Research progress of aqueous Zn–CO₂ battery: design principle and development strategy of a multifunctional catalyst

Abstract

Aqueous Zn–CO₂ battery possesses a large theoretical capacity of 820 mAh g^-1 (5855 mAh cm^-3) and high safety, showing a unique position in carbon neutrality and/or reduction and energy conversion and storage, which has developed rapidly in recent years. However, obstacles such as low value-added products, low current density, high overvoltage, and finite cycles impede its practical application. Cathode catalysts, as a key component, have a significant influence on gas cell performance. Despite many updated papers on cathode materials for aqueous Zn–CO₂ batteries, a systematic summary has rarely been reported, and even less is mentioned about the design principle and development strategy for efficient catalysts. Relying on the structure and mechanism of the Zn–CO₂ battery, this review discusses the research progress and existing challenges, and, more importantly, the design strategies and preparation methods of the efficient cathode are proposed, centering on material structure, charge distribution, and coordination environment. Finally, in this review, the opportunities for the development of a high-performance Zn–CO₂ battery are highlighted, which enables enlightening the future exploration of next-generation energy storage systems.

1 Introduction

Excessive carbon dioxide (CO₂) emissions caused by consumption of fossil fuels further aggravate the global energy crisis and the greenhouse effect (Chang et al., 2017; Asadi et al., 2018; Zhou et al., 2020). In recent years, the carbon capture, utilization, and storage (CCUS) (Chen et al., 2022c; de Oliveira Maciel et al., 2022; Jiang et al., 2022; Pfeiffer et al., 2022) technology has become a hot topic of research. Among them, metal–CO₂ batteries adopt CO₂ catalytic conversion and an energy storage solution where chemical energy is converted into green renewable electricity while reducing CO₂ emissions (Chu et al., 2016; Ahmadiparidari et al., 2019; Eskezia Ayalew, 2021). Compared with traditional ion batteries, metal–CO₂ batteries possess a higher specific capacity and energy density (Xiang Li et al., 2016; Qiao et al., 2017; Hu et al., 2019). For example, the energy density of a lithium–carbon dioxide (Li-CO₂) battery is as high as 1876 Wh kg^-1 (Zhang et al., 2017; Cai et al., 2018), approximately five times that of a lithium-ion (Li-ion) battery (387 Wh kg^-1 (Khurram et al., 2018; Wu et al., 2021)). However, an alkali metal (Li, Na, and K)–CO₂ battery using a toxic organic electrolyte is not environmentally friendly, and their solid products from carbon dioxide reduction (CO₂RR) are chemically inert, resulting in over-accumulation and substances not subjectable to decomposition (Xie and Wang, 2019; Mu et al., 2020). Furthermore, the slow kinetics during the electrochemical process and thermodynamic instability hinder their application. For other metal–CO₂ batteries with aqueous electrolytes, including Mg–CO₂ and Al–CO₂ batteries, their theoretical capacity densities are 6815 and 8076 Wh kg^-1 (Ma et al., 2018), respectively, higher than that of Zn–CO₂ units (984 Wh kg^-1) (Aslam et al., 2023). However, the products of these metal–CO₂ batteries are all metal carbonates and carbon, which cannot generate other value-added chemicals. Zinc metal reserves are abundant, and the price is low. In the derived Zn-gas batteries, Zn–CO₂ batteries show no obvious advantages in energy density compared to Zn-air (1353 Wh kg^-1) or Zn-O₂ batteries but pose significant importance to CO₂ utilization and conversion (Zhou et al., 2021). Relying on environment-friendly, higher safety and wider products, the Zn–CO₂ battery is expected to be an ideal substitute for other metal batteries.

Based on its significant contribution to carbon neutrality (Chu et al., 2016; Ahmadiparidari et al., 2019; Eskezia Ayalew, 2021) and superiority in metal–gas batteries, the groundbreaking work of Zn–CO₂ batteries is exhibited in Figure 1. Aqueous Zn–CO₂ batteries originated from solar-powered CO₂ splitting batteries and later developed into a rechargeable unit, subsequently to a dual-model and self-driven system, toward the extent of a reversible one. Nevertheless, the research on aqueous Zn–CO₂ batteries is still in its infancy, and certain problems should be overcome: 1) the lack of effectively active catalysts with high selectivity leads to limited discharge products, mainly generating CO and formate, while rarely HCOOH or even less of other high value-added products (such as methanol, ethanol, and ethylene), causing the discharge–charge shuttle to be irreversible and potential application to be confined (Wang H. F. et al., 2020; Chen Y. et al., 2021). 2) The lack of stable catalytic behavior in the electrolyte environments. Most rechargeable Zn–CO₂ batteries couple CO₂RR and the oxygen evolution reaction (OER), in which CO₂RR preferably proceeds in acidic/neutral solutions and OER favors basic environments, presenting a significant challenge to balance cathodic reduction and oxidation behavior (Guo Y. et al., 2022), while a completely reversible Zn–CO₂ battery accompanied by CO₂RR and formic acid oxidation reaction (FAOR) is built in neutral or slightly acidic electrolytes, triggering electrochemical corrosion or other undesired side reactions and significantly shortening the battery life. 3) The high overpotential and low current density (≤10 mA cm^-2) severely compromise the power and energy density, where increasing energy consumption and poor long-term reversibility pose a significant obstacle toward practical application. These mentioned problems of a Zn–CO₂ battery are mainly attributed to the high stability of the C=O bond (dissociation energy as high as 806 kJ mol^-1) in CO₂ (Zhou and Sun, 2017; Kim et al., 2020), finite product types, poor selectivity in CO₂RR, and low electrochemical activity in the cathodic oxidation reaction. Indeed, it is tricky to obtain efficient long-term reversible cycles without effective catalysts (Hao et al., 2020; Hao et al., 2021). Moreover, the aqueous electrolytes present low operation potential and anode instability (like Zn self-corrosion or dendrite), resulting also in limited running cycles, but these topics are beyond our discussion in this review.

