CO2 Conversion Toward Real‐World Applications: Electrocatalysis versus CO2 Batteries

Electrochemical carbon dioxide (CO2) conversion technologies have become new favorites for addressing environmental and energy issues, especially with direct electrocatalytic reduction of CO2 (ECO2RR) and alkali metal‐CO2 (M–CO2) batteries as representatives. They are poised to create new economic drivers while also paving the way for a cleaner and more sustainable future for humanity. Although still far from practical application, ECO2RR has been intensively investigated over the last few years, with some achievements. In stark contrast, M–CO2 batteries, especially aqueous and hybrid M–CO2 batteries, offer the potential to combine energy storage and ECO2RR into an integrated system, but their research is still in the early stages. This article gives an insightful review, comparison, and analysis of recent advances in ECO2RR and M–CO2 batteries, illustrating their similarities and differences, aiming to advance their development and innovation. Considering the crucial role of well‐designed functional materials in facilitating ECO2RR and M–CO2 batteries, special attention is paid to the development of rational design strategies for functional materials and components, such as electrodes/catalysts, electrolytes, and membranes/separators, at the industrial level and their impact on CO2 conversion. Moreover, future perspectives and research suggestions for ECO2RR and M–CO2 batteries are presented to facilitate practical applications.


Introduction
Humanity has built a rich material civilization through the overconsumption of fossil fuels (coal, natural gas, and petroleum products) since the industrial revolution while simultaneously imposing serious energy shortages, greenhouse effects, and other issues. [1,2] Renewable energy sources, such as solar and wind power, are environmentally friendly and sustainable alternatives to fossil fuels for growing energy consumption. [3][4][5] hybrid metal-CO 2 batteries can also store intermittent renewable electricity by using the metal as an energy storage carrier upon charging while supplying electricity and converting CO 2 to valuable carbon-containing chemicals during discharging. [25,26] This discharge process is quite similar to ECO 2 RR which is driven by electrical input. But it can directly utilize the stored intermittent renewable electricity to convert CO 2 into chemical fuels and feedstocks, that is, energy conversion can be achieved in a single device that is obviously independent of the ECO 2 RR ( Figure 1). Regrettably, the research on M-CO 2 batteries, especially aqueous or hybrid M-CO 2 batteries is still in its infancy, despite the fact that they have the potential to realize dual functions of energy storage with high energy density and CO 2 conversion in a single device and are even anticipated to play a significant role in daily life, industrial production, and astronomical exploration. [25][26][27] A comprehensive and in-depth reference and suitable strategies can contribute to significant progress in CO 2 conversion and energy storage for this emerging energy technology.
In the past, diverse aspects of ECO 2 RR and M-CO 2 batteries have been reviewed by some eminent research groups. [28][29][30][31][32][33][34][35][36][37][38][39][40][41] However, hardly any work has reviewed and compared the electrocatalytic CO 2 electrolyzers and CO 2 batteries in detail, which are two different but intrinsically linked devices of CO 2 conversion. A comprehensive and clear review of the relationship of components and their functionality between CO 2 electrolyzers and CO 2 batteries is lacking. Therefore, the working principles and recent advances in CO 2 electrolyzers and CO 2 batteries will be summarized in this review from a materials and chemical perspective, with special emphasis on the rational design of CO 2 electrolyzers and CO 2 batteries for practical applications to meet the requirements of "zero-carbon" network and industrialscale, long-term efficient energy conversion and storage. The main characteristics of ECO 2 RR (CO 2 electrolyzer) and M-CO 2 batteries toward CO 2 conversion are summarized in Figure 2. We anticipate that this review will provide readers with sufficient inspiration in the fascinating field of CO 2 conversion and a comprehensive and in-depth reference for ECO 2 RR and emerging M-CO 2 battery technologies.

Electrolyzer Configurations and Components
Typically, ECO 2 RR is realized by an electrically driven process, where the external circuit connects the power supply, cathode, and anode through electric wires with charge transfer and an internal circuit connects the two electrodes through an electrolyte (and membrane) with mass transfer. [42,43] It primarily functions with CO 2 RR at the cathode and water oxidization (i.e., oxygen evolution reaction, OER) at the anode. Currently, electrochemical studies of CO 2 RR have been conducted in a variety of cell types, including H-cells, microfluidic flow electrolyzers, polymer membrane electrode assembly electrolyzers, and solidoxide electrolyzers, corresponding schematic representation of these four electrolyzers is shown in  Schematic diagram of ECO 2 RR (CO 2 electrolyzer) and M-CO 2 battery driven by renewably sourced electricity, and the corresponding potential applications, working toward a zero-carbon network. For the CO 2 electrolyzer, the OER and CO 2 RR take place at the anode and cathode, respectively. For M-CO 2 batteries, the dissolution and deposition of metal occur on the anode to achieve energy release and storage, while the CO 2 RR (discharge process) and water oxidation (OER, charge process) both occur on the cathode.

Solid-Oxide Electrolyzer (SOE)
In SOE, solid oxides with ion conductivity serve as the electrolytes separating the anode from the cathode, where gaseous CO 2 diffuses directly and is reduced at the interface of solid electrode and solid electrolyte to gaseous products like CO or CH 4 , typically operating at high temperatures (>600 °C). [57] Typically, oxygen ion-conducting and proton-conducting electrolytes have been implemented for SOEs. [57,58] For SOE with an oxygen ion-conducting electrolyte (Figure 3d), CO 2 is reduced at the cathode to produce CO and oxygen ions (O 2− ). The generated O 2− is transported through the electrolyte to the anode and is oxidized to produce O 2 . While for the SOE with protonconducting electrolyte, the H 2 O molecules are first oxidized to oxygen at the anode, simultaneously producing H + ions. The H + ions then travel through the electrolyte to the cathode, where they interact with the adsorbed CO 2 to create CO or other value-added chemicals. The high operating temperature is favorable for accelerating the electrochemical CO 2 RR, thus CO 2 electrolysis in SOE demonstrates higher current density, energy efficiency, and stability under the same electrolysis voltage compared to other low-temperature electrolyzers with aqueous electrolytes. [58,59] Nevertheless, SOE is severely constrained by sophisticated electrocatalytic chemistry and challenging working environments.

Cathodic CO 2 Reduction Reaction
A typical heterogeneous ECO 2 RR occurs at the electrocatalystelectrolyte interface and is a complex multi-step proton-electron transfer reaction process. [8,[60][61][62][63] There are generally three steps, chemisorption of CO 2 onto the electrocatalyst surface; transfer of electrons and protons to dissociate CO bonds, coupling of CC bonds, and formation of CH bonds; and product rearrangement, desorption from the electrocatalyst surface and diffusion. At the thermodynamic level, the first step is the rate control step of the overall reaction, where a very large thermodynamic standard equilibrium potential is required to activate the adsorbed CO 2 molecules, allowing adsorbed CO 2 to reach an electron and then generate CO 2 · − radical anion. [61,64] CO 2 · − can generate various products through subsequent proton-coupled electron transfer processes, which seem to be more efficient as these are thermodynamically more advantageous than CO 2 activation steps. [65,66] Products are closely related to the number of electrons transferred (including 2, 4, 6, and 8 electrons, etc.). A mixture of products is usually obtained after CO 2 RR due to the high activity of CO 2 · − radical anion and the close equilibrium potentials of different products, and the selectivity of products is largely determined by the surface and active sites of the catalyst. [67,68] At the kinetic level, the product selectivity is influenced by the concentration of protons and other cations in the electrolyte, the number, and rate of multiple electron transfer, and the rate of reduction is significantly inhibited if the electron and mass transport rate are limited. [69,70] In addition, the equilibrium potential for CO 2 RR is very similar to that of the hydrogen evolution reaction (HER), so CO 2 RR is always accompanied by the production of H 2 , with HER being the main side reaction. [71,72] In the practical investigation of the mechanism of CO 2 catalysis, considering the low solubility of CO 2 in aqueous media, the limited availability of CO 2 will severely limit the CO 2 RR rate. Thus, additional stirring is usually added to augment the CO 2 concentration on the catalyst surface in conventional studies for half-cell, whereas to minimize the effect of mass transfer effects, rotating disc electrodes are employed to support the CO 2 RR catalyst. [44,[73][74][75]

Anodic O 2 Evolution Reaction
As part of an electrolyzer, an oxygen evolution reaction (OER) usually takes place at the anode, contributing electrons to cathodic CO 2 RR for maintaining charge balance in overall electrochemical reactions. [76,77] OER also undergoes a multistep electronic reaction, while the formation of the OO bond requires the overcoming of a high reaction potential. [78,79] In general, varying OER processes that involve four electrons transfer will be brought about by electrolytes with different pH environments. [80,81] In neutral/acidic conditions, oxygen is provided by H 2 O molecules as the reactant, involving the overall reaction of 2H 2 O → O 2 + 4H + + 4e − ; while alkaline condition concerns the overall reaction of 4OH − → O 2 + 2H 2 O + 4e − . Specifically, in acidic media, water molecules first undergo a single-electron oxidation process and adsorb on the surface of the catalyst active site (M * ), on which an adsorbed * OH is formed, (M * + H 2 O → M * OH + H + + e − ); whilst in alkaline media, the interaction of M * with the hydroxyl group can form M*OH intermediate (M * + OH − → M * OH + e − ). [80,81] Afterward, * OH passes through proton coupling and electron removal for the formation of * O species. There are two different methods of obtaining oxygen from * O intermediates, one is through the direct combination of two * O to produce O 2 and the other involves the formation of an * OOH intermediate with subsequent decomposition to O 2 and restoration of the initial M * active site. [80,81] The * OH, * O, and * OOH are considered crucial intermediates, where the bonding interaction between the catalyst and the intermediate is evaluated in the rate-determining step of the overall electrocatalytic OER process, optimization of the adsorption energy of the reaction intermediates is essential to accelerate the OER process. [80,82] The pH-neutral or nearneutral electrolytes make up one of the most commonly used media in CO 2 reduction, as they can not only contribute to the necessary proton source for CO 2 RR but also inhibit unwanted malignantly competing HER in acidic media and undesired reactions between OH − and CO 2 in alkaline media. [77,83] Considering that both CO 2 RR and OER follow a multi-step fundamental reaction and energy barrier accumulation, there is a great need to examine high-performance and economical OER electrocatalysts appropriate for CO 2 RR conditions, which are quite distinctive from those extensively researched under acidic or alkaline circumstances.

