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Heterogeneous Electrocatalysts for CO2 Reduction
Chao Yang, Yuhang Wang, Linping Qian, Abdullah M. Al-Enizi, Lijuan Zhang, and Gengfeng Zheng*
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ABSTRACT: The electrocatalytic CO2 reduction reaction (CO2RR)
has been a fast developing and innovative topic in recent years. It may
enable efficient reduction of carbon emission, as well as store renewable
electricity into chemical bonds in fuels or other chemicals. However, due
to the complexity of reaction factors and mechanisms, the activity and
selectivity of CO2RR have yet to be further optimized in order to realize
commercial applications. Specifically, the high selectivity of CO2
reduction products (especially for multi-electron-transfer and multicarbon reduction products), lower overpotentials, high energy
efficiencies, good stability, and low fabrication cost are required. In
this Spotlight, we focus on the design of active sites in heterogeneous
catalysts for electrochemical CO2 reduction, and we provide several
strategies to target those challenges in this field. First, as the major side
reaction in aqueous solutions, the hydrogen evolution reaction can be
inhibited by increasing the energy barrier for H2 formation and tuning the electrolyte diffusion toward the electrocatalyst surface,
thus enhancing the Faradaic efficiencies of CO2RR products. Second, the proximity of adjacent catalytic sites suggests a strong
capability for tuning the coupling efficiency of multiple carbon atoms. Third, the active sites of heterogeneous electrocatalysts can be
generated by different defects, including single-atom doping, anion vacancy, alloy formation, and lattice defects, which can lead to
distinctively different production distributions. Finally, a solar-driven electrocatalytic CO2 reduction system was developed, enabling
a high solar-to-fuel photoconversion efficiency. Further investigations of heterogeneous electrocatalysts and reaction systems are
expected to enhance the electrochemical CO2RR performances toward the application level.
KEYWORDS: electrochemical CO2 reduction, heterogeneous catalyst, adsorption, copper, Faradaic efficiency
1. INTRODUCTION
Influenced by anthropogenic activities, the content of carbon
dioxide (CO2) in the atmosphere is continuously increasing.1
Among many different CO2 utilization pathways, the electrocatalytic conversion of CO2 into chemical products represents
a unique and promising strategy.2 In this process, using
electricity energy as the input, CO2 can be potentially
transformed into a range of carbon-based molecules, capable
of reducing CO2 emission and enabling storage of chemical
energy.3 In addition, it is ideal to use renewable resources such
as solar or wind energies to produce electricity for potential
scale-up applications.4
Using technoeconomic analysis methods,5 the feasibility and
limitations of the electrocatalytic CO2RR (CO2 reduction
reaction) have been studied. Carbon monoxide (CO) and
formate (HCOOH) are among the highest efficiency CO2RR
products under the current electrolysis conditions.5,6 Nonetheless, the low electrochemical performances toward C2
products, such as ethylene, ethanol, and n-propanol, should be
improved drastically to fulfill a reasonably profitable level,
including high partial current densities of >300 mA cm−2, high
Faradaic efficiencies (FEs) of >80−90%, low full cell operating
© 2021 American Chemical Society
voltage, and excellent stability (>1000 h). Recently, the
increasing use of gas-diffusion-layer (GDL)-based flow-cell
systems and membrane−electrode assemblies (MEAs) has
suggested attractive capabilities for realizing these metrics.7 In
contrast to the conventional H-type electrolysis cells, a solid−
liquid−gas triphase interface formed near the GDL and
electrocatalysts helps to solve the challenge of CO2 mass
transfer in aqueous solutions and achieve high current densities
that can be implemented commercially.8
2. ELECTROLYTE EFFECT
In spite of the attractive potential in the field of CO2RR, the
complexity of this reaction still restricts an understanding of
the overall process. The electrocatalytic CO2RR is typically
Received: October 28, 2020
Accepted: December 24, 2020
Published: January 21, 2021
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Figure 1. Schematic of several possible reaction pathways of electrochemical CO2 reduction reaction toward methane, ethylene, and ethanol,
respectively.
conducted in aqueous electrolytes, which is attributed to their
abundant proton resources and excellent conductivity at room
temperature.9 However, the understanding of local environments near the interface of the solid−liquid−gas triphase
remains quite limited.10 First, protons are consumed by both
CO2RR and hydrogen evolution reaction (HER), resulting in
an increasing local pH near the electrode surface. Second, the
reaction of CO2 and water generation are also influenced by
the multiequilibrium of CO2/bicarbonate/carbonate, and
simultaneously, the formation of carbonates/bicarbonates at
the interface leads to a change in catalyst performance.11 The
difference in pH can lead to the change in specific product
selectivity through the interactions with the H* intermediate,
which is regarded as the Langmuir−Hinshelwood mechanism
(i.e., surface recombination reaction).12 In this process, a
general pH-dependence of onset potentials for methane
formation has been observed.13 A higher Faradaic efficiency
for producing CH4 was also demonstrated in a concentrated
bicarbonate electrolyte, such as in 1 M KHCO3 (where the
surface pH was ca. 8.9) versus in 0.05 M KHCO3 (with pH
9.8).14 Indeed, the role of the CO2/bicarbonate/carbonate
equilibrium reaction may be not only as a pH buffer, but also
as a carrier of reaction species. Shao et al. proposed the role of
bicarbonate anions in CO2RR by fast real-time infrared (IR)
spectroscopy.15 When the potential was stepped from 0.2 to
−0.6 V in 12CO2-saturated KH12CO3 and KH13CO3 solutions,
the IR band from 12CO was increased gradually due to the
conversion of H13CO32− into H12CO32−. Thus, it was
suggested that, under reaction conditions, CO2 molecules
cannot reach the outer Helmholtz plane densely packed by the
hydrate anions; instead, carbonate anions can release CO2
molecules near the surface for CO2RR. The mechanism was
proposed as the fast equilibrium below: CO2 + H2O + OH− ⇌
HCO3− + H2O ⇌ H2CO3 + OH−. In another study, the
bicarbonate anion can also serve as a proton donor.16 Thus, it
is critical to investigate the role of the electrolyte on the
electrochemical CO2RR.
In addition to the equilibrium of CO2/HCO3−/H2CO3, the
hydrated cations can also affect the interfacial interactions at
the surface.17 For example, K+ can stabilize the adsorbed
intermediates to lower the thermodynamic energy barrier.
Sargent and co-workers reported the field-induced K +
enrichment effect by a synthesized gold (Au) nanoneedle
catalyst.18 With the increasing size of the alkali cations, the pKa
of the cations decreases, leading to the increase in CO2
concentration and decrease in pH.19
Controlling the transport process to tune the reaction
pathways is another useful method. A membrane coated
electrode can generally enhance the activity and selectivity of
electrochemical CO2RR; this is attributed to both their
functions, such as the hydrophilic,20 gas adsorption capability,
and the capability for stabilizing intermediates for multielectron-transfer reduction products.21
3. COMPLEXITY OF HETEROGENEOUS CATALYSTS
FOR REACTION PATHWAYS
The complexity of CO2RR is attributed to a variety of
heterogeneous catalysts, reaction mechanisms, electrolysis
conditions, and target products.22,23 The thermodynamic
potentials of electrochemical CO2RR to different products
are comparable to that of HER.24,25 For a two-electron-transfer
reaction with a suitable electrocatalyst, the CO2RR typically
starts to occur at low overpotentials. The two-electron-transfer
products, CO or HCOOH, can go through different pathways.
