![(A) Market size and price of CO 2 RR products; (B) Faradaic efficiency and corresponding overpotential; (C) Maximum faradaic efficiency and current density. This figure is quoted with permission from Kibria et al. Copyright (2019) John Wiley and Sons [22] . CO 2 RR: CO 2 reduction reaction.](https://www.researchgate.net/publication/389354151/figure/fig1/AS:11431281312147325@1740589962235/A-Market-size-and-price-of-CO-2-RR-products-B-Faradaic-efficiency-and-corresponding_Q320.jpg)
![Overview of the reaction routes of CO 2 RR for both C 1 and C 2+ products. This figure is quoted with permission from Trogadas et al. Copyright (2023) John Wiley and Sons [31] . CO 2 RR: CO 2 reduction reaction.](https://www.researchgate.net/publication/389354151/figure/fig2/AS:11431281312154576@1740589962398/Overview-of-the-reaction-routes-of-CO-2-RR-for-both-C-1-and-C-2-products-This-figure-is_Q320.jpg)
![(A) Graphic representation of the Cu/CeO 2-x -4 catalyst synthesis; (B) HRTEM images of the Cu/CeO 2-x -4 catalyst; (C) FEs of bare CeO 2 ; (D) Cu/CeO 2-x -1; and (E) Cu/CeO 2-x -4. This figure is quoted with permission from Xue et al. Copyright (2024) Elsevier [58] ; (F) Schematic representation of the synthetic process for Cu SAs/UIO-H 2 ; (G) The corresponding FEs for the reduction products of Cu SAs/UIO and (H) Cu SAs/UIO-H 2 ; (I) Long-term electrolysis of Cu SAs/UIO-H 2 at -0.66 V vs. RHE. This figure is quoted with permission from Bie et al. Copyright (2024) Elsevier [62] . FE: Faradaic efficiency; HRTEM: high-resolution transmission electron microscopy; SAs: single atoms; RHE: reversible hydrogen electrode.](https://www.researchgate.net/publication/389354151/figure/fig3/AS:11431281312154577@1740589962583/A-Graphic-representation-of-the-Cu-CeO-2-x-4-catalyst-synthesis-B-HRTEM-images-of_Q320.jpg)
![(A) FEs on GB-Cu; (B) FE C2+ of GB-Cu; (C) In situ ATR-SEIRAS spectra of GB-Cu. This figure is quoted with permission from Chen et al. Copyright (2020) American Chemical Society [65] ; (D) Electrochemical deposition procedure for different Cu catalysts; (E) FFT-refined aberration-corrected HAADF-STEM image of the GB-Cu 29.6 ; (F) FEs for C 2+ and (G) ECSA-normalized j C2+ of Cu with different GB densities; (H) Spatially resolved electrochemical map derived from the individual linear sweep voltammetry (LSV) curves. This figure is quoted with permission from Zhang et al. Copyright (2024) American Chemical Society [66] . FE: Faradaic efficiency; HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy; GBs: grain boundaries; ATR-SEIRAS: attenuated total reflectance-surface-enhanced infrared absorption spectroscopy; FFT: fast Fourier transform; ECSA: electrochemically active surface area.](https://www.researchgate.net/publication/389354151/figure/fig4/AS:11431281312147326@1740589962758/A-FEs-on-GB-Cu-B-FE-C2-of-GB-Cu-C-In-situ-ATR-SEIRAS-spectra-of-GB-Cu-This_Q320.jpg)
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DOI: 10.20517/energymater.2024.237 Energy Materials
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Open AccessReview
Development of catalysts and reactor designs for
CO2 electroreduction towards C2+ products
Joonhee Ma, Soo Young Kim*
Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea.
*Correspondence to: Prof. Soo Young Kim, Department of Materials Science, Korea University, 145 Anam-ro, Seongbuk-gu,
Seoul 02841, Republic of Korea. E-mail: sooyoungkim@korea.ac.kr
How to cite this article: Ma, J.; Kim, S. Y. Development of catalysts and reactor designs for CO2 electroreduction towards C2+
products. Energy Mater. 2025, 5, 500052. https://dx.doi.org/10.20517/energymater.2024.237
Received: 30 Oct 2024 First Decision: 27 Nov 2024 Revised: 10 Dec 2024 Accepted: 26 Dec 2024 Published: 25 Feb 2025
Academic Editor: Hao Liu Copy Editor: Ping Zhang Production Editor: Ping Zhang
Abstract
Recent research on the electrocatalytic CO2 reduction reaction (eCO2RR) has garnered significant attention given
its capability to address environmental issues associated with CO2 emissions while harnessing clean energy to
produce high-value-added products. Compared to C1 products, C2+ products provide greater energy densities and
are highly sought after as chemical feedstocks. However, the formation of the C-C bond is challenging due to
competition with the formation of H-H and C-H bonds. Therefore, to elevate the selectivity and yield of C2+ fuels, it
is essential to develop more advanced electrocatalysts and optimize the design of electrochemical cell
configurations. Of the materials investigated for CO2RR, Cu-based materials stand out due to their wide availability,
affordability, and environmental compatibility. Moreover, Cu-based catalysts exhibit promising capabilities in CO2
adsorption and activation, facilitating the formation of C2+ compounds via C-C coupling. This review examines
recent research on both electrocatalysts and electrochemical cells for CO2 electroreduction to C2+ compounds,
introducing the core principles of eCO2RR and the reaction pathways involved in generating C2+ products. A key
focus is the categorization of Cu-based catalyst designs, including defect engineering, surface modification,
nanostructure engineering, and tandem catalysis. By analyzing recent studies on eCO2RR with Cu-based catalysts,
we aim to elucidate the mechanisms behind enhanced selectivity for C2+ compounds. Additionally, various types of
electrolytic cells are discussed. Lastly, the prospects and limitations of utilizing Cu-based materials and
electrocatalytic cells for CO2 reduction are highlighted for future research.
Keywords: CO2 reduction, electrocatalysis, C2+ products, Cu, electrocatalytic cells
Page 2 of Ma et al. Energy Mater. 2025, 5, 500052 https://dx.doi.org/10.20517/energymater.2024.237
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INTRODUCTION
The significant release of CO2 from the use of fossil fuels has brought about its accumulation in the
atmosphere, disrupting the global carbon cycle and causing serious environmental and climate issues,
including the greenhouse gas effect and rising sea levels[1-7]. Consequently, there is a growing interest in
converting CO2 into valuable carbon-based compounds to reduce atmospheric CO2 concentrations and
tackle energy challenges[8,9]. Various approaches have been investigated by researchers to transform CO2 into
valuable carbon-based compounds, including chemical conversion[10], biotransformation[11],
photocatalysis[12,13], and electrocatalysis[14-16]. Among these approaches, the eCO2RR stands out as a promising
technique, leveraging renewable electricity to mitigate CO2-related environmental problems while
generating high-value-added products[17-19].
In aqueous electrolyte media, electrocatalytic CO2 reduction reaction (eCO2RR) products can be classified
into two main categories: (i) 2e- products, such as carbon monoxide (CO) and formate (HCOO-), and (ii)
multi-electron transfer products, including methanol (CH3OH), methane (CH4), ethanol (C2H5OH), and
ethylene (C2H4)[20,21]. Compared to 2e- products, multi-electron transfer products are preferred due to their
higher economic value, serving as crucial feedstock and specialized chemicals [Figure 1A][22].
However, achieving selective CO2 reduction into C2+ multi-carbon products remains challenging due to
several factors: (1) the competitive and undesirable H2O reduction, (2) the large C-C coupling activation
barrier, and (3) the difference in overpotential for the formation of crucial CO intermediates and C2+
species, resulting in lower current densities and selectivity compared to C1 products [Figure 1B and C][23-27].
Therefore, developing advanced electrocatalysts is essential for enhancing both the selectivity and activity
for C2+ production[28,29].
This review provides a detailed examination of strategies designed to control C-C coupling and
multi-electron transfer product processes, specifically focusing on promising electrocatalysts for eCO2RR
that produce C2+ compounds. We first introduce the fundamentals of electrocatalytic CO2 reduction,
discussing the mechanisms and possible reaction pathways for converting CO2 into C2+ compounds. Various
approaches to optimize the selectivity of Cu-based electrocatalysts for C2+ production are then explored.
Additionally, recent advancements in electrochemical cell design for CO2 reduction are reviewed. The paper
concludes by highlighting the primary challenges that remain and offering future insights into the
development of Cu-based catalysts and electrochemical cells for CO2-to-C2+ conversion.
FUNDAMENTAL INSIGHTS INTO CO2 REDUCTION REACTION MECHANISMS
CO2 is a highly stable molecule, primarily due to its strong C=O bond, which has a bond dissociation energy
of 750 kJ mol-1[30]. Converting CO2 into desired products requires very challenging conditions and
substantial energy input. This transformation involves multi-electron transfer steps, such as 2-, 4-, 6-, 8-, or
even 18-electron processes, resulting in a range of possible products [Figure 2][31].
These products can be categorized into two groups according to the quantity of carbon atoms: C1 products,
including CO, CH4, HCOOH, and CH3OH; C2 products, including acetate, C2H4, and C2H5OH; C3 products,
including propanol (C3H7OH) and acetone (CH3COCH3); and C4+ products[32]. Table 1 summarizes the
thermodynamic half-reaction of CO2 conversion and the associated standard potential to produce specific
products[33]. The reaction mechanism for eCO2RR involves several key steps: CO2 adsorption onto the
catalyst surface, electron and proton transfer, the formation of reactive intermediates, and the desorption of
the final products. The performance and selectivity of this process are heavily dependent on factors such as
the choice of catalyst material, the composition of the electrolyte, and the applied potential[34]. A critical
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35
Figure 1. (A) Market size and price of CO2RR products; (B) Faradaic efficiency and corresponding overpotential; (C) Maximum faradaic
efficiency and current density. This figure is quoted with permission from Kibria et al. Copyright (2019) John Wiley and Sons[22]. CO2RR:
CO2 reduction reaction.
Figure 2. Overview of the reaction routes of CO2RR for both C1 and C2+ products. This figure is quoted with permission from Trogadas et
al. Copyright (2023) John Wiley and Sons[31]. CO2RR: CO2 reduction reaction.
aspect of determining reaction pathways and product distributions lies in understanding the binding
energies of intermediates on the catalyst surface. Additionally, operating conditions, including the pH and
temperature of the electrolyte, play a significant role in influencing reaction dynamics and product
selectivity.
MECHANISM FOR C2+ PRODUCTS
C2+ products typically possess higher energy densities than C1 products, making them essential feedstocks in
numerous chemical industries [33]. Consequently, the CO2 reduction reaction (CO2RR) into valuable C2+
products is highly desirable. A critical step in achieving multi-carbon products during CO2RR is the
coupling of two C1 intermediates to form a C-C bond. The *CO intermediate plays a pivotal role in this
process: it can either undergo C-C coupling with another *CO species to produce C2+ products, such as
ethanol, ethylene, and n-propanol, or it can strongly adsorb on the catalyst surface and proceed through
hydrogenation to form C1 products such as CH4[35]. However, the selective generation of C2+ products faces
substantial challenges. First of all, a C-C coupling process requires significant energy barriers, and the
formation of C2+ products involves multiple proton-electron transfer steps, which are inherently slow and
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35
kinetically hindered[36-39]. In contrast, competing reactions involving H-H and C-H bond formation occur
more readily due to their lower energy barriers and simpler reaction pathways[40-42]. These results in
decreased activity and selectivity for C2+ product formation during CO2 reduction. Generally, the process of
C2+ product formation involves four key steps: (1) *CO formation; (2) C-C bond formation; (3) C-C
coupling; and (4) product desorption[43]. An important aspect of generating C2+ products is maintaining a
substantial amount of *CO on the surface, as a higher concentration of CO promotes C-C coupling, thereby
boosting the production of C2+ compounds. The C-C bond formation in CO2 reduction can occur via two
distinct mechanisms: the Eley-Rideal (E-R) and the Langmuir-Hinshelwood (L-H) mechanisms[24]. In the
E-R mechanism, a CO molecule from the gas phase interacts directly with CO species already adsorbed on
the catalyst surface, facilitating C-C coupling[44]. This pathway typically arises when one reactant remains in
the gas phase while the other is adsorbed on the surface. Conversely, the L-H mechanism involves the
adsorption of both CO molecules onto the catalyst surface, where they subsequently interact to form C-C
bonds[45]. This process becomes predominant when the catalyst surface is sufficiently populated with
adsorbed CO, allowing interactions between these species. A comprehensive understanding of the dynamic
interplay between these two mechanisms is key to advancing the selective formation of C2+ products in CO2
reduction processes. Additionally, the selectivity and reaction pathways for C2+ formation are strongly
influenced by the properties of Cu-based electrocatalysts and the electrolyte environment[46,47]. Thus, it is
imperative to develop an ideal catalyst that suppresses the hydrogen evolution reaction (HER) while
featuring minimal energy barriers for CO2 activation and C-C bond formation[48-50].
