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2202854 (1 of 9) © The Authors. Advanced Materials published by Wiley-VCH GmbH
Synergistic Cr2O3@Ag Heterostructure Enhanced
Electrocatalytic CO2 Reduction to CO
Huai Qin Fu, Junxian Liu, Nicholas M. Bedford, Yun Wang, Ji Wei Sun, Yu Zou,
Mengyang Dong, Joshua Wright, Hui Diao, Porun Liu,* Hua Gui Yang, and Huijun Zhao*
H. Q. Fu, J. Liu, Y. Wang, Y. Zou, M. Dong, P. Liu, H. Zhao
Centre for Catalysis and Clean Energy
Gold Coast Campus
Grith University
Queensland , Australia
E-mail: p.liu@grith.edu.au; h.zhao@grith.edu.au
DOI: 10.1002/adma.202202854
Although various chemicals and fuels
have been successfully synthesized via
the electrocatalytic CO reduction reaction
(CORR), the selective reduction of CO to
CO is widely recognized as one of the most
economically viable reactions due to its
simplicity and needing the least electrons
transfer to form the product.[] Among the
reported CORR electrocatalysts, metallic
Ag ones have demonstrated high catalytic
selectivity toward CO, however, requiring
high cathodic potentials (e.g., ≥. V vs
RHE).[] Various approaches have been
attempted to improve the CORR perfor-
mance of Ag electrocatalysts. For instance,
Kim and co-workers immobilized small
Ag NPs (≈ nm) on carbon support to
achieve a high faradic eciency (FECO) of
.% at −. V versus RHE.[] Luo and
co-workers reported the use of the domi-
nant Ag () facet exposed at the active
edge of the triangular Ag nanoplates to
attain a high FECO of .% at −. V
versus RHE.[] Recently, the same team
investigated the structure sensitivity of Ag
nanocubes toward CORR and unveiled
that the Ag nanocubes enclosed by {} facets with the edge
lengths below nm can achieve a superb FECO of .% at
−.V versus RHE.[] They attributed the superb FECO to the
increased percentage of edge active sites. These approaches
utilize the edge active sites of Ag nanocrystals to improve
CORR performance, however, such highly active edge sites are
The electrocatalytic CO2RR to produce value-added chemicals and fuels
has been recognized as a promising means to reduce the reliance on fossil
resources; it is, however, hindered due to the lack of high-performance electro-
catalysts. The eectiveness of sculpturing metal/metal oxides (MMO) hetero-
structures to enhance electrocatalytic performance toward CO2RR has been well
documented, nonetheless, the precise synergistic mechanism of MMO remains
elusive. Herein, an in operando electrochemically synthesized Cr2O3–Ag het-
erostructure electrocatalyst (Cr2O3@Ag) is reported for ecient electrocatalytic
reduction of CO2 to CO. The obtained Cr2O3@Ag can readily achieve a superb
FECO of 99.6% at −0.8V (vs RHE) with a high JCO of 19.0mA cm−2. These
studies also confirm that the operando synthesized Cr2O3@Ag possesses high
operational stability. Notably, operando Raman spectroscopy studies reveal that
the markedly enhanced performance is attributable to the synergistic Cr2O3–Ag
heterostructure induced stabilization of CO2
•−/*COOH intermediates. DFT
calculations unveil that the metallic-Ag-catalyzed CO2 reduction to CO requires
a 1.45eV energy input to proceed, which is 0.93eV higher than that of the
MMO-structured Cr2O3@Ag. The exemplified approaches in this work would
be adoptable for design and development of high-performance electrocatalysts
for other important reactions.
ReseaRch aRticle
1. Introduction
The electrocatalytic conversion of CO into value-added chemi-
cals and fuels in an economically viable manner reduces not
only CO emission, but also our dependence on fossil resources
to benefit sustainable energy and the environmental future.[]
N. M. Bedford
School of Chemical Engineering
University of New South Wales
Sydney, NSW , Australia
J. W. Sun, H. G. Yang
Key Laboratory for Ultrafine Materials of Ministry of Education
School of Materials Science and Engineering
East China University of Science and Technology
Shanghai , China
J. Wright
Department of Physics
Illinois Institute of Technology
Chicago, IL , USA
H. Diao
The Centre for Microscopy and Microanalysis
The University of Queensland
St Lucia, QLD , Australia
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/./adma..
