Rational Engineering of 2D Materials as Advanced Catalyst Cathodes for High‐Performance Metal–Carbon Dioxide Batteries

Given the unique characteristic of integrating CO2 conversion and renewable energy storage, metal–CO2 batteries (MCBs) are expected to become the next‐generation technology to address both environmental and energy crises. As involving complex gas–liquid–solid three‐phase interfacial reactions, cathodes of MCBs can significantly affect the overall battery operation, thus attracting much research attention. Compared to conventional materials, 2D materials offer great opportunities for the design and preparation of high‐performance catalyst cathodes, especially showing superior bifunctional electrocatalytic capacity for rechargeable MCBs. The inherent high‐specific‐surface area and diverse structural architectures of 2D materials enable their flexible and rational engineering designs toward kinetically favorable metal–CO2 electrochemistry. Herein this review, the cutting‐edge progresses of 2D materials‐based catalyst cathodes are presented in MCBs. The reaction mechanisms of various MCBs, including both nonaqueous and aqueous systems, are systematically introduced. Then, the design criteria of catalyst cathodes, and the merits and demerits of 2D materials‐based catalyst cathodes are discussed. After that, three representative engineering strategies (i.e., defect control, phase engineering, and heterostructure design) of 2D materials for high‐performance MCBs are systematically described. Finally, the current research advances are briefly summarized and the confronting challenges and opportunities for future development of advanced MCB cathodes are proposed.


Introduction
In recent decades, the explosive carbon emission has aroused serious climate change, further leading to tremendous and irreversible deterioration to the global ecological environment. [1]To curb the greenhouse effect and achieve carbon neutrality, the excessive carbon dioxide (CO 2 ) emission from fossil-fuel combustion should be paid more attention. [2]In current CO 2 fixation/recycle systems, electrochemical CO 2 reduction is considered to be a sustainable and effective approach, which can convert CO 2 into high-value-added chemicals and fuels. [3]Inspired by the traditional rechargeable metal-ion batteries (MIBs), the newly developed metal-CO 2 batteries (MCBs) enable both CO 2 fixation and energy storage at the same time. [4]istinct from the electrochemical CO 2 splitting technology, MCBs allow intermittent electricity input for CO 2 conversion.Therefore, the MCB techniques are believed to have a broad application prospect in the future.
Given the unique characteristic of integrating CO 2 conversion and renewable energy storage, metal-CO 2 batteries (MCBs) are expected to become the next-generation technology to address both environmental and energy crises.As involving complex gas-liquid-solid three-phase interfacial reactions, cathodes of MCBs can significantly affect the overall battery operation, thus attracting much research attention.Compared to conventional materials, 2D materials offer great opportunities for the design and preparation of high-performance catalyst cathodes, especially showing superior bifunctional electrocatalytic capacity for rechargeable MCBs.The inherent high-specificsurface area and diverse structural architectures of 2D materials enable their flexible and rational engineering designs toward kinetically favorable metal-CO 2 electrochemistry.Herein this review, the cutting-edge progresses of 2D materials-based catalyst cathodes are presented in MCBs.The reaction mechanisms of various MCBs, including both nonaqueous and aqueous systems, are systematically introduced.Then, the design criteria of catalyst cathodes, and the merits and demerits of 2D materialsbased catalyst cathodes are discussed.After that, three representative engineering strategies (i.e., defect control, phase engineering, and heterostructure design) of 2D materials for high-performance MCBs are systematically described.Finally, the current research advances are briefly summarized and the confronting challenges and opportunities for future development of advanced MCB cathodes are proposed.
The emergence of MCBs can be traced back to the research of metal-air batteries (MABs). [5]Based on the reaction mechanisms between alkali metal ions and oxygen (e.g.,  2Li MABs with high theoretical energy densities (such as 3500 Wh kg À1 for the Li-O 2 batteries and 1600 Wh kg À1 for the Na-O 2 batteries) have received great attention and development. [6]5b,7] However, the follow-up studies unexpectedly found that the introduction of CO 2 could not only significantly enhance the capacity of metal-O 2 systems, but also enable the battery operation by functioning as the reactant gas alone.MCBs exhibit huge application prospects especially in some situations with high CO 2 concentration, such as submarine operation and Mars exploration. [8]Depending on the activities of metallic anodes, MCBs are usually classified into nonaqueous (e.g., Li/Na/K-CO 2 ) and aqueous (e.g., Al/Zn-CO 2 ) systems.Typically, the Li/Na/K-CO 2 batteries possess high-equilibrium voltages and large theoretical energy densities, whereas forming value-limited carbonaceous products, mainly carbonates. [9]In contrast, the Al/Zn-CO 2 systems compatible with aqueous electrolytes exhibit relatively small equilibrium voltages and low energy densities, but endowing with higher selectivity on the carbonaceous chemical products (e.g., carbon monoxide [CO] and ethylene [C 2 H 4 ]). [10]Considering the sophistication of practical production and application, the in-depth research is critically needed to fully exploit the advantages of each MCB system.
Despite the great breakthroughs made in recent years, some serious challenges still remain.1e,11] The thermodynamically stable C=O double bond in CO 2 molecules leads to an endothermic process for CO 2 conversion, which tends to be energetically unfavorable. [12]oreover, the involved multielectron/proton transfer processes also retard the electrochemical reactions in MCBs.In this sense, the rational cathode material/catalyst design plays a vital role in accelerating reaction kinetics and improving energy conversion efficiency.Since the advent of graphene, the intensive research of 2D materials has injected new vitality into the development of many fields including the MCBs. [13]Specifically, the distinct aspect ratios and broad surfaces with high activity endow 2D materials with anisotropic charge-transport characteristics and impressive plasticity for structure engineering.By reconstructing the chemical microenvironments of surfaces, the intrinsic physicochemical properties of 2D materials can be optimized toward superior catalytic performance for MCBs, including CO 2 capture, anti-agglomeration, and promoted decomposition of discharge products. [14]Moreover, various flexible configurations (e.g., planar, buckling, and pucker) permit the cathode to effectively resist the microstresses caused by the deposition of discharge products, and thus remarkably improve the structural stability for long-term cycling.So far, there have been increasing reports of engineering 2D materials as catalysts to enhance the electrode kinetics of MCBs on multiple scales.
In this review, we mainly focus on the latest research progress in the 2D materials-based cathode catalysts of MCBs.To provide a fundamental insight into the MCBs, we first introduce the essential reaction mechanisms of different metal-CO 2 systems (Figure 1).Then, the design criteria of cathode are discussed, along with highlighting the unique advantages of 2D materialsbased catalysts for enhancing the electrochemical reaction kinetics.Combined with the relevant reported cases for advanced MCBs, some viable strategies and underlying rationales for 2D materials engineering are systematically summarized.Finally, we outline the potential opportunities and challenges for the next-generation 2D materials-based catalysts of advanced MCBs.

Mechanisms of MCBs
Generally, MCBs consist of both metal anodes and catalyst cathodes.During discharge, CO 2 will undergo catalytic reactions involving multiple electron/proton transfer processes to form different carbonaceous products.The cathode plays a critical role in the battery performance, which involves the sophisticated gas-liquid-solid three-phase interfacial reactions.Exploring the inherent reaction mechanisms of cathodes can better stimulate the rational design of advanced metal-CO 2 electrochemical systems. [15]Moreover, due to the varied chemical activities of the metal anodes, the water-and oxygen-proof capabilities of the diverse MCBs are different, thus deriving distinct electrolyte systems for performance matching.In this section, we will generally introduce the reaction rationales of various MCBs.