FIGURE 1

Therefore, it is crucial to design both effective and chemical/electrochemical stable catalysts to solve the aforementioned problems, tackling competitive adsorption–desorption between *COOH/*OCHO, *COH/*CO, *OH/*OOH, or *H in the catalytic process for the target products (Huang et al., 2019). Despite many updated papers on cathode materials (Table 1) for aqueous Zn–CO₂ batteries, a systematic summary has rarely been reported (Wu et al., 2021), and even less is known about the principles of designing efficient catalysts. Herein, this review summarizes the structure and mechanism of Zn–CO₂ battery and discusses the research progress and existing problems of cathode materials, and, more importantly design strategies and preparation methods for efficient cathode catalysts are proposed, centering on catalyst structure with adsorption performance of intermediate products, eventually highlighting the opportunities and challenges for high-performance Zn–CO₂ batteries, which enables to enlighten the future development of the next-generation energy storage system.

The results were obtained from the data by providing directly from literature or estimating from the given information. NOTE: Dis-char means discharge–charge plateau. O_cv means open-circuit voltage. O_time means operation time. C_density means current density. For better performance evaluation, the data in red, blue, and green represent excellent, good, and average, respectively.

2 Structure and reaction mechanism of Zn–CO₂ battery

2.1 Battery structure

The Zn–CO₂ battery typically consists of a catalyst cathode, Zn sheet anode, electrolyte, and diaphragm seen in Figure 2. Its uniqueness is the electrolyte, which is divided into two parts: the anode counterpart utilizes alkaline KOH and Zn(CH₃COO)₂ mixture solution, while the cathode counterpart uses near-neutral KHCO₃ solution, and sometimes, buffer solution HCOO⁻ is added (Asokan et al., 2020). Bipolar membranes are necessary to separate the electrodes, maintaining the pH and avoiding cross-contamination (Xie et al., 2018a).

FIGURE 2

2.2 Reaction mechanism

Based on the Frontier research progress of Zn–CO₂ batteries, we discuss both anodic and cathodic reaction mechanisms and highlight the cathodic CO₂ electrochemical reduction and opposite evolution mechanisms in alkaline and neutral electrolyte conditions.

2.2.1 Anodic reaction mechanism

The anodic reactions are based on Zn/Zn²⁺ redox couple, and the Zn²⁺ state is affected by the electrolyte’s pH shown as follows. In near-neutral electrolytes, Zn²⁺ exists as the charge carrier, while in alkaline solutions, Zn²⁺ initially appears as Zn(OH)₄^2- and is further dehydrated to ZnO (Zhong et al., 2020).

In the neutral electrolyte: E^θ (Zn/Zn²⁺) = −0.76 V vs. SHE, where E^θ means the standard reduction potential

In the alkaline electrolyte: E^θ (Zn/ZnO) = −1.22 V vs. SHE

2.2.2 Cathodic reaction mechanism

CO₂RR mechanism: When Zn–CO₂ batteries discharge, CO₂RR occurs on the gas–liquid–solid three-phase interface, and this complex process involves the adsorption and dissociation of CO₂ together with the transfer of multiple protons and electrons (Li et al., 2017; Yang et al., 2018). CO₂RR products are mainly present as HCOOH/formate and CO through different routes in the adsorption model on the catalytic site. The former tends to adsorb at the O end (*OCHO) to present HCOOH in near-neutral electrolytes and transit to formate in alkaline environments (Han et al., 2018), while the latter adsorbs at the C end (*COOH), as shown in Figure 3A.

FIGURE 3

HCOOH in the neutral electrolyte:

Formate in the alkaline electrolyte:

CO in the alkaline electrolyte:

Relying on the reversibility of a discharge–recharge reaction, the Zn–CO₂ secondary battery is classified into two types: the reversible battery covering the HCOOH product and FAOR charge process and the rechargeable one generating CO or HCOO⁻ discharge products and coupling OER (simplified as Zn–CO₂ battery) (Wang F. et al., 2021). We discuss the charge mechanism as follows.

