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Energy Mater 2022;2:200015. 10.20517/energymater.2022.13 © The Author(s) 2022.
Open Access Review

Carbon-supported single-atom catalysts for advanced rechargeable metal-air batteries

1Key Laboratory for Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, Shandong, China.

2Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China.

3School of Physics, Shandong University, Jinan 250100, Shandong, China

4Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island 627833, Singapore.

#These authors contributed equally.

*Correspondence to: Prof./Dr. Jintao Zhang, Key Laboratory for Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, No. 27, Shanda South Road, China. E-mail: jtzhang@sdu.edu.cn ; Dr. Jun Wang, Key Laboratory for Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, No. 27, Shanda South Road, China. E-mail: jw707@sdu.edu.cn ; Dr. Lili Zhang, Institute of Chemical and Engineering Sciences, A*STAR, 1 Pesek Road, Jurong Island 627833, Singapore. E-mail: zhang_lili@ices.a-star.edu.sg

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    Abstract

    To address the fossil energy crisis and environmental problems, the urgent demand for clean energy has promoted the rapid development of advanced rechargeable metal-air batteries based on the redox reaction couples of gases, such as the oxygen reduction, oxygen evolution, carbon dioxide reduction and carbon dioxide evolution reactions. High-efficiency electrocatalysts are highly desirable to enhance the conversion efficiency of these reactions for enhancing battery performance. Significant advances in single-atom catalysts (SACs) on carbon matrices have been witnessed in recent years as attractive and unique systems to improve the electrocatalytic activities for high-performance rechargeable Zn- and Li-air batteries. This review summarizes the latest achievements in the applications of carbon-supported SACs in metal-air batteries, with a particular focus on the rational design of SACs and their fundamental electrocatalytic mechanism at the atomic level. The future development and perspectives of SACs in the field of metal-air batteries are also discussed.

    INTRODUCTION

    The global shift away from fossil- and carbon-based energy has promoted the development of green and renewable energy conversion and storage systems, such as rechargeable batteries based on a metal with oxygen (metal-O2) or carbon dioxide (metal-CO2)[1-8]. Among them[9-12], benefiting from the low density of Li metal and the stable chemical properties of Zn, metal-air batteries based on Li or Zn as the anode have a very high energy density and outstanding cycling stability and are therefore considered the most likely next-generation energy storage and conversion devices for electric vehicles[13,14]. In addition, in contrast to the current research on the reduction of CO2 into CO, formic acid and other products, which requires electrolysis to consume energy, metal-CO2 batteries, such as aprotic Li-CO2 and aqueous Zn-CO2 batteries, use CO2 as a negative electrode in combination to achieve slow CO2 fixation/utilization[15-18], while generating electricity instead of consuming electricity. However, these batteries face many challenges in actual applications, such as high overcharge potentials, by-products from decomposition of the electrolyte and poor cycle stability[19-22]. Researchers have found that one of the main reasons for these problems is the slow dynamics of the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO2RR) and carbon dioxide evolution reaction (CO2ER). Thus, research has mainly been focused on catalyst development in order to solve the above-mentioned issue of slow reaction kinetics[23-25].

    Precious metals have been found to be the best performing catalysts[26-28]. However, due to their scarcity and high price, there is a constant search for low-cost and sustainable alternatives, such as transition metal oxides[29-32], transition metal chalcogenides[33-35], metal nitrides[36,37] and carbon-based materials[38-40]. In recent years, metal-based nanoparticles (NPs) have received tremendous interest due to their unique mechanical and electrical properties and the synergy of their surface properties, which contribute to their overall behavior. Compared to their bulk counterparts, NPs possess much larger surface areas and quantum size effects. As the size of an NP decreases, the ratio of its exposed surface atoms increases dramatically. The unique surface atomic structure and electronic structure of NPs offer significant potential in catalytic applications[41-43]. More importantly, as the number of atoms of NPs decreases further, nanoclusters are formed, which exhibit enhanced catalytic performance[44-46]. According to the above phenomenon and explanation, the atomic utilization can be maximized with very high catalytic efficiency if the size of the metal is properly controlled down to the atomic level.

    Single-atom catalysts (SACs) are commonly composed of a single metal atom and coordinated heteroatoms on the surfaces of carbon materials, metal oxides and others. In addition to carbon materials, the anchoring of metal atoms on non-carbon supports is also efficient in improving the utilization of metal for enhancing electrocatalysis. For example, metal-supported SACs tend to be formed when the radius and bond formation energy of the supported atoms are close to those of the host metal atoms and their electronic structure can be precisely tuned by properly loading single atoms for an efficient HER, OER and ORR. Metal oxides are effective catalysts for the ORR, and it is possible to obtain high-efficiency bifunctional catalysts after loading SACs on them, which can be used as Li-O2 and Zn-air battery cathodes. In addition, chalcogenides feature higher electrical conductivity and are effective for CO2 reduction, making them potential matrix materials for loading single atoms in Li- and Zn-CO2 batteries. Comparatively, carbon materials, such as graphene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon paper and carbon cloth with high surface areas and superior electrical conductivity are normally employed as SAC supports to boost the ORR, OER, CO2RR and CO2ER by providing abundant channels for mass transfer and thus remarkably improving the electrochemical activities. Moreover, the matrix materials also play a critical role in storing discharge products in the batteries, such as Li2O2 and Li2CO3. For promising practical applications, the energy stored per unit mass or area should be sufficiently high. In this case, carbon matrices exhibit a unique advantage. Compared with metals, oxides, chalcogenides and other compound materials, their mass-specific density is much lower and the preparation processes of porous carbon materials tend to be mature, which can be used to effectively control the pore structures for improving electrocatalytic performance.

    In 2011, Qiao et al.[47] first deposited a single Pt atom on iron oxide, which was used for CO oxidation. Both experiments and density functional theory (DFT) calculations showed that Pt atoms lowered the CO adsorption energy and activation barrier for CO oxidation. SACs have also been shown to be useful in electrochemical energy conversion and storage[48-50], including the ORR[51-53], OER[54-56], HER[57-59] and CO2RR[60-62]. For superior catalytic performance, SACs with separated metal atoms dispersed on a conductive carrier represent promising electrode materials and have been increasingly used in Li- and Zn-air batteries in recent years.

    In this review, we first give a brief introduction regarding Li- and Zn-air batteries and the geometric and electronic structures of SACs on carbon matrices. We then summarize the recent research progress of the applications of SACs in Li- and Zn-air batteries, with a particular focus on their catalytic mechanism. Finally, the future perspectives for the development of SACs are provided.

