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# Toward Complete CO2 Electroconversion: Status, Challenges, and Perspectives

Changfan Xu, Ping Hong, Yulian Dong, Marc Robert, Guosheng Shao, Yong

Lei

► To cite this version:

Changfan Xu, Ping Hong, Yulian Dong, Marc Robert, Guosheng Shao, et al. Toward Complete CO2 Electroconversion: Status, Challenges, and Perspectives. Advanced Energy Materials, 2025, 10.1002/aenm.202406146. hal-05116233

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# Toward Complete CO₂ Electroconversion: Status, Challenges, and Perspectives

Changfan Xu, Ping Hong, Yulian Dong, Marc Robert,* Guosheng Shao,* and Yong Lei*

Electrocatalytic conversion of carbon dioxide (CO₂) into valuable carbon-based fuels and chemicals represents a promising approach to closing the carbon cycle and setting a circular economy. Nevertheless, for current electrocatalytic CO₂ reduction reaction (ECO₂RR) systems, realizing 100% CO₂ conversion with simultaneously high overall CO₂ conversion rate (i.e., single-pass conversion) and high Faradaic efficiency (FE) remains a significant challenge. Enhancing CO₂ conversion rate often results in a decrease in FE, conversely, improving FE may limit the CO₂ conversion rate. Metal–CO₂ (M–CO₂) batteries with CO₂ conversion functions face similar challenges, particularly for reversible M–CO₂ batteries, which do not accomplish net CO₂ reduction because nearly all of CO₂RR products are reoxidized to CO₂ during subsequent charging process. Such electrocatalytic CO₂ conversion system for carbon neutrality poses substantial challenges. This perspective provides an in-depth analysis of state-of-the-art ECO₂RR systems and M–CO₂ batteries, alongside the main strategies employed to address their respective challenges. The critical importance of achieving both a high CO₂ conversion rate and high Faradaic efficiency is underscored for practical applications and to effectively close the carbon cycle. Furthermore, a strategic roadmap that outlines future research directions is presented, thereby facilitating the advancement of comprehensive CO₂ electroconversion technologies.

## 1. Introduction

Setting the path to net-zero carbon dioxide (CO₂) emissions necessitates accelerated decarbonization of energy systems and enhanced efficiency in renewable energy utilization. The electrocatalytic CO₂ reduction reaction (ECO₂RR),[1,2] powered by renewable energy, presents a promising strategy for decarbonization (Figure 1a). At the core of CO₂RR lies a catalyst that converts CO₂

and water into valuable carbon-based fuels and chemicals, including carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), methanol (CH₃OH), ethanol (C₂H₅OH), ethylene (C₂H₄), and other hydrocarbons.[3,4] The feedstock CO₂ can be sourced from industrial processes, biogenic emissions, or directly from the atmosphere. By transforming renewable energy into storable fuels, the ECO₂RR process offers a viable solution to the intermittency challenges associated with renewable energy sources. This approach effectively mitigates discrepancies in electricity supply and demand, both regionally and seasonally. Through this route, ECO₂RR helps to close the carbon cycle, and finally, contributes to a safer environment and an economy compatible with physical limits.

Nevertheless, achieving both a high overall CO₂ conversion rate (i.e., single-pass conversion, SPC) and high Faradaic efficiency (FE) simultaneously remains a significant challenge in existing ECO₂RR systems. Increasing the SPC tends to trigger a considerable decrease in FE, and

vice versa (Figure 1b).[5–19] Most reported ECO₂RR processes exhibit an SPC of less than 60%. This suggests that a significant quantity of unreacted CO₂ is either directly released back into the environment or reacts with OH⁻ to form (bi)carbonate species, which subsequently permeate through the membrane and reconvert to CO₂ on the anode side (i.e., CO₂ crossover), bypassing the electrochemical conversion process (Figure 1c). Despite a few processes achieving SPC exceeding 60%, they often exhibit

C. Xu, P. Hong, Y. Dong, Y. Lei
Fachgebiet Angewandte Nanophysik
Institut für Physik & IMN MacroNano
Technische Universität Ilmenau
98693 Ilmenau, Germany
E-mail: yong.lei@tu-ilmenau.de

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The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202406146

© 2025 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/aenm.202406146

M. Robert
Sorbonne Université

CNRS
Institut Parisien de Chimie Moléculaire
Paris F-75005, France

E-mail: marc.robert@sorbonne-universite.fr

M. Robert
Institut Universitaire de France (IUF)
Paris F-75005, France

G. Shao
School of Materials Science and Engineering
Zhengzhou University
Zhengzhou 450001, China

E-mail: gsshao@zzu.edu.cn
<figure><img src="image_5.png" /><figcaption>**Figure 1.** a) Schematic illustration of a sustainable energy cycle utilizing CO₂ electroconversion. This illustration shows a complete carbon cycle in which waste CO₂ is electrocatalytically converted into useful fuels and chemicals (such as carbon monoxide, hydrocarbons, and oxygenates) using renewable energy sources like solar and wind power. These products are suitable for energy storage and transportation, as well as a wide range of applications in daily life and industry. b) Comparative analysis of reported single-pass conversion (SPC) and Faradaic efficiency (FE) in ECO₂RR based on data from references.[5–19] c) Comprehending the basic trade-off between SPC and FE in ECO₂RR systems, including i) key formulas for calculating SPC, CO₂ crossover, unreacted CO₂, FE, and energy efficiency (EE). ii) Illustrative examples of molar flow rates for CO₂-to-CO conversion, with one scenario highlighting high FE and another emphasizing high SPC. Reproduced with permission.[20] Copyright 2023, Nature Publishing Group. iii) Radar charts comparing the two scenarios, indicating an appropriate balance between high SPC and high FE is vital for bringing ECO₂RR systems toward actual industrial and environmental applications. d) Comparison of nonaqueous and aqueous M–CO₂ batteries with respective function and charge–discharge reactions, respectively.</figcaption></figure>
markedly low FE values. For example, a nearly 90% SPC of CO₂ to CO was observed at a low CO₂ flow rate of 1 sccm; however, the corresponding FE<sub>CO</sub> was only ≈40%.[5] A low FE indicates that only a minor fraction of the input electrical energy is utilized for the production of desired products. The majority of the energy is instead consumed by side reactions, such as hydrogen evolution reaction (HER) competing with CO₂RR, which ultimately results in low overall energy efficiency (EE). Furthermore, an overemphasis on maximizing SPC may result in a reduced concentration of product in the outlet stream. This diminishes the suitability of the product stream for direct use as feedstock in downstream processes and may require additional, energy-intensive separation procedures (Figure 1c).[20] In this scenario, an excessive amount of energy is required to generate only a limited quantity of target products, which is neither a feasible nor a sustainable approach to achieving efficient ECO₂RR. Actually, in order to maintain a steady supply of reactants, the majority of ECO₂RR studies use excess CO₂, which helps to increase FE but regrettably at the expense of SPC. If CO₂ regeneration from the postoperation electrolyte or the environment is required, the energy consumption for this process would exceed the energy stored in the product obtained through electrolysis, which is also not feasible and sustainable. Ideally, any ECO₂RR process (driven by a certain amount of input energy) should maximize its energy conversion to chemical energy by converting as much CO₂ as possible into the desirable products. Moreover, if an ECO₂RR process exhibits a high SPC but necessitates an excessive amount of input electrical energy that significantly surpasses the chemical energy of its CO₂RR products, it cannot be considered a true effective process for contributing decarbonization because of its excess energy consumption. The radar chart visualization highlights the trade-offs between the two scenarios, both of which come with their own specific constraints (Figure 1c). Therefore, a well-established ECO₂RR system for carbon neutrality must have high SPC and high FE simultaneously, which remains a critical challenge in the field.

Additionally, ECO₂RR systems require a stable and continuous power supply, which can be challenging in remote areas where reliable electricity access may not be available. To solve this, the system should be integrated with an energy storage solution, such as a battery. A promising solution is a metal–CO₂ (M–CO₂) battery, which has the capability to store energy during charging and then supply power to CO₂RR during discharge. As shown in Figure 1d, current M–CO₂ batteries can be broadly classified into nonaqueous and aqueous systems. Among these, reversible nonaqueous M–CO₂ (M = Li, Na, and K) batteries are mainly based on the reaction of 4M + 3CO₂ $\leftrightarrow$ 2M₂(CO₃)ₙ + C. These batteries exhibit high operating voltages of 2.80 V versus Li/Li⁺, 2.35 V versus Na/Na⁺, and 2.45 V versus K/K⁺, along with theoretical energy densities of 1876 Wh kg⁻¹ for (Li–CO₂), 1125 Wh kg⁻¹ for (Na–CO₂), and 911 Wh kg⁻¹ for (K–CO₂), respectively,[21] which contribute to their applications in CO₂ reuse and energy storage. However, these batteries do not contribute to a net reduction in CO₂ emissions, as the majority of the CO₂ reduction products generated are reoxidized to CO₂ during the subsequent charging cycle. Alternatively, rechargeable aqueous M–CO₂ batteries, such as Zn–CO₂ and hybrid Li/Na–CO₂ systems, can use metal as an energy storage carrier to store intermittent renewable electricity during charging, while providing power and converting

the CO₂ into valuable carbon-based fuels and chemicals during the discharge process.[22-24] This discharge process of aqueous M–CO₂ batteries offers unique advantages by providing a proton source essential for the multistep hydrogenation pathway, thereby facilitating the conversion of CO₂ into diverse carbon-based compounds, including CO, HCOOH, CH₄, and CH₃OH, which is closely similar to ECO₂RR. Both technologies face many common scientific issues, such as reaction mechanism, catalyst design, electrode and device structure, and system optimization. These aqueous M–CO₂ batteries currently also pose a substantial challenge in simultaneously converting CO₂ at a large scale and producing valuable products with both high SPC and FE. Therefore, further efforts are imperative to improve the efficiency of M–CO₂ batteries for the conversion of CO₂ into high-value chemical products and to facilitate the effective storage and utilization of renewable energy.

In light of the urgent demand for effective carbon management and sustainable energy solutions, this perspective discusses the intricate of effective CO₂ electroconversion in ECO₂RR systems and M–CO₂ batteries. We focus on the major challenge of simultaneously achieving high SPC and high FE, which remains a key bottleneck for the practical application of ECO₂RR technology and M–CO₂ batteries. Through a systematic analysis of state-of-the-art advancements and their limitations, we identify opportunities for innovation and integration in CO₂ electroconversion. Furthermore, we present a forward-looking roadmap that outlines future research priorities and potential innovations essential for bridging the gap between laboratory-scale feasibility and practical application, thereby facilitating the establishment of a sustainable carbon cycle.