Mechanisms of Nonaqueous MCBs
For alkali metal-CO 2 (A-CO 2 ) batteries, aprotic organic electrolytes are usually employed for ion transport.Located in the first major (IA) group of the periodic table of elements, alkali metals possess stronger electron-donating capacity than other metals.Therefore, the alkali metal anode operated in the waterproof electrolyte enables the battery reaction (Equation ( 1)) implemented at relatively high theoretical discharge voltages, further contributing to superior theoretical energy densities compared to the aqueous batteries.Therefore, despite the harsh operating conditions, A-CO 2 batteries exhibit a practical prospect for the energy storage with efficient electricity output.
The studies on A-CO 2 batteries were initially inspired by exploring the influence of CO 2 on MABs operated in the mixture of O 2 and CO 2 .Without the O 2 reactant, the electrochemical mechanism of pure A-CO 2 batteries seems simpler than that of O 2involved CO 2 reduction.The primary Li-CO 2 battery prototype was realized by Archer et al. in 2013. [16]With the progress of related research, the specific reaction principle is gradually revealed.Nemeth et al. claimed that CO 2 dissolved in the aprotic electrolyte preferentially captures electrons at the cathode upon discharging and forms the intermediate C 2 O 4 2À species by double single-electron reduction reactions (Equation ( 2)) (Figure 2a,b). [17]Similar to the disproportionation reaction of LiO 2 implemented in Li-O 2 batteries, Chen et al. inferred that the metastable C 2 O 4 2À undergoes an analogous two-step disproportionation procedure to form thermodynamically favorable CO 3 2À and carbon (Equation ( 3) and ( 4)). [18]The generated CO 3 2À then electrostatically couples with Li þ to form stable Li 2 CO 3 (Equation ( 5)).The whole reaction process for Li-CO 2 electrochemistry can be elucidated by Equation ( 6) (Figure 2c).
3a,19] Based on this point, Wang et al. synthesized a 3D porous fractal model (Figure 3a) by rational morphology design, which could regulate the formation of CO instead of C species within a broad discharge current window (Figure 3b,c). [20]The related reaction mechanism for CO production can be illustrated by Equation ( 7).Moreover, Qiao et al. found the evidence of Li 2 O formation in a classic Li-CO 2 aprotic environment by using in situ Raman and ex situ Fouriertransform infrared (FTIR) spectroscopies (Figure 3d-f ), which could be explained by Equation ( 8). [21]Compared with Li 2 CO 3 , Li 2 O is formed at a relatively low voltage, the accumulation of which may block the diffusion of CO 2 and further hinder the transition to Li 2 CO 3 (Figure 3g).Currently, there still exist some controversies about the specific pathways of the sophisticated CO 2 reactions, which might be explained by the principle of minimum energy transmission as each catalyst cathode design would possess its distinct kinetically favorable reaction pathway.But Li 2 CO 3 with ultrahigh structural stability is generally considered as the final discharge product in the aprotic Li-CO 2 electrochemistry. [22]i Analogous to the diversity of discharge reaction pathways, the evolution process of CO 2 during charging is also elusive, which largely depends on the cathode kinetics.One possibility is the self-decomposition of Li 2 CO 3 with the fixation of CO 2 to carbon, enabling the Li-CO 2 electrochemistry rechargeable but irreversible. [23]The other possibility is the reversible reaction involving carbon species, which exhibits a perfect electro-cycle for Reproduced with permission. [20]Copyright 2018, Royal Society of Chemistry.d-f ) Characterizations of Li-CO 2 batteries (20 μAh) using the 0.5 M LiClO 4 -DMSO electrolyte to verify the formation/decomposition of Li 2 O upon cycling: d) In situ Raman spectra during the discharge (Left panel) and charge (Right panel) processes; e) voltage profile with a charge cutoff voltage of 4.5 V at 100 mA g À1 ; f ) corresponding ex situ Fourier-transform infrared (FTIR) spectra recorded at different discharge/charge states marked by varied colors.g) Schematic illustration of the rechargeable/reversible (left panel) and rechargeable/irreversible (right panel) Li-CO 2 electrochemistry.Reproduced with permission. [21]Copyright 2017, Elsevier.
sustainable energy conversion and storage.Whatever the reaction route, Li 2 CO 3 plays a vital role in the charging process.Nevertheless, due to its high thermal stability and insulating property, the electrochemical decomposition process of Li 2 CO 3 is extremely sluggish, normally driven by a high charge voltage (>4.2 V). [24] Based on current research status, the selfdecomposition process of Li 2 CO 3 is considered to possess four possible pathways, which are all judged to be endothermic reactions by Gibbs free energy calculations. [25]The first reaction pathway involves the generation of CO 2 and O 2 , accompanied by the charge transfer of 2e À per unit of Li 2 CO 3 (Equation ( 9)).However, the actual decomposition process is remarkably affected by the electro kinetics, in which energy will follow the optimal conversion route to ensure that the barrier encountered is minimal.Therefore, with the decrease of charging current density to a certain threshold, O 2 will not be detected (Figure 4a,b), which can be explained by the second reaction pathway (Equation ( 10)). [26]Specifically, the electron has a kinetically favorable condition with less competition to combine with O 2 and form superoxide radical (O 2 À ), which might be further converted into O 2 (Equation ( 11)), or spawn some parasitic reactions with the electrolyte.Recently, Freunberger et al. discovered a unique electrochemical oxidation mode of Li 2 CO 3 , in which the singlet oxygen ( 1 O 2 ) could be formed (Equation ( 12)).But the high reactivity of 1 O 2 will poison the cell components with various parasitic reactions, thus compromising the long-term cycling endurance of batteries.In addition to the self-decomposition, Li 2 CO 3 can also react with the carbon species formed upon discharge (Equation ( 13)), which has been confirmed to be thermodynamically feasible, with an acceptable equilibrium potential of 2.8 V (Figure 4c). [21,25]i 2.1.2.Reaction Mechanisms of Na/K-CO 2 Batteries Similar to lithium-ion batteries (LIBs), the development of Li-CO 2 batteries is impeded by the severe scarcity of global lithium resource.By contrast, belonging to the same group (IA) in the periodic table of elements, the natural crustal reserves of sodium and potassium are extremely abundant, thus providing an excellent opportunity to replace lithium for electrochemical applications. [27]Based on this point, Chen et al. reported the rechargeable room-temperature Na-CO 2 batteries in 2016 (Figure 5a). [28]With the confirmed discharge/charge reaction mechanism (Figure 5b,c) represented by Equation ( 14), the assembled batteries could deliver a large reversible capacity of 60 000 mAh g À1 at 1 A g À1 and maintain 200 cycles with a cutoff capacity of 2000 mAh g À1 operated below 3.7 V.However, like Li 2 CO 3 , Na 2 CO 3 with similar insulating property exhibits robust structural stability, which requires a high charging potential to drive its decomposition.To reduce the overpotential of Na 2 CO 3 decomposition and accelerate the charge-transfer dynamics, the material (structure and composition) and configuration of the cathode side need to be rationally designed. [29]gure 4. a,b) Gas detection during the charge process of Li-CO 2 battery using RuP 2 cathode: a) Charge curve at 200 mA g À1 ; b) Corresponding gas emission upon charge tested by the in situ differential electrochemical mass spectrometry (DEMS).26a] Copyright 2019, Wiley-VCH.c) Schematic diagram of the underlying mechanism of Li 2 CO 3 decomposition, in which the degradation product of tetraglyme was verified by in situ gas chromatography-mass spectrometry (GC-MS).Reproduced with permission. [25]Copyright 2016, Royal Society of Chemistry.
Due to the low price of potassium salt and abundant potassium resource in nature, the K-CO 2 batteries are also expected to be one of the next-generation alternative systems integrating carbon fixation and energy storage.Compared to the Na/Na þ couple (À2.71V vs standard hydrogen electrode [SHE]), the standard potential of K/K þ is lower (À2.93V vs SHE), suggesting that K-CO 2 batteries have greater potential for higher voltage output than Na-CO 2 batteries. [30]More importantly, K þ , possessing outer electron orbitals farther from the nucleus than Li þ and Na þ , shows weaker electron-confining capability and thus lower Lewis acidity.During the battery operation, K þ can migrate and diffuse more easily in the electrolyte and the electrode/electrolyte interface for better electrochemical performance.9d] By in situ observation of electrode material upon cycling, the intrinsic reaction mechanism can be inferred and described as Equation ( 15) and ( 16).During discharge, K 2 CO 3 will form as a hollow ball and gradually inflate radially with the increase of deposition.The generated CO was confirmed by the discovery of plenty of nanobubbles through the analysis of time-lapse discharge reactions, which further explained the hollow structure of K 2 CO 3 .During the charge process, the K 2 CO 3 decomposition was verified by the shrinkage of the spherical discharge product.Meanwhile, the observed consumption of CNTs revealed the involvement of carbon species in the charge reaction.The assembled battery can run for more than 250 cycles at the cutoff capacity of 300 mAhg À1 .The working mechanism (Figure 5d) was inferred as Equation ( 17) and ( 18), and they demonstrated the reaction reversibility of K 2 CO 3 by combining the experimental results and density-functional theory (DFT) calculations.