© The Authors. Advanced Materials published by Wiley-VCH
GmbH. This is an open access article under the terms of the Creative
Commons Attribution-NonCommercial-NoDerivs License, which
permits use and distribution in any medium, provided the original work
is properly cited, the use is non-commercial and no modifications or
adaptations are made.
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unstable under CORR conditions. Additionally, rigorous syn-
thetic processes are needed to obtain such Ag nanocrystals.
It is known that a high-performance CORR electrocata-
lyst must be able to apt adsorb CO and stabilize the carboxyl
intermediate.[] In this regard, sculpturing metal/metal oxides
(MMO) heterostructures have demonstrated to be an eective
strategy.[] As shown in Table S (Supporting Information), the
MMO-structured CORR electrocatalysts have been successfully
applied for the conversion of CO into various carbon products
(e.g., CO, HCCO−, CH, CHOH, CH, and CHCHOH).
Among them, several Ag-based MMO electrocatalysts have
been used to selectively reduce CO to CO. For instance, Bao’s
group created a novel Ag-CeOx interface to realize a high FECO
of .% at −.V (vs RHE) with a CO partial current density
(JCO) of mA cm−.[] Recently, Zhang etal. elegantly impreg-
nated the ultrafine twinned ZnO–Ag NPs inside the nanopores
of carbon nanospheres to achieve .% energy eciency and
a high FECO of . at −.V versus RHE.[] Notably, Kenis’
group synthesized a Ag-TiO electrocatalyst and embedded
in a flow cell to attain a FECO of % with a high JCO of
mA cm− at −. V versus RHE.[] Yuan et al. anchored
MnO NPs on () faceted Ag crystal surface to reach a high
FECO of .% with a JCO of . mA cm− at a low potential
of −.V versus RHE.[] Despite the widespread use of MMO-
structured CORR electrocatalysts, the mechanistic insight of
MMO-heterostructures-induced performance enhancement
remains elusive.
This work reports an in operando electrochemically synthe-
sized CrO–Ag heterostructure electrocatalyst (CrO@Ag)
for ecient electrocatalytic reduction of CO to CO. We couple
CrO with Ag to form MMO because CrO is electrochemically
stable within the potential/pH window required for CORR.[]
The obtained CrO@Ag can readily aord a superb FECO of
.% at −. V (vs RHE) with a high JCO of . mA cm−.
The operando Raman spectroscopy studies depict that the dras-
tically enhanced performance is attributable to the synergistic
CrO–Ag heterostructure prompted stabilization of
CO•−/*COOH intermediates. Density functional theory studies
unveil that the metallic Ag catalyzed CO reduction to CO
requires an energy input of .eV to proceed, which is .eV
higher than that of the MMO-structured CrO@Ag.
2. Results and Discussion
The CrO@Ag was synthesized by direct electrochemical
conversion of the presynthesized AgCrO microcrystals
(AgCrO-MCs) under operando CORR conditions (Figure 1a).
The AgCrO-MCs precursor was synthesized via a facile
room temperature precipitation reaction (Ag++ CrO−→
AgCrO).[] The X-ray diraction (XRD) patterns of the as-
synthesized AgCrO-MCs (Figure S, Supporting Informa-
tion) can be indexed to the orthorhombic AgCrO crystal
(PDF No. -). The Raman spectrum (Figure S, Sup-
porting Information) shows two dominant peaks at and
cm−, assignable to the stretching modes of CrO−.[] The
field-emission scanning electron microscopy (FE-SEM) images
(Figure Sa, Supporting Information) unveil that the obtained
AgCrO-MCs possess an elongated hexagon shape with an
average size of × × µm. Figure Sb (Supporting Infor-
mation) shows the transmission electron microscopy (TEM)
image. The corresponding selected-area electron diraction
(SAED) pattern (inset in Figure Sb in the Supporting Infor-
mation) exhibits a set of diraction spots that match with the
orthorhombic AgCrO crystal viewed from the [] zone axis.
