Metal-organic framework derived carbon supported Cu-In nanoparticles for highly selective CO2electroreduction to CO

Article abstract of DOI:10.1039/d1cy00843a

Cu-based materials are promising electrocatalysts for the CO₂reduction reaction (CO₂RR). However, they still suffer from intense hydrogen evolution, as well as low selectivity and efficiency for the CO₂RR. In this work, Cu-In bimetallic catalysts for the CO₂RR were prepared by the pyrolysis of Cu-In metal-organic framework (MOF) materials. Cu₉₀In₁₀/C with a Cu/In molar ratio of 9/1 exhibited a high CO selectivity of up to 85% at ?0.75 Vversusthe reversible hydrogen electrode, compared with 3.1% on Cu/C and 10.8% on In/C. On the Cu₉₀In₁₀/C catalyst, In was found to be locally distributed on the surface of Cu nanoparticles, forming Cu₄In, changing the geometric and electronic structures of the Cu surface. These modifications were supposed to have modulated the adsorption properties of the catalyst for *H, CO₂and the intermediates during the CO₂RR, therefore the electrochemical conversion of CO₂to CO was enhanced and hydrogen evolution was mitigated. The enlarged electrochemically active surface area and the accelerated charge transfer and reaction kinetics of this Cu-In bimetallic catalyst also contributed much to its excellent CO₂RR performance. This work not only highlights the superior CO₂electroreduction performance of bimetals over monometals, but also provides a new method to develop bimetallic catalysts.

Full text of DOI:10.1039/d1cy00843a

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Metal–organic framework derived carbon
supported Cu–In nanoparticles for highly selective
CO2 electroreduction to CO†
Cite this: DOI: 10.1039/d1cy00843a
Xia Ma,ab Jianjian Tian,ab Min Wang,ab Xixiong Jin,ab
abc
Meng Shenab and Lingxia Zhang
*
Cu-based materials are promising electrocatalysts for the CO2 reduction reaction (CO2RR). However, they
still suffer from intense hydrogen evolution, as well as low selectivity and efficiency for the CO2RR. In this
work, Cu–In bimetallic catalysts for the CO2RR were prepared by the pyrolysis of Cu–In metal–organic
framework (MOF) materials. Cu90In10/C with a Cu/In molar ratio of 9/1 exhibited a high CO selectivity of up
to 85% at −0.75 V versus the reversible hydrogen electrode, compared with 3.1% on Cu/C and 10.8% on In/
C. On the Cu90In10/C catalyst, In was found to be locally distributed on the surface of Cu nanoparticles,
forming Cu4In, changing the geometric and electronic structures of the Cu surface. These modifications
were supposed to have modulated the adsorption properties of the catalyst for *H, CO2 and the
intermediates during the CO2RR, therefore the electrochemical conversion of CO2 to CO was enhanced
and hydrogen evolution was mitigated. The enlarged electrochemically active surface area and the
accelerated charge transfer and reaction kinetics of this Cu–In bimetallic catalyst also contributed much to
its excellent CO2RR performance. This work not only highlights the superior CO2 electroreduction
performance of bimetals over monometals, but also provides a new method to develop bimetallic catalysts.
Received 12th May 2021,
Accepted 20th July 2021
DOI: 10.1039/d1cy00843a
rsc.li/catalysis
overvoltage to produce various carbon-containing chemicals
and the competition between the CO2 reduction reaction
1 Introduction
The consumption of fossil fuels has brought about climate
issues such as global warming due to the excessive emission
of CO2. In order to alleviate environmental pressure and
realize a carbon-neutral cycle, it is of great significance to
develop sustainable approaches for CO2 conversion.1 By
electrochemical reduction under mild conditions with
renewable electricity from solar, wind, tide, etc., CO2 can be
converted to highly valuable chemicals and fuels, such as CO,
HCOOH, CH4, ethanol and other hydrocarbons,2–4 which are
feedstocks of chemical industries. To meet the criteria for
practical applications, the research on CO2 electrochemical
reduction mainly focuses on pursuing high product faradaic
efficiency (FE) and CO2-conversion efficiency.5 However, the
efficiency of the CO2RR is still very low, because of the high
(CO2RR) and hydrogen evolution reaction (HER) at the
cathode.6–8 Therefore, it is urgent to develop efficient catalysts
to boost the CO2RR and effectively inhibit the HER.