Evaluation Metrics for Practical ECO 2 RR
Several metrics are typically used to assess the effectiveness of electrochemical CO 2 RR procedures in the laboratory, including onset potential, current density (j), Faraday efficiency (FE), overpotential (η), Tafel slope (η), and electrocatalyst stability. [84,85] However, it is vital to consider comprehensively the combination of high energy efficiency (EE), current density, selectivity, and singlepass conversion (SPC) to lower capital and operational costs and meet commercial electrochemical CO 2 RR requirements. [19,86] Current density (j), which is calculated by dividing the total current measured at a constant potential by the area value of the working electrode, is a reflection of the reaction rate and consequently has a direct impact on the selectivity (specific product as a percentage of total products) of the contribution of capital costs to the production of chemicals. [19] The level of current density depends primarily on the number of reactive sites in the catalyst, the mass transfer rate of the reaction system, and the system impedance (the rate of electron transfer to the reactants on the electrode surface).
Faraday efficiency (FE) is the proportion of electrons used in the conversion of CO 2 into concerned products, representing the selectivity of a catalyst for a desired product. The formula for calculating FE is FE = (nmF) / Q, where n, m, F, and Q indicate the number of electrons required for the product, the total amount of product in moles, the Faraday constant, and the total accumulated charge during the reduction of CO 2 , respectively. The amount of CO 2 that is converted to the desired product can be increased by FE advancements, resulting in lower electrocatalytic energy losses and costs. Many efforts have recently been made to increase the FE of CO 2 RR products, but the FE values remain relatively low for C 2 and C 2+ products such as ethylene (C 2 H 4 ), ethane (C 2 H 6 ), and higher hydrocarbons. Achieving high FE values for C 2 and C 2+ products is critical for the practical application of CO 2 RR in industry. Therefore, further research is needed to explore new catalysts, reaction conditions, and reactor configurations that can increase the FE of CO 2 RR for these valuable products.
Energy efficiency (EE) is the percentage of energy stored in the desired product compared to the total energy input, which is at the heart of operating cost calculations, and the higher the EE value, the lower the unnecessary losses. The EE of a system can only be accepted if it has a high FE and a high-voltage efficiency (low overpotential).
The single-pass conversion (SPC) and electrolyzer lifetime are crucial metrics in assessing the economic viability of CO 2 RR technology, both of which were often overlooked for decades. SPC is defined as the percentage of CO 2 conversion to total CO 2 input. [19] A significant excess of CO 2 is typically used in most publications to guarantee a sufficient supply of CO 2 , which helps to increase selectivity but is not ideal for SPC. Moreover, the main electrode components, such as the electrocatalyst, separator, binder, and GDEs, and how they are integrated together, as well as the cathode micro-environment have a significant impact on the lifetime of the electrolyzer. [87] The enhancement of SPC is essential to minimize separation costs of CO 2 and products, while the extended life of the electrolyzer is beneficial in reducing operating costs and effective capital expenditure.
With those parameters as metrics for commercial applications, high productivity (high current density), high selectivity (FE of the desired product), and high EE and SPC must be achieved simultaneously during long-term performance stability. [88] However, to date, ECO 2 RR has not been able to meet the commercialization target of long-term stable operation of at least 20 000 h at current densities >200 mA cm −2 with efficiencies over 70%. [89,90] The ECO 2 RR performances of recently reported CO 2 electrolyzers with industrial current density (>200 mA cm −2 ) are summarized in Table 1. High energy consumption and low duration allowable times remain the greatest challenges for ECO 2 RR today. Commercial applications will move more quickly if there is a better understanding of CO 2 RR mechanisms, the influence of the catalyst environment (CO 2 availability, substrates, catalysts, pH, temperature, and electrolyte composition, etc.) on performance, and how the environment can be regulated by the design of the electrolyzer.

Battery Configurations and Components
The common M-CO 2 battery composes of an alkali metal (Li, Na, K, Zn, Al, etc.) anode, a CO 2 cathode with an efficient catalyst, and an electrolyte. Its key functions lie in the deposition and dissolution of alkali metals for energy storage and supply at the anode and the electrochemical conversion of CO 2 (and OER, if there is one) at the cathode. With regard to the type of electrolyte involved, four main types of M-CO 2 batteries have been developed: aqueous, hybrid, aprotic, and solid-state M-CO 2 batteries, which are different from each other and determine the specific electrochemical reactions during energy storage and CO 2 conversion. Figure 4 depicts four types of metal-CO 2 batteries schematically, and some typical functional materials and electrochemical properties of recently reported metal-CO 2 batteries are summarized in Table 2.
Given the high activity of the Li, Na, and K anodes, it is necessary for them to be equipped with an aprotic electrolyte consisting of corresponding metal salt dissolved in a nonaqueous, organic solvent ( Figure 4a). This structure also involves a carbon-based gas diffusion cathode and a glass fiber membrane. This configuration delivers high energy density and working voltage while yielding a limited number of carbonaceous products, mainly insoluble, insulating carbonates. [22,30] The relatively low activity of Zn and Al can be passivated by the corresponding oxides or hydroxides and is to some extent compatible with aqueous electrolytes. [125] Typically, an aqueous M-CO 2 system with a bipolar membrane divides into two compartments ( Figure 4b): an anode compartment with Zn or Al anodes and alkaline anolyte, and a cathode compartment with a catalytic cathode and neutral or near-neutral catholyte. [126] Its cathodic compartment is similar to an ECO 2 RR electrolyzer and thus offers unique advantages for CO 2 conversion. However, aqueous M-CO 2 systems have lower energy density and operating voltage compared to aprotic Li/Na-CO 2 systems.
The hybrid M-CO 2 system is another effort to build an aqueous system (Figure 4c). When highly reactive Li or Na are closely protected by ion-conductive membranes (e.g., NASICON-type ceramic electrolytes, which allow only metal ions transport), the corresponding M-CO 2 battery can operate in an aqueous electrolyte. [127] In this hybrid configuration, the anode compartment with Li or Na anodes and organic anolyte serves as an energy storage carrier with high energy density, while the cathode compartment with aqueous electrolyte can achieve proton-assisted CO 2 reduction. [128] Although this hybrid configuration design leads to more complexity and different diffusion kinetics for the two solutions, it ensures high energy storage and power supply while achieving CO 2 conversion and expanding the applications of M-CO 2 batteries.
All-solid-state M-CO 2 batteries that do not use any liquid electrolytes had also been developed (Figure 4d). This configuration is considered an important way to achieve the goal of developing high energy density and high safety M-CO 2 batteries since there are no safety concerns regarding the leakage or combustion of liquid electrolytes. [129][130][131][132][133] However, this battery is currently challenged by the solid electrolyte with poor room temperature conductivity, high solid-solid interface resistance, and how much discharge products can be stored in the cathode. Achieving this goal requires research not only on solid electrolytes but also on the structure of the CO 2 cathode and the interfacial chemistry at the metal electrode.

Nonaqueous M-CO 2 Batteries
The most unique feature of M-CO 2 batteries, like M-O 2 / air batteries, is their open cathode structure in contrast to "rocking-chair" type metal-ion batteries with an intercalationtype material as the cathode. [198] CO 2 is the true cathode reactant throughout the whole discharge-charge reaction. For aprotic and solid-state M-CO 2 batteries, it is widely accepted that the metal anode loses electrons during the discharge process to form M n+ , which moves to the CO 2 cathode driven by the electric field. CO 2 molecules at the cathode/electrolyte interface capture electrons and combine with M n+ to produce M 2 (CO 3 ) n (alkali carbonates) and carbon, which is the CO 2 RR process. CO 2 evolution will occur during the charging process, where M 2 (CO 3 ) n combines with carbon to release M n+ , electrons, and carbon dioxide, namely, the CO 2 ER process. The CO 2 diffuses through the porous structure to the outside of the cathode, while the M n+ returns to the anode and is deposited by the electrons that return to the anode via the external circuit to form the metal. [199][200][201] The electrochemical mechanism of reversible nonaqueous M-CO 2 batteries is as follows:

Aqueous or Hybrid M-CO 2 Batteries
In general, the metal oxidation reaction occurs at the anode and the CO 2 reduction reaction at the cathode during discharging in aqueous and hybrid systems, and multistep proton-coupled electron transfer process of CO 2 RR might occur since H 2 O in catholyte also participates in the electrochemical reaction,  [124] accompanied by the production of various carbon-containing chemicals. [202,203] Note that the discharge potential of aqueous and hybrid M-CO 2 batteries is related to the CO 2 RR reaction pathway, electrolyte concentration, pH, and the discharge products are primarily gas or liquid, which saves the battery from issues such as clogging of porous electrodes and burying of active sites caused by solid products. The discharge mechanism of aqueous and hybrid systems is proposed as follows: During charging, the anode experiences the reduction reaction of metal ions, whereas, at the cathode, a reversible conversion between the discharge product and CO 2 or oxidation of H 2 O (also known as an OER) will be involved, depending on the kind of CO 2 RR product. Typically, a reversible reaction will occur when the discharge product is HCOOH, which can be readily oxidized to CO 2 , and the corresponding equations are as follows: When the discharge products are gaseous products such as CO, CH 4, or other chemicals that are difficult to oxidize, OER is usually used as the charging process instead of the oxidation of CO 2 RR products, and the charging reactions are illustrated as follows: By comparison, aqueous or hybrid batteries with CO 2 reversible generation are related to the reverse reaction between CO 2 and discharge products, which leads to significantly 1.0 m NaClO 4 /TEGDME RuO 2 @a-MWCNTs ≈1.5 V, 500 mA·g −1 -120 cycles [171] 0.5 m NaCF 3 SO 3 /TEGDME Carbon cloth δ-MnO 2 ≈1.5 V, 200 mA·g −1 -391 cycles [172] PVDF-HFP/ Na 3.2 Zr 1.9 Mg 0.1 Si 2 PO 12 Ru-CNTs ≈2 V, 200 mA·g −1 7720 mAh·g −1 120 cycles [173] smaller charge/discharge voltage gaps but is not favorable for the production of carbonaceous chemicals and carbon cycle closing. M-CO 2 batteries, which involve an OER process, do not release CO 2 again and facilitate the fixation of CO 2 into carbon-containing chemicals, which will help to build a "zero-carbon" network, but this system has a wide voltage gap between charge and discharge. To achieve rechargeability of this M-CO 2 battery system, a bifunctional catalyst is required for catalyzing the reduction of CO 2 during discharge and OER during charging.

Evaluation Metrics for Practical M-CO 2 Batteries
The "three E" principle for constructing battery systems requires batteries with high-energy density (Energy); low cost and long life (Economics); non-toxic and non-hazardous, lowenergy consumption, and easy recycling (Environment). [204][205][206] For evaluating the efficiency of rechargeable M-CO 2 batteries in the laboratory, several metrics are commonly used, such as polarization, discharge capacity, round-trip efficiency (RTE), Coulombic efficiency (CE), and cycle life. [207] To evaluate the practical performance of M-CO 2 batteries, the following criteria should be considered.
In batteries, energy density refers to the volume or mass of the battery corresponding to the amount of electrical energy released, which is proportional to the specific capacity and voltage (voltage gap between anode and cathode). [208] It is a vital indicator to determine how much energy can be stored in the battery. In contrast to metal ion batteries, whose specific capacity depends on the number of reversible deintercalation/ intercalation of metal ions in the electrode material, the specific capacity of M-CO 2 batteries is primarily determined by how much discharge products accumulate on or inside the porous cathode. However, due to the presence of inactive components such as electrolytes, separators, binders, and collectors in the practical M-CO 2 batteries, the battery level energy density is significantly lower with respect to the theoretical values. [208,209] To evaluate the actual energy density of M-CO 2 batteries, the effect of all inactive components on the energy density must be taken into account.
Power density determines how fast batteries can provide stored energy, which is closely related to the internal resistance of the battery. [209,210] Batteries should have a low internal resistance, as the smaller their resistance, the higher their power output. The internal resistance of batteries is driven mainly by the electronic conductivity of the electrode materials, the ions conductivity of M n+ ions in the electrode materials and electrolyte, and the kinetic properties of electrode surface reactions, which can be mapped to the polarization performances. [210] The polarization characteristic provides information on current density at a particular discharge voltage, but it is also a reflection of the peak power density that M-CO 2 batteries can offer. Only by increasing both ionic and electronic conductivity can the power density be increased to meet practical application requirements.
In addition to providing good energy and power characteristics, M-CO 2 batteries also need to have longevity without major problems. The cycle life of M-CO 2 batteries is dependent on the stability of performance parameters, such as RTE and CE. [207] RTE (the ratio of the discharge voltage to the charge voltage) is determined by the overpotential of the charge and discharge reactions, representing how efficiently M-CO 2 batteries utilize energy. CE is defined as the capacity ratio of discharge and charge in a full charge-discharge cycle), indicating the capacity loss after one cycle. The low CE of M-CO 2 batteries reported so far are suffering from low CE because of the irreversible consumption and parasitic reactions of electrodes, which severely restricts their practical application. Additionally, the structure and compositional stability of all components are a guarantee for a long battery service life. For example, metal dendrites, deactivation of electrocatalysts, and decomposition of electrolytes may result in a shortened cycle life. Moreover, the depth of discharge is also closely related to how long a battery lasts, and a shallow discharge can increase battery life.