Figure Sc (Supporting Information) shows the high-resolution
TEM (HRTEM) image of the as-synthesized AgCrO-MCs and
the corresponding inverse fast Fourier transform (IFFT) TEM
(IFFT-TEM) image from the selected area. The unveiled lattice
spacings of . and . Å coincide with {} and {} fac-
eted AgCrO. The aberration-corrected scanning transmission
electron microscopy (STEM) and corresponding energy-disper-
sive X-ray spectroscopy (EDX) images (Figure Sd, Supporting
Information) confirm the uniformly distributed Ag, Cr, and O.
To obtain CrO@Ag, the as-synthesized AgCrO-MCs were
immobilized on a piece of × cm conductive carbon paper
(Figure S, Supporting Information) and subjected to −.V (vs
RHE) cathodic potential in CO-saturated . KHCO for h.
The XRD pattern of the obtained CrO@Ag (Figureb) con-
firms the presence of the metallic phased Ag (PDF#-),
indicating the occurrence of Ag+ reduction, while the observed
weak diraction peaks at .° and .o are assignable to the
rhombohedral CrO (PDF#-), indicating the presence of
low crystallinity CrO. The X-ray photoelectron spectroscopy
(XPS) spectrum of CrO@Ag (Figures S and S, Supporting
Information) further confirms the successful reduction of Ag+
and Cr+ to metallic Ag and Cr+, respectively. Figurec shows
the FE-SEM image of the as-synthesized CrO@Ag on carbon
paper. As disclosed by the magnified FE-SEM image (inset in
Figurec), compared with the AgCrO-MCs precursor, although
the elongated hexagon shape is partially retained, a notably
increased thickness and significantly changed surface mor-
phology are observable. The high-magnification FE-SEM image
of CrO@Ag (Figured) reveals the presence of the aggregated
NPs. In order to further depict the structural and compositional
characteristics of CrO@Ag, the as-synthesized CrO@Ag
samples were sliced using the focused ion beam (FIB) tech-
nique. Figure S (Supporting Information) shows a typical
cross-sectional FE-SEM image of the FIB-sliced CrO@Ag
sample with the corresponding EDX elemental mapping
images. The disclosed uniform distribution of Ag, Cr, and O
throughout the cross section confirms a homogeneous compo-
sition of CrO@Ag at the micrometer-scale. The higher mag-
nification cross-sectional FE-SEM image (Figure e) confirms
that the entire structure of CrO@Ag is formed by the aggre-
gated NPs. However, the magnified TEM image and the corre-
sponding EDX elemental mapping images around a typical pore
structure area of CrO@Ag (Figure S, Supporting Informa-
tion) disclose that at the nanometer-scale, the CrO component
tends to enrich on the pore surfaces. As shown in Figuref, the
pore surface layer is formed by small Ag NPs with an average
diameter of ≈nm that are surrounded by CrO. The HRTEM
image (Figureg) displays the interplanar distances of .,
., and . nm, corresponding, respectively, to (),
(−−), and () planes of Ag viewed from the [−] direc-
tion, while the presence of low-crystallinity CrO surrounding
the Ag NPs can be confirmed by the observed lattice fringe of
.nm, assignable to the () plane of CrO. This can be
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further confirmed by the STEM and the corresponding EDX
elemental mapping images shown in Figureh. It also unveils
by the HRTEM image that the surface Ag NPs are decorated by
a very thin layer of CrO (Figure S, Supporting Information).
Figure S in the Supporting Information shows the line-scan
TEM-EDS elemental distribution profiles on a typical Ag NP.
As can be seen, the distributions of Cr and O on the surface of
the Ag NP are clearly visible. This adding to the HRTEM image
shown in Figure S (Supporting Information) and the EDX ele-
mental mapping images shown in Figureh categorically con-
firm that the Ag NPs are decorated by a very thin layer of CrO.
The nitrogen adsorption–desorption isotherm of the as-syn-
thesized CrO@Ag (Figure S, Supporting Information) dis-
plays a typical type-IV hysteresis loop, suggesting a mesoporous
structure nature with an average pore size of .nm, favorable
for mass transport.
Adv. Mater. 2022, 34,
Figure 1. a) Schematic illustrating electrochemical conversion of AgCrO-MCs to CrO@Ag under operando CORR conditions. b) XRD pattern,
c,d) FE-SEM images, e) cross-sectional FE-SEM image, f) TEM image and corresponding SAED pattern, g) HRTEM image, and h) STEM image and
corresponding element mapping images of CrO@Ag.
