Emerging Cu‐Based Tandem Catalytic Systems for CO2 Electroreduction to Multi‐Carbon Products

Conversion of carbon dioxide (CO2) to valuable chemicals and feedstocks through electrochemical reduction holds promise for achieving carbon neutrality and mitigating global warming. C2+ products are of interest due to their higher economic value. Since the CO2 to C2+ conversion process involves multiple steps, tandem catalytic strategies are commonly employed in the design of electrochemical CO2 reduction reaction (CO2RR) catalysts and systems/reactors. Among the diverse catalysts that are capable of reducing CO2 to CO, Cu stands out for more efficiently further converting CO to C2+ products. In this review, the emerging Cu‐based tandem catalysts and their impact on CO2RR performance, focusing on three positional relationships are summarized. It delves into the integration of tandem catalytic strategies into membrane electrolyzers, utilizing catalyst‐coated substrate (CCS) and catalyst‐coated membrane (CCM) technologies. Several typical examples are presented to illustrate this integration. Finally, the challenges and prospects of applying tandem strategies in the development of CO2RR catalysts/systems, as well as their device‐level implementation are indicated.


Introduction
The excessive release of CO 2 has resulted in severe global warming that poses a potentially irreversible threat to human beings. [1]lectrochemical conversion of CO 2 into valuable fuels or organic substances powered by renewable electricity offers promising solutions in terms of reducing environmental pollution caused by CO 2 and curbing the dependency on non-renewable energy DOI: 10.1002/admi.202301049sources. [2]The CO 2 RR process can yield up to 16 by-products, while C 1 products (e.g., CO and formate) are mostly produced catalyzed by the currently most efficient catalysts, with a Faradaic efficiency (FE) of over 95%. [3]However, efficient production of more desirable C 2+ chemicals, which have a higher demand in chemical and energy storage applications, with high selectivity (>90%) is still a significant challenge. [4]This is due to the complex reaction pathway, which involves several CO 2 molecules arriving and adsorbing onto the surface, progressive conversion, and spatial positioning, to form C 2+ products. [5]The network visualization map shown in Figure 1a was drawn by collecting ≈3000 papers on Cu and CO 2 RR from the Web of Science and extracting high-frequency words from their titles and abstracts using a tool for network visualization named VOS viewer.The visualization mapping shows that multi-carbon products with high values are getting a lot of attention, and electrolytes and reactors are also becoming hot research topics.Fortunately, the emerging tandem catalytic strategy offers a solution to the above-mentioned issues.Yet, there is a clear relative shortage of research on tandem catalysts, as demonstrated by the density visualization map (Figure 1b), where the keyword "tandem" is located near the edge of the map.Moreover, the overlay visualization map of tandem catalysts with local magnification (Figure 1c) reveals that the research on tandem catalysts has been mainly focused on multi-carbon products in recent years, and the current density and product selectivity are highly relevant to it.On this basis, it is crucial to promote the application of tandem systems in designing electrocatalytic catalysts and processes for producing value-added multi-carbons.
Why Cu is preferred by CO 2 RR? Due to its strong C = O bond (750 kJ mol −1 ), CO 2 is highly stable and needs a high activation energy for CO 2 RR. [6]In this reaction, the hollow d orbitals of metallic elements can coordinate with CO 2 and catalyze the process by reducing the C = O bonding energy.Several metallic catalysts, such as Au, Ag, Sn, and Cu, have been extensively studied, with Cu being considered the best electrocatalyst for producing hydrocarbons and/or alcohols. [7]Although a variety of catalysts are capable of converting CO 2 to CO (the first step to produce C 2+ by CO 2 RR), efficient multi-carbon product generation requires further CO reduction and coupling, a process that occurs mainly on Cu catalysts.More importantly, the selectivity of CO 2 RR products depends on the adsorption behaviors of intermediates (e.g., *OCHO, *COOH, and *CO).Theoretical studies have proved that the best catalysts should have moderately strong binding energy, while Cu possesses desirable binding energy with most intermediates, enabling further coupling of these intermediates and generation of multi-carbon products.Therefore, Cu is generally acknowledged as a unique catalyst in CO 2 RR. [8]he merits of tandem catalytic system.Figure 2a illustrates the intricate process of converting CO 2 to C 2+ products through a pathway with multiple electrons and various intermediates. [9]enerally, CO 2 is first adsorbed on the catalyst surface, then with the electron transfer and proton transfer under the condition of the applied electric field, C-O bond breaks, and C-H bond forms.Afterward, the intermediates are transferred from the active site favoring CO formation to the Cu active site for C-C coupling.The formed multi-carbon product will finally desorb on the catalyst surface and diffuse into the electrolyte.Tandem catalysts have revolutionized this process by breaking the linear scaling relationship and catalyzing different steps through multiple active sites (Figure 2b). [10]For instance, it is easy for Au or Ag to catalyze the conversion of CO 2 to CO, but CO tends to desorb instead of being further reduced on the Au or Ag surface.Conversely, CO 2 conversion to CO on the Cu surface is slow, whereas Cu enables further reduction of CO to multicarbons. [11]Tandem catalysts can significantly improve the catalytic efficiency if different catalyst species for the corresponding steps have the optimum binding energy for the relevant intermediates, which will facilitate the reaction to proceed in an or-derly manner.In this regard, tandem catalysts are more thermodynamically and kinetically favorable as compared to traditional catalysts. [12]By optimizing the reaction pathway, Cu-based tandem catalysts have shown enhanced CO 2 reduction FE and current density in comparison to single-component Cu catalysts. [13]n addition to the optimization of the reaction pathway, changes in local environments, such as increased density of active sites, increased local CO concentration, and maintenance of pH, as well as synergistic effect triggered by the combination of other elements in the tandem system, also contribute to the conversion of CO 2 to C 2+ products, thus increasing the acquisition of C 2+ products. [14]