    COMPARISON OF Li- AND Zn-AIR BATTERIES: ENERGY DENSITY, CELL CONFIGURATIONS AND PRINCIPLES

    Energy density

    Li-ion batteries are widely used in mobile phones and electric vehicles due to their large capacity and stable cycle performance[63-65]. However, due to their energy density limitations, the working times of mobile phones and the mileages of electric vehicles are reaching a plateau. Therefore, this has stimulated researchers to search for new battery systems with higher energy density, among which metal-air batteries have received widespread attention[66-68]. Since Abraham and Jiang invented rechargeable Li-air batteries in 1996, they have been divided into four types, namely, aprotic, water, hybrid and solid state, which utilize different electrolytes[69,70]. Among them, because of their excellent reversibility, aprotic Li-O2 batteries have received significant interest. As shown in Figure 1A, Li-air batteries offer the highest theoretical energy density of 11429 Wh kg-1, which is comparable to that of gasoline[71]. In addition, the theoretical energy densities of Al- and Mg-air batteries are also very high, with values of 8076 and 6815 Wh kg-1, respectively, which are five to six times that of Zn-air batteries (1353 Wh kg-1). However, their practical energy density is low due to the large polarization and low voltage. Furthermore, they are difficult to electrochemically charge in alkaline electrolytes, thereby limiting their practical application. In contrast, Zn is more stable and can be charged more successfully in alkaline electrolytes[72-74]. As shown in Figure 1B, the practical energy density of Zn-air batteries has reached 350 Wh kg-1, which is twice that of Li-ion batteries[75]. They also have a very stable working voltage and are the best power sources for advanced hearing aids[76-78], such as BTE, in-ear and ear canal devices. This has resulted in significant industrial interest in the research and development of Zn-air batteries for hearing aids. For example, Revolt recently signed a cooperation agreement with PhonakAG to jointly develop rechargeable Zn-air batteries for hearing aids.

    Figure 1. (A) Theoretical energy densities for different types of metal-air batteries. Reproduced with permission: Copyright 2017, American Chemical Society[71]. (B) Gravimetric energy densities (Wh kg-1) for various types of rechargeable batteries compared to that of gasoline. Reproduced with permission: Copyright 2014, American Chemical Society[75].

    The energy densities of Li- and Zn-CO2 batteries are also higher than that of Li-ion batteries, with the theoretical energy density of Li-CO2 batteries reaching 1876 Wh kg-1. Further research into the corresponding carbon dioxide batteries cannot only provide a deeper understanding of the CO2RR and CO2ER, but can also promote the study of the corresponding metal-oxygen batteries via an in-depth understanding of the formation and decomposition mechanism of their by-products.

    Cell configurations and working principles of Zn- and Li-air batteries

    Zn-air batteries

    As shown in Figure 2A, the structure of Zn-air batteries is mainly composed of four parts, namely, a metal Zn electrode as the anode, a porous separator, an alkaline electrolyte solution and a cathode porous air electrode[79]. The air cathode is composed of a catalytic layer, a current collecting layer and a gas diffusion layer. A mixture of active material powder and PTFE binder is used to fabricate the catalytic layer. For typical alkaline Zn-air batteries, the electrochemical reactions are shown in Eqs. (1)-(5)[80,81]. During discharge, the oxidation of the Zn anode is combined with hydroxide ions to generate Zn(OH)42-, which possibly decomposes into ZnO. Simultaneously, oxygen is reduced in the cathode at the catalyst-oxygen-electrolyte 3-phase boundaries. During the charging process of a rechargeable Zn-air battery, the zinc compounds are reduced and deposited as Zn metal with the OER via the desirable four-electron process under external voltages.

    Figure 2. (A) Zn-air battery cell. Reproduced with permission: Copyright 2019, Wiley[79]. (B) Zn-CO2 battery cell. Reproduced with permission: Copyright 2019, American Chemical Society[82]. (C) Li-O2 battery cell. Reproduced with permission: Copyright 2020, American Chemical Society[87]. (D) Li-CO2 battery cell. Reproduced with permission: Copyright 2019, Royal Society of Chemistry[93].

    Anode: Zn → Zn2+ + 2e- (1)

    Zn2+ + 4OH- → Zn(OH)42- (2)

    Zn(OH)42- → ZnO + H2O + 2OH- (3)

    Cathode: O2 + 2H2O + 4e- → 4OH- (4)

    Overall: 2Zn + O2 → 2ZnO (5)

    Zn-CO2 batteries

    The main components of Zn-CO2 batteries are a metal Zn anode, a porous air cathode, an electrolyte and bipolar membranes. The bipolar membranes can maintain the stability of the alkaline anolyte and neutral catholyte simultaneously, as well as the dynamic characteristics of ion migration and protons[82]. Compared with inert Li-CO2 batteries, aqueous Zn-CO2 batteries provide a proton-coupled electron transfer pathway for CO2 reduction to value-added carbon-containing products[83-85], including CO, C2H4, HCOOH and C2H5OH [Figure 2B], as the products of the cathode. For example, Xie et al.[86] recently reported a reversible aqueous Zn-CO2 battery based on CO2-HCOOH interconversion on a bifunctional Pd cathode. CO2 was converted to HCOOH on the Pd cathode during the discharge process. During charging, the Pd cathode underwent a reversible HCOOH-to-CO2 conversion. For Zn-CO2 batteries, the chemical reactions of different products on the cathode are shown in Eqs. (6)-(10):

    CO2 + 2e- + 2H+ → CO + H2O E = -0.11 V (6)

    CO2 + 2e- + 2H+ → HCOOH E = -0.20 V (7)

    2CO2 + 12e- + 12H+ → C2H4 + 4H2O E = 0.07 V (8)

    2CO2 + 12e- + 12H+ → C2H5OH + 3H2O E = 0.08 V (9)

    The anode reaction is as follows:

    Zn + 4OH- → Zn(OH)42- + 2e- E = 1.20 V (10)

    Li-O2 batteries

    A typical non-aqueous Li-O2 battery consists of three parts, namely, a Li metal anode, an electrolyte solution with a Li salt in an aprotic solution and the cathode consisting of a porous catalyst and a conductive agent [Figure 2C][87]. During the discharge process, the Li in the anode is oxidized to form Li ions, while the oxygen in the cathode is reduced to form superoxide ions at the two-phase interfaces between the electrolyte solution and the cathode. The superoxide ions are then combined with Li ions transported through the electrolyte to form lithium superoxide (LiO2)[88]. The single-electron transfer electrochemical process or disproportionation reaction leads to the form of lithium peroxide (Li2O2)[89]. According to current research on the discharge mechanism of Li-O2 batteries, the morphology of discharge products is largely controlled by the solubility of LiO2 intermediates[70,90]. In strong Li+ solvated solvents, Li2O2 usually forms thin films or small particles through surface mechanisms. In weak Li+ solvating solvents, large-sized Li2O2 particles are formed by solution mechanisms. During the charging process, the products, including LiO2, are oxidized and decomposed into Li ions, electrons and oxygen. The charge-discharge electrochemical equations are shown below[91]:

    Li electrode: Li → Li+ + e- (11)