Electrode/Catalyst Constructing
The nature of ECO 2 RR, a heterogeneous catalytic electrochemical process, dictates that there are two approaches to improve CO 2 conversion: one is to increase the total catalyst surface area in the electrolyzers and the other is to increase the intrinsic reaction rate, represented by the current density. [89] The first is a necessary step for scaling up and long-term electrolyzers design; while the second, which combines the comprehensive effects of catalyst activity and electrode/electrolyte engineering, is also crucial. When paired with CO 2 RR, anodic OER usually occurs for electron compensation, which consumes a significant amount of energy, yet its production of O 2 is relatively inexpensive and the potential formation of reactive oxygen may also destroy the electrolyzer components. [211] Therefore, the appropriate catalyst engineering is an essential part of the electrochemical CO 2 conversion process, as both CO 2 RR and OER exhibit high overpotentials. A catalyst that has been rationally designed can significantly reduce overpotentials and increase the current of the entire reaction by establishing new pathways or playing with intermediate states.
In CO 2 batteries, where the electrochemical conversion of CO 2 (or OER) takes place at the cathode, the requirements for catalysts are more demanding, with multifunctional catalysts often required. As previously mentioned, in nonaqueous M-CO 2 batteries, CO 2 RR is a four-electron transfer process with slow kinetics and high reaction energy barriers, of which the solid-state discharge product M 2 (CO 3 ) n is usually an insulator hardly dissolved in nonaqueous organic electrolyte. The decomposition of M 2 (CO 3 ) n during charging occurs at high potentials, which can lead to side effects such as the decomposition of electrolytes, giving rise to deteriorating cycling stability. [24,26] Therefore, outstanding bifunctional electrocatalysts are needed to provide active sites that contribute to CO 2 RR and CO 2 ER for reducing the overpotential during discharge and charging and thus improving the efficiency of energy conversion. Whereas in aqueous CO 2 batteries, the design of the electrocatalysis requires the ability to both promote highly active and selective CO 2 reduction with low overpotential and also to reduce the overpotential of the OER.
Even though the requirements for catalysts differ between CO 2 electrolyzers and CO 2 batteries, catalyst development strategies are highly similar. Specifically, the various chemical compounds produced in CO 2 RR depend on the structure and composition of the catalytic materials. Similarly, the geometry and properties of the discharge products in CO 2 batteries are greatly influenced by the electrocatalyst. Rational structural and compositional modulation of various catalytic materials has been adopted in recent years to develop high-performance multifunctional CO 2 conversion electrocatalysts. Typically, two design strategies are used to improve the overall catalytic activity: one adjusts the intrinsic electronic structure and the second involves modifying the apparent physical structure. [212,213] The former reflects the intrinsic activity of active materials, such as optimization of selectivity and stability through intercalation, alloying, core-shell structure, heteroatom doping, defects, altered coordination states, modification, or modulation of metal active centers. While the latter determines the number of active sites and reflects the overall reaction rate by modulating the nanostructure, size, morphology, or self-supporting structural design. It should be noted that, in most cases, these strategic effects are interrelated, influencing catalytic activities in an interactive manner. In addition, identifying active sites for CO 2 conversion is also a critical step in understanding the reaction mechanism and improving the performance of the catalyst. Typically, active sites can be identified based on the following: i) Starting with theoretical calculations, for example, density functional theory (DFT) calculations can aid in identifying the most stable adsorption sites for reactants and intermediates on the catalyst surface, which are the most likely active sites. ii) Experimental characterization, especially, some advanced in situ/operando characterization techniques (X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM), Raman, X-ray absorption spectroscopy (XAS), etc.) would be necessary to analyze the surface structure and composition of catalysts based on their unique electronic and structural properties. iii) Testing for activity and selectivity, once the potential active sites have been identified, their activity and selectivity for CO 2 RR can be tested experimentally. This step confirms whether or not the potential active sites are active for the reaction. In this section, the discussion will be given to the many theoretical and experimental investigations related to these strategies for promoting CO 2 electrolyzer and CO 2 battery performances, with a focus on achieving satisfactory full battery performances at the above commercially relevant metrics.

Electrocatalysts for ECO 2 RR
One of the core tasks of ECO 2 RR commercialization is to develop high-efficiency catalysts that achieve low overpotential, high current density, high selectivity, energy efficiency, high single-pass conversions, long-term durability, and the ability to produce valuable products. Catalysts that have been explored include homogeneous, surface-immobilized molecules, and heterogeneous catalysts (Figure 5a). [214] The homogeneous CO 2 RR occurs when dissolved catalysts diffuse to the electrodes, where catalysts are usually molecular catalysts, such as those of complexes that are based on organic ligand scaffolds and transition metals with electron transfer properties. [215] Immobilized homogeneous catalysts are a way of immobilizing homogeneous catalysts on the surface of electrodes or support materials (e.g., graphene, carbon nanotubes) by means of covalent bonds, π-π, stacking, polymerization, etc. [216][217][218] With this method, the mass transfer, stability, and recovery rate of the molecular catalyst can be improved. Although molecular catalysts offer the advantage of synthetic control of spatial and electronic properties in the vicinity of the active site, [219] the inherent low electrical conductivity and limited active sites of molecular catalysts result in low achievable working current densities. [220] In addition, the electrical potential applied in ECO 2 RR operations is often not gentle enough to lead to extensive leaching of reduced metal centers or degradation of the ligand support of the molecular catalyst, which would result in a loss of performance. Therefore, molecular catalysts are not ideally suited for practical applications as a first choice. On the contrary, heterogeneous nanocatalysts have been explored in detail, including possible large-scale applications, because of their high activity and stability.

Size, Morphology, and Facet Modulation
Depending on the strength of the interaction between different catalyst surface sites and intermediate species such as * H, * OCHO, * COOH, * CO, and * CHO, and the type of CO 2 reduction product, single metal catalysts are usually classified into three categories: CO-selective metals (e.g., Au, Ag, Zn, and Pd), formic acid-selective metals (e.g., Sn, Hg, Pb, and In), and Cu for higher value products such as hydrocarbons or alcohols. Early studies on catalysts for ECO 2 RR focused on bulk materials, where the catalytic activity of bulk metal electrodes is usually low due to their low specific surface area, the limited number of active sites and inefficient utilization of metal atoms, as well as severe hydrogen precipitation reactions, making practical applications difficult. Controlling the local microstructure of a material through apparent size and morphology allows for a significant tuning of catalytic activity, stability, and selectivity. The well-known "size effect" is achieved by altering the size and/or shape of active materials, such as metal nanoparticles, to modify surface chemisorption and the associated catalytic activity and selectivity. [221] Especially, the intrinsic activity of nanoparticles <10 nm is strongly influenced by differences in surface topologies and coordination numbers, where surface steps, corners, edges, and step-edge sites exhibit strong size dependence (Figure 5b). [222,223] Catalysts with tailored morphologies might expose more reactive sites and beneficial crystalline surfaces, and even induce surface strain, thereby tuning the electronic structure and improving catalytic performance. [224,225] For instance, the increased roughness and surface area of the dendrites have the potential to facilitate CO 2 adsorption and electron transport; [226] core-shell structures can modulate the concentration of local * CO intermediates via confinement effects, enhancing CC coupling; [108] and sharp needle-like structures with a high curvature can induce strong electric field effects in their vicinity, boosting * CO adsorption and lowering the energy barriers to CC coupling. [227,228] Many studies have attributed the increased activity and selectivity of CO 2 RR to the optimization of the crystal facets, which is the result of modifying the size and morphology of the electrocatalyst. Wang et al. [229] investigated the energetics of copper facets with the surface coverage of the CO 2 RR (Figure 5c-e). Based on this, they prepared copper with exposed and retained (100) crystal facets by in situ electrodeposition under CO 2 RR conditions, where the adsorption strength of the intermediates plays a function similar to that of capping agents. They demonstrated high selectivity for C 2+ products with ≈90% FE at 520 mA cm −2 , a full-cell C 2+ power conversion efficiency of ≈37%, and the nearly constant C 2 H 4 selectivity over 65 h in MEA electrolyzer at 350 mA cm −2 .

Lattice Strain Regulating
Beyond grain boundaries and high index facets correlated with small grain dimensions, lattice strain has been identified as another factor contributing to the CO 2 RR activity of the catalyst by breaking the linear scaling relationship. Creating strained catalysts can be accomplished using these methods, such as dislocation, shape, or defect strain (Figure 5f-i). [230,231] Han et al. [232] analyzed the structural and compositional evolution of Cu-based catalysts during CO 2 RR, discovering that tensile Figure 5. a) Schemes for CO 2 RR by homogeneous, immobilized, and heterogeneous catalysts. Reproduced with permission. [214] Copyright 2020, American Chemical Society. b) Calculated fractions of Au atoms on corners, edges, and crystal faces, inset showing truncated octahedra and the positions of representative corner, edge, and surface atoms. Reproduced with permission. [223] Copyright 2007, Elsevier. c) DFT calculations, Wulff construction clusters of copper without and with adsorption of CO 2 RR and HER intermediates. d,e) CO 2 electroreduction performance comparison. Reproduced with permission. [229] Copyright 2019, Nature Publishing Group. f-i) Illustration of elastic strain in nanomaterials. Reproduced with permission. [231] Copyright 2019, American Chemical Society. j) Experimental approach for investigating microstructure effects in electrocatalysis at Au electrodes, and k) histograms of current densities from all of the pixels. Reproduced with permission. [233] Copyright 2021, Nature Publishing Group. strains in Cu nanocrystals generated from Cu 2 (OH) 2 CO 3 and Cu(OH) 2 contributed to enhanced hydrogenation of * CO and CC coupling, thereby boosting the overall CO 2 RR selectivity and inhibiting HER. By contrast, Kanan et al. [233] used a combination of high-resolution scanning electron microscopy (HR-SECCM) and electron backscatter diffraction (HR-EBSD) to analyze the effect of dislocations on Au electrodes on CO 2 RR (Figure 5j,k). It demonstrates that CO 2 RR on Au is not inherently sensitive to lattice strain, but maps directly onto the dislocation density induced by lattice rotation gradients at grain boundaries (GBs) and slip bands (SBs). This behavior is driven by steps forming at dislocation surface terminations, leading to the creation of more undercoordinated sites, which are considered to be the most active sites for CO 2 RR. In fact, defining the source of active species and high selectivity driving catalysts for CO 2 RR, especially for C 2+ , remains challenging because the modification of grain size, facets, grain boundaries, and defects can alter the coordination number and local electronic structure of active sites, which can result in distinctive catalytic properties.
Alloying, in addition to altering the geometric structure and electronic properties of the parent metal, allows the d-band center to be adjusted, decoupling various intermediates from the catalytic surface and breaking the linear proportional relationship of intermediate adsorption/desorption, giving catalysts enhanced performance. [234,235] Zeng et al. [236] demonstrated a single-atom Pb-alloyed Cu catalyst (Pb 1 Cu SAAs) for exclusively CO 2 RR into formate with industrially relevant activity in excess of 1 A cm −2 (Figure 6a). As a result, the modulated Cu was found to be the real active site for converting CO 2 into HCOOH, rather than the isolated Pb atom. These activated Cu sites control the initial protonation phase step of CO 2 RR and prevent the formation of other products by directing the CO 2 RR toward an HCOO* path rather than a COOH* direction (Figure 6b,c). Finally, the Pb 1 Cu catalyst could operate continuously for 180 h at ≈100 mA cm −2 using a CO 2 RR device with a solid electrolyte, and the FE HCOOH has retained above ≈85% (Figure 6d,e). Apart from the extensively studied binary alloys, high entropy alloys (HEAs) afford great opportunities for tailoring the surface electronic structure, adsorption energy, and thus the catalytic performance of CO 2 RR. [237] Nellaiappan et al. [238] found that AuAgPtPdCu HEAs exhibited ≈100% FE for gaseous products at low voltage (−0.3 V RHE) and that catalytic activity was dominated by redox-active Cu metals (Cu 2+ / Cu 0 ), with other metals providing only synergistic effects. Liu et al. [239] combined the advantages of HEAs and aerogels to fabricate PdCuAuAgBiIn HEAAs for CO 2 RR (Figure 6f), achieving a current density of 200 mA·cm −2 and FE HCOOH of 87% in a flow cell. Impressive CO 2 RR results were ascribed to the powerful interactions between various metals and surface unsaturated sites, whereby the electronic structure of the different metals could be modulated to optimize the adsorption and desorption of HCOO * intermediates on the catalyst surface, thus increasing the yield of HCOOH. Through the synergistic effect of multi-elements, diverse adsorption sites can be generated without applying external force or introducing complex interfacial structures, providing opportunities to circumvent scaling relationships in catalysis, while simultaneously increasing the complexity of material optimization and mechanistic studies.
In the future, density functional theory (DFT) calculations and machine learning models should be combined to optimize the surface structure and composition of HEAs to guide their applications.