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The synchrotron-based X-ray absorption near-edge struc-
ture (XANES) and the extended X-ray absorption fine struc-
ture (EXAFS) spectra were obtained to further characterize the
valence states and coordination environment of CrO@Ag.
The almost completely overlapped Ag K-edge XANES spectra of
CrO@Ag and Ag foil confirm the presence of metallic Ag in
CrO@Ag (Figure 2a). The two intensive absorption peaks cen-
tered at . and . Å observed from the Fourier transform
(FT) k-weighted Ag K-edge EXAFS spectrum of CrO@Ag
are assignable to Ag–Ag bonds, further confirming the pres-
ence of metallic Ag in CrO@Ag (Figureb). As displayed in
Figurec, the absorption edge of Cr K-edge XANES spectrum
of CrO@Ag very closely approximates to that of CrO, high-
lighting the existence of Cr+ in an octahedral coordination with
O of chromium oxide or hydroxide.[] Figured shows the FT
k-weighted Cr K-edge EXAFS spectra of CrO@Ag and the
reference samples. The dominant peak at . Å in the spec-
trum of CrO@Ag confirms the existence of Cr–O coordina-
tion in the first coordination sphere of Cr+ that matches well
with those of CrO or Cr(OH).[] Compared to the CrO refer-
ence sample, the notably enhanced peak intensity of CrO@Ag
spectrum at . Å is attributable to the presence of the
adsorbed OH− due to the strong anity of CrO@Ag with
OH−.[] The O K-edge near-edge X-ray absorption fine struc-
ture (NEXAFS) spectrum of CrO@Ag (Figure e) displays
two peaks centered at . and . eV, corresponding to
Adv. Mater. 2022, 34,
Figure 2. a,b) Ag K-edge XANES and EXAFS spectra (in R-space), c,d) Cr K-edge XANES and EXAFS spectra (in R-space), e) O K-edge NEXAFS spectra,
and f) Cr L-edge XANES spectra of CrO@Ag and reference samples.
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the adsorbed OH− and HO, respectively,[] while the peak at
. and .eV displayed, respectively, by the spectrum of
CrO@Ag and CrO are assignable to the lattice O of Cr–O
bonds,[] indicating the coexistence of lattice oxygen and
adsorbed hydroxide. Compared to the CrO reference sample,
the existence of the peak at . eV and the increased peak
intensity at .eV observed from the spectrum of CrO@Ag
further confirm the strong anity of CrO@Ag with OH− and
the presence of abundant adsorbed OH−. In addition, the Cr
L-edge XANES spectrum of CrO@Ag is closely approximated
to that of CrO (Figuref). This adding to the absent Cr+ char-
acteristic peak at .eV categorically confirms the existence
of Cr+ in CrO@Ag.
The electrochemical measurements were performed using
a three-electrode electrochemical system accommodated in a
Nafion proton exchange membrane separated two-compart-
ment cell with mL of Ar- or CO-saturated . KHCO solu-
tion in each compartment. Other than CrO@Ag, the metallic
Ag NPs (AgNPs, Figure S, Supporting Information) and
the commercial CrO powder (Figure S, Supporting Infor-
mation) were immobilized, respectively, on the carbon paper
and used as working electrodes. As disclosed in Figure S
(Supporting Information), over the investigated potential range,
the linear sweep voltammetry (LSV) response of CrO@Ag in
CO-saturated . KHCO electrolyte shows markedly higher
cathodic current densities than that obtained from the Ar-satu-
rated electrolyte, implying an apparent electrocatalytic activity
toward CO reduction. The influence of cathodic potential on
CORR performance was investigated. For a given potential,
the chronoamperometric curve was recorded over h period
and the gas chromatograph (GC) was employed to quantify the
gaseous products. Figure 3a shows the dependency of the par-
tial CO current density (JCO) on the cathodic potential. Within
the investigated potential range, CrO@Ag exhibits higher