FAOR mechanism: The reversible Zn–CO₂ battery forms a closed loop between CO₂ and HCOOH (El Sawy and Pickup, 2016; Xiong et al., 2020; Liang M. et al., 2021). FAOR generally occurs in the direct, indirect, or formate pathways (Wang et al., 2004; Zhu et al., 2021a), as seen in Figure 3B. The direct route follows the dehydrogenation of HCOOH with the C-H bond breaking and release of CO₂, which occur quickly and effectively under low potential. As for the indirect route, the initial step is dehydration with C-O and C-H bonds breaking, and the adsorbed CO (CO^*) is subsequently oxidized to CO₂ at high potential. The residual CO^* covers active sites and seriously represses the direct route, leading to catalyst poisoning or even inactivation (Calderón-Cárdenas et al., 2021). Different from the breaking sequence of the direct route, the formate route first breaks the O-H bond of HCOOH to HCOO* (i.e., *OCHO) and further dehydrogenates to CO₂, requiring less binding energy than the aforementioned two paths, thus occurring at even lower potential (Xiong et al., 2020).

Direct route:

Indirect route:

Formate route:

To date, CO₂RR products in the battery are mostly CO or formate and catalysts with high FAOR activity are rarely reported. The reversible Zn–CO₂ batteries following the formate path are only achieved under bifunctional catalysts of porous Pd nanosheets (Xie et al., 2018a) and PdBi alloy (Gao S. et al., 2022).

OER mechanism: O₂ is generated from H₂O molecules in acidic electrolytes or from -OH groups in alkaline media during the OER, as shown in Figure 3C (Suen et al., 2017; Xie et al., 2019). Its symbiotic hydrogen evolution reaction (HER) seriously affects the battery’s Coulombic efficiency and should be inhibited.

To sum up, the reaction mechanisms of the Zn–CO₂ battery are listed in Table 2. The continuous exploration of mechanisms enlightens the direction of efficient catalysts (Wang K. et al., 2020; Wang F. et al., 2021).

3 Cathode catalysts

The catalytic properties and electrolyte environment play a significant role in the electrochemical mechanism, influencing product state and reaction rate and ultimately determining its battery performance (Wang J. et al., 2021). Cathode catalysts (as the core component) are divided into three categories in this review: carbon-based metal-free catalysts, metal-based catalysts, and metal–carbon composites. When the battery discharges, CO₂ is mostly reduced to CO and formate and rarely to HCOOH, depending on reaction intermediates. Specifically, accelerating the formation of *COOH tends to produce CO while reducing the reaction energy barrier of *OCHO, which promotes the generation of HCOOH in near-neutral electrolytes and formate in alkaline electrolytes (Xie et al., 2019). In the opposite charge process, it requires cathodes to show pertinent desorption catalysis of *COOH/HCOO*/CO* in FAOR or *OH/*OOH in the OER. The adsorption energy of intermediates reflects the catalytic activity and is influenced by the electronic structure and chemical environment of the catalytic sites (Selvakumaran et al., 2019; Peng et al., 2022b).

3.1 Carbon-based catalyst

This catalyst has the advantages of high conductivity, large specific surface area, superior stability, and low costs and involves carbon nanotubes, graphene, and other analogs (Lankone et al., 2017; Yu et al., 2020; Zhao et al., 2020; Chen B. et al., 2021). The catalytic performance is further regulated via doping N, F, Si, or P heteroatoms into the electronic structure and introduces defects (Xue et al., 2019; Wang C. et al., 2021; Gao Y. et al., 2022; Wang J. et al., 2022). Generally, the doped heteroatoms with strong electronegativity facilitate the combination with positively charged *COOH intermediates (Shi et al., 2021; Sawant et al., 2022; Thakur et al., 2022). N-doped carbon materials are widely used in this field. As reported in literature, N-doped ordered mesoporous carbon (NOMC) (Gao et al., 2021b) (Figures 4A, B) using the SBA-15 template is prepared via the pyrolytic-etching method, possessing ultra-high specific surface area, highly exposed N-C sites to promote the transport and storage of reactants/intermediates (Shakeri et al., 2021). The Faraday efficiency of the CO product (FE_CO) on NOMC is close to 100% at a low overpotential of 0.36 V, and a Zn–CO₂ battery assembled with the NOMC cathode achieves long, stable cycles of 300 (100 h) even at 1.0 mA cm^-2 (Figures 4C, D), indicating high catalytic performance and super stability. The presence of more electronegative F atoms induces positrons and arouses asymmetric spins, thereby rearranging the electron density of adjacent atoms (Wang G.-D. et al., 2021).

FIGURE 4

A few F atoms were adopted into carbon matrices forming F-doped carbon (FC) (Figures 4E–H) by the one-step pyrolysis method, and the FC catalyst was applied in Zn–CO₂ for the first time (Xie et al., 2018b). Through density functional theory (DFT) calculation (Figures 4I–K), it was shown that *COOH is inclined to adsorb on the fourth C atom next to the CF₂ bond with the lowest Gibbs free energy (ΔG) of 0.64 eV rather than the F-free doped C atom. Simultaneously, it endows the adjacent C atoms with higher positive charge density and asymmetric spin sites, effectively inhibiting the HER with strong binding ability to H*. As a result, its FE_CO is increased to 89.6%, and the assembled solar-driven CO₂ battery achieves 13.6% photoelectric conversion efficiency, higher than that of SiNC materials (Ghausi et al., 2018) (12.5%). Such energy conversion–storage Zn–CO₂ self-powered devices conform to the trends in the development of self-powered integrated devices.