CATALYST DESIGN FOR PROMOTING THE REACTION PATHWAY FOR C2+ PRODUCTS
To date, Copper (Cu) stands out as one of the rare metal catalysts capable of efficiently converting CO2 into
valuable multi-carbon products[51]. This is attributed to its distinctive 3d electronic structure, which provides
optimal *CO binding energy and suppresses HER activity, thereby promoting C-C coupling[52].
Furthermore, Cu-based catalysts stand out as the most promising option for CO2 reduction to C2+ products,
offering an optimal balance of environmental sustainability, economic feasibility, and superior selectivity.
While alternative catalysts may offer lower costs, none rival the unique ability of Cu to facilitate the
production of high-value multi-carbon products. However, challenges remain, including limited selectivity
for certain products and insufficient electrochemical durability. To address these issues, substantial efforts
have been dedicated to improving Cu-based catalysts[53-55]. In the following part, we explore various design
strategies to enhance the efficiency of Cu-based catalysts for the selective conversion of CO2 into C2+
[Table 2].
Defect engineering
Defects are crucial surface features that significantly influence a catalyst’s performance by modifying its
electronic properties and disturbing the periodic structure of the crystal, resulting in unique chemical and
electronic characteristics[56,57]. Here, we highlight recently recent electrocatalysts that utilize defect
engineering to enhance performance toward C2+ products and examine the influence of these defects in
CO2RR.
Vacancy
Enhancing the performance of Cu-based electrocatalysts has largely centered on investigating the role of
vacancies, the most prevalent type of defect. These studies aim to explore how vacancies can be leveraged to
optimize catalytic activity toward C2+ products. Xue et al. designed Cu/CeO2-x catalysts featuring different
levels of oxygen vacancies, investigating the interaction between Ce3+ and Cu species and its impact on
catalytic performance[58]. The introduction of Cu into CeO2-x generated a significant number of oxygen
vacancies. Various Cu/CeO2-x electrocatalysts were synthesized via the hydrothermal-reduction method
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Table 2. Summary of catalysts for the reduction of CO2 to C2+ products
Catalyst Main C2products (FE) Current density Reactor type Ref.
Cu/CeO2-x-4 C2H4: 30%
@-1.2 VRHE
- H-cell [58]
Cu/CeO2C2H4: 78.3%
@ -1.0VRHE
-16.8 mA cm-2
@-1.0VRHE
H-cell [59]
1.0% I-CuO C2H4: 50.2%
@-1.2 VRHE
JC2H4: -9 mA cm-2
@-1.2VRHE
H-cell [60]
Sn-doped CuO(VO) C2H4: 48.5% ± 1.2%
@-1.1 VRHE
- H-cell [61]
Cu SAs/UIO-H2C2H5OH: 46.28%
@-0.66 VRHE
- Flow cell [62]
GB-Cu C2H4: 38%
@-1.2 VRHE
JC2H4: -37 mA cm-2
@-1.2 VRHE
Flow cell [65]
GB-Cu29.6 C2+products: 73.2%
@-3.8 VRHE
-303.61 mA cm-2
@-3.8 VRHE
MEA cell [66]
GB-Cu-IV C2+products: 68.2%
@-1.28 VRHE
JC2+: -0.768 A cm-2
@-1.28 V
Flow cell [67]
Defect engineering
RGBs-Cu C2+products: 77.3%
@400 mA cm-2 JC2+: -353.6 ± 7.5 mA cm-2
@400mA cm-2 Flow cell [68]
3DOP Cu2O–CO C2+products: 77.0% ± 0.3%
@-0.88 VRHE
JC2+: -513.7±0.7 mA cm-2
@-0.88 VRHE
Flow cell [76]
OD-Cu NWs C2+products: 77.7%
@-0.51 VRHE
JC2+: -233.2 mA cm-2
@-0.51 VRHE
Flow cell [77]
C-CuO nanosheets C2H4: 56.2%
@-0.871 VRHE
JC2H4: -171 mA cm-2
@-0.871 VRHE
Flow cell [78]
Cu2O-NS C2+products: 81.32%
@-1.2 VRHE
-187.6 mA cm-2
@-1.2 VRHE
Flow cell [79]
f-Cu2O C2H5OH: 52.6%
@-0.8 VRHE
jC2H5OH: -9.1 mA cm-2
@-0.9 VRHE
H-cell [80]
Cucub C2H4: 57%
@-0.75 VRHE
- Flow cell [81]
NS-D Cu C2+products: 67.5%
@-1.5 VRHE
JC2+: -25.1 mA cm-2
@-1.5 VRHE
H-cell [82]
c-Cu2O C2H4: 61.3%
@-1.2 VRHE
JC2H4: -2.5 mA cm-2
@-1.2 VRHE
H-cell [83]
F-Cu2O@ZIF-8 C2H4: 74.1%
@-1.2 VRHE
- H-cell [84]
Copper NPs (25 nm) C2+products: 92.8%
@-1.7 VRHE
- MEA cell [85]
Cu45.2/GDY C2+products: 91.2%
@-1.0 VRHE
JC2+: -312 mA cm-2
@-1.0 VRHE
Flow cell [86]
HKUST-1-derived Cu clusters C2H4: 45%
@-1.07 VRHE
-262 mA cm-2@-1.07 VRHE Flow cell [87]
Nanostructure engineering
Cu2-CuN3C2H5OH: 51%
@-1.1 VRHE
jC2H5OH: -14.4 mA cm-2
@-1.1 VRHE
H-cell [88]
Cu foil+Gly C2H4: 24%
@-1.9 VRHE
- H-cell [92]
Cu/PANI C2H4: 43.8%
@-1.08 VRHE
-28.3 mA cm-2
@-1.08 VRHE
H-cell [93]
PANI/CuO NSs-25 C2+products: 50%
@-1.6 VRHE
JC2H4: -200.1 mA cm-2
@-1.6 VRHE
H-cell [94]
Hydrophobic Cu dendrite C2H4: 56%
@-30 mA cm-2 - Flow cell [95]
TP-Cu C2H4: 18.35%
@-1.1 VRHE
- H-cell [96]
Cu-D C2+products: 64% ± 1.4%
@-0.68 VRHE
JC2+: -255 mA cm-2
@-0.68 VRHE
Flow cell [97]
Surface modification
S-Cu C2+products: 78%
@-0.9 VRHE
JC2+: -1.81 Acm-2
@-0.9 VRHE
Flow cell [98]
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35
Cu@Ag/C C2H4: 58.03%
@-0.85 VRHE
JC2H4: -49.41 mA cm-2
@-0.85 VRHE
Flow cell [105]
Cu90Ag10 C2H4: 33.6%
@2.2 VRHE
JC2H4: -42.70 mA cm-2 MEA cell [106]
CuAg4/EDTA C2+products: 86.56%
@-1.23 VRHE
JC2+: -10 mA cm-2
@-1.23 VRHE
H-cell [107]
Cu/Ag C2H5OH: 57.5%
@-1.1 VRHE
jC2H5OH: -356.7 ± 9.5 mA cm-2
@-1.1 VRHE
Flow cell [108]
Ag1–Cu1.1NDs C2H4: 38%
@-1.1VRHE
-1 mA cm-2
@-1.1 VRHE
H-cell [109]
Ag65–Cu35 JNS-100 C2H4: 54%
@-1.2 VRHE
JC2H4: -2 mA cm-2
@-1.2 VRHE
H-cell [110]
Cu needle-Ag C2+products: 70%
@300 mA cm-2 JC2+: -245 mA cm-2
@300 mA cm-2 Flow cell [111]
Au/Cu - -15.29 mA cm-2
@-1.08 VRHE
Flow cell [112]
S-2.0-Au@OD-Cu C2H4: 40%
@-300 mA cm-2 JC2H4: -115 mA cm-2
@-0.920 VRHE
Flow cell [113]
Au0.02Cu2O C2H4: 24.4 %
@-1.3 VRHE
-5.7 mA cm-2
@-1.3 VRHE
H-cell [114]
Au@Cu-AuCu 0.9 μmol cm-2 h-1
@-1.0 VRHE
- H-cell [115]
Cu99.3Au0.7 NWs C2+products: 65.3%
@-1.25 VRHE
JC2+: -12.1 mA cm-2
@-1.25 VRHE
H-cell [116]
Au-Cu Janus NSs C2+products: 67%
@-0.75 VRHE
JC2+: -0.29 A cm-2
@-0.75 VRHE
Flow cell [117]
Cu2O NCs-C-Copc C2H4: 70.31 %
@-0.76 VRHE
JC2H4: -226.18 mA cm-2
@-0.76 VRHE
Flow cell [121]
Cu–CoPc C2+products: 82%
@480 mA cm-2 JC2+: -394 mA cm-2
@480 mA cm-2 Flow cell [122]
Cu2O@CM C2+products: 73.2%
@-1.1 VRHE
JC2+: -52.9 mA cm-2
@-1.1 VRHE
H-cell [123]
Cu NPs/Ni–N–C C2H4: 14.27 %
@-0.9 VRHE
JC2H4: -1.67 mA cm-2
@-0.9 VRHE
H-cell [125]
CuO/Ni C2+products: 81.4%
@-1.1 VRHE
JC2+: -1220.8 mA cm-2
@-1.1 VRHE
Flow cell [126]
Ni Pc + Cu-R C2H4: 62 %
@-500 mA cm-2 JC2H4: -370 mA cm-2
@-500 mA cm-2 Flow cell [127]
PTF(Ni)/Cu C2H4: 57.3%
@-1.1 VRHE
JC2+: -3.1 mA cm-2
@-1.1 VRHE
H-cell [128]
Tandem strategy
a-Ni/Cu-NP@CMK C2H4: 72.3%
@-1.1 VRHE
JC2H4: -294 mA cm-2
@-1.1 VRHE
Flow cell [129]
Ni3Al film 1-propanol: 1.9%
@-1.38 VRHE
- H-cell [130]
Ni5Ga3C2H6: 1.3%
@-0.88 VRHE
- H-cell [131]
Non-cu based
SnS2/Sn1-O3G C2H5OH: 82.5%
@-0.9 VRHE
jC2H5OH: -14 mA cm-2
@-0.9 VRHE
H-cell [132]
RHE: Reversible hydrogen electrode; FE: faradaic efficiency; MEA: membrane electrode assembly; GBs: grain boundaries; OD-Cu: oxide-derived
Cu; NSs: nanosheets; PANI: polyaniline; EDTA: ethylenediaminetetraacetic acid; NDs: nanodimers; PTF: porphyritic triazine framework; CMK:
carbons mesostructured from Korea; RGBs: grain boundaries.
[Figure 3A]. High-resolution transmission electron microscopy (HRTEM) analysis demonstrated the
presence of structural defects in the Cu/CeO2-x-4 catalyst, as depicted in Figure 3B. The pristine CeO2
catalyst in Figure 3C only produces H2 and CO, regardless of the applied potential. However, the
Cu/CeO2-x-4 catalyst, which has the highest oxygen vacancy concentration, showed improved selectivity for
C2H4 and CH4 [Figure 3D and E]. Fang et al. investigated the integration of Cu into CeO2 (Cu/CeO2) using
an impregnation-calcination method for CO2RR[59]. The amount of Cu loading was controlled by adjusting
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Figure 3. (A) Graphic representation of the Cu/CeO2-x-4 catalyst synthesis; (B) HRTEM images of the Cu/CeO2-x-4 catalyst; (C) FEs of
bare CeO2; (D) Cu/CeO2-x-1; and (E) Cu/CeO2-x-4. This figure is quoted with permission from Xue et al. Copyright (2024) Elsevier[58];
(F) Schematic representation of the synthetic process for Cu SAs/UIO-H2; (G) The corresponding FEs for the reduction products of Cu
SAs/UIO and (H) Cu SAs/UIO-H2; (I) Long-term electrolysis of Cu SAs/UIO-H2 at -0.66 V vs. RHE. This figure is quoted with
permission from Bie et al. Copyright (2024) Elsevier[62]. FE: Faradaic efficiency; HRTEM: high-resolution transmission electron
microscopy; SAs: single atoms; RHE: reversible hydrogen electrode.