Mechanisms of Aqueous MCBs
In recent years, the research and development of aqueous MCBs have also attracted much attention due to their advantages of safety, reliability, and affordability.Compared to the organic electrolytes used in aprotic MCBs which are environmentally sensitive and flammable, the electrolytes in aqueous MCBs typically contain much H 2 O with more than 70 wt% and thus show superior flame-retardant property.Importantly, the metal elements used as the anodes are all abundant in the earth's crust.Therefore, the minimal requirements for manufacturing environments, cheap raw materials, and limited demand for battery management, and protection systems ultimately contribute to the acceptable cost of aqueous systems.Moreover, because of low viscosity and high ion dissociation, aqueous electrolytes usually exhibit superior charge-transfer capability than nonaqueous electrolytes under the same conditions, which can accelerate the kinetics upon battery operation for enhancing power output. [31]2.1.Mechanism of Zn-CO 2 Batteries Aqueous ZnÀCO 2 batteries show great potential for application in simultaneous CO 2 fixation and energy storage due to the large theoretical energy density, affordable raw material, and high safety reliability.Compared with the aprotic A-CO 2 systems, ZnÀCO 2 batteries feature with the distinct working mechanism of proton-coupled charge transfer, thus enabling CO 2 conversion to much more value-added chemicals upon discharging. [32]For example, based on a conversion mechanism from CO 2 to CO, Zheng et al. designed a redox-medium-assisted system for primary ZnÀCO 2 electrochemistry. [33]The energy storage driven by zinc/zincate redox couple (Equation ( 19)) achieved an efficient CO 2 reduction at the discharge voltage of 0.2 V, which delivered a high Faradaic efficiency (%90%) for CO formation at the current density of 5 mA cm À2 (Figure 6a,b).In recent years, the technological breakthrough of aqueous Zn-air batteries provides a paradigm for the research of rechargeable ZnÀCO 2 batteries. [34]10a,35] Typically, A-CO 2 batteries always suffer from sluggish electrode reaction kinetics mainly resulting from the deposition and accumulation of solid reduction products, which severely impede the reverse reaction.By comparison, the discharge products of ZnÀCO 2 batteries are not limited to the solid phase and thus provide theoretical support for the realization of superior rechargeable MCBs.
In particular, in the unique internal environment of the cell, the liquid-phase products may be relatively favorable, since it does not present either an unreliable supply of mobile gas flow or a dense deposition on the active surface of cathode. [20,36]ased on this, Wang et al. proposed a rechargeable ZnÀCO 2 prototype based on the reversible conversion between CO 2 and liquid-phase HCOOH (Equation ( 20)) using a 3D interconnected Pd nanosheet (NS) net as the cathode (Figure 6c). [37]Such exquisitely designed bifunctional catalyst exhibited impressive electrocatalytic activity and selectivity for both CO 2 reduction reaction (CO 2 RR) to HCOOH and formic acid oxidation to CO 2 under low overpotentials.To realize bifunctional catalysis in aqueous ZnÀCO 2 electrochemistry, the O 2 evolution reaction (OER) activity of cathode material upon charge should be paid more attention.Recently, Wang et al. prepared an Ir@Au bimetallic catalyst by combining chemical and electrochemical deposition, which could induce the OER process (Equation ( 21)) when charged to %2 V (Figure 6d). [38]

Mechanism of Al-CO 2 Batteries
As the most abundant metal element in the earth's crust (%7.45 wt%), Al with much affordable cost is also regarded as a potential anode for aqueous MCBs, which shows a much negative standard potential (À2.33 V vs SHE) of Al/Al(OH) 4 À redox couple.10c,32b] Inspired by the successes of O 2 -involved Al-CO 2 batteries and Zn-CO 2 batteries, Ding et al. recently reported a rechargeable pure Al-CO 2 batteries employing Pd-coated nanoporous gold (NPG@Pd) as the catalyst cathode (Figure 6e). [39]The specific working mechanism is revealed and described as Equation ( 22), in which the discharge products of Al 2 (CO 3 ) 3 and C will reversibly decompose upon charge, indicating the feasibility of CO 2 fixation/utilization in Al-CO 2 electrochemistry.With a small gap of only 0.091 V between the discharge and charge plateaus at 333 mA g À1 (Figure 6f ), such a proof-of-concept battery exhibited an energy efficiency up to 87.7%, even comparable to some previously reported A-CO 2 batteries.

Design Criteria of Catalyst Cathodes
For metal-CO 2 electrochemistry, there could be more than one thermodynamically feasible discharge and/or charge reaction pathway, while the reaction that actually occurs largely depends on electrode reaction kinetics.Especially, the cathode catalyzing the critical three-phase reaction for CO 2 fixation and/or utilization deserves more attention. [40]Therefore, to achieve highperformance MCBs, the composition, structure, and morphology of cathode material need to be reasonably designed.Ideally, the cathode should possess high catalytic performance for efficient CO 2 capture and fixation, enabling homogeneous distribution and easy decomposition of discharge products.Since the catalytic performances rely on materials surface, the kinetics of target reaction can be significantly accelerated by designing morphology, for example, high-specific-surface area and suitable pore structure. [41]The former provides abundant active sites, while the latter generates sufficient space for facilitating CO 2 dissolution, uniform deposition of discharge products, and mass transfer. [42]In addition, great efforts should be also devoted to improving the energy-storage capacity.The power density of the energy-storage system remarkably hinges on charge-transfer process, namely the facile ion migration and rapid electron conduction in the electrode structure, which is the key step toward fast charging-discharging MCBs. [43]Meanwhile, in the pursuit of high energy density of MCBs, the maximization of chargestorage capacity within appropriate electro-redox formats is particularly demanded. [44]Indeed, the higher discharge capacity reflects superior CO 2 fixation ability, which can be quantified by the reduction products mediated by the catalyst cathode.
Moreover, cathode material is expected to catalyze the generation and decomposition of discharge products under suitable potentials, that is, driven by smaller overpotential. [45]However, the selection of practical electrochemical window should focus on the reliable and coordinated operation of various components inside battery.Especially, the voltage tolerance of cathode demands much attention, in which ultrahigh voltage may lead to material decomposition or transformation toward redox inactive phase.In addition, a robust structure configuration is also pivotal to avoid material pulverization or even collapse upon long-term cycling for extending battery service life. [46]eproduced with permission. [33]Copyright 2018, Springer Nature.c) Schematic view of the aqueous ZnÀCO 2 battery with a discharge mechanism of CO 2 RR-to-HCOOH.Reproduced with permission. [37]Copyright 2018, Wiley-VCH.d) Discharge/charge polarization profiles of ZnÀCO 2 batteries using Ir@Au bimetallic catalyst as the cathode.Reproduced with permission. [38]Copyright 2019, Wiley-VCH.e) Schematic diagram for the reaction mechanism