    Air electrode: O2 + e- → O2- (12)

    LiO2 + Li+ + e- → Li2O2 (13)

    2LiO2 → Li2O2 + O2 (14)

    Overall: 2Li + O2 → Li2O2 (EOCV = 2.96 V) (15)

    Li-CO2 batteries

    Initial Li-CO2 battery investigations originated from the effects of CO2 in Li-air batteries. With O2/CO2 combination operating gases, the high discharge capacity is three times that of pure O2-based batteries[92]. Because of their potential to trap CO2 and deliver power, further investigations have focused on Li-CO2 batteries. The configuration of liquid-free Li-CO2 batteries is shown in Figure 2D. These batteries consist of two electrodes, namely, a Li metal anode and a porous air cathode[93]. During the discharge process, the Li anode loses electrons and releases Li+ electrolytes, while CO2 molecules trap electrons and Li ions on the cathode surfaces to form the mixed products of Li2CO3 and carbon. During the charging process, Li2CO3 and carbon are reversibly decomposed to produce CO2 and Li ions[94-96]. The electrochemical reaction equations are as follows:

    Anode: 4Li → 4Li+ + 4e- (16)

    Cathode: 4Li + 3CO2 + 4e- → 2Li2CO3 + C (17)

    Overall: 3CO2 + 4Li → Li2CO3 + C (18)

    SINGLE-ATOM CATALYSTS

    Features

    In 2011, Zhang and colleagues[47] first proposed the term “monoatomic catalysis” in their seminal work. In recent years, this has gradually been accepted by the catalytic community and has become a new field in catalysis. SACs, with their catalytic sites dispersed at the atomic level, utilize metal active sites to the greatest extent and usually exhibit ultrahigh activity and selectivity[97,98]. However, as shown in Figure 3, as the particle size of a catalyst is reduced to the atomic scale, the surface free energy increases rapidly and the single atoms tend to aggregate[99,100]. To understand and solve these problems, we need to develop a deeper understanding of the structures of SACs.

    Figure 3. Schematic illustrating the changes in surface free energy and specific activity per metal atom with metal particle size and the support effects on stabilizing single atoms. Reproduced with permission: Copyright 2013, American Chemical Society[100].

    Geometric and electronic structures

    The strong interaction between a single atom and the surrounding ligands forms a catalytically active site[101]. This structure can be observed by AC-TEM HAADF-STEM AFM[102-104]. As displayed in Figure 4, the factors influencing the geometric structures of SACs on graphene substrates include the number of N atoms, the number of metal atoms, the bonding of other atoms (including the axial addition of atoms), the structural change of the matrix and the modification (doping) of the matrix.

    Figure 4. Schematic illustration of different geometric structures of carbon-supported SACs and the function of adjusting the electronic structure.

    When the geometric structure and chemical bond are established, charge transfer between the metal atom and the surrounding atoms occurs due to the difference in chemical potential[105,106]. The adsorption behavior of the catalyst is mainly controlled by the electronic properties of the active sites. Therefore, adjusting the electronic structure of the metal atom by altering the surrounding environment is the most common method to improve the inherent catalytic ability of a SAC[107]. For example, Luo and co-workers[108] conducted a computational search of 17 metal elements based on the model of the coordination of transition metals with four pyridyl nitrogen atoms in a graphene matrix and found that most M-N-Cs showed poor HER catalytic performance because the Gibbs free energy (ΔH) of their hydrogen adsorption is far from 0 eV. Only M-N-Cs with Co, Cr, Fe, Rh and V transition metals were found to be promising candidates, among which Co-N-C materials show the smallest |ΔGGH| (0.13 eV) and are therefore predicted to have the best HER activity.

    Wang et al.[109] demonstrated via a CO2RR test that a Co-N2 catalyst with the highest FECO of 95% at -0.68 VRHE has a much better performance than Co-N4, which exhibited negligible activity. Furthermore, Pan et al.[110] established two FeN4-type active sites surrounded by different numbers of local carbon atoms, denoted as FeN4-C10 and FeN4-C8, respectively. DFT calculations showed that the oxygen adsorption energy increased from -0.95 to -1.67 eV and the OOH dissociation process at the FeO4-C8 position only requires an activation energy of 0.2 eV, which is 0.36 eV lower than the activation energy at the FeN4-C10 position. Yuan et al.[111] proved that the atomically dispersed Fe-Nx species on B-doped porous carbons have better ORR performance than undoped electrodes through linear sweep voltammetry polarization curves. Ren and co-workers[112] prepared an atom-dispersed Ni-Fe bimetallic center catalyst that showed better CO2RR performance than the corresponding Ni-Fe single center. These examples undoubtedly illustrate that the electronic structure of SACs is affected by multiple factors, and only when certain conditions are met synergistically can SACs with high catalytic performance be obtained.

    APPLICATION OF SINGLE-ATOM CATALYSTS IN METAL-AIR BATTERIES

    Application in Zn-air batteries

    M-Nx-C structure (M = Fe, Co or Cu)

    In recent research reports, it was found that many transition metal center atoms have good ORR/OER properties. In 2017, Specchia et al.[113] compared and studied the kinetics of different M-Cs (M = Fe, Co or Cu) in detail and concluded that the oxygen adsorption strength follows the trend of Cu-NC > Fe-NC > Co-NC. Duan et al.[114] compared the OER activities of the same active sites (M-N4C4) (M = Fe, Co or Ni) and discovered that they follow the order of Ni-N4C4 > Fe-N4C4 > CO-N4C4. Du and co-workers[115] dispersed Fe atoms on N-doped carbon through a “confined recrystallization” self-template strategy and the mass fraction of Fe reached 1.5%. The high angle annular dark-field STEM image showed a uniform distribution of single atoms [Figure 5A]. The fitting lines of the Fe-NC SACs in R-space confirmed the coordination of Fe-N4 [Figure 5B]. The authors also studied the reaction path further using DFT calculations. The calculations showed that the ORR follows the association mechanism of O2 / *OOH / *O / *OH / OH-, while the OER follows the mechanism of OH- / *OH- / *O / *OOH / O2 [Figure 5C]. In fact, this is also the reaction pathway of almost all SACs in Zn-air batteries. It is noteworthy that in flexible solid Zn-air batteries, Fe-NC SACs exhibit a high discharge voltage of 1.25 V and excellent cycle performance over 160 cycles at 1 mA cm-2. In addition, Fe-N4 supported on carbon materials, such as SA-Fe-NHPC[116], Fe/N/C-DT[116] and 3DOM Fe-N-C-900[117], have been synthesized and applied to Zn-air batteries with large peak power density and superior long-term stability.

