Shape-Dependent Electrocatalytic Reduction of CO2 to CO on Triangular Silver Nanoplates

Article abstract of DOI:10.1021/jacs.6b12103

Electrochemical reduction of CO₂ (CO₂RR) provides great potential for intermittent renewable energy storage. This study demonstrates a predominant shape-dependent electrocatalytic reduction of CO₂ to CO on triangular silver nanoplates (Tri-Ag-NPs) in 0.1 M KHCO₃. Compared with similarly sized Ag nanoparticles (SS-Ag-NPs) and bulk Ag, Tri-Ag-NPs exhibited an enhanced current density and significantly improved Faradaic efficiency (96.8%) and energy efficiency (61.7%), together with a considerable durability (7 days). Additionally, CO starts to be observed at an ultralow overpotential of 96 mV, further confirming the superiority of Tri-Ag-NPs as a catalyst for CO₂RR toward CO formation. Density functional theory calculations reveal that the significantly enhanced electrocatalytic activity and selectivity at lowered overpotential originate from the shape-controlled structure. This not only provides the optimum edge-to-corner ratio but also dominates at the facet of Ag(100) where it requires lower energy to initiate the rate-determining step. This study demonstrates a promising approach to tune electrocatalytic activity and selectivity of metal catalysts for CO₂RR by creating optimal facet and edge site through shape-control synthesis.

Full text of DOI:10.1021/jacs.6b12103

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Communication
Shape-dependent Electrocatalytic Reduction
of CO2 to CO on Triangular Silver Nanoplates
Subiao Liu, Hongbiao Tao, Li Zeng, Qi Liu, Zhenghe Xu, Qingxia Liu, and Jing-Li Luo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12103 • Publication Date (Web): 02 Feb 2017
Downloaded from http://pubs.acs.org on February 3, 2017
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Shape-dependent Electrocatalytic Reduction of CO2 to CO on
Triangular Silver Nanoplates
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Subiao Liu, Hongbiao Tao, Li Zeng, Qi Liu, Zhenghe Xu, Qingxia Liu∗, JingꢀLi Luo∗
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada
Supporting Information Placeholder
ly enhance the catalytic activity for CO2RR over metalꢀbased
catalysts, such as Au,5ꢀ7 and Ag8ꢀ13, where FE and current density
reach maximum values. Zhu et al.5 reported CO2RR on Au nanoꢀ
particles (NPs) and found that the 8 nm Au NPs show FE up to 90%
at ꢀ0.67 V (vs reversible hydrogen electrode, RHE). Ag NP, an
attractive alternative for noble metal electrode, shows selectivity
as high as that of Au at a lower cost. The size effects have also
been observed by Cheonghee group8 and they noted that 5 nm
Ag/C has the best CO2RR performance and achieves a maximum
FE of 79.2% at ꢀ0.75 V vs RHE. It has been demonstrated that
the enhanced sizeꢀdependent FE and current density are related to
the ratio of edgeꢀtoꢀcorner. Density functional theory (DFT) simuꢀ
lations also suggest that edge sites are more preferred for CO
evolution than corner sites on sizedꢀcontrolled noble metal NPs.5, 8,
ABSTRACT: Electrochemical reduction of CO2 (CO2RR) proꢀ
vides great potentials for intermittent renewable energy storage.
This study demonstrates a predominant shapeꢀdependent electroꢀ
catalytic reduction of CO2 to CO on triangular silver nanoplates
(TriꢀAgꢀNPs) in 0.1 M KHCO3. Compared with similarly sized
Ag nanoparticles (SSꢀAgꢀNPs) and bulk Ag, TriꢀAgꢀNPs exhibitꢀ
ed an enhanced current density and significantly improved Faraꢀ
daic efficiency (96.8%) and energy efficiency (61.7%), together
with a considerable durability (7 days). Additionally, CO starts to
be observed at an ultralow overpotential of 96 mV, further conꢀ
firming the superiority of TriꢀAgꢀNPs as a catalyst for CO2RR
towards CO formation. Density functional theory (DFT) calculaꢀ
tions reveal that the significantly enhanced electrocatalytic activiꢀ
ty and selectivity at lowered overpotential originate from the
shapeꢀcontrolled structure. This not only provides the optimum
edgeꢀtoꢀcorner ratio, but also dominates at the facet of Ag (100)
where it requires lower energy to initiate the rateꢀdetermining
step. This study demonstrates a promising approach to tune elecꢀ
trocatalytic activity and selectivity of metal catalysts for CO2RR
by creating optimal facet and edge site through shapeꢀcontrol
synthesis.