*COOH is an important intermediate for CO formation by
proton-coupled electron-transfer (PCET) reactions, while
*OCOH is likely to be the intermediate for formate.26 The
reaction pathway selectivity of these two species is strongly
determined by the catalysts. For example, the major CO2RR
product of Bi and In is formate, while for Au and Ag it is CO.27
However, for multi-electron (>2e)-transferred products, the
kinetics of CO2RR becomes more sluggish, leading to larger
overpotentials and lower selectivity. In addition, the multielectron-transfer pathways can have various reaction mechanisms and subsequent product selectivities.28,29 Various
theoretical methods, such as density functional theory
(DFT) and molecular dynamics (MD) simulations, have
been utilized to guide CO2RR experiments.30 Using a solvation
method, the pathways of C1 and C2 products have been
studied.31 *CO−COH is formed by the coupling of *CO and
*COH at a neutral pH. However, the dimerization of *CO to
form *CO−CO is a critical pathway toward C2 products at
alkaline pHs. Further selectivity on ethylene and ethanol can
involve the surface-adsorbed H2O (Figure 1). In combination
with the isotope labeling and spectroscopy methods, multiple
CO2RR pathways have been suggested.32 Particularly, *CO has
been proposed as an indispensable intermediate in the multielectron-transfer reductions.33 The density and capability of
the CO adsorption active sites in the electrocatalysts are
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Figure 2. Schematic of different research methods on electrochemical CO2 reduction.
Figure 3. (a) Schematic illustration of further passivation of HER activity by reduction of the number of active sites via Li+ association. The brown,
green, red, blue, and purple spheres represent C, H, O, N, and Li atoms. (b) CV curves of PNFE/CNT networks in 0.5 M Li2SO4 and 0.5 M
H2SO4 aqueous solutions, respectively. Reproduced with permission from ref 37. Copyright 2016 Wiley. (c) CO partial current densities versus
KHCO3 concentration at different constant potentials for the Sn-OD-Cu in CO2-saturated KHCO3 solution with different concentrations.
Reproduced with permission from ref 38. Copyright 2019 American Chemical Society. (d) Summary of N atomic contents in CN-CNTs and CNH−CNTs. (e) Faradaic efficiency for CO, H2, and CH4 versus potential on CN-H−CNTs. Reproduced with permission from ref 39. Copyright
2017 Wiley Publications. (f) The FEs for electrochemical reduction of CO2 measured on MNC-D. Reproduced with permission from ref 40.
Copyright 2019 Springer Nature Publications. (g) XRD patterns of TNS-0.9-SnO2, TNS-2.0-SnO2, and TNS-3.0-SnO2. (h) Faradaic efficiencies for
HCOOH production of TNS-0.9-SnO2, TNS-2.0-SnO2, and TNS-3.0-SnO2 at each applied potential for 2 h. Reproduced with permission from ref
41. Copyright 2018 Wiley Publications.
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In addition to the interaction between cations and
electrocatalysts, the concentration effect of KHCO3 electrolyte
in CO2RR was also systematically investigated. Cu2O@SnOx
was used as an example to first show a stable selectivity toward
CO with good FECO (>85%). The concentration effect of
KHCO3 was then studied.38 The activity for H2 production
was initially suppressed with the increasing KHCO3 concentration over low overpotentials, while the production of CO
was promoted. The Cu2O@SnOx catalyst presented a
maximum FECO of 98.1% in 0.7 M KHCO3 at a moderate
overpotential of 490 mV. A linear correlation was also found
between log(jCO) and log[HCO3−], which suggested that the
CO2RR rate had a first-order dependence on [HCO3−] (Figure
3c). Combined with the Tafel analysis, the CO2RR mechanism
in this system was proposed as CO2 + HCO3− + e− + * →
COOH* + CO32−, where * denotes an active site. Thus, the
enhancement of CO2RR to produce CO should be attributed
not only to the pH effect, but also to the concentration effect.
Furthermore, the catalyst design can also influence the
competitivity between CO2RR and HER. For instance, metalfree carbon materials have been reported by many research
groups to be one of the most promising catalysts for CO2RR,
due to their distinct inherent characteristics, such as porous
structures, high surface area, and tunable structural configurations.55 Although pristine (i.e., undoped) carbon materials
show limited catalytic activity toward CO2 reduction,
introducing nitrogen atoms is a promising approach. A
nitrogen atom has a larger electronegativity than carbon, and
it can create electroactive sites when it is doped into carbon
materials. Nitrogen exists in carbon materials in four types:
pyridinic, pyrrolic/pyridonic, quaternary or graphitic, and
pyridine-N-oxide.56 Among them, pyrrolic nitrogen presents
a good activity for CO2RR but is sluggish toward HER, while
pyridinic and graphitic nitrogen atoms are active HER
catalysts. In 2017, our group reported a steam-etching method
to modulate the ratio of different nitrogen doping effects.39
Due to their different affinities for water molecules, the carbon
atoms adjacent to pyridinic and graphitic nitrogen atoms were
more prone to be etched away by steam. As a result, the
nitrogen-doped carbon network exhibited an increasing
content of pyrrolic nitrogen from 22.1% to 55.9% among the
total N species, which were designated as CN-CNT and CNH-CNT, respectively (Figure 3d). Cyclic voltammetry (CV)
tests conducted in both Ar-saturated and CO2-saturated
electrolytes showed that, compared to an unetched CN-CNT
sample, the CN-H-CNT (i.e., after steam etching) presented a
higher overpotential for HER and a lower overpotential for
CO2RR. The FE of CO formation (FECO) reached 88% for
CN-H-CNT, while it is only ∼60% for unetched CN-CNT
(Figure 3e). Our observation indicated that the HER
inhibition can benefit the performance of the CO2RR. In a
subsequent study, we further demonstrated that nitrogen atom
configurations were also tuned by the secondary doping
process. The edge sites of mesoporous N-doped carbon
frameworks were easier to attack by a second nitrogen doping
source (such as N,N-dimethylformamide, DMF) than inner
doping sites. This secondary N-doping process led to an
increase in the content of the nitrogen dopant level and FECO
of ∼92% (Figure 3f).40
It is also important to note that, in the studies of metal-free
catalysts, trace metal contaminants may dominate the performance of CO2RR, instead of the intrinsic activity of metal-free
catalysts. Meanwhile, the trace amount of contaminants may
important. Several experimental results have reported the
combined effects of both *CO surface density and adsorption
energy.34−36 Thus, it is critical to rationally design and tune the
active sites on heterogeneous electrocatalysts for CO2RR.