    Figure 5. (A) High angle annular dark-field STEM image of Fe-NC SACs. (B) Fitting lines of Fe-NC SACs in R-space with an inset schematic of the Fe-N4 structure. (C) Schematic of ORR and OER reaction mechanisms on Fe-NC SACs. Reproduced with permission: Copyright 2020, Royal Society of Chemistry[115]. (D) Co-N4/C and Co cluster|Co-N4/C models. (E) Free-energy diagram of two models at U = 0 and 1.23 V. Reproduced with permission: Copyright 2019,Wiley[118]. (F) Polarization curves and corresponding power densities of Co-NPs@NC, Co-ACs@NC, Co-SAs@NC and Pt/C-based Zn-air batteries. Reproduced with permission: Copyright 2019, Wiley[119]. (G) HAADF-STEM image of Cu-NrC-60. (H) STM simulation images of Cu-NrC-60. (I) Volcano plot of the relationship between ORR activity and DEO of Cu-N2, Cu-N3, Cu-N4, a CuPc molecule and N-doped graphene. Reproduced with permission: Copyright 2016, Royal Society of Chemistry[120].

    Ji and co-workers[118] embedded CO atoms on an N-doped carbon flake array (CoSA@NCF/CNF) using a dipping-carbonation-acidification process. Promising results were obtained when CoSA@NCF/CNF was used in wearable Zn-air batteries. Compared with Co nanoclusters, CoSA@NCF/CNF delivered better ORR and OER performance. The authors established the model [Figure 5D] and the free-energy diagram [Figure 5E] of the CoSA and Co cluster-based ORR through DFT calculations. Under the equilibrium voltage (U = 1.23 V), it was found that the minimized free energy change step was the rate-limiting step (RLS) that defines the limiting potential of the ORR for both catalysts as OH*-OH- but with a much lower energy required for CO-N4/C. At U = 0 V, the RLS was found to be endothermic. These results indicate that monoatomic materials can improve the catalytic performance by reducing the overall oxygen reduction/release reaction barrier. Similarly, Han et al.[119] synthesized Co nanoparticles, atomic Co clusters and Co single atoms on N-doped carbon by adjusting the Zn dopant content in a bimetallic ZnCo-ZIF precursor. It can be found that the performance of Co SACs is much better than that of the corresponding nanoparticles and nanoclusters [Figure 5F].

    As discussed above, modifying the number of surrounding atoms changes the electronic structure of the central atom, thereby affecting the catalytic activity. Wu et al.[120] obtained highly doped (8.5 wt.%) and exposed copper(I)-nitrogen [Cu(I)-N] active sites in graphene by paralyzing the coordinated saturated copper phthalocyanine. The HAADF-STEM image is shown in Figure 5G. It can be seen that a large number of Cu atoms are embedded in the graphene matrix. The STM simulation [Figure 5H] shows that the Cu atoms are bonded to two N atoms. The electronic density of states of Cu-N2, Cu-N3 and Cu-N4 were calculated by DFT. The authors found that the Cu-N2 structure has a moderate binding strength with oxygen species, resulting in good adsorption and desorption capabilities for the ORR. This finding was supported by a volcano plot of the relationship between the ORR activity and ΔEO of different models. We can see that Cu-N2 has the best activity with moderate ΔEO, which compromises the reaction barriers during the adsorption and desorption steps [Figure 5I]. The authors also calculated the reaction path and energy barrier of Cu-N2. The removal of *OH needs to overcome an energy barrier of 0.83 eV, which is similar to the theoretical results previously reported on the Pt (111) surface. Yang et al.[121] also successfully embedded Cu-N2 and Cu-N4 into graphene. Through calculations, they found that Cu-N2 and Cu-N4 have potential barriers similar to those of Pt (111) at voltages of 1.23 and 0.4 V, respectively. However, overall, Cu-N2 exhibits higher activity than Cu-N4.

    Single-atom catalysts on modified carbon matrices

    The electronic structure of a SAC can be effectively modified by doping the matrix (heteroatoms or functional groups) to improve the catalytic performance. Sun et al.[122] improved the intrinsic activity of the Fe-N4 active site by introducing a P-O bond, as shown in Figure 6A. The P atom is connected to the N bond on the active site. A water-based Zn-air battery pack equipped with this catalyst showed a significant peak power density of 232 mW cm-2 and an excellent cycle performance of 450 stable cycles. The authors also investigated the high catalytic activity through DFT calculations. Figure 6B shows the free-energy diagram of the ORR and OER at 1.23 V on different active sites. Unlike other reports of the limiting step being the first electron transfer (OO* + H2O + e- = *OOH + OH-) step, the limiting step on the P-O/FeN4-CNS catalyst is the fourth step (OH* + 3OH- + e- = 4OH-). In contrast, in the OER process, the limiting steps are all O* + 2OH- = HOO* + OH- + e-, with P-O/FeN4-CNS still exhibiting the minimum energy barrier. Figure 6C shows the free energy of the ORR and OER limiting steps of different models. It can be seen that the performance of the OER/ORR increases in the order of FeN4-CNS < P/FeN4-CNS < P-OH/FeN4-CNS < P-O/FeN4-CNS. These results show that P impurities changed the charge density distribution and electronic structure of Fe active sites, and this demonstrated that the P-O group and FeN4 displayed a synergistic coupling effect, thereby creating a good surface electronic environment that led to the improved catalytic performance.

    Figure 6. (A) Top and front views of the different models for P-doped FeN4-CNS. (B) Calculated free-energy diagram of ORR and OER at the equilibrium potential (1.23 V). (C) Free energy of ORR and OER limiting steps for the models. Gray, red, blue, purple, white and pink spheres represent C, O, N, Fe, H and P atoms, respectively. Reproduced with permission: Copyright 2016, American Chemical Society[122]. (D) Schematic model of Fe-SAs/NPS-HC with Fe (orange), N (blue), P (green), S (yellow) and C (gray) atoms. (E) Calculated charge density differences of Fe-SAs/ NPS-C with yellow and blue areas representing charge density increase and reduction, respectively. (F) Free-energy diagram of ORR on different catalysts. (G) The linear relationship between OH* binding energy and Bader charge of different catalysts. Reproduced with permission: Copyright 2018, Springer Nature[123]. (H) Optimized Mn-N3O1 structure with ORR intermediate states OH*. The configurations are illustrated both from top and side views. Black, blue, cyan, red and white balls represent C, N, Mn, O and H atoms, respectively. (I) Free-energy diagram of ORR on Mn-N1O3, Mn-N2O2 and Mn-N3O1 surfaces under 0.4 V. (J) Local density of states of Mn d-orbitals in Mn-N1O3, Mn-N2O2 and Mn-N3O1 structures. Reproduced with permission: Copyright 2018,Wiley[124].

