Recent Progress of Non-Noble Metal Catalysts for Oxygen Electrode in Zn-Air Batteries: A Mini Review

Abstract

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play crucial roles in energy conversion and storage devices. Particularly, the bifunctional ORR/OER catalysts are core components in rechargeable metal–air batteries, which have shown great promise in achieving "carbon emissions peak and carbon neutrality" goals. However, the sluggish ORR and OER kinetics at the oxygen cathode significantly hinder the performance of metal–air batteries. Although noble metal-based catalysts have been widely employed in accelerating the kinetics and improving the bifunctionality, their scarcity and high cost have limited their deployment in the market. In this review, we will discuss the ORR and OER mechanisms, propose the principles for bifunctional electrocatalysts design, and present the recent progress of the state-of-the-art bifunctional catalysts, with the focus on non-noble metal-based materials to replace the noble metal catalysts in Zn–air batteries. The perspectives for the future R&D of bifunctional electrocatalysts will be provided toward high-performance Zn–air batteries at the end of this paper.

1. Introduction

With the rapid development of modern society, the resource exhaustion and environmental problems caused by the continuous increase of energy consumption have attracted more and more attention [1,2]. At present, 70% of the world’s energy consumption is provided by non-renewable fossil fuels including coal, oil, and natural gas [3]. Although the energy released from their combustion can satisfy the large requirements during human activities, their scarce resources push people to explore promising renewable energy applications. On the other hand, from the viewpoint of global climate change, the fossil fuels are undesired because greenhouse gases (e.g., CO₂) are produced by fossil fuel combustion, primarily leading to climate warming [4,5,6,7].

In order to curb global warming, many countries have set goals in terms of "carbon emissions peak and carbon neutrality"; China strives to achieve peak carbon dioxide emissions by 2030 and carbon neutrality by 2060 [8,9]. As we all know, electricity is an ideal energy form with high efficiency and environmental friendliness. Therefore, it is of far-reaching significance to vigorously promote and develop technologies and industries related to electrical energy, such as electrochemical energy conversion and storage applications [10,11].

Nowadays, various types of electrochemical energy storage and conversion devices have been widely used in our daily life, including wearable batteries, electric vehicles, portable electronic devices, and more [12,13,14,15]. Meanwhile, next-generation high-energy and economic systems such as rechargeable batteries including metal–air batteries, metal–sulfur batteries, and solid-state batteries are on their way to commercialization [16]. In this paper, we are focused on metal–air batteries.

Metal–air batteries (MABs) have attracted great attention because of their high theoretical energy density, cost-effectiveness, light weight, and air atmosphere operation [17]. MABs are generally composed of four major components [18]: air electrode, metal electrode, electrolyte, and separator, as illustrated in Figure 1a. At the air electrode side, the reactant O₂ can be obtained directly from the surrounding air instead of being encapsulated in the battery, resulting in a substantial reduction in battery weight and the cost [19]. The air electrode consists of electrocatalyst layer and gas-diffusion layer (GDL): the former is used to catalyze the electrochemical reaction and reduce the overpotential of oxygen reduction/evolution reactions (ORR/OER); the latter can promote the oxygen diffusion between the ambient air and the catalyst surface [19].

Depending on different anode metals, MABs can be divided into Li–air batteries [20], Na–air batteries [21], Al–air batteries [22], Mg–air batteries [23], Zn–air batteries [24] and so on. Since highly active alkali metals (e.g., Li, Na, and K) are extremely sensitive to water and air, non-aqueous electrolytes are required from the perspective of safety. In the Li–, Na– and K–air batteries, oxygen diffuses into the electrolyte through the GDL, and oxygen receives electrons and converts into oxygen anions (O₂⁻), as shown in Figure 1b. For relatively inert metals such as Zn, Al, Mg, etc. as the anode, the choice of alkaline aqueous electrolytes would be ideal, because alkaline aqueous electrolytes have higher oxygen diffusion coefficients and lower viscosity, which can promote the diffusion of oxygen and electrons transportation. At this time, oxygen enters the batteries and accepts electrons to form hydroxyl groups, as shown in Figure 1c.

In the field of MABs, Li–air batteries and Zn–air batteries have received the most extensive attention because of their respective outstanding advantages. Although the Li–air batteries has an ultra-high theoretical energy density, the safety concerns using metallic lithium and the scarcity of lithium significantly limit their wide utilization. In this regard, Zn–air batteries are one of the most promising metal–air batteries due to their many competitive advantages, such as relatively high theoretical energy density (1086 Wh kg⁻¹), high rechargeability, abundant resources, low cost (below $100 per kWh), low equilibrium potential, and flat discharge voltage [25]. At present, Zn–air batteries can be used in some high-end fields, such as hearing aids, railway signaling devices, and military facilities [26,27].