Multifarious metal catalysts have been applied in the CO2-
RR, such as Pd, Au, Ag, and Cu.9 Among them, Cu, as one of
the non-noble metals, can electrochemically reduce CO2 to
various products, including C1, C2 and even long-chain
chemicals, at modest overpotentials.10,11 Unfortunately, Cu
shows poor selectivity to single products because of the
multiple proton–electron coupled transfer pathways of the
CO2RR on it.12 Furthermore, because the edge-site Cu atoms
prefer to adsorb protons,13 single metal Cu tends to show
intense activity for the HER rather than for the CO2RR. In
order to improve the CO2RR activity and the product
selectivity on Cu-based catalysts, many modification
strategies have been reported, such as oxide-derivative
preparation,14 hetero-element doping,15 and size and
morphology regulation.16,17 Coupling Cu with another metal
is an effective way to change the pathway of the CO2RR and
improve the FE of a specific product by optimizing the
surface adsorption energy of intermediates.18,19 For instance,
Mun et al.20 synthesized Cu–Pd bimetallic nanoparticles,
which had low affinity for *H and suppressed the
protonation of CO. As a result, the CO selectivity on this
a State Key Laboratory of High Performance Ceramics and Superfine
Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295
Ding-xi Road, Shanghai, 200050, P. R. China. E-mail: zhlingxia@mail.sic.ac.cn
b Center of Materials Science and Optoelectronics Engineering, University of
Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, P. R. China
c School of Chemistry and Materials Science, Hangzhou Institute for Advanced
Study, University of Chinese Academy of Sciences, 1 Sub-lane Xiangshan, Hangzhou
310024, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cy00843a
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catalyst was up to 89%. However, considering the high cost
of noble metals, it is more feasible to couple Cu with other
non-precious metals to improve the CO2RR activity.
solvent of ethanol and deionized water (volume ratio = 1 : 1).
Then Cu(NO)3·3H2O and In(NO3)3·xH2O with different molar
ratios were ultrasonically dissolved in the above solution,
followed by stirring for 30 min at room temperature to form a
homogeneous solution. The solution was heated at 85 °C under
constant stirring for 6 h, and then transferred to a Teflon-lined
autoclave and kept at 120 °C for 12 h. After centrifugation and
washing with ethanol and water several times, CuxIn100−x MOFs
were collected and dried at 60 °C under vacuum. Cu or In MOFs
were also synthesized with the same procedure without adding
In(NO3)3·xH2O or Cu(NO)3·3H2O, respectively.
Post-transition metals, such as In, Sn, and Bi, have low
affinity towards *H and high overpotentials for the HER,21,22
and are regarded as promising catalysts to enhance the CO2-
RR and mitigate the HER. However, they show weak
adsorption ability towards CO2 and intermediates in the CO2-
RR,23 which leads to low activity and slow kinetics for the
CO2RR. Through alloying Cu with post-transition metals, the
binding energy of CO2 and intermediates (*COOH, *OCHO,
*H, etc.) can be modulated by tuning the local electronic and/
or geometric structures, and the activity of Cu-based catalysts
in the CO2RR can be further boosted.20,24 For example, Cu–
Sn alloy nanoparticles achieved 93% selectivity to C1 products
(HCOOH and CO account for 53% and 40%, respectively) at
−1.0 V versus the reversible hydrogen electrode (RHE),25
which was considerably higher than that on the single Cu
and Sn counterparts. However, the selectivity to a target
product was still low. Cu–In alloy materials were synthesized
and ran into the same problem: the selectivity to CO was only
63%.19 Therefore, it is highly desired to develop new
bimetallic catalysts to promote the selectivity to a single
target product.