Cu-Based Tandem Catalyst
Cu-based tandem catalysts for the electrochemical conversion of CO 2 to value-added C 2+ products are gaining increasing attention, a comparison of the performance of Cu-based catalysts in alkaline and/or neutral environments over the past five years is summarized in Table 1, where the tandem catalysts present excellent efficiency toward the reductive production of C 2+ products and show great potential for application in real electrolyzers/devices.Formation of multi-carbon products requires the use of copper, however, its efficiency is marred by the high overpotential and low selectivity (coupled by the hydrogen evolution side reaction) toward the generation of C 2+ products. [26,58]Therefore, tandem catalysts which generally consist a catalyst A for CO 2 -CO and a Cu catalyst for the sequential CO-C 2+ transition at complementary active sites are developed to address this issue (Figure 2b).It is also worth noting that the C-C coupling step is very sensitive to the structure of the catalyst and the local environment of the active site, [59] while the position of the active sites affects the transport and concentration of intermediates, ultimately impact-ing the activity and selectivity. [60]Therefore, the active site position relationship is the key for tandem catalyst design.As shown in Figure 2c, three spatial architectures are commonly used for developing tandem catalysts: i) catalyst A is bonded to the Cu catalyst surface (building heterogeneous interfaces); ii) catalyst A is mixed with Cu catalyst; and iii) catalyst A and Cu catalyst form a core-shell structure.To avoid ambiguity, we use * plus species to Reproduced with permission. [60]Copyright 2018, Springer Nature.b,c) Schematic diagram of the synthesis of three Ag-Cu JNS-100 and the CO 2 RR mechanism on Ag 65 -Cu 35 JNS-100.Reproduced with permission. [66]Copyright 2022, Wiley-VCH.d,e) Comparison of Faraday efficiency and partial current density for Au NBPs, Cu Cu NSs, the mixture of Au NBPs with Cu NSs, Au NBP@Cu core@shell nanostructures, and Au NS-Cu JNCs, Au NBPS-Cu (JNCs).d)Faraday efficiency; e) Partial current density.f) Schematic diagram of CO 2 RR mechanism of Au-Cu tandem catalysts with different structures.Reproduced with permission. [44]Copyright 2021, Wiley-VCH.g) Reaction process for the reduction of CO 2 to C 2 H 4 on Cu-Ag Tandem catalyst.Reproduced with permission. [49]Copyright 2022, Wiley-VCH.
denote active substances in this paper, e.g., *CO (adsorbed CO.Meanwhile, CO denotes free CO (g).

Catalyst A is Bonded to the Cu Catalyst
Catalyst A bounding to Cu forms a tandem catalyst, the introduction of catalyst A facilitates the conversion of CO 2 to CO, thus leading to the enrichment of CO on the Cu surface and promoting further C-C coupling.Morales-Guio et al. deposited gold nanoparticles onto polycrystalline Cu foil, observing tandem Au-Cu bimetal are more effective in generating C 2+ alcohols than pure copper, gold, and/or Au-Cu alloys, as shown in Figure 3a. [60]e authors discovered that the tandem catalyst enhances the concentration of CO on the Cu surface, leading to the promotion of the C-C coupling step.However, when using Au@Cu coreshell nanoparticles with different thicknesses of Cu shell, the catalyst is not selective for alcohols, but only for hydrocarbons and formate. [61]The study tells a story of how the arrangement of different components in the bimetallic catalyst affects the selectivity of CO 2 RR.On the other hand, the selectivity toward hydrocarbon products also relies on the crystalline facets of catalysts. [62]For instance, Cu(100) is preferred over Cu (111) due to its decreased C-C coupling energy barrier. [63]12c,64] Iyengar et al. investigated Cu-Ag bimetallic catalysts for producing ethanol in CO 2 reduction through a combination of experimental and computational methods.They found that the bimetallic catalyst and structural sensitivity had a significant impact on the electrochemical reaction selectivity.Cu nano-octahedra (Cu oh ) coated with Cu(111) surfaces were favored for methane production, while Cu nanocubes (Cu cub ) coated with Cu(100) surfaces were favored for ethylene production.They created Cu oh -Ag and Cu cub -Ag bimetallic catalysts by combining Cu oh and Cuc ub with Ag nanoparticles to enhance the selective generation of ethanol in CO-enriched environments. [65]a et al. recently synthesized three tandem catalysts by combining Cu with Ag, namely, Ag-Cu Janus (JNS-100) nanostructures, with successful facet modulation (Figure 3b). [66]These JNS-100 were able to selectively reduce CO 2 to C 2+ products, with the highest selectivity of 54% and 72% for C 2 H 4 and other C 2+ products, respectively, on Ag 65 -Cu 35 JNS-100.As is visualized by the catalytic mechanism shown in Figure 3c, the density functional theory calculations reveal that the appropriate electronic structure and tandem effect from CO spillover are responsible for the high selectivity.This work demonstrates an efficient crystal design strategy for Cu-based tandem catalysts.Jia and others proposed a method of synthesizing Au-Cu Janus nanocrystals (JNCs) as tandem catalysts for CO 2 RR through seedmediated growth. [44]Due to the large lattice mismatch between Au nanobipyramids (Au NBPs) and Cu nanodomains, and regulated by an appropriate concentration of surfactants, Cu selectively overgrows on the side of Au nanocrystals, forming the spatially separated Au-Cu heterostructure.As shown in Figure 3d,e, compared with Au NBPs, Cu nanospheres (Cu NSs), the mixture of Au NBPs with Cu NSs, Au NBP@Cu core@shell nanostructures, and Au NS-Cu JNCs, Au NBPS-Cu (JNCs) obviously has the maximum Faraday efficiency and partial current density.In order to explain this phenomenon, Jia et al. proposed the mechanism of C 2 generation for hybrid catalysts of Au NBP and Cu NS, Au@Cu core@shell nanostructured catalysts and Au NBP-Cu JNC catalysts.As shown in Figure 3f, because of the poor solubility of CO and the distance between Au and Cu in the mixture catalysts, which resulted in a weak tandem process, Au and Cu catalyzed individually.And in the core-shell structure, Au is wrapped by Cu, so it hinders the catalytic reaction on Au too.Compared with the former, the spatially separated hybrid nanostructure of Au-Cu JNCs and the close contact of the two domains promoted the crosstalk process and favored the generation of C 2 products.The enhanced catalytic activity of CO 2 reduction to C 2+ products may be attributed not only to the tandem mechanism but also to the formation of a heterogeneous interface and electronic effect. [67]Yang et al. developed a Ag-Cu tandem catalyst by in situ electroreduction of AgI-CuO nanocomplex. [49]As shown in Figure 3g, *CO species on Cu + and Cu 0 sites have opposite charges, causing *CO on Cu + -Cu 0 sites more likelihood to be dimerized.On the other hand, the residual iodine ions on Ag-NPS act as electron modulators, leading to 40% Cu present Cu + status.This accounts for the higher C 2+ product selectivity at −1.0 V versus RHE, and a higher C 2+ product FE of 68.9%.