the concentration of the copper precursor during the impregnation process. The electrocatalytic
performance for CO2RR was analyzed in an H-cell with 0.1 M CsI as the electrolyte solution. Due to the
strong electronic interaction of the Ce4+-O2--Cu+ structure of Cu/CeO2 and the appearance of oxygen
vacancies, the optimized catalyst featuring a Cu doping of 9.77 wt% achieved a C2+ Faradaic efficiency (FE)
of 78.3% and a current density of 16.8 mA cm-2 at -1.0V vs. reversible hydrogen electrode (RHE). Recently,
Shen et al. enhanced the performance of Cu catalysts for C2H4 production by introducing oxygen vacancies
through iodine doping in CuO (I-CuO)[60]. Advanced characterization demonstrated that iodine doping
facilitated the formation of Cu+ species in CuO and increased the density of oxygen vacancies. Additionally,
the introduction of iodine significantly enhanced the hydrophobicity of the catalyst, as evidenced by a
contact angle of 131.1°, compared to the 0° of bare CuO. Electrocatalytic CO2 reduction was assessed in an
H-type cell, and liquid products were analyzed using hydrogen nuclear magnetic resonance spectroscopy
(1H NMR). Among the catalysts, 1.0% I-CuO presented the highest FE for C2H4 and the lowest for H2 across
the broad potential range of -1.0 V to -1.4 V vs. RHE. Notably, the FEC2H4 production reached up to 50.2%,
which is 3.43 times greater than that of bare CuO. Additionally, the catalyst demonstrated stable
performance over 15 h without any decline. Jiang et al. promoted ethylene production by optimizing the Sn
doping and oxygen vacancies in CuO. The synergistic interaction between the Sn dopant and the oxygen
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vacancies significantly facilitated the CO2 electroreduction to C2H4[61]. Specifically, Sn-doped CuO (oxygen
vacancies) achieved the highest FEC2H4 of 48.5% ± 1.2%, accompanied by a jC2H4 of 10.9 mA cm-2, and
demonstrated long-term durability over a period of 24 hours. Bie et al. structured atomically dispersed Cu
atoms anchored on hydrogen-reduced UiO66-NH2, characterized by abundant oxygen vacancies[62]. This
catalyst, referred to as Cu single atoms (SAs)/UIO-H2, was constructed through a hydrothermal process
followed by a H2-spillover treatment [Figure 3F]. As illustrated in Figure 3G and H, the FE for C2+ products
on Cu SAs/UIO reached a maximum of 27.71% at -1.06 V vs. RHE, which is significantly reduced compared
to Cu SAs/UIO-H2, highlighting the crucial role of oxygen vacancies in accelerating C-C coupling process.
Furthermore, the Cu SAs/UIO-H2 catalyst demonstrated a maximum FE of 46.28% for C2H5OH and 12.34%
for acetic acid, with an overall FE for C2+ products reaching 58.62%. The catalyst also exhibited excellent
stability over 12 hours, maintaining an ethanol FE of approximately 46% at -0.66 V vs. RHE [Figure 3I].
Grain boundary
It is well established that grain boundaries (GBs) influence the selectivity of CO2RR products[63], and a
quantitative correlation has been observed between the density of GBs in Cu-based electrocatalysts and
CO2-to-C2+ reduction[64]. As a result, GBs are regarded as a crucial factor in enhanced selectivity for C2+
products of Cu-based catalysts. Enhancing GB density in Cu catalysts has been shown to significantly
enhance C2+ production. Chen et al. demonstrated the Cu electrodeposition with polyvinylpyrrolidone
(PVP) as an additive allows for controlled grain growth[65]. The inert PVP adsorbed onto the Cu surface,
accelerating the nucleation rate and reducing the size of crystal, resulting in a Cu electrode rich in GBs. The
Cu electrode deposited without PVP is referred to as ED-Cu. When the GB-rich Cu electrode (GB-Cu) was
used for eCO2RR, it produced C2H4 and C2H6O across a broad potential range of -1.0 V to -1.3 V vs. RHE,
achieving a high FEC2+ of 73%. Notably, GB-Cu showed C2H4 selectivity of ~38% at -1.2 V vs. RHE and a
C2H6O-to-C2H4 ratio of 0.85 [Figure 4A and B]. In situ infrared absorption spectroscopy revealed that the
GBs in Cu were key in facilitating the adsorption of *CO intermediates, thereby promoting CO2RR
[Figure 4C]. Dendritic Cu catalysts with tunable GB were prepared by Zhang et al. using a pulsed
electrochemical deposition method on the gas diffusion electrode (GDE)[66]. The GB density was adjusted by
varying the deposition times to 50 ms, 100 ms, 200 ms, and 500 ms, with a constant pulse duration of
250 ms, all under a constant current density of 100 mA cm-2 [Figure 4D]. The high-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) image shown in Figure 4E revealed distinct
high-contrast lines aligned parallel to the growth direction of the Cu dendrites, indicating a symmetrical
lattice structure on either side, a typical feature of GBs. Notably, the selectivity for C2+ products increased
linearly with the GB density across all applied potentials. The GB-Cu29.6 catalyst exhibited the highest FEC2+,
reaching 46.2%, along with a jC2+ of ~20 mA cm-2 at a potential of -1.1 V vs. RHE [Figure 4F and G].
Figure 4H shows activity mapping at a defined potential based on cyclic voltammetry, where GB-Cu29.6
demonstrated distinct patterns of locally enhanced activity compared to Cu catalysts without GBs. This
suggests that GBs facilitate electron transfer, thereby improving performance toward C2+ productions.
Ding et al. introduced an innovative and straightforward salt-assisted annealing technique that induced
unconventional grain fragmentation, significantly increasing the density of GBs[67]. The optimized
electrocatalysts attained a maximum FEC2+ of 70.0% for the H-type cell and 68.2% for the flow cell, achieving
a jC2+ of 0.768 mA cm-2 at -1.28 V vs. RHE, surpassing the performance of most previously reported
counterparts. To investigate the correlation between GBs and the selectivity of the catalysts for C2+ products,
in situ Raman spectroscopy and Density functional theory (DFT) calculations were conducted. The results
indicated that a greater GB density generates additional active sites for CO2RR, resulting in a higher
concentration of *CO intermediate. This, in turn, promotes C-C coupling and ultimately increases C2+
production. Kong et al. prepared small Cu nanoparticles (NPs) with enhanced small GBs (RGBs-Cu) using
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Figure 4. (A) FEs on GB-Cu; (B) FEC2+ of GB-Cu; (C) In situ ATR-SEIRAS spectra of GB-Cu. This figure is quoted with permission from
Chen et al. Copyright (2020) American Chemical Society[65]; (D) Electrochemical deposition procedure for different Cu catalysts; (E)
FFT-refined aberration-corrected HAADF-STEM image of the GB-Cu29.6; (F) FEs for C2+ and (G) ECSA-normalized jC2+ of Cu with
different GB densities; (H) Spatially resolved electrochemical map derived from the individual linear sweep voltammetry (LSV) curves.
This figure is quoted with permission from Zhang et al. Copyright (2024) American Chemical Society[66]. FE: Faradaic efficiency;
HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy; GBs: grain boundaries; ATR-SEIRAS:
attenuated total reflectance-surface-enhanced infrared absorption spectroscopy; FFT: fast Fourier transform; ECSA: electrochemically
active surface area.
spatial confinement and in situ electroreduction[68]. Figure 5A illustrates the schematic of the synthesis
process for RGBs-Cu electrocatalysts, while HRTEM images reveal the numerous GBs present in the
catalysts. CO2RR electrolysis using the RGBs-Cu electrode was conducted using flow cells with different
concentrations of KOH. Notably, elevating the KOH concentration to 2 and 3 M resulted in an additional
enhancement of the FE for C2+ products, achieving 77.3% at a current density of 400 mA cm-2 in 2 M KOH
[Figure 5B]. Additionally, RGBs-Cu reached a maximum C2+-to-C1 (C2+/C1) ratio of 2.57 at 400 mA cm-2,
which notably exceeded the control sample’s ratio of 0.75 [Figure 5C]. Moreover, RGBs-Cu demonstrated
exceptional stability, maintaining consistent C2+ selectivity of 60% over 134 h at a total current density of
500 mA cm-2 [Figure 5D].
Nanostructure engineering
Bulk Cu-based electrocatalysts suffer from limited exposed active sites and poor electronic structure,
resulting in low CO2RR performance towards C2+[69,70]. To address these challenges, nanostructure
engineering approaches-such as morphology control, facet optimization, and confinement effects-have
appeared as powerful strategies to strengthen their catalytic efficiency for CO2RR.
Confinement effect
Confinement structures in Cu-based electrocatalysts can enhance C2+ selectivity by increasing the retention
time of intermediates such as *CO/*COH by facilitating C-C coupling reactions[71,72]. Consequently,
significant research has focused on tailoring the Cu surface with specialized confinement morphologies to
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Figure 5. (A) Overview of the fabrication process of RGBs-Cu sample; (B) FEC2+ of RGBs-Cu with different electrolyte concentrations;
(C) C2+/C1 product selectivity of RGBs-Cu and PGBs-Cu; (D) Evaluation of the long-term stability of the RGBs-Cu electrode at
500 mA cm-2 in a flow cell. This figure is quoted with permission from Kong et al. Copyright (2024) American Chemical Society[68]; (E)
Schematic representation of Cu2O HoMSs fabrication process; (F) Distribution of CO2RR products on Cu2O HoMSs catalysts 1-3 shell;
(G) C2+/C1 product selectivity on HoMSs; (H) Long-term durability of 3-shell HoMSs for 8 h. This figure is quoted with permission from
Liu et al. Copyright (2022) John Wiley and Sons[76]. FE: Faradaic efficiency; CO2RR: CO2 reduction reaction; HoMSs: hollow multi-shell
structures; RGBs-Cu: Cu nanoparticle catalyst comprising abundant grain boundaries.
optimize both the activity and selectivity for C2+ product formation. Fan et al. fabricated three-dimensional
(3D) ordered porous cuprous oxide cuboctahedra (3DOP Cu2O-CO) through a hard templating method[73].
This method successfully created a structure characterized by ordered macropores that are interconnected
with mesoporous channels throughout the Cu2O cuboctahedra. The 3DOP-Cu2O-CO configuration offers
the benefits of uniformly distributed and interconnected pore channels. Consequently, these electrocatalysts
demonstrated superior performance in CO2RR compared to control catalysts lacking ordered porous
architectures. Notably, 3DOP-Cu2O-CO achieved a significant electrochemical double-layer capacitance of
2.9 mF cm-2, with a maximum C2+ FE of 73.4% at -1.4 V vs. RHE in an H-cell, and 81.7% at -1.0 A cm-2 using
a flow-type cell. Finite element method (FEM) simulations demonstrated that the structured pore
architecture of 3DOP Cu2O-CO capably confines and holds adequate *CO for adsorption during the CO2
reduction process, thereby facilitating the strong dimerization required for C2+ product formation. Liu et al.
designed distinctive Cu-based catalysts with a cavity structure, achieved through in situ electrochemical
reduction of Cu2O cavities[74]. Both transmission electron microscopy (TEM) and scanning electron
microscopy (SEM) analyses confirmed the presence of cavity architecture. FEM simulations also confirmed
that the cavity structure significantly enhances the concentration of CO intermediates at the reaction site,
thereby promoting the C-C coupling reaction. These Cu cavity catalysts demonstrated excellent C2+ FE of
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75.6 ± 1.8% and achieved jC2+ of 605 ± 14 mA cm-2 in a microfluidic flow cell. Moreover, the experimental
C2+/C1 of Cu cavity catalysts reached the maximum of 5.4 ± 0.6. Pan et al. investigated the use of hollow
mesoporous carbon spheres (HMCS) to explore the influence of confinement on the production of C2+
products[75]. They prepared a series of Cu-incorporated HMCS with copper loadings of 10%, 20%, and 30%,
designated as Cu/HMCS5-10%, Cu/HMCS5-20%, and Cu/HMCS5-30%, respectively. The results revealed
that Cu/HMCS5-20% achieved the greatest FEC2+ of 88.7%, outperforming 69.3% and 80.1% FEC2+ recorded
for Cu/HMCS5-10% and Cu/HMCS5-30%, respectively, at a potential of -1.0 V vs. RHE in a 1.0 M KOH
solution. Notably, Cu/HMCS5-20% exhibited remarkable stability, maintaining its performance over 20 h at
-1.0 V vs. RHE without significant degradation. The exceptional C2+ selectivity of Cu/HMCS5-20% was
attributed to the unique confinement effects provided by HMCS, along with the synergistic interactions
among Cu atoms with varying oxidation states. Liu et al. described the development of Cu2O hollow
multi-shell structures (HoMSs) with adjustable shell numbers[76]. The synthesis of Cu2O HoMSs was
accomplished using the Ostwald ripening method, as depicted in Figure 5E. Electrocatalytic CO2 reduction
was conducted in a flow reactor using 0.5 M KHCO3. Prior to testing, Cu2O HoMSs underwent
electroreduction at -0.82 V vs. RHE for ten minutes. Notably, the Cu2O structure with three shells reached a
peak C2+ FE of 77.0 ± 0.3%, outperforming the 1-shell (40.3 ± 1.0%) and 2-shell (62.2 ± 0.3%) counterparts at
-0.88 V vs. RHE [Figure 5F]. The increase in shell number clearly enhanced C2+ production [Figure 5G].