4. OUR WORK
The CO2RR research in our group can be generalized in the
following aspects (Figure 2). First of all, the competition with
the major side reaction, HER, restricts the performance of
many catalysts. By limiting the surface adsorption site density
and adsorption capability of the H atoms, both thermodynamics and kinetics of HER can be inhibited,37−41 thus
potentially leading to the enhancement of CO2RR products.
Second, the CO2RR selectivity can be tuned by the surface
density of the active sites,42−44 in which the adjacent two active
sites promote the carbon bond formation between two
adsorbed *CO intermediates.45 Third, the nanostructured
catalysts have been designed and fabricated with different
defect engineering, strain, and alloy effects, which allow high
performances for C2 products to be exhibited.42,46,47 Last but
not least, pushing the activity of electrocatalytic CO2 reduction
reaction by system engineering has also been investigated. In
addition to the use of flow-cell and MEA systems,48 we
developed a redox-mediated electrolysis system to efficiently
convert solar-generated electricity to CO2RR products with an
attractive energy efficiency and solar-to-fuel efficiency.49
4.1. Competitive Reaction of CO2RR and HER. As a
simple 2e−-transfer reaction, HER (i.e., the water reduction) is
universal for electrochemical reductions in aqueous electrolytes.50 Water reduction generally takes place at a lower (less
negative) thermodynamic potential compared to that of
CO2RR. To improve the performance of CO2RR, the
inhibition of HER should be an effective strategy, which is
correlated with two aspects: the electrolyte environment and
the electrocatalyst structure.51−53
Combined with the aspects mentioned above, a series of
studies have been reported. For instance, polyimides are
generally regarded as an anode candidate for Li and Na
batteries due to their reversible redox capability,54 while the
voltage outputs are still far from optimum without sufficient
surface engineering for HER passivation. In 2016, our group
demonstrated the work of inhibiting HER on a polyimide
surface, i.e., poly(naphthalene four formyl ethylenediamine)
(PNFE) as an example.37 The four oxygen atoms in each
repeating unit of PNFE were first verified as the major
adsorption sites for hydrogen atoms, with the lowest
adsorption energy barrier (Had) compared to the carbon and
nitrogen atoms. DFT calculations demonstrated the intrinsic
sluggish HER of PNFE in a 0.5 M H2SO4 electrolyte without
Li+ (Figure 3a). To further passivate the HER activity, a Li+containing electrolyte was used. The strong affinity for Li+ ions
led to the blocking of the adsorption sites (i.e., O atoms) for
hydrogen atoms, leading to a higher energy barrier and more
sluggish kinetics for H2 formation. Thus, both the Tafel
(2Hadsorption → H2) and Heyrovsky (Hadsorption + H2O + e− →
H2 + OH−) pathways were inhibited. When coating on a
carbon nanotube (CNT) substrate, the PNFE/CNT electrode
exhibited a large HER onset overpotential of 820 mV vs a
reversible hydrogen electrode (RHE) in 0.5 M Li2SO4 aqueous
solution (Figure 3b). Li+ ions blocking the H-adsorption sites
may further improve the concentration of cations to tune the
performance of CO2RR.
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Figure 4. (a) Structure of synthesized Cu-N-C-800, -900. (b) Faradaic efficiencies (top panel, left y-axis), partial current densities of CH4 and C2H4
(top panel, right y-axis), and the ratios of CH4/C2H4 (bottom panel) for Cu-N-C-T (T = 800, 900, 1000, and 1100 °C) catalysts at −1.6 V vs RHE.
Reproduced with permission from ref 45. Copyright 2020 American Chemical Society. (c) Structure models of CeO2(110) doped with one single
Cu site, with three oxygen vacancies. (d) Faradaic efficiency comparison of samples with different Cu doping levels. Reproduced with permission
from ref 63. Copyright 2018 American Chemical Society.
not be realized, as it is hard to be quantified by XPS or EDX
techniques.57 In an electrochemical system, the source of
impurities can be quite diverse, such as from reactants,
electrolyte, counter electrode, reference electrode, and so on.58
In our work, in order to avoid challenges from impurities, a
series of rational control experiments should be conducted to
exclude the effects from potential contamination sources
during synthesis or electrochemical measurements.
Moreover, from the view of material design, the construction
of a hydrophobic surface has been a common way to inhibit
electrolyte diffusion,20 while at the cost of limiting gas
diffusion. An alternative approach to regulate the mass transfer
is to tune the confined space of the electrocatalyst environment. In 2018, our group developed a two-dimensional (2D)
assembly of active catalyst and inert spacer components, with
tunable interspacing distances.41 In brief, atomically thin titania
nanosheets (TNS) and SnO2 nanoparticles were assembled
into highly ordered 2D nanosheets via molecular interaction
between TNS and cationic surfactants. The interspace distance
between adjacent TNS layers was tuned by different types of
cationic surfactants, ranging from 1 to 3 nm. The X-ray
diffraction patterns of (00k) planes confirmed the structure of
lamella assemblies with different interspacing distances (i.e.,
0.9, 2.0, and 3.0 nm, respectively) and the existence of the
SnO2 nanoparticles inside adjacent TNS layers (Figure 3g).
The electrochemical CO2 performances were analyzed for
these samples. Due the capability of differentiating masstransfer rates between CO2 and H2O, the TNS-2.0-SnO2 (with
2.0 nm interlayer spacing) presented the highest FEHCOO− of
73% at −1.6 V vs RHE, while TNS-0.9-SnO2 (without ordered
assembly of SnO2 nanoparticles inside 2D confined space) had
the lowest FEHCOO− of 39% (Figure 2h). In addition, the
largest interlayer spacing in TNS-3.0-SnO2 (i.e., 3.0 nm) was
unable to differentiate the diffusion rates of CO2 and H2O
molecules, so the HER production was still high, thus
substantiating the effect of the confined space of the 2D
assembly on CO2RR.
4.2. Promotion of CO2 Deep Reduction. Copper (Cu) is
known as a unique electrocatalyst to obtain deep reduction
(i.e., multi-electron-transfer) products. The common deep
reduction products are methane (CH4), ethylene (C2H4),
ethanol (C2H5OH), and propanol (C3H7OH).27 This unique
feature of Cu can be attributed to its moderate adsorption for
both carbon and hydrogen.59 The current research on Cubased CO2RR electrocatalyst designs involves various aspects,
such as doping,60 surface facets,61 defect effects,42 and so on.
However, many challenges still exist with a variety of different
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Figure 5. (a) Schematic illustration of intermediate adsorption controlling which copper facets are exposed, with the chemisorbed intermediates
*CO acting as capping agents, leading to a high portion of Cu(100) in the forming catalyst. Reproduced with permission from ref 64. Copyright
2019 Springer Nature. (b) Schematic of the synthesis of CuO nanoparticles at different cooling rates. Reproduced with permission from ref 46.