    Chen and co-workers[123] dispersed a single iron atom on a hollow carbon polyhedron co-doped with phosphorus and sulfur (Fe SAs/NPS-HC) through the Kirkendall effect. A schematic diagram of the structure is shown in Figure 6D. Moreover, Zn-air batteries based on Fe-SAs/NPS-HC showed low overpotentials and stable cycle performance. Figure 6F shows the free-energy diagram of the ORR for Fe-SAs/NPS-C, Fe-SAs/NS-C and Fe-SAs/NC. When U = 0 V, the limiting step on all three active sites is the fourth step. Compared with Fe-SAs/NS-C and Fe-SAs/N-C, the energy barrier on Fe-SAs/NPS-C is the smallest, indicating weaker binding of the OH* intermediate and easier OH- formation on Fe-SAs/NPS-C. According to the charge density differences [Figure 6E] and the linear relationship between OH* binding energy and Bader charge [Figure 6G], it can be seen that the electron donation of the surrounding sulfur and phosphorus can reduce the positive charge of the metal center Fe (Fe δ+) of Fe-SAs/NPS-C, resulting in the reduced bonding strength of OH*.

    Unlike the SACs with a general M-N-C structure in previous reports, Yang et al.[124] proved that coordinated oxygen atoms can also play an important role in regulating the inherent catalytic activity of transition metals and the corresponding catalysts are effective in the ORR under alkaline conditions, exhibiting good performance as electrodes in Zn-air batteries. The authors also found through calculations that the Mn-N3O active center showed the best ORR activity, with its structure shown in Figure 6H. Figure 6I shows the free-energy diagram of the ORR on Mn-N1O3, Mn-N2O2 and Mn-N3O1 surfaces under U = 0.4 V. The rate determination step on the three model surfaces is OH* desorption and the Mn-N3O1 model shows the lowest energy barrier (0.73 eV), which is very close to the value of the Pt (111) surface. Figure 6J shows the corresponding density of states (DOS) diagram. The downward shift of the center and the adjacent position of the d-band in the Mn-N3O1 model first peak make it an ideal ORR active site. The downshifted position of the d-band center and the adjacent position of the first peak relative to the Fermi level in the Mn-N3O1 model showed that the Mn-N3O1 surface would have either stronger or weaker adsorption with OH* intermediates, thus endowing itself with a perfect ORR active site.

    Bimetallic single-atom catalysts

    SACs offer significant potential for many catalytic systems due to their high activity, stability and selectivity regarding the unique electronic structure of their active center with nearly 100% atomic utilization. Recent studies have shown that the introduction of secondary metal atoms can further enhance the activity of SACs, resulting in enhanced performance.

    Han et al.[125] embedded binary Co-Ni sites onto N-doped hollow carbon nanocubes (CoNi-SAs/NC) [Figure 7A] through pyrolysis of the primary amine-coated metal-organic framework. Compared with Ni single-atom and N-doped carbon catalysts, the limiting ORR potential of Co-Ni-N is 0.88 eV when U = 0 V [Figure 7B], greater than those of NC (0.14 eV) and Ni-N (0.60 eV). At the equilibrium potential of 1.23 V, the RLS on NC and Ni-N is the hydrogenation of molecular O2, with energy barriers of 1.09 and 0.63 eV, respectively. While the maximum upslope value of Co-Ni-N is further reduced to 0.35 eV, the RDS becomes the protonation of OH*, which shows that Co-Ni-N has the best ORR capability. During the OER process with a potential of 1.57 V [Figure 7C], OH* adsorption is the RDS on NC and Ni-N with potential barriers of 1.43 and 0.97 eV, respectively, while co-Ni-N can effectively promote the reaction by reducing the energy barrier to 0.69 eV, indicating that Co-Ni-N has the best OER capability. When Co-Ni-N was used as an electrode material in rechargeable Zn-air batteries, it showed lower overpotentials (1.16 and 2.05 V at 10 mA cm-2) than those of Pt/C-IrO2 (1.17 and 2.12 V at 10 mA cm-2).

    Figure 7. (A) Structure of CoNi-SAs/NC. Free-energy diagrams of (B) two models at 0 and 1.23 V and (C) OER processes on NC, Ni-N and Co-Ni-N at 1.57 V. Reproduced with permission: Copyright 2019, Wiley[125]. (D) Overall bifunctional performance for different catalysts based on Tafel slope in 1 M KOH/0.1 M O2-saturated KOH. Proposed mechanisms for (E) ORR and (F) OER. Black, blue, green, golden, red and pink balls represent C, N, Ni, Fe, O and H atoms, respectively. Reproduced with permission: Copyright 2020, Elsevier[126]. (G) Free-energy diagram of ORR on Fe, Mn-N/C under an 0.33 V overpotential. (H) Differential charge density illustrating the increase (olive color) and decrease (cyan color) in electron distributions. (I) Bader charge analysis. Reproduced with permission: Copyright 2018, Royal Society of Chemistry[127]. ORR: oxygen reduction reaction; OER: oxygen evolution reaction.

    In addition, Zhu et al.[126] explained the superior performance of SACs with bimetallic atoms from the perspective of atomic bonds. They used a simple one-step two-solvent impregnation route to embed Fe-Ni bimetallic pairs into nitrogen-doped carbon hollow spheres (Fe-NiNC). The Fe-NiNC catalyst showed better overall bifunctional performance than NC, Fe-NC and Ni-NC [Figure 7D]. The authors compared the ORR and OER activities of Fe-NiNC and Fe-NC through DFT calculations and found an interesting phenomenon. The ORR activity of Fe-NC is similar to that of Fe-NiNC, but the OER activity is much lower. The reason for this is shown in Figures 7E and F. For Fe-NiNC, in the first electron transfer in the ORR process, the O-O bond is disconnected from the OH adsorbed on the Ni atom and O is chemically adsorbed on the Fe atom. The latter process is the same as in the single-atom Fe-NC. While in the OER process, in the presence of the OH ligand bridging Fe-Ni atoms, all intermediates are preferentially adsorbed on Ni. In addition, the physically mixed FeNC+NiNC samples were used for ORR and OER performance testing and showed similar performance to Fe-NC. This indicates that Fe and Ni act as the active centers for the ORR and OER, respectively. Therefore, the catalyst containing bimetallic metal atoms exhibited better bifunctional performance (ORR and OER).

    Gong and co-workers[127] dispersed Fe and Mn atoms on porous carbon through the one-step pyrolysis of a metal-organic framework (ZIF-8) and delivered excellent ORR performance when used as an electrode. The discharge curve shows that the change in output voltage is negligible at 20 mA cm-2 for 23000 s (6.4 h). As shown in the free-energy diagram of Figure 7G, the RLS of Fe, Mn-N/C is the protonation of O* and lower energy (0.22 eV) is required than that of Fe -N/C (0.36 eV). According to Bader charge analysis [Figures 7H and I], the Fe and Mn atoms act as electron donors, while N atoms act as electron acceptors. This unique electron donor-acceptor characteristic can alter the electronic structure of Fe, Mn-N/C, thereby reducing the ORR overpotentials.