## 2. Advancements in ECO₂RR

Over more than a century, ECO₂RR research has achieved major advances. From early investigation in the 1870s using metal electrodes to make formic acid to the landmark discovery in 1986 that copper might accelerate the creation of hydrocarbons, this field has evolved significantly.[25] Catalyst design has evolved from simple bulk metal electrodes, such as zinc, indium, and mercury, to nanoscale catalysts, including single-atom, molecular, and tandem catalysts, which considerably improve reaction efficiency and selectivity.[26-28] Advanced in situ and operando characterization techniques, such as Fourier transform infrared spectroscopy (FTIR), Raman, and X-ray absorption spectroscopy (XAS), have revealed the dynamic evolution of reaction intermediates and structural effects. Moreover, progress in catalyst and electrode structure optimization, electrode-electrolyte interface engineering, and electrolyzer design has significantly enhanced the performance of lab-scale ECO₂RR, especially in terms of current density and the FE of certain specific products such as CO and HCOOH, thus bringing ECO₂RR closer to practical applications. Herein, we present a detailed analysis of the evolution of ECO₂RR systems, methodically assessing these essential characteristics and their connections to the latest advancements in the field (Figure 2).

a) CO₂RR mechanism. Generally, CO₂RR takes place at the complex three-phase interface of solid catalysts, liquid electrolytes, and gaseous CO₂. Insights from theoretical modeling and experimental studies indicate that this process involves three
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Overview of advancements in $ECO_2RR$ systems, spanning a) reaction mechanisms, b) catalyst design and regulation, c) gas diffusion electrodes, d) electrode-electrolyte interfaces, and e) electrolyzer designs, including e1) liquid flow cell electrolyzer, e2) membrane electrode assembly (MEA) electrolyzer, e3) all-solid-state electrolyzer, and e4) $CO_2R-COR$ tandem electrolyzer.</figcaption></figure>

key stages: $CO_2$ chemisorption, surface reactions, and product desorption.[2,3] Proton-coupled electron transfer (PCET) is widely recognized as the fundamental mechanism of $CO_2RR$, wherein protons and electrons participate either independently or cooperatively at each reaction step.[2,29] The PCET process triggers various surface reactions, such as C=O bond cleavage, C-C bond formation, and the creation of C-H or C-O bonds, ultimately resulting in a wide range of products. The selectivity and efficiency of $CO_2RR$ are largely determined by the stability of intermediates formed during PCET, which is greatly impacted by the active sites on various catalyst surfaces. For example, the binding configurations of COOH intermediates (*COOH vs *OCOH) exhibit significant variations across different catalyst materials,

which in turn influence the final product distribution. Specifically, *COOH intermediates on Au, Ag, and Zn surfaces predominantly lead to CO formation, while *OCOH intermediates on Bi, In, and Sn surfaces favor the generation of formate. Moreover, *CO serves as a critical intermediate in $CO_2RR$, which plays a pivotal role in dictating product outcomes. On surfaces where *CO binds weakly (e.g., Au, Ag, Zn), CO readily desorbs as the primary product. On Cu surfaces, however, the moderate *CO binding affinity allows subsequent reactions, such as C-C coupling (*CO-*CO) and deeper hydrogenation, leading to the formation of hydrocarbons (e.g., $CH_4$) and multicarbon products (e.g., $C_2H_4$, $C_2H_5OH$). Within these complicated processes, the HER provides a competitive challenge, lowering the
FE of CO₂RR. Briefly, the product selectivity of CO₂RR is fundamentally connected to how strongly key intermediates (*COOH, *OCHO, *CO, and *H) bind to the catalyst surface. The way these intermediates adhere to the surface influences their subsequent reaction pathways, which in turn dictates the nature and distribution of the final products. Furthermore, kinetic factors such as the solution pH, cation concentration in the electrolyte, and spatial arrangement of active sites further influence product distribution. For example, variations in pH can alter the PCET pathway selectivity, whereas cations affect the adsorption and activation of CO₂ and modify the local solvation network and delivery of protons, as well as local electrical fields,¹⁰,³¹ all of which modify reaction rates and product outcomes.

b) Regulating catalysts. As previously stated, electrocatalysts are essential in determining the final product's distribution and efficiency as well as the binding affinity of CO₂RR intermediates. Significant advancements have been achieved in the development of catalysts that exhibit high activity, enhanced selectivity, long-term stability, and the ability to yield valuable products. Numerous engineering techniques, such as manipulating size and morphology, introducing defects and vacancies, controlling facet orientation or lattice strain, utilizing spatial confinement, and adjusting band center properties, have been used to improve catalytic performance in CO₂RR applications.

Size, morphology, and single-atom. Bulk materials generally exhibit low catalytic performance due to limited surface area, scarce active sites, low metal atom utilization, and competing HER. By altering the size and shape, the surface chemisorption properties can be influenced, leading to substantial enhancements in catalytic activity, stability, and selectivity. Very recently, Kibsgaard's group explored the influence of nanoparticle size on the catalytic selectivity of gold nanoparticles for CO₂ electroreduction (Figure 3a).[³²] Notably, an optimal particle size of ≈3 nm can achieve up to 60% FE at lower potentials. It is remarkable that the particle morphology significantly influences performance, with the multitwin structure demonstrating particular efficacy for CO production. More interestingly, particles with multiple twin structures form unique eightfold coordinated active sites on their surfaces, which are proposed to enhance catalytic activity. These results highlight how CO₂ conversion might be efficiently enhanced by adjusting the size and shape of nanoparticles by increasing the density of high-coordination sites. Notwithstanding this, the well-defined structure of metal nanoclusters (NCs), though highly desirable, faces considerable challenges in terms of stability and catalytic performance. To address this, a co-electropolymerization strategy was employed to integrate single-atom sites into well-defined Au₈ NCs (Figure 3b).[³³] Interestingly, the electronic structure of the Au₈ clusters was changed by the addition of isolated Fe SAs, thereby improving the adsorption of crucial intermediates such as *COOH. It was remarkable that, in comparison to isolated Au₈ clusters, the electrocatalytic activity for CO₂ reduction to CO was boosted by around 18 times. Markedly, integrating SA sites with NCs into a single system can create an asymmetric electron distribution, reducing the free energy of intermediate adsorption and optimizing the catalytic activity of NCs,[³⁴,³⁵] which has proven to be highly effective for enhancing CO₂RR catalytic performance, and significant progress has been made in this area.

Composition. Binary/multimetallic nanoparticles are increasingly important in catalysis because of their ability to fine-tune surface electronic and geometrical properties beyond traditional size, support, or facet effects.[³⁶] For example, Zhong and co-workers[³⁷] used alloying strategies to create a series of Zn-incorporated Cu catalysts to address the inefficiencies of C–C coupling in the conversion of CO₂ to C₂⁺ products. It was found that the incorporation of Zn created neighboring binary sites that facilitated asymmetric CO binding, boosting surface CO coverage and enhancing C–C coupling efficiency. It was shown that the optimized nanointerfaces could suppress HER and improve CO₂ utilization efficiency in mildly acidic conditions, with 31% full-cell C₂⁺ energy efficiency, and 24% single-pass CO₂ conversion in a flow cell system over 150 h. Another typical example is Luo and co-workers[³⁸] developed a low-temperature galvanic replacement method to synthesize nonequilibrium nanosurface alloys (NSAs), addressing challenges in mixing immiscible elements like Cu and Ag while controlling shape, size, and surface composition. This approach effectively prevents thermodynamic phase separation, a common issue in conventional synthesis methods, and enables precise regulation of the morphology, dimensions, and surface composition of the resulting nanoparticles. It can produce nanoparticles with various shapes, such as tetrahedral, cubic, and truncated octahedral structures. Notably, the Cu–Ag(100) and Cu–Ag(111)/Cu–Ag(100) faceted cubic and truncated octahedral-NSAs demonstrated a high FE of ≈58% toward C₂⁺ products, demonstrating shape-dependent selectivity for liquid fuels (Figure 3c). They further enhanced phase stability and product selectivity through the introduction of Pd as a ternary element, an approach that was both informed by supervised machine-learning predictions and subsequently validated experimentally.

Facet and lattice strain. Facet engineering is a recognized method for tailoring catalytic characteristics through alterations in surface atomic structure, intermediate adsorption energies, and reaction mechanisms. It was discovered that the (100) facet of Cu generally exhibits the lowest barrier for *CO dimerization, which favors C₂ product formation.[³⁹,⁴⁰] For example, Cu surfaces with (100) facets exhibit a high selective toward C₂ products, such as ethylene, with C₂H₄:CH₄ ratios significantly greater than those on (111) facets, which predominantly promote the formation of CH₄.[³⁹,⁴⁰] Moreover, the distinctive surface structure of high-index facets, characterized by a high density of steps, kinks, and reactive unsaturated bonds, offers a multitude of active sites that significantly boost catalytic performance.[⁴¹] These structural characteristics facilitate enhanced control over the reaction pathways, thereby improving selectivity for the desired products and enhancing overall efficiency in CO₂RR.[⁴²,⁴³] On the other hand, lattice strain-induced changes in the local electronic structure and coordination of active sites also can significantly influence reaction pathways and product selectivity. For example, strain in catalysts can be induced via dislocation, shape modifications, or defect engineering.[⁴⁴⁻⁴⁶] One such example is the tensile strains in Cu nanocrystals derived from precursors like Cu₂(OH)₂CO₃ and Cu(OH)₂, which improve C₂⁺ selectivity by enhancing *CO hydrogenation and C–C coupling.[⁴⁷] Similar work was also reported by Han and co-workers,[⁴⁸] in which lattice tensile strain in Cu catalysts reduces the dipole–dipole repulsion between adsorbed *CO intermediates, facilitating higher
<figure><img src="image_7.png" /><figcaption>**Figure 3.** a) Structural analysis via HRTEM and simulations reveals single-crystal (1.5 nm) and multiply twinned Au NPs (3.5 and 6.5 nm), with quantified surface coordination sites. Reproduced with permission.[32] Copyright 2024, American Chemical Society. b) Schematic of poly-(Au<sub>8</sub>-DCP@M) catalysts fabricated by co-eletropolymerization. Reproduced with permission.[33] Copyright 2024, Nature Publishing Group. c) Product distribution of the cubic (C), tetrahedral (Th), truncated octahedral (TOh) Cu NPs, and the anisotropic Th-Cu<sub>0.97</sub>Ag<sub>0.03</sub>, C-Cu<sub>0.97</sub>Ag<sub>0.03</sub>, and TOh-Cu<sub>0.97</sub>Ag<sub>0.03</sub> NSAs. Reproduced with permission.[38] Copyright 2024, Nature Publishing Group. d) Adsorption energies of *CO at different *CO coverages on the Cu and 3%-Cu surfaces. e) Time-dependent normalized 1D in situ grazing incidence wide-angle X-ray scattering curves of CuO<sub>TPA</sub>. Reproduced with permission.[48] Copyright 2024, Wiley-VCH. f) p-band center of the corresponding Sn-SACs. Reproduced with permission.[65] Copyright 2024, Elsevier. g) Cu–Ag tandem architecture design. Reproduced with permission.[73] Copyright 2020, Elsevier. h) Schematic of a stacked segmented GDE. Reproduced with permission.[74] Copyright 2022, Nature Publishing Group.</figcaption></figure>
*CO coverage (Figure 3d,e). Concurrently, it promotes *OH adsorption, which further aids in lowering the energy barrier for C–C coupling, achieving an FE for C₂⁺ products of 90.9% at –1.25 V versus RHE.

Defect. Defect engineering is a potent technique for modifying the inherent characteristics of nanomaterials and enhancing catalytic activity. The size of defects (such as point, line, planar, and volumetric defects) has a major impact on the material's electrocatalytic behavior by altering the surface activity.[49] To increase catalytic efficiency, synthesis parameters can carefully regulate many elements such as vacancies, doping impurities, dislocations, grain boundaries, and lattice disorders. For example, defects such as nitrogen doping in carbon-based materials such as CNTs and graphene introduce active sites that enhance the adsorption of CO₂ and intermediate species such as *COOH for CO₂-to-CO*,[50,51] N-doped graphene quantum dots with substantially enriched density of N-doping defects at edge sites catalyze CO₂ into multicarbon hydrocarbons and oxygenates.[52] Similarly, plasma-treated silver catalysts with O vacancies alter the coordination environment and reduce overpotential,[53] the post-treatment heated desulfurization of In₄SnS₈ incorporates S vacancies that promote CO₂ adsorption and activation via modulating the local electronic structure,[54] while defects tune the strong metal–support interactions of Cu–TiO₂ interface resulting in impressive CO productivity of 5350 mmol g⁻¹ h⁻¹.[55]