In addition to heteroatom doping, the construction of intrinsic carbon defects causes charge delocalization, activating carbon atoms at defect sites for better catalytic properties. A defect-rich porous carbon (K-defect-C-1100) was synthesized by a K⁺-assisted strategy (Ling et al., 2022) and has 12-vacancy-type defects (V₁₂) (Figures 5A–D). Its negative potential V₁₂ defect attracts electrophilic CO₂ molecules and renders the battery high FE_CO to 99% (Figure 5E) in a wide discharge potential range with excellent cyclic stability of about 200 cycles.

FIGURE 5

Though electron delocalization is induced into carbon-based materials, the influence on the inertia of molecular activation is finite, which hardly meets application requirements.

3.2 Metal-based catalysts

Metal-based catalysts are generally composed of metal or alloy nanoparticles and metal compounds. Noble metal-based catalysts such as Au, Ru, Pd, and Ir have excellent catalytic properties, providing good acid and alkali resistance (Li D. et al., 2022; Zhao D. et al., 2022; Kang T. et al., 2022; Huang et al., 2022); the exposure states of active sites are easily made compatible via structure design and mass transfer capacity (Sui et al., 2021). Additionally, the catalyst’s lattice structure and charge distribution are evolved by introducing other metal or non-metallic atoms, which optimizes the interaction between the catalyst and intermediates, inhibits possible side reactions, and allows an elevation of the product selectivity (Feng et al., 2021).

As reported, Xie et al. (2018a) prepared porous three-dimensional interconnected Pd nanosheets (3D Pd NSs) with a rich pore structure, large specific surface area, sufficient active sites by electrodeposition (which adsorbs the *OCHO intermediate with high selectivity), and presents a good reversibility in conversion between CO₂ and HCOOH under low potential. The assembled reversible Zn–CO₂ battery steadily runs for more than 100 cycles (33 h), achieves a 788 Wh kg^-1 discharge capacity, and shows an energy efficiency of 81.2%. Gao et al. (2021a) prepared coral-like Au catalyst with an irregular surface and initiated a self-driven CO production device that is co-assembled by a Zn–CO₂ battery and H-type CO₂ electrolyzer for the first time. *COOH is easily adsorbed on Au (111), with a lower reaction energy barrier (ΔG = 1.16 eV) than that in the common environment (ΔG = 1.27 eV). Unlike single metals, bimetal composites do achieve enhanced catalytic performance under the strong synergistic effect. Wang et al. (2019) built a dendritic 3D Ir@Au bifunctional catalyst and constructed a Zn–CO₂ battery simulating the two-step plant photosynthesis to achieve CO₂ fixation and H₂O oxidation, which equably circulates for 30 h at 5 mA cm^-2.

Although noble metals exhibit outstanding catalysis in Zn–CO₂ batteries, their commercial application is restrained by high prices and low reserves. Alloys and transition metals with special catalytic properties are becoming their alternatives (Yang X. et al., 2022; Gao and Zhao, 2022; Lichchhavi and Shirage, 2022). Sn is rich in resources with low toxicity. In the electrochemical reaction, the Sn alloy and Sn₂O₃ are utilized as important promoters to reduce CO₂ to C1 products (CO and HCOOH), but their poor conductivity is still an obstacle (Kang J. et al., 2022; Ansari et al., 2022). The hierarchical core-shell Sn–Cu/Sn@SnO_x (Ye et al., 2020) (Figures 6A–C) was optimized with high conductivity by an Sn/SnO_x interface. Its Sn–Cu/Sn core provides a sufficient Sn source to reconstruct the core-shell structure, thereby guaranteeing stability. Its in situ reconstructed Sn/SnO_x shell increases the reaction energy barrier of *H and *COOH to suppress HER and CO formation; conversely, it reduces the *OCHO binding energy, which sets a decisive step toward obtaining a high selectivity of the HCOOH product and presents a high FE_formate (Figures 6D, E). However, its partial current density of formate (j_formate) is still lower than −0.3 A cm^-2, which is far from application requirements. To solve this problem, Sn is incorporated with Li (s-SnLi) (Yan et al., 2021) on the surface, which stimulates local electron rearrangement and lattice strain that correspondingly occurred on the adjacent Sn atoms, showing more negative charge (Figure 6F).

FIGURE 6

Therefore, the s-SnLi with enhanced CO₂ adsorption capacity and restricted ΔG_*COOH and ΔG_*OCHO (Figure 6G) showed both high catalysis and selectivity in reducing CO₂ to formate. Thus, FE_formate reaches 92%, the maximum power density of 1.24 mW cm^-2, j_formate is increased to −1.0 A cm^-2, and the Zn–CO₂ cell operates for over 800 cycles.