Furthermore, the 3-shell HoMS catalysts exhibited excellent long-term stability, maintaining performance
for 8 h at -300 mA cm-2, underscoring the advantages of the nanoconfinement effect in converting CO2 to
C2+ products [Figure 5H].
Morphology/facet effect
Aside from confinement engineering, the morphology and crystal facets of Cu-based electrocatalysts have a
fundamental role in influencing CO2 electroreduction activity and product selectivity. For example, Wu et
al. revealed a correlation between the structural dimensions of oxide-derived Cu (OD-Cu) electrocatalysts
and their ability to selectively produce C2+ products[77]. Their study involved the synthesis of three distinct
nanostructures: one-dimensional nanowires (NWs), two-dimensional nanosheets (NSs), and 3D
nanoflowers (NFs), all of which displayed notable resistance to structural degradation [Figure 6A]. Initial
evaluations of the CO2RR activity and the electrochemical characteristics of the OD-Cu catalysts were
conducted using H-cell systems. As demonstrated in Figure 6B, the product distribution from CO2RR over
the OD-Cu catalysts was evaluated across a potential range from -0.6 V to -1.4 V vs. RHE. Of the three
morphologies, OD-Cu NWs achieved the highest preference for C2+ products, reaching a maximum FE of
50.2% at -1.4 V vs. RHE, which notably surpassed the performance of OD-Cu NSs (35.6%) and NFs (13.3%).
Additionally, the performance of OD-Cu catalysts was additionally evaluated with flow cells. Notably,
OD-Cu NWs, which have the maximum surface Cu-O coordination, exhibited the most favorable CO2RR
kinetics for C2+ production, achieving 77.7% of FEC2+, 233.2 mA cm-2 for jC2+, 701.8 μmol h-1 cm-2 of C2+ yield,
and 51.1% C2+ energy efficiency [Figure 6C]. Additionally, OD-Cu NWs exhibited remarkable stability,
maintaining consistent CO2RR performance at 300 mA cm-2 for 72 h.
Moreover, Cu-based catalysts with a two-dimensional NS morphology are appealing due to their plentiful
low-coordination edge sites, which are frequently regarded as essential for improving catalytic performance.
Xie et al. synthesized Cu NSs derived from CuO during CO2RR, having an average size of approximately
30 nm and thickness of about 20 nm[78]. With an abundant concentration of low-coordination edge sites,
these Cu NSs enabled efficient C-C coupling reactions, reaching a peak FEC2H4 of 56.2% and a jC2H4 of
171.0 mA cm-2 at -0.871 V vs. RHE in a flow cell. Wang et al. developed thin-layered Cu2O NSs having
thickness of 0.9 nm (Cu2O-NS) via coprecipitation method[79]. For comparison, other ultrathin catalysts,
Cu-NS and CuO-NS, were also fabricated. The Cu2O-NS demonstrated a total current density of
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Figure 6. (A) Diagram illustrating the preparation of OD-Cu catalysts with different morphologies; (B) FEC2+ of OD-Cu NWs using
H-cell; (C) FEC2+ of OD-Cu NWs using flow cells. This figure is quoted with permission from Wu et al. Copyright (2024) Elsevier[77]; (D)
Schematic illustration for the synthesis of various Cu2O; (E) LSV curves of all three catalysts; (F) FEs of various liquid products on
f-Cu2O; (G) jC2H6O on f-Cu2O. This figure is quoted with permission from Yang et al. Copyright (2024) Elsevier[80]. OD-Cu:
Oxide-derived Cu; NWs: nanowires; LSV: linear sweep voltammetry; FE: faradaic efficiency.
-187.6 mA cm-2 and a FE of 81% for C2+ products, with FEC2H4 reaching 40%, and stability of ten hours. The
remarkable CO2RR performance is attributed to coordination defects and oxygen vacancies, which
enhanced *CO adsorption and improved C2+ product selectivity by stabilizing the surface oxidation state of
Cu.
Facet engineering is essential in defining the catalyst's activity toward desired products. This can be
achieved through various methods. Yang et al. explored the impact of morphology on the performance of
Cu2O catalysts for the production of liquid products[80]. Cu2O electrocatalysts with various morphologies
were synthesized using PVP as a capping agent. A schematic representation of the synthesized catalysts,
including cube-shaped Cu2O (c-Cu2O), tetrakaidecahedron-shaped Cu2O (t-Cu2O), and flower-like Cu2O
(f-Cu2O), is provided in Figure 6D. Surface coordination-unsaturated Cu atoms on the (111) facet were
observed to improve CO2 reduction by facilitating the adsorption of CO2 molecules. Oxygen vacancies were
confirmed using electron paramagnetic resonance spectroscopy and ultraviolet-visible diffuse reflection
spectroscopy. Figure 6E presents the linear sweep voltammetry (LSV) curves of the Cu2O catalysts in an
H-cell, showing that the f-Cu2O with exposed (111) facets exhibited higher current density and a minimized
onset potential. The selectivity toward liquid products was further assessed using 1H NMR. At -0.8 V vs.
RHE, the f-Cu2O catalyst achieved an ultimate C2H6O FE of 52.6% and a jC2H6O of 9.1 mA cm-2 at -0.9 V vs.
RHE, attributed to the abundance of oxygen vacancies on its surface [Figure 6F and G]. Furthermore, the
ethanol formation rate on f-Cu2O rose markedly from 0.43 μmol h-1 cm-2 at -0.5 V to 28.56 μmol h-1cm-2 at
-1.1 V vs. RHE. Significantly, the ethanol production rate on the f-Cu2O catalyst was 3.4 times higher than
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that of c-Cu2O and 1.6 times greater than that of t-Cu2O. Gregorio et al. demonstrated facet-dependent
selectivity on the Cu electrode, achieved through morphology adjustment via colloidal chemistry[81]. Aligned
with prior findings, Cu nanocubes enclosed by (100) facets show superior selectivity for C2H4, while Cu
electrocatalysts with (111) facets demonstrate the highest selectivity toward CH4. Fu et al. revealed that Cu
catalysts with exposed (100) facets exhibit significantly higher selectivity for C2+ products compared to those
with (111) and (110) facets[82]. They developed three types of catalysts with distinct morphologies-NWs,
NPs, and NSs-each exposing different crystal facets through pre-reduction and reconstruction methods.
Among them, the NS-D Cu catalyst, characterized by its (100) facet, was particularly effective at enhancing
*CO adsorption, increasing surface *CO coverage, and facilitating CO dimerization, thereby boosting its
activity for C2+ product formation. This catalyst achieved a FE of 67.5% towards C2+ products, a jC2+ of
25.1 mA cm-2, and maintained stable performance for ten hours. Additionally, the NS-D Cu showed a
maximum C2+/C1 ratio of 6.2, surpassing NW-D Cu and NP-D Cu, with ratios of 1.7 and 1.9, respectively.
Dong et al. proposed a novel mechanism elucidating the link between the crystal facets of Cu2O and the
formation of distinct reduction products[83]. Through a combination of theoretical calculations and
experimental validation, they supported their findings. The exposed Cu atoms on the (100) facet displayed a
dense Cu4O1 surface coordination, which enhanced the adsorption of *CO intermediates. Consequently,
Cu2O with the (100) facet showed significantly higher selectivity for C2H4 in 0.1 M KHCO3 solution,
achieving a FE for C2H4 of 61.3% at 1.2 V vs. RHE. Compared to other Cu-based catalysts reported in recent
studies, their Cu2O with (100) facets demonstrated superior selectivity toward C2H4. Luo et al. synthesized
six distinct morphologies of Cu2O NPs using NH2OH·HCl as a reductant, each exposing different crystal
facets, and then applied a ZIF-8 shell to improve their stability [Figure 7A][84]. The NPs, labeled A-Cu2O
through F-Cu2O, were obtained by varying the amount of NH2OH·HCl. Figure 7B and C illustrates the
electrochemical performance of these Cu2O catalysts in 0.1 M KHCO3 over a potential range of -0.7 V to
-1.3 V vs. RHE. Among them, F-Cu2O, which primarily exposed the (332) facet, showed the highest FEC2H4
production, reaching 74.1% at -1.2 V vs. RHE. It was also observed that increasing the proportion of (111)
facets significantly enhanced both the selectivity and activity of as synthesized electrocatalysts. DFT
calculations further confirmed that the (332) facet, with its step-like structure, substantially lowered the
Gibbs free energy for *CHO intermediate coupling, leading to improved electrocatalytic performance.
Size effect
Another approach to develop electrocatalysts is controlling the size of electrocatalysts. Reducing the size
increases the surface-to-volume ratio, thereby enhancing the utilization of metal atoms. Additionally, the
number of undercoordinated sites increases, leading to a perturbed electronic structure and enhanced
activity.
Merino-Garcia et al. investigated the selectivity and activity of the catalyst based on the size of the Cu
particles, which were synthesized with sizes ranging from 25 nm to 80 nm[85]. Cu particles of different sizes
were mixed with a binder and isopropanol to prepare a GDE by depositing the mixture onto carbon paper.
The electrode with 25 nm Cu particles exhibited significantly better performance compared to those with
Cu particle sizes of 40-60 nm and 60-80 nm, achieving a remarkably high ethylene FE of 92.8% and a
production rate of 1,148 μmol m-2 s-1. This enhanced performance is attributed to an increase in the fraction
of under-coordinated sites as the particle size decreases. Furthermore, Rong et al. explored the size effects of
Cu catalysts, ranging from SAs to nanoclusters, using a facile acetylenic-bond-directed site-trapping
approach[86]. Three different catalysts of varying sizes were synthesized: single-atoms (~0.1 nm),
subnanometric clusters (0.5~1 nm) and nanoclusters (1~1.5 nm). They demonstrated that increasing the
size of Cu nanoclusters enhanced both catalytic activity and selectivity toward C2+ products. The
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Figure 7. (A) Preparation of Cu2O nanoparticles and Cu2O@ZIF-8 composites; (B) FEC2H4 of Cu2O nanoparticles with different facets;
(C) FE of products on F-Cu2O at different potentials. This figure is quoted with permission from Luo et al. Copyright (2022) John Wiley
and Sons[84]; (D) Schematic illustration for the synthesis of various Cu2O; (E) C2+ and C1 faradaic efficiencies; (F) jC2+ on Cu-D and Cu-P;
(G) Durability analysis for the Cu-D and Cu-P electrodes at 300 mA cm-2.This figure is quoted with permission from Niu et al. Copyright
(2021) American Chemical Society[97]. FE: Faradaic efficiency.
electrocatalytic performance was assessed in a three-compartment flow cell. Among the synthesized
catalysts, the optimized Cu-based nanoclusters with a size of 1-1.5 nm exhibited the best performance,
achieving a FEC2+ of 91.2%, a partial current density of 312 mA cm-2, and moderate stability lasting
approximately 22 h. Nam et al. synthesized Cu clusters using copper benzene-1,3,5-tricarboxylate
(HKUST-1) as a precursor[87]. The calcination time was controlled to remove benzene tricarboxylate
moieties, resulting in Cu dimer distortion and the generation of undercoordinated Cu sites within the Cu
clusters. HKUST-1 calcined at 250 °C for 3 h exhibited the lowest Cu-Cu Coordination number of 9.5 ± 0.9
among the synthesized Cu clusters. The electrocatalytic performance of the catalysts was evaluated in a flow
cell with 1 M KOH. Among the samples, HKUST-1 calcined at 250 °C for three hours showed the highest
selectivity for C2H4, achieving a FEC2H4 of 45%, significantly outperforming the as-prepared HKUST-1
(FEC2H4=10%). Su et al. synthesized electrocatalysts consisting of CuO clusters anchored on N-doped carbon
NSs (Cu/N0.14C)[88]. Cu/NxC with varying nitrogen contents was prepared in two steps: (1) simple mixing of
copper phthalocyanine and dicyandiamide followed by drying, and (2) pyrolysis of the dried mixture at
different temperatures. The optimized Cu/N0.14C catalyst exhibited a superior FEC2+ of 73%, with a selectivity
towards ethanol of 51% and a partial current density of 14.4 mA cm-2 at -1.1 V vs. RHE. Moreover, it
demonstrated excellent long-term durability, maintaining stable FEC2+ and partial current density for over
ten hours.