Confinement effects. The concept of confinement effects has emerged as an innovative approach to improving electrocatalytic performance and can be categorized into three main types: physical confinement, electronic structure perturbation, and reactant enrichment.[56] Physical confinement involves altering mass transfer paths and rates by confining the catalyst's active site to nanoscale spaces like micropores, mesopores, or nanotubes.[57] This may increase reactant concentration near the active site, enhancing reaction rates and selectivity. Electronic structure perturbation occurs when the electronic structure changes of active sites due to nanoconfinement, which will affect catalyst–reactant interactions and alter adsorption and activation energies, thus changing reaction activity and selectivity.[58] Reactant enrichment refers to increased local reactant concentration on the catalyst surface through physicochemical properties of the confined microenvironment (e.g., hydrophilicity, local pH), further boosting reaction rates.[56,58] A typical example is the synthesis of open Cu nanocavity structures with tunable geometries by electroreduction of Cu₂O cavities formed by acid etching. The structures were able to efficiently regulate mass transport, intermediate retention and reactant distribution. During CO electroreduction, the nanocavities exhibited a morphology-driven selective transition from C₂ to C₃ products, which increased the C₃ yield by prolonging the retention time of the C₂ species, and the optimal nanocavity catalysts were able to reduce CO to propanol with an FE of 21% ± 1%.[59] In addition, the confinement effect is produced by the coated nitrogen-doped carbon (N–C) layer on Cu catalyst. The strong electron-loading capacity and confinement properties of the N–C layer promote C–C coupling and increase the reaction energy of HOCCH* deoxygenation, which improves the ethanol selectivity with an FE of 52% and cathodic EE of 31%.[60]

Band center engineering and beyond. The d-band center theory serves as a foundational concept for understanding and predicting the adsorption energies of reaction intermediates on tran-

sition metal catalysts.[61] It links the electronic structure of d-band electrons to the catalytic characteristics of these materials, establishing a basis for enhancing their performance in electrocatalytic applications, such as CO₂RR. Fine-tuning the d-band center enables selective adsorption of desired intermediates while suppressing unwanted pathways, improving both activity and product selectivity.[62–64] In addition, tuning the p-orbital energy levels of main-group metal single-atom catalysts (SACs) provides new insights into improving ECO₂RR efficiency. For example, Zhang and colleagues[65] investigated the correlation between p-orbital energy levels and catalytic activity in ECO₂RR using a series of Sn SACs with varying coordination environments (Sn-N₃S₁, Sn-N₃P₁, Sn-N₃B₁, and Sn-N₄). They found that tuning the coordination environment upshifts the p-band center of Sn, improving intermediate adsorption and catalytic efficiency (Figure 3f). Sn-N₃S₁, with the most positive p-band center, achieved a 100% FE for CO, surpassing other reported SACs. Furthermore, they identified a linear correlation between the p-band center and catalytic performance, thus establishing the p-band center as a predictive descriptor. To be exact, band modulation can be achieved by a variety of methods such as doping, introduction of vacancies, application of strain engineering, alloying, creation of amorphous regions, phase manipulation, single atom dispersion, interface construction, and utilization of metal–support interactions. It is worth noting that in most cases, the effects of these catalyst design and modulation strategies are interrelated, interactively influencing catalytic activity.

c) Designing of gas diffusion electrodes. Gas diffusion electrodes (GDEs) are employed in ECO₂RR to overcome key challenges faced by traditional aqueous-phase systems. These traditional systems are limited by the poor solubility and slow diffusion of CO₂ in water, which hinder the reaction rate. GDEs facilitate the direct introduction of gaseous CO₂ to the catalyst–electrolyte interface, thereby enhancing the mass transport efficiency of reactants and products. This configuration promotes the achievement of high current densities and superior catalytic performance by providing a triple-phase interface of gas, liquid, and solid, which is essential for scaling ECO₂RR to industrial levels.

GDEs have witnessed substantial advancements, with their architectural design and material composition being pivotal in optimizing the ECO₂RR process. A typical GDE comprises a macroporous gas diffusion layer (GDL) for efficient gas transport and a microporous layer treated with hydrophobic polymers like polytetrafluoroethylene (PTFE) to prevent electrolyte flooding.[66,67] Catalyst layers (CLs) are deposited on the GDL to target certain products, and they can be made of a variety of materials such as metals, molecular complexes, or single-atom doped carbons. Maintaining stability remains challenging, especially at high current densities, as the hydrophobic properties of the GDL may deteriorate over time, resulting in flooding and mass transfer limitations. Excitingly, advanced laminated structures, such as PTFE-based GDEs, have shown considerable promise in maintaining both hydrophobicity and catalyst integrity.[68] These arrangements, which sandwich the catalyst layer between PTFE and carbon supports, have proven longer operating lifetimes (e.g., >150 h) and greater mechanical robustness.

Catalyst integration with GDEs has also seen significant innovation. The development of tandem CO₂ reduction
catalysts/electrodes represents a promising strategy to overcome the intrinsic challenges associated with direct CO₂-to-C₂⁺ electroreduction. This approach decouples the reaction into two sequential steps: initial CO₂-to-CO conversion, followed by CO-to-C₂⁺ transformation. For example, advanced bimetallic nanostructures, such as core–shell architectures and surface-decorated nanoparticles, have markedly enhanced the selectivity for ethylene and other multicarbon products.[⁶⁹⁻⁷²] Innovative electrode designs, such as segmented and stacked tandem diffusion electrodes, maintain a CO concentration gradient, allowing for continuous conversion along the electrode length.[⁷³⁻⁷⁵] Recently, a diffusion electrode, composed of a composite of Cu and Ag particles within the CL, has been employed to create a gas environment enriched with CO. This results in a significant enhancement in the production rate of C₂⁺ species, achieving a fourfold increase at a current density of −160 mA cm⁻² compared to using only a bare Cu electrode (Figure 3g).[⁷³] Moreover, by maximizing the residence time of CO and enhancing its interaction with the Cu catalyst, a segmented GDE with Cu/Fe-N–C tandem catalyst layer architecture (Figure 3h) achieved FE exceeding 90% and current densities over 1 A cm⁻² for C₂⁺ products.[⁷⁴] This approach overcomes the limits of direct CO₂ reduction by capitalizing on the high selectivity and reduced overpotential of CO reduction, while also establishing favorable circumstances for subsequent C–C coupling processes.

d) Electrolyte–electrolyte interfaces. The properties of electrolytes, such as the solvent, pH, cations, anions, and ionomers, play crucial roles in determining the activity and selectivity of electrocatalysts. These parameters significantly influence the reaction kinetics, local environment, and overall product distribution. Understanding how these factors influence the CO₂RR process is essential for enhancing performance.

The structure and interactions of water molecules on the catalyst surface have profound impacts on intermediate adsorption, stabilization, and transport. For instance, studies employing explicit solvent models, as used by Rossmeisl and co-workers,[⁷⁶] have shown that water plays a crucial role in stabilizing important intermediates like *OCCO, facilitating the formation of C₂⁺ products.* Chan and colleagues[⁷⁷] demonstrated the unique effects of adsorbate–water interactions on water structuring and solvation, emphasizing their role in CO₂RR dynamics. Additionally, the work of Hirunsit and co-workers[⁷⁸] clarified how water influences key steps like protonation, C–O bond dissociation, and intermediate adsorption during CO₂RR. These findings provide valuable guidance for designing catalysts with enhanced activity and selectivity.

The pH of the solution and the local pH at the electrode–electrolyte interface are critical for modulating CO₂RR pathways. In general, pH levels that are neutral or near-neutral are typically favored to suppress HER while promoting CO₂ reduction. Although alkaline conditions can be advantageous for specific CO₂RR products, they may also lead to carbonate formation, increased resistance, and blockage of gas channels. On the other hand, lower pH levels tend to favor products like methane, while higher pH supports CO and polycarbonate formation.[⁷⁹] Advanced studies have also uncovered the complex relationship between pH and reaction mechanisms. For example, microkinetic modeling has illustrated that local pH affects the energy barriers associated with the formation of C₁ and C₂ products.[³⁰] More-

over, Abild-Pedersen and co-workers[⁸⁰] demonstrated how pH impacts the selectivity between dicarbon oxygenate/hydrocarbon, where higher pH conditions were found to inhibit pathways leading to C₂ hydrocarbon formation. This highlights the complex role of local pH in CO₂RR, as it can have both positive and negative effects depending on the desired products.

In terms of electrolyte composition, the type of cation also plays a significant role in influencing CO₂RR performance. Alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺), with larger ions such as Cs⁺ being more effective, can stabilize reaction intermediates like *CO₂, *CO, and *OCC, thereby enhancing CO₂RR activity.[⁸¹] This effect is attributed to the stabilization of intermediates at the electrode–electrolyte interface, where cations influence the electric field and local pH.[⁸²,⁸³] Moreover, various anions can also enhance CO₂RR efficiency. For instance, halide ions, such as Cl⁻, Br⁻, and I⁻, promote the reduction of CO₂ by facilitating the formation of key intermediates like *COOH on copper-based catalysts.[⁸⁴] Additionally, bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions, which form from CO₂ dissolution, can affect the CO₂ concentration near the electrode. Under alkaline conditions, the concentration of HCO₃⁻ typically reduces, suppressing HER and improving CO₂RR selectivity. However, excessively strong alkaline environments may convert CO₂ into carbonates, which can decrease CO₂ concentrations and hinder CO₂RR performance.[⁸⁵,⁸⁶]

Ionomers are essential in enhancing CO₂RR performance through the modification of the local microenvironment surrounding electrocatalysts.[⁸⁷] Their unique structural features, such as extended hydrophobic backbone chains and charged functional groups, enable them to function as effective binders and interface modifiers. This, in turn, affects ion transport, CO₂ concentration, pH levels, and reaction selectivity. For instance, Nafion-coated Cu electrodes can improve selectivity toward C₂⁺ products by increasing local pH, whereas Sustainion-coated Cu electrodes achieve higher current densities due to better CO₂ transport.[⁸⁸] Functionalized ionomers containing certain chemical groups further improve catalytic efficiency. Anion exchange ionomers containing functional groups such as benzimidazolium tend to simulate the influence of alkali cations in the outer Helmholtz plane, which promotes CO hydrogenation, thereby boosting the production of ethylene.[⁷] Similarly, quaternary ammonium-based ionomers with carbonyl groups can enhance CO₂ activation, resulting in high current densities and selectivity in pure water-fed electrolyzers.[⁸⁹] Collectively, ionic polymers can effectively modify the microenvironment and support high selectivity and activity, which are multifunctional key components that drive the development of ECO₂RR.

e) Electrolyzer. Electrolyzer configuration plays a critical role in determining the reaction rate, EE, and stability of the CO₂RR process. In H-cells, the performance observed can generally be attributed to the catalyst, within the limitations imposed by the system design. However, H-cells are constrained by low current densities, usually below 100 mA cm⁻², due to the limited solubility of CO₂ in the electrolyte. Recent advancements in GDE design and electrolyte management have driven the development of more efficient electrolyzer systems, mainly including flow cells, membrane electrode assembly (MEA), and solid electrolyzers. These systems universally use the OER at the anode and CO₂RR at the
cathode, but they vary in their structural design and methods of electrolyte management.

Liquid-phase flow cell electrolyzers, featuring separate channels for CO₂ gas, catholyte, and anolyte, tackle mass transport challenges by employing GDEs, which enable the separation of reactant and electrolyte transport processes. This configuration enhances current densities by overcoming CO₂ transport limitations. Ion-exchange membranes are used to isolate the electrolyte streams, preventing product crossover and blocking O₂ from accessing the cathode. Nevertheless, these systems encounter issues such as elevated overpotentials caused by ohmic losses in the catholyte and dilution of liquid products by the bulk electrolyte. Additionally, salt precipitation can limit operational stability in flow cell systems. Despite these challenges, liquid-phase electrolyzers have shown significant potential in achieving high current densities and selectivities for valuable products like CO, formate, and multicarbon hydrocarbons.

MEA electrolyzers employ a zero-gap design, with the cathode catalyst in direct contact with a solid polymer electrolyte. Various types of ion-exchange membranes, such as cation-exchange membrane, anion-exchange membrane (AEM), or bipolar membrane (BPM), can be selectively employed based on the desired product. The zero-gap AME design represents a significant step forward in advancing CO₂RR technology. One key advantage of this design is that it eliminates the need for electrolyte pumps and flow fields, simplifying the system and making pressurization easy. Furthermore, removing the catholyte eliminates difficulties such as electrolyte overflow and salt production, hence improving FE, productivity, and stability. Nevertheless, the accumulation of liquid products within the GDE can impede gas diffusion, thereby posing a significant challenge to the maintenance of stability.