Catalyst A is Mixed with Cu Catalyst
Generally, Cu will undergo surface reconstruction and structural evolution during CO 2 electrolysis process. [68]58b,69] For instance, Ma et al. identified essential factors for obtaining high CO 2 reduction selectivity by analyzing the impact of the positional relationship between two elements in a bimetallic catalyst on the products. [70]Bimetallic Cu-Pd catalysts with three different atomic distributions were studied, including ordered, disordered, and phase-separated atomic arrangements (Figure 4a).The ordered CuPd catalyst has the highest C 1 product selectivity.The two-phase separated CuPd catalysts exhibit higher selectivity (>60%) for C 2+ products as compared to the mixed-mode Cu-Pd alloy, suggesting a higher probability of dimerization of C 1 intermediates on the catalyst surface with help from adjacent Cu atoms.Observing from the surface valence band spectra shown in Figure 4b, it is clear that the phase-separated CuPd has the lowest d-band center and should have the weakest binding to CO, while the Cu nanoparticles have the highest d-band center and should have the strongest binding to CO.However, the selectivity and activity of the phase-separated CuPd and Cu nanoparticles are similar, suggesting that geometric/structural effects may play a more important role than electronic effects in the selectivity and activity of the alloys toward CO 2 RR.These results imply that the selectivity for different products can be modulated by geometrical alignment.
The utilization of tandem catalysts with the mode of mixed metals in CO 2 RR has not been extensively studied.Through experimentation, Chen et al. integrated a Cu-Ag tandem catalyst into a gas diffusion electrode (Figure 4c), which yields a 4-fold multi-carbon rate enhancement over Cu in a gas diffusion flow cell. [71]As is evidenced by XRD and XPS analysis (Figure 4d), no peak shift of Cu or Ag was detected for the Cu 500 Ag 1000 tandem catalyst before and after electrolysis, implying that the structure of the tandem catalyst remains unchanged and no electron interaction between Ag-Cu throughout the electrolysis process was evidenced.Moreover, the researchers found that there was no significant performance difference between the Cu-Ag tandem catalyst and Cu toward CO reduction reaction (CORR) in a pure CO atmosphere.This proves that only Cu in the Ag-Cu tandem catalyst is responsible for CORR, implying Ag and Cu work independently toward the multi-step CO 2 electroreduction to C 2+ products.Additionally, in pure CO 2 or CO atmospheres, the intrinsic activities of the tandem catalysts toward the generation of C 2+ products are significantly higher than those of Cu alone.These results demonstrate that the CO-enriched microenvironment generated by Ag in tandem catalysts is advantageous in copper-catalyzed multi-carbon production.In order to control the CO transport distance, as shown in Figure 4e Wei et al. designed and fabricated a 3D tandem catalytic electrode with silver nanoparticles deposited at the bottom of Cu nanoneedle arrays. [72]They optimized the spatial structure by controlling the  [70] Copyright 2017, American Chemical Society.c) Schematic representation and mechanism of Cu-Ag tandem catalyst in CO 2 reduction reaction.d) XRD (left) and XPS (right) comparison of Cu 500 Ag 1000 before and after electrolysis.Reproduced with permission. [71]Copyright 2020, Elsevier Ltd. e) Schematic illustration of the structure of Cu needle-Ag catalyst.Reproduced with permission. [72]Copyright 2023, Wiley-VCH.
mass ratio of Cu needles to Ag to promote the formation of C 2+ products.Moreover, this catalytic electrode was suitable for different types of electrolyzers, and the Faraday efficiency of C 2+ at a current density of 350 mA cm −2 was 64% and 70% in H-type electrolyzer and flow cell, respectively.Single-atom catalysts (SACs), a special type of catalyst used in tandem catalysis, are also noteworthy.It is well known that the activity of a catalyst largely depends on the number of active catalytic sites, which makes single-atom catalysts of particular interest due to their advantages of maximum active site exposure and atom utilization.Besides, the well-defined coordination and unique electronic structure of SACs make them appealing as well. [73]It has been observed that the FE of SACs toward the catalytic reduction of CO 2 to CO or formic acid is nearly 100%.However, the application of these SACs in the production of C 2+ products is still in its infancy, one of the biggest obstacles is the single catalytic environment of SACs that is not suitable for catalyzing processes involving complicated transfer of multiple protons, intermediates, and electrons.For instance, Kim et al. demonstrated that the reaction kinetics of *CO-to-*CHO (C 1 pathway) and *CO dimerization (C 2 pathway) are independent of each other, and the generation of C 1 and C 2 products are on different active sites. [74]Nevertheless, Zheng et al. proved that the concentration and configuration of Cu SAC could be tailored to promote the formation of C 2 H 4 through C-C coupling, [75] where the shortening of the distance of adjacent Cu catalytic sites favors the formation of C 2 H 4 by C-C coupling, while the catalytic sites at a greater distance favor the formation of CH 4 (Figure 5a).As the pyrolysis temperature increases, Cu gradually dissociates from the carbon skeleton, resulting in the remaining Cu single atoms being too far apart for favorable C-C coupling, leading to an increase in the CH 4 /C 2 H 4 ratio (Figure 5b).
The combination of SACs and tandem catalysts provides an opportunity for the application of SACs in electrochemical CO 2 conversion to C 2+ products.For instance, SACs (catalyst A) incorporated Cu tandem catalysts have been found to foster the production of C 2+ products by creating a local environment of high CO concentration through catalyst A. Wang et al. observed that the addition of a mixed feed containing CO and CO 2 promotes the CO 2 RR and increases C 2 H 4 yield by incorporating a singleatom catalyst into a Cu catalyst. [75,76]They identified, for the first time, a *CO (from CO)-*CO (from CO 2 ) cross-coupling pathway during CO 2 RR process.Using isotopic labeling C, they decoded the mechanism map in point reduction processes containing CO and CO 2 feeds, which implies the existence of independent adsorption sites for CO 2 and CO feeds (Figure 5c).In the work, Ni-NC SACs with high surface area act as efficient CO generators, and CuO x nanoparticles act as catalysts for facilitating C-C coupling (Figure 5d).They discovered that the local co-donation of CO using tandem catalyst-stimulated CO enrichment alters the environment of the catalytic sites, hence, leading to an improvement in the yield of ethylene.Furthermore, the experimental results indicate that the C 2 H 4 yield on the CuO x /Ni-NC (1:4) tandem catalyst is similar to that on the pure CuO x catalyst, while this value for the CuO x /Ni-NC (1:2) tandem catalyst is twice that of the pure CuO x catalyst, as shown in Figure 5e.