Copyright 2020 Elsevier. (c) FEH2 and FEalcohol for ECR on a Cu matrix and Cu3Ag1 at different working potentials. (d) Ratio of alcohols among Cx
products at different potentials for Cu3Ag1. Reproduced with permission from ref 70. Copyright 2020 Wiley Publications.
majority of Cu species coordinated with nitrogen atoms
remained in the form of Cu(I)−N2 pairs, while at a higher
temperature of 900 °C (named as Cu-N-C-900), single-atomic
Cu(II)−N4 was the major form of Cu dopants. Fouriertransformed extended X-ray absorption fine structure (FTEXAFS) confirmed both structures and indicated that the
distance between adjacent Cu(I)−N2 species was significantly
shorter than that of individual Cu(II)−N4 species. The
electrochemical CO2RR performance showed that the Cu(I)−N2 pairs promoted the C−C coupling and subsequent
formation of C2H4, while single-atomic Cu(II)−N4 sites
catalyzed CO2RR toward CH4 (Figure 4b).
In addition to the regulation of nitrogen-doped carbon,
metal oxides are also good substrate candidates. For example,
ceria (CeO2) is an attractive substrate by providing strong
metal−support interactions and enhancing the dispersion of
loaded metal species.62 In 2018, we reported a unique structure
of single-atomic Cu-substituted CeO2, which demonstrated
mechanistic pathways. In our study, we have conducted
research on copper-based catalysts from two aspects.
4.2.1. Single-Atomic Cu Configuration for CO2RR. CO is a
crucial intermediate in the deep reduction and C−C coupling
step. The coverage and density of CO can strongly affect the
product selectivity. 8 In catalysts with a single-atomic
distribution of Cu sites, the limited active sites and relatively
far distance between adjacent sites can hinder the coupling of
two adsorbed *CO species. Therefore, CH4 can be the main
product in this process. For example, we investigated the
relationship of CO2RR selectivity and the surface Cu density.45
Here, nitrogen-doped carbon was used as the framework to
promote the dispersion of copper species. The distance
between adjacent Cu species was adjusted by a different
thermal pyrolysis of a Cu-containing metal−organic framework
(MOF). The contents of Cu decreased gradually with the
increase of pyrolysis temperature (Figure 4a). At a lower
calcined temperature of 800 °C (named as Cu-N-C-800), a
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Figure 6. (a) Energy diagram of each part in the redox-medium-assisted system. (b) Photovoltaic and electrocatalytic current−voltage curves of the
GaAs solar cell (blue curve) and two-electrode O2 evolution (red curve). The red dot indicates the maximum power conversion efficiency (PCE)
point of the designed artificial light reaction, based on photoelectrochemical O2 evolution and Zn deposition. (c) FECO (red bars) and
concentration of CO production (blue bars) at different current densities of 2, 5, and 8 mA cm−2. (d) The voltage-versus-time profile, FECO, and
solar-to-CO PCE of the redox-medium-assisted system at a current density of 10 mA cm−2. Reproduced with permission from ref 49. Copyright
2018, Wang et al., under a Creative Commons CC BY license.
A high Faradaic efficiency toward producing C2H4 (FEC2H4) of
63% was reported, substantially exceeding pure Cu or CuOx
without oxygen vacancies.
Designing and engineering electrocatalysts with specific
crystal planes or defects have been the focus of many
studies.42,64 For instance, Cu catalysts with preferentially
exposed Cu(100) facets were generated for CO2 electroreduction, which achieved ∼90% Faradaic efficiency for C2+
products at a partial current density of 520 mA cm−2 (Figure
5a).64 In addition, when a Cu catalyst was electrochemically
deposited under an environment with a high concentration of
CO, defects with a high density were formed, which was
distinctively different from the Cu catalyst synthesized under
otherwise identical conditions but without the CO environment.65 Theoretical calculations suggested that defects were
favored to be formed on the Cu surface with a high density of
surface-adsorbed *CO during catalyst synthesis, which, in turn,
helped to maintain a high surface *CO coverage when this
defect-rich CuDS catalyst was used for CO2RR. As a result, the
CuDS catalyst achieved a high CO2RR selectivity toward
ethanol in both H-type electrochemical cells and flow cells. In
another study, a fast cooling approach was utilized to create a
high density of microstrains and grain boundaries in copper
oxide, which presented a substantial chemisorption capability
toward CO2 and CO (Figure 5b).55 Compared to other CuO
catalysts formed under slower cooling rates, the fast cooled,
strain-rich CuO catalyst enabled an excellent total current
density of >300 mA cm−2, as well as an FE for the combined
C2 products (majority as alcohols) of 78% at −1.0 V vs RHE.
that oxygen vacancies in CeO2 could be spontaneously
enriched upon the introduction of single-atomic Cu sites.63
DFT calculations revealed a stable structure with up to 3
oxygen vacancies that were neighboring to a single Cu site,
which further facilitated the adsorption and activation of CO2
(Figure 4c). Experimentally, different mass loadings of Cu
were synthesized by a facile immersion−precipitation method.
At a low (<5%) doping density, the Cu species remained a
highly dispersive feature of single-atomic sites on CeO2. The
low oxidation states of [Cu0 + Cu1+] further supported the
existence of multiple oxygen vacancies around the Cu sites.
Such a configuration of both single-atomic Cu sites and
multiple oxygen vacancies played a beneficial role in electrochemical CO2RR performance. The Cu-doped CeO2 showed a
high activity of electrochemical CO2 reduction compared to
Cu nanoparticles or undoped CeO2, with an excellent CH4
production FE of 58% at −1.8 V vs RHE (Figure 3d).
4.2.2. Cu-Based Nanomaterials for CO2RR. Distinctive
from single-atomic Cu catalysts, Cu-based nanomaterials can
facilitate the coupling of C−C, ascribed to their adjacent Cu
sites. Oxygen vacancies can play an important role in the Cucontaining oxides as well, which serve as Lewis base sites for
enhanced adsorption of CO2 and subsequent reaction
intermediates. For instance, our group reported the fabrication
of oxygen vacancy-rich CuOx nanodendrites by an in situ
electrochemical conversion method.47 The rich oxygen
vacancies provided a high density of electrons to activate and
reduce CO2, while the high density of Cu sites facilitated the
coupling of two *CO intermediates adsorbed in adjacent sites.
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Alloying copper with other metals has also been proposed as
a potential approach to tuning the electrocatalytic behaviors.59
For most bimetallic catalysts, several effects are potentially
involved to present the structure−performance relationship,
namely, the ensemble effect, ligand effect, and strain effect.66
Cu and Ag are catalyst candidates widely used in CO2RR, but
the selective products can be substantially altered when they
form alloys.67−69 For instance, Cu3Ag1 catalyst was identified
with electron-deficient Cu catalytic sites, which were attributed
to the interphase electron transfer from Cu to Ag.70 Compared
to the pure Cu(100), the electron-deficient Cu sites helped to
stabilize the alcohol pathway intermediates, such as
CH3CH2O*, and thus, the production of alcohol was
enhanced. The FE of alcohols (including both ethanol and
propanol) reached 63% at −0.95 V vs RHE (Figure 5c), and
the selectivity of alcohols among all the CO2RR production
was as high as 92% (Figure 5d).