Despite the received considerable progress, Zn–air batteries still suffer from some serious challenges. For example, the lack of efficient and reliable air electrodes is the biggest impediment for practical applications [28]. The electrocatalyst layer on the air electrode is used to accelerate the kinetic process of oxygen reduction reaction (ORR, in the discharge process) and oxygen evolution reaction (OER, in the charging process), and its catalytic performance directly affects the comprehensive performance of rechargeable Zn–air batteries [29]. Therefore, designing and using efficient bifunctional electrocatalysts with both ORR and OER catalytic behaviors are the keys to high-performance rechargeable Zn–air batteries [30]. At present, it is recognized that the best ORR and OER catalysts are still noble metal catalysts based on Pt, Ru and Ir or their oxides [31]. However, limited by the scarcity and cost of precious metals, people are gradually turning their attention to designing efficient and inexpensive non-precious metal catalysts as alternatives with catalytic activity rival precious metals, such as single-atom catalysts (Fe/Co-N-C) [32,33,34], transition metal-based catalysts and carbon-based catalysts [35,36,37,38], etc.

In this review, we will discuss the operating principles and features of rechargeable Zn–air batteries, the ORR and OER mechanisms of electrocatalysts on air electrodes, and propose the principles for bifunctional electrocatalysts design and present the recent progress of the state-of-the-art bifunctional catalysts, with the focus on non-noble metal-based materials to replace the noble metal catalysts in Zn–air batteries. The perspectives for the future R&D of bifunctional electrocatalysts will be provided toward high-performance Zn–air batteries at the end of this paper.

2. Fundamentals of Zn–Air Batteries

The typical configuration of Zn–air batteries consists of four parts [39]: metal Zn anode, air cathode, electrolyte, and separator.

Zn anode: In Zn–air batteries, metal Zn serves as the active material of the negative electrode, which can determine the capacity of the battery, and is generally used in the form of pure Zn powder, Zn foil or Zn plate. The metallic Zn loses two electrons during discharge process, dissolves into the electrolyte as Zn²⁺, and it plates back onto the anode during charging. With the progress of the electrochemical reaction, side reactions such as a hydrogen evolution reaction (HER) will occur at the anode, and the thermodynamic instability of Zn in the aqueous electrolyte will trigger the self-discharge behavior [40]. Currently, there are two strategies to address this issue: surface engineering [41,42] and electrolyte additives [43].

Aircathode: The air electrode is composed of an electrocatalyst layer and a gas-diffusion layer (GDL). The electrocatalyst layer provides a site for ORR (discharge process) and OER (charge process) to occur, and its bifunctional activity affects the power performance of rechargeable Zn–air batteries to a large extent [44]. The GDL is used to promote the oxygen diffusion between the ambient air and the catalyst surface.

Electrolyte: Choosing the right electrolyte system is critical to achieving breakthroughs in battery performance, as the electrolyte can effectively determine the rechargeability of a battery, as well as the battery voltage it achieves and the energy it delivers. Furthermore, the electrolyte has a great influence on the transport of active species (e.g., Zn ions, oxygen-containing species) and the power density of the battery. In Zn–air batteries, the electrolyte can be divided into non-aqueous electrolytes and aqueous electrolytes according to the sensitivity of the anode metal to air and water [19], as shown in Figure 1b,c. For relatively inert metals such as Zn, Al, Mg, etc. as anodes, alkaline aqueous electrolytes (such as NaOH, KOH) are usually ideal because of their good ionic conductivity, high oxygen diffusion coefficient and low viscosity. In Zn–air batteries, a high-concentration KOH solutions (~7 M) with excellent electrical conductivity is usually used as the electrolyte [12,24,45]. Although the use of alkaline electrolytes can facilitate an ideal performance of metal–air batteries, there are still some problems to be solved, including the formation of carbonates that shield the active sites of air cathode catalysts and the problems of carbon corrosion of catalysts [12]. In view of the above problems, the development and use of neutral electrolytes have gradually attracted people’s attention in recent years. In Zn–air batteries, the use of neutral chloride electrolytes can reduce carbon corrosion and carbonate formation at the air electrodes, enabling the batteries to exhibit good performance. Although neutral chloride electrolytes can largely be used as an alternative to alkaline electrolytes, the relatively poor electrical conductivity requires further breakthroughs [12].

Separator: In Zn–air batteries, a separator is used to separate the metal Zn anode and air cathode, which have good electrochemical stability, corrosion resistance, high ionic conductivity and sufficient mechanical strength. Sufficient mechanical strength is beneficial to reduce short circuits due to penetration of zinc dendrites [46].

Figure 2 shows the mechanism diagram of the electrode reaction in a rechargeable Zn–air battery [47]. During discharge, Zn is oxidized to Zn ions, and the generated electrons flow to the air electrode through an external circuit. The oxygen diffused into the air electrode undergoes ORR at the triple interface of catalyst–electrolyte–oxygen to generate OH⁻, and then migrates to the anode, where it combines with Zn ions to form zincate ions Zn(OH)₄²⁻. After approaching saturation, the zincate ion Zn(OH)₄²⁻ decomposes to ZnO. As for the charging process, the ZnO on the Zn electrode is reduced to metallic Zn, and the OH⁻ at the triple interface of the air electrode undergoes OER to release oxygen [48]. In rechargeable Zn–air batteries, the electrode reactions during discharge and charge are reversible; the reactions can be summarized as follows:

Discharge process:

Anode:

$$\mathrm{Zn}\to {\mathrm{Zn}}^{2+}{+2\mathrm{e}}^{-}$$

(1)

$${\mathrm{Zn}}^{2+}+4{\mathrm{OH}}^{-}\to {\mathrm{Zn}\left(\mathrm{OH}\right)}_4^{2-}$$

(2)

$$\mathrm{Zn}{(\mathrm{OH})}_4^{2-}\to {\mathrm{ZnO}+\mathrm{H}}_2{\mathrm{O}+2\mathrm{OH}}^{-}$$

(3)

$$\mathrm{Zn}+2{\mathrm{OH}}^{-}\to {\mathrm{ZnO}+\mathrm{H}}_2{\mathrm{O}+2\mathrm{e}}^{-} (E^{\mathsf{\theta }}=-1.25 \mathrm{V} \mathrm{vs}. \mathrm{SHE} ).$$

(4)

Cathode:

$${\mathrm{O}}_2{+2\mathrm{H}}_2{\mathrm{O}+4\mathrm{e}}^{-}\to 4{\mathrm{OH}}^{-} (E^{\mathsf{\theta }}=0.4 \mathrm{V} \mathrm{vs}. \mathrm{SHE}).$$

(5)

Charge process:

Cathode:

$$4{\mathrm{OH}}^{-}\to {\mathrm{O}}_2{+2\mathrm{H}}_2{\mathrm{O}+4\mathrm{e}}^{-} (E^{\mathsf{\theta }}=- 0.4 \mathrm{V} \mathrm{vs}. \mathrm{SHE}).$$

(6)

Anode:

$${\mathrm{ZnO}+\mathrm{H}}_2{\mathrm{O}+2\mathrm{e}}^{-}\to {\mathrm{Zn}+2\mathrm{OH}}^{-} (E^{\mathsf{\theta }}=1.25 \mathrm{V} \mathrm{vs}. \mathrm{SHE}).$$

(7)

Overall reaction:

$${2 \mathrm{Zn}+\mathrm{O}}_2\iff 2 \mathrm{ZnO} (E^{\mathsf{\theta }}=- 1.65 \mathrm{V} \mathrm{vs}. \mathrm{SHE}).$$

(8)

The performance of rechargeable Zn–air batteries can be evaluated by the following aspects: open circuit voltage, power density, energy density, stability, and round-trip efficiency [49], etc. Among them, power density and round-trip efficiency are closely related to the activity of electrocatalysts in air electrodes. The ORR and OER performances are limited by their inherently slow kinetic processes and high overpotentials, resulting in a round-trip energy efficiency below 55–65% [50], and poor long-term cycling stability limiting the practical application of Zn–air batteries. Rational regulation of the electronic structure and microscopic morphology of electrocatalysts can optimize the electrical conductivity, specific surface area, energy barrier and mass transfer behavior, and play a positive role in improving the performance of Zn–air batteries. Therefore, designing and developing high-efficiency bifunctional electrocatalysts is a reasonable strategy to promote the further development of Zn–air batteries.

3. Advanced Non-Noble Metal Electrocatalysts for Oxygen Electrode

3.1. Oxygen Reduction Reaction Mechanism in Alkaline System

Oxygen reduction reaction (ORR) is an electrochemical reaction involving multi-step proton/electron transfer, and its complex and inherently slow kinetics is a major obstacle limiting the large-scale development of fuel cells and metal–air batteries. Based on the report of Anastasijevi´c and colleagues [51], the ORR process in alkaline systems generally undergoes the following major steps [52,53]:

Direct 4e⁻ pathway: Four protons and four electrons are transferred to the oxygen molecules adsorbed on the catalyst surface to generate OH⁻:

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4e^{-}\to 4{\mathrm{OH}}^{-} (E^{\mathsf{\theta }}=0.401 \mathrm{V} \mathrm{vs}. \mathrm{SHE}).$$

(9)

2e⁻+2e⁻ pathway:

$${\mathrm{O}}_2^{}+{\mathrm{H}}_2\mathrm{O}+2e^{-}\to {\mathrm{HO}}_2^{-}+{\mathrm{OH}}^{-} (E^{\mathsf{\theta }}=0.065 \mathrm{V} \mathrm{vs}. \mathrm{SHE}).$$

(10)

Proceed to another 2e⁻ electron process:

$${\mathrm{HO}}_2^{-}+{\mathrm{H}}_2\mathrm{O}+2e^{-}\to 3{\mathrm{OH}}^{-}.$$

(11)

or chemical disproportionation of HO₂⁻

$${2\mathrm{HO}}_2^{-}\to 2{\mathrm{OH}}^{-}+{\mathrm{O}}_2.$$

(12)

Overall reaction:

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4e^{-}\to 4{\mathrm{OH}}^{-}.$$

(13)

According to literature reports, hydrogen peroxide (H₂O₂) or HO₂⁻ intermediates have strong oxidizing properties, which will corrode the electrocatalyst. In contrast, the direct 4e⁻ pathway is an ideal route due to the absence of hydrogen peroxide (H₂O₂) or HO₂⁻ intermediate generation, resulting in higher current efficiency and operating voltage, and improved stability of the corresponding electrodes and membranes [54,55], as shown in Figure 3a.