Merits and Demerits of 2D Materials-Based Catalysts Cathodes
The 2D materials, in which electrons can move freely in two dimensions, have attracted wide attention since the advent of graphene owing to their unique physicochemical properties. [47]In the last decade, this vast family of nanomaterials has shown remarkable potential in the field of energy conversion and storage (e.g., rechargeable ion batteries and electrocatalysis). [48]ompared to traditional materials, 2D materials exhibit diverse structural architectures (e.g., planar, buckling, and puckered structures), which can function as ordered ion/moleculetransport channels.Such distinct features can reserve sufficient space to buffer the volume expansion caused by ion intercalation or the generation of new discharge species, thus enhancing structural robustness for endurable cycling.Moreover, the large aspect ratio (lateral to thickness) endows 2D materials with broad specific surface area, which can expose more active sites for adsorption, storage, or redox reaction of ions/molecules. [49]he abundant edges of NSs with high chemical activity provide a perfect platform for catalyzing various reactions.
According to the working mechanism of MCBs, the ideal cathode should not only function as a catalyst for efficient CO 2 fixation and/or utilization with low energy barrier, but also ensure facile electron/ion synergistic transport pathway for high energy/ power output with practical application value.In this regard, 2D materials may be a type of alternative catalyst cathodes of MCBs, which can improve the three-phase reaction kinetics for boosted energy efficiency. [50]By exploiting the unique structural merits of 2D materials, CO 2 RR can be implemented with desired catalytic activity and selectivity under low overpotentials. [51]Recently, inspired by the paradigm of MABs with reversible conversion between O 2 reduction reaction and OER, developing bifunctional catalyst cathodes for efficient rechargeable MCBs has been widely carried out, which are expected to achieve an electrocatalytic oxidation process during charging.However, in most cases, the discharge products are insulating carbonate and oxalate with ultrahigh thermostability, which tend to accumulate densely on the surface of catalysts and thus severely retard the reaction kinetics.Compared to traditional bulk materials, the rich surface of 2D materials effectively mitigate the accumulation of solid discharge products and simultaneously ensure the maximum electro-contact between the electrode and products, which can significantly reduce the energy threshold to induce the product decomposition process. [52]Therefore, 2D materials are also regarded as promising bifunctional cathodes to realize the reversible round-trip reaction upon battery cycling, that is, preferential electrocatalytic conversion between CO 2 and the CO 2 RR products.
Despite numerous attractive advantages of 2D materials for metal-CO 2 electrochemistry, they still suffer from some intrinsic property limitations in practical battery application.First, unlike graphene, the electrical conductivity of most 2D materials is not satisfactory, even inferior to that of their bulk counterparts due to quantum confinement effect.Specifically, the thicknesses of 2D materials are very thin (only less than one to several nanometers for few-layer even monolayer), the dimensions of which may be comparable to the electron de Broglie wavelength.The motions of electrons will be restricted in this low-dimensional space, further leading to the quantized distribution of electronic states.The transformation from continuous bands to discrete energy levels means that electrons need an external energy input for transition, instead of moving freely, thus macroscopically presenting a reduction of carrier mobility.Second, although 2D materials exhibit extremely high-specific-surface area comparing with bulk materials, the quantity of available active sites does not surge proportionally.Actually, the catalytic activities of most 2D materials are attributed to scarce edge sites, while the basal planes with dominated areal portion are chemically inert and thus are hardly utilized for promoting CO 2 RR unless being activated.In addition, the poor structural and chemical stabilities remain to be improved.For example, the broad surface may lead to severe nonnegligible parasitic reactions between the cathode material and electrolyte, which would consume the pristine active material to form chemically inert solid electrolyte interface (SEI) during cycling.The presence of SEI not only retards the ionic migration, but also masks the surface active sites of catalyst cathode, thus hindering the capture and fixation of CO 2 .In addition, minimizing the self-stacking of 2D materials during the cathode preparation and battery operation is also a huge challenge.Good flexibility resulting from dimensionality reduction may compromise the configuration maintenance of cathode especially during the generation and decomposition of discharge products.

Engineering Strategies of 2D Materials for MCBs
Faced with the aforementioned inherent limitations of 2D materials, various engineering strategies have been proposed in recent years for structure refinement and configuration optimization to meet the property demands of specific energy-related applications.Related studies indicated that the existing engineering approaches could provide certain reference for the development of high-performance catalyst cathodes of MCBs (Table 1).However, due to the unique working mechanism of MCBs and the early stage of research, there still lacks a systematic understanding of modification strategies for the 2D materials-based catalyst cathode.In this regard, we focus on three typical strategies of nanomaterials engineering (i.e., defect control, phase engineering, and heterostructure design) for 2D materials and discuss their recent advances in MCBs.