Considering that metal–organic framework (MOF) derived
carbon materials are promising electrochemical catalysts for
the CO2RR due to their ultra-high porosity, high conductivity,
and excellent CO2 adsorption ability,26,27 herein, we
synthesized bimetallic electrocatalysts with different Cu/In
atomic ratios by pyrolyzing MOF precursors. And their CO2
electrochemical performances were investigated in 0.1 M
KHCO3 using a three-electrode cell. It was found that the Cu–
In bimetallic nanocatalyst with an atomic content of 90% Cu
and 10% In exhibited a maximum FECO of about 85%, which
was almost 7.9 and 26.9 times higher than that on single In/
C and Cu/C, respectively. The synergistic effect between Cu
and In changed the surface electronic and geometric
structures of the bimetallic nanoparticles, which tuned the
adsorption energy of *H, CO2 and the intermediates during
the CO2RR, leading to high selectivity to CO.
Carbon supported Cu–In bimetallic catalysts (CuxIn100−x
/
C): 0.5 g of the as-prepared CuxIn100−x MOF powder was
calcined in a tube furnace at 900 °C for 2 h under an Ar
atmosphere with a ramp rate of 5 °C min−1. Cu/C or In/C was
synthesized with the same method. The whole scheme of the
preparation of CuxIn100−x/C catalysts is shown in Scheme 1.
2.3 Characterization
A Rigaku D/Max 2200PC X-ray diffractometer was used to
obtain the X-ray diffraction (XRD) patterns with a scanning
rate of 4° min−1 at an acceleration voltage of 40 kV and
applied current of 40 mA at room temperature. Transmission
electron microscopy (TEM) and energy-dispersive X-ray
spectroscopy (EDS) were carried out on a field-emission JEM-
2010 microscope at 200 kV. Scanning electron microscopy
(SEM) images were collected on a Hitachi S-4800 microscope.
A Micromeritics 3020 system was used to measure the N2
adsorption–desorption isotherms at 77 K. The surface area of
the catalysts was calculated by the Brunauer–Emmett–Teller
(BET) method and the pore size distribution (PSD) was
calculated by the Barrett–Joyner–Halenda (BJH) method.
X-ray photoelectron spectroscopy (XPS) and Auger electron
spectroscopy (AES) were performed on a Thermo Scientific
ESCALAB 250 spectrometer with an Al Kα radiation source
(1486.6 eV), and Ar ion etching was performed with an
acceleration voltage of 2 kV for 10 s to characterize the
chemical states and element contents on the subsurface. The
binding energy was calibrated with C 1s at 284.8 eV. The bulk
Cu/In atomic ratio was measured using an inductively
coupled plasma optical emission spectrometer (ICP-OES,
Agilent 5100) with 0.5 g of samples.
2 Experimental
2.1 Materials
1,3,5-Benzenetricarboxylic acid (H3BTC), copper(II) nitrate
trihydrate (Cu(NO)3·3H2O), indium(III) nitrate hydrate
(In(NO3)3·xH2O), potassium hydrogen carbonate (KHCO3),
dimethyl sulfoxide (DMSO) and isopropyl alcohol were
purchased from Sinopharm Chemical Reagent Co., Ltd. All the
reagents were used as received. Glassy carbon, Ag/AgCl and Pt
plate electrodes were obtained from Aida Corporation, Tianjin.
2.4 Electrochemical experiments
Preparation of the working electrode: 10 mg of catalyst was
dispersed in a mixed solution containing 1 mL of isopropyl
2.2 Preparation of catalysts
CuxIn100−x MOFs: CuxIn100−x MOFs (x is the molar percentage of
Cu in the precursors) were synthesized by a hydrothermal
procedure. 5 mmol of H3BTC was dissolved in 50 mL of a mixed
Scheme 1 Schematic diagram of the synthetic procedure for Cu–In
bimetallic catalysts.