JCO than that of AgNPs, while only trivial JCO is attained by
the commercial CrO powder. At −.V (vs RHE), CrO@Ag
can attain a JCO of . mA cm−, higher than that of AgNPs
(.mA cm−). These infer that compared to AgNPs, CrO@Ag
possesses superior CORR electrocatalytic activity toward
CO, while the commercial CrO powder is electrocatalytically
inert for CORR. As such, one can easily manifest that the
superior CORR electrocatalytic activity of CrO@Ag toward
CO is attributable to the coexistence of the metallic Ag and
CrO. Figure b and Figure S (Supporting Information)
show the eect of cathodic potential on the Faradaic eciency
of CO (FECO) and H (FEH). For the potential range investi-
gated in Figureb and Figure S (Supporting Information),
the sum of the FECO and FEH is ≈%, signifying that CO
is the sole carbon product of CORR. The FECO obtained from
both CrO@Ag and AgNPs increases with the cathodic poten-
tial, reaching their maximum, respectively, at −. and −.V
(vs RHE), then decreases with further increased cathodic poten-
tials due to the intensified competition from hydrogen evolu-
tion reaction (HER).[a] At −.V (vs RHE), CrO@Ag achieves
its maximum FECO of .%, much higher than the maximum
FECO of .% achieved by AgNPs at −.V (vs RHE). As such,
the CrO@Ag outperforms almost all of the reported MMO
CORR electrocatalysts (Table S, Supporting Information)
and the recently reported non-MMO CORR electrocatalysts
(Table S, Supporting Information). As unveiled in Figurec,
Adv. Mater. 2022, 34,
Figure 3. a) JCO and b) FECO of CrO@Ag, AgNPs and CrO under dierent cathodic potentials. c) Tafel plots of CrO@Ag, AgNPs and CrO.
d) Recorded chronoamperometric curve from CrO@Ag at −.V versus RHE over a h period and the corresponding FECO.
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CrO@Ag exhibits a Tafel slope of mV dec−, much lower
than those obtained from AgNPs ( mV dec−) and CrO
(mV dec−), confirming the kinetic superiority of CrO@Ag
over AgNPs and CrO. The stability of CrO@Ag was exam-
ined in CO-saturated . KHCO solution at EApp=−. V
(vs RHE) over a h period (Figured). The recorded chron-
oamperometric profile unveils an increase in JCO from .
to .mA cm−, while FECO> .% is retained throughout
the entire test period, confirming the excellent stability of
CrO@Ag. The excellent CORR stability of CrO@Ag is due
to its excellent structural stability as evidenced by the almost
unchanged XRD pattern (Figure S, Supporting Information),
Raman spectrum (Figure S, Supporting Information), XPS
spectra (Figure S, Supporting Information), and SEM and
TEM images (Figure S, Supporting Information) of CrO@
Ag after the chronoamperometric stability test. It is noteworthy
that the superior stability of CrO@Ag can also be attributed to
the operando synthetic method for which the catalyst is applied
under the same conditions as it was synthesized.
It has been well documented that the electrocatalytic reduc-
tion of CO to CO commonly proceeds via a proton-coupled
electron transfer (PCET) step to form CO•− or *COOH inter-
mediate and followed by another PCET step to convert the
intermediate into CO.[] It has also been well documented
that the first PCET step is likely the rate limiting step of
CO reduction to CO.[] For the st PCET to proceed, the
adsorption strength of CO on the catalyst is critically impor-
tant[] and was therefore investigated. Figure 4a shows the
Adv. Mater. 2022, 34,
Figure 4. a) CO-TPD spectra of CrO@Ag, AgNPs and the commercial CrO. b) LSV curves of CrO@Ag and AgNPs in Ar-saturated . NaOH
solution at a scan rate of mV s−. c–e) Potential-dependent Raman spectra of CrO@Ag, CrO, and AgNPs in CO-saturated . KHCO solution.
f) Free energy diagrams for CO production on CrOH/Ag () and Ag ().