9, 14
Recently, Min Liu et al.15 also reported a fieldꢀinduced reaꢀ
gent concentration that enables highly efficient CO2RR resulting
from local high electric fields. Besides the effects of size and local
electric filed, understanding the shape effects of metal NP on
CO2RR is also worthꢀnoting. There have been limited studies on
CO2RR over metal NPs regarding the influence of particle shape,
but further exploration is warranted since the presence of edge
and corner sites varies as the shape changes.5, 9 Shape control has
received extensive attention for Ag with particular emphasis on
triangular Ag nanoplate (TriꢀAgꢀNP) because of their unique
structureꢀrelated optical properties and potential applications.16
Increased utilization of fossil fuels has brought about recordꢀ
breaking levels of atmospheric carbon dioxide (CO2), which causꢀ
es climate change and environmental issues.1 To attenuate our
reliance on fossil fuels, sustainable and environmentally friendly
alternatives are desirable options. Electrochemical CO2 reduction
reaction (CO2RR) points to a promising direction not only in deꢀ
creasing CO2 accumulation, but also in converting intermittent
renewable electricity into energyꢀdense fuels.2 However, the more
kinetically preferred H2 evolution reaction (HER) always outcomꢀ
petes the CO2RR and consequently, decreases the selectivity of
target product.3 The high overpotential (η) is also required for
CO2RR to reach substantial reaction rates.4 Therefore, it is desiraꢀ
ble to search for novel catalysts capable of efficiently promoting
CO2RR with high selectivity (Faraday efficiency, FE) and catalytꢀ
ic activity (current density, j) at low η.
In this study, TriꢀAgꢀNPs were synthesized using direct chemical
reduction method.17 The composite containing TriꢀAgꢀNPs and
carbon black (CB) was fabricated on a glassy carbon electrode
(GCE) for CO2RR. It was found that TriꢀAgꢀNPs were particularꢀ
ly active for CO2RR towards CO formation at an ultralow η. DFT
calculations were employed to rationalize the increased catalytic
activity and selectivity of TriꢀAgꢀNPs towards CO2RR.
TriꢀAgꢀNPs were synthesized by chemically reducing an aqueous
solution of AgNO3 with NaBH4 in the presence of H2O2 and trisoꢀ
dium citrate (Scheme 1).
Scheme 1. The synthesis process and digital images of TriꢀAgꢀ
NPs.
Various metallic electrocatalysts have been experimentally and
computationally identified for CO2RR since the binding energy of
intermediate (CO*) on the metal surface is relatively weaker than
that of H* derived from HER. This leads to the selective evolution
of CO rather than the competitive H2 on these metal surfaces. The
existence of optimal particle size has also been reported to strongꢀ
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UV/vis spectra were recorded to investigate the transformation
process of TriꢀAgꢀNPs by judging from the spectra change, as
shown in Figure 1a. Upon the rapid injection of NaBH4, the color
of the original transparent solution immediately became pale yelꢀ
low, suggesting the occurrence of Ag reduction. Approximately 2
min after initiation, the pale solution turned deep yellow, an indiꢀ
cation of Ag NP formation, as evidenced in the absorbance of the
characteristic peak at ~400 nm,17 where it quickly increased to the
maximum. Subsequently, the solution quickly changed to blue,
where the corresponding intensity of the characteristic peak of Ag
NPs at ~400 nm quickly decreased, indicating the gradual conꢀ
sumption of Ag NPs during the transformation process. Another
peak emerged at ∼600 nm and gradually redꢀshifted to the waveꢀ
length of 850 nm, indicative of the formation and growth of Triꢀ
AgꢀNPs, as confirmed by the TEM image in Figure 1b. The Ag
nanoplates obtained are enclosed by two (111) facets at both top
and bottom surfaces, and three (100) facets at the side faces which
contain twin planes and stacking faults along the vertical direction,
as demonstrated by previous studies.16, 18
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Figure 2. (a) Cathodic LSV results; (b) FEs of CO at various
applied potentials (the inset shows the CO, CH4 and H2 overall FE
for TriꢀAgꢀNPs) and (c) CO FEs at fixed potential of ꢀ0.855 V;
(d) CO partial current density.