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alcohol and 40 μL of 5% Nafion (DuPont) followed by
ultrasonication for 90 min to form a homogeneous ink. Then
10 μL of ink was dropped onto a glassy carbon electrode and
dried at room temperature.
Electrochemical reduction of CO2: electroreduction
experiments were performed on
a VSP electrochemical
workstation (Bio-Logic, Germany) using a cell with two
compartments separated by an N117 membrane (DuPont) at
room temperature. The counter and reference electrodes were
a Pt plate (10 × 10 × 0.2 mm) and a Ag/AgCl (saturated KCl),
respectively. 30 mL of 0.1 M KHCO3 solution, poured into
each part of the cells, was used as the electrolyte, which was
bubbled with CO2 for 30 min to reach saturation (pH 6.8) at
the beginning of each measurement. Before each
potentiostatic test, linear sweep voltammetric (LSV) curves
were swept four times to activate the catalysts. The reversible
hydrogen electrode (RHE) potentials were converted based on
the following equation: ERHE = EAg/AgCl + 0.21 + 0.0591 × pH.
All potentials cited in this paper were versus RHE unless
stated otherwise.
Fig. 1 LSV curves of Cu90In10/C, Cu/C, and In/C catalysts in Ar- or
CO2-saturated 0.1 M KHCO3 electrolytes (a) and faradaic efficiency of
the products obtained on Cu90In10/C (b), Cu/C (c), and In/C (d) in the
potential range of −0.65–−0.9 V vs. RHE.
2.5 Product analysis
performance in inhibiting hydrogen evolution.32 Cu/C shows
the highest current density in CO2-saturated electrolyte;
however, there is little difference between its current
densities in Ar- and CO2-saturated electrolytes, revealing the
more intense HER than the CO2RR on Cu/C. For Cu10In90/C,
Cu50In50/C and Cu90In10/C, the current density under CO2-
saturated conditions becomes higher with more Cu added
(Fig. 1a and S1a†), which may arise from the gradually
elevated CO2RR activity on them. Among these three
catalysts, Cu90In10/C exhibits the highest current density
under CO2-saturated conditions, as well as the largest
variance between the current densities in Ar- and CO2-
saturated electrolytes, reflecting its superior activity in the
CO2RR to that in the HER.
The gas products (CO, CH4) were analyzed using an offline
gas chromatograph (Shimadzu, GC-2014) equipped with a
flame ionization detector (FID). H2 was analyzed using a gas
chromatograph (Techcomp, GC7900) equipped with
a
thermal conductive detector (TCD) and a 5 Å molecular sieve
column. Both GCs used N2 as carrier gas. The gas products
were sampled by extracting 1 mL of the gas at the cathode in
an H-cell after each potentiostatic test with a gas injector,
and then injected to GC for analysis. The liquid products
were analyzed using
a
nuclear magnetic resonance
spectrometer (NMR, 500 MHz, Bruker) with water
suppression. The solution used for 1H NMR analysis was
mixed with 500 μL of electrolyte after the electrochemical
reduction test and 100 μL of D2O containing 0.78 μmol of
DMSO which was used as the internal standard.
CO, HCOOH and H2 were detected as the products and
their Faradaic efficiency (FE) on the catalysts was calculated.