Catalyst A Forming Core-Shell Structure with Cu Catalyst
The core-shell structure is usually described as two catalysts in a position relationship where one is wrapping around another.Two situations are considered for Cu-based core-shell structures: Cu catalyst is in the core position and Cu catalyst is in the shell position.Cao et al. analyzed the mass transfer process of CO on different core-shell structures. [13]For the core-shell catalyst with CO 2 to CO catalyst as the core and porous Cu catalyst as the shell, CO molecules have to diffuse outward to the Cu shell, so the utilization of CO is more efficient.On the contrary, only part of CO can diffuse to the surface of Cu catalyst when Cu is in the core position, which will result in low CO density that is not favorable for C-C coupling to generate C 2+ products.
Generally, increasing CO concentration is beneficial for C-C coupling, whilst CO concentration can be increased by reducing the shell size.Xiong et al. prepared a series of Ag@Cu 2 O coreshell catalysts with adjustable spacing, and the CO molecules generated on the Ag cores diffuse outward to the porous Cu x O shell. [77]Assuming that the rate of CO generation on Ag cores at a given potential is not affected by the Cu x O shell diameter, the local concentration of CO on the Cu x O shell decreases with increasing shell diameter (Figure 6a).When the Ag@Cu 2 O core-shell catalyst has the largest shell diameter, the CO concentration on the shell is not sufficient to achieve CO dimerization, so the main product is methane.Once the shell diameter decreases, higher CO concentration would be beneficial for the dimerization of CO to produce C 2+ products.However, if the core-shell distance is too small, the concentration of CO is too high for the CO molecules to take part in the subsequent reaction.Instead, the generated CO molecules diffuse directly outside the shell, resulting in a high FE of CO and a low yield of C 2+ products.In addition to core-shell space tuning, Cai et al. developed a novel graded interface Ag-Cu catalyst toward CO 2 RR.The formation of a core-shell catalyst is attributed to the arrangement of atoms in the insoluble Ag-Cu composite that creates a graded interface during annealing. [78]igure 6b illustrates the reaction pathway that occurs on a coreshell structured Ag-Cu catalyst, where C 1 intermediates (e.g., CO) initially enrich on the Ag-dominated core coupled with spilling over to the Cu sites through the core-Cu shell interface, and ultimately forming ethanol at the Cu sites.Notably, the multiple interfaces provide unique spaces for the electron transfer between Cu and Ag, hence, accelerating the charge kinetics and promoting ethanol generation.Remarkably, the best-performed Ag-Cu core-shell catalyst presents a FE of 80.2% for C 2+ products and 52.6% for ethanol (Figure 6c), and a current density of 320 mA cm −2 at the cell voltage of −1.0 V (vs RHE).

Application of Tandem Strategies in Membrane Electrolyzers
Considering the real device application, research should not solely focus on the fundamental studies relating to the activity and selectivity of catalysts, but also on integrating the design of catalysts into electrolyzers.In General, there are three types of CO 2 RR cells: H-cells, flow cells, and membrane electrode assemblies (MEAs), as shown from left to right in Figure 7a. [79]In the H-cell, the electrode is completely immersed in the aqueous electrolyte, CO 2 first dissolves into the electrolyte and then diffuses to the electrode surface.However, the solubility of CO 2 in the aqueous electrolyte is low, [80] and the acidic nature of CO 2 makes it impossible to maintain the pH of the electrolyte, which would lead to the occurrence of HER, thus reducing the efficiency of the reaction. [21,81]In addition, the low current density of H-type cells is another critical issue that limits their large-scale application, while flow cells and MEAs are capable of achieving industrially level current densities. [82]In the flow cell, the gas diffusion electrode (GDE) exposes one side to the feed gas, allowing the coexistence of the liquid and gas phases within the catalyst layer (CL).Reproduced with permission. [75]Copyright 2020, American Chemical Society.(c) Diagram of possible dimerization pathways in the CO 2 feed, co-feed, and CO feed.d) CuO x /Ni-NC (1:4) tandem catalyst.e) Comparison plots of C 2 H 4 generation rates from catalysts with different compositions and overpotentials.Reproduced with permission. [76]Copyright 2019, Springer Nature.Figure 6.a) Schematic of CO 2 reduction products of Ag@Cu 2 O core-shell catalysts with different shell diameters.Reproduced with permission. [77]opyright 2021, Wiley-VCH.b) Schematic illustration of tandem catalysis for CO 2 RR to C 2 over core-shell Ag-Cu catalyst.c) Faraday efficiencies (FEs) of the best-performing catalyst (Ag-Cu 5%) for C 1 , C 2 , C 2 H 4 , and C 2 H 5 OH at various applied potentials.Reproduced with permission. [78]Copyright 2022, Elsevier Ltd.
CO 2 can diffuse directly into the CL through the gas diffusion layer, forming a three-phase interface with the catalyst and the electrolyte.Meanwhile, the alkaline electrolyte could maintain a high pH because of the continuous electrolyte circulating.However, CO 2 losses in alkaline and neutral environments by reacting with OH − to form carbonate, resulting in low single-pass CO 2 utilization. [83]Besides, a relatively high resistance exists within the flow cell because of the presence of electrolytes between the cathode and anode, limiting the energy efficiency of this type of device. [84]An MEA features a zero-gap design with an ion exchange membrane sandwiched between the cathode and anode (Figure 7b), whilst liquid electrolyte is circulated through the porous catalyst layers (CLs).As a result, MEA-based cells perform more efficiently than flow cells due to the shorter charge transfer distance and lower ohmic loss. [85]In addition, the use of bipolar membranes (BPM) in MEAs enables the conversion of carbonate to CO 2 , thereby increasing the utilization rate of CO 2 and reducing CO 2 losses. [86]81b,87] Basically, an industrial-level electrolyzer is required to be operated at low cell potential (E cell < 3 V) with a high reaction rate (j > 200 mA cm −2 ) and high selectivity (FE > 80%), whilst keeping long-term stability. [88]At present, efficient reduction of CO 2 is achieved only at the laboratory level, while reaching industriallevel production of multi-carbon products through CO 2 RR process remains challenging.Under the current circumstances, coordinating the transport of CO 2 , water, electrons, and protons, and maintaining pH stability is necessary for CO 2 RR to meet industrial production requirements.As the most important MEA components to mediate these processes, gas diffusion electrodes (GDEs) and ion exchange membranes have received much attention.However, how to utilize the gas flow characteristics in GDEs to improve CO 2 utilization for efficient production of multi-carbon products still remains a challenge for GDE design. [89]The integration of tandem strategy with membrane Reproduced with permission. [79]Copyright 2021, Elsevier Ltd. b) Schematic illustration of catalyst-coated substrate (CCS) technology and catalyst-coated membrane (CCM) technology (MEA as an example).
electrolyzers is supposed to provide new ideas to solve the "alkaline problem" of electrolytes, improving the utilization of CO 2 and FE of multi-carbon products, and hence, enabling further development of industrial-level CO 2 electrolyzers.This chapter will introduce the catalyst-coated substrate (CCS) technology and catalyst-coated membrane (CCM) technology for membrane electrolyzers.The CCS technology is generally fabricated by depositing catalysts on 3D porous substrates for achieving full contact of CO 2 with the CL.As shown in Figure 7b, three depositing scenarios, namely, mixed distribution, layer-by-layer deposition, and segmented design, are generally applied in the CCS technology.On the contrary, the CCM technology is a direct coating of active catalysts on membranes.By reducing the distance between the catalyst and membrane, and/or designing the interface between membrane and catalyst, the ion transport distance within the cell can be effectively reduced, and thus, the catalytic efficiency would be improved.Finally, the design of tandem catalytic systems by connecting two or more reactors/electrolyzers is also demonstrated to further highlight the importance and effectiveness of tandem strategies in electrochemical CO 2 reduction to manufacture high-value-added multi-carbon products.