Metal oxides behave better in CO₂RR performance than metal substances, but metal self-reduction may occur in the catalytic process. BiO_2-x nanosheets (m-BiO_2-x) with rich oxygen defects (Tan et al., 2022) and favorable localization of Bi p-orbital electrons were utilized to stabilize *OCHO and suppress *H adsorption for high selectivity in CO₂-to-formate conversion. It offered a non-dopant pathway to boost electrochemical CO₂RR. Inspired by the synergetic effect of multi-metals, Tian et al. (2021) embedded SnO₂ nanoparticles on Bi₂O₃ sheets’ surfaces to assemble SnO₂/Bi₂O₃ composites. Different work functions at the SnO₂/Bi₂O₃ interface induce a built-in potential, thereby promoting the interface electron transfer and improving the conductivity of SnO₂. The strong interface on SnO₂/Bi₂O₃ effectively prevents SnO₂ from electrolytic decomposition, which ensures catalyst stability and promotes CO₂ adsorption and activation. In addition, it showed the enhanced adsorption of *OCHO on Bi₂O₃ to accelerate HCOOH generation and the very strong adsorption of *H to inhibit the HER (Figures 7A, B). The bimetallic oxides integrated stability and catalytic activity, achieving a multifunctional catalyst (Guo H. et al., 2022).

FIGURE 7

Moreover, metallic selenides are an option in CO₂RR catalysts due to their low cost and unique physical and chemical properties, but their selectivity in products is poor (Hu H. et al., 2021). As reported, In₂Se₃ nanosheets reduce CO₂ to CO with an FE_CO of 89% (Lü et al., 2019), and Cu_1.63Se_0.33 reduces CO₂ to CH₃OH with an FE_CH3OH of only 77.6% (Yang D. et al., 2019). Both examples show that catalyst activity and product selectivity are in great demand for improvement. Bimetallic selenides v-CuInSe₂ (Wang Y.-X. et al., 2022) (Figure 7C) were prepared to possess the bimetallic orbital, which regulates the adsorption–desorption strength of *COOH being more conducive to CO generation and shows no stable *H adsorption sites, thereby significantly inhibiting the HER (Figure 7D). Thus, FE_CO was increased to 91% at −0.70 V (vs. RHE).

For metal-based catalysts, the structure and morphology of noble/transition metals are designed to avoid aggregation and self-reduction, where the hybridization and alloying strategy is adopted to modify the electron distribution for excellent catalytic behaviors.

3.3 Metal–carbon composite catalysts

The metal-based catalysts are prone to agglomerate, leading to a decrease in catalytic activity (Chen et al., 2022b). It is a strategy to isolate metal nanoparticles on carbon substrates (Lei et al., 2022) for catalytic performance and conductivity, thereby deriving metal-carbon catalysts. Such materials have excellent geometric structures and fully expose the specific active sites (Zhao S. N. et al., 2022). Their catalysis is not only related to the inherent properties of embedded metal and carbon frame but also regulated via the morphology and interface modification (creating defects and heterostructure) (Belotcerkovtceva et al., 2022; Cao et al., 2022; Fan et al., 2022; Su et al., 2022).

Metal Bi can enhance *OCHO adsorption and inhibit *H adsorption, but is easily oxidized due to its low melting point (Deng et al., 2020; Chhetri et al., 2022). To avoid oxidation, Bi is uniformly dispersed in a carbon layer, which creates an abundance of defects (Bi-D) (Wang J. et al., 2022), thereby showing hybrid crystalline–amorphous phases and heterojunctions, that render the FE_formate as high as 90%; the maximum power density was 1.16 mW cm^-2. Bi geometry was changed into nanotubes (Bi₂O₃ NTs) that accelerated the FE_formate higher than 92.7%, and the maximum power density was up to 1.43 mW cm^-2 (Gong et al., 2019). In addition, Bi clusters are deposited on hollow carbon spheres to form atomic dispersed BiC/HCS (Yang M. et al., 2022), improving CO₂ adsorption capacity, increasing the FE_CO to 97% ± 2% (−0.6 V vs. RHE), achieving a record-breaking peak power density of 7.2 ± 0.5 mW cm^-2, and a cycle number of more than 200 times.

The Sn/SnO_x heterojunction presented higher catalytic activity than individual Sn, as previously mentioned. Multivalent Sn provides rich interfaces between Sn(0) and Sn(II) or Sn(IV), showing a special catalytic activity in stabilizing intermediates (Liu K. et al., 2022; Spada et al., 2022; Zhang et al., 2022). The Sn/SnO₂ heterojunction was uniformly decorated on highly conductive N-doped carbon networks to construct Sn/SnO₂@NC (Xue Teng et al., 2021). The FE_formate is increased to 81%, the peak power density of the assembled battery reaches 0.9 mW cm^-2, and the open-circuit voltage is 1.35 V. Furthermore, the carbon network was replaced by potential two-dimensional materials. Low-dimensional SnO₂ quantum dots were spread among ultrathin Ti₃C₂T_x MXene nanosheets (SnO₂/MXene) (Han L. et al., 2022) to reduce the reaction energy of CO₂ hydrogenation to formate. This is carried out by destabilizing water and water dissociation in order to increase the surface coverage of *H and raise a new situation of electrochemical CO₂-to-formate.