4.3. Solar-Driven CO2 Electroreduction. To further
promote this technology into large-scale applications, using
renewable energy-produced electricity should be the reasonable path toward reducing the carbon footprint. For example,
Grätzel and co-workers demonstrated that, by combining a
tandem device from a commercial photovoltaic cell and SnO2modified CuO nanowires as the CO2RR electrocatalyst, the
solar-to-CO conversion efficiency was reported to be 13.4%.71
Nonetheless, the sunlight fluctuation places a challenge when
applying this direct coupling of photovoltaics and electrolyzers.
Inspired by the process of natural photosynthesis where an
ATP/ADP redox medium is used to transfer the photoenergy
to the dark step of carbon fixation, our group developed a Zn/
Zn(OH)42− redox medium to decouple the solar-driven water
oxidation (i.e., the light reaction) and the electrochemical
CO2RR (i.e., the dark reaction) (Figure 6a).49 Functioning like
a Zn battery, this Zn/Zn(OH)42− redox medium assisted to
transfer electrical energy from the light reaction toward the
dark reaction, with >99% Faradaic efficiency of reversibility.
The electrolyte was 1 M KOH with Zn(OH)42−, and the
reaction was driven by a triple-junction InGaP/GaAs/Ge solar
cell. In the light reaction, NiFe hydroxide was used as the water
oxidation electrocatalyst in the anode, and a Zn plate was used
as a cathode (Figure 6c). The electrochemical energy was then
stored at the Zn/Zn(OH)42− redox medium, which allowed
spontaneous reduction of CO2 to CO using a nano-Au catalyst.
The yield and selectivity of CO2RR were tuned by applying
different electrolysis current densities (i.e., 2, 5, and 8 mA
cm−2) under a constant current mode (Figure 6b). The
electrical energy conversion efficiency for CO2RR was up to
63%, and the total solar-to-CO efficiency was maintained at
∼15.6% for over 24 h (Figure 6d).
developed and implemented to investigate surface reactive
intermediates and reaction mechanisms.
From the technoeconomic analysis results, a promising
option for the fuel production by electrocatalytic CO2RR
should require rational consideration as an integral view, which
includes but is not limited to improving highly active/selective
electrocatalytic sites, enhancing mass transfer, and optimizing
the energy efficiency of the whole system from the electricity
input toward the product formation. Last but not least, the
stability of the electrocatalytic systems should also be
reasonably evaluated and improved to guide the rational
design of this potential technology toward large-scale
implementation.
5. CONCLUSIONS
This Spotlight has briefly introduced the electrochemical
CO2RR and classified this into two aspects, including the
electrolyte effect and the complexity of heterogeneous catalysts
for reaction pathways. Several examples of recent progress in
our research group are provided to show some strategies in
tuning and understanding both the electrocatalyst and
electrolyte effects. In spite of the progress by our group and
many other researchers, many more efforts are still needed to
understand and push forward this field of electrochemical
CO 2 RR. In addition, more in situ/operando analysis
techniques and multiscale theoretical models should also be
Notes
■
AUTHOR INFORMATION
Corresponding Author
Gengfeng Zheng − Laboratory of Advanced Materials,
Department of Chemistry, and Shanghai Key Laboratory of
Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200438, China; orcid.org/00000002-1803-6955; Email: gfzheng@fudan.edu.cn
Authors
Chao Yang − Laboratory of Advanced Materials, Department
of Chemistry, and Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Fudan University,
Shanghai 200438, China
Yuhang Wang − Laboratory of Advanced Materials,
Department of Chemistry, and Shanghai Key Laboratory of
Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200438, China
Linping Qian − Laboratory of Advanced Materials,
Department of Chemistry, and Shanghai Key Laboratory of
Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200438, China; orcid.org/00000003-3412-5453
Abdullah M. Al-Enizi − Department of Chemistry, College of
Science, King Saud University, Riyadh 11451, Saudi Arabia;
orcid.org/0000-0002-3967-5553
Lijuan Zhang − Laboratory of Advanced Materials,
Department of Chemistry, and Shanghai Key Laboratory of
Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200438, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsaem.0c02648
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank the following funding agencies for supporting this
work: the National Key Research and Development Program
of China (2017YFA0206901, 2018YFA0209401), the National
Science Foundation of China (22025502, 21773036,
21975051), the Science and Technology Commission of
Shanghai Municipality (19XD1420400), and the Innovation
Program of Shanghai Municipal Education Commission
(2019-01-07-00-07-E00045). The authors also extend their
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Reduction via Field-Induced Reagent Concentration. Nature 2016,
537, 382−386.
(19) Resasco, J.; Chen, L. D.; Clark, E.; Tsai, C.; Hahn, C.; Jaramillo,
T. F.; Chan, K.; Bell, A. T. Promoter Effects of Alkali Metal Cations
on the Electrochemical Reduction of Carbon Dioxide. J. Am. Chem.
Soc. 2017, 139, 11277−11287.
(20) Wakerley, D.; Lamaison, S.; Ozanam, F.; Menguy, N.; Mercier,
D.; Marcus, P.; Fontecave, M.; Mougel, V. Bio-Inspired Hydrophobicity Promotes CO2 Reduction on a Cu Surface. Nat. Mater.
2019, 18, 1222−1227.
(21) Wei, X.; Yin, Z.; Lyu, K.; Li, Z.; Gong, J.; Wang, G.; Xiao, L.;
Lu, J.; Zhuang, L. Highly Selective Reduction of CO2 to C2+
Hydrocarbons at Copper/Polyaniline Interfaces. ACS Catal. 2020,
10, 4103−4111.
(22) Nitopi, S.; Bertheussen, E.; Scott, S. B.; Liu, X.; Engstfeld, A. K.;
Horch, S.; Seger, B.; Stephens, I. E. L.; Chan, K.; Hahn, C.; Norskov,
J. K.; Jaramillo, T. F.; Chorkendorff, I. Progress and Perspectives of
Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte.
Chem. Rev. 2019, 119, 7610−7672.
(23) Seifitokaldani, A.; Gabardo, C. M.; Burdyny, T.; Dinh, C. T.;
Edwards, J. P.; Kibria, M. G.; Bushuyev, O. S.; Kelley, S. O.; Sinton,
D.; Sargent, E. H. Hydronium-Induced Switching between CO2
Electroreduction Pathways. J. Am. Chem. Soc. 2018, 140, 3833−3837.
(24) Ooka, H.; Figueiredo, M. C.; Koper, M. T. M. Competition
between Hydrogen Evolution and Carbon Dioxide Reduction on
Copper Electrodes in Mildly Acidic Media. Langmuir 2017, 33,
9307−9313.