Surface modification
While earlier studies primarily concentrated on aspects such as composition, morphology, size, crystal
structure, and internal electronic properties, recent research has shifted toward optimizing the surface
microenvironment. Surface modification has emerged as a powerful strategy to improve CO2 reduction to
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C2+ products through the adjustment of properties such as hydrophobicity, electronic structure, and the
adsorption energy of CO2RR intermediates on the catalyst surface[89-91]. Introducing molecules such as
long-chain alkyls or polymers to Cu-based catalysts can suppress H2 production and improve catalytic
stability.
Xie et al. introduced a strategy for modifying Cu electrodes using amino acids. After modifying the surface
of Cu NWs with a series of amino acids, the CO2RR performance toward the generation of C2+ products
significantly improved[92]. Theoretical calculations suggested that -COOH and -NH2 groups in amino acids
enhance the C2+ selectivity by stabilizing and transforming *CHO intermediates while inhibiting the HER.
Wei et al. applied polyaniline (PANI) to modify the catalytic performance and selectivity of Cu[93]. By
coating a PANI thin film, approximately 50 nm thick, on the Cu electrode, the chemical environment of the
Cu surface was altered, resulting in an excellent C2+ FE of 60%, which is almost four times higher than that
of the bare Cu electrode at -1.1 V vs. RHE. Similarly, Ma et al. explored the effect of polymer modification
on the CO2 electroreduction performance and C2+ selectivity using PANI[94]. PANI NPs were coated on the
surfaces of CuO NSs by a wet-chemical polymerization method. The PANI/CuO NSs hybrids were
optimized by controlling the loading amount of PANI. The electrocatalytic performance toward CO2RR was
evaluated using a three-electrode H-type cell. Compared to CuO NSs without the PANI, the optimized
catalysts demonstrated excellent selectivity and durability, achieving a FE of up to 66.4% and long-term
stability of 92 h at -1.6 V vs. RHE. Consequently, decorating CuO NSs with PANI NPs not only boosts their
stability but also increases local *CO surface coverage, owing to the distinctive relationship between the
-NH- group and adsorbates. This interaction further promoted superior C-C coupling at the neighboring
Cu active sites, facilitating the formation of C2+ products. Wakerley et al. demonstrated a highly
hydrophobic Cu dendrite electrode using the ‘plastron effect,’ similar to the mechanism used by aquatic
arachnids, through 1-octadecanethiol treatment[95]. Owing to its superhydrophobic characteristics, the
corresponding electrode exhibited low H2 evolution even at -1.6 V vs. RHE. Conversely, the Cu electrode
that underwent surface modification attained a FE of 56% for C2H4 and 17% for C2H6O production, in
contrast to 9% and 4% for its hydrophilic, wettable counterpart. However, the superhydrophobicity nature
of the electrode surface obstructs the active sites participating in the reaction, resulting in a reduction in
current density. Moreover, the stability of superhydrophobicity during the electrochemical CO2 reduction
process has not been clearly elucidated. To address these issues, Shi et al. revealed the effect of interface
evolution on CO2RR performance using 1-octadecanethiol modification. The interface where catalysis
occurs changed from hydrophilic to hydrophobic, ultimately forming a stable triple-phase interface
(TP-Cu)[96]. We examined characteristics of the electrode during the evolution using SEM, contact angle
measurements, X-ray photoelectron spectroscopy (XPS), finding that the Cu dendrite structure was well
preserved and the 1-octadecanethiol surface layer remained on the surface without desorption. However, it
was observed that wettability changed as the operating time increased. Furthermore, the alkane thiol-
modified copper surface eventually transitioned to a robust copper thiolate form under reaction conditions.
Remarkably, this evolution reduced charge transfer resistance and allowed for higher accessibility of active
sites, facilitating optimized gas-liquid-solid interface. Consequently, TP-Cu demonstrated a significant
reduction in H2 production compared to the initial hydrophobic surface. Niu et al. designed a nature-
inspired copper catalyst on a gas diffusion layer, modeled after plants with hydrophobic characteristics, such
as Setaria[97]. A scalable electrodeposition method was used to fabricate a hierarchical Cu catalyst with fine
needles on a gas diffusion layer (GDL) [Figure 7D]. Unlike other reported literature, these CO2RR
electrodes exhibited stable hydrophobicity without requiring a hydrophobic coating. The CO2
electroreduction activity of the Cu electrodes was evaluated by a flow cell operating with 1 M KOH. As the
negative potential increased from -0.53 V to -0.68 V vs. RHE, FE of C1 products decreased, while the
selectivity towards C2+ products increased, due to enhanced C-C coupling. Specifically, the electrode
achieved a high C2+ production rate of 255 ± 5.7 mA cm-2 with a 64 ± 1.4% FEC2+, demonstrating exceptional
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stable performance at 300 mA cm-2 for more than 45 h [Figure 7E-G]. Recently, Liu et al. innovated a facile
surface modification using toluene molecules to mitigate the degradation of catalysis performance[98].
Additionally, stearic acids were functionalized on the surface of the Cu catalyst (S-Cu). Through π-π
interactions, the hydrophobic and conjugated benzene rings of toluene organized into structured
hydrophobic micro-channels with a spacing of about 5.1 Å. In contrast, the spacing between adjacent stearic
acids was calculated to be 2.3 Å, smaller than the sizes of H2O or CO2 molecules, leading to lower selectivity
for C2+ products. Furthermore, molecular dynamics (MD) simulations were performed to examine the
microenvironment on the surface of the catalyst throughout CO2RR. MD simulations indicated that the
three samples exhibited different distributions of CO2 and H2O, resulting in different CO2RR performances.
Notably, T-Cu demonstrated the highest performance, surpassing both pristine and S-Cu, with a FEC2+ of
78%, a jC2+ of 1.81 A cm-2, and excellent stability for 400 h.
Tandem strategy
Among the various strategies to boost the activity of electrochemical CO2 reduction, the tandem strategy has
been acknowledged as a promising strategy. Tandem catalysts are designed with dual active sites, allowing
them to influence intermediates and achieve faster reaction rates compared to Cu monometallic
components[99-101]. Important intermediates formed at one active site can be transported to another site to
engage in subsequent electrochemical reduction. In particular, the *CO intermediate is essential for the
generation of C2+ products[102]. Zn, Ag, and gold (Au) provide additional CO to amplify *CO surface
coverage on Cu active sites, promoting C-C coupling and elevating C2+ product formation. However, if *CO
undergoes further reduction without coupling to form *CHO or *COH, CH3OH or CH4 will be produced.
Therefore, enhancing *CO intermediate coverage and facilitating coupling reactions is a key strategy to
improve selectivity for C2+ products[103,104].
Ag-based
Ag is known for its high activity in CO production and is one of the most commonly used metals as a
component in tandem catalysts. Although its CO selectivity is lower than that of Au, it is suitable for use in
tandem catalysts from a cost perspective.
Recently, Duan et al. fabricated Cu@Ag/C via in-situ electrochemical reconstruction of
Cu2CO3(OH)2/AgCl/C composite[105]. Cu2CO3(OH)2/AgCl/C was first synthesized by precipitating different
amounts of AgCl, followed by electroreduction using an H-type reactor. The performance of the Cu@Ag/C
catalyst was assessed in an H-type cell with 0.1 M KCl. As depicted in Figure 8A, the FEC2H4 achieved 50.41%
at -1.1 V vs. RHE, which is 1.7 times superior to bare Cu/C catalysts (30.41%). We further compared the
FECO of Cu@Ag/C and Cu/C to emphasize the role of Ag in promoting CO production [Figure 8B]. The
incorporation of Ag significantly enhanced CO formation, increased *CO coverage, and consequently
promoted C-C coupling, contributing to the formation of C2H4. Moreover, electrocatalytic performance was
evaluated using flow-type cells to improve CO2 mass transfer. Cu@Ag/C demonstrated a maximum FEC2H4
of 58.03% at -0.85 V vs. RHE, as shown in Figure 8C. Jeon et al. prepared bimetallic catalysts with varied
Cu/Ag ratios by ultrasonic spray pyrolysis process[106]. The electrochemical performance of CuXAg100-X
catalysts was evaluated in a single cell across a voltage range of 1.8 to 2.3 V. As expected, only CO and H2
were produced with Ag100, while C2H4, CH4, CO, and H2 were produced with the Cu-containing catalyst.
Moreover, Cu-containing catalysts showed a decrease in FECO and an increase in FEC2H4 at potentials that
favor C2H4 production. This is because a crucial step in C2H4 formation involves the C-C coupling reaction,
which consumes CO as a reactant. The optimized CuXAg100-X with a ratio of 9:1 demonstrated the highest
FEC2H4 of 33% at a cell voltage of 2.2 V, which is significantly higher than that of bare Cu (22%). Recently, Liu
et al. constructed Cu-Ag tandem catalysts utilizing ethylenediaminetetraacetic acid (EDTA) as a ligand,
leveraging ligand modification in CO2RR[107]. Here, EDTA played a dual role in modulating both the
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Figure 8. (A) FE distribution of C2H4 on Cu/C and Cu@Ag/C catalysts; (B) FECO on Cu/C and Cu@Ag/C catalysts; (C) FE of various
products on Cu@Ag/C at various potentials. This figure is quoted with permission from Duan et al. Copyright (2024) Elsevier[105]; (D)
Schematic illustration of three types of tandem catalysts; (E) FEs of products on layered Cu/Ag; (F) Partial current density of ethanol on
Cu, mixed CuAg, layered Ag/Cu, and layered Cu/Ag catalysts. This figure is quoted with permission from Luan et al. Copyright (2024)
American Chemical Society[108]; (G) Schematic diagrams of Cu needle-Ag tandem catalysts; (H) FEs for H2, C2H4, CH4, HCOO-,
C2H5OH, CO, and propanol on Cu needle-Ag catalysts; (I) FEs for different products of the Cu needle-Ag tested in a flow cell. This
figure is quoted with permission from Wei et al. Copyright (2023) John Wiley and Sons[111]. FE: Faradaic efficiency.
electrode structure and the local pH around the surface Cu atoms during the electrochemical process.
Cu-Ag tandem catalysts modified with EDTA (CuAg4/EDTA) exhibited an excellent FECO at low potentials,
and achieved an FEC2+ of 86.56%, compared to Cu (44.13%), CuAg4 (33.32%) and Cu/EDTA (52.15%). Luan
et al. developed a tandem catalyst designed for C2H6O production. Several electrode configurations were
fabricated, including layered Cu/Ag, layered Ag/Cu, and mixed CuAg structures [Figure 8D][108]. The
layered Cu/Ag catalyst exhibited outstanding electrochemical performance for CO2-to-C2H6O conversion
compared to other types of electrodes and bare Cu catalysts. Remarkably, the best-performed layered Cu/Ag
electrode achieved a FE of 56.5 ± 2.6% for C2H6O and a current density of 356.7 ± 9.5 mA cm-2 at -1.1 V vs.
RHE [Figure 8E and F]. This elevated performance, compared to the inverse layered Ag/Cu structure, can
be credited to the efficient mass transport of CO gas produced in the Ag catalyst layer (CL). Huang et al.
constructed Ag-Cu nanodimers (NDs) by establishing Cu domains onto pre-existing Ag NPs, resulting in
an adjustable interface between the two distinct metals[109]. This arrangement strengthened CO adsorption
on the surface, promoting the conversion of CO into C2H4, resulting from electron depletion in Cu due to
electron migration from Cu to the Ag domains. Consequently, the Ag-Cu NDs with an Ag and Cu in a 1:1.1
mass ratio showed a notable increase in both the FE for C2H4 and the jC2H4 compared to bare Cu NPs.
Similarly, Ma et al. described the preparation of Janus-like Ag-Cu nanostructures, where Cu having exposed
(100) facets was restricted to one of the six equivalent faces of Ag nanocubes[110]. Compared to Cu
nanocubes, the resulting Ag65-Cu35 Janus nanostructures with (100) facets (Ag65-Cu35 JNS-100) demonstrated
a substantially advanced FE for C4H4 production, reaching 54%, with a total C2+ product efficiency of 72%.
Additionally, the selectivity of the optimized Ag65-Cu35 JNS-100 toward C2H4 surpassed that for CH3CH2OH
and CH3COOH, with FEs of 1.7% and 4.2%, respectively. These enhanced results were due to CO induced
on the Ag domains, which could migrate to the Cu (100) facets, promoting dimerization on the Cu surface
owing to electron flow between Ag and Cu.
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In a tandem catalyst system, two main pathways are involved: (1) CO2 is first reduced to CO by a metal with
high CO selectivity, and (2) the generated CO then migrates to neighboring Cu sites, where it undergoes
C-C coupling and hydrogenation to form C2+ products. However, if the initial CO formation occurs too
rapidly, C-C coupling may be limited, leading to higher selectivity for CO over C2+ products. Extending the
residence time of CO on Cu active sites increases the likelihood of CO absorption and further reduction,
thereby decreasing the FECO and enhancing the FEC2+. Thus, controlling CO residence time on Cu sites is a
key factor in optimizing tandem catalyst systems. Wei et al. fabricated a 3D tandem catalyst by depositing
Ag NPs at the base of Cu nanoneedle arrays to control the transport distance of CO [Figure 8G][111]. The
electrocatalytic performance of the obtained tandem catalyst was initially measured in an H-type cell.