    Application in Zn-CO2 batteries

    As shown in Figure 8A, Wang et al.[17] used a gas diffusion strategy and one-step thermal activation method to insert Fe atoms into N-doped porous carbon polyhedra and adjust the pyrolysis temperature to control the number of nitrogen atoms bonded to Fe atoms to obtain optimized Fe1NC/S1-1000 (with 3 N atom bonds) samples. A maximal power density of 0.6 mW cm-2 was obtained when used in a Zn-CO2 battery. The authors found that Fe1NC/S1-1000 has better CO2ER catalytic activity than Fe1NC/S1-800 (the coordination number of N atoms is 3.8) and Fe1NC/S1-900 (the coordination number of N atoms is 3.3). By constructing a model [Figure 8B], the authors calculated the Fe-N4, Fe-N3 and Fe-N3V CO2ER reaction path energies [Figure 8C]. It was found that the Fe-N3 active center can significantly reduce the energy barrier of intermediate *COOH and CO generation, thereby improving its CO2ER performance.

    Figure 8. (A) Schematic illustration of synthetic procedure for gas diffusion strategy. (B) Optimized atomic structures for *CO and *COOH intermediates absorbed on FeN4, FeN3 and FeN3V models. Yellow, blue, grey and red balls represent Fe, N, C and O atoms, respectively. (C) Calculated free-energy diagrams for CO2ER pathway of FeN4, FeN3 and FeN3V moieties. Reproduced with permission: Copyright 2020, Wiley[17]. (D) FT-EXAFS fitting results of Cu-N2/GN. (E) DFT-based free-energy profile for the optimized Cu-N2 and Cu-N4 models during the CO2ER. (F) Geometric surface area-normalized partial current density for CO production of different catalysts. Reproduced with permission: Copyright 2019, Wiley[128].

    Zheng and co-workers[128] obtained a high-efficiency Zn-CO2 battery catalyst with Cu-N2 active sites dispersed on graphene by pyrolyzing the graphene-supported precursor in a high-temperature N2 atmosphere. FT-EXAFS fitting results confirmed the existence of Cu-N2 [Figure 8D] and the electrode with Cu-N2 as the active site has better CO2ER performance than the electrode with Cu-N4 as the active site. Zn-CO2 with Cu-N2/GN as the catalyst can provide a peak power density of 0.6 mW cm-2. According to DFT calculations, Cu-N2 requires less energy to capture CO2 than Cu-N4. In addition, smaller energy is needed for the intermediate step from *CO2 to *COOH [Figure 8E] on Cu-N2 center, explaining the superior CO2ER activity of Cu-N2/GN. A series of single metal atoms on N-doped carbon (SAs-MC-N, M = Fe, Co, Ni or Cu) was screened through a one-step in-situ pyrolysis method with the active center structure of M-N4. The CO2ER activity follows the order of Ni > Fe > Cu > Co [Figure 8F]. An integrated Zn-CO2 battery assembled with SAs-Ni-N-C as the cathode delivered a maximum COFE. of 93.3% with a peak power density of 1.4 mW cm-2. In addition, Yang et al.[129] designed and synthesized Ni-N and P-O co-doped graphene (NiPG) as a multifunctional catalyst. A dual-mode Zn-CO2/Zn-O2 battery was made with NiPG as the electrode, which can function by supplying either CO2 or O2. This advanced the Zn-air battery through the novel design of a multifunctional electrocatalyst and dual-mode batteries.

    Applications in Li-O2 batteries

    Wang et al.[130] successfully embedded cobalt atoms in ultrathin nitrogen-rich carbon (Co-SAs/NC) through a green gas migration-capture strategy, and it showed an ultralow overpotential (0.4 V) in Li-O2 batteries [Figure 9B]. As shown in Figure 9A, the Co-SAs/NC electrode exhibits a very low overpotential because the Co atom bonded to the nitrogen atom has a stronger adsorption capacity for LiO2. Thus, in the ORR process, it will generate uniform nanoscale Li2O2, which is easily decomposed in the OER process. In contrast, Co nanoparticles are prone to form agglomerated large-scale Li2O2, which not only increases the overpotential during decomposition, but also contains residual Li2O2, leading to a decreased electrochemical performance.

    Figure 9. (A) Schematic of working mechanism for Co-SAs/N-C electrodes. (B) Calculated free-energy diagrams for discharge-charge reactions on the active surface of Co-SAs/N-C. Reproduced with permission: Copyright 2020, Springer Nature[130]. (C) Structure of Fe/N/C Composite. Reproduced with permission: Copyright 2012, American Chemical Society[131]. (D) Discharge mechanism of N-HP Co SACs in Li-O2 batteries. Reproduced with permission: Copyright 2020, Springer Nature[132]. (E) TEM and HAADF-STEM images of Ru0.3 SAs-NC with (F) its charge density distribution and (G) total and partial DOS. Reproduced with permission: Copyright 2020, American Chemical Society[133]. (H) DOS of Pt-CNHS. Reproduced with permission: Copyright 2020, Elsevier[134].

    In a report by Shui et al.[131], Fe atoms were evenly distributed on the carbon support and were bound by nitrogen and oxygen [Figure 9C], with low overpotentials obtained because of the higher boundary between the SAC and the reduced electron and mass barrier by lithium oxide precipitation. There is no doubt that single atoms play a strong role in regulating the formation and decomposition of Li2O2. Song and co-workers[132] synthesized a cobalt atom electrocatalyst (N-HP-Co SACs) for Li-O2 batteries through a polymer encapsulation strategy and showed good cycle stability (218 d). The authors found that its discharge product is micron-sized and the growth mechanism of the flower-like Li2O2 is shown in Figure 9D. The Co atom bonded to the nitrogen atom provides more active sites for Li2O2. Under the enhanced solvation-mediated mechanism (Eads = -1.03 eV, the adsorption energy of Li2O2 on Co-N4), the N-HP-Co SACs could provide more nucleation sites, which are conducive to the formation of small nanosheets and further self-assembled to form large sheets. This kind of Li2O2 can be completely decomposed at a lower charge voltage, which is an important reason for the good cycle performance of the electrode.