(25) Lum, Y.; Cheng, T.; Goddard, W. A., 3rd; Ager, J. W.
Electrochemical CO Reduction Builds Solvent Water into Oxygenate
Products. J. Am. Chem. Soc. 2018, 140, 9337−9340.
(26) Cheng, T.; Xiao, H.; Goddard, W. A., 3rd Reaction
Mechanisms for the Electrochemical Reduction of CO2 to CO and
Formate on the Cu(100) Surface at 298 K from Quantum Mechanics
Free Energy Calculations with Explicit Water. J. Am. Chem. Soc. 2016,
138, 13802−13805.
(27) White, J. L.; Baruch, M. F.; Pander, J. E.; Hu, Y.; Fortmeyer, I.
C.; Park, J. E.; Zhang, T.; Liao, K.; Gu, J.; Yan, Y.; Shaw, T. W.;
Abelev, E.; Bocarsly, A. B. Light-Driven Heterogeneous Reduction of
Carbon Dioxide: Photocatalysts and Photoelectrodes. Chem. Rev.
2015, 115, 12888−12935.
(28) Zhuang, T.-T.; Liang, Z.-Q.; Seifitokaldani, A.; Li, Y.; De Luna,
P.; Burdyny, T.; Che, F.; Meng, F.; Min, Y.; Quintero-Bermudez, R.;
Dinh, C. T.; Pang, Y.; Zhong, M.; Zhang, B.; Li, J.; Chen, P.-N.;
Zheng, X.-L.; Liang, H.; Ge, W.-N.; Ye, B.-J.; Sinton, D.; Yu, S.-H.;
Sargent, E. H. Steering Post C−C Coupling Selectivity Enables High
Efficiency Electroreduction of Carbon Dioxide to Multi-Carbon
Alcohols. Nat. Catal. 2018, 1, 421−428.
(29) Luc, W.; Fu, X.; Shi, J.; Lv, J.-J.; Jouny, M.; Ko, B. H.; Xu, Y.;
Tu, Q.; Hu, X.; Wu, J.; Yue, Q.; Liu, Y.; Jiao, F.; Kang, Y. TwoDimensional Copper Nanosheets for Electrochemical Reduction of
Carbon Monoxide to Acetate. Nat. Catal. 2019, 2, 423−430.
(30) Tian, Z.; Priest, C.; Chen, L. Recent Progress in the Theoretical
Investigation of Electrocatalytic Reduction of CO2. Adv. Theory Simul.
2018, 1, 1800004.
(31) Xiao, H.; Cheng, T.; Goddard, W. A., III Atomistic
Mechanisms Underlying Selectivities in C(1) and C(2) Products
from Electrochemical Reduction of CO on Cu(111). J. Am. Chem.
Soc. 2017, 139, 130−136.
(32) Handoko, A. D.; Wei, F.; Jenndy; Yeo, B. S.; Seh, Z. W.
Understanding Heterogeneous Electrocatalytic Carbon Dioxide
Reduction through Operando Techniques. Nat. Catal. 2018, 1,
922−934.
(33) Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. Electroreduction
of Carbon Monoxide to Methane and Ethylene at a Copper Electrode
in Aqueous Solutions at Ambient Temperature and Pressure. J. Am.
Chem. Soc. 1987, 109, 5022−5023.
(34) De Gregorio, G. L.; Burdyny, T.; Loiudice, A.; Iyengar, P.;
Smith, W. A.; Buonsanti, R. Facet-Dependent Selectivity of Cu
appreciation to the Researchers Supporting Program (RSP2020/55) at King Saud University, Riyadh, Saudi Arabia, for
partial funding of this work.
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Spotlight on Applications
REFERENCES
(1) Sanz-Perez, E. S.; Murdock, C. R.; Didas, S. A.; Jones, C. W.
Direct Capture of CO2 from Ambient Air. Chem. Rev. 2016, 116,
11840−11876.
(2) Hepburn, C.; Adlen, E.; Beddington, J.; Carter, E. A.; Fuss, S.;
Mac Dowell, N.; Minx, J. C.; Smith, P.; Williams, C. K. The
Technological and Economic Prospects for CO2 Utilization and
Removal. Nature 2019, 575, 87−97.
(3) Verma, S.; Lu, S.; Kenis, P. J. A. Co-Electrolysis of CO2 and
Glycerol as a Pathway to Carbon Chemicals with Improved
Technoeconomics Due to Low Electricity Consumption. Nature
Energy 2019, 4, 466−474.
(4) Sriramagiri, G. M.; Ahmed, N.; Luc, W.; Dobson, K. D.;
Hegedus, S. S.; Jiao, F. Toward a Practical Solar-Driven CO2 Flow
Cell Electrolyzer: Design and Optimization. ACS Sustainable Chem.
Eng. 2017, 5, 10959−10966.
(5) Jouny, M.; Luc, W.; Jiao, F. General Techno-Economic Analysis
of CO2 Electrolysis Systems. Ind. Eng. Chem. Res. 2018, 57, 2165−
2177.
(6) Chen, A.; Lin, B.-L. A Simple Framework for Quantifying
Electrochemical CO2 Fixation. Joule 2018, 2, 594−606.
(7) Gabardo, C. M.; O’Brien, C. P.; Edwards, J. P.; McCallum, C.;
Xu, Y.; Dinh, C.-T.; Li, J.; Sargent, E. H.; Sinton, D. Continuous
Carbon Dioxide Electroreduction to Concentrated Multi-Carbon
Products Using a Membrane Electrode Assembly. Joule 2019, 3,
2777−2791.
(8) Li, J.; Chen, G.; Zhu, Y.; Liang, Z.; Pei, A.; Wu, C.-L.; Wang, H.;
Lee, H. R.; Liu, K.; Chu, S.; Cui, Y. Efficient Electrocatalytic CO2
Reduction on a Three-Phase Interface. Nat. Catal. 2018, 1, 592−600.
(9) Song, Y.; Zhang, X.; Xie, K.; Wang, G.; Bao, X. HighTemperature CO2 Electrolysis in Solid Oxide Electrolysis Cells:
Developments, Challenges, and Prospects. Adv. Mater. 2019, 31,
1902033.
(10) Burdyny, T.; Smith, W. A. CO2 Reduction on Gas-Diffusion
Electrodes and Why Catalytic Performance Must Be Assessed at
Commercially-Relevant Conditions. Energy Environ. Sci. 2019, 12,
1442−1453.
(11) Jouny, M.; Hutchings, G. S.; Jiao, F. Carbon Monoxide
Electroreduction as an Emerging Platform for Carbon Utilization.
Nat. Catal. 2019, 2, 1062−1070.
(12) Schreier, M.; Yoon, Y.; Jackson, M. N.; Surendranath, Y.
Competition between H and CO for Active Sites Governs CopperMediated Electrosynthesis of Hydrocarbon Fuels. Angew. Chem., Int.
Ed. 2018, 57, 10221−10225.