It is noted that the FE of H2 (FEH ) on Cu/C reaches above
2
50%, which is much higher than the FECO and FEHCOOOH
over all the applied potentials (Fig. 1c), indicating that the
HER is more favorable on Cu/C, agreeing well with the results
of LSV. On In/C (Fig. 1d), FECO is below 10% over all the
applied potentials, while FEHCOOH increases from 5% at −0.7
V to 39% at −0.9 V, which is consistent with the previous
CO2RR reports that CO2 could be electrochemically reduced
to HCOOH by In catalysts.33 Besides, compared with Cu/C,
3 Results and discussion
3.1 CO2 electroreduction performance
The CO2RR activity on CuxIn100−x/C catalysts was tested at
room temperature in 0.1
M
KHCO3 electrolyte. For
consistency, the linear sweep voltammetric (LSV) curves at
the fourth cycle of all the catalysts were used and are shown
in Fig. 1a and S1a.† The current density measured in Ar-
saturated electrolyte corresponds to the HER28 which is a
competition reaction of the CO2RR at the cathode, while the
current density under CO2-saturated conditions corresponds
to the sum of the HER and CO2RR.29 Notably, the current
densities acquired on the as-obtained samples in CO2-
saturated electrolyte are all higher than those under Ar-
saturated conditions, indicating that they all show CO2RR
catalytic performance.30,31 Among them, In/C shows the
lowest current density under CO2-purged condition, which
may arise from the poor conductivity of In and its excellent
In/C shows decreased FEH to about 30%, indicating that the
2
HER has been mitigated, which is believed to originate from
the high hydrogen evolution overpotential on In.34 However,
the selectivity to a single product of the CO2RR is poor on In/
C. Both CO and HCOOH are produced on Cu10In90/C, Cu50-
In50/C and Cu90In10/C, and FECO becomes higher with the
increase of the Cu content (Fig. S1d†). Cu90In10/C exhibits
remarkably improved selectivity to CO (Fig. 1b). As the
applied potential increases, the selectivity to CO on Cu90In10/
C shows a volcanic trend and reaches a maximum of about
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85% at −0.75 V, which is nearly 7.9 times and 26.9 times
higher than that on In/C and Cu/C, respectively (Fig. S2c†).
The partial current densities of CO over the applied
potentials on Cu90In10/C are all higher than those on Cu/C
and In/C (Fig. S2a†), indicating its high CO2RR activity. The
nanorod-like morphologies, respectively.35,36 Containing both
Cu MOF and In MOF, the Cu90In10 MOF precursor shows an
adhesion morphology of octahedra and nanorods (Fig. 3a).
By the pyrolysis of the MOF precursors, CuxIn100−x/C
preserves the morphologies of the MOFs. As can be seen in
Fig. S5a and b,† Cu/C is octahedral, in which there are some
nanoparticles identified as Cu nanoparticles. In/C exhibits a
nanorod morphology with some micropores rather than
nanoparticles on the surface (Fig. S5c and d†). Cu90In10/C
retains the adhesion morphology of octahedra and nanorods
of the Cu90In10 MOF with a mass of irregular nanoparticles
distributed on the surface, and the size of the nanoparticles
is about 200–400 nm (Fig. 3b and c). TEM and EDS were used
to further characterize the composition of the nanoparticles.
As shown in Fig. 3d–g, the EDS elemental mapping images
show that the nanoparticles consist of both Cu and In, that
is, these nanoparticles are Cu–In bimetals. Meanwhile, the
element O mainly coexists with the element C. Only a small
amount of O is distributed on the bimetallic nanoparticles,
which may originate from the surface oxidation of the
metals.
The chemical states of the metals on the catalyst surface
were characterized by XPS and AES. In Fig. 4a, the Cu 2p XPS
spectra of Cu/C and Cu90In10/C show the peaks of Cu 2p3/2 at
about 932.8 and 934.8 eV corresponding to Cu+/Cu0 and
Cu2+, respectively.37 The AES spectra in Fig. 4b present two
characteristic peaks at 568.6 e and 570.5 eV assigned to Cu0
and Cu+, respectively.38 It can be concluded that on the
surface of Cu/C and Cu90In10/C, Cu is in the multivalent state
of 0, +1 and +2. After etching by Ar ions for 10 s, the satellite
peaks of Cu2+ on the subsurface of Cu/C and Cu90In10/C
disappear (Fig. S7a and b†), indicating that the Cu2+ on the
surface is derived from surface oxidation in air. The In 3d
XPS spectrum of In/C shows two characteristic peaks at 445.2
and 452.8 eV relative to In 3d5/2 and In 3d3/2 of In3+,
respectively (Fig. 4c),33 which may be induced by the surface
oxidation of In in air.39 The In 3d binding energy values of
Cu90In10/C negatively shift compared with those of In/C
(Fig. 4c). Meanwhile, the Cu 2p binding energy values of
Cu90In10/C positively shift compared with those of Cu/C (Fig.