Similarly, bimetal or multimetal catalysts are rooted into carbon substrates to play a synergetic role via inducing geometric or electronic reconstructions (Zhang N. et al., 2021; Kong et al., 2022). For example, the In metal can reduce CO₂ to formate, but is faced with many obstacles such as low j_formate, poor stability, and limited coordination ability (Dong et al., 2018; Raknual et al., 2021).

To resolve these problems, In is fixed into Zn-MOF derivatives to prepare In/ZnO@C hollow nanocubes (Teng et al., 2021) (Figures 8A–D), thereby adjusting the adsorption of *OCHO on Zn catalytic sites and improving the product selectivity. Under the hollow nanocube framework, the synergetic effect of both Zn and In makes j_formate as high as 23.5 mA cm^-2, FE_formate increases to 90% at a potential of −1.2 V (vs. RHE), long cycles of 153 (51 h) at the current density of 1 mA cm^-2, and the peak power density of a Zn–CO₂ battery is improved to 1.32 mW cm^-2 (Figures 8E, F). Simultaneously, Fe and Ni nanoparticles are introduced into ZIF to synthesize Fe₁–Ni₁–N–C (Jiao et al., 2021) (Figure 6G), existing as Fe-N₄ and Ni-N₄ forms. Fe and Ni monoatomic pairs play a synergistic effect, where Ni atoms activate the adjacent Fe to increase the electron density between Fe and CO₂ while reducing the free energy of *COOH (Figures 8H–L); thus, its FE_CO reaches 96.2% at −0.5 V. Furthermore, a bifunctional PdBi alloy anchored on the carbon substrate (BiPdC) (Gao S. et al., 2022) was fabricated via doping Pd into Bi-based MOF (CAU-17) (Figures 9A, B), in which Bi aims at conversion of CO₂RR to HCOOH product and the Pd targets FAOR to the highly reversible conversion of CO₂-to-HCOOH. This reversible Zn–CO₂ battery exhibits 52.64% FE_HCOOH (Figure 9C) and 45 h cycling durability. Despite the low FE_HCOOH, it is of great significance to construct a truly reversible Zn–CO₂ battery. In addition, the vital role of metal lattice revealed strains on CO₂RR and strained PdNi alloy (s-PdNi) (Hao et al., 2022) was confined into carbon shells, presenting optimized *COOH adsorption and *CO desorption on s-PdNi-2.3% (111) surfaces through Bader charge analysis (Figures 9D, E).

FIGURE 8

FIGURE 9

For metal-carbon catalysts, introducing heteroatoms with large electronegativity (such as B, P, and S.) is mainly to regulate the local electron density of the metal, thereby providing appropriate adsorption energy for CO₂RR (Maulana et al., 2021; Peng et al., 2022a; Ma et al., 2022). To achieve P doping, Fe–P nanocrystals were in situ implanted into a ZIF template and calcined at high temperatures to prepare Fe-P@NCPs (Liu et al., 2022d). During high-temperature pyrolysis, the ZIF-8 cage separates and encapsulates ferrocene, converts 2-methylimidazole into an N-C skeleton, and the evaporation of Zn ions simultaneously induces Fe reduction and phosphating, thus forming Fe–P nanocrystals with high interfacial charge transfer capability and catalytic performance. Its FE_CO can reach 95% at −0.55 V (vs. RHE). The Zn–CO₂ battery with Fe-P@NCPs operates at ultra-high stability, exceeding 500 cycles (7 days) without obvious voltage attenuation. It was reported that P-doping promotes OER/ORR (Yang J. et al., 2019); Yang R. et al. (2019) co-doped Ni with N and P on graphene, forming Ni/PNG nanomaterials, and designed a dual-mode Zn–CO₂/Zn–O₂ battery realizing CO₂RR/OER/ORR trifunctional catalysis. Ni-N on Ni/PNG mainly catalyzes CO₂RR (He et al., 2020; Leverett et al., 2022) and assists the OER and ORR, while PG mainly works on the OER/ORR and inhibits the HER. The operational routine of dual-model batteries is controlled by supplying gas, which enlightens the integrated multifunctional energy conversion-storage devices.

Metal-carbon composite catalysts combine the superior catalytic performance of metal and the excellent electrical conductivity of carbon frames, presenting significant superiority in product selectivity and showing great potential as bifunctional catalysts.