    Furthermore, Hu et al.[133] used a MOF-assisted space confinement and ion substitution strategy to successfully embed Ru atoms on N-doped porous carbon and optimized the content of Ru atoms (Ru0.3 SAs-NC). The electrode showed good ORR and OER catalytic performance. The TEM and HAADF-STEM images are shown in Figure 9E. The isolated Ru atoms are highly dispersed on the porous carbon. The charge density difference [Figure 9F] of Ru0.3 SAs-NC was found by DFT calculations, indicating that the Ru-N4 showed rich charge transfer. Its DOS diagram [Figure 9G] also shows that Ru significantly contributed to the valence band and conductance of the Ru0.3 SAs-NC system. Therefore, the surface of Ru0.3 SAs-NC is more active/stable than that of the pyrolytic ZIF-8 surface. A similar study involved the loading of single-atom Pt onto porous ultrathin g-C3N4 nanosheets (Pt-CNHS)[134]. Figure 9H shows the DOS spectrum of Pt-CNHS. Compared with CNHS, Pt-CNHS shows a narrower band gap, which is attributed to the existence of Pt 5d electrons. The charge distributions of a single Pt atom and CNHS are both abundant and uniform. However, the charge distribution of a single Pt atom on CNHS changed significantly. The charge distribution of the adjacent CNHS also altered slightly, indicating that a single Pt atom strongly interacted with CNHS to form a stable and highly active structure. Simultaneously, it can be seen that the Pt atom has an unbalanced charge distribution to generate local electric fields, which could cause a local built-in driving force that strongly promotes effective electron and ion transmission in the electrode.

    Applications in Li-CO2 batteries

    In recent years, SACs have also shown promising performance for the conversion of CO2 to CO and thus attracted widespread attention. However, there is only a few relevant reports on the practical application of SACs as cathode catalysts for Li-CO2 batteries. Hu et al.[135] successfully implanted single iron atoms into N- and S-doped porous graphene [Figure 10A], which showed high Li-CO2 battery efficiencies with a low potential gap (1.17 V) and excellent cycle performance (200 cycles). The porous hierarchical architecture features a large surface area and exhibits abundant free space throughout the interconnected framework, in which the pyridine divalent nitrogen atoms at the edge of the graphene anchor the single iron atoms to form many “Fe-Nx” moieties. As shown in Figure 10C, it was calculated and experimentally verified that this composite catalyst delivered the highest catalytic activity compared with those of its counterparts. It is demonstrated that the co-doping of N and S and the presence of the “Fe-N4” moieties in the carbon skeleton [Figure 10B] would lead to the charge and spin redistribution. This could result in the fast charge transfer between the carbon atoms and the heteroatoms, thereby enabling excellent catalytic performance towards CO2 reduction and evolution for Li-CO2 batteries.

    Figure 10. (A) Atomic-resolution STEM images of Fe-ISA/N, S-HG. (B) Spin density distribution of edged “FeN4” moiety in the carbon skeleton. (C) Adsorption energy of *CO (∆G*CO) as a function of the overpotential (ηChargeDisharge) on the above catalysts. Reproduced with permission: Copyright 2020, Wiley[135]. (D) XANES spectra at the Co K-edge of SA Co/GO, adjacent Co/GO, Co3O4, CoO and Co foil. (E) Fourier transforms of Co K-edge of SA Co/GO, adjacent Co/GO, Co3O4, CoO and Co foil. (F) HAADF-STEM image of adjacent Co from adjacent Co/GO. (g) Side views and (h) charge density differences of Li2CO3 adsorbed on graphene oxide, single cobalt atom and dimer cobalt, respectively. Purple, green, gray and red balls represent Co, Li, C and O, respectively. Reproduced with permission: Copyright 2019, Wiley[136].

    Zhang et al.[136] anchored 5.3% Co atoms on graphene oxide (adjacent Co/GO), in which adjacent atomic dispersion was maintained, as clearly depicted in its HAADF-STEM image of Figure 10F. The positive shift of absorption edge peaks in Figure 10D and the Co-Co bond centered at 1.90 Å in Figure 10E of the catalysts also indicates that Co particles appear in the form of distributed atoms. DFT calculations were applied to study the stability of Li2CO3 intermediates on the surface of GO, SA Co/GO and dimer Co/GO. Interestingly, among the three catalysts, dimer Co/GO shows the lowest adsorption energy of -3.79 V [Figure 10G] and the largest equivalent surface in Figure 10H. These findings indicate that the dimer Co could effectively capture Li2CO3 and promote the electrocatalytic reactions during cycling, which result in superior Li-CO2 battery performance of the adjacent Co/GO catalyst.

    Tables 1[17,115-128,137-152] and Tables 2[130,132-136] list the recent research progress of the applications of SACs in Li- and Zn-air batteries.

    Table 1

    Comparison of electrocatalytic performance based on carbon-supported SACs for Zn-air/CO2 batteries reported in this review

    MaterialsPeak power densityOpen-circuit voltageCycles or hours/current
    density
    Ref.
    Zn-air batteries
    P-O/FeN4-CNS232 mW cm-21.50 V450 cycles/25 mA cm-2[122]
    NC-Co SA20.9 mW cm-21.411 V125/-[137]
    Fe SAs/ MC(950)-1.52 V-[138]
    SA-Fe-NHPC266.4 mW cm-21.52 V240 h/20 mA cm-2[116]
    Co SA@NCF/CNF-1.41 V90 cycles/6.25 mA cm-3[118]
    Co-POC78.0 mW cm-2-200 cycles/2.0 mA cm-2[139]
    Ni single atoms/ cluster-C83.8 mW cm-21.35V258 cycles/2.0 mA cm-2[140]
    Co/N/O tri-doped graphene152 mW cm-21.44 V18 cycles/1.0 mA cm-2[141]
    Mn/C-NO120 mW cm-2--[124]
    SCoNC194 mW cm-21.49 V20 h/5.0 mA cm-2[142]
    Co-SAs@NC105.3 mW cm-21.46 V50 cycles/1.0 mA cm-2[119]
    Fe-N4 SAs/NPC232 mW cm-2-108 cycles/2.0 mA cm-2[143]
    CuSA@HNCNx212 mW cm-21.51 V1800 cycles/10.0 mA cm-2[144]
    EA-Co-90073 mW cm-21.37 V100 h/20.0 mA cm-2[145]
    Cu-NrC-60210 mW cm-2-100 h/20.0 mA cm-2[120]
    Fe-NC SA180 mW cm-21.424 V160 cycles/20.0 mA cm-2[115]
    Fe/N/C-DT220 mW cm-21.451 V-[117]
    Fe-NCF145 mW cm-21.48 V100 cycles/2.0 mA cm-2[146]
    CoN4/NG28 mW cm-21.51 V100 h/10.0 mA cm-2[147]
    3DOM Fe-N-C-900235 mW cm-21.45 V100 h/5.0 mA cm-2[148]
    Cu-Nx-C160 mW cm-2--[121]
    Fe-N/C-70073 mW cm-21.53 V-[149]
    Fe-SAs/NPS-HC195.0 mW cm-21.45 V500 cycles/5.0 mA cm-2[123]
    A-Co@CMK-3-D-162 mW cm-2-45 h/10.0 mA cm-2[150]
    CoNi-SAs/NC101.4 mW cm-21.45 V95 cycles/5.0 mA cm-2[125]
    (Fe,Co)/CNT260 mW cm-21.63 V-[151]
    Mn-N/C-900140 mW cm-21.45 V6.4 h/20.0 mA cm-2[127]
    Fe-NiNC-50220 mW cm-21.41 V100 h/2.0 mA cm-2[126]
    105.3 mW cm-2
    Zn-CO2 batteries
    Cu-N2/GN0.62 mW cm-2-120 cycles/1.0 mA cm-2[128]
    Fe1NC/S1-10000.6 mW cm-20.727 V72 cycles/0.5 mA cm-2[17]
    SAs-Ni-N-C1.4 mW cm-2-32 h/1.0 mA cm-2[152]
    Table 2

