNOWLEDGEMENT
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We thank Dr. D. C. Grills for help with preparation of this manuscript. This work was carried out
at Brookhaven National Laboratory (BNL) under Contracts DEꢀAC02ꢀ98CH10886 and DEꢀ
SC0012704 with the U.S. Department of Energy, Office of Science, and supported in part by its
Division of Chemical Sciences, Geosciences, & Biosciences within the Office of Basic Energy
Sciences. The research was initiated with support from the BNL Laboratory Directed Research
and Development Project No. 13ꢀ013. XPS/XRD measurements and electron microscopy were
carried out at the National Synchrotron Light Source and the Center for Functional
Nanomaterials of BNL, which are supported by the U.S. Department of Energy, Office of Basic
Energy Sciences, under Contracts DEꢀAC02ꢀ98CH10886 and DEꢀSC0012704.
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