3.4 Single-atom catalyst (SAC)

Derived from metal–carbon composite catalysts, the SAC is particularly noticeable, and the isolated single-metal atom behaves as an active center without interaction with adjacent metal atoms (Xu et al., 2018; Shah et al., 2021; Shah et al., 2022). Relying on the high atom utilization and unique coordination environment (Liu et al., 2020; Zhuang et al., 2020), the SAC shows great potential for excellent activity, selectivity, and stability (Sun et al., 2019) in CO₂RR and oxidation reactions. However, in actual conditions, the reduction of a catalyst’s particle size leads to a sharp increase in surface energy, hardly maintaining the atomic level dispersion and driving metal atoms to gather, forming nanoparticles (Zhu et al., 2021b; Lin et al., 2021; Lin et al., 2022). Metal oxides, hydroxides, and carbon-based materials are accepted as substrates to separate metal atoms (Yang D. et al., 2020; Wang R. et al., 2021; Wan and Wang, 2021). Summarizing reports on SAC in Zn–CO₂ batteries show that most substrates are carbon-based materials and the SAC exists in metal–nitrogen–carbon (M-N-C) mode (Chen Z. et al., 2022; Yujie Shi and Lou, 2022).

MOF with unique structures is becoming an alternative to preparing SAC and M-N-C catalysts mostly derived from ZIF-8 and ZIF-67 precursors (Ding et al., 2022; Song et al., 2022), which take dimethylimidazole as the organic ligand presenting the dodecahedral structure and fix the Zn²⁺ and Co²⁺ as ligand metals, respectively (Yang H. et al., 2020; Song et al., 2020; Mo et al., 2022). The in situ metal substitution and high-temperature calcination strategy is adopted to allow control over the coordination environment of metal center sites together with altering the porosity and conductivity of the carbon skeleton (Han W. et al., 2022; Fu et al., 2022). The Fe₁NC/S₁-1000 (Wang T. et al., 2020) catalyst with Fe-N₃ sites balances the adsorption of *COOH and *CO, achieving a peak power density of 0.6 mW cm^-2 and FE_CO as high as 96% at a potential of −0.5 V. Similarly, Zhang Y. et al. (2021) proposed the strategy of post-synthetic metal substitution. Zn-SAC was designed in advance to build a controllable coordination environment, and then Zn was replaced with Ni to generate Ni-SAC (Ni-N_x-C), increasing the FE_CO as high as 95.6%, and this Zn–CO₂ battery circulates 100 times under 2 mA cm^-2. Phthalocyanine (Pc) or porphyrin presents a similar plane symmetry structure as the M-N₄-C catalyst and is also accepted as a precursor for the SAC. (Liang Z. et al., 2021; Cruz-Navarro et al., 2021; Ji et al., 2021; Wang H. et al., 2022). CoPc was uniformly distributed in three-dimensional N-doped hollow carbon spheres (NHC) and activated under CO₂ atmosphere to prepare defected CoPc@DNHCS-T (Gong et al., 2022). Its high-density carbon defect with pyridine nitrogen establishes a double-electron absorption effect that regulates the electronic structure of the Co atom, accelerating CO₂ activation and *COOH formation. As a result, the Zn–CO₂ battery obtained a maximum power density of 1.02 mW cm^-2 and circulated for 44 cycles (∼44 h) at 1 mA cm^-2. Its FE_CO was up to 94%.

Importantly, the number and coordination mode of N atoms in the SAC induce a local electron density change around the central metal atom, regulating the adsorption of reaction intermediates and catalytic activity. (Yan et al., 2018; Lu et al., 2020) Liu et al. (2022c) anchored an Fe single atom on a B/N co-doped carbon matrix to form the Fe-SA/BNC catalyst that aroused significant electron transfer of *COOH and reduced the energy barrier of *CO on FeN₃B sites in the desorption process (Figures 10A–C) compared with Fe-N₄ sites based on DFT simulations. The Zn–CO₂ battery assembled with Fe-SA/BNC obtained a high power density of 1.18 mW cm^-2. Affected by the planar structure of M-N₄, the coupling rate of proton–electron is slow in the CO₂RR, and it requires higher reaction-free energy. Thus, the unique Fe-N₅ sites with axial N coordination on defective porous carbon nanofibers (Fe-N₅/DPCF) (Li Z. et al., 2022) (Figures 10D, E) were prepared via the facile dicyandiamide-assisted annealing method. Here, Fe is axially coordinated with N atoms on adjacent N-doped carbon layers, and the unstable pyrrole nitrogen and pyridine nitrogen sites on the CF substrate are removed, which leave intrinsic defects (Figures 10F, G). The theoretical calculations revealed that Fe-N₅/DPCF plays a key role in boosting CO₂RR (Figure 10H). Unlike plane Zn-N₄ sites, highly active asymmetric Zn-N₃₊₁ in Zn/NC NSs (Chen J. et al., 2022) (Figures 11A, B) was developed. The Zn atom center is coordinated with four N atoms at the edges of two adjacent graphite atoms to form a twisted Zn-N₃₊₁ structure, which promotes H₂O dissociation, accelerates proton–electron coupling, and reduces the energy barrier of *COOH (Figure 11C). Thus, its FE_CO is increased to 95% with a maximum battery power density of 1.8 mW cm^-2. It performs stable runs for 100 cycles (∼26 h) under 1.5 mA cm^-2.