Oxidation State Regulating
Besides metal-based catalysts, metal oxides are classically used as electrocatalysts for CO 2 RR, but their inevitable self-reduction might gradually enhance competitive HER and reduce CO 2 RR selectivity. Especially, oxide-derived Cu has exhibited impressive catalytic behavior for C 2+ production, and the oxidation state has been widely discussed in CO 2 RR. [240] Cuenya et al. [241,242] revealed the effect of catalyst surface with distorted multivalent copper oxide species on the activity and selectivity of CO 2 RR by using a pulsed electrolysis strategy to tune the morphology and oxidation state of copper catalysts (Figure 6g). The underlying principle is to promote reactant activation, modulate intermediate adsorption, and induce CC coupling, thereby increasing the yield of C 2+ . Oxidation states are usually dynamic rather than stable, and a well-established example of stabilizing metal oxidation states is the employment of superlattice oxides. More recently, Zhai et al. [243] utilized BiCuSeO nanosheet catalysts to stabilize the metal oxidation state (Figure 6h) Metal oxides/carbon-support interaction materials are also important industrial heterogeneous catalysts. Given the intrinsic tunability of metal nanoparticles, the catalyst activity can be enhanced by designing and optimizing the carrier interface, such as heterojunction surface modulation, introducing vacancies, and modulating the coordination environment of the metal center. [244][245][246] They present the unexpected possibility of enhancing active sites for molecular activation, improving charge distribution at the interface, and increasing intrinsic activity. Xu et al. [118] reported copper catalysts decorated with different alkaline earth metal oxides (MOs), where BaO/Cu demonstrated a twofold improvement in selectivity for alcohol products (ethanol and n-propanol) versus pure Cu at 100≈500 mA cm −2 , and a 30% half-cell energy efficiency for conversing CO 2 to alcohol. The increased selectivity for alcohols was deemed to originate from sites at the MO/Cu interface. In particular, once the size of the metal is reduced to the atomic scale, each atom can act as a potential active site with the highest atomic accessibility, i.e., single atoms, giving rise to unexpected electrocatalytic behavior in CO 2 RR. [247][248][249][250] Therein, the distinctive electronic structure of the support and the low coordination/unsaturated coordination environment of the metal center can effectively coordinate the interaction of the reactant CO 2 molecules and reaction intermediates with the active sites, lowering the reaction barrier for CC coupling and increasing the reaction rate and overall reactivity. [251,252] Liu et al. [253] reported Ni@C 3 N 4 -CN catalyst for CO 2 to CO (Figure 6i), in which the addition of cyano (−CN) inhibited d-π conjugation of single atom Ni sites embedded in C 3 N 4 . The assembled flow cell with FE CO ≥ 90% at 300 mA cm −2 meets the ideal prospects for industrial application. Nevertheless, the vast majority of current single-atom catalysts are still far from the requirements for large-scale applications in terms of energy efficiency, selectivity, and long-term stability, due to their low loadings and inevitable agglomeration, and harsh preparation conditions. Figure 6. a) Schematic illustration of a Pb 1 Cu SAA. b) 1D reaction phase diagram (RPD) for CO 2 RR. c) 2D RPD for CO 2 RR producing HCOOH based on two independent descriptors, the adsorption-free energies of HCOO* and COOH*, on various surfaces considering different adsorption sites. d) FEs of all CO 2 RR products at different current densities and the corresponding j-V curve of Pb 1 Cu SAAs; and e) long-term operation curve. Reproduced with permission. [236] Copyright 2021, Nature Publishing Group. f) Schematic for boosted HCOOH generation over PdCuAuAgBiIn HEAAs. Reproduced with permission. [239] Copyright 2022, Wiley-VCH. g) Schematic illustration of steering the structure and selectivity of Cu 2 O nanocubes by potential pulses. Reproduced with permission. [241] Copyright 2022, Nature Publishing Group. h) Schematic illustration of superlattice consisting of a one-by-one vertically stacked active layer (metal oxides) and conductive layer subunit, HRTEM image, and structural model of BiCuSeO superlattice. Reproduced with permission. [243] Copyright 2022, Nature Publishing Group. i) Structure and adsorption configurations of key intermediates on Ni@C 3 N 4 -CN. Reproduced with permission. [253] Copyright 2022, Nature Publishing Group.

Designing OER Catalysts
The cathodic CO 2 RR is always paired with the anodic OER in traditional CO 2 electrolyzers, however, the OER kinetics in neutral electrolytes is considerably sluggish since there is a low concentration of reactants adhering to the surface of the catalyst, leading to significant overpotentials of neutral OER catalysis (Figure 7a). [254] Currently, most investigations are concentrating on the cathode side to increase the selectivity and activity of CO 2 RR. The paired anode side has received little study, but the overall efficiency of the conversion of electrical energy to chemical fuels in practical applications must take into account the huge energy barrier and the sluggish kinetics of the OER. A significantly large overpotential is needed to cross the OER barrier to permit CO 2 RR to achieve at an appreciable rate and to achieve a certain level of catalytic performance, thus it is necessary to develop high-performance OER catalysts to drive down the reaction barrier. Fortunately, the study of electrochemical energy conversions processes, such as water splitting, fuel cells, and metal-air batteries, has contributed to the vigorous development of OER, especially in acidic and alkaline electrolytes. Regrettably, the OER overpotential in neutral electrolytes (>460 mV at 10 mA cm −2 ) tends to be much higher than that in acidic or alkaline electrolytes, which limits the overall energy efficiency of above applications.
Activity, stability, and cost are considered to be three key points to take into account when designing OER electrocatalysts for practical applications (Figure 7b). [255] To date, huge attention involving research has focused on the development of low-cost, high-activity electrocatalysts to accelerate the oxygen evolution reaction. For example, Song et al. [256] incorporated clusters and single-atom Ir into defect-rich cobalt-based hydroxide nanosheets for OER (Figure 7c-e). Owing to the low-coordinated reconstruction component, the optimized CoIr-0.2 catalyst exhibited far better OER activity than commercial IrO 2 in a neutral electrolyte, with a low overpotential of 373 mV at 10 mA cm −2 and a Tafel slope of 117.5 mV dec −1 . Tuning the surface oxygen environment (lattice oxygen and adsorbed oxygen species) is effective in achieving efficient neutral OER catalysis. Peng et al. [257] incorporated Ca 2+ into Ru-Ir binary oxides for OER (Figure 7f-h), the optimal RuIrCaO x catalyst exhibited appreciably higher mass and specific activities with a low overpotential of 250 mV at 10 mA·cm −2 and no degradation for 200 h of operation. As a result of the incorporation of Ca 2+ , metal-oxygen bonds are deemed more covalent and surface metal-bonded oxygen sites are more electrophilic; simultaneously, H 2 O molecules are more likely to bind to the catalyst surface, accelerating lattice-oxygeninvolved reaction, thereby boosting overall OER performances. In addition to scarce Ir-and Ru-based materials, some earthrich transition metal-derived materials also exhibited considerable OER properties. Dai et al. [258] discovered that nickel-iron hydroxide carbonate (NiFe-HC) electrode in CO 2 -saturated 0.5 m KHCO 3 exhibited OER activity superior to IrO 2 (Figure 7i). They paired NiFe-HC with a CO 2 RR catalyst of cobalt phthalocyanine/ carbon nanotube (CoPc/CNT) in a CO 2 electrolyzer, achieving high overall energy conversion efficiency of ≈58% with > 97% FE of CO 2 to CO.
Increasing the OER activity of catalysts is both scientifically and technically important, but OER stability, perhaps the more important consideration for practical applications, has not been addressed much, especially to match the CO 2 RR commercialization target of at least 20000 hours of stable operation. Accordingly, seeking an OER catalyst with low cost, high catalytic activity, and stability in pH-neutral or near-neutral electrolytes remains challenging. Recently, Sargent et al. [259] incorporated hydration-effect-promoting (HEP) Mg 2+ cations into Ni-Febased catalysts (Ni-Fe-Mg), triggering a more favorable OER performance with an overpotential of 310 mV and over 900 h of continuous stable operation at 10 mA cm −2 (Figure 7j,k). HEP catalysts are useful in the adsorption and dissociation of water molecules in pH-neutral electrolytes, which can improve OER catalytic activity and stability. This idea can, of course, be applied to other electrochemical reactions involving the adsorption of water, such as CO 2 RR to hydrocarbons that shall be made available to speed up the commercialization of CO 2 RR.