Beyond MEA electrolyzers, several innovative approaches to CO₂RR electrolysis are being investigated to address the challenges of mass transport, product purity, low SPC, and operational stability. A startling breakthrough is the emergence of all-solid-state electrolyzers, which address the product purity challenges faced by traditional electrolyzers.⁹⁰⁾ This design uses solid electrolytes that concentrate electrochemically generated cations at the anode and cathode, enabling the formation of pure HCOOH (0.1 M) for ≈100 h without contamination by other ions. In addition, coupled tandem electrolyzer systems have shown significant potential, providing superior kinetic and practical energy benefits compared to single-tank configurations for the direct synthesis of chemicals and fuels from CO₂ feedstock without the need for intermediate separation or purification. Recently, Strasser's group⁹¹⁾ reported a tandem CO₂ electrolyzer cell chain that integrates a Ni-N-C-based CO₂-to-CO₂/CO electrolyzer (cell-1) with a Cu-based CO₂/CO-to-C₂⁺ electrolyzer (cell-2). Under neutral pH conditions, this tandem setup improves ethylene production rates by 50%, alcohol production by 100%, and C₂⁺ energy efficiency by 100% at current densities up to 700 mA cm⁻² compared to single-cell systems. Their research underscores the kinetic and energetic advantages of tandem electrolyzers for producing C₂⁺ chemicals and fuels directly from CO₂ without intermediate separation or purification.

f) Laboratory-scale to real-world performances. To date, laboratory-scale ECO₂RR has made progress in certain properties, driven by advances in catalysts, electrode structures, con-

trol of the local reaction environment at the electrode surface, and electrolyzer design. From laboratory-scale H-type cells and aqueous flow-by cells (AFB) to GDE-type electrolyzers with gaseous CO₂ and neutral electrolytes (GDEN) and with alkaline electrolytes (GDEA), as well as humidified CO₂-fed MEA electrolyzers (GDEM), to flow-through induced dynamic triple-phase boundary (FTDT) electrolyzer,⁹²⁾ current densities have made tremendous breakthroughs to levels that align with commercial benchmarks (Figure 4a). Selectivity and productivity for ECO₂RR products such as CO, formate, ethanol, and ethylene have also improved dramatically. Particularly for CO and formate, FE close to 100% has been achieved at high current densities (Figure 4b). However, the energy efficiency and stability of most reported ECO₂RR systems remain insufficient for commercialization. For instance, during long-term experiments, ECO₂RR systems typically exhibit an EE of 60% or lower. A few systems have been reported to be stable for up to 2000 h,⁹³⁾ with recent demonstrations reaching 5000 h,⁹⁴⁾ but this is far from the ideal feasibility goal (>50 000 h).⁹⁵⁾ At the system level, scaling up ECO₂RR technology poses significant challenges (Figure 4c), such as high selectivity and productivity being achieved with catalyst areas smaller than 10 cm², but these results deteriorate as the catalyst area increases. A breakthrough was reported by Jiao et al.⁹⁶⁾ recently. They presented a tandem CO₂ electrolyzer designed for producing multicarbon products, specifically acetate and ethylene, at the kilowatt (kW) scale (Figure 4d). The authors successfully scaled up a 1000 cm² CO electrolyzer (0.71 kW) and a 500 cm² CO₂ electrolyzer (0.40 kW). The kW-scale CO electrolyzer stack operated at 300 A for 125 h, producing 98 L of 1.2 M acetate at 96% purity, demonstrating resilience against industrial impurities (Figure 4e). A technoeconomic analysis (Figure 4f,g) revealed that power usage is the largest contributor to the production cost of acetic acid (0.79 USD per kg), with the potential for significant cost reduction by improving cell voltage and Faradaic efficiency. With further improvements in auxiliary processes and access to tax credits, the cost could be reduced to 0.35 USD per kg, offering a promising pathway for economically viable acetic acid production via tandem CO₂ electrolysis at an industrial scale. Nevertheless, these dimensions remain considerably smaller than those of mature anion-exchange or proton-exchange membrane water electrolyzers, which feature active areas nearing 3 m².⁹⁷⁾ Ensuring consistent selectivity and further improving stability as the active area increases represent critical challenges at both the catalyst and reactor design levels for scaling up ECO₂RR systems effectively.

Recently, researchers and companies have invested considerable effort in developing pilot-scale systems to advance ECO₂RR technology from laboratory research to industrial applications.¹¹¹⁾ Achieving this goal will require progress in two key areas: first, the establishment of pilot plant demonstrations with detailed performance data; and second, the construction and validation of chemical engineering process models based on lab- and pilot-scale data. Sinton and co-workers¹¹²⁾ developed a semiempirical electrolyzer model in Aspen Custom Modeler and utilized a 5 cm² lab-scale CO₂ electrolyzer for training. They then extended the model to a single cell of 800 cm² and a pilot scale of 10 × 800 cm² stacks and validated the model with experimental results. Combining the electrolyzer model with the pilot-scale data, their work provides a solid basis for further scaling up CO₂
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<figure><img src="image_14.png" /><figcaption>**Figure 4.** a) Reported current density of different electrolyzers utilizing varied CO₂ feedstocks. Reproduced with permission.[92] Copyright 2022, Nature Publishing Group. b) A plot of EE versus FE based on data reported in several articles.[37,60,68,89,98–110] c) Plot of current density versus electrocatalytic cell area and total CO₂ consumed (dashed line). Reproduced with permission.[97] Copyright 2024, Nature Publishing Group. d) A tandem system for acetate and ethylene production. e) Stability of CO electrolysis in a 1000 cm² stack. f) Cost contributions for tandem CO₂ electrolysis in acetic acid production. g) Roadmap for reducing the cost of electrochemical acetic acid synthesis below market price. Reproduced with permission.[96] Copyright 2024, Nature Publishing Group.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Figure 5.** Schematic of challenges in balancing high SPC and high FE in ECO₂RR systems. The illustration highlights the path to high-performance commercialization by addressing key metrics: selectivity (FE), current density, stability, and energy efficiency. Issues are discussed from four aspects: catalyst failure, GDE flooding, (bi)carbonate and product crossover, and anodic oxidation.</figcaption></figure>

treatments[132] also can fine-tune the hydrophobicity of the GDLs to balance the transport of water and the diffusion of CO₂. ii) Improving materials and architectures, for example, graphite/carbon NPs/Cu/PTFE electrodes can effectively mitigate flooding.[68] Furthermore, selecting binders and carbon-based materials that are resistant to oxidative degradation is crucial. In addition, a hollow GDE with a self-supporting diffu-

sion structure can be realized by detailed pore design. This approach eliminates the need for adhesives in the catalytic process and increases the contact area at the gas-liquid-solid interface, thereby improving mass transfer for CO₂-to-CO conversion (Figure 6a).[133] It is important to recognize that GDEs face trade-offs between conductivity and flooding resistance. Carbon paper, although highly conductive, is susceptible to flooding issues.
<figure><img src="image_16.png" /><figcaption>**Figure 6.** a) Silver hollow fiber electrode with hierarchical micro/nanostructures. Reproduced with permission.[133] Copyright 2022, Nature Publishing Group. b) A hierarchically conductive GDE with the ePTFE membrane. The optimal GDE design balances both high hydrophobicity and conductivity. Reproduced with permission.[134] Copyright 2024, Nature Publishing Group. c) Schematic of a PEM electrolyzer for CO₂RR. The SPC and chemical stability of r-Pb catalyst in the PEM reactor are shown, along with stability tests involving start-up/shut-down cycles. Reproduced with permission.[94] Copyright 2024, Nature Publishing Group. d) Operando-formed dual-copper-active-site adlayer on planar copper enhances C₂⁺ product selectivity. Reproduced with permission.[148] Copyright 2024, Nature Publishing Group. e) Integrated electrolyzer coupling CO₂RR with HMFOR. LSV curves of InOOH-O_V show enhanced activity with HMF, and the maximum FE<sub>formate</sub> is closely correlated with the proportion of oxygen vacancies. Reproduced with permission.[157] Copyright 2023, Nature Publishing Group.</figcaption></figure>
Whereas expanded PTFE resists flooding but lacks conductivity, limiting scalability. To overcome these challenges, a hierarchically structured electrode design has been developed, integrating microscale conductors within hydrophobic PTFE (Figure 6b).[134] This design, compatible with various catalysts, achieves ≈75% C₂⁺ FE, reduces cell voltage by up to 0.9 V, and enables scaling to electrode sizes of 50 cm². iii) Microenvironment management, which involves stabilizing the local pH and minimizing (bi)carbonate formation through enhanced CO₂ adsorption and desorption efficiency, thereby preventing localized ionic concentration gradients.[135] iv) Refinement of operating parameters to boost catalyst performance, maintain gas-liquid pressure equilibrium at the GDE interface, and achieve an optimal potential range for ECO₂RR.

c) (Bi)carbonate and product crossover. Current electrolyzer systems that use alkaline or neutral electrolytes experience a considerable crossover of (bis)carbonate species and reaction products.[66,67] This issue mainly arises from the insufficient permselectivity of AEMs, which allows for the transfer of (bis)carbonate and CO₂RR products (e.g., formate) from the cathode to the anode. Thus, this leads to the loss of CO₂ from the anode and reduced product purity. The migration of cations, such as K⁺, from the anode to the cathode leads to the formation of carbonate salts.[136] These salts accumulate in GDEs, impairing CO₂ diffusion and catalyst performance, which ultimately results in GDE failure and diminished electrolysis efficiency. Moreover, lower water uptake can decrease the crossover of (bi)carbonates and other ions, although it may also impair the membrane's conductivity, thereby impacting the overall system performance. Carbonate formation and the associated CO₂ loss impose an unacceptably low upper limit on SPC. In highly alkaline systems (pH > 14), up to 95% of the input CO₂ is entirely lost. Although neutral media MEA electrolyzers exhibit reduced carbonate formation, the SPC for C₁ products remains below 50%, and for C₂ products, it stays below 25%.[137] Additionally, the high energy costs associated with CO₂ separation and electrolyte regeneration further impede efficiency improvements.

A promising approach to reduce (bi)carbonate and product crossover is through the utilization of electrolyzers based on BPMs. The presence of a fixed negative charge within BPM restricts the movement of anionic species like (bi)carbonates, thereby enhancing CO₂ utilization and minimizing product crossover.[138,139] A BPM consists of a cation-exchange layer (CEL) and an anion-exchange layer (AEL), capable of functioning in either forward or reverse bias modes.[137,140] In forward bias configuration (anode//CEL-AEL//cathode), OH⁻ ions from the cathode migrate through the adjacent AEL and recombine with H⁺ ions from the anode at the CEL-AEL interface, creating a neutral or alkaline environment at the cathode for CO₂RR. Conversely, in reverse bias configuration (anode//AEL-CEL//cathode), H₂O is split into H⁺ and OH⁻ ions at the CEL-AEL interface. The H⁺ ions migrate toward the cathode, while OH⁻ ions move to the anode. Regardless of the configuration, BPM-based electrolyzers can achieve higher SPC with low specific energy consumption than AEM-based electrolyzers and have a high potential for the production of C₂⁺ products.[141,142] Current forward-bias BPM-based electrolyzers have the advantage of maintaining an alkaline environment at the cathode (resulting in high FE) and lowering CO₂ losses without significant energy losses. However, the accu-

mulation of H₂O and CO₂ at the CEL-AEL interface can degrade performance by increasing cell potential and damaging the membrane, leading to poor operational stability. A significant challenge in reverse-bias BPM-based electrolyzers is the formation of an acidic cathode environment, which reduces the efficiency of CO₂RR products by promoting competitive HER assisted by an oversupply of H⁺ ions. To establish BPMs as a cutting-edge solution for CO₂ electrolysis, it is imperative to innovate in several key areas to enhance their overall performance. These areas include improving conductivity, accelerating interfacial kinetics, and ensuring superior long-term stability.