S7c†). Based on these results, the electron transfer from Cu
to In has happened, resulting in electronic richness of In,
FEH on Cu90In10/C has been inhibited below 20%, which
2
may be related to the synergistic effect between Cu and In.
The stability test of Cu90In10/C has been carried out at −0.75
V for 4 h (Fig. S3†), during which the current density and
FECO have not displayed obvious decay, demonstrating the
excellent stability of Cu90In10/C. Cu90In10/C showed such an
optimal CO2 electroreduction performance that its structure,
composition and catalytic mechanism were thoroughly
investigated.
3.2 Structure and composition of the catalysts
The structure and composition of the as-obtained catalysts
were further characterized by XRD. As shown in Fig. 2a, the
crystal structures of the Cu MOF and In MOF are consistent
with previous reports,35,36 and the Cu90In10 MOF shows
characteristic peaks of both the Cu-MOF and In-MOF,
indicating that the Cu90In10 MOF precursor has been
successfully prepared. CuxIn100−x/C samples were obtained by
pyrolyzing CuxIn100−x MOFs at high temperature. As shown in
Fig. 2b, the crystal phase composition of In/C is mainly
metallic In (JCPDS card No. 05-0642), accompanied by a
small amount of In2O3 (JCPDS card No. 06-0416) caused by
the surface oxidation of In. Cu/C shows strong characteristic
peaks at 43.3°, 50.4° and 74.1°, corresponding to metallic Cu
(JCPDS card No. 04-0836). Besides metal In and In2O3, Cu10-
In90/C and Cu50In50/C exhibit characteristic peaks centred at
29.4, 41.4 and 42.3, corresponding to Cu11In9 (JCPDS card
No. 41-0883), and no Cu species was detected (Fig. S4†). Cu90-
In10/C, with a high content of Cu, shows the crystal phase of
metallic Cu, In2O3 and Cu4In (JCPDS card No. 42-1477) which
can be identified by the characteristic peaks at 42.6°, 61.9°
and 78.1°. The transformation from Cu11In9 in Cu10In90/C
and Cu50In50/C to Cu4In in Cu90In10/C is considered to be
related to the Cu/In molar ratio in the precursor.
SEM and TEM were used to further characterize the
morphology and composition of the catalysts. According to
previous reports, Cu and In MOFs exhibited octahedral and
Fig. 2 XRD patterns of Cu, In, and Cu90In10 MOF precursors (a) and
Cu/C, In/C, Cu90In10/C catalysts (b).
Fig. 3 SEM images of the Cu90In10 MOF precursor (a) and Cu90In10/C
(b). TEM image (c) and EDS elemental mappings (d–g) of Cu90In10/C.
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potential range with different scan rates (Fig. S10†), and the
charging current density differences between the anode and
cathode voltages of the CV curves were plotted against the
scan rates and are shown in Fig. 5a and S10f.† The double-
layer capacitance (Cdl) values, which are half of the slopes, of
these catalysts are in the order of Cu90In10/C > Cu50In50/C >
Cu10In90/C
> Cu/C > In/C, suggesting that Cu90In10/C
exhibits the largest electrochemically active surface area
(ECSA), due to the positive relationship of ECSA and Cdl.42
Therefore, for Cu90In10/C, the highest catalytic activity is
favored by the most active sites, which may be related to Cu4-
In species distributed on the Cu surface.
Electrochemical impedance spectroscopy (EIS) of the
catalysts was conducted at open circuit potential (Fig. 5b and
S11b†). Among the as-obtained catalysts, the impedance
radius of Cu90In10/C is the smallest, indicating that Cu90In10/
C has the lowest charge-transfer resistance (Rct) and fastest
electron-transfer rate,43 which accelerated the reaction rate of
the CO2 reduction to CO. It is inferred that the lowest
impedance is caused by the electronic effect between Cu and
In, which accelerates the electron transfer from the catalyst
to CO2.