FIGURE 10

FIGURE 11

As reported, the catalytic effect of N-C configurations in CO₂RR is different and ranked as pyridine nitrogen > graphite nitrogen > pyrrole nitrogen, and these configurations can be altered via regulating the metal complex precursor or pyrolysis conditions (Hu et al., 2021b). For richer pyridine nitrogen, nickel 8-hydroxyquinoline complex (Ni-HQ) as a precursor mixed with melamine is pyrolyzed into Ni-doped graphite nanocarbon (Ni-Nx-2D/NPC) (Zeng et al., 2021) (Figures 11D–F). Pyridine nitrogen improves CO₂ capture capacity, and Ni-N sites inhibit the HER, cooperatively promoting CO₂RR selectivity under wide potential and high current density. The FE_CO is continuously raised above 90% and even nearly approaches 100% (Figure 7N).

SACs with fully exposed sites and a unique coordination environment improve the selective adsorption of specific functional groups, activation of reactants, and regulation of catalytic reactions (Chen et al., 2018; Liu L. et al., 2022). They show great potential in the field of catalysis and would play a vital role in future Zn–CO₂ batteries.

4 Summary and outlook

For aqueous Zn-CO₂ secondary batteries, the CO₂RR process captures and transfers CO₂ into value-added products such as COOH⁻/CH₄ while also generating green electricity. In addition to the opposite charge process, it rarely produces CO₂ again, consuming CO₂ as a whole, which is in line with the CCUS strategy with broad prospects. However, problems such as unstable reaction kinetics, limited electrolyte environment, and poor product selectivity impede the application of Zn–CO₂ batteries. Here, catalyst design plays a decisive role in obtaining high-performance Zn–CO₂ batteries.

In this review, the structure and the involved reaction mechanism of CO₂RR/FAOR/OER in the aqueous Zn–CO₂ battery have been discussed; the preparation method and the strategy to develop cathode catalysts have also been summarized, especially for SAC materials. In summary, carbon-based catalysts contain excellent conductivity, large specific surface area, and abundant resources, showing obvious advantages in superior stability (Paul et al., 2019; Yuan and Lu, 2019; Hu et al., 2021a; Wang J. et al., 2021), but they are far from meeting application demands. Metal-based materials are considered effective catalysts with special geometry (Back et al., 2018), lattice defects, dispersion, and electron distribution (Jiang et al., 2018; Wang Y. et al., 2020). Alloys with low cost and toxicity are becoming their alternatives. Furthermore, metal–carbon composite catalysts are being developed to pursue the integration of excellent stability and catalytic activity (Ding et al., 2019; Wang H. F. et al., 2020; Wang and Astruc, 2020). To tap the potential of metal–carbon composite catalysts to the fullest extent possible, the following strategy is adopted: 1) isolating single-metal sites for fully exposed active sites; (Jiao et al., 2020); 2) steady and high-conductivity carbon support (Sun et al., 2021). Thus, single-atom catalysts (SACs) are initiated and applied in Zn–CO₂ batteries.

FIGURE 12

The adjustable number and coordination mode of N atoms in the SAC present an asymmetric or twisted structure that disrupts uniform charge distribution, resulting in highly desired catalytic properties.

In addition, aiming at the problems of self-corrosion, dendrite, and even hydrogen evolution in the anode of Zn–CO₂ batteries, the reported SEI interface engineering strategy can effectively stabilize the charge–discharge reaction process of the anode, inhibit the formation of zinc dendrites, and reduce the HER process. The excessive reaction of zinc can also be slowed by adding additives to the zinc electrode or by using a polymer film to prevent direct contact between zinc and the water-based electrolyte. Furthermore, the presence of Zn can be altered. For example, Zn is deposited into the collectors as graphene or stainless steel mesh, alternatively, or combined with other metals such as Cu, forming alloys to enhance the anode’s stability.

At present, most Zn–CO₂ battery devices are assembled in H-type electrolytic cells. Their open-circuit voltage is 1–1.5 V, the charge voltage is generally below 2.6 V, and the cycle number is limited, which is far from commercial requirements. This limited performance is caused by its special bipolar membrane in a two-sided electrolyte environment and huge mass/volume of the device. In order to further increase the current and voltage of the device, the overall volume of the device should be compressed as much as possible, and the asymmetric structure of the membrane electrode and gas diffusion electrode should be modified to reduce the contact resistance of each component. On one hand, the hydrophilic and hydrophobic properties on both sides of the membrane electrode can be used to extend the service life of the anode and accelerate the charge–discharge reaction at the three-phase boundary. In addition, the gas diffusion layer can be increased to improve the amount of gas dissolution and accelerate the process of proton–electron coupling.

On the aforementioned bases, advanced in situ characterization techniques such as Raman, XPS, XRD, DEMS, and EXAFS and related theoretical calculations are suggested to deeply explore further the electrochemistry mechanism/process in Zn–CO₂ batteries, to guide the catalyst design, and to further increase the development toward high-performance batteries, multi-function devices, and/or multi-mode units with continuous expanding application potential.

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