Based on density functional theory (DFT) calculations for different catalysts, Nørskov [31] established a volcano plot between ORR activity and metal-oxygen bond energy (Figure 3b). The binding energy between metal and oxygen was neither too large nor too small to indicate the best ORR activity. In addition, Adzic [56] established a volcano diagram of the half-wave potential versus the position of the metal d-state relative to the Fermi level, known as the d-band center theory (Figure 3c). Greeley et al. [57] showed that the closer the d-band center is to the Fermi level, the stronger the interaction with the adsorbate, which hinders the subsequent reaction steps and slows down the kinetics of oxygen reduction. Therefore, the catalyst near the top of the volcano exhibits good ORR activity due to the suitable relative position of the d-band center and the Fermi level.

Therefore, establishing the structure–activity relationship between electronic structure and catalytic activity can provide an effective way to design catalysts with efficient ORR.

3.2. Oxygen Evolution Reaction Mechanism in Alkaline System

Similar to ORR, OER is also a complex four-electron-proton coupling reaction, and its slow kinetic process leads to a high overpotential, thus requiring high energy to overcome the kinetic energy barrier of OER.

The reaction mechanism of OER is more complex and varies with different catalysts, but the mechanisms proposed by most researchers include the same intermediates, such as MOH and MO, while the main difference may be the reaction to form oxygen [58,59].

Proposed mechanism under alkaline conditions:

$$\mathrm{M}+{\mathrm{OH}}^{-}\to \mathrm{MOH}+e^{-}$$

(14)

$$\mathrm{MOH}+{\mathrm{OH}}^{-}\to {\mathrm{MO}+\mathrm{H}}_2\mathrm{O}+e^{-}$$

(15)

$$2\mathrm{MO}\to 2{\mathrm{M}+\mathrm{O}}_2$$

(16)

$$\mathrm{MO}+{\mathrm{OH}}^{-}\to \mathrm{MOOH}+e^{-}$$

(17)

$$\mathrm{MOOH}+{\mathrm{OH}}^{-}\to {\mathrm{M}+\mathrm{O}}_2{+\mathrm{H}}_2\mathrm{O}+e^{-}.$$

(18)

Overall reaction:

$$4{\mathrm{OH}}^{-}\to {\mathrm{O}}_2{+\mathrm{H}}_2\mathrm{O}+4e^{-}.$$

(19)

As shown in Figure 4, the OER process in an alkaline system undergoes the formation of various oxygen-containing intermediates (along the black path), such as MOH, MO, MOOH, etc., and finally obtains the product O₂. However, when the intermediate MO undergoes the next reaction, there are two different paths—one is to directly generate O₂ (green path, Equation (16)) and the other is to form a MOOH intermediate, which is subsequently decomposed to oxygen. Despite these differences, the general consensus is that the bonding interactions (M-O) within the intermediate are crucial for the overall electrocatalytic ability [59].

3.3. Design Principles and Recently Research Progress of Bifunctional Electrocatalysts

Efficient rechargeable Zn–air batteries are highly dependent on the catalytic activity and stability of electrocatalysts on the air electrode. ORR that dominates the discharge process and OER that dominates the charging process are reversible reactions to each other; thus, it is usually necessary to use different catalysts for catalysis [60]. Noble metals or their oxides (Pt, IrO₂, RuO₂, etc.) have played an effective catalytic role in accelerating the slow kinetic process of ORR or OER, respectively, but due to their high cost and lack of bifunctional properties to catalyze both ORR and OER simultaneously. It can be seen that noble metal catalysts are not suitable as bifunctional catalysts for application in rechargeable Zn–air batteries [59]. Therefore, it is of great significance to design and develop inexpensive and efficient non-noble metal bifunctional catalysts to accelerate ORR/OER kinetics process and reduce overpotentials, thereby improving the efficiency, power density, and round-trip energy efficiency of Zn–air batteries [61,62]. In bifunctional catalysts, the potential difference (ΔE) between the OER (10 mA/cm²) overpotential and the ORR half-wave potential is usually used as a parameter to measure the bifunctional properties of the catalyst. A smaller potential difference in ΔE means lower efficiency loss and better bifunctionality [63]. Based on the above considerations, accelerating the ORR/OER kinetic process and mitigating the corrosion problem of the catalyst are fundamental ways to improve the performance of rechargeable Zn–air batteries, which can be regulated by the following aspects:

Larger specific surface area:

For electrocatalysts, a larger specific surface area can provide more active sites for the electrocatalytic reaction, which is crucial for the effective expression of electrocatalytic activity. Rational design and construction of electrocatalysts with special morphologies (e.g., porous structures, 2D nanosheets, 3D hollow structures, etc.) [64,65,66] can obtain a large specific surface area, thereby providing abundant active sites for the catalytic reaction, which can significantly promote the reaction. Fu and co-workers [67] annealed the FeNi LDH microsphere precursor at 450 °C for 4 h in NH₃ atmosphere to prepare mesoporous nickel-iron nitride (Ni₃FeN); the morphology is shown in Figure 5a. The catalyst exhibits good bifunctional activity with relatively low OER overpotential (0.355 V @ 10 mA·cm⁻², vs. RHE) and Tafel slope (70 mV·dec⁻¹), and its ORR half-wave potential E_1/2 is 0.78 V vs. RHE. Compared with the noble metal RuO₂ air cathode, the Ni₃FeN catalyst has a smaller charge–discharge voltage gap and better cycling stability (over 300 cycles). Lu et al. [68] proposed a new strategy to prepare FeNi alloy nanoparticles-embedded N-doped carbon nanotube-entangled porous carbon fiber (FeNi/N-CPCF) composite bifunctional electrocatalysts by electrospinning and subsequent annealing treatment (Figure 5b). The FeNi/N-CPCF-950 catalyst annealed at 950 °C exhibits good bifunctional activity due to its unique hierarchical porous structure with bamboo-like CNTs grafted and the interaction between FeNi alloy and N-doped species. Electrochemical tests show that the catalyst enables the zinc–air battery to have a high energy efficiency of 61.5%, a small charge-discharge voltage difference (0.764 V vs. RHE) and excellent cycle performance (640 h, 960 cycles) at room temperature and pressure. For other related literature, please refer to

Table 1. Summary of electrocatalysts application in Zn–air batteries.

Catalyst Details	Electrochemical Activity	References
CoFe/S-N-C	E(ORR) = 0.855 V (vs. RHE), E(OER) = 1.588 V (10 mA cm−2), power density of 120 mW cm−2 with a specific capacity of 814 mAh g−1	[69]
FeCo/NUCSs (N doped Carbon nanosheets)	E(ORR) = 0.89 V (vs. RHE), E(OER-overpotential) = 300 mV, cycling stability (102 h), specific capacity of 791.86 mAh g−1	[70]
CoNi/N-CNN (Carbon nanoparticle/nanotube)	E(ORR) = 0.819 V (vs. RHE), E(OER) = 1.718 V (10 mA cm−2), discharge power density of 209 mW cm−2	[71]
SSM/Co4N/CoNC	E(ORR) = 0.833 V (vs. RHE), E(OER-overpotential) = 275 mV (10 mA cm−2), cycle 300 times (5 mA cm−2) with slight voltage fluctuations, manifesting good reversibility	[72]
FeNCFs	E(ORR) = 0.84 V (vs. RHE), E(OER) = 1.63 V (10 mA cm−2), power density of 173 mW cm−2 with a specific capacity of 717 mAh g−1	[73]

.

In summary, designing a catalyst with a hierarchical porous structure can not only reduce the size of the electrocatalyst but also increase the specific surface area so as to achieve the purpose of exposing more active sites.

Electronic Conductivity

Since the catalytic reaction occurs at the solid–liquid–gas triple-boundary on the catalyst surface, the higher conductivity can facilitate the transfer of electrons to active sites and external circuits in electrochemical reactions, so accelerated charge transfer can reduce polarization (including electrochemical polarization and ohmic polarization of the electrode) and accelerate the kinetic process of the reaction.

The charge transport capability is determined by the intrinsic conductivity and surface/interface properties of the catalyst itself, which can be tuned by optimizing the energy band structure and increasing the carrier density, etc. For example, transition metal oxides usually adjust the energy band structure by adjusting the element ratio or doping other elements to improve the intrinsic conductivity [74,75]. Controllable ionic doping of catalyst surfaces is another feasible approach to improving conductivity. For example, doping non-metallic elements N, S, P, etc. on the catalyst surface can improve the carrier density and enhance the conductivity of the catalyst [76,77,78]. Zheng et al. [79] developed a novel bifunctional oxygen catalyst which was composed of hollow CoFe alloy with N-doped Ketjen black. The optimal CoFe@NC/KB-800 catalyst exhibits excellent ORR and OER bifunctional activities (ΔE = 0.77 V). It exhibits a high peak power density of 160 mW cm⁻² at 285 mA cm⁻² and excellent cycling stability (low voltage gap of 0.65 V after 600 cycles). The introduction of graphitic N into the catalyst can improve the electrical conductivity, which is beneficial for accelerating the reaction kinetics of the catalyst, thereby promoting the improvement of the electrocatalytic activity. Using the NH₃ activation strategy to prepare N-Co₃O₄, the band gap of N-Co₃O₄ after doping was significantly reduced (from 0.78 eV to 0.35 eV) [80]. In addition, UV-Vis absorption spectroscopy tests showed that the optical bandgap energy was significantly reduced after N doping, resulting in an increase in the conductivity and carrier concentration of N-Co₃O₄, as shown in Figure 6c–f. N-doping can effectively optimize the electronic structure of the Co₃O₄ catalyst, enabling the N-Co₃O₄ electrocatalyst with good ORR activity, and the catalyst exhibits an excellent discharge capacity (98.1 mAh·cm⁻³) in a flexible rechargeable Zn–air battery.

Optimize the energy barrier of catalytic reactions:

Appropriate adsorption energy can promote the adsorption of reactants on active sites and the desorption of active intermediates, thereby reducing the energy barriers of ORR and OER reactions, exhibiting higher intrinsic activity, and promoting the charge–discharge process of Zn–air batteries. By introducing heteroatom defects into transition metal-based electrocatalysts, the energy barrier of the rate-determining step can be further reduced, thereby accelerating the electrocatalytic kinetic process and improving the catalytic activity.