Defect Construction
As a common crystallographic phenomenon, defects exist widely in various crystalline materials, which represent the periodic disorder of atomic arrangements and symmetry breakdown of lattice structures.Generally, they are classified into four types (i.e., point, line, planar, and volume defects) according to different arrangement patterns of the atomic disorders in geometric space.Compared to the perfect crystal, the local and short-range configurations of atoms near the defect will change obviously, for example, coordination number and bond length. [53]Intrinsically, the molecular orbital hybridization theory suggests that the electronic structures (including valance state, symmetry, and orbital occupation) of these atoms have significantly evolved, driven by thermodynamics. [54]In this sense, many physicochemical properties of materials can be regulated by crystal defects.In addition, defects, especially the dislocations, have also been proved to be closely related to the deformation behaviors and mechanical properties of metals.For 2D materials, the confined motion of electrons will lead to discretization of the continuous band, that is, the formation of discrete energy levels and wave functions in the form of standing waves.For example, the bandgap widening and blueshift of absorption edge can be observed in superlattice owing to the discretization of energy levels. [55]Compared with bulk materials, defects would affect more remarkably on the electronic reconfiguration of 2D materials, the intrinsic properties of which thus can be tuned on a more macroscopic scale. [56]mportantly, their broad surfaces with relatively high activity provide an ideal condition for the formation of numerous defects.The surface atoms accounting for a dominated proportion in 2D materials can easily escape from their original lattice sites with a low activation energy, which can be directly obtained from external environment rather than the long-range atomic transfer through thermal vibration. [57]In fact, the maximum contact with the external environment greatly improves the plasticity of materials, that is, availability for fine compositional and structural regulation.Up to now, the defect construction in 2D materials has been widely used for improving energy storage and catalysis performances, which can optimize the electronic structure for enhanced electrical conductivity, boost active sites for facile ion/molecule migration, and mitigate energy barrier for accelerated redox reaction. [58]Therefore, defect construction in 2D materials is an effective approach to prepare superior catalyst cathodes for fast metal-CO 2 electrochemical kinetics.
As the fundamental form of defects, point defects widely exist in various 2D materials, which play a vital role in tuning material properties and performances for certain applications. [59]They can be generally divided into intrinsic defects and extrinsic defects hinging on their origins.As a representative of intrinsic defects, vacancy can be easily found in 2D materials, which origins from the energy fluctuations of atomic thermal vibration. [60]ithout harsh synthesis conditions, constructing vacancies in 2D materials could be an effective method to obtain highly active catalyst cathodes of MCBs.For example, Liu et al. employed Ar plasma etching technology to create oxygen vacancies (V O ) in the NiO nanosheet arrays (NAs) (Figure 7a), which are vertically grown on the flexible carbon cloth (denoted as NiO-Vo NAs/ CT). [61]When used as the cathode of Li-CO 2 battery, the CO 2 RR and CO 2 evolution reaction (CO 2 ER) could be initiated at %2.6 and %3.8 V, respectively (Figure 7b), indicating the lower overpotential of NiOVo NAs/CT than the V O -free counterpart.The partial density of states revealed that V O could promote the upward shift of d-band electronic states referred to the Fermi level (E F ) and thus boost the antibonding states near the E F for enhanced carrier transfer (Figure 7c).With a similar function, Tan et al. reported that the construction of V O could optimize surface electron localizations of BiO 2-x NSs for aqueous Zn-CO 2 batteries, the maximum power density of which could deliver 2.33 mW cm À2 with a long cycling lifetime of more than 100 h at 4.5 mA cm À2 (Figure 7d,e). [62]Recently, in the exploration of bifunctional catalysts, Ye et al. found that the topological defectrich graphene (TDG) as metal-free cathode of Li-CO 2 batteries could provide abundant active sites for CO 2 adsorption and Li 2 CO 3 decomposition (Figure 7f ). [63]As a result, the battery delivered a large discharge capacity of 69 000 mAh g À1 at 0.5 A g À1 with impressive cycling stability for 600 cycles at 1.0 A g À1 (Figure 7g).
Assigned to the extrinsic defect, heteroatom doping can remarkably modify the physicochemical properties of 2D materials by the coordination of atoms with different thermodynamic natures.For example, the carrier mobility of graphene would be significantly accelerated by N doping.Specifically, the similar atomic radii to C greatly promote the introduction of N atoms without causing serious lattice distortion, and meanwhile the lone-pair electrons of N with larger electronegativity would induce the charge redistribution of sp 2 -hybridized C and further regulate the configuration of delocalized large π bonds in graphene for fast electron transfer.In the application of MCBs, this approach is commonly employed to accelerate the cathode reaction kinetics with favorable deposition/decomposition behaviors of discharge products.Depending on the coordination pattern of N doped in graphene, the defects can be classified into pyridinic-N, graphitic-N, and pyrrolic-N.Recently, Wang et al. reported that the pyrrolic-N could facilitate the charge transfer from N BiO 2-x nanosheets (Zn) %0-3.25 V (4.5 mA cm À2 ) -300 cycles (4.5 mA cm À2 ) [62]   TDG (Li) %2.7-3.9V (100 mA g À1 ) 69 000 mAh g À1 (500 mA g À1 ) 600 cycles (1.0A g À1 ; 500 mAh g À1 ) [63]   PNCB (Zn) 0-3.2 V (2.0 mA cm À2 ) -120 cycles (5 mA cm À2 ) [64]   Cu-N 2 /GN (Zn) 0.7-2.4V (1.0 mA cm À2 ) -120 cycles (1.0 mA cm À2 ) [66]   Ru to the bonded Bi atoms in carbon NSs (pyrrolic-N-dominated doped carbon nanosheets supported Bi nanoparticles (PNCB)) to stabilize the *OCHO intermediate upon CO 2 RR (Figure 8a,  b). [64]When evaluated in Zn-CO 2 batteries, the PNCB exhibited a discharge reaction of CO 2 RR to formate coupled with a mixed charge mechanism of OER and formate oxidation reaction (Figure 8c).At the current density of 2 mA cm À2 , it could deliver a maximum power density of 1.43 mW cm À2 with low CO emission.Moreover, the lone-pair electrons of N atoms doped in graphene can be exploited to bond with transition-metal single atoms (e.g., Fe, Ni, and Co) to form the M-N-C structure. [65]enefiting from facile formations of *CO and *COOH as well as available *CO desorption sites, graphene with the M-N-C configuration is a promising bifunctional catalyst cathode for rechargeable MCBs.However, due to the limited unoccupied 3d orbitals, the coordination-saturated M-N would chemically passivate the electrostatic bonding with intermediates and thus compromise the CO 2 ER performance.Recently, Feng et al. prepared defective graphene matrix engineered with coordinatively unsaturated Cu-N bond (Cu-N 2 /GN), in which the coordinationweakened Cu-N 2 sites with short bond lengths could facilitate their electron transfer to *CO 2 specie, thereby promoting the formation of *COOH for superior CO 2 ER (Figure 8d,e). [66]As a result, during charging of Cu-N 2 /GN cathode in Zn-CO 2 batteries, the CO 2 ER process with a low energy barrier could be driven by natural solar energy (Figure 8f ).To accelerate reaction kinetics of MCBs during discharge/charge, single atoms can be directly implanted in some 2D carbon-free materials.For example, Lian et al. reported a cation exchange strategy to introduce single Ru atoms onto Co 3 O 4 NS arrays (Ru-Co 3 O 4 ) to improve the chemical affinity of matrix for key intermediates upon CO 2 RR of Li-CO 2 batteries (Figure 8g). [67]According to the Bader charge analysis, the enhanced catalytic performance could be attributed to the active centers generated by the electron-deficient Ru atoms.) Schematic diagrams of the interaction difference of valence levels of NiO-V O and pure NiO.Reproduced with permission. [61]Copyright 2021, Elsevier.d) Partial density of states (PDOS) of Bi atom p orbital of BiO 2Àx nanosheets (NSs).e) Discharge/charge cycling curves at 4.5 mA cm À2 .Reproduced with permission. [62]Copyright 2023, Chinese Chemical Society.f ) Schematic illustration of a Li-CO 2 battery using defect-rich graphene (TDG) as the cathode.g) Discharge/charge curves at 0.5 A g À1 with corresponding Coulombic efficiency (CE) as inset.Reproduced with permission. [63]Copyright 2021, Wiley-VCH.