TriꢀAgꢀNPs, SSꢀAgꢀNPs and bulk Ag were also performed under
the same experimental conditions, and the potentialꢀdependent
FEs for CO formation were shown in Figure 2b. CO (FE of 8.1%)
started to generate at an onset potential of ꢀ0.206 V for TriꢀAgꢀ
NPs, which was only 96 mV lower than the theoretical equilibriꢀ
um potential (ꢀ0.11 V) and remarkably lower than other reported
Agꢀbased catalysts,8, 21 whereas CO was not detectable under this
ultralow η (96 mV) for SSꢀAgꢀNPs and bulk Ag. To achieve the
equivalent FE for CO, SSꢀAgꢀNPs (8.0%) and bulk Ag (8.6%)
required a potential of ꢀ0.456 V (η of 346 mV) and ꢀ0.656 V (η
of 546 mV), respectively. The FEs of CO increased significantly
for TriꢀAgꢀNPs, SSꢀAgꢀNPs and bulk Ag when more negative
potentials were applied. Concurrently, TriꢀAgꢀNPs showed the
best CO2RR performance and reached its maximum value of 96.8%
with a much lower η of 0.746 V as compared to SSꢀAgꢀNPs
(65.4%, 0.846 V) and bulk Ag (57.2%, 0.946 V). When comparꢀ
ing the FEs of all samples at a fixed potential of ꢀ0.856 V (Figure
2c), TriꢀAgꢀNPs showed 3ꢀ and 1.5ꢀfold higher than SSꢀAgꢀNPs
(64.9%) and bulk Ag (31.9%), respectively. This indicates that the
CO2RR is shapeꢀdependent and the TriꢀAgꢀNPs are the most acꢀ
tive for CO formation as compared to other reported Agꢀbased
catalysts.8, 9, 21 Figure 2d shows the potentialꢀdependent CO parꢀ
tial current densities calculated based on the current densities
from potentiostatic measurements and the corresponding CO FEs
for all samples. It clearly reveals the exclusive catalytic activity of
TriꢀAgꢀNPs for the CO2RR towards CO formation, where a 2.2ꢀ
fold increase in the CO partial current densities of TriꢀAgꢀNPs
relative to SSꢀAgꢀNPs further confirms the shapeꢀdependent efꢀ
fects.
Figure 1. (a) UV/vis spectra; (b) TEM image of TriꢀAgꢀNPs.
To examine the CO2RR activity of TriꢀAgꢀNPs, the mixture conꢀ
taining catalysts and CB was quantitatively dropped onto a GCE
(0.785 cm2). The polarization curves of all catalysts were obtained
by a sweeping potential between ꢀ0.056 and ꢀ1.156 V vs RHE at
a scan rate of 50 mV sꢀ1. Figure 2a shows the linear sweep voltꢀ
ammetry (LSV) results of TriꢀAgꢀNPs, SSꢀAgꢀNPs and bulk Ag
to distinguish the shape effects as well as CB and GCE as referꢀ
ences to differentiate their contributions for CO2RR. The TriꢀAgꢀ
NPs exhibited approximately 2ꢀfold higher current densities (over
5.5 mA cmꢀ2, normalized by the geometrical surface area) relative
to bulk Ag (3.0 mA cmꢀ2), and 1.0 mA cmꢀ2 larger than SSꢀAgꢀ
NPs. More importantly, a much more positive onset potential was
observed for the TriꢀAgꢀNPs compared to the SSꢀAgꢀNPs and to
Bulk Ag. The increment in current density, indicative of a proꢀ
moted cathodic kinetics for CO2RR, is not conclusive evidence
since HER and CO2RR are often interconnected. To verify the
occurrence of predominant CO2RR other than HER, and the enꢀ
hanced shapeꢀdependent catalytic activity of TriꢀAgꢀNPs, potentiꢀ
ostatic electrolysis was carried out at different applied potentials.
The achieved current densities are 30% less than the correspondꢀ
ing values in LSV measurements under the same potential load.
This can be attributed to the combination of the presence of extra
current from the interface charging, and the formation of a CO2
and/or CO partially depleted layer in the vicinity of the catalyst
due to the mass transport limitations.19, 20 The outlet gases were
directly vented into a gas chromatography (GC) to quantitatively
analyze the gas composition. To confirm that the catalytic activity
is not derived from bare GCE and CB, both samples were fabriꢀ
cated as the cathodes. GC analyses indicated that H2 was the maꢀ
jor product resulting from the competitive HER on GCE and CB
(see Figure S7).
For comparing the reaction kinetics for CO2RR and subsequently
confirming the increased catalytic activity of TriꢀAgꢀNPs resulted
from the shape control, Tafel plots for TriꢀAgꢀNPs, SSꢀAgꢀNPs
and bulk Ag are shown in Figure 3a.