Catalyst-Coated Substrate Technology
Most electrolytic cells are equipped with GDEs and the design of new electrodes with advanced structures is one of the keys for improving selective C 2+ yields.As shown in Figure 8a, a conventional GDE typically includes a CL and a gas diffusion layer (GDL). [90]The gas-liquid-catalyst interface formed in the GDE  [36] Copyright 2020, Elsevier Ltd. c) Schematic diagram of CO 2 -CO-C 2 H 4 tandem conversion of Ag and Cu catalysts over stacked segmented GDE.d) Schematic diagram of the flow channel geometry and gas concentration variation along the flow channel during CO 2 tandem reduction.Reproduced with permission. [53]Copyright 2022, Springer Nature.
is critical to improving cell performance.GDL assembled with porous materials (usually carbon paper) can provide abundant CO 2 channels and ensure rapid diffusion of electrolytes. [91]Tandem catalysts consist of multiple components providing a large structure design space for GDEs.When they are deposited on the GDL, the gas-phase products generated by the electrolyte-side catalyst can diffuse through the electrode-side catalyst to reach the GDE gas channel and further to be collected.Moreover, the spatial optimization of the gas flow using the GDE enables the CO 2 RR to take place on the matched catalysts sequentially and control the dynamics of the reaction intermediates, then hence, achieving the maximum utilization of CO 2 .
To improve the utilization of CO, Zhang et al. designed a tandem electrode with a layer-by-layer structure by adding a ZnO layer laterally to the Cu CL (Figure 8b). [36]This spatial design significantly contributes to improved CO utilization with a 3.4fold increase in the FE and a 1.8-fold improvement of the partial current density toward the C 2+ formation, as compared to the bare Cu electrode.Effective management of CO intermediates is critical to facilitate the conversion of CO 2 to C 2+ products, while both CO utilization and C 2+ formation FE will decrease when the CO production rate exceeds the C-C coupling rate. [71,92]Therefore, maximizing the FE of the C 2+ formation requires balancing the CO production and consumption rates. [36,42,93]As shown in Figure 8c, Zhang et al. designed another simple and effective longitudinal segmented gas diffusion electrode (S-GDE) to improve the utilization of CO. [53] They placed a section of Ag GL at the gas inlet of the S-GDE to regulate the CO utilization and to extend the residence time of the generated CO in the Cu CL section, hence, enabling C-C coupling.The flow channel geometry and the gas concentration along the channel for the CO 2 →CO→C 2 H 4 tandem reaction are shown in Figure 8d.By optimizing the relative length and loading of Cu and Ag in the Cu/Ag S-GDE, the residence time of CO in the Cu CL section is maximized, resulting in three times increase in CO utilization as compared to the tandem catalyst Cu/Ag distributing over the entire GDE layer.To select the most compatible CO 2 -CO catalyst with Cu, the researchers compared the performance of S-nGDE consisting of Ag, ZnO, and single-atom catalyst Fe-N-C, respectively, combining with Cu after optimizing the loading, electrode structure, and spatial orientation, and the results show that the Cu/Fe-N-C with S-GDE design exhibits the best performance, with a 90% FE to C 2+ products at a j C2+ exceeding 1 A cm −2 at the cell voltage of 2.89 V, and a half-cell energy efficiency of 40.1forCO 2 -to-C 2 H 4 conversion.This electrode design along a concentration gradient to improve CO utilization in tandem catalysts has shown great potential for industrial applications.