Electrocatalysts for Nonaqueous M-CO 2 Batteries
The catalytic material design requirements for nonaqueous M-CO 2 batteries differ from those for CO 2 RR. CO 2 cathodic catalysts have been instrumental in promoting electrochemical reaction kinetics and achieving viable and reversible M-CO 2 batteries, which are critical for battery capacity and energy density. A variety of electrocatalysts have been explored to enhance the performances of nonaqueous M-CO 2 batteries, mainly including solid catalysts (such as carbon materials, [140,174,260] metal complexes [196,261,262] and polymer compounds, [263,264] etc.) and liquids catalysts with redox mediators (RMs), as shown in Figure 8a. Inspired by homogeneous catalysts in CO 2 RR and the application of RMs in M-O 2 batteries, LiBr, [265] binuclear cobalt phthalocyanine (bi-CoPc), [266,267] 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ), [268] lithium iodide in the trimethyl phosphate (TMP), [269] and 2-ethoxyethylamine (EEA), [270] and Phenyl disulfide (PDS) [271] have been incorporated into M-CO 2 systems, mainly Li-CO 2 batteries. RMs function as mobile charge carriers in the electrolyte, which can mediate chemical reactions by managing the electron transfer between the electrode and CO 2 . In spite of the fact that RMs have been demonstrated to be effective in reducing the reaction overpotential and improving energy conversion efficiency by facilitating the decomposition or formation of discharge products, respectively, it does bring complexity and potential instability to the system due to the notorious shuttle effect with metal anodes, selfdecomposition, and other side reactions.
The main focus in CO 2 batteries at present remains the solid-state catalyst. In general, a satisfactory catalytic material for CO 2 cathodes is required to deliver four main functions: i) transmission of metal ions and CO 2 into active sites via porous channels; ii) catalyzing formation and decomposition of discharge products at surface active sites; iii) providing storage space for discharge products; iv) inducing discharge product growth and morphological evolution at the electrode surface. Based on this principle of CO 2 cathode orientation, the solid-state catalysts employed in CO 2 batteries are usually composed of porous carbon with a high specific surface area. Carbon materials such as carbon nanotubes (CNTs), graphene, carbon nanohorns (CNHs), carbon nanofibres, and other 3D   [254] Copyright 2019, Nature Publishing Group. b) Important merits and general strategies for designing OER catalysts. Reproduced with permission. [255] Copyright 2021, Elsevier. c) The mechanism diagram of OER on the CoIr-0.2 sample surface and the transformation of α-Co(OH) 2 to β-CoOOH phase. d) The iR corrected LSV curves and e) the corresponding Tafel plots of CoIr-x and IrO 2 samples. Reproduced with permission. [256] Copyright 2018, Wiley-VCH. f) LSV curves of RuIrCaOx catalyst and controls on GCEs in CO 2 -saturated 0.5 m KHCO 3 aqueous electrolyte. g) Comparison of the mass activities and specific activities of catalysts. d) Schematic illustration of lattice-oxygen emigration during the OER process. Reproduced with permission. [257] Copyright 2020, Wiley-VCH. i) CO 2 electrolyzer with NiFe-HC anode and CoPc/CNT cathode. Reproduced with permission. [258] Copyright 2019, National Academy of Sciences of the United States of America. j) OER polarization curves and k) chronopotentiometric curves obtained from the Ni-Fe-Mg catalyst on Ni foam electrode with constant current densities of 10 mA cm −2 and the corresponding Faradaic efficiency from gas chromatography measurement of evolved O 2 . Reproduced with permission. [259] Copyright 2020, Wiley-VCH.  2 . The high specific surface area affords a large number of exposed reactive sites for interface-related surface reactions, thereby facilitating the kinetics of CO 2 RR (Figure 8b). A defined hierarchical porous structure can allow for better battery performances and extended cycle life. Specifically, porous structures could contribute to metal ion transport, as well as the diffusion of CO 2 to carbon-electrolyte interfaces, where the diffusion of CO 2 within the CO 2 electrode shall be expected to affect battery performances. Large pore volume gives more storage space for the solid product, resulting in high discharge capacities (Figure 8c), whereby mesopores and macropores contribute maximally to battery capacity and micropores allow for efficient CO 2 transport. [23,195] In addition, well-designed porous channels not only facilitate mass transport but also help to regulate active nanoparticle and discharge product size through spatial confinement effects, favoring increased rate capability (Figure 8d). For example, we designed biomass-derived hierarchically porous with Co/Co 9 S 8 active nanoparticles via micromesopore confinement synthetic strategy, which can effectively overcome overgrowth and agglomeration of Co and Co 9 S 8 nano particles, enhance the durability of catalysts and contribute to the overall battery performances. [195] Naturally, overly large pore structures could reduce the volumetric energy density of the cathode, while pores that are too small are susceptible to blockage by solid-state discharge products and make little contribution to capacity, hence requiring careful tailoring in favor of introducing self-adaptability properties that give catalysts the versatility they need.
Given the slow CO 2 RR and CO 2 ER kinetics, pure carbon materials are far from being practical because of their limited activity. It is therefore necessary to modulate the catalytic activity to induce the growth and evolution of discharge products (Figure 8e). The introduction of heteroatom doping (e.g., N, S, B, P, etc.), defects, modification of coordination states, or adjustment of metal active centers can alter the electronic structure of carbon materials at Fermi level and modify CO 2 adsorption energy, [272,273] thus improving the overall battery performances. In addition to pure carbon and its derivatives, metals or their oxides, carbides, sulfides, and carbon-supported metal catalysts have an essential role in CO 2 batteries. For example, bimetallic catalysts, such as RuCu, [274] FeCu, [196] RuCo alloys, [138] can enhance the adsorption of M n+ and CO 2 upon discharge  and facilitate electron interactions between alloy structures and products upon charging. Some specific structures, such as the construction of Ni/Ru core-shell structures, in which the electronic structure of the Ru shell is tuned by biaxial compressive strain engineering, can be facilitated by kinetically sluggish CO 2 RR and CO 2 ER processes. [145] In addition, organic frameworks such as MOFs and COFs with unique physicochemical properties and regulated functional structures, or even metal phthalocyanines, which use metal ions or clusters as bonds and organic ligands as carriers, have broad application prospects in M-CO 2 batteries. [162,275] An ideal catalyst should also possess excellent electrolyte wettability, allowing rapid liquid and electron transport, optimizing the local environment of catalysts, and maximizing the catalytic activity toward CO 2 RR and CO 2 ER (Figure 8f). Carbon materials such as CNTs and graphene tend to be less permeable due to the lack of high-energy defects on their surfaces. Wettability is also facilitated by the functionalization of carbon surfaces to create defects, or by the doping of heterogeneous atoms, metal or oxide-derived materials, emerging catalysts containing metal centers, and organic ligands such as MOFs and single-atom catalysts. Specifically, electrolyte wettability, influenced by surface nano-microstructure and surface energy, can significantly affect the rate of absorption, ionic conductivity, and diffusion between porous materials and electrolytes, and further affect the rate and durability of battery systems. [276] In addition to electronic structure modulation to augment electron transport, physical structure of catalysts is also of considerable importance to induce product growth and morphological evolution, and to improve the specific capacity and cycling stability of batteries (Figure 8g). For example, 1D porous materials can shorten transport paths and reduce the resistance of ion transport, promoting conductivity and reaction kinetics. 2D materials may deliver a higher surface area, while 3D materials might allow the creation of a finer balance between surface area and pore volume. [276][277][278][279] In addition, constructing 3D self-supporting electrode materials is an effective strategy for avoiding issues such as burial of active sites, increased resistance, and easy dislodgement associated with preparing cathodes by coating methods, and is conducive to promoting charge and ion transfer, enhancing contact between the electrocatalyst and electrolyte, and improving the long-term stability and catalytic activity of the electrode. Typically, we prepared 1D rod-like CuCo 2 O 4 encapsulated in polypyrrole shell (CCO/PPy) as Na-CO 2 battery electrocatalysts, which effectively improved ion directional transfer rates and electrode conductivity, gave rise to more oxygen vacancies and catalytically active sites, and also helped to protect the active material from electrolyte corrosion during long-term operation. [197] Recently, 2D MXenes have caught the interest of researchers because of their ability to actively and spontaneously chemically absorb and activate CO 2 . [280,281] Zhang et al. [281] thoroughly examined the stability and CO 2 RR/CO 2 ER catalytic activity of bare MXene (M 2 C), oxygen-functionalized MXene (M 2 CO 2 ), and single-atom (SA) modified M 2 CO 2 in Li-CO 2 batteries. It was found that in bare M 2 C, Mo 2 C exhibited the best catalytic activity, comparable to that of CNTs, while M 2 CO 2 was less active. Overpotential could be dramatically reduced when an SA was introduced to the M 2 CO 2 , even outperforming graphene catalysts. They also explored the effect of anchoring different transition metal SAs on the structure of Mo 2 CO 2 and identified several satisfactory candidates, including Mn, Fe, and Zn (Figure 9a,b). The influence of SA on the adsorption behavior and the generation of negative charge centers were considered to be responsible for SA modification results.
Although numerous catalysts have been developed for M-CO 2 batteries, none seem to have been found so far that meet practical commercial requirements. Because of our insufficient understanding of the reaction mechanism, we always screen the electrocatalyst through trial-and-error methods. The majority of studies that have been published tend to acknowledge that CO 2 RR proceeds via the reaction of 2M 2 (CO 3 ) n + C ↔ 4M n+ + 3CO 2 + 4e − . Although some fundamental investigations have been carried out, using some in situ characterization techniques such as in situ surface-enhanced Raman spectroscopy (SERS) [283,284] and aberration-corrected environmental transmission electron microscope (AC ETEM) [285] to characterize discharge products, and differential electrochemical mass spectrometry (DEMS) [286,287] to quantify the amount of charge and gas involved in CO 2 RR, it is still unclear where electrocatalyst activity and selectivity originate from, what the characterization of products and intermediates is, and the specific chemical routes. Recently, Peng et al. [282] deciphered the CO 2 RR mechanisms of Cu, Au, and Cu ML @Au in DMSO (dimethyl sulfoxide)based electrolytes by using in situ SERS and DFT calculations (Figure 9c-e). Their research was dedicated to elucidating CO 2 RR mechanisms at the molecular level, providing direct spectroscopic evidence for the formation of intermediates and products (i.e., CO 2 − , CO, and Li 2 CO 3 ) on model electrocatalysts, and with the help of DFT calculations, emphasizing the critical role of the near-Fermi-level d-orbital state of electrocatalysts in CO 2 RR activity.

Electrocatalysts for Aqueous M-CO 2 Batteries
Aqueous M-CO 2 batteries require an effective CO 2 RR/CO 2 ER or CO 2 RR/OER bifunctional electrocatalyst on the CO 2 cathode (depending on the discharge mechanism) to minimize overpotential during discharge and charging. In other words, because various aqueous electrolytes with different catalysts are matched in aqueous M-CO 2 batteries, resulting in different discharge products, the catalyst requirements for charging would naturally be varied. For example, we proposed a reversible hybrid Na-CO 2 battery with saturated NaCl solution as catholyte and N-doped single-walled carbon nanohorns (CNHs) as a cathode (Figure 10a), where discharge products were observed to be mainly NaHCO 3 and C. [288] It is obvious that excellent CO 2 ER activity of the catalyst must be required for charging to enable low charging polarization and high reversibility. In addition to benefiting from soluble discharge products to enhance reaction kinetics, CNHs with N dopants, and unique internal and interstitial nanopore structures, deliver a large number of structural defect sites for CO 2 adsorption and electron transfer, and thus the hybrid Na-CO 2 battery demonstrated a low charging voltage. Conversely, Kim et al. demonstrated a rechargeable hybrid Na-CO 2 battery with NaOH as catholyte and Pt/C + IrO 2 catalyst as cathode, [289] in which CO 2 was used as feedstock gas at the cathode and H 2 produced upon discharge, and O 2 produced upon charge. Apparently, high charging voltage leads to a large polarization, and a catalyst with high OER activity is required to promote energy efficiency (Figure 10b,c). Similarly, some reversible hybrid Li-CO 2 batteries and aqueous Zn-CO 2 batteries with HCOOH as the main discharge product have been developed. [126][127][128] The experience with ECO 2 RR has led to the corresponding development of catalysts such as bimetal alloys, atomically dispersed metal-based catalysts, and carbon materials for aqueous M-CO 2 batteries in recent years, where the discharge products are mainly CO. [186,290] To achieve the rechargeability of these M-CO 2 battery systems, the catalysts are required to catalyze not only the CO 2 RR upon discharge but also the OER upon charging. Wang et al. [291] synthesized a 3D Ir@Au CO 2 RR/ OER bifunctional cathode by chemical and electrochemical deposition for Zn-CO 2 battery (Figure 10d-f). The Ir@Au displayed bifunctional electrocatalytic properties, with 85% FE CO at −0.5 V (390 mV overpotential) for CO 2 to CO conversion and an overpotential of 560 mV at 10 mA cm −2 for neutral O 2 evolution. The corresponding Zn-CO 2 battery demonstrated CO 2to-CO reduction with 90% FE at 8.3 mA·cm −2 and an overall energy efficiency of 68% with fuel generation. Fe/Ni diatomic site catalysts have been shown to be highly efficient CO 2 RR/ OER bifunctional cathodes for rechargeable Zn-CO 2 batteries that displayed high FE and excellent rechargeability. [185,292] Adv. Funct. Mater. 2023, 33, 2300926   Figure 9. a) Volcano curves for η CRR and η CER with ∆G 3 and −∆G 4 as descriptors. b) Relationship between CO adsorption energy and η CRR or ∆E in SA-Mo 2 CO 2 systems. Reproduced with permission. [281] Copyright 2022, Wiley-VCH. c) Illustration of the relationship between surface electronic structure and CO 2 RR activity. d) The partial density of states (PDOS) of the d band of the surface atom of Au, Cu, and Cu ML @Au electrocatalysts. e) Illustration of interrogating the mechanism of CO 2 RR in Li-CO 2 batteries with in-situ SERS. Reproduced with permission. [282] Copyright 2022, American Chemical Society.
In-depth electronic structure analysis revealed that the Fe in the NiFe heteroatomic pair served as the catalytic center with a valence below +3, and the oxidation state of Ni was located between 0 and +2. The functional complementarity and synergy were achieved by the orbital coupling of the catalytic Fe center and Ni atom, with the adjacent Fe-N 4 and Ni-N 4 site pairs (Figure 10g-i). Similarly, Zhu et al. [293] reported a diatomic single-atom catalyst (CuNi-DSA/CNFs) consisting of CuN 4 Adv. Funct. Mater. 2023, 33, 2300926   Figure 10. a) Schematic illustration of hybrid Na-CO 2 battery with CNHs catalyst. Reproduced with permission. [288] Copyright 2020, Elsevier. b) Anodic rotating disk electrode profile and c) charge-discharge profiles of Pt/C + IrO 2 catalyst under CO 2 -saturated 0.1 m NaOH and seawater. Reproduced with permission. [289] Copyright 2018, Elsevier. d) SEM and HAADF images, e) LSV curves in CO 2 and non-CO 2 atmosphere, and f) LSV curves for the OER of Ir@Au cathode. Reproduced with permission. [291] Copyright 2019, Wiley-VCH. g) Fe K-edge XANES spectra of Fe 1 -Ni 1 -N-C and Fe 1 -N-C. h) Ni K-edge FT-EXAFS spectra of Fe 1 -Ni 1 -N-C and Ni 1 -N-C. i) Electron density difference analysis of CO 2 adsorbed on Fe−N 4 sites of Fe 1 -Ni 1 -N-C. Reproduced with permission. [292] Copyright 2021, American Chemical Society. j) Schematic construction of a rechargeable Zn-CO 2 battery with a CuNi-DSA/ CNFs cathode. k) Discharge polarization curve and power density curve, and l) discharge and charge cycling stability of the assembled Zn-CO 2 battery. Reproduced with permission. [293] Copyright 2022, Wiley-VCH. and NiN 4 sites, where strong electron interactions caused by electronegativity shifts between Cu and Ni atoms optimized the adsorption strength of *COOH intermediates and promoted the kinetics of CO 2 RR. The Zn-CO 2 battery exhibited a maximum power density of 4.0 mW cm −2 at 5.8 mA·cm −2 and maintained 56 h of continuous cycling with a narrow voltage gap of 0.38 V (Figure 10j-l).
To date, aqueous CO 2 battery batteries have been studied to some extent but have generally exhibited unsatisfactory power densities due to the inherently sluggish kinetics of CO 2 RR and the lack of highly active catalysts. In addition, the only discharge chemicals currently available in aqueous CO 2 batteries are CO, CH 4, CH 3 OH, and HCOOH, and polycarbonate products such as C 2 H 5 OH, and C 2 H 4 have hardly been reported to date. Although many catalysts have been successfully implemented for the half-reaction of anodic OER and cathodic CO 2 RR, simultaneous highly active CO 2 RR/OER bifunctional catalyst cathodes are still very challenging.