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stability test. Moreover, TriꢀAgꢀNPs show no morphological
change after CO2 reduction (Figure S9), further indicating the
excellent stability of TriꢀAgꢀNPs for CO2RR.
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Figure 3. (a) Tafel plot and (b) η as a function of various current
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densities; (c) Maximum energy efficiencies of TriꢀAgꢀNPs, SSꢀ
AgꢀNPs and bulk Ag; (d) longꢀterm stability at a potential load of
ꢀ0.856 V and the corresponding FEs of CO and H2.
The Tafel slope, an indication of kinetics for CO formation, was
153 mV decꢀ1 for TriꢀAgꢀNPs, closer to the value of 118 mV decꢀ1
expected for rateꢀdetermining step at the electrode22 as compared
to a Tafel slope of 177 mV decꢀ1 for SSꢀAgꢀNPs and 197 mV decꢀ
Figure 4. (a) Free energy diagrams for CO2RR to CO on different
facets and Ag55 cluster at ꢀ0.11 V; active adsorption site density
on (b) TriꢀAgꢀNPs and (c) SSꢀAgꢀNPs as a function of particle
size; (d) proposed mechanism for CO2RR to CO on TriꢀAgꢀNPs.
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for bulk Ag. Both values were larger than the one for TriꢀAgꢀ
A better understanding on the origins credited for the ultralow
onset potential, and the high CO selectivity at lower η for CO2RR
over shapeꢀcontrolled TriꢀAgꢀNPs relative to SSꢀAgꢀNPs, DFT
calculations were further explored to investigate the reactivity of
different Ag features based on the computational hydrogen elecꢀ
trode model.5, 9, 14 Figure 4a shows the Gibbs free energy (ꢁG)
diagram for CO2RR on different facets and Ag55 cluster, where
the total ꢁGs required for the proposed four elementary reaction
steps were simulated and calculated (Figure 4d). The required η
initially originates from the formation of COOH* since an uphill
energy barrier of the first protonꢀcoupled electronꢀtransfer step for
all facets and the cluster was observed. Apparently, the required
ꢁG to form the COOH* on Ag(100) is significantly lower than
that on Ag(111) and quite close to the one on Ag(110), suggesting
a higher catalytic activity of Ag(100) for CO2RR. More imꢀ
portantly, the facet of (100) is predominant in TriꢀAgꢀNPs as
compared to that in SSꢀAgꢀNPs since TriꢀAgꢀNPs are only enꢀ
closed by the facets of (100) and (111). The freeꢀenergy step subꢀ
sequently becomes more thermodynamically facile for the second
protonꢀcoupled electron transfer for the adsorbed CO* at all facets.
Although Ag55 cluster is slightly easier for the formation of
COOH* than all facets, it tends to overbind with CO* and conseꢀ
quently decreases the product evolution rate. Thus, both the domꢀ
inance of (100) and the ease in the CO evolution deriving from
the shapeꢀcontrolled effect point to the decreased η and low onset
potential for CO formation. Additionally, studies have demonꢀ
strated that an increased edgeꢀtoꢀcorner ratio yields enhanced
catalytic activity and selectivity for CO2RR since the adsorption
of CO2 and the subsequent formation of intermediate COOH* are
easier on edge sites than corner sites.5, 9, 14 Thus, the edgeꢀtoꢀ
corner ratios of TriꢀAgꢀNPs and SSꢀAgꢀNPs as a function of the
cluster diameter were also analyzed (Figures 4b and 4c). It is
found that TriꢀAgꢀNPs with a diameter range from 0 to 12 nm
maintain a comparably higher edgeꢀtoꢀcorner ratio. Although the
ratio difference between TriꢀAgꢀNPs and SSꢀAgꢀNPs narrows
gradually at larger diameters, the density of catalytically active
edge sites on TriꢀAgꢀNPs always exceeds that on SSꢀAgꢀNPs.
This ensures sufficient active edge sites for CO2 reduction to CO
and consequently leads to superior selectivity of TriꢀAgꢀNPs.