Catalyst-Coated Membrane Technology
Despite its simple and mature fabrication process, the CL of the CCS tends to have poor contact with the membrane, which would reduce the transfer efficiency of ions from the electrode to the membrane and cause their interfacial loss.The CCMequipped MEA can ensure close contact between the CL and the membrane, thus reducing the interfacial resistance and improving cell stability. [90]The research on three main types of membranes, namely, anion exchange membranes (AEMs), cation exchange membranes (CEMs), and bipolar membranes (BPM)) basically focuses on their ion transport capability, chemical stability, and mechanical stability in CO 2 electrolytic cells.Studies relating the application of CCM technology in CO 2 reduction to multi-carbons are very limited, only a few literature have reported the application of CCM technology in CO 2 electro reduction to CO. [94] In this section, insights into the design of CCMs for CO 2 RR are delivered on the basis of water electrolysis and fuel cells.
In the early stages of MEA manufacturing, CCS has been more widely studied because of its simplicity and ability to carry out large-scale production and applications. [95]However, there are problems such as high cost due to high catalyst loading and low mass transport.MEAs fabricated by the catalyst-coated membrane (CCM) method are gradually taking a major role.As compared with CCS, the efficient catalyst utilization of MEAs fabricated by the CCM method can effectively reduce the catalyst loading.Hnát et al. proved that the catalyst loading of CCM-MEA could be reduced by 75% without negatively affecting the performance of alkaline water electrolysis. [96]Apart from that, the tight binding of the CL and membrane layer promotes the electrochemical reaction and reduces the resistive effect of the MEA, thus improving the performance of the cell. [97]For instance, Shahgaldi et al. pointed out that the power density of CCM-MEA was higher than that of CCS-MEA with the same Pt loading. [98]Furthermore, the CCM technology also showed some prospects in the field of CO 2 reduction.94d] The membrane not only functions as a separator to avoid reaction crossover in this reactor but also acts as a cathodic electrolyte, thus avoiding the use of a liquid cathodic electrolyte.As compared to the Sn-GDE, the Sn-CCME has a lower current density, so the production rate of formic acid is also relatively low.However, Sn-CCME assembled reactor achieves higher formic acid concentration with 50% lower energy consumption than that of Sn-GDE assembled reactor.Nevertheless, it remains a challenge to increase the production rate of formic acid using Sn-CCME without increasing energy consumption.
There are also some weaknesses related to CCMs.Proper design and fabrication of CCM are critical to the assembly and performance of MEA.On the one hand, dense catalytic structures could hinder gas/liquid mass transfer, thus leading to poor catalyst utilization. [99]85b] The 3D-ordered structure not only enhances gas/liquid mass transfer but also improves catalyst utilization.Meanwhile, the removal of the ionomer binder improves the electrical conductivity and exposes more active sites.In addition, the interfacial resistance of this CCM structure is also reduced, and hence, the ion migration at the interface is accelerated.This all-in-one MEA with 3D-ordered transport high-ways and integrated CL/membrane interfaces exhibits excellent stability over 1000 h at a high current density of 1000 mA cm −2 at 60 °C for alkaline water electrolysis.It is certainly a valuable reference for the design of CCM-MEA for CO 2 electroreduction processes that require gas-phase mass transfer.In a review article, Breitwieser et al. demonstrated that redesigning the interface between the polymer electrolyte membrane (PEM) and CL can effectively improve the performance of PEM fuel cells by reducing mass transfer resistance. [101]This strategy can be borrowed from the design of CCMs for CO 2 RR in membrane electrolyzers.For instance, adding an additional interlayer between the PEM and CL will increase the active area of the fuel cell as well as enhance the connection between the membrane and the CLs, which in turn will contribute to the durability of the cell.Wang et al. inserted a porous polytetrafluoroethylene (PTFE) layer between the catalytic layer and the proton exchange membrane. [102]This insertion of an intermediate layer not only increases the electrochemical surface area of the cathode but also improves the bonding between the membrane and catalyst, hence, reducing the ohmic loss and improving the mechanical integrity of the MEA.As compared to traditional MEA, the use of PTFE interface results in a three-fold increase in the electrochemical surface area of the cathode and a 20% improvement in MEA performance.
In order to further reduce the internal resistance of the MEA, Klingele et al. deposited Nafion directly onto the cathode and anode catalytic layers using an inkjet printing method, and then heat-press the two electrodes together (Figure 9c). [103]The membrane electrodes fabricated by the above method show a two-fold increase in power density in fuel cells and smaller impedance than those assembled using commercial Nafion membranes.The use of CCM technology can effectively overcome interfacial losses and improve energy efficiency, however, this technique has not been widely used in CO 2 RR due to its complexity [104] Fundamental studies have demonstrated that the microenvironment at the solid/liquid/gas interface has a significant impact on CO 2 RR. [105]Kim et al. developed a two-layer ionomer coating method to regulate the local environment on the catalyst surface (Figure 9d). [106]Different ionomer-coated Cu, namely, Nafion-coated Cu (Naf850/Cu), and Sustainion-coated Cu (Sus/Cu), were obtained by casting the ionomer onto the Cu sputtered PTFE (Cu/PTFE) membranes.Interestingly, the addition of Nafion ionomer to the Sus/Cu surface (Naf850/Sus/Cu) increases the C 2+ yield from 61% to 80% and decreases the H 2 yield from 18% to 5%.It turns out that the two-layer ionomer coating creates a suitable microenvironment for the CO 2 RR, especially when the Nafion ionomer is coated on the outermost layer of the Cu surface (Naf850/Sus/Cu).Because the formed anion exchange ion membrane (Sustainion ionomer coating) increases the solubility of CO 2, and the cation exchange ion membrane (Nafion ionomer coating) promotes the formation of a localized alkaline environment by adsorbing OH − .As a result, the activity and selectivity of Cu toward CO 2 reduction are both improved.In combination with pulsed electrolysis, a further enhancement of the local CO 2 /H 2 O ratio and pH is achieved, and the selectivity for C 2+ products is increased by 250% as compared to the static electrolysis on the bare Cu catalyst.This work demonstrates the contribution of ionomer coating to the catalyst selectivity, but the preparation of a complete ionomer-coated membrane has not yet been achieved. [104,107]Despite being performed  [94d] Copyright 2020, Elsevier Ltd. b) Schematic illustration of the all-in-one MEA structure with 3D-ordered CLs.85b] Copyright 2022, Springer Nature.c) Direct deposition membrane electrode preparation method.Reproduced with permission. [103]Copyright 2015, Royal Society of Chemistry.d) Schematic diagram of the use of ionomer-enhanced CO 2 RR.Reproduced with permission. [106]Copyright 2021, Springer Nature.
at lower current densities, the experience gained from this study lays the groundwork for the application of CCM technology in MEA.

Design of Tandem Electrocatalytic Processes
Although the fundamental research focuses mostly on the design of catalysts for CO 2 reduction reactions, an easily overlooked fundamental chemical problem has become the biggest obstacle to improving the performance of catalysts: the rapid and thermodynamically favorable reaction of CO 2 with hydroxide (OH − ) to form carbonate (CO 3 2− ), which leads to significant voltage and CO 2 losses. [79,108]This problem should be addressed to realize the application of CO 2 reduction in real devices.Rabinowitz et al. demonstrated that a flowing electrolytic cell can achieve the purpose of maintaining the pH of the electrolyte, but the constant input of a source of OH − , which would continuously react with CO 2 , causes unnecessary carbon loss and electrolyte consumption while regenerating the electrolyte requires more energy. [109]herefore, to some extent, the flowing alkaline electrolyzer is a fuel-wasting device (Figure 10a).It is not reliable to use the data of such electrolyzers to analyze the economy of CO 2 reduction in industrial applications. [110]o realize the highly efficient utilization of CO 2 with reasonable electrolyte consumption and energy input, tandem reaction systems designed to facilitate the two-step electrochemical reduction of CO 2 in two separate cells have seen the prospect.Although it is more complex and costly to design, the tandem system is more attractive for overcoming CO 2 loss problem and improving long-term stability.Romero Cuellar et al. designed a tandem system for the two-step electrochemical reduction of CO 2 at high current densities for multi-carbons production. [111]As shown in Figure 10b, the first step of the CO 2 to CO conversion  [109] Copyright 2020, Springer Nature.b) Schematic illustration of the tandem reaction systems for CO 2 RR.Reproduced with permission. [111]Copyright 2020, Elsevier.c) Schematic of a two-step tandem catalytic system for the efficient CO 2 reduction synthesis of n-propanol.Reproduced with permission. [112]Copyright 2022, Wiley-VCH.(d) Schematic illustration of CO 2to-C 4 and CO-to-C 4 systems and gas flow.e) Schematic of electrochemical-thermochemical cascade concept and efficiency definitions.Reproduced with permission. [113]Copyright 2023, Springer Nature.
reaction is carried out in the first flow cell equipped with an Ag CL.Before participating in the dimerizing step, the as-produced gaseous products are introduced into an absorption tower to remove the unreacted CO 2 .Finally, the purified syngas (CO and H 2 ) are fed into a second electrolyzer producing C 2+ products using Cu as catalyst.Notably, the first CO 2 -CO step takes place in a non-alkaline electrolyte and the second CO-C 2+ step occurs in an alkaline environment, thus avoiding the CO 2 loss and heavy KOH consumption problems.The cumulative FE of the multicarbon product is 62% at a total current density of −300 mA cm −2 , which is 30% higher than that of a single-step electrolysis at the same current density.Wu et al. also developed a two-step tandem catalytic system for the efficient synthesis of n-propanol (Figure 10c). [112]3D single-atom nickel (3D Ni-SAG) and multihollow Cu 2 O nanoparticles were used as CO 2 -to-CO and CO-topropanol as catalysts, respectively.The FE of the formation of CO reaches 95.9% in the first step, after separating the unreacted CO 2 by 5 M NaOH, the purified CO is used for the synthesis of propanol in the second step.Remarkably, the FE for n-propanol formation reaches 15.9%, and the corresponding half-cell power conversion efficiency reaches 19.3%.These results demonstrate the feasibility of the tandem system, attributing to the unique advantages of the capability of combining a wide range of catalysts and the easy control of individual steps.
CO 2 conversion to C 2+ products can be realized beyond the use of tandem electrolysis technology.Recently, Lee et al. designed a C 1 -C 2 -C 4 tandem system by connecting an electrochemical reactor and a thermochemical reactor in series (Figure 10d,e). [113]ubsequently, instead of purifying the gaseous products of the first electrolyzer, they upgraded the outlet gas stream by directly introducing them to the second step C 2 H 4 dimerization reactor to produce C 4 H 10 .A copper-based GDE was used to catalyze the electrochemical CO reduction reaction (eCORR), and the Ni-NiO-SiO 2 composite was used for the dimerization of C 2 H 4 .Experiments and theoretical calculations evidence that the simultaneous presence of CO and C 2 can facilitate C 2 H 4 dimerization.In the meantime, increasing CO coverage will facilitate C 2 H 4 dimerization and destabilize the *C 4 H 9 intermediate, hence, hydrogenating the *C x H y adsorbates.As a result, the selectivity of C 4 H 10 is increased to 95%, which is much higher than that of the CO 2 electrolyzer.As compared to existing cascade and single-step electrochemical systems, this electrochemical-thermo-chemical system provides a renewable electricity-powered pathway for selective production of C 4 H 10 under ambient conditions while avoiding the separation of unreacted CO 2 between the two reactors.