Yu et al. [80] controllably prepared N-doped Co₃O₄ mesoporous nanowire electrocatalysts, and the ORR activity was significantly improved after doping, which can be used as an air cathode for flexible solid-state Zn–air batteries. To further understand the mechanism of N doping on the enhanced catalytic activity of Co₃O₄, Yu calculated the changes in the electronic structure and oxygen adsorption energy of Co₃O₄ before and after N doping using density functional theory. First, build a model with the crystal structure as shown in Figure 7a, b. Comparing the total density of states and projected density of states of Co₃O₄ before and after N doping (Figure 7c), it is found that both the valence band and conduction are negatively shifted after N doping. The UV-Vis absorption test showed that the optical bandgap narrowed after N doping, indicating that the electrical conductivity increased with the increase of N dopant. Figure 7d shows that, after N doping, the energy barrier for O₂ absorption on Co₃O₄ surface is reduced by 0.30 eV, which will lead to a stronger and easier O₂ adsorption of the catalyst. It can be proved that N doping can effectively reduce the energy barrier of O₂ adsorption and can accelerate the kinetic process of the ORR reaction. Shao et al. [81] synthesized a series of ORR/OER bifunctional electrocatalysts based on Pt and perovskite Sr(Co_0.8Fe_0.2)_0.95P_0.05O_3-δ with excellent electrochemical activity and durability. The bifunctional exponential potential difference of its sample Pt-SCFP/C-12 is only 0.73 V. The test results show that the strong electron interaction between Pt and SCFP promotes the rapid electron transfer in the catalytic reaction; the spillover effect between Pt and SCFP effectively reduces the reaction energy barrier of the rate-limiting step, thus exhibiting excellent bifunctional activity.

4. Application of Non-Noble Metal Electrocatalysts in Zn–Air Batteries

In order to alleviate the energy crisis, scientists are vigorously developing efficient and inexpensive non-precious metal catalysts as alternatives with catalytic activity rival precious metals. In recent years, carbon-based materials and transition metal (TM)-based materials have received extensive attention due to their high activity and stability [82,83]. Here, we summarize the applications of ORR/OER bifunctional electrocatalysts as air electrodes in rechargeable ZABs, including the following three categories:

4.1. Metal-Free Based Carbon Catalysts

Metal-free carbon materials, such as carbon nanotubes (CNTs), carbon fibers (CFs), carbon nanosheets (CNSs) and graphene, have large specific surface areas and high electrical conductivity, showing great potential in catalytic processes [84,85,86]. However, unmodified pristine carbon materials (e.g., carbon black, carbon nanotubes, and graphene) undergo a two-electron pathway during ORR, and thus have poor intrinsic activity [87]. In order to obtain high catalytic activity, heteroatoms (such as N, P, S, and B) or defects are introduced into metal-free carbon catalysts to optimize the electronic structure of carbon materials, change their charge density distribution, and improve catalytic activity [88,89]. For example, Ma et al. [85] exploited the specific synergistic effect between B, N, and F dopants, which is beneficial both to enhance the specific surface area of BNF-LCF catalysts and to create abundant defect sites, thereby exhibiting excellent ORR/OER bifunctionality (ΔE = 0.728 V). After assembling the BNF-LCFs catalyst into Zn–air batteries (ZABs), the tests showed that the catalyst not only exhibited a high open circuit potential of 1.536 V and a large specific capacity of 791.5 mAh g⁻¹, but also exhibited satisfactory cycling stability.

After heteroatom-doped carbon materials, the electronegativity of heteroatoms is greater than that of carbon atoms, so that adjacent carbon atoms become active sites for adsorbing oxygen, thereby enhancing catalytic activity [90]. Liu et al. [91] used a mixture of melamine and L-cysteine as precursors with a mass ratio of 4:1, followed by two-step carbonization under argon atmosphere to form N-doped graphene nanoribbons (NGRW), as shown in Figure 8. Experiments confirmed the existence of bifunctional active sites in NGRW, in which the graphical-N site is active for ORR, while the pyridinic-N site is catalytically active for OER. When evaluating the performance of its Zn–air battery, it was found that the N-GRW air cathode not only exhibited a high power density (65 mW·cm⁻²), but also exhibited excellent cycling stability (over 30 h) at a current density of 20 mA·cm⁻².

Zhao et al. [92] generated B-N co-doped highly porous carbon (BNPC) bifunctional catalysts by pyrolyzing a Zn-MOF (MC-BIF-1S) precursor under a H₂-Ar atmosphere (Figure 9a). The BNPC catalyst has good ORR activity (E_onset = 0.894 V vs. RHE), and its stability is better than that of the Pt/C catalyst after 10,000 cycles. OER tests were performed on the BNPC catalysts in 6 mol/L KOH electrolyte, which exhibited a potential of 1.55 V (RHE) at a current density of 10 mA·cm⁻². After the catalyst was loaded on the air electrode and the battery was assembled, the cycling performance was tested. At a charge–discharge current density of 2 mA·cm⁻², the battery was continuously tested for 100 h without significant performance loss, and the results are shown in Figure 9b.