NPs, indicating a poor kinetics for CO2RR. Moreover, the forꢀ
mation of an adsorbed ∗COOꢁ intermediate (COꢂ ꢃ eꢁ →∗ COOꢁ)
on catalyst surface exclusively determines the reaction rate for
CO2RR since the anionic radical is highly unstable. The negative
reduction potential of the CO2/∗ COOꢁ redox couple, Eꢄ
ꢅ
ꢀ1.49 vs. RHE at pH 7, not only creates a significant thermodyꢀ
namic and kinetic bottleneck for the overall CO2RR, but also
promotes side reactions (e.g. HER) and consequently decreases
the current efficiency for CO2RR. However, the onset potential
shifted anodically to ꢀ0.206 V on TriꢀAgꢀNPs (Figure 3b), a net
decrease of 0.25 V and 0.35 V in η compared to SSꢀAgꢀNPs and
bulk Ag, respectively. The exchange current density (ꢆꢇ), a reflecꢀ
tion of the free energy barrier required for CO2RR at the reversiꢀ
ble potential as well as a measure of intrinsic rate of electron
transfer between the electrolyte and the electrode, was 1.1 ꢈ 10ꢁꢉ
mA cmꢀ2 for TriꢀAgꢀNPs, one order of magnitude higher than that
for SSꢀAgꢀNPs (3.8 ꢈ 10ꢁꢊ mA cmꢀ2) and bulk Ag (3.2 ꢈ 10ꢁꢊ
mA cmꢀ2). It is also comparable to a recently reported value
(1.0 ꢈ 10ꢁꢊ mA cmꢀ2) on 3 nm Ag/C for CO2RR to CO,8 further
suggesting an improved shapeꢀdependent catalytic activity. As a
critical parameter in benchmarking electrocatalyst for renewable
energy storage, the maximum energy conversion efficiencies of
TriꢀAgꢀNPs, SSꢀAgꢀNPs and bulk Ag were evaluated (Figure 3c).
The low η together with the high FE of TriꢀAgꢀNPs contributed to
an energy efficiency of over 61.7%, much higher than SSꢀAgꢀNPs
(42%) and bulk Ag (34%). This value is also comparable with the
most efficient existing platforms for the formation of CO,7, 23 and
consequently distinguishes TriꢀAgꢀNPs as a promising platform
for CO2RR.
To address the stability of the developed catalyst, a major concern
for CO2RR, the longꢀterm performance of TriꢀAgꢀNPs was evaluꢀ
ated at a constant potential load of ꢀ0.856 V for 7 days. The outꢀ
let gases were analyzed each hour over a period of 14 h by GC.
The corresponding FEs of CO and H2 were also determined (Figꢀ
ure 3d) and the inset exhibits the longꢀterm stability test over 7
days. The current density maintained a steady value at around
ꢀ1.25 mA cmꢀ2 with negligible degradation and the correspondꢀ
ing FEs of CO only fluctuated slightly around 96% throughout the
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(20) Asadi, M.; Kim, K.; Liu, C.; Addepalli, A. V.; Abbasi, P.; Yasaei,
P.; Phillips, P.; Behranginia, A.; Cerrato, J. M.; Haasch, R. Science 2016,
353, 467ꢀ470.
(21) Hsieh, Y.C.; Senanayake, S.D.; Zhang, Y.; Xu, W.; Polyansky,
D.E. ACS Catal. 2015, 5, 5349ꢀ5356.
(22) Zhang, S.; Kang, P.; Ubnoske, S.; Brennaman, M. K.; Song, N.;
House, R. L.; Glass, J. T.; Meyer, T. J. J. Am. Chem. Soc. 2014, 136,
7845ꢀ7848.
In summary, we successfully synthesized TriꢀAgꢀNPs and invesꢀ
tigated its shapeꢀdependent electrocatalytic reduction of CO2 to
CO in 0.1 M KHCO3. TriꢀAgꢀNPs exhibit considerably higher
selectivity as compared to SSꢀAgꢀNPs and bulk Ag. Moreover,
CO can be detected at an ultralow onset potential, confirming the
excellent catalytic activity of TriꢀAgꢀNPs towards CO2RR. The
durability test over 7 days further confirms the excellent perforꢀ
mance of TriꢀAgꢀNPs for efficient CO2RR. DFT calculations
indicate that the high selectivity of TriꢀAgꢀNPs at an ultralow
overpotential is a consequence of both the optimum edgeꢀtoꢀ
corner ratio and the predominant Ag(100) facet in TriꢀAgꢀNPs.
(23) Rosen, B. A.; SalehiꢀKhojin, A.; Thorson, M. R.; Zhu, W.; Whipꢀ
ple, D. T.; Kenis, P. J.; Masel, R. I. Science 2011, 334, 643ꢀ644.
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ASSOCIATED CONTENT
Supporting Information
Experimental details can be found in the Supporting Information.
AUTHOR INFORMATION
Corresponding Authors
∗luoj@ualberta.ca; ∗qingxia2@ualberta.ca
ACKNOWLEDGMENT
This work was supported by the Natural Sciences and Engineering
Research Council of Canada.
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