Another successful strategy to eliminate (bi)carbonate and product crossover is to conduct ECO₂RR in acidic media. This approach not only minimizes the formation of carbonates and CO₂ crossover but also has the potential to achieve a high SPC of up to 90%,[5,94,143] exceeding that obtained in neutral and alkaline media. Nonetheless, challenges remain in acidic conditions, notably the low selectivity of C₂⁺ and high cell potentials due to the dominance of HER. Very recently, Xia and co-workers[94] developed a proton-exchange membrane (PEM) system for CO₂ reduction to formic acid, using a catalyst derived from waste lead-acid batteries with a lattice carbon activation mechanism (Figure 6c). The system achieves over 93% FE, and ≈91% SPC at 600 mA cm⁻², and operates continuously for over 5200 h, offering significant potential for advancing carbon-neutral technologies. Strategy to improve acidic CO₂RR involves controlling alkali cation concentration/local pH gradients at high current densities to enable C-C coupling to reduce competition with proton reduction.[6,144-146] For example, Sargent's team[145] showed that boosting the concentration of K⁺ ions near electroactive sites can enhance CO₂ activation, thereby enabling efficient CO₂ reduction in acidic conditions. Using copper electrodes at pH < 1, they achieved 77% SPC, with 50% conversion to C₂⁺ at 1.2 A cm⁻² and a full-cell voltage of 4.2 V. Similarly, by utilizing Cu nanoneedles to induce a localized concentration of K⁺ (4.22 M), the solubility limit of K⁺ is surpassed, leading to an efficient C-C coupling process in 3 M KCl at pH = 1.[146] The improvement in catalyst design to boost the rate of CO₂RR also inhibits HER, since CO₂RR decreases the number of protons available at the electrode surface for HER. For example, copper electrodes with a high surface coverage of OH species promote the efficient conversion of CO₂ to ethylene.[147] Tandem electrocatalysis provides an economically viable production route that effectively inhibits HER, mitigates salt precipitation, and enhances both SPC and C₂⁺ selectivity.[143,148] As shown in Figure 6d, Sargent's group,[148] designed a tandem electrocatalyst system to boost the formation of C₂⁺ products under acidic conditions. They constructed dual-copper sites on planar-copper with coordinatively unsaturated configurations, which achieved a 46% ethanol FE and a 91% C₂⁺ FE at 150 mA cm⁻², with 78% SPC and an EE of 30%. It is impressive that this system has a low ethanol crossover (5%), high product concentration (4.5%), and an estimated energy cost of 249 GJ t⁻¹ for ethanol production. Furthermore, the application of an electrically nonconductive nanoporous an electrically nonconductive nanoporous SiC-Nafion layer[149] or organic additive-derived films (N-tolyl pyridinium)[150] on Cu electrodes also has shown great suppression of HER in strongly acidic electrolytes.

d) Anodic oxidation reactions. In ECO₂RR systems, cathodic CO₂RR is conventionally coupled with OER at the anode, which
has significant drawbacks. OER electrocatalysts frequently have low stability and significant overpotentials (over 200 mV), particularly in neutral electrolytes. To get over the high energy barriers and make practical applications possible, OER catalysts must be developed in a stable and effective manner. Emerging OER catalysts, such as Ir- and Ru-based materials and earth-abundant transition metal derivatives, have proven promise in neutral electrolytes. For instance, defect-rich cobalt hydroxides with single-atom Ir,[151] Ca-incorporated Ru-Ir oxides,[152] or Ni-Fe catalysts[153,154] demonstrated reduced overpotentials and enhanced durability. Nevertheless, the scarcity of these materials limits their large-scale practical application. In addition, OER is a highly energy-demanding process, characterized by a high thermodynamic potential (1.23 V), accounting for more than 90% of the total energy consumption in CO₂-to-CO||OER systems.[155] The energy demand for O₂ production frequently diminishes efficiency and profitability. This, in turn, frequently compromises the overall performance and economic feasibility, making it challenging to achieve sustainable operation in such systems. By replacing OER with alternative anodic oxidation reactions that exhibit lower onset potentials and generate valuable products, thus the system's overall efficiency and economic viability can be significantly enhanced. Recently, several alternative oxidation reactions have attracted considerable attention,[156] such as glycerol oxidation reaction, urea oxidation reaction, hydrazine oxidation reaction (HzOR), and methane oxidation reaction (MOR). Nevertheless, certain oxidation processes that yield low-value products such as N₂ or CO₂ are economically unattractive. This has led to a greater emphasis on anodic reactions that generate high-value products or provide environmental advantages. For example, the selective oxidation of biomass-derived compounds, such as converting 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA),[157] offers significant economic benefits by producing valuable platform chemicals. As shown in Figure 6e, the oxygen vacancy-engineered InOOH-OV nanosheets displayed excellent bifunctional electrocatalytic activity with a low onset potential (1.30 V) for HMFOR and excellent performance for CO₂-to-formate, reaching ≈90% cathodic FE and 87.5% anodic FDCA yield. Furthermore, wastewater-based anodic reactions not only facilitate environmental remediation but also effectively couple with CO₂RR.[158,159] Advanced systems that utilize highly oxidative species for pollutant removal or chemical synthesis show considerable potential in combining CO₂RR with alternative anodic processes. These approaches improve energy efficiency and generate value at both electrodes, opening pathways for sustainable and economically feasible CO₂ electrolysis systems.

## 4. Advancements in CO₂ Batteries

The concept of M–CO₂ batteries originated as a derivation of metal–oxygen (M–O₂) battery systems. M–O₂ batteries, which use oxygen as the cathode active gas, possess the advantages of high capacity and energy density. However, it has been gradually found that CO₂ in ambient air has a significant effect on the electrochemical performance of M–O₂ batteries. In 2011, Takechi et al.[160] first exhibited that introducing CO₂ into Li–O₂ batteries enhanced their energy density and discharge capacity. This milestone established the foundation for the development of pure M–

CO₂ batteries. Subsequently, Xu et al.[161] introduced the first Li–CO₂ battery that utilized an activated carbon cathode and ionic liquid electrolyte under moderate conditions in 2013. Using a nonaqueous electrolyte and a Ketjen Black cathode, the rechargeable Li–CO₂ battery at ambient temperature was later reported in 2014.[162] These early investigations have proven that CO₂ plays a crucial role in enabling high-capacity reactions, such as Li₂CO₃ formation during discharge. In contrast to M–O₂ batteries, M–CO₂ batteries combine the two functions of effective CO₂ utilization and energy storage. These batteries are thought to be the ideal choice for energy conversion and storage systems as well as CO₂ capture and utilization technologies, and they have garnered a lot of research interest. Building on these advancements, researchers embarked on the exploration of novel anode materials like sodium (Na), potassium (K), magnesium (Mg), aluminum (Al), and zinc (Zn) to optimize CO₂ conversion and energy output. Among these, the first rechargeable M–CO₂ battery using a Zn anode and an aqueous electrolyte that can produce a hydrogenated product was showcased.[163] This configuration successfully generated formate, encouraging promise for adaptable and effective carbon-based chemical synthesis. As mentioned, nonaqueous and aqueous M–CO₂ batteries are classified according to their distinct anode activities, electrolytes, and applications. Figure 7 summarizes the advancements in research on both nonaqueous and aqueous M–CO₂ batteries, with a focus on the latter’s potential to generate valuable carbon-based compounds while achieving meaningful CO₂ reduction. Energy densities and theoretical voltages of various types of M–CO₂ batteries reported were compared. This field of study has attracted considerable attention in recent years, showing promising prospects for practical applications in energy storage and chemical production.

Nonaqueous M–CO₂ batteries are composed of a metal anode, a porous CO₂-breathing cathode, and a separator with a metal-ion-conducting electrolyte. Their energy storage mechanism depends on CO₂ conversion reactions.[176–178] During the discharge process, the metal anode undergoes oxidation, releasing metal ions (M⁰⁺), which subsequently migrate to the cathode. At the cathode, CO₂ is reduced via CO₂RR to form discharge products. These reactions are reversed by charging, which releases CO₂ at the cathode and regenerates the metal at the anode (i.e., CO₂ evolution reaction, CO₂ER). The CO₂RR proceeds via either a two-electron pathway, forming metal oxalates (M₂(C₂O₄)ₙ), or a four-electron pathway, yielding M₂(CO₃)ₙ and carbon. The four-electron mechanism of 4M + 3CO₂ ↔ 2M₂(CO₃)ₙ + C is widely recognized. The two-electron pathway frequently necessitates the use of specialized catalysts or electrolytes, such as Mo–O coupling in Mo₂C to stabilize Li₂C₂O₄.[179] or to enhance the interaction of Li⁺ with the electrolyte,[180] because the metal oxalate intermediate is usually readily converted to the more stable metal carbonate.

Currently, the research focus of nonaqueous M–CO₂ batteries is to reveal the mechanism of CO₂RR and CO₂ER during discharge and charge through a combination of computational and experimental studies. Ex situ methods, such as scanning electron microscope, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), are widely used to analyze the morphology, structure, and chemical composition of discharging products, while spectroscopic tools like Raman and FTIR identify surface
<figure><img src="image_17.png" /><figcaption>**Figure 7.** a) Significant progress in M–CO₂ battery research, categorizing advancements into two major groups: nonaqueous M–CO₂ batteries (M = Li, Na, K, Al, Mg, Zn) focused on solid-state discharge products, and aqueous M–CO₂ batteries (M = Li, Na, Zn) aimed at producing valuable products such as H₂, CO, HCOOH, CH₄, and CH₃OH. Inset images: primary Li–CO₂ battery. Reproduced with permission.[161] Copyright 2013, Royal Society of Chemistry. First reversible Na–CO₂ battery. Reproduced with permission.[164] Copyright 2016, Wiley-VCH. Reversible Al–CO₂ battery. Reproduced with permission.[165] Copyright 2018, Wiley-VCH. K–CO₂ battery with metal-free carbon. Reproduced with permission.[166] Copyright 2019, Wiley-VCH. Moisture-assisted Mg–CO₂ battery. Reproduced with permission.[167] Copyright 2022, Wiley-VCH. “Rocking Chair” Zn–CO₂ battery. Reproduced with permission.[168] Copyright 2024, Wiley-VCH. Hybrid Na–CO₂ cell produces H₂ and electrical energy from CO₂ dissolution in aqueous solution. Reproduced with permission.[169] Copyright 2018, Elsevier. Reversible Zn–CO₂ batteries via CO₂–HCOOH conversion. Reproduced with permission.[163] Copyright 2018, Wiley-VCH. Zn/Al–CO₂ battery with H₂ production. Reproduced with permission.[170] Copyright 2019, Wiley-VCH. CO₂RR–OER coupled irreversible Zn–CO₂ battery. Reproduced with permission.[24] Copyright 2019, Wiley-VCH. Reversible hybrid aqueous Li–CO₂ battery. Reproduced with permission.[171] Copyright 2020, Wiley-VCH. Zn–CO₂ flow battery generating electricity and CH₄. Reproduced with permission.[172] Copyright 2020, Wiley-VCH. CH₃OH. Reproduced with permission.[173] Copyright 2023, Elsevier. Anode-less hybrid Na–CO₂ battery. Reproduced with permission.[22] Copyright 2023, American Chemical Society. Hybrid Na–CO₂ battery for CO₂–HCOOH conversion. Reproduced with permission.[174] Copyright 2024, Wiley-VCH. Zn–CO₂RR/RMOR batteries. Reproduced with permission.[175] Copyright 2024, Nature Publishing Group. b) Annual publication trends on nonaqueous and aqueous M–CO₂ batteries over the past decade, with data retrieved using the keywords “M–CO₂ battery” (M = Li, Na, K, Al, Mg) and “aqueous M–CO₂ battery” (hybrid Li, hybrid Na, Zn–CO₂ battery) from the Web of Science, data valid through December 2024. c) A comparison of the theoretical voltages and energy densities of aqueous and nonaqueous M–CO₂ batteries.</figcaption></figure>
species.[181] In situ and operando methods, including Raman, differential electrochemical mass spectrometry (DEMS), FTIR, gas chromatography-mass spectrometry (GC-MS), and in situ ambient pressure XPS (APXPS), enable real-time monitoring of discharging products and intermediates, providing insights into formation/decomposition dynamics.[182-185] For example, an in situ study of Na-O₂/CO₂ and Na-O₂ batteries with CNTs or Ag nanowires as the cathode medium was carried out under an advanced aberration-corrected ambient TEM to reveal the structure and morphology of the products during the charging and discharging processes.[186] It is found that in Na-O₂/CO₂-CNT nanocells, the discharge reaction is divided into two main steps: 2Na⁺ + 2e⁻ → O₂ → Na₂O₂ and Na₂O₂ + CO₂ → Na₂CO₃ + O₂, accompanied by Na plating reaction. The charging reaction then proceeds via 2Na₂CO₃ + C → 4Na⁺ + 3CO₂ + 4e⁻. Comparatively, Na-O₂/CO₂-CNT nanocells show better cyclability and performance than Na-O₂/CO₂-Ag and Na-O₂-CNT nanocells, highlighting the importance of carbon and CO₂ in facilitating Na-O₂ nanocells. Similarly, in situ environmental TEM was used to monitor the working mechanism of Na-CO₂ cells.[187] Na-CO₂ nanocells were successfully prepared in CO₂ atmosphere using Pt@NCNT cathode. During the discharge process, Na₂CO₃ microspheres were observed to be formed on the surface of the Pt@NCNT cathode; whereas these Na₂CO₃ microspheres decomposed into metallic Na and CO₂ during the charging process. The above research results provide an important experimental basis for an in-depth understanding of the working mechanism of Na-CO₂ batteries. Furthermore, the synchrotron-based in situ APXPS has been utilized to investigate the CO₂RR on porous carbon surfaces within ionic liquid electrolytes.[188] It is shown that the reduction electrochemical reaction of pure CO₂ on porous carbon electrodes is not active at room temperature, whereas H₂O significantly improves the kinetics. With the help of O₂ in an ionic liquid electrolyte, CO₂RR produced low-valent amorphous carbon and Li₂O₂/Li₂O in Li-CO₂ batteries. The formed amorphous carbon, Li₂O₂, and Li₂O exhibited higher chargeability and faster kinetics compared to Li₂CO₃ oxidation. Peng and co-workers[182] combined in situ isotope labeling spectroscopy, DEMS, and attenuated total reflectance surface-enhanced infrared absorption spectroscopy, the Li-CO₂RR pathway at the model Au|dimethyl sulfoxide interface was investigated. The direct spectroscopic evidence obtained suggests that the major CO₂RR pathways are the CO₂ to CO pathway initiated at low overpotentials (i.e., 2Li⁺ + 2CO₂ + 2e⁻ → CO + Li₂CO₃) and the CO₂ to Li₂C₂O₄ pathway initiated at high overpotentials (i.e., 2Li⁺ + 2CO₂ + 2e⁻ → Li₂C₂O₄). These potential-dependent major CO₂RR pathways depend critically on the coverage of the LiCO₂ intermediate on the electrode surface. In addition, the entire CO₂RR process is accompanied by parasitic reactions that produce gaseous C₂H₄ and possibly LiSOCH₃, with COOH* being the key intermediate. These fundamental insights provide important guidance for reversible nonaqueous Li-CO₂ electro-chemistry.