Summary, Challenges and Prospective
Although there have been some review articles on similar topics, for instance, a recent review of tandem catalysts for CO 2 RR applications is delivered from a compositional point of view. [114]We have to emphasize that our review article presents the knowledge of tandem catalytic systems, ranging from Cu-based tandem catalyst design, the application of these tandem catalysts in bipolar membrane electrolyzers assisted by CCS and CCM technologies to the tandem catalytic system design, for achieving highly efficient and selective C 2+ products yield.In the end, the conclusion, as well as the challenges and opportunities of the tandem catalytic systems toward electrochemical CO 2 conversion to C 2+ products are proposed: In the part on tandem catalysts, we discussed the effect of different Cu-based tandem architectures on the activity and selectivity of C 2+ products.In the case of catalyst A bonding to Cu, arranging the spatial positions of the two catalysts in accordance with the direction of gas flow greatly improves the utilization of CO and C-C coupling.In this regard, it is suggested to pay more attention to the adjustment of catalyst A content and the design of coverage of catalyst A on Cu catalyst.When catalyst A is mixed with Cu catalyst, the effect of alloying is considered first.However, as is evidenced by the independent functions of Ag and Cu catalysts in the Ag-Cu hybrid tandem system toward CO 2 electroreduction, the geometric structure is believed to play a more significant role in CO 2 RR selectivity than the electronic effect induced by alloying.Therefore, a reasonable design of mixed catalyst A and Cu tandem structure would be beneficial for achieving high selectivity for C 2+ products, whereas keeping Cu atoms close whilst ensuring their dispersion is a challenge.In addition, the combination of SACs with Cu catalysts has also shown considerable advantages if the positional relationship between the SACs and the Cu catalyst is properly modulated.In the case of catalyst A and Cu forming a core-shell structure, it has been proved that Cu in the shell position structure outperforms Cu at the core position structure.More importantly, the design of the space within the core-shell and the modulation of the interface are the keys to keeping the balance between CO production and CO consumption, hence, improving the FE of the formation of C 2+ product.
Considering real device applications at the industrial level, the integration of tandem catalysts into MEAs needs systematic study.The application of tandem catalysts in MEAs has been well implemented using CCS technology.The distribution of the different components of the tandem catalyst is designed to control the dynamics of the reaction intermediates by the direction of mass transfer of the reactants in the GDE.The segmented electrodes designed along the gas concentration gradient show great advantages and prospects for industrial applications.CCM technology has not been well established in CO 2 RR applications, which is mainly due to the lack of chemically and mechanically stable membranes.Therefore, the development of stable and durable membranes with good processability is of urgency.Furthermore, the operation of a single-flow electrolytic cell faces the problem of additional energy consumption due to the requirement of electrolyte regeneration.The development of tandem catalytic systems where at least two reactors are linked not only solves the high energy-consuming problem but also improves CO 2 utilization, bringing CO 2 electrolyzers closer to the industrial level.
Looking ahead, the Cu-based tandem approach is one of the most promising methods to achieve a high yield of C 2+ products through CO 2 electroreduction.However, ensuring long-term stability remains a challenging task when it comes to applying these strategies in industrial-grade devices.In this case, we refer readers to read the articles devoted to the discussion of stability.For example, Lai et al. demonstrate a comprehensive discussion of the stability of CO 2 RR. [115]Liu et al. provide an overview of the development and current status of in situ/operando techniques to study the stability and degradation mechanisms of heterogeneous electrocatalysts, with a focus on the relationship between active site deactivation and degradation. [116]In view of fundamental perspectives, the CO 2 RR process involves multiple electron and proton transfer steps, and the reaction pathways of different intermediates such as *COOH and *OCHO cross each other.The accurate identification of key intermediates is critical to reveal the intrinsic mechanism of tandem catalysis.Therefore, the use of in situ characterization techniques and theoretical modeling in fundamental is significant.For practical applications, the development and optimization of CCS and CCM technologies, and their integration in MEA are of paramount importance to further enhance the efficiency of the CO 2 RR process on the device level.The design of electrode materials and the development of membranes are key challenges in the current research.The breakthrough of CCM technology applied will bring great improvement to CO 2 RR efficiency in MEA.Furthermore, it is imperative to optimize the cell structure and solve the alkaline electrolyte problem.The separation of various mixed products requires additional energy consumption, which also hinders the application of electrocatalytic CO 2 reduction reactions.Therefore, the development of long-term stable Cu-based tandem catalysts with high activity and selectivity, and the integration of tandem catalysts/systems into membrane electrolyzers that can avoid the separation of electrolyte and liquid products have great prospects.All in all, tandem strategies provide an efficient pathway for CO 2 RR catalyst and cell design.Despite facing multiple challenges, with the development of novel catalysts and the continuous optimization of electrolyzers, the green cycle of CO 2 is expected to be achieved in the near future.

Figure 1 .
Figure 1.a) Network visualization map of CO 2 RR, b) Density visualization map, and c) Overlay visualization map of tandem catalyst.