(13) Schouten, K. J. P.; Pérez Gallent, E.; Koper, M. T. M. The
Influence of pH on the Reduction of CO and CO2 to Hydrocarbons
on Copper Electrodes. J. Electroanal. Chem. 2014, 716, 53−57.
(14) Billy, J. T.; Co, A. C. Experimental Parameters Influencing
Hydrocarbon Selectivity During the Electrochemical Conversion of
CO2. ACS Catal. 2017, 7, 8467−8479.
(15) Zhu, S.; Jiang, B.; Cai, W. B.; Shao, M. Direct Observation on
Reaction Intermediates and the Role of Bicarbonate Anions in CO2
Electrochemical Reduction Reaction on Cu Surfaces. J. Am. Chem.
Soc. 2017, 139, 15664−15667.
(16) Wuttig, A.; Yoon, Y.; Ryu, J.; Surendranath, Y. Bicarbonate Is
Not a General Acid in Au-Catalyzed CO2 Electroreduction. J. Am.
Chem. Soc. 2017, 139, 17109−17113.
(17) Chen, L. D.; Urushihara, M.; Chan, K.; Nørskov, J. K. Electric
Field Effects in Electrochemical CO2 Reduction. ACS Catal. 2016, 6,
7133−7139.
(18) Liu, M.; Pang, Y.; Zhang, B.; De Luna, P.; Voznyy, O.; Xu, J.;
Zheng, X.; Dinh, C. T.; Fan, F.; Cao, C.; de Arquer, F. P.; Safaei, T.
S.; Mepham, A.; Klinkova, A.; Kumacheva, E.; Filleter, T.; Sinton, D.;
Kelley, S. O.; Sargent, E. H. Enhanced Electrocatalytic CO 2
1042
https://dx.doi.org/10.1021/acsaem.0c02648
ACS Appl. Energy Mater. 2021, 4, 1034−1044
ACS Applied Energy Materials
www.acsaem.org
Catalysts in Electrochemical CO2 Reduction at Commercially Viable
Current Densities. ACS Catal. 2020, 10, 4854−4862.
(35) Lum, Y.; Kwon, Y.; Lobaccaro, P.; Chen, L.; Clark, E. L.; Bell,
A. T.; Ager, J. W. Trace Levels of Copper in Carbon Materials Show
Significant Electrochemical CO2 Reduction Activity. ACS Catal. 2016,
6, 202−209.
(36) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.;
Norskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in
Electrocatalysis: Insights into Materials Design. Science 2017, 355,
eaad4998.
(37) Wang, Y.; Cui, X.; Zhang, Y.; Zhang, L.; Gong, X.; Zheng, G.
Achieving High Aqueous Energy Storage Via Hydrogen-Generation
Passivation. Adv. Mater. 2016, 28, 7626−32.
(38) Li, T.; Yang, C.; Luo, J.-L.; Zheng, G. Electrolyte Driven Highly
Selective CO2 Electroreduction at Low Overpotentials. ACS Catal.
2019, 9, 10440−10447.
(39) Cui, X.; Pan, Z.; Zhang, L.; Peng, H.; Zheng, G. Selective
Etching of Nitrogen-Doped Carbon by Steam for Enhanced
Electrochemical CO2 Reduction. Adv. Energy Mater. 2017, 7,
1701456.
(40) Kuang, M.; Guan, A.; Gu, Z.; Han, P.; Qian, L.; Zheng, G.
Enhanced N-Doping in Mesoporous Carbon for Efficient Electrocatalytic CO2 Conversion. Nano Res. 2019, 12, 2324−2329.
(41) Han, P.; Wang, Z.; Kuang, M.; Wang, Y.; Liu, J.; Hu, L.; Qian,
L.; Zheng, G. 2D Assembly of Confined Space toward Enhanced CO2
Electroreduction. Adv. Energy Mater. 2018, 8, 1801230.
(42) Wang, Y.; Han, P.; Lv, X.; Zhang, L.; Zheng, G. Defect and
Interface Engineering for Aqueous Electrocatalytic CO2 Reduction.
Joule 2018, 2, 2551−2582.
(43) Gu, Z.; Shen, H.; Shang, L.; Lv, X.; Qian, L.; Zheng, G.
Nanostructured Copper-based Electrocatalysts for CO2 Reduction.
Small Methods 2018, 2, 1800121.
(44) Guan, A.; Yang, C.; Quan, Y.; Shen, H.; Cao, N.; Li, T.; Ji, Y.;
Zheng, G. One-Dimensional Nanomaterial Electrocatalysts for CO2
Fixation. Chem. - Asian J. 2019, 14, 3969−3980.
(45) Guan, A.; Chen, Z.; Quan, Y.; Peng, C.; Wang, Z.; Sham, T.-K.;
Yang, C.; Ji, Y.; Qian, L.; Xu, X.; Zheng, G. Boosting CO2
Electroreduction to CH4 Via Tuning Neighboring Single-Copper
Sites. ACS Energy Lett. 2020, 5, 1044−1053.
(46) Yang, C.; Shen, H.; Guan, A.; Liu, J.; Li, T.; Ji, Y.; Al-Enizi, A.
M.; Zhang, L.; Qian, L.; Zheng, G. Fast Cooling Induced GrainBoundary-Rich Copper Oxide for Electrocatalytic Carbon Dioxide
Reduction to Ethanol. J. Colloid Interface Sci. 2020, 570, 375−381.
(47) Gu, Z.; Yang, N.; Han, P.; Kuang, M.; Mei, B.; Jiang, Z.; Zhong,
J.; Li, L.; Zheng, G. Oxygen Vacancy Tuning toward Efficient
Electrocatalytic CO2 Reduction to C2H4. Small Methods 2018, 3,
1800449.
(48) Shen, H.; Gu, Z.; Zheng, G. Pushing the Activity of CO2
Electroreduction by System Engineering. Sci. Bull. 2019, 64, 1805−
1816.
(49) Wang, Y.; Liu, J.; Wang, Y.; Wang, Y.; Zheng, G. Efficient SolarDriven Electrocatalytic CO2 Reduction in a Redox-Medium-Assisted
System. Nat. Commun. 2018, 9, 5003.
(50) Zhu, W.; Michalsky, R.; Metin, O.; Lv, H.; Guo, S.; Wright, C.
J.; Sun, X.; Peterson, A. A.; Sun, S. Monodisperse Au Nanoparticles
for Selective Electrocatalytic Reduction of CO2 to CO. J. Am. Chem.
Soc. 2013, 135, 16833−6.
(51) Dinh, C. T.; Burdyny, T.; Kibria, M. G.; Seifitokaldani, A.;
Gabardo, C. M.; Garcia de Arquer, F. P.; Kiani, A.; Edwards, J. P.; De
Luna, P.; Bushuyev, O. S.; Zou, C.; Quintero-Bermudez, R.; Pang, Y.;
Sinton, D.; Sargent, E. H. CO2 Electroreduction to Ethylene via
Hydroxide-Mediated Copper Catalysis at An Abrupt Interface. Science
2018, 360, 783−787.