Compared to a similar structure lacking Ag NPs at the base, the Cu needle-Ag achieved a selectivity of
45.6% for C2H4 at -1.0 V vs. RHE [Figure 8H]. Additionally, Cu needle-Ag catalysts were evaluated in a flow
cell to achieve commercially relevant current densities. As depicted in Figure 8I, the Cu needle-Ag catalyst
demonstrated exceptional selectivity towards C2+ products across applied current densities between
200 mA cm-2 to 350 mA cm-2. Specifically, the FE for C2+ products rose from 58.2% at 100 mA cm-2 to 70% at
350 mA cm-2. The densely packed Cu nanoneedles create significant diffusion resistance for CO as it moves
from the Ag NPs, forcing CO to travel a longer path. This extended diffusion increased the likelihood of CO
adsorption, promoting dimerization into C2+ products and thereby contributing to the enhanced CO2RR
performance.
Au-based
Similar to Ag, Au is particularly noted for its high selectivity towards CO, as it efficiently weakens CO2
bonds and promotes the formation of CO intermediates. Bimetallic Au-Cu tandem catalysts have shown
improved catalytic performance, with Au facilitating efficient CO production and Cu driving the
subsequent C-C coupling, which is necessary to produce higher-value C2+ products. For example,
Morales-Guio et al. employed electron-beam evaporation to arrange Au NPs onto polycrystalline copper
foil (Au/Cu), creating tandem electrocatalysts[112]. These Au/Cu catalysts displayed superior CO2 reduction
activity towards C2+ alcohols compared to recently developed Cu-based bimetallic catalysts. The existence of
Au NPs on the copper foil enhances the local concentration of CO on adjacent copper surfaces, which
subsequently facilitates its reduction to C2+. Notably, the Au/Cu catalysts demonstrated a selectivity for C2+
formation that was 100 times higher than that for CH4 or CH3OH at low overpotentials. Recently, Wang et
al. used Au NPs of varying sizes and densities as a CO-producing component to boost CO2 electroreduction
into high-value chemicals[113]. They developed gold-copper oxide (Au@Cu2O) tandem catalysts through a
galvanic replacement reaction. Through modification of the Au NP size and the concentration of Au
precursors, they explored the relationship between size and performance. The Au@Cu2O samples were
designated as S-0.25, 0.5, and 2.0-Au@Cu2O for smaller Au NPs and L-0.25, 0.5, and 2.0-Au@Cu2O for
larger Au NPs, reflecting the different sizes and concentrations of the Au precursors. The CO2 reduction
performance of L-0.5-Au@OD-Cu, S-0.5-Au@OD-Cu, S-2.0-Au@OD-Cu, and OD-Cu was assessed in a
1 M KHCO3 using a flow cell. Our results showed that catalysts with smaller Au NPs exhibited more
efficient CO2 reduction reactions at a reduced overpotential compared to those with larger Au NPs and
OD-Cu. Additionally, elevating the loading amount of Au NPs in Au@Cu2O catalysts led to a decline in the
selectivity of C2+ products such as C2H4 and C2H6O, while enhancing the FE of n-propanol. Cao et al.
synthesized tandem catalysts consisting of Au and cubic Cu2O through a galvanic replacement reaction
followed by pre-reduction using the LSV method[114]. Unlike bare Cu2O, the AuxCu2O catalysts exhibited a
shift in selectivity, with CO formation becoming more prominent. A significant increase in C2H4 selectivity
was observed at potentials beyond -1.1 V vs. RHE. Among the catalysts, Au0.02Cu2O achieved the highest FE
for C2H4, reaching 24.4% with a total current density of approximately 5.7 mA cm-2 at -1.3 V vs. RHE, which
was 2 to 2.5 times higher than that of the other AuxCu2O catalysts and five times greater than bare Cu2O.
This enhanced selectivity toward C2H4 was attributed to the optimal CO availability provided by Au atoms
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and the improved C-C coupling facilitated by the synergistic effects of Cu+ and Cu0. Zhu et al. introduced an
effective strategy for designing an epitaxial Au-Cu heterostructure aimed at enhancing the CO2-to-C2+[115].
The electrochemical CO2 reduction was conducted in an H-cell using 0.1 M KHCO3 as the electrolyte. The
Au-Cu heterostructure demonstrated notable C2+ alcohols production, with an onset potential
approximately 150 mV more positive than that of pure Cu and a conversion rate for CO2 to C2+ alcohols that
was about 2.5 times higher at -1.0 V vs. RHE [Figure 9A]. As illustrated in Figure 9B and C, the Au-Cu
heterostructure not only produced more alcohol compared to Cu but also effectively suppressed
hydrocarbon formation, achieving a superior ratio of C2+ alcohols/(CO + > 2e- products). Furthermore, a
dynamic restructuring was observed, where Au-Cu bimetals with phase separation transitioned into
alloy-supported Au@Cu core-shell nanoclusters during electrocatalysis, forming Au@Cu-AuCu structures
[Figure 9D]. A unique tandem mechanism was introduced, highlighting the accumulation of *CO as a key
factor for maintaining the durability of C2+ alcohol production. Wei et al. applied a homo-nucleation
method to decorate Au NPs onto the surface of Cu NWs[116]. Cu NWs are well-recognized for their
significant aspect ratio with dense GBs, both of which have been shown to enhance electron transport and
provide abundant active sites. They synthesized three tandem catalysts with varying Cu-to-Au mass ratios,
labeled Cu99.8Au0.2, Cu99.3Au0.7, and Cu96.7Au3.3. Of these, Cu99.3Au0.7 demonstrated the best catalytic
performance, achieving a FEC2+ of 65.3% at -1.25 V vs. RHE, which is a notable advancement over the
pristine Cu NWs (39.7%). Among the C2+ products, selectivity was highest for C2H4 at 35%, followed by
C2H6O at 19.1%. Zheng et al. developed tandem electrocatalysts based on Janus NPs to overcome the
limitations posed by the spatial arrangement of different constituent components[117]. The Janus structure
ensures optimal accessibility of the two linked metals, allowing the NPs to serve as dual-functional agents.
Au-Cu Janus nanostructures (Au-Cu Janus NSs) were synthesized with N-oleyl-1,3-propanediamine
(OPDA) as a capping agent. OPDA, with its amine group, is essential in inhibiting the surface oxidation of
Cu NPs. To highlight the significance of OPDA in the formation of the Janus nanostructures, they also
synthesized Au-Cu tandem catalysts using octadecylamine and oleylamine. When OPDA was replaced with
other capping agents, the electrocatalysts did not display the Janus structure. The optimized Au-Cu Janus
NSs exhibited selective CO2 reduction towards C2+ products, achieving the highest total current density of
nearly 1 Acm-2, FEC2+ of 67% and a jC2+ of -0.29 A cm-2 at -0.75 V vs. RHE, outperforming both Au@Cu
core-shell nanostructures and Cu NPs [Figure 9E-H]. The distinctive structure of the Au-Cu Janus NSs
promoted CO spillovers from the Au sites to neighboring Cu, enhancing CO coverage and facilitating C-C
coupling.
Molecular catalysts and single-atom catalysts based
Bimetallic systems integrating Cu with other CO-producing metals, including Au and Ag, have been
extensively studied as electrocatalysts for C2+ production, as mentioned above[118-120]. However, a significant
challenge with these catalysts is the potential for changes in composition and morphology during operation,
which can alter both their physical and electronic properties. These variations complicate the insights into
catalytic behavior and hinder the development of effective design principles to enhance catalytic
performance. Consequently, substantial studies have concentrated on developing tandem catalysts that
employ CO-producing molecular catalysts and single-atom catalysts (SACs). Liu et al. developed
high-performance tandem catalysts composed of cobalt phthalocyanines (CoPc), acetylene black, and Cu2O
nanocrystals (Cu2O NCs-C-CoPc)[121]. The electrocatalytic performance for C2+ products was evaluated using
both H-cell and flow-cell systems. In the H-cell, Cu2O NCs-C-Copc achieved a maximum FE for C2H4 of
58.4%, with a jC2H4 of -29.74 mA cm-2 and durability of ten hours. When tested in the flow cell, the selectivity
for C2H4 increased significantly to 70.3%, with a corresponding jC2H4 of -226.18 mA cm-2. Similarly, Kong et
al. developed a tandem catalyst by integrating CoPc with a Cu GDE[122]. Initially, Cu was sputtered onto a
polytetrafluoroethylene (PTFE) substrate, followed by spraying a CoPc-methanol solution onto the Cu
GDE, creating the Cu-CoPc GDE. The optimized Cu-CoPc GDE exhibited excellent performance for CO2
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Figure 9. (A) Rate of C2+ production on Au-Cu and Cu electrodes; (B) Dependence of the molar ratio of C2+ alcohols to hydrocarbons
and of (C) alcohols to (CO + > 2e- products) on Au-Cu and Cu as a function of potential; (D) Diagram depicting phase transformation
and structural reconstruction of Au-Cu during the CO2RR process. This figure is quoted with permission from Zhu et al. Copyright
(2022) Elsevier[115]; (E) Total current density of Cu, Au@Cu core-shell, and Au-Cu Janus electrocatalysts; (F) FEC2+ of three different
catalysts. (G) Comparison of optimal FEC2+ obtained by using Au@Cu core-shell, Au-Cu Janus, and Cu NPs; (H) jC2+ of Au@Cu
core-shell, Au-Cu Janus, and Cu NPs. This figure is quoted with permission from Zheng et al. Copyright (2022) John Wiley and Sons[117].
FE: Faradaic efficiency.
conversion to C2+ products, resulting from the high *CO coverage facilitated by the CO-producing CoPc.
Specifically, the Cu-CoPc GDE achieved a FE of 82% for C2+ products at an applied current density of
480 mA cm-2, which is 1.8 times higher than that of the Cu GDE alone. Min et al. utilized metal porphyrins,
recognized for their efficiency in CO2 adsorption and *CO intermediate formation[123]. Using a liquid-phase
approach, they developed self-supporting tandem catalysts by modifying a cuprous oxide NW array
supported on copper mesh decorated with cobalt(II) tetraphenylporphyrin (CoTPP) molecules, resulting in
the CoTPP-Cu2O@CM catalyst [Figure 10A]. The optimized CoTPP-Cu2O@CM catalyst demonstrated
enhanced C2+ selectivity in an H-cell, achieving a FE of 73.2% for C2+ products and a current density of
-52.9 mA cm-2 at -1.1 V vs. RHE, surpassing the performance of recently reported Cu-based catalysts
[Figure 10B and C]. Furthermore, CoTPP-Cu2O@CM catalyst exhibited long-term durability of about 14 h
without significant change in FEC2H4 and current density [Figure 10D].
SACs are a unique type of catalyst with well-defined coordination and distinct electronic structures[124].
Among the SACs, Ni SACs demonstrate excellent conversion of CO2 to CO, making them particularly
appealing for use in tandem catalysts. For instance, Zhang et al. developed a highly C2H4 selectivity tandem
catalyst by simply mixing colloidal Cu NPs and Ni SACs (Cu NPs/Ni-N-C)[125]. The colloidal Cu NPs were
first synthesized using Tetradecylphosphonic acid (TDPA) as a capping agent, allowing precise control over
the size and shape of the NPs. The optimized Cu NPs/Ni-N-C catalyst exhibited excellent performance for
C2H4 production, achieving a FE of 14.52%, a jC2H4 of -1.67 mA cm-2, and a C2H4/CH4 ratio of 20.1 at -0.9 V
vs. RHE. Zhang et al. developed CuO/Ni tandem catalysts, composed of CuO and Ni SAs, aimed at
enhancing C2+ production[126]. They investigated three strategies for fabricating tandem catalytic electrodes:
layer-by-layer spraying, adjacent nanostructures, and physical mixing. Among these, the adjacent
nanostructure electrode demonstrated superior efficiency by promoting more effective CO generation at the
Ni sites, thereby increasing the local CO concentration near the Cu sites and enhancing C2+ product
formation. Consequently, the CuO/Ni SA tandem catalyst attained an impressive jC2+ of 1,220.8 mA cm-2,
along with a high FE for C2+ products at 81.4%. Among the FEC2+, FE towards C2H4 resulted in 54.1% and
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Figure 10. (A) Fabrication of CoTPP-Cu2O@CM NWs array; (B) FEC2+ of CoTPP-Cu2O@CM, Cu2O@CM, CoTPP-CM, and Cu mesh; (C)
jC2+ of CoTPP-Cu2O@CM, Cu2O@CM, CoTPP-CM, and Cu mesh; (D) Stability test of CoTPP-Cu2O@CM for 15 h. This figure is quoted
with permission from Min et al. Copyright (2024) John Wiley and Sons[123]; (E) Graphic representation of PTF(Ni)/Cu; (F) Total current
densities on PTF(Ni)/Cu and PTF/Cu catalysts; (G) FEs for the production of C2H4 and CH4 on PTF(Ni)/Cu and PTF/Cu catalysts at
different potentials; (H) jC2H4 of PTF(Ni)/Cu. This figure is quoted with permission from Meng et al. Copyright (2021) John Wiley and
Sons[128]. CoTPP: cobalt(II) tetraphenylporphyrin; PTF: porphyritic triazine framework.