Phase Engineering
Compared to amorphous materials, crystalline materials featuring with polymorphism exhibit much tunable physicochemical properties and thus attract extensive research. [68]According to the certain symmetry and periodicity of atomic arrangement, the structure/phase of crystalline materials can be divided into 14 kinds of Bragg lattice in 7 crystal systems.Typically, a material would exist in the form of a thermodynamically stable crystal phase, which is largely determined by the nature of atomic bonding and some external factors (e.g., temperature and pressure). [69]nterestingly, many materials have been found to possess metastable phases or transient phases, which may exhibit some intriguing or new properties. [70]Therefore, the discovery and preparation of these unconventional phases (i.e., phase engineering) provide a feasible and effective route for developing a variety of promising applications and opening up novel fields. [71]In most cases, phase engineering origins from the nucleation of the unusual phase in the parent phase due to low energy barrier, and there is usually some crystallographic correlation between the two phases on the site-orientation relation (e.g., congruent or semi-congruent) or the habitus plane. [72]And then, the interface of new phase will gradually migrate to the parent phase driven by free energy.Intrinsically, these two processes should overcome the interfacial energy between the two phases and the strain energy generated by each other due to the mismatch of lattice structures of different phases, which may also explain the incompleteness of most phase transitions.More importantly, this suggests that the growth of unconventional phases is remarkably affected by the interface/surface features of materials, thereby verifying the unparalleled superiority of 2D materials over their bulk counterparts in regulating phase transition. [73]heoretically, by adjusting experimental parameters to create kinetically favorable conditions for the generation of target phase,  102) plane and pyrrolic-N-doped carbon.c) Galvanostatic discharge/charge profiles at 5 mA cm À2 with specific cycling curves from 0 to 20 h (inset).Reproduced with permission. [64]Copyright 2022, Elsevier.d) Fourier transform of extended X-ray absorption fine structure (FT-EXAFS) fitting results of Cu-N 2 /GN.e) Freeenergy diagrams of Cu-N 2 and Cu-N 4 models for CO 2 ER.f ) Current density-voltage ( J-V ) curves of the charged Zn-CO 2 battery and the solar cell for powering.Reproduced with permission. [66]Copyright 2020, Wiley-VCH.g) Schematic view for the fabrication of Ru-Co 3 O 4 NSs on carbon cloth.Reproduced with permission. [67]Copyright 2021, Wiley-VCH.
the thermodynamically unstable structure can be formed more likely in 2D materials.This can be attributed to the mitigation of phase-transition barriers as 2D materials are free from the high energy threshold of phase transition to activate the reconstruction or displacement of atoms in three dimensions. [74]72c,75] Especially in energy-related fields like batteries and electrocatalysis, phase control can essentially solve some critical challenges faced by 2D materials as electrodes, for example, poor electrical conductivity and low chemical activity. [76]Recently, this engineering strategy makes it possible for the fabrication of high-performance MCBs.
Up to now, plentiful research on cathode materials of MCBs has focused on some metals and their alloys (e.g., Ru, [21,24d,77] Ir, [78] Ni, [79] Fe, [80] Co, [81] Cu, [82] RuCo, [83] and RuRh [13a] ), which usually possess higher catalytic activity than common carbon materials.In practical applications of energy conversion and storage, the conventional phases of these nanomaterials can be easily synthesized due to the high intrinsic thermal stability. [84]ecently, it was found that the unusual hexagonal close-packed (hcp) phases of Au [85] and Cu [86] nanomaterials exhibit superior electrocatalytic activity and selectivity than their common facecentered cubic (fcc) phase in aqueous CO 2 RR.Inspired by the emerging phase engineering concept, we explored the feasibility of using unconventional metal nanomaterials as the bifunctional catalyst cathode in aprotic Li-CO 2 batteries. [87]By precisely tuning external parameters to create both thermodynamically and kinetically favorable conditions for nucleation and growth, the 4 H/fcc Au nanorods were synthesized by wet-chemical method, which could further act as a template for the epitaxial growth of 4 H/fcc Ir.The spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image clearly showed the characteristic stacking sequences of "ABCB" and "ABC" along the close-packed directions of [001] 4 H /[111] f , which could be assigned to 4 H and fcc phases, respectively (Figure 9a).The biphasic coexistence in the Ir nanostructures was also verified by the corresponding fast Fourier transform (FFT) patterns along the [110] 4 H and [101] f zone axes (Figure 9b-d).Comparing with the pure fcc Ir, the wavelet transform (WT) of Ir L 3 -edge extended X-ray absorption fine structure (EXAFS) of 4 H/fcc Ir nanostructures showed a broader distribution of maximum intensity for the k range due to its distinct atomic arrangements (Figure 9e-g).For performance test, the 4 H/fcc-Ir cathode demonstrated greatly accelerated round-trip reaction kinetics for both CO 2 RR and CO 2 ER, showing a small discharge/charge potential gap of 0.61 V with a high energy efficiency up to 83.8% (Figure 9h).DFT calculations suggested the facilitated Ir-O coupling on 4 H Ir, which would contribute to the highly reversible formation/decomposition of low-crystalline or amorphous discharge products with lower energy barriers compared to the conventional counterpart (Figure 9i,j).
Benefiting from the unique structure and catalytic property, the preparation of unconventional 2D materials as cathode catalysts is an effective strategy to improve the round-trip reaction kinetics.In our recent study, we synthesized a series of ultrathin bimetallic Ru-M (M = Co, Ni, and Cu) alloy NSs with a simple one-pot approach as the cathode catalysts of aprotic Li-CO 2 batteries (Figure 10a). [88]In a typical synthesis, CO produced by decomposition of Ru 3 (CO) 12 precursor functioned as the capping agent to induce the controlled growth of ultrathin RuCo NSs (Figure 10b) with a thickness of %2.6 nm.Taking the prepared RuCo alloy NSs as a representation, their distinct structure of hcp phase was verified by HAADF-STEM image, in which the FFT pattern presented the lattice fringes with an inter-planar distance of 0.21 nm assigned to the (01-10) facet of hcp RuCo alloy (Figure 10c-e).The corresponding Fourier transform of the Ru K-edge EXAFS spectrum further revealed the existence of Ru-Co bond with a coordination number of 4.0.When mixing RuCo NSs with CNT (RuCo NSs/CNT), the assembled cathode could act as a bifunctional catalyst, which not only promoted the uniform nucleation and growth of Li 2 CO 3 during discharge but also improved the decomposition of Li 2 CO 3 during charge (Figure 10f ).Electrochemical performance tests showed its low charge potential of 3.74 V with a high energy efficiency of 75% (Figure 10g).