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Adv. Mater. 2022, 34,
temperature-programmed desorption (TPD) profiles of
CrO@Ag, AgNPs and the commercial CrO powder under
CO atmosphere. Compared to AgNPs, CrO@Ag shows higher
CO uptake capacity, indicating a superior adsorption capacity
of CrO@Ag over AgNPs. While compared to CrO@Ag
and AgNPs, the CO uptake capacity of the CrO is negli-
gible, indicating the incapacity of CrO to adsorb CO. Impor-
tantly, a high temperature of °C is required to desorb the
adsorbed CO from CrO@Ag, which is °C higher than that
of AgNPs ( °C), demonstrating a stronger CO adsorption
strength on CrO@Ag.[] Considering the extremely low CO
adsorption capability of CrO, the markedly enhanced CO
adsorption capability by CrO@Ag can be attributed to the
coexistence of metallic Ag and CrO. The formation and sta-
bilization of CO•−/*COOH intermediates during the st PCET
largely determine the electrocatalytic activity.[] It was dem-
onstrated that the adsorbed OH− on electrocatalyst stabilizes
CO•−/*COOH intermediates and can be used as a surrogate
to indicate the adsorption of CO•−/*COOH.[] The adsorptions
of OH− on CrO@Ag and AgNPs were therefore investigated
by LSV measurements in Ar-saturated . NaOH solution
(Figureb).[] The OH− adsorption peak on AgNPs appears at
.V (vs RHE), while the OH− adsorption peak on CrO@Ag
is cathodically shifted to . V (vs RHE), confirming a
stronger adsorption strength of CrO@Ag toward OH−, hence
CO•−/*COOH intermediates.[] As confirmed by the Cr K-edge
EXAFS (Figured) and O K-edge NEXAFS spectra (Figuree),
the presence of abundant OH− groups is responsible for the
superior intermediate adsorption/stabilization capability of
CrO@Ag.
Figure S (Supporting Information) shows the operando
Raman spectroscopy apparatus, involving a Raman microscope
equipped with a microscopic lens and a home-made three-
electrode electrochemical cell. The potential-dependent Raman
spectra of CrO@Ag, CrO, and AgNPs were recorded under
operando CORR conditions in CO-saturated . KHCO
electrolyte (Figurec–e). Figurec unveils that for CrO@Ag,
no obvious Raman peak can be observed under open circuit
potential (OCP) conditions. Under an applied potential (EApp)
of −.V (vs RHE), the observed peak at cm− is assignable
to the Ag lattice vibrational modes, while the peaks at and
cm− are related to O–C–O stretching vibrational modes
of the adsorbed CO− on metallic Ag.[] With EApp=−. V,
a broad peak at ≈ cm− appears, which can be assigned to
the Cr–OH stretching vibrations resulting from the adsorbed
OH− on Cr+.[] With −. < EApp<−.V, this peak progres-
sively broadens and evolves into two peaks. One of which
associates with –COO− symmetric bending vibrations[] and
is blue-shifted from ≈ to cm− when EApp is increased
from −. to −.V (vs RHE) due to the higher-cathodic-poten-
tial-induced higher coverages of the adsorbed CO•−/*COOH
intermediates on the metallic Ag.[] The other one assign-
able to Cr–OH stretching vibrations[] is red-shifted from
to cm− when EApp is increased from −. to −. V
(vs RHE). The observed redshifts of the Cr–OH stretching
vibrations indicate the Cr–OH bond lengthening (the greater
interatomic distance), which is resulted from the higher-
cathodic-potential-induced higher coverages of the adsorbed
CO•−/*COOH intermediates on the metallic Ag that interact
strongly with the adsorbed OH− on Cr+ sites.[] As such, the
concurrently shifted Raman peaks of the –COO− symmetric
bending vibrations and Cr–OH stretching vibrations toward
blue and red infer that the CO•−/*COOH intermediates are
eectively stabilized by the surface OH− groups on CrO.[]
Additionally, the peaks at and cm− associated with
the adsorption of CO− on metallic Ag are increased with the
increased cathodic potentials. These operando observations
confirm that the cathodic potential-enhanced OH− adsorption
on the CrO surface promotes and stabilizes the adsorption
of CO•−/*COOH intermediates on the metallic Ag surface.
As unveiled in Figured, the recorded CrO Raman spectrum
under OCP conditions displays a set of peaks at , , ,
and cm−, assignable to Eg modes resulted from crystal
Cr–O vibration,[] while the peak at cm− is originated
from Cr–OH vibration,[] indicating the presence of the sur-
face OH− groups. Notably, with increased EApp to −.V versus
RHE, all characteristic Raman peaks are well retained without
observable shifting in their wavenumbers. Figuree shows the
Raman spectra of AgNPs under OCP and applied potential
conditions. Under the OCP conditions, the displayed peaks at
and cm− are assignable to the adsorbed CO− on
metallic Ag,[] and the increased EApp leads to increased peak
intensities. Importantly, when compared with the Raman
spectra of CrO@Ag (Figure c), the characteristic Raman
peaks related to the OH− adsorption on CrO induced
CO•−/*COOH intermediates stabilization on the metallic
Ag are not observable for the single component metallic Ag
and CrO, which unambiguously confirms that the superior
CORR electrocatalytic activity of CrO@Ag over AgNPs and
CrO is due to the synergetic promotional eect of the coex-
isted CrO and Ag in CrO@Ag towards the adsorption/stabi-
lization of CO•−/*COOH intermediates.