Gas Electrode Architecture Design
GDEs, which are frequently used in the fuel cell community, are composed of a catalytic layer and a gas diffusion layer (GDL), where a porous electrocatalyst layer made of nanoparticles and a binder is deposited on support for the GDL (such as carbon paper/ carbon cloth), allowing for good ion/electron conduction between collector and electrolyte and fast and easy CO 2 transport. [294,295] Because GDEs can boost CO 2 mass transfer and enhance the current density by several orders of magnitude, GDEs used to CO 2 RR have undergone significant research and produced amazing research findings, as discussed in several recent studies. [83,[295][296][297][298][299][300] In particular, record-breaking partial current densities over 1 A cm −2 have been achieved for C 2+ products, approaching performance levels consistent with those anticipated from commercial technology. [94] In contrast, in M-CO 2 batteries there is still a gap in evaluating performances of the overall CO 2 cathode/ GDEs structure design at the battery level, despite significant efforts in developing effective catalytic materials for CO 2 cathode. Due to the low CO 2 availability and current density maintained, the energy density and power density for practical battery scale values in M-CO 2 batteries are far below expectations and are rarely discussed. The focus of designing GDEs, both in CO 2 electrolyzers and CO 2 batteries, is to consistently achieve reliable CO 2 conversion efficiency at high current densities. CO 2 conversion efficiency is not only dependent on activated catalysts but also on the overall structure and composition of GDEs, which affects the behavior of reactants and products transported and thus the overall performance. Tan et al. [301] created a bi-side-diffusion pouch Li-CO 2 battery that can increase the area of CO 2 transport and ensure CO 2 RR efficiently (Figure 11a). It was experimentally proven that the practical specific energy ranged from 313.76-614.65 Wh kg −1 and the specific power corresponded to 2.15-3.08 W kg −1 by varying the wetting state of the GDL. The specific energy and specific power of lean electrolytes increased by 96% and 43% respectively compared to flooded electrolytes (Figure 11b). They then looked into how cell performance was affected by carbon loading and GDL thickness. With constant electrode porosity, it was discovered that for carbon electrodes, the carbon loading matched the variance in electrode thickness. At carbon loadings up to 1.8 mg·cm −2 , a peak in the specific energy range of 450-796 Wh kg −1 was observed (Figure 11c). The specific energy initially exhibited a modest upward trend when the GDL thickness was <140 µm; however, as the battery mass increased more considerably, the specific energy exhibited a downward trend. With increasing GDL thickness the specific power continued to decrease (Figure 11d). This study demonstrated that the amount of electrolyte in the GDL should be minimized without compromising the performance of the battery, and that dry GDLs ensured high CO 2 diffusivity, which facilitated the development of higher specific energies in practical batteries.

Electrolyte Engineering
The electrolyte plays an essential role in both CO 2 electrolyzers and CO 2 batteries. In addition to forming a closed circuit in the electrochemical system and conducting ions between electrodes, it affects the stability of the intermediate products of the CO 2 conversion process. The ideal electrolyte should have, but not be limited to, the following characteristics: high solubility of CO 2 , high ionic conductivity, strong electrochemical stability, good chemical compatibility with electrode materials, easily handled and stored, and green and safe. [23,56,302] Therefore, the selection of the right electrolyte is crucial to the performance of CO 2 electrolyzers and CO 2 battery systems.

Electrolytes for ECO 2 RR
In CO 2 electrolyzers, two main types of electrolytes have been investigated: aqueous electrolytes and nonaqueous electrolytes (including organic solvents and ionic liquids). Although nonaqueous electrolytes are gradually beginning to find favor with researchers, the vast majority of CO 2 RR studies have been carried out in aqueous electrolytes. Apart from the advantages of aqueous electrolytes, such as low cost, large scale, wide availability, safe handling and storage, and stable ionic conductivity, what is most important is that aqueous solutions can act as proton donors and proton acceptors, with unlimited possibilities for the formation of different electrochemically active substances. Bicarbonate solutions of alkali metals are the most commonly used aqueous electrolyte for CO 2 RR, on which current research is mainly focused on the effects of pH, cations, and anions.
Solution pH and local pH at electrode interfaces are important parameters affecting CO 2 RR activity and selectivity that cannot be generalized for different systems of electrocatalysts and different target products. In general, electrolytes for CO 2 RR are usually neutral or nearly neutral because lower pH levels are more conducive to the production of competing HER. Whereas CO 2 may react with OH − ions to result in carbonate/bicarbonate under alkaline conditions, which increases the overall resistance of the electrolyzer and tends to cause salt accumulation blocking gas diffusion channels, and affecting the electrolysis process. It is, however, advantageous to have a lower pH to produce the reduction products of CO 2 hydrogenation (e.g., CH 4 ), while a higher pH is suitable for the formation of CO and polycarbonate products. [303] Liu et al. [304] proposed a pHdependent microkinetic model to assess the kinetics of CO 2 RR to C 1 and C 2 products on Cu (211), in which the impact of local pH is represented by changes in the reaction energy barrier. It was shown that the Tafel slope and overpotential of the C 1 and C 2 products varied with pH. The formation of C 2 was restricted by the rate of the first proton-electron transfer to the OCCO * intermediate; whereas the formation of C 1 was controlled by the later proton-electron transfer to the CHOH * intermediate and the enhancement of its activity with increasing pH was smaller, in contrast to previous studies focusing on CO * protonation. Recently, Pedersen et al. [305] revealed the unique contribution of electrolyte pH in tailoring dicarbonate/hydrocarbon selectivity (Figure 12a). They found that the protonation of C atoms Adv. Funct. Mater. 2023, 33, 2300926   Figure 11. a) Schematic diagram of the pouch-type Li-CO 2 battery, where the folded structure including layered GDL, porous electrode, and separator pieces is folded in half with a shared lithium electrode, and the open ratio of the package film with two CO 2 windows is 30%. b) Model validation and prediction performance band of practical pouch batteries. The cell-level specific energy band (blue shadow) varies with c) the loading of buckypaper and d) the thickness of customized GDL. The insets show the cell-level specific power (red lines). Reproduced with permission. [301] Copyright 2022, Elsevier. Figure 12. a) C 2 O xy /HC molecular ratios for CO 2 R (pH7) and COR (pH13) as a function of U RHE on a broad range of Cu-based catalysts reported in the literature. Reproduced with permission. [305] Copyright 2022, Nature Publishing Group. b) Schematic illustration of CO 2 distributions in an acid-fed CEM-based MEA. c) Comparison of CO FEs and concentrations of the KHCO 3 -fed and acidic MEAs at different CO 2 flow rates, and d) corresponding SPC efficiencies at 60 mA cm −2 . Reproduced with permission. [311] Copyright 2022, American Chemical Society. e) Schematic representation of the SECM experiment. Reproduced with permission. [316] Copyright 2021, Nature Publishing Group. f) Possible paths for CO 2 -to-CO (blue), and CO-to-CH 4 (green). Reproduced with permission. [319] Copyright 2022, Nature Publishing Group. g) Experimental setup for operando pH measurements in a flow cell. g) Assembled zero-gap bicarbonate flow cell used in this study for operando Raman spectroscopy; h) Expanded view of the same flow cell. Inset: Illustration of the HCO 3 − and CO 3 2− concentration gradients near the cathode. Reproduced with permission. [324] Copyright 2020, American Chemical Society.
in CHCO * and the concomitant formation of surfaceO bonds led primarily to ethanol or acetate production, that significant pH effects arising from the additional dehydroxylation step that was required for complete removal of O atoms in the C 2 hydrocarbon pathway and that alkaline conditions inhibited the kinetically viable but thermodynamically less favorable pathway leading to C 2 hydrocarbon at low overpotentials. Indeed, the influence of the local microenvironment, especially the local pH, on CO 2 RR is still puzzling. However, even in acidic media, high current efficiencies for CO 2 RR can be achieved at industrial current densities by designing catalysts/electrodes (e.g., Au, [306,307] PdCu, [308] Cu [309] ) or reaction environments (electrolytes) [310] to boost the reduction rate of CO 2 and suppress proton reduction of HER. Recently, Li et al. [311] demonstrated CO 2 electroreduction to CO by regulating anolyte composition and concentration in an acid-fed MEA made up of an IrO x anode, cation-exchange membrane (CEM), an Ag gas diffusion cathode, and acidic anolyte (H 2 SO 4 + M 2 SO 4 (M + = Li + , Na + , K + , Cs + )). Contrary to the common opinion that high acidity contributed to HER, their study showed that CO 2 RR had higher electrocatalytic activity when the ratio of H + versus Cs + was higher. Under optimized conditions, the acid-fed MEA achieved a CO Faraday efficiency of ≈80% and an extraordinary SPC of ≈90%, and long-term stability of 50 h (Figure 12b-d).
In addition to pH, the anions and cations in electrolytes are also assumed to be important factors affecting CO 2 RR. The effect of alkali metal ions on CO 2 RR properties is widely accepted in that CO 2 RR activity increases with increasing cation radius in electrolytes, following the trend of Cs + >K + >Na + >Li + . Resasco et al. [312] investigated that the concentration of hydrated alkali metal cations at the outer Helmholtz plane (OHP) increases as cation size increases. Hydrated alkali metal cations near the electrode can generate a dipole field at the outer Helmholtz plane that can stabilize intermediates (e.g., * CO 2 , * CO, * OCC), the adsorption energy for * CO 2 is reduced by this field stabilization, thereby promoting the generation of two-electron products and CC coupling products. The understanding of how cations at the interface affect the activity and selectivity of CO 2 RR processes is still elusive. In addition to stabilizing reaction intermediates, different theoretical explanations such as modifying the local electric field [313,314] and buffering pH of the interface [315] have been proposed. To verify the above models, Koper et al. [316] combined scanning electrochemical microscopy (SECM) and ab initio molecular dynamics (AIMD) simulations to confirm the role of different anions in CO 2 RR experiments on polycrystalline gold (Figure 12e). Beyond the medium-range electric field-electric dipole interaction, they explained that the *CO 2 intermediates were stabilized by partially dissolved metal cations through a short-range electrostatic interaction. The observed differences in the activity of alkali cations toward CO 2 reduction were related to their different concentrations in the OHP and their different ability to interact with negatively charged adsorbates. Eventually, their results showed that CO 2 RR occurs only in the accompaniment of alkali metal cations, which also attracted extensive discussion. [317,318] Recently, Choi et al. [319] investigated the coupling ability of alkali metal cations with possible intermediates using quantum mechanicsbased atomic-level simulations. They proposed a cation-coupled electron transfer-based CO 2 RR mechanism to determine the role of cations in regulating the activity and selectivity of CO, CH 4 , and C 2 H 4 formation during CO 2 RR (Figure 12f). Kinetic studies revealed that the cation effect is caused by the cationassociated surface charge density of the electrode.
The anion also has a vital role in CO 2 RR. Cuenya et al. [320] observed that halide ions can trigger nano-structural reconfiguration on CuO surfaces and effectively reduce the overpotential of CO 2 RR in the order Cl − <Br − <I − without significantly affecting the selectivity. The halide ions adsorbed on the oxidized Cu surface can donate charge to the CO 2 molecule, contributing to the formation and stabilization of the * COOH intermediate. Besides, the presence of SO 4 2− and ClO 4 − on the Cu catalyst surface also had proven to be effective in promoting the formation of C 2 H 4 and CH 3 CH 2 OH. [321] As the most prevalent anion, HCO 3 − is thought to speed up the rate of CO 2 RR by improving the effective dissolved CO 2 concentration adjacent to the electrode surface through a quick equilibrium between HCO 3 − and dissolved CO 2 . [322] However, there is some controversy over the true active form of CO 2 during CO 2 RR, with most researchers acknowledging dissolved CO 2 as the active species, but HCO 3 − has also been considered to be the active species (especially formate). [8] In addition, HCO 3 − can be used as a complementary proton donor for CO 2 RR beyond protons or water, [323] where neither the unwanted HER may be promoted nor the conversion of CO 2 to carbonate or bicarbonate lowering the CO 2 concentration and inhibiting CO 2 RR activity, therefore the optimum ratio of HCO 3 − near the electrode should be found. Zhang et al. [324] developed an operando Raman measurements device in a modified CO 2 RR flow electrolyzer (Figure 12g,h). The device allows the pH surface in a flow cell to be resolved as a function of time, current density, and distance from the catalyst surface, which will be of great value in understanding the effect of pH on the electrocatalyst surface for CO 2 RR.