Figure 2 .
Figure 2. a) Possible pathways for the CO 2 RR, b) Tandem reaction mechanism, and c) Schematic diagram of the three spatial positions of the tandem catalyst.

Figure 3 .
Figure3.a) Reduction rates of CO 2 to C 1 and C 2+ products at Cu, Au, and Au/Cu electrodes.Reproduced with permission.[60]Copyright 2018, Springer Nature.b,c) Schematic diagram of the synthesis of three Ag-Cu JNS-100 and the CO 2 RR mechanism on Ag 65 -Cu 35 JNS-100.Reproduced with permission.[66]Copyright 2022, Wiley-VCH.d,e) Comparison of Faraday efficiency and partial current density for Au NBPs, Cu Cu NSs, the mixture of Au NBPs with Cu NSs, Au NBP@Cu core@shell nanostructures, and Au NS-Cu JNCs, Au NBPS-Cu (JNCs).d)Faraday efficiency; e) Partial current density.f) Schematic diagram of CO 2 RR mechanism of Au-Cu tandem catalysts with different structures.Reproduced with permission.[44]Copyright 2021, Wiley-VCH.g) Reaction process for the reduction of CO 2 to C 2 H 4 on Cu-Ag Tandem catalyst.Reproduced with permission.[49]Copyright 2022, Wiley-VCH.

Figure 4 .
Figure 4. a) Illustration of prepared CuPd nanoalloys with different structures.b) Comparison of the surface valence band photoemission spectra of different samples with respect to Fermi energy levels.Reproduced with permission.[70]Copyright 2017, American Chemical Society.c) Schematic representation and mechanism of Cu-Ag tandem catalyst in CO 2 reduction reaction.d) XRD (left) and XPS (right) comparison of Cu 500 Ag 1000 before and after electrolysis.Reproduced with permission.[71]Copyright 2020, Elsevier Ltd. e) Schematic illustration of the structure of Cu needle-Ag catalyst.Reproduced with permission.[72]Copyright 2023, Wiley-VCH.

Figure 5 .
Figure 5. a) Schematic diagram of the products produced at different densities of copper sites in the CO 2 reaction.b) Comparative performance (Faradaic efficiencies, partial current densities of CH 4 and C 2 H 4 , and the ratios of CH 4 /C 2 H 4 ) of catalysts with different pyrolysis temperatures at −1.6 V versus RHE.Reproduced with permission.[75]Copyright 2020, American Chemical Society.(c) Diagram of possible dimerization pathways in the CO 2 feed, co-feed, and CO feed.d) CuO x /Ni-NC (1:4) tandem catalyst.e) Comparison plots of C 2 H 4 generation rates from catalysts with different compositions and overpotentials.Reproduced with permission.[76]Copyright 2019, Springer Nature.

Figure 7 .
Figure 7. a) Schematic configurations of the H-cell, gas diffusion electrode-based flow cell, and membrane electrode assembly (MEA).Reproduced with permission.[79]Copyright 2021, Elsevier Ltd. b) Schematic illustration of catalyst-coated substrate (CCS) technology and catalyst-coated membrane (CCM) technology (MEA as an example).

Figure 8 .
Figure 8. a) Schematic presentation of GDE for CO 2 RR.b) Schematic diagram of the reaction process of Cu&ZnO layer-by-layer catalyst and mixed electrode in series.Reproduced with permission.[36]Copyright 2020, Elsevier Ltd. c) Schematic diagram of CO 2 -CO-C 2 H 4 tandem conversion of Ag and Cu catalysts over stacked segmented GDE.d) Schematic diagram of the flow channel geometry and gas concentration variation along the flow channel during CO 2 tandem reduction.Reproduced with permission.[53]Copyright 2022, Springer Nature.

Figure 9 .
Figure9.a) Electrochemical membrane reactor and electrode configuration.Reproduced with permission.[94d]Copyright 2020, Elsevier Ltd. b) Schematic illustration of the all-in-one MEA structure with 3D-ordered CLs.Reproduced with permission.[85b]Copyright 2022, Springer Nature.c) Direct deposition membrane electrode preparation method.Reproduced with permission.[103]Copyright 2015, Royal Society of Chemistry.d) Schematic diagram of the use of ionomer-enhanced CO 2 RR.Reproduced with permission.[106]Copyright 2021, Springer Nature.

Figure 10 .
Figure 10.a) Schematic diagram of the flow electrolysis cell.Reproduced with permission.[109]Copyright 2020, Springer Nature.b) Schematic illustration of the tandem reaction systems for CO 2 RR.Reproduced with permission.[111]Copyright 2020, Elsevier.c) Schematic of a two-step tandem catalytic system for the efficient CO 2 reduction synthesis of n-propanol.Reproduced with permission.[112]Copyright 2022, Wiley-VCH.(d) Schematic illustration of CO 2to-C 4 and CO-to-C 4 systems and gas flow.e) Schematic of electrochemical-thermochemical cascade concept and efficiency definitions.Reproduced with permission.[113]Copyright 2023, Springer Nature.

Qingqing
Qin is a Master's student in the Faculty of Materials and Manufacturing, at Beijing University of Technology.Her research focuses on the application of tandem catalysts and the design of catalytic electrode structures.Hongli Suo is a distinguished professor and the group leader of the High-temperature Superconductor Lab at the Beijing University of Technology.She worked as a Postdoctoral Researcher in the Department of Condensed Matter Physics and Applied Physics at the University of Geneva, Switzerland.Suo demonstrates a strong multidisciplinary research background in the fields of materials science, metallurgy, chemistry, and condensed matter physics.Her thesis was selected as the top 100 National Excellent Doctoral Dissertations of P.R. China.She has also published more than 200 scientific papers and nearly 140 of them are listed by mainstream ISI Web of Science.Mengmeng Lao obtained her Ph.D. degree in 2020 from the University of Wollongong (UoW), Australia.Then she moved to Switzerland to undertake a CO2 electroreduction project at École polytechnique fédérale de Lausanne (EPFL) in 2021.Since 2022, she has been working as a postdoctoral researcher at the Dutch Institute for Fundamental Energy Research (DIFFER), in the Netherlands.Her current research centers on developing earth-abundant catalysts for anion exchange membrane water electrolyzers.Wei-Hong Lai obtained his Ph.D. in 2019 from UOW and worked as an Associate Research Fellow at the Institute for Superconducting and Electronic Materials (ISEM), the Australian Institute for Innovative Materials (AIIM), UOW.Lai joined the University of Sydney Technology in March 2020 and moved back to UOW in 2022 as a principle investigator.While working and studying during his Ph.D., Lai has carried out research into the universal synthesis and morphological control of diverse materials, and their applications in energy conversion and storage.

Table 1 .
Summary of the performance of Cu-based catalysts over the last 5 years.