Heterostructure Design
As an emerging concept for designing high-specific-energy electrode, the construction of heterostructures has been proposed in energy-related fields and got extensive concern in recent decade.In fact, the discovery of heterostructure can be traced back to the development of condensed matter physics, in which Shockley claimed its application to the wide-gap semiconductor emitters. [89]According to the narrow definition, heterostructure refers to the interface region formed by the contact of two semiconductors with similar atomic spacing, crystal structure, and thermal expansion coefficient. [90]89d,91] Extended to the generalized concept, heterostructure can be seen as the combination of two or more kinds of materials with physical or chemical bonding, morphologically showing chaotic geometry and random junction sites or interfaces. [92]Compared to simple physical mixing, heterostructure pays more attention to the profound influence of the interactions between the building blocks on the reconstruction of electronic configuration and/or electric field distribution of the whole material.92a,93] In general, heterostructure not only simply synthesizes the inherent advantages of each component, but also fine-regulates the physicochemical environments of hybrid materials at the microscale.The contact of different building blocks will promote the automatic calibration of the energy band at the heterointerface, thus inducing a charge redistribution until their E F reaches thermodynamic equilibrium. [94]Once the diffusion of the majority carriers and the drift of the minority carriers reach dynamic equilibrium, a space charge region (SCR) will be formed at the heterointerface, which features with high conductivity at forward bias and insulation at reverse bias.It is worth mentioning that in this process carriers will migrate from the component with high majority carrier concentration to the component with low majority carrier concentration.In addition, the resultant SCR will function as a counterforce to inhibit the carrier diffusion.As a result, the charge redistribution state is finally determined by band structure, major carrier concentration, and SCR.This intriguing physical phenomenon can be well exploited to enhance the electrochemical performances of MCBs.Specifically, the band structure of the heterostructure electrode can be finely adjusted with a small bandgap to improve the electrical conductivity.The electric field generated by SCR at the heterointerface can be exploited to energize cations for accelerating the transfer kinetics with reduced energy barrier. [95]Electrostatic interactions (e.g., chemical bonding and van der Waals force) between different building blocks or components enable the robust structural configuration of electrode, ensuring the long-term cycling stability. [96]In addition, the optimized charge redistribution would excite more active sites in the electrode material for CO 2 capture and catalysis. [97]urely in terms of large-scale heterostructure construction, 2D materials exhibit great inherent advantages over 0D and 1D materials. [98]A little like architecture, 2D materials feature with relatively large size and distinct nature of adjustable exterior styling, which can be well exploited as the robust skeleton to construct the 3D heterostructures.This ensures sufficient contact between the electrolyte and heterostructure when used as the electrode, further shortening the ion-transport pathway for accelerated electrochemical kinetics. [99]For metal-CO 2 electrochemistry, plenty of ion/electron transport channels and ample with the cutoff capacity of 500 mAh g À1 .i) Energy diagrams of different Ir facets during the decomposition of Li 2 CO 3 and j) the corresponding maximum energy barriers.Reproduced with permission. [87]Copyright 2022, National Academy of Science.
reserved space can well disperse the deposition of solid discharge products and effectively buffer the caused volume expansion to avoid the structure collapse or material pulverization of catalyst cathode.Based on this point, Xin et al. applied similar engineering strategy to Zn-CO 2 batteries by chemically coupling SnO 2 quantum dots and MXene (SnO 2 /MXene) (Figure 11a). [100]By combining high-resolution electron tomography with HAADF-STEM and ptychography imaging, the 3D reconstruction for SnO 2 /MXene at different tilted angles (Figure 11b) revealed that SnO 2 quantum dots are uniformly distributed in the interior of MXene NSs by chemical bonding.At the current density of 13.57mA cm À2 , the battery driven by SnO 2 /MXene cathode could deliver a maximum power density of 4.28 mW cm À2 .In addition, the discharge mechanism of CO 2 -to-formate conversion was revealed by DFT, which indicates that SnO 2 /MXene could reduce the corresponding reaction barrier by boosting surface coverage of *H.78a] The ultrathin Ir NSs with  (c).f ) Galvanostatic discharge-charge curves of different cathode materials at 100 mA g À1 .g) Median voltages and energy efficiency of RuCo NSs/CNT upon cycling.Reproduced with permission. [88]Copyright 2022, Wiley-VCH.
high-density wrinkles could not only expose rich active sites for CO 2 fixation/utilization, but also act as a protective layer to avoid the corrosion of carbon matrix by aprotic electrolyte.The charge termination voltage of Ir NSs-CNFs cathode could be reduced to 3.8 V, and such low charge overpotential enabled outstanding battery operation for 400 cycles without the capacity decay (Figure 11c,d).
Moreover, the broad surface area with high surface energy endows 2D materials with impressive physical bonding (mainly van der Waals force) and chemisorption capability to efficiently interact with 0D and 1D materials to form the cross-linked network. [101]By constructing porous structures or ordered channels, the efficiencies of electron transfer and ion intercalation/deintercalation can be remarkably enhanced.For example, Dai et al. reported a 3D metal-free network by integrating N-doped CNTs and N-doped reduced graphene oxide (denoted as N-CNT/RGO) used for K-CO 2 batteries (Figure 12a,b). [102]nherent in varied geometrical configurations (Figure 12c,d), these two types of carbon materials would generate a synergistic effect for accelerating electrochemical kinetics: 2D RGO with strong van der Waals interactions was the key enabler for the construction of heterostructure and the main site of catalytic reactions, while 1D N-CNT could prevent RGO from restacking and enhance the mechanical stability of 3D porous conductive architecture for efficient multi-matter (e.g., electron, ion, electrolyte, and CO 2 ) transport/reaction and strain-free accommodation of discharge products like K 2 CO 3 .The rechargeable battery driven by the N-CNT/RGO cathode could realize more than 250 cycles with a cutoff capacity of 300 mAh g À1 .The DFT calculations indicated that the good reversibility of batteries was attributed to the decomposition/formation of P12 1 /c1-type K 2 CO 3 at the N-CNT/RGO cathode.In practical cathode fabrication, CNT is an ideal material to be hybridized with various NSs to form heterostructures.For example, Pipes et al. prepared a freestanding membrane composed of MoS 2 NSs, single-walled CNT, and multiwalled CNT (MoS 2 -NS@MWNT) (Figure 12e,f ). [103]he assembled Li-CO 2 battery could realize boosted discharge capacity by %50% and reversible charge process with a small overpotential (i.e., a charge voltage below 3.75 V).

Summary and Perspectives
Facing with a series of serious global issues like greenhouse effect, energy crisis, and environmental pollution, the emerging MCB technology has become a feasible and promising solution,  [100] Copyright 2022, National Academy of Science.c) Discharge/charge profiles of Li-CO 2 batteries with Ir NSs-CNFs cathode at 100 mA g À1 .d) Long-term cycling performance at 200 mA g À1 with the cutoff capacity of 1000 mAh g À1 .78a] Copyright 2018, Wiley-VCH.
which can capture and recycle CO 2 while serving as a power source for renewable energy networks.Benefitting from the distinct geometry and ultrahigh-specific-surface area, 2D materials fit well with the design criteria of catalyst cathode, the key enabler of MCBs.However, to realize the practicality of MCBs, it is necessary to tune some of their intrinsic property limitations.In this review, we have introduced three representative engineering strategies (i.e., defect construction, phase engineering, and heterostructure design) of 2D materials for MCBs.Correspondingly, some recent progresses related to engineered 2D materials as catalyst cathodes are systematically summarized, indicating a prospective pathway toward superior energy storage and catalytic performances.
However, it still remains highly challenging for these engineering strategies.
First, the structural stability and performance reliability of 2D materials may be compromised.For example, excessive defect introduction would increase the risk of structural degradation/ collapse or direct decomposition.To maximize the benefit of engineering effect, the battery performance and the structural stability of electrode material need to be balanced.In this sense, the features (e.g., concentration, type, or distribution state) of constructed defects remain to be manipulated by optimizing synthesis parameters.In addition, although constructing defects can boost the catalytic sites for CO 2 RR and/or CO 2 ER, the generated coordination-unsaturated atoms near the defects exhibit ultrahigh redox activity, possibly aggravating the parasitic reactions with aprotic electrolytes or H 2 O in aqueous electrolytes upon MCB operation.These potential problems thus require careful consideration for the compatibility between catalyst  [102] Copyright 2020, Wiley-VCH.e) TEM image of MoS 2 -NS@MWNT.f ) Schematic illustration of the reaction mechanisms of Li þ -mediated CO 2 RR and CO 2 ER at MoS 2 -NS@MWNT cathode.Reproduced with permission. [103]Copyright 2019, American Chemical Society.
cathode material and electrolyte during battery assembly and rational selection of voltage window set for performance test.
Second, for the sophisticated engineering strategies like phase engineering and heterostructure construction, there still lack clear design principles toward high-performance catalyst cathodes of MCBs.This can be essentially ascribed to the vague understanding of the intrinsic mechanisms by which engineered materials improve electrode kinetics.Specifically, revealing the performance enhancement mechanism of phase engineering relies largely on deep understanding of crystal chemistry, while the heterostructure with many theoretical geometric combinations can be seemed as a chaotic system, of which the inherent feature of irregular dynamic evolution leads to tremendous research difficulty on its electrochemical behaviors in MCBs.It is hard to prespecify the desired unconventional phase or heterostructure just according to the analysis of energy storage and electrocatalytic CO 2 mechanisms.By exploring the evolution of atomic or electronic structures, the advanced in situ characterizations and theoretical simulations may provide valuable information for rational catalyst cathode design.
Third, novel engineering strategies for 2D materials can be developed to provide a variety of options for the preparation of advanced MCB cathodes.According to the characteristics of 2D materials with ultrahigh surface area, interface engineering may be a promising research direction.For example, some inorganic small molecules or organic polymers can be grafted to the 2D interfaces to regulate surface chemical affinity.By directional regulation toward kinetically favorable CO 2 capture and catalytic paths of CO 2 RR and CO 2 ER, the energy efficiencies of MCBs would be enhanced with lower reaction overpotentials.Moreover, interlayer engineering is also an intriguing strategy, which can remarkably boost the active storage sites.Referring to intercalation chemistry, the effectiveness of this method has been demonstrated in rechargeable MIBs.Empirically, similar electrochemical mechanism can be used for MCBs to promote the accessibility of cations with a lower diffusion barrier.In addition, larger interlayer spacing could facilitate surfacedependent electrocatalytic reactions for CO 2 capture and fixation.
In general, the emerging MCB systems integrating CO 2 fixation with energy storage have been regarded as a green and efficient route toward carbon neutrality, while the research on the suitable catalyst cathodes involving the complicated three-phase interfacial reactions deserves much more attention.We believe that structural or morphological engineering for 2D materials will continue to be a powerful strategy to push the performance limits of MCB cathodes.In addition, it is foreseeable that future research will focus on the discovery of novel 2D materials and/or engineering strategies for the preparation of kinetically more favorable cathodes, and reveal the intrinsic structureperformance correlations microscopically to reversely guide cathode design for well coordination of various MCB components.