Aqueous Electrolytes
The utilization of aqueous electrolytes in CO 2 batteries not only significantly enhances the charge/discharge reaction kinetics and addresses the high overpotential and low cycling performance due to insoluble discharge products in non-aqueous CO 2 batteries, but also creates a very promising strategy for the generation of carbon-containing products. As mentioned previously, the main aqueous CO 2 batteries currently investigated include hybrid Li(Na)-CO 2 and Zn-CO 2 batteries, whose catholyte is usually a CO 2 -saturated neutral or near-neutral electrolyte consisting of KHCO 3 or chloride or a small amount of acetate, ensuring that the CO 2 can be flexibly fixation by CO 2 batteries to produce a variety of carbonaceous products. [179,186,189] Unfortunately, despite the similarity to aqueous electrolytes in CO 2 RR electrolyzers and the great progress that has been made in the study of aqueous electrolytes in CO 2 RR, very little has been applied to CO 2 batteries due to the complexity of the system. Considering that the application of aqueous electrolytes will be limited by the narrow potential window of H 2 O, Kang et al. [325] demonstrated a hybrid Na-CO 2 battery based on a "water-in-salt" (WiS, NaClO 4 /H 2 O) electrolyte. The WiS displayed a wide electrochemical stability window, with HER being hindered by increasing WiS concentrations, and corresponding hybrid Na-CO 2 batteries showed robust cycle life. Similarly, WIS (LiTFSI/H 2 O) with a stable wide potential window of ≈3.0 V was used as a suitable electrolyte to evaluate reaction mechanisms in Li-CO 2 batteries (Figure 13a). [326] Li-CO 2 batteries using WIS electrolytes and Mo 2 C/CNT cathodes showed high energy efficiency and good cycling stability.

Nonaqueous Electrolytes
Numerous studies have shown that the carbonate-based electrolytes (such as propylene carbonate), commonly associated with ion batteries are not compatible with maintaining reversible M-CO 2 batteries, as they are highly vulnerable to nucleophilic attack by superoxide or peroxide, which may induce premature battery death. [327] At present, ether-based electrolytes, particularly tetraethylene glycol dimethyl ether  [326] Copyright 2021, American Chemical Society. b) Effects of various electrolyte compositions on CO 2 RR activity. Reproduced with permission. [328] Copyright 2019, American Chemical Society. c) Illustration of molten salt Li-CO 2 battery. Reproduced with permission. [330] Copyright 2021, Royal Society of Chemistry. d)The open circuit potential and model demonstration of the co-axial fibrous battery cell. e) Polarization curves of the prepared stretchable, fiber-shaped Li-CO 2 battery. f) The open circuit potentials of a stretchable, fiber-shaped Li-CO 2 battery being bent from 0 to 180°. Reproduced with permission. [141] Copyright 2023, Wiley-VCH.
(TEGDME), have attracted considerable interest due to their stability against superoxide anions and relatively high oxidation potentials. Gallant et al. [328] investigated the influence of electrolyte composition on CO 2 activity (Figure 13b). The findings indicated that TEGDME-based electrolytes with moderate concentrations of Li + salts (≈0.7-2 m) are suitable for CO 2 activation because they have lower desolvation energies for Li + and high salt concentrations tend to enrich the local density of Li + near CO 2 and reduction intermediates. Nevertheless, very limited electrolyte alternatives are available for the majority of reported M-CO 2 batteries, namely TEGDME with NaClO 4 or trifluoromethanesulfonate or bis(trifluoromethanesulfonyl) imide salts, thus further efforts shall be necessary. Interestingly, Peng et al. [329] exhibited a Li-CO 2 battery that could operate at −60 °C by using a 1,3-dioxolane-based electrolyte. This battery with an iridium-based cathode displayed a high discharge capacity of 8976 mAh g −1 and a long life of 150 cycles. By contrast, Zhou et al. [330] demonstrated a LiNO 3 / KNO 3 molten salt electrolyte for Li-CO 2 battery operating at elevated temperature (Figure 13c). This battery with a Ru@ Super P cathode could be stable for 70 cycles and the charging potential was controlled to ≈3.2 V.

Solid/ Quasi-Solid Electrolytes
Solid/quasi-solid polymer electrolytes provide chances to solve safety risks including electrolyte leakage, volatilization, flammability, and the brutal growth of metal dendrites, and hold considerable promise for research into flexible wearable electronics in particular. Both gel polymer electrolytes (GPE) made up of polymer matrix and liquid electrolyte and organic-inorganic hybrid structure of composite polymer electrolyte (CEP) have been widely explored in M-CO 2 batteries. [130,132,[331][332][333][334] Wang et al. [141] constructed a flexible "spring"-like fibrous Li-CO 2 battery based on GPE (Figure 13d-f). The battery not only exhibited excellent electrochemical properties with high energy density, low charge potential of ≈3.7 V, and long-term cycling stability of 525 cycles, but also showed good adaptability to deformations such as bending and stretching, and water/fire resistance. In addition to polymer electrolytes, inorganic ceramic electrolytes have been applied in all-solid-state M-CO 2 batteries. Using a NASICON (Na superionic conductor, Na 3 Zr 2 Si 2 PO 12 ) as an electrolyte, Tong et al. [335] fabricated an all-solid-state Na-CO 2 battery by reducing the interfacial charge transfer resistance through a succinonitrile-based plastic crystal interface, with a life span of 50 cycles. Lu et al. [173] substituted the Zr ions in Na 3 Zr 2 Si 2 PO 12 with Mg ions as a solid electrolyte (NZM1SP) for Na-CO 2 batteries. The NZM1SP achieved a high ionic conductivity of 1.16 mS cm −1 at ambient temperature, and the corresponding battery exhibited a good life span of 120 cycles with a voltage gap of <2 V. It is well known that current solid-solid electrolytes generally suffer from poor ionic conductivity at room temperature and high interfacial impedance. Although quasi-solid electrolytes (with a liquid electrolyte inside) may improve ionic conductivity, the issue of liquid electrolyte volatilization has yet to be resolved and cycle stability is still far from satisfactory, thus further efforts are still required.

Membrane/ Separator Engineering
A separator or membrane is used to prevent direct contact between the anode and cathode of CO 2 electrolyzers or CO 2 batteries and instead allows ions to pass through. Some characteristics of polymer electrolyte membranes (PEMs) are desirable, for instance, insulating properties, wettability to the electrolyte, non-participation in chemical reactions, strong mechanical and thermal qualities, and displaying high ionic conductivity, which are crucial for reducing energy loss and increasing the lifespan of CO 2 electrolyzers and CO 2 batteries. [297,336]

Membranes for CO 2 Electrolyzers
Three types of PEMs that have been examined in CO 2 electrolyzers include anion exchange membranes (AEMs), cation exchange membranes (CEMs), and bipolar membranes (BPMs), all of which have charged functional groups that facilitate the movement of ions. [83,336,337] For AEMs, anions are moved from the basic cathode to the anode by interacting with fixed cationic groups. Whereas CEMs transport mobile cations from the acidic anode to the cathode by interacting with fixed anionic groups. In contrast to AEMs and CEMs, BPMs are made up of anion exchange layers (AELs) and cation exchange layers (CELs). Under reverse bias (i.e., AEL toward the anode and CEL toward the cathode), the BPMs transport H + and OH − to cathode and anode, respectively (Figure 14a,b). CEMs have exhibited high conductivity and low ohmic losses, preventing products and carbonates from entering the anode, but CEMs are not easily used in CO 2 electrolyzers as they provide acidic conditions at both anode and cathode, thus tending to HER rather than CO 2 RR and potentially causing salt precipitation. [297] Whilst adding a buffer layer between the CEM and cathode (e.g., using phosphate buffer [310,338] ) or modulating the anolyte composition and concentration [311] can help to control pH changes at the cathode interface, it can also increase ohmic losses and lead to lower energy efficiency. Compared to AEM and CEM, BPM has a much higher intrinsic resistance because of its thicker membrane layer; moreover, the CO 2 electrolyzer with BPM (compared to the other two systems) results in significant onset potential and energy loss due to hydrolysis reactions at AEL/CEL interface and neutralization reactions between H + and HCO 3 − . [43,337] AEM is most frequently employed in zerogap electrolyzers because it regulates OH − transport optimally, creating the necessary conditions for both electrodes and, when combined with copper catalysts, promoting the generation of C 1 and C 2+ . The most popular commercial AEMs for driving high-performance CO 2 RR are Sustainion and PiperION, [339] using a PiperION membrane in MEA electrolyzers led to current densities of >1000 mA·cm −2 for CO generation from CO 2 . However, the low ion mobility of the (Bi)carbonate crossover leads to increased ohmic losses, resulting in low availability of CO 2 feedstock, as well as acidification of anode electrolyte and increased overpotential of OER. AEMs are also vulnerable to the crossover of CO 2 RR liquid products (such as formate and ethanol) with the anode, which can complicate downstream separations in industrial settings. [297,340] Inspired by solid-state batteries, Wang et al. [341] accomplished stable production of pure formic acid (0.1 m) in ≈100 h using a solid-state electrolyte (SSE) between AEM and CEM to suppress liquid crossover, in which SSE is either an anionic or cationic conductor. Furthermore, the solid electrocatalytic electrolyzer can be upgraded to a symmetrical four-chamber configuration to overcome the challenge of requiring expensive OER catalysts (e.g., iridium and ruthenium) under acidic anode conditions. As illustrated in Figure 14c, the BPM was used to separate the cathode and anode chambers, and AEM and CEM were also used to separate GDEs and the proposed SSE-50 solid ion conductor. Here, the H + ions from the bipolar membrane could neutralize the negatively charged HCOO − in the solid electrolyte layer on the left to Figure 14. a) Illustration of the structure of cation-and anion-exchange membranes, and b) the ion transport through AEM, CEM, and BPM in acidic, basic, and neutral electrolyte environments. Reproduced with permission. [83] Copyright 2020, Royal Society of Chemistry. c) Schematic illustration of the proposed four-chamber CO 2 reduction cell with a solid electrolyte. d) The current densities against cell voltages and the corresponding HCOOH FEs. e) The concentration of pure KOH, which is simultaneously produced using the four-chamber solid cell during CO 2 reduction. Reproduced with permission. [341] Copyright 2019, Nature Publishing Group. f) Corrosion mechanism diagram of NASICON in aqueous solutions with different pH values. Reproduced with permission. [343] Copyright 2020, Elsevier. produce pure HCOOH, which achieved a peak partial current density of ≈150 mA cm −2 at 3.36 V. Simultaneously, 0.66 m high concentration of KOH could be obtained in the solid electrolyte layer on the right through ionic recombination of OH − and K + (Figure 14d,e). It is instructive to implement large-scale coupling of CO 2 RR with other anode reactions based on the solid electrolyte concept to produce high-purity products, but additional advancements in ionic conductivity and stability of solid electrolytes are still required to maximize energy efficiency.