The DFT calculations were then conducted to gain further
mechanistic insights. The atomic models of the clean Ag ()
surface and the Ag () surface with CrO were used for DFT
calculations. Based on the experimentally confirmed CrO@Ag
structure and operando Raman spectroscopy studies, the Ag
() surface with CrO was modelled by decorating a CrOH
cluster (denoted as CrOH/Ag ()) derived from the CrO
crystal using hydrogen to saturate the dangling bonds.[] It is
well known that the formation of carboxyl (*COOH) species
via hydrogenation of chemically adsorbed CO during the first
PCET step is often the rate-determining step for almost all of
the Ag-based catalysts.[a] Figure Sa (Supporting Information)
shows the DFT optimized intermediates adsorption on the
clean Ag (). The calculated adsorption free energy of *COOH
(ΔG*COOH) on the clean Ag () surface is . eV, while the
adsorption free energy for *CO (ΔG*CO) resulting from the subse-
quent PCET step is calculated to be .eV. The *CO desorption
free energy (ΔGCO) is calculated to be .eV. For CrOH/Ag
() surface (Figure Sb, Supporting Information), the calcu-
lated ΔG*COOH, ΔG*CO, and ΔGCO are ., ., and of .eV,
respectively. Figure f shows the free energy diagrams of Ag
() and CrOH/Ag () catalyzed CO reduction to CO via
a PCET reaction pathway. As can be seen, both Ag () and
CrOH/Ag () catalyzed formation of *COOH are uphill
processes. However, CrOH/Ag () requires an input energy
of .eV to form *COOH, .eV lower than that of the clean
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Adv. Mater. 2022, 34,
Ag () surface (.eV). This is resulted from the promoted
CO adsorption and the stabilized CO•−/*COOH intermedi-
ates by the hydroxyl groups on the octahedrally coordinated
Cr+ species in CrOH/Ag (), where, the O atoms in mole-
cular CO are inclined to adsorb on the H atoms of Cr–OH via
hydrogen bonding and followed by transferring an electron to
the adsorbed CO to form CO•− that combines with a proton
from the electrolyte to form *COOH with its C being bonded
to the adjacent Ag site (Figure Sb, Supporting Information).
The DFT depicted synergistic promotional eect by the coexist-
ence of CrO and Ag is well supported by the operando Raman
spectroscopy observations.
3. Conclusion
We have successfully demonstrated an operando electrochem-
ical approach to fabricate an MMO-structured CrO@Ag with
superb activity and stability toward electrocatalytic reduction
of CO to CO. The operando Raman spectroscopy studies are
combined with DFT calculations to undoubtedly reveal the
mechanistic insights of the MMO-heterostructure-induced
performance enhancement. We also exemplified that the
operando synthesis can be an eective means to achieve high
operational stability. The findings of this work provide useful
insights for the design and development of high-performance
electrocatalysts not only for CORR but also other important
reactions.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
This work was financially supported by Australian Research Council
Discovery Project (DP). Ag K-edge and Cr K-edge XAS
measurements were performed at the -ID-B beamline of the Advanced
Photon Source, a U.S. Department of Energy (DOE) Oce of Science
User Facility, operated for the DOE Oce of Science by Argonne National
Laboratory under Contract No. DE-AC-CH. Operations at
-ID-B are further supported by members of the Materials Research
Collaborative Access Team. C and O K-edge measurements were
performed at the SXR beamline of the Australian Synchrotron, part of
the Australian Nuclear Science and Technology Organisation.
Open access publishing facilitated by Grith University, as part of
the Wiley - Grith University agreement via the Council of Australian
University Librarians.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available from the
corresponding author upon reasonable request.
Keywords
carbon dioxide reduction, CrO/Ag heterostructures, metal/metal
oxides (MMO) heterostructures, synergetic promotional eect
Received: March ,
Revised: May ,
Published online: June ,
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