Figure 3 .
Figure 3. a) Scanning electron microscope (SEM) image of the 3D porous fractal Zn (PF-Zn) cathode.b,c) CO-producing performance of the PF-Zn cathode upon discharge: b) max Faradaic efficiency (FE) of CO at different currents for 10 h discharge; c) FE of CO at 0.1 mA.Reproduced with permission.[20]Copyright 2018, Royal Society of Chemistry.d-f ) Characterizations of Li-CO 2 batteries (20 μAh) using the 0.5 M LiClO 4 -DMSO electrolyte to verify the formation/decomposition of Li 2 O upon cycling: d) In situ Raman spectra during the discharge (Left panel) and charge (Right panel) processes; e) voltage profile with a charge cutoff voltage of 4.5 V at 100 mA g À1 ; f ) corresponding ex situ Fourier-transform infrared (FTIR) spectra recorded at different discharge/charge states marked by varied colors.g) Schematic illustration of the rechargeable/reversible (left panel) and rechargeable/irreversible (right panel) Li-CO 2 electrochemistry.Reproduced with permission.[21]Copyright 2017, Elsevier.

Figure 5 .
Figure 5. a-c) Reaction verification (3CO 2 þ 4Na ↔ 2Na 2 CO 3 þ C) of Na-CO 2 batteries: a) schematic illustration of battery internal architecture;b) schematic view of the button cell; c) in situ Raman spectra and the corresponding discharge/charge profiles (inset).Reproduced with permission.[28]Copyright 2016, Wiley-VCH.d) Schematic illustration for the formation/decomposition of K 2 CO 3 at the cathode of K-CO 2 batteries.Reproduced with permission.[9d]  Copyright 2018, Elsevier.

Figure 6 .
Figure 6.a,b) Electrochemical performances of an energy-storage system using the zinc/zincate redox pair: a) chronovoltammetry profile, FE (top panel), and concentration (bottom panel) of CO produced at the current density of 5 mA cm À2 ; b) FE and concentration of CO at different current densities.Reproduced with permission.[33]Copyright 2018, Springer Nature.c) Schematic view of the aqueous ZnÀCO 2 battery with a discharge mechanism of CO 2 RR-to-HCOOH.Reproduced with permission.[37]Copyright 2018, Wiley-VCH.d) Discharge/charge polarization profiles of ZnÀCO 2 batteries using Ir@Au bimetallic catalyst as the cathode.Reproduced with permission.[38]Copyright 2019, Wiley-VCH.e) Schematic diagram for the reaction mechanism of NPG@Pd catalyst cathode in AlÀCO 2 batteries.f ) Corresponding discharge/charge voltage profiles at 333 mA g À1 .Reproduced with permission.[39]Copyright 2018, Wiley-VCH.

Figure 7 .
Figure 7. a) Electron paramagnetic resonance (EPR) spectra of NiO-V O NAs/CT and pure NiO NAs.b) Discharge/charge profiles at 100 mA g À1 with the cutoff capacity of 1000 mAh g À1 .c) Schematic diagrams of the interaction difference of valence levels of NiO-V O and pure NiO.Reproduced with permission.[61]Copyright 2021, Elsevier.d) Partial density of states (PDOS) of Bi atom p orbital of BiO 2Àx nanosheets (NSs).e) Discharge/charge cycling curves at 4.5 mA cm À2 .Reproduced with permission.[62]Copyright 2023, Chinese Chemical Society.f ) Schematic illustration of a Li-CO 2 battery using defect-rich graphene (TDG) as the cathode.g) Discharge/charge curves at 0.5 A g À1 with corresponding Coulombic efficiency (CE) as inset.Reproduced with permission.[63]Copyright 2021, Wiley-VCH.

Figure 8 .
Figure 8. a) High-resolution X-ray photoelectron spectroscopy (XPS) spectra of N 1s for PNCB.b) Charge density variance between Bi (102) plane and pyrrolic-N-doped carbon.c) Galvanostatic discharge/charge profiles at 5 mA cm À2 with specific cycling curves from 0 to 20 h (inset).Reproduced with permission.[64]Copyright 2022, Elsevier.d) Fourier transform of extended X-ray absorption fine structure (FT-EXAFS) fitting results of Cu-N 2 /GN.e) Freeenergy diagrams of Cu-N 2 and Cu-N 4 models for CO 2 ER.f ) Current density-voltage ( J-V ) curves of the charged Zn-CO 2 battery and the solar cell for powering.Reproduced with permission.[66]Copyright 2020, Wiley-VCH.g) Schematic view for the fabrication of Ru-Co 3 O 4 NSs on carbon cloth.Reproduced with permission.[67]Copyright 2021, Wiley-VCH.

Figure 9 .
Figure 9. a-d) Structural characterizations of 4 H/face-centered cubic (fcc) Ir: a) spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image; b,c) fast Fourier transform (FFT) patterns of the corresponding areas (G and H) marked in (a), respectively.d) Atomic models of 4 H (top panels) and fcc (bottom panels)Ir.e) FT-EXAFS spectra at the Ir L 3 edge.f,g) Counter plots of wavelet transform (WT) of f ) 4 H/fcc Ir and g) pure fcc Ir. h) Discharge/charge profiles at 50 mA g À1 with the cutoff capacity of 500 mAh g À1 .i) Energy diagrams of different Ir facets during the decomposition of Li 2 CO 3 and j) the corresponding maximum energy barriers.Reproduced with permission.[87]Copyright 2022, National Academy of Science.

Figure 10 .
Figure 10.a) Schematic view for the wet-chemical synthesis of 2D Ru-M alloy NSs and the subsequent fabrication of aprotic Li-CO 2 batteries using RuCo NSs as the cathode catalyst.b) Transmission electron microscope (TEM) and c) aberration-corrected HAADF-STEM images of RuCo NSs.d) Atomic model of RuCo alloys along the direction of [0001] h .e) FFT pattern of the area marked with white-dashed square in(c).f ) Galvanostatic discharge-charge curves of different cathode materials at 100 mA g À1 .g) Median voltages and energy efficiency of RuCo NSs/CNT upon cycling.Reproduced with permission.[88]Copyright 2022, Wiley-VCH.

Figure 11 .
Figure 11.a) Schematic view for the synthesis process of SnO 2 /MXene.b) Volume renderings for SnO 2 /MXene at different angles based on 3D ptychography imaging technology.Reproduced with permission.[100]Copyright 2022, National Academy of Science.c) Discharge/charge profiles of Li-CO 2 batteries with Ir NSs-CNFs cathode at 100 mA g À1 .d) Long-term cycling performance at 200 mA g À1 with the cutoff capacity of 1000 mAh g À1 .Reproduced with permission.[78a]Copyright 2018, Wiley-VCH.

Table 1 .
A summary of engineered 2D materials used as the catalyst cathodes of metal-CO 2 batteries.