Notwithstanding the advancements made, several challenges remain that cause discrepancies in observed mechanisms, such as sensitivity to localized conditions, reliance on high current densities, and differences between in situ and normal working conditions. Therefore, in order to improve dependability and get a deeper knowledge of M-CO₂ battery processes, further inves-

tigation is required to integrate complementary techniques under realistic settings. Furthermore, to tackle issues such as low efficiency, high overpotential, and limited cycle stability, efforts have focused on optimizing key components, such as developing advanced cathode catalysts, formulating high-performance electrolytes, designing durable separators, and creating stable anodes. This will be discussed in detail in the next section.

Aqueous M-CO₂ batteries consist of a metal anode, a catalytically active cathode, and a separator, which can be either a solid electrolyte for Li/Na-CO₂ batteries or a bipolar membrane for Zn/Al/Mg-CO₂ batteries, effectively dividing the anolyte and catholyte chambers.[189] Metal ions are released when the metal anode oxidizes during discharge, and CO₂ is reduced at the cathode by a proton-coupled electron transfer process. This process generates carbon-based products (CxH_yO₂), such as HCOOH, CO, CH₄, and CH₃OH, described by the overall discharging reaction: M + CO₂ + H⁺ → CₓHᵧO₂ + Mⁿ⁺. For easily oxidized HCOOH products, the charging reaction involves their reversible decomposition, which releases CO₂ once again. Water oxidation releases oxygen (O₂) during charging for gas or more stable products like CO, CH₄, etc. The overall charging reaction can be summarized as follows: HCOOH → CO₂ + 2H⁺ + 2e⁻ or 2H₂O → O₂(g) + 4H⁺ + 4e⁻. Aqueous M-CO₂ batteries exhibit similarities to ECO₂RR during discharging, wherein the selection of cathode materials directly impacts the reaction pathways and stability of intermediates. Since CO₂ re-release during charging is undesirable from a decarbonization standpoint, catalyst adjustment is crucial to sustaining net CO₂ reduction and enhancing efficiency. For aqueous M-CO₂ batteries, rational catalyst design is critical for stabilizing intermediate products, directing reaction pathways, enhancing CO₂ conversion efficiency, and improving overall battery functionality. Current research primarily focuses on exploring catalysts capable of efficiently facilitating both CO₂RR and OER. Although some catalysts have been reported with promising results, the field remains in its early stages, requiring further innovation and development to achieve practical applications.

**5. Challenges and Strategies in CO₂ Batteries**

Currently, nonaqueous M-CO₂ batteries face significant challenges due to the kinetic limitations of CO₂RR and CO₂ER. These limitations result in high overpotentials, with charging voltages often exceeding 4 V, especially at high current densities. Most reported nonaqueous M-CO₂ systems exhibit substantial drawbacks, including high overpotentials with charging voltages exceeding 4 V, low energy efficiencies (far below the 80% achieved by commercial lithium-ion batteries), restricted cycle life (<1000 cycles), and limited current densities (<0.5 mA g⁻¹). Aqueous M-CO₂ batteries, mainly Zn-CO₂ batteries, have made progress in enhancing FE for CO and formic acid production through catalyst engineering. Nevertheless, such performances still fall short of ECO₂RR. Key limitations include low current densities (<10 mA cm⁻²) due to CO₂ diffusion constraints, low power densities (<10 mW cm⁻²), and limited operational stability (<200 h), as well as low and imbalanced FE and SPC. Toward addressing these challenges, it is important to optimize the CO₂RR/CO₂ER or CO₂RR/OER pathways, as well as to ensure the structural and compositional stability of battery components. Several critical
<figure><img src="image_18.png" /><figcaption>**Figure 8.** Schematic of the challenges for M–CO₂ batteries in practical applications. The diagram highlights critical performance metrics, including current density, stability, EE, and selectivity (FE) with detailed discussions from three main aspects: alkali metal anode, electrolyte, and catalyst cathode.</figcaption></figure>

challenges, including electrode dendrite formation, catalyst deactivation, and electrolyte decomposition, must also be addressed (Figure 8).

## 5.1. Challenges and Strategies in Nonaqueous M–CO₂ Batteries

a) Catalyst cathode. M–CO₂ batteries have been reported to suffer from large polarization, low EE, and actual energy densities that fall significantly short of their theoretical values. A key issue lies in the accumulation of solid carbonate products (M₂(CO₃)ₙ) formed during the discharge process on the electrode surface.[190] For example, lithium carbonate (Li₂CO₃) and sodium carbonate (Na₂CO₃), the main discharge products of Li–CO₂ and Na–CO₂ batteries, respectively, are highly thermodynamically stable and

difficult to decompose during the charging process.[191] Additionally, these products are insoluble in the nonaqueous electrolytes typically used, exacerbating the problem. Excessive accumulation of M₂(CO₃)ₙ not only blocks the porous channels required for the diffusion of Mⁿ⁺ ions and CO₂ but also decreases the electrical conductivity of the electrode and passivates the active surface of the CO₂ electrode. This leads to a significant reduction in discharge capacity. Moreover, the insulating nature of these solid products demands a high charging voltage for decomposition, causing a large charge overpotential. This high overpotential accelerates CO₂ electrode corrosion and electrolyte decomposition, further compromising the cycle life of the battery.

To overcome these challenges, developing a robust cathode is crucial. Such a cathode must enhance both CO₂RR and CO₂ER kinetics while exhibiting high stability to prevent catalyst
degradation during operation, which is essential for achieving efficient and durable M-CO₂ batteries. Advances in cathode catalysts for CO₂ batteries have focused on catalyst optimization, including heteroatom doping, defect engineering, surface functionalization, and optimization of morphology, porosity, and size to enhance catalytic activity and interaction with discharge products. These methods load the obtained solid powder catalysts onto conductive substrates with binders, but they limit the exposure of active sites and hinder efficient mass transport and electron transfer. Therefore, it has become imperative to develop efficient electrodes with advanced structures. Remarkably, Xiao et al.[192] synthesized a 3D NCNT/G structure using a template-assisted chemical vapor deposition method, integrating vertically aligned N-doped carbon nanotubes with graphene layers. This design enhanced CO₂ and electrolyte diffusion, enabled fast electron transfer, and provided high catalytic activity, achieving stable cycling stability of Li-CO₂ battery (>3500 h). Very recently, our group put forward a multiscale defective interface concept via atomic misfit-driven reconfiguration to design a CO₂ cathode (Figure 9a).[193] It is pleasing to note that this self-supporting CO₂ cathode regulates the adsorption/decomposition of intermediates during CO₂RR/CO₂ER and ensures exceptional operation of the Na-CO₂ battery for 2400 cycles (4800 h).

The utilization of a soluble catalyst, known as a redox mediator (RM), could potentially offer a fundamental solution to the challenges associated with insoluble M₂(CO₃)ₙ. As exhibited in Figure 9b, a Li₂C₂O₄-based pathway mediated by a Cu(II) metal-organic framework based on benzene-1,3,5-tricarboxylic acid (RM(II)-BTC) improves Li-CO₂ battery performance via reversible Cu(II)/Cu(I) redox coupling.[194] The RM(II)-BTC mediator facilitates the reduction of CO₂ via Cu(I)-catalyzed dimerization, forming Li₂C₂O₄ as the final product, resulting in a discharge voltage of 2.8 V, a charge potential of 3.7 V, and outstanding cycling stability over 400 cycles. Similarly, through the introduction of cobalt phthalocyanine (CoPc) as a homogeneous catalyst (Figure 9c), a Li-CO₂ battery using a Li/LiTFSI-TEGDME (tetraethylene glycol dimethyl ether)-CoPc/Mo₂C configuration demonstrates enhanced performance with high capacity (18 724 mAh g⁻¹), a low voltage gap (0.83 V), and long cycling life (1600 h).[195]

Beyond internal catalysts, the external environment plays a significant role due to the open structure of cathodes. Factors such as light, photothermal effects, and mechanical forces can influence catalytic activity and stability.[196-198] For example, a photocathode enhances charge transfer and photo energy utilization in a Li-CO₂ battery, achieving high round-trip efficiency (98.8%), low voltage hysteresis (0.04 V), and excellent cycling stability, with potential for wearable applications.[199] It has been shown that defective C₃N₄ efficiently absorbs UV-vis light and generates a large number of photoelectrons/holes. Effective charge transfer between C₃N₄ and CNT facilitates the migration of these charge carriers, promotes CO₂RR/CO₂ER, and prevents recombination (Figure 9d). This increases charge carrier production, reduces recombination, increases light energy utilization, and improves cathode dynamics. Moreover, a force-assisted Li-CO₂ battery with a Bi₀.₅Na₀.₅TiO₃ piezoelectric cathode utilizes ultrasonic waves to enhance CO₂RR/CO₂ER kinetics, achieving a low charging voltage of 3.52 V.[197] Integrating external environment strategies with internal catalyst

optimization provides a comprehensive approach to cathode design.