Wang et al. [93] prepared a pyridine-nitrogen-dominated doped graphene with a large number of vacancy defects (Figure 9c). After optimization, the best-performing catalyst has an ultra-high pore volume (3.43 cm³·g⁻¹) and exhibits unprecedented ORR activity (E_1/2 = 0.85 V vs. RHE) under alkaline conditions. The primary Zn–air battery assembled with this catalyst has a maximum power density of 115.2 mW·cm⁻² and a high energy density of 872.3 Wh·kg⁻¹. In rechargeable Zn–air batteries, lower discharge–charge overpotentials and higher stability (>78 h) are exhibited, as shown in Figure 9d,e.

In summary, rational doping or fabrication of defects in metal-free carbon materials is a good strategy to improve their ORR and OER catalytic activities.

4.2. Transition Metal-Based Catalysts, including Single Atom Catalysts, Transition Metal Oxide/Nitride/Sulfides Catalysts

In recent years, in M-N-C (M = Fe, Co, Ni, etc.) catalysts with M-N_x species embedded in the carbon matrix, the M-N₄ site is quite active during ORR of O₂ adsorption and O=O bond cleavage, thus exhibiting excellent ORR activity [94,95,96]. Recently, Mu et al. [97] reported a novel Co-Nx/C nanorod array catalyst formed by pyrolysis using 3D ZIF framework nanocrystals as precursors (Figure 10a). Electrochemical tests show that the catalyst has superior electrocatalytic activity and stability toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) compared to commercial Pt/C and IrO₂, respectively. Pyrolysis of 3D ZIF nanocrystals into metal–nitrogen co-doped carbon nanorod arrays can yield abundant Co-N active centers, abundant porosity, and high surface area, making them highly bifunctional performance (E_{j = 10}(OER) − E_1/2 (ORR) ≈ 0.65 V). The catalyst was loaded on an air electrode to assemble a battery and tested, showing that it can present high cycling stability even under a large current density of 50 mA·cm⁻² for 80 h of continuous cycling, as shown in Figure 10b. In summary, the highly active and stable M-Nx-C materials synthesized by this method can be truly applied in Zn–air batteries (Figure 10c).

Zhang et al. [98] synthesized an (Ni SAs-NC) electrocatalyst with Ni single atoms uniformly dispersed on N-doped carbon nanosheets (Figure 11a). Electrochemical tests showed that the catalyst had good ORR activity.

Huang et al. [99] prepared cobalt single atoms (denoted as SCoNC) on nitrogen-doped graphene-like carbon supports by a salt-template method with a high Co atomic site fraction (15.3%) (Figure 11b). Owing to the highly dispersed Co-N species in the carbon matrix, SCoNCs exhibit high bifunctional catalytic activity with a positive E_1/2 of 0.91 V (RHE) for ORR (Figure 11c) and a low E (10 mA·cm⁻²) of 1.54 V (RHE) for OER. Further tests showed that the SCoNC single-atom catalyst exhibited a high peak power density of 194 mW·cm⁻² and a high energy density of 945 Wh·kg_Zn⁻¹ (Figure 10d). Zhi et al. [100] prepared Fe single-atom catalysts on two-dimensional (2D) highly graphitic porous nitrogen-doped carbon layers (Figure 11e–f), which exhibited excellent ORR/OER catalytic activity in an alkaline system (E_1/2 = 0.86 V vs. RHE for ORR and 390 mV @ 10 mA·cm⁻² for OER), Figure 11g–h. Zhu et al. [101] used the soft templating method to synthesize a double single-atom catalyst (Fe-Co DSAC), in which Fe and Co atoms were stabilized on 2D carbon nanosheets by coordinating with nitrogen (N) and sulfur (S) heteroatoms, respectively. The synergistic effect of Fe-Co bimetallic centers enables the catalyst to exhibit excellent ORR electrocatalytic activity with a half-wave potential of 0.86 V vs. RHE and a maximum power density of 152.8 mW·cm⁻² for Zn–air batteries, which is superior to that of monometallic Fe and Co SACs. The design of DSAC catalysts opens a new avenue for the development of high-performance electrocatalysts.

Transition metal oxides have abundant and tunable electronic structures, and are often of great interest due to their high reserves and high theoretical activity [102]. However, the poor electrical conductivity of transition metal oxides results in poor electron mobility inside and outside the Zn–air battery [103], causing large electrochemical and ohmic polarizations. In order to alleviate the problem of poor electrical conductivity, transition metal oxides can usually optimize their own electronic structures or use them to form conductive networks or by combining with carbon-based materials, thereby exhibiting good electrocatalytic activity [104,105]. Jiang et al. [62] developed an efficient bifunctional catalyst with a core-shell structure in which ultrafine NiFeO nanoparticles (NPs) are embedded in a porous amorphous MnO_x layer. Among them, NiFeO core enhances oxygen evolution reaction (OER) and amorphous MnO_x-shell enhances oxygen reduction reaction (ORR), the bifunctional potential difference ΔE (E_OER (5 mA·cm⁻²) − E_ORR (3 mA·cm⁻²)) is 0.792 V. The electronic effect between the NiFeO core and the MnO_x shell reduces the affinity and adsorption energy of oxygen on the MnO_x shell and accelerates the ORR kinetic process. Chen et al. [106] prepared an electrocatalyst in which a Co/Co₃O₄ mixture was stitched in porous graphite shell Co/Co₃O₄@PGS (Figure 12a); this strategy aimed at establishing the transfer channel between electrons and reactants on the catalyst surface. Electrochemical tests showed that the ORR activity (E_1/2 = 0.89 V vs. RHE) of the Co/Co₃O₄@PGS electrocatalyst was superior to that of commercial Pt/C (Figure 12b), with an OER (10 mA cm⁻²) overpotential of 1.58 V vs. RHE (Figure 12c). The Co/Co₃O₄@PGS electrocatalyst supported on an air electrode exhibits excellent cycling capability over 800 h at 10 mA cm⁻² (Figure 12d).