28.8% for C2H6O. Additionally, Liu et al. developed tandem electrocatalysts featuring nickel SAs, which
exhibited a wide potential window for CO generation when combined with copper (Ni SAC + Cu-R)[127]. To
assess the function of Ni SACs, DFT calculations were conducted to compare the activation barriers for
*OCCO formation, using Ag NPs and Nickel phthalocyanines as control references. To support their
theoretical results, Ni SACs were synthesized through electrostatic self-assembly, while Cu catalysts were
produced using a one-step wet chemical reduction method. The Ni SAC + Cu-R tandem catalysts were then
prepared by thoroughly mixing Ni SAC and Cu-R. The CO2 electroreduction performance was evaluated in
a flow cell with 1M KHCO3 solution. The Ni SAC + Cu-R catalysts showed significantly improved
performance compared to individual Ni SAC and Cu-R catalysts, achieving a jC2H4 of ~370 mA cm-2 for C2H4
with a FE of ~62% and a stability of approximately 14 h at a current density of 500 mA cm-2. Meng et al.
developed a facile approach to construct a tandem catalyst composed of Cu NPs on a porphyritic triazine
framework anchored with atomically isolated nickel-nitrogen sites [PTF(Ni)][128]. PTF(Ni), with abundant
porosity, was fabricated, and Cu NPs were embedded through a reduction reaction step, resulting in
PTF(Ni)/Cu [Figure 10E]. H-type cells with the mixed electrolyte of 0.1 M KHCO3 and 0.1 M KCl saturated
with CO2 were utilized to evaluate the catalytic performance of the catalysts. Compared to PTF/Cu,
PTF(Ni)/Cu demonstrated outstanding performance, achieving a peak FEC2H4 of 57.3% and a jC2H4 of
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3.1 mA cm-2 at -1.1 V vs. RHE [Figure 10F-H]. This suggests that the atomically dispersed Ni sites in
PTF(Ni)/Cu serve a vital function in increasing the selectivity for C2H4 by facilitating the generation of CO,
causing an increased accumulation of *CO intermediate. Similarly, Chen et al. encapsulated Cu NPs in
mesoporous carbon (CMK-8) with doped atomic Ni-N4 moieties, resulting in a-Ni/Cu-NP@CMK[129].
Electrochemical tests indicate that a-Ni/Cu-NP@CMK exhibits remarkable selectivity for C2H4, achieving a
FEC2H4 of 72.3% at a notable current density of 406.1 mA cm-2 when operated in a flow cell under neutral
conditions. Furthermore, when measured using a membrane electrode assembly (MEA) cell,
a-Ni/Cu-NP@CMK demonstrated excellent stability of over 30 h under the 200 mA cm-2 and a FEC2H4 of 63%
at -2.8 V vs. RHE. Furthermore, the optimized a-Ni/Cu-NP@CMK exhibited superior energy efficiency for
C2H4 production of about 28.3%. The authors suggested that the abundance of CO molecules around the
active sites and the hydrophobic environments contributed to the high performance of a-Ni/Cu-NP@CMK
in C2H4 production.
Non-Cu-based catalysts
Cu is the primary metal known to produce C2+ products; however, recent studies indicate that several
non-Cu-based catalysts can also generate C2+ products with careful design. Paris et al. fabricated a novel
Ni3Al deposited glassy carbon electrode by drop-casting and furnace reduction process[130]. The
as-synthesized electrode successfully converted CO2 into C2+ and C3+ products for the first time, with the
maximum FEs for ethane, ethylene, and 1-propanol reaching 1.7%, 0.4%, and 1.9 ± 0.3%, respectively. Torelli
et al. synthesized several Ni-Ga alloys with distinct phases (NiGa, Ni3Ga, and Ni5Ga3), which can convert
CO2 to C2+ at low overpotentials[131]. The Ni5Ga3 alloy achieved a FE of 1.3% for C2H6 at -0.48 V vs RHE,
whereas Cu-based catalysts only produced CH4 at the same low overpotential. Recently, Ding et al. reported
Sn-based electrocatalysts with superior catalytic performance for CO2 electroreduction to ethanol[132].
Sn-based electrocatalyst is composed of SnS2 NSs and Sn SACs (SnS2/Sn1-O3G). The electrocatalytic
performance for CO2RR was evaluated in an H-cell with 0.5 M KHCO3 as the electrolyte. SnS2/Sn1-O3G
exhibited excellent selectivity toward ethanol, with a FE above 70% over a wide potential window, reaching a
maximum FE of 82.5% for ethanol at -0.9 V vs. RHE. Furthermore, it demonstrated exceptional long-term
stability, maintaining 97% of its initial activity after 100 h. This outstanding performance is primarily
attributed to the dual active centers of Sn and O atoms on Sn1-O3G, which facilitate the formyl-bicarbonate
coupling pathway, thereby enhancing selectivity toward ethanol.
ELECTROCHEMICAL CELLS FOR CO2RR
In addition to catalysts, electrochemical cell design is another crucial factor affecting the CO2 reduction.
There are three types of reactors for the eCO2RR conversion, including an H-type cell, flow cell and MEA
cell, as shown in Figure 11[133]. Notable advancements have been made in the design and fabrication of
electrolytic cells to attain highly efficient C2+ production. Here, we briefly explore the advantages and
disadvantages of each electrolytic cell with examples.
H-type cells
H-type cells are widely utilized for lab-based experiments due to their simple assembly, ease of operation,
and low cost. These have distinct cathode and anode chambers, separated by an ion-exchange membrane.
During the reaction, CO2 gas is bubbled through the aqueous catholyte, where dissolved CO2 molecules are
adsorbed onto the electrocatalyst surfaces and subsequently reduced[134,135]. However, a key limitation of
H-type cells is the poor dissolution of CO2 in aqueous electrolytes, reaching only 0.034 M under ambient
conditions, which restricts CO2 reduction current densities to below 100 mA cm-2. Additionally, other
inherent limitations, such as a small electrode surface area and a significant interelectrode distance, hinder
the ability to meet the increasing demands for C2+ product generation[136-138]. Consequently, H-type cells
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Figure 11. (A) H-cell, (B) flow-cell, and (C) MEA-cell configurations for eCO2RR. This figure is quoted with permission from She et al.
Copyright (2022) John Wiley and Sons[133]. eCO2RR: electrocatalytic CO2 reduction reaction; MEA: membrane electrode assembly.
generally show low selectivity for C2+ products. As an example, Kas et al. described a Cu foil with a supreme
C2H4 FE of 33% at -1.1 V vs. RHE, while Zhang et al. documented Cu monolayer-modified Pd NCs
electrocatalysts, achieving an ethanol FE of 20.4% at -0.46 V vs. RHE, primarily due to intense competition
from HER[27,139].
GDE flow cells
Flow cells have been introduced to address the limitations of H-type cells. A standard flow cell comprises
five primary components: flow field plates, support plates, ion exchange membranes, cathode and anode
GDEs, and electrolytes[140]. Catholyte and anolyte are continuously circulated through the peristaltic pump.
Moreover, CO2 gas directly feeds into the cathode, significantly improving mass transport and boosting
production rates[141-144]. As a result, the thermodynamics and kinetics of CO2RR in flow cells differ
significantly from those in traditional H-type cells, making flow cells more favorable for large-scale
commercial applications[145-147]. An important aspect of flow cells is the GDEs, which improve CO2RR
performance by forming porous hydrophobic channels that facilitate the transport of CO2 gas to the
catalyst-electrolyte interface, thus creating a stable gas-solid-liquid three-phase environment[135,147,148]. GDEs
primarily comprise three layers: the GDL, the current collector (CC), and the CL. The GDL offers numerous
porous channels for efficient CO2 transport and serves as a stable support for the catalyst[149]. The CC
enables efficient electron transport and reduces internal resistance, while the CL serves as the primary
reaction zone, where the gas-solid-liquid three-phase interface is established and CO2RR primarily
occurs[150]. Owing to these advantages, the selectivity for C2+ is higher than for an H-cell. For example, Wang
et al. developed a single-atom Ga catalyst anchored on F-doped Cu2O mesoporous catalyst (Ga1-F/Cu2O),
and assessed its CO2RR performance in a flow cell, as depicted in Figure 12A[151]. Using flow cells, the
Ga1-F/Cu2O demonstrated a maximum FE of 72.8 ± 3.2% for C2+ with a C2+ products yield rate of
1.36 mmol h-1 cm-2 at a current density of 600 mA cm-2 [Figure 12B and C]. Furthermore, the
electrochemical CO2 reduction (ECR) performance of 5% B-CuxO was tested in both H-type and flow cells
[Figure 12D and E][152]. In the H-type cell, the optimal FE for C2+ products was 48.44% at -1.0 V vs. RHE. On
the contrary, the FE for C2+ products increased to 55.73% under the same potential in the flow cell. This
significant enhancement in performance with the flow cell is ascribed to better CO2 diffusion and the
suppression of HER, mainly due to the localized gas-electrolyte-catalyst triple-interface configuration.
Alkaline electrolytes are commonly used in flow cells due to their effectiveness in facilitating CO2RR. For
example, electrolytes such as KOH provide abundant OH- ions, which enhance electron transfer to CO2
molecules, reducing the energy barrier for CO2RR and decreasing the overpotential required for CO2
reduction. Furthermore, the high concentration of OH- ions reduces H* availability, effectively suppressing
HER. However, a major drawback of alkaline electrolytes is their reaction with CO2 gas to form bicarbonate,
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35
Figure 12. (A) Flow cell used in electrochemical tests; (B) FE values of CO2RR products on Ga1-F/Cu2O-2; (C) C2+ product yield rates at
different applied current densities of F/Cu2O, Ga1-F/Cu2O-1, Ga1-F/Cu2O-2, and Ga1-F/Cu2O-3. This figure is quoted with permission
from Wang et al. Copyright (2024) John Wiley and Sons[151]; (D) FEs of H 2, C1, and C2+ on CuxO and various B-CuxO samples measured
with H-cell; (E) FEs of CO2RR products on CuxO and 5% B-CuxO measured with flow cells. This figure is quoted with permission from
Yang et al. Copyright (2024) Elsevier[152]; (F) FEC2H4 for the 3D CTPI (tetrahydro-phenanthrolinium/SSC-modified Cu NPs on Cu/PTFE)
as well as 3D CI (SSC modified-Cu NPs on Cu/PTFE); (G) jC2H4 for 3D CTPI (tetrahydro-phenanthrolinium/SSC-modified Cu NPs on
Cu/PTFE) and 3D CI (SSC modified-Cu NPs on Cu/PTFE); (H) Full-cell energy efficiency and jC2H4 with different CO2 concentrations; (I)
Durability measurement of 3D CTPI catalyst at 220 mA cm-2.This figure is quoted with permission from Ozden et al. Copyright (2020)
American Chemical Society[166]. FE: Faradaic efficiency; CO2RR: CO2 reduction reaction; NPs: nanoparticles; 3D: three-dimensional;
PTFE: polytetrafluoroethylene; CTPI: catalyst/tetrahydro-phenanthrolinium/ionomer; SSC: short-side-chain.
which can block CO2 transport channels and reduce CO2RR efficiency[153]. To overcome these challenges,
acidic electrolytes have been explored as a potential alternative. Acidic electrolytes, characterized by their
high concentration of H* ions, prevent the reaction between CO2 and the electrolyte, thereby reducing
carbonate formation and mitigating salt precipitation[154]. This results in improved single-pass carbon
efficiency (SPCE). Furthermore, the abundance of protons in acidic electrolytes enhances the adsorption
and activation of CO2 molecules on the electrode surface, accelerating the CO2RR reaction rate and
enhancing current density.
For example, Wang et al. reported a Pd-doped Cu/Cu2O catalyst designed to enhance C-C coupling for
improved C2+ product formation[155]. eCO2RR tests of the Pd-doped Cu/Cu2O catalyst were conducted in an
acidic electrolyte (pH = 2). The optimized catalyst achieved a maximum FE of 64.0% for C2+ products, with a
corresponding C2+ partial current density of 407.1 mA cm-2 at -2.18 V vs. RHE. Additionally, it exhibited a
high single-pass CO2 conversion efficiency of 73.2% and excellent electrochemical stability, maintaining
performance for approximately 150 h.