b) Unstable alkali metal anodes (Li/Na/K). The anodes in M-CO₂ batteries face significant challenges, including dendrite formation, the crossover of dissolved CO₂/H₂O/O₂, and electrolyte decomposition products. These issues lead to anode depletion, increased polarization, reduced cycling performance, and eventual cell failure. Insufficient SEI mechanical strength and uneven alkali metal deposition exacerbate these problems.[191,200-202] Recently, a dual-salt electrolyte (LiTFSI-LiPF₆/TEGDME) enables the in situ formation of a stable Li₂CO₃/LiF-rich SEI layer, which inhibits lithium dendrites and prevents side reactions with CO₂.[203] This design enhances lithium anode durability in Li-CO₂ batteries, extending the cycling life of Li-CO₂ batteries to 2720 h. Similarly, Mao et al.[204] created a stable NaF-rich SEI layer using fluorinated graphene (FG), achieving a dendrite-free Na/FG composite anode with excellent air stability, low voltage hysteresis, and a cycle life of 391 cycles in Na-CO₂ batteries. Another effective strategy to solve the problem of dendrite growth is to replace metal anodes with alternatives such as metal-free or metal-containing composites. Our research group has developed a “two-in-one” electrode with high sodium affinity, which creates a uniform electric field and enhances robust adsorption for sodium ions (Figure 9a).[193] This design promotes the formation of favorable nucleation sites for efficient Na deposition, resulting in an exceptionally long lifespan for sodium stripping/plating and providing durable anodes that are resistant to dendrite formation in Na-CO₂ batteries. Another notable work was reported by Hu et al.,[205] who developed a reduced graphene oxide Na anode (rGO-Na) by infusing molten sodium into rGO foam, enhancing mechanical strength, toughness, and Na⁺ plating/stripping kinetics while suppressing dendrite formation. As a result, a Na-CO₂ coin cell achieved 400 stable cycles, while a pouch cell delivered 1.1 Ah capacity, 232 Wh⁻¹ energy density, and 50 cycle stability. In addition, solid or quasi-solid electrolytes have shown promise in inhibiting side reactions and dendrites, and improving anode durability, ensuring longer cycle life and stability, as will be discussed later.

c) Concerns with electrolytes. Developing highly stable electrolytes with high ionic conductivity and wide electrochemical windows is a primary focus for M-CO₂ batteries. Among liquid electrolytes, ether-based solvents, such as TEGDME, are relatively stable in M-O₂/CO₂ batteries due to their low volatility and compatibility with reduced CO₂ species. Nevertheless, there are still challenges in enhancing their performance during deep discharge states and over prolonged cycling periods. Conventional liquid aprotic electrolytes suffer from degradation, flammability, and leakage, limiting their application for extended cycles. To address these issues, research has shifted toward more stable alternatives, such as solid-state electrolytes (SSEs), to enhance stability and safety. SSEs in M-CO₂ batteries encompass ceramic, polymer, and inorganic-polymer composite materials. Ceramic SSEs, such as NASICON-type SSEs (Li₁.₅Al₀.₅Ge₁.₅(PO₄)₃),[206,207] Li₁.₄Al₀.₄Ti₁.₆(PO₄)₃,[208] Na₃Zr₂Si₂PO₁₂[209,210], exhibit ionic conductivity (10⁻⁵-10⁻² S cm⁻¹), thermal stability, and mechanical strength but face challenges like poor interfacial contact and brittleness. Polymer SSEs provide flexibility and improved safety but have ionic conductivity (10⁻⁷-10⁻⁵ S cm⁻¹) and limited electrochemical stability.[205,211-216] Inorganic-polymer
<figure><img src="image_19.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_22.png" /><figcaption>(f)</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>(e)</figcaption></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

**Figure 9.** a) Schematic of a "two-in-one" electrode designed to promote CO₂ conversion and regulate the nucleation and growth of sodium. Reproduced with permission.[193] Copyright 2024, Wiley-VCH. b) A schematic of the Li–CO₂ battery with a solid RM(II)-BTC cathode, preventing the shuttle effect and enabling high CO₂ adsorption, fast kinetics, and reversible redox. RM(II)-BTC reduces to Li₂RM(I)-BTC, reacts with CO₂ and regenerates RM(II)-BTC, producing Li₂C₂O₄ as the final discharge product. Reproduced with permission.[194] Copyright 2024, Nature Publishing Group. c) Mechanism of CoPc as homogeneous catalyst mediated CO₂RR in Li–CO₂ battery, and full curves with CoPc and without CoPc at 100 mA g⁻¹ between 2.0 and 4.5 V. Reproduced with permission.[195] Copyright 2024, Wiley-VCH. d) The working mechanism of the light-assisted Li–CO₂ battery based on the CNT@C₃N₄ photocathode. Reproduced with permission.[199] Copyright 2021, Wiley-VCH. e) Schematics illustrating the benefits of RMORs, showing lower charging voltages and improved catalyst stability. Compared to the OER with less than 10 cycles (unstable), ACTO oxidation enhances stability for over 100 cycles (stable), while HzOR achieves ultrastability with more than 1000 cycles. And the corresponding FEₐ₉ of the Zn–CO₂RR/RMOR batteries. Reproduced with permission.[175] Copyright 2024, Nature Publishing Group. f) Schematic diagram of hybrid Na–CO₂ battery with NaFSI-based electrolytes for conversion CO₂ to HCOOH. Reproduced with permission.[174] Copyright 2024, Wiley-VCH.

composite SSEs have emerged as a promising solution, leveraging the complementary advantages of high ionic conductivity, mechanical flexibility, and stability. For instance, a composite SSE with Li₇La₃Zr₁.₄Ta₀.₆O₁₂ and poly(ethylene oxide) demonstrated high conductivity (1.03 mS cm⁻¹ at 70 °C), a wide electrochemical window (5 V vs Li⁺/Li), and excellent mechanical properties.[217]

Lu et al.[210] enhanced Na₃Zr₂Si₂PO₁₂ by doping with Mg, achieving an ionic conductivity of 1.16 mS cm⁻¹ at ambient temperature in Na₃₂Zr₁.₉Mg₀.₁Si₂PO₁₂ (NZM1SP). A Na–CO₂ battery using a composite electrolyte of NZM1SP and poly(vinylidene fluoride-co-hexafluoropropylene) maintained a voltage gap below 2 V for 120 cycles, demonstrating excellent conductivity, stability,
and performance. However, challenges remain, including optimizing ionic transport and interfacial contact, designing flexible architectures, and refining fabrication techniques for thin layers and high mass-loading electrodes. Addressing these will accelerate the development of reliable, high-performance solid-state $M-CO_2$ batteries.

## 5.2. Challenges and Strategies in Aqueous $M-CO_2$ Batteries

a) Catalyst cathode. Aqueous $M-CO_2$ batteries are significantly hindered by the challenges associated with the cathodic $CO_2RR$ and OER reactions. These reactions involve distinct mechanisms and intermediates, requiring different catalytic properties. Moreover, $CO_2RR$ is limited by poor $CO_2$ solubility, high bond dissociation energy, and competition with HER, while OER suffers from sluggish kinetics and requires large overpotentials, especially in near-neutral pH conditions. These issues lead to high overpotentials, low current/power densities, poor energy efficiency, low FE and SPC, and limited discharge products, all of which impair $M-CO_2$ battery performance. A promising solution lies in the design of bifunctional catalysts capable of efficiently catalyzing both $CO_2RR$ and OER in near-neutral electrolytes.[23,24,218–220] Wang et al.[24] synthesized a 3D Ir@Au bifunctional cathode for $Zn-CO_2$ batteries, achieving efficient $CO_2$-to-CO with 85% FE at $4.1 \text{ mA cm}^{-2}$ ($390 \text{ mV overpotential}$) and OER at $560 \text{ mV overpotential}$ ($10 \text{ mA cm}^{-2}$), resulting in a battery energy efficiency of 68%. Zeng et al.[219] developed a NiFe alloy anchored on N-doped graphene as a bifunctional catalyst for $Zn-CO_2$, achieving 94.5% FE for $CO_2$-to-CO in 0.5 M $KHCO_3$ and a low OER overpotential ($310 \text{ mV at } 10 \text{ mA cm}^{-2}$ in 1 M KOH). The $Zn-CO_2$ battery using NiFe-DASC delivered 90.6% FE for CO production at $5 \text{ mA cm}^{-2}$, a peak power density of $1.36 \text{ mW cm}^{-2}$, and high stability over 180 cycles. The superior performance was attributed to optimized binding energies of $CO_2RR$ and OER intermediates via orbital coupling between Fe and Ni. In recent years, metal-based,[221–224] metal-free,[173,225] and metal–carbon composite catalysts,[126,227] including single-atom,[228–230] dual-single-atom,[231] have been extensively studied to improve $Zn-CO_2$ battery performance. Despite progress, reports often lack detailed OER analysis, limiting optimization strategies. Furthermore, challenges remain in achieving balanced $CO_2RR$–OER performance and long-term stability.

Similarly to $ECO_2RR$, in order to overcome the challenges posed by the OER, small-molecule oxidation reaction-assisted aqueous $M-CO_2$ batteries offer a promising approach to improve energy efficiency while generating value-added products. As shown in Figure 9e, Pang and co-workers[175] introduce reducing molecule oxidation reactions (RMORs) as an alternative to OER in $Zn-CO_2$ batteries. The method uses thiourea dioxide, acetone oxime (ACTO), and hydrazine (Hz) to decrease charging voltage and increase battery life. The RMORs can stabilize $Fe-N_4$ active sites, enhance *CO desorption, and suppress HER, which greatly extends the cycling life and stability of the catalyst. It is impressive that the HzOR-based battery achieved more than 1000 h of stable operation at a low charging voltage of 1.2 V with a current of $1.43 \text{ mA cm}^{-2}$ and 90% FE for CO production at 418 h, which is 50 times longer than the OER-based system. Similarly, a hydrazine-assisted $Zn-CO_2$ battery with $FeN_3$ site catalyst was re-

ported by Hu and co-workers.[232] The resulting $Zn-CO_2$ battery exhibits ≈100% CO selectivity and 89% energy efficiency. In situ IR spectroscopy confirms the formation of *$N_2$ and *CO intermediates during HzOR and $ECO_2RR$ processes. These works integrate energy storage, $CO_2$ conversion, and wastewater treatment into a single system, representing a significant breakthrough in battery technology through interdisciplinary innovation.

b) Unstable alkali metal anode (Zn). In aqueous rechargeable $M-CO_2$ batteries, the metal anode, such as Zn, also faces several challenges that hinder their application. One major issue is dendrite growth, caused by uneven Zn ion stripping/plating and electric field distribution on the anode surface, leading to short circuits and capacity fading. Another key issue is self-corrosion, where Zn dissolves into $Zn^{2+}$ during discharge, reacting with $OH^-$ to form passivating byproducts like $Zn(OH)_2$ and $ZnO$. This passivation blocks ion transport, increases impedance, and reduces plating/stripping efficiency. Additionally, parasitic side reactions, such as HER, deplete the electrolyte and produce insulating byproducts, further impairing the anode's performance. These challenges are interrelated: dendrite growth increases surface roughness and promotes side reactions, while corrosion and HER exacerbate dendrite formation by making the surface more uneven. Furthermore, high current densities intensify these issues by promoting nonuniform deposition and fast dendrite growth. To address these problems, strategies like regulating crystal orientation, constructing SEI, and using advanced electrolytes are critical to achieving long-lasting, efficient Zn anodes.[233]

c) Concerns with electrolytes. Currently studied aqueous $M-CO_2$ batteries mainly include $Zn-CO_2$ and hybrid $Li/Na-CO_2$ batteries. In the case of $Zn-CO_2$ batteries, alkaline conditions favor Zn redox reactions but conflict with $CO_2RR$, which thrives under acidic or near-neutral conditions, necessitating the use of dual electrolytes separated by a bipolar membrane. In hybrid $Li(Na)-CO_2$ batteries, the aqueous catholyte is usually separated from the nonaqueous anolyte by a solid electrolyte membrane such as NASICON.[234–236] Nevertheless, their practical application faces major challenges. A major issue is the narrow window of electrochemical stability of water (≈1.23 V), which limits the operating voltage, leading to lower energy density and the risk of $H_2$ and $O_2$ production from electrolyte decomposition. The solubility and reactivity of $CO_2$ in aqueous systems are constrained by pH-dependent carbonate formation, which depletes reactants and fouls electrodes. Additionally, the metal anode suffers from corrosion and instability, especially in the presence of water, while the cathode catalysts face dissolution and degradation. Recent advances, such as “water-in-salt” electrolytes (WiSEs), have widened the potential stability window and enabled robust hybrid $Na-CO_2$ battery cycling.[237] Very recently, Liang and co-workers[174] presented a hybrid $Na-CO_2$ battery using NaFSI-based WiSEs, enabling liquid $HCOOH$ as the discharge product (Figure 9f). Density functional theory (DFT) calculations reveal that the unique environment created by NaFSI-based WiSEs promotes $HCOOH$ formation by reducing H/O concentrations at the electrolyte/catalyst interface. As a result, the battery achieved an ultrahigh discharge capacity of 148.1 mAh $cm^{-2}$, excellent cycling reversibility (50 days at 30 °C), and remarkable low-temperature stability (105 days at –20 °C). However, issues such as low $CO_2$ solubility, evaporation, and
compatibility with dual-electrolyte configurations remain unresolved, highlighting the need for innovative solutions to realize the full potential of aqueous M–CO₂ batteries.