Tang et al. [65] proposed a salt-templated strategy to prepare Co/Co_xM_y (M = P, N) catalysts, which exhibited good OER activity (Figure 13a). As a typical Mott–Schottky electrocatalyst, Co/Co_xM_y exhibits good OER activity in alkaline systems with a low overpotential of 334 mV at a current density of 10 mA cm⁻². In Zn–air batteries, the Co/Co_xM_y + Pt/C catalyst exhibits higher current efficiency and long cycle life compared with the conventional RuO₂ + Pt/C catalyst (Figure 13b). The research in this paper can provide new avenues for the application of Schottky catalysts. Lee et al. [107] in situ grown NiIn₂S₄ on carbon fibers to obtain NiIn₂S₄/CNFs catalysts (Figure 13c,d). Electrochemical tests show that the NiIn₂S₄/CNFs catalyst not only has good ORR activity (half-wave potential 0.81 V vs. RHE), but also has excellent OER activity (overpotential 0.39 V) (Figure 13e,f). For other related literature, please refer to

Table 2. Summary of electrocatalysts application in Zn–air batteries.

Catalyst Details	Electrochemical Activity	References
Mn-CoNx/N-PC	E(ORR) = 0.85 V (vs. RHE), E(OER-overpotential) = 330 mV (10 mA cm−2), power density of 145 mW cm−2	[108]
Co3O4/Mn3O4/rGO	E(ORR) = 0.86 V (vs. RHE), E(OER) = 1.59 V, power density of 194.6 mW cm−2), open circuit voltage of 1.54 V	[109]
Vo-CoFe/CoFe2O4@NC	E(ORR) = 0.858 V (vs. RHE), E(OER-overpotential) = 360 mV (10 mA cm−2), power density of 139.5 mW cm−2, open circuit voltage of 1.53 V	[110]
V2O3/MnS	E(ORR) − E(OER, 10 mA cm−2 ) = 0.758 V, power density of 118 mW cm−2, specific capacity of 808 mAh gZn−1, energy density (970 Wh kgZn−1)	[111]
S-FeCo3P/NPSGs	E(ORR) = 0.83 V (vs. RHE), E(OER-overpotential) = 290 mV (10 mA cm−2), excellent cycling stability (>600 h)	[112]

.

In this part, some representative non-precious metal catalysts are selected for a brief introduction, from their preparation method, principle, morphology, bifunctional activity and their application in Zn–air batteries. According to the above reviewed literature, M-N-C single-atom catalysts represented by Fe and Co-N-C have excellent electrocatalytic activity, while transition metal nitrogen, sulfur, oxides, etc. have good stability. The rational design and development of electrocatalysts with bifunctional activity and stability according to the advantages and disadvantages of the catalysts themselves can promote the commercialization of Zn–air batteries.

5. Conclusions and Outlook

In conclusion, this paper briefly introduces the research background and significance of Metal–air batteries, and introduces the components of Zn–air batteries and the working principles of each component in detail. Limited by the slow kinetics of ORR (discharge process) and OER (charge process), the battery efficiency, power density and round-trip energy efficiency of rechargeable Zn–air batteries are low, which seriously restricts the development of Zn–air batteries. The reaction mechanisms of ORR and OER are briefly described and used as a theoretical guide to design and develop efficient electrocatalysts to accelerate their kinetic processes.

At present, the large-scale development of Zn–air batteries is still constrained by the low catalytic activity of bifunctional catalysts and the insufficient stability of the catalysts. It is well known that in the process of charging and discharging, ORR and OER are inverse reactions to each other, and the ORR electrocatalyst with a good performance will cause electrochemical corrosion due to excessive voltage during the OER process, and it is easy to cause the catalyst to die due to the collapse of the catalyst structure. For example, metal-free catalysts represented by heteroatom-doped carbon materials have excellent ORR activity, but they will be oxidized at high oxygen-rich and high potentials, resulting in catalyst failure. The carbon-based catalysts with the addition of non-precious metals have both ORR and OER activities, showing good bifunctional activity. In order to achieve large-scale commercialization of Zn–air batteries, future work should focus on designing and developing bifunctional catalysts on the air electrode side. Surface/interface engineering (including structural engineering, heteroatom doping, and defect generation, etc.) are used to optimize the electronic structures of carbon-based materials and transition metal-based materials, resulting in bifunctional catalysts with good electrical conductivity, more active sites, higher intrinsic activity and stability; thereby further promoting the commercialization process of Zn–air batteries.