MEA cells
MEA cells are a promising reactor design for industrial-scale CO2 reduction. They are double-electrode
configurations that do not require a reference electrode[156]. Instead of using potential control as in H-type
cells, MEA cells are regulated by current or voltage. Their configuration is similar to that of flow cells, but
with a key distinction: in MEA cells, the anode and cathode are positioned directly on opposite sides of an
ion-exchange membrane, forming a compact, sandwich-like structure[157-159]. This membrane promotes the
transfer of ions and circumvents products’ crossover. Additionally, this cell design allows CO2 to be
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35
continuously delivered to the cathode and the electrolyte to be supplied to the anode, while eliminating the
need for a separate catholyte at the cathode[133,160,161]. CO2 can be delivered to the cathode in two ways: by
pumping a CO2-saturated solution into the cathodic compartment or by directly introducing CO2 gas, with
or without added humidity[162]. This approach increases the concentration of reactants at the catalyst surface,
resulting in higher reaction rates[163-165]. As a result, the MEA configuration simplifies the overall structure of
the electrolytic cell, reduces mass and electron transfer resistance, and enhances energy efficiency, thereby
positioning it as a more practical alternative for industrial applications. For example, Ozden et al. fabricated
high-performance electrodes with a tetrahydro-phenanthrolinium-modified hierarchical adlayer with
Cu NPs[166]. The As-fabricated electrode achieved 66% of C2H4 FE at partial current densities of over
200 mA cm-2 and full cell energy efficiency of 21% in an MEA cell [Figure 12F-H]. Moreover, the electrolysis
system showed remarkably superior durability of over 100 h of continuous operation at a cell voltage of
3.8 V [Figure 12I]. Despite their potential, MEAs in CO2RR still face several unresolved challenges,
including carbonate salt precipitation, cathode flooding, and the crossover of reaction products. Addressing
these issues remains critical for the development of stable and efficient MEA systems.
Microfluidic electrolytic cells
Microfluidic electrolytic cells (MECs) consist of two chambers for electrolyte flow, separated by GDEs. The
reaction products are carried out of the electrocatalytic cells along with the flowing liquid electrolyte. MECs
provide several benefits, including (1) the ability to conveniently and rapidly adjust operating conditions
and (2) a simple and efficient process for collecting final products[146]. These features make MECs highly
suitable for evaluating catalyst performance under various conditions. However, the lack of separation
between the anode and cathode allows products to migrate in opposite directions under electromagnetic
forces, potentially leading to the oxidation of final products at the anode. This can reduce product selectivity
and decrease the overall energy efficiency of the system.
Solid-state electrolytes electrolyzer
Solid-state electrolyzers represent a novel class of electrochemical CO2 cells, ideal for producing liquid
products with high purity and concentration and have recently attracted significant research attention. In
these systems, the cathode and anode are separated by an anion exchange membrane (AEM) and a cation
exchange membrane (CEM), respectively, while a porous solid electrolyte layer with various functional
groups serves as an intermediate channel[167,168]. The ions generated during the CO2RR process recombine
with protons in this intermediate channel to form the desired liquid product, which is then collected by N2
purging. While this design facilitates easy separation of the liquid products, the inclusion of the solid
electrolyte layer introduces additional resistance, leading to a substantial increase in the overall cell voltage.
IN-SITU
/OPERANDO CHARACTERIZATION TECHNIQUES
Cu-based catalysts undergo changes in their oxidation state during the CO2RR, making catalyst
characterization and performance evaluation quite challenging. To investigate the CO2RR mechanism,
ex-situ techniques have traditionally been used; however, the information gathered from these methods is
limited. To address these limitations, in-situ/operando characterization techniques have gained significant
attention. These methods offer advantages such as real-time monitoring, higher realism, and greater
accuracy, making them more effective for elucidating reaction mechanisms and studying the
structure-reactivity/selectivity relationships of catalysts. In-situ/operando techniques are designed to capture
precise data under actual reaction conditions, enabling the study of dynamic catalyst structures across
multiple time–space scales. This section will focus on key in-situ/operando techniques, including infrared,
Raman, and X-ray absorption spectroscopy (XAS).
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In-situ/operando Fourier transform infrared spectroscopy
Fourier Transform Infrared (FT-IR) spectroscopy utilizes infrared light to induce molecular vibrations,
enabling the identification of chemical compounds through their unique infrared absorption profiles. These
absorption patterns result from dipole moment changes caused by molecular vibrations or rotations within
a solution. In in-situ FT-IR applications, two primary approaches are used: thin-layer mode and attenuated
total reflection (ATR) mode[169]. Thin-layer mode offers simplicity in design and operation but is less
sensitive than ATR mode. ATR mode, while more complex and requiring precise optical alignment,
minimizes interference from the bulk solution, producing spectra with higher clarity and signal-to-noise
ratios. In-situ/Operando FT-IR is a highly effective tool for examining CO2RR, as it directly observes
reaction intermediates and provides detailed insights into their adsorption configurations
[Figure 13A][170,171]. This technique also helps uncover the chemical properties of the system and facilitates
the structural analysis of catalysts. By carefully choosing measurement methods, researchers can better
understand chemical composition, phase transitions, and reaction intermediates, deepening their
knowledge of how catalyst structures influence performance[172-174].
In-situ/operando Raman spectroscopy
In-situ/operando Raman spectroscopy is operated in confocal mode within a chemical reaction cell setup
[Figure 13B]. By utilizing this technique to track changes in the appearance, intensity, or disappearance of
specific Raman peaks, it is possible to observe shifts in oxidation states and infer the underlying reaction
mechanism[175]. A significant challenge in in-situ Raman measurements is enhancing detection limits
without compromising the electrochemical responses in liquid electrolytes. Given the weak signals in
conventional Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS) was developed to offer
higher sensitivity and reduce interference from water when detecting surface metal species[176,177]. In general,
Raman scattering from molecules adsorbed onto the rough surfaces of plasmonic metal nanostructures is
significantly enhanced. However, SERS is constrained by the type and morphology of the material, with
effective enhancement occurring only with specific metals and rough surfaces[178]. Moreover, the technique
has limited spatial resolution because of the excitation wavelength. To address these challenges,
shell-isolated NPs enhanced Raman spectroscopy (SHINERS) has been introduced as an alternative.
SHINERS techniques employ chemically inert and ultra-thin SiO2 or Al2O3 shells to isolate Au or Ag NPs,
which can eliminate problems related to impurity interference, and signal deviation in the SERS field[166].
While in-situ/operando Raman spectroscopy has made significant advancements in the field of CO2RR,
several challenges persist: (1) the liquid flow can interfere with detection, and (2) high current densities may
affect the Raman signal.
In-situ/operando XAS
XAS is also an effective technique for investigating the electronic structure and coordination environment
of materials[179]. It is generally divided into two regions: X-ray absorption near-edge structure, which
provides insights into the electronic structure of atomic orbitals, such as the oxidation state of the element
and symmetry, and extended X-ray absorption fine structure, which helps analyze the local coordination
environment, including the coordination number, coordinating elements, and interatomic distances.
Electrochemical reactions typically involve chemical adsorption and electron transfer, which lead to
structural and oxidation state changes in catalysts. These changes are reversible and challenging to track
accurately with ex-situ characterization methods. In contrast, in-situ/operando XAS offers the advantage of
precisely detecting changes in the oxidation state and local structure of catalyst elements, monitoring
catalyst reconfiguration during electrolysis, and identifying the active sites of the catalysts [Figure 13C][180].
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35
Figure 13. Schematic illustrations of in-situ/operando techniques in CO2RR. (A) In-situ ATR-FT-IR spectroscopy; (B) In-situ/operando
Raman spectroscopy. This figure is quoted with permission from Dutta et al. Copyright (2023) Elsevier[175]. (C) In-situ/operando XAS.
This figure is quoted with permission from Song et al. Copyright (2023) John Wiley and Sons[180]. CO2RR: CO2 reduction reaction; XAS:
X-ray absorption spectroscopy; ATR-FT-IR: attenuated total reflectance Fourier-transform infrared.
CONCLUSION AND FUTURE PERSPECTIVES
Utilizing CO2 reduction in combination with renewable energy sources to generate high-energy density C2+
hydrocarbons presents a promising approach for advancing a sustainable society. In this review, we have
provided an overview of the latest developments that facilitate greater selectivity and higher production
rates for C2+ products, through methods such as catalyst tuning and electrochemical reactor design. For the
catalyst-tuning, we divided into four main strategies including defect engineering, nanostructure design,
surface modifications, and tandem catalysis with various CO-producing metals and provided a detailed
examination of the prevailing methods used. Following this, we explored the design of electrochemical
reactors, covering the commonly used H-cell, flow cell, and MEA cell, along with their key characteristics,
advantages, and disadvantages. Despite the progress made, achieving low-cost and high-efficiency CO2
reduction to high-value products remains a significant challenge. Therefore, we conclude with a discussion
on future research directions and the key obstacles to be addressed: (1) Many advanced and efficient
electrocatalysts possess specialized nanostructures and are typically produced through multi-step processes
on a laboratory scale. However, scaling up these techniques with conventional laboratory methods is often
impractical. Consequently, there is an urgent need for innovative technologies to enable the large-scale
synthesis of Cu-based catalysts with high selectivity, activity, and stability, which will involve careful
selection of electrochemical conditions for industrial-scale CO2 electrocatalytic reduction; (2) Achieving
industrial feasibility for CO2 reduction technology requires electrocatalysts to maintain stability for tens of
thousands of operational hours[181]. However, Cu-based catalysts face significant stability issues, particularly
in aqueous electrolytes, where issues such as high atom mobility, particle aggregation, and structural
degradation often restrict their lifespan to less than 100 h under CO2RR conditions[182]. Consequently,
achieving long-term stability with Cu-based catalysts remains a key challenge. Research into effective
strategies for anchoring Cu-based catalysts on specific supports with strong metal-support interactions is
necessary to preserve their structure and enhance long-term durability; (3) Cu-based electrocatalysts have
demonstrated considerable potential for CO2RR, especially in facilitating the production of valuable C2+
products. Despite extensive research aimed at improving the selectivity of Cu-based catalysts for C2+
products, their performance remains limited. Therefore, the rational design of novel and efficient
electrocatalysts with unique electronic structures and catalytic properties is essential. High-entropy alloys
(HEAs), which consist of multiple metal elements, have emerged as promising candidates for CO2RR due to
their tunable surface compositions and high configurational entropy. Despite their theoretical advantages,
experimental evidence demonstrating the performance of Cu-based HEAs in CO2RR remains limited, with
most studies confined to computational simulations. Experimental validation of Cu-based HEAs for
electrochemical CO2 reduction to C2+ products would not only bridge the gap between computational
studies and practical applications but also pave the way for the design of next-generation catalysts with
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35
enhanced C2+ efficiency and selectivity; (4) From a catalyst perspective, a deeper understanding of the
CO2-to-C2+ reaction mechanisms is still needed. Advancing in situ and in operando characterization
techniques to study the dynamic behavior of Cu-based catalysts, key intermediates, interfaces, and
microenvironments during CO2RR is crucial for identifying the intrinsic factors that promote C2+ formation;
(5) At present, electrochemical cells such as flow cells and MEAs face challenges related to long-term
stability. Key issues include catalyst detachment from the GDL and electrolyte flooding or salt buildup
within the GDL. Addressing these problems requires the advancement of innovative binders with strong
mechanical strength and durability, and the precise design of GDE structures that maintain stability at high
current densities[183]; (6) Finally, innovative electrode and reactor designs are needed to increase
electrochemical CO2 reduction production rates to commercially viable levels (> 200 mA cm-2). However,
significant discrepancies in electrocatalytic performance often arise when transitioning from half-cell
studies to full-cell applications. To bridge this gap and move eCO2RR from lab-scale tests to practical
applications, systematic research is essential to better align the performance of half-cell and full-cell systems.
In summary, while notable advancements have been made, there is still considerable work to be done in
developing high-performance CO2-to-C2+ systems. Continued efforts will be crucial in paving the way
toward a carbon-neutral future.
DECLARATIONS
Authors’ contributions
Proposed the topic of this review: Kim, S. Y.
Prepared the manuscript: Ma, J.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This research was supported by the National Research Foundation of Korea (NRF), funded by the Korean
government (2022M3H4A1A04096380, 2022M3H4A1A01012712).
Conflicts of interest
Kim, S. Y. is an Associate Editor of the journal Energy Materials and was not involved in any steps of
editorial processing, notably including reviewer selection, manuscript handling and decision-making. The
other author declares that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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