(52) Clark, E. L.; Hahn, C.; Jaramillo, T. F.; Bell, A. T.
Electrochemical CO2 Reduction over Compressively Strained Cuag
Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity. J.
Am. Chem. Soc. 2017, 139, 15848−15857.
Spotlight on Applications
(53) Hall, A. S.; Yoon, Y.; Wuttig, A.; Surendranath, Y.
Mesostructure-Induced Selectivity in CO2 Reduction Catalysis. J.
Am. Chem. Soc. 2015, 137, 14834−14837.
(54) Zhang, L.; Qin, X.; Zhao, S.; Wang, A.; Luo, J.; Wang, Z. L.;
Kang, F.; Lin, Z.; Li, B. Advanced Matrixes for Binder-Free
Nanostructured Electrodes in Lithium-Ion Batteries. Adv. Mater.
2020, 32, 1908445.
(55) Duan, X.; Xu, J.; Wei, Z.; Ma, J.; Guo, S.; Wang, S.; Liu, H.;
Dou, S. Metal-Free Carbon Materials for CO2 Electrochemical
Reduction. Adv. Mater. 2017, 29, 1701784.
(56) Lee, W. J.; Lim, J.; Kim, S. O. Nitrogen Dopants in Carbon
Nanomaterials: Defects or a New Opportunity? Small Methods 2017,
1, 1600014.
(57) Thome, I.; Nijs, A.; Bolm, C. Trace Metal Impurities in
Catalysis. Chem. Soc. Rev. 2012, 41, 979−87.
(58) Wuttig, A.; Surendranath, Y. Impurity Ion Complexation
Enhances Carbon Dioxide Reduction Catalysis. ACS Catal. 2015, 5,
4479−4484.
(59) Vasileff, A.; Xu, C.; Jiao, Y.; Zheng, Y.; Qiao, S.-Z. Surface and
Interface Engineering in Copper-Based Bimetallic Materials for
Selective CO2 Electroreduction. Chem. 2018, 4, 1809−1831.
(60) Zhou, Y.; Che, F.; Liu, M.; Zou, C.; Liang, Z.; De Luna, P.;
Yuan, H.; Li, J.; Wang, Z.; Xie, H.; Li, H.; Chen, P.; Bladt, E.;
Quintero-Bermudez, R.; Sham, T. K.; Bals, S.; Hofkens, J.; Sinton, D.;
Chen, G.; Sargent, E. H. Dopant-Induced Electron Localization
Drives CO2 Reduction to C2 Hydrocarbons. Nat. Chem. 2018, 10,
974−980.
(61) Huang, Y.; Handoko, A. D.; Hirunsit, P.; Yeo, B. S.
Electrochemical Reduction of CO2 Using Copper Single-Crystal
Surfaces: Effects of CO* Coverage on the Selective Formation of
Ethylene. ACS Catal. 2017, 7, 1749−1756.
(62) Gao, D.; Zhang, Y.; Zhou, Z.; Cai, F.; Zhao, X.; Huang, W.; Li,
Y.; Zhu, J.; Liu, P.; Yang, F.; Wang, G.; Bao, X. Enhancing CO2
Electroreduction with the Metal-Oxide Interface. J. Am. Chem. Soc.
2017, 139, 5652−5655.
(63) Wang, Y.; Chen, Z.; Han, P.; Du, Y.; Gu, Z.; Xu, X.; Zheng, G.
Single-Atomic Cu with Multiple Oxygen Vacancies on Ceria for
Electrocatalytic CO2 Reduction to CH4. ACS Catal. 2018, 8, 7113−
7119.
(64) Wang, Y.; Wang, Z.; Dinh, C.-T.; Li, J.; Ozden, A.; Golam
Kibria, M.; Seifitokaldani, A.; Tan, C.-S.; Gabardo, C. M.; Luo, M.;
Zhou, H.; Li, F.; Lum, Y.; McCallum, C.; Xu, Y.; Liu, M.; Proppe, A.;
Johnston, A.; Todorovic, P.; Zhuang, T.-T.; Sinton, D.; Kelley, S. O.;
Sargent, E. H. Catalyst Synthesis under CO2 Electroreduction Favours
Faceting and Promotes Renewable Fuels Electrosynthesis. Nat. Catal.
2020, 3, 98−106.
(65) Gu, Z.; Shen, H.; Chen, Z.; Yang, Y.; Yang, C.; Ji, Y.; Wang, Y.;
Zhu, C.; Liu, J.; Li, J.; Sham, T.-K.; Xu, X.; Zheng, G. Efficient
Electrocatalytic CO2 Reduction to C2+ Alcohols at Defect-Site-Rich
Cu Surface. Joule 2021, DOI: 10.1016/j.joule.2020.12.011.
(66) Luo, M.; Guo, S. Strain-Controlled Electrocatalysis on
Multimetallic Nanomaterials. Nat. Rev. Mater. 2017, 2, 17059.
(67) Wang, Y.; Wang, D.; Dares, C. J.; Marquard, S. L.; Sheridan, M.
V.; Meyer, T. J. CO2 Reduction to Acetate in Mixtures of Ultrasmall
(Cu)n,(Ag)m Bimetallic Nanoparticles. Proc. Natl. Acad. Sci. U. S. A.
2018, 115, 278−283.
(68) Chen, C.; Li, Y.; Yu, S.; Louisia, S.; Jin, J.; Li, M.; Ross, M. B.;
Yang, P. Cu-Ag Tandem Catalysts for High-Rate CO2 Electrolysis
toward Multicarbons. Joule 2020, 4, 1688−1699.
(69) Lee, S.; Park, G.; Lee, J. Importance of Ag−Cu Biphasic
Boundaries for Selective Electrochemical Reduction of CO2 to
Ethanol. ACS Catal. 2017, 7, 8594−8604.
(70) Lv, X.; Shang, L.; Zhou, S.; Li, S.; Wang, Y.; Wang, Z.; Sham, T.
K.; Peng, C.; Zheng, G. Electron-Deficient Cu Sites on Cu3Ag1
Catalyst Promoting CO2 Electroreduction to Alcohols. Adv. Energy
Mater. 2020, 10, 2001987.
(71) Schreier, M.; Héroguel, F.; Steier, L.; Ahmad, S.; Luterbacher, J.
S.; Mayer, M. T.; Luo, J.; Grätzel, M. Solar Conversion of CO2 to CO
1043
https://dx.doi.org/10.1021/acsaem.0c02648
ACS Appl. Energy Mater. 2021, 4, 1034−1044
ACS Applied Energy Materials
www.acsaem.org
Spotlight on Applications
Using Earth-Abundant Electrocatalysts Prepared by Atomic Layer
Modification of CuO. Nat. Energy 2017, 2, 17087.
1044
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ACS Appl. Energy Mater. 2021, 4, 1034−1044