Membranes for CO 2 Batteries
Aqueous Zn-CO 2 batteries typically employ BPMs to stabilize different pH values of alkaline anolyte (KOH + Zn(CH 3 COO) 2 ) and neutral or near-neutral catholyte, ensuring charging and discharging processes. In terms of current investigations, the effect on the catalytic material seems to be more pronounced than the effect of the membrane on the performances of Zn-CO 2 batteries, so more attention has been paid to the catalytic material and very little to the membrane. However, Zn-CO 2 batteries have not achieved satisfactory energy and power densities, which cannot be separated from their BPMs. As mentioned before, the inherent high ohmic resistance of BPMs and the kinetic losses associated with internal water dissociation reactions lead BPMs to be subjected to high membrane voltages and energy losses. Future efforts in expanding the application of BPMs in Zn-CO 2 batteries will be of interest, and there is a need to develop a BPM with low ohmic resistance, fast aqueous dissociation kinetics, improved water permeability, and chemical/mechanical stability to strengthen stable ion transport and enhance the energy efficiency of Zn-CO 2 batteries.
In nonaqueous M-CO 2 batteries, conventional polypropylene (PP) or glass fibers (GF) are not effective in avoiding potential contamination of anode by H 2 O and CO 2 and the growth of metal dendrites, so the design of membranes or separator materials has recently received considerable attention. A direct strategy is to modify the membrane by applying a polar compound to the PP or GF surface. For instance, Liu et al. [342] utilized a Zn-MOF nanoplate-modified GF separator for Li-CO 2 batteries. The highly ordered micropores in Zn-MOF could induce uniform Li + deposition, delaying the degradation process of anode and thus prolonging the cycling stability of Li-CO 2 batteries. In addition, an effective strategy is to implement a solid electrolyte, as the solid electrolyte itself is a separator. It is worth mentioning that NASICON-type ceramic electrolytes are usually used as a separator in hybrid Li (Na)-CO 2 batteries. In the case of NASICON, for example, the ion transport is caused by the jumping of Na + ions between NASICON lattice gap positions, so that only Na + cations are allowed to be transported. [26] It physically separates the anode chamber from the aqueous cathode chamber, which effectively prevents the anode metal and organic electrolyte from being contaminated and ensures stable and safe operation of batteries. Liang et al. [343,344] investigated the developing trend of the microstructure and macromorphology of NASICON in aqueous solutions with various pH values at ambient temperature (Figure 14f). In acidic solutions, ionic conductivity tends to decrease linearly with decreasing solution pH, whereas, in alkaline environments, conductivity decreases with increasing pH. As the immersion time increases, the increase in surface stress on the electrolyte leads to severe crack and hole formation. Proton exchange between H 3 O + and Na + , grain refinement, and microscopic stress variations are thought to be the key factors contributing to the rise in bulk impedance, grain boundary impedance, and surface crack impedance. Even under neutral conditions, NASICON is not absolutely stable, especially when the conductivity decreases significantly after a long immersion period. However, the physicochemical stability of NASICON is particularly critical for achieving long-term durability of hybrid Li (Na)-CO 2 batteries. Furthermore, although the NASICON is in direct contact with anolyte and catholyte with low interfacial impedance, its inherently low conductivity does not yet meet the requirements of high energy and power density output. Consequently, further improvements in the conductivity, stability, and interfacial compatibility of NASICON in aqueous and organic electrolytes need to be investigated.

Challenges and Perspective for CO 2 Electrolyzers
Despite numerous efforts in ECO 2 RR and the demonstration of some promising electrolyzers at the pilot scale, [311,345,346] further advancements are necessary to enable the large-scale commercial deployment of ECO 2 RR. Here, we will focus on potential challenges that must be addressed and future orientations that favor large-scale development. First, at the catalyst level, an optimal catalyst needs to be produced industrially at a reasonable cost while also meeting high reactivity, stability, and selectivity requirements. It has been suggested that tandem electroreduction with a tandem structure electrode may provide opportunities. In a typical example, the electroreduction of CO to C 2+ products exhibits higher selectivity and a smaller overpotential than direct CO 2 to C 2+ products. Thus, when two independent, selective catalyst layers are deposited sequentially on a GDL to form a tandem electrode, CO 2 can be converted to CO first. Then, the carbon monoxide reduction reaction is carried out to form C 2+ products, so the problems of low FE and large overpotential may be solved. [347][348][349] Alternatively, free-standing GDEs, such as copperbased catalysts anchored to free-standing GDLs with strong interactions, may be a reliable strategy to maintain the structure and morphology of the catalyst and thus provide a longer service life. In particular, a potential research area that requires attention is membrane electrodes, which have the ability to capture and separate CO 2 . [93] Second, among numerous electrolyzers, gas phase MEA with its low ohmic loss has advantages in terms of high current density and stability. Considering CO 2 utilization, the SPC is indeed low in the majority of reported works. On the basis of the MEA cell, by connecting multiple electrolyzers in parallel or serially, not only is a high FE achieved but also an increased SPC, which will facilitate the expansion of ECO 2 RR technology to a practical level. Furthermore, unlike conventional H-type or flow-type electrolyzers, which require additional and expensive separation and purification processes, all-solid-state electrolyzers with multi-chamber configurations at room temperature have been successfully proposed and proven to produce pure liquid fuel solutions and pure gaseous hydrocarbons directly and continuously, enabling ECO 2 RR technology to be brought closer to the market and deserving more attention and efforts.
Third, coupled multifunctional electrolyzers are highly desirable for the practical deployment of ECO 2 RR. For a practical ECO 2 RR system, in addition to CO 2 RR performance and CO 2 conversion efficiency, energy consumption and the economic viability of the whole reaction system should also be taken into consideration. Generally, CO 2 RR is coupled with OER at the anode, where the OER process will consume ≈90% of the electricity in the CO 2 RR system, resulting in low energy efficiency. [211] The replacement of OER at the anode with valuable anodic oxidations with low onset potentials (e.g., methanol, glycerol, and glucose oxidation reactions) is a promising area for industrial applications.

Challenges and Perspective for CO 2 Batteries
M-CO 2 batteries are still nascent in comparison to relatively mature CO 2 electrolyzers, however, they may offer potential applications in the future due to advancements in catalysts, electrolytes, and electrode materials.
Firstly, with exceptional theoretical energy density and capacity, the nonaqueous Li/Na-CO 2 battery systems are expected to be useful for automotive batteries, especially for Mars exploration, where 95% of the atmosphere is CO 2 . However, the dilemma of low practical energy density must be addressed under real-world operating conditions for this system. To achieve the high specific energy promised by nonaqueous Li/Na-CO 2 batteries, practical cell-scale cyclability, and energy density should be assessed and established. In most cases, the specific capacities of Li/Na-CO 2 batteries are reported based on the mass of the active material without taking into account the other necessary components (metal electrodes, separators, electrolyte, and GDL), which are not representative of specific electrochemical performance (e.g., utilization). [301] As the most crucial foundation for practical application, reversibility should be substantially enhanced to obtain high energy conversion efficiencies at realistic cycle durations.
Secondly, in contrast to the energy-consuming ECO 2 RR half-reaction, aqueous and hybrid M-CO 2 battery systems that combine ECO 2 RR and the disposal/dissolution of anode metals offer increased prospects, as they can be freed from electricity consumption and ensure efficient CO 2 conversion. As a result of the low anode price and the high-value discharge product, Zn/Al-CO 2 systems can be developed as primary batteries for industrial waste gas treatment, enabling negative energy carbide production. More significantly, hybrid Li/Na-CO 2 battery systems use a highly active Li (or Na) anode with a high energy density that can reduce CO 2 while also providing a high level of electricity. Although aqueous and hybrid M-CO 2 batteries have seen an increase in interest since 2018, there is still much work to be done before they can be used in real-world applications for energy supply and carbon neutrality, such as optimizing the electrodes/electrolytes and battery configurations. Fortunately, research on CO 2 RR in aqueous batteries is inherently connected to CO 2 electrolyzers, which can be accelerated by taking advantage of advanced experience in optimizing functional materials for CO 2 electrolyzers, including catalysts, GDEs, catholyte, membranes, and other functional materials. Two rational strategies are proposed: i) optimizing the GDE/ catalyst/electrolyte interface, from powder to 3D independent electrodes to facilitate gas mass transport and increase current density; ii) optimizing the battery configuration to reduce internal resistance and extend lifetime, including the local environment, the gas inlet method, and the use of a flow battery configuration from static to flow system.

Conclusions and Suggestions
Considerable progress has been made with ECO 2 RR and M-CO 2 batteries as effective solutions for closing the carbon cycle and storing intermittent renewable energy sources. In particular, the electrically driven membrane electrode assembly CO 2 electrolyzers have developed into the most effective ECO 2 RR configuration to date, in some specific respects matching expectations for commercial technology. In contrast, highly comparable M-CO 2 batteries are still in their early stages. In this review, we discuss how CO 2 electrolyzers and M-CO 2 batteries are intrinsically linked and focus on the impact of functional materials on their performances at the industrial level, which will guide the implementation of large-scale applications in the future. It is particularly emphasized that the development and optimization of hybrid Li (Na)-CO 2 battery systems have significant potential for equilibrium energy storage and CO 2 conversion applications and should be given special consideration. For both potential CO 2 conversion technologies to be widely adopted and accepted by the industry, further basic and applied research should be urgently promoted, notwithstanding the initial efforts and accomplishments at the laboratory level. Some suggestions for upcoming studies are provided while taking into account the existing research conundrum and future advances.

Critical functional materials involved in CO 2 electrolyzers
and M-CO 2 batteries deserve more research attention, including developments of CO 2 RR/OER and CO 2 RR/CO 2 ER multifunctional catalytic materials with durable catalytic activity, new electrolytes, membranes with ion selectivity and cost-effectiveness, as well as dendritic growth, hydrogen precipitation and corrosion-related challenges for metal electrodes in M-CO 2 batteries. Any brand-new functional material should be comprehensively and thoroughly evaluated and reported based on performance metrics at the full-cell level. 2. Combining in situ characterization and theoretical calculations enables benchmarking, demonstration, and optimization of processes, maximizing energy, and carbon efficiency. Advanced in situ characterization techniques allow for careful analysis of products, monitoring the nature of catalytic reactions, and tracking key material evolution and failure mechanisms. Optimizing electrode/reactor structures based on microkinetic multi-site models. Factors such as gas-liquid flow rate, diffusion and migration, current distribution, and electrochemical performance parameters should also be modeled and optimized to guide the actual operating process. 3. Taking new concepts from other energy conversion mechanisms and integrating them into CO 2 electrolyzers and M-CO 2 batteries. A proof-of-concept example is the combination with bioreactors to upgrade CO 2 into energy-rich longchain compounds. This innovative design has the potential to boost electrochemical performance while also lowering capital expenses. 4. Hazard risk is the first consideration when identifying largescale applications, and environmental and safety risks should receive particular attention. For instance, potential contaminants in the environment come from acids, bases, and organic electrolytes. Although highlighting the importance of hybrid Li (Na)-CO 2 batteries, some technical challenges remain, such as safety issues caused by Li (Na) and flammable organic electrolytes, and while the solid electrolyte separator provides strong protection, considerable efforts are expected to be invested in the reliability and safety of solid electrolyte separator technology in both the academic and industrial sectors. 5. Opening up scientific practice. Establishing an equitable, complementary, and sharable data management system for functional material research, reporting the details of testing for each technological process, ensuring discoverability, accessibility, interoperability, and reproducibility of research results, and providing the community with a comprehensive understanding of CO 2 electrolyzers and M-CO 2 batteries.
Without a doubt, there is still a long way to go for industrial CO 2 conversion and energy storage, and more investigation into hybrid Li (Na)-CO 2 batteries in particular shall be required. With continuous efforts, we are optimistic that the practical CO 2 electrolyzer and M-CO 2 batteries for effective CO 2 fixation and significant energy storage will soon materialize. Hopefully, this review will stimulate innovative ideas and provide guidance for the development of CO 2 conversion and energy storage devices.