## 6. Conclusions and Outlook

ECO₂RR technology and M–CO₂ batteries offer promising pathways for the electrochemical conversion of CO₂ into fuels and chemicals, but their future depends on overcoming the major challenges they face. This perspective highlights the critical challenge of balancing high SPC and high FE simultaneously in both CO₂ electroconversion systems. By analyzing current advancements and limitations, it identifies opportunities for innovation and integration to enhance performance. To bridge the gap between lab-scale feasibility and practical applications, we outline a comprehensive research roadmap that addresses the challenges and identifies potential breakthroughs (Figure 10).

1. Understanding and optimizing the fundamental mechanisms of CO₂ electroconversion. Understanding the structure, composition, and catalytic mechanisms of catalysts for CO₂RR at both molecular and atomic levels relies on a combination of in situ and ex situ characterization techniques, as well as computational approaches. The investigation of CO₂RR requires a multidimensional approach, examining the catalyst bulk structure, surface/interface, electrolyte, and gas phase above the electrolyte.[238] For instance, bulk structural analysis using XRD,[239,240] X-ray spectroscopy, and secondary ion mass spectrometry reveal crystal phases, elemental composition, and structural stability during CO₂RR. Surface/interface dynamics are elucidated through in situ Raman, FTIR, XAS,[241] and XPS, which track intermediates, oxidation states, and surface reconstructions in real-time. Moreover, scanning electrochemical microscopy provides nanoscale insights into catalyst-localized features like crystal facets and grain boundaries, revealing intermediates, active sites, and kinetic rates.[242] In the electrolyte, in situ inductively coupled plasma identifies leached species, while nuclear magnetic resonance and high-performance liquid chromatography detect soluble products like formate and methanol. Gas-phase products, such as CO, methane, or ethylene, are quantified by GC-MS/DEMS,[243] with isotopic labeling providing insights into reaction pathways. Complementary computational techniques like DFT and Ab initio molecular dynamics further deepen understanding by predicting adsorption energies, reaction intermediates, and activation barriers, while simulating catalyst behavior under operating conditions. These methods facilitate the identification of key descriptors, such as adsorption energies or the potential of zero charge, for optimizing catalyst design. Integrating experimental data with computational models and machine learning accelerates material discovery, enabling the design of highly efficient and selective electrocatalysts. Together, these approaches form a holistic framework for advancing CO₂RR research.

2. Catalyst design and optimization. Catalysts are the core of CO₂ electroreduction reactions. Achieving both high FE and SPC for desired products, while maintaining stability during long-term operation, remains a key challenge, partic-

ularly for acidic CO₂RR and high-carbon products. Acidic conditions exacerbate HER dominance and accelerate catalyst degradation, while the formation of high-carbon products requires efficient C–C coupling and stabilization of key intermediates like *CO. Moreover, bifunctional catalysts for CO₂RR/OER or CO₂RR/CO₂ER must balance reducing and oxidizing conditions to ensure system efficiency. To address these issues, atomically precise catalysts, such as single-atom or dual-single-atom catalysts with precise control over the coordination environment and electronic structure, can enhance CO₂ adsorption, suppress HER, and improve stability in acidic media. Molecular-based, hybrid catalytic materials, where molecules are appended at conductive surfaces and/or inserted into thin, porous catalytic films (notably carbon films) hold remarkable perspectives as a strategy to achieve such single atom site reactivity, including for the development of CO₂ electrolyzers. Tandem catalysts, which divide reaction steps across two materials (e.g., Au for CO generation and Cu for C–C coupling), may optimize pathways for high-carbon products. Spatial arrangements and local reaction environment control are critical for intermediate transfer and selectivity. In addition, designing adaptive catalysts, which adapt to fluctuating conditions by altering their surface structure or electronic states, stabilizing intermediates, and improving durability. These approaches emphasize using stable, conductive supports, optimizing spatial arrangements for tandem systems, and leveraging machine learning to fine-tune dynamically responsive catalysts. Together, these innovations hold promise for overcoming current limitations.

3. Electrode and membrane and interface optimization. Future research on GDEs and membrane/interface optimization should focus on addressing key challenges such as flooding, poor mass transport, material degradation, product crossover, and interfacial instability. For GDEs, future efforts should prioritize developing advanced porous structures with hierarchical pore distribution to balance gas-liquid transport and prevent flooding, along with introducing graded hydrophobic/hydrophilic materials to optimize wettability. Enhancing the durability of GDEs through hybrid materials or protective coatings and improving catalyst layer adhesion and uniformity using advanced deposition techniques will also be critical. For membranes and interfaces, research should focus on designing ion-selective membranes that minimize product crossover while maintaining high ionic conductivity. Reducing membrane thickness improves conductivity and water permeability, with ultrathin (<30 µm) layers possible using highly conductive polymers like Nafion or PiperION.[140] Developing durable materials that can withstand harsh conditions, such as chemically stable perfluorinated polymers or reinforced nanofibers, will improve long-term performance. Additionally, engineering stable interfaces by optimizing membrane–catalyst contact with surface modifications or buffer layers is essential to mitigate local pH gradients and enhance efficiency. Ultimately, integrating GDE and membrane innovations into scalable reactor designs while leveraging computational modeling and in situ diagnostics for system optimization will be key to bridging the gap between lab-scale feasibility and industrial deployment.
<figure><img src="image_26.png" /><figcaption>**Figure 10.** A research roadmap for CO₂ electroconversion. The overall roadmap includes understanding the mechanism, optimization of functional components such as catalysts, electrodes, and membranes, advanced system integration and battery innovations, and scaling up from the laboratory to real-world applications.</figcaption></figure>
4) Advanced system integration and battery design innovation. The integration of ECO₂RR systems faces challenges such as the inefficiency of OER at the anode, which consumes high energy and generates low-value O₂ byproducts, as well as the complexity of scaling systems with compatible cathode–anode environments. To address these, future research should focus on developing integrated CO₂RR systems with alternative oxidation reactions, such as biomass oxidation, which produce valuable chemicals (e.g., gluconic acid) while reducing energy requirements. All-solid-state electrolyzers with multi-compartment configurations operating at room temperature, designed for the direct and continuous production of pure liquid fuels, and tandem electrolyzers aimed at improving energy efficiency for generating C₂⁺ products, should receive greater attention and research effort. Furthermore, M–CO₂ battery systems, like Na–CO₂ batteries, struggle with sodium dendrite formation, scalability, and the complexity of combining CO₂ reduction, energy storage, and resource extraction. A promising innovation is the anode-less hybrid Na–CO₂ battery,[22] which harvests sodium ions directly from seawater, bypassing the need for metallic sodium and enhancing safety. This system leverages seawater as a dual resource for electrolyte and CO₂ capture, enabling electricity storage alongside the production of chemicals like formate or hydrocarbons. Research must prioritize designing bifunctional electrodes for efficient sodium deposition and CO₂ reduction, optimizing porous structures for ion and gas transport, and addressing seawater impurities to ensure stability and scalability. These advancements could transform CO₂RR technologies into multifunctional platforms, enabling sustainable energy systems and circular carbon cycles.

5) Scaling up from laboratory to practical applications and industrial integration. Scaling up CO₂ electroconversion technologies requires addressing several critical challenges. Integration with low-carbon power sources, such as solar, wind, and hydropower, is essential for reducing the carbon footprint and ensuring sustainability. Efficient coupling of CO₂ electrolysis with renewable energy systems will involve advances in energy storage, power management, and solar-driven electrochemical cells. Concurrently, carbon capture technologies, including direct air capture and point-source capture, must be integrated to provide a reliable CO₂ feedstock. The design of efficient, scalable electrolyzer stacks is critical for high throughput, requiring improvements in electrode materials, electrolytes, and reactor configurations, ensuring efficiency, and minimizing energy loss. Product separation and storage are also crucial, with advancements in membrane separation, cryogenic technologies, and long-term storage systems needed to purify and store products efficiently. Furthermore, comprehensive technoeconomic and life cycle assessments will be pivotal in evaluating the feasibility, cost-effectiveness, and environmental impact of scaling up CO₂ electroconversion systems. By focusing on optimizing resource inputs, reducing costs, and maximizing overall energy and carbon efficiency, these technologies can transition from laboratory successes to large-scale commercial applications, driving sustainable carbon cycles and supporting global decarbonization efforts.

## Acknowledgements

The authors acknowledge support from the German Research Foundation (DFG, Project No. 501766751) and the Sino-German Center for Research Promotion (GZ1579). C.X. appreciates the support from the China Scholarship Council (No. 202106370041). The authors warmly acknowledge financial support in part from Institut Universitaire de France (IUF).

Open access funding enabled and organized by Projekt DEAL.

## Conflict of Interest

The authors declare no conflict of interest.

## Keywords

CO₂ batteries, CO₂ electrochemistry, CO₂RR, single-pass conversion

Received: December 29, 2024

Revised: February 17, 2025

Published online:

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<figure><img src="image_27.png" /></figure>

**Changfan Xu** received his M.S. degree in metallurgical engineering from Central South University in 2021. He is currently a Ph.D. candidate under the supervision of Prof. Yong Lei at the Technical University of Ilmenau in Germany. His research interests focus on the construction and functionalization of nanomaterials for energy storage and conversion.

<figure><img src="image_28.png" /></figure>

**Marc Robert**, long time professor at Université Paris Cité, is now professor at Sorbonne Université and a senior member (Innovation Chair) of the Institut Universitaire de France (IUF). His work is dedicated to electrochemical and photochemical approaches of electron-transfer processes and the catalytic activation of small molecules (CO₂, N₂), in an effort to solve contemporary energy challenges.

<figure><img src="image_29.png" /></figure>

**Guosheng Shao** is Professor and the Director of the State Centre for International Cooperation on Designer Low-carbon and Environmental Materials (CDLCEM). He is also visiting professor to the University of Surrey, UK (2018-). He joined the University of Bolton as Professor of materials modelling and simulation in 2007, and then established the Institute for Renewable Energy and Environmental Technologies (IREET, 2011) as its founding Director, together with the responsibility as Head of Engineering, etc. His academic interest is “designer” materials and application devices. His current focus is mainly on sustainable (renewable) energy systems and environmental technologies.
<figure><img src="image_30.png" /></figure>

**Yong Lei** is Professor and Head of the Department of Applied Nano-Physics at the Technical University of Ilmenau, Germany. He began working in Germany in 2003 as an Alexander von Humboldt Fellow at the Karlsruhe Institute of Technology. From 2006 he worked at University of Muenster as a group leader and junior professor. In 2011, he joined the Technical University of Ilmenau as professor. His research focuses include template nanostructuring, energy conversion and storage devices, and opto-electronic applications of nanostructures.