Fig. 4 XPS spectra of Cu 2p (a). AES spectra of Cu LMM (b). XPS
spectra of In 3d (c). Atomic ratio of Cu and In on the surface and
subsurface of Cu90In10/C (d).
that is, the surface electronic structure of the Cu catalysts
has been modified by the In introduction.40 Considering that
the critical step of the CO2RR (CO2 + e− → *CO2 ) involves an
Based on previous reports,28 the reaction pathways of the
CO2 electrocatalytic reduction to CO are as follows:
−
electron transfer to CO2, for Cu90In10/C, the modification of
its surface electronic structure is beneficial to improving the
reaction rate of this bottleneck step and modulating the
adsorption energy of the intermediates,41 thus improving its
CO2RR activity.
CO2(aq) + e− → *CO2 (aq)
(1)
(2)
(3)
(4)
−
*CO2 (aq) + H+ → *COOH(aq)
−
Ar ion etching was applied to further analyze the content
of Cu and In on the subsurface of Cu90In10/C (Fig. 4d). Before
etching, the Cu/In atomic ratio is 3.78, close to the atomic
ratio of Cu4In shown by XRD. After etching, the Cu/In atomic
ratio increases to 6.24, while the actual bulk Cu/In atomic
ratio measured by ICP-OES is 11.07 (Table S1†), indicating
that the content of Cu on the subsurface is higher than that
on the surface. It can be deduced that in Cu90In10/C particles,
Cu4In is mainly distributed on the surface while Cu mainly
exists inside the particles, which may be caused by the
relatively low melting point and low content of In. The
specific geometric structure of Cu90In10/C may optimize the
binding strength of *H and *COOH,19 further increasing the
selectivity to CO.
*COOH(aq) + H+ + e− → *CO(aq) + H2O
*CO(aq) → CO(g)
where * represents the active site on the catalyst surface
where the intermediate can be bonded to. The Tafel slope is
3.3 Mechanism analysis
As mentioned above, Cu90In10/C exhibits the highest
selectivity to CO, indicating its enhanced CO2RR activity. But
the specific surface area of In/C, Cu10In90/C, Cu50In50/C, Cu90-
In10/C and Cu/C acquired by the BET method is 259.8, 184.5,
156.4, 191.4 and 147.7 cm3 g−1, respectively (Fig. S9†), which
is not consistent with their CO2RR performance. Therefore,
surface area is not the key contributor to the CO2RR
performance of these catalysts.
To explore the intrinsic activity, electrochemical
characterization was performed and analyzed. Cyclic
Fig. 5 Charging current density differences plotted against scan rates
(a), Nyquist plots (b), and Tafel plots for CO (c) of Cu90In10/C, Cu/C
and In/C. Schematic diagram of the CO2RR on Cu90In10/C (d).
voltammetry (CV) curves were tested in
a non-faradaic
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generally used to evaluate the kinetics of the electrocatalytic
reaction. To obtain the Tafel slopes, electrochemical
reactions have been carried out in the low overpotential
region, where the CO2RR was kinetically controlled. As shown
in Fig. 5c and S11a,† the Tafel slopes of Cu90In10/C, Cu50In50/
C, Cu10In90/C, Cu/C and In/C are 147.4, 178.2, 183.7, 254.9,
and 230.1 mV dec−1, respectively, which are close to 118 mV
dec−1, indicating that the rate-determining step of the CO2RR
not only improve the stability of *COOH, but also benefit the
desorption of CO.41,50 Besides, because of the low adsorption
of *H, CO-targeting intermediates are difficult to be further
hydrogenated. As a result, CO, as the main end-product, is
ready to desorb from the surface of Cu90In10/C.
Conclusions
−
on these catalysts is step (1):30 the formation of the CO2
In summary, a carbon supported Cu–In bimetallic catalyst
was synthesized by pyrolyzing CuIn-MOF precursors.