    Comparison of electrocatalytic performance based on carbon-supported SACs for Li-O2/CO2 batteries reported in this review

    MaterialsOverall potential gap/current density1st discharge capacity/current densityCycles or hours/current
    density-fixed capacity
    Ref.
    Li-O2 batteries
    Ru0.3 SAs-NC0.55 V/0.02 mA cm-213424 mA h g-1/0.02 mA cm-260/0.02 mA cm-2-1000 mAh g-1[133]
    Pt-CNHS-13843 mA h g-1/200 mA g-1100/100 mA g-1-600 mAh g-1[134]
    N-HP-Co SACs-14777 mA h g-1/100 mA g-1261/100 mA g-1-1000 mAh g-1[132]
    Co-SAs/N-C0.40 V/200 mA g-117160 mA h g-1/200 mA g-1260/400 mA g-1-1000 mAh g-1[130]
    Li-CO2 batteries
    Adjacent Co/GO-17358 mA h g-1/100 mA g-1100/100 mA g-1-1000 mAh g-1[136]
    Fe-ISA/N,S-G1.17 V/100 mA g-123174 mA h g-1/100 mA g-1200/1000 mA g-1-1000 mAh g-1[135]

    SUMMARY AND PERSPECTIVES

    Compared with nanoparticles and nanoclusters, SACs undoubtedly exhibit superior catalytic performance. In this review, we briefly introduce the structural characteristics of Li- and Zn-air batteries and SACs, focusing on the applications of SACs in Li- and Zn-air batteries and the catalytic mechanism of SACs in Li- and Zn-air batteries. The limitations of current research are also discussed. Finally, a perspective is included highlighting the potential and challenges for the future development of SACs in Li- and Zn-air batteries.

    Most of the recent reports on SACs as cathode catalysts are based on doping single atoms into N-doped carbon-based materials. Among them, in the category of single-metal SACs, most of the research reports employed M-Nx as the active sites with Fe and Co as the central atoms. On the basis of these reports, some reports provide new ideas, such as doping other atoms, such as S, P and O on the N-doped carbon matrix, or obtaining new active sites, or using bimetal doping to better improve ORR, OER and CO2RR performance. Although researchers are trying to alter the electronic structure in these ways to obtain catalysts that facilitate catalytic reactions, it is more likely that success will be achieved in permutations. The construction of a large database of SACs, classified by matrix and single-atom types, and the consolidation of a large amount of electronic structure information would be highly convenient for future SACs applications.

    For SACs, DFT calculations have shown that the ORR follows the association mechanism of O2 - *OOH - *O - *OH - OH, while the OER goes on through the mechanism of OH - *OH - *O - * OOH - O2. SACs can change the ORR, OER and CO2RR RLSs, mostly to reduce the energy barriers of O2, CO2 adsorption and the removal of *OH. In Li-O2 batteries, SACs can effectively tune the structure and morphology of Li2O2 to reduce overpotentials. There is no doubt that SACs in Li- and Zn-air batteries deliver very good performance and researchers have made good progress in the catalytic mechanism of SACs, but there are also some problems due to the lack of effective in situ characterization techniques and the unclear electrochemical conversion mechanism of SAC materials in advanced batteries systems. Most studies on mechanism analysis use calculation methods, including DFT, to obtain the reaction energy barrier and simulate the transformation kinetics of the corresponding device. The parameter setting difference in the calculation process determines the accuracy of the calculation results. If incorrect modeling is encountered, it is difficult to achieve convergence.

    Our vision seems to be limited to carbon-based materials. In fact, single-atom metals can also be doped on different substrates, such as oxides and selenides. The electrical conductivity of the substrate would be sufficiently high for electrocatalytic applications. In fact, some substrates show special functions in battery reactions. For example, when CeO2 is used as the substrate, the oxygen in its lattice can be involved in the ORR and OER, as an oxygen pump to reduce the reaction overpotentials. For example, the layered materials, MoS2 and MoSe2, are considered as excellent carriers and their (002) crystal face edges would contribute to extremely superior electrocatalytic performance. Moreover, the electrical conductivity of such a matrix can be improved by a series of methods, such as phase transformation and the construction of heterostructures. It is proposed that single-atom metals may be developed to form tri-metal doping or even four-atom clusters in the future, and they would eventually be expected to be deposited onto other substrates.

    The lack of mass production technology for SACs has been a major impediment to their industrial applications. Currently, one-step annealing, co-precipitation and impregnation methods are widely used due to their ease of operation and potential for mass production, but the aggregation commonly leads to the formation of nanoparticles and metal clusters during operation. Therefore, it is necessary to explore facile synthesis processes for high-density, high-purity and large-scale SACs. Recently, the large-scale preparation of ultrahigh-density SACs (kilogram scale) for a variety of metals was developed by a combination of impregnation and two-step annealing process and other methods[153]. In particular, the single atoms with high mass loading were also directly anchored onto different carriers, such as carbon skeletons, oxides and nitrides. These advanced synthesis techniques demonstrate that SACs are expected to be used in industrial applications and would also contribute significantly to the development of Zn- and Li-air batteries.

    DECLARATIONS

    Authors’ contributions

    Conception of the study: Zhang J, Wang J, Zhang L

    Analysis and manuscript preparation: Xia Q, Zhai Y

    Performed the analysis with discussions: Li D, Zhao L

    Availability of data and materials

    Not applicable.

    Financial support and sponsorship

    This work was supported for collection, analysis and interpretation of data, and writing of the manuscript by the National Natural Science Foundation of China (22175108), the Natural Scientific Foundation of Shandong Province (ZR2020JQ09), the China Postdoctoral Science Foundation (No.2020M672054), the Guangdong Basic and Applied Basic Research Foundation (No. 2021A1515111124), the Natural Science Foundation of Shandong Province (No. ZR2020QB122), and the Young Scholars Program of Shandong University (No. 2019WLJH21).

    Conflicts of interest

    All authors declared that there are no conflicts of interest.

    Ethical approval and consent to participate

    Not applicable.

    Consent for publication

    Not applicable.

    Copyright

    © The Author(s) 2022.

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    Cite This Article

    Xia Q, Zhai Y, Zhao L, Wang J, Li D, Zhang L, Zhang J. Carbon-supported single-atom catalysts for advanced rechargeable metal-air batteries. Energy Mater 2022;2:200015. http://dx.doi.org/10.20517/energymater.2022.13

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