Compared with the counterpart catalysts of single Cu and
monometallic In, the Cu–In bimetallic catalyst with a Cu/In
mole ratio of 9/1 showed remarkably enhanced CO selectivity
in the CO2RR, which reached about 85% at −0.75 V vs. RHE.
Cu4In was formed on the surface and the electron transfer
from Cu to In locally modified the geometric and electronic
structures of the Cu-based catalyst, thus optimizing the
adsorption of *H, CO2 and the intermediates (e.g., *COOH)
during the CO2RR, tuning the reaction pathway of the CO2RR
and improving the selectivity to CO. The electrochemically
active surface area of the bimetal catalyst was enlarged
compared with that of the monometallic Cu and In
counterparts. And the synthesized Cu–In bimetallic catalyst
also exhibited lowered electron transfer impedance and an
accelerated electron transfer rate and reaction kinetics,
leading to its enhanced CO2RR activity. This work not only
intermediate from CO2 by accepting an electron. Moreover,
the Tafel slope value of Cu90In10/C is the smallest among
these materials, indicating its fastest reaction kinetics, thus
boosting its highest activity for the CO2RR. Hence, the
electronic structure of Cu90In10/C favors the reaction kinetics
and activity for the CO2RR.
As is known, the activity of Cu-based catalysts for CO2
conversion is strongly related to their chemical state,
morphology, crystal facet, etc.44–46 For example, oxide
derived-Cu materials have been reported to be effective in
improving CO selectivity, which may arise from their
electronically unsaturated sites.44,47 Through crystal facet
engineering, the specific facet of Cu could be exposed to
obtain high CO selectivity. By DFT calculation, Cao and
coworkers46 demonstrated that the Gibbs free energy
required for *COOH formation on Cu (110) is much lower
than that on Cu (111), indicating that Cu (110) is more active
for CO formation than the (111) facet. However, in the Cu/C
catalyst in this work, the main phase is metallic Cu rather
than CuxO. And the main facet of Cu is (111), as indicated by
the diffraction peak at 43.3° in the XRD pattern. These two
factors may induce its low CO selectivity.
provides
a new strategy to fabricate Cu–In bimetallic
electrocatalysts for the CO2RR, but also gives an example to
modify the surface structure of Cu-based catalysts to enhance
their CO2RR activity and selectivity.
By coupling Cu with In, the pathway of the CO2RR may
have been changed by optimizing the surface adsorption
energy of the intermediates, thus improving the selectivity to
CO. It has been proved that Cu atoms on edge sites are active
for *H adsorption,13 thus, Cu catalysts exhibit high hydrogen
evolution activity. Meanwhile, In atoms exhibit low
adsorption energy for *H, indicating that In catalysts are
potential candidates for the CO2RR,34 Moreover, Cu atoms on
edge sites prefer to be replaced by In atoms,19 therefore the
HER can be slowed down.32 Considering that the catalytic
reactions mainly happen on the surface of materials,48,49 in
this work, the introduction of In has modified the geometric
structure of Cu90In10/C by forming Cu4In on the surface of
Cu, thus weakening *H adsorption and inhibiting the HER.
Such a specific geometric structure of Cu90In10/C may also
optimize the binding strength of *COOH,19 further
increasing the selectivity to CO. And the optimized electronic
structure arising from the electron transfer between Cu and
In is favorable for enhancing the CO2 adsorption on the
catalysts, further facilitating CO2 transformation to carbon-
containing intermediates.40 On the other hand, the
adsorption energies of *COOH and CO on Cu are −1.5 eV
and −0.7 eV, while those on In are −1.0 eV and 0 eV,
respectively,49 reflecting the more convenient desorption of
CO from In. It can be concluded that the presence of In may
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the financial support
provided by the National Key R&D Program of China
(2017YFE0127400), the National Natural Science Foundation
of China (No. 51872317 and 21835007) and the Science and
Technology Commission of Shanghai (20520711900).
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